J1IIeillll� &NID CCJ (]) J1 (]) lli by CLARENCE RAINWATER Professor of Physics San Francisco State College Original Project Editor
HERBERT S. ZIM Illustrated by
RAYMOND PERLMAN Professor of Art, University of Illinois
�
GOLDEN PRESS
•
NEW YORK
Western Publishing Company, Inc. Racine, Wisconsin
FOREWORD
This Go lden Guide sin g l es out the phenomena of light and color and describes the scientific concepts in easily understood terms. Light and color are intimate ly invo lved in our lives yet a rea l understanding of their n ature is rare. This book presents in simpl e terms the complex physica l, physio logica l, and psychologica l aspects of light a n d color. To condense this subject into this sm a l l book req u ired some sacrifice, so many details a n d qua li fying remarks have been omitted, a n d m uch of the data has been presented in simp lifi ed form. We are grateful to the individua l s and organizations who generously sup plied data and loaned pictures for our i l l u strations a nd to the author s of the many excel lent books wh ich were drawn upon for idea s and i n fo rm ation ( B i bl iography o n page 1 5 6 ) . We are gra tefu l a l so to Ja mes Hath way, Ja mes Ske l ly, and George Fichter for th eir ed itoria l assistance a nd to Dr . Frederick L. Brown for his cri tica l review. Photo CNdits:
MI. Wilson & Palomar Observatories, Copyright b y California Institute of Carnegie Institute of Washington, 7, 74, 75; Clarence Rainwater, 30, 56, 79, 92, 93, 95, 97, 119, 144; Enid Kotchnig, 31, 146; Ealing Corp., 46; 0. C. Rudolph & Sons, Inc., 54; Yerkes Observatory, 72; lnst�ule for International Research, 96; Florida Development Commission, 99; Roger Behrens, 109; ''The Printing Industry" by Victor Strauss, 112; American Optical Co., 116; redrawn from Scientific American, 118; from "An Introduction to Color" by R. M. Evans, 120; Munsell Color Co., 127; Contai· ner Corp. of America, 1 29; painting by Louis M. Condax from "The Science of Color," Optical Society of America, 132; The United Piece Dye Works, 140; Western Electric, 142 (bot. left); Edward Diehl, 143; Original Dufaycolor by Blanche Glasgow, American Museum of Photography, 145; Elizabeth Wilcox (Polaroid), 147 Nati:>nal Gallery of Art, W ashington, D.C., Chester Dale Collection: detai l from Self Portrait, 1889, by Paul Gauquin. 149: Perkin-Elmer Corp., 152 (tap); Optics Technology, 152 (bot.). Technology and
;
GOLDEN,® A GOLDEN GutDE,® GoLDEN PREss® and GoLDENCRAFT"i are trademarks of Western Publishing Company, Inc.
©
Copyright 197 1 by Western Publishing Company, Inc. All rights reserved, including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge retrieval system or device, unless permission in writing is obtained from the copyright proprietor. Produced in the U.S.A. by Western Publishing Company, Inc. Published by Golden Press, New York , N.Y. Library of Congress Catalog Card Number: 69-11967 ISBN 0-307-63540-6
CONTENTS NATURE OF LIG.HT AND COLOR
.
.
4
.
.
18
Measurement, speed of light, electromag netic waves, spectra, rod iation LIGHT SOURCES
.
.
.
.
.
.
.
.
Sun, electric lights, glow tubes, mercury arc, fluorescent lamps, luminescence ILLUMINATION
Sensitivity
of
eyes,
brightness,
metric units, shadows, lightness
Ll G H T BEHAVI 0 R
Transmission,
.
.
.
reflection,
.
.
.
photo
.
refraction,
.
.
29 36
dis
persion, diffraction, interference, scatter ing, absorption, fluorescence, phospho rescence, polarization, double refraction
OPTICAL INSTRUMENTS .
Mirrors,
prisms,
.
.
diffraction
lenses,
aberrations,
scopes,
projectors,
.
.
.
58
gratings,
telescopes, enlargers,
micro
cameras,
photometers, colorimeters
S E E I N G Ll G H T A ND C 0 L 0 R .
.
.
84
Eye, sight, depth perception, illusions
THE
NAT U R E 0 F C 0 L 0 R .
.
.
.
.
98
Hue, brightness, primary colors, comple mentary hues, additive and subtractive colors, color· matching, color blindness. COLOR PERCEPTION
Color constancy, contrast, afterimages
118 125
COLOR SYSTEMS
Munsell, Ostwald, CIE LIGHT AND COLOR AS TOOLS
Harmony and discord, symbolism, paints,
133
pigments, dyes, business and industrial uses, photography, printing, television, lasers, fiber optics MORE INFORMATION INDEX
.
.
.
.
.
.
.
156
157
Natural light dispersion-a double ra inbow
NATURE OF LIGHT AND COLOR
We know the wor l d th ro ugh ou r sen ses: s i g ht, h earing, to uc h , taste, and s m el l . Ea ch sense re spo n d s to pa r ti cu lar sti m u l i , a nd the se nsations we experience give us i n fo rmation a bo ut ou r surro u n d i n g s . Sight is the most importa nt of the sense s . Th ro ug h sight we pe rceive the shape, size, and co lor of ob jects; a l so the i r distance, motion s , and re lationsh ips to ea ch other. Light· i s the sti m u l u s for th e sense of sight- th e raw ma te ria l of vi sion. To u ndersta nd the fa scinati ng sto ry of l ig ht, l et u s ex plore its natu re , its ma nifestation o f colo r, its behavior in l e nses and prism s , and th en its uses i n science and art. This wi l l h e l p i n u nd erstand i ng how th e sen sat ion of see ing affects o u r· action , ou r attitudes, ou r moods, and ou r dai ly experiences. CO LOR i s t h e essence o f l ight; light the essence of li fe . The green pigment o f pla nts plays an essen tial role in susta i n i n g a ll li fe . The colo rs of many an i mals blend with their s u rroundings, h i di ng the animals from thei r enemies. Some, l ik e th i s anole, ca n even cha nge their c olors a s they move from one bockgrou nd to a n oth e r .
4
Man has put l ight a nd color to wo rk i n m any way s . Physicians detect d i seases b y changes i n t h e c o l o r o f eyeba l l s, throat, or ski n . The acid i ty o f a so l ution, the composition of an a l l oy, the tem perature of a fur nace, and the vel ocity of a distant star ca n be de termined by a color or a color cha nge . Decora tors choose restful colors for bed room s , brighter colors for work area s. In adve rti s i ng , a colo r entice s the consumer to c ha nge his b ra nd of b reakfa st food . Light a nd color give m ea ning to everyday contacts betwee n man and h i s wo rld i n many way s . L i g h t and color involve phys i ca l , physiolog i ca l , and psychological facto rs . Physici sts deal with the energies a nd frequ encies of light waves a nd the i nteraction of l ight with matter. Physiologi sts study visual processes and psychologi sts study the effects of visual and color perception. These th ree g roups of scienti sts developed different viewpoints and d i fferent voca bularies i n ta l king about light and color. After tong study, a comm ittee of the Optica l Society of America reconci l ed the d i fferences and set up a c lea rly defined and con s i sten t ter m i no logy.
A-,:{
[ t 0��� ]:1;.J
A prism disperses l ight to form a spectrum in a lobo- op" ·=--·- ' , ratory spectrograph j u s t os light dispersed by ra in, drops forms a rainbow.
··
H I LGER SPE CTR OGR APH
5
SCIENTIFIC MEASUREMENTS in volve large and small numbers, i nterrelated u n its, and great prec ision. Our everyday u n i ts of m easurement come from the E ng l i s h system with its i nche s, gallons, and pounds and are convenient to use only be· cause they are fam il iar. The met· ric system is favored by scientists beca use. the relation s h i p between units of length , volume, and
weight i s simpler. The syste m ' s u s e o f decimals a l s o makes for faster and more accurate computa tion s . larg e num bers can be e xpressed concisely. Metric units of length a re used i n thi s book . The table below l ists some common units, uses, sym bols, c om pa ra tive val ues in meters and in i nche s , a n d common objects of each u n i t ' s approx i mate size.
UNITS O F LE NGTH
Unit
Symbol
Equivalent in
About the
meters, inches
size of
MET E R mea sures rad io wav e s
m
1 m 3 9. 3 7 inches
A small boy
CENTI METER measures ' m ic rowaves
em
. 01 m (10-2 m ) 0 . 3 9 37 i n .
A s unflower seed
MilliMETE R mea sures m ic rowaves
mm
.00 1 m ( 1 o-3 m ) .03937 in.
A gra i n of sand
.000001 m ( 1 0 .0000 3 9 i n .
MI CRON mea sures i n frared MilliM I C RO N mea sures l ig h t waves AN GSTR OM mea sures u ltra v io let and l ig ht waves
6
m)
(.i
A small bacterium
. 00000000 1 m ( 1 0-9 m ) . 0000000 3 9 i n .
A
. 000000000 1 m ( 1 0-10 m ) A hydrogen . 000000003 9 i n . atom
A be"'e"" molecule
0
*
mp.
N u m bers in this book are often g iven as powers of 1 0 . For ex ample, 103 is 1 ,000 ( read 103 as ''1 0 to the third power' ' ;
6
-
(I
t h e 3 is cal led an e xponent), and 1 06 is 1 , 000 ,000 . Negative e xponents a re fractions or dec i mals; 1 0-3 is 1 11 ,000 or . 00 1.
The Andromeda nebula i s so far away tha t l i gh t from it takes about two and a ha lf m i l l ion y ea rs to reach the earth . Studies of l ight from such ce l estial bod i e s give cl u es to the structure of the u n iv erse .
OF LIGHT i n free space ( a vacuum) i s 1 86, 2 8 2 m i l es* p e r second . T h i s seem s to b e th e natural speed l i m i t i n the 1,mive rse . There i s g ood rea so n to bel i eve that noth i ng con ever travel foster. The speed of light in a vacuum i s a con sta nt, a lways denoted by c i n eq uations, as in E i nstei n ' s en ergy eq ua tion, E = m c2• No matter what th e source o f l ight, or how fa st the source and observer ore movi ng with respect to one a nother, the speed of l ight i n free spa ce is a lways the some . Thi s r ema rkable fact- i s bel i eved to be true only of light. Th e s peed of a bul l et, for exampl e , d epen ds i n port on t h e speed of the g u n from wh ich it i s fired and on the speed of the ob server as wel l . The speed of sound varies with the speed of the measu rer but not with the speed of the sourc e . The speed of l ight i s independent of both source a nd m ea surer. It i s a u niversal constant, one of the m ost i m portant constants i n a l l of sc ience . The constancy of the speed of light i s a ba sic postu late of E i nstei n ' s theory of rel a tivity . THE SPE E D
•
Approx imotely 3 X 1 01'( meters per second
7
carry energy in a l l d i rec tions through the universe. Al l objects receive, a bsorb, a nd rad iate these waves, wh ich can be pictured as electric and mag netic fields vi brati ng at right angles to each othe r a nd a l so to the di rection in wh ich the wave is t rave l i ng. Light is on e form of e l ectromagnetic wave . Al l electromagnetit waves tra vel i n space at the same speed-the speed of lig ht. Electromagnetic waves show a continuous ra nge of frequen cies and wa velengths ( pp. 1 0- 1 1 ). Frequ ency i s the num ber of wa ve crests pass i n g a poi n t i n one s econd. E lectromagnetic wa ve freq uenc ies run from a bout one per second to over a tri l l ion-tri l l ion ( 1 024) per second. For l ight, the freq uencies a re fou r to eight hundred tri l l ion ( 4-8 x 1 014) waves per second. The freq uency times the wavele ng th gives the speed of the wa ve. The higher the freq uency the shorter the wave l ength. ELECTROMAGNETIC WAVES
ELECTROMAGNETIC STRUCTURE OF LIGHT WAVES
magnetic fiel d
8
E l ectric a n d magnetic fields ar• a lways pe rpendicula r to eac other and to the d i rection o motion .
A "SNAPSHOT" OF A GREEN LIGHT WAVE
0
1 0,000 A
5 ,000 A
WA VELEN GT H is the di stance from the crest of one wav e to the crest of the next. The height of a wave crest is its ampli tude and
1 5 ,000 A
is re lated to the energ y wav e . T h e wave shown di ag ra m is a g reen l i g h t Di stances are i n Ang stroms
of the i n the wa ve . ( p. 6 ).
with wave lengths less than O 1 A, are pen etrati ng radiations that are absorbed ve ry l i ttle in pa s s i ng through so l id matter . Th e amount of th e a bsorpti o n d epends upon th e d e n s ity o f the mate ria l , so these ray s are u seful for maki ng shadowgram s (X-ray pictu re s ) of th e den ser parts of an ob ject. GAMMA RAYS AND X-RAYS,
ULTRAVIOLET RAYS are produced i n great quanti ty by th e sun and by spec ial ty pe s of l am p s . Thoug h not de tected by the eye, th ey do affect ph otog raph ic fi l m . Th ey a l so ca use sunta n . Ord i na ry glass do es not tran smit much u l traviolet so yo u d o not ta n b ehi nd an o rd i na ry win dow. The wave l e ng th s of u l travi o l et ray s ( 1 0 A to 3,5 00 A) a re longer than th ose of X-ray s bu t shorter than th ose of l i g ht . A bit longer tha n l i g ht are the wave s of infra red rad iation wh i c h we sense as h eat.
9
LIGHT WAVES I N T H E
frequ ency i n cycles per second
i ndu ction . heating
power
3
X
1017
1()8
1()4
102
1015
1013
1010 i nfrared rays
radio waves
1011
1012
109
107
wavelength i n Angstrom un its
is that portion of th e e l ectromagnetic spectru m that norma l l y stimulates t he sense of sight. Electromagnetic wa ves exh i b i t a continuous range of frequen cies a nd wa velengths. In the vi s i b l e part of the spectrum these frequencies a nd wa ve lengths a re what we see a s colors . The wavelength s of l ight ra nge from 3 , 5 00 A to 7,500 A. The wavelengths of i nf rared rays ( 7,500 A - 1 0,000,000 A), longer tha n l ig h t rays, are not detected by the eye, and do not apprec iably affect ord i na ry photog raphic film. They are a l so ca l l ed hea t or the rmal rays and give u s the sensa tion of warmth. Because all electromagnetic wa ves a re basica l l y a l i ke, we can expect them t o behave i n a s i m i la r m a n n e r . Differences are rea l l y b u t a matter o f deg ree a n d a re due to t h e d ifferen ces in freq uency. Light waves a re u nique o n l y in thei r visu a l effects . The con cept of color, for in sta nce, ha s mea ning only i n reference to l i g ht waves.
VISIBLE LIGHT
10
106
ELECTROMAGNET I C SPECT R U M
1Ql4
1Q20
1Ql8
1 Ql6
1Q22
1Q24
LIGHT
yel low g reen
green
blue-green
THE ELECTROMAGNETIC SPE C TRUM, shown above, covers a l l known rad iation o f w h i ch l ig h t is a s m a l l b u t i mportant part.
The expa nded v i s i bl e portion of the el ectromagnetic s pectrum is seen as a continuous g radation i n hue from red through violet.
WAVELENGTH V I SI BLE R AYS
OF
The wavelengths ( .,\ ) of v i s i bl e l ight are as s ee what we colors. Red has the l ongest waves, v iolet the shorte st.
55
00
����������������
7000
1 1
.
.
�,.
�;.., .
..
. �-?r; ;'ib
J
4000 A
._;��
I
5 000 A
4 5 00 A
Spec trum of sunl ight s howing
( p. 5) sepa rate s a bea m of l ig ht into its component wa ve l engths and d i splays th em in a spectr u m . The spectrum of an i ncandescent l a m p is a con ti n uous ba nd of colors ra ng i ng from violet through bl u e, g reen , ye l low, orange, to red . Each color blends into its neig hbors i n a n un broken ba nd of wa ve l engths . Sol i d s , liqu ids , and gases at ve ry h i g h pressure g ive con ti n uous spectra i f they a re made hot e no ug h. Ga ses at low pres s u re have disc rete spectra consi sting of colored l i n es or bands with dark spaces between. A ga s gives a l i ne spectrum if its mo l ecu le con s i sts of a sing l e a tom, a ba nd spectru m if it consists of more th an one ato m . The gas can be identified by the pa ttern of its spe ctral l i n es or bands, a nd its tem pe ra tu re can be dete rm i ned by the i r relative i n tensities.
A SPECT ROGRAPH
TYPES O F SPECTRA
Conti nuous spectrum o f i n candescent lamp " "
'
:;-� .� :"' t...,· > .
"•
�.�:���
. \�:·',.
Line spectru m of barium
:
;
I
I
Band spectrum of carbon a rc in a i r
!
I
I'
: ,
!,
i· II
I
5 5 00 A
-·� '
'
'
�,:"�., ,. .
" .. '"·'
���t: . •.
6000 A
-
- �"'
.
•.
6 5 00 A
7000 A
principal F ra u n hofer l ines ( p . 2 1 ) .
If a l i n e o r narrow band of the spectrum i s i solated by a s l i t or by colored fi l ters, the l ight that comes through is ca l l ed monochromatic l i g h t . It con s i sts of a single wa ve l en gth or a very narrow range of wa vel engths a nd excites in the ob server a sensation o f color, such a s red, g reen , or b l u e . White l ight, such as s un l i ght, i s a m ixture o f a l l visible wavel ength s . An object that re flects a l l wave l engths equa l l y appears white to ou r eye s . Each l ight source e m i t s a cha racteri stic spectrum which can be plotted on a g ra ph showi ng how the re lative energy va ri e s with th e wa ve length. This relation ship i s ca l l ed th e spectra l energy d i str i bution c u rve for th e l i g ht source. Most so urces a l so give off invisible ultravio let a nd i n frared rad iation, so the comp lete spec tr um usu a l l y i n c l u des mo re th an j ust vi s i b l e l i g ht . SPECTRAL E N ERGY D ISTR I B U T I O N C U R V ES
2 00 1 60 >-. Ol
4; 1 2 0
c w
.�
4)
0
Qj
80
a::
40 0
4000 A
5000 A
6000 A
Wavelength ( in Angstroms )
7000 A
13
is a m easure of the ra te of random motion s o f mo l e c u l e s . Abso l ute zero i s th e tempera tu re a t wh ich all s uch m otion s a re at a theoretical mi n i m u m . The Kel vi n , or a bsol ute, te mperature sca le, wi dely used in scientific wo rk, sta rts at absol ute zero. Th e freez ing po i nt of wa ter i s 273 ° K, a nd th e bo i l i ng po int is 373 ° K. Eq uivalent tem pera tu res on th e Fa hren he it ( F) a nd Cel s i u s ( C ) tem pera tu re scales a re shown at left. Tempe ra tu re a nd the color of a hot object a re ofte n c l o sely related. As a piece of i ro n i s heated it changes in color from g ra y to red, to ora ng e, to ye l low, a nd fi n a l l y to wh ite . TE MPERATURE
F
c
K
is be ing cont i n u a l l y ex cha ng ed betwee n every object and its surro un d i n g s . The amount and qua l i ty of this radi ati o n d epends upon th e te mpera tu re a nd material of both th e em itter a nd the ab sorber. When two objects a re at a bout the same te mperature, _ l i tt le heat i s transferred between them . Whe n o ne object i s much hotter tha n th e other, heat fl ows to th e colder one. This occ u rs wh en you h old o ut you r ha nd a nd fee l th e warmth of a hot stove. Yo ur ha nd rad iates less en ergy tha n it recei ves . Th e rate at which an object em its th i s rad iant energy i s proportional to th e fo urth powe r of its Kelvin tempe ratu re . Do ubl i ng thi s tempe ra tu re i ncrea ses the ra te of ra diation 1 6 times.
TEMPERATURE SCALES
14
RADIATION
is a n a b so l utely b l a c k body. It a bsorbs a l l the ra diation that fa l l s on it. Every light source is a radiator, but some a re m o r e efficie nt tha n others . A n o bject that is a good a bsorber of radiation is a lso a good emitter. A s m a l l d ee p h o l e o r cavity in a g ra p hite block serves a s a practica l b l a c k body. Any light that enters the hol e i s reflected m a ny tim es fro m t h e wa l l s a n d is pa rtly a b sorbed at e a ch reflection u- n til no light remains. Th us the hol e a p pe a rs perfectly b l a c k . If the wa l l� of t h e cavity a re heated, however, they give off radiatio n in a l l directio n s . The ra diatio n -that esca pes fro m the hole is ca l l ed b l a c k-body radiatio n . T h e spectra l distribution of radia nt e n e rgy e mitted by a heated b l a c k body d ep e n d s o n ly on its Kelvi n t e m p er ature, and not a t a l l o n the materia l of which it is m a d e . At low te m perature (bel ow 800°K) o n ly infra red radia tio n results. At a bout 6,000°K (th e tem p e rature of the su n's surfa c e), the peak of the spectra l e n e rgy c u rve is nea r the mid point of the visi b l e spectrum. Both ultra vio l et and infra red radiation a lso o c c u r . THE BEST RADIATOR
The standard u n it of l ight sourc e i ntens ity, the cand le, i s 1 I 60 of the intensity of 1 cm2 of a black body at the tempera ture of melting platinum .
Btack Body Radiation EN ERGY DISTR I BUTION
1 0,000 A
O F A BLAC K B O DY
At a g iven tempera ture , there i s a spec ifi c curve that represents the energ y distribution of o black body. At higher tem pera 1 00 tures, the pea k of the curve >E> occurs at s horter wavelengths. 4) &5 5 0 4)
·�
0
�
0
may be a ss igned to any l ight source by m a tc h i ng i t vi s u a l l y aga i nst a b lack-bod y rad iator . The temperature at wh ich the black bod y matches t h e c o l o r of a light source i s s a i d to be the color tem pe ra ture of the sou rce . For incandescent sources, such as an ordi nary ho usehold l ight bu l b, the color tem perature is rel a ted simply to the true tem pe ra ture and i s often a pproxi matel y equ a l to it. An ob server sees the sta r Antares, with a col or tem pe ra tu re a l most 5 ,000 ° K, a s red . Sirius, at about 1 1 ,000 ° K, i s much hotter a nd appears wh i te . The color tem pe ra tu re of some l ight so urces, however, ha s nothing whatever to do with t he actu a l temperature. A ' ' daylight' ' fl uo re scent tube, for example, may have a ra ted color tem perature of 6 , 5 00 ° K a nd yet be so coo l that i t is not un com fortable to touc h .
COLOR TEMPERATURE
CO LOR-TEM PER ATU RE CLASS I F I CA T I O N OF STARS COLO R 25
B lue white
W h ite
Yell ow wh ite
Y e l lo w
8
A
F
G
,."""'··· • i� I
•'
"'
.
��·�··�!}� :�<���.:,=�
THR E E STANDARD IZE D LIGHT SOU RC ES, known a s A, B, a nd C, a r e u sed t o match col o rs . Sou rce A ap prox imates ordi n ary inca ndescent lam p l i g h t, sou rce B noon sun l ight, and source C average day l ight. Th e sta n da rd i n candescent lamp opera tes at a color te mpera tu re of 2 , 854 ° K. The othe r two standa rd sou rce s a re de rived from this same lamp by the use of carefully specified fil ters . Each source may be considered white in pa rticula r i n sta nc e s . White l i g ht i s known to be a mix tu re of a l l colors ( p. 1 3 ) . These sou rce s of white l i g ht diffe r becau se they are made u p of different amounts of th e various colors . THE RELATIVE EN ER GI E S of the th ree internationally accepted l ight sources are pl otted for the visible region of the spec trum .
Wavelength
5000 A
The curve for the standard in cande scent lamp, because of its tungsten filament, is the same as that for a black body a t 2 ,8 5 4°K.
6000 A
7000 A
200
150
50
0
17
E FF I C I E NC Y O F SOME
paraffi n cand le oil lamp ca. 600 A . D. E FF ICIENCY
0 .1 %
0 .1 %
0 .1 %
185 3
0 .1 %
LIGHT SOUR CE S
L ight sources turn other ki nds of energ y into vi s i b l e radiation . Th e sun u ses nuc lea r energy. To rches, candles, gas la mps, and other fla mes use chemica l energy. Most of our modern light sources use electrical energ y. In the process of making light, most sou rces wa ste much energ y in the form of heat. F lames a re very i neffi cient, b ut for many centuries they we re the only contro l l ed so urces o f light. The i nvention of the Wel sbach mantle, i n 1 8 6 6 , inc rea sed grea tly the light o utput o f the com mon ga s lamp. The mantle, a sma l l wh i te wic k l i ke cover placed over the gas flame, is made of tho r i u m oxide to which a l i ttle cerium has been added . When h ea ted, i t emits visible l i g h t b u t n o t i nfra red rad i a ti o n . U n a b l e t o lose ene rgy by i nfra red emi ssion a s d o most hot ob jects, its tem pe ra tu re ri ses to a lmost the temperature of the fla me, a nd the mantle g i ves off a b ri l l ia nt white l i g ht. If a black body ( the most efficien t rad i a tor) were pl aced in t he same flame, it wou l d lose so much energ y by infrared em i ss ion that it wou ld rem a i n re lative l y coo l , g ivi ng o ff m uch less light tha n the Wel s ba c h mantl e . Carbon a rc la mps ( high c u rrent e l ectrica l d i scha rges between ca rbon e l ectrodes) came i nto use for· public street lighting a bout 1 8 7 9 . Carbon arcs today a re used in powerfu l sea rc h l ights a nd commercia l m ovie projector s . I n 1 8 79 , Thomas Edi son invented t h e i n ca ndescent fi lament la mp which, in m uch i mproved form, i s sti l l our 18
H I STOR I C LIGHT SO URCES
open
i ncande scent filament l amp 1 8 79 0.2%
1 . 0 - 2 .0%
2 .0 - 5 . 0 %
Coope r-Hewitt m ercu ry a rc 1 90 1 1 . 5 -4 . 0 %
most com mon source for home l i g ht i ng . The g low tube fi rst appeared i n 1 8 5 0 a s a la bora tory device for the study of e l ectric d i scharges, but it has since become more fa m i l i a r i n the commercial "neon " s ig n . Th e Cooper-Hewitt �ercury arc of 1 9 0 1 , a nother tu be u s i ng electric d i scha rges, wa s on e of the a ncestors of m odern fl uo rescent l a m p s . L i g h t from a n i ncandescent lamp i s m o r e concentrated tha n that from a fluorescent l a mp a nd seem s brig hter, but it is only about one-thi rd a s effi cient. A carbon a rc i s l ess efficient and less conven ient tha n either, but provides a hotter po i nt source and is far brighter.
li ght from a We lsbach mantle appears more l ike daylight than el ectric i ll u minatio n . But e lectr ic lights a re cheaper , last longer, and are easier to u se .
photosphere
SUN
ou r major l ight sou rce , gets its energy from nuclear processes i n its hot i nterior, whi c h is est ima ted to have a tem pera tu re of about 1 3 m i l l i o n deg rees K. The sun i s a b a l l of g l owing gases that diffe r from tho se fou nd on ea rth because of the su n ' s extre m e l y high pressu re s a nd tem peratures . T h e a tom s o f the sun ' s gases a re h i g h l y ionized; that is, ma ny of the e l ectrons which no rm a l l y sur ro und the nuclei of the atom s have been stri pped away, l eaving the a toms. e l ectrica l l y cha rged. The se ioni zed gases at h igh pressu re e m i t a continuous spectru m i nstead of a l i ne spedru m . Th e waveleng th a t wh ich we receive the m ost energ y from ' the sun ( the peak of its energ y d i str i bution c u rve)- is a bout 5 , 400 A. Thi s l ies in the g reen pa rt of the spec trum, c lose to the wavelength at wh i c h the h u m a n eye has its g rea test sensitivity. THE SUN,
20
SCATTER ING O F SUN LIGHT by the atmosphere i s m uch greater fo r short ( b lu e ) waves than for long (red ) on e s . When the s u n is overhead, the atmospheric pa th is re lati ve l y short, and the sun appe a rs b right yellow, while
the scattered l i gh t is blue (p. 50). At sun set, the s u n l ight loses more blue ra ys by scatteri ng in its l onger path through the a tmo s'phere, and the s u n a ppears red . The reflection of th is red light from clouds makes the sky pink.
The photosphere , the o uter su rface of t he s u n ' s cen tra l core, i s the su n ' s major source of l ight. Tem perature of the photosphere ( m ea sured by a n optica l technique ca l l ed rad i a tion pyrometry} averages about 5 ,750 ° K. S u rrou nd i ng the photosphere i s the chromosphere, composed of cool e r ga ses tha t absorb some of the photosph ere ' s rad ia tio n . Th i s causes da rk l i nes, ca l l ed Fra u nh ofer l i n es ( pp . 1 2- 1 3 }, i n th e continuous spec tr um of th e sun . From m ea s uremen ts of these l i nes, gases o f the chromosphere can be ide ntified. 21
the most fam i l i a r man-made light sources, con s i st of a coi led fi l ament of tung sten wire sealed in a glass bu l b fil led with a m i xture of a rgon and n i trog en gases. Pas s i ng a n el ectr i c c u rrent th rough the fi l a ment heats it to a tem perature of a bout 2, 900° K. Th e i n ert gases surround i11g the fi l a m e n t preven t it from b u r n i n g u p and pro l on g i ts l i fe by retarding its eva poration . Th e en ergy d i stri b ut ion of the l ig ht from a n i n can descent lamp d epen ds on the fi lament tem perature, so the light ten d s to be redder than s u n l ight ( 5 , 75 0° K) . I n fact, most of the rad ia tion from a tungsten lamp l ies in the i n frared pa rt of the spectrum . Over 9 5 per cent of the energ y is radia ted as heat and less than 5 per cent a s light. I n spite of th i s wa sted heat en ergy, th e m od ern l i g ht b u l b i s nearly 1 0 t i m e s as efficient as the first commercia l carbo n fi lament la mps . Th e early clea r-gla s s b u l b s have been rep laced with b u l b s of frosted or tra n s l uce nt g lass, ofte n ti n ted. These newer bu l bs provide p l ea santer i l l u m i n ation by diffu s i ng the l ig h t a nd softe n i n g s h ad ows.
INCANDESCENT LAMPS,
Spectral energy distribution o f radiation from tu ng sten l a m p at 3,000° K e lv in. N ote how li ttle en ergy is in the vi s u a l rang e .
>.. 0>
Qj
c w
c .!2
"'0 0 c.::
120
80
40 0
22
Wavelength
10,000 A
20,000 A
30,000 A
Over two and a half bil lion i n ca ndesce nt lamps a re consumed in the U n ited S tates every year . Most of these are of the simple filament lamp type and range in s i ze fro m a tiny surg ical lamp 1 / 3 i n . long to a mam moth
demonstration bu l b 2 0 i n . ( 5 1 em ) by 4 2 i n . ( 1 0 7 em ) . Although some 2 0 , 000 lamp types are actual ly prod u ced, mos t l arge fi l ament l amps are l i k e the one shown below. filament
l am p base
I
'
I ·I
I I
I ,
r
!
j
'
·
,
1 ' · :
'
' ' ''
.
1
I
:.
' I
I
I'
II : ,l·1 I
I '
·.
'
�
1'1
'
'Iii I: •
,
high voltage t ransformer
A neon sign and electrical connections
have many uses i n scientific laboratori es but a re best k nown as commerc ia l n eo n signs. These thin tubes h ave m etal e l ectrodes sealed i nto eac h end . The a i r i s pumped out of the tube wh ich i s then fi l l ed with neon or a nothe r gas a t low pressure. Whe n a high voltage ( 1 ,000- 1 5 ,000 volts) i s appl i ed to the e l ectrodes, stray e l ectrons i n the tube a re ac celerated to high speed s . These col l i d e with the gas atoms, knoc k i n g out other electrons , which i n turn stri ke more ato m s . Thi s cascading of e l ectrons ( shown at right) becomes th e electric cu rrent th at Aows th rough the tllbe . The fast-moving e l ec tron s tra nsfer energy when they col l id e with n eutral gas atom s . The col l i sion may ionize the a tom by knock i ng out one of its elec trons ( 1 ) , or excite the a tom by movi n g a n e l ectron to a pos i tion of higher en ergy in the atom ( 2 ) . Energy i s emitted a s a l ight photon when the atom retu r n s to its norma l sta te ( 3 ) . When a free electron is captu red by a n ionized atom ( 4 ) it g ives u p i t s energy a s a photo n . Pho ton s a re u n i ts o f l ig ht energy that act l i ke pa rticles. The
GLOW TUBES
24
H
I
�
,.
� f
,
Ill
Ch a ra cteristic line spectru m of neon gas
energy of eac h ph oton depends on its frequency. The freq uency of the emitted photon i s p roportional to the en e rgy lost by the electron i n fa l l ing to a l ower energy state or th e energy g ive n up by the e l ectro n captu red by the ion i zed ato m . . Each kind of a tom when excited gives off l ight at frequencies d eterm i ned by i ts structu re . Some gases emit l ight only at a few wave l engths . The spectrum of sod ium, fo r example , shows two bright yel l ow l i nes very close together, a nd a sod i um va por lamp g i ve s out a yel low light s i nc e the b rig htest l i nes i n the spectru m determine the color of the l ight from the g low tu be . S od i u m vapor l amps a re u sed i n stree t l i g hting . N eon tubes g l ow an orange-red . Tu bes fi l led with krypton give a pale blue l i g ht . Other gases or m ixtures of gases a re ofte n u sed , someti mes in ti nted or fl uorescent g lass tubes which modify the color of the l ight. CASCADE
R EACTION
nitrogen-filled
bulb
This high -pre ssure lamp con tains an arc tu be within a nitrog en-fil led
!;>u l b .
i s very much l i ke a g l ow tube, but uses higher c urrent so tha t th e el ectrod e operates at a red heat . If the pres sure of the merc ury va po r i n the tu be is h i g h (from 1 I 1 00 atmosph ere to several h undred atmosph eres ) , the arc wi l l g ive off a n i n tense bl u i sh wh i te l ig h t as wel l as some ultravi o l et . If th e pressu re i s low (from about 1 I 1 00,000 to 1 I 1 ,000 of a tmo sph eric pressure), most of th e ra diation w i l l be in the ultravio let . Sun lamps a re u s ua l l y low-pres s u re merc u ry arcs en c l osed i n bu l bs of fused quartz . H i g h-pressure lamps a re frequently u sed fo r street l i g h t i n g o r fo r ma k ing bl u epri n ts .
THE MERCURY ARC
FLUORESCENT LAMPS u s e a low-pres s u re merc u ry arc within a g lass tube coated on the i nside with a phos phor such a s c a l c i um tung sta te . Pho sphors have the abil ity to absorb u ltraviolet rays and to re-rad iate the energy a s light. The spectrum emitted by a fl uorescent tube depends l a rgely on the mixture of fluorescent chemi cals u sed i n the phosphor coati ng. A ca refu l selection of phosphors wi l l produce rad iation with broad ba nds in th e visible reg ion of the spectru m , and ve ry l i ttle in the u l t ravio let o r i n frared . Proper phosphors can approx i mate th e color of sun l ight without h i gh te mperatures .
26
A F LUORESCE N T TUBE (cross section) glows when ultra violet
rays
excite
the
electrons
of the phosphor coating. As these excited lower emitted.
electrons energy The
drop
levels, color
of
back
to
light
is
the
light
depends on the structure of the phosphor. ultraviolet light
-
visible light
RA PI D -START
fluorescent
lamp , combining light from phos
cuits are
efficient.
phor and
last
THE SPECTRUM
of a fluorescent
that of sun I ight.
mercury,
approximates
in
more the
circuit
cir
A bal
eliminates
flicker.
fluorescent coating
:.1;���l;'·>:· '
-----lf'��..
.
SPECT R AL EN ERGY CURVE Compare the sp ectral energy curve of a 40-watt "daylight"
100
iJ; 80
Qi �
60
fluorescent lam p with those of g;' 40 the average daylight curve ''§ and of a tungsten l amp curve
(� 22) .
�
20
0 Wavelength
4000A
6000A
27
o r the e m i s s ion o f l ig ht, may b e due to causes other than heat ( thermolum i nescenc e ) .
LUMINESCENCE,
C H EM I L U M I N ESCEN CE i s the emission of light during a chemi cal reaction. When a formalde hyde solution is m ixed into an alkaline alcohol solution, chemi cal energy i s changed into light, cau s i n g the mixture to g low. BIOLUM I N ESCEN CE is the pro duction of l i gh t by che m i l u m i n es ce nce i n l iv ing organ i sms, as in ce rtai n fu n g i, bacteria, comb j e l l y fish ( left), fireflies, and fishes. T h e Ra ilroad W o r m , a beetle larva , is biolum inescent in two co lors. FLUORESCENCE i s the produc tion of light when a s ubstance is exposed to u l trav iolet or oth er rad iation ( inc luding beams o f e lectron s o r o t h e r partic les ) . Most cases o f fluorescence are real ly examples of phosphorescence. PHOSPHORESCENCE is delayed fluorescence. The light emission continues for a time after the exciting radiation stops. T ele v i s ion tubes h ave a phosphores cent coating and thus produce pictures without a pparent flicker. E LECTROLU M I N ESCENCE offers a new sou rce of diffuse i l lumina tion for lighting. Alternating c u r rent appl ied to thin conducting pan e l s excite s l u m inescent mate rial sa ndwiched betwee n the m , producing a soft, easily regu lated g low.
28
ILLUMINATION I l l u m i nation i s often u sed as a general te rm that refers to the quantity and q ua l ity of l ight. The i l l u m i nation of a scene may be bright or d i m , harsh or soft, and perhaps even cold or wa rm . These terms refer loose ly to the amount, contrast, and hue ( color} of the l ig h t. In a nar rower sense, i l l um i nance i s the amount of l i ght received on a specified s u rface area . The range of sensitivity of your eyes to light i s so g reat that you are able to see c l ea r l y u nder widely different cond itions of i l l um inatio n . The ratio o f the i l l u m i nance at noonday to that on a moon l e s s night may be as great as ten m i l l ion to o n e . O n a c l ea r day there may be 2 0 times as much i l l u m i nation on the sunny side of a bu i ld i ng a s o n the shaded s i d e . Modern i ndoor l ighting for houses cal l s for i l l u m ination that i s a bout one fifth that fou nd on the shaded s ide of a b u i l d i n g on a clear day. Th i s i s 2 0 times more i l l u m i nation than was once cons idered adequate fo r homes . I m proved i l l u m ina tion g reatly inc reases the ease with which reading o r fine work c a n be done. The human eye can not d i stinguish the component wave length s of a l i g h t bea m , nor can it d etect s m a l l cha nges i n s pectra l d i stributio n . Neither i s the eye equa l l y sen s i tive to a l l wave l ength s . Meas u rements made w i t h many people have prod uced a standard l u m i n o s ity c urve (p. 3 0 } that represents t h e relative sensitivity of the average eye to different wave lengths of light. Rad iant energy, i n c l uding l i g ht, i s a physical quanti ty that can be measu red d i rectly by severa l types of rad ia tion detectors, such as thermopiles, bolometers, and wave meters. Visible l ight can be measu red by a pho tometer or a l i g h t meter. 29
·;;; 0 .s: E
�
:::> ...J
0.8 0.6
Q) >
0.4
0::
0.2
Qj
'"6
-� ..,
0 ·S
§
""
�'b
�?§'
,o ..
Wavelength
0 4000 A
5000 A
LU M I NOSITY i s th e abil ity of l ig h t to exc ite th e sen sation of brig ht ne ss ( p . 32) . Th e standard l u m i -
6000 A
7000 A
nos ity curve peaks at 5 , 5 5 0 A, indicating th at our eyes are most sens itive to yel low-g reen l igh t . VAR I AT I O N S I N
Outd oo r fi l m , outdoor l igh t
AR T I FICI AL LI G H T i ndoor s i s very diff e rent from outdoor l ig ht, yet you rarely notice th a t y o u r green sweater i s of a somewh a t diff er ent color as you come indoo rs . Just a s your eyes adapt to ch ang e s in ligh t intensity, so th ey adapt to ch anges in l ig h t qual ity. Color ph otog ra ph s sh ow the difference because, u n l ike your eyes , th e fi lm ca nnot adapt its sen s itiv ity to th e ch a nged i ll umi nation . F i l m des i gned for indoor use with in ca ndescent lamps produces a very blu ish or cold picture if used outdoors. Film des i gned for out door use res u lts in a very orang e or wa rm picture wh en used with artificial light i n door s .
Outd oo r fi l m , indoor ligh t I ndoor film, ou tdoor l igh t
30
I nd oo r fi l m , indoor ligh t
+
Dawn
Midmorning
I L LU M I NAT I O N DAYLIGHT cha nges color co n stantly from s u n rise t o s u n set. A co lor photog ra ph taken i n early mo rn i ng or in late afternoon wi ll have m uch warmer color s than one ta ken when the sun i s over head . Col or fi lm p roperly record s the diff erences in the co lo r of daylight, but human v i s i on sim ply compe n s ates for t h e diff er ence s . You not ice only extreme cha ng e s in the color of daylig h t, as at sunrise or s u n set.
M idafternoon
+
L ate a fternoon
i s a purely psychological concept. It i s a sensation of the observer a n d cannot b e measured by i n struments . The a b i l i ty of the eye to j udge absol ute va l u es of brig htness i s very poor due to its g reat powers of adaptation . The eye i s a very sensitive detector of brightness d i fferences, however, provided the two fields of view a re presented s i m u l taneously. The measurement of l ight by vi s u a l compari son i s the bas is of the science of photometry. Lig ht-source i n tens ity depends o n the tota l amount of l ight e m i tted and on the s ma l l ness of the con i ca l sol id ang l e i n wh ich i t i s emitted . Sta ted s i m p l y, i t is the amount of l ight emitted in a g iven d i rection . Brightness is a ssociated with the amount o f the l ight stim u l u s . It is the visual sensation corresponding to the perc eption of l uminance. L u m i na nce, the intens ity of l ight-sou rce per u ni t a rea, is a psychophysic a l property and can be m ea s u red. Lum inance a nd l ig ht- source in tens ity, often confu sed , are best de scribed by examples. BRIGHTNESS
T H E T W O LUM I N OUS SPHERES of d ifferent size ( be low left) have ide ntical light bulbs. They are diffuse emitters and so wil l ap pea r as luminous discs fro m any viewpoint. Both e m it th e sam e tota l a mount of light, and both hav e the same intensity . But the sma ller one wi ll appea r b righter to the ob server and wil l have the h igher l uminance.
In order for the two spheres to have the same luminance the larger sphere would require a larger ( more intense) light sou rce ( below right). It wou ld then emit the same amount of light per unit area of s urface as does the smaller sphere. The two spheres would then appear equally bright, but the larger one would have the g reater i n te n s ity.
T H E R ELAT I O N S H I P OF I N TEN S I T Y to con i ca l ang l e i s shown by co mparing a spot l ig ht and a floodl ig ht with identical light bulbs in reflectors of the same di ameter but dissim ilar shape (abov e ) . The spotlig h t bea m is concentrated into a smal l cone with rays almost pora l l el . The flood light, which e m its the same am ou nt of ligh t, s pread s it into a la rg e cone. The spotlight pro vides a m uch g reater i ll uminance but over a m uch smal ler a rea than the fl oodl ig h t .
I f w e look directly into the bea m of the light sou rce, the s potlig ht a ppears to be muc h brig h ter than the floodl ig ht . I n that d i rection on l y , the inten s i ty and the l u m i n ance of the spot light are g reate r than those of the flood light. If we l ook at the l ig h t source from other di rec tio ns out sid e the bea m, the inte n s i ty and lumina nce of the spot l ig h t are much less, because so much l ig ht is concentrated i n to th e beam that there is l ittle l eft to go in the se other directio n s .
are u sed i n the quantitative mea s u rement of i l l u m i nation. A few of th e more i m po rtant ones are g iven below for reference.
PHOTOMET RIC UNITS
LUM I NOUS FLUX i s the quan tity of light. I t i s mea sured i n lumens. I nten sity is luminous flux per u n i t solid ang le. It is measured in ca ndl e s . ( 1 ca ndle 1 lumen pe r u n i t sol id ang le . ) I l l u m inance i s inc ident fl ux p e r u n it a rea . It i s mea sured in lux. ( 1 lux 1 l u men per m eter2 . ) l u m i nance is intensity p e r u n i t area. It is m easu red in ca n dl es per meter2•
1
'
po i nt l ig h t . un1 t ource
=
=
o
1 eng I e I'd
''tJ.rr e, /tJ�
;":
o
s
+-
1 m
f ------�
1m
area
=
1
m2
point l ight
L ig ht th at i l l u minates a one-square-meter sur face at one meter wi l l cover four square me ters at two meters and spreads over nine square meters at th ree meters .
ra diates its energy u n i formly so th at l ig ht rays spread out from it i n a l l d i rectio ns. I l l u m i nation at a poi n t on a surface va ries w i th th e in te nsity and shape of th e l ig ht source and th e d i sta nce of th e su rface from i t . The amou nt of l i g ht fa l l ing on a u n i t a rea (the i l l u m inance) d ecrea ses with th e square of th e d i sta nce (the i nverse square law ) . A POI NT LIGHT SOURCE
SHADOWS a r e formed when l i g ht cannot pa ss through an opaq ue body i n its path . I l l u m ination of the area beh ind the body i s cut off. A sma l l ( p inpoint) or dis ta nt source of l ight casts a sharp shadow. A near, large, or diffuse l ight sou rce produ ces a fuzzy shadow with a centra l dark a rea that receives no l i g h t ( umbra) and a l i ght outer area ( penumbra ) which receives some l ight from part of the source.
- - -���� ---------'•'
point source
\II I -
�
�:�--;�ii.:�;:: d iffuse source
34
hand
------
--
--
i s a term u sed by an observer to d i stin guish between l ightness and darkness of colored ob jects, a s between l ight blue and dark blue pa i nt. It should not be confused with brightness (p. 32 ) . The observer's perception of l ightness is a l so a recogn i tion of a d i fference i n whiteness or g rayness between objects . I t i s a com pa rative term referring to the amount of d iffusely reflected l ight coming to the observer's eye from a surfa c e . The surface wi l l a ppea r white if it i s a good non-sel ective diffu se reflecto r a n d i s wel l i l l u m i na ted by white l ight. I f it i s a poor reflector or i f it receives l ittle or no i l l u m i nation, the su rface wi l l appear g ray, g rayer, or even black. B lack, then, i s the perception of an area from which the l ight i s i n s ufficient for deta i l ed vision . Wh ite i s the perception of a wel l - i l l u m i nated su rface whose reflectance i s high and non-sel ective . Gray i s the perception of a surface between these extremes .
LIGHTNESS
THE PE RCEPT I O N OF GRAYN ESS is influenced by, among othe r thing s, t h e illumination of the su rrou nd ing area. The gray area s at the bottom of the page are iden tical , but the one surrounded by
black appears to be much lighter than the one s u rrounded by white. Because o f the amount of light refl ected , the three areas be low a re seen as blac k , gray , and wh ite .
D
fa
absorbe nd scaUAre '1 lirJh' I ,."J I /
-
.,..,...,
These are the ways in which a light beam may behave. Not shown is polarization of light.
GLASS
LIGHT BE HAVIOR
light behavior i nc l udes tran s m i ssion, a bsorption, reflec tion, refraction, scattering, d i ffraction, interference and polarization, a l l of which are d i scu ssed in th is section . Transmission, a bsorption and reflection account for a l l t h e l i ght energy when l ight strikes a n o b j ect. I n the course of tra n s m iss ion, l ight may be scattered , refracted or po larized . It can a l so be pola ri zed by reflecti o n . The l ight that i s not transmitted or reflected i s a bsorbed and its energy contri butes to the heat energy of the mole c u l es of the absorbing materi a l . The modi fication of l ight through these processes i s respo n s i b l e for a l l that we see. W h e n a bea m of l ight strik es a thick sheet of g lass, part of the light may be r eflected, part abs orbed and scattered, and the remainder transmitted.
36
GA LILEO attem pted to mea sure th e s peed of l igh t but fai led . Ma n at B wa s to sh ow h is l i gh t wh en he saw l igh t from A. Man at A was to record elap sed ti me from fla sh of A' s l igh t to receipt of B ' s
l i gh t . Th i s simple ti me and dis tance meth od fai led beca u se th e ro und tri p travel time of on l y . 0000 1 1 ( 1 1 X 1 0 -6 ) sec. fo r a 1 -mi le separation was l es s th an h uma n rea ction time (0.4 sec . ) .
s o fast tha t for many yea rs scie ntists th ought that its speed wa s i nfin ite . The fi rst observa tions and mea s u rements wh ich gave a fi nite value to the s peed of l i g ht were made by the Da nish a stron omer O l af Roemer in 1 6 75 . Roemer was measuring the pe riod of revo l u tion of one of Jupite r's sate l l ites by timi n g its successive ec l i p ses be hind the pla net. H e fo und th at the m ea s ure ments made while th e earth was reced i n g from J u piter ga ve longe r period s th an th ose made wh i le the earth wa s a pp roach ing J u piter . Roemer conc l u ded th at the difference wa s due to th e fa ct that in the reced i ng posi tion th e light from each succe ssive e c l i pse had to trave l a greate r di stance to reac h th e ea rth a nd therefo re took a longer ti m e ( d iagram , p. 3 8 ) . He ca l c u l a ted th at l ight took about 2 2 m inutes to travel a distance equal to the dia mete r of the earth's orbit a bout the s u n . Roemer's meth od was correct but h i s accu racy wa s poo r . We now kn ow th at th e time requ i red i s n ea rly 1 6 . 6 7 m i n utes , or a bo ut 1 ,000 seconds. Since th e dia mete r of the ea rth's orbit is a bo ut 1 8 6 , 000,000 m i l e s , th e s pe ed of l ight is ca lculated to be a bout 1 8 6,000 m i les pe r secon d . LIGHT TRAVELS
37
R OEMER'S METHOD \
I
I I I I
Ea rth --
--
--
--
sate l l ite
--
-
Eclipse of J upite r ' s satel l ite was about 1 6 m i n utes late d u e to the added distance (diameter of earth ' s orbit) .
of th e speed of l ight a re made in th e laboratory. U s ing many diffe rent tech niques and much e laborate a pparatus/ scienti sts h ave m ea sured th e speed of l ight i n free space agai n and agai n , always striving for m ore acc u racy. The va l ue n ow a ccepted is 2 9 9 , 7 9 3 kilometers, o r 1 8 6, 2 8 2 m i les, pe r seco nd . The error IS bel ieved to be less th an o ne thousa nd th of one per cent. The p recise m ea surem ent of th e speed of l i g ht, a fundamental con sta nt, i s one of th e g reat technical achievements of ou r time . MOD ERN MEASUREMENTS
MICHElSO N ' S METHOD
l ight
octagonal
Miche lson, in 1 8 7 8 , reflected light from a rapidly rotating mi rror to a fixed distant m irror. By the time the reflected l ight retu rned, the rota ting m irror had moved enou gh so light was reflec ted at a d iff erent ang le, ena bl ing its speed to be ca l cu la ted .
38
i s of two kinds -diffuse and reg u l a r . Diffuse reflection i s the kind by which we ord i na ri l y see objects . It g ives us i n formation about their shape, size, color and texture . Regu l a,r reflection i s m i rrorl i ke . We don't see the su rface of the m i rror; i nstead, we see objects that are reflected in it. When l ight strikes a m i rror at an angle, it is reflected at the same angle. In d i ffuse reflec tion, l ight l eaves at many d i fferent angles. The deg ree of su rface roughness determines the proportion of d i ffuse and reg ular reflection that occurs. Reflection from a smooth, po l i shed su rface l i ke a m i rror i s mostly regu lar, wh i l e d iffuse reflection takes place at s u rfaces that are rough compared with the wavel ength of l ight. S i nce the wavelength of l ight is very sma l l ( about 5 ,000 A) , most reflection is diffuse. R E FLECTION
V I EW ED M I CROSCOPICALLY, a l l reflecti on is regu lar . The a ppear ance of d iffuse refl ection is due to the ma ny different angles that light rays encounter when they stri ke a rough surface . The re flection of each single ra y is reg ular- that is, i t is reflected at the same angle at which it
strikes the su rface. A fa irly smooth surface, such as that of a glossy vinyl raincoat, shows both d iffuse and regular reflection, the relative proportions depending on the angle of the incident light. But a rough su rface, such as that of a tweed coat, shows only diffuse reflec tion . I t has no " shiny" surface .
REGULAR REFLECTION
smooth surface
DIFFUSE REFLECTION
rough surface
39
with the type of mater i a l . Po l i shed metal reflects most of th e light that fa l l s on it, absor bs on ly a l ittle, a nd tran smits practica l l y no n e . Pa per is made up of partly tra nsparent fibers. Light striking paper may penetra te severa l fibers, being parfly reflected at ea c h s urface. The l ight th at finally reaches your eyes and lets you kn ow yo u a re looki ng a t paper ha s be en reflected a nd transm itted many ti mes. Al l th e re st of the l ight ha s been ab sorbed a nd added to the heat en ergy of th e molecules of th e paper . Most materi a l s a re q u i te selective i n the way they absorb and reflect the different wavelengths of l ight. A purple dye w i l l transmit blue and red l ight but wi l l absorb g reen l ight. Gold and copper meta l s reflect red and yel low wavelengths more strongly than blue. Si lver reflects a l l colors and therefore appears a l most wh ite . Meta l l ic reflection i s a n example o f pure s u rface color. Nearly all " o bject colors " are due to selective reflection and a bsorption of l ight. Obj ect colors are a n attri bute of the object, though the color seen at any time depends also o n th e color of the i l l u m i n ation . Tota l a bsorption of l ight m akes an object look black. REFLECT ION VA RIES
incident rays woven material
reflection revea ls the co lor and textu re of woven cloth . What we norma lly consider a s reflection i nvolves sel ecti ve ab sorption, selective reflection and refra c tion of light that pa rtia lly 40 penetrates the surfa ce .
r
-----------------
4) 1 V I
��
·;::; .!:
.,
'\�
I . .0 I E I g
.,
7:�
-----------------
I
.._
....,"'
..... 0
angle of incidence r
=
angle of reflection Reflecting
Surface
LAWS OF REFLECT I O N
1 . Ang l e of reflection equ a l s angle o f i n c idence. 2 . I ncident a n d reflected rays l ie i n the same p l a n e .
3 . I n c i d e n t and reflected rays are o n opposite s ides of
the normal -a line perpend i c u l a r to the reflecti ng surface a n d pa ssing through the point of i n c idence .
A PLA N E M I R RO R re ve rses a scene from left to rig h t. O bj e cts held i n the left hand of a su bject a ppear to be in the rig h t hand of the imag e . Al l ob jects seen in the mir ror a ppear to be as fo r be h ind the su rface they actual ly front of it.
A.
air
=
wavele ngth
air
glass
speed frequency x wavelength speed in air wavelength in air refractive index of glass speed in glass wavelength in glass =
=
is th e bendi ng of a l ight ray wh en it c rosses the bo undary between two d ifferen t mate ria l s , a s from a i r i n to water . Th i s change in d i rection i s d u e t o a cha nge i n speed . L i g h t trave l s fastest i n em pty space and s lows down upon entering matter. Its s peed in a i r is al most th e same as its s peed in s pace, but it trave l s o n l y 3/.1 as fa st in wate r a nd o n l y 2/J as fa st i n g lass. The refractive i nd ex o f a substa nce i s the ratio of th e speed of l ight i n space (or i n a i r ) to its speed in the substa nc e . Th i s ratio i s a l ways g reater than one. When a bea m of l ight enters a pane of g lass perpen d icular to the surface ( a bove ), i t s l ows down , and its wavelength in the g l a s s beco mes shorte r in the same p ro portion . The frequency rema i n s the same. Coming out of the g l a ss, the l ight speeds u p agai n , the wave length retu rn i ng to its former s ize. When a l i g ht ray strikes the g lass a t some other a ng le, it cha nges d i rection as wel l as speed . I nside the g lass, th e ray bends toward the perpend i c u l a r or norma l . I f the two sides of the g l a s s are para l l e l , the l ight wi l l return to i ts orig inal d i rection when it l eaves the glass, even tho ugh it has been d i splaced in its passage . I f the two s ides of the g lass are not paral l e l , a s i n t h e c a s e of a p r i s m or a l e n s , t h e r a y emerg es i n a new d i r.ection . R E FRACTI ON
42
surfaces not para llel
surfaces parallel
LAWS O F REFRACTION
1 . I n c ident and refracted rays lie in the same p l a ne .
2 . When a ray of l i g ht passes a t a n a n g l e i nto a denser
mediu m , it i s bent towa rd the norm a l , hence the an g le of refra ction ( r) i s s ma l ler tha n the a ng l e o f in cide nce ( i ) , as be low. 3 . The i ndex of refraction of any med i u m i s th e ratio between th e speed o f l ig ht i n a vac u u m ( or in a i r ) a nd its speed i n th e med i um . THE I N D EX O F RE FRACT I O N (n) determines the amount of bending of a light ray as it crosses the boundary from air into the medium. For example, in any of the diaWATER
grams be low, the ratio between the line x and the line y, ( ) is equal to the refractive inde� ( n ) i f d i s the same length in both air and the medium. •
,
D I AM O N D
GLASS
.:.� _J
\ -t 1 I I
n
=
1. 33
n
=
1.5
n
=
�
o c
2 .4
43
AIR
FR OM A PO I N T SOURCE ( 0 ) under water, the refracted rays in air make larger and larger angles with the normal , as the angles of incidence become larger. At the same ti me, the a mount of ligh t reflected back i n to the water increa ses. Finally, for the ray 08, when the angle of refraction be-
l i gh t pas s i n g into a i r
comes 90°, a l l t h e light i s reflected. The angle of i n cidence OBA, ca lled the critical angle, for pure water is 4 9 ° ; fo r a ray striking a g la ss-ai r s u rface, it i s about 4 0 ° . Beca u se of this, a 45 -90- 4 5 ° pr is m reflects 1 00% of t h e li ght enteri ng it and can there fo re be u sed as a perfect mi rror ( p. 6 0 ) .
REFLECTIO N occu rs whenever a l ig ht ray str i kes th e surface of a med i u m wh ose refractive index is less tha n th at of th e med i um in which the l i g ht is travel ing. The a mount of l ight th at is reflected depends on th e angle at wh ich it hits the s u rface . light from a point sou rce ( above) h its the s u rface at ma ny a ng l es .
I NTER NAL
is the separation of light i nto its com ponent wavel ength s . One method of d i s persing a l ight beam i s to pass it through a g lass prism-a th ick piece of g lass with flat non-pa ra l lel sides ( below). The re fractive i ndex of a l l mater i a l s ( p . 4 3 ) depends s l ightly on the wave l ength of the l ight. Fo r glass and other tran sparent materi a l s the refractive i ndex i s l a rger fo r the short ( bl u e ) wavelengths than for the longer ( red ) ones. Thus, when a beam of white l ight is passed th rough a prism, the blue rays w i l l be bent more tha n the red rays -that is, the l ight s preads out to form a spectrum . The co lors i n the spectrum a ppea r i n the o rder of i n crea s i ng wave length: vio l et, b l u e , green, ye l low, orange, a nd red . Sir I saac N ewton first exp l a i ned the s pectrum . H e showed that, co ntra ry to popu l a r bel ief, the prism d i d not create the beautiful colors, but o n l y made visi b l e the components of white l ight. Scienti sts make use of d i spersion i n the analysis of l i g ht em i tted o r a bso rbed by va rious materi a l s both o n t h e earth and on other bod ies i n s p a c e ( p. 6 3 ) . DISPERSION
WH ITE LIGHT ( below ) entering through a na rrow s l it at the left stri ke s the pri s m a t an acute angl e . T h e longer-wa'lele ngth red ray s are be n t less tha n the shorter wavele ngth blue, s o a pa rtial sepa ra tion occ u rs in t h e glass. The be am then strikes the second sur face of the pr ism, again a t an a cu te
angle, and the rays a re once aga in re fracted. They leave the prism as divergent ray s of different wavelengths. A wh ite screen i s pl aced some d ista n ce from the prism and the di fferent colors appear on it. The spectrum co n sists of many i ma ges of the slit, each of di fferent color.
i s the bending of waves around a n ob stacle. It is easy to see th i s effect for water wave s . They b e n d around t h e corner of a s e a wa l l , or spread as they move out of a channel . Diffraction of l ight waves, however, i s harder to observe . I n fact, d i ffrac tion of l ight waves is so s l ight that it escaped notice for a long time. The amount of bending is p roportional to the s ize o f l ight waves-about one fi fty-thousandth of an inch ( 5 ,000 A)-so the bend i ng is a l ways very sma l l i ndeed . When l i g ht from a d i stant s treet l a m p i s viewed th ro ug h a wi ndow screen it fo rms a c ros s . The fou r sides of ea ch tiny screen hole act as the sides of a s l it and bend l ight in fou r d i rectio ns, prod u c i ng a cross made of fo ur p rongs of l ight. Anoth er way to see th e di ffrac tion of l i g ht waves is to look at a di stant l i ght bulb th ro ug h a very na rrow vertica l s l i t . Light from the bulb DIFFRACTI ON
A PATTER N O F WAVES wil l m ove outwa rd , forming concentric c ir cl es, if smal l pebbles are drop ped reg u la rly from a fixed po int into a qu iet pond . If a board is placed in the po th of these waves, they will be seen to be nd arou nd the edg es of the board, cau s ing an
46
inte re sting patte rn where the waves from the two edges of the board meet and cross each oth er. When an obstruction with a ver tical slit is p laced in the pond in the path of the wave s, the waves s pread out in circ les beyond the s l it.
bend s at both edges of the s l i t a nd a ppea rs to s p read out sideways, forming an elo ngated d i ffraction pattern in a direction perpendic u l a r to th e s l i t . l i g h t can be imagined a s waves whose fronts spread out in expa nd i ng concentric spheres around a source. Each point on a wave front can be thought of a s the source of a new d i sturbance. Each point can act a s a new l ight source with a new series of concentric wave fronts expanding outward from it. Po i nts a re i n fi n itely numerous on the su rface of a wave front a s it c rosses an open i n g . As new wave fro nts fa n o u t from e a c h poi n t of a sma l l opening, such as a pinhole or a na rrow s l i t, th ey re inforce each othe r when they a re i n p h a s e ( p. 4 8 ) a nd cancel ea ch other when they are comp l etely o ut of ph ase . Th us l ig hter a nd da rker a reas form th e banded diffrac tion patte rn s . DI FFRACT ION PATT ER N S are formed when light from a point source passes through pinho les and s l its. A pinhol e gives a c i rc u lar pattern and a sl it giv e s an elonga ted pattern . A sh arp image is not formed by l ight passing through beca use o f d iffract ion.
As the pinhole or s l it gets smaller, the diffraction pattern gets larger but di mmer. In the d iffraction patterns shown below the a lter nate l ight and dark s pa ces are due to interfe ren ce (p. 4 8 ) be tween waves arriving from d if ferent parts of the pinhole or slit.
CONSTRUCTIVE INTER FERENCE
1.
i n-phase waves
2 . i n - phase
waves combined
.:M= +f-1
DESTRUCTIVE I NTER FERENCE 1.
��;�:-pho•e
2 . out-of-phase waves combined
is an effect that occurs when two waves of equ a l frequency a re superimposed . Th i s o ften hap pens when l ight rays from a s i n g l e source travel by d i fferent paths to the same poi nt. I f, at the point of m eeti ng, the two waves are i n phase ( vi brati ng i n u n i s o n , and t h e c rest of one coi nciding w i t h the crest of the other), they will com bine to form a new wave of the same frequency. The ampl itude of the new wave i s t h e sum of t h e a m p l itudes of t h e original waves. The process o f form ing th is new wave is ca l l ed construc tive i nterference . If the two waves meet out of phase ( crest of one co inciding with a trough of the other), the res u l t i s a wave whose a m p l itude i s the d ifference o f the orig i n a l ampl itudes . Th i s process is cal led destructive i nter ference. If the original waves have eq u a l ampl itudes, INTER FERE NCE
I NTERFER E N CE OCCUR S when l i gh t waves fro m a po int sourc e (a si ngl e s l it ) travel b y two d if ferent poths ( th rough the double
s i n gle s l it
slit). Their interference is shown by a pattern of a l ternate light and dark bands when a screen i s placed across t h e i r path. screen in
phase
l ight source
48
i n pha se
---
they may com pletely destroy each other, leaving no wave at a l l . Con structive i nterference res u l ts in a bright spot; destructive interference produces a dark spot. Partia l constructive o r destructive i nterference re s u l ts whenever the waves have a n i ntermed iate phase relation s h i p . I n terference of waves does not c reate or destroy l ight energy, but merely red i stributes it. Two waves i nterfere only i f thei r phase relationship does not change. They a re then said to be coherent. Light waves from two d i fferent sou rces do not inter fe re because radiations from differen t atom s are con sta ntly changing th eir phase re lation s h i p s . Th ey a re non- coherent ( see lasers, p. 1 5 2 . ) wh ich change their appearance with the angle of viewing and the d i rection of the i l l u mination, are due to i n terference. The d e l i ca te hues of soap bubbles and oil fi l m s , the pale ti nts of mother of-pea r l , and the bri l l iant colors of a peacock ' s ta i l are a l l i ridescent colors . I RIDESC E NT COLORS,
A SOA P B U BBLE a ppears irides ce nt u nder white light when the th ickness of the bubble is of the order of a wavelength of l ig h t. Th i s oc curs because light wav es re flected from front and ba ck sur fa ces of the film travel d ifferent di stances. A difference in pha se results that may ca use destructive interference for some particular
wavelength , and the hue or color associated with that wavelength w i l l be absent from the reflected light. If the missing hue i s red, reflected light a ppears blue-green, the complement of red. If film thickness or direction of i l l umina tion changes, interference occurs at d ifferent wavelengths and the reflected light changes color. red reflections cancel if film thi ckness is one-hal f wavelength of red
white
light
source
white
saeen
When a beam of white light passes through milk, the blue compon ents are scattered, the reds are transmitted.
i s the random deflection of l ight rays by fine parti cles. When s u n l ight enters through a crack, scattering by dust pa rticles in the air makes the shaft of light v i s i b l e . Haze i s a resu l t of l ight scattering by fog and smoke particles. Reflecti on, diffraction, and i n terference all play a part i n the complex phenomenon of scattering. If the scattering pa rti cles a re of u n i form size and much sma l ler tha n the wavel ength of l ight, sel ective scattering may occur and the materi a l wi l l appear colored, as shown above . S horter wavel ength s w i l l be scattered much more strong ly than longer ones . I n genera l , scattered light wi l l appear bluish, wh i le the rema i n i n g d i rectly trans m i tted l ight wi l l lack the scattered blue rays and th us appea r orange or red . Many natura l blue tints a re due to sel ective scattering rather than to blue pigments . The blue of skies and ocean s i s due to th i s kind of scatter ing. B l ue eyes a re the resu l t of l ight scattering in the i r i s when a dark pigment is lacki n g . Scattering b y larger particles i s nonselective and produces wh ite . The whiteness of a b i rd ' s feather, of snow, and o f cloud s - a l l a re due to scattering by parti c les which, though sma l l , a re large compared to the wavelength of l ight. SCATTERING
50
ABSORPTION AN D TRANSMISSION BY O PTICAL M ATER I ALS u l tra - v is- infrared v iolet i ble
quartz
I
I
rock salt
l
S ince crown g lass and flint glass absorb ultrav iolet l ight, a quartz
I
to 1 45 ,000 A I
bulb is u sed for the light source of a sun l a mp.
of light a s it passes through matter re s u l ts in a decrease i n intensity. Abso rption, l i ke scatter ing ( p. 5 0 ) , may be general or selective. Selective ab sorption g ives the worl d most of the colors we see . Glass fi l ters which a bsorb pa rt of the vi s i b l e spectrum are used i n research and photog raphy. An a bsorption curve for a fi l ter ( below) shows the amount of l i g h t absorbed at a particu l a r wave l ength . A u n i t th ickness of the absorbing med i u m wi l l always absorb the same fraction of light from a bea m . I f the first m i l l i meter thickness o f a fi l ter absorbs V2 the l ight, the second m i l l i meter a bsorbs 1h th e rem a i n i ng l ight, o r 1/4 o f the tota l . Th e third m i l l i m eter ab sorbs 1h of th e V4 , so on ly Vs of the light is tra n s mitted th rough three m i l l i mete rs o f fi l te r . S e e a l so p . 1 1 1 .
ABSOR PTIO N
ABSORPTION CURVE OF A GREEN PLASTI C F I LTER
A
20
.Q c:
0. 0 "'
-<(
.JJ
� 0
40 20 0
40
60
80
1 00
.Q
c:
"' "'
·e "' c:
e
1-
� 0
willemi te
under white light FLUORESCENT M I N ERALS seen under white light ( left) emit colored l ight ( right) when exposed
to invisible u l traviolet rays. The color of the fluorescence depends on the nature of the minera l .
FLUORESCE NCE AND PHOSPHORESCE NCE are cau sed by l ight stri k i ng atoms . In th e col lision, en ergy i s trans fe rred fro m the light to the e l ec tron s o f th e atom s . This energy may be re-rad iated as light o r d i ssipa ted a s heat. If the emi tted l i g ht i s o f t h e same frequen cy a s th e i n c ident l i g ht, th e effect i s a ki nd o f scatteri ng . In ma ny cases, howeve r, the emitted light i s of a diffe rent ( u s ua l l y l ower ) frequency th an th e i n c ident lig ht, a nd is cha ra cte ri stic of the atom that emitted it. The i m mediate re-rad iation of absorbed light en ergy as l i g h t o f a d i ffer en t co lor i s c a l l ed fl uo rescence. Some materia l s conti nue to emit l ight for a time after the incident rad iation has been cut off. Th i s i s phos phorescence, usua l l y a property of crysta l s or of l a rge organic molec u l e s . Phosphorescence often depends on the presence of m i nute qua ntities of impu rities or i mper fection s i n the c rystal that provide ' ' traps ' ' for excited e lectro n s . These el ectrons have received extra energy from i nc ident rad i atio n . The electro n s rema i n i n the ' ' traps ' ' until shaken loose by the h eat vibrations of the atoms i n the crysta l . Phosphorescent l ight i s emitted as the e l ectrons retu rn to their normal positi o n s . Solid su bstances that prod uce l ight i n th i s way a re ca l led phosphors .
52
T H I N PLATES OF TOURMALI N E tran smit light with vi brations restricted to a s i ngle plane ( po larized l ight) . When one plate is turned so its ax i s is at right angles to the other, no light passes through. Good natura l crysta l s of tou rmaline a re rare. Better polarizing materials made synthetica lly a re now available.
wa ve s a re restricted i n the i r d i r ection of vi b ra ti o n . Normal light waves vi brate i n an infinite nu mber of d i rections perpend icular to th ei r di recti on of travel ( p. 8 ) . For exa m p l e, i n the head-on vi ew of un po larized l i g h t ( below rig ht ) th e l i nes a , b, c, d, and an infi nite number of others are perpend i c u l a r to the ray. At a pa rticul ar i n stant any one of them m i gh t repre sen t th e di rection of th e vi bration s . Th us, from mo ment to moment, the direction of th e l ig ht vi b ra tions changes i n a random fash i on . W h e n compon en ts o f vi bration i n one di rection o n l y are p resent, t h e light i s p l a n e pola rized . POLARIZED LIGHT
CO N CE PT U AL ERR ORS m ay be introduced when i ll u strating polar ized light. Of the va ri ous po ssi-
bi lities, the symbols used be low give the best analogy to the events happe n i n g with po l a ri zatio n .
*
Head-o n unpol arized
a
* d
pa rtial ly polod,ed
-;;? �
;2
�
;12
�
-! l l
•
po la ri zed horizonta lly
•
po la ri zed vertical ly
�
•
I
b
c
�
53
"
/
co unterc lockwise
rotation of plane of vibra tion
y
cl ockwi s e
of a po l a r i zed l ight wa ve is usual l y unaffected in pas sing th rough a tra nspa rent mate ria l - it remai n s pol a r i zed in the same pl a n e . Some op tic a l l y ac tive materi a l s , howeve r, rotate th e plane of vi bration in either a c lockwise or cou ntercloc kwise di rec tio n . Q ua rtz c rysta l s oc cur in both c loc kwise a nd counte r clockwi se va rieties . Sugar sol utions a re a l so optica l l y active . A chemist can dete rm ine the concentration of sugar in a so l ution by m ea suring the rota tion of th e plane of vibration when plane-po larized l i g ht i s pa ssed th ro ug h th e s o l ution . A dextro se sugar s o l u tion cau ses a clockwi se ro tation; l evulose suga r, a cou ntercloc kw i se one. A device fo r measuring the a ng l e o f rotation of th e plane of vibration is cal l ed a pol a riscope . A sac cha ri m eter is a pola riscope used in sugar analyses . T H E PLANE O F VIBRATION
a vertically polarized ray
CD
may be rotated clockwise o r counterclockwise
( side view)
rotation
,r' calci te ( end v iew)
DOUBLY REFRACTING CRYSTALS, such as ca l c i te and quartz, break u p l ight rays i nto two parts, ca l l ed ord i n a ry and extraord i n a ry rays, which are po l a r i zed at right angles to each other. Such a crysta l has a d i fferent refractive index fo r each of the two rays, and they are bent at d ifferent ang les when they enter the c rysta l . Th i s dou b l e refraction wi l l fo rm two i mages when a ca l ci te c rysta l i s placed over a dot on a piece of paper. The dot appears a s two dots a sma l l d i stance apart. Rotating the c rysta l causes one of the dots to rotate about the oth e r . The dot that rema i n s stationary is the image fo rmed by the ordinary ray. Th i s a lways l i e s in the plane of incidence ( plane i nc l uding the normal and the inc ident ray) . The moving dot i s the i mage formed by the extraord i n a ry ray . A N I COL PR ISM i s made by ac curately cutti ng a calcite crystal and cementing the parts back together with Canada ba lsam. Because of the difference in the refractive indices, the ordinary cal cite ray. is reflected out the side of the prism by the Canada balsam layer, while the extraord inary ray passes directly th rough . N icol prisms were used to produce polarized light rays over a hundred years before the d i s covery of Polaroid. They are still among the most efficient polarizing hill� devices. .
* (/ .
<::-0:
'
'
55
W ithout Polaroid filter
With Polaroid filter
i nvented by Edwin Land in 1 9 3 2 , acts upon l ight i n the same way as tou rma l i ne (p. 5 3 ) , transm it ting o n ly those components of l ight that a re vibrating i n one d i recti o n . The other components of the wave are a bsorbed . Polaroid i s convenient to use. Po laroid sun g l asses, for example, transmit only vertic a l vi bration s, and th us e l i m i nate the g lare of l ight po larized by re flection from horizonta l su rfaces . Photog raph ers often use Po laroid fi l ters ( a bove ) to red uce u nwanted reflec tions, and a l so to da rken the sky i n color photographs by removing some of the scattered l ight, which i s partly pola rized . POLAROI D,
STRE SSES in stru ctural pa rts may be s tudied in plastic o r glas s models by placing them between sheets of Po la roid an d photograph· ing them unde r s tres s . The result· in g stra i n s make the p la stic or glas s doubly re fracting ( p . 5 5 ) , bu t to a d i fferent deg ree fo r different wavelengt hs of li ght. When such material is viewed be tween Polaroid sheets, bea utiful co lors appea r, indicating th e degree of stre ss .
a re largely responsi ble for the po larized l i ght that we see. Natura l l y polarized light is often only partly po larized a nd effects are not rea d i l y noticed .
THREE POLA RIZATION PROCESSES
AB SO R PTION of l ight pa s s i n g throug h a natural crystal o f tou rma line can produce polarizatio n . T h e c rystal resolves a l l t h e vi bration s of the u npolarized l ight into two components and absorbs on e of them. I n one form of Pol aroid ( p. 5 6 ) , long, thin mo lec ular chains containing iodine absorb some l ig h t v ib ra tion s and tra nsmit oth ers .
REFLECTION of light at an angle from a n on-m etal li c surface, such as glass or water, makes it pa rtly po la rize d. When the refle cted ra y and refracted ray are at right ang les, the reflected ray is com pl etely pola ri zed . I n th is case, the incident angle is cal l ed Brewster' s angle. Regu larly re flected l i gh t ( g la re ) from road s , beaches a n d water is pa rtly pola ri zed .
SCATTER I N G partly po la ri zes light from the sky. Lig ht rays from the sun exc ite tran sverse e lectri c vi brations in air molec u les, which then scatter pol arized light i n drrectio n s perpend icular to vibra tio n s . Look a t t h e sky 90 d egrees awa y from the sun through a piece of Pol aroi d. N ote change in brightn e s s a s the Polaroid is rotated.
TOUR MALI NE
I
GLASS
I ' I ....'' I Cll1: '
I �-'!:'·� I
Cli O' .
I I I
1l N
'§
&.
57
O PT I C AL IN STRU M E NT S
An optica l i n strument uses m i rrors, len ses, prisms o r gratings, singly· o r i n com bination, to reflect, refract or otherwise mod i fy l i ght rays . Optical i nstruments, especia lly m i c roscopes and telescopes, have probably broadened man ' s inte l l ectua l horizons more tha n any oth er devices he has made. Perhaps th e best way to understand the operation of optical instruments is by geometrica l optics-a m ethod that dea l s with l ight as rays i n stead of waves or pa r ticles. These rays fol low the laws of reflection ( p . 4 1 ) and refraction ( p . 4 3 ) a s wel l as the laws of geo ,.; etry . I MAGES formed by mirrors and lenses may be either real or virtual and of a predictable size a nd l oc ation . A real i mage, as formed by a camera or proj ector, is an actual conven ing of l i ght
rays and ca n be caught on a screen; virtual i mages cannot. The rays from object points do not pas s through corresponding points of a v i rtual image. I mages seen i n binocu lars a re v i rtua l .
are the oldest and mo st widely u sed optica l in stru ments . Th e p lane mi rror i s the simpl est i mage fo rm ing devi c e . Plane mi rro rs a re found in eve ry home. Sph erical or pa ra bo l i c mirrors a re often used in optica l instruments instead o f lenses. MI R RORS
obj ect
/ / / /"/ // Pla ne ;;�/
/�
v irtual im·age
58
Mi rror
A PLANE OR FLAT M I RROR produces a virtual image since the light rays do not come directly from the image. Rays from an object are redi rected by the mir ror so that they appear to come from an image located as far be hind the mirror as the object is i n front of the mirror. Obj ect and image are the same size, but the image i s reversed from left to right.
c u rvature CO NCAVE SPH E R ICAL M I RRORS have an axis of symmetry through their center, called the optic axis. A point on this axis equi distant from every poi n t on the
Co ncave m i rror ' s s u rface i s the center of curvature. An object beyond the center of c u rvature forms a real image between the focal poi n t and center of cu rvatu re .
1 . If a sma l l ob ject is p la ced at the center of curvature, the m i r ror forms a rea l image which co incides with the object but is upside down .
(2) 2. As the object i s mov ed closer to the mirro r, the i mage moves rapidly away, getting larger the farther it goes. 3. When the object rea ches a po int ha lfway between the mi rror and the center of curva ture, the reflected ra ys fro m eac h poi n t became paral le l and do not fo rm an i mage at a l l . The object is then at the foca l poi nt of the mi rror . 4. I f the object i s moved closer to the m irror than the fo ca l po int, the reflected ra ys d iverg e as though they c a m e from a virtual i mage located beh i n d the m irror. This virtual image is upright and larger than the object.
........ . .. - ..... ....
..... �
v irtual i m age
........ ..
are tra n sparent so l ids of g l a s s or other materi a l whose opposite faces a re plane but not neces sari ly para l l e l . Th ey are used to bend l ight rays by re fraction or i nternal reflection . The amount of bend ing depends o n the refractive i ndex of the pri s m , the angle between its faces, a nd the ang l e of incidence of the l ight. Since the refractive i ndex depends a l so on the wavele ngth (p. 4 5 }, prisms are often used to d i s perse a l ight bea m into i ts spectrum .
OPTICAL PRISMS
4 5 ° - 9 0° - 4 5 ° PR ISMS
..
--, I I
/ /
/'
''
� >
/
60
periscope
A 4 5 - 90 - 4 5 DEGR EE PR ISM will reflect l ight rays by tota l intern a l reflection . When the light rays enter perpend icular to one of the short faces of the prism, they are reflected totally from the long face and depart at right angles to the other s hort face ( 1 ) . These pri s m s are more efficient than s i lvered m i rrors . Two such pri sms m ay be used i n peri s copes t o direct t h e light down the tube and into the eye piece ( 2 ) . The 4 5 -90-45 degree pri sm may a l so be turned so that the l ight rays enter and leave perpendicular to the long face ( 3 ) . B i noc ulars ( p . 7 6 ) use such prisms in this way. A DOVE PR ISM is a modification of the 4 5 - 90-4 5 pri s m . The 90 deg ree corner has been removed . The prism inte rchanges the pos ition of two parallel rays, as shown . I f the pri s m i s rotated a round the d i rection of the light, the two rays will rotate about one another at twice the angular s peed of the pri s m rotation . Dove prisms u sed in optical instruments to i nvert an image are ca l l ed erecting pri s m s .
A TRI PLE M I R R O R has the shape of a corner symmetrically sliced from a glass cube. Light enter ing toward the corner from .any direction is reflected back paral lel to the direction from which it came. Tripl e mi rrors are used in bicycle and other reflectors, Placed along roads, they reflect the head lights of cars, warning motorists of curves a nd other ch anges in driving conditions.
A
60-60-60
DEGREE
PRISM
is used most frequently for dis persing light into its component wavelengths. A new or unknown transparent material is often cut into this shape so its optical prop erties can be studied with a spec trometer _ ( below), an instrument designed to measure angles of refraction of light rays. In the simplest and most familiar type of spectrometer, light from an
l ight is reflected back toward source
outside source enters a narrow, adjustable slit in a collimator tube. I t then passes through a lens that renders the rays of light parallel. The rays of light are directed onto a prism that re fracts and disperses them in a spectrum that can be viewed through a telescope. The spec trograph (p. 5 ) is similar in struc ture but is equipped to photograph the spectra .
SPECTROMETER
3rd order
2nd order
1 st order
image
1 st order
2 nd order 3rd order
A d iffraction g rating prod uces several orders of the s pectru m . The central image is white; h ig her order spectra overlap.
may be used i n stead of prisms to d i sperse l ight. Gratings were first used by Joseph Fraunhofer in 1 8 1 9 to observe the spectrum of the sun. I n 1 8 8 2 Henry A. Rowland perfected a m ethod of producing gratings of exception a l l y high q u a l ity. The m odern version con sists of fi ne para l l e l l i n es (up to 30,000 to the i nc h ) ru led i n an a l u m i n u m coating on a pla ne or con cave gla ss su rface . Light wa ves d iffracted from these l i nes interfere so that a l l wa vel en gths but one a re canceled i n any particu lar d i recti o n . Different wa ve l engths l eave th e gra ti ng at differen t a ng les and fo rm a spectru m . Grating s can be u sed i n the u ltraviolet and infra red as We l l a s in the v i s i ble reg ion o f th e spec tr um; spec ia l grating s are u sed with X-rays . D i spersion by a prism is grea ter fo r short wa ve l ength s th an for longer one s . Th e d i spersion o f a g ra ting, how ever, is a l most i ndependent of the wa ve len gth . G ra ti ngs prod uce a no rma l spectru m in which equal d i sta nces co r re spond to equ al wave l ength i nterval s a n d may be su peri o r to a prism in dispersion and reso lution . Grati ngs prod uce more than one spectrum a t the same time. These occ u r as a series of ever-wider spectra o n either side of a b r i g h t centra l i m a g e . T h e fi r s t spec trum on each side is known a s a first-o rder s pectru m and i s due to i n terference by a series o f waves which are out of phase by one wave length . The second ( second order) s pectrum o n each side is twice as long (has twice the d i spersion ) as the first. Each i s formed by a series of waves out of phase by two wave l engths . The third order spectrum overlaps the second order spectrum and DIFFRACTIO N
GRATI NGS
has three ti m es the d i spersion of the first. H i g h e r orders a l so overlap thei r neighbors and are longer but d i mmer. Diffraction grati ngs, l i ke prisms (p. 60), are used in spectroscopes for dispersion of a beam of l ig h t . The largest i n stru ments are of the g rati ng type. A spectro g raph record s the spectrum photogra phica l l y or e l ectron ica l ly . A monoch romator, also either prism or grating, uses a slit to i solate a narrow portio n of the s pectrum for sc ientific study. These instruments a re used to study the ma ny properties of l ight sources, from candles to d i sta nt stars, to learn the kinds of a to m s and m o l ecu les of wh ich th ey a re made, plus such othe r featu res as tem peratu res, velocities, and energy state s . Most of what scientists kn ow about the structure of ato m s was l ea rned with spectrographs. The same is tru e about o u r know l edge of d i sta nt sta rs, ne bu lae a nd ga lax ies - th e i r te mpera tu re s , velocities and chem ical structu re s . A PASC HEN SPECTROGRAPH uses a concave di ffra ction g rating in a simple ma nner. The g ra ting, the photograp h ic plate, and the entra nce s l it a re all arranged around a c ircu l ar trac k. Light
passes through the s l it, strikes the grating, and forms spectra of various orders a l l in focus on the circle. The photograph i c p late is p laced on t h e track record desired wavelength s .
PASCH EN MOUNTI NG
3rd order spectrum
concave grating
2nd order spectrum
1 st order spectrum centra l i mage
63
A LENS forms a n image by refracti ng the l ight rays from an object. Curved g lass lenses were first used a s s i m p l e mag_n ifiers i n the 1 3th century, b u t it w a s not ti l l nearly i 600 that the microscope was devi sed , fo l lowed b y t h e tel escope a decade or so later. Mi rrors, '' which form a n image by reflecti n g light rays, had a l ready been known for several centu ries a n d were easier to understa n d . A lens, however, has a n advantage over a m i rror in that it perm its the observer to be on the op posite side fro m the i ncoming light.
SIMPLE POSITIVE LE NSES ( a l so known as converg ing lenses) are sing le pi eces of glass that are thicker at their centers than at their edges. Each s urface is a sec tion of a sphere, and a l ine through the two centers of cu rva ture (AB) is the optic axi s . light passing through a lens is bent toward the thicker part of the g la s s . light rays parallel to the optic axi s are bent by the lens so as to converge at the focal point ( F ) of the len s . S im i l arly, l ight coming from the opposite direction converges at a second focal point an equa l d istance on the opposite side o f the len s . T h e distance from the center o f t h e lens ( C ) t o the focal point ( F ) is the foca l length of the lens. 1 _ -+---;._ 1
-
focal l ength
All pos itive lenses a re thicker at thei r centers than at their edges. They range from very th i n lenses with su rfaces of little curvature to thick lenses that a re nearly spherical in shape. Most lenses i n general use are "thin" lenses. Best known and widely used as a s i mple magnifier is the double convex lens whose surfaces usually, but not always, have the same curvature. The plano-convex lens has one side flat, the other convex . The posi tive meniscus lens has one con vex surface and one concave s u rface. The convex s u rface has a sma ller radius of curvature than the concave surface. As a resu lt, the lens i s a lways thicker at its center than at its edg e . double convex
planoconvex
positive meniscus
--------�O P.ti c a xis
64
po sitive le ns
focal point eye
I I I I
point
v irtual image
j-focal length -4- focal leng th -i When an object is p laced between a pos itive lens and either focal point, an upright, enlarged i mage wi l l appear on the same side of
1
object d istance
i mage distance
focal
foca l
rea l
the lens but farther away than the object. This virtual image ( p. 5 8 ) can be seen only b y look ing at the object through the lens.
,, I f an object is placed beyond the focal point, its i mage will be rea l , inverted, and located on the opposite side of the lens. When the object i s more than
twi ce the focal length from the lens the image w i l l be smaller tha n the object . With the obj ect c loser than twice the foca l l ength , the i mage i s larger.
MAGN I FICATION i s the ratio of length of image to length of ob ject. I t equals the distance of the image divided by the di stance of the object from the lens. Hence, an image will be larger than the object o n ly if it i s farther from the len s . The shorter the foca l
leng th of a lens, o r the greater it s convex curvature, the greate r its mag nifying power . Th i s power, expressed i n d io pters , is the foca l length of a lens i n meters divi ded into 1 . A l en s with a foca l length o f 25 em . ( 114 m . ) has a magn i fy i ng p ower of 4 diop ters . ma gn ification
eye real i mage on J retina
'1E��:3
I I
I
magn ifier
=
.image di stance ob ject distance
v irtual i mage
'
obj ect
i mage negative lens
eye
�
The eq u iconcave ( o r do uble-concave ) lens is used as a dimini shing gl as s to see how illu strations will redu ce in pri nting. SIMPLE N EGAT IVE LEN SES (also cal led diverging lenses) a re thicker at the edges than at the center. A negative lens alone can not form a real image as a posi tive lens does. l ight passing through a negative lens parallel to the optic axis i s bent away from the axis . . The foca l point of the negative lens i s located
by extending these diverging rays backward until they cross the axis. The image formed by a diverging lens i s always v i rtual, upright, and smaller and closer to the lens than the object. N egative len ses a re used to re duce images ( below), to correct nearsig htedness ( p. 69) and to co n stru c t compou nd lenses ( p . 7 1 ) .
Three type s of negative lenses
negative meniscus
is a fai l u re of a lens or m i rror to form a perfect image. Two of the six most i m po rtant types of a berration a re spherica l and c h romati c . Spherica l aber ration is ca used when pa ra l l el l i g ht rays pa s s i ng th ro ug h a lens a t d i fferent di stances from the optic axis a r e not a l l focu sed at the same po i nt. A d i aphragm that de creases th e aperture of the lens wi l l e l i m i nate the outer ra ys and red u c e spherical aberra ti on . Chromatic a berra tio n is cau sed by th e fa ct th at a lens be nd s eac h col or, or wave l ength, to a different degree .
AN ABERRATION
66
--.f ---i----
lens diaphragm
Sph erical Aberrati on Co rrec ted
Sph erical Aber rati on SPH E R I C AL ABER RATION de pend s on curvature of th e l e n s . light ra ys passing th rough th e outer part of th e lens bend too sh arply to pass th rough th e focal po int and form a fu z zy i mage .
If th e sph e rica l aberration is to be k e pt to a m i n i m u m, th e radii of curva ture of th e lens s u rfaces sh ould be large co mpared to th e di ameter of th e l e n s . A d ia ph rag m limits the lens a perture .
a screen here shows the lea st b l u rred i ma ge
I I
white l ight
CH R OMATIC AB ER RATION re su lts from unwanted d ispersion of l ig ht ( p . 4 5 ) in a le n s , so th at d iff erent colors a re focused at s l ig htly different distance s . It prod uces a blu rring o f th e imag e in o ptica l i n stru m ents .
C h roma tic a be r ra tion ca n be corrected by com bi ning two or mo re s i mp le l enses made of d if fe rent k in d s of g la ss , so th a t th eir di s persions can c e l each oth e r . S uch lenses a re said to be a ch rom ati c .
all
to
colors come focus
a common
FARSIGHTED EYE
Eyeglasses to correct farsighted ness have positive l enses.
lens
obje ct at normal near point
aid a process cal led a ccom modation . The mu scles attached to th e l ens of the eye change the sha pe of the len s so tha t the images of o b j ec ts at di ffer en t d i sta nc es are brought to a fo cus on th e reti n a . The ra nge of thi s ac comm oda tion, in norm a l eyes ight, is from a " near point" at wh ich the eye can see an ob ject i n most detai l (a bout 1 0 i n ches ) to a far po i nt on th e hor izo n . F o r s o m e people, t h e n e a r point i s much farther away than 1 0 inches. They see d i stant objects c l early but a re unable to focus on objects at reading d i stance. Such person s are farsighted ( a bove ) . The defect i n their EY E GLASS ES
Visual accommodation declines with age.
68
N EARSIGHTED EYE
Eyeglasses to correct nea rsighted ness have negative lense s .
distant objed
lens
eyes may be corrected by wearing spectacles with posi tive ( converg i n g ) lenses. An object now held at the norm a l nea r point i s s e e n or read d i stinctl y . With a g e , t h e lens of the eye loses some flexi b i l i ty and the m u sc l es at tach ed to it lose some tone. Th i s loss of accom moda tion va ries among peopl e . I t can be corrected by g lasses, usua l ly g round so the u pper part of the lens gives a correction for d istant visio n . The lower segment of these bifocal g l asses is a s l ightly th icker, positive lens for c lose work or readi n g . A nears ighted person can s e e objects clearly at close range, but can not foc us on those at a d i sta n c e . Th i s difficulty i s remedied b y g l a sses with negative ( d iverg i n g ) lenses. With t h e m , d i stant objects a re s e e n as though they were with i n his range of accommodation . CONY ACT LENSES are a form of eyeg lasses that f1t directly over the cornea of the eye, float ing on the layer of tears that covers its s urface. Such small lenses correct severe refractive eye conditions and have special advantages for ath letes, actors, and others .
contac t len s
12 1 1 1 0� �2 3
9
·� �· 5
7
6
Asti gmati sm test chart . Rota te page a t a rm ' s l ength . Al l lines should a ppear equa lly inten se.
6
For some person s with severe astigmatism, the test chart might look like th i s .
of th e eye i s due to a fa u l t i n the cu rva tu re of th e cornea or lens ( page 8 5 ) . If either curved surface i s not sym metrical, ra ys i n different p l a n es wi l l not be focused a t th e same d i s tance be h i nd th e l en s . Thu s part of t h e i mage w i l l b e o u t of focu s . Th i s defect can u s u a l l y be corrected by wea ring spectacles with cyl i nd rica l lenses ( below) i nstead of spherical ones. For large corrections or i rreg u l a r corneas, contact len ses may be presc ri bed . Almost two out o f th ree persons have at least a m i l d form of astigmati s m . ASTIG MATISM
focus i n one plane ASTIGMATIC GLASSES have cy l in d ri ca l lenses, which a re c u rved about a transverse axis rathe r t h a n a round a point, as in s i mple pos itive lenses. Cylindrical lenses can aid i n correcting asti gma tism, i n which the lens o f the eye does not have sufficient c u rva ture around its vertical a x i s . \,
70
.... ...
---
are u sed in p re c i s ion optica l i n stru ments such as microscope s , te l escope s , a nd expen sive cameras. Th ey con s i st o f two or mo re s i m p l e l e nses (e lements ) com bined i n such a way th at the a be rration s are m i nim i zed. The e lemen ts are sometimes ceme nted to geth er a nd sometimes caref u l l y spaced a pa rt in the same mount. De sira bl e c ha racterist i c s of an optica l i mage a re brig htness, h i g h reso l ution o f fi n e d etai l , ed ge sha rpness o f th e i mage, flatne ss o f fie ld, and good contrast between l i g ht a nd dark porti o n s . A l l of th ese cannot be ac h i eved at the sam e ti m e, and even the be st l e n s is a compromise, achi evi n g one good qual ity pa rt ly at the expense of a noth e r . Comp uters are now used i n lens design to help choose the proper curvatu re s , m ater ia l s , and spa c i ng s of th e elements fo r best cor rection of th e a be rration s . Some i mage-form i ng i n s tru ments u s e both m i rrors a nd len ses . Such sys tem s, c onta i n i ng both re frac ti ng a nd reflecti n g e l em en ts, are ca l l ed catad ioptr i c . COMPOUND LENSES
eyepiec e MAG N I FY I N G EYEPIECE i s u sed in simple viewing and measuring in stru ments. One form i s the pocket doublet magnifier. Carefu l lens grinding and spacing reduces aberration s . TELE PHOTO LEN S for a camera is a compou nd l e n s which gives the effect of a long focal length system with a relatively short len s·to-film distance. M I CROSCOPE OBJECTIVE LEN S h a s a short foca l length a n d re la tively large aperture. Its h igh to magn ification emphasizes aberra- � tions , so that a h i g h order of 1!01E:-++-H-+---++-+--_., eye piece correction is requ i red.
Ray Diag ra m of an Opera G la s s
wa s in vented by a Du tch opti c i a n , Hans Lippershey, i n 1 608 , som e 3 00 years a fter the invention of spectac l e s . Gal i l eo received news of t h i s inve ntion in 1 6 0 9 , a nd without seei ng th e o rig ina l , he con structed a telescope con s i sti n g of o ne pos itive a nd one n egative len s mounted i n a d i scarded orga n p i pe . The same opti cal a rrangement is used m ore effic iently in opera gla sses, wh ich are usually b i nocular. The pos itive lens i s ca l l ed th e objective , and the n egative lens i s the o c u l a r, or eyep i ece . The field of view afforded by opera g l asses is rather sma l l at large mag ni fication s , so opera gla sses are u sua l ly of low power - 3 to 5 mag nifi cation s . S i nce th e eyepiece is a negative len s, the image seen by the viewe r is u pr ight. The spyg lass, or te rrestria l te l escope , p rovides a n up right i m ag e because a third lens l ies between th e ob jective and ocular ( be low ). Th i s make s th e i n str ument q u i te long; so, fo r co nven ience, it i s usually made col laps ible. TH E TELESCOPE
Ray d ia gram shows how i n verted image i s erected in a si mple spyglass .
72
erecting l enses
eyepiece (ocular)
IN THE REFRACT I N G TELE SCOPE the o bjective forms a real , inverted image of a dis tant object at the prime focus . The eyepiece then forms a magni-
fled virtual image. Magn ification depends on the abil ity of the objective to furnish enough light and a good i mage for high mag nification by the eyepiece .
REFRACTING TELESCOPES use pos i tive len ses for both ob j ective a nd eyep iec e. The real i nverted i mage pro du ced by th e objective is vi ewed , e n larg ed , th rough th e eyep iec e . Refra cti n g te lescopes a re l i m i ted to a ma ximum a pe rture of about one m eter bec a u se of th e difficulty of produ cing ve ry l a rge pi eces of flawl ess opti c a l -q u a l i ty g lass , and then grind i ng c oa xi a l curva tu re s on both s i des o f th e l en s . Al so, a s l en ses are made larger, the a mount of light ab sorbed by the g l a s s inc rea ses, a nd th e wei ght of th e lens causes it to sag , i n trod uc i n g optical er ro rs . Th i s type of te l e scope i s u sed to study celesti a l bod i e s . The refra cting tel escope a t Yerkes Observatory in Wiscons i n has an objective lens 40 i n ches in di ameter, the la rgest u s ed success fu l ly by a stro nomers . The tele sco pe has a theoretical magn ify ing power of 4 , 000 diameters .
The Hale telescope on Palomar Moun ta i n in Sou thern Ca lifornia has the largest m i rror of any op tical telescope . The disk of pyrex glass has a diameter of 200 i n . a n d i s 2 7 i n . thick . I t w e i g h s 1 5 ton s . The concave surface is coated with a l u m i n u m oxide. V iew here i s of the " cage " in which the astronomer makes his observatio n s .
use cu rved m i r rors instead of a n obj ective lens. A l l telescopes over 40 i n c h e s i n dia meter are of the reflecti ng type . Since only the surface of a l a rge m i rror needs to be optica l ly perfect, it can be braced from the back to prevent di stortion. A wa ffle pattern mol ded into the back reduces the we ight and m a i n ta i n s rigidity. The front su rface of the mi rro r is ca refu l ly ground and po l i shed to a pa rabo loid . I t is then coated with a very th i n a l um i n u m fl l m, wh ich ox i d izes into a highly reflecti ng and dura ble surface. A 2 00-inch reflector co l l ects four times as much l ight a s a 1 00-inch i n strument. It sees twice a s far into space, and thus su rveys a un iverse eight times a s large as that seen by the s m a l l e r telescope. REFLECTING TELESCOPES
I n the reflecting telescope, light from astronom ical objects is reflected from a curved m irror to a prime focus where an in ve rted rea l image is formed . I n
the Newto nian fo rm , shown be low, a fl at mi rror is p laced i n the bea m to refl ect the pr ime foc u s towa rd o n eyepiece a t t h e s i d e o f t h e instrument.
Roy D iagram of a Reflecting Telescope
�anal
reflecti ng m ir ror
prime focus
74
pol ished first surface mi rror
l ight from sta rs
spherical m irror
curvature
paraboloida l m i rror
rad ius of c u rvature
Light rays striking a spherica l m irror pa ra l l e l to the optic axis are not a l l reflected through the foca l point ( F ) . But in a parabo-
loidal mi rror, the changing cur vature re flects a l l rays through the foca l po int, and a clear, sharp image results.
is s i m i lar to that of spherical len ses ( p. 6 7 ) . Para l l e l l ight rays that str i ke the m i rror at ·d ifferent d i stances from th e center are not reflected through th e focal point. Si nce they do not m eet a t a s i n g l e poi n t, the res u lting image is fuzzy . The defect becomes more seriou s a s the d i amete r of t h e m i rror I s m ade l a rger i n proportion t o the rad i u s of c u rvature. To avoid spherical aber ration , m i rrors a re made with concave pa ra bo l oidal su rface s . Th ese a re u sed in tel escopes and search l ights. SPH ERICAL ABE RRATION OF CONCAVE MI RRORS
SC H M I DT CAMERA i s a form of te lescope used to take pictures of large areas of the sky. Its field of view is wider than other reflectors because of a " correct ing" lens inside. Since the field of foc us is c u rved, this in strument cannot be used for " eye" ob servi ng. Shown is the 72 -inch Sch m idt Camera at Mount Pa lomar.
BINOCULARS a re used, l ike a sma l l te l escope, to view d i stant objects . Th ey emp loy an optica l system of len ses and prisms to prod uce an enlarged erect image. The ocular and the objective lenses provide the magnifica tion and i l l u m inati o n . Between them is a pa i r of 4 5 -90-45 deg ree pr isms (p. 6 0 ) so arranged tha t the light pa ssing th ro ug h th e b i nocula rs i s interna l l y refl ected fo ur times, making the image erect . THREE F ACTORS are invol ved in the usefu l ness of bi noculars - m ag · nifica tio n , field of v iew, and l ight gatheri ng power . Magn ification must be suited to the purpose. Any movement by either the ob server or the observed is magn ified at h igh power. For hand-held uses, binoculars with magnifications of 6x to 8x are best. H igher mag n ifications req u i re a tripod or other support. Field of view is largely determ ined by the ocular lenses. The d iameter of the ob jective lens determ ines the l ight-
B I NOCULARS
76
gathering power-the larger the better if binoculars a re used at n i ght or i n shady woods . Binocu lars have central or individua l focus ( centra l preferred ) . They ra nge in mag n ifi cat ion from about 2 to 2 0 . Each binoc u la r has a n id entification ma rk such as 8 X 30 or 7 X 50. The first number is the mag n ifi ca tion , t h e second t h e objective d ia meter in m i l l imeters . An 8 X 30 g la ss has s l i ghtly g reater m ag n ifi cation but d isti nctly less light-gathering power than a 7 X 50.
MICROSCOPES, PROJECTORS AND ENLARGERS are s i m i l a r i n principle, but they differ i n pu rpose and de sign . In each, a positive lens forms a rea l image of a brightly i l l u m i nated object. With projectors, the i mage i s caught o n a screen; with microscopes, it i s viewed through an eyepiece; and with photog raphic e n l a rgers, the i m age i s p ro jected on l ight sen sitive paper, where it i s reco rded i n semi-permanent form . MICROSCOPES need intense il lumi nation of the object because the i mage is much larger than the object and the same amount of l ight must be spread over a large arec . The i l l u m i nation i s provided either by a tungsten lamp bulb or by an arc lamp, its light concen trated on the object by a lens or by a concave mirror used as a condenser. The microscope ' s ob jective lens has a short focal length ( from 1 em to less than 1 mm ) and prod uces a sharp image of a very smal l field .
THE MAG N I F ICATION a ch i eved with an optical mi croscope is ap prox i ma tely equal to the power of the objective m u ltipl ied by the power of the ocu l ar or eyepiece, prod ucing a n ov era ll magnifi ca tion of up to about 1 , 5 00 di ameters .
IMAGE SHARPNESS depe nds on re solving power rather than on magn i fying power, and is l imited by d i ffract ion effect s that blur the image. Two po ints sepa rated by a di stance less than one half the wavelength of light cannot be re so lved op tica l ly and w i l l appea r as one p o i n t i n s tead of two. With the usual i l lumina tion, this separation is approx i mately 1 I 1 00,000 of a n inch . THE ELECTRON M I CR OSCOPE, by using a b ea m of electro n s with an effective wavele ngth much shorte r than that of l ig h t, obtains over 1 00 times the re solving power o f opti ca l m i c rosco pe s .
virtual image
PROJ ECTORS use high-i ntens ity tungsten or arc lamps for i l l umi nation . Two condensing lenses con centrate the l ight rays through the object ( u sually a film s l ide ). The converging rays pass on through the projection lenses, and the enlarged image is th rown on a screen. This is an inverted image, so s l ides are inserted upside down in a projector. With any given combination of lenses, the fa rther the image is projected the larger it w i l l be , and greater lamp inten sity will be requ i red. With some projectors ( below ) , an opaque object can be reflected on the screen . OPAQ U E PROJECTOR
PHOTOGR A P H I C E N LARGERS a re prec i s ion projectors with ad j u stments for foc using the image and contro l l i ng image size and brightness. Good enlargers pro vide un iform i l l u mination, a good ' lens system, and a rigid mount. I n operation, light from a lamp i� concentrated by a paraboloidal reflector, passes through a dif fusing glass ( or a conden sing len s ) , con tinues through the negative and then through the projection lens, which forms an enlarged i mage of the negative on the ea sel. I mage sharpness is adju sted by moving the projec tion lens relative to the negative. E N LARGER
Path of light in a pinhole camera
A pinhole camera photograph
CAMERA comes from the Lati n phrase cam era obscura, or dark cham ber, fo r a l l pictu re-taking i nstruments have a dark chamber to protect the sens itive fl l m from light. The simplest camera i s a l ight-proof box with a pinhole in one end and a piece of fl l m on the opposite inside wa l l . Light reaches the fl l m only when the pinhole is uncovered, usually fo r a few secon d s . U se o f a l ens instead o f a pinhol e a l lows m uc h m ore light to pa ss th rough, and the same pi ctu re can be ta ken in much sho rter time . If th e area of the lens is 1 ,000 ti m es as l a rge as the pi nh ole, th e pi ctu re which requ i red te n sec ond s can be made i n 1 I 1 00 sec ond , an average speed i n many mode rn cameras . I n ad dition to a l en s a nd fllm, a ca mera u sua l ly ha s a shutter, an adj usta b l e di aphrag m, a nd some ty pe of fo cusing ad j ustment. The sh utte r preve nts l i g ht from strik ing th e fl l m except when a pictu re i s be i ng made. A mec ha n i s m op ens the shutter and closes it automatica l l y afte r a length o f time . Cam era s h u tters m ay have speed s from 1 0 seconds to 1 I 1 , 000 of a secon d . Some cam eras take pictures at a m i l l ionth of a secon d, en ab l i ng man to see the un seeab le. The d iaphrag m can be ad j usted to ad mit va ry i ng amou nts of l i g h t each time th e shutter is open . Th e fo cusing mecha n i s m m oves the lens ba ck a nd fo rth to ach ieve a clear i mage .
79
THE SH UTTER of a camera a l lows light from the subject to enter the camera and strike the film for a time. Shfltters a re located either at the lens or at the film. Lens shutters are usually placed between or just behind the elements of the lens and usually have a set of leaves that snap open for the des ired time and then snap shut. Foca l plane shutters are next to the film and resem bl e a window blind with slots cut in it. A modern ver s ion has two curtai n s . As one moves and uncovers the fi l m , the second fol lows so closely that the open ing between them i s a mere slot. Exposure time is va ried by ad justing the wi dth of the slot.
THE f- N U MBER of a lens refers to the ratio of its diameter (d) to its focal length (f). For example, f I 8 refers to an aperture whose diameter i s 1/s of the focal length . A camera ' s lowest f nu mber corresponds to its wide open diaphragm and i s called the speed of the len s . U s ing a lower f-n u mber shorte n s the required exposure time, but reduces the depth of field ( p. 8 1 ). The dia phragm is set to the h ighest f n u m ber that can be u sed with a g iven shutter s peed in order to get maxi mum depth of field . Many modern camera s have a built-in photoelectric cell that selects auto matically the lens opening for the shutter s peed u sed.
The relation of the dicllltMI>ter lens to its foc al number. For exc:anJH;
f/3.5
the focal times jt,l �I!Mttt;t. ;.�;...;
FOCUS i s achieved when l ight from a n object pas ses th ro ug h a cam era len s and for ms a c lea r, acc urate i mage on th e fil m or viewer . l ight ra ys from an o bject po int diverge s l ightly as th ey ente r a camera . With a wide open len s , the rays pass th rough a l l pa rts of the lens and converge to th e i mage poi nt {below) . To foc u s the camera, the d i sta nce fro m th e lens to th e fi l m is ad j usted so that the point of co nvergence wi l l l i e at the fi l m s u r fa ce- not be fo re or be h i n d it. I t i s i m pos s i b l e to focus all po ints of a three-d i mens i onal scene on the film at th e same ti m e, but a satisfactory sharpness can usua l ly be ac hieved ove r a con sidera bl e de pth of th e scene. Adj u stment i s most critical when foc u s i ng on a nearby ob ject wi th a wide-open len s . It is l east c r i tica l for a di stant object with the sma l lest len s open i ng . The ac cu racy of th e foc u s i ng may be j u dg ed by o bserv i ng th e sharpness of the i mage on a tran s l ucent g l a s s su rface in reflex-type cameras or by super i m pos i n g two images in a ra ng efi nder in some other camera s . DEPTH O F FIELD i s the depth of a scene that is i n focus on the camera ' s fi l m . A large lens aper ture allows on l y a limited depth to be in focu s . The rest of the
scene i s fuzzy. With a smaller open ing, the cone of l i ght rays from lens to film converges and diverges less. Both near and dis tant objects a re i n better focus.
c·
obj ect points
<--- .
81
standard light source
I I ' •4- t-i .. .
\' fr
t-1-+ �-�
�'1
sample l ight
source � I £..... .. . .. �..; I I I
\ �dWK I;,g I \
., I
..., \ \
pamt""
(white mat surface on both sides)
I
\ I \ I
v
observer In this simple photometer, identical white surfaces are illuminated by the two different light sources being compared .
PHOTOM ETE RS are i n stru ments for comparing the in ten sities of two light sources which have approximately the same hue. The two sources are arranged so that they i l l u m inate adjacent parts of the same vi sual field, usually a screen . One of the l ight sources is then moved closer or farther away unti l the two parts of the screen match i n brightnes s . The average observer can make such a match with an error of l ess than two per cent. One l i g ht source i s u sua l l y a standa rd lamp o f known i nten sity, so the i n tensity o f the other source i s measu red i n terms of t h e standard o n e . Another type of photomete r compares t h e shadows c a s t by two adjacent l ight sources and g ives s i m i l a r i n formation . Lig ht-bu l b manufacturers u se photometers to test thei r prod uct. Astro nomers use them to m ea sure the l ight intensity of stars .
82
sample source ., "'ii :.c
... -
.!121
.s:.
standard sour ces ( c ontrol lable intensities)
- · Both sides of the divided screen are viewed simu ltaneou sly i n this colorimeter to determine the sample ' s color characteri stic s .
a re instruments designed t o measure color chara cteri stics other th an i n tensity . I n one type, one part of a divided field i s i l l umi nated by l ight from the sou rce bei ng stud ied; the other part by a m i xture of l ight from th ree standard sources, each of a d i fferent hue (p. 1 0 1 ) . By a d j usting the amounts of l ight from the standard sources, the operator can match the two parts of the field so that they look exactly a l ike. Someti mes one of the standard sources must be u sed on the same part of the field with the test source, ach ieving the match with the oth er two standa rd sources. No more tha n th ree standard sources are ever req u i red . S i m p l e color match ing with indicator chemica l s i s a part of chemical analysis. Th is type of colorimeter i s m o r e properly ca l l ed a c o l o r comparator.
COLORIMETE RS
83
SEE IN G LI G H T AN D C OLO R
Th e eye is ofte n com pa red to a ca mera . It has a lens th at produces a n inve rted image on the reti na, whose surface is sensitive to l i g ht j ust as fi l m i s . In front, the ey e has an i r i s th at changes th e size of the pu p i l , pe r fo rm ing the same fu nction a s th e diaphrag m of a cam era . Th e pu pil is simply th e hole in the i r i s through wh ich l ight enters the eye . As its s i ze changes, the pu pi l adm its m ore or less l i g ht as need ed , de pend ing on th e a mount of i l lum ination p resent. Th is ada ptation by the i ris to the level of i l l u m i nation i s continued by th e retina ( p . 8 6 } . A ray o f l i g ht entering th e eye pa sses through the transpa rent cornea, the aqueous humor, the lens, a nd th e vitreous humor. A l l help focus th e l ight befo re it str i ke s the rods a nd cones, wh ich a re p hoto receptors located on th e reti na . Her'e i s where the a ctu al process of see i ng beg i ns. The grea te st bend i ng o f l ight rays oc curs at th e fi rst surface of the cornea . A g roup of l igaments and m u scles automatica l ly c.o n tro l the shape of the lens to bring objects at d i fferent d i sta nces i n to focus on the r.eti n a . Th i s p rocess i s called accommodation ( p . 6 8 } . As one gets older, the lens g radua l l y loses its flex i b i l ity, and the a b i l ity to accom modate decreases . AS OBJECT COMES CLOSER , SHAPE OF LENS CHANGES FROM THIS .
84
.
.
HORIZONTAL CROSS SECTION OF THE RIGHT EYE
vitreous humor
THE LENS is su pported by a sus pensory l igament that holds it in ten sion with i n the encircling cil iary muscles. When relaxed, the ciliary muscle holds the ligament taut, and the lens i s flattened for viewing distant objects. When focusing on nearby objects the muscle contracts, loosening the
. . . TO T H I S
suspen sory ligament to let the lens assume its natural bulging shape. Thus the lens i s thin for viewing distant objects ( p . 8 4 , bottom) and becomes thicker to focu s on near ones ( below ) . It is impossible to have near and dis tan t objects in sharp focus at the same time.
�, �
c i l iary m us cles
rays from nea r obj ect
85
i s the eye ' s sensitive i n ner su rface. It i s a com p l ex system of nerve end ings fo rmed o f two kinds of l i ght-sensitive cel l s : rods a nd cones, named for the shapes of thei r tips . The rod s are most numerous and predomi nate near the edges of the reti na . Cones are interspersed with the rod s, but nea r the center of the retina is an area cons i sti ng a l most enti rely of cones. Th i s i s th e yel low spot ( macula l utea ) with a smal l depression ( fovea centra l i s ) in its middle. Di stinct vision occurs only for the part of the i mage that fa l l s on the fovea , and s i nce th i s covers an a ng l e only s l ightly larger than one degree, an object may be " seen " in deta i l only b y scan ning it. A t t h e fovea, e a c h cone i s connected to its own optic nerve fiber. E l sewhere in the retina, which conta i n s some 1 1 5 m i l l ion rods and 7 m i l l ion cones, . a bout 8 0 receptors are connected to a s i n g l e nerve fiber. E l ectrical i m pu l ses from the rod and cone cel l s travel a l ong the optic nerve to the optic lobe of the b ra i n , where the menta l pictu re of the scene is regi stered . THE RETINA
·
COLOR V I S I O N ( cone ce l l s ) does not operate at very low levels of i l l umi nation . By moon light, hues van ish, a nd only shades of gray rem a i n . U n l i ke the co nes, the rod s can ada pt to very dim li ght by in creasing their sen sitivity. They th u s prov ide a coarse but useful vis ion even by starligh t. U nde r such condi tions you may detect an obje ct ou t of the corner of your eye ( rod v ision ) that ca nnot be seen when you look directly at it. The rods sense o n ly brightness. Cones sense both brig htness and hue. Rod vision i s much more sens itive to fl icker and motion than is cone vision.
86
THE V I S U A L I M PU LS E i s p ro duced by the chang ing of visual pu rpl e ( rh odo ps i n ) to retinene. Rhodops in undergoes che m ical ch a nges u nder the i nfluence of light photo n s tha t res u l t in bleach ing of the rhodopsin to pale yellow retinene. The strength of the brightness sen sation depend s on the rate of b leach i ng. Since sensitivity of the rods depends on the amount of visual purple present, its regeneration must be fast and sufficient. This regeneration occurs most rapidly i n the dark and is related to the amount of vitamin A p resent. Lack of vitamin A reta rd s regenera tion, c au s i n g n i gh t blindne s s .
reaches the photochem ical ends of the rods and cones only after pas s i n g th rough the network of nerve ce l l s a nd gang l i a that lies a bove the m . These tip ends of the rods and cones are e mbedded in a layer of epith e l i u m conta i n ing pigment gran u l e s that optica l ly i sol ate the rod s and cones . The gang l i a above these layers sort visual i m p u l ses, a n d the ne rve fi bers tra n s m i t them to the bra i n . LIGHT ENTERING THE EYE
STR U CT U R E OF RETINA, head - on v iew ( ri ght) and secti ona l view ( below), cen tered on the fovea.
l ight
l
pigment layer
i s the most importa nt nerve o f the eye because it carries visual signals to the bra i n . Various motor nerves operate muscles contro l l i n g the m ovem ent of the eyeba l l and upper eyelid and the th ickness of the lens. The optic nerve i s a thick bundle of nerve fibers connected to the rear of the retina at a spot s l ightly off-center towa rd the nose. No rods or cones cover this spot so it i s i nsensitive to l ight. To detect th i s bl ind spot close the left eye and stare at the black dot at the lower left corner of th is page, holding the page about 1 8 i nches from the nose . Move the page a round ti l l the dot in the lower rig ht-hand corner (p. 8 9 ) becomes invisible. The left-hand dot wi l l disappea r i f you look with the left eye only at the right-hand dot. No blind spot i s a pparent with both eyes open, because the fields of vision overlap. The optic nerve from each eye leads to the visual area of the bra i n at the extreme back of the head . I n j u ry to th i s a rea can cause b l i ndness . In front of this region, in areas that a re poorly defined, complex visual asso ciation takes place. The optic nerve fi bers from the right eye appear to lead to the left side of the bra in; those from the left eye seem to go to the right side of the bra i n . Where they cross is the optic chiasma. Actua l ly there is not a com plete cro s s i ng that affects all the nerve fibers but an interm ixing. Nerves from the right ha l f of the reti na of both eyes go to the right half of the bra i n and record the l eft h a l f of the field of vision .
THE OPTIC NERVE
•
88
VISUAL F I ELDS An obj ect in the right h a l f of the visual field reg isters on the l eft h a lf of ea ch retina and i n the l eft s ide of the bra i n . In the d iagram , co lored ova ls represent overlappi ng visua l field s . The round, darker cen tra l a rea i s the v i s ual field of the fovea of the reti na. Areas seen by a single eye ( monocular field s ) a re shown in l i ghter colors.
retina THE OPTIC C H I ASMA, where nerve fibers from the inner sides of the retina cross over to the opposite hemisphere of the brain, is just behind the eye s . N erve fibers from the outer sides of the retina also pas s through the optic chiasma, but r�main on outer optic pathways.
THE V I SUAL CENTERS of the bra i n are at the rear of the right and left lo bes. I n fo rm ing a menta l p icture, an object o n an ind iv idual ' s right stim u la tes cel l s i n t h e left lobe. based on Ciba Collection of Medical Illustrations by F. Netter, M. D. © CIBA
projection on left lobe
projection on right lobe •
89
i s the most ve rsati le of all rad iation detectors. With i n th e reti na of th e eye a chemical re spo n s e to radi ation i s tra n s l ated i nto elec tri cal pul ses . Th ese very weak el ectrical mes sages trave l al mos t in sta ntaneo u s l y to the brain al ong the optic nerve fibe rs . Th e sensation of sight occ urs in the bra in . B eca use o f psycho log ical factors, the qual i ty of visua l sen sations can not be tra n s lated i nto phys ical da ta and th ere i s no way to compare the visua l sen sation s of diffe re nt peo ple with acc uracy . Wha t you see i s fo r the m ost pa rt sub jec tive -who l ly with i n the m i nd , a nd th erefore usually no t m ea s urab le. With i n your bra i n , other bod i l y sensations-ta ste, touch, smel l , and sou nd-are a uto matica l ly correl a ted with the visual one. The bra i n compares the resu l t with remem bered sensations from past experience, mod ifies them accord i n g to you r attitude and i ntent, and then produces i n you r conscious m i nd i n formation a bout what yo u see. Th i s i n formation constitu tes you r perception or awa reness o f ob jects . You r perception may be correct or incorrect, depending i n part upon attitudes and ex perience. An i l l u sion is a fa u l ty perceptio n cau sed by some u n u s u a l presentation of th e scene or by some pre j ud ice o r emotion of th e observer. H e may o r may not be awa re that th e scene i s not what it appears to be . The sight of a n object g ives i n formation about its size, shape, color, texture, location, and motion . Just how these features can be determ i n ed from the l ight emitted or reflected from an obj ect and its s u rro undings i s a major problem i n psychology. The phys i c a l processes of seeing are fa irly wel l u nderstood , and c u r rent research i s provi d i ng an swers to the physio logy o f vision . Much rem a i n s to be lea rned about the psychological a spects of an individua l ' s perceptio n . THE HUMAN EY E
90
a re affected by several kinds of cues wh ich are noted a utomatica l ly a nd ofte n uncon s c iously. You u se such cues a s convergence, superposition, e l evation , brightness, d i sti nctness, a nd kn own size to place objects m en ta l l y near or fa r i n a scen e. Th ese sa me cues l end a pa rtial fee l i ng o f real ity to ordi nary two-d i me nsiona l pi ctures a nd are effective even wh en one eye i s c losed. DISTANCE,
D EPTH
AND
CO N V ERGENCE of l ines i s fre quently a u seful cue in the j udg ing of di stances. D ista n t objects form sma l l e r images on the retina of the eye than do nearby objects. The prob lem is to determi ne whether a particu la r object is sma ll and clos e by, or l arge an d far awa y. In a picture, the convergence
MOTIO N
of l in es may indicate rece s s ion from a v iewer. Parallel lines of a high way seem to meet at the horizon. Every artist knows he must draw them th is way to make them look real . The a rt of making a th ree dimensional scene appear real on a two-d imensional su rface 1s cal led perspective.
l eft
Lines often converge i n two di rections i n perspective drawings.
I SU PERPOS ITION, known olso as overlay, i s a powerfu l depth cue. When one object overlaps another and partially obscures it, then the first object appears to be nearer. I n the i l l u stration above, the jack seems c loser
t o t h e camera. The a ctua l a r rangement i s shown at the right. Simply changing the angle of view dis closes the true position, with the clipped queen actually closer to the camera than the jack.
ELEVATION is a nother depth cue. I n a picture of a landsca pe, the horizon i s always h igher than the foreg round. Objects that are higher or farther from the bottom of the picture always appear to be a g reater di stance away. I n
the pictures be l ow, el eva tion ma ke s the tree at the ri gh t seem fa rther away and l arger than the tree to the l eft. The buil di n g ap pea rs larger in the picture to the left, but both trees and bu i ld ings are the same size.
BR IGHTN ESS D I FFEREN CES may give a false depth cue. This can be demonstrated easily, as in the i l lu stration here. When other distance cues are carefu l l y concealed, t h e apparent relative distance of two playing cards depends on their i l l u mi nation . The card that is given more il lumination and seems brighter than the other appears to be the nearer. Both cards, however, are actua lly the same d istance from the viewer. D I ST I NCTNESS-or sharp edges and clear deta i l s - a lso implies closeness. Haze and fog enhance the appearance of depth . They make distant objects less distinct than those nearby, and some not-so-distant objects become blurred . This tends to compress the scale of di stances. Sometimes i n a fog nearby objects appear unnaturally large ( right) because the viewer unconsciously compares them to what he believes a re more distant objects. Distinctness is a ffected also by a factor inherent in the lens of the eye and also in the lenses of camera s . When the lens is fo cused on a particular object, anyth ing nearer or farther away will be s l ightly out of focu s . T h i s effect is m i n i m i zed in a photograph taken with a very small lens open ing ( p. 8 1 ) Ob jects of equal distinctness are then judged to be equally distant, if other cues are absent. The loss of perspective in such cases i s shown here i n the photograph making it appear that the pole i s growing out of the g i rl ' s head. .
KNOWN ( OR IMAGI N E D ) SIZE of objects is p robably the most important cue to di stance. When a card is placed in a spotlight in a darkened room , a v iewer es timates its di stance based on his prior knowledge of the card ' s size. I f a larger card is then substitu ted, he changes his esti mate in d i rect proportion to the change in the card ' s size. Photo graphs that exaggerate the size of flsh employ the association of depth perception with size de termination . In the a bove illus tration, the size of the hand and of the man are cues to the size of the flsh. First cover the hand and then the man. The flsh will fi rst a ppear larger, then smaller.
MOTI O N PARALLAX accou nts for the apparent motion of ob jects seen from a moving car. I f you stare a t o n e obj ect some dis tance from the side of the road, the trees beyond it seem to be mov ing forward in the same di rection as the car, but tele phone poles near the road move back the other way . The whole scene appears to rotate about the object on which you flx your gaze. Th i s i s s hown below. The person is moving from A to B . The post a ppears to move backward, the tree forward. The same ef fect is obtained by turning your head. I t i s an important factor m the visual perception of distance.
to depth perception req u i re the cooperation of both eye s . Al l the cues menti oned so fa r are monoc u l a r cues -that is, they can be seen with either eye a l o n e .
BINOCULAR
CUES
B I NOCU LAR VISION produces a kind of station a ry paral lax-a very s l ight difference in the appearance and apparent location of objects becau s e your eyes view objects from s l ightly different positions . For o bj ects nearer than about 1 00 feet, the i mages formed in the l eft and right eyes are, in fact, s l ightly different views. The mind interprets th i s difference in terms of depth, e n hancing y o u r perception of a rea l , three-dimensional scene in stead of a flat, two-d imensional pic tu re. Binocular parallax is the major factor i n the space per ception of n ea rby objects. This effect i s totally lacking in ordi nary pictu res, but occurs in the images that you see in m irrors . For very close objects the mus cular effort of converging your
eyes on the point of focus gives you another indication of the distances of objects. A stereo camera i l l u strates bin ocular vision. I t has two lenses separated by the same distance as between the eye s , about 2 112 inches. The two pictures obtained with a stereo camera are s l ightly d ifferent and correspond to the views seen by the right and left eye s . I f the pictures a re a rranged so that the left eye sees only the picture taken by the left lens and the right eye only the picture taken by the right lens, the ap pearance of d e pth i s p roduced. To get a stereo view of the pic tures shown below, hold a piece of paper or cardboard between your eyes so that you v iew the left and right pictures with the correspond ing eye.
Th ese two pictu res were made with a stere o ca m e ra .
95
occur when our perception i s d i s torted by an u n u sua l or deceptive presenta tion of an object or by some pre j ud ice or emotion a bout it. O u r fa ith in the usual accuracy of our visual percepti ons i s strong enough to support the adag e : " Seeing i s be l i eving . " But we are often foo l ed by our eyes . What we see is strong ly infl uenced by what we think we see or by what we wan t t o see.
ILLUSIO N S
o....___--0 0 0
The two lines are of equal length but placing the cir cles farther out makes the upper l i n e appear longer.
An e l l iptical frame may appear c i rcu lar because shading and perspec tive lead you to expect it to be circular. In distorted Ames ' room ( below) the left cotner of the room is much farther away than the right.
Both men a re about the same height. A floor plan reveal s 1n part how i ll u sion was c reated. floor
of
plan
Ames ' room
front
Concentric circles appear to make a spiral ( right). Even when the observer i s i n formed that this is an i l l u s ion, he is unable to see it otherwise. A d ra ftsman ' s com pas s can be used to prove that all arcs have a common center.
The two horizontal lines ( below) are parallel, but the arrangement of the other l i nes makes the hori zontal lines seem to pull apart at the center.
I ndentations appear as depressions ( l eft below) and as rai sed sur faces ( right be l ow) because the
observer interprets the light as coming from above. The photos a re identica l ; one is inverted.
97
Raindrops change sun light into a primary rainbow by refraction and by a s i ngle reflection of the rays with in each d rop. Th i s action makes visible the colors already present in the light.
I n side the water d roplets of a secondary bow, the light rays are reflected twice, reversing the order of the spectrum. A viewer sees each color at a different angle relative to the s u n .
THE NAT U RE OF COLO R
I n 1 6 30 the French phi losopher Desca rtes attributed the color of an object to a change i n the light when i t is reflected from the object. Unti l that time it had been thought that l i ght had no color; th at color belonged to objects and that l ight merely made it vi s i b l e . The mod e rn concept o f color fo l lows Descarte s ' poi n t o f view. Color, l i ke l ight, i s a psychophysical con cept, d epending u po n both rad iant energy ( the phys i c a l stimu l u s } and visua l sensations (the psycholog ical respo n s e } . Color and l i g ht are closely re lated terms. By defin i tio n , color in c l udes all a s pects of l ight except variation s i n time and space. For example, the d i stribution of s u n l ight and shadows on a lawn i s not concerned with the color of the l i g ht . N o r does the fl i c kering of a ca nd l e flame change the color of the light. 98
i s one a spect of your v i s u a l experi en ce when yo u look at an ob ject. The color you see depe n d s on the i n ten s i ty a nd wavelengths of the light tha t i l l u m i n ates the object, on the wa ve l engths of l i g ht r efl ected or tra ns m i tted by the object, on the col o r of the surrounding objects, and o n a bsorption or reflection by substa nces in the l ight path . I n a space capsule, the sky appears black. From the ground, it looks b l ue beca u se of scattering by atmospheric parti c l e s . A cloud may be wh ite where the sun l ight i l l u m i n ates it from a bove o r gray where s u n l ig h t d o e s not strike d i rectly, o r it may possess any o f the hues of a tropical s u n set. The cloud does not change color, but our perception of i ts color changes.
COLOR
THE COLOR O F WATER va ries. An observer flying over the F lori da Keys m ig h t we l l see the ad jacent ocean a reas as g reen, blue-green, and blue. A photo g raph, such as the one shown at right, can captu re th is same effect. Shallow, clear water with a sa ndy bottom will produce a l ight g reen because of the com bination of the yel low reflected from the bottom and the reflec tion of the blue sky. As the water gets deeper, it becomes blue green because there i s more scat tered blue light from the sky and less yel low reflected from the bottom . When the water is very deep with no reflection from the bottom , the water i s a deep blue, pa rticularly noti cea ble in the Gulf Strea m . At no time is the color a property of the water. It i s due to many causes and i s best described as a charac teristic of the light received by your eye.
of color sen sation are h ue , sa tu ration a nd brightness. Non e o f these is directly meas urab le. Si nce m easurab l e ph ys i ca l as pec ts do not acc ura tely specify color, the an swers must be fo_und i n a combinatio n of th e two- th e psychophysica l va ria ble s : do minant waveleng th , purity, a n d l u m i n ance. The eye can not d i stinguish the com ponent wavel engths i n a color sample. Two l ights of d i fferent colors when m i xed prod uce a th i rd color, and no human eye can detect its composite nature . Scienti sts working with color try to pred ict the results of such color m ixtu res . They aim to describe and measure colors as acc u rately as possible. Th i s can be done for the physical part of color. The energy di stribution of the l ight can be plotted as shown below. But the color sensation of the observer his respon se to the l ight ente ri n g his eyes -can not be shown i n the same way. TH E
PSYC H OLOGICAL
ASPECTS
low and monochromatic blue. The EN ERGY at various wavelengths is shown on the two graphs below. brain interprets this mixture as A vertical solid line on each white. If the intensities of the represents the same green color daylight and the yellow and blue (in rectangle), which is tinted with light are adjusted with the green white light. In the left graph, the of the solid line to give the same white is that of daylight, a light tint, the eye sees the same tint mixture of many wavelengths. In of green in both cases, even the right g raph, the white is a though the spectral compositions combination of monochromatic yei- are quite different. Energy Energy 1 600
3 00
1 2 00
2 00
800
1 00
400
0
4000
A
5000
A
6000
Wavelength
A 7000 A
-�---=-:::�-t t I
rt
I t I I
I
0
4 000
A
! I
5000
A
6000
Wavelength
A
7000
A
HUE
SATURATION
pale magenta
BRIGHTNESS
.w hite gray
The three types of color sensations
is the color sensation by which you d i sti nguish the d i fferent parts o f the spectr u m - red , bl ue, g reen, yel low, etc . The psychophysical va riable rel ated to h ue i s the domi nant wavel ength of the l ight for each color. Most l ight samples can be color-matched by adding the p roper spectra l l y pure ( monochromatic ) l ight to wh ite l ight. For these l ight samples there i s only one spectra l ly pure l ight that wi l l g ive a perfect match . The wavelength of that l ight i s ca l l ed the dominant wave length of the s a m p l e and i s a mea s u rable quantity. HUE
Twenty h u e s o f t h e some lightness and saturation are shown here. U nder ideal con ditions the human eye con distinguish wave length d ifferences as small as 2 0 Angstro m s . Thus a carefu l observer m i g h t divide the v i s i ble spectrum into more than a hundred d ifferent hues. I n addition to the spectral hues, a series of purples ( magentas ) , which ore non-s pectra l hues, can be obtained by adding together d ifferent amounts of light from the red and blue ends of the spectru m .
Saturation of red from low at the left to high at the right.
SATURATION refers to the deg ree of hue in a color. I t i s th e c o l o r sensation b y wh ich you distinguish a hue as being pa l e or rich, wea k or stron g . Pink, for example, usually d enotes a red of l ow satu ration, while scarlet is a h i g h l y satu rated red . PUR ITY i s the psychophysical qual ity most closely related to saturation . .The purity of a color sample i s the ratio of the amount of monochromatic light to the amount of white light in a mix ture requi red to match the sample. Monochromatic light has 1 00 per cent purity; white light has zero. Colors of the same pur ity do not all have the same saturation . The two a re closely related but not identical . Pure yel l ow, for ex a mple, is much less satura ted
than pure violet. lu m ina nce (p. 3 2 ) also affects saturatio n . Blues, reds, and purp les a ppear mo re satura ted at low lumi nanc e . Yellows a nd cyans ( blue-g ree n s ) need h igher luminance t o ac hieve the same deg ree of saturatio n . Afte r a certa in poi nt a n inc rease in lum ina nce d ecreases saturation . The two reds below are of the same dominant wavelength but of different pu rity and will appear to be of the same hue but of different saturation .
H IGH PU R I TY
� ·�
l-s-ot·t�rcl ti4)1)----L:__+--·-l
� t--sefl:tt'Eti'fE)Il-�'---t------f i5
� �----�--�7'-�--�---4
� t------::1�----�-----1
4 000 A
4 000 A
6000 A Wavelength
1 02
LOW PURITY
6000 A Waveleng th
SCALES of satu ration and hue are combined here in a single diagram. The color blocks in the circular scale show how one hue blends into another. Each hue has the same lightness (p. 3 5 ) and satu ra-
tion . Color blocks in the s pokes show gradations in satu ration from zero at the center to a maximum at the outer block. Other spokes could be added, one s poke for each hue in the circular scale.
BRIGHTNESS ( p . 3 2 } is the primary vi s u a l sensation by which you detect the presence of l ight. It is a ssociated with the qua ntity of the l ight and the inte n s i ty of the visual sensation . Lumi nance, ea s i l y measured, is the psychophysical va riable usua l ly associated with b ri g htnes s . Although h ue, saturation, and brightness may be sepa rately identified a s color sen sation va riables, they are not i ndependent of one another. When one variable is changed the othe r two a re often affected . Decreas i ng the brightness, for example, can cause a change i n the satu ration or even in the hue of the color.
1 03
red
red blue
green PSYCHOLOGIS T'S PRIMARIES
blue ARTI ST ' S PRIMARIES
PRIMARY COLORS a re simply hues you sta rt with to m i x others. Des i g na ting certa i n hues a s primaries i s a n a rbitrary convention that depend s o n who makes th e selection a nd whether l i g hts or object colors are u sed . Bl ue, green, and red a re usua l ly u sed a s the physicist ' s primaries i n light experiments . Howeve r, any th ree widel y sepa rated monoch romatic colors can serve . The psycho logical primaries are blue, green, red , and yel low, for each seems to i nvoke a singular response which does not invo lve any of the other colors. All other co lors may be descri bed i n term s of these . The same color term used by d i fferen t groups of people may, however, refer to markedly d i s s i m i l a r colors. Red, yel l ow, and blue a re the artist ' s primaries. By mixing appropriate amounts of these pigm ents, a l most any other hue can be produced . The a rtist _ i s dea l ing with object colors; the physicist i s working with colored l ig hts; the psycholog i st is interested in both .
Additive red
PHYSI CIST' S PRIMARI ES
Subtractive
are any two hues which pro duce white when m ixed together i n some proportion . ( " White" here refers to a n y h u e l ess, or achromatic, l ight. At low l u m i na nce it wo u l d be ca l l ed g ray. ) Every hue has a complementa ry hue. The two hues of a com ple menta ry pa i r are widely separated i n the spectr u m . The complement of red i s blue-green; the complement of ye l l ow is blue. Compl ementaries of t h e g r e e n s are purples, hues not found i n the spectru m . M ixing o f complementary l ig hts does not prod uce inter mediate hues. With proper adjustment of the i ntensities of two complementa ry l i g hts, white l i ght i s obta i n ed . Thu s white l ig h t can b e prod uced b y many combinations of complementa ry l ights . COMPLEM E NTA RY HU E S
The curve shows complementa ry wavelengths for the standard ob· server . Perpendicu lars extended to the two scales from any point
on the cu rve will reveal the wavelength s of two complementa ry hues. Complementaries to g reen cannot be obtained from the graph .
Wavelength 6000 A 5500 A 6500 A 7000 A 4000 A ,--- •.----.,.-,-------wr------,
perpendiculars 4250 A L
0, c::
Qj
(I)
4 5 00 A
� � 4750 A
5000 A
1 05
may be achieved by either additive or su btractive method s . The blending of colored l ights from more than one sou rce i s add itive, wh i l e the pas sage of l ight through success ive colored a bsorbers ( fi l ters) i s subtractive . Pigment m ixing i s mainly su btractive even thoug h paints a re added to one another. It is s u btractive beca use pigmen ts absorb ( s u btract} some wavelength s of the l ight stri king them and reflect the rema i n i n g wave length s wh ich you see . Additive mi xtu res of red, g reen , and blue l ights of proper relative l u m i na nce prod uce white where a l l overlap, and cyan, magenta, and yel low where they overlap i n pa i rs ( below). COLOR MI XTU R ES
A D D ITIVE COLOR M I X I NG i s ea s i l y accom p l i s h ed by projecting pure ( monochromatic ) l i g hts from two or more proj ectors onto a white screen. Each proj ector i s eq u i pped with a set of fi l ters which perm it it to prod uce l ig h t of any des i red wavel ength . When a beam of red l i g h t is proj ected so that it overlaps a beam of yel low l ight ( 1 , a bove ) , the res u l t i s orange. I f the relative i n tensities of the two proj ectors a re adju sted, it i s pos s i b l e to change g rad u a l l y the hue of the m ixtu re from orange toward either red or yel low. If the red light i s kept constant and the yel low l ight made more g reen , the hue of the m i xture becomes ye l lower ( 2 ) , and the satu ration becomes less and less. Finally, a wave length i n the b l ue g reen wi l l be reached at which the m ixtu re wi l l lose a l l hue, beco m i n g a chromatic, or wh ite ( 3 ) . Th u s red a n d the bl ue-green are comp l ementa ry hues. I f the va riable l ight i s changed from b l u e-green to violet and the red sti l l kept co nstant, a non-spectra l purple ( 4 ) res u l ts .
1 07
yel low STATIONARY WHEELS
SPI N N I NG WHEELS
of m i xing co lors, i n add i ti on t o using two or mo re c olored bea m s fro m separate projectors ( p . 1 07), i n c l ud e the p laci ng of fi l ters i n ra pi d success ion i n front of a s i ng l e proj ector. Th i s makes use of the persi stence of vision -the retention of an image fo r a fraction of a second after the sti m u l u s has ceased . I mages presented i n rapid succession fuse i n to one another to form a compos ite image. A simple device u s i ng this effect i s the color wheel . By a d j u sting the size of the col ored sectors and spinn i ng the wh ee l rapidly, d i fferen t h ue s m a y be prod uced ( a bove) i n c l u d i n g neutral g ra y ( be low ). T h e h u e s a r e rather u nsatu ra ted a nd t h e l u minance norma l ly is not sufficient to make the g ray seem ' ' wh i te. ' '
OTH ER AD DITIVE METHODS
STATIONARY WH EEL
1 08
SPI N N I NG WHEEL
I n the fal l when the leaves have changed color, d istant views of the landscape provide excel lent examples of natural additive mix-
ing . The co lors of individ u a l leaves fu se to give a single co lo r to a tree, and fa rther away all t h e trees blend i n to a sin gle hue.
i s the term u sed to describe the way you r eye m ixes the colors of i ndivi d u a l l eaves i n a n a utumn landscape, t h e dots of a c o l o r te l evision screen, or the i n d ividual colors of sand g ra i n s o n a d i stant bea c h . When there a re th ree or four point sources of colored l ight widely separated , they a re focu sed sepa rately on the reti na, and the eye sees them as d istinct colors . Th i s sepa rate focu s i ng i s ca l l ed reso lving. I f the point sources are moved c loser together, eventu a l ly they wi l l a l l be focu sed with i n the same group of rods and cones on the reti n a . They cannot be resolved any longer, but are seen as a single a rea even though they are d i stinct poi n t sources. The color sen sed i s a mixed color, i ts h ue depending on the spectra l q u a l i ties of the colors o f which it i s composed. I f you i n c rease the resolving power of you r eye by exa m i n i ng a s ma l l area of a page of colored comic strips u nder a hand lens, you will see that there a re n u m erou s dots o f diffe rent hues. Without the l e n s , ad jacent dots b l e nd together and form a s i n g l e h u e . MOSAIC FUS I ON
1 09
occu rs whenever l ight passes through two or more selectivel y a bsorbing mate ria l s . Dyes , and a l so pigments, may be mixed together to form a single coloring agent ( colorant) that wi l l pro duce selective a bso rption. Sepa rate fi l ters which are traversed by the l ight i n a ny order a l so cause s u btractive color m i x i n g . Even su rface fi l m s from which the l ight is successively reflected su btract l ight. The process i s ca l l ed " su btractive" beca use the fi l m s or fi l ters su btract certa i n portions of t h e s pectrum from t h e i ncident l ig h t, l eaving the rema inder with a hue com pl ementary to that sub tracted. A piece of glass wh ich absorbs the blue and green wavel engths of wh ite light a ppea rs red becau se it transm its on ly the longer wavel eng th s . I f the g l a s s absorbs the green , i t tra n s m i ts a m i xture of red and blue magen.ta . At each wave length, the fi l ter obeys the law of ab sorption ( p . 5 1 ) . The fi l ters not only change the color of the l i g ht, but they reduce its intensity . I f addi tional fi l ters of the same type and thickness are placed i n the l ight path, each wi l l a bsorb the same fraction of the l ight. That is, i f one th ickness tra ns m its 1h of th e l ight, two th icknesses wi l l tra n s m i t 1/4 , S U BTRACTIVE COLOR M I X I NG
Examples of su btractive color mixing are shown below with pain ts ( pigments) and filters . In the former the colors that
F I LTERS
PIGMENTS
Yel low
Crimson +
are not absorbed are reflected from the s u rface. I n filters the colors not absorbed are trans mitted th rough the filter.
Yel low
W H ITE LIGHT, represented by its red, blue, and green compo nents, i s shown entering various combinations of fi lters from the left. Severa l combinations of yel low, cyan and magenta filters are shown . Each i ndividual filter
a l lows all but one of the colors entering it to pa s s . Each pair o f filters (as in A , 8 , and C ) allows a different pa rt of the wh ite light to pass th rough . I n the last ex ample ( D ) , n o light pa sses the th i rd filter.
three thicknesses w i l l transmit 1/a , and so on . When a fi l ter con s i sts of a colorant disso lved i n a solvent, dou bl ing the concentration of the colora n t i s equiva l ent to dou b l i ng the thickness. The appearance of a colored fi lter may give l i ttle i ndication of its spectra l a bsorption c u rve. Two fi lters that appear identical may transmit l i g h t of d i fferent colors wh en combi ned with another colo r ed fi l ter. The artist who i s fa m i l iar with h i s paints can more eas i l y pred ict the res u l ts of his m ixtu res . H e knows that mixing Tha lo B l ue, Alizarin Crimson, and white in the proper proportions w i l l produce lavender. For h i m , sub tractive color m i x i n g i s a matter of experience. 1 00 %
- - - 1 00 %
fi lter
fi lter
fi lter
a bso rption curve l ight
l
3 4 fi lter thicknesses l
2
5
1 1 1
i s the physical process of adjust i ng a color mi xture until it a ppears to be visually the same as a sample color. The manufacturers of pa i nts and dyes, of textiles a nd plastics, of toys and cars, and of books and maga zines, a s wel l a s those wh o print color film, are vita l l y concerned with g ood color matc h i ng . E ith &r ad ditive m ixi ng o f colored l i g h ts o r subtra cti ve mi xing u s i ng fil ters m ay be uti l i zed to achieve a color match . Any color of the spectru m can be ma tc h ed by the addition of monoc h romatic l ight to wh i te light. Purple, a m i xture of red and viol et, i s not a col or of th e s pec trum and cannot be matched by th i s m ethod bec ause it lac ks a s ing l e dom i nant wavel ength . But a pu rp l e color can be identified by the wave length of its c om plem enta ry color . Light of th i s color mixed with purple l ight prod uces a colorless l ight. V ari ous col or matc hing system s (p. 1 2 5 ) a nd co lor i m ete rs ( p . 8 3 ) are ava i lable. Color stan da rd s - sets of color pa tches arra nged system atica lly for v isua l com pa ri son -are ofte n preferred to colorimete rs . Color standa rds and colori m eters can be cal i b rated i n s i m i lar un its and da ta can be conve rted from one syste m to the other. COLO R MATC H I NG
C OLOR CHARTS are useful tools i n the matc h i ng of co lors. Print ers use co lor charts that give i n numbe rs or percentage the dot value of different i nks that is requi red to reproduce a given color. T he co lor squares that provide this information usually have punched holes so that the color in a square can be eas i ly compa red to the color in the i llustra tio n to be rep roduced.
I n old color vision tests, the testees sorted wool skeins by hue, trying to select a l l skeins that accurately matched large one.
i s an everyd ay term refe rr i ng to a ny pron ou nced deviation fro m norm a l color vi sion. Th e th ree major forms of color a bn orma l i ty a re a no m a l o u s tric hro ma ti s m (a less s evere depa rture from norm a l vi sion ), di chromatism (partia l co lor bl i ndness ), and monoc hro ma ti sm (complete c olor bl i ndness ) . T h e norm a l observer, known a s a trich romat, c a n match any color with one, two, or three col ored l ights . Under contro l l ed con d i tions, most tric h romats wi l l use about the same combi nation of sel ected l ights to match any particu lar co l o r . An i ndividual who m u s t a rrive at his color match i n a ma rked ly different way from the aver age tri c h romat is said to have a bnorma l color v i sion . Thi s i s u sua l ly an i n h erited defect. I t becomes evident i n many school and home situations and i s confi rmed when the i ndividual i s a s ked to match two colors or to d i sti nguish between d ifferent hues. The cause o f abnorma l color vision i s not fu l l y known . There may be some d efect i n the cones of the reti na, s i n ce it i s the cones that distinguish color. Th ere may be too much or too l ittl e of essenti a l pigments i n the eye . In ra re cases, the optic nerve between the eye and the bra i n may not function properly. About eight per cent of the male popu lation and less than one per cent of females are born with defective colo r v i sion . Ab no rm al color vis ion can a l so be acqu i red.
COLOR BLI N D NESS
1 13
color to be m atched
\
color matching by anomalous trich romat
observer ANOMALOUS T R I C H ROMATISM is found i n about three-fourths of all people with abnormal co lor v ision . An anoma lous trich romat sees colors, but not normally. H e can match any co lor with a mixture of three colored lights, but will requ ire d ifferent amounts of these l ights than the norma l observer. H e is poor at mixing or match ing color s . If h is vis ion i s green weak (the commonest type ), he w i l l need more than the norma l amount of green in a green-red mi xture to match a partic ular yellow. Other anoma lous trichromats have red or blue wea knesses.
MON OCHROMATISM, or com p lete co lor blind ness, is very rare . O n ly about one person in 30,000 is affl icted. In its typical in herited form , equa l l y common in men and women, the monoch romat depends solely on rod vision . Since h i s eye lacks the high reso l ution of the foveal cones, his visual acuity i s l ow . The mono chromat is unable to di stinguish any colors . H e can see on l y dif ferences in brightness and can match a l l lights with a single l ight. Relative lumi nosity is his only criterion a s shown i n the comparison of normal and mono chro matic spectra below.
Hue and saturation as viewed by a normal observer.
0 c:
e
�
4 5 00 A
�
0
5 000 A
5 5 00 A
6000 A.
6500 A
A typi cal monochromat ' s response to the same chart.
"'
� "'
4 5 00 A
1 14
5 000 A
5 5 00 A
�.
-� m
··% """''
�w <
6000 A
6500 A
DICH ROMATISM occurs in about 2 per cent of wh ite males, but i n only 0.03 per cent of fema les. About one-qua rter of all people with defective color vision are dichromats . A dichromat can match all colors with mixtures of two primary lights rather than the three used by a norma l observer. One common form of di chromatism is red -green bli ndness which permits the individual to see o n ly two co lors -yel low and blue. A neutral g ray is seen in-
stead of blu e-green an d pu rp le . Da rk reds, g reen s, and grays are confu sed, and sens itiv ity to brig htness is dec rea sed by a bout hal f. In a s im ilar, equal ly common conditio n , a pe rson sees c o lors l i ke the dich rom at but gets no re ac tion at al l from the lon g-wavelength end of the spectrum. Sti l l rarer are those kinds ot defective color vision in which a person does not see yel low and blue. These conditions may in volve lack of sensitivity to short waves.
HUE A N D SATURATION COLOR WH EELS
as seen by dichromat with red-green bli ndne ss N ormal Male Feli"\o le
D
0
Carrier
•
Color B l i nd
•
•
Generatio n I N H E R I T E D COLOR B L I N D NESS is a recessive condition . I f one pa rent is normal an d the oth er colo r bl ind, ch i ld re n wil l be nor ma l but fema les will be ca rriers. If a perso n ca rrying color bl ind ness (tho ugh appe aring normal ) ma rr ies another carrie r, some ch i ld ren wil l be col or blind and oth e rs will be ca rr iers. The dia gram shows the passibi lities . V
VI
Ill
1 15
D I C H RO M AT I C COLOR B LI N D 1 1 5 ) can be detected n e ss ( p. by the use of specia lly colo red te st plates, each conta ining a patte rn or number made of d ots against a ba ckground of dots of another color or gray . A person with normal color v ision can see the patterns . One with defective color vision will find some of the patterns confu s i ng or absent because of h i s inabil ity to distingu i s h the colors of which the patterns a re made. At the top of this page, two p lates used
1 16
to di sting uish red-g reen color bl in dness from norma l v i s ion are re prod u ced in their a pprop riate colo rs . A pe rson hav i n g re d-green co lor bl indn ess sees on l y g ray dots . The type of plates repro duced at the battom of the page are used to diagnose blue-ye l low defects . These reproductions ca n not be emp loyed for va lid tests because special i n ks and printing tech niques must be used to ach ieve the exact hues seen as g ray by co lor-blind indiv id ual s . A doctor ' s cha rts a re m ore accu rate .
are necess a ry to be sure that a l l testing fo r color vision ta ke s place under sta ndard view i ng con dition s . Th e i l l u m ination is important. For exam ple, the results m ight d iffer depen d i n g o n whethe r the tests took place u nder a rtifi cial l ight or in s un l ight. Two color samples that a ppea r to match under i l l um i na tion A may not match under i l l u m i nation B, even though a normal observer says that i l l u m i nation A matches i l l u m i nation B . T o avoid th i s d i ffi c u l ty, sta ndard l ight sources ( p . 1 7) a re u sed for i l l u m i nation of sa m p l e s . Under such sta ndard l i g hting c onditions, a l l observers wou ld sti l l not see th e same thing. But by averaging the visual charac te ristics of many individuals, a composite ' ' standa rd ob server' ' ha s been deve loped who a lwa ys sees things in th e same wa y. The International Com mission o n I l l u mina tio n (see p . 1 3 0 ) has devi sed a standa rd ob serve r of th i s na tu re . This fi ctitious individual i s s i m p l y a set of vi sual sen s itivity c u rves a nd energy d i str i bution d ata by which th e re sults of color experiments can be ca l c u l a ted a nd sta ndardized . STANDARDS
1 .0 A luminous efficiency curve shows the abil ity of dif ferent wavelengths to stimulate v i s i on . Thus it is the spectra l sens itivity curve for humans. Lumi nous efficiency curves nor mally fa l l betwee n the l i mits s hown here ( cu rves a and b). H umans ra rely match exactly the curve ( c) of the fictitious stan dard observer.
� 0.5
·o If Q)
w
"' ::;) 0 c
·e
::;) .....
0.0
4000
A
6000
A
Waveleng th ( Ang stroms)
1 17
CO LOR PER CE PT ION
Color perception depends l argelx on h u m a n physiology. Man has th ree vi sual pigments, each i n d i fferent cone sha ped receptor cel l s . One pigment senses primarily blue l ight, one primarily green, a nd one pri m a r i l y red. These cone-shaped cel l s a re tig htly packed i n the foveal reg ion but a re i nterm ixed with rod-shaped cel l s e l s ewhere in th e retina . B ecause of these three pigments, man i s able to d i s c r i m i nate among a wide range of colors . Thei r presence wa s postu lated in 1 8 0 1 , but they have only recently been identified with i n the reti na . Previou s l y it was thought that the i m p u l ses from the con es trave l ed three discrete pathways to the bra i n . Evidence now indi cates that i m pu l ses from t h e three types of receptor con es a re somehow combi ned into a coded signal prior to transm ission from the eye to higher visual centers in the bra i n . The combination ta kes place i n the ganglion ce l l s , nerve cel l s located on the opposite s ide of the reti na from the cone cel l s . Changes in wavel ength or intensity of the color sti m u l u s change the patterns of coded signa l s .
5
The three cone pigments that sense blue, green, and red have spectral sensi bil ity cu rves with peaks at 4 , 4 70 A ( b lue-vio let ), at 5 ,400 A ( g reen ), and at 5 , 770 A ( ye l low). Even though the red receptor pigment has its pea k in the yellow, it extends i nto the red so the brain senses red.
1 00
·-g_ 0 .D 75 <( 0
-
c:
u 41
v
a...
1 18
25 0 4000 A
5000 A
6000 A
Waveleng th ( Ang stroms)
2 M ODES OF PER CEPTION Co lor i s percei ved as be l ong ing to a l i g h t source when the l ig h t sou rce is included in t h e field o f v i e w ( 1 ) . Th i s is ca l l ed t h e il luminant mode of co lor pe rceptio n .
I n the illumination mode of color perception, the light source is not in the scene, but the d i rection and qual ity of the light i s evi dent from the pattern and contrast of the shadows ( 2 ) .
3
The object mode of color per ception occurs when l ight i s dif� fusely reflected from an object, and we perceive the color of the surfa ce ( 3 ) . The color perceived depends partly on nearby objects . The volume mode of color per ception occurs when light pa sses through a tra n s l u cent or tra ns parent substance such a s a col ored liquid or glass ( 4 ) . I nternal qualities predominate rather than the su rface ones. The funda menta l or aperture m ode of co lor perception occu rs if an object is vi ewed through an ope n ing tha t exc ludes the sur roundi ngs ( 5 ) . S im plest and most eas i ly reprod uced , th is method is often used in ex perimenta l work .
5
Colors of objects appear differ ent under different illumination even though to the observer the lights appear to be the same. White incandescent light i llumi-
nates objects on left. Complemen tary red and blue-green sources, which appear white to the ob server, produced the illumination on the right.
is a term u sed to descri be the ab i l ity of the h uman eye to com pens ate fo r o rd inary cha nges i n i l lum ina tion and viewing con d i tion s tha t affect th e color of a parti c u l a r object. You can tel l the c olor of a n object by looking at it u n der white light. U nder a d ifferent i l l u m ination you may sti l l be able to tel l the coJor of the object, but perhaps not so easi ly. Light a nd color reflected from the object may vary g reatly with changes in i l l u m i nation . A yel low book in the sunl ight reflects relatively much more blue l ight than does the same book under a n incan descent lamp, but our eyes have l ittl e trouble i n recog nizing th e same book and th e same object color. Th is is col o r constan cy. Becau se of th i s re ma rka b l e property of h uman vision, you recogn ize a nd identify objects in spi te of wide l y differen t con d i tions of i l lum ina ti on . C olor con sta ncy is evi dentl y a form of eye a da ptation to th e preva i l ing i l l um inati on . The eye adapts s i m i larly to changes m brightnes s (p. 3 2 ).
COLOR CONSTANCY
1 20
The blue patches in each of the fou r rectangles a re the same size and color. They appear to be different, however, beca use of simul taneous contrast between them and the colors around them.
i nc rea ses wh en two c olors are plac ed side by s ide. Th eir d ifference seem s exaggerated . I f an orange is presented n ext to a yel l ow, for i n sta nce, th e orange wi l l appear redder a nd th e ye l l ow gree ner. This i s s i m u l ta neou s c o l o r contrast, wh ich acco unts fo r th e fi n e d i s cri m i nation of t h e h uman eye in colo r matc h i ng . Two colo rs that seem to be iden tica l whe n viewed separately wi l l o ften be found qu ite d i fferen t if p resented side by side . A s i m i l a r effect ( successive color contrast) occurs 1 t t h e s a m p l e s a re presented one after t h e o t h e r in q u i c k succession . These two effects ( si m u ltaneo u s a n d successive contrast) a re related . In color match i n g , you r gaze moves rap idly bac k a nd fo rth from one sa m p l e to the other when you view them side by side. All your seeing is done with a natura l sca n n i ng motion, and n ever with a completel y fi xed gaze. B oth effects enhance the con tra st between colors of d ifferent hue, ca u s i ng each to move towa rd the complementary of the other. CONTRAST
1 21
effects occur not o n l y when two s a m p l es are diffe rent i n hue, b ut a l so when the sam p l es are ident ical i n hue but differ in saturation ( above ) . The i r difference i n satu ra tion i s accentu ated , a n d the one of lower saturation on the right tend s to acqu i re th e complementa ry hue. When two l ig ht sources, one wh ite and one of any hue, p roduce sepa rate sh adows of th e same obj ect, then each s hadow i s i l l u m i nated by one of the l ig h ts - the one not producing the shadow . The backgrou nd i s i l l u m i nated by both of them a n d appea rs in the hue and saturation of the combi n ed i l l u m ination. Under such conditions, th e two s h adows wi l l tend to appear complementary to each oth er. The wh i te-i l l u m inated shadow on the l eft ( below) actua l l y a ppears magenta i n contras t with the g reen i l l u m in ated s hadow on the right. Th i s i s a n i n teresti ng exam p l e of s i m ultaneous color contrast.
S IMULTANE OUS CONTRAST
i s the visua l sensation observed afte r a l ight sti m u l u s has been removed . A negative afterimage i s a phenomenon that i s caused essenti a l l y by fati g u e in some part of the visual sys tem . As you r eyes adapt to a parti c u l ar color there is a dec rease in the i r sensitivity to that color. When you r gaze i s s h i fted to a neutra l area, an afterimage a ppears which is complementa ry to the origi n a l sti m u l u s . Th i s is ca l l ed a negative afterimage. Hold th i s book at reading dista nce and stare at the c ross in the green c ircle ( above) . Then look at the dot to the l eft. You wi l l see a magenta ring. Look at the c ross in the red c irc le ( be l ow) and the n a t the dot to its right to see a green negative afterimage. A positive afterimage i s a far more fleeti ng experi ence and d iffic u lt to produce. One way is to stand in bri l liant s u n l ight with you r eyes closed and furth er cov ered by you r hand . Remove your hand and gaze q uickly ( fo r a bout 2 o r 3 seconds) at either of the c i rc les on thi s page ( wa rn i ng : do n o t l o o k at t h e s u n ! ) . C l o s e you r ey � s im mediate l y and you may get the i m pression tha t you a re seeing a circ le of the same color behind you r eye l i d s . This positive afterimage wi l l seldom last longer tha n 5 to 1 0 seconds. It i s cau sed by th e persi stence of color vision .
A N AFTERIMA GE
•
1 23
neither pos i tive nor nega tive, a re seen if you look steadily at an u n s haded light b u l b . After a few seconds, put out the l ight. For a short time afterward, as you sit in the darkness , you wi l l see a succession o f varying bright colors . These colors have no apparent rel ationship to the wh ite l ight stimu lus. I f, i nstead of sitting i n darkness, you turn from the light bulb to ga ze at a uniform white s u rface, the co lored afterim ages yo u see wi l l becom e progressively more com pleme ntary to those seen in the dark. S UCCESSIVE A FTERIMAGES,
SPREADING E FFECT, i l l ustra ted bel ow, i s a seem i ng con trad iction to s i mu lta neou s color contra s t ( p . 1 2 1 ), in wh ich the contra st between two colors i ncrea s es when th ey are placed side by side . The sa me red i n k was u sed th ro ughout the top strip a nd the same b l ue was u sed th ro ughout th e bottom. From what happens d u r ing s i m u l ta neou s contrast, yo u m ight expect th at th e b l a c k ink next to th e red wou ld make th e red a ppea r l i g hter, or th e wh ite n ext to the red wou ld prod uce a darker red . Due to the spread i ng effe ct, h owever, th e oppo s i te occurs. Th e b l ues surrounded by white a ppea r less saturated , too, th an th ose su rrou nded by b l ack. The spread i ng effect, a s we l l as the phenomena o f afterim ages m ay b e t h e res u l t o f t h e bleac h i ng o f cone pigments i n th e ret i n a , a nd their sub seq uent diffus ion i nto neig hbor i ng cone ce l l s .
C O LOR SYSTEMS Experts h ave long sought a foo l proof system for speci fy i n g c o l o r . They wou l d l i ke to identify o r describe the color of an object or l ig h t so that it can be reproduced with accuracy at a nother place or time. To do th i s they m u st be able to state the color in ter m s so unequ ivoca l that a color match ca n be made with reasonable certa i nty with i n l i m its that are visua l l y accepta b l e . Idea l ly, the system wou l d wo rk, whether the object being observed was a ba l l of woo l , an automobi le with a g lossy fi n i s h , o r a l i q u i d dye . The mode of observation ( p . 1 1 9 ) sho u l d n o t affect th e color matc h . O bvi o u s l y , n o color di ctionary ca n h a n d l e th e fu l l range o f colors. The best i n u se today i n c l u d e s less than 4 ,000 color n ames, a lthough some 1 0 m i l l io n colors a re sa id to be d i stinguishable. A vo l u me of named color sa m p l e s m ig ht be u sefu l with i n th e pa int or texti l e i n d u stri e s . But those s a m p l e s probably wou l d not d u p l i c a t e th e m a n y glossy and meta l l ic co lors u s ed on a uto mobi l e s today. O f the th ree system s descri bed on the fo l lowi n g pages , the Munsel l , th e Ostwald and the C I E , the last i s th e most complex but the l east subjective a nd p rovides th e high degree of co lor-m atc hing acc u racy ma ny tec h n i c i a n s req ui re.
SPE CI F YI N G COLOR i s made more di ffic u lt because color i s i n fluenced by textu re . As s e e n i n this i l lu stration , t h e colors of a l l t h e objects match t h e co lo r sam pl e . Under a ctua l condition s , they migh t not because s u rface texture va ri es from object to object, ma king their colors appea rd ifferent.
THE MUNS ELL COLOR SYSTEM
was orig i na l ly devised by Albert H. M u n sel l , a pai nter and art tea cher. It is an ordered a rray of colored paper sam p l e s . Munsel l u sed th ree color va riables : hue, chroma, and va l ue . Chroma correspond s approxi matel y to saturation ( p. 1 0 2 ); va lue i s rel ated to the l ightness ( p . 3 5 ) of the sam ple. H u es are a rranged i n spectral orde r arou nd a c i rc l e. The ax i s of the c i rc le is a te n-step va l ue scale, bl ack at the bottom , th rough n i ne shades of gray, to white a t the top. Chroma var ies along the radi i from a m i n i m um at the central ach ro matic ax i s to a ma x i m um at th e peri meter. I n practice th e M u ns e l l System i s a n atlas of 1 00 sepa rate pages of paper chips, a rranged tree l i ke about the vertical ( va l ue) axi s . The c h i ps o n any one page are a l l o f t h e same hue, but va ry i n c h roma from left to right and i n va l u e from bottom to top . The d ifferences be tween neighbor i n g sam ples have been chosen to repre sent psycho log i c a l l y eq ual interva l s . The M u n se l l System is a n atlas of s urface co lors . Its reliabi l i ty depends somewhat u po n the su rface textu re of the color sa mple bei n g com pa red . For best res u lts, of course, a standard white sou rce of i l l u m i nation m u s t be used .
0 white 9
i - 6� 5 4
Coordinate s of the M unsel l S ystem
3 2 black neutral color 2 4 6
1 26
II
- chroma
�
T H E M U NSELL C O LOR TREE co n sists of 1 00 vertical sec tio n s , o n e o f whi ch i s s h o w n below. Arrow
value verti ca l sec tion --+
points to color chip identified as 5 8 4 / 8 ( h u e - 5 8 , value - 4 , chroma - 8 ) .
+ 9
T H E T E N BA SIC H U ES of the Mu nsel l system a re red, yel low, gree n , blue, and purpl e and combinations of these in pa irs . For each hue th ere are ten gradations, making 1 00 distinct hues. Each ba sic h u e i s number 5 in its g radation scale. N u m ber 1 OY i s fol lowed by 1 GY. Colored sam pl e s for basi � blue a re shown at right. The va lue des i gnation ra nges from 1 at the botto m ( not show n ) through 9 a t the top . Chroma scales a re of d ifferent lengths, dependi n g o n the partic ula r hue and value. For sale by the National Burea u of S tandards, Washington, D . C . , a n d useful in color specification are 1 8 charts covered with glossy colored chips. I s sued with the cha rts is a tab l e that l i sts chip number, col or name, and Munsell notation .
7
6
5
4
3
2
chrom a+
2
4
6
8
10
i s a materia l system u s i ng colo r samples s i m i l a r to th ose of the M u n se l l . like the M u n s e l l system it bears the inherent wea kness of printed c olors that cannot comp letely represent those proposed by the system . This weakness is ba l a n ced by h aving a system that i s we l l keyed to the C I E system ( pp . 1 3 0- 1 3 2 ) and i n wh ich samples can be d i rectly vi ewed and matched . The Ostwa ld system i s based on s u rface colors and i s often preferred by a rtists . The O stwa ld system uses the psychophys i c a l variables of dominant wavel ength, purity and l u m i nance i n stead of the psycholog i c a l va riables of h u e , saturation and brig htnes s, a s approx imated i n the M u n se l l syste m . O st wald a rranged h i s system with h ues of m a x i m u m pu rity form i n g a n equatorial c i rc l e and with complementary colors opposite. The axis of the c irc l e g rades fro m white at the top down to b l ack. I n practice, t h i s for m s a series of 3 0 color tri a n g l es with a ba se of l ightness and with th e m ost satu rated hue at the a pex . Each six-sided sample is identified by th ree n u m bers representing the proportion of black, white and " fu l l color, " adding up to a tota l of 1 00 per cent. S i m i l a r in term inology to the O stwa l d system i s a vo l u m e prepa red by a b i rd expert, Robert Ridgway, and u sed by bio l og i sts for ove r 50 yea r s . Th e Ridgway color d i ctionary shows over 1 , 1 00 colors with names that i nd i cate fu l l color, ti nt a nd s hade . THE OSTWALD COLOR SYSTEM
COMPA R I SON OF OSTWALD A N D M U N SELL SYSTEM S
Munsell
1 28
T H E OSTWALD COLOR TREE
N umbers on each s i x - s ided sample ind icate pe rce ntage of color ( C ) , white ( W } , and b la ck ( B ) n eeded to match the s a mp l e .
VERTICAL SEC T I O N FROM THE OSTWALD C O LOR TREE
1 29
THE C I E SYST E M
makes it pos s i b l e to desc ribe color samples i n m ath ematical te rms and to represent the domi nant wavelength and purity of th e sam p l e on a diagra m . Th e system w a s developed b y t h e Com m i s sion l nterna tiona l e de I ' Ec l airage- C I E ( I nternationa l C o m m i ssion of I l l uminatio n ) . It d ea l s with dom i nant wavelength and purity l i ke th e O stwa l d syste m ( p . 1 2 8 ) and can a l so be re lated to the Munsel l system ( p . 1 26 ) . Both systems have now been keyed in C I E terms , so notations can usually be converted from one system to the other. To d evelop th e CIE system, data wa s n eeded on the co lor-match i n g cha racteri stics of th e ave rage eye. U s i n g a colorimeter, a n u mber o f observers made a series of color matches agai nst a spectrum of monochromatic colors em ployi n g a m i xture of th ree primary colors ( specific wavelengths of red , green, and blue) . Based on the results, three color m ixture c u rves ( be low) were produced . Th ese represent the relative amount of each of the prima ries needed by the standard observer to match any pa rt of the visible spectrum . By a mathemati cal tran sform ation, a re loted set o f c u rves was prod uced in which the g reen c u rve ( c u rve y) a l so expresses th e l u minance observed at each wavel ength and i s thus a l so the sta ndard l u m i nosity c u rve. 2 .0 Cll :;)
1 .6
�
1 .2
Qj
0.8
Cll >
�
0:::
0.4 0 4 00 0 A
1 30
5 000 A
6000 A
Wavelength ( Ang stroms )
7000 A
developed by the CI E, with values from the th ree color-m ixture c u rve s serves as a p ictori a l map on w h i c h any color c a n be s h own. In practice, the color of a n y sam ple i s defined by c iting its coord i na tes on the c hromatic ity d i a g ra m ( be l ow ) and ad d i n g a fig ure for its rela tive l u m i n a n c e . A CH ROMATICITY DIA G RAM,
T o produce t h e chromaticity diagram, read ings were taken from the color- mixture curves at a suffi cient number of selected interv a l s . F rom these were de rived the coord inates for a se ries of points representing a spectrum of fu l ly satu rated col ors. Plotting these points a nd join ing them with a smooth cu rve provided the basic triangular structure of the ch romatic ity dia gram ( fi g . 1 . below ) . A l l vis ibl e colors c a n b e rep resented by points with i n this figure . The entire s pectrum of fully satu rated hues lies on the smooth outer curve, ra nging from viol et ( 4 ,000 A) at the lowest po int through hues of green ( 5 , 200 A) at top to red ( 7, 000 A) at right. N on-spectral purples a re fo und along the l i n e that con-
0.8
�. ·· l t±tf
nects the lower extrem ities of the cu rve. Any straight l ine th rough the center point C w i l l i ntersect the curve at comple m entary wavelength s . Three standard l i ght sources ( p . 1 7 ) have been defined and their coord inates also plotted as three points on the chroma ticity diagram ( fig . 1 ). These points represent the color characteris tics of an ordinary in candes cent lamp ( A) , a pprox imate noon s u n l ight ( B) , and average day l ight ( C ) . A ' s position, closer to long ( red) wavelengths than C ' s, shows that i ndoor lamps appear reddish compa red to day l ight. A typi ca l CIE nota tion for an impure red color sample ( fig . 0.58, 2 ) m ig h t b e g iven a s x y 0 . 3 3 , a n d relativ e lumi nance ( Y ) 0.24. =
=
=
_LU i l '1 '
-
5 2 00
I
0.6 -
r-+--
y
y A
0 . 4 - i-
r i-
c .
....
0.2 --
-r-
-1-.
0
-R4ooo1
t
I
0
0.2
•
B
----
-
•
7000 A � I
.I l I l l I I I
0.4 X
I
0.6
0.8
X
O i l p a i n t i n g by c o p y r i g h t 1 963
1 32
l.
Candax, from by the O p t i c a l
The Science S o c i ety
of
of Co lor, America
T H E C I E C H R OMATIC I TY D I A GRAM ( left) shows colors de term ined by the CIE g raphs and equation s . The CIE system works on form u las, not on color sam ples, but the i l l ustration does g ive a rough visual ind ication . A long the s mooth curved pe rimeter a re the colors of the spec trum with much s pace for green s because the eye i s more sensi tive to them. Purples lie on the straight line connecting the red and blue ends of the curv e . Al l other colors are represented by poi nts enclosed by the curv e .
T h e achro matic point ( wh ite) is near the center. Colors i n crease i n satu ration with thei r di stance from the achromatic poin t . Th us, ova l zones aroun d this point rep resent colors of equal satu ration . Each pie- like sector conta ins a small range of hues, graded by saturatio n . H ue s at opposite ends of any stra ight line through the center are complementary. An additive m ixture of any two colors i s represented by an i n termedi ate poi nt somewhere on the straight l i ne between the points repre senting the two colors.
L I G H T AN D C O LO R AS T O O LS Light and color are so m u c h a part of o u r l i ves that we often overlook their fu ndamenta l i m porta n c e to m a n y busi nesses s u c h a s adverti s i n g , te l evi sion, photography, pa i nt, pri nti ng, opti c s, and many others . Arti sts, deco rators, and desig n ers use l ight and color i n the i r crea tion s . The a rtist ' s task, for example, i s not one of repro d u ction , but of representati o n . By l i n e s , and for m , and color he seeks to represent those featu res of a person , a scene, a n idea, or a n emotion that h e feel s are i nter esti ng or i m portant. H e must know and use the symbo l i s m and emotional effect of h i s colors no matter w h a t h i s style or schoo l . H e m ust keep i n m i n d the effects of color constancy and c o l or contrast, and h e m ust either apply or pu rposefu l l y ig nore the rules of h a rm o n i o u s color combinations. Even the rea l ist does not try to pa i n t a scene exactly as h e sees it. The pi ctu re i s not the actu a l scene, and the viewe r ' s response to the picture may d iffer from h i s atti tude towa rd th e scene itself.
1 33
are descri ptive terms ordi na rily a s soci ated wi th m u s ic, but they a l s o a pp l y to c olor . Harmony and d i scord i n both cases i nd i cate com binations th at are p lea sant or un pleasant within o u r culture. Rules of color h armony a nd d i scord a re derived from ex perience and sanctioned by social acceptance. They have no known ma th ematical or scientific bas i s . . A n a rtist or a dec orator may discuss harmony and d i s cord i n term s of color circles of tints, hues, and tones. Ti nts are prod uced when wh ite i s added to saturated hues. Add i n g black, i nstea d , produces tones or shade s . Adjacent c olors and opposi n g · com plementary h u e s on th e c ircles harmonize fa irly wel l . Widely separated hues, not complementa ry, tend to prod uce d i scord . Al l ful ly satu rated colors do not a ppea r to h ave tlie same d eg re e o f saturati o n . A satu rated yel low a ppea rs HARMONY AND DISCORD
1 34
3
2 HAR M O N I O U S H UES include ye l l ow a nd g reen ( 1 ) , c lose on the hue circle, and co mplemen tary blue and yellow ( 2 ) .
DISCORDAN T H UES, such as magenta a nd yel low ( 3 ), o re for aport on the hue c ircl e , but not fo r enough to be complementary .
m u ch brighter than does its complementa ry saturated b l u e . S i m u ltaneo u s ly, t h e range of colors from a pa l e yel low tint to a ful ly satu rated yel low seems g reater than from a pa le b l u e ti nt to a da rker, fu l ly satu rated b l u e . Nor m a l ly a satu rated blue seem s pleasant nea r a less satu rated yel low. The reverse situation, placing a fu l ly satu rated yel low next to a less satu rated b l ue, appears unpleasant even though the two colors a re complemen tary. Wh i l e adjacent colors of the hue c i rc l e blend fai rly wel l , they tend to make a bland and u n i n teresti ng com position . Artists wi l l ofte n add a splash of color from a d ifferent part of the spectru m , a deli bera te d i scord , to give spice and e m phasis to a scene. DISCORDAN T A PPEARA NCE of two usually harmo n io u s co lors i s d u e t o t h e di ffe rence in satura tio n . T h e yel low is f u l l y satu ra ted and the blue is not ( 4 ).
4
Equal areas of red and cyan ( 5 ) ore d iscordant a s show n . The some colors ore used in ( 6 ) , but the c ya n i s dominant and the co mbi na ti on i s h a rmon i ou s .
Custom decrees that boy babies be dressed in blue, girl babies in pink; that brides wear white and mou rning widows wear black.
has devel oped ove r the years a s certa i n colors have become a s soc iated with special mean ings. I n genera l , dark, saturated col ors give a fee l i ng of r i c hn ess a nd elega nce . Bright, saturated co lors ex press live l iness and gaiety . Da rk, u nsatu ra ted co lors are sad a nd m oody, wh i l e l ight un saturated ones te nd towa rd fr ivo l i ty and c heerfu l nes s . L i terature a n d c u s tom te l l us that redd i s h hues are wa rm ; b l u i s h ones are cold . Red i s the co lor o f a nger and courag e. White stands fo r purity , i n nocen ce, a nd hope . G reen i s th e color of youth a nd vigo r, b ut also of envy and i nexperience. Yel low suggests cowa rd ice, a l so che erfu lness a nd sunshine. B l ue can be purita n ica l , mood y, i nd ece nt, o r tru e . Pa s s ion i s freq uently purple, but vio l et p u r p l e ha s th e d i g n i ty of roya lty, wh i l e red dish pu rp le i s the col o r of rag e. Colors often h ave d ifferent m ea n i n g s i n d ifferent c u l tu res . I n Western c u ltures, b l a c k expresses wickedness, sorrow, and despa i r . It i s a funera l color, while i n China mourners wear white . The Aztecs associated colors with the fou r compass points, North being red . Emotiona l assoc iatio n s with color may be a n i ndividual and not a c u ltu ra l m atter, and two people may often find their tastes in conflict. THE SYMBOLISM OF COLOR
1 36
u sed to protect o r to decorate su rface s , consist of fi ne l y grou nd pigmen t parti c l e s s u s pe n d ed i n a l iqu id, th e ve h i c l e . Th e pig ment parti c l e s { p. 1 3 8 } a re wh ite or colored, · an d a re u s u a l l y opaq ue . The v eh ic le helps to spread the pa int even ly, a n d , when dry, binds o r g l u es the pigment to the s u rface . The o i l pa int u s e d b y artists i s a pigmen t paste i n a veh i c l e o f l i n seed or poppyseed oil tha t is th i n n ed with tu rpenti ne. A fam i l i ar ve h i c l e of househo ld pa i nts i s l i nseed oi l . The pa i nt is th i n n ed , most common ly with turpentine or a petrol e u m deriva tive. The th inner may a l so speed up dry ing . In newe r pa i nts the veh i c l e is a rubbe r- ba sed , fa st dryi n g com po und, or some pla sti c such as a v i ny l or ac ryl ic typ e. Th e ty pe o f veh i c l e used may affect th e degree of gloss or th e flatness of the pa i nt, a nd thu s th e a ppea ra nce or color of the su rface p a i n ted . A g lossy or enamel pa i nt on a smooth-textured su rface is s h i ny d u e to reg u l a r reflectio n of l ig ht . Th e color of a g lossy surfa c e is seen most e a s i l y when viewed from th e di rec tio n of the inc iden t l i ght. When viewed fro m th e oppo site d i rection, the gloss tends to hide th e c o l o r . A flat pa i nt d iffuses ra th er th an reflects the l ig h t reg u l a rly from a s u rface . PAINT S,
To impa rt an a i r of cheerfu lness or effi ci ency to interior wa l l s , paints with h ig h reflecta nce values ( percentage of incident light re· fleeted ) a re often used . Reflect ances of 5 0 - 6 0 % are held de s i rable for schoo l s and offices;
61 %
48%
3 5 - 5 5 % for t h e home. Paints that g ive lower reflectance values ( 3 0 - 4 0 % ) can be u sed for trim, floors, and furniture. The a p proximate reflectance values for fou r flat green pa ints are shown below.
38%
20%
PIGME NTS
provide the color and the covering or hiding power of paint. They a l so contribute to its d u rabi l ity and to the permanency of the color i n weathering. Natu ra l , or m i n era l , pig ments are made of finely ground ea rth materials, such as c i n na bar, ochre, cha rcoa l , and others . Pigm ents may be either organic or i no rgan ic, but most of them are now prod uced synthetical l y . Th e oldest white pigment i s wh ite lead, a ca rbonate of l ead made by corrod ing lead sheets and then c o l l ecting a nd drying the fine powder. Because i t has about n i ne times greater hiding power, and is i nert chem ica l ly, tita n i u m d ioxide is rep l a c i n g white lead . S ome red and many yel low and brown pigments are derived from a n i ron oxide . The common yel low used by artists, however, is cad mium ye l low. A g reen pigment i s produced from c h romium ox ide; bright Prussian blue i s a complex i ro n cyanide. B l ack is made of fi n ely d ivided lampblack ( carbo n ) . Pa int extenders a re white or nearly wh ite pigments th at add l i ttle or no color to the pai nt, but help to give it body . Meta l s i n powder form may be added in the veh i c l e to make various meta l l i c pa i nts . L u m i nous pai nts conta i n pigments or dyes that g low when i l l u m i nated , as by automobi le head l ights. W h e n a beam of white l i g h t strikes a pa inted surface, some light i s re flected , part of it penetrates and
i s a bsorbed . The rema inder is diffusely reflected and i s respon sible for the color you see.
0.8
0
ti
0.6
cadmium yel low
0.4
pigment m ixture
0 ......
u c 0 c
.E
::;) --'
0.2
u l tra marine blue
0 4000 A
5000 A
6000 A
II 7000 A
Wavelength ( Ang stroms ) S pectral reflection curve for a m ixture of two ports of cad mium yel low ( upper curve) with one port of ultramarine blue
( lower curve) does not show pure effects of either additive or sub tractive mixing. Middle curve i s the blended po int.
Pig m ents do not d i sso lve, nor do they u n ite c h e m i c a l l y with t h e pa int veh ic le. I f two pigments a re m ixed i n a pa i nt, the color of the pa int appears to change though it sti l l consi sts of the two d i stinct pigments. Th i s effect i s due to add itive color m i x i ng ( p . 1 0 7 ) . The m ix i n g of pai nts to obta i n desi red colors i s com p l i cated because not only addi tive but a l so su btractive color m i x i ng ( p . 1 1 0 ) i s i nvol ved . A s uccessfu l pred iction o f the res u l ts usua l l y req u i res m u c h experience. I n fact, o n e . of the problems of th e pa int i n d u stry is to d ete r m i n e how to rel ate the color of a pa int m ixture to the s pectra l reflec tion c h aracteri stic s of its component p i g ments . Resu lts cannot be pred i cted on the basis of the c u rves from the com ponent Pigments. A parti a l understa n d i n g comes from , knowledge about the rel ative amou nts of scattering and absorption that occur i n th e pig ments. 1 39
DYES a re colorants. U n l ike pigments, they a re soluble, and the particles of the dye are of m o l ec ul a r s i ze and can not be seen even with the most powerfu l opti cal m i c roscope. Most dyes can be a p p l i ed d i rectly, parti c u l a rly to the a n i m a l fi bers, such as woo l and s i l k . Others are effec tive only i f the fiber i s treated first with a dye fastener, or mordant, a chem ical that causes the dye colors to ad here to the fibers. Cotton and l i nen usua l ly req u i re a mord a nt. Synthetic fibers often i nvolve complex dyeing problems. Nearly a l l dyes a re o rga n i c com po u n d s . Orig i n a l l y they were m a d e from fruits, flowers, bark, roots, in sects , and marine mol l usks. A few used i n earlier days were of m i nera l orig in, but most used today a re pro duced syntheti c a l ly. The fi rst of these wa s an a n i l ine dye, d i s c overed accidenta l ly i n 1 8 6 6 . Th i s beautifu l ma uve dye was the beg i n n i n g of the enormous coa l-ta r dye i n dustry that today produces nearly a l l dyestuffs .
1 40
About 4,000 different dyes a re ava i lable commercia l ly. These fit i nto 22 chemical c lasses and are each identified by dye experts with a five-digit n u m be r . Dyes a re c las sified chem ical ly, but a l so by the i r color, by the materia l s they color, and b y th e d yeing method s used . The u l tra violet rays of s u n l ig ht break down ma ny dyes i nto color less compounds. A good dye res i sts fad i n g . Dyes must a l so resist the rea ctio n o f detergents a nd other house hold chem i ca l s . Dyes that are not ea s i l y removed are known a s fast dye s . To get suitable dyes for a great n u m ber of fa brics and other materia l s w ith a va riety of textu res, computers are now u sed to summarize the variables. Th e prob l e m s of dyei ng are largely practi ca l . The d ifference between the absorption c urve o f the dye and the reflection c u rve of the dyed materi a l attest to the fact that the c olor of the dye sol ution and of th e dyed mate rial a re often q u ite different. I n viewi ng and matc h i n g dyed materials, standard i l l um i nation is i m po rtan t, a s every woman knows who ta kes a skirt to the window where she can see its true daylight color. CH R OMOPHOR ES, g roups of atoms that conta in loosely h e ld electrons capable of selectively absorbing some wavelength s fro m wh ite l i g h t a n d tran s m itting t h e re st, g ive co lor t o orga nic dyes. Si nce only a portion of the wave lengths contained i n white l ig h t remains a n d reaches your eyes, the light from the dye a ppears
+
-N = N-
c hemical
"-/ benzene
b""""'d""s----�""m lecules ;.;.;o=
co lored . Chromophores a re fou nd in al l colored orga n ic co mpou n d s , o r chromog e n s . To convert a chromogen to a dye, a nother grou p of a tom s , the auxochrome, must be presen t . These a tom s fix (attach ) the dye to a fa bric . E l e ments of the structura l formu la of the dye cal led Butter Y e l low are shown below.
0 - N = N -O N ( CH 3 ) z B utter Yel low ( para dimethyl a mi noa zobenzene)
1 41
I I I I 0
1
2
3
4
5
I I I U 6
7
8
9
COLOR IN BUSIN ESS AND INDUSTRY
has many uses. Showing debit figures i n red ink and credits i n black ink i s an example of color cdd i n g . A more elaborate code i s u sed to mark electronic resistors a n d capaci tors . Each n u meral i s assi gned a unique color ( a bove), so that a sequence of bands or dots can be used to indi cate th e va lue of a component. A 2 5 ,000 -ohm resi stor is ma rked as sh own below. Red sta nd s fo r the two, g ree n for the fi ve, a nd orange, i n th i s position, means the n u m ber ends in thr ee zeros. The si lver band i nd icates that the re si sto r va l u e is accurate with i n 1 0 per cent. a color-coded resisto r
)� COLOR CODES may be u sed to disting uish the d ifferent cir· cuits i n e lectrica l wiring . Some codes a re for convenience and ef ficiency, w h i le others a re pri marily for safety. One of the most common color codes in ex istence i s the red , ye l low, and green traffi c l ight. The use of accurate color match ing devices a l lows chemists to
ana lyze samples ra pidly or to control quality. Color is the key to the temperature of furnaces and to the testin g or g rading of food products such a s tomatoes or bananas. Mass prod uction often demands that parts manufactu red .at dif ferent places match in color when finally assembled. This requires elabora te methods of color contro l .
Color coding i s evident i n the electronic circuit at left and i n the wires that make up the tele phone cable below.
I n packaging, what ' s outside counts most. The design and color on the exterior often determine the success or fai lu re of a product. Bold h ues aid i n the " hard sel l . "
COLOR
IN ADVE RTIS I NG
has three uses: to attract attention, to decorate, and to influence by symbo l i s m . Brig ht colors a n d c ontrast a re more i m po rtant i n catch ing the eye of a prospective customer tha n a brand name or s l oga n . The brand na me i s read after the eye has been caught, and often a fter the c u stomer has p i c ked up the package in h i s hand . An attrac tive appearance can often sel l a n i n ferior prod uct. Manufactu rers spend m i l l ions of do l la rs eac h yea r to ensure that their produ cts a re decorated to the cu stom er ' s taste, know ing that many times a s i m p l e change in color has meant the d ifference between com merc i a l success and fai l u re . The powerfu l symbo l i s m of d ifferent colors can strongly affect the decision to buy or reject. A yel l ow soa p, for insta n ce, may be thought stronger than a white soap, perhaps becau se ye l low i s sym bo l i c o f power. A ma n may buy ciga rettes i n a red package, but avoid them if the package is p i n k- beca use the color i s not a " ma sc u l i n e " o n e. I n recent years, a major gasoline company decided that i t needed a more aggressive marketi ng approach and changed its trade colors from a ta me green and white to a bo ld red , white, and blue at a cost of m i l l ions o f dol l a rs . Color may even be the dec i d i ng factor in the choice of a n expensive car. 1 43
depends on a photochemical reaction i n a n em ul sion o f si lver hal ide c rysta l s em bedded i n gela ti n . Light rays prod uce a latent image i n th e emul sion which i s red uced to p u re s i lver by treatment with a developer. After development, the u n reduced s i lver h a l id e is rem oved b y t h e fi x i n g process . T h e resu lting i mage consi sts of a deposit of pure s i lver embedded i n ha rd ened gelati n . The density of t h e deposit i s determ i ned by the exposure. Th i s i s a negative image because it is dark where the orig inal scene wa s l i ght, and l ight where the original scene was dark. To prod uce a positive image, l i g h t i s passed through th i s negative o nto another photographic e m u l sion, either on fi l m or o n paper. If the original developed film i s not fixed, but is bleac hed i nstead, a u n i form exposure of the film to light wi l l then prod uce a posi tive latent i mage which may be deve loped i n the usual way. Th i s reversa l pro cess i s u sed for many amateu r co lor fi l m s , both movi e and sti l l . B l ack and wh ite photograp h i c e m u l s ions are often more sensi tive to one color than to a nother. By dye-sensitizing, they can be made more sensitive to d ifferent wavelength s , as desired . PHOTOGRAPHY
COLOR NEGATIVE shows max i m u m density a n d deepest color where i mage is l ightest. In a subtrac tive fl l m the col ors on the negative are co mp lementary to the co lor of the i mage.
COLOR POS ITIVE is made by placing a sheet of col or fllm or pa pe r beh ind the negative and exposing them to light. W h e n the fll m o r pa per i s developed, a po sitive color image results.
mosaic screen
l
l ight
l
l
Cross-section of additive, three-color, mosaic system camera film
done fo r commercial pu rposes uses d yes or col ored fi l ters , and nea r l y a l l i nvol ve three separate i m ag es made by expos ure to l ight from three different pa rts of the spectrum . Col o r p rocesses diffe r m a i n l y i n m ethods of ma ki ng and co m b i n i n g th e i m ages. The three m a i n meth od s i n use tod ay are th e color nega tive (p. 1 4 4 ), di rect rever sal (whi ch produ ces tran s pa rencies, p. 1 4 6 ), and color separation . The l a st in vo lve s maki n g a sepa rate negative for ea c h colo r, a nd its use is re stricted exc l u s ive l y to the printing a n d com mercia l picture indu stries ( p . 1 4 9 ) . COLOR PHOTOG RAPHY
COLOR emu l s ion s AD DITIVE ( D u faycolor) ofte n u s e a fi n e grai ned mosaic of m ic roscopic red , blue, and g reen dots . light from the image pa sses through these dots to reac h the emu l s io n , a light-sens itive l ayer o f silver hal id e . The s i lver h a l i de crystal s are affected by the pattern of the image coming through the co lored dots. When the film is developed, the darker areas turn to meta l-
The Dufaycolor film used to make the accompa nying photograph was covered with a cro ss-ha tching of fine lines, as many as 1 ,000 to the inch, each dyed red , green, or blue. I t i s an obsolete, add i tive color process.
l ie s i lver and are removed, leav ing a pos itive tran sparency . The transparent dots, too small to be seen individually on pro j ection , fuse v isually into hues a pp rox imating those of the image.
EMULSION SENSITIVE TO
blue
# 1 em ulsion Ioyer yel low filter # 2 emulsion Ioyer # 3 emulsion Ioyer film bose cros s section of Kodac h rome fi lm
COMMON CO LOR FI LMS, such as Kodach rom e , E k tac h rome, and Anscoch rom e , contain three differ ent emulsion l ayers and a yel l ow fi lte r . In Kodach rom e ( abov e ) the fi rst Ioyer of emulsion is sensi tive to b lue l ight, the s econd to b lue and gree n , and the third to blue and red . But th e yel l ow fi lter, between the first and second emu l s io n layers , blocks the blue l i ght, al lowing only red and g reen to pa ss through . As a re sult, the s econd emul s i on Ioyer i s sensitized only by g reen l i ght, and the th i rd Ioyer only by red . In th i s way, three negative i mages representing the three prim ary colors o re formed simultaneously. During developm ent, the s i lver i n each im age is repl aced by the a pp ropriate dye , and the yel l ow fi lter is bleached out. Becau s e t h e i mages o r e n egativ e, each I oyer must be dyed its comple m entary color to form positive i mage s . The blue sensitive emul s i on Ioyer is dyed yel low, the blue and green sens itive Ioyer is dyed magenta, and the blue and red sensitiv e Ioyer is dyed cya n .
Kodachrome photograph In Kodachrome, the d ye-forming chemicals ( cal led couplers) are conta ined in the developer, and processing is qu ite complicated. The final color transparency is normally pleas ing, and resolution of deta i l i s exceptionally good . I n other multilayer films, the dye couplers a re conta ined in the emulsio n s . The th ree-color images con be developed all at once in the same developer, a process wel l within the capa bility of many amateu rs .
i ntroduced i n 1 9 6 3 ,, enables one to obta i n a fi n i shed color print one m i n ute after taking the picture. The m ulti layer Polacolor negative is exposed in a Pol a roid Land Camera . Processing occurs i n side of the camera fo r th e ro l l fi l m s a nd outside of the camera in a l ig ht-tight sandwic h for the pack fi l m . POLACOLOR FILM,
A PO LACOLOR F I LM assemb l y ( 1 ) con s ists of t h e n egative, the pod and the rece iving sheet ( positive ) . The negative contains th ree emulsion layers, each sensi tive to a d ifferent region of the spectrum. Next to each layer is a layer containing yellow, magenta ,,_�---' or cyan dye-developer molecules. I n the pod is a t h i ck , jelly- like proce ssing reagent to activate the dye-developers . The fi n i shed color positive i s formed o n the receiv ing s h eet. After exposu re , the fi lm assem bly i s pulled through a set of rol l ers ( 2 ) that burst the pod and spread the reagent between the negative and the receiving sheet ( 3 ) . Wherever l ight has activated developing rPrlnF•nt. an emulsion l ayer, dye-developer pod molecules become trapped i n the negative. Wherever the emuls ion tab has not been affected by l ight, No. the d ye-developer molecules con tinue through the negative to the receiving sheet where they are locked in place to form a posi tive color print ( below ) .
1 47
PRINTING
i s one of the most importa nt methods of comm u n i cation u s i ng l ight and color. Th i s a ncient art probably began with ha nd-carved wooden blocks pressed against an inky su rface and then appl ied to paper. The use �f movable type put printing on a mass p roduction ba s i s . Now, photo-typesetti ng, often contro l led by a computer, i s repla c i ng type. H igh -speed presses turn out n ewspapers, magazi nes and books at speed s g reater than 1 ,500 feet per m i n ute . About a b i l l ion books and about 2 . 5 bi l l ion magazines are printed a n n ua l ly i n the U . S .
LETTER PRESS, derived from the ori g in a l method of printing from carved blocks, prints from a raised surface. Type is made with elevated letters; photog raphs are tra nslated into pattern s of raised dots . Many newspapers are printed by letterpress, with a type setting dev i ce- the Linotype machine-using raised letters . GRAVURE is the reverse of let terpress. I nk i s picked up from depressions in the plate. Gravure developed from intaglio where the letters or designs were carved into wood . The plate is inked, then wiped so that ink re mains only in the pits until it is picked up by the paper in print ing. Printing off of rotary plates rotogravure - i s very rapid.
doc tor blade
rubber "blanket"
LITHOGRAPHY, original l y print ing from a stone, now im prints both i l l ustration ond text from relatively s mooth metal plates . To reproduce a photograph or other pictu re with light and shade, the i l l u stration i s photographed through a ca mera that has a
Ful l-col or printi ng of Gauguin ' s self portra it req u i res three color plates and one black. Dots a re a l igned at a di fferent angle to be sure they do n ot print on o n e another.
criss The screen . rotatable crossing ru l ed l i n e s of the screen produce a series of dots on the negative. The finer the screen , the better the deta i l . Newspapers use a 6 5 -line screen (65 dots per l inear inch ) . This book uses 1 3 3 . T o make your eyes register the dots as a continuous tone value, the camera screen i s set so that the pa rallel rows of dots on the negative occur at some angle to the horizonta l . For black or any single color halftone, the angle i s 4 5 deg rees. I n fu l l-color reproduction, the photographed is i l l u s tration through blue, green, and red filters, producing a separate negative for each color. The dots are large where the original color i s satu ra ted; smal l where it i s weak. A printing plate is made for each negative, the proc ess being very simila r to print ing a photograph from a negative, but a metal plate coated with a light-sens itive emu l sion is used in stead of photographic pape r . N e ither a ra i sed nor a de pressed su rface is needed on the
metal plates used i n l i thographic printing . I n stead, each plate is waxy or otherwise repe l s the ink except at those places where letters or designs a ppear. Yel low, magenta, and cyan inks, complementary i n color to the fil ters , are used with the plates to duplicate the o riginal i l l u s tration . Sometimes the overlap of these three inks does not give a strong black, so a black-and wh ite halftone i s usually added . Four-color sepa rations were used for many of the i ll ustrations 1n th i s book a s s h own above . I N O F FSET L I T H O GR AP H Y, the ro ll er-mounted plate becomes inked a s i t revo lves . Its image is tra n s ferred ( offset) in reverse onto a rubber ma t or " b la nket" which enci rcles a nother cylinder. As this mat turn s , it tran s fers the now un reversed i mage o nto pa per held fi rm during the moment of im pres s io n by a counter-rota ting third cyl inder, A separate three cyl inder u n i t o perates for each color.
1 49
color TV camera
l u m i nance or B&W signal
adder
adder mirrors
m i rror
color tubes
The color TV ca mera records the scene in three primary colors .
emp loys a mosa ic method of add i tive color m i x ing ( p . 1 09 ) . The image i s prod uced by the action of a bea m of swi ft electro n s on a fl uores cent s c ree n . The fine beam of e l ectrons sweep s across the sc reen 5 2 5 times for each picture, and 30 pictures are presented each secon d . To ach ieve satisfactory defl n ftion, the i nten sity of the el ectron bea m is mod u lated at frequencies u p to 4 . 5 m i l l ion cycles per second ( 4 V2 mega cyc les ) . Th e l a rge ba ndwidth of th i s freq uency means that the tra n s mitti n g station m u st u se carrier wa ve s less tha n 1 0 feet long . { AM radio wave s a re a bout 1 ,000 feet long. ) I n the televi sion stud io, the color camera views a scene th rough a system of color sepa rati ng ( d i c h roic) m i rrors, producing a sepa rate electronic signal for each primary color. About 20 per cent of th e l i g h t that enters the camera bypa sses its th ree color tubes and i s di rected to a monoc h rome tube . The signal from th i s latte r tube conta i n s most of the i n formation needed to produce pic ture deta i l and brightnes s . After the signa l s leave the camera , they combine to modulate the carrier wave, which i s then b roadcast by a transmitter and a nten n a . COLOR TELEVISIO N
1 50
tra n s m i tted from the tele vi s ion station a re picked u p by the antenna o f th e re ceiver, and a re separated i nto th e th ree origina l c olor components and the monoch rome ( black a nd white ) portion by electron i c c i rcuits. The color components a re converted i n to three s ma l l voltages u sed to control the e m i ssion of e l ectrons from three electron g u n s , one fo r each color component. The electron s themselves do not c a rry color character i stics, but represent the vol tages p roduced by the l ight e ntering the camera. The color screen of the receiver is covered by rows of th ree-dot c l usters of phosphors that em it red , b l u e, or green l ight when exci ted by a n elec tron ( p . 2 4 ) . The amount of l ight emitted i s rel a ted to th e n u m be r of elec tro ns tha t stri ke the phos pho r. The dots i n ea ch c l uster are arra ng ed so th e e lec tron s fro m the g reen gun a lwa ys strike the p ho sphor that em its green l i g h t, th ose fro m th e red gun always str i ke th e phosphor that em its red l ight and th ose from the blue gun excite th e phosphor that em its bl u e light. As diffe rent vo ltages are suppl ied to the red , bl u e a nd g reen guns, varying l evels of each color are em i tted by the phos phor c l uster s . Since th e dots a re ve ry s m a l l and close together, they can not be re solved sepa ra te l y by th e eye, but are mixed by mosaic fu sion ( p. 1 0 9 ) i n to the h ue th at was o rig ina l ly seen i n tha t area of th e pictu re by th e camera. As a l l of th e rows of three -dot c l u ste rs a re scanned, a col ored picture i s p roduced . ELECTROMAG NETIC WAVES
I n the R . C.A. -type color tele vision tube , beams fro m three electron guns excite the phos phor d ots on the screen.
screen with phosphor dots
Beam from ruby laser
prod uce an i ntense, monochromatic beam of light th at can be focused to we ld, me lt, or va po rize a sma l l a mo u n t of a ny su bsta nce . They m ay be u sed i n com m u n i ca tions, i n th e acceleration of chemica l reactions, and i n surgery . La ser i s an ac ronym fo r ' ' l i g ht a m p l ifi cation by stimu lated emission o f ra d i a ti o n. " Th e fi rst laser was b u i l t in 1 9 6 0 . Lasers em it coherent ( i n-step) light waves , whereas a n ord i n a ry light sou rce rad iates l i g h t of m ixed wave l engths i n an out-of- step ( no ncoherent) and random m a n ner. The l u m i n ance of the i m ag e of o rd i n a ry l ig ht cannot exceed the source ' s l u m i na nce. But a l a ser can form a very l u m i nant image becau se its para l l e l rays can be foc u sed to a ti ny Sf:>Ot and a re added togethe r in phase . LASERS
Penc il-thin laser beam is u sed in optical and mechanical a lignment.
The l i g ht fro m a sma l l laser, i n fact, can be focu sed to form an i mage of l u m i n ance grea te r th a n th at of the sun ' s s u rfac e . The concentration of en ergy is so g reat th at extreme ly h igh te m peratures are p roduced . l i g ht ra ys fro m a laser can be beamed throu gh space w i th a fraction of a n inch sprea d per m i l e . The l ight i s a l so extrem ely pu re i n color ( m on oc h romati c ) . I N A RU BY LASER ( be l ow ) a ru by crysta l rod has plane parol· lei po l i s h ed ends which are silvered l i ke mi rror s . One end is on ly pa rti al ly si lvered and acts as a wi ndow for the light to get ou t. E nergy is su pplied to the ru by crystal by a powerf u l Aa sh tube lamp. T h is serves to pump the ( c h romium) atom s of the crys tal to a " meta stable" energy state in which they l inger fo r a few thousa ndths of a second before dropping to the g round state with the emission of a photon of l i g h t . M o s t o f t h e s e photpns p a s s ou t o f the crystal wa lls a n d a re lost, bu t soon one photon wil l move direc tly a l ong the rod and is re Aected from the polished end s, pa ssing back and fo rth a long the rod unti l it encounters a n a tom in the exci ted metastable state . The exc ited atom then rad ia tes
co ol ant
its photon in exact pha se with the photon which stru c k it. This second photon may in turn stim u late another atom , and t h i s " ca s code " process continues until the whole c rysta l i s fi l l ed with in phose radiation osci llati ng back and forth ins ide the rod . Pa rt of th is radiation is em itted thro ugh the ha lf-s ilvered end of the rod and becomes the laser bea m . Al l of th i s tok es place with in a few bi ll ionth s of a second, then the Aa sh tube fires ag ain and the process i s repeated . Modern la sers have been made of solid c rysta ls, glass, l iq u id s and ga ses. S om e operate i n pu lses as described, but ma ny emit co n tinuo usly. In a l l , the ra d i ation i s monoc h ro matic a n d coherent. I t i s this h i gh deg ree of coherence that makes laser l ig h t d ifferent from that of a ll other sou rce s .
co olant
1 53
HOLOGRAPHY i s a special kind of photography i n wh ich th e film c a ptu res not th e i ma ge of th e subjec t but the pa tter n of th e wavefronts of l ight reflected fro m th e subject. Invented by De nni s Gabor i n 1 9 4 8 , the pro cess took on n ew i m porta n c e when the l a ser wa s i n ve nted . I n addi tion to its u ses i n th e ente rta i n ment field, holography has many scientific and i nd ustria l appl ications, wh ich a re be i ng deve loped ra pid ly. T O MAKE A H O LOGRAM, a co herent l i gh t beam ( from a laser) is spl it i nto two pa rts-an object beam w h i ch i l l u mi nates the su b.· ject, and a reference beam wh ich is di rected to the photograp h ic fi l m by m irrors. At the fil m the I ight reflected from the sub ject interferes with the reference bea m to cau se a complex pattern of l i ght and da rk fringes in the film when it is deve l oped. W hen a ho logram i s h e l d in a beam of coherent l ig ht, part of
t h e di ffra cted light i s a rep ro· duction of the orig inal l ig h t wav e pa ttern t h a t came from the subj ect . T h u s , a v iewer look ing th rough the holog ra m can see a li fe-like reproduction of the o rigi· nal scene. Th i s v irtual i mage i s tru ly three-d imen s ional , s i nce the vi ewer can move his head and see a different pe rspective of the subj ect. Bes ides th e v irtual image, there is a lso a rea l i mage formed by the rays d iffrac ted i n another directio n .
laser beam
AR RA N G E M E NT F O R PRO D U CI NG A H O LO G RAM
USE OF H O L O G RA M I N PRO D U CI N G R EAL AN D VI RT U A L I MA G E S
rea l i ma ge
FI B E R OPTICS
i s th� res u l t of an i ngen ious a p p l i cation of a simple principle. I magine a l i g h t ray that has en tered the end of a s l i m so l i d glass rod . I f it a lways strikes the surface of the glass at angles greater than the critical a ng le (p. 44 ) , the ray wi l l be tota l l y reflected each tim e and trapped with i n the g l a s s . Reflected from side to side, the l ight ray wi l l be conducted a long the rod like water i n a hose. Final ly, the l igh-t ray hits the end of the rod at a s ma l l angle to the perpend i c u l a r and light ray traveling through fiber
A big advance i n fi ber optics came with the development of very fine clear fibers encased in a th i n coating of l ower refractive index. Total reflection takes place between the fiber and its coating. The fibers a re so thin ( a few micron s in diameter) that they a re flexible, and their coat ings al low them to be bou nd into bundles without i nterfering with each other's action . Such a bun dle can conduct light for severa l feet. When the fibers are lined up so they have the same re l ative pos ition at each end of the bun d le, they can transmit i mage s . This m a k e s pos s i ble a sort o f super-periscope so flexible that physicians can use it to examine the interior of body organ s . A sheath of unaligned fibers a round the bundle can carry i l lumi nation to the area being examined .
1 55
i s able to exit. Th i s explai ns th e success of bent l ucite and g l a s s rods a s l i ght-conductors in a dvertis ing d isplays and in simple i l l umi nati ng devi ces . A bu nd le of fibers prod uces a gra i ny ima ge l i ke a ha l ftone pr i nting proc ess. By drawi ng out the fibers so th at o ne end of the bundl e is sma l l er tha n th e other, the ima ge can be enlarg ed or reduced and its bri ghtness changed . The quality of the image can be improved by sha pi n g the end of th e bu nd le or by a d j u sti ng th e al ign ment of the fibers to elimi nate di sto rtions. M O R E I N FORMAT I ON
Burnham, Hanes, Bartleson, COLOR: A G U I DE TO BAS I C FACTS AND CON CEPTS, New York, N . Y . : John W ile y & Son s, 1 9 6 3 . A com pact handbook of defi n itions and facts about color. Comm ittee on Colorimetry , Optical Society of A merica, THE S C I E NC E O F CO LO R, New York, N . Y . : Thomas Y . Crowell Co . , 1 9 5 3 . A complete authori tative report for the serious student, with extensive reference and glossary. Eastman Koda k, COLOR AS SEEN AN D P H OTOG RAP HED, Roche ster, N . Y . : A Koda k Col or D ata Book, 1 9 6 6 . A brief explanation of color principles and color films. Evans, Ra lph M . , A N I NT RO D U CT I O N TO COLOR, New York , N . Y . : J o h n W i l ey & Sons, 1 94 8 . A carefu l explanation o f all aspects of color in non·technica l l ang uage. Eva ns, Ral ph M., EYE, F I LM, AN D CAMERA I N C OL O R PHOTOG RAP H Y , N ew York, N . Y . : John W iley & Sons, 1 9 5 9 . A thorough discussion of vi sual perception and its relation to photographic representation . Fowles, G rant R. , I N TROD UCTI ON TO MODERN OPT I C S , New York, N . Y . : H olt, Rinehart & Win ston , 1 96 8 . A c ol lege textbook that emphasizes the latest dev elopments in o ptics, i n cluding lasers. Mann, Ida, and A. Pirie, TH E SC I E N CE OF SEE I NG , Ba ltimore, Md . : Pel ican Books, 1 9 50. All about eyes and how they work. Minnaert, M., THE N AT U R E OF L I G H T AND C O L O U R IN THE OPEN A I R, N ew York, N . Y . : Dover Publications, I n c . , 1 9 5 4 . Explanations of shadows, mirages, rainbows and similar phenomena . Neb lette, C. B . , PHOTOGRAPHY, ITS MATERIALS A N D PROCESSES, 6th Edi tion , Van Nostran d-Reinhold Book s, New York, N . Y . : 1 96 2 . An authoritative di scu ssion of a ll phases of photograph ic technology. Sears, F rancis W., O PTICS, 3 rd Edition, Read ing, Ma ss . : Addison Wesley, 1 9 4 9 . A popula r col lege textbook with excellent i ll u strations.
1 56
-
I N DEX
Bol dface n u mera l s i n d icate m a i n covera g e . Aberra t i o n , 6 7 c h r o m a t i c , 66, 6 7 , 7 1 sphe r i ca l , 6 7 , 7 1 , 7 5 Absolute temperature sca l e , 1 4 Absorpt i o n of l i g h t , 36, 40, 5 1 , 57, 99, 1 1 0, 1 39 Acco m m o d a t i o n , 68, 69, 84 A c h r o m a t i c l e n s , 66 Ac h ro m a t i c l i ght , 1 05 1 07, 1 32 Ad d i t i v e c o l o r m i x i n g , 1 06, 1 07-1 08, 1 09, 1 1 2, 1 30, 1 3 1 , 1 32, 1 39, 1 45, 1 50 Ad vert i s i n g , i n f l u e n c e of c o l o r , 5, 1 43 Afte r i m a ges, 1 23, 1 24 Ames' d i storted room, 96 A m p l itude, 9 And romeda, 7 Ang l e of i n c i d e n ce, 44 Ang l e of refra c t i o n , 44 Angstroms, 6 Anoma l o u s t r i c h ro m a t i s m , 1 1 3, 1 1 4 Anscochrome f i l m , 1 46 Aqueous h u mor, 8 4 Arc l a mp, 2 6 , 7 7 , 7 8 Ast i g m a t i s m , 70 Astro n o m i c a l t e l e scope, 73 Atmosphere, 2 1 Auxochrome, 1 4 1 Ba r i u m , l i ne spectru m , 12 B i foca l g l a sses, 69 B i n o cu l a r cues, 95 B i nocu l a r v i s i o n , 95 B i nocu l a rs, 60, 72, 76 B i o l u m i nescence, 28 B l a c k body, 1 5 , 1 6- 1 8 B l i nd spot, 88 B l i nd ness, 88 c ol or , 1 1 3- 1 1 7 B o l o meter, 29 Brewster's a n g l e , 5 7 B r i g htness, 32-33, 9 1 , 93, 1 00, 1 0 1 , 1 03 , 1 20, 1 33 B u n s e n b u rner, 1 9 C a m e ra , 7 1 , 75, 79-8 1 , 92, 95, 1 47
Cand l e m e a s u re, 1 5, 33 C a r b o n a rc l a m p, 1 8, 1 9 Carbon a rc, band spectrum of, 1 2 Carbon b l ock, 1 9 Ca rbon f i l a ment l a m p , 22 Catad i o p t r i c ·instruments, 71 C e l s i u s tempera t u re 1 4 C h e m i c a l energy, 1 8 C h e m i l u m i nescence, 28 C h r o m a t i c a berra t i o n , 66, 67 C h ro mo gens, 1 4 1 C h romop hores, 1 4 1 C h romosphere, 2 1 C I E system, 1 1 7, 1 25, 1 28, 1 30-1 32 c h r o m a t i c i t y d i a g ra m , 1 3 1 , 1 32 Coherent l i g h t waves, 49, 1 52, 1 54 C o l l i m ator, 6 1 Color ad vert i s i n g , 5 , 1 43 b l i nd ness, 1 1 3- 1 1 7 b r i g htness, 1 00, 1 03 b u s i ness a n d i n d u s t r y , 1 42 cha rts, 1 26, 1 27 c h r o m a , 1 26, 1 27 cod es, 1 42 consta n cy, 1 20, 1 33 contra st, 1 2 1 -1 22, 1 24, 1 33 contro l , 1 42 d i ct i o n a ry of, 1 25, 1 28, 1 29 e m ot i o n a l effects of, 1 33, 1 36 harmony and d is cord , 1 34-1 35 i m ages, 1 44, 1 45 match i n g , 83, 1 1 2, 1 1 3, 1 25, 1 30, 1 42 m i xtures 1 00, 1 06-1 1 1 , 1 1 2, 1 39 monochromatic, 1 02, 1 04, mosa i c f u s i o n , 1 09, 1 51 n a t u re of, 98-1 1 7 n a t u re of l i g h t a n d , 4- 1 7 n e g a t i v e , 1 44, 1 45 of l i g ht, 98 packa g i n g , 1 43
pa i nts, 1 06, 1 1 0, 1 37-1 39 pe rcept i o n , 99, 1 1 8- 1 24 photogra p h y , 1 44 - 1 47 pos i t i ve , 1 44 p r i m a r y , 1 04 psycho l o g i c a l response, 98, 1 00, 1 26 psych o p h y s i c a l con cept, 9 8 , 1 0 1 , 1 28 p u r i ty , 1 02, 1 28 reprod u c t i o n , 1 45 sca les, 1 03 s e n sa t i o n , 1 00, 1 0 1 , 1 02 sepa ra t i o n , 1 45, 1 46 s ta n d a rds, 1 1 2, 1 1 7 s y m bo l i s m of, 1 33, 1 36, 1 43 syste m s , 1 25 - 1 32 t e l e v i s i o n , 1 09, 1 50- 1 5 1 temperature, 1 4, 1 6, 17 t i n t s, 1 34 tones, 1 34 tra n s m i s s i o n , 1 1 0, 111 t r a n s p a r e n c y , 1 45, 1 46 v a l u e , 1 26 C o l o r b l i nd ness, 1 1 3-1 1 7 C o l o r p h otogra p h y , 1 44- 1 47 C o l o r te l e v i s i o n , 1 09, 1 50-1 5 1 C o l o ra n t, 1 1 0, 1 1 1 , 1 40 C o l o red afte r i m a ge , 1 23 , 1 24 C o l o red f i l te r 1 1 0, 1 1 1 , 1 45 C o l o r i meters, 83, 1 1 2, 1 30 C o m m i ss i o n l n ter n a t i o n a l e de I ' E c l a i ra g e (CI E ) , see I n tern a t i o na l Commission on I l l u m ination C o m p l e m e n t a ry h u e s , 1 05, 1 07, 1 3 2, 1 34, 1 35 C o m p o u n d l en s e s 7 1 C o m p o u n d m i c roscope, 77
1 57
Cones, eye, 86-88 , 1 1 3 , 118 C o n structive interference, 48-49 Contact lenses, 69, 70 Con vergence, 9 1 C o n verg i n g l e nses, 68, 69 Cooper-Hew itt mercury a rc, 1 9 Cornea, 70, 84, 85 Critical a n g l e , 44, 1 55 C rysta l s , d o u b l e refract i n g , 55 Dark c h a m ber, 79 Darkness, 35 Da y l i g ht, 3 1 , 1 3 1 Depth, 9 1 -93, 94 Desca rtes, 98 Destruct iv e i n terf e renc e, 48-49 Developer, 1 44, 1 46 D i ch r o m a t i s m , 1 1 3, 1 1 5, 1 1 6 D i ff ra ction, 36, 46--4 7 , 50, 77 g r a t i n g s, 58, 62-63 Diffuse reflection, 35, 39, 1 37 D i m i n i s h i n g g l ass, 66 D i opter, 65 D i scord a nt c o l ors, 1 34, 1 35 D i spersion, 45, 62, 63, 66 D i stance, 9 1 , 94, 95 D i sti nctness, 9 1 , 93 D i verg i n g l e nses, 68, 69 Dove p r i s m , 60 Dufayco l o r f i l m, 1 45 Dyes, 1 1 0, 1 40-1 4 1 , 1 44, 1 45, 1 46 Ea rth, 2 1 E d i s o n , T h o m a s, 1 8 · E i n s te i n , energy equation, 7 theory of re l a t i v ity, 7 Ektachrome f i l m , 1 46 E l ectrol u m i nescence, 2 8 E l ectroma g n e t i c w a ves, 8-1 1 , 1 5 1 E l ectron bea m, 77, 1 50 E l ectron m i c roscope, 77 E l evation, 9 1 , 92 E m o t i o n a l effects of col or, 1 33 , 1 36 E m u l s i o n s, 1 44, 1 45 Energy, rad iant, 8, 1 4, 1 5, 27, 29, 98, 1 00, 1 53
1 58
E n l a rgers, 77, 78 Eye, 29, 30, 84-97, 1 09, 1 1 3, 1 1 8 Eyeg l a sses, 68-69, 70 Eyepiece, m i croscope, 77 opera g l a sses, 72 refract i n g te l escope, 73 spy g l a ss, 72 Fahrenheit temperature, 14 Farsi ghted ness, 68 F i be r opti cs, 1 55- 1 56 F i l a m ent l a m p , 1 8, 1 9, 22, 23 F i l m , photogra p h i c , 30, 3 1 , 78, 79, 80, 8 1 , 1 46, 1 47 F i lter, col ored, 1 1 0, 1 1 1 , 1 45 p o l a r o i d , 54 F i x i n g process, photography, 1 44 F l u o rescence, 28, 5 2 F l u o rescent l a m p , 1 9, 23, 25, 26--2 7 f - n u m ber, 80 Foca l l e n g t h , 64; 77, 80 Foca l p o i n t , 59, 64, 65 Focus, 68, 70, 74, 76, 78, 79, 8 1 ' 84, 85 F o ur -co l o r sepa rations, 1 49 Fovea, 86, 87, 89, 1 1 4, 118 F ra u n hofer, J oseph, 62 l i nes, 1 2-1 3, 2 1 Frequency, 8, 1 0, 1 1 F u l l - co l o r rep rod uction, 1 49 Gabor, D e n n i s, 1 54 Ga l i leo, 37, 72 G a m m a rays, 9 Ga n g l i a , eye, 87 Geomet r i c a l opti cs, 58 G l a re, 54 G l a ss f i ber bund l e , 1 55, 1 56 G l o w tu be, 1 9, 24-25, 26 G r a t i n g s , d iffract i o n , 58, 62-63 Gravu re, p r i n t i n g , 1 48 H a l e tel escope, 74 H a r m o n i o u s colors, 1 33 , 1 34, 1 35 Heat ra ys, 1 0 H o l ography, 1 54
H o l og r a m , 1 54 H ue, 1 00, 1 0 1 -1 1 2, 1 1 5, 1 26, 1 27, 1 32, 1 34, 1 35, 1 36 I l l u m i nance, 33, 34 I l l u m i n a t i o n , 29-35 , 76, 77, 78, 93, 99, 1 20, 1 22, 1 4 1 e l ectric, 23 I l l u s i o n s , 90, 96--9 7 I m a g e s h a rpness, 77 I mag e s , 58, 64, 67, 76, 77, 78, 1 08, 1 44, 1 45, 1 55 ; see a lso, rea l i ma g e , v i rt u a l image I n ca ndescent f i l a me n t l a m p, 1 2, 1 8, 1 9, 22-23, 1 3 1 I n c i d e n t f l u x , 33 I n c i d e n t l i g h t, 36, 4 1 , 43, 44, 52, 57 I n fra red r a d i a t i o n , 9, 1 0, 1 5, 27 I n h e r i ta n c e , c o l o r b l i n d ness, 1 1 5 I n ta g l io, 1 48 I n t e n s i ty of l i g h t , 1 5, 30, 32, 33, 82, 99 I n terference, 36, 48-49, 50 I n te r n a l reflect i o n , 44, 60, 76 I nte r n a t i o n a l C o m m i s sion on I l l u m i na tion (Com m i ssion l n ternationa l e d e I ' E cl a i ra g e -C I E ) , 1 1 7, 1 30 ; see a lso C I E system I nte r n a t i o n a l sta n d a r d i zed l i g h t sou rces, 17 I r idescent co l o rs, 49 I r is, 84, 85 Ke l v i n temperatu re, 1 4 K od a c h ro m e f i l m , 1 46 L a n d , Ed w i n , 56 camera ( P o l a ro i d ) , 1 47 Lasers, 1 52-1 53, 1 54 Latent i m a ge, 1 44 Lens, opt i ca l , 58, 64-8 1 a berra t i o n s , 6 7 a c h ro m a t i c , 66 ca mera , 79, 80, 8 1 c h ro m a t i c a b e r r a t i o n , 66, 67 c om p o u n d , 7 1
conde n s i n g , 78 contact, 69, 70 converg i n g , 64 correct i n g , 15 c u r ved g l ass, 64 cylind r i ca l , 70 d i ve rg i n g , 66, 69 d o u b l e -concave, 66 d o u b l e -convex, 64 e q u i co n cave, 66 m i c roscope, 7 1 , 77 n e g a t i v e , 66, 69 n e g a t i ve m e n i scus, 66 o b j ecti ve, 7 1 , 73, 76 ocu l a r, 76, 77 p l a n oconcave, 66 p l a n oconvex, 64 pos i t i ve, 64, 68, 69, 73, 77 pos i t i ve m e n i scus, 64 projector, 78 s p h e r i ca l a be r ra t i o n , 67 t e l e p h oto, 7 1 lens, e y e , 6 8 , 69, 70, 84-85, 88, 93 letterpress, 1 48 light absorption, 36, 5 1 , 54, 99, 1 1 0, 1 39 behavior, 36-57 b r i g htness, 32-33, 9 1 , 93, 1 00, 1 03 , 1 20, 1 33 cond u ctors, 1 55 d i ff ra c t i o n , 36, 46-47, 50, 77 d i s pe r s i o n , 45, 62, 63, 66 e n te r i n g the eye, 87 i n c i d e n t , 36, 4 1 , 43, 44, 52, 57 i n t e n s i t y , 1 5, 30, 32, 33, 82, 99 i n te rference, 36, 48-49, 50 measurement, 6, 29, 30, 33 meter, 29 monochromatic, 1 3, 1 0 1 , 1 02, 1 04, 1 07, 1 1 2 , 1 52, 1 53 photons 24, 25, 86 p o l a r i :j:ed , 53-56, 57 rays, 57, 67, 78, 8 1 , 84, 98 reflected, 35, 36, 39-4 1 , 44, 49, 50, 54, 58, 60, 98, 99 refra ct i o n , 36, 40, 42-43, 57, 58, 60
scatte r i n g , 2 1 , 36, 50, 54, 99 see i n g , 84 sou rces, 1 7, 1 8-28, 63 speed of, 7, 37-38, 42, 43 t ra n s m i s s i o n, 36, 1 55 v i s i b l e, 1 0 waves, 6, 8, 9, 1 0, 1 1 w h ite, 1 3, 1 7, 45, 1 00, 1 02, 1 05, 1 1 1 , 1 1 2 l i g h t a n d c o l or, a s too l s , 1 33- 1 56 n a t u re of, 4 - 1 7 l i g htness, 35, 1 03 l i n otype, 1 48 l i ppershey, H a n s , 72 l i t h o g r a p h y , 1 48- 1 49 l u mens, 33 l u m i n a n ce, 32, 33, 1 02, 1 03, 1 2 8, 1 52, 1 53 l u m i n e scence, 28 l u m i nos i ty, 30, 1 1 4 l u m i n o u s f l ux, 33 l u x mea su re, 33 Macu l a l u tea, eye, 86 M a g n i f i c a t i o n , 65, 7 1 , 73, 76, 77 Ma g n i f i e r, pocket d o u b l et, 7 1 s i m p l e, 64 Measu rements, 6, 29, 30, 33 Mercu ry a rc, 1 9, 26 Metric system, 6 M i c h e l so n, A l bert A . , 38 M i croscope, 6 4 , 7 1 , 77 M i rrors, 4 1 , 44, 58-59, 6 1 , 64, 67, 74, 75,
77
Mo noc h ro m a t i c l i ght, 1 3, 1 0 1 , 1 02, 1 04, 1 07, 1 1 2, 1 52, 1 53 M o n o c h rom a t i s m , 1 1 3, 1 14 Monochromator, 63 Monochrome tube, 1 50 Monocu l a r cues, 95 Moo n l i g h t, 86 Mord a n t, 1 40 Mosa i c add i t i ve c o l o r m i x i n g , 1 50 Mosa i c f u s i o n , 1 09, 1 5 1 Mosa i c system f i l m , 1 45 Motion, 91 p a ra l l a x , 94, 95 M u l t i l a y e r f i l m , 1 46, 1 47 M u n se l l , A l be rt, H . , 1 26 c o l o r system, 1 25, 1 26-1 27, 1 30
M u s c l e s , c i l i a r y , 85 N a t i o n a l B u rea u of Sta n d a rd s , 1 27 Nea r s i g hted n e s s , 66, 69 N e g a t i v e l e n se s , 66, 69 Neon, 1 9, 24-25 New to n, S i r I sa a c, 45 Newto n i a n t e l escope, 74 N ic o l p r i s m , 57 N o n - co h e r e n t l i g h t waves, 49, 1 52 Nuclear energy, 1 8 O b ject i v e l e n s , 7 1 , 76 Offset l i t h o g r a p h y , 1 49 Opera g l a s s, 72 O p t i c a x i s, 59, 64 O p t i c c h i a s m a , 88, 89 O p t i c nerve, 85, 86, 87, 88, 89, 90 Optica l i l l u s i o n , 90, 96-97 O p t i ca l i n s t r u m e n t s 58-83 O p t i ca l Society of A m e r i ca , 5 O r g a n i c d y es, 1 4 1 Ostwa l d c o l o r s y stem, 1 25, 1 28-1 29, 1 30 P a i nts, 1 06, 1 1 0, 1 37-1 39 P a r a bo l i c m i rror, 58, 75 P a r a bo l o i d a l m i rror, 74, 75 P a ra l l a x , 94, 9 5 P a sc h e n spectrog raph , 63 P e n u m bra , 34 Percept i o n , 32, 35, 90, 94, 95, 96, 99, 1 1 8- 1 24 P e r i scopes, 60, 1 55 Perspect i ve , 9 1 P h o s ph orescence, 28, 52 P h ospho r s , 52 P h otoe l e c t r i c ce l l , 80 P h ot o g ra p h y , 56, 7 1 , 78, 79-8 1 , 92, 95, 1 44- 1 47 P h otometers, 29, 82 P h ot o m e t r i c u n i t s , 33 P h otometry, 3 2 P h otons, 24, 25, 86, 1 53 P h ot o s p h e re, 2 1 P h oto-typesett i n g , 1 48 P i g m ents, 87, 1 06, 1 1 0, 1 1 8, 1 37- 1 39 P i n h o l e ca m e r a , 79
1 59
P l a n e m i rror, 58 P l a n e of v i b ra t i o n , 54 P o l a color f i l m , 1 47 P o l a r iscope, 55 P o l a r i zat ion processes, 36, 57 P o l a r i zed l i g ht, 53-56, 57 P o l a ro i d , 54, 56, 57 L a n d camera, 1 47 Positive l e n ses, 64, 68-69 Positive transparency, 1 45 P resses, h i g h - speed, 1 48 P r i m a ry c o l o rs, 1 04 P r i n t i n g , 1 48, 1 49 P r i s m s, 55, 58, 60-61 , 62, 63, 76 Projectors, 77, 78 P u p i l , eye, 84, 85 P u rity of c o l or, 1 02, 1 28
Rad i a t ion, 9, 1 0, 1 1 , 1 4, 1 5, 22, 26, 1 53 d e tectors, 29, 90 pyrometry, 2 1 Ra i n bow, 4 , 5 , 98 Rea l i m age, 58, 59, ,65, 66, 77, 1 54 Reflect i n g t e l escopes, 74 Reflect i o n , 35, 36, 39-4 1 , 44, 49, 50, 57, 58, 60, 98, 99 Refl ectors, 6 1 , 7 1 , 74, 78 Refracti n g t e l escopes, 73 Refract i o n , 36, 40, 42-43, 57, 58, 60, 98 Refractive i nd e x , 42, 43, 45, 57, 60 Refractors, 7 1 Regu l a r ref l ecti o n , 3 9 , 1 37 Reso l v i n g , 1 09 Ret i n a , 84, 86, 87, 88, 89, 90, 9 1 , 1 09, 1 1 8 Retinene, 86 R h odops i n , 86 Ridgway, Robert, 1 28 Rods, eye, 86, 1 1 8 Roemer, O l af, 37, 38 Roto g ravu re, 1 48 Row l a nd , H e n ry A., 62
1 60
Sacc h a r i meter, 54 Satura t i o n , 1 00, 1 0 1 , 1 02, 1 03, 1 1 5, 1 26, 1 34, 1 35, 1 36 Scatte r i n g , 2 1 , 36, 50, 57, 99 Sch m i d t camera, 75 Screen, pro jector, 77, 78 Sensation of s i g h t, 90, 98, 1 03 Senses, 4, 1 0, 90, 1 00 Shadowgrams, 9 Shadows, 34, 98, 1 22, 1 33 S i l ver h a l ide crysta l s , 1 44, 1 45 S i m u l taneo us c o l o r contrast, 1 2 1 , 1 24 S i ze, known ( o r i m a g i n ed } , 94 Sod i u m, spect rum of, 25 vapor l a m ps, 25 Sound, speed of, 7 Spectro graph, 5, 1 2, 6 1 , 63 Spectrometer, 6 1 S pectroscopes, 63 Spectru m , 5, 1 0-1 3, 25, 62, 98, 1 1 7 Spherica l a berra t i o n , 67, 75 Sprea d i n g effect, col ors, 1 24 Spy g l ass, 72 Sta n d a rd i l l u m i n a t i o n , 141 Sta n d a rd l i g h t sou rces, 1 7, 83, 1 1 7 Sta n d a rd l u m ino sity curve, 29, 30 , 1 30 Sta rs, c o l o r - te m perature c l a ssification, 1 6 Stereo camera, 95 Su btract ive color m i x i n g , 1 06, 1 1 0-1 1 1 , 1 1 2, 1 39 Successive afte r i m a ges, 1 24 S u n , 1 5, 20-2 1 S u n l a m p, 26 Su n l i g ht, 1 3, 1 7, 2 1 , 98, 99, 1 3 1 Suntan, 9 Su perpos i t i o n , 9 1 , 92 Su rfa ce i l l u m i na t i o n , 34 Suspensory l i gament, eye, 85
Symbo l i s m of c o l o r, 1 3 3, 1 36, 1 43 Te l e p h oto l e ns, 73 Te l e s cope, 64, 72, 75, 76 refract i n g , 73 refl e ct i n g , 74 Te l e v i s i o n , col o r, 1 09, 1 50-1 5 1 Te m p e r a t u re, 1 4 c o l or, 1 4, 1 6 , 1 7 Terrestr i a l t e l e scope, 72 T h e r m a l rays, 1 0 T h e r m o p i l e , 29 Tou r m a l i n e, 53, . 54, 56 Tra n s m i ss i o n of l i g ht, 36 Transpa rency, c o l o r f i l m , 1 45, 1 46 Trichromat, 1 1 3, 1 1 4 Tri p l e m i r ro r, 6 1 T u n g s ten l a m p s, 22, 77, 78 U l t r a v i o l e t rays, 9, 1 5, 26, 27 U m bra, 34 U n ive rsa l constant, 7 U n sa t u rated c o l o rs , 1 36 V i rt u a l i ma g e , 58, 59, 65, 66, 77, 1 54 V i s i o n , 84-97 co l o r, 98- 1 1 7, 1 1 81 24 V i s u a l p u r p l e , 86 V i treous h u m o r, 84 Water wa ves, d i ff raction of, 46 Wave crests, 8, 9, 48 W a v e l e n g t h , def i n i tion, 9 d o m i n a nt, 1 0 1 , 1 1 2, 1 28, 1 30 of i n f r a red ra y s , 1 1 o f l i g h t, 1 0, 39, 46, 77 o f v i s i b l e rays, 1 1 Wel s b a c h g a s m a n t l e , 1 8, 1 9 W h i te l i g ht, see L i g h t X - ra y s , 9 Ye l l ow spot, eye, 86
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