National tsuibdu ui um,.-.^.^

Ubrary, E-01 Admin. Bldg.

NBS MONOGRAPH 104

COLORIMETRY

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ provides measurement and technical information services

essential to the efficiency and effectiveness of the work of the Nation's scientists and engineers. The

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accomplish this mission, the Bureau is organized into three institutes covering broad program areas of research and services: THE INSTITUTE FOR BASIC STANDARDS . . . provides the central basis within the United

States for a complete and consistent system of physical measurements, coordinates that system with the measurement systems of other nations, and furnishes essential services leading to accurate and uniform

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D. C,

20234.

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3 Located at 5285 Port Royal Road, Springfield, Virginia 2215L
UNITED STATES DEPARTMENT OF COMMERCE

Alexander B. Trowbridge, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Colorimetry

I. Nimeroff

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

National Bureau of Standards Monograph 104

(Supersedes NBS Circular 478)

Issued January 1968

For sale by the Superintendent of Documents, U.S. Government Printing OflSce Washington, D.C. 20402 - Price 35 cents
Foreword

NBS Circular 478, Colorirnetry, was issued 1 March 1950 and has been reprinted several

times in response to a continuing demand. Such have been the advances in colorirnetry that in

1965, instead of authorizing another reprinting of a work obsolete in many details, I asked

Mr. I. Nimeroff to up-date it. I am grateful to him for undertaking this tedious but important

task, and I am much pleased with the result.

Deane B. Judd

Assistant Division Chief,

Metrology Division

II
Contents

Page

1. General considerations 1

1.1. Introduction 1

1.2. Definiiton of color 2

1.3. Tridimensionality of color 2

1.4. Metamerism and macular pigmentation 2

1.5. Grassman's law 3

1 2. The 1931 CIE Standard colorimetric system and

19G4 supplement 4

2.1. General principles 4

2.2. Standard sources 9

2.3. Spectrophotometric colorimetry 13

2.4. Short-cuts and automatic computation 13

2.5. Visual tristimulus colorimeters 14

2.6. Dominant wavelength and purity 16

3. Colorimetry by difference 19

8.1. General principles 19

3.2. Chromaticity .spacing, perceptibility 19

3.8. Visual devices 20

a. Martens photometer 20

b. Chromaticity-difference colorimeter 20

3.4. Photoelectric devices 22

4. Color specification by material standards 22

4.1. General principles 22

4.2. Transparent media 23

Page

a. Lovibond glasses 23

b. Arny solutions 25

4.3. Pigmented or dyed surfaces 25

a. Color dictionaries 25

b. Ostwald system 26

c. Munsell system 27

d. ISCC-NBS system 32

5. One-dimensional color scales 34

5.1. General principles 34

5.2. Color temperature 35

5.3. Grading systems 35

a. Petroleum products 37

b. Vegetable oils 40

c. Rosins and paint vehicles 40

d. Sugar products 41

6. Limitations of the methods 42

6.1. Spectrophotometric colorimetry 42

6.2. Photoelectric tristimulus colorimetry 43

a. Angular conditions 43

b. Source 44

c. Filter-detector combinations 44

6.3. Material standards comparison 45

7. References 45

Library of Congress Catalog Card Number: 67—62079

III

Colorimetry

I. Nimeroff

National Bureau of Standards

Washington, D.C. 20234

The definition of color, as a characteristic of light, and the basic principles of its meas- urement are given. The reduction of spectrophotometric data to three chromaticity coordi- nates by means of the three-function CIE standard observer sj'stem for colorimetry is described. Various methods of direct colorimetry, and visual and photoelectric methods of colorimetry by differences from material standards are treated. The most useful collections of material color standards are described and the most widely used one-dimensional color scales are explained. The limitations of several colorimetric methods : spectrophotometric,

photoelectric, and visual, are discussed. Key Words : Color dictionaries, color vision, colorimeters, photoelectric tristimulus,

spectrophotometric colorimetry, visual colorimetry.

1 . General Considerations

1.1. Introduction

It is common practice to regard color as a property of objects, and in a limited sense this view is justified. AVe have color comparators for solutions

in \vhich the color is taken as an index of the composition of the solution ; and in applying a suitably

prepared set of color standards in a color comparator, the color of the unknown behaves as if it were a property of the solution itself, just as the concentrations of the constituents which it indicates.

However, this document deals with color for its own sake ; and, for this purpose, a broader view is useful. Lights have colors as well as objects. The

flame of a Bunsen burner can be changed from

bluish purple to orange by the introduction of

sodium. And since even objects lose their colors and

become invisible unless they reflect, scatter, or transmit radiant energy, or form a part of an

illuminated scene, this broader view^ is that color is a property of light, and of light alone.

As it is possible to measure with a spectrophotometer the spectral energy distribution of any light

beam, and as the color oi a light correlates closely

with its spectral composition, some of the more

physically minded people have contended that

color is a physical property of radiant energy ; but

this is not the most useful view. The color change

of the Bunsen flame from bluish purple to orange

can be shown by a purely physical measurement to be caused by a change in the spectral composition

of the emitted energy, but it takes more than

physics to decide whether this flame has the same

color as the light reflected from the peel of a given

citrus fruit. Application of the spectrophotometer

to the orange peel will show that the spectral composition of the light reflected from it under daylight illumination is radically different from that

emitted by the sodium flame. It has a continuous

spectrum relatively strong in the long-wave (550 to 770 nm)^ portion of the visible spectrum (380 to 770 nm) . The visible energy of the sodium flame is nearly all confined to two narrow bands (589.0 and

589.6 nm) . Physically, therefore, the two lights are

different, but they have closely the same color. The

two lights must therefore be identical in some other

respect. This identity consists in some aspect of the

response made by a normal observer to the sodium

flame being the same as the corresponding aspect

of the response to the peel of the citrus fruit. The

broader view of color must, therefore, include not

only the spectral composition of the radiant energy

reaching the eye of the observer, but the properties

of the observer as well. These properties have been

evaluated by finding equivalent stimuli, like the

energy of the sodium flame and that reflected from

an orange peel, which have different spectral compositions but still manage to stimulate the same

color response to the normal observer. Such equivalent stimuli are called Tnetamers. In this chapter

there will be presented the standard method for

• This monograph is an updating of NBS Circular C-478,

Colorimetry, by D. B. Judd (1950). With the permission of Judd,

much of the original text has been kept intact where applicable.

1 The unit nanometers (nm), 10-9 meters, was formerly called millimicrons (m/t).

1
finding by computation whether or not any two

lights form equivalent stimuli. The fundamental

method of color specification based upon equivalent stimuli plus spectrophotometry will be described in detail together with other methods of

obtaining the same numbers. And finally some dis- cussion will be given on the use of the Munsell color

system and color dictionaries such as the ISCCNBS (Inter-Society Color Council-National Bureau of Standards) Method of Designating Color

and a Dictionary of Color Names, National Bureau

of Standards Circular 553, 1956 [79] ?

1.2. Definition of Color

The most widely accepted technical definition of

color is that given by the Committee on Colorimetry of the Optical Society of America [15] : "Color consists of the characteristics of light other

than spatial and temporal inhomogeneities; light

being that aspect of radiant energy of which a human observer is aware through the visual sensa- tions which arise from tlie stimulation of the retina

of the eye." It will be noted that this definition re- lates color and light to radiant energy only in so far as the energy produces a visual effect within an

observer. On this account color and light are said

to be psychological entities that can be evaluated

by means of psychophysical quantities, and in their

evaluation it is ordinarily not necessary to pay

attention to energy of wavelength shorter than

380 nm, nor longer than 770 nm because the eye is relatievly insensitive to such energy.

1.3. Tridimensionality of Color

If an observer with normal color vision attempts

to adjust one element of his visual field whose color

is under his control so that it matches a neighboring element, he will ultimately discover that three

independent adjustments have to be made. If he is using tlie red, yellow, and blue paints frequently

found in primary grade schools, only by chance

will he obtain a match from a mixture of two of

them. Even a brown color requires blue in addition

to red and yellow. Within the color gamut of the

three paints, an exact match for a given paint color

is easily possible, but three is the irreducible minimum. Similarly, if he is trying to color-match one

spot of light by shining several spotlights of differ- ent colors onto the same neighboring spot of a screen, he finds, in general, that either three lights

of fixed spectral composition are required, or, if only two lights be added together, not only the

amounts of both but also the spectral composition

of at least one has to be adjustable. The same rule

applies to rotary mixture on a sector disk; four

sectors, giving three independent adjustments, are necessary and sufficient. As the color vision of a normal observer is at

least tridimensional, it follows that there must be

at least three independent excitations in the optic nerve fibers corresponding to each patch of the

- Figures in bracliets indicate the literature references at the end of this paper.

visual field. Theories of color vision have been , derived mostly from speculation as to the character

of these excitations. It also follows that a color

specification is expressible by three numbers. For

normal observers three numbers are necessary ; for

partially color-blind observers only two numbers

are necessary ; and for totally color-blind observers

only one is necessary. In the examples given (paints, spotlights, sector disks) , the observer by ajustment of three variables \ obtains a color match, that is, he has to set up a second stimulus equivalent to the first. Except by

accident, however, the ternary or binary mixture

does not match the unknown in spectral composition. In the usual case the mixture is equivalent to the unkonwn in color but not in spectral composition, and the unknown and the mixture are said to form a metameric pair. There are, however, degrees

of difference in spectral composition. If one painted

panel be matched by a mixture of red, yellow, and

blue paints, the degree of metamerism is likely to be only moderate; but if the paint panel illuminated by daylight be matched by shining on a white card three spotlights each of which contains ; energy restricted to a narrow wavelength band i (such as spectrum red, green, and blue) , the degree i

i of metamerism will ordinarily be large. sj

1.4. Metamerism and the Macular Pigment

Studies of extremely metameric pairs in which j

' mixtures of two parts of the spectrum are set up ' to color-match other two-part spectrum mixtures ^ have yielded our most valuable knowledge regard- ' ing the properties of the average normal eye [1, 44, I* 84, 99, 158]. An outstanding fact derived from ° observation of such metamers is that the center of * the retina (fovea centralis) has somewhat different

properties from that part of the retina immediately r surrounding it; that is, a color match set up for

the central 2 or 3 deg of the retina becomes an

easily detectable mismatch if the eye be turned so j i,, as to allow the stimuli forming the metameric pair 3 j: to affect a portion of the retina, say, 6 deg from \ j the fovea. Furthermore, if the metamers are com- j |j pared in large patches so as to subtend 6 deg or , ^ more at tlie eye of the observer, this mismatch , j causes a central spot, known as the Maxwell spot , jj [98], to appear temporarily on either field even , ^ though the field is physically uniform. After the : ^ spot has faded away, change of fixation to the other j ^^ field will renew the spot. This dependence of meta- , ^ merism on the portion of the retina used arises ,1 |j chiefly from the existence of a spot of brownish or ^ ^ yellowish pigment irregularly covering and inter- - | penetrating the central 3 or 4 deg of the normal , ^ retina; it is called the mncida luten or sometimes ; | the yeJloio spot (see fig. 1). Figui'e 1 shows a horizontal cross section of the eye. Light enters the , jj tear-film ff, passes through the cornea aa, the aque- ous humor B, the pupil bb, the crystalline lens A,

the vitreous humor C, and the macula p before

reaching the retina i. The macular pigment acts as

2
FiGXJKE 1. Horizontal cross-section of the normal human eye.

a selective filter interposed between the vitreous

humor C and the retina i. Metamers set up for one normal observer usually fail to hold strictly for

anyone else. This failure is ascribable to variations

in amount of pigmentation of the eye media

(cornea, lens, humors, macula), the macular pigment being one of the chief variables. The properties of the normal eye derived fi'om a small-field

study of these extreme metamers therefore refer

only to the central 2 deg of the retina, and they

refer to an hypothetical average eye. Nobody has

been found whose eye differs so little from this

average eye that the differences could not be detected. Practically speaking, therefore, nobody has an eye that is colorimetrically laormal.

1.5. Grassman's Law

From a knowledge of spectral metamers, it has

been possible to summarize concisely the properties

of the average normal eye. This summary is made

in accord with the principle known as Grassman's

law [42] foreshadowed by Newton's laws of color

mixture. If a light composed of known amounts of

three components (called primaries) is equivalent

in color to an unknown light, the three known

amounts may be used as a color specification for

this light. These amounts are called the tristimulus

values of the color. Grassman's law states that,

when equivalent lights are added to equivalent

lights, the sums are equivalent. Thus, if an un- known spot of color were matched by shining on

the same spot of a white screen two component

spotlights of tristimulus values, Xj, l^i, Zi, and

Z2, F2, Z2, respectively, by Grassman's law, the

tristimulus values, JT, Y, Z, of the unknown spot

of color would be simply : X = Xi + X2

y = + ^2 (1)

z = + . Any beam of light, whether it originates from a

self-luminous body or comes, by transmission, scat- tering, or reflection, from a nonself-luminous object, may be considered as made up of a large

number of portions of the spectrum. The amounts

of these various portions may be determined by

spectrophotometry. The spectral values, ~x{\),

y(A),'i(A), of each of these portions have been

determined for a number of normal observers, and

average values are given in table 1 in arbitrary

units for a spectrum of unit spectral irradiance.

The principle expressed in Grassman's law has

been established by repeated experiment over a wide middle range of retinal illuminances. It breaks down for very high retinal illuminances

[159] that begin to approach those sufficient to do

the eye permanent harm, and it breaks down if the illumination of the whole retina continues for

several minutes to be so slight that vision by the

retinal rods (twilight vision) intrudes significantly

[84]. Between these two extremes, however, Grassman's law holds independently of the adaptive

state of the eye. Thus, if two stimuli of different

wavelength distributions of energy be found that

are once responded to alike by the eye, they will

be seen alike even after exposure of the eye to an- other stimulus sufficient to change considerably the

appearance of the two equivalent stimuli. For ex- ample, if a portion of the spectrum near 640 nm

(red) be superposed on a portion near 550 nm

(yellowish green), it will be found possible to obtain the color of this combination from an intermediate portion of the spectrum, say, 590 nm

(orange) . If the retina of the eye be highly illuminated by light of wavelength near 640 nm, and its sensitivity to radiant flux of this wavelength region

considerably reduced in this way, it is found that,

although neither of the equivalent stimuli any

longer appears orange, they still give identical

colors ; for example, they may yield identical yellows or identical greenish yellows. The eye thus

cannot be trusted to yield the same color perception

from a given stimulus; simultaneous and successive contrast affect it profoundly. But the eye is still a satisfactory null instrument and obeys Grassman's

law.

By Grassman's law it is possible to test whether

any two beams of light of differing spectral composition form a metameric pair. The condition for

metamerism of tw^o beams of light of spectral ir- radiance. El and E2, is that simultaneously

:

2"o(^i)x«(A)AA= 2=°o(^2)x«(A)AA

S-o(^i)xy(A)AA= S-o(^2)xy(A)AA (2)

2"o(^i)xs(A)AA= S"o(^2)x3(A)AA

where ¥ (A), and (A) characterize the observers' spectral responses. (These equations are written in accord with the CIE notation adopted

in 1963. Symbols with subscripted A, as E\, indi- cate spectral concentration, while symbols with

parenthetical A, as ¥(A) , indicate other spectral

relationships not critically dependent on choice

of wavelength interval.) The wavelength interval.

3
AA, to be used in these summations depends upon

the irregularity of the curve of spectral irradi- ance with wavelength; intervals of 20 nm are sometimes sufficient to yield a significant result; intervals of 10 nm often do ; and intervals of 5 nm

are usually sufficient except for discontinuous spectral distributions such as those characterising

gaseous discharge lamps. De Kerf [27] has re- ported instances where 1-nm intervals have been

required.

The summations of eq (2) form the tristimulus

values of the color and are customarily given the

symbols X, Z, Z, respectively, so that the condition

for a color match would ordinarily be written

:

Xi = X2

Yr = (2a)

Zi — Z2

and would mean that the two colors are identical

since their tristimulus values are identical. Thus, to match color one requires Xi parts of the X primary, i'l parts of the T primary, and Zi parts of

the Z primary; and because to match color two

requires the same amounts of the same primaries

as does color one, the two colors are the same. Any three lights may be used as primaries in a system of tristimulus color specifications, provided

only that no one of them is equivalent to a combination of the other two. Tristimulus specifications

X, Y, Z, expressed relative to one set of primaries,

may be transformed into specifications R, G, B,

relative to any other set, by transformation equations of the form

:

R = KyX -f K^Y -f K^Z

G = /AX + ZsZ + K^Z

B = KjX -f K^Y + K^Z

(3)

The constants K\ to K% may take on any arbitrary

values, positive, negative, or zero, provided they

are not such as to make one of the new primaries

identical to a combination of the other two; that

is, provided that

:

K2 ^3.

^4 ^5 ^6

K-, ^9

0 (3a)

As the exceptions that cause the determinant of the

system to vanish are trivial, the choice of coordi- nate system is very wide. T?he primaries do not even have to correspond to physically realizable

lights. Imaginary lights defined by spectral compositions having negative values for some parts of

the spectrum are admissible, and indeed are pre- ferred for routine colorimetry because by their use the computation of tristimulus values from spec- trophotometric data is somewhat simplified.

I I I

I

! I

I I I I I I I

I M M I I 1 1 I

I I 1 N I I I I I

2. The 1931 CIE Standard Colorimetric System and 1964 Supplement

2.1. General Principles

In 1931, the Commission Internationale de

I'Eclairage (CIE)^ recommended that all subsequent color data be expressed in terms of the same

tristimulus system so that the results would be immediately comparable. The standard observer and

coordinate system recommended [49, 65, 135, 143]

is defined by the tristimulus values of the spectrum

colors given in table la and plotted in figure 2. The

supplementary observer for large fields, adopted

by the CIE in 1964 [59] is given in table lb. It will

be noted that the primaries chosen are such that none of these tristimulus values is less than zero.

It is further true that the green primary chosen,

whose amounts are designated by y, is such as to carry all of the luminosity, the other two primaries

(red, blue) whose amounts are designated by X

and Z, respectively, being unassociated with luminosity. Therefore, the ^'alues of y'for the spectrum

correspond to the standard luminosity function,

and it is convenient and customary to express the Y

value of a luminous area as its luminmice (photometric brightness) in terms of some recognized

unit (such as candles per square meter, milli- lambert, or foot-lambert) . The Y value of an opaque specimen may be conveniently expressed as

its huninous leflectnnce (ratio of reflected to inci- dent luminous fiux) ; and the Y value of a trans- mitting specimen is customarily put in terms of

i-i 800

FiGUKE 2. Spectral tristimulua values according to the 1931 CIE standard observer.

3 This was formerly called ICI for tlie initial letters of the English name of this commission.

luminous transmittaiice (ratio of transmitted to incident luminous flux)

.

If, as is usual, light combinations are not the

chief interest, it is convenient to substitute for the

tristimulus values, A', 1', Z, tlie two ratios, X/ (X

+ Y_ + Z) and T/ (X -f Z + Z) , combined with the

luminous value, Y. The two ratios are known as chromaticUy coordinates., a?,y, because they serve to

4
Tabu: 1(a). The 1931 CIE standard observer (2°)

Spectral tristimulus values of equal-energy Spectral tristimulus values of equal-energy Wave- spGctrum Wave- spGctrum

length, length, nm —1/12

380 0.0014 U.UUUU 0.0065 580 0.9163 0.0017 385 .0022 .0001 .0106 585 .9786 .8163 .0014 390 .0042 .0001 .0201 590 1.0263 .7570 .0011 395 .0076 .0002 .0362 595 1.0567 .6949 .0010 400 .0143 .0004 .0679 600 1.0622 .6310 .0008 405 .0232 .0006 .1102 605 1.0456 .5668 .0006 410 .0435 .0012 .2074 610 1.0026 .5030 .0003 415 .0776 .0022 .3713 615 0.9384 .4412 .0002 420 .1344 .0040 .6456 620 .8544 .3810 .0002 425 .2148 .0073 1.0391 625 .7514 .3210 .0001 430 .2839 .0116 1.3856 630 .6424 .2650 .0000 435 .8285 .0168 1.6230 635 .5419 .2170 .0000 440 .3483 .0230 1.7471 640 .4479 .1750 .0000 445 .3481 .0298 1.7826 645 .3608 .1382 .0000 450 .3362 .0380 1.7721 650 .2835 .1070 .0000 455 .3187 .0480 1.7441 655 .2187 .0816 .0000 460 2908 .0600 1.6692 660 .1649 .0610 .0000 465 .2511 .0739 1.5281 665 .1212 .0446 .0000 470 .1954 .0910 1.2876 670 .0874 .0320 .0000 ^ 1 0 .1421 .1126 1.0419 675 .0636 .0232 .0000 480 .0956 .1390 0.8130 680 .0468 .0170 .0000 485 .0580 .1693 .6162 685 .0329 .0119 .0000 490 .0320 .2080 .4652 690 .0227 .0082 .0000 495 .0147 .2586 .3533 695 .0158 .0057 .0000 600 .0049 .3230 .2720 700 .0114 .0041 .0000 505 .0024 .4073 .2123 705 .0081 .0029 .0000 510 .0093 .5030 .1582 710 .0058 .0021 .0000 515 .0291 .6082 .1117 715 .0041 .0015 .oooc 520 .0633 .7100 .0782 720 .0029 .0010 .0000 625 .1096 .7932 .0573 725 .0020 .0007 .0000 630 .1655 .8620 .0422 730 .0014 .0005 .0000 535 .2257 .9149 .0298 735 .0010 .0004 .0000 640 .2904 .9540 .0203 740 .0007 .0003 .0000 645 .3597 .9803 .0134 745 .0005 .0002 .0000 550 .4334 .9950 .0087 750 .0003 .0001 .0000 655 .5121 1.0002 .0057 755 .0002 .0001 .0000 560 .5945 0.9950 .0039 760 .0002 .0001 .0000 565 .6784 .9786 .0027 765 .0001 .0000 .0000 670 .7621 .9520 .0021 770 .0001 .0000 .0000 675 .8425 .9154 .0018 775 .0000 .0000 .0000 580 .9163 .8700 .0017 780 .0000 .0000 .0000 Totals 21.3713 21.3714 21.3715

5
Table 1(b). The 1964 GIE supplementary observer (10°)

Spectral trlstiraulus values of equal-energy Spectral tristimulus values of equal-energy Wave- spsct rum Wave- spsctruM

length, length, nm —^

— nm — — —

2/10 2'10 2/10

380 u.uuuu A AAA7 580 JL .Ull^ V. oOot7 A AAAA

385 AAA"? .UUUi 585 1 (\1 A.^ 1 .U I rrO AAAA

390 .0024 .UUUo A1 Af^ 590 111 a'^ .UUUO AAAA

395 .0072 .UUUo 595 . 1 ^U4 AAAA .uuuu 400 .0191 .0020 .0860 600 1.1240 .6583 .0000 405 .0434 .UU40 .1971 605 1 AQQ1 .oyoy .0000 410 .0847 .UUOo .3894 610 1 AQ At^ l.UoUO F^OQ A .0000 415 .1406 .0145 .6568 615 0.9507 .0000 420 .2045 .U^iX4; A01 4 .y 4 zo 620 .cSODo .oyoi .0000 425 .2647 .0295 1.2825 625 .7549 .3396 .0000 480 .3147 AQ Q7 1.5535 630 .04 ( 0 .0000 435 .3577 .0496 1.7985 635 .5351 .2283 .0000 440 .3837 .0621 1.9673 640 .4316 .1798 .0000 445 .3867 .0747 O AO '7 O 645 .3437 .1402 .0000 450 .3707 .0895 1.9948 650 .2683 .1076 .0000 455 .3430 .1063 1.9007 655 .2043 .0812 .0000 460 .3023 .1282 1.7454 660 .1526 .0603 .0000

.2541 .1528 1.5549 665 .1122 .0441 .0000 470 .1956 .1852 1.3176 670 .0813 .0318 .0000 475 .1323 .2199 1.0302 675 .0579 .0226 .0000 480 .0805 .2536 0.7721 680 .0409 .0159 .0000

.485 .0411 .2977 .5701 685 .0286 .0111 .0000 490 .0162 .3391 .4153 690 .0199 .0077 .0000 495 .0051 .3954 .3024 695 .0138 .0054 .0000 500 .0038 .4608 .2185 700 .0096 .0037 .0000 505 .0154 .5314 .1592 705 .0066 .0026 .0000 610 .0:-!75 .6067 .1120 710 .0046 .0018 .0000 515 .0714 .6857 .0822 715 .0031 .0012 .0000 520 .1177 .7618 .0607 720 .0022 .0008 .0000 525 .1730 .8233 .0431 725 .0015 .0006 .0000 530 .2365 .8752 .0305 730 .0010 .0004 .0000 535 .3042 .9238 .0206 735 .0007 .0003 .0000 540 .3768 .9620 .0137 740 .0005 .0002 .0000 545 .4516 .9822 .0079 745 .0004 .0001 .0000 550 .5298 .9918 .0040 750 .0003 .0001 .0000 555 .6161 .9991 .0011 755 .0002 .0001 .0000 560 .7052 .9973 .0000 760 .0001 .0000 .0000 5H5 .7938 .9824 .0000 765 .0001 .0000 .0000 570 .8787 .9555 .0000 770 .0001 .0000 .0000 575 .9512 .9152 .0000 775 .0000 .0000 .0000 580 1.0142 .8689 .0000 780 .0000 .0000 .0000 Totals 23.3294 23.3323 23.3343

6
Table 2 (a). Chromaticity coordinates of the spectrum colors (2°

)

Wave- Chromaticity coordinates Wave- Chromaticity coordinates

lengtli, nm nm 1/2

380 0.1741 0.0050 0.8209 580 0.5125 0.4866 0.0009 385 .1740 .0050 .8210 585 .5448 .4544 .0008 390 .1738 .0049 .8213 590 .5752 .4242 .0006 395 .1736 .ou4a .8215 595 .6029 .3965 .0006 400 .1733 .0048 .8219 600 .6270 .3725 .0005 405 .1730 .0048 .8222 605 .6482 .3514 .0004 410 .1726 .0048 .8226 610 .6658 .3340 .0002 415 .1721 .0048 .8231 615 .6801 .3197 .0002 420 .1714 .0051 .8235 620 .6915 .3083 .0002 425 .1703 .0058 .8239 625 .7006 .2993 .0001 430 .1689 .0069 .8242 630 .7079 .2920 .0001 435 .1669 .0086 .8245 635 .7140 .2859 .0001 440 .1644 .0109 .8247 640 .7190 .2809 .0001 445 .1611 .0138 .8251 645 .7230 .2770 .0000 450 .1566 .0177 .8257 650 .7260 .2740 .0000 455 .1510 .0227 .8263 655 .7283 .2717 .0000 460 1440 0297 660 . i ouu 2700 0000

465 ;i355 ;0399 .8246 665 .7311 12689 !oooo 470 .1241 .0578 .8181 670 .7320 .2680 .0000 475 .1096 .0868 .8036 675 .7327 .2673 .0000 480 .0913 .1327 .7760 680 .7334 .2666 .0000 485 .0687 .2007 .7306 685 .7340 .2660 .0000 490 .0454 .2950 .6596 690 .7344 .2656 .0000 495 .0235 .4127 .5638 695 .7346 .2654 .0000 500 .0082 .5384 .4534 700 .7347 .2653 .0000 505 .0039 .6548 .3413 705 .7347 .2653 .0000 510 .0139 .7502 .2359 710 .7347 .2653 .0000 515 .0389 .8120 .1491 715 .7347 .2653 .0000 520 .0743 .8338 .0919 720 .7347 .2653 .0000 525 .1142 .8262 .0596 725 .7347 .2653 .0000 530 .1547 .8059 .0394 730 .7347 .2653 .0000 535 .1929 .7816 .0255 735 .7347 .2653 .0000 540 .2296 .7543 .0161 740 .7347 .2653 .0000 545 .2658 .7243 .0099 745 .7347 .2653 .0000 550 .3016 .6923 .0061 750 .7347 .2653 .0000 555 .3373 .6589 .0038 755 .7347 .2653 .0000 560 .3731 .6245 .0024 760 .7347 .0000 565 .4087 .5896 .0017 765 .7347 .2653 .0000 570 .4441 .5547 .0012 770 .7347 .2653 .0000 575 .4788 .5202 .0010 775 .7347 .2653 .0000 580 .5125 .4866 .0009 780 .7347 .2653 .0000

7
Table 2(b). Chromaticity coordinates of the spectrum colors (10°)

Wave- Chromaticity coordinates Wave- Chromaticity coordinates

length, length, nm VlO nm

VlO 210

0 380 0.1813 0.0197 0.7990 0 580 0.5386 0.4614 0.0000

1385 .1809 .0195 .7996 .585 .5654 .4346 .0000

.390 .1803 .0194 .8003 .590 .5900 .4100 .0000

.395 .1795 .0190 .8015 .595 .6116 .3884 .0000

.400 1784 0187 .8029 .600 .6306 .3694 0000

405 .1771 .0184 .8045 605 .6471 .3529 .0000

1410 .1755 .0181 .8064 .610 .6612 .3388 .0000

.415 .1732 .0178 .8090 .615 .6731 .3269 .0000

.420 .1706 .0179 .8115 .620 .6827 .3173 .0000

.425 1679 0187 8134 .625 6898 3102 0000

430 .1650 .0203 .8147 630 .6955 .3045 .0000

'A35 .1622 .0225 .8153 .635 .7010 .2990 .0000

.440 .1590 .0257 .8153 .640 .7059 .2941 .0000

.445 .1554 .0300 .8146 .645 .7103 .2898 .0000

.450 1510 0364 8126 .650 7137 2863 0000

455 [1459 ".0452 .8088 655 '.7156 ;2844 '.0000 AGO .1389 .0589 .8022 .660 .7168 .2832 .0000

.465 .1295 .0779 .7926 .665 .7179 .2821 .0000

.470 .1152 .1090 .7758 .670 .7187 .2813 .0000

.475 .0957 .1591 .7452 .675 .7193 .2807 .0000

.480 .0728 .2292 .6980 .680 .7198 .2802 .0000

.485 .0452 .3275 .6273 .685 ,7200 .2800 .0000

.490 .0210 .4401 .5389 .690 .7202 .2798 .0000

.495 .0073 .5625 .4302 .695 .7203 .2797 .0000

.500 .0056 .6745 .3199 .700 .7204 .2796 .0000

.505 .0219 .7526 .2256 .705 .7203 .2797 .0000

.510 .0495 .8023 .1482 .710 .7202 .2798 .0000

.515 .0850 .8170 .0980 .715 .7201 .2799 .0000

.520 .1252 .8102 .0646 .720 .7199 .2801 .0000

.525 .1664 .7922 .0414 .725 .7197 .2803 .0000

.530 .2071 .7663 .0267 .730 .7195 .2806 .0000

.535 .2436 .7399 .0165 .735 .7192 .2808 .0000

.540 .2786 .7113 .0101 .740 .7189 .2811 .0000

.545 .3132 .6813 .0055 .745 .7186 .2814 .0000

.550 .3473 .6501 .0026 .750 .7183 .2817 .0000

.555 .3812 .6182 0007 .755 .7180 .2820 .0000

.560 .4142 .5858 .0000 .760 .7176 .2824 .0000

.565 .4469 .5531 .0000 .765 .7172 .2828 .0000

.570 .4790 .5210 .0000 .770 .7169 .2831 .0000

.575 .5096 .4904 .0000 .775 .7165 .2835 .0000

.580 .5386 .4614 .0000 .780 .7161 .2839 .0000

8
.8

.6

y

1 1 1 1 1 1 1

t^550

-

—

~ G -

^500

\ / / / v\

_\ BG y

"V /''OPk TrO^^OO

'Source C — I -^..A, —

\ \, / ^,>!\75onm

\ ^ // —

- \ /^/ P \ / I'^l

1 1 1 1 1 1

X

Figure 3. Th,e {x,y)-chromaticitij diagram, shoioing the spectrum locus and the purple boundary.

Wavelength is indicated in millimicrons. The hue names are those proposed by Kelly (77).

specify the chromatic aspect of the light. The

analogous ratio, Z/ {X + JT + Z) , is also known as a chromaticity coordinate, z, but only two of the

three coordinates, x, y, z, give independent information since by definition the sum of all three is unity. Table 2a gives the chromaticity coordinates,

X, y, 2, of the spectrum colors for 2° and table 2b

for 10° fields [59] . Figure 3 shows the points repre- senting the spectrum colors in the (a?, y) -chromaticity diagram. This diagram is also known as a Maxwell triangle because of Maxwell's first use of

such a diagram [98]. Furthermore, it has aptly

been called a mixture diagram because it indicates

in a very simple way the chromaticity of the color

resulting from the additive combination of any

two lights. The point representing this chx'omaticity is found on the straight line connecting the

points representing two lights. The primary lights are represented by points at the corners of a tri- angle, and every point within the triangle represents the chromaticity of a mixture of the primary

lights whose proportions are indicated by the

chromaticity coordinates, a?, y, z. The spectrum

colors are shown by a smooth curve known as the

spectrum locus. A few points on this locus are

identified by wavelength in nanometers. It will be

noted from figure 3 that the spectrum locus is sub1 stantially straight from 540 nm to the long-wave

extreme. This means that the standard observer

' would find binary mixtures of, say, 540 nm with

640 nm, closely equivalent to some intermediate

portion of the spectrum. But the spectrum locus

from 540 nm to the short-wave extreme is curved

outward. This means that for the standard observer

a binary mixture of 540 nm with, say, 440 nm

would differ importantly in chromaticity from the

intermediate parts of the spectrum. By drawing

straight lines through any central point (such as

£» = y = 1/3, representing the so-called equalenergy stimulus) and extending them until they

cut the spectrum locus, we may find the spectral

complementaries relative to a stimulus represented

by that point ; that is, we may find the two parts of

the spectrum that, when combined in proper proportions, will for the standard observer be equivalent to the central stimulus.

The straight line in figure 3 joining the extremes

of the spectrum locus represents the chromaticities

of the mixtures of the two extremes of the visible

spectrum. The area bounded by the closed curve made up of the spectrum locus and this straight

line is the locus of all physically realizable chromaticities. Note that the points representing the primaries of the CIE coordinate system, the apices of

the triangle {x = l,y = s = 0;y = 1, a? = 3 = 0;

3 = l,a? = y = 0),all fall outside this area ; that is, the primaries are imaginary. Note also that both

the X and Z primaries fall on the line y = 0, which

is unassociated with luminosity and is known as the alychne or liglitJess line. The short-wave ex- treme of the spectrum locus comes close to this

line; this means that, although it has the power

to elicit in the standard observer a considerable X

and Z response, resulting in a vivid bluish purple

color, radiant flux of wavelength 380 to 420 nm is almost unassociated with luminosity. The areas in

figure 3 corresponding to common color designations for lights are those proposed by Kelly [77]

and will be discussed later.

2.2. Standard Sources

At the time of setting up the standard observer

and coordinate system, the International Commission on Illumination [135], Commission International de I'Eclairage (CIE), recommended use of

three standard sovirces for colorimetry; source A,

representative of gas-filled incandescent lamps;

source B, representative of noon sunlight; and

source C, representative of average daylight such

as that from a completely overcast sky. Source A is an incandescent lamp operated at a color temperature of 2854 °K, on the international temperature

scale (c2 = 14,380). Source B is obtained hy using

this same lamp in combination with a two-cell

Davis-Gibson liquid filter designed to give a color

temperature of about 5000 °K. Source C is obtained similarly and results in a source of corre- lated color temperature about 6800 °K. These

sources are i-ecommended for general use, or whenever there is no special reason for using some other

source. Table 3 gives the relative spectral irracli- ance of Sources A, B, C, D5500, Desoo, and D7500. Sources D5500, Dc5oo, D7500 represent several phases

of daylight, closely represented by the subscripted

correlated color temperatures. Tables 4a and 4b

give computation forms for evaluation of the colors

of non-self-luminous specimens that transmit, scatter, or reflect incident light for the 2° standard ob9
Table 3. Relative spectral irradiance E of the six standard sources, A, B, C, D,i„„, Uc-m, U'nd D

Mr flVG- nm

Eb Ec ^5500 .^^6500 1117500 waveI6ngtu nm

Ea Eb .0^5500 .li'esoo Jii7500

380 9.79 22.40 33.00 32.60 50.00 66.70 580 114.44 101.00 97.80 97.70 95.80 94.20

ocso 10 90 26 85 39 92 33 35 52 30 68 35 OoO 118.08 100.07 95 43 94 55 92 25 90 60 12 09 31 30 47 40 38 10 54 60 7000 p;qa 121.73 99 20 93 20 91 40 OO. i V 87 00 13 36 36 18 55^17 49 50 68.70 85.95 Dyo 125.39 98.44 91.22 92.90 89 35 87.10 400 14.71 41.30 63.30 60.90 82.80 101.90 600 129.04 98.00 89.70 94.40 -90.00 87.20 405 16.15 46.62 71.81 64.75 87.15 106.90 605 132.70 98.08 88.83 94.75 89.80 86.65

ill n 17 68 52 10 80 60 68 60 91 50 111 90 «1 A 136 34 98 50 88 40 Qp* 1 A RQ RA ftft OD.lv1 A 19 29 57 70 89 53 70 10 92 45 112 35 DlO 139 99 99 06 88 19 OTt. Oil 21 00 OA 98 10 71 «C\ 93 40 112 80 143*62 99 70 88 10 Qd. 9A ft7 7A fiQ AA 425 22.79 68.37 105.80 69.75 90.05 107.95 625 147.23 100.36 88.06 92.30 85.50 81.15 430 24.67 73.10 112.40 K)i.o\) 86.70 630 150.83 100.00 88.00 •90.40 83.30 78.70 ^oD 1 J .oJ. 117 7^ 7A 7=^

1 D. 1 O 1 1 O 1 PC 635 154 42 101 .o6 R7 RA Q1 Q Q*J ^A 7Q KK l^U Zo. 1 yj QA fin 1 01 f^A 1^1 .OU 1 Ail QA 1 ^ 1. 640 157 98 (27 8A Q1 Q A QQ 7A 7Q AC\ jU.OO l^o.rxU 01 SO 1 1 A Q^l 1. iU.yo 645 1 ox .Ol. lUo.UO 07 QQ OA AA 01. oO 76.60 450 33.09 85.40 124.00 98.00 117.00 133.30 650 165.03 103.90 88.30 88.90 80.00 74.80 455 35.41 86.88 123.60 99.25 114.40 132.85 655 168.51 104.59 88.20 89.60 80.10 74.55 88 30 1 f>Q 1 A J. — o, lU 1 AA PLA 117 111 -OU Sin 1 Q9 d,A 1 OZ.rrU 660 1 71 Qft 105.00 0 7 OA OA QA QA OA 7-4 QA 465 40.30 90.08 123.30 100.20 116.35 129.85 665 175.38 105.08 87.22 92.10 81.25 74.85 470 42.87 92.00 123.80 99.90 114.90 127.30 670 178.77 104.90 86.30 93.90 82.80 75.40 475 45.52 93.75 124.09 101.30 115.40 127.05 675 182.12 104.55 85.30 91.95 80.30 73.50 480 48.25 95.20 123.90 102.70 115.90 126.80 680 184.43 103.90 84.00 90.00 78.30 71.60 485 51.04 96.23 122.92 100.40 112.35 122.30 685 188.70 102.84 82.21 84.85 74.00 69.75 490 53.91 96.50 120.70 98.10 108.80 117.80 690 191.93 101.60 80.20 79.70 69.70 63.00 495 56.85 95.71 116.90 99.40 109.10 117.20 695 195.12 100.38 78.24 81.25 70.65 64.50 500 59.86 94.20 112.10 100.70 109.40 116.60 700 198.26 99.10 76.30 82.80 71.60 65.10 505 62.93 92.37 106.98 100.70 108.60 115.15 705 201.36 97.70 74.36 83.80 72.95 66.60 510 66.06 90.70 102,30 100.70 107.80 113.70 710 204.41 96.20 72.40 84.80 74.30 68.10 515 69.25 89.65 98.81 100.35 106.30 111.20 715 207.41 94.60 70.40 77.50 67.95 62.25 520 72.50 89.50 96.90 100.00 104.80 108.70 720 210.36 92.90 68.30 70.20 61.60 56.40 525 75.79 90.43 96.78 102.10 106.25 109.55 725 213.26 91.10 66.30 74.75 65.75 60.30 630 79.13 92.20 98.00 104.20 107.70 110.40 730 216.12 89.40 64.40 79.30 69.90 64.20 535 82.52 94.46 99.94 103.15 106.05 108.35 735 218.92 88.00 62.80 82.15 72.50 66.70 540 85.95 96.90 102.10 102.10 104.40 106.30 740 221.66 86.90 61.50 85.00 75.10 69.20 545 89.41 99.16 103.95 102.55 104.20 105.60 745 224.36 85.90 60.20 78.45 69.35 63.90 550 92.91 101.00 105.20 103.00 104.00 104.90 750 227.00 85.20 59.20 71.90 63.60 58.60 555 96.44 102.20 105.67 101.50 102.00 102.45 755 229.58 84.80 58.50 62.35 55.00 50.60 560 100.00 102.80 105.30 100.00 100.00 100.00 760 232.11 84.70 58.10 52.80 4fi 40 42.60 565 103.58 102.92 104.11 98.60 98.15 97.80 765 234.59 84.90 58.00 64.35 56.60 52.00 570 107.18 102.60 102.30 97.20 96.30 95.60 770 237.01 85.40 58.20 75.90 66.80 61.40 575 110.80 101.90 100.15 97.45 96.05 94.90 775 239.37 86.10 58.50 73.85 65.10 59.85 580 114.44 101.00 97.80 97.70 95.80 94.20 780 241.67 87.00 59.10 71.80 63.40 58.30

10
Table 4a.

CIE coordinates (2°)

Computation form

Source A (2854 "K.)

Sample Source of Trans. DataSubmitted by

X

(nzn) EaX2 Eay2 T TEaX2 rEay-i TEaZ2

380 1 6 0. 90 5 23 400 19 1 93 10 71 2 340

20 262 8 1256

30 649 27 3167

40 926 61 4647 450 1031 117 5435

60 1019 210 5851

70 776 362 5116

80 428 622 3636

90 160 1039 2324

500 27 1792 1509

10 57 3080 969

20 425 4771 525

30 1214 6322 309

40 2313 7600 162 550 3732 8568 75 60 5510 9222 36

70 7571 9457 21 80 9719 9228 18

90 11579 8540 12 600 12704 7547 10

10 12669 6356 4

20 11373 5071 8

30 8980 3704

40 6558 2562

650 4336 1637

60 2628 972

70 1448 530

80 804 292

90 404 146 700 209 75

10 110 40

20 57 19

30 28 10 40 14 6 750 6 2 60 4 2

70 2

Sums 109828 100000 35547 Sums X = Y = Z =

0.4476 0.4075 0.1449 S = X+Y+Z

X, y, and z (X = X/S, y = Y/S, z = Z/S)

Planck 2854 °K., C2 = 14,380 Computed by Checked by. Planck 2848 °K., C2 = 14,350

11
Table 4b. Cmnputation form

CIE coordinates Source C (Davis-Gibson, 2854 to 6800 °K.)

Sample Source of Trans. DataSubmitted by

_

X EtXi r TEaX2 (nm)

380 4 20 0. 90 19 89 400 85 2 404

10 329 9 1570 20 1238 37 5949 30 2997 122 14628

40 3975 262 19938

450 3915 443 20638

60 3362 694 19299

70 2272 1058 14972

80 1112 1618 9461

90 363 2358 5274

500 52 3401 2864

10 89 4833 1520 20 B76 6462 712 30 1523 7934 388

40 2785 9149 195 560 4282 9832 86 60 5880 9841 89

70 7322 9147 20

80 8417 7992 16 90 8984 6627 10 600 8949 5316 7 10 8325 4176 2 20 7070 3153 2 30 5309 2190

ooyo

650 2349 886 60 1361 504 TO 708 259

80 369 134 90 171 62 700 82 29

10 39 14 20 19 6

30 8 3 40 4 2

750 2 1 60 1 1 70 1

Sums 98041 100000 118103 Sums X = Y = Z =

0.3101 0.3163 0.3736 8 = X+Y+Z

X, y, and z (X = X/S, y - Y/S, z = Z/S)

Planck 2854 "K., Ci = 14,380 Planck 2848 °K., C2 = 14.350 Computed by Checked by.

12
Table 4c.- 10° Distribution coefficients for sources Sx and Sc

For Sa For Sc

1 , (nm) —

£>aXio Eayio —

EttZio —

EcXio —

380 1 1 2 90 3 11 9 1 43

400 25 3 111 103 11 463

10 132 14 605 581 60 2672

20 377 40 1795 1708 J. 1 y 8122

30 682 83 3368 3011 370 14865

40 968 156 4962 3969 343 20349

450 1078 260 5802 3914 945 21058

60 1005 426 5802 3168 1343 18292

70 737 698 4965 2062 13887

80 341 1076 3274 840 2675 8144

90 76 1607 1968 167 3484 4268

500 20 2424 1150 37 4398 2085

10 218 3523 650 327 5284 976

20 750 4854 387 971 501

30 1644 6086 212 1973 7302 255

40 2847 7267 104 3275 8362 119

550 4326 8099 33 4744 8882 36

60 8198 8766 6322 8941

70 8277 9002 7653

80 10201 8740 8444 7235

90 11967 8317 8874 6168

600 12748 7466 8583 5027

10 12349 6327 7756 3974

20 10809 5026 6422

30 8583 3758 4851 2124

40 5992 2496 3226 1344

650 3892 1561 2014 808 60 2306 911 1142 451

70 1277 499 OtfO 233

80 666 259 293 114

90 336 130 136 53 700 167 64 62 24

10 83 33 28 11 20 40 15 13 4 30 19 8 5 3 40 10 4 3 1 750 6 2 2 1 60 2 1 70 2 1

111,159 100,000 32,200 97,298 100,000 116,137

server. Table in refers to Source A; table 4b, to Source C. Source B is relatively little used except

in Great Britain. Table 4c gives corresponding

values for the 10° observer.

2.3. Spectrophotometric Colorimetry

The fundamental nature of the tristimulus

specification of color permits it to be used as a common denominator by means of which colorimeters involving color standards of glass, plastic, or solutions, or systems of material color standards, transparent and opaque, may be intercompared. In order to demonstrate how the CIE

standard observer and coordinate system may be

used for this purpose, four printing inks, red

purple, greenish yellow, greenish blue, and blue,

have been evaluated, and the steps are reproduced

here in detail. Figure 4 shows spectral reflectances

of these four printing inks obtained on a recording spectrophotometer. Table 5a gives the spectral reflectances read from the originals of the curves of figure 4. These reflectances apply to the

specimens measured except for small wavelength-scale and photometric-scale corrections

which have not been applied. Table 5b gives for

the greenish yellow specimen the products indicated on the form for computation of luminous

reflectance, Y, and chromaticity coordinates,

x.y,s, under standard source C ; see table 4b. The

sums of these products are the tristimulus values,

X,Y,Z. The luminous reflectance is found as y/100,000; and the chromaticity coordinates,

x,y,s, are found by dividing X,F, and Z, respectively, by the corresponding sum, X + Y + Z. Table 5c lists these results for all four printingink specimens. Figure 5 is the (a?,?/) -chromaticity

diagram on which have been plotted large dots to represent these chromaticity coordinates, x,y. Comparison of figure 5 with figure 3 shows that

the chromaticity points of the four printing-ink

specimens correspond to the hue designations red

purple, greenish yellow, and blue. This accords

well with the color designations found by visual

inspection of the specimens. Furthermore, it will

be noted that one of the blues is greener than the

other. The position of the chromaticity point for

the greener-ink color is in accord with the greener

hue of this ink. Note also that the greenish yellow is much lighter than the red purple or either

of the blues; this accords with the luminous re- flectance determinations (compare 0.74 with

0.221, 0.242, and 0.246 in table 5c).

2.4. Short-Cuts and Automatic Computation

The labor of computing X^Y^Z, or Y,x,y, cor- responding to pairs of spectrophotometric curves

to see how the colors of the corresponding speci- mens compare is considerably great. Often the

degree of metamerism exhibited by the pair is sufficiently small that the comparison can be made

directly from the curves themselves, and much

product-control work can be handled in this way.

There is still frequent need, in the establishment

of color standards and tests for conformity to those standards, to compute the tristimulus values,

X,Y,Z, by a short-cut method, the selected ordi- nate method, to reduce spectrophotometric data.

In this method the ordinates of the spectrophotometric curve are read at a series of selected

wavelengths different for each source. Instead of

multiplying by the tristimulus values of the spec- trum of the source, {Ex)\^ (E^)\, {Ezjx, the

selected ordinates are spaced proportionately

closer in the wavelength regions where the tristi- mulus values are higher, and the corresponding

readings of spectral reflectance are simply added.

Tables 6 gives [21, 49] selected ordinates for source A (incandescent lamp light) and source C

(average daylight). Table 7 gives the spectral re- flectances of the greenish yellow printing-ink

specimen read from figure 4b for the selected ordi- nates for source C together with the sums of these

spectral reflectances, both for ten ordinates and

for thirty. It will be noted that, after applying the

multiplying factors listed in table 6, the tristimulus values, for the greenish yellow print13
Figure 4. Spectral r'eflectance of printing-ink specimens: (a) red-purple; (h) greenish

yellow; (c) greenisii blue; (d) blue.

ing-ink specimen are found again to a close approximation (compare 0.630, 0.704, and 0.145 from

table 7 with 0.631, 0.704, and 0.145, respectively,

from table 5c. Ten selected ordinates sometimes give signifi- cant information (see table 7) ; thirty selected or- dinates are often snificient (as above) ; and one hundred selected ordinates are sufficient for all but a few very irregular spectral distributions

(such as produced by gaseous discharge tubes).

These wavelengths are available for many sources

in other publications [13, 21, 49]. Nickerson

[114] and De Kerf [27] have published studies of

the reliability of the selected-ordinate method of

computation.

Analog and digital techniques have been de- veloped for use with automatic computing devices

to abbreviate the labor of computation required

by spectrophotometric colorimetry. In the application of the analog technique [25] data are sent

directly from the spectrophotometer to the computer without wavelength-scale and photometricscale corrections. In one application of the digital

technique [12, 95] spectral data are punched di- rectly on cards and fed into the computer, while

in another [75] the spectral data are corrected

and then punched on cards and fed into the

computer.

2.5. Visual Tristimulus Colorimeters

Tristimulus values, may be obtained by

direct comparison of the unknown light with an

optical mixture of three primary lights in a di- vided photometric field. Since the primaries of

the CIE standard colorimetric coordinate system

are imaginery, such a tristimulus colorimeter can- not be made to read directly. It must be cali- brated by measurements of four known stimuli,

and then may yield tristimulus values, X,Y,Z, by r a transformation the reverse of that indicated in

eq. (3). Since the color matches set up in a tristi- mulus colorimeter designed to cover any substantial part of all possible colors with a single set of primaries exhibit serious metamerism, the field has to be relatively small, subtrending about 2 deg

at the observer's eye. This restriction to a small

14
Table 5a. Spectral reflectances of four i)ri)iting-ink specimens

Wave- Spectral reflectance relative to magnesium oxide

length, nm Red Cjreenisii vjreenisn Blue purple yellow blue

880 O.o 1 5 u.uy i 0 230* 390 .375 * .uoy .Xo 1 293* 400 .376 .085 .228 .354 410 .379 .079 .269 .415 420 O Q 1 .U 1 * 306 .458 430 .373 .V 1 D .ouO '505 440 .345 077 A(\'7 ,rrU 1 [568 450 .295 .086 .467 .616 460 .235 .095 .520 .639 470 .174 1 no 645

480 .120 1 d.^ .1 rrO .OOU *635 490 .083 !608 500 .066 .445 .523 .568 610 .061 .635 .483 .508 520 .UO 1 438

580 .054 79^ ,ODo '353 540 .055 7QQ 272

550 .062 .743 .220 .198 560 .071 .752 .162 .145 670 .uy 0

7ftQ . 1 OO .117 580 .220 7(19 *106 590 .440 7(17 1 09 *102

600 .597 .790 .093 .098 610 .676 .793 .088 .097 620 71 Si

. 1 iO 7QQ .103 630 . iOiJ .uy o 122

640 .756 Q AO .ouy 147

650 .768 .814 .124 .172 600 .776 .818 .136 .187 670 . I fSU . J. 10 186

680 7 CO QO/4 172 690 .783 .827 .149 ;i62 700 .788 .829 .160 .169 710 .794 .832 .177 .192 720 .799 .883 .196 .221 730 .805 .885 .218 .256 740 .809 .836 .258 .304 750 .812 .837 .298 .362 750 .815* .838* .338* .422* 770 .817* .839* .375* .484*

*Extrapolated,

angular size of field severely limits the precision

of setting compared to what is possible by direct

comparison of large specimens in daylight. Furthermore, the metamerism also prevents one nor- mal observer from getting the same reading as another except by accident. If a reasonable approximation to the standard values of X,]r, and

Z is to be assured, either the readings of a group

of five or ten observers must be averaged, or a

color standard yielding a spectral composition

similar to that of the unknown specimen must be

used. Because of industrial interest in large-field

color matching Stiles and Burch [146] and Speranskaya [144] determined the color-matching

functions for 10°-field viewing. In this determination either the observers were instructed to ignore

the Maxwell spot [73, 98, 103, 104, 156] or it was

masked. The color-matching functions thus found

are significantly different from the 2°-field functions of the 1931 CIE Standard Observer. The

difference is chiefly that expected from the re- moval of an intervening yellow filter, the macular pigment, from the field of view.

It will be seen that tristimulus colorimeters give

Table 5b. Computation of tristimulus values, X ,Y, Z, and

cJironiaticity coordinates, x, y, for the greenish-yellow

printing-ink specimen under source C

(The computation form given as table 4b has been used.)

Wave- Reflect- —

,

—

,

length, ance pEcX pEcy pEcZ nm (p)

380 0.091 0 2 390 .089 2 8 400 .085 7 0 34 410 .079 26 1 124 420 .077 95 3 458

430 .076 228 9 1112

440 .077 306 20 1535

450 .086 337 38 1775 460 .095 319 66 1883 470 .108 245 114 1617

480 .145 161 235 1372 490 .250 91 590 1319 500 445 23 1513 1274

510 .635 57 3069 965

520 .708 408 4575 504

530 .725 1104 5752 281 540 .733 2041 6706 143 550 743 8182 7305 64 560 !752 4422 7400 29 570 .768 5623 7025 15 580 .782 6582 6250 13 590 .787 7070 5215 8 600 790 7070 4200 6 610 1793 6602 8312 2 620 .798 5642 2516 2 630 .803 4263 1759 0 640 .809 2988 1167 650 814 1912 721 660 ;ri8 1113 412

670 .822 582 213

680 .S24 304 110 690 .827 141 51 700 .829 68 24 710 .832 32 12 720 .833 16 5 730 .835 7 3 740 .836 3 2 750 .837 2 1 760 .838 1 1 770 .839 1 0

Tristimulus

values X,Y,Z 63,076 70,395 14,495 Chromaticity

coordinates- x,y,z 0.4263 0.4758 0.0980

Table 5c. Tristimulus values, X, Y, Z, under source C, luminous reflectance relative to magnesium oxide, Y/Y„,

and chromaticity coordinates, x, y, computed from the spectral reflectance of four printing-ink specimens as in table 5b

Hue

designation of specimen

Tristimulus

values, arbitrary

units

Luminous

reflect- ance, Y/Yo

Chromaticity

coordinates

X V X y Z

Red purple- Greenish

yellow Greenish

blue

39788

63076

19003 21948

22124

70395

24245 24633

80570

14495 54529

69010

0.221

.704

.242 .246

0.430

,426

.194

.190

0.239

.476

.248 Blue .213

only poor information regarding the unknown

specimen. Their application to product-control

problems is negligible, but because of the ease of

calibration and simplicity of the theory they are very useful research tools. Tristimulus colorime15
y

Y

Figure 5. Points representing the colors of four printimj- ink specimens whose spectral reflectances are shown in figure 9.4- The colors of the ideal closed-cavity radiator are also shown, the temperatures of the radiators being indicated in degrees Kelvin. The smooth curve connecting these points is often called the

Plancliian locus.

ters have been built and described by Allen [3],

Donaldson [28], Guild [43], McAdam [96], Newhall [110], Stiles [145], Verbeek [154], and Wright

[157]. The Wright instrument has spectrum primaries; the other four have primaries formed by

combining a light source with glass filters. To the

Guild and Wright instruments we owe our accurate information regarding the properties of the

normal visual system which have been expressed

in terms of the standard observer.

The foregoing instruments make up the

comparison-field mixture by optical combination

of light beams from different sources so that a sum of the separate effects is obtained. A similar

optical effect is obtained if the beams are caused

to fall upon the same portion of the retina in such

rapid succession that a nonflickering spot of color

is seen. The effect is that of a time-weighted aver- age of the separate beams. A very simple and

widely used tristimulus colorimeter is obtained

by taking four disks that have been cut along a radius, interlocking them so as to expose a sector of each, and causing them to rotate on the spindle

of a motor so rapidly that neither the separate

sectors nor even flicker is perceived. Such an ar- rangement for combining colors by rotary mixture

is called a Maxwell disk. The four disks provide

the necessary three degrees of freedom in the adjustment for a match, and if the tristimulus values

of the component disks be known {Xi,Yi,Zi;

X2,Y2^2; XsjZsjZa; Xi,Yi,Z4), the tristimulus

| values of the mixture can be computed from the

fractions of the total area occupied by the respective sectors, /i,/2,/3,/4

:

X = fiXi + /2X2 + fsXs + fiXi

Y = hYy + /2r2 + fzY, + UY, (4) I Z — fiZi + 72^2 + fsZs + f^Zi.

If the disks are chosen anew for each kind of 1 unknown color to be measured so as to be all fairly ' similar in color to the unknown, the spectral composition of the mixture color is usually suffi- ciently nonmetameric that no restriction to the

central 2 deg of the retina is required. Further- more any two normal observers with some expe- i rience at making the adjustment can check each

other closely. The chief drawback of this simple

arrangement for product-control work in color is the time lost in adjustment of the sector disk

areas. The motor must be turned off, brought to a stop, the disks loosened and readjusted, the motor turned on and allowed to resume speed several

times to obtain a final setting of reasonably good , precision.

Nickerson has described a disk colorimeter j [105] that avoids the difficulties of the elementary ; Maxwell disk. Light reflected from the unknown

specimen fills one-half of a photometric field, and

that from a stationary sector disk fills the other.

By having the observer look at the sector disk

through a rapidly rotating glass wedge, each sec- tor is presented to view in sufficiently quick suc- cession that no flicker is produced ; and at the same

time the sectors, since they are stationary, may

be continuously adjusted until a color match is obtained. A further advantage is obtained by ex- tending the rotary scanning to the unknown speci- men. In this way the average color of a notably

j nonuniform specimen such as that made up of c coarse salt crystals may be obtained. The disk , colorimeter has been extensively used by the j United States Department of Agriculture for the r color-grading of food products and is well 1 adapted to product-control colorimetry of many

kinds. '1

2.6. Dominant Wavelength and Purity

Another way to identify a combination of lights ? to specify a color, alternate to the tristimulus 't method, is to determine the luminance (photo- n metric brightness) of one spot of light of fixed i spectral composition (such as average daylight)

and the luminance of a spot of light of continu- t ously variable spectral composition separately f identified (as by wavelength in the spectrum). In 1 this way the requisite three degrees of freedom in : adjustment to a color match are supplied. This

form of identification leads naturally into a speci- j fication in which the luminance [20] of the un- \> known is given and the chromaticity is specified i by two variables in polar coordinates. One of these

' variables is an angle, tlie other a radius, and both
Table 6. Selected Ordinates (In Nanometers) for Computing Tristimuliis Values, X, Y, Z, for Specimens Under Standard Sources A and C [20]

Source A Source C

Number

X Y Z X y Z

12»

3

444.0

516.9*

544.0

487.8

507.7* 517.3

416.4

424.9*

429.4

42iA

435.5*

443.9

465.9 489.4*

500.4

414.1 422.2*

426.3 4

5*

6

554.2 561.4*

567.1

524.1 529.8* 534.8432.9

436.0*

438.7

452.1 461.2*

474.0

508.7 515.1*

520.6

429.4 432.0*

434.3

78*9

572.0 576.3* 680.2

539.4

543.7*

547.8

441.3 443.7* 446.0531.2

544.3*

552.4

525.4 529.8* 533.9

436.5 438.6*

440.6 10

11*

12

583.9 587.2*

590.5

551.7

555.4*

559.1

448.3

450.5*

452.6

558.7 564.1* 568.9537.7 541.4*

544.9

442.5 444.4*

446.3 13

14* 15

593.5 596.5* 599.4

562.7

566.3*

569.8

454.7 456.8* 458.8573.2 577.3*

581.3

548.4

551.8*

555.1

448.2 450.1*

452.1 16

17*

18

602.3

605.2*

608.0

673.3

576.9*

580.5

460.8

462.9*

464.9

585.0 688.7*

592.4

558.5 561.9* 565.3

454.0

455.9* 457.9 1920*

21

610.9

61.9 S* 616.9

584.1 587.9*

591.8

467.0

469.2*

471.6

596.0 599.6*

603.3

568.9 572.5* 576.4459.9 462.0*

464.1 22

23* 24

620.0

623.3*

626.9

595.9 600.1*

604.7

474.1 476.8*

479.9

607.0 610.9*

615.0

580.5 584.8*

589.6

466.3 468.7*

471.4 25

26* 27

630.8 635.3*

640.5

609.7 615.2*

621.5

483.4 487.5*

492.7

619.4

624.2*

629.8

594.8

600.8* 607.7

474.3 477.7*

481.8 28

29*

30

646.9 655.9*

673.5

629.2 639.7* 659.0499.3 508.4*

526.7

636.6

645.9* 663.0

616.1 627.3* 647.4

487.2 495.2*

511.2 Multiplying

factors 30 ordinates _ 10 ordinates _ 0.03661 .10984 0.03333

.10000 0.01185.03555 0.03268.09804 0.03333

.10000 0.03938

.11812

•Values for calculation with 10 selected ordinates.

can be computed from the chromaticity coordi- nates of the fixed .spot of lifi'ht, the variable spot

of light, and the unknown, these coordinates serv- ing to locate the respective positions in a chromaticity diagram. If the fixed light is nearly achromatic, the angle often correlates well with the

hue of the color perception, and the radius fairly

well with its saturation. The most fundamental

way to specify the direction on a chromaticity diagram from the point representing the fixed light

to the point representing the unknown light is by

wavelength of the part of the spectrum required to make the match. If the unknown color can be

matched by adding some part of the spectrum to the fixed light, it is said to have a spectral color,

and the required wavelength is called dominant

Kmvelength. But if a color match is produced for

the fixed light by adding some part of the spec- trum to the unknown color, the unknown is said

to be nonspectral, and the required wavelength is called the complementary wavelength. Either

dominant wavelength or complementary wavelengtli may be obtained for the standard observer

by drawing on a chromaticity diagram a straight

line through the point representing the fixed light

and that representing the unknown color, and then

by reading the wavelength corresponding to the

point at which this line extendecl intersects the

locus of spectrum colors. If the unknown color is plotted between the fixed light and the spectrum,

the intersection gives the dominant wavelength;

but if the fixed light is represented by a point

intermediate to the unknown and the intersection

of the straight line with the spectrum locus, the

intersection indicates the complementary wavelength.

The degree of approach of the unknown color

to the spectrum color is commonly indicated by

the ratio of the amount of the spectrum color to the total amount of the two-part combination ; this

ratio is called purity, and if the amounts are specified in luminance units, the ratio is called lumi- nance (formerly colorimetric) purity. By far the

most common convention, however, is to express

the amounts in units of the excitation sum X + Y + Z; the resulting ratio is called excitation

purity and corresponds simply to distance ratios on the chromaticity diagram of a colorimetric co- ordinate system [49, 64, 133]. Formulas have been

derived by Hardy [49] and MacAdam [91] to

17
Table 7. Spectral Reflectances of Greenish Yellow Print- ing-ink Specimen (see flg. 4b) Read for the Selected Ordinates for Source G (see table 6)

N^nmhPT X r Z

1 0 076 0 102 0.078

2* .076 0.076 .240 0.240 .076 0.076

3 .079 .460 .076 4 088 615 075

5* .096 .096 '.683 .683 .075 .075

6 .117 .711 .076 T 727 720 076

8* '.739 .739 '.725 .725 .077 0.77 9 .745 .728 .077 10 750 731 078

11* !758 .758 1735 .735 ;080 .080 12 .765 .738 .082 13 772 742 084

14* !779 .779 .'745 .745 .086 .086 15 .783 .747 .088 16 785 750 089

17* '.787 .787 !755 .755 .091 .091 18 .788 .760 .093 19 789 765 095

20* !790 .790 '.772 .772 .'097 .097 21 .791 .778 .100 22 .792 .782 .103 23* .793 .793 .785 .785 .106 .106 24 .795 .787 .110 25 .797 .788 .119 26* .800 .800 .790 .790 .133 .133 27 .803 .792 .155

' 28 .807 .796 .210 29* .812 .812 .802 .802 .342 .342 30 .819 .812 .650

Totals Times

factor

19.298 0.630

6.430

0.630

21.136

0.704

7.032

0.703

3.677

0.145

1.163 0.137

Values for calculation with 10 selected ordinates.

convert from luminance purity to excitation purity, and the reverse. Figure 6 indicates how dominant wavelength

and excitation purity of the four printing-ink

specimens would be found from their chromaticity

coordinates auy, relative to source C taken as the

fixed light. Table 8 gives the dominant and complementary (C) wavelengths found as in figure 10

by the intersections of the straight lines with the

spectrum locus. Table 8 also gives the excitation

purities found by dividing the distance from the

fixed point (source C) to the specimen point by

the total distance from the fixed point to the

boundary (spectrum locus plus straight line con- necting its extremes). Large-scale charts for reading dominant wavelength and purity relative to source C are provided in the Hardy Handbook of

Colorimeti'y [49]

.

Apparatus for the direct measurement of dominant wavelength and luminance purity has been

designated by Nutting [125] and by Priest [132].

The degree of metamerism ordinarily obtained

with such apparatus leaves it open to the same

objections as have prevented tristimulus colorimeters with single sets of primaries from being usey

.4

.2

1

1 520 nm '

1

1

1

1

1

(f, 510 ^ 540 — once IN Ng.550 —

\^560

\570

? 500

—4 490

YELLOW /. 0 <6 \

/c? « V590 ^ 0 /«y tv( - 0 2/>"^ 0 0 7*~~^600nm

. 0 CO 0

\. <o/o "V " g 0 V^IO

EQUAL ENERGY " O ^a^ZO /y° fe0^^650

b 0 ^^-^00-780

5 PURPLE y480

\ BLUE \ 470

1 \Jr 1 1 1 1 1 .2 .4 .6 .8 X

Figure G. Dominant ivurclenyth a)id purity evaluated from the (x,y) -chromaticity diagram. The four points indicated by circles represent the colors of the

four printing-ink specimens shown in figures 9.4 and 9.5.

Table 8. Dominant ivavelengths and excitation purities of four pri7iting-ink specimens (see flg. 4)

Chromaticity

coordinates Dominant Excita- Hue designation (from table 5c) wave- tion

of specimen length, purity. nm percent y

Red purple 0.430 0.239 498. 9C* 53.7

Greenisli yellow .426 .476 573.2 74.0 Greenish blue .194 .248 483.6 49.6 Blue .190 .213 479.7 55.1

*C denotes complementary wavelength.

ful for production control. There is a further dis- advantage in the direct measurement of lumi- nance purity in that the luminance of the spectrum

component has to be determined relative to the

luminance of the mixture by separate photometry.

Since there is usually a large chromatic difference

between these two fields, simple equality-of-bright- ness settings are not reliable, and an auxiliary

flicker i^hotometers, as in Priest's apparatus [132],

must be used. This method has been found to ex- aggerate individual-observer differences; oftentimes observers will differ only slightly in the

mixtures of spectrum light and fixed light that

they find to be equivalent to an unknown color,

but they will disagree importantly in their photomerty of the components.

18
3. Colorimetry by Difference

3.1. General Principles

In the fundamental colorimetry of lights and

objects a single standard is used for each class of

specimen. Opaque surfaces are referred to the

ideal perfect ditluser, or to physically realizable

near-perfect diflusers, such as a sufficiently thick

layer of magnesium oxide deposited from the

smoke of magnesium turnings or ribbon [108] or

layers of barium sulfate [141, 6]. Transparent objects, such as gelatin films and crystal or glass

plates, are referred to the equivalent thickness of

air; transparent solutions, to the equivalent thick- ness of distilled water or solvent. Self-luminous

objects, such as fluorescent lamps, cathode-ray

tubes, and incandescent lamps, are measured rela- tive to one of the standard sources, usually source A [19]. The colors of specimens closely resembling the respective standards can be evaluated

quite precisely and accurately; those differing

radically in spectral composition, only with rela- tive uncertainty. That is, near-white specimens,

nearly clear glass plates, and incandescent lamps

nearly equivalent to source A present the simplest

colorimetric problem; highly selective absorbers

and emitters, like the rare-earth glasses and gas- eous discharge tubes, present difficult measurement

problems. In general, the greater the deviation in spectral composition between the unknown speci- men and the standard, the greater the uncertainty

of the result obtained by a visual or a photoelectric

colorimeter.

Automatic spectrophotometry has greatly ex- tended the application of both visual and photoelectric colorimetry. It has supplied a practical

way to calibrate working standards of color. If a fairly large group of specimens is at hand to be

measured, say twenty or more, all of similar spectral composition, the most satisfactory way to measure them in the present state of colorimetric

science is to evaluate one or two of them carefully

by means of the spectrophotometer to serve as working standards, then obtain the color specifications of the remainder by visual or photoelectric

determination of the difference between specimen

and standard.

3.2. Chromaticity Spacing, Perceptibility

In the interpretation of the importance of chromaticity differences based upon separation of the

points representing the two chromaticities in the

(a?,2/) -diagram a warning is necessary. This diagram is considerably expanded in the green portion relative to the other portions, much as the

Mercator projection of the earth's surface is ex- panded near the poles. Thus, two points separated

by a given distance in the green portion of this

diagram correspond to chromaticities that are

y

A .6 .8 X

Figure 7. Chromaticity spaciiu/ in tfie (x,y) -diagram. Between any center and any point on the corresponding ellipse there are approximately 100 just noticeable chromaticity steps

[67].

much harder to distinguish under ordinary viewing conditions than two chromaticities separated

by the same amount in other portions of the diagram. Furthermore, the bluish purple portion of

the diagram is correspondingly compressed. The

system of ellipses shown on figure 7 serves to indi- cate approximately the metric properties of the

(i3?,y) -diagram. Under moderately good observing

conditions, the distances from the central point of

each ellipse to any point on its boundary corre- spond approximately to one hundred times the

chromaticity difference just perceptible with cer- tainty. These ellipses were drawn from a review

of the literature in 1936 [66, 67], and subsequent

extensive work published by Wright [160, 161]

and by MacAdam [92, 94] has corroborated the

essential correctness of the indicated chromaticity

spacing. Figure 7 not only indicates the extent to which the green portion of the diagram is ex- panded, and the bluish purple compressed, but

also indicates that, in general, the chromatic importance of a distance on the -diagram is a function both of the position of the central point

and the direction of the deviation from it. When sets of primaries other than those of the

CIE standard observer system are expressed by

transformations of the form of eq (3) , the chromaticity spacings in the resulting Maxwell triangle

may be made to vary widely. There have been

several attempts to select primary sets that yield

uniform chromaticness scales in which the chromaticity spacing corresponds to perceptibility [14, 55,

66, 90, 144] . The transformation equations for the

chromaticity coordinates, r,g, of the uniform19
65( 700'

Figure 8. Uniform-chromaticity-scale triangle according

toJndd [66]. The length of the straight line connecting the points representing any two chromaticities on this triangle is approximately proportional to the perceptibility of the chromaticity difference.

chromaticnss-scale (UCS) System [66], from the

CIE chromaticity coordinates, x,y, are

2.7760a; + 2.1543y - 0.1129 ^ ~ -1.0000a; + 6.3553V + 1.5405

(5) ^ -2.9446a; + 5.0323y + 0.8283 ^ ~ -1.0000* + 6.3553y + 1.5405

The triangle resulting from this transformation is shown in figure 8. In 1960 the CIE adopted a provisional recom- mendation that the transformation suggested by

MacAdam in 1937 [89] be used whenever a spacing

perceptually more uniform than that of the {x,y)-

diagram is desired. This transformation is: _ 4a; ^ ~ 12y - 2a; + 3 (6) ^ 6y

12y - 2a; + 3

Although the transformation coefficients are simple, the (wj'y) -diagram (fig. 9) has a spacing that

closely resembles the UCS diagram.

3.3. Visual Devices

a. Martens Photometer

One of the most useful visual devices for determining relative luminance is the Martens photometer. Figure 10 shows the Martens photometer

combined with a diffuse illuminator to form the

Priest-Lange reflectometer [134]. This reflecto- meter is intended for the measurement of luminous

reflectance of opaque specimens relative to reflect- ing standards of similar spectral reflectance. The

Priest-Lange instrument is also adaptable to the

measurement of luminous transmittance of trans- parent plates relative to transmitting standards

similar in spectral transmittance to the unknown.

Finally, the Martens photometer, removed from i the mounting, may be used for the determination

of the luminance of an unknown self-luminous sur- ; face relative to a spectrally similar standard of

known luminance. The superior usefulness of the

Martens photometer arises from the convenience of i the adjustment for equality of brightness between

the two halves of the photometer field and from the

fact that the dividing line between the half-fields '

is exceptionally narrow so that it is often invisible

when a brightness match has been set.

b. Chromaticity-Difference Colorimeter

The determination of chromaticity coordinates,

a;,2/, by comparison of the unknown specimen with

a working standard of similar spectral reflectance can be carried out visually with high precision by means of a colorimeter described by Judd [64].

The adjustment of the chromaticity of the comparison field to match the standard field is by two

double wedges, one of greenish and the other of

yellowish glass. Since the light from the comparison field must pass through both the yellow and

the green w^edge, some of the radiant energy being

subtracted by each, it is sometimes called a subtractive colorimeter; see figure 11 which gives a schematic diagram. The standard and comparison

fields are brought into juxtaposition by means of a Lummer-Brodhun cube having a double-ti'apezoid

pattern subtending 9 X 13° at the observer's eye. The adjustment to near equality of brightness to

facilitate detection of chromaticity differences is

j by movement of the projection lamp that illumi- nates both standard and comparison surfaces.

A substitution method is usually employed with

this colorimeter, a match first being set up between

the standard and comparison surfaces by adjust- i ment of the wedges. Then the unknown specimen is substituted for the standard, and the wedges readjusted to restore the match. The differences in

wedge settings can be calibrated in terms of differ- ences in the chromaticity coordinates, x,y, of the

CIE standard coordinate system, provided the

spectral i-eflectances of the comparison surface are known approximately, from the known spectral

transmittances of the wedges. This calibration has

been carried out for about 100 widely differing : comparison surfaces. It has been found that the

calibration is chiefly a function of the chromaticity

coordinates, x,y, of the comparison surface ; so cali- brations for comparison surfaces intermediate in

chromaticity to those already calibrated may usually be found satisfactorily by interpolation.

Because of the large patterned field of high lumi- nance and the convenience of the brightness adjustment this instrument takes full advantage of the

ability of the obsei'ver to detect small chromaticity

difference. If specimen and comparison surfaces

are similar in spectral composition, the settings for ' match may be repeated generally within 0.001 in

chromaticity coordinates, x,y. The chromaticitydifference colorimeter has the disadvantage, how20
0.1 0.2 0.3 0.4 0.5 0.6

u

FiGUEE 9. The (n,\ ) -diagram developed by MacAdam [90].

SPECIMEN

SPECIMENS

MARTENS

PHOTOMETER

FiGUBE 10. Vertical cross-section of the Priest-Lange [134]

reflectometer, showing Martens photometer, diffuse illu- minator, and specimen holder.

Horizontal cross section of the illuminator is above.

LAMP ON CARRIAGE

LENS

COMPARISON

SURFACE

FiGUEE 11. Schematic diagram of chromatieity-difference colorimeter [68].

ever, of requiring a skilled operator. Furthermore

the calibrations, carried out in accord with table

4b, are time-consuming; and the instrument is not

applicable to some highly selective samples because

nearly homogeneous energy is too little changed

in spectral composition by passage through the

wedges. This colorimeter has been used in setting

up a color standard for ruby mica [70] ; for inspection of working standards, transparent and opaque,

for conformity to a master standard ; and for general colorimetry by difference, for both fluorescent

and nonfluorescent specimens [137]

.

21
3.4. Photoelectric Devices

If three photocells could be adjusted, as by glass

filters, so that their responses were proportional

throughout the visible spectrum to some linear

combination [as in eq. (3) of the standard CIE

distribution curves (see fig. 2)], then they could

be used to test whether any two light beams have

the same color according to eq. (2) and could be

made to yield direct measurements of tristimulus

values, X,Y,Z [34, 45]. The X-function filter is the

most difficult to design because it has two lobes, one

short-wave lobe and one long-wave lobe. Two separate filters to cover a portion of one photocell have

been designed for this bilobal function by Barnes

[10] and by NimerofT and Wilson [121]. Figure 12

shows the X-function match achieved in the

Nimeroff-Wilson colorimeter. This function has

also been fitted by a filter-photocell combination to approximate the long-wave lobe to which a portion

of the Z-function is added, either electrically, as in

the Color Difference Meter of Hunter [56, 57] and

arithmetically, as in the Multipurpose refiecto- meter of Hunter [55], the Colormaster of Glasser

and Troy [38] and the Color Eye of Bentley [11].

In these instruments the transformation equations

to obtain X,Y,Z for source C may be represented

as: X = 0.80^ + 0.185

Y = 1.006^ (7)

Z = 1.185

400 450 500 550 600 650 700

X, nm

Figure 12. Sipcctral transmittance of the required (light

line) and the approximate filter (heavy line) for CIE

funetion X in the Nimeroff-Wilson colorimeter.

4. Color Specification

4.1. General Principle

Because of the convenience of material standards

of color, they are often used in commerce in prefer- ence to specification according to the more f undawhere R^G., and B are settings with red, green, and

blue filters, respectively. In some of these instruments the filter for the long-wave lobe of the Xfunction may be amber, hence settings with this

filter may be designated A. Van den Akker [152]

has discussed the incompleteness of success of these

colorimeters to duplicate the CIE standard ob- server system. Recent versions of Color Difference

meters and the Color Eye are shown in figure 13. Figure 14 shows the degree of success achieved

by the filters designed by Hunter [55] to duplicate

the CIE standard observer and simultaneously to adjust the spectral distribution of a projection

lamp to that for CIE source C. Figure 15 shows

the discrepancies that the filters of his multipurpose reflectometer introduce. These discrepancies

are roughly proportional to the distance from the

point representing the magnesium-oxide standard,

and are frequently larger than 0.02 m x ov y; that

is, more than 10 times a reasonable chromaticity

tolerance for most colorimetric work. However, for

the comparison of near-white surfaecs this degree

of duplication is sufficient. Figure 16 refers to the

small rectangle near the center of figure 15 and

indicates that the discrepancies are less than 0.001

in a? or y for comparison of near-white surfaces

with magnesium oxide. In general, a similar agreement can be expected in using this photoelectric

tristimulus colorimeter for the determination of

small chromaticity differences between nonmetameric pairs. And even for measurement of fairly

sizable nonmetameric chromaticity differences,

such as analyzed spectrophotometrically in the upper portion of figure 17 (BPB 8/2 versus MgO, BG

7/4 versus BG 6/4) , and small chromaticity differ- ences with a moderate metameric component, such

as shown in the lower left portion of figure l7 (Ti

versus Y2

)

, the discrepancy is in the neighborhood

of 0.002 in X or y, which is negligible for many purposes. However, for highly metameric pairs, such

as shown in the lower right portion of figure 17,

the discrepancy may be expected to be in the

neighborhood of 0.02, just as it is for large chromaticity differences.

If the limitations of photoelectric tristimulus

colorimetry are appreciated, the method is most

useful in product-control colorimetry of nonfluorescent specimens by difference from a working

standard. The precision of the method is comparable, though perhaps not quite equal, to the best

that can be done by eye. No unusual qualifications or extended special training is required by the

operator; and, compared to visual colorimetry or

to indirect colorimetry by the siDectrophotometer,

the results are obtained very rapidly.

Material Standards

mental CIE system. Material standards may be

carried from place to place, and, if the colors are

sufficiently closely spaced in the neighborhood of

the unknowii color, the nearest match may be found

by visual comparison. The color specification con22
9^1^ f r -p TM

FiGXiBE 13a. Tristimulus

Figure 13b. Tristimulus colorimeters by Hunter Associates Laboratory that give results in nearly uniform color spacing.

sists of identifying the particular member of the

system yielding a match for the color to be specified. "We deal here both with systems of material

standards of such scope that a considerable frac- tion of the colors possible in nonself-luminous objects are represented and with a few special

small groups of material standards for particular

purposes.

4.2. Transparent Media

Color systems based uiDon transparent media

take advantage of the fact that it is possible with

<'s by Gardner Laboratory

.

Figure 13c. Tristimulus colorimeters by Instrument De- velopment Laboratories that provide narroiv-band inter- ference filters to aid in evaluating the spectral character

of a color.

a fixed source to control the color of the trans- mitted light over a wide range by introducing

varying amounts of three absorbing materials.

This is done by permitting the light to pass

tlirough two or more elements of the absorbing

medium instead of through a single element, and

is called subtractive combination or mixture be- cause the action of each element is to subtract a

certain fraction of each part of the spectrum of the

incident light. The color specification consists of

the number of unit elements of each of the three

absorbing components required to produce the

color match.

a. Lovibond Glasses The Lovibond color system consists of three sets of colored glasses, red, yellow, and blue [86, 87,

88], the principal coloring materials being gold,

silver, and cobalt, respectively. The unit of the
400 500 600

Wovelength.nm 700

Figure 14. Curves shoiving the approach of the Huntertristimulus filters [55] combined toith incandescent lamp

and barrier-layer cell to the CIE spectral tristimulus values for source G.

Figure 15. Chromuticitti discrepancies expected from the use of the nuiltipiirposc rcfifctotnctcr with Hitnt<r tristi- mulus filters [551. Moasurements are all referred to magnesium oxide as the stand- ard white. O, spectroiihotoinetric tolorimetry, o, multipurpose

reHecloinster.

FtGUHE 16. Af/rccment between multipurpose rcfleetometer

[55] and the chroniaticitij-differcnce (subtractive) colori- meter for near-white porcelain-enamel specimens measured relative to magnesium oxide.

scale defined by each set is arbitrary, but tlie three

units are rehited by being adjusted so that, for

observation by dayliglit, subtractive combination

of one unit of each of the red, the yellow, and the

blue scales results in a filter perceived as neutral or achromatic. Each scale is exemplified by many

glasses, each glass being marked with the number

of unit glasses to which it is equivalent. Although

the original purpose of the Lovibond color system

was to aid in the color control of beer, these glasses are widely used today for other i)roducts such as vegetable oils, lubricating oils, and paint vehicles.

A spectrophotometric analysis of the Lovibond

color system was made by Gibson and Harris [32],

and a scale of the red glasses used in combination

with the 35-yellow glass has been constructed by

Priest and Gibson [33, 60, 155], having the same

unit as the original Lovibond red scale but embodying a closer approach to the principle that the

24
0.60

S 0.40 CI 0.20

a 0

nja.

a. J

0.80

M 0.60

0.40

1 1 1

- /'~^\BG 7/4

BG 6/40^ y

\ 1 1

-

1 1 1

/ \- — a; =-7* ^=--^ "1

1 1 1 400 500 600 700 400 500 600 700 Wavelength , n m

Figure 18. The chrotnaticities of various two-part combinations of Lovibond red, yelloiv, and blue glasses for source A as computed by Haupt and Douglas [51]. Two-part combinations of Lovibond glasses will produce many

commercially-important colors.

FiGtTRElT. Spectral reflectances of pairs of samples

exhibiting various degrees of inetumerism.

Upper two pairs differ considerably In color, but show little or no metamerism. Lower two pairs are near matches ; left, moderately metameric ; right, strongly.

Lovibond numeral should indicate the number of

unit glasses to which the single glass bearing the

numeral is equivalent. It is possible to adjust the

glasses to slightly lower numerals, if desired, by

reducing their thickness slightly [29]. Tintometer,

Ltd., makers of the Lovibond glasses, have computed from the published spectrophotometric

measurements [32] the chromaticity coordinates,

a?,y, of all the colors of the ideal Lovibond system

[140] produced by illuminating the glasses with

CIEl standard sources B and C. Computation of

these colors for source A has been done at the National Bureau of Standards [51]. Figure 18 shows

the Lovibond network for source A. In addition to calibration of red glasses on the Lovibond scale,

Tintometer Ltd. will also calibrate them on a scale, not precisely the same, set up in 1961 by agreement

with the Color Committee of the American Oil

Chemists' Society, based on the Priest-Gibson scale and known as the AOCS scale. Lovibond red

glasses defining the AOCS scale are on deposit at Tintometer Ltd and at the National Bureau of

Standards. The grading of vegetable oils in this

country is carried out by means of this scale.

b. Arny Solutions

The Arny solutions consist of groups of solutions

whose concentrations are adjusted to produce the

color match. The required concentrations are the

specifications of the color. The most used group is a triad consisting of half-normal aqueous solutions

of cobalt chloride (red), ferric chloride (yellow),

and copper sulfate (blue) in 1 percent hydrochloric acid. This group produces all colors except

deep blue and deep red; it is supplemented by a triad of ammoniated aqueous solutions of potassium permanganate and potassium dichromate [8,

9]. Mellon and Martin [101] have reported the

spectral transmittances for a number of solutions

for colorimetric standards, including the Arny

solutions at three or four concentrations, each for

the spectral range 440 to 700 nm. By extrapolation

of these data it is possible to find approximately

the tristimulus values and chromaticity coordinates, i3?,y, on the standard coordinate system

adopted in 1931 for these solutions, just as was

done by Mellon [102] for the coordinate system

used in America before the international agree- ment. In this way it is possible to transform color

specifications from the standard system into the

required concentrations of the Arny solutions, and

the reverse. The Arny solutions are used in the 11th

edition of the United States Pharmacopoeia as standards for the color of cod-liver oil and in

carbonization tests with sulfuric acid for 28 oi*- ganic compounds.

4.3. Pigmented or Dyed Surfaces

If the material standards are pigmented or dyed

surfaces, no automatically convenient notation,

such as suggested by additive combination of lights or subtractive combination of absorbing elements,

is available. Any systematic aspect of the color

specification must be derived from the method of

identifying the various members of the set of

colored surfaces serving as standards.

a. Color Dictionaries From a color dictionary are obtained definitions

of color names in terms of material standards. The

primary aim is therefore to provide an array of

25
named surface colors adequate for the purpose, and

any arrangement or organization of tlie colors serves only the secondary purpose of assisting the

user to find the one which most nearly matches.

The Maerz and Paul Dictionary of Color [97] is the foremost authority on color names. It contains

about 7000 different samples of color printed on

semiglossy paper, and there are listed about 4000

color names Avliich are keyed to one or another of

the color samples. Tliese names are drawn from

usage in many fields : paint, textile, ceramic, scientific, technical, and artistic. The samples are also identified by plate, column, and row, and because

of their large number and fairly uniform color dis- tribution it is usually possible to find among them

a sample approaching what is called a "commercial

color match" for any giv^en uniform opaque surface. On this account the Maerz and Paul Dictionary finds a considerable application as a collec- tion of color standards quite separate from its primary function of defining color names. There are

noticeable color differences between corresponding

samples in different copies of the Dictionary, but

the differences have been held to a reasonably

small amount by discarding the less satisfactory

printed sheets. The accepted authority for color names in the

textile and allied industries is the Color Association of the United States. This association has

issued nine editions of a standard color card since

1915, the current edition [149] containing 216 color

samples of pure dye silk. Furthermore, the association issues to its members several seasonal cards

each year. All colors of these standard and seasonal

cards are identified by name and cable number.

The standard colors have been measured by spec- trophotometric and colorimetric procedures, and

luminous reflectance, Y/Yo, relative to magnesium

oxide, and chromaticity coordinates, a?,y, for source C have been published [137]

.

A color dictionary much used for the specification of the colors of flowers, insects, and birds was

prepared in 1921 by Eidgway [139] . This outstanding pioneer work contains about 1000 named color

samples of paper painted by hand. Each chart

shows columns of colors of the same dominant

wavelength progressing from each chromatic color

at the middle of the column toward white and the

top, and toward black at the bottom ; and there are

five series of such columns, each one encompassing

the entire hue circuit, but at different purities.

Many of the names were coined at the time of publication to fill in gaps in popular color nomenclature and so have not much descriptive value. Each

sample is arbitrarily identified by column, row, and series, however. In addition, there is an alphabetical list of the color names giving this identifi- cation.

b. Ostwald System

The notation of the Ostwald system is based on

the properties of idealized pigment surfaces having spectral reflectance constant at a certain value

between two complementary wavelengths and re- flectance constant at a certain other value at other

parts of the spectrum [30, 128]. The full colors are those that have the low values of spectral reflec- tance equal to zero and the high ones equal to 100

percent. The difference between these two reflec- tances for other idealized pigment surfaces is the

full color content, the value of the low reflectance

(

is the white content, and the difference between the ' high reflectance and 100 percent is the black content. The complete Ostwald notation consists of a number and two letters. The number indicates

dominant (or complementary) wavelength on an

arbitrary but approximately uniform perceptual

scale, and is called Ostwald hue number. The first letter indicates white content, a being a white content of 89.13 percent, which is as near to 100 per- cent as is practicable for usual pigment-vehicle

combinations, and other letters in alphabetical se- quence indicating decreasing white content on a logarithmic scale. The second letter indicates black

content, a being a black content of 10.87 percent,

which is as near to zero as is practicable, and other

letters in alphabetical sequence indicating increasJ ing black content on a logarithmic scale. The i:

' logarithmic scales were thought by Ostwald to in- sure uniform color scales, but this is true only to ' P a rough approximation. Since the percent white

content, black content, and full-color content must ' " necessarily add up to 100, no explicit indilation of r the latter is required.

The Ostwald ideas have been a considerable aid

in thinking about color relationships on the part of i J those who duplicate colors by mixtures of chromatic pigments with white and black pigments, I and they have served as a guide in the selection of ! combinations of such colors to produce pleasing *

effects. However, the use of these idealized pigment • ^ surfaces as a basis for a system of colorimetry has \ " been hampered by the fact that actual pigment sur-

[; ^jj faces approximate them rather poorly, and by the ' fact that not all actual pigment surfaces can be , color matched by one of these ideal surfaces. Still, ^ ") color charts made up more or less in accord with " the Ostwald principles have been widely used for ' color standards and for the selection of harmoniz- .' * ing colors [63, 127, 129, 148]. Of these, the Jacob- ! J son Color Hannony Manual [63] is pre-eminent r not only because of its technical excellence, but |

' also because Foss [30] has given a clear statement | of which of the somewhat contradictory Ostwald : principles w^ere followed in its construction, and TM Granville and Jacobson [47] have made a spectrophotometric study of the color chips and have ,, ^! published luminous reflectance, F/I^o, relative to ^ TM magnesium oxide, and chromaticity coordinates, . , for every chip. These chips are therefore valuable for use in colorimetry by difference from a i working standard (see sec. 3) , and the fact that the j chip is a member of an orderly arrangement of ; colors facilitates the selection of a working stand- j l ard for any particular purpose. I (l

r
While Colorless

Hue

BIccK Black

Figure 19. Dimensions of the surface-color solid:

(a) opaque surfaces (b) transparent volumes.

c. Munsell System

'.. The basis of the Munsell system is description of

ji colors perceived to belong to surfaces in terms of

|! hue, lightness, and saturation. Each such tridimen1/ sional description can be represented by a point

\l plotted in a space diagram known as the surface1 color solid, as shown in figure 19. In the surfacecolor solid the central axis represents the grays

' extending from black at the bottom to white at the

top. Lightness of a chromatic (nongray) color determines the gray to which it is equivalent on this

J scale. Lightness is represented in the color solid by

distance above the base plane. Hue determines

|j whether a color is perceived as red, yellow, green,

blue, purple, or some intermediate; it is repre- sented in the color solid by angle about the central

axis. Saturation indicates the degree of departure

of a surface-color perception from the gray of the same lightness; it is represented by distance from

' the central gray axis. The Munsell color system specifies a surface color

by giving for usual viewing conditions its position

I on more or less arbitrary hue, lightness, ancf saturaj tion scales having perceptually nearly uniform

I steps [107]. The Munsell term corresponding to lightness is Munsell value, that for saturation is Munsell chroma, and that for hue is Munsell hue.

Munsell value is zero for the ideal black surface

I having luminous reflectance equal to zero, and it is

:l 10 for the ideal white diffusing surface having

luminous reflectance equal to 1. Munsell chroma is

jj expressed in arbitrary units intended to be perij ceptually of the same size regardless of value and

hue. The strongest known pigment colors have

chromas of about 16; neutral grays have zero chroma as do black and white. Munsell hue is ex- pressed on a scale intended to divide the hue circuit

(red, yellow, green, blue, purple, back to red) into

100 perceptually equal steps. According to one con- vention the 100 Munsell hues are identified simply

by a number from 1 to 100, and on this scale hues

that differ by 50 are nearly complementary. The

Figure 20. Diagram of hue circle with

Munsell hue notation.

at LETTER

BY LETTER ^ ^ r NUMBER >- RP 7.5 RP 10 6 7 8 9

96MM97 98 99

97.5 '00 2.5

RP 2.5 R 5 IR2 3 4

y R R-YR

RP 7.5R 10 6 7 8 9

6 7 8 9

7.5

I 2.5R IYR2 3 4

II 12 13 14

10 12.5

FiGUKE 21. Alternate ways of expressing Munsell hue

notation.

most common convention, however, is to divide

these 100 hues into 10 groups of 10 hues each, and

identify each group by initials indicating the central member of the group, thus : red R, yellow red

YR, yellow Y, green yellow GY, green G, blue

green BG, blue B, purple blue PB, purple P, and

red purple RP. The hues in each group are identified by the nmnbers 1 to 10. Thus, the most purplish

of the red hues ( 1 on tlie scale of 100) is designated

as IR, the most yellowish as lOR, and the central

hue as 5R, or often simply as R ; see figures 20 and

21. The transition points (lOR, 10 YR, lOY, and so on) between the groups of hues are also sometimes

designated by means of the initials of the two adjacent hue groups, thus: R-YR = lOR, YR-Y =

lOYR, Y-GY = lOY, and so on; but this convention is little used. The Munsell notation is com- monly written : Hue value/chroma, this is, the hue

notation, such as 6R, then the value, such as 7, and

finally the chroma, such as 4, the latter two being

separated by a solidus : 6R 7/4. More precise designations are given in tenths of the arbitrary steps of

the scales, thus: 6.2R 7.3/4.4. The grays are indicated by the symbol N for neutral followed by the

value notation, thus: N 7/ or N 7.3/ ; the chroma

being zero for neutrals is not specifically noted.

Three representations of the Munsell system

27
have been published, the original Atlas in 1915

[105], now chiefly a collector's item, the Munaell

Book of Color in 1929 and 1942 [106], and the

Munsell Color Standards in High-Gloss Surface

in 1957. The book consists of rectangles of matfinish liandpainted paper permanently mounted on

charts in a loose-leaf binding. The high-gloss color

standards are detachably mounted. The neutrals

form a one-dimensional color scale extending from

N 1/ to N 9/. Eacli chromatic sample, of which

there are about 1000, takes its place on three color

scales: a hue scale, a value scale, and a chroma

scale; and the spacing of these scales is intended

to be perceptually uniform. The pocket edition,

adapted for determining Munsell notation of

unknown colors by visual comparison, consists of

forty constant-hue charts, so called because all the

samples on each chart have the same Munsell hue.

These samples are arranged in rows and columns,

the rows being chroma scales at constant Munsell

value, the coiumns bemg value scales at constant Munsell chroma. Comparison of an unknown color

with these two families of scales gives by interpolation the Munsell value and Munsell chroma of

the unknown. Interpolation between adjacent

constant-hue charts gives the Munsell hue. Unknowns not too far outside the range of the Munsell charts may be evaluated with some reliability

by extrapolation along the value and chroma

scales. Table 9 gives Munsell Book notations of the

four printing;-ink specimens reproduced on figure

4. The greenish yellow specimen was evaluated by

interpolation, the other three by extrapolation of

varying degrees of uncertainty, the red-purple

specimen being furthest outside the range of the

Munsell charts and hence the least certainly evaluated by visual estimate.

To facilitate the comparison of the unknown

color with those of the permanently mounted paper

rectangles on the Munsell charts, either the un- known color must be brought into juxtaposition

with the rectangle and held in nearly the same

plane, or, if the form of the unknown prevents

such juxtaposition, two masks of thin cardboard

having a rectangular opening to fit the paper

rectangles should be used. One mask should be

placed over the unknown color ; the other over one

or another of the Munsell colors in succession to obtain the interpolated Munsell Book notation.

Kelly [69, Y6] made effective use of a form of mask

with three rectangular openings particularly

adapted to comparisons involving powdered chemicals and drugs viewed through a cover glass. It is advantageous to have the color of the mask fairly

close to that of the unknown, particularly in Munsell value ; that is, if the unknown color is dark, the

mask should be of a dark color also. Use of a light

mask for a dark color prevents the observer from

making as precise a visual estimate as he can make

with a mask more nearly a color match for the

unknown.

Because of the visual uniformity of the scales,

the estimates of Munsell notation for unknown

colors within the color range of the charts have a

reliability corresponding to the use of a much

larger collection of unequally spaced color standards. On this account the pocket edition of the

Mv/nsell Book of Color is widely used as a practical

color standard for general purposes.

The standard or library edition shows the same

colors as the pocket edition, but it shows them on

constant-value charts in a polar-coordinate system,

and on constant-chi"oma charts in a rectangular

coordinate system, as well as on contant-hue charts.

This edition has full explanatory matter in the

text and is adapted particularly for teaching color. It is too bulky for convenient practical use in determining the Munsell notation of an unknown

color. The samples of the 1929 Munsell Book of Color

have been measured by means of the spectrophotometer twice independently with generally concor- dant results [39, 78]. In both these studies lumi- nous reflectance, Y/Yo-, relative to magnesium

oxide and chromaticity coordinates, cc,^/, of the

roughly 400 samples were computed for source C. Glenn and Killian [39] have also published the

dominant wavelength and purity for each of the

colors; Kelly, Gibson, and Nickerson [78] have

published specifications {Y,x,y) for three additional sources (source A, Macbeth daylight, and

limit blue sky. Nickerson and Wilson [118] have

extended them to nine sources. Furthermore, they

have published [78] a series of (x,y) chromaticity

diagrams showing the position of the Munsell

colors for each of the Munsell values from 2/

through 8/. From these diagrams it is possible to

find the chromaticity coordinates, x,y, correspondTable 9. Munsell renotations, luminous reflectances, chromatieity coordinates, Munsell Book Notations,

and ISCC-NB8 color designations of four printing-ink specimens

Hue desiRnation

of specimen

Luminous

reflectance (Y/Yo)

Chromaticity

coordinates Munsell renotation (hue value/ chroma)

Munsell book

notation (hue value/ chroma)

ISCC-NBS

color designation V

Red Purple

Greenish Yellow

Greenish Blue Blue

0.221

.704

.242

.246

0.430

.426

.194

.190

0.239

.476

.248

.213

5.5RP 5.25/I6.1 8.3Y 8.60/10.6 5.6B 5.48/8.5 0.8PB 5.5o/9.6

6.0RP 4.8/16 7.5Y 9.0/9.5 3.0B 5.4/9 lO.OB 5.4/11Vivid purplish red

Brilliant greenish yellow

Strong greenish blue Strong blue

28
.5 V / 1 / V

-/ / /

' / /V \

.4 / XT //I2 Vl6 / / j^^'^Jl^——r^/—

y //-^'A^/^ / 5R / y

.3

75B<? / /

.2

I .2 .3 .4 .5 6

X

Figure 22. Chromaticities of ideal Mtinsell colors, value 5/, shown on the {x.y ) -diar/rain. This chart serves to define JIunsell renotation hue and chroma

for colors having Y/Yo = 0.198. (Prepared by Color Measure- ment Laboratory, War Food Administration, U.S.D.A.)

ing to any Munsell Book notation ; and the reverse transformation is also possible. The samples of the

1942 supplement to the Munsell Book of Color to- gether with many special Munsell standards have

been measured spectrophotometrically by Granville, Nickerson, and Foss [46] and by Nickerson,

Tomaszewski, and Boyd [119]. These Munsell

i standards, together with those of the 1929 Munsell

Book of Color, number nearly 1500 and comprise

the largest systematic set of color standards of

known luminous reflectance and chromaticity co- ordinates ever made. These standards are commercially available separately in disk form and on

large sheets, and they make practical the general

colorimetry of opaque specimens not only by disk

mixture, but also by difference from a standard.

From the charts themselves, luminous reflectance

and chromaticity coordinates of an unknown color

may also be found quickly and with an accuracy

sufficient for many purposes by obtaining first the

Munsell Book notation of the unknown and then

transforming it by reference to {x,y) interpolation

charts ba.sed on the complete set of standards. Two

methods by which to make sure transformations

have been described. One, an ASTM method [7a],

makes u.se of tables and charts of corresponding

CIE chromaticity coordinates and Munsell notations. The other is an automatic-computer program

[75] by which CIE notations are transformed to Munsell notations.

The Munsell color standards may also be used,

though less conveniently, in the colorimetry of

I light-transmitting [69, 76] elements (gelatin films,

crystal and glass plates, solutions, and so on) ; and,

conversely, such elements may be given Munsell

Book notations from their CIE specifications by means of interpolation charts. Table 12, to be dis- cussed presently in another connection, shows

Munsell Book notations so derived from the lumi- nous transmittances, J', and chromaticity coordinates, of the glass standards of the ASTM

Union colorimeter. The last four book notations

given are relatively uncertain because the colors to be specified are far outside the range of the Munsell standards.

The spacing of the Munsell colors has been ex- amined in detail by a subcommittee of the Colorimetry Committee of the Optical Society of America [111] . This committee work confirmed the many

local irregularities in spacing revealed by the

spectrophotometric studies and established the

need for some more general but minor adjustments

to make the colors of the Munsell charts correlate more nearly perfectly under ordinary conditions

(adaption to daylight, gray surrounding field, and

so on) with the surface-color solid. The subcommittee found it possible from this study to recom- mend specifications {Y,x.y) on the standard coor- dinate system defining an ideal Munsell system

[112]. It has also given to every Munsell standard

a revised notation, called the Munsell renotation,

indicating exactly in what way and how much

each color chip deviates from the ideal. Furthermore, the recommended definition of the ideal

Munsell system has been extended beyond the color

ranges covered by the present Munsell charts so as to include all colors theoretically producible from

nonfluorescent materials [83] under source C. The

connection between luminous reflectance, Y, and

Munsell renotation value is given in Table 10. Note

that N 0/ corresponds to Y — 0, N 9.91/ corre- sponds to magnesium oxide (Y = 1.00), and

N 5.0/ corresponds to Y = 0.198 (quite different

from 0.50, the halfway point) . Figure 22 shows one

of the (a?,?/) -chromaticity charts (that for Munsell

renotation value equal to 5.0) defining the ideal

system. From these charts combined with the data

given in table 10 it is possible to find the ideal

Munsell notation for any color specified in terms of

luminous reflectance, Y, and chromaticity coordinates, a?,y. Table 9 shows these Munsell renotations

for the four printing-ink specimens of figure 4. Note that they agree in a general way, though not

perfectly, with the notations found by direct visual

comparison with the Munsell Book of Color. Some

of the discrepancies are ascribable to local irregularities of the color spacing of the Munsell charts,

but most of them may be laid to the uncertainty of

the visual estimates, all but that for the greenish

yellow specimen requiring extrapolation over a considerable color range.

Munsell renotations, such as these, have a unique

usefulness as color specifications. Because of their

definition in terms of the standard CIE coordinate

system they are capable of nearly the precision of

the Y,x,y form of specification, and like that form

they may be extended to apply to all object colors,

both opaque and transparent objects. For opaque

surfaces Y/Y„ is luminous reflectance relative to magnesium oxide; for transparent objects Y/Yo is luminous transmittance relative to an equivalent

thickness of air; for solutions Y/Y^ is luminous

29
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t--*i-IOOCD C0Ot-Tt<M O0500?ClO '»JHC0iHO05 cococococo cococoeoco

tOCOi-IOOtD -^i-IOitO-* <MO[-10C0

r-(005t-sO lO-jHcj-HO o:oq«)io-* ^^cococo COCOCOCOCO cificifjci COCOCOCOCO COCOCOCOCO COCOCOCOCO

THO5 00t-CO rHOOdd

COCOCOCOCO

<NOoot-in

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COCOCOCOCO

s

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CO w

CO ^ CD

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isZt irt ^ ^ ^ ^ ^ ^ ^ ^ ''J^ ^ ^ ^ Tt^ tJh tji tj< ^i

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'3 t-00050iH CacO-^CDt- 00 05,HO)CO lOtO00O5iH (N-*!O00O5 iHCOlOt-OO OM10t-05 iHCOlOOOO

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O500t-!OK5 T)<C0<NiHO 05cct-;Din O500t-COlO -^eOMlHO rooOt-CDlO T)<C0C4rHO IHCOMrHO o

00

incocjoos QOOio-*co

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t>(>t>t-t- t- 1- 1- t't>t-t>l> t-t-t>t-t- t-COCOCDCO COOCDCDCO

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*N05t-lO C0O00CD-* MOOOt-in COi-IOOOCD

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c-ic^c-icirH thihi-Jtho <^'Z^Saioi doidodod

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05 05

0500I>CDIO -^COCItHO OSOOt-CDin

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c-i M 1-i 1-i T-H ddodd OOOOO 000005

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0505050505 0505050505 0505050505 0505050505

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si

30
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rsOOt-CDlO <}ICO?1'HO OSOOt-Olffl *MNr-l© ^CONtHO C^CCt-fOia -^CCMr-lO

1-lr-lr-lOO e * CO e-i o ooooo OXCOlOrlH o

•

»HXCDCOtH XCDtJ^C^IOS t-WC^iHOi Mthoo:x CO in Tt^ CO i-i oosxt^io CS0505XX XXXXX XI>t-t-I:-

t-l«fOiHC5 b-lOCOr-tO *coiMtho5 xt-om-* Xt-lOCOrH

C-l tH O 03 »

CDCDCDlOin

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ClCOUOt-03 O X O M

003 0 03 03 oci-*ox

r-<03t^lrtC0 03XXXX

»HC0O0iC<i MOXCDKO

xxt-c-tLOXr-l-^t- COtHOXO

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*COCOCOCO COCOCOC-IM t-COXTfO

OlOCOCl i-l C^rHrHrHrH rH t-( iH rH rH i-i rH iH T iHtHiHt-ItH rHrHrHi-lTH T-t tH i-t tH tH rif-lrHr

OTXt-COlO 03X[-;OlO T)<C0MrHO 03«t-®lO TjH0)<Ni-IO 03Xt-«0lO -^COClrHO 03Xt-C0m t)HC0iMiHO

OOOinO liiOiai-fb- COClOiHX LOMXWW OXOCOtH 03b-ir3CO(N TH003Xt* COOCOCOO COCDCCDCD t-X030i-H Ot^OCIO t-ir^CIOt- OC'IOXO COrnXCO-^ O-IOSt-WCO OXCO-^C^ OXiOCOtH a3t^»OC0W C3t*t^C0rH Oit-lC^Cl t-hOOOO 03030303X XXXt-t- t-t-CO«OCO OlOlOWirS lf5 -"SH TJICOCOCOCO tMC^M(NC^ iH fH rH i-( iH OOOOO

eococccoco cioioioici oicidoic^ mnncsm riffidc^'ci ciciiNoici (Ncioicioi c^imc^mm cic^riwci iM"?iNc4iN

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OCOM030 C0Ot-'*Cl X-*r1t--* rlX-^i-lX OXO^M (MO03XX

OrlXlOC-l C3COO)03CD

C^MiHi-Hr-( 000030!

Tti Tit Tji ^Tii'^coco cococococi cccccoojeo cccocicoco

Xt>b-t-t^

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f-X03OiH X iro c~i O

t- 1- t- 1'- o

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03 03 X 00

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31
transmittance relative to the same thickness of dis- tilled water or solvent. For any of these objects

Munsell renotation value, F, may be found from

Y/Yo in accord with table 10. Because of their close correlation with the color solid, Munsell renotations are capable of being quickly understood. Thus

the renotation 8.dY 8.6p/10.c for the greenish

yellow printing-ink specimen indicates froin the

letter Y that the specimen is a yellow, from the

value 8.60 that it is a relatively light color, being

close to the top of the value scale, 0 to 10, and from

the chroma lO.c, that it is a strong or nongrayish

color, being more than 10 Munsell chroma steps

away from the gray of the same Munsell value.

Munsell renotation hue and chroma serve more

adequately for object colors the purposes formerly

served by dominant wavelength and purity. Munsell renotation hue correlates significantly better

under ordinary conditions of daylight observation

with the hue of tlie perceived color than does dominant wavelength, and Munsell renotation chroma

is by far superior to purity in its correlation to saturation. This correlation with the color-perception solid does not, however, necessarily hold under

all observing conditions. Ordinarily this printingink specimen will be perceived to have a light,

strong greenish yellow color, but it is not so per- ceived under all conditions. If tliis specimen be

viewed next to a brilliant yellow-green area such

as is provided by a fluorescent fabric, it will be per- ceived to take on a darker color of yellowish orange

hue and moderate saturation. Thus the lightness,

hue, and saturation of the color perception depend

upon the surroundings and upon the adaptive state of the eye; and lightness, liue, and saturation are taken correctly to be psychological terms. But the

Munsell renotation refers only to the light that is reflected from the specimen and stays constant re- gardless of these changes in observing conditions.

It is therefore a psychopliysical characterization of

the specimen according to the light reflected from

it, just as are luminous reflectance, F, and chromaticity coordinates, x.y from wliich it can be derived.

Another advantage of expressing spectrophotometric results in the form of the Munsell renotation

is that the amount aiul kind of the color difference

between two specimens can be found immediately

from the two renotations in an easily understandable form. Thus, from the luie difference between

the two blue printing-ink specimens (5.fi^ compared to 0.8/*/)), the former is seen to be more

greenish (less purplish) by about five Munsell hue

steps. From the value difi'erence (5.4c compared to

5.5o) the two are seen to be of the same value to the

nearest one-tenth Munsell value step; and from

the chroma difference (S.,5 compared to 9.6) the

greenish blue is seen to be more grayisli by one Munsell chroma step. Such differences as these in terms of Munsell hue, value, and chroma may be

combined info a single index, /, of color difference

[115,117] :

/ = - {2An) + 6AF + ^AO (8).

' I 1) - 0 where G is Munsell chroma, and A^, AF, AO, are : : the differences between the two colors in Munsell S hue, value, and chroma, respectively. The differ- 1 ' ence between the colors of the two blue printing- ' I ink specimens would be found by this formula as: , 90 ^ I / = ^

(2 X 5.2) + 6 X 0.04 + 3 X 1.1 j I = 18.7 + 0.2 + 3.3 = 22.2 units. I

'

These units are of such size that color differences of

less than one unit would ordinarily not be of commercial importance ; that is, pairs of colors exhibiting such differences would be considered to be commercial matches. Note that these two blue printing- , ink specimens are far from being a commercial

match ; also note that the hue difference is far more

important than the chroma difference, and that

the value equivalence is well within commercial

toleration.

d. ISCC-NBS System ! A method devised at the request of the American

Pharmaceutical Association and the United States i Pharmacopoeial Convention for designating the i colors of drugs and chemicals is coming into use

for general purposes. The general plan of the

method was Avorked out by the Inter-Society Color

^ Council, and the details were developed at the National Bureau of Standards; the method is there- ' fore referred to as the ISCC-NBS method of designating colors [69, 79]. This method provides a 1, designation for every color perceived as belonging

to an object (either an opaque surface, or a light- ij' transmitting layer), and it has been extended to 1' the colors of self-luminous areas by Kelly [77] ; I

see figure 3. The number of color designations was

purposely made small, 267, for the sake of sim- V

plicity. Since about fen million surface colors can jl'' be distinguished by the normal human observer m" with optinunn observing conditions, the ISCC- w

NBS method falls far short of supplying a differ- p

ent designation for each distinguishable color, or 'i even for all colors (numbering perhaps half a ! million) considered to be commercially different.

The plan of the method is to divide the surfacecolor solid (see fig. 19) arbitrarily into 267 com- f partments, and assign a designation to each in as good conformity as possible to color nomenclature ^ currently used in art, science, and industry. The

compartments embracing the black-white axis are ^ given the following designations: black, dark ^ gray, medium gray, light gray, and white. The ii compartments adjacent to these are given similar 3 designations formed by adding an adjective indi- „ eating the hue, such as yellowish white, dark purplish gray, or greenish black. All other compart- i ments take designations consisting of a hue name ;^ (red, orange, yellow, green, blue, purple, pink,

32
white -ish

wh

very pale very light

o

D> ^

>*o "

o> ^

•5

(v. p.) (v. I. )

bri 1 1 iont

light light-ii (

1

-is pole

( P ) light

(brill.

)

m

gray

Gy.

)

gray Gy.)

light groyisti

(

i gy )

( 1.)

(

med -ish (

-

ish

gray i sti moderate stron g

o

o>

>. - o ^^

( n V 1

I y y. ^

1 m ^

( s ,

)

T5 dork

-is (

d-is dork grayish (d.gy.)

dork

(d.) deep

block

(Bl

-Ish

block

(

-

ish

Bl)

blocki sh

( hi ^

very dork

( V. a. J

very deep

( V. d e e p

)

Table 11. Abbreviations for the hue names used in the ISCC-NBS system

vivid

( v.)

Soturotion (Munsell Chroma)

Figure 23. The modifiers used in the color designations of the ItiCC-NBS system [75].

i brown, olive) preceded by modifiers (Ugbt, dark,

grayish, strong) indicating the liglitness and satu- ration of the perceived color. Figure 23 shows the

complete list of modifiers used in the ISCC-NBS

I system; note that vivid is a substitute for very

strong; pale, a substitute for light grayish; deep,

for dark and strong, and so on. The boundaries between the groups of colors

known by these designations have been adjusted to accord as closely as possible with common usage

and have been expres.sed in terms of Munsell Book

; notation. Figure 23 also shows these boundaries

jifor colors of purple hue.

I, The ISCC-NBS designations are not to be conisidered a substitute for numerical designation of

Icolor resulting from application of a suitable

icolorimetric method, but they do supply a certain

precision to verbal color designations that has previously been lacking. Table 11 gives the ISCCNBS designations of the four prmting-ink speci- mens shown on figure 4 and the ISCC-NBS hue

designations liave l)een used throughout the text of

this chapter. These color desigiuxtions are used in ithe current edition (N.F. VII) of the National

.Formulary, and they are being inserted into the

'United States Pharmacopoeia and into a textbook

of qualitative analysis [130]. They have been used

for describing the colors of building stone [83]

and soils [138], and for a considerable variety of

research purposes such as the description of mica

colors after heat treatment [53]. Kelly [80] has

found the Munsell notation for the centroid color

Name

Red

Keddish orange Orance

Orange yellow Yellow

Greenish yellow Yellow green

Yellowish green Green

Bluish green Greenish blue Blue

Purplish blue

Violet

Abbreviation

R

rOO

OY

Y

gYYG

yG

G

bGKBB

PBV

Name

Purple Reddish purple

Purplish red Purplish pink

Pinii Yellowish pink Brownish pink

Brownish orange Reddish brown Brown

Yellowish brown

Olive brown

Olive

Olive green

Abbreviation

P

rPpR

pPk

Pk

yPk

brPk

brO

rBr BryBr

OlBr

01OIG

of each compartment assigned an ISCC-NBS color

designation and has recommended a system of

abbreviations; see table 11. The ISCC-NBS designations are generally unsuited for use in sales promotion. The method has

been approved for color description of drugs and

chemicals by the delegates of nine national societies, and it has been recommended for general use by the United States of America Standards Institute [161a].

In 1958 the method was recommended by the

Subcommittee on the Expression of Historical

Color Usage [60], Inter-Society Color Council, for

the statistical expression of color trends. The adaptation of the method for this purpose involved the

tabulation of six distinct, but correlated, degrees or

levels of accuracy in color description [82]. In the

first level the color solid is divided into just 13

parts, ten described by a generic hue name and

three neutrals, white, gray and black. In the second

level, the color solid is divided into 29 parts, ten of the original 13 parts being further divided and

assigned intermediate hue names. In the third level each part of the color solid described by a generic

or intermediate hue name and the appropriate

modifier descriptive of its lightness and saturation; that is, in the third level of accuracy, the 267

color designations of the ISCC-NBS method are used without abridgement. Level four is illustrated

by the Munsell Book of Color [100] with its more

than 1,000 uniformly spaced color samples. Level

five is illustrated by the interpolated Munsell book

notation by which about 100,000 different colors can be reliably specified. Level six, the level in which the greatest accuracy of color identification

is possible, is illustrated by two basic methods : the

CIE method, and the interpolated Munsell renotations. This degree of accuracy is realizable only

through tJie measurement of the color with a spec- irophotometer, and is equivalent to the division of

the color solid into about 5,000,000 parts. Figure 24

gives further information about the six levels : (A)

level of fineness of color identification, (B) number

of divisions of color solid, (C) type of color designation, (D) example of color designation, and (E)

alternate color-order systems usable in that level. Through the cooperation of the Inter-Society

33
13 29 267 943 to 7056 About 100.000 About 5,000,000

Generic hue names

and neutrals

(circled)

All hue names

and neutrals ISCC-NBSAll hue names and

neutrals with modifiers (NBS C553)

Color-order system

(Collections of color standards sampling

the color solid systematically)

Visually interpolated Munsell notation from Munsell Book of Color

CIE(x,y,Y)

x= 0.395

y= 0.382

Y = 35.6

or Interpolated Munsell

renotation brown yellowish brown

light yellowish brown

(centroid No.76) lOYR 6/4

Munsell 1548 9^YR 6.4/4-5 9.6 YR 6.45/4.3

I \i pPk

pR

yPk

rO

rSr OY

©

V

PB

ySr

01 Br^

(g) gY

OIG@

TCCA (9th Std.) 216 70128

TTC-595 (1st Ed) 187 F23IO

H.C.C. 800

H407

MSP (1st Ed) 7056

I2H6

Plochere 1248

ISO 0 5-d Ridgway 1115 XXIX 13" b C.H.M. (3rd Ed) 943

3gc

yG

©

gB bG

Direction for increased fineness of color definition

Direction for stotisticol expression of color usage (roll up)

A. Level of fineness of color identification

B. Number of divisions of color solid

C. Type of color designation

D. Example of color designation

E. Alternate color-order systems usable in that level

Figure 24. Basis of Universal Color Language.

Color Council and the Tobey Color Card Co., St. Louis, Mo., the National Bureau of Standards

commenced issuing the ISCC-NBS Centroid Color

Charts [109] in February 1965. Each set consists of

18 charts displaying samples closely approximating 215 of the 267 centroid colors, and fair approximations of 36 others, making 251 color samples in

all. The samples are in the form of one-inch squares

of paper coated with glossy-finish paint affixed to a variable gray background so that each color is on

a neutral background of approximately its own

lightness. The Munsell renotation of each color

sample is supplied in the cover pages to the charts.

Duplicates of each of the 251 ISCC-NBS centroid

colors can be obtained in 9 by 12 in sheets from the

Munsell Color Company, Inc. of Baltimore, Md.

These centroid colors have been used as standards

for a wide variety of purposes. They facilitate

color description at level three, and have found

their most important application so far in the re- cording of data for establishing trends of public

acceptance of color in various lines of merchandise.

5. One-Dimensional Color Scales

5.1. General Principles

There are many tests analogous to the comparison of a solution of an unknown amount of a constituent with a series of suitably prepared standard

solutions to find the concentration of the speciinen.

In these tests the colors of the unknowns exhibit a one-dimensional change with concentration; and,

although this change may be complicated in terms

of luminous transmittance and chromaticity coor- dinates [102], a suitably spaced series of standards

over this range of colors will yield the desired concentration either by actual match with one of the

standards, or by visual interpolation among them..

Such a series of standards is said to constitute a' color scale. The ideal material from which to make'

the standards is the constituent of the unknown it- self ; in this way there is guaranteed not only a

perfect color match at some point along the scaled but also a perfectly nonmetameric match so that

variation of the illuminant or individual-observer

variations are generally unimportant. | However, if the unknown is impermanent, it may

become necessary to try to duplicate the desired)

34
colors in a more nearly permanent medium. Glass

is a frequent choice because of its generally supel{ rior permanence. Some degree of metamerism has

jthen to be tolerated because the standards have

I coloring constituents not a perfect spectral match

I for the unknown. It is also rare that a perfect job

I of color matching for any standai'd illuminant and

I observer is done. The observer is then faced with

i what is often a difficult, and sometimes an imposj sible, task. He must estimate the position of the

I unknown color on the scale, and often it will seem

! to him tliat the unknown color is not equal to any

I of the standard colors, nor intermediate between

any two of them. The concepts in terms of which

; the observer perceives these color differences then

I come into play. If he judges the color difference

I between the two luminous areas presented to him

I in terms of hue, brightness, and saturation, as is

1 fairly common, he could estimate the position of

the unknown color on the color scale as the point

j on the scale yielding the same hue, or as that yieldl ing the same brightness or the same saturation; or

; he could try to estimate the point on the scale

i yielding the closest color match; or he could disii regard brightness differences and try to estimate

\ the point on the scale yielding the closest chi-omaI ticity match. The determination becomes an esti- mate based on what criterion of equivalence is used

by the observer, and it depends upon his mental

capability in an essentially indescribable way. In

1 spite of these drawbacks, a good color scale is a

' useful time-saver, as long as it is not used in at1 tempts to provide a one-dimensional solution to

^1 what is essentially a multidimensional problem.

< Judgments of position on the color scale accord-

' I ing to equality of brightness can be expected to

' correspond to luminous transmittance. Judgments

' according to equality of hue agree well with the

' Munsell renotation hue; loci of constant hue for

• ordinary conditions of observation are indicated on

the (a?,?/) diagram by the curved lines of figure 3

ij separating the areas corresponding to the various

' hue luimes. Judgments according to equality of

saturation agree well with Munsell renotation

\ chroma (see fig. 22). If there is only secondary

\ brightness variation along the scale, judgments of

i nearest chi'omaticity match may be found approximately by taking the shortest distance on a uniform-chromaticity-scale diagram [14, 23, 55, 66,

^ 141, 142]. Figure 8 shows the uniform-chroma-

' ticity-scale triangle according to Judd [66]. If the

I chromaticity coordinates of a color are {x,y) in the

J standard CIE system, the color would have chroImaticity coordinates {r,g) in this uniform-chromaticity-scale triangle in accord with eq (5). On

this diagram the ellipses of figure 7 would be equal

3 tangent circles. If there is primary variation of

J both luminance and chromaticity, no reliable way

' of estimating the nearest color match has yet been

developed. According to the OSA Colorimetry

; Committee [24] , "The complete experimental clari-

' fication of this problem is one of the major programs yet to be undertaken in the field of colorimetric research."

5.2. Color Temperature

Perhaps the most widely used one-dimensional

color scale is that of color temperature for classifying light sources. The color temperature of a light source is the temperature at which the walls of a furnace must be maintained so that light from a small hole in it will yield the chromaticity of the

source to be specified. The color scale thus consists

of the series of lights producible by closed-cavity

radiation and is specified by temperature on the

absolute scale (degrees Kelvin). Working standards of color temperature may consist of an incandescent lamp operating at a fixed voltage combined

with a series of amber or blue glasses, like the

Lovibond blue glasses ; but by far the most common

way of producing these chromaticities over moderate ranges of color temperature is by variation of

the voltage applied to an incandescent lamp. The

locus of these chromaticities (the so-called

Planckian locus) is shown on figures Y and 8. If

the chromaticity of the light source is close to, but

not exactly equal to, any of the Planckian chromaticities, still it is possible to correlate a color temperature with the source by taking the nearest chromaticity match. Figure 25 shows this correlation [67, 81]. The isotemperature lines, which cut the Planckian locus at varying angles, are all such

as to be perpendicular to this curve on figure 26. The x,]/ and u^v chromaticity coordinates of a number of Planckian radiators {c2 = 14,380) and

standard sources in the 2° and 10° observer systems

have been computed by Nimeroff [123]. These are

listed in table 12. Since color temperature specifies

only the chromaticity of a light, there are many

spectral compositions corresponcling to the same

color temperature. Color temperature of a source is therefore an incomplete and unreliable indication

of the rendering of the colors of objects illuminated by it or of the photographic effect of the

source. To make color temperature a meaningful

and useful basis for comparing two lights it must

also be known that they are spectrally similar.

Thus, incandescent lamps may be usefully intercompared by means of color temperature, and

fluorescent lamps with about the same admixture

of mercury spectrum may also be so intercompared,

but incandescent lamps may not be intercompared

with fluorescent.

5.3. Grading Systems

When the degree of refinement and quality of

such products as oils, rosin, and sugars may be

characterized on similar one-dimensional color

scales which range from dai'k red through yellow

to perfectly colorless, the development of a complex color specification for these products is re- dundant. These perceived color changes correlate

35

1
.4000

.3500

V

o o

in

\ \ Wo 1 \ o

\\\\o\\\ \M

U

T 1 O w i

\> rvA

^

II ^

' \

IN A

?s 1

1

C2= 14380-

.1500 .2000 .2500 .3000 .3500 U

Figure 26. Isotemperature lines of correlated color temperature shown perpendicular on the Planckian locus on the u,v chromaticity diagram [81].

36
Table 12. Chronmticity coordinates of Planckian radiators (c, = H,380) and standard sources

2° Observer system 10° Observer system

Source

(°K)

1/2 «2 ^10

1000 0.6526 0.3447 0.4482 0.3547 0.6474 0.3504 0.4383 0.3555 2000 .5266 .4133 .3050 .3590 .5300 .4122 .3078 .3591 3000 .4368 .4041 .2506 .3476 .4403 .4026 .2532 .3476 4000 .3804 .3767 .2251 .3344 .3827 .3759 .2266 .3345 5000 .3450 .3516 .2115 .3231 .3464 .3515 .2119 .3234 6000 .3220 .3317 .2034 .3141 .3227 .3323 .2030 .3146 7000 .3063 .3165 .1982 .3070 .3066 .3176 .1973 .3078 8000 .2952 .3048 .1947 .3014 .2951 .3063 .1934 .3022 9000 .2869 9956 .2970 .2866 9975 1905 .2979 10000 .2806 ^2883 '.1904 .2933 .2802 !2905 .1884 .2944

oo .2399 .2342 .1800 .2636 .2394 .2366 .1786 .2648 A .4476 .4075 .2560 .3495 .4512 .4059 .2588 .3496 B .3485 .3517 .2137 .3235 .3498 .3527 .2138 .3241 C .3101 .3163 .2009 .3073 .3104 .3191 .1994 .3086 E .3333 .3333 .2105 .3158 .3333 .3333 .2105 .3158

2./:6,-.*l 5)

Figure 27. Chromaticities of some standards for grading

petroleum products (solid circles), rosin (crosses), and

honey (squares).

! The solid curve is generated by an ideal absorption-band sweep-

' ing across the spectrum from the short-wave to the long-wave band [40]. The standards have broad absorption bands of differf-nt sharpness.

witli a sliift in tlie wide absorption band toward

tlie ultraviolet as brown pi<rnients are removed on

reHnin«)f. The diffeieuces in scales among these

products correspond to differences in sharpness of

rhe absorption band, with oils possessing' the

sharpest bands, rosins next, and sugars least sharp.

Figure 27 compares, on the 1960 CIE-UCS [80]

i-hromaticity diagram, the chromaticity locus

irenerated by varying an ideal absorption band

' 40) with a few color standards used for petroleum

>olid dots), rosin (crosses), and sugar (squares)

products.

It is .sometimes convenient to state the color of a product in terms of its etjuivalent on another scale and modified by an adjective that describes the off- locus position. Thus T^. S. Rosin Standard X

fe.xtra water white), which is equivalent to ASTM

petroleum color 1.4 but slightly 011 the i)urple side,

will be designated \Ap. The letter g will be used

to designate departures toward green.

a. Petroleum Products

For more than 30 years the color of lubricating

oils and petrolatum has been graded by comparison with the colors of 12 glass standards [7]. The

petroleum product in a 33-mm layer and the standard are illuminated by artificial daylight produced

by combining an incandescent lamp of color temperature approximately 2750 °K with a filter of

Corning Daylite glass specially selected to have

spectral transmittances within specified tolerances

and further to have for standard source A lumi- nous transmittance, t^, and chromaticity coordinates, within the limits

:

0.107 to 0.160

X .314 to .330 y .337 to .341

'z .349 to .329 The specimen holder, the magazine containing the

glass color standards, the artificial daylight assembly, and a viewing diaphragm defining the direction of view are mounted together to form a portable instrument known as the Union colorimeter.

Table 13 gives the Lovibond analysis of the glass

color standards (6), the luminous transmittance,

Tf, and chromaticity cordinates, for source C

[141], the color names used by the Is^ational Petroleum Association, and the nearest chromaticity

match on the new ASTM color scale, which was

adopted in 1957. This new scale consists of a set of

glasses which define a scale that is closer to the

range of petroleum colors and with improved step

uniformity [48,72].

Table 14 gives for these glasses the luminous

transmittances and chromaticity coordinates for source C in the defining UCS (r,^) -system as well

as in the standard CIE (a?.?/) -system. Also listed are the luminous transmittance tolerances for the

glass standards and their equivalents on three

other scales. The grading of najDhthas, kerosines, and so on, has for many years been carried out by comparison

of the color of rather thick layers (up to 20 in) of

the refined oil with the colors of a set of three color

standards made of yellowish glass. The Saybolt

37
Table 13. A8TM Union colorimeter standards, Lovihond analysis, luminous transmittance, tv, chromaticity coordinates,

x,y, for CIE source C, NPA color )ia»i('S, nearest chromaticity match on the new AS'l'M color scale, and Munsell

Book notations

Union

color Number

Lov

Red

ibond anal

Yellow

5'sis

Blue

Luminous

transmittance,

Tv

Chrom

coord

X

aticity mates

y

National Petroleum

Association names

Nearest

chromaticity match on ASTM scale Munsell Book

notations

1.0 0.12 2.4 0.751 0.349 0.382 Lily white 0.55g lOY 9.6/3.8

1.5 .60 8.0 .654 .400 .446 Cream white 1.05g 8Y 8.8/8.0

2.0 2 5 26 0 .443 .472 .476 Extra pale 1.85p 3Y 7.2/12

2.5 4.6 27.0 .365 .498 .457 Extra lemon pale 2.35p lOYR 6.0/12

3.0 6.9 32.0 .287 .525 .440 Lemon pale 2.85p 7YR 5. 7/13.

E

o.\) 211 556 423 XL* A, Lltt \)L Ciilf^xi \}a.Lr; 3 5p

4.0 14.0 50.0 0.55 .096 !591 ^400 Orange pale 4'.2p 2YR 3,2/12

4.5 21.0 56.0 .55 .065 .620 .376 Pale 4.9 lYR 2.6/12

5.0 35.0 93.0 .036 .653 .347 Light red 5.8 lYR 2.2/9.5

6.0 60.0 60.0 .55 .017 .676 .323 Dark red 6.5 lYR 1.3/8.5

7.0 60.0 106.0 1.8 .0066 .684 .316 Claret red 6.75 lYR 0.6/4.5

8.0 166.0 64.0 .0020 .714 .286 7.8 lYR 0.2/1.4

Table 14. ASTM color standards for petroleum products. Chromaticity coordinates in the CIE and VC8 system with , tolerances, luminous transmittances xcith tolerances, and nearest chromaticity matches on the Union scale, the 35- yellow plus N-red Lovibond scale used for vegetable oils, and the Gardner scale used for paint vehicles

ASTM

color number

Chromaticity coordinates Luminous

transmittance

Tv

Nearest chromaticity match on :

'

CIE-system UCS-system* Union

color

scale

Lovibond 35Y + NR

scale N

Gardner

color

scale X y r g

0.5 0.3469 0.3739 0.462 0,473 0.86 -t- 0.06 0.9 p 5.3p

1.0 .3945 .4359 .489 .475 .77 .06 1.45p 8.0p

1.5 .4473 .4781 .521 .454 .67 + .06 1.8 g 1.5p 10.0

2.0 .4876 .4826 .552 .442 .55 ± .06 2.15g 3.3 11.4g

2.5 .5192 .4711 .582 .416 .44 + .04 2.65g 5.3g 12.3g

3.0 .5437 .4517 .611 .388 .31 + .04 3.1 g 7.7g 13.2

3.5 .5648 .4308 .640 .359 .22 + .04 3.5 g 10.3g 13.9

4.0 .5855 .4102 .671 .328 .152 + .022 3.85g 13.5g 14.8

4.5 .6053 .3906 .703 .296 .109 + .016 4.2 g 17.5g 15.6

5.0 .6257 .3742 .736 .264 .081 + .012 4.55 23 g 16.3

5.5 .6435 .3565 .770 .230 .058 + .010 4.85 30 17.2

6.0 .6604 .3395 .805 .195 .040 + .008 5.3 41 18.1

6.5 .6765 .3235 .841 .159 .026 + .006 6.0 57

7.0 .6913 .3086 .877 .123 .016 .004 7.2 82

7.5 .7059 .2941 .915 .085 .0081 + .0016 7.7 8.0 .7204 .2796 .956 .044 .0025 + .0006

Tolerances on chromaticity coordinates, r,g,h (b = 1-r-g), are ±0.006.

chromometer is a device for carrying out tliis comparison. It consists of an artificial daylight lamp

meeting the same requirements as given above for

the ASTM Union colorimeter, a graduated tube

for the oil specimen, a holder for the glass color

standards, and a prism eyepiece to bring into

juxtaposition the two fields to be compared. There

is an open ungraduated tube beneath the holder for

the glass standards that serves to duplicate to a degree on the standard side of tlie instrument the

effect of light multiply reflected within the speci- men tube. The glass standards consist of two whole

disks and one "one-half" dislv ; these standards

must have luminous transmittances, t„, and chromaticity coordinates, ,«,2/, for source C, as follows:

Whole di.sks One-half disks

Tv 0.8G0 to 0.86.5 0.888 to 0.891 X .342 to .350 .327 to .331 y .367 to .378 .344 to .350

.272 to .291 .319 to .330

Oils having colors closely resembling that of distilled water are graded on the Saybolt chromometer by comparison with the half disk; more

yellowish oils by one or two whole disks. The depth

of oil yielding the closest color match with the

glass disk is found by a prescribed procedure [5],

38
and the color of the oil specimen is designated by a

! jiiumber defined from the disc used and the re- Jquired depth of oiL Table 15 gives this definition

"[[5] in terms of Saybolt number, Union color, m, land ASTM color, a.

I In spite of the fact that the prism eyepiece brings

ithe fields to be compared into juxtaposition and so

i'lfacilitates the detection of differences in brightness, the change in luminous transmittance with

thickness of these refined oils is so slight that it is iijmuch less easily detected than the corresponding

jchromaticity change. The settings of the jiayboxt

jchromometer therefore probably depend essentially on nearest chromaticity match. If the oil sample is turbid, however, not even an approximate match can be obtained, and the method may

be inapplicable. In these cases a thiCKuess ot uie

'specimen yielding a chromaticity match is much

darker than the standard, and no reliable setting of

depth of sample yielding nearest match can be

';found.

• Figure 28 shows a portion of the (a?,i/) chromaIticity diagram on which have been plotted the

•rectangles corresponding to the chromaticity toler- ances for Saybolt half disks and whole disks (see

•above). It will be noted that the tolerances are

fairly wide so that variations of ±10 percent of the

chromaticity change caused by introduction of the

idisk into the daylight beam are permissible. The

ijrelation between the Union color numbers, m, and

y

.32 .34

"IFiGUEE 28. Chromaticities of the Saybolt color standards compared to that for Union No. 1 color, and to those of the yellowish wedge of the chromaticity-difference colori1 meter (arrows).

\

Table 15. Saybolt numbers and con espondi7ig Union color,

u, and ASTM color, a, defined in terms of specimen depth

and disk number

Number

of disks n

Deptli of

oil in Saybolt number

Union

color u ^STM

color a

20.00 -1-30 0.030 0.015

18.00 -1-29 .033 .017 16 00 4-28 .019

y2 14100 + 27 .042 .022

Vi 12.00 -1-26 .049 .025

1 20.00 + 25 .059 .030

1 18.00 + 24 .066 .034

1 16 00 + 23 074 .Uoo

1 14;00 + 22 !084 .043

1 12.00 + 21 .098 .051

1 10.75 + 20 .110 .056

1 9.50 + 19 .124 .064

1 8 25 + 18 143 .074

1 7^25 + 17 !l63 .084

1 6.25 + 16 .189 .097

2 10.50 + 15 .225 .116

2 9.75 + 14 .243 .125

2 9 00 + 13 262

2 8!25 + 12 !286 .147

2 7.75 + 11 .305 .157

2 7.25 + 10 .326 .167

2 6.75 + 9 .350 .180

2 6 50 -4- ft 1 Q7 .lot

2 6^25 + 7 .376 .194

2 6.00 + 6 .393 .202

2 5.75 + 5 .411 .211 2 5.50 + 4 .430 .221

2 5 25 4- Q 2 5100 + 2 .474 .243

2 4.75 + 1 .498 .256

2 4.50 0 .526 .270

2 4.25 - 1 .557 .286

2 4 00 2

2 3175 - 3 .631 .324

2 3.625 - 4 .652 .335

2 3.50 - 5 .677 .347

2 3.375 - 6 .701 .360

2 3 25 Y 797 • O 1 *

2 3^125 - 8 .758 .388

2 3.00 - 9 .789 .405 2.875 —10 .823 .422

2 2.75 -11 .860 .441 2 2.625 -12 .901 .462 2 2.50 -13 .947 .486

2 2.375 -14 .996 .511 2 2.25 -15 1.05 .539

2 2.125 -16 1.11 .671

the Saybolt color expressed in terms of number of

whole disks, n, and depth, D, in inches, _ 1.182ri u — D

"> may be derived by reference to figure 28. Two vectors,, each corresponding to 300 wedge units define

the steps from source C to Saybolt V2 disk to Saybolt whole disk, or 600 wedge units in all. The

chromaticity point representing Union Color No. 1 does not fall exactly on the extended line. The in- tersection of the dotted line and the extended line

gives the closest chromaticity match, and shows

that the chromaticity of Union Color No. 1 approximates 1.10 Saybolt disk. An oil of Union

Color No. 1 will therefore match 1.00 Saybolt disk

in a thickness of 33.0/1.10 or 30.0 mm (1.182 in)

and will match two whole disks in a thickness of

60.0mm (2.365 in).

The corresponding relation for ASTM color

number, a, is
a = 0.607n/D , based on the facts tliat the ghass that defines ASTM

color number 0.5 is closely equivalent to 1.03 Saybolt disk (as shown on fig. 28) and that thickness on the ASTM scale has been changed to 1.25 in from 33.0 mm [7].

b. Vegetable Oils In addition to the brown pigments found in

petroleum products, edible vegetable oils (corn,

cottonseed, olive, peanut, rapeseed, sesame, soy- bean) usually contain some chlorophyll. Although

metamerism exists between the oils and Lovibond

glasses [100], the oils are graded, for commercial

purposes, by the number (N) of Lovibond red

units, addecl to Lovibond 35-yellow, that are re- quired to match a 5.25-in layer of the oil. Table 16

Table 16. Lovibond 35-yelloiv plus N-red scale used to color-grade edible veyetable oils. CIE chroinuticity coor- dinates, x,y, and nearest chromaticity match on the ASTM and Gardner color scales

Chromaticity Nearest chromaticity Lovibond coordinates match on : 35Y + NR

scale y ASTM Gardner X color scale color scale

0 0.441 0.521 1.3g 9.2g

1 .455 .508 1.4g 9.9g 2 .469 .496 1.6g 10.7g 3 .482 .485 1.9 11.3 4 .494 .474 2.1p 11.7 5 .506 .464 2.4p 12. 2p

6 .517 .455 2.6p 12.6p 7 .527 .446 2.8p 12.9p 8 .537 .438 3.0p 13.2p 9 .546 .431 3.3p 13.6P 10 .555 .424 3.5p 13.8P

12 .570 .412 3.8p 14.4p 14 .583 .402 4.0p 14.9p 16 .594 .393 4.3p 15. 3p

18 .603 .385 4.5p 15.6P 20 .612 .378 4.7p 15. 9p 24 .625 .367 5.1p 16.5p 28 .636 .358 5.4p 17. Op

32 .645 .351 5.6 17.4 36 .652 .345 5.8 17.7 40 .658 .340 6.0 18.0 45 .665 .334 6.1 50 .670 .329 6.3 60 .679 .321 6.6 70 .685 .314 6.8 80 .691 .309 7.0 90 .695 .305 7.1 100 .699 .301 7.3

shows for values of N from 0 to 100 on the 353^ +

NjR scale the CIE source C chromaticity coordinates, x.y, and the nearest chromaticity match on

the ASTM scale and on the Gardner scale (the

Gardner scale is described in the next section).

Thomson [50] showed that spectral transmit- tance of oil at wavelengths 460, 550, 620, and 670

nm T460, T550, T6205 and tcto) can be expressed in an

index that gives excellent correlation with color

grading by the Lovibond red units. This index,

1.29 log (1/t4go) + 69.7 log (1/t55o)

+41.2 log (1/t62o) - 56.4 log (I/toto),

has been found useful for commercial grading of ^ vegetable oils.

c. Rosins and Paint Vehicles

Gum rosin has been graded by color for more

than 50 years. Up to 1914 the color standards were made of rosin itself in spite of the relative imper- '

' manence of its color, and from 1914 to 1936 standards composed of combinations of Lovibond

glasses were used. Bi'ice [15] has described the"'

selection of the present twelve official standards { composed of two components of colored glass com- , billed with one component of clear glass, all three | cemented together with Canada balsam. The vari- ous combinations are given letter designations denoting the grades of rosin delimited by them and ; have legal status under the Naval Stores Act. The i cemented face of the clear glass in each combina- | tion is fine ground so as to duplicate the slight J

.

turbidity characteristic of molded samples of

rosin, which commonly contain traces of fine dirt. The chromaticity spacing was adjusted by means I „ of the uniform-chromaticity-scale triangle of ]

' figure 8 so as to progress regularly from small steps \ for yellow rosins to steps of about four times thel_

initial size for reddish orange rosins. Osborn audi

Kenyon have studied rosin colors spectrophotometrically [124]. Table 17 gives the names associ-i ated with letter designations, luminous transmittance, T, and chromaticity coordinates, x,y, for

CIE source C, together with the nearest chromaticity match on the ASTM, 35^ + Nli, and Gardner color scales. Many special color scales have been set up for the

specification of paint vehicles (varnishes, linseed

oil, tung oil, etc.). A solution of nickel sulfate and

iodine [16] is used to define the darkest color per- missible for spar varnish. A color comparator having eighteen glass color standards made by Hellige,

Inc., has been used for similar purposes. The Pfund

color grader compares a variable thickness of the

unknown specimen with a variable thickness of an amber colored glass [131]. The standard is wedge

shaped, and the cell for the specimen is likewise

wedge shaped. The Parlin (or Cargille) color

standards consist of a set of thirty-five solutions.

The first ten are Hazen platinum-cobalt solutions,

[52], developed originally to measure the color ofj'

^| natural water and still used for that purpose under!

the name of APHA (American Public Health! ,j Association) standards [4]. The remainder of the! Parlin color standards are caramel solutions. Theyj

have been adopted by the ASTM ( Designationj

D365-39) for testing the color of soluble nitrocellulose base solutions. The Pratt and Lambert

color standards are varnish mixtures calibrated^

^ against the Pfund color grader. The DuPontj,

colorimeter employs six plates as color standards,r ^ together with a wedge of the same glass permitting | a continuous variation of color between the stand-i \ ards. The Gardner color standards consist of;i eighteen combinations of the red and yellow Arny||i

solutions. Gardner has determined the Arny andlij

40
[ijlTABT-K IT. U.S. st(i)idnrds for rosi». Luminous transmittance, ~t,., chromaticity coordinates, x,y, /or CIE source C, and

II nearest chromaticity match on the ASTM color scale, the 35-yeUoiv plus N-red Lovibond scale, and the Gardner color

scale

Designation Luminous

transmittance Chromaticity coordinates Nearest chromaticity match on :

Letter Name X y ASTM

color

scale

35Y + NR

scale N

Gardner

color

scale XWW

WG

N

M

E.xtra water white Water white Window glass Nancy

Mary

0.609

.531 466

.396

.322

0.4339

.4579

.4785

.5001 .5212

0.4663

.4732

.4741 .4704

.4619

1.4p 1.7p

1.9p

2,2p

2.6p

1.2p 2.2p

3.2p

4.4 6.0g

9.7p 10.8p

11. 4p

11.9

12.5 K

IH

G

P

Kate

Isaac Harry

Georce Frank

.245

.178 .1114

.0723

.0398

.5430

.5649

.5879

.6116

.6364

.4483

.4310

.4102

.3874

.3632

3.0p

3.5 4.0

4.6

5.3

7.8g 10.3k

13.8g

18.7g 27 K

13.2p

13.9

14.9

15.7 16.9 E

D

Edward

Dolly

.0131

.0021 .6640.6943 .3358

.3057

6.1 7.2 44

88

18.2

1 1; Table 18. The Gardner color standards. Chromaticity coordinates. x,y, for CIE source C, nearest chromaticity matches on

''I the .[STM color ^calc and the 35-yclloii: plus S-rcd Lovibond scale, and both nearest lightness matches and nearest

< i| chromaticity matches on the Union color scale

Chromaticity coordinates Nearest chromaticity match on : Nearest liRhtness Gardner match on Union

color 35Y + NR ASTM Union color scale from number V scale color color Gardner (30) N scale scale

1 0.313 0.324 0.06g 0.12 -1

2 .318 .333 .12g .22 1 +

3 .322 .339 .18g .33 1-1

4 .330 .349 .28 .51p 1-1 Vo 5 .342 .371 .47g .86 IVa-Z

6 .354 .387 .e 1.05P iy2-2

7 .372 .410 .8 1.25P

8 .394 .440 1.0 g 1.45

9 .419 .462 0.7p 1.2 1.6 10 .444 .475 1.4p 1.5 1.75 11 .467 .482 2.3p 1.75 1.9 K 3-3

12 .504 .471 4.6 2.3 p 2.45g 3 ¥2 -4

13 .535 .454 7.1g 2.85 3.0 g 4-4%

14 .567 .427 10.6g 3.55 3.55k 15 .590 .407 14.2g 4.1 3.9 K -5

16 .619 .380 21 g 4.8 4.45 6 17 .638 .361 28 g 5.35 4.75 7-8 18 .659 .340 40 6.0 5.25 8 +

if Lovibond specification for the nearest matches for

' all the above-mentioned sets of color standards and

I" has also oljtained tlie nearest equivalents in terms

of potassium dichromate solutions [31]. From

'I these nearest equivalents it is possible to express

|t color specifications given by any of these means in

i" terms of any other of them, as shown in Table 18

i which gives the chromaticity coordinates, x,y, of

I each of the eighteen Gardner standards for CIE

H source C, nearest chromaticity match on the LoviP bond 35Y + NR scale, the ASTM scale and the

•r Union color scale, together with the nearest lightf

ness match on the Union scale. The Gardner standi; ards define a locus of chromatic ities that agrees

P closely with that of the ASTM standards with only

I

6 of the 18 standards departing by more than 0.001

^ as indicated by the letters and

The British Paint Research Station has recom- mended [151] combinations of Lovibond glasses

for color-grading oils and varnishes. Some of the

combinations involve colorless or blue glasses to be

combined with the oil to match red and yellow

glasses, and a device facilitating the setting up of

such combinations is also recommended. The Lovibond glasses are mounted in a slide, and the two

photometric fields to be compared are brought into

juxtaposition by mirrors.

d. Sugar Products

For regulatory purposes, the classification of

sugar products (honey, maple syrup, molasses,

sugarcane) is conducted visually by comparison

with glass chromaticity standards. The source C

chromaticity coordinates, a?,y, are listed in table 19

41
Table 19. U.S. Department of Agriculture chromaticity standards for sugar products. Chromaticity coordinates, x,y, for CIE source C, and nearest chromaticity matches on theASTM, the Lovibond 35Y -f NR, and the Gardner color scales

Designation and Name

Chromaticity coordinates Nearest chromaticity match on ASTM

color

scale

Lovibond 35Y + NR

scale

Liquid sugar WW(ls), water white

W(ls), white EW(h), light amber

Extracted honey

WW(h), water white EW(h), extra white W(h), white 1 ELA(h), extra light amber 2 LA(h), light amber 3 A(h), amber

Maple sirup

LA(ms), light amber MA(ms), medium amber DA(ms), dark amber

Sugarcane molasses

No. 1 No. 2 No. 3

0.3381

.3772

.4169

0.3818

.4169

.4786 .5317

.6141

.6711

0.4947

.5567

.6041

0.5183

.6301 .6815

0.3529

.3937

.4245

0.3982

.4245

.4531 .4450

.3845

.3279

0.4509

.4352

.3943

0.4489

.3691 .3179

0.35p

.75p

1.3 p

0.8

1.3

2.1

2.9

4.7

6.3

2.35p

3.35

4.4

2.7 p

5.15

6.65

4.0p

7.3 19.5g 51

4.9p 9.4k 16.8g

6.3 25 g

64

1 Same as Sugarcane Sirup No. 1.

2 Same as Sugarcane Sirup No. 2.

3 Same as Sugarcane Sirup No. 3.

for the glasses developed by Brice [15] and

adopted by the U. S. Department of Agriculture

for this purpose. The nearest chromaticity matches

on three one-dimensional color scales (ASTM,

35Y+NR, and Gardner) are also given for each

standard.

Increasing use is being made of one-dimensional

photometric methods [86, 37, 162, 16,3] for more

precise designation of sugar colors. Deitz [26] has

developed a one-dimensional specification that is

defined as the color difference between the solvent

and the sugar solution. The color difference is evaluated in Adams chromatic-value space [2] and

is expressed in terms of a so-called NBS unit of

sugar color. Measurements of the attenuancy at two wavelengths (420 nm and 560 nm) are used

in this evaluation to a good approximation. Brice

[16] developed a similar one-dimensional specification that is based on chromaticity differences

expressed in MacAdam units [19].

6. Limitations of the Methods

6.1. Spectrophotometric Colorimetry

Spectrophotometric colorimetry, the most fundamental color-measurement technique, sufi'ers from two major sources of difficulty. First, the 1931

standard observer color-matching data were obtained for only some 17 observers [61] under 2°

angular subtense and (^uite low illumination levels. Extrapolation of the use of this system for all ob- servers and conditions may require justification.

Secondly, there is usually some lack of precision

and accuracy of spectral measurement of exitance,

transmittance, and reflectance. Spectrophotometrs

are subject to errors of wavelength and photometric scales, stray-energy and slit-width effects,

multiple reflections, and errors due to samples that

are temperature dependent, wedge-shaped, and

translucent. A compilation of these errors and

recommendation for tlieir accounting has been

published by Gibson [35]

.

Althougli the CIP^ standard observer system for

colorimetry has been generally satisfactory since

its reconunendaf ion in 1931, there have been some

difficulties with the system. Most notable is its; inability to resolve the chromaticity difference of

anafase and rutile titanium dioxide [62, 71] usually viewed with angular subtense much greater

than 2°. These difficulties have led the CIE to seek a new^ system (the 1964 CIE supplementary observer) through the Stiles and Burch [146] and

Speran.skaya [144] color-matching data of some 75 j observers for a 10° photometric field. A complete'

analysis of these data permits the establishment of

estimates of within- and between-observer variability of the system. Estimation of the variability

in the color-matching functions, r(red), g(green),

and b(blue) requires information about the

between-observer deviations, ar, a-g, at, the withinobserver deviations, lea,-, too-g, wo-f,, and the covariances, a,gj a-rb, <rbg- The covariances, o-;;, are the

products of the correlation coefficients, pu, and the

deviations, o-;, a; ; thus o-j; = paa-iaj where i and j' refer to the color-matching functions. Figure 29

shows the between-observer deviations and the cor-

;

relation coefficients determined by Nimeroff [123] ^

42
o 0

1 1 i

- ';i

/

1 rg ^ ^

/

1 1 1 500 600

Wavelength , nm

700

fl Figure 29. TJie standard between-observer deviations (up- per graph) for 10°-pcld color-matching functions by

Stiles and Biirch [1-H>] and Hperanskarja [144] and the

correlation-coefficients (lower graph) derived by Nimeroff [123].

ijfrom the Stiles-Burch and Speranskaya data. The

laverage ratio of the between-ob.server deviations,

io-j, to within-observer deviations, ,cO-,, for the three

coloi--inatcliing functions was estimated as 5.7. Judd and Kelly [74] have transformed the aver-

.age color-matching data, r.g.h, to the tristimulus

' values, x.y^, by the transformation equations

:

X = ki/- + kog + ksb

y — kiV + Jc^g + keb (9) z — kiv + kgg + koh. (The variances V{^l) and the covariances C(uv) in

. the transformed system are computed by applying

' the propagation of error theory, thus

:

1

Viu)

C'iu) + 2 +

(10)

In table 20 are listed the tristimulus values derived by Judd and Kelly. Also listed are the

lietween-observer variances and covariances in

tliese values derived by Ximeroff [123] on the assumption that the variance and covariance terms

involving the constants ki of the transformation

are zero. These data permit estimation of the

ranges of color Avithin which matches set by all observers for any one color will fall. Use of the

factor 0.175 may be useful in estimating ranges by

one observer from the ranges by many observers.

Propagation of error theory has also been applied [120] to solve the problem of finding the

limitations of spectral measurements used in evaluating chromaticity coordinates. On the assumption

that the standard observer is free from error, uncertainty ellipses for specimens illuminatecl by

CIE source C may be derived from the equation

{x-Xn)^a^y -f (x-xn) {y-y„)(T„j + (y-y„)

2 (11)

where Te depends on the level of significance, x and

y are coordinates of the ellijjse, a?„ and y„ are the

chromaticity coordinates computed from the mean

spectral data of a specimen, a^x and a^y, are the

variances, and a^y is the covariance of the chromaticity coordinates. The results of this investigation are shown in figure 30 compared with the perceptibility ellipses of MacAdam [91] . This error theory

has been applied also to the problem of estimating

the uncertainty ellipses resulting from imprecision

in measuring spectral exitance of fluorescent lamps

[122].

Errors introduced by using summation as in eq

(2) instead of integration to evaluate tristimulus

values X.Y,Z, have been treated by Nickerson

[114] and by De Kerf [27]. Integration is not possible Avith the present CIE system because the

spectral tristimulus values are specified for small

but finite intervals. De Kerf compares colorimetric

results from summation with intervals of 1, 5, 10,

and 20 nm. The conclusion reached by both investigators is that the size of the interval is determined

by the type of problem. The most spectrally selec- tive specimens require the smallest summation

intervals.

6.2. Photoelectric Tristimulus Colorimeters

Errors in measurement by photoelectric tristi- mulus colorimeters result mainly from three fail- ings [58, 153] : (1) to duplicate the geometric conditions of the real situation, (2) to illuminate the

specimen with light of applicable spectral character, and (3) to duplicate the required filter- detector spectral sensitivity for the color-matching

functions.

a. Angular Conditions

Many photoelectric colorimeters use the standard 45°0° conditions, or the equivalent 0°45° conditions. Errors expected for nonglossy si)ecimens

by .substitution of 0°-hemispherical condition for

the standard 45°0° conditions have been analyzed

by Budde and Wyszecki [18]

.

43
Table 20. Spectral tristimulus values and between-observer variances and covariances

WavelenRth nm

— — V{x) X 104 y(y)

X 104 V(2)X 104 X 104 G{xz)

X 10-4

y

X 104 11

1

\ 400 0.0191097 0.0020044 0.0860109 1.26 0.012 25.58 0.10 0.68 0.47 ! 410 r\Q A <-j £* .Uo47oo .00875D QQAQ(»f» .ooyooo 6.61 .11 132.11 .71 29.38 3.21 \ 420 OA/1 A no AO -f O A ! Q OR .zo 193.19 70 . 4 O 42.06 3.50

' 430 AO O CfT a 71 .O071 A7 154.07 32.93 5.09 ,1 440 .0d2077 J. . yo 1 ji<5 o.oO .40 97.76 18.93 3.55 ! 450 .370702 .089456 1.99480 3.53 .56 81.53 .41 16.14 1.85 i 460 1 O QO At 1 75437 10.93 l.Uv OAC AT .ID 45.40 1.36 1 470 .195618 .185190 1 31756 O TO Z. i Z 180.40 l.U JL 42.31 1.32 i 480 .080507 o er o cr o A

.ii53589 TTO 1 Of^ 7.16 C AO 60.63 1.08 18.24 .08 490 AT G-i '70 O O A 1 O O 415254 6.74 1 u.*±o 23.38 .Do 9.73 .76 500 .003816 .460777 .218502 4.14 12.87 5.93 .006 2.79 2.45 f 510 AO T /I Ct^ a f\C7 A A .oOd741 1 1 Oftdd . J. J.iiU'TT: 3.25 Q OQ 1.91 1.12 1.00 520 .117749 TCI T

T

.7d1 /u7 AC AT AA T CO .65 i.uy oo .18 530 .875211 1 zlO 1 .^o OT .z 1 .UOO .020 / 540 .o7d i i Z ACI O Q Q .yDiyoo .vlOD 1 O oo o.yu 3.01 1 o .lo .28 550 .529826 .991761 .003988 14.31 6.68 1.05 5.28 .53

I'l .70 , 560 AA 7 O 1 A AAAAAfi OQ OQ ft d7 1.00 9.60 1 AO .51 570 O fT Q £• K e .955552 43.72 y.ys .y4 14.20 1.48 .11 V 580 1.01416 QGQCIO A .oDoyo4 56.88 J. l.^O TQ . 1 O

1 C 1 9 1.84 .38 590 1.11852 T7TA AK .77740O Do.oD y.*i * .65 19.41 1.94 .81 i 600 1.12399 .658341 49.31 7.31 .38 16.98 1.48 .72 610 K O T O C O .oZ / ybo 32.43 .25 11.63 1.02 .52 " 620 O A O A F; T 15.87 O A(\ .12 6.01 .45 .22 630 a A "7 A .zoo4yo 5.75 .yz Q9 .059 2.26 .12 .059 * 640 .431567 1 T A Q O Q .i7yoZo .75 .lo .003 .30 .001 .0007 1 650 .268329 .107633 .47 .079 .0005 .19 .001 .0003^ 660 .152568 .46 .071 .0004 .18 .002 .0009 670 AQ1 '^CiC\£i. .Uo I ZdUO 1 Q .019 .0002 .049 .00041* 680 .0408508 .0159051 .043 .006 .00002 .016 .0001 .0001^ 690 .0199413 .0077488 .009 .001 .000004 .003 .00003 .0000

!

700 .00957688 .00371774 .004 .0005 .000002 .001 9 X 10-6 5 X 10- 710 .00455263 .00176847 .001 .0002 4 X 10-7 .0004 1 X 10-6 9 X 10- 720 .00217496 .00084619 .0002 .00003 4 X 10-8 7 X 10-5 3 X 10-S 5 X 10-.

FiGUKE 30. Uncertainty ellipses compared tvith MacAdam

perceptibility ellipses. Correlated uncertainty, solid line ellipses ; MacAdam perceptibility, dotted line ellipses. (All ellipses are plotted on a times-ten

scale.)

b. Source Most photoelectric colorimeters are desi^ied for one standard source, usually CIE source C. Such

colorimeters do not indicate how closely specimen

and standard matcli for other sources, which depend on the degree of metamerism. Several color

imeters now have provision for inserting an auxili

ary filter to approximate a source other than th one supplied.

c. Filter-detector combinations

Most photoelectric tristimulus colorimeters us( three filter-detector combinations and approximati

the short-Avave lobe of the a;-function by a suitabh

fraction of the z-funrtion. Errors encountered ir this type of colorimeter have been treated by Vai

den Akker [152J.

There is some degree of failure to duplicate th(

CIE color-matching functions even in colorimeters

with four filters, one for y, one for s, and two foi the two-lobed x. This failure restricts utility oJ these colorimeters to measurement of the color oJ specimens of spectral characteristics similar to thai

of the standard.

If the limitations are appreciated, methods us

ing photoelectric tristimulus colorimeters are use

ful in product-control colorimetry of nonfluores

cent specimens. The National Bureau of Standards^

has developed three sets of color standards, Non

selective standards, KB-Chromatic standards, ano S-Chromatic standards, for use with these meth

ods. These standards are now available, either in dividuaHy or in sets, from the Gardner Labora

tory. Inc., Bethesda, Md. 20014, and Huntei,

Associates Laboratory, Inc., Fairfax, Va. 22030.

44
6.3. Material Standards of Color

I Limitations of visual comparisons result from

inability to make precise interpolation between

pwo colors. Use of the Munsell color system re- —quires interpolation of hue, value, and chroma. Ex-

! ^erience shows that the best a trained observer

does in such interpolations is correct within 0.5 hue

$tep for chromas -t or more, within 0.2 value step,

and 0.4 chroma step. r Visual comparisons are limited also because of

the difficulty of choosing permanent materials that

[lave suitable spectral characteristics. The most

Ipermanent of these are vitreous enamel and glass,

i nvhich, however, are so limited in variety of spec-

'I'tral characteristics that metamerism cannot be

avoided. This limitation requires control of the

spectral character of the light source and disqualifies observers with abnormal vision. ^ Material standards of glass used in one-dimenpional color scales cause difficulty because the

luminous-transmittance match with the specimen

)jimay not match in chromaticity ; see, for example,

''i' table 17. Hence, one-dimensional scales as repre-

'j^jsented by material standards are not always an

stpidequate solution to a problem.

K Gross errors in visual judgment may result from

Jmarked dissimilarity of surrounds. For example,

t the orchid standard of the KB-chromatic set issued

tby the NBS is perceived to have a moderately

"Ijsaturated light reddish-purple color when viewed

[jjagainst a middle gray surround. When it is viewed

l,[against a brilliant magenta surround it is seen as

Jfigreenish gray. Spectrophotometric colorimetry

pees not account for the influence of surround.

%2.]

lip.] m

il8.]

[10. [11

[12. (13 il4

•[15 [16

[17

[18.

7. References

Abney, W. de W., Phil. Trans. Rov. Soc. 193A, 259

(1900) ; also Researches in Colour Vision and the Trichromatic Theory, pp. 223-247, Longmans,

Green, London, 1913. Adam.s, E. Q., .1. Opt. Soc. Am. 32, 168 (1942).

Allen, F., J. Opt. Soc. Am. 8, 3.39 ( 1924 )

.

American Public Health Association, Standard Methods for the Examination of Water and Sewage, 8th

ed.. New York, 1936. American Society for Testing and Materials, D 156-

64, Book of ASTM Standards. American Society for Testing and Materials, E 259-

66, Book of ASTM Standards. American Society for Testing and Materials, Method

D 1500-04, Book of ASTM Standards.

] American Society for Testing and Materials, Method

D 1.535, Book of ASTM Standards. Arny, H. V., Proc. 8th Internat. Congr. Applied Chem.

26,319 (1912).

Arny, H. V., and C. H. Ring, J. Franklin Inst. 180, 199 (1915), and Arny, H. V., J. Ind. Eng. Chem. 8, 309 (1916).

] Barnes, B. T., J. Opt. Soc. Am. 29, 448 (1939).

] Bentley, G. P., Electronics 24, 102 ( 1951 )

.

Billmeyer, F. W., .Jr., .1. Opt. Soc. Am. 50, 137 (1960).

] Bowditch, F. T., and M. R. Null, ibid. 28, 500 (1938).

] Breckenridge, F. C, and W. R. Schaub, ibid. 29, 370

( 1939

)

] Brice, B. A., ibid. 30, 152 (1940)

.

] Brice, B. A., ibid. 50, 49 ( 1960 )

.

] British Engineering Standard, 2.57 (1936).

Budde, W., and G. Wyszecki, Die Farbe 4, 15 (1955).

[19.] Byler, W. H., and C. C. Carroll, J. Opt. Soc. Am. 35, 258 (1945).

[20.] Committee on Colorimetry, ibid. 34, 246 (1944).

[21. J Committee on Colorimetry, ibid. 34, 648 (1944).

[22.] Committee on Colorimetry, ibid. 34, 666 (1944).

[23.] Committee on Colorimetry, ibid. 34, 677 (1944).

[24.] Committee of Colorimetry, ibid. 34, 688 (1944).

[25.] Davidson, H. R., and L. W. Imm. ibid. 39, 942 (1949).

[26.J Deitz, V. R., J. Res. Natl. Bur. Std. 57, 159 (1956).

[27.] DeKerf, J., J. Opt. Soc. Am. 48, 334 (1958)

.

[28.] Donaldson, R., Proc. Phys. Soc. 47, 1068 (1935) ; also A. Hilger, London, Bull. 250 (1936)

.

[29.] Estey, R. S., Oil & Soap 12, 135 (1935).

[30.] Foss, C, D. Nickerson, and W. C. Granville, J. Opt.

Soc. Am. 34, 361 (1944).

[31.] Gardner, H. A., Physical and Chemical Examination

of Paints, Varnishes, Lacquers, and Colors, 10th

Ed., Henry A. Gardner Laboratory, Bethesda,

Maryland, 1946.

[32.] Gibson, K. S., F. K. Harris, and I. G. Priest, BS Sci. Pap. 547, (1917).

[33.] Gibson, K. S., and G. W. Haupt, J. Res. NBS, 13, 433 (1934) RP718.

[34.] Gibson, K. S., Instruments 9, 309, 335 (1936) ; also NBS Letter Circular LC-545, 1939.

[35.] Gibson, K. S., NBS Circular 484 (Sept. 1949).

[36.] Gillett, T. R., Principles of Sugar Technology, Elsevier, N.Y., 1953.

[37.] Gillet, T. R., and W. D. Heath, Anal. Chem. 26, 1780

(1954).

[38.] Glasser, L. G., and D. J. Troy, J. Opt. Soc. Am. 42, 652 (1952).

[39.] Glenn, J. J., and J. T. Killian, ibid. 30, 609 (1940).

[40.] Godlove, I. H., ibid. 37, 778 (1947).

[41.] Goebel, D. G., H. K. Plammond III, and B. P. Caldwell, ibid. 56, 783 (1966).

[42.] Grassman, H., Ann. Physik Chem. [2], 89, 69 (1853) ; also Phil. Mag. [4], 7, 254 (1853).

[48.] Guild, J., Trans. Opt. Soc. (London) 27, 106 (1925-

1926).

[44.] Guild, J., Phil. Trans. Roy. Soc. A230, 149 (1931).

[45.1 Guild, J., J. Sci. Instr. 11, 69 (1934)

.

[46.] Granville, W. C, D. Nickerson, and C. E. Foss, J. Opt.

Soc. Am. 33, 376 (1943).

[47.] Granville, W. C, and E. Jacobson, ibid. 34, 382

(1944) . [48.] Hancock, H. M., and J. J. Watt, ASTM Bulletin, No. 211,31 (Oct. 1954).

[49.] Hardy, A. C, Handbook of Colorimetry, Technology

Press, Cambridge, Mass., 1936.

[50.] Haupt, G. W., Oil & Soap 15, 282 (1938).

[51.] Haupt, G. W., and F. L. Douglas, .7. Res. Natl. Bur.

Std., 39, 11 (1947).

[52.] Hazen, Allen, Am. Chem. J. 14, 300 (1892).

[53.] Hidnert, P., and G. Dixon, J. Res. NBS, 35, 309

(1945) RP1675.

[54.] Hunter, R. S., J. Opt. Soc. Am. 30, 536 (1940) ; also

J. Res. NBS 25, 581 (1940) RP1345.

[55.] Hunter, R. S., "Photoelectric Tristimulus Colori- meter with Three Filters, NBS Circular C429, 1942.

[56.] Hunter, R. S., J. Opt. Soc. Am. 38, 661A (1948).

[57.] Hunter, R. S., ibid. 48,985 (1958).

[58. ] Institute of Paper Chemistry, Paper Trade J., TAPPI

Section 105, 285, 293, 356, 380 (1937) ; 107, 231, 275 (1938).

[59.] International Commission on Illumination, Proceedings 15th Session, Vienna (CIE Bureau Central,

Paris, 1964) llA, 35.

[60.] Inter-Society Color Council, Interim Report of the ISCC Subcommittee on Problem 23, Expression of Historical Color Usage, Nov. 20, 1960; also Building Research Institute, Inc., Identification of Colors for Building 1962, Washington, D. C. Publi- cation No. 1001, App., pp. 49-63.

[61.] Ishak, L G. H., J. Opt. Soc. Am. 42, 442 (1952).

[62.] .Tacobsen, A. E., ibid. 38, 422 (1948).

[63.] Jacobsen, E., Color Harmony Manual, Container

Corporation of America, Chicago, 1942.

45
[64.] Jiirld, D. B, J. Res. NBS 7, 827 (1931) RP377 ; also

J. Opt. Soc. Am. 21, 729 (1931).

[65.] Judd. D. B., J. Opt. Soc. Am. 23, 359 (1933).

[66.] Judd, D. B., ibid. 25, 24 (1935) ; also J. Res., NBS

14, 41 (1935) RPY56.

[67.] Judd, D. B., J. Res. NBS, 17, 771 (1936) RP944 also

J. Opt. Soc. Am. 26, 421 (1936)

.

[68.] Judd, D. B., Textile Research 9, 253 (1939).

[69.] Judd, D. B., and K. L. Kelly, J. Res. NBS, 23, 355

(1939) RP1239

[70.] Judd, D. B., ibid. 25,245 (1945).

[71.] Judd, D. B., J. Res. NBS, 43, 227 (1949) RP2024

;

and J. Opt. Soc. Am. 39, 945 (1949) with errata in 40,52 (1959).

[72.] Judd, D. B., L. Plaza, and M. A. Belknap, J. Res.

Natl. Bur. Std., 44, 559 ( 1950)

.

[73.] Judd, D. B., Eutopic color perceptions . . .

, Coloquio

Sobre Problemas Opticos de la Vision, Madrid II, 197 (1953)

[74.] Judd, D. B., and K. L. Kelly, CIE Proceedings 14th

Session, Brussels, Belgium (1959), Report of CIE

Committee W-1.3.1, Colorimetry.

[75.] Keegan, H. J., W. C. Reinboldt, J. C. Schleter, J. P. Menard, and D. B. Judd, J. Opt. Soc. Am. 48, 863

(1958).

[76.] Kelly, K. L., Bull. National Formulary Committee

8, 359 (1940).

[77.] Kelly, K. L., J. Res. NBS, 31, 271 (1943) RP1565

;

also J. Opt. Soc. Am. 33, 027 ( 1943 )

.

[78.] Kelly, K. L., K. S. Gibson, and D. Nickerson, J. Res. NBS, 31, 55 (1943) RP1549 ; also J. Opt. Soc. Am.

33 355 (1943)

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[80.] Kelly, K. L., J. Res. NBS, 61, 427 (1958) RP2911.

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[83.] Kessler, D. W., A. Hockman, and R. B. Anderson,

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[107.] Munsell, A. H., A Color Notation, 9th Ed., Munsell

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[108.] NBS Letter Circular LC-547, March 17, 1939. j [109.] National Bureau of Standards, ISCC-NBS Centroid

:

Color Charts, Standard Sample No. 2106, obtain- able from the Othce of Standard Reference Mate-

'

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251 centroid and near-centroid colors. [110.] Newhall, S. M., Psychol. Monogr. 47, 199 (1936). i [111.] Newhall, S. M., J. Opt. Soc. Am. 30, 617 (1940).

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[137.] Reimann, G., D. B. Judd, and H. J. Keegan, J. Opt.

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[144.1 Speranskaya, N. I., Visual Problems of Colour,

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[14.5.1 Stiles, W. S., Optica Acta 2, 168 (1955).

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'

[147.1 Sziklai, G. C, J. Opt. Soc. Am. 41, 321 (1951).

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46
[149.] Textile Color Card Association of the United States,

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i [151a.] United States of America Standards Institute, Z44-1942.

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I [154.1 Verbeek, H. P. J., Physica 13, 77 (1934).

[155.] Walker, G. K., J. Res. NBS 12, 269 (1934) RP653.

1

I

1

U.S. GOVERNMENT PRINTING OFFICE: 1967 O 272-747

47

[156.] Walls, G. L., and R. W. Mathews, New Means of Studying Color Blindness and Normal Foveal

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[157.] Wright, W. D., Trans. Opt. Soc. (London) 29, 225

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[158.] Wright, W. D., ibid. 30, 141 (1928-1929).

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[160.] Wright, W. D., Proc. Phys. Soc. 53, 93 (1941).

[161.1 Wright, W. D., J. Opt. Soc. Am 33, 632 (1943).

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[163.] Zerban, F. W., J. IMartin, and C. Erb, ibid. 24, 168

(After Helmholtz.) Note that the macular pigment acts as a

yellow filter interposed between vitreous humor 0 and retina t.


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