Field Trials of Color-Mixture Functions

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Field Trials of Color-Mixture Functions*W. S. STILES AND G. WYSZECKI

Divion of Applied Physics, National Research Council, Ottawa, Canada(Received July 31, 1961)

The field trials consist essentially of comparisons between the instrumental readings of the three primariesof the Donaldson colorimeter in matches on a number of filter colors (a) as obtained by actual observersand (b) as computed from the spectral energy distributions of the stimuli provided by the filter colors on thebasis of various sets of color-mixture functions. In particular, the field trials were directed to test the pro-posed 100 standard color-mixture functions derived by Judd from the Stiles-Burch and Speranskaya data.It was found that the agreement between the chromaticities of 18 filter colors, as measured on the Donaldsoncolorimeter and as predicted from the proposed 100 standard color-mixture functions, was in some respectsunsatisfactory. Various possible causes of the discrepancies were followed up by additional measurementsand computations. However, for several filter colors, especially'the blue to blue-green and near-white colors,the discrepancies remain unexplained. It seems conceivable that Grassmann's laws of color mixture do notstrictly apply to large-field color matching, although no specific experiments were included in the presentinvestigation.

INTRODUCTION

ONE of the major tasks of the CIE Expert Com-mittee E-1.3.1 Colorimetry is to investigate the

possibility of establishing a standard observer for large-field (100) color matching which would supplement theexisting 1931 CIE standard observer for 2 color match-ing. In accordance with this task, 100 color-mixturefunctions were determined by Stiles-Burch' andSperanskaya 2 for a number of observers. Judd 3 derivedfrom these data a mean set of color-mixture functions(cmf) which consequently were proposed by the CIE as100 standard cmf. However, the final adoption of themean data was deferred until it could be shown by fieldtrials that these 10° cmf correctly predict tristimulusvalues of object colors observed in practice.

The purpose of this paper is to outline the methodused in field trials conducted at the National ResearchCouncil of Canada and to report on various additionalexperiments and theoretical considerations related tothe results of the field trials.

EXPERIMENTAL

The initial field trials consist essentially of colormatches obtained by ten observers on a Donaldsoncalorimeter in conjunction with a set of 18 test filters,and a comparison of these color matches with thosecomputed from the spectral energy distributions of thestimuli provided by the filter colors and either the pro-posed 10° standard cmf or other sets of cmf. Both theDonaldson clorimeter and the set of test filters arebriefly described as follows:

Donaldson Colorimeter.

A schematic diagram of the Donaldson clorimeteris given in Fig. 1. There are two coiled-filament lamps

*These field trials were carried out in 1958-1959, a periodduring which Dr. W. S. Stiles of the National Physical Laboratory,Teddington, Middlesex, England, was a visiting scientist with theNational Research Council of Canada.

1 W. S. Stiles and J. M. Burch, Optica Acta 6, 1 (1959).2 N. I. Speranskaya, 10° color-mixture functions supplied by the

Committee for the USSR Participation in International PowerConferences, in a letter to Dr. Judd (March, 1958).

3 D. B. Judd, Proc. CIE, Vol. A (1959), 91, Brussels.

(1), operated at voltages to give the color temperatureof CIE standard source "A". One lamp (250 w) servesas source for the six primaries (2) of the colorimeterwhich are glass filter combinations to give a blue, blue-green, green, yellow, orange, and red color, respectively.Only the red, green, and blue primaries were used in thefield trials, the other three being kept closed perman-ently. Right behind the primary filters the shutters (3)are located which the observer can control by measure-able amounts. The light passing through the primaryfilters and shutters is then imaged by a condenser (4)into an integrating sphere (5) the inside walls of whichare covered with a layer of magnesium oxide. Theadditive mixture of the primary colors appears in onehalf of the visual field (15) produced in the observer'seye (14) by a Lummer-Brodhun cube (8) and other

1 2a

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FIG. 1. Schematic diagram of Donaldson colorimeter. 1. Lightsource; 2. primary filters (mixing primaries); 2a. primary filters(desaturation primaries); 2b. test filter (also position of duplicateprimaries for calibration purposes); 3. shutters; 4. condenser;5. integrating sphere; 6. 450 prism; 7. variable sector; 8. Lummer-Brodhun cube; 9. field lens; 10. artificial pupil; 11. auxiliary lens;12. magnesium oxide surface; 13. semireflecting mirror; 14.observer's eye; 15. dimensions of visual field (contrast field).

58

VOLUME 52, NUMBER JANUARY, 196i2

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i- /�/_ 7 11T1% 1y+ 9 i

FIELD TRIALS OF COLOR-MIXTURE FUNCTIONS

suitable optics (6), (9), (11). The dimensions of thevisual field (a contrast field) are given at the right-handside of the Lummer-Brodhun cube.

The instrument source also serves to illuminateanother set of similar primary filters (2a) to providedesaturation of the test color by one or the other pri-mary color.

The sample source (500 w) illuminates a magnesium-oxide surface (12) which reflects the light through thetest filter (2b) filling out the other half of the visualfield (15) by means of suitable optics (9) (11). The eyeof the observer is placed in front of an artificial pupil(10) 3 mm in diameter. A suitable lens (11) can beswitched into the optical axis to suit the observer's eyeto give good focusing onto the dividing lines of thecontrast field. Both the test color and the mixture of theprimary colors appear in Maxwellian view.

The flux of each of the three beams of light originatingat the two sources can be controlled by a variable sector(7). In addition, the sample source can be moved for-ward and backward on a photometer bench.

TABLE I. Test filters.

Manufacturer's Manufacturer'sNo. specification No. specification

1 S-BG/2 10 C-4014/32 C-5551/3 11 C-3384/33 C-5433/2 12 C-3382/44 C-4303/5 13 C-2408/35 C-5900/6.5 14 S-RG6/3.56 C-4407/5 15 S-RG6/1.57 S-BG26/4 16 Comb. of No. 5 and No. 148 C-3965/3 17 Comb. of No. 5 and No. 159 C-4010/2.5 18 Comb. of No. 3 and No. 14

a Letters C and S indicate Corning and Schott glass, respectively. Numberfollowing stroke () gives thickness of filter in millimeters.

Test Filters

In Table I, 15 glass filters are listed which as singlecomponents and in combinations of two provide 18 testcolors when used in conjunction with the sample sourceof the Donaldson colorimeter as indicated in Fig. 1.

Each filter was carefully measured on a Cary spec-trophotometer to give spectral transmittances for therange of 380 to 760 mAt at intervals of 5 m/A at a tempera-ture of 21'C.

On the basis of the spectral transmittances of the 18test filters, the spectral energy distribution of standardsource "A", and the proposed 100 standard cmf, thechromaticity coordinates x,y for each filter color havebeen computed and are shown in Fig. 2. Figure 2 illus-trates the chromaticity diagram for the proposed 100standard observer showing the spectrum locus with afew wavelengths and the purple line. Besides the loca-tion of the 18 test colors, the locations of the primarycolors red (R), green (G), and blue (B) are also shown.The yellow primary (Y) will be referred to later on.

y

xFIG. 2. Diagram of x, y chromaticity (10° observer) showing

locations of 18 test colors and red (R), green (G), blue (B), as wellas yellow (Y) instrument primaries.

Calibration of the Donaldson Colorimeter

For the computation of predicted instrumental read-ings red= W1, green= W2, and blue= W 3 , a calibrationof the Donaldson colorimeter is required. This wasobtained by making matches in the instrument on three"duplicate primaries" whose color stimuli have relativespectral energy distributions closely in agreement withthose of the actual mixing primaries of the instrument.If the agreement in relative spectral energy distributionwere perfect, these matches would be obtainable byusing for each duplicate primary only the correspondingmixing primary, and the match would consist of a pureluminance match. In fact, matches on all the duplicateprimaries were made in this way, and for the blue andred primaries there was no detectable difference ofchromaticity. For the green primary, the very smalldifference of color observable at brightness match wasjudged to be too small to make any correction necessary.For each different set of cmf, different predicted instru-mental readings Wi are computed, and these may becompared with the actual settings made by the severalobservers. It is more informative, however, to make thecomparison not directly in terms of instrumental read-ings, but in terms of a colorimetric system very similarto the CIE system. This was made possible by trans-forming the instrumental readings, whether observed orpredicted, in exactly the same way: that is to say, if theinstrumental readings are represented as the compon-ents Wi of a vector W, this vector is multiplied by afixed matrix D. D was chosen so that for the particularcase when the predicted instrumental readings werederived from the proposed 100 standard cmf, the com-

59January 1962

60 W. S. STILES AND

ponents of the transformed vector (X= WD) weresimply predicted tristimulus values in the CIE-typesystem in which Judd has expressed the proposed stand-ard. The same transformation matrix D was used evenwhen the computation of the predicted instrumentalreadings-vector W-was based on other sets of cmf.This is quite legitimate for the comparison being made,as all that transformation D does is translate differencesbetween observed and predicted instrumental readingsinto a familiar coordinate system. The components ofX which represent tristimulus values observed or pre-dicted were finally converted into chromaticities (xy),and it is these chromaticities that will be considered inthe comparison.

400 - - : 6 00 700

X [me]J

FIG. 3.

G. WYSZECKI Vol. 52

A match between any one of the 18 test colors and thecolor of the corresponding mixture of the instrumentalprimaries will be a metameric match; that is, althoughthe colors of the two halves of the visual field have thesame tristimulus values, their respective spectral energydistributions will be different. It is evident that thehigher the "degree of metamerism"4 of the two colors,the better the chance of detecting a possible differencein the cmf between the proposed 10° standard observerand an actual observer making direct matches on thecalorimeter. It is essential for the field trials that, ineach case, a sufficient metamerism be obtained; that is,the differences in corresponding spectral energy distri-butions should be representative of practical cases. The

A [mp]

FIG. 4.

FIGS. 3-6. Eighteen pairs of relative spectral energy distributions (EA) producing metameric color matchesfor proposed 100 standard observer.

4 W. S. Stiles and G. Wyszecki (unpublished).

400 500 600 700

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4 0 0 = 0 6 0 0 7 0 0

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January 1962 FIELD TRIALS OF COLOR-MIXTURE FUNCTIONS

000

X [my']

FIG. 5.

results of any field trials are thus to be judged withrespect to the differences in spectral energy distributionspresent in a color match.

The pairs of energy distributions which would pro-

y4+

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FIG. 6.

duce metameric matches for the proposed 100 standardobserver have been computed for each of the 18 testcases and are shown in Figs. 3-6. In each of these figures,the solid line represents the energy distribution associ-

.3501-

y

.3001

5

,, f+A00 a o

0

. _ .x I , 30

x . 300FIG. 7. Distribution of chromaticities (x,y) of test colors 4 and 5 as measured by 10 observers. Color matches

of an individual observer are identified by small symbol, e.g., open square.

.300

. . * * .

. . . _. .

W. S. STILES AND G.

ated with the proper mixture of the instrument pri-maries, the dashed line the energy distribution associ-ated with the corresponding filter color. All 18 cases areconsidered to exhibit sufficient metamerism to makeeach filter color an acceptable test color.

Observations and Results

In the initial field trials, 10 observers made sixmatches (two on each of three occasions) on each of the18 test colors. Each of the 60 settings W. obtained for

.200

each test color was then transformed into chromaticitycoordinates x,y in accordance with transformationsmentioned earlier. Two examples, test colors 4 and 5,of the distribution of chromaticities in the x,y diagramare shown in Fig. 7. The color matches of an individualobserver are identified by a small symbol (e.g., opensquare) plotted at the proper place in the diagram. Thelarge solid dot represents the color match obtained bycomputation for the proposed 10° standard observer.From the figure, it may be noted that the spread ob-tained for an individual observer is usually confined to

.3501-

y

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y

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I I.200x

.300

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XFIG. 8.

FIGS. 8-12. Portions of the x,y chromaticity diagram showing predicted and observed chromaticitiesfor 18 test colors related to a 10° matching field. For point symbols, see text.

.250

62 WYSZECKI Vol. 52

I

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FIG. 9. (Caption on page 62.)X

a smaller portion of the total spread obtained for thewhole group of 10 observers. Although only two cases

are produced here in detail, it may be mentioned thatthe location of individual spreads relative to each otheris amazingly consistent throughout the 18 test colors.Very often a grouping of observers into two distinctgroups can be observed, leaving a slight gap in chro-maticities between them. Figure 7 illustrates also howthe boundary of the total spread was found. The area ofthe total spread contains all, or nearly all, the points (afew isolated points were ignored) obtained by the 10

observers. A simple convex polygon was adopted as

sufficiently representative for the spread.The results obtained for the whole set of 18 test colors

are illustrated by total spreads and are compared withthe computed chromaticities for the proposed 10°standard cmf as well as various other sets of 10° cmfwhich illustrate individual variations in cmf. Figures8-12 illustrate the results. The point symbols used havethe following meanings: Solid dot: Chromaticity pre-dicted by the proposed 100 standard cmf. Cross:Chromaticity predicted by Stiles-Burch mean cmf

.400 F-

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y

.300 H

.300

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63January 1962

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W. S. STILES AND G. WYSZECKI

10 11

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.400

X.450

9

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.500 .550

x

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12

y

.400

.300 .350

X.600

FIG. 10. (Caption on page 62.)

(uncorrected for rod intrusion). Backward-slopinghatched area: Spread of the chromaticities derived from53 individual Stiles-Burch sets of cmf on which theStiles-Burch mean set is based. Open square: Chroma-ticity predicted by Speranskaya mean cmf (uncorrectedfor rod intrusion). Open triangle: Chromaticity pre-dicted by Speranskaya mean cmf (corrected for rodintrusion). Forward-sloping hatched area: Spread of theobserved chromaticities (Donaldson measurements) for10 observers. Dashed line through cross point: Pointstoward chromaticity of blue instrument primary. Doublecross: Chromaticity of instrument primary (only B andR occur).

X.650

The Stiles-Burch uncorrected and rod-corrected cmfgave only slightly different chromaticities and the cor-rected values could not be usefully plotted separately.

The area of the observed or computed chromaticityspread is hatched in accordance with concentration ofpoints, that is, only the places which contain a numberof points are hatched.

The main points brought out by Figs. 8-12 are asfollows:

Test Color 1: Blue

This color is not very different from the blue instru-ment primary, and the close agreement of the observedand predicted chromaticities is to be expected.

I

I

64 Vol. 52

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III

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Test Colors 2 to 4: Blue to Blue-Green

The Speranskaya predicted chromaticity point liesconsiderably closer to the center of the spread of theobserved chromaticities than does the Stiles-Burchpredicted chromaticity point, the latter correspondingin each case to a relatively high y value. For colors 2 and3, the Speranskaya points show a similar but muchsmaller displacement of the y value from the center ofthe observed spread, but for color 4 it lies very close tothe center. The spread of the Stiles-Burch predictedpoints is generally similar in shape to the spread of theobserved values, but in going from 2 to 4, the Stiles-

I I I I I.650

x

Burch spread becomes noticeably larger relative to theobserved spread.

Test Colors 5 to 8: Near-"Whites"-Low-Saturation Colors

The Speranskaya and Stiles-Burch chromaticitiesagree fairly well, except for color 6, but there is somedisplacement of the predicted chromaticities from thecenter of the observed spread in the general direction ofyellowish-green, the displacement for color 6 beingappreciably less for the Speranskaya chromaticitypoint. The shapes of the observed and Stiles-Burchspreads are similar.

I I I .700

X.750


65January 1962


.250 -

17

18

.300 .350

XFIG. 12. (Caption on page 62.)

.400 .450

Test Colors 9 to 13: Green-to-Red Series

Moving from green to red, a stretching-out of theobserved spread approximately along the dashed linecommences at color 11 and increases to color 13, whichis a red differing little from the red instrument primary(R). Where this stretching out occurs, the uncorrectedSperanskaya chromaticity tends to lie well within theobserved spread, while the corrected point approachesthe Stiles-Burch chromaticity. A suitable compressionand displacement of the observed spread in the directionof the blue primary would bring it into fair agreementwith the Stiles-Burch spread. For the green color 9,there is a displacement towards the green of the pre-dicted points away from the center of the observedspread: for the greenish-yellow color 10, the Stiles-Burch and corrected Speranskaya points fall fairly nearthe center of the observed spread.

Test colors 14 to 16: Reddish Purples

A marked difference in shape and extent of the ob-served and Stiles-Burch spreads appears for thesecolors, the observed spread being stretched out approx-imately in the direction of the dashed line pointingtoward the blue instrument primary. The uncorrectedSperanskaya chromaticity point lies nearer than theStiles-Burch point to the center of the observed spread,but for colors 14 and 15 the rod correction brings the twopredicted values materially closer.

Test colors 17 and 18: Bluish Purples

Rod-corrected Speranskaya and Stiles-Burch chro-maticities are in good agreement and the displacementfrom the center of the observed spread is small.

Additional Experiments and Computations

The results of the initial field trials show that theagreement between the chromaticities of the 18 testcolors, as measured on the Donaldson colorimeter andas predicted from the proposed 100 standard cmf as wellas a number of individual 100 cmf, is in some respectsunsatisfactory.

The differences between the observed and the variouspredicted chromaticities of the 18 test colors are con-sidered under the following headings:

(i) Differences caused by possible errors in thecalibration of Donaldson colorimeter;

(ii) differences believed to be associated with theparticipation of the rod mechanism in the ob-servations, and affecting mainly test colors11-16;

(iii) the relatively high y values of the predictedcompared with the observed chromaticities ofblue-green test colors 2, 3, and 4, shown particu-larly by the Stiles-Burch chromaticities and toa much smaller extent by the Speranskayachromaticities.

(iv) displacements of the predicted chromaticitiesfrom the center of the observed spread for testcolors where the Stiles-Burch and Speranskayapredictions agree fairly well, that is for colorsfrom neutral to green, i.e., test colors 5, 6, 7,and 9.

(v) differences due to lens and macular pigments.

(i) Since the calibration of the Donaldson colorim-eter depends to a large extent on visual luminancematches between the instrument primaries and cor-

y

.2001-

66 Vol. 52

I I I I .I


A 5354

300

.150x

0~

x .300

FIG. 13. Examples for test colors 4 and 5 of the effect of 4%errors in luminance matches required in calibration of Donaldsoncalorimeter.

responding duplicate primaries, a number of luminancematches differing from the average luminance matchused in deriving the transformation matrix (D) werestudied as to the effect of chromaticity changes for thesame test colors. A typical example is given in Fig. 13which shows the predicted chromaticities of colors 4and 5 compared with the observed spread. Inside thespread a point is plotted connected with an arrow whichgives direction and amount of shift produced by 4%changes in the luminances of the red and blue primariesrelative to that of the green primary. These ratherlarge changes would improve somewhat the agreementon test color 4 but at the same time would make thedisagreement on color 5 worse. On the basis of suchstudies of propagation of errors in luminance matches,it was concluded that slight errors of that type are mostunlikely to be the reason for the disagreement foundbetween observed and predicted chromaticities.

200

y

150

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30C

2 350

y

300

A

55C

y

. . . . . .

.150 x .200

6

.250 x .300

Y

40C

Spectral radiometric measurements of the light pass-ing through the Donaldson colorimeter were made inorder to determine the change in spectral energy dis-tribution of the two light sources due to the integratingsphere and other optics in the instrument. The obtainedmodification in the energies was used to recalculatesome of the predicted chromaticities. Only small dis-placements were found, none of them of the order of thedisplacement obtained between observed and originallypredicted chromaticities.

About six months after the original field trials wereconcluded, four of the 10 observers and one new ob-server repeated the observations in the same fashionas before. The instrument was completely recalibratedfor this purpose. However, only the colors with evennumbers and No. 5 were retested. The results for colors2-6 and 8-14 are shown in Figs. 14 and 15, respectively.The originally observed spread, for the four observersonly, is shown by a solid line and compared with thenewly observed spread, for the same four observers plusthe new one, shown by a dashed line. Only small changeswith perhaps slight improvements on colors 2, 4, and 6were found between the two sets of observations.

(ii) The stretching out of the observed spread in thegeneral direction of the blue primary (dashed line)suggested that rod intrusion was occurring and a similardevice to that used to reduce such effects in the Stiles-Burch measurements was tried. For the green instru-ment primary, the yellowish-green primary was sub-stituted and the matches on three test colors thatshowed the stretching-out (12, 14, and 16), and onethat did not (18), were repeated for four observers. Theresults were then plotted in a CIE-type chromaticity

10

450

40C

FIG. 14.

12

. I . . . I

.600 x .650-

14

. . . .

x .650

FIG. 15.

FIGS. 14 and 15. Comparison of spreads of original field trials (solid lines) with those of repeated field trials(dashed lines) demonstrated on test colors 2, 4, 5, and 6 and 8, 10, 12, and 14.

.150 x

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x .300

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67January 1962


.300

y

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.350

600

14

y

.300

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I I I ..650

594

.400 H

.600X

FIG. 16. Comparison of spreads of original field trials (solid lines) with those of field trialswhich incorporate a yellowish-green primary instead of a green primary.

diagram, the calibration of the Donaldson colorimeterfor the new primary system being carried out in thesame way as for the old, and the new D matrix againbeing chosen so that for predicted chromaticities basedon the proposed standard cmf, the final values wereobtained in Judd's particular CIE-type system. Figure16 shows for each of the four test colors, the spread ofthe results of the four observers (a) as originally ob-tained with the green primary and (b) as obtained withthe yellowish-green primary. The use of the yellowish-green primary has materially reduced the spread for thecolors 12, 14, and 16 and for the first two, the agreementwith the rod-corrected predicted values is muchimproved.

The scotopic luminances of the two fields of theDonaldson colorimeter during the main matches withthe green primary were computed on the basis of theCIE scotopic luminous efficiency function V'. Thedifferences (scotopic luminance of test field)-(scotopicluminance of comparison (mixture) field) or (ST-SC),expressed as a fraction of the geometric mean scotopicluminance, (STSC)1, are given in Table II. The ratio(ST-SC)/(STSC)5 is an indicator of the possibility ofdistortion of the color match by rod vision, particularlywhen the ratio is taken in relation to the smallestperceptible scotopic difference visible on a field ofscotopic luminance (STSC)i. Under the best conditionsfor rod discrimination the least perceptible value of

68 Vol. 52

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TABLE II. Data for estimating the effect of rod intrusion.

Test colornumber

123456789

101112131415161718

(ST-SC)/(STSC)

-0.011+0.132+0.107+0.224a-0.034+0.078-0.058-0.161-0.016-0.213a-0.378a-0.889a-0.745a-0.784a-0.333a-0.264a-0.090+0.017

Retinal illuminance(photopic trolandsbased on 100 Vx

curve, i.e., the pro-posed 10° standard

yak function)

14388479

11612712176

1031155637212732154511

a Test colors for which rod vision is expected to have a disturbing effect.

ST-SC is probably not below 0.2 (STSC) 1. One shouldexpect therefore that rod vision would in any case havelittle disturbing effect for test colors not marked with across in the table. Of the marked test colors, six showpronounced stretching out of the observed spread inthe blue primary direction. For color 10, for which(ST-SC)! (STSC) 1 only just exceeds 0.2, there is little ifany effect. With respect to color 4, see remarks underheading (iii). The extent to which a high value of(ST-SC)!(STSC)' can lead to distortion of a matchby rod intrusion, one should expect to depend not onlyon the smallest perceptible scotopic luminance differenceunder the prevailing scotopic field luminance, but alsoon the discrimination thresholds for the cone mecha-nisms under their respective stimulation levels in thefield.

Thus, if (ST-SC)!(STSC)1 is below 0.2, one may

conclude that rod distortion of a match will be small butthere seems at present no satisfactory theory for pre-dicting what happens when this is not the case. Thepresent field trials would indicate that at fairly lowphotopic brightnesses of rather saturated colors in theyellow, orange, red, and reddish-purple regions, the roddistortion takes the form of a displacement away fromthe blue primary, the amount depending presumably onthe particular observer's rod "sensitivity" under theconditions of the tests. This appears to be variable, andit is worth noting that repeated observations by thesame observer tend to be drawn out in the same direc-tion as the whole observed spread.

The effects of rod intrusion in the present field trialsare very similar to those observed in the N.P.L. PilotInvestigation.5 When in the main Stiles-Burch meas-

I W. S. Stiles and J. M. Burch, Optica Acta 2, 168 (1955).

LENS PIGMENT

MAOLLAR PIGMENT

450 \lm&) 500

FIG. 17. Relative spectral density distributions of lens pigmentand macular pigment.

urements the device of a yellow primary for measure-ments on long wavelengths was introduced, thereappeared to be a small residual rod distortion that wascorrected for by a method already explained.' The samemethod was applied by Judd to correct the Speranskayadata for rod intrusion. The effect of the correction onthe predicted chromaticities of the 18 filters, is to dis-place the predicted chromaticity toward a point cor-responding, approximately, to the equi-energy white-the white that is identified in the correction methodwith the color produced by rod stimulation. It wasalready suspected that the rod distortions were morenearly in the direction of a blue lying below the equi-energy white but for the very small corrections to beapplied to the Stiles-Burch data, the use of the equi-energy white-a well-defined point-rather than achromaticity in the blue region, made no real difference.For the Speranskaya data, however, the corrections arelarger and extend to colors further from the red cornerof the chromaticity diagram. The new field trials makeit probable that the rod intrusion correction should be

350

y

.30C

\M ;-

+E

* <--'' M,.. 2M

4 .350

-Y

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5

-Mt

ML.9 LM

350 x x .3.X)

FIG. 18. Scaled loci drawn through the chromaticities of testcolors 4 and 5 showing the effects of lens (L) and macular (M)pigment changes.

.

.; .....

69January 1962

W. S. STILES AND

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FIG. 19.

FIGS. 19-23. Portions of the 2 0-x,y chromaticity diagram showing predicted and observed chromaticities for 18 test colors related toa 2 matching field. Hatched area: observed spread; solid dot: predicted chromaticity for CIE 2 standard observer; cross: predictedchromaticity for N.P.L. 2 pilot observer.

approximately in the direction of the blue primary, andit can be seen from Figs. 10 and 11 that a correction inthis direction would in general improve the agreementbetween the Stiles-Burch and rod-corrected Speran-skaya chromaticities.

(iii) For the test colors 2-4 (6 may also be includedhere) where the Speranskaya chromaticities givesuperior agreement with the observed chromaticities,it is only for No. 4 that the ratio (ST-SC)/(STSc)Oexceeds 0.2, and only for this color would rod distortion

appear to be a possible or likely factor. It is of interestto note, however, that the line from the Stiles-Burchchromaticity to the blue primary passes through or veryclose to the Speranskaya chromaticities and that theobserved spread is displaced from the Stiles-Burchspread along the same line. Additional Donaldsoncalorimeter measurements for three observers on testcolors 3-6 at a field luminance increased by a factor ofabout three produced only insignificant shifts of thespread of these observers' results.

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On the whole, rod intrusion does not appear to be alikely cause of the discrepancies for colors 2, 3, 4, and 6

and other possibilities were examined. These colors and,particularly test color 4, are those for which the Max-well spot is most in evidence and this may be a compli-cation of a different kind for a contrast-type matchingfield than for a simple bipartite (Stiles-Burch) or center-occluded bipartite field (Speranskaya). Repeat matcheswere therefore made on test colors 3-6 using a simplebipartite field (ellipse of vertical and horizontal diam-eters 10.5° and 8.50 divided vertically). No significantdifference depending on type of field was found. The

pronounced color breakup at the fovea that was in

evidence in matches on spectral stimuli with spectralprimaries was certainly more pronounced than theMaxwell spot obtained with any of the 18 test colors.Simultaneous contrast between the central (foveal)area and the surrounding field-a conceivable cause ofmatch distortion when there is severe color breakup-would be eliminated by Speranskaya's method and mayaccount for the better agreement of her predictedchromaticities in this region, with those observed.Speranskaya6 describes comparative measurements with

6 N. I. Speranskaya, Nall. Phys. Lab. Symposium No. 8, VisualProblems of Colour (Her Majesty's Stationery Office, London,1958), Vol. 1, p. 317.

January 1962

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the occluded and nonoccluded bipartite field and con-cludes that the results are not significantly different. Itis not clear, however, how large a difference would havehad to be present to show up significantly. Her data,plotted on a rather small scale, seem to show somedifferences.

It has already been noted that the field luminancesused by Speranskaya were considerably lower thanthose used in the main Stiles-Burch investigation(generally less than ). For saturated colors in the blueand blue-green, too high a field luminance reduces colordiscrimination and in fact for this reason the fieldluminance used for spectrum colors between 420 and500 m approximately, in the main Stiles-Burch

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investigations was reduced below the values used in thepilot investigations.5 Comparison of the pilot data withthe main data indicates that the luminance change hasnot materially altered the cmf in the spectral region inquestion. Fry7 has emphasized particularly the loss ofcolor discrimination in the blue and greenish-blue, andin a discussion with the authors, he suggested that thiswas a factor which might be distorting the color-match-ing data. This suggestion has not been fully exploredyet. A related factor may be the completeness of adap-tation to the matching field. In the Stiles-Burch inves-tigation, the fixing of the subject's head by the dentalimpression method-to ensure central entry of the

I G. A. Fry, J. Opt. Soc. Am. 49, 1156 (1959).

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stimuli in the pupil-tended to facilitate and encourageprolonged viewing of the field. This and the use of asurround of the same color as the bipartite area extend-ing the total field diameter to 140 were both likely toproduce more complete adaptation than in the otherinvestigations concerned.

A direct comparison of the Stiles-Burch and Speran-skaya cmf brings out that the difference would cor-respond very approximately to a smaller macularpigmentation of the Stiles-Burch subjects. However,one subject, Madame Yustova, made measurements onthe N.P.L. trichromator as well as on Madame Speran-skaya's apparatus. Comparison of her two sets of resultsshows a difference very similar to that of the mean

Stiles-Burch and mean Speranskaya groups. This makesit extremely unlikely that the differences arise fromsubject sampling errors. Apart from luminance level andfield pattern, the Stiles-Burch and Speranskaya inves-tigations differ in that spectral primaries were used inthe former and the stimuli were introduced into the eyeby images of exit slits formed in the center of the actualeye pupil, while the latter employed filter primaries anda normal type of artificial pupil. In these respects,the present field trials resembled the Speranskayainvestigation.

The authors served as observers on both the N.P.L.trichromator and the Donaldson colorimeter. It wasfound that the two sets of results of W. S. S. as well as

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those of G. W. showed differences very similar to thoseof the mean Stiles-Burch data and the mean observeddata of the field trials. The inference is again thatsubject sampling errors are not mainly responsible forthe discrepancies in question.

(iv) For the test colors 5, 7, and 9 where the Stiles-Burch and Speranskaya predictions agree closely butare displaced from the center of the observed spread,the possible effect of rod intrusion as judged by theratio (ST-SC)/(STSC)' would appear to be completelynegligible. Also, there were no difficulties attributableto Maxwell spot in the Donaldson measurements whichwere probably the easiest to make of all the test colorsstudied. No reason for these discrepancies, apart fromundetected instrumental errors or small departures fromGrassman's laws-factors that may also play a part inthe other comparisons-occurs to the authors.

(v) It was thought useful to see exactly how thepredicted chromaticities would be modified by an in-crease or reduction in lens or macular pigment. For therelative spectral density of lens pigment, use was madeof a mean curve based on measurements by Ludvighand McCarthy et al. (see Stiles8 ) and for the macularpigment, the relative spectral density curve of xantho-phyll in chloroform, which Wald9 identifies as macu-lar pigment, was adopted (Fig. 17). Seven cases werestudied: (1) An increase in macular density, measuredat the wavelength of maximum density 455 mgu of 0.70;(2) an increase in macular density of half this amount,viz. 0.35; (3) a reduction of macular density of 0.35;(4) an increase in lens pigment density of 1.0, measuredat the wavelength 403 mu; (5) a reduction in lens pig-ment density of 1.0; (6) changes 2 and 4 together; and(7) changes 2 and 5 together. The cmf taken as basiswere the 100 Stiles-Burch data, but the chromaticitydisplacements produced by these pigment changeswould be substantially the same for the Speranskaya orproposed standard data.

The calculations for cases 1-4 enable scaled loci to bedrawn through the chromaticity of each test colorshowing the effects of various pigment changes. Figure18 gives the results for test colors 4 and 5. In the case oftest color 4, the displacement of the Stiles-Burchpredicted chromaticity and the observed spread is aboutin the direction of a change of lens pigment or almostequally closely in the direction of a change of macularpigment. The magnitude of the displacement corre-sponds approximately to a difference of 0.2 in macularpigment density or 0.5 in lens-pigment density. Theseamounts appear improbably large to reconcile with asampling error origin of the difference.

For test color 5, the effect of lens-pigment change ispractically nil. The displacement of the predicted point(Stiles-Burch or Speranskaya) from the observedspread is not in the correct direction for a change ofmacular density.

8 W. S. Stiles, Ned. T. Natuurk. 15, 125 (1949).9 G. Wald, Science 101, 653 (1945).

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Some further breakdown of the various chromaticityshifts into apparent pigment changes may be worth-while, but this was not done.

Field Trials with a 20 Matching Field

It was considered useful to supplement the 100 colormatches by color matches made with a 2 field. Thevisual field of the Donaldson colorimeter was dia-phragmed down to give a simple vertically divided20-diam bipartite matching field and measurements onall the 18 test colors were made for five observers. Theobserved spread and the predicted chromaticities on thebasis of the CIE and the 20 N.P.L. pilot data' werederived in the same way as for the 100 data, the fixedD matrix being chosen to give final chromaticities in thestandard 2 CIE system. The results of this work aregiven in Figs. 19-23.

The observed spread is larger despite the inclusion offive instead of 10 observers than that for the correspond-

74 Vol. 52

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ing large field measurements, with the exception of twoor three test colors where rod intrusion has stretchedout the large field spread.

With one exception-2° pilot data chromaticity fortest color 4-all the predicted chromaticities lie withinthe observed spread as drawn, although they are notalways located at the center. As far as can be judgedwith tests on five observers, the CIE data adequatelypredict the chromaticities of the 18 test colors. Thepredictions of the N.P.L. 20 pilot data correspond to anobserver of lower macular pigmentation than the CIEobserver, as has already been noted.5 This differencemay have arisen in the, same way as the similar differ-ence between the 100 Stiles-Burch and the Speranskayadata, although the greater density of macular pigmentin foveal vision gives more scope for subject samplingerrors in 2° results obtained for fairly small groups.

CONCLUSIONS

While the CIE 2 standard cmf seem to predictadequately color matches made with a 2° visual field,

the recently proposed 10° standard cmf failed in somerespects to predict color matches made with a 100 field.The disagreement between predicted and observedchromaticities was especially evident for blue, blue-green, near-white and green test colors, and no explana-tion for these discrepancies was found other than tosuspect the validity of Grassmann's laws for large-fieldcolor matching. The possibility of undetected instru-mental errors causing the discrepancies is consideredonly a remote possibility despite the complexity of theexperiments. This view is strongly supported by thefact that repeated experimental checks did not bear outany such errors and independent field trials conductedby Wright-Wyszecki'0 largely confirmed the resultsreported here.

It appears that further field trials on the 100 datawould be valuable. However, more direct checks onGrassmann's laws for large-field color matching mightalso prove to be highly instructive.

1 H. Wright and G. Wyszecki, J. Opt. Soc. Am. 50, 647 (1960).

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 52, NUMBER 1 JANUARY, 1962

Comparison of Monocular and Binocular Color Matching*

CARL S. HOFFMANtFlorida State University, Tallahassee, Florida

(Received March 21, 1961)

The binocular mixture of 631 mg and 533 m/u resulting in a match to 582 mg was compared with monocularmixtures in normal and color-deviant subjects. A haploscopic type of color-mixing apparatus was usedwhich provided for easy, natural binocular fusion. A photopic adaptation field was also provided. The resultswere (1) All subjects were able to match 582 muA binocularly. (2) The monocular mixture contained more533 ma than the binocular, the ratio being 1.47:1. (3) The binocular match was on the average 17 timesmore variable than the monocular. (4) Variability was not due to binocular rivalry, as highly trained sub-jects who experienced little or no rivalry showed no decrease in variability. From these data it was concludedthat (1) Binocular matches are different from monocular, possibly implying different physiological mecha-

nisms for each, and (2) The basic Hering and Young-Helmholtz theories are inadequate to account forthe data.

INTRODUCTION

THE present study is an extension of earlier work',2

T investigating the differences between binocularand monocular color matches. While the earlier researchwas concerned with a variety of mixtures matchingyellow, purple, and white, the present study is only

* Based in part on a doctoral dissertation submitted to theGraduate School of Florida State University in partial fulfillmentof the requirements for the Ph.D. degree. The writer wishes tothank Dr. Howard D. Baker for his guidance and supervision inconnection with this study. This study was supported in part bythe Physiological Psychology Branch of the Office of NavalResearch, under contract.

f Now at Bendix Radio Corporation, Baltimore, Maryland.I W. Trendelenberg, Arch. ges. Physiol. Pilulgois 201, 235-246

(1923).2 G. F. Rochat, Arch. n6erl. physiol. 10, 448-453 (1925).

concerned with a match to luminance in the yellowportion of the spectrum.

Trendelenberg,l using a modified Helmholtz color-mixing apparatus, found that a greater proportion ofgreen to red was used monocularly than binocularly.For the mixture 671 mu plus 535 mu to match 589 mu,the monocular-green: binocular-green ratio ranged from4:1 to 14:1. Rochat 2 was unable to obtain a match to

sodium in a stereoscope apparatus, although he didobtain some of Trendelenberg's results in the mixture ofcomplementary colors. It is interesting to note that bothTrendelenberg and Rochat reported extreme difficultyin obtaining binocular mixtures due to binocular rivalry,although Trendelenberg was able to eliminate rivalryby reducing the field to 30 min. In addition, neither con-

75January 1962

Documents

Field Trials of Color-Mixture Functions