JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

The Illuminant in Textile Color Matching*

DOROTHY NICKERSON**Cotton Branch, United States Department of Agriculture, Production and Marketing Administration, Washington, D. C.

(Received February 5, 1948)

A subcommittee of the Inter-Society Color Council was established in 1944 to study twothings: "What color temperature of natural daylight is used by textile color matchers?" and"What color temperature of artificial daylight is preferred by textile color matchers?" ThePhiladelphia Army Quartermaster Depot and three large textile companies-Forstmann,Botany, and Pacific-cooperated in the work. As a result of its study, summarized here, thesubcommittee recommends that artificial daylight lamps for textile color matching be aimedat 7500'K with an illuminance of more than 75 and less than 300 footcandles. (By artificial day-lighting, the subcommittee means an illuminant that has an energy distribution as similar tothat of daylight of equivalent color temperature as it is possible to obtain commercially.)

THE color of daylight, both natural andartificial, has been of interest to this so-

ciety since its formation in 1916, and even beforethat to the Illuminating Engineering Society,for in 1910 Dr. Herbert E. Ives presented to theNew York Section a paper that discussed colormeasurement of daylight and artificial daylight.In 1914 papers were published on artificial day-light and daylight glass by Luckiesh and Cadyand by Brady. Since then the names of Macbeth,Gage, Gibson, Davis, and Taylor have beenclosely connected with work involved in deter-mining the color of sunlight and daylight andof its reproduction.

In 1938 results were reported to this society ofwork done in the United States Department ofAgriculture to obtain a factual basis for settingspecifications for artificial daylight for use in thegrading of agricultural products. The presentreport is a logical extension of all such previouswork. It is made by a subcommittee of the Inter-Society Color Council, of which the Optical So-ciety of America is one of 15 national memberassociations.

* Summary of Report on I.S.C.C. Problem No. 13;(presented to the Optical Society of America, October23-25, 1947, Cincinnati, Ohio).

** Chairman of I.S.C.C. subcommittee on Problem 13.Members of subcommittee: Constantin Monego, JuliusForstmann and Company, Inc. and Philadelphia TextileInstitute; Frank J. O'Neil, Pacific Mills; P. M. Otto, Jr.,Botany Worsted Mills; Frederick T. Simon, SidneyBlumenthal and Company, Inc.; Stanley Backer, Phila-dephia Quartermaster Depot; E. I. Stearns, Calco Chemi-cal Division; Norman Macbeth, Macbeth Corporation;I. H. Godlove (Chairman of AATCC Color Committeeduring time of this study); John J. Hanlon, MohawkCarpet Mills.

In 1944, at the request of the American Asso-ciation of Textile Chemists and Colorists, an-other of the Council's member associations, asubcommittee was appointed to study twothings: "What color temperature of natural day-light is used by textile color matchers?" and"What color temperature of artificial daylight ispreferred by textile color matchers?" The kindof natural daylight that is preferred came as aby-product of answers to these two questions.

Because of its nature, the full report of thiswork, including extended tables of data, has beenpublished by the Illuminating Engineering So-ciety, in Illuminating Engineering,' followingpresentation to their September 1947 nationaltechnical conference. However, since a reportof such work holds interest for other I.S.C.C.member associations than the A.A.T.C.C. andthe I.E.S., summaries are planned for promptpresentation and publication by each interestedmember body of the Council. Detailed data willbe found in the I.E.S. report for those who wishmore than the present summary.

To avoid a possible misunderstanding of thesubcommittee's purpose, it should be stated thatthis study was not proposed to answer the ques-tion: "What illuminant is best for making ac-curate color matches?" Its purpose was to ascer-tain the color temperature of daylight used andpreferred, so that a color temperature specifica-tion could be recommended for artificial day-lighting. By artificial daylighting, the subcom-

I D. Nickerson, "The illuminant in textile color match-ing, an illuminant to satisfy preferred conditions of day-light-match," Illum. Eng. 43, 416 (1948).

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mittee means an illuminant that has an energydistribution as similar to that of daylight ofequivalent color temperature as it is possible toobtain commercially.

For the benefit of those not familiar with colormatching requirements, perhaps it should be ex-plained that for accurate color matching-when amatch must hold under all conditions of illumina-tion-no single illuminant is sufficient, not evendaylight. To match in color under all illuminants,the spectral reflectance of two samples must beidentical. An illuminant that may prove bestfor seeing a color difference between one pair ofsamples may differ completely from that whichis best for another pair. The answer depends uponthe spectral reflectance curve of the samplesunder examination, and that is why color match-ing units usually contain lamps of two widelydifferent color temperatures.

The work of the subcommittee began infor-mally in the spring of 1943 in connection withdiscussion of color tolerances. Representatives ofseveral textile mills and the Philadelphia ArmyQuartermaster Depot agreed that before dis-cussing standards for color tolerance specifica-

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tions, illumination for inspection should bestandardized.

At the first meeting in June 1943, there werelaid before the group results of studies made inthe United States Department of Agriculturewhich recommended 7500'K as a minimum colortemperature and 60 to 80 footcandles as a mini-mum illuminance for color grading of agriculturalproducts. The meeting also considered theA.A.T.C.C. Tentative Specification for StandardIlluminants in Textile Color Matching (June 26,1942 revision), as developed by an A.A.T.C.C.Color Committee under the chairmanship ofWalter M. Scott. This tentative specificationrecommends two standards of color matching,one a color temperature generally comparableto I.C.I. Illuminant A, the other to I.C.I.Illuminant C, a minimum illuminance of 60 to80 footcandles for grading light colored ma-terials, and 150 footcandles for dark coloredmaterials. Discussion brought out the need foradditional information regarding amounts andcolor temperatures of natural daylighting in usefor textile color matching, and on artificial day-lighting to give a maximum as well as minimumilluminance for preferred conditions.

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FIG. 1. Footcandle and color temperature taken at selected color matching stations in four large textilemills. Conditions for color matching called "very good" shown against those called "very poor."

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AMOUNT AND QUALITY OF NATURAL DAYLIGHT

FIG. 2. Portable color matching booth pr(lamp unit in which filters and bulbs may be c]a series of different color temperatures, amount and geometry of illumination to ren

Representatives of Forstmann, IBotany mills agreed to cooperate in data if plans and equipment could l

for the study. Others present agreedthe necessary planning and equipmei

In the several months following thmeeting, plans were developed and tlmethod of study agreed upon:

(1) To collect and study hourly recordperiods, on the amount and quality of naiused by color matchers at color matching statlarge textile mills, by using a Weston footcaiamount, and an Ansco-Sweet densitomete:read color temperature, for quality.

(2) To have color matchers examine agroups of textile samples under low to hightures of controlled Macbeth illuminationfluorescent and daylight illumination, todifferences, and also to express their opinionthe artificial illumination liked most, (b) wence corresponds to color of present naturnand (c) which illuminant gives results thatresults now obtained under natural daylight

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11-". 0 0 0 0 0FIG. 3. Diagrammatic representation of ill.

On the basis of over one thousand observationsof illuminance and color temperature of naturaldaylight made at color matching stations in thecooperating mills,2 data are recorded in the fullreport that bring out differences caused by thehour, season, and sky conditions. Results indi-cate that color temperature is high in January,decreasing until May and June; illuminance islow in January, increasing to May and June.Except where reflections interfere, there is aregularity of change in illuminance with thetime of day; color temperature shows no con-sistent change with the time of day.L If Illuminating conditions called "very good" forcolor matching averaged 224 footcandles at9400'K for clear sky; 211 footcandles at 88500Kfor slightly overcast sky; 332 footcandles at6500'K for overcast sky. Conditions called"very poor" for color matching averaged 450

wvided with a footcandles at 78500K for clear sky; 500 foot-,et allow the candles at 6700'K for slightly overcast sky;iain constant. 120 footcandles at 5800'K for overcast sky; 100

footcandles at 5800'K for very overcast sky; andPacific, and 40 footcandles at 5300'K for a very dark sky.getting such The preference of color matchers for natural

be provided daylighting is answered in two ways. First, ato provide nearly unanimous preference for a light or

it. * moderately overcast or "covered" sky was ex-

e June 1943 pressed early in the test, when each color matcherhe following encountered on a tour of the color stations of

each mill was asked what kind of a sky he pre-s, for 30-day ferred. Only two matchers, out of several dozen,tural daylight expressed a preference for a clear sky. Second,:ions in several measurements of the illuminance required to

r, adapted to yield conditions called "very good" showed aminimum of 100 footcandles. This minimum

series of five requirement in illuminance is independent ofcolor tempera- color temperature from 25,0000 K down to abouti, and under 7500'K. Below this color temperature down to

r(e)ord pared about 6000'K, the increase in yellowness of the

'hether prefer- incident light seemed to cause the minimumil daylighting, requirement in illuminance to rise to about 250correspond to footcandles. Figure 1 is a plot of illuminance inconditions. footcandles against reciprocal color temperature

NTURAL in mireds, showing the distribution of conditionsATURAL judged to be very good compared to those judged

DAYLIGHT2 Two Forstmann mills are included, those at Garfield

iminants used. and Passaic.

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to be very poor. As may be seen from Fig. 1,conditions called very poor are those which com-bine low color temperature and low illuminance,and those which include very high illuminanceat any color temperature.

COLOR TEMPERATURE OF ARTIFICIAL DAYLIGHTPREFERRED BY TEXTILE COLOR MATCHERS

For the part of the study which concerns thedetermination of the color temperature of arti-ficial daylight preferred by textile color matchers,it was necessary to give considerable thought tostandardization of as many conditions as pos-sible. This meant standardization of the illumina-tion used and of the samples examined underthose illuminants. The only important variabledesired in the illuminant was color temperature.Therefore the introduction of other variableswas avoided as far as possible.

Artificial daylight as it is provided by tungstenlamps filtered through Corning 590 filters is al-

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ready so well established as the best practicaland available source of artificial daylight for thecolor temperature range above that of averagenoon sunlight, that it was adopted withoutquestion for use in this work. It was agreed thatall color matching work done under these lampswould be done also under each color matcher'sselection of natural daylight, and under fluo-rescent daylight wherever this was available inthe cooperating mills. However, since this studywas aimed only at providing a series of colortemperatures under which color matchers mightwork, most thought was given to providing acomparable series of illuminants in which colortemperature was the one important variable.

This was accomplished by working out a light-ing layout on a miniature scale in which back-ground conditions and amount and geometry ofillumination could be held constant. A portablebooth, painted neutral gray (about MunsellN 6/) was made so that it might be set on a table.

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FIG. 4. Distribution of color of lamps used, plotted on an I.C.I. (xy) diagram relative to iso-temperature lines.

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A regular 500-watt size Macbeth lamp washung in this booth within a specially constructedmetal cylinder built to enclose the lamp, and tohold, in the bottom, a piece of sandblasted Crys-talex glass to diffuse the light. The position ofthe cylinder housing and the diffusing glass wasintended to remain constant, thus providingconstant geometry for illuminating conditionsthroughout the test. Within the cylinder pro-vision was made for raising and lowering thelamp so that the amount of illumination on thetable surface might be held as near constant aspossible. A change in color temperature was pro-vided by a selection of filters to be used with aseries of standard lamps.

A specially built portable unit, with lamps,filters, a Variac, and voltmeter, was provided

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for this work. It is illustrated, with the samplesused in the test, in Fig. 2. This unit was movedfrom one mill to another for use in all tests.

Whenever fluorescent lamps were used in themills, test work was done under them also. Nocontrol was kept of these lamps, their color or theilluminance provided by them, the units beingused just as they were available at the mill.

Thus, matching tests were made under fivesteps of filtered daylight, one condition of naturaldaylighting, and fluorescent daylight wheneverit was available. A diagrammatic representationof the illuminants used is shown in Fig. 3, andtheir colorimetric representation is shown inFig. 4 on an I.C.I. (x, y) diagram relative tolines of constant nearest-color temperature.

The samples used in this test were to be given

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to the color matchers who would examine themunder this series of illuminants. Since the choiceof color temperature probably would dependupon the samples examined, it was of the utmostimportance that a representative set be used.Without being too numerous they had to repre-sent all materials, all fabrics, and all colors. Threematerials were selected: wool, cotton, and vis-cose rayon. These represent a large proportionof textile usage. Three fabrics were used: cloth,yarn, and a pile fabric. Five colors were chosen:red, yellow, blue, brown, and black.

It is practically impossible for a color matcherto decide what illumination he likes best if he isgiven only one sample of a particular type. Apiece of red wool cloth is red under all the lampsused in this study. His normal daily task is tosee differences between a sample and a standardand make a decision based on the difference.

The necessity of preparing samples with. smallcolor differences introduced additional difficultyin the choice of average samples. If all the colorsconsisted of one pair differing slightly in hue,a different illuminant might be found better thanif each pair differed simply in depth of the dyeing.In the case of each color it was necessary to pre-pare all kinds of color departure from standard.Only then would the choice of lamps be based onaverage samples.

The method selected was to vary slightly afundamental dye formula, usually, by the addi-tion of a small aemount of toning dye. The extentof the differences chosen was intended to repre-sent differences slightly greater than those foundin commercially satisfactory matches. Nearly allwould be rejected on final inspection but wouldrequire some study to decide on correctivemeasures. -.

Twenty-nine samples were used in the test.This number appeared to be ample for thecolorists to make a choice of illuminants; theuse of additional samples would have made thetest too time consuming. Figure 5 shows-spectro-photometric curves of the standards used ineach series of samples.

The color examiners who took part in thisstudy are men who earn their living by coloringor, examining the color of fabrics. These menwere asked to examine the 29 samples in daylightand under several; conditions of artificial day-

lighting. Within the five sets of samples eachsample was numbered. Number one of each serieswas used as a standard, the other samples beingcompared with it.

To encourage uniformity in describing thecolor differences, the observers were suppliedwith forms on which they could check the kindof difference observed. This tended to give stand-ardized terminology for description of color dif-ferences and made it easier to analyze the re-sults. They were, however, not limited to theseterms but could use others if they preferred.

The study was set up to discover the colortemperature of artificial light- preferred by colormatchers, yet an analysis of the results will de-pend upon the standard to which they are com-pared. The committee made three comparisonswith the following results. If the preference is forartificial daylighting that provides results closestto those obtained under preferred conditions ofnatural daylighting, then, based on the averageof these observations, the color temperatureanswer is 7500'K. If, however, the preference isfor an illuminant that will provide results closestto those expected on a basis of the differencesintroduced by dye formulation, then the averageresults for color temperature is 8400'K. Further,if the preference is for an illuminant to provideresults closest to those obtained by spectrophoto-metric measurement of the sample, the averagecolor temperature result is also 8400'K (althoughIlluminant C, about 6700'K, was used in thecolorimetric calculations).

Perhaps the most important fact brought outby this part of the study is that within a fairlywide range of color temperature, and with il-luminance not more than 60 to 90 footcandlesunder the lamps, the color matching results allcome very close to those obtained under naturaldaylight. In only one mill was there a consistentlybetter showing for color matching results undernatural as compared with artificial daylight.Even there the margin of difference was verysmall. Outstanding differences between resultsunder the various color temperatures did notoccur.

The mills varied in degree of agreement withexpected differences from a high of 75 percent atone mill to 40 percent at another; all showedabout 10 percent more agreement for compari-

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DOROTHY NICKERSON

ResuLts for First Sri-s of Matlhcs FIG. 6. Percentage of agree-ment among color matchers atone mill in their ability to call thematches in a common terminol-ogy.

Reu.Lt f Second Seres of Mtches

sons with dye formulation data than with colori-metric measurements. The average agreementfor all mills with dye formulation data wasabout 55 percent, varying only about 2 percentfrom this for the various illuminants. The averageagreement with computed clorimetric data wasabout 45 percent, with about 2 percent variationfor the several illuminants.

Perhaps it should be noted here that dif-ferences in results between mills may be due toone of several factors. Among these, the abilityto break down the difference seen into at leastthree factors and for color matchers to use acommon terminology in describing these dif-ferences, is probably the most important. Un-doubtedly, the color matchers in one mill couldsee the differences as well as matchers in anothermill. But seeing a color difference does not meanthat one matcher can analyze this difference andrecord it in a terminology understood by other

matchers. It takes training to do this, and whilecolor matchers in all mills usually looked for andrecorded a hue difference, they varied by millsin their ability to record other types of differ-ences. The record sheet reminded each matcherto look for differences in lightness and saturationas well as hue. The subcommittee found, afterthe work was done, that the probable reasons forgreater agreement in one mill than in the otherswas the fact that a training course in color hadbeen given in this mill the year before duringwhich their matchers had learned to analyzecolor differences and describe them in a commonterminology.

Agreement among color matchers in this mill,after eliminating the practice eflect found in thefirst series, as shown in Fig. 6, was almost 50 per-cent for differences called in terms of all threeattributes of hue, lightness, and saturation. Agree-ment on single attributes was as high as 75 to 80

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percent. The highest agreement on all three attri-butes was about 52 percent under daylight, witha variation under other illuminants of 2 to 6 per-cent. This mill averaged 75 percent in recogniz-ing and agreeing with color differences due tochange by dye formulation. Such a high degreeof agreement sets quite a record, and showswhat can be expected with training and practice.

In addition to the record of differences forsamples matched under various color tempera-tures, color matchers also answered three ques-tions asked at the end of the matching studies.

The distribution of answers to the first ques-tion regarding illuminant preference is shown inFig. 7. In mills A and B the preference was clearlyfor 7500'K. No such definite preference appearsfor mill C, although in its first selection 7500'Krepresented the highest percentage preference,10,300'K was its second selection. In mill D thegreatest preference was for 6750'K. Answers toquestions as to whether the choice corresponds topresent conditions or whether it gives the sameresults as daylight now used showed that for alarge majority of cases the preference indicated

was thought to be similar, or nearest to thecondition preferred for natural daylight, andthat it was thought to give results similar tothose obtained under natural daylight.

RECOMMENDATION

On the basis of this study in all its details,and of information available through previousstudies, our subcommittee is unanimous in recom-mending that artificial daylight lamps for textilecolor matching be aimed at 7500'K, with anilluminance of more than 75 and less than 300footcandles.

We realize that the choice of 7500'K is some-what arbitrary since it is not outstandingly betterthan color temperature somewhat higher thanthis. The considerations which suggest 7500'Kare as follows: (1) At an illuminance of 100 foot-candles, 7500'K is the minimum for naturaldaylighting conditions called very good formatching, and at 200 footcandles it falls nearthe average of such conditions; (2) when coloristswere asked which of several lamps they pre-ferred, their choice on the average was near

FIG. 7. Distribution of prefer-ences for color temperature ex-pressed by color matchers afterthey had matched under the en-tire series of lamps in test.

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75001K; (3) color differences observed by color-ists under natural daylight chosen by themagreed best with color differences observed bythem under 7500'K; (4) it is understood thatmanufacturers of artificial light units for colormatching can produce 7500'K sources with thefootcandle range indicated. While the recom-mendation itself makes no mention of illumina-tion geometry, it is assumed that this shouldfollow good lighting practice.

ACKNOWLEDGMENT

The subcommittee wishes to thank the follow-ing: Curt Forstmann and W. von Bergen ofJulius Forstmann and Company, P. M. Otto, Sr.(who has since died) and Frederick R. Haigh of

Botany Worsted Mills; and Harold W. Leitchand Raymond Schuster of Pacific Mills for theircooperation in encouraging, as well as permitting,this study to be made in these mills. The sub-committee also wishes to acknowledge and thankthe color matchers and technical workers whoparticipated in making the color judgments.Without their careful and time-consuming co-operation in making hundreds of check measure-ments, and without the wholehearted coopera-tion of the men in charge of each department,this study could not have been made. Of themore than 80 participants, 75 percent wereactively engaged in color discrimination at thetime of these studies; the other 25 percent wereexperienced persons although they were notactively engaged in color discrimination.

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