Fluorescence and Gray Content of Surface Colors

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Fluorescence and Gray Content of Surface Colors

RALPH M. EVANSEastman Kodak Company, Color Technology Division, Rochester, New York

(Received April 25, 1959)

An investigation of the colors in the Munsell 5R plane and an extension of this study to colors producedin a small aperture in a large white illuminated surround have led to the discovery of some interesting andnovel relationships. It is found that under these conditions the domain of surface color perception includesthe whole of the range from V=0 to 10 and p, =0 to 1.0 and under some conditions more. For a color of agiven dominant wavelength there is a locus lying wholly within this space along which lie colors that do notappear to contain gray. If luminance or purity is increased above a point on this line, the sample takes onthe appearance characteristic of a fluorescent material. If either is decreased below a point on this line, thecolor is perceived as having a gray component added to the purely chromatic component in increasingamounts until at P, = 0, there is no chromatic component perception of the color or at low values of V thesample appears black. Above a point somewhat higher than surround luminance, the appearance of fluo-rescence ceases and the surface mode changes to the illuminant mode, the saturation of the perceived colordecreasing with increasing luminance above this point. An hypothesis is suggested to explain the facts andit is pointed out that more than one kind of "brightness" is necessarily involved.

INTRODUCTION

Munsell System

THE Munsell System was devised as a meansTwhereby all possible simple surface colors could

be arranged systematically in a three-dimensional space.The coordinates are the variables Munsell hue (H),chroma (C), and value (V), correlating for an observerwith normal vision adapted to daylight with the percep-tual attributes hue, saturation, and lightness. V is arelatively simple function of relative luminance and isconstant for constant luminance. It is represented alongthe axis of the cylindrical coordinates used by the systemand gives rise to a series of approximately equally spacedsteps of gray when small samples are seen against awhite background. The scale runs from 10 for the idealperfectly reflecting, perfectly diffusing white to 0 forthe perfectly absorbing surface, perceived as black. C isthe radial coordinate of the system and correspondsapproximately on a white background to the saturationof the perceived color. The scale for C is arbitrary, butthe system has been adjusted empirically so that thesame number of steps away from the equivalent gray isperceived as corresponding to the same saturation re-gardless of hue. H is defined by its angular positionabout the axis and has been spaced empirically at all Vand C levels, so that any radial plane starting from theaxis contains only those colors perceived to have thesame hue. In general, dominant wavelength, ND, is notconstant over a constant Munsell hue plane, the amountof variation depending on H but becoming very largefor some hues, notably in the Munsell hue range yellow-red to red.

This color order system has been exemplified to agood approximation by the samples of the Munsell Bookof Color.

Gray Content

When a constant H plane series of these samples isconsidered carefully, it can be seen that along any hori-

zontal line where V (and, hence, luminance) is constant,gray decreases as saturation increases; i.e., the color isperceived as gray at C=0 and the gray component ofthe perception is gradually displaced by the purelychromatic component. It is equally true that along anyvertical line in which C is constant, the amount of grayperceived in the colors is seen to increase from verylittle, if any, gray at the highest Munsell value to blackas V approaches zero, the gray in this case increasing,while the purely chromatic component, or hue content,of the perception remains essentially constant.

It follows from this that it should be possible, startingwith gray on the axis at any V level, to find a series ofcolors each of which appears to contain the same amountof gray as the achromatic axis sample. This is in fact thecase, and experimental results on actual samples of thered hue Munsell 5R are described in this article underthe heading "Results."

Illuminated Aperture

The domain of surface colors may also be investigatedwithout the use of actual reflecting samples. If a largewhite surface illuminated from the front contains a smallaperture (say 20) and behind this is placed a light sourceof any color, over a range in which the luminance of thissource is approximately the same or less than that of thewhite surround the aperture will appear in the surfacemode, i.e., the color will be seen as though it belonged,for example, to a piece of paper pasted onto the whitebackground. (The aperture must be uniformly illumi-nated if head movements or binocular viewing are per-mitted.) Since such an aperture may be made to have acolor of any dominant wavelength or relative luminancewith respect to the surround and any purity up to unity,it is possible with this to investigate all colors that canbe seen in the surface as well as relative aperture modes ofappearance. Such a device was constructed. With it, ofcourse, it is more convenient to work with the psycho-physical variables calorimetric purity, dominant wave-

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VOLUME 49, NUMBER 11 NOVEMBER, 1959

RALPH M. EVANS

FIG. 1. Front view of Munsell apparatus with sidesremoved to show Munsell patches on sliding arms.

length, and luminance but the results can be converted,if desired, to Munsell notation from existing conversicntables.

With this apparatus the threshold of colors perceivedto have zero gray content was found, roughly parallel tothe series of constant gray content. Above this threshold,at either higher luminance or purity, it was found thatthe colors seen were in the surface mode of appearancebut gave the subjective impression normally associatedwith colors that are known to fluoresce physically. It isnecessary therefore to define a new region of surfacecolors which contain no gray and appear to fluoresce.It is new to the writer that this region exists for alldominant wavelengths and purities and particularlythat one of the requirements is that the luminance of thecolor must lie near or below that of the surround for theappearance to present itself. Actually it was discoveredindependently by Newhall and Burnham during theseobservations that even samples perceived as achromaticalso appear to fluoresce for a short distance above theluminance of the surround and that substantially moreluminance than that of the surround was required forall colors before the surface mode of color perceptionchanged to the illuminant mode.

This, with other observations, will be discussed afterthe descriptions of apparatus and experimental results.It should be noted here that over a rather large part ofthis apparently fluorescent region the colors appearbrighter than the white surround, in many cases so muchbrighter that there is an unpleasant feeling of possibledanger to the eyes. After-images in roughly comple-mentary hues are also very strong indeed.

Like Stokes who invented the words fluorescence andfluorescent in 1852 and 1853,1 the writer is also led tosuggest the words "fluorence" and "fluorent" for thecorresponding appearance phenomena. The need for the

1 Shorter Oxford English Dictionary of Historical Principles,edited by J. A. H. Murray (Oxford University Press, New York,1933).

new words arises from the fact that colors that do notfluoresce may appear to, as did those of the presentinvestigations, and many colors that fluoresce physicallydo not appear to do so.2 In terms of the new words,fluorent perceptions do not necessarily require fluores-cent specimens for their production, and the fluorescentspecimens do not necessarily produce the fluorence im-pression. The words will be used in this article with thehope that they will clarify rather than confuse the sub-ject for the reader and because they call attention to thefact that none of the colors used for observation wasproduced by physical fluorescence.

DESCRIPTION OF APPARATUS

Munsell Apparatus

The apparatus for the Munsell study was designed toaccommodate all of the samples in a Munsell constanthue-plane at 7 value levels. Each of the seven slidingarmss hown in Fig. 1 was made to hold a cardboardstrip on which one-in. square Munsell samples at a singlevalue level were butt-mounted to produce a saturationscale graded in one-half chroma steps (Munsell renota-tion spacing) from a neutral gray to maximum availablechroma. A single knob was located below the constanthue-plane, as shown in Fig. 1. Each sliding arm couldbe moved laterally with the knob when the correspond-ing push button on the control box shown at the rightwas depressed. The control box in use was actuallyplaced to the observer's left and below the top edge ofthe table.

The sliding arms were covered, as shown in Fig. 1, bya surround containing seven circular apertures, in. indiam, so that only a single sample at each value levelwas exposed at a time. Strips such as that shown to theleft were removable and used to carry comparison sam-ples when needed (as in chroma determination). Thestrips, when used, had the same background value asthat of the surround. Beyond the central rectangularsurround, which was 12 in.X 16 in., a mat black surroundwas fastened that entirely concealed the rest of thesliding arms. The sliding arms were indexed on the backso the experimenter could record which Munsell sampleappeared in each aperture.

Four Macbeth 6500 K, 200-w lamps were arranged ina square bank, disposed around the observer's head to the

Table 1. CIE specifications of Macbeth 6500 K lamps.

CIELamp no. sourceCIE 1 2 3 4 Mean C

x 0.3093 0.3103 0.3156 0.3110 0.3115 0.3101y 0.3243 0.3238 0.3298 0.3242 0.3255 0.3163

2 Deane B. Judd, Color in Business, Science and Industry (JohnWiley & Sons, Inc., New York, 1952). On page 154 it is statedthat more than 60% of the TCCA color standards fluoresce de-tectably. Almost none of them appears to.

1050 Vol. 49

November1959 FLUORESCENCE AND GRAY

left and right, and tilted to illuminate the central sur-round area uniformly. Spectrophotometric curves of thefour Macbeth filters were obtained, and a calculationmade to determine at what color temperature each lampshould be operated to give the best approximation toCIE source C. Actual specifications of the four lamp-filter combinations, and the mean specification, aregiven in Table I and may be compared to the specifica-tion also given for CIE source C. Luminance of aMiddleton-Sanders3 barium sulfate white surround wasvery closely 50 ft-l. Because of its near-perfect reflec-tance, this surround was assumed to be of Munsell 10/value. Two other neutral surrounds were also used whichwere nearly 5/ and 2.5/ value (gray and black). Thevalue 5/ surround with no comparison samples is shownin use in Fig. 1.

Aperture Colorimeter

The apparatus shown in Fig. 2 for the threshold studywas essentially one that has been described in an earlierpaper.' The observer viewed a white baryta surround,21 in. high by 26 in. wide from a distance of 30 in. Thesurround was illuminated by an approximation to CIEsource A at 90 ft-l. The test field was located approxi-mately at the center of the surround, and consisted of arectangular aperture in the surround, 8 in. wide by 1 in.high, subtending a vertical visual angle of 20. Normalbinocular vision was used to view a calorimetric mixtureseen as filling the aperture. No chin rest was used. Theobserver simply looked at the test field, and varied itsappearance as instructed by rotating one of two knobs.

Filters were prepared for use in an integrating-barcolorimeter in such a manner as to provide variation inpurity at near-constant dominant wavelength and lumi-nance. Luminance could be varied independently bymeans of a carbon wedge and balancer. Rotation of oneof the calorimetric knobs produced variation from zeroto maximum available purity of the test color and rota-tion of the second knob produced a 100 to 1 range ofluminance of the test color. Luminance range could beextended by using neutral metal screens in the system.Nominal specifications of the test colors at maximumpurity are given in Table II.

TABLE II. Nominal specifications for CIE source Aof colors produced by filters used.

Kodak Wratten Filters#23A #12 #55 #44A #74B

Dominantwavelength (D) 605.5 583.6 524.1 491.9 452.7

Excitationpurity (Pe) 1.00 0.96 0.62 0.76 0.99

Colorimetricpurity (P,) 1.00 0.97 0.76 0.65 0.85

Luminance (Y) 0.36 0.81 0.25 0.10 0.0023

3W. E. K. Middleton and C. L. Sanders, Illum. Eng. Soc.(N. Y.) 48, 254-256 (1953).

4 Newhall, Burnham, and Evans, J. Opt. Soc. Am. 48, 976-984(1958). Figure 2 is taken from this article.

FIG. 2. Side view of aperture calorimeter (retouched to showsurround illuminator). The light source for the variable filterassembly is seen at the far right.

EXPERIMENTAL OBSERVATIONS

With the Munsell apparatus described in the fore-going, four observers made four sets of observations,each of which consisted of several series of observations,each made on white, gray, and black backgrounds, andeach repeated five times. For each observation, theexperimenter set a value 6/ sample of hue 5R, and theobserver selected samples according to one of the fourseries of observations: 1. constant chroma series forseveral chroma levels under the definition, "all colorsin the series are to be equally different from a gray ofthe same value"; 2. constant saturation series for severallevels under the definition, "all colors in the series areto appear to have the same hue content"; 3. constantcontrast with the background series for several levelsunder the definition, "all colors in the series are to havethe same contrast against the background"; 4. constantgray content series for several value levels of gray underthe definition, "all colors in the series are to contain anamount of gray equal to a sample displayed whileselecting the colors."

RESULTS

Munsell Apparatus

All results in these series were so consistent amongobservers that they could be averaged without appreci-able distortion. The results of this work are summarizedin the following for the four series of observations andare shown in the figures.

1. Chroma

As anticipated, the scales obtained for contrast withgray on a white background confirmed the Munsellchroma spacing with a slight deviation at high chroma.

CONTENT OF SURFACE COLORS 1051

RALPH M. EVANS

9

9

6

Chroma0 00 0 000 O - - - - -SOturaion

0 0 0 0 0 0 0 0o 0990 00 0 0

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0 0 0 O c E ° ° t ° o c 0 0 0 0 0 00o

,* & o 4 0o o 0

, 0

0 ~ 0 00 0 0 00 0

9

1 2 3 4 6 7 8 9 10 I I I2

Chromo

Vol. 46

00 0

- Chomo- --- - Soturotion

0 0 0 00

00 0 0 0 0

000o

1 2 3 4 5 6

Chromo

FIG. 3. Chroma and saturation as set by the observers, plottedusing actual instrument chroma and value as coordinates. Whitebackground (Va= 10/).

The results are shown by the solid lines of Fig. 3. In thisfigure Munsell value (V) is plotted against Munsellchroma (C). The actual samples of the instrument areplotted as small circles; the points on the curves are theaveraged observations. Perfect agreement with theMunsell spacing would have resulted in vertical straightlines.

On gray or on black backgrounds the results are verydifferent from the Munsell notations of the samples(Figs. 4 and 5), requiring far less purity as value de-creases, the effect being greater on the black backgroundthan on the gray. In other words the contrast with grayof a Munsell chip is greater on a gray or a black back-ground than its chroma notation indicates. In the ex-treme case (on black) for SR this difference was approxi-mately 4 chroma steps between the lightest and thedarkest sample on that background. Note that fromthe data it cannot be deduced how great the differenceis for the same sample when changed say from the whiteto the gray or black background. No such study wasmade. In this study both the comparison gray and thesample were on the same background and hence bothchanged.

From a somewhat different point of view these resultsindicate that when an observer is asked to select a colorthat has a given "difference from a gray of the samevalue" he sees more difference for a given sample when

FIG. 5. Same as Fig. 3 but black background (V=2.5/).

they are both on a black background than on a whiteone, and on gray the amount is intermediate. The effectincreases as darker samples are used.

2. Saturation

For this series no gray comparisons were used. Theobservers were shown a 6/ value sample and asked tofind colors at the other value levels that "contained thesame amount of hue." (V 6/ was chosen because it wasthe center of the 7 used.) This definition led to resultsthat were slightly, though significantly and systematic-ally, different from those for chroma. The results areshown by the dotted lines of Figs. 3-5. Since the differ-ences are very small, amounting to little over a singlechroma step, for most practical purposes the two defini-tions could be used interchangeably. (The curves crossat value 6/ because that is the point set by the experi-menter in each case.) The difference is, however, realand important for theoretical considerations because itcalls our attention to the possibility that more than onevariable is involved in the difference between a givencolor and a gray of the same luminance. In psycho-physical terms, less purity is required to match theamount of hue in another (lighter) sample than is re-quired to produce the same contrast with a gray of

9

9

8

6

8Chromn- ---- - Saluation

w,6

1U

0 0

0 0 0 0

4

4j. 0

0

1 2 3 4 5 6 7

Chrmo8 9 10 If 12

FIG. 4. Same as Fig. 3 but gray background (V=5/).

EXCITATION PURITY p

FIG. 6. Data of Fig. 3 (white background) plottedwith excitation purity as abscissa,

1052

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0 .04 .08 J2 36 .20 .24 .28 .32 .36 AO 44 .4- . . .

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November1959 FLUORESCENCE AND GRAY CONTENT OF SURFACE COLORS 1053

2 S 4 5 6Chromo

* * 10 I 1 12

0 .04 .08 .12 .16 .20 .24 .28 .32 .36 AO .44 .48 FIG. 9. Same as Fig. 8 but gray background (V=5/).EXCITATION PURITY e

FIG. 7. Data of Fig. 5 (black background) plottedwith excitation purity as abscissa.

matched luminance; i.e., purity is not the only variableinvolved in the difference from the gray.

The really striking result obtained from these datais seen, however, only when purity is used as a variablerather than chroma. It is then found that purity, whichon a white background is approximately related tochroma by the equation C= ap0V (where a is a constantdepending on hue and pc is clorimetric purity), on ablack background became the only variable involved.The results are plotted in Figs. 6 and 7 where the chromascales of the previous figures are replaced by those forexcitation purity. Figure 6 shows the series for the whitebackground with the solid line for the chroma seriesand the dashed lines for the saturation series. Figure 7shows the series on the black background. To the extentthat the lines are vertical, saturation is independent ofvalue. It should be recalled also that luminance is con-stant for constant value. We can say, then, for thishue (5R) under these conditions on a black background,that two colors that have the same saturation also havethe same excitation purity independent of luminance.Since for a given dominant wavelength, clorimetricpurity is constant when excitation purity is constant,the same statement may be made about clorimetricpurity. (For the papers of this instrument the dominant

6

I 0 0 0 0 0 0 /

0 0 0 0 0 0 0 0 0 0 0

. 0 0 0 0 0 0 0 0 0 00 0 0

0 0 a 0 0 000 ~0 0O0 0 .000

* 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0/00 0000

O 0

FIG. 8. The samples along any curve were judged to have equalcontrast with the background. White background (V= 10/).

wavelength is approximately 605 and clorimetric andexcitation purity are only slightly different in any case.)

3. Contrast with Background

It was found that when a color was arbitrarily set atvalue 6/ other colors could be selected from other valuelevels that would have the same contrast with the back-ground. These curves form regular series, having differ-ent shapes on the three backgrounds, and are shown inFigs. 8-10. It can be seen that they may be describedroughly as figures of revolution around long axes whichcoincide with that for the value of the surround. Thismeans that for colors of either higher or lower luminancethan the surround less purity is needed for the samecontrast than when the color has the same luminanceas the surround. This result is, perhaps, not surprising;it indicates that relative luminance and purity are inter-changeable (in proper amounts) in producing contrastwith a background. The data show another effect, how-ever, that is quite important for the present study. Therelationship of the shape of the curves to the back-ground value changes appreciably for higher purities onthe black background. These colors act as though thebackground were lighter. It is consistent with the hy-pothesis which will be used later in this paper that thesurround sets the adaptation level of the eye only aslong as it is brighter than the stimulus color. If thestimulus color is brighter than the surround, it tends to

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6 - 0 .0 0 0 0 0 . 0 00 .4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 /

2 3 4 5 6 7 8 9 10 1I 1 2

Chromo

FIG. 10. Same as Fig. 8 but black background (V=2.5/).

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15 RIALPH Ai. EVANS

take over the adaptation and raise it to higher levels.This assumption was used by Judd5 in determining his"lightness index."

4. Gray Content

In the settings for these serieswere used, including the author, ato the observations completelywhat we meant by "gray content,were (a) that gray is a continuspace, varying from zero to a maas black, (b) that gray is a mixtutions of white and black, and (cmedian gray, a white, and a blacreases both toward white and to

To allow for these concepts theseries of colors that contained theas an achromatic sample of givefront of the apparatus throughoutthe question of whether this wasamount of gray than some othermatic series was not raised. Thethe same amount. In spite of thproach, we all came out with resuthat they are again averaged.

In this experiment gray stripsjacent to all the openings of the3 in. wide were mounted besideinches to the left. For the white stwere used. For the gray and blackwere mounted on another white on all sides. The observers weregray strips, to find all the colors t]same gray content. Since gray dmum at zero chroma down to thseries, this was possible to do quilevel provided there were enougcourse, impossible at value levelsgray, since for these all samples cc

9/

0 00 0 0

0 6/

* 00 0,6 0 o 0 0

QO0 0 0 0 0 0

.0

I 2 3 4 5 6

Chromo

FIG. 11. Samples along any curve vcontent equal to the value indicated atWhile background.

6 Deane B. Judd, J. Research Natl. ](1940).

only three observersnd each of us broughtdifferent concepts of." Very roughly theseous variable in colorLximum which is seen.re in varying propor-c) that there exists ack and that gray de-waTrl hlacL

r oo o9/

00 00 0 0 o O o

o o o o o o o 0 0 o

0 0 o o 0 0 0 0 0

0000 0 000 o

0 0 0 0 o

I 2 3 4 5 6 7 a 9 10 I 2

Chromo

FIG. 12. Same as Fig. 11. Gray background.

task set was to find a results for six levels of gray are shown for the threesame amount of gray backgrounds in Figs. 11-13. For each curve the value

i V displayed on the level of the comparison gray is given by the numberthe setting. Note that on the curve.

the same or a different It is seen that for the white background the result ismember of the achro- a series of essentially parallel straight lines. The turningcolors were to contain down at the ends of the curves is thought due in part to.ese differences of ap- the failure at high chromas of the Munsell notation usedlts so closely the same for this set of colors as shown in Fig. 3. The ends of the

curves at zero chroma coincide with the value of thelong enough to be ad- comparison gray as would be expected. The results plotinstrument and about as about equally good straight lines if excitation puritye the openings a few is used rather than chroma as the abscissa. The resultsirround the strips only indicate that decreasing either value (luminance) orbackgrounds the strips chroma (purity) of the color increases the gray contentstrip about 1 in. larger of the color perception. A change of one value step isasked, for each of six seen to be equivalent to some 3 or 4 chroma steps forhey could that had the these colors.ecreases from a maxi- The gray content changes according to the value scalehe end of each chroma (since the curves are parallel) and so it seems reasonablete easily at each value to extrapolate the series to value 10/. Since value 10/ ish samples. It was, of a very pure white not seen as containing any gray, thishigher than that of the extrapolated curve would be the locus of colors not con-)ntained less gray. The taining gray. If such a curve is imagined one step above

the top curve, it will be seen that this still leaves a largearea of the V-C diagram unaccounted for. At chroma 8,for example, there would be two value steps from 8 to 10lying above the line of zero gray content. The question

00 naturally arose as to what the perceptions of the colorsin that region would be like. Before considering it, how-ever, it is necessary to look at the results on the grayand black surrounds.

\ Figure 12 shows the results when the gray surround,ON 0 o o o was used and Fig. 13 when the black was used. A com-

parison of the results indicates the rather surprising factthat the gray background dropped the gray content of

.I .2 the colors by about 2 value steps but that the black7 9 10 ,1 2 background dropped it only slightly further. The curves

'cre judged to have a gray remain essentially parallel. These results are believedthe beginning of the curve. heavily dependent on the particular setup for this ex-

periment. Changing the value of the surround from 10/

3ur. Standards 24, 293-333 to 5/ has dropped the gray content 2 value steps, whilechanging the background from 5/ to 2.5/ produces little

9I

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1054 Vol. 49

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November 1959 FLUORESCENCE AND GRAY

further effect. The comparison gray, however, was al-ways surrounded by a relatively narrow white border.It is felt that this white border was not sufficient tomaintain the gray content of te comparison gray againstthe much larger area of the surround and, furthermore,that the presence of this white prevented the lowerbrightness of the surround from controlling the eyeadaptation. In any case, here again the backgroundacted as though it were much lighter than it actually was.

Some years ago the writer published6 the statementthat gray appeared to be the perception of relativeluminance. Since luminance is constant along any hori-zontal line in these diagrams, this statement is obviouslynot correct except for achromatic colors. It seemed im-portant, therefore, to find out what lay above the lineof zero gray content in an effort to learn more about theperception of gray. It was largely for this purpose thatthe aperture colorimeter was assembled.

Aperture Colorimeter

With the equipment described it was possible, overthe range of purity of existing Wratten filters, to in-vestigate the whole region from V=0 to well over 10(luminance match to the background) and all colori-metric purities from 0 to the maximum for the filterused. For many regions of the spectrum it is possibleto obtain purities of unity or only slightly below.

To show the magnitude of the area being investigatedand for comparison with later results it is instructiveto consider the gray content series plotted on a V-p 6diagram. This is shown in Fig. 14 for the white surroundonly. It is seen that a large percentage of the colors inthe range from V= 0/ to V= 10/ and pe= 0 to p,= 1 arenot represented. It is known, of course, that not allthese colors are possible with nonfluorescent pigments,although a considerable part at least are theoreticallypossible if fluorescence is introduced. It was not knownwhether all colors in this range would appear in thesurface mode, although it seemed likely since all therelative luminances would be unity or less.

When this region was explored it was found, as statedabove, that all colors in this region took on the appear-

9

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4'Ralph M, Evans, J. Opt. Soc. Am. 39, 774-779 (1949).

to

9 9/

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45 X / X

3 ,

3 X X X

.10 .20 .30 .40 .50 .60 .70 .80 .90 L.0

EXCITATION PURITY Pe

FIG. 14. Data of Fig. 11, plotted with excitation purity as abscissa.

ance normally associated with fluorescent materials, orto use the new word, were perceived as fluorent. It wasfound possible, in fact, for all the observers to makesettings such that the color observed was perceivedneither to contain gray nor to show fluorence. With theapparatus used and the inexperience of the observerswith this new kind of observation, the precision ofsetting was poor and there were quite large differencesbetween observers. Since time was not available to im-prove this precision, it was thought better to investigatethe whole field in a qualitative manner to learn thenature of its boundaries rather than to try to locatethem accurately. Thus, the data that follow should beconsidered as descriptively rather than quantitativelyaccurate. There were no inconsistencies in the data forthe different observers, and there is no question butthat the data describe the nature of the phenomenainvolved.

In addition to this threshold between grayness andfluorence, it was also found possible to determine threeother loci. If a gray patch was present near the aper-ture, the colors containing the same amount of graycould be found. The luminance point could be found foreach purity at which the appearance changed from thesurface mode to the illuminant mode. Finally, and mostsurprising at some purities, it was found possible tomake the usual heterochromatic brightness match be-tween the color and the surround.

All four of these loci were widely different and differ-ent for different hues but always came in the same order.That is, with increasing luminance for a given puritythe four thresholds were passed in the order, same graycontent, zero gray or fluorence, brightness match, andilluminant mode. The results for the author's eye forfive filters are shown in Figs. 15 through 19.

6 2 3 4 5 6 7 e 9 0 I I ISChrOmo

FIG. 13. Same as Fig. 11. Black background.


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RALPH M. EVANS

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COLORIMETRIC PURITY PC

FIG. 15. Aperture colorimeter data for dominant wavelength452.7 my. White surround. Curve IL=illuminant mode threshold;Curve B="Brightness" match with surround; Curve FL=zerogray (or fluorence) threshold; Curve Gy=constant gray con-tent = 6/; Curve MA L = MacAdam limit curves for the dominantwavelengths shown.

Neglecting for the moment the line on each markedMAL, we may interpret them as follows. Each diagramshows value (V) plotted against colorimetric purity (,)for a given dominant wavelength as shown. At zerovalue all colors appear black at all values of p,. As valueor luminance increases along any purity ordinate, blackquickly changes to gray, which then decreases until theamount present appears to match that in a value 6/achromatic sample on the same background. Thesecurves are marked Gy. Continuing on to higher valuesa point is reached at which gray disappears from thecolor perception. This is the zero gray (or fluorence)threshold marked FL. Above this a point is reached(in some cases a very short distance, in others quitelong) where it is possible to decide that the brightnessof the patch equals that of the surround. These curvesare marked B. Some distance above this, at a pointrather widely different for different observers (but al-ways above) is a poorly defined point at which theappearance changes from that of a surface color to thatof a luminous aperture (illuminant mode threshold).These curves are marked IL. From the curve represent-ing zero gray content (FL) to the curve representing achange from the surface mode (IL) all colors are per-ceived as fluorent (have the characteristic appearanceof fluorescent materials). Above this line they not onlydo not have this appearance but also are perceived as

rapidly decreasing in saturation. In most observationsthere is a range on the neutral axis above V= 10 wherethe white is fluorent before the IL threshold is reached.

MacAdam Limits

It was mentioned earlier that it is known that thereare limits to the purity and relative luminance that canbe obtained with real pigments. These are known as theMacAdam Limits7 and are shown on Figs. 15-19 as thelines marked MAL. It is apparent that they do not inany case coincide with the zero gray loci, although theirgeneral shape is the same and they show the sametendencies with change in dominant wavelength. Thelow precision of the data make it not impossible thatthey coincide but a number of considerations make itappear unlikely. Among others, the MAL curves haveto pass through V=0 when pc= 1.0; it is unlikely thatthe FL curves do. These MAL lines represent colorsthat can be exceeded for purity or relative luminanceonly by fluorescent materials. It is apparent that if thelines lie below the FL threshold the colors lying betweenthem and FL will not show fluorence even though theyfluoresce. If the MAL lies above the FL locus, the colorsbetween the two will be perceived as fluorent eventhough the specimens do not fluoresce.

Exploratory Experiments

To check certain hypotheses that had been suggestedto explain these results, a number of small experiments

U,

-'6

.1 .2 .3 .4 .5 .6 .7

COLORIMETRIC PURITY C

.8 .9 1.0

FIG. 16. Same as Fig. 15 but dominant wavelength 491.9 mL.

I D. L. MacAdam, J. Opt. Soc. Am. 25, 361 (1935).

1056 Vol. 49

7vember 1959 FLUORESCENCE AND GRAY

were run to determine the effect of some of the variablesthat had been held constant. (1) The aperture was de-creased to a ° square. (2) The surround was madecomplementary to the aperture color at high purity.(3) The surround was made the same color as the aper-ture at high purity. (4) A low luminance surround levelwas used. (5) A deuteranomolous observer (error score12 on the AO Charts) was used for colors of the threedominant wavelengths 524.1, 583.6, and 632.7. The re-sults may be described as follows.

1. With the small aperture the heterochromaticmatching was somewhat more nearly the same as aluminance match at high purity. Contrary to expecta-tion, the change to illuminant mode occurred consis-tently at somewhat lower luminances. It had been ex-pected that the surround would control more completelyand hence would hold the surface mode to higher in-tensities. The FL thresholds were the same within theprecision of the data.

2. For the complementary-colored surround neitherthe IL nor the B thresholds were changed appreciably,and the FL threshold could have been an extension ofthat obtained with the white surround, i.e., acted asthough the colors had purities higher than 1.0.

3. When the surround was the same color, the IL andB thresholds remained much the same but the FL locuschanged completely. The aperture appeared pink ormagenta at all purities much lower than that of thesurround, and fluorence appeared in this pink increasingin strength as purity was decreased, i.e., acted likepurities less than zero. The results are shown in Fig. 20.

9

7

4j6

5

4

3

2

.2 .3 .4 .5 .6 .7


12 8.m

10 ' 9

- 6

4

3

2

.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0


FIG. 18. Same as Fig. 15 but dominant wavelength 583.6 mu.

4. The results for low surround luminance were es-sentially the same with respect to this luminance as atthe higher level.

12

90

9

a

7

5

4

3

2

.8 .9 1.0 .2 .3 .4 .5 .6 .7

COLORIMETRIC PURITY C

.8 .9 9.0

FIG. 17. Same as Fig. 15 but dominant wavelength 524.1 mp.

6 05.5m,

I

Gy


FIG. 19. Same as Fig. 15 but dominant wavelength 605.5 m,4.

RALPH M. EVANS

12 52 4 1 m

II IL

10

9

-J 6

P N

4

3

j1 .2 .3 .4 .5 .6 .7 .S .9 1.0


FIG. 20. Aperture colorimeter data for surround and sampleboth of dominant wavelength 524.1 myt and surround at maximumpurity (0.76) throughout. Samples appeared pink to left of neutraland fluorescent to left of FL.

Note that changing surround luminance is the samething relative to the aperture as putting a Munsell sample

on a different neutral background. The fact that noneof the loci changed relative to the background can be

interpreted as meaning that if the gray or black sur-round had controlled adaptation as completely as didour aperture surround, gray would have been removedfrom the Munsell colors in an amount equal to thedifference in values of the backgrounds; i.e., from 10/

to 5/ gray content should have decreased five valuesteps and 2.5 more for the black.

5. The results for the deuteranomolous observer were,

within the variability of the normal observers, the samefor these three filters and his description of the appear-ances were the same. His variability may have been

somewhat higher.

GENERAL CONSIDERATIONS

It is to be regretted, of course, that time did not

permit refinement of these data so that the results withthe aperture instrument would have the same fairlyhigh precision as those from the Munsell instrument.

On the other hand it had become apparent toward theend that much more control of the way in which anobserver looked at the patch would be necessary to obtain

this result. An observer could vary his setting for a

particular threshold considerably by looking directly atthe patch steadily, by looking steadily to one side or by

looking back and forth from surround to color. Thesewould need to be investigated before good precisioncould be attained. It is likely, in fact, that some flash

exposure technique such as was used in an earlier study8

to prevent change of adaptation would be necessary.The facts already uncovered, however, give much foodfor thought.

The writer, after much thought and study (the ex-periments were completed over a year ago), has notbeen able to find any reasonably simple relationshipbetween the results and the psychophysics of the stimuli.The results, however, are consistent with some suchhypotheses as the following.

If the effects of all wavelengths are represented by theluminosity curve and may be summed, then it is to beexpected that it is possible to set the B locus since thisrepresents the usual heterochromatic brightness matchwith the surround. The fact that the required luminancefalls off at high purities is in line with other recent work,

notably that by MacAdam,9 by Wyszecki and Sanders, 0

and by Breneman."1The fact that the surface mode of appearance persists

up to luminances considerably higher than that of thesurround can be explained if it is assumed that thebackground holds the eye adaptation level because of itsmuch greater area and it is postulated that the surfacemode will appear for all colors at or below eye adaptationlevel. There is considerable evidence that this is a

reasonable assumption. The loss of saturation abovethis level was to be expected. There is considerable evi-dence that colors are perceived at maximum saturationat approximately the same luminance level as thesurround.

There remain the questions of grayness and fluorence.The similarity of the threshold in a first order sort ofway to the MacAdam Limits is suggestive, and con-sideration of the way in which they are calculated leadsto a rather definite hypothesis which it is not possibleto check at the present time. If it is assumed that theeye has three receptors (the argument would be thesame for more) and that the sum of the responses ofthese is luminance, then for any given color each indi-

vidual receptor is responding less than this sum. Sincethe color does appear of a definite hue, however, one

(or possibly a pair) of the receptors is responding muchmore than the rest. The hypothesis is that when this

receptor is, or these receptors are, stimulated to the same

level as that produced by the surround, the zero gray

threshold is reached. Above this, to the point where theluminance of the patch starts to control the eye adapta-tion, colors are seen in the fluorent mode and below thisdown to black are seen with a series of gray components.

8 Burnham, Evans, and Newvhall, J. Opt. Soc. Am. 42, 597-605(1952).

9 D. L. MacAdams, J. Opt. Soc. Am. 40, 589-595 (1950).10 C. L. Sanders and G. Wyszecki, J. Opt. Soc. Am. 47, 398-404

(1957). G. Wyszecki and C. L. Sanders, ibid. 47, 840-842 (1957).11 E. J. Breneman, J. Opt. Soc. Am. 48, 228-232 (1958).

Vol. 491058

November1959 FLUORESCENCE AND GRAY

On this hypothesis the appearance of fluorescence(fluorence) is due to an excess of stimulation of thatreceptor above the state to which it is held by luminanceadaptation to the surround. Gray is then due to lack ofstimulation of this receptor compared to the surround.(It might almost be called negative fluorence.) In otherwords, to paraphrase the statement of the earlier publi-cation, 6 fluorence is the perception of the relative lumin-ous stimulation of the most stimulated receptor whenthe stimulation is higher than that due to the surround(or adaptation level), and gray is the perception of thisrelative stimulation when it is lower than that of thesurround (or adaptation level). For the achromatic colorsthe stimuli are the same when the luminances are thesame and hence, for this case only, we may write thatgray is the perception of relative luminance. Perhaps itis overstressing the point to note again that the percep-tion of gray and now also of fluorence has no counterpartin the stimulus per se. They arise in the relationship ofthis stimulus to the surround or the adaptation condi-tion of the observer's eyes. It is for this reason thatneither of them can be seen in homogenous unrelated(i.e., single with dark surround) fields.

The point of view may be made a little clearer byconsidering a particular case. For the Wratten No. 23Afilter (XD=605.5) the luminance (Y) is 0.11 and colori-metric purity is 1.00 (Fig. 19). Almost all of the lighttransmitted by this filter lies in roughly one-third of thespectrum. In the instrument this filter is paired withan achromatic one of the same luminance. When thisachromatic filter is in the beam and the luminance isadjusted, a perfect match can be obtained with thesurround, which is both a visual and a physical match.Now as the No. 23A filter is gradually introduced inplace of the neutral there is no change in luminance butthe spectral component corresponding to the red filtertransmission of the beam increases steadily above thecorresponding spectral component of the surround. Inaccord with the hypothesis such colors should all befluorent and in fact they are. At some lower luminancea higher purity would be necessary to make the mostexcited receptor receive the same stimulus as from thesurround, and so the point will move downward and tothe right. It would follow from this line of reasoningthat colors of different dominant wavelengths shouldhave quite different curves and this also is found. If theactual eye receptor sensitivity curves were known infact, the curves could be calculated quite easily and thehypothesis checked. Since the CIE distributions are notlikely to be those of the eye, the hypothesis probablycannot be checked by CIE computations, but, as amatter of interest, for several of the filters the ratios ofthe X's or Z's (depending on the color) were calculated.They were found not to agree, as expected, but did showthe same general sort of agreement as is shown by theMacAdam Limits in the figures; i.e., low in the blue,

high in the yellow, and straight and moderately low forred. The ratios did show positive values for pc= 1 andin this sense are a closer fit than the MacAdam Limits,but the data are not sufficiently precise for a test of thehypothesis. It appeared likely that from true distribu-tions good agreement might have been obtained. Itmight not be amiss to suggest that the reverse may alsobe true. It might be possible to deduce the true sensi-tivity distributions if sufficiently precise data on the FLlimits could be obtained with monochromatic light.

There still remains the question of brightness. It wasnoted earlier that the fluorent colors of high luminanceand purity had in all cases an extraordinary dazzlingbrilliance, which must be seen to be believed. The OSAhas defined brightness as the psychological correlate ofluminance and, in fact, even under the conditions usedhere a characteristic of the perceived color could be setcorresponding to this definition. The fact remains thatwhile this setting was being made the colors at high purityoften appeared so "bright" compared to the surroundas to be uncomfortable. We have no word for this kindof "brightness" and no psychophysical correlate, exceptas one is suggested by the hypothesis above. The con-clusion is inescapable that there are two kinds of bright-ness, one of them due to luminance or something ap-proaching it, and perhaps setting adaptation levels, andthe other, perhaps identifiable with what has been calledfluorence in this paper and perhaps due to the excessstimulation of a color receptor above its adaptationlevel. The writerprefers to leave the matter at this point,but no one who has seen a light almost too dazzling forthe eye to stand comfortably, knowing that it is of lowerluminance than a comfortably white surround, can be-lieve that luminance is an even approximate correlate,for colors of high purity, with what is called brightnessin ordinary speech.

It might be well to point out one further fact thatseems clearly indicated by this work. Although thedomain of surface color perceptions seems to be three-dimensional in the sense that hue, saturation and light-ness are sufficient to locate a color perception in perceivedsurface color space, they are not really adequate todescribe the perception. To do this it is necessary to statealso where the perception lies along the gray content-fluorence axis. It is this fact, perhaps, which has causedso much difference of opinion among people who haveworked with perceptual color order systems.

ACKNOWLEDGMENTS

It is a pleasure to acknowledge the important assis-tance in this work given by Dr. Sidney M. Newhall,Dr. Robert Burnham, and Miss Ruth Kanis, all ofwhom helped on all parts of the problem and acted asobservers in obtaining the data. The manuscript hasbeen much improved by the kind assistance of Dr.Deane B. Judd and Dr. David L. MacAdam.


Documents

Fluorescence and Gray Content of Surface Colors