Spectral Sensitivity of the Fovea III Heterochromatic Brightness and Chromatic Adaptation

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Spectral Sensitivity of the Fovea. III. Heterochromatic Brightnessand Chromatic Adaptation

LEO M. HURVICH AND DOROTHEA JAMESONColor Technology Division, Eastman Kodak Company, Rochester, New York

(Received October 23, 1953)

A series of experiments is reported that extends an earlier study of the foveal threshold luminosity func-tion and its dependence on chromatic adaptation. Heterochromatic brightness matches were made by alarge-step, equality-of-brightness method at three luminance levels: 10 mL, 20 mL, and 40 mL. Results arecompared for chromatically neutral white, and blue, green, yellow and red adapting-surround fields at threelevels of adapting luminance: 10 mL, 20 mL, and 40 mL.

The results for suprathreshold photometric equations confirm the dependence of foveal luminosity onchromatic adaptation demonstrated earlier for threshold sensitivities. The foveal (1°) luminosity functionis shown to undergo regular and consistent changes with changes in chromatic adaptation and to be furtherdependent on the ratio of surround/standard luminance. Implications of these results for the CIE systemare touched upon.

INTRODUCTION

W E have demonstrated, in an earlier study, thatthe spectral sensitivity of the 1 fovea is de-

dependent on the chromatic adaptation of the visualsystem. In that study, spectral sensitivity was meas-ured by the reciprocal of the minimal perceptiblestimulus energy, and controlled changes in the stateof chromatic adaptation were found to produce con-sistent and significant changes in the form of thethreshold luminosity function. Blue and green adapta-tions resulted in a relative loss in threshold sensitivityto the shorter-wavelength stimuli, whereas yellow andred adaptations reduced sensitivity to the longer-wave-length spectral regions. The experimental resultsprovided a strong basis for the conclusion that themechanism responsible for the luminosity, or simplebrightness, response cannot be entirely independentof the physiological processes responsible for chromaticsensations.*1 4

This conclusion is of both theoretical and practicalimportance and immediately raises a number ofquestions concerned with the generality of the ex-perimental findings. The study reported here conse-quently extends the area of investigation to examinethe influence of chromatic adaptation on photometricmatches (direct heterochromatic brightness com-parisons) at suprathreshold levels.

APPARATUS AND CALIBRATION

The apparatus used in this study is essentially thesame as that used in our previous investigation andhas been described in detail in an earlier report. With

' D. Jameson and L. M. Hurvich, J. Opt. Soc. Am. 43, 552(1953).

* A brief review of the pertinent literature, and a discussion ofthe experimental findings in relation to color theory have beenpresented in references 1 and 2. To the references cited there,references 3 and 4 should be added.

2 L. M. Hurvich and D. Jameson, J. Opt. Soc. Am. 43, 485(1953).

R. Siebeck, Z. Psychol. 41, 89 (1907).E. Engelking and F. Poos, Z. Psychol. 56, 22 (1925).

this apparatus, as arranged for the present experiments,the observer sees a slightly elliptical, bipartite fieldapproximately in the center of a large illuminatedsurround. The central, bipartite field is 1X0.8 invertical and horizontal diameters, and the diameterof the surround is about 370. The wavelength com-ponents of the bipartite field are independently variableand are controlled by Farrand monochromators, andthe radiant flux of each of these stimuli is independentlycontrolled by calibrated neutral density Wrattenfilters and wedges at the exit slits of the two mono-chromators. The wavelength composition and radiantflux of the surround and adapting illumination arecontrolled by a double monochromator of zero dis-persion. The exposure duration of the standard andcomparison stimuli in the bipartite field was controlledby a calibrated photographic shutter.

The energy and wavelength calibrations have beenfully described. The instrument settings for the lowest(= 10 mL) luminance of the 550-mju standard stimulusand of the different adapting-surround fields weredetermined by binocular photometric matches usinga Macbeth illuminometer. The settings for the twohigher luminances (=20 mL and 40 mL) were madein terms of calibrated neutral density filters and wedgepositions, and were also checked by photometricreadings with the Macbeth instrument.

METHOD AND PROCEDURE

Two observers made monocular, equality-of-bright-ness matches between the two halves of a foveallyfixated, 1, bipartite field seen approximately in thecenter of a 370 circular surround and adapting field.Direct brightness comparisons were made between fivepairs of stimulus wavelengths. The standard stimulus,which filled the right half of the bipartite field, wasfixed at 550 m/u throughout the entire series of com-parisons. The comparison wavelengths, seen in theleft half of the bipartite field, were 450 mis, 500 mu,550 myu, 600 mit, and 650 m/. The choice of a direct

213

VOLUME 44, NUMBER 3 MARCH, 1954

L. M. HURVICH AND D. JAMESON

comparison technique with large wavelength intervalfor the heterochromatic brightness measurements wasmotivated, at least in part, by the fact that suchdirect visual comparisons of differently colored stimuliare most common in everyday experience. For a neutralstate of adaptation, the standard appeared yellow-green to our observers, and the hues of the comparisonstimuli were, approximately, reddish-blue, green,yellow-green, reddish-yellow, and yellowish-red. Thesehues, of course, suffer striking modifications withchanges in chromatic adaptation.

Brightness equations were made to the 550-mustandard at three standard luminances, namely, 10 mL,20 mL, and 40 mL. All equations were repeated forfive different adapting chromaticities and for threedifferent adapting luminances. The adapting conditionsinclude the bright-adapted neutral state (white), andbright-adapted chromatically tuned states induced byfour chromatic adapting and surround stimuli. Theadapting-surround stimuli are perceptually uniquein hue to the given observer: a blue, a green, a yellow,and a red. The three levels of adapting luminance were10 mL, 20 mL, and 40 mL. For observer H the wave-length bands of the chromatic fields were centered atapproximately 468 mu, 489 m, and 589 m. Theextra-spectral binary red for this observer is comple-mentary to 489 mg4 for a neutral state of adaptation.The values for observer J are 475 m, 498 m, and579 myu. The extra-spectral binary red for this observerwas complementary to 498 m. These values are thesame as those used in the threshold study and therationale underlying the use of chromatic adaptingfields that evoke pure hue sensations that are alsoinvariant with intensity (i.e., no Bezold-Brticke hueshift) has been discussed earlier. The method for theselection of such chromatic stimuli for each observerhas also been presented.' The procedure used to deter-mine for each observer an absolute white (chromaticallyneutral) adapting stimulus, that satisfies the condi-tions of both perceptual and physiological neutrality,has been given in detail in previous reports.2

In a single experimental session, direct brightnesscomparisons were made for the fifteen standard andcomparison stimuli (five wavelength pairs, three lumi-nances) for a single adapting luminance and chromatic-ity. Each experimental session began with a 10-minutepreliminary period of dark adaptation followed by a 5-minute adaptation to the specified adapting field.This field was present as an illuminated surroundthroughout the experiment, and was continuouslyviewed by the observer between test exposures. Upona signal from the experimenter immediately prior toa test exposure, the observer fixated the center of the10 dark area located approximately in the center of the370 surround. Following each exposure of the bipartitefield within this 1 central area, the observer reportedhis brightness judgment, and again fixated somewhereon the surround field until another preparatory signal

was iven. The duration of the test exposure wasabout 1.3 seconds.

A modified method of limits (minimal change) wasused to determine the point of subjective brightnessequality. With the radiant flux of the 550-mg standardfixed at one of the three predetermined levels, the fluxof the comparison wavelength was varied in successiveexposures of the bipartite field until the observer re-ported no difference in brightness. For each brightnesscomparison in a single experimental session, the radiantflux of the comparison wavelength was varied in oneseries of ascending and one of descending stimuluspresentations. The two values for a given brightnessequation resulting from the darker-to-equal and thebrighter-to-equal series were recorded as separatemeasurements. A second set of values for the samestimulus pairs and the same condition of adaptationwas determined at a later date. The average relativeenergy value for each match is based on the four in-dividual measurements thus obtained, and the measureof variability about each mean is also based on thefour individual settings. The root-mean-square devia-tions consequently express the combined variabilityof the within-session measurements for the ascendingand descending series of stimulus presentations, andthe variation of the brightness equations from oneexperimental session to the next.

Results for the five adapting chromaticities and thethree levels of adapting luminance were obtained inseparate experimental sessions, and were taken inrandom sequence. The sequence of wavelength pairsand standard luminances was varied at random withina single session for a single state of adaptation.

REStJLTS

The difficulties for purposes of exact heterochromaticphotometry of using the "large-step" equality-of-brightness method are well known. Although themethod has been variously criticized as insensitive,erratic, unreliable, etc.,' 8 a number of investigationshave demonstrated the usefulness of the method.914Our own experimental results confirm the usefulnessof the method and make it clear that reasonably con-sistent measurements of luminosity can be made evenby direct comparison of spectral stimuli that differas much as 50 to 100 mu in wavelength.

5 H. v. Helmholtz, Jlandbuclz der Physiologischen Optik (L. Voss,Hamburg and Leipzig, 1896), second edition, p. 440.

6 H. E. Ives, Phil. Mag. 24(6), 149 (1912).7 W. W. Coblenz and W. B. Emerson, Natl. Bur. Standards

(U. S.) Bull. 14, 167 (1918).8 K. S. Gibson and E. P. T. Tyndall, Natl. Bur. Standards

(U. S.) Sci. Papers 19, 131 (1923-24).9W. Abney and E. R. Festing, Trans. Roy. Soc. (London)

177, 423 (1886).10 A. Konig and R. Ritter (1891). In K6nig's Gesammelte Ab-

Ihandlungen zur pysiologisczen Oplik (J. A. Barth, Leipzig, 1903),p. 144.

"E. Gruber, Phil. Studien 9, 429 (1894).12 H. S. Langfeld, Z. Psychol. 53, 113 (1909).13 L. L. Sloan, Psychol. Monog. 38, No. 1, 1 (1928).14 W. D. Wright, Researches in Normal and Defective Colour

Vision (C. V. Mosby Company, St. Louis, 1947), pp. 75 ff.

214 Vol. 44

SPECTRAL SENSITIVITY OF FOVEA

On the average, the variability of the heterochromaticbrightness matches increases with increasing wave-length separation between the two stimuli being com-pared. This result is not surprising, and has beenreported by others.8 1

2 5 For observer H the averagestandard deviation for the 550-mui to 550-mg brightnessmatch is 8.2 percent, and the average for all the hetero-chromatic wavelength pairs for all conditions is 19.6percent. The average values for observer J are 10.5percent for the 550-mA pair, and 12.5 percent for theheterochromatic comparisons.

Precision measures of various magnitude have beenreported in the literature for equality-of-brightnessmeasurements. Knig's average value for normalobservers we infer to be generally somewhat largerthan 5.8 percent.'6 Ives' results show precision errorsranging from 1 percent to 25 percent, with the averagefor five observers and all wavelength comparisonscoming to about 5 percent. Kohlrausch has reportedthe probable error of a single observation as about10 percent'8 and in a later paper he presents valuesfor two observers as 2.4 percent and 2.3 percent.'9 Asmeasures of consistency (reproducibility) Ives reportsvalues ranging from 1 percent to 7.5 percent for twoobservers,6 and, most recently, Schiess has reportedreproducibility errors as high as 38 percent.2 0 Schiessalso cites Dresler's data which, in specific cases,exhibit values as high as 40 percent.2 '

A number of factors might be expected to yieldrelatively high variability in our experiments. First,the number of measurements is small (N=4). More-over, since the four individual measurements on whichboth means and root-mean-square deviations werecalculated were measured in pairs on separate days,within-session and session-to-session variability aredeliberately combined. Thus, the standard deviationis, in a sense, a composite expression of precision andconsistency. Furthermore, the method used in theseexperiments to obtain an "equal-brightness" measure-ment, whereby the experimenter alternately reduced orincreased the radiant flux of the comparison stimulusuntil it was no longer judged brighter or darker thanthe standard, probably means that two values ob-tained at the upper and lower limits of the interval ofuncertainty are averaged. This procedure under-standably produces a greater scatter than would beobtained by permitting the observer to bracket abrightness difference between two test fields. Moreover,the short (1.3-sec) duration of stimulus exposure usedto maintain a controlled state of adaptation may alsoinfluence the magnitude of the variability.

This deliberate use of procedures that maximize

5 L. T. Troland, Psychol. Rev. 25, 305 (1918).6 Reference 10, p. 166 and p. 184." Reference 6, Figs. 2 and 6, Plate III.18 A. Kohlrausch, PflUgers Arch. ges. Physiol. 200, 210 (1923).19 A. Kohlrausch, Das Licht 5, 259 and 275 (1935).20 J. Schiess, Optik 9, 312 (1952).21 A. Dresler, Das Licht 7, 203 (1937).

the measures of variability would completely obscuresystematic changes of only minor significance. On theother hand, it tends to maximize the reality and im-portance of any systematic adaptation effects that doemerge.

The detailed results of the relative brightness matchesfor the various adaptations and the different testluminances are shown graphically in Figs. 1 through4.22 The results for observer H are in the upper halfof each figure and those for observer J in the lowerhalf. In each graph, the solid lines connecting thecrosses are the average results for white adaptation atthe specified luminance, and the dashed lines con-necting the open circles are the results for a specifiedchromatic adaptation at the same luminance. Theresults for white and blue adaptations are comparedin Fig. 1. Log relative luminosity is plotted as ordinate;wavelength, in millimicrons, is abscissa. The adapting-surround luminance is specified in the upper left ofeach graph, and the luminance of the 550-mux standardis shown under each pair of curves comparing neutraland chromatic adaptations. N= 4 for each plotted point.All curves are fitted by visual inspection.

The general effect of adaptation to blue, as comparedwith adaptation to chromatically neutral white light,is a reduction in the brightness of the 450- and 500-mAstimuli relative to the 550-m/u standard, and an in-crease in the relative brightness of the 600- to 650-mutest stimuli. The magnitude of the effect varies bothwith the adapting-surround luminance and the lumi-nance of the heterochromatic comparison stimuli. Whenthe adapting luminance is equal to 10 mL, for ex-ample, and the central test stimuli are matched to astandard luminance of 40 mL, the brightness matchesshow little or no change with change from white toblue adaptation, although a consistent effect is evidentfor this same 10-mL adapting level when the teststimuli are matched to a standard luminance whichis also equal to 10 mL. On the other hand, the adapta-tion effect for the 10-mL brightness matches is mostmarked when the luminance of the adapting-surroundfield is increased to 40 mL.

The results for the two observers differ in thatthe reduction in relative brightness for wavelengthsshorter than the 550-mu standard is greater for ob-server X, and the increase in relative brightness forwavelengths longer than the standard is greater forobserver J. This inter-observer difference would resultfrom a greater reduction in brightness sensitivity withblue desensitization in the region of the 550-mA standardfor observer J than for observer H. That such a dif-ference does exist between the observers has alreadybeen demonstrated in the threshold sensitivity func-tions for blue and white adaptations that were re-ported earlier.23

2 Complete numerical data with standard deviations areavailable from the authors upon request.

23 Reference 1, Figs. 3 and 4, p. 554.

215March 1954


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Upper graphs - observer H; lower graphs - observer J.

The results for green and white adaptations are chromatic states is, in gelshown in Fig. 2. Again, the brightness matches for contrast to the blue and greach chromatic state of adaptation yield luminosity tendency for a relative irfunctions which differ in a consistent manner from shorter wavelengths and a those for the chromatically neutral white adaptations. wavelengths. On the wholEThe result is similar to the changes effected by blue tions for the chromatic andadaptation, namely, there is a reduction in the relative larger for observer H thabrightness of stimuli of shorter wavelength than the This difference, again, is c550-mi standard, and an increase in the relative bright- luminosity results for th(ness of the wavelengths longer than the standard stim- yellow and white adaptalulus. Again, the magnitude of the adaptation effect with the findings of othervaries for different combinations of adapting-surround shifts in the spectral hue-iuand test field luminances although not so regularly as in to slightly saturated, nethe case of blue adaptation. The paired functions for fields which appeared slipwhite and yellow adaptations are shown in Fig. 3. In servers induced large shiftthis case, the difference between the achromatic and 24 See reference 1. Figs. 7 anc

lines, blue adaptation.

leral, rather small. Here, in*en effects, the results show aicrease in brightness at theelative decrease at the longere, the differences in the func-I neutrally adapted states arean they are for observer J.onsistent with the threshold

same two observers with-ions.2 4 It is also consistentexperiments concerned withivariant loci with adaptationar-white stimuli. Adaptingghtly yellowish to both ob-s in the spectral loci of the

1 8, p. 555.

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invariant hues for observer H and only minor shiftsin the corresponding loci for observer J.25

For both observers, the changes brought about byyellow as compared with white adaptation are as great(or greater) for the 10-mL adapting level at all testfield luminances, as they are for the 40-mL adaptations.This inversion of the luminance dependency is probablyrelated to differences in the relative saturations of thevarious spectral regions at different luminances.

The detailed results for white and red adaptationsare shown for both observers in Fig. 4. At all luminancesof the test fields and for all adapting-surround lumi-nances, there is a reduction, with red adaptation, ofthe relative brightness of the longer-wave stimuli.

25 D. Jameson and L. M. Hurvich, J. Exptl. Psychol. 41, 455(1951).

Wavelengths shorter than the 550-mu standard -show,on the whole, either no change in relative brightnesswith red adaptation for some luminances, or a bright-ness decrease, although this decrease at the short wave-lengths is very small in comparison with the reductionin relative brightness suffered by the longer-wavestimuli. These brightness losses are no doubt relatedto the presence of a red component in the sensory re-sponses at both ends of the spectrum.

Analysis of the results for the various combinationsof standard and adapting luminances shows the changesbrought about by chromatic adaptation to be sys-tematically related to the surround/standard luminanceratio. For the condition of adapting-surround lumi-nance equal to 1/4 of the 550-mu standard luminance(surround= 10 mL, standard=40 mL) the average

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results for the two observers show little or no effect forblue and green adaptations. With yellow and redadaptations, on the other hand, there are consistentincreases for the shorter wavelengths and decreasesfor the longer wavelengths. Thus, although no im-portant changes in relative luminosity are broughtabout by blue and green adapting chromaticities ofmuch lower luminance than the test stimuli, this can-not be generalized to include the yellow or red adapta-tions.

As the ratio of surround to standard luminance isprogressively increased, the relative luminosity changesare also increased for blue, green, and red adaptations,and they tend to decrease for yellow adaptations.Finally, when the adapting-surround luminance isequal to four times that of the standard stimulus

500 600

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(surround= 40 mL, standard= 10 mL), the adaptationeffects are maximal for blue, green, and red adaptations,and minimal for yellow adaptation. The inversionfor yellow adaptation is probably related to the pres-ence of a yellow component in the 550-mg standardstimulus, and to the complex functions relating spectralsaturation and wavelength, and changes in saturationwith changes in luminance. For a discussion of someaspects of the spectral saturation problem, the paper byD. M. Purdy.2"

It is of interest to compare the present data withthe threshold sensitivity functions obtained earlier.Direct comparison of results for a neutral white adapt-ing-surround field at an adapting luminance of 10 mLindicates that the equal-brightness judgments yield

26 D. M. Pdrdy, Brit. J. Psychol. 21, 283 (1931).

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luminosity functions that are broader than the func- the corresponding threshotions based on threshold measurements for the same and short-wave spectralcondition of adaptation, same field size, and same ob- however, is not primarily iservers. hold luminosity and hete

The tendency for equality-of-brightness measures to rather in the relation bet,exceed the threshold luminosity values, accords with occurring in the two situathe frequently reported observation that in making Figures 5 through 8 corheterochromatic comparisons, there is a tendency to chromatic adaptation on eqoverestimate the relative brightness of the more highly with the effects previouslysaturated stimulus.72 7'28 In our experiments, large technique.' Log luminositydifferences in both hue and saturation are perceived matic states based on tbetween the 550-mA standard and the various other on average heterochromawavelengths matched to it in brightness and the relative are shown for both observ(luminosity values for equal brightness are higher than for the threshold determin

27 H. Pieron, L'Annee Psychol. 41-42, 125 (1940-41). The paired functions for t28 S. M. Newhall, J. Opt. Soc. Am. 30. 617 (1940). ments are based on average

lines, red adaptation.

ld values at both the long-regions. Our interest here,a the relation between thres-rochromatic brightness butween the adaptation effectstions.spare the average effects ofuality-of-brightness matchesy obtained by the thresholdr functions for paired chro-ireshold measurements and,tic brightness comparisonsers. The adapting luminanceations was equal to 10 mL.he equal brightness measure-es of the settings for all three

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ao0 , , made at wavelength intervals of 50 n whereas the

--- YELLOW-ADAPTED average threshold luminosity functions are based on.5 -BLUE-ADAPTED EQUAL BRIGHTNESS measurements obtained at 10-mu intervals throughout

1.5 - -BUJEXA \PTED the spectrum. The intersection locus for the equalbrightness functions is, of course, fixed in these experi-

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FIG. 5. Log relative luminosity. Yellow and blue adaptations. /Threshold and equal-brightness functions. Observer H. /

> 1.0 -

(10-mL, 20-mL, and 40-mL) adapting luminances and o//for the three stimulus levels (10 mL, 20 mL, and 40 mL) E ,5_ /- THRESHOLD \at which brightness comparisons were made. \

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shown in Figs. 5 and 6 for observers H and J re- ,/spectively. The differences in the paired functions for \the two contrasted chromatic states are clearly similar o -

for the equal brightness and threshold luminosity 0 / curves. In comparing these functions no special signi-/ficance should be attached to the absence of discon- 3.0 tinuities in the equal brightness functions. The bright-ness matches to the 550-mg standard stimulus were l

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--- YELLOW-ADAPTED EQUAL BRIGHTNESS FIG. 7. Log relative luminosity. Red and green adaptations.1.5 -BLUE-ADAPTED Threshold and equal-brightness functions. Observer H.

The average results for paired red and green adapta->- Tso A/< By \ \ tions are shown in Figs. 7 and 8. Again, the equality-o' of-brightness functions for the two chromatic states

E 2.5 _-X / THRESHOLD differ from each other in a manner similar to the dif--J / / /2 \ferences measured for threshold luminosities. The

maxima are located at longer wavelengths for greeni 2.0 _ // \ _ than for red adaptation, sensitivity is reduced at the

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The effect of chromatic adaptation upon spectral3.0 luminosity appears, on the average, to be at least as

significant for direct heterochromatic brightness as itis for threshold luminosity responses.

4.5 400 500 600 700 CONCLUDING REMARKS

WAVELENGTH - mg

FIG. 6. Log relative luminosity. Yellow and blue adaptations. The results of the series of experiments reportedThreshold and equal-brightness functions. Observer J. here confirm the generality of the experimental finding


that the form of the photopic foveal luminosity func-tion is dependent upon the state of chromatic adapta-tion. Similar changes in this function are found to occurwhether luminosity is measured by the threshold lightresponse or by direct heterochromatic brightnessequations. The heterochromatic brightness comparisonsare characterized by considerable variability, but thelow precision of the method does not obscure realchanges in relative brightness resulting from changesin the chromatic state.

For our observers, the large-step, equality-of-bright-ness functions are not identical to the threshold func-tions for the same neutrally adapted state, but arebroader than the comparable threshold sensitivitycurves. For a single level of neutral bright adaptation,no systematic differences occur among equal-bright-ness functions at different luminances.

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FIG. 8. Log relative luminosity. Red and green adaptations.Threshold and equal-brightness functions. Observer J.

The effects of chromatic adaptation on the luminosityfunction usually increase with an increase in thesurround-to-standard luminance ratio, although in ourexperiments an exception to this trend occurs when theadapting illuminant is yellow, rather than blue, green,or red.

Since the dependence of luminosity on chromaticadaptation is demonstrated to be a general effect, andnot restricted to threshold light responses, the im-portance of this dependence for photometry and color-imetry deserves some attention. The curves shown inFig. 9 indicate the way in which chromatic adaptationsof the sort measured here might be expected to causethe luminosity function to depart from the CIE stand-ard luminosity function. The curve drawn as a solidline is the CIE function for the standard observerand may be assumed to represent spectral luminosity

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for a chromatically neutral bright-adapted state.The other curves were obtained by applying to theCIE values our experimentally determined ratios ofchromatic to neutral luminosity values at specifiedwavelengths. The ratios used for the derived functionsare averages for both observers for all conditions inwhich the adapting luminance is equal to the luminanceof the photometric standard (surround= 10 mL, stand-ard= 10 mL; surround=20 mL, standard=20 mL;surround=40 mL, standard=40 mL). The curverepresented by long dashes is the luminosity functioncalculated for blue adaptation, the one represented byshort dashes is the function for red adaptation, andthat shown as a dotted line is the curve derived foryellow adaptation." All three curves are locked at550 mu, and all changes illustrated are relative to thefixed luminosity value at this wavelength. Similarluminosity curves could be derived from the CIEluminosity function by applying ratios based on adapta-tion effects measured by the threshold response. (SeeFigs. 5, 6, 7 and 8.)

These curves indicate that the spectral luminosityfunctions applying to the chromatically adapted ob-server may depart considerably from the standardluminosity function. At some wavelengths there aretwo to threefold differences in the relative luminosityvalues for different adaptation states. At 600 mu, forexample, the relative luminosity value is 31 percentfor red adaptation and 70 percent for blue, and at650 my the values for these two adaptations are 4 per-cent and 12 percent, respectively. For adapting lumi-nances less than the standard luminance the luminosityfunction applying to chromatically adapted observers

29 The green curve is omitted simply to permit a clearer graphicalsummary.

--- RED-ADAPTED

- - GREEN-ADAPTED

.1 -

221March 1954

EQUAL BRIGHTNESS

2.5

3.0

i.5 _


would depart from the standard luminosity functionto a lesser extent, and for adapting luminances higherthan that of the standard the departures would, ingeneral, be considerably greater than those shown inFig. 9.

These remarks should not, of course, be construedas specific recommendations for changes in the standardluminosity values for different chromatic states. 0

The luminosity functions illustrated in Fig. 9 are onlyapproximate indications of the influence of chromaticadaptation, since the luminosity ratios used for thecomputations are based on experimental results foronly two observers measured at only five wavelengths.They are illustrated here simply to indicate that noone luminosity curve can be assumed to apply, withinthe limits of experimental error, to any and everychromatic state of adaptation.

SUMMARY

A series of experiments is reported which extends anearlier study of the spectral sensitivity of the foveaand its dependence on chromatic adaptation. Theearlier study was confined to measures of threshold

3 We are indebted to Dr. D. B. Judd of the U. S. NationalBureau of Standards for suggesting specific rephrasings of ourremarks in order to avoid such an implication.

sensitivities, whereas in the present experiments fovealluminosity measurements were obtained by a large-step, equality-of-brightness method.

Results for monocular, heterochromatic brightnesscomparisons are presented for two observers. Brightnessequations were made to a standard wavelength of550 mu, at each of three standard luminances, 10 mL,20 mL, and 40 mL, for each of five comparison wave-lengths, 450 mu, 500 mg, 550 mu, 600 mu, and 650 mz.Equations were made in a 1 bipartite field, centeredin a 370 illuminated surround. Exposure duration was1.3 sec. Results were obtained for adapting-surroundfields of five chromaticities, white, blue, green, yellow,and red, and three adapting luminances, 10 mL, 20 mL,and 40 mL.

The direct heterochromatic brightness comparisonsconfirm the dependence of foveal luminosity on chro-matic adaptation demonstrated earlier for thresholdsensitivities. The main effect of adaptation to a givenhue is a relative decrement in the luminosity of thespectral stimuli which evoke that hue.

The effect of chromatic adaptation on relativeluminosity is different in magnitude for different ratiosof surround-to-standard luminance. These differences,in turn, are specifically related to the hue of the chro-matic adapting field.

222 Vol. 44

Documents

Spectral Sensitivity of the Fovea III Heterochromatic Brightness and Chromatic Adaptation