JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Light Adaptation and the Brightness of Brief Foveal Stimuli*

JUDITH WHEELER ONLEYUniversity of Rochester, Rochester, New York

(Received December 13, 1960)

Brightness scales for differing conditions of adaptation andcontrast have been established by the combined procedures ofratio production and equal-brightness judgments under adaptingconditions which differ systematically for the two eyes. Understimulus conditions in which light adaptation alone is systemati-cally varied, the equivalence of brightness ratios at differing levelsof adaptation is demonstrated for a wide range of adaptingluminances. Preliminary findings suggest that this equivalencealso holds for chromatic adapting and test stimuli. Changes inthe brightness of brief foveal stimuli due to light adaptation may

be describable within the framework of a general power law, if alltest-stimulus luminances are expressed as luminance abovepsychological zero brightness (that is, as luminance abovethreshold for each specific condition).

Changes in brightness due to light adaptation may be measuredeither by the use of a preadapting stimulus or by presentation oftest stimuli on a prevailing surround. When adaptation is definedin terms of the production of a criterion luminance threshold, theadaptive effects of the two procedures may be shown to beequivalent.

INTRODUCTION

THE human eye is capable of responding to lightintensities over a range of nearly 10 billion to 1.

The instantaneous functional range of the eye is muchsmaller, and depends on the adaptation of the eye to theprevailing level of illuminance. Within any givenadaptive state, the brightness sensation can vary fromjust visible to a brilliant white. These sensations maybe qualitatively similar for differing adapting levels,yet the luminance required to evoke a given sensationwill vary greatly for different adaptive states.

The brightness we perceive when looking at a teststimulus of fixed luminance L is a function not only ofthe characteristics of that stimulus, but also of theparticular conditions of stimulation prior to, during,and even after the presentation of the test stimulusitself. For any given conditions of adaptation andsurround illumination, we may determine a psycho-physical scale for brightness to answer the question:How does the sensation of brightness vary with lumi-nance? For the dark-adapted observer, the brightnessscale obtained by several ratio scaling methods and inseveral laboratories here and abroad'-4 has been foundto follow a power law of the form

B=kLn,

where B represents brightness, L luminance, and n theexponent of the power function which has been observedto vary between about 0.30 and 0.40.

The present research was designed to study changes

*These experiments were conducted in the Department ofPsychology, University of Rochester, under the support of a grantfrom the National Institute of Neurological Diseases and Blind-ness, United States Public Health Service. This report is based onresearch presented in partial fulfillment of the requirements forthe degree of Doctor of Philosophy at the University of Rochester.A brief report of this research was presented at the Spring meetingof the Optical Society of America in 1960. The author expressesher sincere appreciation to Dr. Robert M. Boynton, under whosesponsorship the research was undertaken.

I R. M. Hanes, J. Exptl. Psychol. 39, 438 (1949).2 R. M. Hanes, J. Exptl. Psychol. 39, 719 (1949).3 S. S. Stevens, Psychol. Rev. 64, 153 (1957).4 R. G. Hopkinson, Nature 178, 1065 (1956).

in the brightness scale as a function of light adaptation,and to determine the relations among scales derivedindependently for differing adaptive states. The psycho-physical evaluation of the effects of light adaptation onthe brightness sensation has in the past been carried outfrom two separate methodological positions: the deriva-tion of psychological scales under standard conditionsof adaptation'-' and the determination of luminancerequired for equal brightness sensations under differingconditions of adaptation. 7 -8 Because of crucial differ-ences in the experimental conditions utilized in theseearlier studies, and because not all of the researchesadequately controlled the effects of light adaptation tothe test stimuli themselves, it has not been feasible tocombine existing data of the two procedures to generatea general set of brightness scales. It is the attempt ofthe present research to apply a single analogous experi-mental technique to both scaling and equal-brightnessprocedures, to explore one possible procedure for deter-mining a generalizable set of brightness scales.

The principle of a standard reference system for thespecification of brightness under all conditions of lightadaptation is not a new one. It was described in detailby Wright in 1938.9 The technique of binocular match-ing, which provides a basis for analogous scaling andequal-brightness procedures, has been in use at leastsince the time of Hering.'0 ' 2 The present study employsthis technique in (1) independent determination of theform of the brightness scale for specific levels of adapta-tion using standard ratio-production procedures and

I R. G. Hopkinson, W. R. Stevens, and J. M. Waldram, Illum.Eng. 6, 37 (1941). See also R. G. Hopkinson, ibid. 52, 211 (1957).

'J. C. Stevens and S. S. Stevens, ONR Symposium Rept.ACR-30, 41 (1958).

7 K. J. W. Craik, Proc. Roy. Soc. (London) B128, 232 (1940).8 F. H. G. Pitt, Proc. Phys. Soc. (London) 51, 817 (1939).9 W. D. Wright, The Perception of Light (Blackie and Son,

Limited, London, England, 1938).E. Hering, Graefe-Saemisch Handbuch (1905).N. Inouye and S. Oinuma, Arch. Ophthalmol. Graefe's 79,

145 (1911).12 For a discussion of the application of binocular matching

techniques to studies of light adaptation, see J. F. Schouten andL. S. Ornstein, J. Opt. Soc. Am. 29, 168 (1939).

667

VOLUME 51, NUMBER 6 JUNE, 1961

JUDITH WHEELER ONLEY

THE OBSERVER SEES:

ADAPTING: TEST:

VARIARLE(IE)

X. -STANDARSRE)

=mJ--- -

.275 ...

FIG. 1. Experimental conditions: what the observer sees.

haploscopic"3 stimulation to avoid monocular simul-taneous contrast effects, and (2) haploscopic equal-brightness judgments to determine the luminancerequired for sensations of equal brightness for adaptinglevels which differ systematically for the two eyes.

The study explores stimulus conditions which allowfor the variation of light adaptation alone, and those inwhich both adaptation and contrast may influence thebrightness sensation. Brightness scales obtained fromratio-production judgments are tested by the equal-brightness estimates to reveal the extent to whichbrightness ratios are equivalent for differing adaptinglevels.

METHODS

Experimental Conditions

Changes in the form of the brightness scale as a func-tion of adapting luminance have been studied underthe conditions outlined in Fig. 1. The adapting field inall cases subtends 100. Standard and comparison testluminances are presented in a 1 haploscopic field,fixated centrally to assure foveal stimulation. Temporalrelations of test and adapting stimuli are shownschematically in the figure. All test stimuli are of 0.275sec duration: brief enough to preclude significantchanges in adaptation due to the test stimulus, and longenough to avoid the complex brightness enhancementrange observed in studies of the Broca-Sulzer phenom-enon. Test stimuli appear once every 6 sec, and ob-servers may view as many flashes as are necessary toreach each required judgment.

In the preadapted case, the adapting stimulus isturned off 0.3 sec prior to the onset of each test stimulusand turned on again 0.2 sec after the cessation of thatstimulus. In this case, only the level of light adaptation

13 Haploscopy has been defined [F. B. Hofmann, Graefe-Saemisch Handbuch Ges. Augenheil, 8, part II 520 (1925)] as thebinocular fusion of visual fields presented independently to eacheye. '1'he term "haploscopic" is used to distinguish betweeni thebinocular-matching stimulus situation, which employs inde-pendent stimulation of the two eyes, and the viewing conditionsof normal binocular vision.

can vary. In the superimposed case, test stimuli appearsuperimposed on a prevailing adapting luminance. Thetest stimulus is always brighter than the adaptingstimulus which surrounds it, which should preclude anymajor effects on test stimulus brightness due to induc-tion by the surround field. The most complex case understudy is the replacement case. Here, test stimuli arepresented in the center segment of the adapting field,in an area previously occupied by a filler luminanceequal to that adapting stimulus. The brightness of teststimuli which are dimmer than the adapting surroundmay in this case be influenced both by light adaptationand by induction due to the presence of the surround.

Procedures

Ratio Production

After a five-minute period of binocular preadaptationto a specific luminance level, the observer adjusts thevariable test stimulus to the least luminance which canbe detected for the specific conditions of adaptation andcontrast being investigated. Left-eye thresholds weredetermined in each session. To obtain the brightnessscale, observers adjust the variable test stimulus tomatch, look half as bright and look twice as bright aseach of a series of standard test stimuli, with left andright adapting fields maintained at a constant adaptinglevel. An observer's adjustments of the variable tomatch each standard are accepted as an index for himof the equivalent standard luminance of the variablefield. This allows treatment of all results as independentof possible response differences in the two eyes.

Determinations of Equal Brightness

For five minutes prior to the presentation of testflashes, the observer views adapting fields which differin luminance for the two eyes. A standard adaptingluminance of -0.76 log ml is in all cases presented tothe right eye. Each luminance level investigated by themethod of ratio production is in turn adopted as the"variable" adapting luminance presented to the left eye(see Fig. 1). After this differential preadaptation, theobserver is asked to adjust the luminance of the variabletest stimulus to match the brightness of each of thestandard test-stimulus luminances investigated above.

Data to be presented are based on median judgmentsof each of three trained observers; each made a total ofeight judgments for each of the conditions to be de-scribed. The exploratory investigation of chromaticadapting and test stimuli in the preadapted case wascarried out for only one of these observers.

APPARATUS

Optical System

The experimental conditions require the presentationof six light fields which may be simultaneously or

RATIO PRODUCTION ADAPTING FIELDS;LE

PRE- ADAPTED CASE (

RE

SUPERIMPOSED CASE C REPLACEMENT CASE T

EQUAL BRIGHTNESS VARIABLE STANDARD

668 Vol. 51

LIGHT ADAPTATION 669

Upper Beams:

ADAPTING FIELD

ADAPTING FIELD

FILLER d

LE RE

SHp B Lp

0 0 [L : : :S g Lower:PT LI

"L2 ,TEST FIELDSSHT

7L3

I ;

,w|

Fu F

PRISM PRISM

FIG. 2. Schematic diagram of the Maxwellian view optical system.

independently varied in luminance. A six-channelMaxwellian view optical system, shown in Fig. 2, wasdesigned to meet these requirements. The light beamsfor the six stimulus fields originate from two sources(G. E. No. 1507 auto bulbs, 12 v, 50 cp) selected formatched characteristics from a large population ofbulbs. The sources (SA and ST) were intermittentlymonitored by photocells (PA and PT) and were main-tained at constant current throughout these experi-ments.

Light beams are obtained from each source in sym-metrical pairs. Each beam is collimated by a lens (LI)and subsequently brought to focus in the plane of ashutter (SH) by lens L2. Beams are then recollimatedby lens L3 and passed by means of prisms and beam-splitters through the apertures at A and T to lens L4,which forms an image of the source filament at thecenter of the pupil of the observer. The eye thus seeslens L4 as filled with light, in a Maxwellian view.Apertures A and T are placed at the first focal point oflens L4, so that the eye placed at the second focal pointof this lens perceives these images at infinity.

The adapting stimuli were introduced from source SA

through aperture A; adapting field fillers for the re-placement case were introduced from the same source,through the alternate paths illuminating aperture T.Test stimuli were produced from source ST, entering themajor optical path from below, and being viewedthrough aperture T. Filters at F in each beam wereused to control stimulus luminance. Test-stimulusbeams were controlled in addition by continuouslyvariable, neutral-density wedges (WL paired wedges

provided a density range of 5.0 and WR provided arange of 2.5).

Test-flash duration and intertrial intervals were con-trolled by an electromechanical timer, activating rotarysolenoid shutters to control each beam as shown inFig. 1. The duration of test-flash presentation and thetiming of field substitutions were checked by cathode-ray oscilloscope records from a photocell placed in theposition of each eye.

Luminance Calibrations

Relative luminance calibrations were obtained foreach light beam by a method suggested by Alpern.1 4 Bythe use of an auxiliary lens and aperture, the Maxwellianview field is made to fill the test field of the Macbethilluminometer so that matching judgments may bemade in the same manner as for a diffuse test source.All filters and wedges were calibrated by this methodin the beam positions in which they were usedexperimentally.

Bipartite field matches between each Maxwellianview field and a calibrated diffuse field of the sameangular subtense were used to establish absoluteluminance calibrations for each field. A 2-mm artificialpupil was used for these measurements. 15

RESULTS AND DISCUSSION

It is necessary to assume some arbitrary unit orstarting point in order to plot brightness scales based

14 M. Alpern, A Study of Some Aspects of Metacontrast, Doctoraldissertation, The Ohio State University, Columbus, Ohio, 1950.

15 H. Leibowitz, Am. J. Psychol. 67, 530 (1954).

669June 1961

J7 t T DI TH \V H F. FLE R ON LEY

on ratio-production data. The functions at the top oJFig. 3 show, brightness scales for several conditions ofadaptation, with the same arbitrary starting pointassumed for each. As plotted, each of these functionsindividually reflects relative brightness sensations forits specific level of adaptation; the functions do notindicate any information about relative brightnesses atdiffering levels of adaptation. For this example, threeof the functions have been adjusted to agree with equalbrightness judgments at two standard luminance levels(0.0 and 2.0 log ml). The curves have been adjustedvertically to agree with the points based on medianequal-brightness estimates. The extent of this agree-ment for all brightness levels provides an empirical testof the equivalence of brightness ratios under varyingconditions of light adaptation.

If brightness ratios are shown experimentally to beequivalent (that is, if fractions and multiples of equalbrightnesses are also judged to be equally bright), itshould be possible to generate a family of brightnessscales for "all" levels of adaptation based on a singlestandard scale and a series of brightness matches. Toobtain the scale for any level of adaptation other thanthe standard one, it should be necessary only to deter-mine the changes in test-stimulus luminance requiredto maintain equal brightness of test stimuli presentedafter differential preadaptation to the standard leveland the second level in question. Sensory scales for awide variety of conditions could thus be generated by

1000 SCALES EQUATED ARBITRARILY

ADAPTING LUMINANCE:-0.76 log ml

0 2.24100 3.24*4.24

10/

U,

w 1000Z SCALES ADJUSTED BY EQUAL B JUDGMENTSF-M

100 l

10 x

-2 -I 0 I 2 3 4

LUMINANCE (log ml)FIG. 3. Sample brightness scales based on ratio-production data.

Top: scales derived with arbitrary starting point. Bottom: scalesadjusted vertically by judgments of equal brightness at twostandard stimulus levels.

qualitative judgments of the equivalence of sensations,rather than by the more complex ratio procedures whichrequire the observer to make a quantitative estimate ofhis sensory experience.

Based on the data of the present experiments, theequivalence of brightness ratios holds for all conditionsin which induction or contrast effects are ruled out, thatis, for those cases in which light adaptation is theprimary determinant of the brightness sensation. Figure4 shows the families of brightness scales obtained fortwo observers in the three experimental cases. Curvesare based on the median ratio-production judgmentsof half as bright and twice as bright; the points arebased on the median haploscopic judgments of lumi-nance required for equal brightness under differentialpreadaptation to these levels. As in the example (Fig. 3),the curves have been adjusted vertically to reflectrelative brightnesses between adapting levels. Theagreement between brightness matches and judgmentsof sensory ratios is good for both the preadapted andthe superimposed cases. This agreement should lendsupport to those who champion the validity of quantita-tive judgments of sensory experience.

In the replacement case, where both light adaptationand induction may influence the brightness sensation,brightness estimates based on the two procedures agreeonly for test stimulus luminances above the luminanceof the adapting surround. (These conditions are anal-ogous to the superimposed case.) For stimulus lumi-nances below the level of the surround, equal-brightnessjudgments show greater decrements in brightness thanthose shown by the ratio-production judgments. Theestimates based on equal-brightness judgments areconsistent with the findings of other authors using themethod of magnitude estimation. 6" 6 The viewing condi-tions of the replacement case are complex: it may bethat the use of the binocular-matching technique in thiscase introduces binocular interactions which obscurethe actual form of the brightness scale.

The generality of ratio equivalence has been exploredfurther for the preadapted case. Figure 5 shows higheradapting levels for two observers. Note that the equiv-alence continues to hold, even at adapting luminancesof the order of 10 000 ml. Further investigation of thisproperty for one observer and chromatic stimuli isshown in Fig. 6. The chromatic characteristics of boththe test and adapting stimuli were varied as shown.Stimuli for these conditions were obtained by usingWratten filters 29 (red), 65 (green), and 48 (blue) in thepath of the standard tungsten sources (of about 28500).For conditions in which adaptation alone varies, bright-ness estimates based on scaling procedures and thosebased on haploscopic brightness matches appear toagree for chromatic as well as achromatic test andadapting stimuli.

16 D. Jameson and L. M. Hurvich, J. Opt. Soc. Am. 49, 890(1959).

670 Vol. 51

LIGHT ADAPTATION 67

PRE-ADAPTED TO! SUPERIMPOSED ON: REPLACED IN:

1000

.I)

(-a)

(DIa:1

100

10

Il

-- 0.76 log ml1.24

--- 224

+1

0 +

O:NMI I I I

-I 0 1 2 3 -I 0 1 2 3 -I 0 1 2 3

LUMINANCE (log ml)FIG. 4. Families of brightness scales for the three experimental cases. Curves from ratio-production data have been adjusted vertically

to reflect equal brightnesses at differing adapting levels. Points represent median haploscopic brightness matches after differentialpreadaptation of the two eyes.

The form of the brightness scale for the dark-adaptedobserver has generally been found to follow a powerlaw: a plot of brightness as a function of luminance onlog-log coordinates approximates a straight line. Figure7 is a summary of brightness scales for the dark-adaptedobserver, obtained by several scaling procedures andunder a variety of stimulus conditions. 2' 3'1 7 All scalesadhere to a power-law form, and under the presentconditions of brief foveal stimulation, the form of thefunction is verified by both ratio-production proceduresand the use of direct-magnitude estimations.'

The curvilinearity of functions in Figs. 4-6 suggeststhat a power-law prediction cannot be generalized toinclude conditions in which light adaptation is varied.However, these scales may become nearly linear if weexpress all test stimulus luminances as "luminanceabove threshold," taking as the zero point of eachstimulus scale the luminance threshold which has beendetermined experimentally for that condition. Figure 8shows brightness scales for one observer reevaluated in

17 J. W. Onley, Science 132, 1668 (1960).18 The generality of this finding has been extended to the viewing

of chromatic test stimuli by the dark-adapted observer: seereference 17.

100

Co

U)

C,)

zI-D

it

m0

10

100 r

PRE-ADAPTED TO;STD. -0.76 log mlo 1.24+ 2.24A 3.24* 4.24

10 F

I

/f O:RBI I I I I

-I 0 1 2 3 4

LUMINANCE (log ml)

FIG. 5. Equivalence of brightness ratios for the preadapted case,tested for higher adapting levels.

W' / +0./ +

O:NM

. . . . . . .

2

671J une 1961

l

JUDITH WHEELER ONLEY

GREEN

TEST STIMULUS

r BLUE

Vol. 51

ADAPTINGSTIMULUS.

RED

GREEN

BLUE

WHITE

-2 -I 0 1 2 3-2 -I 0 1 2 3-2 -I 0 1 2 3-2 -I 0 1 2 3 4

LUMINANCE (log ml)

FIG. 6. Equivalence of brightness ratios for the preadapted case for one observer and conditions of chromatic preadaptation and teststimulation. (Stimulus conditions were obtained by the use of Wratten filters 29, 65, and 48 in the path of the standard tungsten sourceto provide the red, green, and blue stimuli, respectively.) Double judgments (-1.10 and 1.90 log ml).

terms of these stimulus magnitudes. These data are forthe two experimental cases in which adaptation was theprimary determinant of the brightness sensation. These

1000

100

Ul3

=1

U)wzl-:

I

10

1000 r PRE - ADAPTED CASE

100 -

ca

0

10 -

I'J'Ju *

U/)U/)IIz

CD

Ft

100 -

10

-2 -I 0 1 2 3

LUMINANCE (log ml)

FIG. 7. Summary of brightness scales determined for the dark-adapted observer. Conditions of these experiments are describedin the references of footnotes 2, 3, and 19.

IL-2

FiG. 8. Brightness scales for one observer; all stimulus magnitudesexpressed as "luminance above threshold."

I I I L I

SUPERIMPOSED CASE

672

I

. _

(I)I-

z

m

ADAPTING STIMULI :STD. -0.76 log ml

o 1.24+ 2.24a 3.24

4.24

/ A/ /o0:JO

/ I I … I I-1 0 1 2 3 4

LUMINANCE (log ml above Threshold)

- |

-

s ffi

IVuui

2

LIGHT ADAPTATION

2

Et)

0

0-J0n

II

PRE-ADAPTED CASE

(nen

ICO

a:m

10

0: JO

-I 0 1 2 3 4

ADAPTING LUMINANCE (log ml)

5

FIG. 9. Luminance thresholds for the preadapted and super-imposed cases. Dashed arrows define the effective adaptingluminance for the nominal conditions of the superimposed case.

functions may be described by the general expression

1000 _ TEST STIMULUS LUMINANCE: (log ml)

100 _-

-I I I - - I I

50 I 2 3 4

EFFECTIVE ADAPTING LUMINANCE

(Ioq ml)

FIG. 10. The reduction in brightness of brief foveal stimuli as afunction of increasing light adaptation. The equivalent preadapt-ing luminance has been selected as the criterion of adaptive effect.(The gradient of brightness decrease for these functions is deter-mined by the choice of a criterion case.)

where B and L are defined as in the Introduction, andLo is the luminance required for a threshold responseunder each specific condition of test stimulus viewing.The exponent n varies with the level of adaptation. Forthe preadapted case, the exponent increases from 0.41to 0.57 under conditions of increasing light adaptation.For the superimposed case it increases from 0.41 to 0.48.

The simplicity of the functions in Fig. 8, and theirsimilarity for conditions involving previous and pre-vailing adapting stimuli, suggest that the results ofthese two experimental procedures may merely repre-sent differing measures in time, of the same adaptiveeffects. In each case, test stimuli are delivered to thefoveal retina which is in a specifiable state due to theadaptive effects of a previous or present photic stimulus.If we assume that the test-stimulus threshold measuredunder the conditions in which suprathreshold stimuliare to be evaluated is an index of the effective adaptivestate of the eye, we may compare the adaptive effectsof the preadapted and superimposed cases by relatingthe thresholds measured under the two conditions.Figure 9 shows test-stimulus thresholds for each nominal

adapting luminance for the two cases. Effective adapt-ing luminances for the superimposed case are defined bydetermining that adapting luminance which wouldproduce the same threshold if presented as a preadaptingstimulus. (Designation of the preadapting stimulus asthe criterion case is arbitrary.)

Changes in the brightness of specific test-stimulusluminances as the effective adapting luminance is in-creased are shown in Fig. 10. These curves suggest thatstudies of the effects of light adaptation which utilize aprevailing adapting stimulus may be shown to beexperimentally equivalent to those in which an adaptingstimulus precedes the test stimuli to be evaluated, 9 aslong as the adaptive effects of the two cases are experi-mentally assessed by the determination of test flashthresholds. When the concept of level of adaptation isdefined in terms of the production of a criterion thresh-old, those conditions which govern level of adaptationby either a previous or a prevailing adapting stimulusappear to provide consistent estimates of the effects oflight adaptation on the brightness sensation.

19 R. M. Boynton, Arch. Ophthalmol. (Chicago) 60, 800 (1958)

673Julle 1961