JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Relation of Threshold Criterion to the Functional Receptors of the Eye*JOHN L. BROWN, Aviation Medical Acceleration Laboratory, Jolinsville, Pennsylvania

AND

MARGARET P. KuHNs AND HELMUT E. ADLER, Department of Psychology, Columbia University, New York, New York(Received September 6, 1956)

Luminance thresholds for the resolution of parallel line grating test patterns were determined by themethod of constant stimuli. Measurements were made in the dark-adapted eye. Thresholds were determinedwith each of eight selected color filters and also with Wratten neutral tint filters. Seven gratings were usedwhich required visual acuities from 0.042 to 0.625. With gratings requiring a high order of visual acuity,minimum luminances for resolution of a grating are very nearly the same for all the color filters. As visualacuity requirements are decreased, however, luminance thresholds determined with red filters becomehigher relative to thresholds determined with other filters. Luminance thresholds with blue filters becomelower relative to thresholds obtained with other filters. These results are interpreted to indicate that changesin the threshold criterion may result in changes in the character of visual function from rod function throughmesopic function to cone function even though the eye remains dark-adapted. In situations where anindividual is adapted to a visual field of low over-all luminance and must periodically, in short glimpses,read visual displays at higher levels of illumination, the appropriate specification of the relative effectivenessof illumination wavelengths will depend on the visual acuity required to read the display.

INTRODUCTION

THE relationships between a number of visualT functions and test field luminance are bestrepresented by curves having two distinct branches.Such functions include visual acuity,' flicker fusionfrequency,2 stereoscopic acuity,3 and brightness dis-crimination.4 The experimental procedures used todetermine these "duplex" curves characteristicallyrequire that the eye be adapted to a given luminancelevel before measurements are made at that level.

Dark-adaptation curves also assume a duplex formunder certain conditions. Such is the case when lumi-nance of the light-adaptation field is relatively high andthreshold measurements during dark adaptation arebased on the detection of light in a region of the retinawhich contains both rods and cones.5

The two branches of duplex curves are usuallyinterpreted to represent two kinds of receptors. Thebranch associated with the lower range of luminances issaid to represent rod function and the branch associatedwith the higher range of luminances is said to representcone function.'

In virtually all instances where duplex curves havebeen reported to represent visual function, experimentalprocedures have resulted in a change in the condition

* This report was prepared at Columbia University under USAFContract No. AF33(038)-22616 covering work on Visual Factorsin Cathode Ray Tube Data Presentation. The contract wasinitiated under a project identified by Task No. 720W-7186-71544,"Presentation of Data on Radar Scopes," and was administered bythe Psychology Branch of the Aero Medical Laboratory, Directo-rate of Research, Wright Air Development Center, with Dr.Kenneth T. Brown acting as Project Engineer.

I Shlaer, Smith, and Chase, J. Gen. Physiol. 25, 553-569 (1942).2 S. Hecht and S. Shlaer, J. Gen. Physiol. 19, 965-977 (1936).3 C. G. Mculler and V. V. Lloyd, Proc. Nati. Acad. Sci. U. S.

34, 223-227 (1948).4Hecht, Peskin, and Patt, J. Gen. Physiol. 22, 7-19 (1938).6 G. Wald and A. B. Clark, J. Gen. Physiol. 21, 93-105 (1937).6 S. Hecht, Physiol. Rev. 17, 239-290 (1937).

of adaptation of the eye with changes of luminance. Itis therefore not surprising that the question of whetherrod function or cone function predominates with agiven set of conditions is frequently answered on thebasis of the adaptation of the eye.

Several recent experiments in which the course ofdark adaptation was measured in terms of luminancethresholds for the resolution of a series of visual acuitygratings have emphasized the importance of the crite-rion of threshold as well as the level of adaptation of theeye in determining whether rod or cone responsesoccur.7-9 When gratings were used which required avisual acuity of 0.25 or higher, there was no break inthe dark-adaptation curve. This was interpreted toindicate that luminance thresholds measured in thecompletely dark-adapted eye depend exclusively on conefunction if the criterion of threshold requires a suffi-ciently high order of visual acuity.

It would be of value to obtain information as to thenature of the transition from dependence on rod func-tion to dependence on cone function which occurs whenan increasingly higher order of visual acuity is requiredby the threshold criterion. Such information would aidin the appropriate specification of spectral sensitivityof the eye with changes in the nature of visual tasks.

One device which is frequently employed in order todistinguish rod and cone function is the use of severalspectral distributions of light for the determination ofthresholds. 0 Determination of the luminance range inwhich divergence of threshold curves obtained withdifferent colors is found has been employed by a number

7 J. L. Brown, J. Opt. Soc. Am. 44, 48-55 (1954).8 Brown, Graham, Leibowitz, and Ranken, J. Opt. Soc. Am. 43,

197-202 (1953).9 A. L. Diamond and A. S. Gilinsky, "Luminance thresholds for

the resolution of visual detail during dark adaptation followingdifferent durations of light adaptation," Tech. Rept. 52-257,Wright Air Development Center, U. S. Air Force, April, 1952.

10 C. S. Bridgman, J. Opt. Soc. Am. 44, 394-396 (1954).

198

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VOLUME 47, NUMBER 3 MARCH, 1957

THRESHOLD CRITERION

of investigators as a method of demarcating the mesopicrange."-' 4 It has been pointed out that such divergencymay result from rod-cone interaction or it may resultfrom the fact that either rods or cones determinethreshold in the range of divergence depending on thewavelength distribution of the test light. 5

It was the purpose of the present experiment toinvestigate in terms of luminance thresholds the relativeimportance of rods and cones and their possible inter-action for a series of visual tasks ranging from lightdetection and the detection of very coarse patterns tothe detection of very fine patterns of spatial detail.

Threshold determinations were made with a varietyof wavelength distributions of the test light. Gratingpatterns ranging from very coarse to very fine lines wereused. Measurements were based on short flash presenta-tions of the test pattern to the dark-adapted eye oftwo observers.

APPARATUS

The apparatus used was a Hecht-Shlaer adaptometerequipped with a holder for gratings and a small fixationcross, F, of adjustable brightness which appeared in thecenter of a circular area, 6 in diameter, in which testflashes were presented.7 The elements of this apparatusare illustrated schematically in Fig. 1. The light source,M, was a 40-w tungsten filament bulb operated at 120 vdc. The source was located behind the opal glass diffus-ing screen, G. Color temperature was measured with anEastman Kodak color temperature meter as approxi-mately 2800'K. For the present experiment the subjectwas provided with a switch which operated the shutter,S, and enabled him to present the test flashes to himself.Test flashes were approximately rectangular and wereof 0.016 sec duration. Duration was measured with theaid of photographs of a photocell response to lightflashes. Flashes were presented monocularly to theright eye. Eight color filters ranging from a deep red toa deep blue were used. These included two Farrandinterference filters (550, 650), and six Corning colorfilters (2030, 2412, 3480, 3486, 3387, 5543). The spectraldensity characteristics of these filters were measuredwith a Beckman spectrophotometer. Gratings havingequal lines and spaces of seven different widths wereused throughout the experiment. Acuity required forresolution of the gratings was determined in the follow-ing manner: A scale was placed against the condensinglens, L2 , of the apparatus on the subject's side andilluminated with "white" light. A grating having 30lines per in. was then placed in the apparatus at T andthe lines and spaces included in a distance of onecentimeter on the scale were counted. Visual angle

11 W. de W. Abney and F. L. Festing, Proc. Roy. Soc. (London)50, 371-372 (1891-1892).

12 A. H. Taylor, Illum. Eng. 38, 89-98 (1943).

13 K. S. Weaver, J. Opt. Soc. Am. 39, 278-291 (1949).14 W. D. Wright, Researches on Normal and Defective Color

Vision (C. V. Mosby and Company, St. Louis, 1947).15 C. S. Bridgman, J. Opt. Soc. Am. 43, 733 (1953).

EP - |

Fez ---- ---- 1--

FIG. 1. Schematic diagram of the apparatus.See text for explanation of symbols.

and visual acuity were calculated for the distance fromthe eye of the observer to the scale as well as the appar-ent thickness of a single line of the grating on thescale.Calibration measurements were made for each of threeconditions of illumination of the grating: 2030 filter(deep red), "white" light, and 5543 filter (deep blue).Chromatic aberration of the lenses in the apparatusresulted in slightly higher visual acuity requirementswith longer wavelength distributions of illumination.The greatest difference was that between the deep redand the deep blue with the finest grating. Visual acuityrequired with red was 0.629 while that required withblue was 0.614. The number of lines per inch on each ofthe gratings was known, and acuity values for all of thegratings were calculated from the one set of measure-ments made with the 30-line-per-in. grating. Visualacuities for the seven gratings with white illuminationwere 0.042, 0.083, 0.125, 0.250, 0.385, 0.500, and 0.625.A special holder, IF, for the two interference filters, wasattached to the back of the field stop, B, between lensesL, and L2 so that they could be inserted in the opticalsystem where the beam of light was made up of parallelrays. Other color filters were positioned at NN.

Control of accommodation is an extremely importantfactor in experiments where fairly high visual acuity isrequired and threshold measurements are based on shortflash duration presentations. The fixation cross, F,which could be viewed continuously, was provided asa cue for proper accommodation. It was possible toadjust the position of the image of the fixation crosswithin fairly narrow limits along the optical axis of theapparatus in order to bring fixation cross image andgrating image into the same plane.

The position of the grating on the axis of the opticalsystem was such that the grating image could beresolved even under the extreme conditions of completedark adaptation of the eye and a deep blue illuminant,conditions under which the eye is effectively myopic.'6

The required accommodation was approximately 1.75diopters. A 3-mm artificial pupil, EP, was used underall conditions of the experiment in order to reducepossible effects of differences between subjects in spheri-cal aberration, and to minimize variability due tofluctuating accommodation.

Luminance was varied by means of a calibrated

16 B. O'Brien, "A study of night myopia," Tech. Rept. 53-206,Wright Air Development Center, U. S. Air Force, May, 1953.

March 1957 199

BROWN, KUHNS, AND ADLER

TABLE I. Values employed in the calculationof threshold luminances.

Fovealthresholds(log relative

energy)Subject

JB PK

4,43a 5.64a4.28a 5.30a4.16a 4.97a4.02a 4.66a3.91a 4.38a3.78 4.14a3.68 3.943.57 3.773.43 3.613.31 3.543.11 3.312.94 3.042.89 2.952.68 2.692.46 2.502.32 2.262.26 2.192.28 2.222.20 2.202.24 2.232.22 2.222.29 2.262.28 2.332.31 2.292.27 2.342.38 2.442.46 2.632.65 2.852.87 3.043.17 3.263.45 3.703.78 4.034.11 4.314.59 4.754.82 4.985.20 5.335.35 5.505.64 5.73

Derived luminousefficiencycoefficients

(logKx)Subject

JB PK

0.57 -0.640.72 -0.300.84 0.030.98 0.341.09 0.621.22 0.861.32 1.061.43 1.231.57 1.391.69 1.461.89 1.692.06 1.962.11 2.052.32 2.312.54 2.502.68 2.742.74 2.812.72 2.782.80 2.802.76 2.772.78 2.782.71 2.742.72 2.672.69 2.712.73 2.662.62 2.562.54 2.372.35 2.152.13 1.961.84 1.741.55 1.301.22 0.970.89 0.690.41 0.250.18 0.02

-0.20 -0.33-0.35 -0.50-0.64 -0.73

Logrelativeenergy oftungstensourcelogEx

-0.60-0.43-0.26-0.10

0.020.110.220.310.410.500.590.660.710.750.790.830.870.930.961.001.011.031.041.061.071.081.091.101.111.141.151.171.191.201.231.251.281.31

(logEx-1.03)logEX'

-1.63-1.46-1.29-1.13-1.01-0.92-0.81-0.72-0.62-0.53-0.44-0.37-0.32-0.28-0.24-0.20-0.16-0.10-0.07-0.03-0.02

0.000.010.030.040.050.060.070.080.110.120.140.160.170.200.220.250.28

a Extrapolated.

neutral density wedge, W, and Wratten neutral filtersat NN. Test field luminance was calibrated by amonocular matching procedure which is described inan earlier report.7 Measured luminance with the wedgeset for maximum transmission and with no fixed filtersin the optical system was 6900 mL.

Two subjects were used, one male and one female,both of whom had normal color vision and normalvisual acuity on the basis of clinical standards. Both ofthese subjects had served in dark-adaptation experi-ments during the previous year and yielded data whichwere in the normal range.

PROCEDURE

At the beginning of each experimental session sub-jects were dark-adapted. In most instances duration ofdark adaptation was 30 mn or more. A minimum of 20min was sometimes used prior to the measurement ofthresholds for fine gratings with red light. At the endof the dark-adaptation period the subject fixated the

cross and, being careful to achieve optimum possibleaccommodation, presented the first test flash. For theinitial presentation, the experimenter attempted to setthe luminance of the test field at some point nearthreshold for resolution of the particular grating beingused. Subjects were required either to state the apparentposition of the grating or to say "no" if unable to makea judgment. Sufficient preliminary training was givenso that only one or two incorrect judgments were madeon the average during any experimental session. Thesewere treated as negative responses. The experimenter soadjusted the luminance of the test flash from onepresentation to the next that 10 flashes were presentedat each of 5 luminances in a range of luminances suchthat for the highest luminance the subject did notcorrectly identify the position of the grating 100% ofthe time, while for the lowest luminance the subject didcorrectly identify the grating position at least once.Two grating positions were used which were 900 apartwith the lines inclined 450 from the vertical. The gratingwas changed in a random fashion from one position tothe other on successive flashes, and it was presented anequal number of times in each position. Luminancesteps of 0.1 log mL were used. It was usually possible tocover the range from 10% correct identification ofgrating position to 90% correct identification in arange of 0.5 log mL, although sometimes a wider rangewas necessary. The 60% points, as determined by inter-polation on frequency of seeing curves, were taken asthreshold values. A minimum interval of 15 sec wasobserved after the presentation of each test flash so thatthe eye would recover maximum sensitivity beforepresentation of the next flash.

Additional thresholds were determined in the dark-adapted eye for simple detection of light with thecoarsest grating in the apparatus, and for foveal lightdetection with a uniform circular field 30 min in diameter.For the foveal threshold measurements the centralfixation cross was replaced by four dimly illuminatedpoints of light which were spaced at 900 intervals alongthe circumference of an imaginary circle, one above, onebelow, and one on each side of the test field, and eachat a distance of approximately 1.50 from the center ofthe test field. The brightness of these points could beadjusted by a rheostat controlled by the observer. Theobserver was instructed to fixate the geometric centerof the four points. Light detection thresholds weredetermined by the method of constant stimuli.

Threshold luminances were calculated on the basis ofthe following equation:

LUM(THRESH.)= TNX T, E TxKxEx'AX,

where TN trangmiggion of neutral ftlters; Ti,- =ratio ofthe wedge transmission at threshold to the maximumwedge transmission; Tx=spectral transmission of colorfilters used; Kx= luminous efficiency values for the

Wave-length

X

3708090

40010203040

45060708090

50010203040

55060708090

60010203040

65060708090

70010203040

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200 Vol. 47

THRESHOLD CRITERION

TABLE II. Logarithm of threshold luminance in millilamberts for the detection of light and for the identification of parallel line gratingorientations. Gratings represented visual acuities from 0.042 to 0.625. Thresholds were obtained with eight different color filters and withneutral filters. Observers were PK and JB. Filters are identified by manufacturer's number and color appearance.

Light detection30' With 0.042

foveal grating in Grating visual acuityarea 60 area 0.042 0.083 0.125 0.250 0.385 0.500 0.625

Filter JB PK JB PK JB PK JB PK JB PK JB PK JB PK JB PK JB

Neutral -1.26 -2.40 -2.89 -2.46 -2.52 -2.01 -2.17 -1.79 -1.86 -1.08 -1.02 -0.56 -0.67 -0.37 .- 0.28650a

Red Inter- -1.21 -1.08 -1.39 -1.43 -1.34 -1.18 -1.24 -1.18 -1.19 -1.03 -1.09 -0.56 -0.69 -0.46 . -0.37ference

2030 -1.42 -1.79 -1.84 -1.84 -1.84 -1.69 -1.74 -1.74 -1.64 -1.49 -1.49 -1.13 -1.19 -0.83 . -0.64Dark Red

2412 -1.25 ... -1.46 -1.49 -1.41 ... -1.26 -1.29 -1.21 -1.19 -1.01 ... . -0.69 . -0.32Red

3480 -1.40 -1.77 -1.99 -1.92 -1.79 -1.57 -1.64 -1.46 -1.39 -1.20 -1.18 -0.75 -0.88 -0.40 . -0.33Orange

3486 -1.14 ... -2.76 -2.96 -2.46 ... -2.01 -1.64 -1.71 -1.14 -1.16 ... .. * . -0.16Yellow-Orange

3387 -0.97 -2.41 -2.83 -2.53 -2.53 -2.07 -2.18 -1.78 -1.88 -1.06 -1.08 -0.47 -0.38 . -0.19Yellow

550a -1.27 -2.64 -3.05 -2.83 -2.80 -2.22 -2.44 -1.98 -1.95 -1.20 -1.35 -0.72 * -0.55 . * -0.30Green

5543 -0.89 -3.16 -3.46 -3.17 -3.16 -2.67 -2.51 -2.28 -2.11 -1.39 -0.53 -0.68 *- -0.36 . ±.. +0.11Dark Blue

a These filters are Farrand Interference Filters; other color filters are Corning Filters. Neutral filters are Kodak Wratten Filters.

observer; Ex'= adjusted log relative energy value of thesource; and AX= a 10-m/g interval.

The transmission of the neutral filters varied slightlyas a function of wavelength. However, it was found thatresults of calculations which were based on the actualtransmission of neutral filters at each wavelength didnot yield results appreciably different from thoseobtained when a single value, TN, was used.

In order to develop individual luminous efficiencyvalues for each observer, foveal threshold measurementswere made in the dark-adapted right eye by Dr. YunHsia'7 of the Columbia Psychology Laboratory. Thesemeasurements were based on a 42-min field, centered inthe fovea, which was presented in 0.004-sec flashes.Thresholds were measured in terms of relative energyrequirements in narrow spectral bands at 10-mu inter-vals from 420 to 740 m/u. These values are presented inlogarithmic form in columns two and three of Table I.Luminous efficiency values, Kx, were derived from thefoveal threshold data for each observer by takingreciprocals of the threshold relative energy values andthen multiplying by a constant in order to equate theluminous efficiency coefficients at 550 mu with theCIE coefficient at that wavelength. These data arepresented in logarithmic form in columns four and fiveof the table. The spectral energy of the tungsten sourceat 2800°K was estimated from available data on thespectral emission of tungsten sources at various colortemperatures.' 8 Logarithms of relative energy valuesof the source are presented as a function of wavelengthin column six of Table I. The values in column sixrepresent log relative energy of the source Ex, aftercorrecting for the spectral selectivity of the wedge andbalance. In order to facilitate calculation of luminances,

17 Y. Hsia and C. H. Graham, Proc. Natl. Acad. Sci. U. S. 38,80-85 (1952).

181 . E. S. Lighting Handbook (Illuminating Engineering Society,New York, 1947).

a constant was added to each of the values in column sixin order to adjust for the measured luminance of thetest field. This constant was determined by multiplyingeach of the corrected log relative energy values of thesource, Ex, by the corresponding value of Kx for JB,and summing between 370 and 740 m/.:

740

E KxExAX.370

The luminance of the test field as measured photometri-cally by JB (6900 mL) was then divided by the resultingnumerical value. The logarithm of the quotient was thedesired constant. The adjusted values, Ex' are presentedin column seven.

RESULTS

The results of the present experiment are presentedin Table II. Values entered in the table represent thelogarithms of luminance thresholds calculated in ac-cordance with the procedure outlined in the foregoing.

The luminance thresholds for grating resolution showgood agreement between observers. This is true in allcases where interindividual comparisons can be made,with the exceptions of the luminance threshold for the0.250 acuity with the dark blue filter and the 0.42acuity with the yellow-orange filter. It was not possibleto obtain reliable luminance thresholds for PK with thegrating which required a visual acuity of 0.625. Addi-tional luminance thresholds were therefore determinedfor observer PK with a grating requiring a visual acuityof 0.500.

Luminance thresholds with the deep red filter (2030)are lower throughout the entire range of visual acuitythan comparable thresholds obtained with other colorfilters. These differences in the luminance threshold fordeep red light are probably not indicative of any in-herent superiority of deep red light for visual acuity,

201March 1957

BROWN, KUHNS, AND ADLER

-0.4 -0.2 -1.4 -1.2 -1.0 -08 -0.6 -0.4 -0.

LOG VISUAL ACUITYFIG. 2. Log threshold luminance as a function of the logarithm of visual acuity required for resolution of the grating test objects.

Curves have been labeled in terms of the spectral selective filters which were used. Curves for both observers, PK and JB, havebeen shifted along the ordinate in amounts indicated at the right-hand side of the figure.

however, since luminance thresholds for light detectionthresholds are also lower with the 2030 filter.

It is of interest to compare the threshold luminancesfor identification of position of the 0.042 acuity gratingwith threshold luminances for the same test patternwhen observers were required to report only on thedetection of light. In the results for JB, simple lightdetection required slightly lower thresholds, in general,than did the identification of grating position. Theamount of the difference appears to be greater for shortwavelength distributions than it is for longer wavelengthdistributions of the test light.

A comparison of the light detection and gratingposition identification thresholds of PK with the 0.042visual acuity grating shows a rather unexpected result.In all cases, more light was required for the thresholddetection of light for the correct identification of gratingposition. When this result was first observed, it wasattributed to chance variability, but the same result wasconsistently obtained upon repeated redeterminationsof these thresholds. It would appear that observer PKadopted a stringent subjective criterion for the detectionof light where there was no objective basis for a crite-rion, but was able to achieve lower thresholds in asituation where there was an objective check on thecorrectness of her responses.

Luminance thresholds for the visual acuity gratingsare presented graphically in Fig. 2 for both observers,

PK and JB. The logarithm of threshold luminance foreach grating is plotted as a function of the logarithm ofthe visual acuity required to see the grating. There is aseparate curve for each of the spectral conditions in-vestigated. The yellow-orange (3486) curve for PK hasbeen omitted since only three thresholds were measuredon this curve. For purposes of clarity, the curves havebeen arbitrarily positioned with respect to the ordinateon the basis of wavelength distribution. Curves for thelonger wavelength distributions have been displacedupward above the curve which represents the use ofneutral filters only, and curves for shorter wavelengthdistributions have been displaced downward below theneutral curve. The amount of displacement of eachcurve is the same for both subjects and is indicated atthe right side of the graph.

The curves for the three red filters (650, 2030, 2412)are simple, positively accelerated curves. They areassumed to represent cone function. Curves with twobranches have been used to fit the data for the othercolor filters and for neutral filters. The upper segmentsof these duplex curves, which relate the higher range ofacuity and threshold luminance, are assumed to repre-sent cone function while the lower segments areprobably representative of rod function. The "cone"portions of the curves for both subjects and for all ofthe filters have been fitted by the same smooth curve.This was accomplished by visually fitting a smooth

202 Vol. 47

THRESHOLD CRITERION

6.C

(I)Z 5.C

0F 4.CZ'LLJ

> 3.CDa

L1J 2.C

< 1. C~IcLUWr

r-'3.0 -2.5 -2.0 -1.5 -1.0 -. 5 -3.0 -2.5 -2.0 -1.5 -1.0 -. 5

LOG THRESHOLD LUMINANCE (NEUTRAL LIGHT)-ML

FIG. 3. Relative equivalent density for each of the color filters as a function of the logarithm of threshold luminance measuredwith neutrally filtered light. The derivation of relative equivalent density is described in the text. Curves have been positionedalong the ordinate axis in accordance with the positions of curves in Fig. 2. Data of observers PK and JB.

curve to each set of datum points, tracing all of thesefitted curves on a transparent overlay, and then drawinga single curve which was estimated to afford the bestrepresentation of all of the traced curves. A similarprocedure was followed in fitting the lower or "rod"segments of the duplex curves.

It would appear that a single curve affords a reason-able description of all of the "cone" data of both ob-servers. Similarly, with the exception of JB's data forthe dark blue (5543) filter, a single curve affords areasonable description of the "rod" data.

The data of Fig. 2 have been replotted in Fig. 3 inorder to emphasize the way in which the sensitivity ofthe eye to different wavelengths changes with changesin the threshold luminance. The curve in Fig. 2 whichrepresents thresholds for neutrally filtered light hasbeen taken as a reference. The points plotted in Fig. 3were obtained by subtracting the log luminance thresh-old representing neutrally filtered light for a givenvisual acuity from the log luminance threshold repre-senting a given color filter for the same acuity. Thesedifferences were then plotted as a function of thethreshold luminances obtained with neutrally filteredlight. The relative positions of the curves with respectto the ordinate axis are based on the arbitrary position-ing in Fig. 2. The ordinate scale in Fig. 3 does notrepresent the actual density of any of the color filters.The curves may be interpreted to illustrate the way inwhich the density of each of the color filters wouldhave to change over the range of luminances on theabscissa in order for the threshold luminance for each

of the gratings to be a constant, independent of thespectral character of the test flash. The thresholdluminance obtained with neutrally filtered light hasbeen selected as a reference standard because its spectraldistribution is closest to that of the standard of equiva-lent luminance,'9 i.e., a full radiator at 2042°K, andbecause it can be calculated readily on the basis ofphotometric measurements. In this report, the referencestandard may be considered to define a scale of "equiva-lent luminance" in which the criterion of equivalenceis not a brightness match, but rather a luminancethreshold for detection of a grating pattern.

The curves in Fig. 3 appear to be diverging for bothobservers from a luminance of approximately - 1.2 logmL down to the lowest luminance. It is evident that asvisual acuity requirements are increased and thresholdluminance increases, at least to - 1.2 log mL, there isa tendency for the relative threshold luminance withlong wavelength distributions to decrease with respectto neutral luminance thresholds, while relative thresh-olds with short wavelength distributions tend to in-crease. For luminances above the region of - 1.2 log mLthere is no further systematic change in the relativeequivalent densities for different colors as luminance isfurther increased. This may be taken to indicate thatthe spectral sensitivity characteristic of the eye remainsessentially constant for visual tasks which requirethis particular luminance value or higher.

19 J. W. T. Walsh, J. Opt. Soc. Am. 39, 278-291 (1949).

I L P.K.R.I. (650)

D. (2030)

R.(2412)

0.(3480

D _ YO.(3486)

Y. (3387)

) - G.550

.8. (554 3)

V.A. .042 .083 .125 .250 .385.500%I I _ I II

Y.0. - _A. 0 0

Iy.

_ G.

42 .083 5 .250 .385 .625

203March 1957

BROWN, KUHNS, AND ADLER

DISCUSSION

The results of the present experiment as presented inTable II and Fig. 2 are clearly typical of many situa-tions in which visual function passes from dependenceon rods to dependence on cones. One important differ-ence in the present experiment is the fact that alldeterminations were made in the dark-adapted eye.The results therefore illustrate, at least in the dark-adapted eye, that the character of the functionalreceptor population may change with changes in thenature of the visual task which the eye is called upon toperform while adaptation remains constant. This is inno sense a startling result. It is of importance, nonethe-less, in view of the widespread tendency to associate thetype of receptors which are functioning with the level ofadaptation of the eye. It is probably true that the conesalone are of importance when the eye is adapted to highlevels of illumination. However, the present experimentillustrates that either rods, or cones, or possibly bothmay function when the eye is dark-adapted, dependingon the nature of the visual task.

It seems likely that in many practical situations,certainly in night flying, the level of adaptation of theeye may be considerably below a level which corre-sponds to the level of instrument illumination. If this isthe case, it is important to investigate visual functionsin such a way that level of adaptation and luminance oftest presentations are kept independent and not con-founded as they have been in much of the classicalresearch.

It is evident from the tabulated data of Table II thatthe "cone" branches of Fig. 2 would all be nearlysuperimposed if the data presenting different coloredtest flashes had not been shifted along the log luminanceaxis. The luminance scale has been constructed on thebasis of cone spectral sensitivity, and we would there-fore expect threshold luminances for cones to beindependent of test flash color.

The luminance scale is not independent of test flashcolor for specification of "rod" thresholds. This isillustrated by the relatively lower "rod" branches whichwere obtained as test flash spectral distribution wasrestricted to shorter wavelengths. Rod thresholds,expressed in luminance, will vary with changes inspectral distribution of the test flash in relation to thedifference in relative sensitivity between rods and

cones. Rod threshold luminances are therefore lowestfor test flashes restricted to shorter wavelengths, andincrease as the wavelength distribution of the testflash is shifted to longer wavelengths.

The two branches of duplex curves such as thoseillustrated in Fig. 2 are commonly referred to as "rod"and "cone" branches. A question may be raised as towhether or not the "rod" branch is exclusively repre-sentative of rods and the "cone" branch is exclusivelyrepresentative of cones.

We would expect any relation between log luminanceand some visual function (e.g. acuity) to be of a definiteform, independent of the spectral character of the testflash, over any range in which the relation is dependenton a single class of receptors which have a fixed spectralsensitivity characteristic." Thus, if the cone branches ofcurves in Fig. 2 are solely representative of cones, itshould be possible to fit the cone branches for each of thedifferent color filters with a single curve. There is noa priori reason why the same curve should fit cone datafor two different observers; nevertheless, a single curveseems to provide an equally good fit of the results ofboth observers in this experiment.

If the "rod" branches of curves in Fig. 2 are solelyrepresentative of rod function, it should also be possibleto fit all of these branches with a single curve. Since rodthresholds expressed in terms of luminance vary withchanges in the spectral character of the test flash, it willbe necessary to shift the curve along the log luminanceaxis for changes in spectral distribution, but the same.curve should fit all of the rod branches.

A single curve does fit all of the rod branches ofobserver PK reasonably well. No single curve could befound which would fit all of the rod branches of observerJB. The rod curve which was used to fit the data of PKappeared to provide as good a fit as any single curvewhich could be drawn to fit the data of JB, so thissame curve was used for the data of JB. The fit is goodfor data which were obtained with yellow-orange (3486),neutral, yellow (3387), and green (550) filters, but thecurve is too steep for the two datum points obtainedwith the orange (3480) filter and not steep enough forthe data which were obtained with the dark blue(5543) filter. This result is suggestive of the possibilitythat the "rod" branches for observer JB do not repre-sent rod function exclusively but may represent acombination of rod and cone function.

204 Vol. 47