JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Dark Adaptation Following Light Adaptation to Red and White Lights*

SELIG HECHT AND YUN HSIA2

Laboratory of Biophysics, Columbia University, New York, New York

(Received January 20, 1945)

I. PHOTOPIC AND SCOTOPIC VISION

THE double structure of the vertebrateT retina involves separate receptors for nightvision and for day vision. In man this is re-sponsible for the classical Purkinje phenomenon(Purkinje, 1825) in terms of which the relativeluminosity of the spectrum varies with thebrightness (Koenig and Ritter, 1891), and findsits quantitative expression in the scotopic andphotopic luminosity curves shown in Fig. 1. Theduality of human vision was first formulated bySchultze (1866) and later independently byParinaud (1881) and by von Kries (1895). Sincethen it has served as theoretical matrix for ahost of visual phenomena (cf. summary byHecht, 1937).

One of its more practical consequences hasbeen the use of red light for achieving and pre-serving dark adaptation under conditions per-mitting the eyes to be occupied with tasks re-quiring relatively high visual acuity and bright-ness (Miles, 1943). This has been commonpractice in fluoroscopy rooms, and has come tohave extensive use in the armed forces.

Recently Lowry (1943) has taken exception tosuch use of red light and has presented data toshow that the important factor in determiningthe time necessary for dark adaptation is not thespectral composition but the brightness of thelight to which te eye has been adapted. Inorder to resolve this contradiction we havere-examined the problem theoretically and ex-perimentally.'

*Publication assisted by the Ernest Kempton AdamsFund for Physical Research of Columbia University.

1A report of these findings was given to the OpticalSociety of America at its meeting on October 21, 1944 inNew York.

2 Fellow of the China Institute of Physiology andPsychology.

I After our work was completed and reported, a paperappeared by Rowland and Sloan (1944) in which theydemonstrated experimentally the value of red over whitelight for preserving dark adaptation. In addition, theymade an approximate determination of the numerical ad-vantage of red over white. Our own approach is sufficientlydifferent to constitute an independent confirmation of thetheory and practice of red light pre-adaptation.

II. REASON FOR RED LIGHT

In explaining the use of red light for achievingdark adaptation, it is usually said that the retinalrods which mediate scotopic vision are practi-cally insensitive to red light by comparison withthe cones which mediate photopic vision. Asevidence, one generally refers to the scotopic andphotopic luminosity curves of Fig. 1 from whichsuch a conclusion might seem reasonable.

This, however, is not correct. Each of theluminosity curves is drawn independently, anddescribes the comparative effectiveness of thespectrum for its own system only. The relativeluminosities of the two systems are not given,because the maximum of each curve is arbi-trarily put at 100.

Moreover, it is not even true that, comparedto the cones, the rods are practically insensitiveto red light. Actually in peripheral vision, therods are slightly more sensitive to red light thanthe cones. This becomes clear from the com-parison of the two luminosity curves on a realenergy basis such as is shown in Fig. 2 (Hecht,Shlaer, and Smith, 1935). At any wave-length,as the energy is increased, it is first recognized

/00 5 600 70

00~~~~~~~~~~~~

60-

40~~~~~~

0

400 500 600 700

W/ave/engf/h-m~u

FIG. 1. Luminosity curves for scotopic (rod) vision andfor photopic (cone) vision. The scotopic data are themeasurements of Hecht and Williams (1922); the photopicdata are those of Gibson and Tyndall (1923). Since themaxima are arbitrarily set at 100, these curves give noinformation about the relative sensitivity of rods andcones for red, or for any other part of the spectrum.

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S. HECHT AND Y. HSIA

colored (cones)

3-

IQ4 C olorl- ess ( o s

400 600 600 700

£Vave/enfih -nlu.

FIG. 2. Spectrum sensibility curves for rod and conevision on a real energy basis. The data for the separatecurves are from the same sources as in Fig. 1. The positionof the two curves on the ordinates corresponds to the factthat after complete dark adaptation, any region of theretina outside the fovea sees red light of 650 m.u as colorlessat the threshold, and as colored only above the threshold.The precise energy increment above the threshold for theappearance of color (cone function) varies for differentparts of the retina; in the parafovea it lies between 0.1 and1.0 log unit.

by the rods as colorless light, and then at higherenergies by the cones as colored light. The energydifference between the two systems at differentwave-lengths is the photochromatic interval ofCharpentier (1880) and Parinaud (1898) andshows up in many visual functions such as darkadaptation, instantaneous threshold, flicker, in-tensity discrimination, and visual acuity (Hecht,1937).

The fact that the rods and cones have prac-tically identical sensibilities for red light is con-firmed by the measurements of Mandelbaum(1941) who found that from fovea to extremeperiphery, covering all variations of rod andcone population, the sensitivity of the retina tored light remains practically constant. Thussince the rods in scotopic vision are at least assensitive as the cones and usually more so, this

cannot be the reason for using red light or redtransmitting glasses for achieving dark adap-tation.

The real reason is somewhat more subtle, anddepends on the relative positions of the twoluminosity curves on the wave-length scale of thespectrum. What is required is the evaluation ofthe relative effectiveness for the two systems ofdifferent lights of specific energy and spectralcomposition.

For simplicity assume a light which emitsequal energy throughout the visible spectrum,and a filter which has a sharp cut-off, so that ittransmits light only beyond 620 mAu. The visualeffectiveness of the whole white light for thecone system is proportional to the area underthe photopic visibility curve in Fig. 1, while theeffectiveness of the transmitted red light aloneis the area under the curve to the right of avertical line at 620 mj. The relative brightnessof whole light compared to the red light will bethe ratio of these two areas. For the photopiccurve this ratio is very nearly 10, which meansthat if for the cones whole white light is to bemade equal in brightness to the red light trans-mitted from it through the red filter, the wholelight has to be reduced to 1/10 its intensity bymeans of a neutral filter or any other devicewhich decreases it evenly along the spectrum.

Now consider the scotopic curve for the rods.Because it is about 50 m,4 to the left of the conecurve the ratio of the total area under it to thearea beyond 620 mA will be much greater than10. Actually it is greater than 100, which meansthat, for the rods, if whole light is to be madeequal in effectiveness to the red light trans-mitted by the filter, the whole light has to bereduced to less than 1/100 by a neutral device.It follows that when whole white light and redlight have been equalized for cone vision, thenfor rod vision the white light will till be 10times brighter than the red light. Thus by com-parison to white, red light has a tenfold ad-vantage in the dark adaptation of the rods foran equal effectiveness in cone vision.

The ratios here used are approximate. Theprecise values will depend not only on thephotopic and scotopic luminosity curves in Fig. 1-call these P and S-but on the energy distri-bution E in the spectrum of the lamp and on the

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transmission T of the filter. At any wave-lengthX these values will be PA, S, E, and Tx. Thusin the visible spectrum between 400 and 700 mAthe brightness of whole white light W forphotopic vision is

700

W.=f Ep PX, (1)

and for the filtered red light R it is

700

Rp=f ExPxTxdX, (2)400

while for scotopic vision the comparable value forwhite is

700

Ws= ExSxdX, (3)400

and for the transmitted red

700

Rs= EXSXT)\dX. (4)40

For photopic vision the ratio of white to redis BP = WP/RP, while for scotopic vision theratio is Bs= Ws/Rs. The relative advantage inrod vision of red over white when both havebeen equalized in brightness for the cones isgiven by the ratio Bs/Bp.

The advantage ratio Bs/Bp will depend notonly on the luminosity curves in Fig. 1, but onthe energy distribution in the spectrum of thesource and on the spectral transmission of thered filter. For daylight and for tungsten lampsand for common red cut-off filters, computationsshow that red has a dark adaptation advantageover white of 10 to 50 times.

These values are subject to variation becauseof individual differences in visibility curves. Inaddition, the visibility curves themselves, suchas the accepted I.C.I. values or even the originaldata of Gibson and Tyndall (1923) are open torevision (see summary by Federov, Federova,Plakhov, and Seletzkaya, 1940). Similarly thescotopic luminosity function (Hecht and Wil-liams, 1922) may be too high in the red. How-ever, for present purposes they are adequate.

In his argument Lowry has ignored theexistence of all this knowledge of scotopic andphotopic vision and has tacitly assumed that

two lights matched in brightness with thephotopic cone system will be equally effectiveon the scotopic rod system.

III. RED AND WHITE LIGHT ADAPTATION

Our experiments were conducted in threestages. Our first desire was to make certain thatthe common experience with dark adaptationfollowing red and white illumination is correct.For this purpose we measured dark adaptationwith a standard adaptometer (Hecht and Shlaer,1938) following light adaptation to white lightand to red light of approximately equal photopicbrightness.

The white adaptation field was illuminated bythe regular 40-watt lamp of the adaptometer. On115 volts, its color temperature is 2600'K asmeasured with the Eastman Color TemperatureMeter. With neutral filters the brightness of theadaptation field was adjusted to 26.3 milli-lamberts.

For red light we used the dark-adaptor plasticmaterial made by the Polaroid Corporation. Its

0.8

0.6

.0

N 0.4

LI

0.2

0.0

500 600

Wale/eqth -mlz700

FIG. 3. Transmission of the red plastic material used inthese experiments. This is the common filter materialsupplied by the Polaroid Corporation in the red gogglesused for dark adaptation.

I

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S. HECHT AND Y. HSIA

S

4

46

. 3

.5

4

SR S~RO

341 =

3C~~ly./

JWL42t

4<:gS~Al

o/0 20 0 /0 Z0

TIme in Dark- tjnu4es

FIG. 4. Individual measurements of dark adaptationfollowing adaptation to white light (clear circles) or tored light (filled circles) of approximately equal photopicbrightness. Five subjects were tested twice; two only once.In the left column the red experiment preceded the white;in the right column white preceded red.

spectral transmission is shown in Fig. 3 and wasmeasured with Shlaer's (1938) photoelectricspectrophotometer. From Eqs. (1) and (2) wecomputed the photopic transmission of the red

filter to be 0.129. Visual heterochromic measure-

ment of its transmission made with the Koenig-Martens polarization photometer at high bright-ness confirmed the computed value. The filterwas placed in the path of the adapting light andits photopic brightness adjusted to be 38.9millilamberts. The brightnesses of the red andwhite were planned to be approximately equal,but owing to a computational error, discoveredafter the experiments had been made, theyturned out to be different but still of the sameorder of magnitude. As will be apparent, theerror is of no importance.

Each subject was first dark adapted. His righteye was then exposed to the light adapting fieldfor five minutes, after which its dark adaptation

was measured using a 30 circular blue field placed70 off-center exposed for flashes of 0.2 second.

Figure 4 shows the results with seven subjects.Five of them were tested twice, once with redadaptation preceding white and then with whiteadaptation preceding red. The other two subjectswere tested only once.

It is obvious from Fig. 4 as well as from theaverage data in Fig. 5 that dark adaptation fol-lowing red light adaptation is much more rapidlyattained than following white adaptation. If wechoose as a measure of the speed of dark adap-tation the time required to reach a level ofsensitivity 0.5 log unit above the final threshold,the speed of adaptation subsequent to red ismore than three times as great as that sub-sequent to white. The average time to reachthis level after red is 1.9 minutes; after white, itis 6.2 minutes.

IV. LIGHT ADAPTATION AND DARK ADAPTATION

The second series of experiments was designedto clarify the situation further. It is known fromprevious work (Hecht, Haig, and Chase, 1937)that the speed of dark adaptation varies inverselywith the brightness of the preceding light adap-tation. The relationship is such that the time inthe dark required to reach a given threshold is asigmoid function of the logarithm of the pre-adap-tation brightness. We decided to measure forwhite light and for red light this relation betweenpre-adaptation brightness and subsequent darkadaptation.

As before the experiments were made with theHecht-Shlaer adaptometer with the self-con-tained adaptation light whose brightness and

.s .6 0 Wlhl/fe-26.3 ml

i.3

0 /0 / s ZoVrlne in Oar -linvfes

FIG. 5. Average measurements from Fig. 4. It is plainthat following white adaptation, dark adaptation (whichhere is purely rod) is slower than following red adaptation.

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spectral composition may be varied by neutraland color filters. The subject was first thoroughlydark adapted. He was then light adapted for 5minutes to a specified light, after which thecourse of his dark adaptation was measured withblue light for the same retinal area as before. Forlight adaptation we used three intensities ofwhite light, and three intensities of red lightarranged in approximately equal pairs of whiteand red. Because of the same error, the red andwhite lights were not quite the same, but this isof no importance since the two groups of meas-urements are to be treated independently.

The results obtained with one individual areshown in Fig. 6. At each intensity level for redand for white the measurements were repeatedonce. They were so unequivocal that we foundit unnecessary to use additional subjects. FromFig. 6 it is apparent that the speed of darkadaptation decreases as the intensity of the pre-adaptation increases. Moreover, adaptation fol-lowing white exposure is always slower than thatfollowing red adaptation. The differences in rateof dark adaptation following red and followingwhite pre-adaptation appears to be greater atthe higher intensities. However, this may beonly because of the scale of plotting the ordinates.

To compare the speed of adaptation one maychoose a point on the dark adaptation curvewhich can be determined with good precision.

-A I -- I I$ at W 6hj/e-324m, _

2 Red-.3s8 z

zI I

I 11 I IL I

I-I

K I I I

0 /0 20 0 /0 207Rime in Pron- Ilinules

FIG. 6. Dark adaptation following adaptation to whitelight or red light at three brightness levels. Two runs areshown for each color and brightness for one subject. Re-gardless of brightness level, dark adaptation is alwaysfaster after red light adaptation than after white of com-parable brightness.

/4

'1,01 /0

6

.1

z .

0 / 2 3

Adaptaon r!ihfess -og Ai//i/asber-.s

FIG. 7. Relation between the brightness of the precedinglight adaptation and the time taken during the subsequentdarkness for the eye to reach a threshold of 0.5 log unitabove the final threshold. The two curves are roughlyparallel, with the red displaced nearly 1.5 log units to theright. This means that to secure the same dark adaptationin the same time, the eye may be exposed to 30 times asmuch red light as white light.

Such a value is 0.5 log unit above the finalthreshold because this is approximately wherethe curves have the maximum inflection.

The average of two such determinations on thedata of Fig. 6 are shown in Fig. 7. This showsthat for white and for red pre-adaptation the timeto reach a given threshold level in the subsequentdark adaptation increases with the intensity ofthe pre-adaptation. The two curves look like thebeginning portions of a sigmoid relationship.

The important point is that the red functionis well to the right of the white function. The twocurves are approximately parallel and the dis-placement between them is about 1.5 log unit.This means that to achieve a given degree ofdark adaptation one may be adapted to redlight which is approximately 30 times brighterthan white light. It will be recalled that this isabout the expected ratio computed in an earlysection of this paper.

V. LOWRY'S EXPERIMENT REPEATED

In an effort to reconcile our findings with thosedescribed by Lowry we repeated his experiment.We arranged a large sheet of opal glass illu-

4 Rowland and Sloan found in two experiments that tomatch the effect of 58 millilamberts of red light theyneeded between 2.0 and 1.1 millilamberts of white light.This is almost the same ratio as ours.

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. .. . I . .1 . . . . I~~~~~~~~~~~~~~~~~~~~~~~

. .. . . . W. 1. . I I .

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S. HECHT AND Y. HSIA

minated from behind by a lamp whose colortemperature was 28000K. The brightness of thefield was adjusted to 4.3 millilamberts withneutral filters. Similarly with the dark adaptorplastic filter and neutral filters we adjusted thefield to yield red light of 4.4 millilamberts. Thesetwo photopically equal brightnesses were usedfor light adaptation, and were as near as wecould conveniently come with our filters to the3.15 millilambert field used by Lowry.

Before an experiment the subject was thor-oughly dark adapted. He was then light adaptedfor 3 minutes to either the red or white field.When the light was turned off the speed of hisdark adaptation was measured by a methodsimilar to Lowry's, in which we determined thetime required to see binocularly a white testfield 3 in diameter, 7 below center, set at0.035 microlambert, and flashed for 1/5 secondevery second. The brightness of the test field is4.54 on the log micromicrolambert scale inFigs. 3-5. Lowry's test field was a 120 diskfixated in the center. However, it has been wellestablished (Hecht, Haig, and Wald, 1935) thatthe threshold of such a large field corresponds tothe threshold of its outer edge only. This isbecause the threshold of the retina for whitelight decreases rapidly from center to periphery.Thus a small field placed 6 or 7 from the foveagives about the same threshold as a 12° fieldfixated centrally.

We measured three subjects three times toeach light adaptation. Their results are given inTable I, where each datum is a single deter-mination. Following white light adaptation theaverage time for all subjects to reach the pre-ar-ranged threshold of 0.035 microlambert is 42seconds. This is the same order of magnitude asLowry's results and shows that we were operatingunder nearly the same conditions. Following redadaptation, however, the measurements showthat distinctly less time is required to reach thepre-arranged threshold than before. For all sub-jects the average is 30 seconds. White light thuscauses a 40 percent increase in recovery timeover red.

The values of 42 and 30 seconds for adaptationtime subsequent to white and red light adapta-tions are in keeping with the information gainedin the preceding section and are indeed to be

TABLE I. Recovery after light adaptation. Time in darkto reach fixed threshold of 0.035 microlambert afteradaptation to white or red lights of equal photopic bright-ness, 4.4 millilamberts.

Subject Y. H. M. D. R. W.

Color white red white red white redTime in seconds 40 38 29 25 42 30

41 35 38 25 51 2960 41 47 24 42 2848 34 - - 30 27

Average 47 37 38 25 41 29

expected from the curves in Fig. 6. Note thatthese curves are shallow at low values of pre-adaptation brightness, and steep for high values.For a pre-adaptation of 4 millilamberts red light(log value of 0.6) the recovery time in dark is ofthe order of a minute for red light. A lengtheningof this time by 40 percent calls for a relativelylarge increase in brightness, approximately alog unit or more.

VI. POSSIBLE EXPLANATION

The mystery of Lowry's results may lie in thesecond paragraph on page 620 of his paper inwhich he states that the two adapting fields wereadjusted to equality by means of a flicker pho-tometer. The brightness and size of the photo-metric field are not given. But if the photometricfield were larger than 2 or 30 -which is likely-and it were dimmer than the adapting fieldbeing measured-which is also likely under theconditions-then Lowry's results are under-standable. The photometric measurements willthen have been made at low brightness levels atwhich the Purkinje phenomenon is effective. Atsuch low levels the rods are maximally functionalwhile the cones are only partly functional. Andsince the rods and cones summate brightness(Koenig and Ritter, 1891) the photometry itselfwill be mainly scotopic rather than photopic.

If this is the correct diagnosis, then the reasonthat Lowry got nearly the same results followingred and white adaptation is that his red andwhite lights were of nearly equal stimulatingpower to the rods and not to the cones. It isalso to be noted that there is a small differenceeven in Lowry's data; the averages differ by 1.1seconds in the right direction, which for thisshallow portion of the curve in Fig. 6 may cor-respond to a fair photopic brightness difference

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between red and white. Lowry's experiments arecertainly correct as performed, but they areirrelevant to the phenomenon they were designedto clarify.

SUMMARY

1. Red and white lights, when equated forphotopic vision and used for light adaptation,influence subsequent dark adaptation unequally.Following red, dark adaptation is much fasterthan following white.

2. From the experimental relation betweenbrightness of light adaptation and speed of sub-sequent dark adaptation, it is found that toproduce the same dark adaptation in the sametime, red light adaptation may be held 30 timesas bright as white.

3. These results conform to theoretical ex-pectation from knowledge of night and dayvision, and confirm extensive experience. Ex-periments which apparently contradict suchtheory and practice are shown to be in error.

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