JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

The Effect of a Peripheral Glare Source upon the Apparent Brightness of an Object

GLENN A. FRY * AND MATHEW ALPERN *School of Optometry, Ohio State University, Columbus, Ohio

(Received August 8, 1952)

The decrease in perceived brightness of a foveal test object produced by a peripheral glare source can beaccounted for in terms of a veiling luminance produced by stray light falling on the fovea. The same effectcan be produced by an artificial patch of veiling luminance superimposed on the test object. The effect ofincreasing the angle between the glare source and the test object is equivalent to reducing the brightness ofthe glare source in accordance with the stray light hypothesis. The measured amount of veiling luminanceconforms to that found by previous investigators. The changes in brightness that occur immediately follow-ing the onset of a peripheral glare source as well as the changes which occur following removal of the glaresource can all be accounted for in terms of the veiling luminance produced by stray light falling on the fovea.

INTRODUCTION

SCHOUTEN and Ornstein' showed that a glare sourceL in the periphery of the field of view of one eyereduces the apparent brightness of a foveally fixated testobject as determined by comparison with a similar ob-ject seen by the opposite eye. They considered twopossible mechanisms which might account for thiseffect: (a) A physiological effect initiated by the glaresource could be transmitted by the retina to the fovea;or (b) the glare source might produce stray light in theeye which casts a veiling illuminance over the image ofthe foveally fixated test object.

As a result of their investigations, they concludedthat the former explanation is more satisfactory thanthe latter.

However, Schouten and Ornstein' made the observa-tion that the depression of brightness still persists whenthe image of the glare source falls on the optic nervehead, and this fact makes it necessary to assume eitherthat the "blind spot is not blind" or that the effect ismediated by the stray light.

They investigated the possibility of differential effectsfrom spots of light focused on the sclera in front of andbehind the ora serrata, and did find a difference whichthey used as evidence for a physiological effect trans-mitted through the retina. However, Fry and Alpern2

have repeated their basic experiment and have failed toconfirm their finding.

In our present study of this effect, we have attempted,(a) To find out if feeble veiling luminance superimposedon the image of a test object could produce a reductionin apparent brightness commensurate with that pro-duced by a glare source. (b), To ascertain whether theeffects obtained with peripheral glare sources could be

*Acknowledgment is made to the Illuminating EngineeringSociety for a grant which has provided a graduate fellowship forthe junior author and also technical assistance. The senior authoris indebted to the Bausch and Lomb Optical Company for a grantfor research in physiological optics which has made possible hisparticipation in the investigation.

I J. F. Schouten and L. S. Ornstein, J. Opt. Soc. Am. 29, 168(1939).

2 G. A. Fry and M. Alpern, The effect on foveal vision producedby a spot of light on the sclera near the margin of the retina (tobe published).

accounted for in terms of stray light produced by theglare source and falling in the region of the image of thetest object. (c) To study the temporal characteristics ofthe effect.

APPARATUS

Figure 1 shows the apparatus used for the experimentsdescribed in the subsequent parts of this report. A andB represent the centers of the entrance pupils of theleft and right eyes, respectively. The diaphragm at Econtains two rectangular openings (a and b) as shown inFig. 2, which are designated a and b. A narrow mirrorY permits each of the two eyes to observe the mirrorimage Z' of the bright point at Z. This mirror imageZ' falls in the same plane as a and b and midway betweenthem, as shown in Fig. 1. The diaphragm III screens thefield of view so that each of the two eyes can see thefixation point Z', but the right eye only sees the rectan-gular opening b which constitutes the test object, andthe left eye only sees a, which constitutes the compari-son object. The penta prism H and the right-angleprism G are arranged in two tiers with the right-angleprism above the penta prism. The lenses, J and F, forman image, 2.2 mm in diameter, of the small aperture atM at the center of the entrance pupil A of the left eye.They also form an image, 2.2 mm in diameter, of the

P 0

FIG. 1. Apparatus for investigating theSchouten-Ornstein effect.

189

VOLUME 43, NUMBER 3 MARCH, 1953

G. A. FRY AND M. ALPERN

.50°

_ IU . 2 FIG. 2. Pattern for inves-

TT sources.d b 2,.50 tigating the effect of glare

small aperture at N in the center of the entrance pupilof the right eye. The hypotenuse face of the right-angleprism reflects the beam from M to the left eye, and thepenta prism by means of two internal reflections directsthe beam from N to the right eye.

The beam-splitting prism D before the left eye servesno useful purpose except to make the optical distancebetween A and E equal to that between B and E. Thebeam-splitting prism C in front of the right eye per-mits the right eye to see the opening or openings in thediaphragm U which are illuminated by light from thesmall aperture V, an image of which is focused by lensIV at the center of the entrance pupil of the right eye.The diameter of this image is 2.3 mm. The small aper-ture at V is adjustable in a vertical and horizontaldirection so that its image can be made to fall exactly atthe same point as the image of N.

In the following experiments, three types of patternswere used for the openings in the diaphragm at U: (a)Two rectangular openings, c and c, which are seenflanking the retangular opening b (Fig. 2). (b) A singlecircular opening e centered around the fixation point(Fig. 3).

The subject sinks his teeth into a previously preparedbiting board and this brings his head into an approxi-mate position. He is then directed to fixate the fixationpoint Z'. The biting board which controls the position ofthe head is then adjusted until the center of the entrance

aHe 90

c 9,50 ,b

FIG. 3. Pattern for investigating the effect of a patch ofveiling luminance.

pupil B of his right eye coincides with the images of Nand V, until the center of the entrance pupil of the lefteye lies in the same horizontal plane as that of the righteye, and until the cornea of the left eye lies at an equaldistance from Z' as the cornea of the right eye. To aidthis operation, a diaphragm is mounted between dia-phragm III and the beam-splitting prisms, C and D,which contains a small circular hole for the right eyeand a horizontal slit for the left. The assembly support-ing the right-angle prism G and the penta prism I isthen rotated until the image of M falls at the center ofthe entrance pupil of the left eye. The rotation of thepenta prism does not affect the position of the image ofN.

The lens W forms an image of the ribbon filament Xat the aperture in the diaphragm V. The rays from theribbon filament T are collimated by the lens S, and thelens Q focuses an image of the filament at the apertureN and the lens R focuses an image at the aperture M.The optical centers of lenses Q and R are separated byan amount equal to the separation of the two aperturesN and M. In some of the early experiments both N andM were illuminated by a piece of opal glass which in

TABLE I. Data for TH obtained with the stimu.us pattern inFig. 3 showing the effect of varying Be. Be and B b are expressedin foot-Lamberts.

B. Bb V B. Bb V

0 4.98* 0 55.8 17.06 2.421.116 5.30 0.064 111.6 22.26 3.472.49 5.22 0.048 223.0 22.12 3.424.70 5.86 0.18 335.0 25.0 4.03

11.16 7.28 0.46 558.0 39.4 6.9322.32 9.68 0.94 1245.0 61.0 11.2527.9 10.62 1.13 2350.0 110.5 21.2

* Value for B. used in computing values of V.

turn was illuminated from the rear by a single lamp.Also, in some of the early experiments a 100-wattfrosted bulb was used at X instead of the ribbonfilament.

Experiments involving simultaneous momentaryexposure of a and b preceded or followed by a moment-ary exposure of e (see Fig. 3) were made possible bymeans of electromagnetic shutters and rotating disks.The rotating disk II exposes e for 5 sigma once eachrevolution and also has an opening which prevents itfrom interfering with the exposure of a and b. By ad-justing one disk with respect to the other, a and b canbe made to precede or follow e by any interval up to300 sigma. A cam on a shaft rotating one-fourth as fastas the shaft carrying the two disks controls the twoelectromagnetic shutters which permit exposure of thetargets once every four revolutions of disks I and II.A system of relays was also worked out to permit theoperator to give a single exposure by pressing a key.

The luminance of the targets seen by reflection at Cwas varied by Wratten neutral density filters and by apair of crossed polaroids placed at V. The luminance of

190 Vol. 43

APPARENT BRIGHTNESS OF AN OBJECT

b seen by transmission through the beam splitter Ccould be varied independently from that of a seenthrough the beam splitter D by a pair of crossed polaroidfilters L and P. The luminance of the target a seenthrough D could be varied independently of that of bseen through C by Wratten neutral density filtersplaced at 0. The luminance of the targets a and b couldbe varied together by means of Wratten neutral densityfilters placed at K. The entire apparatus was suppliedwith suitable baffles so that no extraneous light couldenter the eyes.

EFFECT OF A PATCH OF VEILING LUMINANCE UPONTHE APPARENT BRIGHTNESS OF THE

TEST OBJECT

The stimulus pattern for this experiment is shown inFig. 3. A patch of veiling luminance (e) having a diam-eter of 9.5° is superimposed upon the test object (b)with its center falling at the fixation point Z'. Thefixation point, the test object b, and the patch of veilingluminance superimposed upon it are seen by the righteye. The left eye sees only the fixation point and thecomparison stimulus a. The luminance of a(Ba) waskept fixed; the luminance of b (B b) was varied in orderto make it match a in brightness. For various levels ofluminance for the veiling glare (Be), one of the subjectsTH made five settings of Bb. The results are presentedin Table I and Fig. 4. In Fig. 4 the data are expressedin terms of V and Be. The symbol V represents a quan-tity which is called the glare index by Schouten andOrnstein' and is defined as follows:

V= (B b/Ba) -1. (1)

The important point is that B b continually increases asBe increases. If we start with a match between Ba andBb and then increase the veiling glare, this cuts downthe perceived brightness of b in spite of the fact that itadds to the luminance of b, and the luminance of b mustbe further supplemented to reestablish the match witha.

With much simpler equipment, this type of experi-ment was carried out with three other subjects whoobtained the same type of result. The slope of the bestfitting straight line varies from day to day and variesbetween about 0.7 and 1.0. The value used for Ba incalculating V is based on a binocular match between band a when the veiling glare is off, and any error in thedetermination of this value produces a marked effect onthe slope of the best fitting straight line. Values of V arepractically meaningless for small values of Be where thedifference between Ba and B b is no longer significant.Variations in the relative sensitivities of the two foveaswill also affect the results.

An explanation of how a patch of veiling luminancecan reduce the apparent brightness of an object hasbeen attempted in a previous paper. 3

I G. A. Fry and M. Alpern, "The effect of veiling luminance uponthe apparent brightness of an object" (to be published).

W I zu.5 -

-J

.2 O

0

-I 0 1 2 3 4LOG Be (FOOTLAMBERTS)

FIG. 4. Results for subject TH obtained with the stimuluspattern in Fig. 3 showing the effect of varying the luminance of thepatch of veiling luminance, e.

EFFECTS OBTAINED WITH GLARE SOURCES

The stimulus pattern for this part of the investigationis illustrated in Fig. 2. In this pattern the two rectanglesdesignated by c and c' constitute the glare sources andthese are symmetrically placed on opposite sides of bwhich is the test object. The distance from the center ofb to the center of either of the glare sources is designatedby 0. The experimental procedure consisted in setting 0at various fixed values, and for each value of 0 the sub-ject made five separate settings for the luminance of bwhich made the apparent brightness of b equal to thatof a which is seen by the opposite eye. Six values of 0

TABLE II. Data for GAF obtained with the stimulus pattern inFig. 2 showing the effect of varying 0 and B, on the luminance ofb required to make b match a. Luminance of a constant. B b andB, are expressed in foot-Lamberts.

Bb

c and c' 0B. removed 0.750 10 1.50 2.50 3.50 4.50

2350.0 6.74 126.00 53.8 30.0 16.94 10.76 10.641245.0 6.34 65.0 32.7 22.3 15.78 11.5 9.42558.0 7.34 47.5 21.4 . 13.04 8.96 8.68 8.2335.0 5.83 33.0 17.3 12.95 7.95 6.6 6.95223.4 6.48 .30.1 14.94 11.0 7.77 6.66 8.52111.7 .78 19.3 10.2 9.14 7.98 7.62 6.255.8 5.46 11.48 9.6 8.16 7.3 6.14 5.9827.9 6.24 13.2 7.66 6.06 6.56 7.02 5.34

191March 1953

G. A. FRY AND M. ALPERN

TABLE III. Data for TH obtained with the stimulus patternin Fig. 2 showing the effect of varying 0 and B0 on the luminance ofb required to make b match a in brightness. Luminance of a con-stant. Bb and B0 expressed in foot-Lamberts.

Bb

c and c' 0B. removed 0.750 10 1.5° 2.5° 3.50 4.50

6.65 5.81 5.04 4.92 6.28 6.46 5.74 5.516.63 4.71 5.84 5.22 4.7 4.66 4.72 5.133.25 4.73 9.96 8.38 7.36 6.3 6.06 5.7466.5 5.07 8.94 6.06 5.28 4.8 4.89 5.54

166.3 4.68 7.12 5.3 5.16 4.84 4.95 4.80322.5 4.67 9.68 6.7. 5.46 5.1 4.93 4.64665.0 4.74 13.2 8.54 6.56 5.72 5.28 5.4

1663.0 5.12 22.6 16.06 11.32 8.74 8.56 6.983325.0 4.75 18.74 11.34 9.14 7.68 6.08 5.086650.0 4.76 35.6 25.5 20.26 15.04 9.96 7.04

16 630.0 4.73 62.2 42.2 28.8 18.62 12.46 9.8233 250.0 5.36 95.4 64.6 40.4 31.8 20.18 15.9274 100.0 5.6 684.0 91.8 58.8 39.4 30.0 18.24

139 900.0 5.47 - 678.0 107.8 99.8 58.2 30.0Mean 5.0

were used: 40, 30, 2-0, 1, 10, and 3 In each runthe order of 0 values was from large to small. Prior toeach run, a set of data was obtained with the inhibitingstimuli removed from the field of view.

The data for two separate subjects are summarizedin Tables II and III and are plotted in Figs. 5 and 6.

In order to compare this type of data with thatshown in Fig. 4 glare index values (V) were computed

w-J0CD0

0:

0

-.ta-

FIG. 5. Resultspattern shown invalues of B in3= 111.7; 4=223.,

for the data shown in Fig. 6 and Table III in accordancewith Eq. (1).

The values of V are summarized in Table IV. Valuesof V less than 0.1 have little meaning because of theindeterminateness of the data. It should be noted thatthe value of B. was determined by making a binocularmatch between Bb and Ba when the glare sources wereremoved from the field. This makes the results in-dependent of any imbalance which might exist betweenthe too eyes. The balance between the two eyes variesconsiderably from observer to observer and from oneexperimental session to another. A redetermination ofB. was made for each brightness level, and these varyingvalues of B0 were used in computing values of V.

Figure 7 shows values of log V plotted against log B,for various values of 0. The variation in the slopes of thebest fitting straight lines may be dependent in partupon the variations in balance between the two eyes ora small error in the determination of B. a. If the variationin V were completely determined by stray light fallingon the fovea, the slope of the line in these graphs as wellas that in Fig. 4 should be the same.

By using the graphs in Fig. 7 to interpolate the data,one can determine for the various values of 0 the value ofB, required to yield a constant value of V. In Fig. 8 areshown the values of B, for different values of 0 whichare required for a value of V equal to unity. This meansthat the amount of stray light falling on the fovea isconstant, and according to Fig. 4 the equivalentartificial veiling luminance (Be) is equal to 30 foot-Lamberts. According to the data in Fig. 8,

Be= kB 0/0 , (2)

where kg= 0.00545, and n= 2.5. The illuminance at theeye E produced by the two glare sources is given by thefollowing equation 1.

E= B/7rw, (3)

where w is the number of steradians of solid anglessubtended by the two rectangular glare sources, and Eis expressed in foot-candles, and B, in foot-Lamberts.

Since w= 7.656X 10-4 steradian,

B - 22.4E/0 2 5, (4)

!i.\ ~ \e where B is expressed in foot-Lamberts and E in foot-candles.

It should be noted that the form of Eq. (4) is identical4\ with that of the equations used by Holladay,4 Stiles,6

and Stiles and Dunbar6 to express the relationship be-tween the intensity of the glare source, the glare angle,and the equivalent veiling luminance. The values of the

.____________________________ _ .. constants are also in reasonable agreement with thosejO 2° 30 40 50 found by these investigators when allowance is made

for the units used. The equation given by Stiles and

obtained by subject GAF with the stimulus 4 L. L. Holladay, J. Opt. Soc. Am. 12, 271 (1926).Fig. 2. The different curves represent different 5 W. S. Stiles, Proc. Roy. Soc. (London) B104, 322 (1928-1929).foot-Lamberts as follows: 1 = 27.9; 2 = 55.8; 6 W. S. Stiles and C. Dunbar, Illum. Res. Comm. (Great

1; 5=335; 6=558; 7=1245; 8= 2350. Britain) Tech. paper No. 16, (1935).

192 Vol. 43

APPARENT BRIGHTNESS OF

Dunbar isBe= OE/02, (5)

where Be is expressed in candles per unit area and E inlumens per unit area.

In the experiment -described above, the beam of lightentering the eye was confined to an area smaller thanthe pupil and was not affected by variations in pupilsize. In this respect, the experiment differs somewhatfrom those performed by the previous investigators. Itshould also 'be noted that the data presented by theauthors is limited to glare angles from ° to 4.50, and thedata presented by Stiles and Holladay4 and Stiles5 andDunbar6 cover much larger glare angles.

CHANGES IN FOVEAL SENSITIVITY IMMEDIATELYFOLLOWING THE ONSET OF A GLARE SOURCE

Schouten and Ornstein' used the stimulus patternshown in Fig. 9 for studying the changes in fovealsensitivity which occur immediately after the onsetof a glare source. They exposed the glare source forvarious durations and during the last 10o- of this ex-

TABLE IV. Data from Table III expressed in terms of V.

0B0 0.750 10 1.50 2.50 3.50 4.50

6.65 - - 0.0812 0.112 - -16.63 0.219 0.1082 - - 0.0021 0.082833.25 1.106 0.763 0.557 0.333 0.282 0.21466.5 0.764 0.195 0.041 - - 0.0927

166.3 0.522 0.1327 0.1005 0.0342 0.0577 0.0256332.5 1.072 0.435 0.169 0.092 0.0557 -665.0 1.78 0.8 0.384 0.2062 0.114 0.139

1663.0 3.41 2.13 1.21 0.707 0.6725 0.3633325.0 2.95 1.38 0.925 0.617 0.280 0.06956650.0 6.48 4.35 3.25 2.16 1.091 0.479

16 630.0 12.12 7.94 5.09 2.94 1.632 1.07433 250.0 16.82 11.04 6.53 4.94 2.76 1.9774 100.0 121.0 15.38 9.49 6.04 4.36 2.26

139 900.0 - 122.8 18.9 17.25 9.63 4.46

posure also exposed a and b. The semicircle b and theglare source were seen by the right eye, and a was seenby the left eye. The luminance of a was kept constantand the luminance of b was varied from exposure toexposure to determine the luminance required to makeb match a. In this way they demonstrated that thesensitivity falls gradually during the first 200 and afterthis remains constant. No overshooting occurs.

The maintenance of a constant level after the first200o they designate as the law of constant level. Thisempirical law is of great theoretical significance informulating and testing hypotheses concerning themechanism subserving the effect, but the practicalbearing upon the present investigation is that the find-ing of Schouten and Ornstein can be used as justificationfor using glare sources continuously exposed and makingmeasurements without reference to the duration ofexposure. Care has been exercised, however, in startingoff each experimental session with the subject darkadapted and working always from the lower to thehigher luminance levels.

Schouten and Ornstein attached a great deal ofsignificance to the fact that no "overshooting" occurs.

500

w-J'I 2000u)00-J 100

I-.cn

-J

00.0

so

1 2 3e- DEGREES

4 5

FIG. 6. Results obtained by subject TH with the stimuluspattern shown in Fig. 2. The different curves represent differentvalues of B0 in foot-Lamberts as follows: 1=6.65; 2=16.63;3 =33.25; 4=66.5; 5 = 166.3; 6=332.5; 7= 665; 8= 1663; 9=3325;10=6650; 11=16630; 12=33 250; 13=74100; 14=139 900.

This points to the possibility that the effect may be tiedup with component III of the retinal potential asanalyzed by Granit. However, if stray light falling onthe fovea is responsible for the loss in sensitivity, onewould expect this to be brought about by a lowering ofthe concentration of the photosensitive substance in the

e- 4.5 ea 2. er 000

Xo 3.5'o e<700 0 0

LOG B, Ft.L.

FIG. 7. Same data as in Fig. 6 replotted in terms oflog V against log B.

.

AN OBJECT , 193March 1953

I a I I ___ I

G. A. FRY AND M. ALPERN

4

-J3

00-J

2

0 0.2 0.4

LOG -DEGREES

0.6 0.8

FIG. 8. Values of B,, required to yield a value of unity for Vfor various values of . Data interpolated from the curves inFig. 7.

photoreceptors until this concentration reaches itsequilibrium. No overshooting is to be expected in thisprocess.7

EFFECTS OBTAINED WITH ASYNCHRONOUSMOMENTARY STIMULI

Schouten and Ornstein' used the stimulus patternshown in Fig. 9 to study the effects of a momentary ex-posure of the glare source. They exposed the glare sourcefor 10o- and after an interval exposed a and b simultane-ously for lo-. They reported that the inhibitory effectof the glare source on b increases as the interval in-creases up to about 50ao, and then decreases and disap-pears completely when the interval between the twostimuli is as long as 150o.

Since the glare angle employed was 100, it appearedto us that this effect might depend upon stray lightsuperimposed upon the test object and its immediatesurround. If this is true one should be able to producethe same effect with a patch of artificial stray light. Inorder to study this type of effect first hand, the stimuluspattern illustrated in Fig. 3 was employed. The arti-ficial stray light e was presented for an exposure of 5a

7 G. A. Fry and M. Alpern, Am. J. Optom. 23, 509 (1946).

and at a certain interval of time prior to or succeedingthis exposure of e, a and b were exposed simultaneouslyfor Scr. This cycle of exposures was repeated once every3 sec. The effect of e upon b was determined by varyingthe luminance of b to make it match a in brightness.The subject started with the luminance of b at a levelwhich made it appear brighter than a and from onecycle to the next reduced the luminance of b until itappeared to be equally bright as a. He then started witha luminance of b which made it appear darker than aand increased the luminance to obtain a match. Thisprocedure was repeated three times, and an average ofthe six settings was taken as representing a match.

Similar matches were made for other time intervalsbetween the exposure of e and the exposure of a and b.The results are shown in Fig. 10 and are summarizedin Table V.

It should be noted that after 300ao the retina is justrecovering from the veiling luminance. The maximuminhibitory or adaptation effect occurs when the glaresource precedes the test object by 50f. It should benoted also that the veiling glare can affect the test ob-ject even when it follows the test object by as much as100oa. A similar effect upon the threshold of visibilityhas been noted by Crawford.8

RECOVERY FROM INDIRECT ADAPTATION

Schouten and Ornstein' showed that the rate ofrecovery from adaptation to a peripheral glare sourcedepended upon the length of exposure of the eye to theglare source in spite of the fact that the inhibitory effectreaches a constant level 200o- after the beginning of theexposure and retains this level during the rest of theexposure. They showed that the effects were analogousto those obtained by direct adaptation.

As a matter of fact, it is possible from the data pre-sented in Figs. 9 and 10 in their paper' to show that therecovery curve following direct adaptation for 60 sec toa surface having a luminance of 270 c/m2 would beapproximately the same as the recovery curve followingindirect adaptation for 60 sec to a glare source at aglare angle of 3 and producing an illuminance of 130lux at-the eye.

According to Eq. (5), a glare source at a glare angleangle of 3 and producing an illuminance of 130 lux atthe eye should produce an amount of stray lightequivalent to a veiling luminance of 144 c/m2 . This

9 XGLARESOURCE

FIG. 9. Stimulus pattern employed by Schouten and Ornstein.

8 B. H. Crawford, Proc. Roy. Soc. (London) B134, 283 (1947). X

194 Vol. 43

APPARENT BRIGHTNESS OF AN OBJECT

calculated value is close enough to the empirical valueof 270 c/M2 to lead one to suspect that indirect adapta-tion is dependent upon stray light falling on the fovea.

If this is true, the mechanism of adaptation is essen-tially the same in both direct and indirect adaptationand can probably be explained in terms of the primaryphotochemical mechanism in the photoreceptors.

EFFECTS DEPENDENT UPON WAVELENGTHCOMPOSITION

Schouten and Ornstein' and Ivanoff9 have investi-gated the effects of varying the wavelength composition.If the effects are dependent indirectly upon stray lightin the eye, the differential reflectance of the retina andthe differential transmittance of the sclera and choroidfor the various wavelengths would contribute to thevariations that are found.

CONCLUSIONS

The decrease in perceived brightness of a foveal testobject produced by a peripheral glare source can beaccounted for in terms of the veiling luminance pro-

, 30,000

BCI= 9170 ft- L

(/) 20000 Ife 5930 ft-LES 15,000 0 N

F I <~aOnset ofo 10.0o a and b

! 800 Im -3 -.2 -.1 0 .1 ,2 .3

TIME OF ONSET OF e (SEC)

FIG. 10. The effect of asynchronous exposure of e and a-b (Fig.3). Zero on the time scale represents the onset of a and b which areexposed simultaneously for S. The patch of veiling glare (e) isalso exposed for 5. When e is turned off, b matches a whenBb=9170 foot-Lamberts.

9 A. Ivanoff, Rev. optique 26, 479 (1947).

TABLE V. The effect on V of asynchronous flash (5a) exposuresof e and a-b (see Fig. 3). Be=5930 foot-Lamberts. Ba constant.Bb varied from one cycle of exposures to the next to obtain amatch with a. Bb in foot-Lamberts.

a ~~Bb No veiling glare 9170

300 12 510250 11 800

e precedes a-b by 150 11700200 11 200t00 1720050 28280

e and a-b together 19 400[50 15 800100 14 630

e follows a-b by 1200 9550250 9410300 9200

duced by stray light falling on the fovea. The same effectcan be produced by a patch of veiling luminance.

The changes in brightness that occur immediatelyfollowing the onset of a peripheral glare source as wellas the changes which occur following removal of theglare source can all be accounted for in terms of theveiling luminance produced by stray light falling on thefovea.

The implications of this study for practical problemsof illuminating engineering are as follows: The demon-stration that the effect of a glare source on any aspectof foveal vision is dependent upon stray light lendssupport to the principle that the combined effect of anumber of glare sources in different parts of the fieldcan be predicted by summating the increments of straylight falling on the fovea from the individual sources.This lays the foundation for calculating the effect ofany brightness distribution in the field of view. A pro-cedure for doing this has already been outlined by Moonand Spencer."'"

10 P. Moon and D. E. Spencer, J. Opt. Soc. Am. 33, 444 (1943).11 P. Moon and D. E. Spencer, J. Opt. Soc. Am. 35, 233 (1945).

195March 1953