(HELMHOLTZ OPHTHALMOLOGICAL I N S T I T U l ’ E , M O S C O W . DIRECTOR: P R O F . D R . M . I . A W E R B A C H I .

ILLUMINATION AND VISUAL ACUITY.

By Prof. Dr. S. V . Kravkoa.’)

I. In cases of discrimination of light objects on a dark ground, the curve, representing the influence of illumination on visual acuity, shows a maximum corresponding to compara- tively low degrees of illumination. This fact has been established beyond doubt by the experiments of Wilcox’, Klenova2 and Musylev?. For an explanation of it Wilcox turned his attention to the role of light irradiation in the eye. Owing to irradiation, light objects seem enlarged. In the case of a discrimination of the minimum dark interval between two light objects, i t is only natural that this interval becomes more perceptible with a decrease of irradiation and less perceptible with an incrase of the latter. Kravkov4 considers that the acuity curve can not be explained by the influence of ir- radiation alone. Kravkov’s5 data concerning the influence of illumination on irradiation clearly show a constant increase of irradiation together with that of illumination. Therefore, should we explain the changes in visual acuity merely by irradiation we ought to expect with an increase of the latter a constant reduction of visual acuity in the case of dis- crimination of light objects on a dark ground. But practically we do not observe these phenomena.

Therefore Kravkov considered that visual acuity depends not only on irradiation but also on a second factor, namely, the contrast-sensibility of the eye. A s we know the latter

* ) Received the 5th of February 1938.

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constantly increases with the illumination. Visual acuity must improve together with the contrast-sensibility of the eye, as the dark interval between two light objects becomes more perceptible. Thus visual acuity observed at a certain degree of illumination is determined by two factors, - the increase of irradiation with a negative effect and the increase of contrast-sensibility with a positive one. With corresponding curves showing the development of either factor with an in- crease of illumination, we may obtain a final curve with a maximum which fairly accurately represents the changes of visual acuity observed in the experiment. In 1936 WilcoxD again returned to the question of the relation between visual acuity and illumination and again stated that irradiation alone can provide sufficient explanation. In a series of special ex- periments, undertaken by him with the aim to establish the in- fluence of illumination on irradiation, he showed that with an increase of illumination, the effect of irradiation of light objects diminished at first and increased later on. Thus Wilcox's irradiation curve revealed the existence of a certain minimum depending on illumination. It is natural that with such an irradiation curve, the changes in visual acuity could be explained by irradiation alone. Thus Kravkov and Wilcox in their experiments concerning the changes in the effect of irradiation with an increase of illumination came to contradictory results. According to Kraukou, irradiation is constantly increasing. According to Wilcox there exists a minimum. What is the explanation of this discrepancy? Obviously different results were caused by different methods. The latter could only be the dlifferent size of the objects on which irradiation had been measured. Wilcox took light bars, whose width was seen at the angle of 2'22". Kraukov used bars with a corresponding width of about 6"30'.

2. In view of that fact L. P. Galochkina, a collaborator of our laboratory, undertook, on one suggestion, experiments with the aim to elucidate the changes in the degree of ir- radiation with the increase of illumination, both in the case of small and large light objects.

The experiments were performed with small white bars

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of a small size, whose width was visible at the angle of 3p27 and with large white bars visible at the angle of 6#53". The test persons looked with both eyes, through artificial pupillae, 3 mm. in diameter. Their eyes were adapted to dim light. The task set before the test subjects was to establish the width of the black interval between the white bars so, that it would correspond to the total width of both white bars. The coef- ficient of reflection of the white bars was about 80 %.

I 0 /a 20 60 70 4tz.

Fig. 1.

The difference between the width of the interval to be established and the total width of the two white bars was taken as the value characterizing the irradiation of these bars. The experiments were performed on two persons. Each underwent several series of experiments (2-3) with various degrees of illuminat~ion. The illumination measured from 0,5 to 71 lux.

The results obtained (average for both persons) are shown on fig. 1 and fig. 2. On fig. 1 are represented the results of the experiments with smaller objects, while fig. 2 shows the ex- periments with larger objects.

The values of illumination are shown on the abciss, the degree of irradiation (in conditioned units) on the ordinate. The curves clearly indicate that the influence of illumination on the degree of irradiation varies a great deal, depending on that whether we determine irradiation on small or large objects. In the first case we obtain a curve with a minimum,

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as the one obtained in Wilcox’s experiments, in the second, irradiation invariably increases with the illumination as i t was described by Kravkov. We shall consider the question of the difference in the irradiation curves at the end of our paper. A t present we shall proceed further with our analysis of the factors affecting visual acuity. Experiments in visual acuity in discerning light objects on dark ground were per-

Fig. 2.

formed by Wilcox on small objects (less than 3’). We have already seen that for objects of such size the irradiation curve shows a minimum. Therefore the changes in visual acuity with illumination could be explained here by the influence of irradiation alone. But how are we to explain the curve of visual acuity in the case of light objects of a bigger size? In that case we have a constant increase of irradiation with a heightening of illumination (see fig. 2 ) . Moreover the ex- periments of Klenova were performed with objects visible at the angle of 6’ and still they showed a maximum in the curve, indicating the relation between visual acuity and illumination.

To verify this regularity on still larger objects, a col- laborator of our laboratory, F. J. Musylev, undertook, on our suggestion, additional experiments in order to determine the

3.

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relation between visual acuity and illumination in discerning light objects of a comparatively large size.

The test persons looked at two movable white bars with both eyes through artificial pupillae about 3 mm. in diameter. The width of the bars was visible at the angle of 7’. The coef- ficient of reflection of the white bars was very close to 80 $4.

The illumination varied from 1.5 to 700 lux. The test persons had to mark the moment of the appearance of a just perceptible interval between the white bars. The eyes

Fig. 3.

of the test persons were adapted to dim light. The ex- periments were performed on 4 persons. The results obtained with all the persons are shown on fig. 3. Illumination is marked on the absciss, visual acuity on the ordinate. The curves clearly indicate that (for comparatively large white objects used by Musylev), the values of visual acuity show a maximum.

For objects of that size, the curve of the changes in ir- radiation with the increase in illumination does not show any minimum, (see fig. 2, above and also the previous com- minications by Kraukou) .

4. Therefore we cannot explain the changes in visual acuity in connection with illumination by the action of a single factor - irradiation, as it is done by Wilcox. We must assume the participation of a second factor. As such we consider the contrast-sensibility of the eye. The contrast- sensibility of the eye does not remain constant at any degree of illumination; with the heightening of illumination the

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contrast-seisibility of the eye increases, while the value of the discrimination threshold is correspondingly lowered. This fact must also affect the perceptibility of the minimum interval between the objects, i. e. visual acuity. The higher is the cont- rast sensibility of the eye, the less may, evidently, be the perceptible interval. Therefore we must consider our previous explanation of visual acuity curve correct, and Wilcox's ex- planation insufficient. Visual acuity in discrimination light

p-; j'

+ Fig. 4.

objects on a dark ground will vary from the effect of ir- radiation, depending on the 'fact whether we have to deal with quite small objects or with objects of comparatively large size. The scheme in fig. 4 shows how the two factors mentioned above determine visual acuity in the case of small and large light objects. Illumination is marked on the absciss, visual acuity on the ordinate.

The I-curves show the changes in visual acuity under the influence of a single factor - irradiation, the E-curves the changes under the influence of contrast-sensibility alone. The dotted line, V, represents the curve of visual acuity, i f under the latter we understand the function ( a product) of the or- dinates of the two curves mentioned above. A s one can see, we actually obtain a curve with a maximum - for both the experiments with small and large objects.

5. The last question to solve is why the relation between irradiation and illumination differs in the case of small and large objects (see fig. 1 & 2 ) . To answer this question we must

39 1

take into consideration a factor, which may affect the cont- rast-sensibility of the eye. This factor may be called the xontrast of perceptibility(<. It consists in the fact that the just perceptible difference in one region of the visual field increases in the presence of another clearly perceptible one.

Fig. 5.

This phenomenon was described by F r y and Bartley' and quite recently by F r y and Cobbh. According to that regularity, the perceptibility of the border bb (see figure 5 ) in the presence of another clearly perceptible border aa will be worse than in the absence of the latter. In the lower part of the fig. 5 we see the approximate distribution of illumination on the retina corresponding to the border zone bb. It means that the edge of the white bar near the border bb will be seen not at the point I but at point 2 (see the curve on fig. 5 ) . In other words under these conditions the irradiation of the white bar will be decreased. The nearer is the border aa the stronger

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is its influence on the perceptibility of the border bb. A more perceptible border aa produces a worse contrast-sensibility for bb. The border aa will be more perceptible with the increase of the brightness of the light bar and therefore the irradiation of the bar, ascribed to its apparent widening along the border bb, will be still smaller. If we mark illumination along the axis of the absciss and the degree of irradiation along the axis

I 1

Fig. 6.

of the ordinate, then on the basis of the factor considered above, the degree of irradiation must constantly decrease with an increase of illumination.

On the other hand, with the increase of brightness the rate of the rise of retina-illumination curve increases at the transition from the dark field image to the light bar image. Owing to this factor the effect of irradiation must constantly grow. In result, the final irradiation value of a narrow light bar will be formed under the influence of two factors, the >>contrast of perceptibility<< and an increasing illu- mination of the retina. Accepting some constants which characterize the curves of the influence of these two fac- tors, we may obtain by the multiplication of the or- dinates of the two curves a curve with a minimum. See

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fig. 6 in which the ideal curve Ii shows the influence of the >>contrast of perceptibilitycc ; the curve G, the influence of an increasing illumination in the irradiation zone and I the resulting curve of the irradiation values. Illumination is marked on the absciss and the degree of irradiation on the ordinate. No minimum .was observed in the case of compara- tively large light objects (see fig. 2) . This may be explained by the fact that in the case of large objects the influence of the >>contrast of perceptibilitycc, is absent. This is due,to a comparatively great distance between the borders aa (see fig. 5) and the border ( b b ) ; the first does not affect the per- ceptibility of the second.

Fig. 7.

6. In conclusion we shall give a short account of some experiments undertaken by us with the special aim to in- vestigate the influence of the >>contrast of perceptibilitycc on the contrast-sensibility of the eye, described by Fry, Barthley and Cobb.

The experiment consisted in the following. The test persons had to establish the discrimination-threshold for a darker disk, appearing in the centre of a white one. The experiment had several variations. In the first, the darker disk appeared on a plain white ground. In the second, a black ring was drawn near the border of the appearing disk ( a t the distance of 2.5O). In the third, a grey ring was drawn near the appearing disc and in the fourth the black ring was placed at a greater distance from the edge of the appearing disc (about 3.7O).

These variations are shown on fig. 7, A designates the darker disc, the appearance of which on the ground B had to be established by the test person.

The disc A was formed by a revolving grey sector which could be enlarged or reduced when desired.The disk A becomes

26 Acta Ophthalmol., Vol. 16. 2-3.

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Variations Test persons

S K P

darker in the )>in<( - position of the grey sector and lighter on its removal. The thresholds of perceptibility of the disc were measured in degrees of the movable grey sector. The thresholds, thus established, for all the three persons are given in table 1.

111 IV

2 20 3 2O 2 70 240

25O 410 3 50 340

1 60 220 180 17O

Tuble I .

Conclusions.

1. The changes of visual acuity with the illumination show (in the case of discerning light objects on a dark ground) a curve with a maximum.

2. Such a character of the changes in visual acuity with illumination has been established both with respect to large and small light objects.

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3. The changes of the degree of irradiation with illumina- tion show a minimum only when the effect of irradiation i s measured on light objects of a small size.

4. Therefore i t is impossible to explain (as i t is done by W i k o x ) the curve of the changes of visual acuity by irra- diation alone.

5. Besides irradiation, the changes in visual acuity are produced by the contrast-sensibility of the eye, which in- variably grows with the increase of brightness.

6. The curve characterizing the changes in the irradiation of small objects with the increase of illumination, may be explained i f we take into consideration >>the contrast of per- ceptibility<< which affects the irradiation of light objects of a small size.

REFERE:NCES.

2. Klenova, E.: Sehscharfe beim Unterscheiden weisser Objekte auf schwarzem Gruiid und ihre Abhangigkeit von der Beleuch- tungsintensitat. v. Graefes Archiv f . Ophthalmologie, 1935, Bd. 133.

4. Kravkov , S . V.: Sehscharfe und Beleuchtung beim Unterscheiden weisser Objekte auf schwarsem Grunde. v. Graefes Archiv f . Ophthalmologie, 1933, Bd. 131.

5 . - u. a. Der Irradiationseffekt im Auge in seiner Abhangig- keit von der Lichtintensitat, Konstrast und Nebenreizwir- kung. v. Graefes Archiv f . Ophthalmologie, 1934, Bd. 133.

3. Musylev, F . I . : Visual acuity and illumination. ))Psycho-physiology of vision((. Moscow, 1938 (Russian, in print).

1. Wilcox, W . W.: The Basis of the Dependence of Visual Acuity on Illumination. Proceed. Nat. Acad. Sci. of USA, 1932, Vol. 18, N I.

6. Wilcox, W . W.: An interpretation of the relation between visuaI acuity and light intensity. Journ. gener. Psychology, 1936, v. 15, N 2.

7. F r y , G. and Bart ley , S. H.: The effect of one border in the visual field upon the threshold of another. Amek. Journ. Physiol. 1935, vol. 112, p. 414.

8. F r y , G . and Cabb, P.: Visual discrimination of the parallel bright bars in a dark field. Amer. Journ. Psychol. 1937, vol. 49, N I.