Visual Acuity

VISUAL ACUITY represents the eye's ability to separately perceive two points between which there is a certain distance. Visual acuity is one of the most important functions of the visual organ, and our ability to orient ourselves in the surrounding space depends mainly on its state. Physiological basis of visual acuity. When viewing two points, for example, two stars in the night sky, we obtain their inverted, diminished image on the retina in the region of the macula lutea thanks to the refractive apparatus of the eye. Depending on the distance between both stars, the image of both luminous points may fall on two adjacent cones or between two cones onto which the images fall, one or several cells unexcited by light will be located. Separate perception of both stars is possible only in the latter case. If the light stimulus captures two adjacent cones, then both perceived objects merge. Thus, the separate perception of two objects of the external world depends on the size of the image produced on the retina and is possible only if this image is not less than a certain limit. The size of the image on the retina in...

the eye. In figure 1, rays are drawn from the edges of the object a₁ through the center of the pupil, the so-called principal rays. If we designate the object by a and its distance from the eye by d₁, the size of the image l produced on the retina will depend on the angle ω, which is called the visual angle. The magnitude of this angle is determined by the following simple trigonometric formula: tg ω = l/d₁. Since we are usually dealing here with very angles, we can also replace the tangent with the angle itself, whence visual acuity is greater the smaller the visual angle at which the eye is still able to separately perceive two objects, i.e., visual acuity is inversely proportional to the visual angle. If visual acuity is denoted by v (from the Latin visus - vision), then v = 1/ω, i.e., visual acuity is directly proportional to the distance at which we are still able to distinguish the object, and inversely proportional to the size of the object. Through a series of studies, it was established that the normal eye is still able to separately perceive two objects if the distance between them is not less than one minute (1'). At the same time, however, one should not consider that 1' is the limit of the resolving power of the eye. Eyes are frequently encountered that are capable of separately perceive two objects if the distance between them is less than 1'. Therefore, if a visual angle of 1' is taken as the basis for normal visual acuity, this must be understood only in the sense that this angle represents the most frequent value obtained when examining a number of normal eyes. However, the ability of the eye to separately perceive two objects depends not only on their mutual distance. The following three factors also matter here: the visual angle under which the objects themselves are seen, illumination, and the contrast between the object and the background. Aubert, on the basis of experimental research, came to the following conclusions in this regard. Visual acuity is higher the larger the observed objects are in themselves, the brighter their illumination, and the sharper the contrast between the object and the background. The influence of illumination on visual acuity is especially great. If one starts from a very weak illumination, barely making it possible to distinguish visible objects, and then gradually increases it, one can establish the following regular relationship between the degree of illumination and visual acuity. At the beginning of the increase, visual acuity rises rapidly, then as illumination is further increased, the rise of the visual acuity curve slows down more and more, and the curve gradually turns into a horizontal line. Finally, as illumination is further increased, blinding phenomena set in, and visual acuity begins to drop again. The same data are obtained if, instead of white light, monochromatic light is used, and it turns out that various parts of the spectrum have an unequal effect on visual acuity. In addition to a number of external factors, visual acuity is also influenced by a number of factors depending on the eye itself. These include the state of adaptation of the eye, the sharpness of the image on the retina, and the pupil width. Regarding adaptation, the following is noted: upon transition from a dark room to a bright one, the eye is initially so blinded by bright light that its visual acuity at first appears greatly reduced and only gradually, as it adapts to the new lighting conditions, does visual acuity begin to increase. Similar relationships are observed upon transition from a bright room to a dark one. Therefore, for precise determinations of visual acuity, it is necessary that the eyes adapt for at least 15 minutes to the lighting conditions under which the determination of visual acuity will be made. The sharpness of the image produced by the observed object on the retina is one of the decisive factors determining visual acuity. In emmetropic eyes when looking into the distance in the region of the macula lutea, a sharp image of the fixed object is obtained, because here, thanks to the optical device of the eye, the main focus of the refractive media lies on the retina. Completely different relationships are observed in various refraction anomalies. Here the main focus lies in front of or behind the retina, and therefore from any distant point of the external space, an image in the form of a circle of diffusion is obtained on the retina. As a result, the images turn out to be unclear, blurred, which leads to a decrease in visual acuity. The size of the circles of diffusion on the retina, apart from refraction, also depends on the width of the pupil: the wider the pupil, the larger, other things being equal, the diameter of the circles of diffusion, and vice versa. On the other hand, too sharp a narrowing of the pupil also leads to an increase in the circles of diffusion due to the fact that phenomena of light deviation associated with the passage of light through narrow openings (light diffraction) begin to manifest here. Thus, both a too wide and a too narrow pupil lowers visual acuity due to the increase in the diameters of the circles of diffusion. Brailovsky, based on mathematical calculations and observations, was able to show that the best pupil width in this respect is 3 mm. Methodology of visual acuity research. To study visual acuity, special tables are used on which signs of various sizes are printed (letters, numbers, hooks, rings). The signs used to determine visual acuity are called optotypes. The most essential and fundamental issue in determining visual acuity is the nature of the optotypes. This issue is extremely important, because the same person gives different visual acuity depending on which tables are used for the determination, and sometimes this difference reaches quite significant proportions. Currently used tables are all constructed according to Snellen's principle. Snellen in 1862 proposed drawing optotypes in such a calculation that in each sign, regardless of whether it is a number, a letter, or some signs for the illiterate, a detail of the sign would be seen at a visual angle of 1', and the entire sign at a visual angle of 5'; here, a detail of the sign is understood as both the thickness of the strokes of which the given optotype is composed and the gap between individual strokes included in the composition of the sign. Snellen's principle is best understood from Fig. 2. It can be seen here that in the given letter "E", the size of the letter is 5 times the thickness of the stroke, and that the gap between individual strokes corresponds to the thickness of the strokes themselves. Snellen proposed three kinds of tables: of letters for the literate, of numbers for the semi-literate, and of special signs, hooks, for the illiterate. The shape of Snellen's hook can be seen from Fig. 3. When examined with these hooks, the subject must indicate in which direction the ends of the sticks are pointing. We have wide distribution

Figure 2. Snellen's letters. m Figure 3. Snellen's hooks. is based not only on the clear vision of the individual strokes making up their composition and on the distinction of the distance between the strokes, but a whole series of other factors also matters here, such as, for example, the character of the contours of a given letter, the alternation of light and dark spaces within it. Furthermore, when drawing certain numbers and letters, it is impossible to precisely observe Snellen's principles. Finally, purely psychological factors play an even greater role in the recognition of numbers and letters, as a result of which the subject, if well-literate, can correctly name a given letter or figure even in cases where they do not see it quite clearly, and thus an element of guessing is mixed into the determination of visual acuity. Due to all these circumstances, different letters of the alphabet included in the tables for determining visual acuity possess varying recognizability, which understandably creates great difficulties for the precise determination of visual acuity. How great the importance of the general configuration of a sign is for the recognizability of its individual detail can be seen from Fig. 4. In all these figures there is a white gap or clearance of identical size on a black background (reduced in Fig. 4). Below each of these signs are numbers which indicate at what maximum distance (in meters) this white clearance is correctly recognized. At the same time, it is seen that despite the identical size of this clearance, it is recognized in different signs from various distances, which ranges from 63 to 123 m. In view of the difficulties presented by optotypes in the form of letters and numbers for the precise determination of visual acuity, it was quite natural to strive to find some form of sign devoid of all these shortcomings. According to Landolt, an optotype must satisfy the following requirements. 1. The test object for examining visual acuity must represent a figure the recognition of which would be based on the separate perception of two visual impressions. 2. It must be applicable to an equal degree to literate and illiterate people of various nationalities. 3. It must make few demands on the intelligence of the patient. 4. It must exclude the possibility of guessing. 5. It must provide the opportunity to quickly and easily verify the patient's statements. Landolt's ring satisfies all these requirements (Fig. 5). It represents a black ring on a white background, in which in one place there is a pro-

stantly used tables by Donberg, Kryukov, and others, all based on Snellen's principle, but differing from each other in the selection of letters and figures included in their composition. Although the examination of visual acuity using numbers and figures presents certain purely practical conveniences, it is at the same time significantly imprecise. When determining visual acuity, we aim to establish the ability of the eye to separately perceive two objects between which there is a certain distance. Meanwhile, the recognition of letters and figures is bas-

break limited by parallel edges. The size of the break corresponds to the thickness of the ring's contours, and it is seen from a certain distance at a visual angle of 1'; the diameter of the ring itself is taken 5 times larger and is seen at a visual angle of 5'. On Landolt's charts, rings of various sizes are arranged, with the breaks in them pointing in different directions. Landolt's ring was adopted as the international optotype at the XI International Ophthalmological Congress in 1909. All tables for determining visual acuity, independently 159

VISUAL ACUITY

what characters they consist of, are compiled in the following manner. Each chart consists of a series of rows, mostly of 10, with several characters in each row. The size of the characters is the same in each row, but when moving from row to row, the size of the characters gradually decreases. To the right of each row there is a figure indicating at what distance an eye with normal visual acuity should recognize the given row. There are two systems of charts: foot and metric. In the former, the distance is indicated in feet, in the latter in meters. Foot charts are currently almost never used. In metric charts, the top row is designed for 50 m, and the bottom row for 5 m. Sometimes there are 1-2 additional rows at the bottom, designed for a closer distance. The examination is usually carried out in a dark room with artificial lighting. For the latter purpose, special illuminating devices (Roth apparatus) are used. They are a wooden rectangular frame, the walls of which are lined from the inside with mirror glass. Charts for examining visual acuity are inserted into the back of this frame, and an electric bulb is inserted in the front, covered on the side of the subject by an opaque screen. During the examination, the patient is seated at a distance of 5 m from the chart. Each eye is examined separately. To close the second eye, the patient is put on a trial spectacle frame, into one cell of which a frosted glass or a black metal plate is inserted. In this position, the patient reads the characters of the chart from top to bottom, and for an accurate determination of visual acuity, one cannot be limited to one or several characters of each row, but it is necessary that the subject names all the characters of the given row. The results of the visual acuity examination when using metric charts are always expressed as a decimal fraction. In this case, the following formula ¹ D' is used, in which V denotes visual acuity, d is the distance at which the patient reads the last row that he is still able to make out, and D is the distance at which the normal eye should see this row. Thus, for example, if the patient, sitting at a distance of 5 m from the chart, reads only the top row designed for 50 m, then his visual acuity will be ^=0.1. If, however, at this distance he also reads the last row designed for 5 m, then his visual acuity will be |=1.0. Thus, normal visual acuity is designated as 1.0, lowered as a decimal fraction less than unity (tenths or hundredths), and increased visual acuity as a fraction above unity. For these cases, some charts serve with additional rows designed for a distance of less than 5 m. If the patient sees nothing from a distance of 5 m, he is gradually brought closer to the chart until he sees the largest character. Thus, for example, if he sees it only from a distance of 2 m, then his visual acuity will be ^ = 0.04. It is even simpler to make the subject count fingers shown at various distances, assuming that the size of our fingers corresponds to the characters of the largest row of the charts. If the eye has only light perception, its V is conventionally designated as +, and complete blindness is designated as V = 0. Below are the most accurate charts for examining visual acuity. The first of them (Fig. 6 - reduced approx. 4 times) is an international chart adopted as a standard for determining visual acuity at the XI International Ophthalmological Congress, published by Hess and modified somewhat by Hegener. It consists of two halves: in one - Landolt rings, in the other - digits. The presence of digits is explained by the fact that the majority of ophthalmologists, recognizing the advantages of Landolt rings, did not want, however, for a number of practical reasons, to abandon digits as characters for determining visual acuity. 7 1 O o 4 O 7 O G Л 7 4 O ooo 7 O 4 7 o o c 1 4 7 O 4 7 O 1 O 4 -4 7 ooo o o o c o o o o c 1 O 7 O 4 7 1 4 Л O 4 O 7 4- 1 7 O 1 7 O «- 7 1 Figure 6. o o o o o o o o o c o o o o o э o o o c The size and design of these digits were selected purely empirically so that their recognizability coincides with the recognizability of Landolt rings. Thanks to this, the visual acuity determined both by rings and by digits turns out to be the same. - The second of the most accurate charts is the Golovin and Sivцев charts. They also consist of two halves: in the first of them are Landolt rings, in the second - Russian letters. This chart was constructed in a purely empirical way based on the following considerations. Different letters of the alphabet, as already indicated above, are not recognized equally easily. There are letters that are difficult to recognize, such as e.g. 3, and letters that are easily recognized, like A, O, T. The former are recognized (at the same size) from a significantly closer distance than the latter. If both easily and difficultly recognized letters are included in the rows of the chart at the same time, this makes the determination of visual acuity inaccurate, because the patient, for example, without recognizing all the letters of the 5th row, at the same time still recognizes some letters of the 6th and 7th rows. The recognizability of certain letters was preliminarily established by examining 400 emmetropes, and only letters with identical recognizability were introduced into the composition of the chart, and those were selected which, according to some recognition features, came closest to Landolt rings. - This chart represents the first attempt to compile charts for determining visual acuity, in which the letters were selected not by chance, but after a detailed study of their recognizability based on the statistical method. In view of this, the use of these charts guarantees significantly greater accuracy in determining visual acuity than other charts consisting of letters or digits. In most currently used charts, the top row when examined at a distance of 5 m corresponds to a visual acuity of 0.1, and the bottom, tenth row - to a visual acuity = 1.0, and when moving from one row to another, visual acuity increases by 0.1. Thus, a series in the form of an arithmetic progression is obtained, namely - 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0. - The recent introduction of new forms of corrective glasses consisting of magnifying systems (telescopic glasses and magnifiers; see Glasses) has necessitated the creation of new charts with a slightly different gradation of visual acuity. The fact is that these glasses, along with their beneficial action, also possess a side effect expressed in the fact that they narrow the field of vision, and this side effect is expressed more sharply the stronger the magnifying power of the glasses. Therefore, it is advantageous to prescribe the weakest magnifying system that still gives a sufficiently good visual acuity. The selection of these glasses is based on an accurate determination of visual acuity, and since they are intended for persons with sharply weakened visual acuity, there is a need for charts that make it possible to accurately determine visual acuity precisely within low values. For this purpose, the Henker chart serves. This chart is compiled according to the type of the international one, but within the range from 0.1 to 0.3 a number of additional rows are introduced, namely: 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.25 and 0.3. All the considered charts are intended for determining visual acuity for distance. There are, in addition, special charts for examining visual acuity for near. They mostly consist not of separate digits or letters, but of some printed text. On each such chart there are several texts differing from each other in the sizes of the letters. In view of the fact that individual letters differ in unequal recognizability and, moreover, a word can be read correctly even in the case when not all the letters included in its composition are seen clearly enough, it is quite natural that charts of this kind cannot claim any significant accuracy. However, since they are very convenient for checking whether the glasses prescribed to the patient are really suitable for reading, they are very widespread. The most satisfactory chart in terms of accuracy for determining visual acuity for near is Hegener's chart. In the manufacture of charts for near, one major difficulty is encountered. Due to the fact that these charts are designed for a close distance, the individual characters on them have a very small size, and therefore, to comply with the angle of vision in V, their details must be very small, sufficiently clear, but at the same time retain the size defined for a given detail. In their manufacture by ordinary printing methods, this is almost impossible to achieve, because various defects of paper and printing inks entail a certain degree of spreading of the ink on the paper, which affects the size of individual details. In view of this, Hegener made his charts photographically, reducing the usual international type charts in a corresponding manner as applied to examination at a distance of 25 cm. Hegener's chart is a positive slide mounted in a frame equipped with a handle. The entire frame can slide along a graduated ruler, one end of which rests against the patient's forehead above the eye during examination, and against the forehead above the bridge of the nose during binocular examination.

Thanks to its sliding movement along a graduated ruler, Gæger's test-card is very convenient for determining the far point of distinct vision in high myopia. The general appearance of the test-card is shown in Fig. 7. If visual acuity for distance is examined first for each eye separately and then for both eyes simultaneously, it turns out that binocular visual acuity is higher than monocular, i.e., when looking with both eyes, a visual acuity is obtained that exceeds the visual acuity of the better-seeing eye.

To explain this circumstance, a number of contradictory and poorly substantiated theories have been proposed. Ditinsky and Ilyina, in investigating this issue, were able to ascertain that the reason for this phenomenon lies in the unequal width of the pupil during monocular and binocular determination of visual acuity. During monocular examination, when one eye is closed and does not participate in the act of vision, the pupil in that eye dilates due to darkness. This reflexively causes a slight dilation of the pupil in the eye being examined, which, through an increase in the circles of light diffusion on the retina, entails a decrease in visual acuity during monocular determination.