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 chiefly on its state. Physiological basis of visual acuity. When viewing two points, for example, two stars in the night sky, thanks to the refractive apparatus of the eye, we obtain their inverted, diminished image on the retina in the region of the macula lutea. Depending on the distance between both stars, the image of both luminous points can fall on two adjacent cones or between two cones, onto which the images fall, there will be one or several cells not stimulated by light. Separate perception of both stars is possible only in the latter case. If the light stimulus encompasses 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...

eye. In figure 1, from the edges of the object a₁ rays are drawn 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, then the size of the image β 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: tgω = β / d₁. Since we usually deal here with very small 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 designated by v (from the Latin visus - vision), then v = d / B, 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 often encountered that are capable of separately perceiving 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 eye's ability to separately perceive two objects depends not only on their mutual distance. The following three factors also matter here: the visual angle at which the objects themselves are seen, the illumination, and the contrast between the object and the background. Aubert, based on experimental research, came to the following conclusions in this regard. Visual acuity is higher the larger the objects being viewed 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 that barely makes it possible to distinguish visible objects and then gradually intensifies it, one can establish the following regular relationship between the degree of illumination and visual acuity. At the beginning of intensification, visual acuity grows rapidly; then, as illumination is further intensified, the rise of the visual acuity curve slows down more and more, and the curve gradually transitions into a horizontal line. Finally, as illumination is further intensified, blinding phenomena occur, 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, a number of factors depending on the eye itself also affect visual acuity. These include the state of adaptation of the eye, the distinctness of the image on the retina, and the width of the pupil. 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 strongly reduced and only gradually, as it adapts to the new illumination conditions, does visual acuity begin to rise. 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 illumination conditions under which the determination of visual acuity will be performed. The distinctness of the image produced by the viewed object on the retina is one of the decisive factors determining visual acuity. In emmetropic eyes, when looking into the distance, a distinct image of the fixed object is obtained in the region of the macula lutea, 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 refractive anomalies. Here, the main focus lies in front of or behind the retina, and therefore an image in the form of a circle of diffusion is obtained on the retina from any distant point of the external space. As a consequence of this, the images turn out to be unclear and 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 light diffraction phenomena associated with the passage of light through narrow openings begin to manifest here. Thus, both too wide and too narrow a pupil lowers visual acuity due to an 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 regard is 3 mm. Methods of investigating visual acuity. To investigate visual acuity, special tables are used on which signs of various sizes are printed (letters, numbers, hooks, rings). The signs serving to determine visual acuity are called optotypes. The most essential and fundamental question in determining visual acuity is the question of the nature of the optotypes. This question is extremely important, because the same person gives different visual acuity depending on what tables the determination is carried out with, and sometimes this difference reaches quite significant proportions. The tables currently in use are all constructed according to the principle of Snellen. 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 visible at a visual angle of 1', and the entire sign at a visual angle of 5'; wherein a detail of the sign is understood to be 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 a given letter "E", the size of the letter exceeds the stroke thickness by 5 times and that the gap between individual strokes corresponds to the thickness of the strokes themselves. Snellen proposed tables of three kinds: 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 examining with these hooks, the subject must indicate in which direction the ends of the sticks are pointing. We have a large distribution

Figure 2. Snellen's letters. m Figure 3. Snellen's hooks. is based not only on the clear vision of individual strokes making up their composition and on the distinction of the distance between strokes, but a whole series of other factors also matters here, such as the character of the contours of a given letter, the alternation of light and dark spaces in it. Moreover, when drawing certain numbers and letters, it is impossible to precisely observe Snellen's principles. Finally, in recognizing numbers and letters, purely psychic moments play an even greater role, as a result of which the subject, if well literate, can correctly name a given letter or number even if 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 naturally 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 the same size on a black background (in Fig. 4, reduced). 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 evident 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 that optotypes in the form of letters and numbers present for the precise determination of visual acuity, it was completely 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 be a figure whose recognition 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 check the patient's statements. All these requirements are met by Landolt's ring (Fig. 5). It is a black ring on a white background, in which in one place there is a pro-

stantly used the tables of Donberg, Kryukov, and others, all based on Snellen's principle, but differing from each other in the selection of letters and numbers included in their composition. Although the study of visual acuity using numbers and letters presents certain purely practical conveniences, it is at the same time to a significant degree imprecise. When determining visual acuity, we strive 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 numbers 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 juxtaposed, with the breaks in them facing in different directions. Landolt's ring was adopted as the international optotype at the XI International Ophthalmological Congress in 1909. All charts for determining visual acuity, regardless of 159

VISUAL ACUITY

what characters they consist of, are compiled as follows. Each chart consists of a series of lines, for the most part of 10, with several characters in each line. In each line the size of the characters is the same, but when moving from line to line the size of the characters gradually decreases. To the right of each line there is a figure indicating at what distance an eye with normal visual acuity should recognize the given line. There are two systems of charts: the foot system and the metric system. 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 line is calculated for 50 m, and the bottom line for 5 m. Sometimes there are 1-2 additional lines at the bottom, calculated for a closer distance. The examination is usually carried out in a dark room under artificial lighting. For the latter purpose, special illuminating devices (Roth apparatuses) serve. They are a wooden rectangular frame, the walls of which are lined inside with mirrored glass. From the back, charts for examining visual acuity are inserted into this frame, and from the front an electric light bulb, closed from the side of the examinee 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 ground 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 line, but it is necessary that the examinee name all the characters of the given line. The results of visual acuity examination when using metric charts are always expressed as a decimal fraction. In doing so, the following formula is used ¹ D' in which V denotes visual acuity, d is the distance at which the patient reads the last line that he is still able to make out, and D is the distance at which the normal eye should see this line. Thus, for example, if a patient, sitting at a distance of 5 m from the chart, reads only the top line calculated for 50 m, then his visual acuity will be V=0.1. If he reads at this distance also the last line calculated for 5 m, then his visual acuity will be V=1.0. Thus, normal visual acuity is designated as 1.0, decreased as a decimal fraction less than unity (tenths or hundredths), and increased visual acuity as a fraction above unity. For these cases, in some charts, additional lines calculated for a distance of less than 5 m serve. 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, his visual acuity will be V = 0.04. It is even simpler to make the examinee 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 total blindness is designated as V = 0. Below are the most accurate charts for examining visual acuity. The first of them (Fig. 6 - reduced approximately 4 times) is an international chart adopted as a standard for determining visual acuity at the XI International Ophthalmological Congress, published by Hess and slightly modified by Hegener. It consists of two halves: in one - Landolt rings, in the other - numbers. The presence of numbers 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 numbers as characters for determining visual acuity. 7 1 O o 4 O 7 O G L 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 L 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 e o o o c The size and design of these numbers 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 numbers turns out to be the same. - The second of the most accurate charts are the Golovin and Sivtsev charts. They also consist of two halves: in the first of them are Landolt rings, in the second - Russian letters. This chart was constructed purely empirically on the basis of the following considerations. Different letters of the alphabet, as already indicated above, are recognized with unequal ease. There are letters that are difficult to recognize, such as e.g. Z, and letters that are easily recognized, such as 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 lines of the chart at the same time, this makes the determination of visual acuity inaccurate, since the patient, for example, without recognizing all the letters of the 5th line, nevertheless simultaneously recognizes some letters of the 6th and 7th lines. The recognizability of certain letters was preliminarily established by examining 400 emmetropes, and only letters with equal recognizability were introduced into the composition of the chart, and those were selected which according to certain 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 numbers. In the majority of currently used charts, the top line when examined at a distance of 5 m corresponds to a visual acuity of 0.1, and the bottom, tenth line - to a visual acuity = 1.0, and when moving from one line to another, the visual acuity increases by 0.1. Thus, a series is obtained in the form of an arithmetic progression, 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 correcting glasses consisting of magnifying systems (telescopic glasses and magnifiers; see Glasses) has necessitated the creation of new charts with a somewhat different gradation of visual acuity. The fact is that these glasses, along with their beneficial action, also possess a side action, expressed in the fact that they narrow the visual field, and this side action is the sharper, the stronger the magnifying ability 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 arises the 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, however, in the range from 0.1 to 0.3, a number of additional lines 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 for the most part consist not of individual numbers 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 correctly read even in the case when not all the letters making up its composition are visible with sufficient clarity, it is quite natural that charts of this kind cannot claim any significant accuracy. However, since they are very convenient for checking how well the glasses prescribed to the patient are actually 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 maintain the angle of vision in V, their details must be very small, sufficiently clear, but at the same time retain a size specific to the given detail. In their manufacture by ordinary printing methods, this is almost impossible to achieve, since various defects of paper and printing inks entail to a certain extent the spreading of ink on the paper, which affects the size of individual details. In view of this, Hegener made his charts by photographic means, reducing ordinary charts of the international type in a corresponding manner as applied to examination at a distance of 25 cm. Hegener's chart is a diapositive mounted in a frame equipped with a handle. The entire frame can slide along a graduated ruler, one end of which rests during the examination of the patient against the forehead above the eye, and during binocular examination - against the forehead above the bridge of the nose. Thanks to sliding along a graduated rule, Hegener's card is very convenient for determining the far point of distinct vision in high myopia. The general appearance of visual acuity is shown in Fig. 7. If visual acuity for distance is tested first for each eye separately and then for both eyes simultaneously, it turns out that binocular visual acuity is higher than monocular, that is, 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, when investigating this issue, were able to find 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 this eye dilates due to darkening. This reflexively causes a slight dilation of the pupil in the eye being examined, which, due to the increase in circles of light diffusion on the retina, entails a decrease in visual acuity during monocular determination.