# Visual acuity

 Diagram illustrating the concept of visual acuity

Visual acuity is the ability of the eye to perceive separately two points located from each other at a distance. The measure visual acuity is the angle of view, i.e., the angle formed by the rays emanating from the edges of the considered object or from two points (a And b) to anchor point (K) of the eye (scheme; a and b - displays the points a and B on the retina). Visual acuity is inversely proportional to the angle of view, i.e., the smaller, the visual acuity above. In the normal human eye is able separately to perceive objects, the angular distance between them not less than 1' (one minute).
Visual acuity is one of the most important functions of the organ of vision. It depends on the size of cone cells in the macula, the retina, as well as from a number of factors: eye refraction, the width of the pupil, the transparency of the cornea, lens, vitreous body, the condition of the retina and optic nerve, age. Definition of visual acuity is one of the main methods of research of a condition of the organ of vision.
To study visual acuity using tables consisting of 12 rows of letters, rings or drawings (tables for children) of a specific size. The table is built so that the thickness of the stroke letters or mark the tenth row is visible from a distance of 5 m at an angle of view in 1'; this corresponds to a visual acuity 1.0. Discrimination of letters, digits on the top row of the table corresponds to the visual acuity 0.1; second - 0,2; third - 0,3 etc. Discernment of characters in 11 and 12 rows corresponds to the visual acuity 1.5, and 2.0.
When visual acuity table should be well lit, for which they are placed in the lighting apparatus Company (see Company of the device). The study was conducted from a distance of 5 m, visual acuity of each eye examine separately, closing while the other eye opaque shield. The signs in the table show the end of the black pointer. Offer studied to read the signs in the table, beginning with the largest. Visual acuity corresponds to the last row, the signs of which investigated reads fully or does not distinguish it 1-2 sign. Figure about this number indicates the severity of view. If the study does not read the signs of the first row, visual acuity at it less than 0.1. In these cases, visual acuity fail investigated to the table or to bring him some symbols (ring with a gap, black sticks on a white background), size equal signs of the top row of the table, noting the distance from which he begins to distinguish them. Every 0,5 m corresponding visual acuity of 0.01. So define the visual acuity from 0.09 to 0.01. With lower visual acuity propose to distinguish between the fingers or hand movement investigating. Discernment movements of the hand at the distance of 30 cm in front of the eye corresponds to the visual acuity of 0.001. If the patient feels only the light, the visual acuity is designated as light sensation. When complete blindness visual acuity is equal to 0. Research results are recorded separately for the right and left eyes.

Visual acuity is the degree of perceived boundaries and parts of the considered subjects. O. C.- one of the most important functions of the visual analyzer, has considerable influence on the ability to navigate in the environment. The physiological basis O. C. is a contrast, or differential, sensitivity, i.e. the ability of the eye to notice the difference in brightness fields being compared. So, a bright spot of light on the background becomes first noticeable only at known changing its brightness in comparison with the brightness of the background.
O. C. usually characterize the minimum interval between two objects or points, in which the eyes can see them separately. For separate distinguish between two points need to between their images on the retina was a space, agitation which causes the feeling different than the excitement of those places, which are projected image of these points. in the Final process of recognition of details and properties of the considered object is caused differencirovannoe activity of the cerebral cortex. Along with fractional analysis of light irritation this process involves and communication (synthesis) last with other irritations, especially tactile and proprioceptive. In the act of seeing "to our eye join not only the other senses, but also the activity of our thinking" (F. Engels). This explains the fact that after the birth of O. C. the child develops gradually and usually only to 6-8 years reaches the normal level.
Practical measure visual acuity is the inverse of an angle of view (Fig. 1), i.e. the corner formed by the rays coming in from the edges of the considered object or considered from two points (L) to anchor the eye (s'); in this case the angle ACB. The smaller the angle of view, allowing separate the vision of two points, the more O. C. Most people have a minimum value of this angle is equal to one minute (G), so this value is accepted as the norm, and O. C. eyes that have the smallest angle of view in 1'is a normal O. C.

Fig. 1. Scheme of perspective.
Fig. 2. The dependence of visual acuity from the retina: the solid line - the sharpness cone (day) of view; dashed - acuity rod (twilight) vision; dashed rectangle - region "blind spots.

The most advanced feature distinguishing parts of the objects possesses the Central hole mesh shell (see). As the distance from the center to the periphery of the retina O.Z. for cone of vision steeply falling (Fig. 2). Significantly affects O. C. the state of refraction of the eye. Astigmatism, myopia, high hyperopia O. C. significant decreases in comparison with visual acuity emmetropic eye, as on the retina get a clear picture of the considered subjects. The clarity of images on the retina and, therefore, O. C. several worsen too broad and too narrow pupil; optimal O. C. notes when the pupil diameter 3 mm (C. K. Verbitsky, S. M. Brailovski). Some improvement O. C. observed when monochromatic illumination of the object (for example, yellow), not mixed with the light (Century Weinberg and E. A. Lapinskaya).
Practical importance has the effect of light intensity on the O. C.
In distinguishing black objects on a white background O. C. reaches the highest level only when illuminated in thousands suites. When using white objects on a black background there are other ratios: O. C. reaches its maximum at illumination of only 5-6 Luke, and at the further increase of illumination is reduced (I.e. Century maple).
Visual acuity impact and adaptation of the eye (see). O. C. worse, if the eyes had adjusted to the brightness, less than or significantly greater than the brightness of the test field. C. Century Kravkov al. determined the dependence of O. C. from the Central nervous system. This is indicated by the changes O. C. when covering one eye, under the influence of auditory stimuli and the conditioned reflex of its change.
For research O. C. enjoy special tables that bear the test marks of various sizes (letters, numbers, hooks, rings) - the optotypes. In the basis of modern tables on the principle proposed by Selenom (N. Snellen) in 1862, the Optotypes done so, to a certain distance of the part of the sign would be visible from the perspective of a 1', and the whole character - from the perspective of a 5'; in this case, the part of the sign is understood as the thickness of the strokes that made this optotype and the interval between the individual touches that are part of the token (Fig. 3). The table consists of several rows, with each row are signs of equal size. The side of each series marked the distance from which a bar of the sign is visible from the perspective of 1'. Knowing this distance and the distance from which the investigated recognize this symbol, it is easy to determine O. C. according to the formula: V=d/D, where V is the visual acuity, d is the distance at which there is studied from the table D - the distance from which a bar of the sign is visible from the perspective of 1'.
In the USSR, the most common tables Golovin - Sivtseva (Fig. 4), which has 12 rows of characters - letters and rings with a gap (the optotypes Landolt). In the study at a distance of 5 m upper row of these tables corresponds to the visual acuity of 0.1, and the tenth range - normal visual acuity 1.0. Thus, when passing down from one row to another value O. C. increase in arithmetic progression- 0,1; 0,2; 0,3; 0,4; 0,5; 0,6; 0,7; 0,8; 0,9 and 1.0. Two of the bottom row is the eleventh and twelfth - are used to determine O. C. than 1.0 (1.5 and 2.0).

Fig. 4. Table Golovin - Sivtseva for determination of visual acuity in the apparatus for lighting, designed in the Institute of eye diseases named. Helmholtz.

Table placed in the lighting apparatus of the Company in the modification of the Institute of eye diseases. Helmholtz with electric lamp 40W, closed by the monitored by an opaque shield. This creates a relatively uniform illumination of tables 700 LK. Illuminator strengthen on a wall so that its lower edge was at a distance of 120 cm from the floor. During the study, the patient should keep your head straight, eyelids, both eyes need to be open. The inscrutable eyes covered with an opaque shield of white color. In 2-3 seconds show pointer sign on the table and ask investigated to call it. When evaluating the results of the study are the notions of full-time and part visual acuity. When full O. C. all characters to the corresponding number of investigated calls correctly. If the rows of the table, the corresponding visual acuity 0,3; 0,4; 0,5; 0,6 not recognized by one character, and in the ranks of the corresponding visual acuity 0,7; 0,8; 0,9; 1,0,- two signs, O. C. estimated this number as incomplete.
To determine O. C. less than 0.1 patient gradually approaching to the table (intervals of 0.5 m) until the call correctly signs of the top row. O. C. evaluate using the above formula. For example, if the study sees signs from a distance of 3 m, O. C. is 0.06(3M/50m). But it is better to determine O. C. less than 0.1 to use a set of optotypes B. L. Pole. The set consists of 6 ring and 6 three of optotypes of various sizes, pasted on cardboard sheets. The size of the optotypes is designed so that the thickness of the strokes and the width of gaps match the visual acuity 0,09; 0,08; 0,07; 0,06; 0,05 and 0.04 for a distance of 5 meters and If O. C. in the analyzed below 0,04, you test it from a distance of 2.5 m In the determination of O. C. below 0.1 optotype placed in the apparatus for lighting tables.
In the tables for research O. C., compiled on the basis of an arithmetic progression, the difference in O. C. when moving from one row to another very uneven. So, the transition from the first number to the second (0.1 and 0.2) O. C. increases in 2 times, and in the transition from the fifth row to the sixth (0,5 and 0,6) - only 1.2 times. In connection with the proposed table, based on the principle exponentially. The most practical application has a table C. E. Sheveleva.

Fig. 5. Table for determination of visual acuity in children of preschool age.

For definition of visual acuity in children of preschool age are tables with pictures (Fig. 5). Before the examination usually bring baby to the table and asked to name depicted objects so that he can adapt to the fact that it will require. During the determination of O. C. children get tired quickly. Therefore, in the study of O. C., starting with the top row of the table show the child in each row, only one picture. If he can't call her, then point to recognize all other pictures of this number, then the above number, and so on, until it is properly called most of the characters in a row. This number will determine O. C. in a given child.
Using reviewed tables define O. C. for Dali. There are, in addition, the table for the study of visual acuity at close range. They are mostly not of the individual digits or letters, and from several printed texts, which differ from each other by the size of the letters. These tables are usually used in assigning points for reading. A number of authors [Ohm (J. Ohm), Gunther (Gunther G.) and others] have proposed an objective method of determining O. C. on optokinetic nystagmus. The latter occurs only when moving in front of the eye objects differ. This method is particularly precious in case of detection of aggravatio or examination, when subjective research methods O. C. insufficiently reliable.