VISUAL ACUITY — a measure of ability of an eye to find, distinguish and learn objects on a surrounding background. Respectively speak about O. z. by definition of the smallest visible, the smallest divided and smallest recognizable. O.'s decrease z. at the person it can be observed at disturbances both in optical, and in a nervous link of visual system. One of the most frequent reasons of decrease in O. z. disturbances of focusing of the image on a retina are — ametropias (see). Lake z. without correction especially considerably decreases at a myopia (see. Short-sightedness ); at a hypermetropia (see. Far-sightedness ) it can remain normal thanks to tension accommodations of an eye (see). O.'s decrease z. it is observed at opacification of the refracting environments of an eye — corneas, a crystalline lens and a vitreous. At last, O. z. it can be reduced owing to diseases of a retina, optic nerve, and also overlying departments of the visual analyzer.
When any of the specified reasons of decrease in O. z. it is not possible to find, speak about amblyopias (see).
Physiological bases of visual acuity
Research O. z. is the basic funkts, the test at assessment of a condition of the central sight of the person also pathologies are normal. Lake z. it is measured by size, the return to a visual angle, under the Crimea the smallest visible subject is perceived. R. Hooke in 17 century established that 2 stars are visible with the naked eye separately if the angular distance between them is equal or more than 1 min. If an eye distinguishes the details of objects seen at an angle 1 min., O. z. is accepted equal 1,0.
It is necessary for separate perception of 2 objects that between their projections on a retina of an eye there was a certain interval. Long time was considered that 2 shining points are perceived separately if they excite 2 non-adjacent flasks (i.e. divided at least by one unexcited flask). Really, the limit angle of distinguishing of fine details in 1 min. corresponds to distance on a retina of a schematic eye in 4,9 nanometers, a cut is approximately equal to 2 diameters of a flask of a retina. However O. z., measured by various methods, corresponds to various corners of permission, quite often considerably smaller, than 1 min. therefore the hypothesis of two flasks is exposed to essential amendments.
Lake z. depends on two processes — formations of the image on a retina of an eye and its transfer to the relevant departments of a brain. Respectively it is influenced by 2 groups of factors connected with «optical» and «nervous» links of visual system.
To the optical factors influencing O. z., a state the wedge, refractions belong (see. Refraction of an eye ) and accommodations of an eye, existence of an aberration in its optical system (see. Aberration of an eye ), width of a pupil, and also transparency of the refracting environments of an eye.
Treat nervous factors density photoreceptors (see) in the central pole of a retina, a threshold of their sensitivity, a condition of all elements retinas (see), optic nerve (see), the subcrustal and cortical centers of sight (see. Visual centers, ways ). Also the condition of light adaptation, movement of eyes, and also irritation of other systems have a certain value.
A technique of a research of visual acuity
At the person about O. z. judge by recognition of specially shown signs, usually black on a white background. Introduction them in practice is connected with names of Becker (Becker, 1805), Chevalier (A. Chevalier, 1815) and Kyukhlera (H. Kuchler, 1844). Signs can be in the form of letters, figures, silhouette pictures or special figures, in to-rykh it is necessary to find a certain detail, napr, a gap in a ring. From the moment of emergence of signs for a research O. z. the tendency to their standardization is noted. Snellen (N. of Snellen, 1862) entered the principle of a constant ratio of the size of a sign and thickness of its line equal 1: 5, and by this principle constructed alphabetic tables. He offered a formula of definition of O. z. according to tables, a cut use still:
V = d/D,
where V — O. z., expressed by simple or decimal fraction; d — distance, with to-rogo the research is conducted; D — distance, with to-rogo the distinguished sign it is visible investigated with the visual acuity equal 1,0.
Signs for a research O. z. it is possible to divide into two basic groups: simple, or signs of distinguishing, and difficult, or signs of recognition. To to the first group rings with gaps, hooks like letter «Sh» and silhouette figures of various orientation belong (only one sign shown in various provisions is used). A number of signs of distinguishing is applied:
1. Landolt's ring — a ring, black on a white background, with a gap (fig. 1, a). Thickness of the line of a ring and width of a gap are equal its 1/5 outer diameters. A subject of distinguishing is the provision of a gap. It can have four options (above, below, on the right and at the left) or eight (the same provisions plus four slanting). On the XI International ophthalmologic congress in Naples in 1909 Landolt's ring was accepted as a standard sign for a research O. z.
2. Snellen's hook — the letter «Sh», the free ends a cut are turned to the right, to the left, up (fig. 1, b) or down. In tables and devices for a research O. z. the option of this sign when the average strip is shortened (fig. 1, c) is especially often applied — a so-called hook of Pflyuger. These signs are convenient for O.'s definition z. at children, and also at deaf-mutes and foreigners. Investigated in these cases give the model of a test sign in hands and ask to specify the provision of the shown signs with its help.
3. A cross with a break of one of its strips (fig. 2). A subject of distinguishing is the provision of a strip with a break. Lake z. it is measured on the minimum visible shift of a strip. Determined by this test O. z. it is accepted to call vernier. It is much higher, than O. z., determined by other signs.
4. Signs with periodic structure — black-and-white lattices and chess cells. Treat them Gilleri's world — the lattice from black and white strips of equal width shown in four provisions and Goldmann's world — a black-and-white chess cell in the form of the square divided by white intervals into four square sections (in three of them the side of the chess field is less than limit distinguishing, in the fourth — it is more). A subject of recognition is the provision of section with larger fields (fig. 3).
To to the second group letters, figures and silhouette pictures of various contents belong.
1. Letters and figures. Choose usually those from them, to-rye well fit into a square. At their tracing respect for the principle of Snellen is obligatory: thickness of the line shall be 5 times less than the party of a square. Recognition of various letters and figures is not identical. D. A. Sivtsev (1925) suggested to use for a research O. z. the Russian letters (NKIBMShY) having approximately identical probability of recognition. Hess (S. of Hess, 1910) applied figures (0 1 4 7).
2. Pictures for a research O. z. at children. Them the set as in the Soviet Union, and abroad is offered. Signs for a research O. z. show one by one, by groups or in the form of tables, in to-rykh group of signs of one size are combined in one line (fig. 4).
Tables of the Russian letters in combination with other optotypes are offered by A. A. Kryukov (1902), S. S. Golovin and D. A. Sivtsev (1925), I. M. Averbakh, S. V. Krav-kov and S. Ya. Friedman (1939), V. E. Shevalev (1946). Golovin's table — Sivtseva (fig. 5) is most distributed in the USSR. It consists of two half: in left — the letters H K I B M Sh Y, in right — Landolt's rings in four provisions. That and others are randomized on 12 lines containing signs of the identical size. Their sizes (from top to down) correspond to visual acuity 0,1; 0,2; 0,3; 0,4; 0,5; 0,6; 0,7; 0,3; 0,9; 1,0 — 1,5 — 2,0. To the left of every line often designate size O. z., on the right — distance in meters, with to-rogo this line it is visible at normal O. z. (1,0). The table is expected a research from distance of 5 m.
There is a number of special tables for O.'s definition z. at visually impaired: Landolt's table, Gen-kera (O. of Henker, 1932), B. L. Polyak's optotypes (1951). They have signs for O.'s definition z. from 0,05 to 0,2-0,4.
In the majority of countries of Western Europe and the USA tables are expected a research from distance of 20 feet (6), and lines are designated by distance in feet or meters, with to-rogo they differ normal. Lake z. it is measured by the relation, in numerator to-rogo 20 (or 6), and in a denominator — the number standing at the line read investigated. So, at designation in feet the line with an index 40 corresponds to visual acuity 0,5, and with an index 200 — visual acuity 0,1. At designation in meters to a line 24 there corresponds visual acuity 0,25, and a line 120 — visual acuity 0,05.
Size O. z. depends on color structure and background brightness, on Krom signs, and their contrast with a background are shown. The monochromatic background gives big O. z., than mixed. Lake z. comes nearer to maximum at the wavelength of light of 540 — 600 nanometers, background brightness about 300 cd/m 2 and contrast about 60 — 80%. Essential value has also illumination of the room, in Krom the research is made. During the use of tables with increase of illumination of O. z. increases, at presentation of signs by means of transparantny devices and especially projectors — decreases.
Research O. z. it has to be carried out with observance of reference conditions. At a research on tables their illumination of 700 lx, illumination indoors at the level of 0,8 m from a floor not less than 75 lx (is required at filament lamps) and not less than 150 lx (at fluorescent lamps); contrast of signs with the field of the table shall be not less than 80%. It is much more difficult to meet reference conditions at researches by means of projectors. It is necessary that without additional lighting indoors they created background brightness not less than 100 cd/m 2 and contrast of signs with a background not less than 70%.
Usually O. z. investigate at presentation of a motionless object on unlimited (about 1 — 2 sec.) time. For judgment of a fast rosti of visual reactions show test the sign (usually the same ring of Landolt) moving with constant speed on an arch with the center in the studied eye. Lake z., received in these conditions, is called dynamic. Lake z., received at presentation of objects on set short (less than 1 sec.) time, is called exposition. O.'s dependence z. from time of presentation of an object it is especially expressed in the range from 15 to 100 ms.
Allocate retinal O. z., to-ruyu define at opacification of the refracting environments of an eye. The bunch of laser radiation focused by means of optical elements in two dot images near a nodal point goes to an eye eyes (see). The bunches of coherent monochromatic radiation dispersing from them form interference bands, to-rye are projected on area of the central pole of a retina (investigated sees a lattice from ghost and dark lines). At reduction of distance between sources of radiation width of strips increases. Changing the provision of two points of rather nodal point by means of optical elements, investigating changes orientation of strips and asks at the same time investigated to specify orientation. On the minimum distance between strips of an interferential lattice, at Krom investigated distinguishes its orientation, judge retinal O. z. This method allows to predict a possibility of recovery of O. z. at opacification of the refracting environments.
Research O. z. it is made also by methods of a nistagmovizometriya and an electrophysiologic research on evoked potentials of a cerebral cortex.
Nistagmovizometriya is based on calling of optokinetic nystagmus (see) at the object investigated at continuous motion. Investigated the black-and-white lattice or a chess cell moving with a certain speed is shown in a special window of a nistagmoapparat. Emergence of a railroad nystagmus is registered by means of a slit lamp or other optical device. By the extreme size of a lattice or cell, at Krom the railroad nystagmus is still noted, objective O. z is defined.
Research O. z. on evoked potentials carry out by means of the device for presentation of a visual incentive (the pulsing or quickly lit chess cell with various size of squares) and the electroencephalograph for registration of capacities of the area of visual bark. Record EEG at the different sizes of a square is carried out. EEG process by means of the COMPUTER and calculate that minimum size of a square, on to-ry there is an accurate reaction.
Devices for definition of visual acuity are counted on presentation of all above-stated test objects of strictly certain angular sizes under the set conditions of illumination and contrast.
On a way of presentation of test objects devices for definition of visual acuity can be divided into 4 basic groups: the tables lit by an external light source in quality to-rogo the general lighting in the room or special lighters is used (tables are made in the typographical way on paper); transparantny devices, in to-rykh the test objects applied on a translucent plate are lit with the light source located in the device; projective devices — projectors, with the help to-rykh test objects in the form of slides are projected on the reflecting screen; collimator devices — the devices containing test objects in the form of slides and the special optical system creating their image in infinity that allows to dispose the shown test objects in close proximity to the studied eye (in devices of the first three groups test objects have at distance of 5 — 6 m from the examinee for an exception of influence of accommodation on results of a research).
In the USSR are issued the lighter of tables (see. Roth device ), Golovin's table — Sivtseva, Orlova's table for a research O. z. children, devices have figurative POZD-1 and POZB-1, the automated APZ-1 device.
The device transparantny portable POZD-1 is intended for a research of visual acuity for a distance (fig. 6), represents the rectangular case, perednyaya back walls to-rogo are made of translucent plexiglas and contain test objects in the form of Landolt's rings. On one party signs for a research of visual acuity from 0,1 to 0,4, and on another — from 0,5 to 2,0 are located (for distance of a research — 5 m). Illumination of test objects is carried out by the filament lamps located in the case and providing brightness of a background surface not less than 90 cd/m2. The case of the device can turn around a horizontal axis for serial presentation of test objects.
The POZB-1 device is intended for a research of visual acuity close (fig. 7). It is transparantny and consists of the case with the lamps which are built in inside and a translucent window, and also a set of replaceable cartridges with test objects. As test objects in it Landolt's rings, Pflyuger's hooks, letters of the Russian alphabet (BIMNGTSh) and the printing text can be used. Range of a research of visual acuity from 0,1 to 1,0. Also test objects for definition are included in the package of the device astigmatism of an eye (see) and researches solid vision (see). All test objects are calculated for distance of 35 cm.
The automated transparantny device with distance steering of APZ-1 consists of a board of test objects, the block of processing and two operating consoles (for the doctor and for the patient). On a board of test objects randomly to exclude a possibility of storing, Landolt's rings corresponding to visual acuity from 0,1 to 2,0 at distance of a research of 5 m are located. Test objects are serially lit with the filament lamps which are built in inside. The device can work in the managed and automatic mode. In the first case the doctor, pressing the corresponding keys, includes necessary test objects on a board, and the patient, concerning touch contacts on the panel, specifies the direction of ring scission seen to it. On the panel at the doctor at the same time the corresponding answer lights up («truly» or «incorrectly»). In the second case inclusion of objects on a board is made automatically and depending on the answer of the patient to the previous sign larger or smaller sign is shown.
At the end of a research on the indicator of the panel of the doctor O. z is specified.
Abroad various projective devices were widely adopted. Except test objects for a research O. z., special tests for a research of an astigmatism, solid and stereoscopic vision are usually provided in them. Are issued as well collimator devices for a research O. z. Existence in them of optical system for examining of the shown test objects can distort several results of a research and therefore such devices are used usually to approximate bystry identification of vision disorders.
Bibliography: V. V. wolves, A. I. Gorban and D and l and and sh in both l and O. A. Klinicheskaya and refractometry, JI., 1976, bibliogr.; Kowalewski E. I., Lomakin L. N. and Cases B, P. The automated device for definition of visual acuity, the Medical technician, No. 6, page 6, 1979; The Multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 459, M., 1962; Rozenblyum Yu. 3. The main tendencies in a research of visual acuity, in book: New methods of functional diagnosis in Ophthalmolum., under the editorship of K. V. Trutneva, page 161, M., 1973; F e y and A. A N. Psychometric research of visual acuity, Vestn, oftalm., No. 1, page 46, 1980; Der Augenarzt, hrsg. y. K. Veilhagen, Bd 2, S. 167, Lpz., 1972; B en da H. Verfahren der Sehscharfebestimmung, Stuttgart, 1971; B o r i s h I. M. Clinical refraction, v. 1, p. 345, Chicago, 1970; System of ophthalmology, ed. by S. Duke-Elder, v. 5, p. 420, L., 1970.
BB. 3. Rozenblyum; A. D. Noskova (tekhn.).