VISUAL ADAPTATION

From Big Medical Encyclopedia

VISUAL ADAPTATION (synonym adaptation of an eye) — adaptation of an eye to various levels of brightness (from 10 - 6 to 10 4 nit) due to change of absolute light sensitivity of the visual analyzer. This sensitivity (see. Photoperception ) sharply raises (in tens and even hundreds of thousands of times) in the dark that does possible perception of very weak light sources, and goes down upon transition from smaller illumination to bigger. Respectively distinguish And. h. dark and light.

And. h. it is caused by photochemical processes in rhodopsins of a retina, functional interaction of its receptive fields, and also dynamics of excitement and braking in the central departments of visual system.

At And. h. to darkness absolute light sensitivity of an eye increases most the first 30 min. and reaches a fixed level by 50 — 60 min. The electric sensitivity of an eye (measured on the minimum current causing a feeling of a weak posvechivaniye) the first 30 min. goes down a little, and then remains to a constant. At And. h. to light sensitivity of an eye goes down in the first seconds and becomes a constant in 5 — 7 min.

And. h. it is broken at some general and eye diseases, e.g. at a hemeralopia, glaucoma, a pigmental degeneration of a retina, amotio of a retina.

K A. h. belongs also color And. h. (synonym: the color asthenopia, a color adisparopiya, color exhaustion), edges is expressed that during the examining during nek-ry time of two different color tones they begin to seem identical. The physiological mechanism of this phenomenon is a little studied.

See also Sight .

Fig. 1. ADM adaptometer: 1 — the handle for installation of replaceable light filters in a sphere; 2 — a ball for preliminary light adaptation; 3 — a tube for illumination of a fixating point; 4 — a disk with the circular neutral wedge changing brightness of a fixating point; 5 — a measuring drum; — the switch of an oslepitel; 7 — a plate for record; 8 — the general switch of the device; 9 — a stop watch; 10 — the switch including a lamp of the adaptometer or a lamp of a sphere of preliminary adaptation; 11 — a drum for turning on of replaceable neutral light filters in the metering device; 12 — the handle for turning on of the additional neutral light filter; 13 — the gate of an opening for the observation of eyes inspected.
Fig. 2. Recording adaptometer of Hartinger: 1 — inclusion of illumination of a scale of sensitivity; 2 — the button for inclusion of a photo cell of an upper sphere; 3 — the microampermeter; 4 — a scale of sensitivity; 5 — the handle for management of a measuring diaphragm; 6 — a key for turning on of the observed screen; 7 — the switch; 8 — the button for inclusion of a photo cell of the lower sphere; 9 — the switch of a fixating point; 10 — the handle of inclusion of the lower sphere; 11 — a time scale; 12 — the chin rest; 13 — a roulette for determination of distance to eyes inspected; 14 — a window for observation; 15 — a lamp for preliminary adaptation.

Devices for a research of visual adaptation. The adaptometer — the device for a research dark And. h. by measurement of a threshold of light sensitivity of the visual analyzer. On a curve of the schedule of change of light sensitivity depending on time it is possible to judge adaptation at inspected.

Until recently Nagel's adaptometer, Kravkov and Vishnevsky's camera and the first model of the adaptometer of Belostotsky and Hoffman were eurysynusic.

In a crust. time the modifitsironanny model of the adaptometer of Belostotsky and Hoffman — the ADM adaptometer is produced. By means of the ADM adaptometer it is possible to investigate adaptation of an eye to darkness after preliminary (10 — 20 minute) adaptations to light of various brightness, and also to determine visual acuity at the lowered illumination and after dazzle by bright light. Approximately adaptation is defined within 3 min., however it is necessary for a complete definition of dark adaptation up to 60 min.

The device (fig. 1) consists of the metering device, a ball for preliminary light adaptation and the operating console. As test objects three figures (a circle, a square and a cross) of the identical area and three tables with various arrangement of signs for a research of visual acuity at a short distance are used. Brightness of test objects can be reduced in 400 million times in comparison with maximum by means of a measuring diaphragm and additional light filters.

From the ophthalmologic devices released abroad the recording adaptometer of Hartinger (fig. 2) produced in GDR gained distribution. Reduction of brightness of the observed field in 100 million times in comparison with maximum is carried out by two measuring diaphragms without light filters that provides an invariance of spectral composition of light.

For control of light conditions special photo cells are built in the device. Existence of the fixating point rotating around the observed field gives the chance to investigate light sensitivity of both the center, and peripheral departments of a retina. Registration of results of a research is carried out on the special schedule moved with the clockwork by means of the marked pin. Some foreign firms release the devices intended for a research of a recovery rate of light sensitivity after dazzle by a bright light source; niktometr «Carla Tseyss Yen» (GDR), niktomat Rodenshtok (Germany) and so forth. These devices are used generally to definition of professional suitability of drivers of transport.


Bibliography: B. G. O box of visual dark adaptation, L., 1958; Cranes C. B. Eye and its work, page 133, X. — L., 1950; The Multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 401, book 2, page 105, M., 1962; Urmakher L. S. The reference book in ophthalmologic optics and to devices, M., 1971; Physiology of touch systems, vzd edition of G. V. Gershuni, p.1, page 221, D., 1971, bibliogr.; With omberg W. UBtersucbuHfismetboden «LichtsJnn», in book: Angenarzt, firsg. v. To. Velhagen, Bd 2&. 147, Lpz., 1959, Bibliogr.

E. S. Avetisov; A. D. Noskova (tekhn.).

Яндекс.Метрика