ABERRATION OF A EYE

From Big Medical Encyclopedia

ABERRATION OF A EYE — distortion of images on a mesh cover of an eye as a result of imperfections of its optical system.

And. it can be caused by various reasons: irregular shape of surfaces of a cornea and crystalline lens, imperfection of their tsentrirovka, heterogeneity of eye environments (especially a crystalline lens) and the phenomena of diffraction arising in an eye on the way of passing of a ray of light (rounding by light waves of obstacles, etc.).

All types of an aberration of optical systems are inherent in optical system of an eye of the person in a varying degree: spherical, chromatic, and also diffraction aberrations and an astigmatism (see the Aberration, the Astigmatism of an eye).

Spherical aberration of an eye is caused by a heterogeneous structure of a crystalline lens. It is defined as a difference between extent of refraction by optical system of the beams passing through peripheral and central sites of a pupil of an eye and is measured in dioptries. One dioptry (1 dptr) — the refracting force of a lens with focal length of 1 m. Spherical And. it is considered positive if peripheral beams refract stronger central and their focus is closer to a crystalline lens, than to a mesh cover, and negative if focus of peripheral beams is closer to a mesh cover, than to a crystalline lens. Lack of uniform focus for the central and peripheral beams falling on a pupil leads to the fact that the considered shining points are projected on a mesh cover of an eye in the form of spots (circles of light scattering). As a result of it visual acuity decreases.

Spherical And. it korrigirutsya to a certain extent by decrease in a kra of a vizna of surfaces of a cornea and crystalline lens in process of transition from their central areas to peripheral. Spherical And. also width of a pupil depends on a condition of accommodation of eyes (see). Usually at day lighting (diameter of a pupil of 3 — 4 mm) the aberration of eyes equals 0,5 — 1 dptr.

Chromatic aberration of an eye is caused by unequal refraction by optical system of an eye of light rays with various length of waves (see the Refraction of an eye). At different people it is not identical. Chromatic aberration is in number characterized by a difference between the refracting force of an eye for yellow radiation with the wavelength of 587,6 nanometers (5876A) and the refracting force of an eye for this wave and expressed in dioptries.

As a result of chromatic aberration of the image of objects on a mesh cover of an eye are surrounded with the fringe. However because of selective sensitivity of a mesh cover of an eye to radiations of various length of waves of people does not notice the painted contours of objects.

Chromatic And. inability of an eye has a talk with a normal refraction (see the Emmetropia) to see deleky blue or violet objects, as well as the phenomena of «speakers» and the «receding» flowers. In many cases chromatic And. features of the receptions used by artists in landscape and the tailor of painting speak.

On use of the phenomena chromatic And. a number of the methods and devices used in ophthalmology to measurement of size of an ametropia of an eye is based. Diffraction aberrations of an eye are called distortions on a mesh cover of an eye as a result of the diffraction arising during the passing of light rays through a pupil of small diameter. At diffraction And. dot objects are represented on a mesh cover not in the form of points, and in the form of the round spots surrounded with ranks of light and dark rings. Diffraction And. it is shown the more sharply, than diameter of a pupil is less.

The greatest image sharpness of objects on a mesh cover of an eye and consequently, and the best sight of an eye takes place with diameters of a pupil of an eye, equal 2 — 4 mm. Further increase in diameter of a pupil is followed by decrease in visual acuity.

L. H. Gassovsky.

Яндекс.Метрика