OPHTHALMOMETRY (grech, ophthalmos of eyes + metreo to measure, measure; synonym keratometriya) — the method of measurement of radius of curvature of a front surface and the refracting force of a cornea which is carried out by means of special devices — oftalmometr.
The lake is used for diagnosis of an astigmatism, a keratoconus (see. Cornea ), for calculations at selection of contact lenses, at a refraction keratoplasty — to a keratomileza (see), keratofakiya (see), and also at through optical keratoprotezirovaniya (see) and implantations of artificial crystalline lens (see). Unlike ophthalmometry, edge gives the chance to define degree of an astigmatism quantitatively, the keratoskopiya and a ke-ratografiya is applied to qualitative test of an astigmatism. The principle O. is based that the front surface of a cornea works as a spherical mirror therefore the size of the image of an object depends on its curvature. Radius of curvature of a front surface of a cornea (r) is calculated on a formula: r = 2Ba/A, where And yes and — sizes of an object and its image; In — distance from an object to a cornea. At the person the radius of curvature of a front surface of a cornea is on average equal to 7,7 mm. The small difference between radiuses of curvature in different meridians which is not exceeding in size of 0,1 mm (corresponds to the refracting force of a cornea approximately in 0,5 dptr), is considered as fiziol, a corneal astigmatism (see. Astigmatism of an eye ).
The island is carried out in the darkened room; investigated is in a sitting position, his head not movably is established on a special support of an oftalmometr. At the direction of a look of the oftalmometr investigated in a lens as a result of reflection from its cornea images of two objects, so-called brands turn out. The doctor, observing in an eyepiece, tries to obtain by means of the special screw of an oftalmometr of combination of working points of brands. This moment corresponds to certain indicators of a graduated scale, edges is attached to an eyepiece of an oftalmometr. Determine the radius of curvature of a cornea in millimeters by a scale and the refracting force of a cornea in dioptries.
Action of an oftalmometr is based on geometrical interrelation between the sizes of an object, to-rogo the special measuring brand, and its image received at reflection from a front surface of a cornea taking into account removal of an object and its image from a cornea serves.
For determination of radius of curvature of a front surface of a cornea measure or the size of an object (distance between brands), or the size of the image (distance between images of brands). The optical system of an oftalmometr (fig. 1) is arranged in such a way that allows to perform measurement on a mobile eye thanks to the special elements doubling the image of measuring brand.
The first oftalmometr was offered by G. Helmholtz, but in view of complexity of calculations of practical application did not receive. Everything modern oftalmometra can be divided into two types. Devices belong to one type, in to-rykh distance between images of brands changes by movement of the brands connected with a graduated scale, on a cut determine the radius of curvature of a cornea. Such principle was carried out in Zhaval's oftalmometra — Shettsa. Devices belong to other type, in to-rykh measuring brands are installed not movably, and the distance between their images changes by means of the elements of doubling which are also connected with a graduated scale. On this type all modern models of oftalmometr are constructed: OF-3 (USSR) — fig. 2, Opton (Germany), «Tseyss-Yena» (GDR) and others, in to-rykh are used the measuring brands allowing to perform measurements in the presence of a corneal astigmatism consistently in each of eye meridians. Use of the measuring brands offered by Sutcliff allows to perform measurement at the same time in two meridians.
Main characteristics of an oftalmometr of OF-3: range of measurement of the radiuses of 5 — 11,8 mm, scale value of the radius of 0,05 mm, range of measurement of a refraction of a cornea 66 — 28 dptr, scale interval of a refraction 0,25 dptr.
Oftalmometra, supplied with a nozzle for fixing of a look, allow to determine the radius of curvature of a cornea in its various zones, and also coordinates of these zones that gives information on a form of a cornea.
Bibliography: The multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 1, page 298, M., 1962; Soft T. N. and Dikov A. A. «Oftalmometr of OF-3», Is new. medical technicians, No. 1, page 40, 1965; Soft T. N., Alekseev D. E. and Etko JI. I. Analysis of gages of parameters of a front surface of a cornea of an eye, in the same place, No. 3, page 40, 1979; The Guide to eye surgery, under the editorship of M. L. Krasnov, page 121, M., 1976; System of ophthalmology, ed. by S. Duke-Elder, v. 5, L., 1970.
Yu. B. Slonimsky; T. N. Soft (tekhn.).