FOTOKERATOMETRYYa (Greek phos, photos light + keras, keratos a horn, horn substance + metreo to measure, measure) — a method of a research of topography of a front surface of a cornea by measurement of radius of its curvature by means of the special device — the photokeratometer.
The research allows to reveal different types of an astigmatism of an eye (see), in early terms to diagnose a keratoconus (see the Cornea) and to carry out dynamic on -
Fig. 1. Photokeratometer: 1 — kontsentr
chesky rings (a modified disk of Placido); 2 — a support for a chin; 3 — the «Polaroid» camera.
a blyudeniye behind its development for the purpose of the timely solution of a question of an operative measure. T. apply in modern laboratories of contact correction of sight to selection and adjustment of individual contact lenses (see). Ophthalmosurgeries use F. for preparation and carrying out difficult calculations at the refraction operations undertaken for the purpose of change of curvature of a front surface of a cornea — a kerato.mileza (see), ke-ratofakiya (see), epikeratofakn, a radial keratotomy (see the Cornea).
The forefront of the photokeratometer (fig. 1) represents a concave surface, on a cut the illuminated concentric rings (a modified disk of Placido) are located. In the center there is a fixating tag for designation of a visual axis.
The patient is seated in front of the device, establish his chin on a special support and suggested to be looked at a fixating tag. The front surface of a cornea serves as a convex mirror, in Krom the lit rings are reflected. This image is photographed the camera which is built in the device providing bystry receiving fotokeratogramm.
The arrangement of rings on a fotokera-togramma corresponds to curvature of a cornea: the central three rings — areas of a cornea diameter apprx.
3 m, edges it can be explored by a usual oftalmometr (see Of-taljmometr iya); the last outside ring — area of a cornea to dia. 6,5 — 8 mm depending on the radius of its curvature.
On a fotokeratogramma correctly located concentric circles (fig. 2) are normal visible. At any patol. the processes leading to change of a geometrical form of a front surface of a cornea, the correct concentric arrangement of rings is broken (fig. 3).
Fig. 2. Fotokeratogramma is normal: 1 —
evenly located concentric rings of the correct form reflected in a cornea; 2 — shadows from eyelashes.
Fig. 3. Fotokeratogramma at the wrong astigmatism owing to keratokonu-
sa: 1 — rings of the distorted form with uneven intervals between them;
2 — shadows from eyelashes.
fotokeratogramm use the comparator to calculation — the device, to-ry increases a picture several times and projects it on the special screen. The image is compared to a set of reference rings and determine the radius of curvature of a cornea in any set zone.
406 PHOTOCOAGULATION of an EYE
of Contraindications to use are not present a method. К). B. Slonimsky.