The central-peripheral radius of the normal corneal curvature. A photokeratoscopic study.

Both eyes of 40 normal persons were examined with a photokeratoscope. A method for analyzing the photokeratoscopic data based on the principle of least square fitting is presented. It is demonstrated that the corneal flattening expressed as the relative change in the radius of the corneal curvature is proportional with the square of the chord distances from apex. The factor of proportionality RV is defined as the coefficient of radius variation, which, together with the radius of the apex curvature K, are characteristic constants for a given meridian in a given eye. Expressed as mean and SD, the study demonstrates for the horizontal meridian K = 7.86 mm (0.25) and RV = 0.71 X 10(-2) mm-2 (0.26 X 10(-2)), for the vertical meridian the corresponding dimension are K = 7.65 mm (0.24) and RV = 0.70 X 10(-2) mm-2 (0.38 X 10(-2)). These results imply a change in the radius of the corneal curvature 1 mm and 5 mm from apex at, respectively, 0.7% and 17.7%. The precisions expressed as SD, by which the 2 parameters may be determined, are for K 0.03 mm and RV 0.12 X 10(-2) mm-2.