CORNEAL TOPOGRAPHY - IMAGE-PROCESSING AND NUMERICAL-ANALYSIS OF KERATOSCOPY

Keratoscope photograph negatives of one normal cornea and calibration steel spheres were digitized by a CCD video camera connected to a fram e grabber. The centroid of the innermost reflected mire was determined and used for conversion of the image to polar coordinates. The distan ce to each reflected ring was computed. Calculations of the radius of curvature describing the reflecting surface locally were performed by an iterative procedure based on differential equations. The calculated mean radius for steel spheres in the range of 5.5 to 11 mm in diamete r conformed with the known values. The average difference between obse rved and expected values was 0.2% and the average coefficient of varia tion was 0.1%. The standard deviations on observed ring means decrease d inversely with ring number with an average coefficient of variation of 0.2%. Likewise, on a corneal surface SD also decreased inversely wi th ring number from 0.080 to 0.032 mm. In terms of refractive power th is meant, that outwards from the third ring 95% of observations were w ithin +/-0.4 diopters from the average value.