Corneal topography: Image processing and numerical analysis of keratoscopy

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Abstract
Keratoscope photograph negatives of one normal cornea and calibration steel spheres were digitized by a CCD video camera connected to a frame grabber. The centroid of the innermost reflected mire was determined and used for conversion of the image to polar coordinates. The distance 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 diameter conformed with the known values. The average difference between observed and expected values was 0.2% and the average coefficient of variation was 0.1%. The standard deviations on observed ring means decreased inversely with ring number with an average coefficient of variation of 0.2%. Likewise, on a corneal surface SD also decreased inversely with ring number from 0.080 to 0.032 mm. In terms of refractive power this meant, that outwards from the third ring 95% of observations were within ±0.4 diopters from the average value.