ASSESSMENT OF RADIAL ASPHERES BY THE ARC-STEP ALGORITHM AS IMPLEMENTED BY THE KERATRON KERATOSCOPE

PURPOSE: To assess the accuracy with which the Keratron (Optikon 2000, Rome, Italy) measured rotationally symmetric, radially aspheric test surfaces according to an are-step profile reconstruction algorithm and to discriminate between error caused by the algorithm and error from other sources. METHODS: Height, local power, and axial power calculate d from radius of curvature centered on the instrument's axis were repo rted by the Keratron for four surfaces that had radial profiles simila r to normal corneas. The Keratron profile reconstruction algorithm was simulated by using ray tracing. Keratron measurements were compared w ith the surfaces' formulas and the ray-traced simulations. RESULTS: Th e heights reported by the Keratron were within 0.25 mu m from the four surfaces at less than 3 mm from the keratoscope axis and generally wi thin 1 mu m of the height calculated from the surfaces' formulas. The Keratron's axial powers were within +/- 0.1 diopter of the simulation of the axial solution between 1 and 4 mm of the axis but were greater central to 1 mm and peripheral to 4 mm. The Keratron's local powers we re within -0.25 diopters at less than 4 mm from the axis and periphera lly were between +1.75 diopters and -0.75 diopter of power calculated from the surface's instantaneous radii of curvature. Height error beca use of the are-step algorithm was less than -0.2 mu m. CONCLUSIONS: Th e Keratron's arc step profile reconstruction algorithm contributed to its ability to measure height more accurately than keratoscopes that u se spherically biased algorithms and provided measurement of local pow er.