HEIGHT MEASUREMENT OF ASTIGMATIC TEST SURFACES BY A KERATOSCOPE THAT USES PLANE GEOMETRY SURFACE RECONSTRUCTION

PURPOSE: To assess the accuracy with which the Keratron keratoscope (O ptikon 2000, Rome, Italy) measured astigmatic test surfaces by a profi le reconstruction algorithm within a plane geometry model and to discr iminate between error caused by the model and error caused by other fa ctors. METHODS: Height was reported by the Keratron for eight surfaces with central astigmatism ranging from 4 to 16 diopters. A three-dimen sional ray tracing simulation produced theoretic reflected ring patter ns on which the Keratron's reconstruction algorithm was performed, The Keratron's measurements were compared with the surfaces' formulas and the ray-traced simulations. RESULTS: With a new mathematical filter f or smoothing ring data, now part of the Keratron's software, maximum e rror was 0.47% of the total height and was usually less than 1% of loc al power for surfaces with 4 diopters of astigmatism. For surfaces wit h 16 diopters of astigmatism, maximum error was as high as 2.9% of tot al height and was usually less than 2.5% of local power. The reconstru ction algorithm accounted for 40% and 70% of height error, respectivel y. CONCLUSIONS: The efficacy of keratoscopes cannot be assumed from th eir design theories but must be tested. Although plane geometry surfac e reconstruction contributed greatly to total height error, total erro r was so small that it is unlikely to affect clinical use.