Purpose. To compare the performance of the Orbscan II topographer with a videokeratoscope for a set of tilted test buttons with known aspheric surface profile characteristics, and for a series of measurements made on normal human corneas.
Methods. Measurements of apical radius and p-value were obtained from 12 aspheric test surfaces using the Orbscan II, the EyeSys videokeratoscope, and Form Talysurf analysis. Measurements from the corneas of 18 human subjects were assessed by comparison with data from the EyeSys videokeratoscope. Repeatability of Orbscan measurement was also assessed for both the test surfaces and the human corneas.
Results. Measurement of the aspheric surfaces produced small but significant differences between the three instruments. For human corneas, the Orbscan apical radius measure under-read from 0.020 to 0.070 mm, and the p-value measure under-read from 0.011 to 0.085 compared with the EyeSys results.
Conclusions. The Orbscan appears to under-read slightly for both apical radius and p-value when compared to both the Talysurf and the EyeSys on both the aspheric surfaces and the corneas. The differences are small but statistically significant. The corneal shape factor provided by the Orbscan instrument display is of limited value.
Department of Optometry, University of Bradford, Bradford, West Yorkshire, United Kingdom
Received November 3, 2006; accepted March 6, 2007.