Accuracy of the EyeSys 2000 in measuring surface elevation of calibrated aspheres

This article examines the accuracy of the EyeSys 2000 videokeratoscope in measuring aspheric surfaces for calculation of the wavefront aberrations of the anterior cornea. Six rotationally symmetric aspheric surfaces were measured. The surface elevation was computed from the normal measurement files. Both elevation error and relative error were computed from the average of the three maps. The root-mean-square errors for the various surfaces ranged from 1.48 to 6.55 microns, with less error on the oblate and spherical surfaces. The error found was very systematic, increasing monotonically toward the periphery. The article includes a strategy to compensate for the systematic error to meet the required 0.5-micron accuracy needed. An equation was developed that used only the apical radius and shape factor, which improved the accuracy to the required 0.5-micron level.