Abstract

Placido-disk based videokeratoscopes measure the slope of the corneal surface in the meridional direction. The derivative of this slope data is related to the optical power distribution of the cornea. While the power map is widely used for clinical evaluation, information about the sag or height of the cornea is useful for determining the optical effects of the cornea. This sag data is calculated by integrating the slope data obtained by the videokeratoscope. A significant drawback to viewing the surface sag data is proper display of the pertinent height information. Small height variations on the surface are obscured by the axial symmetric aspheric sag corresponding to the base power of the cornea and a possible cylindrical sag corresponding to corneal astigmatism. In this paper, we will examine a method for decomposing corneal height data into Zernike polynomials, and show how to obtain base and astigmatic curvatures and powers from the expansion coefficients. These coefficients are used to calculate the parabolic and cylindrical components of the topography data. By removing these terms from the original height data, the residual higher-order height variations of the cornea are shown more clearly. Topographies of an eye with corneal astigmatism and an eye that underwent radial keratotomy are displayed using this technique.

© 1995 Optical Society of America