Corneal topography.

Computer-assisted videokeratography systems are becoming the prevailing method for analysis of the corneal surface. A realistic insight into the limitations of these topography systems is critical for proper interpretation of the data. Methods based on reflections have limited accuracy when measuring aspherical surfaces. The error may become quite extensive when abrupt changes of curvature exist, eg, at the edge of a photorefractive keratectomy. In this case, raster stereographic methods have a theoretical advantage because they determine a true corneal topography, but additional means to visualize the corneal surface must be used and enough sensitivity is more difficult to achieve. Improved calculations have provided a better approximation of the ideal contour a cornea should have to provide optimal visual performance. New methods are proposed to evaluate the optical aberration induced by a given cornea and to discern focusing errors that are correctable with eyeglasses from irregular astigmatism that is not.