USING CORNEAL HEIGHT MAPS AND POLYNOMIAL DECOMPOSITION TO DETERMINE CORNEAL ABERRATIONS

Purpose. To review the use of corneal videokeratoscopic height data, e laborate on the advantages and disadvantages of such data, describe te chniques for overcoming the limitations of height data, and demonstrat e its use in quantifying the optical properties and aberrations of the cornea. Methods, The steep sag of the cornea hides fine variations in corneal height that arise naturally or due to disease or surgery. The dynamic range, or ratio of the overall sag to the feature height, is the primary limitation of videokeratoscopic height data. Techniques fo r removing single or multiple reference surfaces are described in deta il, and applications of the methodology to wavefront and raytracing an alysis of corneal aberrations arising from radial keratotomy (RK), pho torefractive keratectomy (PRK), and keratoconus are described. Results . Removing a single reference surface from the raw corneal height data begins to reveal subtle variations in corneal height. However, expans ion of surface height data into a complete set of basis functions prov ides a sophisticated method for extracting high-order corneal variatio ns. Choosing an orthogonal basis set provides a robust least-squares f it and forms unique expansions of the surface. The resulting coefficie nts are uncorrelated and form a simple measure of the optical quality. Conclusion. Videokeratoscopic height data are useful for analyzing an d quantifying corneal deformity arising from disease or refractive sur gery and they provide a sophisticated alternative or complement to dio ptric power maps.