Descriptors of corneal shape

With the advent of videokeratoscopy and similar technology, we are now provided with more information about corneal contour than ever before. One of the common problems in this area is which parameters should be used to quantify the corneal shape. From the time of Helmholtz, the ellipse has been used as a first-order approximation for the normal corneal profile, and in the optometric literature one or another of the conic shape parameters p, Q, or e have been used to describe the general shape of the corneal surface. The purpose of this technical note is to clarify the relationships between these various descriptors of corneal shape.