The locus of photokeratoscope target points that yield images lying in a plane perpendicular to the camera axis was evaluated for a 15.8-mm radius spherical convex reflector. The linear transform of an ellipse was fitted to sets of theoretical and experimental points by the method of least squares. The semimajor and semiminor axes for the theoretical and experimental points were 103.1, 40.0, 95.0, and 42.9 mm, respectively. The theoretical points conformed to the linear transform of the ellipse with a coefficient of correlation departing from unity by less than 4 parts in 10 000. For the combined experimental points, the coefficient of correlation was 0.998. The theoretical locus was also calculated for an elliptical reflector simulating the gross dimensions of the cornea and found to be essentially elliptical, closely approximating that for the spherical reflector.
SIDNEY WITTENBERG and WILLIAM M. LUDLAM, "Planar Reflected Imagery in Photokeratoscopy," J. Opt. Soc. Am. 60, 981-985 (1970)