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Uniqueness of Corneal Shape From Placido Ring Images

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Abstract

The measurement of corneal shape has become a rapidly growing enterprise in the past few years. Corneal shape is typically measured using the Placido ring technology. A series of concentric rings (the Placido rings) are flashed, the rays reflected off the cornea are captured by a CCD camera, and the data are analyzed to reconstruct the corneal shape. All present commercially available algorithms do the reconstruction one meridian at a time and make the assumption that rays lie in the meridional plane (van Saarloos & Constable, 1991). This assumption is equivalent to assuming the corneal normal lies in the meridional plane. This assumption is not correct for real corneas since astigmatism or other deviations from axial symmetry are typically present. When the normal is allowed to deviate from the meridional plane an ambiguity results since each CCD point could correspond to an infinity of points on the continuous ring. Therefore multiple corneal shapes seem possible (Applegate, 1992). This ambiguity has prompted the suggestion that instead of using rings as the target, checkerboard patterns or rings with gaps should be used to guarantee the uniqueness of the solution (Klein, 1993).

© 1996 Optical Society of America

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