Optical instrument designed to measure the radii of curvature of the crystalline lens. The method is based on a comparison of the sizes of the third h′3 and fourth h4′ Purkinje images (typically determined by photography) with that of the first image h′1, that is the anterior corneal surface, the characteristics of which are easily measured. The radius of curvature of, say the fourth Purkinje image r′4 can be calculated using the approximate equation
r′4 = r1 (h4′/h′1)where r1 is the radius of curvature of the front surface of the cornea. If the test object is near the eye a correction factor is necessary. See Purkinje-Sanson image; ophthalmophakometer; phacoscope.