phakometer

phakometer 

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 h3 and fourth h4′ Purkinje images (typically determined by photography) with that of the first image h1, that is the anterior corneal surface, the characteristics of which are easily measured. The radius of curvature of, say the fourth Purkinje image r4 can be calculated using the approximate equation
r4 = r1 (h4′/h1)
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.