High precision Twyman-Green interferometer for the measurement of ophthalmic surfaces.

PURPOSE We developed a method for measuring ophthalmic surfaces based on a Twyman-Green interferometer. METHODS To demonstrate the accuracy, we measured the anterior surface of four used hard contact lenses and evaluated the interference pattern with phase shift technique. The obtained interferograms were evaluated by an image processing system. RESULTS The interferometric resolution of the measurement is up to lambda/20 (30 nm) which refers to a dioptric resolution of about delta D = 10(-3) m(-1) for typical anterior radii of curvature. The dioptric maps were calculated from the height distribution. As we used a focus tracking distance measurement setup with an accuracy of 10 microns we could determine the absolute value of the dioptric number to be delta D approximately 0.04 m-1. To demonstrate the accuracy we tested contact lenses which were specified to have a spherical anterior surface. Obviously, the topography of contact lenses changes when worn. We measured astigmatic deviations up to 0.6 D in the central zone of the lenses. We briefly discuss the possibilities of this method concerning a topographer for measuring the fine corneal surface structure. CONCLUSIONS The presented Twyman-Green interferometer with distance measurement and image processing system is a very accurate method for measuring the topographic properties of ophthalmic surfaces. The method may also be useful for studying the distribution of the tear layer on the contact lens, or the corneal topography.