A keratoscope is a standard device for measuring the shape of the cornea. Unfortunately, it cannot take measurements from diffuse corneal surfaces after photorefractive keratectomy (PRK) or during laser assisted in situ keratomileusis (LASIK), when such measurements are needed. The classical keratoscope can operate only with specular surfaces. To meet the National Institutes of Health's need for new methods of measuring the optical properties of the cornea, Physical Optics Corporation (POC) proposes a nonscanning speckle interferometer (NSI) that will measure corneal shape and radius during and immediately following PRK and LASIK. The NSI, based on a known principle for optical testing of rough surfaces, incorporates important improvements, including an acousto-optic deflector for creating a beam rake with frequency shift for nonscanning measuring, fiber optic wavelength division-multiplexing technology and principles of absolute distance interferometry developed by implementing the Chines remainders theorem. The NSI is flexible, allowing measurement of the corneal shape in diffuse surface zones after PRK and LASIK; smooth zones are also accessible because of the special position of the exit pupil in the focusing lens. Shape measuring precision is about 0.1 UM, and is defined mostly by surface roughness.
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