Abstract

A new and highly sensitive system, based on the three-slit interferometer, is described for the scanning measurement of optical path length differences over large objects. The system relieves the positional constraints of previous systems by separating the two lateral slits from the central slit using a double-exposure hologram. The lateral slit wavefronts are holographically reconstructed from an off-axis reference beam and added to light from the central slit to form a live intensity distribution pattern. Optical path length measurements can be made by comparing the magnitude of adjacent maxima as an object is translated across the central slit. The apparatus can be used for a number of practical applications including the measurement of flatness for an optical flat, thickness for transmissive or reflective films, and refractive-index differences of materials.

© 1988 Optical Society of America

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