Optical EngineeringGeometrically desensitized interferometry for shape measurement of flat surfaces and 3-D structures
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We construct an interferometer for flatness testing of precision-engineered objects such as rigid disk drive platters, pump parts, and fuel injectors. A pair of phase diffraction gratings illuminate an object simultaneously at two different angles of incidence, resulting in an equivalent wavelength of 12.5 µm. This system operates in standard phase-shifting mode for a height resolution of 0.01 µm with up to 150 µm of surface departure. The <1 s measurement speed, 96-mm viewing aperture, simple mechanical part alignment, and 50-mm working distance are consistent with high-volume production testing. A recently developed coherence scanning mode accommodates even larger departures and discontinuous regions such as step heights.