A two-wavelength heterodyne interferometer (2λHI) allows measuring the contour of a rough surface and the difference between two points of measurement may be greater than the light wavelength.1 A special probe offers the chance to measvae in a small and deep drilling with a thin, focused laser beam. It is shown that, according to the theory of diffraction, the measurements with the 2λHI are useful whenever the laser beam hits the surface vertically. Comparative measurements with the 2λHI and a focusing sensor are presented.

© 1990 Optical Society of America

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