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An optical, noncontacting surface sensor (optical probe) breadboard with a tungsten light source and silicon detectors has detected a change of about 0.1 μm. in the axial position of a small surface area from a distance of 4 cm. The optical probe utilizes two diametrically opposed, converging beams from one lens to form temporarily sequential images of a small illuminated spot on the surface whose axial location is to be determined. With the surface at the external probe focus, the sequential spot images are precisely coincident, but they separate in opposite directions as the surface approaches or departs from the probe. Two detectors are arranged to detect the extremely small imbalance in reflected light resulting from image separation, the difference in their outputs providing the distance error signal. The sum of their outputs provides a measure of the probe's departure from normal to the surface. The calculated distance error detectivity (DED) of the probe on a diffuse surface is about 10 nm with a tungsten source, on the order of 0.1 nm with a xenon arc, and even better with a laser. A specular surface provides increased gain and improved DED. Possible applications of the probe are discussed, and the restrictions imposed by the normality adjustment mechanism required on nonflat surfaces are considered.
© 1969 Optical Society of America
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