We propose a simple confocal system that uses a laser Gaussian beam as a probe to measure microstructure features of curved or turned surfaces. The probe beam is focused by a lens with a low numerical aperture to maintain the beam unclipped throughout the detection process and to limit the vertical resolution to several microns. This resolution is suitable for examining the microstructure of machined surfaces. The size of the pinhole in the plane of detection is characterized for optimal depth discrimination. The analytic description is based on the propagation of Gaussian beams by using the scalar Fresnel diffraction integral.
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