Three-dimensional range sensing and surface profilometry using lateral shearing interferometry

Abstract

A distance measuring system and surface profiler with extended range based on collimation testing technique using lateral shearing interferometry is proposed. The sensor works on the principles of focus sensing and interferometry and the depth gating is achieved by collimation testing. A wedge-shaped shearing plate was used for the interferometer and when the object surface is exactly at the focus of the collimating lens the interference fringes are perfectly parallel to a reference line with a finite spatial frequency. If the object surface is out of focus of the collimating lens the fringe pattern is oriented with a change in the spatial frequency. The orientation of the interference fringes and their spatial-frequency increases as we move the object farther from the focused position. Fourier transform method for fringe analysis is used and from the maximum of the Fourier spectrum the distance and three-dimensional surface profile of the objects is measured. Since the surface profile is reconstructed from the maxima of the first-order Fourier spectrum, a large range of measurement without any fringe ambiguity problem can be achieved using the system. The system works both for smooth as well as diffuse objects and is compact, robust and inexpensive. A high-depth of resolution of the order of 10 micrometer achieved with a range measurement 10mm.