High Speed Measurement of Discontinuous Surface Profiles

Abstract

The measurement of surface profile by projecting phase-stepped fringe patterns at an angle to the observation direction is well known. Ambiguous range data results, however, when the object has discontinuities in its profile. The ambiguities can be prevented by projecting fringes of varying spatial frequency. In this paper we describe an approach which combines high accuracy and reliability. The spatial frequency is reduced exponentially from the maximum value. The sequence of phase values at a given pixel is then unwrapped independently of the other image pixels, and all the intermediate phase values contribute in a least squares sense to the final range estimate for the pixel. The algorithm has been implemented on a pipeline image processing system. The fringe patterns are projected at 30 frames s⁻¹ using a high resolution data projector. Images are acquired and analysed in real time, at the same framing rate. A total acquisition and processing time of 0.75 s has been achieved for a maximum spatial frequency of 16 fringes across the field of view.