High-speed three-dimensional shape measurement for isolated objects based on fringeprojection

Abstract

A method for high-speed measurement of the three-dimensional (3D) shape of spatiallyisolated objects is proposed. Two sinusoidal fringe patterns with phase differenceπ and an encoded pattern are used to measure the 3D shape. A modified Fourier transformprofilometry (FTP) method is used for phase retrieval and obtaining high-quality texture.The measurable slope of the height variation is larger than for methods based ontraditional FTP and the same as that for methods based on phase measurementprofilometry (PMP). The number of patterns is less than for the high-speed methodsbased on PMP, using which isolated objects can be measured. Consequently,this approach is less sensitive to object motion. In the proposed method, theencoded pattern consists of vertical stripes with width the same as the period of thesinusoidal fringe. Three gray levels are used to form the stripes. Six symbols areencoded with these three gray levels. Then, a pseudorandom sequence is constructedwith an alphabet of these six symbols. The stripes are arranged according to thesequence to form the pattern. In the procedure of phase unwrapping, the strings(subsequences) are constructed with symbols corresponding to three neighbor periods ofthe deformed fringe. The position of the subsequence is worked out by stringmatching in the pseudorandom sequence. The ranking number of the fringe isidentified and then the absolute phase of the deformed fringe is obtained. The 3Dshape of the objects is reconstructed with triangulation. A system consisting of aspecially designed digital light processing projector and a high-speed camera ispresented. The 3D capture speed of 60 frames per second (fps), with a resolution of640 × 480 points, and that of 120 fps, with a resolution of320 × 240 points, were achieved. Preliminary experimental results are given. If the control logic of thedigital micromirror device was modified and a camera with higher speed was employed, themeasurement speed would reach thousands of fps. This makes it possible to analyzedynamic objects.