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

A method for high-speed measurement of the three-dimensional (3D) shape of spatially isolated objects is proposed. Two sinusoidal fringe patterns with phase difference pi and an encoded pattern are used to measure the 3D shape. A modified Fourier transform profilometry (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 on traditional FTP and the same as that for methods based on phase measurement profilometry (PMP). The number of patterns is less than for the high-speed methods based on PMP, using which isolated objects can be measured. Consequently, this approach is less sensitive to object motion. In the proposed method, the encoded pattern consists of vertical stripes with width the same as the period of the sinusoidal fringe. Three gray levels are used to form the stripes. Six symbols are encoded with these three gray levels. Then, a pseudorandom sequence is constructed with an alphabet of these six symbols. The stripes are arranged according to the sequence to form the pattern. In the procedure of phase unwrapping, the strings (subsequences) are constructed with symbols corresponding to three neighbor periods of the deformed fringe. The position of the subsequence is worked out by string matching in the pseudorandom sequence. The ranking number of the fringe is identified and then the absolute phase of the deformed fringe is obtained. The 3D shape of the objects is reconstructed with triangulation. A system consisting of a specially designed digital light processing projector and a high-speed camera is presented. The 3D capture speed of 60 frames per second (fps), with a resolution of 640 x 480 points, and that of 120 fps, with a resolution of 320 x 240 points, were achieved. Preliminary experimental results are given. If the control logic of the digital micromirror device was modified and a camera with higher speed was employed, the measurement speed would reach thousands of fps. This makes it possible to analyze dynamic objects.