A study on multispectral projection moire measurement

Abstract

Projection moire topography measurement is a useful method of non-contacting 3-D shape measurement in the engineering fields. Presently in projection moire topography measurement, an image of the long period grating lighted by a white light source is projected onto the object by projection lens so that the binary shadow fringes are obtained by viewing it through a second grating in front of the camera. The fringes form the binary moire topography containing the information of surface profile. The 3-D shape of the object is rebuilt by analyzing the moire topography. Due to using a long grating period and error of the projection lens the resolution and precision is lower. And because the binary fringes in a frame of topography image contained fewer the altitude information of the object, the distortion existed in the 3-D shape rebuilt. For all above problems, a novel method of multi-spectral projection moire topography measurement is proposed in this paper. The method uses the virtual grating with alterable period and polychromatic light source. The color fringes are generated by polychromatic light interferometry and projected onto the objection. The color fringes are modulated by surface profile of the object. The image of the curved color fringes is seen on the color CCD camera through the receiving optical system. In the receiving optical system the color fringes are selected according to the colorimetric principle. Thus the different wavelength color fringes form the color multi-channel. The image received by the color CCD camera is compound multi-channel color moire topography. The compound color moire topography image is separated into single color channel topographies by image processing. The different color channels provide more altitude information of the object than the binary moire topography. The 3-D altitude information that rebuilds the 3-D shape of the object finally is obtained by fusion of multi-channel topography. In the paper the principle of the method is proposed. And the effect on selecting grating period for the color interferometry fringes and the wavelength interval is discussed. The result of experimentation proves that the resolution of the projection moire topography reconstruction was effectually improved.