The research on the moire fringe image preprocessing

Based on Talbot effect and Moiré fringes of Ronchi gratings which is defined as Moiré Deflection Technology (MDT), lenses' power is calculated. Moiré Deflection Technology can be used for non-contact measurement of phase objects and surfaces and MDT is more widely used in the lenses' power measurement. The power of measured lenses can be figured out accurately according to the relationship between moiré fringes tilting angle and the tested lens's power. Area array CCD recorded moiré fringe image generated by two gratings mechanical interference to measure lenses' power. After appropriate digital image processing such as gray scale equality, image enhancement, image thinning etc, the moiré fringes image is processed by thinning to single pixel width. The pixels of the moiré fringes are labeled to fitting lines to calculate the slope coefficients. However, the moiré fringe image appears to have an unequal light distribution. Based on this phenomenon, a binary processing method, which is based on background gray-scale extension, was proposed. At first, a statistical method is used to calculate gray values from a sampling image with block-based processing. Then the linear interpolation is employed to generate new gray values instead of image pixels to obtain a background image and the moiré fringe image should be corrected by the obtained background image. At last, binarized the image by 2D OTSU threshold algorithm. The experimental results show that the method is simple and effective to segment the moiré pattern from the original moiré fringe image. This method can be used to improve precision of lenses measurement.