Morphological image processing in the speckle metrology

Abstract

In the speckle metrology, the processing of speckle image is very important. A method of morphological image processing is proposed in this paper. The theory of mathematical morphology and some basic morphological operations are introduced. In the experiment of laser speckle double-exposure, we get the specklegram which contains the information of micro-displacement. Then we employ the device of point-to-point analysis and get the image of laser speckle pattern interference fringes. Using the theory of speckle, the formula of micro-displacement is obtained. In the image processing, we first use grey-level transformation and histogram equalisation. Then we use the method of dynamic local threshold to get the binary image of laser speckle pattern interference fringes. In order to extract the fringe spacing, the image is processed by the operations of mathematical morphology in this work, such as the operations of opening, closing, skeletonisation and deburring. After that, the image can be used to extract the average fringe spacing by which the micro-displacement can be calculated finally. We get a more accurate result than traditional methods. The deviation may be less than 1% and the order of magnitude can reach 1 μm. The method of morphological image processing used in the speckle metrology has some advantages such as fast processing, easiness and high precision.