A Hybrid Optical-Digital Image Processing Method for Surface Inspection

A hybrid measurement technique is proposed for high-precision surface inspection. The technique uses an interferometer to image microscopic surface defects. In order to quantify the degree of various surface defects, the interferograms are scanned, digitized, and subsequently converted to a binary image by using an adaptive thresholding technique which takes into account the inhomogeneity of the imaging system. A new misalignment measure for binary patterns identifies the “straightness” of the fringe lines. It is shown that the resulting percentages of misaligned picture elements conform fairly well with the degree of various surface defects.