Abstract: The matched filtering has great prospect in the field of crack detection based on digital image processing. The choose of the filter is very important for the result of the matched filtering. In this paper, a demonstration about the superiority of the matched filter used in crack detection is given first of all, and then three principles are given about the method for constructing the matched filter using in crack detection, including the principles of the form, the dimension and the width of the nonzero element of the filter matrix. At last, the principles are testified by simulation experiment, and the crack image edges matched filtering is drew on the original image. 

Key words: crack, matched filter, principles