Minimal distance measure method based on an absolute difference measure is applied to the similarity measure among the optical correlation resultant images, and then the similarity conclusion between the image to be tested and the reference image can be deduced. By this method, the false recognition which often occurs when the traditional correlation peak thresholding method is used, can be avoided and the recognition accuracy can be greatly improved. Even the low quality correlation output between two gray scale images can be processed so that the complex preprocessing of the gray scale images, such as the binarization and encoding, can be omitted. The optical experimental results implemented on an incoherent optical correlator are given.