An optical pattern recognition apparatus comprising means (1, 2, 3, 5) employing coherent light for converting at least one reference image and at least one input image into a joint coherent image, means (6) for transforming the joint coherent image to produce a Fourier transform image of the at least one reference image and the at least one input image, means (7) for providing an intensity distribution recording on the basis of the Fourier transform image, means (8, 9, 10, 14, 15) responsive to the intensity distribution recording for supplying correlation information for determining a correlation between the at least one reference image and the at least one input image, and characterised by means (4, 12, 13) operable in response to the supplied correlation information for varying the intensity of a coherent light beam applied in the converting means to the at least one reference image whereby to effect correction of the correlation information.