1. A nonlinear joint transform image correlator comprising: a joint image production means for producing a collimated joint image of a reference image and an input image; a first Fourier transform lens device disposed to receive said collimated joint image at an object plane thereof and to produce the interference between the Fourier transforms of said reference image and said input image at a Fourier plane thereof; an image detector means located at said Fourier plane of said first Fourier transform lens device, said image detector means including a plurality of detection cells disposed in a planar array at said Fourier plane, each of said detection cells generating an indication of the light intensity of said interference of said Fourier transforms of said reference image and said input image at said detection cell; a threshold device connected to each detection cell of said image detector means for determining if the light intensity detected by each detection cell is greater than a threshold value, thereby generating a thresholded indication of the light intensity of said interference of said Fourier transforms of said reference image and said input image at said detection cell; and inverse Fourier transform means connected to said thresholding means for producing the inverse Fourier transform of said thresholded indication of the light intensity of said interference of said reference image and said input image, said inverse Fourier transform being the correlation between said reference image and said input image.