1. An image comparison system comprising: means for forming a composite image composed of a reference image and a test image; means for generating a two-dimensional image spectrum from said composite image; means for whitening said two-dimensional image spectrum by setting the magnitude of every point of the two-dimensional image spectrum to a uniform level; means, responsive to said means for whitening, for constructing the phase-only image of said composite image; and means for detecting a value of the phase-only image exceeding a predetermined threshold representative of the location of a difference between the said reference and test images.