Extraction of difference betwen two images

The two photographs to be compared are modulated by a diffuser containing high spatial frequencies and are successively recorded on a photographic plate. The diffuser is given a small translation in its plane between the two exposures. After processing under the usual conditions of linearity, the negative is illuminated by a parallel beam of coherent light. The spectrum observed at the focus of a lens is modulated by straight parallel fringes. These fringes are broad if the translation of the diffuser between the exposures is small. A simple slit placed in the Fourier plane eliminates the light due to identical regions of the two photographs and transmits the difference, which can be seen in the image of the negative. Since the fringes in the Fourier spectrum are as wide as two centimeters, one can employ a broad source. This assures good quality reconstructions. Further as the diffuser has a structure finer than the smallest details on the photographs, the diffuser does not lower the quality of the images.