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By placing a moire´ pattern behind the first field lens of an optical spatial processor its diffraction spectrum will be formed at the first image plane of the system. This diffraction spectrum is filtered to give two images only along a selected direction. The third field lens collects the selected orders to form the Moire´ pattern at the second image plane. However, if a recording photographic plate is placed at a certain distance away from the second image plane, a double image occurs which is shifted along the selected direction. The resulting moire´-of-moire´ pattern depicts the partial derivative contours of the model.