Optical image subtraction using multiple-imaging effect of photographic diffusers

This method of optical image subtraction makes use of the extremely high demagnification and the large image densities involved in the multiple-imaging effect obtained with photographic diffusers. A coarse grid of period equal to 25 mm is multiply imaged by the diffuser, which represents a photographic recording of a fine-grain speckle distribution. An in-plane displacement of the coarse grid produces an equivalent motion in each of the multiple-image gratings that modulate the input transparencies. This means that the π-rad phase shift between them can be obtained by operating on the coarse grid, thus increasing the accuracy of phase shifting by a factor equal to the demagnification of the imaging process. A description of the method, supported by experimental results, is given.