The setup used in many optical data processing schemes is a coherent optical image forming system. The most important lement in this setup is the complex spatial filter. It can perform a large variety of linear operations upon the object or input. In general, it is difficult to produce complex filters, since both amplitude transmission and phase delay may vary across the filter plane in a complicated manner. Our own filters which are very similar to binary holograms, consist of many little transparent rectangles on opaque background. They can easily be drawn on a large scale by a computer-guided plotter, and then photographically reduced in size. We show that our filters, despite containing only amplitude values zero and one, can perform any data processing operation which could be performed by any complex filter. After explaining the principle, we present three groups of applications. First, we describe new versions of some classical methods: schlieren observation and phase contrast. Next, we report on spatial which perform differential operations upon the object in order to enhance gradients or corners. Finally, we use our binary filters for signal detection.
A. W. Lohmann and D. P. Paris, "Computer Generated Spatial Filters for Coherent Optical Data Processiing," Appl. Opt. 7, 651-655 (1968)