Symbolic substitution is a method of performing parallel optical logic by recognition and substitution of structural elements. With spatial filtering it is possible to perform logic operations without an active logic gate. This paper describes how symbolic substitution can be implemented optically by use of spatial filtering. Earlier implementations used an additive logic, where shifted versions of the input pattern were added and then thresholded. Our new method is a multiplicative method that sequentially tests the pattern elements.
Adolf W. Lohmann,
Thomas M. Merklein,
"Symbolic Substitution Implemented By Spatial Filtering Logic," Optical Engineering 28(4), 284390 (1 April 1989). http://dx.doi.org/10.1117/12.7976966