There are many schemes and designs of optical correlator which are used in pattern recognition. In practice, a hybrid version where one FT is done electronically  and the other FT is done optically using FT lenses has been used. However, it is my considered opinion that a full digital version is more useful that all optical or hybrid type. This is because of the problem associated with human interfacing optical system for real time h data / image entry and display thereof. 

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Feature Selection for Classification