Optical matched filtering for character recognition

An ordinary matched filter responds with a peak signal to the input. The peak corresponds to the location of a target. In some pattern recognition tasks it is desirable not to find the location of a target but to identify it (for example, a printed character). It is therefore proposed that the concept of a matched filter is modified in such a way that the output pattern consists of vertical fringes. These fringes are quite suitable as a "recognition signature" because the now uninteresting vertical position of the input character becomes irrelevant: when the input is moving horizontally, the output fringes move correspondingly. A slit detector will therefore produce an electronic single-frequency signal. An even better performance can be expected from a detector with a fringe mask in front of it. Such a detector system is tuned resonantly to a particular moving fringe pattern. This modified matched filtering concept may incorporate such properties as multiplexing, principal component recognition, pupil replication, and incoherent inputs.