Storage capacity of an optically formed spatial filter for character recognition

Optical spatial filtering has been proposed as a means of character recognition. The cross correlations between the unknown character and a number of stored masks are performed optically. In this paper an estimate is derived for the capacity of such a system, i.e., the number of masks one can store. Two estimates are made for the capacity. One holds for a noiseless optical system. The derivation of the second estimate takes into account noise of the photographic plate. Noise measurements of Kodak 649F plates are given. A numerical example shows the order of magnitude of the capacity. While, in our specific example, a capacity of several hundred thousand is computed for the noiseless system, this figure is reduced by two orders of magnitudes for the noisy system.