Optical pattern recognition using holographic films

Abstract

Pattern recognition (PR) is one of the most widely pursued application processing. The purpose of a PR system is not only to determine the presence and location of a given pattern but also to determine the degree ofsimilarity between different patterns. Its basic operation is the comparison (correlation) between input and reference (or stored) functions. Based on the implementation of the correlator, pattern recognition methods can be grouped into three major categories based on the implementation of the correlator, i.e.: 1) all-optical; 2) all-electronic; or 3) hybrid optical-electronic. In particular, all-optical correlators are of special interest since they offer speed and high capacity in performing correlation tasks. Two well-known all-optical correlator designs are based on the joint transform and Vanderlugt architectures.
The Vanderlugt offers the advantage of having the object plane fully accessible to the placement of input functions (or target image). Although Vanderlugt architecture greatly simplifies the optical correlation by synthesizing a Fourier hologram, now called Vanderlugt filter, it still remains very sensitive to exact repositioning of the hologram during the correlation process. This sensitivity to hologram repositioning demands for the use of photorefractive materials or spatial light modulators. The latter may permit near real time input of the function, however, this is currently very expensive. In using the low-cost silver halide holographic emulsions, the wet chemical processing involved creates a serious problem in repositioning the developed Vanderlugt filter. The allowed positional tolerance of the filter is given by ΔX = λf/4W, where λ is the wavelength, f is the focal length of the lens used and W is the width of the input object. For typical values of λ = 632 nm (HeNe wavelength), f = 10 cm, and W = 1 cm, this translates to a tolerance of around 1.5 μm which makes the repositioning of the film extremely difficult. Added to this, the shrinkage of the film after developing makes pattern recognition using silver halide films quite complicated.
In this paper, we report a novel protocol for optical pattern recognition using silver halide films. We demonstrate its successful implementation and special emphasis is given to the practical aspects of a pattern recognition system.