Optical EngineeringExperimental assessment of a photorefractive bandpass joint transform correlator
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An experimental implementation of a joint transform correlator based on a nondegenerate four-wave mixing interaction in the photorefractive material bismuth silicon oxide is described. An investigation is made of the pattern recognition performance of the correlator by assessing its ability to maintain a correlation response despite orientation changes of a model road vehicle while simultaneously discriminating against a similar vehicle. The full-bandwidth performance, essentially implementing an inverse filtering operation, was found to give good discrimination but to be highly sensitive to changes of the vehicle orientation from that of the reference function. A bandpass filtering strategy, based on the difference of Gaussian wavelet filter, was implemented in the correlator by exploiting the Gaussian amplitude profile of the TEM00 mode of the helium neon readout laser. An extensive experimental evaluation of the performance of the modified filtering scheme is presented, demonstrating considerably enhanced intraclass distortion tolerance while maintaining good interclass discrimination ability.