Real-time joint transform correlation by partially coherent readout illumination.

A real-time joint Fourier transform image correlator that uses partially coherent readout illumination to perform signal detection in complex amplitude is described. The interference between the Fourier transforms of input images is obtained by coherent light. However, the correlation signal is produced by partially coherent illumination, which is used to read out the Fourier transforms interference intensity from the output of the spatial light modulator (liquid crystal image transducer). The advantage of this technique is the reduction in the spatial phase variations in the output wavefront of the image transducer. Such nonuniformities may arise from nonflatness and mismatch of various output layers in some devices that can degrade the performance of the correlator. We present experimental results to show that the phase distortion effects present in the output surface of the device can be reduced by using partially coherent readout illumination. It is also shown that the correlation bandwidth obtained by the proposed system is smaller compared to VanderLugt filtering under quasi-monochromatic illumination. As a result, a brighter correlation peak intensity is produced, and a better correlation SNR is obtained. Pattern recognition experiments using naturally illuminated real objects are presented, and the effects of relaxed coherence on the real-time joint transform correlator is determined.