Joint transform correlator performing pure phase correlation for optical pattern recognition

A joint transform correlator, performing pure phase correlation by a phase spatial light modulator in the input to the second stage optical setup, is proposed. It can deliver excellent correlation performance in the presence of non-overlapping noise or additive noise. The proposed method extracts the interference fringe phase information from the joint power spectrum by simple subtraction and division operations; the extracted phase is then encoded onto a phase spatial light modulator to obtain the correlation results. To extract the fringe phasors, the proposed scheme uses an 50/50 beamsplitter and an lambda/4 plate in the optical setup, and two CCDs to capture the joint power spectrum and the phase shifted joint power spectrum respectively This system is found to be equivalent to correlation between only the Fourier phases of the reference and input images; however, there are practical difficulties in implementing this type of pure phase correlation using the VanderLugt architecture. Computer simulations show that the proposed joint transform correlator out-performs binary joint transform correlation, using the binarization methods of multilevel threshold function and adaptive windowing threshold, when the targets are embedded in either non-overlapping noise or scenes with additive noise; performance is maintained even when the target has a very low light contrast ratio in a severely noise corrupted input scene.