We describe a technique for reducing false signals from optical correlators using binary phase-only filters (BPOF's). Since the impulse-response function of the BPOF contains both the target object and the inverted version of it, the output consists of the correlation of the input object with both of these objects. In our research a Fresnel phase plate lens is encoded onto the BPOF. As a result the correlation with the desired target is on a converging beam, and the correlation with the undesired inverted image of the target is on a diverging beam. Experimental results show that these filters reject inverted input objects.
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