An improved optical security system based on two phase-only computer-generated masks is proposed. The two transparencies are placed together in a 4f correlator so that a known output image is received. In addition to simple verification, our security system is capable of identifying the type of input mask according to the corresponding output image it generates. The two phase masks are designed with an iterative optimization algorithm with constraints in the input and the output domains. A simulation is presented with the resultant images formed by the two phase-only elements. Various mask combinations are compared to show that a combination is unique and cannot be duplicated. This uniqueness is an advantage in security systems.
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