We describe an electro-optical method of deciphering a watermark from a recently invented encoded image termed a concealogram. The watermark is revealed as a result of spatial correlation between two concealograms, one containing the watermark and the other containing the deciphering key. The two are placed side by side on the input plane of a modified joint-transform correlator. When the input plane is illuminated by a plane wave, the watermark image is reconstructed on part of the correlator's output plane. The key function deciphers the concealed watermark from the visible picture only when the two specific concealograms are matched. To illustrate the system's performance, both simulation and experimental results are presented. (c) 2005 Optical Society of America.
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