Security holograms using an encoded reference wave

A new kind of security hologram using an encoded reference wave is presented. The hologram is produced with the interference of an encoded reference wave and a convergent object wave. The encoded reference wave is formed from a plane wave that has passed through a random phase plate. When the hologram is illuminated with a duplicate of the reference beam, a verification pattern (a bright spot) is produced in the output plane, Only if the random phase plate matches the hologram, does sharp focus emerge, Because the verification pattern is simply a spot peak rather than an image pattern, this kind of security hologram is suitable for machine inspection. Therefore, it can be used for better counterfeit-resistant security codes in the form of embossed easy-to-copy holograms. Initial experimental results are given. (C) 1996 Society of Photo-Optical Instrumentation Engineers.