Gratings of constantly varying depth for visual security devices

Sinusoidal gratings of locally varying profile depth are incorporated into diffractive optically variable image devices (DOVIDs) for document security . The variation in profile depth is tailored to specific visual effects tha t can be readily authenticated. While the diffractive characteristics of th ese gratings depend very sensitively on the depth, the security of these DO VIDs is inherent to the diffractive structures insofar as the exact reconst ruction of the original profile is required for the realization of the orig inal visual effects. Sinusoidal gratings of locally varying profile depth a re very resistant against copying by standard holographic techniques since these techniques are shown to lead to a loss of fidelity in profile form or depth. (C) 1999 Society of Photo-Optical Instrumentation Engineers. [S0091 -3286(99)00101-4].