Gratings of constantly varying depth for visual security devices

Abstract

Sinusoidal gratings of locally varying profile depth are incor- porated into diffractive optically variable image devices (DOVIDs) for document security. The variation in profile depth is tailored to specific visual effects that can be readily authenticated. While the diffractive characteristics of these gratings depend very sensitively on the depth, the security of these DOVIDs is inherent to the diffractive structures insofar as the exact reconstruction of the original profile is required for the realization of the original visual effects. Sinusoidal gratings of locally varying profile depth are very resistant against copying by standard ho- lographic techniques since these techniques are shown to lead to a loss of fidelity in profile form or depth. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00101-4)