Holographic optical elements encoded security holograms with enhanced features

A simple and cost-effective two-step method for forming encoded security holograms with enhanced features is described in this paper. These security holograms contain enhanced encoded/concealed anti-counterfeit security features, which can only be decoded using a key hologram in the final reading process. The encoded key hologram and the security hologram are in the form of special encoded complex holographic optical elements. When the security hologram is illuminated with the decoding beam, specific moire-like fringe patterns are formed on the security hologram and in addition several spatially separated bright focused spots are also generated from the security hologram. A careful spatial filtering of these bright spots results in specific moire patterns at different locations in the observation plane and moreover these patterns contain variable interferometric features. Further, these moire patterns disappear when the security hologram is perfectly repositioned and only the variable interferometric features are formed. Since these security holograms contain variable interferometric features in addition to the specific moire patterns and bright focused spots, thus making these holograms suitable for both visual and as well as machine inspection.