A multilayer optical safety device has a resonant waveguide grating (RWG) layer on a bottom layer having a hologram. The RWG layer has a plurality of RWGs for generating one or more patterns. Each RWGhas first and second RWG portions, respectively, having diffractive gratings of different grating periods for optical coupling in and out, respectively. By using the RWGs, the interference to the holographic image generated by the bottom layer can be reduced to the maximum extent. This advantage is particularly useful when the holographic image carries digital data, such as in digital holographicencryption, and when the encrypted hologram is used at the bottom layer. The digital data may contain biometric information, such as a fingerprint. In each RWG portion of a single RWG, the scatteringelements in the grating are curved, thereby improving the angular tolerance of the azimuth angle during pattern observation by the observer, while the single RWG remains selective at elevation.
MULTILAYER OPTICAL SAFETY DEVICE