Diffractive moire features for optically variable devices

摘要

We present diffractive second-line security features based on the moire phenomenon, that are designed for use in Optically Variable Devices (OVDs). After a short introduction of our 2D and 1D moire methods, we first present the integration of line-based, 1D animated moire patterns into OVDs. These covert features are verified using a printed, high-resolution screen, which causes the covert information to become visible. When the screen is moved back and forth, the covert information, for example a text, appears to move dynamically in a well defined way. We then present diffractive OVDs where specially designed 2D moire features have been integrated into graytone images. Such an integration has the advantage that the area in which the second-line security feature appears can be used simultaneously for a visually attractive first-line effect rather than just having a homogeneous background. The integrated diffractive moire features are verified with a 2D microlens array through which the OVD is viewed; as the verifier or the OVD together with the verifier is moved, one observes dynamic visual effects. A special form of integrating a diffractive moire-feature into an OVD is shown in the last part of the paper, where the 2D microlens verifier is used in a fixed combination together with the information layer that consists of diffractive microstructures. Such a diffractive moire-magnifier feature is characterized by the unique visual impression that it creates where projected images appear to move as the sample is tilted.