<title>Zero-order gratings for optically variable devices</title>

Abstract

We present the results of the application of zero-order diffraction gratings for optical variable devices (OVD's) for document security. Zero-order gratings have periods which are smaller than the wavelength of light; to describe accurately the optical properties of the zero-order gratings, we have applied rigorous electromagnetic theory, which we have compared to experimental measurements. We studied the diffractive behavior of zero-order grating varies locally in a predetermined manner. In the latter case, the resulting surface profile can exhibit variations in the diffraction properties, for example, a moire pattern. Furthermore, we have developed diffractive surface-reliefs which are a combination of a high-frequency, zero-order grating with large-period gratings, the addition of the zero-order grating to a large-period grating results in a surface relief with novel diffractive properties.