We present diffractive second-line security features based on the moiré 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.
KEYWORDS: Printing, Superposition, CRTs, Security printing, CMYK color model, 3D acquisition, Detection and tracking algorithms, Color reproduction, Color imaging, Visualization
Printing with custom inks is of interest both for artistic purposes and for printing security documents such as banknotes. However, in order to create designs with only a few custom inks, a general purpose high-quality gamut reduction technique is needed. Most existing gamut mapping techniques map an input gamut such as the gamut of a CRT display into the gamut of an output device such as a CMYK printer. In the present contribution, we are interested in printing with up to three custom inks, which in the general case define a rather narrow color gamut compared with the gamut of standard CMYK printers. The proposed color gamut reduction techniques should work for any combination of custom inks and have a smooth and predictable behavior. When the black ink is available, the lightness levels present in the original image remain nearly identical. Original colors with hues outside the target gamut are projected onto the gray axis. Original colors with hues inside the target gamut hues are rendered as faithful as possible. When the black ink is not available, we map the gray axis G into a colored curve G' connecting in the 3D color space the paper white and the darkest available color formed by the superposition of the 3 inks. The mapped gray axis curve G'(a) is given by the Neugebauer equations when enforcing an equal amount a of custom inks c1, c2 and c3. Original lightness values are mapped onto lightness values along that curve. After lightness mapping, hue and saturation mappings are carried out. When the target gamut does not incorporate the gray axis, we divide it into two volumes, one on the desaturated side of the mapped gray axis curve G' and the other on the saturated side of the G' curve. Colors whose hues are not part of the target color gamut are mapped to colors located on the desaturated side of the G' curve. Colors within the set of printable hues remain within the target color gamut and retain as much as possible their original hue and saturation.
KEYWORDS: Printing, Visualization, Superposition, Halftones, Chemical elements, Image resolution, Large screens, Calibration, Color imaging, Graphic arts
Printing with custom inks is useful for extending the gamut of printed images, for creating artistically appealing design or for providing protection against counterfeiting. The basic colors we consider, consist of the custom inks, their superpositions and the white paper. Color separation for custom inks requires to determine the relative amounts of the basic colors allowing to render each desired input color. To achieve this goal, one may tetrahedrize the set of basic colors on a given 3D color space. However, even for a few basic colors, there are many different tetrahedrizations.
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