OPTICAL COATINGS FOR DOCUMENT SECURITY

Optical coatings that shift color with viewing angle have been adapted into optically variable foils, pigments, and inks to prevent color co pying by color copiers, printers, cameras, or lithographic reproductio n. Metal-dielectric multilayer thin-film structures have large color s hifts with angle, high chroma, a large color gamut, and light fastness that make them uniquely suited for security inks. World currencies ar e now protected by these structures. (C) 1996 Optical Society of Ameri ca