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 copying by color copiers, printers, cameras, or lithographic reproduction. Metal-dielectric multilayer thin-film structures have large color shifts with angle, high chroma, a large color gamut, and light fastness that make them uniquely suited for security inks. World currencies are now protected by these structures.