Optically variable ink

The United States fifty-dollar bill, for example, uses color shifting ink for the numeral 50 so that it displays copper at one angle and bright green in another.

One major manufacturer is a Swiss company called SICPA (Société Industrielle et Commerciale de Produits pour l'Agriculture).

Additional suppliers include German company Gleitsmann Security Inks, Sun Chemical (through their Brand Protection Division based in Manchester, UK), and the Swiss company Printcolor AG, located in Berikon, Switzerland.

Color-shifting inks reflect various wavelengths in white light differently, depending on the angle of incidence to the surface.

When the document is tilted, movement of a bright light stripe occurs and the colour changes.