Optically variable ink
The ink displays two distinct colors depending on the angle the bill is viewed at. 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.
OVI is particularly useful as an anti-counterfeiting measure as it is not widely available. One major manufacturer is a Swiss company called SICPA (Société Industrielle et Commerciale de Produits pour l'Agriculture). Additional suppliers include Sun Chemical (through their Brand Protection Division based in Manchester, UK) and the Brazilian company Sellerink, located in São Paulo, Brazil.
Color-shifting inks reflect various wavelengths in white light differently, depending on the angle of incidence to the surface. An unaided eye will observe this effect as a change of color while the viewing angle is changed. A color copier or scanner can copy a document only at one fixed angle relative to the document’s surface.
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