To most people, counterfeiting has little to do with holography and everything to do with ersatz $20 bills. Far more well known than researchers` efforts to make security holograms is the US Treasury`s undertaking to make US currency harder to duplicate: its new $100 and $50 bills, modernized to thwart ever-improving inkjet-printer technology, have been in circulation for awhile, and the new $20 bill was released in October. Although they do not contain holograms, the new bills do incorporate a type of optical variable device (OVD) in the form of color-shifting ink.
When viewed straight on, this ink appears green; when viewed obliquely, it is black (see figure). Made by Sicpa SA (Prilly, Switzerland), the ink contains multilayer interference filters in the form of myriad tiny flakes, all oriented in the same direction within the ink film.1 Layers of metal within the filters give the ink a metallic sheen. Other color-shift combinations can be produced, as well: for example, green-to-blue, green-to-magenta, and magenta-to-green. A variant of the ink is visible in the near-infrared if viewed from straight on and invisible if viewed obliquely, a property ideal for use with covert optical sensors. The tiny filters are manufactured by vacuum deposition onto a roll-coater.
REFERENCE
1. A. F. Bleikolm, Proc. SPIE 3314, 223 (Jan. 1998).
