Abstract

There has been considerable work on developing optical interference coatings that exhibit a change in color when viewed at different angles, since Dobrowolski (1) successfully demonstrated this technique for anticounterfitting. His group developed a multilayer, all-dielectric coating with a HL³HL³H design for Canadian currency. Today, many countries' currencies, valuable documents, or other important items, e.g., credit or identification cards, use optically variable devices (OVD) for security. OVD based on Fabry-Perot filters with metal reflectors and dielectric spacer layers have been demonstrated and successfully commercialized. OVD can be made as color shift films on a substrate ("foil") or deposited on a release layer or directly on a coating drum, then removed as flakes, ground into small particles and used as pigments in printing inks (2).

© 1998 Optical Society of America