Thin film optically variable article and method having gold to green color shift for currency authentication

摘要

Thin film optical variable article to be used in a reflection mode having a gold to green color shift with angle for currency authentication and adapted to be carried by a substantially opaque currency sheet which serves as a substrate. The article includes a substantially transparent, optically thick element carrying a subtractive colorant and having first and second surfaces and a multilayer interference coating carried on one of said first and second surfaces. The article is adapted to be carried by the currency sheet so that the coating faces the currency sheet and the colorant carrying element serves as a superstrate facing the incident light. The multilayer interference coating is comprised of a substantially opaque layer of aluminum nearest the substrate, followed by a layer of magnesium fluoride, and then by a layer of chromium having substantial transmission. The colorant has a yellow hue. At normal incidence of light the article has a coppery-gold color shifting towards a vivid green at non-normal incidence of light. The colorant operates in an essentially subtractive mode in combination with the multilayer interference coating to provide the color shift from gold to green at two different angles of incidence and substantially no color at higher angles of incidence to cause a modification of the normal incidence gold color and the color shift with angle properties as seen by reflection.