Optical Interference Coatings for Inhibiting of Counterfeiting

Abstract

Counterfeiting of valuable papers and identification documents made of paper or plastics, such as banknotes, credit cards, passports, etc., is presently inhibited through the use of inks of many colours, intricate engraved designs on special papers which sometimes contain watermarks or embedded coloured paper platelets or metal threads. These methods have not always offered adequate protection. It is proposed to use optical multilayer coatings to validate such documents. One property of such coatings is that their spectral characteristics vary with the angle of incidence of the light. For most applications this variation is a decided disadvantage and efforts are often made to minimize it. But by this property optical multilayers differ essentially from all known types of dyes, inks, pigments and paints used in printing, photography and colour-copying machines. This angular variation can be made use of to enable the man in the street to distinguish at a glance an authentic document bearing such a coating from even the best counterfeit lacking it. With the same coating it is possible also to provide ever more discriminating tests of the authenticity of the document for the shop or bank, and for the crime laboratory where more sophisticated equipment can be used. The calculated and actual performance of a number of coatings designed for this purpose are described. These include some that permit positive identification not dependent on colour vision.