Patent US 4,455,039

Documents are encoded with at least one thin, transparent coating, normally invisible, but having a readily detectable physical characteristic such as, electrical conductivity, electrical impedance, electrical capacitance, electroluminescence. Each document consists of a substrate on which is applied (1) at least one of the thin coatings, and (2) conventional printing, preferably intaglio. The thin coating may be applied before or after the printing. The coating includes particles driven into the surface to a substantial depth. The uncoated substrate is free of the detectable physical characteristic. The coating is confined to a limited area of the substrate surface and itself constitutes a code or is laid down in a pattern for identification. A second coating can be applied so that all areas of the surface have the same appearance to mask visual detection of the first coating. The first coating can be of such a nature as to defeat electrostatic copying, particular, color copying, by effecting a discernible difference between the original and the copy. Each coded coating is detectable by a machine which is capable of quickly distinguishing a genuine document from a counterfeit document.