Patent Document ID: 4455039
Application ID: 06391690
Patent Status: 1

Claim One:
1. A security document of value having a mark detectable by apparatus responsive to electrical capacitance as a particular physical characteristic of the mark and thereby distinguishable from a counterfeit document lacking such a mark, comprising: a substrate of a first material free from said physical characteristic, a mark formed as a coating of a second material having said physical characteristic, said coating consisting of a particulate generally invisible very thin surface layer sputter-deposited on a portion only of one surface of said substrate and a sub-surface region below said very thin layer formed of discrete particles of said second material embedded within said first material arrayed in nonelectrically conductive relationship one relative to the others as a continuation of said thin surface layer, there being a greater number of particles of said second material in the sub-surface region than in the surface layer, the physical characteristic of said particulate material being electrical capacitance, said mark being nondetectable except by said apparatus and visually observable matter overprinted on said document.