Marked, difficult-to-counterfeit documents

A method for creating marked, difficult-to-counterfeit documents, comprising labeling printed documents produced by solid toner apparatus, such as a laser printer or a copying machine. The toner comprises a material the presence of which can be detected using spectrophotometric techniques. The toner is marked with a single compound, emitting light in a known spectrum. Two or more compounds having different spectra are mixed, so as to produce a characteristic "code" or "signature" as the result of their excitation with light. The amount of marked toner present at a given point on the document is preser as a threshold value, below which a document is considered not authentic. The position of the marked toner on the document is specifically selected, and the authenticity of the document is determined by detecting the presence of the marked toner at a specific location on the document.