Proceedings PaperInteractive paper as security substrate
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The threat of counterfeiting Bank Notes and other security documents is steadily increasing with the continuous improvement of printing and copying technologies. The perfect reproduction of a document is indeed reachable with the right combination of technique and economics. Our only protection is to make it difficult by introducing artifacts which appear only through the counterfeiting process or reduce significantly the efficiency of that process. A number of methods are available to defeat counterfeiting none of which are completely satisfactory. The difficulty is not as much the prevention of the duplication than doing it while safeguarding the intrinsic properties (durability) of the documents as far as permanent readability, resistance to wear, etc. Basic approaches are: (1) The light switch: a transparent coating which becomes opaque during any attempt of duplication. (2) An alternative to this first approach is an ink which becomes transparent at the time of duplication. (3) The use of a substrate (paper stock) whose optical density will rise to that of the printed graphic on the bill at the time of copying. This would be a true interactive paper. (4) Luminescent materials that would blind the photoreceptor during exposure. Such material could be in the substrate or in the printing ink. (5) Introduction of an artefact in the printed copy through a hologram hidden in a transparent coating or embossed in the substrate. All approaches have been investigated to a certain extent with variable success. We report hereafter the results of investigations made by the author on alternatives (1) and (2) in this laboratory and at the University of Texas at El Paso under contract from the National Bank of Belgium and with the collaboration of the CNRS in Paris and the University of Marseille France.