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Watermark detection

  • US 4,296,326 A
  • Filed: 03/05/1980
  • Issued: 10/20/1981
  • Est. Priority Date: 03/06/1979
  • Status: Expired due to Term
First Claim
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1. A method of rejecting sheets which do not have a mould watermark (as herein defined), comprising the steps of:

  • measuring for each sheet, in an area in which a watermark is expected to be present, the absorption of ultraviolet radiation by the sheet;

    measuring for each sheet, in the area in which a watermark is expected to be present, the transmittance of visible light by the sheet;

    and rejecting sheets for which the said absorption measurement does not show a substantially constant value and also sheets for which the said transmittance measurement shows a substantially constant transmittance of visible light over the said area.

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