Watermark detection
First Claim
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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Accused Products
Abstract
In a method of detecting sheets which do not have a genuine watermark (i.e. watermarks which result from variations in fibre distribution introduced during manufacture) the absorption of ultra-violet radiation is measured for each sheet in the area in which the watermark is expected to be present, and the transmittance of light by this area of the sheet is also measured. Sheets for which the absorption measurement does not show a substantially constant value, and sheets for which the light-transmittance shows a substantially constant value over the said area, are rejected. The absorption measurement is preferably effected by measuring the reflectance of ultra-violet radiation by the sheet, but the transmittance of ultra-violet radiation or the fluorescence of the sheet in the presence of ultra-violet radiation can also be measured. The measured value for the area can be compared with a reference value obtained by a measurement of the same parameter outside the watermark area.
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Citations
19 Claims
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1. A method of rejecting sheets which do not have a mould watermark (as herein defined), comprising the steps of:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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- 10. Apparatus for examining sheets passing along a flow line and generating a reject signal in response to the passage of a sheet which does not have a mould watermark (as herein defined), comprising a source of ultra-violet radiation arranged to direct such radiation into the path of each sheet, means for measuring the absorption of the ultra-violet radiation of each sheet in an area of the sheet in which a watermark is expected to be present, means for comparing the measured absorption value with a reference value, means for measuring the transmittance of the said area of each sheet to visible light, means for comparing the light-transmittance value with a reference value, and means for rejecting sheets for which the said absorption measurement does not have a substantially constant relationship to the said reference value and also sheets for which the said light transmittance measurement has a substantially constant relationship to the transmittance reference value.
Specification