Watermark detection

Details Watermark detection Watermark detection

1980-03-05

1981-10-20

Smith, Alfred E.

Radiant energy

Invisible radiant energy responsive electric signalling

Ultraviolet light responsive means

250461R, 283 8R, 356 71, 356 72, G01J 142, G01N 2138, B42D 1500, G06K 974

Patent

active

042963265

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.

REFERENCES:
patent: 3774046 (1973-11-01), Hoch et al.
patent: 3842281 (1974-10-01), Goodrich

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