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...http://www.google.com.au/patents/US4296326?utm_source=gb-gplus-sharePatent US4296326 - Watermark detection
