Stationery, or other printable media, is encoded with a digital watermark. The watermark is not conspicuous to a human observer of the media, yet conveys plural bits of auxiliary information when optically scanned and digitally processed. The watermark can be formed by ink-jet printing, or otherwise....http://www.google.com.au/patents/US7136502?utm_source=gb-gplus-sharePatent US7136502 - Printing media and methods employing digital watermarking
