Machine-readable information provided on a printed document in the form of printed marks which are not visually perceptible to the human eye. The printed marks are of a combination of sufficiently small size and selected color which renders the printed marks visually imperceptible to the human eye, yet...http://www.google.de/patents/US6753977?utm_source=gb-gplus-sharePatent US6753977 - Machine-readable information embedded on a document
