A method of embedding an optical signature in a document and document produced therefrom. The optical signature uses an invisible optical pattern or covering that can reflect light in non-visible wavelengths, such as light in infrared, or fluoresces when exposed to a particular wavelength of light, such...http://www.google.fr/patents/US7837117?utm_source=gb-gplus-shareBrevet US7837117 - Embedded optical signatures in documents
