The invention relates to security feature for identification documents, and in particular to A security feature, comprising a first covert layer comprising a first covert material that is not visible to a human eye under except under a first condition, the first covert layer further comprising a material...http://www.google.ca/patents/US20050067497?utm_source=gb-gplus-sharePatent US20050067497 - Optically variable security features having covert forensic features
