A counterfeit-resistant document comprises a contrasting layer and a dynamic camouflaging layer. The contrasting layer is highly contrastive and includes a latent message that can be reproduced over a broad range of copy device control settings. The dynamic camouflaging layer is applied over the contrasting...http://www.google.fr/patents/US6396927?utm_source=gb-gplus-shareBrevet US6396927 - Variable density verification
