A document protection system is disclosed which employs a thermochromic pantograph and a validation mark. Also disclosed is a method to print thermochromic pantographs onto a substrate. A thermochromic pantograph according to the present invention has a latent image which is rendered visible when sufficient...http://www.google.de/patents/US5873604?utm_source=gb-gplus-sharePatent US5873604 - Document security system having thermo-activated pantograph and validation mark
