Copy-Protected Documents

文件类型：PDF文档
文件大小：509K
The invention relates to documents of value incorporating features that prevent them being accurately copied by image reproduction means such as a scanner or photocopier. A document of value (10) bears an image comprising a material doped with a rare-earth element wherein the image has a first appearance under daylight and has a second appearance under synthetic white light. A copy (14) of the original document (10) is not colour identical to the original document. Typical colour changes are from yellow/green to yellow or orange to pink and purple to pale green when reproduction is attempted. Preferred rare-earth elements are erbium, neodymium and holmium which are doped into an yttium lattice. An appropriate printing ink is also disclosed.