Fluorescence-based correlation mark for enhanced security in printed documents

A device for authenticating a printed color document, said device comprising:

• a frame;
• a transparency overlay key supported by said frame, said transparency overlay key comprising a transparency on which a halftone pattern is printed using yellow colorant;
• a UV light source connected to said frame and arranged to direct UV radiation onto and through said transparency overlay key;
• wherein said frame defines a slot adapted for selective insertion of a printed document to be authenticated so that said printed document is positioned beneath and adjacent said transparency overlay key for subjecting said transparency overlay key and said printed document to UV radiation from said UV light source.

A method of enhanced document security, said method comprising:

• providing a substrate that will fluoresce when subjected to UV radiation;
• receiving digital image data defining a document to be printed in terms of at least cyan, magenta, yellow and black image separations;
• receiving input defining a watermark to be printed as part of the document;
• deriving cyan, magenta, black, and yellow halftone patterns from the cyan, magenta, black, and yellow image separations, respectively;
• modifying the yellow halftone pattern using phase shifts to define a phase-shifted yellow halftone pattern that encodes the watermark as a correlation mark;
• printing the document on the substrate according to the cyan, magenta, black, and phase-shifted yellow halftone patterns, wherein the printed document is adapted to be authenticated by overlaying a transparency key on the printed document and simultaneously subjecting the transparency key and the printed document to UV illumination to reveal the correlation mark.