A fluorescence-based correlation mark is included in a printed document by encoding the correlation mark as phase shifts in the yellow halftone image only of a printed color document. The correlation mark transparency key is likewise printed using only yellow colorant or can be printed in black or with another colorant or mixture of colorants that will appear dark or black when subjected to UV illumination. UV illumination of the document without use of the transparency key, and use of the transparency key without UV illumination of the document are insufficient to reveal the fluorescence-based correlation mark. UV illumination of the document while the transparency key is overlaid with the document will allow the correlation mark to be perceived.