Invisible Ink from Xerox.

This article reports that researchers at Xerox have developed a way to add fluorescent words and images to documents like checks, coupons and transcripts using standard printers. The technique makes it possible to create everyday documents that have telltale marks visible only under ultraviolet light. Bright white paper is often fluorescent to begin with; the new process exploits that fact by printing the same shades of color in different ways, leaving more or less paper exposed. The necessary software will be incorporated into high-end commercial printers.