COLOUR printers are now so good that crooks can use them to counterfeit bank notes and share certificates. To foil them, a manufacturer has developed a forensic marking technology that will give each colour printer an invisible fingerprint, making it as identifiable as the print hammers on a manual typewriter.

Patents filed by Hewlett-Packard (GB 2361211) explain how the operation of a printer's yellow laser drum—or ink-jet print head—can be microscopically marked with a unique code that can be spread over several printed characters. As only one pixel in each character cell is altered, and the eye is least sensitive to yellow, the mark is all but invisible. But detectives could use a reading device tuned to receive yellow light to tie counterfeit artefacts to a suspect printer.

While Hewlett-Packard won't say whether it plans to use the new technology, it has confirmed to New Scientist that it is now ...