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Forensic experts track printer fingerprints

Counterfeiters who make $10 bills on printers could find the cops on their trail if tracking techniques pan out.

Researchers at Purdue University have developed image analysis techniques that may one day help tie counterfeit money and forged documents to the printers that produced them.

In lab experiments, the researchers examined documents that came from 12 different models of printers and were able to correctly link a document to its printer 11 times. The techniques currently let forensic investigators match a document with only a specific printer model, but will be honed so that a document can be matched to a particular printer.

"That means we will be able to tell the difference between counterfeit bills created on specific printers even if they are the same model," Edward Delp, professor of electrical and computer engineering at Purdue, said in a statement.

Delp and other professors and graduate students who worked on the project will present papers detailing the technology in November at the International Conference on Digital Printing Technologies in Salt Lake City. The group will also work with the U.S. Secret Service to develop new methods for tracing documents and counterfeit bills.

Software developed by Delp can identify "intrinsic signatures" of printers, or the subtle differences in the output of printers based on the small differences in their mechanics. To cut costs, printer manufacturers use plastic gears and other parts that create variations in printed sheets. These variations could be attenuated, but it would raise manufacturing prices considerably.

The group also exploits "banding," or the horizontal layers of ink that make up printed characters. Ink is laid down in horizontal lines or bands, which will vary in width and intensity because of the mechanical operation of the printer and the speed of the internal drum.

Banding, however, will change when the toner cartridge is changed. To counteract that effect, Purdue researchers are working with printer manufacturers to create a watermarking technology that would insert an "extrinsic signature" into the document. These signatures could not be recognized by the human eye but could be ferreted out through image analysis.

Jan Allebach, a professor of electrical and computer engineering, and George Chiu, a professor of mechanical engineering, have been working on a technology to reduce banding. The same technology can be used to insert the watermark.