The technique may become a standard in the printing and verification of important printed items such as airline boarding passes, passports, commercial checks, travelers' checks and currency. It also may be used to trace correspondence and printed matter with possible links to terrorist activities.
The research, which was funded by the National Science Foundation, is detailed in three papers that will be presented Nov. 5 at the International Conference on Digital Printing Technologies in Salt Lake City.
Printing defects can come in all forms, from uneven lines and letters to different ink shades and ink marks. The researchers looked at all of these imperfections, but focused on laser printers and banding, or horizontal printing imperfections.
Laser printer cartridges contain a photoconductor drum. Laser beams scan back and forth across the drum, which is coated with a charged material that releases upon exposure to specific rhythms of light.
Toner attracts to uncharged areas to create letters or images. Drum rotation varies for every printer. This leads to banding imperfections, where some printed areas look darker or lighter than others.
"Although they (the banding imperfections) may not be obvious to the human eye, they can be identified if one knows where and how to find the noises/artifacts that are the intrinsic signatures," said George Chiu, associate professor of mechanical engineering at Purdue.
Chiu told Discovery News that intentionally embedded signatures probably could be detected quickly and with relatively minor investment at public places like airports.
"Currently we are assuming a minimal set of tools: a good flatbed scanner with a computer running the specific detection/identification algorithm," Chiu explained. "However, with more sophisticated instruments we will be able to detect more finer details."
Chiu and his team are aware that clever forgers and terrorists may try to crack the banding code and either attempt to duplicate or change it.
"But the bad guys need to literally have the technology and know-how to change the printer design as well as the electronics and firmware of the printer, so there is quite a high point of entry to achieve this," Chiu said.
He added that the existence or nonexistence of signatures could provide valuable clues to investigators.
Discovery News contacted the U.S. Secret Service, which is working with Purdue on the new detection methods. Rich Dusak, a supervisory document analyst in the SS's Forensics Services Division, said the new methods are not yet in use.
"We are, however, following the research as closely as we can," he said.
Dusak did reveal that similar signatures within copiers have helped agents to solve cases. In one instance, an individual was utilizing a full color copier to make some extra money after hours.
The copier's signature enabled agents to trace the money back to the machine. The individual's fingerprints were found on the notes, and he was caught.
Dusak added, "In one typical ongoing case, researchers were able to read the codes of suspect financial documents. The codes were traced to a large color copier within a business."
In other cases, he said printers have been found in places such as prisons and mobile homes that have been linked to suspects. Once the unique signature of these printers is determined, agents could then try to match questionable documents to the printers.
"The criminals are a step ahead of us one day, and we're ahead on another," Dusak said. "We're working hard and looking at new methods to effectively adapt to the changes in technology and to stay ahead of the computer savvy criminals."
