Most kinds of criminal behavior can be detected, investigated, and punished. But counterfeiters and document fraudsters often escape prosecution because it is difficult to connect them to their printing equipment in a way that will satisfy the courts. The problem is exacerbated by the wide range of inexpensive and high-quality laser and inkjet printing machines available on the market. To meet these challenges, the London College of Communication (LCC; London, England) forensic research program is using high-resolution digital image analysis to improve the methodology of “fingerprinting” printing machines.
The sophistication of the method used to resolve fraud and counterfeit cases depends on the quality of the reproduction and whether visible distinguishing marks are present. The naked eye or magnification is sufficient in many cases. If the reproduction is expertly done or there are no obvious distinguishing marks such as heavy print-roller scratch marks then special techniques must be applied.
Chemical analysis is frequently used in the forensic analysis of laser print. However, disposable toner cartridges create a problem because vital evidence can be destroyed if a cartridge has been replaced. A replacement toner or ink cartridge may have a different chemical composition, rendering chemical analysis inconclusive. Furthermore, paper evidence is destroyed during the process of chemical analysis.
A technique under development by the United States Secret Service Forensic Science Division involves the use of electrostatic detection to reveal faint paper-feed-roller marks. A voltage of up to about 10 kV is applied by corona discharge to the document. Then the roller marks, which vary from machine to machine, can be revealed and measured by applying toner to the paper. However, there is no guarantee that the roller marks will always be conclusive evidence. Also, the toner applied to the paper substrate can harm the evidence
Digital image analysis
Jack Tchan and his colleagues at the LCC have begun applying digital-image-analysis techniques to this task by analyzing patterns generated in print due to irregular movements by the print engine. These irregular movements cause lines to be printed across a page when solid smooth print is required, known as banding. Fine banding can be caused by imbalance of the rotor component of the polygon mirror or mechanical weaknesses of the laser scanning unit; rough banding can be caused by unsteady motion of the photoconductor drum or the fuser unit.
Banding patterns vary from printer to printer and can be used to link a document to the printer that produced it. Banding can cause the size of print to vary across a page in patterns unique to the printer used. Size variations, while imperceptible to the eye, can be quantified by precise measurement using digital image analysis.
Tchan says that a high-spatial-resolution digital-image-analysis system has been built that comprises a Hamamatsu C4742-95-12NRB monochrome digital CCD camera. The important feature of the camera is that the CCD chip is Peltier-cooled to increase its signal-to-noise ratio. A lens magnifies the object image and enhances the resolution of the images produced by the camera. The accuracy of the measurements is aided by a red LED light source.
The image data from the camera are digitized to 8-bit resolution using a Matrox Imaging Meteor II frame-grabber board and MIL-Lite software. Algorithms were developed so that length measurements on individual graphic objects and text characters could be made.
For their initial results, the researchers produced sets of squares from two Hewlett-Packard laser printers. The horizontal length of each square was measured by edge extraction and thinning. Distinct patterns in the horizontal length of each square exist across the page and can vary for different print engines. The results demonstrated that it is possible to apply this method to analyze documents in fraud or counterfeiting cases. More significant results may even be found by also analyzing the print in the vertical direction.
The researchers hope to investigate inkjet printing as well. The success of the pilot investigation suggests that a more advanced camera could be used. In the longer term a scanner-based system may be developed to automate and speed up the data-acquisition process.
