Crimes in print

Oct. 1, 2007
Digital imaging performs forensic analysis of fraudulent laser-printed documents.

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.

About the Author

Conard Holton | Editor at Large

Conard Holton has 25 years of science and technology editing and writing experience. He was formerly a staff member and consultant for government agencies such as the New York State Energy Research and Development Authority and the International Atomic Energy Agency, and engineering companies such as Bechtel. He joined Laser Focus World in 1997 as senior editor, becoming editor in chief of WDM Solutions, which he founded in 1999. In 2003 he joined Vision Systems Design as editor in chief, while continuing as contributing editor at Laser Focus World. Conard became editor in chief of Laser Focus World in August 2011, a role in which he served through August 2018. He then served as Editor at Large for Laser Focus World and Co-Chair of the Lasers & Photonics Marketplace Seminar from August 2018 through January 2022. He received his B.A. from the University of Pennsylvania, with additional studies at the Colorado School of Mines and Medill School of Journalism at Northwestern University.

Sponsored Recommendations

Request a quote: Micro 3D Printed Part or microArch micro-precision 3D printers

April 11, 2024
See the results for yourself! We'll print a benchmark part so that you can assess our quality. Just send us your file and we'll get to work.

Request a Micro 3D Printed Benchmark Part: Send us your file.

April 11, 2024
See the results for yourself! We'll print a benchmark part so that you can assess our quality. Just send us your file and we'll get to work.

Request a free Micro 3D Printed sample part

April 11, 2024
The best way to understand the part quality we can achieve is by seeing it first-hand. Request a free 3D printed high-precision sample part.

How to Tune Servo Systems: The Basics

April 10, 2024
Learn how to tune a servo system using frequency-based tools to meet system specifications by watching our webinar!

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!