Scientific & Technical

The use of drum defects to link laser-printed documents to individual laser printers

Trash marks are unintentional markings observed on printed, scanned, or photocopied documents that result from permanent defects or transient material in office machines and can be used for source attribution of questioned documents. Trash mark examinations have been in use in forensic laboratories for decades, yet the method remains relatively untested and relies on training, experience, and anecdotal information to support its validity. This study generated and harnessed objective data to empirically test one of the foundational theories for assessing the origin of photocopied documents: provided trash marks are present in sufficient quantity and/or quality, no two machines will exhibit a constellation of trash marks that is indistinguishable from another. In this project, objective trash mark location and size data was generated for 50 known photocopiers using both a traditional and a novel, automated method. Inter-machine comparisons were conducted using a novel variant of the Hausdorff distance algorithm to generate a quantitative assessment of how similar or different the 2450 pairs of trash mark constellations were from one another. This study found that each of the machines bearing one or more trash marks exhibited objective differences in their trash mark constellations, ultimately providing support for the tested hypothesis

This review embodies a comprehensive study pertaining to forensic examination of photocopied documents. A critical discussion on the trends, advances of analytical techniques implemented in the past four decades, and their respective edge over each other has been explicitly presented in the study. The field of interest has witnessed pronounced development in the examination procedures, particularly in the methodology and sample preparation. Analytical methods including spectroscopic, microscopic, and chromatographic techniques for the examination of printing toner have been variedly reported. Furthermore, studies focusing on the physical and optical examination of toner and printed document have also been discussed. The majority of studies have shown the efficacy of ATR-FTIR in identifying the binders present in toners. Raman spectroscopy has been largely employed to identify the type of pigment used in the printed sample. Techniques like SEM-EDX, XRF, LIBS, LA-ICPMS, etc. have been used for the elemental profiling of the toners. Recent literature shows the advent of techniques that are non-destructive, require nil to minimal sample preparation, and sustain the integrity of the sample as evidence. Another significant development is the use of data fusion techniques and chemometric methods to enhance the discrimination and identification of the sample. This is plausibly the only detailed review that features all the techniques that have ever been used for the forensic investigation of photocopiers and their toners.

With continuous innovations in laser printing technology, the number of cases of falsification using laser printers is increasing annually. Difficulties in identifying printing alternations, especially laser-printed alternations by the same printer are increasing. Effectively, repeated printing on the same page by the same printer means the original toner on the document was fused repeatedly. Thus, this study mainly investigates the effects of second fusing on the microscopic morphology of toner to examine whether documents have been tampered with. To detect the influence of fusing on the microscopic morphology of toner, 170 documents printed by 17 different models of laser printers from 5 brands were studied. It was found that, according to microscopic observations, the micrographs of toner may become brighter, darker or both with repetition of the fusing process. By calculating the average gray value of the micrographs of toner, it was found that, in some laser printers, the gray value of the thermoplastic accumulation area will increase after a second fusing, and the gray value of the toner particle dispersion area will decrease. In conclusion, by comparing the micrographs of toner from suspicious and reliable contents on the same page or in the same document, together with the measurement and analysis of average gray values, it is possible to examine the once- and twice-fused document contents, and further determine whether the document has been altered.