Technical Note: Analyzing the effect of repeated fusing on toner to examine printing alterations made by the same laser printer

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.