Printing is a significant innovation in commerce and daily life, with printed materials found in various products. Printed documents are often used in crimes and require an understanding of the printing process to infer evidence. When the observations are properly interpreted, the document itself and its scientific analysis will offer sufficient evidence. Traditional inspection methods, which rely on specialized equipment, knowledgeable workers, and chemicals, can be costly, wear out easily, and potentially harm papers. Before document forensics in the digital domain emerged, specific equipment and trained staff were required to verify document integrity.
There are various printing methods, including laser printing, inkjet printing, offset lithographic printing, electrostatic printing, flexography, and screen printing. Offset lithography is widely used in commercial printing for reproducing papers and creating fakes, using photography to manufacture plates. Offices use offset duplicators to create small runs of typewritten documents. Gravure, or intaglio printing, uses an image carrier below the surface being printed, resulting in high-quality products. Screen printing uses ink squeezed through a mesh to print on various surfaces, such as ceramics, glass, metal, wood, and circuit boards. This technique allows for thick ink coatings to be transferred to delicate surfaces. Non-impact printers have emerged, requiring little or no touch between the plate and paper. Xerography, closely related to laser printing, transfers images from a drum to paper by changing static electric charges. Inkjet printing fires ink at the right areas on the paper, guided by electrostatic forces.
Experts analyze laser and inkjet printers and electrostatic copiers to identify signatures from various processes. They compute features like noise energy, contour roughness, and average gradient. This is the first attempt to differentiate papers using attributes retrieved from individual characters in documents, enabling the identification and localization of document forgeries.
Document forensics involves identifying the origin of documents, including the device's brand and model. Statistical properties of printed characters are used to identify source brands and models. Forgeries, such as pasting and reprinting, involve modification, addition, or cancellation information on the document or replacing an entire page with a fabrication. Character position distortion is commonly used in these activities. Beusekon et al. developed a method for extracting text lines and alignment lines for document inspection, allowing tampered characters to diverge from ascender, descender, and base lines. To expose altered documents, matching frequencies between all pairs of documents are used. When a page is altered or reprinted, location displacement occurs when a fraudulent page or tampered region is compared to an authentic document. The fabricated page or manipulated region is discovered by computing the matching quality of two page pictures.
Laser printing uses six electrophotographic steps: charging, exposure, developing, transferring, fusing, and cleaning. The toner image is melted by the fuser, resulting in glossy characters with clear contours. Electrostatic copiers use CCD/CMOS image sensors and perform the same process. An electrostatic copier, on the other hand CCD/CMOS image sensors are used to scan the document, and the analog signal is converted to a digital signal and then electrophotographs it. The print head, carriage, and paper-advance mechanism are the three main components of an inkjet printer. The carriage is attached to the print head, which shoots ink into paper as the carriage moves back and forth in the scan direction.
Then comes the method name pre-processing, it uses features from individual characters in tilt-corrected scanned images. The characters are segmented, a threshold is generated, and the image is divided into text, edge, and background regions.
The following step is feature extraction: The backdrop region is not studied in this study because it contains no information about the character. Instead, the text and edge regions are used to extract the four features listed below:
1. Noise energy in the text region
2. Noise energy in the edge region
3. Contour roughness on the character
4. Average gradient on the edge region of the character.
In conclusion, the forensic assessment of the printed paper gave useful insights regarding the document's origin and probable manipulation. Forensic professionals can determine the legitimacy of a document and uncover any traces of tampering or fraud by meticulously inspecting ink patterns, paper fibers, and printing procedures. The incorporation of new technologies and skills in handwriting analysis improves the investigative process even further. As technology advances, so does the ability to unearth concealed details within printed documents, thereby preserving the integrity and correctness of evidence presented in legal and investigative procedures.
By Namrata Biswas