Sequence Discrimination of Laser-printed and Handwritten Intersections through Coaxial Microscopy

Background: Determining the chronological sequence at print-handwriting intersections remains a persistent challenge in forensic document examination. Methods: In this study, we developed a coaxial light microscopy approach based on a six-feature taxonomy to assess stroke order. A total of 3600 intersections were prepared under two scenarios - handwriting-before-printing and printing-before-handwriting - using three types of laser printers and twelve commercial pens. Six morphological features were systematically evaluated: Internal ink loss, gloss reduction, stroke contraction, central bright trace, edge ink loss, and stroke deformation. Results: In handwriting-before-printing samples, gloss reduction and internal ink loss were the most prominent indicators, whereas central bright trace emerged as the key feature in printing-before-handwriting samples. Significant differences were observed across printer classes and pen families, with loose-toner printers and rollerball/ballpoint pens yielding clearer feature expression than fused-toner printers and fiber-tip pens. Temporal testing at 0 and 30 days showed that gloss reduction, stroke contraction, central bright trace, and stroke deformation were time-robust, while internal ink loss and edge ink loss exhibited greater variability. Conclusion: In a blinded case validation, three independent examiners reached unanimous conclusions using the taxonomy. These findings demonstrate that coaxial light microscopy provides a reproducible and accessible method for establishing stroke sequence at print-handwriting intersections, with clear potential for forensic application.