A fast, low-cost, and nondestructive method using digital images to detect crossing pen ink lines in documents with old and recent writing

In forensic document examination, the analysis of pen ink is crucial for detecting fraud, particularly by evaluating crossing ink lines. In this study, a fast, simple, non-destructive method was developed for detecting crossing lines of blue pen inks in documents with old and recent writing, using smartphone-based digital images. Digital images of three different regions of interest (ROI) in the areas with pen writing were acquired for twenty-six real documents containing blue pen inks, dated between the years 1994 and 2023. The ROIs were defined as follows: I – Document Pen Ink (DPI), II – Intersection Pen Ink (IPI), and III – Overlay Pen Ink (OPI), representing different areas of the original and crossed pen ink lines. ColorGrab® free smartphone software was used to capture and process the values of the Red (R), Green (G), Blue (B), Hue (H), Saturation (S), and Value (V) channels for the digital images of the pen inks for each ROI. The R, G, B, and V channels provided the best analytical responses for identifying crossing lines in the OPI regions of the documents, enabling distinction of the regions after data normalization and principal component analysis (PCA). Application of k-means cluster analysis showed that the method was able to effectively distinguish between older and more recent writing at the crossing pen ink lines. The development of this nondestructive method offers a promising way to identify pen ink line crossing in old documents that may have been tampered with by the introduction of more recent writing.