Nondestructive Vis-NIR Reflectance Spectroscopy as a Forensic Tool for Ink Discrimination: A Preliminary Study

Visible light reflectance spectra were taken from signatures inscribed with five different inks of five different ball-point pens by a modular portable Vis-NIR spectrophotometer with PC. The ink-analysis procedure was nondestructive, consisting of visible reflectance spectra acquisition from multiple designated sampling spots of each signature, using a customized reflection probe holder and a tungsten white light source. Sampling spots were preselected to partly cover the signature ink-line on the white background (paper). The repeatable reflectance spectra (R%), recorded and plotted for each ink, which is subject of this study, appeared with typical spectral features (extrema) for each tested ink. To determine another characteristic spectral feature for ink discrimination, such as inflexion points specific to the absorption edges, a numerical derivative of the first order was calculated for the average R% spectrum of each ink. The wavelengths at which the typical features appear were used for Vis-NIR codification of each ink for discrimination purposes. A different code could be assigned for each ink, using the description of the "general feature," the extrema, and the inflexion points from the reflectance spectrum. This method can be used as a nondestructive tool for ink identification for forensic purposes.