Blue Ballpoint Pen Inks Differentiation using Multivariate Image Analysis of Digital Images Captured with PhotoMetrix PRO®

In Forensic Documentoscopy, it is frequently questioned if a particular document was written with one or more pens. Different methods have been developed to distinguish pen inks from each other, but some of these techniques require the ink extraction, destructing the document, and other techniques uses high cost instruments. PhotoMetrix PRO®, an app for mobile devices, is a qualitative and colorimetric analysis tool that applies uni- and multivariate analysis. Amongst them, Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA) and Partial Least Squares Discriminant Analysis (PLS-DA) can be obtained from digital images’ decomposing data. It is a non-destructive and a simple method, of easily use and low cost. Chemometric knowledge is important for results interpretation. This study aims to evaluate the PhotoMetrix PRO® capacity on blue ballpoint pens differentiation. Three experiments were performed with different ballpoint pens, including colorful pens as an app functionality control. The results showed appropriate differentiation between colorful ballpoint pens, and there was a satisfactory tendency of separation for different brands of blue ballpoint pens, most used in Brazil. This method is interesting to confirm subjective results, eliminating visual differences, intrinsic for each observer, which can be useful in places with an instrument deficiency, like the Video Spectral Comparator®. New studies to evaluate writing pressure’s influence on data collection and the inks’ age, as well as different mobile’s camera quality, are recommended. This study introduces a new technology that might be further studied for practical application in Forensic Documentoscopy and other Forensic areas.