Visual detection and enhancement of alterations in questioned documents by Mueller polarimetry

Abstract

In this article, we have explored the use of Mueller polarimetry for the detection andenhancement of alterations in questioned documents. Erasures, obliterations (withliquid paper and by pasting an additional layer of paper), and insertions (made withseveral inks) were studied in both regular and glossy paper. Promising results wereobtained, which depend on the type of paper and the relation between the color ofthe ink and that of the illuminating light source. Erasures are easier to detect in glossypaper than in regular paper. Obliterations with liquid paper produced successful resultsin both types of paper, while detection of obliterations made with an additionallayer of paper led to higher contrast for regular paper. Regarding the insertions, theblack ball-pointink could be differentiated from roller-balland gel-penink, which isoften difficult to achieve visually. The contrast observed between the two inks washigher for regular paper than for glossy paper. Although the results shown in this articleare promising, a wider variety of papers and pen types must be tested to furtherdevelop the procedure. It has the advantage of being non-destructiveand far moreeconomic than other methods. In some cases, the results can be complementary tothose obtained by other methods (e.g., fluorescence with UV excitation and illuminationwith transmitted and oblique light), while in other cases the method offers uniqueadvantages.