Novel imaging solution to quantify, evaluate and compare gunshot residue patterns

The analysis of gunshot residue (GSR) patterns is crucial for the reconstruction of shooting incidents. Established chemographic methods, such as the Modified Griess and Sodium Rhodizonate tests, are effective but also destructive, hazardous and not readily quantifiable. This paper presents proof-of-principle for a new quantifiable method to compare GSR patterns and evaluate the performance of a flatbed scanner, modified by the manufacturer to operate in blue-green fluorescence (BGF, 450 nm excitation/630 nm barrier) and infrared reflectance (IR) modes (850 nm). The effects of 9 mm Luger ammunition brand as well as fabric color and type were evaluated using the spectroscopic scanner and Weber-contrast method developed. The unenhanced Weber contrast among ammunition brands ranged from 0.28 to 0.85 when viewed upon a single cotton fabric background. Darkening caused by smoke deposits appeared to dampen propellant-particle brightness, at least for certain brands of ammunition, but did not fully explain the difference in contrast between brands. Analysis of 248 representative fabric swatches revealed that suitable contrast could be achieved after enhancement for 92 % (BGF mode) and 91 % (IR mode) of those evaluated, with median Weber contrast values of 10.3 (BGF) and 4.83 (IR) achieved following enhancement. These were well above the Weber threshold required for human visual perception (∼0.1) and demonstrate its potential use as a comparison tool in forensic practice and research. Evaluation of muzzle-to-target distance estimation using this scanner will be the subject of a future study.