Non-destructive detection of soot-obscured latent fingermarks using robotic optical coherence tomography

Fires frequently obliterate or contaminate physical evidence, complicating scene reconstruction. Latent fingermarks can survive combustion but are often masked by soot, making conventional powdering or chemical enhancement both ineffective and potentially destructive to ridge detail and DNA. This study evaluates optical coherence tomography (OCT) as a rapid, non-contact alternative for visualizing soot-covered fingermarks on substrates commonly encountered at fire scenes, including stainless-steel door handles and window glass. A spectral-domain OCT probe was mounted on a six-axis robotic arm, enabling automated raster scanning over irregular items. Natural fingermarks were deposited on three stainless-steel door handles with different surface curvatures and on tempered window glass, then exposed to open flame for five minutes to produce uniform soot layers. Volumetric OCT data were rendered into en face projections in under one minute (∼37 s for the acquisition of OCT data, ∼20 s for the analysis) per specimen. On flat or gently curved steel surfaces, OCT resolved Level-1 and Level-2 ridge detail through soot; pronounced curvature reduced contrast but still yielded identifiable minutiae. Glass produced the highest-quality images, revealing sweat pores (Level 3). Because the technique is non-destructive and contact-free, both the soot layer and underlying residue remained intact, permitting subsequent DNA profiling and accelerant analysis. The results demonstrate that robotic-OCT can serve as a first-line screening tool at post-fire scenes, enhancing the recovery of comparison-quality fingermarks while preserving downstream evidential value.