Visualization of latent blood stains using visible reflectance hyperspectral imaging and chemometrics

The detection of latent traces is an important aspect of crime scene investigation. Blood stains on black backgrounds can be visualized using chemiluminescence, which is invasive and requires a darkened room, or near-infrared photography, for which investigators need to change filters manually to optimize contrast. We demonstrated the performance of visible reflectance hyperspectral imaging (400-720 nm) for this purpose. Several processing methods were evaluated: single wavelength bands, ratio images, principal component analysis (PCA), and "SIMPLe-to-use Interactive Self-modeling Mixture Analysis" (SIMPLISMA). Using these methods, we were able to enhance the contrast between blood stains and 12 different fabrics. On black cotton, blood dilutions were visible with a minimal concentration of 25% of whole blood. The hyperspectral camera system used in this study is portable and wireless, which makes it suitable for crime scene use. The described technique is noncontact and nondestructive, so all traces are preserved for further analysis.