Understanding the visibility of blood on dark surfaces: A practical evaluation of visible light, NIR, and SWIR imaging.

Bloodstains on dark surfaces are often difficult to detect due to a lack of contrast. Infrared photography is in many cases a solution as it enhances the contrast between blood and background. Still, on some surfaces bloodstains cannot be visualized. In this study, we investigate why bloodstains on certain surfaces are not detected and how visibility can be improved. Bloodstains on 166 dark fabrics were photographed by four different cameras and for each, the visibility of the bloodstains was scored. The spectral properties of the dark fabrics were examined as well as the properties of the dyes used to colour the textiles. In addition, spreading of the blood within the textile and the roughness of the material were taken into account. In the investigated set of textiles, visibility of blood is mainly determined by the spectral properties of the textile dye. In addition, a high surface roughness of the textile reduces the visibility.