Contamination detection Rodents under black light is based on the phenomenon of rodent urine exhibiting white-blue to white-yellow fluorescence when viewed under black light. The fresher the stain, the more fluorescent blue it becomes. Urine stains on food bags, containers, floors, etc., which are invisible under white light, retain their fluorescence and are easily discernible under black light. Additionally, rodents urinate when they are on the move, and their paths are visible under black light in characteristic «droplets» or trails. Rodent hairs exhibit a white-blue fluorescence. Rodents have oily hair that is UV reactive, leaving smudge marks wherever they travel.