Why UV works
Components in dried urine fluoresce under ultraviolet light, showing as a dull yellow-green glow against the surrounding carpet. The room has to be genuinely dark for it to read properly, which usually means working at night or blacking out windows. What appears is often a shock to the owner: a spot that looked like a dinner-plate mark on the surface can show as a contaminated zone several feet across, because urine spreads outward as it soaks down.