What causes urine to glow under UV light?

Urine is a complex organic chemical mixture containing primarily phosphocreatine – a chemical containing phosphorous – as well as urea and creatinine. After the urine is deposited and biological breakdown begins, these chemicals create ammonia and other nitrogen-based compounds that deliver the strong smell associated with urine. Our process can eliminate all three compounds and removes the odors generated as well as the pigments excreted with the urine. However, an area treated may still glow when observed under a black light, This glow is not due to the presence of urine in that spot, but from phosphorous left over from the breakdown of phosphocreatine. During that process the phosphorous becomes chemically bonded to the fibers as a dye would, and is thus very strongly attached to the fibers. Under a UV light, the phosphorous glows. But because the phosphorous is now part of the surface of the fibers, it is quite difficult to remove it without damaging the existing dyes on those fibers.  Fortunately, there is little need to remove the phosphorus. The chemistry of the original urine deposit has been altered so that no urine odors can be produced. Think of it this way: phosphorous is used in LEDs and fluorescent light bulbs to cause them to glow and produce light.

Reference: Legend Brands Cleaning