A black light, found in pet supply stores, is a type of flashlight that produces ultraviolet light. This light causes organic substances, including dog urine, to fluoresce or glow when you shine it on them, due to the proteins they contain. These proteins reflect ultraviolet light under low-light conditions. If your dog is not completely housebroken or has marked your carpet with urine, he will likely want to return to those spots to mark them again. With the help of a black light, you can locate urine stains on your carpet, even older stains, to completely clean and eliminate them.
Shut off most of the lights in your home so there is only enough light for you to find your way around. The less light the better; if you need to, use a normal flashlight to illuminate your way to the area of the carpet you want to inspect. Turn off the flashlight.
Turn on the black light and shine it over the carpet. Focus on the areas where you suspect your dog has urinated. Urine stains will glow brightly under the light.
Smell the carpet for signs of urine odors and focus on those that have a strong aroma of ammonia from the dog's urine. Shine the light on any particularly stinky areas to see if there is urine present that glows under the ultraviolet light.
Mark carpet stains that glow under the black light by placing a tissue or paper towel over the area. When you turn the lights back on, clean the areas you have marked with a dog urine cleaner that contains enzymes to break down the chemicals in the urine, such as uric acid. Other household substances, including vinegar and rubbing alcohol, can help eliminate the uric acid as well.
Re-check the areas you had previously found and cleaned with a black light a few days after the spots have dried. If you still notice that the stains glow under the light in the dark, clean them again.