It is to find urine on the carpeting or walls. It works well but a black light will light up EVERYTHING else too. Some cleaners, you will be shocked at how "dirty" your house is!!
I had fun with it. Then the next day I scrubbed everything down, cabinets and everything. Any oil sprays or perfume or air freshners, it will all show up. It is scary. But what you are looking for is large glowing splotches on the carpeting. Then that is where you soak with Natures Miracle or Nok-Out.