The Stink Free Stink-Finder Ultra-Violet Light and Flashlight is for people who suspect their pet is "doing their business" where they shouldn't, but they can't find the source of the smell. In other words, Kitty is surreptiously peeing on the carpet, making the entire house smell, but the offending toilet area cannot be found and cleaned.
Ultra-violet light, when used in a dark room, makes natural fluids glow. If you happen to have a black light bulb, you can use it for the same effect, although it will lack the portability of this nifty little device.
The Stink Free Stink-Finder is very good at what it does. It's actually too good sometimes. It will find any spot on your carpet that has ever, in the entire history of its existence, had any kind of natural fluid (water excluded) spilled on it, even if it has since been cleaned. If you have a two-year old who spills milk when you're not looking, you'll know all about it now. If the young bachelors who lived in your apartment before you dribbled beer from kitchen to bedroom, you'll see the trail. If there was a murder in your house, the Stink-Finder will discover the gruesome evidence.
I didn't realize this when I first got it and thought my cat was peeing everywhere, including the ceiling. The key to using the Stink-Finder is to use it on new carpet or to check it out before Kitty misbehaves so you already know where the old "invisible stains" are. You can also use it to watch and see which areas are growing new spots, and then Kitty's secret pee zone has been outed. A good rule of thumb is that stains in corners, especially near doors and windows, are generally from cats.
The Stink Free Stink Finder also comes with a flashlight, so when you're walking around in the ultra violet near-dark freaking out because your carpet looks like a fluid war zone, you can turn on the flashlight and see that it's ok. This will also keep you from bumping your knees on table corners and getting lost in your own house.
Overall, the Stink-Finder is a useful tool if you have a pet that isn't going potty where it's supposed to. It's not foolproof and can't be used in a court of law to convict Kitty, but it can help you figure out where the problem areas are and get them cleaned. Good luck!