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For the best results, look for a handheld LED light with 9-12 bulbs. It should be rated between 365-385 nm, which stands for nanometers. A lower rating won't be strong enough to light up the cat urine, and a higher rating is too similar to natural light to have an effect. You can find these lights online or at a local home improvement store. You can also find UV lights with fluorescent bulbs. As long as it's rated between 365-385 nm, this will work as well, although the LED lights tend to be stronger. While you could try to draw all the curtains in the room as tightly as possible, it will be easier to wait until it gets dark outside before you try to use the UV light. When you're ready to start looking, turn off the lights in the room, as well as the lights in any adjacent rooms or hallways. If the room isn't dark enough, your eyes won't be able to detect the glowing urine. Typically, the urine spot will be somewhere in the area where you can smell it, although sometimes you'll have to look around a little before you find it. To make the search more efficient, start searching near where you think the cat may have urinated, then work outward. When the UV light comes into contact with the urine, it should start to glow. Depending on how much urine there was, and where it is located, the stain may look like a spot, puddle, splatter, or drips. For example, if your male cat sprayed to mark his territory on the wall, it may look like a splatter, with a few drips running down the wall. If the cat peed on the floor, you might just see a large round-ish spot. Some cleaning products and other household materials, including wallpaper glue, may glow under a blacklight, so don't panic if your whole room lights up when you turn on the UV light. Other substances, including some bodily fluids and tonic water, may also glow. Use the location, size, shape, and smell of the spots to determine if it's cat urine. Cats may sometimes urinate on a variety of different surfaces, so don't just check the floor. Slowly move the light from side to side, checking along the walls and door frames, on top of and on the sides of furniture, and on any bedding in the area. If you don't spot the stain right away, slowly move outward from the source of the odor. It can be hard to remember exactly the size and shape of the stain when the lights come back on. To ensure you know exactly where you're supposed to clean, use something like tape or chalk to mark a perimeter around the stain. It's a good idea to clean further outside of the stain than what you can actually see, in case the stain soaked in and spread out, so don't worry about perfectly defining the edges. Just make a small mark at the top, bottom, and sides of the stain so you'll remember where it was.
Purchase or borrow a UV light between 365-385 nm. Wait until evening and get the room as dark as possible. Go to the area where you suspect the urine is located and turn on the UV light. Look for a glowing yellow or neon green spot. Sweep the light back and forth, checking various surfaces. Mark the edges of the stain so you'll know where to find it.