How to Find Pet Urine With a Black Light (Step by Step)
Our customer asked:
How do you find pet urine with a black light, especially the stains you can smell but can't see?
TL;DR
Wait until night, make the room completely dark, give your eyes a few minutes to adjust, and sweep a UV flashlight slowly one to two feet above the floor. Dried pet urine glows a dull yellow-green. Mark each spot with painter's tape, treat it with an enzymatic cleaner, and check again after it dries. Bright blue-white glow is not urine. That is detergent.
I run product innovation at LUXPRO. Pet stain detection is one of the most common reasons people buy a UV flashlight. It is also the use where people most often tell us that the light does not work, when the real problem is technique. A black light is a simple tool, and a few things decide whether it helps you. Here is the method that works.
Why urine glows, and why fresh urine barely does
As urine dries, the compounds that fluoresce concentrate into a crust in the carpet fibers. A dried stain glows much more than a wet one for that reason. If the accident happened an hour ago, use your nose and a paper towel. The black light is for stains that are days, weeks or years old. Those are the ones you can smell but cannot find.
Cat urine usually glows more strongly than dog urine. Old stains that have been shampooed over can still glow, because shampoo spreads the residue and does not break it down.
What you need
- A real UV flashlight. A phone with tape over the flash will not do this (here is why). We make the LP32UV for this job: 32 UV LEDs in a wide head, $16.99, batteries included.
- Painter's tape or sticky notes to mark spots.
- An enzymatic pet stain cleaner.
- Your phone, for photos if you need to document anything for a landlord.
Step by step
1. Make it completely dark. Skipping this step causes nine failures out of ten. Wait until after sunset, close the blinds, turn off the hallway light, and cover the TV standby LED if you have to. A consumer UV flashlight produces a faint glow from the stain. Any ambient light will drown it out.
2. Let your eyes adapt for three to five minutes. Your night vision is what detects the glow here. This matters most with a 385 to 395 nm light like ours, because the beam itself has some visible violet in it. Dark-adapted eyes pick the yellow-green stain out from the purple background much more easily.
3. Hold the light one to two feet from the surface. UV intensity falls off quickly with distance. Waving the light around from standing height is the second most common mistake. Get low. With a wide flood head, one to two feet covers a useful patch and keeps the stain bright enough to see.
4. Sweep slowly, in lanes. Work the room as if you were mowing a lawn. Overlap your passes. Stains are often smaller and fainter than you expect, and sometimes only the edge of the ring shows.
5. Check the vertical surfaces. Male cats and marking dogs spray upward. Check baseboards, the lower foot of walls, furniture legs, curtain hems, the sides of the couch, and door frames. People who look only at the floor miss most of the marking.
6. Mark every spot before you turn on the lights. Tape the outline. When the room lights come on, the stain disappears again, and you will not remember where the edges were.
7. Treat a wider area than what glows. On carpet, urine spreads in the pad underneath and can cover two to three times the area of the surface stain. Saturate past the tape line with enzymatic cleaner and leave it for as long as the label says. Enzymes need time. Do not blot it up after thirty seconds.
8. Check again after it is fully dry. Give it a day or two. A faint glow may remain even after the odor is gone, and some cleaners fluoresce themselves. Let your nose be the final judge.
Reading the colors: what is urine and what is not
| What you see | Likely cause |
|---|---|
| Dull yellow-green patch or ring with an irregular edge | Dried urine |
| Drip streaks running down a wall or furniture leg | Spray marking |
| Intense electric blue-white | Laundry detergent, carpet shampoo or spot-cleaner residue (optical brighteners) |
| Tiny bright specks everywhere | Lint, paper fibers, dust from laundered fabric |
| Bright splash near a table or bar cart | Tonic water, energy drinks, some vitamins (B2 glows strongly) |
| Whole carpet glows evenly | The fiber itself, or a recent professional cleaning. Look for patches that differ from the background. |
Look at the shape as well as the color. Urine leaves a puddle, a ring, or a drip trail. Cleaner residue shows up as wipe marks and scrub circles. After a few stains you will be able to tell them apart quickly.
What our light does well and what it does not
The LP32UV runs at 385 to 390 nm. On medium and dark carpet, hardwood, tile and baseboards, stains show clearly in a dark room. On very light or white carpet, contrast is lower, because pale fibers reflect more of the beam's visible violet. Steps 1 and 2 above make up for most of that. If white carpet is your main problem, or you do this professionally and cannot always get a room fully dark, a filtered 365 nm light will give you better contrast. I recommend specific ones in my review of the 7 best UV flashlights.
For everyone else, I want coverage from a pet-stain light. A single-emitter UV penlight lights a circle about the size of a saucer, so a living room takes a long time. A 32-LED head lights a patch closer to the size of a doormat. That was the design goal: a wide, even flood, a simple tail switch, and AAA batteries that you can replace at any gas station.
One maintenance note from the engineering side. If the light will sit in a drawer for six months between uses, take the alkaline batteries out. Leaking alkalines destroy more flashlights than drops do.
Frequently Asked Questions
Q: Why can't I see anything with my black light?
In order of likelihood: the room is not dark enough, you are holding the light too far away, the stain is still wet, or the light is a cheap 400-plus nm unit that is closer to a purple LED than a UV one. Here is how to tell the difference.
Q: Does dog urine glow under a black light?
Yes, once it has dried, though it usually glows more faintly than cat urine.
Q: Will the stain still glow after I clean it?
Sometimes. Residue and some cleaners fluoresce. If the odor is gone after the area has fully dried, the stain has been dealt with.
Q: Is it safe to use a UV flashlight around my pets?
Yes. Keep the beam out of their eyes, and out of yours. Animals will not look away from a UV beam, because it does not appear bright to them. More: Are UV flashlights safe?
Q: What else can I use the light for once the carpet is sorted?
Scorpions, checking cash, antiques, leak dye, and fishing tackle. Here is the full list: What is a black light flashlight used for?