How to find cat urine stains with a black light

UV lamp can help to find cat urine spots

Is it possible to find cat urine using a black light? Photo: Tarvo Metspalu, CC BY-SA 3.0

How can you find cat urine in your home? One important part of solving your cat’s litter box problems is to clean all the locations where you cat has urinated. The problem is that your cat can still detect urine in those locations long after the urine has dried. But how do you know where your cat peed during the past month or two?

In this article, we are going to teach you how to find cat urine stains, including those that have long since dried, using a black light.

What is a black light, and how can it find cat urine?

Black light is also know as ultraviolet, or UV, light. UV light, simply, is light with a frequency higher than violet visible light. However, it’s not necessary to learn physics right now. Just think of a rainbow.

Water crystals break visible light into a spectrum where red has a lower visible frequency and violet has a more highly visible one. Anything lower than red is called infrared (which can be sensed as heat but is not visible), and anything higher than violet is ultraviolet, also known as black light, because it’s not visible either.

Black light has many applications. For example, most money bills and some documents have markings that are visible under black light only, to prevent counterfeiting. UV light is used in the medical field for sterilization and disinfection purposes; even drinking water is often disinfected under UV lighting. Tanning beds use black light to stimulate additional Vitamin D production in your skin, eventually creating a tan… or cancer.

UV light is also used in art to create drawings visible only under UV lighting. Body fluids, such as blood, urine, and semen, are easier to detect under UV light; for this reason, UV lights are used in crime scene investigations. Have you ever seen a purple toilet? It’s so the staff can see where one has urinated over the edge.

See where this is going? Cat urine literally glows when exposed to the black light. Even long after urine has dried, its scent-creating chemicals will still be visible, thereby making cat urine stains (including older ones) easy to find.

What type of black light devices should you use to find cat urine?

Many types of black light devices are available on the market, with varying prices and sizes. The most popular are bar-type lamps, which use fluorescent UV bulbs, and small LED flashlights.

LED UV lamp - best tool to find cat urine.

LED UV lights outperform fluorescent lights. But you also need to take a look at wavelength.

Although light strength can vary from make to make, LED lamps are generally stronger. They are considerably stronger if you need to find cat urine. By stronger, we don’t mean the actual visible light, which does not make cat urine glow, but rather the UV light it emits.

The most important feature to look for is wavelength! UV light wavelengths may vary from 10 to 400 nm (nano meters, or billionth parts of a meter). Most flashlights are marked to indicate what wavelength they emit. You are most likely to see LED lamps with 400 nm, 385 nm, or 365 nm in general stores. DO NOT buy lamps with a wavelength of 395 or 400 nm. They are almost useless in finding cat urine because they are too close to being visible light. For best results, we recommend going with 365 or 385 nm.

The number of LED bulbs also makes a difference in how easy it is to find cat urine. For household use, you can easily go with 9 or 12 LED bulbs in a lamp. Any additional bulbs will increase the price.

You can find black light lamps in hardware stores, in pet stores, and online. Click here to buy UV light on amazon.com (we do earn commission if you buy via our link).

A good UV LED flashlight should cost no more than $20 to $30, which means that the technology is affordable as a household item and is not high-end technology that is only available to scientists. You can find cheaper ones, too, but most of them perform poorly because the UV light they emit is not optimal for finding cat urine.

How do you find cat urine using a black light?

You now have the equipment, and you are ready to investigate a crime. It’s as simple as point and look, but following are a few tips to improve your search.

It will be easier to find cat urine if you dim the light in the room. Search in the evening, or with blinds closed and lights out. Cat urine will still glow under UV in broad daylight; however, you will have to point the light a short distance away from the surface to notice anything.

Turn the black light on and shine it on different surfaces. In general, if you are looking for house-soiling locations, you must look on all the horizontal surfaces, including bed sheets, furniture tops, bookshelves, your clothes, and even in sinks and bathtubs. If your cat is spraying urine, look on vertical surfaces as well, especially near doorways, passages, and corners. Find more about urine spraying here.

Turning on the black light the first time might be scary because everything may be glowing. Actually, there are several substances that glow in black light conditions, including some types of wallpaper glue.

All you have to do is determine whether what you are seeing looks like something else. Generally, you are looking for yellow splatter or individual spots. On vertical surfaces, you are likely to observe a single drop that has run down the wall before drying out. This video shows what urine looks like when viewed under a black light:

If your cat has been urinating outside of the litter box for a while, you will see bright spots and weaker spots. The weaker spots are older and contain less of a concentration of chemicals that glow; however, your cat is still able to sense them.

Mark the locations where you find spots, as you won’t be able to see them when you turn the black light off. If you find just a few locations, you can try to remember them. However, if you are dealing with a seriously bad kitty, you can apply tape to those locations so you can find them later.

Next, treat all the discovered locations with a detergent intended for pet urine removal. Remember to clean an area somewhat larger than the area you discovered with the flashlight. First, doing so leaves no room for error. Second, cat urine tends to soak into surfaces (especially carpets), and what you see under a black light may just be the tip of the iceberg.

Remember that knowing how to find cat urine is not the hardest part. Finding the urine is just a single step in a path to making your house free of litter box problems. For additional help, you can check out our article series about cat litter, litter boxes, and cat house-soiling problems here.

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