How Black Lights Work: The Science Behind the Glow

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Portrait of woman under black lights with glowing makeup. Powerofforever / Getty Images

You have probably seen black lights at amusement parks, science museums and Halloween displays. Black lights may look just like normal fluorescent lamps or incandescent light bulbs, but they do something completely different. Switch one on, and white clothes, teeth and various other things glow in the dark.

For example, if you have a fluorescent poster and shine a black light on it in a dark room, the poster will glow brightly. You may have also seen pieces of paper that look blank in regular light but spell out a glowing message under a black light. Many amusement parks use hand stamps that are invisible until you view them under black light.

In this article, we'll find out exactly what's going on here. We'll also see why black lights make some objects glow but not others, and we'll look at some interesting black light applications.

What Is "Black Light"?

If you turn on a black light bulb in a dark room, what you can see from the bulb is a purplish glow. What you cannot see is the ultraviolet light that the bulb is also producing.

Our eyes can see visible light in a spectrum ranging from red through orange, yellow, green, blue and violet. Above violet is ultraviolet light, which we cannot see. How Sunburns and Sun Tans Work discusses ultraviolet light and its effects on our skin. A black light bulb produces UVA light (as opposed to UVB light, which is much more harmful).

Ultraviolet light frequencies are above violet on the spectrum and are invisible to the human eye.

What you see glowing under a black light, whether on a fluorescent poster or an invisible hand stamp or a newly washed white T-shirt, are phosphors.

A phosphor is any substance that emits visible light in response to some sort of radiation. A phosphor converts the energy in the UV radiation from a black light into visible light.

In the next section, we'll see how phosphors are used in regular fluorescent lighting and in black lights.

Why Is My Shirt Glowing?

White T-shirts and socks normally glow under a black light because modern detergents contain phosphors that convert UV light into white light. This makes whites look "whiter than white" in normal sunlight. What you are seeing in sunlight is the normal reflection of visible white light from the cloth, as well as the emission of white light that the phosphors create from UV light in sunlight. The T-shirt really is whiter than white!

Black Light Designs

Black lights come in both tube and bulb form.

The conventional black light design is just a fluorescent lamp with a couple of important modifications. Fluorescent lamps generate light by passing electricity through a tube filled with inert gas and a small amount of mercury. (See How Fluorescent Lamps Work for more information.)

When energized, mercury atoms emit energy in the form of light photons. They emit some visible light photons, but mostly they emit photons in the ultraviolet (UV) wavelength range. Because UV light waves are invisible to the human eye, fluorescent lamps have to convert this energy into visible light. They do this with a phosphor coating around the outside of the tube.

Phosphors are substances that give off light -- or fluoresce -- when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level, causing the atom to vibrate and create heat. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum -- the phosphor gives off white light we can see.

Black lights work on this same principle. There are actually two different types of black light, but they work in basically the same way.

A tube black light is a basically a fluorescent lamp with a different sort of phosphor coating. This coating absorbs harmful shortwave UV-B and UV-C light and emits UV-A light (in the same basic way the phosphor in a fluorescent lamp absorbs UV light and emits visible light). The "black" glass tube itself blocks most visible light, so in the end only benign long-wave UV-A light, along with some blue and violet visible light, passes through.
An incandescent black light bulb is similar to a normal household light bulb, but it uses light filters to absorb the light from the heated filament. It absorbs everything except the infrared and UV-A light (and a little bit of visible light).

In both of these light designs, the emitted UV light reacts with various external phosphors in exactly the same way as the UV light inside a fluorescent lamp reacts with the phosphor coating. The external phosphors glow as long as the UV light is shining on them.

In the next section, we'll see what kinds of objects contain phosphors, and we'll look at some interesting uses for black lights.

Black Light Uses

If you walked around all night with a portable black light, you would discover that there are phosphors all over the place. There are lots of natural phosphors, in your teeth and fingernails, among other things. There also a lot of phosphors in manmade material, including television screens and some paints, fabric and plastics. Most fluorescent colored things, such as highlighters, contain phosphors, and you'll find them in all glow-in-the-dark products. Clubs and amusement parks use special black light paint that glows different colors. You can also buy fluorescent black light bubbles, invisible black light ink, fluorescent black light carpet and even fluorescent black light hair gel.

Ordinary highlighters work well as "black light pens." Under the black light, the fluorescent ink glows!

In addition to making people and fluorescent posters look cool, black lights have some practical applications. For example:

Appraisers use them to detect forgeries of antiques. Many paints today contain phosphors that will glow under a black light, while most older paints do not contain phosphors.
Repairmen use them to find invisible leaks in machinery -- they inject a little fluorescent dye into the fuel supply and illuminate it with a black light. For example, they might detect an invisible air conditioner leak by adding fluorescent dye to the refrigerant. Black lights can be used to detect counterfeit bills.
Law enforcement officers can use them to identify counterfeit money. The United States and many other countries include an invisible fluorescent strip in their larger bills that only shows up under a black light.
Amusement parks and clubs use them to identify invisible fluorescent hand stamps for readmission.
Forensic scientists use them to analyze crime scenes. To pick out fingerprints, for example, they often dust with fluorescent dye under a black light. This makes it easier to pick the fingerprints out from surrounding dirt. Black lights can also identify semen and other bodily fluids that naturally fluoresce.

Most of these uses, as well as dozens of others, follow a common theme -- the black lights make the invisible visible or isolate one specific substance from everything around it. When you think about it, there are dozens of situations where you could put this phenomenon to work. The applications are potentially endless!

For more information on black lights and other sorts of light energy, check out the links on the next page.

Originally Published: Jul 18, 2002

Black Light FAQ

Are black lights bad for you?

Limited exposure shouldn’t cause any issues, but prolonged eye contact without eye protection can cause retina damage.

Is UV light the same as a black light?

Black light is a type of UV light that emits ultraviolet radiation. This radiation has a shorter wavelength than violet light, which is the shortest wavelength of light in the visible component of the electromagnetic spectrum.

Can black light detect germs?

No, black light cannot reveal bacteria or other germs.

Why do they call it a black light?

Black light shows mostly ultra violet or “black” light, hence its name, but “leaks” purplish light.

What shows under black light?

Thiamine, riboflavin, niacin, fluids and vitamins shine under black light. Urine, semen and blood contain fluorescent molecules, so they also show up under black light. Interestingly, some cleaners and laundry detergents, scorpions, tonic water and antifreeze and teeth whiteners all also glow under black light.