Black lights 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, while the bulb itself only emits faint purple light.
These devices are all around us -- in clubs, science museums, amusement parks and teenagers' bedrooms, among other places -- but to most people, they're a total mystery.
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.
How They Work
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. UV light waves are too short for us to see -- they are completely invisible -- so 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 and some blue and violet visible light pass 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).
Black lights come in both tube and bulb form.
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 find out about some of the more common phosphor materials and look at some common black light applications.
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!
The first thing most people notice when you switch on a black light is that some of their clothing glows. This is because most laundry detergents contain phosphors to make whites appear brighter in sunlight. Sunlight contains UV light that makes the whites glow "brighter than white." Dark clothes don't glow because the dark pigments absorb the UV light.
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.
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.