This article covers the basics of how a black light works. It covers how the ultra-violet light reacts with materials to produce visible light.

Black lights are somewhat of a mystery to the general public. Most people have been exposed to a black light at least once in their life, possibly without even knowing it. They have a wide range of uses. The most common use is for lighting effects. Stage productions sometimes utilize their unique effects and many people like to have them in their home just for fun. Black lights are also used to detect counterfeit currency. Some theme parks use invisible hand stamps that are only visible under a black light. Nightclubs and prisons may also use this form of ID for their visitors. The ink used only shows up under the black light, making the stamp hard to reproduce. Hotels use black lights to make sure the rooms are being cleaned properly. Traces of urine and other fluids that cannot be normally seen, show up under a black light. Special ink can be used to mark merchandise to protect a business from theft and fraud.

To understand how a black light works, there are a few things to understand about light in general. Our eyes see a broad spectrum of light, ranging from red to blue. Beyond these colors are the light rays that we cannot see, infrared and ultra-violet. Infrared light rays generally have a warming effect. This type of light is used for keeping food warm in the buffets featured in some restaurants. Ultra-violet light is what is produced by a black light.
 
A black light will make anything containing phosphors seem to glow in the dark. Phosphors are substances that will produce a visible light when exposed to energy. The ultra-violet light emitted by a black light finds these phosphors, and turns them into visible light.

Black light bulbs are a lot like the florescent bulbs found in classrooms and office spaces. They contain small amounts of mercury that react with the energy sent through the bulb to create light. Fluorescent light bulbs are made with a thin layer of phosphors coated on the inside of the bulb. These phosphors take the energy produced by the mercury and transform it into light. Black light bulbs do not have this coating of phosphors inside the bulb. Most black light bulbs are made with a dark glass to block off most of the visible light given off by the bulb, so that only the ultra-violet rays are emitted. A perfect black light would give off no visible light, but this has not been made possible yet. The purple color you see is the small amount of visible light that is given off by the bulb. The ultra-violet light produced by the bulb finds the phosphors in your clothes and other items, causing them to illuminate.

Florescent paints often react strongly under a black light. They seem to glow when exposed. Another thing you might notice is white clothing also seems to glow. Most detergents contain phosphors. These are deposited in the material when the clothes are washed. The ultra-violet light leaves the bulb, finds the phosphors and reacts with them. The resulting light is emitted, giving your white clothing a bright glow. Black clothes will not glow because they absorb all the light produced. However if you look closely at a black shirt under the effects of a black light, you will most likely see the tiniest amounts of lint glowing from the surface of the shirt.

If you wish to purchase a black light, here is a useful tip. The best kind of black light is the kind that comes in tube form. This is a true black light. Incandescent light bulbs, like those you put in a lamp, do not work as well. In tube form, the ultra-violet light is produced from the gas inside and emits the light rays. For incandescent bulbs, the ultra-violet light is produced by a paint applied to the bulb itself. It will not work as well as the tube form.