What is a Black Light?

A black light is a light bulb with an inner coating that restricts the wavelengths that can pass through the glass. It releases very little light visible to the human eye, giving it its contrary name. Instead, a black light emits mostly ultra-violet (UV) or black light radiation, which cannot be seen. Certain colors and pigments do absorb the invisible light, however, and then discharge it, which causes a glowing effect. These lights are often used for entertainment purposes, but those that produce medium- and short-wave UV rays are used by forensic and other types of scientists, and medical professionals.

The wavelengths of a black light start at one extreme end of the visible light spectrum, in the high-energy visible light (HEV) range. HEV waves are detected by the human eye as deep blues and violets. These wavelengths eventually become so short and scattered that they are no longer visible. These lights shine dimly with a deep blue or violet glow, which is sometimes referred to as a black blue light.

Traditionally, black lights were made with a special glass, called Wood's glass, that only allowed UV and infrared (IR) light to pass through it. This type of glass is not very strong, however, so special coatings are now more commonly used on light bulbs to prevent the visible light from passing. Both incandescent and fluorescent black lights are available, although incandescent bulbs usually burn much hotter and for a shorter amount of time than standard light bulbs. Mercury vapor lamps are more commonly used for theatrical displays because they produce the necessary type of UV light more efficiently.

Fluorescent black lights are some of the most common types because they are almost identical to regular fluorescent bulbs. The tubes contain a small amount of mercury, which releases mostly UV light when it changes from a liquid to a gas. Standard lights have a phosphor coating, which is a compound that reacts to the UV by emitting visible light. In black lights, a different type of phosphor is used that increases the UV. A purple-blue coating is added to block the light that is visible.

Phosphors are any compounds that glow or emit light in certain conditions. Not only are they used to coat fluorescent light bulbs, they create pictures on cathode ray tube (CRT) television screens and make glow-in-the-dark toys glow. Certain types react with black light, and they can be used in paint and on posters, causing images to glow. Some of the compounds that make up many types of detergent also absorb UV radiation and expels it as visible light, explaining why white shirts and socks shine so brilliantly under a black light.

Black lights can be used for different purposes depending on their wavelengths within the UV range. Long wave lights are often used for entertainment purposes or to diagnose illnesses; medium wave can help investigators find body fluids or counterfeit items; short wave can kill germs. Wavelengths are measured in nanometers (one billionth of a meter). The UV spectrum generally falls between 100 and 380 nanometers.

A long wave UV-A black light emits wavelengths between 380 and 315 nanometers. Individuals often use these types of lamps for entertainment purposes, such as at concerts and in clubs. The lights make materials glow in the dark and helps create atmosphere.

Bug zapper lights also emit UV-A light. Insects are able to see light in the UV range, so they are strongly attracted to the devices. They usually do not include the purple-blue coating since it's not important that they block the visible light.

A medical tool called a Wood's lamp also uses light in this range to detect skin diseases and other conditions. Some organic compounds, fungi, and bacteria glow when exposed to this light, helping medical professionals see the extent of some types of infections. The lamp has also been used to diagnose other skin conditions.

A light that emits medium wave UV-B rays of between 314 and 280 nanometers is known as a medium wave black light. These types can cause certain bodily fluids to glow (also called luminescence). Crime scene investigators and forensic labs often use such lights to detect blood, saliva, semen, and other fluids. Some people use them to locate places where pets have urinated so that the area can be cleaned.

Medium wave black lights are also used to identify anti-counterfeit markers in currency and detect art forgeries, among other security uses. Pet snakes and lizards, for example, also benefit from them because they provide the vitamin D that reptiles need. Treatments with UV-B may also help people with some skin conditions, like psoriasis and vitiligo.

If the wavelengths lay between 279 and 200 nanometers, it is called a short wave UV-C black light. Such lights emit radiation that is destructive enough to kill germs and other harmful organisms. The radiation may be used to purify water of living contaminants or sterilize medical equipment.

Because of the possibility of eye damage from unseen radiation, individuals should not stare into black lights. Professionals with prolonged and repeated exposure, particularly to UV-B and UV-C lights, should wear protective eyewear with yellow-tinted lenses. The yellow tint blocks blue light waves in the HEV or near-UV spectrum, which are the waves that medical professionals have associated with macular degeneration. UV light can also cause skin damage, and the B and C wavelengths can directly damage DNA, potentially leading to skin cancer. Experts do not believe that occasional exposure to black lights is harmful, however.