A black light releases very little light that is visible to the human eye, giving it it’s contrary name. A black light’s wavelengths 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. From there, the wavelengths become so short and scattered we can no longer detect them, creating ultra-violet (UV) or “black light” radiation.

A common black light is a fluorescent tube with an inner coating that restricts the wavelengths that can pass through the glass. A black light can use the same lightbar and ballast as a normal fluorescent light. Since most of the light emitted from a black light is not visible, it shines dimly with a deep blue or violet glow and is sometimes referred to as a “black blue light.”

While we can’t see much of the radiation coming from a black light, certain colors and pigments absorb the invisible light then discharge it, causing a glowing effect. Black light posters are one example. Phosphor in detergent also absorbs UV radiation and expels it as light, explaining why white shirts and socks shine so brilliantly under a black light.

Though all black lights emit UV rays, they can be used for different purposes depending on their wavelengths within the UV range. Wavelengths are measured in nanometers (one billionth of a meter), with the UV spectrum generally falling between 100 and 380 nanometers (nm). This spectrum is divided into three categories:

The terms “long wave” and “medium wave” are relative here, as all UV waves are short compared to the red end of the visible light spectrum. Regardless, these terms are sometimes used to differentiate between various types of UV devices and their applications.

A long wave UV-A black light is the type used for entertainment purposes, such as at concerts and in clubs. It makes materials glow and creates atmosphere.

A black light that emits medium wave UV-B rays causes certain bodily fluids to luminesce. Crime scene investigators and forensic labs use these to detect blood, saliva, semen and other fluids. Medium wave black lights are also used to detect anti-counterfeit markers in currency and have many other security applications.

A short wave UV-C black light emits radiation that is destructive enough to purify water of living contaminants and to sterilize medical equipment. It has a variety of industrial uses and is considered “germicidal.”

Because of the potential harm to the eyes from unseen radiation, one should not stare into a black light. Professionals who work with black lights, particularly UV-B and UV-C lights, wear UV-protective eyewear with yellow-tinted lenses. The yellow tint blocks blue light waves in the HEV or near-UV spectrum, which doctors have associated with macular degeneration. Barring staring into a black light, experts do not believe that occasional exposure to black lights as used for entertainment purposes is harmful. However, those with prolonged or repeated exposure to black light should wear UV-protective eyewear that includes HEV protection.