What Is Black Light? How Black Light Works

Black light emits is ultraviolet radiation, which is invisible or "black."
Black light emits is ultraviolet radiation, which is invisible or “black.” (photo: Kallemax)

Black light is ultraviolet light or a lamp that emits most of its light in the ultraviolet part of the spectrum. Black light is beyond the range of human vision, so a room illuminated with a black light appears dark. Other names for black light are ultraviolet light or UV-A light. A device that releases ultraviolet light is called a black light, blacklight, or Wood’s lamp. The name “Wood’s lamp” honors Robert Williams Wood, the inventor of glass UV filters.

There are many types of black lights, including special fluorescent lamps, LEDs, incandescent lamps, and lasers. These lights are not created equal, as each produces a unique spectrum of light.

Black lights are used to observe fluorescence, in tanning beds, to attract insects, for artistic effects, for disinfection, and to cure plastics.

Why Is Black Light Called “Black” Light?

Black light is called “black” because ultraviolet light is invisible to the human eye. If a black light only emitted ultraviolet light, it would be invisible. Most black lights appear blue or violet because they emit short wavelength visible light (blue to violet). This makes it possible to tell when the light is on. However, some ultraviolet lamps and lasers only emit invisible radiation.

The photoreceptors of the human retina can detect black light. People who have eye surgery sometimes see further into the ultraviolet part of the spectrum than they did before. Artificial corneas and lenses change the perceived spectrum, giving people vision more like what a raptor or insect sees.

Types of Black Lights

Black lights come in many different forms. There are incandescent lights, fluorescent lamps, light-emitting diodes (LEDs), lasers, and mercury-vapor lamps. Incandescent lights produce very little ultraviolet light, so they actually make poor black lights.

Incandescent black lights are inefficient because the glass is coated with a filter to block light that isn't ultraviolet.
Incandescent black lights are inefficient because the glass is coated with a filter to block light that isn’t ultraviolet. (photo: Anakin101)

An incandescent black lights has a filter to block visible light but permit the passage of ultraviolet wavelength. This type of bulb or filter generally produces light with a dim violet-blue cast, so the lighting industry designates these devices as “BLB,” which stands for “blacklight blue.”

Fluorescent black lights typically cost a bit more than incandescent bulbs, but they are energy-efficient, don’t overheat, and bright. A good example is the type of fluorescent bulb used in “bug zappers.” This type of lamp is designated “BL,” which stands for “black light.” Fluorescent lamps generate black light by energizing mercury atoms. Energy kicks mercury atom electrons into a higher energy level and photons in the ultraviolet range are released when the electrons return to a lower energy, stable state. The interior of a fluorescent bulb tube has a phosphor coating, which absorbs harmful UV-B and UV-C light, allowing UV-B to pass. The glass of the bulb also blocks most harmful radiation.

Black light or ultraviolet lasers produce coherent, monochromatic radiation that is completely invisible to the human eye. It’s particularly important to wear eye protection when working with such devices because the light can cause immediate and permanent blindness and other tissue damage.

Black Light Uses

Black lights have many uses. Ultraviolet light is used to observe fluorescent dyes, improve the brightness of phosphorescent materials, cure plastics, attract insects, promote melanin production (tanning) in skin, and illuminate artwork. There are multiple medical applications of black lights. Ultraviolet light is used for disinfection; diagnosing fungal infections, bacterial infections, acne, melanoma, ethylene glycol poisoning; and treating neonatal jaundice.

Black Light Safety

Most black lights are relatively safe because the UV light they emit is in the longwave UVA range. This is the region closest to that of visible light. UVA has been linked to human skin cancer, so extended exposure to black light radiation should be avoided. UVA penetrates deeply into skin layers, where it can damage DNA. UVA does not cause sunburn, but it can destroy vitamin A, suppress immune function, damage collagen, and promote skin aging. Ultraviolet light passes through the cornea of the eye and can damage the lens and cause cataracts.

Some black lights emit more light in the UVB or UVC ranges. These lights can cause skin burns. Because this light has a higher energy than UVA or visible light, it can damage cells more quickly.

Ultraviolet lamps and lasers are extremely powerful. They emit oxidizing radiation that kills microbes and burns human tissue.


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