How Do Black Lights Work?


Whether you’re shooting at friends in a laser tag arena, venturing through the narrow passageways of a haunted house, or simply watching your favorite episode of CSI, black lights never cease to amaze. There’s something fun and mysterious about revealing secrets and hidden messages with that strange purple light. But how do black lights work? (Hint: It’s not magic.)

The signature glow produced by black lights requires two separate things: A source of ultraviolet light and a surface coated with UV-reactive phosphors. The source of the light is a bulb, whether incandescent, fluorescent, CFL, or LED. You can find phosphors—a loose grouping referring to special compounds and minerals—in and on all kinds of things: Highlighters, soap, rocks, glow-in-the-dark toys, and even your teeth. When ultraviolet light hits a phosphor, the phosphor glows in a phenomenon called luminescence.

Incandescent Black Lights

You may remember from our previous article on yellow bug lights that the short-wavelength spectrum of light beyond visible light is ultraviolet light. All light sources produce UV in varying quantities. For example, the simplest form of black light, an incandescent black light, produces very little ultraviolet light but uses a special filtering glass called Wood’s Glass to block most visible light produced by the bulb filament, thus enhancing the effect of the UV spectrum. However, the relatively small amount of UV light incandescent bulbs produce makes incandescent black lights the least impressive of black lights.

Fluorescent Black Lights: BL vs. BLB

Fluorescent light sources naturally emit much more of the ultraviolet light spectrum, making the technology ideal for use in black lights. Fluorescent black lights fall into two different categories, black light (BL) and black light blue (BLB). Fluorescent black lights use special phosphor coatings on the inside of the bulb to filter out visible light and enhance the emission of ultraviolet light. In both BL and BLB technologies, this ultraviolet light causes external phosphors in its surroundings to glow, just like an incandescent black light does. However, because fluorescent technology produces much more of the UV spectrum, fluorescent black lights are more effective than incandescent black lights.

While both a fluorescent black light and a black light blue use UV light and phosphor coatings to create luminescence, the difference between them is how much invisible ultraviolet light they emit in relation to visible white light. A fluorescent black light, which appears similar to any ordinary white fluorescent lamp, emits a relatively large amount of white light mixed with ultraviolet light. The light from a fluorescent black light looks similar to what we are used to from ordinary fluorescent sources, yet still causes limited luminescence of external phosphors.

Black light blue fluorescents are much more commonplace. Like other fluorescent black lights, they use a special phosphor coating to filter white light; however, for more complete blocking of white light, black light blue bulbs also are made of a purple-colored filtering glass. This combination allows them to emit a greater amount of ultraviolet light than white light. The result is the familiar purple-colored light and a very pronounced luminescence of phosphors in black light reactive objects.

Other Black Light Technologies

Of course, fluorescent black light technology lends itself equally well to compact fluorescent black lights. Though compact, CFL black lights work according to the same principles of black light or black light blue fluorescent lighting. LED black lights are less common, though they are starting to emerge, especially in stage and nightclub lighting. Other niche applications include HID lighting, especially mercury vapor, and “bug zapper” lights like the Paraclipse Mosquito Eliminator.

We hope we haven’t completely destroyed the mystique of black lights for you. But if we have, be sure to let us know in the comments, or drop us a line on Facebook, Google+, or Twitter page.

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Benjamin is a writer for 1000Bulbs.com.