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Black Lights All Aglow

Updated on February 15, 2013
Black lights look black when unlit and the things they light up don't appear spectaculary interseting.
Black lights look black when unlit and the things they light up don't appear spectaculary interseting. | Source
When the black light is turned on, it glows a dark purple and the fluoreschent objects glow.
When the black light is turned on, it glows a dark purple and the fluoreschent objects glow. | Source

By Joan Whetzel

Fluorescent posters, rocks, and other "glowing" items re-emit low frequency, long-wavelength light in the presence of black lights. While these lights are often reminiscent of the psychedelic 60s, they are also used around Halloween to produce eerie effects for parties and haunted houses. They are used in museum exhibits to help produce the glow on fluorescent rocks. Many amusement parks and clubs stamp hands with an ink that glows under black lights. They are used by police to locate stains bodily fluids and tiny spots of blood at crime scenes. Those bug zappers people like to place in their back yards are also a form of black light. Fluorescent materials and black lights have their uses. But what is a black light, and how does it work?

What Is Black Light?

Black lights (a.k.a. Wood's lamps) give off an electromagnetic radiation in the near-ultraviolet range. Invented by William H. Byler, blacks lights are more commonly linked to Physicist Robert Wood, who invented the Wood's glass that is used in black lights. Black lights are also called ultraviolet (UV) lights. Their light is barely visible because it's at the far end of the visible EMF range for human vision.

The fluorescent black light tubes that are so popular are produced in the same manner as regular fluorescent light tubes with one exception. They are made with phosphor that, rather than emitting white light, gives off UV light. In addition, the tubes are coated with an optical filter that's a dark bluish purple in color. This optical filter blocks almost all of the visible light and converts the UV rays to that familiar purple glow we're all familiar with. The bug zapper lights, used to control mosquitoes and flies, don't have the filtering material, making them more visible to the eye.

How Black Light Works

Normally, people can't see UVA light (not UVB); we can only see fluorescent glow and phosphorescence of items like those psychedelic black light posters, so popular in the 60s and the glow-in-the-dark stars and paints that kids like these days. While the UV light is virtually invisible to the human eye, tiny amount do pass through the optical filter, which have a wavelength in the 400 to 410 nanometers range.

When turned on, black lights give off a weak, dark-purple glow in a darkened room. The items that glow when the black lights are on usually contain phosphors, whose mercury atoms get excited or energized by the UV ray, causing them to re-emit a lower frequency light in different colors. These phosphors are the stuff that gives of the glow, or fluorescence, when exposed to the black light.

Objects containing phosphors, glow-in-the-dark when exposed to black lights. The phosphors have absorbed atoms of a shorter wavelength, or higher frequency, from the black light, which are then re-emitted as longer wavelength photons, or UV rays with a lower frequency.

Black Lights

Black lights may be constructed from fluorescent lamps, mercury vapor lamps, LEDs, or incandescent lamps, though most black lights these days come in the fluorescent lamp variety. With most black lights, the blue-purple optical filter is a coating on the outside of the bulb. Some, however, have the filter in the lamp housing. Most black lights are lower power UV lights. The higher power UV light sources are harmful to the eyes and skin. Anyone using these lights are required to wear protective eye equipment.


Harris, Tom. How Stuff Works. How Black Lights Work. Downloaded. 1/7/2012.

Pickover, Clifford. A. The Physics Book. New York: Sterling Publishing, 2011.

Wikipedia. Black Lights. Downloaded 1/7/2012.

Attraction Black Light Theater

Black Light Stop Motion Short

What glows with a Blacklight UV - Are all blacklights the same?

V 115 fluorescence of various materials 1 - Fluoreszenz

Fluorescent Jellyfish (Glow in the Dark)

Fluorescent Mineral display -Geofair 2009


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    • joanwz profile image

      Joan Whetzel 5 years ago from Katy, Texas

      I don't know the answer to that one. I'll have to look for the answer myself.

    • profile image

      Tom 5 years ago

      How dark does it need to be for black lights to effect phosphorous surfaces to make them glow?