lights look just like normal fluorescent
lamps or incandescent
light bulbs, but they do something completely different.
Switch one on, and white clothes, teeth and various other
things glow in the dark, while the bulb itself only
emits faint purple light.
These devices are all around us -- in clubs, science
museums, amusement parks and teenagers' bedrooms, among other
places -- but to most people, they're a total mystery.
In this edition of HowStuffWorks,
we'll find out exactly what's going on here. We'll also see
why black lights make some objects glow but not others, and
we'll look at some interesting black light applications.
How They Work The conventional black light
design is just a fluorescent
lamp with a couple of important modifications. Fluorescent
lamps generate light by
passing electricity through a tube filled with inert gas and a
small amount of mercury. (See How
Fluorescent Lamps Work for more information.)
When energized, mercury atoms emit
energy in the form of light photons. They emit some
visible light photons, but mostly they emit photons in the ultraviolet
(UV) wavelength range. UV light waves are too short
for us to see -- they are completely invisible -- so
fluorescent lamps have to convert this energy into visible
light. They do this with a phosphor coating around the
outside of the tube.
Phosphors are substances that give off light -- or
fluoresce -- when they are exposed to light. When a
photon hits a phosphor atom, one of the phosphor's electrons
jumps to a higher energy level and the atom heats up. When the
electron falls back to its normal level, it releases energy in
the form of another photon. This photon has less energy than
the original photon, because some energy was lost as heat. In
a fluorescent lamp, the emitted light is in the visible
spectrum -- the phosphor gives off white light we can see.
Black lights work on this same principle. There are
actually two different types of black light, but they work in
basically the same way.
A tube black light is a basically a fluorescent
lamp with a different sort of phosphor coating. This coating
absorbs harmful shortwave UV-B
light and emits UV-A
light (in the same basic way the phosphor in a fluorescent
lamp absorbs UV light and emits visible light). The "black"
glass tube itself blocks most visible light, so in the end
only benign long-wave UV-A light and some blue and violet
visible light pass through.
An incandescent black light bulb is similar to a
household light bulb, but it uses light filters to
absorb the light from the heated filament. It absorbs
everything except the infrared and UV-A light (and a little
bit of visible light).
Black lights come in both tube and bulb
In both of these light designs, the emitted UV light reacts
with various external phosphors in exactly the same way
as the UV light inside a fluorescent lamp reacts with the
phosphor coating. The external phosphors glow as long as the
UV light is shining on them.
In the next section, we'll find out about some of the more
common phosphor materials and look at some common black light
What Glows? If you walked around all night
with a portable black light, you would discover that there are
phosphors all over the place. There are lots of natural
phosphors, in your teeth and fingernails, among other things.
There also a lot of phosphors in manmade material, including
screens and some paints, fabric and plastics. Most
fluorescent colored things, such as highlighters,
contain phosphors, and you'll find them in all glow-in-the-dark
products. Clubs and amusement parks use special black light
paint that glows different colors. You can also buy fluorescent
black light bubbles, invisible black light ink,
fluorescent black light carpet and even fluorescent black
light hair gel.
Ordinary highlighters work well as "black
light pens." Under the black light, the fluorescent ink
The first thing most people notice when you switch on a
black light is that some of their clothing glows. This is
because most laundry detergents contain phosphors to make
whites appear brighter in sunlight. Sunlight contains UV light
that makes the whites glow "brighter than white." Dark clothes
don't glow because the dark pigments absorb the UV light.
In addition to making people and fluorescent posters look
cool, black lights have some practical applications. For
Appraisers use them to detect forgeries of antiques.
Many paints today contain phosphors that will glow under a
black light, while most older paints do not contain
Repairmen use them to find invisible leaks in machinery
-- they inject a little fluorescent dye into the fuel supply
and illuminate it with a black light. For example, they
might detect an invisible air
conditioner leak by adding fluorescent dye to the
Law enforcement officers can use them to identify
counterfeit money. The United States and many other
countries include an invisible fluorescent strip in their
larger bills that only shows up under a black light.
Amusement parks and clubs use them to identify invisible
fluorescent hand stamps for readmission.
Forensic scientists use them to analyze crime scenes. To
pick out fingerprints, for example, they often dust with
fluorescent dye under a black light. This makes it easier to
pick the fingerprints out from surrounding dirt. Black
lights can also identify semen and other bodily fluids that
Most of these uses, as well as dozens of others, follow a
common theme -- the black lights make the invisible visible or
isolate one specific substance from everything around it. When
you think about it, there are dozens of situations where you
could put this phenomenon to work. The applications are
For more information on black lights and other sorts of
light energy, check out the links on the next page.