...if you turn on a
black light bulb in a dark room, what you can see from the bulb is a purplish
glow. What you cannot see is the ultraviolet light that the bulb is also
producing. Our eyes can see visible light in a spectrum ranging from red
through orange, yellow, green, blue and violet. Above violet is ultraviolet
light, which we cannot see. A black light bulb produces UVA light (as opposed
to UVB light, which is much more harmful). A "black light" or
Ultraviolet lamp is almost the same as the flourescent lights used in most
classrooms. The difference is that "UV" lights lack the white
phosphor coating, and they are made with dark blue glass. Flourescent tubes
contain a near vacuum with a little mercury vapor.
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What you see glowing
under a black light, whether on a fluorescent poster or an invisible hand stamp
or a newly washed white T-shirt, are phosphors. A phosphor is any substance
that emits visible light in response to some sort of radiation. A phosphor
converts the energy in the UV radiation from a black light into visible light.
In the next section, we'll see how phosphors are used in regular fluorescent
lighting and in black lights. White T-shirts and socks normally glow under a
black light because modern detergents contain phosphors that convert UV light
into white light. This makes whites look "whiter than white" in
normal sunlight. What you are seeing in sunlight is the normal reflection of
visible white light from the cloth, as well as the emission of white light that
the phosphors create from UV light in sunlight. The T-shirt really is whiter
than white!
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There are
lots of natural phosphors, in your teeth and fingernails, among other things.
There also a lot of phosphors in manmade material, including television 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. |
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
phosphors. 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 refrigerant. Law enforcement officers can use
them to identify counterfeit money. |
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The United States and
many other countries include an invisible fluorescent strip in their larger
bills that only shows up under a black light.musement 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 naturally fluoresce…
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