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Incandescent or
Fluorescent?
For many years the pros and cons of incandescent versus
fluorescent lights have been described. With the addition of
CFL (Compact Fluorescent Lights), LED and other technologies
the debate has shifted somewhat. But, first, some elementary
science...
Incandescent bulbs acquired the name because what they do is,
in essence, burn (i.e. incandesce, or glow with heat). A thin
tungsten filament sits inside a partial vacuum. Electricity
passes through the wire and, because it offers resistance to
the flow of electrons, it heats up. Quickly, the temperature
rises enough to make the metal glow.
The interior of the incandescent bulb is usually coated with a
diffusing material to cut down on the bright glare of a clear,
glass bulb. Though, of course, there are many clear glass
lightbulbs around. That doesn't alter the basic way the bulb
operates, but it does change the look.
That look is chiefly the result of the lightbulb's ability to
closely emulate the wavelengths of light given off by the sun.
That's determined to a large degree by the temperature of its
surface, about 6,000 Kelvin. That's roughly equal to 5,700
Celsius, since K = C + 273. That's about 10,300F. That
similarity is measured by something called color
temperature.
Interior bulbs will often be a bit lower in color temperature,
more toward the bluish than the yellow of sunlight. But don't
confuse the color temperature with the actual temperature.
Incandescent bulbs typically glow at anywhere from 120F-200F
(49C-93C) or higher. Water boils at 220F or 100C, so that gives
you an idea how hot one can be.
By contrast, a fluorescent bulb is much cooler because it
generates light in an entirely different way.
Inside the fluorescent bulb are electrodes and a thin vapor,
usually mercury and a few trace gases. A voltage is applied to
one of the electrodes at one end in a long tube and electrons
are shot out toward the other end, bumping into the mercury
atoms along the way. When they do, they give energy to its
electrons. When the mercury atoms release that energy again, it
appears in the form of light.
That process generates much less waste energy in the form of
heat. That provides the primary reason that fluorescent bulbs
are so much more efficient. In other words they can produce
about the same amount of light (measured in lumens) for the
same energy put into them because so much less is wasted in the
form of heat.
Remember that incandescent bulbs glow because the filament is
white hot. That produces a lot of heat for the amount of light
given off. The result to you, the consumer, is you're paying
for a great deal more electricity over the long run to heat a
wire in the case of incandescents.
On the other hand, fluorescents don't produce the same color
temperature or spectrum as incandescents. Modern CFLs come
close, but they still don't have quite the same look as an
'ordinary' light bulb. As a result, fluorescents have, for
decades, been reserved for areas where that look isn't regarded
as so important, such as the garage.
Still, the initial cost of a fluorescent bulb is 3-10 times a
standard 75 or 100-watt bulb. So, the trade off becomes
electricity cost savings over the lifetime of the bulb (about
6,000-15,000 hours for fluorescent, 750-1,000 hours for
incandescents) versus up front investment costs. Over their
lifetime, a fluorescent saves an average of about 70% on the
total cost, including purchase price and electricity bill.
While recent discussion about the environment has entered the
debate, in fact the contribution of light bulbs is so small
it's hardly worth mentioning. Much larger factors, such as
industrial gases, auto exhaust and other outputs are many
thousands of times larger than the total produced by home
lighting.
So, the old pros and cons still apply. At least, they did until
LEDs entered the picture... But that's another
topic.
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