Campfires and Ultraviolet Radiation

name         James
status       educator
grade        9-12
location     OH

Question -   Do you know, does a campfire produce a significant
amount of UV radiation, if any, at all?
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Everything with a temperature above absolute zero (0.0K) emits
electromagnetic radiation. The amounts and the wavelength range are
determined by the temperature. You are at ~ 300K and emit most energy
around 10,000nm. People cannot see wavelengths that long so you do not
glow in the dark (except to pit vipers). A camp fire is ~ 2,000K so you
see it, but it emits little if any UV (as I, a psychophysicist,
understand it. Maybe double check with a nonpsycho physicist)

Michael S. Loop
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The primary radiation from a campfire is in the infrared part of the
electromagnetic spectrum with, obviously, some visible radiation.  It is
very unlikely that there is detectable ultraviolet radiation from a
campfire, particularly at any reasonable distance from the fire.

David Kupperman
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No, they are not hot enough.

The hotter something is, the more of the energy it emits will be in the
form of short-wavelength radiation.  The sun, for instance, emits most
of its energy in the form of visible light, with about 5% of its energy
as UV and about 20% (very rough numbers, I do not have the actual figures
with me) as infrared.  The sun has a surface temperature of about 
6000 K, which is much, much higher than a campfire flame, which 
might reach about 1200 K.  So a campfire will emit most of its energy as IR and
basically none as UV.

If you are worried about why skin exposed to campfire radiation appears
flushed, it is not from UV burns.  Basically, it is because the water in
your body absorbs the infrared from a campfire very efficiently, so your skin
gets very warm

Richard Barrans
Department of Physics and Astronomy
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Campfires are not hot enough to emit a significant amount of  UV 
radiation. The temperature of a campfire is of the order of a 
hundreds of degrees C. The temperature of a body has to be of the 
order of thousands of degrees C. (e.g. the Sun) to emit a 
significant amount of UV radiation. The web site:

http://hea-www.harvard.edu/~efortin/thesis/html/Black_body.shtml 

is very graphic and can provide you with more details.

Vince Calder
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Gee, I have never heard of anybody getting a tan or sunburn from a
campfire...
It is clearly a much lower percentage than in sunlight.
Do not forget, a campfire is not really all that bright, compared with
sunlight.
And it emits from one smallish spot, diminishing with distance by the
inverse-square law.
Sunlight covers all the ground evenly, so there is much more of it,
scattering around the sky and illuminating you from all directions.
So even if fire did have a substantial percentage UV,
you might never get any noticeable effects.

I believe there are fire-detectors that sense UV from flame.
Visible-blind photodiodes of of wide-band gap semiconductors
SiC (silicon carbide) or GaN (gallium nitride).
(They would be insulators, except they are perfectly crystallized
so as to allow mobile electrons to not get trapped at defects.)
So I think there is some noticeable amount of UV.
Not yet clear why they think UV is a clearer signal than IR.
A candle is really bright in the infra-red.

Jim Swenson
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