Name: Rick H.
My understanding is, that any object whose temperature
is above absolute zero, emits infrared radiation. This radiation travels
outward at the speed of light. My question is; is there a formula or
ratio that predicts the dissipation of that radiation over distance? The
example which really interests me is, since a person emits infrared
radiation, how large is their infrared "aura" (sorry), and how far away
could that be measured and by what instrument?
The strength of the emitted radiation drops by distance squared. That means
that going from 1 to 2 m away from a body, the detected radiation strength
drops by a factor of 4. This is because a given pencil of the emitted
radiation spreads over a larger area as one moves away from the source.
Radiation emitted from a warm body may be PREDOMINANTLY in the infra red
region but other wavelengths are also present. The distribution of radiation
emitted from an ideal "black body" is dependent on its temperature only and
is given by Plank's function. Radiation emitted from a real body is
additionally dependent on a property of the surface of the body called
"emissivity" which itself can be wavelength dependent.
If you are interested in the amount of radiation (energy) contained in a
particle range of wavelengths, e.g., in the IR range, you can integrate the
area under the emission curve for that surface between the wavelengths of
interest. This curve is essentially the Plank's function at the particular
temperature of the body multiplied by the spectral emissivity.
You can read more about this by looking at an introductory text book on
radiation. Simple formulas are also available whereby you can avoid the
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
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Update: June 2012