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_What is Emissivity?_

_The surface temperature means just what you think it does, but you
may not have encountered the word emissivity before. Emissivity is the
ratio of thermal energy emission of the target object, over the
thermal energy emission of a true blackbody at the same temperature._

_Everything in nature emits electromagnetic radiation that is a
function of its temperature, and not (in theory) a function of what it
is made out of. That radiation is what we call thermal radiation. In
the case of a perfect black body, which absorbs 100% of all radiation
that hits it, the thermal radiation emitted is a function of the
temperature only. Furthermore, no object will emit purely thermal
radiation with more intensity than a true blackbody, so its emission
is a maximum. But most of the real stuff in the real world, as you
probably already know, is not perfect. In the case of real world
stuff, what it is made out of does make a difference, and nature
expresses that difference by a sort of efficiency; some things will
emit more thermal radiation, even though they are at the same
temperature as something else. High emissivity material emits more
thermal radiation, and low emissivity material emits less thermal
radiation, at any given temperature. Emissivity is always expressed as
a number between zero and one._

_The ASTER TIR instrument will measure the thermal radiation coming
out of the top of the atmosphere. My atmospheric correction algorithms
will remove the effect of the atmosphere, as best we can (it would be
better to call this atmospheric compensation, but atmospheric
correction is already the officially enshrined title). We will then be
left with the thermal radiation leaving the ground surface. From that
we will derive both the temperature and the emissivity._

_Back to [1]Tim Thompson's ASTER Page_
_Back to [2]Tim Thompson's Home Page_
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References

1. http://www.geocities.com/CapeCanaveral/8851/aster.html
2. http://www.geocities.com/CapeCanaveral/8851/