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706. Problem 49.12 (RHK) A thermograph is a medical instrument used to measure
radiation from the skin. For example, normal skin radiation at a
temperature of
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and
the skin over a tumour radiates at a slightly higher temperature. (a) We
have to derive an approximate expression for the fractional difference
in
the radiant intensity between adjacent areas of the skin that are at slightly
different temperatures
and
. (b)
We have to evaluate this expression for a temperature difference of
.
We may assume that skin radiates with a constant emissivity.|
Solution: Click For PDF Version (a) We use the Stefan-Boltzmann law for the radiant intensity
where
Therefore,
(b) We next workout the expression
We find
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:
,
is the
emissivity and the Stefan constant
.
and
.