Real objects do not radiate as much heat as a
perfect black body. They radiate less heat than a black
body and are called gray bodies. To take into account the
fact that real objects are gray bodies,
Equation 2-12 is modified to be of the following form.
Emissivity is simply a factor by which we
multiply the black body heat transfer to take into account
that the black body is the ideal case. Emissivity is a
dimensionless number and has a maximum
value of 1.0.
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