There is an emission point, origin of radiation with elementary solid angle d, and absorption surface. Observer has a viewing angle throughout an elementary surface dS and there is an elementary spectrum d through which we assume the radiation emission.

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There is an emission point, origin of radiation with elementary solid angle d $\Omega \protect$ , and absorption surface. Observer has a viewing angle $\Theta \protect$ throughout an elementary surface dS and there is an elementary spectrum d $\nu \protect$ through which we assume the radiation emission.

$\includegraphics[]{intro.eps}$

Observer receive a radiation quantity calculated as follows:

Amaury LATAILLADE 2002-11-04