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Added the relationship between photon flux density and irradiance in the Equations section. |
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For a black body surface, the spectral radiance density (defined per unit of area normal to the propagation) is independent of the angle <math>\theta</math> of emission with respect to the normal. However, this means that, following [[Lambert's cosine law]], <math> B_\nu(T) \cos \theta</math> is the radiance density per unit area of emitting surface as the surface area involved in generating the radiance is increased by a factor <math> 1/\cos \theta</math> with respect to an area normal to the propagation direction. At oblique angles, the solid angle spans involved do get smaller, resulting in lower aggregate intensities.
The emitted energy flux density or irradiance <math>B_\nu(T,E)</math>, is related to the photon flux density <math>b_\nu(T,E)</math> through <ref>{{cite
|title=The Physics of Solar Cells
|url=https://doi.org/10.1142/p276
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|year = 2002
|page = 19
|doi=10.1142/p276
|isbn=978-1-86094-340-9
}}</ref>
:<math>B_\nu(T,E) = Eb_\nu(T,E)</math>
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