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Geiner's Quantum Mechanics - An Introduction has an unnumbered equation above Eq. 6 in section 2.4 discussing density -- not sure if it is energy density -- of radiation:

... $$dE/dV = E/V =a T^4$$ where $$a = \frac{\pi^2 k_B^4}{15 \hbar^3 c^3} = 7.56 \times 10^{-15} erg cm^{-3} K^{-4}$$ This gives rise to an homogeneous, isotropic,radiation of density K, where K is given by

$$K = \frac{c}{4 \pi} \frac{dE}{dV} erg \: cm^{-2}$$ ...

I'm particularly puzzled by the factor of $\frac{c}{4\pi}$. How is it from? Hopefully this is enough info for the question to be answerable.

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    $\begingroup$ It's probably due to the stupid units. Erg cm etc $\endgroup$ – Your Majesty Nov 1 '14 at 6:48

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