Planck photon distribution
= Planck photon distribution
{c}
{title2=$dn/d\nu$}
= Planck distribution
{c}
{synonym}
For <photons> in <thermal equilibrium> at <temperature> $T$, <Planck's law> gives the photon <number density> per frequency interval. In terms of the <Planck function>, it is $dn/d\nu=4\pi B_\nu(T)/(ch\nu)$, hence
$$
\frac{dn}{d\nu}=\frac{8\pi\nu^2}{c^3}\frac1{e^{h\nu/(k_BT)}-1}.
$$