Planck Intensity Density -- from Eric Weisstein's World of Physics

Thermodynamics "> Blackbody Radiation v

Planck Intensity Density

In terms of frequency, the energy flux from blackbody radiation is given by

where is the average energy density, is Planck energy, N is the number density of oscillators, h is Planck's constant, k is Boltzmann's constant, and T is the temperature. Here,

is the number of cells per unit phase space. The factor of 2 must be added since two electrons with opposite spins may occupy the same element of phase space. The momentum of a photon is given by

so

and

where is an element of solid angle. Plugging in,

Planck Energy, Planck Law