text: This section discusses bremsstrahlung emission and the inverse absorption process (called inverse bremsstrahlung) in a macroscopic medium. We start with the equation of radiative transfer, which applies to general processes and not just bremsstrahlung:
   is the radiation spectral intensity, or power per (area × × photon frequency) summed over both polarizations.  is the emissivity, analogous to defined above, and  is the absorptivity.  and  are properties of the matter, not the radiation, and account for all the particles in the medium - not just a pair of one electron and one ion as in the prior section. If  is uniform in space and time, then the left-hand side of the transfer equation is zero, and we find
  If the matter and radiation are also in thermal equilibrium at some temperature, then  must be the blackbody spectrum:
  Since  and  are independent of , this means that  must be the blackbody spectrum whenever the matter is in equilibrium at some temperature – regardless of the state of the radiation. This allows us to immediately know both  and  once one is known – for matter in equilibrium.