The Radiative Stress Tensor
  We use the transfer equation in relativistic form to develop an expansion of the one-photon distribution for a medium with constant photon mean free path, $\epsilon$. The resulting radiative viscosity tensor may be expressed either as a simple integral operator acting on the Thomas stress tensor or as the solution of an inhomogenous, linear partial differential equation. The expression obtained for the radiative viscosity tensor applies for media with long, as well as short, photon mean free paths. We also develop results applicable for relatively smooth flows by using the form of the Thomas stress tensor with generalized transport coefficients derived by the application of a suitable operator to the bare Thomas coefficients. 
Xinzhong Chen and Edward A. Spiegel