Satisfied qualification of radiative heat conduction approximation when radiation transferred in light media

Based on the radiation transfer equation, the radiation energy density flux with higher terms was obtained using iterative method. Using the formula, the satisfied qualification of the radiative heat conduction approximation was analyzed when radiation transferred in light media. The approximation always required the quotient between varying scale of temperature and Rosseland mean free path to be far larger than some value, as called citerion h, it indicated the hardness of satisfaction of the approximation: as the criterion increased, the approximation will be hardly satisfied. The results indicated that the criterion is between 3 and 60 when the bremsstrahlung and scattering process are both concerned. And the influence of two factors, the changing scale of density in space and the radiu