RADIATIVE TRANSFER: ASYMPTOTIC SOLUTION OF THE KINETIC EQUATION OF RADIATION PROPAGATION, N-th ORDER ASYMPTOTIC APPROXIMATION AND IMPROVED BOUNDARY CONDITIONS

In the article, new asymptotic approximation of the $n$-th order is obtained and proposed to be used in calculations of radiation propagation in optically thick media without scattering; the asymptotic approximation is simpler and more precise than the known diffusion approximation. It is shown, that for optically thick media the asymptotic solution of the kinetic equation of radiation propagation without scattering is asymptotic expansion of the exact integral solution of that kinetic equation. The rigorous derivation of the diffusion approximation equation is obtained. Improved boundary conditions, which are essential for practical application in calculations of radiation propagation, are derived.