Nonequilibrium corrections in the pressure tensor due to an energy flux

The form P=a(u,v)U+b(u,v)(J) over right arrow (J) over right arrow for the pressure tensor for a system submitted to an energy flux (J) over right arrow (U being the identity matrix, u the specific energy, and v the specific volume) widely used for anisotropic radiation and proposed to be more general by Dominguez and Jou [Phys. Rev. E 51, 158 (1995)] has been recently questioned by R. E. Nettleton [Phys. Rev. E 53, 1241 (1996)]. We provide a physical basis, in a completely different way, for;this expression for anisotropic radiation and ultrarelativistic gases and we criticize some previous physical interpretations. We recall the necessity of an understanding of this kind of expression in a thermodynamic framework.