A new engineering method is presented for the analysis of radiation exchange within an open enclosure filled with an isothermal radiation gas. It is shown that the general class can be formulated of an open enclosure composed of black, diffusive-grey or spectral quantity. A number of new relations are derived between temperature, heat flux and effective radiation energy flux of surfaces. The method of determination of transfer configuration factors and dissipation losses is also presented. Whereas in previously-published methods, errors were possible if an equilibrium state were assumed, the current approach does not have this drawback, but enables the radiation gas temperature to be explicitly predicted. A special emphasis is put on the problems of radiation in enclosures composed of adiabatic surfaces which affect the measurement technique of surface emissivity. Calorimetric emission measurements of thermal radiation properties of opaque surfaces are also presented.