The importance of radiation on the temperature distribution in a semi-transparent solid is reported. The first-order differential approximation of radiation is combined with conduction analysis to investigate the temperature profiles in a plane slab and a rectangular region. The coupled nonlinear partial differential equations are solved numerically by either a standard implicit or an implicit alternating direction method. Results obtained for opaque boundaries are in good agreement with exact formulations found in the literature. An extension to partially transparent boundaries is made and results presented.