The local aspect of the thermalization of thermal radiation due to the interaction of thermal radiation with matter is explored. It is shown that for absorption and emission interactions some well-known phenomenological statements of irreversible thermophysics are indeed relevant. For scattering interactions the entirely different picture that emerges is also briefly discussed. The local analysis is based on the concepts of temperature and entropy of nonequilibrium thermal radiation fields, as originally introduced by Planck, and shows that these concepts are susceptible (...) of a very natural thermodynamic interpretation. (shrink)