The entropy calculation for radiation fluxes is reviewed and applied to simple radiation-surface interactions. A plate interacting with radiation from a hot object in the zenith of the hemisphere surrounded by a colder atmosphere is analyzed in detail. The entropy generation rate upon absorption and reflection of the incoming radiation is calculated and discussed. The plate is adiabatic in a first version (thermal equilibrium), then its temperature is fixed by allowing a heat flux to or from the plate. This analysis prepares the way towards an entropy generation minimization analysis of more complex radiation settings.