How much work can be extracted from a radiation reservoir?

Radiation reservoirs are more complex than heat reservoirs. They depend on the intensive thermodynamic parameters (such as temperature, pressure, and chemical potential) as well as on other state parameters (such as the geometric factors). The paper refers to work extraction from a high temperature radiation reservoir (the pump), the sink being a heat reservoir, respectively a radiation reservoir. The simplest case of radiation reservoir (i.e. blackbody isotropic radiation) is considered. Reversible and endoreversible conversion is analyzed. The upper bound for reversible work extraction is not Carnot efficiency. All upper bound efficiencies obtained here depend on the pump geometric factor.