This paper describes a general method to calculate the semi-gray radiation heat transfer that occurs within an enclosure comprised of diffuse surfaces. The method is implemented in an existing solar power tower cavity receiver model in the System Advisor Model (SAM, NREL, 2011). The semi-gray radiation model is used to find an optimal distribution of emissivities for the thermal- and solar radiation wavelength bands for surfaces that comprise the solar central cavity receiver. The optimal distribution of emissivities maximizes the overall thermal efficiency of a cavity receiver. The model shows an effective way to reduce heat loss from the cavity is to minimize the temperatures of the passive surfaces through manipulation of their radiative surface properties. For the cavity receiver design considered, an optimal emissivity distribution for the active absorber surfaces of the cavity is a selective surface with high absorptivity in the solar wavelength band and low emissivity in the thermal wavelength band. Passive surfaces within the cavity should be highly reflective for radiation over the full spectrum. For absorber surfaces with solar absorptivity of 0.95, thermal emissivity of 0.1 and reflective passive surfaces with emissivities of 0.1, for the full spectrum, the thermal efficiency of the receiver can be increased by about 0.7% in comparison to gray surfaces having an emissivity of 0.95 for all wavelengths. (C) 2012 Elsevier Ltd. All rights reserved.