Radiation heat transfer within a high-temperature solar cavity-receiver containing a windowed aperture exposed to concentrated solar radiation is solved using the gray-band approximated radiosity method for semitransparent enclosures. Spectrally selective quartz and sapphire are examined for window materials. Window and cavity temperatures are calculated as a function of the incoming radiative flux and solar energy absorption efficiency. For validation and comparability, a windowless cavity is analyzed. Due to its relatively high reflectance in the visible spectrum, the sapphire window requires higher inlet solar radiative flux than the quartz window to obtain the same reactor temperature and energy absorption efficiency.