RANSPARENT heat mirrors are thin films that transmit visible light but reflect infrared radiation. This paper investigates the potential utilization of this wavelength selective property in the process of trapping incident solar energy and its conversion to heat in a high-temperature (5401650°C) windowed cavity. The heat mirror film permits the solar energy to enter the cavity but reflects part or all of the internally generated blackbody (heat) radiation as it attempts to escape. The characteristics and durability of a number of transparent heat mirror films developed at the MIT Lincoln Laboratory for solar energy applications have been reported. l'3 These transparent heat mirror film types include metal films1 (e.g., Au and Ag), semiconductors2 (e.g., Sn-doped In2O3), composite layer films2 (e.g., TiO2/Ag/TiO2), and thin-film conducting microgrids.3 From a simplistic viewpoint, heat mirror films pass nearly all of the radiation below a given wavelength and reflect nearly all of the radiation above it; the separation point (transmission vs reflection) edge wavelength being determined by the materials and construction methods used to fabricate the heat mirror. Although in the early development stage, heat mirror films offer the potential advantage of reducing the window heat load because of their reflective properties. A simplified model of a heat mirrored cavity receiver is considered herein and used to predict cavity operating efficiency as a function of incident sun level, cavity operating temperature, and heat mirror reflectance edge position. The results are compared with those for a windowed receiver where escape of reradiated energy is prevented by a process of radiation absorption in the window, rather than by reflection back to the cavity interior as in the case of the heat mirror. Experimental data on some existing heat mirrors are summarized, and achievable performance improvements with existing heat mirror films are calculated and compared with idealized predictions. The results reported here should be considered as an initial inquiry into the usefulness of the heat mirror approach in a cavity receiver with the objective of determining whether the use of such films is sufficiently promising to warrant more detailed investigations. Though evaluations are made for three experimental heat mirrors, these heat mirrors have not been optimized for the cavity receiver application, and