A thermo-photovoltaic power generator for efficiently converting thermal energy into electric energy including a selective thermal emitter for receiving thermal energy and emitting thermal radiation with black body emissivity over a range of wavelengths, low-bandgap photovoltaic cells responsive to thermal radiation at wavelengths within a particular band of said range of wavelengths and operative to convert such thermal radiation to electric energy, and a band pass filter disposed between the thermal emitter and the photovoltaic cells for transmitting thermal radiation from the emitter at wavelengths within the particular band to the photovoltaic cells, and for reflecting thermal radiation from the emitter at wavelengths outside the particular band back to the emitter.