Silicon operating in a vacuum is a good candidate thermal emitter since it has a high melting point (1680 K). The semiconductor bandgap, which can provide selective emission, adds to the potential for high operating temperature and, therefore, high radiated power. We present the detailed emitter theory, along with both theoretical and experimental results for spectral emittance of thin (∼1 &mgr;m) silicon films on sapphire substrates with a platinum backing. These results show the importance of temperature and film thickness in determining the selective spectral emittance and, with the proper material parameters, can be readily extended to other materials and systems. © 2003 American Institute of Physics