As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study develops a spectral emittance model for films of rare earth containing materials. Although there are several possible rare earth doped high temperature materials, this study was confined to rare earth aluminum garnets. Good agreement between experimental and theoretical spectral emittances was found for erbium, thulium and erbium-holmium aluminum garnets. Spectral emittances of these films are sensitive to temperature differences across the film. Emitter efficiency is also a sensitive function of temperature. For thulium aluminum garnet the efficiency is 0.38 at 1700 K but only 0.19 at 1262 K.Chubb, Donald L. and Pal, AnnaMarie and Patton, Martin O. and Jenkins, Phillip P.Glenn Research CenterCERAMICS; RARE EARTH ELEMENTS; DOPED CRYSTALS; HIGH TEMPERATURE; EMITTERS; EARTH (PLANET); SPECTRA; EMITTANCE; ALUMINUM; GARNETS; SPECTRAL THEORY; THERMOPHOTOVOLTAIC CONVERSION; OPTICAL PROPERTIES; TEMPERATURE GRADIENTS; FILM THICKNESS; SPECTRAL EMISSION...