Deviations from perfect absorption and perfect black body behavior lead to light losses. For selective emitters, any light emitted at wavelengths not matched to the bandgap energy of the photovoltaic may not be efficiently converted, reducing efficiency. In particular, emissions associated with phonon resonances are difficult to avoid for wavelengths in the deep infrared, which cannot be practically converted. An ideal emitter would emit no light at wavelengths other than at the bandgap energy, and much TPV research is devoted to developing emitters that better approximate this narrow emission spectrum.