The spectral emissivity of Yb3+ in a series of compounds has been investigated to study the effects of crystal structure type, chemical environment, ytterbium concentration, impurity concentration and temperature on the spectral radiant intensity of the selective emitter peak and emitted power from the material. A figure of merit has been defined which enables the compounds to be ranked for application in a practical thermophotovoltaic energy conversion device. It is shown that significant selective emission can be achieved from compounds in which the Yb3+ concentration is as low as 10 mol%. Apart from pure Yb2O3, the compounds Yb3Al5O12, YbNbO4 and Y0.9Yb0.1O1.5 are found to have emission spectra suitable for efficient matching to silicon photovoltaic cells.