A simple model of an emission Fourier-transform spectrometer is provided for a beam splitter with infinitesimal thickness but with general optical properties otherwise. Using the principle of conservation of energy, we derived the rigorous expressions that relate the interferograms from three different signals (source, reference, and beam-splitter emission), which are simultaneously observed by the detector. The relationship among phase and amplitude of the three interferograms is evaluated. The results indicate that the behavior of a Fourier-transform emission spectrometer may be more complicated than what is assumed in some models. Calibration procedures should reflect the implications of this rigorous approach.