  Electromagnetic sources relying on spontaneous emission are difficult to
characterize without a proper framework due to the partial spatial coherence of
the emitted fields. In this paper, we propose to characterize emitters of any
shape through their natural emitting modes, i.e. a set of coherent modes that
add up incoherently. The resulting framework is very intuitive since any
emitter is regarded as a multimode antenna with zero correlation between modes.
Moreover, for any finite emitter, the modes form a compact set that can be
truncated. Each significant mode corresponds to one independent degree of
freedom through which the emitter radiates power. The proposed formalism is
implemented using the Method of Moments (MoM) and applied to a lossy sphere and
a lossy ellipsoid. It is shown that electrically small structures can be
characterized with a small number of modes, and that this number grows as the
structure becomes electrically large.
