  In many applications of absorbing structures it is important to understand
their spatial response to incident fields, for example in thermal solar panels,
bolometric imaging and controlling radiative heat transfer. In practice, the
illuminating field often originates from thermal sources and is only spatially
partially coherent when reaching the absorbing device. In this paper, we
present a method to fully characterize the way a structure can absorb such
partially coherent fields. The method is presented for any 3D material and
accounts for the partial coherence and partial polarization of the incident
light. This characterization can be achieved numerically using simulation
results or experimentally using the Energy Absorption Interferometry (EAI) that
has been described previously in the literature. The absorbing structure is
characterized through a set of absorbing functions, onto which any partially
coherent field can be projected. This set is compact for any structure of
finite extent and the absorbing function discrete for periodic structures.
