  Using the theory of imaging with partially coherent light, we derive general
expressions for different kinds of interferometric setups like double slit,
shift and mirror interference. We show that in all cases the interference
patterns depend not only on the point spread function of the imaging setup but
also strongly on the spatial emission pattern of the sample. Taking typical
experimentally observed spatial emission patterns into account, we can
reproduce at least qualitatively all the observed interference structures,
which have been interpreted as signatures for spontaneous long range coherence
of excitons, already for incoherent emitters. This requires a critical
reexamination of the previous work.
