  We propose and analyze a k-filtering effect which gives rise to the drastic
difference between the actual spatial coherence length of quasi-two-dimensional
(quasi-2D) excitons or microcavity (MC) polaritons in planar nanostructures and
that inferred from far-field optical measurements. The effect originates from
the conservation of in-plane wavevector k in the optical decay of the particles
in outgoing bulk photons. The k-filtering effect explains the large coherence
lengths recently observed for indirect excitons in coupled quantum wells (QWs),
but is less pronounced for MC polaritons at low temperatures, T<10K.
