Generation of highly directional beams from a globally incoherent source

We have carried out a series of observations of the far field intensity distribution produced by a class of partially coherent planar sources. Our results confirm that, under the Collett-Wolf quasi-homogeneity conditions, highly directional beams can be produced even from a source which is globally incoherent. The far-zone pattern is shown to be largely independent of the intensity profile in the source plane and that the beam radius varies as predicted by Foley and Zubairy in the paraxial approximation. We show experimentally how the coherence length in the source plane can be measured directly from the far zone intensity distribution.