Gaussian Schell Source as Model for Slit-Collimated Atomic and Molecular   Beams
  We derive the spatial coherence and intensity profiles of beams emerging from two consecutive collimating apertures, and compare our results with data. We show how to make a Gaussian Schell-model (GSM) beam by assuming Gaussian apertures. Then we compare the intensity profile, the transverse coherence width and the divergence angle of a GSM beam with those same properties of a beam that is collimated with two hard-edged slits. The GSM beam formulae are simpler, and offer an intuitive way to understand how partially coherent beams interact with interferometers. 
Ben McMorran, Alex Cronin