Scattering from spherical particles is described by the Mie model. This model has been implemented in ZEMAX as a user-defined DLL, to be used with a bulk scattering volume in non-sequential mode. The model provides an accurate description of the scattering process, for both the general case of arbitrary sphere size and when the sphere size is much smaller than the wavelength, i.e. in the Rayleigh limit.

References

¹http://en.wikipedia.org/wiki/Mie_theory
²Craig F. Bohren and Donald R. Huffman, "Absorption and Scattering of Light by Small Particles", John Wiley & Sons (1983).
³http://en.wikipedia.org/wiki/Sun
⁴http://hyperphysics.phy-astr.gsu.edu/Hbase/atmos/blusky.html