Polarisation effects in multiple scattering photon calculations using the Boltzmann vector equation

Studies on radiative transfer lead to a clean formulation of polarised phot on transport in terms of the vector Boltzmann equation whose solution gives the four Stokes components of the flux, from which the full polarisation s tate of the photons can be determined at any given position, wavelength (en ergy) and solid angle. One of the relevant results observed during the form ulation of the vector transport equation is the partial coverage of the wav e properties of the photons with this model. In fact, even if the Boltzmann vector equation is an important step forward for the description of radiat ive transfer with respect to the scaler approach used to describe "particle "-like photons, it is Still insufficient to provide a whole description of an important phase-related property like coherence. In this sense. the abov e vector equation seems to be appropriate for describing photon beams which add incoherently among them, but not for describing coherent interference. Up to now, coherence has been described independently to the transport equ ation. as an additive term to the vector solution valid for diffuse incoher ent radiation. New approaches are being investigated in order to include th e concept of coherence into the vector transport equation. In this article, a summary view of the evolution of the Boltzmann transport equation will b e provided, first from scalar to vector, to lake care of the description of the evolution of the polarization state, Secondly the present state of the treatment of coherence will be considered. Finally, the state-of-the-art d escription of multiple scattering involving Rayleigh scattering will be dis cussed, in the framework of both reflection and transmission experiments. ( C) 1999 Elsevier Science Ltd. All rights reserved.