  The high gradient of magnification across the source during both small impact
parameter events and caustic crossings offers a unique opportunity for
determining the surface brightness profile of the source. Furthermore, models
indicate that these events can also provide an appreciable polarisation signal
-- arising from differential magnification across the otherwise symmetric
source. In a previous paper, polarimetric variations from point lensing of a
circumstellar envelope were considered, as would be suitable for an extended
envelope around a red giant. In this work we examine the polarisation in the
context of caustic crossing events, the scenario which represents the most
easily accessible situation for actually observing a polarisation signal in
Galactic microlensing.
