  We derive a simple expression for the photon helicity and polarisation-flip
probabilities in arbitrary background fields, in the low energy regime. Taking
the background to model a focused laser beam, we study the impact of pulse
shape and collision geometry on the probabilities and on ellipticity signals of
vacuum birefringence. We find that models which do not account for pulse
duration can overestimate all signals in near head-on collisions by up to an
order of magnitude. Taking pulse duration into account, the flip probability
becomes relatively insensitive to both angular incidence and the fine details
of the pulse structure.
