  We consider signals of photon-photon scattering in laser-based, low energy
experiments. In particular, we consider the ellipticity induced on a probe beam
by a strong background field, and compare it with a recent worldline expression
for the photon polarisation flip amplitude. When the probe and the background
are plane waves, the ellipticity is equal to the flip amplitude. Here we
investigate the ellipticity-amplitude relation for more physical fields.
