  We extend a recently proposed Quantum Field Theory (QFT) approach to the
Lifshitz formula, originally implemented for a real scalar field, to the case
of a fluctuating vacuum electromagnetic (EM) field, coupled to two flat,
parallel mirrors. The general result is presented in terms of the invariants of
the vacuum polarization tensors due to the media on each mirror. We consider
mirrors that have small widths, with the zero-width limit as a particular case.
We apply the latter to models involving graphene sheets, obtaining results
which are consistent with previous ones.
