  We address two issues in the quantum electrodynamical description of photonic
media with some disorder, neglecting material dispersion. When choosing a gauge
in which the static potential vanishes, the normal modes of the medium with
disorder satisfy another transversality condition than the modes of the ideal
medium. Our first result is an integral equation for optical modes such that
all perturbation-theory solutions automatically satisfy the desired
transversality condition. Secondly, when expanding the vector potential for the
medium with disorder in terms of the normal modes of the ideal structure, we
find the gauge transformation that makes the static potential zero, thereby
generalizing work by Glauber and Lewenstein [Phys. Rev. A 43, 467 (1991)]. Our
results are relevant for the quantum optics of disordered photonic crystals.
