Field theoretic description of partially reflective surfaces
  The issue of electric charges in interaction with partially reflective surfaces is addressed by means of field theoretic methods. It is proposed an enlarged Maxwell lagrangian, describing the electromagnetic field in the presence of a semitransparent surface, and its corresponding photon propagator is computed exactly. The amended Green function reduces to the one for a perfect conductor in the appropriate limit, and leads to the interaction between charges and surfaces with varying degrees of transparency, featured by a phenomenological parameter. The interaction found via image method is recovered, in the limiting case of perfect mirrors, as a testimony to the validity of the model. 
F.E. Barone, F.A. Barone
hep-th