Light propagation in non linear electrodynamics
  Working on the approximation of low frequency, we present the light cone conditions for a class of theories constructed with the two gauge invariants of the Maxwell field without making use of average over polarization states. Different polarization states are thus identified describing birefringence phenomena. We make an application of the formalism to the case of Euler-Heisenberg effective Lagrangian and well know results are obtained. 
V. A. De Lorenci, R. Klippert, M. Novello and J. M. Salim