  Within the geometric optics approximation, we derive the light cone condition
for a class of homogeneous non-trivial QED vacua using the effective action
approach. Our result generalizes the ``unified formula'' suggested by Latorre,
Pascual and Tarrach which turns out to be a low-energy limit. We calculate the
velocity shifts induced by electromagnetic fields and finite temperature and
especially analyze the high-energy limit consistent with the loop
approximation.
