  Anisotropy in heavy-ion collisions leads to polarization of both direct and
virtual photons, the latter detected via internal conversion to dileptons
pairs. Thus measurement of photon polarization probes the anisotropy at all
stages of collisions. In order to characterize the polarization of virtual
photons, we derive here the general structure of the photon polarization
operator, rho^(mu nu), in an anisotropic medium, in terms of the four spectral
functions, two transverse and one longitudinal, as usual, plus a new spectral
function that reflects anisotropy in asymmetric collisions; and derive the
local production rate of dilepton pairs in terms of these four functions.
