  A theory of Bose-Einstein condensation (BEC) of light in a dye microcavity is
developed. The photon polarization degeneracy and the interaction between dye
molecules and photons in all of the cavity modes are taken into account. The
theory goes beyond the grand canonical approximation and allows one to
determine the statistical properties of the photon gas for all numbers of dye
molecules and photons at all temperatures, thus describing the microscopic,
mesoscopic, and macroscopic light BEC from a general perspective. A universal
relation between the degrees of second-order coherence for the photon
condensate and the polarized photon condensate is obtained. The photon
Bose-Einstein condensate can be used as a new source of nonclassical light.
