  We review direct photons in the phenomenology of nuclear collisions at
relativistic energies. Direct photons carry information about the space-time
evolution of nuclear collisions because the QCD medium is transparent against
colorless particles. After a status summary of theoretical and experimental
studies, transverse momentum spectra and azimuthal momentum anisotropies of
direct photons are studied in the context of the hydrodynamic modeling of
relativistic nuclear collisions with emphasis on pre-equilibrium photons. It is
implied that pre-equilibrium photons can be as important as thermal and prompt
photons for comprehensive understanding of direct photon production.
