  The WA98 collaboration in the CERN SPS has reported an excess of photons over
those originating from the decays of final hadrons in the lead-lead collisions.
These photons can originate either from primary interactions of partons from
colliding nuclei or from secondary interactions among produced particles.
Photons produced in the secondary interactions, often called thermal photons,
can be calculated by using thermal production rates and equilibrium
hydrodynamics for the evolution of the expanding matter. I will review the main
features of hydrodynamical studies for the WA98 data. The data can be
reproduced both with or without a phase transition to the QGP, but high initial
temperature, over the values predicted for the phase transition temperature, is
required by the data. I will also show a prediction for the photon excess for
central gold-gold collisions at the Brookhaven RHIC collider. In this
prediction, the initial state for the hydrodynamical expansion is obtained from
a perturbative QCD calculation.
