  We discuss the transverse momentum distribution of thermal and prompt photons
in ultrarelativistic heavy ion collisions. The prompt photons are mostly
produced by fragmentation of quark jets (bremsstrahlung). If these quark jets
suffer a large energy loss in heavy-ion reactions, prompt photons are
significantly suppressed. Thermal electromagnetic radiation from the
quark-gluon plasma and the hadron gas might then dominate the intermediate
k_T-range. In central Au+Au collisions at \sqrt{s}=200A GeV, and in the range 2
GeV < k_T < 4 GeV, the inverse slope of the thermal radiation is 360-460 MeV,
while that of the QCD photons is 500-1000 MeV. This large difference might
allow to disentangle the various sources of direct photons experimentally. At
\sqrt{s}=5.5A TeV, nuclear shadowing and quark-jet quenching suppresses the
QCD-photons strongly for k_T<5 GeV. Thermal radiation dominates in this
k_T-region.
