  We compute the spectrum of thermal photons created in Au+Au collisions at
$\sqrt{s_{NN}}=200$ GeV, taking into account dissipative corrections in
production processes corresponding to the quark--gluon plasma and hadronic
phases. To describe the evolution of the fireball we use a viscous fluid
dynamic model with different parametrizations for the temperature--dependence
of $\eta/s$. We find that the spectrum significantly depends on the values of
$\eta/s$ in the QGP phase, and is almost insensitive to the values in the
hadronic phase. We also compare the influence of the temperature--dependence of
$\eta/s$ on the spectrum of thermal photons to that of using different
equations of state in the fluid dynamic simulations, finding that both effects
are of the same order of magnitude.
