  The linear sigma model with linearized fluctuations of all involved fields
facilitates the onset of a sequence of first-order phase transitions at a
critical point. This phase structure has distinctive imprints on the photon
emission rates. We argue that analogously a critical point in the QCD phase
diagram manifests itself by peculiarities of the photon spectra, in particular
when the dynamical expansion path of matter crosses the phase transition curve
in the vicinity of the critical point.
