  We present a model of coherent Bremsstrahlung emission from the compression
phase of relativistic heavy ion collisions based on a hydrodynamical
description of the collision process. The decceleration of the matter through a
shock front, calculated in 1-D relativistic dissipative hydrodynamics, is at
the origin of a copious Bremsstrahlung emission. The interferences between the
Bremsstrahlung emitted by two receeding shock fronts result in deviations from
the usual $1/\omega$ law, with in particular a strong suppression of low energy
photons in symmetric collisions. We show how it might be possible to obtain
information on the equation of state, possible transition to a quark gluon
plasma and dynamical properties of the matter from the study of the
interference pattern. We show that the optimal region for the observation of
coherent photons is in SIS - AGS energy range.
