  We revisit the medium-induced coherent radiation associated to hard and
forward (small angle) scattering of an energetic parton through a nuclear
medium. We consider all $1\to 1$ hard forward processes ($g \to g$, $q \to q$,
$q \to g$ and $g \to q$), and derive the energy spectrum of induced coherent
radiation rigorously to all orders in the opacity expansion and for the
specific case of a Coulomb scattering potential. We obtain a simple general
formula for the induced coherent spectrum, which encompasses the results
corresponding to previously known special cases.
