  We compute the spectrum of emitted radiation by a generic quantum system
interacting with an external classic noise. Our motivation is to understand
this phenomenon within the framework of collapse models. However the
computation is general and applies practically to any situation where a quantum
system interacts with a noise. The computation is carried out at a perturbative
level. This poses problems concerning the correct way of performing the
analysis, as repeatedly discussed in the literature. We will clarify also this
issue.
