Intensity of the radiation field in the positive (leftpanel) and negative (rightpanel) direction for the 1D radiative transfer model discussed in Sect. 3.3. All models have the same scattering cross section C_sca = 1 and the different lines correspond to models with different absorption cross sections C_abs, ranging from 2 to −2.5 in steps of −0.25. The thick orange and red curves correspond to pure scattering and critical net stimulated emission, respectively.