Variation with frequency of the normalized source function at τ ≈ 1, for different values of the velocity-changing elastic collision parameter ζ (indicated in the top-left frame). The model-atmosphere is the same as the one considered in Sect. 6 and Hummer (1969). As expected, successive solutions range between standard PRD-R_I − A values (blue) and the limit of the CRD solution (dashed line) for increasing values of ζ.