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tokens: in a scattering process the frequencies of the incoming and outgoing photons are completely uncorrelated. Therefore, it is very important to clarify when this limit can be safely applied. As mentioned before, the hypothesis of CRD is generally suitable for treating weak spectral lines, and it is a good approximation for modeling the core region of strong lines of the intensity spectrum. On the other hand, it turns out to be completely unsuitable for modeling the extended wings of strong resonance lines. In light of this, we provide a detailed comparison between the results obtained through the full PRD calculation and in the limit of CRD. In Figure 3 , the emergent intensity and Q / I profiles calculated in the absence of a magnetic field are shown, for the Sr ii 4078 Å line and for the Sr i 4607 Å line, comparing the full PRD calculation (solid line) to the calculation in the CRD limit (dashed–dotted line). This limit has been obtained by setting the elastic collision rate , while keeping all other parameters, including the depolarizing collision rates, as they were. In other words, we artificially modify the branching ratios so that all scattering processes occur through the redistribution matrix. For both lines considered, the intensity is greater at line center when using the CRD approximation than when PRD is considered. This can be qualitatively understood as follows. In the limit of CRD, the scattered radiation at a given frequency depends on a weighted average of the incoming radiation over the whole line profile, and it thus takes into account that, for an absorption line, the intensity increases going from the core to the wings. On the other hand, in the limit of CS the scattered radiation is related only to the incoming radiation at that same frequency. PRD phenomena relax such coherency, relating the scattering radiation to the incoming one, but over an interval that is generally smaller than the one considered in the CRD case. This same reasoning explains why, in PRD, the intensity is instead larger in the wings. Figure 3. Intensity (top row) and Stokes Q / I (bottom row) profiles of the emergent radiation calculated for an LOS with in the absence of a magnetic field, for the Sr ii line at 4078 Å (left column) and for the Sr i line at 4607 Å (right column), considering the FAL-C atmospheric model. In each figure, the results for the full PRD calculation (solid line) and the results obtained in the CRD limit (dashed–dotted line) are plotted. The reference direction of positive Q / I is taken perpendicular to the vertical direction. Figure 3 clearly shows that the difference between CRD and PRD calculations is, as expected, rather small in the Sr i 4607 Å line. In particular, we expect that, once the Stokes profiles are smeared to properly account for the effects of the atmosphere’s macroturbulent velocity and of the finite spectral resolution of a typical instrument, the differences between both calculations will be negligible. The