Centre-to-limb variation of $\overline{Q}/\overline{I}$ (left panel) and $\overline{U}/\overline{I}$ (right panel) signals as function of μ = cos(θ). The results are obtained for χ = 0 and with the FAL-C atmospheric model, considering uniform horizontal (θ_B = π/2 and χ_B = 0) magnetic fields of different strengths (see the legend). Dash-dotted, dashed, and solid lines correspond to CRD, PRD–AA, and PRD–AD calculations, respectively.