Emergent Stokes I (first row), Q/I (second row), U/I (third row), and V/I (fourth row) profiles for the Ca I 4227 Å line at μ = 0.034 (left column) and μ = 0.966 (right column) calculated in the FAL-C atmospheric model in the presence of a horizontal (θ_B = π/2,  χ_B = 0) magnetic field with B = 20 G. Calculations take into account PRD effects considering the exact expression of R^III (blue lines) and the R^III−CRD approximation (green marked lines). The reference direction for positive Q is taken parallel to the limb. The red dotted lines report the error between R^III and R^III−CRD calculations, given by Eq. (15).