Emergent Stokes I (top panels) and Q/I (bottom panels) profiles of the Ca I 4227 Å line at μ = 0.034 (left panels) and μ = 0.966 (right panels) calculated in the FAL-C atmospheric model in the absence of magnetic fields. 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).