Polarization diagrams of the Ca I 4227 line-center emergent radiation at μ = 1, obtained from CRD, PRD–AA, and PRD–AD calculations in the FAL-C atmospheric model, in the presence of a height-independent horizontal (θ_B = π/2) magnetic field of 20 G. The circular markers indicate the effective calculations, carried out for the azimuths χ_B = n π/8 (with n = 0, ..., 15). Left panel: Comparison between CRD and PRD–AA calculations. Right panel: Comparison between CRD, PRD–AA, and PRD–AD calculations (in the PRD–AA and CRD diagrams the circular marker indicates a magnetic field direction with azimuth χ_B = 0, whereas the arrow refers to χ_B = π/4). The reference direction for positive Stokes Q is parallel to the y-axis of the considered reference system.