Examples of critical intensity in Na I D₁. Left panel: three background intensities (green lines) and their corresponding emergent intensities (black lines) on top of the critical intensity spectra computed for different values of atomic orientation (see discrete color bar) and normalized to S_line = min(S). The gray lines are references corresponding to . Middle panel: Stokes V signals corresponding to the deepest intensity profile. The case is made more visible by the dashed black line. Right panel: same signals as in middle panel but plotted in 2D.