Comparison of the emergent intensity and the source function of an EB He I D₃ emission profile. Top panel: emergent intensity. Three vertical lines in black, blue, and red correspond to three selected frequencies ν_cont, ν_abs, and ν_em. Bottom panel: total source function at three different frequencies (black), (blue), and (red). The indications h_cont, h_abs, and h_em correspond to the heights at which τ(ν_cont), τ(ν_abs), and τ(ν_em) are equal to one.