Stokes V normalized to the continuum intensity in the Na ID₁ line for B = 0, constant w₁, and for different constant boundary illuminations (α₂ = I₀/S) and positive values of α₁ = S_V/S. Top (T_ν0 = 1): opacity is not enough to create the peaks. Middle (T_ν0 = 5): peaks are clearly formed. Bottom: (T_ν0 = 10): peaks are fully contrasted and start to saturate. The level of the central dip depends on the Stokes V emissivity.