Departure coefficients for non-LTE spectral lines. This plots shows the dependence with optical depth log τ_c of the ratio of upper and lower level departure coefficients β_u/β_l. Color lines correspond to each of the analyzed spectral lines formed under non-LTE conditions: Mg I b₂ (blue), Na I D₁ (green) and Ca II (red). They were determined for the VALC model and used to initialize the inversion (see Sect. 3.1). The horizontal dashed line indicates the region where β_u = β_l and therefore the source function S(λ) corresponds to Planck’s function B(λ) (Sect. B).