Frequency variation of normalised source function S₁₂ of 1 ↔ 2 line at different optical depths τ for a two-level atom. Horizontal dashed black lines show CRD solutions for τ = 0, 1, 10, 100, 10³, 10⁴. We compare FNLTE results (coloured lines) with Hummer’s PRD results (black open circle). CRD and PRD solutions were computed using the ALI and FBF methods, respectively. Clearly, we accurately recover the results presented by Hummer (1969) and PSP23.