Optical depth variations of normalised source functions for the 1 ↔ 3 line of a three-level hydrogen atom in the frame of XRD approximation. We compare FNLTE results (coloured lines) for various frequencies x = 0, 1, 2, 3, 4 with XRD reference solutions (open black circle) computed using the MALI-XRD method of Sampoorna et al. (2013). The dashed black line shows the CRD solution computed using the MALI-CRD method.