Spherically symmetric test models for the corrected Feautrier coefficients. In all models the incident intensity at the core was set to I⁺ = 4, while at the outer boundary no incoming radiation was assumed. The upper panel shows the uncorrected model, and the lower panel shows the same setup with the correction applied. In this model we assumed an optically thick line (τ_line = 50), and a low background true opacity and source function.