Variation with frequency of the normalized source function, for different values of the optical depth at line center across the atmosphere. Dashed lines indicate the (constant with frequency) CRD values at τ = 0,  1,  10,  100,  10³,  10⁴ for comparison, where both S_CRD and S(x = 0) increase with τ. It satisfactorily reproduces the standard PRD results using Hummer’s R_I − A (see also Hummer 1969, Fig. 1c).