A plane-parallel semi-infinite homogeneous atmosphere is considered. An algorithm for the determination of effective optical depth of absorption line formation is given. The anizotropic scattering case is studied in detail. Our results of numerical calculations of the value τ_e(μ₀) are given for the case of scattering angle γ= π for various values of single scattering albedo, λ, and the parameter of the Heyney — Greenstein phase function, g. A refined formula for the function T^m(-μ,μ₀) is presented.