Disk-center (μ = 1) equivalent width contribution functions, ℬ(log τ_c), of a weak (artificial) Fe ii line with excitation potential χ = 10 eV, at wavelengths λ 850 nm (top) and λ 1600 nm (bottom), evaluated according to the weak line approximation (Eqs. (B.5), (B.7)), for a single snapshot of the 3D model, the corresponding ⟨ 3D ⟩ average model, and the associated 1D LHD model used in this work. The contribution functions, originally defined on the monochromatic optical depth scale, have been transformed to the Rosseland optical depth scale as a common reference.