Multi-level source function description of the Ly-α spectral line synthesized from the FALC atmosphere. Panel (a) shows the computed Ly-α source function from RH ($S_{{\nu }_0}^l(T_{\text{exec}})$) as well as the three components describing the multi-level source function, the Planck function B_ν0(T_e), the mean radiation field $\overline{J}(T_{\mathrm{rad}})$, and the interlocking source function B_ν0(T^⋆) in units of brightness temperature. The black dashed line (top of the atmosphere) shows the normalized contribution function to intensity for the wavelength position indicated with a black vertical in Panel (c). Panel (b) illustrates the scattering probability σ, destruction probability ϵ, and interlocking probability η. Panel (c) shows the emergent line profile in units of brightness temperature as well as the optical depth unity height with the axis to the right. The icon in the top left corner indicates that two-level processes dominate the Ly-α source function.