Top panels: departure coefficients β for the lower (solid lines) and the upper 4p ³P₂ (dashed lines) levels of the Si i 10 827 Å line plotted against the atmospheric height. Bottom panels: the line source function S_lu for the Stokes parameter I of the Si i 10 827 Å line versus height. The left and right panels show results for the granular and intergranular 1D models corresponding, respectively, to the spatial grid points (ix = 28, iy = 10) and (ix = 59, iy = 10) in the ⟨ | B_Z | ⟩ = 80 G snapshot. Arrows mark the formation heights of the line core. The curves of different colors indicate the β and S_lu values calculated using three different atomic models of the silicon atom. The black dotted curve shows the line source function for the two-level atom approximation, which is equal to the mean intensity averaged over the line profile. The black thin solid curve shows the Planck function. The scaling factor S_H = 0.1.