Departure of the VDF of the naturally broadened upper level (f₂(u, τ)) of a two-level atom from the Maxwellian equilibrium distribution f ^M(u) at different line center optical depths within the atmosphere (indicated in the figure legend). For comparison, the corresponding quantity at τ = 1 for the CS case is shown as a dashed line. There is clearly a greater overpopulation of f₂ at large u in the CS case than in the present case of a two-level atom with a naturally broadened upper level.