Line formation plot with all factors that make up the contribution function (Eq. (3)) of the EB He I D₃ emission profile at location L_em (see Fig. 5) and at a heliocentric viewing angle of μ = 1. Top left: opacity χ_ν divided by optical depth τ_ν. The h_max, ℓ surface (Eq. (2)) is shown as a solid red line and the vertical velocity v_z as a solid white line. Top right: total source function (Eq. (4)). The vertical velocity v_z is shown as a solid white line, the temperature T as a dot-dashed line, and the line source function S^l in temperature units as a dotted line. Bottom left: τ_νe^−τ_ν, which displays the height range where τ_ν is close to 1. The τ_ν = 1 surface is shown as a solid red line and the vertical velocity v_z as a solid white line. Bottom right: total contribution function. The emergent intensity is shown on a temperature scale as a yellow solid line.