The Planck function B_ν (solid), mean radiation J_ν (dotted), and source function S_ν (dashed) averaged over the first 100 Å of the Mg I edges with thresholds at λ = 2512.4 and 1621.5 Å. Left: Kurucz model. Right: FCHHT-B model. Formal representative temperatures are plotted for these quantities: the electron temperature for B_ν, radiation temperature for J_ν, and excitation temperature for S_ν. They are obtained by applying the inverse Planck function to B_ν,J_ν,S_ν, respectively. The dot-dashed distributions at the bottom are the intensity contribution functions j_ν   exp( − τ_ν), with j_ν the emissivity and τ_ν the optical depth,near the thresholds wavelengths. The lefthand deeper-formed curve is for the 2512 Å edge. Each curve is scaled to its maximum value.