Ratios f₂/f^M as a function of the normalized velocity u, for different radiation field profiles, and for ε = 0 and ζ = 0. The same linestyle convention as in Fig. 1 was adopted. The magnitude of these departures from a Maxwellian velocity distribution depends on both (i) the distance of the peaks of I vs. line center, and (ii) the amount of its central reversal characterized by I_max./I(0).