SB extinction α^l for Hα (solid), Ca II 8542 Å (solid), He II 304 Å (dashed), Si IV 1394 Å (dot-dashed), Fe IX 171 Å (dot-dashed) and Fe XII 193 Å (dotted) as function of temperature, for total hydrogen densities N_H = 10¹⁴ and 10¹² cm^-3 (upper and lower panel, respectively). The two cross-over solid curves near the bottom are the competing neutral hydrogen and electron density, with scales on the right. The extinction scales at left and the density scales at right shift upward between the panels to compensate for the density decrease. The horizontal line at log α^l = −7 marks optical thickness unity for a slab of 100 km geometrical thickness.