Comparison of predicted column density ratios to the observations for various UV background models from the literature. The adopted ionizing spectra are from Haardt & Madau (2001, HM01), Faucher-Giguère et al. (2009, FG09), Madau & Haardt (2009), assuming equilibrium conditions (MH09/HM+S) or delayed helium reionization with He ii/H i boosted by factors 10, 30, and 500 (MH09/DR10, MH09/DR30, MH09/DR500), respectively, and our recovered SEDs (see text). Shaded areas mark the observed ratio of Si ii/Si iii (upper panel, wider range) and Al ii/Al iii (upper panel, narrow range) for the z = 1.7529 system and C iii/C iv for the systems at z = 2.3799 (middle panel) and z = 2.8266 (lower panel).