Left panel: upper limit at 95% CL on the optical depth as a function of energy (solid line) as derived from Eq. (1). The optical depth distribution derived with different EBL models at specific redshifts is shown for comparison (dash-dotted line: Kneiske & Dole 2010; dashed line: Gilmore et al. 2009, “low” model; dotted line: Franceschini et al. 2008). Right panel: the minimum difference δ_min,E0 between the maximum optical depth derived here and the optical depths predicted by the same EBL models is shown as a function of the redshift. E₀ is the energy for which, at a given redshift, the difference between the derived and predicted optical depths is minimized. The upper limit on the redshift, for each EBL model under consideration, corresponds to δ_min,E0 = 0.