Example of a BOSS quasar spectrum of at z = 2.91. The spectrograph resolution at λ ~ 400 nm is ~0.2 nm. The red and blue lines cover the forest region used here, λ_RF ∈ [104,120] nm. This region is sandwiched between the quasar’s Lyβ and Lyα emission lines at 102.572 nm and 121.567 nm respectively. The blue line is the model of the continuum for this particular quasar, C_q(λ_RF), and the red line is the product of the continuum and the mean absorption, , as calculated by the method described in Sect. 3.