Oke & Korycansky (1982), studying a sample of high redshift QSOs, defined two depression factors, and , characterizing the amount of absorption between Ly and Ly and between Ly and the Lyman limit respectively: , where and are the observed and intrinsic fluxes per unit wavelength in the QSO restframe. is defined in the same way as. We adopted the estimates given by Madau (1995) for the attenuation of the continuum due to line blanketing as a function of redshift. He found the flux decrements averaged over all the lines of sight to be
where Å and Å. The coefficient relative to the Ly forest contribution is equal to . The main contribution to , representing the flux decrement caused by all Lyman series lines, belongs to Ly, Ly (Å) and Ly (Å), whose coefficients are , and . Figure 5 shows the depression factors and as computed from equations 2 and 3. The fact that implies that Ly and higher order lines contribute significantly to line blanketing. We have applied Madau's prescriptions to compute the opacity of IGM through the mean factors and , whereas we approximated the Lyman continuum absorption setting for both observed and synthetic SEDs below , where Å is the Lyman limit.