  A computational procedure is presented to estimate the spectral shape of the
ionizing background between 1 and 10 Ryd by analyzing optically thin absorption
systems in the spectra of high redshift quasars. The procedure is based on the
response surface methodology from the theory of experimental design. The shape
of the recovered UV background at z~3 shows a significant intensity decrease
between 3 and 4 Ryd compared to the metagalactic spectrum of Haardt & Madau
(1996). This decrease is interpreted as produced by HeII Gunn-Peterson effect.
There are no features indicating a contribution from galaxies to the UV
background which is therefore dominated by QSOs at z~3.
