  The shape of the UV ionizing background is reconstructed from optically thin
metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143,
and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are
analyzed by means of the Monte Carlo Inversion method completed with the
spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 <
z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z <
1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua
and the variability of the spectral shape comes from diversity of spectral
indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered
spectral shapes show intensity depression between 3 and 4 Ryd due to HeII
Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium
(true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the
depth of this depression corresponds within 1-2sigma to the values directly
measured from the HI/HeII Ly-alpha forest towards the quasars studied. The
observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and
eta can be explained by the combined action of variable spectral softness and
differences in the mean gas density between the absorbing clouds. Neither of
the recovered spectral shapes show features which can be attributed to the
putative input of radiation from soft sources like starburst galaxies.
