  Using our photoionization code CIRRUS, we show that the study of proximity
effect on metal elements provides a possibility of placing constraints on the
spectral shape of the metagalactic UV background (MUVB). From theoretical point
of view, the most optimum indicator of the spectral shape of the MUVB is the
ratio OIV/OIII. Unlike previous thought, the ratio SiIV/CIV is not a good
indicator of the spectral shape because of its strong dependence on an
ionization parameter as well. We also find that the observed excess of CIV
systems in luminous QSOs may be a photoionization effect, but not a
gravitational lensing effect.
