  The ionizing spectral energy distribution of quasars exhibits a steepening of
the distribution short-ward of 1200 A. The change of the power-law index from
approximately -1 to -2 so far has been interpreted as being intrinsic to
quasars. We study an alternative interpretation, in which a tenuous absorption
screen is responsible for the change of index. We find that the most successful
function of HI with z requires that the ionization fraction be independent of
the metagalactic background radiation, that is frozen in. This implies that the
putative component should be of sufficiently low density that it could not
recombine (while the background is subsiding) and/or cool from a high
temperature state.
