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We constrain the spectrum of the cosmic ultraviolet background radiation byfitting the observed abundance ratios carbon ions at $z\sim 2\hbox{--}3$ withthose expected from different models of the background radiation. We use therecently calculated modulation of the background radiation between 3 and 4 Ryddue to resonant line absorption by intergalactic HeII, and determine the ratiosof CIII to CIV expected at these redshifts, as functions of metallicity, gasdensity and temperature. Our analysis of the observed ratios shows that'delayed reionization' models, which assume a large fraction of HeII at$z\sim3$, is not favored by data. Our results suggest that HeII reionizationwas inhomogeneous, consistent with the predictions from recent simulations.

Carbon ionization states and the cosmic far-UV background with HeII absorption