Fraction of obscured sources as a function of the source flux for three surveys: SwiftFT (red solid points), ELAIS-S1 (magenta open points), C-COSMOS (blue open dots). The black solid line represents the X-ray background synthesis model by Gilli et al. (2007). The red dotted line represents the fraction of obscured sources for the SwiftFT survey, by zeroing the flux limits in the hard band. Upper panel: S sample. Lower panel: H sample.