Our adopted IR spectra of various galaxy populations. The short-dashed cyan line corresponds to our adopted spectrum for the moderate-luminosity LIRG star-forming population, while the red long-dashed curve is the spectrum of high-luminosity ULIRG sources. In both cases the spectra are similar to that of the prototype star forming galaxy M 82 (in the range from 5 to 18 μm it is precisely the ISOCAM CVF spectrum of M 82). The lower dotted line corresponds to a low-luminosity inactive spiral (see Sect. 3.2 and AF2010), while the upper dotted line is closer to that of ULIRGs. The lower dot-dash green line is the average type-I AGN spectrum. The boundaries of the MIPS 24 μm filter are also shown in the source rest-frames at various source redshifts. The open squares are the average fluxes estimated by Fadda et al. (2010) for a faint sample of LIRGs at z ~ 1 and ULIRGs at z ~ 2. The two blue filled circles at 127 and 182 μm and the red filled datapoint at 100 μm are the average fluxes for LIRGs and ULIRGs from Herschel observations.