Rest-frame SEDs of Herschel-SWIRE 500 μm sources with extreme starburst luminosities (SFR > 5000 M _⊙ yr⁻¹), labelled with the redshift, whose optical through near-infrared SEDs are best-fitted by a QSO template. Alternative SWIRE associations are shown plotted below the extreme starburst solution. Possible redshift aliases (z _comb)are shown plotted above the preferred z _phot solution. Dotted loci: M 82 starburst, dashed loci: Arp 220 starburst, long-dashed loci: AGN dust torus. The constant C = 0 except in cases where SEDs are shown for both the photometric redshift and for a lower redshift alternative SWIRE association. In these cases C = −2 for the lower SED and +1 for the upper one.