Lockman-SWIRE field is even deeper and the 5-$\sigma$ noise level is 13.6, 11.2 and 16.2 mJy at 250, 350 and 500 $\micron$, respectively.

![Three-colour SPIRE images of the 11.3 deg$^2$ Boötes filed (top), the 15.2 deg$^2$ Lockman-SWIRE field (middle) and the 21.6 deg$^2$ [*XMM*]{}-LSS field (bottom).[]{data-label="fig:lss14_sky"}](fig2_image.eps){height="8.9in" width="3.35in"}

The CIB at sub-mm wavelengths is dominated by emission from DSFGs over a wide range of redshift, while other potential sources of signal, e.g., the cosmic microwave background (CMB), Sunyaev-Zel’dovich (SZ) effect, and radio galaxies, are subdominant. One exception is the emission from Galactic dust (cirrus) which cannot be ignored at FIR/sub-mm wavelengths. Therefore, the SPIRE map signal can be modelled, to first order, as the sum of the following three terms: the emission from DSFGs; Galactic cirrus emission; and instrument noise. Thus we can write $$I_{\rm sky} = I_{\rm DSFG} + I_{\rm cirrus} + I_{\rm noise}.$$ We expect that only the emission from DSFGs have intrinsic correlation with the quasars. This is because some quasars could be strong IR/sub-mm emitters from dust heated by the accreting black hole, as well as star formation in the host galaxy (e.g. Lutz et al. 2008; Shi et al. 2009; Clements et al. 2009; Serjeant et 