Upper panel: 1-σ confusion noise as a function of the wavelength for different telescope diameters. We use the confusion noise given by the P(D) method (see Sect. 7.1) for this plot. Red triangles: Frayer et al. (2009)Spitzer/MIPS confusion measurements. Black diamonds: Nguyen et al. (2010): Herschel/SPIRE confusion noise measurements (5-σ_conf cut). Lower panel: resolved fraction of the CIB by sources brighter than 5-σ confusion noise (fluctuations) and the source density limit. Both panel: Red dotted line: telescope with a diameter of 0.85 m such as Spitzer. Black solid line: 3.29 m telescope such as Herschel. Green dashed line: 12 m telescope such a Atacama pathfinder experiment (APEX). Blue dot-dashed line: 15 m telescope such as the JCMT. Purple three dot-dashed line: 25 m such as the CCAT project. Asterisks: transition between the source density limitation (short wavelengths) and the fluctuation limitation (long wavelengths).