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Gravitational lensing by massive galaxy clusters allows study of thepopulation of intrinsically faint infrared galaxies that lie below thesensitivity and confusion limits of current infrared and submillimetertelescopes. We present ultra-deep PACS 100 and 160 microns observations towardthe cluster lens Abell 2218, to penetrate the Herschel confusion limit. Wederive source counts down to a flux density of 1 mJy at 100 microns and 2 mJyat 160 microns, aided by strong gravitational lensing. At these levels, sourcedensities are 20 and 10 beams/source in the two bands, approaching sourcedensity confusion at 160 microns. The slope of the counts below the turnover ofthe Euclidean-normalized differential curve is constrained in both bands and isconsistent with most of the recent backwards evolutionary models. Byintegrating number counts over the flux range accessed by Abell 2218 lensing(0.94-35 mJy at 100 microns and 1.47-35 mJy at 160 microns, we retrieve acosmic infrared background (CIB) surface brightness of ~8.0 and ~9.9 nW m^-2sr^-1, in the respective bands. These values correspond to 55% (+/- 24%) and77% (+/- 31%) of DIRBE direct measurements. Combining Abell 2218 results withwider/shallower fields, these figures increase to 62% (+/- 25%) and 88% (+/-32%) CIB total fractions, resolved at 100 and 160 microns, disregarding thehigh uncertainties of DIRBE absolute values.

Herschel deep far-infrared counts through the Abell 2218 cluster-lens