Gravitational lensing by massive galaxy clusters allows study of the population of intrinsically faint infrared galaxies that lie below the sensitivity and confusion limits of current infrared and submillimeter telescopes .
We present ultra-deep PACS 100 and 160 \mu m observations toward the cluster lens Abell 2218 to penetrate the Herschel confusion limit .
We derive source counts down to a flux density of 1 mJy at 100 \mu m and 2 mJy at 160 \mu m , aided by strong gravitational lensing .
At these levels , source densities are 20 and 10 beams/source in the two bands , approaching source density confusion at 160 \mu m. The slope of the counts below the turnover of the Euclidean-normalized differential curve is constrained in both bands and is consistent with most of the recent backwards evolutionary models .
By integrating number counts over the flux range accessed by Abell 2218 lensing ( 0.94 - 35 mJy at 100 \mu m and 1.47 - 35 mJy at 160 \mu m ) , we retrieve a cosmic infrared background surface brightness of \sim 8.0 and \sim 9.9 nW m ^ { -2 } sr ^ { -1 } , in the respective bands .
These values correspond to 55 \pm 24 % and 77 \pm 31 % of DIRBE direct measurements .
Combining Abell 2218 results with wider/shallower fields , these figures increase to 62 \pm 25 % and 88 \pm 32 % CIB total fractions , resolved at 100 and 160 \mu m , disregarding the high uncertainties of DIRBE absolute values .