We present results of ultra-deep ISOCAM observations through a cluster-lens at 7 \mu m and 15 \mu m with the Infrared Space Observatory ( ISO ) satellite .
These observations reveal a large number of luminous Mid-Infrared ( MIR ) sources .
Cross-identification in the optical and Near-Infrared ( NIR ) wavebands shows that about half of the 7 \mu m sources are cluster galaxies .
The other 7 \mu m and almost all 15 \mu m sources are identified as lensed distant galaxies .
Thanks to the gravitational amplification they constitute the faintest MIR detected sources , allowing us to extend the number counts in both the 7 and 15 \mu m bands .
In particular , we find that the 15 \mu m counts have a steep slope \alpha _ { 15 \mu { m } } = -1.5 \pm 0.3 and are large , with N _ { 15 \mu { m } } ( > 30 \mu { Jy } ) = 13 \pm 5 arcmin ^ { -2 } .
These numbers rule out non-evolutionary models and favour very strong evolution .
Down to our counts limit , we found that the resolved 7 \mu m and 15 \mu m background radiation intensity is respectively ( 2 \pm 0.5 ) \times 10 ^ { -9 } and ( 5 \pm 1 ) \times 10 ^ { -9 } W m ^ { -2 } sr ^ { -1 } .