archive data.

The resulting survey, CFBDSIR, identifies astrophysical sources on $J$-band WIRCam [@Puget.2004] images and selects ultracool brown dwarfs candidates depending upon their $z'-J$ colours, using $z'$ magnitudes from CFBDS MegaCam [@Boulade.2003proc] images. The $z'-J$ colour has a wide range for brown dwarfs, varying from 2.6 for mid-L types to over 4.5 for late-T types (Fig. \[zJspT\]). It therefore provides (at least at a good signal-to-noise ratio) a good selection criteria to identify ultracool brown dwarfs. They usually have $z'-J>$3.8, as confirmed by atmosphere models such as BT-Settl [@Allard.2007], synthetic MegaCam/WIRCam colours derived from known objects [@Delorme.2008b] and direct observational data [@Delorme.2008a; @Burningham.2008; @Burningham.2009]. Models suggest that cooler objects, not yet discovered (T$<$500K), are even redder. Figure  \[zJspT\] however shows that the colour-spectral type relation for late T-dwarfs has a wide spread, meaning that this $z'-J$ colour range also contains some warmer mid-T dwarfs. Follow-up photometry of candidates in $H$ and $K_{\rm s}$ band easily distinguishes the relatively blue very late T from the redder mid-T-dwarfs [see @Lodieu.2007 for instance]. Only very rare astrophysical objects share the same $z'JHK$ colour range: very high-redshift starburst-galaxies, extremely red Balmer break galaxies, and atypical broad absorption-lines quasars at moderately high redshift ($z\geq 2$), 