We present the results of a multiband survey for brown dwarfs in the Chamaeleon~I dark cloud with the Wide Field Imager ( WFI ) camera at the ESO/MPG 2.2-m telescope on La Silla ( Chile ) .
The survey has revealed a substantial population of brown dwarfs in this southern star forming region .
Candidates were selected from R , I and H \alpha 
imaging observations .
We also observed in two medium-band filters , M855 and M915 , for the purpose of spectral type determination .
The former filter covers a wavelength range containing spectral features characteristic of M-dwarfs , while the latter lies in a relatively featureless wavelength region for these late-type objects .
A correlation was found between spectral type and ( M855–M915 ) colour index for mid- to late M-type objects and early L-type dwarfs .
With this method , we identify most of our object candidates as being of spectral type M5 or later .
Our results show that there is no strong drop in the number of objects for the latest spectral types , hence brown dwarfs may be as abundant as low-mass stars in this region .
Also , both kind of objects have a similar spatial distribution .
We derive an index \alpha = 0.6 \pm 0.1 of the mass function in this region of dispersed star formation , in good agreement with the values obtained in other star forming regions and young clusters .
Some of the brown dwarfs have strong H \alpha 
emission , suggesting mass accretion .
For objects with published infrared photometry , we find that strong H \alpha emission is related to a mid-infrared excess , indicative of the existence of a circumstellar disk .