  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.
