  We present the first results of our followup programme of optical imaging and
spectroscopy of a deep 12 micron survey conducted with the ISO satellite
(Clements et al. 1999). We find that the objects are typically of fairly low
redshift (z ~ 0.1-0.2), but with a tail that extends to high redshifts. The
highest redshift object is a previously unknown quasar at z=1.2. The sample of
objects for which spectroscopy has been obtained forms a complete subsample of
the original survey with an R band magnitude limit of 19.6. We are thus able to
use accessible volume methods to determine the luminosity function. We find
that the luminosity function is close to the low redshift 12 micron luminosity
function determined by IRAS (Fang et al., 1998; Rush et al., 1993), but with a
possible excess at the highest luminosities, which also correspond to the
highest redshifts. This excess is compatible with the rapid luminosity
evolution of L ~ (1+z)^{4.5} claimed by Xu (2000) and suggested by Elbaz (1999)
on the basis of number counts. These conclusions remain tentative because of
small number statistics at the highest redshifts. Almost all of the galaxies in
this survey are emission line objects. We use the spectra to determine emission
line diagnostics for the underlying ionisation source. We find that the
majority of objects have HII region-like spectra (roughly 2/3) but there is a
significant fraction (about 1/3) that contain an AGN. This confirms suspicions
that deep mid-IR surveys will be a powerful way to find obscured AGN. Part of
the survey area has been imaged deeply by the INT Wide Field Camera. Nearly all
12 micron sources in this area have good optical identifications. The prospects
are thus good for obtaining complete spectroscopy.
