  We discuss optical associations, spectral energy distributions and
photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman
Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24,
70, 160 mu) data have been associated with optical UgriZ data from the INT Wide
Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The
spectral energy distributions of selected ELAIS sources in N1 detected by
SWIRE, most with spectroscopic redshifts, are modelled in terms of a simple set
of galaxy and quasar templates in the optical and near infrared, and with a set
of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN
dust torus) in the mid infrared. The optical data, together with the IRAC 3.6
and 4.5 mu data, have been used to determine photometric redshifts. For
galaxies with known spectroscopic redshifts there is a notable improvement in
the photometric redshift when the IRAC data are used, with a reduction in the
rms scatter from 10% in (1+z) to 7%. The photometric redshifts are used to
derive the 3.6 and 24 mu redshift distribution and to compare this with the
predictions of models. For those sources with a clear mid infrared excess,
relative to the galaxy starlight model used for the optical and near infrared,
the mid and far infrared data are modelled in terms of the same dust emission
templates. The proportions found of each template type are: cirrus 31%, M82
29%, Arp 220 10%, AGN dust tori 29%. The distribution of the different infrared
sed types in the L_{ir}/L_{opt} versus L_{ir} plane, where L_{ir} and L_{opt}
are the infrared and optical bolometric luminosities, is discussed.
