  We present a re--analysis of the European Large Area ISO Survey (ELAIS) 90mum
observations carried out with ISOPHOT, an instrument on board the ESA's
Infrared Space Observatory (ISO). With more than 12 sq. deg., the ELAIS survey
is the largest area covered by ISO in a single program and is about one order
of magnitude deeper than the IRAS 100mum survey. The data analysis is presented
and was mainly performed with the Phot Interactive Analysis software but using
the pairwise method of Stickel et al. (2003) for signal processing from ERD
(Edited Raw Data) to SCP (Signal per Chopper Plateau). The ELAIS 90mum
catalogue contains 229 reliable sources with fluxes larger than 70 mJy and is
available at www.blackwell-synergy.com. Number counts are presented and show an
excess above the no-evolution model prediction. This confirms the strong
evolution detected at shorter(15mum) and longer (170mum) wavelengths in other
ISO surveys. The ELAIS counts are in agreement with previous works at 90mum and
in particular with the deeper counts extracted from the Lockman hole
observations. Comparison with recent evolutionary models show that the models
of Franceschini et al. and Guiderdoni et al. which includes a
heavily-extinguished population of galaxies give the best fit to the data.
Deeper observations are nevertheless required to better discriminate between
the model predictions in the far-infrared and are scheduled with the Spitzer
Space Telescope which already started operating and will also be performed by
ASTRO-F.
