Gaia is an ESA mission which will measure the positions, distances, space motions, and many physical characteristics
for one billion stars in our Galaxy and beyond. The satellite will have three instruments on board: the
Astrometric Field for the astrometry; the Blue and Red Photometers for the low-resolution spectroscopy and
the Radial Velocity Spectrometer which will allow measuring radial velocities. This paper will give an overview
of the processing of the dispersed images for BP and RP: the data will be corrected for CCD related effects in
the pre-processing, where also the sky background and the flux contamination from neighbouring sources will be
removed; the data will be then internally calibrated to the same "mean instrument" and externally calibrated
to obtain spectrum and flux in physical units, which will be stored in the final catalogue.