  This paper provides an overview of the PACS photometer flux calibration
concept, in particular for the principal observation mode, the scan map. The
absolute flux calibration is tied to the photospheric models of five fiducial
stellar standards (alpha Boo, alpha Cet, alpha Tau, beta And, gamma Dra). The
data processing steps to arrive at a consistent and homogeneous calibration are
outlined. In the current state the relative photometric accuracy is around 2%
in all bands. Starting from the present calibration status, the
characterization and correction for instrumental effects affecting the relative
calibration accuracy is described and an outlook for the final achievable
calibration numbers is given. After including all the correction for the
instrumental effects, the relative photometric calibration accuracy
(repeatability) will be as good as 0.5% in the blue and green band and 2% in
the red band. This excellent calibration starts to reveal possible
inconsistencies between the models of the K-type and the M-type stellar
calibrators. The absolute calibration accuracy is therefore mainly limited by
the 5% uncertainty of the celestial standard models in all three bands. The
PACS bolometer response was extremely stable over the entire Herschel mission
and a single, time-independent response calibration file is sufficient for the
processing and calibration of the science observations. The dedicated
measurements of the internal calibration sources were needed only to
characterize secondary effects. No aging effects of the bolometer or the
filters have been found. Also, we found no signs of filter leaks. The PACS
photometric system is very well characterized with a constant energy spectrum
nu*Fnu = lambda*Flambda = const as a reference. Colour corrections for a wide
range of sources SEDs are determined and tabulated.
