The absolute flux calibration of the photometer is based on models of standard stars in the 0.6-15 Jy flux range in the three filters: αBoo (Arcturus), αTau, αCMa (Sirius), αCeti & γDra (see Dehaes et al. 2010) and on thermophysical models for a set of more than 10 asteroids (Müller & Lagerros 1998, 2002), building up on similar approaches for ISOPHOT (Schulz et al. 2002) and Akari-FIS (Shirahata et al. 2009). Together they cover a flux range from below 100 mJy up to 300 Jy. Both types of sources agree very well in all 3 PACS bands, and the established absolute flux calibration is consistent within 5%. Neptune and Uranus with flux levels of several hundred Jansky are already close to the saturation limits, but have been used for flux validation purposes. At those flux levels, a reduction in response of up to 10% has been observed. For comparison, the latest FIR flux model of Neptune is considered to be accurate to better than 5% (Raphael Moreno, priv. comm. & Fletcher at al. 2010).

For the 5 primary PACS standard stars, the measured absolute flux accuracy is within 5% of predicted values (Aussel et al. 2010) in the blue and green filters, and within 10% of the predicted values in the red filter. For the full set of primary and secondary standards, the measured absolute flux accuracy is within 10% in the blue and green, and better than 20% in the red filter.

Absolute flux calibration uncertainties should improve over the mission, with better statistics of available celestial calibration observations.

The calibration itself comprises: (i) flat-fielding, (ii) responsivity correction -- conversion of engineering units, volts, to Jy/pixel, and (iii) gain drift correction to account to small drifts in gain with time.

The flux calibration of the PACS photometer assumes: