The Infrared Space Observatory (ISO) was not designed for all-sky surveys. It was an observatory performing pointed observations of mainly point sources. However, the cold focal plane allowed to perform absolute sky brightness measurements with ISOPHOT, and the two and half year mission resulted in a large set of individual observations of the extended sky brightness at mid- and far-infrared wavelengths, in both photometric and spectrophotometric modes. The beam of <=3' was considerably smaller than the beam of COBE/DIRBE (42'), and could efficiently avoid point sources and cirrus structure. ISO's good filter coverage in the 3-200 um range as well as the possibility of performing mid-infrared spectrophotometry provide better spectral coverage than the previous satellites. The rejection of straylight from the Sun, Earth, and Moon has been proved to be excellent. Via its absolute photometric flux calibration, ISOPHOT can give an independent determination of the absolute brightness level of the zodiacal light (there is an unresolved zero point difference between the IRAS and DIRBE absolute calibrations in the far-infrared)

The philosophy of the ISOPHOT surface brightness observations and their general calibration scheme is described in Héraudeau et al. (2003a). However, the calibration implemented in the final version of the Off-Line Processing Software, was not the final word of ISOPHOT's absolute surface brightness calibration. After checking the consistency between the ISOPHOT and COBE/DIRBE brightness values (Héraudeau et al. 2003b) a complex calibration programme was defined in order to improve the reliability and accuracy of the ISOPHOT results (see the list of individual calibration investigations below). The newly developed algorithms and corrections have been implemented in PIA V11, a new version of the Phot Interactive Analysis software.

Transient correction

Reset interval correction at low signal level

Re-analysis of dark signal

Bypassing skylight in FCS measurements

Footprint analysis

Formalism of the surface brightness calibration

Systematic comparison with DIRBE

The following people contributed to the refinement of the absolute surface brightness calibration (in alphabetical order): Péter Ábrahám, Carlos del Burgo, Philippe Héraudeau, Uwe Herbstmeier, Mika Juvela, Csaba Kiss, Ulrich Klaas, Dietrich Lemke, Kalevi Mattila, L. Viktor Tóth

Implementation of the new results in PIA V11

Documentation of the new calibration results