  All observations by the aperture photometer (PHT-P) and the far-infrared
(FIR) camera section (PHT-C) of ISOPHOT included reference measurements against
stable internal fine calibration sources (FCS) to correct for temporal drifts
in detector responsivities. The FCSs were absolutely calibrated in-orbit
against stars, asteroids and planets, covering wavelengths from 3.2 to 240
micron. We present the calibration concept for point sources within a
flux-range from 60 mJy up to 4500 Jy for staring and raster observations in
standard configurations and discuss the requisite measurements and the
uncertainties involved. In this process we correct for instrumental effects
like nonlinearities, signal transients, time variable dark current,
misalignments and diffraction effects. A set of formulae is developed that
describes the calibration from signal-level to flux densities. The scatter of
10 to 20 % of the individual data points around the derived calibration
relations is a measure of the consistency and typical accuracy of the
calibration. The reproducibility over longer periods of time is better than 10
%. The calibration tables and algorithms have been implemented in the final
versions of the software for offline processing and interactive analysis.
