All targets used for flux calibration are point sources in the ISOPHOT wavelength range. For an extended source which is much larger than the aperture or pixel, the surface brightness is derived by involving the effective solid angle of the aperture or pixel.
For the P subsystems, the effective solid angle for each aperture corresponds to the area of the aperture in the focal plane projected on the sky. The areas were determined from the measured physical dimensions of the apertures.
The conversion from point source flux 
in Jy to a surface
brightness can be written for PHT-P as follows:
where 
is the obscuration factor for the ISO secondary mirror,
in sr is the effective solid angle of the aperture,
for a given filter 
. The PHT-C surface brightness is derived from the
in-band powers of each pixel. The computation for PHT-C is as follows
(with the same notation):
A model of detector footprints (or beam profile) is used to
determine the effective solid angles of the C100 and C200 detector
pixels
for each filter band. The model takes into
account the ISO primary mirror, secondary mirror, the tripod supporting
the secondary, and the linear sizes of the detectors as measured
pre-flight.
For PHT-S the values of
for each pixel were determined
by combining in-flight calibration observations of the detector footprints
inside the aperture and model calculations for the shape of the footprint
outside the aperture. These values are included in the conversion from
signal to extended source flux density.
![\begin{displaymath}
I_{\nu}^f =
\frac{F_{\nu}^f{f_{PSF}^f}} {(1-\epsilon^2){{\Omega}_{eff}^f}}
~~~~{\rm [MJy\,sr^{-1}]},
\end{displaymath}](img316.gif)
![\begin{displaymath}
I_{\nu}^f =
\frac{{k^f}{P^f(i)}}{C1^f(1-\epsilon^2){{\Omega}_{eff}^f}}
~~~~{\rm [MJy\,sr^{-1}]},
\end{displaymath}](img319.gif)