For calculation of the integration time per polariser the user gives
the error in the polarisation measurement 
and the expected
polarisation
from which the error in polarisation angle 
is
determined.
For polarimetric measurements the error in the
measurement of the polarisation degree 
is used as an indicator for the quality of the measurement rather than
the S/N ratio. is described as
is the polariser efficiency (see Tab. 14
and 15) and 
the S/N
ratio of the measurement for one polariser setting.
The error of the polarisation angle is:
where is the effective polarisation of the source, which is defined
by:
is the polarisation degree of the source, 
the solid
angle (see Tab. 9), 
the background flux
assumed to be unpolarised,
the total source flux, and 
the fraction of the
point spread
function (see Tab. 2 for PHT-P and Tab. 4 for PHT-C).
The measurement procedure for polarimetry is different from that of
normal photometry and, therefore, is calculated in a
different way. In order to measure the polarisation of the source a
sequence of three measurements is performed each with a polariser put
into the beam. The polariser angles are 0, 120, and 240 degrees,
respectively. From these three measurements the Stokes parameters can be
determined and the combined polarisation degree of the source and the
underlying background. If the same measurement is performed on a
background reference position the polarisation of the background can be
determined independently. The polarisation of the source can be measured
more accurately if the background is faint.
This leads to the following description of :
Note that the difference of the S/N ratios is calculated here since no differential measurement with background subtraction is performed. This would require summation of the NEPs in order to obtain the differential S/N.
The S/N ratio of the source plus background is:
The power from a point source and its background incident on the detector is
and for extended sources:
e is the obscuration factor of the secondary mirror of the ISO
telescope, where 
, and 
is the
polariser transmission (see Tab. 14 and 15).
and are the source and the background
fluxes, respectively.
with the following noise term for point sources:
and for extended sources:
The background noise is:
The constants C1 and C2 are tabulated in Tab. 10.
The background S/N ratio is:
where 
is the power of the background incident on the
detector:
and
The integration time is:
Note that the considerations above only apply to relatively weakly polarised
sources (
).
