In the first data that was obtained with the LWS instrument it
became clear that there were fringes in the spectra. During the
Performance verification phase the fringing was seen in a number
of spectra of different sources. It has been shown that the
fringes are strongest in extended sources and in point sources
that are observed off-axis. Also sources on a strong background
will have strong fringing. The exact cause of the fringing is
not yet known. The period of the fringes is constant in
wavenumber in all sources at about 3.6 cm 
.
The fringe period can be
described with an interference formula:
where d is the gap that causes the fringes, n is the order
of the fringe and 
is the wavelength of the fringes.
For two adjacent fringe peaks at wavelengths 1 and
2 we get:
The gap that was derived from the data taking in PV phase that contained fringes was 1.455 mm. The observers can use this gap to identify the fringes. the amplitude of the fringes depends on the wavelength they are observed at, the extend of the source, the strength of the source and the position of the source in the beam.
Currently a procedure is being written that can correct the data for fringes. This procedure may later be implemented in the ISO Spectrometer Analysis Package (ISAP), see also Chapter refch-post.