In order to make sure that the line shape is reproduced correctly by the instrument, cell measurements are made and analysed with LINEFIT. The knowledge of the instrumental line shape (ILS) is very important, otherwise wrong gas concentrations are deduced from the atmospheric measurements.
In principle, the ILS can be determined either with a
single mode laser or with cell measurements. While a laser beam is tricky
to use correctly, cell measurements are made quite easily. But, for the
analysis of cell spectra a software is needed, for example LINEFIT. Particularly
for ILS investigations a cell has been designed by Frank
Hase 
. Please have a look on cell.pdf giving
recommendations for the use of gas cells.
Similar to the analysis of atmospheric spectra cell spectra
are analysed by fitting synthetic spectra to measured ones. In the case
of cell spectra the gas volume probed is well known (p,T, composition),
and ILS parameters like modulation efficiency and line assymmetry are fitted.
LINEFIT includes a simple and an extended ILS modelling: While the simple
model assumes a linear decrease of modulation eff. and constant phase error
along the optical path difference (OPD) the extended version fits these
parameters as function of OPD. For further details please see linefit.pdf.
Fig.1: ILS of a slightly misaligned FTIR.
Furthermore, LINEFIT package includes ALIGN.
ALIGN calculates the ILS and the Haidinger fringes in
dependance of adjustment parameters like lateral displacement of the retroreflector
or of the entrance aperture. The use of ALIGN is
LINEFIT has been written by Frank
Hase . It is available and widely used within the NDSC IR community
and by BRUKER. If you're also interested, please contact us.
 |
Atmospheric
Trace Constituents and Remote Sensing
Atmosphärische Spurenstoffe und Fernerkundung |