Generation of synthetic remote FTIR interferograms

Development of chemical detection algorithms for open-path FTIR is complicated due to the restricted release of the target compounds. The testing and verification of these algorithms are often impossible since the toxic target compounds can never be released outside the laboratory. A method of generating synthetic FTIR data by introduction of laboratory absorbance spectra into previously collected clean reference backgrounds facilitates this algorithm testing. A method of synthesizing `field grade' spectra has several additional advantages over actual open air releases. The cost of synthetic data is insignificant compared to the cost of data collected at actual open air releases. Unlike open air releases, the synthetic data's concentration pathlength (CL) and cloud temperature (Tc) are precisely known and easily modified. This capability makes it possible to evaluate the precise limit of detection for each of the algorithms being considered. This method works equally well for passive FTIR (using natural radiation as the source) or active FTIR (using a hot source). It is shown that synthetic spectra and real spectra of a target cloud at the same concentration pathlength and cloud temperature are in good agreement.