Non-linear FTIR open path data reduction algorithm

A nonlinear spectral curve fitting computer algorithm for the reduction of Fourier transform spectrometer data is being developed at the Air Force's Arnold Engineering Development Center. The algorithm is an adaptation of an existing algorithm utilized for plume signature data and optical gas diagnostics data reduction. The nonlinear algorithm is not constrained by the linear absorbance assumptions required for classical least squares (CLS). Therefore, the nonlinear algorithm allows for the determination of gas concentrations from high absorption level spectra as well as determination of other nonlinear parameters such as spectra shift and spectral resolution. In addition, the currently implemented algorithm has the ability to simultaneously determine the background spectra or I degrees. These capabilities could make the non-linear algorithm a welcome addition as a quality assurance and data reduction tool. Open path Fourier transform infrared (FTIR) spectra from various sources have been analyzed utilizing this algorithm in an attempt to determine its capabilities and limitations. As various data sets are analyzed the algorithm is further refined to make its application easier and faster. A brief description of the algorithm and the results of a comparative study between the non-linear algorithm and CLS is presented.