Passive Fourier-transform infrared spectroscopy of chemical plumes: an algorithm for quantitative interpretation and...

We present a ratioing algorithm for quantitative analysis of the passive Fourier-transform infrared spectrum of a chemical plume. We show that the transmission of a near-field plume is given by τ(plume) = (L(obsd) - L(bb-plume))/(L(bkgd) - L(bb-plume)), where τ(plume) is the frequency-dependent transmission of the plume, L(obsd) is the spectral radiance of the scene that contains the plume, L(bkgd) is the spectral radiance of the same scene without the plume, and L(bb-plume) is the spectral radiance of a blackbody at the plume temperature. The algorithm simultaneously achieves background removal, elimination of the spectrometer internal signature, and quantification of the plume spectral transmission. It has applications to both real-time processing for plume visualization and quantitative measurements of plume column densities. The plume temperature (L(bb-plume)), which is not always precisely known, can have a profound effect on the quantitative interpretation of the algorithm and is discussed in detail. Finally, we provide an illustrative example of the use of the algorithm on a trichloroethylene and acetone plume.