Determination of trace-gas amounts in plumes by the use of orthogonal digital filtering of thermal-emission spectra

The thermal emission of gases in a plume can be measured by a Fourier-transform spectrometer that is located some distance from the plume. In order to measure quantitatively the amount of a particular gas of interest, in general a large spectrally structured background must be removed. Differencing techniques, in which a measured background spectrum is subtracted from a measured spectrum believed to contain a target, often do not remove background spectral features adequately. The inadequacy of two-spectrum differencing techniques is due to the spatial and the temporal variations in a scene. We present a method by which to reduce spatial and temporal spectral clutter to instrument random noise, allowing the measurement of gas amounts in an effluent plume. The method is applied to simulated data and field data to show its effectiveness.