Determination of Fit Basis Functions

U. S. Patent 7,092,852, issued August 15, 2006

When sensitive absorption techniques like wavelength modulation or cavity-enhanced spectroscopy are used to measure spectra, the concentrations that can be inferred from the spectra are rarely limited by white noise. Instead, artifacts from optical interference fringes produce a structured baseline that slowly varies in time. This invention describes a way to infer the spectral background by measuring a series of spectra over a time interval that encompasses some of the drift of the background. Singular value decomposition is used to extract the background vectors from this time series; these background vectors are then incorporated into a least squares fit to determine the concentration. The result is greater stability of the measurement. The figure shows results obtained when this method was applied to the determination of water vapor at part-per-billion levels. With the dynamic background correction, the variance was significantly reduces and showed less trending. The same spectra were analyzed in each case, but with the baseline correction the detection limit of the instrument was improved through a software modification.

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