Effects of stray light in FTIR instruments on open-path measurements

摘要

The single-beam spectra recorded with two commercially available monostatic FT-IR open-path monitors exhibited nonzero signal intensities in the wave number regions where water vapor and CO$-2$/ totally absorb. Each instrument yielded a measureable spectral response with the transmitting/receiving telescope blocked. This response is attributed to stray light within the FT-IR system. In some cases the stray light contributed up to 17% of the total return intensity measured along a 414-m path length. Although the baseline region of the single-beam spectra can be corrected for stray light, its effect on the spectral absorbance is not uniform over the range of absorbance values recorded in long-path measruements. Thus, the concentration of species measured along the path cannot be easily corrected, which has a deleterious effect on the accuracy of the spectral data.