Determination of the Stray Light Levels in a Dispersive Near-Infrared Spectrometer with Trichloromethane

Abstract

The stray light level in a dispersive near-infrared (NIR) spectrometer was determined at 1152, 1410, 1692, and 1861 nm with the sharp absorption bands of trichloromethane. The stray light level in the dispersive spectrometer was found to be a function of wavelength and was greatest at the lower and upper extremes of the free spectral range of the instrument. Stray light measurements with and without a blocking filter showed that the majority of the stray light in the instrument was due to the overlap of higher diffraction orders produced by the diffraction grating. A theoretical model to predict the stray light in the instrument was developed. When compared with the model, the measured stray light levels were found to be consistent with those expected if the primary source of stray light was due to the overlap of higher diffraction orders in the monochromator of the spectrometer. Measurements of absorbance as a function of pathlength were made with the four absorption bands of trichloromethane (1152, 1410, 1692, and 1861 nm). After correction for the results for stray light, the linear regression data for the absorbance vs. pathlength plots were determined and reported. The measured slopes should be useful in checking the photometric accuracy of NIR spectrometers over the wavelength range from 1100 to 1900 nm.