Stray light is an inherent problem with spectrometers. Stray light is light that arrives at the focal plane at a wavelength position shifted from the primary wavelength position. The amount of stray light generally decreases with the difference between the shifted wavelength and the primary wavelength (Wardle et al., 1997). Use of holographic gratings reduces this problem; however, for single monochromators, stray light from nearby wavelengths is typically 10 to 10 times that of the primary wavelength.
Stray light is an important consideration for making UV measurements. The rapid increase of ozone absorption with decreasing wavelength (Fig. 6.3) causes UV radiation at the earth's surface to increase by several orders of magnitude over a relatively short wavelength range, particularly when the sun is low in the sky. This means that stray light from longer wavelengths can make up a significant fraction of the signal measured at shorter wavelengths where there is relatively little radiation.
It is possible to apply a correction for the stray light present in a single monochromator (Kerr, 2002). This is done by measuring the distribution of light from a monochromatic source (laser) that is dispersed in the spectrometer and scattered to nearby wavelengths from the primary wavelength. Once this dependence is known, it is possible to calculate the spectrum of stray light and subtract it from the measured spectrum.
The problem of stray light is significantly reduced with the double Brewer monochromator. In this case, the second pass through the recombining spectrometer effectively reduces stray light to about 10 to 10 times that of the primary wavelength (Wardle et al., 1997). Use of the double Brewer instrument increases the accuracy of ozone and UV measurements and extends the range of operation to lower sun angles.
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