A series of five neutral density filters is located in a filter wheel of the fore-optics and can be inserted into the optical path should the intensity of incoming radiation require reduction. The filters increase in density by factors of about 3 up to a density of about 300 (Kerr, 2002). For operational measurements of ozone using the standard DOAS technique (described in Section 6.5), there is no need to account for the presence of a filter. This is because the filters are nearly neutral, so they attenuated all wavelengths nearly equal, and there is essentially zero differential optical absorption from wavelength to wavelength. In reality, the filters are not exactly neutral; however, optical density gradients are linear with wavelength and any effects linear with wavelength are removed by the Brewer DOAS technique for measuring ozone (see Section 6.5).
Knowledge of the attenuation characteristics of the neutral density filters is essential for absolute spectroscopy applications and for measurements of aerosol optical depth. Absolute applications include the measurement of global or direct spectral UV irradiance. Routine measurements are made of global spectral UV irradiance (Kerr and McElroy, 1993) and special measurements have been developed to measure direct irradiance (Bais, 1997; Grobner and Kerr, 2001). For measurements of aerosol optical depth (e.g., Bais, 1997; Kerr, 2002), accurate knowledge of the optical thickness of the neutral density filters is crucial since it is difficult to distinguish between the nearly neutral optical depth of a filter with that of atmospheric aerosols.
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