Ball Aerospace uses several techniques in radiance calibrations of the SBUV/2 instruments. The instrument Primary Test Fixture (PTF) and Normal Incidence Test Fixture (NITF) both use Spectralon diffusers as radiance targets. Diffuser BRDF (Bidirectional Reflectance Distribution Function) is measured for a central spot at several scatter angles and at several wavelengths. Weighted BRDF is then calculated across the instrument FOV, based on diffuser BRDF measurements, spatial uniformity test data, instrument vignetting, and test geometry. This weighted BRDF curve is then fitted spectrally to determine BRDF at each wavelength of the SBUV/2 instrument. The PTF and NITF have their own BRDF curves, since each fixture has a unique diffuser plate and test geometry. A third test fixture is used for the last SBUV/2 instrument radiance calibration, using a Labsphere Uniform Source System (USS) and an external source for reference. The large aperture of the sphere provides a uniform radiance target with no need for BRDF knowledge. Comparison of instrument calibrations from all three radiance targets shows a small discrepancy of about ±1% among these calibration methods, which indicates that BRDF calculations for both PTF and NITF test diffusers are acceptable.
In recent years, the consistency of ultraviolet solar spectral irradiance measurements has improved to the extent that broad band averages of solar irradiances agree to within a few percent over ranges of a few hundred nanometers. Over spectral ranges from tens to tenths of nanometers significantly larger disagreements in spectral irradiances are observed. Some well known factors which contribute to measurement uncertainties over smaller wavelength intervals are wavelength calibration errors, uncertainties in radiometric standards especially in the transition region from one standard to another, and differences and uncertainties in slit scattering function between instruments. Extensive pre-launch radiometric calibration of SBUV-2 ozone monitoring instruments in air and vacuum have indicated new sources of radiometric calibration uncertainties. These are effects of Woods anomalies in grating efficiency and wavelength dependent changes (sometimes with significant structure) in the reflectance of MgF2 overcoated aluminum surfaces which seems to be associated with the absorption and desorption of water vapor by the MgF2 thin film. These effects combine to produce wavelength dependent radiometric calibration differences between air and vacuum conditions of as much as 10 percent or more. These results indicate that one should not assume that an accurate radiometric calibration in air is valid in space for the wavelength region of 200 - 400 nm.
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