BRDF measurement and calculation for SBUV/2 radiance calibration test diffuser

Abstract

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.