BRDF Characterization of Solar Diffuser for JPSS using PASCAL

The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite launched on October 28, 2011 as well as the upcoming Joint Polar Satellite System (JPSS). VIIRS collects radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the visible and infrared spectrum from 0.4 to 12.5 mm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 mm wavelength range (M1 – M11, I1 – I3) range is performed by measuring the sunlight reflectance from Solar Diffuser assembly (diffuser is Spectralon™). The reflected sun light is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon™. This paper presents the BRDF measurements of the Spectralon™ for JPSS in the 0.4 – 0.9 mm wavelength using PASCAL (Polarization And Scatter Characterization Analysis of Lambertian materials) with an uncertainty better than 1.2%. PASCAL makes absolute measurements of BRDF in an analogous fashion National Institute of Standards and Technology (NIST) Spectral Tri-function Automated Reflectance Reflectometer (STARR) facility. The entire incident light beam is collected by the detector assembly. With the sample in place, a precision circular aperture with a viewing region larger than the illuminated spot is used to collect the light. BRDF is calculated on the basis of the incident power and geometric factors eliminating the need for a standard characterized at another laboratory. Spectral definition is achieved with band pass filters. As in typical BRDF instruments, the detector rotates about the sample in the plane defined by the source beam and the azimuth rotation of the sample. Unique additional features of this instrument include the ability to vary the sample elevation and roll / clock the sample about its normal. PASCAL is set up to mount the flight Solar Diffuser assembly (Spectralon™ is 23.7 X 28.6 cm). The ability to change elevation and roll the sample about the surface normal enables measuring the BRDF in the as use configuration of VIIRS including source, Solar Diffuser, and detector geometry. A description of PASCAL, validation and the BRDF results of the Solar Diffuser will be presented