Long-term comparison of spectralon BRDF measurements in the ultraviolet

Abstract

Long-term (i.e. multi-year) measurements of the Bidirectional Reflectance Distribution Function (BRDF) of three laboratory Spectralon diffuse targets in the ultraviolet are presented. The Spectralon targets were used in the pre-launch radiance calibration of the Solar Backscatter Ultraviolet/2 (SBUV/2) satellite instruments on NOAA 14 and 16. The BRDF data were obtained between 1994 and 2003 using the scatterometer located in the National Aeronautics and Space Administration's Goddard Space Flight Center (NASA's GSFC) Diffuser Calibration Facility (DCaF). The targets were measured at 13 wavelengths between 230 nm and 425 nm and at incident and scatter angles used in the SBUV/2 pre-launch calibration. With the exception of a spurious measurement in 1995, the percent difference in the measured BRDF of the first target, designated H1, was within ±0.7 % from 252nm to 425nm between 1994 and 2000. The percent difference in the measured BRDF of the second target, designated H2, was also within ±0.7 % over the same spectral range between 1997 and 2003. At 230 nm, the H1 and H2 BRDF measurements show larger differences primarily due to reduced signal to noise in the measurements. The combined measurement uncertainty of the reported BRDF measurements is 1.0% (k=1). The comparison also shows how the ultraviolet BRDF of these Spectralon samples changed over time under cleanroom deployment conditions.