Comparison of Bidirectional Transmission Distribution Function (BTDF) Measurements on Fused Silica and Sintered Polytetrafluoroethylene Diffusers
Catherine C. Cooksey, James J. Butler, Georgi T. Georgiev
A number of satellite-based, Earth remote sensing instruments have incorporated transmissive diffusers for the purpose of on-orbit spectral radiance calibration with the majority of these diffusers being comprised of single or multi-paned ground quartz. For these applications, accurate determination of the bidirectional transmissive distribution function (BTDF) of the diffusers is critical. While the comparison of bidirectional reflectance distribution function (BRDF) measurements on reflective diffusers has been the subject of a number of publications, comparison of BTDF measurements on transmissive diffusers has been infrequent if non-existent. This study presents the results of the comparison of BTDF measurements by the NASA Goddard Space Flight Center (GSFC) Diffuser Calibration Laboratory (DCL) and the National Institute of Standards and Technology (NIST) Spectral Tri-function Automated Reference Reflectometer (STARR) facility on two transmissive diffuse samples: HOD-500*, a synthetic fused silica sample manufactured by Hereaus Quarzglass, and Spectralon-250*, a sintered polytetrafluoroethylene sample manufactured by Labsphere, Incorporated. BTDF measurements were acquired at seven wavelengths from 290 nm to 740nm, at incident angles of 0° and 30°, and at transmissive scatter angles from 165° to 179°. For all measurements on both samples, the NASA GSFC DCL data agreed with the NIST STARR data to within their combined measurement uncertainties.