An illuminated Solar Diffuser is the calibration source for the VIS/NIR bands on the NPOESS/VIIRS sensor. We
completed a set of BRF measurements to fully characterize the distribution of scattered light from the solar diffuser.
NPOESS/VIIRS has an overall VIS/NIR radiometric calibration uncertainty requirement of 2%(1 sigma), of which
1.32% was allocated to the characterization of the BRF. In order to meet this requirement, we modified the existing
goniometer and measurement procedure used on MODIS. Modifications include sample yoke redesign, periodic
measurements of the lamp polarization coupled with stability measurements, modifications to source optics, and stray
light reduction. We measured BRF in 6 spectral wavebands for 9 out-of-plane illumination angles and 2 view angles.
We achieved NIST traceable measurements with an uncertainty ranging from 1.09% to 1.32%. Our measurements of a
smaller Spectralon<sup>TM</sup> sample match NIST measurements of the same sample to better than 0.5%. These requirements
are nominally the same as achieved on MODIS. As a result of instrument upgrades, we currently meet this overall
uncertainty while having included additional uncertainty terms.