NPL scales for radiance factor and total diffuse reflectance

Abstract

The National Reference Reflectometer has been developed by the National Physical Laboratory (NPL) to realize scales for radiance factor and total diffuse reflectance, the latter scale being obtained through the spatial integration of goniometric measurements of reflectance. Analysis of its performance in the spectral region from 400 nm to 1000 nm shows that for spectrally neutral white materials such as matte white tiles and SpectralonTM plaques the instrument is capable of realizing 0/45 radiance factor measurements with an uncertainty of 0.2% (k = 2) and 0/d total diffuse reflectance measurements with an uncertainty of approximately 0.25% (k = 2). The uncertainties depend on the variation of reflectance with angle, and there will therefore be some dependence on the material being measured. This instrument is now used to establish UK diffuse reflectance scales in the visible part of the spectrum.