Deviation of white diffuse reflectance standards from perfect reflecting diffuser at visible and near-infrared spectral ranges

The assumption that the reflectance of white diffuse reflectance standards is identical to that of the PRD allows these standards to be used to characterize reflectance or radiance factors of any surface at any irradiation/collection geometry simply by comparison. However, this assumption is only true within certain limits, and, for some applications, requirements may be out of those limits. PTB and IO-CSIC have studied the variation of the reflectance with respect to the bidirectional geometry for the four most typical white diffuse materials (barium sulfate, opal glass, ceramic and Spectralon). We have defined descriptors in order to more clearly quantify the spectral reflectance variation with the bidirectional geometries. The values obtained for these descriptors have been separately presented for the visible and near-infrared spectral ranges. In both spectral ranges, deviations of white diffuse reflectance standards with respect to the PRD were found, regarding both Lambertian behaviour and spectral constancy.