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

B Bernad,K-O Hauer,C Strothkämper,A Pons,A Schirmacher,J Campos,T Quast,A Ferrero

doi:10.1088/1681-7575/ab3351

Abstract

The assumption that the reflectance of white diffuse reflectance standards is identical to that of the perfect reflecting diffuser (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), at in- and out-of plane geometries and at spectral range from 380 nm to 1700 nm. 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. The observed deviation from the BRDF is in general very large for high incidence and collection angles (reaching in many cases 20%). Therefore, it is not possible to assume Lambertianity in standards at those geometries when calibrating measuring systems.

Highlights

White diffuse reflectance standards are supposed to realize the reflectance properties of the perfect reflecting diffuser (PRD), defined as a diffuser exhibiting isotropic diffuse reflection with a reflectance equal to one [1]
The values obtained for these descriptors have been separately presented for the visible and near-infrared spectral ranges
As in [15], the bidirectional reflectance distribution function (BRDF) spectra [ fr(θi, φi; θr, φr; λ)] were nor malized with respect to the BRDF spectrum at the conventional bidirectional geometry 0◦ : 45◦ [θi = 0◦, θr = 45◦, φi = 0◦, φr = 180◦], which will be used as reference geometry:

Summary

Introduction

White diffuse reflectance standards are supposed to realize the reflectance properties of the perfect reflecting diffuser (PRD), defined as a diffuser exhibiting isotropic diffuse reflection with a reflectance equal to one [1]. 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 by comparison. This assumption is only true within certain limits, and, for some applications, Metrologia 56 (2019) 055005 requirements may be out of those limits. For this reason, it is a very common practice to provide measurements of reflectance factors of standards at directional-hemispherical or bidirectional geometries, whose values are the more different the less Lambertian is the material. It seems very convenient to investigate the variation of the reflectance factor of the most widely-used white diffuse reflectance standards for a number of representative bidirectional geometries, and at 0°:45° or 45°:0°

Results

Discussion

Conclusion