Three-dimensional appearance characterization of diffuse standard reflection materials

The basis for the description of diffuse reflecting materials is the concept of the radiance factor beta, or as a quite similar depiction, the bidirectional reflectance distribution function f(r). Both characterizations use the concept of the perfectly reflecting diffuser (PRD), which reflects, by definition, the incoming radiation loss-free, completely diffuse and with Lambertian direction characteristics. The PRD is a theoretical concept only, which cannot be realized materially. Since there is no material with these characteristics, the realization is carried out with physical methods, i.e. by the measuring apparatus itself, in the context of an absolute measurement. For practical purposes, radiance factor measurements are predominantly accomplished relative to commercially available reflection standards. In the present investigation, different widely used diffuse reflection materials were measured for the first time in a multi-geometry configuration with the robot-based gonioreflectometer of the Physikalisch-Technische Bundesanstalt (PTB) in order to characterize their three-dimensional reflection behaviour. For a set of four distinct incident angles, the full hemispherical reflection indicatrix was determined at a wavelength of 550 nm. The angle-resolved reflection data are an important reference for manufacturers, providers and users of radiometric and photometric products. This paper attempts to give users in research and industry an overview of the strongly non-Lambertian reflection behaviour of standard reflection materials, because it is a widespread false assumption that commonly used standard reflection materials have only minor deviations from the ideal specification of the PRD.