Investigations and Applications of Angle-Resolved Measurements of Spectral Reflectance and Transmittance

This doctoral thesis explores the quantification of a material's appearance through angle-resolved measurements of spectral reflectance and transmittance. The appearance of a material often determines the quality of a product across various sectors of industry, entertainment, and research. Therefore, there is a need for accurately characterized reference standards, traceable to the international system of units (SI). Accordingly, this thesis focuses on bidirectional reflectance distribution function (BRDF) and bidirectional transmittance distribution function (BTDF) measurements of diffuse material in the visible and near-infrared wavelength range. Moreover, the thesis presents the development of an SI-traceable facility for the accurate determination of angle-resolved scattering quantities.

One application of BRDF includes the verification of proper operation of ray tracing models (RTM) using calibrated reference standards. The first part of the thesis introduces an SI-traceable 3D gonioreflectometer for BRDF measurements of an artificial sample. To find a suitable sample for the verification process, the 3D instrument systematically measured several material candidates, evaluating their BRDF characteristics. Subsequently, the instrument's uncertainty in BRDF measurements is characterized, and the reliability of RTM is checked by comparing simulated and measured BRDF data of an artificial sample based on the best material candidate.

The following sections of the thesis are dedicated to exploring the practical applications of BTDF. The sections present the development of a facility for SI-traceable BTDF measurements, featuring an absolute gonioreflectometer extended to BTDF measurements in the visible and near-infrared wavelength range. The instrument specializes in measurements of matte surfaces that uniformly transmit light in all directions. Validation of the absolute instrument's capabilities in BTDF measurements was conducted through a detailed uncertainty evaluation and also a comparison measurement with a commercial spectrophotometer. Furthermore, the thesis discusses the adaptation of the 3D instrument for out-of-plane measurements of BTDF, recognizing the utility of materials that have anisotropic scattering distributions.

The fourth part of the thesis offers considerations on the definition of BTDF in the context of thick sample measurement. The thesis presents a method for correcting measured BTDF as a function of viewing zenith angle based on the geometry of the facility. Furthermore, the method was validated by using two instruments exhibiting varying degrees of sensitivity to sample thickness. This work provides tools for unambiguous treatment of the BTDF of thick samples.