gonioreflectometry

Gonioreflectometry is the measurement of how light is reflected from a surface as a function of the geometry of illumination and observation. It characterizes the angular distribution of reflected light, typically in terms of the bidirectional reflectance distribution function (BRDF), which describes how reflectance varies with incident and viewing directions. The technique is used for materials characterization, optical engineering, and computer graphics, among other fields.

In a typical gonioreflectometric setup, a light source projects a beam onto the sample at a specified incident angle and azimuth, while a detector or camera measures the reflected radiance at a defined viewing angle and azimuth. By varying the angles over a chosen grid, the angular reflectance map is obtained. Some systems incorporate polarization analysis, spectral selection, or imaging capabilities to capture wavelength- or polarization-dependent BRDFs. Measurements may be performed on flat samples or with more complex geometries, and can be either specular-dominated or diffuse-dominated depending on the material.

Data from gonioreflectometry are often analyzed by fitting physical or semi-empirical reflectance models, or by extracting diffuse and specular components. The resulting BRDF/BSDF data are used in applications such as physically based rendering, remote sensing, quality control of coatings and pigments, and material science research. Imaging-based variants extend the approach to map angular reflectance across a surface, producing spatially resolved reflectance information.

Equipment typically includes a motorized goniometer or turntable, a controllable light source, a detector or spectroradiometer, and, in some configurations, polarization analyzers and cameras.