Robot-based Gonioreflectometer

Measurements of diffuse reflection are important for a variety of applications in optical metrology. In reflectometry both the spectral and the spatial distribution of radiation diffusely reflected by solid materials are measured and characterised with the indication of classification numbers. In practice, reflection measurements of diffuse reflecting materials are accomplished predominantly relative to a standard. As primary standard for this purpose the perfectly reflecting diffuser (PRD) is established, which reflects the incoming radiation loss-free, completely diffuse and with a lambertian direction characteristic. The PRD is a theoretical concept only, which cannot be realised experimentally respectively materially. Since there is no material with these characteristics, the realisation of primary standards is carried out with physical methods, i.e. by the measuring apparatus itself, in the context of an absolute measurement. In contrast to the directed, specular reflection, the incoming light in diffuse reflection is distributed over all directions in space. If the radiance L of the reflected radiation is independent of the direction, one speaks of a lambertian emitter. The classification numbers, in the case of reflection are the reflectance ρ, the radiance factor β and the BRDF “bidirectional radiance distribution function” fr. These classification numbers are not material-specific but depend on a multiplicity of parameters, e.g. the wavelength of the radiation, the direction of irradiation and reflection as well as the aperture angle. There are, in principle, nine different measuring geometries for a reflection measurement. They consists of all possible combinations, where the incident and the reflected radiation is in each case hemispherical, conical or directional. Therefore it is always necessary to specify the measurement geometry for a material classification number. The hemispherical measuring methods can be accomplished thereby with an integrating sphere whereas directional geometries are realised with a gonioreflectometer. The name gonioreflectometer is used in this context for a device which allows the precise control of the angles of the incident and reflected optical beams in a reflection measurement. This publication deals with the first time utilization of a commercial 5-axis industrial robot as a sample holder in a gonioreflectometer.