Two surfaces may look different despite the same colour and brightness. The appearance is characterized by additional optical and mechanical properties such as gloss, texture, transparency, granularity or glitter. The interaction of these visual attributes and in addition the illumination results in the appearance of a surface.
The appearance of a product is decisive for the choice of a customer and thus central to a variety of industries, including automotive, cosmetics, paper, printing, packaging, coatings and plastics industries. There is also an increased development of surfaces that create specific effects such as iridescence, translucence or gonio-chromaticity (colour change with angle).
In most cases, the assessment and quality assurance is performed by visual methods, which have several disadvantages (objectivity, communication between customers and suppliers, expenses, etc.). An objective assessment is therefore desirable in many areas.
For an objective evaluation of the appearance of surfaces gonio-reflectometry methods (i.e. angle-dependent backscatter measurements, ie BRDF measurements) are usually applied. A basic problem is the multi-dimensionality of the measured variable, as the quantity depends on the direction of observation the light incidence angle and the optical wavelength.
In addition, polarization sensitivities may be observed mainly in the direction of specular re-flection. For textured surfaces, the BRDF is position-dependent. In fluorescent materials, the backscattered light must be spectrally analysed. The main difficulty is to reduce the multi-dimensionality (which expresses itself in several terabytes of data and long measurement times) to a set of few parameters, which are chosen and known with sufficient accuracy that visual perception can be described objectively.
