The concept of “anisotropy” in connection with thermally toughened architectural glass is of concern to architects, glass finishers, experts, façade constructors and building owners. At national and international specialist conferences, this term, which refers to the optical appearance of glass façades in the form of grey, white or rainbow-colored spots, rings or stripes, is a focal point of seminars and presentations. The reason for this is that clients, façade consultants and architects have long been placing very high demands on the transparency of thermally toughened glass in windows and façades. Innovative online measurement methods (scanners) also allow the recording and visualization of anisotropies.

The aim of the “Anisotropies” working group in the façade technology working group of the Fachverband Konstruktiver Glasbau e. V. is to develop an initial guide to the complex subject of anisotropies and thus to provide architects, glass finishers, experts, façade constructors and building owners with a tool that describes the boundary conditions and influencing parameters as well as their effect on the evaluation of anisotropies. In addition, the aim is to work out what increases anisotropy effects, as well as which factors in the manufacturing process and which factors independent of the production process contribute to the visual perceptibility of these effects.

To this end, well over 100 monolithic glass panes from FKG members were measured by the research institutes involved in the working group using suitable, statistically quantifiable measurement methods based on the physical principles of stress optics and the further development of digital image processing and then visually evaluated by the working group. The measurement methods include pixel-precise recording of the path difference (delay) in the glass surface and evaluation using statistical methods. The aim is to gradually pave the way for the objective evaluation of anisotropies. In addition, the measurement of the glass samples should enable glass finishers to evaluate a correlation between the process parameters and the presence of anisotropies and to work on modifying the corresponding process parameters.