Anisotropies indicate a disturbing effect with thermally toughened safety glass. The pre-stressing process introduces different stresses into the glass which cause birefringence in the glass. These birefringences, also called anisotropies, are perceived in polarised light as disturbing optical effects. Polarised light is present in normal daylight.
Usually, such anisotropies are perceived as grey rings, stripes or leopard patterns. As the glass thickness increases, the effect becomes more pronounced. This can also be accentuated when occurring in tempered glass with modern solar protection coatings because coloured effects result from the grey patterns.
The topic of anisotropy has gained importance in specifications of glass for the architectural field. Although they are not a flaw according to current standards, architects, facade designers and builders are affected by this visible effect as it seriously impairs the visual appearance of a glass facade. This edition of ISOLAR® Compass raises the issue of the visual assessment of anisotropies and describes an objective measurement procedure that allows a robust and fair assessment of glass for anisotropies.