Iridescence effects, quench marks, leopard marks… The names given to optical anisotropy in toughened and heat-strengthened glass are diverse and widely used by facade contractors, architects and glass suppliers. The general term –and technically more accurate- comprising all previous definitions “anisotropy” is not fully understood from the fundamental physical and optical reasons governing it. The principles of anisotropy are presented as to provide a general overview of this phenomenon aimed to introduce later the latest technology enabling on-line monitoring through automated polariscopic techniques in tempering lines. The difficulties to objectively quantify and evaluate anisotropy in real glass units are exposed and a study about a spatial-statistical approach based in textural analysis of photoelastic images by means of Grey Level Co-occurrence Matrices is also introduced. This approach, in combination with actual first order statistic assessment using p-quantiles  could provide an additional tool for benchmarking and comparison purposes during production of heat treated glass.
In the last decades the architectural glazing industry has experienced an enormous technical evolution. New challenging energetic, structural and visual demands increase progressively the complexity of glazing products and the
Anisotropy is the term used in physics to characterize an object or phenomenon having different properties in different directions, opposite to isotropy. In building glass, anisotropic effects appear in particular
The number of variables influencing anisotropy visibility in heat treated glass makes tremendously impractical and inefficient any attempt to predict the potential visual appearance of glass at the intended building
Accounting for the specific data processing, optical measurement techniques and the image acquisition system used, actual approaches followed by industry and research universities differ significantly although, as remarked previously, they
As seen, different methods rely on different techniques to assess anisotropic effects. The most accurate provide precise measurements of light retardation measured all over the glass surfaces. However, the photoelastic
The Grey Level Co-Occurrence Matrix (GLCM)
The co-occurrence matrix is generally understood as a two-dimensional histogram of the number of times that pairs of intensity values occur in a given
The overview of anisotropic effects, their causes and impact in architectural glass have been presented together with the problematic related with the quantification and evaluation of such a complex and
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