The stress embedded in the glass plate as a result of the heat treatment process is the cause of the phenomenon

The term "nisotropy" is used in the façade industry to describe the appearance of patterns and colorful areas in heat-treated glass when viewed under specific lighting and viewing conditions, such as sunlight. Stresses embedded in the glass plate as a result of the heat treatment process are responsible for this phenomenon. In order for a display cabinet door to be bi-refractive or doubly-refractive, the level and distribution of stresses must be controlled. 

This is accomplished through (non-uniform) temperature distribution and heat transfer, which are affected by, for example and without limitation, contact between a glass plate and supporting rollers, oscillation of the insulated glass (IGU) plate, the layout and operation of heating pipes and quenching nozzles (note that tempering by air floatation technology was not reviewed during this study).

An earlier study by Redner and Bhat revealed that the non-uniform distribution of stresses as a result of the tempering process was a result of the process itself. An intricate layout of glass areas that alter the behavior of (polarised) light in a relatively different manner results in a specific (anisotropic) pattern as a result of a complex thermodynamic process and intricate layout of display cabinet insulated glass door areas

FIGURES 1 and 2 depict an example of an in-house modification to the temperature control system that was implemented by a leading display cabinet door supplier in his quest to reduce the visibility of anisotropy. It was discovered that making changes to the rollers' design resulted in a more uniform heat distribution, which eliminated the central longitudinal band (indicated by the red arrow in Figure), and thus resulted in an anisotropy pattern that was more homogeneous and, more importantly, less disturbing.

For example, CWCT Technical Note 35 (Assessing the appearance of glass) and the British standard governing the heat strengthened glass process both recognize anisotropy as an unavoidable characteristic of heat-treated glass when exposed to specific light conditions.

In the past, façade specifications have always been in accordance with such standards; however, specifications have become increasingly stringent in recent years, frequently requiring  with reduced anisotropy or anisotropy-free display cabinet door and effectively redefining the phenomenon as a defect for which insulated glass (IGU) may be rejected. On the other hand, the façade industry is struggling to meet such demands due to the limitations of glass processing, which limits the amount of material that can be produced by the industry.

Furthermore, acceptance and rejection of glass due to anisotropy at the factory and on-site are primarily based on subjective inspection criteria and supported by extensive sampling, as it is unclear whether and how anisotropy can be measured or controlled. When it comes to this phenomenon, there are often disagreements that can lead to disputes between the cladding supply chain, which does not consider it a defect, and designers/specifiers and their clients, who do consider it a flaw and want to avoid it.