A PHENOMENON OF GROWING IMPORTANCE
Architects are always looking for perfectly flat glass in their façade, where no optical distortion or color variations are visible. Although roller-wave, overall bow and edge lift are well known phenomena limited by the standards, the situation is different with anisotropy.
The same standards which regulate maximum allowable optical distortion, are accepting anisotropy as inherent in all heat-strengthened and fully tempered glass. Although anisotropy is a physical phenomenon, its appearance is sometimes hardly accepted. Actual specifications are trying to limit the appearance of anisotropy on the façade.
Saint-Gobain Glass has dedicated itself to this topic since 2015 to understand the causes of anisotropy and its effects, develop knowledge to limit the eventual optical phenomenon, inform project owners and architects of the issue and help to attain tailored anisotropy quality.
A summary of the knowledge around anisotropy has been done in the HANDBOOK ON ANISOTROPY EFFECTS ON GLASS FAÇADES from September 2023 is available below.
TOWARDS A UNIVERSAL MEASUREMENT
A lot of efforts have been made by the glass industry to minimize the visual appearance of anisotropy. On-line measurement equipment has been developed allowing to improve the homogeneity of the heating and cooling of the glass during the heat treatment process by modifying the settings of the tempering oven parameters. A quantification based on the photo-elastic theory allows calculation of the optical retardation. All these developments have resulted in the ASTM standard C1901-21e2, but all of this is related to only the monolithic heat-treated glass.
MEASURING IS NECESSARY, BUT NOT SUFFICIENT
Current anisotropy inspection systems qualify monolithic – uncoated as well as coated – glass panes giving information on the retardation map of the heat-treated glass coming out of the tempering line. This enables operators to adjust the tempering furnace parameters if necessary.
Although the in-line scanners allow considerable improvement in the homogeneity of the appearance of each single heat-treated glass, they still don’t allow us to predict and limit the visibility of the anisotropy. The actual visibility or perception of the anisotropic effects on the façade depends not only on the level of optical delay measured by the scanner, but also on the optical properties of the measured glass. For example, a coating might decrease or enhance the visibility of anisotropic effects.
PERCEPTION GREATLY DEPENDS ON THE COATING PROPERTIES…
Saint-Gobain Glass has been working on providing the right products for the market. Thanks to its knowledge on glass coatings, a new way to uniquely characterize their effect on quench mark visibility was developed. The σQM (sigma-QM) parameter is Saint-Gobain Glass’ solution to quantify the impact of coatings on the perceived anisotropy quality. The σQM considers the way light interacts with the coated glass, including the interferometric effects occurring into the coating. It is computed considering the lighting conditions that most enhance the visibility of anisotropy effects (see WHAT MAKES ANISOTROPY EFFECTS VISIBLE). The σQM is an indication of the shift between the
nominal colour of a coated glass and the actual colour perceived in the presence of quench marks on the coated glass. By combining the σQM with the results of anisotropy scanners, Saint-Gobain Glass created a new quality criterion for monolithic coated glass after heat-treatment. Designed to be integrated in industrial anisotropy scanners, this “monoscore” enables processors to select the coated glass according to their full anisotropy quality right after the tempering furnace.
A detailed article has already been published on GlassOnWeb by Saint-Gobain Glass in July 2022 and is available here.
… AND OF THE IGU COMPOSITION ALSO
Furthermore, most heat-treated glass is further processed into laminated glass and/or insulating glass units (IGU). The presence of multiple glass panes, potentially coated, can affect the actual visibility of the anisotropy of the final Insulated glazing unit (IGU) assembly.
With the current equipment, operators receive real-time information on the level of anisotropy of each single heat-treated glass but this isn’t sufficient to answer the real questions: What is the influence of the solar reflective coating? Will the coating mask the anisotropy of the inner pane of the IGU? Or will the inner pane, with or without a thermal low-E coating increase the level of anisotropy of the outer pane? The interaction of the inner pane with the outer pane of an IGU is more important when coatings with low outside light reflection and a high light transmission are used. Measurements combined with a survey, where people graded the level of anisotropy, allow us to link the optical properties of a coated glass with the appearance of anisotropy.
A survey was organized by Saint-Gobain Glass among people of different backgrounds i.e. sales & marketing, technical people, R&D and so-called specifiers who have daily contacts with architects. This is discussed in the technical paper INFLUENCE OF COATINGS ON THE PERCEPTION OF THE ANISOTROPY PHENOMENON available below.
The outcome of a survey showed that the coating has a major influence on the appearance of the finished unit. Solar control reflective coatings with a high light transmission and low outside light reflection are showing less anisotropy for a same optical retardation. On the other hand, these solar control coatings are more sensitive to the quality of the tempering of the inner pane. So depending on the chosen solar control reflective coating, the assembly into an IGU becomes more or less critical. Hence simply limiting the optical retardation of the single heat-treated glass doesn’t guarantee a certain level of anisotropy of the finished IGU.
GOING BEYOND: QUALIFYING THE VISIBILITY OF ANISOTROPY EFFECT ON FINAL PRODUCT
Further work needs to be done to predict the effective level of anisotropy of the finished unit based on the optical retardation measured on the single heat-treated glass and the type of solar control (reflective) coating used.
In the future, inspection systems would have to grade the anisotropy quality of final products such as IGU. An “IGU anisotropy score” would predict how visible the quench marks will be once the glazing unit is installed in its target façade, whatever the IGU composition. Such a scoring system would consider all the elements of an IGU: the different glass substrates but also the applied coatings.
This would make it possible to address the uncertainty about anisotropy when purchasing an IGU for a facade project. Not only would it be possible to know in advance what the perceived quality of the
glazing installed, it would also be possible to define the anisotropy quality class required for the most
demanding projects. These IGU quality classes would guarantee the beauty of the building’s façade in all weathers, in all places and over time.