Understanding birefringence in architectural glass
In modern architecture, the visual and structural integrity of glass facades is essential. Birefringence, a phenomenon that affects tempered glass, can significantly alter the appearance of a building, especially under certain lighting conditions. This article examines the causes, effects, and simulations of birefringence in tempered glass.
The tempering process and its impact on glass properties
The tempering process strongly increases the resistance of glass to breakage by creating mechanical stress within the material, and is therefore widely used for meeting safety standards. It consists in heating the glass to a temperature where it becomes soft, then quickly cooling it down so that the outer shell solidifies before the core. This lets the outer shell in a very tough compressive state, making the glass harder to break, while the core is in an unstable tensile state, causing the glass to split into many harmless fragments, if nevertheless the glass breaks.
What is Glass Anisotropy?
Tempered glass anisotropy is a dreaded phenomenon which may look very unpleasant on a glazed facade. The mechanical stress created by the tempering process slightly modifies optical properties of the glass : its refractive index is slightly modified, and depends on light direction. Such a material is called a birefringent material, and has strong effects on polarized light, such as blue sky or light reflected on the glass surface under angle.
The phenomenon can have a very significant impact on the visual appearance of a building, mostly in clear sky conditions:
Glazings of the Vancouver Convention Center, observed in clear sky conditions
Simulating birefringence effects in glass with Ocean™ software
A new material model was developed in our virtual prototyping software Ocean™ for reproducing the phenomenon. The anisotropic refractive index of the glass material is physically described using birefringence coefficients, and the model also allows coatings to be applied on both sides of the glass sheet.
Modeling birefringence coefficients for realistic results:
In order to reproduce the non-uniform nature of the effect, the birefringence coefficients may depend on position, using any function. In this work, the birefringence coefficients were modulated according to random vertical stripe functions, corresponding to glass traveling under quenching nozzles in the tempering furnace. However, the defined shapes are only qualitatively similar to real tempered anisotropy patterns.
Case study: Evaluating birefringence under different sky conditions
Simulations were performed:
- Under clear sky, using the Ocean polarized sky model
- Under overcast sky, using an environment map
Tests were made with and without a low-E coating. The results are presented below
Simulation of a glass facade without low-E coating
No coating, cloudy sky:
The anisotropy is already visible on this picture, but is quite low. Overcast skies are almost non-polarized, so the polarization comes only from reflection and refraction under angle on the glass.
No coating, clear sky​:
The effect is now very strong, and unpleasant. This confirms usual observations, where glass anisotropy is mostly seen by reflection of blue sky.
Influence of low-E coating on birefringence
Low-E coating, cloudy sky:​
The patterns are now slightly colored, but not more visible than without coating.
Low-E coating, clear sky:
The anisotropy effect is now colored : the stripes are not only lighter and darker, but vary between purple and cyan. This is similar to what was observed in the real picture.
Practical considerations and future research
In the last simulated image, we can observe some differences with the real picture. This can be due to multiple factors:
- The amount of birefringence: we have set it to 5.10-4Â arbitrarily. This value strongly varies between tempering processes, in order to make a quantitative validation, it should be measured on the glass sample to reproduce.
- The sky conditions: the effect is stronger with very clear skies, and low haze. In a future article, we will present the results of using a real polarized sky capture, instead of a model.
- The angle of observation, and orientation of the glass in respect to the sky
- The coating: an arbitrarily simulated low-E coating was used in the simulation. As the nature of the coating has an influence on reflection colors, it should be defined from real measurements of a sample identical to the glass used in the real building.
Conclusion: Managing birefringence for optimal building aesthetics
In conclusion, understanding and simulating birefringence in tempered glass is imperative to maintaining the visual integrity of modern buildings. Future research will focus on more accurate simulations and real-world comparisons to refine our approaches.
To our knowledge, this was the first simulation of birefringence on architectural glass. This could be used for assessing optimal tolerance on birefringence, depending on glass configuration, coatings, and environment, so that the visual appearance of a building is not affected.
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Q&A
What causes birefringence in tempered glass?
Birefringence in tempered glass is caused by the mechanical stresses introduced during the tempering process, which alter the optical properties of the glass and create anisotropic effects.
Ocean™ is able to accurately predict the effect of birefringence on the appearance of the facade, as well as other optical distortions due to glass treatment.
How does birefringence impact the visual appearance of a building?
Birefringence can create visible patterns, especially under polarized light conditions, such as clear skies, which may result in unwanted visual distortions on glass facades.
Can birefringence be reduced or eliminated?
While birefringence cannot be entirely eliminated, its effects can be minimized through careful control of the tempering process, the use of coatings, and strategic glass design.Â
Ocean™ can be used in R&D processes to test different coatings that could achieve the desired corrective effect.
How can birefringence be simulated for design purposes?
Advanced simulation tools like Ocean™ software allow architects and engineers to predict birefringence effects by modeling anisotropic refractive indices and applying coatings to glass samples.
Is birefringence only visible under clear skies?
No, but it is most pronounced under clear skies with polarized light. It can also be observed under other lighting conditions but to a lesser extent.
With Ocean™’s Sky Importer, users can test their materials under various real-world weather scenarios to evaluate the final look of their projects.
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