Anisotropy in Tempered Glass: Causes, Iridescence and Quality Control
1. The Ubiquitous Iridescence (Anisotropy Effect)
If you have ever strolled through the business district of a modern city or visited a large airport or museum, you have likely seen this scene: on large-area glass facades, faint, irregular colored patterns appear, sometimes like pale rainbows, sometimes like an oil slick spreading on water, and other times like pale yellow clouds. Its technical name is iridescence (also known as anisotropy-induced stress pattern).
Image Source: https://glassinspector.com/did-you-know-what-anisotropy-is-in-tempered-glass/
2. What is Iridescence (Anisotropy)?
Iridescence refers to a visible optical interference phenomenon produced by uneven internal stress distribution in tempered glass when illuminated by polarized light. It appears on the glass surface as irregular colored stripes, patches, or cloud-like patterns.
Iridescence is an inherent characteristic of tempered glass. Neither Chinese nor international standards classify it as a safety defect; it primarily affects the aesthetic appearance of the glass.
The “anisotropy” of tempered glass is the physical origin of the iridescence. Anisotropy refers to the phenomenon in which a material exhibits different optical properties in different directions. In tempered glass, rapid cooling creates a permanent stress distribution inside the glass, transform the glass from isotropic to anisotropic. It is precisely this anisotropy that causes birefringence when polarized light passes through the glass, resulting in the visible colored fringes — i.e., the iridescence. In other words, without anisotropy, there would be no iridescence; the iridescence is the “visual manifestation” of anisotropy under polarized light.
3. Physical Causes: The "Dance" of Light and Force
Tempered glass exhibits optical anisotropy (birefringence). When polarized light enters tempered glass, it splits into two perpendicularly vibrating polarization components (ordinary and extraordinary rays). These two components propagate at different speeds within the glass, creating an optical retardation (path difference). During the actual tempering process, factors such as cooling air pressure fluctuations and heating temperature variations lead to uneven stress distribution across the glass plane. As a result, different areas produce different retardations, forming rainbow-like colored stripes—what we see as iridescence.
A prerequisite for observing iridescence is that the incident light must be polarized. Scattered light from the sky and reflected light from water or glass surfaces are inherently significantly polarized. For example, when we stand on the ground and look up at a glass facade, the incident light is partially polarized. After this polarized light passes through the tempered glass, the interference colors become clearly visible.
4. Quality Control: "Quantifying" and "Controlling" the Anisotropy Effect
In the past, the visibility of iridescence in tempered glass was mostly judged by human experience and intuition, leading to subjective debates ("I think it's obvious," "I think it's fine"). To bring quality control of iridescence into a systematic framework, this subjective issue must be transformed into comparable and verifiable objective parameters.
Completely eliminating iridescence is difficult, but scientific testing methods and quantifiable evaluation criteria can be used to measure and control its visibility. Existing measurement instruments place tempered glass in a polarized field to capture the iridescence pattern. By processing these images, parameters such as the optical retardation at various points on the glass can be obtained, thereby quantifying the visibility of iridescence.
Measurement instruments cannot directly eliminate iridescence, but they can tell us where the tempering equipment is malfunctioning, how the process is drifting, and which glass panels should be intercepted before leaving the factory. The goal of iridescence quality control is to establish a "knowable, controllable, and predictable" production system using scientific measurement instruments, ensuring that the anisotropy effect is constrained within a reasonable and mutually agreed-upon range.
Glass Anisotropy Measurement: OpticStress®
OpticStress® is an online measurement system for anisotropy of tempered glass, including retardation and iridescence (also known as stress pattern). The system adopts a non-contact polarization optical measurement method to obtain the retardation and relative intensity of iridescence, and provides glass quality classes based on the retardation distribution. The assessment parameters include: retardation and iridescence pseudo-color maps and their statistical data; cumulative distribution curves of retardation and iridescence; 2D data distribution curves; glass quality classes based on retardation; and simulation images of iridescence from different observation angles.
