Iridescence, aka “quench pattern/mark”, “strain pattern” or “anisotropy”, may be visible in heat-strengthened and fully tempered glass under certain polarized lighting conditions. NGA Glass Technical Paper (GTP) FB20-08 (2022) Iridescence in Heat-Treated Architectural Glass further explains this optical phenomenon.
The accepted industry consensus standard for heat-treated glass, ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass says the strain pattern does not affect any physical properties or performance values of the glass and is not a glass defect nor a cause for rejection.
ASTM C1901 Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass provides a method to quantify (in nanometers) the amount of iridescence that may be visible under certain polarized lighting conditions in heat treated flat monolithic glass. However, it does not define an acceptable/unacceptable amount of iridescence.
In contrast, DIN Spec 18198 Measurement and evaluation methods for optical anisotropic effects in thermally toughened glass gives objective criteria to classify the optical quality (Quality A, B or C) of monolithic glass based on two measurement methods of anisotropy effects. Annex B of the specification discusses the variables impacting potential visibility of anisotropy, including glass thickness, coatings, and viewing conditions at the building site.
While ASTM does not rate glass quality, only a way to measure it, the European DIN spec provides a “good, better, best” rating for the quality of the glass. NGA will update its current GTP to include the DIN information for reference purposes.
To learn more about anisotropy, watch a recording of NGA Thirsty Thursday Standards for Anisotropy, presented by Louis Moreau of AGNORA in May 2021. This 45-minute session is accredited for 0.5 AIA LU and addresses the most current research and technical updates surrounding anisotropy, including a review of ASTM C1901.