What is a strain pattern?
Strain patterns are also called quench marks. The terms describe an optical phenomenon inherent to heat-treated (heat-strengthened or fully tempered) glass that appears as a series of light and dark areas. The strain pattern is especially visible when viewed at an angle of (45º-60º) to the glass surface, and it is more noticeable when you are wearing polarized sunglasses. It is not a defect but a characteristic of heat-treated glass.