The toughening process produces areas of different stress in the cross section of the glass. These areas of stress produce a bi‑refringent effect in the glass, which is visible in polarized light. When thermally toughened soda lime silicate safety glass is viewed in polarized light, the areas of stress show up as coloured zones, sometimes known as ‘leopard spots’, or anisotropy.

Polarized light occurs in normal daylight. The amount of polarized light depends on the weather and the angle of the sun. The bi‑refringent effect is more noticeable either at a glancing angle or through polarized spectacles.

Anisotropy is not generally considered a defect but a visible effect.