Birefringence – or double refraction – can occur in several materials such as those that are made up of liquid crystals that have irregular shapes, and also in plastics under mechanical stress.
When a plastic optical lens is being fitted into the frame, the mechanical stress of this process can cause birefringence. The result is ‘stress marks’ at the edges for the glasses frame.
The stress marks on the lens cause light rays to be split into two parts – one an ordinary ray and the other an extraordinary ray. The result is the appearance of ‘warped’ areas in the lens; typically at the edges of the lens near the frame.
In most instances the wearers’ vision is not impacted as the birefringence is at the very edge of the lens. However, if impacted further across the lens it can make wearer adaptation to the lens prescription difficult.