5.4 Photoelastic Constant ()

Optical glass is usually free of strain, but when mechanical or thermal stress is exerted upon it, glass shows birefringence. Stress F (Pa), optical path difference (nm) and thickness of glass d (cm) have the following relationship:

In this case, proportional constant is called the photoelastic constant. It is listed in this catalogue at a unit of (nm/cm/10⁵ Pa). The photoelastic constant is the material constant that will change by glass type. By using it, optical path difference can be computed from given stress. Internal stress can also be computed from optical path difference.