Thermally tempered glass surface stress measurement by critical ray

A critical ray was excited along the glass surface by laser beam incidence. Part of the critical ray was refracted during propagation. The angle of refraction depended on the refractive index of the glass sirface. When the glass surface was birefringent due to the photoelastic effect, the angles of refraction differed from each other between two linearly polarized beams. This difference allows surface stress determination.