Surface stress measurement by the optical waveguide effect in thermally-tempered float glass

Float glasses have tin-diffused layers on one side. These layers show an optical waveguide effect caused by refractive index gradients in the layer. Surface stress, caused by tempering, gives different refractive index distribution for TM and TE waves via the stress birefringence effect. Non-destructive and rapid stress determination is possible by observing the effective index distributions of propagating modes in TM and TE waves.