This document discusses birefringent coatings that can indicate stress and strain levels. Birefringent coatings have different refractive indices that produce a photoelastic fringe pattern related to coating stresses. The brittle coatings will crack if stresses exceed a threshold. The document also explains that birefringence is caused by differing refractive indices in a transparent material when stresses are present. Birefringent coatings allow for 2D and 3D non-contact investigation of residual stresses.