Photoelasticity has been used for decades to accurately measure surface strains in a strucutre. In this method the test part, complex or not, is first coated with a strain-sensitive plastic coating and then subjected to an external load. The strains which exist throughout the part and over its surface are transferred to the coating and observed as optical interference fringes with a reflection polariscope. Two different paterns are produced with the polariscope -- isochromatics via circular polarization and isoclinics via linear polarization. Isochromatic fringes appear as a series of successive and contiguous different-colored bands each representing a different degree of birefringence corresponding to the underlying strain. The patterns can be read like a topographic map to visualize the stress distribution over the surface of the coated test part. The isoclinic fringes appear as black bands providing the direction of the principal strain.
