When a polarized laser beam transmitted into a stressed glass block, a sort of optical birefringence occurs. However, since the optical sensitivity of a glass is not so high, it is somewhat difficult to measure photoelastic fringe pattern by the ordinary methods. It is well known that the Photo-Elastic Modulator method can measure not only minute retardation of birefringence less than a quarter wavelength but the orientation of principal birefringence simulaneously. Taking the distinctive feature of the method, a system of PEM will be used effectively for the simultaneous measurement of principal stress difference and principal stress direction in a transparent specimen which involves residual stresses. It is, however, difficult to determine the maximum or minimum values of stress or strain just near specimen surface accurately and precisely. An additional measurement of in-plane stresses on the surface is examined on the basis of the reflection technique.
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