We show a non destructive method for obtaining the isochromatic and isoclinic fringes in three dimensional photoelastic specimens. The basic idea is to delimit a slice between two plane laser beams. The properties of polarization of the scattered light (Rayleigh law) and the interference possibilities of the diffused beams are used. By introducing speckle pattern properties we can analyse the correlation factor between two scattered beams as we can for the illumination in a plane polariscope in the classical frozen-stress investigation of a slice. We use a monochromatic laser beam, a CCD camera and a PC. As we cannot directly obtain the correlation factor, we do a statistical analysis of the speckle patterns. The variance (a function of the correlation factor) is computed from the light intensities of three images corresponding to the speckle pattern for plane 1 alone, plane 2 alone, and both planes together.
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