Contrast of crystal defects under polarized light

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Abstract
The contrast of some crystal lattice defects in polarized light has been calculated using a previously developed theory (Fathers and Tanner 1973a,b). It is shown that it is possible to distinguish between screw, edge and mixed dislocations in an isotropic material. When viewed from the side the screw dislocation becomes invisible when the dislocation line is at an angle of π/4 to the crossed polarizer and analyser, the edge dislocation becomes invisible when it is parallel to the polarizer or analyser, and the mixed dislocation never becomes invisible for any setting. Experimental results on dislocations in gadolinium gallium garnet confirm these predictions. The image of a spherical precipitate in an isotropic material has been calculated and compared with experiment. Excellent agreement is found. Calculated images of end-on edge dislocations in an isotropic medium subjected to a plane strain show that the sign of the contrast in some lobes may be reversed by the strain. The symmetry of the image is not affected. Very good agreement is found between experimental and computed images.