A systematic study of the variations of the contrast of a dislocation in silicon on section topographs with the depth of the line was performed both experimentally and with computer simulations. Mo Kalpha radiation and 33-3 and -3-33 symmetric reflections were used. The crystal thickness was 440 micron so that the value of mu.t was 0.64. The influence of the orientation of the dislocation was studied for values of the angle between the line and its Burgers vector ranging between 60 and 90 in the glide plane. It was observed that when the dislocation lies close to the entrance surface, whatever its orientation, its image is centred around the trace of the plane of incidence passing through the intersection of the dislocation with the direct beam, while when the dislocation lies close to the exit surface its image is centred around the projection of the dislocation on the section pattern. The variation of the orientation of the image for intermediate depths of the dislocation is interpreted by means of the geometrical construction of the dynamical image. The values of the orientation of the image calculated according to this simple model are in good agreement with those measured on both experimental and simulated topographs. The same geometrical model enables the difference in the relative positions of the direct and dynamic images of stereo pairs to be explained. A new feature was observed in the simulated images of the experimental ones, namely a concentration of intensity along the projection of the dislocation in the reflected direction. Slit width taken into account in the simulations for a better fit with experimental topographs but not polarization, which was taken to be normal to the plane of incidence. Because of the small value of the crystal thickness anf of mu.t, the variation of the contrast with the Burgers vector is very small, making its determination very difficult.

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