Bloch waves and weak-beam imaging of crystals

The influence of the number of diffracted beams on weak-beam contrast simulations of thickness contour lines and dislocation images is investigated. For large deviation parameters s(g)-->thickness contour lines from two-beam simulations are similar to those from many-beam simulations. In many-beam simulations of wedge-shaped bent samples extra thickness contour lines appear at locations with ((g)-->, 3(g)-->). These extra lines occur between the imaging condition ((g)-->, -(g)-->) and ((g)-->, 3(g)-->). Therefore, in the case of a more symmetrical imaging condition many-beam simulations are mandatory. In bent samples the contributions of different Bloch waves to weak-beam images change as a function of the imaging conditions ((g)-->, x(g)-->). Near ((g)-->, 3.5(g)-->) two Bloch waves dominate. In the case of x <3 two other Bloch waves with different wavelengths are most important for the image contrast. The 'classical' ((g)-->, 3(g)-->) weak-beam condition is not suitable to determine signs and magnitudes of Burgers vectors from terminating thickness contour lines. Higher deviation parameters s(g)--> are necessary, especially for dense dislocation arrangements.