AbstractThe contrast effects at dislocations in very thin bi‐layered single crystals of polyethylene are discussed on the basis of the dynamical theory. The crystal thickness is smaller than one extinction distance. In such crystals the dislocation lines can be imaged either as bright or as dark lines in both the bright and the dark field image. In one area of the bright field image one family of dislocations may be observed as bright lines and another family as dark lines. Image “inversion” takes place in the bend contours, i.e., dark lines become bright and vice versa. The dark field images due to the diffraction vectors +g and −g may be either similar or complementary. All these observations can be accounted for by comparing them with profiles computed on the basis of the dynamical theory.Although one would expect the crystals to be thin enough for the kinematical theory to be applicable, it is found that the image profiles, as deduced by Hirsch, Howie, and Whelan[2] on the basis of this theory, cannot account for the observations. It is shown that this is a result of the approximations used in computing the integrals occuring in this theory.The observed dislocation networks are discussed in terms of Burgers vectors. The contrast effects are used to establish this analysis unambigeously. It is found that ribbons of partial dislocations occur in the networks. From the partial separation an estimate is made for the ratio of the stacking fault energy over the shear modulus (γ/μ b).Singularities in the networks are analysed and shown to result from the presence of partial dislocations ending in the foil surfaces.