Article révisé par les pairs
Résumé : The kinematical theory developed by Hirsch et al. (1960) for the diffraction contrast of electron microscope images of perfect screw dislocations parallel to the surface of the crystal foil is extended to an arbitrary perfect dislocation with slip plane parallel to the surface.The line profiles are numerically calculated and the variation of the line width, the position of the maximum and the value of this maximum with character of the dislocation line are discussed. It is argued that eventual dynamical corrections, supposing the kinematical case to be valid for the perfect material, would not alter the qualitative predictions. If the dislocation bisects the acute (obtuse) angle formed by the Burgers vector and the diffraction vector, its image has the smallest (largest) possible width. To all other orientations correspond profiles with intermediate values for the widths. To a broader profile corresponds also a position of the image more displaced away from the real position of the line.It is also found that for n = g.b = 0, a broad, double contrast may arise for orientations near to the 1/4π orientation if the order of the reflection considered is high.