The elastic displacement field of a sharply angular dislocation with its two legs parallel to a planar free surface is given in a practical analytical form. Its expression gives access, in transmission electron microscopy (TEM), to computed images of numerous interacting dislocations all located at a distance h close to the free surface. As an application, this field is used repeatedly to study, in dark-field TEM and a g(3g) diffraction mode with g{220}, the contrast of dissociated triple nodes of a low-angle twist boundary in silicon extended over a (111) plane. It is shown that free surface elastic effects can influence the contrasts of some 30° partials if h is at nanometer scale.