The invention relates to an illumination system for combining two light beams. A prism bounded by two entrance faces that form a dihedron. An integrator is coupled to an exit face (20.3) of the prism. Each beam penetrates the prism via one of the entrance faces and is reflected by internal reflection on the other entrance face toward the exit face. These beams are substantially focused onto the entrance face of the integrator in such a way that the latter delivers an almost uniform exit beam. Applications: Projection and backprojection apparatus.