2 workarounds did not work.
1) created light fog which gave a nice shading inside the lamp, but killed all the reflections on the outside. in addition, alltho with raytracing activated and a point light inside I wasn´t able to create those significant rays within the fog where the light comes through the deleted faces.
2) tried it with particles too, but that just dips everything into a homogenous nebula...
everything starts and ends in the right place at the right time.