An illumination unit is described having a light source (24), in particular a high-intensity discharge lamp or an ultra high performance lamp, a main reflector (1) and a back reflector (3) with an aperture opposite the main reflector, through which aperture light is reflected from the light source onto the main reflector. The illumination unit is in particular characterized in that the center of the light source (24) and the back reflector (3) are located or shaped relative to each other such that a first sector angle (L2-L2') enclosed between the light source center and the edge of the back reflector (3) aperture is smaller than 180°. The efficiency of light emission can be considerably increased thereby. Preferred embodiments of the invention relate to various shapes of the back reflector (3) and the inner walls of the gas discharge space, as well as the shape of the part of the glass bulb that surrounds the gas discharge space, where each of these can cause a further increase in light output.