An illumination unit has a light source, in particular a high-intensity discharge lamp or an ultra high performance lamp, a main reflector and a back reflector with an aperture opposite the main reflector. Light is reflected from the light source through the aperture onto the main reflector. The centers of the light source and the back reflector 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 aperture is smaller than 180.degree.. The efficiency of light emission is considerably increased. Preferred embodiments, each of which can cause a further increase in light output, relate to various shapes of the back reflector and the inner walls of the gas discharge space, as well as to the shape of the part of the glass bulb that surrounds the gas discharge space.