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.