pivoted about their longitudinal axes by actuation of the drive 28 such that the light transmitted by the discharge lamp 18 can pass onto the reflector 20 and from there through the cover 12 0027 fig2 shows a further embodiment of the invention with the same reference numerals being used for the same components with the embodiment shown in fig2 an at least partly transparent cylinder 32 which has a pre determined transmittance for example of 50 is provided as a diaphragm means to change the illuminance in the area of the site of the operation the cylinder 32 is arranged coaxial with respect to the longitudinal axis of the discharge lamp 18 and is movable axially along the double arrow shown in the direction of the longitudinal axis of the discharge lamp 18 the cylinder 32 is arranged on a holder 34 which is axially movable about a spindle drive 36 the cylinder 32 is moved in the direction of the arrow by actuating the spindle drive 36 such that the cylinder 32 can cover the discharge lamp 18 with different widths in the axial direction the cylinder 32 can for example be moved completely upwardly such that no dimming effect is provided in the position shown in fig2 the complete dimming effect is achieved this amounts for example to 50 in dependence on the transmittance of the cylinder 0030 fig3 shows a side view of a further embodiment of a lamella 42 which consists of a substantially rectangular metal sheet which is covered by black lacquer for the avoidance of unwanted reflections and which has a.