like extension application_number US 50076090 A description a high pressure discharge lamp 1 of 2 kw rating and an overall length of 19 cm is adapted to be associated with a reflector r shown only schematically and representing an optical system the lamp is fitted in the reflector r in axial direction which makes short length of the lamp of substantial importance see for example also fig3 of the referenced u s pat no 4 686 419 the discharge vessel 2 is made of quartz glass it is quite close to an isothermal vessel the wall thickness of the quartz glass is about 2 mm or may be 2 5 mm the overall structure is essentially barrel shaped with a generatrix having a radius of 38 25 mm the wall thickness in the central region 3 of the barrel shaped vessel 2 is thicker than at the end portions 4 and increases from the end portions to about 3 mm the wall loading due to the convection bending of the discharge arc is the highest in the central region 3 about 50 w cm 2 the largest outside diameter of the discharge vessel is 36 mm and its axial length about 51 mm the outer diameter at the ends 4 of the barrel to which at each end a connecting shaft like extension 5 is unitarily joined is about 16 mm the overall discharge volume.