METAL HALIDE DISCHARGE LAMP

1,253,948. Metal-vapour lamps. GENERAL ELECTRIC CO. 13 April 1970 [19 May, 1969], No. 17467/70. Heading HID. An arc lamp comprises a filling which includes an inert gas (20 torr argon, for example), AlCl 3 and Al in which as stated the aluminium getters free chlorine and oxygen and also combines with chlorine from the thermal dissociation of AlCl 3 . Preferably included in the filling are Hg, and AlI 3 which reduces attack on the tungsten electrodes. The lamp envelope 2 is made of sintered high density polycrystalline alumina or of fused silica or quartz with an internal protective layer of Al 2 O 3 formed for example by exposing the hot tube to a mixture of AlCl 3 vapour, CO 2 and argon. The ends of the envelope are closed by niobium caps 3, 31 hermetically sealed to the alumina by an alumina-calcia sealing composition 4. A niobium tube 5 which supports one electrode 6, penetrates into the cap 3 for evacuation of the lamp after which the tube is closed off. The lamp is mounted in an evacuated outer jacket (not shown).