GB1353683A - Discharge lamp having cavity electrodes - Google Patents

Abstract

1353683 Metal vapour lamps GENERAL ELECTRIC CO 25 May 1971 [25 May 1970] 17072/71 Heading H1D A high pressure metal vapour lamp for diffused hollow cathode mode operation comprises at least one self-heating hollow cathode having an inner cavity member 36 whose walls are substantially gas-tight and made of a refractory metal and an outer shield 34 surrounding member 36 in which thermal coupling between members 36 and 34 is better at the front region 36a, electron-emitting material located within the lower portion 36b of the cavity member, and no part of the shield projects beyond the cavity member. The gas-tight cavity walls promote injection of plasma into the cavity during A.C. reignition after zero current and aids penetration of the arc into the electrode cavity. The cavity member and shield are made of W by moulding or pressing W powder the upper part 36a consists of pure W having a high work function and the lower part 36b consists of W admixed with ThO 2 or Y 2 O 3 having a low work function. Thermal contact of the inner member 36 with the outer shield only in the top region 36a produces a low temperature region which reduces diffusion of emission material. Increased taper of the inner member 36 or provision of one or more layers of metal foil in the space between members 36 and 34 provides additional thermal insulation. The lamp is otherwise as described in Specification 1,240,069 and comprises a bulbous envelope (Fig. 3, not shown) in which is mounted a quartz arc tube containing two cavity electrodes and a filling of Hg at 1-15 atmospheres together with a saturated vapour of NaI with ThI and InI added for improved colour rendition. The ends of the tube have heat reflecting coatings to produce a minimum temperature which is above 500‹ C.