GB1337134A - Metal halide lamp - Google Patents

Abstract

1337134 Metal-vapour lamps GENERAL ELECTRIC CO 1 Jan 1971 [26 Jan 1970] 179/71 Heading H1D A high pressure metal halide lamp for vertical operation, which includes Hg and an inert starting gas and has an electrode sealed at each end of the discharge tube, is constructed asymmetrically such that more heat is generated and/or less heat is lost from the lower end of the tube so that the temperature of the lower end is substantially above 670‹ C' without driving the temperature of the upper end above 1000‹ C. This is achieved by having a smaller electrode 14a which generates more heat, and a heat reflective coating 18 of zirconium oxide at the lower end. In an alternative embodiment (Fig. 6a, not shown) the ends of the discharge tube which may be made of quartz or fused silica are constructed asymmetrically, the lower end being substantially conical and the upper end hemispherical. The outer envelope is preferably evacuated in small lamps (< 400 W) and has an inert gas filling in large lamps. A preferred filling for an arc tube of 18 c.c. volume and arc tube gap of 4 cm. includes: 90 mg Hg; 16 mg NaI; 0À9 mg TlI; and 0À16 mg InI. The mercury filling is preferably completely vaporized and there are excess halides present. Argon may be used as the starting gas at a pressure, e.g. of 25 torr. In series with the starter electrode 15 is a current limiting resistor 32 and on heating up a bimetallic switch 33 connects electrodes 13 and 15 together.