Metal halide discharge lamp having optimum electrode location

#This invention concerns high intensity arc discharge lamps. It particularly relates to metal halide lamps having an arched arc tube. Such lamps are shown in U.S. Pat. No. 3,858,078, the disclosure of which is incorporated herein by reference.

#In such a lamp, the arc tube is generally made of high silica glass, for example, fused quartz, has an electrode at each end and contains a fill including mercury, metal halide and an inert gas. The lamp is made for horizontal operation, arch uppermost, and the arc tube is arched to substantially conform to the shape of the arc discharge during operation.

#We have found that improved lamp operation is obtained when the electrodes are located about midway between the lower wall of the end cavity of the arc tube and the axis of the arc tube.

#The single FIGURE in the drawing is an elevational view of an arc tube in accordance with this invention.

#As shown in the drawing, one embodiment of an arched arc tube 1 in accordance with this invention has press seals 2 and electrodes 3 at each end thereof. Electrode 3 can be the usual type of electrode used in metal halide arc discharge lamps, namely, a tungsten coil 4 mounted on a thoriated tungsten rod 5. One end of rod 5 is embedded in press seal 2 and is connected therewithin to a molybdenum ribbon 6 which is connected to external lead-in wire 7.

#The axis of the arc tube is indicated by broken line 8. The distance from axis 8 to the arc tube wall is shown as 2d. Each electrode 3 is located about midway between axis 8 and the lower arc tube wall, that is, a distance of about d from either.

#The advantage of such a location for the electrode is that, during operation, the hot electrode heats the end cavity of the arc tube substantially uniformly so as to minimize condensation of the metal halides thereat, without being so close to the arc tube wall as to cause devitrification of the quartz or as to promote reaction between the metal halides and the quartz.