EP0530318A1

EP0530318A1 - Arc discharge lamp having reduced sodium loss

Info

Publication number: EP0530318A1
Application number: EP19910911587
Authority: EP
Inventors: Scott R. Hunter
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1990-05-22
Filing date: 1991-05-22
Publication date: 1993-03-10
Also published as: CA2081573A1; WO1991018413A1

Abstract

Un tube à régime d'arc (10) dans lequel sont renfermées des électrodes (20, 22) au niveau d'extrémités opposées, comprend une ampoule à décharge (12) ainsi que des passages d'électrode. Le tube à régime d'arc (10) renferme une matière de remplissage contenant du sodium ou un composé de sodium. On supprime la migration du sodium dans le tube à régime d'arc (10) en interconnectant électriquement une première région du tube en régime d'arc (10) et une seconde région de ce dernier (10). Les première et seconde régions s'étendent à partir d'environ la moitié du passage d'électrode jusqu'à la pointe de l'électrode (20, 22). Selon un mode de réalisation préféré, une bride conductrice (36, 38) est fixée de manière à assurer un contact électrique à chaque passage d'électrode adjacent au puits terminal de l'ampoule à décharge (12), et les brides (36, 38) sont connectées ensemble par un fil isolé de mise en court-circuit (40).An arc tube (10) in which electrodes (20, 22) are enclosed at opposite ends, includes a discharge bulb (12) and electrode passages. The arc tube (10) contains a filler containing sodium or a sodium compound. The migration of sodium in the arc tube (10) is suppressed by electrically interconnecting a first region of the arc tube (10) and a second region of the latter (10). The first and second regions extend from about half of the electrode passage to the tip of the electrode (20, 22). According to a preferred embodiment, a conductive flange (36, 38) is fixed so as to ensure electrical contact at each electrode passage adjacent to the terminal well of the discharge bulb (12), and the flanges (36, 38) are connected together by an insulated short-circuiting wire (40).

Description

ARC DISCHARGE LAMP HAVING REDUCED SODIUM LOSS

Field of the Invention

This invention relates to arc discharge lamps having sodium as a component of a fill material and, more particularly, to arc tubes in which sodium loss is suppressed by maintaining a substantially uniform electrical potential on the surface of the arc tube during arc discharge.

Background of the Invention

Sodium is an important constituent in most high intensity metal halide arc discharge lamps, usually in the form of sodium iodide or sodium bromide. Sodium is used to improve the efficacy and color rendering properties of these lamps. A metal arc lamp of this type is disclosed in U.S. Patent No. 3,407,327, issued October 22, 1968 to Koury et al. The presence of sodium in such lamps enhances the emission in the red spectral region, lowers the correlated color temperature and acts as a so-called "arc fattener", thereby increasing the radiating volume and establishing a more stable arc. Sodium compounds are also used in high pressure sodium lamps. High pressure sodium lamps typically utilize an alumina or yttria arc tube, while metal halide lamps typically use a quartz arc tube.

It has long been recognized that arc tubes containing sodium lose sodium during discharge lamp operation. Sodium is lost by the movement, or migration, of sodium ions through the arc tube wall. The iodine originally present in a metal halide lamp as sodium iodide is freed by sodium loss, and the iodine combines with mercury in the arc tube to form mercury iodide. The mercury iodide leads to increased reignition voltages, thereby causing starting and lamp maintenance problems. Previous measurements have shown that the rate of sodium loss is dependent on the temperature of the arc tube wall and on the magnitude of the electric field across the arc tube and between the arc tube and surrounding metallic support structures.

One prior attempt to reduce sodium loss from metal halide lamps was the use of a so-called "frameless construction" described in U.S. Patent No. 3,424,935 issued January 28, 1969 to Gungle et al. In the frameless construction, there are no frame members close to the arc tube. The electrical connection to the upper electrode is a fine tungsten wire spaced as far away from the arc tube as possible. Although this configuration reduces sodium loss in high wattage, single-ended metal halide arc discharge lamps, sodium loss is still evident near the end of the life of such lamps.

In the newer, lower wattage metal halide lamps, other techniques have been employed to reduce the rate of sodium loss. In U.S. Patent No. 4,281,274, issued July 28, 1981 to Bechard et al, a miniature arc tube containing sodium iodide is located within a gas-filled outer envelope. The arc tube is mounted within a glass sleeve which is non-transparent to ultraviolet radiation emanating from the discharge within the arc tube. The glass sleeve is electrically biased so as to repel the sodium ions released from the arc tube. However, the presence of the glass sleeve leads to excessive heat loss by convection in these lamps. This technique is not suitable for AC operation of an arc tube, since the positive bias is provided on the glass sleeve only during one half of the AC voltage cycle.

An attempt to overcome such limitations is disclosed in U.S. Patent No. 4,499,396, issued February 12, 1985 to Fohl et al. A low wattage, metal halide arc tube is enclosed in a domed shroud contained within a gas-filled envelope. Heat loss by convection is suppressed, and sodium loss is reduced by shielding the arc tube from the surrounding support structures. Another technique for reducing the rate of sodium loss from metal halide and high pressure sodium discharge lamps is disclosed in U.S. Patent No. 4,614,890 issued September 30, 1986 to Meyer et al. The arc tube, which is hermetically sealed within the outer envelope of the discharge lamp, has a wall member with a portion of increased thickness. The portion of increased thickness is positioned adjacent to a metal conductor disposed within the outer envelope, whereby the loss of sodium from the arc tube is inhibited.

Another technique for reducing sodium loss is disclosed in U.S. Patent Nos. 4,620,125 issued October 28, 1986 to Keeffe et al and 4,625,141 issued November 25, 1986 to Keeffe et al. A low wattage, metal halide discharge lamp includes an evacuated envelope containing a heat reducing member and an arc tube within the heat reducing member. The heat reducing member and the arc tube have a metal band and an outer strap adjacent one another and adjacent to one electrode. The metal band, outer strap and electrode are all electrically connected to an electrical lead of one polarity, whereby sodium loss from the arc tube is reduced.

Yet another technique for reducing sodium loss from low wattage metal halide arc discharge lamps is discussed by Keeffe et al in Journal of Illumination Engineering Society. Summer 1988, pages 39-43. An arc tube and a quartz shield are enclosed within an evacuated outer jacket. The shield and support members are constructed according to a "floating frame" design, which provides electrical isolation from the arc tube circuit. The electrolysis circuit is interrupted, thereby reducing the rate of sodium loss from the arc tube.

Japanese Patent No. 60-40138 published July 30, 1976 discloses a lamp wherein biased return leads in the outer jacket attract and remove electrons produced by photoemission from the electrical inleads. However, no frame or shield is provided to suppress electrical extraction of positive sodium ions from the arc tube. U.S. Patent No. 4,843,266, issued June 27, 1989 to Szanto et al, discloses a discharge lamp wherein metal components within the lamp envelope are insulated to reduce sodium migration.

It is a general object of the present invention to provide improved arc discharge lamps.

It is another object of the present invention to provide arc discharge lamps wherein sodium migration from the arc tube is suppressed.

It is a further object of the invention to provide arc discharge lamps wherein migration of alkali metal ions from the arc tube is suppressed.

It is yet another object of the present invention to provide arc discharge lamps which have long operating lives.

It is a further object of the present invention to provide means for suppressing sodium migration from arc tubes which is simple in construction and low in cost.

Summary of the Invention

According to the present invention, these and other objects and advantages are achieved in a lamp assembly comprising an arc tube having electrodes sealed therein, the arc tube including a discharge bulb and electrode feedthroughs, the arc tube enclosing a fill material containing an alkali metal or an alkali metal compound, a sealed lamp envelope enclosing the arc tube, means for coupling electrical energy to the electrodes, and potential control means for maintaining a substantially uniform electrical potential on the discharge bulb of the arc tube during arc discharge. The arc tube typically comprises a quartz arc tube having electrodes mounted in opposite ends and containing a fill material including one or more metal halides. The electrodes are sealed into the arc tube in flattened press seals. The invention is particularly useful in suppressing loss of sodium ions from the arc tube. The potential control means preferably includes means for electrically interconnecting first and second regions of the arc tube on or near the end wells of the discharge bulb. Each of the first and second regions preferably extends from about the middle of the press seal region to the tip of the electrode. In a preferred embodiment, the potential control means comprises first and second conductive straps surrounding the arc tube in the first and second regions, respectively, and a wire interconnecting the first and second straps. Preferably, the arc tube has a conductive coating to enhance electrical contact with the conductive straps. The conductive straps are preferably wrapped around the respective press seal regions as close to the end wells of the discharge bulb as possible.

According to another aspect of the invention, an arc lamp comprises an arc tube having electrodes sealed therein at opposite ends, the arc tube including a discharge bulb and electrode feedthroughs and enclosing a fill material containing an alkali metal or an alkali metal compound, means for coupling electrical energy through the electrode feedthroughs to the electrodes, and potential control means for maintaining a substantially uniform electrical potential on the discharge bulb of the arc tube during discharge.

According to another embodiment of the invention, the potential control means can comprise a transparent conductive coating which covers a sufficient portion of the discharge bulb surface to maintain a substantially uniform electrical potential thereon.

By maintaining the surface of the discharge bulb at a substantially uniform electrical potential, the maximum potential difference between the center of the discharge and the outer surface of the discharge bulb is minimized, and sodium migration is thereby suppressed. The invention is most effective in suppressing sodium loss when high temperature, low resistance regions of the arc tube near each electrode are electrically connected together. Brief Description of the Drawings

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the accompanying drawings which are incorporated herein by reference and in which:

FIG. 1 shows a metal halide arc lamp having means for suppressing sodium loss in accordance with the present invention;

FIG. 2 is a mass spectrum showing emitted sodium ion intensity of the lamp of FIG. 1 with the shorting wire connected and with the shorting wire disconnected;

FIG. 3 is a mass spectrum showing emitted sodium ion intensity when shorting straps are connected to the outermost ends of the press seals;

FIG. 4 is a cross-sectional view of a double-ended metal halide discharge lamp assembly in accordance with the present invention;

FIG. 5 is a cross-sectional view of a frameless metal halide discharge lamp assembly in accordance with the present invention; and

FIG. 6 is a cross-sectional view of a floating frame metal halide discharge lamp assembly in accordance with the present invention.

Detailed Description of the Invention

A metal halide arc discharge lamp in accordance with the present invention is shown in FIG. 1. A quartz arc tube 10 includes a discharge bulb 12 and press seals 14 and 16. Electrodes 20 and 22 are sealed in opposite ends of the discharge bulb 12. The discharge bulb 12 encloses a discharge region containing a fill material including mercury and one or more metal halides. Sodium, usually in the form of a halide such as sodium iodide or sodium bromide, is a constituent of the fill material. Electrodes 20 and 22 are connected through press seals 14 and 16, respectively, typically by thin molybdenum foils 24 and 26. Techniques for the formation of press seals 14 and 16 are well known in the art. The seals 14 and 16 constitute electrode feedthroughs for conducting electrical energy to electrodes 20 and 22. As used herein, the phrase "electrode feedthrough" refers to a portion of the arc tube in which electrical power is carried on an electrical conductor from the exterior of the arc tube to an electrode in the interior of the arc tube. The arc tube is sealed to the electrical conductor in the electrode feedthrough region.

A reflective paint 30, such as zirconium oxide, is typically applied to the outer surface of the arc tube to reflect heat and to raise the temperature of the arc tube in these regions. The paint 30 is typically applied to regions of the arc tube 10 extending from about the middle of foil 24 to the tip of electrode 20 and from about the middle of foil 26 to the tip of electrode 22. Portions of the discharge bulb adjacent to electrodes 20 and 22 are called end wells.

As discussed above, sodium loss from the arc tube 10 due to migration of positive sodium ions through the wall of the discharge bulb 12 degrades lamp performance and reduces its operating life. In accordance with the present invention, the discharge lamp is provided with means for controlling the electrical potential on the surface of the discharge bulb 12 during arc discharge. The potential control means includes means for maintaining a substantially uniform electrical potential on the discharge bulb 12 during arc discharge.

In the embodiment of FIG. 1, the potential control means includes conductive straps 36 and 38 and a wire 40. The conductive strap 36 is wrapped around press seal 14, and conductive strap 38 is wrapped around press seal 16. The straps 36 and 38 are located as close as possible to discharge bulb 12. The straps 36 and 38 are attached to the respective press seals so as to make electrical contact with the quartz of arc tube 10. The wire 40 is electrically connected between straps 36 and 38. The wire 40 can be insulated or uninsulated. When the wire 40 is insulated, emission of photoelectrons from the wire is suppressed. It has been found that this configuration substantially reduces loss of sodium from the discharge bulb 12.

FIG. 2 shows a mass spectrum for a mass range of approximately 19-26 atomic mass units for the ions emitted from a type 100 M100/U watt metal halide arc tube mounted in a high vacuum system and electrically isolated from the surrounding chamber. The mass spectrum was measured with a quadrupole mass spectrometer. A peak in the spectrum occurs at mass 23, which corresponds to sodium ions. Curve 44 shows the sodium emission when the wire 40 is disconnected from straps 36 and 38. Curve 46 shows the sodium emission when the straps 36 and 38 are connected by wire 40. These measurements show that the rate of sodium loss is reduced by a factor of greater than two in accordance with the present invention. Measurement of other arc tubes electrically configured in the same manner have consistently shown a factor of 2-5 reduction in the rate of sodium loss from the arc tubes.

FIG. 3 shows a mass spectrum of sodium emission when conductive straps are positioned on the arc tube near the outermost end of each of the press seals. Curve 48 shows the sodium emission when the straps are disconnected, whereas curve 50 shows the sodium loss when the straps are electrically connected to each other. The reduction in sodium loss when the straps are electrically connected is only about 10% in this case. FIGS. 2 and 3 demonstrate that the straps 36 and 38 should be placed on the press seals 14 and 16 as close as possible to the discharge bulb 12.

The configuration including straps 36 and 38 and wire 40 maintains the outer surface of discharge bulb 12 at a substantially uniform electrical potential. In the absence of straps 36 and 38 and wire 40, electric field gradients exist in the region of the discharge bulb 10. The electric field gradients are caused by the voltage applied between the electrodes 20 and 22. Such electric field gradients cause a potential difference between the inside and the outside of the arc tube. Since the quartz arc tube 10 is known to have a finite conductivity at the elevated operating temperatures of the arc lamp, the electric potential across the quartz causes migration of positive sodium ions from the interior to the exterior of the arc tube. The present invention is believed to reduce the electric potential between the inside and outside of the arc tube, thereby reducing sodium loss.

During arc discharge, the arc tube 10 reaches operating temperatures on the order of 800°C to 900°C at the end wells of the discharge bulb 12 and 500°C to 600°C at the outer ends of the press seals. The outer ends of the press seal regions 14 and 16 experience much lower operating temperatures since they are further removed from the discharge region. As indicated above, the electrical conductivity of quartz increases as its temperature increases. In order to effectively reduce sodium loss in accordance with the present invention, the straps 36 and 38 should be electrically attached to the arc tube 10 in regions that are at or close to the lamp operating temperature to thereby insure electrical connections at regions of relatively high conductivity. The outer ends of the press seals 14 and 16 remain at relatively low conductivity during arc discharge since these regions have lower operating temperatures. This is demonstrated in FIG. 3 by the fact that shorted straps attached to the outer ends of the press seals provide very little reduction in sodium loss.

Another requirement of the invention is to minimize light loss. Thus, the diameter of wire 40 should be minimized, and the straps 36 and 38 should be positioned on the arc tube 10 so as to minimize light loss.

A balance of the above requirements indicates that the straps 36 and 38 are preferably positioned on arc tube 10 in regions which extend from about the middle of molybdenum foil 24 to the tip of electrode 20 and from about the middle of molybdenum foil 26 to the tip of electrode 22. Thus, with reference to FIG. 1, strap 36 is preferably positioned on arc tube 10 in a region indicated by the letter A, while strap 38 is preferably positioned on the arc tube 10 within a region indicated by the letter B. The regions A and B can also be defined as including the end wells of the discharge bulb 12 and about the inner half of press seals 14 and 16. When the straps 36 and 38 are positioned on a portion of the discharge bulb 12, a part of the light output of the lamp is blocked. When the straps 36 and 38 are positioned close to molybdenum foils 24 and 26, the effectiveness of the invention is somewhat reduced due to the lower operating temperatures. Thus, preferred positions of straps 36 and 38 are on press seals 14 and 16 immediately adjacent to the ends of discharge bulb 12.

It will be understood that other potential control means can be utilized within the scope of the invention. For example, straps 36 and 38 can be replaced with conductive wires of various shapes and sizes. The wire 40 is preferably insulated with quartz or a ceramic insulating tube to suppress generation of photoelectrons from the wire. However, a noninsulated wire can be utilized.

Preferably, a conductive layer is applied to the surface of the arc tube 10 at least in the regions where straps 36 and 38 contact the arc tube 10. The conductive layer provides a relatively low resistivity electrical contact between the straps 36 and 38 and the quartz arc tube. In a preferred embodiment, the zirconium oxide paint normally used to reflect heat at the ends of the arc tube also provides low resistance contacts between straps 36 and 38 and arc tube 10. The zirconium oxide is conductive at the elevated operating temperatures of the lamp. Preferably, zirconium oxide or another conductive layer is also applied to the surfaces of straps 36 and 38 to provide low resistance contacts between straps 36 and 38 and arc tube 10. In accordance with another embodiment of the invention, the straps 36 and 38 and the wire 40 are replaced with a transparent conductive coating on the outer surface of discharge bulb 12. An example of a transparent conductive coating is indium tin oxide. The transparent coating covers a sufficient portion of discharge.bulb 12 to maintain a substantially uniform electric potential on the surface of the discharge bulb. The transparent conductive coating can be continuous or patterned.

A preferred embodiment of a lamp assembly in accordance with the invention is shown in FIG. 4. A double-ended metal halide discharge lamp 60 includes an evacuated quartz envelope

62 containing a metal halide arc tube 63. Electrical conductors 64 and 65 are sealed into and pass through hermetic seals 66 and 67, respectively, at opposite ends of the quartz envelope 62. The arc tube 63 is coated with zirconium dioxide paint in regions 68 and 69. The paint regions 68 and 69 extend approximately from the middle of the seals of arc tube

63 to the tips of electrodes 76 and 77. Metal straps 70 and 71 are tightly wrapped around the press seals of arc tube 63 as close as possible to discharge bulb 73. Preferably, the straps 70 and 71 are also coated with zirconium dioxide. A wire 74 located within a quartz or ceramic insulating tube 75 electrically interconnects straps 70 and 71.

An embodiment of the invention utilizing the so-called "frameless construction" is shown in FIG. 5. A metal halide arc discharge lamp assembly 80 includes a lamp envelope 82 filled with an inert gas. The preferred gas fill is nitrogen at a pressure of approximately 400 torr. However, the pressure can be within a range from 100 torr to approximately 1 atmosphere depending on the lamp type. The lamp envelope 82 is hermetically sealed to a stem member 83. A lamp base 84 provides means for coupling electrical energy through the lamp envelope 82 to energize an arc tube 85. The arc tube 85 is mechanically supported by lower frame members 86 and 87 connected to lamp stem 83. The arc tube 85 is further supported by an upper frame member 88 and bulb spacers 90. Metal straps 92 and 93 are tightly wrapped around press seals of the arc tube 85 as close as possible to the discharge bulb. Zirconium dioxide paint 94 is preferably used to enhance electrical contact between the arc tube 85 and the straps 92 and 93. A wire 96 electrically connects straps 92 and 93. The wire 96 is preferably placed within a quartz or ceramic insulating tube 97 to suppress the generation of photoelectrons from wire 96. The arc tube 85 receives electrical power through leads 98 and 99. The lamp assembly shown in FIG. 5 differs from the frameless construction disclosed in the aforementioned Patent No. 3,424,935 since the frame members 86, 87 and 88 are electrically isolated from lamp power leads 98 and 99.

A metal halide arc discharge lamp utilizing a "floating frame" design is shown in FIG. 6. A metal halide arc tube 102 is mounted within a quartz shroud 104. The arc tube 102 and the shroud 104 are supported within a lamp envelope 106 by a frame 108. The frame 108 extends from a lamp stem 110 to a dimple 112 at the top of the lamp envelope 106. The shroud 104 is secured to frame 108 by straps 114 and 116. The arc tube 102 is secured to frame 108 by straps 120 and 122 tightly wrapped around the press seals of the arc tube 102 as close as possible to the discharge bulb. Zirconium dioxide is preferably used as discussed hereinabove to enhance electrical contact between straps 120 and 122 and arc tube 102. The frame 108 provides the electrical connection between straps 120 and 122.

In each of the embodiments of FIGS. 3-5, conductive straps are electrically attached to the arc tube as close as possible to the end wells of the discharge bulb portion of the arc tube. The straps are connected together to provide a substantially uniform electrical potential on the discharge bulb. The invention has been described thus far in connection with controlling loss of sodium from an arc tube. The present invention can also be utilized to control loss of other alkali metal ions, including cesium, lithium and potassium, from arc tubes. The alkali metals are usually added to the arc tube as compounds such as metal halides, but in some cases may be added in metallic form. The present invention can be utilized to control sodium loss from high pressure sodium arc lamps.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A lamp assembly comprising: an arc tube having electrodes sealed therein, said arc tube including a discharge bulb and electrode feedthroughs, said arc tube enclosing a fill material which emits light during discharge, said fill material containing an alkali metal or an alkali metal compound; a sealed lamp envelope enclosing said arc tube; means for coupling electrical energy to said electrodes; and potential control means for maintaining a substantially uniform electrical potential on said discharge bulb of said arc tube during arc discharge.

2. A lamp assembly as defined in claim 1 wherein said arc tube contains one or more alkali metal halides and wherein said arc tube comprises quartz.

3. A lamp assembly as defined in claim 2 wherein said electrodes are mounted in opposite ends of said discharge bulb and wherein said potential control means comprises first and second conductors electrically connected to said feedthroughs adjacent to said discharge bulb and a third conductor electrically connected between said first and second conductors.

4. A lamp assembly as defined in claim 3 wherein said first and second conductors comprise first and second straps around said feedthroughs and wherein said third conductor comprises an insulated wire.

5. A lamp assembly as defined in claim 4 further including a coating on at least a portion of said arc tube to enhance electrical contact between said first and second straps and said arc tube.

6. A lamp assembly as defined in claim 5 wherein said coating comprises zirconium oxide.

7. A lamp assembly as defined in claim 2 wherein said electrodes are mounted in opposite ends of said discharge bulb, wherein said feedthroughs comprise press seals at opposite ends of said discharge bulb and wherein said potential control means comprises means for electrically interconnecting portions of said press seals adjacent to said discharge bulb.

8. A lamp assembly as defined in claim 2 wherein said electrodes are mounted in opposite ends of said discharge bulb, wherein said feedthroughs comprise press seals at opposite ends of said discharge bulb and wherein said potential control means comprises first and second conductors electrically connected to said press seals, respectively, and a third conductor electrically connected between said first and second conductors, said first and second conductors being electrically connected to said press seals in regions that are at or near the operating temperature of said discharge bulb during arc discharge.

9. A lamp assembly as defined in claim 2 wherein said potential control means comprises a transparent conductive coating on a surface of said discharge bulb.

10. A lamp assembly as defined in claim 1 wherein said sealed lamp envelope comprises a quartz tube having feedthroughs at opposite ends thereof for coupling electrical energy to said electrodes.

11. A lamp assembly as defined in claim 4 further including upper and lower frame members for supporting said arc tube in said lamp envelope, said upper frame member being connected between said first strap and an upper end of said lamp envelope, said lower frame member being connected between said second strap and a lower end of said lamp envelope.

12. A lamp assembly as defined in claim 1 further including a frame member extending between opposite ends of said lamp envelope for support of said arc tube, said potential control means comprising first and second straps around said feedthroughs adjacent to said discharge bulb, said first and second straps being electrically connected to said frame member such that said frame member electrically interconnects said first and second straps.

13. A lamp assembly as defined in claim 12 further including a transparent shroud substantially surrounding said arc tube and mechanically supported by said frame member.

14. A lamp assembly comprising: an arc tube having electrodes sealed therein at opposite ends, said arc tube including a discharge bulb and electrode feedthroughs, said arc tube enclosing a fill material which emits light during discharge, said fill material containing an alkali metal or an alkali metal compound; a sealed lamp envelope enclosing said arc tube; means for coupling electrical energy through said lamp envelope to said electrodes; and potential control means for electrically interconnecting a first region of said arc tube and a second region of said arc tube, said first and second regions each comprising a portion of said arc tube between about the middle of said electrode feedthrough and the tip of said electrode.

15. A lamp assembly as defined in claim 14 wherein said arc tube comprises quartz and contains one or more alkali metal halides.

16. A lamp assembly as defined in claim 15 wherein said potential control means comprises first and second conductive straps electrically connected to the first and second regions, respectively, of said arc tube and a conductor electrically connected between said first and second straps.

17. A lamp assembly as defined in claim 16 further including a conductive coating on at least a portion of said arc tube to provide low resistance contacts between said first and second straps and said arc tube.

18. A lamp assembly as defined in claim 16 wherein said conductor is insulated to suppress emission of electrons.

19. A lamp assembly comprising: an arc tube having electrodes sealed therein at opposite ends, said arc tube including a discharge bulb and electrode feedthroughs, said arc tube enclosing a fill material which emits light during discharge, said fill material containing an alkali metal or an alkali metal compound; a sealed lamp envelope enclosing said arc tube; means for coupling electrical energy through said lamp envelope to said electrodes; and potential control means for electrically interconnecting a first region of said arc tube and a second region of said arc tube, said first and second regions each comprising a portion of said arc tube in proximity to one of said electrodes and selected to have a sufficiently low resistance during arc discharge to maintain a substantially uniform electrical potential on said discharge bulb when said first and second regions are electrically interconnected.

20. A lamp assembly comprising: an arc tube having electrodes sealed therein at opposite ends, said arc tube including a discharge bulb and press seals, said electrodes and said press seals being located on a longitudinal axis of said arc tube, said arc tube enclosing a fill material containing one or more sodium halides; a sealed lamp envelope enclosing said arc tube; means for coupling electrical energy through said lamp envelope to said electrodes; and conductive means electrically connected between a first region of said arc tube and a second region of said arc tube, each of said regions comprising a portion of said arc tube between about the middle of said press seal and a tip of said electrode.

21. An arc lamp comprising: an arc tube having electrodes sealed therein at opposite ends, said arc tube including a discharge bulb and electrode feedthroughs, said arc tube enclosing a fill material which emits light during discharge, said fill material containing an alkali metal or an alkali metal compound; means for coupling electrical energy through said electrode feedthroughs to said electrodes; and potential control means for maintaining a substantially uniform electrical potential on said discharge bulb of said arc tube during discharge.

22. An arc lamp as defined in claim 21 wherein said potential control means comprises first and second conductors electrically connected to said electrode feedthroughs adjacent to said discharge bulb and a third conductor electrically connected between said first and second conductors.