Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J61/00—Gas-discharge or vapour-discharge lamps
  • H01J61/82—Lamps with high-pressure unconstricted discharge having a cold pressure > 400 Torr
  • H01J61/825—High-pressure sodium lamps
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J61/00—Gas-discharge or vapour-discharge lamps
  • H01J61/02—Details
  • H01J61/24—Means for obtaining or maintaining the desired pressure within the vessel
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J61/00—Gas-discharge or vapour-discharge lamps
  • H01J61/02—Details
  • H01J61/30—Vessels; Containers
  • H01J61/35—Vessels; Containers provided with coatings on the walls thereof; Selection of materials for the coatings

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• FIG. 6 is a cross-sectional view of a floating frame metal halide discharge lamp assembly in accordance with the present 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the electrical conductivity of quartz increases as its temperature increases.
• 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.
• 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.
• 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.
• strap 36 is preferably positioned on arc tube 10 in a region indicated by the letter A
• 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.
• 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.
• a noninsulated wire can be utilized.
• 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.
• 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.
• 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.
• the straps 36 and 38 and the wire 40 are replaced with a transparent conductive coating on the outer surface of discharge bulb 12.
• 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
• arc tube 63 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
• Metal straps 70 and 71 are tightly wrapped around the press seals of arc tube 63 as close as possible to discharge bulb 73.
• 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.
• 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.
• 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.

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• 1991
  • 1991-05-22 EP EP19910911587 patent/EP0530318A1/de not_active Withdrawn
  • 1991-05-22 CA CA 2081573 patent/CA2081573A1/en not_active Abandoned
  • 1991-05-22 WO PCT/US1991/003591 patent/WO1991018413A1/en not_active Application Discontinuation

Non-Patent Citations (1)

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