Abstract:
An electroded high watt ceramic metal halide lamp assembly is provided which comprises a light transmissive arc-tube surrounding at least one electrode, a fill disposed in the arc-tube that includes at least one metal halide component and at least one metallic halide getter. The metallic halide getter has a Gibbs Free Energy greater than mercury halide and less than thallium halide, vapor pressure less than mercury halide, free energy of formation of oxide less than Aluminum oxide.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electroded ceramic metal halide lamp (CMH) assembly, and more particularly, to the lumen maintenance of a CMH. 
     CMH lamps can have severe degradation in 100 hr lumens. Such degradation has been known to occur quickly, for example, reduction of 100 hr lumens by 25% in the first 1000 hours of lamp operation. Degradation is believed to arise from wall blackening of the arc-tube. 
     Lumen degradation is primarily due to the transport of tungsten to the walls of the discharge tube, by sputtering during starting, and by chemical transport as halides of tungsten in steady state operation. While halides are necessary components of the arc discharge fill, the transport of tungsten during steady state operation is greatly enhanced by the formation of excess halides, such as iodine. Excess iodine found in most high-intensity discharge (HID) lamps is bound by mercury, which forms a mercury iodide. 
     In order to optimize lumen maintenance, a design choice is sometimes made that shifts the lamp into a design space that sub-optimizes other key critical-to-quality (CTQ) factors such as color rendering index (CRI), correlated color temperature (CCT), color control, etc. However, it would be desirable to reduce the impact of lumen degradation without sacrificing other CTQ factors. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accord with a first embodiment of the present invention, an electroded ceramic metal halide lamp is provided including metallic halide getters to allow optimization of lumen maintenance without sacrificing a region of the arc-tube design space that allows optimization of other key CTQ factors. The metallic halides formed from the getters have a Gibbs free energy lower than mercury iodide, but higher than dysprosium iodide, holmium iodide, thulium iodide, sodium iodide and thallium iodide. 
     Preferably, metallic halides formed from the getters of the present invention have a vapor pressure lower than mercury iodide. 
     In addition, the metallic halides formed from getters create a metallic oxide getter that is less stable than aluminum oxide. The use of metallic halide getters reduce lumen degradation. In another aspect of the present invention, an electroded ceramic metal halide lamp assembly is provided which comprises a light transmissive arc-tube surrounding at least one electrode, a fill disposed in the arc-tube that includes at least one metal halide component; and at least one metallic halide getter wherein the metallic halide getter has a Gibbs free energy value of between about higher than mercury iodide and lower than thallium iodide. 
     In another aspect, an electroded ceramic metal halide lamp assembly is provided which comprises a light transmissive arc-tube surrounding at least one electrode. A fill disposed in the arc-tube includes at least one metal halide component and at least one metallic halide getter, wherein the metallic halide getter has a vapor pressure less than the vapor pressure of mercury iodide. 
     In yet another aspect of the present invention, an electroded ceramic metal halide lamp assembly is provided which comprises a light transmissive arc-tube surrounding at least one electrode, a fill disposed in the arc-tube that includes at least one metal halide component; and at least one metallic halide getter, wherein the metallic halide getter has a free energy of oxide formation less than the free energy of formation of aluminum oxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangement of components, and in various steps and arrangement of steps. The drawings are only for purposes of illustrating a specific embodiment are not to be construed as limiting the invention. 
         FIG. 1  is a schematic diagram of a ceramic metal halide lamp assembly according to the present invention. 
         FIG. 2  graphically compares the Gibbs free energies of various potential getter iodides, including iodides dosed in ceramic metal halide lamps, at a temperature of 1000K; 
         FIG. 3  graphically compares the vapor pressure of various potential getter iodides, including iodides dosed in ceramic metal halide lamps, at a temperature of 1000K; and 
         FIG. 4  graphically compares the free energy of the formation of oxides for potential getters. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Lumen degradation is primarily due to the transport of tungsten to the walls of the arc discharge tube, by sputtering during starting, and by chemical transport as halides of tungsten during steady state operation. Halides contemplated by the present invention include bromide, chloride, iodide and other such halides. The transport of tungsten during steady state operation is greatly enhanced by the formation of excess iodine. For example, excess iodine is found in high intensity discharge (HID) lamps bound by mercury, forming mercury iodide, as opposed to the iodine bound to rare earth and sodium. By the addition of metallic halide components, or getters, to the arc discharge tube, the excess iodine is largely removed from the system, minimizing the formation of mercury iodide, and thereby minimizing tungsten transport in steady state operation. 
       FIG. 1  shows an electroded ceramic metal halide lamp assembly  100  according to the present invention. The walls of the arc discharge tube  102  can consist of a silica glass, as is known in the art. Preferentially, the discharge vessel walls are comprised of a ceramic, transparent or translucent material which can withstand high thermal conditions. For example, the discharge walls of the arc-tube can consist substantially of a monocrystalline metal oxide, such as sapphire, a polycrystalline sintered metal oxide, such as a polycrystalline sintered metal oxide (PCA), yttrium aluminum garnet or yttrium oxide, or of a polycrystalline non-oxidative material, such as aluminum nitride. Such materials allow for wall temperatures of 1500-1600K and resist chemical attacks by halides and sodium. The arc-tube is preferably tubularly shaped having annularly shaped end surfaces and cylindrically shaped outer and inner surfaces. The wall thickness can be of any suitable size. 
     The end caps  104  are formed from a suitable polycrystalline ceramic material, preferably polycrystalline alumina, which is in an unsintered or “green state”. The end caps  104  must preferably include about 0.02 to about 0.2 percent by weight magnesium oxide with polycrystalline alumina powder. Each end cap  104  has a disc-shaped main wall  110 , a cylindrically shaped skirt or flange, and a tubularly shaped extension or flange  106 . The main wall  110  has a planar inner surface facing the end surface of the arc-tube and a planar outer surface facing away from the end surface of the arc-tube. 
     The “green” end caps  104  are initially heated to a prefiring or presintering temperature to remove organic or binder material and to develop green strength. The prefiring temperature is relatively low compared to the sintering temperature. Preferably, the prefiring temperature is in the range of about 900° C. to about 1100° C. The prefiring is preferably preformed in air but alternatively can be any other suitable oxidizing atmosphere for burning-off the organic material. 
     Once cooled, the presintered end caps  104  are placed over the ends of the arc-tube  102  with the end surfaces of the arc-tube engaging the inner surfaces of the end cap main walls and the outer surface of the arc-tube engaging the inner surfaces of the end cap flanges. The end caps, therefore, close the open ends of the arc-tube. 
     The end caps  104  are preferably formed by cold die pressing a mixture of fine ceramic powder into a desired shape. The end caps  104 , however, can alternatively be formed by compressing ceramic powder into a body or block and machining the desired shape from the block, by injection molding, or by any other suitable process. 
     The flange  106  extends axially inward toward the arc-tube from the outer periphery of the main wall  110 . The flange  106  has a cylindrically shaped inner surface which has a diameter sized to form a sufficient monolithic seal with the outer surface of the arc-tube  102 . The length of the flange inner surface is sized to provide a sufficient sealing area between the end cap  104  and the arc-tube  102 . 
     The flanges  106  extend axially outward from the outer surface of the main wall  110  and is located generally at the center of the main wall  110 . The flange  106  and the main wall  110  cooperate to form an axially extending aperture or hole which passes entirely though the end cap  104 . The aperture is sized and shaped to form a sufficient hermetic seal between the electrode assembly  108  and the end cap  104 . Preferably, the aperture is cylindrically shaped. The length of the extension is sized to provide sufficient support for the electrode assembly  108  and to provide a sufficient sealing area between the end cap  104  and electrode assembly  108 . 
     The electrode assembly  108  is of standard construction having a generally straight support and a coil secured to the inner end of the support. The support and the coil are each formed from a high temperature and electrically conductive metal such as molybdenum or tungsten. 
     The arc-tube  102  contains a metal halide fill which provides suitable efficacy and color rendition. As an example, a fill in the present invention comprises a combination of a sodium halide and a cerium halide along with xenon gas. Useful sodium and cerium halides can be selected from the group consisting of bromides, chlorides and iodides, including mixtures thereof such as sodium iodide and cerium chloride. The weight proportion of cerium halide is maintained no greater than the weight proportion of sodium halide in the fill, with a reservoir of these fill materials in the arc-tube being desirable to compensate for any loss of the individual constituents during lamp operation. A typical fill may also include an inert ignition gas, for example argon, and mercury, as well as other metal halide additives. 
     In choosing a metallic halide getter in accord with the present invention, an important aspect is to choose a getter that has a free energy of formation less than the free energy of formation of the mercury halide. Lumen maintenance is optimized by reducing the amount of mercury iodide located within the arc-tube. Thus, the role of the getter is to remove the halide, most typically iodine, from the arc-tube before the free energy of formation of mercury iodide is reached. As can be seen in  FIG. 2 , iodides of zinc, manganese, indium, cadmium, lead and silver satisfy the criteria of metallic halide getters of the present invention having a free energy of formation less than mercury iodide. 
     The getter material can be incorporated into the lamp in the same manner as traditional getter materials are. These include, for example, as a strip of metal on a mounting tab associated with the electrode mechanism or attached to a frame secured to the ends of the arc tube. U.S. Pat. Nos. 7,057,350 and 6,586,878 provide teachings of this and are herein incorporated by reference. 
     Another important aspect in choosing a metallic halide getter in accordance with the present invention is the vapor pressure of the metallic getter with respect to the vapor pressure of mercury iodide. Getter materials that satisfy this criterion will remove excess iodine formed in the arc-tube from the discharge environment. Metallic iodide getters that meet this requirement are shown in  FIG. 3 , and include sodium, tin, lead, indium, copper, manganese, cadmium, zinc and silver. 
     A final desirable attribute for the metallic getter is the stability of the oxides of the metallic getters relative to aluminum oxide. Preferably, the free energies of formation of the metallic getter oxides be lower than that of aluminum oxide, otherwise the metallic iodide getter material could cause degradation of the main discharge body of the CMH lamp. As can be seen by the graph in  FIG. 4 , copper, thallium, lead, cadmium, tin, indium, zinc and manganese. 
     The use of metallic halide getters within the fill of the arc-tube in electroded high watt ceramic metal halide lamps according to the preceding will reduce the formation of mercury iodide, and therefore tungsten transport should be inhibited. This will achieve better lumen maintenance. 
     Zinc, manganese, indium, cadmium and lead, which satisfy each criteria may be particularly good selections as a getter material. Furthermore, it is noted that the present invention may be particularly beneficial in conjunction with high wattage CMH lamps. For example, lamps operating at above about 150 watts. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.