Abstract:
A tipless arc tube for a high intensity discharge lamp and method of manufacture wherein the arc tube may remain open to an uncontrolled atmosphere during the step of hermetically scaling the arc tube. The novel arc tube and method obviate the need to perform any process steps within a controlled atmosphere. The pressure of the fill gas sealed within the arc tube may be controlled by controlling the temperature of the fill gas during the step of hermetically sealing the arc tube. The novel arc tube and method obviate the need to use a pump to control the fill gas pressure.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/800,669 filed Mar. 8, 2001, assigned to the assignee of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to high intensity discharge (“HID”) lamps, arc tubes, and methods of manufacture. More specifically, the present invention relates to HID lamps, arc tubes, and methods of manufacture wherein the pressure of the fill gas in the arc tube is greater than one atmosphere at substantially room temperature. 
     HID lamps have been developed as a point source and are particularly suited for fiber optic lighting systems, projection display, and automotive headlamps. Metal halide lamps with xenon as a fill gas have been favored in many applications because of the instant light capability, relatively long life, and relatively high efficiency in producing white light with good color rendition. 
     In the manufacture of HID lamps for point sources, it is desirable to obtain a final fill gas pressure which is greater than one atmosphere at substantially room temperature. Final fill gas pressures greater than about five atmospheres are common and fill gas pressures may be as high as about six hundred atmospheres. 
     In the manufacture of xenon metal halide lamps, it is known to obtain a superatmospheric xenon pressure by freezing an amount of xenon into the light emitting chamber of the lamp prior to sealing the chamber. The volume of xenon frozen into the chamber (when at substantially one atmosphere and room temperature) is larger than the volume of the chamber so that the pressure of the xenon sealed within the chamber is greater than one atmosphere when the temperature of the xenon returns to substantially room temperature. The pressure (in atm) of the fill gas sealed within the chamber at substantially room temperature equals the ratio of the volume of gas frozen into the chamber (at substantially one atmosphere and room temperature) relative to the volume of the chamber. 
     In the known methods of making superatmospheric arc tubes, the prior art teaches that the interior of the arc tube body must be isolated from an uncontrolled atmosphere once the solid fill material and mercury are introduced into the interior of the arc tube body and the second electrode lead assembly is positioned in the remaining open end portion to prevent oxidation of the metallic components of the second electrode lead assembly during the sealing process of the second end portion. 
     The prior art teaches that the interior of the arc tube may be isolated from an uncontrolled atmosphere by either (i) placing the arc tube body in a controlled atmosphere such as a glove box as taught in U.S. Pat. No. 5,108,333 to Heider et al. dated Apr. 28, 1992 or (ii) connecting the open end to a vacuum system which provides the necessary seal as taught in U.S. Pat. No. 5,505,648 to Nagasawa et al. dated Apr. 9, 1996. 
     As disclosed in Heider et al., one end portion of the arc tube body must be long enough to enclose the entire electrode lead assembly when the assembly is positioned within the end portion. Once the arc tube is placed within the controlled atmosphere of the glove box, the body is filled with xenon and then the end portion is fused closed so that the entire electrode lead assembly is enclosed within the body. The arc tube may then be removed from the glove box so that the xenon may be frozen into the chamber and then sealed by shrinking or pinching the second end portion. The excess portion of the end portion is then removed to expose the outer lead of the electrode lead assembly. 
     The prior art methods suffer from the significant disadvantage of the requirement for isolating the arc tube body from the uncontrolled atmosphere. This has generally required the use of a glove box or vacuum system. Such methods are complex and difficult to automate. 
     Accordingly, it is an object of the present invention to obviate many of the deficiencies of the prior art and provide a novel HID lamp, arc tube and method of making arc tubes. 
     It is another object of the present invention to provide a novel arc tube and method of making arc tubes for HID lamps which obviates the need to perform any process steps within a controlled atmosphere. 
     It is a further object of the present invention to provide a novel arc tube and method of making tipless arc tubes for HID lamps in which the arc tube remains open to an uncontrolled atmosphere during the step of finally sealing the arc tube. 
     It is yet another object of the present invention to provide a novel arc tube and method of making tipless arc tubes for HID lamps in which communication of an inert fill gas with an uncontrolled atmosphere such as air is maintained until the arc tube is hermetically sealed. 
     It is yet a further object of the present invention to provide a novel arc tube and method of making arc tubes for HID lamps which obviates the need to remove a portion of the end portion to expose the outer portion of the electrode lead assembly. 
     It is still another object of the present invention to provide a novel arc tube and method of making arc tubes for HID lamps in which each end portion of the arc tube body has substantially the same length as the end portions of the finished arc tube. 
     It is still a further object of the present invention to provide a novel apparatus for extending the tubular opening formed by the end portion of an arc tube body and method of making arc tubes for HID lamps. 
     It is still another object of the present invention to provide a novel arc tube and method of making arc tubes for HID lamps in which the temperature of the fill gas is controlled prior to sealing the arc tube in an uncontrolled atmosphere. 
     It is yet another object of the present invention to provide a novel arc tube and method of making arc tubes for HID lamps having superatmospheric fill pressure in which there is no pressure differential at the time of sealing. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an arc tube body having a bulbous light emitting chamber. 
     FIGS. 2 a-e  illustrate the prior art process steps for forming the arc tube body illustrated in FIG.  1 . 
     FIG. 3 a  illustrates the step of heating the end portion of an arc tube body while flushing the interior of the body with an inert gas during the pinch sealing process. 
     FIG. 3 b  is a cross-sectional view of an arc tube body having an electrode lead assembly pinch sealed in one end. 
     FIG. 4 is a schematic illustrating an electrode lead assembly. 
     FIG. 5 illustrates the step of introducing the solid lamp fill material and mercury into the interior of the chamber. 
     FIG. 6 is a cross-sectional view of a prior art arc tube body having its elongated end portion tipped off beyond the electrode lead assembly. 
     FIG. 7 illustrates the step of heating the upper end portion of an arc tube body while maintaining the interior of the body open to the surrounding atmosphere. 
     FIG. 8 is a cross-sectional view of an arc tube made by one method of the present invention. 
     FIG. 9 is a cross-sectional view of one embodiment of an arc tube body according to the present invention. 
     FIG. 10 is a cross-sectional view of an arc tube made from the arc tube body illustrated in FIG.  9 . 
     FIG. 11 a  illustrates the step of flushing and filling the arc tube body with the final fill gas according to the present invention. 
     FIG. 11 b  illustrates the steps of positioning the electrode lead assembly and pinch sealing the second end portion of the arc tube according to one aspect of the present invention. 
     FIG. 12 illustrates the steps of positioning the electrode lead assembly and pinch sealing the second end portion of the arc tube according to another aspect of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention finds utility in arc tubes for all types and sizes of HID lamps and methods of manufacture of such lamps generally. By way of example only, certain aspects of the present invention will be described in connection with tipless quartz formed body arc tubes for double-ended metal halide lamps. 
     FIG. 1 illustrates a prior art arc tube body which has been formed from a quartz tube. The arc tube body  10  comprises a bulbous light emitting chamber  12  intermediate open tubular end portions  14 , 16 . The arc tube body  10  may be formed using any suitable conventional method. 
     Formed body arc tubes may be manufactured in the manner described in the Lamouri et al. copending patent application Ser. No. 09/597,547 filed Jun. 19, 2000, and entitled “Horizontal Burning HID Lamps And Arc Tubes” assigned to the assignee of the present invention. FIGS. 2 a-e  illustrate such a method of forming arc tubes from quartz tubing (FIG. 2 a ) by loading the tubing on a lathe and heating the tubing (FIG. 2 b ), gathering the heated tube by axial movement of the tube (FIG. 2 c ), and expanding with internal pressure the gathered tube against a mold (FIG. 2 d ) to obtain the desired shape of the arc tube body (FIG. 2 e ). The thickness of the arc tube body may be adjusted by the amount of quartz accumulated in the gathering process and the shape of the arc tube body is determined by the shape of the mold. 
     As shown in FIGS. 3 a  and  3   b , a first electrode lead assembly  18  is positioned within the open tubular end portion  14  and the end portion  14  is sealed using a conventional pinch sealing process. During the pinch sealing process, a portion of the end portion  14  is heated to soften the quartz, and then the softened portion is pressed together and around the portion of the electrode lead assembly  18  positioned therein using conventional pinch jaws (not shown) forming pinch seal  20 . The pinch seal  20  fixes the position of the assembly  18  relative to the arc tube body  10  and provides a hermetic seal between the interior of the chamber  12  and the exterior of the body  10  through the end portion  14 . 
     The electrode lead assembly  18  may be a conventional lead assembly comprising several metallic components including a tungsten electrode  22 , a molybdenum foil  24 , and a molybdenum outer lead  26  as shown in FIG.  4 . During the pinch sealing process, the metallic components may reach temperatures as high as 2000° C. or more when the quartz is softened. At such high temperatures, the metallic components are highly susceptible to corrosion if exposed to moisture in a reactive atmosphere such as air. To prevent such corrosion, an inert gas is introduced into the chamber  12  through the remaining open tubular end portion  16  and flows past the lead assembly  18  during the pinch sealing process. The gas may be introduced by any conventional means such as insertion of a probe  28  as shown in FIG. 3 a  or the connection of a hose (not shown) to the open end portion  16 . The gas may be any inert gas such as nitrogen or argon or mixtures thereof. 
     The next step is to dose the arc tube body with the desired fill material by introducing the material into the chamber  12  through the remaining open end portion  16 . The solid lamp fill material  30  may be introduced into the chamber  12  through the remaining open end portion  16  by any conventional means such as a pin type dispenser of lamp fill pellets manufactured by APL Engineered Materials, Inc. Mercury 31, if desired, may also be introduced into the chamber  12  through the end portion  16  by any conventional means. FIG. 5 illustrates an arc tube body  10  having lamp fill pellets  30  and mercury  31  within the chamber  12 . 
     The remaining steps in the process include the flushing and filling of the chamber with the final fill gas, the positioning of the second electrode lead assembly in the remaining open end portion, and the sealing of the remaining open end portion. As discussed with respect to the pinch sealing of the first end portion, it is important to prevent the exposure of the metallic components of the electrode lead assembly to a corrosive atmosphere at high temperature. 
     The prior art methods teach the necessity to isolate the components from an uncontrolled atmosphere by either (i) placing the arc tube body in a glove box, or (ii) connecting the open end of the arc tube body to a vacuum system prior to filling the interior of the arc tube body with the final fill gas and positioning the second electrode lead assembly. As shown in FIG. 6, the open end portion  16  may be fused closed outside the lead assembly  32  once the final fill pressure is obtained to isolate the interior of the chamber  12  containing an inert atmosphere. Thus the prior art prevents corrosion of the metallic components of the lead assembly during the pinch sealing of the end portion  16  by isolating the components in an inert atmosphere within the interior of the arc tube body. 
     It has been discovered that the isolation of the interior of the arc tube from an uncontrolled atmosphere by use of a glove box or vacuum system may be obviated by orienting the arc tube body  10  so that the open end portion  16  extends upwardly as shown in FIGS. 5 and 7, and relying on the relative weight of the fill gas to air to maintain a fill of inert gas within the arc tube body. The final inert fill gas may be introduced into the interior of the chamber  12  by insertion of a suitable conventional probe  34 . The fill gas may be any inert gas such as argon, neon, xenon, krypton, or a combination thereof. In the preferred embodiment of the present invention, the fill gas is xenon or a mixture of xenon and argon, both of which are heavier than air and will tend to remain within the interior of the arc tube body  10  so long as the body remains in a substantially vertical orientation, thus retarding the influx of the lighter contaminated air of the uncontrolled atmosphere surrounding the arc tube. 
     The interior of the arc tube body  10  is flushed and filled with the fill gas to the tip  38  of the end portion  16  so that all other gases are displaced. Once the arc tube body is flushed and filled, the probe  34  may be removed and the second electrode lead assembly  32  is positioned within the end portion  16  as shown in FIG.  7 . The end portion  16  must extend sufficiently above the lead assembly  32  so that the lead assembly  32  will remain immersed in the column of fill gas within the end portion  16  despite some mixing of the fill gas with the uncontrolled atmosphere surrounding the arc tube body near the tip  38  of the end portion  16 . 
     As shown in FIGS. 7 and 8, the second end portion  16  may then be sealed by a conventional pinch sealing process. A portion of the end portion  16  is heated to soften the quartz, and then the softened portion is pressed together and around the portion of the electrode lead assembly  32  positioned therein using conventional pinch jaws (not shown) forming pinch seal  36 . The pinch seal  36  fixes the position of the assembly  32  relative to the arc tube body  10  and provides a hermetic seal between the interior of the chamber  12  and the exterior of the body  10  through the end portion  16 . In another embodiment, the end portion may be sealed by a shrink sealing process. 
     As further illustrated in FIG. 8, the chamber  12  is now hermetically sealed from the exterior of the arc tube body  10 . The excess portion of the end portion  16  may then be removed to expose the outer lead  42  of the electrode lead assembly  32 . 
     FIGS. 9 and 10 illustrate another embodiment of the present invention. The arc tube body  50  may be formed having a chamber  52  intermediate the open end portions  54 , 56 . The end portions  54 , 56  may have substantially the same length. In the preferred embodiment, the length of the end portions  54 , 56  of the arc tube body  50  may be substantially the length of the end portions of the finished arc tube so that the step of trimming the excess portion of the second end portion once the chamber is sealed may be eliminated. However, it remains necessary to provide a column of fill gas which is sufficiently long so that the second electrode lead assembly  58  positioned within the second end portion  56  is completely immersed in fill gas during the pinch sealing process of the second end portion. 
     In one embodiment of the present invention, the column of fill gas may be extended beyond the length of the end portion by communication of the open end portion with a mechanical means forming an elongated shaft having substantially the same diameter as the outside diameter of the end portion. In the embodiment shown in FIGS.  11   a  and  11   b , a flush and fill block  60  forms a main shaft  62  which communicates with the open end portion  56  of the arc tube body  50  during the steps of positioning the electrode lead assembly  58 , flushing/filling the body  50  with the final fill gas, and pinch sealing the end portion  56 . 
     The block  60  forms the main shaft  62  and one or more auxiliary shafts  64  which provide communication between the main shaft  62  and the surrounding atmosphere. The open end of the end portion  56  may be positioned relative to the block  60  to effect communication of the main shaft  62  with the tubular opening formed by the end portion  56 . The interior of the arc tube chamber  52  and open end portion  56  may be flushed and filled with the final fill gas by insertion of a conventional probe  66  into the chamber  52  as shown in FIG. 11 a.    
     Once the arc tube body  50  is flushed and filled with the final fill gas, the probe  66  may be removed. The fill gas now fills the end portion  56  and the main shaft  62  and tends to remain within the shaft  62  as a result of the relative weight of the fill gas to the surrounding atmosphere. The electrode lead assembly  58  may then be positioned within the end portion  56  and main shaft  62  using a conventional assembly holder  68  as shown in FIG. 11 b . With the fill gas filling the shaft  62  to the top, the electrode lead assembly  58  may be completely immersed in the fill gas to prevent corrosion during the pinch sealing process. 
     In one aspect of the present invention, the fill gas may be cooled at the time the chamber is hermetically sealed to obtain a superatmospheric fill gas pressure at substantially room temperature. Care must be given to prevent contamination, e.g., by continuing to introduce fill gas into the arc tube during the cooling process. 
     FIG. 12 illustrates the embodiment of the present invention wherein the fill gas is xenon. With reference to FIG. 12, a blanket of argon may be placed over the xenon which now fills the shaft  52  to the top. The temperature of a portion of the chamber  52  may then be reduced to a temperature below the freezing point of xenon, i.e., temperatures of about −112° C. or lower, by any conventional means such as by a liquid nitrogen spray  61 . Once the entire volume of xenon within the interior of the body  50  and shaft  62  is frozen into the chamber, the end portion  56  may be sealed by any conventional sealing process such as pinch or shrink sealing. The electrode lead assembly  58  will remain immersed in a non-reactive gas during the sealing process by maintaining the argon blanket over the xenon which will fill the voids within the interior of the body  50  and shaft  62  created by the freezing of the xenon into the chamber  52 . Small amounts of argon may be sealed within the chamber  52 , but will not affect the performance of the lamp. 
     In this embodiment, the final fill pressure of the xenon in the sealed arc tube at substantially room temperature, is determined by the ratio of the volume of the interior of the arc tube body  50  and the shaft  62 , to the volume of the sealed chamber  52 . The volume of the shaft  62  may be varied to obtain the desired final fill pressure. 
     In another embodiment of the present invention, a flow of gas comprising a mixture of at least two non-reactive gases may be introduced into the chamber  52 . The temperature of the chamber may be reduced below the freezing point of one of the gases, but remain above the freezing point of the other gas so that one of the gases will freeze and remain in the chamber while the gas will continue to flow. The final fill pressure may be determined by controlling the composition of the gas mixture and the flow rate of the gas. Once the desired amount of gas has been frozen into the chamber, the flow may be stopped and the end portion  58  may be sealed to thereby hermetically seal the chamber  52 . 
     For example, the gas may comprise xenon and argon. If the temperature of the chamber is reduced to below the freezing point of xenon but remain above the freezing point of argon, the xenon will freeze in the chamber while the argon will continue to flow to provide a non-reactive atmosphere surrounding the second electrode lead assembly during the pinch or shrink sealing of the remaining open end portion of the arc tube. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.