Abstract:
The present application discloses high intensity discharge (“HID”) lamps, arc tubes, and methods of manufacture. The application relates to HID lamps, arc tubes, and methods of manufacture wherein a precise amount of fill gas may be contained in the light-emitting chamber of the arc tube so that the pressure of the fill gas contained in the arc tube at substantially room temperature may be precisely controlled at pressures greater than about one-half atmosphere.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of U.S. Provisional Application Nos. 60/669,380 and 60/587,048, the disclosures of which are hereby incorporated by reference. 
       RELATED APPLICATIONS 
       [0002]    The present application is related to commonly assigned U.S. Pat. No. 6,612,892 entitled “HIGH INTENSITY DISCHARGE LAMPS, ARC TUBES, AND METHODS OF MANUFACTURE,” issued Sep. 2, 2003, commonly assigned U.S. Pat. No. 6,517,404 entitled “HIGH INTENSITY DISCHARGE LAMPS, ARC TUBES, AND METHODS OF MANUFACTURE,” issued Feb. 11, 2003; and copending and commonly assigned U.S. patent application Ser. No. 10/457,442, entitled “HIGH INTENSITY DISCHARGE LAMPS, ARC TUBES, AND METHODS OF MANUFACTURE,” filed Jun. 10, 2003, the disclosures of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    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 about one-half atmosphere at substantially room temperature. 
         [0004]    Short arc gap metal halide lamps are particularly suited for fiber optic lighting systems, projection display, and automotive headlamps. Metal halide lamps with high pressure fill gas have been favored in many applications because of the fast warm-up, relatively long life, and relatively high efficiency in producing white light with good color rendition. 
         [0005]    In the manufacture of such lamps, 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 two hundred atmospheres. 
         [0006]    In the manufacture of metal halide lamps, it is known to obtain a superatmospheric fill gas pressure by freezing an amount of the fill gas (heretofore xenon) into the light emitting chamber of the lamp prior to sealing the chamber. The volume of gas 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 gas sealed within the chamber is greater than one atmosphere when the temperature of the gas returns to substantially room temperature. The pressure 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. 
         [0007]    In the manufacture of superatmospheric arc tubes, it is difficult to control the amount of fill gas contained in the sealed arc tube due to the difficulty in preventing the escape of fill gas from the arc tube during the sealing process when the open tubular end portion of the arc tube is heated to about 2000° C. prior to pinch or shrink sealing the end portion. 
         [0008]    Applicant has discovered a novel method for making superatmospheric arc tubes containing a fill gas such as xenon or krypton wherein the amount of the fill gas contained in the arc tube may be precisely controlled. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  illustrates the step of heating a pre-formed arc tube body. 
           [0010]      FIGS. 2   a,    2   b,  and  2   c  illustrate the steps of flushing the arc tube body, injecting and freezing the fill gas, and pinch sealing the second end portion of the arc tube. 
           [0011]      FIG. 3  illustrates the steps of positioning the first electrode lead assembly, flushing the arc tube, and pinch sealing the first end portion. 
           [0012]      FIG. 4  illustrates the steps of evacuating the arc tube body, injecting the fill gas, evacuating excess fill gas, and pinch sealing the second end portion. 
       
    
    
     DETAILED DESCRIPTION  
       [0013]    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. 
         [0014]      FIG. 1  illustrates an arc tube body which has been formed from a quartz tube. The arc tube body  80  comprises a bulbous chamber  83  intermediate open tubular end portions  82 , 84 . The arc tube body  80  may be formed using any suitable conventional method. 
         [0015]      FIGS. 2   a,    2   b,  and  2   c  illustrate several steps according to one embodiment of the present invention. With reference to  FIGS. 2   a,    2   b,  and  2   c,  the fill gas may be injected into the chamber  83  through a probe  89 . While maintaining a blanket of inert gas over the electrode assembly  87 , the temperature of the chamber  83  may be reduced to a temperature below the freezing point of the fill gas by any conventional means such as by the application of liquid nitrogen  90  (for example, by dip or spray). Once the desired temperature is reached, a volume of fill gas is injected and frozen into the chamber. The end portion  84  may then be hermetically sealed by any conventional sealing process such as pinch or shrink sealing. A cover gas may be applied to the open end during these process steps. The process according to this aspect of the invention results in acceptable levels of variability and a greatly reduced amount of time required to freeze the fill gas into the chamber, i.e., by several seconds or more. 
         [0016]    It has been discovered that in the manufacture of superatmospheric arc tubes having a krypton fill, due to the lower freezing temperature of krypton with respect to xenon (i.e., −157° C. vs. −112° C.), it is difficult to precisely control the amount of krypton sealed within the arc tube due to evaporation losses during the sealing process. In the embodiment described above the open end of the arc tube may be heated to temperatures as high as 2000° C. in preparation for sealing while simultaneously reducing the temperature in the arc tube chamber to freeze the fill gas injected into the chamber. It is suspected that heat from the sealing process is transferred to the frozen fill gas via three primary means. First, radiative heat may be transferred from the arc tube heating apparatus, although this effect is understood to be minimal. Second, the quartz arc tube body may conduct heat into the arc tube chamber, although this effect is minimized due to the low thermal conductivity of quartz. Third, the gaseous fill in the chamber may conduct heat via convection from the heat source to the frozen fill gas. 
         [0017]    It has been discovered that the amount of fill gas (for example, krypton) may be precisely controlled by evacuating the gaseous fill from the interior of the arc tube prior to heating the end portion for the sealing process. The evacuation of the gaseous fill eliminates the convective transfer of heat from the sealing process to the frozen fill gas, and thus significantly reduces the loss of fill gas by evaporation during the sealing process. 
         [0018]    In an embodiment of the present invention, an arc tube having a superatmospheric pressure of fill gas (for example, argon, xenon, krypton, or mixtures thereof) may be obtained by using a vacuum pump flush process prior to freezing the fill gas into the arc tube chamber. 
         [0019]    According to this embodiment, the pre-formed arc tube body  80  may be superheated using conventional techniques such as exposure to a flame as shown in  FIG. 1 . A flow of inert gas such as nitrogen (not shown) may be used to clean the surface of the arc tube after the temperature of the arc tube has been elevated. 
         [0020]    An electrode lead assembly  85  may then be positioned within the open tubular end portion  82  of the arc tube  80  by conventional means such as an insertion probe (not shown) as shown in  FIG. 3 . A flush gas assembly  86  connected to an arc tube holder  81  may be used to inject flush gas into the other open tubular end portion  84  of the arc tube  80  to provide an inert blanket around the electrode lead assembly  85  during the sealing process. The end portion  82  may be immediately sealed by any conventional sealing process such as pinch or shrink sealing once the electrode lead assembly  85  is fully inserted into the end portion  82  and blanketed by inert gas. 
         [0021]    Next, the arc tube  80  having the electrode lead assembly  85  sealed in the end portion  82  may be dosed with the desired fill material by introducing the material into the arc tube chamber  83  through the open end portion  84 .  FIG. 4  illustrates an arc tube body  80  having lamp fill pellets  91  and mercury  92  within the arc tube chamber  83 . 
         [0022]    Once the arc tube  80  is dosed with the desired solid fill material, the open end portion  84  of the arc tube  80  may be mated with a pump flush block  100  as shown in  FIG. 4 . With reference to  FIG. 4 , the pump flush block  100  includes a central shaft  102  that communicates with the open end portion  84 . The electrode lead assembly  87  may be inserted into the end portion  84  using the probe  104 . The pump flush block  100  may include multiple ports  106 ,  108 , and  110  for connection to a vacuum pump assembly (not shown), a source of inert gas (not shown), and a source of fill gas (not shown) at a pressure greater than greater than about one torr. The pressure of the fill gas is chosen to: optimize fill speed, provide a measurable pressure drop, and minimize the amount of fill gas for cost-effectiveness. For typical applications, pressures between 50 and 350 torr have been found suitable for a 5 cc source of fill gas. 
         [0023]    Once the arc tube  80  is mated to the pump flush block  100 , impurities in the arc tube may be removed by several methods. In one method, the arc tube may be thoroughly evacuated using a vacuum pump assembly through vacuum pump port  106 . In another method, the impurities may be removed using a pump/flush process. In the pump/flush process, the arc tube is evacuated using the vacuum pump assembly, filled with an inert gas via fill port  108 , and then evacuated again. The arc tube may be pump/flushed several times during which a pre-heat of the arc tube body and electrode assembly for a predetermined amount of time may be performed. When the impurities in the arc tube have been diluted to the desire level, the fill gas may be injected from the source of fill gas into the arc tube via the fill gas port  108  to fill the arc tube body and head volume of the pump flush block. The fill gas may then be frozen into arc tube chamber  83  by reducing the temperature below the freezing point of the fill gas by any conventional means such as by the application of liquid nitrogen  90  to the chamber  83 . The amount of fill gas deposited in the arc tube may be precisely controlled by calculating the desired pressure drop in the system volume. For example, it may be determined that the amount of fill gas required to be frozen into the chamber is obtained by obtaining a pressure drop in the arc tube from 200 torr to 190 torr. In this example, the fill gas is introduced into the arc tube at 200 torr. The arc tube and head are isolated and the chamber is cooled by the application of liquid nitrogen until the pressure drops to 190 torr. 
         [0024]    When the desired pressure differential is achieved, the arc tube may be evacuated again to remove the gaseous content of the chamber leaving only the frozen fill gas in the chamber. When a vacuum is drawn in the chamber, the end portion  84  may be hermetically sealed by any conventional sealing process such as pinch or shrink sealing. 
         [0025]    The processes according to the present invention are also applicable to arc tubes where the electrodes are sealed in a single end of the arc tube. The arc tube may be flushed and dosed and then the two electrode lead assemblies may be inserted into the end portion of the arc tube. The evacuation, pump/flush, freezing of the fill gas, evacuation, and sealing steps may then be performed. 
         [0026]    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.