Abstract:
The disclosure has described a sub-miniature arc lamp and a method to make a sub-miniature arc lamp. An embodiment of the sub-miniature arc lamp includes a sapphire body having a first end and a second end, the first end being coupled to a first cap and the second end being coupled to a second cap to define a sealed envelope, wherein a first electrode being mounted in the first cap and a second electrode being mounted in the second cap are enclosed within the envelope. Other embodiments are described and claimed.

Description:
FIELD OF INVENTION 
     The present invention relates to arc lamps, and more particularly, to sub-miniature arc lamps. 
     BACKGROUND 
     In optical systems involving the generation and controlled radiation of long or continuous pulses of light, such as spectroscopy, or solar simulation, where high intensity, color correct illumination of sensitive working areas is required, such as in fiber optics illumination devices, it is advantageous to have a light source capable of producing the highest possible light flux density. Products utilized in such applications include short arc inert gas lamps. An existing short arc lamp includes a sealed quartz chamber containing a gas pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. A window provides for the transmission of the generated light, and a reflector may be positioned surrounding the arc gap. 
     Various applications require small short arc lamps, such as in video projectors and medical and dental equipments. Sub-miniature arc lamps are produced to meet the needs of these applications. In an existing design of a sub-miniature arc lamp, an anode and a cathode are mounted inside a quartz tube with a top and a base. The anode and the cathode are separated by a short arc gap. The joint between the quartz tube and the top and the joint between the tube and the base are sealed. The quartz tube is filled with inert gas. During operation, the breakdown voltage is exceeded across the short arc gap between the anode and the cathode, an illuminating flow of electrons is discharged from the cathode to the anode. 
     Generally speaking, there are four major reasons for lamp failure, including electrode erosion, contamination of the fill gas, cracked glass to metal seals, and explosion caused by devitrification or cracking of the quartz tube. Erosion of the electrodes causes a reduction in light output and, potentially, failure of the quartz tube. Devitrification of the quartz tube, caused by the high temperature inside the quartz tube during operation, is the removal or destruction of the glassy quality of the quartz tube. In addition to devitrification, the high temperature inside the quartz tube can also lead to the cracking of the quartz tube. Eventually, the devitrification and cracking of the quartz tube will lead to breakage of the quartz tube. Besides damaging the lamp, breakage of the quartz tube can cause user injuries as well. 
     Moreover, high peak currents discharged through the lamp during operation generate instantaneous high temperature on the inner wall of the quartz tube. The high temperature on the inner wall of the quartz tube causes the silicon oxide in the quartz tube to reduce to silicon and oxygen, which causes contamination of the fill gas. In addition to high temperature, devitrification will also lead to oxygen generation from the quartz tube. The electronegative nature of the oxygen inhibits the electron flow and effectively raises the breakdown voltage of the lamp. An increased breakdown voltage impedes ignition and triggers reliability problems with the lamp. 
     A prior solution to reduce the contamination inside the quartz tube is to use gas additives to reduce tungsten wall coverage inside the quartz tube. However, the gas additives also make processing the sub-miniature arc lamps at high temperature difficult. 
     Another prior solution is to operate the lamp in a vertical position to minimize devitrification of the quartz tube. Horizontal operation in high pressure quartz lamps tends to cause early failures due to tube devitrification problems. However, having to operate the arc lamp in vertical orientation complicates the design of the optical equipment using the arc lamp. 
     SUMMARY 
     A sub-miniature arc lamp and a method to make a sub-miniature arc lamp are described. An embodiment of the sub-miniature arc lamp includes a sapphire body having a first end and a second end, the first end being coupled to a first cap and the second end being coupled to a second cap to define a sealed envelope, wherein a first electrode being mounted in the first cap and a second electrode being mounted in the second cap are enclosed within the envelope. Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the appended claims to the specific embodiments shown, but are for explanation and understanding only. 
         FIG. 1A  shows a side view, a top view  102 , and a bottom view  103  of an embodiment of a sub-miniature arc lamp. 
         FIG. 1B  shows a cross-section view of the embodiment of the sub-miniature arc lamp along the axis A as shown in  FIG. 1A . 
         FIG. 1C  shows a full size view of an embodiment of a mercury xenon 150 Watt lamp. 
         FIG. 2  shows an embodiment of a cathode assembly. 
         FIG. 3  shows an embodiment of an anode assembly. 
         FIG. 4  shows an alternate embodiment of a sub-miniature arc lamp. 
         FIG. 5  shows an embodiment of a sub-miniature arc lamp. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known components, structures, and techniques have not been shown in detail in order not to obscure the understanding of this description. 
       FIG. 1A  shows a side view  101 , atop view  102 , and a bottom view  103  of an embodiment of a sub-miniature arc lamp. The embodiment includes a sapphire tube  150 , a first cap on top of the sapphire tube  150  (also referred to as a top)  110 , a tubulation  114 , an anode  120 , a second cap on the base of the sapphire tube  150  (also referred to as a base)  130 , a cathode  140 , and a number of getters  160 . The tubulation  114  is inserted into the top  110 . The anode  120  is mounted in the top  110  through the tubulation  114 . The cathode  140  is mounted in the base  130 . The top  110  and the base  130  are attached to the top and bottom of the sapphire tube  150  respectively to form a sealed envelope. The sealed envelope inside the sapphire tube is filled with an inert gas. Replacing the quartz tube with a sapphire tube reduces devitrification of the tube during operation, and hence, helps to prolong lamp life. Moreover, using a sapphire tube also reduces contamination of the inert gas inside the sapphire tube because, unlike the quartz tube, the inner wall of the sapphire tube does not release oxygen during operation at high temperature. It should be apparent to one of ordinary skill in the art that sapphire bodies of other shapes, such as a sphere, can be used to build a sub-miniature arc lamp. The sapphire tube in  FIG. 1A  is used only for illustrative purposes, and should not be construed to limit the scope of the appended claims. 
     Furthermore, a number of getters are mounted along the anode  120  and the cathode  140  to absorb or remove impurities inside the sapphire tube. Along the cathode  140 , a retainer ring  165  is put on top of the getters  160  to hold the getters in place along the cathode. In one embodiment, the getters include one or more mercury (“Hg”) dispensing getters. The mercury-dispensing getters, made from a mixture of titanium mercury alloys marketed by SAES GETTERS S.p.A. in Milano, Italy under the trade names St 505® and St 101®, are non-evaporable. The alloy mixture can be compressed into various shapes, such as, pills, rings, pellet strips, or slotted strips. The combination of alloys dispenses a controlled quantity of mercury and absorbs the impurities within the inert gas inside the sealed sapphire tube. In an alternate embodiment, the getters are mounted along only the cathode. In another embodiment, the getters are mounted along only the anode. 
       FIG. 1B  shows the cross-section view of the embodiment of the sub-miniature arc lamp along axis A in  FIG. 1A . The embodiment includes a top  110 , a tubulation  114 , an anode  120 , a base  130 , a cathode  140 , a sapphire tube  150 , and a number of getters  160 . The tubulation  114  is inserted into the top  110 . The anode  120  is pressed into the top  110  at  113  through the tubulation  114 , i.e. the anode  120  is coupled to the top  110  by press fit. Similarly, the cathode  140  is press-fitted in the base  130  at  133 . However, it should be apparent to one of ordinary skill in the art that other mounting techniques can be used to mount the anode to the top and to mount the cathode to the base. The base  130  is attached to the bottom of the sapphire tube  150  at the welded ends  135 . The top  110  is attached to the top of the sapphire tube  150  to form a sealed envelope. The sealed envelope inside the sapphire tube is filled with an inert gas via the gas entry hole  112  in the top  110 . The embodiment further includes a set of getters  160  mounted along the anode and the cathode. 
     During operation of the lamp, the sealed envelope of the sapphire tube  150  is filled with an inert gas at a pressure of several atmospheres. In one embodiment, the envelope is filled with xenon. When the breakdown voltage is exceeded across the short arc gap between the anode  120  and the cathode  140 , an illuminating flow of electrons is discharged from the cathode  140  to the anode  120 . 
       FIG. 1C  shows a full size view of an embodiment of a mercury xenon 150 Watt lamp. The lamp shown in  FIG. 1C  has a height of 1.43 inches and a diameter of 0.46 inches. It should be understood that the embodiment shown in  FIG. 1C  is for illustrative purpose only. Other embodiments of a mercury xenon lamp can have different dimensions and power. 
       FIG. 2  shows an embodiment of a base and cathode assembly of a sub-miniature arc lamp. The assembly includes a base  230 , a number of mercury dispensing getters  260 , a number of spacers  261 , and a cathode  240 . On the cathode  240 , there is a ridge  241  near the top of the cathode  240  to accommodate a retaining ring (not shown). The retaining ring holds the getters  260  in place when the getters  260  are mounted along the cathode  240 . The components in  FIG. 2  are shown separated from each other in order to provide the reader with an unobstructed view of every component. In practice, the getters  260  are mounted along the cathode  240 , and the lower end of the cathode  240  is pressed into the hole  235  in the middle of the base  230 . Each of the spacers  261  is mounted along the cathode in between two getters. 
       FIG. 3  shows an embodiment of a top and anode assembly of a sub-miniature arc lamp. The assembly includes an anode  320 , a number of mercury dispensing getters  360 , a tubulation  314 , and a top  310 . The components in  FIG. 3  are shown separated from each other in order to provide the reader with an unobstructed view of each component. In practice, the getters  360  are mounted along the anode  320  and the lower end of the anode  320  is inserted into the tubulation  314 , which is attached to the top  310 . In one embodiment, the anode  320  is press fitted into the top  310 . 
       FIG. 4  shows a cross-section view of an embodiment of a sub-miniature arc lamp. The embodiment includes a top  410 , an anode  420 , a base  430 , a cathode  440 , a sapphire tube  450 , and a number of getters  460 , an airtight housing  470 , a seal  478 , a spring  480 , a glass window  490 , an “O” ring seal  479 , a window seal  492 , a cathode socket connection  445 , and an anode socket connection  425 . The anode  420  is mounted in the top  410 . The cathode  440  is mounted in the base  430 . The top  410  and the base  430  are attached to the top and bottom of the sapphire tube  450  respectively to form a sealed envelope. The sealed envelope inside the sapphire tube  450  is filled with an inert gas. A number of getters  460  are mounted along the anode  420  and the cathode  440  to absorb or remove impurities inside the sapphire tube. In one embodiment, the getters include one or more mercury-dispensing getters. 
     The assembly of the sapphire tube  450 , the top  410 , and the base  430  is mounted inside the airtight housing  470 , which has a bottom, a top, and a curved surface in between. The bottom of the housing is coupled to the base  430  at the seal  478 . The bottom of the housing  470  is further coupled to a cathode socket connection  445 . The top of the housing  470  is coupled to the glass window  490  and the junction between the window  490  and the housing  470  is sealed with the window seal  492 . The top  410  is coupled to the glass window  490  via the “O” ring seal  479  and the spring  480 . The top  410  is further coupled to an anode socket connection  425  through the glass window  490 . The cavity  475  inside the housing  470  is filled with an inert gas. The inert gas surrounds the seal between the sapphire tube and the top and the one between the sapphire tube and the bottom. In one embodiment, the housing  470  is filled with argon. Surrounding the seals with inert gas prevents oxidation of the seals in order to prolong the lamp life. It is because oxidation weakens the seals and makes the seals more susceptible to leakage. 
       FIG. 5  shows an embodiment of a sapphire body with sapphire to metal seals and an embodiment of the anode and cathode assemblies before being coupled to the sapphire body. On the left side of  FIG. 5 , a sapphire body  550  in the shape of a tube is coupled to a sapphire-to-metal seal  551  at the bottom of the tube and another sapphire-to-metal seal  552  at the top of the tube. On the right side of  FIG. 5 , a tubulation  514  is inserted and brazed into a top  510  to accommodate an anode  520  mounted in the top  510 . A number of getters  560  are mounted along the anode  520 . In one embodiment, the getters  560  include one or more mercury-dispensing getters. The anode  520  is aligned with a cathode  540 , which is mounted in a base  530 . A second set of getters  565  are mounted along the cathode  540 . A number of spacers  566  are mounted along the cathode  540 , one between every two getters. The assembly of anode and cathode on the right side of  FIG. 5  can be mounted to the top and bottom of the sapphire body  550  respectively to form a sealed envelope, which will be filled with an inert gas. 
     The foregoing discussion merely describes some exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, the accompanying drawings and the claims that various modifications can be made without departing from the spirit and scope of the invention.