Abstract:
A water-cooled arc lamp comprises a two concentric cylindrical glass envelopes. A circulation of high purity water and ethylene glycol is maintained between the envelopes which form a water jacket. Such water mixture is highly transparent to light at the relevant wavelengths. A pair of anode and cathode electrodes in a xenon atmosphere is disposed inside the inner envelope. The cooling water mixture is pumped at a sufficiently high flow rate to prevent water from boiling at the glass to water surfaces and thereby suppress bubbles. A safety interlock flow switch is able to interrupt arc lamp operating power if the water circulation fails. An external parabolic reflector compensates for the light path diffraction distortions that occur as the light passes through the water jacket. In alternative embodiments, the water mixture is color doped to color filter the output light.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to arc lamps, and specifically to water-cooled lamps that can be operated at high extreme power levels and that are physically much smaller than conventional types of the same power.  
           [0003]    2. Description of the Prior Art  
           [0004]    Short-arc lamps provide intense point sources of light that allow light collection in reflectors for applications in medical endoscopes, instrumentation and video projection. Also, short-arc lamps are used in industrial endoscopes, for example in the inspection of jet engine interiors. More recent applications have been in color television receiver projection systems and dental curing markets.  
           [0005]    A typical short-arc lamp comprises an anode and a sharp-tipped cathode positioned along the longitudinal axis of a cylindrical, sealed concave chamber that contains xenon gas pressurized to several atmospheres. U.S. Pat. No. 5,721,465, issued Feb. 24, 1998, to Roy D. Roberts, describes such a typical short-arc lamp.  
           [0006]    Conventional short-arc lamps have reached power levels of five kilowatts already, but such lamps are relatively large and expensive to produce. A typical five kilowatt quartz lamp is three inches in diameter and is sixteen inches long. Prior art quartz lamps also have a relatively short life.  
         SUMMARY OF THE PRESENT INVENTION  
         [0007]    It is therefore an object of the present invention to provide a multi-kilowatt short-arc lamp that is more compact than conventional designs.  
           [0008]    It is another object of the present invention to provide a multi-kilowatt short-arc lamp that separates out and disposes of the infrared heat generated.  
           [0009]    Briefly, a water-cooled arc lamp embodiment of the present invention comprises two concentric cylindrical glass envelopes. A circulation of high purity water and ethylene glycol is maintained between the envelopes which form a water jacket. Such water mixture is highly transparent to light at the relevant wavelengths. A pair of anode and cathode tungsten electrodes in a xenon atmosphere is disposed inside the inner envelope. The cooling water mixture is pumped under pressure through the water jacket to increase the boiling point of the water mixture and thereby suppress bubbles. A safety interlock flow switch is able to interrupt arc lamp operating power if the water circulation fails. An external parabolic reflector compensates for the light path diffraction distortions that occur as the light passes through the water jacket. In alternative embodiments, the water mixture is color doped to color filter the output light.  
           [0010]    An advantage of the present invention is that a tubular-sapphire arc lamp is provided that is more compact than non-watercooled lamps of similar power levels.  
           [0011]    Another advantage of the present invention is that a tubular-sapphire arc lamp is provided that is simple in design.  
           [0012]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the drawing figures. 
       
    
    
     IN THE DRAWINGS  
       [0013]    [0013]FIG. 1 is cross sectional view of a water-cooled short-type arc lamp in a first embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is cross sectional view of a water-cooled short-type arc lamp in a second embodiment of the present invention;  
         [0015]    [0015]FIG. 3 is a cross section view illustrating a water-cooled arc-lamp illumination system embodiment of the present invention; and  
         [0016]    [0016]FIG. 4 is cross sectional view of a water-cooled short-type arc lamp in a third embodiment of the present invention that includes glass rods that help circulate cooling water to the distal end of the lamp. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    [0017]FIG. 1 illustrates a xenon short-arc lamp embodiment of the present invention, and is referred to herein by the general reference numeral  100 . The xenon arc lamp  100  comprises an inner hollow cylinder of glass  102 , an outer hollow cylinder of glass  104  in which the inner cylinder is coaxially disposed, a water jacket  106  disposed between the inner and outer cylinders, a xenon atmosphere  108  disposed within the inner cylinder, and a cathode  110  and anode  112  short-arc pair of electrodes disposed coaxially in the inner cylinder and also in the xenon atmosphere. A circulation of liquid, transparent coolant is maintained in the water jacket to cool the heat dissipated during operation by the cathode and anode short-arc pair of electrodes.  
         [0018]    A mixture of deionized water and ethylene glycol is disposed and able to circulate within the water jacket  106 . Such mixture naturally filters out ultraviolet (UV) light that would otherwise be output by the lamp. An infrared (IR) filter coating  114  is preferably applied to the outside surfaces of the hollow cylinder of glass  104  to suppress IR output.  
         [0019]    The water jacket  106  supports a pressurization of the coolant to 30-90 PSI to allow for a minimum flow of four gallons per minute (GPM) that is required to provide for adequate heat transfer for a five kilowatt lamp  100 . A pair of liquid-coolant supply  116  and return  118  ports are provided at the anode end of the water jacket near a stem  120  of the anode electrode.  
         [0020]    A radially finned heat exchanger  122  receives circulating liquid coolant, and is coaxially disposed about the base stem  120 . Any number of fin designs are appropriate, e.g., radial fins as shown, longitudinal fins, turbine-blade type, etc. The object is to couple as much heat as possible out of the anode stem  120  and heatsink  122  into the circulating coolant. Another object is to spread the heat as uniformly as possible to reduce thermal distortions and stresses.  
         [0021]    A set of four bases  124 - 127  are provided at each end of each of the inner and outer cylinders and provide for the separate containment of the water jacket  106  and the xenon atmosphere  108 . Plugs  124  and  125  are typically comprised of kovar and are brazed to an inner cylinder  102  of sapphire glass. The respective coefficients of thermal expansion are therefore closely matched and an appropriate seal can be maintained over the operational life of the lamp. Bases  126  and  127  are made of metal and are sealed with rubber O-rings against pressurized water leaks against the outer cylinder  104  of quartz glass. Bases  124  and  126  are penetrated by a cathode stem  128  of the cathode electrode. This provides for a first electrical connection  130  to operate the lamp. Conversely, bases  125  and  127  are penetrated by the anode stem  120 , and this provides for a second electrical connection  132  to operate the lamp.  
         [0022]    In commercial production, it is preferable to construct lamp  100  such that the inner cylinder  102  and all its working parts inside can be replaced as a single assembly. The bases  126  and  127  are therefore made to be removable from both the outer cylinder  104  and the electrode stems  120  and  128 .  
         [0023]    [0023]FIG. 2 illustrates a xenon short-arc lamp embodiment of the present invention, and is referred to herein by the general reference numeral  200 . Lamp  200  is similar to lamp  100  (FIG. 1), and differs principally in the orientation of the internal heatsink fins and the coolant piping. The xenon arc lamp  200  comprises sapphire-glass inner envelope  202 , a quartz-glass outer envelope  204  in which the inner envelope is coaxially disposed, a water jacket  206  disposed between the inner and outer envelopes  202  and  204 , a xenon atmosphere  208  disposed within the inner envelope, and a cathode  210  and anode  212  pair of short-arc electrodes disposed coaxially in the inner envelope, and also in the xenon atmosphere. A circulation of liquid, transparent coolant is maintained in the water jacket to cool the heat dissipated during operation by the cathode and anode short-arc pair of electrodes. A hot-mirror coating  214  is preferably applied to the outside surfaces of the quartz glass envelope  204  to suppress IR output.  
         [0024]    In one embodiment of the present invention that appears to be economically producible, the sapphire-glass inner envelope  202  was 1.5″ in diameter and 2.697″ long. The quartz-glass outer envelope  204  was 2.185″ in diameter and 5.205″ long. The cathode  210  and anode  212  electrodes were substantially comprised of tungsten. A coolant mixture of deionized water and ethylene glycol (20% volume) filled and circulated within the water jacket  206 . The coolant was pressurized within the water jacket  206  to prevent boiling and concomitant bubbles, e.g., to 60-90 PSI. A minimum flow of four gallons per minute (GPM) was maintained for a five kilowatt lamp  200 .  
         [0025]    A pair of liquid-coolant supply  216  and return  218  ports are provided at the anode end of the water jacket near a stem  220  of the anode electrode. These are shown with straight in approaches that neck down to smaller diameters as they pass into the lamp. A longitudinally finned heat exchanger  222  receives circulating liquid coolant, and is coaxially disposed about the base stem  220 .  
         [0026]    A set of four bases  224 - 227  provided at each end of each of the inner and outer envelopes and providing for the separate containment of the water jacket  206  and the xenon atmosphere  208 . Bases  224  and  225  are typically comprised of kovar and are fused to an inner envelope  202  of sapphire glass. The respective coefficients of thermal expansion are therefore closely matched and an appropriate seal can be maintained over the operational life of the lamp. Bases  226  and  227  are made of metal and are sealed with rubber O-rings against pressurized water leaks against the outer envelope  204  of quartz glass. Bases  224  and  226  are penetrated by a stem  228  of the cathode electrode. This provides for a first electrical connection  230  to operate the lamp. Conversely, bases  225  and  227  are penetrated by the anode stem  220 , and this provides for a second electrical connection  232  to operate the lamp.  
         [0027]    In order to reduce operational costs, it is preferable to construct lamp  200  such that the inner envelope  202  and all its working parts inside can be replaced as a single assembly, e.g., by remanufacturing. The bases  226  and  227  are therefore made to be removable by the factory from both the outer envelope  204  and the electrode stems  220  and  228 .  
         [0028]    The bare lamp assembly, comprising the inner sapphire glass envelope  202 , the electrodes  210  and  212 , and the kovar bases  224  and  225 , is therefore preferably all bonded together. The stems  230  and  232  can be threaded so nuts or other fasteners can be used to retain the outside base ends  225  and  226  against the expansion pressures generated inside the water jacket  206 .  
         [0029]    Lamps  100  and  200  can be scaled up and operated at much higher power levels, e.g., ten, fifteen, and twenty kilowatts.  
         [0030]    [0030]FIG. 3 represents an illumination system embodiment of the present invention, and is referred to herein by the general reference numeral  300 . The system  300  comprises a water-cooled arc lamp  302  that is essentially equivalent to lamp  100  (FIG. 1) and lamp  200  (FIG. 2). As with all these lamps, the several diffraction interfaces within the lamp between sapphire glass, liquid coolant, quartz glass, and air longitudinally distort the output light. A near-parabolic reflector  304  produces a conventional light-output beam  306  from lamp  302  by correction for the internal lamp distortions. Of course, the reflector can be shaped to bring either near or infinity focus for different applications. But the common theme in all such reflectors  304  will be to correct for the internal distortions of the coaxially disposed water-cooled lamp  302 .  
         [0031]    A water pump  308  is used to force a circulation of liquid coolant into a supply pipe  310 . Heated coolant is collected in a return pipe  312  and operates a flow switch  314 . An electrical circuit  316  can be used to interrupt operating power to the lamp  302  whenever the flow rate is too slow or the temperature is too high. A radiator  318  is used to cool the liquid coolant, e.g., in a heat transfer to forced air. A radiator return line  320  completes the cooling circuit back to the pump  308 . Such cooling circuit is preferably pressurized to at least sixty PSI, and a pressure relieve valve common to boilers and water heaters may be necessary for safe operation.  
         [0032]    [0032]FIG. 4 illustrates a xenon short-arc lamp embodiment of the present invention, and is referred to herein by the general reference numeral  400 . Lamp  400  is similar to lamps  100  (FIG. 1) and  200  (FIG. 2). The xenon arc lamp  400  comprises sapphire-glass inner envelope  402 , a quartz-glass outer envelope  404  in which the inner envelope is coaxially disposed, a number of glass rods  405  to direct water flow, a water jacket  406  disposed between the inner and outer envelopes  402  and  404 , a xenon atmosphere  408  disposed within the inner envelope, and a cathode  410  and anode  412  pair of short-arc electrodes disposed coaxially in the inner envelope, and also in the xenon atmosphere. A circulation of liquid, transparent coolant is maintained in the water jacket to cool the heat dissipated during operation by the cathode and anode short-arc pair of electrodes. A hot-mirror coating  414  is preferably applied to the outside surfaces of the quartz glass envelope  404  to suppress IR output.  
         [0033]    The glass rods  405  help channel cooling water flow out to the distal end of the lamp. These help prevent short-path currents that don&#39;t contribute much to lamp cooling. Any number of other styles and kinds of water channeling can be used. The point is to get circulating water down to the distal end so as-uniform-as-possible cooling can progress all along the length and diameter of the lamp.  
         [0034]    A pair of liquid-coolant supply  416  and return  418  ports are provided at the anode end of the water jacket near a stem  420  of the anode electrode. These are shown with straight in approaches that neck down to smaller diameters as they pass into the lamp. A longitudinally finned heat exchanger  422  receives circulating liquid coolant, and is coaxially disposed about the base stem  420 .  
         [0035]    A set of four bases  424 - 427  provided at each end of each of the inner and outer envelopes and providing for the separate containment of the water jacket  406  and the xenon atmosphere  408 . Bases  424  and  425  are typically comprised of kovar and are fused to an inner envelope  402  of sapphire glass. The respective coefficients of thermal expansion are therefore closely matched and an appropriate seal can be maintained over the operational life of the lamp. Bases  426  and  427  are made of metal and are sealed with rubber O-rings against pressurized water leaks against the outer envelope  404  of quartz glass. Bases  424  and  426  are penetrated by a stem  428  of the cathode electrode. This provides for a first electrical connection  430  to operate the lamp. Conversely, bases  425  and  427  are penetrated by the anode stem  420 , and this provides for a second electrical connection  432  to operate the lamp.  
         [0036]    In order to reduce operational costs, it is preferable to construct lamp  400  such that the inner envelope  402  and all its working parts inside can be replaced as a single assembly, e.g., by remanufacturing. The bases  426  and  427  are therefore made to be removable by the factory from both the outer envelope  404  and the electrode stems  420  and  428 .  
         [0037]    The bare lamp assembly, comprising the inner sapphire glass envelope  402 , the electrodes  410  and  412 , and the kovar bases  424  and  425 , is therefore preferably all bonded together. The stems  430  and  432  can be threaded so nuts or other fasteners can be used to retain the outside base ends  425  and  426  against the expansion pressures generated inside the water jacket  406 . Lamp  400  can be scaled up and operated at high power levels, e.g., ten, fifteen, and twenty kilowatts.  
         [0038]    In general, embodiments of the present invention exhibit very high heat transfer coefficients. This, without large water pressure increases that suppress boiling. The water jackets and channels are kept thin, and has relatively high flow rates, e.g., six GPM for the lamp.  
         [0039]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.