Abstract:
A discharge lamp of the short arc type has a bulb with an arc tube and sealing tubes extending at opposite sides of the arc tube and which contains a discharge gas and a pair of opposed electrodes supported on lead pins which protrude from the outer end of the sealing tubes, the lead pins being affixed to graded glass in the sealing tube. A cooling fin surrounds the outer surface of one of the sealing tubes; and is formed of a pair of plate-shaped bodies each of which has a curved portion that contacts an outer surface of the sealing tube a strip-shaped portion extending radially from each of opposite edges of the curved portion. The strip-shaped portions of the plate-shaped bodies positionally overlap, and cooling openings are formed in the strip-shaped portions of only one of the plate-shaped bodies. Preferably, a gap is formed between the plate-shaped bodies.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a light source device comprising a discharge lamp of the short arc type used as a light source for a projection apparatus wherein light is applied to a light modulation device and an image is produced by the reflected light. 
         [0003]    2. Description of Related Art 
         [0004]    As known from commonly owned U.S. patent application Ser. No. 11/340,456 published as Publication No. 2006/0170318, a discharge lamp A of the short arc type (hereinafter simply a “lamp”) having xenon gas in an arc tube B, as shown in  FIG. 13 , is a known light source for projection apparatus, such as projectors. The lamp A, comprising a bulb D having arc tube B, a sealing tube C formed at both ends of the arc tube B, has a anode E and cathode F paired and placed opposite each other inside the arc tube B, Lead pins G support the anode E and the cathode F and are structured so as to protrude outward from the outer edge of sealing tubes C. 
         [0005]    Due to the very high pressure resulting inside the arc tube B when lit, to increase radiance, this lamp must be structured so that the sealing tubes C will not break even under high internal pressure, and the lead pins G must protrude from the outer end of the sealing tubes C for electric power to be supplied to the lamp A. Therefore, sealing parts I are formed in the lamp A by using graded glass H on the lead pins G and sealing tubes C. 
         [0006]    In recent years, however, due to the trend toward carrying projection apparatus, such as projectors incorporating a lamp such as lamp A, and using them in various locations, there is a demand for smaller projection apparatus, and therefore, smaller lamps. In order to make lamps smaller, the lamp length must be shortened, and in a lamp such as that shown in  FIG. 13 , the distance L between the back end portion EA of the anode E and the sealing part I must be shortened. However, it has been found that, when shortening this distance L, the sealing part I and the anode E become closer and the temperature of the sealing part I increases due to the high temperature of the anode E when the lamp is lit, thereby causing a problem in which the sealing part I is damaged. 
         [0007]    The anode E and the cathode F are placed opposite each other inside the arc tube B, and the lead pins G which support this anode E and cathode F pass through cylindrical retaining bodies J. The portion of the sealing tube C where these cylindrical retaining bodies J are located is heated up, thereby decreasing the diameter to form a pinched part K. 
         [0008]    In this pinched part K, the space between the inner face of the openings through which the lead pins G of the cylindrical retaining bodies J pass and the outer face of the lead pins G is not completely welded. The internal space of the arc tube B and the internal space of the sealing tubes C is continuous. A problem occurred in which the gas in the internal space of the arc tube A, which is in a high temperature state, flowed into the sealing tube C, and came into contact with the sealing parts I of the graded glass H, damaging the sealing parts I. 
         [0009]    Taking such situations into consideration, to make it possible to cool the discharge gas which flows into the sealing tube C, even if the above-mentioned distance L is shortened in the discharge lamp of the short arc type disclosed in the above noted U.S. Patent Application Publication No. 2006/0170318, a sealing tube cooling component M has been provided on the outside face of the sealing tube C and a lead pin cooling component N has been provided around the circumference of the lead pin G which protruded from the outer edge of the sealing tube C. In so doing, cooling air is blown onto the sealing tube cooling component M and sealing tube C is indirectly cooled through the sealing tube cooling component M, which is cooler than the sealing tube C, thereby decreasing the temperature of the discharge gas which flows inside the sealing tube C. Therefore, it is believed that damage to the sealing part I can be prevented. 
         [0010]    However, when the above-mentioned lamp A was actually produced and operated, the temperature of the discharge gas flowing through the inside of the sealing tube C could not be sufficiently decreased for the following reasons, and the sealing part I was occasionally damaged. 
         [0011]    As shown in  FIG. 14 , the sealing tube cooling component M, comprising a plate-shaped body MA having an arc-shaped curved portion MA 1  which comes into contact with the sealing tube C and strip-shaped part MA 2  which is continuous from the edge of curved part MA 1 , and another plate-shaped body MB having an arc-shaped curved portion MB 1  and plate-shaped part MB 2  which is continuous from the edge of curved part MB 2 , is structured such that the pinnated parts MA 2 , MB 2  overlap. When one strip-shaped body MA is placed in the upstream direction of the airflow, the cooling air only blows directly onto the strip-shaped body placed in the upstream direction of the airflow. The cooling air never directly comes into contact with the other plate-shaped body MB. Therefore, the other plate-shaped body MB is only cooled indirectly by thermal conduction from the plate-shaped body MA through the strip-shaped bodies MA 2 , MB 2  which are mutually adjacent. Therefore, the plate-shaped body MB will be in a higher temperature state than the plate-shaped body MA. As a result, the temperature at the locations in the sealing tube C with which the curved part MB 1  of the other plate-shaped body MB comes into contact will not decrease, so it is assumed that the temperature of the discharge gas flowing inside the locations was not sufficiently reduced. 
         [0012]    The above-mentioned problem can conceivably be solved by placing both plate-shaped bodies in the upstream direction of the airflow, or more specifically, using a plurality of cooling mechanisms between which both plate-shaped bodies are placed, and having cooling air blow onto both plate-shaped bodies from both directions, thereby cooling both plate-shaped bodies. However, providing a plurality of cooling mechanisms has the disadvantages that the projection apparatus in which the discharge lamp of the short arc type is mounted consequently becomes larger, going against the above-mentioned demand for smaller size, and the noise generated by the projection apparatus becomes excessive, thereby annoying the user. 
       SUMMARY OF THE INVENTION 
       [0013]    Accordingly, an object of the present invention is to prevent damage to the sealing part in the sealing tube by reliably cooling the sealing tube and sufficiently lowering the temperature of the discharge gas flowing inside the sealing tube in a discharge lamp of the short arc type wherein a cooling component is attached to the outer surface of sealing tubes continuing to both ends of the arc portion. 
         [0014]    Another object is to prevent damage to the sealing part of a sealing tube in a light source device having such a discharge lamp of the short arc type. 
         [0015]    The present invention is characterized in that, in a discharge lamp of the short arc type comprising a bulb having an arc tube and sealing tubes continuously extending from a respective end of the arc tube, with discharge gas and a pair of opposed electrodes inside the arc tube, lead pins which support the electrodes protruding from the outer end of the sealing tube being affixed to graded glass in the sealing tube, a cooling fin that surrounds the outer surface of the sealing tube and has curved portions that contact the outer surface of the sealing tube and plate-shaped bodies, which are placed adjacent to each other extending radially from the edges of the curved portions, are provided with cooling openings in only one of the plate-shaped bodies. 
         [0016]    Furthermore, a gap is formed between the pair of plate-shaped bodies to allow cooling air to pass through the cooling openings to the arc tube. Preferably, the gap gradually increases in the tube axial direction of the bulb as the distance to the arc tube decreases. 
         [0017]    The present invention is further characterized in that the surface areas of the curved portions of the plate-shaped bodies that are in contact with the sealing tube are covered with a heat absorbing material. Advantageously, the at least part of the outer surface of the sealing tube is covered by a heat absorbing material in addition or instead. 
         [0018]    A light source device comprising the discharge lamp of the short arc type of the invention is characterized in that it has a casing comprising a light exit opening for outputting light from the short arc discharge lamp and a cooling air inlet for introducing cooling air, wherein the discharge lamp of the short arc type is arranged so that the tube axis of the lamp extends along the light output path; and the plate-shaped body having the openings faces opposite the cooling air inlet to allow the cooling air introduced into the casing to pass through the cooling openings. 
         [0019]    With the discharge lamp of the short arc type of present invention, the cooling fin has cooling air directly blown onto the first plate-shaped body provided with the cooling openings and, since the cooling air passes through the openings, also directly onto the second plate-shaped body, so that both plate-shaped bodies can be reliably cooled. The sealing tube can be reliably cooled by having the curved portion of each cooled plate-shaped body come into contact with the outer surface of the sealing tube and the temperature of the discharge gas which flows inside the sealing tube can be lowered, thereby reliably preventing damage to the sealing part. 
         [0020]    Furthermore, because a gap for introducing cooling air which passed through the cooling air openings towards the arc tube has been formed between the pair of plate-shaped bodies, the arc tube, which operates at high temperature when the lamp is lit, can be reliably cooled by having cooling air which passes through the gap contact the outer surface of the arc tube. 
         [0021]    Because the gap between the pair of plate-shaped bodies gradually widens in the axial direction of the bulb closer to the arc tube, cooling air which passed through the cooling air openings can be reliably directed towards the arc tube. 
         [0022]    The face of the curved portion of the second plate-shaped body which does not have any cooling air openings and comes into contact with the sealing tube is covered by a heat absorbing material, thereby making it possible to reliably achieve a sufficient cooling effect on the sealing tube. 
         [0023]    Because the outer surface of the sealing tube is at least partly covered by heat absorbing material, a sufficient cooling effect of the sealing tube can be reliably achieved. 
         [0024]    By having a discharge lamp of the short arc type placed such that the tube axis of the bulb extends in the light output direction and the discharge lamp of the short arc type is placed inside a casing having a cooling air inlet for introducing cooling air, and placing a plate-shaped body having cooling air openings opposite the cooling air inlet, the cooling air introduced inside the casing will directly blow onto that first one of the plate-shaped bodies then the cooling air that passes through the opening of the first plate-shaped body will also directly blow onto the plate-shaped portion of the second plate-shaped body. Therefore, both plate-shaped bodies can be reliably cooled. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is side view of the overall structure of a discharge lamp of the short arc type according to the present invention. 
           [0026]      FIG. 2  is a longitudinal cross-sectional view of the bulb structure relating to the discharge lamp of the short arc type according to the present invention. 
           [0027]      FIG. 3  is a cross-sectional view taken along the line A-A′ in  FIG. 1 . 
           [0028]      FIG. 4  is cross-sectional view taken along the line B-B′ in  FIG. 1 . 
           [0029]      FIG. 5  is a longitudinal cross-sectional view of the overall structure of a light source device according to the present invention. 
           [0030]      FIG. 6  is an enlarged view of important parts required to explain a second example of the discharge lamp of the short arc type according to the present invention. 
           [0031]      FIG. 7  is a cross-sectional view of the cooling fin and sealing tube along the line A-A′ shown in  FIG. 6 . 
           [0032]      FIG. 8  is a cross-sectional view of the cooling fin along the line B-B′ shown in  FIG. 6 . 
           [0033]      FIG. 9  is a side view of the discharge lamp of the short arc type viewed in a direction perpendicular to that of  FIG. 6 . 
           [0034]      FIG. 10  is a conceptual view of the cooling air flow in the discharge lamp of the short arc type according to the present invention. 
           [0035]      FIG. 11  is a cross-sectional view of another example of a discharge lamp of the short arc type of the present invention corresponding to that of  FIG. 7 . 
           [0036]      FIG. 12  is a cross-sectional view of another example of a discharge lamp of the short arc type of the present invention in which a heat absorbing material is disposed on the sealing tube. 
           [0037]      FIG. 13  is a longitudinal cross-sectional view of the structure of a known discharge lamp of the short arc type. 
           [0038]      FIG. 14  is a cross-sectional view of the structure of the cooling component in the sealing tube relating to the discharge lamp of the short arc type according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]      FIGS. 1 &amp; 4  are used to explain a first example of a discharge lamp of the short arc type relating to the present invention. The discharge lamp  10  of the short arc type (hereinafter “lamp  10 ”) has an arc-shaped arc tube  11 , sealing tubes  12  continuing to both ends of the arc tube  11 , and a bulb  1  composed of quartz glass, for example. A noble gas, such as xenon, argon, or krypton or a mixture of these gasses is disposed inside of the arc tube  11  as a discharge gas, and an anode  13  and cathode  14  composed of a metal with a high melting point, such as tungsten, are placed opposite each other in the arc tube  11 . 
         [0040]    The tip of the lead pins  15  made from tungsten support the base of the anode  13  and the cathode  14 , each of the lead pins  15  extends axially along the tube axis of the bulb  1  inside a respective one of the sealing tubes  12  with the base portion thereof protruding from the outer end of the respective sealing tube  12 . 
         [0041]    Cylindrical retaining bodies  16  composed from cylindrical quartz glass are placed at locations inside of the sealing tube  12  closer to the arc tube  11 . The location at which the cylindrical retaining bodies  16  are located in the sealing tubes  12  is constricted through thermal compression to form pinched parts  12 A, thereby securing the cylindrical retaining bodies  16 . By inserting the lead pins  15  through the openings provided in the center of the cylindrical retaining bodies  16 , the anode  13  and the cathode  14  are fixed in a predetermined position inside of the bulb  1 . However, the space between the inner face of the cylindrical retaining bodies  16  defining the openings and the outer face of the lead pins  15  is not completely welded so that the interior space of the arc tube  11  and the interior space of the sealing tubes  12  are connected. Therefore, discharge gas inside the arc tube  11  that reaches a high temperature when the lamp is lit flows into the sealing tubes  12 . 
         [0042]    Graded glass  17  is placed inside the sealing tubes  12 . The graded glass  17  is sealed to the outer end of the sealing tubes  12  at an end which has a thermal expansion coefficient roughly equal to that of the quartz glass from which the sealing tube  12  is formed, and the other end forms a sealed portion  17 A that has a thermal expansion coefficient roughly equal to that of the tungsten from which the lead pins  15  is made to which it is sealed. 
         [0043]    As shown in  FIG. 3 , a cooling fin  2  comprising an overlapping pair of plate-shaped bodies  2 A,  2 B, both of which are made of copper, is provided on the outside of the one of the sealing tubes  12  that is on the side of the anode  13 . The cooling fin  2  comprises a plate-shaped body  2 A having a semi-circularly curved portion  21 A which comes into contact with the outer surface of the sealing tube  12  and is matched to the shape of the outside surface of the sealing tube  12 , and a pair of strip-shaped portions  22 A extending radially from the sealing tube  12 . The strip-shaped portions  22 A are connected to opposite edges of the curved portion  21 A. The cooling fin  2  also comprises another plate-shaped body  2 B having a curved portion  21 B which comes into contact with the outside surface of the sealing tube  12  and which is curved into a semicircular shape matched to the shape of the outside surface of the sealing tube, and a pair of strip-shaped portions  22 B extending radially relative to the sealed portion  12  at opposite edges of the curved portion  21 B. The strip-shaped portions  22 A,  22 B overlap and are connected to each other at narrow portions  25 A,  25 B. The narrow portions  25 A are narrower in the axial direction of the bulb than strip-shaped portions  22 A and protrude from the outer edge of the strip-shaped portions  22 A. Similar to strip-shaped portions  22 A, narrow portions  25 B are also formed on strip-shaped portions  22 B. In order to keep the strip-shaped portions  22 A and the strip-shaped portions  22 B close together without separating, the plate-shaped body  2 A and the second plate-shaped body  2 B are fastened together by screws  23  passing through holes formed in the narrow portions  25 A and the narrow portions  25 B. 
         [0044]    The strip-shaped portion  22 A of the first plate-shaped body  2 A has a plurality of spaced apart cooling air openings  24 , to allow cooling air to pass through them, whereas the second plate-shaped body  2 B has no cooling air openings on the strip-shaped portion  22 B thereof. Forming a large number of the cooling air openings  24  makes it easier for the cooling air to pass through first plate-shaped body  2 A so as to strike against the second plate-shaped body  2 B. However, when the ratio of the surface area of the first plate-shaped body  2 A to the total aperture area combining the area of all cooling air openings  24  is too great, the cooling effect of the sealing tube  12  by plate-shaped body  2 A is lessened. Conversely, if the ratio is too small, the amount of cooling air striking the second plate-shaped body  2 B decreases, so an area ratio within the range of 20% to 30% is preferred. 
         [0045]    The cooling fin  2  such as this is provided on the sealing tube  12  of the anode  13  as explained below, for example. 
         [0046]    The first plate-shaped body  2 A is placed such that the semicircular curved portion  21 A thereof comes into contact with the outer surface of the sealing tube  12 , the second plate-shaped body  2 B is placed such that the semicircular curved portion  21 B thereof comes into contact with the outer surface of the sealing tube  12 , the strip-shaped portions  22 A and the strip-shaped portions  22 B thereof are kept close together, and the screws  23  are inserted and tightened to fasten the narrow parts  25 A to the narrow parts  25 B through the holes formed therein. 
         [0047]      FIG. 5  is a longitudinal cross-sectional view of the overall structure of a light source device according to the present invention. As shown in the figure, a light source device  30  comprises a casing  3  having a circular light exit opening  31  and a cooling air inlet  32 , wherein a discharge lamp  10  of the short arc type is placed such that the tube axis of the bulb  1  extends in the light output direction, two reflecting mirrors  4 ,  5  are placed facing the light exit opening  31  to reflect the light emitted from the lamp  10 , and a support body  6  which is attached to one of the sealing tubes  12  of the lamp  10  for supporting a base  18 . 
         [0048]    The reflecting mirror  4 , which is placed behind the reflecting mirror  5  with respect to the direction of the light output, is an ellipsoidal condensing mirror wherein the first focal point thereof matches that of the arc spot which is formed between the anode  13  and the cathode  14  in the bulb  1 . The reflecting mirror  5 , which is placed in front of the reflecting mirror  4  with respect to the direction of the light output, is a spherical reflection mirror wherein the focal point thereof matches that of the arc spot which is formed between the anode  13  and the cathode  14  in the bulb  1 . By providing two reflection mirrors such as reflection mirrors  4 ,  5 , either some of the light is directly radiated from the bulb  1  or is reflected from the reflecting mirror  4  and radiated outward from the light exit opening  31 . In addition, the light which is directed forward and outward from the outward edge of the reflecting mirror  4  is returned to the arc spot by the second reflecting mirror  5 , then is collected by the reflecting mirror  4  and is radiated outward from the light exit opening  31 . 
         [0049]    The lamp  10  is fastened inside the casing  3  such that the plate-shaped body  2 A in which cooling air openings  24  are formed in the cooling fin  2  thereof is oriented opposite the cooling air inlet  32  which is formed in the casing  3 , and the rear sealing tube  12  with respect to the light exit direction is supported by the support body  6  and fastened on one side thereof. In other words, the cooling fin  2  is placed such that the first plate-shaped body  2 A having the cooling air openings  24  and the second plate-shaped body  2 B not having the cooling air openings are placed under the air flow. The support body which supports the lamp  10  can also support a pair of sealing tubes  12 . 
         [0050]    In the light source device  30 , while the lighting of the lamp  10  is driven by a power supply device, cooling air is introduced from the cooling air inlet  32  to inside the casing by a cooling air supply device not shown here. As shown by the arrows in  FIGS. 3 &amp; 4 , the cooling air is directly blown onto the plate-shaped body  2 A which is positioned in the upstream direction of the airflow. Part of the cooling air is directly blown onto the second plate-shaped body  2 B through the cooling air openings formed in the plate-shaped body  2 A. 
         [0051]    According to the present invention, by placing the discharge lamp of the short arc type  10  such that a first plate-shaped body  2 A having cooling air openings  24  is positioned in the upstream direction of the airflow, the cooling air from the cooling air inlet  32  is introduced into the casing  3 , thereby directly cooling the first plate-shaped body  2 A and the second plate-shaped body  2 B of the cooling fin  2 . Therefore, cooling can be efficiently carried out through both the plate-shaped body  2 A which is in contact with the sealing tube  12  of the bulb  1  and the second plate-shaped body  2 B. Also, because the temperature of the discharge gas flowing inside the sealing tube  12  can be lowered more than was previously possible, damage to the sealed portion  17 A which is formed in the sealing tube  12  can be reliably prevented. 
         [0052]    Next,  FIGS. 6 &amp; 9  will be used to explain a second example of a discharge lamp of the short arc type in accordance with the present invention.  FIG. 6  is an enlarged view of the main parts needed to explain a discharge lamp of the short arc type in accordance with the present invention.  FIG. 7  is a cross-sectional view of the cooling fin and sealing tube taken along the line A-A′ shown in  FIG. 6 .  FIG. 8  is a cross-sectional view of the cooling fin taken along the line B-B′ shown in  FIG. 6 .  FIG. 9  is a side view of the discharge lamp of the short arc type as seen in a direction perpendicular to that shown in  FIG. 6 . 
         [0053]    As shown in  FIG. 7 , a cooling fin  60  comprises: a plate-shaped body  60 A having a curved portion  61 A which comes into contact with the outer surface of the sealing tube  12  having an arc-shaped curvature to fit the outer shape of the sealing tube  12 , and a pair of strip-shaped parts  62 A which extend from both ends of the curved portion  61 A and extend radially outward relative to the sealing tube  12 ; and another plate-shaped body  60 B having a curved portion  61 B which comes into contact with the outer surface of the sealing tube  12  and has an arc-shaped curvature to fit against the outer shape of the sealing tube  12 , and a pair of strip-shaped parts  62 B which extend from both ends of the curved portion  61 B in a radially outward direction with respect to the sealing tube  12 . 
         [0054]    As shown in  FIGS. 7-9 , a gap G is formed in the cooling fin  60  between the plate-shaped bodies  60 A,  60 B, the gap G extending along the tube axis of the bulb  1 . A plurality of cooling air openings  64 , which are mutually separated, are formed on the strip-shaped portion  62 A of the plate-shaped body  60 A to allow cooling air to pass through, while no cooling air openings are been formed on the strip-shaped portion  62 B of the second plate-shaped body  60 B. 
         [0055]    The strip-shaped portion  62 A of plate-shaped body  60 A comprises joining parts  621 A extending parallel to the tube axis of the bulb  1 , and slanted portions  622 A which are continuous with the joining parts  621 A and extend diagonally outward (upward as seen in  FIGS. 8 &amp; 9 ). The strip-shaped portion  62 B of the second plate-shaped body  60 B comprises joining parts  621 B extending parallel to the tube axis of the bulb  1 , and slanted portions  622 B which are continuous with the joining parts  621 B and extend diagonally outward (downward as seen in  FIGS. 8 &amp; 9 ). 
         [0056]    As shown in  FIG. 6 , both ends of the joining portions  621 A in a radial direction of the bulb  1  are provided with portions  65 A having a smaller width in the tube direction than the width of the slanted portions  622 A, and the narrow parts  65 A are formed so as to protrude from the outer edge of the strip-shaped portion  62 A. Similar to the joining portions  621 A, narrow parts  65 B are formed on joining portions  621 B. 
         [0057]    A V-shaped cross section is formed between the plate-shaped bodies  60 A,  60 B in such a structure by bringing the joining portions  621 A,  621 B close together, having the slanted portions  622 A,  622 B mutually separate, fastening the joining portions together using the screws  63  which pass through holes formed in the narrow parts  65 A,  65 B, thereby forming the V-shaped cross section by the slanted portions  622 A,  622 B as shown in  FIG. 8 . In other words, as shown in  FIGS. 8 &amp; 9 , the gap G which is formed between the slanted portions  622 A,  622 B widens in the direction along the tube axis of the bulb  1  toward the arc tube  11 . 
         [0058]    The discharge lamp  20  of the short arc type of the second embodiment in accordance with the invention can be expected to have the same effect as the discharge lamp  10  of the short arc type of the first embodiment. As shown by the arrows in  FIG. 10 , because the cooling air passing through the plate-shaped body  60 A of the cooling fin  60  is directed toward the arc tube  11  of the bulb  1  and the sealing tube  12  is cooled through the cooling fin  60 , the arc tube  11  can be cooled. Therefore, the lamp  20  in the second example makes it possible to lower the temperature of the discharge gas inside the arc tube  11  positioned upstream from the sealing tube  12 , and makes it possible to cool the discharge gas passing through the inside of the sealing tube  12  which is downstream by cooling the sealing tube  12  between the cooling fin  60 . In other words, the lamp  20  in the second embodiment can further lower the temperature of discharge gas flowing inside the sealing tube  12  through the synergistic effect between cooling the sealing tube  12  through the cooling fin  60  and cooling by bringing cooling air into contact with the arc tube  11 . 
         [0059]    The discharge lamp of the short arc type relating to the first and second embodiments explained above can further improve the cooling effect of the sealing tube  12  by having a cooling fin structured as shown in  FIGS. 11 &amp; 12 . The parts in  FIGS. 11 &amp; 12  which are the same as for the cooling fin shown in  FIGS. 3 &amp; 4  are given the same reference characters. 
         [0060]    The cooling fin  2  shown in  FIG. 11  comprises an inner surface which comes into contact with the outer surface of the sealing tube  12  in the curved portion  21 A of first plate-shaped body  2 A and an inner surface which comes into contact with the outer surface of the sealing tube  12  in the curved portion  21 B of the second plate-shaped body  2 B, both of which are covered by a heat absorbing material  27  composed of carbon, for example. The cooling effect that the cooling fin  2  has on the sealing tube  12  is thereby improved, so the temperature of the discharge gas inside the sealing tube  12  can be lowered further. 
         [0061]    As shown in  FIG. 12 , the outer surface of the sealing tube  12  is covered across the entire outer surface thereof by a heat absorbing material  28  composed of carbon, for example. By bringing the heat absorption material  28  between the outer surface of the sealing tube  12  and the inner face of the curved portions  21 A,  21 B, the cooling effect which the cooling fin  2  has on the sealing tube  12  can be improved, thereby making it possible to farther lower the temperature of the discharge flowing inside the sealing tube  12 . The cooling effect which the cooling fin  60  structured as shown in  FIG. 7  and  FIG. 8  has on the sealing tube  12  can of course be improved by adopting the structure shown in  FIG. 11  and  FIG. 12 . 
         [0062]    A discharge lamp of the short arc type was manufactured according to the following specifications, then a test was performed in which the temperature near the sealed portion of the graded glass shown by X in  FIG. 1  was checked one hour after the discharge lamp of the short arc type was lit up. 
       &lt;Lamp Specifications&gt; 
       [0000]    
       
         
           
             Bulb  1 : Overall length 235 mm 
             Sealing tube  12 : Quartz glass, outer diameter 24 mm, wall thickness 2.5 mm 
             Linear distance from graded glass  17  sealed portion  17 A to anode  13 : 59.5 mm 
             Lead pins  15 : Tungsten, diameter 4.0 mm 
             Distance between electrodes: 4.0 mm 
             Lamp power: 2 kW 
             Sum of surface area of plate-shaped body  2 A and surface area of plate-shaped body  2 B (including the contact area with the sealing tube  12 ): 10600 mm 2    
             Sum of the contact area between the plate-shaped body  2 A and the sealing tube  12  and the contact area between the plate-shaped body  2 B and the sealing tube  12 : 2030 mm 2    
             Material for plate-shaped bodies  2 A and  2 B: copper 
           
         
       
     
         [0000]    
       
         
               
               
             
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Temperature of Graded Glass Sealed Portion 
               
               
                   
                 (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Lamp 1 (Conventional) 
                 482 
               
               
                 Lamp 2 (Invention) 
                 471 
               
               
                   
               
             
          
         
       
     
         [0072]    Table 1 shows the test results. In Table 1, the lamp  1  is a conventional lamp wherein no cooling air openings where formed in the cooling fin, and the lamp  2  as shown in  FIG. 1  or  FIG. 4  is a lamp in accordance with the present invention wherein cooling air openings are formed in one plate-shaped body which comprises the cooling fin. 
         [0073]    As shown in Table 1, the temperature of the conventional lamp  1  was 482° C. and temperature of the lamp  2  in the present invention was 471° C. Therefore, the lamp  2  according to the present invention was confirmed to have a temperature more than 10° C. lower than the conventional lamp  1  at sealed portion X of the graded glass.