CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2010-021354 filed on Feb. 2, 2010, the entire contents of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a discharge lamp for a vehicle. More particularly, the invention relates to a technical field for reducing a length of a narrow tube portion on a cathode side to be smaller than that of a narrow tube portion on an anode side and disposing an cathode side electrode at an opposite side to a socket with an anode side electrode interposed therebetween, thereby enhancing a light distribution performance and a luminous efficiency. 
       RELATED ART 
       [0003]    Unlike a general illuminating lamp, a headlamp for a vehicle requires a fine light distribution control. For this reason, there is demanded a light emitting form which is uniform, is bar-shaped and has a high brightness ratio. Filaments of an incandescent lamp and a halogen lamp have the feature. Therefore, they have been used as light sources of a headlamp for a vehicle for a long period of time. 
         [0004]    On the other hand, a headlamp for a vehicle which uses a discharge lamp as a light source has an advantage that a luminance can be enhanced because the discharge lamp has a larger light quantity as compared with an incandescent lamp and a halogen lamp, and the discharge lamp has a longer lifetime as compared with the incandescent lamp and the halogen lamp. 
         [0005]    Thus, the discharge lamp has a higher luminance and a longer lifetime as compared with the incandescent lamp and the halogen lamp. In recent years, therefore, a headlamp for a vehicle including a discharge lamp using quartz glass has spread. 
         [0006]    In the discharge lamp, generally, a light emitting tube which is formed of the quartz glass, holds a pair of electrodes and is filled with a gas such as an inert gas is disposed in an external tube which is intended for protecting a light emitting tube and stabilizing a temperature. 
         [0007]    The light emitting tube is constituted by a light emitting portion for carrying out a discharge and a pair of narrow tube portions provided on sides opposite to each other with the light emitting portion interposed therebetween. The light emitting portion serves to generate an arc when the discharge is carried out, and has a larger diameter than that of the narrow tube portion. 
         [0008]    Some light emitting tubes are formed of ceramic (for example, see Patent Document 1). 
         [0009]    In the discharge lamp, a predetermined starting voltage is applied to the pair of electrodes and the discharge is carried out in a light emitting portion of a light emitting tube so that lighting is initiated. 
         [0010]    In the discharge lamp, generally, an AC lighting method is used as a lighting method. The main reason is that mercury to be a filling substance and the other iodides are separated from each other in an arc so that a color separation is caused in the arc in case of a DC lighting method. In recent years, the DC lighting method is used in a discharge lamp which does not utilize mercury to be the main cause of the color separation according to some examples (for instance, see Patent Document 2). 
         [0011]    In the discharge lamp using the DC lighting method, a pair of electrodes is set to be a cathode side electrode and an anode side electrode respectively, and a thermal electron discharged from the cathode side electrode collides with the anode side electrode in a generation of an arc.
   [Patent Document 1] JP-T-2007-521620 Publication   [Patent Document 2] JP-A-2007-250225 Publication   
 
         [0014]    However, a discharge lamp having a light emitting tube formed of ceramic seals the light emitting tube with low melting glass referred to as frit glass. For this reason, it is necessary to reduce a temperature of a sealed part. Consequently, there is employed a structure in which a length of a narrow tube portion is increased to seal a tip part thereof. 
         [0015]    Accordingly, a total length of the light emitting tube is great. Therefore, there is an increased possibility that a light emitted from a light emitting portion might be shielded by a light emitting tube. As a result, a light distribution performance might be deteriorated in a structure of a headlamp for a vehicle. 
         [0016]    Moreover, a luminous efficiency of the discharge lamp depends on the coldest point temperature in a light emitting tube. In the case in which the DC lighting method is used, however, a temperature on a cathode side is lower than that on an anode side. For this reason, it is necessary to raise the temperature on the cathode side which reaches the coldest point temperature, thereby enhancing the luminous efficiency. 
       SUMMARY 
       [0017]    Exemplary embodiments of the present invention provide a discharge lamp for a vehicle in which a light distribution performance and a luminous efficiency can be enhanced. 
         [0018]    In order to solve the above mentioned problem, a discharge lamp for a vehicle in which a light emission is carried out by a DC lighting method, according to an exemplary embodiment, comprises: 
         [0019]    an external tube attached to a socket; 
         [0020]    a light emitting tube disposed in the external tube and formed of ceramic, the light emitting tube including a light emitting portion and a pair of narrow tube portions linked to the light emitting portion at opposite sides with the light emitting portion interposed therebetween; 
         [0021]    a cathode side electrode and an anode side electrode which are disposed in the light emitting tube; and 
         [0022]    two lead wires connected to the cathode side electrode and the anode side electrode respectively and having at least a part disposed in the external tube respectively, 
         [0023]    wherein a length of the narrow tube portion on the cathode side being set to be smaller than that of the narrow tube portion on the anode side, and 
         [0024]    wherein the cathode side electrode being disposed on an opposite side to the socket with the anode side electrode interposed therebetween. 
         [0025]    In the discharge lamp for a vehicle, accordingly, the length of the narrow tube portion on the cathode side which is disposed on the opposite side to the socket with the anode side electrode interposed therebetween is smaller than that of the narrow tube portion on the anode side. 
       ADVANTAGE OF THE INVENTION 
       [0026]    A discharge lamp for a vehicle in which a light emission is carried out by a DC lighting method, according to the exemplary embodiment, comprises: 
         [0027]    an external tube attached to a socket; 
         [0028]    a light emitting tube disposed in the external tube and formed of ceramic, the light emitting tube including a light emitting portion and a pair of narrow tube portions linked to the light emitting portion at opposite sides with the light emitting portion interposed therebetween; 
         [0029]    a cathode side electrode and an anode side electrode which are disposed in the light emitting tube; and 
         [0030]    two lead wires connected to the cathode side electrode and the anode side electrode respectively and having at least a part disposed in the external tube respectively, 
         [0031]    wherein a length of the narrow tube portion on the cathode side being set to be smaller than that of the narrow tube portion on the anode side, and 
         [0032]    wherein the cathode side electrode being disposed on an opposite side to the socket with the anode side electrode interposed therebetween. 
         [0033]    Accordingly, the cathode side electrode is rarely influenced by a thermal conduction from the lead wire so that a heat radiation is hard to perform. By disposing the cathode side electrode at the opposite side to the socket with the anode side electrode interposed therebetween, therefore, it is possible to suppress a reduction in a temperature, thereby enhancing a luminous efficiency. 
         [0034]    By setting the length of the narrow tube portion on the cathode side to be smaller than that of the narrow tube portion on the anode side, moreover, a heat capacity in the narrow tube portion of the cathode side electrode is reduced. Consequently, a reduction in a temperature on the cathode side is suppressed so that the luminous efficiency can be enhanced still more. 
         [0035]    Furthermore, by setting the length of the narrow tube portion on the cathode side to be smaller than that of the narrow tube portion on the anode side, a distance from the light emitting portion to a front end of the discharge lamp for a vehicle is shortened so that a light emitted from the light emitting portion and required for a light distribution pattern is prevented from being intercepted at the front end of the discharge lamp. Thus, it is possible to ensure an excellent light distribution performance. 
         [0036]    Further, since the discharge lamp for a vehicle further comprises a heat insulating film formed in a position on an external surface of the light emitting tube where the cathode side electrode is covered, a temperature in a portion on the cathode side having the heat insulating film formed thereon in the light emitting tube is raised by a radiant heat from an arc. Consequently, it is possible to enhance a luminous efficiency. 
         [0037]    Furthermore, in the discharge lamp for a vehicle, the heat insulating film is disposed in a position in which one end of the heat insulating film on the anode side does not protrude toward the anode side electrode from a tip of the cathode side electrode. 
         [0038]    Accordingly, a light emitted from the light emitting portion and required for the light distribution pattern is prevented from being intercepted by the heat insulating film. Consequently, it is possible to prevent deterioration in a light distribution performance. 
         [0039]    Furthermore, in the discharge lamp for a vehicle, the light emitting tube includes a thick portion which has a thickness greater than thicknesses of the other parts of the light emitting tube and is formed in a position of the light emitting tube where the cathode side electrode is covered. Therefore, strength of the light emitting tube is increased so that a leakage can be prevented from being caused over the light emitting portion in an early stage. 
         [0040]    Furthermore, in the discharge lamp for a vehicle, the thick portion is disposed in a position in which one end of the thick portion on the anode side does not protrude toward the anode side electrode from a tip of the cathode side electrode. 
         [0041]    Accordingly, the light emitted from the light emitting portion and required for the light distribution pattern is not transmitted through the thick portion. Consequently, it is possible to prevent deterioration in the light distribution performance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  is a schematic sectional view showing a headlamp for a vehicle, illustrating the best mode of a discharge lamp for a vehicle according to the invention together with  FIGS. 2 to 4 , 
           [0043]      FIG. 2  is an enlarged side view showing the discharge lamp, a part of which is taken away, 
           [0044]      FIG. 3  is an enlarged sectional view showing a part, and 
           [0045]      FIG. 4  is an enlarged sectional view showing an example in which a thick portion is formed on a light emitting tube. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    The best mode for carrying out a discharge lamp for a vehicle according to the invention will be described below with reference to the accompanying drawings. The discharge lamp for a vehicle is provided in a headlamp for a vehicle. 
         [0047]    A headlamp  1  for a vehicle is attached and disposed on both left and right ends at a front end of a car body. 
         [0048]    As shown in  FIG. 1 , the headlamp  1  for a vehicle includes a lamp housing  2  having a concave portion which is opened forward and a cover  3  for closing an open surface of the lamp housing  2 , and a lighting device outer casing  4  is constituted by the lamp housing  2  and the cover  3 . An internal space of the lighting device outer casing  4  is formed as a lighting chamber  5 . 
         [0049]    An inserting hole  2   a  penetrating longitudinally is formed on a rear end of the lamp housing  2  and is closed with a back cover  6 . An arranging hole  2   b  penetrating vertically is formed on a lower end of the lamp housing  2 . 
         [0050]    A reflector  7  is tiltably supported in the lighting chamber  5  by means of an optical axis adjusting mechanism which is not shown. An attaching hole  7   a  penetrating longitudinally is formed on a rear end of the reflector  7 . 
         [0051]    A discharge lamp (a discharge lamp for a vehicle)  8  is attached into the attaching hole  7   a  of the reflector  7  in a horizontal state. 
         [0052]    A discharge lamp lighting device  9  is attached into the arranging hole  2   b  of the lamp housing  2 . The discharge lamp lighting device  9  is formed with a lighting circuit (not shown) accommodated in a case body  10 . An input side connector  11  is provided on an outer peripheral surface of the case body  10 , and an output side connector  12  is provided on an upper surface of the case body  10 . 
         [0053]    The input side connector  11  is connected to a power supplying circuit (not shown) through a connecting cord (not shown). 
         [0054]    The output side connector  12  is connected to a starting device  14  through a feeding cord  13 , and a connector  14   a  of the starting device  14  is connected to a socket of the discharge lamp  8  which will be described below. 
         [0055]    The discharge lamp  8  is turned on by raising a source voltage of the power supplying circuit through the lighting circuit of the discharge lamp lighting device  9  and applying a starting voltage to the discharge lamp  8  through the feeding cord  13  and the starting device  14  to commence a discharge. As a lighting method for the discharge lamp  8 , a DC lighting method is used. 
         [0056]    An extension  15  is disposed in the lighting chamber  5 . The extension  15  serves to shield a part of each portion disposed in the lighting chamber  5 . A shade (not shown) for intercepting a part of a light emitted from the discharge lamp  8  is disposed in the lighting chamber  5 . 
         [0057]    The discharge lamp  8  is constituted by connecting a body  16  to a socket  17  (see  FIG. 2 ). 
         [0058]    The body  16  has an external tube  18  and a light emitting tube  19  disposed in the external tube  18 . The light emitting tube  19  is formed of ceramic. 
         [0059]    The external tube  18  is constituted by integrally forming a closing portion  18   a  for covering the light emitting tube  19  and a holding portion  18   b  protruded forward from a front end of the closing portion  18   a.    
         [0060]    The light emitting tube  19  is formed by a light emitting portion  20  and narrow tube portions  21  and  22  which are linked to both front and rear ends of the light emitting portion  20 , respectively. The narrow tube portions  21  and  22  are formed almost cylindrically and extended longitudinally respectively and have outside diameters which are smaller than an outside diameter of the light emitting portion  20 . The narrow tube portion  21  positioned on a front side has a smaller length in a longitudinal direction than that of the narrow tube portion  22  positioned on a rear side. 
         [0061]    An iodide and an inert gas such as xenon or argon are filled in the light emitting portion  20  and each of the ends at the light emitting portion  20  side of the narrow tube portions  21  and  22 . An intermediate portion in a longitudinal direction of the light emitting portion  20  is provided as a flat portion  20   a  extended longitudinally and taking an almost cylindrical shape (see  FIG. 3 ). A length L 1  in the longitudinal direction of the flat portion  20   a  is set to be equal to or greater than 2.0 mm. 
         [0062]    A cathode side electrode  23  and an anode side electrode  24  which are formed to be longitudinally long respectively are disposed apart from each other in a longitudinal direction in the narrow tube portions  21  and  22 . A rear end  23   a  of the electrode  23  on the cathode side and a front end  24   a  of the electrode  24  on the anode side are positioned in the light emitting portion  20  of the external tube  18 . 
         [0063]    As described above, in the discharge lamp  8 , a DC lighting method is used as a lighting method, and a thermal electron discharged from the cathode side electrode  23  on the collides with the anode side electrode  24  in a generation of an arc so that a temperature of the anode side electrode  24  is raised. Consequently, it is possible to prevent erosion of an electrode by increasing an electrode diameter to be greater than that of the cathode side electrode  23  or increasing a heat capacity. 
         [0064]    A protrusion amount S 1  of the cathode side electrode  23  from the narrow tube portion  21  toward the light emitting portion  20  side is set to be equal to or smaller than 2.0 mm. A protrusion amount S 2  of the anode side electrode  24  from the narrow tube portion  22  toward the light emitting portion  20  side is set to be equal to or greater than 2.0 mm. 
         [0065]    A space is formed between an inner peripheral surface of the narrow tube portion  21  and an outer peripheral surface of the cathode side electrode  23 , and a length L 2  in a longitudinal direction of the space is set to be equal to or smaller than 1.0 mm. 
         [0066]    A heat insulating film  25  is formed on an outer peripheral surface from a front end of the light emitting portion  20  to a rear end of the narrow tube portion  21 . The heat insulating film  25  has an effect for absorbing or reflecting a visible light or an infrared light, thereby suppressing a reduction in a temperature of a portion of the light emitting tube  19  in which the cathode side electrode  23  is disposed, and has a rear end which is coincident with a rear end of the cathode side electrode  23 , for example. The heat insulting film  25  may be formed in such a manner that the rear end is positioned on a front side of the rear end of the cathode side electrode  23 . 
         [0067]    A first lead wire  26  is connected to a front end of the cathode side electrode  23  (see  FIGS. 2 and 3 ). The first lead wire  26  has a portion which is protruded forward from the narrow tube portion  21  on the front side of the light emitting tube  19 , penetrates through the holding portion  18   b  and is protruded toward an outside of the external tube  18 . The portion of the first lead wire  26  which is protruded toward the outside of the external tube  18  is bent in a predetermined direction and a rear end is connected to a first connecting terminal (not shown) which is provided on the socket  17 . 
         [0068]    The first lead wire  26  is constituted by a first portion  26   a  disposed in the narrow tube portion  21  and connected to the cathode side electrode  23  and a second portion  26   b  linked to a front end of the first portion  26   a , and the first portion  26   a  is formed of molybdenum, for example, and the second portion  26   b  is formed of niobium, for example. 
         [0069]    A molybdenum coil  26   c  is wound around the first portion  26   a . A portion of the second portion  26   b  which is disposed in the narrow tube portion  21  is sealed with frit glass (low melting glass). 
         [0070]    The first lead wire  26  is partially held in the holding portion  18   b  of the external tube  18 , and a portion excluding a part in an outward protruded portion of the external tube  18  is covered with an insulating sleeve  27 . The insulating sleeve  27  is formed by an insulating material such as glass or ceramic. 
         [0071]    A second lead wire  28  extended almost longitudinally is connected to a rear end of the anode side electrode  24 . The second lead wire  28  is protruded rearward from the narrow tube portion  22  at a rear side of the light emitting tube  19 . A rear end of the second lead wire  28  is connected to a second connecting terminal (not shown) provided on the socket  17 . 
         [0072]    The second lead wire  28  is constituted by a first portion  28   a  disposed in the narrow tube portion  22  and connected to the anode side electrode  24 , and a second portion  28   b  linked to a rear end of the first portion  28   a . The first portion  28   a  is formed of molybdenum, for example, and the second portion  28   b  is formed of niobium, for example. 
         [0073]    A molybdenum coil  28   c  is wound around the first portion  28   a . A portion of the second portion  28   b  which is disposed in the narrow tube portion  22  is sealed with frit glass. 
         [0074]    A getter  29  attached to the second lead wire  28  is disposed in the external tube  18 . The getter  29  has a function for adsorbing an impurity (an impurity gas) which can be present in the external tube  18  and preventing a reduction in a luminous efficiency during a lifetime. 
         [0075]    In the discharge lamp  8 , an interval H 1  between an inner peripheral surface of the external tube  18  and an outer peripheral surface of the flat portion  20   a  in the light emitting portion  20  is set to be equal to or smaller than 1.0 mm (see  FIG. 2 ). Moreover, an argon gas is filled as a shroud gas in a space on an outside of the light emitting tube  19  in the external tube  18 . The argon gas is set to have an atmospheric pressure of 0.3 atm to 0.9 atm. 
         [0076]    In a discharge lamp (a discharge lamp for a vehicle), generally, it is necessary to enhance a luminous efficiency and to suppress arc bending generated in a light emitting portion in order to maintain an excellent light distribution performance. For this reason, a diameter of the light emitting portion is set to be small. When a diameter of the light emitting portion is reduced, however, a temperature in an upper part of the light emitting portion is raised so that a lifetime is inhibited from being prolonged due to a devitrification of a light emitting tube or a generation of a crack. 
         [0077]    In the discharge lamp  8 , therefore, the interval H 1  between the inner peripheral surface of the external tube  18  and the outer peripheral surface of the flat portion  20   a  in the light emitting portion  20  is set to be equal to or smaller than 1.0 mm to enhance a heat radiating efficiency from the light emitting portion  20  to the external tube  19 , thereby suppressing a rise in the temperature of the light emitting portion  20  as described above. 
         [0078]    By filling the argon gas in the space on the outside of the light emitting tube  19  in the external tube  18 , moreover, it is possible to ensure an excellent thermal transmitting property from the light emitting portion  20  to the external tube  19 . In the discharge lamp  8 , the atmospheric pressure of the argon gas is set to be 0.3 atm to 0.9 atm, thereby preventing a generation of a discharge in the space having the shroud gas filled therein and ensuring an air tightness of the space. 
         [0079]    As described above, in the discharge lamp  8 , the electrode  23  on the cathode side is positioned on the front side of the electrode  24  on the anode side, and the length in the longitudinal direction of the narrow tube portion  21  positioned on the front side is set to be smaller than that in the longitudinal direction of the narrow tube portion  22  positioned on the rear side. 
         [0080]    The cathode side electrode  23  is rarely influenced by the thermal conduction from the lead wire and is hard to radiate a heat. Therefore, by disposing the cathode side electrode  23  at the front side it is possible to suppress a reduction in a temperature, thereby enhancing a luminous efficiency. 
         [0081]    Further, more by setting the narrow tube portion  21  on the cathode side to be shorter than the narrow tube portion  22  on the anode side, it is possible to decrease a heat capacity in the narrow tube portion  21 , thereby suppressing a reduction in a temperature on the cathode side to enhance a luminous efficiency still more. 
         [0082]    For example, it is preferable that a length L 3  of the narrow tube portion  21  on the cathode side and a length L 4  of the narrow tube portion  22  on the anode side should be 0.4×L 4 ≦L 3 ≦0.8×L 4  and 10 mm≦L 4 ≦15 mm, respectively. 
         [0083]    In other words, thermal stress of the sealed part at the transition region is increased when L 3  is smaller than 0.4×L 4 . As a result, a crack or a leakage is generated so that lifetime durability is reduced. Moreover, a temperature of the sealed part is raised. Therefore, a filled substance such as an iodide reacts to frit glass and thus disappears so that an acting performance (a luminous flux maintenance rate) is also reduced. 
         [0084]    On the other hand, when L 3  is greater than 0.8×L 4 , the coldest point temperature in the light emitting tube is reduced. Therefore, a light emission of the filled substance is adversely influenced so that the luminous efficiency is reduced. 
         [0085]    If the relationship between the length L 3  of the narrow tube portion  21  on the cathode side and the length L 4  of the narrow tube portion  22  on the anode side satisfies 0.4×L 4 ≦L 3 ≦0.8×L 4  and 10 mm≦L 4 ≦15 mm, a temperature of the sealed part is optimized, and accordingly it is possible to carry out a reaction of the filled substance such as an iodide and frit glass and a prevention of a crack or a leakage from being caused by a thermal stress. Furthermore, it is also possible to enhance a luminous efficiency through a rise in the coldest point temperature. 
         [0086]    By setting the diameter of the narrow tube portion  21  to be smaller than that of the narrow tube portion  22  to reduce the heat capacity in the narrow tube portion  21 , it is also possible to suppress a reduction in a temperature on the cathode side still more. 
         [0087]    By setting the narrow tube portion  21  on the cathode side to be shorter than the narrow tube portion  22  on the anode side, moreover, it is possible to reduce the distance from the light emitting portion  20  to the front end of the discharge lamp  8 , thereby intercepting a light emitted from the light emitting portion  20  and reflected by the reflector  7  with difficulty through the discharge lamp  8 . Consequently, it is possible to ensure an excellent light distribution performance. 
         [0088]    In the discharge lamp  8 , as shown in  FIG. 1 , a light emitted at a rearward angle θ of approximately 55° or less with respect to a vertical line P is used as a light irradiated forward. It is desirable that the narrow tube portion  21  should be set to have such a length as not to intercept, through the discharge lamp  8 , a light emitted within the range of the angle θ and reflected by the reflector  7 . 
         [0089]    As described above, the iodide is filled in the light emitting portion  20  and the ends of the narrow tube portions  21  and  22  at the light emitting portion  20  side. However, it is desirable that a filling amount of the iodide in the space on the cathode side should be increased and the iodide should be thus present at a high density in order to raise the coldest point temperature. 
         [0090]    Therefore, in the discharge lamp  8 , the length L 2  in the longitudinal direction of the space which is present between the inner peripheral surface of the narrow tube portion  21  and the outer peripheral surface of the electrode  23  on the cathode side is set to be equal to or smaller than 1.0 mm as described above. Consequently, a volume of the space between the narrow tube portion  21  and the electrode  23  on the cathode side is reduced to cause the iodide to be present on the cathode side at a high density. Thus, it is possible to raise the coldest point temperature, thereby enhancing a luminous efficiency. 
         [0091]    Moreover, the protrusion amount S 1  of the cathode side electrode  23  from the narrow tube portion  21  toward the light emitting portion  20  side is set to be equal to or smaller than 2.0 mm. Consequently, a temperature of an internal wall in the light emitting tube on the cathode side is raised, resulting in an increase in the density of the iodide which is present in the space on the cathode side. 
         [0092]    In the discharge lamp  8 , as described above, the heat insulating film  25  is formed over the outer peripheral surface from the front end of the light emitting portion  20  to the rear end of the narrow tube portion  21 . Accordingly, a temperature of the portion on the cathode side in the light emitting tube  19  where the heat insulating film  25  is formed is raised by the radiant heat from the arc. Thus, it is possible to enhance the luminous efficiency still more. 
         [0093]    Furthermore, the range of formation of the heat insulating film  25  is set to be a range in which the rear end is coincident with the rear end of the electrode  23  on the cathode side. Consequently, a light emitted from the light emitting portion  20  and required for a light distribution pattern is not intercepted by the heat insulating film  25 . Thus, it is possible to prevent a reduction in a light distribution performance. 
         [0094]    In the discharge lamp  8 , as described above, the density of the iodide which is present in the space on the cathode side is increased. However, there is a possibility that alumina (ceramic) to be a material for forming the light emitting tube  19  might be corroded by the iodide depending on the filling amount of the iodide to cause a leakage over the light emitting portion  20  in an early stage. 
         [0095]    In the discharge lamp  8 , therefore, it is desirable that the portion in the light emitting tube  19  which covers the cathode side electrode  23  should be formed as a thick portion  19   a  having a greater thickness than the thicknesses of the other parts of the light emitting tube  19  as shown in  FIG. 4 . Also in the case in which the thick portion  19   a  is formed, it is also possible to form the heat insulating film  25  on an outer periphery thereof. 
         [0096]    By forming the thick portion  19   a  on the light emitting tube  19 , strength of the light emitting tube  19  is thus increased. Consequently, it is possible to prevent the leakage from being caused over the light emitting portion  20  in the early stage. 
         [0097]    Moreover, it is desirable that the range of formation of the thick portion  19   a  should be set into a range in which a rear end is coincident with the rear end of the cathode side electrode  23  in the same manner as the range of formation of the heat insulating film  25 , and the light emitted from the light emitting portion  20  and required for the light distribution pattern should be prevented from being transmitted through the thick portion  19   a.    
         [0098]    In the discharge lamp  8 , as described above, the light emitted at the rearward angle θ of approximately 55° or less with respect to the vertical line P is used as the light irradiated forward. 
         [0099]    In the molding of the discharge lamp  8 , in respect of a processing, a thickness of a bent part in the light emitting tube  19  is apt to be varied. Consequently, there is a possibility that a light transmitted through a portion having a thickness varied might not advance in a desirable direction, resulting in deterioration in the light distribution performance. 
         [0100]    Therefore, in the discharge lamp  8 , the light emitting portion  20  is provided with the flat portion  20   a  which is extended longitudinally by 2.0 mm or more and the protrusion amount S 2  of the electrode  24  on the anode side from the narrow tube portion  22  is set to be equal to or greater than 2.0 mm, and the light emitted from the light emitting portion  20  at the angle θ of 55° or less is transmitted through the flat portion  20   a  to enhance the light distribution performance as described above. 
         [0101]    Any shape and structure of each portion described in the best mode for carrying out the invention is only illustrative for concreteness in an execution of the invention. Consequently, the technical scope of the invention should not be construed to be restrictive.
A discharge lamp for a vehicle has an external tube attached to a socket, a light emitting tube disposed in the external tube and formed of ceramic, the light emitting tube including a light emitting portion and a pair of narrow tube portions linked to the light emitting portion at opposite sides with the light emitting portion interposed therebetween, a cathode side electrode and an anode side electrode which are disposed in the light emitting tube, and two lead wires connected to the cathode side electrode and the anode side electrode respectively and having at least a part disposed in the external tube respectively. A length of the narrow tube portion on the cathode side being set to be smaller than that of the narrow tube portion on the anode side. The cathode side electrode being disposed on an opposite side to the socket with the anode side electrode interposed therebetween.