Patent Publication Number: US-2002002988-A1

Title: Method and apparatus for cleaning translucent tube for discharge lamp, and discharge lamp

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a method and an apparatus for cleaning a translucent tube for a discharge lamp. Furthermore, the present invention relates to a discharge lamp produced using a translucent tube that has been cleaned by such a cleaning method.  
       [0002] In recent years, an image projection apparatus such as a liquid crystal projector and a DMD projector has been widely used as a system for realizing large-scale screen images, and a high pressure discharge lamp having a high intensity (e.g., ultra high pressure mercury lamp and a metal halide lamp) has been commonly and widely used as the light source of such an image projection apparatus. Such a high pressure discharge lamp is produced by inserting an electrode assembly in which tungsten electrodes and metal foils are connected to each other into a glass tube for a discharge lamp (e.g., Japanese Laid-Open Patent Publication No. 10-321135).  
       [0003]FIG. 11 is a schematic cross-sectional view of a glass tube  100  for discharge lamp that is a translucent tube for a discharge lamp used to produce a high pressure discharge lamp. The glass tube  100  includes a substantially spherical luminous bulb portion  10  and a side tube portion  20  extending from the luminous bulb portion  10 . The luminous bulb portion  10  is a portion that becomes the luminous bulb of the high pressure discharge lamp, and the side tube portion  20  is a portion that becomes the sealing portion (seal portion) of the high pressure discharge lamp. The glass tube  100  is formed of quartz glass from which alkaline components are removed as much as possible. The alkaline components are removed as much as possible for the following reasons. When an alkaline component (e.g., Na) is present in the glass tube  100  (in particular, the luminous bulb portion  10 ), the alkaline component becomes the seed of crystallization of the quartz glass, and crystallization of the quartz glass (phase transition to cristobalite) proceeds at a high temperature during lamp operation. As a result, the quartz glass becomes opaque. This phenomenon of becoming opaque is referred to as devitrification, and this is one factor that shortens the life of the discharge lamp. In order to suppress devitrification to prolong the life of the discharge lamp, the concentration of the alkaline components contained in the quartz glass constituting the glass tube for a discharge lamp is, for example, 1 ppm or less.  
       [0004] Since the glass tube  100  for a discharge lamp is handled with care so that impurities are not attached thereto, it conventionally has been believed that substantially no alkaline component is attached to the glass tube  100  supplied from, for example, a manufacturer. In measurement of the inventors of the present invention, the alkaline component attached to the glass tube  100  is in an undetectable level. Therefore, in the past, for example, when cleaning the glass tube  100 , the glass tube  100  was immersed in a container  170  to which pure water  160  was constantly supplied to clean the glass tube  100 , as shown in FIG. 12.  
       [0005] However, the observation of the inventors of the present invention confirmed that even if the glass tube  100  having an alkaline component concentration of 1 ppm or less is used to produce a discharge lamp, an influence of the alkaline component cannot be avoided. Although the inventors of the present invention handled the glass tube  100  under an inert atmosphere (e.g., argon) in order to prevent impurities from being mixed as much as possible in a production process of a discharge lamp, an influence of the alkaline component could not be eliminated. For this reason, when the glass tube  100  whose inner face was believed to have no alkaline component attached was tentatively cleaned and the impurity concentration of the alkaline component contained in the cleaning liquid was analyzed, the inventors of the present invention found that the alkaline component in a concentration of more than the alkaline component concentration in the quartz glass was attached on the inner face of the glass tube  100 . In other words, it was found that the level of the alkaline component of the glass tube  100  was higher than the alkaline component level of the original material (quartz glass) because of attachment of the alkaline component on the glass tube  100 .  
       [0006] In order to remove the alkaline component attached on the glass tube  100  to return the alkaline level to that of the original material (e.g., an alkaline component concentration of 1 ppm or less), it is not sufficient that the glass tube  100  is simply immersed in a flowing pure water, and a special cleaning is required. However, the glass tube  100  does not have a simple shape (e.g., not a disk shape as a semiconductor wafer), but has a complicated shape, so that it is difficult to clean the inner face of the glass tube  100 , especially the inner face of the luminous bulb portion  10 . This problem will be described further with reference to FIG. 13.  
       [0007] In the case where the glass tube  100  is immersed in a container  170  filled with pure water  160 , generally an air bubble  32  is generated in the glass tube  100 , as shown in FIG. 13. As shown in FIG. 12, the glass tube  100  lies in the container  170 , an air bubble  32   a  is present in an upper portion of the internal portion of the luminous bulb portion  10  of the glass tube  100 . This air bubble  32   a  remains in the upper portion of the internal portion of the luminous bulb portion  10  of the glass tube  100 , even if the glass tube  100  is rotated or moved. Therefore, the air bubble  32   a  prevents the pure water  160  from being in contact with the inner face of the luminous bulb portion  10 , so that the impurities present in the inner face of the luminous bulb portion  10  cannot be removed. Furthermore, also in the case of an air bubble  32   b  that is present in a portion other than the luminous bulb portion  10 , as long as the air bubble  32   b  is present, the pure water  160  cannot be in contact with the inner face of the glass tube  100  in that portion, so that the impurities cannot be removed.  
       [0008] Furthermore, when a large air bubble  32   c  that blocks the internal portion of the glass tube  100  is present, the pure water  160  cannot flow in the internal portion of the glass tube  100 , even if the pure water  160  continues to flow in the container  170 . Therefore, the inner face of the glass tube  100  substantially cannot be cleaned. The reason is as follows. When the pure water  160  in the internal portion of the glass tube  100  stop flowing, even if the impurities present in the inner face of the glass tube  100  are dissolved in the pure water  160 , the impurities cannot go out from the glass tube  100 . Therefore, when glass tube  100  is dried after cleaning, the impurities are attached on the inner face of the glass tube  100  again. Therefore, unless the pure water  160  flows in the internal portion of the glass, the inner face cannot be cleaned, although the outer face of the glass tube  100  can be cleaned. In addition, even if there is no air bubbled  32  in the glass tube  100 , the inner diameter of the glass tube  100  is not so large that the flow of the pure water  160  in the glass tube  100  is not satisfactory, or rather the pure water  160  hardly flows.  
       [0009] This is a problem not only in the case where the pure water  160  flows in the container  170 , but this problem also occurs when cleaning the glass tube  100  immersed in the container  170  with ultrasonic waves. In other words, even if the impurities are ready to be dissolved in the pure water by applying ultrasonic waves, the impurities are eventually attached on the inner face of the glass tube  100  after drying, unless the impurities go out from the glass tube  100 . Furthermore, a method of cleaning by inserting a rotating brush into the glass tube  100  causes scratches on the inner face of the glass tube  100 , and therefore this is not practical.  
       SUMMARY OF THE INVENTION  
       [0010] Therefore, with the foregoing in mind, it is a main object of the present invention to provide a method for cleaning away the impurities attached on the inner face of a translucent tube for a discharge lamp.  
       [0011] A method for cleaning a translucent tube for a discharge lamp having a luminous bulb portion includes the steps of introducing a cleaning fluid from one end of the translucent tube; and allowing the cleaning fluid to flow while bringing the cleaning fluid in contact with at least an inner face of the luminous bulb portion of an inner face of the translucent tube, thereby removing impurities attached on the inner face of the luminous bulb portion.  
       [0012] It is preferable that in the step of removing impurities, the cleaning fluid is allowed to flow while being in contact with an outer face of the translucent tube in the same step, thereby removing impurities attached on the outer face of the translucent tube.  
       [0013] It is preferable that the step of introducing a cleaning fluid includes the steps of disposing the translucent tube in a container for containing a cleaning liquid as the cleaning fluid such that ends of the tube are positioned in a substantially vertical direction; and injecting the cleaning liquid into the container. The step of removing impurities includes the steps of elevating a liquid surface of the cleaning liquid above an upper portion of the luminous bulb portion of the translucent tube for a discharge lamp; and lowering the liquid surface of the cleaning liquid below a lower portion of the luminous tube portion.  
       [0014] It is preferable that the step of elevating the liquid surface of the cleaning liquid and the step of lowering the liquid surface of the cleaning liquid are repeated.  
       [0015] It is preferable that the step of lowering the liquid surface of the cleaning liquid is performed such that the liquid surface of the cleaning liquid is lowered below a lower end of the translucent tube.  
       [0016] It is preferable that the step of elevating the liquid surface of the cleaning liquid is performed such that the liquid surface of the cleaning liquid is elevated above an upper end of the translucent tube.  
       [0017] It is preferable that the method further includes the step of discharging the cleaning liquid in the container from the container.  
       [0018] In one embodiment, the ending point of cleaning is determined by monitoring the concentration of the impurities contained in the cleaning liquid in the container.  
       [0019] It is preferable that the translucent tube is disposed using a holding tool for holding a plurality of translucent tubes such that ends of the tubes are positioned in a substantially vertical direction.  
       [0020] In one embodiment, the cleaning liquid is one selected from the group consisting of ultrapure water (the value of resistance: e.g., about 10MΩ or more), pure water (the value of resistance: e.g., about 1MΩ or more), deionized water (the value of resistance: e.g., about 1MΩ-10MΩ or more), hydrofluoric acid aqueous solution and hydrogen peroxide aqueous solution, and a combination of one of these cleaning liquids and cleaning fine particles.  
       [0021] In one embodiment, a plurality of kinds of impurities are attached on the inner face of the luminous bulb portion as the impurities, and the method includes at least a first step of introducing a first cleaning liquid as the cleaning liquid from the one end of the translucent tube with respect to a first kind of impurities of the plurality of kinds of impurities, and a second step of introducing a second cleaning liquid other than the first cleaning liquid as the cleaning liquid from the one end of the translucent tube with respect to a second kind of impurities other than the first kind of impurities of the plurality of kinds of impurities.  
       [0022] In one embodiment, the cleaning fluid is either one of gas, liquid, or fine particle powder, and the step of removing impurities attached on the inner face of the luminous bulb portion is performed by discharging the cleaning fluid introduced from one end of the translucent tube for a discharge lamp from the other end of the translucent tube for a discharge lamp.  
       [0023] In one embodiment, the cleaning fluid is inert gas (e.g., argon gas or nitrogen gas).  
       [0024] According to another aspect of the present invention, an apparatus for cleaning a translucent tube for a discharge lamp includes a container for accommodating a translucent tube for a discharge lamp having a luminous bulb portion and for containing a cleaning liquid; an injection pipe or an injection tube through which the cleaning liquid is injected into the container; a discharge pipe or a discharge tube through which the cleaning liquid in the container is discharged; and a concentration monitoring meter for monitoring a concentration of impurities that was attached on the translucent tube and is contained in the cleaning liquid in the container.  
       [0025] It is preferable that the concentration monitoring meter has a function to determine an end of cleaning by comparing an electrical conductivity of the cleaning liquid in the container or the cleaning liquid discharged from the container with a reference value, an electrical conductivity of the cleaning liquid injected to the injection pipe or the injection tube being used as the reference value.  
       [0026] In one embodiment, the container has an airtight structure that prevents the cleaning liquid in the container from being in contact with air outside the container.  
       [0027] According to another aspect of the present invention, a discharge lamp comprising a luminous bulb and sealing portions extending from the luminous bulb, the luminous bulb enclosing a luminous material and including a pair of electrodes opposed to each other therein, wherein the discharge lamp is produced by a method for producing a discharge lamp including the steps of preparing a translucent tube for a discharge lamp including a luminous bulb portion that becomes a luminous bulb of the discharge lamp, and side tube portions extending from the luminous bulb portion, the translucent tube having been subjected to a cleaning process; inserting an electrode assembly including metal foils and electrodes connected to the metal foils into the side tube portion such that the heads of the electrodes are positioned inside the luminous bulb portion; and forming the sealing portions by tightly attaching the metal foils of the electrode assembly to the side tube portions. The cleaning process includes the steps of (a) introducing a cleaning fluid from one end of a translucent tube for a discharge lamp including a luminous bulb portion that becomes a luminous bulb of the discharge lamp, and side tube portions extending from the luminous bulb portion; and (b) allowing the cleaning fluid to flow while bringing the cleaning fluid in contact with at least the inner face of the luminous bulb portion of the inner face of the translucent tube, thereby removing impurities attached on the inner face of the luminous bulb portion. The step (a) includes a step (a-1) of disposing the translucent tube in a container for containing a cleaning liquid as the cleaning fluid so that ends of the translucent tube are positioned in a substantially vertical direction; and a step (a-2) of injecting the cleaning liquid into the container. The step (b) includes a step (b-1) of elevating a liquid surface of the cleaning liquid above an upper portion of the luminous bulb portion of the translucent tube; and a step (b-2) of lowering the liquid surface of the cleaning liquid below a lower portion of the luminous tube portion.  
       [0028] In one embodiment of the discharge lamp, as the luminous material, at least mercury, a rare gas and halogen are enclosed in the luminous bulb, and the luminous bulb is made substantially of quartz glass, and the electrodes are made substantially of tungsten, and the mole number of the halogen is larger than the sum of the total mole number of metal elements that have the property of bonding to the halogen and are present in the luminous bulb (except for tungsten and mercury), and the mole number of the tungsten present in the luminous bulb after evaporated from the electrodes during lamp operation. In addition to that, when each kind of the metal elements (except for tungsten element and mercury element) is Mi, the mole number of the metal element Mi is mi, and the stoichiometric coefficient of the metal element Mi is ni, the mole number of the halogen is larger than the sum of the total number (Σ(mi×ni)) obtained by adding the numbers obtained by multiplying the mole number mi of the metal element Mi by the stoichiometric coefficient ni with respect to each kind of the metal elements Mi, and the mole number of the tungsten.  
       [0029] In one embodiment, the mole number of the halogen enclosed in the luminous bulb is not less than five times the total mole number of sodium (Na), potassium (K), lithium (Li), chromium (Cr), iron (Fe) and nickel (Ni) present in the luminous bulb.  
       [0030] In one embodiment, the discharge lamp is a mercury lamp in which a bulb wall load of the luminous bulb is 80 W/cm 2  or more.  
       [0031] According to the present invention, a cleaning fluid is introduced from one end of a translucent tube for a discharge lamp (e.g., glass tube or ceramic tube for a discharge lamp), and the cleaning fluid is allowed to flow while bringing the cleaning fluid in contact with at least the inner face of the luminous bulb portion. Thus, the impurities attached on the inner face of the luminous bulb portion can be removed satisfactorily by constantly introducing new cleaning fluid to the inner face of the luminous bulb portion.  
       [0032] Furthermore, when the cleaning fluid is allowed to flow while bringing the cleaning fluid in contact with, not only the inner face of the luminous bulb portion, but also the outer face of the translucent tube, the impurities attached on the outer face of the translucent tube as well as the impurities attached on the inner face thereof can be removed. As a result, the impurities (especially, fingerprints or oils and fats of workers) attached on the outer face of the translucent tube can be removed, so that the devitrification of the luminous bulb that may occur during lamp lighting due to the impurities can be prevented.  
       [0033] In the case where a cleaning liquid is used as the cleaning fluid, the cleaning liquid is allowed to flow while being in contact with the inner face of the luminous bulb portion in the following manner. A translucent tube is disposed in a container in which a cleaning liquid is to be poured so that the ends of the tube are positioned in a substantially vertical direction, and then the liquid surface of the cleaning liquid is elevated above the upper portion of the luminous bulb portion. Thereafter, the liquid surface of the cleaning liquid is lowered below the lower portion of the luminous bulb portion. In this case, it is preferable to repeat the process of elevating the liquid surface of the cleaning liquid and the process of lowering the liquid surface of the cleaning liquid. When the liquid surface of the cleaning liquid is lowered below the lower end of the translucent tube, the cleaning liquid having a high content of impurities can be discharged from the tube, so that the impurities attached on the inner face of the translucent tube can be removed satisfactorily. Also when the liquid surface of the cleaning liquid is elevated above the upper end of the translucent tube, the cleaning liquid having a high content of impurities can be discharged from the tube. Furthermore, this is also preferable, because the entire inner face of the translucent tube can be in contact with the cleaning liquid. It is more preferable to discharge the cleaning liquid in the container from the container, because the cleaning liquid containing impurities can be replaced by a cleaning liquid that has not been subjected to cleaning yet. Furthermore, when cleaning is performed while monitoring the concentration of the impurities contained in the cleaning liquid, it is possible to easily determine the time at which the impurities are removed completely (end of cleaning).  
       [0034] In the case where a holding tool for holding a plurality of translucent tubes for a discharge lamp is used, the plurality of translucent tubes can be cleaned, so that working efficiency can be improved. As the cleaning liquid, ultrapure water, pure water, deionized water, hydrofluoric acid aqueous solution, and hydrogen peroxide aqueous solution can be used, and a combination of one of these cleaning liquids and cleaning fine particles can be used. As the cleaning liquid, for example, ultrapure water is used, an alkaline component can be removed satisfactorily. In the case where a plurality of kinds of impurities are attached on the inner face of the luminous bulb portion, a first cleaning liquid is used with respect to a first kind of impurities, and a second cleaning liquid is used with respect to a second kind of impurities to clean the translucent tube.  
       [0035] Furthermore, as the cleaning fluid, either gas, liquid, or fine particle powder can be used to remove the impurities attached on the inner face of the luminous bulb portion by discharging the cleaning fluid introduced from one end of the translucent tube for a discharge lamp from the other end of the translucent tube for a discharge lamp. As the cleaning fluid, for example, inert gas such as argon gas and nitrogen gas can be used.  
       [0036] An apparatus for cleaning a translucent tube for a discharge lamp of the present invention includes an injection pipe (or injection tube) through which a cleaning liquid is injected into a container for accommodating the translucent tube for a discharge lamp, a discharge pipe (or discharge tube) through which the cleaning liquid is discharged from the container, and a concentration monitoring meter for monitoring the concentration of the impurities contained in the cleaning liquid in the container. Therefore, the impurity concentration can be monitored while cleaning, which can ensure the cleaning of the translucent tube for a discharge lamp. If the concentration monitoring meter has a function to determine the end of cleaning, the ending point of cleaning can be determined easily. In the case where the concentration monitoring meter is used, for example, the time at which the electrical conductivity of the cleaning liquid in the container or the cleaning liquid discharged from the container is substantially equal to a reference value can be set as the ending point of cleaning. Furthermore, in the case where the container has an airtight structure so that the cleaning liquid in the container is not in contact with the air outside the container, it is possible to prevent impurities contained in the air outside the container from being mixed with the cleaning liquid in the container.  
       [0037] In a discharge lamp produced with the translucent tube for a discharge lamp cleaned by the method for cleaning a translucent tube for a discharge lamp of the present invention, the impurities are removed satisfactorily, so that the discharge lamp has higher performance (e.g., longer lifetime) than those in the prior art.  
       [0038] According to the present invention, cleaning fluid is introduced from one end of a translucent tube for a discharge lamp (e.g., glass tube for a discharge lamp), and the cleaning fluid is allowed to flow while bringing the cleaning fluid in contact with at least the inner face of the luminous bulb portion. Therefore, the impurities attached on the inner face of the luminous bulb portion can be removed satisfactorily. As a result, the impurities (e.g., alkaline components) attached on the inner face of the luminous bulb portion can be cleaned away to the same concentration level as that of the impurities of the material (quartz glass) constituting the glass tube for a discharge lamp. Therefore, the present invention can produce a discharge lamp having a long lifetime in which devitrification or the like can be prevented.  
       [0039] This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0040]FIGS. 1A to  1 C are cross-sectional views for illustrating a method for cleaning a translucent glass tube for a discharge lamp of an embodiment of the present invention.  
     [0041]FIGS. 2A and 2B are views for illustrating a method for cleaning a translucent tube for a discharge lamp of Embodiment 1.  
     [0042]FIG. 3 is a perspective view showing a structure of a cleaning apparatus  70 .  
     [0043]FIG. 4 is a perspective view showing a structure of a holding tool  90 .  
     [0044]FIG. 5 is a cross-sectional view showing a variation of the cleaning apparatus  70 .  
     [0045]FIG. 6 is a flowchart showing each process of a cleaning method of Embodiment 2.  
     [0046]FIG. 7 is a flowchart showing the production process of a discharge lamp of Embodiment 3.  
     [0047]FIGS. 8A to  8 C are cross-sectional views for illustrating a lamp production process S 300 .  
     [0048]FIG. 9 is a cross-sectional view schematically showing a structure of a discharge lamp  1000  of Embodiment 3.  
     [0049]FIG. 10 is a cross-sectional view schematically showing a structure of a lamp provided with a mirror  1200 .  
     [0050]FIG. 11 is a cross-sectional view of a glass tube for a discharge lamp.  
     [0051]FIG. 12 is a view for illustrating a method for cleaning a glass tube for a discharge lamp.  
     [0052]FIG. 13 is a view for illustrating a problem of a method for cleaning a glass tube for a discharge lamp. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0053] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following drawings, for simplification, elements having substantially the same functions have the same reference numerals.  
     [0054]FIGS. 1A to  1 C are cross-sectional views schematically showing each process of a method for cleaning a translucent tube for a discharge lamp of en embodiment of the present invention.  
     [0055] First, a glass tube  100  for a discharge lamp that is a translucent tube for a discharge lamp having a substantially spherical luminous bulb portion  10  and side tube portions  20  is prepared. The inner diameter and the glass thickness of the luminous bulb portion  10  of the glass tube  100  prepared in this embodiment are 6 mm and 3 mm, respectively. The inner diameter and the length in the longitudinal direction of the side tube portion  20  is 3.4 mm and 250 mm, respectively. The glass tube  100  is formed of quartz glass, for example, having an alkaline component concentration of 1 to 2 ppm, preferably 1 ppm or less. Impurities  30  are attached on the inner face (e.g., inner face  10   a  of the luminous bulb portion  10 ) of the prepared glass tube  100  before cleaning. Examples of the impurities  30  include alkaline components (Na, K etc.), silica powder, and organic substances. In this embodiment, a glass tube for a discharge lamp is used, but the glass tube for a discharge lamp can be replaced by a ceramic tube for a discharge lamp.  
     [0056] As shown in FIG. 1A, cleaning fluid  50  is introduced from one end  100   a  of the glass tube  100 . The introduced cleaning fluid  50  flows while being in contact with the inner face  10   a  of the luminous bulb portion  10 , as shown in FIG. 1B, and impurities  30  attached at least on the inner face  10   a  of the luminous bulb portion  10  are removed, as shown in FIG. 1C. Then, the cleaning fluid  50  is discharged from the other end  100   b  of the glass tube  100 , and thereafter new cleaning fluid  50 ′ is introduced from the one end  100   a  of the glass tube  100 , and the impurities  30  are removed by the cleaning fluid  50 ′. This introduction of cleaning fluid is repeated, and at the point where all the attached impurities  30  are removed, the glass tube  100  has been cleaned to the same concentration level as that of the impurities contained in the material (quartz glass) constituting the glass tube  100 .  
     [0057] According to the cleaning method of this embodiment, the cleaning fluid  50  flows while being in contact with the inner face  10   a , so that the cleaning fluid  50  washes away the impurities  30 . Furthermore, it is possible to remove the impurities  30  in an infinite dilution manner by replacing the cleaning fluid  50  in the glass tube  100  by new cleaning fluid. On the other hand, in the cleaning method shown in FIG. 12, even if the glass tube  100  is moved more or less, the pure water in the glass tube  100  is hardly moved, and the pure water in the glass tube  100  can hardly be replaced. For this reason, in the cleaning method shown in FIG. 12, removal of impurities is performed by diffusing the impurities in substantially stationary pure water. Therefore, the cleaning method of this embodiment can remove the impurities  30  from the tube more satisfactorily.  
     [0058] As the cleaning fluid  50  in this embodiment, either gas (argon gas), liquid (cleaning liquid) or fine particle powder (quartz beads) can be used. The cleaning fluid introduced from one end  100   a  of the glass tube  100  is discharged only from the other end  100   b , as shown in FIG. 1C, but also from the one end  100   a  when the cleaning fluid is, for example, a cleaning liquid.  
     [0059] Hereinafter, an embodiment will be described in the case where a cleaning liquid is used as the cleaning fluid, and a glass tube for a discharge lamp is used as the translucent tube for a discharge lamp. However, the present invention is not limited by the following embodiments. For example, a ceramic tube for a discharge lamp can replace the glass tube for a discharge lamps  
     [0060] Embodiment 1  
     [0061] A method for cleaning a glass tube for a discharge lamp with a cleaning liquid as the cleaning fluid will be described with reference to FIGS. 2A and 2B. FIGS. 2A and 2B schematically show each process of a cleaning method of Embodiment 1.  
     [0062] First, as shown in FIG. 2A, a glass tube  100  for a discharge lamp is disposed in a container  72  of a cleaning apparatus  70  provided with the container  72  for containing a cleaning liquid  60 . The glass tube  100  is disposed so that the ends of the glass tube  100  are in a substantially vertical direction. One end  100   a  of the glass tube  100  is positioned in a lower portion of the container  72 .  
     [0063] Next, the cleaning liquid  60  is injected into the container  72  through an injection pipe (or injection tube)  74  for injection of the cleaning liquid  60  into the container  72 . The amount of the cleaning liquid  60  to be injected can be adjusted by a valve  75 . As the cleaning liquid  60  is injected into the container  72 , the liquid surface  60   a  of the cleaning liquid  60  is elevated. Therefore, after the cleaning liquid  60  is introduced from one end  100   a  of the glass tube  100 , the cleaning liquid  60  in the glass tube  100  flows upward while being in contact with the inner face of the glass tube  100 . The injection of the cleaning liquid  60  continues and the cleaning liquid  60  in the glass tube  100  is allowed to flow while being in contact with the inner face  10   a  of the luminous bulb portion  10 , so that the impurities (not shown) attached on the inner face  10   a  of the luminous bulb portion  10  are removed.  
     [0064] In this case, the cleaning liquid  60  flows upward while in contact with not only the inner face of the glass tube  100 , but also the outer face of the glass tube  100 . Therefore, not only the impurities attached on the inner face  10   a  of the luminous bulb portion  10 , but also the impurities attached on the outer face (especially, outer face of the luminous bulb portion  10 ) of the glass tube  100  are removed. In the outer face of the glass tube  100 , finger prints or oils and fats of workers containing much of impurities such as sodium are readily attached. It is not preferable that they remain on the luminous bulb of a discharge lamp, because this may cause devitrification. More specifically, since the temperature is increased not only in the inner face of the luminous bulb, but also the outer face during lamp lighting, and therefore finger prints or oils and fats are burned and attached thereto, which may cause devitrification. In this embodiment, the outer face of the glass tube  100  as well as the inner face thereof can be cleaned, so that such devitrification can be prevented effectively. In addition, the inner face and the outer face of the glass tube  100  can be cleaned at the same time, so that working efficiency is good.  
     [0065] At the point where the liquid surface  60   a  of the cleaning liquid  60  is elevated above the upper portion of the luminous bulb portion  10 , the entire inner face  10   a  of the luminous bulb portion  10  has been cleaned with the cleaning liquid  60 . Thereafter, the injection of the cleaning liquid  60  from the injection pipe  74  continues so that the liquid surface  60   a  of the cleaning liquid  60  is elevated until the liquid surface reaches a predetermined height in the container  72 , preferably, above the upper end of the glass tube  100 . When the liquid surface  60   a  is elevated above the upper end of the glass tube  100 , the cleaning liquid  60  can be in contact with the entire inner face of the glass tube  100 . In addition, the cleaning liquid  60  can be replaced by discharging the cleaning liquid  60  in the tube from the upper end of the glass tube  100 . Furthermore, for example, when dust is present on the liquid surface  60   a , it is possible to prevent the dust from being attached and remaining on the inner face of the tube.  
     [0066] Next, as shown in FIG. 2B, the cleaning liquid  60  in the container  72  is discharged through a discharge pipe (discharge tube)  76  for discharge of the cleaning liquid  60  in the container  72  to lower the liquid surface  60   a  of the cleaning liquid  60 . In this embodiment, the cleaning liquid  60  is injected into the container  72  through the injection pipe  74  in the state where both the valve  75  of the injection pipe  74  and the valve  75  of the discharge pipe  76  are open, so that the liquid surface  60   a  of the cleaning liquid  60  is elevated to the height of the uppermost portion  76   h  of the discharge pipe  76 . When the liquid surface  60   a  reaches the uppermost portion  76   h , the cleaning liquid is discharged (drained) automatically on the principle of siphon. When the inner diameter of the discharge pipe  76  is larger than that of the injection pipe  74 , the discharge liquid  60  can be completed without stopping the injection of the cleaning liquid  60 . Thus, the up and down of the liquid surface  60   a  can be repeated infinitely. Furthermore, the up and down of the liquid surface  60   a  can be performed by adjusting the valve  75  of the injection pipe  74  and the valve  75  of the discharge pipe  76 . Alternatively, the discharge pipe (liquid removing pipe) is provided in a lower portion of the container  72  or on the bottom face of the container  72 , so that the cleaning liquid  60  can be discharged from the discharge pipe. In this case, after the injection of the cleaning liquid  60  is stopped, the cleaning liquid  60  is discharged.  
     [0067] When the liquid surface of the cleaning liquid  60  is lowered below the lower portion of the luminous bulb portion  10  by the discharge of the cleaning liquid  60 , the cleaning liquid  60  moves while being in contact with the inner face  10   a  of the luminous bulb portion  10 . In this embodiment, the cleaning liquid  60  in the tube is removed by lowering the liquid surface  60   a  below the lower end  100   a  of the glass tube  100 , and the liquid surface  60   a  is elevated again by new cleaning liquid  60  that is constantly injected from the injection pipe  74 . Thus, cleaning can be performed while replacing the entire cleaning liquid  60  in the glass tube  100 .  
     [0068] After the liquid surface  60   a  of the cleaning liquid  60  is lowered below the lower portion of the luminous bulb portion  10 , the liquid surface  60   a  can be elevated again. In this case, the inner face of the glass tube  100  can be cleaned while replacing a part of the cleaning liquid  60  in the glass tube  100  by a part of the cleaning liquid  60  outside the tube. Furthermore, it is possible to combine the method of partially replacing the cleaning liquid  60  in the tube and the method of entirely replacing the cleaning liquid  60  in the tube. In this embodiment, the liquid surface  60   a  is elevated and lowered by the injection and the discharge of the cleaning liquid  60 . However, the liquid surface  60   a  can be elevated and lowered by physically moving the glass tube  100  up and down (e.g., mechanically or manually).  
     [0069] The glass tube  100  is cleaned by repeating elevating and lowering the liquid surface  60   a  a plurality of times. In this embodiment, for example, a set of elevating and lowering the liquid surface  60   a  is performed over about 5 minutes, and cleaning is carried out by performing the total of five sets.  
     [0070] When it is desired to clean the glass tube  100  sufficiently within a necessity minimum time while confirming that cleaning is completed, the end of cleaning can be determined by monitoring the concentration of the impurities contained in the cleaning liquid  60  in the container  72 . For example, cleaning can be performed while monitoring with a concentration monitoring meter whether or not the concentration of the impurities in the cleaning liquid  60  to be injected to the injection pipe  74  and the concentration of the impurities in the cleaning liquid  60  in the container  72  are within a predetermined range (or substantially equal). The concentration of the impurities in the cleaning liquid  60  discharged from the container  72  can be monitored instead of the concentration of the impurities of the cleaning liquid  60  in the container  72 .  
     [0071] The concentration of the impurities contained in the cleaning liquid  60  can be measured, for example, with an electrical conductivity meter. More specifically, as shown in FIGS. 2A and 2B, using an electrical conductivity meter  82  for measuring the electrical conductivity of the cleaning liquid  60  in the container  72  and a concentration monitoring meter  80  provided with a monitor  84  showing the indicated value of the electrical conductivity meter  82 , the end of cleaning can be determined. First, before disposing the glass tube  100  in the container  72 , the cleaning liquid (e.g., pure water or ultrapure water)  60  to be injected to the injection pipe  74  is poured, for example, in the container  72 , and the electrical conductivity of the cleaning liquid  60  is measured with the electrical conductivity meter  82 , and the indicated value obtained by the measurement is used as the reference value. Thereafter, when cleaning the glass tube  100 , the electrical conductivity of the cleaning liquid  60  in the container  72  is compared with the reference value, so that the end of cleaning can be determined. Furthermore, the electrical conductivity meter is placed in a cleaning liquid supply tank (not shown) of the cleaning liquid  60  to be injected to the injection pipe, and the indicated value from the electrical conductivity meter can be used as the reference value.  
     [0072] Next, an apparatus and a tool suitably used in the cleaning method of Embodiment 1 will be described with reference to FIGS. 3 and 4. FIG. 3 is a perspective view showing the structure ofe the cleaning apparatus  70  shown in FIG. 2. FIG. 4 is a perspective view of a holding tool  90  for holding a plurality of glass tubes  100  for a discharge lamp positioned substantially in the vertical direction.  
     [0073] The cleaning apparatus  70  shown in FIG. 3 includes a container  72  for containing a cleaning liquid  60  into the container  72 , an injection pipe  74  through which the cleaning liquid  60  is injected, and a discharge pipe  76  through which the cleaning liquid  60  in the container  72  is discharged. The cleaning apparatus  70  further includes a concentration monitoring meter  80  (see FIG. 2) for monitoring the concentration of the impurities contained in the cleaning liquid  60  in the container  72  (impurities attached on the glass tube  100 ), which is not shown in FIG. 3. The container  72 , the injection pipe (or injection tube)  74 , and the discharge pipe (or discharge tube)  76  of the cleaning apparatus  70  are made of polyvinylchloride (thickness: for example, about 5 mm) so as to prevent impurities of the materials constituting these members from being mixed with the cleaning liquid  60 . Instead of polyvinylchloride, PTFE (e.g., Teflon (registered trademark)) can be used.  
     [0074] The upper face of the container  72  is open, and glass tube  100  is inserted from the opening on the upper face so that glass tube  100  can be disposed in the container  72 . After disposing the glass tube  100  in the container  72 , it is desired to set a lid  78  on the upper face of the container  72  so as to shut off dust or the like from the container  72 . The upper face and the lower face of the container  72  are squares having a side of about 310 mm, and the height of the container  72  is, for example, about 505 mm. The shape of the container  72  may be a rectangle having rectangular top and bottom faces, or may be a cylinder having circular top and bottom faces.  
     [0075] The injection pipe  74  has one end  74   a  from which the cleaning liquid (e.g., pure water)  60  is introduced, and the other end  74   b  from which the cleaning liquid  60  is supplied to the container  72 . The one end  74   a  is positioned outside the container  72 , and the other end  74   b  is positioned in the container  72 . When injecting the cleaning liquid  60 , for example, a hose made of polyvinylchloride can be connected to the one end  74   a . In this embodiment, the injection pipe  74  penetrates the side of the container  72  at a predetermined portion  74   c , and a valve  75  for adjusting the amount of the cleaning liquid  60  to be injected is provided between the one end  74   a  and the other end  74   b  of the injection pipe  74 .  
     [0076] The discharge pipe  76  has one end  76   a  positioned in the container  72  and the other end  76   b  positioned outside the container  72 . The one end  76   a  of the discharge pipe  76  is provided in a lower portion of the container  72 , preferably in a portion lower than the lower end of the glass tube  100  disposed in the container  72 . When the one end  76   a  of the discharge pipe  76  is provided in a portion lower than the lower end of the disposed glass tube  100 , the following advantage is provided. When the cleaning liquid  60  is discharged, the liquid surface  60   a  of the cleaning liquid  60  can be lowered below the lower end of the glass tube  100 . As a result, the entire cleaning liquid  60  in the tube can be let out from the glass tube  100 . Furthermore, it is preferable to provide the uppermost portion  76   h  of the discharge pipe  76  in a portion higher than the upper portion of the luminous bulb portion  10  of the disposed glass tube  100 , when discharging the cleaning liquid  60  from the discharge pipe  76  using the principle of siphon. It is more preferable to provide the uppermost portion  76   h  in a portion higher than the upper end of the disposed glass tube  100 . Since the valve  75  is provided in the discharge pipe  76 , discharge can be adjusted and controller by the valve  75 . In this embodiment, apart from the discharge pipe, a pipe  77  for liquid removal provided with a valve  75  for the purpose of removing the cleaning liquid  60  is provided in a lower portion of the side of the container  72 . Therefore, the cleaning method of this embodiment can be performed by using the pipe  77  for liquid removal as the discharge pipe without using the discharge pipe  76 .  
     [0077] Next, a holding tool  90  for holding a plurality of glass tubes  100  for discharge lamps will be described. The holding tool  90  of FIG. 4 includes a holding plate  94  having openings  98  through which the side tube portions  20  of the glass tube  100  pass and the luminous bulb portions  10  do not pass, and a supporting rod  92  for supporting the holding plate  94 . The holding plate  94  is attached to the supporting rod  92  that the holding plate  94  is substantially horizontal, so that the glass tubes  100  can be positioned in the substantially vertical direction by inserting the glass tubes  100  into the openings  98  of the holding plate  94 . About one hundred openings  98  are provided in the holding plate  94 , and therefore, using the holding tool  90 , about one hundred glass tubes  100  can be cleaned at one time.  
     [0078] Below the holding plate (upper plate)  94 , there are a middle plate  95  that prevents the side tube portions  20  of the glass tubes  100  from moving freely, and a lower plate  96  that is provided under the middle plate  95  and is to be disposed on a stand  97 . In addition to the holding plate (upper plate)  94 , the middle plate  95  and the lower plate  96  are also supported by the supporting rod  92 , and when the glass tubes  100  are set into the holding tool  90 , the lower ends of the glass tubes  100  are positioned between the middle plate  95  and the lower plate  96 . It is not necessary that the lower plate  96  and the stand  97  are provided with openings through which the glass tubes pass, but it is desired that the lower plate  96  and the stand  97  are provided with the openings  98  so that the cleaning liquid  60  can be let in and out satisfactorily. Furthermore, this is preferable, because the same member as those of the upper plate  94  and the middle plate  95  can be used to produce the tool.  
     [0079] A handle  92   a  used to lift the holding tool  90  is formed in an upper portion of the supporting rod  92 , and the holding tool  90  can be disposed in the container  72  of the cleaning apparatus  70  with the handle  92   a . Therefore, after a plurality of glass tubes  100  are set in the holding portion  90 , the stand  97  is disposed on the bottom face of the container  72 , and then the holding tool  90  is moved into the container  72  with the handle  92   a , so that each of the plurality of glass tubes  100  can be disposed in the container  72 , each positioned in a substantially vertical direction. When the cleaning liquid  60  is poured in the container  72 , the holding tool  90  is stabilized in the container  72  by its self-weight, and thus the glass tubes  100  can be positioned stably in the container by the holding tool  90 .  
     [0080] The stand  97  supporting the holding tool  90  is larger that the lower plate  96  of the holding tool  90 , and a notch  97   a  is provided in the stand  97  so that a portion in the vicinity of the one end  76   a  of the discharge pipe  76  is not overlapped with the stand  97 , when the stand  97  is disposed on the bottom face of the container  72 . When disposing the holding tool  90  on the stand  97 , it is preferable that an opening provided in the lower plate  96  of the holding tool  90  is matched with the opening provided in the stand  97 , because the cleaning liquid  60  can be let in and out satisfactorily.  
     [0081] The members (e.g., the holding plate  94  and the supporting rod  92 ) constituting the holding tool  90  are formed of, for example, polyvinylchloride so as to prevent impurities from being mixed in the cleaning liquid  60 . In this embodiment, a gap hi between the upper plate  94  and the middle plate  95  is for example, about 100 mm. A gap h 2  between the upper plate  94  and the lower plate  96  is for example, about 165 mm, and a gap h 3  between the upper plate  94  and the handle  92   a  is for example, about 200 mm. After the plurality of tubes  100  are set in the holding tool  90 , the holding tool  90  is disposed in the container  72 . Then, elevating and lowering the liquid surface  60   a  is repeated a plurality of times, and thus the plurality of glass tubes  100  can be cleaned. The end of cleaning can be determined with the concentration monitoring meter  80 .  
     [0082] In order to prevent impurities (e.g., alkaline components) contained in the ambient air surrounding the cleaning apparatus  70  from entering the container  72 , as shown in FIG. 5, the container  72  can be of an airtight structure. More specifically, after the holding tool  90  in which glass tubes  100  are set is disposed in the container  72 , an inert gas (e.g., Ar)  84  is filled in the container  72 , and then a lid  78  is attached so that the opening on the top face of the container  72  is sealed. Also in order to prevent the ambient air from entering the container from the discharge pipe  76 , a discharged liquid receptacle  86  is provided in an outlet (the other end)  76   b  of the discharge pipe  76  and an inert gas  84  is filled in the discharged liquid receptacle  86 . Alkaline components are contained in the air in some concentration, and the concentration is larger in seaside areas. Therefore, it is preferable to use the container  72  of an airtight structure to clean the glass tubes  100  in order to obtain the glass tubes  100  whose impurities level is maintained at the level of the original material (quartz glass).  
     [0083] Embodiment 2  
     [0084] In Embodiment 1, a method for cleaning a glass tube for a discharge using one kind of cleaning liquid (e.g., pure water) has been described, but the present invention can apply to a method for cleaning a glass tube for a discharge lamp to remove a plurality of kinds of impurities with a plurality kinds of cleaning liquid. Hereinafter, a cleaning method of Embodiment 2 will be described with reference to FIG. 6. The same description as in Embodiment 1 will be omitted or simplified.  
     [0085]FIG. 6 is a flowchart showing each step of the cleaning method of this embodiment. First, a glass tube  100  for a discharge lamp is set in the holding tool  90  shown in FIG. 4 (step S 110 ), and the holding tool  90  in which the glass tube  100  is set is disposed in a container  72  of a first cleaning apparatus  70  (see FIG. 3). A hydrofluoric acid aqueous solution is supplied as the cleaning liquid  60  to the container  72  of the first cleaning apparatus  70 , and the liquid surface  60   a  (liquid surface of hydrofluoric acid aqueous solution) in the container  72  of the first cleaning  
     [0086] After cleaning at step S 150 , in order to protect the glass tube  100  from contamination by impurities contained in the ambient air surrounding the glass tube  100 , argon gas is introduced from one end of the glass tube  100 , and allowed to flow while being in contact with the inner face of the glass tube  100 . Then, argon gas is discharged from the other end of the glass tube  100 . Such flow of argon allows the air in the glass tube  100  to be purged by the argon gas, so that the cleaned state after step S 150  can be maintained.  
     [0087] Furthermore, when the air in the glass tube  100  is purged by argon, even if the glass tube  100  is moved, argon stays in the glass tube  100 , and argon in the glass tube  100  is not replaced by the ambient air for a comparatively long time (e.g., about several ten minutes to several hours). Therefore, the purge of the air in the glass tube  100  by argon is also advantageous in that handling of the glass tube  100  becomes easy. In addition to purging the air in the glass tube  100  by argon, fine powder particles (e.g., quartz beads) can be introduced from one end of the glass tube  100 , and the fine powder particles are brought in contact with the inner face of the glass tube  100 , so that impurities or dust attached on the inner face of glass tube  100  (especially inner face  10   a  of the luminous bulb portion  10 ) can be removed.  
     [0088] Embodiment 3  
     [0089] The translucent tube for a discharge lamp that has been cleaned by the cleaning method of the above described methods is used to produce a discharge lamp. Hereinafter, a discharge lamp produced with the translucent tube for a discharge lamp that has been cleaned by the cleaning method of the above described methods will be described with reference to FIGS.  7  to  11 . The same description as in the above embodiments will be omitted or simplified.  
     [0090]FIG. 7 is a flowchart showing a production process of a discharge lamp of this embodiment.  
     [0091] First, a quartz glass tube for a discharge lamp is prepared as a translucent tube for a discharge lamp (step S 100 ). The glass tube prepared in this embodiment is the same as that of Embodiment 1 (see FIGS. 1, 2A and  2 B or the like). In this embodiment, a glass tube for a discharge lamp is used, but a ceramic tube for a discharge can replace the glass tube for a discharge lamp.  
     [0092] Next, the glass tube is cleaned in the manner as described in the above embodiments (step S 200 ). This step provides a glass tube that has been cleaned to the level of the impurities contained in the material (quartz glass) constituting the glass tube.  
     [0093] Then, using the cleaned glass tube, a known lamp production process (step S 300 ) is performed, so that a discharge lamp is completed. The lamp production process primarily includes an electrode insertion process (step S 310 ) and a sealing portion formation process (step S 320 ). These processes will be described briefly with reference to FIGS.  8 A and  8 C.  
     [0094] First, as shown in FIG. 8A, an electrode assembly  50  including a metal foil (Mo foil)  24  and an electrode  12  connected to the metal foil  24  is inserted into the side tube portion  20  of the cleaned glass tube  100  (step S 310 ). In this process, the electrode assembly  50  is inserted so that the head of the electrode  12  is positioned inside the luminous bulb portion  10  and the metal foil  24  is positioned inside the side tube portion  20 . A coil is wound around the head of the electrode  12 , and the coil has a function to reduce the temperature at the electrode head during lamp operation. An external lead  26  is connected to the metal foil  24  of the electrode assembly  50  on the side opposite to the side connected to the electrode  12 .  
     [0095] Next, as shown in FIG. 8B, the pressure in the glass tube  100  is reduced (e.g., less than one atmospheric pressure), and the side tube portion  20  of the glass tube  100  is heated and melted with a burner  54 , so that the side tube portion  20  and the metal foil  24  are attached so that the sealing portion  22  is formed (sealing portion formation process S 320 ). Thereafter, a rare gas, halogen and mercury are enclosed in the luminous bulb portion  10 , and then the other side tube portion  20  is subjected to the electrode insertion process S 310  and the sealing portion formation process S 320 , and a pair of sealing portions  22  seals the luminous bulb  10 . Then, as shown in FIG. 8C, a discharge lamp  1000  can be obtained. In the thus produced discharge lamp  1000 , the impurities are removed more satisfactorily than an uncleaned glass tube or a glass tube cleaned by the cleaning method shown in FIG. 12, so that the discharge lamp of this embodiment can exhibit more excellent characteristics (e.g., longer lifetime) than those of conventional lamps  
     [0096]FIG. 9 shows the structure of the discharge lamp  1000  of this embodiment. The discharge lamp shown in FIG. 9 is a mercury lamp (e.g., high pressure mercury or ultra high pressure mercury lamp), and includes a luminous bulb  10  made substantially of quartz glass and a pair of electrodes  12  and  12 ′ disposed in a space (discharge space)  15  in the luminous bulb  10  and made substantially of tungsten. In the discharge space  15  of the luminous bulb  10 , at least mercury  18 , and a rare gas and halogen are enclosed. The main purposes for this enclosure are as follows. The mercury  18  is enclosed as a luminous material, and the rare gas is enclosed as a gas for smooth start, and halogen is enclosed to activate the halogen cycle.  
     [0097] In this embodiment, the mole number of halogen enclosed in the luminous bulb  10  is larger than the sum of the total mole number of metal elements that have the property of bonding to halogen (except for tungsten and mercury) and are present in the luminous bulb  10 , and the mole number of tungsten that is evaporated from the electrodes  12  and  12 ′ during lamp operation and is present in the luminous bulb  10 . Typical examples of metal elements having the property of bonding to halogen are alkali metal elements (Na, K, etc.), when excluding tungsten element and mercury element. The reason that the mole number of halogen is defined as above is described in detail in International Application No PCT/JP00/04561 (International filing date Jul., 6, 2000, Applicant: Matsushita Electric Industrial Co. Ltd.), which is incorporated herein by reference.  
     [0098] The reason that the mole number of halogen is defined will be described briefly. When the mole number of halogen is defined as above, halogen that can contribute to the halogen cycle can be constantly present in the luminous bulb  10 , so that the activation of the halogen cycle can be ensured. Thus, blackening occurring in the luminous bulb  10  can be prevented. This prevention of blackening prolongs the life of the lamp (e.g., 5000 hours to 10000 hours or more), even if the lamp is used under high output conditions (the bulb wall load of the luminous bulb is for example, 80 W/cm 2  or more) that may end the lamp life early in the prior art. The following is a possible reason why the mole number of halogen was not defined in the prior art. The mechanism of blackening was not examined based on the phenomenon of actual lamp operations under high output conditions. In addition, as described above, it was conventionally believed that alkaline components were hardly attached on the glass tube  100  for a discharge lamp that was handled with care so as not to attach impurities.  
     [0099] In the discharge lamp of this embodiment, impurities present in the inner face of the luminous bulb are removed satisfactorily by the cleaning method of the above embodiments. Therefore, with the amount level of enclosed halogen that does not interfere with lamp operation, it can be easily realized that the mole number of halogen is larger than the sum of the total mole number of the above-described metal elements and the mole number of evaporated tungsten. On the other hand, in the case of a discharge lamp produced with a uncleaned glass tube or a glass tube that has been cleaned by the cleaning method shown in FIG. 12, there are a large amount of impurities that have not been removed in the luminous bulb  10 , so that it is virtually difficult that the amount level of enclosed halogen that does not interfere with lamp operation satisfies the halogen mole number condition defined as above, which was experimentally confirmed by the inventors of the present invention.  
     [0100] In view of the case where one metal atom can be bonded to a plurality of halogens, a preferable mole number of halogen to be enclosed is as follows. When each kind of metal elements (except for tungsten element and mercury element) having the property of bonding to halogen is Mi, the mole number of halogen of a metal element Mi is mi, and the stoichiometric coefficient of the metal element Mi is ni, it is preferable that the mole number of halogen is larger than the sum of the total number (Σ(mi×ni)) obtained by adding the numbers obtained by multiplying the mole number mi of the metal element Mi by the stoichiometric coefficient ni with respect to each kind of the metal elements Mi, and the mole molybdenum is electrically connected to one end of the metal foil  24 . The rated power is 150 W (corresponding to a bulb wall load of about 85 W/cm 2 ).  
     [0101] The discharge lamp  1000  of Embodiment 3 can be formed into a lamp  1200  provided with a reflecting mirror in combination with a reflecting mirror  180 , as shown in FIG. 10. If the lamp  1200  provided with a reflecting mirror is combined with a housing (not shown) for supporting the lamp, a lamp unit can be formed. FIG. 10 schematically shows a cross-section of the lamp  1200  provided with a reflecting mirror.  
     [0102] The lamp  1200  provided with a reflecting mirror includes the discharge lamp  1000  including a substantially spherical luminous portion  10  and a pair of sealing portions  22 , and a reflecting mirror  180  for reflecting light emitted from the discharge lamp  1000 .  
     [0103] The reflecting mirror  180  is designed to reflect the radiated light from the discharge lamp  1000  so that the light becomes, for example, a parallel luminous flux, a condensed luminous flux converged on a predetermined small area, or a divergent luminous flux equal to that emitted from a predetermined small area. As the reflecting mirror  180 , a parabolic reflector or an ellipsoidal mirror can be used, for example. In the example shown in FIG. 10, a lamp base  55  is attached to one of the sealing portion  22 ′ of the discharge lamp  1000 , and an external lead  26  extending from the sealing portion  22  and the lamp base  55  are electrically connected. The sealing portion  22  attached with the lamp base  55  is adhered to the reflecting mirror  180 , for example, with an inorganic adhesive (e.g., cement) so that they are integrated. A lead wire  185  is electrically connected to the external lead  26  of the sealing portion  22  positioned on the front opening side of the reflecting mirror  180 . The lead wire  185  extends from the external lead  26  to the outside of the reflecting mirror  180  through an opening  182  for a lead wire of the reflecting mirror  180 . For example, a front glass can be attached to the front opening of the reflecting mirror  180 .  
     [0104] Such a lamp unit can be attached to an image projection apparatus such as a projector employing liquid crystal or DMD, and is used as the light source for the image projection apparatus. The discharge lamp and the lamp unit of the above embodiments can be used, not only as the light source for image projection apparatuses, but also as a light source for general illumination or ultraviolet steppers, or a light source for an athletic meeting stadium, a light source for headlights of automobiles or the like.  
     [0105] In Embodiment 3, the lamp enclosing mercury in an amount of 150 mg/cc has been described as an example. However, the amount is not limited thereto, and can be larger or smaller. In other words, in Embodiment 3, the case where the mercury vapor pressure is about 20 MPa (the case of so-called ultra high pressure mercury lamp) has been described. However, the present invention can apply to a high pressure mercury lamp in which a mercury vapor pressure is about 1 MPa. Furthermore, the gap (arc length) between the pair of electrodes  12  can be short, or can be longer than that. The high pressure discharge lamps of Embodiment 3 can be used by either lighting method, alternating current lighting or direct current lighting.  
     [0106] Furthermore, instead of mercury or with mercury, a metal halide can be enclosed. More specifically, in Embodiment 3, a mercury lamp employing mercury as a luminous material has been described as an example of a high pressure discharge lamp. However, the present invention can apply to a high pressure discharge lamp such as a metal halide lamp in which a metal halide is enclosed. However, in the configuration of the discharge lamp of Embodiment 3, it is preferable that the amount of enclosed mercury is about 200 mg/cc or less. This is preferable because with an amount of enclosed mercury of more than that, the pressure in the luminous bulb  10  is too high during operation, so that airtightness cannot be maintained in a portion of the molybdenum foil  24  of the sealing portion  22  and it is highly possible that the lamp is damaged. If it is ensured that the airtightness can be maintained, the amount of enclosed mercury can be larger than 200 mg/cc. When the amount of enclosed mercury exceeds 200 mg/cc, the thermal conductivity of the gas in the luminous bulb  10  becomes high. Therefore, the heat of discharge plasma easily propagates to the electrodes  12  or the luminous bulb  10  (quartz glass), which further raises the temperature, so that impurities more significantly leak from the glass or the electrodes. Therefore, in the case where mercury is enclosed in an amount of more than 200 mg/cc, the advantage of the lamp  1000  of Embodiment 3 produced with a glass tube made of a material having a high purity that has been cleaned satisfactorily is highly distinguished.  
     [0107] Furthermore, in Embodiment 3, the case where the bulb wall load is about 80 W/cm 2  has been described as an example, but the bulb wall load is not limited thereto. The bulb wall load can be smaller or larger than that. In the case of a higher load, a lamp is operated in a high temperature, and therefore impurities significantly leak from the glass or electrodes. Therefore, the advantage of the lamp  1000  of Embodiment 3 produced with a glass tube made of a material having a high purity is highly distinguished. However, in the configuration of the discharge lamp of Embodiment 3, it is preferable that the bulb wall load is about 100 W/cm 2  or less. This is preferable because with a load of more than that, the temperature of the luminous bulb  10  is too high, so that deformation or deterioration due to heat may occur. In this case, if these problems can be avoided by adding another means for cooling the luminous bulb  10 , the bulb wall load can be larger than 100 W/cm 2 .  
     [0108] Furthermore, in Embodiment 3, the case where the rated power is 150 W has been described as an example, but the rated power is not limited thereto, and can be more than 150 W or less than 150 W. However, the configuration of the discharge lamp of Embodiment 3 is particularly suitable for a lamp having a comparatively large power of 50 W or more. Since a lamp having a large power is operated in a higher temperature, impurities significantly leak from the glass or the electrodes. Therefore, the advantage of the lamp  1000  of Embodiment 3 made of a material having a high purity is highly distinguished. As the discharge lamp of Embodiment 3, a lamp enclosing bromine (Br) as halogen has been described as an example, but halogen can be replaced by chlorine (Cl) or iodine (I).  
     [0109] The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.