Patent Publication Number: US-2002005695-A1

Title: Lamp unit and image projection apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to lamp units. In particular, the present invention relates to lamp units used as a light source for an image projection apparatus such as a liquid crystal projector or a digital micromirror device (DMD) projector.  
       [0002] In recent years, an image projection apparatus such as a projector using a liquid crystal projector or a DMD has been widely used as a system for realizing large-scale screen images. A high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. For the light source of the image projection apparatus, light is required to be concentrated on an imaging device included in the optical system of the projector, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source. Therefore, among high-pressure discharge lamps, a short arc ultra high-pressure mercury lamp that is close to a point light and has a high intensity has been noted widely as a promising light source. The short arc ultra high-pressure mercury lamp can be used as a light source for projectors in the form of a lamp provided with a mirror in combination with a reflecting mirror.  
       [0003] Referring to FIG. 7, a conventional lamp  1200  provided with a mirror including a short arc ultra high-pressure mercury lamp  1000  will be described. FIG. 7 is a schematic top view of a lamp  1200  provided with a mirror including a combination of an ultra high-pressure mercury lamp  1000  and a reflecting mirror  60 .  
       [0004] The lamp  1200  provided with a mirror includes a lamp  1000  and a reflecting mirror  60  for reflecting light emitted from the lamp  1000 . The lamp  1000  includes a substantially spherical luminous bulb  110  made of quartz glass, and a pair of sealing portions (seal portions)  120  and  120 ′ made of also quartz glass and connected to the luminous bulb  110 . A discharge space  115  is inside the luminous bulb  110 . A mercury in an amount of, for example, 150 to 250 mg/cm 3  as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space  115 . A pair of tungsten electrodes (W electrode)  112  and  112 ′ are opposed with a certain distance (e.g., about 1.5 mm) in the discharge space  115 .  
       [0005] The W electrode  112  is welded to a molybdenum foil (Mo foil)  124  in the sealing portion  120 , and the W electrode  112  and the Mo foil  124  are electrically connected. The sealing portion  120  includes a glass portion  122  extended from the luminous bulb  110  and the Mo foil  124 . The glass portion  122  and the Mo foil  124  are attached tightly so that the airtightness in the discharge space  115  in the luminous bulb  110  is maintained. An external lead (Mo rod)  130  made of molybdenum is joined to one end of the Mo foil  124  by welding, and the Mo foil  124  and the external lead  130  are electrically connected. The configurations of the W electrode  112 ′ and sealing  120 ′ are the same as those of the electrode  112  and sealing  120 , so that description thereof ill be omitted.  
       [0006] Next, the operational principle of the lamp  1000  will be described. When a start voltage is applied to the W electrodes  112  and  112 ′ via the external leads  130  and the Mo foils  124 , discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space  115  of the luminous bulb  110 , and thus the mercury is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes  112  and  112 ′. As the mercury vapor pressure of the lamp  1000  is higher, the emission efficiency is higher, so that the lamp having a higher mercury vapor pressure is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb  110 , the lamp  1000  is used at a mercury vapor pressure of 15 to 25 MPa.  
       [0007] The light emitted from the discharge lamp  1000  is reflected at the reflecting mirror  60  and emits in the emission direction  50 . The reflecting mirror  60  has a front opening  60   a  on the side of the emission direction  50 . As described above, the mercury vapor pressure of the lamp  1000  is set to be within the range that is permitted by the physical strength against pressure of the luminous bulb  110  to prevent the damage of the lamp  1000 . However, for the purpose of preventing scattering, if the lamp should be broken, or preventing foreign matter from being directed toward the mirror, a front glass  170  is attached at the front opening  60   a . In other words, the lamp  1200  provided with a mirror is of an airtight structure, and scattered matters (glass pieces or mercury) generated, if the lamp should be broken, are prevented from going out. A lead wire  65  for external interconnection is electrically connected to the external lead  130  of the sealing portion  120 . The lead wire  65  for external interconnection is extended to the outside of the reflecting mirror  60  through an opening  62  for lead wire and electrically connected to an external circuit (e.g., ballast). The reflecting mirror  60  is attached to the sealing portion  120 ′ of the discharge lamp  1000 , and a lamp base  55  is attached to one end of the sealing portion  120 ′.  
       [0008] When combining this lamp  1200  provided with a mirror with an optical system of an image projection apparatus (projector), as shown in FIG. 8A, it is general to use a lamp unit  1500  in which the lamp  1200  is integrated with a lamp house  180  for holding the lamp  1200  provided with a mirror.  
       [0009]FIG. 8A is a schematic view of the configuration of an image projection apparatus including the lamp unit  1500  and an optical system  190  ( 191  to  193 ), the lamp unit  1500  being partially cut away. FIG. 8B is a perspective view viewed from the front of the lamp house  180  of the lamp unit  1500 .  
       [0010] The lamp house  180  is a holding member provided with an opening  180   a  for emitting light at front, and has a non-airtight structure (having a L-shape in the example of FIGS. 8A and 8B). The lamp house  180  is attached to a predetermined position of the image projection apparatus, so that the lamp unit  1500  can be combined with the optical system  190  of the image projection apparatus. The light emitted from the lamp unit  1500  first reaches an image display device  192  (e.g., DMD or liquid crystal device (LCD)) of the optical system  190  through a lens  191 , and then is projected with magnification to a screen (not shown) through a projection lens  193 .  
       [0011] Since the conventional lamp  1200  provided with a mirror has an airtight structure, heat generated from the lamp during lamp operation is contained inside the lamp  1200  provided with a mirror, so that the temperature inside the lamp  1200  provided with a mirror is increased. In other words, when the lamp is damaged, the scattered matters of the lamp may come out of the lamp, and therefore, in order to prevent the scattered matters from coming out and ensure the security of the lamp without fail, the lamp  1200  provided with a mirror is required to have an airtight structure. As a result, the temperature in the atmosphere in the inside  61  of the lamp  1200  provided with a mirror is increased during operation, which is accompanied by an increase in the temperature of the sealing portion  120 . Molybdenum constituting the Mo foil  124  of the sealing portion  120  has a property that it is oxidized at 350° C. or more. Therefore, the high temperature of the lamp  1200  provided with a mirror causes oxidation of the Mo foil  124  (in particular, a welded portion between the Mo foil  124  and the external lead  130 ), which causes the conductivity of the Mo foil  124  to be lost, so that the lamp  1200  provided with a mirror stops operating.  
       [0012] In the past, partially because the size of the lamp  1200  provided with a mirror was large, the inside  61  of the lamp  1200  provided with a mirror was comparatively large, so that the temperature increase in the inside  61  of the lamp  1200  provided with a mirror did not cause a large problem in most cases. Furthermore, because of a comparatively short lifetime of the lamp due to deterioration of the luminous portion  110  of the lamp or a comparatively low output of the lamp, the reliability of lamp operation comparatively can be guaranteed, even if the temperature increase occurs in the inside  61  of the lamp  1200  provided with a mirror.  
       [0013] However, nowadays, since the size of the lamp  1200  provided with a mirror is small, the extent of the temperature increase of the inside  61  of the lamp  1200  with a mirror is becoming large. In addition, with improvement of the characteristics of the luminous portion  110  of the lamp, a longer lamp lifetime (e.g., several thousands hours or more) can be provided on the product base. Therefore, in order to guarantee the reliability of lamp operation during a long period, the problem of the temperature increase of the inside  61  of the lamp  1200  provided with a mirror cannot be ignored. Furthermore, under the circumstances that the lamp having a higher output is under development, the temperature of the lamp  1200  provided with a mirror tends to be significantly increased by increasing the output of the lamp. Therefore, it seems that the problem of the temperature increase of the inside  61  of the lamp  1200  provided with a mirror becomes increasingly serious.  
       [0014] The inventors of the present invention found the following phenomenon. When the lamp  1200  provided with a mirror is incorporated into an optical system of a projector using, for example, a DMD, as a light source of the projector, a part of the light emitted from the lamp  1200  provided with a mirror is reflected by the optical system and is incident to the lamp  1200  provided with a mirror, which increases the temperature of the lamp  1200  provided with a mirror. In the case where such a phenomenon occurs, the reliability of lamp operation cannot be guaranteed, even if the lamp  1200  provided with a mirror has been designed with an estimation of the internal temperature of the lamp  1200  provided with a mirror based on the output of the lamp.  
       [0015] Furthermore, the inventors of the present invention examined the approach of making a hole in a part of the reflecting mirror  60  for the purpose of replacing the air in the inside  61  of the lamp  1200  provided with a mirror by the outside air. However, When a hole is made in a part of the reflecting mirror  60 , the luminous flux emitted from the lamp  1200  provided with a mirror is reduced because of a reduction in the area that reflects the light emitted from the lamp  1000 , and thus the optical performance of the lamp is degraded. Moreover, when a hole is made in a part of the reflecting mirror  60 , the lamp  1200  provided with a mirror is not of an airtight structure, which causes a problem in security.  
       SUMMARY OF THE INVENTION  
       [0016] Therefore, with the foregoing in mind, it is a main object of the present invention to provide a lamp unit with improved reliability of its operation in which the temperature of the inside of the lamp provided with a mirror is suppressed.  
       [0017] A lamp unit of the present invention includes a lamp provided with a mirror and a house for holding the lamp provided with a mirror. The lamp provided with a mirror includes a discharge lamp including a luminous bulb in which a luminous material is enclosed and a pair of electrodes are opposed to each other in the luminous bulb; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively; and a reflecting mirror for reflecting light emitted from the discharge lamp and having a front opening for emitting the reflected light. The lamp provided with a mirror is formed so as to have a non-airtight structure. The house includes a transmission window made of a material for transmitting light emitted from the front opening and positioned forward in the emission direction of the front opening of the reflecting mirror.  
       [0018] It is preferable that the lamp provided with a mirror has a non-airtight structure with the front opening of the reflecting mirror being open.  
       [0019] It is preferable that the house has a structure that can accommodate scattered matters when the discharge lamp is scattered to prevent the scattered matters from coming out.  
       [0020] It is preferable that the house includes an opening for replacing gas inside the house by gas outside the house.  
       [0021] It is preferable that the house has an airtight structure.  
       [0022] It is preferable that the house further includes a convection apparatus for cooling.  
       [0023] The transmission window can be made of glass or reinforced plastics.  
       [0024] It is preferable that the house is made of a metal.  
       [0025] In one embodiment of the present invention, the lamp unit is a lamp unit for image projection apparatus in which the optical axis of the discharge lamp coincides with the optical axis of the reflecting mirror.  
       [0026] In one embodiment of the present invention, the lamp unit is constituted as a replaceable unit as a light source for an image projection apparatus.  
       [0027] According to another aspect of the present invention, an image projection apparatus includes the above-described lamp unit, and an optical system using the lamp unit as a light source. The optical axis of the discharge lamp included in the lamp unit coincides with the optical axis of the lamp unit and the optical axis of the optical system.  
       [0028] In one embodiment of the present invention, the lamp unit is constituted as a replaceable unit as a light source for an image projection apparatus, and the optical system includes at least a lens and an image display device selected from the group consisting of digital micromirror device and a liquid crystal display device.  
       [0029] In the lamp unit of the present invention, the lamp provided with a mirror is formed so as to have a non-airtight structure, and a transmission window is provided in a house (housing) for holding the lamp provided with a mirror. Therefore, it is possible to move the gas inside the lamp provided with a mirror into other portions throughout the house, so that the temperature of the inside of the lamp provided with a mirror during lamp operation can be suppressed over the prior art. As a result, a lamp unit having improved reliability of lamp operation can be provided. Furthermore, since the temperature increase of the lamp provided with a mirror can be suppressed, a lamp unit having a long lamp lifetime can be provided. Furthermore, the transmission window is provided forward in the emission direction of the front opening of the reflecting mirror, so that the transmission window prevents the scattered matters from coming out, even if the scattered matters (e.g., glass pieces or mercury) generated at lamp breakage comes out from the front opening of the reflecting mirror. The lamp provided with a mirror included in the lamp unit of the present invention has a non-airtight structure with the front opening of the reflecting mirror being open, for example.  
       [0030] In the case where the house has a structure that can accommodate the scattered matters, the scattered matters generated at lamp breakage can be prevented from coming out from the lamp unit. Therefore, the security of the lamp unit can be improved further. When an opening for replacing gas in the inside by gas in the outside of the house is provided at least in an upper portion of the house in the vertical direction, the temperature increase of the inside of the lamp provided with a mirror can be suppressed more effectively. When the house has an airtight structure, no scattered matter generated at lamp breakage can come out. When the convection apparatus for cooling is provided in the house, the gas in the house can be convected forcefully, so that the temperature increase of the lamp provided with a mirror can be suppressed more effectively. The transmission window can be made of glass or reinforced plastics. When the house is made of a metal, the heat release properties of the lamp unit can be improved, so that the temperature increase of the lamp provided with a mirror can be suppressed more effectively.  
       [0031] According to the lamp unit of the present invention, the temperature increase of the lamp provided with a mirror during lamp operation can be suppressed. As a result, a lamp unit having improved reliability of lamp operation can be provided. Furthermore, since the temperature increase of the lamp provided with a mirror can be suppressed, a lamp unit having a long lamp lifetime (e.g., of five thousands hours to one million hours) can be provided.  
       [0032] 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  
     [0033]FIG. 1 is a schematic view showing the configuration of a lamp unit  500 .  
     [0034]FIG. 2 is a view from the back of a reflecting mirror  60  of the lamp unit  500 .  
     [0035]FIG. 3 is a schematic cross-sectional view showing the configuration of a lamp unit  600 .  
     [0036]FIG. 4 is a schematic cross-sectional view showing the configuration of a lamp unit  700 .  
     [0037]FIG. 5 is a schematic cross-sectional view showing the configuration of the lamp unit  800 .  
     [0038]FIG. 6 is a schematic cross-sectional view showing the configuration of a lamp unit  900 .  
     [0039]FIG. 7 is a schematic view showing the configuration of a conventional lamp  1200  provided with a mirror.  
     [0040]FIG. 8A is a schematic view showing the configuration of an image projection apparatus including a conventional lamp unit  1500  and an optical system  190 .  
     [0041]FIG. 8B is a schematic perspective view showing the configuration of a conventional lamp house  180 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0042] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following drawings, for simplification, the elements having substantially the same functions bear the same reference numeral.  
     [0043] Embodiment 1  
     [0044] Embodiment 1 of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic view showing the configuration of a lamp unit  500  of Embodiment 1.  
     [0045] The lamp unit  500  includes a lamp  200  provided with a mirror and a house (lamp house)  80  for holding the lamp  200  provided with a mirror. The lamp  200  provided with a mirror includes a discharge lamp  100  and a reflecting mirror  60  for reflecting light emitted from the discharge lamp  100 . The lamp  200  provided with a mirror has a non-airtight structure in which a front glass is not provided at a front opening  60   a  of the reflecting mirror  60 . In other words, the lamp  200  provided with a mirror has a non-airtight structure in which the front opening  60   a  of the reflecting mirror  60  is open. Furthermore, the house  80  for holding the lamp  200  provided with a mirror has a transmission window  70  made of a material that transmits light emitted from the front opening  60   a  in a portion forward in the emission direction  50  of the from opening  60   a  of the reflecting mirror  60 . The house  80  serves to protect the lamp  200  provided with a mirror, in addition to serving to hold the lamp  200  provided with a mirror.  
     [0046] The discharge lamp  100  included in the lamp unit  500  includes a luminous bulb  10 , and a pair of sealing portions  20  and  20 ′ connected to the luminous bulb  10 . A discharge space  15  in which a luminous material  18  is enclosed is provided inside the luminous bulb  10 . A pair of electrodes  12  and  12 ′ are opposed to each other in the discharge space  15 . The luminous bulb  10  is made of quartz glass and is substantially spherical. The outer diameter of the luminous bulb  10  is, for example, about 5 mm to 20 mm. The glass thickness of the luminous bulb  10  is, for example, about 1 mm to 5 mm. The volume of the discharge space  15  in the luminous bulb  10  is, for example, about 0.01 to 1 cc. In this embodiment, the luminous bulb  10  having an outer diameter of about 13 mm, a glass thickness of about 3 mm, a volume of the discharge space  15  of about 0.3 cc is used. As the luminous material, mercury is used. For example, about 150 to 200 mg/cm 3  of mercury, a rare gas (e.g., argon) with 5 to 20 kPa, and a small amount of halogen are enclosed in the discharge space  15 .  
     [0047] The pair of electrodes  12  and  12 ′ in the discharge space  15  is arranged with a gap (arc length) of, for example, about 1 to 5 mm (preferably about 1 to 3 mm). As the electrodes  12  and  12 ′, for example, tungsten electrodes (W electrodes) are used. In this embodiment, the W electrodes  12  and  12 ′ are arranged with a gap of about 1.5 mm. The electrode axis (W rod) of the electrode  12  is electrically connected to the metal foil  24  in the sealing portion  20 . Similarly, the electrode axis of the electrode  12 ′ is electrically connected to the metal foil  24 ′ in the sealing portion  20 ′.  
     [0048] The sealing portion  20  includes a metal foil  24  electrically connected to the electrode  12  and a glass portion  22  extended from the luminous bulb  10 . The airtightness in the discharge space  15  in the luminous bulb  10  is maintained by the foil-sealing between the metal foil  24  and the glass portion  22 . The glass portion  22  of the sealing portion  20  is made of quartz glass, for example. The metal foil  24  is a molybdenum foil (Mo foil), for example, and has a rectangular shape, for example. The sealing portion  20  has a circular shape in section, for example. The metal foil  24  is positioned substantially in the center of the sealing portion  20 . The metal foil  24  of the sealing portion  20  is joined to the electrode  12  by welding, and the metal foil  24  has an external lead  30  on the side opposite to the side that is joined to the electrode  12 . The external lead  30  is formed of, for example, molybdenum, and connected to the metal foil  24  at a connection portion  32  by welding. The configuration of the sealing portion  20 ′ is the same as that of the sealing portion  20 , so that the description thereof is omitted. One sealing portion  20  is arranged on the side of the front opening  60   a  of the reflecting mirror  60  (on the side of the emission direction  50 ), and the other sealing portion  20 ′ is fixed to the reflecting mirror  60 . A lamp base  55  is attached to an end of the sealing portion  20 ′. The sealing portion  20 ′ and the reflecting mirror  60  are attached with, for example, an inorganic adhesive (e.g., cement) to form one unit.  
     [0049] The reflecting mirror  60  fixed to the sealing portion  20 ′ is designed to reflect the light radiated from the discharge lamp  100  such that the light becomes 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  60 , a parabolic reflector or an ellipsoidal mirror can be used, for example. An opening  62  for a lead wire is provided in the reflecting mirror  60 , and the lead wire  65  for external interconnection is drawn out of the reflecting mirror  60  through the opening  62  for a lead wire. The lead wire  65  for external interconnection that is drawn out of the reflecting mirror  60  is electrically connected to a terminal  84  provided in the house  80 , and the terminal  84  is electrically connected to an external circuit (e.g., ballast). The lamp base  55  of the lamp  100  also is electrically connected to the terminal  84  through a lead wire  66  for external interconnection.  
     [0050] The reflecting mirror  60  is fixed to the house  80  with a mirror holding member  82 . There is no limitation regarding the mirror holding member  82 , as long as it can hold the reflecting mirror  60 . For example, the reflecting mirror  60  can be fixed to the house  80  with a connecting member (screw, bolt, nut, etc.). Alternatively, the reflecting mirror  60  can be fitted into the mirror holding member  82 . Furthermore, the reflecting mirror  60  and the mirror holding member  82  can be attached or adhered to each other, or the reflecting mirror  60  can be fixed to the house  80  with magnetic force.  
     [0051] In this embodiment, for the purpose of simplifying the configuration of the mirror holding member  82 , the reflecting mirror  60  is pressed to a part of the house  80 , utilizing the force of a band  86 , to constitute the mirror holding member  82 , as shown in FIG. 2. FIG. 2 is a schematic view of the reflecting mirror  60  viewed from its back.  
     [0052] As shown in FIG. 2, the band (e.g., wire)  86  is fixed at both ends with a band fixture  87 , and has a circular (ring-shaped) structure. A part of the band  86  can be hooked to a band fastener (buckle)  88 . With this configuration, the band  86  is set along the back surface of the reflecting mirror  60 , and the band  86  is hooked to the band fastener  88 , so that the reflecting mirror  60  easily can be fixed to the house  80 . The mirror holding member  82  shown in FIG. 2 easily can fix the lamp  200  provided with a mirror with a simple configuration, so that the mirror holding member  82  has a large advantage in assembling a lamp unit. It is preferable to provide a hook  89  for movement prevention for preventing the reflecting mirror  60  from moving after the reflecting mirror  60  is fixed.  
     [0053] Referring back to FIG. 1, the transmission window  70  included in the house  80  is constituted by, for example, glass or reinforced plastics. The transmission window  70  is provided forward in the emission direction  50  of the front opening  60   a  of the reflecting mirror  60 , and therefore even if the scattered matters(e.g., glass pieces or mercury) generated at lamp breakage comes out from the front opening  60   a  of the reflecting mirror  60 , the transmission window  70  prevents the scattered matters from coming out. Therefore, in the lamp unit  500  of this embodiment, even if the lamp  200  provided with a mirror that is not provided with the front glass at the front opening  60   a  of the reflecting mirror  60  is used, the transmission window  70  ensures the security of the lamp. In the lamp unit  500  of this embodiment, the temperature of the house  80  during lamp operation can be lower than that of the mirror  60  of the conventional lamp  1200  provided with a mirror shown in FIG. 7, so that another advantage is that not only glass, but also reinforced plastics can be used preferably as the material for the transmission window  70 . In this embodiment, an opening is formed at the front of the house  80  positioned forward in the emission direction  50 , and the transmission window  70  is provided so as to cover the opening from the outside of the house  80 , but the present invention is not limited thereto, and the transmission window  70  is provided so as to cover the opening from the inside of the house  80 . Furthermore, the transmission window  70  may be provided in a part (e.g., central portion) or the entire portion of the front of the house  80  positioned forward in the emission direction  50 . In this embodiment, the house  80  is designed to have an airtight structure, so that even if the lamp  100  is broken and scattered matters (glass pieces or mercury) are generated, the scattered matters are prevented from coming out from the lamp unit  500 . In other words, the configuration of the house  80  can accommodate the scattered matters so as to prevent the scattered matters from coming out, so that the security of the lamp can be ensured further.  
     [0054] The house  80  is formed of, for example, a metal (e.g., aluminum, stainless steel, iron, etc.). Metals typically have high heat conductivity, so that metals can improve the heat release properties of the house  80  (lamp  200  provided with a mirror). Furthermore, in the case of the house  80  formed of metal, the house  80  can be reused easily so that this is advantageous in view of recycle of the resources. The volume of the inside  90  of the house  80  in this embodiment is, for example, about 800 to 2000 cm 3 . On the other hand, the volume of the inside  61  of the reflecting mirror  60  is, for example, about 200 cm 3 . Thus, the volume of the inside  90  of the house  80  can be four to  10  times as large as the volume of the inside  61  of the reflecting mirror  60 . According to the configuration of the lamp unit  100  of this embodiment, it is possible to reduce the temperature by about 10 to 50° C. lower than that of the inside  61  of the conventional lamp  1200  provided with a mirror during lamp operation. In FIG. 1, the inside  90  of the house  80  forward of the reflecting mirror  60  is in communication with the inside  90  of the house  80  backward of the reflecting mirror  60 , and the air in the inside  90  of the house  80  can move freely throughout the house  80 .  
     [0055] In this embodiment, the lamp  100  and the reflecting mirror  60  in the lamp unit  500  are designed so that their optical axes coincide with each other, so that the lamp unit  500  can be used preferably as the light source of an image projection apparatus. It is known that when the optical axis alignment is not satisfactory, image forming by the image projection apparatus is poor. For example, a dislocation of only 0.4 mm in the optical axes reduces the brightness on the screen by up to about 60%. When the lamp unit  500  is used as a headlight of an automobile, strict alignment of the optical axes is not necessarily required because it only needs to illuminate forward.  
     [0056] Furthermore, when combining the lamp unit  500  with the optical system  190  ( 191  to  193 ) shown in FIG. 8 to constitute an image projection apparatus, the lamp unit  500  is designed so as to form a replaceable unit as the light source for an image projection apparatus, so that the lamp unit  500  very easily can be attached to the image projection apparatus or replaced. Furthermore, when setting the lamp unit  500  in a position for a lamp unit installation in the image projection apparatus, in the case where the optical axis of the lamp unit  500  is designed to coincide with the optical axis of the optical system  190 , simply attaching or replacing the lamp unit  500  can complete the alignment of the optical axes.  
     [0057] According to the present invention, the lamp unit  500  is provided with the lamp  200  provided with a mirror having a non-airtight structure in which a front glass is not provided at the front opening  60   a  of the reflecting mirror  60 , and therefore the air in the inside  61  of the lamp  200  provided with a mirror whose temperature is increased during lamp operation can convect (move), not only in the inside  61  of the lamp  200  provided with a mirror, but also in a wide range throughout the inside  90  of the house  80 . Therefore, the temperature increase in the lamp  200  provided with a mirror during lamp operation can be suppressed more than in the case of the conventional lamp  1200  provided with a mirror in which convention is caused only in the inside  61  of the reflecting mirror  60 . As a result, the reliability of the lamp operation can be improved further. Furthermore, since the lamp can be used in the state where the temperature increase in the lamp  200  provided with a mirror is suppressed, the lifetime of the lamp can be prolonged. Furthermore, the house  80  having the transmission window  70  can ensure the security of the lamp. In addition, the lamp unit  500  is a replaceable unit as a light source for an image projection apparatus, so that the lamp unit  500  can be attached to the image projection apparatus or replaced very easily. In the case where the lamp unit  500  is designed with the optical axis alignment taken into consideration when setting the lamp unit  500 , the optical axis alignment can be completed by attaching or replacing the lamp unit  500 .  
     [0058] In the lamp unit  500  of this embodiment, the house  80  having an airtight structure is used. However, it is possible to use the house  80  provided with the opening  81 , if the house  80  has a structure in which the scattered matters from the lamp  100  when the lamp is scattered are accommodated so as not to come out, as shown in FIG. 3. In the lamp unit  600  shown in FIG. 3, a lid portion  81   a  covering the upper portion of the opening  81  so as to prevent the scattered matters from coming out from the opening  81  is formed in the house  80 .  
     [0059] The lid portion  81   a  is spaced away from the outer wall of the house  80 , so that the air in the inside  90  of the house  80  is in communication with the outside air through the opening  81  and the gap between the lid portion  81   a  and the house  80 . Therefore, even if the temperature of the air of the inside  90  of the house  80  is increased as a result of the temperature increase of the air in the inside  61  of the lamp  200  provided with a mirror during lamp operation, the air can be replaced by the outside air through the opening  81 . For this reason, the temperature increase of the lamp  200  provided with a mirror can be suppressed further. The air having a high temperature moves upward in the vertical direction by convection. Therefore, in order to replace the air in the inside  90  of the house  80  by the outside air efficiently, it is preferable to provide the opening  81  at least in an upper portion in the vertical direction of the house  80 .  
     [0060] It is sufficient to provide at least one opening  81 , but it is preferable to form a plurality of openings  81  in order to increase the efficiency of the replacement of the air in the inside of the house  80  by the outside air. In the case where the opening  81  is formed in a lower surface and/or a side, in addition to the upper surface of the house  80 , the configuration can be that the openings  81  are provided in a portion having the lowest temperature and in a portion having the highest temperature, so that convection can be caused efficiently. As a result, it is possible to replace the air in the inside  90  more effectively.  
     [0061] In the lamp unit  600 , the lid portion  81   a  is provided in the opening  81  of the house  80  to form a configuration that accommodates scattered matters to prevent the scattered matters from coming out. However, there is no particular limitation regarding the configuration of the house  80 . For example, a net that prevents the scattered matters from coming out may be provided.  
     [0062] Embodiment 2  
     [0063] The lamp unit  500  of Embodiment 1 can be modified to form a lamp unit  700  in which a heat sink  56  is provided in the lamp base  55  of the lamp  100  for the purpose of further reducing the temperature increase of the lamp  200  provided with a mirror. FIG. 4 is a schematic view of the configuration of the lamp unit  700  of this embodiment.  
     [0064] The heat sink  56  attached to the lamp  100  of the lamp unit  700  is thermally coupled to the lamp  100  and has a function to suppress the temperature increase of the lamp by enlarging the surface area. The heat sink  56  is, for example, a fin for radiation, and is made of a material having a high heat conductivity (e.g., metal materials such as Al and Cu). The temperature increase of the lamp  200  provided with a mirror during lamp operation can be suppressed more effectively by providing the heat sink  56 . It is possible to provide the opening  81  for replacing the air in the inside  90  of the house  80  by the outside air, as in the lamp unit  600  shown in FIG. 3, also in the case where the heat sink  56  is provided.  
     [0065] Furthermore, when further effective suppression of the temperature increase of the lamp  200  provided with a mirror is desired, as shown in the lamp unit  800  of FIG. 5, a convection apparatus  95  for cooling can be provided in the house  80  of the lamp unit  500  of Embodiment 1. The convection apparatus  95  for cooling is, for example, a cooling fan for forcefully causing the air in the inside  90  of the house  80  to convect. The convection apparatus  95  for cooling is coupled to the house  80  via, for example, a pipe  92 , and the air in the inside  90  of the house  80  is forcefully convected and cooled by the convection apparatus  95  for cooling. As a result, the temperature increase of the lamp  200  provided with a mirror can be suppressed more effectively. In the lamp unit  800 , it is possible to reduce the temperature by about 50° C. to about 100° C. from that of the inside  61  of the conventional lamp  1200  provided with a mirror during lamp operation. Although one pipe  92  is provided in FIG. 5, separate pipes for drawing-in and drawing-out can be used. The convection apparatus  95  for cooling has a function to cool by forcefully convecting the air of the inside  90  of the house  80 , so that the convection apparatus  95  for cooling can be attached to the house  80  of either the lamp unit  600  or  700 .  
     [0066] The configuration in which the temperature of the gas is directly cooled by providing a cooler in the convection apparatus  95  for cooling, as well as cooling by the cooling fan is preferable to suppress the temperature increase of the lamp  200  provided with a mirror. Furthermore, it is possible to use, for example, an inert gas (N 2 , etc.) instead of the air in the inside  90  of the house  80 . Furthermore, it is possible to provide a pipe  92  connected to the convection apparatus  95  for cooling on the back of the reflecting mirror  60  of the lamp  200  provided with a mirror and to allow a coolant (e.g., water) to flow through the pipe  92  for the purpose of directly reducing the temperature increase of the lamp  200  provided with a mirror. In other words, it is possible to forcefully reduce the temperature of the lamp  200  provided with a mirror by the approach of allowing a coolant to flow. It seems that such an approach of forcefully suppressing the temperature increase of the lamp provided with a mirror is more effective when used for the lamp provided with a mirror that has been developed for higher wattage.  
     [0067] Other embodiments  
     [0068] The lamp units of the above embodiments can reduce the temperature in the inside of the lamp provided with a mirror from that of the conventional configuration, so that it is possible to reduce the length of the metal foil  24  in the sealing portion  20  that also serves to release the heat in the lamp  100 . This can reduce the size of the lamp  100 , and therefore it is possible to provide a lamp unit including a smaller lamp  200  provided with a mirror. Furthermore, the temperature in the inside of the lamp provided with a mirror during lamp operation can be reduced from that in the conventional lamp, which may make it possible to constitute the metal foil made of a material other than molybdenum.  
     [0069] Furthermore, the above embodiments have been described by taking the non-airtight lamp  200  provided with a mirror without the front glass in the front opening  60   a  of the reflecting mirror  60  as an example. However, a non-airtight lamp  200 ′ provided with a mirror as shown in FIG. 6 can be used, where a front glass  170  is provided in the front opening  60   a , and an opening (through hole)  60   c  for drawing the air in and out is provided in a part of the reflecting mirror  60 . In the case of the configuration FIG. 6, the opening  60   c  is provided in a position that is most distant from the luminous bulb  10  of the lamp  100  and where the efficiency of the light reflection is not reduced very much. For example, a plurality of openings are formed in a position near the front opening  60   a  of the reflecting mirror  60 . In the case of the lamp unit  900  having the configuration shown in FIG. 6, there are substantially two front glasses, that is, the transmission window  70  of the house  80  and the front glass  170  of the lamp  200  provided with a mirror, so that the effect of preventing scattering forward can be large.  
     [0070] In the above embodiments, mercury lamps employing mercury as the luminous material have been described as an example of the discharge lamp. However, the present invention can apply to any discharge lamps in which the airtightness of the luminous bulb is maintained by the sealing portion (seal portion). For example, the present invention can apply to a discharge lamp enclosing a metal halide such as a metal halide lamp.  
     [0071] Furthermore, in the above embodiments, 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 where the mercury vapor pressure is about 1 MPa or a low pressure mercury lamp where the mercury vapor pressure is about 1 kPa. Furthermore, the lamp can be of a short arc type where the distance (arc length) between the pair of electrodes  12  and  12 ′ is short, or the distance can be longer than that. The discharge lamps of the above embodiments can be used by either alternating current lighting or direct current lighting.  
     [0072] The lamp units in the above embodiments can be used preferably as the light source of a projector. In addition, the lamp units also can be used as the light source for ultraviolet ray steppers, the light source for sports stadiums, the light source for headlights for automobiles or floodlights illuminating road signs.  
     [0073] 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.