Patent Publication Number: US-2009224667-A1

Title: Auxiliary light source and lighting system having the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an auxiliary light source for lowering voltage which is necessary to start a high-pressure discharge lamp, and also relates to a lighting system including the auxiliary light source and a high-pressure discharge lamp. 
     2. Description of the Background Art 
     A high-pressure discharge lamp is mainly provided for a lighting system which is used for a liquid crystal projector and an optical device such as an exposure device. The high-pressure discharge lamp includes an arc tube which has enclosed in its internal space light-emitting material such as mercury, or halide which generates halogen cycle, or the like, and also includes a pair of main discharge electrodes which are situated inside the arc tube so as to face each other. To start the high-pressure discharge lamp, high voltage is applied between the main discharge electrodes, and discharge is caused between the main discharge electrodes by dielectric breakdown, whereby the light-emitting material is excited and emits light. 
     In recent years, in order to downsize a light-emitting area in a high-pressure discharge lamp so as to improve its light-emitting efficiency, an amount of light-emitting material enclosed inside an arc tube has been increased, and a capacity of an internal space of the arc tube has been decreased. As a result, a pressure inside the arc tube is extremely increased at the time of starting the high-pressure discharge lamp. The pressure there inside, according to a recently reported example, is approximately 200 atmospheres or more. Further, in the optical device, not only a reduction in an initial start (cold start) time, but also a reduction in a restart (hot start) time is required. 
     Particularly, the higher the pressure inside the arc tube is, the higher is the voltage which is necessary to start discharging. Accordingly, at the time of restarting (hot start) where a temperature inside the arc tube is high, a high voltage needs to be applied. In addition, restarting is delayed until the temperature of the high-pressure discharge lamp decreases to a certain extent. Further, even at the time of initial starting (cold start), a high voltage (e.g., ten-odd kV) needs to be applied. 
     However, problems are caused when high-voltage is applied at the time of starting the high-pressure discharge lamp. For example, dielectric breakdown is caused not only between the main discharge electrodes but also at unexpected portions (e.g., dielectric breakdown in an insulated cable coating, a creeping discharge in a connector or in a connection terminal, or the like), and consequently an electric shock is caused. In another case, due to a noise caused by application of a high-voltage, an electrical circuit mounted in the optical device malfunctions. 
     Then, a lighting system for starting the high-pressure discharge lamp by applying lower voltage is developed (e.g., Patent document 1: Japanese Laid-Open Patent Publication No. 2003-203605). As shown in  FIG. 13 , a lighting system  1  disclosed in Patent document 1 includes a high-pressure discharge lamp  2  and an auxiliary light source  3  which is formed independently of the high-pressure discharge lamp  2 . The high-pressure discharge lamp  2  is composed of; an arc tube  5  which includes a light-emitting portion  5   a  having a light-emitting material M 1  such as mercury enclosed in its internal space, and also includes a pair of sealing portions  5   b  for sealing the internal space of the light-emitting portion  5   a ; a pair of main discharge electrodes  6   a  situated within the light-emitting portion  5   a  so as to face each other; metal foils  6   b  which are electrically connected to the main discharge electrodes  6   a  and which are embedded inside the sealing portion  5   b ; and external lead rods  6   c  each having one end which is electrically connected to each of the metal foils  6   b  and which is embedded inside the sealing portion  5   b , and also having the other end which protrudes outward from the arc tube  5 . 
     The auxiliary light source  3  has discharge space, and discharge medium M 2  is enclosed in the discharge space. When the discharge medium M 2  is excited by discharge, the discharge medium M 2  generates ultraviolet rays. Further, the auxiliary light source  3  has a discharge chamber  7  situated so as to be adjacent to one of the sealing portions  5   b , and a starting electrode  8  situated so as to be in parallel with one of the metal foils  6   b  via the discharge chamber  7 , the metal foil  6   b  being embedded inside the one of the sealing portions  5   b . A conductive wire  9  for applying a high-frequency voltage between the one of the metal foils  6   b  and the starting electrode  8  is electrically connected to the starting electrode  8 . 
     In order to start the high-pressure discharge lamp  2  in the lighting system  1 , the high-frequency voltage is applied between the one of the metal foils  6   b  and the starting electrode  8 . The dielectric barrier discharge is then generated between the one of the metal foils  6   b  and the starting electrode  8  via the discharge space of the discharge chamber  7 . The discharge medium M 2  in the discharge space is excited by the dielectric barrier discharge, whereby ultraviolet rays UV is generated. The ultraviolet rays UV irradiates the light-emitting material M 1  enclosed in the light-emitting portion  5   a  in the high-pressure discharge lamp  2 , whereby the light-emitting material M 1  is ionized. As a result, discharge between the main discharge electrodes  6   a  is accelerated, whereby it is possible to start the high-pressure discharge lamp  2  by applying lower voltage. 
     In order to generate the dielectric barrier discharge between the metal foil  6   b  and the starting electrode  8 , a capacitive coupling needs to be established between the one of the metal foils  6   b  and the starting electrode  8  via the discharge space having the discharge medium M 2  enclosed therein. A high-frequency voltage (e.g., 10 kHz to 1 MHz) therefore needs to be applied between the one of the metal foils  6   b  and the starting electrode  8 . The lighting system  1  disclosed in Patent document 1 has the following problems. 
     (i) A high-frequency voltage generation circuit inevitably needs to be arranged in a feeder circuit for feeding the power to the lighting system  1  so as to generate a high-frequency voltage. Particularly, even if the high-pressure discharge lamp  2  is a DC-powered high-pressure discharge lamp which does not require an AC voltage, the high-frequency voltage generation circuit needs to be arranged so as to actuate the auxiliary light source  3 .
 
(ii) A transformer or the like having a preferable frequency characteristic is required for the high-frequency voltage generation circuit. Since such a transformer is expensive, overall costs of the feeder circuit increase.
 
(iii) A countermeasure against noises generated from the high-frequency voltage generation circuit is required. Such a countermeasure also increases the overall costs of the feeder circuit.
 
     SUMMARY OF THE INVENTION 
     A main object of the present invention is to provide an auxiliary light source which is capable of lowering voltage necessary to start the high-pressure discharge lamp without applying a high-frequency voltage. 
     A first aspect of the present invention is directed to an auxiliary light source  14 . The auxiliary light source  14  includes a vacuum chamber, a pair of electrodes, and fluorescent material. The vacuum chamber has an internal space which is evacuated. The pair of electrodes is situated inside the vacuum chamber so as to face each other. The fluorescent material is filled inside the vacuum chamber and emits light including ultraviolet rays by receiving electrons emitted when voltage is applied between the electrodes. An arc tube of a high-pressure discharge lamp is situated within an irradiation range of the light, and the light is emitted at least from a time just before the high-pressure discharge lamp is turned on until the high-pressure lamp emits light. 
     In the auxiliary light source  14  according to the present invention, a pair of electrodes  54  is situated inside the vacuum chamber  40 . Consequently, when voltage is applied between the electrodes  54  to generate an electric field between the electrodes, electrons e are easily emitted (field emission) from one electrode  54  to the other electrode  54 , even if the voltage is too low to cause dielectric breakdown between the main discharge electrodes  34  in the high-pressure discharge lamp  12 . By receiving the electrons e, the fluorescent material  44  filled in the vacuum chamber  40  emits light L including ultraviolet rays. 
     At least from just before the high-pressure discharge lamp  12  is turned on until the same emits light, the light L including the ultraviolet rays irradiates the arc tube  26  of the high-pressure discharge lamp  12 , the arc tube  26  being situated within an irradiation range of the auxiliary light source  14  (at this stage the voltage is being applied between the main discharge electrodes  34 ). The main discharge electrodes  34  situated in the arc tube  26  receive the ultraviolet rays included in the light L. Consequently, electrons are apt to be emitted from the main discharge electrodes  34  (photo-electric effect). Otherwise, the light-emitting material  30  enclosed in the arc tube  26  is ionized by receiving the ultraviolet rays included in the light L, whereby a path (discharge route) for causing discharge between the main discharge electrodes  34  is provided. As a result, not only at the time of cold start, but also at the time of hot start, it is possible to start the high-pressure discharge lamp  12  instantaneously even with low voltage (e.g., 1.5 kV). 
     In other words, in the auxiliary light source  14  according to the present invention, since the voltage is applied to the electrodes  54  only so as to generate the electric field, high voltage is not required. Further, it is possible to apply low frequency AC voltage which is not capable of generating the dielectric barrier discharge (high-frequency is not necessary). Moreover, it is possible to apply a DC voltage to the electrodes  54 . 
     The “vacuum” of the vacuum chamber  40  represents a pressure level lower than the atmospheric pressure (e.g. ≦10 −5  Pa), and is not limited to an absolute vacuum. Further, the degree of the vacuum in the vacuum chamber  40  is set appropriately in accordance with a value of the voltage applied to the electrodes  54 , shapes of the electrodes  54 , and the like. 
     Preferably, in the auxiliary light source  14 , an emitter may be putted on a surface of at least one of the electrodes so as to induce the electrons to be emitted easily. 
     Due to the function of the emitter  46 , electrons e are emitted from the electrodes  54  in a lower electric field, and thus it is possible to emit the light L steadily even when low voltage is applied. 
     A second aspect of the present invention is directed to a lighting system  10 . The lighting system  10  includes a high-pressure discharge lamp, a reflector, and the auxiliary light source. The high-pressure discharge lamp  12  includes a sealed chamber  22  which is composed of an arc tube  26  having a light-emitting material  30  enclosed in an internal space thereof, and one or two sealing portions  28  extending from the arc tube  26 , and also includes a pair of main discharge electrodes  34  situated inside the arc tube  26  so as to face each other. The reflector  16  has a concave reflecting surface  58  which is situated inside the reflector  16 , and a high-pressure discharge lamp fixing hole  59  which is formed at a central portion of the concave reflecting surface  58  and which has the sealing portion  28  of the high-pressure discharge lamp  12  inserted and fixed thereto. The auxiliary light source  14  is arranged at the back side of the reflector, and irradiates the arc tube via the sealing portion fixed to the high-pressure discharge lamp fixing hole  59  of the reflector  16 . 
     In the lighting system  10  according to the present invention, the auxiliary light source  14  is arranged outside the reflector  16 , that is, at the back side of the reflector  16 . When the DC voltage or the low-frequency AC voltage is applied to the electrodes  54  in the auxiliary light source  14  at least during a time period from just before the high-pressure discharge lamp  12  is turned on until the same emits light, and the light L including the ultraviolet rays irradiates the arc tube  26  via the sealing portion  28  of the high-pressure discharge lamp  12 , then it is possible to start/restart the high-pressure discharge lamp  12  with low voltage on the ground of the phenomena as aforementioned. Further, since the auxiliary light source  14  which is arranged at the back side of the reflector  16  does not interrupt a light path from the high-pressure discharge lamp  12 , an amount of light irradiated from the lighting system  10  is not decreased. 
     Preferably, in the lighting system  10 , the high-pressure discharge lamp  12  and the auxiliary light source  14  may be connected in parallel to each other. 
     As above described, the DC voltage or the low-frequency AC voltage is applied to the auxiliary light source  14 , whereby it is possible to cause the light L including the ultraviolet rays to be emitted. When the lighting system  10  is configured so as to connect the auxiliary light source  14  to the high-pressure discharge lamp  12  in parallel, required is only the DC voltage or the power feeding unit  18  for supplying the low-frequency AC voltage which is necessary to start the high-pressure discharge lamp  12  and to light steadily. That is, a power feeding unit for the high-pressure discharge lamp  12  and a power feeding unit for the auxiliary light source  14  need not be provided individually. 
     In the auxiliary light source and the lighting system including the same according to the present invention, instead of using high voltage and high-frequency AC voltage for starting and for generating the dielectric barrier discharge, respectively, it is possible to lower the voltage which is necessary to start the high-pressure discharge lamp, and also possible to improve a start time of the high-pressure discharge lamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a lighting system according to the present invention; 
         FIG. 2  is a diagram showing a high-pressure discharge lamp according to the present invention; 
         FIG. 3  is a diagram showing an auxiliary light source according to the present invention; 
         FIG. 4  is a schematic circuit diagram showing a power feeding unit using DC voltage; 
         FIG. 5  is a diagram showing a procedure for manufacturing the high-pressure discharge lamp; 
         FIG. 6  is a diagram showing a procedure for manufacturing the auxiliary light source; 
         FIG. 7  is a diagram showing an auxiliary light source according to another embodiment of the present invention; 
         FIG. 8  is a diagram showing a single-ended auxiliary light source; 
         FIG. 9  is a diagram showing a lighting system in which the single-ended auxiliary light source is used. 
         FIG. 10  is a diagram showing an auxiliary light source according to another embodiment of the present invention; 
         FIG. 11  is a diagram showing an auxiliary light source according to another embodiment of the present invention; 
         FIG. 12  is a diagram showing an exemplary AC-powered auxiliary light source; and 
         FIG. 13  is a diagram showing a conventional art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1 to 3 , the lighting system  10  includes a high-pressure discharge lamp  12 , an auxiliary light source  14 , and, where necessary, a reflector  16  having the high-pressure discharge lamp  12  mounted thereto. The high-pressure discharge lamp  12  and the auxiliary light source  14  are connected to each other in parallel, and the power is fed to the high-pressure discharge lamp  12  and the auxiliary light source  14  from the power feeding unit  18  via feeders  20 . The present invention may be applied to any type of the high-pressure discharge lamp, regardless of whether a single-ended type or a double-ended type, and regardless of whether a DC-powered type or an AC-powered type. Hereinafter, first embodiment where a double-ended DC-powered high-pressure discharge lamp  12  is used will be described, and then second embodiment where a double-ended type AC-powered high-pressure discharge lamp  12  is used will be described mainly regarding those points which are different from the DC-powered case. 
     The high-pressure discharge lamp  12  is composed of a sealed chamber  22  and a pair of main discharge mounts  24 . The sealed chamber  22  is composed of an arc tube  26 , which has an approximately spherical shape or a rugby-ball shape and which also has an internal space, and sealing portions  28  which extend from both sides of the arc tube  26 . The sealed chamber  22  is made of silica glass which is insusceptible to thermal expansion and thermal contraction. 
     Enclosed in the internal space in the arc tube  26  are, light-emitting material  30  such as inert gas (including an argon gas, a xenon gas, and the like) or mercury vapor, and halide which caused halogen cycle, and the like. In the internal space, a pair of main discharge electrodes  34  (to be described later) are situated so as to be distanced from each other and so as to face each other. Voltage is applied between the main discharge electrodes  34 , and discharge is caused by dielectric breakdown, whereby the light-emitting material  30  is excited and emits light. 
     Each of the main discharge mounts  24  includes a metal foil  32  made of molybdenum, a main discharge electrode  34  made of tungsten whose one end is situated in the internal space in the arc tube  26  and whose the other end is fixed to one end of the metal foil  32  by welding or the like, an external lead rod  36  whose one end is fixed to the other end of the metal foil  32  and whose the other end protrudes outward from the sealing portion  28 , and a preseal glass  38  which is used as necessary (the preseal glass  38  being described subsequently in detail). As shown in the diagram, in the case of the DC-powered high-pressure discharge lamp  12 , the anode main discharge electrode  34   a  is formed larger than the cathode main discharge electrode  34   b.    
     The preseal glass  38  is a member which encloses therein the metal foil  32 , a second end (a portion welded with the metal foil  32  and its adjacent portion) of the main discharge electrode  34 , and one end (a portion welded with the metal foil  32  and its adjacent portion) of the external lead rod  36 . The preseal glass  38  is made of the silica glass which is also used for the sealed chamber  22 , and a thickness of the preseal glass  38  is thinner than that of the sealed chamber  22 . An end of the preseal glass  38  at the electrode side is molded in a truncated cone shape, and the end of the truncated-cone shape is firmly shrink-sealed, when the preseal glass is welded inside the sealing portion  28  and integrated therewith. 
     The auxiliary light source  14  is composed of a vacuum chamber  40 , an auxiliary light source mount  42 , fluorescent material  44 , and an emitter  46 . 
     The vacuum chamber  40  is composed of a light-emitting portion  49  having a vacuum internal space  48 , and sealing portions  50  provided at both ends of the light-emitting portion  49 . As with the sealed chamber  22  of the high-pressure discharge lamp  12 , the vacuum chamber  40  is molded with silica glass which is insusceptible to thermal expansion and thermal contraction. Here, the “vacuum” of the vacuum chamber  40  is not limited to an absolute vacuum representing zero pressure, but also indicates a state where a pressure level is lower than atmospheric pressure (e.g., ≦10 −5  Pa). 
     The auxiliary light source mount  42  is molded with molybdenum, and are composed of a pair of metal foils  52  which are embedded inside the sealing portions  50  of the vacuum chamber  40 , a pair of tungsten auxiliary light source electrodes  54  of cylindrical shapes (or of another shape, alternatively), which respectively have first ends situated inside the vacuum chamber  40  so as to face each other, and which also have second ends respectively fixed to one ends of the metal foils  52 , a pair of external lead rods  56  which respectively have one ends fixed to the other ends of the metal foils  52  of the auxiliary light source, and which also have the other ends protruding outward from the sealing portions  50  of the auxiliary light source. A current flowing, during field emission, between the electrodes  54  of the auxiliary light source is 1 mA or less, and thus the anode electrode  54   a  need not be formed larger than the cathode electrode  54   b  even in the case where a direct current is supplied to the auxiliary light source  14 . 
     As shown in  FIG. 3 , by receiving electrons e which are emitted from the auxiliary light source electrodes  54  when a voltage is applied thereto, the fluorescent material  44  emits light L including ultraviolet rays. The fluorescent material  44  is applied so as to cover a tip of the first end of the anode electrode  54   a , or applied on an inside surface of the vacuum chamber  40  (particularly at a portion adjacent to the anode electrode  54   a ). Another embodiment will be described later. 
     Generally, the “fluorescent material” represents a material which efficiently emits/discharges light ranging from ultraviolet rays to infrared rays including visible rays by absorbing energy of electron beam, X-ray, ultraviolet rays, electric field and the like and by using a part of the absorbed energy. An exemplary fluorescent material is made by mixing a matrix such as halphosphate, silicate, oxide or the like with a few percent of an activator element for emitting light and by causing a chemical reaction therebetween. The fluorescent material  44  of the present embodiment is boron nitride which receives electrons e and irradiates the light L including the ultraviolet rays. 
     The emitter  46  is provided to the cathode electrode  54   b  as necessary so as to cause the electrons to be emitted easily. In the present embodiment, a pasty material including carbon nanotube as the emitter  46  is applied on a surface of the cathode electrode  54   b . Consequently, a large number of projections made of the carbon nanotube can be formed on the surface of the cathode electrode  54   b . Since the diameter of the carbon nanotube is extremely small (approximately 2 to 3 nm), and electric field concentration is likely to occur, it is considered to be possible to emit the electrons e from the cathode electrode  54   b  through the carbon nanotube when lower voltage is applied thereto. 
     As shown in  FIG. 1 , the reflector  16  is a concave shape member, accommodates the high-pressure discharge lamp  12  extending from its central portion, and causes the light generated from the arc tube  26  to be reflected forward therefrom. 
     A reflecting surface  58  having a concave shape is formed on an inner surface of the reflector  16 , and a high-pressure discharge lamp fixing hole  59  is formed at the central portion of the reflector  16 . The anode sealing portion  28  of the high-pressure discharge lamp  12  is inserted into the high-pressure discharge lamp fixing hole  59  and fixedly attached with cement C. The anode sealing portion  28  is exposed to the back side of the high-pressure discharge lamp fixing hole  59 , and the auxiliary light source  14  is arranged in the vicinity of the exposed anode sealing portion  28 . Consequently, the light L from the auxiliary light source  14  reaches the arc tube  26  passing through the anode sealing portion  28 . Although not shown in the diagram, the auxiliary light source  14  is permeably covered with protective ceramics. 
     Although inexpensive borosilicate glass is used as the material of the reflector  16 , various materials such as glass, metal, and aluminum silicate may be used instead thereof. As the cement C, an aluminum-silica (Al 2 O 3 —SiO 2 ) system, an aluminum (Al 2 O 3 ) system, or a silicon carbide (SiC) system may be used. 
     The power feeding unit  18  is composed of an AC power supply  60  (which may be replaced with a DC power supply), a main starting circuit  100 , and a starting circuit  150 . 
     When voltage is supplied from the AC power supply  60 , the main starting circuit  100  supplies a constant power, which is necessary for the high-pressure discharge lamp  12  to emit the light continuously, to the main discharge electrodes  34  of the high-pressure discharge lamp  12 , in accordance with fluctuations and temporal changes in the voltage supplied to the high-pressure discharge lamp  12  and the auxiliary light source  14 . As shown in  FIG. 4 , the main starting circuit  100  includes a pulse width control circuit  102  for outputting a pulse width control signal corresponding to a current for starting the high-pressure discharge lamp  12 , an FET switching section  104  for performing a switching operation in accordance with the pulse width control signal outputted from the pulse width control circuit  102 , a reactor  105  and a smoothing capacitor section  106  which smooth a switching pulse current outputted from the FET switching section  104  and which stably supply the smoothed switching pulse current to the high-pressure discharge lamp  12 , and a sense resistor  108  for detecting the current for starting the high-pressure discharge lamp  12  as sense voltage. 
     When starting the high-pressure discharge lamp  12 , the starting circuit  150  increases voltage fed from the main starting circuit  100  to a level higher than an electric field discharge is generated between the electrodes  54  of the auxiliary light source  14 , but lower than dielectric breakdown is not caused between the main discharge electrodes  34 . The starting circuit  15  then applies the increased voltage between the main discharge electrodes  34  of the high-pressure discharge lamp  12  and also between the electrodes  54  of the auxiliary light source  14 . The starting circuit  150  includes a starting diode  152 , a branch line  154 , a resistor  156 , a trigger element  158 , a boosting transformer  160 , a pulse-generating capacitor  162 , and a boosted output diode  164 . The starting diode  152  is connected to a positive output of the main starting circuit  100 . The positive output of the main starting circuit  100  leads to a positive output line  166 , and the branch line  154  branches off therefrom. The resistor  156  and the trigger element  158  are placed in the branch line  154 . The branch line  154  is connected to one end of a primary side of the boosting transformer  160  via the resistor  156  and the trigger element  158 . A zero voltage line  168  is connected to the other end of the primary side of the boosting transformer  160 . The resistor  156  is connected to one end of the pulse-generating capacitor  162  in series. The other end of the pulse-generating capacitor  162  is connected to the zero voltage line  168  of the main starting circuit  100 . One end of a secondary side of the boosting transformer  160  is connected to an output of the starting diode  152  via the boosted output diode  164 . The other end of the secondary side of the boosting transformer  160  is connected to a positive input of the starting diode  152 . 
     (Procedure for Manufacturing High-Pressure Discharge Lamp) 
     With reference to  FIG. 5 , an exemplary procedure for manufacturing the high-pressure discharge lamp  12  will be described. The second end of the anode main discharge electrode  34   a  is fixed to one end of the metal foil  32  by spot welding. One end of the external lead rod  36  is fixed to the other end of the metal foil  32  by spot welding. A serially formed structure composed of the anode main discharge electrode  34   a , the metal foil  32  and the external lead rod  36  is inserted inside the preseal glass  38  having a thickness t of 0.5 to 0.8 mm (a). The preseal glass  38  is heated at 2000° C. or more (a softening point of the silica glass is about 1650° C., accordingly, the heating temperature is set to 2000° C. or more) so as to cause thermal contraction, thereby enclosing thereinside the entirety of the metal foil  32  as well as portions adjoining to its ends which respectively are welded with the anode main discharge electrode  34   a  at one end and with the external lead rod  36  ( b ) at the other end. Finally, the preseal glass  38  is cut at its predetermined portion, whereby the main discharge mount  24  is created (c). The thinner the thickness t of the preseal glass  38  is, the shorter is the heating time of the preseal glass  38 . Thus, by thinning the thickness t of the preseal glass  38 , it is possible to prevent the preseal glass  38  from peeling off from a surface of the metal foil  32 , the peeing off being caused by a difference in the thermal contraction rate between the preseal glass  38  and the metal foil  32 . The cathode side electrode  34   b  is also manufactured in a similar manner. 
     Under an argon (Ar) atmosphere, the anode main discharge mount  24  formed in the manner above is inserted into an internal space in one of the sealing portions  28  of the sealed chamber  22 , the sealing portions  28  protruding from both sides of the arc tube  26  (the arc tube  26  being yet to be sealed at this stage). By utilizing resilience of a ring R temporarily engaged with the external lead rod  36  which is extracted from the main discharge mount  24 , the anode main discharge mount  24  is positioned in the internal space in the one of the sealing portions  28  ( d ). The sealing portion  28  is then heated at 2000° C. or more for 10 to 12 seconds, for example, so as to be shrunk, whereby the preseal glass  38  on the anode side is embedded inside the sealing portion  28  ( e ). It is understood that in addition to the above-described shrink-sealing, pinch-sealing may be applied, in which the sealing portion  28  having been heated and softened is pinched with a mold (pincher). 
     After the metal foil  32  and the portions adjoining to its ends, which are included in the anode main discharge mount  24 , are enclosed and embedded inside the one of the sealing portion  28 , predetermined processing such as washing of the arc tube  26  is performed. Next, a light-emitting material  30  such as an inert gas or mercury vapor is introduced to fill in the internal space of the arc tube  26 . In the same procedure as described above, the metal foil  32  and portions adjoining to its ends, which are included in the cathode main discharge mount  24 , are enclosed and embedded inside the other one of the sealing portions  28 . Then, the high-pressure discharge lamp  12  is completed. 
     (Procedure for Manufacturing Auxiliary Light Source) 
     With reference to  FIG. 6 , an exemplary procedure for manufacturing the auxiliary light source  14  will be described. To one end of the metal foil  52 , a second end of the anode electrode  54   a  is fixed by spot welding. The anode electrode  54   a  having fluorescent material  44  applied to a first end thereof in advance (or after the auxiliary light source mount  42   a  is manufactured). One end of the external lead rod  56  is fixed to the other end of the metal foil  52  by spot welding. Then, the anode auxiliary light source mount  42  is completed. In a similar manner, the cathode auxiliary light source mount  42   b  is manufactured. An emitter  46  is attached to the cathode electrode  54   b  in advance (or after the auxiliary light source mount  42   b  is manufactured). 
     The anode auxiliary light source mount  42   a  manufactured in this manner is inserted inside a silica tube  40   a  as the vacuum chamber  40  having a thickness t of 0.5 to 0.8 mm (a). Thereafter, under an inert atmosphere composed of an inert gas such as Ar or nitrogen, while the inert gas is flowing through the silica tube  40   a , a part of the silica tube  40   a , which corresponds to the metal foil  52  having been inserted in the silica tube and its adjacent portions in the anode auxiliary light source mount  42   a  are heated at 2000° C. or more so as to cause thermal contraction (or may be subject to pinch-sealing). Then, an anode sealing portion  50   a  is formed (b). The cathode auxiliary light source mount  42  including the cathode electrode  54   b , which has the emitter  46  attached in advance, is prepared. A predetermined degree of vacuum is produced in the internal space  48  of the vacuum chamber  40  by using a vacuum pump or the like, and in the same manner as above described, the metal foil  52  and its adjacent portions in the cathode auxiliary light source mount  42  are enclosed and embedded inside the sealing portion  50  ( c ). 
     (Procedure for Starting High-Pressure Discharge Lamp) 
     Hereinafter, a procedure for starting the high-pressure discharge lamp  12  will be described (see  FIG. 4 ). When a switch (not shown) of the power feeding unit  18  is switched on, pulse width control is performed at the FET switching section  104  in the main starting circuit  100 . An output from the FET switching section  104  is smoothed by the reactor  105  and the smoothing capacitor section  106 , and then outputted to the positive output line  166 . The voltage on the positive output line  166  is about 300V when the high-pressure discharge lamp  12  is started, and becomes equal to a predetermined voltage (e.g., 80V) when the high-pressure discharge lamp  12  emits light steadily. 
     In this manner, when the high-pressure discharge lamp  12  steadily emits light, a current outputted from the main starting circuit  100  flows along the zero voltage line  168  through the high-pressure discharge lamp  12 , and causes the sense resistor  108  to generate voltage. The pulse width control circuit  102  detects the voltage across the sense resistor  108 , thereby detecting a starting current flowing through the high-pressure discharge lamp  12 . The pulse width control circuit  102  also detects the voltage on the positive output line  166  thereby controlling the FET switching section  104  such that a constant power is supplied to the high-pressure discharge lamp  12 . 
     The state where the high-pressure discharge lamp  12  is steadily illuminated has been described above. Hereinafter, a state where the high-pressure discharge lamp  12  is started will be described. DC output outputted from the main starting circuit  100  flows through the positive output line  166  and the branch line  154  in a divided manner. On the branch line side, the DC output flows through the resistor  156  and charges the pulse-generating capacitor  162 . When voltage of the pulse-generating capacitor  162  reaches a predetermined trigger voltage (e.g., about 100V) for the trigger element  158 , the trigger element  158  is activated such that a pulse current flows through the primary side of the boosting transformer  160 . Consequently, a boosted pulse current generated in the primary side steadily raises a voltage downstream of the boosted output diode  164  (to 1.2 kV, for example). The voltage is overlapped with the voltage on the positive output line  166  (about 300V), DC voltage of about 1.5 kV is applied to the high-pressure discharge lamp  12  and the auxiliary light source  14 . 
     The dielectric breakdown between the main discharge electrodes  34  of the high-pressure discharge lamp  12  is not caused by the DC voltage only. On the other hand, the auxiliary light source  14 , to which the DC voltage has been applied, has the vacuum chamber  40  which is in a vacuum state, and which has the electrodes  54  situated thereinside so as to face each other. Electric field is generated, with such low DC voltage, between the electrodes  54 , whereby electrons e are emitted from the cathode electrode  54   b  to the anode electrode  54   a  via the emitter  46 . By receiving the electrons e, the fluorescent material  44 , which is attached to and covers the first end of the anode electrode  54   a , emits the light L including the ultraviolet rays. 
     The light L emitted from the auxiliary light source  14  is led from one end face of the high-pressure discharge lamp  12 , the end face facing the auxiliary light source  14  (and being exposed from the high-pressure discharge lamp fixing hole  59  of the reflector  16 ), to the arc tube  26  through the sealing portion  28  (optical fiber effect), and irradiates the light-emitting material  30  and the main discharge electrodes  34  (or either of the light-emitting material  30  and the main discharge electrodes  34 ) enclosed in the arc tube  26 . As a result, the dielectric breakdown is caused between the main discharge electrodes  34  of the high-pressure discharge lamp  12 , whereby the high-pressure discharge lamp  12  is started. 
     In this manner, the reason why the high-pressure discharge lamp  12  can be started only with the low DC voltage, with which it is impossible to generate the dielectric breakdown between the main discharge electrodes  34 , is considered to be as follows. That is, when the light L including the ultraviolet rays irradiates the light-emitting material  30 , the ultraviolet rays ionizes the light-emitting material  30 , and thus a path (discharge route) for causing discharge between the main discharge electrodes  34  is formed. As a result, it is possible to start the high-pressure discharge lamp  12  even with the low voltage. Further, when the light L including the ultraviolet rays irradiates the main discharge electrodes  34 , the electrons e can be emitted easily (photo-electric effect) from the main discharge electrodes  34 , and the discharge between the main discharge electrodes  34  is accelerated, whereby it is possible to start the high-pressure discharge lamp  12  with the low voltage. 
     After the high-pressure discharge lamp  12  is started in this manner, a glow discharge is produced, and then an arc discharge is initiated. When the high-pressure discharge lamp  12  then shifts to emit light steadily, voltage of the lamp increases gradually, and returns to a predetermined level of voltage (e.g., 80V). The predetermined level of voltage is maintained thereafter. In this case, output voltage of the main starting circuit  100  is lowered inevitably, and thus a charging voltage to the pulse-generating capacitor  162  becomes equal or lower than the trigger voltage for the trigger element  158 . Then, the trigger element  158  is deactivated. As a result, the starting circuit  150  is deactivated. When the voltage of the high-pressure discharge lamp  12  is lowered as above described, the electric field strength in the auxiliary light source  14  is also decreased concurrently, and thus the electrons e stop being emitted from the cathode electrode  54   b . Consequently, light emission from the auxiliary light source  14  also stops automatically. 
     The auxiliary light source  14  is not limited to that described above. In the case where the degree of vacuum of the internal space  48  in the vacuum chamber  40  is low (i.e., close to the atmospheric pressure), and the airtightness need not be increased by using the metal foil  52 , then the auxiliary light source  14  may have a configuration as shown in  FIG. 7 , in which the auxiliary light source mount  42  is composed of the electrodes  54  only, and both ends of the vacuum chamber  40  are shrunk respectively centering around the lengths of the electrodes  54 . In this case, the vacuum chamber  40  is made of hard glass whose linear expansion coefficient is substantially the same as that of the tungsten which is used for the electrodes  54 . This is to prevent lack of air-tightness in the sealing portion  50 , which is caused with a large difference in the linear expansion coefficient between the vacuum chamber  40  and the electrodes  54 . 
     As shown in  FIG. 8 , the auxiliary light source  14  may be a single-ended type in which the sealing portion  50  is formed at only one side of the light-emitting portion  49  in the vacuum chamber  40 . Particularly, as shown in  FIG. 9 , in the case of the single-ended auxiliary light source  14 , the auxiliary light source  14  can be easily inserted into and fixed to the high-pressure discharge lamp fixing hole  59  of the reflector  16  such that the light-emitting portion  49  is viewed from the side of the reflecting surface  58 . Thereafter, one of the sealing portions  28  of the high-pressure discharge lamp  12  is inserted from the side of the reflecting surface  58  of the reflector  16  and fixed to the high-pressure discharge lamp fixing hole  59 , and wiring is arranged as necessary, whereby it is possible to form a compact lighting system  10  in which the auxiliary light source  14  is accommodated inside the high-pressure discharge lamp fixing hole  59 . 
     Further, as shown in  FIG. 10 , the auxiliary light source.  14  may have a configuration in which the first end of the anode electrode  54   a  is formed in a disc shape facing the cathode electrode  54   b , and have fluorescent material  44  applied to a surface thereof. Further, as shown in  FIG. 11 , without applying the fluorescent material  44  to the anode electrode  54   a , the fluorescent material  44  may be applied at an anode side portion of an interior surface of the vacuum chamber  40 . The electrons e emitted from the cathode electrode  54   b  do not travel linearly toward the anode electrode  54   a , but travel toward the anode side while drawing rather unlimited trajectories due to electric field generated between the electrodes  54 . Consequently, the fluorescent material  44  applied on the interior surface of the vacuum chamber  40  is capable of receiving the electrons e. Then, the light L including the ultraviolet rays is emitted from the fluorescent material  44 . 
     Further, the high-pressure discharge lamp  12  and the auxiliary light source  14  may be supplied by individual power feeding units, respectively. In this case, it is noted that even if the high-pressure discharge lamp  12  comes to emit light steadily, light emission from the auxiliary light source  14  does not stop automatically. Accordingly, the auxiliary light source needs to have a power feeding unit which is capable of detecting a decrease in the voltage supplied from the power feeding unit of the high-pressure discharge lamp and also capable of stopping the power supply to the auxiliary light source  14 . 
     In the case of the AC-powered high-pressure discharge lamp  12 , the first ends of the main discharge electrodes  34 , which are situated in the arc tube  26  and which face each other, have the same shapes as each other. When the AC voltage is applied to the electrodes  54  of the auxiliary light source  14 , the electrons e are emitted from and to each of the electrodes  54  in accordance with alternating current cycles. As shown in  FIG. 12 , the fluorescent material  44  may be arranged at both ends of the interior surface of the vacuum chamber  40  (or may be arranged on the entire interior surface of the vacuum chamber  40 ). With this arrangement, the electrons e are emitted from both of the electrodes  54  to the fluorescent material  44 . It is possible to configure an auxiliary light source  14  which is capable of emitting the light L including the ultraviolet rays at any time in the alternating current cycles. It is understood that in the same manner as the case of the DC-powered type, the fluorescent material  44  may be applied to one of the electrodes  54 . In this case, the light L is emitted, during the alternative current cycles, only when the electrons e are emitted toward the one of the electrodes  54  having the fluorescent material  44  applied thereto. 
     Further, the power feeding unit  18  for the AC-power is the same as that for the DC-power, except that a main starting circuit  100  which is capable of outputting an alternate current is used. The starting circuit  150  boosts the AC voltage outputted from the main starting circuit  100  such that an electric field, which causes the electrons e to be emitted from the electrodes  54  of the auxiliary light source  14 , is generated. Accordingly, a high-frequency generation circuit and the like need not be provided to the power feeding unit  18 . 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed. 
     The disclosure of Japanese Patent Application No. 2008-56175 filed Mar. 6, 2008 including specification, drawings and claims is incorporated herein by reference in its entirety.