Patent Publication Number: US-6710521-B2

Title: Discharge bulb with infrared transmitting film

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a discharge bulb having an arc tube that is a sealed bulb acting as a light-emitting discharge section formed with pinch seal portions on both ends thereof. 
     2. Description of the Related Art 
     A conventional discharge bulb is illustrated in FIG. 9. A cylindrical shroud glass  3  having an ultraviolet blocking effect is integrally welded to an arc tube  1 . The cylindrical shroud glass  3  covers pinch seal portions  1   a  and  1   b  and a sealed glass bulb  2  that is a light-emitting discharge section. The cylindrical shroud glass  3  is used, in part, to prevent broken pieces of the arc tube  1  from spreading as a result of an explosion, and to eliminate ultraviolet light in a waveband that is harmful to human bodies and the like from the light emitted by the sealed glass bulb  2 . The reference symbol “a” represents electrodes provided in a face-to-face relationship with the sealed glass bulb  2  as a light-emitting section, and the reference symbol “b” represents molybdenum foils sealed to the pinch seal portions  1   a  and  1   b . The electrodes a and a and the lead wire c are connected to the molybdenum foil b. 
     The lead wire c extracted from the pinch seal portion  1   a  on the front end of the arc tube  1  is supported by a lead support  6  extending in front of an insulated base  5 , and the rear end of the arc tube  1  is secured to the front side of the insulated base  5  with an adhesive  4  to integrate the arc tube  1  with the insulated base  5 . 
     The symbol  10  represents a reflector for forming a meeting beam from, for example, a headlamp of an automobile, and symbol  9  represents a shade having a light-blocking section in a configuration adapted to an effective reflecting surface  10   a  of the reflector and is typically provided to guide light emitted by the sealed glass bulb  2  only to the effective reflecting surface  10   a . The shade  9  also has a function of forming clear-cut lines of a luminous distribution pattern. 
     In the above-described conventional discharge bulb, since a very large quantity of light is emitted by the sealed glass bulb  2  compared to an incandescent light, a large quantity of light is also guided to the pinch seal portions because of a light-guide effect. For reasons including the fact that the molybdenum foils b that reflect light are normally provided on the pinch seal portions  1   a  and  1   b , and that the surface configuration of the pinch seal portions is uneven, light can exit the pinch seal portions  1   a  and  1   b  and be reflected by the reflector  10  in the forward direction, which could result in glare. 
     SUMMARY OF THE INVENTION 
     In an intention to prevent light from exiting the pinch seal portions  1   a  and  1   b , the inventors applied light-blocking films, that had generally been used for forming a luminous distribution, to the pinch seal portions  1   a  and  1   b  and to the shroud glass  3 . However, using this technique, new problems arose in that an intended color temperature could not be achieved because of a large increase of the temperature of the pinch seal portions  1   a  and  1   b  and the sealed glass bulb  2 , although light from the pinch seal portions  1   a  and  1   b  is prevented to some degree. In addition, the pinch seal portions  1   a  and  1   b  become likely to crack which reduces the durability of the arc tube. Further, the light-blocking films can come off because of the high temperature in their environment. 
     The inventors thought that the great increase in the temperature of the arc tube was attributable to the fact that the conventional light-blocking films for forming a luminous pattern blocked infrared light in addition to visible light to accumulate heat in the arc tube. Then, they thought that the accumulation of heat in the arc tube could be avoided by transmitting infrared light while blocking visible light. In experiments, the inventors confirmed that the above-described problems will not occur when infrared transmitting films for blocking visible light and transmitting infrared light are used as the light-blocking films to prevent the overheating of the pinch seal portions  1   a  and  1   b  and the sealed glass bulb  2 , and accordingly, the present invention is being presented based on such a finding. 
     It is an object of the present invention to provide a discharge bulb that does not produce glare and that has excellent durability by applying infrared transmitting films for blocking visible light and transmitting infrared light to at least pinch seal portions of an arc tube. 
     In order to achieve the above-described object, a discharge bulb according to a first aspect of the invention includes an arc tube that is a sealed bulb, for example, a glass bulb, as a light emitting discharge section formed with pinch seal portions at both ends thereof, in which infrared transmitting films for blocking visible light and transmitting infrared light are applied to at least the pinch seal portions of the arc tube. 
     The infrared transmitting films prevent visible light from exiting the pinch seal portions, and therefore suppress generation of light that can result in glare. 
     Since the infrared transmitting films do not prevent infrared light from exiting the pinch seal portions, no heat is accumulated in the arc tube (in particular, the pinch seal portions and sealed glass bulb). 
     In a second aspect of the invention, the arc tube of the present invention may be provided in a lighting chamber of a vehicle front light lamp, and the infrared transmitting films may be applied in predetermined ranges extending from the bottom of the sealed glass bulb of the arc tube provided in the lighting chamber to left and right lateral surfaces thereof. 
     Since substances such as mercury and a metal halide may be enclosed in the sealed glass bulb in a saturated state, the enclosed substances may be deposited in a liquid state on the bottom of the sealed glass bulb. As a result, light exiting the sealed glass bulb downward becomes yellow light that is colored by the enclosed substances, and that light is mixed with white light that should be emitted by the sealed glass bulb, which is not preferable. The infrared transmitting films provided to extend from the bottom of the sealed glass bulb to left and right lateral surfaces thereof prevent the colored light (yellow light) from exiting the sealed glass bulb. 
     In a third aspect of the present invention, the discharge bulb may be an arc tube that has a sealed glass bulb as a light emitting discharge section formed with pinch seal portions on both ends thereof and a cylindrical shroud glass integrally joined, such as by welding, to the arc tube to enclose and seal the arc tube. Infrared transmitting films for blocking visible light and transmitting infrared light are applied to at least the pinch seal portions of the arc tube and/or at least regions of the shroud glass associated with the pinch seal portions. 
     The infrared transmitting films provided on the pinch seal portions of the arc tube and/or the shroud glass prevent visible light from exiting the pinch seal portions and prevent visible light that has exited the pinch seal portions from exiting the shroud glass, which suppresses generation of light that can result in glare. 
     The infrared transmitting films provided on the pinch seal portions of the arc tube and/or the shroud glass do not prevent infrared light from exiting the pinch seal portions and do not prevent infrared light that has exited the pinch seal portions from exiting the shroud glass, which prevents accumulation of heat in the arc tube (the pinch seal portions and sealed glass bulb). 
     Especially, when the infrared transmitting film is applied only to the shroud glass, accumulation of heat in the arc tube is less likely to occur than when the infrared transmitting film is applied to the pinch seal portions only or to both of the pinch seal portions and the shroud glass because the temperature of the shroud glass is lower than the temperature of the arc tube (pinch seal portions) when the discharge bulb is turned on. 
     When the infrared transmitting film is applied to both of the pinch seal portions and shroud glass, visible light is prevented from exiting two times, which reliably prevents generation of light that can result in glare. 
     Further, in a discharge bulb according to a fourth aspect of the present invention, the arc tube may be provided in a lighting chamber of a vehicle headlamp, and the infrared transmitting films are applied in predetermined ranges extending from the bottom of the sealed glass bulb of the arc tube provided in said lighting chamber to left and light lateral surfaces thereof and/or in predetermined ranges extending from the bottom of the shroud glass to left and right lateral surfaces thereof. 
     Since substances such as mercury and a metal halide are enclosed in the sealed glass bulb in a saturated state, the enclosed substances are deposited on the bottom of the sealed glass bulb in a liquid state. As a result, light exiting the sealed glass bulb downward becomes yellow light that is colored by the enclosed substances, and the light is mixed with white light that should be emitted by the sealed glass bulb, which is not preferable. The infrared transmitting film on the bottom of the sealed glass bulb and/or the bottom of the shroud glass prevents the colored light (yellow light) from exiting the sealed glass bulb and/or shroud glass. 
     In a fifth aspect of the invention, the discharge bulb may be used as a light source of a reflection type headlamp for forming a predetermined luminous distribution with light reflected by a reflector provided behind the same. Infrared transmitting films to serve as linear light-blocking sections for forming clear-cut lines of the luminous distribution pattern are applied to left and right lateral surfaces of the shroud glass. 
     Since clear-cut lines of a luminous distribution pattern are formed by the linear light-blocking sections extending before and behind the infrared transmitting films applied to the left and right lateral surfaces of the shroud glass form clear-cut lines of a luminous distribution pattern, there is no need for a shade for forming clear-cut lines. 
     Further, in a sixth aspect of the invention, the linear light-blocking sections for forming clear-cut lines provided on the left and right lateral surfaces of the shroud glass are constituted by infrared light/visible light blocking films extending in the form of strings. 
     A luminous distribution having sharp clear-cut lines can be formed by forming the linear light-blocking sections for forming clear-cut lines of a luminous distribution pattern using infrared light/visible light blocking films which can be formed with high accuracy compared to infrared transmitting films. 
     In a seventh aspect of the invention, the discharge bulb may be used as a light source of a reflection type headlamp for forming a predetermined luminous distribution with light reflected by a reflector provided behind the same. Infrared transmitting films applied to the shroud glass are applied in regions of the shroud glass other than a region associated with an effecting reflecting surface of said reflector contributing to the formation of the luminous distribution. 
     Visible light included in light exiting the sealed glass bulb that is a light-emitting discharge section passes through regions of the shroud glass where the infrared transmitting films are not applied, and it is reflected forward by the effective reflecting surface of the reflector to form a predetermined luminous distribution. Infrared light included in the light exiting the sealed glass bulb that is a light-emitting discharge section exits the shroud glass in all regions thereof without being blocked by the infrared transmitting films on the shroud glass, which improves radiation of the arc tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical sectional view of a discharge bulb of an embodiment of the invention inserted in a reflector for forming a driving beam from an automobile headlamp. 
     FIG.  2 ( a ) is a horizontal sectional view of an arc tube of a discharge bulb. 
     FIG.  2 ( b ) is a cross sectional view of the arc tube (sectional view taken along the line II—II shown in FIG.  2 ( a )). 
     FIG. 3 is a vertical sectional view of the discharge bulb inserted in a reflector for forming a meeting beam. 
     FIG. 4 is a vertical sectional view of a discharge bulb of another embodiment of the invention inserted in a reflector for forming a meeting beam from an automobile headlamp. 
     FIG. 5 is a cross sectional view of an arc tube of the discharge bulb (sectional view taken along the line V—V shown in FIG.  4 ). 
     FIG. 6 is a vertical sectional view of an arc tube that is a major part of a discharge bulb that is another embodiment of the invention. 
     FIG. 7 is a cross sectional view of the arc tube of the discharge bulb (sectional view taken along the line VI—VI shown in FIG.  6 ). 
     FIG. 8 is a vertical sectional view of a discharge bulb that is a fourth embodiment of the invention inserted in a reflector for forming a meeting beam from an automobile headlamp. 
     FIG. 9 is a vertical sectional view of a reflector having a discharge bulb according to the prior art inserted therein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described using the aforementioned drawings. FIGS. 1 through 3 show a first embodiment of the invention. In FIGS. 1 through 3, an arc tube  20  is a structure in which a sealed glass bulb  22  as a light-emitting discharge section is formed in the middle of a glass tube in the longitudinal direction thereof; pinch seal portions  23   a  and  23   b  having a rectangular cross section are formed before and behind the sealed glass bulb  22 ; and cylindrical portions  24   a  and  24   b  that are non-pinch-seal portions extend before and behind the cylindrical portions  24   a  and  24   b , respectively. The structure as a whole has a rod-like configuration. 
     Electrodes a, a are provided in a face-to-face relationship in the sealed glass bulb  22  that is sealed with the pinch seal portions  23   a  and  23   b , and substances such as a rare gas for starting, mercury and a metal halide may be enclosed in the bulb  22 . Lead wires c, c connected to molybdenum foils b, b are extracted from the pinch seal portions  23   a  and  23   b  on both ends of the sealed glass bulb  22  and are extended through the cylindrical portions  24   a  and  24   b  in the longitudinal direction thereof. 
     Front and rear ends of a cylindrical shroud glass  30  for blocking ultraviolet light are integrally joined, for example, by welding to the cylindrical portions  24   a  and  24   b  of the arc tube  20  to provide a structure in which the sealed glass bulb  22  and the pinch seal portions  23   a  and  23   b  are covered with the shroud glass  30 . As a result, ultraviolet light in a waveband harmful to human bodies is eliminated from light emitted by the sealed glass bulb  22 , and broken pieces of the glass do not spread even in the case of an explosion of the sealed glass bulb  22 . 
     Further, the shroud glass  30  forms a sealed space  34  that is isolated from the atmosphere and is formed around the arc tube  20  (sealed glass bulb  22 ). The sealed space  34  is evacuated and charged with argon gas having a minimized moisture concentration, and the pressure in the sealed space  34  is adjusted to about 0.5 atm when the arc tube is off (at the normal temperature) such that the pressure will be about 1 atm when the arc tube is on or when it is at a high temperature. Since this ensures air-tightness of the sealed space  34  in which substantially no moisture exists, there is no possibility of devitrification of the arc tube. 
     Infrared transmitting films  40  for blocking visible light and transmitting infrared light are applied to outer surfaces of the pinch seal portions  23   a  and  23   b  of the arc tube  20  having a rectangular cross section to prevent visible light from exiting the pinch seal portions  23   a  and  23   b . The infrared transmitting films  40  may be formed by, for example, depositing a titanium oxide layer, silica layer and the like to provide the property of transmitting infrared light while blocking visible light. Of course, other ways of creating the transmitting films are not limited to those described. 
     Since the quantity of light emitted by the sealed glass bulb  22  of the arc tube  20  in the discharge bulb is much greater than that of an incandescent bulb, a large quantity of light is guided to the pinch seal portions  23   a  and  23   b  due to a light guide effect. The light guided to the pinch seal portions can exit the pinch seal portions  23   a  and  23   b  as a result of reflection at the molybdenum foils b, b or the like and can be reflected forward by the reflector  60  to produce glare. However, since the infrared transmitting films  40  applied to the outer surfaces of the pinch seal portions  23   a  and  23   b  absorb and block visible light that otherwise exits the pinch seal portions  23   a  and  23   b , no light exits the pinch seal portions  23   a  and  23   b.    
     Further, since the infrared transmitting films  40  allow infrared light to pass, infrared light exiting the pinch seal portions  23   a  and  23   b  is transmitted by the infrared transmitting films  40  without being blocked, which prevents accumulation of heat in the pinch seal portions  23   a  and  23   b.    
     Therefore, the temperature of the pinch seal portions  23   a  and  23   b  and the sealed glass bulb  22  does not increase excessively, which prevents problems attributable to a temperature rise at the arc tube. 
     The arc tube  20  is provided such that it horizontally extends into a lighting chamber S of a vehicle headlamp in the fore and aft direction as shown in FIG. 1, and an infrared transmitting film  40   a  may also be applied to the bottom of the sealed glass bulb  22  provided in the lighting chamber of the arc tube to prevent colored light other than white light from exiting the sealed glass bulb  22  downward. 
     Specifically, since substances such as mercury and a metal iodide may be enclosed in the sealed glass bulb  22  in a saturated state, a yellow liquid enclosed substance  26  may become deposited on the bottom of the sealed glass bulb  22 . As a result, light exiting the sealed glass bulb  22  downward becomes yellow light that has the color of the enclosed substance, and as such, the yellow light is mixed with the white light that should be emitted by the sealed glass bulb  22 . This effect, of course, degrades the performance of the lamp, which is not preferable. Since the infrared transmitting film  40   a  provided on the bottom of the sealed glass bulb  22  prevents the yellow-colored light from exiting the sealed glass bulb  22 , only the white light exits the arc tube  20  (sealed glass bulb  22 ). Further, since infrared light is transmitted by the infrared transmitting film  40   a , a large increase of the temperature of the sealed glass bulb  22  can be prevented by providing the infrared transmitting film  40   a  on the sealed glass bulb  22 . 
     The infrared transmitting film  40   a  may be provided in a range covering the bottom and left and right lateral surfaces at an angle of 120 deg. (60 deg. each for the left and right surfaces) as shown in FIG.  2 ( b ), and this prevents yellow visible light from exiting the sealed glass bulb  22  downward. 
     As shown in FIG. 1, the discharge bulb is comprised of the arc tube  20  and an insulated base  51  for supporting the arc tube  20 . The front end of the arc tube  20  is supported by a single lead support  52  that protrudes in front of the insulated base  51 , and the rear end of the arc tube  20  is gripped by a metal support member  54  secured to a front face of the insulated base  51 , which integrally secures the arc tube  20  to the insulated base  51 . 
     A front end lead wire c extracted from the arc tube  20  is secured to the lead support  52  through welding, and a rear end lead wire c is extended through a bottom wall  51   b  formed with a recess  51   a  and secured to a terminal  56  provided on the bottom wall  51   b  through welding. Of course, while welding is described in this embodiment, other means for securing the lead wire may be used without departing from the scope of the invention. 
     When the discharge bulb is inserted in a bulb insertion hole  64  of a reflector  60  for forming a driving beam of an automobile headlamp, light emitted by the arc tube  20  (sealed glass bulb  22 ) is reflected forward by the reflector  60  to form a white driving beam. 
     FIG. 3 shows a case in which the discharge bulb is used as a light source of a meeting beam from an automobile headlamp. 
     A shade  70  for controlling a luminous distribution is disposed under the discharge bulb such that it covers lower, front, and upper front regions of the arc tube  20 . An upper rear edge  71  of the shade  70  agrees with an upper parting line  61 A 1  of an effective reflecting surface  61 A, and a front edge  40   1  of the rear side infrared transmission film  40  agrees with a lower parting line  61 A 2  around the bulb insertion hole  64  of the effective reflecting surface  61 A. Further, the shade  70  is formed with linear light-blocking sections (not shown) for forming clear-cut lines. 
     As a result, light exiting the sealed glass bulb  22  is restricted (blocked) by the infrared transmitting film on the rear side of the arc tube and the shade  70 , and is guided to the effective reflecting surface  61 A. The light is then reflected by the effective reflecting surface  61 A forward to form a white meeting beam having predetermined clear-cut lines. 
     While the infrared transmitting film  40   a  is applied to the bottom of the sealed glass bulb  22  to prevent yellowish visible light from exiting the sealed glass bulb  22  downward in the above-described embodiment, it is not essential to provide the infrared transmitting film  40   a  on the bottom of the sealed glass bulb  22  when the discharge bulb is used as a light source for forming a driving beam (see FIG.  1 ). 
     Specifically, although yellowish light certainly exits the sealed glass bulb  22  downward if the infrared transmitting film  40   a  is not provided, the yellow color is substantially unnoticeable in the luminous distribution and creates no problem in forming a driving beam because it is mixed with white light which exits the sealed glass bulb  22  in directions other than the downward direction and which is reflected and distributed by the reflector  60 . 
     FIGS. 4 and 5 show a second embodiment of the invention. FIG. 4 is a vertical sectional view of a discharge bulb as a second embodiment of the invention inserted in a reflector for forming a meeting beam from an automobile headlamp. FIG. 5 is a cross sectional view of an arc tube of the discharge bulb (sectional view taken along the line V—V shown in FIG.  4 ). 
     While the infrared transmitting films  40  and  40   a  are applied to outer surfaces of the pinch seal portions  23   a  and  23   b  of the arc tube  20  and across the bottom and lateral surfaces of the sealed glass bulb  22  in the above-described first embodiment, in the second embodiment, infrared transmitting films  40   b  and  40   c  may be applied to the front end and rear end of the shroud glass  30  in association with the pinch seal portions  23   a  and  23   b  of the arc tube instead of providing the infrared transmitting films  40  and  40   a  on the arc tube  20 . Therefore, visible optical components of light that have exited the pinch seal portions  23   a  and  23   b  of the arc tube are blocked by the infrared transmitting films  40   b  and  40   c  on the shroud glass  30 . 
     An infrared transmitting film  40   d  may also be applied between the front and rear infrared transmitting films  40   b  and  40   c  to substantially cover the lower half of the shroud glass  30 , and visible optical components of yellow light which exit the sealed glass bulb  22  downward are blocked by the infrared transmitting film  40   d.    
     A front edge  40   cl  of the rear side infrared transmitting film  40   c  agrees with a lower parting line  61 A 2  of a bulb insertion hole  64  of an effective reflecting surface  61 A of a reflector  60 A. Further, the infrared transmitting film  40   d  contributes to the formation of clear-cut lines of a meeting beam that is formed by light reflected by an upper effective reflecting surface  61 A of the reflector  60 A. 
     Specifically, the infrared transmitting films  40   b ,  40   c  and  40   d  are applied to the entire region of the shroud glass  30  excluding the rectangular region associated with the sealed glass bulb  22  at the upper side of the same; light (visible light) emitted by the sealed glass bulb  22  exits the shroud glass  30  through the rectangular region where no infrared transmitting film is formed; and the light (visible light) is guided by a shade  70 A only to the effective reflecting surface  61 A of the reflector  60 A. However, unlike the shade  70  described in the first embodiment, the shade  70 A is formed with no linear light-blocking sections for forming clear-cut lines of a luminous distribution pattern. Instead, a longitudinally extending linear upper edge  40   dl  of the infrared transmitting film  40   d  provided on the bottom of the shroud glass  30  substantially in the middle thereof in the longitudinal direction may serve as a part for forming clear-cut lines of a meeting beam. In other words, the upper edge  40   dl  of the infrared transmitting film  40   d  that extends from a bottom surface of the shroud glass  30  facing the sealed glass bulb  22  to left and right lateral surfaces of the same is provided in a position associated with clear-cut lines of a meeting beam. 
     The second embodiment is otherwise the same as the first embodiment, and therefore, like reference numbers are used to avoid repetition of the description. 
     According to the second embodiment, as thus described, visible optical components of light which have exited the pinch seal portions  23   a  and  23   b  are blocked by the infrared transmitting films  40   b  and  40   c  and are blocked from exiting the shroud glass  30  to prevent glare. 
     Visible optical components of yellow light which have exited the sealed glass bulb  22  downward can not exit the shroud glass  30 , which makes it possible to form an adequate luminous distribution for a meeting beam consisting of only white light as intended. 
     In the automobile headlamp having a reflector unit shown in FIG. 4, since infrared optical components are distributed in a dark region where no visible light is distributed above clear-cut lines of a luminous distribution pattern of a meeting beam, the region as a dark part above the clear-cut lines which is invisible to naked eyes can be recognized on a monitor by photographing the scene in front of the automobile with an infrared noctovision camera and projecting it on the monitor, which improves safety of driving. 
     FIGS. 6 and 7 show a third embodiment of the invention. FIG. 6 is a vertical sectional view of an arc tube which is a major part of a discharge bulb as a third embodiment of the invention, and FIG. 7 is a cross sectional view of the arc tube (sectional view taken along the line VII—VII shown in FIG.  6 ). 
     The third embodiment is the same as the second embodiment in that the infrared transmitting films  40   b ,  40   c  and  40   d  are applied to the shroud glass  30  to block visible optical components of light which have exited the pinch seal portions  23   a  and  23   b  of the arc tube with the infrared transmitting films  40   b  and  40   c , and optical components of yellowish light which have exited the sealed glass bulb  22  downward are blocked with infrared transmitting films  40   e.    
     While the linear side edges  40   dl  of the infrared transmitting film  40   d  extending in the longitudinal direction on the left and right lateral surfaces of the shroud glass  30  serve as parts for forming clear-cut lines of a meeting beam in the above-described second embodiment, only string-shaped edge regions along the longitudinally extending linear side edges  40   dl  of this embodiment are constituted by infrared light/visible light blocking films  40   e  in the present embodiment. Specifically, there is provided a configuration in which the infrared transmitting film  40   d  is provided so as to extend from the bottom surface of the shroud glass  30  facing the sealed glass bulb  22  to the left and right lateral surfaces thereof. Also, the infrared light/visible light blocking films  40   e  are provided in the string-shaped edge regions associated with clear-cut lines of a meeting beam, where linear side edges  40   el  of the infrared light/visible light blocking films  40   e  contribute to the formation of clear-cut lines of a meeting beam. 
     Since the infrared light/visible light blocking films  40   e  can be formed with high accuracy compared to the infrared transmitting film  40   d , regions associated with clear-cut lines may be formed with accurate linearity, which makes it possible to obtain a luminous distribution having sharp clear-cut lines. 
     The third embodiment is otherwise the same as the second embodiment shown in FIGS. 4 and 5, and therefore, like reference numbers are used to avoid repetition of the description. 
     FIG. 8 is a vertical sectional view of a discharge bulb as a fourth embodiment of the invention inserted in a reflector for forming a meeting beam from an automobile headlamp. 
     The fourth embodiment is similar to the second embodiment in that side edges  40   dl  of a longitudinally extending linear infrared transmitting film on left and right lateral surfaces of a shroud glass serve as parts for forming clear-cut lines of a meeting beam, side edges  40   bl  and  40   cl  before and behind the infrared transmitting films  40   b  and  40   c  associated with the pinch seal portions  23   a  and  23   b  of the arc tube agree with upper and lower parting lines  61 A 1  and  61 A 2  of the effective reflecting surface  61 A of the reflector  60 A respectively to serve as parts for defining light traveling from the sealed glass bulb  22  to the effective reflecting surface  61 A. 
     Therefore, the present embodiment eliminates the need for the shades  70  and  70 A for controlling a luminous distribution which are required in the above-described second and third embodiments, which simplifies the configuration of a lighting device. 
     The embodiment is otherwise the same as the second embodiment, and like reference numbers are used to avoid repetition of the description. 
     While infrared transmitting films are provided on either arc tube  20  or shroud glass  30  in the first through third embodiments, infrared transmitting films may be provided on both of the arc tube  20  and shroud glass  30 . In such a configuration, visible optical components are eliminated two times, i.e., when they exit the arc tube and when they exit the shroud glass, and this completely eliminates the possibility of glare and makes it possible to obtain adequate white light with reliability. 
     While the above embodiments referred to configurations in which the shroud glass  30  is integrally welded to the arc tube  20 , the invention can be equally applied to a discharge bulb having a structure in which an open base section of an ultraviolet blocking shroud glass in the form of a cap having a closed-end that is separate from an arc tube is secured to an insulated base  51  to cover the ac tube and lead support as a whole with the cap type shroud glass. 
     As apparent from the above description, according a first aspect of the invention, a luminous distribution can be easily controlled because visible light that can produce glare does not exit the pinch seal portions of the arc tube. 
     Since infrared light that has nothing do to with a luminous distribution is allowed to exit the pinch seal portions of the arc tube, there is no possibility of overheating of the arc tube attributable to accumulation of heat in the arc tube (pinch seal portions and sealed glass bulb). 
     According to second aspect of the present invention, since light exiting a sealed glass bulb is not affected by the color of enclosed substances deposited on the bottom of the sealed glass bulb, adequate white light can be obtained from the arc tube. 
     According to a third aspect of the present invention, since no visible light that can produce glare exits the arc tube, control of a luminous distribution is simplified. Especially, when the infrared transmitting films are applied to both of the pinch seal portions and the shroud glass, control of a luminous distribution is further simplified because visible light that can produce glare is reliably prevented. 
     Since infrared light is allowed to exit the arc tube while visible light that can produce glare is disallowed to exit the same, there is no possibility of overheating of the arc tube attributable to accumulation of heat in the arc tube (pinch seal portions, sealed glass bulb and shroud glass). 
     In fact, when the infrared transmitting film is applied only to the shroud glass, since the temperature of the shroud glass is lower than the temperature of the pinch seal portions, the infrared transmitting film is less vulnerable to heat, which improves the durability of the infrared transmitting film and makes it possible to use a discharge bulb for a long time. 
     According to a fourth aspect of the present invention, since light exiting the sealed glass bulb (shroud glass) is not affected by the color of enclosed substances that reside in the sealed glass bulb, adequate white light can be obtained from the arc tube. 
     According to a fifth aspect of the present invention, since there is no need for a shade for forming clear-cut lines, the configuration of a lighting device can be simplified. 
     In particular, when the infrared transmitting films are applied to the shroud glass such that infrared light will be distributed to a dark region above clear-cut lines of a luminous distribution pattern, the dark region above the clear-cut lines can be monitored with an infrared noctovision camera. Specifically, even a dark region above the clear-cut lines which is invisible to naked eyes can be recognized on a monitor by photographing the scene in front of the automobile with an infrared noctovision camera and projecting it on a monitor, which improves safety of driving. 
     According to a sixth aspect of the present invention, since sharp clear-cut lines are formed on a luminous distribution, visibility is improved. 
     According to a seventh aspect of the present invention, since the infrared transmitting film on the shroud glass serves as a shade for blocking light traveling toward regions other than the effective reflecting surface that contributes to the formation of a luminous distribution of the reflector, there is no need for a shade for controlling a luminous distribution.