High pressure discharge lamp

Abstract: A high pressure discharge lamp (1) includes: a quartz glass bulb (2) having an expanded portion (21) and sealing portions (22); conductive elements (5,5'), and a pair of electrodes (3,4). The conductive elements (5,5') are airtightly sealed at the sealing portions (22) of the quartz glass bulb (2). Each electrode (3,4) of the pair of electrodes (3,4) is disposed so as to be opposite the other and is connected to one of the conductive elements (5,5'). An angle  1 between a tangent along the inner surface of the expanded portion (21) at a position 0.5mm away from an origin of one of the sealing portions (22) along the length direction of each electrode (3,4) and the direction of the length of each electrode (3,4) is at least about 40° .

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high pressure discharge lamp. More specifically, the present invention relates to a high pressure discharge lamp wherein problems such as blowouts of bulbs made of quartz glass or leaking of contained gas from the quartz glass bulb are prevented. 2. Description of Related Art In general, in a high pressure discharge lamp shown in FIG. 3 each electrode of a pair of electrodes (i.e., an anode 3 and a cathode 4) is disposed so as to be opposite the other in a quartz glass bulb 2, which includes an expanded portion for luminescence 21 and sealing portions 22. The quartz glass bulb 2 is formed by welding the sealing portions 22. The anode 3 and the cathode 4 are joined by, for instance, welding with molybdenum foils 5 and 5'. Also, the sealing portions 22 of the quartz glass bulb 2 are airtightly sealed by, for example, welding with molybdenum foils 5 and 5'. Gas for assisting an electric discharge is contained in the expanded portion for luminescence 21 of the quartz glass bulb 2 which has been airtightly sealed. The temperature inside such a high pressure discharge lamp 1 tends to be increased when the luminous efficacy of the lamp 1 is enhanced by increasing the pressure of a light emitting material or a gas contained in the lamp 1. However, since each of the sealing portions is airtightly sealed by welding the two portions of the quartz glass, problems such as leaking, cracking, or even breaking at the welding portions may occur when the internal pressure of the bulb 2 is increased. Accordingly, one of the objectives of the present invention is to provide a high pressure discharge lamp which is capable of maintaining the airtightness even if the pressure of light emitting material or gas contained in the lamp is increased and to prevent problems as leaking or blowout of the quartz glass bulb. The inventors of the present invention, after pursuing diligent studies to achieve the above-mentioned objectives, made observations of the angle between the quartz glass bulb and the direction along the length of an electrode in the vicinity of the sealing portion and have discovered that the internal pressure of the high pressure discharge lamp acts strongly in the direction of detachment of the welded portion of the quartz glass bulb when this angle is small, and that the internal pressure of the lamp, which acts in the direction detaching the welded portion of the quartz glass, becomes smaller as the angle increases. Accordingly, the airtightness of the high pressure discharge lamp may be maintained when the angle formed by the quartz glass bulb and the direction along the length of an electrode in the vicinity of the sealing portion is large and, hence, problems such as a leaking of the contained gas or a blowout of the bulb may be prevented. SUMMARY OF THE INVENTION The present invention provides a high pressure discharge lamp including: a quartz glass bulb having an expanded portion and sealing portions; conductive elements, which are airtightly sealed at the sealing portions of the quartz glass bulb; and a pair of electrodes, each electrode being disposed so as to be opposite the other and each electrode being connected to one of the conductive elements, wherein an angle  1 between the tangent along the inner surface of the expanded portion at a position 0.5 mm away from an origin of one of the sealing portions along the length of each electrode and the direction along the length of each electrode is at least about 40°, with the origin of one of the sealing portions defined as a boundary point between the end of one of the sealing portions and an inner surface of the expanded portion; and the tangent along the inner surface of the expanded portion at a position 0.5 mm away from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each electrode, which passes through a point 0.5 mm away from the origin of one of the sealing portions toward the expanded portion along the length of each electrode, and the inner surface of the expanded portion and makes contact with the inner surface of the expanded portion. Advantageously, the conductive elements may be molybdenum foils. Preferably, an angle  2 between a tangent along the inner surface of the expanded portion at an optional point between more than 0.5 mm and 1.5 mm from the origin of one of the sealing portions along the length of each electrode and the direction along the length of each electrode may be at least about with the tangent along the inner surface of the expanded portion at an optional point between more than 0.5 mm and 1.5 mm from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each electrode, which passes through an optional point between more than 0.5 mm and 1.5 mm from the origin of one of the sealing portions toward the expanded portion along the length of each electrode, and the inner surface of the expanded portion and makes contact with the inner surface of the expanded portion. Expediently, an angle  3 between a tangent along the inner surface of the expanded portion at an optional point between more than 1.5 mm and 3 mm from the origin of one of the sealing portions along the length of each electrode and the direction along the length of each electrode is at least about 50°, with the tangent along the inner surface of the expanded portion at an optional point between more than 1.5 mm and 3 mm from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each electrode, which passes through an optional point between more than 1.5 mm and 3 mm from the origin of one of the sealing portions toward the expanded portion along the length of each electrode, and the inner surface of the expanded portion and makes contact with the inner surface of the expanded portion. In yet another aspect of the invention, the relationship among  1 ,  2 and  3 is  1 <  2 <  3 . Preferably,  1 is at least about 45° . Preferably,  2 is at least about 60° . Preferably,  3 is at least about 70° . Preferably, mercury vapor is contained in the high pressure discharge lamp in an amount between about 0.12 and 0.3 mg/mm 3 . Preferably, halogen gas is contained in the high pressure discharge lamp in an amount between about 10 -8 and 10 -2 µmol/mm 3 . Preferably, inert gas is contained in the high pressure discharge lamp at a pressure of about 6 kPa or greater. Preferably, the bulb wall loading in the high pressure discharge lamp is about 0.8 W/mm 2 or greater. Preferably, the pair of electrodes comprise tungsten containing potassium oxide. Preferably, the relationship between internal pressure P (atm) of the high pressure discharge lamp and the angle  1 is:  1 ≧ 0.25P + 5 (wherein P ≧ 140). According to the present invention, it becomes possible to provide a high pressure discharge lamp which is capable of maintaining airtightness even if the pressure of a light emitting material or gas contained in the lamp is increased and to prevent problems as leaking or blowout of the quartz glass bulb. BRIEF DESCRIPTION OF THE DRAWINGS Some of the features and advantages of the invention have been described, and others will become apparent from the detailed description which follows and from the accompanying drawings, in which: FIG. 1 is a diagram showing a schematic cross-sectional view of a high pressure discharge lamp according to an embodiment of the present invention; FIG. 2 is an enlarged schematic cross-sectional view for explaining the vicinity of an origin of a sealing portion of the high pressure discharge lamp according to the embodiment of the present invention; FIG. 3 is a diagram showing a schematic cross-sectional view of a conventional high pressure discharge lamp; and FIG. 4 is a diagram showing a schematic cross-sectional view of a high pressure discharge lamp according to another embodiment of the present invention which is manufactured by using a prefabricated quartz glass bulb. DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide a high pressure discharge lamp in which the above-mentioned problems have been solved. Another object of the present invention is to provide a high pressure discharge lamp which, even after being lit for a considerably long time, does not have problems such as blowout of the bulb made of quartz glass and leaking of the contained gas from the quartz glass bulb. The invention summarized above and defined by the following claims may be better understood by referring to the following detailed description, which should be read with reference to the accompanying drawings. This detailed description of a particular preferred embodiment, set out below enabling to build and use a particular implementation of the invention, is not intended to limit the scope of the invention, but to serve as a particular example thereof. In FIG. 1, a high pressure discharge lamp 1 includes a quartz glass bulb 2, an anode 3, a cathode 4, and molybdenum foils 5 and 5'. The quartz glass bulb 2 has an expanded portion 21. The shape of the expanded portion 21 is not particularly limited and may be spherical or oval-spherical. The quartz glass bulb 2 may be formed by using a natural or synthetic quartz glass. Also, the quartz glass bulb 2 may be a single layer bulb formed as a one-piece unit or a two or more layer multi-layered bulb. The shape of the anode 3 and that of the cathode 4 may be the same or can be different. The distance between the anode 3 and the cathode 4 is not particularly limited. The anode 3 and the cathode 4 are joined to the molybdenum foils 5 and 5' by, for example, a welding means. The quartz glass bulb 2 is airtightly sealed with the molybdenum foils 5 and 5' at sealing portions 22. A gas for assisting a discharge, such as mercury vapor, is contained and sealed in the expanded portion 21. It is essential, that the angle  1 , between a tangent along the inner surface of the expanded portion at a position 0.5 mm away from the origin of the sealing portion along the length of each electrode and the direction along the length of the electrode, be at least about 40°. This will be explained with reference to FIG. 2. The dotted line 221 indicates an end of the sealing portion 22. In this specification, the term "the origin of the sealing portion" means a boundary point between the end of the sealing portion 22 and the inner surface of the expanded portion 21. As shown in FIG. 2, the quartz glass of the expanded portion 21 of the quartz glass bulb 2 starts separating away from the electrodes 3 and 4 at the origin of the sealing portion 22. Point A is defined as a point located on the surface of the electrode 4 (or 3), 0.5 mm away from the origin of the sealing portion 22 along the length direction of the electrode 4. The line A-A' is a straight line which passes through point A so as to be perpendicular to the direction of the length of the electrode 4. Tangent k is a straight line along the inner surface of the expanded portion 21 which makes contact with the inner surface of the expanded portion 21 at the point A'. Thus, the term "a tangent along the inner surface of the expanded portion " means a straight line along the length of an electrode which passes through a point of intersection formed by a straight line perpendicular to the direction along the length of the electrode, which passes through a point 0.5 mm away from the origin of the sealing portion toward the expanded portion 21 along the length of the electrode, and the inner surface of the expanded portion 21 and makes contact with the inner surface of the expanded portion 21 of the quartz glass bulb 2. The angle  1 is defined as an angle formed by the above-mentioned tangent and the direction along the length of the electrode. According to the present invention, the angle  1 is preferably about 45° or larger, more preferably about 50° or larger, and most preferably about 55° or larger. If angle  1 is about 40° or larger, it becomes possible to prevent such problems as a blowout of the bulb made of quartz glass or leaking of the contained gas from the quartz glass bulb during the operation of the high pressure discharge lamp. Also, angle  2 , which is an angle between a tangent at an optional point between more than 0.5 mm and 1.5 mm from the origin of the sealing portion along the length of the electrode (i.e., the tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of the electrode, which passes through an optional point between more than 0.5 mm and 1.5 mm from the origin of the sealing portion toward the expanded portion 21 along the length of the electrode, and the inner surface of the expanded portion 21 and makes contact with the inner surface of the expanded portion 21 of the quartz glass bulb 2) and the direction along the length of the electrode, is preferably about 45° or larger and more preferably about 60° or larger. If angle  2 is about 45° or larger, problems such as a blowout of the bulb made of quartz glass or leaking of the contained gas from the quartz glass bulb during the operation of the high pressure discharge lamp may be prevented in a more efficient manner. The same definition of  1 is applied to  2 except that "a point 0.5 mm away from the origin" is changed to "an optional point between more than 0.5 mm and 1.5 mm away from the origin". Moreover, an angle  3 , which is an angle between a tangent at an optional point between more than 1.5 mm and 3 mm from the origin of the sealing portion along the length of the electrode and the direction along the length of the electrode, is preferably about 50° or larger, and more preferably about 70° or larger. If angle  3 is about 50° or larger, problems such as a blowout of the bulb made of quartz glass or leaking of the contained gas from the quartz glass bulb during the operation of the high pressure discharge lamp may be prevented in a more efficient manner. The same definition of  1 is applied to  3 except that " a point 0.5 mm away from the origin" is changed to "an optional point between more than 1.5 mm and 3 mm away from the origin". Further, the relationship among  1 ,  2 and  3 is preferably  1 <  2 <  3 in which case problems such as a blowout of the bulb made of quartz glass or a leaking of the contained gas from the quartz glass bulb during the operation of the high pressure discharge lamp may be prevented more efficiently since the internal pressure of the high pressure discharge lamp, which acts in the direction of detachment of the welded portion of the quartz glass bulb, becomes smaller. In addition, it is preferable that mercury vapor be contained and sealed in the high pressure discharge lamp. The amount of mercury vapor is preferably between about 0.12 and 0.3 mg/mm 3 and more preferably between about 0.18 and 0.24 mg/mm 3 . If the amount of mercury vapor is between about 0.12 and 0.3 mg/mm 3 , it becomes possible to improve the luminous efficacy of the lamp and prevent such problems as leaking or a blowout during the operation of the high pressure discharge lamp. Also, it is preferable that halogen gas is contained and sealed in the high pressure discharge lamp. The amount of halogen gas is preferably between about 10 -8 and 10 -2 µmol/mm 3 and more preferably between about 10 -6 and 10 -4 µmol/mm 3 . If the amount of halogen gas is between about 10 -8 and 10 -2 µmol/mm 3 , it becomes possible to improve the luminous efficacy of the lamp and prevent such problems as leaking or a blowout during the operation of the high pressure discharge lamp. Examples of halogen gas include chlorine gas, bromine gas, and iodine gas, and these may be used in combination. In case two or more halogen gases are used in combination, it is preferable that the total amount of the gases be between about 10 -8 and 10 -2 µmol/mm 3 . Moreover, it is preferable that inert gas is contained and sealed in the high pressure discharge lamp. The pressure of inert gas is preferably about 6 kPa or greater, and more preferably between about 20 and 50 kPa. If the pressure of the inert gas is 6 kPa or greater, it becomes possible to improve the luminous efficacy of the lamp and prevent such problems as leaking or a blowout during the operation of the high pressure discharge lamp. Examples of inert gas include helium gas, neon gas, argon gas, krypton gas, and xenon gas, and these may be used in combination. In case two or more inert gases are used in combination, it is preferable that the total pressure of the gases be about 50 kPa or less. Further, the bulb wall loading in the high pressure discharge lamp is preferably about 0.8 W/mm 2 or greater, and more preferably in the range between about 1.2 and 1.8 W/mm 2 . If the bulb wall loading is about 0.8 W/mm 2 or greater, the luminous efficacy of the high pressure discharge lamp may be enhanced and problems such as leaking or a blowout during the operation of the high pressure discharge lamp may be prevented. The materials used for an anode and a cathode are preferably tungsten, molybdenum, and tantalum. The use of tungsten is more preferable and that of tungsten containing potassium oxide is especially preferable. The amount of potassium oxide in tungsten is preferably about 30 ppm or less. If tungsten containing potassium oxide is used, the luminous efficacy of the high pressure discharge lamp may be enhanced and problems such as leaking or a blowout during the operation of the high pressure discharge lamp may be prevented. The effect of preventing problems such as leaking or a blowout during the operation of the high pressure discharge lamp is especially remarkable when the relationship between the internal pressure P (atm) and  1 is  1 ≧ 0.25P + 5 (wherein P ≧ 140). That is, when conventional techniques are used, problems such as the leaking of a contained gas or the blowout of the bulb during the operation of the high pressure discharge lamp are frequently caused although the luminous efficacy may be improved by increasing the internal pressure of the lamp. The occurrence of these problems may be significantly decreased by using the above-defined angle of  1 when the internal pressure of the lamp is as described above. As shown in FIG. 4, the high pressure discharge lamp according to another embodiment may be manufactured by prefabricating, firstly, an extruding portion (i.e., a swelling or convex portion) by processing the quartz glass bulb 2 and then using a conventional method such as collapsing or natural fusing (melting). Alternatively, the high pressure discharge lamp according to yet another embodiment may be produced by applying pressure along the length of an electrode when the sealing portion 22 is formed. The characteristics of an embodiment of the high pressure discharge lamp according to the present invention are described as follows: Electric power of the discharge lamp 120-200 W Voltage of the discharge lamp 50-100 V Distance between the electrodes 1.0-2.0 mm Luminous efficacy 40-70 lm/W Bulb wall loading 0.8-1.5 W/mm 2 Radiation wavelength 360-700 nm The high pressure discharge lamp according to the present invention may be used in the same manner as a conventional high pressure discharge lamp. That is, when the high pressure discharge lamp of the present invention is connected to a power supply, a trigger voltage is applied to the cathode and the anode to start the discharge. In this manner, a desired brightness of the lamp may be obtained. Having thus described exemplary embodiments of the invention, it will be apparent that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements, though not expressly described above, are nonetheless intended and implied to be within the scope of the invention. Accordingly, the foregoing discussion is intended to be illustrative only; the invention is limited and defined only by the following claims.

Claims: A high pressure discharge lamp, (1) comprising: a quartz glass bulb (2) having an expanded portion (21) and sealing portions (22) conductive elements (5, 5') airtightly sealed at said sealing portions of said quartz glass bulb; and a pair of electrodes, (3, 4) each electrode thereof being disposed so as to be opposite the other and connected to one of said conductive elements; characterised in that an angle  1 between a tangent along the inner surface of said expanded portion at a position 0.5 mm away from an origin of one of said sealing portions along the length of each of said electrodes and the direction along the length of each of said electrodes is at least about 40°, with said origin of one of said sealing portions defined as a boundary point between the end of said one of said sealing portions and an inner surface of said expanded portion; and said tangent along the inner surface of said expanded portion at a position 0.5 mm away from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each of said electrodes, which passes through a point 0.5 mm away from the origin of said one of said sealing portions toward said expanded portion along the length of each of said electrodes, and the inner surface of said expanded portion and makes contact with the inner surface of said expanded portion. A high pressure discharge lamp according to claim 1, wherein said conductive elements are molybdenum foils. A high pressure discharge lamp according to claim 1, wherein an angle  2 between a tangent along the inner surface of said expanded portion at an optional point between more than 0.5 mm and 1.5 mm from the origin of one of said sealing portions along the length of each of said electrodes and the direction along the length of each of said electrodes is at least about 45°, with said tangent along the inner surface of said expanded portion at an optional point between more than 0.5 mm and 1.5 mm from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each of said electrodes, which passes through an optional point between more than 0.5 mm and 1.5 mm from the origin of said one of said sealing portions toward said expanded portion along the length of each of said electrodes, and the inner surface of said expanded portion and makes contact with the inner surface of said expanded portion. A high pressure discharge lamp according to claim 1 or 3, wherein an angle  3 between a tangent along the inner surface of said expanded portion at an optional point between more than 1.5 mm and 3 mm from the origin of one of said sealing portions along the length of each of said electrodes and the direction along the length of each of said electrodes is at least about 50°, with said tangent along the inner surface of said expanded portion at an optional point between more than 1.5 mm and 3 mm from the origin defined as a tangent which passes through a point of intersection defined by a straight line perpendicular to the direction along the length of each of said electrodes, which passes through an optional point between more than 1.5 mm and 3 mm from the origin of said one of said sealing portions toward said expanded portion along the length of each of said electrodes, and the inner surface of said expanded portion and makes contact with the inner surface of said expanded portion. A high pressure discharge lamp according to claim 4, wherein the relationship among  1 ,  2 and  3 is  1 <  2 <  3 . A high pressure discharge lamp according to claim 1, wherein  1 is at least about 45° . A high pressure discharge lamp according to claim 3, wherein  2 is at least about 60° . A high pressure discharge lamp according to claim 4, wherein  3 is at least about 70° . A high pressure discharge lamp according to claim 1, wherein mercury vapor is contained in the high pressure discharge lamp in an amount between about 0.12 and 0.3 mg/mm 3 . A high pressure discharge lamp according to claim 1, wherein halogen gas is contained in the high pressure discharge lamp in an amount between about 10 -8 and 10 -2 µmol/mm 3 . A high pressure discharge lamp according to claim 1, wherein inert gas is contained in the high pressure discharge lamp at a pressure of about 6 kPa or greater. A high pressure discharge lamp according to claim 1, wherein the bulb wall loading in the high pressure discharge lamp is about 0.8 W/mm 2 or greater. A high pressure discharge lamp according to claim 1, wherein said pair of electrodes comprise tungsten containing potassium oxide. A high pressure discharge lamp according to claim 1, wherein the relationship between an internal pressure P (atm) of the high pressure discharge lamp and the angle  1 is:  1 ≧ 0.25P + 5, with P ≧ 140. A high pressure discharge lamp according to claim 3, wherein the relationship between an internal pressure P (atm) of the high pressure discharge lamp and the angle  1 is:  1 ≧ 0.25P + 5, with P ≧ 140. A high pressure discharge lamp according to claim 4, wherein the relationship between an internal pressure P (atm) of the high pressure discharge lamp and the angle  1 is:  1 ≧ 0.25P + 5, with P ≧ 140.

Inventors: AKIYAMA YASUHIRO, NISHIDA KAZUHISA, AKIYAMA, YASUHIRO, NISHIDA, KAZUHISA
Applicants: NEC MICROWAVE TUBE LTD, NEC MICROWAVE TUBE, LTD.
Application year: 2001
Main classification: H01J61
IPC codes: H01J61, H01J11