Discharge lamp

Abstract

To provide a discharge lamp that can be lit stably for a desired period of time while suppressing the usage amount of an emitter material.SOLUTION: A discharge lamp in which a cathode and an anode are arranged opposite each other in a first direction within an arc tube filled with mercury, and the cathode includes a body portion made of a high-melting point metal, and a tip portion made of a high-melting point metal containing an emitter material, which has a first end and a second end in the first direction and whose cross-sectional shape when cut on a plane perpendicular to the first direction gradually decreases in diameter from the first end side to the second end side, and the first end is surface-bonded to the body portion, where the length of the tip portion in the first direction is 5 mm or less, and the diameter of the circumscribing circle of the second end when viewed in the first direction is 0.8 mm or more and 4 mm or less.SELECTED DRAWING: Figure 2A

Description

[Technical field]

[0001] The present invention relates to a discharge lamp. [Background technology]

[0002] Conventionally, short-arc discharge lamps containing mercury have a short distance between the tips of a pair of electrodes arranged opposite each other inside the light-emitting tube, making them similar to a point light source. When combined with an optical system, these lamps have been used as light sources for exposure equipment with high light-collection efficiency.

[0003] In the discharge lamps described above, in order to improve the electron emission characteristics, a cathode having a tip made of a material containing an emitter material (sometimes called a "material that easily emits electrons") for improving the electron emissivity is often adopted. The following Patent Document 1 discloses a cathode structure consisting of a main body and a tip containing an emitter material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5704301 Summary of the Invention [Problem to be solved by the invention]

[0005] The material used as the emitter is, for example, thorium oxide (ThO 2 ) and lanthanum oxide (La 2 O 3 Although these substances are excellent emitter materials, they are difficult to handle because they are harmful to humans and the environment, or radioactive, and there are many cases where restrictions are imposed on their transportation and use.

[0006] In recent years, the range of uses has been expanding, and so there is a demand for higher output from discharge lamps. For this reason, studies have been conducted on making the electrodes of discharge lamps larger so that they can withstand the input of higher power, and as a result, there is a tendency for the amount of emitter material used in one discharge lamp to increase.

[0007] However, as described above, the substances used as the emitter material are often difficult to handle and are subject to regulations regarding their transportation and use, so it is not preferable to use an unnecessary amount of them.

[0008] Therefore, the inventors of the present invention have conducted extensive research to minimize the amount of emitter material used in the manufacture of the cathode of a discharge lamp, and have arrived at the configuration of the present invention.

[0009] In view of the above problems, an object of the present invention is to provide a discharge lamp that can be stably lit for a desired period of time while suppressing the amount of emitter material used. [Means for solving the problem]

[0010] The discharge lamp according to the present invention comprises: A discharge lamp in which a cathode and an anode are disposed opposite each other in a first direction within an arc tube filled with mercury, The cathode is A main body made of a high melting point metal; a tip portion having a first end and a second end in the first direction, the cross-sectional shape of which, when cut along a plane perpendicular to the first direction, gradually decreases in diameter from the first end side toward the second end side, the first end being surface-bonded to the main body portion, the tip portion being made of a high-melting point metal containing an emitter material; The length of the tip portion in the first direction is 5 mm or less, and the diameter of the circumscribed circle of the second end when viewed in the first direction is 0.8 mm or more and 4 mm or less.

[0011] The discharge lamp is The main body may be made of pure tungsten, and the tip may be made of thoriated tungsten containing thorium oxide as an emitter material.

[0012] Here, the "diameter of the circumscribing circle of the second end" refers to the diameter of the circumscribing circle of the second end when the cathode is viewed in the first direction from the second end side, but if the surface of the cathode on the second end side relative to the first direction is curved, it corresponds to the diameter of the circumscribing circle at the boundary between the curved portion and the straight portion when the cathode is viewed in the radial direction. Details will be described later in the section "Mode for carrying out the invention" with reference to Figures 2A to 2C.

[0013] As is common in the technical field of discharge lamps, "pure tungsten" refers to a material with a purity of 99% or more tungsten (W).

[0014] Conventionally, in order to realize stable lighting for a desired period of time, the tip of the cathode mounted on a discharge lamp is almost always made by incorporating a sufficient amount of emitter material into a high melting point metal. However, in light of the background in which the above-mentioned problems have become apparent, the inventors of the present invention have conducted extensive research into limiting the amount of emitter material used to as much as possible, and have come up with the discharge lamp of the above configuration.

[0015] In a discharge lamp having an anode and a cathode arranged opposite each other in an arc tube, the emitter material contained in the tip of the cathode diffuses as the cathode becomes hot during lighting. The emitter material present near the surface of the tip moves to the second end, which has a higher temperature, due to diffusion that occurs mainly on the surface of the tip (called "surface diffusion"). When the diffused emitter material reaches the vicinity of the second end, the work function near the second end of the cathode decreases, and good electron emission characteristics are achieved.

[0016] Here, the inventors prepared a tip of a cathode having emitter material distributed all over it, and after lighting the discharge lamp for a period of time sufficiently longer than the expected lifespan of the discharge lamp, they confirmed the extent to which the emitter material had been reduced by surface diffusion in a first direction at the tip of the cathode, thereby determining the necessary and sufficient amount of emitter material to be contained in the tip.

[0017] As described in detail in the section "Mode for Carrying Out the Invention," the inventors' confirmation showed that the portion in which the emitter material present on the surface of the tip is reduced by surface diffusion is almost always located up to a position approximately 5 mm or less from the second end in the first direction. In other words, it was confirmed that in a cathode having a tip whose length in the first direction is greater than 5 mm, emitter material that does not contribute to surface diffusion often remains on the surface of the tip even when the cathode is used until the end of the discharge lamp's life. Note that "within a range of 5 mm or less from the second end" means a range in which the distance from the second end at the beginning of the discharge lamp is 5 mm or less.

[0018] Therefore, by adopting the above-mentioned configuration, the discharge lamp can be stably lit until the end of its life, and the amount of emitter material used in the cathode can be reduced as much as possible to the minimum amount required.

[0019] Furthermore, when checking the remaining amount of emitter material at the time when the discharge lamp reached the generally set lifespan, it was found that while the amount of emitter material had decreased within a range of 5 mm or less from the second end of the tip, there was still a sufficient amount of emitter material remaining toward the center, away from the tip surface, to enable the lamp to continue to be lit.

[0020] A discharge lamp in which mercury is sealed in the arc tube (hereinafter referred to as a "mercury lamp") must have a relatively large diameter at the second end in order to meet the expectations of high output and long life. Specifically, the diameter of the second end is 0.8 mm or more and 4 mm or less. From the viewpoint of obtaining the effects of higher output and longer life, the diameter of the second end is preferably 1.0 mm or more and 4.0 mm or less. If the diameter of the second end is too thin, it becomes too hot during lighting, and wears out quickly, resulting in a short life. If the diameter of the second end is too thick, the volume of the tip increases, the amount of emitter material used increases, and the position of the arc generated from the second end tends to fluctuate over time, making it difficult to maintain stable lighting. For example, a discharge lamp in which xenon gas is sealed in the arc tube (also referred to as a "xenon lamp") has a diameter of the second end of at most about 0.6 mm even if it is a high-output lamp.

[0021] In addition, from the viewpoints that electron emission occurs at low energy, i.e., that the work function is low and that it is difficult to evaporate or melt at high temperatures, the tip of the cathode is made of pure tungsten, a high melting point metal, and thorium oxide (ThO 2 It is preferable to use thoriated tungsten having tungsten ions dispersed therein.

[0022] The temperature of the tip of the cathode made of thoriated tungsten is estimated to be about 2700° C. by the Richardson-Dushman equation. Pure tungsten has a melting point of about 3400° C. under normal pressure, making it a preferred material as a high-melting-point metal for forming the cathode of a discharge lamp.

[0023] The discharge lamp is The lighting power (W) was calculated based on the contact area (mm 2 ) (W / mm 2 ) is preferably 500 or more.

[0024] Inside the tip of the cathode of the discharge lamp, the emitter material also moves due to diffusion that occurs mainly at the grain boundaries of the high melting point metal (called "grain boundary diffusion"), although this diffusion is slower than "surface diffusion".

[0025] Like surface diffusion, grain boundary diffusion is a phenomenon in which the emitter material diffuses toward the second end when the cathode becomes hot during lighting. It is presumed that whether the emitter material moves toward the second end by grain boundary diffusion depends on the internal temperature of the tip of the cathode.

[0026] The temperature inside the cathode, particularly in the vicinity of the central axis along the first direction, is presumably determined by the arc heat input from the second end of the tip, and the temperature rise of the cathode due to the arc heat is presumably dependent on the power density in a cross section taken along a plane perpendicular to the first direction at a certain distance from the second end of the tip.

[0027] Here, the inventors confirmed how much the shape of the emitter material at the tip of the cathode has changed in the first direction after the discharge lamp has been lit up to almost the end of its life. It is presumed that when the emitter material becomes sufficiently hot, it moves toward the second end by grain boundary diffusion and changes its shape to become closer to a sphere. On the other hand, when the temperature does not rise sufficiently, it is presumed that the emitter material hardly moves and that its shape hardly changes from the shape elongated in one direction.

[0028] And, at the tip of the cathode, within a range of about 4 mm or less from the second end, the emitter material was confirmed to have deformed to approach a spherical shape, but in the part farther than 4 mm, the shape that was elongated in one direction was maintained almost the same as the state before lighting. Furthermore, these results were hardly changed even when the power input to the discharge lamp was changed.

[0029] From the above, the inventors have determined that the grain boundary diffusion for moving the emitter material to the vicinity of the second end of the tip portion occurs within a range of a distance of approximately 4 mm or less from the second end, and further that, with respect to the first direction, the threshold at which the correlation between the power density and the cathode temperature changes in a cross section cut along a plane perpendicular to the first direction at a position 4 mm from the second end is 500 (W / mm 2 ) was found.

[0030] When grain boundary diffusion does not occur, most of the emitter material that contributes to the discharge lamp is emitter material that moves to the second end side by surface diffusion. In contrast, when grain boundary diffusion occurs, the emitter material that contributes to the discharge lamp includes emitter material that moves to the second end side by grain boundary diffusion in addition to emitter material that moves to the second end side by surface diffusion. In other words, when grain boundary diffusion occurs, not only emitter material dispersed near the surface of the tip part but also emitter material dispersed throughout the tip part contributes to lighting. In other words, compared to the case of surface diffusion only, the amount of emitter material that does not contribute to lighting is reduced, which leads to a relative reduction in the amount of emitter material used. Therefore, the power density is 500 (W / mm 2 ), which is achieved by the effect of migration to the second end side by surface diffusion of the emitter material and the effect of migration to the second end side by grain boundary diffusion. 2 ) or more.

[0031] The discharge lamp is When the distance between the anode and the cathode is D (mm), the length of the tip portion in the first direction is preferably (D×0.15) mm or more and 5 mm or less.

[0032] Optical devices such as exposure devices that are equipped with this discharge lamp are optically designed to be considered as a point light source over the life of the discharge lamp. However, the tips of the anode and cathode of the discharge lamp wear out over the life of the lamp, and the distance D between the anode and cathode gradually increases. Usually, the optical design allows the initial distance D to increase by about 15%.

[0033] Therefore, since the tip of the cathode is allowed to wear down by about 15% of the separation distance D from the beginning during its life, if the emitter material at the tip of the cathode remains at the tip even if it wears down by about 15%, the function of the discharge lamp will be maintained (the lamp will continue to light stably). Therefore, it is preferable that the length of the tip of the cathode is (D x 0.15) mm or more. Effect of the Invention

[0034] According to the present invention, a discharge lamp is realized that can be lit stably for a desired period of time while suppressing the amount of emitter material used. [Brief description of the drawings]

[0035] [Figure 1] 2 is a diagram showing a schematic view of an embodiment of a discharge lamp when viewed in the Z direction. [Figure 2A] FIG. 2 is an enlarged view of the vicinity of the tip of the cathode. [Figure 2B] This is a drawing of the cathode as seen from the -X side. [Figure 2C] FIG. 2 is an enlarged view of the vicinity of the tip of the cathode. [Figure 3A] This is an image showing the dispersion state of thorium oxide (ThO2) when the tip is cut on the XY plane. [Figure 3B] This is an image showing the dispersion state of thorium oxide (ThO2) when the tip is cut on the XY plane. [Figure 3C] This is an image showing the dispersion state of thorium oxide (ThO2) when the tip is cut on the XY plane. [Figure 3D] This is an image showing the dispersion state of thorium oxide (ThO2) when the tip is cut on the XY plane. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The discharge lamp of the present invention will be described below with reference to the drawings. Note that the following drawings are all schematic illustrations, and the dimensional ratios and numbers in the drawings do not necessarily match the actual dimensional ratios and numbers.

[0037] Fig. 1 is a schematic diagram showing an embodiment of a discharge lamp 1 as viewed in the Z direction. As shown in Fig. 1, the discharge lamp 1 of this embodiment includes an arc tube 2, a pair of lead rods (3a, 3b), an anode 4 supported by one lead rod 3a, and a cathode 5 supported by the other lead rod 3b.

[0038] 1, the direction in which the anode 4 and the cathode 5 face each other is defined as the X direction, the direction in which the tip portion 2c formed on the tube wall 2b is located as viewed from the tube axis 2a of the arc tube 2 as described later is defined as the Y direction, and the direction perpendicular to the X direction and the Y direction is defined as the Z direction. The X direction corresponds to the first direction.

[0039] As described above, in this specification, when expressing a direction, if there is a need to distinguish between positive and negative directions, the direction is described with a positive or negative sign, such as "+Z direction" and "-Z direction." When expressing a direction without distinguishing between positive and negative directions, the direction is simply described as "Z direction."

[0040] The discharge lamp 1 is a short arc type discharge lamp in which the distance in the X direction between the tip of the anode 4 and the tip of the cathode 5 (at room temperature without thermal expansion) is 40 mm or less. The short arc type discharge lamp is a discharge lamp in which the luminous material sealed inside the arc tube 2, the power to be input, etc. are selected according to the application.

[0041] The arc tube 2 is a tube made of a material that is transparent to the light generated inside when power is applied and has a shape that bulges toward the center compared to both ends in the X direction. Here, being transparent to the light generated inside means that the transmittance for light in the ultraviolet range (300 nm to 400 nm) is 80% or more in the intensity spectrum of the light generated inside the arc tube 2. Also, having high heat resistance means that the material has a heat resistance that does not melt due to the heat generated during lighting, and specifically, it is preferable that the melting point is 1400°C or higher.

[0042] The material of the arc tube 2 in this embodiment is quartz glass, but other than quartz glass, for example, translucent ceramics, etc. may also be used.

[0043] Mercury is sealed inside the arc tube 2 as a light-emitting material. When the lamp is not lit, the mercury inside the arc tube 2 is liquid, but when the lamp is lit, it vaporizes due to the heat generated inside the arc tube 2 and spreads inside the arc tube 2.

[0044] A tip portion 2c is formed on the tube wall 2b of the arc tube 2 on the +Y side as viewed from the tube axis 2a. The tip portion 2c is a protruding portion formed when closing a portion of the arc tube 2 in which mercury is placed during the manufacturing process of the discharge lamp 1. In a typical discharge lamp 1, the tip portion 2c remains on the tube wall 2b of the arc tube 2, but in the case of the discharge lamp 1, it is not essential that the tip portion 2c is formed.

[0045] The lead rods (3a, 3b) are cylindrical members provided at both ends in the X direction of the light emitting tube 2. An anode 4 is connected to the end of the lead rod 3a, and a cathode 5 is connected to the end of the lead rod 3b. The lead rods (3a, 3b) support the anode 4 and the cathode 5 at the center of the light emitting tube 2, respectively, so that the anode 4 and the cathode 5 are disposed opposite each other in the X direction.

[0046] The ends of the lead rods (3a, 3b) located on both ends in the X direction of the light emitting tube 2 are electrically connected to the bases (6, 6) via molybdenum conductive foils (not shown) mounted on the stems. With this configuration, when power is supplied through the power supply wires connected to the bases (6, 6) or through the electrodes of the device in which the discharge lamp 1 is mounted, a discharge occurs between the anode 4 and the cathode 5, and light is emitted.

[0047] The anode 4 is an electrode into which electrons flow from the discharge plasma when the lamp is lit, and is larger than the cathode 5. By way of example only, the anode 4 in this embodiment is an electrode made of pure tungsten and formed so as to have a length of 60 mm in the X direction and a diameter of 40 mm at its thickest part when viewed in the X direction.

[0048] The cathode 5 is an electrode that emits electrons when lit, and is an electrode having a cross-sectional shape that gradually decreases in diameter toward the anode 4 when cut in a plane perpendicular to the X direction (YZ plane).

[0049] Fig. 2A is an enlarged view of the vicinity of tip portion 11 of cathode 5, and Fig. 2B is a drawing of cathode 5 as viewed from the -X side. Cathode 5 includes a main body portion 10 and a tip portion 11, as shown in Fig. 2A.

[0050] The main body 10 is a member having one end 10a connected to the lead rod 3b and the other end 10b surface-bonded to a first end 11a of the tip 11. In the present embodiment, the main body 10 has a portion on the other end 10b side where the cross section of the main body 10 when cut in the YZ plane gradually decreases in diameter.

[0051] The shape of the main body 10 is formed by cutting the tip 11 into the above-mentioned shape after surface-bonding the main body 10 and the tip 11. In the tip 11 of the created cathode 5, the end surface-bonded to the other end 10b of the main body 10 becomes the first end 11a, and the end on the anode side (-X side) becomes the second end 11b.

[0052] The main body 10 is a member made of pure tungsten and formed so as to have a length in the X direction of 35 mm and a diameter of 20 mm at its thickest part when viewed in the X direction. The size and shape of the main body 10 are arbitrary.

[0053] The material for the main body 10 may be any high melting point metal, and molybdenum (Mo) may also be used. However, due to its high melting point and the fact that thoriated tungsten is considered to be the preferred material for the tip 11 as described above, it is preferable to use pure tungsten.

[0054] The tip portion 11 is a member extending from the main body portion 10 toward the anode 4, and has a shape in which the cross section when cut in the YZ plane gradually decreases in diameter toward the anode 4. As shown in Figs. 2A and 2B, the tip portion 11 has a planar shape and a second end 11b that is circular when viewed in the X direction. In this embodiment, the diameter r1 of the circumscribing circle of the second end 11b is 1.2 mm. In this embodiment, as described above, the second end 11b of the tip portion 11 has a circular shape when viewed in the X direction, so that the shape of the outer edge of the second end 11b coincides with the circumscribing circle of the second end 11b.

[0055] The material of the tip portion 11 may be a high melting point metal containing an emitter material. The emitter material is, for example, thorium oxide (ThO 2 ) and lanthanum oxide (La 2 O 3 As the high melting point metal, molybdenum (Mo), tungsten (W), etc. may be used.

[0056] However, as described above, from the viewpoint of electron emission occurring at low energy, i.e., having a low work function and being unlikely to evaporate or melt at high temperatures, the tip 11 of the cathode 5 is made of pure tungsten, which is a high melting point metal, and thorium oxide (ThO 2It is preferable to use thoriated tungsten (sometimes called "thoriated tungsten") in which thorium oxide is dispersed. The concentration of thorium oxide in the tip portion 11 is about several percent (for example, 3%).

[0057] The temperature of the tip 11 of the cathode 5 made of thoriated tungsten is estimated to be about 2700° C. according to the Richardson-Dushman equation. Pure tungsten has a melting point of about 3400° C. under normal pressure, and is a preferred material as a high-melting point metal for forming the cathode 5 of the discharge lamp 1.

[0058] Here, the "diameter of the circumscribing circle of the second end 11b" will be described in the case where the surface of the tip portion 11 on the second end 11b side is curved. FIG. 2C is an enlarged view of the vicinity of the tip portion 11 of the cathode 5 having a shape different from that of this embodiment. When the second end 11b is curved, as shown in FIG. 2C, the diameter r2 of the circumscribing circle in the cross section when the boundary 11c between the curved portion and the straight portion is cut by the YZ plane when the tip portion 11 is viewed in the Z direction (radial direction), corresponds to the "diameter of the circumscribing circle of the second end 11b". As an example of the shape of the cathode 5 shown in FIG. 2C, if the shape and size are similar to those of the main body portion 10 and the tip portion 11 of this embodiment described above, the length d2 from the second end 11b to the boundary 11c is 0.4 mm. Although not shown in the figure, a cross section of the boundary 11c of the cathode 5 shown as an example in FIG. 2C cut in the XY plane has a circular shape when viewed in the X direction, and therefore the shape of the outer edge of the second end 11b coincides with the circumscribing circle of the second end 11b.

[0059] [Verification experiment] Here, with respect to the tip portion 11, to what extent in the X direction is thorium oxide (ThO 2 We conducted a verification experiment to confirm whether the ion beam moves, and we will explain the details here.

[0060] (Verification method) A cathode 5 having a tip 11 made of thoriated tungsten with a length of 7 mm in the X direction was prepared, and the tip 11 was continuously lit for 500 hours with a lighting power of 5 kW. Then, the thorium oxide (ThO 2 The dispersion state of the second end 11b was confirmed at positions 3 mm, 4 mm, 5 mm, and 6 mm away from the second end 11b. The information indicating these positions is the initial distance from the second end 11b before the verification experiment, and is the sum of the distance from the second end 11b relatively close to the end of its life and the distance of the second end 11b that was worn away during the verification experiment. The worn distance in this verification experiment was approximately 0.2 mm.

[0061] (result) 3A to 3D are graphs showing the distribution of thorium oxide (ThO 2 3A is an image taken at a position 3 mm from the second end 11b, FIG. 3B is an image taken at a position 4 mm from the second end 11b, FIG. 3C is an image taken at a position 5 mm from the second end 11b, and FIG. 3D is an image taken at a position 6 mm from the second end 11b. Note that in FIGS. 3A to 3C, thorium oxide (ThO 2 ) is presumed to be depleted, and thorium oxide (ThO 2 ) are marked with dashed lines at the borders of the areas where the

[0062] As can be seen from FIG. 3A to FIG. 3D, thorium oxide (ThO 2 ) decreases near the surface up to a position 5 mm from the second end 11b. 2 It can be seen that the surface diffusion of thorium oxide (ThO 2It is confirmed that a distance of 5 mm or less, at which surface diffusion of the cathode may occur, is sufficient. In fact, a discharge lamp having a cathode with a tip 11 length of 5 mm lit stably over the lifespan, similar to a discharge lamp having a cathode with a tip 11 length of 7 mm.

[0063] As described above, the above-mentioned configuration allows the discharge lamp 1 to be stably lit until the end of its life, and the thorium oxide (ThO 2 ) can be kept as close to the minimum necessary as possible.

[0064] In addition, the thorium oxide shown in Figures 3A and 3B has a shape close to a sphere. This allows the occurrence of grain boundary diffusion of thorium oxide to be confirmed. That is, at the positions shown in Figures 3A and 3B, the occurrence of grain boundary diffusion as well as the occurrence of surface diffusion of thorium oxide is confirmed, so this is a preferable length for the length of tip portion 11 in the X direction.

[0065] On the other hand, in FIG. 3D, as described above, almost no surface diffusion has occurred, and the thorium oxide appears to be elongated in one direction, so it cannot be said that grain boundary diffusion has occurred.

[0066] In this embodiment, taking into consideration the effect of the grain boundary diffusion described above, the lighting power is set to 7500 W, and the contact area of ​​the portion where the tip portion 11 and the main body portion 10 are in surface contact is set to 13.3 mm 2 In this case, the lighting power divided by the contact area was 564W / mm 2 This gives 500W / mm 2 If we take into account the combined effect of surface diffusion and grain boundary diffusion in the emitter material, the lighting power divided by the contact area is 500W / mm 2 However, as described above, if the length from the second end 11b of the tip portion 11 is 5 mm or less, the effect of surface diffusion can be obtained, and the value obtained by dividing the lighting power by the contact area is preferably 500 W / mm 2 It is acceptable to use less than this.

[0067] The length of tip 11 in the X direction can be any length up to 5 mm, but considering that tip 11 gradually wears away due to heat generated during lighting, the allowable wear amount is D×0.15 and the length of tip 11 of the cathode is D×0.15 or more from the viewpoint of satisfying the optical design of an optical device including a discharge lamp. That is, the length of tip 11 in the X direction is preferably (D×0.15) mm or more and 5 mm or less. In the above-mentioned embodiment, the separation distance D is 9 mm and the length of tip 11 in the X direction is 1.35 mm.

[0068] The configuration of the discharge lamp 1 described above is merely an example, and the present invention is not limited to the illustrated configurations. [Explanation of symbols]

[0069] 1: Discharge lamp 2: Arc tube 2a: Tube shaft 2b: Pipe wall 2c: Tip part 3a: Lead rod 3b: Lead rod 4: Anode 5 : Cathode 6: Base 10: Main body 10a: One end 10b: Other end 11: Tip 11a : First end 11b: Second end 11c : Boundary

Claims

1. A discharge lamp in which a cathode and an anode are disposed opposite each other in a first direction within an arc tube filled with mercury, The cathode is A main body made of a high melting point metal; a tip portion having a first end and a second end in the first direction, the cross-sectional shape of which, when cut along a plane perpendicular to the first direction, gradually decreases in diameter from the first end side toward the second end side, the first end being surface-bonded to the main body portion, the tip portion being made of a high-melting point metal containing an emitter material; A discharge lamp characterized in that the length of the tip portion in the first direction is 5 mm or less, and the diameter of the circumscribed circle of the second end when viewed in the first direction is 0.8 mm or more and 4 mm or less.

2. 2. The discharge lamp according to claim 1, wherein the body portion is made of pure tungsten, and the tip portion is made of thoriated tungsten containing thorium oxide as an emitter material.

3. The lighting power (W) was calculated based on the contact area (mm 2 ) divided by (W / mm 2 3. The discharge lamp according to claim 1, wherein the value of the refractive index is 500 or more.

4. 3. The discharge lamp according to claim 1, wherein the length of the tip portion in the first direction is equal to or greater than (D×0.15) mm and equal to or less than 5 mm, where D (mm) is a distance between the anode and the cathode.

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