Discharge lamp

Abstract

The present invention provides a discharge lamp capable of sufficiently suppressing consumption of an anode. This discharge lamp is provided with: a light-emitting tube in which a light-emitting substance is sealed; and an anode and a cathode that are disposed inside the light-emitting tube so as to face each other. The anode has a substantially columnar shape in which the outer peripheral edge of the tip, which faces the cathode, is chamfered or rounded. The anode has, on the inside of the chamfered or rounded outer peripheral edge, a circular tip surface when viewed from the column axis direction. The tip surface is provided with a recess. The recess is formed on the inside of a concentric circle having a diameter of at least 70% the diameter of the tip surface, but is not formed at the center of the tip surface.

Description

discharge lamp

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

[0002] A conventional short-arc discharge lamp is known to be composed of a quartz glass arc tube with a bulging center, and an anode and a cathode arranged facing each other inside the bulging portion of the arc tube. When current is applied to this discharge lamp, electrons emitted from the cathode collide with the luminous material sealed in the arc tube, generating charged particles. These charged particles repeatedly collide, turning the luminous material sealed in the arc tube into a plasma state, which then flows to the anode, forming an arc between the two electrodes.

[0003] When a discharge lamp is lit, an arc is formed by the collision of electrons emitted from the cathode, and the anode is heated by the collision of electrons in the arc column, reaching a high temperature, evaporating and being consumed. Furthermore, when the anode is consumed by the electron collision, the evaporated tungsten adheres to the inner wall of the arc tube, blackening the inner wall surface. When a discharge lamp is used for a long period of time, the tip of the anode wears out and the radiation intensity of the discharge lamp gradually decreases. When this decrease in radiation intensity exceeds the usage limit, the discharge lamp must be replaced with a new one.

[0004] Therefore, various measures have been studied to suppress the consumption of the anode (for example, Patent Documents 1 and 2 listed below), but it remains difficult to sufficiently suppress the consumption of the anode.

[0005] JP 2011-14248 A JP 2003-346709 A

[0006] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a discharge lamp that can sufficiently suppress the consumption of the anode.

[0007] The discharge lamp according to the present invention comprises an arc tube in which a luminous substance is sealed, and an anode and a cathode arranged opposite each other inside the arc tube, wherein the anode is approximately cylindrical with a chamfered or rounded outer edge at the tip facing the cathode, and the anode has a circular tip surface inside the chamfered or rounded outer edge when viewed from the cylindrical axial direction, and the tip surface has a recess, and the recess is formed inside a concentric circle having a diameter that is at least 70% of the diameter of the tip surface, and is not formed in the center of the tip surface.

[0008] With this configuration, the portion where the recess is formed is farther from the arc, and therefore the temperature is lower than the portion where the recess is not formed. As a result, the temperature of a portion of the tip surface is lowered, which prevents the anode from evaporating and being consumed. In this case, by forming the recess inside a concentric circle having a diameter that is at least 70% of the diameter of the tip surface, the temperature of a portion of the tip surface can be effectively reduced, thereby sufficiently preventing the consumption of the anode.

[0009] In the discharge lamp according to the present invention, the recess may be formed inside a concentric circle having a diameter that is at least 50% of the diameter of the tip end surface.

[0010] According to this configuration, the temperature of a portion of the tip surface can be more effectively lowered.

[0011] In the discharge lamp according to the present invention, the recess may be formed outside a concentric circle having a diameter that is 20% of the diameter of the tip end surface.

[0012] Since the arc gathers at the center of the tip face, if a recess is formed at the center of the tip face, the initial light intensity will be reduced. However, by forming the recess outside a concentric circle with a diameter that is 20% of the diameter of the tip face, it is possible to ensure an appropriate initial illuminance.

[0013] In the discharge lamp according to the present invention, the recess may be formed in a circular shape surrounding the center of the tip end face. In this specification, "formed in a circular shape surrounding the center of the tip end face" specifically refers to shapes such as those shown in Figures 3, 4B, 4C, 12, and 13, which will be described later.

[0014] With this configuration, a recess is formed in any direction (Y-Z directions in Figures 3, 4B, 4C, 12 and 13) from the center of the tip surface, so that tungsten evaporated from the tip surface in only a specific direction does not adhere to the inner wall of the arc tube and blacken the inner wall surface in only a specific direction.

[0015] In the discharge lamp according to the present invention, the recess may be formed in a circular ring shape centered on the center of the tip surface. In this specification, the circular ring shape surrounding the center of the tip surface specifically refers to the shapes shown in Figures 3, 4B, 4C, and 12, which will be described later.

[0016] According to this configuration, the recesses are formed at positions equidistant from the center of the tip face, so that the directional dependency of the amount of tungsten evaporated from the tip face becomes more uniform, and the directional dependency of the blackening of the inner wall face is improved.

[0017] In the discharge lamp according to the present invention, the luminous material may be mercury, and the amount of the mercury enclosed may be less than 20 mg / cc.

[0018] When the mercury content is less than 20 mg / cc, the arc diameter becomes narrow (thin), so energy is concentrated at the center of the anode, causing it to heat up and evaporate easily. Therefore, the present invention is particularly useful for discharge lamps with a mercury content of less than 20 mg / cc.

[0019] In the discharge lamp according to the present invention, the maximum depth of the recess may be 0.5 mm or more.

[0020] According to this configuration, the temperature drop in the portion where the recess is formed is large, which is highly effective in suppressing evaporation of the anode.

[0021] In the discharge lamp according to the present invention, the area of ​​the recess may be 2% to 25% of the area of ​​the tip end face when viewed in the cylindrical axial direction.

[0022] According to this configuration, while ensuring the initial irradiance, it is possible to lower the temperature of a portion of the tip end surface, and to sufficiently prevent the anode from evaporating and being consumed.

[0023] In the discharge lamp according to the present invention, the tip surface may include an inner tip surface located inside the recess and an outer tip surface located outside the recess, and the inner tip surface may be closer to the cathode than the outer tip surface.

[0024] According to this configuration, while ensuring the initial irradiance, it is possible to lower the temperature of a portion of the tip end surface, and to sufficiently prevent the anode from evaporating and being consumed.

[0025] FIG. 1 is an explanatory diagram showing the configuration of a discharge lamp according to the present embodiment; FIG. 1 is a cross-sectional view including the cylindrical axis of the anode; FIG. 2 is a view of the tip of the anode from the cathode side; FIG. 3 is a view of the tip surface of the anode of Comparative Example 1 from the cathode side; FIG. 4 is a view of the tip surface of the anode of Example 1 from the cathode side; FIG. 5 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 6 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 7 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 8 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 9 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 10 is a view of the tip of the anode of Example 1 from the cathode side; FIG. 11 is a view of the tip surface of the anode of Example 2 from the cathode side; FIG. 12 is a view of the tip of the anode of Example 1 from the cathode side;

[0026] An embodiment of a discharge lamp according to the present invention will be described with reference to the drawings. Note that the following drawings are schematic illustrations, and the dimensional ratios in the drawings do not necessarily correspond to the actual dimensional ratios, and the dimensional ratios between the drawings do not necessarily correspond to the actual dimensional ratios.

[0027] In the following, the XYZ coordinate system will be referred to as appropriate. Furthermore, in this specification, when expressing a direction, if a distinction is made between positive and negative directions, the direction will be described with a positive or negative sign, such as "+X direction" and "-X direction." Furthermore, when a direction is expressed without distinguishing between positive and negative directions, it will simply be described as "X direction." In other words, in this specification, when simply referring to "X direction," both the "+X direction" and the "-X direction" are included. The same applies to the Y direction and the Z direction.

[0028] 1 is an explanatory diagram showing the configuration of a discharge lamp according to this embodiment. The discharge lamp 1 comprises an arc tube 2, and an anode 3 and a cathode 4 disposed opposite each other inside the arc tube 2. The anode 3 and the cathode 4 are each supported by a lead rod 5.

[0029] The arc tube 2 is formed by expanding the center of a glass tube. The arc tube 2 has bulging portions 20 whose inner diameter increases from both ends in the X direction toward the center. The external shape of the bulging portions 20 is an axially rotating body such as a sphere or an ellipsoid.

[0030] The arc tube 2 has a pair of sealed tube portions 21 that extend continuously in opposite directions from both ends of the bulge portion 20 in the X direction. The sealed tube portions 21 are formed integrally with the bulge portion 20 from, for example, quartz glass. The central axes of the pair of sealed tube portions 21 overlap each other and are indicated by axis X1 in FIG. 1 .

[0031] A light-emitting space S1 is formed inside the bulge 20. Mercury is enclosed in the light-emitting space S1 as a light-emitting material. When mercury is enclosed as a light-emitting material, a rare gas (argon, krypton, xenon, etc.) is also enclosed in the light-emitting space S1. The amount of mercury enclosed as a light-emitting material in the arc tube 2 may be less than 20 mg / cc. If the amount of mercury enclosed is less than 20 mg / cc, the arc diameter becomes narrow (thin), energy is concentrated at the center of the anode 3, the temperature rises, and the anode 3 is likely to evaporate. Therefore, the present invention is particularly useful for discharge lamps 1 with a mercury enclosed amount of less than 20 mg / cc. The light-emitting material may be a gaseous substance, and for example, only xenon gas may be enclosed.

[0032] The anode 3 and the cathode 4 are disposed inside the bulge 20 facing each other in the X direction. In this embodiment, the anode 3 is made of tungsten, and the cathode 4 is made of thoriated tungsten. The anode 3 is generally cylindrical, and the cylindrical axis of the anode 3 preferably overlaps with the axis X1. The cathode 4 is generally cylindrical with a smaller diameter than the anode 3, and the cylindrical axis of the cathode 4 preferably also overlaps with the axis X1.

[0033] The lead rod 5 is connected to the anode 3 and the cathode 4 and extends in the X direction within the sealed tube portion 21. The anode 3 and the cathode 4 are fixed to the tip of the lead rod 5. The central axis of the lead rod 5 should coincide with the axis X1. The lead rod 5 is made of a material containing a high-melting point metal, such as tungsten.

[0034] The base 6 covers the side of the sealed tube portion 21 that faces away from the anode 3 and the cathode 4. The base 6 is electrically connected to the lead rod 5.

[0035] Fig. 2 is a cross-sectional view including the cylindrical axis X1 of the anode 3. Fig. 3 is a view of the tip 3a of the anode 3 as seen from the cathode 4 side.

[0036] The anode 3 has a generally cylindrical shape with a chamfered or rounded outer periphery 31 of the tip 3 a facing the cathode 4. As shown in Fig. 2 , the anode 3 of this embodiment has a tapered shape with the outer periphery 31 of the tip 3 a facing the cathode 4 chamfered. The anode 3 has a circular tip surface 32 inside the chamfered outer periphery 31 when viewed from the direction of the cylindrical axis X1 (X direction).

[0037] 2 and 3, the tip surface 32 has a recess 7. The recess 7 is formed so as to exclude the center of the tip surface 32. The recess 7 may be formed in a circular shape surrounding the center of the tip surface 32. In this embodiment, the recess 7 is formed in a ring shape centered at the center of the tip surface 32, as shown in FIG.

[0038] The portion of the tip surface 32 where the recess 7 is formed is farther from the arc, and therefore its temperature is lower than that of the portion where the recess 7 is not formed. This reduces the temperature of a portion of the tip surface 32, thereby preventing the anode 3 from evaporating and being consumed. As a result, the degree of blackening of the inner surface of the arc tube 2 is reduced, and the illuminance maintenance rate is improved.

[0039] As shown in Fig. 3, the recess 7 is formed inside a concentric circle c1 (the concentric circle c1 of the tip surface 32) having a diameter that is at least 70% of the diameter d of the tip surface 32. If the recess 7 is formed outside the concentric circle c1 (i.e., if the recess 7 is formed at a position that is too far from the center of the tip surface 32), the recess 7 will be formed in a location where the arc discharge is weak, making it difficult to achieve the effect of cooling a portion of the tip surface 32. Therefore, by forming the recess 7 inside the concentric circle c1 of the tip surface 32, it is possible to effectively cool a portion of the tip surface 32, thereby sufficiently suppressing wear of the anode 3.

[0040] The recess 7 is preferably formed inside a concentric circle (not shown) having a diameter that is at least 50% of the diameter d of the tip surface 32. This allows a portion of the tip surface 32 to be cooled more effectively.

[0041] However, since the arc gathers at the center of the tip face 32, if the recess 7 is formed at the center of the tip face 32, the initial light intensity will decrease. However, by forming the recess 7 so as to exclude the center of the tip face 32, the initial irradiance can be ensured. As shown in Figure 3, the recess 7 is preferably formed outside a concentric circle c2 having a diameter that is 20% of the diameter d of the tip face 32. This allows the initial irradiance to be adequately ensured.

[0042] Furthermore, the maximum depth 7a of the recess 7 (see FIG. 2) is preferably 0.5 mm or more. If the maximum depth 7a is 0.5 mm or more, the temperature drop in the portion where the recess 7 is formed is large, which is effective in suppressing evaporation of the anode 3. If the maximum depth 7a of the recess 7 is too deep, the arc will concentrate on the tip surface 32 other than the recess 7, which will actually accelerate evaporation. Therefore, the maximum depth 7a of the recess 7 is preferably 2 mm or less.

[0043] Furthermore, when viewed from the direction of the cylindrical axis X1 (X direction), the area of ​​the recess 7 is preferably 2% to 25% of the area of ​​the tip face 32. By making the area of ​​the recess 7 2% or more of the area of ​​the tip face 32, the effect of reducing the temperature by the recess 7 can be obtained. Furthermore, if the recess 7 formed in the tip face 32 is too wide, it becomes equivalent to a state in which the distance between the anode 3 and the cathode 4 is long, and the initial irradiance decreases, but by making the area of ​​the recess 7 25% or less of the area of ​​the tip face 32, the initial irradiance can be ensured.

[0044] As described above, the discharge lamp 1, as in this embodiment, comprises an arc tube 2 in which a luminous material is sealed, and an anode 3 and a cathode 4 arranged opposite each other inside the arc tube 2, the anode 3 is approximately cylindrical with a chamfered or rounded outer periphery 31 of the tip 3 a facing the cathode 4, the anode 3 has a tip surface 32 that is circular when viewed from the cylindrical axial direction inside the chamfered or rounded outer periphery 31, the tip surface 32 has a recess 7, and the recess 7 is formed inside a concentric circle c1 having a diameter that is at least 70% of the diameter d of the tip surface 32, but is not formed in the center of the tip surface 32.

[0045] According to this configuration, the portion where the recess 7 is formed is farther from the arc, and therefore the temperature is lower than the portion where the recess 7 is not formed. As a result, the temperature of a portion of the tip surface 32 is lowered, which makes it possible to prevent the anode 3 from evaporating and being consumed. In this case, by forming the recess 7 inside a concentric circle c1 having a diameter that is at least 70% of the diameter d of the tip surface 32, the temperature of a portion of the tip surface 32 can be effectively lowered, and therefore consumption of the anode 3 can be sufficiently prevented.

[0046] Examples that specifically illustrate the configuration and effects of the present invention will be described below.

[0047] First, the effect of the recess 7 formed on the tip end surface 32 of the anode 3 was verified. Specifically, the following discharge lamps were fabricated, and a comparative test was carried out on the illuminance maintenance rate depending on the lighting time.

[0048] Comparative Example 1 A discharge lamp having a tip end surface 32 with a diameter of 12 mm and no recess formed on the tip end surface 32 was used as Comparative Example 1.

[0049] 4A, a discharge lamp in which the diameter of the tip surface 32 was set to 12 mm and an annular recess 70 having a width of 1 mm was formed on a circumference having a diameter of 1 mm (i.e., an annular recess 70 having a diameter of 2 mm from the center of the tip surface 32) was formed was used as Comparative Example 2. The recess 70 finally formed was in the shape of a disk having a diameter of 2 mm.

[0050] (Example 1) As shown in FIG. 4B , Example 1 was a discharge lamp 1 in which the diameter of the tip surface 32 was 12 mm and an annular recess 7 with a width of 1 mm was formed on a circumference with a diameter of 3 mm (annular recess 7 with an inner diameter of 2 mm and an outer diameter of 4 mm).

[0051] (Example 2) As shown in Figure 4C, Example 2 was a discharge lamp 1 in which the diameter of the tip surface 32 was 12 mm and an annular recess 7 with a width of 1 mm was formed on a circumference with a diameter of 5 mm (annular recess 7 with an inner diameter of 4 mm and an outer diameter of 6 mm).

[0052] The arc tube 2 was filled with 2.5 mg / cc of mercury and 3.5 atm of argon as a rare gas, the distance between the tip surface 32 of the anode 3 and the tip surface of the cathode 4 was 9 mm, and the lamp power was 7.5 kW.

[0053] Fig. 5 shows the results of comparing the illuminance maintenance rate with the lighting time, with the vertical axis representing the illuminance maintenance rate when the illuminance at the beginning of lighting is set to 100%, and the horizontal axis representing the lighting time. Fig. 6 shows the illuminance maintenance rate after 800 hours of lighting, showing the illuminance maintenance rate of each discharge lamp when the illuminance maintenance rate of the discharge lamp of Comparative Example 1 is set to 1. As shown in Figs. 5 and 6, Examples 1 and 2 and Comparative Example 2, in which the recesses 7 and 70 are formed, were able to maintain the illuminance better than Comparative Example 1, in which the recesses 7 and 70 are not formed.

[0054] Next, the position of the recess relative to the tip surface 32 of the anode 3 was examined. Specifically, a comparative test was conducted on the initial light intensity when annular recesses each having a width of 1 mm were formed at different positions on the tip surface 32 having a diameter of 12 mm. Mercury was sealed in the arc tube 2 at 2.5 mg / cc and argon as a rare gas at 3.5 atm. The distance between the tip surface 32 of the anode 3 and the tip surface of the cathode 4 was 9 mm, and the lamp power was 7.5 kW.

[0055] 7 shows the relationship between the recess position and the initial light intensity. Here, the recess position (mm) on the horizontal axis is the diameter of the circle located at the width center of the annular recess. For example, a recess position of 1 mm indicates that the recess is located at the position shown in FIG. 4A, and a recess position of 3 mm indicates that the recess is located at the position shown in FIG. 4B.

[0056] 7, when the recessed portion is located at 1 mm, i.e., when the recessed portion 70 is also located at the center of the distal end surface 32 as in the above-described comparative example 2, the initial light intensity decreases by about 3%. On the other hand, by forming the recessed portion 7 so as to exclude the center of the distal end surface 32 as shown in FIG. 7, the initial illuminance can be ensured.

[0057] 8 shows the relationship between the position of the recess and the illuminance maintenance rate after 800 hours of lighting. The vertical axis shows the illuminance maintenance rate, where the illuminance maintenance rate of Comparative Example 1, in which no recess is formed, is set to 1, and the horizontal axis shows the position of the recess (mm) (i.e., the distance from the center of the tip surface 32 to the center of the recess). The diameter of the tip surface 32 was 12 mm, and the width of the recess was 1 mm. The lamp power was 7.5 kW.

[0058] Similar to FIG. 8 , FIG. 9 shows the relationship between the position of the recess and the illuminance maintenance rate after 800 hours of lighting, with the vertical axis showing the illuminance maintenance rate, where the illuminance maintenance rate of Comparative Example 1, in which no recess is formed, is taken as 1, and the horizontal axis showing the position of the recess as a ratio to the diameter of the tip surface 32.

[0059] Fig. 10 is a graph corresponding to Fig. 9 when the diameter of the tip surface 32 is 7 mm and the width of the recess is 0.5 mm. The lamp power was 3.5 kW.

[0060] As shown in FIGS. 8 and 9 , when a recess is formed at a position (for example, 10 mm) away from the center of tip face 32, that is, when a recess is formed outside a concentric circle c1 having a diameter that is 70% of the diameter d of tip face 32 (corresponding to the case where the groove is positioned at 9 to 11 mm when tip face 32 has a diameter of 12 mm in FIG. 11 described later), the outer periphery of tip face 32 is a location where the arc discharge is weak (a location where the temperature is low), and therefore the recess does not have much effect in improving the illuminance maintenance rate.

[0061] Similarly, as shown in FIG. 10 , when a recess is formed at a position away from the center of tip face 32 (for example, 5.5 mm: the rightmost data in FIG. 10 ), that is, when a recess is formed outside a concentric circle c1 having a diameter that is 70% of the diameter d of tip face 32 (corresponding to the case where the groove is positioned 5 to 6 mm when tip face 32 has a diameter of 7 mm in FIG. 11 described later), the outer periphery of tip face 32 is a location where the arc discharge is weak (a location where the temperature is low), and therefore the recess does not have much effect in improving the illuminance maintenance rate.

[0062] Next, the position of the recess relative to the tip surface 32 of the anode 3 was examined for each of three discharge lamps: one with a diameter d of the tip surface 32 of 7 mm, a width of the recess of 0.5 mm, and a lamp power of 3.5 kW; one with a diameter d of the tip surface 32 of 9 mm, a width of the recess of 1 mm, and a lamp power of 5 kW; and one with a diameter d of the tip surface 32 of 12 mm, a width of the recess of 1 mm, and a lamp power of 7.5 kW. Specifically, the illuminance maintenance rate was evaluated by changing the position of the recess in each discharge lamp. The results are shown in Fig. 11.

[0063] In Figure 11, the vertical axis indicates the position of the recess (mm), and the horizontal axis indicates the diameter (mm) of the tip surface 32. Here, the position of the recess on the vertical axis is plotted for both the inner diameter and the outer diameter of the annular recess. For example, if the diameter d of the tip surface 32 is 12 mm and an annular recess with a width of 1 mm is formed on a circumference with a diameter of 5 mm, the position of the recess is set to 4 to 6 mm.

[0064] The evaluation of the illuminance maintenance rate is shown in FIG. 11, where discharge lamps that achieved an improvement of 5% or more compared to discharge lamps that do not have a recess formed on the tip end surface 32 are marked with "Good", discharge lamps that achieved an improvement of 2% or more are marked with "Average", and discharge lamps that achieved an improvement of less than 2% are marked with "Poor".

[0065] As shown in FIG. 11 , when the recess was formed inside a concentric circle c1 having a diameter that was 70% of the diameter d of the tip surface 32, an improvement of 2% or more was obtained, and when the recess was formed inside a concentric circle having a diameter that was 50% of the diameter d of the tip surface 32, an improvement of 5% or more was obtained.

[0066] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configurations should not be considered to be limited to these embodiments. The scope of the present invention is defined not only by the description of the above embodiments but also by the claims, and further includes all modifications within the meaning and scope of the claims.

[0067] The structures employed in the above-described embodiments can be employed in any other embodiment. The specific configurations of the components are not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention.

[0068] [Other Embodiments] (1) In the discharge lamp 1 according to the above embodiment, the recesses 7 are configured to be formed continuously in a circumferential shape surrounding the center of the tip surface 32. However, the discharge lamp 1 is not limited to this configuration. For example, as shown in Fig. 12, the recesses 7 may be configured to be formed intermittently in a circumferential shape surrounding the center of the tip surface 32. With this configuration, processing can be performed using a simple processing device (such as a drill press) rather than a processing device for forming continuous grooves (such as a milling machine), and the recesses 7 can be easily formed in the tip surface 32.

[0069] (2) In the discharge lamp 1 according to the above embodiment, the recess 7 is formed in a circular ring shape centered on the center of the tip surface 32. However, the discharge lamp 1 is not limited to this configuration. For example, the recess 7 may be formed in a rectangular frame shape centered on the center of the tip surface 32, as shown in Fig. 13. With this configuration, the recess 7 is formed at positions over a wide range of distances from the center of the tip surface 32, so that the precision in the position of the recess 7 is less required than with the circular recess 7 (Figs. 3, 4B, 4C, and 12), and variations in yield and lamp performance can be suppressed.

[0070] (3) The recesses 7 do not have to be arranged in a circular pattern around the center of the tip surface 32, as shown in Fig. 14. This configuration allows processing using a simple processing device and reduces variations in yield and lamp performance. Although not shown, the recesses 7 may also be arranged asymmetrically, for example, in a spiral pattern.

[0071] (4) In the discharge lamp 1 according to the above embodiment, the anode 3 has a generally cylindrical shape with a chamfered outer periphery 31 at the tip 3 a facing the cathode 4. However, the discharge lamp 1 is not limited to this configuration. For example, as shown in Fig. 15 , the anode 3 may have a generally cylindrical shape with rounded corners at the tip 3 a facing the cathode 4. Note that when the outer periphery 31 is generally cylindrical with rounded corners as shown in Fig. 15 , the diameter d of the tip surface 32 does not include the rounded corners of the outer periphery 31.

[0072] (5) In addition, in the discharge lamp 1 according to the above embodiment, the tip surface 32 is configured to be disposed on a single plane perpendicular to the cylindrical axis X1. That is, the tip surface 32 is located on the same plane inside and outside the recess 7. However, the discharge lamp 1 is not limited to this configuration. For example, as shown in Fig. 16 , the tip surface 32 may be configured to include an inner tip surface 32a located inside the recess 7 and an outer tip surface 32b located outside the recess 7, with the inner tip surface 32a being closer to the cathode 4 than the outer tip surface 32b.

[0073] (6) When viewed from the cylindrical axial direction, the area of ​​the recess 7 is preferably 2% to 25% of the area of ​​the tip face 32. Fig. 17 is a diagram showing the area ratio of the recess 7 to the area of ​​the tip face 32 for discharge lamps in which the diameter d of the tip face 32 is 12 mm, the lamp power is 7.5 kW, and the width of the recess 7 is changed. When the illuminance maintenance rate was evaluated under the conditions shown in Fig. 17, an improvement of 2% or more was obtained under all conditions compared to a discharge lamp in which no recess 7 was formed on the tip face 32.

[0074] (7) In the discharge lamp 1 according to the above embodiment, the entire recess 7 is formed inside the concentric circle c1 having a diameter that is 70% of the diameter d of the tip surface 32, excluding the center of the tip surface 32. However, the discharge lamp 1 is not limited to this configuration. The recess 7 may be formed such that a portion of it is outside the concentric circle c1 having a diameter that is 70% of the diameter d of the tip surface 32.

[0075] REFERENCE SIGNS LIST 1: Discharge lamp 2: Arc tube 3: Anode 3a: Tip 4: Cathode 5: Lead rod 6: Base 7: Recess 7a: Maximum depth 20: Bulging portion 21: Sealed tube portion 31: Outer periphery 32: Tip surface 32a: Inner tip surface 32b: Outer tip surface 70: Recess S1: Light-emitting space X1: Cylindrical axis c1: Concentric circle c2: Concentric circle

Claims

1. A discharge lamp comprising: an arc tube in which a luminous substance is sealed; and an anode and a cathode arranged opposite each other inside the arc tube; wherein the anode is approximately cylindrical with a chamfered or rounded outer periphery at the tip facing the cathode; and the anode has a circular tip surface inside the chamfered or rounded outer periphery when viewed from the axial direction of the cylinder; and the tip surface has a recess, and the recess is formed inside a concentric circle having a diameter that is at least 70% of the diameter of the tip surface, but is not formed in the center of the tip surface.

2. The discharge lamp according to claim 1, wherein said recess is formed inside a concentric circle having a diameter that is at least 50% of the diameter of said tip surface.

3. A discharge lamp according to claim 1 or 2, wherein the recess is formed on the outside of a concentric circle having a diameter that is 20% of the diameter of the tip surface.

4. A discharge lamp according to claim 1 or 2, wherein the recess is formed in a circular shape surrounding the center of the tip surface.

5. A discharge lamp according to claim 1 or 2, wherein the recess is formed in an annular shape centered on the center of the tip surface.

6. A discharge lamp according to claim 1 or 2, wherein the luminescent material is mercury and the amount of mercury filled is less than 20 mg / cc.

7. A discharge lamp according to claim 1 or 2, wherein the maximum depth of said recess is 0.5 mm or more.

8. A discharge lamp according to claim 1 or 2, wherein the area of ​​the recess is 2% to 25% of the area of ​​the tip surface when viewed from the axial direction of the cylinder.

9. A discharge lamp according to claim 1 or 2, wherein the tip surface comprises an inner tip surface located inside the recess and an outer tip surface located outside the recess, and the inner tip surface is closer to the cathode than the outer tip surface.

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