Discharge lamp

A discharge lamp includes an airtight tube including a light-emitting unit in which a space is formed and seal portions formed at least on one end of the light-emitting unit, a discharge medium including a metal halide and a rare gas sealed in the light-emitting unit, a metal foil sealed into the seal portion, and a pair of electrodes one ends of which are overlapped and connected to the metal foil and the other ends of which are provided such that they are led into the space of the light-emitting unit and arranged in opposition to each other. A concavity is formed on at least a portion of the back surface side of the metal foil on which the electrode is overlapped, and a compression distortion is formed on the seal portion in the vicinity of the concavity.

TECHNICAL FIELD

The present invention relates to a discharge lamp used for vehicle headlights, projectors and the like.

BACKGROUND ART

The discharge lamp used for vehicle headlights is known from JP-A 2007-87683 (KOKAI) (Patent Reference 1) and WO 2007/086527 A1 (Patent Reference 2), and it has a discharge medium which is comprised of a metal halide of sodium, scandium, zinc or the like and a rare gas such as xenon, sealed into a discharge space of an airtight tube with both ends of which are sealed and generates a predetermined light by applying a voltage to electrodes connected to metal foils which are attached to sealing portions by sealing to excite the discharge medium.

But, this type of discharge lamp has a problem that the metal halide sealed in the discharge space reaches to the metal foil through a small gap between the electrode axis and the glass to exfoliate the glass and the metal foil configuring a seal portion, resulting in easily causing crack leak (hereinafter called as foil leak). This problem becomes more conspicuous when a coil is wound around the electrode axis.

According to Patent Reference 1, a hole is formed in the metal foil, and the glass which configures the seal portion is entered into the hole to improve the adhesiveness between the metal foil and the seal portion, thereby suppressing occurrence of foil leak. According to Patent Reference 2, the metal foil surface is fabricated to have an irregular shape by laser to improve the adhesiveness with the seal portion, thereby suppressing the occurrence of foil leak.Patent Reference 1: JP-A 2007-87683 (KOKAI)Patent Reference 2: WO 2007/086527 A1

DISCLOSURE OF INVENTION

Technical Problem

But, the above-described measures against the foil leak cannot meet a demand for a longer life of the discharge lamp, and additional improvement is necessary.

The object of the present invention is to provide a discharge lamp capable of suppressing the occurrence of foil leak.

Technical Solution

According to an aspect of the present invention, there is provided a discharge lamp, comprising an airtight tube including a light-emitting unit having a space formed therein and a seal portion formed on at least one end of the light-emitting unit; a discharge medium containing a metal halide and a rare gas sealed in the light-emitting unit; a metal foil sealed into the seal portion; and a pair of electrodes, the electrode having one end being overlapped and connected to the metal foil and the other end being led into the space of the light-emitting unit so as to be arranged to face each other, wherein a concavity is formed on at least a portion of the back surface side of the metal foil on which the electrode is overlapped, and a compression distortion is formed on the seal portion in the vicinity of the concavity.

Advantageous Effect

The invention can adequately suppress the occurrence of foil leak.

MODE FOR CARRYING OUT THE INVENTION

First Embodiment

A metal halide lamp according to one embodiment of the discharge lamp of the invention is described below with reference to the drawings.FIG. 1is a side view illustrating a first embodiment of the metal halide lamp according to the invention, andFIG. 2is a top view illustrating the first embodiment of the metal halide lamp according to the invention.

The metal halide lamp has an airtight tube1as a main portion. The airtight tube1has an elongate shape in a lamp tube axis direction with an almost elliptical light-emitting unit11formed at its approximate center. Seal portions12aand12bwhich are pinch sealed into a plate-like shape are formed at both ends of the light-emitting unit11. The airtight tube1is desirably made of, for example, a material such as quartz glass having heat resistance and translucency.

A discharge space14which has an almost cylindrical shape at the center and its both ends tapered is formed within the light-emitting unit11. The discharge space14has preferably a volume of 10 mm3to 40 mm3when it is used for vehicle headlights.

A discharge medium comprising a metal halide2and a rare gas is sealed in the discharge space14.

The metal halide2is constituted by sodium iodide (NaI), scandium iodide (ScI3), zinc iodide (ZnI2) and indium bromide (InBr). But, the metal halide2is not limited to the above combination. It may be constituted by adding halides of tin and/or potassium, or changing the combination of halogens to be bonded to the metal.

As the rare gas, xenon which has high luminous efficiency just after the startup and functions mainly as a starting gas is sealed. The xenon has a pressure of not less than 5 atm at normal temperature (25° C.) and desirably 10 to 20 atm when its use is designated for vehicle headlights. As the rare gas, neon, argon, and krypton can be used in addition to the xenon, and they can also be used in combination.

The discharge space14does not substantially contain mercury. This “does not substantially contain mercury” means that it is optimum to contain no mercury but it is allowed to contain mercury in an amount equivalent to substantially no enclosure in comparison with a conventional mercury-containing metal halide lamp, e.g. less than 2 mg, or preferably not more than 1 mg of mercury per 1 mL.

Electrode mounts3are sealed in the seal portions12aand12b. The electrode mount3comprises a metal foil31, an electrode32, a coil33and a lead wire34.

For example, the metal foil31is a thin metal plate made of molybdenum, and a worked portion311is formed on its front and back surfaces on the side of the light-emitting unit11. The worked portion311has plural hemispherical recesses arranged by the laser irradiation (for details, see WO 2007/086527 A1). Diffusion of the metal halide2to the ends of the metal foil31in its width direction is delayed by the worked portion311, so that the foil leak is suppressed.

The electrode32is a thoriated tungsten electrode which has thorium oxide doped to tungsten. A diameter R of the electrode32can be determined to be, for example, not less than 0.30 mm but not more than 0.40 mm in practical use. One end of the electrode32is connected to the metal foil31on the side of the light-emitting unit11, and the other end is arranged within the discharge space14to face the opposed end of the other electrode32with a prescribed interelectrode distance between them. For the vehicle headlights, the prescribed interelectrode distance is desirably about 4.2 mm in appearance, namely it is not an actual distance but an appearance distance in the lamp.

The electrode is not limited to the straight rod shape as in this embodiment but may have a non-straight rod shape having a large diameter at a leading end or a shape having a different size between a pair of electrodes of a direct current lighting type. And, the electrode32may be a doped tungsten electrode or a rhenium-tungsten electrode.

The coil33is made of, for example, doped tungsten and wound in a spiral shape around the shaft portion of the electrode32which is sealed in the seal portions12aand12b. But, the coil33is not wound around the shaft portion of the electrode32connected to the metal foil31. To design the coil33, the coil pitch is not more than 300%, and the coil-wound length is desirably not less than 60% with respect to the electrode sealing length.

For example, the lead wire34is made of molybdenum and its one end is connected to the metal foil31. On the other hand, the other end of the lead wire34is extended to the exterior of the airtight tube1along the tube axis. And, one end of an L-shape support wire35made of nickel is connected to the lead wire34which is extended toward the front end of the lamp. The other end of the support wire35is extended toward a socket6described later, and the support wire35parallel to the tube axis is covered by a sleeve4made of ceramics.

A cylindrical outer tube5is disposed concentrically with the above-configured airtight tube1along the tube axis to cover the exterior of the airtight tube1. They are connected by melting both ends of the airtight tube1and the outer tube5. And, for example, one or a mixture of nitrogen and a rare gas such as neon, argon, xenon or the like can be sealed into the space between the airtight tube1and the outer tube5. The outer tube5is desirably provided with an ultraviolet shielding characteristic by adding an oxide of titanium, cerium, aluminum or the like to a quartz glass.

The socket6is connected to one end of the airtight tube1to which the outer tube5is connected. They are connected by attaching a metal band71to the outer circumferential surface of the outer tube5which is arranged close to the socket6and pinching the metal band71with metal tongue-shaped pieces72which are formed at an open end of the socket6on the airtight tube1holding side. And, the socket6has a bottom terminal8aon its bottom and a side terminal8bon its side, and they are respectively connected with the lead wire34and the support wire35.

The above-configured metal halide lamp is lit by connecting a lighting circuit to the bottom terminal8aand the side terminal8b. This lamp for vehicle headlights is arranged with the tube axis in a substantially horizontal state and lit with electric power of about 35 W at a stable time and about 75 W at the time of start up which is not less than two times in comparison with the power of the stable time.

A bonded portion and its vicinity between the metal foil31and the electrode32are described in detail with reference toFIG. 3andFIG. 4.FIG. 3is a diagram illustrating a cross section along the tube axis direction of the bonded portion and its vicinity between the metal foil and the electrode, andFIG. 4is a diagram illustrating a cross section perpendicular to the tube axis direction of the bonded portion and its vicinity between the metal foil and the electrode.

It is apparent fromFIGS. 3 and 4that the metal foil31and the electrode32are connected by forming melted portions36at portions (overlapped portions) where they are partly lapped over mutually. The melted portions36are huge metal crystals which are formed by so called laser welding which irradiates the electrode32from the back side of the metal foil31with laser beam by using a YAG laser or the like.

Concavities312are formed on the back side of the metal foil31, namely the melted portions36, at the overlapped portion of the electrode32and the metal foil31. As a result, compression distortions9are formed in the vicinity of the concavities312of the seal portions12aand12b. The compression distortions9suppress the seal portions12aand12band the metal foil31from exfoliating, and foil leak is suppressed.

In this case, it is desirable that an overlapped length (length of the overlapped portions) L1of the metal foil31and the electrode32and a tube-axis-direction length L2(a sum of respective lengths L2′ when plural concavities312are formed) of the concavity312satisfy 0.2≦L2/L1. Thus, the above-described action and effect become high, and the effect of suppressing the foil leak is improved.

In this embodiment, two positions are undergone the laser welding, so that the concavity312is formed at two positions. Therefore, the compression distortion9is also formed at two positions in accordance with the concavities312. Therefore, a tensile stress resulting from tensile distortion caused in the vicinity of the compression distortions9is dispersed, and occurrence of a crack or the like due to the tensile stress can be suppressed. To form the plural compression distortions9as described above, it is adequate to form the individual concavities312separately from one another so that they do not overlap.

Here, the concavities312are formed by a process of sealing the electrode mount3into the seal portions12aand12b. It is confirmed that if the concavities312are relatively large and have a depth, they can be formed easily.

Specifically, when the electrode32has a diameter R of 0.30 mm or more and 0.40 mm or less and the concavities312are approximately circular-shaped recesses (indicating the inclusion of an elliptical shape which is a substantially perfect circule) as in this embodiment and have a depth d of 0.01 mm or more and a length L3of 0.1 mm or more (preferably, d≧0.05 mm and L3≧0.2 mm), the compression distortion9tends to remain in the seal portions12aand12b. The cause is considered that the formation of the compression distortion9is related to the flow of the glass into the concavities321at the time of sealing.

The upper limit of the depth d is limited by the thickness of the metal foil31, and the upper limit of the width L3is limited by the length L1of the overlapped portion.

The formation of the compression distortion9is somewhat influenced by the thickness of the seal portions12aand12band a seal pressure. Incidentally, the compression distortion9is not formed by the recesses which are based on the melted portion having substantially no depth as in JP-A 2006-196267 (KOKAI). And, in this embodiment, the worked portion311which is configured to have the arrangement of the hemispherical recesses on the front and back surfaces of the metal foil31is formed, but the compression distortion9is not formed on the seal portions12aand12bwhen the recesses have a size and depth of the level described above.

Examples

FIG. 5is a diagram illustrating an example of the metal halide lamp ofFIG. 1. The following test is performed with the size and materials according to the same specifications unless otherwise specified.

Electric discharge tube1: Made of quartz glass, discharge space14has an inner volume of 27.5 mm3, inner diameter A of 2.5 mm, outer diameter B of 6.2 mm, and sphere length C in longitudinal direction of 7.8 mm,

Compression distortion9: Remained in seal portions12aand12bin the vicinity of the concavity312, tube-axis-direction length L2(≈L2′×2)=0.6 mm, compression stress=50 kg/cm2.

The lamp of this example can suppress the occurrence of foil leak up to about 3000 hours, and a long-life metal halide lamp could be realized. The cause is considered that the compression distortion9is formed on the seal portions12aand12bin the vicinity of the concavities312formed when the metal foil31and the electrode32are welded, the adhesiveness to the glass is enhanced on the back surface side of the overlapped portion of the metal foil31, and it became difficult to exfoliate the seal portions12aand12band the metal foil31.

Then, the depth d and the tube-axis-direction length L2of the concavity312were changed, and the generation time of foil leak was tested while an intensity of compression stress was varied. The results are shown inFIG. 6. The test condition is a flash on and off cycle on EU120-minute mode specified in JEL215 which is a standard of HID light sources for vehicle headlights. Thirty lamps were tested, and the foil leak generation time means time when one of the thirty lamps had first foil leak. And, the types and stress values of the compression distortions9were checked according to a sensitive color plate method (a method of discriminating a state of distortion caused in the glass by an optical path difference of light).

It is apparent from the results that there is a tendency that the foil leak generation time becomes long as the compression stress is larger, particularly 10 kg/cm2or more, and when the compression stress is 10 kg/cm2or more, a high effect against the foil leak can be obtained. This tendency does not change even if the pitch of the coil33is changed in order to change the ease of entry of the metal halide2. Therefore, it is adequate to form such that a stress value of the compression distortion9becomes 10 kg/cm2or more. But, if the compression stress is excessively large, the tensile stress also increases, so that a crack is caused starting from the boundary between the compression stress and the tensile stress, possibly resulting in leakage. Therefore, the compression stress is desirably not more than 300 kg/cm2.

When a relationship L2/L1between the overlapped length L1between the metal foil31and the electrode32and the tube-axis-direction length L2of the concavity312meets 0.2≦L2/L1, and desirably 0.5≦L2/L1, high adhesiveness can be normally maintained over a wide range on the overlapped portion where the adhesiveness to the glass tends to become low, so that it is also effective against the foil leak.

Therefore, in this example, the concavities312are formed on the back surface of the metal foil31on which the electrode32is lapped, and the compression distortion9is formed in the seal portions12aand12bin the vicinity of the concavities312. Thus, the seal portions11aand12band the metal foil31become difficult to exfoliate, and the occurrence of foil leak can be suppressed.

And, a high effect against the foil leak can be obtained by determining the compression stress generated in the seal portions12aand12bto be 1.0 kg/cm2or more. In addition, when it is determined that the overlapped length between the metal foil31and the electrode32is L1and the tube-axis-direction length of the compression distortion9is L2and 0.2≦L2/L1is satisfied, high adhesiveness can be maintained over a wide range on the overlapped portion where adhesiveness tends to become low, and a high effect can be obtained against the foil leak.

When it is determined that the concavity312is an approximately circular shaped recess, has a depth d and a length L3and satisfies d≧0.01 mm and L3≧0.1 mm, the compression distortion9can be easily formed in the seal portions12aand12b, and the compression stress can also be enhanced.

Second Embodiment

FIG. 7is a diagram illustrating the metal halide lamp according to a second embodiment of the invention. In the second embodiment and following, like or equivalent component parts corresponding to those of the metal halide lamp of the first embodiment described above are denoted by like reference numerals, and their descriptions will be omitted.

In the second embodiment, the metal foil31and the electrode32are mutually connected by resistance welding to form the concavity312which is long in the tube axis direction on the back side of the overlapped portion between the electrode32and the metal foil31, thereby forming the compression distortion9on the seal portions12aand12bin the vicinity of the concavity312. In this case, when it is determined that the concavity312has a depth d and a diameter L3and satisfies d≧0.005 mm and L3≧0.2 mm (preferably, d≧0.01 mm and L3≧0.4 mm), the compression distortion9long in the tube axis direction tends to be formed. By forming the above large concavity312, a large and strong compression distortion9can be formed, and an effect of suppressing foil leak becomes high.

Therefore, this embodiment can suppress the occurrence of foil leak similar to the first embodiment.

Although the present invention has been described in detail above by reference to the specific embodiment of the invention, the invention is not limited to the embodiment described above. It is to be understood that modifications and variations of the embodiment can be made without departing from the spirit and scope of the invention.

For example, the concavity312is formed on the overlapped portion between the metal foil31and the electrode32by the laser welding in the first embodiment and by the resistance welding in the second embodiment. But, the concavity312may be formed separately by mechanical means after the metal foil31and the electrode32are connected by performing a connection method, which does not form the concavity312on the overlapped portion between the metal foil31and the electrode32, such as the laser welding method described in, for example, JP-A 2000-288755 (KOKAI).