High pressure discharge lamp

A high pressure discharge lamp includes an outer tubular bulb whose inner space is exhausted to a vacuum; an arc tube contained in the outer tubular bulb; and a voltage reducing switching element for reducing a voltage applied to the arc tube. The voltage reducing switching element includes a heat sensitive switching element and a starting-voltage reducing element which are connected in series. During a period in which the heat sensitive switching element has a predetermined temperature or higher, the heat sensitive switching element is closed to electrically connect the arc tube and the voltage reducing switching element in parallel, so that the starting-voltage reducing element is operated. During a period in which the heat sensitive switching element has a temperature lower than the predetermined temperature, the heat sensitive switching element is opened so that the starting-voltage reducing element is not operated.

TECHNICAL FIELD

The present invention relates to a high pressure discharge lamp, such as a metal halide lamp.

BACKGROUND ART

Recently, metal halide lamps have been actively developed and promoted, using a tube made of translucent polycrystalline alumina ceramic (PCA) in place of quartz as an arc tube material. Although quartz has a heat resistance up to about 1,000° C., the PCA tube has a higher heat resistance up to about 1,200° C., thereby allowing the tube wall load to be set in a higher range. Therefore, the use of the PCA tube makes it possible to provide a metal halide lamp with a higher lamp efficiency. At first, metal halide lamps (ceramic metal halide lamps) using the PCA tube were developed and commercialized for indoor lighting applications, such as in shops, which require a high color rendering. Recently, high efficiency metal halide lamps have been under development for general outdoor lighting applications.

With respect to high efficiency metal halide lamps for outdoor lighting applications using a conventional quartz arc tube, there has been a proposal to use, as light emitting substances, a combination of sodium halide (NaX) and a halide of a lanthanoid rare-earth metal, such as cerium halide (CeX3) and praseodymium halide (PrX3), which emit light whose spectrum is in a wavelength region of high spectral luminous efficiency (see Japanese Laid-Open Patent Publication No. Sho 57-92747, for example).

Meanwhile, with regard to a recently developed ceramic metal halide lamp, there has been a proposal to use cerium iodide (CeI3) and sodium iodide (NaI) as light emitting substances. This metal halide lamp has a high efficiency, for example, 115 lm/W with a 300 W type. It also has a sufficient color rendering for outdoor lighting, with a color rendering index Ra of 70 and a rated life time of 12,000 hours.

Also, a conventional metal halide lamp is structured such that its arc tube is contained in an outer tubular bulb of, for example, a drop shape, and that an inert gas such as nitrogen (N2) is sealed in the outer tubular bulb. The arc tube is held by stem leads and the like in the outer tubular bulb.

A higher efficiency ceramic metal halide lamp using the above-mentioned light emitting substances (CeI3, NaI) has also been proposed (see Japanese Unexamined Patent Publication No. 2000-501563, for example). In this metal halide lamp, the tubular shape parameter (the ratio of the electrode interspacing Le to the internal diameter φi of the central tubular part) is greater than 5, and the arc tube is relatively long and narrow. Also, the molar ratio of the light emitting substances: NaI/CeI3is 3 to 25, and the tube wall load (we) is set to 30 W/cm2or lower. This construction provides a high lamp efficiency of 130 lm/W with a 150 W type, and a color rendering of Ra 53, for example.

DISCLOSURE OF INVENTION

Problem that the Invention is to Solve

In a conventional metal halide lamp, an inert gas is sealed in the space inside the outer tubular bulb, as described above. However, if the PCA tube is held in the outer tubular bulb with the inert gas sealed therein, heat conduction loss from the outer wall of the arc tube increases, and the resultant decrease in the tube wall temperature (Tw) causes a reduction in the vapor pressure of the light emitting substance inside the tube. As a result, it becomes difficult to achieve the object of enhancing lamp efficiency. In an attempt to address this problem, the present inventors changed the pressure of the inert gas sealed in the outer tubular bulb from the commonly used pressure of about 30 kPa to a vacuum (e.g., 1 Pa or lower).

However, they found that in the above-mentioned ceramic metal halide lamp, an arc discharge occurs in the outer tubular bulb during a lamp life test under certain conditions. For example, in cases the metal halide lamp is operated by means of an electronic ballast and a starting voltage of about 5 kV is applied to the arc tube to start the metal halide lamp, an arc discharge occurs between the stem leads. Occurrence of an arc discharge may cause the outer tubular bulb to break on rare occasion. In order to commercialize ceramic metal halide lamps, prevention of such an arc discharge is desired.

Means for Solving the Problem

The present invention relates to a high pressure discharge lamp including: an outer tubular bulb whose inner space is exhausted to a vacuum; an arc tube contained in the outer tubular bulb; and a voltage reducing switching element for reducing a voltage applied to the arc tube. The voltage reducing switching element comprises a heat sensitive switching element and a starting-voltage reducing element which are connected in series. During a period in which the heat sensitive switching element has a predetermined temperature or higher, the heat sensitive switching element is closed to electrically connect the arc tube and the voltage reducing switching element in parallel, so that the starting-voltage reducing element is operated. During a period in which the heat sensitive switching element has a temperature lower than the predetermined temperature, the heat sensitive switching element is opened so that the starting-voltage reducing element is not operated.

The arc tube can be fixed in the outer tubular bulb by a first lead and a second lead which supply the voltage applied to the arc tube. In this case, the voltage reducing switching element can be fixed to at least one of the first lead and the second lead.

Effects of the Invention

According to the present invention, particularly when a high pressure discharge lamp of high temperature fails during lamp operation, the starting voltage applied to the arc tube can be reduced to a level at which an arc discharge does not occur in the outer tubular bulb. It is therefore possible to prevent occurrence of an arc discharge in the outer tubular bulb in a lamp life test.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring toFIGS. 1 to 7, embodiments of the present invention are described below.

FIG. 1is a side view of an arc tube1of a 150 W metal halide lamp according to one embodiment of the present invention, in which a luminous vessel2is sectionally illustrated in order to show its internal structure.

The luminous vessel2of the arc tube1has a central tubular part3, which forms a discharge space19, and side tubular parts4and5, which extend from opposite ends of the central tubular part3. The central tubular part3and the side tubular parts4and5are sintered integrally. The luminous vessel2is made of, for example, a translucent polycrystalline alumina ceramic (PCA) material.

The side tubular parts4and5contain electrodes6and7, respectively. The electrode6or7comprises an electrode rod8or9made of tungsten, and a coil10or11that is made of tungsten and fixed to one end of the electrode rod8or9. These ends of the electrode rods8and9are positioned in the discharge space.

The other end of the electrode rod8or9is connected to one end of a current supplier12or13made of conductive cermet. The current supplier12or13extends in the hollow of the side tubular part12or13, and the other end of the current supplier12or13is connected to one end of an electrode lead15or16. As the conductive cermet constituting the current suppliers12and13, an Al2O3—Mo type material is used, for example. Also, niobium (Nb) or the like is used for the electrode leads15and16.

The opening ends of the side tubular parts4and5, from which the electrode leads15and16extend, are sealed with glass frit14, so that the discharge space19is sealed gas-tightly. As the glass frit, for example, Dy2O3—Al2O3—SiO2type glass is used. In view of suppressing the erosion of the glass frit14by a light emitting substance17during the operation of the metal halide lamp, it is preferable to fill the glass frit14up to the joint of the current supplier12or13with the electrode rod8or9.

The discharge space19of the arc tube1has the light emitting substance17sealed therein. As the light emitting substance17, metal halide and sodium are used, for example. Although conventional metal halide lamps use cerium iodide (CeI3) and sodium iodide (NaI) as the light emitting substance17, it is preferable in the present invention to use praseodymium iodide (PrI3) and sodium iodide (NaI) as the light emitting substance17. When the former iodides are used, the color of light emitted by the lamp tends to be shifted to the green region. However, the use of the latter iodides makes it possible to improve the color of light emitted by the lamp. Also, the discharge space19has mercury (Hg)18that is sealed therein as a buffer substance. The discharge space19further has xenon (Xe) or argon (Ar) that is sealed therein as a buffer/auxiliary-starting gas.

FIG. 2is a front view of an exemplary 150 W metal halide lamp having the arc tube1ofFIG. 1, in which an outer tubular bulb21is sectionally illustrated in order to show its internal structure.

A metal halide lamp20includes the outer tubular bulb21whose inner space is exhausted to a vacuum, the arc tube1contained in the outer tubular bulb21, and a voltage reducing switching (VRS) element27that reduces the voltage applied to the arc tube1.

The outer tubular bulb21is made of a material such as hard glass. The ends of the outer tubular bulb21are tapered, and a base22is fitted to one end thereof. The lower electrode lead16extending from the side tubular part of the arc tube1is fixed and electrically connected to a stem lead24, which is partially sealed in a stem glass23protruding from the above-mentioned end of the outer tubular bulb21. The upper electrode lead15is fixed and electrically connected to a supporting lead32, which extends from a stem lead25partially sealed in the stem glass23. A getter ring26made of barium (Ba) is sealed in the stem glass23while being insulated from the electrode lead15and the electrode lead16.

In the case of, for example, the 150 W metal halide lamp20using the arc tube made of a PCA material (PCA tube), the arc tube1is preferably long and narrow. Such an example is one in which the distance Le between the electrodes6and7is 32 mm, the internal diameter φi of the central tubular part3is 4 mm, and the tubular shape parameter Le/φi is 8.

Also, in the case of the above-mentioned arc tube1, it is preferred, for example, that the amount of the light emitting substances17(PrI3, NaI) sealed therein be 9 mg, the composition ratio: NaI/PrI3of the light emitting substances17be 10, the amount of the mercury18sealed therein be 0.7 mg, and the xenon pressure be about 25 kPa.

In operating the 150 W metal halide lamp20that has not been started, a high-frequency starting voltage of about 5 kV with a frequency of 80 to 400 kHz, for example, about 100 kHz, is applied for about 50 msec per second (50 msec ON/950 msec OFF) until the metal halide lamp20is started. When the metal halide lamp20is subjected to a life test, an arc discharge occurs in the outer tubular bulb21if the metal halide lamp20has no voltage reducing switching element. In the metal halide lamp20with the structure as illustrated inFIG. 2, an arc discharge occurs particularly between the stem leads24and25that are partially sealed in the stem glass23, which may results in breakage, etc., of the outer tubular bulb21, although on rare occasion.

Such an arc discharge is probably caused by re-application of the starting voltage to the arc tube1of high temperature when the metal halide lamp20fails during operation with the gas pressure inside the outer tubular bulb21being higher than 1 Pa.

In the absence of gas leakage into the outer tubular bulb21, application of the starting voltage to the arc tube1of the failed metal halide lamp20does not result in occurrence of an arc discharge. Also, even in the event of gas leakage into the outer tubular bulb21, if the arc tube1is in a low temperature condition, application of the starting voltage does not induce an arc discharge.

The gas pressure inside the outer tubular bulb21is probably increased by the following reasons: cracking of the arc tube1causes the Xe gas sealed in the arc tube1to leak in the outer tubular bulb21; or, defective sealing of the stem leads24and25or microcracks of the outer tubular bulb21cause outside air to leak in. For example, when the metal halide lamp20fails during operation, the arc tube1becomes cracked due to thermal stress upon cooling, thereby causing leakage of the Xe gas. Also, with the gas pressure inside the outer tubular bulb21being high, the metal halide lamp20can be started when it cools, but it may fail during lamp operation.

It is noted that conventional metal halide lamps in which about 30 kPa nitrogen is sealed in the outer tubular bulb21are free from such an arc discharge. This indicates that if the gas pressure inside the outer tubular bulb21is 30 kPa or higher, application of the starting voltage to the arc tube1of high temperature after failure does not result in occurrence of an arc discharge.

In cases where the gas pressure inside the outer tubular bulb21is in the range of 1 Pa to 30 kPa, an arc discharge occurs in the outer tubular bulb21, because the gas ionization is increased by thermo electrons. This phenomenon is dependent on the so-called Paschen's law. The probability of occurrence of an arc discharge in the outer tubular bulb21becomes particularly high when the gas pressure inside the outer tubular bulb21is 5 kPa to 10 kPa.

Also, only when the metal halide lamp20has high temperature, an arc discharge occurs in the outer tubular bulb21. This is because high-temperature components such as the stem leads24and25release thermo electrons, thereby causing an increase in electron concentration in an early stage and therefore an increase in ionization frequency.

In the arc tube1of the metal halide lamp20of high temperature after failure, the vapor pressure in the discharge space19is high. Thus, even if a starting voltage of 5 kV level is applied thereto, it is difficult to restart it. In order to efficiently restart it, a cooling time of 5 to 10 minutes is necessary for lowering the vapor pressure. Accordingly, during the period the metal halide lamp20of high temperature after failure is not operated, it is desirable to reduce the high-frequency starting voltage to a level at which an arc discharge does not occur.

The voltage reducing switching (VRS) element27according to the present invention performs the function of reducing the high-frequency starting voltage applied to the arc tube1of high temperature down to a level at which an arc discharge does not occur. The operations of the metal halide lamp20equipped with the VRS element27are detailed below.

FIG. 3shows a circuit diagram including the VRS element27and the arc tube1. The VRS element27comprises a heat sensitive switching element28and a starting-voltage reducing element29that are connected in series. The VRS element27is designed such that when the heat sensitive switching element28has a predetermined temperature or higher, the heat sensitive switching element28closes to electrically connect the arc tube1and the voltage reducing switching element29in parallel. As a result, the starting-voltage reducing element29is operated, so that the high-frequency starting voltage applied to the arc tube1is reduced to a level at which an arc discharge does not occur.

On the other hand, if the heat sensitive switching element28has a temperature lower than the predetermined temperature, the heat sensitive switching element28is designed to open. As a result, the starting-voltage reducing element29is not operated, so that the high-frequency starting voltage is applied to the arc tube1without being reduced.

The heat sensitive switching element28may be, for example, a bimetal element composed of two kinds of metal plates with different thermal expansion rates that are bonded together, or a temperature sensitive lead switch that is turned on at a predetermined temperature or higher. Also, the starting-voltage reducing element29may be, for example, a capacitor such as a ceramic capacitor, or a surge absorber element.

The temperature sensitive lead switch comprises a lead switch, a magnet that supplies a magnetic flux to the lead switch, and temperature sensitive ferrite jointed to the magnet. The saturated magnetic flux density of the temperature sensitive ferrite decreases sharply around the Curie temperature. The temperature sensitive ferrite performs the function of controlling the amount of magnetic flux supplied from the magnet to the lead switch by temperature, and the ON/OFF of the lead switch is controlled by change in the amount of magnetic flux.

FIG. 4is an enlarged view of the main part of the metal halide lamp20ofFIG. 2, in which the heat sensitive switching element28is a bimetal element28aand the starting-voltage reducing element29is a ceramic capacitor29a. The bimetal element28aand the ceramic capacitor29aare connected in series via a connection lead27c. One terminal of the ceramic capacitor29ais connected to the stem lead24via a connection lead27a, and the joint of the connection lead27aand the stem lead24constitutes a fixed terminal A. InFIG. 4, the fixed terminal A is positioned on a part of the stem lead24near the stem glass23, but the position of the fixed terminal A is not to be limited thereto.

Meanwhile, one end of the bimetal element28afunctions as a movable terminal B. For example, in a low temperature condition of less than 100° C., the movable terminal B of the bimetal element28ais positioned away from the stem lead25, with a gap formed between the movable terminal B and the stem lead25.FIG. 5(a) shows a circuit diagram of the VRS element27in such a state. If the temperature of the bimetal element28arises to 100° C. or higher, deformation of the bimetal element28abrings the movable terminal B into contact with the stem lead25, so that the movable terminal B is electrically connected to the stem lead25.FIG. 5(b) shows a circuit diagram of the VRS element27in such a state. Of course, the connecting positions of the bimetal element28aand the ceramic capacitor29amay be reversed such that the fixed terminal A is provided on the stem lead25and the movable terminal B is provided on the stem lead24.

The capacity of the ceramic capacitor29amay be set such that the high-frequency starting voltage can be reduced from the normal value down to a level at which an arc discharge does not occur in the outer tubular bulb21when the movable terminal B is connected to the stem lead25. It is preferred that the capacity of the ceramic capacitor29abe, for example, 100 to 5,000 pF. If the capacity is less than 100 pF, the starting voltage may not be sufficiently reduced. Also, if the capacity exceeds 5,000 pF, the metal halide lamp20may exhibit a malfunction, such as flicker, during lamp operation.

From the viewpoint of preventing the ceramic capacitor29afrom being heated by the radiant heat from the arc tube1during lamp operation in the above-described construction, it is preferred, for example, to attach a shielding plate30to the stem lead24. Ceramic is suited as the material of the shielding plate30.

The operations from the failure of the metal halide lamp20to the restart of the metal halide lamp20by the VRS element27are as follows. First, the VRS element27of the metal halide lamp20that failed during the steady-state operation is in a high temperature condition where the bimetal temperature (Tb) is, for example, about 400° C., so that the movable terminal B is connected to the stem lead25. Thus, due to the function of the ceramic capacitor29aof the VRS element27connected to the arc tube1in parallel, the high-frequency starting voltage is reduced. It is therefore possible to reliably prevent an arc discharge from occurring in the outer tubular bulb21even if the gas pressure in the outer tubular bulb21has risen to a level that induces an arc discharge in a life test of the metal halide lamp20.

Then, after the lapse of a predetermined cooling time, the temperature (Tb) of the bimetal element28alowers to, for example, about 100° C., so the movable terminal B moves to a position away from the stem lead25. As a result, the normal high-frequency starting voltage is applied to the arc tube1without being reduced, so that the metal halide lamp20can be promptly restarted.

FIG. 6is an enlarged view of the main part of the metal halide lamp20ofFIG. 2, in which the heat sensitive switching element28is a temperature sensitive lead switch28band the starting-voltage reducing element29is a surge absorber element29b. The temperature sensitive lead switch28band the surge absorber element29bare connected in series via a connection lead27c′. In the same manner as inFIG. 4, one terminal of the surge absorber element29bis connected to the stem lead24via the connection lead27a, and the joint of the connection lead27aand the stem lead24constitutes a fixed terminal A. InFIG. 6, the fixed terminal A is also positioned on a part of the stem lead24near the stem glass23, but the position of the fixed terminal A is not to be limited thereto.

One terminal of the temperature sensitive lead switch28bis connected to the stem lead25via a connection lead27b, and the joint of the connection lead27band the stem lead25constitutes a fixed terminal C. That is, geometrically a VRS element27′ is always connected in parallel to the arc tube1, but the electric connection between the surge absorber element29band the stem lead25is dependent on the behavior of the temperature sensitive lead switch28b.

For example, under a low temperature condition of less than 100° C., the temperature sensitive lead switch28bis turned off, i.e., the electrical connection between the surge absorber element29band the stem lead25is intercepted (OFF).FIG. 7(a) shows a circuit diagram of the VRS element27′ in such a state. If the temperature of the temperature sensitive lead switch28brises to 100° C. or higher, the temperature sensitive lead switch28bis turned on, so that the electrical connection between the surge absorber element29band the stem lead25is established (ON).FIG. 7(b) shows a circuit diagram of the VRS element27′ in such a state. Therefore, during the period from the failure of the metal halide lamp20to the restart of the metal halide lamp20, the VRS element27′ using the temperature sensitive lead switch28boperates in the same manner as the VRS element27using the bimetal element28a. Of course, the connecting positions of the temperature sensitive lead switch28band the surge absorber element29bmay be reversed.

In such constructions as inFIGS. 4 to 7, it is clear that as the starting-voltage reducing element29, the surge absorber element29bthat becomes short-circuited at high voltage that is equal to or higher than a predetermined value can be used in place of the ceramic capacitor29a, and conversely, that the ceramic capacitor29acan be used in place of the surge absorber element29b. The surge absorber element29bis roughly classified into a semiconductor type and a discharge type. In the present invention, even the use of either type produces essentially the same effects.

The VRS elements27and27′ alike function effectively even when the voltage applied to start the metal halide lamp20is a high-frequency voltage or an intermittent pulse voltage. Also, the waveform of the high-frequency voltage is not to be particularly limited, and various waveforms such as a rectangular wave or a sinusoidal wave can be used. Further, it is also possible to superimpose a high-frequency voltage on a pulse voltage for application.

The present invention is described more specifically below by way of Examples and Comparative Example.

COMPARATIVE EXAMPLE 1

(a) Production of 150 W Metal Halide Lamp

The arc tube1for a 150 W metal halide lamp was produced. A PCA material was used as the material of the arc tube1. The resultant arc tube1(PCA tube) was long and narrow, with the distance Le between the electrodes6and7being 32 mm, the internal diameter φi of the central tubular part3being 4 mm, and the tubular shape parameter Le/φi being 8. The discharge space19of the arc tube1contained 9 mg of praseodymium iodide (PrI3) and sodium iodide (NaI) (composition ratio: NaI/PrI3=10), which served as the light emitting substances17, and 0.7 mg of the mercury18. Also, xenon was sealed therein at a pressure of about 25 kPa.

Using this arc tube1, the 150 W metal halide lamp20as illustrated inFIG. 2was fabricated. However, the voltage reducing switching element27was not mounted.

A life test was performed by applying a rectangular wave voltage with a frequency of 150 Hz to the arc tube1by means of an electronic ballast. When the metal halide lamp20was not in operation, for example, before being started, a high-frequency starting voltage of about 5 kV with a frequency of about 100 kHz was applied for about 50 msec per second (50 msec ON/950 msec OFF) until the metal halide lamp20was started.

During the life test, an arc discharge occurred in the outer tubular bulb21. The arc discharge occurred particularly between the stem leads24and25near the stem glass23of the outer tubular bulb21. However, the occurrence of an arc discharge in the outer tubular bulb21was limited only to conditions where the gas pressure inside the outer tubular bulb21exceeded 1 Pa. Also, the occurrence of an arc discharge was found only when the starting voltage was reapplied to the arc tube1of high temperature after the failure of the metal halide lamp20during lamp operation.

On the other hand, in conditions where the outer tubular bulb21was exhausted to a high degree of vacuum, an arc discharge did not occur in the outer tubular bulb21. Further, even in conditions where the gas pressure inside the outer tubular bulb21exceeded 1 Pa, if the temperature of the arc tube1was less than 100° C., application of the starting voltage did not result in occurrence of an arc discharge in the outer tubular bulb21.

(a) Production of 150 W Metal Halide Lamp

The metal halide lamp20was produced in the same manner as in Comparative Example 1, except that the VRS element27was mounted as illustrated inFIG. 4. The VRS element27used in this example comprised the bimetal element28aand the ceramic capacitor29athat were connected in series. The capacity of the ceramic capacitor29awas set to about 3,500 pF. Also, the bimetal element28aused was configured such that it was turned on at 100° C. or higher and turned off below 100° C. This VRS element27is capable of reducing the high-frequency starting voltage from the normal value of about 5 kV down to a level of about 1 kV or less at which an arc discharge in the outer tubular bulb21can be prevented from occurring.

The resultant metal halide lamp20exhibited excellent lamp characteristics suitable for outdoor lighting applications, with the initial luminous flux being 19,700 lm, the lamp efficiency 131.3 lm/W, and the color rendering index Ra70.

A life test was performed in the same manner as in Comparative Example 1.

Immediately after the failure of the metal halide lamp20, the bimetal element28ahad a high temperature of about 400° C., so that the movable terminal B of the VRS element27was connected to the stem lead25. Thus, due to the function of the ceramic capacitor29aof the VRS element27connected in parallel to the arc tube1, the high-frequency starting voltage was reduced from the normal value to about 1 kV. Accordingly, upon the failure of the metal halide lamp20due to increased gas pressure inside the outer tubular bulb21, the starting voltage was prevented from being applied to the arc tube1of high temperature without being reduced, so that an arc discharge did not occur in the outer tubular bulb21.

About 8 minutes after the failure of the metal halide lamp20, the temperature of the bimetal element28alowered to about 100° C., so that the movable terminal B of the VRS element27was disconnected from the stem lead25. As a result, the normal high-frequency starting voltage was applied to the arc tube1without being reduced, so that the metal halide lamp20was promptly restarted. At this time, since the temperature of the arc tube1was sufficiently low, occurrence of an arc discharge in the outer tubular bulb21was not observed.

As described above, during the life test, no arc discharge was observed in the outer tubular bulb21, and the rated life time was as long as 12,000 hours.

(a) Production of 150 W Metal Halide Lamp

The metal halide lamp20was produced in the same manner as in Comparative Example 1, except that the VRS element27′ was mounted as illustrated inFIG. 6. However, instead of the surge absorber element, the ceramic capacitor29awas used. Specifically, the VRS element used in this example comprised the temperature sensitive lead switch28band the ceramic capacitor29athat were connected in series. The capacity of the ceramic capacitor29awas set to about 3500 pF. Also, the temperature sensitive lead switch28bused was configured such that it was turned on at 100° C. or higher and turned off below 100° C. This VRS element27′ is also capable of reducing the high-frequency starting voltage from the normal value of about 5 kV down to a level of about 1 kV or less at which an arc discharge in the outer tubular bulb21can be prevented from occurring. The initial characteristics of the resultant metal halide lamp20were the same as those of Example 1.

A life test was performed in the same manner as in Comparative Example 1.

Immediately after the failure of the metal halide lamp20, the temperature sensitive lead switch28bhad a high temperature of about 400° C. Therefore, the temperature sensitive lead switch28bwas turned on, and the ceramic capacitor29afunctioned so that the high-frequency starting voltage was reduced from the normal value to about 1 kV. Accordingly, upon the failure of the metal halide lamp20due to increased gas pressure inside the outer tubular bulb21, the starting voltage was prevented from being applied to the arc tube1of high temperature without being reduced, so that an arc discharge did not occur in the outer tubular bulb21.

About 8 minutes after the failure of the metal halide lamp20, the temperature of the temperature sensitive lead switch28blowered to about 100° C., so that the temperature sensitive lead switch28bwas turned off. As a result, the normal high-frequency starting voltage was applied to the arc tube1without being reduced, so that the metal halide lamp20was promptly restarted. At this time, since the temperature of the arc tube1was sufficiently low, occurrence of an arc discharge in the outer tubular bulb21was not observed.

As described above, during the life test, no an arc discharge was observed in the outer tubular bulb21, and the rated life time was almost the same as that in Example 1.

Thereafter, about 7 kPa air was forced into each of the outer tubular bulbs21of the metal halide lamps20of Examples 1 and 2. Then, when the metal halide lamp20failed, it was forcedly restarted, to perform a forced life test. In this test, no arc discharge was also observed in the outer tubular bulb21.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, in a metal halide lamp that is started by applying a high starting voltage to an arc tube contained in an outer tubular bulb whose inner space is exhausted to a vacuum, an arc discharge in the outer tubular bulb is prevented from occurring in a lamp life test. It is therefore possible to provide a metal halide lamp with high quality and high efficiency. The present invention is preferably applicable particularly to a metal halide lamp having an arc tube made of a PCA material or quartz, but it is also applicable to various high pressure discharge lamps other than the metal halide lamp.