JP2000149868A - Direct-current discharge lamp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct-current discharge lamp having a small fusion loss or deformation of a cathode head, even if a cathode luminescent spot having high luminance is obtained by increasing a current density at the cathode head, and having the luminance or the position of the cathode luminescent spot, stabilized for long hours, and a long lamp life. SOLUTION: In this direct-current discharge lamp in which a cathode 21 and an anode 22, composed of tungsten, are arranged oppositely in an arc tube 11, and an emitter, composed of a single substance of a barium oxide or a calcium oxide or a strontium oxide or of a sintered body of tungsten powder including a complex thereof, is arranged on the head of the cathode 21, the volume of the cathode 21 is set as 70-200% of the volume of the anode 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct current discharge lamp used as a light source for projecting an image or performing a photochemical reaction.

[0002]

2. Description of the Related Art In a direct current discharge lamp, a cathode and an anode are arranged opposite to each other in an arc tube made of quartz glass. The cathode surface receives the current due to positive ions and the current due to electrons on the cathode surface, while the cathode receives energy when the positive ions collide with the cathode. When you emit electrons, you lose energy. That is, since the energy received by the cathode is smaller than the energy received by the anode, the volume of the cathode is designed to be smaller than the volume of the anode. However, when the volume of the cathode is set to about 20 to 30% of the volume of the anode, the temperature of the cathode and the anode is reduced. Is experimentally known to maintain equilibrium at, for example, 2800 ° C. For this reason, in the conventional DC discharge lamp, the volume of the cathode is 20 to 30% of the volume of the anode.
It was designed to a degree.

[0003]

By the way, a direct current discharge lamp has a cathode bright spot formed at the tip of the cathode when it is turned on. However, a light source for projecting an image or performing a photochemical reaction is required to have a high brightness point light source. Therefore, a xenon lamp, a xenon mercury lamp, an ultra-high pressure mercury lamp, or the like, in which a high-luminance cathode luminescent spot is formed at the tip of the cathode, is used. Recently, the demand for high brightness of the light source has become more and more intense. For this reason, the current density at the tip of the cathode has been increased to 20,000 to 30,000 A / c.
Although a high-brightness light source has been obtained by setting it to about m ^2, it is necessary to make the temperature of the cathode extremely high in order to flow such a high-density current.

The cathode of a DC discharge lamp is provided with an emitter for facilitating the starting of the lamp and for reducing the deterioration of the luminous flux. Usually, tungsten doped with several wt% of thorium oxide (hereinafter referred to as triated) is used. The cathode is often formed from tungsten). Then, the tip of the cathode is formed in a conical shape to form a cathode luminescent spot of discharge at the forefront, but since the work function of thoriated tungsten is high, in order to obtain the high current density as described above,
The temperature at the tip of the cathode needs to be about 2,800 ° C.

For this reason, in the initial stage of lighting, as shown by the dotted line in FIG. 2, the tip of the cathode, which has a small tip area and is sharp, is melted and deformed as the lighting time elapses, and after about 300 hours of lighting. Is, as shown by the solid line in FIG.
The shape of the tip becomes a distorted trapezoid with a large tip area. For this reason, the current density at the tip of the cathode decreases and the luminance decreases. Then, the position of the cathode tip recedes by the amount of melting, the distance between the electrodes increases, and the position of the cathode luminescent spot also changes, so that the cathode luminescent spot is displaced from the focal position of the reflecting mirror and the light distribution characteristics change. Resulting in. Furthermore, the vapor of the melted thoriated tungsten adheres to the inner wall of the arc tube and becomes black, so that the light transmittance is reduced and the life of the discharge lamp is shortened.

[0006] A cathode in which an alkaline earth type emitter obtained by sintering tungsten powder containing barium oxide, calcium oxide and strontium oxide is added to the tip of a cathode made of tungsten is also used. Since the emitter has a work function lower than that of thoriated tungsten, a high current density of about 20,000 to 30,000 A / cm ² can be obtained at a temperature of the cathode tip of about 2,300 ° C. For this reason, the cathode with an alkaline earth emitter added has less evaporation during lighting and has a longer life than the cathode made of thoriated tungsten, but the cathode tip deforms with thoriated tungsten. The current density at the cathode tip is small, the brightness is reduced, and the position of the cathode luminescent spot is also displaced, so that the light distribution characteristic is changed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a direct current discharge lamp which has little erosion or deformation at the cathode tip, and has a long and stable lamp luminance and position for a long time.

[0008]

According to the present invention, a cathode and an anode made of tungsten are opposed to each other in an arc tube, and a barium oxide, a calcium oxide, a strontium oxide alone, or a composite thereof is provided. In a DC discharge lamp in which an emitter made of a sintered body of tungsten powder containing a body is arranged at the tip of the cathode, the volume of the cathode is set to 70 to 200% of the volume of the anode. In the case of a DC discharge lamp in which a cathode and an anode are attached to the tip of an electrode rod, respectively, the volume of the cathode and the volume of the anode are:
Refers to the volume including the electrode rod.

As described above, in a conventional DC discharge lamp, the volume of the cathode is designed to be about 20 to 30% of the volume of the anode. However, the present inventor has proposed that barium oxide, calcium oxide and strontium oxide be used. In a direct current discharge lamp in which an emitter made of a single body or a sintered body of a tungsten powder containing a composite thereof is added to a cathode, the volume of the cathode is set to 70 to 200% of the volume of the anode.
Even if the temperature at the cathode tip is 1,900-2,000 ° C., the current density at the cathode tip is 20,000-30,000.
The present invention was completed by finding that it can be reduced to about A / cm ² .

Barium (Ba), calcium (Ca),
Strontium (Sr) is an alkaline earth metal, and although their properties are similar, their oxides are substances having a low work function. If the temperature of the cathode tip is high in the early stage of lighting, the evaporation of the material that lowers the work function of the cathode tip, such as barium oxide, contained in the alkaline earth-based emitter in the cathode is fast, and this material covers the cathode tip. Becomes smaller. Therefore, even when an emitter containing a material having a low work function is used, the temperature at the tip of the cathode must increase with the lapse of the lighting time.

However, in the present invention, since the temperature of the cathode tip is low in the initial stage of lighting, the evaporation of the substance which lowers the work function of the cathode tip is slow, and the rate at which this substance coats the cathode tip is kept high. That is, since the work function of the tip of the cathode is kept low for a long time, the electron emission is stable in a state where the temperature of the cathode is low. Therefore, even when the lamp is lit for a long time, the blackening of the inner wall of the arc tube is small, the shape of the cathode tip is kept sharp, the brightness of the cathode luminescent spot is stable, the light distribution characteristics are stable, and the lamp life is long. It can be a DC discharge lamp.

[0012]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows that the rated power consumption is 500 W, the lamp voltage is 25 V, and the lamp current is 20 A.
2 shows a xenon mercury lamp. The inner diameter of the electrode core rod 31 is about 11 cm @ 3,
Body 21 and electrode core 32 attached to the tip of
The anode body 22 attached to the tip of the
mm. A predetermined amount of mercury and xenon gas are sealed in the arc tube 11. The tail ends of the electrode core rods 31 and 32 are molybdenum foil 4
The molybdenum foil 40 is buried and sealed in a sealing tube 12 connected to both ends of the arc tube 11. In addition, bases 50 are respectively attached to the tail ends of the sealing tubes 12.

The anode body 22 has a diameter of 6 m.
m, a cylindrical body of tungsten having a length of 20 mm, and a tip portion is formed in a tapered shape to have a trapezoidal shape.
The electrode core rod 32 for the anode has a diameter of 3 mm and a length of 22 m.
m, and the sum of the volumes of the anode main body 22 and the electrode core rod 32 (the volume of the anode in the present invention) is 6
90 cm ³ . On the other hand, the cathode body 21 has a diameter of 7 mm.
mm, a 18 cm long tungsten cylinder,
The tip portion is formed into a tapered shape and has a sharp trapezoidal shape with a small tip area. The electrode rod 31 on the cathode side is a tungsten rod having a diameter of 3 mm and a length of 17 mm, and the sum of the volumes of the cathode main body 21 and the electrode rod 31 (the volume of the cathode in the present invention) is 812 cm ³ . Therefore, the volume of the cathode is 117% of the volume of the anode. Then, 10% barium oxide is applied to the tip of the cathode main body 21.
The emitter which sintered the tungsten powder containing is embedded. In addition, several types of discharge lamps having the same configuration and different in volume ratio between the cathode and the anode were manufactured at the same time.

The xenon mercury lamp was actually turned on continuously, and the decay rate of the radiation intensity of light having a wavelength of 365 nm, the extension of the distance between the electrodes, and the shape change of the cathode tip were examined. As a result, in the discharge lamp in which the volume of the cathode was 117% of the volume of the anode, the elongation of the distance between the electrodes after lighting for 1000 hours was 0.05 mm, and the shape of the tip of the cathode was hardly changed. Then, the attenuation rate of the radiation intensity of the light having a wavelength of 365 nm was 15% as shown in FIG.

In the discharge lamp in which the volume of the cathode was 70% of the volume of the anode, the distance between the electrodes was increased by 0.08 mm after 1000 hours of operation, and a slight change in the shape of the cathode tip was recognized. Further, the attenuation rate of the radiation intensity of the light having a wavelength of 365 nm was 20% as shown in FIG. Further, in the discharge lamp in which the volume of the cathode was 50% of the volume of the anode, the extension of the distance between the electrodes after lighting for 1000 hours was 0.2 mm, and a slight change in the shape of the cathode tip was recognized. Further, the attenuation rate of the radiation intensity of the light having a wavelength of 365 nm is
As shown in FIG. 3, it was 35%.

On the other hand, as a comparative example, when a discharge lamp in which the cathode was formed of thoriated tungsten and the volume of the cathode was 21% of the volume of the anode was turned on, the elongation of the distance between the electrodes when the lamp was turned on for 1000 hours was 0.1 mm. 3 mm, and the shape of the cathode tip changed as shown by the solid line in FIG. Then, the attenuation rate of the radiation intensity of the light having a wavelength of 365 nm was 45% as shown in FIG. As can be seen from the graph, the DC discharge lamp of the embodiment of the present invention has almost no change in the shape of the cathode tip, has a very small extension of the distance between the electrodes, has a small attenuation rate of light emission intensity, and has a long lamp life. Has advantages.

Next, a sintered body of a tungsten powder containing 10% of barium oxide and a sintered body of a tungsten powder containing 10% of calcium oxide are used as emitters. %
From 200% to 200%, and the decay rate (%) of the radiation intensity at 365 nm, the elongation of the distance between the electrodes (nm), the change in the shape of the cathode tip, and the temperature (° C.) of the cathode tip during 1000 hours of operation were examined. Table 1 shows the results.

[0018]

[Table 1]

As can be seen from Table 1, regardless of the type of emitter, the tip temperature of the cathode, the extension of the distance between the electrodes, and the change in the shape of the cathode tip are the same, but the ratio of the volume of the cathode to the anode is 70% or more. Then, the temperature at the tip of the cathode is 2,
When the temperature was lower than 000 ° C., the shape of the tip of the cathode did not change, and the state shown by the dotted line in FIG. The elongation between the electrodes was 0.08 mm or less, and it was confirmed that the position of the tip of the cathode hardly changed. The decay rate of the 365 nm radiation intensity is slightly different depending on the type of the emitter. The decay rate of the barium type emitter is somewhat smaller than that of the barium type emitter. This decay rate is smaller than that of the conventional discharge lamp in which the cathode is formed of thoriated tungsten and the decay rate is 45%, indicating that the lamp life is extremely long.

On the other hand, when the volume ratio of the cathode to the anode is 50%, the temperature at the tip of the cathode is 2,400.
At ℃, the shape of the cathode tip was deformed to the state shown by the solid line in FIG. The elongation between the electrodes is 0.2 mm,
The position of the cathode tip changed relatively largely. Also, 365
The attenuation rate of the radiation intensity in nm is 35% or more, and the attenuation is relatively large.

From this, it is understood that the volume ratio of the cathode to the anode needs to be 70% or more. On the other hand, if the ratio of the volume of the cathode to the anode becomes too large, the number of steps in the manufacture of the lamp increases,
Problems such as formation of a low-temperature portion on the cathode and adhesion of unevaporated mercury occur. Therefore, the ratio of the volume of the cathode to the volume of the anode needs to be 200% or less. Therefore, after all, the ratio of the volume of the cathode to the anode is 70-200.
%, Less erosion and deformation of the cathode tip,
The brightness and position of the cathode luminescent spot are stable for a long time, the lamp life is long, and the discharge lamp can be easily manufactured.

As described above, the smaller the ratio of the volume of the cathode to the volume of the anode, the smaller the number of steps in manufacturing the lamp, and the lower the cost. The temperature of the cathode tip is affected by the taper angle of the cathode tip as well as the ratio of the volume of the cathode to the anode, and the taper angle of the cathode tip is determined by the required distribution of light emission intensity.
As shown in FIG. 4 (A), when the distribution width of the light emission intensity is narrow, the taper angle at the tip of the cathode is large, and FIG.
As shown in (2), when the distribution width of the light emission intensity is wide, the taper angle at the cathode tip becomes small. The larger the taper angle at the tip of the cathode, the lower the temperature at the tip of the cathode, and the smaller the ratio of the volume of the cathode to the anode. Therefore, as shown in FIG. 4A, when the distribution width of the required light emission intensity is narrow and the taper angle at the tip of the cathode is large, the ratio of the volume of the cathode to the anode is set to, for example, 70 to 70%.
When the distribution width of light emission intensity is wide and the taper angle at the tip of the cathode is small as shown in FIG. 4B, the ratio of the volume of the cathode to the anode is set to, for example, 120 to 120%.
The ratio of the volume of the cathode to the anode is preferably selected depending on the taper angle at the tip of the cathode.

Although barium oxide and calcium oxide are used as emitters in the above embodiments, similar results can be obtained by using strontium oxide or a mixture thereof.

[0024]

As described above, the direct current discharge lamp of the present invention uses an emitter made of a sintered body of tungsten powder containing a simple substance of barium oxide, calcium oxide, and strontium oxide, or a composite thereof. Since the volume of the cathode is set to 70 to 200% of the volume of the anode, even if the current density at the cathode tip is increased to obtain a high-brightness cathode luminescent spot, there is little erosion or deformation of the cathode tip, and The brightness and position of the point are stable for a long time, and a DC discharge lamp with a long lamp life can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a state of erosion of a cathode.

FIG. 3 is a relationship diagram between a lighting time and a light intensity decay rate.

FIG. 4 is a diagram showing the relationship between the light intensity distribution and the shape of the cathode tip.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Arc tube 12 Sealing tube 21 Cathode main body 22 Anode main body 31 Cathode-side electrode core 32 Anode-side electrode core 40 Molybdenum foil 50 Base

Claims (1)

[Claims] A cathode and an anode made of tungsten are opposed to each other in an arc tube, and barium oxide, calcium oxide,
In a DC discharge lamp in which an emitter made of a strontium oxide alone or a sintered body of a tungsten powder containing a composite thereof is arranged at the tip of the cathode, the volume of the cathode is 70 to 200% of the volume of the anode. A DC discharge lamp characterized by the above-mentioned.