Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Reexamination Certificate
2001-05-25
2003-07-29
Patel, Nimeshkumar D. (Department: 2874)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S632000, C313S633000
Reexamination Certificate
active
06600268
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a short arc mercury lamp and a lamp unit. In particular, the present invention relates to a short arc mercury lamp and a lamp unit used as a light source for an image projection apparatus such as a liquid crystal projector and a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector or a projector using a DMD has been widely used as a system for realizing large-scale screen images, and a high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. In the image projection apparatus, light is required to be concentrated on a very small area of a liquid crystal panel or the like, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source.
As high pressure discharge lamps that can meet this need, the research and development of metal halide lamps was conducted first of all. However, it was found that when the arc length was reduced to achieve a light source close to a point light and high intensities, the arc width is increased in the case of metal halide lamps. Therefore, nowadays, a short arc ultra high pressure mercury lamp that is closer to a point light and has a high intensity has been noted widely as a promising light source. In the ultra high pressure mercury lamp, 90% of the entire luminous flux emit light in an effective region, whereas in the metal halide lamps having a large arc width, only 50% of the entire luminous flux emit light in an effective region. This occurs for the following reasons. In the case of the metal halide lamps, the average excitement potential of the enclosed metal is comparatively as low as 4 to 5 eV, and therefore emission occurs in the vicinity of the arc so that the arc width is large. On the other hand, in the case of the ultra high pressure mercury lamps, since mercury has a higher average excitement potential (7.8 ev) than that of the enclosed metal for the metal halide lamp, emission occurs in the central region of the arc, and thus the arc width is small. Therefore, the average intensity of the arc in the ultra high pressure mercury lamp can be higher than that of the metal halide lamp.
Referring to
FIGS. 12A and 12B
, a conventional short arc ultra high pressure mercury lamp
1000
will be described.
FIG. 12A
is a schematic view of an ultra high pressure mercury lamp
1000
. The lamp
1000
includes a substantially spherical luminous bulb
110
made of quartz glass, and a pair of sealing portions (seal portions)
120
and
120
′ also made of quartz glass and connected to the luminous bulb
110
. A discharge space
115
is inside the luminous bulb
110
. A mercury
118
in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm
3
as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space
115
.
A pair of tungsten electrodes (W electrode)
112
and
112
′ are opposed with a certain distance D (e.g., about 1.5 mm) in the discharge space
115
. Each of the W electrodes
112
and
112
′ includes an electrode axis (W rod)
116
and a coil
114
wound around the head of the electrode axis
116
. The coil
114
has a function to reduce the temperature at the head of the electrode. The respective electrode axes
116
of the W electrodes
112
and
112
′ are matched to be on the same axis to maintain the optical symmetry, and therefore, the electrode central axes
119
of the W electrodes
112
and
112
′ are matched to each other.
The electrode axis
116
of the W electrode
112
is welded to a molybdenum foil (Mo foil)
124
in the sealing portion
120
, and the W electrode
112
and the Mo foil
124
are electrically connected by a welded portion
117
where the electrode axis
116
and the Mo foil
124
are welded. The sealing portion
120
includes a glass portion
122
extended from the luminous bulb
110
and the Mo foil
124
. The glass portion
122
and the Mo foil
124
are attached tightly so that the airtightness in the discharge space
115
in the luminous bulb
110
is maintained. In other words, the sealing portion
120
is sealed by attaching the Mo foil
124
and the glass portion
122
tightly for foil-sealing. The sealing portions
120
have a circular cross section, and the rectangular Mo foil
124
is disposed in the center of the inside of the sealing portion
120
. The Mo foil
124
of the sealing portion
120
includes an external lead (Mo rod)
130
made of molybdenum on the side opposite to the side on which the welded portion
117
is positioned. The Mo foil
124
and the external lead
130
are welded with each other so that the Mo foil
124
and the external lead
130
are electrically connected at a welded portion
132
. The structures of the W electrode
112
′ and sealing portion
120
′ are the same as those of the W electrode
112
and sealing
120
, so that description thereof will be omitted.
As shown in
FIG. 12B
, the lamp
1000
is electrically connected to a ballast
1200
for lighting. When the ballast
1200
is operated in the state where the external lead
130
is connected to the ballast
1200
, the lamp
1000
turns on.
Next, the operational principle of the lamp
1000
will be described. When a start voltage is applied to the W electrodes
112
and
112
′ from the ballast
1200
via the external leads
130
and the Mo foils
124
, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space
115
of the luminous bulb
110
, and thus the mercury
118
is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes
112
and
112
′. The higher the mercury vapor pressure of the lamp
1000
is, the higher the emission efficiency is, so that the higher mercury vapor pressure is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb
110
, the lamp
1000
is used at a mercury vapor pressure of 15 to 25 MPa.
The conventional lamp
1000
sometimes failed to turn on when the lamp was turned on again after turning off, although the lamp was used properly. The cause of the failure of lamp lighting was conventionally not clear. However, as a result of in-depth research, the inventors of the present invention found that this was caused by the fact that, as shown in
FIG. 13
, a bridge (mercury bridge)
140
of mercury
118
occurs between the W electrodes
112
and
112
′, so that the W electrodes
112
and
112
′ are short-circuited.
When a start voltage is applied to the lamp
1000
in a state where the electrodes are short-circuited by the mercury bridge
140
, a large amount of current flows in the lamp
1000
. As a result, the ballast
1200
detects operation abnormality and automatically stops the start of the lamp lighting. After the start of the lamp lighting is stopped, the mercury bridge
140
still remains, so that the lamp
1000
is not turned on, even if the ballast
1200
starts operating for lighting again.
It seems that the mercury bridge
140
is formed in the following manner. When turning on the lamp
1000
, the temperature at the W electrodes
112
and
112
′ causing discharge is about 3000° C., and the temperature at the luminous bulb
110
positioned around the W electrodes is about 1000° C. When the lamp
1000
is turned off, the W electrode
112
made of a metal is cooled faster than the luminous bulb
110
made of glass. Therefore, mercury vapor in the discharge space
115
is condensed more on the W electrode
112
than on the inner wall of the luminous bulb
110
, so that the mercury vapor is likely to precipitate as a mercury ball (Hg ball) in the W electrode
112
.
When the W electrode
112
is cooled and the condensation of the mercury vapor proceeds, as shown in
FIG. 14A
, the Hg ball
118
Horiuchi Makoto
Ichibakase Tsuyoshi
Kai Makoto
Sasaki Ken-ichi
Seki Tomoyuki
Berck Kenneth A
Harness & Dickey & Pierce P.L.C.
Patel Nimeshkumar D.
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