Electric lamp and discharge devices: systems – Pulsating or a.c. supply
Reexamination Certificate
2000-03-01
2002-11-12
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
C313S639000
Reexamination Certificate
active
06479946
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method and system for driving a high-pressure mercury vapor discharge lamp having a pair of discharge electrodes in an arc tube in which mercury and a rare gas are enclosed. The invention further relates to an image projector employing the lamp.
(2) Description of the Prior Art
High-pressure mercury vapor discharge lamps are known to have a high luminance and therefore utilized for a light source of such optical devices as a liquid crystal projector and the like, with the lamp being positioned so that the light emitting region (arc) is located at the focal point of a reflector mirror such as a parabolic mirror. When such a high-pressure mercury vapor discharge lamp is used as a component in such optical devices, it is necessary that loss of the light emitted from the lamp be minimized so that a high illuminance on the screen is achieved. For this reason, it is preferable that a lamp used for such a light source of liquid crystal projectors have a light emitting region as close as possible to a point source. More specifically, so-called short arc lamps, which have a short light emitting region, are preferable, and in addition, high-pressure mercury vapor discharge lamps are more preferable than metal halide lamps.
In metal halide lamps, the metals enclosed therein as metal halides have a low average excitation energy, and therefore the lamps operate at a relatively low arc temperature. As a result, the light emitting region is not restricted in a region between the electrodes but is spread over a wide region from the center of the arc tube towards the arc tube wall, and accordingly, a point source light is difficult to be achieved by using a metal halide lamp. By contrast, in high-pressure mercury vapor discharge lamps, mercury, which has a high average excitation energy, is enclosed as a fill material thereof. Therefore, in high-pressure mercury vapor discharge lamps, the light emitting region is restricted in a small region, and thus, a light emitting region close to a point source is readily achieved. An example of such a short arc high-pressure mercury vapor discharge lamp is disclosed in Japanese Unexamined Patent Publication No. 2-148561. In this lamp, a mercury vapor pressure during lamp operation is made to be 200 atm. or higher, and continuous radiation components in the visible light range are thus generated by mercury molecule emission. The lamp has a lamp power of 50 W, and exhibits a high luminance and improved color rendering property, and is therefore suitable for the light source of liquid crystal projectors.
As for methods of driving such high-pressure mercury vapor discharge lamps used for a liquid crystal projector and the like, special operating conditions such as an instant restarting and an instant starting of light rays necessary for motor vehicle headlights are not required, and therefore, the driving methods are seldom described in publications. Generally, for driving a high-pressure mercury vapor discharge lamp, alternating voltage having a frequency of several tens to several hundreds hertz is used, since within the frequency range, circuit designing is rendered relatively easy in view of the response speed of large power semiconductor devices.
With a recent trend toward a larger screen size and a higher resolution of liquid crystal projectors, a lamp having a large lamp power that can achieve a higher illuminance on the projection screen has been increasingly demanded. In order to achieve such a lamp, it may be possible to increase a lamp power by raising a mercury vapor pressure during lamp operation and thereby increasing a lamp voltage. However, when rising a mercury vapor pressure is difficult because of the constraint imposed by a wall tube strength against pressure, a lamp current must be increased to increase the lamp power.
In the case in which a high-pressure mercury vapor discharge lamp is operated with a large lamp current, however, there arises a problem that flicker is caused when the lamp is operated by the foregoing conventional driving method with alternating voltage having a frequency of several tens to several hundreds hertz. Flicker is a phenomenon in which an illumination of a projected image is varied from moment to moment. Such flicker induces poor quality in the projected image in liquid crystal projectors and the like employing a high-pressure mercury vapor discharge lamp.
SUMMARY OF THE INVENTION
In view of the foregoing and other drawbacks in prior art, it is an object of the present invention to provide a method and a system for driving a high-pressure mercury vapor discharge lamp for use in a liquid crystal projector and the like, which method and system are free from flicker in the projected image even when the lamp is operated with a large lamp current.
It is another object of the invention to provide an image projector using such a driving system.
In order to achieve the above and other objects, the inventors have studied the cause of such flicker, and as a consequence discovered that wandering of a cathode luminescent spot generated in the vicinity of a tip of each electrode is the cause of the flicker. Referring now to
FIG. 1
showing a luminance distribution of the light emitting region between the electrodes
11
and
12
, it is noted that a cathode luminescent spot refers to a spot at which the highest luminance is observed (the reference numerals
11
a
and
12
a
in
FIG. 1
) in the vicinity of a point at which electrons are emitted when each of the electrodes
11
and
12
is turned to be a cathode. Now, discussed below is a process in which the wandering of a cathode luminescent spot occurs.
For example, as shown in
FIG. 2A
, when a negative voltage is applied to the electrode
11
and a positive voltage to the electrode
12
, a minute region
11
a
adjacent to the tip of the electrode
11
is heated to a high temperature and thus turned to be a cathode luminescent spot at which arc is generated by thermionic emission effect. By emission of electrons, such a state of the minute region
11
a
being a high temperature is further maintained. Thereafter, as shown in
FIG. 2B
, when the polarity of the applied voltage is reversed, the electrons emitted from the electrode
12
enters a wide region in the tip of the electrode
11
. By the entry of the electrons, the energy from the electrons is transferred to the electrode
11
, and the wide region of the tip of the electrode
11
is heated as well as the minute region
11
a.
Here, in the case of a lamp current being relatively small, although the electrode
11
is heated, the state of the minute region
11
a
having a higher temperature than the rest of the region is still maintained. As a consequence, when the polarity of the applied voltage is again reversed, the minute region
11
a
is again turned to be a cathode luminescent spot. Therefore, once a cathode luminescent spot forms at the minute region
11
a
, the spot does not easily move to another region but stays at a relatively stable position. By contrast, in the case of a lamp current being large, more specifically as shown in
FIG. 2C
, in the case where a lamp current is so large that the electrode
11
is heated in a wide region
11
b
in the electrode tip and, viewed on a so-called microscopic level, the electrode
11
is fused and deformed by the heat from moment to moment, the minute region
11
a
is not necessarily in the state of a higher temperature than that of the rest of the region. In other words, any spot in the wide region adjacent to the tip of the electrode
11
can become a temperature that can result in a cathode luminescent spot. Consequently, it can occur that, when the polarity of the applied voltage is again reversed, another minute region
11
c
, not the minute region
11
a
, is turned to be a cathode luminescent spot. That is to say, influenced by convection in the arc tube or surface roughness of the tip of the electrode
11
caused by the heat deformation, a position of a cathode luminescen
Horiuchi Makoto
Kai Makoto
Ogura Toshiaki
Takeda Mamoru
Tsutsumi Takeharu
Dinh Trinh Vo
Parkhurst & Wendel L.L.P.
Wong Don
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