Microwave-excited discharge lamp having inner and outer cases for providing impedance match conditions

A microwave-excited discharge lamp (2) is constructed with a double-case structure consisting of an outer case (3) disposed in a microwave electromagnetic field, and an inner case (4) disposed inside the outer case.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
1. Field of the Invention 
The present invention relates to a microwave-excited discharge lamp which 
emits light by discharge under a microwave electromagnetic field. 
2. Description of the Related Art Statement 
In recent years, in accordance with desires to save energy etc., high 
intensity discharge lamps have been attracting attention. The reason is 
that high intensity discharge lamps can easily provide large light output 
compared with fluorescent lamps which give light output through phosphors. 
The high intensity discharge lamps are divided into two types: an 
electrode type discharge lamp having electrodes, such as a metal halide 
lamp and a mercury lamp; and an electrodeless type discharge lamp such as 
a microwave-excited lamp. 
In the microwave-excited lamp, a predetermined microwave electromagnetic 
field is formed by a magnetron or a microwave generator, and plasma 
discharge caused by a microwave electromagnetic field is used as a light 
source. The microwave-excited lamp has a long life compared with the 
electrode type discharge lamp which are limited by deterioration of the 
electrodes. Furthermore, in the microwave-excited lamp, an emission 
spectrum of the light output does not deteriorate after long periods of 
service because of electrodeless configuration. 
Moreover, since change of impedance between operating condition and 
extinguished condition is small, the microwave-excited discharge lamp 
provides far greater advantages than the electrode type discharge lamp 
with regard to characteristics of flashing operation, starting, and 
restarting. The microwave-excited discharge lamp also has a remarkable 
advantage over the electrode type discharge lamp because of environmental 
protection concerns. The reason is that, as has been described in the 
above, the microwave-excited discharge lamp has a long life, thereby 
components of the microwave-excited discharge lamp need not be changed for 
a long time. Furthermore, the microwave-excited discharge lamp gives light 
output having the brightness and efficacy comparable to those of the 
electrode type discharge lamp without use of environment-harmful mercury. 
A conventional microwave-excited lighting apparatus comprises a magnetron 
for generating a microwave, a waveguide for transmitting the generated 
microwave to a cavity, the cavity connected to the waveguide, and a 
microwave-excited lamp disposed inside the cavity. 
The microwave-excited lamp is formed into a bulb of substantially spherical 
shape or an elongated cylindrical shape by quartz glass or like material 
having a transparent or translucent property, and is supported by a glass 
supporting rod in the internal space of the cavity. A rare gas such as 
argon, a small amount of mercury, and metal halides such as thallium 
iodide are sealed inside the microwave-excited lamp as luminescent 
material. The internal pressure of the microwave-excited lamp in the 
extinguished condition is adjusted to a range between about 13 kPa and 27 
kPa in order to easily perform a starting operation, namely, the beginning 
of the below-mentioned plasma discharge of the rare gas. 
In the conventional microwave-excited lighting apparatus, when a high 
voltage is supplied to the magnetron from a high voltage power supply, for 
example, a microwave of 2,450 MHz is radiated into the internal space of 
the waveguide from an antenna of the magnetron. The microwave is 
propagated through the waveguide, and is radiated into the cavity through 
an aperture opened in the waveguide. Thereby, the predetermined microwave 
electromagnetic field is formed in the internal space of the cavity. When 
the microwave electromagnetic field is applied, dielectric breakdown of 
the rare gas occurs, and thereby, the plasma discharge of the rare gas is 
started. According to this plasma discharge, the temperature on the inside 
walls of the microwave-excited lamp rises, as a result of which the 
mercury and the metal halides are vaporized, thereby the internal pressure 
of the microwave-excited lamp increases. In a steady state operating 
condition in which the internal pressure and the temperature at the 
coldest point on the inside walls are stabilized at respective 
predetermined values (for example, at 101.3 kPa to 202.6 kPa and at 500 to 
600.degree. C., respectively), light having a predetermined emission 
spectrum is generated in the microwave-excited lamp by the plasma 
discharge of metal vapor. Thereby, the light is radiated outside as light 
output from the cavity through a metal mesh plate. In the above-mentioned 
steady state operating condition, pressure caused by the metal vapor 
occupies a larger volume than pressure caused by the rare gas in the 
internal pressure of the microwave-excited lamp. 
Furthermore, in the steady state operating condition, the impedance 
matching condition between the waveguide and a resonator consisting of the 
cavity and the microwave-excited lamp is satisfied. In other words, a 
value of load of the resonator (hereafter referred to as an "input 
impedance of the resonator") becomes larger than a value of input 
impedance of the resonator in the extinguished condition, and reaches a 
value substantially equal to an inherent impedance of the waveguide 
(hereafter referred to as an "impedance on the power supply side"). The 
input impedance of the resonator is dependent on the following types of 
power loss, (1) and (2). 
(1) Power loss due to the plasma discharge in the microwave-excited lamp. 
(2) Power loss due to eddy currents generated on an inside wall of the 
cavity. 
In the steady state operating condition, therefore, the microwave is 
radiated toward the cavity with hardly any reflections at the aperture of 
the waveguide, and thereby, the plasma discharge is produced efficiently 
in the microwave-excited lamp. 
The conventional microwave-excited lamp takes several seconds until it 
reaches the steady state operating condition from start of lighting 
operation. However, as disclosed in unexamined and published Japanese 
patent application TOKKAI (SHO) No. 57-63768, for example, there is a type 
of microwave-excited lamp wherein an necessary time from the start of the 
lighting operation until reaching the steady state operating condition is 
reduced to about 1 second by sealing two or more kinds of halogens in the 
microwave-excited lamp. 
In the above-mentioned conventional microwave-excited lamp, the plasma 
discharge occurring inside the microwave-excited lamp changes due to 
changes in environmental conditions such as an ambient temperature. 
Thereby, there is a problem that the power loss due to the plasma 
discharge also change. As a result, the input impedance of the resonator 
is not equal to the impedance on the power supply side by influence of the 
change of environmental conditions. There is a problem that the impedance 
matching condition between the resonator and the waveguide cannot be 
satisfied. This has also lead to further problems that the microwave is 
reflected at the aperture of the waveguide toward the magnetron, and that 
the microwave leaks outside from the mesh plate. As a result, it is 
impossible to form a predetermined microwave electromagnetic field in the 
internal space of the cavity. There occurs a problem that the light output 
cannot be radiated efficiently. In the case that the microwave is 
reflected back toward the magnetron, the reflected microwave is 
superimposed on the microwave radiated from the antenna, so that the 
microwave is distorted. As a result, the predetermined microwave 
electromagnetic field cannot be formed in the internal space of the 
cavity. Furthermore, there is a liability that the reflected microwave 
incident on the antenna damages the magnetron, and shortens the life of 
the microwave-excited lighting apparatus. 
On the other hand, in the case that the microwave leaks outside from the 
mesh plate, the predetermined microwave electromagnetic field cannot be 
formed in the internal space of the cavity. This has also led to a problem 
in which the microwave induces high-frequency noise, so that the 
high-frequency noise adversely affects electronic apparatus in the 
vicinity of the microwave-excited lighting apparatus. 
In the conventional microwave-excited discharge lamp, when the 
microwave-excited discharge lamp attains to a small size in order to 
reduce size of the light source, the aforementioned problem about the 
impedance matching condition is improved. By miniaturization of the 
microwave-excited discharge lamp, the power loss due to the plasma 
discharge decreases, and the input impedance of the resonator also 
decreases. 
On the other hand, the impedance on the power supply side can be reduced, 
for example, by reducing the internal volume of the waveguide. However, 
the waveguide has a high-pass filtering characteristic, and its cutoff 
frequency is dependent on the shape of the waveguiding channel inside the 
waveguide. Hence, another problem arises that microwaves cannot be 
propagated through the waveguiding channel if the waveguiding channel is 
made too small. Therefore, it is practically impossible to reduce the 
impedance on the power supply side in corresponding relationship to the 
input impedance of the resonator. As has been explained in the above, when 
the conventional microwave-excited discharge lamp attains to the small 
size, the impedance matching condition between the resonator and the 
waveguide can not be satisfied. 
Furthermore, in the case that the light output from the light source is 
used by means of converging the light through a lens or a reflecting 
mirror, reducing the size of the light source is highly recommended in 
order to efficiently extract the light output. As described in "Small 
Long-Lived Stable Light Source for Projection- Display Applications," 
International Symposium Digest, Technical Report, Vol. 24, pp. 716-719, 
for example, when a microwave-excited discharge lamp is used as a 
backlight source for a projection display, it is highly recommended that 
the microwave-excited discharge lamp be small in size in order to 
efficiently extract the light output from the backlight source. 
However, in the conventional microwave-excited discharge lamp, when the 
conventional microwave-excited discharge lamp attains to the small size, 
there is the problem that the impedance matching condition between the 
resonator and the waveguide can not be satisfied. Thereby, it is difficult 
to use the conventional microwave-excited discharge lamp for the backlight 
source of the projection display. 
OBJECTS AND SUMMARY OF THE INVENTION 
The object of the present invention is to provide a microwave-excited 
discharge lamp that can solve the aforementioned problems. 
In order to achieve the above-mentioned object, a microwave-excited 
discharge lamp in accordance with the present invention comprises: 
an outer case disposed in a microwave electromagnetic field, and the outer 
case sealing at least a discharge gas, and 
an inner case disposed inside the outer case, and the inner case sealing at 
least a discharge gas. 
According to the present invention, the microwave-excited discharge lamp is 
constituted with a double-case construction consisting of the outer case 
disposed in the microwave electromagnetic field and the inner case 
disposed inside the outer case. Thereby, a plasma discharge as prescribed 
in the design can be produced within the inner case without being affected 
by the environmental conditions such as temperature, wind, humidity, etc., 
to which the microwave-excited discharge lamp is subjected. In this way, 
the effects of the environmental conditions on the microwave-excited 
discharge lamp can be reduced. 
While the novel features of the invention are set forth particularly in the 
appended claims, the invention, both as to organization and content, will 
be better understood and appreciated, along with other objects and 
features thereof, from the following detailed description taken in 
conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereafter, a preferred embodiment of the present invention is described 
with reference to the accompanying drawing. 
FIG. 1 is a schematic plan view showing a basic construction of a 
microwave-excited lighting apparatus equipped with a microwave-excited 
discharge lamp in a first embodiment of the present invention. 
In FIG. 1, a microwave-excited lighting apparatus 1 comprises a magnetron 
unit 10 for generating a microwave, a waveguide 12 for transmitting the 
microwave to a cavity 13, and a microwave-excited discharge lamp 2 
disposed inside the cavity 13. 
The magnetron unit 10 comprises a magnetron 10a for generating a microwave, 
for example, at a frequency of 2,450 MHz and with an output power of 250 
to 400 W, an antenna 10b for radiating a generated microwave, and a fan 
10c for cooling the magnetron 10a. The magnetron 10a is connected to a 
high voltage power supply 11 which drives the magnetron 10a. 
The waveguide 12 is a metal box having, for example, a rectangular cross 
section. The antenna 10b is contained in one end part of the waveguide 12, 
and an aperture 12a is opened in the other end part of the waveguide 12. 
The waveguide 12 is constructed in accordance with, for example, the EIA 
(Electronic Industries Association) standards. In order to propagate the 
microwave in the range of 2,170 MHz to 3,300 MHz efficiently, for example, 
length of the waveguide 12 is 100 cm, and rectangular cross-sectional 
dimensions of the waveguide 12 are 86.36 mm.times.43.18 mm. 
The cavity 13 is constructed with a metal enclosure substantially 
cylindrical in shape, and mounted at one open end thereof on a surface of 
the waveguide 12 so as to encircle the aperture 12a opened in the 
waveguide 12. The other open end is a light output extraction port on 
which a metal mesh plate 13a is mounted. Microwave energy is stored in the 
internal space of the cavity 13. When a microwave is radiated through the 
aperture 12a, a predetermined microwave electromagnetic field is formed in 
the internal space of the cavity 13. When the impedance matching condition 
described later is satisfied, the cavity 13 does not allow the microwave 
to leak outside through the mesh plate 13a, so that a plasma discharge can 
be produced efficiently in the microwave-excited discharge lamp 2. 
Thereby, the light output can be radiated outside through the mesh plate 
13a without wastage. Furthermore, a visible light reflecting mirror (not 
shown) is provided on the inside walls of the cavity 13 in order to direct 
the light output efficiently. 
The microwave-excited discharge lamp 2 of the present invention will be 
elucidated with reference to FIG. 2. FIG. 2 is an enlarged perspective 
view showing the microwave-excited discharge lamp in the first embodiment 
of the present invention. 
As shown in FIG. 2, the microwave-excited discharge lamp 2 comprises an 
airtight outer case 3 shaped in a bulb form and disposed in the 
predetermined microwave electromagnetic field, and an airtight inner case 
4 shaped in a bulb form and enclosed inside the outer case 3 in concentric 
relationship therewith. The outer case 3 and the inner case 4 are both 
supported on a supporting rod 5, made of quartz glass or the like, inside 
the internal space of the cavity 13 as shown in FIG. 1. The supporting rod 
5 supports the outer case 3 and the inner case 4 so that the supporting 
rod 5 does not affect the airtightness of the outer case 3. The outer case 
3 and the inner case 4 are each made of a material having a transparent or 
translucent property such as quartz glass or alumina. The diameter of the 
outer case 3 is, for example, 30 mm (approximately the same as a 
conventional single-bulb lamp designed to similar standards). The diameter 
of the inner case 4 is 1 mm to 10 mm, about 1/30 to 1/3 the diameter of 
the conventional single-bulb lamp. The outer case 3 is filled with a rare 
gas such as argon, along with a material, such as mercury, that does not 
contribute to the emission of light but emits light in the invisible 
region of the spectrum when a plasma discharge is produced. The inner case 
4 is filled with a rare gas such as argon, along with a material, such as 
metal halides, that contributes to the emission of light and emits light 
in the visible region of the spectrum when a plasma discharge is produced. 
Specific examples of the material contributing to the emission of light 
include sodium iodide that by itself emits light over the entire visible 
region of the spectrum, and a compound of a plurality of metal halides 
such as gadolinium iodide, lutetium iodide, and thallium iodide. Instead 
of the metal halides, sulphur that has an emission spectrum close to that 
of sunlight may be used as the material that contributes to the emission 
of light. The internal pressures of the outer case 3 and the inner case 4 
in an extinguished condition are adjusted at a few kPa to a few dozen kPa 
in order to easily perform a starting operation, namely, the beginning of 
the below-mentioned plasma discharge of the rare gas. 
It will be recognized, however, that the shape of the outer case 3 and 
inner case 4 is not limited to the bulb form. Furthermore, the outer case 
3 and inner case 4 need not be formed in geometrically similar shapes as 
long as the inner case 4 is enclosed inside the outer case 3 with an 
appropriate spacing provided therebetween. 
In the as shown in FIG. 1, microwave-excited lighting apparatus 1, the 
microwave-excited discharge lamp 2 is constructed with a double-case 
structure consisting of the outer case 3 and the inner case 4. Thereby, 
when the plasma discharge occurs under the microwave electromagnetic 
field, the plasma discharge can be produced inside the inner case 4 
without being affected by environmental conditions such as the ambient 
temperature of the microwave-excited discharge lamp 2. As a result, it is 
possible to reduce effects of the environmental conditions on the 
microwave-excited discharge lamp. Furthermore, if any of the 
above-mentioned materials contributing to the emission are sealed in the 
outer case 3, this will not cause any problems that will adversely affect 
the operation and the life of the microwave-excited discharge lamp 2. 
In this way, in the microwave-excited discharge lamp 2 of the present 
invention, optimum kinds of the materials can be selected for the outer 
case 3 and the inner case 4 in order to satisfy the performance required 
as a light source. 
Operation of the microwave-excited lighting apparatus 1 will be explained 
with reference to FIG. 1. 
When a high voltage is supplied to the magnetron 10a from the high-voltage 
power supply 11, the magnetron unit 10 is activated, so that a microwave 
of 2,450 MHz is radiated from the antenna 10b into the interior of the 
waveguide 12. The microwave propagates through the waveguide 12, and is 
fed into the cavity 13 through the aperture 12a, thereby, the 
predetermined microwave electromagnetic field is formed in the internal 
space of the cavity 13. The microwave electromagnetic field firstly causes 
dielectric breakdown of the rare gas in the inner case 4, so that the 
plasma discharge of the rare gas is generated in the inner case 4. 
Consequently, the microwave electromagnetic field causes dielectric 
breakdown of the rare gas in the outer case 3, so that the plasma 
discharge of the rare gas is generated in the outer case 3. According to 
the plasma discharge, temperatures on the respective inside walls of the 
outer case 3 and the inner case 4 rise, as a result of which the mercury 
and the metal halides are vaporized, thereby the respective internal 
pressures of the outer case 3 and the inner case 4 increase. In a steady 
state operating condition in which the internal pressure and the 
temperature at the coldest point on the inside wall of the inner case 4 
are stabilized at respectively predetermined values (for example, at 101.3 
kPa to 202.6 kPa and at 500 to 600.degree. C.), light having an emission 
spectrum defined by the sealed metal halides is produced in the inner case 
4 by the plasma discharge of the metal vapor. Thereby, the light is 
radiated outside as light output through the mesh plate 13a of the cavity 
13. In this steady state operating condition, pressure caused by the metal 
vapor occupies a larger volume than pressure caused by the rare gas in the 
respective internal pressures of the outer case 3 and the inner case 4. 
Furthermore, in the steady state operating condition, the impedance 
matching condition between the waveguide 12 and the resonator consisting 
of the cavity 13 and the microwave-excited lamp 2 is satisfied. In other 
words, a value of load of the resonator, which is dependent on the power 
losses due to the plasma discharges within the outer case 3 and the inner 
case 4 and on the moderate power losses due to eddy currents generated on 
an inside wall of the cavity 13, becomes larger than a value of the load 
of the resonator in the extinguished condition. Furthermore, in the steady 
state operating condition, the value of load of the resonator reaches a 
value substantially equal to an inherent impedance of the waveguide 12. 
In the steady state operating condition, therefore, the microwave is 
radiated toward the cavity 13 with hardly any reflections at the aperture 
12a of the waveguide 12, and thereby, the plasma discharge is produced 
efficiently inside the microwave-excited discharge lamp 2. As a result, 
the microwave-excited discharge lamp 2 of the present invention can 
radiate its light output with high efficiency toward the outside through 
the mesh plate 13a. 
Apart from the aforementioned explanation, wherein the magnetron unit 10 
generates the microwave electromagnetic field, an alternative construction 
may be such that a high-frequency current may be fed through a coil to 
form a microwave electromagnetic field. Thereby, the microwave-excited 
discharge lamp 2 of the present invention may be constructed to produce 
light through the discharge by the microwave electromagnetic field. 
Furthermore, as has been explained above, the microwave-excited discharge 
lamp 2 of the present invention comprises the same material in outer case 
3 that does not contribute to the emission of the light while the material 
that contributes to the emission of light from the inner case 4. Thereby, 
it is possible that size of the light source is reduced. As a result, the 
impedance matching condition between the waveguide 12 and the resonator 
can be easily satisfied even when the microwave-excited discharge lamp 2 
be small in size. More specifically, in the microwave-excited discharge 
lamp 2 of the present invention, the power loss due to the plasma 
discharge formed in the outer case 3 and not contributing to the emission 
of the light is added to the power losses due to the plasma discharge 
formed in the inner case 4 and the eddy currents generated on the inside 
wall of the cavity 13. Thereby, it is possible that the load of the 
resonator is made substantially equal to the intrinsic impedance of the 
waveguide 12. In this way, the impedance matching condition between the 
waveguide 12 and the resonator can be easily satisfied. 
Thus, the microwave-excited discharge lamp 2 of the present invention is 
formed by the double-case construction consisting of the outer case 3 and 
the inner case 4. Thereby, the microwave-excited discharge lamp 2 enables 
a predetermined plasma discharge to be formed inside the inner case 4 
without being affected by environmental conditions such as the ambient 
temperature of the microwave-excited discharge lamp 2. Furthermore, in the 
microwave-excited discharge lamp 2, it is possible to reduce the effects 
of the environmental conditions on the microwave-excited lamp 2. Moreover, 
since the material that does not contribute to the emission of the light 
is sealed into the outer case 3, the size of the light source can be 
reduced while satisfying the impedance matching condition between the 
waveguide 12 and the resonator. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that such disclosure is not 
to be interpreted as limiting. Various alterations and modifications will 
no doubt become apparent to those skilled in the art to which the present 
invention pertains, after having read the above disclosure. Accordingly, 
it is intended that the appended claims be interpreted as covering all 
alterations and modifications as fall within the true spirit and scope of 
the invention.