U.S. patent number RE32,626 [Application Number 06/932,416] was granted by the patent office on 1988-03-22 for microwave generated plasma light source apparatus.
This patent grant is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Hiroshi Ito, Hitoshi Kodama, Hirotsugu Komura, Yoshibumi Minowa, Kenji Yoshizawa.
United States Patent |
RE32,626 |
Yoshizawa , et al. |
March 22, 1988 |
Microwave generated plasma light source apparatus
Abstract
A microwave generated plasma light source including a microwave
generator, a microwave cavity having a light reflecting member
forming at least a portion of the cavity, and a member transparent
to light and opaque to microwaves disposed across an opening of the
cavity opposite the feeding opening through which the microwave
generator is coupled. An electrodeless discharge bulb is disposed
at a position in the cavity such that the cavity operates as a
resonant cavity at least when the bulb is emitting light. In the
bulb is encapsulated at least one discharge light emissive
substance. The bulb has a shape and is sufficiently small that the
bulb acts substantially as a point light source.
Inventors: |
Yoshizawa; Kenji (Hyogo,
JP), Kodama; Hitoshi (Kanagawa, JP),
Minowa; Yoshibumi (Hyogo, JP), Ito; Hiroshi
(Kanagawa, JP), Komura; Hirotsugu (Fukuoka,
JP) |
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
12289166 |
Appl.
No.: |
06/932,416 |
Filed: |
November 20, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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242075 |
Mar 9, 1981 |
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Reissue of: |
625565 |
Jul 2, 1984 |
04498029 |
Feb 5, 1985 |
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Foreign Application Priority Data
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Mar 10, 1980 [JP] |
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55-29911 |
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Current U.S.
Class: |
315/39;
313/231.31; 315/111.21; 315/248 |
Current CPC
Class: |
H01J
61/54 (20130101); H01J 65/044 (20130101) |
Current International
Class: |
H01J
65/04 (20060101); H01J 007/46 (); H01J
017/80 () |
Field of
Search: |
;315/39,248,111.21,111.31 ;313/231.31,231.41 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Chatmon; Saxfield
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Parent Case Text
This application .Iadd.is a reissue of U.S. Pat. No. 4,498,029,
which .Iaddend.is a continuation of application Ser. No. 242,075,
filed 3/9/81, now abandoned.
Claims
What is claimed is:
1. A microwave generated plasma light source apparatus comprising:
a microwave generator; a microwave cavity having a light reflecting
member forming at least a portion of said cavity, said microwave
generator being coupled through a feeding opening in said cavity to
said cavity; a member transparent to light and opaque to microwaves
disposed across an opening of said cavity opposite said feeding
opening; a waveguide for guiding microwaves generated by said
microwave generator to said feeding opening of said cavity; and an
electrodeless discharge bulb disposed at a position in said cavity
such that said cavity operates as a resonant cavity at least when
said bulb is emitting light, said bulb encapsulating at least one
discharge light emissive substance and having a shape and being
sufficiently small that said bulb functions substantially as a
point light source, said bulb being made of transparent quartz
glass and having an inner diameter such that a ratio of an outer
surface area of said bulb to the microwave input power is from 1.5
mm.sup.2 /W to 15 mm.sup.2 /W, the weight of said bulb with respect
to the microwave input power being no more than 3.0.times.10.sup.-2
g/W, said microwave generator comprising means for generating
microwaves intermittently with a rest interval of no more than 5
msec.
2. The microwave generated plasma light source apparatus as claimed
in claim 1 wherein said light reflecting member comprises a light
reflecting shell having rotational symmetry.
3. The microwave generated plasma light source apparatus as claimed
in claim 1 wherein said light reflecting member comprises a center
reflecting shell having at least a portion thereof of the same
shape as said bulb and having a wing portion extending from a
peripheral edge of said center reflecting shell, an inner surface
of said wing portion being non-reflective to light.
4. The microwave generated plasma light source apparatus as claimed
in claim 1 further comprising lens means for one of collecting and
scattering light passing through said member transparent to light
and opaque to microwaves.
5. The microwave generated plasma light source apparatus as claimed
in claim 1 wherein said non-electrode discharge bulb comprises an
electrically conductive discharge start assisting member disposed
at least in the vicinity of said bulb for concentrating a magnetic
field.
6. The microwave generated plasma light source apparatus as claimed
in claim 5 wherein said electrically conductive discharge start
assisting member is encapsulated in said bulb.
7. The microwave generated plasma light source apparatus as claimed
in claim 5 wherein said discharge start assisting member is
disposed on a side of said bulb facing said feeding opening.
8. The microwave generated plasma light source apparatus as claimed
in claim 5 wherein said discharge start assisting member is in the
shape of wire.
9. The microwave generated plasma light source apparatus as claimed
in claim 5 wherein said start assisting member comprises an
electrically conductive member and a dielectric cover covering said
electrically conductive member.
10. The microwave generated plasma light source apparatus as
claimed in claim 9 wherein a space is provided between said
conductive member and said dielectric cover, said space being at a
reduced pressure.
11. The microwave generated plasma light source apparatus as
claimed in claim 1 wherein said light reflecting member is formed
with a pair of opposed cutoff sleeves into which a pair of
supporting members of said bulb are inserted to support said
bulb.
12. The microwave generated plasma light source apparatus as
claimed in claim 11 wherein said bulb has said supporting members
integrally formed therewith.
13. The microwave generated plasma light source apparatus as
claimed in claim 1 wherein said discharge light emissive substance
encapsulated in said bulb comprises mercury of 7.times.10.sup.-6
gram atom/cc to 60.times.10.sup.-6 gram atom/cc, gallium of at
least 1.times.10.sup.-7 gram atom/cc and halogen of
1.5.times.10.sup.-7 to 50.times.10.sup.-7 gram atom/cc.
14. The microwave generated plasma light source apparatus as
claimed in claim 1 wherein said microwave generator comprises a
magnetron and full-wave voltage doubler power supply means for
driving said magnetron.
15. The microwave generated plasma light source apparatus as
claimed in claim 1 wherein the length of said waveguide is selected
such that the operation of said magnetron is within a phase width
of a quarter wavelength with respect to a sink region immediately
after said bulb is ignited. .Iadd.
16. A microwave generated plasma light source apparatus comprising:
a microwave generator; a microwave cavity having a light reflecting
member forming at least a portion of said cavity, said microwave
generator being coupled through a feeding opening in said cavity to
said cavity; a member transparent to light and opaque to microwaves
disposed across an opening of said cavity; a waveguide for guiding
microwaves generated by said microwave generator to said feeding
opening of said cavity; and an electrodeless discharge bulb
disposed at a position in said cavity such that said cavity
operates as a resonant cavity at least when said bulb is emitting
light, said bulb encapsulating at least one discharge light
emissive substance and having a substantially spherical shape and
being sufficiently small that said bulb functions substantially as
a point light source, said bulb being made of transparent quartz
glass and having an inner diameter such that a ratio of an outer
surface area of said bulb to the microwave input power is not more
than 15 mm.sup.2 /W, the weight of said bulb with respect to the
microwave input power being no more than 3.0.times.10.sup.-2 g/W,
said microwave generator comprising means for generating microwaves
intermittently with a rest interval of no more than 5
msec..Iaddend. .Iadd.
17. The microwave generated plasma light source apparatus as
claimed in claim 16 wherein said light reflecting member comprises
a light reflecting shell having rotational symmetry..Iaddend.
.Iadd.18. The microwave generated plasma light source apparatus as
claimed in claim 16 wherein said light reflecting member comprises
a center reflecting shell having at least a portion thereof of the
same shape as said bulb and having a wing portion extending from a
peripheral edge of said center reflecting shell, an inner surface
of said wing portion being non-reflective to light..Iaddend.
.Iadd.19. The microwave generated plasma light source apparatus as
claimed in claim 16 further comprising lens means for one of
collecting and scattering light passing through said member
transparent to light and opaque to microwave..Iaddend. .Iadd.20.
The microwave generated plasma light source apparatus as claimed in
claim 16 wherein said electrodeless discharge bulb comprises an
electrically conductive discharge start assisting member disposed
at least in the vicinity of said bulb for concentrating a magnetic
field..Iaddend. .Iadd.21. The microwave generated plasma light
source apparatus as claimed in claim 20 wherein said electrically
conductive discharge start assisting member is encapsulated in said
bulb..Iaddend. .Iadd.22. The microwave generated plasma light
source apparatus as claimed in claim 20 wherein said discharge
start assisting member is disposed on a side of said bulb facing
said feeding opening..Iaddend. .Iadd.23. The microwave generated
plasma light source apparatus as claimed in claim 20 wherein said
discharge start assisting member is in the shape of wire..Iaddend.
.Iadd.24. The microwave generated plasma light source apparatus as
claimed in claim 20 wherein said start assisting member comprises
an electrically conductive member and a dielectric cover covering
said electrically conductive member..Iaddend. .Iadd.25. The
microwave generated plasma light source apparatus as claimed in
claim 24 wherein a space is provided between said conductive member
and said dielectric cover, said space being at a reduced
pressure..Iaddend. .Iadd.26. The microwave generated plasma light
source apparatus as claimed in claim 16 wherein said light
reflecting member is formed with a pair of opposed cutoff sleeves
into which a pair of supporting members of said bulb are inserted
to support said
bulb..Iaddend. .Iadd.27. The microwave generated plasma light
source apparatus as claimed in claim 26 wherein said bulb has said
supporting members integrally formed therewith..Iaddend. .Iadd.28.
The microwave generated plasma light source apparatus as claimed in
claim 16 wherein said discharge light emissive substance
encapsulated in said bulb comprises mercury of 7.times.10.sup.-6
gram atom/cc to 60.times.10.sup.-6 gram atom/cc, gallium of at
least 1.times.10.sup.-7 gram atom/cc and halogen of
1.5.times.10.sup.-7 to 50.times.10.sup.-7 gram atom/cc and halogen
of 1.5.times.10.sup.-7 to 50.times.10.sup.-7 gram atom/cc..Iaddend.
.Iadd.29. The microwave generated plasma light source apparatus as
claimed in claim 16 wherein said microwave generator comprises a
magnetron and full-wave voltage doubler power supply means for
driving said magnetron..Iaddend. .Iadd.30. The microwave generated
plasma light source apparatus as claimed in claim 16 wherein the
length of said waveguide is selected such that the operation of
said magnetron is within a phase width of a quarter wavelength with
respect to a sink region immediately after said bulb is
ignited..Iaddend. .Iadd.31. The microwave generated plasma light
source apparatus as claimed in claim 16 wherein said microwave
cavity has a rotationally symmetrical shape..Iaddend. .Iadd.32. The
microwave generated plasma light source apparatus as claimed in the
claim 16 wherein said microwave cavity has at least one spherical
surface portion whose center is common to the center of said
bulb..Iaddend. .Iadd.33. The microwave generated plasma light
source apparatus as claimed in claim 32 further comprising lens
means disposed before said light transparent member..Iaddend.
.Iadd.34. The microwave generated plasma light source apparatus as
claimed in claim 16 wherein said opening of said cavity across
which said member transparent to light and opaque to microwaves is
disposed is located opposite to said feeding opening..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a light source utilizing a
microwave generated plasma discharge.
Recently, a light source utilizing high frequency discharge,
particularly, a microwave generated plasma discharge, has been
considered in view of the long life thereby provided, which is
significantly longer than the life of a conventional light source
having electrodes which are relatively easily consumed.
A light source using high frequency discharge has essentially no
electrode and thus there is no thermal loss such as is inherent to
a light source having electrodes. Further, the discharge impedance
thereof at the time when discharge starts is not significantly
different from that during the stable discharge. In addition to
these advantages, since discharge power is localized around an
envelope of the lamp bulb, it is easy to couple power to the light
source at the discharge starting time. Thus, the time required to
achieve the maximum lamp output is short.
FIG. 1 shows, in vertical cross section, a conventional microwave
generated plasma light source constructed by incorporating the
above features and FIG. 2 is a cross section of the light source
taken along a line II--II in FIG. 1.
In these Figures, a magnetron 1 enclosed by an envelope 10 and
cooled by a cooling fan 7 generates microwave energy which is
radiated through a magnetron antenna 2 into a waveguide tube 3. The
microwave energy propagates along the waveguide tube 3 and is
radiated through a feeding opening 5 to a cavity 49 having a
semicircular cross section and defined by a mesh 9 and a
semicircular light reflector 4 having a plurality of gas passages
formed therein to thus establish a microwave electromagnetic field
therein. A discharge occurs in a noble gas encapsulated in a
discharge bulb 6 due to the microwave electromagnetic field to thus
heat the bulb wall or envelope and to thereby evaporate a metal
such as mercury also encapsulated in the bulb. Then, discharge in
the gaseous metal takes place. With this gaseous metal discharge,
the microwave energy is caused to be absorbed by the discharge bulb
6 substantially completely during its propagation along the length
of the discharge bulb 6 through several reflections within the
cavity 49 so that the microwave energy is converted into discharge
energy substantially completely. That is, the bulb is excited in a
non-resonance state.
The reflector 4 defining a portion of the cavity 49 reflects light
directed rearwardly of the lamp bulb so that all the light from the
bulb is directed to pass through an open end of the cavity which is
covered by a mesh member 9 which is transparent to light but only
translucent to microwaves to thereby utilize the light produced by
the gaseous metal discharge effectively.
Cooling air supplied by the fan 7, after cooling of the magnetron
1, passes through the air passages 8 of the reflector 4 to cool the
discharge bulb 6 and is discharged from the cavity 49 through the
mesh member 9.
In the conventional microwave generated plasma light source
constructed as above, the microwave electromagnetic waves are
distributed in the cavity 49 having a semicircular cross section as
shown in FIG. 3. However, the distribution is not uniform.
Therefore, the discharge in the discharge bulb 6 is not uniform and
thus the light intensity distribution is not uniform in the axial
direction of the bulb.
One approach of eliminating the non-uniformity of light intensity
distribution is to alter the shape of, for example, the reflector
4. This approach, however, is not practical because it is difficult
as a practical matter to provide a reflector of a shape
corresponding to the microwave electric field distribution in the
cavity. Another approach is to use as small a discharge bulb as
possible to thereby obtain a uniform discharge. Since, in this
case, however, the cavity 49 is used in the non-resonance state, it
is impossible to supply sufficient power to the discharge bulb 6 to
excite it resulting in a low discharge efficiency. Therefore, the
device thus constructed is not suitable for use as an ultraviolet
ray source for photographic plate making where a high light
intensity and a highly uniform illumination distribution over an
area to be illuminated are required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a microwave
generated plasma light source apparatus with which a uniform
illumination distribution is provided and which is suitable for use
as an ultraviolet ray source, for example for a photographic plate
making device.
The above object is achieved by the present invention by the use of
a microwave cavity as a resonator when the discharge bulb is
operated and by selecting the shape of the discharge bulb so that
it functions as a point light source.
More specifically, the above and other objects of the invention are
met by a microwave generated plasma light source apparatus
including a microwave generator, a microwave cavity having a light
reflecting member forming at least a portion of the cavity with the
microwave generator being coupled through a feeding opening in the
cavity to the cavity, a member transparent to light and opaque to
microwaves disposed across an opening of the cavity opposite the
feeding opening, a waveguide for guiding microwaves generated by
the microwave generator to the feeding opening of the cavity, and a
non-electrode discharge bulb disposed at a position in the cavity
such that the cavity operates as a resonant cavity at least when
the bulb is emitting light. The bulb encapsulates at least one
discharge light emissive substance and has a shape and is
sufficiently small that the bulb functions substantially as a point
light source, taking into consideration the size and shape of the
light reflecting member.
Preferably, the light reflecting member is a light reflecting shell
having rotational symmetry. At least a portion of the light
reflecting shell may conform to the shape of the bulb and a wing
portion may be provided extending from a peripheral edge of the
shell and an inner surface of the wing portion is made
non-reflective to light. A lens may be provided for collecting or
scattering light passing through the member which is transparent to
light and opaque to microwaves. An electrically conductive
discharge start assisting member may be disposed at least in the
vicinity of the bulb for concentrating a magnetic field with the
start assisting member in one preferred embodiment being
encapsulated in the bulb. The start assisting member is preferably
disposed on a side of the bulb facing the feeding opening. The
discharge start assisting member may have the shape of a wire or
may be an electrically conductive member having a dielectric
covering therearound. A space may be provided between the
conductive member and the dielectric cover which is at a reduced
pressure.
The light reflecting member may be formed with a pair of opposed
cut-off sleeves into which a pair of supporting members of the bulb
are inserted to support the bulb. The bulb may have the supporting
members formed integrally therewith.
Preferably, the discharge light emissive substance encapsulated in
the bulb is mercury of 7.times.10.sup.-6 gram atom/cc to
60.times.10.sup.-6 gram atom/cc, gallium of at least
1.times.10.sup.-7 gram atom/cc and a halogen of 1.5.times.10.sup.-7
to 50.times.10.sup.-7 gram atom/cc. The bulb may be made of
transparent quartz glass and should have an inner diameter such
that a ratio of an outer surface area of the bulb to input
microwave power is in a range of 1.5 to 15 mm.sup.2 /W. The weight
of the discharge bulb with respect to the microwave input power
should be no more than 3.0.times.10.sup.-2 g/W. The microwaves
generator may generate the microwave intermittently in which case
the rest interval should be no more than 5 msec. The microwave
generator may be a magnetron with a full-wave voltage doubler power
supply used to drive the magnetron. The length of the waveguide
should be selected such that the operation of the magnetron is
within a phase width of a quarter wavelength with respect to a sink
region of the magnetron immediately after the bulb is ignited.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section of the conventional microwave discharge
light source;
FIG. 2 is a cross section of the microwave discharge light source
taken along a line II--II in FIG. 1;
FIG. 3 shows a microwave electric field distribution in a cavity of
the conventional microwave discharge light source;
FIG. 4 is a cross sectional view of a preferred embodiment of the
present invention;
FIG. 5 is a cross section of the present microwave discharge light
source taken along a line V--V in FIG. 4;
FIG. 6 is a perspective view of a modification of the cavity
portion of the embodiment in FIG. 4;
FIG. 7 is a cross section of a cavity of a second embodiment of the
present invention;
FIG. 8 is a cross section of a modification of the cavity of the
second embodiment of the present invention;
FIGS. 9 through 11 show a third embodiment of the present invention
in which FIG. 9 is a cross section thereof, FIG. 10 is a cross
section of the cavity portion thereof and FIG. 11 is an enlarged
view of an essential portion thereof;
FIGS. 12 through 14 show modifications of the discharge bulb of the
third embodiment of the present invention, respectively;
FIG. 15 is similar to FIG. 11, showing a modification of the third
embodiment of the present invention;
FIGS. 16 through 18 are graphs showing characteristic curves which
are plots of the relative light output of a non-electrode,
spherical discharge bulb 4 according to a fourth embodiment of the
present invention;
FIG. 19 is a graph showing a relation of the weight of the
non-electrode spherical discharge bulb of the fourth embodiment to
a light amount stabilizing time at the start of discharge;
FIGS. 20 and 21 show a fifth embodiment of the present invention in
which FIG. 20 is a circuit diagram of a power source of the
microwave generator and FIG. 21 shows a microwave waveform
generated by the microwave generator in FIG. 20; and
FIGS. 22 and 23 show a sixth embodiment of the present invention in
which FIG. 22 is an example of the Rieke disgram of a magnetron and
FIG. 23 is a graph showing the impedance shift of the cavity
thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described in detail with reference to
preferred embodiments thereof.
Describing a first embodiment with reference to FIGS. 4 and 5, the
first embodiment includes a magnetron 1 equipped with a magnetron
antenna 2, a waveguide 3 having one end connected to the magnetron
1 and the other end connected to a microwave feeding opening 5
formed in a wall of a light reflecting member 4 which is formed as
a shell in the shape of a rotationally symmetrical cup or dome, and
an envelope 10 housing these components. In the wall of the
waveguide 3 are formed a plurality of air holes 8.
The microwave discharge light source apparatus further includes a
spherical discharge bulb 6 of quartz glass which has no electrode
and which has a sufficiently small size so as to make the light
emissive portion thereof approximate a point light source. The bulb
6 is filled with mercury as a discharge photoemissive substance and
argon gas as a starter noble gas and fixedly supported by the light
reflecting member 4.
The magnetron 1 is cooled by air flow provided by a fan 7 supported
by an upper wall of the envelope 10 and the open end of the cup
shaped reflection member 4 is covered by a mesh plate 9 which is
transparent to light but opaque to microwaves. The light reflecting
member 4 forms together with the mesh plate 9, a resonance cavity
49 which serves as a resonator when the bulb 6 is lit.
In operation, when electric power is supplied to the magnetron 1,
the magnetron 1 radiates microwave energy through the waveguide 3
and the feeding opening 5 into the microwave cavity 49. However,
since the discharge bulb 6 is not ignited immediately after the
start of microwave oscillation, the cavity 49 is in the
non-resonance state and only a microwave electromagnetic field is
established in the cavity 49 due to microwaves leaking from the
feeding opening 5. Then, the discharge of the bulb 6 is started by
the electromagnetic field. When the discharge of the bulb 6 is
started, the cavity 49 becomes resonant and a resonance
electromagnetic field is established therein. Sufficient microwave
energy to maintain the discharge of the discharge bulb 6 is
supplied by the resonance electromagnetic field.
Therefore, according to the invention, since the microwave cavity
49 acts as a resonator, it is possible to inject a sufficient
amount of microwave energy into the small spherical discharge bulb
so that the efficiency of the device is increased over prior art
constructions. Since the shape of the discharge bulb 6 can be
considered as a point source, it is possible to dispose it at a
position within the cavity 49 where the variation of microwave
electromagnetic field distribution is negligible. Therefore,
unevenness of the discharge in the discharge bulb 6 is eliminated
as is unevenness of light emission. Furthermore, since the light
reflecting member 4 has a rotationally symmetrical shape, the
manufacture thereof is very easy so that, in determining the shape
of the cavity 49 serving as a resonator, which is quite difficult
to do analytically, it is easy to select an optimum size by
changing the size thereof experimentally.
If the mesh plate 9 is formed by, for example, etching a thin metal
plate of a material such as a stainless steel plate having a
thickness of 0.1 mm, the microwave loss thereof is much smaller
than a mesh plate made of metal wire because there is contact
between metal portions. Therefore, the microwave energy can be more
effectively converted into discharge energy and thus the light
emission efficiency is considerably improved.
Further, as shown in FIG. 6, it is possible to form the cavity 49
of the light reflecting member as a truncated pyramid. With this
shape, a desired illumination distribution is obtained within the
square area. In this case, the microwave electromagnetic field
distribution is approximately in a square mode, while with the
rotationally symmetrical cavity, the mode is approximately circular
or cylindrical.
EMBODIMENT 2
A second preferred embodiment will be described with reference to
FIG. 7 in which the same or similar components as those in FIG. 6
are depicted by the same reference numerals. A light control member
11 including a lens is disposed below a mesh plate 9 and a cavity
43 is formed by a reflecting member 41 having a center of curvature
at the point at which the discharge bulb 6 is disposed and a wing
portion 42 extending from the reflection member 41. An inner
surface of the wing portion 42 is coated with graphite or provided
with an anti-reflection layer 43 so that there is substantially no
reflection from the inner surface of the wing portion 42. With this
structure, in addition to the ease of cavity design, all of the
light incident on the light control member 11 can be considered as
emerging from the discharge bulb 6. Thus, it is possible to control
the illumination distribution by suitably setting the light
collection characteristics of the light control member 11.
The light control member 11 may be of the light scattering type
and, regardless to say, it is to be capable of regulating the
illumination distribution.
Further, this embodiment can be modified as shown in FIG. 8. In
FIG. 8, the light reflecting member 4 is rotationally symmetrical
and has a peripheral portion inner surface which is provided with
the light absorption layer 43 which may be a coated layer of a
material such as graphite. The light control member 11 may be
fixedly secured by screws 21 through an annular mounting plate 20
to the light reflecting member 4 with an annular spacer 22. With
the modification of FIG. 8, the illumination distribution control
of the illuminating area is performed in a similar manner to that
in FIG. 7. In addition, since the cavity 49 can be formed as a
resonance cavity by suitably determining the area where the light
absorption layer 43 is provided, the ease of design is much
improved.
EMBODIMENT 3
A third embodiment will be described with reference to FIGS. 9
through 11 of which FIG. 9 is a schematic cross-sectional
illustration thereof. FIG. 10 is an enlarged view of the cavity
portion thereof and FIG. 11 is a further enlarged view of the
cavity portion. In these figures, the same reference numerals used
in FIGS. 4 and 5 indicate the same or corresponding components as
those used in FIGS. 4 and 5. Here, the electrodes discharge bulb 6
of quartz in the shape of a sphere is formed with a pair of quartz
protrusions 61 extending from an outer surface 62 thereof
oppositely. As before, the bulb 6 is filled with mercury and argon
gas. The bulb 6 is further formed with a quartz tube 63 extending
from the outer surface 62 thereof in which a start assisting member
64 in the form of a tantalum wire is encapsulated.
A mesh plate 9 which is transparent to light but opaque to
microwave is fixed by bolts 15 between a flange portion 16 of the
light reflecting member 4 formed by bending the peripheral edge
thereof and an annular press plate 17 corresponding in shape to the
flange portion 16 which covers the open end of the light reflecting
member 4 to thereby define the microwave cavity 49.
Free ends of the protrusion 61 of the bulb 6 are inserted into
inner ends of supporter cylinder 12 which is made of quartz. The
outer ends of the supporter cylinder 12 are inserted into cut-off
sleeves 13 formed on the outer surface of the light reflecting
member 4 and supported thereby by stopper screws 14, respectively.
The position of the bulb 6 supported in this way is determined such
that when the bulb 6 is energized the cavity 49 becomes a
resonator.
This arrangement can provide not only the same effects as provided
by the first embodiment but also an effect that, due to the
provision of the start assisting member 64, the intensity of the
electromagnetic field around the opposite ends of the start
assisting member 64 is quite high, as indicated by E in FIG. 11,
and thus the electromagnetic field strength within the bulb 6 is
higher than the discharge starting electromagnetic field strength
even when the electromagnetic field strength within the cavity 49
immediately before the discharge starting of the bulb 6 is low thus
assuring the ignition of the bulb 6 without increasing the
microwave input energy. Further, the bulb 6 is easily detachable
and positioning of the bulb 6 can be easily performed by merely
adjusting the position of the supporting cylinder 12.
Furthermore, since the start assisting member 64 is embedded in the
exhausted tube 63 of dielectric material such as quartz and the
breakdown voltage of the dielectric material in a vacuum is much
higher than in air, the possibility of discharge outside of the
bulb 6 is such reduced and the possibility of melting the starting
assisting member 64 is eliminated. The electromagnetic field
distribution within the cavity 49 prior to the ignition of the bulb
6, i.e., in the state where impedance matching is not yet
established, is most dense the near the feeding opening 5.
Therefore, if the start assisting member 63 is positioned such that
one end thereof is in the vicinity of the feeding opening 5, it is
possible to further strengthen the electromagnetic field within the
bulb 6 to thereby cause the discharge starting to be accomplished
easier.
Further, since the impedance of the start assisting member 64
during stable discharge of the bulb 6 is negligible compared with
the impedance of the bulb 6 itself, the existence of the start
assisting member does not affect the stability of the operation of
the bulb 6.
The length l of the cut-off sleeve 13 should be determined such
that the leakage of microwaves from the cavity resonator is
restricted to be at or below a level (1 mW/cm.sup.2) at which there
is no safety problem. The power density (P) of leaked microwaves
can be expressed by the following equation: ##EQU1## where,
##EQU2## where .lambda. is the free space wavelength of the
microwaves in centimeters, P.sub.o is microwave input energy in
watts, a is the inner diameter of the cut-off sleeve 13 is
centimeters and E.sub.r is the specific dielectric constant of the
supporting member.
Therefore, in order to make the leakage power P equal to or smaller
than 1 mW/cm.sup.2, the length l must be equal to or longer than:
##EQU3##
As a typical example, when P.sub.o =1 kW, a=0.4 cm, .lambda.=12.24
cm and E.sub.r 32 4, the length l should be 1.6 cm or longer.
In the embodiment shown in FIGS. 9 through 10, the start assisting
member 64 is embedded in the cylinder member 63 which protrudes
from the outer surface of the discharge bulb 6. Alternatively, it
may be housed directly in the discharge bulb 6 as shown in FIG. 12
or it may be covered by a dielectric material such as quartz to
provide a reduced pressure atmosphere therefor so that the member
64 does not react with other materials filling the bulb 6 and then
be housed in the bulb 6 as shown in FIG. 13. Alternatively, a pair
of start assisting members 64 may be used for this purpose as shown
in FIG. 14. In FIG. 14, a pair of start assisting members 64 are
encapsulated and coupled in series by a common evacuated tube
member 63 made of a dielectric material with opposing ends of the
members 64 being slightly separated so that the electromagnetic
field intensity is high around the space therebetween and the tube
member 63 housed in the bulb 6.
FIG. 15 shows a modification of the discharge bulb 6 in which the
supporting thereof is somewhat simplified. In FIG. 15, the cylinder
member 63 in which the start assisting member 64 is positioned is
used as a supporting portion thereof which is held by a
corresponding supporting member provided around the feeding opening
5 of the light reflecting member 4. To this effect, a thread 65 is
formed on the outer surface of the cylinder member 63 and a bulb
support member 66 of the low loss dielectric material such as
quartz glass having one end suitably fixed to the light reflecting
member 4 and the other end threaded correspondingly to the thread
65 of the cylinder member 63 is provided. The discharge bulb 6 is
fixedly supported by screwing the cylinder member 63 into the
thread of the bulb supporting member 66. In this case, there is no
need of providing the protrusions 61 and the cut-off sleeves 13 and
therefore the manufacture of the device is considered quite
simple.
EMBODIMENT 4-1
In accordance with a fourth embodiment, in addition to mercury used
in the preceding embodiments, gallium is provided as a light
emitting substance so that emitted light includes waves of the
gallium atom spectrum of 403 nm and 417 nm as well as the mercury
atom spectrum of 365 nm, 405 nm and 436 nm. The purpose of this
embodiment is to make the apparatus of the present invention also
applicable to an exposing light source for a diazo type
photosensitive material which is sensitive to wavelengths of 403 nm
and 417 nm. The discharge bulb 6 and the light reflecting member 4
used in this embodiment can be any of those of the preceding
embodiments.
An actual device was assembled using the construction shown in
FIGS. 4 and 5 with microwave output power of the magnetron 1 being
700 W and with the inner surface of the light reflecting 4 being
completely covered by carbon black so as to eliminate the effects
of reflection from the light reflecting member so that measurement
could be made of only the direct light from the bulb 6. The
materials filling the bulb 6 were mercury, gallium and iodine as a
halogen. A light output having wavelengths from 350 nm to 450 nm
was measured for bulbs containing various amounts of these
materials.
FIG. 16 shows a plot of relative light output on the ordinate for
wavelengths from 350 nm to 450 nm with respect to the amount of
mercury encapsulated in the bulb 6 on the abscissa. Here, the inner
diameter of the spherical discharge bulb 6 was 30 mm and the bulb 6
also contained argon gas at 60 mm Hg, 1 mg of gallium, 4 mg of
mercury iodide and a variable amount of mercury. As will be clear
from FIG. 16, when the amount of mercury is increased with the
amounts of gallium and mercury iodide held constant, the light
output reaches a maximum when the amount of mercury is about 100
mg. The arc is stable up to mercury amounts of about 150 mg and
then the light emission becomes unstable with larger amounts. This
may be considered due to the fact that when the amount of mercury
is increased beyond 150 mg, the mercury vapor pressure in the
discharge bulb 6 becomes saturated and the excess amount of mercury
is deposited on the inner wall of the discharge bulb 6. This
phenomenon can also be observed when the amount of mercury is
varied with the amounts of gallium and mercury iodine being other
constant values.
EMBODIMENT 4-2
FIG. 17 shows a plot of relative optical output of the discharge
bulb 6 in a wavelength range from 350 nm to 450 nm with the amount
of mercury iodide on the abscissa for a case where the spherical
bulb 6 has an inner diameter of 30 mm and contains argon gas at 60
mmHg, 60 mg of mercury, 0.5 mg of gallium and various amounts of
mercury iodide. As is clear from FIG. 17, the light output of the
bulb 6 increases substantially with increased amounts of mercury
iodide reaching a maximum when the amount of mercury iodide is
about 2 mg, i.e., when the atom ratio of gallium to iodide is
around 1:1.2. With a further increase in the amount of mercury
iodide, the output decreases gradually. This tendency can also be
observed when the amount of mercury iodide is varied while the
amounts of mercury and gallium are other constant values. It has
been observed that the maximum light output is obtained when the
gallium to mercury iodide ratio is 1:4, i.e., for a gallium atom to
iodide atom ratio of about 1:1.2.
EMBODIMENT 4-3
FIG. 18 shows plots of relative light outputs in a wavelength range
from 350 nm to 450 nm of three spherical discharge bulbs 6 which
have an inner diameter of 30 mm and which contain argon gas at 60
mmHg and a variable amount of a mixture of gallium and mercury
iodide with fixed ratio of 1:4 together with mercury in amounts of
60 mg, 80 mg and 150 mg. As is clear from FIG. 18, regardless of
the amount of mercury, the light output increases with an increased
amount of the mixture and becomes a maximum with the amount of
gallium at about 0.5 mg to about 2.0 mg and then decreases with a
further increase of gallium. When the amount of gallium is
increased beyond 2.5 mg, i.e., when the amount of mercury iodide is
greater than 10 mg, the arc becomes astable even when there is no
residual mercury. This may be considered as due to the fact that
iodine in the arc affects the latter adversely. This is confirmed
by the fact that when only the amount of gallium is increased with
the amount of mercury iodide restricted to be 10 mg or less, there
is no turbulence of the arc while gallium is deposited on a portion
of the inner wall of the bulb in operation.
It will be clear from a consideration of Embodiments 4-1 through
4-3 that in order to obtain an intense light output in a wavelength
range of from 350 nm to 450 nm by using microwave excitation, a
non-electrode discharge light source having a spherical discharge
bulb having an inner diameter of 30 mm with amounts of mercury,
gallium and mercury iodide encapsulated in the bulb of 20 mg to 170
mg, 1510 mg, 0.1 mg or more and 0.5 mg to 15 mg, respectively,
should be used. These values can be represented in gram atomic
weight per unit inner volume of the bulb as 7.times.10.sup.-6
-60.times.10.sup.-6, 1.times.10.sup.-7 or more and
1.5.times.10.sup.-7 to 50.times.10.sup.-7, respectively. In this
case, mercury iodide includes mercury of 0.75.times.10.sup.-7 to
25.times.10.sup.-7 gram atomic weight/cc. However, since the amount
of mercury contained in the mercury iodide is very small in
comparison with the required amount of mercury, that amount may be
considered negligible. It should be noted again that, with these
substances except for gallium, with less than the specified values,
it is impossible to obtain a required light output and in, the
specified wavelength range with these substances, except for
gallium if greater quantities are used, the light output decreases
and the arc becomes astable. As to the amount of gallium, since it
does not become halogenized gallium and the saturating vapor
pressure of metal gallium at the temperature of the inner wall of
the operating bulb is low, metal gallium which is not converted
into gallium iodide is deposited on the bulb inner wall. However,
since this metal gallium does not affect the arc adversely, there
is no need of defining of an upper limit on the amount thereof.
Although the above Embodiments 4-1 to 4-3 relate specifically to a
spherical bulb having an inner diameter of 30 mm, substantially the
same results can be obtained by using a bulb having an inner
diameter of 20 mm to 50 mm. However, with a bulb having an inner
diameter smaller than 20 mm and for a microwave input of 700 W, the
bulb tends to break within a short time due to the high temperature
even if the amount of cooling air is increased. On the contrary,
with a bulb having an inner diameter larger than 55 mm, the
temperature of the bulb wall will be too cool even if the cooling
air supply is stopped and thus it is impossible to obtain the
necessary vapor pressures of the substances encapsulated in the
bulb in operation resulting in a reduced light output. Therefore,
in order to obtain a required light output within the desired
wavelength range, it is preferable to select the surface area of
the bulb per unit microwave input within the range from 1.5
mm.sup.2 /W to 15 mm.sup.2 /W. This range is also preferable for
Embodiments 1 to 3 in which only mercury is used as the discharge
emissive substance.
EMBODIMENT 4--4
FIG. 19 shows plots of time required to stabilize the light
emission of spherical bulbs 6 having an inner diameter of 30 mm for
different wall thicknesses, and hence total bulb weights, for bulbs
containing argon at 60 mmHg, 80 mg of mercury, 1 mg/gallium, and 4
mg of mercury iodide. The graph of FIG. 19 also contains similar
plots of the time required to stabilize the light emission of bulbs
having inner diameters of 25 mm and 40 mm, respectively, with each
bulb containing suitable amounts of argon, mercury, gallium and
mercury iodide in the same ratio as the 30 mm diameter bulb to
estabilish the same physical and chemical conditions within the
bulbs for comparison purposes. In this embodiment, the stabilizing
time required to stabilize the light output is defined as the time
until the light output reaches 80% of the light output after the
bulb is completely stabilized.
As is clear from FIG. 19, the stabilizing time increases
substantially linearly with increases of bulb weight beyond about 4
g, while for weights of less than 4 g, the effect of shortening the
stabilizing time is not substantial.
With a bulb weight greater than 20 g, the stabilizing time becomes
longer than 1 minute and thus the merit of a microwave discharge
light source apparatus having a short stabilizing time disappears.
It should be noted that the data shown in FIG. 19 was obtained by a
magnetron having a microwave output of 700 W. Since the stabilizing
time depends mainly upon the correlation between the microwave
output and the thermal capacity of the transparent quartz glass
forming the outer wall of the bulb, a larger the microwave input to
the bulb results in a shorter stabilizing time which is
proportional to the thermal capacity of the quartz glass forming
the outer wall of the bulb 6. Therefore, in order to restrict the
stabilizing time within desirable limits, the weight of the bulb 6
for a given microwave input thereto should be set within
predetermined limits. For example, a stabilizing time shorter than
1 minute can be obtained with a bulb 6 having a weight of about
3.0.times.10.sup.-2 g/W or lighter. This is also applicable to
Embodiments 1-3 which are bulbs containing only mercury as emission
substance.
EMBODIMENT 5
A specific circuit of a power source for the magnetron 1 used in
the Embodiments 1 to 4 will now be described with reference to FIG.
20.
In FIG. 20, a transformer T has a primary winding 1P connected
across an A-C supply E and a secondary winding 1S is connected in
parallel with a series circuit of a capacitor C.sub.11 and a diode
D.sub.11. A series circuit of a capacitor C.sub.12 and a diode
D.sub.12 is connected in parallel with the series connection of the
capacitor C.sub.11 and diode D.sub.11. The capacitors C.sub.11 and
C.sub.12 and diodes D.sub.11 and D.sub.12 form a full wave voltage
doubler rectifier whose output voltage is applied to an anode of
the magnetron 1. The transformer T has a further secondary winding
2S having terminals connected to a cathode of the magnetron 1.
By using the full-wave voltage doubler rectifier circuit shown in
FIG. 20, it is possible to restrict the rest period of microwaves
to 5 msec or shorter economically. Further, if a leakage
transformer is used as the transformer T, a microwave output having
a waveform shown in FIG. 21 can be obtained. In FIG. 21, a time
period 181 is a microwave generating period and a time period 191
is the microwave rest period. When this rest period 191 is on the
order of 1 msec, the ionized gas does not extinguish so that a
discharge can be restarted immediately thereafter. Thus, there is
no termination of discharge so long as the rest period is
sufficiently short.
Since with the circuit of FIG. 20 the rest period can be made 5
msec or shorter by using a full-wave voltage doubler rectifier and
a leakage transformer for applying the anode voltage to the diode
of the magnetron 1, there is no termination of discharge caused by
a longer rest period. This is another important effect of the
present invention in comparison with a conventional power source
for a magnetron 1 using a half-wave voltage doubler rectifier in
which the rest period is usually 8 to 10 msec and for which there
is a disadvantage that the discharge may stop after a period of
several to several tens of seconds after discharge initiation
depending on the types of metals encapsulated in the discharge bulb
6. This phenomenon of the conventional power supply can be
considered to be due to the fact that since the metals encapsulated
in the bulb are vaporized and the metal gas atom density in the
bulb after the discharge commences is high so that the amount of
energy derived from the microwave energy injected into the bulb
before collision of electrons with atoms is small, the ionization
probability is lowered below a level necessary to maintain the
discharge. Further, the prior art power supply used to drive the
magnetron to thereby cause it generate microwaves continuously is
expensive. As to the microwave generator itself, there is a
disadvantage that if it first generates microwaves continuously and
then the operation thereof is shifted to the sink region, it is
very difficult to recover the normal operation. There is no such
defect in the apparatus of the invention.
Although the circuit of FIG. 20 has been described as being used
with a single magnetron 1, it is possible to use a pair of
magnetrons for this purpose. In such a case, the magnetrons may be
driven by power supplies having half-wave voltage doubler rectifies
shifted in phase by 180.degree. with respect to each other.
EMBODIMENT 6
In the microwave discharge light source apparatus of any of
Embodiments 1 to 4, it is advantageous to further shorten the
stabilizing time from the discharge initiation of the discharge
bulb 6 through the metal gas discharge to the stabilized discharge
state. The stabilizing time depends upon the evaporation rate of
the metal encapsulated in the bulb 6 and that rate, in turn,
depends upon the rate of temperature increase of the inner wall of
the bulb 6. An increase of the temperature increase rate can be
brought about by a larger discharge energy, i.e., microwave
energy.
In view of these facts, as well as the operational characteristics
of the magnetrons, it has been found that a shortening of the
stabilizing time can be achieved by suitably selecting the length
of the waveguide 3.
In general, the operation of the magnetron can be represented by a
Rieke diagram on an impedance chart as shown in FIG. 22. In FIG.
22, the distance from the center of the chart and the angular
position respectively represent the microwave reflection
coefficient .sigma. and the phase. Lines A to F are equi-output
power lines of the oscillation output of the magnetron with the
line A corresponding to the highest output power line and with the
output gradually decreasing toward the line F. .sigma. indicates
the sink region of the magnetron where the oscillation thereof
becomes abnormal.
FIG. 23 shows an example of the impedance of the cavity 4 after the
discharge bulb 6 is ignited. The impedance of the cavity
immediately after the bulb is ignited is indicated by a point LA.
After the ignition, the impedance of the cavity 4 varies with
variations of the discharge state due to vaporization of the metals
in the bulb becoming constant in the stable state. By matching the
impedance, i.e., regulating the resonance frequency of the cavity
and the dimensions of the feeding opening such that the
characteristic impedance is at the center of the impedance chart,
the impedance varies from the point LA through L to the center of
the chart.
As mentioned above, the output power of the magnetron 1 is largest
at the side of the sink region. Therefore, by making the load
impedance (here, the cavity impedance) seen from the magnetron 2
larger at the sink side, a greater amount of microwave energy can
be provided. When the waveguide 3 is connected to the cavity 4, the
impedance seen from the free end of the waveguide 3 becomes equal
to the cavity impedance rotated around the center of the impedance
chart by an angle corresponding to the length of the waveguide 3.
Therefore, in order to position the line L.sub.1 in FIG. 23 at the
sink side in FIG. 22 by rotation, a line L.sub.2 may be obtained by
rotating the line L.sub.1 by, for example, 0.25 .lambda.g, where
.lambda.g is the wavelength of the waveguide 3. That is, the length
of the waveguide 3 may be 0.25 .lambda.g. As is clear from FIG. 23,
the same effect can be obtained by selecting the length of the
waveguide to be 0.25 .lambda.g+n.times.0.5 .lambda.g where n is an
integer because 0.5 .lambda.g corresponds to one complete rotation
of the line L.sub.1.
Accordingly, in this example, by varying the length of the
waveguide 3 between the cavity 4 and the magnetron 1, the load
impedance for the magnetron 1 varies along the line L.sub.2 so that
the magnetron output power follows the lines A-B shown in FIG. 22
resulting in a larger output power. Therefore, the stabilizing time
can be shortened.
The above description relates to the case where the cavity
impedance is moved from the point LA along the line L.sub.1.
However, the same is also applicable to other impedance conditions
of the cavity which may depend on the shape of the cavity, the
position of the discharge bulb therein and the content of the bulb,
etc. In any case, the length of the waveguide is to be selected so
as to meet these conditions. It should be noted that, generally,
the magnetron output is large where the operating point thereof is
at a position within a quarter wavelength phase width of the
waveguide on the sink side and therefore the length of the
waveguide can be determined by the latter condition.
In the microwave generated plasma light source apparatus described
with reference to Embodiments 1 to 6, it is possible to use a small
discharge bulb 6 and thus the microwave power per unit surface area
of the bulb 6 can be made large even if the microwave power
supplied thereto is relatively small resulting in a high light
emission efficiency. For example, a magnetron having a small
output, such as magnetron used for an electronic cooking range for
home use, may be utilized for this purpose.
As mentioned hereinbefore, the microwave generated plasma light
source apparatus according to the present invention includes a
microwave generator, a microwave cavity serving as a resonance
cavity having a light reflecting member and a member transparent to
light and but opaque to microwaves, a waveguide for guiding
microwave generated by the microwave generator to a feeding opening
of the cavity, and a small non-electrode discharge bulb which is
disposed in a position in the cavity such that the cavity operates
as a resonant cavity at least when the bulb is lit with the bulb
being sufficiently small that it can be approximated as a point
light source and discharge light emissive substances therein are
encapsulated therein. With this construction, it is possible to
supply microwaves to the bulb efficiently, the optical design is
facilitated and a illumination distribution in achieved or another
desired illumination distribution can be provided.
Particularly, if gallium is added as the discharge emissive
substance to the discharge bulb, the light emission characteristics
of the light source apparatus are suitable for use in a
photographic plate making which requires a highly uniform
illumination distribution in a specific area.
The light source apparatus of the invention can be used also as a
light source such as a spotlight source, which requires a small
light source of high output, by modifying the shape of the light
reflecting member constituting the microwave resonance cavity.
* * * * *