Note: Descriptions are shown in the official language in which they were submitted.
~ ~t7~
The present invention relates to a metal vapor dis-
charge lamp having a translucent ceramic luminous tube and also
to a method of producing such a metal vapor discharge lamp. More
particularly, -the inventlon is concerned with a luminous tube of
the type mentioned above, provided wi-th heramic electrode sup--
porting tubes attached to both ends thereof, as well as method of
producing such a luminous tube.
The present invention wi:ll be illustrated by way of the
accompanying drawings in which:-
Fig. 1 is a plan view of a luminous tube incorporatedin a conventional metal vapor discharge lamp;
Fig.s 2A and 2B are an enlarged plan view and a sec-
tional view of a hermetically sealed portion of an electrode sup-
porting tube in the luminous tube shown in Fig. l;
Fig. 3 is a plan view of a luminous tube of a known
metal vapor discharge lamp proposed by the present inventors;
Fig. 4 is a partly-sectioned plan view of an example of
the luminous tube incorporated in a metal vapor discharge lamp in
accordance with an embodiment of the invention;
Fig. 5 is a sectional view of an end cap holding an
exhaust electrode supporting tube;
Fig. 6 is a sectional view of an end cap holding a non-
exhaust electrode supporting tube;
Fig. 7 is a partly-sectioned plan view of a luminous
tube assembly; and
Fig. 8 is an illustration of a system suitable for use
in the sealing of the luminous tube assembly.
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~J ~ 1 --
In general, metal vapor discharge lamps having high
luminous efficiency such as a high~pressure sodium lamp have a
translucent ceramic luminous tube which is composed of a cylin-
drical ceramic tube member and ceramic or metallic end caps
heremetically closing both ends of the ceramic tube member. The
interior of the ceramic tube is charged with a metal such as mer-
cury and sodium after evacuation.
Broadly, the method for heremetically sealing the lumi-
nous tube after charging with a met:al can be sorted into twotypes: a method which does not make use of an exhaust tube and a
method which makes use of exhaust tube.
In the first-mentioned method which employs no exhaust
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tube, a series of the s-teps such as evacuation of the
interior of the ceramic tube member, charging with a metal
and attaching of end caps to the ceramic tube member are
conducted in a hermetic assembly room such as a bell-jar,
employing a complicated assembly system. This method,
therefore, is extremely difficult to conduct and can provide
only a low efficiency of work.
For this reason, the second-mentioned method relying
upon exhaust tube is more popular. Fig. 1 exemplarily
shows a luminous tube with an exhaust tube. The luminous
tube is composed of a ceramic tube member 1 and alumlna end
caps 2 and 3 attached to both ends of the ceramic tube
member 1 by means of frit. Electrode supporting tubes 4 and
5 made of a heat-resistant metal such as niobium are fitted
to the center of the end caps 2 and 3, respectively. The
electrode supporting tubes 4 and 5 support respective
electrodes at their inner ends which are projected into the
tube member 1, thus serving as conductors for supplying
electric power to the electrodes. One of the electrode
supporting tubes, e.g., the electrode supporting tube 4, is
intended ~or use as the exhaust tube, through which the
interior of the luminous tube is evacuated and charged with
a metal such as mercury and sodium. This electrode
supporting tube 4, therefore, will be referred to as
"exhaust electrode supporting tube", hereinunder.
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56136
This l.uminous tube, having an exhaust tube constituted
by one of the electrode supporting tubes, is fabricated by
the following method. As the first step, the electrode
supporting tubes 4 and 5 are lnserted into central holes
formed in the alumina end caps 2 ancL 3. The electrode
supporting tubes 4 and 5 are hermetically fixed to the
alumina end caps 2 and 3 by means of a frit, simultaneously
with the fixing of the alumina end caps 2 and 3 to the
ceramic tube member 1. Subsequently, the outer end of the
electrode supporting tube 5, which is not intended for use
as the exhaust tube (referred to as "non-exhaust electrode
supporting tube",hereinunder), is cut after a cold
press-bonding followed by arc welding of the cut end as
necessitated, thus forming an end seal 5' having a shape as
shown in Fig~ 1. Subsequently, the evacuation of the
interior of the ceramic tube member 1 and the charging of
the same with a metal are conducted through the exhaust tube
constltuted by the exhaust electrode supporting tube 4 and,
thereafter, the outer end of the electrode supporting tube 4
is cold press-bonded and cut to form an end seal 4' in the
same way as the sealing of the outer end of the electrode
supporting tube 5.
When a luminous tube for a metal vapor discharge lamp
such as a high-pressure sodium lamp is produced through the
aid of the exhaust tube, the outer ends of the electrode
5~
supporting tubes are sealed by cold press-bonding followed
by cutting, so that the end extremities 6 of these sealed
ends have the form of blades as shown in Fig. 2A and are
tipped to have reduced thickness as shown in Fig. 2B. In
particular, both widthwise ends 6a r 6a of each sealed end
have an extremely small thickness ancl, hence, are liable to
be damaged, causing a risk of leak. Therefore, the
evacuation and sealing operation, as well as the mounting of
the luminous tube in an outer bulb, have to be done with
greatest care.
When the end seals of the electrode supporting tubes is
conducted by cold press-bonding, the sealing operation has
to be done after mounting, on the alumina end caps, not only
the exhaust electrode supporting tube but also the
non-exhaust elextrode supporting tube. In consequence, a
laborious work is required for hermetically fixing the end
caps to the ends of the ceramic tube member.
In order to obviate the above-described problems, the
present inventors have already proposed an improved metal
vapor discharge lamp in Japanese Utility Model Laid-Open No.
182359/1983. In this metal vapor discharge lamp, as shown
in Fig. 3, the outer end of the non-exhaust electrode
supporting tube 5 is sealed hermetically by fusing the tube
material by other means than the cold press-bonding, e.g.,
by an arc discharge, thereby forming a hermetic sealed end
5". According to this method, the work for fusing and
sealing the end of the non-exhaust electrode supporting tube
5 can be conducted without substantial diEficulty and
independently of the evacuation and charging of the interior
of the ceramic tube member. In addition, since the
attaching of the electrode supporting tube 5 to the
associated end cap can be done after the sealing of the end
of the tube 5, the tube 5 can be handled easily after it is
hermetically attached to -the end cap ~ of the ceramic tube
member. In addition, the work for the assembly of the
electrode supporting tube 5, the end cap 3 and the ceramic
tube member 1 is facilitated, thus contributing greatly to
the improvement in the efficiency of the work.
When the present inventors proposed this improved metal
vapor discharge lamp, and even thereafter, it has been
considered to be quite difficult to adopt the proposed
sealing method to the sealing of the exhaust electrode
supporting tube 4 and, therefore, the proposed sealing
method has been applied onl~ to the sealing of the end of
the non-exhaust electrode suporting tube 5.
More specifically, the exhaust electrode supporting
tube 4 has to b~ sealed in the final step of the production
process after the charging with the metal in the ceramic
tube member. In other words, this electrode supporting tube
4 cannot be sealed before the mounting on the associated end
~.~7~;6~3~
cap, unlike the non-exhaust electrode supporting tube 5.
When the exhaust electrode supporting tube is sealed by
the fusing-type sealing method in the final step of the
production process, the heat of the arc welding for fusing
the end of the electrode suppoxting l:ube adversely affects
the glass frit by which the end cap is hermetically fixed to
the end of the ceramic tube member and also the charged
metal carried by the electrode supporting tube. This
problem is sexious particularly in the case where the
exhaust electrode supporting tube serves also as a metal
reservoir in which the charged metal is reserved. Namely,in
such a case, the charged metal is evaporated and scattered
by the heat generated during the sealing operation, and is
mixed into the fused end of the electrode supporting tube,
causing troubles such as leak during operation of ~he
luminous tube. In addition, since the charged metal absorbs
impurity gases evaporated from the material of the electrode
supporting tube~ the purity of the charged metal is impaired
to adversely affect the operation characteristics of the
product luminous tube.
E'or these reasons, the sealing of the exhaust electrode
supporting member has been conducted by cold press-bonding
followed by cutting. The problern therefore still remains
in connection with the likelihood of damaging of the sealed
end 4' of the exhaust electrode supporting tube 4 and
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75~
reliability is still low with reyard to the sealed end 4',
requiring a greatest care in the handling of the luminous
tube after the sealing.
Another problem is encountered when the exhaust
electrode supporting tube is used also as the metal
reservoir. Namely, a temperature gradient appears during
operation of the metal vapor discharge lamp such that the
outermost end of the exhaust electrocle supporting tube
experiences the lowest temperature. The vapor pressure in
the luminous tube and, hence, the lamp voltage are changed
in relation to a change in this lowest temperature. This
means that the length of projection of the exhaust electrode
supporting tube beyond the end cap, which affects the
temperature of the coldest outer end extremity of this
electrode supporting tube, is a significant factor which
determines the lamp voltage. Thus, the projection length
has to be designed and selected in due consideration of the
lamp voltage.
This, however, goes quite contrary to the demand from
the view point of production. Namely, when the sealing of
the exhaust electrode supporting tube is conducted by cold
press-bonding after the mounting on the end cap, a
considerable length of the electrode supporting tube has to
be projected beyond the end cap, in order to prevent the
- 7 -
juncture between the end cap and the electrode supporting
tube from being afEected by the deformation of the electrode
tube end caused by the cold press-bonding. In addition, in
order to make sure of the tight seal of the end of the
electrode supporting tube by the colcl press-bonding, the
press-bonding has to be done over a substantial length. This
means that the electrode supporting tube has to be prepared
in a large length, resulting in an inefficient use of the
expensive ma-terial such as niobium. Thus, the length of
projection of the exhaust electrode supporting tube has to
be determined also taking these factors into account.
Thus, when the electrode supporting tube is sealed by
the cold press-bonding, it is quite difficult to determine
projection length of the electrode supporting tube beyond
the end cap on the basis of the lamp voltage solely, and the
actual determination of the projection length encounters
various restrictions.
It is to be pointed out also that, when the sealing is
conducted by the cold press-bonding, the projection length
and the shape of the electrode supporting tube fluctuate
largely, resulting in 1uctuation of the temperature at the
coldest end of the electrode supporting tube, which in turn
causes a variation in the lamp voltage of the metal vapor
discharge lamp as the product.
- 8 --
The sealing of the end of the exhaust electrode supporting tube 4
by cold press-bonding causes also a problem in connection with a
minute gap 7 which is formed in the sealed portion 4' as sho~n in
Fig. 2B. Namely, during the operation of the lamp, the region
around this minute gap 7 constitutes the col.dest portion in the
luminous tube, so that the charged metal such as sodium amalgam
tends to invade this minute gap 7. The sodium amalgam thus
trapped in the minute gap tries -to evaporate as the lamp is
started agaln but cannot evaporat~ perfectly. In consequence,
the operation characteristics tend to be degraded particularly in
the case of lamps in which the amount of the charyed metal is
small or in the case of so-called unsaturated-type sodi.um lamp.
Accordingly the present invention provides a metal vapor
discharge lamp having a ceramic luminous tube which suffers from
only a small lamp voltage fluctuation and which exhibits improved
starting characteristics, higher reliabllity of the seal of the
electrode supporting tube and higher rate of utilization of
expensive material, as well as a method for producing such a
metal vapor discharge lamp, thereby overcoming the above-
dsscribed problems of the prior art.
The invention in its one aspect provides
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~-r
~i~
a metal vapor discharge lamp having a luminous tube
constituted by a translucent ceramic tube member, end caps
hermetically fixed to both ends of the translucent ceramic
tube member, and el.ectrode supporting tubes hermetically
inserted into respective end caps such as to partly project
outwardly from the translucent ceram:ic tube, one of the
electrode supporting tubes being an exhaust electrode
supporting tube which serves also as an exhaust tube for
evacuation and also as a reservoir for a me-tal charyed in
the luminous tube, the outer end extremity of the one of the
electrode supporting tubes constituting the coldest portion
of the metal vapor discharge lamp during the operation of
the tube, wherein improvement comprises that the outer end
of at least the one of the electrode supporting tubes is
hermetically sealed through fusion by application of heat.
roJ'c~ti~n
With this arrangement, the ~ e~ length of the
electrode supporting tube which constitutes the coldest
portion of the luminous tube can be determined freely in
consideration of the lamp voltage, without substantially
taking into account other factors. By virture of this
feature, the fluctuation of the lamp voltage is reduced and
the lampstarting characteristics are improved
advantageously. In addition, the reliability of the seal on
the end of the electrode supporting tube is improved because
there is no thin-walled blade end portion on the electrode
-- 10 --
supportlng tube, unlike the discharge lamp produced by the
cold press-bonding.
The present invention provides in its another aspect a
method of producing a metal vapor discharge lamp comprising
the s-teps ofO preparing a first electrode supporting tube
having an electrode fixed to the inner end thereof and
unsealed at its outer end, the first electrode supportiny
tube serving also as an e~haust tube for evacuatinon and as
a reservoir for storing a charged metal, and preparing also
a second electrode supporting tube having an electrode fixed
to the i.nner end thereof and hermetically sealed at its
outer end; inserting the first and second electrode
supporting tubes to respective end caps, hermetically fixing
the end caps to respective ends of a translucent ceramic
tube; placing the assembly of the translucent ceramic tube,
the end caps and the electrode supporting tubes in a
hermetic vessel and then evacuating the interiors of the
hermetic vessel and the translucent ceramic tube, followed
by charging of an inert gas and charging of a metal in the
first electrode supporting tube with the unsealed end;
expelling the inert gas and charging the hermetic vessel and
the translucent ceramic tube with a lamp starting gas up to
a predetermined pressure; and sealing the unsealed outer end
of the first electrode supporting tube -through fusion by
application of heat, with a heat-shieldingtabsorbing plate
~5~
held in close contact with the pro;ected outer end portion of the
first electrode suppor-ting tube within the atmosphere o~ the
lampstarting gas.
According to this method, when the end of the exhaust
electrode supporting tube is fused by application of heat dur~ng
sealing process, the end of the electrode supporting tube can be
cooled and solidified without delay by virtue of the presence of
heat-shielding/absorbing plate which effectively absorbs the
heat. The heat-shielding/absorbing plate effectively shields and
absorbs the heat applied during the sealing operation, so that
the undesirable evaporation of the charged metal in the electrode
supporting tube can be prevented, thereby obviating various trou-
bles which may otherwise be caused during the lamp operation,
such as the leak attributable to the fusion of the evaporated
material into the sealed portion of the tube and the deteriora-
tion of the operation characteristics of the luminous tube. For
the same reason, the reliability of the hermetic seal of the tube
is enhanced and the fluctuation of the lamp voltage is suppressed
advantageously.
In one embodiment of the present invention of the lamp
the other electrode supporting tube does not serve as the exhaust
tube, and is sealed at its outer end through fusion by applica-
tion of heat. Desirably, the length of pro;ection of said one ofsaid electrode supporting tubes beyond said end cap is greater
than that of the other electrode supporting tube. Suitably, the
length of projection of said one of said electrode supporting
tubes beyond said end cap is greater than that of the other elec-
trode supporting tube. Desirably, said metal charged in saidluminous tube is sodium amalgam. Suitably, said metal charged in
said luminous tube is sodium amalgam.
~ - 12 -
7~
The present invention further provides a metal vapor
discharge lamp comprising an elongate translucent ceramlc tube
having a wall at each end thereof; an aperture through each
respective end wall; and a pair of electrode supportlng tubes
with the extPrior surface of each supporting tube hermetically
sealed in one of said apertures with one end of each electrode
supporting tube projectins outward from said translucent ceramic
tube, one of said electrode supporting tubes comprising an
exhaust tube ~or evacuation of said lamp and a reservoir for
metal charged in said lamp and havi:ng its outer end hermetically
sealed through fusion by the application of heat.
The present invention again provides a method of pro-
ducing a sealed metal vapor discharge lamp of the type which
includes an elongate luminous tube having an electrode supportlng
tube pro~ecting from each end thereof comprising the steps of
providing an elongate translucent ceramic tube having an aperture
at each end thereof; preparing a first electrode supporting tube
having an electrode fixed to the inner end thereof and unsealed
at its outer end, said first electrode supporting tube serving
also as an exhaust tube for evacuation and as a reservoir for
storing a charged metal; preparing a second electrode supporting
tube having an electrode fixed to the inner end thereof and her-
metically sealed at its outer end; inserting said first and sec-
ond electrode supporting tubes to respective ones o~ said aper-
tures at each end of said translucent ceramic tube; hermetically
fixing said electrode supporting tubes to respective ones of said
apertures at each end of said translucen~ ceramic tube; placing
the assembly of said translucent ceramic tube, and said electrode
supporting tubes in a hermetic vessel; evacuating the interior of
said hermetic vessel and said translucent ceramic tube; charging
said hermetic vessel with an inert gas; charging a metal in said
first electrode supporting tube; expelling said inert gas and
charging said hermetic vessel and said translucent ceramic tube
with a lamp starting gas up to a predetermined pressure; provid-
ing a heat-shielcling~absorbing plate in close contact with the
- 13 -
~ ~t7~
projected outer end portion of said first electrode supporting
tube; and sealing said unsealed outer end of said first electrode
supporting tube through fusion by application of heat while main-
taining said heat-shielding/absorbing plate in close contact with
said projected outer end portion of said ~irst electrode support-
ing tube within the atmosphere of said lamp starting gas.
The invention will be descrlbed hereinunder with refer-
ence -to the accompanying drawings.
- 13a -
~ ~S6~3~
Refexring first to E'ig. 4 which is a par-tly-sectioned
plan view of an example of a luminous tube used in a metal
vapor discharge lamp embodying the present invention, the
luminous tube generally designated by a reference numeral 10
has a translucent ceramic tube member 11 made of translucent
alumina. End caps 12 and 13 which also are made of alumina
are hermetically attached to both ends of the ceramic tube
member 11, through the intermediary of frit. The end caps
12 and 13 are provided with central holes which receive,
respectively, electrode supporting tubes 14 and 15 made of
niobium. The electrode supporting tubes 14 and 15 are
hermetically fixed to the end caps through frit. Electrodes
16 and 17 are supported by the inner ends of the electrode
supporting tubes 14 and 15, respectively.
One of the electrode supporting tubes, the supporting
tube 14 in this case, is utilized as an exhaust tube through
which the interior of the ceramic tube member 11 is
evacuated and then charged with a charged metal. The
evacuation and charging are conducted through an exhaust
hole 18 and the outer end opening of the tube 14. ~fter the
evacuation and charging, the outer end of the exhaust
electrode supporting tube 14 is closed by fusion such as to
form a hermetic sealed end 14a~ In this state, the exhaust
electrode supporting tube 14 has a tubular form with a
closed bottom. The exhaust electrode suppoxting tube 14
- 14 -
~.2~6~3~
projec-ts outwardly beyond the end cap 12 by a distance which
is greater than the length of projection of the other
electrode supporting tube 15 beyond the end cap 13, so that
the coldest portion is formed on the outer end of the
electrode supporting tube 14.
The other electrode supporting tube 15 is not designed
for use as an exhaust tube. Be~ore the evacuation through
the exhaust electrode supporting tube 14, the non-exhaust
electrode supporting tube 15 is subjected to the same
sealing operation as the exhaust electrode supporting tube,
i.e., closing by fusion such as to form a hermetic sealed
end 15a, thus having a tubular form with a closed bottdm\. A
reference numeral 19 denotes a hole which is formed in th'e~
wall of the non-exhaust electrode supporting tube 15 for
allowing the air in the tube 15 to escape, thus preventing
trapping of air in the electrode supporting tube 15, while a
numeral 20 designates a charged metal which in this case is
sodium amalgam. The charged metal is charged in the
electrode supporting tube 14 in advance of the sealing
operation. When the lamp is not operating, -the sodium
amalgam is accummulated in the electrode supporting tube 14.
During the operation of the lamp, the sodium amalgam is
evaporated and diffused into the luminous tube 10 by an
amount corresponding to the temparature of the outer end of
the electrode supporting tube 14.
- 15 -
7~86
This luminous tube 10 is mounted in an outer bulb (not
shown) known per se by a known measure, thus forming a metal
vapor discharge lamp.
As will be understood from the foregoing description,
according to the invention, at least one of the electrode
supporting tubes which serves also as an e~haust tube and a
reservoir for the charged metal is closed by fusing at its
outer end, thus forming a hermetic sealed end. Therefore,
the fragile thin-walled blade end, which here-tofore has bee.n
formed when the sealing is conducted by cold press-bonding,
is eliminated such as to ensure a high reliability of the
sealed end. For the same reason, the fluctuation in the
projection length of the electrode supporting tube is
suppressed advantageously. It is to be understood also that
the operation characteristics of the metal vapor discharge
lamp, particularly in the unsaturated-type lamp, is improved
remarkably because of elimination of the minute gap which is
inevitably formed in the coldest sealed end of the electrode
supporting tube in the conventional luminous tube sealed by
cold press-bonding.
A description will be made hereinunder as to the method
of producing a metal vapor discharge lamp of the invention
having the described embodiment. As the first step of the
production process, the electrode supporting tube 14 made of
niobium, intended for use also as an exhaust tube, is
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~ ~75~
inserted into the central through hole 12a in the alumina
end cap 12 having a disk-like form, through an intermediary
of a frit, thus comple-ting one end cap assembly 21 as shown
in Fig. 5. The electrode supporting tube 14 is beforehand
provided with the exhaust hole 18 formed therein and with
the electrode 16 attached thereto.
Subsequently, as shown in Fig. 6, the other electrode
supporting tube 15 which is not intended for use as the
exhaust tube also is formed from niobium, with the electrode
17 fixed to one end thereof and with its outer end 15a
hermetically sealed by fusion through, for example, an arc
welding such as TIG welding conducted in argon gas. As
stated before, the electrode supporting tube 15 is provided
with a hole lg for preventing air from being trapped in the
tube 15. However, this hole 19 may be omitted provided that
the juncture between the electrode 17 and the electrode
suporting tube 15 is hermetically sealed to such a degree as
not to permit air in the tube 15 from escaping into the
luminous tube. This electrode suporting tube 15 is inserted
into the central throu~h hole 13a of the disk-shaped alumina
end cap 13, through the intermediary of a frit, thus
completing the other end cap assembly 22. The end cap
assemblies 21 and 22 are then hermetically fixed to both
ends of the ceramic tube member 11 by fusion through a frit
as shown in ~ig. 7,
thereby closing both ends of the ceramic tube member 11.
Meanwhile, the electrode supporting tubes 1~ and 15 also are
hermetically fixed by fusion throuyh the frit to respective
end caps 12 and 13 r such that the electrode supporting tubes
14 and 15 project over predeterminecl lengths beyond the end
caps 12 and 13. More specifically, the projection length of
the electrode supporting tube 15 is selected to be smaller
than the projection length of the electrode supporting tube
14 which is determined such that the projection length after
the sealing by fusion corresponding to the lamp voltage to
be obtained.
The ceramics tube 11, end caps 12, 13 and the electrode
supporting tubes 14, 15 hermtically assembled together
constitute a luminous tube assembly which is generally
designated at a numeral 30. The luminous tube assembly 30
thus formed i9 placed in a hermetic vessel 31 which is shown
in Fig. 8. A discharge electrode 33 connected to one of the
output terminals of an arc generator 32 of an arc welder is
disposed in the vessel 31 such as to oppose the outer end of
the exhaust electrode supporting tube 14 of the luminous
tube assembly 30. A heat-shielding/absorbing plate 34 is
disposed to tightly fit on the projected portion of the
electrode supporting tube 14 such as to be held in close
contact with the same. The heat-shielding/absoxbing plate 34
is connected to the other o~ltput terminal of the arc
- 18 -
generator 32. The heat-shielding/absorbiny plate 34 is
preferably made of a material which has a high heat
conductivity, as well as high resistance both to heat and
arc. A typical example of such a material is molybdenum.
When the heat-shielding/absorbing plate 3~ is made of an
electrically non-conductive material, the other output
termlnal of the arc generator 32 is connected ~irectly to
the electrode supporting tube 14.
Subsequently, the interior of the hermetic vessel 31 is
evacuated and is charged with argon gas. Then, a
predetermined amount of mixture of sodium and mercury, i.e.,
sodium amalgam, is charged into the unsealed electrode
supporting tube 14. Then, after evacuating the interior of
the hermetic vessel 31 to a high degree of vacuum, the
interior of te hermetic vessel 31 and, hence, the interior
of the luminous tube assembly, are charged with xenon gas
which is a starting gas for the luminous tube up to a
pressure of 15 to 350 Torr. The xenon gas is bound to
remain in the luminous tube after the sealing of the tube.
After the charging with the xenon gas, arc generator 32 is
actuated to effect an arc discharge between the exhaust
electrode supporting tube l~ and the opposing discharge
electrode 33, using the xenon gas as a discharge gas. In
consequence, the outer end of the electrode supporting tube
1~ is molten and solidified, such as to form a hermetic
-- 19 --
56~
sealed end 14a similar to the hermetic sealed end 15a of the
non-exhaust electrode supporting tube 15, thus completing a
luminous tube 10 as shown in Fig. 4 The thus formed lumi-
nous tube 10 is mounted in an outer bulb (not shownl known
per se by a ]cnown method, whereby a metal vapor discharge
lamp is completed.
During the sealing of the outer end of -the exhaust
electrode supporting tube 14 by arc discharge, the melting
of the outer end of the electrode suppor-ting tube 14 does
not propagate beyond the heat-shielding/absorbing plate 34
which is held in contact with the electrode supporting tube
14 and, as the arc discharge is ceased, the molten end
portion of the electrode supporting tube 14 is solidified
without delay, thus forming a hermetic sealed end 14a. It
is, therefore, possible to constantly obtain a desired
projection length of the electrode supporting tube 14 after
the sealing, by suitably selectir.g the position of the
heat-shielding/absorbing plate 34 with respect to the
electrode supporting tube 14 on which it is tightly fitted.
The heat-shielding/absorbing plate 34 offers another
advantage in that it effectively ahsorbs the heat produced
by the arc discharge so as to prevent the heat from
adversely affecting the glass frit be-tween the electrode
supporting tubes 14, 15 and th eassociated end caps 12, 13,
as well as the glass frit between the end caps 12, 13 and
- 20 -
5~
adjacent ends of the ceramic tube 11. The
heat-insulating/absorbing plate 3~ also prevents heating and
evaporation of the sodium amalgam as the charging metal so
as to avoid the undesirable fusion of the evaporated sodium
amalgam into the fused portion of the electrode supporting
tube 14. For the same reason, any impediment on the sealing
arc discharge, due to contamination of the inner wall of the
hermetic vessel 31 by sodium amalgam attaching thereto, is
avoided conveniently.
In the described embodiment of the method of the
invention, the outer end of the electrode supporting tube is
directly fused and sealed by arc discharge without any
mechanical processing, this is not exclusive and the end of
the electrode supporting tube may be sealed in two steps:
namely, a mechanical work for collapsing and flattening the
tube end for facilitating a subsequent sealing by fusion,
and the fusion for sealing the tube end. Obviously, the
sealing of the electrode supporting tube may be conducted by
other means than the described arc discharge, e.g~, by means
of a laser~ The sealing of the ou-ter end of the
non-exhaust electrode supporting tube 15 may be effected
under atmospheric pressure by means of, for example, a
commercially available torch.
The described embodiment of the production method in
- 21 -
~7s~
accordance with the invention shows only the basic form of
the invented method in which only one luminous tube assembly
is procesed at one time within the hermetic vessel. This,
however, is not exclusive and the arrangement may be such
that a mul-tiplicity of luminous tube assemblies 30 are
disposed in the hermetic vessel 31 and corresponding
discharge electrodes 33 are placed in face-to-face relation
to the exhaust elec-trode supporting tubes 14 of the luminous
tube assemblies 30 or, alternatively, such that a single
discharge electrode is movable to ~ace the exhaust electrode
supporting tube 14 of successive luminous tube assemblies.
With such an arrangement, it is possible to conduct the
evacuation and sealing operation on a multiplicity of
luminous tube assemblies concurrently or successively.
Table 1 shows the result of an experiment which was
conducted to examine the fluctuation of lamp voltage in the
metal vapor discharge lamps incorporating the luminous tubes
produced by the method described hereinbefore, in comparison
with the lamp voltage fluctuation in the conventional metal
vapor discharge lamps in which the sealing of the exhaust
electrode supporting tube is carried out by cold
press-bonding.
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75~i~tii
Table l
L onventional lamps Lamps of invention
lot 1 lot 2 lot 1 lot 2
. ,_
n 97 99 99 98
V~ 129 131 131 129
~_ 5.79 5.63 r 3-32 3.~3
_
In Table 1 above, a symbol n represents the number of
the discharge lamps employed in the test, while V~represents
the mean value of the lamp voltages. The fluctuation of the
lamp voltage is expressed in terms of fluctuation factor ~.
~ rom table 1, it will be understod that the metal vapor
discharge lamps in accordance with the invention exhibits
much smaller lamp voltage fluctuation as compared with the
conventional metal vapor discharge lamps. This owes to the
facts that ~he shape of the sealed end of the exhaust
electrode supporting tube is simplifie~ by virture of the
adoption of fusion type sealing method, and that the length
of projection from the luminous tube is regulated thanks to
the provision of the heat-shielding/absorbing plate which
permits the control of position where the hermetic seal is
formed on the end of the exhaust electrode supporting
tube.
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These advantageous ef~ects are de.rived from the
elimination of fluctuation of the projection length of the
electrode supporting tube serving also as an exhaust tube
and a metal reservoir, the end extremity of the projected
end of this tube constituting the coldest portion of the
metal vapor discharge lamp. Thus, these advantageous
effects have nothing to do with the .lengkh of the projection
of the other electrode supporting tube which does not
constitute the coldest portion. That is, the advantage of
the invention i.s never impaired even when the other
non-exhaust electrode supporting tube is sealed by other
means than the fusion by application of heat, although the
sealing of this tube by fusion as in the described
embodiment is preferred from the view point of reliability
of the seal.
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