Ceramic arc tube of metal vapor discharge lamps and a method of producing the same

The disclosed ceramic arc tube of metal vapor discharge lamps has an arc discharge portion with electrode-holding end portions integrally formed at opposite ends thereof. The outside diameter of the arc discharge portion is larger than that of the electrode-holding end portions. The ceramic arc tube is made by placing a tubular green body in a fusiform cavity of a die, inflating the middle portion of the green body more than end portions thereof, and firing the thus shaped green body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a ceramic arc tube of metal vapour discharge 
lamps such as high-pressure metal vapour discharge lamps and a method of 
producing the same. More particularly, the invention relates to a ceramic 
arc tube of discharge lamps which arc tube has an arc discharge portion 
integrally formed with electrode-holding end portions, the arc discharge 
portion having a larger outside diameter than that of the end portions, 
and a method of producing such ceramic arc tube. 
2. Description of the Prior Art 
High pressure metal vapour discharge lamps using recently-developed 
translucent polycrystalline alumina ceramic arc tubes, which arc tubes 
withstand vapours of such metals as sodium or metal halides, have a high 
luminous efficiency, so that such discharge lamps have attracted much 
attention from the standpoint of energy saving. In the description of the 
invention, the metal vapour discharge lamp refers to the high pressure 
sodium vapour discharge lamp, the metal halide vapour discharge lamp, or 
the like. 
The metal vapour discharge lamp comprises an arc tube holding metal vapour 
and a protective envelope surrounding the arc tube. Accordingly, the arc 
tube is required to have both a good translucency of light and a high 
corrosion resistivity against the light-emitting material sealed therein 
such as sodium vapour or the metal halide vapour. Only translucent alumina 
ceramics has been found to meet the need of high corrosion resistivity 
against the light-emitting material and the good translucency, so that the 
alumina ceramics has been used almost exclusively for the arc tube of the 
high-pressure metal vapour discharge lamps. 
The transluscent alumina ceramics, however, has a lower thermal 
malleability than the quartz. Thus, although the quartz arc tube for 
mercury-vapour lamps can be melted and sealed simply by heating it to a 
high temperature, the sealing of the alumina ceramics arc tube with the 
light-emitting material disposed therein requires a comparatively 
complicated process. 
In a typical conventional process of sealing a translucent alumina ceramic 
arc tube, the opening ends of the fired alumina arc tube is sealed by 
means of glass frit material with the mounting caps made of either a 
heat-resistant metal or alumina ceramic which have a coefficient of 
thermal expansion similar to that of the alumina arc tube. 
Furthermore, heat-resistant metallic electrodes provided with the 
through-holes for introducing the light emitting materials are sealed at 
the center portion of the above caps by glass frit. 
The conventional sealing process has shortcomings in that the process is 
difficult to carry out because of the requirements of heating at the high 
temperature of 1,300.degree. to 1,400.degree. C. and in vacuo. 
Moreover, in the glass frit sealed arc tube, the light emitting materials 
enclosed in the ceramic tube is susceptible to leakage due to the 
comparatively widely sealing area of glass frit, exposure to the high 
operating temperature and thermal shock caused by on-off operations of the 
lamp. 
Especially, when being used in the improved discharge lamp provided with a 
high luminous efficiency and high colour rendering, the alumina tube 
sometimes fail to meet the required reliability including the errosion 
resistivity at a high temperature under high pressure. Furthermore, the 
use of the caps made of metal or ceramics results in an increased number 
of parts and requirement of severe dimensional accuracy, whereby the 
manufacturing cost becomes high and the products tend to be uneconomical. 
To obviate the aforesaid shortcomings, the so-called semi-closed type 
alumina arc tubes have been proposed, in which ceramic caps are applied to 
opposite ends of each alumina tube before firing in such a manner that the 
caps are integrally secured to the alumina tube when they are fired 
together. More specifically, such semi-closed type alumina arc tube is 
generally produced by a method comprising steps of preparing a tubular 
green body having opposite ends thereof open by using an alumina series 
material whose firing shrinkage is fully known, preparing cap green bodies 
by using an alumina series material whose firing shrinkage is smaller than 
that of said tubular green body, fitting the cap green bodies in end 
openings of the tubular green body, and firing the tubular green body 
having the cap green bodies in vacuo or in hydrogen atmosphere. Whereby, a 
translucent alumina arc tube with caps integrally secured thereto is 
produced by the firing. This method of making the semi-closed type alumina 
arc tube has shortcomings in that the step of applying the cap green 
bodies to the tubular green body tends to cause deformation and damage of 
the green bodies, that the control of the firing shrinkages of the tubular 
green body and the cap green bodies is difficult, and that cracks are 
sometimes occured at end portions of the alumina arc tube to cause 
incomplete joint of the caps with the alumina arc tube which lead to 
possible leakage of the sealed light-emitting material. 
In another method of the prior art to produce an arc tube having an alumina 
tube with caps integrally formed therewith by using the same material for 
the tube and the caps, a molding core made of a metal or organic substance 
having a low melting point is placed in the cavity of a die, and an 
integral body of the alumina tube with caps is formed in the space between 
the inner surface of the die and the molding core by applying pressure 
from the outside of the die. The molding core is then melted away by 
heating, out of the alumina arc tube. This method of using the molding 
core has technical difficulties in that pressing of the tubular alumina 
green body to the molding core tends to contaminate the tubular alumina 
green body with the material of the molding core, that the molten material 
of the molding core sometimes permeates into the alumina arc tube, and 
that traces of the molding core material left on the alumina arc tube 
become defects. Accordingly, this method of using the molding core has not 
commercially been used in industries due to the aforesaid technical 
difficulties. 
The shape of the alumina arc tube for metal vapour discharge lamps has been 
limited to straight tube, because the malleability of the alumina arc tube 
is not so high as the quartz arc valve used in mercury-vapour lamps. The 
quartz valve can be easily shaped simply by heating it to a high 
temperature. Although the salient feature of the metal vapour discharge 
lamp depends on the high luminous efficiency, it is hard to further 
improve the luminous efficiency if the shape of the arc tube is limited to 
straight one. More specifically, the transmittance of the translucent 
alumina ceramics has been improved to 94 to 96% already, so that there is 
not much room left to improve the luminous efficiency by raising the 
transmittance of the alumina arc tube. 
Theoretically, the luminous efficiency may be improved by raising the 
vapour pressure, i.e., by raising the wall loading of the arc tube, as 
confirmed by experiments. In practice, however, when the wall loading 
exceeds the currently used level such as 20 W/cm.sup.2 in the case of the 
high-pressure sodium vapour discharge lamp, the temperature at the center 
portion of the arc tube becomes very high, e.g., 1,200.degree. to 
1,300.degree. C., so that the metal vapour in the arc tube such as sodium 
vapour reacts with the alumina tube resulting in the blackening phenomenon 
which shortens the service life of the discharge lamp. Accordingly, the 
improvement of the luminous efficiency by raising the wall loading of the 
arc tube is not practicable. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to obviate the aforesaid 
shortcomings and technical difficulties of the prior art, by providing an 
improved method of producing the arc tube. In a method of the present 
invention, a tubular green body is made by using mixture material of 
alumina fine particles and plasticizer whose main ingredient is 
non-thermoplastic organic substance, and the tubular green body is placed 
in the fusiform cavity of a die and fluid pressure is applied to the 
inside of the tubular green body, so as to shape the tubular green body by 
inflating the middle portion thereof more than end portions thereof. The 
thus shaped green body is removed from the die and fired. The shaped green 
body may be pre-fired to remove the plasticizer therefrom. 
Another object of the present invention is to provide a ceramic arc tube of 
metal vapour lamps, which ceramic tube has an arc discharge portion with 
electrode-holding end portions integrally formed at opposite longitudinal 
ends thereof, the arc discharge portion having a larger outside diameter 
than that of the end portions. 
In a preferred embodiment of the present invention, the wall thickness 
t.sub.1 of the ceramic arc tube at the arc discharge portion is thinner 
than the wall thickness t.sub.2 at the electrode-holding end portions 
thereof, preferably in a range of 1.5t.sub.1 &lt;t.sub.2 &lt;5t.sub.1. 
A further object of the present invention is to provide a ceramic arc tube 
of any of the aforesaid types, in which the two electrode-receiving end 
portions have different inside diameters for electrodes. 
A still further object of the invention is to provide a ceramic arc tube of 
any of the aforesaid types, in which the cross section of the arc 
discharge portion along the longitudinal center line of the ceramic arc is 
of ellipse with a major axis b along the longitudinal central axis and a 
minor axis a perpendicular thereto, the ratio a:b being in a range of 1:4 
to 1:8. 
In another preferred embodiment of the ceramic arc tube of the invention, 
the cross section of the electrode-holding end portions perpendicular to 
the longitudinal central axis of the ceramic arc tube is circular, while 
the cross section of the arc discharge portion perpendicular to said 
longitudinal central axis at the midpoint of the two end portions is of 
ellipse with a minor axis to major axis ratio of 1:1.5 to 1:4. 
Another object of the invention is to provide a ceramic arc tube of any of 
the aforesaid types, in which the inside surface of the arc discharge 
portion is curved with different radii of curvature r.sub.1 and r.sub.2 at 
opposite ends thereof, the ratio r.sub.1 :r.sub.2 being in a range of 
1:1.5 to 1:2.

Throughout different views of the drawing, 1 is an alumina tube, 2 is a 
cap, 3 is a heat-resistant metal electrode, 4 is glass frit, 5 is a 
tubular green body, 6 is a die, 7 is a cavity, 8 is a pressure-supply 
terminal, 9 is an end member, 10 is a shaped body, 11 is a coupling 
portion, 12, 21, 31, 41, 51, and 61 are ceramic arc tubes, 13, 14, 23, 33a 
33b, 43, 53, 63a, and 63b are end portions, 15, 22, 32, 42, 52, and 62 are 
arc discharge portions, and 53 is a ceramic cap. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before entering the details of the invention, a ceramic arc tube of the 
prior art will be briefly reviewed by referring to FIG. 1. An alumina arc 
tube 1 is fired with opposite ends left open, and the open ends are closed 
by applying caps 2 thereto while inserting glass frit 4 therebetween to 
seal out the end openings. The cap 2 is made of niobium or alumina 
ceramics, so that the cap has a similar coefficient of thermal expansion 
as that of the alumina tube. Heat-resistant metallic electrodes 3, one of 
which introduces a light-emitting material 3a, are inserted into the 
alumina tube 1 through holes of the caps 2, and the holes of the caps 2 
are sealed by glass frit 4. The sealing by the glass frit 4 is effected by 
heating at a high temperature in vacuo. 
As pointed out above, the alumina arc tube of the prior art has 
shortcomings of possible leakage of the light-emitting materials at the 
joint of the alumina arc tube 1 and the caps 2, the need of high 
dimensional accuracy of the caps 2 and those portions of the alumina arc 
tube 1 which engage the caps 2, and the high manufacturing cost. 
The present invention obviates the shortcomings of the prior art by 
providing an improved ceramic arc tube comprising an arc discharge portion 
having end portions integrally formed therewith. The integral formation of 
the end portions with the arc discharge portion eliminates the caps and 
the requirement of dimensional accuracy of the caps and the cap-engaging 
surfaces of the arc discharge portions. Besides, the number of sealings is 
minimized by the aforesaid integral formation of the invention. 
In a method of the invention to produce a ceramic arc tube, a tubular green 
body is formed by using a mixture material containing fine alumina 
particles and plasticizer, and the tubular green body is inflated in the 
fusiform cavity of a die and then fired to make the desired ceramic arc 
tube. To fulfil the object of the invention, the mixture material to form 
the tubular green body must produce alumina ceramics with a desired 
transmittance. Accordingly, the mixture material must contain fine 
particles of alumina with a high purity and a high activity, and 
platicizer whose major ingredient is non-thermoplastic organic substance. 
The organic substance of the plasticizer may be decomposed or may 
sublimate by pre-firing. The mixture material may further contain a 
suitable sintering auxiliary agent and a mixing auxiliary agent such as 
water. The ingredients of the mixture material must be weighed so as to 
provide a proper mixture ratio, and thoroughly mixed by a wet procedure, 
and kneaded or dried so as to provide molding body with a suitable 
plasticity. 
The fine particles of alumina and the sintering auxiliary agent can be of 
those of the prior art, and for instance can be selected from the group of 
.alpha.-alumina, .gamma.-alumina, magnesium compounds, and rare earth 
element compounds, while considering such factors as the required 
transmittance, the expected firing conditions, and the required mechanical 
properties. 
The preferred non-thermoplastic organic substance to be used in the present 
invention are polyvinyl alcohol or methyl cellulose. The kind and the 
amount of the non-thermoplastic substance should be so selected as to 
provide suitable plasticity of the molding body while considering the 
configuration and the size of the final product. Thus, there is no 
specific restrictions on the non-thermoplastic organic substance. 
Although thermoplastic organic substance such as polypropylene or 
polyethylene may be used as a part of the plasticizer to prevent 
deformation of the product due to re-softening during the preheating for 
removing the organic substance, the major ingredients of the plasticizer 
must be non-thermoplastic organic substances. 
The mixing auxiliary agent is required to wet well the ingredients being 
mixed, to act as a solvent, and to be removed during the after-process of 
drying and firing. Water is generally used as the mixing auxiliary agent, 
but non-aqueous solvent may be also used as the mixing auxiliary agent 
depending on the configuration of the final product. 
A de-airing pug-mill may be advantageously used to mix the aforesaid 
mixture material so as to provide the molding body with a proper 
plasticity, because the elimination of air from the molding body by such 
de-airing pug mill is effective in achieving the desired plasticity. 
A tubular green body 5 (FIG. 2) is formed by shaping the molding body thus 
prepared with a molding machine or a wet process. Preferably, the inside 
diameter of the tubular green body should be such that, when fired, the 
corresponding inside diameter is the same or slightly larger than the 
diameter of the electrode of the discharge lamp on which the ceramic arc 
tube is mounted. 
Referring to FIG. 2, the tubular green body 5 is placed in a die 6 having a 
cavity 7 of fusiform shape. The cavity 7 is defined by the fusiform inside 
surface of the die 6. A pressure-applying terminal 8 of a pressure source 
(not shown) is connected to one end of the die 6, so as to apply fluid 
pressure to the inside of the tubular green body 5 through that end. An 
end member 9 is applied to the opposite end of the die 6, so as to close 
the opposite end opening of the tubular green body 5. When the fluid 
pressure is applied to the inside of the tubular green body 5 from the 
pressure source (not shown), the middle portion of the green body 5 
between opposite end portions thereof is inflated more than the end 
portions, whereby a shaped body 10 is produced as shown by dash-dot-dot 
lines of FIG. 2. After stopping the application of the pressure from the 
pressure source (not shown), the pressure-applying terminal 8 and the end 
member 9 are removed from the die 6, and the shaped body 10 is removed 
from the die 6. 
To facilitate the removal of the shaped body 10, the die 6 is made of two 
halves 6a and 6b, by dividing the die 6 substantially along a plane 
perpendicular to the longitudinal axis thereof at the center thereof while 
forming coupling portions 11 at the abutting portions of the halves 6a and 
6b, as shown in FIG. 2. The halves 6a and 6b are so shaped as to minimize 
burrs on the outer surface of the shaped body 10 while ensuring the easy 
removal of the shaped body 10. 
Although the die 6 of FIG. 2 has only one cavity 7, it is possible to form 
two or more cavities 7 in one die 6. For instance, FIG. 3 shows a die 6 
which has three cavities 7a, 7b, and 7c disposed in a row. With the die 6 
of FIG. 3, three shaped bodies 10 will be formed as connected each other, 
and such three shaped bodies 10 may be separated at the adjacent end 
portions either immediately after the removal from the die 6 or after 
firing as will be described later. What is required to the fusiform shape 
of the cavity 7 of the die 6 is to ensure the production of the desired 
shape of the final ceramic arc tube suitable for the desired properties of 
the discharge lamp and to facilitate the removal of the shaped body. Thus, 
the fusiform shape of the cavity 7 includes a variety of modifications. 
As regards the fluid pressure to inflate the tubular green body, pneumatic 
pressure is preferable, but oil pressure may be used too. The fluid as a 
carrier of the pressure should not corrode the tubular green body 5. If 
there is a possibility of such corrosion, a thin resilient film such as a 
rubber film may be disposed on the inner surface of the green body 5 which 
is exposed to the fluid pressure. 
The shaped body 10 thus formed may be pre-heated, for instance, to remove 
the plasticizer therefrom which plasticizer is added in the molding body 
to facilitate the shaping of the tubular green body 5. However, the 
pre-heating is not essential in the present invention. The pre-heating 
conditions may be determined depending on the type of the plasticizer used 
and the size of the final product. The preferable temperature for the 
pre-heating is 1,200.degree. C. or lower, which temperature will not 
deteriorate the activity of the particles in the shaped body 10. 
Then, final firing is carried out on the shaped body 10 at a high 
temperature either immediately after removal from the die 6 or after the 
aforesaid pre-heating, so as to produce the desired ceramic arc tube 12 as 
shown in FIG. 4 or FIG. 5. The temperature, the duration, and the 
atmosphere of the final firing are determined depending on the chemical 
composition of the starting mixture material, the size of the final 
product, the required transmittance, and the required mechanical 
properties. 
Referring to FIGS. 4 and 5, the ceramic arc tube 12 thus produced has 
opposite end portions 13 and 14, which have openings of suitable size to 
hold electrodes of the metal vapour discharge lamp. An arc discharge 
portion 15 at the middle of the ceramic arc tube 12 is integrally formed 
with the end portions 13 and 14 without any discontinuous joints 
therebetween. The arc discharge portion 15 houses light-emitting material 
therein and it becomes luminous upon energization of the light-emitting 
material. The aforesaid integral formation of the discharge portion 15 
with the end portions 13 and 14 minimizes the number of portions to be 
sealed. Since the aforesaid process of invention is free from any 
contamination of the inside surface of the ceramic arc tube 12, excellent 
transmittance of the ceramic arc tube 12 is ensured. 
The embodiment of FIG. 4 has a cylindrical arc discharge portion 15 with a 
larger outside diameter than that of the end portions 13 and 14. In the 
embodiment of FIG. 5, the arc discharge portion 15 has a smoothly curved 
sidewall with a maximum outside diameter which is larger than the outside 
diameter of the end portions 13 and 14. The shape of the ceramic arc tube 
12 of the invention, however, is not restricted to those of FIGS. 4 and 5. 
EXAMPLE 
A mixture material was prepared by using fine alumina particles with a 
purity of 99.99% and a grain size of 0.1 to 0.2 micron, which mixture 
contained additives including 0.05 weight % of magnesium oxide and 0.05 
weight % of yttrium oxide, 3 weight % of methyl cellulose as an organic 
binder, 1 weight % of polyethylene glycol (with a trademark of POLYNON) as 
a lubricant, 25 weight % of water as a mixing auxiliary agent, and the 
remainder of the aforesaid fine particles of alumina. The mixture material 
was thoroughly blended by a kneader and molding body was prepared by 
milling it by a de-airing pug-mill. 
A tubular green body 5 with an outside diameter of 6.5 mm and an inside 
diameter of 2.5 mm was prepared by extruding the molding body by a piston 
type extruder, which tubular green body 5 was immediately placed in the 
fusiform cavity 7 of a die 6 as shown in FIG. 2. One end of the tubular 
green body 5 was closed by the end member 9, and air was forced into the 
inside of the tubular green body 5 through the opposite end opening 
thereof, whereby the tubular green body 5 was transformed into the shaped 
body 10 whose outer surface was in contact with the inner surface of the 
die 6 defining the fusiform cavity 7. The outside diameter of the shaped 
body 10 at the middle portion thereof was about 10 mm and the wall 
thickness there was about 1.3 mm. 
After completion of the shaped body 10 by forcing air under pressure 
therein, the die 6 carrying the shaped body 10 was dried by heating for 
about two minutes by an induction type electric drier to harden the outer 
surface of the substantially tubular shaped body 10, and the dried shaped 
body 10 was removed from the die 6. 
The shaped body 10 was heated for three hours in air at 800.degree. C. to 
completely remove organic substances therefrom, and then the shaped body 
10 was fired in a vacuum furnace for six hours at 1,800.degree. C. 
Whereby, a ceramic tube 12 was produced. 
A gastightness test was carried out on the alumina ceramic arc tube 12 by a 
helium leak detector, which showed at leak-rate of 10.sup.-10 atm. He 
CC/sec. The ceramic arc tube 12 withstood a spalling test of heating at 
200.degree. C. in the air immediately followed by dipping in water of 
20.degree. C. When measured by an integrating sphere type photometer, the 
ceramic arc tube of this Example shows a total light transmittance of 93%. 
Thus, excellent properties of the ceramic arc tube 12 for use as an arc 
tube in a metal vapour discharge lamp were demonstrated. 
As proven in the Example, the method of the invention to produce the 
ceramic arc tube of metal vapour discharge lamps eliminates the 
application of the caps to the tubular green body as required in the prior 
art, and the ceramic arc tube of the invention has the arc discharge 
portion integrally formed with the end portions thereof so as to ensure 
excellent gastightness. Thus, the method of the invention simplifies the 
manufacture of the ceramic arc tube, and a wide variety of shapes of the 
ceramic arc tube can be easily produced by the method of the invention. 
Especially, the wall thickness of the luminous arc discharge portion can 
be made thin to provide a high transmittance. 
Referring to FIG. 6, illustrating another embodiment of the invention, a 
ceramic arc tube 21 has an arc discharge portion 22 disposed at the middle 
thereof to hold light-emitting material therein, which material becomes 
luminous upon energization, and end portions 23 integrally formed with the 
arc discharge portion 22 at opposite end thereof. The end portions 23 hold 
discharge electrodes to be mounted thereon. 
The arc discharge portion 22 has an outside diameter D.sub.1 and a wall 
thickness t.sub.1, while the end portions 23 have outside diameters 
D.sub.2 and a wall thickness t.sub.2. In the present invention, the 
outside diameter D.sub.1 of the arc discharge portion 22 must be larger 
than the outside diameter D.sub.2 of the end portions, i.e., D.sub.1 
&gt;D.sub.2. Preferably, the wall thickness t.sub.2 of the end portions 23 is 
1.5 to 5 times the wall thickness t.sub.1 of the arc discharge portion 22, 
i.e., 1.5t.sub.1 &lt;t.sub.2 &lt;5t.sub.1. 
The reasons for choosing the aforesaid dimensional relationship in the 
present invention are as follows. The arc discharge portion 22 becomes the 
hottest part of the high-pressure metal vapour discharge lamp when the 
lamp is energized. If the temperature of the arc discharge portion 22 
becomes too high, the light-emitting material namely, metal vapour sealed 
therein tends to chemically react with the ceramics constituting the arc 
discharge portion 22, so as to deteriorate the luminous efficiency and the 
service life of the metal vapour discharge lamp. Accordingly, to suppress 
the temperature rise of the wall of the arc discharge portion 22, the 
outside diameter D.sub.1 of the arc discharge portion 22 is selected to be 
larger than the outside diameter D.sub.2 of the end portions 23 of the 
ceramic arc tube 21, which end portions 23 hold electrodes to be mounted 
thereon and are located close to the coldest spot. 
The wall thickness t.sub.1 of the arc discharge portion 22 is selected to 
be comparatively thin, e.g., 0.2 to 1 mm, so as to ensure a high overall 
transmittance of the discharge lamp. On the other hand, the wall thickness 
t.sub.2 of the end portion 23 is so selected as to provide the required 
sealing strength of the discharge electrodes and to withstand against 
thermal stress at the time of on-off operations of the metal vapour 
discharge lamp. If the wall thickness t.sub.2 of the end portions 23 is 
thinner than 1.5 time the wall thickness t.sub.1 of the arc discharge 
portion 22, the strength becomes insufficient to withstand against the 
thermal stress at the time of turning on the discharge lamp, so that the 
high durability or long service life of the ceramic arc tube 21 cannot be 
ensured. On the other hand, if the wall thickness t.sub.2 of the end 
portions 23 is thicker than 5 times the wall thickness t.sub.1 of the arc 
discharge portion 22, the heat capacity of the portions 23 becomes too 
large to ensure the desired coldest spot temperature at the end portions 
23, and the excessive thickness difference between the arc discharge 
portion 22 and the end portions 23 tends to cause an undue thermal stress 
in the ceramic arc tube 21, which may lead to breakage of the ceramic arc 
tube 21. Thus, the preferable wall thickness t.sub.2 of the end portions 
23 is selected to be in a range of 1.5 time to 5 times the wall thickness 
t.sub.1 of the arc discharge portion 22. 
The arc discharge portion 22 in the embodiment of FIG. 6 is substantially 
of straight cylinder, and the opposite ends of the arc discharge portion 
22 are tapered toward the end portions 23 for connection therewith. It is 
also possible to reduce the outside diameter of the arc discharge portion 
22 from its maximum value D.sub.1 gradually to the outside diameter 
D.sub.2 of the end portions 23 as shown in FIG. 7. 
The dimensions of the arc discharge portion 22 of the ceramic arc tube 21, 
such as the length, the outside diameter, and the shape thereof, can be 
determined depending on given specifications of the metal vapour discharge 
lamp on which the ceramic arc tube 21 is mounted. For instance, the cross 
section of the arc discharge portion 22 taken along the longitudinal axis 
of the ceramic arc tube 21 can be of ellipse as shown in FIG. 7, or of egg 
shape with the maximum outside diameter located toward one of the two end 
portions 23 as shown in FIG. 8, or of cocoon shape with two maximum 
outside diameter portions on opposite sides of the longitudinal center 
thereof as shown in FIG. 9. 
The dimensions of the end portions 23, such as the length, the outside 
diameter, and the inside diameter of the electrode-holding hole thereof, 
can be determined so as to provide heat radiation characteristics suitable 
for given specifications of the metal vapour discharge lamp on which the 
ceramic arc tube 21 is to be mounted; namely, the material and dimensions 
of the electrodes to be secured to the end portions 23, the required 
strength of sealing, and the temperature and the location of the coldest 
spot. 
In the embodiments of FIGS. 6 through 9, the outside diameter of the arc 
discharge portion at the middle of the ceramic arc tube is larger than the 
outside diameter of the end portions thereof, so that the luminous 
efficiency and the colour rendition of the metal vapour discharge lamp, 
especially those of the high-pressure discharge lamp, can bimproved while 
ensuring a long service life of the ceramic arc tube. The aforesaid 
relationship of the wall thicknesses between the arc discharge portion and 
the end portions also ensure a high sealing strength of the discharge 
electrode and a high strength against thermal stress, while maintaining 
the required transmittance. 
Referring to FIG. 10 a translucent ceramic arc tube 31, representing 
another embodiment of the present invention, has an arc discharge portion 
32 integrally formed with electrode-holding end portions 33a and 33b of 
the ceramic arc tube 31 are integrally formed with the arc discharge 
portion 32. The outside diameter D.sub.1 of the arc discharge portion 32 
is larger than the outside diameter D.sub.2 of either one of the end 
portions 33a and 33b. One end portion 33a has a hole with an inside 
diameter d.sub.1 to hold and seal a discharge electrode, which inside 
diameter d.sub.1 is larger than an inside diameter d.sub.2 of another hole 
at the other end portion 33b for holding and sealing a discharge 
electrode. 
When the metal vapour lamp on which the ceramic arc tube 31 is mounted is 
energized, if the temperature of the arc discharge portion 32 becomes too 
high, the ceramics constituting the arc tube 31 tends to chemically react 
with the light-emitting material sealed therein, so that the luminous 
efficiency of the discharge lamp may be deteriorated and the service life 
of the discharge lamp may be shortened. To suppress the temperature rise, 
the outside diameter D.sub.1 of the arc discharge portion 32 at the middle 
of the ceramic arc tube 31 is selected to be larger than the outside 
diameter D.sub.2 of the electrode-holding end portions 33a and 33b located 
close to the coldest spot. The inside diameter d.sub.1 of the hole at one 
end portion 33a is so selected as to hold and seal a metallic niobium tube 
providing a passage to insert a tungsten electrode introducing the 
light-emitting material mounted at the tip thereof and to enclose the 
light-emitting material therein. The inside diameter d.sub.2 of the hole 
at the other end portion 33b is selected so as to hold and seal a tungsten 
rod electrode having a smaller outside diameter than that of the aforesaid 
niobium tube. 
The integral formation of the arc discharge portion 32 at the middle of the 
arc tube 31 and the end portions 33a and 33b can be achieved for instance 
by making the arc discharge portion 32 straight tubular and tapering the 
opposite ends of the arc discharge portion 32 toward the end portions 33a 
and 33b, as shown in FIG. 10. It is also possible to gradually reduce the 
outside diameter of the arc discharge portion 32 as it extends from the 
central part thereof toward the opposite end portions 33a and 33b, as 
shown in FIG. 11. 
The configuration of the arc discharge portion 32 of the ceramic arc tube 
31 is not restricted to those shown in FIGS. 10 and 11. The length, the 
outside diameter, and the shape of the arc discharge portion 32 of the 
ceramic arc tube 31 can be selected so as to meet given specifications of 
the metal vapour discharge lamp on which the ceramic arc tube 31 is to be 
mounted. 
Referring to FIG. 11, the wall thickness t.sub.1 of the central part of the 
arc discharge portion 32 is rather thin to provide a high transmittance of 
the discharge lamp. On the other hand, the wall thickness t.sub.2 of the 
end portions 33a and 33b is rather thick to ensure proper sealing strength 
of the discharge electrodes and to provide a high mechanical strength 
against thermal stress due to on-off operations of the discharge lamp. In 
general, it is preferable to keep the relationship of t.sub.1 &lt;t.sub.2, 
namely, to keep the wall thickness of the central part of the arc 
discharge portion 32 thinner than the wall thickness of the end portions 
33a and 33b. 
In the embodiments of FIGS. 10 and 11, the structure of keeping the outside 
diameter of the luminous arc discharge portion larger than that of the end 
portions improves the luminous efficiency and colour rendition of a 
high-pressure metal vapour discharge lamp and ensures a long service life 
of the ceramic arc tube. Besides, one of the two electrode-holding end 
portions has a hole adapted to seal only a bar-like tungsten electrode, 
while the other end portion has a hole adapted to seal only a metallic 
niobium tube, so that the amount of the expensive and less 
corrosion-resistant niobium is minimized and the small sealing area to 
hold the bar-like tungsten electrode results in an improved reliability of 
the sealing. 
The integral formation of the luminous arc discharge portion and the 
electrode-holding end portions eliminates the need of sealing caps which 
have been used in the prior art, provides a high corrosion resistance 
against the light-emitting material sealed therein, so as to improve the 
luminous efficiency and the colour rendition, and gives strong sealing 
strength of the electrodes and a high strength against the thermal stress 
while maintaining the necessary transmittance. 
In another embodiment of the invention as shown in FIG. 12, a translucent 
ceramic arc tube 41 has a luminous arc discharge portion 42 formed at the 
middle of the arc tube 41 and electrode-holding end portions 43 integrally 
formed at the opposite ends of the arc discharge portion 42. The outside 
diameter D.sub.1 of the arc discharge portion 42 at the central part 
thereof is larger than the outside diameter D.sub.2 of the end portions 
43. The cross section of inside space of the arc discharge portion 42 
taken along a longitudinal central axis of the ceramic arc tube 41 is of 
ellipse having a major axis b along said central axis and a minor axis a 
perpendicular thereto with an a:b ratio of 1:4 to 1:8. 
When a metal vapour discharge lamp having the ceramic arc tube 41 is 
energized, if the temperature of the central part of the arc discharge 
portion 42 is raised too high, the ceramics consituting the arc discharge 
portion 42 tends to chemically react with the sealed metal vapour, so as 
to reduce the luminous efficiency and the service life of the discharge 
lamp. To suppress the temperature rise, the outside diameter D.sub.1 of 
the central part of the arc discharge portion 42 is larger than the 
outside diameter D.sub.2 of the end portions 43 close to the coldest spot. 
In general, the inside diameter a of the central part of the arc discharge 
portion 42 and the length b of the inside space of the arc discharge 
portion 42 along the longitudinal axis of the ceramic arc tube 41 can be 
determined so as to meet given specifications of the metal vapour 
discharge lamp, such as the radiant flux (output) and the light-emitting 
material sealed therein. However, to improve the luminous efficiency and 
the colour rendition and to ensure a high strength against thermal stress 
at the time of turning on the discharge lamp and a long service life of 
the discharge lamp, the aforesaid a:b ratio is preferred. If the length b 
relative to the inside diameter a is shorter than that to satisfy the 
ratio a:b of 1:4, the wall temperature at the central part of the arc tube 
tends to be low and the distance from the center of discharge to the arc 
tube center tends to be large, whereby the radiant flux is absorbed by the 
light-emitting material sealed therein before reaching the wall of the arc 
tube so as to reduce the luminous efficiency, and the temperature of the 
coldest spot at the end of the arc tube tends to increase, so that stable 
discharge cannot be expected. On the other hand, if the length b relative 
to the inside diameter a is longer than that to satisfy the ratio a:b of 
1:8, the shortcomings of the arc tube of the prior art cannot be obviated, 
and the arc tube is weak to thermal stress and the luminous efficiency of 
and service life of the arc tube are low and short. 
The wall thickness of the central part of the luminous arc discharge 
portion 42 at the middle of the ceramic arc tube 41 having the 
light-emitting material sealed therein is preferably thinner than the wall 
thickness of the end portions 43 thereof, so as to provide a high 
transmittance. The shapes of through holes at the end portion 43 can be 
determined so as to suit the configurations of discharge electrodes to be 
inserted and sealed therein. The through holes at the two end portions 43 
need not be identical. 
In the ceramic arc tube of the embodiment of FIG. 12, the outside diameter 
of the luminous central portion thereof is larger than that of the end 
portions thereof and the arc discharge portion thereof has an inner space 
of ellipsoidal shape, so that the ceramic arc tube improves the luminous 
efficiency and the colour rendition of the metal vapour discharge lamps 
and provides a high strength against thermal stress to ensure a high 
durability and a long service life of the arc tube. 
Referring to FIGS. 13 and 14, a translucent ceramic arc tube 51 
representing another embodiment of the present invention has a 
cocoon-shaped luminous arc discharge portion 52 at the middle of the arc 
tube 51 to hold the light-emitting material sealed therein and 
electrode-holding end portions 53 integrally formed at opposite ends of 
the arc discharge portion 52. The outside diameter D.sub.1 of the arc 
discharge portion 52 at the central part thereof is larger than the 
outside diameter D.sub.2 of the end portions 53. The inside cross section 
of the central part of the arc discharge portion 52 at right angles to a 
longitudinal center line of the ceramic arc tube 51 is of ellipse having a 
minor axis c and a major axis d with a ratio c:d of 1:1.5 to 1:4. The 
inside cross section of the end portions 53 perpendicular to the aforesaid 
longitudinal center line is circular. 
When a metal vapour discharge lamp having the ceramic arc tube 51 is 
energized, if the temperature of the central part of the luminous arc 
discharge portion 52 is raised too high, the ceramics constituting the arc 
discharge portion 52 tend to chemically react with the sealed metal 
vapour, so as to reduce the luminous efficiency and the service life of 
the discharge lamp. To suppress the temperature rise and to prevent the 
emitted light from being absorbed before leaving the arc tube, the outside 
diameter D.sub.1 of the central part of the arc discharge portion 52 is 
larger than the outside diameter D.sub.2 of the electrode-holding end 
portions 53 and the inside cross section of the central part of the 
ceramic arc tube perpendicular to the longitudinal center line thereof is 
elliptic. As compared with a circular cross section, the aforesaid 
elliptic cross section reduces the absorption of the emitted light by 
vapour ions in the arc tube before being radiated through the tube wall 
toward the outside, so as to improve the luminous efficiency. The elliptic 
cross section provides different distances between the discharge arc and 
the tube wall depending on radial directions such as directions of the 
major axis and the minor axis, so that when the arc tube has a directivety 
of light emanation as in the case of a reflector lamp, the major axis of 
the elliptic cross section can be so oriented as to coincide with that 
direction in which a high temperature rise is likely to occur, and the 
wall loading of the arc tube can be virtually improved so as to rise the 
luminous efficiency. 
In general, the dimension of the elliptic inside cross section at the 
central part of the ceramic arc tube can be determined by considering 
given specifications of the metal vapour discharge lamp, such as radiant 
flux (output) and the light-emitting material sealed therein. However, to 
improve the luminous efficiency and the service life of the ceramic arc 
tube, the aforesaid ratio c:d in the range of 1:1.5 to 1:4 is preferred. 
If the major axis d relative to the minor axis c is shorter than that to 
satisfy the ratio c:d of 1:1.5, the wall temperature at the central 
portion of the arc tube tends to becomes too high and the light absorption 
by the vapour in the arc tube tends to increase, so that the desired 
improvement of the luminous efficiency cannot be expected. On the other 
hand, if it is intended to make the major axis d relative to the minor 
axis c longer than that to satisfy the ratio of 1:4, the alumina ceramics 
of such dimensions is difficult to make and the internal stress will 
remain in the translucent alumina ceramics after the firing and the 
residual internal stress may result in breakage of the arc tube during use 
of the discharge lamp. 
The circular cross section is preferable at the inside surface of the end 
portion 53, because in the case of the ceramic arc tube 51 having the arc 
discharge portion integrally formed with the end portions as shown in FIG. 
13, it is economical to use discharge electrodes of round bar type or 
tubular type, and the circular through holes at the end portions 53 are 
suitable for holding such discharge electrodes. Furthermore, if ceramic 
caps 54 holding and sealing the discharge electrodes are used to seal the 
opposite ends of the ceramic arc tube 51 as shown in FIG. 15, the ceramic 
caps 54 are easy to make if they are circular and the circular ceramic 
caps 54 have uniform distribution of thermal stress. 
The length of the end portions 53 integrally formed with the arc discharge 
portion 52 can be determined so as to gastightly seal the discharge 
electrode fitted therein and the ceramic caps 54 applied thereto. 
The wall thickness of the central part of the ceramic arc tube 51 having 
the light-emitting material sealed therein is preferably thinner than the 
wall thickness of the end portions 53 thereof, so as to provide a high 
transmittance. The shapes of the through holes at the end portions 53 can 
be modified so as to suit the configurations of the electrodes to be held 
and sealed thereby. The through holes at the two end portions 53 need not 
be identical. 
In the embodiment of FIGS. 13 through 15, the outside diameter of the 
luminous central portion thereof is larger than that of the end portions 
thereof and the inside cross section of the central part of the ceramic 
arc tube is elliptic, so that the ceramic arc tube improves the luminous 
efficiency of the high-pressure metal vapour discharge lamps and ensures a 
long service life of the arc tube. 
Referring to FIG. 16, a translucent ceramic arc tube 61 representing 
another embodiment of the present invention has a luminous arc discharge 
portion 62 at the middle thereof to hold light-emitting material to be 
sealed therein and electrode-holding end portions 63a and 63b integrally 
formed at opposite ends of the arc discharge portion 62. The outside 
diameter D.sub.1 of the arc discharge portion 62 at the central part 
thereof is larger than the outside diameter D.sub.2 of the end portions 
63a and 63b. The inside surface of one end of the arc discharge portion 62 
adjacent the end portion 63a is curved at a radius of curvature r.sub.1, 
while the inside surface of the opposite end of the arc discharge portion 
62 adjacent the opposite end portion 63b is curved at a radius of 
curvature r.sub.2, and the radii of curvature r.sub.1 and r.sub.2 
preferably have a ratio r.sub.1 :r.sub.2 of 1:1.5 to 1:2. 
In general, the radii of curvature r.sub.1 and r.sub.2 of the opposite ends 
of the inside surface of the arc discharge portion 62 can be determined by 
considering given specifications of the metal vapour discharge lamp, such 
as radiant flux (output) and light-emitting material to be sealed therein. 
However, to improve the luminous efficiency and the colour rendition and 
to ensure a long service life of the discharge lamp by providing a high 
strength against thermal stress at the time of turning on the discharge 
lamp, the aforesaid ratio r.sub.1 :r.sub.2 in the range of 1:1.5 to 1:2 is 
preferable. If the ratio r.sub.1 :r.sub.2 is larger than 1:1.5, the 
temperature control function of the coldest spot is lost, and dispersion 
is caused in the luminous efficiency and colour rendition and the service 
life of the discharge lamp tends to be shortened. On the other hand, if 
the ratio r.sub.1 :r.sub.2 is less than 1:2, the temperature difference 
between the opposite electrode-holding end portions becomes too large and 
the luminous efficiency tends to be reduced. 
The wall thickness of the central part of the ceramic arc tube 61 having 
the light-emitting material sealed therein is preferably thinner than the 
wall thickness of the end portions 63a, 63b thereof, so as to provide a 
high transmittance. The shapes of through holes at the end portions 63a, 
63b can be determined so as to suit the configurations of the discharge 
electrodes to be inserted therein. The through holes at the two end 
portions 63a and 63b need not be identical. 
In the embodiment of FIG. 16, the outside diameter of the luminous arc 
discharge portion at the middle of the ceramic arc tube is larger than 
that of the end portions thereof and the inside surfaces at opposite ends 
of the arc discharge portion are curved at different radii of curvature, 
so that the temperature of the coldest spot can be raised or lowered 
simply by modifying the radius of curvature of the inside surface at the 
end of the arc discharge portion. Accordingly, the temperature of the 
coldest spot can be accurately controlled by selecting proper shape of the 
end parts of the arc discharge portion irrespectively of the configuration 
of the electrodes and the direction of the lamp energization, whereby the 
luminous efficiency and colour rendition of the discharge lamp are 
stabilized to facilitate the design of the discharge lamp. Furthermore, 
excessive temperature rise at the end parts of the arc discharge portion 
is prevented, and hence pressure rise in the arc tube is prevented, and 
breakage of the arc tube and excessive reduction of the luminous 
efficiency are prevented. Consequently, a high strength against thermal 
stress is ensured to provide a long service life of the arc tube. 
Although the invention has been described with a certain degree of 
particularity, it is understood that the present disclosure has been made 
only by way of example and that numerous change in details of construction 
and the combination and arrangement of parts may be resorted to without 
departing from the scope of the invention as hereinafter claimed.