Discharge arc lamp

In a discharge arc lamp, an arc tube comprises an arc tube body and a main electrode at each end inside of the arc tube body, between which in the operating condition of the lamp a discharge takes place. At each end of the arc tube is an end closure comprising an end plug joined to respective each end and a cap member, the end plug and the cap member having facing surfaces. A seal provided by sealing means to seal the cap member to the end plug includes an annular seal portion between the facing surfaces of the end plug and the cap member. One or both of the facing surfaces is so shaped that the annular seal portion has a predetermined and uniform thickness t.

This invention relates to a discharge arc lamp, particularly, though not 
exclusively, to a ceramic metal halide (CMH) lamp. In particular, this 
invention relates to the sealing of discharge arc tubes in such lamps. 
FIG. 1 is a schematic representation of a known construction for the end of 
a discharge arc tube for a CMH lamp. At one end of an arc tube body 10 of 
ceramic material is sintered an end plug 12. A cermet cap 14 is sealed to 
the end plug 12 by a glass seal 15. In the cermet cap 14 is set an 
electrode shank 16 carrying an electrode structure (not shown) and a 
lead-in wire 18. 
A problem has arisen with such an end construction in that if, in the 
sealing process, the cermet cap 14 is firmly pressed onto the end plug 12, 
the resulting glass seal 15 is formed of a very thin fillet of glass. Any 
bubbles trapped within the glass seal are therefore elongated and can 
extend from the outside edge of the seal 15 to the inside of the arc tube 
body 10, thus creating a leakage path for the contents of the arc tube. 
One solution to this problem has been to not fully load the cermet cap 14 
onto the end plug 12, and so produce a glass seal 15 having a greater 
thickness. However, if the loading is not applied precisely along the arc 
tube axis, cap tilting can occur, producing a wedge-shaped seal which, at 
its thinnest edge, can give rise to a leaky seal. Furthermore, the 
thickness of the seal produced, and consequently the height of the cermet 
cap relative to the arc tube end and the separation of the electrodes, is 
variable, giving rise to a variable lamp performance. 
It is an object of the present invention to provide a different form of end 
construction for a discharge arc tube which at least alleviates the 
problems described herein. 
According to a first aspect of the present invention, there is provided an 
arc tube for a discharge arc lamp, said arc tube comprising an arc tube 
body and a main electrode at each end inside of said arc tube body, 
between which in the operating condition of the lamp a discharge takes 
place; 
said arc tube further comprising at each end an end closure: said end 
closure comprising an end plug joined to respective said end and a cap 
member, the end plug and the cap member having facing surfaces; 
wherein a seal provided by sealing means to seal said cap member to said 
end plug includes an annular seal portion between the facing surfaces of 
said end plug and said cap member, one or both of said facing surfaces 
being so shaped that said annular seal portion has a predetermined and 
uniform thickness. 
A discharge arc tube provided in accordance with the present invention has 
end closures including a hermetic seal of a predetermined and uniform 
thickness. Any small inclusions (bubbles) in the sealing means are not 
sufficiently elongated by too great an applied pressure in the sealing 
process, and so a leakage path is not created. Furthermore, the cap member 
and end plug may be firmly pressed together when the glass seal is formed. 
Preferably said cap member is made of a cermet material. 
Preferably the facing surface of said end plug has a step defining an 
annular projection. The thickness of the annular projection of the end 
plug determines the height of the cap relative to the arc tube end and 
hence the separation of the electrodes which may be kept constant from 
tube to tube, giving rise to a less variable performance in lamps 
produced. 
The present invention also provides a discharge arc lamp comprising an arc 
tube in accordance with the first aspect of the present invention.

FIG. 2 shows a ceramic metal halide discharge arc lamp having an outer bulb 
30 and lamp cap 32. Within the outer bulb 0 is situated a discharge arc 
tube 34 of circular cross-section provided with two main electrodes 36, 
38. The main electrode 36 is connected to a lead-through element 40 which 
is electrically connected through a flexible conductor 42 to a rigid 
current conductor 44. The main electrode 38 is connected to a lead-through 
element 46 which is electrically connected through an auxilliary conductor 
48 to a rigid current conductor 50. 
FIG. 3 shows the construction of the end of the arc tube 34, indicated 
generally by the box in FIG. 2, in greater detail. The arc tube 34 
comprises an arc tube body 52 of ceramic material to which is sintered an 
end plug 54, also of ceramic material. Facing surfaces 55, of a cermet cap 
56, and 57, of the end plug 54, are sealed together by a glass seal 58. 
Set into the cermet cap 56 is the lead-through element 40 and an electrode 
shank 60 carrying the main electrode 36. 
The glass seal 58 includes an annular seal portion 62 between the facing 
surfaces 55 and 57 of the cermet cap 56 and the end plug 54, the annular 
seal portion 62 being of a predetermined and uniform thickness t (as 
indicated in FIG. 3). It has been found that for a leak-free seal, made of 
a magnesium alumino silicate glass, which may include a titania dopant, 
the optimum seal thickness is in the range of from 100 .mu.m to 300 .mu.m, 
preferably in the range of from 130 .mu.m to 160 .mu.m. The thickness of 
the annular seal portion 62 is defined by a step presented by an annular 
projection 64 extending from the facing surface 57 of the end plug 54. The 
annular projection 64 of the end plug 54, is radially outwards of the 
annular seal portion 62. The glass seal 58 also includes an outer seal 65. 
FIG. 4 shows, schematically, stages in the formation of the glass seal 58 
in a preferred, but non-limiting, method. A frit ring 66, as shown in FIG. 
4a is placed in position on a cermet cap 56, and heated to produce what is 
termed a `premelt cap` 68 as shown in FIG. 4b. Finally, as shown in FIG. 
4c, the premelt cap 68 is then firmly pressed onto the end plug 54 of an 
arc tube while heat is applied to form the glass seal 58. The annular 
projection 64 of the end plug 54 ensures that the seal 58 includes an 
annular seal portion of predetermined and uniform thickness. 
The details of construction and formation of the other end of the discharge 
arc tube 34 enclosing the other main electrode 38 are similar to those 
outlined above for the end of the discharge arc tube 34 enclosing the main 
electrode 36. 
In a second embodiment shown in FIG. 5, parts corresponding to those in 
FIG. 3 are designated by like reference numerals. The glass seal 70 
includes an annular seal portion 72 of predetermined and uniform thickness 
t between the facing surfaces 55, of the cermet cap 56, and 71, of an end 
plug 73. In this embodiment a step presented by an annular projection 74 
formed as part of the end plug 73 is radially inwards of the annular seal 
portion 72. The thickness of the annular projection 74 of the end plug 73 
defines the thickness of the annular seal portion 72. The glass seal 70 
also includes an outer seal 75. 
The production of a glass seal 70 in which the annular seal portion 72 of 
predetermined and uniform thickness is external of the annular projection 
74 of the end plug 73 has advantages when a premelt cap is used. FIG. 6 
shows schematically stages in the formation of such a seal 70. As shown in 
FIG. 6a, when a premelt cap 76 is applied to an end plug 73, because of 
the shape of the meniscus of the glass 79 around the cap 56, there is a 
gap 78 between the glass 79 and the annular projection 74 of the end plug 
73 and hence the possibility of a gas bubble being trapped in the glass of 
the seal. However, because of the shape of the end plug 73, as the premelt 
cap 76 is applied to the end plug 73, the gap 78 is moved outward of the 
cap as shown in FIG. 6b and hence there is less likelihood of a gas bubble 
being trapped. FIG. 6c shows the fully assembled end closure with the gap 
78 eliminated. 
In a third embodiment shown in FIG. 7, parts corresponding to those in FIG. 
3 are designated by like reference numerals. The glass seal 80 includes an 
annular seal portion 82, between facing surfaces 55, of the cermet cap 56, 
and 83, of an end plug 84, and an outer seal 85. The end plug 84 has an 
annular projection 86 to define the thickness t of the annular seal 
portion 82 and another annular projection 88 partially enclosing the cap 
56. Accordingly the outer seal 85 produced is longer and more reliable 
than the outer seal 65 of the first embodiment shown in FIG. 3. 
FIG. 8 shows a fourth embodiment, parts corresponding to those in FIG. 5 
being designated by like reference numerals. The glass seal 94 includes an 
annular seal portion 96 between the facing surfaces 55, of the cermet cap 
56, and 99, of an end plug 97, and an outer seal 98. The end plug 97 has 
an annular projection 100 to define the thickness t of the annular seal 
portion 96 and another annular projection 102 partially enclosing the cap 
56. Accordingly the outer seal 98 produced is longer and more reliable 
than the outer glass seal of the embodiment shown in FIG. 5. 
The annular projections 64, 74, 86, 88, 100, 102 of the end plug 54, 73, 
84, 97 may be simply produced by using a suitably contoured press tool 
face. 
It is envisaged that the thickness of the annular seal portion may be 
defined by one or more steps in the facing surface 55 of the cermet cap 
56, instead of by the step or steps in the end plug 54, 73, 84, 97 as 
described, or by provision of steps in the facing surfaces of both the cap 
and the end plug. 
It will be appreciated that although the present invention has been 
described with reference to a ceramic metal halide discharge arc lamp in 
which the arc tube is closed by cermet caps, the invention has wider 
applicability to discharge arc lamps generally.