GB2191337A

GB2191337A - A process for sealing the ceramic envelope of a high pressure discharge lamp, especially a sodium lamp, and a lamp made by this process

Info

Publication number: GB2191337A
Application number: GB08708894A
Authority: GB
Inventors: Miklos Csapody; Ferenc Nagel
Original assignee: Tungsram Rt
Current assignee: Tungsram Rt
Priority date: 1986-04-29
Filing date: 1987-04-14
Publication date: 1987-12-09
Anticipated expiration: 2007-04-14
Also published as: BE1001637A4; NL8700905A; HUT43764A; GB8708894D0; SU1590052A3; GB2191337B; DE3713371A1; JPS62283529A; DE3714350A1; HU195029B

Description

1 GB2191337A 1 SPECIFICATION which body fixedly carries the current conduc

tors and which is united with the envelope by A process for sealing the ceramic envelope means of an annular enamel bond.

of a high pressure discharge lamp, espe- However, such a construction cannot be cially a sodium lamp, and a lamp made by 70 used in an economic manner for the manufac this process ture of high pressure lamps. This is because The invention concerns a process for sealing when the bulb and a closing body are to be the ceramic envelope (discharge vessel) of a fused together, this annular enamel part has to high pressure discharge lamp, primarily sodium be melted. The heat required to accomplish lamp, which envelope is integrally closed at 75 this causes an increase in the pressure of the one end with its own material, in the course filling in the bulb and the filling blows through of which process the other open end of the the fused enamel, that is to say, in the course envelope is united with a ceramic disc carrying of manufacture a portion of the filling material the current lead-in conductors by means of an is lost and the outflowing filling gas addition enamel bond. 80 ally may even displace the enamel from its The invention also concerns a high pressure desired position.

discharge lamp, primarily a sodium lamp, man- Our aim was to provide a process for seal ufactured by the process which has a ceramic ing the ceramic envelope of a discharge lamp envelope or bulb (these terms are used inter- which is 'pinch-sealed' and which is free of changeably herein) integrally closed at one end 85 the above-mentioned disadvantage, i.e. in with its own material, which contains a filling which the full volume of the filling in.the en of inert gas and additive(s) and a ceramic disc velope remains in it during the formation of secured to the bulb by means of an enamel the enamel bond between the ceramic envel bond, the disc carrying the current lead-in ope and the ceramic closure body.

conductors. 90 We have recognised that if the sealing body It is well-known that a problem exists with is made from a ceramic disc with a cross- actual embodiments of currently known high section larger than the internal cross-section pressure discharge lamps, or rather with their of the envelope and expediently identical with manner of manufacture. As the lamp dimen- the external cross-section of the envelope, sions and power are reduced, there is a diffi- 95 then during assembly the disc can be placed culty when the bulb is to be exhausted on the envelope and in this way the manufac (pumped out) and sealed: as the lamp has no ture of the envelope is simplified. Furthermore, exhaust tube, it is difficult to maintain a tem- we have also recognised that if a packing ring perature of 1200-1500'C in the sealing zone, made from a resilient metal of high melting which temperature is required to melt the 100 point is placed between the abutting surfaces enamel material uniting the discharge vessel of the disc and the open end of the envelope, and the ceramic disc closing the vessel. Simul- then such a ring can serve as a packing under taneously, at the other end of these lamps of the influence of a suitable clamping force be ever-decreasing size, one must maintain a tween the disc and the envelope while an temperature of 80' to prevent evaporation of 105 enamel bond is formed between the envelope the additive(s). The temperature gradient re- and the disc. As a consequence of the sealing quired to fulfil these requirements is very diffi- function of the metallic ring the filling in the cult to assure for lamps of approximately 35 envelope remains captive despite the fact that mm length and nominal rating of 35 W, be- its pressure increases while the enamel bond cause the ceramic material is at the limit of its 110 is formed. The filling cannot blow through the tolerance of such a temperature gradient. molten enamel and cannot dislocate it from its In embodiments utilizing an exhaust tube position.

this problem does not exist, but on the other In order that the metallic ring should be. a hand the relatively large-dimensioned exhaust good seal, it must be resilient because only tube is the source of a significant heat loss 115 then can it lie on and bear in a stable manner and the resulting external 'cold point' necessi- against the end surface of the envelope as tates an overheating of the ceramic parts. well as the disc. This resilient property must Known constructions seeking to assure the re- be preserved even at the temperature of the quired 'cold point' temperature necessitate the melting point of the enamel. In addition, its use of very wide heat-reflectors yokes which 120 coefficient of thermal expansion must match significantly reduce the luminous efficiency of the coefficient of thermal expansion of the the lamps. materials of the envelope and of the disc. It Japanese Publication No. 60-127634 dis- must also be resistant to the corrosive effect closes a discharge lamp which seeks to re- of the filling in the envelope. These demands duce the dimensions of the lamp and at the 125 are most favourably met by a closed (un same time dispenses with an exhaust tube, broken) metallic ring made from niobium. If and in which, similarly to fiat halogen lamps, the materials in the discharge space corrode there is provided a ceramic bulb which is the metal, then the packing ring is provided 1 pinch-sealed' and which contains a ceramic with a corrosion-resistant layer on its surface body for sealing the open end of the bulb, 130 facing the discharge space.

2 GB2191337A 2 On the basis of the above recognitions we sealing may be achieved with a relatively have worked out a process for sealing a cerasmaller compressive force.

mic, expediently alumina, envelope of a high Advantageously, however, a closed niobium pressure discharge lamp, primarily a sodium ring made from sheet material may also be lamp, which envelope is closed at one end 70 used as the packing ring. Such a ring may be integrally with its own material. In the pro- placed between concentric ribs projecting to cess, with the aid of an enamel ring the open wards each other from the end surfaces of end of the envelope is sealed to a ceramic, the envelope and the disc in such a manner expediently alumina, disc which carries the that these ribs sandwich the sealing ring. This current conductors passing through it in an 75 is particularly advantageous because a niobium expediently vacuum-tight manner, the enamel ring can be made relatively cheaply from sheet ring being melted under the effect of heat and material and thus its continuity (freedom from then solidified under appropriate cooling. Ac- gaps or breaks) is inherently solved.

cording to the invention, the process is further If the task is to seal a bulb with a filling developed in that a closed, i.e. gap-free, resiliwhich contains a metal halide as an additive, ent metallic packing ring is placed on the in- then the sealing ring should preferably be a ternal end surface of the open end of the niobium ring which is provided on its surfaces envelope, (internal in the sense of facing to- in contact with the interior of the bulb with a wards the interior of the bulb) and concentri- per se known halide- resistant layer, e.g. a cally with the packing ring an external (in the 85 mixture of rare earth oxides. This layer may sense of not being in contact with the interior be formed simultaneously with a formation of of the envelope) enamel ring is placed on the the enamel bond if the enamel ring is made open end of the envelope, and a disc carrying from a halide-resistant enamel material. In or the current lead-in conductor is placed on der to assure that during the heating required these rings, the disc having a cross-section 90 to achieve a good bond, the enamel bond be which exceeds the internal cross-section of tween the disc and the current lead-in conduc the bulb and expediently equals the outer tors should not become loose and thus the cross-section of the latter; and then the envel- integrity of the seal of the disc should be ope (containing the filling of inert gas and admaintained, it is expedient to seal the current ditive) and the disc are compressed together 95 conductors into the disc by means of an with a force suitable for the deformation of enamel with a melting point higher than the the packing ring, expediently a force of 0.5 to melting point of the enamel ring used to fuse 1000 N, and while maintaining the comprestogether the bulb and the disc. To prevent the sive force an enamel bond between the envel- enamel that fixes the current conductors into ope and a disc is formed with the aid of the 100 the disc from melting, it is also possible to enamel ring, and then the compressive force remove heat through these sealed-in conduc is terminated. tors: thus, for instance, the conductors may As a result, the envelope and the disc will also be fixed in place with an enamel which is be sealingly united together. The thus pro- identical with the enamel of the enamel ring.

duced discharge lamp may be used without an 105 According to an advantageous feature of the external envelope, but may alternatively be invention, the conductor- carrying disc can be provided with an external envelope closed in a subjected to the effect of the ambient atmos vacuum-tight manner. In that case, there is phere at the operating temperatures without either a vacuum or an inert gas filling in the damage. Suitable current conductors are those space between the bulb of the discharge lamp 110 made, for instance, from high-alloy steels.

and the outer envelope. The sealed discharge lamp may, if desired, Expediently, in the manufacture of high pres- be placed within an outer light-transmitting, sure discharge lamps the ceramic envelope preferably glass, envelope. In the space be and the ceramic disc are of circular cross- tween the discharge lamp and the outer envel- section, or are of a so-called racetrack-shape, 115 ope either vacuum is formed or an inert gas i.e. of a cross-section like a flattened circle, or filling is employed, and the outer envelope is oval or elliptical. The advantage of an envelvacuum-tightly sealed off.

ope of circular cross-section is that it is sim- For high pressure discharge lamps having an ple to manufacture, while the 'racetrack'- outer envelope tantalum current lead-in con shaped envelope has the advantage that it 120 ductors may expediently be used. In that makes it easier to position the electrodes and case, the material of the electrodes is also hence a more uniform heat distribution and tantalum and thus the usually required weld wall stabilisation effect can be achieved. between the conductors and the electrodes An essentially diamond-shaped cross-see- can be obviated. When the electrode is tanta tion, closed niobium ring may be used with 125 lum, a tantalate-based cathode-activing ma advantage as the packing ring. This ring is terial is employed.

position such that its diagonal, opposite edges The invention furthermore also concerns a bear respectively against the end surface of high pressure discharge lamp, primarily a so the ceramic disc and the end surface of the dium lamp, made by the above- described pro ceramic envelope; in this way, vacuum-tight 130 cess. The high pressure discharge lamp. expe- 3 GB2191337A 3 diently sodium lamp, has a ceramic envelope discharge lamp may be manufactured without closed at one end by means of its own ma- an external envelope and can be marketed in terial, and is expediently made of alumina; the that way. However, in this case the material open end of the envelope is secured to a of the lead-in conductors fused into the cera- ceramic, expediently alumina, disc by means 70 mic disc cannot be the same as the material of an annular enamel bond; and the ceramic of the lead-in conductor projecting from the disc is vacuum-tightly united with the current disc, but instead the two metals must be lead-in conductors passing through it. Such a welded together externally of the envelope lamp is further improved in the sense of the and in the vicinity of the disc. This is because invention in that an internal, closed packing 75 the coefficient of thermal expansion of a metal ring made of a resilient metal and disposed which can without damage be subjected to concentrically with the enamel ring is arranged the effect of the air does not match the coe between the open end of the envelope and fficient of thermal expansion of the material of the disc and the cross-section of the disc is the disc; thus, for instance, a high-alloy steel chosen to be greater than the internal cross- 80 lead-in conductor cannot be fused into the section of the envelope, and is expediently disc.

equal to the external cross-section of the en- The invention will be described, purely by velope. way of example, with reference to the accomThe envelope of the high pressure discharge panying schematic drawings, in which:

lamp according to the invention is expediently 85 Figure 1 is a diagrammatic cross-section of of circular cross-section or has a cross-section a preferred embodiment of a high pessure dis of flattened circular shape, e.g of the so-called charge lamp according to the invention; racetrack' shape. Naturally the shapes of the Figure 2 illustrates a cross-section of a high packing ring, the enamel ring and the disc are pressure discharge lamp of flattened circular chosen to match the cross-section of the en- 90 cross-section in side elevation, with its 'foot velope. print' or contour being shown also in plan; In a preferred embodiment of the discharge Figure 3 is a view similar to Fig. 1 but lamp according to the invention, the packing showing an embodiment having a packing ring ring is a closed niobium ring of essentially bent from sheet material, and diamond-shaped cross-section. This niobium 95 Figure 4 is a diagrammatic cross-section of ring bears against the end surface of the en- a high pressure metal halide discharge lamp.

velope and the disc along its opposite diago- Throughout the drawings the same reference nal edges. On sealing, the ring is somewhat numbers designate the same or functionally flattened along the edges and this is why it is similar components or parts.

called 'essentially' of diamond-shaped cross- 100 Current lead-in conductors 6, 7 forming a section in this preferred embodiment. single unit with electrodes 9, 10 are placed In Fig. 3, the packing ring 3 is a niobium into the bores of a prefabricated ceramic disc ring made from sheet material which is sand- 5 and are fixed in these bores by means of wiched on two sides by two concentric ribs an enamel bond 8, whereby to make the cera- projecting towards each other from the end 105 mic disc gas-tight. The filling, which contains surface of the envelope and from the adjacent primarily a sodium-mercury amalgam (but surface of the disc 5, respectively. which may additionally or alternatively be any An advantageous discharge lamp according other mixture, such as a metal halide one suit to the invention has a disc into which the able from the point of view of discharge phy- lead-in conductors are fixed by means of an 110 sics for use in light discharge technology), is enamel with a melting point higher than that placed into a bulb 1 disposed in an upside of the material of the enamel ring. The ma- down position.

terial of this enamel is calcium aluminate modi- If the filling contains metal halide, then a fied with 2-10 mass percent of rare earth halide-resistant layer 11 is provided on the oxide and strontium or barium oxide. The 115 bulb surface which is to come into contact melting point of the enamel varies as a func- with the discharge space so that this layer 11 tion of this modification. is put on the internal surface of a packing In one preferred embodiment of a high pres- ring. Furthermore, the enamel used for sealing sure discharge lamp having an external envel- should expediently be halide-resistant.

ope, both the current conductors and the elec- 120 A packing ring 3 and an enamel ring 4 are trodes are made of tantalum, which is advan- placed on the end surface 2 of the envelope tageous because tantalum has a coefficient of 1, then the disc carrying the current conduc thermal expansion which matches that of the tors 6, 7 is placed on these rings, the bulb is material of the disc. evacuated and then an inert gas is filled into In an advantageous embodiment of a dis- 125 the bulb 1. Thereafter the bulb 1 and the disc charge lamp according to the invention, the 5 are compressed together in the directions current lead-in conductors projecting from the shown by the arrows in the Figure so that the disc to the surroundings can be subjected to packing ring 3 deforms and effects a seal.

the effect of the air at the operating tempera- The magnitude of the compressive force is tures without risk of damage. In this case the 130typically 0.5 to 1000 N, related to 1 cm of 4 GB2191337A 4 the edge of the packing ring in contact with According to Figs. 1 and 4 the packing ring the end surface of the bulb. The compressive 3 is essentially a curciform- section, gapless force is maintained while the enamel ring 4 is niobium ring. The diagonal deformed edges of heated and melted. The thus assembled dis- the packing ring bear against the bulb 1 and charge lamp is cooled, with due regard to the 70 the disc 5, respectively. An enamel ring 4 is desired crystal structure and the tension or disposed concentrically with the packing ring stress in the ring 4. In this way the bulb 1 is 3 between the end surface 2 of the bulb 1 sealed to the disc 5 in a vacuum-tight manner, and the disc 5 to provide the bond between that is to say en enamel bond is formed. At the discharge lamp bulb 1 and the disc 5.

the end of this operation, the compressive 75 Lead-in current conductors 6, 7 pass force is removed. The packing ring 3 ensures through the disc 5. They are made of a metal that during the sealing process, that is to say, lic material (Fig. 1) which permits the conduc during the formation of the enamel bond, the tor ends outwardly of the disc to be exposed high pressure prevailing within the bulb 1 can- to the atmosphere at the operating tempera- not blow the fused material of the enamel ring 80 ture of the discharge lamp without fear of 4 away from its position and that the additive damage due that temperature, namely a high material and inert gas cannot exit from the alloy steel. The current conductors 6, 7 are bulb 1. sealed into the disc 5 by way of an enamel In the described sealing process, the melting joint 8 of a melting point higher than that of point of the enamel 8 is higher than the melt- 85 the enamel ring 4.

ing point of the enamel ring 4, hence the disc The further demand is made on the lamp remains sealed. according to Fig. 4 that the enamel should, if When the material of the enamel 8 is the possible, also be halideresistant. The current same as that of the enamel ring 4, then durconductors 6, 7 passing through the disc 5 ing the formation of the joint heat must be 90 are mutually parallel but the tips of the elec removed through the current conductors 6, 7 trodes 9, 10 face each other by virtue of hav in order to prevent the enamel bond 8 from ing been bent by 90'. The electrodes 9, 10 melting. do not contain radiators. Naturally, there is no The discharge lamp produced according to reason why radiators should not be used, if the invention may be provided with an exter- 95 that is desired. The material of the current nal, expediently glass, bulb conventional for conductors 6, 7 is not always the same as high pressure discharge lamps. in the space the material of the electrodes 9, 10 and in between the discharge lamp and the external such cases, as known per se, they must be envelope there is either a vacuum or an inert coupled together by way of a welded bond.

gas filling. 100 In the case where the bulb 1 is of a flat The bulb sealing process according to the tened circular cross-section, e. g. of a 'race invention is advantageous mainly for the pro- track' cross-section (Fig. 2), naturally the duction of low power, high pressure discharge packing ring 3, the enamel ring 4 and the disc lamps but may also be used for high output 5 are also of the same shape so as to form a or high power light sources, even with an out- 105 matching shape with the bulb. When the di put of the order of magnitude of kilowatts. By mensions and output are predetermined, the utilising the process according to the invention electrodes 9, 10 may be so placed in the bulb the practice of fully utilising the heat gradient that a wall-stabilising effect in the discharge tolerance of the ceramic material during manu- lamp also prevails.

facture is discontinued because the only open 110 The discharge lamp according to the inven end of the bulb can be sealed reliably in a tion may with advantage be used in all situa single operation. tions where a low output or small-sized lamp The high pressure discharge lamp produced is required. For instance, in combination with by the process according to the invention has a suitable electronic ballast circuit it may be a bulb of alumina one end of which has a 115 used to replace conventional incandescent fila closed bottom made from its own material. ment lamps. It is known that the filling in the The shape of the bulb may be similar to the bulb can readily attack the enamel and thus known cylindrical incandescent filament lamp after a certain time the bond fails. However, glass envelope, but it may also be of a flat- the lifetime of the discharge lamp according to tened cross-section shown in Fig. 2. 120 the invention has been extended relative to In Fig. 2 the bulb 1 has an open end with a the conventional lamps containing only enamel planar end surface 2 connected via an internal bonds because the materials in the discharge sealing ring 3 and an external enamel ring 4 space cannot come into contact with the with the alumina disc 5. enamel because of the interposed internal In Fig. 3, the packing ring 3 is a niobium 125 metallic packing ring.

ring made from sheet material which is sand

Claims

wiched on two sides by two concentric ribs CLAIMS projecting towards each

other from the end 1. A process for sealing a ceramic bulb of surface of the envelope and from the adjacent a high pressure discharge lamp which bulb is surface of the disc 5, respectively. 130 closed at one end by means of its own ma- GB2191337A 5 terial, comprising placing a closed, flexible claim, wherein the current conductors are metallic packing ring and an enamel ring con- made of tantalum.

centric with and external to the packing ring 13. A process for sealing a ceramic bulb are placed on the end surface of the open end of a high-pressure discharge lamp substantially of the bulb provided with a filling of additive 70 as herein described with reference to and as and inert gas; placing a ceramic disc sealedly shown in any one of the Figs. of the accom carrying current lead-in conductors on the said panying drawings.

rings, the disc having a cross-section greater 14. A high pressure discharge lamp, pri than the internal cross-section of the bulb, gas marily a sodium lamp, produced by the pro- tightly joining the disc and the bulb by apply- 75 cess of any of the preceding claims, compris ing a compressive force effective to deform ing ceramic envelope integrally sealed by the sealing ring; maintaining the compressive means of its own material at one end and force while an enamel bond is formed with containing a filling of additive and inert gas; a the aid of heat and the enamel ring to fuse ceramic disc which is fixed to the envelope by the bulb and the disc together, and then re- 80 an annular enamel bond so as to produce a moving the compressive force. vacuum-tight seal and which carries current
2. A process as claimed in claim 1, conductors passing through the disc, the disc wherein the magnitude of the compressive fused to the end surface of the open end of force is of 0.5-1000 N. the bulb has a cross-section which is greater
3. A process as claimed in clim 1 or 2, 85 than the internal cross- section of the bulb and wherein the cross-section of said disc is iden- between them there is an inner, closed, resili tical with the outer crosssection of the bulb. ent, metallic packing ring concentric with an
4. A process according to claim 1, charac- outer enamel. ring.

terised in that the bulb is of circular cross- 15. A discharge lamp according to claim section or of flattened circular cross-section, 90 14, wherein the ceramic ring, the enamel ring and the packing ring, enamel ring and disc are and the ceramic disc are of shape-lockingly shaped similarly. similar cross-section to the ceramic envelope.

5. A process according to any preceding 16. A discharge lamp according to claim claim, wherein the packing ring is a closed 14 or claim 15, wherein the metallic sealing niobium ring of essentially rectangular dia- 95 ring is essentially of a rectangular cruciform or mond-shaped cross-section such that the dia- diamond-shaped cross-section, closed niobium gonal edges of the ring bear against the bulb ring the diagonal edges of which bear against and the disc, respectively. the ceramic disc.

6. A process according to any of claims 1 17. A discharge lamp according to claim to 4, wherein the packing ring is a closed 100 14 or claim 15, wherein the packing ring is a niobium ring made from sheet material placed closed niobium ring made from sheet material between concentric ribs projecting towards sandwiched on two sides by concentric ribs each other from the mutually opposed end projecting towards each other respectively surfaces of the bulb and the adjacent surface from the end surface of the ceramic envelope of the disc. 105 and the ceramic disc.

7. A process according to any preceding 18. A discharge lamp according to any of claim, wherein the packing ring has a halogen- claims 14 to 17, wherein the enamel material resistant layer on its surface in contact with sealing the current conductors into the cera the interior of the bulb. mic disc has a melting point which exceeds 8. A process according to any preceding 110 that of the material of the enamel ring.

claim, wherein the current conductors are 19. A high pressure discharge lamp, e.g. a fixed into the disc by means of an enamel the sodium lamp, according to any of claims 14 melting point of which is higher than the melt- to 18, substantially as herein described with ing point of the enamel ring. reference to and as shown in any one of the 9. A process according to any preceding 115 Figures of the accompanying drawings.

claim, wherein during the formation of the enamel bond, heat is removed via the current CLAIMS conductors. Amendments to the claims have been filed, 10. A process according to any preceding and have the following effect:- claim, wherein the current conductors are 120 textually amended claims have been filed as made of a metal which can be subjected withfollows:

out damage to the free atmosphere at the 1. A process for sealing a ceramic bulb of operating temperature of the lamp. a high pressure discharge lamp which bulb is 11. A process according to any preceding closed at one end by means of its own ma- claim, wherein the sealed discharge lamp is 125 terial, comprising placing a closed, resilient placed in an outer envelope, the space be- metallic packing ring and an enamel ring con tween the outer envelope and the discharge centric with and external to the packing ring lamp is exhausted of air, and the outer envel- are placed on the end surface of the open end ope is vacuum-tightly sealed. of the bulb provided with a filling of additive 12. A process according to any preceding 130and inert gas; placing a ceramic disc sealedly GB2191337A 6 carrying current lead-in conductors on the said companying drawings.

rings, the disc having a cross-section greater 14. A high pressure discharge lamp, pri than the internal cross-section of the bulb, gas marily a sodium lamp, produced by the pro tightly joining the disc and the bulb by apply- cess of any of the preceding claims, compris ing a compressive force effective to deform 70 ing ceramic envelope integrally sealed by the packing ring; maintaining the compressive means of its own material at one end and force while an enamel bond is formed with containing a filling of additive and inert gas; 5 the aid of heat and the enamel ring to fuse ceramic disc which is fixed to the envelope by the bulb and the disc together, and then re- an annular enamel bond so as to produce a moving the compressive force. 75 vacuum-tight seal and which carries current 2. A process as claimed in claim 1, conductors passing through the disc, the disc wherein the magnitude of the compressive fused to the end surface of the open end of force is of 0.5-1000 N. the bulb has a cross-section which is greater 3. A process as claimed in claim 1 or 2, than the internal cross-section of the bulb and wherein the cross-section of said disc is iden- 80 between them there is an inner, closed, resiii tical with the outer cross-section of the bulb. ent, metallic packing ring concentric with an 4. A process according to claim 1, charac- outer enamel ring.

terised in that the bulb is of circular cross- 15. A discharge lamp according to claim section or of flattened circular cross-section, 14, wherein the packing ring, the enamel ring and the packing ring, enamel ring and disc are 85 and the ceramic disc are of shape-lockingly shaped similarly. similar cross-section to the ceramic envelope.
5. A process according to any preceding 16. A discharge lamp according to claim claim, wherein the packing ring is a closed 14 or claim 15, wherein the metallic packing niobium ring of essentially rectangular dia- ring is essentially of a rectangular cruciform or mond-shaped cross-section such that the dia- 90 diamond-shaped cross- section, closed niobium gonal edges of the ring bear against the bulb ring the diagonal edges of which bear against and the disc, respectively. the ceramic disc.
6. A process according to any of claims 1 17. A discharge lamp according to claim to 4, wherein the packing ring is a closed 14 or claim 15, wherein the packing ring is a niobium ring made from sheet material placed 95 closed niobium ring made from sheet material between concentric ribs projecting towards sandwiched on two sides by concentric ribs each other from the mutually opposed end projecting towards each other respectively surfaces of the bulb and the adjacent surface from the end surface of the ceramic envelope of the disc. and the ceramic disc.
7. A process according to any preceding 100 18. A discharge lamp according to any of claim, wherein the packing ring has a halogen- claims 14 to 17, wherein the enamel material resistant layer on its surface in contact with sealing the current conductors into the cera the interior of the bulb. mic disc has a melting point which exceeds
8. A process according to any preceding that of the material of the enamel ring.

claim, wherein the current conductors are 105 19. A high pressure discharge lamp, e.g. a fixed into the disc by means of an enamel the sodium lamp, according to any of claims 14 melting point of which is higher than the melt- to 18, substantially as herein described with ing point of the enamel ring. reference to and as shown in any one of the
9. A process according to any preceding Figures of the accompanying drawings.

claim, wherein during the formation of the Printed for Her Majesty's Stationery Office enamel bond, heat is removed via the current by Burgess & Son (Abingdon) Lid, Dd 8991685. 1987.

conductors. Published at The Patent Office. 25 Southampton Buildings,
10. A process according to any preceding London, WC2A l AY, from which copies may be obtained.

claim, wherein the current conductors are made of a metal which can be subjected without damage to the free atmosphere at the operating temperature of the lamp.
11. A process according to any preceding claim, wherein the sealed discharge lamp is placed in an outer enveope, the space between the outer envelope and the discharge lamp is exhausted of air, and the outer envelope is vacuum- tightly sealed.
12. A process according to any preceding claim, wherein the current conductors are made of tantalum.
13. A process for sealing a ceramic bulb of a high-pressure discharge lamp substantially as herein described with reference to and as shown in any one of the Figures of the ac-