Discharge lamp with a mercury - indium iodide fill

Abstract

 The invention relates to a discharge lamp with a bulb (10) made of an arc tube part (11) and hermetically sealing tube parts (12) in the form of rod-like tubes which are integrally adjacent to the arc tube part on two sides, with a pair of electrodes (21) being arranged opposite each other within the arc tube part and mercury as well as indium iodide being filled in the same. An electrode structural body (22) is inserted into each of the hermetically sealing tube parts in such a way that a gap (K) is present between the electrode structural body and the inside wall of the hermetically sealing tube part. The electrodes (21) are arranged on a respective tip of the electrode structural body (22) in the arc tube part, and an end section of the respective hermetically sealing tube part (12) which is averted from the arc tube is hermetically sealed by means of a hermetically sealing structure (23,30).

Description

[0001] The invention relates to a discharge lamp which emits blue light and is used for illuminating water tanks.

[0002] Lamps are known for illuminating water tanks which emit blue light by using indium as the emission material, so that beautiful colours of fish and colours of corals, sea algae and the like can be displayed in a brilliant fashion (see JP-A HEI 11-67148 for example). Recently, tropical fish are generally observed by families, with a lamp for illuminating water basins as shown in JP-A HEI 11-67148 for example being arranged and used above a water tank.

[0003] Specifically, the arc tube consists of silica glass in the aforementioned lamp for illuminating water tanks, at the two ends of which there is a hermetically sealing tube each being produced by melting and compressing silica glass. The internal volume is close to 2 cm³. The lamp is operated at 150 W.

[0004] From the structural context of the hermetically sealing tube with a compressed structure of the arc tube, the coolest point of the arc tube has 700 °C to 800 °C.

[0005] Fig. 6 shows an arrangement of an example of the aforementioned lamp for the illumination of a water tank by using silica glass for the arc tube. As is shown in Fig. 6, a pair of electrodes 21 is arranged within the arc tube consisting of silica glass. A hermetically sealing tube 12 with a compressed structure is formed at each of the two ends of the arc tube 11. The electrodes 21 are each connected via a molybdenum foil 29 to the outer leads 24, which foil is placed in the hermetically sealing tube 12. Mercury and indium iodide are filled into the arc tube 11.

[0006] As also described in JP-A HEI 11-67148, a lamp is conventionally used as a lamp emitting blue light in which mercury and indium iodide are filled into a bulb consisting of silica glass and which is operated with approximately 150 W.

[0007] Recently, there has been a demand for an energy-saving lamp in which less power is consumed and the output is concretely less than/equal to 50 W.

[0008] In order to save energy in the lamp shown in JP-A HEI 11-67148 with the arc tube consisting of silica glass, there is a method in which the inside pressure of the arc tube and thus the voltage is reduced. Specifically, the filling quantity of the mercury which determines the inside pressure of the arc tube is reduced, the inside pressure is reduced, the voltage is reduced and thus the power consumption is reduced.

[0009] If the voltage is lowered by a reduction of the inside pressure, the discharge can no longer be maintained in the case of an excessive reduction of the voltage. The reduction of the voltage therefore has limits. In reality, the power can be reduced by 1 percent at most, but not more.

[0010] On the other hand, it is also imaginable that by increasing the internal volume of the arc tube and without changing the filling quantity of the mercury which determines the inside pressure of the arc tube it is possible to reduce the inside pressure of the arc tube and the output and to thus save power. However, the lamp is enlarged in this case and the voltage decreases excessively, so that the discharge can no longer be maintained. Power can thus not be saved.

[0011] Independent of the lamp described in JP-A HEI 11-67148, it is also possible to produce a lamp in which current and voltage are set to a low level and power is thus saved when the arc tube consists of silica glass. In this case, the internal volume of the arc tube is reduced, as a result of which the quantity of the filled material is reduced.

[0012] In the case of a lamp with an arc tube consisting of silica glass, the coolest point of the arc tube still has a lower temperature than the lamp shown in JP-A HEI 11-67148 as a result of the structural correlation of the hermetically sealing tube with a compressed structure of the arc tube, but it still has approximately 700 °C. This means that the material filled into the arc tube evaporates to a high extent in relation to the small internal volume of the arc tube because the temperature of the coolest point lies close to approximately 700 °C. As a result, the ratio of the evaporated indium which is filled into the arc tube is high. In order to obtain a predetermined evaporation quantity of the indium, a small quantity of indium is sufficient.

[0013] When the quantity of indium becomes too high for the volume of the arc tube, the radiation with 451 nm which represents the main emission line (peak) of the indium emission is no longer emitted as a result of self-absorption or a radiation with approximately 451 nm becomes a continuous emission. As a result of this, the blue colour of the emitted light will become brighter up to nearly white.

[0014] In the case of the lamp with 150 W as described in JP-A HEI 11-67148, the quantity of indium iodide is approximately 0.1 mg, as is described in this specification. Since it is an exceptionally small quantity, indium is usually filled in by filling pellet-like indium iodide into the arc tube. In the case of a lamp with an output of less than/equal to 150 W, the quantity of the indium to be filled in is smaller than the same. The currently used smallest pellet comprises approx. 0.05 mg.

[0015] If in order to obtain a lamp with an output smaller than/equal to 150 W efforts are made to reduce the filling quantity of the indium as compared with the quantity as stated above, it will become necessary to comminute the pellets for example, to weigh the indium iodide and to fill in the same. Since the quantity of indium is very small, the filling of a suitable quantity is very difficult. When it is weighed and filled, the processability is very bad.

[0016] As already described above, there are various disadvantages in the production of a discharge lamp with an output of less than/equal to 150 W which emits blue light. No discharge lamps have conventionally been produced in practice which emit blue light with an output of less than/equal to 150 W.

[0017] The invention was made to eliminate the disadvantages of the state of the art as described above. It is the object of the present invention to provide a discharge lamp with a small power consumption in which indium is used as the emission material and which emits blue light.

[0018] The object is solved with the discharge lamp as claimed in present claim 1. Preferred embodiments are disclosed in the subclaims.

[0019] As described above, it is difficult to realise a lamp with a low power consumption with a lamp comprising an arc tube consisting of silica glass, in which indium is used as the emission material and which emits blue light. The reason for this is that, as described above, the filling of an exceptionally small quantity of indium iodide into the arc tube is difficult.

[0020] A lamp with an arc tube made of glass usually comprises at either end of the arc tube a hermetically sealing tube with a compressed structure, as was shown above by reference to Fig. 6.

[0021] On the other hand, a lamp is known as shown in Fig. 1 (a) and (b) in which on the two sides of the arc tube there is a hermetically sealing tube each in the form of a rod-like tube. In the case of a discharge lamp by using a translucent ceramic material it is not possible to seal by melting as is the case with glass. A lamp is thus usually used with the arrangement as shown in Fig. 1 (a) and (b).

[0022] The inventors have produced a discharge lamp with a low power consumption which emits blue light by using a lamp with the shape as shown above by reference to Fig. 1 (a) and (b). They have found that an emission of blue light will occur even then when the filling quantity of the indium iodide is increased more than when using a lamp of the shape as shown in Fig. 6.

[0023] It is expected that the reason for this is the following.

[0024] In the case of a lamp whose two sides comprise hermetically sealing rod-like tubes, the filled indium iodide remains mostly in the region of a lower temperature of the hermetically sealing rod-like tubes on both sides. The quantity of the indium iodide which is present in the arc tube is smaller than/equal to the reciprocal of the multiple quantity of the filled indium iodide.

[0025] Based on the above facts, the above problem is solved by the invention as follows:

(1) A bulb comprises an arc tube part which is integrally adjacent to hermetically sealing tube parts in the form of rod-like tubes. A pair of electrodes is arranged opposite of one another in the arc tube part. Electrode structural bodies whose tips comprise the above electrodes are each pushed into the above described hermetically sealing tube parts with a gap to the inside wall of the respective hermetically sealing tube part. Mercury and indium iodide are filled into the arc tube part of the lamp in which an airtight sealing structure is formed between the region on the side of the outer end of said hermetically sealing tube part and the electrode structural body. Blue light is emitted at low power consumption.

(2) A quantity of smaller than/equal to 0.1 mg of indium is filled in the bulb as described above. The output of the discharge lamp is determined to be more than/equal to 20 W and less than/equal to 50 W.

[0026] As a result of the measure that an arc tube part and hermetically sealing tube parts in the form of rod-like tubes are arranged which are integrally adjacent to the arc tube part, and that further electrode structural bodies whose tips contain the aforementioned electrodes are pushed into the hermetically sealed tube parts, each with a gap towards the inside wall of the hermetically sealed tube part, and that in the lamp in which an air-tight sealing structure is formed between the region on the side of the outside end of said hermetically sealing tube part and the electrode structural body, and that mercury and indium iodide are filled in, blue light can be emitted in a stable fashion even in the case of a larger filling quantity of the indium iodide as compared with a conventional lamp with hermetically sealing tubes which are produced by melting of silica glass and by compression of the same.

[0027] A correct quantity can be filled into the arc tube without comminution of the indium iodide pellets. It is thus enabled to produce in a relatively simple way a discharge lamp with a low power consumption which performs an emission in the blue colour and which conventionally was very difficult to produce.

[0028] The invention is now explained below in closer detail by reference to embodiments of a discharge lamp shown in the drawings, wherein:

Figs. 1 (a) and (b)

each show a schematic representation of the arrangement of an embodiment of the discharge lamp in accordance with the invention;

Figs. 2 (a) and (b)

each show a schematic representation of the arrangement of a further embodiment of the discharge lamp in accordance with the invention;

Fig. 3

shows a schematic representation (1) of a test result;

Fig. 4

shows a schematic representation (2) of a test result;

Fig. 5

shows a schematic representation (3) of a test result, and

Fig. 6

shows a schematic representation of the arrangement of an example of a conventional discharge lamp made of silica glass which performs an emission in the blue colour.

[0029] Although embodiments of the invention are described below in which a translucent ceramic material is used as an arc tube, the subject matter of the invention is not limited however to merely a ceramic lamp.

[0030] Figs. 1 (a) and (b) each show a schematic representation of an arrangement of an embodiment of a discharge lamp in accordance with the invention which is made of ceramic material and which is operated by using alternate current and is arranged as a metal halide lamp. Fig. 1 (a) shows a cross-sectional view. Fig. 1 (b) shows a schematic representation of a boundary region between its arc tube and its hermetically sealing tube on an enlarged scale.

[0031] In the discharge lamp made of ceramic material as shown in Figs. 1 (a) and (b), the bulb 10 comprises a substantially spherical arc tube part 11 which encloses a discharge space S and hermetically sealing rod-like tube parts 12 which are adjacent to the two ends of said arc tube part 11 in the manner that they project outwardly from the same. The bulb 10 consists of a translucent ceramic material.

[0032] A body made of polycrystalline translucent aluminium oxide, polycrystalline translucent yttrium aluminium garnet or polycrystalline translucent yttrium oxide can be used as the ceramic material which bulb 10 consists of. A body of polycrystalline aluminium oxide is especially advantageous.

[0033] The maximum outside diameter of the arc tube part 11 usually is 5.0 to 8.5 mm in the bulb 10, the internal volume is 0.07 cm³ to 0.23 cm³, the outside diameter of the hermetically sealing tube part 12 is 1.5 mm to 2.5 mm, the inside diameter is 0.7 mm to 0.9 mm and the length is 6 mm to 12 mm.

[0034] In bulb 10, a pair of electrodes 21 is arranged opposite of each other in the arc tube part 11. Electrode 21 is formed in the example as shown in Figs. 1 (a) and (b) in such a way that the tip region of an electrode rod 22 is wound with a metallic coil. An outer lead 24 extending in the same direction is integrally connected with the base of the electrode rod 22 via a rod-like metal ceramic 23 which extends in the same direction as the electrode rod 22 and is electrically connected to the same. Tungsten or the like is used for the electrode rod 22 and the metallic coil. Molybdenum or the like is used for the outer lead 24.

[0035] In the inside region of the hermetically sealing tube part 12 which is close to the arc tube part 11, the electrode rod 22 is inserted into a sleeve 26 made of ceramic material. A body made of polycrystalline aluminium oxide, silica glass or the like can be used for the material of said sleeve 26. It is advantageous however that it should consist of the same material as the bulb 10. It is desirable that the outside diameter of the sleeve 26 corresponds to the inside diameter of the hermetically sealing tube part 12 and that at the same time the inside diameter thereof corresponds with the outside diameter of the electrode rod 22.

[0036] In practice, the difference between the outside diameter of the sleeve 26 and the inside diameter of the hermetically sealing tube 12 normally is 0.03 mm to 0.10 mm and the difference between the inside diameter of the sleeve 26 and the outside diameter of the electrode rod 22 is 0.03 mm to 0.07 mm. By using the sleeve 26, the gap between the electrode rod 22 and the inside wall of the hermetically sealing tube part 12 is reduced and the quantity of the filling material penetrating and condensing in the same is thus kept at a low level.

[0037] As is shown in Fig. 2 (a), one can also make an arrangement in which the electrode rod 22 is inserted into the sleeve 26 and in which the electrode rod 22 is tightly wound with a coil 27 made of tungsten on the electrode side of the sleeve 26.

[0038] As is shown in Fig. 2 (b), an arrangement can also be provided in which an outer lead 28 made of niobium is connected to the end of the electrode rod 22, in which no sleeve component is used, in which the electrode rod 22 is wound around with a coil 27 made of tungsten and in which sealing is made hermetically with frit glass 30 without using a metal ceramic material.

[0039] In Figs. 1 (a) and (b), an electrode structural body made of the electrode rod 22 and the sleeve 26 is pushed into the hermetically sealing tube part 12 of the bulb 10. The electrode rod 22 and the sleeve 26 are inserted concretely in such a way into the hermetically sealing tube part 12 that the electrode 21 is located in the arc tube part 11. Furthermore, an annular frit glass 30 is attached between the metal ceramic 23 and the outside end of the hermetically sealing tube part 12. The frit glass is heated and molten in this state, as a result of which an air-tight sealing arrangement is produced in the region on the side of the outside end of the hermetically sealing tube part. As a consequence, the electrode structural body made of electrode rod 22 and sleeve 26 comprises a gap k towards the inside wall of the hermetically sealing tube part 12.

[0040] As is shown in Fig. 1 (b), the distance of the gap K between the boundary between the arc tube part 11 and the hermetically sealing tube part 12 and the end of the frit glass 30 which represents the inside boundary of the air-tight sealing arrangement within the hermetically sealing tube part 11 is designated with L (mm).

[0041] Frit glass 30 for the hermetical sealing can advantageously be used on the basis of an oxide of aluminium oxide/silicon oxide/rare earths or frit glass on the basis of aluminium oxide/CaO.

[0042] Such a discharge lamp made of ceramic material can be operated by regulating the current and the voltage with a lower power consumption, e.g. with less than/equal to 50 W. The specifications of the lamps described above with power consumptions of 20 W and 50 W are stated below.

Table 1

Type	Filling material	internal volume of arc tube	Inl quantity per internal volume of arc tube	Hg quantity per internal volume of arc tube
Ceramic metal halide lamp 20 W, 70 V, 0.3 A	Mercury: 2.0 mg Indium iodide (Inl): 0.1 mg	0.078 cm3	1.28 mg/cm3	25.6 mg/cm3
50 W, 90 V, 0.6 A	Mercury: 5.0 mg Indium iodide (Inl): 0.1 mg	0.223 cm3	0.45 mg/cm3	22.4 mg/cm3

[0043] In the case of such a discharge lamp made of ceramic material, the hermetically sealing tube parts cannot be subjected to any melting deformation during the process of producing the air-tight sealing arrangement in the hermetically sealing tube part because the bulb consists of a translucent ceramic material. The gap between the hermetically sealing tube part and the electrode structural body inserted therein is therefore filled with the frit glass 30 for hermetical sealing, as a result of which the air-tight sealing arrangement is produced. As a consequence of this, the gap K between the electrode structural body and the inside wall of the hermetically sealing tube part 12 is formed.

[0044] Since the frit glass 30 does not have a sufficiently high heat resistance temperature, an overheated state during the operation of the discharge lamp must be avoided.

[0045] As a result of such a requirement, a hermetically sealing tube part of a suitable length adjacent to the arc tube part is required in the hermetically sealing tube part of the discharge lamp made of ceramic material in order to insulate the middle section of the arc tube part which reaches an exceptionally high temperature during operation from the air-tight sealing section. As a result, the gap K between the electrode structural body and the inside wall of the hermetically sealing tube part 12 is narrow.

[0046] Since the narrow gap K is formed within the hermetically sealing tube part adjacent to the arc tube part between the hermetically sealing tube part and the electrode structural body, the coolest point of the arc tube has a lower temperature in the vicinity adjacent to the hermetically sealing tube part 11 as a result of the structural correlation between such a hermetically sealing tube part and the arc tube part than the coolest point of a conventional discharge lamp (approximately 700 °C), specifically 600 °C.

[0047] In the discharge lamp with the above arrangement, the temperature is lower than the coolest point of the conventional lamp made of silica glass with hermetically sealing tubes with a compressed structure. The mercury and the indium which are filled into the arc tube therefore influence the temperature of the coolest point, so that in comparison with conventional lamps made of silica glass less of it is evaporated. The filling quantity of the indium needs to be enlarged as a result of the difficult evaporation.

[0048] In the case of a conventional lamp made of silica glass it is necessary to reduce the pellet-like configuration of the indium oxide in order to achieve savings in output. It was however not possible to further reduce the pellet-like configuration of the indium iodide, but it is possible in the arrangement in accordance with this embodiment to use pellet-like indium iodide with the current size because the filling quantity of the indium is increased.

[0049] A narrow gap is formed within the hermetically sealing tube part between the tube part and the electrode structural body. A suitable quantity of indium can be filled into the arc tube in the case of a ceramic discharge lamp with a temperature of less than/equal to 700 °C of the coolest point of the arc tube part and a lamp is thus obtained which reliably emits blue light and a line spectrum at 451 nm.

[0050] The filling quantity of indium iodide is close to 0.1 mg for example in the lamp described above by reference to Table 1 and is thus equivalent to the conventional discharge lamp made of silica glass with approximately 150 W.

[0051] On the other hand, the mercury filled into the arc tube regulates the inside pressure within the arc tube, as a result of which a voltage state is achieved in which the arc discharge can be maintained, ensuring that the current value is set to the predetermined range.

[0052] As the next step, the relation between the filling quantity of mercury and the filling quantity of indium iodide was examined by a test. Moreover, the relation of length L of the gap of the hermetically sealing tube part was examined (distance from the boundary between the arc tube part and the hermetically sealing tube part and the inner boundary of the air-tight sealing arrangement within the hermetically sealing tube part; see Fig. 1 (b)). In this test, the test was performed by using the metal halide lamps which are shown in Table 1 and consist of a ceramic material and are of the 20 W/50 W type. The inside diameter of the arc tube and the materials filled into the arc tube are shown in Table 1.

[0053] In the test as described above, by using lamps in which the value of the length L of the above gap was changed in a range of 2 mm to 12 mm the relation between mercury and indium iodide was determined by mercury (mg) / indium iodide (mg) mercury, the relation of mercury (mg) / indium iodide (mg) was changed in a range of 10 to 150 and the state of the frit glass, the colouring of the emitted light, the state of the arc tube, the lamp power (W), the mercury quantity (mg) and the indium iodide quantity (mg) were examined in every single case. Since the weight of the smallest currently used pellets of indium iodide is close to 0.05 mg as described above, the filling quantity of the indium iodide was chosen from 0.15 mg, 0.1 mg or 0.05 mg, the filling quantity of the mercury was changed according to output and the test was performed. In this test, the colouring of the emitted light after 500 hours of examination and the state of the arc tube were examined. The result is shown in Figs. 3 to 5. At a value of mercury (mg) / indium iodide (mg) of 150, the filling quantity of mercury was 7.5 mg and voltage increased to 60 W. This result is also shown in Fig. 5.

[0054] As is shown in Figs. 3 to 5, the frit glass from which the air-tight sealing arrangement is formed is moved too close to the arc tube, with the length (gap length L) lying closer than 3 mm in lamp 1 from the boundary between the arc tube part and the hermetically sealing tube part towards the inner boundary of the air-tight sealing arrangement within the hermetically sealing tube part. The frit glass melted here and the air-tight properties of the arc tube were impaired. Evaluation could therefore not be performed.

[0055] In the case of lamps 11, 12 in which the length (gap length L) from the boundary between the arc tube and the hermetically sealing tube part towards the inner boundary of the hermetically sealing arrangement within the hermetically sealing tube part is larger than 10 mm, the volume of the gap within the hermetically sealing tube part will become large. The quantity of the indium iodide penetrating said gap will become too large. The quantity of the halogen (iodine) present within the arc tube will decrease. The halogen cycle within the arc tube becomes insufficient and the arc tube will be blackened. The emitted light still has a blue coloration. The light flux of the lamp decreases by the blackening of the arc tube, thus leading to the disadvantage of a reduction of the illuminance of the illumination region.

[0056] It is understood that this disadvantage is eliminated when the filling quantity of the indium iodide is increased according to the volume of the gap within the hermetically sealing tube part. In the case of a lamp with a large length of the hermetically sealing tube, the length of the tube will also increase. It is understood that this is therefore not advantageous in practice.

[0057] On the other hand, in the case that the value of mercury (mg) / indium iodide (mg) is larger than 120, the filling quantity of indium iodide is smaller than the filling quantity of the mercury at a filling quantity of the indium iodide of 0.05 mg. In this case, the quantity of the indium iodide decreases in lamps 2 to 11, so that also the quantity of the halogen (iodine) present within the arc tube will decrease. The halogen cycle within the arc tube becomes insufficient and the arc tube will be blackened. Since further the indium quantity will decrease within the arc tube, the emitted light has a stronger white colour, namely a blue colouring which is approximated to white. No expected colouring is obtained.

[0058] On the other hand, in the case of the value of mercury (mg) / indium iodide (mg) of less than 15 at a filling quantity of indium iodide of 0.15 mg, the filling quantity of mercury is smaller than indium iodide. In the lamps 2 to 4 the quantity of indium iodide will be relatively large, so that the quantity of the indium present within the arc tube will increase. The radiation close to 451 nm will become a continuous emission and the blue colour of the emitted light will become brighter.

[0059] In lamps 5 to 9, an excessive quantity of indium penetrates the gap within the hermetically sealing tube part. According to this amount, the quantity of the indium is optimised within the arc tube and the emitted light of the lamp has a blue colour.

[0060] In the lamps 10 and 11, the gap within the hermetically sealing tube part will become too large, into which indium will penetrate in the state of indium iodide. The quantity of the halogen (iodine) present within the arc tube will decrease. The halogen cycle within the arc tube will become insufficient and the arc tube will be blackened. Although the emitted light has a blue coloration, the blackening of the arc tube reduces the light flux of the lamp, leading to the disadvantage of a reduction of the illuminance in the illumination area.

[0061] The result of the tests as described above show that the frit glass will not melt in a lamp in which the length L from the boundary between the arc tube part and the hermetically sealing tube part towards the inner boundary of the air-tight sealing arrangement within the hermetically sealing tube part is at 3 mm to 10 mm, in which further the value of mercury (mg) / indium iodide (mg) is at 15 to 120 and in which the filling quantity of the mercury is at 1.5 mg to 6 mg and the filling quantity of the indium iodide is at 0.1 to 0.05 mg, despite a lower power consumption of less than / equal to 50 W, that there will not be any reduction in the light flux as a result of a blackening of the arc tube and blue light will be emitted reliably. By using the lamp with the arrangement as shown in Figs. 1 (a) and (b) it is possible, by filling an amount larger than / equal to 0.05 mg and less than / equal to 0.1 mg of indium, to obtain a discharge lamp with a power consumption of more than / equal to 20 W and less than 50 W which performs an emission in the blue colour.

Claims

1. A discharge lamp with a bulb made of an arc tube part and hermetically sealing tube parts in the form of rod-like tubes which are integrally adjacent to the arc tube part on two sides, with a pair of electrodes being arranged opposite each other within the arc tube part and mercury as well as indium iodide being filled in the same,
characterised in that
an electrode structural body each is inserted into the hermetically sealing tube parts in such a way that a gap is present between the electrode structural body and the inside wall of the hermetically sealing tube part, that the electrodes are arranged on a respective tip of the electrode structural body in the arc tube part, and that an end section of the respective hermetically sealing tube part which is averted from the arc tube is hermetically sealed by means of a hermetically sealing structure.

2. A discharge lamp according to claim 1, characterised in that the electrode structural body consists of an electrode rod whose part situated in the hermetically sealing tube part is inserted at least in sections in a sleeve or coil wound about the outside circumference of the electrode rod.

3. A discharge lamp according to claim 2, characterised in that the electrode rod protrudes into the arc tube part.

4. A discharge lamp according to claim 2 or 3, characterised in that the sleeve or coil has a length of at most the length of the hermetically sealing tube part.

5. A discharge lamp according to one of the claims 2 to 4, characterised in that the sleeve consists of a ceramic material, especially polycrystalline aluminium oxide, or silica glass and preferably of the same material as the bulb.

6. A discharge lamp according to one of the claims 2 to 4, characterised in that the coil consists of a metal wire, especially a tungsten wire.

7. A discharge lamp according to one of the claims 1 to 6, characterised in that the bulb consists of a ceramic material, especially of polycrystalline translucent aluminium oxide, polycrystalline translucent yttrium aluminium garnet or polycrystalline translucent yttrium oxide.

8. A discharge lamp according to one of the claims 1 to 7, characterised in that the air-tight sealing structure in the region of the outer ends of the hermetically sealing tube parts comprises molten frit glass.

9. A discharge lamp according to one of the claims 1 to 8, characterised in that the air-tight sealing structure comprises a body made of metal ceramics which is arranged adjacent to the facing outer end of the hermetically sealing tube parts.

10. A discharge lamp according to one of the claims 1 to 9, characterised in that the gap between the electrode structural body and the inside wall of the hermetically sealing tube part has a thickness of 0.03 to 0.1 mm.

11. A discharge lamp according to one of the claims 1 to 10, characterised in that the bulb is filled with a maximum of 0.1 mg of indium.

12. A discharge lamp according to one of the claims 1 to 11, characterised in that the power of the discharge lamp is between 20 W and 50 W.

Drawing