The invention relates to a gas discharge lamp comprising a substantially cylindrical discharge vessel, containing an ionizable filling comprising a rare gas and a salt, and first and second electrodes positioned near a first and a second end of the discharge vessel, respectively, wherein the first end of the discharge vessel has a cylindrically shaped side wall and a substantially disc-shaped end wall near the tip of the first electrode, and wherein the second end of the discharge vessel has a tapered side wall near the tip of the second electrode.

A lamp of this type is described in JP-A-11329353. The tapered side wall in the known lamp is provided in order to prevent cracks being developed at one end of the lamp vessel during production or during operation.

The current invention concerns the stability of the lamp during operation, and in particular the stability of lamp operation when the lamp is used in a controlled variable color temperature driver. Such a lamp driver or ballast applies a direct current component, along with the usual high-frequency alternating current to the lamp, in order to segregate the ionized filling during operation, thereby changing the color temperature of the lamp. Due to the unidirectional character of the direct current component, the location of the precipitated excess of salt present in the discharge vessel influences the operational stability of the lamp, and the lamp may completely fail to operate if too much of the salt is present on the wrong side of the discharge vessel.

The tapered shape of the wall as shown in JP-A-11329353 as such would potentially help to increase the wall temperature because of the smaller distance to the electrode, thereby providing a slightly colder spot at the opposite end of the vessel. The invention aims at stabilizing the operation of the lamp to a greater extent.

To that end, the tapered side wall has a substantially constant thickness in the lamp according to the invention. The mass of the end portion of the lamp vessel near the

second electrode is considerably less than the mass of the end portion shown in JP-A- 11329353 as a result of this. Since the discharge vessel material, and in particular ceramic material such as aluminum oxide, has a high heat radiation capacity, the presence of less of such material results in a lower heat radiation and a higher temperature of said second end portion. This increases the temperature difference between the two end portions. Preferably, the wall of the second end of the discharge vessel has a bottleneck shape.

Preferably, the wall of the first end of the discharge vessel has a substantially greater mass near the tip of the electrode than the wall of the second end. This feature can be considered to constitute a separate invention, irrespective of the shape of the wall at either end of the vessel. To that end, the invention relates to a gas discharge lamp comprising a substantially cylindrical discharge vessel, containing an ionizable filling comprising a rare gas and a salt, and first and second electrodes positioned near a first and a second end of the discharge vessel, respectively, wherein the wall of the first end of the discharge vessel has a substantially greater mass near the tip of the electrode than the wall of the second end.

Preferably, the thickness of the end wall of the first end of the discharge vessel is more than 1.5 times the thickness of the side wall of the discharge vessel. The lamp is typically an HID lamp, preferably a metal halide lamp, and the salt preferably comprises NaI and CeI3.

The invention also relates to a lamp system comprising a gas discharge lamp according to any of the appended claims and a driving apparatus for driving said gas discharge lamp, said apparatus comprising current generating means for generating a current through the lamp having an AC current component and an adjustable DC current component.

The invention will be explained in more detail with reference to the Figures, wherein: Fig. 1 shows a cross-section of a first embodiment of a lamp; Fig. 2 shows a cross-section of a second embodiment of a lamp ; and Fig. 3 shows a cross-section of a third embodiment of a lamp.

In Figure 1, the electric discharge lamp has a tubular, light-transmissive ceramic lamp vessel 1, of polycrystalline aluminum oxide, and first and second current conductors 2,3 which enter the lamp vessel 1 opposite each other, each supporting a tungsten

electrode 4,5 in the lamp vessel 1, which electrodes are welded to the respective current conductors 2,3. A ceramic sealing compound 6, containing 30% by weight of aluminum oxide, 40% by weight of silicon oxide, and 30% by weight of dysprosium oxide provided in a melting process, seals the lamp vessel 1 around the current conductors 2,3 in a gastight manner. As an alternative, the lamp vessel 1 could be made from quartz.

The lamp vessel has an ionizable filling comprising argon as a rare gas, such as xenon and/or argon, and a metal halide, for example a mixture of sodium iodide and cerium iodide. Bromides or other halides are also possible, however.

In a practical example, where the overall pressure inside the vessel 1 is of the order of 10-25 at during operation, the vessel 1 may contain mercury and a relatively small amount of argon.

The metal halides are provided as a saturated system comprising an excess amount of salt, such that a salt pool of melted salt will be present inside the lamp vessel 1 during operation of the lamp.

The current conductors 2,3 have first halide-resistant parts 21,31 extending within the lamp vessel 1 and, extending from the ceramic sealing compound 6 to the exterior of the lamp vessel, second parts 22,32 which are connected to the first parts 21,32 by welding.

The first parts 21,31 of the first and second current conductors 2,3 are made from molybdenum aluminide. The second parts 22,32 of the two current conductors 2,3 consist of niobium.

The lamp vessel 1 has narrow end parts 11,12 in which the respective current conductors 2,3 are enclosed. The end parts 11,12 each have a free end 111, 121, where the lamp vessel 1 is sealed by the ceramic sealing compound 6.

The lamp vessel 1 is enveloped by an outer envelope (not shown) which is sealed in a gastight manner and is evacuated or filled with an inert gas in order to protect the niobium second parts 22,32 of the current conductors 2,3.

In operation, a discharge will extend between the electrodes 4,5. The high temperature of the discharge will cause the halides to be ionized and to produce light. The color of the light of the discharge is different for different substances. For example, the light produced by sodium iodide is red whereas light produced by cerium iodide is green.

Typically, the lamp will contain a mixture of suitable substances, and the composition of this mixture, i. e. the identity of said substances as well as their mutual ratio, will be chosen so as

to obtain a specific overall color. In a typical example, the molar ratio between sodium iodide and cerium iodide is 5: 1.

Furthermore, a direct current component through the lamp can force the ionized halide mixture can to one side of the lamp vessel 1 to a certain extent, whereby also the different halides are segregated to a certain extent, leading to an overall different color temperature of the light generated by the lamp. In this way, a continuously variable color temperature control of the lamp can be obtained by the use of a suitable lamp driver. It was found to be of importance, however, that the excess amount of said halides should always precipitate at one side of the lamp vessel 1 in order to be able to have a stable control of the color temperature of the lamp.

According to Figure 1, the lamp vessel 1 is comprised of a central cylindrical part 10 which is connected by sintering to the narrowed cylindrical end parts 11,12 via ceramic disc shaped end walls 13. The thickness of these end walls 13 is about twice the thickness of the side wall of the vessel formed by cylindrical part 10. Since the lamp 1 is symmetrical, the side of the lamp where the excess amount of salt will precipitate cannot be controlled. If this type of lamp is used with a continuously variable color temperature driver, therefore, the lamp will be unstable in practice, resulting in lamp failure, if too much salt precipitates at the wrong side of the lamp.

According to Figure 2, the lamp vessel is comprised of a central cylindrical portion 10 and two tapered end portions 14 forming the connections to the narrowed cylindrical end portions 11, 12. The thickness of the wall of the central cylindrical portion 10, the tapered end portions 14, and the narrowed end portions 11,12 is substantially equal. Both end portions thus have a typical bottleneck shape. Since it is possible to apply more heat to the end portions of the vessel 1 without the risk of cracking in comparison with the end portions shown in Figure 1, the melted seal material 6 can penetrate deeper into the narrowed cylindrical end portions 11,12, thereby leaving a smaller gap behind the electrodes 4,5. In a saturated metal halide lamp such a gap, acting as a cold spot, is a typical place for salt to precipitate during operation. Since this type of lamp is symmetrical, however, it will have the same disadvantages as the lamp type shown in Figure 1 with a continuously variable color temperature driver.

According to Figure 3, the lamp vessel is comprised of a central cylindrical portion 10 which is connected by sintering to the narrowed cylindrical end part 11 via ceramic disc shaped end wall 13 at a first end. The thickness of end wall 13 is again about twice the thickness of the side wall of the vessel formed by cylindrical part 10. At the second

end, the vessel comprises a tapered end portion 14 forming the connection to the narrowed cylindrical end portion 12. The thickness of the wall of the central cylindrical portion 10, the tapered end portion 14, and the narrowed end portion 12 is substantially equal. This gives the second end portion a typical bottleneck shape.

In the configuration of Figure 3, the second end portion of the lamp vessel 1 near the second electrode 5 will have a higher temperature during operation than the first end portion near the first electrode 4 owing to the greater mass which is present near the first electrode, which result in a better heat radiation capacity of the first end portion.