The invention relates to a lamp comprising a lamp vessel of quartz glass and having a discharge room filled with a filling, two electrodes extending into said discharge room, two external current conductors partially extending outside said lamp vessel, two metal foils embedded in quartz glass, wherein a first end part of a metal foil is connected to an electrode and wherein a second end part of a metal foil is connected to an external current conductor.

Such a lamp is known from United States patent document no. 3,205, 395 (Buchwald). As can be seen in figure 4 thereof, a vacuum tight joint is made by heating the outer wall of the lamp vessel of quartz glass in a region indicated with reference numeral 15 to cause it to collapse down upon and seal to a guide disc indicated with reference numeral 12. However, quartz glass of the outer wall is not collapsed onto nor fused to the electrode of this prior art lamp, as the electrode is not sealed to the guide disc and a tube rivet is provided as a bracing means. With reference to figure 4 and 5 of the above-mentioned United States patent publication, the quartz glass of the outer wall is not collapsed onto nor fused to the quartz glass embedding the metal foils either, i. e. the latter quartz glass is located free of the region 15 mentioned above.

A disadvantage of the lamp as described in United States patent document no.

3,205, 395 (Buchwald) is that due to a high pressure and a high temperature of a gas (mercury or xenon, for example) filling the discharge room, the quartz glass at the location of the connection between the first end part of the metal foil and the electrode may easily break (explode) and therefore lead to failure of the lamp. In this respect it is important to note that the quartz glass and the metal foil have mutually differing coefficients of thermal expansion resulting in high thermal stresses.

It is an object of the present invention to obviate this disadvantage and in order to accomplish that objective a lamp referred to in the introduction according to the invention is characterized in that at least one metal foil embedded in quartz glass is spaced apart, at least along a portion of its length, from an outer wall of the lamp vessel, wherein the first end part of said metal foil, along said portion of its length, is connected to an electrode. Said

portion of its length particularly varies between 0,5 mm and 10 mm, preferably between 0,5 mm and 5 mm. As the quartz glass embedding said metal foil is spaced apart along the indicated portion of its length from the outer wall of the lamp vessel, a space being part of the discharge room is present at that location between said quartz glass and the outer wall of the lamp vessel, so that a hot gas (mercury or xenon, for example) filling that space exerts a inwardly directed pressure against the quartz glass embedding said metal foil, thereby compensating an oppositely (outwardly) directed pressure at the connection between the (quartz glass of the) first end part of the metal foil and the electrode. This will result in a long lifetime of the lamp. It is noted that the invention relates to an embodiment wherein the two electrodes extending into the discharge room are interconnected by an incandescent filament (halogen lamp), as well as to an embodiment wherein the two electrodes extending into the discharge room ionize an ionizable filling of the discharge room.

In one preferred embodiment of a lamp according to the invention said metal foil embedded in quartz is sealed near its second end part to the outer wall of the lamp vessel.

In another preferred embodiment of a lamp according to the invention said metal foil together with an electrode and an external current conductor connected thereto is manufactured as a single unit and sealed to the outer wall of the lamp vessel. This enables a precise positioning of the electrode within the lamp, as well as the use of heavy electrodes.

In another preferred embodiment of a lamp according to the invention an end part of at least one of said external current conductors, which extends outside said lamp vessel, is covered by a cap in non-operative condition of the external current conductor. Said cap ensures that said end part of the external current conductor is maintained in a vacuum space or a space filled with a protective gas in order to prevent oxidation/corrosion of said end part. In operative condition of the lamp the cap is removed.

The invention also relates to a method for manufacturing a lamp, wherein two electrodes are connected to two external current conductors by means of metal foils, wherein a first end part of each metal foil is connected to an electrode and wherein a second end part of each metal foil is connected to an external current conductor, wherein a lamp vessel is made of quartz glass such that a discharge room is formed which is filled with an ionizable filling, wherein said metal foils are embedded in quartz glass, wherein said electrodes are extending into said discharge room and wherein said external conductors are partially extending outside said lamp vessel, characterized in that at least one metal foil embedded in quartz glass is spaced apart, at least along a portion of its length, from an outer wall of the

lamp vessel, wherein the first end part of said metal foil, along said portion of its length, is connected to an electrode.

The invention will now be explained in more detail with reference to three figures illustrated in a drawing, showing a preferred embodiment in a side elevation, partly in cross-section.

The electric lamp of figure 1 has a lamp vessel 1 which is closed in a vacuumtight manner and which has a quartz glass wall 2 and an electric element 3, and a filling inside. Metal foils 4 are embedded in quartz glass and connected to respective internal current conductors 5 extending to the electric element 3 and to respective external current conductors 6 made of molybdenum. The lamp shown is an electric discharge lamp in which the free end portions of the internal current conductors 5 form the electric element 3, i. e. tungsten electrodes. The lamp vessel 1 has a filling of mercury, sodium iodide and scandium iodide, and xenon, for example, with a pressure of 100 bar at operating conditions of the lamp, i. e. when burning.

In the electric lamp shown, the metal foils 4, made of molybdenum in the figures, are embedded in quartz glass 7. The lamp vessel 1 is fixed in a lamp cap which has contacts (not shown). The lamp consumes a power of approximately 35 W during operation and is suitable for use in an optical system, such as a projector, for example, or as a vehicle headlamp.

As shown in figure 1 the quartz glass 7 embedding the metal foils 4 is spaced apart from the outer wall 2 of the lamp vessel 1 so as to form a space 8 being at least substantially circular in cross section between said quartz glass 7 and said outer wall 2. The space 8 is part of the discharge room 9 of the lamp vessel 1 and is thus filled with the above- indicated filling.

Referring to figures 2 and 3 the lamp vessel 1 (figure 2) on the one hand and the metal foil 4 together with the internal current conductor 5 and the external current conductor 6 (figure 3) on the other hand form each single units so that the latter unit can be precisely positioned into the former unit and sealed at the end 10 (figure 1). The end parts of both external current conductors 6 are covered by a cap 12 in non-operative condition of the lamp so as to ensure that said end parts are held in a vacuum space 11 or a space 11 filled with a protective gas in order to prevent corrosion of said end parts. Obviously, the cap 12 is removed before using the lamp. Furthermore, sidewardly extending protrusions 13 are

provided in order to improve positioning of the units shown in figure 3 into the unit depicted in figure 2.

As the quartz glass 7 embedding said metal foils 4 is spaced apart along its entire length from the outer wall 2 of the lamp vessel 1, the space 8 being part of the discharge room 9 is present at the location of the connection between the metal foils 4 and the internal current conductor 5 between said quartz glass 7 and the outer wall 2 of the lamp vessel 1, so that a hot gas (mercury or xenon, for example) filling that space 8 exerts a inwardly directed pressure against the quartz glass 7 embedding said metal foils 4, thereby compensating an oppositely (outwardly) directed pressure at the connection between the first (quartz glass of the) end part of the metal foils 4 and the electrodes 5.