Dielectric barrier discharge lamp

Details Dielectric barrier discharge lamp Dielectric barrier discharge lamp

2002-05-23

2003-08-12

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Combined load device or load device temperature modifying...

Discharge device load

C315S059000, C313S623000, C313S625000

Reexamination Certificate

active

06605899

ABSTRACT:

TECHNICAL FIELD
The invention relates to a dielectric barrier discharge lamp in accordance with the preamble of claim
1
.
This is a discharge lamp in which either the electrodes of one polarity or all the electrodes, i.e. of both polarities, are separated from the discharge by means of a dielectric layer (known as a one-sided or two-sided dielectric barrier discharge). In the text which follows, electrodes of this type are also referred to as “dielectric electrodes” for short. In operation, it is quite possible that the polarity of the electrodes may also change, i.e. each electrode alternately functions as an anode and a cathode. In this case, however, it is advantageous if all the electrodes have a dielectric barrier. For further details, reference is made to EP 0 733 266 B1, which describes a particularly preferred mode of operation for dielectric barrier discharge lamps.
The abovementioned dielectric layer may be formed by the wall of the discharge vessel itself, if the electrodes are arranged outside the discharge vessel, for example on the outer wall. On the other hand, the dielectric layer may also be produced in the form of an at least partial encapsulation or coating of at least one electrode arranged inside the discharge vessel, which is also referred to as an internal electrode for short in the text which follows. This has the advantage that the thickness of the dielectric layer can be optimized with a view to the discharge properties. However, internal electrodes require gas-tight current lead-throughs. This requires additional manufacturing steps.
Lamps of the generic type are used in particular in appliances for office automation (OA), e.g. color photo copiers and scanners, for signal lighting, e.g. as brake and direction indicator lights in automobiles, for auxiliary lighting, for example the interior lighting of automobiles, and for background lighting of displays, e.g. liquid-crystal displays, as edge type backlights.
These technical application areas require both particularly short start-up phases and also light fluxes which are as far as possible temperature-independent. Therefore, these lamps do not usually contain any mercury. Rather, these lamps are typically filled with noble gas, preferably xenon, or noble gas mixtures. While the lamp is operating, in particular excimers, for example Xe
2
*, which emit a molecular band radiation with a maximum at approximately 172 nm, are formed within the discharge vessel. Depending on the application, this VUV radiation is converted into visible light by means of phosphors.
PRIOR ART
The document WO98/49712 has disclosed a tubular barrier discharge lamp with at least one internal electrode in strip form. One end of the tubular discharge vessel of the lamp is closed off in a gas-tight manner by a stopper which is fused to a part of the inner wall of the discharge vessel by means of soldering glass. The strip-like internal electrode is guided outward through the soldering glass as a supply conductor. A drawback is that a layer of soldering glass as a gas-tight joining means is required between the stopper and the vessel wall.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid the abovementioned drawback and to provide a dielectric barrier discharge lamp in accordance with the preamble of claim
1
which has an improved closure technique which does not involve the use of joining means.
In a lamp having the features of the preamble of claim
1
, this object is achieved by the features of the characterizing part of claim
1
. Particularly advantageous configurations are given in the dependent claims.
Furthermore, protection is claimed for a process for producing this lamp in accordance with the features of the process claim.
According to the invention, the discharge tube of the dielectric barrier discharge lamp is closed off in a gas-tight manner, at at least one of its two ends, with the aid of a disk-like closure element but without the use of joining means, as a result of the or each of the two closure elements being arranged at the respective end, inside the discharge tube, and being joined in a gas-tight manner, over its entire circumference, directly to the inner wall of the discharge tube. As is explained in more detail below, this gas-tight joining takes place as a result of the inner wall and the edge of the disk-like closure element being heated to the respective softening point. The term “fusing” is also used as a shortened way of describing this operation, although this term is to be understood in a general sense as meaning that the materials of the two elements which are to be joined do not necessarily have to be intimately fused together. It is only essential that a gas-tight join be formed by heating the two elements which are to be joined to the respective softening points and then bringing them into contact with one another, without additional joining means.
Moreover, the discharge tube is constricted along its entire circumference in the region of the fusion, in such a manner that the constriction surrounds the edge of the disk-like closure element in the form of a ring. In this context, the term “disk-like closure element” is to be understood, in a general sense, as meaning that this closure element merely has to be suitable for being pushed into the discharge tube and being able to close off the end of the tube in the manner described. In the most simple case, it is a circular plate. However, other designs are also suitable, provided only that they have a circular circumference, for example a cylindrical stopper or the like.
The process according to the invention for the production of this discharge lamp involves providing the disk-like closure element, the diameter of which is selected to be slightly smaller than the internal diameter of the discharge tube. At an end of the discharge tube which is to be closed off, this disk-like closure element is introduced in such a manner that initially an annular gap remains, typically of a few hundred micrometers, for example approx. 100 &mgr;m to 300 &mgr;m. An appropriate gap width results firstly from the requirement that it should be as easy as possible for the disk-like closure element to be introduced into the discharge tube, and secondly that the gap must also be closed again in a gas-tight manner at the end of the production of the discharge vessel. To this extent, it is advantageous if the gap is not excessively wide, since otherwise the constriction has to be made correspondingly deep. Moreover, it is advantageous for both the disk-like closure element and that end of the discharge tube which is to be closed off to be preheated in advance. Then, the closure element and the discharge tube are heated in the region of the closure element to the softening point. When the softening point is reached, the discharge tube is finally constricted in such a manner that the entire edge of the closure element is joined to the discharge tube wall in a gas-tight manner in the region of the constriction.
For the purpose of constriction, by way of example, a roller made from a material with a high melting point, for example a graphite roller, is used to press the softened part of the wall of the discharge tube onto the edge of the closure element, with the roller rotating with respect to the circumference of the discharge tube. For the typical gap width described above, a radial depth of the constriction of a few tenths of a millimeter, typically in the range from approx. 0.1 mm to 1 mm, preferably between 0.2 mm and 0.8 mm, particularly preferably between 0.4 mm and 0.6 mm, for example 0.5 mm, has proven sufficient.
It is preferable for the same type of glass to be used for the discharge tube and the disk-like closure element. The fact that the coefficients of expansion are consequently identical means that the stresses are lower than when using an additional joining means as in the prior art. In the latter case, the risk of inevitable stresses is correspondingly high on account of the different coefficients of expansion of joining means, for e

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