Flat light emitter

Details Flat light emitter Flat light emitter

1998-11-17

2001-06-26

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

With gas or vapor

Having a particular total or partial pressure

C313S485000, C313S491000, C313S483000

Reexamination Certificate

active

06252352

ABSTRACT:

TECHNICAL FIELD
The invention proceeds from a flat radiator in accordance with the preamble of claim
1
. Furthermore, the invention relates to a system composed of this flat radiator and a voltage source in accordance with the preamble of claim
10
.
The designation “flat radiator” is understood here to mean radiators having a flat geometry and which emit light, that is to say visible electromagnetic radiation, or ultraviolet (UV) or vacuum ultraviolet (VUV) radiation.
Depending on the spectrum of the emitted radiation, such radiation sources are suitable for general and auxiliary lighting, for example home and office lighting or background lighting of displays, for example LCDs (
L
iquid
C
rystal
D
isplays), for traffic lighting and signal lighting, for UV irradiation, for example sterilization or photolysis.
At issue here are flat radiators which are operated by means of dielectrically impeded discharge. In this type of radiator, either the electrodes of one polarity or all electrodes, that is to say of both polarities, are separated from the discharge by means of a dielectric layer (discharge dielectrically impeded at one end or two ends), see, for example, WO 94/23442 or EP 0 363 832. Such electrodes are also designated as “dielectric electrodes” below for short.
PRIOR ART
DE-A 195 26 211 discloses a flat radiator in which strip-shaped electrodes are arranged on the outer wall of a discharge vessel. The radiator is operated with the aid of a train of active power pulses separated from one another by pauses. Consequently, a multiplicity of individual discharges, which are delta-like (&Dgr;) in top view, that is to say at right angles to the plane in which the electrodes are arranged, burn in each case between neighbouring electrodes. These individual discharges are lined up next to one another along the electrodes, widening in each case in the direction of the (instantaneous) anode. In the case of alternating polarity of the voltage pulses of a discharge dielectrically impeded at two ends, there is a visual superimposition of two delta-shaped structures. The number of the individual discharge structures can be influenced, inter alia, by the electric power injected.
In accordance with the equidistantly arranged strips, the individual discharges are—assuming an adequate electric input power—distributed virtually uniformly inside the planar-like discharge vessel of the radiator. However, it is disadvantageous in this solution that the surface luminous density drops sharply towards the edge. The reason for this is, inter alia, the missing contributory radiation at the edge from the neighbouring regions outside the discharge vessel.
A further disadvantage is that the individual discharges preferentially are formed between the anodes and only one of the two respectively directly neighbouring cathodes. Evidently, individual discharges do not form simultaneously on both sides of the anode strips independently of one another. Rather, it cannot be predicted by which of the two neighbouring cathodes the discharges will be formed in each case. Referring to the flat radiator as a whole, this results in a non-uniform discharge structure, and consequently in a temporally and spatially non-uniform surface luminous density.
A uniform surface luminous density is, however, desirable for numerous applications of such radiators. Thus, for example, the back lighting of LCDs requires a visual uniformity whose depth of modulation does not exceed 15%.
REPRESENTATION OF THE INVENTION
It is the object of the present invention to provide a flat radiator having strip-like electrodes in accordance with the preamble of claim
1
and whose surface luminous density is virtually uniform up to the edge.
This object is achieved by means of the characterizing features of claim
1
. Particularly advantageous embodiments are to be found in the dependent claims.
The term “strip-like electrode” or “electrode strip” for short is to be understood here and below as an elongated structure which is very thin by comparison with its length and is capable of acting as an electrode. The edges of this structure need not necessarily be parallel to one another in this case. In particular, substructures along the longitudinal sides of the strips are also to be included. Moreover, a strip can also have a pattern, for example a zig-zag pattern or square-wave pattern.
The basic idea of the invention consists in using an adapted electrode structure to balance the fall, typical for flat radiators, in luminous density from the middle to the edges. The electrode structure is configured for this purpose to the effect that the electric power density increases towards the edges of the flat radiator.
In a first embodiment, the strip-shaped electrodes are arranged next to one another on a common wall of the discharge vessel (type I). This produces in operation an essentially planar-like discharge structure. The advantage is that shadows owing to the electrodes on the opposite wall are avoided. Instead of a single anode strip, as previously, two mutually parallel anode strips, that is to say an anode pair, are arranged in each case between the cathode strips. The result of this is to eliminate the problem outlined at the beginning that, in the quoted prior art, in each case only individual discharges of one of two neighbouring cathode strips burn in the direction of the individual anode strips situated therebetween.
In the following explanation of the principle of a first realization according to the invention of an electrode structure for a flat radiator of type I, reference is made to the diagrammatic representation in FIG.
1
. In order to be able to discern the details more effectively, only a section of the electrode region is shown. The aim to be achieved is to construct the individual discharges in operation in a spatially more dense fashion towards the edges
1
-
3
of the flat radiator than in the remaining part of the discharge vessel. For this purpose, the cathode strips
4
are specifically shaped in such a way that they have spatially preferred root points for the individual discharges. These preferred root points are realized by nose-like extensions
6
facing the respectively neighbouring anode
5
. Their effect is locally limited intensifications of the electric field, and consequently that the delta-shaped individual discharges
7
ignite exclusively at these points. The extensions
6
are arranged more densely in the direction of the narrow sides of the cathodes
4
,
4
′, that is to say in the direction of the edges
1
,
3
oriented perpendicular to the electrode strips
4
,
5
. Typically, the mutual spacing between the extensions
6
at the edges
1
,
3
is only half as large as in the middle. In the direct vicinity of the corner points of the flat radiator, the spacing between the extensions
6
is finally reduced to about a third. An individual anode strip
5
′ is preferably arranged in each case in the direct neighbourhood of the edges
2
orientated parallel to the electrode strips
4
,
5
(the corresponding opposite second edge of the flat radiator is not represented in the selected detail of FIG.
1
). Consequently, during operation the base sides of the delta-shaped (&Dgr;) individual discharges lined up along these individual anode strips
5
′ are in each case in the direct neighbourhood of the corresponding edges
2
. As a result, the drop in luminous density is also relatively slight as far as the vicinity of these edges
2
. Furthermore, to provide support it is additionally possible for the extensions
8
, facing the two individual anode strips
5
′, of the directly neighbouring cathode strips
4
′ to be arranged more densely overall than in the case of the remaining cathode strips
4
. However, the mean power density is less than the maximum achievable power density. Consequently, with this solution, as well, it is not possible to achieve a maximum luminous density averaged over the entire flat radiator.
The second principle for realizing an electrode structure for a flat ra

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