Note: Descriptions are shown in the official language in which they were submitted.
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Flat light emitter
Technical field
The invention relates to a flat radiator having an
at least partially transparent discharge vessel which is
closed and filled with a gas filling or opeiz and flowed
through by a gas filling and consists of electrically non-
conducting material, and having strip-like electrodes
comprising anodes and cathodes arranged on the wall of the
discharge vessel, at least the anodes being separated in each
case from the interior of the discharge vessel by a
dielectric material. Furthermore, the invention relates to a
system composed of this flat radiator and ~. voltage source.
The designation "flat radiator" is understood here
to mean radiators having a flat geometry and which emi t
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 (Liquid
Crystal Displays), 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.
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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 (D) 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
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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
According to the invention there is provided flat
radiator having an at least partially transparent discharge
vessel which is closed and filled with a ga.s filling or open
and flowed through by a gas filling and consists of
electrically non-conducting material, and having strip-like
electrodes comprising anodes and cathodes arranged on the
wall of the discharge vessel, at least the anodes being
separated in each case from the interior of the discharge
vessel by a dielectric material, characterized in that for
the purpose of specifically influencing them electric power
density distribution in the discharge, the electrodes are
specifically shaped in such a way that in operation the
surface luminous density of the flat radiator is largely
constant up to its edges.
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 iri this case. In particular, substructures
along the longitudinal sides of the strips are also to be
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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
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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
Figure 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
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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 Figure 1). Consequently, during
operation the base sides of the delta-shaped
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 neigh-
bouring 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 radiator of type I aims to
increase the luminous density of the individual
discharges to a greater extent the nearer they are
arranged to the edge. This is achieved (compare the
partial diagrammatic representation of the principle in
Figure 2) by virtue of the fact that the two anode
strips 9a,9b of each anode pair 9 are widened in the
direction of the edges 10,11 orientated perpendicular
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thereto, of the flat radiator. Typical values for the
widening amount to a factor of approximately two for
the edge regions of the flat radiator and to a factor
of about three for the corner regions.
- In a first variant, the anode strips are widened
asymmetrically with respect to their longitudinal axis
in the direction of the respective anodic partner strip
9b or 9a. Owing to this measure, the respective spacing
d from the neighbouring cathode 12 remains constant
throughout despite widening of the anode strips 9a,9b.
Consequently, during operation the ignition conditions
for all the individual discharges (not represented) are
also the same along the electrode strips 9,12..It is
ensured thereby that the individual discharges are
formed in a fashion lined up along the entire electrode
length (assuming an adequate electric input power).
In a second variant (not represented), the anode strips
are widened in the direction of the respective
neighbouring cathode. However, in this case the
widening is only relatively weakly formed. This
prevents the discharges from forming exclusively at the
point of maximum width of the anode strip, that is to
say at the point of the striking distance which is
shortest in this case. The widening is distinctly
smaller than the striking distance, typically
approximately one tenth of the striking distance.
Furthermore, both widening variants can also be
combined, that is to say the widening is formed both in
the direction of the respective anode partner strip and
in the direction of the neighbouring cathode.
An increasing electric current density, and thus also
an increasing luminous density of the individual
discharges is achieved along the widening, with the
result that it is possible effectively to balance the
luminous density distribution up to the edges 10,11.
However, it is no longer possible to realize the
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maximum luminous density in the middle region of the
flat radiator owing to the increase in luminous density
in the edge regions thereof. The advantage by
comparison with the first solution is, however, that -
assuming an adequate electric input power - it is
possible to achieve the maximum spatial density of the
individual discharges everywhere inside the discharge
vessel, that is to say in this case the individual
discharges are essentially directly adjacent to one
another.
Moreover, the two principles for realizing the specific
electrode shaping can also be combined with one another
(compare Figure 3a).
In the case of the anode widening, the cathodes need
not necessarily be provided with extensions, as is
shown merely by way of example in Figure 2. Rather, the
cathodes can also be designed as simple parallel strips
in the case of the widened anode strips.
In order to minimize the drop in the surface luminous
density at the edge, an experimental optimization of
the dense packing of the extensions and/or of the anode
widening is required in the concrete individual case.
In a further embodiment, the anode strips and cathode
strips are arranged on mutually opposite walls of the
discharge vessel (type II). During operation, the
discharges consequently burn from the electrodes of one
wall through the discharge chamber to the electrodes of
the other wall. In this arrangement, each cathode strip
is assigned two anode strips in such a way that, viewed
in cross-section with respect to the electrodes, the
imaginary connection of cathode strips and cor-
responding anode strips respectively yields the shape
of a "V". The result of this is that the striking
distance is greater than the spacing between the two
walls. As has been shown, it is possible using this
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arrangement to achieve a higher W yield than if anodes
and cathodes are arranged alternately next to one
another on only one common wall. According to the
present state of knowledge, this positive effect is
ascribed to reduced wall losses. The double anode
strips are preferably arranged on the top plate, which
serves primarily to couple out light, and the cathode
strips are arranged on the base plate of the flat
radiator. The advantage is the low shading of the
useful light emitted by the top plate, since the anode
strips are designed to be narrower than the cathode
strips. For the purpose of as small as possible a drop
in luminous density at the edge, as in the case of the
type I flat radiator the cathode strips have extensions
which are arranged increasingly more densely towards
their narrow sides. As an addition or an alternative to
this, the widening of the anode strips, already
likewise explained in the case of the type I flat
radiator, towards the edge of the flat lamp is also
advantageous.
Description of the drawings
The invention is to be explained below in more detail
with the aid of an exemplary embodiment. In the
figures:
Figure 1 shows a diagrammatic representation for
explaining the principle of a first shaping of the
electrodes according to the invention,
Figure 2 shows a diagrammatic representation for
explaining the principle of a second shaping of the
electrodes according to the invention,
Figure 3a shows a diagrammatic representation of a
partially cut away top view of a flat radiator
according to the invention, and
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Figure 3b shows a diagrammatic representation of a side
view of the flat radiator of Figure 3a.
Figures 3a,3b show in a diagrammatic representation a
top view and side view [sic] of a flat fluorescent
lamp, that is to say a flat radiator, which emits white
light during operation. This flat radiator is suitable
for normal lighting or for background lighting of
displays, for example LCD (Liquid Crystal Display).
Features similar to those in Figures 1 and 2 are
denoted below by means of the same reference numerals.
The flat radiator 13 comprises a flat discharge vessel
14 with a rectangular base face, four strip-like
metallic cathodes 12, 15 (-) and dielectrically impeded
anodes (+), of which three are constructed as elongated
double anodes 9 and two are constructed as individual
strip-shaped anodes 8. The discharge vessel 14 for its
part comprises a base plate 18, a top plate 19 and a
frame 9. The base plate 18 and top plate 19 are
connected in a gas-tight fashion to the frame 20 by
means of glass solder 21 in such a way that the
interior 22 of the discharge vessel 14 is of cuboid
construction. The base plate 18 is larger than the top
plate 19 in such a way that the discharge vessel 14 has
a free-standing circumferential edge. The inner wall of
the top plate 19 is coated with a mixture of
fluorescent materials (not visible in the
representation), which converts the UV/WV radiation
generated by the discharge into visible white light. In
one variant (not represented), in addition to the inner
wall of the top plate, the inner wall of the base plate
and of the frame are additionally also coated with a
mixture of fluorescent materials. Furthermore, one
light-reflecting layer each, made from A1z03 and TiOz,
respectively, is applied to the base plate.
The cutout in the top plate 19 serves merely
representational purposes and reveals the view onto a
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part of the anodes 8,9 and cathodes 12,15. The anodes
8,9 and cathodes 12,15 are arranged alternately and in
parallel on the inner wall of the base plate 18. The
anodes 8,9 and cathodes 12,15 are in each case extended
at one of their ends and are guided to the outside on
the baseplate 18 from the interior 22 of the discharge
vessel 14 on both sides in such a way that the
associated anodic or cathodic feedthroughs are arranged
on mutually opposite sides of the baseplate 18. At the
edge of the baseplate 18, the electrode strips
8, 9, 12, 15 merge in each case into a cathode-side 23 or
anode-side 24 bus-like conductor track. The two
conductor tracks 23,24 serve as contacts for connecting
with an electric voltage source (not represented). In
the interior 22 of the discharge vessel 14, the anodes
8,9 are completely covered with a glass layer 25 (see
also Figures 1 and 2), whose thickness is approximately
250 um.
The double anodes 9 respectively comprise two mutually
parallel strips, as already represented in detail in
Figure 2. In the direction of the edges 26,27
orientated at right angles to them, the two anode
strips 9a,9b of each anode pair 9 are widened at one
end in the direction of the respective partner strip 9b
or 9a. The anode strips 9a,9b are approximately 0.5 mm
wide at the narrowest point, and approximately 1 mm
wide at the widest point. The mutually largest spacing
gmaX (compare Figure 2) of the two strips of each anode
pair 9 is approximately 4 mm, while the smallest
spacing g,t,i" is approximately 3 mm. The two individual
anode strips 8 are in each case arranged in the direct
vicinity of the two edges 29,30 of the flat radiator 13
which are parallel to the electrode strips 8,9,12,15.
The cathode strips 12; 15 have nose-like extensions 28
which face the respectively neighbouring anode 8; 9. As
a result of them, there are locally limited
intensifications in the electric field and,
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consequently, the delta-shaped individual discharges
(not represented in Figure 3a, 3b but compare Figure 1)
ignite exclusively at these points. The extensions 28
of the two cathodes 15, which are the direct neighbours
of the edges 29, 30 of the flat radiator 13 which are
parallel to the electrode strips 8,9,12,15, are
arranged more densely along the respective longitudinal
sides, facing the said edges 29, 30, in the direction
of the narrow sides of the cathodes 15. The spacing d
(compare Figure 2) between the extensions 28 and the
respective directly neighbouring anode strip is
approximately 6 mm.
The electrodes 8,9,12,15 including the feedthroughs and
supply leads 23,24 are constructed respectively as
cohering cathode-side or anode-side structures
resembling conductor tracks. The structures are applied
directly to the base plate 18 by means of the silk-
screen printing technique.
A gas filling of xenon with a filling pressure of
10 kPa is located in the interior 22 of the flat
radiator 13.
One variant (not represented) differs from the flat
radiator represented in Figures 3a, 3b merely in that
not only the anodes but also the cathodes are separated
from the interior of the discharge vessel by a
dielectric layer (discharge dielectrically impeded at
3 0 both ends )
In a complete system, the anodes 8,9 and cathodes 12,15
of the flat radiator 13 are connected via the contacts
24 and 23, respectively, to one pole each of a pulsed
voltage source (not represented in Figures 3a,3b).
During operation, the pulsed voltage source supplies
unipolar voltage pulses which are separated from one
another by pauses. In this case, a multiplicity of
individual discharges are formed (not represented in
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Figures 3a,3b), which burn between the extensions 28 of
the respective cathode 12;15 and the corresponding
directly neighbouring anode strip 8;9.
The invention is not restricted to specified exemplary
embodiments. It is also possible in addition, to
combine features of different exemplary embodiments.
