Discharge lamp with dielectrically impeded electrodes

Details Discharge lamp with dielectrically impeded electrodes Discharge lamp with dielectrically impeded electrodes

1999-12-13

2001-10-30

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Pulsating or a.c. supply

C313S620000, C313S607000, C315S260000

Reexamination Certificate

active

06310442

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge lamp and an illuminating system containing such discharge lamp.
The invention relates to a discharge lamp, in particular also to a fluorescent lamp, in which all electrodes are arranged on the external wall of the discharge vessel. The external wall serves in this case, inter alia, as a dielectric layer which separates the electrodes from the discharge during operation of the lamp. This type of discharge is therefore also termed a bilaterally dielectrically impeded discharge.
The spectrum of the electromagnetic radiation emitted by such as lamp can, in this case, comprise both the visible region and the UV (ultraviolet)/VUV(vacuum ultraviolet) region and the IR (infrared) region. Furthermore, a fluorescent layer can also be provided for converting invisible radiation into visible radiation.
Furthermore, the invention relates to a discharge lamp having a tubular discharge vessel sealed at both ends. The cross section of the discharge vessel is preferably circular. However, even only approximately circular cross sections, for example regular polygons such as, for example, hexagons, etc are also suitable. The term “tubular” is not restricted here to straight tubular discharge vessels, but likewise comprises bent, for example angled, tubular discharge vessels. Since the discharge direction runs essentially perpendicular to the lamp longitudinal axis, the length of the lamp is also not limited in principle.
Such lamps are used, in particular, in equipment for office automation (OA), for example color copiers and color scanners, for signal lighting, for example as brake lights and direction indicator lights in automobiles, for auxiliary lighting, for example interior illumination of automobiles, and for background lighting of displays, for example liquid crystal displays, and so-called “edge-type backlights”.
These technical fields require both particularly short start-up phases, and luminous fluxes which are as independent as possible of temperature. Consequently, these lamps contain no mercury. Rather, these lamps are usually filled with inert gas, preferably xenon, or inert gas mixtures.
The said applications require both a high luminous density and a luminous density which is uniform over the length of the lamp. To increase the luminous density, lamps for OA use are normally provided with an aperture along the longitudinal axis. Increasing the power injected into previous systems does not suffice to raise the luminous density further, since the loading of a lamp cannot be raised at will for lasting and reliable operation. A further difficulty is, that with the systems used so far in copiers and scanners, the efficiency of the discharge decreases with increasing injected power.
2. Background Information
U.S. Pat. No. 5,117,160 discloses an inert gas discharge lamp for OA equipment. There are two strip-shaped electrodes arranged along the lamp longitudinal axis on the outer surface of the wall of a tubular discharge vessel. The lamp is operated with AC voltage at a preferred frequency of between 20 kHz and 100 kHz. During operation, the 147 nm xenon line is excited. The efficiency of the useful radiation which can be achieved with the employed mode of operation, and consequently the resulting luminous density are relatively low.
It is known, furthermore, from U.S. Pat. No. 5,604,410 that the efficiency of dielectrically impeded discharges can be raised substantially compared with the dielectrically impeded discharges excited with AC voltage (see U.S. Pat. No. 5,117,160) with the aid of a pulsed operation (pulsed, dielectrically impeded discharge) adapted to the particular conditions (striking distance, electrode configuration, electrode geometry, filling gas and filling pressure).
SUMMARY OF THE INVENTION
It is the object of the present invention to eliminate the said disadvantages and provide a discharge lamp, in particular also a fluorescent lamp, with an improved luminous density.
This object is achieved by means of the present invention.
The term “electrode pair” is firstly introduced for the purpose of better comprehension of what follows. It is understood here as two elongated, mutually parallel electrodes having different polarities during operation, between which a “discharge plane” burns during operation. In the case of the preferred pulse-type injection of active power in accordance with U.S. Pat. No. 5,604,410, the discharge plane comprises a flat discharge structure which comprises a multiplicity of individual discharges.
According to the invention, the discharge lamp has three or more elongated electrodes which are arranged on the external wall of the tubular discharge vessel of the lamp and parallel to the longitudinal axis of the tubular discharge vessel in such a way that the following relationship is satisfied:
s
a
≥
0.1
,
preferably
s
a
≥
0.2
and more preferably
s
a
is greater than 0.25
s defining the maximum spacing between the imaginary connecting line of an electrode pair and the most closely neighboring wall of the discharge vessel, and a defining the mutual spacing between the electrodes of this electrode pair (measured centrally starting from the electrodes). Reference is also made in this connection to
FIG. 6
which shows diagrammatically on the example of a discharge lamp
1
and three electrodes
3
,
4
and
5
the maximum spacing s between the imaginary connecting line
20
of an electrode pair
3
,
4
or
3
,
5
and the most closely neighboring wall of the discharge vessel
2
.
Thus, during operation, at least two discharge planes are generated which extend between corresponding electrode pairs and along the longitudinal axis of the discharge vessel. A multiplicity of individual discharges are lined up in this plane next to one another along the electrodes and merge in the limiting case into a type of form of discharge resembling a curtain.
In this case, the discharge planes can also have a common electrode, for example in the case of three electrodes, in which two electrodes of the same polarity have only one common counter electrode of opposite polarity. In other words, in this case two electrode pairs share a common electrode. In the case of unipolar voltage pulses, this is preferably the cathode, the two other electrodes being connected as anodes. Further discharge planes can be generated inside the discharge vessel in order to increase the luminous density of the lamp still further.
In the case of three electrodes, these are preferably—seen in cross section—arranged at least approximately at the corner points of an imaginary isosceles or equilateral triangle. The latter case has the advantage that the lamp can be produced fairly simply, since the lamp need in each case be rotated only by 120° in order to mount the second and third electrodes. Moreover, it may be shown with the aid of simple geometrical considerations that in this case the quotient s/a always assumes the value 1/(2.{square root over (3)})≈0.29, independently of the lamp diameter, and consequently satisfies the relationship described previously. By contrast, the arrangement in the form of an isosceles triangle has the advantage that it is possible thereby to realize larger striking distances (and thus higher injected electric powers, see further below) for the two discharge planes, if the angle formed by the two discharge planes is selected to be less than 120°.
With four electrodes, it is possible either to realize two independent discharge planes, or else three discharge planes with a common electrode, depending on whether in the case of unipolar excitation the four electrodes are connected as two cathodes and two anodes, or as one cathode and three anodes (or one anode and three cathodes).
In principle, it is also possible for more than three discharge planes to be generated in this way. However, it basically depends on the diameter of the discharge tube whether it is possible at all in the case of three and more discharge planes for there still t

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