Light source using dielectric barrier discharge lamp

Details Light source using dielectric barrier discharge lamp Light source using dielectric barrier discharge lamp

1999-08-06

2001-05-29

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

Automatic regulation

C315S057000, C315S111210, C315S219000, C315S276000

Reexamination Certificate

active

06239559

ABSTRACT:

TECHNICAL FIELD
The invention relates to a light source device which comprises a so-called dielectric barrier discharge lamp. This dielectric barrier discharge lamp is a type of discharge lamp which is used as a UV light source for a photochemical reaction and in which light emitted from excimer molecules which are formed by a dielectric barrier discharge is used.
DESCRIPTION OF RELATED ART
Generic art with respect to the dielectric barrier discharge lamp is for example the radiator from the patent disclosure document of Japanese patent application HEI 2-7353 in which a discharge vessel is filled with a discharge gas which forms excimer molecules and in which excimer molecules are formed by a dielectric barrier discharge which is also called an ozonizer discharge or a corona, as is described in the “Discharge Handbook”, Elektroassociation, Jun. 1989, 7th edition, page 263, Japan. In the radiator light is emitted from the above described excimer molecules.
A dielectric barrier discharge lamp between the electrodes which surround a discharge plasma space has one dielectric or two dielectrics.
FIG. 1
shows a dielectric barrier discharge lamp
1
in which two dielectrics
5
and
6
are present. In
FIG. 1
the lamp bulb
9
acts as the dielectrics
5
and
6
.
In the operation of the dielectric barrier discharge lamp
1
an AC voltage with a high frequency of for example 10 kHz to 200 kHz and 2 kV to 10 kV is applied to the electrodes
3
,
4
of their two poles. Due to the dielectrics
5
and
6
between the discharge plasma space
2
and the electrodes
3
and
4
the current flows from the electrodes
3
and
4
, not directly into the discharge plasma space
2
, but current flows due to the fact that the dielectrics
5
and
6
act as capacitors. This means that on the surfaces of the dielectrics
5
and
6
on the side of the discharge plasma space
2
by the polarization of the dielectrics an equivalent electric charge is induced as according to the respective electrode
3
and
4
which however has the opposite sign. Between the dielectrics
5
and
6
which surround the discharge plasma space and which are arranged opposite one another, a discharge occurs.
Since along the surfaces of the dielectrics
5
and
6
on the side of the discharge plasma space
2
only little current flows, in the area in which the discharge takes place the electrical charge induced on the surfaces of the dielectrics
5
and
6
on the side of the discharge plasma space
2
is neutralized by the electrical charge moved by the discharge, by which the electrical field of the discharge plasma space is made smaller. The discharge current therefore soon stops even if voltage continues to be applied to the electrodes
3
and
4
. But in the case in which the voltage applied to the electrodes
3
and
4
rises, the discharge current remains uninterrupted.
When the discharge stops, after a discharge has taken place once, there is no repeated discharge until the polarity of the voltage applied to the electrodes
3
and
4
is reversed.
In the case of a dielectric barrier discharge lamp for example which is filled with xenon gas, the xenon gas is separated by the discharge into ions and electrons, by which a xenon plasma forms. In this plasma xenon which has been excited to a certain energy level is bound, by which excimer molecules are formed. The xenon excimers are dissociated after a certain lifetime. The energy released in this process is emitted as photons with vacuum UV wavelengths. It is necessary to form these excimer molecules with high efficiency so that the dielectric barrier discharge lamp is operated as a vacuum UV light source with high efficiency.
Here a major factor which prevents formation of the excimer molecules with high efficiency during discharge is that the discharge plasma is excited to an energy level which does not contribute to formation of the excimer molecules.
Electron motion of the discharge plasma immediately after starting the discharge takes place in groups. The energy is high, but the temperature is low. In this state there is a great probability that the discharge plasma will pass into a resonant state which is necessary for formation of the excimer molecules. When the discharge duration becomes longer, the electron motion of the plasma gradually passes into a thermal state, i.e. into the thermal equilibrium state which is called the “Maxwell-Boltzman distribution”. This raises the plasma temperature, and the probability of passage to a more highly excited state in which no excimer molecules can be formed becomes greater.
Also, when excimer molecules arc formed are there furthermore cases in which the excimer molecules are destroyed by a subsequent discharge before they emit the photons expected after the lifetime expires and are dissociated in a natural manner. In the case of the xenon excimer, in practice after starting the discharge until photon emission with the vacuum UV wavelengths a time interval of about 1 microsecond is necessary. A subsequent discharge and repeated discharge in this time interval reduce the efficiency of the excimer emission.
It becomes apparent that it is most important to reduce as much as possible the energy of the subsequent discharge once the discharge has started.
Also in the case of a short discharge duration the probability of passage to a more highly excited state likewise becomes greater when the energy added in this discharge time interval is too great. The plasma which has passed into a more highly excited state emits only IR radiation, relaxes, and increases the lamp temperature. But it does not contribute to excimer emission.
This means that discharge driving must be done by which excitation of the discharge plasma is suppressed to the energy level which does not contribute to formation of the excimer molecules. In this respect a conventional light source device with a dielectric barrier discharge lamp is not satisfactory.
To achieve high efficiency of the excimer emission by all pulse discharges including of a dielectric barrier discharge lamp, Japanese patent disclosure document HEI 1-243363 was advanced. Here the above described condition is met that the energy of the following discharge is reduced as much as possible once a discharge has started. However it has only been described in this suggestion which parameters must be controlled to achieve high efficiency of the excimer emission. But here effective conditions of the parameter values are not specifically shown. Especially in the case of a dielectric barrier discharge lamp is it very difficult to find optimum conditions because the application of a voltage to the discharge plasma space and introduction of a current via the dielectrics must be done and the degree of freedom is low in the control of this voltage and current.
To improve the efficiency of the dielectric barrier discharge lamp, for example Japanese patent disclosure document HEI 8-508363 was advanced. In this proposal however no specific circumstances are described which, to achieve suppression of the excitation of the discharge plasma to the energy level which does not contribute to formation of the excimer molecules, is truly effective to effect the above described formation of the excimer molecules with high efficiency.
Japanese patent disclosure document HEI 1-163006 was named for example as an improvement proposal with respect to the driving waveform of a fluorescent lamp using a dielectric barrier discharge. Here it is described that the radiance of the fluorescent lamp is increased by driving with trains of square-wave pulses with a positive and a negative polarity or with triangular waves using an alternating current. In this publication the result of a test is described in which for the trains of square-wave pulses or triangular waves in conjunction with one frequency and a pulse duty height the change of radiance was studied with respect to the change of the applied voltage. Here the increase of the efficiency compared to conventional sinusoidal driving is described.
A feed device in practice compri

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