Electric lamp and discharge devices: systems – Pulsating or a.c. supply
Reexamination Certificate
2002-01-15
2003-11-11
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
C315S250000
Reexamination Certificate
active
06646391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a light source device for a dielectric barrier discharge lamp. In particular, the invention relates to a light source device for a dielectric barrier discharge lamp which is used as an ultraviolet (UV) light source for a photochemical reaction, and in which light radiated from excimers formed by the dielectric barrier discharge is used.
2. Description of the Related Art
Conventionally, when controlling light from a light source device of a dielectric barrier discharge lamp, two processes are typically performed, either controlling the voltage applied to the lamp or controlling the frequency of the voltage applied to the lamp. In the process in which the voltage applied to the dielectric barrier discharge lamp is controlled, there is an upper limit with respect to the radiant efficiency of the UV radiation and a lower limit with respect to the uniformity of emission. For example, if a dielectric barrier discharge lamp in which a fluorescing material has been applied to the inside of the discharge vessel is used for purposes of image processing, the region between the these upper and lower limits is narrow. Therefore, adequate light control by this process is not possible.
In the process in which the frequency of the voltage applied to the lamp is controlled, light control is possible in a relatively wide range. There is a disadvantage, however, of reducing emission uniformity when the frequency is reduced and the amount of emission is decreased. In order to avoid this reduction in the uniformity of emission, the voltage applied to the lamp is controlled such that uniformity of emission is ensured in a state in which the emission amount is small. Conversely, when the frequency is raised and the amount of emission is increased there is a disadvantage of reducing the radiant efficiency of the UV radiation.
Japanese Patent Disclosure Document HEI 11-233071 describes a light source device for a dielectric barrier discharge lamp in which a supply means is provided, the supply means being divided into two parts and in which the frequency of the AC voltage and the DC source voltage are controlled. The object of this device is to change the voltage in the area in which reduction of the radiant efficiency of the excimer emission is allowable as a result of the increase of the voltage applied to the lamp. This is performed by uniform irradiation of a body to be irradiated with a large area by several dielectric barrier discharge lamps, with respect to voltage control. In addition, with respect to the frequency setting, the device carries out precision adjustment of the nonuniformity as a result of scattering of the radiant efficiency of the individual lamps by several lamps. In this device, the voltage applied to the lamp and the frequency are controlled without any connection to one another and independently of one another. Moreover, problems with respect to the uniformity of emission and the radiant efficiency in the individual dielectric barrier discharge lamps are neither recognized nor indicated.
The following is a description of problems associated with light control for a light source device of a dielectric barrier discharge lamp. Normally, in a dielectric barrier discharge lamp, an electrical charge forms an electrical field which is moved by a discharge in a discharge space and has been deposited on a dielectric. The effect is used by superposition of this electrical field on an electrical field which is formed by a voltage applied from outside the lamp, the outside voltage necessary for starting the discharge is reduced essentially by half.
In a case, however, in which the period after formation of a discharge and movement of the electrical charge until starting of the next discharge, the voltage applied from the outside to the dielectric barrier discharge lamp is changed, the electrical charge present on the dielectric is moved by electrical conduction of residual plasma in the discharge space after completion of the discharge and neutralized. This phenomenon is inevitable in the light source device of a dielectric barrier discharge lamp with a feed device by which an AC high voltage applied to the dielectric barrier discharge lamp is generated by a step-up transformer. The reason for this is that a step-up transformer cannot produce a strict DC voltage.
Normally, the voltage formed on the secondary side of a step-up transformer has a tendency to be continuously attenuated in the direction to zero voltage. Moreover, the voltage begins with an oscillation at the resonant frequency which is fixed by an inductance of the step-up transformer and the electrostatic capacitance of the dielectric barrier discharge lamp. The voltage is also changed in an oscillating manner by the voltage applied from the outside to the dielectric barrier discharge lamp as a result of the “ringing” phenomenon, when the resonant frequency is higher than the control frequency.
Thus, in a case of reducing the emission amount by a reduction of the control frequency for light control, the time interval of a discharge compared to non-light control increases. Accordingly, the amount of electrical charge increases which is moved by electrical conduction of the residual plasmas of the discharge space after completion of the discharge which is present on the dielectric and is neutralized. The intensifying action of the electrical field which is formed by the electrical charge adhering to the dielectric, with respect to the electrical field which is formed by the voltage applied from outside the lamp, is changed. This means that the discharge intensity for non-light control and for light control changes even if the voltage amplitude of the voltage applied from outside the lamp for non-light control and light control does not change. This situation is described in
FIGS. 10
,
11
(
a
) and
11
(
b
).
FIG. 10
shows a schematic of one example of a light source device of a dielectric barrier discharge lamp. This device includes an invertor of a full bridge system. Reference number
1
labels a dielectric barrier discharge lamp to which a chopper voltage generated by switching devices Q
91
through Q
94
and a step-up transformer T
91
is applied. In the switching devices Q
91
through Q
94
, a voltage supplied by a power source US is subjected to gate control by gate voltages Vg
1
and Vg
2
. Thus, a dielectric discharge is carried out.
FIGS.
11
(A) and
11
(B) each show the voltage waveform on the two ends of the dielectric barrier discharge lamp
1
. FIG.
11
(B) shows a case in which the control frequencies of the gate voltages Vg
1
and Vg
2
are made lower than those shown in FIG.
11
(A). In this case, the time interval T
1
in which the switching devices Q
91
through Q
94
are in the ON state does not change. With respect to the time interval in which all switching devices Q
91
through Q
94
are in the OFF state, as shown in FIG.
11
(A), there is a short time interval T
2
a which changes in FIG.
11
(B) into a long time interval T
2
b.
If the voltage of the power source US does not change, the voltage waveforms in the time interval T
1
in FIGS.
11
(A) and
11
(B) have similar shapes. The amplitude Vp of a lamp voltage Ve shown in FIG.
11
(A), therefore, has roughly the same value as that shown in FIG.
11
(B). However, Vta shown in FIG.
11
(A) and Vtb (Vta is larger than Vtb) shown in FIG.
11
(B) label a voltage immediately prior to the lamp voltage Ve becoming negative by the switching devices Q
92
and Q
93
being turned on in the next half period. This is because in the interval in which all the switching devices Q
91
through Q
94
are in the OFF state, as a result of the LC resonant phenomenon, the lamp voltage Ve changes due to the electrostatic capacitance of the dielectric barrier discharge lamp
1
and to the inductance on the secondary side of the step-up transformer T
91
. Thus, in FIGS.
11
(A) and
11
(B), the interval T
2
b is larger than the interval T
2
a, and therefore, the amount of change of the lamp volt
Hiraoka Takahiro
Okamoto Masashi
Okamoto Toshio
A Minh D
Nixon & Peabody LLP
Safran David S.
Ushiodenki Kabushiki Kaisha
Wong Don
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