Electric lamp and discharge devices – With gas or vapor – Having electrode exterior to envelope
Reexamination Certificate
2001-02-07
2003-09-02
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode exterior to envelope
C313S594000
Reexamination Certificate
active
06614185
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a discharge lamp, a discharge lamp device using the discharge lamp, and a reader using the discharge lamp device.
BACKGROUND ART
Conventionally, an external electrode type discharge lamp that uses no mercury and is superior in the rise characteristics of a beam of light is known as a discharge lamp used in, for example, an image reader, such as a copying machine or an image scanner, as mentioned in, for example, Published U.S. Pat. No. 2,969,130.
In this discharge lamp, the interior of a tube-shaped light emitting body, such as a glass tube, is filled with a discharge medium, such as xenon, and the outer face of the light emitting tube is provided with a pair of external electrodes in such a way as to face each other, and, by applying a voltage between the pair of external electrodes and passing an electric current through them, the discharge medium discharges in the light emitting tube, thereby outwardly emitting luminous light generated by an electric discharge. A fluorescent layer is formed on the inner wall surface of the light emitting tube, except for the area of an aperture portion between the pair of external electrodes. The fluorescent materials of the fluorescent layer are excited by ultraviolet rays emitted by the electric discharge of the discharge medium, the ultraviolet rays are then converted into visible light, and the visible light is projected outwardly through the aperture portion.
However, in the discharge lamp that uses the pair of external electrodes, two tube walls on both sides of the light emitting tube are placed between the pair of external electrodes, and the tube walls serve to limit an electric current flowing between the external electrodes. Therefore, in order to obtain current by which an electric discharge is activated or lighting is maintained, a high voltage of about 2 to 3 kV and a high frequency of several tens of kilohertz to several hundred kilohertz, for example, are needed for lamp input. Thus, there is a problem in that a high-pressure proofing constituent element needs to be used as a lighting circuit part when the lamp input becomes a high voltage and, in addition, the electrodes to which the high voltage is applied must be coated to be fully insulated. Further, there is another problem in that a high frequency increases the emission of electromagnetic waves and thereby exerts a noise influence upon other electronic equipment although it might be a possible solution to raise a lighting frequency instead of considerably raising the lamp input voltage.
As mentioned in Japanese Unexamined Patent Publication No. 27269 of 1995, there is a discharge lamp in which a shaft-shaped internal electrode is disposed at the center of a cross section of a tube-shaped light emitting body, which has been sealed, along the longitudinal direction of the light emitting tube, and a high frequency voltage is applied between the internal electrode and an external electrode disposed on the outer face of the light emitting tube, thereby discharging between the internal and external electrodes. As the external electrode, use is made of a wire mesh or a metal film impervious to light that is disposed at an area excluding an aperture portion.
However, in the discharge lamp that uses the shaft-shaped internal electrode, only one tube wall of the light emitting tube is placed between the shaft-shaped internal electrode disposed at the center of the cross section of the light emitting tube and the external electrode, and therefore the starting voltage can be lowered more than in a case where a pair of external electrodes are used, but there is a need to dispose the internal electrode at the center of the light emitting tube that is to be sealed. Therefore, the processing accuracy of the sealing must be improved, and, if the processing accuracy is low, characteristic fluctuations will easily occur. Additionally, since there is a need to dispose the shaft-shaped internal electrode at the center of the cross section of the bulb, the path length of an electric discharge cannot be lengthened to ½ or more of the inner diameter of the tube. Therefore, there is a problem in that it is difficult to greatly increase efficiency, and it is difficult to form the internal electrode into a desired shape.
In the discharge lamp that uses the pair of external electrodes and in the discharge lamp that uses the shaft-shaped internal electrode and the metal film impervious to light serving as an external electrode, the fluorescent layer is formed in the area excluding the aperture portion so as to emit light from the aperture portion. Therefore, there is a problem in that the conversion efficiency of ultraviolet rays into visible light is lower than a case where a fluorescent layer is formed on the whole of the inner wall surface of a light emitting tube so as to emit light from the whole of the light emitting tube as in fluorescent lamps used generally, and, in addition, the luminous efficiency is so low that only about 65% of visible light generated in the light emitting tube is emitted out of the light emitting tube.
Accordingly, a possible solution for improving the luminous efficiency of the light emitting tube is to use a transparent conductive film as an external electrode. However, in a conventional discharge lamp whose starting voltage is high, the film thickness of the transparent conductive film needs to be thickened because the electric resistance and loss of the transparent conductive film must be reduced. This leads to a drop in the visible light transmittance of the transparent conductive film, so that the luminous efficiency cannot be expected to be fully improved. And, in the discharge lamp mentioned in Japanese Unexamined Patent Publication No. 27269 of 1995, the metal mesh is used as an external electrode. However, the metal mesh is at a disadvantage in that adhesion between the glass of the light emitting tube and the metal mesh is bad, and a slight electric discharge occurs in the outer face of the light emitting tube, and, in addition, dust adheres to the mesh.
The present invention was made in view of these respects, and it aims to provide a discharge lamp capable of being easily manufactured, capable of reducing lamp voltage, such as a starting voltage or a discharge maintaining voltage, and capable of improving the luminous efficiency, to provide a discharge lamp device using the discharge lamp, and to provide a reader using the discharge lamp device.
DISCLOSURE OF THE INVENTION
A discharge lamp of the present invention comprises a tube-shaped light emitting body, a discharge medium enclosed in the light emitting tube, an internal electrode formed on an inner wall surface of the light emitting tube along a longitudinal direction of the light emitting tube, and an external electrode disposed outside the light emitting tube along the longitudinal direction of the light emitting tube. Since the internal electrode is formed on the inner wall surface of the light emitting tube, and the external electrode is disposed outside the light emitting tube, only one tube wall of the light emitting tube lies between the internal electrode and the external electrode, and therefore the limitation of an electric current running between the internal electrode and the external electrode can be reduced, and a lamp voltage, such as a starting voltage or a discharge-maintaining voltage, can be lowered. Further, since the internal electrode is formed on the inner wall surface of the light emitting tube, it can be easily processed with high accuracy. Further, since the internal electrode is disposed on the inner wall surface of the light emitting tube, the internal electrode and the external electrode can be arranged to create a desired relationship of being close to or away from each other.
Presumably, an electric discharge first starts where the distance between the internal electrode and the external electrode is shortest, and an electric-discharge path length is gradually lengthened when a voltage is applied therebetween. Th
Honda Kouichi
Nishimura Kiyoshi
Shibuya Masuo
Takahara Yuichiro
Darby & Darby
Patel Vip
Toshiba Lighting & Technology Corporation
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