Patent Publication Number: US-7586261-B2

Title: Rare gas fluorescent lamp

Description:
RELATED APPLICATION 
   The disclosure of Japanese Patent Application No. 2005-026854, filed Feb. 2, 2005, including the specification, claims and drawings thereof, is incorporated herein by reference in its entirety. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a rare gas fluorescent lamp and, in particular, to a rare gas fluorescent lamp that includes a light-emitting tube whose inner surface is coated by a fluorescent material and which is filled with a rare gas, a plurality of external electrodes which is provided on an outer surface of the light-emitting tube, and a conductive material, which is provided on an inner surface of an end portion of the light-emitting tube that corresponds to a portion on which the external electrodes are arranged. 
   DESCRIPTION OF RELATED ART 
   Conventionally, as a fluorescent lamp used for a light source of a business machine or a backlight of a liquid crystal display panel, a rare gas fluorescent lamp has been widely used, in which a plurality of strip-shaped external electrodes is provided on an outer surface of a light-emitting tube, and a high frequency voltage is applied to the external electrodes to turn on the rare gas fluorescent lamp. 
     FIGS. 7A and 7B  show an example of a conventional rare gas fluorescent lamp.  FIG. 7A  is a cross-sectional view of the rare gas fluorescent lamp in an axial direction, and  FIG. 7B  is a cross-sectional view of the rare gas fluorescent lamp taken along a line A-A of  FIG. 7A . The rare gas fluorescent lamp includes a light-emitting tube  101 , an external electrode  102 , and a fluorescent substance  103 . In the rare gas fluorescent lamp, the light-emitting tube  101  is filled with a rare gas, such as Xenon gas, and the external electrode  102  provided on an outer surface of the light-emitting tube  101  applies high frequency voltage through the light-emitting tube  101  made of a dielectric material to generate discharge in the light-emitting tube  101 . Ultraviolet rays are radiated by the discharge. The ultraviolet ray excites the fluorescent substance  103  coated on an inner surface of the light-emitting tube  101  to generate visible light so that the visible light is emitted to the outside. 
   Even though the external electrode  102  is made from, for example, an aluminum tape, it is not limited to the strip-shaped tape. The external electrodes  102  may be formed in a line shape or a mesh shape. In addition, the external electrodes  102  may be made from a metal tape such as a copper tape, or a conductive pigment such as a silver paste, instead of the aluminum tape. 
   A conductive material  104  is provided in a peripheral direction of an inner surface at an end portion of the light-emitting tube  101  to form a short circuit over a region on an inner surface of the light-emitting tube  101  on which both the external electrodes  102  are arranged. Examples of the conductive material  104  include a carbon paste and a silver paste. 
   Functions of the conductive material  104  will be described. The conductive materials  104  provided in the glass tube  101  are provided over inner sides of both of the external electrodes  102 . Since the areas occupied by the conductive materials are almost equal to each other, it has the same effect that capacitors having a substantially same capacitance are shorted by the same conductive materials  104 . Thus, the conductive material  104  has an electric potential that is almost half of the potential of both of the external electrodes  102 . On the other hand, since a discharge space has very large impedance before the discharge is initiated in the space, an inner wall of the light-emitting tube  101  provided on an inner side of the external electrode  102  has almost the same potential as that of the external electrode  102 . As a result, since very high electric field is applied between the conductive material  104  and the vicinity of the conductive material  104  on the inner wall of the light-emitting tube  101  provided on the inner side of the external electrode  102 , a desired preliminary discharge is generated. Thus, it becomes easy to generate main discharge. Since the preliminary discharge causes a lamp to be started, it is possible to generate the main discharge without failure of starting even when a low voltage is applied. Refer to, for example, Japanese Patent No. 3149780 Japanese Unexamined Patent Application Publication No. 10-188910. 
   SUMMARY OF THE INVENTION 
   In general, a rare gas fluorescent lamp includes a region (hereinafter referred to as ‘effective light-emitting region’) for ensuring a predetermined output, and the remaining region (hereinafter referred to as ‘dead space’). The rare gas fluorescent lamp is preferably configured such that the effective light-emitting region is wide and the dead space is narrow in terms of space-saving design. 
   However, there is a problem in the conventional rare gas fluorescent lamp in that an undesired creeping discharge is extensively formed on an inner surface of the light-emitting tube due to the conductive material provided on the light-emitting tube for improving starting performance, thereby increasing a dead space and reducing an effective light-emitting region. That is, in the case the conductive material is provided, the starting performance is improved due to generation of preliminary discharge, but extensive creeping discharge is generated on end portions of the light-emitting tube, thereby reducing the effective light-emitting region. 
   A cause of the above-mentioned problem will be described with reference to  FIG. 8 . 
   When high voltage is applied to the external electrode  102 , high voltage is generated in a discharge space within the light-emitting tube  101  to cause discharge. As shown in  FIG. 8 , as the discharge proceeds, electrons within the discharge space are accumulated on an inner wall of the light-emitting tube  101  located on a positive (+) potential side of the external electrode  102 , and positive ions are accumulated on an inner wall of the light-emitting tube  101  located on a negative (−) potential side of the external electrode  102 . Consequently, an electric field generated by the accumulated charges offsets an electric field generated by the external electrode  102 , thereby stopping the discharge. A large amount of accumulated charges try to stay on the inner wall of the light-emitting tube  101 , while the accumulated charges adjacent to the conductive material  104  try to move through the conductive material  104  having small resistance within the light-emitting tube  101 . When the charges adjacent to the conductive material  104  are removed, neighboring charges are drawn, such that charges move along the inner wall of the light-emitting tube  101  that is, creeping discharge occurs. Since charges are not stored on a region on which the creeping discharge occurs, it is difficult for discharge to occur in the next discharge cycle. As a result, it is not possible to generate ultraviolet rays required for exciting the fluorescent substance, thereby reducing the effective light-emitting region. 
   The present rare gas fluorescent lamp can prevent reduction of an effective light-emitting by securely suppressing extensive spread of creeping discharge on an inner surface of a light-emitting tube even when a conductive material is provided so as to improve the starting performance. 
   In order to solve the above-mentioned problems, the rare gas fluorescent lamp having a light-emitting tube whose inner surface is coated by a fluorescent material and which is filled with a rare gas, a plurality of external electrodes which are provided on an outer surface of the light-emitting tube, and a conductive material, which is provided at an end portion of the light-emitting tube that corresponds to a portion on which the external electrodes are arranged, the rare gas fluorescent lamp comprises a creeping discharge prevention unit that is provided inward of the conductive material in an axial direction to prevent diffusion of creeping discharge occurring between the conductive material and electrical charges stored on the inner surface of the light-emitting tube. 
   Accordingly, it is possible to prevent an effective light-emitting region from being reduced by securely suppressing the creeping discharge from being extensively diffused on an inner surface of the light-emitting tube even when the conductive material is provided to improve starting performance. 
   The creeping discharge prevention unit may be formed such that at least one of the external electrodes protrudes toward the outside of the light-emitting tube in a vicinity of an end of the at least one of the external electrodes. 
   Accordingly, dielectric polarization is suppressed in dielectric under the light-emitting tube in which the external electrode is protruded, such that only weak barrier discharge is formed. As a result, since the amount of negative and positive charges, which are respectively accumulated on an inner surface of dielectric on high voltage side and on an inner surface of dielectric on low voltage side under the light-emitting tube, is very small, it is possible to suppress the creeping discharge from being diffused between the charges and the conductive material. 
   The creeping discharge prevention unit may be formed such that a thickness of a wall of the light-emitting tube, which corresponds to at least one of the external electrodes is larger than that of the other portion, in a vicinity of an end of the at least one of the external electrodes. 
   Accordingly, it is possible to make electrostatic capacitance small under the thick portion of the light-emitting tube. As a result, since it is possible to make charges stored on an inner surface of a dielectric very small, thereby suppressing the creeping discharge from being diffused between the charges and the conductive material. 
   The creeping discharge prevention unit may be formed such that an additional member is interposed between the light-emitting tube and at least one of the external electrodes, in a vicinity of an end of the external electrode. 
   Accordingly, since electrostatic capacitance is reduced under part of the light-emitting tube in which the additional member is provided, it is possible to make charges stored on an inner surface of a dielectric very small, thereby suppressing the creeping discharge from being diffused between the charges and the conductive material. 
   The creeping discharge prevention unit may be formed such that a surface area per unit length of at least one of the external electrodes in an axial direction is less than a surface area per unit length of the other portion in the axial direction, in a vicinity of an end of the external electrode. 
   Accordingly, since electrostatic capacitance is reduced under the part of the light-emitting tube which corresponds to the portion having a small surface area, it is possible to make charges stored on an inner surface of a dielectric very small, thereby suppressing the creeping discharge from being diffused between the charges and the conductive material. 
   The creeping discharge prevention unit is formed such that a wall of the light-emitting tube which corresponds to at least one of the external electrodes protrudes toward inside or outside of the light-emitting tube, in a vicinity of an end of the at least one of the external electrode. 
   Accordingly, since a creeping distance from the conductive material to charges stored on an inner surface of the light-emitting tube is increased by a distance protruded along the inner surface, the creeping distance is longer compared to the conventional lamp. As a result, barrier discharge is formed under the protruded part of the light-emitting tube, such that charges are accumulated. However, since the distance from the charges stored on the inner surface of the light-emitting tube to the conductive material is elongated compared to the conventional structure, it is possible to prevent the effective light-emitting region from being reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present rare gas fluorescent lamp will be apparent from the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to a first embodiment of the present invention; 
       FIG. 2  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to a second embodiment of the present invention; 
       FIG. 3  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to a third embodiment of the present invention; 
       FIGS. 4A and 4B  show a rare gas fluorescent lamp according to a fourth embodiment of the present invention; 
       FIGS. 5A and 5B  are partial cross-sectional views of a rare gas fluorescent lamp in an axial direction; according to a fifth embodiment of the present invention; 
       FIG. 6  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to a sixth embodiment of the present invention; 
       FIGS. 7A and 7B  are views showing an example of a conventional rare gas fluorescent lamp; and 
       FIG. 8  is a view explaining the reason that creeping discharge is extensively generated on an end portion of a light-emitting tube. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A rare gas fluorescent lamp according to the present invention is divided into two types as set forth below. 
   A first-type rare gas fluorescent lamp includes a creeping discharge prevention unit  4  for preventing electrical charges from being accumulated on an inner surface of a light-emitting tube  1  (under external electrodes) corresponding to portions at which external electrodes  21  and  22  are arranged, as shown in the following first embodiment ( FIG. 1 ) to fourth embodiment ( FIG. 4 ). A second-type rare gas fluorescent lamp includes a creeping discharge prevention unit  4  that extends a creeping distance, as shown in the following fifth embodiment ( FIGS. 5A and 5B ) and sixth embodiment ( FIG. 6 ). 
   The creeping discharge prevention unit  4  shown in the sixth embodiment ( FIG. 6 ) includes a unit for preventing electrical charges from being accumulated on an inner surface (under the external electrodes) of the light-emitting tube  1  corresponding to portions at which external electrodes  21  and  22  are provided, and a unit for extending the creeping distance. 
   A first embodiment of the present invention will be described with reference to  FIG. 1 . 
     FIG. 1  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment. The rare gas fluorescent lamp includes a light-emitting tube  1 , on its inner surface of which a fluorescent material  5  is coated and in which rare gases is filled, external electrodes  21  and  22 , which are made from an aluminum tape or the like provided on an outer surface of the light-emitting tube  1 , a conductive material  3 , which is provided on an inner surface of an end portion of the light-emitting tube  1  in an O-ring or C-ring shape so as to correspond to portions on which the external electrodes  21  and  22  are provided, a creeping discharge prevention unit  4 , which is provided closer to a central portion than the conductive material  3  in the vicinity of the conductive material  3  to prevent diffusion of creeping discharge occurring between the conductive material  3  and electrical charges stored on an inner surface of the light-emitting tube  1 , a notch  41 , which is provided on part of the external electrode  21 , a conductive protrusion  42 , which is provided over the notch  41  of the external electrode  21  and does not contact the light-emitting tube  1 , and a fluorescent material  5 , which is coated on an inner surface of the light-emitting tube  1 . 
   As shown in  FIG. 1 , at least one of the external electrodes  21  includes the conductive protrusion  42  having approximately a ‘U’ shaped section that is not in contact with the light-emitting tube  1  in the vicinity of a portion corresponding to the conductive material  3 . For example, the ‘U’-shaped protrusion  42  made from a copper sheet is closely in contact with the external electrode  21  made from an aluminum tape. As shown in the drawing, the protrusion  42  may be formed to be connected to the external electrode  21 , crossing the notch  41 . Alternatively, the protrusion may be formed as part of the external electrode  21  made from a thick aluminum tape instead of providing the notch  41 . In addition, the protrusion  42  may be provided on both of the external electrodes  21  and  22  instead of providing on any one of the external electrodes  21  and  22 . 
   In the creeping discharge prevention unit  4  according to the present embodiment, when the external electrode  21  on a side on which the protrusion  42  is provided is at a high voltage, since the external electrode  21  is not in contact with the light-emitting tube  1  at the protrusion  42 , dielectric polarization is suppressed in a dielectric on the high voltage side under the protrusion  42 , such that only weak barrier discharge is formed. As a result, since the amount of negative and positive charges, which are respectively accumulated on an inner surface of the dielectric on high voltage side and on an inner surface of a dielectric on low voltage side, is very small on the inner surface of the light-emitting tube  1  under the protrusion  42 , it is possible to suppress the creeping discharge from being spread between the charges and the conductive material  3 . 
   A second embodiment of the present invention will be described with reference to  FIG. 2 . 
     FIG. 2  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment. The rare gas fluorescent lamp includes light-emitting tube  1  having a thick portion  43 , in which the thickness of the thick portion  43  is larger than other portion of the light-emitting tube in a direction perpendicular to a tube axis and in a direction opposite to a light-emitting space, and a protrusion  44 , which is a protruding portion of each external electrode and is provided on an outer surface of the thickness portion  43 . The other structural elements of the lamp are the same as those of  FIG. 1  and the same reference numerals are used therefor. The thickness portions  43  are formed by performing a heating process on part of the light-emitting tube  1  by means of, for example, a burner. 
   As shown in the above drawing, each of the external electrodes  21  and  22  includes the protrusion  44 , which has approximately a ‘U’-shaped portion in a cross-section view including a tube axis and is formed to cover the thickness portion  43  in the light-emitting tube  1 , and is provided to cover the thickness portion  43  and other portion of the light-emitting tube  1 . 
   In the creeping discharge prevention unit  4  according to the present embodiment, it is possible to make electrostatic capacitance of a portion under the thickness portion  43  small, thereby making electrical charges stored on an inner surface of a dielectric small. Accordingly, it is possible to prevent diffusion of the creeping discharge between the charges and the conductive material  3 . 
   In addition, the thick portion  43  of the light-emitting tube  1  may be formed toward the light-emitting space side instead of the opposite direction to the light-emitting space. In this case, it is possible to obtain the same effect as that obtained from the creeping discharge prevention unit shown in  FIGS. 4 and 5 . 
   A third embodiment of the present invention will be described with reference to  FIG. 3 . 
     FIG. 3  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment. In  FIG. 3 , an additional member  45  is interposed between the external electrode  21  and the light-emitting tube  1  in the vicinity of an end portion of the external electrode  21 . The other structural elements are the same as those shown in  FIG. 2  and the same reference numerals are used therefor. 
   As shown in  FIG. 2 , an additional member  45 , which is made of an insulation member different from the light-emitting tube  1  is provided on the light-emitting tube  1 , in a direction perpendicular to a tube axis and opposite to a light-emitting space. The additional member  45  is made of a high-resistive material, such as resin, ceramic, or semiconductor, and is fixed to the light-emitting tube  1  by the use of an adhesive. The external electrode  21  includes the protrusion  44 , which has approximately a ‘U’-shaped section and is formed to cover the additional member  45 , and other portion of the light-emitting tube  1 . 
   In the creeping discharge prevention unit  4  according to the present embodiment, since electrostatic capacitance is reduced under the portion of the light-emitting tube  1  in which the additional member  45  is provided, it is possible to make charges stored on an inner surface of a dielectric very small, thereby suppressing the creeping discharge from being diffused between the charges and the conductive material  3 . 
   A fourth embodiment of the present invention will be described with reference to  FIG. 4 . 
     FIG. 4A  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment.  FIG. 4B  is a top plan view of the rare gas fluorescent lamp according to the present embodiment. In the  FIGS. 4A and 4B , a narrow portion  46  is formed in the external electrode  21 . The other structural elements are the same as those shown in  FIG. 1  and the same reference numerals are used therefor. 
   As shown in  FIGS. 4A and 4B , at least one of the external electrodes  21  has the narrow portion  46 , which is decreased in width compared to other portion of the electrode, in the vicinity of a portion corresponding to the conductive material  3 . Thus, in the narrow portion  46 , a surface area per unit length of the light-emitting tube  1  in an axial direction is smaller than that of the other portion in the direction. The narrow portion  46  is formed by cutting out part of the external electrode  21  made of, for example, an aluminum tape along a longitudinal direction of the external electrode  21 . If the width of the external electrode  21  corresponding to a position overlapping with the conductive material  3  is too small, it is not possible to securely generate preliminary discharge. Thus, the external electrode  21  needs to have the width required to ensure starting performance. 
   In the creeping discharge prevention unit  4  according to the present embodiment, since electrostatic capacitance is reduced under the narrow portion  46 , it is possible to make charges stored on an inner surface of a dielectric very small, thereby suppressing the creeping discharge from being diffused between the charges and the conductive material  3 . 
   In addition, the same effect as that in the present embodiment can be obtained by making holes in a portion where a surface area needs to be reduced. 
   A fifth embodiment of the present invention will be described with reference to  FIGS. 5A and 5B . 
     FIG. 5A  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment. In  FIG. 5A , a protrusion  47  is formed by protruding inward part of the light-emitting tube  1  in the vicinity of an end portion of the external electrodes  21  and  22 , and a protrusion  48  of each of the external electrodes  21  and  22  corresponds to the protrusion  47 . The other structural elements are the same as those shown in  FIG. 1  and the same reference numerals are used therefore. 
   As shown in  FIG. 5A , the light-emitting tube  1  includes the protrusion  47  having a ‘U’ shaped section which is protruded to a light-emitting space in a direction perpendicular to a tube axis and is provided around the light-emitting tube  1 . Each of the external electrodes  21  and  22  includes the protrusion  48  having a ‘U’ shaped section on a portion which corresponds to the protrusion  47  and is provided along an outer surface of the light-emitting tube  1 . The protrusion  47  is formed by performing a heating process on part of the outer surface of the light-emitting tube  1  by means of, for example, a burner. 
   In the creeping discharge prevention unit  4  according to the present embodiment, since a creeping distance from the conductive material to charges stored on an inner surface of the light-emitting tube on the inner surface of the light-emitting tube  1  is increased by a distance protruded along the inner surface of the protrusion  47 , the creeping distance is longer compared to the conventional lamp. As a result, barrier discharge is formed under the protrusion  47 , such that charges are accumulated. However, since the distance from the charges stored on the inner surface of the light-emitting tube  1  to the conductive material  3  is elongated compared to the conventional structure, it is possible to prevent reduction of the effective light-emitting region. 
   In addition, as shown in  FIG. 5B , the protrusion  47  in the light-emitting tube  1  may be formed toward an opposite direction of a light-emitting space instead of being formed toward the light-emitting space side. 
   A sixth embodiment of the present invention will be described with reference to  FIG. 6 .  FIG. 6  is a partial cross-sectional view of a rare gas fluorescent lamp in an axial direction according to the present embodiment. The other structural elements are the same as those shown in  FIG. 5A  and the same reference numerals are used therefor. 
   As shown in  FIG. 6 , the light-emitting tube  1  includes the protrusion  47  having a ‘U’ shaped section which is protruded to a light-emitting space in a direction perpendicular to a tube axis and is provided around the light-emitting tube  1 . Each of the external electrodes  21  and  22  is formed in a straight section and is provided on an outer surface of portions other than the protrusion  47  without contacting the protrusion  47  provided on the light-emitting tube  1 . That is, a space is interposed between the protrusion  47  and the external electrodes  21  and  22  in the light-emitting tube  1 . 
   In the creeping discharge prevention unit  4  according to the present embodiment, when the external electrode  21  with the protrusion  47  provided is at a high voltage, the external electrode  21  is not in contact with the light-emitting tube  1  at the protrusion  47 . Accordingly, dielectric polarization is suppressed in a dielectric on the high voltage side under the protrusion  47 , such that only weak barrier discharge is formed. As a result, since the amount of negative and positive charges, which are respectively accumulated on an inner surface of a dielectric on high voltage side and on an inner surface of a dielectric on low voltage side, is very small on the inner surface of the light-emitting tube  1  under the protrusion  47 , it is possible to suppress the creeping discharge from being diffused between the charges and the conductive material  3 . In addition, since the light-emitting tube  1  includes the protrusion  47  and the distance from the charges stored on the inner surface of the light-emitting tube  1  to the conductive material  3  is elongated compared to the conventional structure, it is possible to prevent reduction of the effective light-emitting region. 
   Experimental results of the rare gas fluorescent lamp according to the present invention will be described. 
   EXAMPLE 1 
   According to the configuration shown in  FIG. 1 , four types of rare gas fluorescent lamps were prepared. In more detail, the light-emitting tubes  1  have an outer diameter of 8 mm or 10 mm, the emitting gas is Xe—Ne mixture gas having a ratio of Xe:Ne=2:8, and Xe has a partial pressure of 8 kPa or 12 kPa. 
   The light-emitting tube  1  has a length of 500 mm, and a thickness of 0.4 mm. Each of the external electrodes  21  and  22  is made from an aluminum tape. Each of the external electrodes  21  and  22  has almost the same length as the light-emitting tube, and has a width of 1 mm. 
   The conductive material  3  corresponds to the external electrodes  21  and  22 . The conductive material  3  is provided on an end portion of the external electrodes  21  and  22  and has a width of about 1 mm. 
   EXAMPLE 2 
   Finally, According to the configuration shown in  FIG. 3 , four types of rare gas fluorescent lamps were provided to have the same specification as Example 1. A phenolic resin is used as the additional member  45  and is provided at a position which is 4 to 5 m distant from the conductive material  3 . 
   EXAMPLE 3 
   According to the configuration shown in  FIG. 4 , four types of rare gas fluorescent lamps were provided to have the same specification as Example 1. The narrow portion  46  has a width of 0.5 mm. 
   EXAMPLE 4 
   According to the configuration shown in  FIG. 5 , four types of rare gas fluorescent lamps were provided to have the same specification as Example 1. 
   COMPARATIVE EXAMPLE 
   According to the configuration shown in  FIG. 7A , four types of rare gas fluorescent lamps were prepared to have the same specification as Example 1. 
   Comparing the above-mentioned examples 1 to 4 and the comparative example, when the rare gas fluorescent lamps according to Examples 1 to 4 were turned on with an input power of 5 W to 10 W, the starting performance was not deteriorated when the creeping discharge prevention unit  4  on the light-emitting tube has a length of 3 to 9 mm. In addition, the light intensity was not reduced in a region which is more than 15 mm distant from both ends of the light-emitting tube. On the other hand, in the rare gas fluorescent lamp according to the comparative example, the light intensity was decreased in a region which is 40 mm distant from both ends of the light-emitting tube. 
   Although only some exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages. Accordingly, all such modifications are intended to be included within the scope.