Abstract:
In an image display apparatus including: an electron source; a target having a phosphor and an anode electrode, the target emits light for display by being illuminated with electrons from the electron source; and an intermediate electrode disposed in the midpoint between the electron source and the target, the intermediate electrode is applied with a potential greater than that applied to the anode electrode. Thereby, halation caused by back scattering electrons reentering a phosphor is reduced.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image display apparatus using an electron source. 
   2. Related Background Art 
   Japanese Patent Application Laid-Open H03-261024 discloses a spontaneous light emitting type flat display, which displays an image by illuminating a phosphor with an electron beam emanated from an electron source to generate fluorescence. The flat display is a thin image display apparatus constituted by placing an electron-emitting device for generating an electron beam within a vacuum panel sandwiched between a face plate and a rear plate. In the image display apparatus, a surface conduction electron-emitting device is employed as the electron-emitting device, and the electron beam is accelerated and irradiated onto the phosphor to cause the phosphor to emit light for displaying an image. 
   Japanese Patent Application Laid-Open H11-250839 discloses an image display apparatus with reduced halation, which is caused by back scattering electrons, generated by a phosphor illuminated with an electron beam, reentering the phosphor and causing it to emit light in unwanted portions; providing high-definition, high-contrast and purer spectral colors. 
     FIG. 4  is a schematic sectional view illustrating a planar image display apparatus disclosed in Japanese Patent Application Laid-Open H11-250839. 
   In this image display apparatus, an electron-emitting device  202  is formed on an insulating substrate  201 . A grid  204  is a modulating electrode having a passage hole for the electron beam, and is mounted on an insulating layer  203 . A transparent conductive ITO (indium tin oxide) film  211 , a phosphor  206  and an aluminum film  210  provided for improving luminous efficiency are formed on the panel side of a face plate substrate  205 , over which a graphite film  207  is formed to avoid back scattering electrons. 
   An electroconductive capturer  213  has an opened portion  214  for passing an electron ray emanated from the surface conduction electron-emitting device  202 , and an unopened portion  215  for capturing the back scattering electrons from the face plate substrate  205  side, and is maintained at a predetermined distance from the face plate by means of a partition member  216 . 
   Using glass frit  208 , the face plate substrate  205  and the substrate  201  are sealed, having an outer frame  209  in-between, to constitute a vacuum enclosure. A surface conduction electron-emitting device  202  is connected to an outer drive circuit (not shown), and the graphite film  207 , aluminum film  210  and ITO film  211  are connected to a high voltage power supply (not shown) by a high voltage cable which is not shown. 
   In the image display apparatus described above, the internal pressure is maintained at vacuum of approximate 10 −4  Pa, and electrons are emanated in the form of an electron beam when driving pulse voltage is applied to the surface conduction electron-emitting device  202  by the outer drive circuit. The electron beam passes the grid  204 , and is accelerated by positive high voltage applied to the phosphor  206  and the aluminum film  210  from the high voltage power supply to emit fluorescence upon impinging on the phosphor  206 . 
   As an electron source, in addition to using a surface conduction electron-emitting device, it is known to use a thermal electron source using a hot cathode, a field emission type electron-emitting device or a metal/insulating layer/metal type electron-emitting device. 
   In a planar image display apparatus as described above, the smaller opened portion of the electroconductive capturer increases the capture rate of the back scattering electrons, and as a result, improves the effect of reducing halation. However, the opened portion also functions to pass an electron beam (primary electron) emanated from the electron source, and the smaller opened portion prevents more primary electrons from passing through, reducing brightness and luminous efficiency. For this reason, a problem arises in that it has been difficult to make the opened portion smaller to a width such that enough back scattering electrons can be captured, which results in poor reduction of halation. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image display apparatus capable of reducing halation caused by back scattering electrons reentering a phosphor. The invention is an image display apparatus, comprising: an electron source; a target having a phosphor and an anode electrode, the target being illuminated with electrons from the electron source; and an intermediate electrode disposed between the electron source and the target, wherein the intermediate electrode is applied with a potential greater than that applied to the anode electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an embodiment of an image display apparatus according to the invention; 
       FIG. 2  is a sectional view illustrating a first example of an image display apparatus according to the invention; 
       FIG. 3  is a sectional view illustrating essential parts of a second example of an image display apparatus according to the invention; and 
       FIG. 4  is a schematic sectional view illustrating a conventional planar image display apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An image display apparatus according to the present invention comprises an electron source, a target having a phosphor and an anode electrode that are irradiated with electrons from the electron source, an intermediate electrode disposed between the electron source and the target, in which a voltage is applied to the intermediate electrode that is higher than the voltage applied to the anode electrode. 
   The image display apparatus according to the present invention described above can reduce the halation caused by a back-scattered electron reentering the phosphor. 
   Now, an embodiment of the present invention will be described with reference to the drawings. 
     FIG. 1  is a schematic diagram showing an image display apparatus according to an embodiment of the present invention. 
   The image display apparatus according to this embodiment has an insulating substrate  1011  and a transparent substrate  1021  facing each other and spaced apart from each other. 
   The insulating substrate  1011  has a plurality of electron sources  1012  on a surface thereof. The electron sources  1012  are not limited to a particular type and may be any electron source suitable for image display apparatus, such as a thermoelectron source using a thermal cathode, a field emission electron-emitting element, a metal/insulator/metal (semiconductor) electron-emitting element, and a surface conduction electron-emitting element. 
   On the other hand, the transparent substrate  1021  has a phosphor  1022  on a surface thereof facing to the insulating substrate  1011  and an anode electrode  1025  overlaid on the phosphor  1022 , and the phosphor  1022  and the anode electrode  1025  constitute a target  1020 . The transparent substrate  1021  is desirably made of an insulating material, and the anode electrode  1025  is desirably made of a material that is electroconductive and has a high visible-light reflectivity and a high electron transmittance. 
   While the anode electrode  1025  is formed on the surface of the phosphor  1022  in the example shown in  FIG. 1 , the anode electrode may be formed on the surface of the transparent substrate  1021 . In that case, the anode electrode is desirably made of an electroconductive transparent material. Alternatively, an anode electrode made of an electroconductive material having a high visible light reflectivity and a high electron transmittance may be formed on the surface on the phosphor  1022 , and at the same time, an anode electro made of an electroconductive transparent material may be formed on the surface of the transparent substrate  1021 . In other words, anode electrodes may be formed both on the surface of the phosphor  1022  and the surface of the transparent substrate  1021 . 
   Furthermore, the image display apparatus according to this embodiment has an intermediate electrode  1030  having an electron-passing opening  1031  that is disposed at a predetermined distance from the anode electrode  1025  between the insulating substrate  1011  and the transparent substrate  1021 . For example, the intermediate electrode  1030  is preferably made of a conductive material, such as Fe and Invar, and the thermal expansion coefficient thereof is preferably as close to that of the transparent substrate or insulating substrate as possible. 
   In the image display apparatus according to this embodiment, a voltage equal to or higher than the lowest voltage required to make the phosphor  1022  emit light is applied to the anode electrode  1025 , and a voltage higher than the voltage applied to the anode electrode  1025  is applied to the intermediate electrode  1030 . Consequently, a back-scattered electron produced by irradiation of the phosphor  1022  with an electron beam having been emitted from the electron source  1012  and passed through the electron-passing opening  1031  in the intermediate electrode  1030  is attracted and collected by the intermediate electrode  1030 . Thus, the halation is reduced that can be caused by the back-scattered electron reentering the phosphor  1022 . However, the voltage applied to the intermediate electrode  1030  is preferably limited up to 1.2 times as high as the voltage applied to the anode electrode  1025 , because an excessively great voltage difference between the intermediate electrode  1030  and the anode electrode  1025  may cause discharge between the electrodes. In other words, supposing that the voltage applied to the anode electrode  1025  is denoted by Va, and the voltage applied to the intermediate electrode  1030  is denoted by Vb, it is preferred that a relation “Va&lt;Vb≦Va*1.2” is satisfied. 
   Furthermore, the target  1020  may have a supporting member (not shown), and the intermediate electrode  1030  may be formed on the supporting member. In that case, the supporting member is preferably made of an insulating material or a high resistance material. 
   Furthermore, the intermediate electrode  1030  according to this embodiment is not limited to the planar shape with the electron-passing opening  1031  and may be ribbon-like shaped or wire-like shaped, for example. Furthermore, in order to facilitate patterning of the intermediate electrode  1030 , the intermediate electrode  1030  may be formed in the shape of a thin film. 
   EXAMPLE 
   In the following, the present invention will be described in mode detail with reference to examples. 
   First Example 
     FIG. 2  is a cross-sectional view of an image display apparatus according to a first example of the present invention. 
   As shown in  FIG. 2 , the image display apparatus according to this example has a rear plate  2010  and a face plate  2020  facing each other and spaced apart from each other with an outer frame  2040  interposed therebetween. 
   The rear plate  2010  comprises a rear plate substrate  2011  made of high strain point glass and a surface conduction electron-emitting element  2012  disposed thereon. On the other hand, the face plate  2020  has a face plate substrate  2021  made of high strain point glass, an ITO film  2024 , which is a transparent electroconductive film, overlaid on an inner surface of the face plate substrate  2021  (a surface thereof facing to the rear plate substrate  2011 ), and a phosphor  2022  overlaid on the ITO film  2024 . Furthermore, in order to improve light emission efficiency, a metal back  2023  is formed on the surface of the phosphor  2022 . The ITO film  2024  and the metal back  2023  constitute an anode electrode  2025 . Alternatively, the anode electrode  2025  may be constituted by one of the ITO film  2024  and the metal back  2023 . 
   The image display apparatus according to this example also has an intermediate electrode  2030  having an electron-passing opening  2031  between the rear plate  2010  and the face plate  2020 . The intermediate electrode  2030  is fixed using an adhesive to the rear plate  2010  via a spacer (not shown) at a distance of about 2 mm from the rear plate  2010 . Alternatively, the intermediate electrode  2030  may be fixed to the face plate  2020  via a space (not shown). 
   Between the face plate  2020  and the rear plate  2010 , there is interposed the outer frame  2040  having a thickness that allows the intermediate electrode  2030  and the face plate  2020  to be spaced apart from each other by about 2 mm. The periphery of the outer frame  2040  and the plates  2010  and  2020  are sealed with frit glass  2050 . The inner space defined by the plates  2010  and  2020  and the outer frame  2040  is maintained substantially under vacuum (at a pressure of about 10 −4  Pa). In this way, the plates  2010  and  2020  and the outer frame  2040  constitute a vacuum envelope. 
   The surface conduction electron-emitting element  2012  is connected to an external driving circuit (not shown) provided outside the vacuum envelope. In addition, the intermediate electrode  2030  is connected to a high voltage power supply (not shown) via a high voltage cable (not shown), the anode electrode  2025  is connected to the intermediate electrode  2030  via a resistor (not shown), and the intermediate electrode  2030  and the anode electrode  2025  are fixed at their respective predetermined voltages. According to this configuration, the voltage of the anode electrode  2025  is lower than the voltage of the intermediate electrode  2030  because of the presence of the resistor, so that the voltage can be applied to the intermediate electrode  2030  that is higher than the voltage applied to the anode electrode  2025 . 
   In this example, specifically, a voltage of 10 kV is applied to the anode electrode  2025 , and a voltage of 10.5 kV is applied to the intermediate electrode  2030 . If the voltage difference between the anode electrode  2025  and the intermediate electrode  2030  is excessively great, a discharge occurs between the electrodes and damages the phosphor  2022 . Thus, in this example, the voltage difference between the anode electrode  2025  and the intermediate electrode  2030  is set at 0.5 kV, in order to prevent occurrence of such a discharge. Here, it is to be noted that the voltages applied to the electrodes  2025  and  2030  are not limited to the values described above. The voltage applied to the intermediate electrode  2030  can be readily adjusted by adjusting the high voltage power supply, and the voltage applied to the anode electrode  2025  can be readily adjusted by changing the value of resistance of the resistor. 
   In the configuration described above, one high voltage power supply and one resistor are used. However, in an alternative configuration, a high voltage power supply for applying a voltage to the anode electrode  2025  may be provided in addition to the high voltage power supply for applying a voltage to the intermediate electrode  2030 . In that case, the resistor described above can be omitted. 
   An electric signal is transmitted from the external driving circuit to the image display apparatus fabricated as described above to drive the image display apparatus, thereby making the image display apparatus display an image. In the image display apparatus according to this example, because a back-scattered electron is attracted to the intermediate electrode  2030 , the back-scattered electron is prevented from reentering the phosphor  2022 . Therefore, the image display apparatus according to this example reduces the halation intensity by about 30% or more, depending on the voltage difference between the anode electrode  2025  and the intermediate electrode  2030 , the distance between the face plate  2020  and the intermediate electrode  2030  or the like. Furthermore, it is recognized that the color purity is improved as a result of the reduction of halation intensity. 
   Second Example 
     FIG. 3  is a cross-sectional view showing essential parts of an image display apparatus according to a second example of the present invention. The rear plate and the outer frame of the image display apparatus according to this example are the same as those according to the first example shown in  FIG. 2  and, therefore, will not be further described below. 
   In this example, a supporting member  3060  made of an insulating material is formed on a surface of a face plate  3020  facing the rear plate (not shown), and an intermediate electrode  3030  is formed on the supporting member  3060 . The intermediate electrode  3030  according to this example is composed of a thin film of aluminum deposited on the supporting member  3060  by mask deposition, for example. 
   In this example also, the intermediate electrode  3030  is connected to a high voltage power supply (not shown) via a high voltage cable (not shown), an anode electrode  3025  is connected to the intermediate electrode  3030  via a resistor (not shown), and thus, the intermediate electrode  3030  and the anode electrode  3025  are fixed at their respective predetermined voltages. Alternatively, the supporting member  3060  may be made of a high resistance material, and the electrical resistance of the supporting member  3060  can be appropriately changed to adjust the voltage applied to the anode electrode  3025  formed from electrodes  3023  and  3024 . 
   It is recognized that the image display apparatus according to this example also can reduce the halation by reducing the number of back-scattered electrons that reenter a phosphor  3022 . 
   This application claims priority from Japanese Patent Application No. 2004-310738 filed Oct. 26, 2004, which is hereby incorporated by reference herein.