Abstract:
The present invention provides a plasma tube array including: plural light-emitting tubes; a front supporting member and a back supporting member which spread over the front and back of the light-emitting tubes; plural display electrode pairs provided on the surface of the front supporting member facing the light-emitting tubes; and plural signal electrodes provided on the surface of the back supporting member facing the light-emitting tubes. Each display electrode constituting the display electrode pair is a display electrode which is made of a metal thin wire, provided with plural openings formed in a distributed manner and includes a first metal thin wire facing a discharge slit and extending along the discharge slit, and the first metal thin wire is a metal thin wire thicker than a second metal thin wire which forms a region closer to a non-discharge slit side than the first metal thin wire.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a plasma tube array which includes an array of two or more light-emitting tubes incorporating fluorescent substance layers therein, produces discharge inside the two or more light-emitting tubes and causes the fluorescent substance layers inside the light-emitting tubes to emit light and thereby displays an image.  
         [0003]     2. Description of the Related Art  
         [0004]     As a large image display device which performs self light emission, there is a proposal on a technique (see Japanese Patent Laid-Open No. 61-103187) of displaying an image by using the principle of plasma display with an array of multiple light-emitting strings made up of glass tubes incorporating fluorescent substance layers therein and controlling light emission by parts of the respective light-emitting strings (see Japanese Patent Laid-Open No. 61-103187).  
         [0005]     Each light-emitting string includes a protective film such as an MgO film and a fluorescent substance layer in the glass tube filled with a discharge gas composed of, for example, Ne and Xe. The fluorescent substance layer is formed on a supporting member, a mounted part, called a “boat”, which has a substantially semicircular cross-section and the supporting member (boat) is inserted into the glass tube. Then, the glass tube is heated in a vacuum chamber and the gas is exhausted, the tube is filled with a discharge gas and then both ends of the glass tube are sealed. Multiple light-emitting strings created in this way are arranged in parallel and fixed and the light-emitting strings are provided with electrodes and a voltage is applied to the electrodes to thereby provoke discharge in the light-emitting strings and cause the fluorescent substance to emit light.  
         [0006]      FIG. 1  is a perspective view showing a basic structure of a plasma tube array.  
         [0007]     The plasma tube array (PTA)  100  shown here has a structure in which light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . containing fluorescent substance layers which emit red (R), green (G), blue (B) fluorescence filled with a discharge gas are arranged in parallel and in a planar shape as a whole, and a transparent front supporting member  20  and a transparent back supporting member  30  are placed on the front and back of the light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . respectively and the array of multiple light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . are held between the front supporting member  20  and back supporting member  30 .  
         [0008]     On the front supporting member  20  are multiple display electrode pairs  21  each made up of two display electrodes  211 ,  212  arranged parallel to each other forming a discharge slit in between in the direction of the array of the multiple light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . that is, the direction in which the display electrodes extend across the multiple light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . . These display electrode pairs  21  are arranged in two or more rows in the longitudinal direction of the light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . forming a non-discharge slit between the neighboring display electrode pairs  21 . Furthermore, the two display electrodes  211 ,  212  making up one display electrode pair  21  consist of metallic (e.g., Cr/Cu/Cr) bus electrodes  211   a ,  212   a  on the mutually far sides (non-discharge slit sides) and transparent electrodes  211   b ,  212   b  each made up of an ITO thin film on the mutually near sides (discharge slit sides). The bus electrodes  211   a ,  212   a  are intended to reduce the electric resistance of the display electrodes  211 ,  212  and the transparent electrodes  211   b ,  212   b  are designed so as to allow light emitted from the light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . to pass through up to the front supporting member  20  side without being intercepted and thereby realize brighter display.  
         [0009]     Furthermore, on the back supporting member  30  are multiple metallic signal electrodes  31  which are associated with and extend parallel to the multiple light-emitting strings  10 R,  10 G,  10 B,  10 R,  10 G,  10 B, . . . respectively.  
         [0010]     When the PTA  100  having such a structure is viewed two-dimensionally, the intersections between the signal electrodes  31  and display electrode pairs  21  become unit light-emitting regions (unit discharge regions). Display is realized by using either one of the display electrode  211 ,  212  as a scanning electrode, producing a selective discharge at the intersection between the scanning electrode and signal electrode  31  to select a light-emitting region, using wall charge formed on the inner surface of the light-emitting string of the region accompanying the discharge and thereby generating a display discharge between the display electrodes  211 ,  212 . A selective discharge is an opposed discharge produced in the light-emitting string between the opposed scanning electrode and signal electrode  31  in vertical direction, while a display discharge is a planar discharge produced in the light-emitting string between the display electrodes  211 ,  212  arranged in parallel on a plane. Such an electrode arrangement causes two or more light-emitting regions to be formed inside the light-emitting string in the longitudinal direction.  
         [0011]      FIG. 2  is a schematic view showing the structure of light-emitting strings making up the PTA  100  shown in  FIG. 1 .  
         [0012]     Here, three light-emitting strings  10 R,  10 G,  10 B are shown. Each of the light-emitting strings  10 R,  10 G,  10 B has a structure in which a protective film  12  of MgO, etc., is formed on the inner surface of a glass tube  11  and a boat  13  which is a supporting member on which fluorescent substance layer  14 R,  14 G,  14 B emitting R, G, B fluorescence is formed is inserted in the glass tube  11  (see Japanese Patent Laid-Open No. 2003-86141).  
         [0013]      FIG. 3  shows the boat on which the fluorescent substance layer is formed.  
         [0014]     The boat  13  has a semicircular or U-figured cross-section or the like and has an elongated shape as with the glass tube  11  (see  FIG. 2 ) and each of three types of fluorescent substance layers  14 R,  14 G,  14 B (see  FIG. 2 : here represented by the fluorescent substance layer  14 ) corresponding to the three type of light-emitting strings  10 R,  10 G,  10 B shown in  FIG. 1 ,  FIG. 2  is formed on the inside thereof.  
         [0015]     Returning to  FIG. 2 , the explanation will be continued.  
         [0016]     Each of the light-emitting strings  10 R,  10 G,  10 B shown in  FIG. 2  has a structure in which the boat  13  having the shape shown in  FIG. 3  is inserted in the glass tube  11 .  FIG. 2  shows the display electrode pair  21  made up of the two display electrodes  211 ,  212 , between which a discharge slit is formed, is arranged on the light-emitting strings  10 R,  10 G,  10 B. The two display electrodes  211 ,  212  are made up of metallic bus electrodes  211   a ,  212   a  and transparent electrodes  211   b ,  212   b.    
         [0017]     Here, in the case of the structure shown in  FIG. 2 , a region D 1  defined by one set of the three light-emitting strings  10 R,  10 G,  10 B provided with the three types of fluorescent substance layers  14 R,  14 G,  14 B respectively and one display electrode pair  21  made up of the two display electrodes  211 ,  212  constitutes one pixel which is a unit for displaying a color image. The diameter of each light-emitting string  10 R,  10 G,  10 B is typically on the order of 1 mm, and therefore in the case of the structure shown in this  FIG. 2 , the size of the region D 1  of one pixel is about 3 mm×3 mm.  
         [0018]     In the PTA having the basic structure described above, instead of arranging light-emitting strings two-dimensionally, it is also possible to form a curved image display plane by arranging the light-emitting strings along a curve (see Japanese Patent Laid-Open No. 2003-92085) or make the image display plane flexibly modifiable into various types of curved surfaces.  
         [0019]     In such a case, a flexible substrate, for example, PET (polyethylene terephthalate) substrate is used as the front supporting member  20  and back supporting member  30  and the display electrodes  211 ,  212  formed on the front supporting member  20  are also required to have a structure resistant to bending. In this case, when the display electrodes  211 ,  212  combining the metallic bus electrodes  211   a ,  212   a  and transparent electrodes  211   b ,  212   b  each made up of an ITO thin film as explained with reference to  FIG. 1 ,  FIG. 2  are used, since the ITO thin film has poor ductility, it may be cracked or subject to breaks when the flexible substrate is bent. For this reason, instead of the transparent electrodes  211   b ,  212   b  made of the ITO thin film, there is a proposal on an electrode structure with metal thin wires wired in a mesh, ladder stitch, or comb-like pattern (Japanese Patent Laid-Open No. 2003-338244). The electrodes having wiring of these metal thin wires are more appropriate in the sense that they are resistant to bending of the substrate and that the image display plane is formed on a flexible curved surface.  
         [0020]      FIG. 4  is a schematic diagram showing an example of display electrodes using metal thin wires.  
         [0021]     The figure shows a display electrode pair  21  made up of two display electrodes  211 ,  212  with a discharge slit  210  formed in between and the respective display electrodes  211 ,  212  are made up of bus electrodes  211   a ,  212   a  which are also provided for the display electrodes shown in  FIG. 1 ,  FIG. 2  and branched electrodes  211   c ,  212   c  made up of mesh-like metal thin wires  611 ,  612  instead of the transparent electrodes  211   b ,  212   b  shown in  FIG. 1 ,  FIG. 2 . In these branched electrodes  211   c ,  212   c , multiple openings  621 ,  622  surrounded by the metal thin wires  611 ,  612  are formed, distributed over all the branched electrodes  211   c ,  212   c.    
         [0022]     When the display electrodes  211 ,  212  having the structure shown in  FIG. 4  as well as the display electrodes using the transparent electrodes  211   b ,  212   b  shown in  FIG. 1 ,  FIG. 2 , a discharge produced in the discharge slit  210  provokes a discharge inside the light-emitting string  10  (called light-emitting string  10  as representative of light-emitting strings  10 R,  10 G,  10 B) shown in  FIG. 2  and causes the fluorescent substance  14  therein to emit light.  
         [0023]     Light emitted from this fluorescent substance passes through the discharge slit  210  or the openings  621 ,  622  of the branched electrodes  211   c ,  212   c  and appears as an image when the entire display surface is viewed.  
       SUMMARY OF THE INVENTION  
       [0024]     The problem to be solved when using the display electrodes having a structure with wiring of metal thin wires as illustrated in  FIG. 4  is to make compatible reducing the resistance of the display electrodes and allowing light emitted from the fluorescent substance to pass through the structure with a high degree of efficiency.  
         [0025]     Wiring relatively wide (or thick) metal thin wires or wiring metal thin wires relatively densely can reduce the electric resistance of the display electrodes but this causes the ratio of the area of the openings  621 ,  622  to the total area of the branched electrodes  211   c ,  212   c  (hereinafter referred to as “opening ratio”) to decrease, which causes the light emitted form the fluorescent substance to be shielded, reducing the transmission coefficient and resulting in a dark image.  
         [0026]     On the contrary, using thinner metal thin wires and using more coarse wiring of metal thin wires will increase the opening ratio and improve the transmission coefficient accordingly, but the electric resistance of the display electrodes  211 ,  212  is also increased, which requires a higher drive voltage to be applied to the display electrodes to obtain necessary light-emission intensity, etc., leading to deterioration of the discharge characteristic.  
         [0027]     The present invention has been made in view of the above circumstances and provides a plasma tube array including display electrodes adopting more flexible wiring of metal thin wires and having an electrode structure which makes both a discharge characteristic and a high opening ratio compatible at a high level.  
         [0028]     A plasma tube array according to the present invention includes: 
        a plurality of light-emitting tubes arranged parallel to one another, each containing a fluorescent substance layer;     a front supporting member and a back supporting member which spread over the front and back of the plurality of light-emitting tubes;     a plurality of display electrode pairs provided on the surface of the front supporting member facing the light-emitting tubes, each made up of two display electrodes extending parallel to each other in a direction extending across the plurality of light-emitting tubes between which a predetermined discharge slit is interposed, with one display electrode pair neighboring the other with a non-discharge slit interposed in between; and     a plurality of signal electrodes provided on the surface of the back supporting member facing the light-emitting tubes, formed associated with the plurality of light-emitting tubes, which extend along the light-emitting tubes,     wherein at least one of the display electrodes constituting the display electrode pair has a plurality of openings and metal wires forming the openings have different widths depending on area.        
 
         [0034]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair has a metal wire facing the discharge slit and extending along the discharge slit, the metal wire being larger in width than other metal wires.  
         [0035]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair has a plurality of metal wires extending in a direction orthogonal to the light-emitting tubes, and the metal wires are substantially parallel to the other of the display electrodes constituting the pair.  
         [0036]     In the plasma tube array according to the present invention, preferably, among the metal wires, a metal wire having the largest width is twice in width than a metal wire having the smallest width.  
         [0037]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair has a first region facing the discharge slit, the first region having a smaller opening ratio than a second region disposed closer to the non-discharge slit than the first region.  
         [0038]     In the plasma tube array according to the present invention, preferably, the opening ratio of the first region is 50% or less.  
         [0039]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair has a plurality of metal wires extending in a direction orthogonal to the light-emitting tubes and forming openings.  
         [0040]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair has a metal wire facing the non-discharge slit and extending along the non-discharge slit, the metal wire being larger in width than other metal wires.  
         [0041]     In the plasma tube array according to the present invention, preferably, at least one of the display electrodes constituting the display electrode pair includes an opening adjacent to the non-discharge slit, which is open to the non-discharge slit.  
         [0042]     In the plasma tube array according to the present invention, preferably, the display electrodes constituting the display electrode pair excluding extension lines thereof are shaped symmetrically.  
         [0043]     According to the present invention, it is possible to provide a plasma tube array including display electrodes having an electrode structure which makes both a discharge characteristic and a high opening ratio compatible at a high level, capable of forming a curved image display screen and bending the image display screen flexibly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]      FIG. 1  is a perspective view showing a basic structure of a plasma tube array;  
         [0045]      FIG. 2  is a schematic view showing a structure of a light-emitting string making up a plasma tube array;  
         [0046]      FIG. 3  illustrates a boat in which a fluorescent substance layer is formed;  
         [0047]      FIG. 4  is a schematic diagram showing an example of display electrodes adopting metal thin wires;  
         [0048]      FIG. 5  illustrates display electrodes of a plasma tube array according to a first embodiment of the present invention;  
         [0049]      FIG. 6  illustrates display electrodes of a plasma tube array according to a second embodiment of the present invention;  
         [0050]      FIG. 7  illustrates an experiment result;  
         [0051]      FIG. 8  illustrates display electrodes of a plasma tube array according to a third embodiment of the present invention;  
         [0052]     FIGS.  9 (A) and  9 (B) illustrate display electrodes of a plasma tube array according to a fourth embodiment of the present invention;  
         [0053]      FIG. 10  illustrates a discharge characteristic of the electrode structure shown in FIGS.  9 (A) and  9 (B);  
         [0054]      FIG. 11  illustrates a relationship between a drive voltage and brightness of light emission in the electrode structure in FIGS.  9 (A) and  9 (B);  
         [0055]      FIG. 12  illustrates display electrodes of a plasma tube array according to a fifth embodiment of the present invention; and  
         [0056]      FIG. 13  illustrates display electrodes of a plasma tube array according to a sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0057]     Embodiments of the present invention will be described below.  
         [0058]     Various embodiments which will be described below differ from the conventional techniques explained so far only in the electrode structure of the display electrode, and therefore the overall structure in the respective embodiments will be explained with reference to the explanations given so far and the electrode structure of the display electrode specific to the respective embodiments will be mainly described here.  
         [0059]      FIG. 5  illustrates display electrodes of a plasma tube array according to a first embodiment of the present invention.  
         [0060]     This figure shows a display electrode pair  21  made up of two display electrodes  211 ,  212  between which a discharge slit  210  is formed and the respective display electrodes  211 ,  212  are constructed of bus electrodes  211   a ,  212   a  and branched electrodes  211   c ,  212   c  made up of mesh-like metal thin wires  611 ,  612  as in the case of the conventional example shown in  FIG. 4 . Multiple openings  621 ,  622  surrounded by the metal thin wires  611 ,  612  are formed over the entire surfaces of the branched electrodes  211   c ,  212   c.    
         [0061]     What has been explained so far about the display electrodes shown in  FIG. 5  are similar to the display electrodes in the conventional example shown in  FIG. 4 , but unlike the conventional example shown in  FIG. 4 , the display electrodes  211 ,  212  shown in  FIG. 5  adopts metal thin wires having a greater width, as metal thin wires  611   a ,  612   a  facing the discharge slit  210  and extending the discharge slit, than metal thin wires  611   b ,  612   b  forming regions close to the non-discharge slit sides formed in a space with the neighboring display electrode pair. For example, the metal thin wires  611   a ,  612   a  are metal thin wires having a wire width of 20 μm and the metal thin wires  611   b ,  612   b  are metal thin wires having a wire width of 5 μm.  
         [0062]     As shown in this  FIG. 5 , when thicker metal thin wires are used only for the parts facing the discharge slit  210 , the discharge characteristic is improved a great deal. In this case, the overall opening ratio of the branched electrodes  211   c ,  212   c  decreases slightly, but it is possible to maintain a sufficiently high opening ratio.  
         [0063]      FIG. 6  illustrates display electrodes of a plasma tube array according to a second embodiment of the present invention.  
         [0064]      FIG. 6  shows three light-emitting strings  10 . In this  FIG. 6 , the same components as those of the display electrodes of the first embodiment shown in  FIG. 5  are assigned the same reference numerals as those in  FIG. 5 .  
         [0065]      FIG. 6  as well as  FIG. 5  shows a display electrode pair  21  made up of two display electrodes  211 ,  212  between which a discharge slit  210  is interposed. As in the case of the display electrodes shown in  FIG. 5 , the respective display electrodes  211 ,  212  are constructed of bus electrodes  211   a ,  212   a  and branched electrodes  211   c ,  212   c . However, unlike  FIG. 5 , the branched electrodes  211   c ,  212   c  making up the display electrodes  211 ,  212  shown in this  FIG. 6  have an electrode structure with ladder-like wiring of metal thin wires  611 ,  612 .  
         [0066]     An experiment is conducted here by creating the display electrodes  211 ,  212  using a wire width W of 12 μm and 20 μm as metal thin wires  611   a ,  612   a  facing the discharge slit  210  and extending along the discharge slit  210 . The wire width of metal thin wires  611   b ,  612   b  other than the metal thin wires  611   a ,  612   a  facing the discharge slit  210  out of the metal thin wires  611 ,  612  making up the branched electrodes  211   c ,  212   c  is 5 μm.  
         [0067]      FIG. 7  illustrates the experiment result.  
         [0068]     This  FIG. 7  shows the result of continuously applying a high-voltage square wave voltage to the display electrodes  211 ,  212 , so-called sustained drive, and graphs A, B, C, D show a last ON voltage, first ON voltage, first OFF voltage and last OFF voltage, respectively.  
         [0069]     Here, the voltage at which at least one of pixels originally controlled so as not to emit light discharges (emits light) when the voltage applied to the display electrodes  211 ,  212  is increased gradually from a sufficiently low voltage is the first ON voltage B and the voltage at which all pixels including even pixels originally controlled so as not to emit light discharge (emit light) is the last ON voltage A. Furthermore, the voltage at which at least one of pixels which should originally emit light stops emitting light when the voltage is increased gradually from a state in which all pixels originally controlled so as to emit light are discharging (emitting light) is the first OFF voltage C and the voltage at which all pixels which should originally emit light stop emitting light when the voltage is further decreased is the last OFF voltage D.  
         [0070]     Therefore, it is necessary to drive the display electrodes at a voltage not higher than the first ON voltage B and not lower than the first OFF voltage C (region with hatching in  FIG. 7 ) and this region between the first ON voltage B and first OFF voltage C corresponds to an operation allowance.  
         [0071]     In the case of the display electrodes  211 ,  212  shown in  FIG. 6 , the first ON voltage B exceeds the first OFF voltage C when the wire width W of the metal thin wires  611   a ,  612   a  facing the discharge slit  210  is approximately 13 μm or above. The wire width at the intersection between the first ON voltage B and the first OFF voltage C varies depending on the electrode structure, etc., but the wire width of the metal thin wires  611   a ,  612   a  facing the discharge slit  210  needs to be 13 μm or above when the electrode structure shown in  FIG. 6  is adopted.  
         [0072]     As shown in this  FIG. 7 , it is possible to change the discharge characteristic drastically by only adjusting the wire width of the metal thin wires  611   a ,  612   a  facing the discharge slit  210  and secure a sufficient operation allowance and realize a stable discharge (light emission, image display) using the metal thin wires  611   a ,  612   a  having a relatively large wire width. Furthermore, since the discharge characteristic can be improved by only adjusting the wire width of the metal thin wires  611   a ,  612   a  facing the discharge slit  210 , it is possible to secure a sufficient opening ratio, maintain a high transmission coefficient and provide bright display.  
         [0073]      FIG. 8  illustrates display electrodes of a plasma tube array according to a third embodiment of the present invention.  
         [0074]     This  FIG. 8  shows a display electrode pair  21  made up of two display electrodes  211 ,  212  between which a discharge slit  210  is interposed and the respective display electrodes  211 ,  212  are constructed of bus electrodes  211   a ,  212   a  and branched electrodes  211   c ,  212   c.    
         [0075]     The branched electrodes  211   c ,  212   c  in this  FIG. 8  are formed of metal thin wires  611 ,  612  arranged parallel to the discharge slit  210  and slit-like openings  621 ,  622  are formed between these metal thin wires  611 ,  612 . Here, all the metal thin wires  611 ,  612  making up the branched electrodes  211   c ,  212   c  have the same wire width. However, first regions D 11 , D 21  enclosed by one metal thin wire  6111 ,  6121  facing the discharge slit  210  and neighboring metal thin wire  6112 ,  6122  have a narrower space so as to have a smaller opening ratio than that of second regions D 12 , D 22  of the branched electrodes  211   c ,  212   c  other than the first regions D 11 , D 21 . The opening ratio here refers to the ratio of the area of the openings except the area covered with metal thin wires to the area of the region, and a higher opening ratio means a higher transmission coefficient of light from the light-emitting strings. Furthermore, instead of the opening ratio, a “coverage rate” representing the ratio of the area covered with metal thin wires represented by (1−opening ratio) may also be used below.  
         [0076]     As shown in this  FIG. 8 , reducing only the opening ratio of the first regions D 11 , D 21  in the vicinity of the discharge slit  210  also improves the discharge characteristic, eliminates the necessity for drastically reducing the overall opening ratio of the branched electrodes  211   c ,  212   c  and can balance the discharge characteristic and the opening ratio at a high level.  
         [0077]     FIGS.  9 (A) and  9 (B) illustrate display electrodes of a plasma tube array according to a fourth embodiment of the present invention. FIGS.  9 (A) and  9 (B) show three light-emitting strings  10  respectively.  
         [0078]     In these FIGS.  9 (A) and  9 (B), the same components as those of the display electrodes of the third embodiment shown in  FIG. 8  are assigned the same reference numerals as those in  FIG. 8 .  
         [0079]     FIGS.  9 (A) and  9 (B) as well as  FIG. 8  show a display electrode pair  21  made up of two display electrodes  211 ,  212  between which a discharge slit  210  is interposed and the respective display electrodes  211 ,  212  are constructed of bus electrodes  211   a ,  212   a  and branched electrodes  211   c ,  212   c.    
         [0080]     The branched electrodes  211   c ,  212   c  in FIGS.  9 (A) and  9 (B) are formed of metal thin wires wired parallel to the discharge slit  210  and metal thin wires wired diagonal to the discharge slit  210  and rhombic openings  621 ,  622  are formed between their metal thin wires  611 ,  612 . As with  FIG. 8 , the metal thin wires  611 ,  612  making up the branched electrodes  211   c ,  212   c  also have the same wire width here.  
         [0081]     Here,. in the case of the electrode structure shown in  FIG. 9 (A), the widths between metal thin wires  6111 ,  6121  and metal thin wires  6112 ,  6122  are set so that the coverage rate (1−opening ratio) of first regions D 11 , D 21  (including their respective metal thin wires  6111 ,  6121 ;  6112 ,  6122 ) enclosed by the metal thin wires  6111 ,  6121  facing the discharge slit  210  and neighboring metal thin wires  6112 ,  6122  extending parallel thereto becomes 57%. In the case of  FIG. 9 (B), the coverage rate is set to 33% in the entire region of the branched electrodes  211   c ,  212   c  including the first regions D 11 , D 21 . In  FIG. 9 (A), the coverage rate of the branched electrodes  211   c ,  212   c  except the first regions D 11 , D 21  is set to 33%, too.  
         [0082]      FIG. 10  illustrates the discharge characteristic of the electrode structure shown in FIGS.  9 (A) and  9 (B). This  FIG. 10  as well as  FIG. 7  shows a result of continuously applying a high-voltage square wave voltage to the display electrodes  211 ,  212 , a so-called sustained drive. The horizontal axis shows a coverage rate (%) (1−opening ratio) of the regions D 11 , D 21  and the vertical axis shows voltage (V) and graphs A, B, C, D represent last ON voltage A, first ON voltage B, first OFF voltage C and last OFF voltage D respectively as in the case of  FIG. 7 .  
         [0083]     As is evident from this  FIG. 10 , the discharge characteristic is also improved by increasing only the coverage rate (reducing the opening ratio) of the regions D 11 , D 21  in the vicinity of the discharge slit  210 . That is, both the first ON voltage B and first OFF voltage C decrease, and therefore it is possible to reduce the drive voltage and increase the brightness of light emission when the same drive voltage is applied.  
         [0084]      FIG. 11  illustrates a relationship between a drive voltage (V) and brightness of light emission (cd/m 2 ) in the electrode structure in FIGS.  9 (A) and  9 (B).  
         [0085]     As shown in  FIG. 9 (A), graph A corresponds to the case where the opening ratio of only the first regions D 11 , D 21  adjacent to the discharge slit  210  is reduced (shield factor is reduced to 57%) and as shown in  FIG. 9 (B), graph B corresponds to the case where the opening ratio of the overall region of the branched electrodes  211   c ,  212   c  is the same (coverage rate is 33%).  
         [0086]     In order to obtain the same brightness of light emission, the graph A when only the coverage rate in the vicinity of the discharge slit  210  is increased requires a lower drive voltage than the graph B when the coverage rate is uniform, and therefore the graph A can obtain higher brightness of light emission when driven at the same drive voltage.  
         [0087]     Thus, as shown in  FIG. 8  and  FIG. 9 (A), reducing the opening ratio (increasing the coverage rate) of the region in the vicinity of the discharge slit  210  improves the discharge characteristic, does not require the opening ratio to be reduced considerably for the entire region of the branched electrodes  211   c ,  212   c  and can thereby balance the discharge characteristic and opening ratio at a high dimension.  
         [0088]      FIG. 12  illustrates display electrodes of a plasma tube array according to a fifth embodiment of the present invention.  
         [0089]     In this  FIG. 12 , bus electrodes  211   a ,  212   a  are formed at positions facing a discharge slit  210  and branched electrodes  211   c ,  212   c  are formed at positions farther from the discharge slit  210  (non-discharge slits formed between neighboring display electrode pairs (not shown)) than the bus electrodes  211   a ,  212   a.    
         [0090]     Furthermore, the branched electrodes  211   c ,  212   c  are formed of metal thin wires  611 ,  612  of the same wire width extending vertically and horizontally and rectangular openings  621 ,  622  are formed between the metal thin wires  611 ,  612 . However, the sides farthest from the discharge slit  210 , that is, openings  621   a ,  622   a  adjacent to the non-discharge slit have no metal thin wire which would partition the neighboring non-discharge slit and are open to the non-discharge slit.  
         [0091]     In the case of the display electrodes having the electrode structure shown in this  FIG. 12 , the bus electrodes  211   a ,  212   a  are formed at positions adjacent to the discharge slit  210 , which produces the same effect as that of the first embodiment shown in  FIG. 5 , that is, using a thick metal thin wire only for the metal thin wire adjacent to the discharge slit  210  produces the effect of improving the discharge characteristic. Furthermore, the absence of a metal thin wire extending laterally on the non-discharge slit side to be formed in spaces with the neighboring display electrode pairs can narrow the slit width of the non-discharge slit and expand the areas of the branched electrodes  211   c ,  212   c  accordingly to thereby balance the high opening ratio and the improvement of the discharge characteristic as a whole at a further higher level.  
         [0092]      FIG. 13  illustrates display electrodes of a plasma tube array according to a sixth embodiment of the present invention.  
         [0093]     The figure shows a display electrode pair  21  made up of two display electrodes  211 ,  212  between which a discharge slit  210  is interposed and the respective display electrodes  211 ,  212  are constructed of bus electrodes  211   a ,  212   a  and branched electrodes  211   c ,  212   c  made up of mesh-like metal thin wires  611 ,  612 . Multiple openings  621 ,  622  enclosed by the metal thin wires  611 ,  612  are formed over the entire surfaces of the branched electrodes  211   c ,  212   c.    
         [0094]     Here, the display electrodes  211 ,  212  shown in this  FIG. 13  adopt thicker metal thin wires.