Abstract:
A plasma display panel includes a first substrate, and a second substrate facing the first substrate. The first substrate includes a transparent substrate, a scanning electrode and a common electrode both formed on the transparent substrate, and a dielectric layer covering the transparent substrate, the scanning electrode and the common electrode therewith. The second substrate includes an electrically insulating substrate, data electrodes formed on the electrically insulating substrate, partition walls formed on the electrically insulating substrate, and a phosphor layer covering the electrically insulating substrate and the data electrodes therewith between adjacent partition walls. The phosphor layer includes a blue-phosphor layer composed of phosphor which emits a blue light. The blue-phosphor layer is composed of a mixture of two or more phosphors each of which emits a blue light and has an initial brightness and variation of brightness with the lapse of time both different from one another.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a plasma display panel, and more particularly to a phosphor layer as one of parts constituting a plasma display panel, in particular, a phosphor layer which emits a blue light. 
   2. Description of the Related Art 
   A plasma display panel (PDP) is recently popularized as a flat display because of its advantages that a plasma display panel is thin, a big screen can be readily fabricated with a plurality of plasma display panels, a plasma display panel has a broad viewing angle, and a plasma display panel has a high response speed. 
     FIG. 1  is a perspective view of a display cell in a conventional three-electrode surface-discharge AC type plasma display panel  300 . 
   As illustrated in  FIG. 1 , the display cell includes a front substrate  351  and a rear substrate  352  arranged in parallel with each other. In use of the plasma display panel  300 , the front substrate  351  is directed to a viewer. 
   The front substrate  351  is comprised of an electrically insulating substrate  302  composed of transparent material such as glass, a plurality of scanning electrodes  303  and common electrodes  304  (only one of them is illustrated in  FIG. 1 ) formed on a surface of the electrically insulating substrate  302  facing the rear substrate  352 , trace electrodes  305  formed on the scanning electrodes  303 , trace electrodes  306  formed on the common electrodes  304 , a dielectric layer  312  formed on the electrically insulating substrate  302 , covering the scanning electrodes  303 , the common electrodes  304  and the trace electrodes  305  and  306  therewith, and a protection layer  313  formed on the dielectric layer  312 . 
   The scanning and common electrodes  303  and  304  are alternately arranged and spaced away from each other by a certain distance in parallel with each other. 
   The trace electrodes  305  and  306  reduce electrical resistances of the scanning and common electrodes  303  and  304 . 
   The protection layer  313  protects the dielectric layer  312  from discharges, and is composed of magnesium oxide (MgO), for instance. 
   The rear substrate  352  is comprised of an electrically insulating substrate  301  composed of transparent material such as glass, a plurality of data electrodes  307  (only one of them is illustrated in  FIG. 1 ) formed on a surface of the electrically insulating substrate  301  facing the front substrate  351  such that the data electrodes  307  extend perpendicularly to the scanning and common electrodes  303  and  304 , a dielectric layer  314  formed on the electrically insulating substrate  301 , covering the data electrodes  307  therewith, a partition walls  315  formed on the dielectric layer  314 , and a phosphor layer  311  formed on both an exposed surface of the dielectric layer  314  and sidewalls of the partition wall  315 . 
   In the display cell illustrated in  FIG. 1 , through the rear substrate  352  is designed to include the transparent substrate  301 , it is not always necessary for the electrically insulating substrate  301  to be transparent. 
   The partition wall  315  defines a discharge gas space and a display cell (a pixel)  308 . 
   When viewed perpendicularly to the electrically insulating substrate  301 , the partition wall  315  is in the form of a grid. The partition wall  315  is comprised of vertical partition walls  315   a  extending in parallel with the data electrodes  307 , and horizontal partition walls  315   b  extending perpendicularly to the vertical partition walls  315   a.    
   The vertical and horizontal partition walls  315   a  and  315   b  are equal in height to each other. A height of the vertical and horizontal partition walls  315   a  and  315   b  from a surface of the electrically insulating substrate  301 , that is, a total thickness of the dielectric layer  314  and the partition wall  315  is 120 micrometers, for instance. 
   The display cell  308  is filled with a discharge gas composed of a mixture of helium, neon, xenon or other noble gases singly or in combination. The phosphor layer  311  receives ultra-violet rays resulted from discharge of the discharge gas, and resultingly, emits a visible light  310  towards a viewer. 
     FIGS. 2A to 2F  illustrate steps to be carried out in a method of fabricating the plasma display panel  300 .  FIGS. 2A ,  2 C and  2 E are plan views of the rear substrate  352 , and  FIGS. 2B ,  2 D and  2 F are cross-sectional views taken along the lines  2 B— 2 B,  2 D— 2 D and  2 F— 2 F in  FIGS. 2A ,  2 C and  2 E, respectively. 
   Hereinbelow is explained a method of fabricating the plasma display panel  300  with reference to  FIGS. 2A to 2F . 
   As illustrated in  FIG. 1 , the scanning and common electrodes  303  and  304  are formed on the electrically insulating substrate  302  such that they are alternately arranged and extend in parallel with each other. 
   Then, the trace electrodes  305  and  306  are formed on the scanning and common electrodes  303  and  304 , respectively. 
   Then, the dielectric layer  312  is deposited on the electrically insulating substrate  302  such that the scanning and common electrodes  303  and  304  and the trace electrodes  305  and  306  are fully covered with the dielectric layer  312 . 
   Then, the protection layer composed of MgO is formed on the dielectric layer  312 . 
   Thus, there is completed the front substrate  351 . 
   With respect to the rear substrate  352 , as illustrated in  FIGS. 2A and 2B , a plurality of the data electrodes  307  are formed on the electrically insulating substrate  301  so that they extend in a common direction. 
   Then, as illustrated in  FIGS. 2C and 2D , the dielectric layer  314  is formed on the electrically insulating substrate  301  so as to cover the data electrodes  307  therewith. 
   Then, as illustrated in  FIGS. 2E and 2F , the partition wall  315  is formed on the dielectric layer  314 . 
   The partition wall  315  may be formed by sand-blasting or printing. For instance, the partition wall  315  is formed by sand-blasting, as follows. 
   First, there is made partition-wall paste comprised of a mixture of filler, glass powder, binder and solution. 
   Then, the partition-wall paste is coated onto the dielectric layer  314 . Then, solution contained in the partition-wall paste is evaporated. Thus, there is formed a paste layer (not illustrated). 
   Then, a dry film is adhesively put on the paste layer, and then, is patterned. 
   Then, the paste layer is sand-blasted with the thus patterned dry film being used as a mask. Thus, the paste layer is patterned in accordance with a pattern of the dry film. 
   Then, the dry film is removed, and the paste layer is baked. 
   Thus, binder contained in the paste layer is evaporated, and simultaneously, glass powder is melted and solidified again. As a result, there is formed the partition wall  315  composed of filler and glass. 
   The partition wall  315  is formed in the form of a grid such that the vertical and horizontal partition walls  315   a  and  315   b  are equal in height to each other. 
   Then, as illustrated in  FIG. 1 , the phosphor layer  311  is formed on an exposed surface of the dielectric layer  314  and sidewalls of the partition wall  315 . 
   Thereafter, the electrically insulating substrate  301  is caused to overlap the electrically insulating substrate  302  such that the protection layer  313  formed on the electrically insulating substrate  302  makes contact with the partition wall  315  formed on the electrically insulating substrate  301 , and that the data electrodes  307  extend perpendicularly to the scanning and common electrodes  305  and  304 . 
   Then, thermal treatment is applied to the electrically insulating substrates  301  and  302  overlapping each other to thereby adhere them to each other at their ends by virtue of flits. Thus, a space defined by the electrically insulating substrates  301  and  302  and a seal layer (not illustrated) comprised of the flits is hermetically sealed. 
   Thereafter, the space is evacuated of air, and then, filled with discharge gas. 
   Thus, there is completed the plasma display panel  300  illustrated in  FIG. 1 . 
   When the plasma display panel  300  is fabricated as a color plasma display panel, RGB phosphor layers which emit red (R), green (G) and blue (B) lights are arranged in every three display cells. Specifically, a phosphor layer composed of phosphor which emits a red light, a phosphor layer composed of phosphor which emits a green light, and a phosphor layer composed of phosphor which emits a blue light are vertically and horizontally arranged in this order. 
   Among those three phosphor layers, a phosphor layer which emits a blue light is accompanied with a problem that an initial brightness and a lifetime collide with each other, and this problem remains unsolved. Specifically, if a phosphor layer emitting a blue light is composed of a material providing a desired initial brightness, the phosphor layer could not have a sufficient lifetime, and in contrast, if a phosphor layer emitting a blue light is composed of a material providing a desired lifetime, the phosphor layer could not have a sufficient initial brightness. 
   Hence, many attempts have been made to solve the above-mentioned problem in a phosphor layer emitting a blue light. 
   For instance, Japanese Patent Application Publication No. 2002-332481(A) has suggested phosphor composed of silicate and being excited by ultra-violet rays in vacuum condition, as a material of which a phosphor layer emitting a blue light is composed. 
   Specifically, the Publication suggests the phosphor composed of mM 1 O.nM 2 O.2M 3 O 2  containing at least one of Eu and Mn as an additive, wherein M 1  indicates at least one of Ca, Sr and Ba, M 2  indicates at least one of Mg and Zn, M 3  indicates at least one of Si and Ge, m is equal to or greater than 0.5, but equal to or smaller than 3.5 (0.5≦m≦3.5), n is equal to or greater than 0.5, but equal to or smaller than 2.5 (0.5≦m≦2.5), and M 1  indicates at least two of Ca, Sr and Ba, or Sr, or Ba when m and n are equal to one (m=n=1). 
   Japanese Patent Application Publication No. 2001-84911(A) has suggested a plasma display panel including a phosphor layer composed of a mixture of phosphors emitting different-colored lights. 
   Specifically, at least one of a phosphor layer emitting a red light, a phosphor layer emitting a green light, and a phosphor layer emitting a blue light is composed of a mixture of phosphor emitting a red, green or blue light and phosphor emitting a different-colored light. 
   The phosphor suggested in the firstly mentioned Publication has an advantage that variation in a brightness with the lapse of time is small, but is accompanied with a problem that an initial brightness is low. 
   The firstly mentioned Publication has an object of suppressing variation in a brightness wit the lapse of time, caused by thermal treatment or exposure to plasma. The object can be performed by selecting phosphor composed of silicate and excited by ultra-violet rays in vacuum condition, as a material of which a phosphor layer emitting a blue light is composed. However, the suggested phosphor is accompanied with a problem that it can suppress variation in a brightness wit the lapse of time, but it unavoidably sacrifices an initial brightness. 
   The secondly mentioned Publication has an object of controlling a color coordinate (color temperature) when a white light is to be emitted, that is, when red, green and blue lights are emitted. This object can be performed by composing a phosphor layer of a mixture of first phosphor emitting a certain color light, and second phosphor emitting another color light. However, the suggested plasma display panel is accompanied with a problem that a color degree is unavoidably reduced when a blue light is singly emitted. 
   SUMMARY OF THE INVENTION 
   In view of the above-mentioned problems in the conventional plasma display panels, it is an object of the present invention to provide a plasma display panel which is capable of suppressing variation in a brightness with the lapse of time without reduction in an initial brightness. 
   It is also an object of the present invention to provide a plasma display unit including the above-mentioned plasma display panel. 
   Hereinbelow is described a plasma display panel in accordance with the present invention through the use of reference numerals used in later described embodiments. The reference numerals are indicated only for the purpose of clearly showing correspondence between claims and the embodiments. It should be noted that the reference numerals are not allowed to interpret of claims of the present application. 
   In one aspect of the present invention, there is provided a plasma display panel comprising a first substrate ( 351 ) facing a viewer, and a second substrate ( 352 ) arranged in facing relation to the first substrate ( 351 ). The first substrate ( 351 ) is comprised of a transparent substrate ( 322 ), a scanning electrode ( 303 ) and a common electrode ( 304 ) both formed on the transparent substrate ( 302 ) in facing relation to the second substrate ( 352 ), and a dielectric layer ( 312 ) covering the transparent substrate ( 302 ), the scanning electrode ( 303 ) and the common electrode ( 304 ) therewith. The second substrate ( 352 ) is comprised of an electrically insulating substrate ( 301 ), a plurality of data electrodes ( 307 ) formed on the electrically insulating substrate ( 301 ), a plurality of partition walls ( 315 ) formed on the electrically insulating substrate ( 301 ), and a phosphor layer ( 311 ) covering the electrically insulating substrate ( 301 ) and the data electrodes ( 307 ) therewith between adjacent partition walls ( 315 ). The phosphor layer ( 311 ) includes at least a blue-phosphor layer composed of phosphor which emits a blue light. The blue-phosphor layer is composed of a mixture of two or more phosphors each of which emits a blue light and has an initial brightness and variation of brightness with the lapse of time both different from one another. 
   In the plasma display panel in accordance with the present invention, the blue-phosphor layer constituting the phosphor layer is comprised of a mixture of two or more phosphors which emit a blue light and which have initial brightnesses different from one another, and variations of a brightness with the lapse of time, different from one another. 
   Herein, variation of a brightness with the lapse of time is equivalent to a lifetime of a phosphor layer composed of phosphor which emits a blue light. That is, a phosphor layer having greater variation of a brightness with the lapse of time would have a shorter lifetime, and a phosphor layer having smaller variation of a brightness with the lapse of time would have a longer lifetime. 
   In general, an initial brightness collides with variation of a brightness with the lapse of time in a phosphor layer which emits a blue light. That is, a phosphor layer having a higher initial brightness would have greater variation of a brightness with the lapse of time, in other words, would have a shorter lifetime. In contrast, a phosphor layer having a lower initial brightness would have smaller variation of a brightness with the lapse of time, in other words, would have a longer lifetime. Accordingly, it would be possible to control both an initial brightness and variation of a brightness with the lapse of time by mixing at a predetermined ratio first phosphor having a high initial brightness and hence having great variation of a brightness with the lapse of time, and second phosphor having a low initial brightness and hence having small variation of a brightness with the lapse of time. 
   Two phosphors may be mixed with each other, or three or more phosphors may be mixed with one another. 
   For instance, the blue-phosphor layer may be composed of a mixture of first phosphor which emits a blue light and second phosphor which emits a blue light, the second phosphor having an initial brightness smaller than the same of the first phosphor, and smaller variation of brightness with the lapse of time than the same of the first phosphor. 
   By mixing such first and second phosphors with each other, it would be possible to have a phosphor layer emitting a blue light and having a desired initial brightness and desired variation of brightness with the lapse of time. A mixture ratio may be determined in dependence on a use. For instance, if an initial brightness is made much of, the first phosphor is mixed in a higher ratio than a ratio of the second phosphor. In contrast, if variation of brightness with the lapse of time is made much of, the second phosphor is mixed in a higher ratio than a ratio of the first phosphor. 
   It is preferable that the blue-phosphor layer includes the first phosphor in the range of 10 to 90 weight % both inclusive. 
   According to the results of the experiments having been carried out by the inventor, when the first and second phosphors are mixed to each other for composing a phosphor layer emitting a blue light, it is preferable that a mixture ratio of the first phosphor is in the range of 10 to 90 weight % relative to a weight of the phosphor layer (accordingly, a mixture ratio of the second phosphor is in the range of 10 to 90 weight % relative to a weight of the phosphor layer). By setting a mixture ratio of the first phosphor in the range of 10 to 90 weight %, it would be possible to have a desired initial brightness and variation of brightness with the lapse of time. 
   For instance, there may be selected phosphor which emits a blue light and includes aluminate having bivalent europium (Eu 2+ ), as the first phosphor. As phosphor which emits a blue light and includes aluminate having bivalent europium (Eu 2+ ), there may be selected BaMgAl 10 O 17 :Eu 2+ , BaMgAl 12 O 19 :Eu 2+  or BaMgAl 14 O 23 :Eu 2+ , for instance. 
   For instance, there may be selected phosphor which emits a blue light and includes silicate having bivalent europium (Eu 2+ ), as the second phosphor. As phosphor which emits a blue light and includes silicate having bivalent europium (Eu 2+ ), there may be selected CaMgSi 2 O 6 :Eu 2+  or CaSrMgSi 2 O 6 :Eu 2+ , for instance. 
   The first phosphor may be selected one or more among phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ). 
   Similarly, the second phosphor may be selected one or more among phosphors which emit a blue light and include silicate having bivalent europium (EU 2+ ). 
   When the first and second phosphor layers are designed to be comprised of two or more phosphors emitting a blue light, it is preferable that the phosphors are in the range of 10 to 90 weight % both inclusive relative to a weight of the blue-phosphor layer. 
   When the first and second phosphor layers are designed to be comprised of two or more phosphors emitting a blue light, it is preferable that one of the phosphors is BaMgAl 10 O 17 :Eu 2+  or CaMgSi 2 O 6 :Eu 2+ . 
   In another aspect of the present invention, there is provided a plasma display unit including the above-mentioned plasma display panel as a display panel. 
   There is further provided a plasma display unit including an analog interface which converts a received analog image signal into a digital image signal, and outputs the digital image signal, and a plasma display module including the above-mentioned plasma display panel for outputting mages in accordance with the digital image signal received from the analog interface. 
   The advantages obtained by the aforementioned present invention will be described hereinbelow. 
   In general, an initial brightness collides with variation of a brightness with the lapse of time in a phosphor layer which emits a blue light. That is, a phosphor layer having a higher initial brightness would have greater variation of a brightness with the lapse of time, in other words, would have a shorter lifetime. In contrast, a phosphor layer having a lower initial brightness would have smaller variation of a brightness with the lapse of time, in other words, would have a longer lifetime. Accordingly, it would be possible to control both an initial brightness and variation of a brightness with the lapse of time by mixing at a predetermined ratio first phosphor having a high initial brightness and hence having great variation of a brightness with the lapse of time, and second phosphor having a low initial brightness and hence having small variation of a brightness with the lapse of time. 
   Thus, in a plasma display panel or a plasma display unit, if an initial brightness is made much of, the first phosphor is mixed in a higher ratio than a ratio of the second phosphor. In contrast, if variation of brightness with the lapse of time is made much of, the second phosphor is mixed in a higher ratio than a ratio of the first phosphor. By controlling a mixture ratio of the first and second phosphors, it would be possible to have a blue-light emitting phosphor layer having a desired initial brightness and desired variation of a brightness with the lapse of time. 
   The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective exploded view of a display cell in a conventional plasma display panel. 
       FIG. 2A  is a cross-sectional view of a rear substrate, showing a step to be carried out in a method of fabricating the plasma display panel illustrated in  FIG. 1 . 
       FIG. 2B  is a cross-sectional view taken along the line  2 B— 2 B in  FIG. 2A . 
       FIG. 2C  is a cross-sectional view of a rear substrate, showing a step to be carried out in a method of fabricating the plasma display panel illustrated in  FIG. 1 . 
       FIG. 2D  is a cross-sectional view taken along the line  2 B— 2 B in  FIG. 2C . 
       FIG. 2E  is a cross-sectional view of a rear substrate, showing a step to be carried out in a method of fabricating the plasma display panel illustrated in  FIG. 1 . 
       FIG. 2F  is a cross-sectional view taken along the line  2 B— 2 B in  FIG. 2E . 
       FIG. 3  is a graph showing brightness characteristics of the first to fifth embodiments. 
       FIG. 4  is a graph showing brightness characteristics of the sixth embodiment. 
       FIG. 5  is a block diagram of a plasma display unit in accordance with the seventh embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings. 
   [First Embodiment] 
   A phosphor layer emitting a blue light in accordance with the first embodiment of the present invention was comprised of a mixture of first phosphor emitting a blue light, and having a relatively high initial brightness and relatively great variation of a brightness with the lapse of time, and second phosphor emitting a blue light, and having a lower initial brightness than the same of the first phosphor and smaller variation of a brightness with the lapse of time than the same of the first phosphor. 
   As the first phosphor, there was selected BaMgAl 10 O 17 :Eu 2+  (hereinafter, referred to as “BAM”) among phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ), and as the second phosphor, there was selected CaMgSi 2 O 6 :Eu 2+  (hereinafter, referred to as “CMS”) among phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). 
   There was made a mixture of BAM at 90 weight % and CMS at 10 weight %. The blue-light emitting phosphor layer in accordance with the first embodiment was composed of the mixture. 
   Hereinbelow is explained a method of fabricating the blue-light emitting phosphor layer. 
   First, BAM, CMS and a solution was mixed with one another to thereby make phosphor paste. The solution mainly contains BCA (Butyl Carbitol Acetate) including resin such as ethyl cellulose. 
   Then, the phosphor paste is coated by screen-printing or through the use of a dispenser onto an exposed surface of the dielectric layer  314  and sidewalls of the partition wall  315  both formed on the electrically insulating substrate  301  of the rear substrate  352 . 
   Then, the phosphor paste is dried and baked. As a result, there is formed the blue-light emitting phosphor layer  311  having a thickness of about 10 micrometers. 
   Hereinafter, there is fabricated a plasma display panel including the blue-light emitting phosphor layer in accordance with the first embodiment, in accordance with the method of fabricating the plasma display panel illustrated in  FIG. 1 . 
   A predetermined drive voltage was applied to the plasma display panel including the thus fabricated blue-light emitting phosphor layer in accordance with the first embodiment, to cause the blue-light emitting phosphor layer to emit a blue light. After the plasma display panel has been driven continuously for 1000 hours, a brightness of a blue light emitted from the blue-light emitting phosphor layer was measured. 
   Table 1 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               BAM 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
               First 
                90 wt % 
                10 wt % 
               124 
               ◯ 
               82 
               ◯ 
             
             
               Embodiment 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
   A threshold for an initial brightness was set equal to 100 cd/m 2 , and a threshold for a brightness after continuous 1000-hour operation was set equal to 80 cd/m 2 . Specifically, if a brightness measured after the plasma display panel has been driven continuously for 1000 hours was reduced by more than 20%, the estimate was “No good” (X), and if a brightness measured after the plasma display panel has been driven continuously for 1000 hours was reduced by 20% or smaller, the estimate was “Good” (◯). 
   As shown in Table 1, the blue-light emitting phosphor layer composed singly of BAM had an initial brightness of 125, and a brightness of 78 measured after the plasma display panel has been driven continuously for 1000 hours. Thus, the blue-light emitting phosphor layer composed singly of BAM could have an initial brightness higher than a target initial brightness of 100 cd/m 2 , but could not have a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   As shown in Table 1, the blue-light emitting phosphor layer composed singly of CMS had an initial brightness of 98, and a brightness of 96 measured after the plasma display panel has been driven continuously for 1000 hours. Thus, the blue-light emitting phosphor layer composed singly of CMS could have brightness higher than a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours, but could not have a target initial brightness of 100 cd/m 2 . 
   In contrast, the blue-light emitting phosphor layer comprised of a mixture of BAM at 90 weight % and CMS at 10 weight %, in accordance with the first embodiment, had an initial brightness of 124 cd/m 2  and a brightness of 82 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layer in accordance with the first embodiment could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layer in accordance with the first embodiment has desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time. 
   [Second Embodiment] 
   A blue-light emitting phosphor layer in accordance with the second embodiment was comprised of a mixture of BAM at 80 weight % and CMS at 20 weight %. The measurement was made in the same way as the first embodiment. 
   Table 2 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               BAM 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
               Second 
                80 wt % 
                20 wt % 
               122 
               ◯ 
               83 
               ◯ 
             
             
               Embodiment 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
   As shown in Table 2, the blue-light emitting phosphor layer comprised of a mixture of BAM at 80 weight % and CMS at 20 weight %, in accordance with the second embodiment, had an initial brightness of 122 cd/m 2  and a brightness of 83 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layer in accordance with the second embodiment could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layer in accordance with the second embodiment has desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time. 
   [Third Embodiment] 
   A blue-light emitting phosphor layer in accordance with the third embodiment was comprised of a mixture of BAM at 50 weight % and CMS at 50 weight %. The measurement was made in the same way as the first embodiment. 
   Table 3 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 3 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               BAM 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
               Third 
                50 wt % 
                50 wt % 
               115 
               ◯ 
               88 
               ◯ 
             
             
               Embodiment 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
   As shown in Table 3, the blue-light emitting phosphor layer comprised of a mixture of BAM at 50 weight % and CMS at 50 weight %, in accordance with the third embodiment, had an initial brightness of 115 cd/m 2  and a brightness of 88 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layer in accordance with the third embodiment could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layer in accordance with the third embodiment has desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time. 
   [Fourth Embodiment] 
   A blue-light emitting phosphor layer in accordance with the fourth embodiment was comprised of a mixture of BAM at 30 weight % and CMS at 70 weight %. The measurement was made in the same way as the first embodiment. 
   Table 4 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 4 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               BAM 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
               Fourth 
                30 wt % 
                70 wt % 
               110 
               ◯ 
               90 
               ◯ 
             
             
               Embodiment 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
   As shown in Table 4, the blue-light emitting phosphor layer comprised of a mixture of BAM at 30 weight % and CMS at 70 weight %, in accordance with the fourth embodiment, had an initial brightness of 110 cd/m 2  and a brightness of 90 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layer in accordance with the fourth embodiment could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layer in accordance with the fourth embodiment has desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time. 
   [Fifth Embodiment] 
   A blue-light emitting phosphor layer in accordance with the fifth embodiment was comprised of a mixture of BAM at 10 weight % and CMS at 90 weight %. The measurement was made in the same way as the first embodiment. 
   Table 5 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 5 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               BAM 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
               Fifth 
                10 wt % 
                90 wt % 
               102 
               ◯ 
               92 
               ◯ 
             
             
               Embodiment 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
   As shown in Table 5, the blue-light emitting phosphor layer comprised of a mixture of BAM at 10 weight % and CMS at 90 weight %, in accordance with the fifth embodiment, had an initial brightness of 110 cd/m 2  and a brightness of 90 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layer in accordance with the fifth embodiment could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layer in accordance with the fifth embodiment has desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time. 
     FIG. 3  is a graph showing an initial brightness (◯) and a brightness (●) observed after the plasma display panel has been driven continuously for 1000 hours in each of the blue-light emitting phosphor layer composed only of BAM, the blue-light emitting phosphor layer composed only of CMS, and the blue-light emitting phosphor layers in accordance with the first and fifth embodiments. 
   As is obvious in view of  FIG. 3 , if a weight ratio of BAM relative to a total weight of a blue-light emitting phosphor layer is in the range of 10 to 90 weight % (that is, a weight ratio of CMS is in the range of 90 to 10 weight %), it is possible to have a blue-light emitting phosphor layer having desired characteristics with respect to an initial brightness and variation of a brightness with the lapse of time. 
   [Sixth Embodiment] 
   In the above-mentioned first to fifth embodiments, as the first phosphor, there was selected BaMgAl 10 O 17 :Eu 2+  (BAM) among phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ), and as the second phosphor, there was selected CaMgSi 2 O 6 :Eu 2+  (CMS) among phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). Namely, each of the first and second phosphors is composed of single blue-light emitting phosphor. 
   However, phosphor to be selected as the first or second phosphor is not to be limited to single phosphor. Two or more phosphor may be selected as the first or second phosphor. 
   An example of the first phosphor comprised a plurality of phosphors is explained hereinbelow as the sixth embodiment. 
   In the sixth embodiment, as the first phosphor, there were selected two phosphors, namely, BaMgAl 10 O 17 :Eu 2+  and BaMgAl 14 O 23 :Eu 2+  among phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ), and as the second phosphor, there was selected CaMgSi 2 O 6 :Eu 2+  (CMS) among phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). 
   Similarly to the first to fifth embodiments, there were made six examples 1 to 6 each including BaMgAl 10 O 17 :Eu 2+  and BaMgAl 14 O 23 :Eu 2+  at 100, 90, 80, 50, 30 and 10 weight %, respectively, relative to a total weight of a blue-light emitting phosphor layer. Then, the measurement was made for each the examples 1 to 6 in the same way as the first embodiment. 
   Each of the examples 1 to 6 contains BaMgAl 10 O 17 :Eu 2+  at 60 weight %, and BaMgAl 14 O 23 :Eu 2+  at 40 weight %. 
   Table 6 shows the results of the measurement. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 6 
             
           
           
             
                 
                 
             
             
                 
                 
                 
               Before 1000-hour 
                 
             
             
                 
                 
                 
               Operation 
               After 1000-hour 
             
           
        
         
             
                 
               BAM 
               CMS 
               Initial 
                 
               Operation 
             
           
        
         
             
                 
               mixture 
               mixture 
               Brightness 
                 
               Brightness 
                 
             
             
                 
               ratio 
               ratio 
               100 or 
                 
               80 or 
                 
             
           
        
         
             
               required characteristic 
               higher 
               Estimate 
               higher 
               Estimate 
             
             
                 
             
           
        
         
             
               Example 1 
               100 wt % 
                0 wt % 
               125 
               ◯ 
               78 
               X 
             
             
               (BAM) 
             
             
               Example 2 
                90 wt % 
                10 wt % 
               122 
               ◯ 
               81 
               ◯ 
             
             
               Example 3 
                80 wt % 
                20 wt % 
               120 
               ◯ 
               82 
               ◯ 
             
             
               Example 4 
                50 wt % 
                50 wt % 
               113 
               ◯ 
               85 
               ◯ 
             
             
               Example 5 
                30 wt % 
                70 wt % 
               107 
               ◯ 
               90 
               ◯ 
             
             
               Example 6 
                10 wt % 
                90 wt % 
               102 
               ◯ 
               93 
               ◯ 
             
             
               CMS 
                0 wt % 
               100 wt % 
               98 
               X 
               96 
               ◯ 
             
             
                 
             
             
               (Brightnesses are expressed in a unit of [cd/m 2 ]. 
             
           
        
       
     
   
     FIG. 4  is a graph showing the results of Table 6. That is,  FIG. 4  shows an initial brightness (◯) and a brightness (●) observed after the plasma display panel has been driven continuously for 1000 hours in each of the examples 1 to 6 and the blue-light emitting phosphor composed only of CMS. 
   As shown in Table 6, the blue-light emitting phosphor layer (example 1) comprised of a mixture of BaMgAl 10 O 17 :Eu 2+  and BaMgAl 14 O 23 :Eu 2+  had an initial brightness of 125 cd/m 2  and a brightness of 78 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   That is, the blue-light emitting phosphor layer in accordance with the example 1 could have a target initial brightness of 100 cd/m 2 , but could not have a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   In contrast, the blue-light emitting phosphor layers (examples 2 to 6) comprised of a mixture of BaMgAl 10 O 17 :Eu 2+  and BaMgAl 14 O 23 :Eu 2+  at 10 to 90 weight % and CMS at 90 to 10 weight % had an initial brightness of 122, 120, 113, 107 and 102 cd/m 2 , respectively, and a brightness of 81, 82, 85, 90 and 93 cd/m 2 , respectively, to be measured after the plasma display panel has been driven continuously for 1000 hours. That is, the blue-light emitting phosphor layers in accordance with the examples 2 to 6 could have both a target initial brightness of 100 cd/m 2  and a target brightness of 80 cd/m 2  to be measured after the plasma display panel has been driven continuously for 1000 hours. 
   Thus, it was found out that the blue-light emitting phosphor layers in accordance with the examples 2 to 6 have desired characteristics with respect to both an initial brightness and variation of a brightness with the lapse of time, similarly to the above-mentioned first to fifth embodiments. 
   As is obvious in light of the sixth embodiment, the first phosphor may be comprised of two or more blue-light emitting phosphors selected among phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ). Similarly, the second phosphor may be comprised of two or more blue-light emitting phosphors selected among phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). 
   When a blue-light emitting phosphor layer is comprised of two or more blue-light emitting phosphors, a total weight of the blue-light emitting phosphors is set in the range of 10 to 90 weight % both inclusive relative to a weight of a blue-light emitting phosphor layer. 
   In the above-mentioned first to sixth embodiments, BaMgAl 10 O 17 :Eu 2+  and BaMgAl 14 O 23 :Eu 2+  were used as blue-light emitting phosphor selected from phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ). However, it should be noted that another phosphor may be selected from phosphors which emit a blue light and include aluminate having bivalent europium (Eu 2+ ). For instance, as blue-light emitting phosphor, there may be selected BaMgAl 20  μg:Eu 2+ . 
   In the above-mentioned first to sixth embodiments, CaMgSi 2 O 6 :Eu 2+  (CMS) was used as blue-light emitting phosphor selected from phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). However, it should be noted that another phosphor may be selected from phosphors which emit a blue light and include silicate having bivalent europium (Eu 2+ ). For instance, there may be selected CaSrMgSi 2 O 6 :Eu 2+ . 
   The blue-light emitting phosphor layer in accordance with the first to sixth embodiments is used as a part of a plasma display panel or a part of a plasma display unit. 
   [Seventh Embodiment] 
   In the seventh embodiment, there is explained an example of a plasma display unit including a plasma display panel including a blue-light emitting phosphor layer in accordance with one of the first to fifth embodiments and the examples 2 to 6 in the sixth embodiments. 
     FIG. 5  is a block diagram of a plasma display unit  60  including a plasma display panel including the blue-light emitting phosphor layer. 
   As illustrated in  FIG. 5 , the plasma display unit  60  has a modularized structure. Specifically, the plasma display unit  60  is comprised of an analog interface  20  and a plasma display module  30 . 
   The analog interface  20  is comprised of a Y/C separating circuit  21  including a chroma-decoder, an analog-digital (A/D) converting circuit  22 , a circuit  23  for controlling a synchronization signal, including a phase-lock loop (PLL) circuit, a circuit  24  for converting an image format, an reverse-gamma converting circuit  25 , a system control circuit  26 , and a PLE control circuit  27 . 
   In brief, the analog interface  20  coverts a received analog image signal into a digital image signal, and then, outputs the digital image signal to the plasma display module  30 . 
   For instance, an analog image signal transmitted from a television tuner (not illustrated) is separated into luminance signals for RGB colors in the Y/C separating circuit  21 , and then, converted into a digital signal in the A/D converting circuit  22 . 
   Then, if a pixel configuration in the plasma display module  30  is different from a pixel configuration of the image signal, necessary conversion is carried out in the image-format converting circuit  24 . 
   A characteristic of a luminance to a signal input to a plasma display panel is linear. Image signals are usually compensated for, specifically, gamma-converted in advance in accordance with characteristics of a cathode ray tube (CRT). Hence, after the image signals are A/D-converted in the A/D converting circuit  22 , reverse-gamma conversion is applied to the image signals are in the reverse-gamma converting circuit  25  for producing digital image signals having linear characteristics. The digital image signals are output to the plasma display module  30  as RGB image signals. 
   Since an analog image signal does not include a sampling clock signal and a data clock signal used for A/D conversion, the PLL circuit included in the control circuit  23  produces a sampling clock signal and a data clock signal, based on a horizontal synchronization signal provided together with the analog image signal, and outputs the clock signals to the plasma display module  30 . 
   The PLE control circuit  27  carries out luminance control. Specifically, if an average picture level is equal to or smaller than a threshold level, a luminance for displayed images is raised, and if an average picture level is greater than a threshold level, a luminance is reduced. 
   The system control circuit  26  outputs various control signals to the plasma display module  30 . 
   The plasma display module  30  is comprised of a digital signal processing and controlling circuit  31 , a panel section  32 , and a power source circuit  33  including a DC/DC converter. 
   The digital signal processing and controlling circuit  31  is comprised of an interface signal processing circuit  34 , a frame memory  35 , a memory control circuit  36 , and a driver control circuit  37 . 
   The interface signal processing circuit  34  receives various control signals transmitted from the system control circuit  26 , an RGB image signal transmitted from the reverse-gamma converting circuit  25 , a synchronization signal transmitted from the control circuit  23 , and a data clock signal transmitted from the PLL circuit. 
   For instance, an average picture level (APL) of an image signal input into the interface signal processing circuit  34  is calculated in an APL calculating circuit (not illustrated) included in the interface signal processing circuit  34 , and output as 5-bit data, for instance. The PLE control circuit  27  arranges PLE control data in accordance with the calculated average picture level, and outputs the PLE control data to a picture level control circuit (not illustrated) included in the interface signal processing circuit  34 . 
   The digital signal processing and controlling circuit  31  processes those signals in the interface signal processing circuit  34 , and then, transmits a control signal to the panel section  32 . The memory control circuit  36  transmits a memory control signal to the panel section  32 , and the driver control circuit  37  transmits a driver control signal to the panel section  32 . 
   The panel section  32  is comprised of a 50-size plasma display panel  50 , a scanning driver  38  for driving a scanning electrode, data drivers  39  for driving data electrodes, pulse-generating circuits  40  for applying a pulse voltage to the plasma display panel  50  and the scanning driver  38 , and a circuit  41  for collecting excess power from the pulse-generating circuits  40 . 
   The plasma display panel  50  is designed to have 1365×768 pixels. In the plasma display panel  50 , the scanning driver  38  controls a scanning electrode, and the data drivers  39  control data electrodes, thereby a light is emitted from selected display cells for displaying images. 
   A first power source supplies power to the digital signal processing and controlling circuit  31  and the panel section  32 . A power source circuit  33  receives DC power from a second power source, converts a DC voltage into a desired voltage, and supplies the desired voltage to the panel section  32 . 
   Hereinbelow is explained a method of fabricating the plasma display unit  60 . 
   First, the plasma display panel  50 , the scanning driver  38 , the data drivers  39 , the pulse-generating circuits  40 , and the power-collecting circuit  41  are arranged on a substrate to thereby fabricate the panel section  32 . 
   Apart from the panel section  32 , there is fabricated the digital signal processing and controlling circuit  31 . 
   The panel section  32 , the digital signal processing and controlling circuit  31  and the power source circuit  33  are assembled as a module. Thus, the plasma display module  30  is completed. 
   Apart from the plasma display module  30 , there is fabricated the analog interface  20 . 
   After the plasma display module  30  and the analog interface  20  have been fabricated separately from each other, they are electrically connected to each other. Thus, there is completed the plasma display unit  60  illustrated in  FIG. 5 . 
   By modularizing the plasma display unit  60 , the plasma display panel  50  can be fabricated independently of other parts constituting the plasma display unit  60 . For instance, if the plasma display panel  50  went wrong in the plasma display unit  60 , the plasma display module  30  including the plasma display panel  50  having gone wrong can be exchanged into new one, ensuring simplification in repair and reduction in time for repair. 
   Since the plasma display unit  60  includes the plasma display panel  50  having a blue-light emitting phosphor layer in accordance with one of the first to fifth embodiments and the examples 2 to 6 in the sixth embodiments, the plasma display unit  60  can suppress variation of a brightness with the lapse of time without reduction in an initial brightness with respect to emission of a blue light. 
   While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 
   The entire disclosure of Japanese Patent Application No. 2003-349749 filed on Oct. 8, 2003 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.