Patent Publication Number: US-7906909-B2

Title: Plasma display panel

Description:
This application claims the benefit of Korean Patent Application No. 10-2007-0066540 filed on Jul. 3, 2007, which is hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     An exemplary embodiment of the invention relates to a plasma display panel. 
     2. Description of the Related Art 
     A plasma display panel includes a phosphor layer inside discharge cells partitioned by barrier ribs and a plurality of electrodes. 
     A driving signal is supplied to the electrodes, thereby generating a discharge inside the discharge cells. When the driving signal generates a discharge inside the discharge cells, a discharge gas filled inside the discharge cells generates vacuum ultraviolet rays, which thereby cause phosphors formed inside the discharge cells to emit light, thus displaying an image on the screen of the plasma display panel. 
     SUMMARY 
     An exemplary embodiment of the invention provides a plasma display panel capable of improving a contrast characteristic by reducing the reflection of light caused by a phosphor layer. 
     In one aspect, a plasma display panel comprises a front substrate, a rear substrate facing the front substrate, a barrier rib that is positioned between the front substrate and the rear substrate and partitions a discharge cell, and a phosphor layer formed inside the discharge cell, the phosphor layer including a first phosphor layer emitting first color light, a second phosphor layer emitting second color light, and a third phosphor layer emitting third color light, wherein the first phosphor layer includes a first pigment, and the second phosphor layer includes a second pigment, an average particle size of the second phosphor layer is larger than an average particle size of the first phosphor layer, and a content of the second pigment is more than a content of the first pigment. 
     In another aspect, a plasma display panel comprises a front substrate, a rear substrate facing the front substrate, a barrier rib that is positioned between the front substrate and the rear substrate and partitions a discharge cell, and a phosphor layer formed inside the discharge cell, the phosphor layer including a first phosphor layer emitting first color light, wherein the first phosphor layer includes a first pigment, wherein a content of the first pigment satisfies the following equation: 0.005≦C 1 /L 1 ≦6, where C 1  is a content of the first pigment in units of parts by weight, and L 1  is an average particle size of the first phosphor material in units of micrometer. 
     In still another aspect, a plasma display panel comprises a front substrate, a rear substrate facing the front substrate, a barrier rib that is positioned between the front substrate and the rear substrate and partitions a discharge cell, and a phosphor layer formed inside the discharge cell, the phosphor layer including a second phosphor layer emitting second color light, wherein the second phosphor layer includes a second pigment, wherein a content of the second pigment satisfies the following equation: 0.01≦C 2 /L 2 ≦8, where C 2  is a content of the second pigment in units of parts by weight, and L 1  is an average particle size of the second phosphor material in units of micrometer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompany drawings, which are included to provide a further understanding of the invention and are incorporated on and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIGS. 1A to 1D  show a structure of a plasma display panel according to an exemplary embodiment of the invention; 
         FIG. 2  illustrates an operation of the plasma display panel according to the exemplary embodiment; 
         FIG. 3  is a table showing a composition of a phosphor layer; 
         FIGS. 4A and 4B  are graphs showing reflectances depending on compositions of first and second phosphor layers, respectively; 
         FIGS. 5A and 5B  are diagrams for explaining a distribution characteristic of pigment particles depending on the size of phosphor particles; 
         FIG. 6  is a diagram for explaining a relationship between the size of a first phosphor particle and a content of a red pigment; 
         FIG. 7  is a diagram for explaining a relationship between the size of a second phosphor particle and a content of a blue pigment; 
         FIG. 8  is a diagram for explaining a difference between a content of a red pigment and a content of a blue pigment; 
         FIG. 9  shows a color coordinate characteristic of the plasma display panel; 
         FIGS. 10A and 10B  are graphs showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a red pigment, respectively; 
         FIGS. 11A and 11B  are graphs showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a blue pigment, respectively; 
         FIG. 12  is a diagram for explaining a difference between a particle size of a red pigment and a particle size of a blue pigment; 
         FIGS. 13A and 13B  illustrate another example of a composition of a phosphor layer; 
         FIGS. 14A and 14B  are a table and a graph showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a green pigment, respectively; and 
         FIGS. 15A to 15C  show another structure of a plasma display panel according to the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail embodiments of the invention examples of which are illustrated in the accompanying drawings. 
       FIGS. 1A to 1D  show a structure of a plasma display panel according to an exemplary embodiment of the invention. 
     As shown in  FIG. 1A , a plasma display panel  100  according to an exemplary embodiment of the invention includes a front substrate  101  and a rear substrate  111  which coalesce with each other using a seal layer (not shown) to face each other. On the front substrate  101 , a scan electrode  102  and a sustain electrode  103  are formed parallel to each other. On the rear substrate  111 , an address electrode  113  is positioned to intersect the scan electrode  102  and the sustain electrode  103 . 
     An upper dielectric layer  104  covering the scan electrode  102  and the sustain electrode  103  is positioned on the front substrate  101  on which the scan electrode  102  and the sustain electrode  103  are positioned. 
     The upper dielectric layer  104  limits discharge currents of the scan electrode  102  and the sustain electrode  103 , and provides electrical insulation between the scan electrode  102  and the sustain electrode  103 . 
     A protective layer  105  is positioned on the upper dielectric layer  104  to facilitate discharge conditions. The protective layer  105  may include a material having a high secondary electron emission coefficient, for example, magnesium oxide (MgO). 
     A lower dielectric layer  115  covering the address electrode  113  is positioned on the rear substrate  111  on which the address electrode  113  is positioned. The lower dielectric layer  115  provides electrical insulation of the address electrodes  113 . 
     Barrier ribs  112  of a stripe type, a well type, a delta type, a honeycomb type, and the like, are positioned on the lower dielectric layer  115  to partition discharge spaces (i.e., discharge cells). A red discharge cell, a green discharge cell, and a blue discharge cell may be positioned between the front substrate  101  and the rear substrate  111 . In addition to the red, green, and blue discharge cells, a white or yellow discharge cell may be formed. 
     Each discharge cell partitioned by the barrier ribs  112  is filled with a discharge gas including xenon (Xe), neon (Ne), and so forth. 
     A phosphor layer  114  is positioned inside the discharge cells to emit visible light for an image display during the generation of an address discharge. For instance, first, second and third phosphor layers respectively emitting red, blue, and green light may be positioned inside the discharge cells. In addition to the red, green, and blue light, a phosphor layer emitting white or yellow light may be positioned in the discharge cell. 
     A thickness of the phosphor layer  114  inside at least one of the red, green, and blue discharge cells may be different from thicknesses of the phosphor layers  114  inside the other discharge cells. For example, a thickness of the third phosphor layer  114  inside the green discharge cell and a thickness of the second phosphor layer  114  inside the blue discharge cell may be larger than a thickness of the first phosphor layer  114  inside the red discharge cell. The thickness of the third phosphor layer  114  inside the green discharge cell my be equal to or different from the thickness of the second phosphor layer  114  inside the blue discharge cell. 
     In the plasma display panel  100  according to the exemplary embodiment of the invention, widths of the red, green, and blue discharge cells may be substantially equal to one another. However, a width of at least one of the red, green, and blue discharge cells may be different from the widths of the other discharge cells. For example, a width of the red discharge cell is the smallest, and widths of the green and blue discharge cells are larger than the width of the red discharge cell. The width of the green discharge cell may be equal to or different from the width of the blue discharge cell. 
     Widths of the phosphor layers  114  inside the discharge cells change depending on the widths of the discharge cells. For example, a width of the second phosphor layer  114  inside the blue discharge cell and a width of the third phosphor layer  114  inside the green discharge cell are larger than a width of the first phosphor layer  114  inside the red discharge cell. Hence, a color temperature characteristic of an image can be improved. 
     The plasma display panel  100  according the exemplary embodiment may have various forms of barrier rib structures as well as a structure of the barrier rib  112  shown in  FIG. 1A . For instance, the barrier rib  112  includes a first barrier rib  112   b  and a second barrier rib  112   a.  The barrier rib  112  may have a differential type barrier rib structure in which heights of the first and second barrier ribs  112   b  and  112   a  are different from each other. 
     In the differential type barrier rib structure, a height of the first barrier rib  112   b  may be smaller than a height of the second barrier rib  112   a.    
     While  FIG. 1A  has been shown and described the case where the red, green, and blue discharge cells are arranged on the same line, the red, green, and blue discharge cells may be arranged in a different pattern. For instance, a delta type arrangement in which the red, green, and blue discharge cells are arranged in a triangle shape may be applicable. Further, the discharge cells may have a variety of polygonal shapes such as pentagonal and hexagonal shapes as well as a rectangular shape. 
     While  FIG. 1A  has shown and described the case where the barrier rib  112  is formed on the rear substrate  111 , the barrier rib  112  may be formed on at least one of the front substrate  101  or the rear substrate  111 . 
     It should be noted that only one example of the plasma display panel according to the exemplary embodiment has been shown and described above, and the exemplary embodiment is not limited to the plasma display panel with the above-described structure. For instance, while the above description illustrates a case where the upper dielectric layer  104  and the lower dielectric layer  115  each have a single-layered structure, at least one of the upper dielectric layer  104  or the lower dielectric layer  115  may have a multi-layered structure. 
     While the address electrode  113  positioned on the rear substrate  111  may have a substantially constant width or thickness, a width or thickness of the address electrode  113  inside the discharge cell may be different from a width or thickness of the address electrode  113  outside the discharge cell. For instance, a width or thickness of the address electrode  113  inside the discharge cell may be larger than a width or thickness of the address electrode  113  outside the discharge cell. 
       FIG. 1B  shows another structure of the scan electrode  102  and the sustain electrode  103 . 
     The scan electrode  102  and the sustain electrode  103  may have a multi-layered structure, respectively. For instance, the scan electrode  102  and the sustain electrode  103  each include transparent electrodes  102   a  and  103   a  and bus electrodes  102   b  and  103   b.    
     The bus electrodes  102   b  and  103   b  may include a substantially opaque material, for instance, at least one of silver (Ag), gold (Au), or aluminum (Al). The transparent electrodes  102   a  and  103   a  may include a substantially transparent material, for instance, indium-tin-oxide (ITO). 
     Black layers  120  and  130  are formed between the transparent electrodes  102   a  and  103   a  and the bus electrodes  102   b  and  103   b  to prevent the reflection of external light caused by the bus electrodes  102   b  and  103   b.    
     The transparent electrodes  102   a  and  103   a  may be omitted from the scan electrode  102  and the sustain electrode  103 . In other words, the scan electrode  102  and the sustain electrode  103  may be called an ITO-less electrode in which the transparent electrodes  102   a  and  103   a  are omitted. 
     As shown in  FIG. 1C , the plasma display panel  100  may be divided into a first area  140  and a second area  150 . 
     In the first area  140 , a plurality of first address electrodes Xa 1 , Xa 1 , . . . , Xam are positioned parallel to one another. In the second area  150 , a plurality of second address electrodes Xb 1 , Xb 1 , . . . , Xbm are positioned parallel to one another to be opposite to the plurality of first address electrodes Xa 1 , Xa 1 , . . . , Xam. 
     For example, in case the first address electrodes Xa 1 , Xa 1 , . . . , Xam are positioned parallel to one another in turn in the first area  140 , the second address electrodes Xb 1 , Xb 1 , . . . , Xbm respectively corresponding to the first address electrodes Xa 1 , Xa 1 , . . . , Xam are positioned parallel to one another in the second area  150 . In other words, the first address electrode Xa 1  is opposite to the second address electrode Xb 1 , and the first address electrode Xam is opposite to the second address electrode Xbm. 
       FIG. 1D  shows in detail an area A where the first address electrodes and the second address electrodes are opposite to each other. 
     As shown in  FIG. 1D , the first address electrodes Xa(m-2), Xa(m-1) and Xam are opposite to the second address electrodes Xb(m-2), Xb(m-1) and Xbm with a distance d therebetween, respectively. 
     When the distance d between the first address electrode and the second address electrode is excessively short, it is likely that a current flows due to a coupling effect between the first address electrode and the second address electrode. On the other hand, when the distance d is excessively long, a viewer may watch a striped noise on an image displayed on the plasma display panel. 
     Considering this, the distance d may lie in a range between about 50 μm and 300 μm. Further, the distance d may lie in a range between about 70 μm and 220 μm. 
       FIG. 2  illustrates an operation of the plasma display panel according to the exemplary embodiment. The exemplary embodiment is not limited to the operation shown in  FIG. 2 , and a method for operating the plasma display panel may be variously changed. 
     As shown in  FIG. 2 , during a reset period for initialization of wall charges, a reset signal is supplied to the scan electrode. The reset signal includes a rising signal and a falling signal. The reset period is further divided into a setup period and a set-down period. 
     During the setup period, the rising signal is supplied to the scan electrode. The rising signal sharply rises from a first voltage V 1  to a second voltage V 2 , and then gradually rises from the second voltage V 2  to a third voltage V 3 . The first voltage V 1  may be a ground level voltage GND. 
     The rising signal generates a weak dark discharge (i.e., a setup discharge) inside the discharge cell during the setup period, thereby accumulating a proper amount of wall charges inside the discharge cell. 
     During the set-down period, a falling signal of a polarity opposite a polarity of the rising signal is supplied to the scan electrode. The falling signal gradually falls from a fourth voltage V 4  lower than a peak voltage (i.e., the third voltage V 3 ) of the rising signal to a fifth voltage V 5 . 
     The falling signal generates a weak erase discharge (i.e., a set-down discharge) inside the discharge cell. Furthermore, the remaining wall charges are uniform inside the discharge cells to the extent that an address discharge can be stably performed. 
     During an address period following the reset period, a scan bias signal, which is maintained at a sixth voltage V 6  higher than a lowest voltage (i.e., the fifth voltage V 5 ) of the falling signal, is supplied to the scan electrode. A scan signal, which falls from the scan bias signal to a scan voltage −Vy, is supplied to the scan electrode. 
     A width of a scan signal supplied during an address period of at least one subfield may be different from a width of a scan signal supplied during address periods of the other subfields. For instance, a width of a scan signal in a subfield may be larger than a width of a scan signal in the next subfield in time order. Further, a width of the scan signal may be gradually reduced in the order of 2.6 μs, 2.3 μs, 2.1 μs, 1.9 s, etc., or in the order of 2.6 μs, 2.3 μs, 2.3 μs, 2.1 μs, . . . , 1.9 μs, 1.9 μs, etc. 
     As above, when the scan signal is supplied to the scan electrode, a data signal corresponding to the scan signal is supplied to the address electrode. The data signal rises from a ground level voltage GND by a data voltage magnitude ΔVd. 
     As the voltage difference between the scan signal and the data signal is added to the wall voltage generated during the reset period, the address discharge occurs within the discharge cell to which the data signal is supplied. 
     A sustain bias signal is supplied to the sustain electrode during the address period to prevent the address discharge from unstably occurring by interference of the sustain electrode Z. 
     The sustain bias signal is substantially maintained at a sustain bias voltage Vz. The sustain bias voltage Vz is lower than a voltage Vs of a sustain signal and is higher than the ground level voltage GND. 
     During a sustain period following the address period, a sustain signal is alternately supplied to the scan electrode and the sustain electrode. The sustain signal has a voltage magnitude corresponding to the sustain voltage Vs. 
     As the wall voltage within the discharge cell selected by performing the address discharge is added to the sustain voltage Vs of the sustain signal, every time the sustain signal is supplied, the sustain discharge, i.e., a display discharge occurs between the scan electrode and the sustain electrode. 
     A plurality of sustain signals are supplied during a sustain period of at least one subfield, and a width of at least one of the plurality of sustain signals may be different from widths of the other sustain signals. For instance, a width of a first supplied sustain signal among the plurality of sustain signals may be larger than widths of the other sustain signals. Hence, a sustain discharge can be more stable. 
       FIG. 3  is a table showing a composition of a phosphor layer. 
     As shown in  FIG. 3 , a first phosphor layer emitting red light includes a first phosphor material having a white-based color and a red pigment. 
     The first phosphor material is not particularly limited except the red light emission. The first phosphor material may be (Y, Gd)BO:Eu in consideration of an emitting efficiency of red light. 
     The red pigment has a red-based color. The first phosphor layer may have a red-based color by mixing the red pigment with the first phosphor material. The red pigment is not particularly limited except the red-based color. The red pigment may include an iron (Fe)-based material in consideration of facility of powder manufacture, color, and manufacturing cost. 
     The Fe-based material may exist in a state of iron oxide in the first phosphor layer. For instance, the Fe-based material may exist in a state of αFe 2 O 3  in the first phosphor layer. 
     A second phosphor layer emitting blue light includes a second phosphor material having a white-based color and a blue pigment. 
     The second phosphor material is not particularly limited except the blue light emission. The second phosphor material may be (Ba, Sr, Eu)MgAl 10 O 17  in consideration of an emitting efficiency of blue light. 
     The blue pigment has a blue-based color. The second phosphor layer may have a blue-based color by mixing the blue pigment with the second phosphor material. The blue pigment is not particularly limited except the blue-based color. The blue pigment may include at least one of a cobalt (Co)-based material, a copper (Cu)-based material, a chrome (Cr)-based material or a nickel (Ni)-based material in consideration of facility of powder manufacture, color, and manufacturing cost. 
     At least one of the Co-based material, the Cu-based material, the Cr-based material or the Ni-based material may exist in a state of metal oxide in the second phosphor layer. For instance, the Co-based material may exist in a state of CoAl 2 O 4  in the second phosphor layer. 
     A third phosphor layer emitting green light includes a third phosphor material having a white-based color, and may not include a pigment. 
     The third phosphor material is not particularly limited except the green light emission. The third phosphor material may include Zn 2 SiO 4 :Mn +2  and YBO 3 :Tb +3  in consideration of an emitting efficiency of green light. 
       FIGS. 4A and 4B  are graphs showing reflectances depending on compositions of first and second phosphor layers, respectively. 
     First, a 7-inch test model on which a first phosphor layer emitting red light from all discharge cells is formed is manufactured. Then, light is directly irradiated on a barrier rib and the first phosphor layer of the test model in a state where a front substrate of the test model is removed to measure a reflectance of the test model. 
     The first phosphor layer includes a first phosphor material and a red pigment. The first phosphor material is (Y, Gd)BO:Eu. The red pigment is an Fe-based material, and the Fe-based material in a state of αFe 2 O 3  is mixed with the first phosphor material. 
     In  FIG. 4A , {circle around ( 1 )} indicates a case where the first phosphor layer does not include the red pigment. {circle around ( 2 )} indicates a case where the first phosphor layer includes the red pigment of 0.1 part by weight. {circle around ( 3 )} indicates a case where the first phosphor layer includes the red pigment of 0.5 part by weight. 
     In case of {circle around ( 1 )} not including the red pigment, a reflectance is equal to or more than about 75% at a wavelength of 400 nm to 750 nm. Because the first phosphor material having a white-based color reflects most of incident light, the reflectance in {circle around ( 1 )} is high. 
     In case of {circle around ( 2 )} including the red pigment of 0.1 part by weight, a reflectance is equal to or less than about 60% at a wavelength of 400 nm to 550 nm and ranges from about 60% to 75% at a wavelength more than 550 nm. 
     In case of {circle around ( 3 )} including the red pigment of 0.5 part by weight, a reflectance is equal to or less than about 50% at a wavelength of 400 nm to 550 nm and ranges from about 50% to 70% at a wavelength more than 550 nm. 
     Because the red pigment having a red-based color absorbs incident light, the reflectances in {circle around ( 2 )} and {circle around ( 3 )} are less than the reflectance in {circle around ( 1 )}. 
       FIG. 4B  is a graph showing a reflectance of a test module depending on a wavelength. First, a 7-inch test model on which a second phosphor layer emitting blue light from all discharge cells is formed is manufactured. Then, light is directly irradiated on a barrier rib and the second phosphor layer of the test model in a state where a front substrate of the test model is removed to measure a reflectance of the test model. 
     The second phosphor layer includes a second phosphor material and a blue pigment. The second phosphor material is (Ba, Sr, Eu)MgAl 10 O 17 . The blue pigment is a Co-based material, and the Co-based material in a state of CoAl 2 O 4  is mixed with the second phosphor material. 
     In  FIG. 4B , {circle around ( 1 )} indicates a case where the second phosphor layer does not include the blue pigment. {circle around ( 2 )} indicates a case where the second phosphor layer includes the blue pigment of 0.1 part by weight. {circle around ( 3 )} indicates a case where the second phosphor layer includes the blue pigment of 1.0 part by weight. 
     In case of {circle around ( 1 )} not including the blue pigment, a reflectance is equal to or more than about 72% at a wavelength of 400 nm to 750 nm. Because the second phosphor material having a white-based color reflects most of incident light, the reflectance in {circle around ( 1 )} is high. 
     In case of {circle around ( 2 )} including the blue pigment of 0.1 part by weight, a reflectance is equal to or more than about 74% at a wavelength of 400 nm rises to about 72% at a wavelength more than 650 nm. 
     In case of {circle around ( 3 )} including the blue pigment of 1.0 part by weight, a reflectance is at least 50% at a wavelength of 510 nm to 650 nm. 
     Because the blue pigment having a blue-based color absorbs incident light, the reflectances in {circle around ( 2 )} and {circle around ( 3 )} are less than the reflectance in {circle around ( 1 )}. A reduction in the reflectance can improve the contrast characteristic, and thus the image quality can be improved. 
       FIGS. 5A and 5B  are diagrams for explaining a distribution characteristic of pigment particles depending on the size of phosphor particles. 
     As shown in  FIG. 5A , in case the size of phosphor particles  500  is relatively large, a relatively wide space is provided between the phosphor particles  500 . 
     Pigment particles  510  mixed with the phosphor particles  500  may be positioned between the phosphor particles  500  in the relatively wide space. An area of the surface of the phosphor layer  114  which the pigment particles  510  occupy may decrease, and thus the reflectance may increase. 
     On the other hand, as shown in  FIG. 5B , in case the size of the phosphor particles  500  is relatively small, the plurality of pigment particles  510  may be positioned on the surface of the phosphor layer  114  even if the pigment particle  510  of the same content and the same size as the pigment particle  510  of  FIG. 5A  are used. Accordingly, the reflectance can be sufficiently reduced. 
     Considering the description of  FIGS. 5A and 5B , as the size of the phosphor particles  500  increases, a content of the pigment increases so as to sufficiently reduce the reflectance. 
       FIG. 6  is a diagram for explaining a relationship between the size of a first phosphor particle and a content of a red pigment. 
     More specifically,  FIG. 6  is a graph showing a reflectance and a luminance depending on changes in a ratio C 1 /L 1  of a content C 1  of the red pigment to an average particle size L 1  of particles of the first phosphor material. The average particle size L 1  is measured in units of micrometer (μm), and the content C 1  of the red pigment is measured in units of parts by weight. 
     While the ratio C 1 /L 1  ranges from 0.001 to 8.0, the reflectance and the luminance of the displayed image are measured. In this case, particles of the red pigment have the substantially equal size. 
     In  FIG. 6 , ⊚ indicates an excellent state in which the reflectance is sufficiently low or the luminance is sufficiently high, ◯ indicates a good state, and X indicates a bad state in which the reflectance is excessively high or the luminance is excessively low. 
     As shown in  FIG. 6 , when the ratio C 1 /L 1  is 0.001 to 0.003, the reflectance is bad (X). Because the average particle size L 1  of the first phosphor material is excessively larger than the content C 1  of the red pigment, the most particles of the red pigment are positioned between the particles of the first phosphor material. Hence, the reflectance of the first phosphor may be excessively low and the contrast characteristic may worse. For example, supposing that the content C 1  of the red pigment is 0.1 part by weight, the average particle size L 1  of the first phosphor material has an excessively large value between 34 μm and 100 μm. 
     On the other hand, when the ratio C 1 /L 1  is 0.005 to 0.007, the reflectance is good (◯). In this case, because the reflectance is low, the contrast characteristic may be reduced. However, a reduction width of the contrast characteristic is small. 
     When the ratio C 1 /L 1  is equal to or larger than 0.01, the reflectance is excellent (⊚). Because the average particle size L 1  of the first phosphor material is sufficiently smaller than the content C 1  of the red pigment, the reflectance of the first phosphor layer is sufficiently high because of the reason described in  FIG. 5B . 
     When the ratio C 1 /L 1  is 0.001 to 3.0, the luminance is excellent (⊚). Because the average particle size L 1  of the first phosphor material is sufficiently larger than the content C 1  of the red pigment, the most particles of the red pigment are positioned between the particles of the first phosphor material, and thus an area of the surface of the first phosphor layer which the red pigment occupies may be sufficiently small. 
     When the ratio C 1 /L 1  is 5.1 to 6.0, the luminance is good (◯). In this case, because the luminance is low, a viewer may perceive that a displayed image is dark. However, a dark level of the displayed image is small. 
     When the ratio C 1 /L 1  is equal to or larger than 8.0, the luminance is bad (X). Because the average particle size L 1  of the first phosphor material is excessively smaller than the content C 1  of the red pigment, the area of the surface of the first phosphor layer covered by the red pigment is excessively wide. For example, supposing that the content C 1  of the red pigment is 2 parts by weight, the average particle size L 1  of the first phosphor material is an excessively small value of about 0.25 μm. 
     Considering the table of  FIG. 6 , the content C 1  of the red pigment may satisfy the following equation 1.
 
0.005 ≦C 1 /L 1≦6  [Equation 1]
 
     Furthermore, the content C 1  of the red pigment may satisfy the following equation 2.
 
0.01 ≦C 1 /L 1≦3  [Equation 2]
 
       FIG. 7  is a diagram for explaining a relationship between the size of a second phosphor particle and a content of a blue pigment. 
     More specifically,  FIG. 7  is a graph showing a reflectance and a luminance depending on changes in a ratio C 2 /L 2  of a content C 2  of the blue pigment to an average particle size L 2  of particles of the second phosphor material. The average particle size L 2  is measured in units of micrometer (μm), and the content C 2  of the blue pigment is measured in units of parts by weight. 
     While the ratio C 2 /L 2  ranges from 0.005 to 10.0, the reflectance and the luminance of the displayed image are measured. In this case, particles of the blue pigment have the substantially equal size. 
     In  FIG. 7 , ⊚ indicates an excellent state in which the reflectance is sufficiently low or the luminance is sufficiently high, ◯ indicates a good state, and X indicates a bad state in which the reflectance is excessively high or the luminance is excessively low. 
     As shown in  FIG. 7 , when the ratio C 2 /L 2  is 0.005, the reflectance is bad (X). Because the average particle size L 2  of the second phosphor material is excessively larger than the content C 2  of the blue pigment, the most particles of the blue pigment are positioned between the particles of the second phosphor material. Hence, the reflectance of the second phosphor may be excessively low, and the contrast characteristic may worse. 
     On the other hand, when the ratio C 2 /L 2  is 0.01 to 0.03, the reflectance is good (◯). In this case, because the reflectance is low, the contrast characteristic may be reduced. However, a reduction width of the contrast characteristic is small. 
     When the ratio C 2 /L 2  is equal to or larger than 0.05, the reflectance is excellent (⊚). Because the average particle size L 2  of the second phosphor material is sufficiently smaller than the content C 2  of the blue pigment, the reflectance of the second phosphor layer is sufficiently high because of the reason described in  FIG. 5B . 
     When the ratio C 2 /L 2  is 0.005 to 4.0, the luminance is excellent (⊚). Because the average particle size L 2  of the second phosphor material is sufficiently larger than the content C 2  of the blue pigment, the most particles of the blue pigment are positioned between the particles of the second phosphor material, and thus an area of the surface of the second phosphor layer which the blue pigment occupies may be sufficiently small. 
     When the ratio C 2 /L 2  is 4.2 to 8.0, the luminance is good (◯). In this case, because the luminance is low, the viewer may perceive that a displayed image is dark. However, a dark level of the displayed image is small. 
     When the ratio C 2 /L 2  is equal to or larger than 10.0, the luminance is bad (X). Because the average particle size L 2  of the second phosphor material is excessively smaller than the content C 2  of the blue pigment, the area of the surface of the second phosphor layer covered by the blue pigment is excessively wide. 
     Considering the table of  FIG. 7 , the content C 2  of the blue pigment may satisfy the following equation 3.
 
0.01 ≦C 2 /L 2≦8  [Equation 3]
 
     Furthermore, the content C 2  of the blue pigment may satisfy the following equation 4.
 
0.05 ≦C 2 /L 2≦4  [Equation 4]
 
       FIG. 8  is a diagram for explaining a difference between a content of a red pigment and a content of a blue pigment. 
     As shown in  FIG. 8 , (a) shows particles  800  of a second phosphor material constituting a second phosphor layer  114 B, and (b) shows particles  810  of a first phosphor material constituting a first phosphor layer  114 R. 
     The size of the second phosphor particles  800  is larger than the size of the first phosphor material particles  810 . 
     A reason why the size of the second phosphor particles  800  is larger than the size of the first phosphor material particles  810  may be caused by a difference between a composition of the second phosphor material and a composition of the first phosphor material and a difference between a processing process of the second phosphor material and a processing process of the first phosphor material. 
     As described above, in case the size of the phosphor particles increases, a content of the pigment has to increase. As a result, the reflectance can be reduced, and the contrast characteristic can be improved. 
     Accordingly, because the size of the second phosphor particles  800  is larger than the size of the first phosphor material particles  810 , a content of a blue pigment constituting the second phosphor layer  114 B is more than a content of a red pigment constituting the first phosphor layer  114 R. 
       FIG. 9  shows a color coordinate characteristic of each of an A-type panel in which a content of a blue pigment constituting a second phosphor layer is more than a content of a red pigment constituting a first phosphor layer, and a B-type panel in which a content of a blue pigment constituting a second phosphor layer is equal to or less than a content of a red pigment constituting a first phosphor layer. 
     For example, the A-type panel including the blue pigment of 1 part by weight and the red pigment of 0.2 part by weight and the B-type panel including the blue pigment of 0.1 part by weight and the red pigment of 0.3 part by weight were manufactured, and then color coordinates of the A-type and B-type panels were measured using MCPD-1000 in a state where the same driving signal is supplied to the A-type and B-type panels. 
     As shown in  FIG. 9 , in the B-type panel, a green coordinate P 1  has X-axis coordinate of about 0.276 and Y-axis coordinate of about 0.660, a red coordinate P 2  has X-axis coordinate of about 0.642 and Y-axis coordinate of about 0.368, and a blue coordinate P 3  has X-axis coordinate of about 0.158 and Y-axis coordinate of about 0.103. 
     In the A-type panel, a green coordinate P 10  has X-axis coordinate of about 0.275 and Y-axis coordinate of about 0.655, a red coordinate P 20  has X-axis coordinate of about 0.635 and Y-axis coordinate of about 0.337, and a blue coordinate P 30  has X-axis coordinate of about 0.130 and Y-axis coordinate of about 0.060. 
     Because the content of the red pigment is more than the content of the blue pigment in the B-type panel, the most particles of the blue pigment are positioned between particles of the second phosphor material having the relatively large size in the second phosphor layer as shown in  FIG. 5A , and the most particles of the red pigment are positioned on the surface of the first phosphor layer as shown in  FIG. 5B . Hence, because the viewer visually perceives the red pigment, the viewer may perceive that the displayed image is close to red. This case means that the color temperature is relatively low. 
     On the other hand, because the content of the blue pigment mixed with the second phosphor material having the relatively large particle size is more than the content of the red pigment in the A-type panel, the viewer can perceive both the blue pigment and the red pigment. Accordingly, a sharp reduction in the color temperature can be prevented. 
     It can be seen from  FIG. 9  that a triangle connecting the three coordinates P 10 , P 20  and P 30  of the A-type panel further moves in a blue direction as compared with a triangle connecting the three coordinates P 1 , P 2  and P 3  of the B-type panel. This means that the color temperature of the A-type panel is higher than the color temperature of the B-type panel. Accordingly, the viewer may think that an image displayed on the A-type panel is clearer than an imaged displayed on the B-type panel. 
       FIGS. 10A and 10B  are graphs showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a red pigment, respectively. 
     In  FIGS. 10A and 10B , the first phosphor layer is positioned inside the red discharge cell, the second phosphor layer is positioned inside the blue discharge cell, and the third phosphor layer is positioned inside the green discharge cell. Further, a reflectance and a luminance of the plasma display panel are measured depending on changes in a content of the red pigment mixed with the first phosphor layer in a state where the blue pigment of 1.0 part by weight is mixed with the second phosphor layer. In this case, the reflectance and the luminance of the plasma display panel are measured in a panel state in which the front substrate and the rear substrate coalesce with each other. 
     The first phosphor material is (Y, Gd)BO:Eu. The red pigment is an Fe-based material, and the Fe-based material in a state of αFe 2 O 3  is mixed with the first phosphor material. 
     The second phosphor material is (Ba, Sr, Eu)MgAl 10 O 17 . The blue pigment is a Co-based material, and the Co-based material in a state of CoAl 2 O 4  is mixed with the second phosphor material. 
     In  FIG. 10A , {circle around ( 1 )} indicates a case where the first phosphor layer does not include the red pigment in a state where the second phosphor layer includes the blue pigment of 1.0 part by weight. {circle around ( 2 )} indicates a case where the first phosphor layer includes the red pigment of 0.1 part by weight in a state where the second phosphor layer includes the blue pigment of 1.0 part by weight. {circle around ( 3 )} indicates a case where the first phosphor layer includes the red pigment of 0.5 part by weight in a state where the second phosphor layer includes the blue pigment of 1.0 part by weight. 
     In case of {circle around ( 1 )} not including the red pigment, a panel reflectance rises from about 33% to 38% at a wavelength of 400 nm to 550 nm. The panel reflectance falls to about 33% at a wavelength more than 550 nm. In other words, the panel reflectance has a high value of about 37% to 38% at a wavelength of 500 nm to 600 nm. 
     Because the first phosphor material having a white-based color reflects most of incident light, the panel reflectance in {circle around ( 1 )} is relatively high although the blue pigment is mixed with the second phosphor layer. 
     In case of {circle around ( 2 )} including the red pigment of 0.1 part by weight, a panel reflectance is equal to or less than about 34% at a wavelength of 400 nm to 750 nm, and has a relatively small value of about 33% to 34% at a wavelength of 500 nm to 600 nm. 
     In case of {circle around ( 3 )} including the red pigment of 0.5 part by weight, a panel reflectance ranges from about 24% to 31.5% at a wavelength of 400 nm to 650 nm and falls to about 30% at a wavelength of 650 nm to 750 nm. Further, the panel reflectance has a relatively small value of about 27.5% to 29.5% at a wavelength of 500 nm to 600 nm. 
     As above, as a content of the red pigment increases, the panel reflectance decreases. 
     There is a relatively great difference between the panel reflectance in {circle around ( 1 )} not including the red pigment and the panel reflectances in {circle around ( 2 )} and {circle around ( 3 )} including the red pigment at a wavelength of 500 nm to 600 nm, for instance, at a wavelength of 550 nm. 
     Because a wavelength of 500 nm to 600 nm is mainly seen as red, orange and yellow light in visible light, the high panel reflectance at a wavelength of 500 nm to 600 nm means that a displayed image is close to red. In this case, because a color temperature is relatively low, the viewer may easily feel eyestrain and may feel that the image is not clear. 
     On the other hand, the low panel reflectance at a wavelength of 500 nm to 600 nm means that absorptance of red, orange and yellow light is high. Hence, a color temperature of a displayed image is relatively high, and thus an image can be clearer. 
     Accordingly, the relatively great difference between the panel reflectance in {circle around ( 1 )} and the panel reflectance in {circle around ( 2 )} and {circle around ( 3 )} at a wavelength of 500 nm to 600 nm means that an excessive reduction in the color temperature can be prevented although the red pigment is mixed with the first phosphor layer. Hence, the viewer can watch a clearer image. 
     Considering this, the color temperature of the panel can be improved by setting the panel reflectance to be equal to or less than 30% at a wavelength of 500 nm to 600 nm, for instance, at a wavelength of 550 nm. 
       FIG. 10B  is a graph showing a luminance of the same image depending on changes in a content of the red pigment included in the first phosphor layer in a state where a content of the blue pigment included in the second phosphor layer is fixed. 
     As shown in  FIG. 10B , a luminance of an image displayed when the first phosphor layer does not include the red pigment is about 176 cd/m 2 . 
     When a content of the red pigment is 0.01 part by weight, the luminance of the image is reduced to about 175 cd/m 2 . The red pigment can reduce the luminance of the image, because particles of the red pigment cover a portion of the particle surface of the first phosphor material and thus hinder ultraviolet rays generated by a discharge inside the discharge cell from being irradiated on the particles of the first phosphor material. 
     When a content of the red pigment ranges from 0.1 to 3 parts by weight, a luminance of the image ranges from about 168 cd/m 2  to 174 cd/m 2 . 
     When a content of the red pigment ranges from 3 to 5 parts by weight, a luminance of the image ranges from about 160 cd/m 2  to 168 cd/m 2 . 
     When a content of the red pigment is equal to or more than 6 parts by weight, a luminance of the image is sharply reduced to a value equal to or less than about 149 cd/m 2 . In other words, when a large amount of the red pigment is mixed, the particles of the red pigment cover a large area of the particle surface of the first phosphor material and thus the luminance is sharply reduced. 
     Considering the graphs of  FIGS. 10A and 10B , when a content of the red pigment ranges from 0.01 to 5 parts by weight, a reduction in the luminance can be prevented while the panel reflectance is reduced. A content of the red pigment may range from 0.1 to 3 parts by weight. 
       FIGS. 11A and 11B  are graphs showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a blue pigment, respectively. A description of  FIGS. 11A and 11B  overlapping the description of  FIGS. 10A and 10B  is briefly made or entirely omitted. 
     In  FIGS. 11A and 11B , the first phosphor layer is positioned inside the red discharge cell, the second phosphor layer is positioned inside the blue discharge cell, and the third phosphor layer is positioned inside the green discharge cell. Further, a reflectance and a luminance of the plasma display panel are measured depending on changes in a content of the blue pigment mixed with the second phosphor layer in a state where the red pigment of 0.2 part by weight is mixed with the first phosphor layer. In this case, the reflectance and the luminance of the plasma display panel are measured in a panel state in which the front substrate and the rear substrate coalesce with each other. 
     The other experimental conditions in  FIGS. 11A and 11B  are substantially the same as the experimental conditions in  FIGS. 10A and 10B . 
     In  FIG. 11A , {circle around ( 1 )} indicates a case where the second phosphor layer does not include the blue pigment in a state where the first phosphor layer includes the red pigment of 0.2 part by weight. {circle around ( 2 )} indicates a case where the second phosphor layer includes the blue pigment of 0.1 part by weight in a state where the first phosphor layer includes the red pigment of 0.2 part by weight. {circle around ( 3 )} indicates a case where the second phosphor layer includes the blue pigment of 0.5 part by weight in a state where the first phosphor layer includes the red pigment of 0.2 part by weight. {circle around ( 4 )} indicates a case where the second phosphor layer includes the blue pigment of 3 parts by weight in a state where the first phosphor layer includes the red pigment of 0.2 part by weight. {circle around ( 5 )} indicates a case where the state where the first phosphor layer includes the red pigment of 0.2 part by weight. 
     In case of {circle around ( 1 )} not including the blue pigment, a panel reflectance rises from about 35% to 40.5% at a wavelength of 400 nm to 550 nm. The panel reflectance falls to about 35.5% at a wavelength more than 550 nm. In other words, the panel reflectance has a high value of about 39% to 40.5% at a wavelength of 500 nm to 600 nm. 
     Because the second phosphor material having a white-based color reflects most of incident light, the panel reflectance in {circle around ( 1 )} is relatively high although the red pigment is mixed with the first phosphor layer. 
     In case of {circle around ( 2 )} including the blue pigment of 0.1 part by weight, a panel reflectance is equal to or less than about 38% at a wavelength of 400 nm to 750 nm, and has a relatively small value of about 34% to 37% at a wavelength of 500 nm to 600 nm. 
     In case of {circle around ( 3 )} including the blue pigment of 0.5 part by weight, a panel reflectance ranges from about 26% to 29% at a wavelength of 400 nm to 650 nm and falls from about 28% to 32.5% at a wavelength of 650 nm to 750 nm. Further, the panel reflectance has a relatively small value of about 28% to 29% at a wavelength of 500 nm to 600 nm. 
     In case of {circle around ( 4 )} including the blue pigment of 3 parts by weight, a panel reflectance ranges from about 22.5% to 29% at a wavelength of 400 nm to 650 nm and ranges from about 29% to 3% at a wavelength of 650 nm to 750 nm. Further, the panel reflectance has a relatively small value of about 26.5% to 28% at a wavelength of 500 nm to 600 nm. 
     In case of {circle around ( 5 )} including the blue pigment of 7 parts by weight, a panel reflectance ranges from about 25% to 28% at a wavelength of 400 nm to 700 nm and ranges from about 28% to 30% at a wavelength more than 700 nm. 
       FIG. 11B  is a graph showing a luminance of the same image depending on changes in a content of the blue pigment included in the second phosphor layer in a state where a content of the red pigment included in the first phosphor layer is fixed. 
     As shown in  FIG. 11B , a luminance of an image displayed when the second phosphor layer does not include the blue pigment is about 176 cd/m 2 . 
     When a content of the blue pigment is 0.01 part by weight, a luminance of the image is about 175 cd/m 2 . 
     When a content of the blue pigment is 0.1 part by weight, a luminance of the image is about 172 cd/m 2 . 
     When a content of the blue pigment ranges from 0.5 to 4 parts by weight, a luminance of the image has a stable value of about 164 cd/m 2  to 170 cd/m 2 . 
     When a content of the blue pigment ranges from 4 to 5 parts by weight, a luminance of the image ranges from about 160 cd/m 2  to 164 cd/m 2 . 
     When a content of the blue pigment exceeds 6 parts by weight, a luminance of the image is sharply reduced to a value equal to or less than about 148 cd/m 2 . In other words, when a large amount of the blue pigment is mixed, particles of the blue pigment cover a large area of the particle surface of the second phosphor material, and thus the luminance is sharply reduced. 
     Considering the graphs of  FIGS. 11A and 11B , when a content of the blue pigment ranges from 0.01 to 5 parts by weight, a reduction in the luminance can be prevented while the panel reflectance is reduced. A content of the blue pigment may range from 0.5 to 4 parts by weight. 
       FIG. 12  is a diagram for explaining a difference between a particle size of a red pigment and a particle size of a blue pigment. 
     As shown in  FIG. 12 , (a) shows particles  1200  of a second phosphor material constituting a second phosphor layer  114 B and particles  1210  of a blue pigment constituting the second phosphor layer  114 B, and (b) shows particles  1220  of a first phosphor material constituting a first phosphor layer  114 R and particles  1230  of a red pigment constituting the first phosphor layer  114 R. 
     In  FIG. 12 , the size of the particles  1200  of the second phosphor material is larger than the size of the particles  1220  of the first phosphor material, and the size of the particles  1210  of the blue pigment is larger than the size of the particles  1230  of the red pigment. 
     As above, because the particles  1210  of the blue pigment mixed with the second phosphor material whose the size of the particles  1200  is relatively large are relatively large, the particles  1210  of the blue pigment are positioned not between the particles  1200  of the second phosphor material but on the surface of the second phosphor layer  114 B. Accordingly, in case the size of the particles  1200  of the second phosphor material is relatively large, the reflectance can be reduced. 
     In other words, because the size of the particles  1210  of the blue pigment is larger than the size of the particles  1230  of the red pigment, an effect similar to the fact that a content of the blue pigment is more than a content of the red pigment can be obtained. 
       FIGS. 13A and 13B  illustrate another example of a composition of a phosphor layer. A description in  FIGS. 13A and 13B  overlapping the description in  FIG. 3  is briefly made or entirely omitted. 
     As shown in  FIG. 13A , the third phosphor layer emitting green light includes a third phosphor material having a white-based color and a green pigment. 
     A description in  FIG. 13A  may be substantially the same as the description in  FIG. 3  except that the third phosphor layer includes the green pigment. 
     The green pigment has a green-based color. The third phosphor layer may have a green-based color by mixing the green pigment with the third phosphor material. The green pigment is not particularly limited except the green-based color. The green pigment may include a zinc (Zn) material in consideration of facility of powder manufacture, color, and manufacturing cost. 
     The Zn-based material may exist in a state of zinc oxide, for instance, in a state of ZnCO 2 O 4  in the third phosphor layer. 
       FIG. 13B  is a graph showing a reflectance of a test model depending on a wavelength. 
     Similar to  FIGS. 4A and 4B , a 7-inch test model on which a third phosphor layer emitting green light from all discharge cells is formed is manufactured. Then, light is directly irradiated on a barrier rib and the third phosphor layer of the test model in a state where a front substrate of the test model is removed to measure a reflectance of the test model. 
     The third phosphor layer includes a third phosphor material and a green pigment. The third phosphor material includes Zn 2 SiO 4 :Mn +2  and YBO 3 :Tb +3  in a ratio of 5:5. The green pigment is a Zn-based material, and the Zn-based material in a state of ZnCO 2 O 4  is mixed with the third phosphor material. 
     In  FIG. 13B , {circle around ( 1 )} indicates a case where the third phosphor layer does not include the green pigment. {circle around ( 2 )} indicates a case where the third phosphor layer includes the green pigment of 0.1 part by weight. {circle around ( 3 )} indicates a case where the third phosphor layer includes the green pigment of 0.5 part by weight. {circle around ( 4 )} indicates a case where the third phosphor layer includes the green pigment of 1.0 part by weight. 
     In case of {circle around ( 1 )} not including the green pigment, a reflectance is equal to or more than about 75% at a wavelength of 400 nm to 750 nm and is equal to or more than about 80% at a wavelength of 400 nm to 500 nm. 
     Because the third phosphor material having a white-based color reflects most of incident light, the reflectance in {circle around ( 1 )} is high. 
     In case of {circle around ( 2 )} including the green pigment of 0.1 part by weight, a reflectance is equal to or less than about 75% at a wavelength of 400 nm to 550 nm and ranges from about 66% to 70% at a wavelength of 550 nm to 700 nm. 
     In case of {circle around ( 3 )} including the green pigment of 0.5 part by weight, a reflectance is equal to or less than about 73% at a wavelength of 400 nm to 550 nm and ranges from about 63% to 65% at a wavelength more than 550 nm. 
     In case of {circle around ( 4 )} including the green pigment of 1.0 part by weight, a reflectance is similar to the reflectance in {circle around ( 3 )} at a wavelength of 400 nm to 750 nm. 
     Because the green pigment having a green-based color absorbs incident light, the reflectances in {circle around ( 2 )}, {circle around ( 3 )} and {circle around ( 4 )} are less than the reflectance in {circle around ( 1 )}. 
     The fact that the reflectances in {circle around ( 3 )} and {circle around ( 4 )} are similar to each other means that a reduction width of the panel reflectance is small although a content of the green pigment increases. 
       FIGS. 14A and 14B  are a table and a graph showing a reflectance and a luminance of a plasma display panel depending on changes in a content of a green pigment, respectively. 
     In  FIGS. 14A and 14B , the first phosphor layer is positioned inside the red discharge cell, the second phosphor layer is positioned inside the blue discharge cell, and the third phosphor layer is positioned inside the green discharge cell. Further, a reflectance and a luminance of the plasma display panel are measured depending on changes in a content of the green pigment mixed with the third phosphor layer in a state where the blue pigment of 1.0 part by weight is mixed with the second phosphor layer and the red pigment of 0.2 part by weight is mixed with the first phosphor layer. In this case, the reflectance and the luminance of the plasma display panel are measured in a panel state in which the front substrate and the rear substrate coalesce with each other. 
     The first phosphor material is (Y, Gd)BO:Eu. The red pigment is an Fe-based material, and the Fe-based material in a state of αFe 2 O 3  is mixed with the first phosphor material. 
     The second phosphor material is (Ba, Sr, Eu)MgAl 10 O 17 . The blue pigment is a Co-based material, and the Co-based material in a state of CoAl 2 O 4  is mixed with the second phosphor material. 
     The third phosphor material includes Zn 2 SiO 4 :Mn +2  and YBO 3 :Tb +3  in a ratio of 5:5. The green pigment is a Zn-based material, and the Zn-based material in a state of ZnCO 2 O 4  is mixed with the third phosphor material. 
       FIG. 14A  is a table showing a reflectance at a wavelength of 550 nm. 
     As shown in  FIG. 14A , when a content of the green pigment is 0, a panel reflectance is a relatively high value of 28%. 
     When a content of the green pigment is 0.01 part by weight, a panel reflectance is about 26.5%. When a content of the green pigment is 0.05 part by weight, a panel reflectance is about 26.2%. 
     When a content of the green pigment is 0.1 part by weight, a panel reflectance is about 26%. When a content of the green pigment is 0.2 part by weight, a panel reflectance is about 25.9%. 
     When a content of the green pigment greatly increases to 2.5 parts by weight, a panel reflectance falls to about 24.3%. 
     When a content of the green pigment is 3 parts by weight, a panel reflectance is about 24%. 
     When a content of the green pigment is 4, 5 and 7 parts by weight, respectively, a panel reflectance is about 23.8%, 23.5% and 22.8%, respectively. 
     As can be seen from  FIG. 14A , when a content of the green pigment is equal to or more than 4 parts by weight, a reduction width of the panel reflectance is small. 
       FIG. 14B  is a graph showing a luminance of the same image depending on changes in a content of the green pigment included in the third phosphor layer in a state where a content of each of the red pigment and the blue pigment is fixed. 
     As shown in  FIG. 14B , a luminance of an image displayed when the third phosphor layer does not include the green pigment is about 175 cd/m 2 . 
     When a content of the green pigment is 0.01 part by weight, a luminance of the image is reduced to about 174 cd/m 2 . The green pigment can reduce the luminance of the image, because particles of the green pigment cover a portion of the particle surface of the third phosphor material, and thus hinder ultraviolet rays generated by a discharge inside the discharge cell from being irradiated on the particles of the third phosphor material. 
     When a content of the green pigment ranges from 0.05 to 2.5 parts by weight, a luminance of the image has a stable value of about 166 cd/m 2  to 172 cd/m 2 . 
     When a content of the green pigment is 3 parts by weight, a luminance of the image is about 164 cd/m 2 . 
     When a content of the green pigment is equal to or more than 4 parts by weight, a luminance of the image is sharply reduced to a value equal to or less than about 149 cd/m 2 . In other words, when a large amount of the green pigment is mixed, the particles of the green pigment cover a large area of the particle surface of the third phosphor material and thus the luminance is sharply reduced. 
     Considering  FIGS. 14A and 14B , when a content of the green pigment ranges from 0.01 to 3 parts by weight, a reduction in the luminance can be prevented while the panel reflectance is reduced. A content of the green pigment may range from 0.05 to 2.5 parts by weight. 
     A reduction width in the panel reflectance when a content of the green pigment increases is smaller than a reduction width in the panel reflectance when the red pigment and the blue pigment are mixed. Accordingly, a content of the green pigment may be smaller than a content of each of the red pigment and the blue pigment. Further, the green pigment may not be mixed. 
       FIGS. 15A to 15C  show another structure of a plasma display panel according to the exemplary embodiment. 
     As shown in  FIG. 15A , a black matrix  1000  overlapping the barrier rib  112  is formed on the front substrate  101 . The black matrix  1000  absorbs incident light and thus suppresses the reflection of light caused by the barrier rib  112 . Hence, a panel reflectance is reduced and a contrast characteristic can be improved. 
     In  FIG. 15A , the black matrix  1000  is formed on the front substrate  101 . However, the black matrix  1000  may be positioned on the upper dielectric layer (not shown). 
     Black layers  120  and  130  are formed between the transparent electrodes  102   a  and  103   a  and the bus electrodes  102   b  and  103   b . The black layers  120  and  130  prevent the reflection of light caused by the bus electrodes  102   b  and  103   b , thereby reducing a panel reflectance. 
     As shown in  FIG. 15B , a common black matrix  1010  contacting the two sustain electrodes  103  is formed between the two sustain electrodes  103 . The common black matrix  1010  may be formed of the substantially same materials as the black layers  120  and  130 . In this case, since the common black matrix  1010  can be manufactured when the black layers  120  and  130  is manufactured, time required in a manufacturing process can be reduced. 
     As shown in  FIG. 15C , a top black matrix  1020  is directly formed on the barrier rib  112 . Because the top black matrix  1020  reduces a panel reflectance, a black matrix may not be formed on the front substrate  101 . 
     As described above, when a pigment is mixed with the phosphor layer, the panel reflectance can be further reduced. For instance, the first and second phosphor layers may include the red and blue pigments, respectively. 
     The black layers  120  and  130 , the black matrix  1000 , the common black matrix  1010  and the top black matrix  1020  may be omitted from the plasma display panel. Because the pigment mixed with the phosphor layer can sufficiently reduce the panel reflectance, a sharp increase in the panel reflectance can be prevented although the black layers  120  and  130 , the black matrix  1000 , the common black matrix  1010  and the top black matrix  1020  are omitted. 
     A removal of the black layers  120  and  130 , the black matrix  1000 , the common black matrix  1010  and the top black matrix  1020  can make a manufacturing process of the panel simpler, and reduce the manufacturing cost. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.