Patent Publication Number: US-8531351-B2

Title: Multi plasma display device

Description:
This application claims the benefit of Korean Patent Application No. 10-2009-0115959 filed on Nov. 27, 2009, the entire contents of which is incorporated herein by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to a multi plasma display device. 
     2. Discussion of the Related Art 
     A multi plasma display device is a display device displaying an image on a plurality of plasma display panels positioned adjacent to one another. The multi plasma display device may display a large screen image using a plurality of small-sized plasma display panels 
     SUMMARY OF THE INVENTION 
     In one aspect, there is a multi plasma display device comprising a first panel, a second panel positioned adjacent to the first panel, and a lens unit positioned so that the lens unit commonly overlaps a portion of a front surface of the first panel and a portion of a front surface of the second panel in a boundary portion between the first panel and the second panel, wherein each of the first panel and the second panel includes a front substrate, a rear substrate positioned opposite the front substrate, a barrier rib that is positioned between the front substrate and the rear substrate to partition a discharge cell, and a seal layer between the front substrate and the rear substrate, wherein the lens unit overlaps the seal layer of the first panel and the seal layer of the second panel and does not overlap the discharge cell of the first panel and the discharge cell of the second panel. 
     The lens unit may allow a size of the boundary portion between the first and second panels to seem to be smaller than an actual size of the boundary portion through an optical operation of the lens unit. 
     One end of the lens unit may be positioned between the seal layer and an outermost barrier rib of the first panel, and the other end may be positioned between the seal layer and an outermost barrier rib of the second panel. 
     One end of the lens unit may be positioned in a portion overlapping an outermost barrier rib of the first panel, and the other end may be positioned in a portion overlapping an outermost barrier rib of the second panel. 
     The lens unit may include a plurality of protrusions on the surface of the lens unit. 
     Each of the plurality of protrusions may have substantially a triangle shape. 
     The lens unit may include a first portion overlapping the first panel and a second portion overlapping the second pane. A shape of each of the protrusions formed in the first portion may be different from a shape of each of the protrusions formed in the second portion. 
     A width of the lens unit may be greater than the size of the boundary portion. 
     The lens unit may include a plurality of first prisms in a first portion overlapping the first panel and a plurality of second prisms in a second portion overlapping the second panel. An angle between a first surface of the second prism adjacent to the first portion and a base of the lens unit may be less than an angle between a second surface of the second prism opposite the first surface and the base of the lens unit. An angle between a first surface of the first prism adjacent to the second portion and the base of the lens unit may be less than an angle between a second surface of the first prism opposite the first surface and the base of the lens unit. 
     A distance between a top of an outermost first prism of the first prisms and a top of an outermost second prism of the second prisms may be greater than a distance between tops of two adjacent first prisms of the first prisms and a distance between tops of two adjacent second prisms of the second prisms in a boundary portion between the first portion and the second portion. 
     The first prisms and the second prisms may be arranged in opposite directions. 
     The multi plasma display device may further comprise a black layer positioned in the boundary portion between the first panel and the second panel. 
     The black layer may be positioned at the side of at least one of the first panel and the second panel. 
     A width of the lens unit may be greater than a thickness of the lens unit. 
     A ratio of the width to the thickness of the lens unit may be 10:1 to 10:8. The ratio of the width to the thickness of the lens unit may be 10:2 to 10:6. 
     In another aspect, there is a multi plasma display device comprising a first panel, a second panel positioned adjacent to the first panel, a black layer positioned in a boundary portion between the first panel and the second panel, and an optical sheet on the black layer, the optical sheet including a plurality of prisms, wherein a width of the optical sheet is greater than a width of the black layer. 
     The black layer may be formed of an electrically conductive material. 
     The multi plasma display device may further comprise a first auxiliary frame positioned at the side of the first panel in a boundary portion between the first panel and the second panel, and a second auxiliary frame positioned at the side of the second panel in the boundary portion between the first panel and the second panel. The optical sheet may be positioned so that the optical sheet commonly overlaps the first auxiliary frame and the second auxiliary frame. 
     A first film filter including a first electromagnetic shielding layer may be positioned on a front surface of the first panel, and a second film filter including a second electromagnetic shielding layer may be positioned on a front surface of the second panel. The first auxiliary frame may be connected to the first electromagnetic shielding layer, and the second auxiliary frame may be connected to the second electromagnetic shielding layer. 
     The black layer may be positioned at the side of at least one of the first panel and the second panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in 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: 
         FIG. 1  illustrates a configuration of a multi plasma display device according to an embodiment of the invention; 
         FIGS. 2 to 4  illustrate a structure and a driving method of a plasma display panel; 
         FIGS. 5 to 24  illustrate an optical sheet; and 
         FIGS. 25 and 26  illustrate a method of manufacturing a multi plasma display device according to an embodiment of the invention; 
         FIGS. 27 to 31  illustrate another configuration of a multi plasma display device according to an embodiment of the invention; and 
         FIGS. 32 to 34  illustrate another configuration of a black layer. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail embodiments of the invention examples of which are illustrated in the accompanying drawings. 
       FIG. 1  illustrates a configuration of a multi plasma display device according to an embodiment of the invention. 
     As shown in  FIG. 1 , a multi plasma display device  10  according to an embodiment of the invention includes a plurality of plasma display panels  100 ,  110 ,  120 , and  130  positioned adjacent to one another. 
     Among the plurality of plasma display panels  100 ,  110 ,  120 , and  130 , a 1-1 driver  101  and a 1-2 driver  102  supply driving signals to the first plasma display panel  100 . The 1-1 driver  101  and the 1-2 driver  102  are integrated into an integrated driver. Further, a 2-1 driver  111  and a 2-2 driver  112  supply driving signals to the second plasma display panel  110 . In other words, the plasma display panels  100 ,  110 ,  120 , and  130  may be structured so that a different driver supplies a driving signal to each of the plasma display panels  100 ,  110 ,  120 , and  130 . 
     Seam portions  140  and  150  are formed between two adjacent plasma display panels of the plurality of plasma display panels  100 ,  110 ,  120 , and  130 . The seam portions  140  and  150  may be called regions between the two adjacent plasma display panels. 
     In the multi plasma display device  10 , because an image is displayed on the plurality of plasma display panels  100 ,  110 ,  120 , and  130  positioned adjacent to one another, the seam portions  140  and  150  may be formed between two adjacent plasma display panels. 
       FIGS. 2 to 4  illustrate a structure and a driving method of a plasma display panel. 
     A plasma display panel may display an image in a frame including a plurality of subfields. 
     More specifically, as shown in  FIG. 2 , the plasma display panel may include a front substrate  201 , on which a plurality of first electrodes  202  and  203  are formed, and a rear substrate  211  on which a plurality of second electrodes  213  are formed to cross the first electrodes  202  and  203 . 
     In  FIGS. 2 to 4 , the first electrodes  202  and  203  may include scan electrodes  202  and sustain electrodes  203  substantially parallel to each other, and the second electrodes  213  may be called address electrodes. 
     An upper dielectric layer  204  may be formed on the scan electrode  202  and the sustain electrode  203  to limit a discharge current of the scan electrode  202  and the sustain electrode  203  and to provide insulation between the scan electrode  202  and the sustain electrode  203 . 
     A protective layer  205  may be formed on the upper dielectric layer  204  to facilitate discharge conditions. The protective layer  205  may be formed of a material having a high secondary electron emission coefficient, for example, magnesium oxide (MgO). 
     A lower dielectric layer  215  may be formed on the address electrode  213  to provide insulation between the address electrodes  213 . 
     Barrier ribs  212  of a stripe type, a well type, a delta type, a honeycomb type, etc. may be formed on the lower dielectric layer  215  to partition discharge spaces (i.e., discharge cells). Hence, a first discharge cell emitting red light, a second discharge cell emitting blue light, and a third discharge cell emitting green light, etc. may be formed between the front substrate  201  and the rear substrate  211 . Each of the barrier ribs  212  may include first and second barrier ribs each having a different height. 
     The address electrode  213  may cross the scan electrode  202  and the sustain electrode  203  in one discharge cell. Namely, each discharge cell is formed at a crossing of the scan electrode  202 , the sustain electrode  203 , and the address electrode  213 . 
     Each of the discharge cells partitioned by the barrier ribs  212  may be filled with a predetermined discharge gas. 
     A phosphor layer  214  may be formed inside the discharge cells to emit visible light for an image display during an address discharge. For example, first, second, and third phosphor layers that respectively generate red, blue, and green light may be formed inside the discharge cells. 
     While the address electrode  213  may have a substantially constant width or thickness, a width or thickness of the address electrode  213  inside the discharge cell may be different from a width or thickness of the address electrode  213  outside the discharge cell. For example, a width or thickness of the address electrode  213  inside the discharge cell may be larger than a width or thickness of the address electrode  213  outside the discharge cell. 
     When a predetermined signal is supplied to at least one of the scan electrode  202 , the sustain electrode  203 , and the address electrode  213 , a discharge may occur inside the discharge cell. The discharge may allow the discharge gas filled in the discharge cell to generate ultraviolet rays. The ultraviolet rays may be incident on phosphor particles of the phosphor layer  214 , and then the phosphor particles may emit visible light. Hence, an image may be displayed on the screen of the plasma display panel  100 . 
     A frame for achieving a gray scale of an image displayed on the plasma display panel is described with reference to  FIG. 3 . 
     As shown in  FIG. 3 , a frame for achieving a gray scale of an image may include a plurality of subfields. Each of the plurality of subfields may be divided into an address period and a sustain period. During the address period, the discharge cells not to generate a discharge may be selected or the discharge cells to generate a discharge may be selected. During the sustain period, a gray scale may be achieved depending on the number of discharges. 
     For example, if an image with 256-gray level is to be displayed, as shown in  FIG. 3 , a frame may be divided into 8 subfields SF 1  to SF 8 . Each of the 8 subfields SF 1  to SF 8  may include an address period and a sustain period. 
     Furthermore, at least one of a plurality of subfields of a frame may further include a reset period for initialization. At least one of a plurality of subfields of a frame may not include a sustain period. 
     The number of sustain signals supplied during the sustain period may determine a gray level of each of the subfields. For example, in such a method of setting a gray level of a first subfield at 2 0  and a gray level of a second subfield at 2 1 , the sustain period increases in a ratio of 2 n  (where, n=0, 1, 2, 3, 4, 5, 6, 7) in each of the subfields. Hence, various gray levels of an image may be achieved by controlling the number of sustain signals supplied during the sustain period of each subfield depending on a gray level of each subfield. 
     Although  FIG. 3  shows that one frame includes 8 subfields, the number of subfields constituting a frame may vary. For example, a frame may include 10 or 12 subfields. Further, although  FIG. 3  shows that the subfields of the frame are arranged in increasing order of gray level weight, the subfields may be arranged in decreasing order of gray level weight or may be arranged regardless of gray level weight. 
     At least one of a plurality of subfields of a frame may be a selective erase subfield, or at least one of the plurality of subfields of the frame may be a selective write subfield. 
     If a frame includes at least one selective erase subfield and at least one selective write subfield, it may be preferable that a first subfield or first and second subfields of a plurality of subfields of the frame is/are a selective write subfield and the other subfields are selective erase subfields. 
     In the selective erase subfield, a discharge cell to which a data signal is supplied during an address period is turned off during a sustain period following the address period. In other words, the selective erase subfield may include an address period, during which a discharge cell to be turned off is selected, and a sustain period during which a sustain discharge occurs in the discharge cell that is not selected during the address period. 
     In the selective write subfield, a discharge cell to which a data signal is supplied during an address period is turned on during a sustain period following the address period. In other words, the selective write subfield may include a reset period during which discharge cells are initialized, an address period during which a discharge cell to be turned on is selected, and a sustain period during which a sustain discharge occurs in the discharge cell selected during the address period. 
     A driving waveform for driving the plasma display panel is illustrated in  FIG. 4 . 
     As shown in  FIG. 4 , a reset signal RS may be supplied to the scan electrode Y during a reset period RP for initialization of at least one of a plurality of subfields of a frame. The reset signal RS may include a ramp-up signal RU with a gradually rising voltage and a ramp-down signal RD with a gradually falling voltage. 
     More specifically, the ramp-up signal RU may be supplied to the scan electrode Y during a setup period of the reset period RP, and the ramp-down signal RD may be supplied to the scan electrode Y during a set-down period following the setup period SU. The ramp-up signal RU may generate a weak dark discharge (i.e., a setup discharge) inside the discharge cells. Hence, the wall charges may be uniformly distributed inside the discharge cells. The ramp-down signal RD subsequent to the ramp-up signal RU may generate a weak erase discharge (i.e., a set-down discharge) inside the discharge cells. Hence, the remaining wall charges may be uniformly distributed inside the discharge cells to the extent that an address discharge occurs stably. 
     During an address period AP following the reset period RP, a scan reference signal Ybias having a voltage greater than a minimum voltage of the ramp-down signal RD may be supplied to the scan electrode Y. In addition, a scan signal Sc falling from a voltage of the scan reference signal Ybias may be supplied to the scan electrode Y. 
     A pulse width of a scan signal supplied to the scan electrode during an address period of at least one subfield of a frame may be different from pulse widths of scan signals supplied during address periods of the other subfields of the frame. A pulse width of a scan signal in a subfield may be greater than a pulse width of a scan signal in a next subfield. For example, a pulse width of the scan signal may be gradually reduced in the order of 2.6 μs, 2.3 μs, 2.1 μs, 1.9 μs, etc. or may be reduced in the order of 2.6 μs, 2.3 μs, 2.3 μs, 2.1 μs, . . . 1.9 μs, 1.9 μs, etc. in the successively arranged subfields. 
     As above, when the scan signal Sc is supplied to the scan electrode Y, a data signal Dt corresponding to the scan signal Sc may be supplied to the address electrode X. As a voltage difference between the scan signal Sc and the data signal Dt is added to a wall voltage obtained by the wall charges produced during the reset period RP, an address discharge may occur inside the discharge cell to which the data signal Dt is supplied. In addition, during the address period AP, a sustain reference signal Zbias may be supplied to the sustain electrode Z, so that the address discharge efficiently occurs between the scan electrode Y and the address electrode X. 
     During a sustain period SP following the address period AP, a sustain signal SUS may be supplied to at least one of the scan electrode Y or the sustain electrode Z. For example, the sustain signal SUS may be alternately supplied to the scan electrode Y and the sustain electrode Z. Further, the address electrode X may be electrically floated during the sustain period SP. As the wall voltage inside the discharge cell selected by performing the address discharge is added to a sustain voltage Vs of the sustain signal SUS, every time the sustain signal SUS is supplied, a sustain discharge, i.e., a display discharge may occur between the scan electrode Y and the sustain electrode Z. 
       FIGS. 5 to 24  illustrate an optical sheet. 
     As shown in (a) of  FIG. 5 , optical sheets  500  and  510  may be positioned in adjacent two boundary portions, i.e., the seam portions  140  and  150 . The seam portions  140  and  150  seem to be smaller than the actual size of the seam portions  140  and  150  because of an optical operation of the optical sheets  500  and  510  (i.e., because the optical sheets  500  and  510  refract incident light). Considering that the optical sheets  500  and  510  refract the incident light, the optical sheets  500  and  510  may be called lens units. 
     For example, as shown in  FIG. 7 , when the optical sheet  500 ( 510 ) is not formed in an area A 2 , an observer may perceive a width of the seam portion  140 ( 150 ) as W 3 . On the other hand, when the optical sheet  500 ( 510 ) is formed on the seam portion  140 ( 150 ) in an area A 1 , the observer may perceive the width of the seam portion  140 ( 150 ) as W 2  smaller than W 3 . 
     Further, the optical sheet  500 ( 510 ) may partially overlap each of two plasma display panels adjacent to the optical sheet  500 ( 510 ), so that the seam portions  140  and  150  seem to be smaller than the actual size of the seam portions  140  and  150 . 
     The optical sheets  500  and  510  may be formed of a transparent material that is easy to mold. For example, the optical sheets  500  and  510  may be formed of acrylic material. 
     It is assumed that the multi plasma display device  10  includes the first panel  100 , the second panel  110  positioned adjacent to the first panel  100 , the third panel  120  positioned adjacent to the first panel  100 , and the fourth panel  130  positioned adjacent to the second and third panels  110  and  120 , as shown in  FIG. 5 . In this case, the first optical sheet  500  is positioned on the first seam portion  140  between the first and second panels  100  and  110  and between the third and fourth panels  120  and  130 , and the second optical sheet  510  is positioned on the second seam portion  150  between the first and third panels  100  and  120  and between the second and fourth panels  110  and  130 . 
     An image displayed on the two adjacent plasma display panels seems to be discontinuous because of the first and second seam portions  140  and  150 . 
     In the embodiment, because the first and second seam portions  140  and  150  seem to be smaller than the actual size of the first and second seam portions  140  and  150  by respectively positioning the first and second optical sheets  500  and  510  on the first and second seam portions  140  and  150 , the image displayed on the two adjacent plasma display panels seems to be more smoothly. Hence, the quality of the image displayed by the multi plasma display device  10  may be improved. 
     When the first to fourth panels  100  to  130  shown in (a) of  FIG. 5  are the plasma display panels, the optical sheet  500 ( 510 ) may be positioned in a portion overlapping seal layers  520  and  530  of the two adjacent plasma display panels as shown in (b) of  FIG. 5 . 
     The seal layers  520  and  530  are used to attach front substrates  201 A and  201 B and rear substrates  211 A and  211 B of the two adjacent plasma display panels to each other, respectively. The image is not displayed on formation portions of the seal layers  520  and  530 . 
     Thus, portions between the seal layers  520  and  530  of the two adjacent plasma display panels may be called the seam portions  140  and  150 . 
     Although (b) of  FIG. 5  shows that a space between the two adjacent plasma display panels is empty, an attaching layer or a buffer layer may be further positioned in the space. 
     Further, although  FIG. 5  shows the multi plasma display device  10  is comprised of the four plasma display panels  100  to  130 , the multi plasma display device  10  may be comprised of two plasma display panels. For example, as shown in  FIG. 6 , when the multi plasma display device  10  is comprised of two plasma display panels  600  and  610 , a seam portion  620  is positioned in a space between the two plasma display panels  600  and  610  and an optical sheet  630  is positioned on the seam portion  620 . 
     The optical sheets  500  and  510 , as shown in  FIG. 8 , may include a plurality of protrusions  501  and  502  on the surfaces of the optical sheets  500  and  510 , respectively. 
     The plurality of protrusions  501  and  502  may refract incident light at a predetermined angle. For this, the protrusions  501  and  502  may have a triangle shape. The triangle shape of the protrusions  501  and  502  may mean that the protrusions  501  and  502  have a substantial triangle shape as well as a mathematically perfect triangle shape. 
     For example, as shown in  FIG. 8 , it is assumed that the seam portion  140 ( 150 ) having a width of W 3  is positioned under the optical sheet  500 ( 510 ). In this case, light starting from a first position P 1  of the seam portion  140 ( 150 ) travels along a first path PT 1  through the first protrusions  501 , and light starting from a second position P 2  of the seam portion  140 ( 150 ) travels along a second path PT 2  through the second protrusions  502 . Hence, the observer perceives the width of the seam portion  140 ( 150 ) as W 2  smaller than W 3 . 
     Because the first and second protrusions  501  and  502  refract light at a predetermined angle, the first and second protrusions  501  and  502  may be called prisms. 
     As shown in  FIG. 9 , the optical sheet  500 ( 510 ) may include a first overlapping portion S 1  between the optical sheet  500 ( 510 ) and a front substrate  201 A of a first panel of two adjacent plasma display panels and a second overlapping portion S 2  between the optical sheet  500 ( 510 ) and a front substrate  201 B of a second panel of the two adjacent plasma display panels. The first protrusions  501  are formed in the first overlapping portion S 1 , and the second protrusions  501  are formed in the second overlapping portion S 2 . 
     The first and second protrusions  501  and  502  may have different shapes, so that the first and second protrusions  501  and  502  refract incident light in different directions. 
     For example, as shown in  FIGS. 9 and 10 , an angle θ 10  between a first surface PUS 1  of the second protrusion  502  adjacent to the first portion S 1  and the base of the optical sheet  500 ( 510 ) may be smaller than an angle θ 20  between a second surface PUS 2  opposite the first surface PUS 1  and the base of the optical sheet  500 ( 510 ). Further, an angle θ 1  between a first surface PUS 1  of the first protrusion  501  adjacent to the second portion S 2  and the base of the optical sheet  500 ( 510 ) may be smaller than an angle θ 2  between a second surface PUS 2  opposite the first surface PUS 1  and the base of the optical sheet  500 ( 510 ). 
     In the embodiment, the angles θ 2  and θ 20  may be substantially equal to each other, and the angles θ 1  and θ 10  may be substantially equal to each other. A maximum difference between the angles θ 2  and θ 20  may be 4° and a maximum difference between the angles θ 1  and θ 10  may be 10° in consideration of an error in a manufacturing of the optical sheet. 
     When the angles θ 1  and θ 10  each have an excessively small value, a reduction effect in the visible size of the seam portions may be greatly reduced. Further, when the angles θ 1  and θ 10  each have an excessively large value, the observer may perceive the seam portion or the image through the first surface PUS 1  of the protrusion when the observer observes the multi plasma display device at the side of the multi plasma display device. In other words, when the observer observes the multi plasma display device at the side of the multi plasma display device, the observer may look a striped pattern resulting from the optical sheet. Considering this, the angles θ 1  and θ 10  may be approximately 25° to 35°. 
     When the angles θ 2  and θ 20  each have an excessively small value, it is difficult to form the protrusions. Further, when the angles θ 2  and θ 20  each have an excessively large value, an image may run on the screen. Considering this, the angles θ 2  and θ 20  may be approximately 88° to 92°. 
     If the angles θ 1  and θ 10  are equal to each other and the angles θ 2  and θ 20  are equal to each other, the first and second protrusions  501  and  502  may be symmetric with respect to a Y-axis when a straight line perpendicular to the optical sheets  500  and  510  is called the Y-axis. In other words, the first and second protrusions  501  and  502  may be arranged in opposite directions. 
     When a width W 10  of each protrusion has an excessively large value, the slight optical effect is obtained and it is difficult to form the first and second protrusions  501  and  502 . Hence, the width W 10  of each protrusion may be equal to or less than approximately 100 μm. 
     Because an outermost first protrusion SOIL and an outermost second protrusion  502 L face each other in a portion where the first and second protrusions  501  and  502  are adjacent to each other, a distance D 1  between a top of the outermost first protrusion SOIL and a top of the outermost second protrusion  502 L may be greater than a distance D 2  between tops of two adjacent first protrusions  501  and a distance D 3  between tops of two adjacent second protrusions  502 . 
     A thickness and a width of the optical sheet are described below. 
     As shown in  FIG. 11 , a thickness T of the optical sheet  500 ( 510 ) may be smaller than a width W 1  of the optical sheet  500 ( 510 ). 
       FIG. 12  is a graph illustrating a width of the seam portion and a striped pattern of the optical sheet depending on a ratio W 1 /T of the width W 1  to the thickness T of the optical sheet  500 ( 510 ). 
     When the ratio W 1 /T of the width W 1  to the thickness T of the optical sheet  500 ( 510 ) changes from 10:1 to 10:10, many observers observed and evaluated changes in the width of the seam portion in the front of the multi plasma display device  10  (for example, a position “A” in  FIG. 13 ). 
     Further, when the ratio W 1 /T of the width W 1  to the thickness T of the optical sheet  500 ( 510 ) changes from 10:1 to 10:10, the many observers observed and evaluated the generation of the striped pattern of the optical sheet  500 ( 510 ) at a position moving from the front to the side of the multi plasma display device  10  by 60° (for example, a position “B” in  FIG. 13 ). 
     In  FIG. 12 , X, ◯, and ⊚ represent bad, good, and excellent states of the characteristics, respectively. 
     As shown in  FIG. 12 , when the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) is 10:2 to 10:10, the state of the width of the seam portion was excellent. In other words, as the thickness of the optical sheet  500 ( 510 ) increases, the width of the seam portion the observers felt becomes smaller. 
     For example, as shown in (a) of  FIG. 14 , if the thickness of the optical sheet  500 ( 510 ) is T 1 , the width of the seam portion the observer feels may be B 1 . Further, as shown in (b) of  FIG. 14 , if the thickness of the optical sheet  500 ( 510 ) is T 2  greater than T 1 , the width of the seam portion the observe feels may be B 2  smaller than B 1  because a travel distance of light inside the optical sheet  500 ( 510 ) is longer than a travel distance of light in (a) of  FIG. 14 . 
     When the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) is 10:1, the state of the width of the seam portion was good. 
     Further, when the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) is 10:1 to 10:6, the generation state of the striped pattern of the optical sheet  500 ( 510 ) was excellent. In other words, when the thickness T of the optical sheet  500 ( 510 ) has a sufficiently small value, it is difficult for the observer to perceive the striped pattern resulting from the optical sheet  500 ( 510 ) even if the observer observes the optical sheet  500 ( 510 ) at a position moving from the front to the side of the multi plasma display device  10  by 60°. 
     On the other hand, when the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) is 10:9 to 10:10, the generation state of the striped pattern of the optical sheet  500 ( 510 ) was bad. 
     For example, as shown in  FIG. 15 , if the thickness of the optical sheet  500 ( 510 ) is T 3  and is excessively greater than the width W 1  of the optical sheet  500 ( 510 ), light incident on the optical sheet  500 ( 510 ) at an angle of 60° at a position “B” may be transmitted from one side to the other side of the optical sheet  500 ( 510 ). Hence, the observer at the position “B” may perceive that an image is displayed on the one side of the optical sheet  500 ( 510 ). In other words, the observer may perceive that the striped pattern appears in the side of the optical sheet  500 ( 510 ). In this case, the quality of an image displayed by the multi plasma display device  10  is reduced. 
     When the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) is 10:7 to 10:8, the generation state of the striped pattern of the optical sheet  500 ( 510 ) was good. In this case, only some observers may perceive the striped pattern appears in the side of the optical sheet  500 ( 510 ). 
     Considering the description of  FIG. 12 , the width to thickness ratio W 1 /T of the optical sheet  500 ( 510 ) may be 10:1 to 10:8, and preferably, 10:2 to 10:6. 
     As shown in  FIG. 16 , the optical sheet  500 ( 510 ) may overlap seal layers  520  and  530  of two plasma display panels adjacent to the optical sheet  500 ( 510 ). In  FIG. 16 , the seal layer  520  is called a first seal layer, and the seal layer  530  is called a second seal layer. Further, it is assumed that first and second plasma display panels include the first and second seal layers  520  and  520 , respectively. 
     A width W 1  of the optical sheet  500 ( 510 ) may be greater than a distance L 1  between an end adjacent to a barrier rib  212 A of the first panel among both ends of the first seal layer  520  and an end adjacent to a barrier rib  212 B of the second panel among both ends of the second seal layer  530 . Further, the width W 1  of the optical sheet  500 ( 510 ) may be smaller than a distance L 2  between an outermost barrier rib  212 A of the first panel and an outermost barrier rib  212 B of the second panel. Thus, the optical sheet  500 ( 510 ) may extend further than the first seal layer  520  by a length E 1  in a middle direction of the first panel and may extend further than the second seal layer  530  by a length E 2  in a middle direction of the second panel. 
     Further, while the optical sheet  500 ( 510 ) overlaps the first seal layer  520  of the first panel, the optical sheet  500 ( 510 ) may not overlap the discharge cell of the first panel. Preferably, while the optical sheet  500 ( 510 ) overlaps the first seal layer  520  of the first panel, the optical sheet  500 ( 510 ) may not overlap the phosphor layer formed in the discharge cell of the first panel. In addition, while the optical sheet  500 ( 510 ) overlaps the second seal layer  530  of the second panel, the optical sheet  500 ( 510 ) may not overlap the discharge cell of the second panel. 
     For this, one end EDGE 1  of the optical sheet  500 ( 510 ) may be positioned between the outermost barrier rib  212 A and the first seal layer  520  of the first panel, and the other end EDGE 2  of the optical sheet  500 ( 510 ) may be positioned between the outermost barrier rib  212 B and the second seal layer  530  of the second panel. In this case, the size of a boundary portion between the first panel and the second panel may be visually reduced while a distortion of the image in the boundary portion between the first panel and the second panel is suppressed. 
     Alternatively, as shown in  FIG. 17 , while the optical sheet  500 ( 510 ) overlaps the first and second seal layers  520  and  530 , the optical sheet  500 ( 510 ) may overlap at least one of an outermost barrier rib  212 A of the first panel and an outermost barrier rib  212 B of the second panel. Preferably, one end of the optical sheet  500 ( 510 ) may be positioned in a portion overlapping the outermost barrier rib  212 A of the first panel, and the other end of the optical sheet  500 ( 510 ) may be positioned in a portion overlapping the outermost barrier rib  212 B of the second panel. In this case, the width W 1  of the optical sheet  500 ( 510 ) may be greater than a distance L 2  between the outermost barrier rib  212 A of the first panel and the outermost barrier rib  212 B of the second panel. 
     Alternatively, as shown in  FIG. 18 , while the optical sheet  500 ( 510 ) overlaps the first and second seal layers  520  and  530 , the width W 1  of the optical sheet  500 ( 510 ) may be greater than a distance L 3  between the first and second seal layers  520  and  530 . In this case, the width W 1  of the optical sheet  500 ( 510 ) may be smaller than a distance L 1  between an end adjacent to the barrier rib  212 A of the first panel among both ends of the first seal layer  520  and an end adjacent to the barrier rib  212 B of the second panel among both ends of the second seal layer  530 . Thus, the first seal layer  520  may extend further than the optical sheet  500 ( 510 ) by a length E 3  in a middle direction of the first panel, and the second seal layer  530  may extend further than the optical sheet  500 ( 510 ) by a length E 4  in a middle direction of the second panel. 
     Considering the descriptions of  FIGS. 16 to 18 , a boundary portion between the first panel and the second panel may mean a portion between the first seal layer  520  and the second seal layer  530 , and the width W 1  of the optical sheet  500 ( 510 ) may be greater than a length of the boundary portion between the first panel and the second panel. 
     As shown in  FIG. 19 , the multi plasma display device  10  may include the first panel  100 , the second panel  110  positioned adjacent to the first panel  100 , the third panel  120  positioned adjacent to the first panel  100 , and the fourth panel  130  positioned adjacent to the second and third panels  110  and  120 . Further, the first optical sheet  500  may be positioned on the first seam portion  140  between the first and second panels  100  and  110  and between the third and fourth panels  120  and  130 , and the second optical sheet  510  may be positioned on the second seam portion  150  between the first and third panels  100  and  120  and between the second and fourth panels  110  and  130 . 
     Further, at least one of the first and second optical sheets  500  and  510  may be divided in a common boundary portion of the first to fourth panels  100  to  130 . For example, as shown in  FIG. 19 , the second optical sheet  510  may be divided into a 2-1 optical sheet  510 A and a 2-2 optical sheet  510 B with the first optical sheet  500  interposed between the 2-1 optical sheet  510 A and the 2-2 optical sheet  510 B. In this case, after the first optical sheet  500  is positioned, the 2-1 optical sheet  510 A and the 2-2 optical sheet  510 B may be sequentially positioned. 
     Alternatively, as shown in  FIG. 20 , the first optical sheet  500  and the second optical sheet  510  may overlap each other in a common boundary portion of the first to fourth panels  100  to  130 . In this case, a formation process of the first and second optical sheets  500  and  510  may be simplified. 
     Alternatively, as shown in  FIG. 21 , while the first optical sheet  500  and the second optical sheet  510  overlap each other, at least one of the first and second optical sheets  500  and  510  may have a groove in an overlapping portion between the first and second optical sheets  500  and  510 . For example, as shown in  FIG. 21 , the first optical sheet  500  has a first groove  501 , the second optical sheet  510  has a second groove  511 , and the first groove  501  of the first optical sheet  500  and the second groove  511  of the second optical sheet  510  may be engaged with each other. In this case, an overlapping portion between the first optical sheet  500  and the second optical sheet  510  may be prevented from being excessively thick. 
     Alternatively, as shown in  FIG. 22 , the multi plasma display device may include a first filter  2220  on the front substrate  201 A of the first panel and a second filter  2230  on the front substrate  201 B of the second panel. The first filter  2220  and the second filter  2230  may be film filters. Although it is not shown, each of the first filter  2220  and the second filter  2230  may include an electromagnetic shielding layer for reducing electromagnetic interference. The electromagnetic shielding layer may be formed of a metal material. 
     Further, the multi plasma display device  10  may include first and second auxiliary frames  2200  and  2210  for grounding the electromagnetic shielding layers of the first filter  2220  and the second filter  2230 . The first and second auxiliary frames  2200  and  2210  may be formed of a metal material with excellent electrical conductivity, for example, aluminum (A 1 ). The first auxiliary frame  2200  may be positioned at the side of the first panel, and the second auxiliary frame  2210  may be positioned at the side of the second panel. 
     In addition, one end of the first auxiliary frame  2200  may be connected to the electromagnetic shielding layer of the first filter  2220 , and the other end of the first auxiliary frame  2200  may be connected to a main frame positioned in the rear of a rear substrate  211 A although it is not shown. One end of the second auxiliary frame  2210  may be connected to the electromagnetic shielding layer of the second filter  2230 , and the other end of the second auxiliary frame  2210  may be connected to a main frame positioned in the rear of a rear substrate  211 B although it is not shown. 
     Thus, the electromagnetic shielding layer of the first filter  2220  may be grounded by the first auxiliary frame  2200 , and the second filter  2230  of the second filter  2230  may be grounded by the second auxiliary frame  2210 . 
     In such a structure, the optical sheet  500 ( 510 ) may be positioned on the first and second auxiliary frames  2200  and  2210 , so that the optical sheet  500 ( 510 ) commonly overlaps the first and second auxiliary frames  2200  and  2210 . In this case, the width W 1  of the optical sheet  500 ( 510 ) may be greater than a distance L 4  between a connection portion between the first auxiliary frame  2200  and the first filter  2220  and a connection portion between the second auxiliary frame  2210  and the second filter  2230 . 
     Alternatively, as shown in  FIG. 23 , a black layer  2300  may be positioned in a boundary portion between the first panel and the second panel to commonly overlap the front substrate  201 A of the first panel and the front substrate  201 B of the second panel. In this case, the black layer  2300  may improve contrast characteristic of the multi plasma display device  10 . Further, the width W 1  of the optical sheet  500 ( 510 ) may be greater than a width L 5  of the black layer  2300 . 
     Alternatively, as shown in  FIG. 24 , a black layer  2300  may be positioned on the first and second auxiliary frames  2200  and  2210 , and the optical sheet  500 ( 510 ) may be positioned on the black layer  2300 . The black layer  2300  may be formed of a material with excellent electrical conductivity. In this case, the black layer  2300  may allow the electromagnetic shielding layer of the first filter  2220  to be more efficiently connected to the first auxiliary frame  2200  and may allow the electromagnetic shielding layer of the second filter  2230  to be more efficiently connected to the second auxiliary frame  2210 . 
       FIGS. 25 and 26  illustrate a method of manufacturing the multi plasma display device according to the embodiment of the invention. 
     As shown in (a) of  FIG. 25 , a seal layer  520 ( 530 ) may be formed at an edge of at least one of a front substrate  201  and a rear substrate  211  on which an exhaust hole  200  is formed. Thus, as shown in (b) of  FIG. 25 , the front substrate  201  and the rear substrate  211  may be attached to each other through the seal layer  520 ( 530 ). 
     Subsequently, an exhaust tip (not shown) may be connected to the exhaust hole  200 , and an exhaust pump (not shown) may be connected to the exhaust tip. The exhaust pump may exhaust an impurity gas remaining in a discharge space between the front substrate  201  and the rear substrate  211  to the outside and may inject a discharge gas, such as argon (Ar), neon (Ne), and xenon (Xe), into the discharge space. The discharge space between the front substrate  201  and the rear substrate  211  may be sealed through the above-described method. 
     Subsequently, as shown in (a) of  FIG. 26 , at least one of the front substrate  201  and the rear substrate  211  may be cut along predetermined cutting lines CL 1  and CL 2  and may be ground in a state where the front substrate  201  and the rear substrate  211  are attached to each other. In this case, the seal layer  520  ( 530 ) may be cut and ground together with the at least one substrate. 
     As a result, as shown in (b) of  FIG. 26 , at least one of the front substrate  201  and the rear substrate  211  may be prevented from excessively protruding in a cutting and grinding portion. Further, the size of a portion SA on which an image is not displayed may be reduced. Even if a plurality of plasma display panels are successively positioned, the size of the seam portion may be prevented from excessively increasing. 
     As described above, because the size of the seam portion is reduced by reducing a length of at least one of the front substrate  201  and the rear substrate  211  through cutting and grinding processes in a state where the front substrate  201  and the rear substrate  211  are attached to each other using the seal layer  520 ( 530 ), it may be preferable that the plasma display panel is used as the multi plasma display device compared with other display panels. 
       FIGS. 27 to 31  illustrate another configuration of a multi plasma display device according to an embodiment of the invention. Structures and components identical or equivalent to those illustrated above are designated with the same reference numerals, and a further description may be briefly made or may be entirely omitted. 
     AS shown in  FIG. 27 , a multi plasma display device according to an embodiment of the invention may include a first main frame  2700  positioned in the rear of a first panel  100  (i.e., in the rear of a rear substrate of the first panel  100 ), a second main frame  2710  positioned in the rear of a second panel  110  (i.e., in the rear of a rear substrate of the second panel  110 ), a third main frame  2720  positioned in the rear of a third panel  120  (i.e., in the rear of a rear substrate of the third panel  120 ), and a fourth main frame  2730  positioned in the rear of a fourth panel  130  (i.e., in the rear of a rear substrate of the fourth panel  130 ). A driving board may be positioned in each of the first to fourth main frames  2700  to  2730  to supply a driving signal to each of the first to fourth panels  100  to  130 . 
     In the multi plasma display device according to the embodiment of the invention, at least one of a plurality of adjacent plasma display panels may include a dummy discharge cell in a dummy area. 
     For example, as shown in  FIG. 28 , each of adjacent first and second panels may include a dummy discharge cell DMC in a dummy area DA. The dummy discharge cell DMC indicates a discharge cell in which an image is achieved. Phosphor layers  214 A and  214 B may not be formed in the dummy discharge cells DMC of the first and second panels as shown in  FIG. 28 . Alternatively, the phosphor layers  214 A and  214 B may be formed in the dummy discharge cells DMC of the first and second panels. A data signal may not be supplied to the dummy discharge cells DMC. Hence, even if the phosphor layers  214 A and  214 B are formed in the dummy discharge cells DMC, a discharge may not occur in the dummy discharge cells DMC. In other words, the dummy discharge cells DMC may allow a discharge to more stably occur in an outermost active discharge cell. 
     An optical sheet  500 ( 510 ) may overlap at least one discharge cell of each of adjacent plasma display panels. Preferably, the optical sheet  500 ( 510 ) may overlap seal layers  520  and  530  of adjacent first and second panels and a dummy discharge cell DMC in a dummy area DA of each of the adjacent first and second panels. The optical sheet  500 ( 510 ) may not overlap an active discharge cell ACC in an active area AA inside the dummy area DA. 
     For example, one end of the optical sheet  500 ( 510 ) may be positioned in a portion overlapping a barrier rib  212 A of an outermost discharge cell in an active area AA of the first panel, and the other end of the optical sheet  500 ( 510 ) may be positioned in a portion overlapping a barrier rib  212 B of an outermost discharge cell in an active area AA of the second panel. 
     As shown in  FIGS. 29 and 30 , the optical sheet  500 ( 510 ) may include a plurality of depressions  2900  and  2910 . The optical sheet  500 ( 510 ) shown in  FIGS. 29 and 30  has a prism of depression form, compared with the optical sheet  500 ( 510 ) having a prism of protruding form shown in  FIG. 8 . 
     Even in the optical sheet  500 ( 510 ) shown in  FIGS. 29 and 30 , a boundary portion of at least two adjacent plasma display panels seem to be visually smaller than an actual size of the boundary portion. 
     In  FIGS. 29 and 30 , the first depression  2900  may correspond to the first protrusion  501  of  FIG. 8 , and the second depression  2910  may correspond to the second protrusion  502  of  FIG. 8 . 
     In the optical sheet  500 ( 510 ) shown in  FIGS. 29 and 30 , a formation portion of the depressions  2900  and  2910  may be positioned toward a seam portion. 
     Alternatively, as shown in (a) of  FIG. 31 , the optical sheet  500 ( 510 ) may include a first layer  3100  and a second layer  3110  having a stack structure. The first layer  3100  may include a first prism whose a surface is depressed, and the second layer  3110  may include a second prism whose a surface protrudes. The first layer  3100  and the second layer  3110  may be stacked, so that the first prism and the second prism engage each other. 
     Even in the optical sheet  500 ( 510 ) shown in  FIG. 31 , a boundary portion of at least two adjacent plasma display panels seem to be visually smaller than an actual size of the boundary portion. 
     For this, as shown in (b) of  FIG. 31 , the second layer  3110  may be positioned in a boundary portion between the first and second panels, and the first layer  3100  may be positioned on the second layer  3110 . Further, a refractive index of the first layer  3100  may be less than a refractive index of the second layer  3110 . 
       FIGS. 32 to 34  illustrate another configuration of a black layer. 
     As shown in  FIG. 32 , a black layer positioned in a boundary portion between a plurality of panels may be positioned at the side of at least one panel of the plurality of panels. For example, a first black layer  3200 A may be positioned at the side of a first panel, and a second black layer  3200 B may be positioned at the side of a second panel. The black layers  3200 A and  3200 B are attached to the sides of the first and second panels in form of sheet, respectively. 
     The fact that the black layers  3200 A and  3200 B are positioned at the side of the panel may indicate the black layers  3200 A and  3200 B are positioned at the sides of front substrates  201 A and  201 B, at the sides of rear substrates  211 A and  211 B, and at the sides of seal layers  520  and  530 . Hence, light may be prevented from being reflected from the sides of the front substrates  201 A and  201 B, the sides of the rear substrates  211 A and  211 B, and the sides of the seal layers  520  and  530 . As a result, the image quality may be improved. 
     Further, the black layers  3200 A and  3200 B may contain an electrically conductive material. In this case, the electromagnetic shielding layer on the front surface of the panel may be electrically connected to the main frame on the rear surface of the panel. In other words, the black layers  3200 A and  3200 B may ground the electromagnetic shielding layer. 
     Alternatively, as shown in  FIG. 33 , a width of a black layer  3200  may be smaller than a thickness of the panel. Hence, one end of the black layer  3200  may be positioned at the side of the front substrate  201 , and the other end of the black layer  3200  may be positioned at the side of the rear substrate  211 . In this case, a conductive layer may be positioned on the black layer  3200 . For example, as shown in  FIG. 34 , a first black layer  3200 A may be positioned at the side of a first panel, and a first conductive layer  3400 A may be positioned on the first black layer  3200 A. Further, a second black layer  3200 B may be positioned at the side of a second panel, and a second conductive layer  3400 B may be positioned on the second black layer  3200 B. 
     In this case, the first conductive layer  3400 A may electrically connect an electromagnetic shielding layer (not shown) on a front surface of the first panel to a main frame (not shown) on a rear surface of the first panel, and the second conductive layer  3400 B may electrically connect an electromagnetic shielding layer (not shown) on a front surface of the second panel to a main frame (not shown) on a rear surface of the second panel. In other words, the conductive layers  3400 A and  3400 B may ground the electromagnetic shielding layers. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.