Patent Publication Number: US-6335591-B1

Title: Luminescence display panel using discharge gas

Description:
FIELD OF THE INVENTION 
     The present invention relates to a luminescence display panel using discharge gas for use with television and computer systems, for example. 
     BACKGROUND OF THE INVENTION 
     FIG. 6 illustrates a conventional luminescence display panel using discharge gas, generally indicated by reference numeral  20 , for use preferably with a plasma display system. The panel  20 , including transparent front and rear plates,  21  and  22 , spaced apart from each other, is depicted so that one surface  23  of the front plate, away from the rear plate  22 , is oriented upward. 
     Also, FIG. 7 illustrates an enlarged partial schematic view of an AC plasma display panel, which is an example of such luminescent display panels. As shown, the panel  20  includes a discharge chamber  3  between the front and rear plates,  21  and  22 . The other surface of the front plate  21 , opposing the rear plate  22 , supports a plurality of pairs of elongated scanning and maintaining electrodes,  6  and  7 , extending in a parallel fashion. The electrodes,  6  and  7 , are covered with a dielectric layer  4  and further with a protection layer  5 , positioned away from the front plate  21 . The rear plate  22  supports a plurality of elongated data electrodes  8  each extending perpendicular to the scanning and maintaining electrodes,  6  and  7 , in a parallel fashion. Also supported on the rear plate  22  are a plurality of elongated partitions  9  each extending in parallel to and spaced a certain distance from the data electrodes  8 , so that the discharge chamber  3  is formed between neighboring partitions  9 . A phosphor  10  is provided between the neighboring partitions  9  so that it covers both data electrode  8  and opposing side surfaces of the partitions  9  in the discharge chamber  3 . For clarity of the drawing, the phosphor  10  is illustrated only in part. Each discharge chamber  3  is filled with a gas mixture having xenon and at least one inert, as such as helium, neon, or argon. 
     In operation, an electric discharge is generated between the scanning and maintaining electrodes,  6  and  7 , in the discharge chamber  3 . This excites the phosphor  10  to emit visible light, which is used for displaying an image to be viewed on the front plate  21 . 
     Referring to FIG. 8, which is a cross-sectional view taken along lines VIII—VIII in FIG. 7, descriptions will be made to the light emission. As shown, three neighboring phosphors  10  construct different color elements of each pixel, red light element  10 R, green light element  10 G and blue light element  10 B for emitting red, green, and blue lights, respectively. 
     When the electric discharge  1  has occurred in the discharge chamber  3 , the ultraviolet light  2  generated by the discharge  1  excites the phosphor  10 . This allows the color elements  10 R,  10 G, and  10 B to emit red, green, and blue light, respectively, as shown by dotted lines in FIG.  8 . It should be understood that the light passes are provided by dotted lines in FIG. 8, as well as in other drawings, only for illustration. 
     FIG. 9, which is also a cross-sectional view taken along lines VIII—VIII in FIG. 7, illustrates passes of red light emitted only from the color element  10 R. In this instance, the emitted red light RO and R 1  passes through the front plate  21  and then projects out toward a viewer. Simultaneously, the red light R 1  projected obliquely to the front plate  21  is in part reflected at an inner surface  21   a  of the front plate  21  and then at the neighboring green element  10 G. The reflected red light is then projected in part through the front plate  21 , which is shown at R 2 . Remaining red light is reflected at the surface  21  a of the front plate  21  and then at the neighboring blue element  10 B and, afterwards, projected through the front plate  21 . Likewise, the light emitted obliquely is transmitted transversely and reflected on the green and blue elements,  10 G and  10 B, which results in an undesirable halation of the red light being projected through the front plate  21  to the viewer. 
     Also, the red light emitted from the back surface of the red element  10 R is reflected at the inner surface of the rear plate  22  and then transmitted in part through the neighboring green element  10 G, which is finally projected through the front plate  21  as shown at R 4 . Further, the red light reflected at the green element  10 G is further reflected at the surfaces  22   b  and then  22   a  of the backing and then transmitted through the blue element  10 B, which is finally projected through the front plate  21  as shown at R 5 . As such, another undesirable halation of the red light is projected through the front plate  21  to the viewer. This results in a degradation of a color contrast of the plasma display panel. 
     In addition, as shown in FIG. 10, which is also a cross-sectional view of taken along lines VIII—VIII in FIG. 7, when the red and green elements,  10 R and  10 G, simultaneously emit respective lights, the green lights GO and G 1  are merged with the red light halation, R 2  and R 4 , which degrades purity of the green color. At this moment, the red light is also merged with the green light, which also results in a degradation of the red color. 
     The halation can be evaluated. For example, as shown in FIG. 11A, in the evaluation, all the color elements on the left side of the AC plasma display panel are turned on to present a white image and, on the other hand, all the color elements on the right side are turned off to present a black image. Then, measured is a variation of brightness in a boundary zone between the left turned-on and right turned-off regions. When no halation is assumed to occur, the left side region (L&lt;0) would provide 100% brightness in white and the right side region 0% brightness in black (L&gt;0). Contrary to this, practically, as shown in FIG. 11B, although in the conventional AC plasma display 100% brightness is obtained in the left side turned-on region (L&lt;0), the brightness in the right side turned-off region decreases gradually from the boundary line and then 0% brightness in black is obtained at a portion spaced a certain distance P away from the boundary line. Also, the distance P in which the brightness decreases from 100% to 0% is significantly large in the conventional AC plasma display panel. Therefore, the boundary line between the white and black regions is unclear due to the halation. This in turn deteriorates a color contrast purity of each color. 
     SUMMARY OF THE INVENTION 
     To overcome this problem, a luminescence display panel using discharge gas of the present invention includes dark front and rear plates. Preferably, darkness of each plate is equal to at least about 20%. Instead, transparency of each plate may be equal to at most about 80%. This allows the halation to be decreased considerably in the luminescence display panel of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of an AC plasma display panel according to the present invention; 
     FIG. 2 is an enlarged cross-sectional view of the AC plasma display panel in FIG. 1, illustrating passes of light emitted from a red color luminescent element; 
     FIG. 3 is an enlarged cross-sectional view of the AC plasma display panel in FIG. 1, illustrating red and green light simultaneously emitted from red and green color luminescent elements, respectively; 
     FIG. 4 is a graph illustrating a relationship of distance versus brightness for describing an effect of halation; 
     FIG. 5 is a graph illustrating a relationship of darkness or transparency versus halation ratio; 
     FIG. 6 is a perspective view of a conventional gas discharge display panel; 
     FIG. 7 is a partial enlarged perspective view of a conventional AC plasma display panel; 
     FIG. 8 is a cross-sectional view taken along lines VIII—VIII in FIG. 7 for illustrating a light emitting mechanism for display; 
     FIG. 9 is also a cross-sectional view taken along lines VIII—VIII in FIG. 7 in which only a red color element is energized to emit red light; 
     FIG. 10 is also a cross-sectional view taken along lines VIII—VIII in FIG. 7 in which red and green color elements are energized to emit red and green light, respectively; 
     FIG. 11A is a front view of the plasma display panel in which a left side portion of the panel is turned on and a right side portion is turned off; and 
     FIG. 11B is a graph showing a halation or a relationship of distance versus brightness in a boundary zone of left and right sides shown in FIG.  11 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown an AC plasma display panel, generally indicated by reference numeral  30 , which is an embodiment of a luminescence display panel using discharge gas of the present invention. The AC plasma display panel  30  illustrated with its display surface  33  oriented upward includes a front plate  31  and a rear plate  32 , both of which are in dark color. The plates,  31  and  32 , are spaced apart from each other, forming a discharge chamber  3  between them. The AC plasma display panel  30  is similar to the above-described conventional AC plasma display panel  20  with an exception that the surface and rear plates,  31  and  32 , have dark color. Therefore, in the following description, like reference numerals are provided for like elements and a detailed structural description of such elements is eliminated therefrom. Likewise, a physical structure of the display, except for darkness of the plates, is also identical to that described above and, therefore, no further description is made to this in the following description. 
     FIG. 2, which is similar to FIG. 9 described above, is a cross-sectional view of the AC plasma display panel  30 , illustrating light emitted only from a red luminescent element  10 R. As shown, light R 0  and R 1  emitted form the red luminescent element  10 R presents a fluorescence of red color to human beings. Most of the obliquely emitted light R 0  is projected through the front plate  31 , though; it is reflected in part at a surface portion  31  a of the front plate  31 . The reflected light is then transmitted to a surface of a green element  10 G. The transmitted light is considerably weak since it has been reduced considerably due to the darkness of the front plate  31 . Then, the reduced light is reflected at the green element  10 G and then transmitted to the front plate  31  again where it is further reduced considerably, allowing no or only a small part of the reduced light to pass through the front plate  31 . Further, the reduced light may be reflected again at the front plate  31 , then transmitted to a blue element  10 B, and finally transmitted again to the front plate  31 ; however, the transmitted light is so weak that no light can be projected through the front plate  31 . 
     Another red light emitted from the back surface of the red element  10 R adjacent to the rear plate  32  is reflected at a surface  32   a  of the rear plate  32  and then transmitted to the green element  10 G. In this instance, the light transmitted to the green element  10 G has already been reduced much due to the darkness of the rear plate  32 . In addition, the reduced light may be transmitted through the green element  10 G and then to front plate  31  where it is again reduced considerably, allowing no or only a part of the reduced light to project through the front plate  31 . Therefore, no reduced light will be transmitted to the neighboring blue element  10 B. As such, when the red luminescent element  10 R is energized, only red light such as R 0  and R 1  will be projected through the front plate  31 , which ensures that no halation is generated due to the possible light reflection at the green and blue elements,  10 G and  10 B. 
     As shown in FIG. 3, in the event that the red and green luminescent elements are energized at the same time, the light emitted from the green element  10 G will induce no color mixing with the light emitted from the red element  10 R, due to the darkness of the front and rear plates,  31  and  32 , and thus results in considerable reduction of red light halation, which ensures the purity of the green color. Likewise, although not shown, no halation of green light is projected through the front plate  31 , which prevents the green light from being mixed with the red light. This in turn ensures that the red light, R 0  and R 1 , from the red element  10 R is viewed clearly by the viewer without any color mixing. 
     Experiments were made to evaluate a halation using, an AC plasma display panel of 42 inches, having a resolution of 640 by 480 pixels, with each pixel having a size of 1.08 mm by 1.08 mm. Two plates were prepared for the front and rear plates, one plate P(d 8 -t 92 ) having darkness of 8% (transparency of 92%) and the other plate P(d 30 -t 7 O) having darkness of 30% (transparency of 70%). As is known in the art, the darkness and transparency were defined by the following equations (1) and (2), respectively: 
     
       
         D=100 (LT / LP) (%)  (1) 
       
     
     D: Darkness 
     LT: Quantity of light transmitted through transparent glass plate 
     LP: Quantity of light projected from light source 
     
       
         T=100−D (%)  (2) 
       
     
     T: Transparency 
     Three display-panels were fabricated using the plates. Combinations of the plates are shown in the following table: 
     
       
         
           
               
             
               
                 TABLE 
               
             
            
               
                   
               
               
                 Combination of Plates 
               
            
           
           
               
               
               
            
               
                 Plate Combination 
                 Front plate 
                 Rear plate 
               
               
                   
               
               
                 A 
                 P(d8-t92) 
                 P(d8-t92) 
               
               
                 B 
                 P(d30-t70) 
                 P(d8-t92) 
               
               
                 C 
                 P(d30-t70) 
                 P(d30-t70) 
               
               
                   
               
            
           
         
       
     
     For each panel, as shown in FIG. 11A, the left side light elements were tuned on and the right side light elements were tuned off. In this state, a variation of the brightness was measured in the boundary zone indicated by a dotted-circle indicated at C in FIG.  11 B. 
     The test result is illustrated in FIG  4 . This shows that the brightness decreases exponentially as the distance L increases, which means that the darkness is effective for reducing the adverse affect of the halation. Specifically, the plate combination A, in which the plate having the darkness of 8% (transparency of 92%) is used for both the front and rear plates, still presents much halation. In the plate combination B, halation is further decreased relative to the plate combination A but it is still strong. Contrary to this, the plate combination C decreases the halation considerably. This means that it is more effective for decreasing halation considerably to use plates having greater darkness for both the front and rear plates, than to use one plate having greater darkness for either of the front and rear plates and another plate having less darkness for the remaining plate. 
     FIG. 5 shows the relationship between darkness (transparency) of the front and rear plates versus halation ratio. The halation ratio was defined by a ratio of brightness measured at a point indicated by Q in FIG. 11B, a certain distance away from the boundary line, relative to a reference brightness obtained at the same point in the plate combination A in which the plate having darkness of 8% (transparency of 92%) was used for both the front and rear plates. The result shows that, when the darkness (transparency) of both the front and rear plates was more than 20% (less than 80%), the halation ratio was 0.2 or less, in which condition no halation was recognized. This means that, by using a plate having darkness of more than 20% for both the front and rear plates, the halation is reduced so that the viewer will not recognize the halation. It should be noted that excessive increase in darkness of the plates,  31  and  32 , will reduce brightness of the panel and therefore great care should be taken with respect to the brightness of the panel when deciding the darkness of the plates. It can be said that, in considering the brightness of the panel, the darkness of the front and rear plates should be less than about 60%. 
     Although in the previous experiments the suitable result was obtained when the front and rear plates have the same darkness of 30%, this is not restrictive to the present invention and the front and rear plates having different darknesses may be employed. It should be noted, however, using the same plate for both the front and rear plates is more economical in manufacturing the plates. 
     Although the description has been made to the AC plasma display panel  30 , the present invention is not limited thereto and can be applied equally to other types of AC and DC plasma display panels. In this instance, the above-described advantages can be obtained to the same extent. 
     Accordingly, it is apparent from the above description that the present invention using darker front and rear plates decreases the halation considerably, and thereby prevents the mixing of color. This in turn allows the gas discharge display panel to provide a high contrast and clear images to viewers.