Patent Publication Number: US-7710036-B2

Title: Filter and plasma display device

Description:
This application claims priority from Korean Patent Application No. 10-2006-0108675 filed on Nov. 6, 2006, in the Korean Intellectual Property Office, the entirety of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     This disclosure relates to a filter and a plasma display device using the filter in which an external light shield sheet for shielding external light incident upon a plasma display panel (PDP) is disposed at a front of the PDP, so that the bright room contrast of the PDP can be improved and so that the luminance of the PDP can be uniformly maintained. 
     2. Description of the Related Art 
     Generally, plasma display panels (PDPs) display images including text and graphic images by applying a predetermined voltage to a number of electrodes installed in a discharge space to cause a gas discharge and then exciting phosphors with the aid of plasma that is generated as a result of the gas discharge. PDPs can be manufactured as large-dimension, light and thin flat displays. In addition, PDPs can provide wide vertical and horizontal viewing angles, full colors and high luminance. 
     External light incident upon a PDP may be reflected by an entire surface of the PDP due to white phosphors that are exposed on a lower substrate of the PDP. For this reason, PDPs may mistakenly recognize black images as being brighter than they actually are, thereby causing contrast degradation. 
     SUMMARY 
     In one general aspect, a display apparatus comprises a plasma display panel (PDP) having a display surface. The display device further comprises a filter having a panel side facing the display surface of the PDP and an opposing viewer side facing away from the display surface of the PDP. The filter includes an external light shield having a first base unit and first pattern units. The first pattern units have boundaries defined by intersections of the first pattern units and the first base unit. The first pattern units absorb external light from the viewer side. The first pattern units are substantially parallel to a first axis. 
     An electromagnetic interference (EMI) shield overlaps the external light shield. The EMI shield includes a second base unit, second pattern units substantially parallel to a second axis and having boundaries defined by intersections of the second pattern units and the second base unit, and third pattern units substantially parallel to a third axis and having boundaries defined by intersections of the third pattern units and the second base unit. The second and third pattern units are conductive and intersect in a mesh configuration. 
     The second axis is more aligned with the first axis relative to an alignment of the third axis with the first axis. An interior angle between the first axis and a longitudinal axis of the external light shield is 20 degrees or less. An interior angle between the second axis and a longitudinal axis of the EMI shield is within a range of 25 to 60 degrees. An interior angle between the third axis and the longitudinal axis of the EMI shield is within a range of 27.5 to 60 degrees. An interior angle between the first axis and the second axis is within a range of 20 to 60 degrees. An interior angle between the first axis and the third axis is within a range of 28 to 65 degrees. An exterior angle between the second axis and the third axis is within a range of 60 to 127.5 degrees. 
     Implementations can include one or more of the following features. For example, the interior angle between the first axis and the longitudinal axis of the external light shield can be 5 degrees or less. The interior angle between the second axis and the longitudinal axis of the EMI shield can be within a range of 30 to 55 degrees. The interior angle between the third axis and the longitudinal axis of the EMI shield can be within a range of 32.5 to 55 degrees. 
     The interior angle between the first axis and the second axis can be within a range of 40 to 50 degrees. The interior angle between the first axis and the third axis can be within a range of 40 to 50 degrees. The exterior angle between the second axis and the third axis can be within a range of 70 to 117.5 degrees. 
     In some implementations, the display apparatus further comprises black matrices disposed at the PDP. The black matrices are substantially parallel to a fourth axis. The interior angle between the first axis and the longitudinal axis of the external light shield is the same as an interior angle between the first axis and the fourth axis. 
     In another general aspect, a display apparatus comprises a plasma display panel (PDP) having a display surface. The display apparatus further comprises a filter having a panel side facing the display surface of the PDP and an opposing viewer side facing away from the display surface of the PDP. The filter includes an external light shield having a first base unit and first pattern units. The first pattern units have boundaries defined by intersections of the first pattern units and the first base unit. The first pattern units absorb external light from the viewer side and are substantially parallel to a first axis. The first axis intersects a longitudinal axis of the external light shield. 
     An electromagnetic interference (EMI) shield overlaps the external light shield. The EMI shield includes a second base unit and second pattern units. The second pattern units are conductive and substantially parallel to a second axis. The second pattern units have boundaries defined by intersections of the second pattern units and the second base unit. An interior angle between the first axis and the second axis is within a range of 40 to 50 degrees. 
     Implementations can include one or more of the following features. For example, a refractive index of the first pattern units can be higher than a refractive index of the first base unit. The boundaries of at least one of the first pattern units can define a width of a pattern top disposed toward one of the panel side and the viewer side and can define a width of a pattern bottom disposed toward the other of the panel side and the viewer side, the pattern bottom being wider than the pattern top. A distance between the pattern top and the pattern bottom can define a first pattern height, and a thickness of the external light shield can be 1.01-2.25 times greater than the first pattern height. 
     A distance between a pair of adjacent first pattern units can be 1.1 to 5 times greater than the width of the pattern bottom. A distance between the pattern top and the pattern bottom can define a first pattern height. The first pattern height can be 0.89 to 4.25 times greater than a distance between adjacent boundaries, of a pair of adjacent first pattern units, at one of the panel side and the viewer side. 
     In another general aspect, a display apparatus comprises a plasma display panel (PDP) having a display surface. The display apparatus further comprises a filter having a panel side facing the display surface of the PDP and an opposing viewer side facing away from the display surface of the PDP. The filter includes an external light shield having a first base unit and first pattern units. The first pattern units have boundaries defined by intersections of the first pattern units and the first base unit. The first pattern units absorb external light from the viewer side and are substantially parallel to a first axis. The first axis intersects a longitudinal axis of the external light shield. 
     An electromagnetic interference (EMI) shield overlaps the external light shield. The EMI shield includes a second base unit, second pattern units substantially parallel to a second axis and having boundaries defined by intersections of the second pattern units and the second base unit, and third pattern units substantially parallel to a third axis and having boundaries defined by intersections of the third pattern units and the second base unit. The second and third pattern units are conductive and intersect in a mesh configuration. The second axis is more aligned with the first axis relative to an alignment of the third axis with the first axis. An interior angle between the first axis and the second axis is within a range of 20 to 60 degrees. 
     Implementations can include one or more of the following features. For example, the interior angle between the first axis and the second axis can be within a range of 27 to 53 degrees. The interior angle between the first axis and the second axis can be within a range of 27.5 to 52.5 degrees. 
     The mesh configuration can include: an interior angle between the second axis and a longitudinal axis of the EMI shield within a range of 25 to 60 degrees, an interior angle between the third axis and the longitudinal axis of the EMI shield within a range of 27.5 to 60 degrees, and an exterior angle between the second axis and the third axis within a range of 60 to 127.5 degrees. 
     In another general aspect, a display apparatus comprises a plasma display panel (PDP) having a display surface. The display apparatus further comprises a filter having a panel side facing the display surface of the PDP and an opposing viewer side facing away from the display surface of the PDP. The filter includes an external light shield having a first base unit and first pattern units. The first pattern units have boundaries defined by intersections of the first pattern units and the first base unit. The first pattern units absorb external light from the viewer side and are substantially parallel to a first axis. The first axis intersects a longitudinal axis of the external light shield. 
     An electromagnetic interference (EMI) shield overlaps the external light shield. The EMI shield includes a second base unit, second pattern units substantially parallel to a second axis and having boundaries defined by intersections of the second pattern units and the second base unit, and third pattern units substantially parallel to a third axis and having boundaries defined by intersections of the third pattern units and the second base unit. The second and third pattern units are conductive and intersect in a mesh configuration. The second axis is more aligned with the first axis relative to an alignment of the third axis with the first axis. An interior angle between the first axis and the third axis is within a range of 28 to 65 degrees. 
     Implementations can include one or more of the following features. For example, the interior angle between the first axis and the third axis can be within a range of 33 to 58 degrees. The interior angle between the first axis and the third axis can be within a range of 40 to 50 degrees. The interior angle between the first axis and the third axis can be within a range of 30 to 62.5 degrees. The interior angle between the first axis and the third axis can be within a range of 35 to 57.5 degrees. 
     The mesh configuration can include: an interior angle between the second axis and a longitudinal axis of the EMI shield within a range of 25 to 60 degrees, an interior angle between the third axis and the longitudinal axis of the EMI shield within a range of 27.5 to 60 degrees, and an exterior angle between the second axis and the third axis within a range of 60 to 127.5 degrees. 
     In some implementations, a refractive index of the first pattern units is higher than a refractive index of the first base unit. 
     Other features and advantages will be apparent from the following description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an example plasma display panel (PDP). 
         FIG. 2  is a cross-sectional view of an example external light shield sheet. 
         FIGS. 3 through 6  are cross-sectional views of external light shield sheets and illustrate optical characteristics of external light shield sheets. 
         FIG. 7  is a cross-sectional view of example pattern units of an external light shield sheet. 
         FIGS. 8 and 9  are plan views of example pattern units of an external light shield sheet. 
         FIGS. 10A and 10B  are plan views illustrating structures of black matrices that can be formed on an upper substrate of a PDP. 
         FIGS. 11 and 12  illustrate an example electromagnetic interference (EMI) shield sheet. 
         FIGS. 13 and 14  illustrate a filter in which an EMI shield sheet and an external light shield sheet overlap each other. 
         FIG. 15  is a plan view illustrating a structure of bus electrodes that can be formed on an upper substrate of a PDP. 
         FIGS. 16 and 17  are plan views of various barrier rib structures that can be formed on a lower substrate of a PDP. 
         FIGS. 18 and 19  are cross-sectional views of external light shield sheets having pattern units with various shapes. 
         FIGS. 20 through 25  are cross-sectional views of pattern units with recessed bottoms and illustrate optical characteristics of the pattern units. 
         FIG. 26  is a cross-sectional view for explaining the relationship between a distance between a pair of adjacent pattern units of an external light shield sheet and a height of the pair of adjacent pattern units. 
         FIGS. 27 through 30  are cross-sectional views of filters. 
     
    
    
     DETAILED DESCRIPTION 
     In some implementations, a plasma display device can improve the bright room contrast and the luminance of a plasma display panel (PDP) by effectively shielding external light incident upon the PDP. In at least one implementation, the plasma display device can reduce the probability of occurrence or perception of a moire phenomenon. 
       FIG. 1  is a perspective view illustrating an implementation of a PDP. As shown in  FIG. 1 , the PDP includes an upper substrate  10  and a plurality of electrode pairs formed on the upper substrate  10 , each electrode pair including a scan electrode  11  and a sustain electrode  12 . The PDP of  FIG. 1  also includes a lower substrate  20  and a plurality of address electrodes  22  that are formed on the lower substrate  20 . 
     Each electrode pair  11  and  12  includes transparent electrodes  11   a  and  12   a  and bus electrodes  11   b  and  12   b . The transparent electrodes  11   a  and  12   a  may be made of indium-tin-oxide (ITO). The bus electrodes  11   b  and  12   b  may be made of a metal such as silver (Ag) or chromium (Cr) or may be made with a stack of chromium/copper/chromium (Cr/Cu/Cr) or a stack of chromium/aluminium/chromium (Cr/Al/Cr). The bus electrodes  11   b  and  12   b  are respectively formed on the transparent electrodes  11   a  and  12   a  and reduce a voltage drop caused by the transparent electrodes  11   a  and  12   a , which have high resistance. 
     In some implementations, each electrode pair  11  and  12  may be comprised of the bus electrodes  11   b  and  12   b  only. In this case, the manufacturing cost of the PDP can be reduced by omitting the transparent electrodes  11   a  and  12   a . The bus electrodes  11   b  and  12   b  may be formed of various materials, e.g., a photosensitive material, in addition to those described above. 
     Black matrices can be formed on the upper substrate  10 . The black matrices perform a light shied function by absorbing external light incident upon the upper substrate  10  so that light reflection can be reduced. In addition, the black matrices can enhance the purity and contrast of the upper substrate  10 . 
     In detail, the black matrices can include a first black matrix (BM)  15 , which overlaps a plurality of barrier ribs  21 , a second black matrix  11   c , which is formed between the transparent electrode  11   a  and the bus electrode  11   b  of each of the scan electrodes  11 , and a second black matrix  12   c , which is formed between the transparent electrode  12   a  and the bus electrode  12   b . The first black matrix  15  and the second black matrices  11   c  and  12   c , which can also be referred to as black layers or black electrode layers, may be formed at the same time and may be physically connected. Alternatively, the first black matrix  15  and the second black matrices  11   c  and  12   c  may not be formed at the same time and may not be physically connected. 
     If the first black matrix  15  and the second black matrices  11   c  and  12   c  are physically connected, the first black matrix  15  and the second black matrices  11   c  and  12   c  may be formed of the same material. On the other hand, if the first black matrix  15  and the second black matrices  11   c  and  12   c  are physically separated, the first black matrix  15  and the second black matrices  11   e  and  12   c  may be formed of different materials. 
     The bus electrodes  11   b  and  12   b  or the barrier ribs  21  may have a dark color and may thus serve the functions of the black matrices, e.g., a light shield function and a contrast enhancement function. Alternatively, it is possible for one or more components to operate as or to achieve results earlier attributed to the black matrices. For example, a first element (for example, the dielectric layer  13 ) on the upper substrate  10  and a second element (for example, the barrier ribs) on the lower substrate  20  may have complementary colors so that the overlapping area of the first and second elements can appear black as viewed from the front of the PDP. In this case, the overlapping area of the first and second elements may serve the functions of the black matrices. 
     An upper dielectric layer  13  and a passivation layer  14  (or a protective film) are deposited on the upper substrate  10  on which the scan electrodes  11  and the sustain electrodes  12  are formed in parallel with one other. Charged particles generated as a result of a discharge accumulate in the upper dielectric layer  13 . The upper dielectric layer  13  may protect the electrode pairs. The passivation layer  14  protects the upper dielectric layer  13  from sputtering of the charged particles and enhances the discharge of secondary electrons. 
     The address electrodes  22  intersect the scan electrodes  11  and the sustain electrodes  12 . A lower dielectric layer  24  and the barrier ribs  21  are formed on the lower substrate  20  on which the address electrodes  22  are formed. 
     A phosphor layer  23  is formed on the lower dielectric layer  24  and the barrier ribs  21 . The barrier ribs  21  include a plurality of vertical barrier ribs  21   a  and a plurality of horizontal barrier ribs  21   b  that form a closed-type barrier rib structure. The barrier ribs  21  define a plurality of discharge cells and prevent ultraviolet (UV) rays and visible rays generated by a discharge in one cell from leaking into adjacent discharge cells. 
     Referring to  FIG. 1 , a filter  100  may be disposed at the front of the PDP. The filter  100  may include an external light shield sheet, an anti-reflection (AR) sheet, a near infrared (NIR) shield sheet, an electromagnetic interference (EMI) shield sheet, a diffusion sheet, and an optical sheet. 
     If the distance between the filter  100  and the PDP is 10-30 μm, the filter  100  can effectively shield external light incident upon the PDP and can emit light (hereinafter referred to as panel light) generated by the PDP. In order to protect the PDP against external impact such as pressure, the distance between the filter  100  and the PDP may be 30-120 μm. An adhesive layer, which can absorb impact, may be disposed between the filter  100  and the PDP in order to further protect the PDP against external impact. 
     Various barrier rib structures can be used other than those mentioned herein. Example structures include a differential-type barrier rib structure in which the height of vertical barrier ribs  21   a  is different from the height of horizontal barrier ribs  21   b , a channel-type barrier rib structure in which a channel that can be used as an exhaust passage is formed in at least one vertical or horizontal barrier rib  21   a  or  21   b , and a hollow-type barrier rib structure in which a hollow is formed in at least one vertical or horizontal barrier rib  21   a  or  21   b . In the differential-type barrier rib structure, the height of horizontal barrier ribs  21   b  may be greater than the height of vertical barrier ribs  21   a . In the channel-type barrier rib structure or the hollow-type barrier rib structure, a channel or a hollow cavity may be formed in at least one horizontal barrier rib  21   b.    
     In some implementations, red (R), green (G), and blue (B) discharge cells may be arranged in a straight line. This is an example only, and the discharge cells may be arranged in other ways. For example, R, G, and B discharge cells may be arranged as a triangle or a delta-type shape. Alternatively, R, G, and B discharge cells may be arranged as a polygon such as a rectangle, a pentagon, or a hexagon. 
     The phosphor layer  23  is excited by UV rays that are generated upon a gas discharge. As a result, the phosphor layer  23  generates one of R, G, and B rays. A discharge space is provided between the upper and lower substrates  10  and  20  and the barrier ribs  21 . A mixture of inert gases, e.g., a mixture of helium (He) and xenon (Xe), a mixture of neon (Ne) and Xe, or a mixture of He, Ne, and Xe is injected into the discharge space. 
       FIG. 2  is a cross-sectional view of an external light shield sheet that can be included in a filter. Referring to  FIG. 2 , the external light shield sheet includes a base unit  200  and a plurality of pattern units  210 . 
     The base unit  200  may be formed of a transparent plastic material, e.g., a UV-hardened resin-based material, enabling light to smoothly transmit therethrough. Alternatively, the base unit  200  may be formed of a rigid material such as glass in order to enhance the protection of an entire surface of a PDP. 
     Referring to  FIG. 2 , the pattern units  210  may be triangular (e.g., a triangular-prism-type shape). The pattern units  210  may be formed in various other suitable shapes, other than a triangular shape. The pattern units  210  may be formed of a darker material than the base unit  200 . In particular, the pattern units  210  may be formed of a black material. For example, the pattern units  210  may be formed of a carbon-based material or may be dyed black so that the absorption of external light can be increased. 
     The pattern units  210  can have boundaries (e.g., surfaces) defined by intersections (e.g., where the pattern units  210  interface the base unit  200 ) of the pattern units  210  and the base unit  200 . The boundaries of the pattern units can define the widths of pattern tops and the widths of pattern bottoms. For example, two boundary surfaces of a pattern unit can define a pattern top and a pattern bottom. Each of the boundary surfaces of the pattern unit can define an edge of the pattern top and the pattern bottom defined between the two surfaces. The pattern tops can be disposed toward one of the panel side and the viewer side, the pattern bottoms can be disposed toward the other of the panel side and the viewer side. 
     The boundaries of the pattern units can be sloped, and the pattern bottoms can be wider than the pattern tops. Whichever of an upper side and a lower side of each of the pattern units  210  is wider than the other will hereinafter be referred to as the bottom of a corresponding pattern unit  210 . 
     Referring to  FIG. 2 , the bottoms of the pattern units  210  may face a PDP side (e.g., a side facing a display surface of the PDP), and the tops of the pattern units  210  may face a viewer on the opposite side of the PDP (e.g., a side facing away from the PDP display surface). Alternatively, the bottoms of the pattern units  210  may face a viewer, and the tops of the pattern units  210  may face a PDP. 
     In general, an external light source is located above a PDP and therefore external light is highly likely to be diagonally incident upon a PDP from above within a predetermined angle range. At least partially because the external light is diagonally incident, it can be absorbed in the pattern units  210 . 
     Each of the pattern units  210  may contain light absorption particles. The light absorption particles may be stained resin particles. In order to improve the absorption of light, the light absorption particles may be stained a specific color, such as black. 
     The light absorption particles may have a size of 1 μm or more. In this case, it is possible to facilitate the manufacture of the light absorption particles and the insertion of the light absorption particles into the pattern units  210  and to increase the absorption of external light. If the light absorption particles have a size of 1 μm or more, each of the pattern units  210  may contain 10% or more of the light absorption particles, by weight. In this fashion, it is possible to effectively absorb external light refracted into the pattern units  210 . 
       FIGS. 3 through 6  illustrate external light shield sheets and illustrate optical characteristics of the external light shield sheets. 
     More specifically,  FIG. 3  illustrates the situation in which the tops of a plurality of pattern units  305  face toward a user and the refractive index of the pattern units  305 , and particularly, the refractive index of slanted surfaces of the pattern units  305 , is lower than the refractive index of a base unit  300  so as to absorb and shield external light and to enhance the reflection of panel light through the reflection of visual rays. As described above, external light which reduces the bright room contrast of a PDP is highly likely to be incident upon a PDP from above. Referring to  FIG. 3 , according to Snell&#39;s law, external light that is diagonally incident upon an external light shield sheet, as indicated by dotted lines, is refracted into and absorbed by the pattern units  305  which have a lower refractive index than the base unit  300 . External light refracted into the pattern units  305  may be absorbed by light absorption particles in the pattern units  305 . 
     Also, panel light for displaying an image is reflected toward a user by the slanted surfaces of the pattern units  305 , as indicated by solid lines. More specifically, since the angle between panel light and the slanted surfaces of the pattern units  305  is greater than the angle between external light and the slanted surfaces of the pattern units  305 , external light is refracted into and absorbed by the pattern units  305 , whereas panel light is reflected by the pattern units  305 . 
     The external light shield sheet of  FIG. 3  can absorb external light so that external light can be prevented from being reflected toward a user. Also, the external light shield sheet of  FIG. 3  can enhance the reflection of light emitted from a PDP  310 , increasing the bright room contrast of images displayed by the PDP  310 . 
     In order to increase the absorption of external light and the reflection of light emitted from the PDP  310 , the refractive index of the pattern units  305  may be configured to be 0.3-1.0 times higher than the refractive index of the base unit  300  in consideration of the incidence angle of external light with respect to the panel  310 . In particular, in order to increase the reflection of panel light by the slanted surfaces of the pattern units  305 , the refractive index of the pattern units  305  may be 0.3-0.8 times higher than the refractive index of the base unit  300  in consideration of a vertical viewing angle of the PDP  310 . 
     When the refractive index of the pattern units  305  is lower than the refractive index of the base  300 , light emitted from the PDP  310  is reflected by the slanted surfaces of the pattern units  305  and thus spreads out toward the user, thereby resulting in unclear, blurry images, i.e., a ghost phenomenon. 
       FIG. 4  illustrates the situation in which the tops of a plurality of pattern units  325  faces toward a user and the refractive index of the pattern units  325  is higher than the refractive index of a base unit  320 . Referring to  FIG. 4 , when the refractive index of the pattern units  325  is higher than the refractive index of the base unit  320 , external light incident upon the pattern units  325  and light emitted from a PDP  330  are both absorbed by the pattern units  325 . 
     Therefore, it is possible to reduce the probability of occurrence or perception of the ghost phenomenon. In order to absorb as much panel light as possible and thus to prevent the ghost phenomenon, the refractive index of the pattern units  325  may be 0.05 or more higher than the refractive index of the base unit  320 . 
     When the refractive index of the pattern units  325  is higher than the refractive index of the base unit  320 , the transmissivity and bright room contrast of an external light shield sheet may decrease. In order not to considerably reduce the transmissivity of an external light shield sheet while preventing the ghost phenomenon, the refractive index of the pattern units  325  may be 0.05-0.3 higher than the refractive index of the base unit  320 . Also, in order to uniformly maintain the bright room contrast of the PDP  330  while preventing the ghost phenomenon, the refractive index of the pattern units  325  may be 1.0-1.3 times greater than the refractive index of the base unit  320 . 
       FIG. 5  illustrates the situation in which the bottoms of a plurality of pattern units  345  face toward a user and the refractive index of the pattern units  345  is lower than the refractive index of a base unit  340 . Referring to  FIG. 5 , external light is absorbed by the bottoms of the pattern units  345 , thereby enhancing the shielding of external light. The distance between a pair of adjacent pattern units  345  may be widened compared to the distance between a pair of adjacent pattern units  325  illustrated in  FIG. 4 . Therefore, it is possible to enhance the aperture (or opening) ratio of an external light shield sheet. 
     According to the implementation shown in  FIG. 5 , panel light emitted from a PDP  350  is reflected by the slanted surfaces of the pattern units  345  and is thus concentrated together with panel light that directly transmits through the base unit  340  without being reflected by the slanted surfaces of the pattern units  345 . Therefore, it is possible to reduce the probability of occurrence or perception of the ghost phenomenon. 
     In order to further prevent the ghost phenomenon, a distance d between the PDP  350  and an external light shield sheet may be 1.5-3.5 mm. 
       FIG. 6  illustrates the situation in which the bottoms of a plurality of pattern units  365  face toward a user and the refractive index of the pattern units  365  is higher than the refractive index of a base unit  360 . Referring to  FIG. 6 , when the refractive index of the pattern units  365  is higher than the refractive index of the base unit  360 , panel light incident upon the slanted surfaces of the pattern units  365  is likely to be absorbed by the pattern units  365 . Accordingly, images are displayed only by panel light that transmits through the base unit  360 . Thus, it is possible to reduce the probability of occurrence or perception of the ghost phenomenon. 
     Also, since the refractive index of the pattern units  365  is higher than the refractive index of the base unit  360 , it is possible to enhance the absorption of external light. 
       FIG. 7  is a cross-sectional view of an external light shield sheet. Referring to  FIG. 7 , when a thickness T of an external light shield sheet is 20-250 μm, it is possible to facilitate the manufacture of an external light shield sheet and provide an external light shield sheet with an increased transmissivity. More specifically, the thickness T may be set to be 100-180 μm. In this case, it is possible to effectively absorb and shield external light using a plurality of pattern units  410  and to ensure the durability of an external light shield sheet. 
     Referring to  FIG. 7 , the pattern units  410  are formed in a base unit  400  as triangles, particularly, equilateral triangles. A bottom width P 1  of the pattern units  410  may be 18-36 μm. In this case, it is possible to secure a sufficient aperture ratio to properly emit panel light toward a user and increase the absorption of external light. 
     A height h of the pattern units  410  may be 80-170 μm. The slopes of the slanted surfaces of the pattern units  410  may be determined in consideration of the bottom width P 1  and the height h so that the absorption of external light and the reflection of panel light can be increased, and that the pattern units  410  can be prevented from being short-circuited. 
     A distance D 1  between adjacent boundaries of a pair of adjacent pattern units  410  at adjacent pattern bottoms may be 40-90 μm, and a distance D 2  between the adjacent boundaries of the pair of adjacent pattern units  410  at adjacent pattern bottoms may be 90-130 μm. In this case, it is possible to achieve a sufficient aperture ratio to display images with increased luminance through the emission of panel light toward a user and provide a number of pattern units having slanted surfaces with an optimum slope for enhancing the absorption of external light and the emission of panel light. 
     The distance D 1  may be 1.1-5 times greater than the bottom width P 1 . In this case, it is possible to secure an optimum aperture ratio for displaying images. In particular, the distance D 1  may be 1.5-3.5 times greater than the bottom width P 1 . In this case, it is possible to optimize the absorption of external light and the emission of panel light. 
     The height h may be 0.89-4.25 times greater than the distance D 1 . In this case, it is possible to prevent external light from being incident upon a PDP. In particular, the height h may be 1.5-3 times greater than the distance D 1 . In this case, it is possible to prevent the pattern units  410  from being short-circuited and to optimize the reflection of panel light. 
     The distance D 2  may be 1.0-3.25 times greater than the distance D 1 . In this case, it is possible to secure a sufficient aperture ratio to display images with optimum luminance. In particular, the distance D 2  may be 1.2-2.5 times greater than the distance D 1 . In this case, it is possible to optimize the total reflection of panel light by the slanted surfaces of the pattern units  410 . 
       FIGS. 8 and 9  are plan views of a plurality of pattern units of an external light shield sheet. A plurality of pattern units may be formed in a base unit as stripes, and are a predetermined distance apart from each other. 
     A moire phenomenon may occur when a plurality of pattern units of an external light shield sheet that are a predetermined distance apart from each other overlap black matrices, a black layer, bus electrodes, and barrier ribs that are formed on a PDP. The moire phenomenon refers to low-frequency patterns that are generated by overlapping similar types of grating patterns. For example, when mosquito nets are overlaid each other, ripple patterns appear. 
     Referring to  FIGS. 8 and 9 , a plurality of pattern units  510 ,  520 , and  530  are formed diagonally with respect to the lengthwise (longitudinal) direction of an external light shield sheet, thereby reducing the probability of occurrence or user perception of the moire phenomenon. The pattern units  510 ,  520 , and  530  can be substantially parallel to one or more axes that are diagonal with respect to the longitudinal axis of the external light shield and that form one or more angles (e.g., θ 1 , θ 2  and θ 3 ) with the longitudinal axis of the external light shield. 
     Referring to  FIG. 10A , black matrices  610  are parallel to horizontal barrier ribs which are formed on a lower substrate of a PDP, and are also parallel to an upper side or lower side of the external light shield sheet illustrated in  FIGS. 8 and 9 . Therefore, angles θ 1 , θ 2  and θ 3  between the upper side of the external light shield sheet and the pattern units  510 ,  520  and  530  are the same as the angles between the pattern units  510 ,  520  and  530  and black matrices. 
     A plurality of pattern units of an external light shield sheet may form an angle of 20 degrees or less with black matrices on a PDP, thereby reducing the probability of occurrence or perception of the moire phenomenon. Given that external light is highly likely to be incident upon a PDP from above, the pattern units may form an angle of 5 degrees or less with the black matrices, thereby reducing the probability of occurrence or perception of the moire phenomenon, securing an optimum aperture ratio, increasing the reflection of panel light, and effectively shielding external light. 
       FIG. 9  is an enlarged view of a portion  500  of the external light shield sheet illustrated in  FIG. 8 . Referring to  FIG. 9 , the pattern units  510 ,  520 , and  530  may be parallel to each other. Even if the pattern units  510 ,  520 , and  530  are not parallel to each other, the angles between the pattern units  510 ,  520  and  530  and black matrices may fall within the above-described range. 
     As described above, the angles θ 1 , θ 2  and θ 3  may be 20 degrees or less. In this case, it is possible to reduce the probability of occurrence or perception of the moire phenomenon. Also, given that external light is highly likely to be incident upon a PDP from above, the angles θ 1 , θ 2  and θ 3  may be 5 degrees or less. In this case, it is possible to reduce the probability of occurrence or perception of the moire phenomenon, secure an optimum aperture ratio, increase the reflection of panel light, and effectively shield external light. 
     Referring to  FIGS. 8 and 9 , the pattern units  510 ,  520 , and  530  are formed diagonally in a direction from a lower right portion of an external light shield sheet to an upper left portion of the external light shield sheet. Alternatively, the pattern units  510 ,  520 , and  530  may be formed diagonally in a direction from an upper left portion of an external light shield sheet to a lower right portion of the external light shield sheet. 
       FIGS. 10A and 10B  illustrate black matrices that can be formed on a PDP. Referring to  FIG. 10A , black matrices  610  may overlap respective corresponding horizontal barrier ribs which are formed on a lower substrate  600 . Also, the black matrices  610  may overlap respective corresponding scan electrode-sustain electrode pairs, which are formed on an upper substrate (not shown), so that the scan electrode-sustain electrode pairs can be hidden from view by the black matrices  610 . 
     When a width b of the black matrices  610  is 200-400 μm and a distance a between a pair of adjacent black matrices  610  is 300-600 μm, it is possible to secure an optimum aperture ratio for optimizing the luminance of images displayed by a PDP and to increase the efficiency of shielding external light and the efficiency of enhancing the purity and contrast of an upper substrate. 
     Referring to  FIG. 10B , black matrices  650  may be spaced apart from respective corresponding electrode pairs, each electrode pair comprising a scan electrode  630  and a sustain electrode  640 . 
     A width d of the black matrices  650  is 70-150 μm, and a distance c between a pair of adjacent black matrices  650  is 500-800 μm. In this configuration, it is possible to increase the efficiency of shielding external light and the efficiency of enhancing the purity and contrast of an upper substrate. 
     As described above, the moire phenomenon may occur when pattern units of an external light shield sheet overlie black matrices on an upper substrate. 
     When a width of black matrices is 3-15 times greater than the bottom width P 1  of pattern units, it is possible to secure an optimum aperture ratio for a PDP and increase the efficiency of shielding external light while reducing the probability of occurrence or perception of the moire phenomenon. Also, when the distance between a pair of adjacent black matrices is 4-12 times greater than the distance D 1  between a pair of adjacent pattern units, it is possible to optimize the reflection of panel light and reduce the probability of occurrence or perception of the moire phenomenon by enabling panel light to be reflected through black matrices by the slanted surfaces of pattern units of an external light shield sheet. 
     When the black matrices  610  overlap respective corresponding scan electrode-sustain electrode pairs, as illustrated in  FIG. 10A , the width b of the black matrices  610  may be 10-15 times greater than the bottom width P 1  of pattern units of an external light shield sheet. In this case, it is possible to reduce the occurrence or perception of the moire phenomenon, secure an optimum aperture ratio for a PDP, and increase the efficiency of shielding external light. In addition, the distance a between a pair of adjacent black matrices  610  may be 4-9 times greater than the distance between a pair of adjacent pattern units. In this case, it is possible to optimize the reflection of panel light and reduce the probability of occurrence or perception of the moire phenomenon. 
     When the black matrices  650  are spaced apart from respective corresponding scan electrode-sustain electrode pairs, the distance d of the black matrices  650  may be 3-7 times greater than the bottom width P 1  of pattern units of an external light shield sheet. In this case, it is possible to reduce the probability of occurrence or perception of the moire phenomenon, secure an optimum aperture ratio for a PDP, and increase the efficiency of shielding external light. In addition, the distance a between a pair of adjacent black matrices  650  may be 7-12 times greater than the distance between a pair of adjacent pattern units. In this case, it is possible to optimize the reflection of panel light and reduce the probability of occurrence or perception of the moire phenomenon. 
       FIGS. 11 and 12  illustrate an example configuration of an EMI shield sheet. Referring to  FIGS. 11 and 12 , an EMI shield sheet includes a base unit on which a plurality of metallic patterns are disposed as a mesh. The metallic patterns can be formed of a conductive metal, such as copper (Cu). The angles between the metallic patterns and the upper boundary of the EMI shield sheet, i.e., an angle θ 5  and θ 4 , are the same as the angles between the metallic patterns and black matrices formed on a PDP. 
       FIG. 12  is an enlarged view of a portion  700  of the EMI shield sheet illustrated in  FIG. 11 . Referring to  FIG. 12 , first mesh patterns  720  are formed in a diagonal direction from upper right to lower left. Second mesh patterns  710  are formed in a diagonal direction from upper left to lower right and cross the first mesh patterns  720 . The first mesh patterns  720  form the angle θ 5  with black matrices, and the second mesh patterns  710  form the angle θ 4  with the black matrices. The first mesh patterns  720  form an angle θ 8  with the second mesh patterns  710 . 
     The first mesh patterns  720  can be arranged substantially parallel to an axis running diagonally through upper right to lower left. The angle θ 4  may represent an interior angle between this axis and the longitudinal axis of the EMI shield. The second mesh patterns  710  can be arranged substantially parallel to an axis running diagonally through upper left to lower right. The angle θ 5  may represent an interior angle between this axis and the longitudinal axis of the EMI shield. The angle θ 8  may represent an exterior angle between the respective axes of the first mesh patterns and the second mesh patterns. 
     The width of the first and second mesh patterns  720  and  710  may be within the range of 5-15 μm. In this case, it is possible to effectively prevent the occurrence or perception of the moire phenomenon, to properly shield EMI, to secure an optimum aperture ratio for a plasma display device, and to maintain an optimum luminance for images displayed by a plasma display device. 
     In some implementations, the EMI shield sheet illustrated in  FIGS. 11 and 12  may be attached to an external light shield sheet of a plasma display device. The structure of an external light shield sheet with an EMI shield sheet attached thereon will be described in detail with reference to  FIGS. 13 and 14 . 
     Referring to  FIGS. 13 and 14 , in order for the EMI shield sheet to effectively shield EMI and reduce the probability of occurrence or perception of the moire phenomenon, the angles between the first mesh patterns  720  and black matrices and between the second mesh patterns  710  and the black matrices, i.e., the angles θ 5  and θ 4 , may be within the range of 20 to 60 degrees. In this case, the angle between the first mesh patterns  720  and the second mesh patterns  710 , i.e., the angle θ 8 , may be within the range of 60-130 degrees. 
     In order to prevent the moire phenomenon from being caused by patterns diagonally formed on an external light shield sheet, the angles θ 5  and θ 4  may be within the range of 30-55 degrees. In this case, the angle θ 8  may be within the range of 70-118 degrees. 
     When the angles θ 5  and θ 4  are within the range of 35-45 degrees, it is possible to facilitate the manufacture of the first and second mesh patterns  720  and  710 , which intersect each other, and to secure an optimum aperture ratio for a plasma display device. 
       FIGS. 13 and 14  illustrate an external light shield sheet  800  with an EMI shield sheet  810  attached thereon. The EMI shield sheet  810  may be attached to the external light shield sheet  800  on which a plurality of pattern units  840  are diagonally formed in order to reduce the occurrence or perception of the moire phenomenon. 
       FIG. 14  illustrates an enlarged view of portions  820  and  830  of the external light shield sheet  800 . Referring to  FIG. 14 , the pattern units  840  overlap first and second mesh patterns  850  and  860  which are formed on the EMI shield sheet  810 . 
     When an angle θ 6  between the pattern units  840  and the first mesh patterns  850  is within the range of 20-60 degrees, the external light shield sheet  800  can effectively shield EMI and reduce the probability of occurrence or perception of the moire phenomenon. In order for the external light shield sheet  800  to shield external light and effectively prevent the moire phenomenon, the angle θ 6  may be within the range of 27-53 degrees. The angle θ 6  may represent an interior angle between respective axes to which the pattern units  840  and the first mesh patterns  850  are substantially parallel. 
     The angle θ 6  may be within the range of 40-50 degrees, in order to increase the ease of fabrication of the pattern units  840  and the first and second mesh patterns  850  and  860 , secure an optimum aperture ratio of a plasma display device and provide wide viewing angles. 
     When an angle θ 7  between the pattern units  840  and the second mesh patterns  860  is within the range of 28-65 degrees, the external light shield sheet  800  can properly shield EMI and reduce the probability of occurrence or perception of the moire phenomenon. The angle θ 7  may represent an interior angle between respective axes to which the pattern units  840  and the second mesh patterns  860  are substantially parallel. 
     The angle θ 7  may be within the range of 33-58 degrees, in order for the external light shield sheet  800  to shield external light incident upon a PDP from above and effectively prevent the moire phenomenon. 
     The angle θ 7  may be within the range of 40-50 degrees, in order to increase the ease of fabrication of the pattern units  840  and the first and second mesh patterns  850  and  860 , secure an optimum aperture ratio of a plasma display device and provide wide viewing angles. 
     Table 1 below presents experimental results obtained by setting an angle θ 1  between the pattern units  840  and black matrices to 2.5 degrees and continuously varying the angles θ 4 , θ 5 , θ 6 , θ 7 , and θ 8 . Table 1 illustrates the relationships between the occurrence of the moire phenomenon and the angles θ 4 , θ 5 , θ 6 , θ 7 , and θ 5 . 
     Referring to Table 1, reference character ∘ indicates the situation when the moire phenomenon has occurred, reference character Δ indicates the situation when the probability of occurrence of the moire phenomenon has been reduced to 50% or less, and reference character x indicates the situation when the moire phenomenon has not occurred. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 θ 1   
                 θ 5   
                 θ 4   
                 Moire 
                 θ 8   
                 θ 6   
                 θ 7   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 2.5 
                 5 
                 5 
                 ∘ 
                 170 
                 2.5 
                 7.5 
               
               
                 2.5 
                 5 
                 7.5 
                 ∘ 
                 167.5 
                 2.5 
                 10 
               
               
                 2.5 
                 10 
                 10 
                 ∘ 
                 160 
                 7.5 
                 12.5 
               
               
                 2.5 
                 10 
                 12.5 
                 ∘ 
                 157.5 
                 7.5 
                 15 
               
               
                 2.5 
                 15 
                 15 
                 ∘ 
                 150 
                 12.5 
                 17.5 
               
               
                 2.5 
                 15 
                 17.5 
                 ∘ 
                 147.5 
                 12.5 
                 20 
               
               
                 2.5 
                 20 
                 20 
                 ∘ 
                 140 
                 17.5 
                 22.5 
               
               
                 2.5 
                 20 
                 22.5 
                 ∘ 
                 137.5 
                 17.5 
                 25 
               
               
                 2.5 
                 25 
                 25 
                 ∘ 
                 130 
                 22.5 
                 27.5 
               
               
                 2.5 
                 25 
                 27.5 
                 Δ 
                 127.5 
                 22.5 
                 30 
               
               
                 2.5 
                 30 
                 30 
                 Δ 
                 120 
                 27.5 
                 32.5 
               
               
                 2.5 
                 30 
                 32.5 
                 x 
                 117.5 
                 27.5 
                 35 
               
               
                 2.5 
                 35 
                 35 
                 x 
                 110 
                 32.5 
                 37.5 
               
               
                 2.5 
                 35 
                 37.5 
                 x 
                 107.5 
                 32.5 
                 40 
               
               
                 2.5 
                 40 
                 40 
                 x 
                 100 
                 37.5 
                 42.5 
               
               
                 2.5 
                 40 
                 42.5 
                 x 
                 97.5 
                 37.5 
                 45 
               
               
                 2.5 
                 45 
                 45 
                 x 
                 90 
                 42.5 
                 47.5 
               
               
                 2.5 
                 45 
                 47.5 
                 x 
                 87.5 
                 42.5 
                 50 
               
               
                 2.5 
                 50 
                 50 
                 x 
                 80 
                 47.5 
                 52.5 
               
               
                 2.5 
                 50 
                 52.5 
                 x 
                 77.5 
                 47.5 
                 55 
               
               
                 2.5 
                 55 
                 55 
                 x 
                 70 
                 52.5 
                 57.5 
               
               
                 2.5 
                 55 
                 57.5 
                 Δ 
                 67.5 
                 52.5 
                 60 
               
               
                 2.5 
                 60 
                 60 
                 Δ 
                 60 
                 57.5 
                 62.5 
               
               
                 2.5 
                 60 
                 62.5 
                 ∘ 
                 57.5 
                 57.5 
                 65 
               
               
                 2.5 
                 65 
                 65 
                 ∘ 
                 50 
                 62.5 
                 67.5 
               
               
                 2.5 
                 65 
                 67.5 
                 ∘ 
                 47.5 
                 62.5 
                 70 
               
               
                 2.5 
                 70 
                 70 
                 ∘ 
                 40 
                 67.5 
                 72.5 
               
               
                 2.5 
                 70 
                 72.5 
                 ∘ 
                 37.5 
                 67.5 
                 75 
               
               
                 2.5 
                 75 
                 75 
                 ∘ 
                 30 
                 72.5 
                 77.5 
               
               
                 2.5 
                 75 
                 77.5 
                 ∘ 
                 27.5 
                 72.5 
                 80 
               
               
                 2.5 
                 80 
                 80 
                 ∘ 
                 20 
                 77.5 
                 82.5 
               
               
                 2.5 
                 80 
                 82.5 
                 ∘ 
                 17.5 
                 77.5 
                 85 
               
               
                 2.5 
                 85 
                 85 
                 ∘ 
                 10 
                 82.5 
                 87.5 
               
               
                 2.5 
                 85 
                 87.5 
                 ∘ 
                 7.5 
                 82.5 
                 90 
               
               
                 2.5 
                 90 
                 90 
                 ∘ 
                 0 
                 87.5 
                 92.5 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, when the angle θ 5  is within the range of 25-60 degrees, the probability of occurrence or perception of the moire phenomenon can be reduced. When the angle θ 5  is within the range of 30-55 degrees, the probability of occurrence or perception of the moire phenomenon can be further reduced. When the angle θ 4  is within the range of 27.5-60 degrees, the probability of occurrence or perception of the moire phenomenon can be reduced. When the angle θ 4  is within the range of 32.5-55 degrees, the probability of occurrence or perception of the moire phenomenon can be further reduced. 
     When the angle θ 8  is within the range of 60-127.5 degrees, the probability of occurrence or perception of the moire phenomenon can be reduced. When the angle θ 8  is within the range of 70-117.5 degrees, the moire phenomenon can be further reduced. 
     When the angle θ 6  is within the range of 22.5-57.5 degrees, the moire phenomenon can be reduced. When the angle θ 6  is within the range of 27.5-52.5 degrees, the moire phenomenon can be reduced. 
     When the angle θ 7  is within the range of 30-62.5 degrees, the moire phenomenon can be reduced. When the angle θ 7  is within the range of 35-57.5 degrees, the moire phenomenon can be further reduced. 
       FIGS. 15 through 19  are cross-sectional views of external light shield sheets illustrating various shapes of pattern units. 
     Referring to  FIG. 15 , a plurality of pattern units  900  may be asymmetrical with respect to their respective horizontal axes. In other words, a pair of slanted surfaces or boundaries of each of the pattern units  900  may have different areas or may form different angles with the bottom of an external light shield sheet. A pair of slanted surfaces of each of the pattern units  900  may have different areas or may form different angles with the bottom of a corresponding pattern unit  900 . In general, an external light source is located above a PDP. Thus, external light is highly likely to be incident upon a PDP from above at a certain range of angles. One of a pair of slanted surfaces of each of the pattern units  900  upon which external light is directly incident will hereinafter be referred to as an upper slanted surface, and the other slanted surface will hereinafter be referred to as a lower slanted surface. In order to enhance the absorption of external light and the reflection of light emitted from a PDP, the upper slanted surfaces of the pattern units  900  may be less steep than the lower slanted surfaces of the pattern units  900 . That is, the slope of the upper slanted surfaces of the pattern units  900  may be less than the slope of the lower slanted surface of the pattern units  900 . 
     Referring to  FIG. 16 , a plurality of pattern units  910  may be trapezoidal. As illustrated in  FIG. 16 , a distance D 1  between a pair of adjacent boundaries of the pattern units  910  at adjacent pattern bottoms can be less than a distance D 2  between the adjacent boundaries at adjacent pattern tops. In  FIG. 16 , a top width P 2  of the pattern units  910  is less than a bottom width P 1  of the pattern units  910 . The top width P 2  may be 10 μm or less. The slope of the slanted surfaces of the pattern units  910  can be appropriately determined according to the relationship between the bottom width P 1  and the top width P 2  so that the absorption of external light and the reflection of light emitted from a PDP can be increased. 
     Referring to  FIGS. 17 through 19 , a pair of slanted surfaces of each of a plurality of pattern units  920 ,  930 , and  940  may have curved lateral surfaces or boundaries with a predetermined curvature. In order to further shield external light diagonally incident upon a PDP, the slope of the slanted surfaces of the pattern units  920 ,  930 , or  940  may lessen (or become more gentle) from the bottoms to the tops of the pattern units  920 ,  930 , or  940 . 
     Each of the pattern units  920 ,  930 , and  940  illustrated in  FIGS. 17 through 19  may have curved edges with a predetermined curvature. 
       FIG. 20  is a cross-sectional view of an external light shield sheet including a plurality of pattern units  1010  with recessed (or concave) bottoms. Referring to  FIG. 20 , the bottoms  1015  of the pattern units  1010  are recessed. Thus, it is possible to reduce image smear caused by panel light reflected from the bottoms  1015  of the pattern units  1010 . In addition, since the external light shield sheet illustrated in  FIG. 20  has a relatively large surface area, the external light shield sheet can be firmly attached onto another function sheet or a PDP. 
     The bottoms  1015  of the pattern units  1010  may be recessed so that the height of the pattern units  1010  becomes less at the center of each of the pattern units  1010  than on either side of the bottom  1015  of each of the pattern units  1010 . 
     The pattern units  1010  may be formed by forming a plurality of grooves in a base unit  1000  and filling the grooves—at least partially and, in some implementations, not completely—with a light absorption material so that the bottoms  1015  of the pattern units  1010  can be slightly recessed. 
       FIG. 21  illustrates a pattern unit  1030  with a flat bottom. Referring to  FIG. 21 , since the bottom of the pattern unit  1030  is flat, panel light diagonally incident upon the pattern unit  1030  may be reflected back toward a PDP by the bottom of the pattern unit  1030 , thereby causing image smear and reducing the sharpness of an image displayed by a PDP. 
     Referring to  FIGS. 21 and 22 , an incidence angle θ 2  of panel light which is diagonally incident upon a pattern unit  1010  with a recessed bottom is less than an incidence angle θ 1  of panel light which is incident upon the pattern unit  1030 . Thus, the pattern unit  1010  can absorb panel light incident thereupon due to its recessed bottom, whereas the pattern unit  1030  reflects panel light incident thereupon. Therefore, by using the pattern unit  1010  with a recessed bottom, it is possible to reduce image smear and thus to improve the sharpness of an image. 
       FIG. 23  is a cross-sectional view of an external light shield sheet including a pattern unit  1110  with a recessed bottom. Referring to  FIG. 23 , the external light shield sheet may be disposed so that the bottom of the pattern unit  1110  can face a viewer. In this case, it is possible to increase the range of incidence angles of external light that is can be absorbed by the bottom of the pattern unit  1110 . In other words, it is possible to increase the incidence angle of external light with respect to the bottom of the pattern unit  1110  and thus to improve the absorption of external light by the pattern unit  1110 . 
       FIG. 24  is a cross-sectional view of a pattern unit  1210  with a recessed bottom. Table 2 presents experimental results indicating the relationships between a depth a of grooves, a bottom width d of pattern units with recessed bottoms, and the ability of the pattern units to reduce image smear. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Depth of 
                 Bottom Width of 
                 Smear 
               
               
                 Grooves (a) 
                 Pattern Units (d) 
                 Reduction 
               
               
                   
               
             
            
               
                 0.5 μm 
                 27 μm 
                 x 
               
               
                 1.0 μm 
                 27 μm 
                 x 
               
               
                 1.5 μm 
                 27 μm 
                 ∘ 
               
               
                 2.0 μm 
                 27 μm 
                 ∘ 
               
               
                 2.5 μm 
                 27 μm 
                 ∘ 
               
               
                 3.0 μm 
                 27 μm 
                 ∘ 
               
               
                 3.5 μm 
                 27 μm 
                 ∘ 
               
               
                 4.0 μm 
                 27 μm 
                 ∘ 
               
               
                 4.5 μm 
                 27 μm 
                 ∘ 
               
               
                 5.0 μm 
                 27 μm 
                 ∘ 
               
               
                 5.5 μm 
                 27 μm 
                 ∘ 
               
               
                 6.0 μm 
                 27 μm 
                 ∘ 
               
               
                 6.5 μm 
                 27 μm 
                 ∘ 
               
               
                 7.0 μm 
                 27 μm 
                 ∘ 
               
               
                 7.5 μm 
                 27 μm 
                 x 
               
               
                 8.0 μm 
                 27 μm 
                 x 
               
               
                 9.0 μm 
                 27 μm 
                 x 
               
               
                 9.5 μm 
                 27 μm 
                 x 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, when the depth a is within the range of 1.5-7.0 μm, it is possible to reduce image smear and thus to increase the sharpness of an image. 
     In order to prevent the pattern unit  1210  from being damaged by an external shock and to facilitate the manufacture of the pattern unit  1210 , the depth a may be within the range of 2-5 μm. 
     As described above with reference to  FIG. 7 , when a width d of the pattern unit  1210  is within the range of 18-35 μm, it is possible to secure an optimum aperture ratio for an effective emission of panel light and to increase the efficiency of shielding external light. Thus, the width d may be 3.6-17.5 times greater than the depth a. 
     When a height of the pattern unit  1210  is 80-170 μm, the slopes of a pair of slanted surfaces of the pattern unit  1210  can become suitable enough to effectively absorb external light and to effectively reflect panel light. Thus, the height c may be 16-85 times greater than the depth a. 
     When a thickness b of an external light shield sheet is 100-180 μm, it is possible to facilitate the transmission of panel light, to effectively absorb and shield external light and to enhance the durability of an external light shield sheet. Thus, the thickness b may be 20-90 times greater than the depth a. 
     Referring to  FIG. 25 , a pattern unit  1230  may be trapezoidal. In this case, a top width e of the pattern unit  1230  may be less than a bottom width d of the pattern unit  1230 . When the top width e is less than 10 μm, the slopes of a pair of slanted surfaces of the pattern unit  1230  can become suitable enough to effectively absorb external light and to effectively reflect panel light. Thus, the relationship between the depth a and the bottom width d may be the same as the relationship between the depth a and the width d of  FIG. 24 . 
       FIG. 26  is a cross-sectional view illustrating a structure of an external light shielding sheet for explaining the relationship between a thickness T of the external light shield sheet and a height h of a plurality of pattern units of the external light shield sheet. 
     Referring to  FIG. 26 , in order to enhance the durability of an external light shield sheet comprising a plurality of pattern units and secure the transmission of visible light emitted from a PDP for displaying images, the thickness T may be set to 100-180 μm. 
     When the height h is within the range of 80-170 μm, the manufacture of an external light shield sheet can be facilitated, an optimum aperture ratio can be obtained, and the shielding of external light and the reflection of light emitted from a PDP can be increased. 
     The height h can be varied according to the thickness T. In general, external light that considerably affects the bright room contrast of a PDP is highly likely to be incident upon a PDP from above. Therefore, in order to effectively shield external light, the height h may be within a predetermined percentage range of the thickness T. 
     Referring to  FIG. 14 , as the height h increases, the thickness of a base unit decreases, and thus, dielectric breakdown is more likely to occur. On the other hand, as the height h decreases, more external light is likely to be incident upon a PDP at a predetermined range of angles, and thus it becomes more difficult for an external light shield sheet to properly shield such external light. 
     Table 3 presents experimental results obtained by testing a plurality of external light shield sheets having the same thickness T and different pattern unit heights (h) for whether they cause dielectric breakdown and whether they can shield external light. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Thickness (T) of External 
                 Height (h) 
                 Dielectric 
                 External Light 
               
               
                 Light Shield sheet 
                 of Pattern Units 
                 Breakdown 
                 Shielding 
               
               
                   
               
             
            
               
                 120 μm 
                 120 μm  
                 ∘ 
                 ∘ 
               
               
                 120 μm 
                 115 μm  
                 Δ 
                 ∘ 
               
               
                 120 μm 
                 110 μm  
                 x 
                 ∘ 
               
               
                 120 μm 
                 105 μm  
                 x 
                 ∘ 
               
               
                 120 μm 
                 100 μm  
                 x 
                 ∘ 
               
               
                 120 μm 
                 95 μm 
                 x 
                 ∘ 
               
               
                 120 μm 
                 90 μm 
                 x 
                 ∘ 
               
               
                 120 μm 
                 85 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 80 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 75 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 70 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 65 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 60 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 55 μm 
                 x 
                 Δ 
               
               
                 120 μm 
                 50 μm 
                 x 
                 x 
               
               
                   
               
            
           
         
       
     
     Referring to Table 3, when the thickness T is 120 μm and the height h is greater than 115 μm, pattern units of an external light shield sheet are highly susceptible to dielectric breakdown, thereby increasing defect rates. When the height h is less than 115 μm, the pattern units are less susceptible to dielectric breakdown, thereby reducing defect rates. When the height h is less than 85 μm, the external light shielding efficiency of the pattern units is likely to decrease. When the height h is less than 60 μm, external light is likely to be directly incident upon a PDP. 
     When the thickness T is 1.01-2.25 times greater than the height h, it is possible to prevent dielectric breakdown of the upper portions of the pattern units and to prevent external light from being incident upon a PDP. In order to prevent dielectric breakdown of the pattern units and infiltration of external light into a PDP, to increase the reflection of light emitted from a PDP, and to secure optimum viewing angles, the thickness T may be 1.01-1.5 times greater than the height h. 
     Table 4 presents experimental results obtained by testing a plurality of external light shield sheets having different pattern unit bottom width-to-bus electrode width ratios for whether they cause the moire phenomenon and whether they can shield external light, when the width of bus electrodes that are formed on an upper substrate of a PDP is 70 μm. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Bottom Width of 
                   
                   
               
               
                 Pattern Units/Width of 
                   
                 External light 
               
               
                 Bus Electrodes 
                 Moire 
                 shielding 
               
               
                   
               
             
            
               
                 0.10 
                 Δ 
                 x 
               
               
                 0.15 
                 Δ 
                 x 
               
               
                 0.20 
                 x 
                 Δ 
               
               
                 0.25 
                 x 
                 ∘ 
               
               
                 0.30 
                 x 
                 ∘ 
               
               
                 0.35 
                 x 
                 ∘ 
               
               
                 0.40 
                 x 
                 ∘ 
               
               
                 0.45 
                 Δ 
                 ∘ 
               
               
                 0.50 
                 Δ 
                 ∘ 
               
               
                 0.55 
                 ∘ 
                 ∘ 
               
               
                 0.60 
                 ∘ 
                 ∘ 
               
               
                   
               
            
           
         
       
     
     Referring to Table 4, when the bottom width of pattern units is 0.2-0.5 times greater than the width of bus electrodes, the moire phenomenon can be reduced and the amount of external light incident upon a PDP can be reduced. In particular, the bottom width of pattern units may be 0.25-0.4 times greater than the width of bus electrodes. In this case, it is possible to reduce the moire phenomenon, to effectively shield external light, and to secure a sufficient aperture ratio to discharge light emitted from a PDP. 
     Table 5 presents experimental results obtained by testing a plurality of external light shield sheets having different pattern unit bottom width-to-vertical barrier rib width ratios for whether they cause the moire phenomenon and whether they can shield external light, when the width of vertical barrier ribs that are formed on a lower substrate of a PDP is 50 μm. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Bottom 
                   
                   
               
               
                 Width of Pattern 
               
               
                 Units/Top Width of 
                   
                 External 
               
               
                 Vertical Barrier Ribs 
                 Moire 
                 Light shielding 
               
               
                   
               
             
            
               
                 0.10 
                 ∘ 
                 x 
               
               
                 0.15 
                 Δ 
                 x 
               
               
                 0.20 
                 Δ 
                 x 
               
               
                 0.25 
                 Δ 
                 x 
               
               
                 0.30 
                 x 
                 Δ 
               
               
                 0.35 
                 x 
                 Δ 
               
               
                 0.40 
                 x 
                 ∘ 
               
               
                 0.45 
                 x 
                 ∘ 
               
               
                 0.50 
                 x 
                 ∘ 
               
               
                 0.55 
                 x 
                 ∘ 
               
               
                 0.60 
                 x 
                 ∘ 
               
               
                 0.65 
                 x 
                 ∘ 
               
               
                 0.70 
                 Δ 
                 ∘ 
               
               
                 0.75 
                 Δ 
                 ∘ 
               
               
                 0.80 
                 Δ 
                 ∘ 
               
               
                 0.85 
                 ∘ 
                 ∘ 
               
               
                 0.90 
                 ∘ 
                 ∘ 
               
               
                   
               
            
           
         
       
     
     Referring to Table 5, when the bottom width of pattern units is 0.3-0.8 times greater than the width of vertical barrier ribs, the moire phenomenon can be reduced and the amount of external light incident upon a PDP can be reduced. In particular, the bottom width of pattern units may be 0.4-0.65 times greater than the width of vertical barrier ribs. In this case, it is possible to reduce the moire phenomenon, to effectively shield external light, and to secure a sufficient aperture ratio to discharge light emitted from a PDP. 
       FIGS. 27 through 30  are cross-sectional views of filters. A filter may be disposed at the front of a PDP, and may include an AR/NIR sheet, an EMI shield sheet, an external light shield sheet, and an optical sheet. 
     Referring to  FIGS. 27 and 28 , an AR/NIR sheet  1310  includes a base sheet  1313 , which is formed of a transparent plastic material; an AR layer  1311 , which is attached onto an entire surface of the base sheet  1313  and reduces glare by preventing the reflection of external light incident upon a PDP; and an NIR shield layer  1312 , which is attached onto a rear surface of the base sheet  1313  and shields NIR rays emitted from a PDP so that signals provided by a device (such as a remote control transmitting signals using infrared rays) can be smoothly transmitted. 
     An EMI shield sheet  1320  can include a base sheet  1322  which is formed of a transparent plastic material and an EMI shield layer  1321  which is attached onto a surface of the base sheet  1322  and shields EMI generated by a PDP so that the EMI can be prevented from being released externally (to the outside). The EMI shield layer  1321  can be formed of a conductive material in a mesh form. In order to properly ground the EMI shield layer  1321 , an invalid display zone on the EMI shield sheet  1320  where no images are displayed can be covered with a conductive material. 
     An external light source is generally located over the head of a user regardless of an indoor or outdoor environment. An external light shield sheet  1330  effectively shields external light so that black images can be rendered even blacker by a PDP. 
     An adhesive layer  1340  is interposed between the AR/NIR sheet  1310 , the EMI shield sheet  1320 , and the external light shield sheet  1330 , so that the filter  1300  including the AR/NIR sheet  1310 , the EMI shield sheet  1320 , and the external light shield sheet  1330  can be firmly attached onto a PDP. In order to facilitate the manufacture of the filter  1300 , the base sheets  1313  and  1322  may be formed of the same material. 
     Referring to  FIG. 27 , the AR/NIR sheet  1310 , the EMI shield sheet  1320 , and the external light shield sheet  1330  can be sequentially deposited or stacked. Alternatively, the AR/NIR sheet  1310 , the external light shield sheet  1330 , and the EMI shield sheet  1320  may be sequentially deposited or stacked, as illustrated in  FIG. 28 . The order in which the AR/NIR sheet  1310 , the EMI shield sheet  1320 , and the external light shield sheet  1330  are deposited is not restricted to those set forth herein and illustrated in the figures. At least one of the AR/NIR sheet  1310 , the EMI shield sheet  1320 , and the external light shield sheet  1330  may be optional. 
     Referring to  FIGS. 29 and 30 , a filter  1400 , which is disposed at the front of a PDP, includes an AR/NIR sheet  1410 , an EMI shield sheet  1430 , an external light shield sheet  1440 , and an optical sheet  1420 . The AR/NIR sheet  1410 , the EMI shield sheet  1430 , and the external light shield sheet  1440  may be the same as their respective counterparts illustrated in  FIGS. 27 and 28 . The optical sheet  1420  enhances the color temperature and luminance properties of light incident upon a PDP from above. The optical sheet  1420  includes a base sheet  1422  formed of a transparent plastic material, and an optical sheet layer  1421  which is formed of a dye and an adhesive on a front or rear surface of the base sheet  1422 . 
     At least one of the base sheets  1313  and  1322  illustrated in  FIGS. 27 and 28  and at least one of a base sheet  1413 , a base sheet  1412 , and the base sheet  1422  illustrated in  FIGS. 29 and 30  may be optional. One of the base sheets  1313  and  1322  illustrated in  FIGS. 27 and 28  and one of the base sheets  1413 ,  1412 , and  1422  illustrated in  FIGS. 29 and 30  may be formed of a rigid material such as glass, instead of being formed of a plastic material, so that the protection of a PDP can be enhanced. Whichever of the base sheets  1313  and  1322  illustrated in  FIGS. 27 and 28  and the base sheets  1413 ,  1412 , and  1422  illustrated in  FIGS. 29 and 30  is formed of glass may be a predetermined distance apart from a PDP. 
     A filter may also include a diffusion sheet. The diffusion sheet can diffuse light incident upon a PDP so that the brightness of the PDP can be uniformly maintained. In addition, the diffusion sheet can widen vertical and horizontal viewing angles of a display screen by uniformly diffusing light emitted from a PDP. Moreover, the diffusion sheet can hide patterns formed on an external light shield sheet. Furthermore, the diffusion sheet can uniformly enhance the front luminance of a PDP through collection of light in a direction corresponding to a vertical viewing angle, and can enhance the antistatic property of a PDP. 
     The diffusion sheet may be comprised of a transparent or reflective diffusion film. In general, the diffusion sheet may be comprised of a polymer base sheet containing small glass particles. In some examples, the diffusion sheet may be comprised of a polymethyl-methacrylate (PMMA) base sheet. In this case, the diffusion sheet is thick and highly heat-resistant and can thus be applied to large-scale display devices, which can generate a considerable amount of heat. 
     As described above, the filter may be disposed at the front of a PDP. The filter may also be used in various display devices such as a liquid crystal display (LCD) and an organic light emitting diode (OLED). 
     It is possible to effectively realize black images and to improve the bright room contrast of a PDP by disposing an external light shield sheet for absorbing and shielding external light at the front of the PDP. Also, it is possible to reduce the probability of occurrence or perception of the moire phenomenon. 
     Various changes in form and details may be made in the example implementations described and shown, and other implementations are within the scope of the following claims.