Patent Publication Number: US-2007121027-A1

Title: Liquid crystal display

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims priority from Korean Patent Application No. 10-2005-0115851 filed on Nov. 30, 2005 in the Korean Intellectual Property Office, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Technical Field  
      The present disclosure relates to a liquid crystal display, and more particularly, to a liquid crystal display capable of improving a viewing angle and brightness and providing a symmetric viewing angle in all directions.  
      2. Discussion of the Related Art  
      A liquid crystal display is one of the most widely used flat panel displays. A liquid crystal display includes two glass substrates (plates) provided with electrodes and a liquid crystal layer interposed therebetween. The liquid crystal display displays images by applying voltages to the electrodes to generate an electric field in the liquid crystal layer, which rearranges liquid crystal molecules in the liquid crystal layer to adjust transmittance of incident light.  
      A liquid crystal display may include thin film transistors (TFT) for switching voltages applied to electrodes on one of the plates. In addition to the TFTs, such a TFT plate further includes gate and data lines, and gate and data pads receiving gate and data signals from an external source and transmitting the received gate and data signals to the gate and data lines. Pixel electrodes are disposed at pixel areas defined by intersections between the gate lines and the data lines and are electrically connected to the TFTs.  
      Various methods for improving a response speed and a viewing angle of a liquid crystal display have been suggested. One method for improving the response speed and the viewing angle of a liquid crystal display is an optically compensated bend (OCB) mode liquid crystal display.  
      The OCB mode liquid crystal display includes two opposing panels including various electrodes, a liquid crystal layer inserted between the two panels, alignment films disposed on respective inner surfaces of the two panels, which allow liquid crystal molecules to be aligned horizontally with respect to the two panels, and polarization plates disposed on respective exterior surfaces of the two panels.  
      In the OCB mode liquid crystal display, a white display is observed by applying a slight voltage to a space between two panels, while a black display is observed by applying a voltage higher than the voltage applied to the space between the two panels. In the OCB mode liquid crystal display, the alignment films of the two panels are rubbed in the same direction, and an initially applied high voltage allows liquid crystal molecules to be perpendicular to the surfaces of the two panels and to have a bend alignment.  
      The OCB mode liquid crystal display has a wide and symmetrical viewing angle in a direction perpendicular to the rubbing direction, while having a narrow viewing angle in the rubbing direction.  
      In such a liquid crystal display including two panels, an electric field is generated at a liquid crystal layer by respectively applying a data voltage and a common voltage to a pixel electrode and a common electrode of the two panels. The transmittance of light passing through the liquid crystal layer is controlled by adjusting the intensity of the electric field to create desired images. To prevent image deterioration due to long-time application of a unidirectional electric field to the liquid crystal layer, the polarity of the data voltage with respect to the common voltage is reversed every frame, every row, or every pixel.  
      However, when the polarity of the data voltage is reversed, the response speed of liquid crystal molecules is decreased. Thus, it takes a long time for a liquid crystal capacitor to reach a target voltage level, thereby causing an image blurring phenomenon. To solve the problem, impulsive driving that inserts a black image between normal images for a short time was developed.  
      The impulsive driving includes a backlight switching type driving that periodically turns off a backlight lamp to yield black images and a liquid crystal switching type driving that periodically applies a black data voltage, in addition to a normal data voltage substantially participating in image display, to pixels.  
      According to the backlight switching type driving or the liquid crystal switching type driving, a brightness reduction inevitably occurs due to the “OFF” period of the backlight or a duty ratio.  
      In order to overcome the above-described viewing angle and brightness problems, a conventional liquid crystal display includes optical sheets. However, there is still a limitation to an improvement in viewing angle although a slight increase in the brightness is achieved. In this regard, when more optical sheets are used to improve the viewing angle, manufacturing costs are increased, making liquid crystal displays ineffective in terms of the manufacturing costs.  
     SUMMARY OF THE INVENTION  
      An exemplary embodiment of the present invention includes a liquid crystal display including a backlight assembly emitting light, a liquid crystal panel, disposed on the backlight assembly, displaying an image using the light emitted from the backlight assembly and having at least one alignment film, optical sheets, interposed between the backlight assembly and the liquid crystal panel, having a surface which has a prismatic pattern of prisms focusing the light emitted from the backlight assembly, wherein a long edge direction of the prismatic pattern is substantially the same as a rubbing direction of the at least one alignment film, and upper and lower housing units receiving the backlight assembly, the optical sheets, and the liquid crystal panel.  
      An exemplary embodiment of the present invention includes a liquid crystal display including a backlight assembly emitting light, a liquid crystal panel, disposed on the backlight assembly, displaying an image using the light emitted from the backlight assembly, and having a pair of display plates with alignment films, and an optically compensated bend mode liquid crystal layer interposed between the display plates, optical sheets, interposed between the backlight assembly and the liquid crystal panel, having a surface which has a prismatic pattern of prisms focusing the light emitted from the backlight assembly, wherein a long edge direction of the prismatic pattern is substantially the same as the rubbing directions of the alignment films, and upper and lower housing units receiving the backlight assembly, the optical sheets, and the liquid crystal panel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Exemplary embodiments of the present invention can be understood in more detail from the following descriptions taken in conjunction with the attached drawings in which:  
       FIG. 1  shows an exploded perspective view illustrating a liquid crystal display according to an embodiment of the present invention;  
       FIG. 2A  shows a layout of a liquid crystal panel of  FIG. 1 ;  
       FIG. 2B  shows a sectional view taken along a line IIb-IIb′ of  FIG. 2A ;  
       FIG. 3A  shows a schematic cross-sectional view of the liquid crystal display of  FIG. 1 ;  
       FIG. 3B  schematically illustrates rubbing directions of alignment films, transmission axes of polarization films, and the long edge direction of a prismatic pattern in a liquid crystal panel of  FIG. 3A ;  
       FIG. 4A  is a graph illustrating a brightness distribution of light emitted from the liquid crystal display of  FIG. 3A ;  
       FIG. 4B  is a graph illustrating brightness with respect to viewing angle in the transverse and longitudinal directions of a liquid crystal panel of  FIG. 4A ; and  
       FIG. 4C  is a graph illustrating brightness with respect to viewing angle in the left and right diagonal directions of the liquid crystal panel of  FIG. 4A . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Exemplary embodiments of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like reference numerals refer to like elements throughout the specification.  
       FIG. 1  shows an exploded perspective view illustrating a liquid crystal display according to an embodiment of the present invention. Referring to  FIG. 1 , the liquid crystal display  100  includes a liquid crystal panel assembly  130 , a backlight assembly  140 , an upper housing unit  110 , and a lower housing unit  160 .  
      The liquid crystal panel assembly  130  includes a liquid crystal panel  136  including a thin film transistor plate  133  and a common electrode plate  134 , a liquid crystal layer (not shown), a gate tape carrier package  131 , a data tape carrier package  132 , and a printed circuit board  135 .  
      The thin film transistor plate  133  includes gate lines (not shown), data lines (not shown), a thin film transistor array (not shown), and pixel electrodes (not shown). The common electrode plate  134  includes a black matrix (not shown), a common electrode (not shown), and faces the thin film transistor plate  133 .  
      The gate tape carrier package  131  is connected to the gate lines formed in the thin film transistor plate  133 , and the data tape carrier package  132  is connected to the data lines formed in the thin film transistor plate  133 .  
      Various driving elements for providing a gate driving signal and a data driving signal to the gate tape carrier package  131  and the data tape carrier package  132 , respectively, are mounted onto the printed circuit board  135 .  
      The backlight assembly  140  includes optical sheets  141 , an optical plate  142 , lamps  143 , and a reflective plate  144 .  
      The lamps  143  may be light emitted diodes (LEDs), cold cathode fluorescent lamps (CCFLs), or external electrode fluorescent lamps (EEFLs). The lamps  143  receive a lamp driving voltage from an external source and generate light. The lamps  143  may be separated from each other by a predetermined distance and be connected in parallel in the same phase. The lamps  143  may be direct-type lamps. The lamps  143  may be arranged in the transverse direction of the liquid crystal panel  136  to maintain a uniform discharge gas distribution and thus achieve brightness uniformity. Although not shown, lamp holders are positioned at the outermost lamps  143  to support the lamps  143 .  
      The optical plate  142  may be disposed at the upper side of the lamps  143 , and enhances the brightness uniformity of light emitted from the lamps  143 .  
      The reflective plate  144  is disposed at the lower side of the lamps  143  to reflect a light beam emitted through the lower side of the lamps  143 . The reflective plate  144  may be integrally formed with the bottom of the lower housing unit  160 . That is, when the lower housing unit  160  is made of a highly reflective material such as aluminum (Al) or its alloy, it can provide a reflective function. The optical sheets  141  are placed at the upper side of the optical plate  142 , and diffuse and focus the light emitted from the lamps  143 . The optical sheets  141  include a diffusion sheet, a first prism sheet, and a second prism sheet.  
      The diffusion sheet is disposed at the upper side of the lamps  143 , and enhances the brightness and brightness uniformity of the light emitted from the lamps  143 .  
      The first prism sheet is disposed on the diffusion sheet, and a surface of the first prism sheet has a prismatic pattern composed of a predetermined array of trigonal prisms for focusing the light diffused from the diffusion sheet and outputting the focused light. The first prism sheet may be a brightness enhancement film. The brightness and viewing angle of a liquid crystal display can be enhanced along the long edge direction of the prisms of the prism pattern formed on a surface of the first prism sheet.  
      The second prism sheet is disposed on the first prism sheet, and is a multi-layered, reflective-type polarization prism sheet for focusing, polarizing, and outputting light. The second prism sheet may be a dual brightness enhancement film. If the brightness and viewing angle of a liquid crystal display can be sufficiently assured by using only the first prism sheet, the second prism sheet may be omitted.  
      The liquid crystal panel assembly  130  is disposed at the upper side of the optical sheets  141 . The liquid crystal panel assembly  130 , together with the backlight assembly  140 , is received in the lower housing unit  160  while being supported by a receiving frame  150 . The receiving frame  150  is a rectangular frame having sidewalls. Inner portions of the sidewalls of the receiving frame  150  have stepped portions or protrusions for supporting the liquid crystal panel assembly  130  and the backlight assembly  140 . The lower housing unit  160  is rectangular, and has sidewalls along upper edges and thus receives the backlight assembly  140  and the liquid crystal panel assembly  130  in a receiving space defined by the sidewalls. The lower housing unit  160  also serves to prevent the bending of the backlight assembly  140  including a number of sheets. The printed circuit board  135  of the liquid crystal panel assembly  130  is folded along outer portions of the sidewalls of the lower housing unit  160  to be fitted in the bottom of the lower housing unit  160 . The shape of the lower housing unit  160  can be changed according to a method of placing the backlight assembly  140  and the liquid crystal panel assembly  130  in the lower housing unit  160 .  
      The lower housing unit  160  is coupled with the upper housing unit  110  to cover an upper surface of the liquid crystal panel assembly  130  received in the lower housing unit  160 . A window (not shown) exposing the liquid crystal panel assembly  130  to the outside is formed at the upper surface of the upper housing unit  110 .  
      The upper housing unit  110  may be hooked and/or screwed to the lower housing unit  160 .  
       FIG. 2A  shows a layout of a liquid crystal panel of  FIG. 1 , and  FIG. 2B  shows a sectional view taken along a line IIb-IIb′ of  FIG. 2A . Referring to  FIGS. 2A and 2B , together with  FIG. 1 , the liquid crystal panel  136  includes the thin film transistor plate  133 , the common electrode plate  134 , a liquid crystal layer  3  inserted between the thin film transistor plate  133  and the common electrode plate  134 , compensation films  210  attached to respective exterior surfaces of the thin film transistor plate  133  and the common electrode plate  134 , and polarization films  212  attached to respective exterior surfaces of the compensation films  210 .  
      In the thin film transistor plate  133 , a gate line  22  is formed on an insulating substrate  10  in a transverse direction, and a gate electrode  26  is connected to the gate line  22  in the formed of a protrusion. The gate line  22  and the gate electrode  26  constitute a gate wire.  
      The gate wire is preferably made of Al containing metal such as Al and Al alloy, Ag containing metal such as Ag and Ag alloy, Cu containing metal such as Cu and Cu alloy, Mo containing metal such as Mo and Mo alloy, Cr, Ti or Ta. In addition, the gate wire may have a multi-layered structure including two conductive films (not shown) having different physical characteristics. One of the two films is preferably made of a low resistivity metal including Al containing metal, Ag containing metal, and Cu containing metal for reducing signal delay or voltage drop in the gate wire. The other film is preferably made of material such as a Mo containing metal, Cr, Ta or Ti, which have good physical, chemical, and electrical contact characteristics with other materials such as indium tin oxide (ITO) or indium zinc oxide (IZO). Examples of combinations of the two films are a lower Cr film and an upper Al containing film and a lower Al containing film and an upper Mo containing film. However, the gate wire may be made of various metals or conductors.  
      In addition, a storage electrode wire (not shown) extending in a transverse direction in parallel with the gate line  22  may be formed on the insulating substrate  10 . A predetermined portion of the storage electrode wire overlaps with the pixel electrode  82 , thereby forming a storage capacity. The shape and arrangement of the storage electrode may vary. The pixel electrode  82  may overlap the previous gate line  22  to form a storage capacity, which is called a separate wire type.  
      A gate insulating layer  30  is formed on the gate wire. The gate insulating layer  30  is made of an insulating material such as silicon nitride (SiNx) or silicon oxide (SiOx).  
      A semiconductor layer  40  made of hydrogenated amorphous silicon or polycrystalline silicon is formed on the gate insulating layer  30 . The semiconductor layer  40  may be formed in various shapes such as an island shape or a stripe shape, and, for example, may be formed in an island shape extending over the gate electrode  26  under the data line  62 . When the semiconductor layer  40  is formed in a stripe shape, it may be disposed under the data line  62  and extend up to the gate electrode  26 .  
      Ohmic contact layers  55  and  56 , which are made of silicide or n+ amorphous silicon hydride in which an n-type impurity is highly doped, are on the semiconductor layer  40 . The ohmic contact layers  55  and  56  may have a variety of shapes such as an island shape or a stripe shape. For example, the ohmic contact layers  55  and  56  may be formed under the drain electrode  66  and the source electrode  65  in an island shape. When the ohmic contact layers  55  and  56  are formed in a stripe shape, they may be disposed and extend under the data line  62 .  
      The data line  62  and the drain electrode  66  are formed on the ohmic contact layer  55  and  56  and the gate insulating layer  30 . The data line  62  extends in a longitudinal direction and intersects the gate line  22 . The source electrode  65  extending over the semiconductor layer  40  is formed as a branch of the data line  62 . The drain electrode  66  is separated from the source electrode  65  and is formed on the semiconductor layer  40  opposite the source electrode  65  in view of the gate electrode  26 . The drain electrode  66  includes a bar-type pattern on the semiconductor layer  40  and a drain electrode extension portion having a large area extending from the bar-type pattern and a contact hole  76  therein. The data line  62 , the source electrode  65 , and the drain electrode  66  constitute a data wire.  
      The data wire is preferably formed as a single layer or a multiple layer made of at least one material selected from the group consisting of aluminum (Al), chromium (Cr), molybdenum (Mo), tantalum (Ta), and titanium (Ti). For example, the data wire and the storage electrode  67  are preferably made of refractory metal such as Cr, a metal containing Mo, Ta, or Ti. Alternatively, the data wire and the storage electrode  67  may have a multi-layered structure including a lower film (not shown) made of a lower refractory metal film and a low-resistivity upper film (not shown). Examples of the multi-layered structure include a double-layered structure having a lower Cr film and an upper Al containing film, a double-layered structure having a lower Mo containing film and an upper Al containing film, and a triple-layered structure having a lower Mo film, an intermediate Al film, and an upper Mo film.  
      At least a portion of the source electrode  65  overlaps the semiconductor layer  40 , and the drain electrode  66  is opposite to the source electrode  65  in view of the gate electrode  26  and at least a portion of the drain electrode  66  overlaps the semiconductor layer  40 . Here, the ohmic contact layers  55  and  56  are interposed between the underlying semiconductor layer  40  and the source electrode  55  and the drain electrode  66  to reduce the contact resistance between them.  
      A passivation layer  70  is formed of an organic insulating layer on the data wire, the storage electrode  67  and an exposed portion of the semiconductor layer  40 . Here, the passivation layer  70  is preferably made of an inorganic insulator such as silicon nitride or silicon oxide, a photosensitive organic material having a good flatness characteristic, or a low dielectric insulating material such as a-Si:C:O and a-Si:O:F formed by plasma enhanced chemical vapor deposition (PECVD).  
      A contact hole  74  exposing the data line pad  54  and a first contact hole  72  exposing the drain electrode  66  is formed on the passivation layer  70 .  
      A pixel electrode  82  is formed along the shape of a pixel on the passivation layer  70 . The pixel electrode  82  is electrically connected to the drain electrode  66  via the contact hole  76 . The pixel electrode  82  is made of a transparent conductor such as ITO or IZO or a reflective conductor such as Al.  
      An alignment layer (not shown) capable of aligning the liquid crystal layer  3  is coated on the pixel electrode  82 .  
      In the common electrode plate  134 , a black matrix  94  for preventing light leakage is disposed on an insulating substrate  96  made of a transparent insulating material such as glass. The black matrix  94  partially covers a gate line  22 , a data line  62 , and a thin film transistor of the thin film transistor plate  133 .  
      A color filter  98  is disposed on portions of the insulating substrate  96  and the black matrix  94  corresponding to each pixel, and is comprised of sequentially arranged red, green, and blue components.  
      A common electrode  90  made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO) is disposed on the color filter  98 . Here, the common electrode  90  is common to a plurality of pixels.  
      An alignment film (not shown) allowing liquid crystal molecules of the liquid crystal layer  3  to be aligned in a predetermined direction with respect to the surface of the insulating substrate  96  is disposed on the common electrode  90 .  
      When the common electrode plate  134  and the thin film transistor plate  133  are coupled with a predetermined gap therebetween, the liquid crystal layer  3  inserted between the common electrode plate  134  and the thin film transistor plate  133  has a predetermined cell gap.  
      Liquid crystal molecules of the liquid crystal layer  3  are aligned so that a liquid crystal display is driven in an optically compensated bend (OCB) mode. That is, in the OCB mode, nematic liquid crystals are set to a splay alignment at an initial state, and when a voltage is applied to a liquid crystal cell, the splay alignment is transferred to a bend alignment, and light transmittance is controlled by adjusting the applied voltage. Alignment films (not shown) are formed on surfaces of a pixel electrode  82  and the common electrode  90  and rubbed so liquid crystal molecules are aligned in a predetermined direction. The alignment films formed on the surfaces of the pixel electrode  82  and the common electrode  90  are rubbed in the same direction so liquid crystal molecules are set to a splay alignment. The alignment films are substantially rubbed in about a 45 degree or about a 135 degree direction with respect to the transverse direction of the liquid crystal panel  136  to reduce the manufacturing costs of the polarization films  212  and to achieve the symmetric viewing angle in all directions.  
      The compensation films  210  are disposed on respective exterior surfaces of the thin film transistor plate  133  and the common electrode plate  134 , and the polarization films  212  are disposed on exterior surfaces of the compensation films  210 .  
      The polarization axes (or transmission axes) of the two polarization films  212  are perpendicular to each other and form an angle of about 45 degrees or about 135 degrees with respect to the rubbing direction of the alignment films.  
      The compensation characteristics of the compensation films  210  are optimized based on green light. Each of the compensation films  210  is composed of a support and a discotic layer.  
      The support is a layer that plays a major role in maintaining the shapes of the compensation films  210 . Thus, a triacetate cellulose (TAC) film is mainly used as the support. The discotic layer is a compensation layer having a hybrid structure to compensate for the effect of hybrid-aligned liquid crystal molecules.  
      A relation among the rubbing direction of the alignment films, the transmission axes of the polarization films  210 , and the long edge direction of the prisms of the prismatic pattern of the first prism sheet will now be described with reference to  FIGS. 3A through 4C .  
       FIG. 3A  shows a schematic cross-sectional view of the liquid crystal display of  FIG. 1 , and  FIG. 3B  schematically illustrates rubbing directions of alignment films, transmission axes of polarization films, and the long edge direction of a prismatic pattern in a liquid crystal panel of  FIG. 3A .  FIG. 4A  is a graph illustrating a brightness distribution of light emitted from the liquid crystal display of  FIG. 3A ,  FIG. 4B  is a graph illustrating brightness with respect to viewing angle in the transverse and longitudinal directions of a liquid crystal panel of  FIG. 4A , and  FIG. 4C  is a graph illustrating brightness with respect to viewing angle in the left and right diagonal directions of the liquid crystal panel of  FIG. 4A .  
      Referring to  FIGS. 3A and 3B , a liquid crystal display includes a liquid crystal panel  136  composed of a thin film transistor plate  133  and a common electrode plate  134  that face to each other. As described above, alignment films are respectively disposed on a pixel electrode and a common electrode. The alignment films are rubbed in about a 45 degree direction with respect to the transverse direction of the liquid crystal panel  136 , as indicated by a dotted arrow labeled “R”.  
      A liquid crystal layer (not shown) having positive dielectric anisotropy is interposed between the thin film transistor plate  133  and the common electrode plate  134 . At a voltage-OFF state, liquid crystal molecules have a predetermined pretilt angle toward the rubbing direction R and are aligned horizontally with respect to the thin film transistor plate  133  and the common electrode plate  134 . On the other hand, at a voltage-ON state, the liquid crystal molecules have a bend alignment. That is, when an electric field is sufficiently applied between the thin film transistor plate  133  and the common electrode plate  134 , the long axes of the liquid crystal molecules are aligned substantially parallel to the electric field, i.e., substantially perpendicular to the thin film transistor plate  133  and the common electrode plate  134 , due to the positive dielectric anisotropy of the liquid crystal molecules.  
      Polarization films  212  are attached to exterior surfaces of the thin film transistor plate  133  and the common electrode plate  134 . The transmission axes P of the two polarization films  212  are substantially perpendicular to each other, and substantially form an angle of about 45 degrees or about 135 degrees with respect to the rubbing direction R of the alignment films of the thin film transistor plate  133  and the common electrode plate  134 . That is, the transmission axes P are substantially parallel to the transverse or longitudinal direction of the liquid crystal panel  136 . The polarization films  212  may be formed using an elongation process. A polarization film having a transmission axis parallel to the transverse or longitudinal direction of a rectangular liquid crystal panel can reduce manufacturing costs, compared to a polarization film having a transmission axis parallel to the diagonal direction of a liquid crystal panel.  
      Optical sheets  141 , e.g., first prism sheets, are disposed below the polarization film  212  at the lower side of the thin film transistor plate  133 . In a conventional OCB mode liquid crystal display, the viewing angles and the symmetrical property of the viewing angle in a direction perpendicular to the rubbing direction of an alignment film (i.e., in about 135 degree and about 315 degree directions with respect to the transverse direction of a liquid crystal panel) are excellent, whereas the viewing angle in the rubbing direction is reduced. Referring to  FIGS. 3A and 3B , a first prism sheet is disposed so the long edge direction, as indicated by an arrow labeled “BEF”, of prisms of a prismatic pattern formed at the first prism sheet is substantially the same as the rubbing direction R. By doing so, light beams diffused from a diffusion sheet are focused on an area of the first prism sheet substantially perpendicular to the rubbing direction R, but are not focused on an area of the first prism sheet parallel to the rubbing direction R. Therefore, a wide viewing angle can be achieved, thereby achieving the symmetric viewing angle. As described above, a liquid crystal display according to the present invention can yield a sufficient viewing angle using only a single first prism sheet, thereby resulting in a reduction in manufacturing costs.  
      Furthermore, when brightness of a white display does not reach a desired level, a brightness increase can be induced using a second prism sheet. When brightness of a white display reaches a desired level, the second prism sheet is omitted, thereby leading to a further reduction in manufacturing costs.  
      Referring to  FIG. 4A  illustrating a brightness distribution of a liquid crystal display according to an embodiment of the present invention, symmetrical viewing angles in all directions can be achieved. A rubbing direction R is the substantially same as a long edge direction of prisms of a prismatic pattern of a first prism sheet, and thus, it is possible to yield a sufficient viewing angle in the rubbing direction R.  
      Referring to  FIG. 4B , a brightness distribution A for the longitudinal direction (e.g., 90 degree direction) of a liquid crystal panel is substantially the same as a brightness distribution B for the transverse direction (e.g., 0 degree direction) of the liquid crystal panel. Both the brightness distributions A and B are symmetrical with respect to a viewing angle of about 0 degrees. Therefore, a liquid crystal display according to the present invention can provide a wide and symmetrical viewing angle in horizontal and vertical directions.  
      Referring to  FIG. 4C , a brightness distribution C for the upper-left diagonal direction (e.g., 135 degree direction) of a liquid crystal panel is substantially the same as a brightness distribution D for the upper-right diagonal direction (e.g., 45 degree direction) within an effective viewing angle range. Both the brightness distributions C and D are symmetrical with respect to a viewing angle of about 0 degrees. Therefore, a liquid crystal display according to the present invention can provide a wide and symmetrical viewing angle in left and right diagonal directions.  
      The above-described embodiment of the present invention has been illustrated in terms of the viewing angle and the symmetric viewing angle of an OCB mode liquid crystal display. However, the present invention is not limited to the above-illustrated example, and can also be applied to liquid crystal displays having an asymmetrical viewing angle in a particular direction.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.