Patent Publication Number: US-7916252-B2

Title: In-plane switching liquid crystal display including viewing angle compensation film using +A-plate

Description:
This application is a divisional application of application Ser. No. 12/219,626, filed on Jul. 24, 2008, now U.S. Pat. No. 7,847,896, which is a divisional of application Ser. No. 11/090,301, filed Mar. 28, 2005, now U.S. Pat. No. 7,420,636, which claims priority to Korean Patent Application No. 10-2004-0021202, filed Mar. 29, 2004, each of which are incorporated by reference for all purposes as if fully set forth herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a liquid crystal display (LCD). More particularly, the present invention relates to an in-plane switching liquid crystal display (IPS-LCD) including a compensation film employing at least one A-plate while adjusting an optical axis direction and a retardation value thereof in order to improve a viewing angle characteristic of the IPS-LCD filled with liquid crystal of positive dielectric anisotropy (Δ∈&gt;0) or negative dielectric anisotropy (Δ∈&lt;0). 
     BACKGROUND ART 
     IPS-LCDs are disclosed in U.S. Pat. No. 3,807,831, but this patent does not disclose the use of viewing-angle compensation films. IPS-LCDs including no viewing-angle compensation films have a disadvantage in that they have a low contrast ratio due to a relatively great amount of light leakage in the dark state at inclination angle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a basic structure of an IPS-LCD. 
         FIG. 2  is a view illustrating the arrangement of the absorption axes of polarizer plates and the optical axis of liquid crystals of an IPS-LCD panel in the basic structure of  FIG. 1 . 
         FIG. 3  is a view illustrating a refractive index of a retardation film. 
         FIG. 4  is a view illustrating a structure of a first IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 5  is a view illustrating a structure of a second IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 6  is a view illustrating a structure of a third IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 7  is a view illustrating a structure of a fourth IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 8  is a view illustrating a structure of a fifth IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 9  is a graph representing a simulation result obtained from the first IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
         FIG. 10  is a graph representing a simulation result obtained from the second IPS-LCD including a viewing angle compensation film according to one embodiment of the present invention. 
     
    
    
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide an IPS-LCD representing a superior contrast characteristic and a low color shift at a front and at a predetermined inclination angle of the IPS-LCD by minimizing light leakage in a black state of the IPS-LCD at the predetermined inclination angle. 
     The viewing angle characteristic of the IPS-LCD may be lowered due to a dependency of an orthogonality between the absorption axes of two polarizer plates to the viewing angle and a dependency of a birefringence of an IPS-LCD panel to the viewing angle. 
     The present inventors have found that +A-plates with an optical axis direction and a retardation value adjusted according to an alignment order thereof are necessary to solve the above problems lowering the viewing angle characteristic of the IPS-LCD. On the basis of this finding, the present invention has been completed. 
     Accordingly, the present invention provides an in-plane switching liquid crystal display comprising: a first polarizer plate; a liquid crystal cell, which is filled with liquid crystal of positive dielectric anisotropy (Δ∈&gt;0) or negative dielectric anisotropy (Δ∈&lt;0), an optical axis of the liquid crystal filled in the liquid crystal cell being aligned in-plane in parallel to a polarizer plate; and a second polarizer plate, wherein an absorption axis of the first polarizer plate is perpendicular to an absorption axis of the second polarizer plate, and the optical axis of the liquid crystal filled in the liquid crystal cell is parallel to the absorption axis of the first polarizer plate, wherein at least one A-plate is interposed between the polarizer plate and the liquid crystal cell in order to compensate for a viewing angle, and an optical axis direction and an in-plane retardation value of the A-plate are adjusted according to an alignment order of the A-plate. 
     The present invention is characterized by using upper and lower polarizer plates and at least one A-plate with the optical axis direction and the retardation value thereof adjusted according to the alignment order to the A-plate in order to compensate for the viewing angle of the IPS-LCD in the black state. 
     The contrast ratio value is an index representing a degree of definition for an image, and a higher contrast ratio value allows a higher definition image. The IPS-LCD presents the lowest contrast characteristic at an inclination angle of 70°. If the contrast characteristic of the IPS-LCD can be improved at the inclination angle of 70°, the contrast characteristic of the IPS-LCD can be improved at all viewing angles. When the IPS-LCD exclusively uses the polarizer plate, a minimum contrast ratio value of the IPS-LCD at the inclination angle of 70° is identical to or less than 10:1. The present invention can improve the minimum contrast ratio value by using the A-plate. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention. 
       FIG. 1  is a view illustrating a basic structure of an IPS-LCD. 
     The IPS-LCD includes a first polarizer plate, a second polarizer plate and a liquid crystal cell. An absorption axis of the first polarizer plate is aligned in perpendicular to the an absorption axis  5  of the second polarizer plate and the absorption axis  4  of the first polarizer plate is parallel to an optical axis  6  of liquid crystal of an IPS-panel. In  FIG. 2 , two absorption axes  4  and  5  of two polarizer plates and one optical axis  6  of the liquid crystal are shown. 
     The liquid crystal display using a compensation film according to the present invention includes the first polarizer plate  1 , the liquid crystal cell  3 , which is Homogeneously aligned between two glass substrates and filled with liquid crystal of positive dielectric anisotropy (Δ∈&gt;0) or negative dielectric anisotropy (Δ∈&lt;0), and the second polarizer plate  2 . The optical axis  6  of the liquid crystal filled in the liquid crystal cell is aligned in-plane in parallel to the first and second polarizer plates  1  and  2 . The absorption axis  4  of the first polarizer plate  1  is aligned in perpendicular to the absorption axis  5  of the second polarizer plate  2  and the absorption axis  4  of the first polarizer plate  1  is parallel to the optical axis  6  of the liquid crystal filled in the IPS-panel. In addition, according to the liquid crystal display of the present invention, one of first and second substrates  15  and  16  includes an active matrix drive electrode having a pair of electrodes, which is formed on a surface of the substrate adjacent to a liquid crystal layer. 
     A retardation value of the liquid crystal layer formed in the liquid crystal cell of the IPS-LCD according to the present invention is preferably 200 nm to 350 nm at a wavelength of 550 nm. 
     In order to obtain the white state when voltage is applied to the IPS-panel, light which is linearly polarized at an angle of 90° by passing through the first polarizer plate must be linearly polarized at an angle of 0° by passing through the liquid crystal layer. To this end, the retardation value of the liquid crystal layer of the IPS-panel must be set to a half of 589 nm, wherein 589 nm is a wavelength of monochromatic light providing highest brightness for people. In order to obtain a white color, the retardation value of the liquid crystal layer can be adjusted slightly shorter than or longer than the half of the 589 nm. Therefore, preferably, the liquid crystal layer has the retardation value of about 295 nm, which is about a half of the 589 nm. 
     The LCD according to the present invention may align the liquid crystal in multi-domains, or the liquid crystal may be divided into multi-domains as voltage is applied thereto. 
     The LCDs can be classified into IPS (In-Plane Switching) LCDs, Super-IPS (Super-In-Plane Switching) LCDs and FFS (Fringe-Field Switching) LCDs according to modes of the active matrix drive electrode including a pair of electrodes. In the present invention, the IPS-LCD may include the Super-IPS LCD, the FFS LCD, or a reverse TN IPS LCD. 
       FIG. 3  illustrates a refractive index of a retardation film used for compensating for a viewing angle of the IPS-LCD. Referring to  FIG. 3 , an in-plane refractive index in an x-axis direction having a higher refractive index is n x  ( 8 ), an in-plane refractive index in a y-axis direction having a smaller refractive index is n y  ( 9 ), and a thickness refractive index in a z-axis direction is n z  ( 10 ). Depending on the magnitudes of the refractive indexes, the characteristics of the retardation films will be determined. 
     A film where the refractive indexes in the two-axis directions among the refractive indexes in the three-axis directions are different from each other is referred to as an uniaxial film. A film with n x &gt;n y =n z  is referred to as a positive A-plate, the in-plane retardation value of which is defined using the difference between two refractive indexes lying in a plane, and the thickness of the film as given in the following equation 1.
 
 R   in   =d ×( n   x   −n   y )  (Equation 1)
 
wherein d represents the thickness of the film.
 
       FIGS. 4 to 8  show structures of viewing angle compensation films including the A-plate according to the present invention. 
     An IPS-panel  3  is interposed between two orthogonal polarizer plates  1  and  2 , wherein liquid crystal molecules  7  of the IPS-panel are aligned in parallel to an IPS-LCD panel substrate in a rubbing direction, which is formed on the substrate by surface-treating the substrate such that liquid crystal molecules are aligned in one direction. 
     In order to obtain the viewing angle compensation function, the retardation film must be interposed between the liquid crystal cell  3  and the polarizer plates. 
     An optical axis (or a slow axis) of the retardation film is determined according to the structure of the retardation film. The optical axis of the retardation film can be aligned in perpendicular to or parallel to an absorption axis of an adjacent polarizer plate. 
     The retardation value of the retardation film is determined according to an alignment order of the retardation film. 
       FIGS. 4 to 7  show structures of the IPS-LCDs including the viewing angle compensation films according to the present invention. Herein, it should be noted that a relative position between a backlight unit and an observer must not be inter-changed. The A-plate can be represented with a non-diagonalized matrix at a predetermined inclination angle because the non-diagonalized matrix may output different results depending on a multiplication order. 
     According to a first embodiment of the present invention, there is provided an LCD including an A-plate  11  interposed between a second polarizer plate  2  and a liquid crystal cell  3 , wherein an optical axis  12  of the A-plate  11  is parallel to an absorption axis  5  of the second polarizer plate  2  and the A-plate  11  has an in-plane retardation value in a range of 250 nm to 450 nm at a wavelength of 550 nm. 
     In order to allow the A-plate to compensate for the retardation caused by the IPS-liquid crystal cell, the optical axis of the IPS-liquid crystal cell must be aligned perpendicularly to the optical axis of the A-plate in a black state of the IPS-LCD. Therefore, the optical axis  12  of the A-plate  11  must be aligned in parallel to the absorption axis  5  of the second polarizer plate  2 . 
     The A-plate  11  has the in-plane retardation value in a range of 250 nm to 450 nm at a wavelength of 550 nm due to the following reason. 
     The IPS-liquid crystal cell interposed between the orthogonal polarizer plates is converted into the white state only when the light, which has been linearly polarized at an angle of 0°, is linearly polarized at an angle of 90° after passing through the liquid crystal cell when voltage is applied to the IPS-panel under the conditions in which the retardation value of the liquid crystal cell is a half (λ/2) of 589 nm and the optical axis of the liquid crystal cell is inclined at an angle of 45° with respect to the absorption axis of the polarizer plate. However, since the design value may vary depending on wavelength dispersion characteristics of the liquid crystal, the above function can be obtained only when the retardation value of the A-plate is in a range of 250 nm to 450 nm. 
     The first embodiment of the present invention is shown in  FIG. 4 . 
     Table 1 shows a simulation result obtained from the first IPS-LCD structure as shown in  FIG. 4  when the design value of the retardation film is actually applied to the first IPS-LCD structure. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Internal 
                   
                 A-plate 
                 Internal 
                 Minimum 
               
               
                 protective 
                   
                 (nm) (flat 
                 protective 
                 contrast ratio 
               
               
                 film of 1 st   
                   
                 wavelength 
                 film of 2 nd   
                 value at 
               
               
                 polarizer 
                   
                 dispersion 
                 polarizer 
                 inclination 
               
               
                 plate 
                 IPS-panel 
                 characteristic) 
                 plate 
                 angle of 70° 
               
               
                   
               
             
            
               
                 COP 
                 IPS-panel 
                 260 
                 COP 
                 10 
               
               
                 COP 
                 290 nm 
                 370 
                 40 μm TAC 
                 25 
               
               
                 COP 
                   
                 395 
                 80 μm TAC 
                 30 
               
               
                 COP 
                   
                 415 
                 PNB, −130 nm 
                 10 
               
               
                 40 μm TAC 
                   
                 400 
                 40 μm TAC 
                 20 
               
               
                 40 μm TAC 
                   
                 412 
                 80 μm TAC 
                 75 
               
               
                 80 μm TAC 
                   
                 420 
                 80 μm TAC 
                 15 
               
               
                   
               
            
           
         
       
     
     When the IPS-LCD exclusively uses the polarizer plate, the minimum contrast ratio value of the IPS-LCD is identical to or less than 10:1 at an inclination angle of 70°. Since the minimum contrast ratio value can be obtained in all viewing angles at the inclination angle of 70°, the improvement of the contrast ratio value at the inclination angle of 70° means the improvement of the contrast ratio value in all viewing angles. 
     Table 1 shows the improvement result of the viewing angle characteristic (contrast characteristic) by using the A-plate, in which the most superior viewing angle characteristic can be obtained when the minimum contrast ratio at the inclination angle of 70° has a maximum value. 
     According to a second embodiment of the present invention, there is provided an LCD including an A-plate  11  interposed between a second polarizer plate  2  and a liquid crystal cell  3 , wherein an optical axis  12  of the A-plate  11  is perpendicular to an absorption axis  5  of the second polarizer plate  2  and the A-plate  11  has an in-plane retardation value in a range of 50 nm to 150 nm at a wavelength of 550 nm. 
     Herein, the optical axis  12  of the A-plate  11  must be perpendicular to the absorption axis  5  of the second polarizer plate  2  and the A-plate  11  must have the in-plane retardation value in a range of 50 nm to 150 nm at a wavelength of 550 nm due to the following reason. 
     When the optical axis of the IPS-liquid crystal cell is aligned in parallel to the optical axis of the A-plate, the total retardation value (sum of retardation value of IPS-Panel and A-Plate) is ¾λ at a wavelength of 550 nm. In addition, if a ¼λ retardation film or a ¾λ retardation film is aligned perpendicularly to the absorption axis of the polarizer plate, light leakage generated at a predetermined inclination angle caused by the polarizer plates can be minimized. Therefore, in order to allow the A-plate to perform the above function together with the IPS-Liquid crystal cell, the A-plate is designed such that the total retardation value of the A-plate and the IPS-liquid crystal cell is set to ¾λ at the wavelength of 550 nm. That is, since the design value may vary depending on the wavelength dispersion characteristics of the IPS-liquid Crystal cell and the A-plate, the retardation value of the A-plate is set to the above range in order to obtain the total retardation value of ¾λ. 
     The second embodiment of the present invention is shown in  FIG. 5 . 
     Table 2 shows a simulation result obtained from the second IPS-LCD structure as shown in  FIG. 5  when the design value of the retardation film is actually applied to the second IPS-LCD structure. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Internal 
                   
                 A-plate 
                 Internal 
                 Minimum 
               
               
                 protective 
                   
                 (nm) (flat 
                 protective 
                 contrast ratio 
               
               
                 film of 1 st   
                   
                 wavelength 
                 film of 2 nd   
                 value at 
               
               
                 polarizer 
                   
                 dispersion 
                 polarizer 
                 inclination 
               
               
                 plate 
                 IPS-panel 
                 characteristic) 
                 plate 
                 angle of 70° 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 40 μm TAC 
                 290 nm 
                 110 
                 COP 
                 20 
               
               
                 40 μm TAC 
                   
                 70 
                 40 μm TAC 
                 15 
               
               
                 80 μm TAC 
                   
                 117 
                 COP 
                 25 
               
               
                 80 μm TAC 
                   
                 80 
                 40 μm TAC 
                 20 
               
               
                 80 μm TAC 
                   
                 50 
                 80 μm TAC 
                 15 
               
               
                   
               
            
           
         
       
     
     According to a third embodiment of the present invention, there is provided an LCD including an A-plate  11  interposed between a first polarizer plate  1  and a liquid crystal cell  3 , wherein an optical axis  12  of the A-plate  11  is parallel to an absorption axis  4  of the first polarizer plate  1  and the A-plate  11  has an in-plane retardation value in a range of 40 nm to 150 nm at a wavelength of 550 nm. 
     Herein, the optical axis  12  of the A-plate  11  must be parallel to the absorption axis  4  of the first polarizer plate  1  and the A-plate  11  must have the in-plane retardation value in a range of 40 nm to 150 nm at a wavelength of 550 nm due to the following reason. 
     When the optical axis of the IPS-liquid crystal cell is aligned in parallel to the optical axis of the A-plate, the total retardation value is ¾λ at a wavelength of 550 nm. In addition, if a ¼λ retardation film or a ¾λ retardation film is aligned perpendicularly to the absorption axis of the polarizer plate, light leakage generated at a predetermined inclination angle caused by the polarizer plates can be minimized. Therefore, in order to allow the A-plate to perform the above function together with the IPS-Liquid Crystal cell, the A-plate is designed such that the total retardation value of the A-plate and the IPS-liquid crystal cell is set to ¾λ at the wavelength of 550 nm. That is, since the design value may vary depending on the wavelength dispersion characteristics of the IPS-liquid Crystal cell and the A-plate, the retardation value of the A-plate is set to the above range in order to obtain the total retardation value of ¾λ. 
     The third embodiment of the present invention is shown in  FIG. 6 . 
     Table 3 shows a simulation result obtained from the third IPS-LCD structure as shown in  FIG. 6  when the design value of the retardation film is actually applied to the third IPS-LCD structure. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Internal 
                 A-plate 
                   
                 Internal 
                 Minimum 
               
               
                 protective 
                 (nm) (flat 
                   
                 protective 
                 contrast ratio 
               
               
                 film of 1 st   
                 wavelength 
                   
                 film of 2 nd   
                 value at 
               
               
                 polarizer 
                 dispersion 
                   
                 polarizer 
                 inclination 
               
               
                 plate 
                 characteristic) 
                 IPS-panel 
                 plate 
                 angle of 70° 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 40 μm TAC 
                 85 
                 290 nm 
                 COP 
                 15 
               
               
                 40 μm TAC 
                 60 
                   
                 40 μm TAC 
                 12 
               
               
                 80 μm TAC 
                 105 
                   
                 COP 
                 20 
               
               
                 80 μm TAC 
                 70 
                   
                 40 μm TAC 
                 20 
               
               
                 80 μm TAC 
                 45 
                   
                 80 μm TAC 
                 15 
               
               
                   
               
            
           
         
       
     
     According to a fourth embodiment of the present invention, there is provided an LCD including a first A-plate  11  interposed between a first polarizer plate  1  and a liquid crystal cell  3  and a second A-plate  13  interposed between the liquid crystal cell  3  and a second polarizer plate  2 , wherein an optical axis  12  of the first A-plate  11  is parallel to an absorption axis  4  of the first polarizer plate  1 , an optical axis  14  of the second A-plate  13  is parallel to an absorption axis  5  of the second polarizer plate  2 , the first A-plate  11  has an in-plane retardation value in a range of 100 nm to 150 nm at a wavelength of 550 nm, and the second A-plate  13  has an in-plane retardation value in a range of 350 nm to 450 nm at a wavelength of 550 nm. 
     Herein, the optical axis  12  of the first A-plate  11  must be parallel to the absorption axis  4  of the first polarizer plate  1 , the optical axis  14  of the second A-plate  13  must be parallel to the absorption axis  5  of the second polarizer plate  2 , the first A-plate  11  must have the in-plane retardation value in a range of 100 nm to 150 nm at a wavelength of 550 nm, and the second A-plate  13  must have the in-plane retardation value in a range of 350 nm to 450 nm at a wavelength of 550 nm due to the following reason. 
     In order to minimize light leakage caused by the orthogonal polarizer plates at a predetermined inclination angle by using the A-plate, a ¼λ retardation film or a ¾λ retardation film at the wavelength of 550 nm is used. To this end, the first and second A-plates have the above retardation values such that the total retardation value of the IPS-liquid crystal cell, which is a kind of the A-plates, and the A-plates is set to ¼λ at the wavelength of 550 nm. In the wavelength of 550 nm, the first A-plate plays a role of a ¼λ A-plate, the IPS-liquid crystal cell plays a role of a ½λ A-plate, and the second A-plate plays a role of a −½λ A-plate. Accordingly, the total in-plane retardation value is set to ¼λ. That is, according to the fourth embodiment of the present invention, after forming an A-plate unit including the first and second A-plates and the IPS-liquid crystal cell such that the A-plate unit has the total in-plane retardation value of ¼λ, the A-plate unit is aligned perpendicularly to the absorption axis of the first polarizer plate. 
     The fourth embodiment of the present invention is shown in  FIG. 7 . 
     Table 4 shows a simulation result obtained from the fourth IPS-LCD structure as shown in  FIG. 7  when the design value of the retardation film is actually applied to the third IPS-LCD structure. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Internal 
                 1 st  A-plate 
                   
                 2 nd  A-plate 
                 Internal 
                 Minimum 
               
               
                 protective 
                 (nm) (flat 
                   
                 (nm) (flat 
                 protective 
                 contrast ratio 
               
               
                 film of 1 st   
                 wavelength 
                   
                 wavelength 
                 film of 2 nd   
                 value at 
               
               
                 polarizer 
                 dispersion 
                 IPS- 
                 dispersion 
                 polarizer 
                 inclination 
               
               
                 plate 
                 characteristic) 
                 panel 
                 characteristic) 
                 plate 
                 angle of 70° 
               
               
                   
               
             
            
               
                 40 μm TAC 
                 105 
                 IPS 
                 380 
                 40 μm TAC 
                 55 
               
               
                 40 μm TAC 
                 140 
                 panel 
                 385 
                 80 μm TAC 
                 45 
               
               
                 80 μm TAC 
                 115 
                 290 nm 
                 380 
                 80 μm TAC 
                 20 
               
               
                 80 μm TAC 
                 120 
                   
                 380 
                 40 μm TAC 
                 50 
               
               
                   
               
            
           
         
       
     
     According to a fifth embodiment of the present invention, there is provided an LCD including first and second A-plates  11  and  13  interposed between a second polarizer plate  2  and a liquid crystal cell  3 , wherein the second A-plate film  13  is adjacent to the second polarizer plate  2 , an optical axis (n x )  12  of the first A-plate  11  is perpendicular to an absorption axis  5  of the second polarizer plate  2 , an optical axis  14  of the second A-plate  13  is parallel to an absorption axis  5  of the second polarizer plate  2 , the first A-plate  11  has an in-plane retardation value in a range of 100 nm to 150 nm at a wavelength of 550 nm, and the second A-plate  13  has an in-plane retardation value in a range of 350 nm to 450 nm at a wavelength of 550 nm. 
     Herein, the optical axis of the first A-plate must be perpendicular to the absorption axis  5  of the second polarizer plate  2 , the optical axis  14  of the second A-plate  13  must be parallel to the absorption axis  5  of the second polarizer plate  2 , the first A-plate  11  must have the in-plane retardation value in a range of 100 nm to 150 nm at a wavelength of 550 nm, and the second A-plate  13  must have the in-plane retardation value in a range of 350 nm to 450 nm at a wavelength of 550 nm due to the following reason. 
     The first and second A-plates are designed such that the total retardation value of the IPS-liquid crystal cell and the first and second A-plates is set to ¾λ at the wavelength of 550 nm. That is, at the wavelength of 550 nm, the IPS-liquid crystal cell plays a role of a ½λ A-plate, the first A-plate plays a role of a −¼λ A-plate, and the second A-plate plays a role of a ½λ A-plate. That is, according to the fifth embodiment of the present invention, after forming an A-plate unit including the first and second A-plates and the IPS-liquid crystal cell such that the A-plate unit has the total in-plane retardation value of ¾λ, the A-plate unit is aligned perpendicularly to the absorption axis of the first polarizer plate, thereby minimizing light leakage at a predetermined inclination angle caused by the polarizer plates. 
     The fifth embodiment of the present invention is shown in  FIG. 8 . 
     Table 5 shows a simulation result obtained from the fifth IPS-LCD structure as shown in  FIG. 8  when the design value of the retardation film is actually applied to the third IPS-LCD structure. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Internal 
                   
                 1 st  A-plate 
                 2 nd  A-plate 
                 Internal 
                 Minimum 
               
               
                 protective 
                   
                 (nm) (flat 
                 (nm) (flat 
                 protective 
                 contrast ratio 
               
               
                 film of 1 st   
                   
                 wavelength 
                 wavelength 
                 film of 2 nd   
                 value at 
               
               
                 polarizer 
                 IPS- 
                 dispersion 
                 dispersion 
                 polarizer 
                 inclination 
               
               
                 plate 
                 panel 
                 characteristic) 
                 characteristic) 
                 plate 
                 angle of 70° 
               
               
                   
               
             
            
               
                 40 μm TAC 
                 290 nm 
                 125 
                 395 
                 40 μm TAC 
                 75 
               
               
                 40 μm TAC 
                   
                 125 
                 395 
                 80 μm TAC 
                 43 
               
               
                 80 μm TAC 
                   
                 125 
                 390 
                 80 μm TAC 
                 15 
               
               
                 80 μm TAC 
                   
                 125 
                 385 
                 40 μm TAC 
                 37 
               
               
                   
               
            
           
         
       
     
     The polarizer plates may include protective films used for protecting a polarizer element made from stretched PVA (polyvinyl alcohol) doped with iodine. In addition, the protective film can be made from TAC (triacetate cellulose) having a thickness retardation value, such as 40 μm TAC or 80 μm TAC, PNB (polynorbornene) or COP (cyclo olefin) having no thickness retardation value. The viewing angle compensation characteristic of the IPS-LCD may be influenced by the protective film used for protecting the polarizer element. 
     According to the present invention, the A-plate can be made from polymer or a cured liquid crystal film. 
     In addition, according to the present invention, the A-plate film can be used as an internal protective film for at least one polarizer plate. Since the internal protective film of the polarizer plate is used for protecting the polarizer element, a transparent material having the polarizer element protecting function is used as the internal protective film. That is, since the A-plate is made from the transparent material having the polarizer element protecting function, the A-plate film can be used as a retardation film having the polarizer element protecting function. 
     EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described. However, it is noted that the preferred embodiments described below are used for illustrative purpose only and the present invention is not limited thereto. 
     Embodiment 1 
     The IPS-LCD shown in  FIG. 4  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =395 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate is made from non-oriented COP (COP without stretching) having no retardation value, and the internal protective film for the second polarizer plate is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm.  FIG. 9  shows the simulation result for the contrast ratio value of the IPS-LCD at all inclination angles for all azimuthal angles when the A-plate is used as the viewing angle compensation film under the above circumstance. 
     Referring to  FIG. 9 , a center of a circle corresponds to an inclination angle of 0, and the inclination angle increases as a radius of the circle becomes enlarged. Numerals 20, 40, 60 and 80 marked along the radius of the circle in  FIG. 9  represent the inclination angles. 
     In addition, numerals 0 to 330 marked along a circumference of the circle represent the azimuthal angles.  FIG. 9  shows the contrast characteristic in all viewing directions (inclination angles of 0° to 80° and azimuthal angles of 0° to 360°) when an upper polarizer plate is aligned in a direction of an azimuthal angle of 0°, and a lower polarizer plate is aligned in a direction of an azimuthal angle of 90°. The conventional IPS-LCD, which exclusively uses the polarizer plates, may represent a contrast ratio equal to or less than 10:1 at an inclination angle of 70°. However, the IPS-LCD of the present invention represents a superior contrast ratio above 30:1 at an inclination angle of 70° as shown in  FIG. 9  and Table 1. 
     Embodiment 2 
     The IPS-LCD shown in  FIG. 4  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =412 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 75:1 (see, Table 4). 
     Embodiment 3 
     The IPS-LCD shown in  FIG. 5  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =110 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from COP having a retardation value of about 0. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 20:1 (see, Table 2). 
     Embodiment 4 
     The IPS-LCD shown in  FIG. 5  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =117 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from COP having a retardation value of about 0. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 25:1 (see, Table 2). 
     Embodiment 5 
     The IPS-LCD shown in  FIG. 5  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =50 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 15:1 (see, Table 2). 
     Embodiment 6 
     The IPS-LCD shown in  FIG. 6  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =85 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from COP having a retardation value of about 0. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 15:1 (see, Table 3). 
     Embodiment 7 
     The IPS-LCD shown in  FIG. 6  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =105 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from COP having a retardation value of about 0. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 20:1 (see, Table 3). 
     Embodiment 8 
     The IPS-LCD shown in  FIG. 6  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate  11  is fabricated by using stretched m-PC (modified-polycarbonate) and has an in-plane retardation value R in =70 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 20:1 (see, Table 3). 
     Embodiment 9 
     The IPS-LCD shown in  FIG. 7  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =105 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =380 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm.  FIG. 10  shows the simulation result for the contrast ratio value at inclination angles for all azimuthal angles. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 55:1 (see, Table 4). 
     Embodiment 10 
     The IPS-LCD shown in  FIG. 7  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =115 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =380 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 20:1 (see, Table 4). 
     Embodiment 11 
     The IPS-LCD shown in  FIG. 7  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =120 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =380 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 50:1 (see, Table 4). 
     Embodiment 12 
     The IPS-LCD shown in  FIG. 8  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =125 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =395 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 75:1 (see, Table 5). 
     Embodiment 13 
     The IPS-LCD shown in  FIG. 8  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =125 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =395 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 40 μm and a thickness retardation value R th =−32 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 43:1 (see, Table 5). 
     Embodiment 14 
     The IPS-LCD shown in  FIG. 8  includes the IPS-liquid crystal cell  3  filled with the liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°, dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1 at a wavelength of 550 nm. The A-plate is fabricated by using stretched m-PC (modified-polycarbonate) and includes a first A-plate  11  having an in-plane retardation value R in =125 nm at a wavelength of 550 nm and a second A-plate  13  having an in-plane retardation value R in =390 nm at a wavelength of 550 nm. The internal protective film for the first polarizer plate  1  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm, and the internal protective film for the second polarizer plate  2  is made from TAC (Triacetate Cellulose) having a thickness of about 80 μm and a thickness retardation value R th =−65 nm. When the above viewing angle compensation film and polarizer plates are employed, the minimum contrast ratio value of the IPS-LCD at an inclination angle of 70° for all azimuthal angles is 15:1 (see, Table 5). 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims. 
     INDUSTRIAL APPLICABILITY 
     As described above, the in-plane switching liquid crystal display using at least one A-plate according to the present invention can improve the contrast characteristic at a front and at a predetermined inclination angle thereof by adjusting the optical axis direction and the retardation value of the A-plate while minimizing a color shift according to viewing angles in the black state.