Patent Publication Number: US-2016246096-A1

Title: Liquid crystal panel compensation structure and liquid crystal display apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to a field of liquid crystal display technique, and more particularly to a liquid crystal panel compensation structure and liquid crystal display apparatus. 
     BACKGROUND OF THE INVENTION 
     A liquid crystal display (LCD) is a thin flat display apparatus which comprises a certain amount of color or monochrome pixels and is disposed before a light source or a reflecting surface. Because the liquid crystal display has low power consumption and is characterized in high display quality, small size and light weight, it is favored by peoples and becomes a main display apparatus. Nowadays, the major one of the liquid crystal display is the thin film transistor (TFT) liquid crystal display. 
     As the area of the TFT-LCD being enlarged, a viewing angle is increased,and therefore the contrast and the definition of the image is decreased due to a result of birefringence variation of the liquid crystal molecules in a liquid crystal layer following on variation of the viewing angle. For a normal liquid crystal display panel, the brightness is rapidly decreased (i.e. become darker) and the color is changed when the normal liquid crystal display panel is watched from a certain angle. A conventional liquid crystal display usually provides a 90 degree viewing angle, i.e. 45 degree viewing angle in the left and right side, respectively. The nematic liquid crystal is a substance having birefringence Δn. The light passing through the liquid crystal molecule is divided into an ordinary ray and an extraordinary ray. If the light is obliquely incident into the liquid crystal molecule, two refracted light would be generated. The birefringence of the light is Δn=ne−no, wherein ne represents the refractive index of the liquid crystal molecule for the ordinary ray, and no represents the refractive index of the liquid crystal molecule for the extraordinary ray. Therefore, after the light passing through the liquid crystal clamped between the top and bottom glasses, a phase retardation would be occurred on the light. The light characteristic of a liquid crystal cell is measured by the phase retardation Δn·d, which is so called as optical path difference, wherein Δn is the birefringence and d is the depth of the liquid crystal cell. Different phase retardation under different viewing angles is the reason why the problem of viewing angle occurs. A good phase retardation of an optical compensation film could cancel the phase retardation of the nematic liquid crystal, such that the visual angle of the liquid crystal panel can be increased. 
     The principle of compensation of the optical compensation film is to correct the phase difference generated from the liquid crystal at different viewing angles to symmetrically compensate the birefringence nature of the liquid crystal molecule. By applying the optical compensation film for compensating, light leakage of the dark image can be effectively reduced and the contrast of an image in a certain range of viewing angle can be greatly increased. In view of the function and object of the optical compensation film, the optical compensation film can be divided into a phase difference film that purely changes phases, a chromatic compensation film, a visual angle enlargement film, etc. By applying the optical compensation film, light leakage of the dark image can be effectively reduced, the contrast and chromaticity of an image in a certain range of viewing angle can be greatly improved, and a part of the gray level inversion can be overcome. The main parameters for measuring the characteristic of the optical compensation film comprises an in-plane compensation value Ro in the plane direction, a depth compensation value Rth in the depth direction, the refractive index N and the film depth D, which satisfies the following equation: 
         Ro =( Nx−Ny )· D;  
 
         Rth =[( Nx+Ny )/2− Nz]·D  
 
     Wherein, Nx is the refractive index along a slow axis (an axis with a biggest refractive index, i.e. the vibration direction of the light with slower transmitting speed) in the plane of the film, Ny is the refractive index along a fast axis (an axis with a smallest refractive index, i.e. the vibration direction of the light with faster transmitting speed, and is perpendicular to Nx) in the plane of the film, and Nz is the refractive index in the plane direction of the film (perpendicular to Nx and Ny). 
     For different liquid crystal display mode, i.e. different type of liquid crystal cell, the applied optical compensation film should be different and the values of Ro and Rth should be adjusted to an adequate value. The optical compensation films used in the large scale liquid crystal TV nowadays are mostly for VA (vertical alignment) display mode. The one used in early days is the N-TAC of Konica, and then is continuously developed to the Zeonor of OPTES, the F-TAC series of Fujitsu, the X-plate of Nitto, etc. 
     For different optical path differences of the liquid crystal, it is necessary to design different optical compensation modes. For a liquid crystal panel with optical path difference in a range between 324.8˜361.4 nm, it can be seen from  FIG. 1  and  FIG. 2 , wherein  FIG. 1  is an isoluminance contour diagram for wide viewing angle in dark state of a conventional liquid crystal panel after being compensated by a known compensating structure, and  FIG. 2  is an equal contrast ratio contour diagram in wide viewing angle of the liquid crystal panel after being compensated by the bilayer-biaxial compensating structure described above. It can be seen from  FIG. 1  and  FIG. 2  that there is serious light leakage at the position of which the horizontal viewing angles phi are 20˜40°, 140˜160°, 200˜220°, and 310˜330° when applying the conventional compensation structure for compensating. The viewing angles where serious light leakage in dark state are much closer to the horizontal viewing angles, and the contrast ratio and definition in these viewing angles are low. However, the relative position between the viewers and the TV determines that the area near the horizontal viewing angles are easier to be seen by the viewers, and therefore the contrast ratio and the definition at these viewing angles make most affection to the viewing effect. 
     SUMMARY OF THE INVENTION 
     In view of the drawbacks existed in the conventional technique, the present invention provides a liquid crystal panel compensation structure which can reduce the problem of light leakage in dark state of the liquid crystal panel and increase contrast and definition at large viewing angle through setting compensation value for the liquid crystal panel with optical path difference in a range between 324.8˜361.4 nm. 
     In order to achieve the object described above, the present invention adopts the technique solution as below: 
     A liquid crystal panel compensation structure comprises a liquid crystal panel, and a first compensation film and a second compensation film disposed at two sides of the liquid crystal panel, respectively; a liquid crystal layer comprises a plurality of liquid crystal molecules being disposed in the liquid crystal panel, a refractive index anisotropy of the liquid crystal layer is Δn, a depth of the liquid crystal layer is d, and a pretilt angle of the liquid crystal molecules is θ; the first compensation film is a biaxial compensation film, an in-plane compensation value thereof is Ro1, and a depth compensation value thereof is Rth1; the second compensation film is an uniaxial compensation film, and the depth compensation value thereof is Rth2, wherein 
       342.8 nm≦Δ n·d≦ 361.4 nm;
 
       85°≦θ&lt;90°;
 
       52 nm≦Ro1≦78 nm;
 
       196 nm≦Rth1≦283 nm;
 
       Y1 nm≦Rth2≦Y2 nm;
 
         Y 1=0.000632·( Rth 1) 2 ·1.3299· Rth 1+367.51; and
 
         Y 2 =−0.00617·( Rth 1) 2 +1.864· Rth 1+47.39.
 
     Wherein, 58 nm≦Ro1≦71 nm and 220 nm≦Rth1≦269 nm. 
     Wherein, 64 nm≦Rth2≦103 nm. 
     Wherein, a first polarizing film and a first protecting film are further disposed above the first compensation film sequentially, and a second polarizing film and a second protecting film are further disposed on the second compensation film sequentially. 
     Wherein, a material of the first polarizing film and the second polarizing film is polyvinyl alcohol. 
     Wherein, a material of the first protecting film and the second protecting film is triacetyl cellulose. 
     Wherein, an angle between an absorption axis of the first polarizing film and a slow axis of the first compensation film is 90°, and the angle between the absorption axis of the second polarizing film and the slow axis of the second compensation film is 90°. 
     Wherein, a first adhesive layer is disposed between the liquid crystal panel and the first compensation film, a second adhesive layer is disposed between the liquid crystal panel and the second compensation film, and a material of the first adhesive layer and the second adhesive layer is pressure sensitive adhesive. 
     Wherein, the liquid crystal panel is a vertical alignment mode liquid crystal panel. 
     Another aspect of the present invention is to provide a liquid crystal display apparatus comprising a liquid crystal panel and a backlight module, the liquid crystal panel being disposed opposite to the backlight module, and a display light source being provided to the liquid crystal panel from the backlight module so as to display an image by the liquid crystal panel, wherein the liquid crystal panel adopts the liquid crystal panel having the compensation structure stated above. 
     Compared to the conventional technique, through setting a compensation value of a bilayer compensation film for the liquid crystal panel with an optical path difference in a range of 342.8˜361.4 nm in the present invention, the problem of light leakage in the dark state of the liquid crystal panel can be effectively reduced, the contrast and definition at large viewing angle is increased, and the degree of visual range at large viewing angle is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isoluminance contour diagram for wide viewing angle in dark state of a conventional liquid crystal panel. 
         FIG. 2  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel shown in  FIG. 1 . 
         FIG. 3  is an example diagram of a liquid crystal display apparatus provided by the embodiment of the present invention. 
         FIG. 4  is an example diagram of a liquid crystal panel provided by the embodiment of the present invention. 
         FIG. 5  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 342.8 nm. 
         FIG. 6  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 352.1 nm. 
         FIG. 7  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 361.4 nm. 
         FIG. 8  is an isoluminance contour diagram for wide viewing angle in dark state of a liquid crystal panel after being compensated in an embodiment. 
         FIG. 9  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel shown in  FIG. 7 . 
         FIG. 10  is an isoluminance contour diagram for wide viewing angle in dark state of a liquid crystal panel after being compensated in another embodiment. 
         FIG. 11  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel shown in  FIG. 10 . 
         FIG. 12  an isoluminance contour diagram for wide viewing angle in dark state of a liquid crystal panel after being compensated in an other embodiment. 
         FIG. 13  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel shown in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to make the objects, technique solutions and advantages of the present invention be clearer, the present invention will now be described more specifically with reference to the following embodiments with the attached drawings. 
     As shown in  FIG. 3 , the liquid crystal display apparatus comprises a liquid crystal panel  100  and a backlight module  200 . The liquid crystal panel  100  is disposed opposite to the backlight module  200 , and a display light source is provided to the liquid crystal panel  100  from the backlight module  200  so as to display an image by the liquid crystal panel  100 , wherein the liquid crystal panel  100  is a liquid crystal panel adopts a compensation structure having bilayer compensation film for proceeding compensation. 
     Specifically, the liquid crystal panel compensation structure is shown in  FIG. 4 . The liquid crystal panel compensation structure comprises liquid crystal panel  10  and a first compensation film  11  and a second compensation film  12  disposed at two sides of the liquid crystal panel  10 . A first polarizing film  13  and a first protecting film are further disposed above the first compensation film  11  sequentially, and a second polarizing film  14  and a second protecting film  16  are further disposed above the second compensation film  12  sequentially. A first adhesive layer  17  is further disposed between the liquid crystal panel  10  and the first compensation film  11 , and a second adhesive layer  18  is further disposed between the liquid crystal panel  10  and the second compensation film  12 . Wherein, the liquid crystal panel  10  is with vertical alignment cell (VA Cell); a material of the first polarizing film  13  and the second polarizing film  14  is polyvinyl alcohol (PVA), an angle between an absorption axis of the first polarizing film  13  and a slow axis of the first compensation film  11  is set to 90°, and the angle between the absorption axis of the second polarizing film  14  and the slow axis of the second compensation film  12  is set to 90°; a material of the first protecting film  15  and the second protecting material  16  is triacetyl cellulose (TAC), the TAC protecting film  15  and  16  are mainly for protecting the PVA polarizing film  13  and  14  in order to improve a mechanical behavior of the PVA polarizing film  13  and  14  and prevent the PVA polarizing  13  and  14  from retracting; and the material of the first adhesive layer  17  and the second adhesive layer  18  is a pressure sensitive adhesive (PSA). A liquid crystal layer having a plurality of liquid crystal molecules is disposed in the liquid crystal panel  10 , a refractive index anisotropy of the liquid crystal layer is Δn, a depth of the liquid crystal layer is d, and a pretilt angle of the liquid crystal molecules is θ. In the compensation structure stated above, the first compensation film  11  is a biaxial compensation film, an in-plane compensation value thereof is represented by Ro1, and a depth compensation value thereof is represented by Rth1; and the second compensation film  12  is an uniaxial compensation film, and the depth compensation value thereof is represented by Rth2. 
     The object of the structure stated above is to achieve the object of effectively reducing the problem of light leakage in the dark state of the liquid crystal panel and increasing the contrast and definition at large viewing angle for the liquid crystal panel with an optical path difference in a range of 342.8˜361.4 nm through reasonably setting the compensation values of the first compensation film  11  and the second compensation film  12 . 
     During simulation, the settings are proceeded as follows: 
     First, the liquid crystal layer is set as:
         1. The pretilt angle θ is 85°≦θ&lt;90°;   2. The tilt angles of the liquid crystal in the four quadrants are 45°, 135°, 225° and 315°, respectively;   3. The optical path difference Δn·d is 342.8 nm≦Δn·d≦361.4 nm.       

     Second, the backlight source is set as:
         1. The light source: blue light-Yttrium aluminum garnet light emitting diode (Blue-YAG LED) spectrum;   2. A brightness in the center of the light source is defined to be 100 nit;   3. The distribution of the light source is Lambert&#39;s distribution.       

     Refer to  FIG. 5 - FIG. 7 ,  FIG. 5  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 342.8 nm and the pretilt angle is 89°;  FIG. 6  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 352 nm and the pretilt angle is 89°; and  FIG. 7  is a trend diagram of light leakage following on variation of compensation value in dark state of the liquid crystal display apparatus provided by the embodiment of the present invention when the optical path difference of the liquid crystal is 361.4 nm and the pretilt angle is 89°. Therefore, by simulating under accompanying different compensation values with different pretilt angles through the same method, the range of the compensation values corresponding to the first compensation film  11  and the second compensation film  12  when the light leakage in the dark state is smaller than 0.2 nit and in the range of 342.8 nm≦Δn·d≦361.4 nm and 85°≦θ&lt;90° can be obtained as: 52 nm≦Ro1≦78 nm; 196 nm≦Rth1≦283 nm; and Y1 nm≦Rth2≦Y2 nm; wherein 
         Y 1=0.000632·( Rth 1) 2 −1.3299· Rth 1+367.51; and
 
         Y 2=−0.00617·( Rth 1) 2 +1.864· Rth 1+47.39.
 
     Because a relationship between the compensation values Ro and Rth, refractive index N and depth D of the compensation film is as follows: 
         Ro =( Nx−Ny )· D;  and
 
         Rth =[( Nx+Ny )/2− Nz]·D;  
 
       Nx=Ny, i.e. Ro=0, for an uniaxial compensation film. 
     Therefore, the compensation values can be changed through the following three method:
         1. Changing the depth D to change the compensation values when the refractive index N of the compensation film remains unchanged;   2. Changing the refractive index N to change the compensation values when the depth D of the compensation film remains unchanged; and   3. Changing the depth D and the refractive index N at the same time to change the compensation values when the range of the compensation values is ensured.       

     Some specific compensation values are selected in the following, and the corresponded compensation results are tested for further describing the technique effects obtained by the technique solutions of the present invention. 
     Refer to  FIG. 8  and  FIG. 9 ,  FIG. 8  is an isoluminance contour diagram for wide viewing angle in dark state of a liquid crystal panel after being compensated in a specific embodiment, and  FIG. 9  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel after being compensated in the specific embodiment. The settings in the  FIG. 8  and  FIG. 9  are: the optical path difference Δn·d=342.8 nm, the pretilt angle θ=89°, Ro1=71 nm, Rth1=269 nm, Rth2=64 nm. The measured maximum light leakage in the dark state is 0.11 nit. By comparing  FIG. 8  and  FIG. 1 , it can be directly observed that the light leakage in the dark state of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is far less than the light leakage in the dark state of the conventional liquid crystal panel. By comparing  FIG. 9  and  FIG. 2 , it can be directly observed that the contrast distribution for wide viewing angle of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is better than the contrast distribution for wide viewing angle of the conventional liquid crystal panel. 
     Refer to  FIG. 10  and  FIG. 11 ,  FIG. 10  is an isoluminance contour diagram for wide viewing angle in dark state of the liquid crystal panel after being compensated in a specific embodiment, and  FIG. 11  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel after being compensated by the specific embodiment. The settings in the  FIG. 10  and  FIG. 11  are: the optical path difference Δn·d=325.1 nm, the pretilt angle θ=89°, Ro1=65 nm, Rth1=244 nm, Rth2=103 nm. The measured maximum light leakage in the dark state is 0.13 nit. By comparing  FIG. 10  and  FIG. 1 , it can be directly observed that the light leakage in the dark state of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is far less than the light leakage in the dark state of the conventional liquid crystal panel. By comparing  FIG. 11  and  FIG. 2 , it can be directly observed that the contrast distribution for wide viewing angle of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is better than the contrast distribution for wide viewing angle of the conventional liquid crystal panel. 
     Refer to  FIG. 12  and  FIG. 13 ,  FIG. 12  is an isoluminance contour diagram for wide viewing angle in dark state of the liquid crystal panel after being compensated in a specific embodiment, and  FIG. 13  is an equal contrast ratio contour diagram for wide viewing angle of the liquid crystal panel after being compensated by the specific embodiment. The settings in the  FIG. 12  and  FIG. 13  are: the optical path difference Δn·d=361.4 nm, the pretilt angle θ=89°, Ro1=58 nm, Rth1=220 nm, Rth2=103 nm. The measured maximum light leakage in the dark state is 0.18 nit. By comparing  FIG. 12  and  FIG. 1 , it can be directly observed that the light leakage in the dark state of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is far less than the light leakage in the dark state of the conventional liquid crystal panel. By comparing  FIG. 13  and  FIG. 2 , it can be directly observed that the contrast distribution for wide viewing angle of the liquid crystal panel after being compensated by the compensation structure of the present embodiment is better than the contrast distribution for wide viewing angle of the conventional liquid crystal panel. 
     The values of optical path differenceΔn·d, the pretilt angle θ, Ro1, Rth1, Ro2 and Rth2 in the three specific embodiments are only examples for explanation. It is proved that when the values of the parameters are in the ranges below, i.e. 52 nm≦Ro1≦78 nm, 196 nm≦Rth1≦283 nm, Y1 nm≦Rth2≦Y2 nm, Y1=0.000632·(Rth1) 2 −1.3299·Rth1+367.51, and Y2=−0.00617·(Rth1) 2 +1.864·Rth1+47.39, the technique effect the same as or near the technique effect of the specific embodiments above can be reached. 
     In summary, through setting the compensation value of the bilayer-biaxial compensation film for the liquid crystal panels having smaller optical path difference in the present embodiment, the light leakage problem in the dark state of the liquid crystal panel can be effective reduced, the contrast and definition at large viewing angle is increased, and the degree of visual range at large viewing angle is improved. 
     It should be noted that, in the present disclosure, the relative terms such as first and second are only for distinguishing one object or operation from another object or operation, but not for requiring the real relationships or orders between these objects or operations. Furthermore, the terms of “comprising”, “including” or other variations are meant to cover nonexclusive including, such that the processes, methods, objects or equipment including a plurality of elements do not only include these elements but also include other elements which are non-obviously listed, or include the basic elements which have to be existed in the processes, methods, objects or equipment. Under the situation of no more limitations, the processes, methods, objects or equipment including an element limited by the term of “comprising a . . . ” do not exclude the possibility of other existence of the same element therein. 
     Those described above are the embodiments of the present application. It is noted that various improvements and modifications can be made within the theory of the present application by those with ordinary skill in the technique field, and these improvements and modifications should be included in the protection scope of the present application.