Abstract:
An LCD includes a first TAC film, a first optical uniaxial phase compensating film, an LC cell, a second optical uniaxial phase compensating film and a second TAC film from the incident surface to the emitting surface. The first optical uniaxial phase compensating film is used for providing a first compensating value and a second compensating value by adjusting thickness and by adjusting a first refractive index, a second refractive index, and a third refractive index. The second optical uniaxial phase compensating film is used for providing a third compensating value by adjusting thickness and by adjusting a fourth refractive index, a fifth refractive index, and a sixth refractive index. Leakage of light is controlled according to the first compensating value, the second compensating value, and the third compensating value in the LCD.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD comprising an optical uniaxial phase compensating film. 
         [0003]    2. Description of the Prior Art 
         [0004]    Owing to their low-profile, thin, and lightweight features, LCDs have become the mainstream display devices in recent years. Liquid crystal screens are widely used in electronic devices such as cellphones, personal digital assistants (PDAs), digital cameras, computers, notebooks, etc. 
         [0005]    An LCD comprises an LC cell. An alignment of LC molecules in the LC cell is determined by variation of an electric field applied on the LC cell, and the transmission of light in the LC cell is adjusted accordingly. An LC material has a property of birefringence, which means that the refractive index of light in the direction of the long axis of the molecules is different from the refractive index of light in the direction of the short axis of the molecules. Therefore, linearly polarized incident light has diverse phases through different paths in the LC cell in the polarized direction. The feature of color and the transmission of light at a slant viewing angle are different from those at a front viewing angle. 
         [0006]    Birefringence index of the LC molecules in the LC cell varies with an observation inclination. With the observation inclination increases, both of the contrast ratio of an image and image resolution decrease. To enhance the contrast ratio of the image obviously at a specific viewing angle and to reduce leakage of light in dark state on the LCD, a compensating film is attached to the LC panel of a conventional LCD. The birefringence of the LC molecules can be symmetrically compensated because the retardation value of light in different directions is compensated using the compensating film. 
         [0007]    Please refer to  FIG. 1  and  FIG. 2 ,  FIG. 1  shows a simulation of a distribution of light leakage in dark state after being compensated by a conventional uniaxial retardation film.  FIG. 2  shows a simulation of a distribution of contrast over all viewing angles after being compensated by the conventional uniaxial retardation film. The optical path difference of liquid crystal Δn×d is set at 315 nm. The retardation values Ro and Rth of the A plate retardation film are 58 nm and 220 nm, respectively, and the retardation value Rth of the C plate is 40 nm. As can be seen from  FIG. 1  and  FIG. 2 , under these circumstances there is severe light leakage problem in a horizontal viewing area. Generally, since the horizontal viewing area at is more visible than a vertical viewing area, contrast ratio and clarity in the horizontal viewing area affects viewing quality for observers than the vertical viewing area. 
         [0008]    As a result, it is necessary to restrict the area of leakage of light in dark state within the vertical viewing area, rather than the horizontal viewing area. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide an LCD adopting an optical uniaxial phase compensating film. The compensating value of the optical uniaxial phase compensating film can be adjusted by adjusting thickness of the optical uniaxial phase compensating film or the refractive index of the optical uniaxial phase compensating film. The advantage of the LCD adopting the optical uniaxial phase compensating film is that the area of leakage of light in dark state is restricted to the vertical viewing area. 
         [0010]    According to the present invention, a liquid crystal display (LCD) comprises a backlight source for generating light; a first triacetate cellulose (TAC) film; a first polyvinyl alcohol (PVA) film; a first optical uniaxial phase compensating film, for providing a first compensating value and a second compensating value by adjusting thickness of the first optical uniaxial phase compensating film and by adjusting a first refractive index, a second refractive index, and a third refractive index corresponding to light in a first direction, the light in a second direction, and the light in a third direction, respectively; a liquid crystal (LC) cell; a second optical uniaxial phase compensating film, for providing a third compensating value by adjusting thickness of the second optical uniaxial phase compensating film and by adjusting a fourth refractive index, a fifth refractive index, and a sixth refractive index corresponding to the light in the first direction, the light in the second direction, and the light in the third direction, respectively; a second PVA film; and a second TAC film. Light leakage in dark state at a wide viewing angle being controlled according to the first compensating value, the second compensating value, and the third compensating value in the LCD. The first compensating value is determined by an equation as follows: Ro A =(Nx A −Ny A )×D A  where RthA indicates the first compensating value, NxA and NyA indicate refractive indexes corresponding to the X- and Y-axes of three-dimensional Cartesian coordinates for the first optical uniaxial phase compensating film, respectively, and DA indicates thickness of the first optical uniaxial phase compensating film. An optical path difference of the LC cell is determined by (ne−no)×d, the optical path difference is between 305.8 nm and 324.2 nm, where ne and no indicate an extraordinary refractive index and an ordinary refractive index of the LC cell, respectively, d indicates thickness of the LC cell, the first compensating value of the first optical uniaxial phase compensating film is between 55 nm and 78 nm, and the second compensating value of the first optical uniaxial phase compensating film is between 208 nm and 293 nm, the third compensating value of the second optical uniaxial phase compensating film is between the Y 1  nm and Y 2  nm where Y 1 =0.004174x2−3.119x+555.5 and Y 2 =−0.005882x2+1.733x+25.9 stand, and x indicates the second compensating value. 
         [0011]    In one aspect of the present invention, the second compensating value is determined by an equation as follows: Rth A =[(Nx A +Ny A )/2−Nz A ]×D A  where Rth A  indicates the second compensating value, NxA, NyA, and NzA indicate refractive indexes corresponding to the X-, Y-, and Z-axes of three-dimensional Cartesian coordinates for the first optical uniaxial phase compensating film, respectively, and DA indicates thickness of the first optical uniaxial phase compensating film. 
         [0012]    In another aspect of the present invention, a pretilt angle of LC molecules in the LC cell is 89 degrees. 
         [0013]    In another aspect of the present invention, the third compensating value is determined by the fourth refractive index, the fifth refractive index, the sixth refractive index, and thickness of the second optical uniaxial phase compensating film. 
         [0014]    In another aspect of the present invention, the first optical uniaxial phase compensating film is an A-plate compensating film, an optical axis of the first optical uniaxial phase compensating film and a surface of the first optical uniaxial phase compensating film are in parallel, the second optical uniaxial phase compensating film is a C-plate compensating film, and an optical axis of the second optical uniaxial phase compensating film is vertical to a surface of the second optical uniaxial phase compensating film. 
         [0015]    In still another aspect of the present invention, the LCD further comprises a first pressure sensitive adhesive (PSA). The first PSA is disposed between the first optical uniaxial phase compensating film and the LC cell. 
         [0016]    In yet another aspect of the present invention, the LCD further comprises a second PSA. The second PSA is disposed between the second optical uniaxial phase compensating film and the LC cell. 
         [0017]    According to the present invention, a liquid crystal display (LCD) comprises a backlight source for generating light; a first triacetate cellulose (TAC) film; a first polyvinyl alcohol (PVA) film; a first optical uniaxial phase compensating film, for providing a first compensating value and a second compensating value by adjusting thickness of the first optical uniaxial phase compensating film and by adjusting a first refractive index, a second refractive index, and a third refractive index corresponding to light in a first direction, the light in a second direction, and the light in a third direction, respectively; a liquid crystal (LC) cell; a second optical uniaxial phase compensating film, for providing a third compensating value by adjusting thickness of the second optical uniaxial phase compensating film and by adjusting a fourth refractive index, a fifth refractive index, and a sixth refractive index corresponding to the light in the first direction, the light in the second direction, and the light in the third direction, respectively; a second PVA film; and a second TAC film. Light leakage in dark state at a wide viewing angle is controlled according to the first compensating value, the second compensating value, and the third compensating value in the LCD. The first compensating value is determined by an equation as follows: Ro A =(Nx A −Ny A )×D A  where RthA indicates the first compensating value, NxA and NyA indicate refractive indexes corresponding to the X- and Y-axes of three-dimensional Cartesian coordinates for the first optical uniaxial phase compensating film, respectively, and DA indicates thickness of the first optical uniaxial phase compensating film. The first compensating value of the first optical uniaxial phase compensating film is between 55 nm and 78 nm 
         [0018]    In another aspect of the present invention, an optical path difference of the LC cell is determined by (ne−no)×d, the optical path difference is between 305.8 nm and 324.2 nm, where ne and no indicate an extraordinary refractive index and an ordinary refractive index of the LC cell, respectively, d indicates thickness of the LC cell. 
         [0019]    In another aspect of the present invention, the second compensating value is determined by an equation as follows: Rth A =[(Nx A +Ny A )/2−Nz A ]×D A  where RthA indicates the second compensating value, NxA, NyA, and NzA indicate refractive indexes corresponding to the X-, Y-, and Z-axes of three-dimensional Cartesian coordinates for the first optical uniaxial phase compensating film, respectively, and DA indicates thickness of the first optical uniaxial phase compensating film. 
         [0020]    In another aspect of the present invention, the second compensating value of the first optical uniaxial phase compensating film is between 208 nm and 293 nm. 
         [0021]    In another aspect of the present invention, a pretilt angle of LC molecules in the LC cell is 89 degrees. 
         [0022]    In another aspect of the present invention, the third compensating value is determined by the fourth refractive index, the fifth refractive index, the sixth refractive index, and thickness of the second optical uniaxial phase compensating film. 
         [0023]    In another aspect of the present invention, the third compensating value of the second optical uniaxial phase compensating film is between the Y 1  nm and Y 2  nm where Y 1 =0.004174x2−3.119x+555.5 and Y 2 =−0.005882x2+1.733x+25.9 stand, and x indicates the second compensating value. 
         [0024]    In another aspect of the present invention, the first optical uniaxial phase compensating film is an A-plate compensating film, an optical axis of the first optical uniaxial phase compensating film and a surface of the first optical uniaxial phase compensating film are in parallel. The second optical uniaxial phase compensating film is a C-plate compensating film, and an optical axis of the second optical uniaxial phase compensating film is vertical to a surface of the second optical uniaxial phase compensating film. 
         [0025]    In still another aspect of the present invention, the LCD further comprises a first pressure sensitive adhesive (PSA). The first PSA is disposed between the first optical uniaxial phase compensating film and the LC cell. 
         [0026]    In yet another aspect of the present invention, the LCD further comprises a second PSA. The second PSA is disposed between the second optical uniaxial phase compensating film and the LC cell. 
         [0027]    Compared with prior art, the present invention comprises an LCD comprising an optical uniaxial phase compensating film. When optical path difference of the LC cell is between 305.8 nm and 324.2 nm (the optical path difference which the wavelength of 550 nm corresponds to) and that the pretilt angle of LC molecules is 89 degrees, the first compensating value Ro A  of the first optical uniaxial phase compensating film needs to be between 55 nm and 78 nm and the second compensating value Rth A  of the first optical uniaxial phase compensating film  123  needs to be between 208 nm and 293 nm. Besides, the third compensating value Rth C  of the second optical uniaxial phase compensating film needs to be between the Y 1  nm and Y 2  nm where Y 1 =0.004174x2−3.119x+555.5 and Y 2 =−0.005882x2+1.733x+25.9 stand, and x indicates the second compensating value Rth A . The present invention properly adopts the first compensating value Ro A  of the first optical uniaxial phase compensating film, the second compensating value Rth A  of the first optical uniaxial phase compensating film, and the third compensating value Rth C  of the second optical uniaxial phase compensating film. Serious light leakage in dark state in the area at the horizontal viewing angle in the conventional optical uniaxial phase compensating film is effectively improved if the present invention is adopted. Besides, both of the contrast ratio and the clarity in the area at the horizontal viewing angle are improved as well. 
         [0028]    These and other features, aspects and advantages of the present disclosure will become understood with reference to the following description, appended claims and accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  shows a simulation of a distribution of light leakage in dark state after being compensated by a conventional uniaxial retardation film. 
           [0030]      FIG. 2  shows a simulation of a distribution of contrast over all viewing angles after being compensated by the conventional uniaxial retardation film. 
           [0031]      FIG. 3  shows a schematic diagram of an LCD according to a preferred embodiment of the present invention. 
           [0032]      FIGS. 4 to 6  respectively show relationship diagrams between leakage values and different first compensating values Ro A  of the first optical uniaxial phase compensating film, different second compensating values Rth A  of the first optical uniaxial phase compensating film, and different third compensating values Rth C  of the second optical uniaxial phase compensating film on condition that the optical path difference of the LC cell is 305.8 nm, 315 nm, and 324.2 nm. 
           [0033]      FIG. 7  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell of 305.8 nm, the first optical uniaxial phase compensating film with the first compensating value Ro A  of 71 nm and the second compensating value Rth A  of 269 nm and by the second optical uniaxial phase compensating film with the third compensating value Rth A  of 24 nm. 
           [0034]      FIG. 8  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 7 . 
           [0035]      FIG. 9  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell of 315 nm, the first optical uniaxial phase compensating film with the first compensating value Ro A  of 65 nm and the second compensating value Rth A  of 244 nm and by the second optical uniaxial phase compensating film with the third compensating value Rth A  of 75 nm. 
           [0036]      FIG. 10  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 9 . 
           [0037]      FIG. 11  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell of 324.2 nm, the first optical uniaxial phase compensating film with the first compensating value Ro A  of 58 nm and the second compensating value Rth A  of 220 nm and by the second optical uniaxial phase compensating film with the third compensating value Rth A  of 95 nm. 
           [0038]      FIG. 12  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
         [0040]    Please refer to  FIG. 3 .  FIG. 3  shows a schematic diagram of an LCD  10  according to a preferred embodiment of the present invention. The LCD  10  comprises a liquid crystal (LC) cell  16 , a first polarizer  12 , a second polarizer  14 , and a backlight source  18 . The backlight source  18  is used for generating light. The LC cell  16  is glued to an area between the first polarizer  12  and the second polarizer  14  with a pressure sensitive adhesive (PSA)  2 . The first polarizer  12  and the second polarizer  14  are used for deflecting an incident light. A first optical axis of the first polarizer  12  is perpendicular to a second optical axis of the second polarizer  14 . 
         [0041]    The first polarizer  12  comprises a first polyvinyl alcohol (PVA) film  121 , a first triacetate cellulose (TAC) film  122 , and a first optical uniaxial phase compensating film  123 . The PVA film  121  is inserted between the first TAC film  122  and the first optical uniaxial phase compensating film  123 . The second polarizer  14  comprises a second PVA film  141 , a second optical uniaxial phase compensating film  142 , and a third TAC film  143 . The second PVA film  141  is inserted between the second optical uniaxial phase compensating film  142  and the third TAC film  143 . In this embodiment, the first optical uniaxial phase compensating film  123  is an A-plate compensating film. The optical axis of the first optical uniaxial phase compensating film  123  and the surface of the first optical uniaxial phase compensating film  123  are in parallel. The second optical uniaxial phase compensating film  142  is a C-plate compensating film. The optical axis of the second optical uniaxial phase compensating film  142  is vertical to the surface of the second optical uniaxial phase compensating film  142 . The first optical uniaxial phase compensating film  123  is used for providing a first compensating value Ro A  and a second compensating value Rth A . The second optical uniaxial phase compensating film  142  is used for providing a third compensating value Rth C . The slow axis of the first optical uniaxial phase compensating film  123  forms a 90 degree angle with the absorption axis of the first PVA film  121 . The slow axis of the second optical uniaxial phase compensating film  142  forms a zero degree angle with the absorption axis of the second PVA film  141 . The method of determination of the first compensating value, the second compensating value, and the third compensating value will be detailed in the following description. The optical path difference of the LC cell  16 , the compensating value of the first optical uniaxial phase compensating film  123 , and the compensating values of the second optical uniaxial phase compensating film  142  are values corresponding to a wavelength of 550 nm in the following embodiment. 
         [0042]    Please refer to  FIGS. 4 to 6 .  FIGS. 4 to 6  respectively show relationship diagrams between leakage values and different first compensating values Ro A  of the first optical uniaxial phase compensating film  123 , different second compensating values Rth A  of the first optical uniaxial phase compensating film  123 , and different third compensating values Rth C  of the second optical uniaxial phase compensating film  142  on condition that the optical path difference of the LC cell  16  is 305.8 nm, 315 nm, and 324.2 nm. For a simpler description, the incident light generated by the backlight source  18  belongs to Lambertian distribution in this embodiment. Luminance in the center of the incident light is defined as 100 nit. The pretilt angle of the LC molecules in the LC cell  16  is 89 angles. The optical path difference of the LC cell  16  is determined by Δn×d. The optical path difference of the LC cell  16  is between 305.8 nm and 324.2 nm where ne indicates an extraordinary refractive index of the LC cell  16 , ne and no indicate an extraordinary refractive index and an ordinary refractive index of the LC cell  16 , respectively, and d indicates thickness of the LC cell  16 . 
         [0043]    In  FIGS. 4-6 , Ro A  indicates the first compensating value of the first optical uniaxial phase compensating film  123  in the X-Y plane. Rth A  indicates the second compensating value of the first optical uniaxial phase compensating film  123  in the Z-axial direction. Rth C  indicates the third compensating value of the second optical uniaxial phase compensating film  142  in the Z-axial direction. Ro A , Rth A , and Rth C  are determined by values plugged into the following equations: 
         [0000]        Ro   A =( Nx   A   −Ny   A )× D   A    Equation 1,
 
         [0000]        Rth   A =[( Nx   A   +Ny   A )/2 −Nz   A   ]×D   A    Equation 2,
 
         [0000]        Rth   C =[( Nx   C   +Ny   C )/2 −Nz   C   ]×D   C    Equation 3,
 
         [0044]    where Nx A , Ny A , and Nz A  indicate refractive indexes of the light generated by the backlight source  18  corresponding to the X-, Y-, and Z-axes of three-dimensional Cartesian coordinates, respectively, when the light passes through the first optical uniaxial phase compensating film  123 . Nx C , Ny C , and Nz C  indicate refractive indexes of the light generated by the backlight source  18  corresponding to the X-, Y-, and Z-axes of three-dimensional Cartesian coordinates, respectively, when the light passes through the second phase compensating film  142 . D A  and D C  indicate thickness of the first optical uniaxial phase compensating film  123  and thickness of the second phase compensating film  142 , respectively. 
         [0045]      FIGS. 4-6  show that in different LC optical path differences, the compensating value of the first optical uniaxial phase compensating film  123  and the compensating value of the second optical uniaxial phase compensating film  142  have similar influential tendencies on the light leakage in dark state. In other words, in different LC optical path differences, the range of the compensating value is the same for the minimum light leakage in dark state. 
         [0046]    As shown in  FIGS. 4 to 6 , different pretilt angles of LC molecules and the different compensating values are simulated and the pretilt angle of 89 degrees is calculated. In the range of 305.8 nm≦Δn×d≦324.2 nm and the light leakage in dark state is smaller than 0.2 nit, the range of compensating value of the first optical uniaxial phase compensating film  123  and the range of compensating value of the second optical uniaxial phase compensating film  142  are calculated. That is, on condition that the optical path difference of the LC cell is between 305.8 nm and 324.2 nm and that the pretilt angle of LC molecules is 89 degrees, the LCD  10  can still refrain light from leaking based on the first compensating value Ro A  of the first optical uniaxial phase compensating film  123 , the second compensating value Rth A  of the first optical uniaxial phase compensating film  123 , and the third compensating value Rth C  of the second optical uniaxial phase compensating film  142 . It is necessary to control the first compensating value Ro A  of the first optical uniaxial phase compensating film  123  to be between 55 nm and 78 nm and the second compensating value Rth A  of the first optical uniaxial phase compensating film  123  to be between 208 nm and 293 nm. Further, it is necessary to adjust the third compensating value Rth C  of the second optical uniaxial phase compensating film  142  based on the adjusted the second compensating value Rth A  to control the third compensating value Rth C  to be between the Y 1  nm and Y 2  nm where Y 1 =0.004174x2−3.119x+555.5 and Y 2 =−0.005882x2+1.733x+25.9, and x indicates the second compensating value Rth A . 
         [0047]    Therefore, the first compensating value Ro A  of the first optical uniaxial phase compensating film  123 , the second compensating value Rth A  of the first optical uniaxial phase compensating film  123 , and the third compensating value Rth C  of the second optical uniaxial phase compensating film  142  are all for the incident light with the wavelength of 550 nm. When a compensating value is within the above-mentioned range, the LCD will obtain the best compensation and the minimum light leakage in dark state. 
         [0048]    Please refer to  FIGS. 7  through  FIG. 12 ,  FIG. 7  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell  16  of 305.8 nm, the first optical uniaxial phase compensating film  123  with the first compensating value Ro A  of 71 nm and the second compensating value Rth A  of 269 nm and by the second optical uniaxial phase compensating film  142  with the third compensating value Rth A  of 24 nm.  FIG. 8  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 7 .  FIG. 9  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell  16  of 315 nm, the first optical uniaxial phase compensating film  123  with the first compensating value Ro A  of 65 nm and the second compensating value Rth A  of 244 nm and by the second optical uniaxial phase compensating film  142  with the third compensating value Rth A  of 75 nm.  FIG. 10  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 9 .  FIG. 11  shows a simulation of a distribution of light leakage in dark state upon conditions that optical path difference of the LC cell  16  of 324.2 nm, the first optical uniaxial phase compensating film  123  with the first compensating value Ro A  of 58 nm and the second compensating value Rth A  of 220 nm and by the second optical uniaxial phase compensating film  142  with the third compensating value Rth A  of 95 nm.  FIG. 12  shows a simulation of a distribution of contrast over all viewing angles based on the conditions illustrated in  FIG. 11 . 
         [0049]    When comparing  FIGS. 7, 9, 11  with  FIG. 1 , it is observed that, after being compensated by the first optical uniaxial phase compensating film  123  and the second optical uniaxial phase compensating film  142  according to the embodiment of the present invention, the light leakage in dark state is much less than that after being compensated by the prior art, and the light leakage area is restricted to a narrower area at vertical viewing angle. When comparing  FIGS. 8, 10, 12  with  FIG. 2 , it is observed that the contrast distribution over all viewing angles after being compensated by the compensation system according to the embodiment of the present invention is superior that after being compensated by the prior art, especially in the area at horizontal viewing angle. 
         [0050]    The person skilled in the art can adjust the refractive index or the thickness of the first optical uniaxial phase compensating film  123  and the refractive index or the thickness of the second optical uniaxial phase compensating film  142  using Equation 1, Equation 2, and Equation 3 after obtaining the first compensating value Ro A  of the first optical uniaxial phase compensating film  123 , the second compensating value Rth A  of the first optical uniaxial phase compensating film  123 , and the third compensating value Rth C  of the second optical uniaxial phase compensating film  142 . 
         [0051]    Compared with the prior art, the present invention properly adopts the first compensating value Ro A  of the first optical uniaxial phase compensating film  123 , the second compensating value Rth A  of the first optical uniaxial phase compensating film  123 , and the third compensating value Rth C  of the second optical uniaxial phase compensating film  142 . Serious light leakage in dark state in the area at the horizontal viewing angle in the conventional optical uniaxial phase compensating film is effectively improved if the present invention is adopted. Besides, both of the contrast ratio and the clarity in the area at the horizontal viewing angle are improved as well. 
         [0052]    While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements made without departing from the scope of the broadest interpretation of the appended claims.