Abstract:
A polarizer with built-in retarder is accomplished by employing the retarder directly inside the polarizer to replace one of the transparent substrates of the polarizer, such that the polarizer is substantially built-in with the retarder. Not only the polarizer has larger visible ranges and better displaying quality because of the effect of optic compensation, the thickness of the polarizer is also smaller, and its transparency and optic characteristics are better than prior art polarizer.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a retarder film, polarizer with built-in retarder, and liquid crystal display device, in particular a kind of polarizer structure directly built in with retarder film containing a light retardation layer that provides dual compensation for visual range and chromatic polarization, and its process.  
         [0003]     2. Description of the Prior Art  
         [0004]     Liquid crystal display (LCD) is now used by all kinds of electronic devices, such as television, computer, mobile handset, and personal digital assistant (PDA). Due to its characteristics of fast response and high contrast ratio of direct viewing angle, thin-film resistor LCD (TFT-LCD) has become the mainstream LCD technology.  
         [0005]      FIG. 1A  depicts the sectional view of a conventional LCD  10 , which typically comprises a liquid crystal element  11  and two polarizers  12 ,  13  disposed respectively on each surface of liquid crystal element  11 . The liquid crystal element  11  is constituted by a glass substrate and a plurality of liquid crystal particles adhered to both surfaces of the glass substrate. Polarizer  12  (or  13 ) is made of a polarizing film  123  (or  133 ) sandwiched between two transparent substrates  121 ,  122  (or  131 ,  132 ) that provides compensation for polarization.  
         [0006]     If we look at the contrast curve of the visible range of conventional LCD  10  ( FIG. 1A ) as shown in  FIG. 1B , it is clear that conventional LCD offers good visual effect in vertical and horizontal directions only. At 45° or 135° angle, the contrast ratio drops and the hues shift, which seriously affects the display quality of LCD.  
         [0007]     Later on LCDs are added with a retarder film to enhance the visual effect of oblique angles.  FIG. 2  shows the sectional view of a conventional LCD  20  laminated with a retarder plate  24 . The retarder plate  24  is adhered between a top surface of the liquid crystal element  21  and the polarizer  22 . The retarder plate  24  consists of a transparent substrate  241  and one or multiple layers of phase retarders  242 ,  243 . The phase retarder works to retard certain wavelengths at predetermined angles and directions, thereby improving the oblique-angle display quality of LCD. Similarly, the two polarizers  22  and  23  disposed on top and bottom of liquid crystal element  21  are made of a polarizing film  223 ,  233  sandwiched between two transparent substrates  221 ,  222 ,  231 ,  232 . For example, U.S. Pat. No. 6,717,642 discloses a technology of improving the visible angle and display quality of LCD by adding a retarder plate.  
         [0008]     In the prior art LCD  20  as shown in  FIG. 2 , the polarizers  22 ,  23 , and the retarder plate  24  are separately produced and then adhesively laminated together. In light that the separately produced polarizers  22 ,  23  and retarder plate  24  require respectively at least one transparent substrate  222 ,  232 ,  241  to provide adequate structural strength and rigidity, and polarizers  22 ,  23  more so need at least two transparent substrates  221 ,  222 ,  231 ,  232  to achieve protection for polarizing films  223  and  233  and the effect of scratch resistance. However, the use of many substrates and the presence of many lamination layers increase the thickness of LCD and affect adversely its transparency and optic characteristics.  
       SUMMARY OF INVENTION  
       [0009]     The primary object of the present invention is to provide a retarder film, which is formed by applying a light retardation layer on a transparent polymer film and satisfies the following conditional formulas: 
 
220 nm&gt; Ro ( a )+ Ro ( b )&gt;0.1 nm 
 
−270 nm&lt; Rth ( a )+ Rth ( b )&lt;110 nm 
 
−300 nm&lt; Rth ( a )&lt;−10 nm 
 
         [0010]     where Ro(a) and Rth(a) are respectively the in-plane retardation (Ro) and out-of-plane retardation (Rth) of light retardation layer  3142 ; Ro(b) and Rth(b) are respectively the Ro and Rth of transparent polymer film  3141 ; nx denotes the refractive index along x-axis of surface; ny denotes the refractive index along y-axis of surface; nz is thicknesswise refractive index along z-axis; Ro=(nx−ny)*d; Rth={(nx+ny)/2−nz}*d; and d is film thickness.  
         [0011]     Another object of the present invention is to provide a polarizer with built-in retarder, which is accomplished by directly employing a retarder film containing light retardation material to replace one of the transparent substrates. As such, the polarizer achieves better visible range and display quality due to the effect of optic compensation, and is reduced in thickness with at least one less layer of transparent substrate, hence offering better transparency and optic characteristics.  
         [0012]     Yet another object of the present invention is to provide a liquid crystal device, comprising a polarizer with built-in retarder. By constructing a plurality of light retardation layers with specific orientation in the structure of polarizer, the liquid crystal display device will exhibit better visible range. Even from an oblique viewing angle of 45 degree or 135 degree, the liquid crystal display device also offers better contrast and color performance.  
         [0013]     To achieve the aforesaid objects, the present invention provides a retarder film and a polarizer with built-in retarder, which comprises a first transparent substrate, a polarizing film, and at least a retarder film, the first transparent substrate being made of triacetyl cellulose (TAC) plate to provide strength and rigidity to the polarizer structure.  
         [0014]     The polarizing film is a polyvinyl alcohol (PVA) film, which provides polarizing effect. The retarder film is directly disposed on a surface of the polarizing film. Therefore, the first transparent substrate, polarizing film and retarder film together constitute one body. The retarder film is made of a transparent polymer film with light retardation material formed thereon. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The details of the present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures.  
         [0016]      FIG. 1A  shows the sectional view of a conventional LCD.  
         [0017]      FIG. 1B  shows the contrast curve of the visible range of conventional LCD in  FIG. 1A .  
         [0018]      FIG. 2  shows the sectional view of a conventional LCD added with a retarder plate.  
         [0019]      FIG. 3  shows the sectional view of an embodiment of retarder film according to the present invention.  
         [0020]      FIG. 4A  shows the sectional view of polarizer with built-in retarder in the first embodiment according to the present invention.  
         [0021]      FIG. 4B  shows the contrast curve of visible range of polarizer with built-in retarder in the first embodiment as shown in  FIG. 4A .  
         [0022]      FIG. 5A  shows the sectional view of polarizer with built-in retarder in the second embodiment according to the present invention.  
         [0023]      FIG. 5B  shows the contrast curve of visible range of polarizer with built-in retarder in the second embodiment as shown in  FIG. 5A .  
         [0024]      FIG. 6A  shows the sectional view of polarizer with built-in retarder in the third embodiment according to the present invention.  
         [0025]      FIG. 6B  shows the contrast curve of visible range of polarizer with built-in retarder ( 31   c ) in the third embodiment as shown in  FIG. 6A .  
         [0026]      FIG. 7  shows the sectional view of polarizer with built-in retarder in the fourth embodiment according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]      FIG. 3  depicts the sectional view of an embodiment of retarder film  314  according to the present invention. By disposing a light retardation layer  3142  on a transparent polymer film  3141 , the resulting film can retard specific wavelengths at predetermined angles and directions to achieve the purpose of compensating the display quality of LCD from oblique viewing angles. In this embodiment, the polymer film  3141  is transparent polymer film commonly used in the industry and preferably thermoplastic resin, and more preferably thermoplastic resin with excellent mechanical strength, moisture penetrability, transparency, thermal stability and optic characteristics. Examples of this kind of transparent polymer film include cellulose resin, such as triacetyl cellulose, propionyl cellulose, and transparent resin, such as polyamide, polycarbonate, polyester, polystyrene, polyacrylate, norbornene-based polymer, and polyethyl acetate. In consideration of the optic characteristics and weather resistance properties (heat, moisture, etc.) of the polarizer, triacetyl cellulose (TAC) that has been surface treated with alkaline and saponified is the preferred choice.  
         [0028]     In this embodiment, the transparent polymer film  3141  and the light retardation layer  3142  respectively satisfies the following optic conditional formulas: 
 
220 nm&gt; Ro ( a )+ Ro ( b )&gt;0.1 nm 
 
−270 nm&lt; Rth ( a )+ Rth ( b )&lt;110 nm 
 
−300 nm&lt; Rth ( a )&lt;−10 nm 
 
         [0029]     where Ro(a) and Rth(a) are respectively the in-plane retardation (Ro) and out-of-plane retardation (Rth) of light retardation layer  3142 ; Ro(b) and Rth(b) are respectively the Ro and Rth of transparent polymer film  3141 ; nx denotes the refractive index along x-axis of surface; ny denotes the refractive index along y-axis of surface; nz is thicknesswise refractive index along z-axis; Ro=(nx−ny)*d; Rth={(nx+ny)/2−nz}*d; and d is film thickness.  
         [0030]     Retarder film  314  made according to the aforesaid conditional formulas is commonly referred to as C-plate in the industry. After retarder film  314  is built into the polarizer, it provides light retardation effect of predetermined angles and directions to achieve the purposes of optic compensation and improvement of visible range and display quality. Because the retarder film  314  of the present invention can provide support and protection for the polarizing film in polarizer, it can be directly built inside the polarizer to replace one of the transparent substrates originally disposed on the side of polarizer, thereby reducing the overall thickness of polarizer (for at least one less transparent substrate is used as compared to prior art) and enhancing its optic characteristics. Below are detailed descriptions of the implementation method.  
         [0031]      FIG. 4A  and  FIG. 4B  are respectively sectional view of polarizer  31  with built-in retarder in the first embodiment of the invention and the contrast curve of visible range of polarizer  31  with built-in retarder in the first embodiment as shown in  FIG. 4A .  
         [0032]     As shown in  FIG. 4A , the polarizer  31  with built-in retarder may be used in conjunction with a liquid crystal element  32 . In this embodiment, the liquid crystal element  32  is an in-plane switching (IPS) LCD element. It can also be a MVA LCD or TN LCD element. The composition and functions of liquid crystal element  32  are not elaborated here for it is a prior art and not a major feature of the invention. The polarizer  31  with built-in retarder film  314  comprises mainly a first transparent substrate  311 , a first polarizing film  312 , a first phase retarder  313  and the retarder film  314 . The first transparent substrate  311  is made of triacetyl cellulose (TAC), which has sufficient structural strength and rigidity to support the entire polarizer  31  and protect the first polarizing film  312  from scratch. The first polarizing film  312  is a polyvinyl alcohol (PVA) film. The first polarizing film  312  has specific polarizing effect and is prepared by stretching the PVA film after it is absorbed with iodine or dichromatic substance, such as dichromatic dye. Because the composition and effects of the first transparent substrate  311  and the first polarizing film  312  are the same as the prior art, their composition and effects are not elaborated here.  
         [0033]     The main feature of this embodiment is that the first phase retarder  313  is directly built inside the polarizer  31 . The first phase retarder  313  is also an optic compensation film, only its optic characteristics and process are different from those of retarder film  314 . As shown in  FIG. 4 , the first phase retarder  313  is directly formed on the first polarizing film  312  such that the first transparent substrate  311 , first polarizing film  312  and first phase retarder  313  are in one body, and the first transparent substrate  311  and the first phase retarder  313  respectively constitutes a protective layer on the two opposing surfaces of first polarizing film  312 . Thus the polarizer  31  comprised of first phase retarder  313 , first polarizing film  312  and first transparent substrate  311  is a single element that stands independently and can be independently sold, preserved and shipped. In this embodiment, the retarder film  314  is made of transparent polymer film  3141  with a light retardation layer  3142  formed thereon as shown in  FIG. 3 , and the retarder film  314  and polarizer  31  built in with a first phase retarder  313  are laminated in sequence onto liquid crystal element  32 .  
         [0034]     The first phase retarder  313  and retarder film  314  can retard wavelengths at predetermined angles and directions, thereby improving the oblique angle display quality of LCD  30 . In this embodiment, the first phase retarder  313  is a polymer film (called A-Plate) that satisfies the conditions of nx&gt;ny=nz and 60 nm&lt;Ro&lt;250 nm. That is, the first phase retarder  313  acts as an optical compensation film, also a protective layer. The optic conditions for retarder film  314  (C-Plate) have been described earlier and will not be reiterated here.  
         [0035]     Polarizer  31  with built-in retarder can be disposed on the top surface (the side with an eye in the figure) or the bottom surface (the side with a light bulb in the figure) of liquid crystal element  32 . In the embodiment as shown in  FIG. 4A , polarizer  31  with built-in retarder is superimposed over the top surface of liquid crystal element  32 , while the bottom surface of liquid crystal element  32  is adhered with a polarizing plate  35  of prior art consisting of a second polarizing film  352  sandwiched between a third transparent substrate  351  and a fourth transparent substrate  353 . Generally, the polarizing directions of the first polarizing film  312  and the second polarizing film  352  are perpendicular to each other.  
         [0036]     As shown in  FIG. 4A  and  FIG. 4B , the polarizer  31  with built-in retarder in LCD  30  contains a first phase retarder  313  (A-Plate) and a retarder film  314  (C-Plate). In comparison with the contrast curve of conventional polarizer  12 ,  13  ( FIG. 1B ) which is free of retarder, the polarizer  31  with built-in retarder as disclosed herein provides better contrast and color performance in terms of visible range from oblique angle (as shown in  FIG. 4B ), and achieves the effect of optic compensation. Also, in comparison with prior art polarizer  22  and prior art LCD  20  with retarder as shown in  FIG. 2 , the polarizer  31  with built-in retarder disclosed herein uses at least one less transparent substrate, which not only reduces its thickness, but also improves its transparency and optic characteristics.  
         [0037]     The other embodiments of the invention to be described have basically the same or similar elements as the embodiment described above. Thus those elements are given the same numbers with an English alphabet suffix for distinction purpose and their compositions will not be elaborated again.  
         [0038]      FIG. 5A  and  FIG. 5B  are respectively sectional view of polarizer  31   b  with built-in retarder in the second embodiment of the invention and the contrast curve of visible range of polarizer  31   b  with built-in retarder in the second embodiment as shown in  FIG. 5A . In this embodiment, a first polarizing film  312   b  is sandwiched between a first transparent substrate  311   b  and a retarder film  314  (C-Plate), and the side of retarder film  314   b  formed with light retardation material faces down (i.e. away from the first polarizing film  312   b ). As such, the polarizer  31   b  with built-in retarder is in one body consisting of the first transparent substrate  311   b , the first polarizing film  312   b  and the retarder film  314   b . Furthermore, the first phase retarder  313   b  (A-Plate) is adhesively disposed on the bottom surface (i.e., the side having the retarder film  314   b ) of polarizer  31   b , which is then adhered to the top surface of liquid crystal element  32   b . In this second embodiment, a second phase retarder  316  (A-Plate) is formed on the surface of a third transparent substrate  351   b , and a second polarizing film  352   b  is sandwiched between the third transparent substrate  351   b  and the fourth transparent substrate  353   b , where the second phase retarder  316 , the third transparent substrate  351   b , the second polarizing film  352   b  and the fourth transparent substrate  353   b  together form another polarizer  35   b  with built-in retarder disposed on the bottom surface of liquid crystal element  32   b .  
         [0039]      FIG. 6A  and  FIG. 6B  are respectively sectional view of polarizer  31   c with built-in retarder in the third embodiment of the invention and the contrast curve of visible range of polarizer  31   c  with built-in retarder in the third embodiment as shown in  FIG. 6A . In this embodiment, a first polarizing film  312   c  is sandwiched between a first transparent substrate  311   c  and a retarder film  314   c  (C-Plate) to form a polarizer  31   c . The side of retarder film  314   c  formed with light retardation material faces down. A first phase retarder  313   c  (A-Plate) is formed on the bottom surface of retarder  314   c  in one body with polarizer  31   c , which is subsequently adhered to the top surface of liquid crystal element  32   c . Also in this embodiment, on the bottom surface of liquid crystal element  32   c , there are disposed of in sequence: a transparent substrate  354 , a second polarizing film  352   c , and a fourth transparent substrate  353   c . Said transparent substrate  354  in particular is a transparent polymer substrate with low birefringence, which may be a cyclic olefin polymer (COP), cyclic olefin copolymer (COC) or metallocene catalyzed cyclic olefin copolymer (mCOC) having lower phase difference, i.e. its Ro and Rth approximate zero.  
         [0040]      FIG. 7  is a sectional view of polarizer  31   d  with built-in retarder in the fourth embodiment of the present invention, where a first polarizing film  312   d  is sandwiched between a first transparent substrate  311   d  and a retarder film  314   d  (C-Plate) and forms into one body with polarizer  31   d . Furthermore, polarizer  31   d  is adhered to the top surface of liquid crystal element  32   d . On the bottom surface of liquid crystal element, there are in sequence a second phase retarder  316   d  (A-Plate), a third transparent substrate  351   d , a second polarizing film  352   d , and a fourth transparent substrate  353   d.    
         [0041]     Preferred embodiments of the present invention have been disclosed in the examples. However the descriptions made in the examples should not be construed as a limitation on the actual applicable scope of the present invention, and as such, all modifications and alterations without departing from the spirits of the invention shall be deemed as further embodiment of the invention and remain within the protected scope and claims of the invention.