Abstract:
A polarization direction measuring apparatus includes: a first polarizing plate having an unknown polarization direction about a reference axis; a sample whose polarization direction is to be measured; a rotatable sample holder on which the sample is mounted in a first direction and a second direction opposite to the first direction, wherein the sample holder rotates the sample along a reference axis in the azimuth direction; a light source that generates light passing though the first polarizing plate and the sample; and a light detector detecting light generated by the light source that passes though the first polarizing plate and the sample.

Description:
[0001]     This application claims the benefit of Korean Patent Application No. 10-2006-0060963 filed on Jun. 30, 2006, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This document relates to a display apparatus, and more particularly, to an apparatus and a method for measuring a polarization direction of a polarizing plate for a liquid crystal display.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Liquid crystal displays are generally devices displaying an image using optical anisotropy and birefringence properties of liquid crystal molecules.  
         [0006]     In a liquid crystal display, two substrates each having electrodes for generating electric field face each other so that the surfaces of the two substrates on which the electrodes are formed are opposite to each other, and a liquid crystal material is injected between the two substrates.  
         [0007]     Arrangement of the liquid crystal molecules is changed by an electric field obtained by applying a voltage to the electrodes, and thus the liquid crystal display displays an image by controlling the quantity of light transmitted through the substrates.  
         [0008]     A thin film transistor-liquid crystal display (TFT-LCD) using thin film transistors (TFT) as switching devices has been generally used.  
         [0009]     In the TFT-LCD, white light generated in a backlight unit passes through liquid crystal pixels such that transmittance of the white light is controlled. Then, a color image is displayed using additive color mixtures of light transmitted through red (R), green (G) and blue (B) color filter layers positioned on the liquid crystal pixels.  
         [0010]     A display level of the TFT-LCD is determined by controlling the quantity of transmitted or reflected light depending on the intensity of electric field in each pixel.  
         [0011]     Because characteristics of the liquid crystal display depend on the polarization state of a first polarizing plate and a second polarizing plate set in a cross-Nicol configuration, optimum designs of the first polarizing plate and the second polarizing plate are necessary and the polarization directions of the first and second polarizing plates have to be accurately known.  
         [0012]     However, when the polarization directions of the first polarizing plate and the second polarizing plate are not accurately controlled, the amount of light passing through a liquid crystal layer of the liquid crystal display is not accurately controlled. This leads to a reduced image quality of the liquid crystal display.  
         [0013]      FIG. 1  illustrates a configuration of an apparatus for measuring a polarization direction of a polarizing plate of a related art liquid crystal display.  
         [0014]     Referring to  FIG. 1 , the related art polarization direction measuring apparatus includes a light source  71 , a first polarizing plate  72 , a sample holder  73 , a sample  74 , and a light detector  75 . Light is generated by the light source  71 , and the light detector  75  detects the quantity of light passing through the first polarizing plate  72  and the sample  74 .  
         [0015]     Referring to  FIG. 1 , the related art polarization direction measuring apparatus includes the first polarizing plate  72 , of which a polarization direction is accurately known, and the sample holder  73  capable of accurately positioning a sample  74  along a reference axis thereof.  
         [0016]     In other words, the polarization axis of the sample  74  is inclined relative to the polarization axis of the first polarizing plate  72  at a predetermined angle, and the sample  74  is placed along the reference axis of the sample holder  73 . Then, while rotating the sample  74  in the azimuth direction, the light detector  75  detects when the minimum quantity of light is detected. Therefore, the angle (i.e., a polarization direction) of the sample  74  inclined relative to the reference axis is obtained.  
         [0017]     The related art polarization direction measuring apparatus can accurately measure the polarization direction of the sample  74  when the polarization direction of the first polarizing plate  72  is accurately known.  
         [0018]     Further, the reference axis of the sample holder  73  has to be known accurately to mount the sample  74  into the sample holder  73 . In other words, the polarization direction of the first polarizing plate  72  and the reference axis of the sample holder  73  have to be known accurately for accurate measurement of the polarization direction of the sample  74 . Accordingly, it is difficult to accurately measure the polarization direction of the sample  74 .  
       SUMMARY OF THE INVENTION  
       [0019]     Accordingly, the present invention is directed to an apparatus and method for measuring polarization direction of a polarizing plate that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
         [0020]     An advantage of the present invention is to provide an improved apparatus and method for simply measuring the polarization angle of a polarizing plate.  
         [0021]     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0022]     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a polarization direction measuring apparatus includes: a first polarizing plate having an unknown polarization direction about a reference axis; a sample whose polarization direction is to be measured; a rotatable sample holder on which the sample is mounted in a first direction and a second direction opposite to the first direction, wherein the sample holder rotates the sample along a reference axis in the azimuth direction; a light source that generates light passing though the first polarizing plate and the sample; and a light detector detecting light generated by the light source that passes though the first polarizing plate and the sample.  
         [0023]     In another aspect of the present invention, a method of measuring a polarization direction includes: aligning a light source, a first polarizing plate, and a light detector; inserting a sample into a sample holder in a first direction, wherein a reference axis of the sample holder aligns with the light source and the light detector; rotating the sample holder in an azimuth direction to measure a first angle at which a minimum quantity of light is detected; inserting the sample into the sample holder in a second direction opposite to the first direction; rotating the sample holder in the azimuth direction to measure a second angle at which a minimum quantity of light is detected; and obtaining a value corresponding to one half a difference between the first angle and the second angle to measure a polarization direction of the sample.  
         [0024]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The accompany drawings, which are included to provide a further understanding of the invention and are incorporated on and constitute a part of this specification illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0026]      FIG. 1  illustrates a configuration of an apparatus for measuring a polarization direction of a polarizing plate of a related art liquid crystal display;  
         [0027]      FIG. 2  is a perspective view of a liquid crystal display;  
         [0028]      FIG. 3  schematically illustrates a configuration of a liquid crystal panel;  
         [0029]      FIGS. 4A and 4B  illustrate polarization modes of a polarizing plate of a liquid crystal display;  
         [0030]      FIG. 5  illustrates a configuration of an apparatus for measuring a polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention;  
         [0031]      FIG. 6  illustrates an example of a sample holder of a polarization direction measuring apparatus according to an embodiment of the present invention;  
         [0032]      FIGS. 7A and 7B  are cross-sectional views of the sample holder of  FIG. 6 ;  
         [0033]      FIGS. 8A and 8B  illustrate a process of measuring the polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention; and  
         [0034]      FIG. 9  is a flow chart of a method for measuring a polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0035]     Reference will now be made in detail embodiments of the invention examples of which are illustrated in the accompanying drawings.  
         [0036]      FIG. 2  is a perspective view of a liquid crystal display.  
         [0037]     Referring to  FIG. 2 , a liquid crystal panel positioned inside the liquid crystal display includes a first substrate  10  and a second substrate  20  coupled to each other with a predetermined gap therebetween, and a liquid crystal layer  30  injected between the first substrate  10  and the second substrate  20 . The first substrate  10  includes a TFT area having a TFT that performs a switching function, a pixel area (Pixel), and a storage CST.  
         [0038]     The first substrate  10  includes a transparent glass substrate  11  and a plurality of gate lines  12  and a plurality of data lines  16  positioned on the transparent glass substrate  11 . The plurality of gate lines  12  are arranged in one direction with a substantially constant distance therebetween. The plurality of data lines  16  are arranged with a substantially constant distance therebetween in a direction substantially perpendicular to the gate lines  12 . The pixel area (Pixel) is defined by the gate lines  12  and the data lines  16 .  
         [0039]     A pixel electrode  18  is formed in each pixel area (Pixel), and a thin film transistor is formed at each crossing of the gate lines  12  and the data lines  16 . The thin film transistor applies a data signal on the data line  16  to the pixel electrode  18  in response to a scan signal applied on the gate line  12 .  
         [0040]     The second substrate  20  includes a transparent glass substrate  21  and a black matrix layer  22 . The black matrix layer  22  is formed on the transparent glass substrate  21  to block light transmitted outside the pixel area (Pixel) of the first substrate  10 . Red (R), green (G) and blue (B) color filter layers  23  are formed to express color corresponding to each pixel area. A common electrode  24  is formed on each of the color filter layers  23 .  
         [0041]     A charge capacitor is formed over a portion of the gate line  12  and is connected in parallel to the pixel electrode  18 . A first electrode of the charge capacitor uses a portion of the gate line  12 , and a second electrode uses a metal pattern with an island shape made of the same material as a source electrode and a drain electrode.  
         [0042]     The liquid crystal layer  30  of the liquid crystal display is aligned by an electric field between the pixel electrode  18  and the common electrode  24 . The quantity of light transmitted through the liquid crystal layer  30  is controlled based upon an alignment direction of the liquid crystal layer  30 , thereby displaying a desired image.  
         [0043]      FIG. 3  schematically illustrates a configuration of a liquid crystal panel.  
         [0044]     Referring to  FIG. 3 , the liquid crystal panel includes the first substrate  10 , the second substrate  20 , the liquid crystal layer  30 , a first polarizing plate  50 , a second polarizing plate  60 , and a backlight unit  40 . A first alignment layer  15  and a second alignment layer  25  are formed on the first substrate  10  and the second substrate  20 , respectively, using a rubbing method.  
         [0045]     The liquid crystal panel of  FIG. 3  is a liquid crystal cell with anti-parallel alignment between the first and second polarizing plates. Examples of the anti-parallel alignment include an in-plane switching (IPS) mode liquid crystal cell and an electrically controllable birefringence (ECB) mode liquid crystal cell. The first polarizing plate  50  and the second polarizing plate  60  are set in a cross-Nicol configuration.  
         [0046]      FIG. 3  shows the various elements of the liquid crystal cell spaced apart from one another. However, in fact, all the elements contact one another. Electrical and optical characteristics of the liquid crystal display are determined by alignment directions of the first and second alignment layers  15  and  25  and polarization directions of the first and second polarizing plates  50  and  60 . In other words, because the electrical and optical characteristics of the liquid crystal display are determined by the combination of the alignment directions of the first and second alignment layers  15  and  25  and the polarization directions of the first and second polarizing plates  50  and  60 , the alignment directions and the polarization directions (i.e., alignment axes and polarization axes) need to be accurately known.  
         [0047]      FIGS. 4A and 4B  illustrate a polarization mode of a polarizing plate of a liquid crystal display according to an embodiment of the present invention.  
         [0048]     Referring to  FIGS. 4A and 4B , a polarizing plate may be classified as an extraordinary mode (e-mode) polarizing plate or an ordinary mode (o-mode) polarizing plate. The liquid crystal has both properties of the e-mode polarizing plate and properties of the o-mode polarizing plate.  
         [0049]     In the e-mode, a transmissive axis of the first polarizing plate  50  and a long axis the liquid crystal are in an extraordinary state. In an o-mode, the transmissive axis of the first polarizing plate  50  and the long axis the liquid crystal are perpendicular to each other.  
         [0050]     The first polarizing plate  50  is spaced apart from the second polarizing plate  60  with the liquid crystal layer  30  therebetween. The first polarizing plate  50  functions as a polarizer, and the second polarizing plate  60  functions as an analyzer.  
         [0051]      FIG. 4   a  illustrates the e-mode in which the polarization direction of the polarizer  50  is horizontal to the plane and the polarization direction of the analyzer  60  is perpendicular to the plane.  
         [0052]      FIG. 4   b  illustrates the o-mode in which the polarization direction of the polarizer  50  is perpendicular to the plane and the polarization direction of the analyzer  60  is horizontal to the plane.  
         [0053]     The e-mode and the o-mode are polarization modes for absorbing light vibrating in a selected direction or transmitting only light vibrating in a selected direction.  
         [0054]      FIG. 5  illustrates a configuration of an apparatus for measuring a polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention.  
         [0055]     Referring to  FIG. 5 , the apparatus for measuring the polarization direction of the polarizing plate for the liquid crystal display includes a light source  100 , a first polarizing plate  200 , a sample holder  310 , a sample  320 , and a light detector  400  that are positioned in the order named.  
         [0056]     The light source  100  provides light passing through the first polarizing plate  200  and the sample  320 , and generally uses a laser light source.  
         [0057]     The polarization direction of the first polarizing plate  200  is set relative to a reference axis of the sample holder  310 .  
         [0058]     The sample  320  is a polarizing plate to be measured, and it corresponds to the first polarizing plate  200 . A polarization direction of the sample  320  is obtained by one half ((X2−1)/2) a difference (i.e., X2−X1) between a first angle X1, where the minimum quantity of light is detected when rotating the sample holder  310  when a first surface of the sample  320  faces the light source, i.e., a first direction, and a second angle X2, where the minimum quantity of light is detected when rotating the sample holder  310  when a second surface of the sample  320  faces the light source, i.e., a second direction. The sample  320  put into the sample holder  310  rotates along the reference axis in the azimuth direction.  
         [0059]     The light detector  400  detects the light that is produced by the light source  100  and passes through the first polarizing plate  200  and the sample  320 . The light detector  400  uses a photodetector or a luminance meter.  
         [0060]     Accordingly, the reference axis is in a reference plane of the sample holder  310 . Although the polarization direction of the first polarizing plate  200  is not accurately known, the polarization direction of the sample  320  is accurately measured using this simple method.  
         [0061]      FIG. 6  illustrates an example of a sample holder of a polarization direction measuring apparatus according to an embodiment of the present invention.  FIGS. 7A and 7B  are cross-sectional views of the sample holder of  FIG. 6 .  
         [0062]     Referring to  FIG. 6 , the sample holder  310  includes a housing into which the sample  320  is inserted, a first insertion opening  330 , and a light transmission opening  340 . The first insertion opening  330  is formed in one direction of the housing, and fastens the sample  320  inserted in the first direction. The light transmission opening  340  passes through the housing and transmits light passing through the first polarizing plate  200 .  
         [0063]     Referring to  FIGS. 7A and 7B , the sample holder  310  may additionally include a second insertion opening  331 . The shape of the second insertion opening  331  is symmetrical to the shape of the first insertion opening  330 , and the second insertion opening  331  fastens the sample  320  inserted in the second direction.  
         [0064]     In  FIG. 7A , the sample holder  310  includes only the first insertion opening  330 . In  FIG. 7B , the sample holder  310  includes the first and second insertion opening  311  and  331  whose shapes are symmetrical to each other.  
         [0065]     Further, the sample holder  310  may additionally include a grip  350  that is positioned at one side of the housing to rotate the housing.  
         [0066]     The polarization direction measuring apparatus does not need to know the polarization direction of the first polarizing plate  200 . Instead, the sample  320  may be inserted into the insertion opening of the sample holder  310  in a reverse direction. A method for measuring the polarization direction of the sample  320  is as follows.  
         [0067]     First, after the sample  320  is put into the sample holder  310 , the sample  320  is fastened in the insertion opening  330  along the reference axis.  
         [0068]     Next, while the sample holder  310  rotates, the first angle X1 is measured at which the minimum quantity of light is detected using the light detector  400 .  
         [0069]     After the sample  320  is again put into the sample holder  310  in a second or reverse direction, the second angle X2 is measured at which the minimum quantity of light is detected using the light detector  400 .  
         [0070]     Then the polarization direction of the sample  320  to the reference axis is θ1, and θ1 is equal to (X2−X1)/2.  
         [0071]      FIGS. 8A and 8B  illustrate a process of measuring the polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention.  
         [0072]      FIG. 8A  illustrates a case where the reference axis of the sample holder  310  is not inclined.  FIG. 8B  illustrates a case where the reference axis of the sample holder  310  is inclined at x°.  
         [0073]     At the right of  FIGS. 8A and 8B , the first angle X1 is measured when inserting the sample  320  into the sample holder  310  in the first direction, and the second angle X2 is measured when inserting the sample  320  into the sample holder  310  in the second direction.  
         [0074]     When the polarization direction of the sample  320  is θ1, the following equations are satisfied: X1=−θ1+x, X2=+θ1+x, and X2−X1=2θ1. In other words, the polarization direction θ1 of the sample  320  is obtained irrespective of the inclination angle x° of the reference axis of the sample holder  310 .  
         [0075]     In other words, although the polarization direction of the first polarizing plate  200  is not known, the polarization direction of the sample  320  is obtained using the above-described measuring method by inserting the sample  320  into the sample holder  310  facing opposite directions.  
         [0076]      FIG. 9  is a flow chart of a method for measuring a polarization direction of a polarizing plate of a liquid crystal display according to an embodiment of the present invention.  
         [0077]     Referring to  FIGS. 5 and 9 , in the method for measuring the polarization direction of the polarizing plate of the liquid crystal display, the light source  100 , the first polarizing plate  200  and the light detector  400  are aligned, and the sample  320  is then put into the sample holder  310  along the reference axis in the first direction in step S 110 .  
         [0078]     The sample holder  310  is aligned between the first polarizing plate  200  and the light detector  400 . The sample  320  is a second polarizing plate corresponding to the first polarizing plate  200  and is fastened in the sample holder  310  in each of the first direction and the second direction. The polarization direction of the first polarizing plate  200  is optionally set, and the reference axis of the sample holder  310  is in a reference plane of the sample holder  310 .  
         [0079]     While the sample holder  310  rotates in the azimuth direction, the first angle X1 at which the minimum quantity of light is detected is measured in step S 120 .  
         [0080]     Next, the sample  320  is again put into the sample holder  310  in the second direction opposite to the first direction in step S 130 . The sample holder  310  is again aligned between the first polarizing plate  200  and the light detector  400 .  
         [0081]     While the sample holder  310  rotates in the azimuth direction, the second angle X2 at which the minimum quantity of light is detected is measured in step S 140 .  
         [0082]     Finally, a relationship (X2−X1)/2 between the first angle X1 and the second angle X2 is calculated to obtain the polarization direction of the sample  320 .  
         [0083]     As described above, the method and the apparatus for measuring the polarization direction may obtain the polarization direction of the sample using only the reference axis of the sample holder without knowing the polarization direction of the reference polarizing plate. Accordingly, the polarization direction of the sample can be accurately measured using this simple method.  
         [0084]     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.