Abstract:
A LCD panel having the large cell gap tolerance includes: an LC (Liquid Crystal) having properties changed by input voltage and changing a transmittance rate change of light incident from the outside; electrodes for applying voltage to the LC; base plates on which the electrodes are formed, each base plate having an LC layer located at prescribed intervals to inject the LC between the electrodes; a split pattern or a floating electrode formed inside each electrode, changing voltage applied to the LC and compensating a cell gap change; and a micro-lens attached on one side of one of the base plates and gathering lights, which are incident from the outside, on a central symmetric line of the slit pattern or the floating electrode. The LCD panel of large cell gap tolerance and the LCD projector using it include LCD panels having large LCD cell gap tolerance.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an LCD panel of large cell gap tolerance and an LCD projector using it, and more particularly, to an LCD panel of large cell gap tolerance and an LCD projector using it, in which a brightness change in each gray level and a transmittance rate change due to an alignment error are small by compensating a cell gap change. 
   2. Background of the Related Art 
   Recently, an Liquid Crystal Display(LCD) projector is utilized in a large display device for an HDTV or a large display device used for an announcement conference such as a seminar because of being small in its volume and easily adjusted in its projection screen size. In general, the LCD projector includes dichroic mirrors for dividing white light outputted from a light source into red, green and blue lights colors, LCD panels for modulating the divided lights with the dichroic mirrors and a projection lens for adding and magnifying lights outputted from the LCD panels. 
   A conventional LCD panel used in the LCD projector includes an Liquid Crystal (LC), of which properties are changed according to input voltage, pixel electrodes and a-common electrodes for applying the input voltage to the LC, and base plates on which the electrodes are formed. Furthermore, the base plates, on which the pixel electrodes are formed, further include TFTs (Thin Film Transistors) for applying/blocking voltage to/from LC layers every pixel. Here, the TFT is most used in the LCD panel of the LCD projector as being easy in multi-gray and fast response. 
   However, such LCD panel shows different properties according to its thickness. That is, according to the LCD panel thickness, a cell transmittance rate is changed small in high gray level but large in low gray level. Additionally, the LCD panel brightness is gradually lowered because an aperture ratio is lowered when the resolution of the LCD panel becomes gradually high. 
   Even though the LCD is manufactured by highly controlled processes, the LCD thickness is not uniform to depending on the position even in the same LCD panel. Furthermore, the different LCD panel thickness causes a different thermal expansion to depending on the position (or point) in the same LCD panel because the LCD panel receives lots of infrared rays and visible rays from the light source. For example, if there is a difference of temperature of 1° C. between glass base plates of the LCD panel, the cell gap is changed about 0.1 μm. 
     FIG. 1  illustrates a cross-sectional view of an LCD panel on which a conventional microlens is attached. 
   As shown in  FIG. 1 , the LCD panel, on which the micro lens is attached, includes a first glass base plate  1 , TFTs  2  and pixel electrodes  3  formed on the first glass base plate  1 , a second glass base plate  6  formed in a predetermined interval from the first glass base plate  1 , a common electrode  5  directing the TFTs  2  and the pixel electrodes  3  and formed on the second glass base plate  6 , an LC layer  4  filled with LC and formed between the pixel electrodes and the common electrode  5 , and a micro lens  7  attached on an opposite side of the side the second glass base plate  6 , on which the common electrode  5  is attached. 
   The micro-lens  7  transmits light entering a BM(Black Matrix), a signal line or a scan line (, which are non-modulated areas) toward the pixel electrodes and increase effective aperture ratio. 
     FIG. 2  illustrates a detailed cross-sectional view of the pixel electrode of  FIG. 1. A  gate electrode  9  of the TFT  2  is connected to the scan line of the LCD panel, a source electrode  8  is connected to the signal line of the LCD panel, and a drain electrode  10  is connected to the pixel electrode  5  of the LCD panel. 
   An operation method of the LCD panel on which the micro lens is attached will be described as follows. 
   In case of a selection period of time: 
   If voltage of the gate electrode  9  connected to the scan line is larger than that of the source electrode  8  connected to the signal line, a connection resistance of a channel formed between the drain electrode  10  and the source electrode  8  becomes small. Therefore, voltage of the source electrode  8  connected to the signal line is formed between the pixel electrode  3  and the LC layer  4 . 
   In case of a non-selection period of time: 
   If voltage of the gate electrode  9  connected to the scan line is smaller than that of the source electrode  8  connected to the signal line, the connection resistance of a channel formed between the drain electrode  10  and the source electrode  8  becomes larger, and thereby the drain electrode  10  and the source 
   electrode  8  are electrically isolated. Therefore, the LC layer  4  keeps electric charge accumulated during the selection period of time. 
   If root means square (rms) voltage, which is applied to the LC layer  4  formed between the pixel electrode  3  and the common electrode  5 , is controlled when linearly polarized light emitted from a polarizer (not shown) mounted on the outside of the micro lens  7  passes the LC layer  4  through the micro lens  7 , the polarized state of the light is changed. The LCD pixel brightness is changed by the changed light selectively passing the analyzer mounted to the outside of the first glass base plate  1  of the LCID panel, and thereby the pixel brightness change as data information. 
   Meanwhile, the LCD projector according to the prior arts, according to LC mode, uses a 90° TN mode in case of a transmission type, a parallel oriented ECB (Electric Controlled Birefringence) mode in case of a reflection type, or a TN mode having a twist angle less than 90°. 
   Recently, the LCD panel used in the LCD projector shows resolution of 0.7 inch XGA level and may show resolution of 0.5 inch XGA level in the future. 
   However, the conventional LCD, on which the micro lens is attached, and the LCD projector using it has still several problems that the brightness change in each gray level is large and the transmittance rate change due to alignment error is large, and thereby the video quality is deteriorated and the production yield is low. The problems will be described in more detail, taking examples as follows. 
     FIG. 3  illustrates a graph showing a relative transmittance change in each gray level of the LCD panel according to the cell gap of the prior art. 
   As shown in  FIG. 3 , G 0  indicates the reference, so the transmittance change is zero when the thickness is 4.0 μm, G 1  indicates the relative transmittance change when the cell gap is 4.4 μm, and G 2  indicates the relative transmittance change when the cell gap is 3.6 μm. Therefore, because the cell relative transmittance change differs about 40% or more according to the cell gap, the brightness change in each gray level is still large. 
   Therefore, even though the conventional LCD panel, on which the micro-lens is attached, places the focus on the pixel electrode, the brightness change in each gray level is large and the transmittance rate change due to the alignment error is large, thereby deteriorating the video quality and lowering the production yield. 
   Meanwhile, the LCD projector using the conventional LCD panel, on which the micro lens is attached, also has the above problems that the brightness change in each gray level is large, the transmittance rate change due to the alignment error is large, thereby deteriorating the video quality and lowering the production yield. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to an LCD panel of large cell gap tolerance and an LCD projector using it that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide an LCD panel of large cell gap tolerance and an LCD projector using it, which can minimize a transmittance rate change in each gray level by compensating the thickness of the LCD panel, to improve the video quality and the production yield. 
   Another object of the present invention is to provide an LCD panel of large cell gap tolerance and an LCD projector using it, which can minimize a transmittance rate change due to brightness and alignment errors in each gray level by compensating the thickness of the LCD panel, to improve the video quality and the production yield. 
   A further object of the present invention is to provide an LCD panel of large cell gap tolerance and an LCD projector using it, which minimize a transmittance rate change due to brightness and alignment errors in each gray level by placing focus of a micro lens on a central symmetric line of a slit pattern or a floating electrode and compensating the thickness of the LCD panel, to increase the video quality and the production yield. 
   A still further object of the present invention is to provide an LCD panel of large cell gap tolerance and an LCD projector using it, which can minimize a transmittance rate change due to a cell gap change and an alignment error using a side electric field property according to the cell gap change. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an LCD (Liquid Crystal Display) panel having large cell gap tolerance includes: an LC (Liquid Crystal) having properties changed by input voltage, the LC changing a transmittance rate change of light incident from the outside; electrodes for applying voltage to the LC; base plates on which the electrodes are formed, each base plate having an LC layer located at prescribed intervals to inject the LC between the electrodes; a slit pattern or a floating electrode formed inside each electrode, changing voltage applied to the LC and compensating a cell gap change; and a micro-lens attached on one side of one of the base plates, the micro lens gathering lights, which are incident from the outside, on a central symmetric line of the slit pattern or the floating electrode. 
   In another aspect of the present invention, to achieve these objects and other advantages and in accordance with the purpose of the invention, an LCD projector includes: dichromatic filters for dividing light output form a light source into red, green and blue colors; LCD panels for modulating the lights by minimizing a transmittance rate change of the lights output from the dichromatic filters by compensating a value that multiplies a anisotropic refractive index(Δn) of LC and d(cell gap) in relation to a cell gap deviation in a gray level condition; and a projection lens for gathering and magnifying the lights output from the LCD panels. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings; 
       FIG. 1  illustrates a cross-sectional view of a conventional LCD panel on which a microlens is attached; 
       FIG. 2  illustrates a detailed cross-sectional view of a pixel electrode of  FIG. 1 ; 
       FIG. 3  illustrates a graph showing a relative transmittance change in each gray level of the conventional LCD panel according to the cell gap; 
       FIG. 4  illustrates a cross-sectional view of an LCID panel, on which a micro-lens is attached, according to a first preferred embodiment of the present invention; 
       FIG. 5  illustrates a detailed cross-sectional view of a pixel electrode of  FIG. 4 ; 
       FIG. 6  illustrates a view showing a structure of an equivalent circuit of a slit pattern of  FIG. 4 ; 
       FIG. 7  illustrates a graph showing a relative transmittance change in each gray level according to the cell gap of the LCD panel, on which the micro-lens is attached, according to of the present invention; 
       FIG. 8  illustrates a view showing a relative transmittance change due to alignment error of the micro-lens; 
       FIG. 9  illustrates a cross-sectional view of an LCD panel using a micro-lens according to a second preferred embodiment of the present invention; 
       FIG. 10  illustrates a detailed cross-sectional view of a pixel electrode including a floating electrode of  FIG. 9 ; and 
       FIG. 11  illustrates a view showing a structure of an LCD projector of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 4  illustrates a cross-sectional view of an LCD panel, on which a micro-lens is attached, according to a first preferred embodiment of the present invention. 
   As shown in  FIG. 4 , the LCD_(Liquid Crystal Display) panel, on which the micro-lens is attached, includes a first glass base plate  11 , TFTs  12 , pixel electrodes  14  and slit patterns  15 , which are formed on the first glass base plate  11 , a second glass base plate  18  formed in a predetermined interval from the first glass base plate  1 , a common electrode  17  directing the TFTs  12  and the pixel electrodes  14  which is formed on the second glass base plate  18 , an LC(Liquid Crystal) layer  16  filled with LC between the pixel electrodes  14  and the common electrode  17 , and the micro-lens  19  attached on an second glass base plate  18 . Here, the micro-lens  19  is positioned opposite side of the common electrode  17  with reference to the second glass base plate  18  is. 
     FIG. 5  illustrates a detailed cross-sectional view of the pixel electrode of FIG.  4 . 
   A gate electrode  21  of the TFT  12  is connected to a scan line of the LCD panel, a source electrode  20  is connected to a signal line of the LCD panel, and a drain electrode  22  is connected to the pixel electrode  14  of the LCD panel. Furthermore, the slit pattern  15  formed in the pixel electrode  14  is designed in such a manner that a cross-sectional center of a light spot  23  passing the micro-lens  19  is located at the center of the slit pattern  15 , i.e., a point where an X-axis symmetric line and a Y axis symmetric line of the slit pattern meet with each other. Here, a central symmetric line of the slit pattern  15  is a symmetric line, which divides the slit pattern  15  into two in a longitudinal direction. 
   An operation method of the LCID panel according to the present invention will be described as follows. 
   First, referring to the drawings, when voltage is applied to the LC layer, the relationship between an electric field induced to the silt pattern  15  and a voltage  5  distribution will be described as follows. 
     FIG. 6  illustrates a view showing a structure of an equivalent circuit of the slit pattern of FIG.  4 . 
   As shown in  FIG. 6 , V1 and V3 indicate voltage formed on the pixel electrodes, and V2 indicates voltage formed on the common electrode. At this time, 10 assuming that there is a microelectrode at a part A in the slit pattern, C 1 , C 2  and C 3  indicate capacitances formed among the pixel electrodes, the common electrode and the microelectrode. 
   Therefore, induction voltage_(V(A)) induced to the microelectrode(A) of the slit pattern is obtained through the following equation(1):
 
 V ( A )= C   1   V 1 +C   2   V 2 +C   3   V 3 +. . . /C   1   +C   2 + C   3 +. . .   (1) 
 
   In the relationship between the voltage distribution_(V1, V3; V1=V3) induced to the microelectrode of the part A and the voltage distribution of the pixel electrodes, if the voltage distribution_(V1, V3; V1=V3) induced to the microelectrode of the part A is different from the voltage distribution of the pixel electrodes, the horizontal electric field of voltage corresponding to a difference between the voltage distribution(V1, V3; V1=V3) induced to the microelectrode of the part A and the voltage distribution of the pixel electrodes is applied between the microelectrode of the part A and the pixel electrode. 
   In the relationship between the cell gap change and the induction voltage distribution, the V(A) moves toward the voltage distribution of the common electrode because the capacitance_(C 2 ) between the microelectrode and the common electrode is increased if the cell gap is reduced, and the V(A) moves toward the voltage distribution of the pixel electrode because the capacitance_(C 2 ) is lowered if the cell gap is increased. That is, if dielectric anisotropic(A C) of LC is positive and a lateral electric field and a vertical electric field are applied at the same time, liquid crystal molecules increase a strength oriented horizontally if the lateral electric field becomes strong, but increase a strength oriented vertically if the lateral electric field becomes weak. 
   In the relationship between the cell gap change and LC anisotropic refractive index (Δn), LC anisotropic refractive index (Δn)is increased because the horizontal electric field is increased if the cell gap_(d) is reduced, but reduced because the vertical electric field is increased if the cell gap is increased. 
   Therefore, the cell transmittance is proportional to a value that multiplies anisotropic refractive index (A n) of LC and the cell gap_(d). If one of anisotropic refractive index (A n) of LC and the cell gap_(d) is increased, the other is reduced because anisotropic refractive index (A n) of LC and the cell gap_(d) are acted in opposite directions to each other. Therefore, the transmittance rate change of the LCD panel according to the cell gap change is reduced because the value that multiplies anisotropic refractive index (A n) of LC and the cell gap_(d) is changed small. 
     FIG. 7  illustrates a graph showing a relative transmittance change in each gray level according to the cell gap of the LCD panel, on which the micro lens is attached, according to the present invention. G 10  is a graph showing the transmittance rate change in case that a reference cell gap is 4.011 m, G 11  is a graph showing the transmittance rate change in case that the cell gap is 4.4 Fun, and G 12  is a graph showing the transmittance rate change in case that the cell gap is 3.6 μm. 
   The following table 1 shows the maximum transmittance rate when the cell gap is changed ±10% in case that a diameter of light spot gathered on a symmetric central line of the slit pattern through the micro-lens is 4μ like the width of the slit pattern, and the width of the pixel electrode is 4 μm. 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Graylevel 
               32 
               64 
               96 
               128 
               160 
               192 
               224 
               256 
             
             
                 
             
           
           
             
               Maximum 
               16.4 
               4.2 
               8.2 
               14.1 
               15.5 
               17.8 
               15.4 
               3.1 
             
             
               Brightness 
             
             
               change (%) 
             
             
                 
             
           
        
       
     
   
   As shown in the drawing, the width of the transmittance rate change according to the cell gap deviation of the LCD panel, on which the micro lens is attached, of the present invention is narrower in each gray level than that of the conventional LCD panel of FIG.  3 . That is, the LCID panel, on which the micro-lens is attached, according to the present invention is reduced in the alignment error and minimized in the light transmittance rate change. 
     FIG. 8  illustrates a view showing a relative transmittance change due to alignment error of the micro lens in a gray level  96  of FIG.  7 . The graph shows the relative transmittance rate that the transmittance rate within the range of light spot radius of 2 μm is integrated every point of 0.25 μm from the central symmetric line of the slit pattern. 
   For example, assuming that at a point x away from the central symmetric line and an integrated value of the transmittance rate of the light point radius of 2 μm from x is I(x), the integrated value of the transmittance rate of radius of 2 μm from the point 1 μm away from the x is I(x+1), and thereby the relative 5 transmittance rate(R(x)) is obtained by the following equation (2).
 
 R ( x )= I ( X+ 1)/ I ( X ) X 100  (2) 
 
   Therefore, the relative transmittance rate change due to the alignment error is smallest when light passing the micro lens is incident on the central symmetric line of the slit pattern. That is, because an effect of the slit pattern is not shown when the light passing the micro lens is separated from the slit pattern, the micro lens is designed in such a manner that the micro lens places the focus on the central symmetric line of the slit pattern. 
   Meanwhile, the slit pattern is designed in such a manner that the width of the slit pattern is smaller than the cell gap because the operation voltage is increased and the light and dark contrast ratio is lowered if the width is increased. For example, if the cell gap is about 4 μm, the operation voltage is lowered and the light and dark contrast ratio is increased only when the width of the slit pattern is 4 μm. Therefore, to make the light, which passed the micro-lens, satisfy the above conditions and pass the slit pattern, the light must be located within 2 μm from the central symmetric line. 
     FIG. 9  illustrates a cross-sectional view of an LCD panel using a micro-lens according to a second preferred embodiment of the present invention. 
   As shown in  FIG. 9 , the LCD panel, on which the micro lens is attached, includes a first glass base plate  11 , TFTs  12  and pixel electrodes  14 , which are formed on the first glass base plate  11 , floating electrodes  15 - 1  formed inside the pixel electrodes  14 , a second glass base plate  18  formed in a predetermined interval from the first glass base plate  11 , a common electrode  17  directing the TFTs  12  and the pixel electrodes  14 , which is formed on the second glass base plate  18 , an LC layer  16  filled with LC between the pixel electrode  14  and the common electrode  17 , and a micro-lens  19  attached on an opposite side of the side of the second glass base plate  18 . Here, the micro-lens  19  is positioned opposite side of the common electrode  17  with reference to the second glass base plate  18 . 
     FIG. 10  illustrates a detailed cross-sectional view of the pixel electrode including the floating electrode of FIG.  9 . 
   A gate electrode  21  of the TFT  12  is connected to a scan line of the LCD panel, a source electrode  20  is connected to a signal line of the LCD panel, and a drain electrode  22  is connected to the pixel electrode  14  of the LCD panel. Furthermore, the floating electrode  15 -I formed in the pixel electrode  14  is designed in such a manner that a cross-sectional center of a light spot  23  passing the micro-lens  19  is located at the center of the floating electrode  15 - 1 , i.e., a point where an X axis symmetric line and a Y axis symmetric line of the floating electrode  15 - 1  meet with each other. If the floating electrode  15 - 1  is designed to satisfy the above conditions, the transmittance rate change due to the alignment error is minimized like the slit pattern  15  of the first embodiment. 
   Meanwhile, because of the structure of the pixel electrode and the micro-lens, if the focus of the micro lens is not placed on the point where the symmetric lines of the X axis and the Y axis meet with each other, the focus is placed on one point of the central symmetric line. 
   In the LCD panel, in which the floating electrode  15 - 1  is formed inside the pixel electrode, voltage induced to the floating electrode is induced like the above equation  1  relative to the LCD panel, on which the slit pattern is formed, of the first embodiment. That is, because the floating electrode serves as the microelectrode (A) of the slit pattern of the first embodiment, the capacitance formed between the surrounding electrodes is equal to that formed between the microelectrode and the surrounding electrodes in an equivalent circuit. 
   Therefore, in case that there is the floating electrode, because voltage induced to the floating electrode is moved toward voltage of the common electrode if the cell gap is reduced, the horizontal electric field is increased, and thereby the cell refraction anisotropy is increased. To the contrary, because voltage induced to the floating electrode is moved toward voltage of the pixel electrode if the cell gap is increased, the anisotropic refractive index (A n) of LC is reduced. 
   The cell brightness is a function of a value that multiplies anisotropic refractive index (A n) of LC and the cell gap_(d). Here, because the cell refraction anisotropy_(Δn) and the cell gap(d) act in opposite directions to each other, the LCD panel including the floating electrodes formed on the pixel electrodes operates like the LCD panel having the slit pattern of the first embodiment. That is, the transmittance rate of the LCD panel including the floating electrodes is insensible to the cell gap change. 
   Therefore, the LCD panel, on which the micro lens is attached, according to the present invention can reduce the alignment error generated when the micro-lens is attached. Furthermore, the LCD panel according to the present invention can minimize the transmittance rate change by designing in such a manner that the focus of the micro lens is placed on the symmetric point of the slit pattern or the floating electrode. Thereby, the LCD panel can have improved color uniformity, color purity and color reproducibility. 
   Till now, the LCD panel according to the present invention is described in relation to the embodiments having the slit pattern and the floating electrode formed on the first glass base plate, but the slit pattern and the floating electrode may be formed on the common electrode. At this time, in case that the slit patterns or the floating electrodes are formed in the pixel electrodes, they are simultaneously formed when the pixel electrodes are formed. However, in case that the slit patterns or the floating electrodes are formed on the common electrode, a step of exposing the common electrode is added. 
   The LCD panel constructed as the above may be utilized as an LCD element of an LCD projector requiring high brightness and quality like conference data or HDTVs. 
     FIG. 11  illustrates a view showing a structure of the LCD projector of the present invention. The LCID projector includes a light source  40  generating and outputting white light, diachromatic filters or diachromatic mirrors  41   r ,  41   b - 1 ,  41   b - 2  and  41   g  receiving the white light and dividing the white light into red, green and blue lights LCD panels  42 R,  42 G and  42 B for modulating (i.e., controlling the transmittance rate) and outputting the red, green and blue lights, refraction mirrors  43   1  and  43 - 2  for regulating a optical path of the lights, and a projection lens  44  receiving and magnifying the lights passing the LCD panels  42 R,  42 G and  42 B and outputting the magnified lights to a screen  45 . 
   Here, as described above, the LCD panels  42 R,  42 G and  42 B place the focus of the micro-lens on the center of the slit patterns or the floating electrodes formed on the pixel electrodes. 
   Therefore, the LCD projector according to the present invention can have excellent color uniformity, color purity and color reproducibility displayed on the screen. 
   The forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.