Abstract:
A liquid crystal display having one or more pixels, each of which includes a first sub-pixel operating at a first threshold voltage, and a second sub-pixel neighboring the first sub-pixel, and operating at a second threshold voltage. The first sub-pixel is divided into two separate portions by the second sub-pixel to form at least four domains of liquid crystal molecules illuminating at various gray levels for improving viewing angle characteristics of the liquid crystal display.

Description:
CROSS REFERENCE 
   The present application claims the benefits of U.S. Provisional Patent Application Ser. No. 60/782,885, which was filed on Mar. 15, 2006 and entitled “LIQUID CRYSTAL DISPLAY DEVICE.” 

   BACKGROUND 
   The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display with compensated pixel arrays. 
   A liquid crystal display is a thin and flat display device comprised of a number of pixels arrayed in front of a light source or reflector. Each pixel contains a layer of liquid crystal molecules between two electrodes. The liquid crystal molecules have electric charges on them. Applying biases to the electrodes creates electrostatic forces that twist the molecules. This twists the light passing through the molecules, and allows varying degrees of light to pass (or not to pass) through the filters. An image can therefore be displayed by those rows and columns of pixels. 
     FIG. 1  illustrates a conventional pixel array  100 , in which each pixel  102  is formed by a first sub-pixel  104  and a second sub-pixel  106  divided by a slit  120  therebetween. In order to improve viewing angle characteristics, the first and second sub-pixels  104  and  106  are designed with different threshold voltages, such that the two sub-pixels  104  and  106  would be charged at different saturation voltage levels when they illuminate. The different saturation voltage levels of the two sub pixels  104  and  106  cause the liquid crystal molecules therein to have different orientation directions. Thus, this improves the viewing angle characteristics. 
   In order to further improve the viewing angle characteristics, each sub-pixel  104  is divided into a number of domains, in which the liquid crystal molecules have various orientation directions. For example, the sub-pixel  104  is divided by protrusions  108  and  110  into three portions. Due to the geometry of the three portions, the molecules of the upper-left portion have an orientation direction represented by an arrow  112 , the molecules of the lower-left portion have an orientation direction represented by an arrow  114 , and the molecules of the right portion have one orientation direction represented by an arrow  116  for its upper half and another orientation direction represented by an arrow  118  for its lower half. Each portion of an orientation direction defines a domain. Thus, the sub-pixel  102  has four domains. 
   These various domains improve the viewing angle characteristics.  FIG. 2  illustrates a cross-sectional view  200  of the sub-pixel  104  along line A-A. When the electrodes  202  and  204  are charged, the protrusion  108  causes the molecules at the right to orient along one direction, and the molecules at the left to orient along another direction. This allows the top position  206 , upper-right position  208 , and upper left position  210  to receive the same amount of light. In other words, the sub-pixel  104  can be viewed from various angles with relatively uniform light transmittance. 
   One drawback of the conventional pixel array  100  is that the protrusions and slits reduce its aperture ratio, which refers to the ratio between the area of a pixel that can transmit light and the actual area of the pixel. It is understood by people skilled in the art that more protrusions and slits lead to a lower aperture ratio. As shown in  FIG. 1 , the sub-pixels  104  and  106  are divided by a slit  120 , and each of them includes two protrusions. This reduces the aperture ratio of the pixel  102 . 
     FIG. 3  illustrates another conventional pixel array  300 , in which each pixel  302  is formed by a first sub-pixel  304  of a lower threshold voltage and a second sub-pixel  306  of a higher threshold voltage. As shown in the drawing, each sub-pixel  304  or  306  only has one protrusion. Thus, the aperture ratio of the pixel  302  is improved. 
   One drawback of the conventional pixel array  300  is that its pixel arrangement is often susceptible to the “mura” issue, which refers to the non-uniformity of an image over a large area of pixels. Referring to  FIGS. 3 and 4  simultaneously, when viewing from the right side of the conventional pixel array  300 , certain rows of sub-pixels  402  would appear to be darker because of their molecule orientations. The sub-pixel  306  has a higher threshold voltage, so that it appears to be darker when it illuminates. The sub-pixel  304  has a lower threshold voltage, so that it appears to be brighter when it illuminates. As a result, columns  404  would appear to be darker because they are formed by the high threshold voltage sub-pixels  306 , while columns  406  would appear to be brighter because they are formed by low threshold voltage sub-pixels  304 . This causes bright and dark stripes interwoven with each other, which is the “mura” defect. 
   As such, what is needed is a liquid crystal display with a pixel array that provides a high aperture ratio, while being free from the “mura” defect. 
   SUMMARY 
   The present invention discloses a liquid crystal display having one or more pixels. In one embodiment of the present invention, each pixel includes a first sub-pixel operating at a first threshold voltage, and a second sub-pixel neighboring the first sub-pixel, and operating at a second threshold voltage. The first sub-pixel is divided into two separate portions by the second sub-pixel to form at least four domains of liquid crystal molecules illuminating at various gray levels for improving viewing angle characteristics of the liquid crystal display. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional pixel array of a liquid crystal display. 
       FIG. 2  illustrates a cross-sectional view of the conventional pixel. 
       FIG. 3  illustrates another conventional pixel array of a liquid crystal display. 
       FIG. 4  illustrates the conventional pixel array having the “mura” defect when viewing from a certain angle. 
       FIG. 5  illustrates a pixel array in accordance with one embodiment of the present invention. 
       FIG. 6  illustrates the pixel array when viewing from a first direction in accordance with the embodiment of the present invention. 
       FIG. 7  illustrates the pixel array when viewing from a second direction in accordance with the embodiment of the present invention. 
       FIG. 8  illustrates another pixel array in accordance with another embodiment of the present invention. 
   

   DESCRIPTION 
     FIG. 5  illustrates a pixel array  500  in accordance with one embodiment of the present invention. The pixel array  500  can be used in various modes of liquid crystal displays, such as the vertically-aligned (VA) mode LCD devices and the twisted-nematic (TN) mode LCD devices. The pixel array  500  is comprised of a plurality of pixels in a rectangular shape where each pixel is divided into two sub-pixels. Each pixel is further divided into a number of domains where the crystal molecules are oriented along different directions. For example, the pixel  502  is divided into three portions  508 ,  510  and  512  by slits  514  and  516 , wherein the slit  514  extends from a mid-point of one side of the pixel  502  to a corner of an opposite side of the pixel  502 , and the slit  516  extends from the mid-point of one side of the pixel to another corner of the opposite side of the pixel  502 . The portions  508  and  510  are electrically connected to each other, whereas the portion  512  is electrically disconnected from the other portions  508  and  510 . Thus, the portions  508  and  510  form a first sub-pixel, and the portion  512  forms a second sub-pixel. The liquid crystal molecules of the portion  508  are oriented along a direction represented by an arrow  518 , and the liquid crystal molecules of the portion  510  are oriented along a direction represented by an arrow  520 . Due to the geometry of the slits  514  and  516 , the liquid crystal molecules of the upper half of the portion  512  are oriented along a direction represented by an arrow  522 , and the liquid crystal molecules of the lower half of the portion  512  are oriented along a direction represented by an arrow  524 . The area where the liquid crystal molecules have the same orientation direction is defined as a domain. Thus, the pixel  502  has four domains. These domains improve the viewing angle characteristics for the pixel  502 . Further, the pixel  502  has only two slits  514  and  516 . Compared to the conventional pixel  102  of  FIG. 1  that has four protrusions and one slit, the aperture ratio of the pixel  502  is significantly improved. 
   The portions  508  and  510  are designed to have a lower threshold voltage, and the portion  512  is designed to have a higher threshold voltage. Such arrangement of the high and low threshold voltage portions is repeated for all the pixels in the pixel array  500 . As shown in  FIG. 5 , the shaded areas represent the high threshold portions, and the un-shaded areas represent the low threshold portions. The pixels with areas shaded by horizontal lines are charged by a positive polarity, and the pixels with areas shaded by vertical lines are charged by a negative polarity. The polarity of the charges may be switched in order to extend the life spans of the pixels. Due to the difference of the polarity, the positively charged pixels and the negatively charged pixels may have slightly different gray levels. 
     FIG. 6  illustrates the pixel array of  FIG. 5  when viewing from its left side. For each pixel  602 , due to the orientations of the liquid crystal molecules, the upper domains  604  and  606  appear to be brighter than the lower domains  608  and  610 . Since the domains  604  and  610  are disposed in the first sub-pixel, and the domains  606  and  608  are disposed in the second sub-pixel, which are driven by different voltages, the domain  604  has a lower threshold voltage, it is brighter than the domain  606 . Likewise, since the domain  610  has a lower threshold voltage, it is brighter than the domain  608 . As such, each pixel has four domains of various gray levels. The un-shaded areas represent the brightest domains. The areas shaded with horizontal or vertical lines represent the mid-bright domains. The areas shaded with checker patterns represent the darkest domains. The areas shaded with slashes represent the mid-dark domains. As shown in  FIG. 6 , the brightest, mid-bright, mid-dark, and darkest domains are mixed over the whole pixel array  600 . Thus, the pixel array  600  would not have the interwoven bright and dark stripes as shown in  FIG. 4 . As a result, the “mura” phenomenon is eliminated. 
     FIG. 7  illustrates the pixel array of  FIG. 5  when viewing from its right side. For each pixel  702 , due to the orientations of the liquid crystal molecules, the upper domains  704  and  706  appear to be darker than the lower domains  708  and  710 . Since the domains  704  and  710  are disposed in the first sub-pixel, and the domains  706  and  708  are disposed in the second sub-pixel, which are driven by different voltages, the domain  704  has a lower threshold voltage, it is brighter than the domain  706 . Likewise, since the domain  710  has a lower threshold voltage, it is brighter than the domain  708 . As such, each pixel has four domains of various gray levels. More specifically, the un-shaded areas represent the brightest domains. The areas shaded with horizontal or vertical lines represent the mid-bright domains. The areas shaded with checker patterns represent the darkest domains. The areas shaded with slashes represent the mid-dark domains. As shown in  FIG. 7 , the brightest, mid-bright, mid-dark, and darkest domains are mixed over the whole pixel array  700 . Thus, the pixel array  700  would not have the interwoven bright and dark stripes as shown in  FIG. 4 . As a result, the “mura” phenomenon is eliminated. 
     FIG. 8  illustrates a pixel array  800  in accordance with another embodiment of the present invention. The pixel array  800  is comprised of a plurality of pixels in a rectangular shape where each pixel is divided into two sub-pixels. Each pixel is further divided into a number of domains where the crystal molecules are oriented along different directions. For example, the pixel  802  is divided into three portions  808 ,  810  and  812  by silts  14  and  816 , wherein the slit  814  extends from a mid-point of one side of the pixel  802  to a corner of an opposite side of the pixel  802 , and the slit  816  extends from the mid-point of one side of the pixel to another corner of the opposite side of the pixel  802 . The portions  808  and  810  are electrically connected to each other, whereas the portion  812  is electrically disconnected from the other portions  808  and  810 . Thus, the portions  808  and  810  form a first sub-pixel, and the portion  812  forms a second sub-pixel. The liquid crystal molecules of the portion  808  are oriented along a direction represented by an arrow  818 , and the liquid crystal molecules of the portion  810  are oriented along a direction represented by an arrow  820 . Due to the geometry of the slits  814  and  816 , the liquid crystal molecules of the upper half of the portion  812  are oriented along a direction represented by an arrow  822 , and the liquid crystal molecules of the lower half of the portion  812  are oriented along a direction represented by an arrow  824 . The area where the liquid crystal molecules have the same orientation direction is defined as a domain. Thus, the pixel  802  has four domains. These domains improve the viewing angle characteristics for the pixel  802 . Further, the pixel  802  has only two slits  814  and  816 . Compared to the conventional pixel  102  of  FIG. 1  that has four protrusions and one slit, the aperture ratio of the pixel  802  is significantly improved. 
   The portions  808  and  810  are designed to have a lower threshold voltage, and the portion  812  is designed to have a higher threshold voltage. Such arrangement of the high and low threshold voltage portions is repeated for all the pixels in the pixel array  800 . As shown in  FIG. 8 , the shaded areas represent the high threshold portions, and the un-shaded areas represent the low threshold portions. The pixels with areas shaded by horizontal lines are charged by a positive polarity, and the pixels with areas shaded by vertical lines are charged by a negative polarity. The polarity of the charges may be switched in order to extend the life spans of the pixels. Due to the difference of polarity, the positively charged pixels and the negatively charged pixels may have slightly different gray levels. 
   The pixel array  800  of  FIG. 8  differs from the pixel array  500  of  FIG. 5  in that the arrangement of the high threshold portions and the low threshold portions are reversed for every two neighboring pixels. The pixel array  800  is similar to the pixel array  500  in the sense that when the pixel array  800  is viewed from a certain angle, each pixel will have four domains of various gray levels. As such, the pixel array  800  will not have the interwoven bright and dark stripes as shown in  FIG. 4 , and therefore, the “mura” phenomenon is eliminated. 
   It is noted that the above embodiments are mere examples for purposes of description. Any alternative of the embodiments capable of eliminating the “mura” defect without reducing the aperture ratio is within the spirit of the invention. For example, the slits discussed above can be curves instead of straight lines. As another example, the locations of the high threshold and low threshold portions can be switched. 
   The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.