Patent Publication Number: US-2002003598-A1

Title: Liquid crystal display with improved field of view

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates generally to the field of liquid crystal displays (LCDs). More particularly, the present invention relates to techniques for improving the field of view of a liquid crystal display.  
       BACKGROUND OF THE INVENTION  
       [0002] In general, it is desirous to provide improved contrast, gray-scale linearity, and hue-angle (i.e., chromaticity) performance over the field of view of conventional displays. These parameters are particularly important in avionic systems and other high-definition viewing applications, such as, twisted nematic active matrix, back panel liquid crystal displays (LCDs).  
       [0003] In conventional display systems, such as, LCDs, a matrix of pixels is controlled by electrical signals to provide a static or a dynamic graphic image. If the display is a color display, each pixel can be comprised of a red element, a green element, and a blue element. An element can be comprised of a single liquid crystal domain or a dual liquid crystal domain or many liquid crystal domains. The liquid crystal domain is controlled by a thin-film transistor (TFT). The domains (i.e., partitions) include a collection of nematic molecules which are manipulated by electrical signals provided by the thin-film transistors. Thus, each thin-film transistor causes a particular amount of light to be transmitted through its associated element.  
       [0004] In conventional systems, contrast, gray-scale stability and or linearity, and hue-angle performance degrade significantly as the viewing angle changes. The contrast in conventional LCDs is usually a maximum within a narrow viewing angle centered about a normal angle and diminishes quickly as the angle of view is changed. This loss of contrast at large viewing angles is caused by light leaking through pixel elements. In color LCDs, such leakage can also cause severe color shifts for both saturated and gray-scale colors (e.g., poor hue-angle performance).  
       [0005] The field of view in typical twisted-nematic LCDs is often severely limited because, in addition to color degradation caused by dark leakage, the optical anisotrophy of the nematic liquid crystal molecules results in large variations of gray-scale transmission (i.e., a shift in the brightness-voltage curve), as a function of viewing angle. The variation is often so severe that some pixels reverse their transmission levels at extreme viewing angles. These limitations are particularly disadvantageous in applications requiring a very high-quality display, such as, in avionics, where cockpit displays must be viewed from both pilot and co-pilot seating positions. Also, high information content displays, such as, aviation displays, require that the relative gray-scale transmission be as invariant as possible with respect to viewing angle.  
       [0006] As shown in FIG. 1, a conventional full-color, single-tilt domain display  100  includes a polarizer  105 , an analyzer  110 , a liquid crystal cell  115 , and one or more compensator layers  120 . Liquid crystal cell  115  includes an active matrix substrate  125 , a color matrix substrate  130 , and liquid crystal material  135 . Polarizer  105  and analyzer  110  both polarize electromagnetic energy. However, the term “polarizer” typically refers to a polarizer element that is closest to the source of light, while the term “analyzer” refers to a polarizer element that is closest to a viewer of the LCD.  
       [0007] Substrate  125  includes an array of TFTs, transparent electrodes, address lines, and an alignment layer. The address lines activate individual liquid crystal display elements via the TFTs. The color matrix substrate  130  can include a black matrix coating, a color filter matrix, a transparent electrode, and an alignment layer. The alignment layers on the active matrix substrate layer  125  and color matrix substrate layer  130  act in combination to induce twisted nematic orientation in liquid crystal material  135 .  
       [0008] With reference to FIG. 2, a dual domain LCD system  150  is similar to the system  100  described with reference to FIG. 1. System  150  includes a collection of nematic molecules  200  disposed between a pair of substrates  205  and  210 . Substrate  205  has been rubbed in a direction  215  to the right and a direction  220  to the left. Direction  215  is directly opposite (i.e., 180° degrees apart) to direction  220 . Similarly, substrate  210  is rubbed in a direction  225  into the page and in a direction  230  out of the page. Direction  225  is directly opposite to direction  230 . Directions  215  and  220  are arranged at 90° angles with respect to directions  225  and  230 , respectively. This rubbing configuration produces two twist domains  235  and  240 .  
       [0009] Rubbing directions  215  and  225  cause twist domains  235  to have a twist angle of 90° and to have an angle of orientation of 0°. Twist directions  220  and  230  cause domains  240  to have a twist angle of 90° and to have an angle of orientation of 180°. Accordingly, domains  235  and  240  have directly opposite angles of orientation (e.g., 180° apart).  
       [0010] System  150  can average the gray-scale behavior of the display over positive and negative vertical viewing directions. Such averaging can produce improved gray-scale linearity over the field of view. Additionally, system  150  can include a compensator structure incorporating an oblique-oriented, positively birefringent compensator element referred to as an O-plate compensator. The O-plate compensator structure significantly improves gray-scale linearity and provides high contrast over large variations in viewing directions for single domain twisted-nematic LCD architectures. However, O-plate compensators can be expensive and difficult to manufacture. Further, conventional dual-domain systems often utilize a multi gap color filter for achieving improved chromaticity stability. However, such devices are difficult to produce and are costly.  
       [0011] Thus, there is a need for a liquid crystal display having improved contrast and/or gray-scale linearity and/or hue-angle performance over the field of view. Further still, there is a need for a liquid crystal display which does not require a multi-gap color filter.  
       SUMMARY OF THE INVENTION  
       [0012] The present invention is related to a twisted nematic liquid crystal display having a field of view. The liquid crystal display includes a liquid crystal cell having a plurality of pixel elements. The pixel elements are comprised of a first domain and a second domain. The first domain has a first angle of orientation and a first twist. The second domain has a second angle of orientation. The first angle of orientation is not opposite the second angle of orientation or the first twist is not equal to 90° to improve the performance over the field of view.  
       [0013] The present invention further relates to a liquid crystal display including a liquid crystal cell that has an array of elements. The elements are comprised of a first partition and a second partition. The first partition has a first angle of orientation and a first twist. The second partition has a second angle of orientation. The first angle of orientation is not opposite the second angle of orientation or the first twist is not equal to 90° to improve the gray-scale linearity over a field of view.  
       [0014] The present invention still further relates to a method of obtaining an optimized field of view for a liquid crystal display including an array of elements. The method includes providing a first partition for each element, which has a first angle of orientation and a first twist, and providing a second partition for each element, which has a second angle of orientation. The first angle of orientation is not opposite the second angle of orientation or the first twist is not equal to 90° to provide the optimized field of view.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The present invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements and:  
     [0016]FIG. 1 is a cross-sectional view of a conventional, single-domain liquid crystal display system;  
     [0017]FIG. 2 is a cross-sectional view of a conventional, dual-domain, twisted nematic liquid crystal display system; and  
     [0018]FIG. 3 is a cross-sectional view of a liquid crystal display in accordance with an exemplary embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS  
     [0019] With reference to FIG. 3, a dual-domain LCD system  300  is configured for use as an avionic display system. System  300  has an optimized field of view where contrast, gray-scale stability and/or linearity, and hue-angle performance are improved relative to conventional systems as viewing angle increases. System  300  advantageously can operate without the use of a multi-gap color filter or expensive compensator layers, such as, layer  205  (FIGS. 2 and 3).  
     [0020] System  300  is similar to system  150  described with reference to FIG. 1. However, system  300  has rubbing directions changed to compensate for contrast, gray-scale stability and/or linearity, and hue-angle performance over the field of view. Additionally, system  300  can be comprised of a number of different partitions instead of only partitions  235  and  240 , as shown in FIG. 3.  
     [0021] System  300  can include dual domains for each of red, green, and blue elements for a total of six domains. Each domain can have its own twist angle and rub orientation (e.g., the rub is the liquid crystal alignment parameter). Such a concept can eliminate the need for multi gap color filters. Alternatively, the twist angle and alignment parameters can be configured utilizing other methods.  
     [0022] As shown in FIG. 3, system  300  includes a collection of nematic molecules  200  disposed between a pair of substrates  205  and  210 . Substrate  205  has been rubbed in a direction  315  generally to the right and a direction  320  generally to the left. Unlike conventional systems, directions  315  and  320  are not necessarily directly opposite (e.g., more than or less than 180° apart). Similarly, substrate  210  is rubbed in a direction  325  generally into the page and a direction  330  generally out of the page. These rubbing configurations produce two twist domains  235  and  240 .  
     [0023] Directions  315  and  320  can be arranged at 90° angles with respect to directions  325  and  330 , respectively. Alternatively, directions  315  and  320  can be arranged at slightly more than or less than 90° angles with respect to directions  325  and  330 , respectively. For example, by having an angle of twist (e.g., defined by directions  315  and  325 ) less than 90°, domain  235  can compensate for domain  240  to improve contrast, hue-angle, and gray-scale stability and/or linearity performance over the field of view. Similarly, by having directions  315  and  320 , which define the angle of orientation of domain  235  and  230 , different than 180° apart (e.g., not directly opposite), system  300  can compensate for contrast, gray-scale linearity, and hue-angle degradation over the field of view. Preferably, the angle of twist of domains  235  and  240  is not equal to 90° and is between 75° and 105°. Preferably, the angle of twist is between 85° and 95°. Also, the difference between the angle of orientation of domains  235  and  240  is between 165° and 205°; preferably, the difference is between 175° and 185°.  
     [0024] System  300  can also be configured so that one of domains  235  and  240  has a conventional angle of orientation and an angle of twist, while the other does not. For example, although domain  235  can have an angle of twist of not equal to 90°, domain  240  can have an angle of twist equal to 90° if the compensation scheme so requires. Any number of rubbing directions and angles can be utilized to achieve the advantages of the present invention. Table 1 below defines exemplary examples of angles for directions  315  and  325 . Table 1 shows the difference in the orientation angles with angles of twists for various rubbing directions.  
                   TABLE 1                       Parameter   Value                                                                                Direction 315   0   0   0   5   0   0   5   0   15   0   0       Direction 320   180   175   180   180   185   165   180   190   180   205   180       Direction 325   88   90   90   95   90   90   95   100   90   90   105       Direction 330   270   265   272   270   275   270   270   270   280   295   270       Domain 235-Twist   88   90   90   90   90   90   90   100   75   90   105       Domain 240-Twist   90   90   92   90   90   105   90   80   110   90   90       Domain 235-Orientation   44   45   45   500   45   45   50   50   52.5   45   52.5       Domain 240-Orientation   225   220   226   225   230   217.52   225   230   235   250   225       Difference in   181   175   181   175   185   172.5   175   180   183.5   205   173.5       Orientation                  
 
     [0025] As can be seen from Table 1, a variety of values can be utilized to configure system  300  for various viewing angles. For example, differences in the angle of orientation can vary from 165° to 205°, and angles of twist can range from 75° to 105°.  
     [0026] It is understood that, while the detailed drawings, specific examples, and particular components given describe a preferred exemplary embodiment of the present invention, they are for the purpose of illustration only. The apparatus of the invention is not limited to the precise details and conditions disclosed. For example, although a twisted nematic display is described, other types of liquid crystal display systems may utilize the principles of the present invention. Various changes may be made to the details disclosed without departing from the spirit of the invention, which is defined by the following claims.