Abstract:
An improved liquid crystal display includes overlapped electrical conductors in a picture element (pixel) region of a liquid crystal material. The conductors are selectively defined so as to control the electric field applied to the liquid crystal material in a region associated with the liquid crystal material. By controlling the electric field in the region, the degree to which the molecules in the liquid crystal material rotate in response to the electric field can be controlled. In this manner, the contrast of the liquid crystal material in the region may be selectively controlled to, for example, improve the contrast of the liquid crystal display in multiple axes.

Description:
TECHNICAL FIELD 
     The invention relates generally to liquid crystal displays, and, more particularly, to a liquid crystal display having improved isocontrast performance and a method for producing same. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays (LCDs) have been in use for quite some time and are useful for displaying information. Although LCDs are attractive for use in displays for portable devices in which power consumption is a concern, LCDs are also commonly used in other display applications, such as test and monitoring equipment. Advantageously, LCDs typically consume a relatively small amount of power. 
     An LCD can be fabricated either as a reflective display, in which light passes through the display and is reflected back through the display for viewing, or as a transmissive display, in which a light source is located behind the display and a viewer directly views the display. 
     FIG. 1A is a cross-sectional view illustrating an example of a conventional reflective LCD  10 . Briefly described, an LCD is fabricated by locating a liquid crystal (LC) material  11  between glass substrate elements  16  and  17 , to which transparent indium tin oxide (ITO) electrode lines  12  and  14  have been applied. The ITO lines  12  and  14  are applied to the glass substrate elements  16  and  17  in a manner such that the ITO lines form a matrix of intersecting lines. The ITO lines  12  and  14  are typically oriented orthogonal to each other such that a picture element (pixel) is formed in the liquid crystal material at the intersection of each ITO line  12  and  14 . 
     A polarizer  18  is applied to a surface of glass substrate element  16  opposite that to which the ITO line  12  is applied. Similarly, analyzer  19  is applied to a surface of glass substrate element  17  opposite to which the ITO line  14  is applied. A diffuser  21  is applied over the analyzer and a reflector  22  is applied over the diffuser. In the case of a transmissive display, a light source is located in place of the reflector  22 . The polarizer  18  is a thin film applied to the glass to serve as a filter that polarizes the impinging light so that the entering light beam is polarized in one direction. The analyzer is a thin film that aids in the polarization process. The diffuser  21  is also a thin film that diffuses, or smears, the light beam so that annoying birefringence does not occur when the display is viewed. Birefringence can be explained by understanding the refraction of a plane wave of light at the boundary between an isotropic medium (such as air) and an anisciropic medium (such as a crystal). The wavefronts of the incident wave and the refracted wave should be matched at the boundary. Because the anisotropic medium supports two modes of distinctly different phase velocities, for each incident wave there are two refracted waves with two different directions and different polarizations. This effect is known as birefringence. 
     Upon the application of an electrical potential between the ITO line  12  and the ITO line  14 , an electric field is established between the ITO lines  12  and  14  and passes through the LC material. In accordance with known principles, the molecules in the LC material, in response to the electric field, become mobile and. depending upon the type of LC material, will rotate, twist, or otherwise change state, thereby preventing light, by the cross polarizing of the traveling light wave. from passing through the display and appearing dark to a viewer. The display may be normally white or black. Upon the application of the electric field, the LC material will change state. In other words, if the material is “black” it will become “white” and if the material is “white” it will become “black.” Importantly, the LC material changes state in response to the electric field applied by the ITO lines  12  and  14 . 
     FIG. 1B is a plan view schematically illustrating a conventional pixel  15  formed at the intersection  23  (referred to as a “pixel junction”) of ITO lines  12  and  14  of the LCD  10  of FIG.  1 A. Some of the elements have been omitted for clarity. Pixel  15  includes the LC material  11  located at the intersection of, and disposed between ITO lines  12  and  14 . 
     FIG. 1C is a cross-sectional view of the conventional pixel  15  of FIG.  1 B. In response to the electric, or e field  25 , created in the region of the pixel junction  23  between ITO line  12  and ITO line  14 , the molecules that make up LC material  11  will change state, or rotate, thereby becoming visible to a viewer ( 24  of FIG.  1 A). 
     FIG. 1D is a cross-sectional view of the conventional reflective LCD  10  of FIG. 1A illustrating the difference between an “addressed” pixel and a “non-addressed” pixel. In FIG. 1D, the LC material  11  is illustrated as comprising individual molecules, an example of which is indicated by reference numeral  13 . The voltage source “Vs”  27  corresponding to pixel  33  indicates that the LC material  11  within pixel  33  is selected or addressed. When addressed, the orientation of the individual molecules  13  within the LC material  11  sandwiched between alignment layer  28  and alignment layer  29  change state, or twist, and appear to “straighten out”. Alignment layers  28  and  29  are each thin films which have been physically rubbed in specific directions so as to assist the LC molecules  13  adjacent to these layers to pre-rotate in favorable directions. For example, if it is desirable for an LCD to have a preferred viewing angle, these rubbed layers enhance that angular view. The aligned molecules  13  (associated with pixel  33 ) allow the light from light source  24  to pass through the LC material  11  with a specific polarization. The light from light source  24  can be reflected back to the viewer  26  through glass substrate element  16  and polarizer  18 . 
     The molecules  13  within pixel  35 . associated with voltage source “Vna”  26 , have not been addressed. The random molecular orientation of these molecules  13  suppresses the light from light source  24  and prevents the light from passing through the LC material  11  associated with pixel  35 . Hence, pixel  35  is non-addressed and would appear dark to a viewer  26 . 
     FIG. 1E is a graphical representation  31  of the isocontrast curves of pixel  15  of FIGS. 1B and 1C. When LCDs are viewed at angles normal to, or nearly normal to, the surface of the LCD display, the rotated liquid crystal material is easy to discern. However, when viewed at off angles, the polarizing effect of the twisted liquid crystal material on the traveling light wave quickly becomes indiscernible. This is caused by the crystalline nature of the liquid crystal material. This condition is illustrated in FIG. 1E, which is a graphical representation of a contrast curve (referred to as an isocontrast curve) for a conventional pixel  15 . A contrast curve which has the same contrast ratio (light returning from the addressed pixel/light returning from a non-addressed pixel) at every point on its curve is called an isocontrast curve. As shown in FIG. 1E, the liquid crystal material in the region of pixel  15  clearly has better contrast at some angles that at other angles. For example, isocontrast line  34  shows that the pixel has a higher contrast when viewed at approximately 180 or 360 degrees, than it does when viewed at 90 or 270 degrees. For example, arrow  37  indicates a viewing angle in which a viewer would see limited contrast. 
     Therefore there is a need in the industry for a liquid crystal display in which the contrast of the liquid material may be controlled and maximized depending on the viewing angle desired. 
     SUMMARY OF THE INVENTION 
     The invention is a liquid crystal display having improved and controllable isocontrast and a method for producing same. 
     In architecture, the invention can be conceptualized as a liquid crystal display, comprising a liquid crystal material disposed between a pair of transparent plates. The display includes a first electrical conductor and a second electrical conductor associated with the liquid crystal material and configured to form a picture element in an overlap region in which the first electrical conductor and the second electrical conductor overlap. The first electrical conductor and the second electrical conductor are configured to apply an electric field to the liquid crystal material. The electric field causes molecules in the liquid crystal material to change state in response to the electric field in the overlap region associated with the picture element. The overlap region is selectively defined to alter the electric field so that a degree to which the liquid crystal molecules change state in response to the electric field is controlled by the selectively defined overlap region. 
     The invention can also be conceptualized as a method for controlling contrast in a liquid crystal display, the method comprising the steps of: forming a liquid crystal material between a pair of transparent plates and associating a first electrical conductor and a second electrical conductor with the liquid crystal material. The method also includes the step of forming a picture element in a region in which the first electrical conductor and the second electrical conductor overlap. The first electrical conductor and the second electrical conductor apply an electric field to the liquid crystal material. The electric field causes molecules in the liquid crystal material to change state in response to the electric field in the region associated with the picture element. The method also includes the step of selectively defining the overlap region to alter the electric field so that a degree to which the liquid crystal molecules change state in response to the electric field is controlled by the selectively defined overlap region. The invention allows control over the shape of the electric field so that the change in state, or twist, of the liquid crystal material is controllable so as to allow favorable viewing of the display from any angle, thereby reducing, and possibly eliminating, blind spots. 
     An advantage of the invention is that it allows control over the contrast of a liquid crystal display. 
     Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production. 
    
    
     Other features and advantages of the invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     FIG. 1A is a cross-sectional view illustrating an example of a conventional reflective LCD; 
     FIG. 1B is a plan view schematically illustrating a conventional pixel formed at the intersection of the ITO lines of the LCD of FIG. 1A; 
     FIG. 1C is a cross-sectional view of the conventional pixel of FIG. 1B; 
     FIG. 1D is a cross-sectional view of the conventional reflective LCD of FIG. 1A illustrating the difference between an “addressed” pixel and a “non-addressed” pixel; 
     FIG. 1E is a graphical representation of the isocontrast curves of the pixel of FIGS. 1B and 1C; 
     FIG. 2 is a cross-sectional view illustrating a liquid crystal display constructed in accordance with an aspect of the invention; 
     FIG. 3A is a plan view illustrating a pixel constructed in accordance with an aspect of the invention; 
     FIG. 3B is a cross-sectional view of the pixel of FIG. 3A; 
     FIG. 4A is a plan view illustrating an alternative embodiment of the pixel of FIGS. 3A and 3B; 
     FIG. 4B is a cross-sectional schematic view illustrating the pixel of FIG. 4A; and 
     FIG. 5 is a graphical representation of the isocontrast curves of the pixel of FIGS.  3 A and  3 B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description applies to all liquid crystal (LC) displays, and furthermore, applies to all systems and methods in which the state of a liquid crystal material is altered by the application of an electric field to the LC material. 
     Turning now to the drawings, FIG. 2 is a cross-sectional view illustrating a liquid crystal display constructed in accordance with an aspect of the invention. LCD  100  includes LC material  11  disposed between glass substrate elements  16  and  17 . ITO electrode line  112  is applied to the surface of glass substrate  16  that faces LC material  11  and ITO electrode line  114  is applied to the surface of glass substrate  17  that faces LC material  11 . The ITO lines  112  and  114  are applied to the glass substrate elements  16  and  17  in a manner such that the ITO lines form a matrix of intersecting lines. For example, the ITO lines  112  may be applied in one direction and the ITO lines  114  may be applied in a direction orthogonal to the direction of the ITO lines  112 . However, and in accordance with an aspect of the invention, the ITO lines  112  and  114  are selectively defined and configured so as to selectively alter the electric field created between the lines  112  and  114  so as to control the change of state of the LC material  11  there between. The ITO lines  112  and  114  may also be applied to the glass substrate elements  16  and  17 , respectively, at angles other than orthogonal to each other. Furthermore, other conductive material may be used in place of ITO. 
     A polarizer  18  is applied to the surface of glass substrate element  16  opposite that to which the ITO line  112  is applied. Similarly, analyzer  19  is applied to the surface of glass substrate element  17  opposite that to which ITO line  114  is applied. A diffuser  21  is applied over the analyzer and a reflector  22  is applied over the diffuser. 
     In accordance with an aspect of the invention. and to be described below, an electric field created between the ITO line  112  and the ITO line  114  passes through the LC material  11  at the point where ITO line  112  crosses, or overlaps, ITO line  114 . In accordance with an aspect of the invention, the electric field causes the LC material located between ITO line  112  and ITO line  114  to change state to a further degree than previously possible. In this manner, and with respect to a reflective display as shown in FIG. 2, light from a light source  124  travels through LC material  11  and is reflected back to a viewer  126  such that the viewer  126  may observe the selected pixels from a wider viewing angle than previously achievable. 
     FIG. 3A is a plan view illustrating a pixel  115  constructed in accordance with an aspect of the invention. The pixel  115  includes LC material  11  (illustrated with a dotted line for clarity), located between ITO line  112  and  114 . The region in which ITO lines  112  and  114  intersect, or overlap, is known as the pixel junction  123 , and is the region in which the pixel  115  is formed. In accordance with an aspect of the invention, the ITO line  112  is formed so that in the overlap region of pixel junction  123 , a circular portion  124  of the ITO line  112  intersects and overlaps the ITO line  114 , thus increasing the area of the overlap region in which the ITO lines  112  and  114  form the pixel  115 . Similarly, the ITO line  114  could be made in this circular fashion, while the ITO line  112  remains rectangular. Furthermore, both ITO lines  12  and  114  can be constructed using circular portion  124 . Importantly, the circular portion  124  increases the area over which the ITO lines  112  and  114  intersect area and favorably changes the shape of the “twisted” LC material  11  over which ITO lines  112  and  114  intersect. 
     FIG. 3B is a cross-sectional view of the pixel  115  of FIG.  3 A. As shown in FIG. 3B, ITO line  112  applies a positive charge to the LC material  11  and ITO line  114  applies a negative charge to the LC material  11 . Upon application of a voltage potential to ITO lines  112  and  114 , an electric field  125  is created between the circular portion  124  of ITO line  112  and ITO line  114 . The electric field  125  passes through LC material  11 , causing the molecules in the LC material to change state. In accordance with the invention, and shown within region  127  of FIG. 3B, there is a favorable bunching of the electric field  125  in the region  127 , caused by the selective application of the ITO line  112  having the circular portion  124 , thereby applying a more favorable and greater electromotive force (emf) field in the region  127 . The greater emf field passes through LC material  11 , thereby causing the liquid crystal molecules within LC material  11  to change state to a higher degree than if the ITO line  112  was conventionally formed. In this manner, the more favorably shaped emf field results in a greater twist of the molecules within LC material  11  that shapes the isocontrast curve as desired (to be described below with respect to FIG.  5 ). This greater twist results in the LC material having a higher contrast at pixel location  115 . 
     FIG. 4A is a plan view illustrating an alternative embodiment of the pixel of FIGS. 3A and 3B. Pixel  130  is formed such that ITO line  132  is selectively defined so as to describe a square profile  136  at the pixel junction  133  at which ITO line  132  intersects ITO line  134 . In this manner, the selective formation of the ITO line  132  increases the area over which the ITO lines  132  and  134  cover LC material  11 . 
     FIG. 4B is a cross-sectional schematic view illustrating the pixel  130  of FIG.  4 A. In accordance with this aspect of the invention, the ITO line  132  applies a positive charge to the LC material  11  and the ITO line  134  applies a negative charge to the LC material  11 . As shown, the electric field  135  created between the square portion  136  of ITO line  132  and ITO line  134  causes a “focusing” of the electric field  135  in the region indicated by arrow  137 . Focusing the electric field  135  at the pixel junction  133  results in a greater contrast ratio, thereby providing a pixel having a broader viewing angle. It should be understood that various ITO line configurations can be used with equal effectiveness to both improve the isocontrast ratio and shape of the isocontrast curve. 
     FIG. 5 is a graphical representation  150  of the isocontrast curves of pixel  115  of FIGS. 3A and 3B. As shown in FIG. 5, isocontrast curve  151  is significantly more circular in shape than isocontrast curve  34  of FIG.  1 E. In this manner, the ability to view the pixel represented by curve  151  is improved at the 90 and 270 degree viewing angles as compared to the viewing angle of pixel  15  (FIG.  1 E). The arrow  152  indicates that the blind spot illustrated using arrow  37  of FIG. 1E is substantially reduced in the pixel  115  corresponding to the isocontrast curve  151  of FIG.  5 . 
     It will be apparent to those skilled in the art that many modifications and variations may be made to the preferred embodiments of the invention, as set forth above, without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the invention, as defined in the claims that follow.