Patent Publication Number: US-7916108-B2

Title: Liquid crystal display panel with color washout improvement and applications of same

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a liquid crystal display (LCD), and more particularly to an LCD apparatus having an LCD panel with color washout improvement. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays (LCDs) are commonly used as a display device because of its capability of displaying images with good quality while using little electrical power. An LCD apparatus includes an LCD panel formed with liquid crystal cells and pixel elements with each associating with a corresponding liquid crystal cell and having a liquid crystal (LC) capacitor and a storage capacitor, a thin film transistor (TFT) electrically coupled with the liquid crystal capacitor and the storage capacitor. These pixel elements are substantially arranged in the form of a matrix having a number of pixel rows and a number of pixel columns. Typically, scanning signals are sequentially applied to the number of pixel rows for sequentially turning on the pixel elements row-by-row. When a scanning signal is applied to a pixel row to turn on corresponding TFTs of the pixel elements of a pixel row, source signals (image signals) for the pixel row are simultaneously applied to the number of pixel columns so as to charge the corresponding liquid crystal capacitor and storage capacitor of the pixel row for aligning orientations of the corresponding liquid crystal cells associated with the pixel row to control light transmittance therethrough. By repeating the procedure for all pixel rows, all pixel elements are supplied with corresponding source signals of the image signal, thereby displaying the image signal thereon. 
     Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. The orientations of liquid crystal molecules in liquid crystal cells of an LCD panel play a crucial role in the transmittance of light therethrough. For example, in a twist nematic LCD, when the liquid crystal molecules are in its tilted orientation, light from the direction of incidence is subject to various different indexes of reflection. Since the functionality of LCDs is based on the birefringence effect, the transmittance of light will vary with different viewing angles. Due to such differences in light transmission, optimum viewing of an LCD is limited within a narrow viewing angle. The limited viewing angle of LCDs is one of the major disadvantages associated with the LCDs and is a major factor in restricting applications of the LCDs. 
     Several approaches exist for increasing the viewing angles of LCDs, such as in-plane switching (IPS) mode, and multi-domain vertical alignments. IPS mode uses comb-like inter-digitized electrodes to apply electrical fields in the plane of the substrates, thereby aligning the liquid crystal molecules along the substrates and providing wide viewing angles for use in wide viewing angle monitors or other applications. However, although IPS provides wide viewing angles, it requires high voltages and has low aperture ratios. In addition, due to the planar electric field structure, IPS mode inherently suffers from severe image sticking. A multi-domain arrangement is achieved by introducing a protruding structure that forces the liquid crystal molecules to tilt in different directions. However, such a multi-domain vertical alignment requires an extra photolithography step during fabrication. 
     Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF THE INVENTION 
     The present invention, in one aspect, relates to an LCD panel with color washout improvement. In one embodiment, the LCD panel includes a common electrode; a plurality of scanning lines, {G n }, n=1, 2, . . . , N, spatially arranged along a row direction; a plurality of data lines, {D m }, m=1, 2, . . . , M, spatially arranged crossing the plurality of scanning lines {G n } along a column direction perpendicular to the row direction; and a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, each pixel P n,m  defined between two neighboring scanning lines G n  and G n+1  and two neighboring data lines D m  and D m+1 . Each pixel P n,m  comprises at least a first sub-pixel, P n,m ( 1 ), and a second sub-pixel, P n,m ( 2 ). Each of the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) comprises a sub-pixel electrode, a liquid crystal (LC) capacitor and a storage capacitor both electrically connected between the sub-pixel electrode and the common electrode in parallel, and a transistor having a gate electrically connected to the scanning line G n , a source electrically connected to the sub-pixel electrode and a drain. 
     In one embodiment, the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the first sub-pixel P k,m ( 1 ) of the pixel P k,m , where k=1, 2, . . . , N, and k≠n. For example, k=n+1 or n−1. 
     In another embodiment, the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the second sub-pixel P k,m ( 2 ) of the pixel P k,m , where k=1, 2, . . . N, and k≠n. In one embodiment, k=n+1 or n−1. 
     In one embodiment, the sub-pixel electrode of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  has an area A 1 , and the sub-pixel electrode of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  has an area A 2 , and the ratio of A 1 /A 2  is in a range of about 0.2-5.0. 
     For such an LCD panel, when a scanning signal is applied to a scanning line G n  to turn on the corresponding transistors connected to the scanning line G n , a plurality of data signals is simultaneously applied to the plurality of data lines {D n }, respectively, so as to charge the corresponding LC capacitors and storage capacitors of each pixel of the corresponding pixel row for aligning states of corresponding liquid crystal cells associated with the pixel row to control light transmittance therethrough. 
     The plurality of data signals comprises a plurality of gray level voltages, each gray level voltage being associated with a gray level, g, of an image to be displayed on a pixel in the pixel row such that when the gray level voltage is applied the pixel, a potential difference, ΔV 12 (g), is generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel, which varies with the gray level g of the image to be displayed on the pixel, where g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). 
     In one embodiment, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of a pixel varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g is less than the potential difference ΔV 12 (g+1) for the gray level (g+1); and (ii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g is greater than the potential difference ΔV 12 (g+1) for the gray level (g+1), where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero. 
     In another embodiment, the potential difference ΔV 12 (g) varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V c ; (ii) when the gray level g is in the range from g a  to g b , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V b ; and (iii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V c , where V a &gt;V b &gt;V c . 
     In another aspect, the present invention relates to a method of driving a liquid crystal display (LCD) with color washout improvement. In one embodiment, the method includes the step of providing an LCD panel. The LCD panel has a common electrode; a plurality of scanning lines, {G n }, n=1, 2, . . . , N, spatially arranged along a row direction; a plurality of data lines, {D m }, m=1, 2, . . . , M, spatially arranged crossing the plurality of scanning lines {G n } along a column direction perpendicular to the row direction; and a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix. Each pixel P n,m  is defined between two neighboring scanning lines G n  and G n+1  and two neighboring data lines D m  and D m+1 . Each pixel P n,m  includes at least a first sub-pixel, P n,m ( 1 ), and a second sub-pixel, P n,m ( 2 ), where each of the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) comprises a sub-pixel electrode, a liquid crystal (LC) capacitor and a storage capacitor both electrically connected between the sub-pixel electrode and the common electrode in parallel, and a transistor having a gate electrically connected to the scanning line G n , a source electrically connected to the sub-pixel electrode and a drain. 
     In one embodiment, the drain of the transistor of the first sub-pixel P n,m (a) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the first sub-pixel P k,m ( 1 ) of the pixel P k,m . In another embodiment, the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the second sub-pixel P k,m ( 2 ) of the pixel P k,m , where k=1, 2, . . . , N, and k≠n. 
     Furthermore, the method includes the steps of generating the plurality of driving signals; and applying a plurality of driving signals to the LCD panel so as to generate a potential difference, ΔV 12 (g), in the sub-pixel electrodes of the first and second sub-pixels of each pixel, respectively. In one embodiment, the plurality of driving signals comprises a plurality of scanning signals sequentially applied to the plurality of scanning lines, a plurality of data signals simultaneously applied to the plurality of data lines, and a common signal applied to the common electrode, respectively. 
     In one embodiment, the plurality of data signals comprises a plurality of gray level voltages. Each gray level voltage is associated with a gray level, g, of an image to be displayed on a pixel in the pixel row. When the gray level voltage is applied the pixel, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel varies with the gray level g of the image to be displayed on the pixel, where g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). 
     In one embodiment, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of a pixel varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g is less than the potential difference ΔV 12 (g+1) for the gray level (g+1); and (ii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g is greater than the potential difference ΔV 12 (g+1) for the gray level (g+1), where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero. 
     In another embodiment, the potential difference ΔV 12 (g) varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V a ; (ii) when the gray level g is in the range from g a  to g b , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V b ; and (iii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V c , where V a &gt;V b &gt;V c . 
     In yet another aspect, the present invention relates to an LCD panel. In one embodiment, the LCD panel has a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, n=1, 2, . . . , N, and m=1, 2, . . . , M, and N, M being an integer greater than zero, each pixel P n,m  comprising at least a first sub-pixel, P n,m ( 1 ), having a sub-pixel electrode, and a second sub-pixel, P n,m ( 2 ), having a sub-pixel electrode. 
     In one embodiment, the plurality of pixels {P n,m } is configured such that when a gray level voltage associated with a gray level, g, of an image to be displayed on a pixel P n,m  is applied to the pixel P n,m , a potential difference, ΔV 12 (g), is generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel P n,m , and varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g is less than the potential difference ΔV 12 (g+1) for the gray level (g+1); and (ii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g is greater than the potential difference ΔV 12 (g+1) for the gray level (g+1). g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h is an integer greater than zero and R=(2 h −1). Additionally, 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero. 
     The LCD panel also has a common electrode; a plurality of scanning lines, {G n }, n=1, 2, . . . , N, spatially arranged along a row direction; and a plurality of data lines, {D m }, m=1, 2, . . . , M, spatially arranged crossing the plurality of scanning lines {G n } along a column direction perpendicular to the row direction, where each pixel P n,m  of the the plurality of pixels {P n,m } is defined between two neighboring scanning lines G n  and G n+1  and two neighboring data lines D m  and D m+1 . 
     In one embodiment, each of the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) of each pixel P n,m  further comprises a liquid crystal (LC) capacitor and a storage capacitor both electrically connected between the sub-pixel electrode and the common electrode in parallel, and a transistor having a gate electrically connected to the scanning line G n , a source electrically connected to the sub-pixel electrode and a drain. In one embodiment, the drain of the transistor of the first sub-pixel P n,m (a) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the first sub-pixel P k,m ( 1 ) of the pixel P k,m . In another embodiment, the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the second sub-pixel P k,m ( 2 ) of the pixel P k,m . k=1, 2, . . . , N, and k≠n. 
     In a further aspect, the present invention relates to an LCD panel. In one embodiment, the LCD panel includes a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, n=1, 2, . . . , N, and m=1, 2, . . . , M, and N, M being an integer greater than zero, each pixel P n,m  comprising at least a first sub-pixel, P n,m ( 1 ), having a sub-pixel electrode, and a second sub-pixel, P n,m ( 2 ), having a sub-pixel electrode. The plurality of pixels {P n,m } is configured such that when a gray level voltage associated with a gray level, g, of an image to be displayed on a pixel is applied to the pixel P n,m , a potential difference, ΔV 12 (g), is generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel P n,m , and varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g 3 , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V 3 ; (ii) when the gray level g is in the range from g 3  to g ba , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V b ; and (iii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V c . g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h is an integer greater than zero and R=(2 h −1). Additionally, 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero, and V a &gt;V b &gt;V c . 
     In yet a further aspect, the present invention relates to an LCD panel. In one embodiment, the LCD panel includes a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, n=1, 2, . . . , N, and m=1, 2, . . . , M, and N, M being an integer greater than zero, each pixel P n,m  comprising at least a first sub-pixel, P n,m ( 1 ), having a sub-pixel electrode and a second sub-pixel, P n,m ( 2 ), having a sub-pixel electrode. In one embodiment, the plurality of pixels, {P n,m }, is configured such that when a gray level voltage associated with a gray level, g, of an image to be displayed on a pixel is applied to the pixel P n,m , a potential difference, ΔV 12 (g), is generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel P n,m , which varies with the gray level g of the image to be displayed on the pixel, where g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). 
     In one embodiment, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel satisfies the following relationships of:
         (1). when 0≦g≦g a , ΔV 12 (g)&lt;ΔV 12 (g+1); and   (2). when g b ≦g≦R, ΔV 12 (g)&gt;ΔV 12 (g+1),
 
where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero.
       

     In another embodiment, the potential difference ΔV 12 (g) satisfies the following relationships of:
         (i). when 0≦g≦g a , ΔV 12 (g)=V a ;   (ii). when g a ≦g≦g b , ΔV 12 (g)=V b ; and   (iii). when g b ≦g≦R, ΔV 12 (g)=V c ,
 
where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero, and V a , V b  and V c  are constant voltages with V a &gt;V b &gt;V c .
       

     In one aspect, the present invention relates to a method of driving a liquid crystal display (LCD) with color washout improvement. In one embodiment, the method includes the steps of providing an LCD panel comprising a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, n=1, 2, . . . , N, and m=1, 2, . . . , M, and N, M being an integer greater than zero, each pixel P n,m  comprising at least a first sub-pixel, P n,m ( 1 ), having a sub-pixel electrode, and a second sub-pixel, P n,m ( 2 ), having a sub-pixel electrode; and applying a plurality of driving signals to the LCD panel so as to generate potential difference, ΔV 12 (g), in the sub-pixel electrodes of the first and second sub-pixels of each pixel, respectively, which varies with a gray level g of an image to be displayed on the pixel, where g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). 
     In one embodiment, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel satisfies the following relationships of:
         (1). when 0≦g≦g a , ΔV 12 (g)&lt;ΔV 12 (g+1); and   (2). when g b ≦g≦R, ΔV 12 (g)&gt;ΔV 12 (g+1),
 
where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero.
       

     In another embodiment, the potential difference ΔV 12 (g) satisfies the following relationships of:
         (ii). when 0≦g≦g a , ΔV 12 (g)=V a ;   (iii). when g a ≦g≦g b , ΔV 12 (g)=V b ; and   (iv). when g b ≦g≦R, ΔV 12 (g)=V c ,
 
where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero, and V a , V b  and V c  are constant voltages with V a &gt;V b &gt;V c .
       

     These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate one or more embodiments of the invention and, together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment, and wherein: 
         FIG. 1  partially shows schematically an equivalent circuit diagram of an LCD panel according to one embodiment of the present invention; 
         FIG. 2  shows schematically (a) waveform charts of driving signals applied to an LCD panel according to one embodiment of the present invention, and (b) a layout view of the LCD panel, where the transistors electrically connected to the gate lines G 1  and G 2  are turned on, and the transistors electrically connected to the gate line G 3  are turned off, respectively; 
         FIG. 3  shows schematically (a) waveform charts of driving signals applied to the LCD panel shown in  FIG. 2   b , and (b) the layout view of the LCD panel, where the transistors electrically connected to the gate line G 1  are turned on, and the transistors electrically connected to the gate lines G 2  and G 3  are turned off, respectively; 
         FIG. 4  shows schematically (a) waveform charts of driving signals applied to the LCD panel shown in  FIG. 2   b , and (b) the layout view of the LCD panel, where the transistors electrically connected to the gate line Glare turned off, and the transistors electrically connected to the gate lines G 2  and G 3  are turned on, respectively; 
         FIG. 5  shows schematically (a) waveform charts of driving signals applied to the LCD panel shown in  FIG. 2   b , and (b) the layout view of the LCD panel, where the transistors electrically connected to the gate lines G 1  and G 3  are turned off, and the transistors electrically connected to the gate line G 2  are turned on, respectively; 
         FIG. 6  shows the relationship of voltages of the first and second sub-pixel electrodes of a pixel of an LCD panel and the grey level for an image to be displayed on the pixel of the LCD panel according to one embodiment of the present invention, (a) a simulation result, and (b) an experimental result; 
         FIG. 7  shows the relationship of voltages of the first and second sub-pixel electrodes of the pixel of the LCD panel according to the embodiment of the present invention in  FIG. 6 , (a) a simulation result, and (b) an experimental result; 
         FIG. 8  shows the relationship of the voltage difference in the first and second sub-pixel electrodes of the pixel of the LCD panel and the grey level for an image to be displayed on the pixel of the LCD panel according to the embodiment of the present invention in  FIG. 6 , (a) a simulation result, and (b) an experimental result; 
         FIG. 9  shows the gamma curve the LCD panel according to the embodiment of the present invention in  FIG. 6 , (a) a simulation result, and (b) an experimental result; 
         FIG. 10  shows a simulation result of the relationship of voltages of the first and second sub-pixel electrodes of a pixel of an LCD panel and the grey level for an image to be displayed on the pixel of the LCD panel according to one embodiment of the present invention; 
         FIG. 11  shows a simulation result of the relationship of voltages of the first and second sub-pixel electrodes of the pixel of the LCD panel according to the embodiment of the present invention in  FIG. 10 ; 
         FIG. 12  shows a simulation result of the relationship of the voltage difference in the first and second sub-pixel electrodes of the pixel of the LCD panel and the grey level for an image to be displayed on the pixel of the LCD panel according to the embodiment of the present invention in  FIG. 10 ; and 
         FIG. 13  shows a simulation result of the gamma curve the LCD panel according to the embodiment of the present invention in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Various embodiments of the invention are now described in detail. Referring to the drawings, like numbers indicate like components throughout the views. As used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. Additionally, some terms used in this specification are more specifically defined below. 
     As used herein, the terms “gamma” and/or “gamma curve” refer to the characterization of brightness of an imaging display system, for example, an LCD device, versus grey levels (scales). Gamma summarizes, in a single numerical parameter, the nonlinear relationship between grey level and brightness of the imaging display system. 
     As used herein, the terms “grey level” and “grey scale” are synonyms in the specification and refer to one of (discrete) shades of grey for an image, or an amount of light perceived by a human for the image. If the brightness of the image is expressed in the form of shades of grey in h bits, n being an integer greater than zero, the grey level takes values from zero representing black, up to (2 h −1) representing white, with intermediate values representing increasingly light shades of grey. In an LCD device, the amount of light that transmits through liquid crystals is adjusted to represent the gray level. 
     As used herein, the term “grey level voltage” or “driving voltage” refers to a voltage generated from a data driver in accordance for driving a particular area or pixel of an LCD panel, in accordance with a grey level of a frame of an image to be displayed at the particular area or pixel of the LCD panel. 
     The terms “light transmittance/transmission”, “brightness” and “luminance”, as used herein, are synonym in the specification and refer to the amount of light that passes through a particular area of an LCD panel. 
     It has been known that the orientations of liquid crystal molecules in liquid crystal cells of an LCD panel play a crucial role in the transmittance of light therethrough. For example, in a twist nematic LCD, when the liquid crystal molecules are in its tilted orientation, light from the direction of incidence is subject to various different indexes of reflection. Since the functionality of LCDs is based on the birefringence effect, the transmittance of light will vary with different viewing angles. Due to such differences in light transmission, optimum viewing of an LCD is limited within a narrow viewing angle. Additionally, at different grey levels, liquid crystals have different response times in an LCD panel. For example, liquid crystals usually have the shortest response time at the grey level  255 , for 8-bit data signals for example, compared to that at other grey levels. The difference between the response times at different grey levels may result in deviations of the gamma curves for different grey levels at different areas of the LCD panel. 
     Therefore, one aspect of the present invention provides methods to overcome the drawbacks of a color sequential LCD device. 
     The description will be made as to the embodiments of the present invention in conjunction with the accompanying drawings in  FIGS. 1-13 . In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to an LCD panel with color washout improvement. In one embodiment, the LCD panel includes a plurality of pixels spatially arranged in the form of a matrix. Each pixel includes at least a first sub-pixel having a sub-pixel electrode and a second sub-pixel having a sub-pixel electrode. The plurality of pixels is configured such that when a gray level voltage associated with a gray level, g, of an image to be displayed on a pixel is applied to the pixel, a potential difference is generated in the sub-pixel electrodes of the first and second sub-pixels of the pixel, which varies with the gray level g of the image to be displayed on the pixel, where g=0, 1, 2, . . . , (2 h −1) corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero. That is the potential difference in the sub-pixel electrodes of the first and second sub-pixels of the pixel that results in different alignments of the LC molecules in the first and second sub-pixels of the pixel, thereby improving color washout of the LCD panel. 
     Referring to  FIG. 1 , an LCD panel according to one embodiment of the present invention is partially and schematically shown. The LCD panel  100  includes a common electrode  160 , a plurality of scanning lines, G 1 , G 2 , . . . , G n−1 , G n , G n+1 , . . . , G N , that are spatially arranged along a row (scanning) direction  130 , and a plurality of data lines, D 1 , D 2 , . . . , D m−1 , D m , D m+1 , . . . , D M , that are spatially arranged crossing the plurality of scanning lines G 1 , G 2 , . . . , G n−1 , G n , G n+1 , . . . , G N  along a column direction  140  that is perpendicular to the row direction  130 . N and M are integers greater than one. The LCD panel  100  further has a plurality of pixels, {P n,m },  110  that are spatially arranged in the form of a matrix. Each pixel P n,m    110  is defined between two neighboring scanning lines G n  and G n+1  and two neighboring data lines D m  and D m+1 . For the purpose of illustration of embodiments of the present invention,  FIG. 1  schematically shows only four scanning lines G n−1 , G n, G   n+1  and G n+2 , two data lines D m  and D m+1 , and three corresponding pixels of the LCD panel  100 . 
     Furthermore, each pixel P n,m    110  is configured to have two or more sub-pixels. As shown in  FIG. 1 , a pixel P n,m    110  located, for example, between two neighboring scanning lines G n  and G n+1  and two neighboring data lines D m  and D m+1  crossing the two neighboring scanning lines G n  and G n+1  has a first sub-pixel, P n,m ( 1 ),  111   a  and a second sub-pixel, P n,m ( 2 ),  111   b . Each of the first sub-pixel P n,m ( 1 )  111   a  and the second sub-pixel P n,m ( 2 )  111   b  comprises a sub-pixel electrode  115   a / 115   b , a liquid crystal (LC) capacitor  113   a / 113   b  and a storage capacitor  114   a / 114   b , and a transistor  112 / 116 . Each pixel is capable of displaying h bits of image data. 
     Both the LC capacitor  113   a  and the storage capacitor  114   a  of the first sub-pixel P n,m ( 1 )  111   a  of the pixel P n,m    110  are electrically connected between the sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 )  111   a  of the pixel P n,m    110  and the common electrode  160  in parallel. The transistor  112  of the first sub-pixel P n,m ( 1 )  111   a  of the pixel P n,m    110  has a gate  112   g  electrically connected to the scanning line G n , a source  112   s  electrically connected to the sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 )  111   a  of the pixel P n,m    110  and a drain  112   d  electrically connected to the data line D m . The sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 )  111   a  of the P n,m    110  is in turn electrically connected to the drain  116   d  of the transistor  116  of the second sub-pixel P n+1,m  ( 2 ) of the pixel P n+1,m . 
     Furthermore, both LC capacitor  113   b  and the storage capacitor  114   b  of the second sub-pixel P n,m ( 2 )  111   b  of the pixel P n,m    110  are electrically connected between the sub-pixel electrode  115   b  of the second sub-pixel P n,m ( 2 )  111   b  of the pixel P n,m    110  and the common electrode  160  in parallel. The transistor  116  of the second sub-pixel P n,m ( 2 )  111   b  of the pixel P n,m    110  has a gate  116   g  electrically connected to the scanning line G n , a source  116   s  electrically connected to the sub-pixel electrode  115   b  of the second sub-pixel P n,m ( 2 )  111   b  of the pixel P n,m    110  and a drain  116   d  electrically connected to the sub-pixel electrode  115   a  of the first sub-pixel P n−1,m ( 1 ) of the pixel P n−1,m . 
     In one embodiment, the sub-pixel electrodes  115   a / 115   b  of the first sub-pixel P n,m ( 1 )  111   a  and the second sub-pixel P n,m ( 2 )  111   b  of each pixel P n,m    110  are deposited on a first substrate (not shown), while the common electrode  160  is deposited on a second substrate (not shown) that is spatially apart from the first substrate. The LC molecules are filled into cells between the first and second substrates. Each cell is associated with a pixel P n,m    110  of the LCD panel  100 . Voltages (potentials) applied to the sub-pixel electrodes control orientational alignments of the LC molecules in the LC cells associated with the corresponding sub-pixels. 
     The transistor  112  and the transistor  116  in one embodiment are field-effect TFTs and adapted for activating the first sub-pixel P n,m ( 1 )  111   a  and the second sub-pixel P n,m ( 2 )  111   b , respectively. Other types of transistors may also be utilized to practice the present invention. When the transistor  112  and the transistor  116  are selected to be turned on by a scanning signal applied through the scanning line G n  to which the gate  112   g  of the transistor  112  and the gate  116   g  of the transistor  116  are electrically coupled, a data signal applied through the corresponding data line D m  is incorporated into the first sub-pixel P n,m ( 1 )  111   a  and the second sub-pixel P n,m ( 2 )  111   b  by means of charging the corresponding LC capacitors  113   a  and  113   b , and storage capacitors  114   a  and  114   b  of the first sub-pixel P n,m ( 1 )  111   a  and the second sub-pixel P n,m ( 2 )  111   b , respectively. The charged potentials of the LC capacitors  113   a  and  113   b  of the first and second sub-pixels  111   a  and  111   b  of the pixel  110  are corresponding to the electrical fields applied to corresponding liquid crystal cells between the first and second substrates. The storage capacitor  114   a  and the storage capacitor  114   b  are adapted for providing coupling voltages to the corresponding LC capacitors  113   a  and  113   b , respectively, to compensate for charge leakages therefrom. The storage capacitors  114   a  and  114   b  of the first and second sub-pixels  111   a  and  111   b  can be identical or different. 
     In one embodiment, the driving signals include a plurality of scanning signals, a plurality of data signals and a common signal. For such an LCD panel  100  shown in  FIG. 1 , when a scanning signal is applied to a scanning line G n  to turn on the corresponding transistors  112  and  116  connected to the scanning line G n , a plurality of data signals is simultaneously applied to the plurality of data lines {D n } so as to charge the corresponding LC capacitors  113   a  and  113   b  and storage capacitors  114   a  and  114   b  of each pixel P n,m    110  of the corresponding pixel row for aligning states of corresponding liquid crystal cells associated with the first and second sub-pixels P n,m ( 1 )  111   a  and P n,m ( 2 )  111   b  of the pixel P n,m    110  to control light transmittance therethrough. Accordingly, the voltage, Vp 1 , generated in the sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 ) and the voltage, Vp 2 , generated in the sub-pixel electrode  115   b  of the second sub-pixel P n,m ( 2 ) of the P n,m  are different, due to the coupling between the sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 ) of the P n,m  to the sub-pixel electrode  115   b  of the second sub-pixel P n+,m ( 2 ) of the P n+1,m . In other words, the LC molecules associated with the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) of the P n,m  may be aligned at different orientations responsive to a voltage difference, ΔV 12 =(Vp 2 −Vp 1 ), in the sub-pixel electrode  115   a  of the first sub-pixel P n,m ( 1 ) and the sub-pixel electrode  115   b  of the second sub-pixel P n,m ( 2 ) of the P n,m . 
     Practically, the plurality of data signals includes a plurality of gray level voltages. Each gray level voltage is associated with a gray level, g, of an image to be displayed on a pixel P n,m . g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). When such a gray level voltage is applied the pixel P n,m , the potential difference ΔV 12 (g)=(Vp 2 −Vp 1 ) in the sub-pixel electrodes of the first and second sub-pixels of the pixel is generated, and varies with the gray level g. In one embodiment, the potential difference ΔV 12 (g) in the sub-pixel electrodes  115   a  and  115   b  of the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) of the pixel P n,m  satisfies the following relationships of:
         (1). when 0≦g≦g 3 , ΔV 12 (g)&lt;ΔV 12 (g+1); and   (2). when g b ≦g≦R, ΔV 12 (g)&gt;ΔV 12 (g+1),
 
where 0≦g≦g b &lt;R, g 3  and g b  each being an integer greater than zero.
       

     In another embodiment, the potential difference ΔV 12 (g) satisfies the following relationships of:
         (i). when 0≦g≦g 3 , ΔV 12 (g)=V 3 ;   (ii). when g a ≦g≦g b , ΔV 12 (g)=V b ; and   (iii). when g b ≦g≦R, ΔV 12 (g)=V c ,
 
where 0≦g≦g b ≦R, g 3  and g b  each being an integer greater than zero, and V 3 , V b  and V c  are constant voltages with V 3 &gt;V b &gt;V c .
       

     Another aspect of the present invention relates to an LCD panel having a common electrode, a plurality of scanning lines, G 1 , G 2 , . . . , G n−1 , G n , G n+1 , . . . , G N , that are spatially arranged along a scanning direction, and a plurality of data lines, D 1 , D 2 , . . . D m−1 , D m , D m+1 , . . . , D M , that are spatially arranged crossing the plurality of scanning lines G 1 , G 2 , . . . , G n−1 , G n , G n+1 , . . . , G N  along a direction that is perpendicular to the scanning direction, and a plurality of pixels, {P n,m }, that are spatially arranged in the form of a matrix. N and M are integers greater than one. Each pixel P n,m  includes at least a first sub-pixel P n,m ( 1 ) and a second sub-pixel P n,m ( 2 ). Each of the first sub-pixel P n,m ( 1 ) and the second sub-pixel P n,m ( 2 ) comprises a sub-pixel electrode, a liquid crystal (LC) capacitor and a storage capacitor both electrically connected between the sub-pixel electrode and the common electrode in parallel, and a transistor having a gate electrically connected to the scanning line G n , a source electrically connected to the sub-pixel electrode and a drain. 
     In one embodiment, the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the first sub-pixel P k,m ( 1 ) of the pixel P k,m , where k=1, 2, . . . , N, and k≠n. For the exemplary embodiment shown in  FIG. 1 , k=n−1. 
     In another embodiment, the drain of the transistor of the second sub-pixel P n,m ( 2 ) of the pixel P n,m  is electrically connected to the data line D m , and the drain of the transistor of the first sub-pixel P n,m ( 1 ) of the pixel P n,m  is electrically connected to the sub-pixel electrode of the second sub-pixel P k,m ( 2 ) of the pixel P k,m , where k=1, 2, N, and k≠n. 
     Referring to  FIGS. 2-5 , waveform charts of the driving signals  201  applied to the LCD panel  200  and charging in the corresponding sub-pixel electrodes  215   a  and  215   b  of the LCD panel  200  are shown according to one embodiment of the present invention. In the exemplary embodiment, the LCD panel  200  is shown schematically and partially with 3×3 pixels, where the pixels, for example, in the first column of the 3×3 pixel matrix are referenced by P 1,1 , P 2,1  and P 3,1 , respectively. Each pixel has a first sub-pixel electrode  215   a , a second sub-pixel electrode  215   b , a first transistor (switching device)  211  and a second transistor (switching device)  216 , each transistor  211  or  216  having a gate, a source and a drain. The gates of both the first transistor  211  and the second transistor  216  of each pixel are electrically connected to a corresponding scanning line by which the pixel is defined, such as G 1 , G 2  or G 3 . The sources of the first transistor  211  and the second transistor  216  of each pixel are electrically connected to the first sub-pixel electrode  215   a  and the second sub-pixel electrode  215   b  of the pixel, respectively. The drain of the second transistor  216  of each pixel is electrically connected to a corresponding data line by which the pixel is defined, such as D 1  or D 2 , and the drain of the first transistor  212  of each pixel is electrically connected to the sub-electrode  215   b  of its next neighboring pixel in the same column of the pixel. For example, the drain of the first transistor  212  of the pixel P 1 , is electrically connected to the sub-electrode  215   b  of the pixel P 2,1 , the drain of the first transistor  212  of the pixel P 2,1  is electrically connected to the sub-electrode  215   b  of the pixel P 3,1 , and so on, as shown in  FIGS. 2-5 . 
     In the exemplary embodiment, the driving signals  201  include three scanning signals  271 ,  272  and  273  sequentially applied to the scanning lines G 1 , G 2  and G 3 , and two data signals  281  and  282  simultaneously applied to the data lines D 1  and D 2 , and a common signal Vcom  290  applied to the common electrode (not shown), respectively. Each of the scanning signals  271 ,  272  and  273  is configured to have a high voltage potential, Vh, and a low voltage potential, Vl, for effectively turning on and off the corresponding transistors of a corresponding pixel row. The common signal Vcom  290  has a constant potential (voltage). The data signals  281  and  282  are generated according to an image to be displayed on these pixels such that when the data signals  281  and  282  are applied to corresponding pixels, a potential (voltage) difference between the potentials of the first and second electrodes  215   a  and  215   b  of a pixel is generated. The potential difference is a function of the grey level for the image to be displayed. 
     As shown in  FIG. 2 , in the time period  221  of (t 1 -t 0 ), the transistors  212  and  216  electrically connected to the scanning lines G 1  and G 2  are turned on, while the transistors  212  and  216  electrically connected to the scanning line G 2  are turned off, respectively. Accordingly, a potential (voltage), Vp 2 , of the second sub-pixel electrode  215   b  of the pixels P 1 , and P 2,1  is generated directly by application of the data signal  281  to the drain of the second transistor  216  of the pixels P 1 , and P 2,1 , respectively, while a potential (voltage), Vp 1 , of the first electrode  215   a  of the pixel P 1,1  is generated by application of the generated voltage Vp 2  of the second sub-pixel electrode  215   b  of the pixel P 2,1  to the drain of the first transistor  212  of the pixel P 1,1 . The latter charging process is indicated by arrow  218   a . In this case, the voltage difference, ΔV 12 =Vp 2 −Vp 1  is generated in the first and second electrodes  215   a  and  215   b  of the pixel P 1,1 . 
     In the time period  222  of (t 2 -t 1 ), as shown in  FIG. 3 , the transistors  212  and  216  electrically connected to the scanning line G 1  are turned on, while the transistors  212  and  216  electrically connected to the scanning lines G 2  and G 3  are turned off, respectively. Accordingly, a voltage, Vp 2 , of the second sub-pixel electrode  215   b  of the pixel P 1 , is generated directly by application of the data signal  281  to the drain of the second transistor  216  of the pixel P 1,1 , while no voltage of the first electrode  215   a  of the pixel P 1 , is generated since the transistor  216  of the pixel P 2,1  is turned off. The charging process of the second electrode  215   b  of the pixel P 1 , is indicated by arrow  218   b . Accordingly, the voltage difference, ΔV 12 , in the first and second electrodes  215   a  and  215   b  of the pixel P 1,1  is corresponding to Vp 2 . 
     As shown in  FIG. 4 , in the time period  223  of (t 3 -t 2 ), the transistors  212  and  216  electrically connected to the scanning lines G 2  and G 3  are turned on, while the transistors  212  and  216  electrically connected to the scanning line G 1  are turned off, respectively. Accordingly, a potential (voltage), Vp 2 , of the second sub-pixel electrode  215   b  of the pixels P 2,1  and P 3,1  is generated directly by application of the data signal  281  to the drain of the second transistor  216  of the pixels P 2,1  and P 3,1 , respectively, while a potential (voltage), Vp 1 , of the first electrode  215   a  of the pixel P 2,1  is generated by application of the generated voltage Vp 2  of the second sub-pixel electrode  215   b  of the pixel P 3,1  to the drain of the first transistor  212  of the pixel P 2,1 . The latter charging process is indicated by arrow  218   c . In this case, the voltage difference, ΔV 12 , in the first and second electrodes  215   a  and  215   b  of the pixel P 2,1  is corresponding to (Vp 2 −Vp 1 ). 
     In the time period  224  of (t 4 -t 3 ), as shown in  FIG. 5 , the transistors  212  and  216  electrically connected to the scanning line G 2  are turned on, while the transistors  212  and  216  electrically connected to the scanning lines G 1  and G 3  are turned off, respectively. Accordingly, a voltage, Vp 2 , of the second sub-pixel electrode  215   b  of the pixel P 2,1  is generated directly by application of the data signal  281  to the drain of the second transistor  216  of the pixel P 2,1 , while no voltage of the first electrode  215   a  of the pixel P 2,1  is generated since the transistor  216  of the pixel P 3,1  is turned off. The charging process of the second electrode  215   b  of the pixel P 2,1  is indicated by arrow  218   d . Accordingly, the voltage difference, ΔV 12 , in the first and second electrodes  215   a  and  215   b  of the pixel P 2,1  is corresponding to Vp 2 . 
     In the embodiment as shown in  FIGS. 2-5 , the first sub-pixel electrode  215   a  of a pixel has an area A 1  and the second sub-pixel electrode  215   b  of the pixel has an area A 2 . The ratio of Δ 1 /A 2  is in a range of about 0.2-5.0, in one embodiment. 
     Referring to  FIG. 6 , the simulation and experimental results for the voltages Vp 1  and Vp 2  of the first and second sub-pixel electrodes of a pixel of an LCD panel against the grey level for an image to be displayed on the pixel of the LCD panel are shown according to one embodiment of the present invention, where the area ratio of A 1 /A 2 =1/1.6, and the grey level is expressed in an 8 bit. In  FIG. 6 , Vp′ 1 =(Vp 1 −Vcom), and Vp′ 2 =(Vp 2 −Vcom), where Vcom is the voltage applied to the common electrode. The voltage difference in the first and second sub-pixel electrodes of the pixel is ΔV 12 =(Vp′ 2 −Vp′ 1 )=(Vp 2 −Vp 1 ).  FIG. 7  shows the simulation and experimental results for the voltages Vp 1  and Vp 2  of the first and second sub-pixel electrodes of the pixel of the LCD panel according to the embodiment of the present invention in  FIG. 6 . In this embodiment, the first sub-pixel electrode has a lower voltage and a larger area, comprising with those of the sub-pixel electrode. 
     Accordingly, the voltage difference ΔV 12  in the first and second sub-pixel electrodes of the pixel varies with the grey level, as shown in  FIG. 8 . When the grey level g increases from 0 to g a , the voltage difference ΔV 12  increases, i.e., ΔV 12 ( g )&lt;ΔV 12 ( g +1), for 0≦g≦g a , while the voltage difference ΔV 12  decreases as the grey level g increases from g b  to R=255, i.e., ΔV 12 (g)&gt;ΔV 12 (g+1) for g b ≦g≦R. Both g a  and g b  that is larger than g a  are larger than zero but less than R, and may vary with the characteristic of the liquid crystals and the area ratio of the first sub-pixel electrode to the second sub-pixel electrode. 
       FIG. 9  shows the simulation and experimental results of the gamma curve of the LCD panel, where Gamma_ 0  is set to be 2.4 and the first sub-pixel electrode has a lower voltage and a larger area, comprising with those of the sub-pixel electrode. For the simulation of the gamma curve, as shown in  FIG. 9   a , the area ratio of A 1 /A 2 =1/1.6, while the area ratio of A 1 /A 2 =1/1.2 for the experiment result of the gamma curve, as shown in  FIG. 9   b . In the simulation of the gamma curve, the driving signals are configured such that when the grey level g is in the range of 0-96, the first sub-pixel transmits no light, or a little amount of light, where the gamma curve in this range of the grey level is indicated by reference numeral  910 , while the first sub-pixel transmits a large amount of light when the grey level g is greater than 96. Furthermore, when the grey level g is in the range of 176-255, the second sub-pixel transmits the most amount of light, where the gamma curve in this range of the grey level is indicated by reference numeral  920 . The brightness of the second sub-pixel is reduced for g&lt;176. 
       FIGS. 10 and 12  respectively show the simulation result for the voltages Vp 1  and Vp 2  of the first and second sub-pixel electrodes of a pixel of an LCD panel and its voltage difference ΔV 12 =(Vp 2 −Vp 1 ) against the grey level for an image to be displayed on the pixel of the LCD panel are shown according to another embodiment of the present invention. It is clear that the voltage difference ΔV 12  in the first and second electrodes of a pixel varies with the grey level g. In this embodiment, ΔV 12 (g)=V a  for 0≦g≦g a , ΔV 12 (g)=V b  for g a ≦g≦g b , and ΔV 12 (g)=V c  for g b ≦g≦R=255, where V a =1.2V, V b =1.1V and V c =0.8V.  FIG. 11  shows the simulation for the voltages Vp 1  and Vp 2  of the first and second sub-pixel electrodes of the pixel of the LCD panel.  FIG. 13  shows the simulation of the gamma curve of the LCD panel. 
     One aspect of the present invention provides a method of improving color washout of an LCD device. In one embodiment, the method includes the step of providing an LCD panel having a plurality of pixels, {P n,m }, spatially arranged in the form of a matrix, n=1, 2, . . . , N, and m=1, 2, . . . , M, and N, M being an integer greater than zero. Each pixel P n,m  has at least a first sub-pixel, P n,m ( 1 ), having a sub-pixel electrode and a second sub-pixel, P n,m ( 2 ), having a sub-pixel electrode, The method also includes the step of applying a plurality of driving signals to the LCD panel so as to generate potential difference, ΔV 12 (g), in the sub-pixel electrodes of the first and second sub-pixels of each pixel, respectively, which varies with a gray level g of an image to be displayed on the pixel, where g=0, 1, 2, . . . , R corresponding to one of the shades of grey of the image expressed in h bits, h being an integer greater than zero and R=(2 h −1). 
     In one embodiment, the potential difference ΔV 12 (g) generated in the sub-pixel electrodes of the first and second sub-pixels of a pixel varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g is less than the potential difference ΔV 12 (g+1) for the gray level (g+1); and (ii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g is greater than the potential difference ΔV 12 (g+1) for the gray level (g+1), where 0&lt;g a ≦g b &lt;R, g a  and g b  each being an integer greater than zero. 
     In another embodiment, the potential difference ΔV 12 (g) varies with the gray level g, such that (i) when the gray level g is in the range from 0 to g a , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V a ; (ii) when the gray level g is in the range from g a  to g b , the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V b ; and (iii) when the gray level g is in the range from g b  to R, the potential difference ΔV 12 (g) for the gray level g has a constant voltage, V c , where V a &gt;V b &gt;V c . 
     The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. 
     The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.