Abstract:
A transflective liquid crystal display is set forth that comprises first and second substrates disposed opposite one another and a liquid crystal layer disposed between the substrates. The first substrate includes a red pixel region, a green pixel region, a blue pixel region and a white pixel region defined thereon. Each of the red, green, blue and white pixel regions has a respective transmission region and a respective reflection region associated with the pixel. An offset brightness is applied to the display at the white pixel region. The offset brightness of the white pixel region may be operable, for example, to compensate for differences in appearance of the transflective liquid crystal display that would otherwise occur between operation of the display in a transmission mode of operation and a reflective mode of operation. The red, green, blue and white pixel regions may be organized into color regions that are arranged for use in generating individual colors that, in turn, are used in the generation of a display image. A method for operating such a transflective liquid crystal display device is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to a transflective type LCD device and a method of operating the same.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, flat panel displays can be classified as light receiving types or light emitting types. Light emitting displays use materials that can be stimulated to generate the light that produces the image, while light receiving displays use an external light source to generate an image using an external light source. Examples of light emitting displays include plasma display panels (PDPs), field emission displays, and electro-luminescence displays. Light receiving displays include LCD devices.  
         [0005]     LCD displays are often superior in resolution, color display, picture quality and the like, when compared to many other types of display devices. Consequently, LCD displays are widely used as monitors for notebook computers, desktop computers, high-definition television screens, etc.  
         [0006]     There are a number of different ways to construct an LCD display. Most LCD displays include two substrates that are each provided with respective electrodes formed thereon so that the electrodes of the substrates face one another when the LCD display is assembled. A liquid crystal layer is interposed between the two substrates. When voltages are applied to the electrodes of the two substrates, an electric field is generated and applied to the liquid crystal layer. The LCD device generates images when the liquid crystal molecules of the liquid crystal material are manipulated using the electric field to control the transmittance of light through individual portions of the liquid crystal layer.  
         [0007]     As noted above, LCD displays utilize an external light source to generate an image since the liquid crystal material itself does not generate light. The external light may come from an ambient source or may be integrated with the LCD display. To this end, many LCD displays are provided with a backlight unit disposed proximate the liquid crystal material. The LCD displays an image by controlling the light emanating from the backlight unit through the liquid crystal panel.  
         [0008]     LCD displays that employ a backlight or the like as the external light source are generally referred to as transmission type LCD displays. Since such transmission type LCD displays employ an artificial backlight source, they can provide a bright image even in a dark environment. However, such transmission type LCD displays consume a substantial amount of power, which can be a disadvantage in various situations.  
         [0009]     LCD displays that employ ambient light as the extra light source are known as reflection type LCD displays. Such reflection type LCD displays generate an image by reflecting external natural light or artificial light and adjusting the transmittance of that light according to the alignment of liquid crystal molecules. Since the reflection type LCD devices do not need a backlight unit, they do not consume as much power as their transmission type counterparts. In these reflection type LCD displays, pixel electrodes on the lower substrate are formed of a conductive material having a high reflectivity. A common electrode on the upper substrate is formed of a transparent conductive material to allow the transmission of external light.  
         [0010]     Although reflection type LCD devices have the advantage of low power consumption, the generated image may be difficult or impossible to view if the ambient light does not have a sufficient intensity. Consequently, such LCD devices frequently cannot be used in low-light conditions.  
         [0011]     Transflective type LCD displays have been proposed in an attempt to overcome some of the disadvantages associated with both transmission type LCD displays and reflection type LCD displays. The transflective type LCD display may operate in multiple modes. More particularly, transflective type LCD displays can be selectively driven in either the transmission mode or the reflection mode of operation.  
         [0012]      FIG. 1  is a sectional view of an exemplary transflective type LCD display. As shown, the LCD display includes a lower substrate  1 , an upper substrate  70 , and a liquid crystal layer  60  interposed between the lower substrate  1  and the upper substrate  70 . In the lower substrate  1 , a gate electrode  6  and a gate line (not shown) are formed on a transparent substrate  2 . A gate insulating layer  10  is formed on the resulting transparent substrate  2 , including on the gate electrode  6  and the gate line. An active layer  13  and an ohmic contact layer  16   a ,  16   b  are disposed over the gate insulating layer  10  on the gate electrode  6 . Source and drain electrodes  23  and  26  are formed on the ohmic contact layer  16   a ,  16   b  such that they are spaced apart from each other. In this manner, a thin film transistor (Tr) is formed, at least in part, by the gate electrode  6 , the active layer  13 , the ohmic contact layer  16   a ,  16   b , and the source and drain electrodes  23  and  26 . A data line  20  is also formed on the gate insulating layer  10  in the same layer as the source and drain electrodes  23  and  26 . The data line  20  is formed integrally with the source electrode  23  so that they are electrically connected with one another. The data line  20  intersects the gate line (not shown) to define a pixel region ‘SP’.  
         [0013]     A first passivation layer  30  is formed on the resulting transparent substrate  2  including the thin film transistor (Tr). Passivation layer  30  may be formed using an organic insulator having a low dielectric constant. A reflection plate  40  is formed of a metal material having a high reflectivity on the first passivation layer  30  within a reflection area ‘RA’. A second passivation layer  45  is formed on the first passivation layer  30 , including on the reflection plate  40 . The second passivation layer  45  may be formed using an inorganic insulating material. A pixel electrode  50 , which electrically contacts the drain electrode  26  of the thin film transistor (Tr) through a drain contact hole  55 , is formed on the second passivation layer  45  within the pixel region ‘SP’.  
         [0014]     In the upper substrate  70 , a black matrix  75  shaped in a lattice configuration is formed on a transparent substrate  71 . Red (R), green (G) and blue (B) color filters  80   a ,  80   b  and  80   c  are formed on the transparent substrate  71  including the black matrix  75 . On the color filters  80   a ,  80   b  and  80   c , an over coat layer  85  and a transparent conductive common electrode  90  are formed. Each of the color filters  80   a ,  80   b  and  80   c  is formed at a position so that it is aligned with corresponding pixel electrodes  50 . The black matrix  75  is formed at positions corresponding to the thin film transistor (Tr) region, the gate line and the data line  20 .  
         [0015]     A liquid crystal layer  60  is interposed between the pixel electrode  50  and the common electrode  90 . Liquid crystal molecules of the liquid crystal layer  60  are realigned by an electric field generated when a voltage difference is applied between the pixel electrode  50  and the common electrode  90 .  
         [0016]     In the exemplary transflective type LCD display device, cell gap ‘d 1 ’ of the reflection area ‘RA’ and cell gap ‘d 2 ’ of the transmission area ‘TA’ have substantially the same thickness. As a result, neither the cell efficiency at the reflection area ‘RA’ or the cell efficiency at transmission area ‘TA’ are optimal, and the transmittance and the brightness may be less than desired. Also, in the reflection mode operation, external light passes through the color filters twice—the first being before it is incident onto the reflection plate and the second after it is reflected by the reflection plate. In contrast, light emitting from the backlight unit disposed below the liquid crystal panel passes through the color filters only one time in the transmission mode of operation. A difference in color characteristics may exist between the reflection mode and the transmission mode. Such differences may be problematic.  
         [0017]     An exemplary transflective type LCD display that attempts to address the color differences is shown in  FIGS. 2A, 2B  and  3 .  FIGS. 2A and 2B  are partial plan views of a transflective type LCD display having a transmission hole and a double cell gap, while  FIG. 3  is a sectional view taken along the line A-A of  FIG. 2B .  
         [0018]     As shown in  FIGS. 2A and 2B , the transflective type LCD display includes a plurality of gate lines  3  that are arranged in a horizontal direction (with respect to the orientation of the figure), and a plurality of data lines  20  that are arranged in a vertical direction. Pixel regions ‘SP’ are defined where the plurality of gate lines  3  and the plurality of data lines  20  cross one another. Thin film transistors (Tr) are formed at intersecting points of the plurality of gate lines  3  and the plurality of data lines  20 . Red (R), green (G), and blue (B) color filters are formed corresponding to the respective pixel regions ‘SP’. Each of the pixel regions includes a transmission area ‘TA’ formed at a center thereof, and a reflection area ‘RA’ around the transmission area ‘TA’. The reflection area ‘RA’ is formed with a reflection plate (not shown). Although not shown in the drawings, the passivation layer has been removed in the region of the transmission area ‘TA’ to form a step height difference with respect the reflection area ‘RA’. As a result, the cell gap of the reflection area ‘RA’ is different from the cell gap of the transmission area ‘TA’. In the reflection area ‘RA’, the color filter is partially removed to form a circular transmission hole ‘TH’.  FIG. 2A  shows that the transmission hole ‘TH’ from which the color filter is removed is formed above and below the transmission area ‘TA’.  FIG. 2B  shows another embodiment in which a plurality of transmission holes ‘TH’ enclose each transmission area ‘TA’ and are formed on substantially the whole reflection area ‘RA’.  
         [0019]      FIG. 3  is a cross-sectional view through the transflective type LCD display shown in  FIG. 2A . Like reference numerals in  FIGS. 1 and 3  denote like elements, and thus their description will be omitted.  
         [0020]     In the transmission area ‘TA’ of the lower substrate  1 , the first passivation layer  30  is removed so that the cell gap ‘d 4 ’ of the transmission area ‘TA’ is twice the thickness of the cell gap d 3  of the reflection area ‘RA’. When the cell gap of the transmission area is different from that of the reflection area, the cell operates in an electrically controlled birefringence (ECB) mode. Whenever the cell gap is increased by a factor of two, the transmittance curve can be periodically repeated to obtain the same cell efficiency of the transmission area ‘TA’ as that of the reflection area ‘RA’. Consequently, it is possible to concurrently maximize both the cell efficiency of the reflection area ‘RA’ and the cell efficiency of the transmission area ‘TA’.  
         [0021]     In the upper substrate  70 , the black matrix  75  is formed so that it is aligned with the data line  20  of the lower substrate  1  on the transparent substrate  71 . Likewise, the red, green and blue color filters  80   a ,  80   b  and  80   c  are formed so that they are aligned with the respective pixel regions ‘SP’ of the lower substrate  1  on the black matrix  75 . A portion of the color filter layer corresponding to the reflection area ‘RA’ having the reflection plate  40  is removed to form the transmission hole ‘TH’, and the transmission hole ‘TH’ is filled with a transparent organic material constituting the over coat layer  85  formed on the red, green and blue color filters  80   a ,  80   b  and  80   c . The common electrode  90  is formed on the over coat layer  85 .  
         [0022]     The circular transmission holes ‘TH’ are formed within the reflection area ‘RA’. Either the area of the circular transmission holes ‘TH’ or their number may be adjusted to decrease the color purity in the reflection area so that the color purity of the transmission area ‘TA’ substantially matches the color purity of the reflection area ‘RA’. Such adjustments may also be used to enhance the brightness characteristics of the reflection area ‘RA’. However, if the transmission holes ‘TH’ are formed in the manner shown in  FIG. 3 , the LCD display may become difficult to design and/or manufacture.  
       SUMMARY OF THE INVENTION  
       [0023]     A transflective liquid crystal display is set forth that comprises first and second substrates disposed opposite one another and a liquid crystal layer disposed between the substrates. The first substrate includes a red pixel region, a green pixel region, a blue pixel region and a white pixel region defined thereon. Each of the red, green, blue and white pixel regions has a respective transmission region and a respective reflection region associated with the pixel. An offset brightness is applied to the display at the white pixel region. The offset brightness of the white pixel region may be operable, for example, to compensate for differences in appearance of the transflective liquid crystal display that would otherwise occur between operation of the display in a transmission mode of operation and a reflective mode of operation. The red, green, blue and white pixel regions are organized into individual color regions that are arranged for use in generating individual colors that, in turn, are used in the generation of a display image. A method for operating such a transflective liquid crystal display device is also disclosed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0025]      FIG. 1  is a sectional view of an exemplary LCD display;  
         [0026]      FIGS. 2A and 2B  are partial plane views of an exemplary transflective type LCD display according to the related art;  
         [0027]      FIG. 3  is a sectional view of the exemplary transflective type LCD display as taken along the line A-A of  FIG. 2B ;  
         [0028]      FIG. 4  is a partial plane view of one embodiment of a transflective type LCD display constructed in accordance with the teachings of the present invention;  
         [0029]      FIG. 5  is a sectional view of the transflective type LCD display taken along the line B-B of  FIG. 4 ;  
         [0030]      FIG. 6  is a chromaticity diagram using the CIE system of color specification to adjust the white offset used by the transflective LCD display of  FIGS. 4 and 5 ; and  
         [0031]      FIGS. 7A and 7B  are graphs showing transmittance according to wavelength in a transflective type LCD device constructed in the manner shown in  FIGS. 4 and 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]      FIG. 4  is a partial plan view of one embodiment of a transflective type LCD display, while  FIG. 5  is a sectional view taken along the line B-B of  FIG. 4 . With reference to these figures, the transflective type LCD display of this embodiment includes a plurality of horizontally oriented gate lines  103  that spaced apart from one another at, for example, constant intervals. A plurality of vertically oriented data lines  120  are arranged to cross the plurality of gate lines  103  and define a plurality of pixel regions ‘SP’ therebetween. The pixel regions ‘SP’ may be arranged as single color regions that each include a red (R) pixel region, a green (G) pixel region, a blue pixel region (B) and a white (W) pixel region. The single color region can display a desired color by combining the colors expressed in the respective pixel regions R, G, B and W. A thin film transistor (Tr) is disposed at the crossing of each gate line  103  and data line  120 .  
         [0033]     Red (R), green (G), blue (B) and white (W) color filters  180   a ,  180   b ,  180   c  and  180   d  are formed to overlie the respective pixel regions R, G, B and W. The white (W) color filter  180   d  can be formed of a thick over coat layer, a transparent insulating buffer layer, or the like.  
         [0034]     Each of the pixel regions R, G, B and W includes a transmission area ‘TA’ and a reflection area ‘RA’ that encloses the transmission area ‘TA’. The transmission area ‘TA’ is formed proximate a center of the respective pixel region and has a predetermined area. The area ratio between the reflection area ‘RA’ and the transmission area ‘TA’ of each of the R, G and B pixel regions may be equal to or not equal to the area ratio between the reflection area ‘RA’ and the transmission area ‘TA’ in the W pixel region.  
         [0035]     The transflective type LCD device also includes a lower substrate  101  having a thin film transistor (Tr) that operates as a switching element and a pixel electrode  150  connected with the thin film transistor. An upper substrate is also included and has red, green, blue and white color filter layers  180   a ,  180   b ,  180   c  and  180   d  and common electrode  190  formed therein. A liquid crystal layer  160  is interposed between the pixel electrode  150  of the lower substrate  101  and the common electrode  190  of the upper substrate  170 .  
         [0036]     The lower substrate  101  includes a transparent substrate  103 . A gate electrode  106  and a gate line (not shown) are formed on the transparent substrate  101 . A gate insulating layer  110  is formed on the transparent substrate  101 , including over the gate electrode  106  and the gate line. Layer  110  may be formed from an inorganic insulator, such as silicon dioxide (SiO2) or silicon nitride (SiNx). An amorphous active layer  113  is formed corresponding to the gate electrode  106  on the gate insulating layer  110 . An impurity-doped ohmic contact layer (not shown) is formed on the active layer  113  in the form of patterns spaced apart from each other. Metallic source and drain electrodes  123  and  126  are formed on the ohmic contact layer. Thin film transistor ‘Tr’ is formed, at least in part, by the gate electrode  106 , the active layer  113 , the ohmic contact layer and the source and drain electrodes  123  and  126 . A data line  120  is formed integrally with the drain electrode  126  of the thin film transistor (Tr) and may be disposed in the same layer as the source and drain electrodes  123  and  126 .  
         [0037]     A first passivation layer  130  formed of an organic insulator, such as benzo cyclo butene (BCB) or photo acryl, is disposed on the source and drain electrodes  123  and  126  and the data line  120  within the reflection area ‘RA’. In the transmission area ‘TA’, the first passivation layer  130  is etched to expose the gate insulating layer  110  disposed below the first passivation layer  130 , and also has a transmission hole  156  with a step height difference from the reflection area ‘RA’.  
         [0038]     A reflection plate  140  formed of a metal having high reflectivity is disposed on the first passivation layer  130  within the reflection area ‘RA’. In the illustrated embodiment, the reflection plate  140  is formed on side surfaces and a predetermined portion of the upper surface of layer  130 . The reflection plate  140  is partially removed from the reflection area ‘RA’ at the drain electrode  126 . A second passivation layer  145  formed of an inorganic insulator, such as silicon dioxide (SiO 2 ) or silicon nitride (SiNx) is disposed on the reflection plate within the reflection area ‘RA’.  
         [0039]     The first passivation layer  130  and the second passivation layer  145  are partially removed from an upper surface of the drain electrode  126  of the thin film transistor (Tr) to form a drain contact hole  155 . A pixel electrode  150  is formed of a transparent conductive material, such as ITO or IZO, and is disposed on the second passivation layer  145  within the pixel region ‘SP’. The pixel electrode  150  contacts the drain electrode  126  through the drain contact hole  155 .  
         [0040]     The upper substrate  170  includes a transparent substrate  171  and a black matrix  175  formed on the transparent substrate  171 . The red (R), green (G), blue (B) and white (W) color filters  180   a ,  180   b ,  180   c  and  180   d  are also disposed in the upper substrate  170 . An over coat layer  185  and a common electrode formed of a transparent conductive material are disposed proximate the red (R), green (G), blue (B) and white (W) color filters  180   a ,  180   b ,  180   c  and  180   d,.    
         [0041]     The red (R), green (G), blue (B) and white (W) color filters  180   a ,  180   b ,  180   c  and  180   d  have a one-to-one correspondence with the pixel regions. The black matrix  175  partially overlaps edges of the pixel electrodes  150  and is aligned with and/or formed by the data line  120 .  
         [0042]     The red (R), green (G), and blue (B) color filters  180   a ,  180   b , and  180   c  have red, green and blue color, respectively and the white (W) color filter  180   d  does not have any color. In particular, the white color filter  180   d  may be formed of a thick over coat layer or a transparent insulation buffer layer rather than a separate material so as to compensate for the step height difference from the red (R), green (G), and blue (B) color filters  180   a ,  180   b , and  180   c.    
         [0043]     A liquid crystal layer  160  is disposed between the pixel electrode  150  and the common electrode  190 . The liquid crystal molecules of the liquid crystal layer  160  are realigned when an electric field is applied between the pixel electrode  150  and the common electrode  190 . This electric field is generated when a voltage differential is applied between the pixel electrode  150  and the common electrode  190 .  
         [0044]     The transflective LCD device may operate in multiple modes. When the amount of ambient light is high, the transflective type LCD device operates in the reflection mode. When the amount of ambient light is low, the transflective type LCD device operates in the transmission mode using a backlight or the like to enhance the brightness of the LCD display. In one or both modes, the white (W) pixel may be driven to an offset value in an effort to reduce any differences in the display of colors that would otherwise occur between the transmission mode of operation and the reflection mode of operation.  
         [0045]      FIG. 6  is a chromaticity diagram using CIE system of color specification in the transflective LCD device. In general, the color reproduction range of the LCD device can be represented by the chromaticity diagram using the CIE system of color specification, in which each color can be described using a mixing ratio of an X tristimulus value (representing the degree of red stimulation by a light source), a Y tristimulus value (representing the degree the green stimulation by a light source), and a Z tristimulus value (representing the degree of blue stimulation by a light source). The X, Y, and Z values can be correlated with the measurements taken by a spectrophotometer.  
         [0046]     The stimulus value is given by an integral value of a product of a spectrum of light generated from the backlight unit, a spectrum of light transmitting the color filter and a color matching function. Accordingly, the tristimulus values may be described by the following equations:  
         X   =     k   ⁢       ∫   380   780     ⁢       ϕ   ⁡     (   λ   )       ⁢       x   _     ⁡     (   λ   )       ⁢     ⅆ   λ             ;       
         Y   =     k   ⁢       ∫   380   780     ⁢       ϕ   ⁡     (   λ   )       ⁢       y   _     ⁡     (   λ   )       ⁢     ⅆ   λ             ;           ⁢   and       
         Z   =     k   ⁢       ∫   380   780     ⁢       ϕ   ⁡     (   λ   )       ⁢       z   _     ⁡     (   λ   )       ⁢     ⅆ   λ             ,       
 
         [0047]     where φ (λ) is the spectrum of the source and where  x (λ),  y (λ), and  z (λ) represents the corresponding red, green, and blue spectral wavelength distribution energies.  
         [0048]     The ratio of the X, Y, Z tristimulus values may be used to define chromaticity coordinate for a given color. The chromaticity coordinate values x, y, z satisfy the relationship of x+y+z=1, and are respectively expressed by the following equations:  
         x   =     X     X   +   Y   +   Z         ;       
         y   =     Y     X   +   Y   +   Z         ;           ⁢   and       
       z   =       Z     X   +   Y   +   Z       .         
 
         [0049]     By using the above equations, all the colors can be expressed by three values of x, y and Y. Herein, Y is a brightness value and x and y are combined to one combination to represent the chromaticity and correspond to properties of color except for the brightness.  
         [0050]     Using the exemplary diagram of  FIG. 6 , each color in the reflection mode and the transmission mode may be expressed as a single point within the saddle shape. As can be seen from the diagram, the range of color reproduction obtained solely by the use of the red, green, and blue pixels in the reflection mode differs from the range of color reproduction that is obtained solely by the use of the read, green, and blue pixels in the transmission mode. To compensate for this difference, the transflective LCD device drives the white (W) pixels of the display to one or more offset values so that the differences in the color reproduction ranges between the reflection mode and the transmission mode are substantially reduced.  
         [0051]     The white offset used in the foregoing compensation scheme can be determined in a number of different manners. In the following example, the white offset value W 0  may be selected in accordance with the following equations:
 
 W   0 =Offset R   ×R   1 +Offset G   ×G   1 +Offset B   ×B   1 ;
 
R 0 , G 0 , B 0 =R 1 , G 1 , B 1  
 
         [0052]     where 
 
Offset R +Offset G +Offset B ≦1. 
 
         [0053]     Red, green and blue offset (Offset R , Offset G , Offset B ) values are determined according to the color filter spectrum and the targeted color reproducing range. The targeted color reproduction range in the example shown in  FIG. 6  is the color reproduction range of the transflective display when it operates in the transmission mode.  
         [0054]      FIGS. 7A and 7B  are graphs showing light transmittance (T) as a function of wavelength (λ) in the reflection and transmission modes of operation of the transflective LCD display.  FIG. 7A  illustrates the difference in light transmittance between the transmission mode and reflection mode that occurs without any white offset compensation applied to the white (w) pixels of the single color regions of the display. In contrast,  FIG. 7B  illustrates the light transmittance in the transmission mode and reflection mode that occurs when white offset compensation is applied to the white (W) pixels of each of the single color regions of the display. As seen from the graph of  FIG. 7B , as the offset brightness of the white pixel region increases, the light transmittance increases compared with the uncompensated operation of the transflective LCD device shown in  FIG. 7A . As such, the offset brightness produced by the white pixel regions reduced the differences in the light transmittance that would otherwise occur in the transflective LCD display between the transmission and reflection modes of operation.  
         [0055]     The transflective type LCD shown above makes several advantages available to a designer wishing to exploit them. For example, the driving of the white pixels may be used in either the transmission mode or the reflection mode to enhance the overall brightness of the display. Further, the driving of the white pixels may be used in either mode, but particularly the reflection mode, to ensure that the range of colors experienced by a user appears substantially the same whether the transflective LCD display is in the transmission mode or the reflection mode. Still further, since the functionality previously provided by the conventional transmission holes is replaced by functionality provided by the white pixel regions, the design and/or manufacture of the transflective LCD display may be simplified and/or more readily enhanced with other design features.  
         [0056]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.