Abstract:
An LCD device with an LCD cell. The cell has a liquid crystal layer, a base panel adjacent to the liquid crystal layer and a top panel adjacent to the liquid crystal layer but opposing the base panel. The base panel has a first polarizer arranged to polarize incident light into a first direction. The top panel has a color filter with one or more color filter portions so as to produce light with a predetermined color, a second polarizer on a side of the color filter opposite to the liquid crystal layer and designed to polarize incident light into a second direction perpendicular to the first direction, and a third polarizer located between the color filter and the liquid crystal layer and designed to polarize incident light into the second direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/017,170 filed Dec. 27, 2007 and claims the priority of European Patent Application No. 08164707.5, filed on Sep. 18, 2008, the entirety of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to LCD with improved contrast ratio and apparatus comprising such a LCD. 
         [0004]    2. Description of the Related Art 
         [0005]    A liquid crystal display (LCD) is one of the most widely used flat panel displays. An LCD includes two panels provided with field-generating electrodes such as pixel electrodes and a common electrode and a liquid crystal (LC) layer sandwiched between those panels. The LCD displays images by applying voltages to field-generating electrodes to generate an electric field in the LC layer, which determines orientations of LC molecules in the LC layer to adjust polarization of incident light. 
         [0006]    The need to increase the color gamut of displays makes multi-primary displays interesting. One of the recent developments is a RGBY display which has a red (R), green (G), blue (B) and yellow (Y) color filter portion in the color filter of the LCD pixel. Such RGBY displays are interesting for color rendering since they have a wider color gamut than RGB or RGBW LCDs. Moreover, they have a lower power consumption than RGB LCDs. 
         [0007]    The materials and processes of the red, green and blue color filter portions have been optimised to make sure the diffusion of light through the color filter is being reduced. Some depolarization occurs in the red, green and blue color filter portions but to an acceptable level. 
         [0008]    However no yellow pigments are available to render a suitable yellow color filter portion. Nowadays, they show a substantial amount of diffusion which results in depolarization of incident polarized light. This reduces the contrast ratio (CR) and the color gamut drastically. 
         [0009]    Still, RGBY or RGyGcB are among the most promising multi-primary displays to allow a good rendering of natural surface colors. For instance, Sanyo-Epson presented an RGyGcB display (ChromaRich technology) which showed improved color gamut. This display has been used in the Epson P-3000 and P-5000 line of photo viewers. However, tests have shown that the yellow color filter is still depolarising the light and therefore may be unacceptable for LCD applications. 
         [0010]    It is observed that E. Peeters, e.a., disclose various materials that can be used for in-cell polarizers such as: polarizers based on lyotropic liquid crystalline dyes or in-situ photo-polymerization of highly ordered (smectic) guest-host systems. Such guest-host systems may be based on reactive liquid crystal diacrylates. Photo-polymerization may be obtained by doping such diacrylates with dichroic dye molecules, e.g., as present in dichroic azo dye. E. Petters, J. Lub, A. A. Stenbakkers and D. J. Broer, Advanced Materials, 2006, 18, 2412-2417. E. Petters, J. Lub, W P M Nijssen, J. Stenbakkers and D J. Broer, EuroDisplay 2005, 165-167. E. Peeters, e.a., disclose the idea to use such in-cell polarizers as a replacement for traditional polarizers on the outside of an LCD cell. They do not disclose or suggest to use such in-cell polarizers in addition to traditional outside polarizers. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    An exemplary embodiment of a LCD device comprises at least one LCD cell. The LCD cell comprises a liquid crystal layer, a base panel, and a top panel. The base panel is adjacent to said liquid crystal layer and comprises a first polarizer arranged to polarize incident light into a first direction. The top panel is adjacent to said liquid crystal layer but opposing said base panel, and comprises a color filter, a second polarizer, and a third polarizer. The color filter comprises at least one color filter portions so as to produce light of a predetermined color. The second polarizer is arranged on a side of said color filter opposite to said liquid crystal layer and arranged to polarize incident light into a second direction perpendicular to said first direction. The third polarizer is arranged between said color filter and said liquid crystal layer and arranged to polarize incident light into the second direction. 
         [0012]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention can be more fully understood by referring to the following detailed description and examples with references made to the accompanying drawings, wherein: 
           [0014]      FIG. 1  is a block diagram of a LCD device; 
           [0015]      FIG. 2   a  exemplarily shows the profile of a LCD pixel cell; 
           [0016]      FIG. 2   b  exemplarily shows the profile of components of a LCD pixel cell; 
           [0017]      FIGS. 3   a  and  3   b , respectively, schematically show one pixel (or cell) of a prior art color LCD device; 
           [0018]      FIGS. 4   a  and  4   b  show embodiments of a pixel of an LCD device in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0020]      FIG. 1  is a diagram of an electronic device  1  with an LCD  10  according to an embodiment of the present invention. The electronic device  1  also has a power supply  20  connected to the LCD  10  to supply power to the LCD  10 . In this embodiment, the LCD  10  is a color image display integrated into the electronic device  1 . As known to those skilled in the art, the electronic device  1  can be a mobile phone, a personal digital assistant (PDA), a notebook computer, a desktop computer, a television, a car media player, a portable video player, digital camera, global positioning system (GPS), car navigation system, avionics display, etc. 
         [0021]    According to an embodiment of the present invention,  FIG. 2   a  further illustrates the profile of LCD  10 , which includes a liquid crystal (LC) layer  100 , a common electrode  102 , and a pixel electrode  104 . The electrode  104  is arranged on a substrate  105 . LCD  10  may have many cells, but  FIG. 2   a  illustrates only one cell of LCD  10  to explain the present invention. In this example, the pixel cell, corresponding to a sub pixel, can have a size of 40 μm×40 μm and a thickness of 4.15 μm. Note that the pixel cell can have other sizes like 20 μm×20 μm, 30 μm×30 μm, 39.5 μm×39.5 μm, or 30 μm×61 μm (or other unsquare designs) and the thickness can be any suitable one greater than 1.5 μm. As shown, the LC layer  100 , where the LC molecules are vertical aligned (not shown in  FIG. 2   a ; “vertical” is to be understood in the orientation of the drawing shown in  FIG. 2   a ), is sandwiched between the common electrode  102  and the pixel electrode  104 . The electrode  104 , placed on the TFT (TFT=thin film transistor, not shown) side, is provided for switching the liquid crystal layer  100 . Note that the LCD  10  may include other components, such as substrates  130 , color filters (not shown in  FIG. 2   b ), and TFT (thin film transistors)  150 , see  FIG. 2   b.    
         [0022]    The common electrode  102 , the LC layer  100 , and the pixel electrode set  104  form a liquid crystal capacitor, which stores applied voltages after turn-off of the TFT(s) (not shown). The pixel electrode set  104 , supplied with the data voltages, generates electric fields in cooperation with the common electrode  102 , which reorients liquid crystal molecules of the liquid crystal layer  100 . The common electrode  102 , which can be a conventional common electrode, can be made of ITO or IZO. The pixel electrode  104  can be made of ITO or IZO too. 
         [0023]      FIGS. 3   a  and  3   b , respectively, schematically show one pixel (or cell) of a prior art color LCD device to further illustrate the problem to be solved by the present invention.  FIG. 3   a  shows the pixel having a base panel with a polarizer POL 1 , a glass layer GL 1  on top of the polarizer POL 1  and an electrode EL 1  on top of the glass layer GL 1 . Moreover, the pixel has a top panel with a polarizer POL 2 , a glass layer GL 2  below the polarizer POL 2 , a color filter CF below the glass layer GL 2  and an electrode EL 2  below the color filter CF. Polarizer POL 1  has a polarizing direction perpendicular to the polarizing direction of polarizer POL 2 . In the example shown, polarizer POL 1  polarizes light in the x-direction and polarizer POL 2  in the y-direction. The color filter CF, as shown in  FIG. 3   a , comprises 4 portions, i.e., a red portion R, a green portion G, a blue portion B and a yellow portion Y. It is observed that the terms “on top of” and “below” refer to the orientation as shown in the drawing and are not intended to limit the scope of the present invention. 
         [0024]    The power supply  20  is connected between the electrode EL 1  and electrode EL 2 . In the embodiment shown in  FIG. 3   a , the LCD device is of the so-called “vertical alignment” (VA) type, i.e., the molecules in the liquid crystal  100  are vertically aligned when the power supply voltage as provided by power supply  20  is low, e.g., 0 V. “Vertically” refers to a direction perpendicular to the surface of the electrodes EL 1 , EL 2 . In this state, the pixel should be “black”, i.e., should not be transparent for any light. It is observed that the technique of the invention is not restricted to VA but can also be applied for other LC modes, like FFS (Fringe Field Switching), IPS (In-Plane Switching), ECB (Electrically Controlled Birefringence), OCB (optically compensated bend). 
         [0025]      FIG. 3   a  shows that light L falls on the bottom of the base panel. Light L falls on polarizer POL 1  and becomes linearly polarized in the x-direction, as shown at the right hand side of the schematic pixel. The light L then passes glass layer GL 1  and electrode EL 1  and remains linearly polarized in the x-direction. 
         [0026]    The light L then passes liquid crystal  100  unobstructed due to the molecules in liquid crystal  100  being vertically aligned. After having passed liquid crystal layer  100 , the light L is still linearly polarized in the x-direction. 
         [0027]    Then, the light L passes electrode EL 2  and enters color filter CF. The portions of the light passing the red R, green G, and blue B portions, respectively, of the color filter CF are filtered to render a red, green, blue light portion, respectively. These three portions are still substantially linearly polarized in the x-direction because, nowadays, the materials used for these red R, green G, and blue B portions do not substantially alter the polarization. 
         [0028]    However, materials used to date for the yellow portion Y are such that they diffuse passing light, resulting in a depolarizing effect. This is schematically shown at the right hand side of the pixel where, at the junction between yellow color filter portion Y and glass layer GL 2 , the polarization of the light having passed yellow color filter portion Y has both a x-component and a small y-component. I.e., there light L has become elliptically polarized. 
         [0029]    All light L passes glass layer GL 2  unaltered and arrives at the junction between glass layer GL 2  and polarizer POL 2 . Polarizer POL 2  blocks all light polarized in the x-direction. Therefore, all light that has passed the red R, green G, and blue B portions, respectively, will be completely blocked by polarizer POL 2 . I.e., downstream from these red R, green G, and blue B portions, respectively, the pixel is “black” (does not transmit any light). However, polarizer POL 2  passes the y-component of the depolarized portion of light L that has passed yellow color filter portion Y. So, the pixel will transmit a small amount of (polarized) yellow light and will not appear entirely “black”. This is unacceptable for most applications. 
         [0030]      FIG. 3   b  shows the same pixel as  FIG. 3   a . However, in the state shown in  FIG. 3   b , the power supply  20  now supplies a voltage sufficient to horizontally align the molecules in liquid crystal  100 . This may, e.g., be 5 V. The effect of this horizontal alignment is that the direction of polarization of light L, which is polarized in the x-direction when it enters liquid crystal  100 , is rotated by 90° (π/2). So when leaving liquid crystal  100  light L is polarized in the y-direction, as indicated at the right hand side of the pixel in  FIG. 3   b.    
         [0031]    Again, the red R, green G, and blue B portions of color filter CF will substantially not alter the polarization direction of light L and will, thus, transmit red, green and blue light portions respectively all polarized in the y-direction. Polarizer POL 2  will pass these red, green and blue light portions without altering them. Yellow color filter portion Y will, again, diffuse the light portion passing this portion resulting in some depolarization. Thus, the light having passed yellow portion Y will have a small component in the x-direction. However, most of the yellow light portion will be polarized in the y-direction. The latter portion will also pass polarizer POL 2  without being altered. Only the yellow portion that is polarized in the x-direction will be obstructed by polarizer POL 2 . Still, most of the light will be transmitted by the pixel rendering the pixel a white color. 
         [0032]    It will be understood by the person skilled in the art that, in reality, electrode EL  2  will be split in several electrode portions, i.e., one portion for each color filter portion R, G, B, Y. Each one of these electrode portions will be connected to a distinct TFT that is arranged to separately switch each one of these electrode portions on a separate voltage in order to switch each one of the color filter portions on and off by either vertically or horizontally aligning the molecules in the respective portions of the liquid crystal  100 . The amount of light passed through the respective portions of the liquid crystal  100  can be controlled by controlling the voltage level applied to electrodes EL 1  and EL 2 . Thus, the pixel can transmit any desired color. 
         [0033]      FIGS. 4   a  and  4   b  show embodiments of a pixel of an LCD device that solves the problem of insufficient black level, and therefore unacceptable contrast ratio. 
         [0034]    Components of the pixel shown in  FIGS. 4   a  and  4   b  that are the same as in  FIGS. 3   a  and  3   b  are referenced with the same reference numbers. The difference between the pixel in  FIGS. 4   a ,  4   b  and the pixel in  FIGS. 3   a ,  3   b  is that the pixel in  FIGS. 4   a ,  4   b  comprises an additional polarizer POL 3  in the top panel upstream from the color filter CF. Such a polarizer is called an “in-cell polarizer”. 
         [0035]    Ideally, additional polarizer POL 3  should have the same polarizing direction as polarizer POL 2 . I.e, polarizer POL 3  should only transmit light portions polarized in the y-direction and block the x-component of incident light completely. However, to date, in-cell polarizers having a 100% polarizing capacity in one direction are not yet known. So, in practice, some light polarized in the x-direction will still pass additional polarizer POL 3 . 
         [0036]    In the embodiment shown in  FIG. 4   a , the additional polarizer POL 3  is provided between electrode EL 2  and color filter CF, and covers all color filter portions R, G, B, and Y. 
         [0037]    The way the pixel of  FIG. 4   a  operates is as follows. If power supply  20  provides an operating voltage of 0 V, all molecules in liquid crystal  100  are vertically aligned and liquid crystal  100  does not alter the passing light that is polarized in the x-direction. In that state, all light after having passed transparent electrode EL 2  is at least partly blocked by additional polarizer POL 3 . The amount of light polarized in the x-direction and passing additional polarizer POL 3  depends on the polarizing capacity of additional in-cell polarizer POL 3 . Some light polarized in the x-direction will still pass additional polarizer POL 3 . Portions of that remaining light passing the red R, green G, and blue B color filter portions will arrive at polarizer POL 2  substantially without being altered as to their polarization direction. I.e., they are still polarized in the x-direction. Then, these remaining portions will be entirely blocked by polarizer POL 2 . However, the portion of the light entering the yellow color filter portion Y will, again, be diffused rendering a yellow, elliptically polarized light portion with both a yellow x-component and a yellow y-component. The yellow y-component will pass the polarizer POL 2 . So, still some amount of yellow light may be transmitted by the pixel. However, compared to the pixel of  FIGS. 3   a ,  3   b  the contrast ratio is improved drastically. 
         [0038]    In the on-state, the power supply  20  will provide a high voltage, e.g. 5 V which renders the molecules in liquid crystal  100  to become horizontally aligned. The effect of this horizontal alignment is that the direction of polarization of light L, which is polarized in the x-direction when it enters liquid crystal  100 , is rotated by 90° (π/2). So when leaving liquid crystal  100  light L is polarized in the y-direction, As explained with reference to  FIG. 3   b , in this state, all light will not be obstructed when passing any of the color filter portions R, G, B, Y and polarizer POL 2 . However, also polarizer POL 3  will not obstruct this light. So, in the on-state, pixel will show a white light. 
         [0039]    Again, in practice, as will be understood by the person skilled in the art that, in the embodiment of  FIG. 4   a , electrode EL 2  will be split in several electrode portions, i.e., one portion for each color filter portion R, G, B, Y. Each one of these electrode portions will be connected to a distinct TFT that is arranged to separately switch each one of these electrode portions on a separate voltage in order to switch each one of the color filter portions on and off by either vertically or horizontally aligning the molecules in the respective portions of the liquid crystal  100 . The amount of light passing through the respective portions of the liquid crystal  100  can be controlled by controlling the voltage level applied to electrodes EL 1  and EL 2 . Thus, the pixel can transmit any desired color. 
         [0040]    Thus, the invention provides an LCD of which the pixels have an improved black level. However, also the white level is improved and therefore the LCD according to the invention increases the contrast ratio and the color gamut. 
         [0041]    The embodiment shown in  FIG. 4   a  has one polarizer POL 3  below all four color filter portions R, G, B, Y. Such a single polarizer may be made of a broadband polarizer material, i.e., a material that is transparent for a broad frequency spectrum of the incident light L. Of course, such a broadband polarizer material should at least be transparent for the colors red, green, blue and yellow. Such materials could be selected from the group of materials comprising: polarizers based on lyotropic liquid crystalline dyes or in-situ photo-polymerization of highly ordered guest-host systems. Such guest-host systems may be smectic guest-host systems and may be based on reactive liquid crystal diacrylates. Photo-polymerization may be obtained by doping such diacrylates with dichroic dye molecules, e.g., as present in dichroic azo dye. More information about these materials can be obtained from E. Petters, J. Lub, A. A. Stenbakkers and D. J. Broer, Advanced Materials, 2006, 18, 2412-2417 and E. Petters, J. Lub, W P M Nijssen, J. Stenbakkers and D J. Broer, EuroDisplay 2005, 165-167. 
         [0042]    In an alternative embodiment, the polarizer POL 3  is split into at least two portions per pixel, as shown in  FIG. 4   b .  FIG. 4   b  shows an example where polarizer POL 3  has been split into four different portions: a portion POL 3 R, a portion POL 3 G, a portion POL 3 B, and a portion POL 3 Y. Each one of these polarizer portions POL 3 R, POL 3 G, POL 3 B, POL 3 Y is designed as a limited bandwidth polarizer. I.e., polarizer portion POL 3 R is designed to substantially transmit only light in the red frequency range. Polarizer portion POL 3 G is designed to substantially transmit only light in the green frequency range. Polarizer portion POL 3 B is designed to substantially transmit only light in the blue frequency range. Polarizer portion POL 3 Y is designed to substantially transmit only light in the yellow frequency range. 
         [0043]    By the arrangement shown in  FIG. 4   b , each color filter portion will transmit light with a more limited bandwidth. This attributes to a better color gamut. 
         [0044]    Instead of providing a separate polarizer portion POL 3 R, POL 3 G, POL 3 B, POL 3 Y for each one of the color filter portions R, G, B, Y, alternatively, two or three such separate polarizer portions may be provided. For instance, the polarizer portions POL 3 R, POL 3 G, POL 3 B may be one single portion made of a material being transparent to a broadband frequency spectrum including red, green and blue. However, other combinations are possible, depending on the available materials which may, for instance, be selected on ease of patterning into the proper configuration. 
         [0045]    In the above embodiments, the polarizer POL 3  has been shown to be located between the electrode EL 2  and the glass layer GL 2 . However, alternatively, the polarizer POL 3  may be located between the liquid crystal  100  and the electrode EL 2 . 
         [0046]    The invention has been explained with reference to a R, G, B, Y LCD device but can equally be applied in any color LCD device using color filters. I.e., the invention as described here focuses on a yellow color filter, but the idea of making use of in-cell polarizers for any wavelength could also be of interest to traditional RGB (high contrast) displays or RGBX displays, where “X” means, e.g., Y=yellow, W=white, or any other color of the fourth color filter. 
         [0047]    The LCD device according to the invention can be applied in mobile phones, personal digital assistants (PDA), notebook computers, desktop computers, televisions, car media players, portable video players, digital cameras, global positioning systems (GPS) as used in car navigation systems, avionics displays, etc. However, other applications may be true-color wide gamut displays, such as photo-viewers and photo-printer pre-viewers, where it is important that the gamut of the display matches the gamut of the photo paper, and that of the camera. 
         [0048]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.