Abstract:
A liquid crystal display (LCD) is disclosed including a first cholesteric liquid crystal (LC) layer between a first and second substrate containing a plurality of adjacent alternately arranged pixels of a first and second primary color. A second cholesteric LC layer between the second substrate and a third substrate contains a plurality of adjacent alternately arranged pixels of the first and a third primary color. The first primary color pixels in the first and second LC layer, and the second primary color pixels in the first LC layer and the primary color pixels of the third primary color in the second LC layer, are arranged behind one another. Two first primary color pixels in the first and second LC layer interact with an adjacent second primary color pixel in the first LC layer and an adjacent third primary color pixel in the second LC layer for representing a display pixel.

Description:
FIELD OF THE INVENTION 
   The invention relates to a color liquid crystal display device wiht improved resolution. 
   BACKGROUND OF THE INVENTION 
   Color liquid crystal display devices with cholesteric liquid crystals are particularly well suited for reflective color LCD display devices. Such color liquid crystal display devices are known in a multiplicity of embodiments. Relevant reference is made, for example, to printed documents U.S. Pat. No. 5,493,430 B, US 2004/0239830 A1 and US 2004/0041163 A1. In these known color liquid crystal display devices, the various colors are represented by mixing the three primary colors red, green and blue. In consequence, three separate pixels in the above three primary colors, which are arranged in one plane, are needed for representing a single color pixel. This limits the resolution of the color liquid crystal display device. 
   From U.S. Pat. No. 6,791,512 B1, a color LCD display with two liquid crystal layers arranged behind one another is known. In this arrangement, the individual primary color pixels are arranged next to one another in one liquid crystal layer. The second liquid crystal layer is only used for representing primary color pixels from the first liquid crystal layer either offset or not offset. This results in a sharper display. 
   From printed documents U.S. Pat. Nos. 6,377,321 B1, 6,654,080 B1 and US 2005/0036077 A1, a color liquid crystal display device is in each case known which have two liquid crystal layers arranged behind one another. One liquid crystal layer in this arrangement is used for reflecting visible light and the other liquid crystal layer is used for reflecting IR light. This does not result in an improvement in the resolution. 
   From JP 10-054996 A, a color liquid crystal display device is known which has a first and a second cholesteric liquid crystal layer which are arranged behind one another in the direction of viewing. The first cholesteric liquid crystal layer is arranged between a first and a second substrate and the second cholesteric liquid crystal layer is arranged between the second and a third substrate. In the first liquid crystal layer, green and red primary color pixels are arranged alternately and in the second liquid crystal layer blue and green primary color pixels are arranged alternately. 
   A further disadvantage of known color LCD displays consists in that their legibility is unsatisfactory in bright sunlight or in any other bright environment. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention, therefore, to provide a color liquid crystal display device which has an improved resolution. 
   This object is achieved by the features of Claim  1 . 
   Due to the fact that primary color pixels located both next to one another and behind one another are used for representing one display pixel, only the areas for two instead of the usual three primary color pixels are needed for each display pixel in one plane or, respectively, in one liquid crystal layer. This results in the improved resolution. In addition, the legibility of the display in bright sunlight or in any other bright environment is also improved. 
   The advantageous embodiment according to Claim  3  simplifies the drive or control since the two primary color pixels of the first primary color are driven by means of a common control line. 
   The advantageous embodiment according to Claim  4  provides a color liquid crystal display which is suitable both for daytime and for night time operation. In daytime operation, light incident from the direction of viewing, e.g. sunlight, is reflected by the primary color pixels arranged in two liquid crystal layers, in such a manner that an overall pixel in a particular color is obtained. In night time operation, the illumination is produced by a backlighting device which emits light by means of spacing webs between the individual primary color pixels to reflectors on the front of the color LCD display device. The reflectors then reflect the light onto the individual primary color pixels so that a similar appearance as in daytime operation is obtained. The primary color pixels driven for displaying a display pixel with a particular color then reflect colored light in the required composition in the direction of the observer. The absorber layer provides for a defined dark state of the display device. 
   The absorber layer or absorber elements, respectively, arranged in the area of the primary color pixels can protrude slightly into the spacing webs or correspond exactly to the lateral extent of the primary color pixels or can also be slightly smaller. The reflector elements provided in the area of the spacing webs on the front can be smaller, equal to or greater than the respective spacing webs in the lateral direction. The absorber elements and the reflector elements are dimensioned in such a manner that an optically correct impression of the display device is obtained. 
   The advantageous embodiment according to Claim  5  increases the luminance of the light reflected by the primary color pixels of the first primary color so that the area of the primary color pixels with the first primary color can be reduced in comparison with the primary color pixels with the second and third primary color. This results in a further improvement in the lateral resolution. 
   Distributing the primary colors blue, green and red according to Claim  6  and  7  results in better white balance and dark state of the display. 
   The advantageous embodiment according to Claims  8  to  10  increases the color space in the red domain, i.e. colors can be represented with higher color saturation. 
   The advantageous embodiment according to Claim  11  prevents light dispersed on the display from passing forward to the observer and thus impairing the quality of the display. In particular, this macroscopically improves the dark state in bright incident light. 
   The advantageous embodiment according to Claim  12  results in improved efficiency of the backlighting. For example, only every second spacing web can be used for the backlighting whilst the narrower spacing webs are only used for separating the individual primary color pixels. 
   The advantageous embodiment o according to Claim  13  to  16  takes into account that the primary color pixels of the first primary color are present twice per display pixel as a result of which the area of the two primary color pixels of the first primary color can be reduced compared with the area of the primary color pixels of the other two primary colors. This also reduces the area needed for a display pixel which leads to improved resolution. In addition, this provides for very accurate white balance of the color LCD display. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details, features and advantages are obtained from the description of an illustrative embodiment of the invention which follows. 
       FIG. 1  shows a diagrammatic sectional view of a first embodiment of the invention, 
       FIG. 2  shows a top view of the primary color pixels of the first liquid crystal layer of the embodiment according to  FIG. 1 , 
       FIG. 3  shows a top view of the primary color pixels of the second liquid crystal layer of the embodiment according to  FIG. 1 , 
       FIG. 4  shows the embodiment according to  FIG. 1  wherein the primary color pixels are connected in such a manner that blue and green light is reflected towards the observer, 
       FIG. 5  shows the embodiment according to  FIG. 1  wherein the primary color pixels are connected in such a manner that green and red light is reflected towards the observer, 
       FIG. 6  shows the embodiment according to  FIG. 1  wherein the primary color pixels are connected in such a manner that purple and yellow light is reflected towards the observer, 
       FIG. 7  shows the basic structure of a second embodiment of the invention which can be illuminated both with ambient light and by a backlighting device, 
       FIG. 8  shows the second embodiment according to  FIG. 7  in daytime operation when displaying a yellow display pixel, and 
       FIG. 9  shows the embodiment according to  FIG. 7  in night time operation with activated backlighting for displaying a yellow display pixel. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a diagrammatic sectional view of a first illustrative embodiment of the invention. The color LCD display shown comprises a first liquid crystal layer  2  which is enclosed between a first substrate  4  and a second substrate  5 . In the direction of viewing  8  behind the first liquid crystal layer  2 , a second liquid crystal layer  10  is arranged which is enclosed between the second substrate and a third substrate  12 . The first, second and third substrate  4 ,  6  and  12  consist preferably of glass. In the first and second liquid crystal layer  2  and  10 , a plurality of primary color pixels  14  in the three primary colors blue, green and red are in each case arranged at a distance from one another. The first primary color B is blue, the second primary color G is green and the third primary color R is red. 
     FIG. 2  diagrammatically shows that blue and green primary color pixels  14 -B and  14 -G are arranged alternately in a row  16  in the first liquid crystal layer  2 . In the individual primary color pixel columns  17 , primary color pixels  14  of the same color are thus in each case arranged.  FIG. 3  shows a top view of the matrix arrangement of the primary color pixels  14  in the second liquid crystal layer  10  with a plurality of blue primary color pixels  14 -B and red primary color pixels  14 -R, wherein blue and red primary color pixels  14 -B and  14 -R are arranged alternately. In the individual primary color pixel columns  17 , primary color pixels  14  of the same color are thus arranged in each case. The blue primary color pixels  14 -B in the first liquid crystal layer  2  have a helicity which is opposite to the helicity of the blue primary color pixels  14 - 2  in the second liquid crystal layer  10 . For example, the blue primary color pixels  14 -E in the first liquid crystal layer  2  are dextrorotatory whilst the blue primary color pixels  14 -B in the second liquid crystal layer  10  are laevorotatory. The helicity of the green and red primary color pixels  14 -G and  14 -R is not decisive and can thus be selected arbitrarily. As shown in  FIG. 2 and 3 , the red and green primary color pixels  14 -R and  14 -G are of equal size with regard to their surface extent and the blue primary color pixels  14 -B are approximately half as large. 
   The individual primary color pixels  14  are driven in a familiar manner via transparent electrodes which are not shown in the drawing for reasons of clarity. Four immediately adjacent primary color pixels  14  interact in each case for displaying a single display pixel  18 - i . These are in each case two immediately adjacent primary color pixels  14  in the first liquid crystal layer  2  and in each case two primary color pixels  14  arranged immediately underneath these in the second liquid crystal layer  10 . In each display pixel  18 - i , a blue primary color pixel  14 -B and a green primary color pixel  14 -G in the first liquid crystal layer  2  and a blue primary color pixel  14 -B and a red primary color pixel  14 -R in the second liquid crystal layer  10  thus interact. In  FIGS. 2 and 3 , four display pixels  18 - 1 ,  18 - 2 ,  18 - 4  and  18 - 5  are in each case shown completely and two display pixels  18 - 3  and  18 - 6  are shown only partially. In the remaining  FIGS. 1 and 4  to  9 , two display pixels  18 - 1  and  18 - 2  are in each case shown completely and one display pixel  18 - 3  is in each case shown partially. 
     FIGS. 4 to 6  then diagrammatically show the illumination of the color liquid crystal display according to the first embodiment with light, e.g. with sunlight which is incident on the color LCD display from the direction of viewing  8 . In this first embodiment, the blue primary color pixel  14 -B in the first liquid crystal layer  2  has a dextrorotatory helicity and the blue primary color pixel  14 -B in the second liquid crystal layer  10  has a laevorotatory helicity. The individual primary color pixels  14  are switched by means of the drive electrodes, not shown, in such a manner that the color display pixel  18 - 1  glows blue and the color display pixel  18 - 2  glows green in  FIG. 4 . In  FIG. 5 , the color display pixel  18 - 1  glows green and the color display pixel  18 - 2  glows red. In  FIG. 6 , the color display pixel  18 - 1  glows purple and the color display pixel  18 - 2  glows yellow. 
   As can be seen from  FIG. 4 , the laevorotatory component from the white light  20  incident on the dextrorotatory, blue-reflecting primary color pixels  14 -B in the first liquid crystal layer  2  is reflected as blue light  20 -B to the observer. The remaining part of the white light  20  penetrates through the dextrorotatory blue primary color pixels  14 -B and is incident on the laevorotatory blue primary color pixels  14 -B in the second liquid crystal layer  10 . The laevorotatory blue primary color pixel  14 -B in the second liquid crystal layer  10  reflects the dextrorotatory blue component as blue light  20 -B towards the observer. The remaining light  20 -D penetrates through the laevorotatory blue primary color pixel  14 -B and is absorbed, for example, in an absorber, not shown. Due to the different helicity of the blue primary color pixels  14 -B in the first and second liquid crystal layer  2  and  10 , both blue primary color pixels  14 -B arranged underneath and behind one another, respectively, contribute to the color reflection so that the size of the blue primary color pixels  14 -B can be reduced in comparison with the size of the green and red primary color pixels. The green and red primary color pixel  14 -G and  14 -R in the first and second liquid crystal layer  2  and  10 , respectively, are switched to transmittance so that the white light  20  which is incident on the green and red primary color pixels  14 -G and  14 -R is transmitted without reflection and thus does not contribute to the display. The first display pixel  18 - 1  therefore glows blue. 
   White light  20  incident on the second display pixel  18 - 2  is transmitted, or not reflected, respectively, by the blue primary color pixels  14 -B both in the first liquid crystal layer  2  and in the second liquid crystal layer  10 . Thus, no blue light is reflected towards the observer. Of the white light  20  incident on the green primary color pixel  14 -G, the green component  20 -G is reflected towards the observer. Light components  20 -D which are incident on the red primary color pixel  14 -R in the second liquid crystal layer  10  are transmitted without reflection. The second display pixel  18 - 2  therefore glows green. 
   In the representation in  FIG. 5 , the individual primary color pixels  14  are switched in such a manner that the first display pixel  18 - 1  generates green light whilst the second display pixel  18 - 2  generates red light. 
   White light  20  incident on the first display pixel  18 - 1  is thus transmitted, or not reflected, respectively, by the blue primary color pixels  14 -B both in the first liquid crystal layer  2  and in the second liquid crystal layer  10  so that no blue light passes to the observer. Of the white light  20  incident on the green primary color pixel  14 -G, the green component  20 -G is reflected towards the observer, the remaining light  20 -D passes through the green primary color pixel  14 -G. These light components  20 -B which are incident on the red primary color pixel  14 -R in the second liquid crystal layer  10  are transmitted without reflection and absorbed. The first display pixel  18 - 1  thus glows green. 
   White light  20  incident in the second display pixel  18 - 2  is transmitted, or not reflected, respectively, by the blue primary color pixels  14 -B both in the first liquid crystal layer  2  and in the second liquid crystal layer  10 . Light incident in the area of the green primary color pixel  14 -G is transmitted and is incident on the red primary color pixel  14 -R arranged behind it in the second liquid crystal layer  10 . This red primary color pixel  14 -R reflects the red component  20 -R towards the observer through the green primary color pixel  14 -G switched to transmittance, the remaining light  20 -D passes through the red primary color pixel  14 -R and is absorbed. The second display pixel  18 - 2  thus glows red. 
   In the representation in  FIG. 6 , the individual primary color pixels  14  are switched in such a manner that the first display pixel  18 - 1  generates purple light whilst the second display pixel  18 - 2  generates yellow light. 
   In consequence, the four primary color pixels  14  in the first display pixel  18 - 1  are switched in such a manner that of the incident white light  20 , the blue component  20 -B is reflected towards the observer by the two blue primary color pixels  14 -B as in the case of  FIG. 3 , the remaining light  20 -D passes through the two blue primary color pixels  14 -B. White light  20  incident in the area of the green primary color pixel  14 -G is transmitted and is incident on the red primary color pixel  14 -R arranged behind it in the second liquid crystal layer  10 . This red primary color pixel  14 -R reflects the red component  20 -R of the light towards the observer through the green primary color pixel  14 -G switched to transmittance, the remaining light  20 -D passes through the red primary color pixel  14 -R. Mixing the blue and red light results in a purple color impression for the first display pixel  18 - 1 . 
   White light  20  incident on the second display pixel  18 - 2  is transmitted, or not reflected, respectively, by the blue primary color pixels  14 -B both in the first liquid crystal layer  2  and in the second liquid crystal layer  10  so that no blue light passes to the observer. Of the light incident in the area of the green primary color pixel  14 -G, the green component  20 -G is reflected towards the observer. Of the light components  20 -D incident on the red primary color pixel  14 -R arranged behind them in the second liquid crystal layer  10 , the red component  20 -R is reflected towards the observer through the green primary color pixel  14 -G, the remaining light  20 -D passes through the red primary color pixel  14 -R. Mixing the red and green light components  20 -R and  20 -G results in yellow light. The second display pixel  18 - 2  thus glows yellow. 
   In the text which follows, a second embodiment of the invention is described with reference to  FIGS. 7 to 10 . The second embodiment corresponds to the first embodiment in its basic structure. Differences consist in that in the second embodiment, the second substrate  6  is split into a first part-substrate  6 - 1  and a second part-substrate  6 - 2  and that a color filter  21  for improving the color quality of the red primary color pixels  14 -R is provided between the two part-substrates  6 - 1  and  6 - 2 . Between the individual primary color pixels  14 , spacing webs  22  and  23  are provided, narrower spacing webs  22  and wider spacing webs  23  being provided alternately. On the front  5  of the first substrate  4 , reflectors  24  are provided in the area of the spacing webs  22  and  23 . On the side of the reflectors  24  facing the observer, an absorber layer  26  is applied. On the back  13  of the third substrate  12 , a further absorber layer  28  is provided in the areas outside the spacing webs  22  and  23 , i.e. behind the primary color pixels  14 . In the direction of viewing  8  behind this further absorber layer  28 , a backlighting device  30  is arranged, e.g. in the form of LED backlighting or an OLED layer. 
   In the text which follows, the operation of this second embodiment in daytime operation, i.e. with ambient or sunlight, and in night time operation with backlighting device  30  switched on is described with reference to  FIGS. 8 and 9 . 
   The representation in  FIG. 8  shows daytime operation, i.e. with backlighting device  30  switched off. The individual primary color pixels  14  are switched in such a manner that both the first display pixel  18 - 2  and the second display pixel glow yellow. White light  20  incident from the direction of viewing  8  passes through the blue primary color pixels  14 -B and is absorbed in the further absorber layer  28 . From the white light  20  incident on the green primary color pixels  14 -G, the green component  20 -G is reflected in the direction of the observer. The transmitted component  20 -D of the white light  20  passes through the color filter  21  which essentially only transmits red light, and is incident on the red primary color pixels  14 -R where it is reflected almost completely back through the color filter  21  and also through the green primary color pixels  14 -G towards the observer. In the aggregate, this results in a yellow display pixel. 
   In  FIG. 9 , too, both the first and the second display pixel  18 - 1  and  18 - 2  glow yellow. However, the color display device is not illuminated with ambient light but by light  32  from the activated backlighting device  30 . The white light  32  from the backlighting device  30  is conducted through the spacing webs  22  and  23  to the reflectors  24  and is there reflected onto the individual primary color pixels  14  in the same way as the ambient light in daytime operation so that a yellow display pixel is displayed in the same way as in daytime operation. The reflectors  24  are arranged in such a manner that light incident perpendicularly is reflected back conically. This results in light rays  34  which are incident on a blue primary color pixel  14 -B, on the wide spacing web  22  and on the green primary color pixel in each case in the first liquid crystal layer  2 . The light  34  incident on the blue primary color pixel  14 -B passes through the blue primary color pixel  14 -B in the first liquid crystal layer  2  and also the blue primary color pixel  14 -B in the second liquid crystal layer  10 . The light  34  incident on the wide spacing web  22  is incident on the red color filter  21  and subsequently on the red primary color pixel  14 -R and is reflected towards the observer as red light  20 -R. From the light  34  incident on the green primary color pixel  14 -G, the green component  20 -G is reflected towards the observer. The residual transmitted light  20 -D is incident on the red color filter  21  and then on the red primary color pixel  14 -R. The red primary color pixel  14 -R reflects the incident light  20 -D almost completely as red light  20 -R towards the observer. Residual light  20 -D passing through individual primary color pixels  14  is absorbed in the further absorber layer  28  on the back  13  of the third substrate  12 . 
   In the second embodiment, the illumination by the backlighting device  30  in night time operation is effected both by the smaller spacing webs  23  and the larger spacing webs  22  and the reflectors  24 . The illumination webs  22 ,  23  consist of transparent material, e.g. of a transparent adhesive in the area of the first and second liquid crystal layer  2 ,  10  and of transparent potting material, e.g. of silicone, in the area of the red color filter  21  between the first and second part-substrate  6 - 1 ,  6 - 2 . 
   As an alternative, the illumination can be effected essentially only via every second spacing web  22 , i.e. the spacing webs  22  are much wider than the spacing webs  23 . The spacing webs  23  are then used only for separating adjacent primary color pixels  14  and the display is illuminated by the wide spacing webs  22 . To a certain extent, the amount of light available for illuminating the primary color pixels  14  can also be controlled via the width of the spacing web transversely to the direction of viewing  8 . 
   In the liquid crystal layers, spacing elements, so-called spacers, can be arranged as is known, e.g., from EP 2009704.4 A1. 
   It is pointed out that the representations relating to the two illustrative embodiments are diagrammatic and, in particular, the individual beam paths are not always drawn strictly in accordance with the geometric optics.