Patent Publication Number: US-2003227585-A1

Title: Multi-color liquid crystal display device

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a display and, more particularly, to a multi-color liquid crystal display (LCD) device having a low manufacturing cost.  
       BACKGROUND OF THE INVENTION  
       [0002] Along with enhancement of scientific technology and living quality, information products have gone deep into every aspect of life. As compared to conventional cathode ray tube (CRT) displays, because the liquid crystal displays (LCDs) have the advantages of small volume, no glittering, low radiation, and power saving, they are more and more popular in the market. In general portable products like personal digital assistants (PDAs), mobile phones, or digital still cameras (DSCs), the panels thereof usually adopt the design of black-and-white display, hence having limited effect. Therefore, in order to achieve multi-color or full-color displaying effect, it is usually necessary to dispose a color filter in an LCD. Through the help of the color filter to display the three primary colors of red (R), green (G), and blue (B), full-color displaying mode can then be achieved by mixing different ratios of the three primary colors.  
       [0003] However, the color filter is expensive and has a high manufacturing cost. Because the required number of colors of the above small LCD panel is less, the price of this kind of products will go through the roof if the expensive color filter is applied to this kind of portable products, hence deteriorating the sell. Moreover, these small products have much limited display screens, and only need multi-color displaying effect.  
       [0004] Accordingly, the present invention aims to propose a multi-color LCD device, which utilizes combinations and variations of a plurality of liquid crystal layers and several polarizers to achieve multi-color displaying object so as to resolve the above problems.  
       SUMMARY OF THE INVENTION  
       [0005] The primary object of the present invention is to propose a multi-color LCD device, wherein at least two liquid crystal panels and several polarizers are matched to achieve the effect of color display, hence applying to some portable products requiring less number of colors.  
       [0006] Another object of the present invention is to provide a multi-color LCD device having a low price.  
       [0007] According to the present invention, an LCD device comprises at least two liquid crystal panels stacked up and down. A first polarizer, a second polarizer, and a third polarizer or a reflector are disposed above, between, and below the two liquid crystal panels, respectively. Each of the polarizers has a transmission axis and an absorption axis orthogonal to each other. The transmission axis of each of the polarizers lets white light be transmitted. The absorption axis of at least one polarizer absorbs light in the wavelength range of a single color plates, and the absorption axes of other polarizers absorb white light.  
       [0008] The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawings, in which: 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a structure diagram of a four-color LCD of the present invention;  
     [0010]FIG. 2 is an action diagram of a color polarizer of the present invention;  
     [0011]FIGS. 3 a  to  3   d  show diagrams of four driving modes of FIG. 1;  
     [0012]FIG. 4 is a structure diagram according to another embodiment of the present invention; and  
     [0013]FIG. 5 is a structure diagram according to yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0014] The present invention adopts the design of more than two liquid crystal panels matched with color polarizers to let an LCD device achieve multi-color displaying object without any color filter. A four-color LCD device using twisted nematic (TN) mode will be described below to illustrate characteristics of the present invention.  
     [0015] As shown in FIG. 1, a four-color LCD device  10  comprises a first liquid crystal panel  12  and a second liquid crystal panel  14 . Each of the two liquid crystal panels  12  and  14  is composed of a pair of opposite transparent substrates  18  and  18 ′ with a liquid crystal layer  16  sandwiched between them. Transparent electrode layers  20  and  20 ′ are disposed on two opposite inner surfaces of the two transparent substrates  18  and  18 ′. The first and second liquid crystal panels  12  and  14  and stacked up and down. A first polarizer  22 , a second polarizer  24 , and a third polarizer  26  are also provided. Each of the three polarizers has a transmission axis and an absorption axis (not shown) orthogonal to each other. The first polarizer  22 , the second polarizer  24 , and the third polarizer  26  are disposed above the first liquid crystal panel  12 , between the first and second liquid crystal panels  12  and  14 . And below the second liquid crystal panel  14 , respectively. The first and second polarizers  22  and  24  are color polarizers. In other words, when light passes through the polarizer  22  or  24 , the transmission axis thereof will let light be completely transmitted, while the absorption axis thereof will absorb light in the wavelength range of a single color. A red polarizer  30  is exemplified in FIG. 2. The transmission axis of the red polarizer  30  is horizontal, and the absorption axis thereof is vertical. When a light is split into a red light  32  and a cyan light  34  of complementary color, both the red light  32  and the cyan light  34  can pass through the transmission axis of the red polarizer  30 . The absorption axis of the red polarizer  30  will absorb the cyan light  34  and let the red light  32  be transmitted. The third polarizer  26  is a common polarizer. That is, when light passes through the third polarizer  26 , the transmission axis thereof will let light be completely transmitted, while the absorption axis thereof will completely absorb light.  
     [0016] When there is no voltage applied (off state), liquid crystal molecules of the liquid crystal layer  16  will align toward a certain direction according the direction of thin trenches of an orientation film, and twist 90 degrees between the upper and lower transparent electrodes  20  and  20 ′. When a voltage is applied across the transparent electrode layers  20  and  20 ′ (on state), the alignment direction of liquid crystal molecules will be altered to be parallel to the electric field. In the present invention, presentation of different colors is controlled by whether a voltage is applied onto the two liquid crystal panels  12  and  14 . FIGS. 3 a  to  3   d  show four driving modes of the two liquid crystal panels  12  and  14 . The transmission axis and the absorption axis of the first polarizer  22  are parallel (denoted by  ) and normal (denoted by  ) to the paper, respective. The absorption axis thereof absorbs cyan light (i.e., only letting red light be transmitted, denoted by AC). The transmission axis and the absorption axis of the second polarizer  24  are normal and parallel to the paper, respective. The absorption axis thereof absorbs red light (i.e., only letting cyan light be transmitted, denoted by AR). The transmission axis and the absorption axis of the third polarizer  26  are parallel and normal to the paper, respective. The transmission axis thereof lets white light be transmitted and the absorption axis thereof completely absorbs white light, which are denoted by TW and A, respectively.  
     [0017] When there is no voltage applied onto the first liquid crystal panel  12  (off state), as shown in FIG. 3 a , because the transmission axis (parallel to the paper) of the first polarizer  22  is TW and the absorption axis thereof (normal to the paper) is AC, after an incident light enters the first polarizer  22 , a white polarized light parallel to the paper and a red polarized light normal to the paper will be obtained. These two polarized lights will rotate 90 degrees along with twist of liquid crystal molecules of the first liquid crystal panel  12  to become a while polarized light normal to the paper and a red polarized light parallel to the paper, which then enter into the second polarizer  24 . Because the transmission axis (normal to the paper) of the second polarizer  24  is TW and the absorption axis thereof (parallel to the paper) is AR, the red polarized light parallel to the paper will be absorbed, while the white polarized light normal to the paper will pass through the transmission axis to still be a white polarized light normal to the paper, which then enters into the second liquid crystal panel  14 . If there is no voltage applied onto the second liquid crystal panel  14  (off state), the white light will rotate 90 degrees along with twist of liquid crystal molecules of the second liquid crystal panel  14  to become a while polarized light parallel to the paper, which then enters into the third polarizer  26 . Because the transmission axis of the third polarizer  26  is parallel to the paper, the white polarized light will be transmitted successfully to form a white color display. If a voltage is applied onto the second liquid crystal panel  14  (on state), as shown in FIG. 3 b , the white polarized light normal to the paper will not rotate along with liquid crystal molecules. The white polarized light thus cannot pass through the third polarizer  26 , hence forming a black color display.  
     [0018] When a voltage is applied onto the first liquid crystal panel  12  (on state), as shown in FIG. 3 c , a white polarized light parallel to the paper and a red polarized light normal to the paper obtained after an incident light passes through the first polarizer  22  will not rotate along with twist of liquid crystal molecules to be still a while polarized light parallel to the paper and a red polarized light normal to the paper, which then enter into the second polarizer  24 . Because the transmission axis (normal to the paper) of the second polarizer  24  is TW and the absorption axis thereof (parallel to the paper) is AR, the red component of the white polarized light parallel to the paper will be absorbed when passing through the absorption axis to obtain a cyan polarized light parallel to the paper. The red polarized light parallel to the paper will successfully pass through the second polarizer  24  to simultaneously enter into the second liquid crystal panel  14  with the cyan polarized light. If a voltage is applied onto the second liquid crystal panel  14  (on state), these two polarized lights will still be a cyan polarized light parallel to the paper and a red polarized light normal to the paper after passing through the second liquid crystal panel  14  and then enter into the third polarizer  26 . The third polarizer  26  will absorb the red polarized light normal to the paper and only let the cyan polarized light parallel to the paper be transmitted, hence forming a cyan display. If there is no voltage applied onto the second liquid crystal panel  14  (off state), the cyan polarized light parallel to the paper and the red polarized light normal to the paper will rotate 90 degrees along with twist of liquid crystal molecules to become a cyan polarized light normal to the paper and a red polarized light parallel to the paper after passing through the second liquid crystal panel  14 . The third polarizer  26  will absorb the cyan polarized light normal to the paper and only let the red polarized light parallel to the paper be transmitted, hence forming a red display.  
     [0019] The present invention uses two liquid crystal panels  12  and  14  matched with the three polarizers  22 ,  24 , and  26  to let the LCD device  10  have white, black, red, and cyan displaying modes. Through proportional mixing of colors and matched with gray-scale driving, an LCD device will have a colorful displaying effect without any expensive color filter. The LCD device thus has the advantage of low cost and applies to display panels of portable electronic products.  
     [0020] The above two liquid crystal panels  12  and  14  can also use super twisted nematic (STN) mode, film super twisted nematic (FSTN) mode, or thin film transistor (TFT) mode. In addition to the above transmitive mode LCD device, a reflector  28  can be disposed on the lower surface of the third polarizer  26  to form a reflective mode LCD device, as shown in FIG. 4. The reflector  28  can also be a semi-reflector having partly reflective and partly transmitive function to form a transflective mode LCD device.  
     [0021] Besides, the above third polarizer  26  can also be replaced with a reflector, as shown in FIG. 5. In other words, a reflector  28  is disposed on the lower surface of the lower transparent substrate  18 ′ of the second liquid crystal panel  14 . A light passing through the second liquid crystal panel  14  is reflected by the reflector  28  and then acted on in order by the second liquid crystal panel  14 , the second polarizer  24 , the first liquid crystal panel  12 , and the first polarizer  22  to form different color displays, hence having a multi-color displaying effect. The reflector can also be disposed on the upper surface of the lower transparent substrate.  
     [0022] Although the present invention has been described with reference to the preferred embodiments thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.