Patent Publication Number: US-2007097504-A1

Title: Dual panel display

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a structure of a dual-display LCD, and more particularly, to a structure of a dual-display LCD to prevent a non-uniform brightness distribution on the main display panel.  
      2. Description of the Prior Art  
      An LCD is a thin and electricity-saving flat display, which uses liquid crystal rotation angle and a polarized light layer to control light transmittance. Brightness is controlled by the transmittance. When color is constant, a liquid crystal remains in a steady state, and this is why the LCD has stable appearance without flicker. Because of the above-mentioned advantages, LCDs are applied in mobile phones, PDAs, notebook computers, and digital cameras. Recently, dual-display LCDs have been applied to various products.  
      For example, many shell-type mobile phones use dual-display LCDs. The dual-display LCD has a main display panel and a sub-display panel. The sub-display panel emits light from a rear cover of the mobile phone, shows simple words or images, and typically communicates basic call information to users before they answer the phone. The main display panel positioned in the mobile phone shows the main output of the phone.  
      One prior art dual-display LCD is made by combining two LCD modules back to back. This method increases the weight and thickness of the end product, and therefore does not allow for light, thin, and small products. Another prior art lets light exit on both sides of a backlight module. When assembling a main panel and a sub panel, they are fixed on two sides of a main housing for positioning the panels. The backlight module is positioned between the main housing and the main panel, and it provides light for the sub panel via the main housing. Since a light guide plate is shared between the main panel and the sub panel, there is another problem. Some light enters the sub panel via a light-exiting opening, and other light is reflected to the main panel completely by the white main housing because the sizes of the sub panel and the light-exiting opening are smaller than the main panel. Therefore there is a non-uniform distribution of light on the main panel, i.e., light at the light-exiting opening of the main housing is dim and forms a shadow on the light-exiting opening. This causes a difference of light intensity and color on the main panel.  
      There are some methods to improve the problems mentioned above. For example, it is possible to modify the pattern of the light guide plate in the backlight module to make the light distribution more uniform, but there is a high threshold in the critical dimension of the pattern design because some light is absorbed in the rear optical source. There is still a non-uniform brightness distribution on the main panel even after fine tuning. Using diffusers can improve this phenomenon, but the result is not good enough. Using composite films like brightness enhancement films or diffusers to overcome this phenomenon substantially increases the cost and thickness of the dual-display LCD. Therefore, the present invention teaches a method of overcome the non-uniform brightness distribution phenomenon, reducing cost, and enhancing quality.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a dual panel display that adjusts light-exiting capacity on both sides of the backlight module and prevents the problem mentioned above.  
      The claims of the present invention discloses a dual panel display comprising a backlight module, a first display panel, a second display panel, and plurality of reflective polarizers. The backlight module comprises a light guide plate having a light-incidence face, a first light-exit face, and a second light-exit face, the first light-exit face and the second light-exit face positioned parallel with each other and adjacent to the light-incidence face, and at least a light source being positioned at the light-incidence face for producing natural light passing into the light guide plate. A first display panel is positioned at a side of the first light-exit face of the light guide plate, and a second display panel is positioned at a side of the second light-exit face of the light guide plate. Of the plurality of reflective polarizers, at least two of the first reflective polarizers are simultaneously positioned between the first light-exit face of the light guide plate and the first display panel, or between the second light-exit face of the light guide plate and the second display panel.  
      Since the dual display structure of the present invention uses the plurality of reflective polarizers positioned between the light guide plate of the backlight module and the first display panel or the second display panel, light can be controlled by rotating the included angle of the transmission axis of the reflective polarizers to provide uniform light on the first display panel and overcome the non-uniform brightness distribution phenomenon.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  and  FIG. 2  are diagrams of a dual panel display according to the present invention.  
       FIG. 3  is an exploded diagram of a dual panel display according to the present invention.  
       FIG. 4  is a three-dimensional diagram of two reflective polarizers shown in  FIG. 3 .  
       FIG. 5  and  FIG. 6  are respectively diagrams of second and third embodiments of a dual panel display according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      Please refer to  FIGS. 1 and 2 .  FIG. 1  is a front-view diagram of a dual panel display  10  according to the present invention, and  FIG. 2  is a rear-view diagram of a dual panel display  10 .  FIG. 3  is an exploded diagram of a dual panel display  10 . As shown in  FIG. 1 , the dual panel display  10  of the present invention comprises a first display panel  12  which is an LCD panel, and the first display panel  12  comprises a first surface  14  which is the display face of the first display panel  12 . A dashed-line area in  FIG. 1  is the display area of the first display panel  12 . The dual display structure  10  of the present invention also comprises a housing  18  which is used to hold and steady the backlight module  80  shown in  FIG. 3  and the first display panel  12 .  FIG. 2  shows a reverse side of the dual display structure  10  in  FIG. 1 . As shown in  FIG. 2 , the dual display structure  10  of the present invention further comprises a second display panel  32  which is an LCD panel, and the second display panel  32  comprises a second surface  16  which is the display face of the second display panel  32 . A dashed-line area in  FIG. 2  is the display area of the second display panel  32 .  
      As shown in  FIG. 1 , the dual panel display  10  comprises a first display panel  12 , a brightness enhancement film  56 , a diffuser  58 , a light guide plate  54 , a diffuser  50 , two reflective polarizers  72 ,  74 , a brightness enhancement film  52 , a housing  18 , and a second display panel  32 . The size of the first display panel  12  is bigger than the second display panel  32  since the first display panel  12  is the main display panel and the second display panel  32  is the sub-display panel. The backlight module  80  comprises a light source  34 , the diffusers  50 ,  58 , the brightness enhancement films  52 ,  56 , the light guide plate  54 , and the reflective polarizers  72 ,  74 . The light source  34  comprises at least a light generator such as light emitting diode (LED). The light guide plate  54  is of plastic and has a light-entering surface  36 , and two parallel surfaces adjacent to the light-entering surface  36  are defined as a first light-exiting surface  37  and a second light-exiting surface  38 . The light source  34  is positioned at side of the light-entering surface  36  to generate light to enter into the light guide plate  54 . The diffuser  58  and the brightness enhancement films  56  are positioned between the first light-exiting surface  37  and the first display panel  12 . Furthermore, in order to provide better backlight for the second display panel  32 , the diffuser  50  and the brightness enhancement films  52  are positioned between the second light-exiting surface  38  and the housing  18  in sequence in the dual panel display  10 . The reflective polarizers  72 ,  74  are positioned between the light guide plate  54  and the brightness enhancement films  52 . The housing  18  has a fixing surface  19  with a light-exiting opening  22 , and there are one first fixing surface  20  and one second fixing surface  24  on both sides of fixing base  19  of the housing  18 . The first display panel  12  is positioned on the first fixing surface  20  of the housing  18 . The backlight module  80  and the reflective polarizers  72 ,  74  are positioned between the first display panel  12  and the first fixing surface  20 . The second display panel  32  is positioned on the second fixing surface  24  of the housing  18 . The size of the light-exiting opening  22  is approximately the same as the size of the second display panel  32 .  
      When light is emitted from the light source  34 , it comes into the light guide plate  54  through the light-entering surface  36  first. Then, part of the light travels to one side near the first display panel  12  and spreads uniformly on the first display panel  12  by way of the diffuser  58  and the brightness enhancement films  56 . In the meantime, another part of the light travels to another side of the light guide plate  54  through the diffuser  50  and the brightness enhancement films  52  to the fixing base  19 , and the light nearby light-exiting opening  22  enters the second display panel  32  via the light-exiting opening  22  of the housing  18  and spreads uniformly on the second display panel  32  to result in dual-panel display. In the prior art, light traveling to the fixing base  19  is completely reflected to the first display panel  12  by the first fixing surface  20  of the white housing  18 . However, the light nearby the light-exiting opening  22  directly passes through the light-exiting opening  22  and enters the second display panel  32 . Therefore, light nearby the light-exiting opening  22  will not be reflected by the first fixing surface  20  of the white housing  18  to the first display panel  12  and will spread out in a non-uniform distribution of light on the first display panel  12 . This results in a shadow having a shape near the light-exiting opening  22  on the first display panel  12  and is known as the non-uniform brightness distribution phenomenon. The non-uniform brightness distribution phenomenon can also arise from poor design of the light guide plate  54 .  
      However, the reflective polarizers  72 ,  74  are positioned at the side of the second light-exiting surface  38  of the light guide plate  54 , so the light intensity through the light guide plate  54  to the fixing surface  19  can be adjusted, and part of light emitted from the second light-exiting surface  38  is reflected to the light guide plate  54  increasing brightness on the first light-exiting surface  37  in order to overcome the non-uniform brightness distribution phenomenon on the first display panel  12 . Please refer  FIG. 4 .  FIG. 4  is a three-dimensional diagram of the reflective polarizers  72 ,  74  shown in  FIG. 3 . The reflective polarizers  72 ,  74  are added between the second display panel  32  and the second light-exiting surface  38  of the light guide plate  54  in the present invention, and two transmission axes A, B of the adjacent reflective polarizers  72 ,  74  have an included angle defined as a comparative angle of the two reflective polarizers. According to Malus&#39;s Law: 
 
I=I 0  cos 2 α   (1) 
 
      (I 0 : incident light intensity, I: transmission light intensity, α: comparative angle)  
      The coupling polarization character of the adjacent reflective polarizers  72 ,  74  can be changed by adjusting the comparative angle of the two reflective polarizers. The light intensity of one side of the adjacent reflective polarizers  72 ,  74  can be different from the light intensity of the other side. For example, the comparative angle of the two reflective polarizers  72 ,  74  can be adjusted to reflect more light emitting from the second light-exiting surface  38  back to the light guide plate  54  in order to increase the light intensity on the first light-exiting surface  37 . The light emitting from the light guide plate  54  and entering into the fixing surface  19  becomes less, and therefore the brightness of the light-exiting opening  22  on the second display panel  32  is relatively lower. The light reflected from the light guide plate  54  travels through the diffuser  58  and the brightness enhancement films  56  to provide uniform brightness on the first display panel  12  and overcome the non-uniform brightness distribution phenomenon effectively. In another embodiment, the comparative angle of the transmission axes A, B of the adjacent reflective polarizers  72 ,  74  is less than or equal to 80 degrees and more than or equal to 0 degrees, as shown in  FIG. 4 . Furthermore, the adjacent reflective polarizers  72 ,  74  can also be positioned on the fixing surface  19  as shown in  FIG. 5 , and the diffuser  50  and the brightness enhancement films  52  can be positioned between the light-exiting opening  22  and the second display panel  32 .  
      As shown in  FIG. 6 , in another embodiment, the diffuser  50  and the brightness enhancement films  52  can be positioned between the adjacent reflective polarizers  72 ,  74 . Moreover, the reflective polarizers  72 ,  74  can be positioned between the first display panel  12  and the first light-exiting surface  37  of the light guide plate  54  to overcome the non-uniform brightness distribution phenomenon on the first display panel  12 .  
      The structure disclosed by the present invention is a dual panel display  10 , and therefore when the structure is assembled, miscellaneous optical components must be set up between the first display panel  12  and the second display panel  32  in order to transmit light to the display surfaces of the first display panel  12  and the second display panel  32 , i.e., the first surface  14  and the second surface  16 . Since the design details of the miscellaneous optical components are not critical to the present invention, they will not be discussed further.  
      Furthermore, the reflective polarizers  72 ,  74  are not limited to be adjacent. For example, other optical films such as diffusers can be positioned between the reflective polarizers  72 ,  74 . The comparative angle of the transmission axes A, B of the adjacent reflective polarizers  72 ,  74  is the only concern to provide better light intensity for the first display panel  12  and the second display panel  32 . Moreover, more than two reflective polarizers can be used in the dual panel display  10 , and the light intensity in two panels of the dual panel display  10  still can be controlled with Malus&#39;s Law. The light guide plate can be positioned between the reflective polarizers.  
      Compared to the dual display structure of the prior art, the reflective polarizers  72 ,  74  are included between the second display panel  32  and the second light-exiting surface  38  in the dual panel display according to the present invention, and two transmission axes A, B of the adjacent reflective polarizers  72 ,  74  have an included angle defined as a comparative angle of the two reflective polarizers. The light intensity of one side of the adjacent reflective polarizers  72 ,  74  can be different from the light intensity of the other side by adjusting the comparative angle, and light emitted from the light-exiting opening  22  is reduced to overcome the non-uniform brightness distribution phenomenon on the first display panel  12  effectively. Therefore, the present invention can adjust the brightness of the first display panel and the second display panel and overcome non-uniform brightness distribution phenomenon by using two reflective polarizers only. The developing time of the light guide plate is reduced effectively, and redundant films, such as brightness enhancement films and diffusers, can be removed to economize cost and reduce thickness of the dual panel display. The process yield can be improved by collocation of the pattern of the light guide plate. Furthermore, the light intensity can be controlled by adjusting the comparative angle of the plurality of reflective polarizers.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.