Abstract:
Disclosed is an edge type backlight unit having a local dimming function realized by adjusting the distance between a light guide plate and a plurality of optical systems disposed under the light guide plate. The edge type backlight unit includes a light guide plate guiding light emitted from a light source, and a luminance control unit including a plurality of optical systems reflecting light guided by the light guide plate and emitting the light to a liquid crystal panel, and controlling luminance by controlling the reflectance of each of the plurality of optical systems according to an input image signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 10-2008-0119938 filed on Nov. 28, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an edge type backlight unit, and more particularly, to an edge type backlight unit, which is capable of local dimming through the adjustment of the distance between a light guide plate and a plurality of optical systems disposed under the light guide plate. 
         [0004]    2. Description of the Related Art 
         [0005]    With the increasing use of information apparatuses, slim and lightweight flat panel display devices using small amounts of power have recently drawn much attention. 
         [0006]    Among those flat panel display devices, liquid crystal displays (LCDs) are being widely used for notebook computers, desktop monitors, and televisions because of their excellent resolution, color display and image quality. 
         [0007]    In general, LCDs employs separate backlight units for light emission because liquid crystal panels are unable to emit light. 
         [0008]    In LCDs, the backlight unit is installed under the liquid crystal panel to emit light to the liquid crystal panel. The LCD displays an image by controlling the amount of light according to the alignment of liquid crystals, thereby displaying an image. 
         [0009]    Backlight units may be grouped into a direct type and an edge type. Direct type backlight units include light sources under the liquid crystal panel to emit light directly onto the entire surface of a substrate. Edge type backlight units include light sources disposed at one or both edges of a light guide plate to reflect the light emitted from the light sources toward the liquid crystal panel. 
         [0010]    LCDs are being widely used to display moving pictures such as movies and dramas, and require a local dimming function for the vivid expression of images with varying luminance. 
         [0011]    To realize local dimming, direct type backlight units, directly emitting light to the bottom of the liquid crystal panel, may employ light emitting diodes (LEDs) as light sources. However, the LEDs need to be spaced apart from the liquid crystal panel at a predetermined interval in order to achieve uniform luminance, adversely affecting the tendency toward slim and lightweight displays. 
         [0012]    In contrast, edge type backlight units use less space because light falling upon the edges is reflected by light guide panels, which are contributive to achieving slim and lightweight displays. However, the edge type backlight units are unable to perform local dimming because light falls upon the edges of the light guide plate and is reflected toward the bottom of the liquid crystal panel. 
       SUMMARY OF THE INVENTION 
       [0013]    An aspect of the present invention provides an edge type backlight unit, which can realize local dimming through the adjustment of the distance between a light guide plate and each of a plurality of optical systems disposed under the light guide plate. 
         [0014]    According to an aspect of the present invention, there is provided an edge type backlight unit having a local dimming function, including: a light guide plate guiding light emitted from a light source; and a luminance control unit including a plurality of optical systems reflecting light guided by the light guide plate and emitting the light to a liquid crystal panel, the luminance control unit controlling luminance by controlling the reflectance of each of the plurality of optical systems according to an input image signal. 
         [0015]    The luminance control unit may further include a controller supplying control voltages, each having a voltage level varied according to the contrast of the image signal, to the plurality of optical systems, respectively. 
         [0016]    The optical systems may each have a variable distance from the light guide plate, the distance being controlled by piezoelectric effects. 
         [0017]    The luminance control unit may further include a plurality of first electrodes disposed under the light guide plate in one-to-one correspondence with the plurality of optical systems. The controller may apply the control voltages to the first electrodes and the optical systems, respectively, and the optical systems may each have a variable distance from the light guide plate, the distance being controlled by an electrostatic force with a corresponding electrode of the first electrodes. 
         [0018]    The luminance control unit may further include a plurality of second electrodes disposed under the light guide plate in one-to-one correspondence with the plurality of optical systems. The controller may apply the control voltages to the first electrodes, the optical systems and the second electrodes respectively, and the optical systems may be disposed between the first electrodes and the second electrodes, and each have a variable distance from the light guide plate, the distance being controlled by an electrostatic force with respective corresponding electrodes of the first and second electrodes. 
         [0019]    The luminance control unit may further include a spacer spacing the optical system from the light guide plate at a preset distance. 
         [0020]    The optical system may be one of a reflection plate reflecting light, a scattering pattern scattering light, and a diffusion sheet diffusing light. 
         [0021]    The optical system may include a coating layer increasing light reflectance, on a surface thereof. 
         [0022]    The first electrode may be formed of a transparent material. 
         [0023]    The first electrode or the second electrode may be formed of a transparent material. 
         [0024]    The edge type backlight unit may further include an optical member directing light, emitted from the light source to the light guide plate, to a preset direction. 
         [0025]    The light source may control the quantity of incident light in association with the luminance control of the luminance control unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0027]      FIG. 1  is a schematic view of an edge type backlight unit according to the present invention; 
           [0028]      FIGS. 2A and 2B  are views of an edge type backlight unit according to an exemplary embodiment of the present invention; 
           [0029]      FIGS. 3A and 3B  are views of edge type backlight units according to other exemplary embodiments of the present invention; and 
           [0030]      FIG. 4  is a view illustrating local dimming of an edge type backlight unit according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
         [0032]      FIG. 1  is a schematic view of an edge type backlight unit according to the present invention. 
         [0033]    Referring to  FIG. 1 , the edge type backlight unit  100 , according to the present invention, includes a light guide plate  110  and a luminance control unit  120 . 
         [0034]    The light guide plate  110  is disposed under a liquid crystal panel, and light from light sources ‘ 1 ’ falls onto the edges of the light guide plate  110 . The light guide plate  110  guides the incident light to illuminate the liquid crystal panel. The light sources ‘ 1 ’ may emit light at one or both edges of the light guide plate  110 . Luminance is controlled according to the amount of light being emitted to the liquid crystal panel. 
         [0035]    The luminance control unit  120  is disposed under the light guide plate  110 , and includes optical systems reflecting light inside the light guide plate  110 . The luminance control unit  120  controls luminance by adjusting the reflectance of each of the optical systems. Here, the luminance is controlled by adjusting the distances between the optical systems and the light guide plate  110 , and the optical systems are disposed in plurality under the light guide plate  110 . By controlling luminance levels selectively for the coverage area of each of the optical systems, luminance can be adjusted locally, that is, local dimming can be performed. 
         [0036]    Hereinafter, edge type backlight units according to embodiments of the present invention will be described in detail with reference to the drawings. 
         [0037]      FIGS. 2A and 2B  illustrate an edge type backlight unit according to an exemplary embodiment of the present invention.  FIGS. 2A and 2B  are enlarged views of a coverage area of an optical system in the edge type backlight unit according to this embodiment. 
         [0038]    Referring to  FIG. 1  and  FIG. 2A , the luminance control unit  120  of the edge type backlight unit  100 , according to the present invention, may include a controller  121 , optical systems  122 , and spacers  123 . 
         [0039]    The controller  121  transmits control voltages, which have voltage levels varying with the contrast of an image signal sent from the outside, to the respective optical systems  122 . 
         [0040]    The optical systems  122  may each include any one of a reflector plate that reflects light diffused to a downward direction of the light guide plate  110 , a scattering pattern that scatters light, and a diffusion sheet that diffuses light. Furthermore, the optical system  122  may include a coating layer to increase light reflectance properties. 
         [0041]    The optical systems  122  are formed of a piezoelectric body. Thus, the optical systems  122  each have a variable distance from the light guide plate  110 , which is controlled by piezoelectric effect when receiving the control voltage, thereby controlling luminance. 
         [0042]    That is, when the optical system  122  moves closer to the bottom of the light guide plate  110 , light is reflected to increase the amount of light being emitted to the liquid crystal panel, thus increasing luminance. 
         [0043]    In contrast, as shown in  FIG. 2B , as the optical system  122  is spaced farther apart from the bottom of the light guide plate  110 , light reflection is reduced to decrease the amount of light being emitted to the liquid crystal panel, thereby decreasing luminance. 
         [0044]    The spacer  123  spaces the light guide plate  110  and the optical systems  122  apart from each other at a preset distance. 
         [0045]    The luminance control unit  120  employed in the edge type backlight unit  100  of the present invention may have various embodiments. These will now be described in detail with reference to the drawings. 
         [0046]      FIGS. 3A and 3B  illustrate edge type backlight units according other embodiments of the present invention.  FIGS. 3A and 3B  are enlarged views of the coverage area of one optical system in the edge type backlight unit illustrated in  FIG. 1 . 
         [0047]    Referring to  FIG. 3A , the luminance control unit employed in an edge type backlight unit  200 , according to another exemplary embodiment of the present invention, may include a plurality of first electrodes  222   a  disposed under a light guide plate  210  in one-to-one correspondence with a plurality of optical systems  222   b.  As shown in  FIG. 3A , the first electrodes  222   a  may be disposed under the optical systems  222   b  or between the light guide plate  210  and the optical systems  222   b.    
         [0048]    A controller  221  applies control voltages to the optical systems  222   b  and the first electrodes  222   a,  respectively. Control voltages of the same or opposite polarities may be applied to the optical system  222   b  and the first electrode  222   a.    
         [0049]    That is, in order to increase luminance, control voltages of the same polarity are applied to the optical system  222   b  and the first electrode  222   a,  so that the optical system  222   b  can be moved closer toward the bottom of the light guide plate  210  by the electrostatic force generated by the electrically charged optical system  222   b  and first electrode  222   a.    
         [0050]    In contrast, to decrease luminance, control voltages of opposite polarities are applied to the optical system  222   b  and the first electrode  222   a,  so that the optical system  222   b  can be spaced farther apart from the bottom of the light guide plate  210  by the electrostatic force generated by the electrically charged optical system  222   b  and first electrode  222   a.    
         [0051]    If the first electrode  222   a  is disposed between the optical system  222   b  and the light guide plate  210 , the controller  221  may apply control voltages of opposite polarities to the optical system  222   b  and the first electrode  222   a  in order to increase luminance. In this case, the controller  221  may apply control voltages of the same polarity to the optical system  222   b  and the first electrode  222   a  to decrease luminance. 
         [0052]      FIG. 3B  illustrates an edge type backlight unit  300  according to another exemplary embodiment of the present invention. 
         [0053]    Referring to  FIG. 3B , a luminance control unit employed in the edge type backlight unit  300 , according to this embodiment, may further include a plurality of second electrodes  322   c  in one-to-one correspondence with a plurality of optical systems  322   b.    
         [0054]    A controller  321  applies control voltages to the optical systems  332   b,  the first electrodes  322   a  and the second electrodes  322   c,  respectively. The control voltages of the same or opposite polarities may be applied to the optical systems  322   b,  the first electrodes  322   a  and the second electrodes  322   c.    
         [0055]    To increase luminance, the controller  321  applies control voltages of the same polarity to the optical system  322   b  and the second electrode  322   c,  and applies a control voltage of the polarity, which is opposite to that of the control voltage of the optical system  322   b,  to the first electrode  322   a,  so that the optical system  322   b  can be moved closer to the bottom of the light guide plate  310  by electrostatic force. 
         [0056]    In contrast, to decrease luminance, the controller  321  applies control voltages of the same polarity to the optical system  322   b  and the first electrode  322   a,  and applies a control voltage of the polarity, which is opposite to that of the control voltage of the optical system  322   b,  to the second electrode  322   c,  so that the optical system  322   b  can be spaced farther apart from the bottom of the light guide plate  310  by the electrostatic force generated by the electrically charged optical system  322   b  and first electrode  322   a.    
         [0057]    Furthermore, the first electrode  322   a  may be made to be small enough to minimize optical inference, and may be formed of an optically transparent material. 
         [0058]    The edge type backlight units  200  and  300 , according to the present invention, may further include optical members  230  and  330  respectively. The optical members  230  and  330  serve to concentrate light so that light being incident onto the light guide plates  210  and  310  from the light sources ‘ 1 ’ is directed to a preset direction, respectively. 
         [0059]    The optical members  230  and  330  may each be constructed as a prism or lens. 
         [0060]      FIG. 4  is a view illustrating local dimming of an edge type backlight unit according to the present invention. 
         [0061]    It can be seen from  FIG. 4  that the edge type backlight unit performs local dimming by controlling the distance between an optical system and a light guide plate. 
         [0062]    In the case of an image signal which is predominantly dark with a predetermined bright portion, all the light in the light guide plate, except for the light lost by total reflection, is emitted in an upward direction of the light guide plate only by an optical system drawn near to the light guide plate. The quantity of light being emitted becomes excessive without controlling the quantity of incident light, resulting in excessively high luminance. Therefore, light sources ‘ 1 ’ control the quantity of incident light in association with the luminance control of the luminance control unit. Thus, the power consumption of the backlight unit can be reduced, since not many images require the maximum luminance at the entire liquid crystal panel. 
         [0063]    Also, the optical member directs light to a specific direction so that the quantity of light can be controlled locally, and thus power use can be further reduced. 
         [0064]    As set forth above, according to exemplary embodiments of the invention, the edge type backlight unit having smaller volume than the direct type backlight unit can perform local dimming by controlling the distance between the light guide plate and a plurality of optical systems disposed under the light guide plate. 
         [0065]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.