Abstract:
A backlight unit, which can emit one of s-polarized light and p-polarized light using wire-grid polarizer, and a liquid crystal display apparatus employing the backlight unit are provided. The backlight unit includes: a light source radiating light; a light guide plate disposed with the light source at a lateral side of the light guide plate, guiding the light incident on the lateral side from the light source; a wire-grid polarizer formed on a predetermined layer of the light guide plate transmitting a first polarized component of the light through the predetermined layer and reflecting a second polarized component of light; a reflector reflecting the light to un upper surface of the light guide plate; and a polarization converter changing a polarization of the light not emitted through the upper surface of the light guide plate.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims priority from Korean Patent Application No. 10-2005-0079989, filed on Aug. 30, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Apparatuses and methods consistent with the present invention relate to a backlight unit and a liquid crystal display (LCD) apparatus, and more in particular, to a backlight unit in which one of s-polarized light or p-polarized light can be emitted by using wire-grid polarizer and an LCD apparatus employing the backlight unit.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  illustrates a conventional liquid crystal display (LCD) apparatus. Referring to  FIG. 1 , the LCD apparatus includes an LCD panel  10  and a backlight unit  20  installed on a rear substrate of the LCD panel  10  to radiate light to the LCD panel  10 . The LCD itself does not generate light and thus the backlight unit  10  provides light to produce an image. The LCD panel  10  includes a liquid crystal layer  13 , transparent electrodes  12  and  14  controlling the orientation of the liquid crystal of the liquid crystal layer  13 , and rear and front polarization panels  11  and  15  converting the incident light into polarized light polarized in a predetermined direction. In such a configuration, the light emitted from the backlight unit  20  is polarized in a particular direction by the rear polarization panel  11  and passes through the liquid crystal layer  13 . When passing through the liquid crystal layer  13 , the direction of polarization of the light is changed or not changed according to the voltage applied to the transparent electrodes  12  and  14 , and the light passes through the front polarization panel  15  or is blocked by the front polarization panel  15 . Thus, the pixels of the LCD apparatus are respectively turned on or off to produce an image.  
         [0006]      FIGS. 2A and 2B  are cross-sectional views of examples of the backlight unit  20 . Referring to  FIG. 2A , the backlight unit  20  can include a light guide plate  22  having a hologram pattern on its upper surface, a light source  21  placed at a lateral side of the light guide plate  22 , and reflection panels  24  and  23  respectively placed on the lower surface and on an opposite lateral side of the light guide plate  22 . Referring to  FIG. 2B , the backlight unit  20  can include a light guide plate  25  with a sloped lower surface, a light source  21  placed at a lateral side of the light guide plate  25  and a reflection panel  24  attached to the lower surface of the light guide plate  25 . In the backlight unit in  FIG. 2A , light emitted from the light source  21  to the upper surface of the light guide plate  22  is diffracted by the hologram pattern to be emitted almost vertically, and light emitted to the lower surface of the light guide plate  22  is reflected by the reflection panel  24  and is incident on the upper surface of the light guide plate  22 . In the backlight unit in  FIG. 2B , light incident on the light guide plate  25  is emitted at an angle through the upper surface of the light guide plate  25  and reflected from the lower surface of the light guide plate  25  and proceeds to the upper surface of the light guide plate  25 .  
         [0007]     However, in the conventional backlight unit  20 , since almost equal amounts of p-polarized light and s-polarized light are mixed and emitted, only half of the light emitted from the backlight unit is transmitted in the rear polarization panel  11  and the other half of the light is absorbed. Accordingly, the efficiency is low, and heat is generated as light is absorbed in the rear polarization panel  11 . Also, there is a limit to the increase in the brightness of the LCD apparatus.  
         [0008]     To solve these problems, the use of a dichroic polarizer sheet to emit a light having the same polarization direction as the rear polarization panel have been proposed. However, when a dichroic polarizer sheet is used, efficiency varies according to the incident angle and the wavelength of the light and light is still absorbed. A dual brightness enhancement film (DBEF) is highly efficient in that polarized light, which is not transmitted, is reflected and recycled. However, the cost is high and additional manufacturing processing is needed for the DBEF to be attached to the backlight unit.  
         [0009]     U.S. Patent Publication No. 2003/0210369 discloses an LCD apparatus, shown in  FIGS. 3A and 3B , in which brightness is improved by using a wire-grid polarizer. Referring to  FIGS. 3A and 3B , the LCD apparatus includes a wire-grid polarizer  16  on a portion of a lower electrode  12 .  
         [0010]     The wire-grid polarizer  16  is made of thin metal wires disposed at regular intervals. The wire-grid polarizer  16  acts like a diffraction lattice when the distance between the metal wires is greater than the wavelength light, and acts like a polarizer when the distance between the metal wires is less than the wavelength of light. When acting like a polarizer, polarized light parallel to the metal wires, that is, s-polarized light, is reflected and polarized light perpendicular to the metal wires, that is, the p-polarized light, is transmitted.  
         [0011]     According to the operation of the LCD apparatus disclosed in US Patent Publication No. 2003/0210369, referring to  FIG. 3A , when a liquid crystal layer  13  is turned off, light emitted from a backlight unit passes through a rear polarization panel  11  and the liquid crystal layer  13  and is blocked by a front polarization panel  15 . Also, external light passes through the front polarization panel  15 , the liquid crystal layer  13 , and the wire-grid polarizer  16 , and is blocked by the rear polarization panel  11 . On the other hand, referring to  FIG. 3B , when the liquid crystal layer  13  is turned on, the light emitted from the backlight unit passes through the front polarization panel  15  and is emitted to the outside. On the other hand, external light passes through the front polarization panel  15 , and is reflected by the wire-grid polarizer  16  as the polarization is changed by the liquid crystal layer  13 , the polarization of the reflected light is changed again by the liquid crystal layer  13  and the light is emitted to the outside through the front polarization panel  15 . Accordingly, not only the light emitted from the backlight unit but also the external light can be used, thereby improving brightness.  
         [0012]     However, the brightness of the LCD apparatus cannot be improved without external light, and additional manufacturing process is needed to form the wire-grid polarizer when manufacturing LCD panels. Also, since the structure of the backlight unit is the same as in other conventional LCD panels, light is still absorbed by the rear polarization panel  11 .  
         [0013]     Accordingly, a backlight unit, which can be manufactured at low costs in a simple way without changing the structure of an LCD panel, which is relatively costly, and efficiently emit light of a particular polarization is needed.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a backlight unit which can emit s-polarized light or p-polarized light efficiently and without loss using a wire-grid polarizer.  
         [0015]     The present invention also provides an LCD apparatus employing a backlight unit using a wire-grid polarizer.  
         [0016]     According to an aspect of the present invention, there is provided a backlight unit comprising: a light source radiating light; a light guide plate disposed with the light source at a lateral side of the light guide plate and guiding the light incident on the lateral side into the light guide plate; a wire-grid polarizer, formed on a predetermined layer of the light guide plate, transmitting a first polarized component of the light through the predetermined layer and reflecting a second polarized component of the light; a reflector reflecting the light to an upper surface of the light guide plate; and a polarization converter changing a polarization of the light which is not emitted through the upper surface of the light guide plate. Here, the predetermined layer comprises at least one of the upper surface and a lower surface of the light guide plate.  
         [0017]     According to another aspect of the present invention, the reflector comprises a lower reflection panel disposed below the light guide plate. Further, a lateral reflection panel may be disposed at an opposite lateral side of the light guide plate to reflect light which is emitted through the opposite lateral side of the light guide plate, and not through the upper surface of the light guide plate, to the light guide plate.  
         [0018]     According to still another aspect of the present invention, the polarization converter may be placed between the opposite lateral side of the light guide plate and the lateral reflection panel. The polarization converter may also be placed between the lower surface of the light guide plate and the lower reflection panel. The polarization converter may be also a diffuse reflection panel facing the second lateral side of the light guide plate.  
         [0019]     According to still another aspect of the present invention, the wire-grid polarizer comprising a plurality of parallel wires may be placed side by side with an interval of 200 nm or greater and functions as a diffraction lattice diffracting the light incident on the upper surface of the light guide plate.  
         [0020]     According to still another aspect of the present invention, a micro-structured pattern may be formed on the upper or lower surface of the light guide plate, which changes a proceeding direction of the light incident on the micro-structured pattern to be substantially vertical. The micro-structured pattern may be a hologram diffraction pattern. The micro-structured pattern may be also a prism pattern.  
         [0021]     According to still another aspect of the present invention, a coating layer may be formed on the micro-structured pattern which is formed on the upper surface of the light guide plate, wherein the wire-grid polarizer may be formed on the coating layer.  
         [0022]     According to still another aspect of the present invention, there is provided an LCD apparatus including an LCD panel and a backlight unit installed on a rear surface of the LCD panel to illuminate the LCD panel, the backlight unit comprising: a light source radiating light; a light guide plate disposed with the light source at a lateral side of the light guide plate and guiding the light incident on the lateral side into the light guide plate; a wire-grid polarizer formed on a predetermined layer of the light guide plate, transmitting a first polarized component of the light through the predetermined layer and reflecting a second polarized component of the light; a reflector reflecting the light to an upper surface of the light guide plate; and a polarization converter changing a polarization of the light which is not emitted through the upper surface of the light guide plate. Here, the predetermined layer comprises at least one of the upper surface and a lower surface of the light guide plate.  
         [0023]     According to still another aspect of the present invention, there is provided a method for generating a backlight comprising: guiding light into a light guide plate from a lateral side to an opposite lateral side of the light guide plate; transmitting a first component of the light through a predetermined layer of the light guide plate and reflecting a second polarized component of the light; reflecting the light, emitted through a lower surface of the light guide plate, to the light guide plate; and changing a polarization of the light not emitted through an upper surface of the light guide plate so that the light with a changed polarization is emitted through the upper surface of the light guide plate.  
         [0024]     The method for generating backlight may further comprise reflecting the light, emitted through the opposite lateral side of the light guide plate, to the light guide plate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:  
         [0026]      FIG. 1  is cross-sectional view of a conventional LCD apparatus;  
         [0027]      FIGS. 2A and 2B  are cross-sectional views of examples of a backlight unit of the LCD apparatus of  FIG. 1 ;  
         [0028]      FIGS. 3A and 3B  are cross-sectional views of a conventional LCD apparatus using a wire-grid polarizer;  
         [0029]      FIGS. 4A and 4B  are cross-sectional views of a backlight unit using wire-grid polarizer according to an exemplary embodiment of the present invention;  
         [0030]      FIGS. 5A through 5C  are plan views illustrating various placements of the wire-grid polarizer in the backlight unit according to exemplary embodiments of the present invention;  
         [0031]      FIG. 6  is a cross-sectional view of a backlight unit using a wire-grid polarizer according to another exemplary embodiment of the present invention;  
         [0032]      FIGS. 7A through 7C  are cross-sectional views of backlight units using wire-grid polarizer according to other exemplary embodiments of the present invention; and  
         [0033]      FIGS. 8A and 8B  are cross-sectional views of backlight units using wire-grid polarizer according to other exemplary embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]      FIG. 4A  is a cross-sectional view of a backlight unit  30  to using a wire-grid polarizer  33  according to an exemplary embodiment of the present invention. Referring to  FIG. 4A , the backlight unit  30  includes a light source  31  placed at a side of a light guide plate  32 , a wire-grid polarizer  33  formed on the upper surface of the light guide plate  32 , and a polarization converter  34  and a lower reflection panel  35  placed below the light guide plate  32 . The light source  31  may be a point light sources such as a light emitting diode (LED) or a linear light source such as a cold cathode fluorescent lamp (CCFL). The light guide plate  32  can be formed of a plastic with a high light transmittance such as polymethyl methacrylate (PMMA).  
         [0035]     Light is emitted from the light source  31  at a predetermined angle, is incident on a side of the light guide plate  32  and proceeds inside the light guide plate  32 . Since the light guide plate  32  has a high refractive index and a high light-transmittance, portions of the light which are incident at angles on the upper and lower surfaces of the light guide plate  32  are totally reflected and proceed to the other side of the light guide plate  32 . A portion of the light which is incident on a side of the light guide plate  32  almost perpendicularly proceeds directly to the other side of the light guide plate  32 .  
         [0036]     A hologram pattern is formed on the upper surface of the conventional light guide plate in  FIG. 2A , and a portion of the light incident on the upper surface of the light guide plate is diffracted by the hologram pattern and emitted almost perpendicularly to the upper surface of the light guide plate. In the present exemplary embodiment, as shown in  FIG. 4A , the wire-grid polarizer  33  are formed on the upper surface of the light guide plate  32 , instead of a hologram pattern. The wire-grid polarizer  33  is parallel metal wires placed side by side regular intervals, and reflect s-polarized light which is parallel to the metal wires and transmit p-polarized light perpendicular to the metal wires.  
         [0037]      FIGS. 5A through 5C  are plain views illustrating various arrangements of the wire-grid polarizer  33  of the backlight unit  30  according to exemplary embodiments of the present invention. Referring to  FIGS. 5A through 5C , the metal wires forming the wire-grid polarizer  33  may be placed perpendicularly, horizontally, or sloped at a predetermined angle to the light path, according to the polarization direction of light transmitted by the rear polarization panel of the LCD apparatus. As described above, with predetermined interval between the metal wires, the wire-grid polarizer  33  can have the characteristics of both a diffraction lattice and a polarizer at the same time, depending on the wavelength of the light. For example, when the interval of the metal wires is greater than 200 nm, the wire-grid polarizer  33  may have the diffraction characteristics for visible light.  
         [0038]     Accordingly, when the period of the metal wires is greater than 200 nm, as shown in  FIG. 4A , p-polarized light among the light incident on the upper surface of the light guide plate  32  is diffracted and transmitted by the wire-grid polarizer  33 , and can be emitted almost vertically. The s-polarized light, however, is reflected by the wire-grid polarizer  33  and proceeds to the lower surface of the light guide plate  32  to be emitted through the lower surface of the light guide plate  32 . The light emitted to the lower surface of the light guide plate  32  passes through the polarization converter  34  and is reflected by the lower reflection panel  35 , and then passes through the polarization converter  34  to be incident again on the lower surface of the light guide plate  32 .  
         [0039]     At this time, s-polarized light is changed into p-polarized light by the polarization converter  34 . The polarization converter  34  may be a ¼ wave plate, for example. A ¼ wave plate is an optical device which delays the phase of light by one fourth of the wavelength of the light. The s-polarized light passes through the ¼ wave plate twice, resulting in the phase of the s-polarized light being delayed by one half of the wavelength and changing into p-polarized light. Thus, the light which is reflected by the lower reflection panel  35  and incident again on the lower surface of the light guide plate  32  is changed into p-polarized light. The light which is changed into p-polarized light and incident again on the lower surface of the light guide plate  32  is then incident on the upper surface of the light guide plate  32  and passes through the wire-grid polarizer  33  to be emitted.  
         [0040]     Consequently, in the present exemplary embodiment, most of the light emitted through the upper surface of the light guide plate  32  is p-polarized. For example, the backlight unit  30  provides light which is highly polarized such that the amount of p-polarized light is 150 to 1000 times as much as that of s-polarized light.  
         [0041]     Moreover, in the backlight unit  30 , p-polarized light is emitted and s-polarized light is reflected, and then the reflected s-polarized light is changed into a p-polarized light. Thus, light emitted from the light source  31  can be emitted to the outside as particularly polarized without light absorption or loss. Accordingly, brightness is improved in the backlight unit  30  of the present exemplary embodiment.  
         [0042]     Among the light incident through a lateral side of the light guide plate  32  and proceeding inside the light guide plate, a portion of the light which is not emitted through the upper surface of the light guide plate  32  is finally emitted to the other lateral side of the light guide plate  32 . The light emitted to the other lateral of the light guide plate  32  can be recycled since a lateral reflection panel  36  is installed at the other lateral side of the light guide plate  32 . Referring to  4 A, when the lateral reflection panel  36  is sloped slightly, the reflected light is totally reflected onto the lower and upper surfaces of the light guide plate  32  and proceeds inside the light guide plate  32  and most of the light can be emitted through the upper surface of the light guide plate  32 .  
         [0043]     In the backlight unit shown in  FIG. 4A , the wire-grid polarizer  33  are formed on the upper surface of the light guide plate  32  so that p-polarized light can be emitted. Alternatively, referring to  FIG. 4B , the wire-grid polarizer  33  can be formed on the lower surface of the light guide plate  32 . Referring to  FIG. 4B , in the backlight unit  30  of the present embodiment, the light source  31  is installed at a lateral of the light guide plate  32 , and the wire-grid polarizer  33  is formed on the lower surface of the light guide plate  32 , and the polarization converter  34  and the lower reflection panel  35  respectively face the lower surface of the light guide plate  32 . The only difference between the backlight units of  FIGS. 4A and 4B  is that the backlight unit  30  in  FIG. 4B  includes the wire-grid polarizer  33  formed on the lower surface of the light guide plate  32 .  
         [0044]     In this configuration, a portion of the light emitted from the light source  31  and proceeding inside the light guide plate  32  is incident on the lower surface of the light guide pate  32 . Among the light incident on the lower surface of the light guide plate  32 , s-polarized light is reflected by the wire-grid polarizer  33  and emitted almost vertically through the upper surface of the light guide plate  32 . P-polarized light is diffracted and transmitted almost vertically by the wire-grid polarizer  33  through the lower surface of the light guide plate  32 . The p-polarized light emitted through the lower surface of the light guide plate  32  passes through the polarization converter  34  and is reflected by the lower reflection surface panel  35 , and then passes through the polarization converter  34  and is incident on the lower surface of the light guide plate  32 . At this time, the p-polarized light is changed into s-polarized light. When the wire-gird polarizer  33  has only the characteristics of a polarizer, the s-polarized light is reflected again by the wire-grid polarizer  33 . When the period of the metal wires of the wire-grid polarizer is about 420 nm, s-polarized light can also partially pass through the wire-grid polarizer  33  and be emitted to the upper surface of the light guide plate  32 . The rest of the s-polarized light is reflected by the wire-grid polarizer  33 , passes through the polarization converter  34 , is reflected by the lower reflection panel  35 , passes through the polarization converter  34  again, and is incident on the lower surface of the light guide plate  32 . At this time, the s-polarized light is changed into p-polarized light, passes through the wire-grid polarizer  33  and is emitted through the upper surface of the light guide plate  32 .  
         [0045]     Accordingly, in the backlight unit shown in  FIG. 4B , not only s-polarized light but p-polarized light can also be emitted in part. Since a portion of the p-polarized light, which is transmitted by the wire-grid polarizer  33 , is emitted through the upper surface of the light guide plate  32 , and the rest is changed into s-polarized and emitted, the ratio of the s-polarized light to the p-polarized light can be maintained high.  
         [0046]      FIG. 6  is a cross-sectional view of a backlight unit  40  using wire-grid polarizer  43  according to another exemplary embodiment of the present invention. Referring to  FIG. 6 , the backlight unit  40  in the present exemplary embodiment includes a light source  41  placed at a lateral side of a light guide plate  42  which has a cross-section shaped like a right-angled triangle, the wire-grid polarizer  43  formed on the upper surface of the light guide plate  42 , and a polarization converter  44  and a lower reflection panel  45  facing the lower surface of the light guide plate  42 . The lower surface of the light guide plate  42  is sloped relative to the upper surface of the light guide plate  42 . The cross-section of the light guide plate  42  is shaped like a right-angled triangle or a wedge in which the thickness decreases in the light proceeding direction. Also, in the present exemplary embodiment, the interval between metal wires of the wire-grid polarizer  43  may be less than  200  nm so that the wire-grid polarizer  43  acts only like a polarizer, not a diffraction lattice.  
         [0047]     In such a configuration, the light incident on the lateral side of the light guide plate  42  is reflected at the upper and lower surfaces of the light guide plate  42 . In the present exemplary embodiment, since the wire-grid polarizer  43  does not act like a diffraction lattice, when the light is incident on the upper surface of the light guide plate  42  at an angle greater than the critical angle, both s-polarized light and p-polarized light are totally reflected. Also, since the lower surface of the light guide plate  42  is sloped relative to the upper surface of the light guide plate  42 , the incident angle of the light on the upper surface of the light guide plate  42  gradually decreases. When the incident angle on the upper surface of the light guide plate  42  is smaller than a critical angle, p-polarized light is transmitted by the wire-grid polarizer  43  and emitted. The s-polarized light, on the other hand, is reflected by the wire-grid polarizer  43  to the lower surface of the light guide plate  42 .  
         [0048]     The s-polarized light reflected to the lower surface of the light guide plate  42  is transmitted by the polarization converter  44 , is reflected by the lower reflection panel  45 , and passes through the polarization converter  44  again, and is incident on the lower surface of the light guide plate  42 . At this time, s-polarized light is changed into p-polarized light. Accordingly, the light passes through the wire-grid polarizer  43  and is emitted.  
         [0049]     In the present exemplary embodiment, since the metal wires has an interval less than about  200  nm, the wire-grid polarizer  43  does not act like a diffraction lattice and thus the ratio of p-polarized light to s-polarized light is high among the light emitted from the upper surface of the light guide plate  42 .  
         [0050]     To emit the light vertically to the upper surface of the light guide plate and at the same time increase the ratio of p-polarized light to s-polarized light, the period of the metal wires may be less than 200 nm, and a micro-structured diffraction pattern can be formed on the lower surface of the light guide plate.  FIGS. 7A through 7C  are cross-sectional views of backlight units  30  using a wire-grid polarizer according to other exemplary embodiments of the present invention.  
         [0051]     First, referring to  FIG. 7A , the backlight unit  50  according to the present exemplary embodiment includes a light source  51  at a lateral side of a light guide plate  52  having a micro-structured pattern  57 , a wire-grid polarizer  53  on the upper surface of the light guide plate  52  on a lower surface thereof, and a polarization converter  54  and a lower reflection panel  55  facing the lower surface of the light guide plate  52 . Furthermore, a reflection panel  56  may be further included to recycle the light emitted to the other side of the light guide plate  52 . The period of the metal wires may be less than about 200 nm so that the wire-grid polarizer  53  acts like a polarizer, not a diffraction lattice.  
         [0052]     In such a configuration, light emitted from the light source  51  and incident on the lateral side of the light guide plate  52  proceeds inside the light guide plate  52 . When the light is incident at an angle on the lower surface of the light guide plate  52  as the light proceeds inside the light guide plate  52 , the light is refracted by the micro-structured pattern  57  formed on the lower surface of the light guide plate  52  and is vertically incident on the upper surface of the light guide plate  52 . For example, the micro-structured pattern  57  may be a prism pattern as shown in  FIG. 7A . Among the light vertically incident on the upper surface of the light guide plate  52 , p-polarized light is transmitted by the wire-grid polarizer  53  and is emitted vertically, and s-polarized light is reflected by the wire-grid polarizer  53  to the lower surface of the light guide plate  52 . The s-polarized light reflected to the lower surface of the light guide plate  52  passes through the polarization converter  54 , which may be a ¼ wave length plate, is reflected by the lower reflection panel  55 , passes through the polarization converter  54 , and is incident on the lower surface of the light guide plate  52 . At this time, the s-polarized light is changed into p-polarized light. Accordingly, the p-polarized light is transmitted by the wire-grid polarizer  53  and is emitted vertically.  
         [0053]     Thus, the backlight unit  50  shown in  FIG. 7A  emits light vertically through the upper surface of the light guide plate  52  and at the same time increases the ratio of the p-polarized light to the s-polarized light among the emitted light, thus providing highly polarized light.  
         [0054]      FIGS. 7B and 7C  illustrate variations of the backlight unit  50  in  FIG. 7A . In the backlight unit  50  in  FIG. 7B , the polarization converter  54  is not between the lower surface of the light guide plate  52  and the lower reflection panel  55 , but placed between the other lateral side of the light guide plate  52  and the lateral reflection panel  56 . Accordingly, in the backlight unit in  FIG. 7B , s-polarized light which is not emitted through the upper surface of the light guide plate  52  but to the other lateral side of the light guide plate  52  is changed into p-polarized light and enters on the other lateral side of the light guide plate  52 . The p-polarized light proceeds inside the light guide plate  52  in the opposite direction and is emitted vertically to the upper surface of the light guide plate  52 .  
         [0055]     In the backlight unit  50  in  FIG. 7C , the polarization converter  54  and the lateral reflection panel  56  in  FIG. 7B  are replaced with a diffuse reflector  58 . The diffuse reflector  58  diffuses and reflects incident light in various directions, and its polarization changes into various directions as well. Accordingly, s-polarized light which is emitted from the light source  51  and incident on a lateral side of the light guide plate  52  and not emitted through the upper surface of the light guide plate  52  but emitted through the other lateral side of the light guide plate  52  is diffused at various angles by the diffuse reflector  58  and incident again on the other lateral side of the light guide plate  52 . The polarization of the light which is incident again on the other lateral side of the light guide plate  52  is changed into various directions and the light includes not only s-polarized light but also p-polarized light. Among the light incident again on the other lateral of the light guide plate  52  and proceeding inside the light guide plate  52 , p-polarized light is vertically emitted through the upper surface of the light guide plate  52 .  
         [0056]     The backlight units  50  in  FIGS. 7A through 7C  use a prism pattern as a micro-structured pattern. Alternatively, a hologram can also be used for a diffraction pattern as shown in  FIGS. 8A and 8B . A backlight unit  60  in  FIG. 8A  has the same structure as the backlight unit  50  in  FIG. 7A , except that a hologram diffraction pattern is used as a micro-structured pattern  67  instead of a prism pattern. Accordingly, the backlight unit  60  in  FIG. 8A  operates in the same manner as the backlight unit  50  in  FIG. 7A . Also, although not shown, the backlight unit  60  in  FIG. 8A  may be modified in the same manner as the backlight units  50  shown in  FIGS. 7B and 7C . That is, a polarization converter  64  may not be placed between the lower surface of a light guide plate  62  and a lower reflection panel  65  but between the other lateral side of the light guide plate  62  and a lateral reflection panel  66 . Also, the polarization converter  64  and the lateral reflection panel  66  placed on the other lateral side of the light guide plate  62  can be replaced with a diffuse reflector.  
         [0057]     In the backlight unit in  FIG. 8B , the micro-structured pattern  67  is formed on the upper surface of the light guide plate  62 , not on the lower surface. Since wire-grid polarizer  63  cannot be formed on the curved micro-structured pattern  67 , a transparent coating layer  68  having a planar upper surface is formed on the micro-structured pattern  67 , and the wire-grid polarizer  63  are formed on the coating layer  68 .  
         [0058]     In such a configuration, light emitted from the light source  61  and incident on a lateral side of the light guide plate  62  is diffracted by the hologram diffraction pattern  67  formed on the upper surface of the light guide plate  62  and is vertically emitted through the upper surface of the light guide plate  62 . Then the light passes through the transparent coating layer  68  and is incident on the wire-grid polarizer  63  formed on the upper surface of the coating layer  68 . Among the light incident on the wire-grid polarizer  63 , p-polarized light is transmitted by the wire-grid polarizer  63  vertically. S-polarized light, on the other hand, i,s reflected by the wire-grid polarizer  63  to the lower surface of the light guide plate  62 . The s-polarized light reflected to the lower surface of the light guide plate  62  is transmitted by the polarization converter  64 , is reflected by the lower reflection panel  65 , passes through the polarization converter  64  again, and is incident on the lower surface of the light guide plate  62 . During this process, the s-polarized light is changed into p-polarized light. Accordingly, the p-polarized light can transmit through the wire-grid polarizer  63 .  
         [0059]     The backlight unit according to various embodiments of the present invention can be employed in an LCD apparatus without changing the structure of an LCD panel. That is, a backlight unit of the present invention can be placed on the rear substrate of the LCD panel  10  in  FIG. 1 , thereby providing an LCD apparatus with high brightness in a simple way. The polarization of the light emitted from the backlight unit of the present invention and the polarization of the rear polarization panel  11  of the LCD panel  10  can be the same. As described with reference to  FIGS. 5A through 5C , the polarization of the light emitted from the backlight unit can be adjusted by the orientation of the metal wires of the wire-grid polarizer.  
         [0060]     As described above, the backlight unit according to the present invention emits s-polarized light or p-polarized light, reflects the other polarized light, and changes the polarization of the reflected light. Thus all the light emitted from the light source can have a certain polarization without light absorption or loss to the outside. Accordingly, most of the light emitted from the backlight unit transmitted from the rear polarization panel of the LCD apparatus and the brightness of the LCD apparatus is improved.  
         [0061]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.