Abstract:
A display device includes a liquid crystal display panel that varies a phase of polarized light via liquid crystal molecules, wherein second recessed patterns of a second light guide plate are directly faced with first recessed patterns of a first light guide plate to form a first empty space, and wherein first flat surfaces of the first light guide plate are directly faced with second flat surfaces of the second light guide plate to form a second empty space narrower than the first empty space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Divisional of co-pending U.S. application Ser. No. 12/879,549 filed Sep. 10, 2010, which claims priority to Korean Patent Application No. 10-2009-0087442, filed on Sep. 16, 2009. The contents of all of these applications are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This disclosure relates to a display device, and more particularly to a transparent display device with a liquid crystal display (LCD) panel. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Most recently, transparent display devices are actively being researched which allow not only an image to be displayed thereon but also allows objects at their rear side to be viewed. These transparent display devices can be ordinarily realized employing any one of an organic light emission panel and a plasma display panel which use self-generated light. 
         [0006]    However, the transparent display devices cannot ordinarily be realized employing an LCD panel which does not self generate light and must use light applied from the rear side. This results from the fact that an opaque backlight assembly must be disposed at the rear side of the LCD panel and polarizing plates for controlling the penetration of light must be attached to the front and rear surfaces of the LCD panel. More specifically, the polarizing plates on the front and rear surface of the LCD panel allow light to pass through them when a liquid crystal material within the LCD panel is driven. On the contrary, when the liquid crystal material is not driven, the polarizing plates are in an opaque state. Due to this, it is impossible to implement the transparent display device using the LCD panel. 
         [0007]    Nevertheless, transparent display devices can be applied to the front glasses of vehicles, household glasses, and so on, in order to provide desired information to users. Also, it is evident that the possibility of applying the transparent display device in a variety of fields will rapidly increase. Moreover, since the LCD panel provides a wide viewing-angle, a high brightness, a high contrast ratio, and a full color display, it is very necessary to develop a transparent display device with the LCD panel. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, embodiments of the invention are directed to a transparent display device that substantially obviates one or more of problems due to the limitations and disadvantages of the related art. 
         [0009]    An object of the invention is to provide a transparent display device that is adapted to selectively drive an LCD panel in transparent and opaque modes. 
         [0010]    Additional features and advantages of the embodiments of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the embodiments will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
         [0011]    According to an embodiment of the invention, a display device includes a liquid crystal display panel that varies a phase of polarized light via liquid crystal molecules, and having first and second polarizing plates respectively disposed on opposite sides of the liquid crystal display panel; a first light guide plate including a surface having a plurality of recessed patterns, to reflect light input through a side surface of the first light guide plate, and to output two-dimensional light towards the liquid crystal display panel; and a second light guide plate including a surface having a plurality of recessed patterns, the plurality of recessed patterns of the second guide plate facing the plurality of recessed patterns of the first light guide plate. 
         [0012]    According to another embodiment of the invention, a display device includes a liquid crystal display panel that varies a phase of polarized light via liquid crystal molecules; a first light guide plate including a surface having a plurality of recessed patterns, and a first polarizing plate disposed at an incident surface of the first light plate; a second light guide plate including a surface having a plurality of recessed patterns, the plurality of recessed patterns of the second guide plate facing the plurality of recessed patterns of the first light guide plate; and a second polarizing plate disposed on the liquid crystal display panel. 
         [0013]    According to another embodiment of the invention, a display device includes a liquid crystal display panel that varies a phase of polarized light via liquid crystal molecules; a first light guide plate including a planar first surface, and a second surface having a plurality of recessed patterns; and a second light guide plate including a planar first surface, and a second surface having a plurality of recessed patterns, wherein amounts of light respectively emitted from the first and second surfaces of the first light guide plate are different, and amounts of light respectively emitted from the first and second surfaces of the second light guide plate are different. 
         [0014]    Other systems, methods, features and advantages will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. Nothing in this section should be taken as a limitation on those claims. Further aspects and advantages are discussed below in conjunction with the embodiments. It is to be understood that both the foregoing general description and the following detailed description of the present disclosure are exemplary and explanatory and are intended to provide further explanation of the disclosure as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the disclosure. In the drawings: 
           [0016]      FIG. 1  is a perspective view showing a transparent display device according to a first embodiment of the invention; 
           [0017]      FIGS. 2A and 2B  are cross-sectional views illustrating the transparent display device of  FIG. 1  driven in transparent and opaque modes, respectively; 
           [0018]      FIG. 3  is a waveform diagram sequentially illustrating lights from respective light sources being polarized in the transparent display device of  FIG. 1 ; 
           [0019]      FIGS. 4A and 4B  are views showing the transparent display device of  FIG. 1  driven in the transparent and opaque modes, respectively; 
           [0020]      FIG. 5  is a perspective view showing of a transparent display device according to a second embodiment of the invention; 
           [0021]      FIGS. 6A and 6B  are cross-sectional views illustrating the transparent display device of  FIG. 5  driven in transparent and opaque modes, respectively; 
           [0022]      FIG. 7  is a waveform diagram sequentially illustrating lights from respective light sources being polarized in the transparent display device of  FIG. 5 ; 
           [0023]      FIG. 8  is a cross-sectional view showing a transparent display device according to a third embodiment of the invention; and 
           [0024]      FIG. 9  is a cross-sectional view showing a transparent display device according to a fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings. These embodiments introduced hereinafter are provided as examples in order to convey their spirits to the ordinary skilled person in the art. Therefore, these embodiments may be embodied in a different form, so are not limited to these embodiments described here. Also, the size and thickness of the device might be expressed to be exaggerated for the sake of convenience in the drawings. Wherever possible, the same reference numbers will be used throughout this disclosure including the drawings to refer to the same or like parts. 
         [0026]    Transparent display devices according to the example embodiments of the invention will now be explained in detail with reference to the attached drawings. 
         [0027]      FIG. 1  is a perspective view showing a transparent display device according to a first embodiment of the invention. As shown in  FIG. 1 , the transparent display device according to the first embodiment of the invention includes an LCD panel  10  configured to display images, and first and second light sources  40  and  41  each configured to emit light. The transparent display device further includes first and second light guide plates  20   a  and  20   b  each configured to derive two dimensional light from light which is generated in the respective one of the first and second light sources  40  and  41 , and first and second polarizing plates  11   a  and  11   b  disposed above and under the LCD panel  10  and each configured to polarize light. Both the first and second light guide plates  20   a  and  20  are formed to have a cross-section of a planar shape. 
         [0028]    The LCD panel  10  is configured with a thin film transistor (TFT) array substrate and a color filter substrate which are combined with a liquid crystal layer being between the TFT array and color filter substrates. The TFT array substrate includes a plurality of TFTs, pluralities of gate and data lines configured to cross each other and to define pixel regions, and a plurality of pixel electrodes arranged in the pixel regions. The color filter substrate includes a latticed black matrix used to shield light, and red, green and blue color filter layers formed between the black matrix. 
         [0029]    The first and second light sources  40  and  41  are disposed to face first and second incident surfaces  31  and  32  of the first and second light guide plates  20   a  and  20   b , respectively. Each of the first and second light sources  40  and  41  can include one of a lamp, such as a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), or others, and a light emission diode array. The light emission diode array can include either red, green, and blue light emission diodes or a plurality of white light emission diodes. Alternatively, the light emission diode array can include light emission diode packages each configured with red, green, and blue light emission diodes which are combined in a single package. The light emission diode array may also include a ultra-violet light emission diode. Such first and second light sources  40  and  41  emit visible light which includes first and second polarization components (for example, vertical and horizontal polarization components). 
         [0030]    The first light guide plate  20   a  with the first incident surface  31  facing the first light source  40  is configured to include a plurality of recessed patterns  22  formed in its lower surface (the surface facing the second light guide plate  20   b ). The plurality of recessed patterns  22  enables light entered from the first light source  40  to be totally reflected, refracted, and scattered before converted into two-dimensional light progressing toward the upper surface of the first light guide plate  20   a  facing the LCD panel  10 . In other words, the plurality of recessed patterns  22  functions to derive two-dimensional light from light generated in the first light source  40 . 
         [0031]    Such recessed patterns  22  can be formed by irradiating laser light on the lower surface of the first light guide plate  20   a  and the upper surface of the second light guide plate  20   b  using a separate mask with the same patterns as the recessed patterns. In this instance, the irradiating energy and period of laser light depend upon the specifications of a transparent display device, for example, the size and kind of the LCD panel  10 . Alternatively, the recessed patterns  22  can be formed through any one among an etching process, a printing process, a prominence depressing process, and others. Also, the transparency and brightness of the transparent display device are varied along the size of the recessed pattern  22  and the distance between the recessed patterns  22 . As such, in order to provide desired transparency and brightness, it is necessary to adjust the size of the recessed pattern  22  and the distance between the recessed patterns  22 . In embodiments of the invention, the recessed patterns  22  may have various shapes, such as hemispherical, columnar, cone, and so on. Further, a mix of different shaped recessed patterns may be formed on the relevant surfaces of the first and second light guide plates  20   a  and  20   b , for example. 
         [0032]    Similarly, the second light guide plate  20   b  with the second incident surface  32  facing the second light source  41  is configured to include a plurality of recessed patterns  22  formed in its upper surface (the surface facing the first light guide plate  20   a ). The plurality of recessed patterns  22  enables light entered from the second light source  41  to be totally reflected, refracted, and scattered before being converted into two-dimensional light that progresses toward the lower surface of the second light guide plate  20   b , i.e., in an opposite direction to the LCD panel  10 . In other words, the plurality of recessed patterns  22  functions to derive two-dimensional light from light generated in the second light source  41 . 
         [0033]    However, each of the first and second light guide plates  20   a  and  20   b  allows about 30% of light to be output toward the surface where the recessed patterns  22  are formed. More specifically, about 70% of the converted two-dimensional light is output toward the surface not having the recessed patterns  22 , and about 30% of the converted two-dimensional light is output toward the surface having the recessed patterns  22 . The two-dimensional light output from each of the first and second light guide plates  20   a  and  20   b  is non-polarized light. 
         [0034]    Although the plurality of recessed patterns  22  is formed in the upper surface of the second light guide plate  20   b , the recessed patterns  22  can be formed in the lower surface of the second light guide plate  20   b  like those of the first light guide plate  20   a . As shown in the embodiment of  FIG. 1 , the first and second light guide plates  20   a  and  20   b  can be disposed to allow the surfaces each provided with the recessed patterns  22  to face each other. 
         [0035]    The transparent display device according to the invention can realize a transparent display, which projects objects at its rear side, using only the first light guide plate  20   a , because of the lack of a reflective sheet. On the other hand, the transparent display device of the invention further using two-dimensional light derived by the second light guide plate  20   b  can realize a transparent display with an improved definition (a higher definition that that of the former instance). 
         [0036]    More specifically, If light enters the second light guide plate  20   b , 70% of two-dimensional light progresses in a downward direction of the second light guide plate  20   b , but 30% of two-dimensional light progresses in a upward direction of the second light guide plate  20   b . The downwardly progressing two-dimensional light is irradiated to objects which are positioned under the second light guide plate  20   b . The upwardly progressing two-dimensional light is input to the LCD panel  10  via the upper surface of the second light guide plate  20   b  with the recessed patterns  22  in such manner as to have a phase difference with respect to two-dimensional light which is converted by the first light guide plate  20   a . As such, the objects at the rear of the transparent display device can be more definitively viewed to a user. 
         [0037]    In this transparent mode, two-dimensional light derived by the first light guide plate  20   a  is used for displaying an image on the LCD panel  10 , two-dimensional light derived by the second light guide plate  20   b  is used for distorting a white light component of the image displayed on the LCD panel  10 . Due to this, light reflected by the objects at the rear of the second light guide plate  20   b  is projected to the eyes of a user through the LCD panel  10 . If an image including only black color components is displayed on the LCD panel  10 , all of lights applied from the rear of the LCD panel  10  are thoroughly shielded by the first and second polarizing plates  11   a  and  11   b . On the contrary, when an image with the brightness of above a fixed degree (or a fixed gray level) is displayed on the LCD panel  10 , two-dimensional light derived by the first light guide plate  20   a  is transmitted through the LCD panel  10 . Then, white light or high brightness light of the image is distorted by light derived by the second light guide plate  20   b . As such, light reflected by the objects at the rear of the LCD panel  10  is transmitted through the LCD panel  10  and the first and second polarizing plates  11   a  and  11   b.    
         [0038]    In accordance therewith, the second light guide plate  20   b  can be used to control the LCD panel  10  to be driven in either a transparent mode (a first mode) or an opaque mode (a second mode). In embodiments of the invention, reference to a transparent mode indicates that light is separately provided to the second light guide plate  20   b , while reference to an opaque mode indicates that light is not separately provided to the second light guide plate  20   a . In embodiments of the invention, when the light is separately provided to the second light guide plate  20   b , an observer looking at the LCD panel  10  is better able to see a scene or an object that is positioned at or beyond an opposite side of the LCD panel  10  with respect to the observer. Accordingly, there is an improvement in brightness, clarity, etc, of the scene or object relative to when the light is not separately provided to the second light guide plate  20   b.    
         [0039]    In detail, almost all incident light to the second light guide plate  20   b  is output in the form of two-dimensional light which progresses in the lower surface direction of the second light guide plate  20   b , when the second light source  41  is turned-on (or activated). The downward progressing two-dimensional light is irradiated to objects positioned at the rear of the display device. On the other hand, the rest of the incident light corresponding to 30% is output in the form of two-dimensional light which progresses toward the upper surface of the second light guide plate  20   b  provided with the recessed patterns  22 . Two-dimensional lights progressing from the first and second light guide plates  20   a  and  20   b  toward the LCD panel  10  have a fixed phase difference from each other due to a difference between their traveling distances. Also, two-dimensional light derived by the second light guide plate  20   b  distorts the white light components of an image while it passes through the second polarizing plate  11   b , LCD panel  10  and first polarizing plate  11   a . To this end, the light reflected by objects at the rear of the second light guide plate  20   b  can be transmitted through the second polarizing plate  11   b , LCD panel  10  and first polarizing plate  11   a . Therefore, the objects positioned at the rear of the second light guide plate  20   b  can be projected while images are displayed on the LCD panel  10 . In other words, the LCD panel  10  is driven in the transparent mode. The second light source  41  is preferably configured to emit 1.3˜1.5 times brighter light than that of the first light source  40 . 
         [0040]    On the contrary, when the second light source  41  is turned-off (or non-activated), the second light guide plate  20   b  can not provide independent light including the first and second polarization components. As such, the LCD panel  10  is driven in the opaque mode. In other words, the turned-off second light source  41  forces the LCD panel  10  to enter the opaque mode, so that an image is displayed on the LCD panel  10  by only two-dimensional light derived in the first light guide plate  40 . 
         [0041]      FIGS. 2A and 2B  are cross-sectional views illustrating the transparent display device of  FIG. 1  driven in transparent and opaque modes, respectively.  FIG. 3  is a waveform diagram sequentially illustrating lights from light sources being polarized in the transparent display device of  FIG. 1 . As shown in  FIGS. 2A and 2B , the transparent display device of the first embodiment allows the LCD panel  10  to be driven in either a transparent display mode or an opaque display mode. 
         [0042]    First, if the transparent display device of the first embodiment is driven in the transparent mode, the first and second light sources  40  and  41  disposed to face the incident surfaces of the first and second light guide plates  20   a  and  20   b  are in a turned-on state, as shown in  FIG. 2A . The operation of the transparent display device in the transparent mode will be explained in detail with reference to  FIGS. 2A and 3 . 
         [0043]    While the first light source  40  disposed to face the incident surface of the first light guide plate  20   a  is in the turned-on state, light progresses from the first light source  40  to the inner side of the first light guide plate  20   a  through the incident surface of the first light guide plate  20   a . At this time, the light traveling within the first light guide plate  20   a  includes the first and second polarization components and is non-polarized light. The light with the first and second polarization components is entirely (totally) reflected (more specifically, are reflected, refracted, and scattered) by the recessed patterns  22  formed at the lower surface of the first light guide plate  20   a  and converted into two-dimensional light which progresses toward the LCD panel  10  disposed above the first light guide plate  20   a.    
         [0044]    Similarly, light entered from the turned-on second light source  41  into the second light guide plate  20   b  is entirely (or totally) reflected by the recessed patterns  22 , which are formed at the upper surface of the second light guide plate  20   b , and output in the form of two-dimensional light progressing toward the downward direction of the second light guide plate  20   b . At this time, about ⅓ of the two-dimensional light converted by the recessed patterns  22  of the second light guide plate  20   b  is output toward the first light guide plate  20   a . Faint two-dimensional-light output from the second light guide plate  20   b  is mixed with two-dimensional light derived in the first light guide plate  20   a  in a space between the second polarizing plate  11   b  and the first light guide plate  20   a . Two-dimensional lights derived by the first and second light guide plates  20   a  and  20   b  have a fixed phase difference from each other due to a difference between their traveling paths (distances). 
         [0045]    The mixed two-dimensional lights each having the first and second polarization components are polarized in the second polarizing plate  111   b  and then are transmitted through the LCD panel  10  and the first polarizing plate  11   a . The transmitted two-dimensional light is used to form white image components of an image displayed on the LCD panel  10 . This white light is basically configured with the two-dimensional light converted by the first light guide plate  20   a . However, the white light further includes two-dimensional light converted by the second light guide plate  20   b  in the transparent mode. At this time, the white light is distorted by two-dimensional light derived in the second light guide plate  20   b . The distortion of the white image components displayed on the LCD panel  10  allow non-polarized light components, which are reflected by the objects at the rear of the LCD panel  10 , to be transmitted through the first and second polarizing plates  11   a  and  11   b  and LCD panel  10 . Accordingly, the objects positioned at the rear of the LCD panel  10  can be viewed to the eyes of a user. 
         [0046]    In this manner, two-dimensional light converted by the second light guide plate  20   b  is transferred to users through the LCD panel  10 . As such, the objects positioned at the rear of the transparent display device can be viewed to the users. Two-dimensional lights progressing upwardly and downwardly from the second light guide plate  20   b  enable the objects at the rear of the transparent display device to be more definitively viewed to the users. In other words, the LCD panel  10  not only displays an image using two-dimensional light derived by the first light guide plate  20   a  under it, but also projects the objects positioned at the rear of the second light guide plate  20   b  using two-dimensional light derived by the second light guide plate  20   b.    
         [0047]    Secondly, when the transparent display device of the first embodiment is driven in the opaque mode, the second light source  41  disposed to face the incident surface of the second light guide plate  20   b  is in a turned-off state, as shown in  FIG. 2B . Therefore, the LCD panel  10  can display an image using only two-dimensional light derived by the first light source  40  and the first light guide plate  20   a . In this opaque mode, the transparent display device is driven in the same manner as the image display operation of an ordinary LCD device 
         [0048]    When the LCD panel  10  is driven in the transparent mode, the transparent display device has a light variation as shown in  FIG. 3 . As both the first and second light sources  40  and  41  are turned-on in the transparent mode, the first and second light sources  40  and  41  emit non-polarized light including the first and second polarization components. As such, two-dimensional light output from the upper surface of the first light guide plate  20   a  includes two kinds of non-polarized lights. However, the non-polarized two-dimensional lights have a fixed phase difference from each other because they are disposed at different positions from each other. 
         [0049]    Non-polarized two-dimensional lights converted by the first light guide plates  20   a  and  20   b  are polarized and mixed with each other while passing through the second polarizing plate  11   b . Also, the polarized and mixed two-dimensional lights interfere with each other. Such polarized and mixed two-dimensional lights enable white image components displayed on the LCD panel  10  to be distorted when they pass through the LCD panel  10  and first polarizing plate  11   a . In accordance therewith, light reflected by the objects positioned at the rear of the second light guide plate  20   b  can be projected. In order to project more definitively the objects positioned at the rear of the display device, the second light source  41  included in the first embodiment is designed to emit 1.3˜1.5 times brighter light than that of the first light source  40 . 
         [0050]      FIGS. 4A and 4B  are views showing the transparent display device of  FIG. 1  driven in the transparent and opaque modes, respectively. 
         [0051]    Both the first and second light sources  40  and  41  are in the turned-on state in the transparent mode. In this instance, the objects positioned at the rear of the display device is viewed to users by light progressing from the second light guide plate  20   b  toward the LCD panel  10 , as shown in  FIG. 4A . 
         [0052]    On the contrary, the second light source  41  is in the turned-off state and the first light source  40  is in the turned-on state, in the opaque mode. As such, two-dimensional light is not derived by the second light guide plate  20   b . Therefore, the LCD panel  10  displays an image using only two-dimensional light converted in the first light guide plate  20   a . In other words, the display device can display only an image without or weakly projecting the objects positioned at its rear side, as shown in  FIG. 4B . As a result, the objects positioned at the rear of the display device are not viewed or less clearly viewed to the eyes of a user. More specifically, because the second light guide plate  20   b  is disposed under the first light guide plate  20   a , the objects positioned at the rear of the display device are thoroughly or partially shielded in the opaque mode. 
         [0053]      FIG. 5  is a perspective view showing a transparent display device according to a second embodiment of the invention. 
         [0054]    Referring to  FIG. 5 , the transparent display device according to the second embodiment of the invention includes an LCD panel  110  configured to display images, and first and second light sources  140  and  141  each configured to emit light. The transparent display device further includes first and second light guide plates  120   a  and  120   b  each configured to derive two dimensional light from light which is generated in the respective one of the first and second light sources  140  and  141 , a first polarizing plate  1111   a  disposed on an incident surface  31  (first incident surface) of the first light guide plate  120   a  and configured to polarize light, and a second polarizing plate  111   b  disposed above the LCD panel  110  and configured to polarize light. The reference number of “ 32 ” indicates an incident surface (second incident surface) of the second light guide plate  120   b.    
         [0055]    The LCD panel  110  is configured with a TFT array substrate and a color filter substrate which are combined with a liquid crystal layer being between the TFT array and color filter substrates. The TFT array substrate includes a plurality of TFTs, pluralities of gate and data lines configured to cross each other and to define pixel regions, and a plurality of pixel electrodes arranged in the pixel regions. The color filter substrate includes a black matrix used to shield light, and red, green and blue color filter layers formed between the black matrix. 
         [0056]    The first light source  140  is disposed to face the first polarizing plate  111   a  on the first incident surface  31  of the first light guide plate  120   a . The second light source  141  is disposed to face the second incident surface  32  of the second light guide plate  120   b . The first and second light sources  140  and  141  may be configured in the same manner as those described in the first embodiment of  FIG. 1 . 
         [0057]    The first and second light guide plates  120   a  and  120   b  and the recessed patterns  122  are formed in the same structures as those described in the first embodiment. Also, the first and second light guide plates  120   a  and  120   b  and the recessed patterns  122  have the same functions as those described in the first embodiment. Therefore, the detailed description of the first and second light guide plates  122   a  and  122   b  and recessed patterns  122  will be omitted. 
         [0058]      FIGS. 6A and 6B  are cross-sectional views illustrating the transparent display device of  FIG. 5  driven in transparent and opaque modes, respectively.  FIG. 7  is a waveform diagram sequentially illustrating lights from light sources being polarized in the transparent display device of  FIG. 5 . As shown in  FIGS. 6A and 6B , the transparent display device of the second embodiment allows the LCD panel  110  to display an image in transparent and opaque modes. 
         [0059]    First, if the transparent display device of the second embodiment is driven in the transparent mode, the second light sources  141  disposed to face the incident surface  32  of the second light guide plates  120   b  is in a turned-on state. Also, the first light source  140  disposed to face the first polarizing plate  111   a  on the incident surface  31  of the first light guide plate  120   a  is in the turned-on state. The operation of the transparent display device in the transparent mode will be explained in detail with reference to  FIGS. 6A and 7 . 
         [0060]    As the first light source  140  disposed to face the first polarizing plate  111   a  on the incident surface  31  of the first light guide plate  120   a  is in the turned-on state, light including the first and second polarization components is emitted from the first light source  140  and polarized by the first polarizing plate  111   a . The light emitted in the first light source  140  includes both the first and second polarization components, but the polarized light output from the first polarizing plate  111   a  includes only the first polarization component. 
         [0061]    At the same time, light entered from the turned-on second light source  141  into the second light guide plate  120   b  is output in the form of non-polarized two-dimensional light which progresses in the downward direction of the second light guide plate  120   b  and toward the first light guide plate  120   a . In this instance, the amount of two-dimensional light progressing toward the first light guide plate  120   a  corresponds to about ⅓ (i.e., 30%) of that of two dimensional light progressing in the downward direction of the second light guide plate  120   b . In this instance, it is preferable, though not required, for the second light source  141  to emit light of the same brightness as light emitted from the first light source  140 . 
         [0062]    As such, two-dimensional light passing through the upper surface of the first light guide plate  120   a  includes two-dimensional light with only the first polarization component polarized by the first polarizing plate  111   a  and two-dimensional light with both the first and second polarization components converted in the second light guide plate  120   b . Among two-dimensional lights, the polarized two-dimensional light converted by the first light guide plate  120   a  is used to display an image on the LCD panel  110 . On the other hand, the non-polarized two-dimensional light of the first and second polarization components, which is converted by the second light guide plate  120   b , is used to project the object positioned at the rear of the display device by interfering with the polarized two-dimensional light, which is converted by the first light guide plate  120   a . Consequently, because the polarized and non-polarized two-dimensional lights interfere with each other when they are mixed with each other, it can be substantially realized the transparent mode which enables not only an image to be displayed on the LCD panel  110  but also objects positioned at the rear side of the display device to be projected. 
         [0063]    When the transparent display device of the second embodiment is driven in the opaque mode, the second light source  141  is in the turned-off state. As such, the LCD panel  110  can display an image using only two-dimensional light of first polarization component which is derived by the first light source  140  and the first light guide plate  120   a . Therefore, the objects positioned at the rear of the second light guide plate  120   b  are not displayed to users. In this opaque mode, the transparent display device is driven in the same manner as the image display operation of an ordinary LCD device. 
         [0064]      FIG. 8  is a cross-sectional view showing a transparent display device according to a third embodiment of the invention, and  FIG. 9  is a cross-sectional view showing a transparent display device according to a fourth embodiment of the invention. 
         [0065]    The transparent display device of the third embodiment shown in  FIG. 8  has a structure modified from that of the first embodiment. The transparent display device of the third embodiment performs the same operation as that of the first embodiment in the transparent and opaque modes. Also, the transparent display device of the fourth embodiment shown in  FIG. 9  has a structure modified from that of the second embodiment. The transparent display device of the fourth embodiment performs the same operation as that of the second embodiment in the transparent and opaque modes. Therefore, the descriptions of the third and fourth embodiments overlapping those of the first and second embodiments will be omitted. 
         [0066]    Referring to  FIG. 8 , the transparent display device includes first and second polarizing plates  11   a  and  11   b  that are disposed to face an LCD panel  10  disposed in between the two polarizing plates  11   a  and  11   b . The transparent display device further includes a first light guide plate  220   a  disposed under the second polarizing plate  11   b  and formed to have a cross-section of a wedge shape and a surface with recessed patterns  222 , and a second light guide plate  220   b  formed to have a cross-section of the wedge shape and a surface with the recessed patterns  222  which faces the recessed pattern surface of the first light guide  220   a . Furthermore, the transparent display device includes a first light source  40  disposed to face an incident surface (first incident surface)  231  of the first light guide plate  220   a , and a second light source  41  disposed to face an incident surface (second incident surface)  232  of the second light guide plate  220   b.    
         [0067]    The first light guide plate  220   a  is configured to have an edge portion that includes the first incident surface  231  to have the thickest thickness. In detail, the first light guide plate  220   a  is formed to progressively have a thinner thickness further away from the first incident surface  231 . Similarly, the second light guide plate  220   b  is configured to have an edge portion that includes the second incident surface  232  to have the thickest thickness. In other words, the second light guide plate  220   b  is formed to progressively have a thinner thickness further away from the second incident surface  232 . 
         [0068]    The pluralities of recessed patterns  222  are formed in a lower surface of the first light guide plate  220   a  and an upper surface of the second light guide plate  220   b , respectively. The recessed patterns  222  formed in the first light guide plate  220   a  are arranged to face the recessed patterns  222  formed in the second light guide plate  220   b.    
         [0069]    Also, the first incident surface  231  of the first light guide plate  220   a  and the second incident surface  232  of the second light guide plate  220   b  are disposed at opposite directions to each other. In other words, the first and second incident surfaces  231  and  232  are disposed to face each other with respect to a middle portion of the bodies of the light guide plates  220   a  and  220   b.    
         [0070]    As such, the first and second light sources  40  and  41  are disposed to face each other in the center of the first and second light guide plates  220   a  and  220   b . The second light source  41  is configured to emit 1.3˜1.5 times brighter light than that emitted from the first light source  40 , like the second light source  41  included in the transparent display device according to the first embodiment. 
         [0071]    In this manner, the first and second light guide plates  220   a  and  220   b  are formed in the wedge shape. As such, the thickest portion of the first light guide plate  220   a  can be positioned to overlap the thinnest portion of the second light guide plate  220   b  and vice versa. Therefore, a slimed transparent display device can be implemented. 
         [0072]    As shown in  FIG. 9 , the transparent display device of the fourth embodiment modified from that of the second embodiment, and includes an LCD panel  110 , and first and second light guide plates  220   a  and  220   b  disposed under the LCD panel  110 . The transparent display device further includes a first polarizing plate  111   a  attached to a first incident surface of the first light guide plate, and a second polarizing plate  111   b  disposed above the LCD panel  110 . 
         [0073]    The first and second light guide plate  220   a  and  220   b  are formed in the cross-sectional structures of a wedge shape. In detail, the thicknesses of the first and second light guide plates  220   a  and  220   b  progressively become thinner further away from the respective incident surfaces  231  and  232 . Also, the first and second incident surfaces  231  and  232  of the first and second light guide plates  220   a  and  220   b  are disposed to face each other with respect to a middle portion of the bodies of the first and second light guide plates  220   a  and  220   b , like the first and second incident surfaces  231  and  232  described in the transparent display of  FIG. 8 . 
         [0074]    Furthermore, the transparent display device of the fourth embodiment includes first and second light sources  140  and  141  disposed to face the first and second incident surfaces of the first and second light guide plates  220   a  and  220   b . The first and second light sources are disposed on the opposite sides to each other with respect to a middle portion of the light guide lights  220   a  and  220   b , respectively. 
         [0075]    Such a transparent display device according to the fourth embodiment is driven in the same manner as that of the second embodiment in the transparent and opaque modes. Therefore, the operations of the transparent display device according to the fourth embodiment will be omitted. 
         [0076]    As described above, the transparent display devices according to the invention enable objects positioned at their rear sides to be selectively viewed or not viewed by users while images are displayed. As such, the transparent display devices according to the invention are implemented using an LCD panel which is normally not realized as a transparent display. Therefore, the applicable field of the LCD panel can be expanded. Furthermore, a variety of content images can be displayed using the LCD panel. 
         [0077]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.