STEREOSCOPIC IMAGE DISPLAY APPARATUS Technical Field [1] The present invention relates to a stereoscopic image display apparatus, and more particularly, to a stereoscopic image display apparatus capable of reducing a viewing distance by using a front type parallax barrier. Background Art [2] In general, a stereoscopic image display apparatus is classified into a stereoscopic image display apparatus using a polarized goggle and a stereoscopic image display apparatus not using a polarized goggle. The stereoscopic image display apparatus using the polarized-goggle has problems in that it is inconvenient to wear the polarized goggle. In addition, wearing the polarized goggle for a long time, for example, one or more hours, may cause eye-associated diseases. Therefore, the stereoscopic image display apparatus using the polarized-goggle has not been widely used. The stereoscopic image display apparatus not using the polarized-goggle is classified into a lenticular lens type, a hologram type, and a parallax barrier type. The lenticular lens and hologram types of stereoscopic image display apparatuses have highly complicated structures and high production costs, so that these stereoscopic image display ap¬ paratuses may be used for special purposes. At recent, the actively researched and developed one is the parallax barrier type of stereoscopic image display apparatus. [3] Although it was proposed at the beginning of the 20's century, the parallax barrier stereoscopic image display apparatus has been developed and used at recent. The parallax barrier stereoscopic image display apparatus have been actively researched and developed since LCD, PDP, and organic EL display apparatuses were invented. As a result, rear type parallax barrier stereoscopic image display apparatuses are com¬ mercially provided. The rear type parallax barrier stereoscopic image display apparatus denotes a stereoscopic image display apparatus of which a parallax barrier is disposed behind an image display panel. [4] The conventional rear type parallax barrier stereoscopic image display apparatuses have problems of low brightness, complicated production processes, and high production cost. The conventional rear type parallax barrier stereoscopic image display apparatuses and problems thereof are described in detail in PCT/KR03/01415 and PCT/KR03/01537 which are Patent Applications filed by the inventor of the present invention. [5] In order to solve the problems of the conventional rear type parallax barrier stereoscopic image display apparatus, front type parallax barrier stereoscopic image display apparatuses have been researched and developed. However, the front type parallax barrier stereoscopic image display apparatuses have a critical problem in that a viewing distance (a distance where a stereoscopic image can be seen with obverse's eyes) is too long to see the stereoscopic image. Although theoretically having high brightness and simple production process, the front type parallax barrier stereoscopic image display apparatuses cannot be commercially provided due to the problems. However, the inventor of the present invention invented a front type parallax barrier stereoscopic image display apparatus which are disclosed in the aforementioned Patent Applications. In other words, any practical front type stereoscopic image display appa ratus was not provided before the inventor invented the front type parallax barrier stereoscopic image display apparatus disclosed in the aforementioned Patent Ap¬ plications. Hereinafter, for the convenience of description, a conventional front type stereoscopic image display apparatus is an apparatus having some problems which the inventor tried to solve. Disclosure of Invention Technical Problem [6] Now, problems of the conventional front type parallax image display apparatus will be described. [7] FlG. 1 is a schematic cross sectional view showing a general two-dimensional image liquid crystal display panel. [8] The two-dimensional image liquid crystal display panel 10 includes a first substrate 12 having a rear polarizing layer 11 stacked on a rear surface thereof and a first switching device layer 13 stacked on a front surface thereof, a second substrate 16 having a color filter layer 15 stacked on a rear surface thereof and a front polarizing layer 18 stacked on a front surface thereof, and a liquid crystal layer 14 interposed between the first switching device layer 13 and the color filter layer 15. [9] The two-dimensional image liquid crystal display panel 10 may be constructed with a TFT (thin film transistor) LCD panel. In this case, firs switching devices of the first switching device layer 13 are TFTs. Since structure and operations of the TFT LCD panel are well known to the ordinarily skill in the art, the structure and operation thereof are simply described. [10] The rear polarizing layer 11 has a function of polarizing white light emitting from a backlight (not shown). The first and second substrates 12 and 16 are made of a glass. The first substrate 12 has a function as an underlying layer for the first switching device layer 13 and a pixel electrode layer (not shown). On the first switching device layer 13, the TFTs are arrayed in a shape of matrix. In response to an image signal, the TFTs are driven to form an image (two-dimensional image). Although not shown in the figure, a pixel electrode layer made of ITO or the like is disposed under the first switching device layer 13. [11] The second substrate 16 has a function as an underlying layer used to from the color filter layer 15 or the like. In addition, the second substrate 16 also has a function of protecting liquid crystals from an external impact. In addition, the second substrate also has a function of preventing external oxygen and moisture from penetrating into the liquid crystals. For the reason, the second substrate is made of a thick glass material having a thickness of about 0.5 to 0.7 mm. [12] The color filter layer 15 stacked on the rear surface of the second substrate 16 has a predetermined color pattern of RGB color filters. [13] Although not shown in the figure, a common electrode layer made of ITO or the like is disposed under the color filter layer 15. When a voltage supplied through a wire 17 is applied between the pixel electrode layer and the common electrode layer, liquid crystals interposed therebetween is aligned, and an image is represented according to On and Off of the first switching devices of the first switching device layer 13. [14] As described above, the first liquid crystal layer 14 formed by injection liquid crystals are interposed between the first switching device layer 13 and the color filter layer 15, more specifically, between the first switching device layer 130 and the common electrode layer. Although not shown, an alignment layer for aligning the liquid crystals is provided. Light passing through the first liquid crystal layer 14 has a predetermined color according to the RGB color filter which the light passes through. [15] By the construction, a two-dimensional image is formed on the two-dimensional image display panel 10. In order to form a stereoscopic image formed by the stereoscopic image display apparatus, the two-dimensional image needs to have a left eye image and a right eye image. [16] Now, a general parallax barrier liquid crystal display panel will be described with reference to FIG. 2. [17] FIG. 2 is a schematic cross sectional view showing a general parallax barrier liquid crystal display panel. [18] Referring to FIG. 2, the parallax barrier liquid crystal display panel 20 includes at least a third substrate 22 having a rear polarizing layer 21 stacked on a rear surface thereof and a transparent electrode layer 23 stacked on a front surface thereof, a protective layer 25, a second liquid crystal layer 24 interposed between the transparent electrode layer 23 and the protective layer 25. A counter electrode layer (not shown) is disposed under the protective layer 25. On the other hand, a front polarizing layer 28 is disposed above the protective layer 25. [19] A plurality of transparent electrodes are disposed in stripe in the transparent electrode layer 230. Similarly, a plurality of counter electrodes are disposed in the counter electrode layer to face the plurality of the transparent electrodes. When a voltage supplied through a wire 270 is applied between the transparent electrodes and the counter electrodes, liquid crystals in the second liquid crystal layer 240 are aligned. According to the alignment of the liquid crystals, light from the TFT LCD panel is blocked or passed. Therefore, the liquid crystal layer has a function of a parallax barrier. [20] The elements disposed in stripe and having the light blocking and passing function i s called a parallax barrier. Due to the parallax barrier, the left and right eye images displayed on the two-dimensional image liquid crystal display panel 10 are incident on the left and right eyes, so that the stereoscopic image can be seen by observer's eyes. [21] The parallax barrier liquid crystal display panel 200 may be constructed with an LCD panel using TN twisted nematic liquid crystals. Alternatively, the parallax barrier liquid crystal display panel 200 may be constructed with an LCD panel using STN (super-twisted nematic) liquid crystals or FTN (film compensated super twisted nematic) liquid crystals. [22] Here, the protective layer 25 must be constructed with a thick layer in order to endure an external impact. Typically, the protective layer 25 has a thickness in a range of from 0.55 to 1.1 mm. [23] FIG. 3 is a schematic cross sectional view showing a conventional front type parallax barrier stereoscopic image display apparatus. [24] The conventional stereoscopic image display apparatus is constructed by attaching the parallax barrier liquid crystal display panel 20 to a front portion of the two- dimensional image liquid crystal display panel 10. The stereoscopic image display apparatus having the construction is called a front type parallax barrier stereoscopic image display apparatus. [25] Referring to FIG. 3, although the conventional stereoscopic image display apparatus may be constructed by attaching the front polarizing layer 18 of the two-dimensional image liquid crystal display panel 10 shown in FIG. 1 and a rear polarizing layer 21 of the parallax barrier liquid crystal display panel 20 shown in FIG. 2, since the front polarizing layer 18 and rear polarizing layer 21 has the same functions and operations, one of them may be removed. In FIG. 3, the polarizing layers are shown with an in¬ termediate polarizing layer 18'. [26] Now, a viewing distance of the conventional stereoscopic image display apparatus having such a construction will be described. [27] Referring to FIG. 3, a distance ε between the image and the parallax barrier corresponds to a distance from the first liquid crystal layer 14 to the second liquid crystal layer 24. As shown in the figure, between the first liquid crystal layer 14 and the second liquid crystal layer 24, a color filter layer 15, a second substrate 16, an in- termediate polarizing layer 18', a third substrate 22, and a transparent electrode layer 23 are disposed. Here, since the color filter layer 15 is a thin layer (for example, a deposited Cr layer) deposited on the second substrate, the thickness of the color filter layer is negligible in an approximate calculation of the distance ε between the image and the parallax barrier. Similarly, since the ITO layers are very thin, the protective layer on which the ITO layers are stacked is collectively called a protective layer. Therefore, hereinafter, in calculation of the distance ε between the image and the parallax barrier, the thickness of the color filter layer 15 is included in the thickness of the second substrate 16. [28] In a case where a monitor is constructed with the stereoscopic image display apparatus, the second substrate 16, the intermediate polarizing layer 18', and the third substrate 22 have thicknesses of 0.7 mm, 0.3 mm, and 0.7 mm, respectively. The total thickness is typically about 1.7 mm. On the other hand, in a case where a mobile terminal is constructed with the stereoscopic image display apparatus, the second substrate 16 has a thickness of 0.5 mm, the intermediate polarizing layer 18' has a thickness of 0.1 to 0.3 mm, and third substrate 22 has a thickness of 0.5 mm. The total thickness is typically about 1.3 mm. The distance ε between the image and the parallax barrier cannot be less than 1.3 mm. [29] An observer can see the stereoscopic image by positioning two eyes at the focusing point of the stereoscopic image. A distance between the focusing point of the stereoscopic image and the image is defined as a viewing distance L [30] The viewing distance L is determined with the distance ε between the image and the parallax barrier, an inter-eye distance E, and a pixel width P by using the following Equation 1. [31] L ~ ε E/P ... (l) [32] Here, the inter-eye distance E is slightly different among observers, but the average inter-eye distance is about 6.5 cm. The pixel width P is a width of a pixel of the image display apparatus. As can be understood from Equation 1, the pixel width P is inversely proportional to the viewing distance L. If the pixel width P is reduced, image resolution increases, but the viewing distance L is too long. In other word, the pixel width P and the viewing distance L have a trade-off relation. Typically, the pixel width P of about 100 μm is utilized for the image display apparatus for the monitor, and the pixel width of about 60 μm is utilized for the image display apparatus for the mobile terminal. [33] By substituting the distance ε between the image and the parallax barrier for Equation 1, the viewing distance L of the stereoscopic image display apparatus for the monitor is about 1.1 m, and the viewing distance L of the stereoscopic image display apparatus for the mobile terminal is about 1.3 m. Like this, the viewing distance L of the front type parallax barrier stereoscopic image display apparatus has the viewing distance is about 1.2 m. [34] The viewing distance of 1.2 m means that an observer can see the stereoscopic image at a position separated by 1.2 m from the image display apparatus. [35] The viewing distance of 1.2 m may not cause a problem in case of a TV set constructed with the front type parallax barrier stereoscopic image display apparatus. Since the observer is generally located at a relatively distant position from the TV set, the observer can watch a 3D stereoscopic image display on the TV set constructed with the front type parallax barrier stereoscopic image display apparatus. [36] However, the viewing distance of 1.2 m causes a critical problem in case of a computer monitor constructed with the front type parallax barrier stereoscopic image display apparatus. In general, a gamer plays a 3D game on the computer monitor while operating a joystick and typing commands on the keyboard. Since an average reach of human beings is 70 cm or less, it is impossible for the gamer to play the 3D game with accurately seeing and discriminating the 3D stereoscopic image displayed by the front type parallax barrier stereoscopic image display apparatus having the viewing distance L of 1.2 m. [37] In addition, for the same reason, the viewing distance of 1.2 m causes a critical problem in case of a mobile terminal such as a mobile phone and a PDA constructed with the front type parallax barrier stereoscopic image display apparatus. In other word, it is impossible for the user of the mobile terminal to look over the stereoscopic image information or play a 3D game on the mobile terminal. In particularly, since the size of the screen of the mobile terminal, it is impossible to discriminate the 3D stereoscopic image displayed on the screen of mobile terminal constructed with the front type parallax barrier stereoscopic image display apparatus having the viewing distance L of 1.2 m. [38] In addition, since the mobile terminal requires the viewing distance L of 30 to 40 cm, there is a critical problem in that desired stereoscopic image can be seen by the conventional front type parallax barrier stereoscopic image display apparatus ir¬ respective of tacking into consideration a diamond zone which is described later. Technical Solution [39] In order to solve the aforementioned problems, the present invention provides a front type parallax barrier stereoscopic image display apparatus capable of reducing a viewing distance. [40] According to an aspect of the present invention, there is provided a stereoscopic image display apparatus comprising: a two-dimensional image liquid crystal display panel comprising at least a first substrate having a rear polarizing layer stacked on a rear surface thereof and a first switching device layer stacked on a front surface thereof, a second substrate having a color filter layer stacked on a surface thereof, and a first liquid crystal layer interposed between the first switching device layer and the color filter layer; a parallax barrier liquid crystal display panel comprising at least a third substrate having a front polarizing layer stacked on a front surface thereof and a transparent electrode layer stacked on a rear surface thereof, a protective layer, and a second liquid crystal layer interposed between the transparent electrode layer and the protective layer; and an intermediate polarizing layer interposed between the protective layer of the parallax barrier liquid crystal display panel and the second substrate of the two-dimensional image liquid crystal display panel. [41] In the aspect of the present invention, the two-dimensional image liquid crystal display panel may be a TFT liquid crystal display panel. [42] In addition, the parallax barrier liquid crystal display panel may be a TN liquid crystal display apparatus. [43] In addition, the parallax barrier liquid crystal display panel may be an STN liquid crystal display apparatus. [44] In addition, the parallax barrier liquid crystal display panel may be FTN liquid crystal display apparatus. [45] In addition, the protective layer may have a thickness of about 0.3 mm or less. Advantageous Effects [46] According a stereoscopic image display apparatus of the present invention, a distance between an image and a parallax barrier is reduced, so that it is possible to reduce a viewing distance. [47] In addition, according to a stereoscopic image display apparatus of the present invention, unnecessary layers between the image and the parallax barrier are removed, it is possible to simplify production processes and reduce production cost. Brief Description of the Drawings [48] FlG. 1 is a schematic cross sectional view showing a general two-dimensional image liquid crystal display panel. [49] FlG. 2 is a schematic cross sectional view showing a general parallax barrier liquid crystal display panel. [50] FlG. 3 is a schematic cross sectional view showing a conventional stereoscopic image display apparatus. [51] FlG. 4 is a schematic cross sectional view showing a stereoscopic image display apparatus according to an embodiment of the present invention. Best Mode for Carrying Out the Invention [52] [Embodiments] [53] Now, a stereoscopic image display apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings. [54] FlG. 4 is a schematic cross sectional view showing a stereoscopic image display apparatus according to an embodiment of the present invention. The stereoscopic image display apparatus includes a two-dimensional image liquid crystal display panel 100 and a parallax barrier liquid crystal display panel 200 which is disposed in front of the two-dimensional image liquid crystal display panel 100. The stereoscopic image display apparatus having such a structure is referred to as a front type parallax barrier stereoscopic image display apparatus. [55] The two-dimensional image liquid crystal display panel 100 has a first substrate 120 having a rear polarizing layer 110 stacked on a rear surface thereof and a first switching device layer 130 stacked on a front surface thereof, a color filter layer 150, and a first liquid crystal layer 140 interposed between the first switching device layer 130 and the color filter layer 150. [56] The two-dimensional image liquid crystal display panel 100 is preferably constructed with a TFT (thin film transistor) LCD panel. In this case, firs switching devices of the first switching device layer 130 are TFTs. Since structure and operations of the TFT LCD panel are well known to the ordinarily skill in the art, the structure and operation thereof are simply described. [57] The rear polarizing layer 110 has a function of polarizing white light emitting from a backlight (not shown). The first substrate 120 is made of a glass and has a function as an underlying layer for the first switching device layer 130 and a pixel electrode layer (not shown). On the first switching device layer 130, the TFTs are arrayed in a shape of matrix. In response to an image signal, the TFTs are driven to form an image (two-dimensional image). Although not shown in the figure, a pixel electrode layer made of ITO or the like is disposed under the first switching device layer 130. [58] The color filter layer 150 has a predetermined pattern of RGB color filters. [59] Although not shown in the figure, a common electrode layer made of ITO or the like is disposed under the color filter layer 150. When a voltage supplied through a wire 170 is applied between the pixel electrode layer and the common electrode layer, liquid crystals interposed therebetween is aligned, and an image is represented according to On and Off of the first switching devices of the first switching device layer 130. [60] As described above, the first liquid crystal layer 140 formed by injection liquid crystals are interposed between the first switching device layer 130 and the color filter layer 150, more specifically, between the first switching device layer 130 and the common electrode layer. Although not shown, an alignment layer for aligning the liquid crystals is provided. [61] Light passing through the first liquid crystal layer 140 has a predetermined color according to the RGB color filter which the light passes through. By the construction, a two-dimensional image is formed on the two-dimensional image display panel 10. In order to form a stereoscopic image formed by the stereoscopic image display apparatus, the two-dimensional image needs to have a left eye image and a right eye image. [62] The parallax barrier liquid crystal display panel 200 includes at least a third substrate 220 having a front polarizing layer 210 stacked on a front surface thereof and a transparent electrode layer 230 stacked on a rear surface thereof, a protective layer 250, a second liquid crystal layer 240 interposed between the transparent electrode layer 230 and the protective layer 250. A counter electrode layer (not shown) is disposed on the protective layer 250. [63] A plurality of transparent electrodes are disposed in stripe in the transparent electrode layer 230. Similarly, a plurality of counter electrodes are disposed in the counter electrode layer to face the plurality of the transparent electrodes. When a voltage supplied through a wire 270 is applied between the transparent electrodes and the counter electrodes, liquid crystals in the second liquid crystal layer 240 are aligned. According to the alignment of the liquid crystals, light from the TFT LCD panel is blocked or passed. [64] The elements disposed in stripe and having the light blocking and passing function is called a parallax barrier. Due to the parallax barrier, the left and right eye images displayed on the two-dimensional image liquid crystal display panel 100 are incident on the left and right eyes, so that the stereoscopic image can be seen by observer's eyes. [65] The parallax barrier liquid crystal display panel 200 may be constructed with an LCD panel using TN twisted nematic liquid crystals. Alternatively, the parallax barrier liquid crystal display panel 200 may be constructed with an LCD panel using STN (super-twisted nematic) liquid crystals or FTN (film compensated super twisted nematic) liquid crystals. [66] In the stereoscopic image display apparatus according to the embodiment of the present invention, the protective layer 250 needs not to be constructed with a thick layer. [67] As descried above, in the conventional front type parallax barrier stereoscopic image display apparatus, since the conventional protective layer 25 (see FIG. 3) is provided as an upper layer of the stereoscopic image display apparatus, the protective layer 25 needs to endure an external impact, so that the protective layer must be constructed with a thick layer. Typically, the conventional protective layer 25 has a thickness in a range of from 0.55 to 1.1 mm. Unlike the conventional front type parallax barrier stereoscopic image display apparatus, in the present invention, the protective layer 250 is provided as an intermediate layer of the stereoscopic image display apparatus. In other words, the protective layer 250 needs not to be constructed with a thick layer for enduring an external impact. In the present invention, the external impact is prevented by using the second substrate 220. As a result, in the embodiment of the present invention, the protective layer 250 can be constructed with a thin layer having a thickness of 0.3 mm or less. [68] Referring to FlG. 4, a distance ε between the image and the parallax barrier corresponds to a distance from the first liquid crystal layer 140 to the second liquid crystal layer 240. As shown in the figure, between the first liquid crystal layer 140 and the second liquid crystal layer 240, a color filter layer 150, a second substrate 160, an intermediate polarizing layer 180', and a protective layer 250 are disposed. Comparing with the conventional one shown in FlG. 3, in the stereoscopic image display apparatus according to the embodiment of the present invention, the conventional third substrate 22 is removed between the first liquid crystal layer 140 and the second liquid crystal layer 240, and a thin layer, that is, the protective layer 250 is interposed between the between the first liquid crystal layer 140 and the second liquid crystal layer 240. In addition, unlike the conventional one, the wire 270 for the parallax barrier liquid crystal display panel 200 are provided to the third substrate 220 formed on a front surface side thereof. As seen in FlG. 4, in the stereoscopic image display apparatus according to the embodiment of the present invention, since the wire 270 for the parallax barrier liquid crystal display panel 200 is provided to the front surface side of third substrate 220, the separation distance of the wire 220 from the wire 170 for the two-dimensional image liquid crystal display panel 100 further increases, so that it is possible to more surely prevent occurrence of short-circuit between the wires. [69] On the other hand, the thickness of the conventional third substrate is about 0.7 mm for a monitor and about 0.5 mm for a typical mobile terminal. However, the protective layer 250 according to the embodiment of the present invent has a thickness of 0.3 or less. [70] Therefore, in a case where a monitor is constructed with the stereoscopic image display apparatus according to the embodiment of the present invention, the second substrate 160, the intermediate polarizing layer 180' and protective layer 250 have thicknesses of 0.7 mm, 0.1, and 0.3 mm, respectively. The total thickness is about 1.1 mm. As a result, the distance ε between the image and the parallax barrier becomes 1.1 mm. By using Equation 1, the viewing distance L of the stereoscopic image display apparatus for the monitor is 70 cm. Therefore, the stereoscopic image display apparatus according to the embodiment of the present invention can be used for the monitor. [71] On the other hand, in a case where a mobile terminal is constructed with the stereoscopic image display apparatus according to the embodiment of the present invention, the second substrate 160, the intermediate polarizing layer 180' and protective layer 250 have thicknesses of 0.3 mm, 0.1 mm, and 0.3 mm, respectively. The total thickness is about 0.7 mm or less. As a result, the distance ε between the image and the parallax barrier becomes 0.7 mm or less. By using Equation 1, the viewing distance L of the stereoscopic image display apparatus for the monitor is 70 cm. Although the distance of 70 cm is not short, since the screen of the mobile terminal is small, the focus of the stereoscopic image display apparatus can be formed in a range of from 40 to 90 cm, which is called a "diamond zone". If the focus is formed in the diamond zone, an observer, that is, a user of the mobile terminal can see the stereoscopic image displayed on the screen of the mobile terminal. Sine the viewing distance of 70 cm obtained by the stereoscopic image display apparatus for the mobile terminal according to the present invention belongs to the diamond zone, the observer can see the stereoscopic image. The diamond zone is firstly contrived and proposed by the inventor of the present invention. [72] Like this, according to the stereoscopic image display apparatus of the present invention, the viewing distance can be greatly reduced in comparison to the con¬ ventional technique. Therefore, it is possible to commercially a front type parallax barrier stereoscopic image display apparatus. [73] In addition, according to the stereoscopic image display apparatus of the present invention, since the wire 270 for the parallax barrier liquid crystal display panel 200 is provided to the third substrate 220 formed at the front surface side thereof, a separation distance from the wire 270 to the wire 170 for the two-dimensional image liquid crystal display panel 100 further increase, it is possible to prevent occurrence of short- circuit between the wires. [74] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention. Industrial Applicability [75] According a stereoscopic image display apparatus of the present invention, a distance between an image and a parallax barrier is reduced, so that it is possible to reduce a viewing distance. [76] In addition, according to a stereoscopic image display apparatus of the present invention, unnecessary layers between the image and the parallax barrier are removed, it is possible to simplify production processes and reduce production cost.