Abstract:
Disclosed is an ultra slimmed LCD capable of being applied to a hand-held terminal such as a cellular phone. In the LCD, a first substrate includes a pixel electrode. A switching element is formed on an inner surface of the first substrate, and turns on or off the pixel electrode. A first polarizing plate is attached on an outer surface of the first substrate. A second substrate faces the first substrate, and includes a color filter layer for displaying a color image. A common electrode is formed on the color filter layer. The second substrate converts a linear light input through one side face of the second substrate into a planar light. A liquid crystal layer is interposed between the first substrate and the second substrate. A second polarizing plate is arranged between the liquid crystal layer and the second substrate, and converts the planar light converted through the second substrate into a light vibrating in a predetermined direction. The second polarizing plate includes a polyvinyl alcohol layer of polymer polarization medium arranged at a middle layer, and two supporter layers of triacetyl cellulose arranged at upper and lower outer faces of the polyvinyl alcohol layer, respectively.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD in which a light guiding plate is removed from an LCD panel and a polarizing plate is placed in the LCD panel.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, LCDs are widely being used as portable information terminals because of their slim in thickness and light in weight characteristics compared with cathode ray tubes (CRTs). An LCD panel is a passive device that does not emit light by itself, and is provided with a reflection plate at a rear side thereof. LCD panel types vary by light source and include a reflection type LCD panel, a projection type LCD panel, and a semi-projection type LCD panel. The reflection type LCD panel uses an external light as the light source. Each of the projection type LCD panel and the semi-projection type LCD panel has a back light assembly at a rear side thereof.  
           [0005]    Since the LCD panel can be driven at a very low voltage, for instance, a few volts, the reflection type LCD has a low power consumption characteristic, but it cannot be used in a dark environment. Also, since a color LCD panel using a color filter has a very low efficiency of light, there is a drawback in that the reflective type LCD cannot display bright colors.  
           [0006]    Both of natural and artificial light sources can be used in the LCD. However, regardless of the type of the light source, a light vibrating in only one direction is used. To this end, a polarizing plate is attached to both of outer surfaces of the LCD panel.  
           [0007]    Light is a kind of electromagnetic wave, vibrates in a sine function, and has a vibration direction normal to an advancing direction thereof. This vibration direction is omni-directional. The polarizing plate passes only the light vibrating in a selected direction, and absorbs or reflects the remaining light vibrating in unselected directions using a proper medium. Therefore, the polarizing plate functions to make a light vibrating in a specific direction.  
           [0008]    In order to enhance the light efficiency, the polarizing plate is attached to both of outer surfaces of the LCD panel. As the polarizing plates, a urea-based polarizing plate or a dye-based polarizing plate is used.  
           [0009]    The urea-based polarizing plate is prepared by adsorbing a urea-based solution having a high anisotropy to polyvinyl alcohol, and drawing the resultant polyvinyl alcohol in a predetermined direction. The urea-based film has advantages such as a superior polarization property, and a high contrast ratio, but it may cause a problem due to a high sublimation of iodine molecules at a high temperature and a high humidity.  
           [0010]    The dye-based polarizing plate is made by a similar method to the manufacturing method of the urea-based polarizing plate, i.e., by using polyvinyl alcohol as a polarization medium, adsorbing a dye having a dichroism to polyvinyl alcohol, and drawing the resultant polyvinyl alcohol such that chloride molecules are arranged in the drawing direction. As the dye, a direct dye or an acid dye is used. This dye-based polarizing plate, however, has a low polarity because of a low anisotropy of the dye itself compared with the urea-based polarizing plate, but it has a strong resistance against an environment of a high temperature and a high humidity.  
           [0011]    [0011]FIG. 1 is a schematic view showing a conventional LCD in which a polarizing plate is used. Referring to FIG. 1, polarizing plates  40  and  50  are respectively disposed on outer surfaces of an upper glass substrate  10  and a lower glass substrate  20 . A backlight assembly  90  is arranged near a side surface of a light guiding plate  60  disposed below the lower glass substrate  20 . A liquid crystal layer  30  is interposed between the upper glass substrate  10  and the lower glass substrate  20 .  
           [0012]    In FIG. 1, a reference numeral  32  denotes a sealant, a reference numeral  70  denotes a reflecting plate, and a reference numeral  80  denotes an LCD panel.  
           [0013]    [0013]FIG. 2 is a sectional view showing a structure of the polarizing plate used in the LCD of FIG. 1. Referring to FIG. 2, a polymer polarization medium layer  43  of polyvinyl alcohol is centered at the polarizing plate, and supporter layers  42  and  44  of triacetyl cellulous (TAC) are attached on upper and lower surfaces of the polyvinyl alcohol layer  43 , respectively. An upper protection film  41  is attached on the upper supporter layer  42 , and an adhesive layer  45  and a lower protection film  46  in this order are attached on the lower supporter layer  44 .  
           [0014]    The supporter layers  42  and  44  are used for securing durability, mechanical strength, heat resistant property, humidity resistant property, etc., and the adhesive layer  45  is used for adhering the polarizing plates  40  and  50  to the glass substrates  10  and  20 .  
           [0015]    Among the aforementioned elements, the light guiding plate and the polarizing plate are important factors in determining an overall thickness of the LCD panel.  
           [0016]    Therefore, it is necessary to decrease the total thickness of the LCD panel by decreasing the thickness of the light guiding plate and the polarizing plate.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention provides an LCD in which the overall thickness is decreased by removing a light guiding plate and arranging a polarizing plate inside an LCD panel.  
           [0018]    In one aspect, there is provided a liquid crystal display (LCD) including an LCD panel. The LCD panel includes a pair of transparent substrates, and a liquid crystal layer interposed therebetween. The LCD panel changes an alignment of liquid crystal molecules of the liquid crystal layer by a voltage applied to electrodes formed on inner surfaces of the substrates facing with each other and thereby displays an image. An illumination unit supplies a light to the LCD panel. At least one of the pair of substrates further has a light guiding plate function for converting a linear light output from a light source into a planar light.  
           [0019]    According to another aspect of the invention, there is provided a liquid crystal display comprising an LCD panel. The LCD panel includes a pair of transparent substrates, and a liquid crystal layer interposed therebetween. The LCD panel changes an alignment of liquid crystal molecules of the liquid crystal layer by a voltage applied to electrodes formed on inner surfaces of the substrates facing with each other and thereby displays an image. An illumination unit supplies a light to the LCD panel. At least one of the pair of substrates has a light guiding plate function for converting a linear light output from a light source into a planar light, and a first polarizing plate is interposed between the substrate having the light guiding plate function and the liquid crystal layer.  
           [0020]    According to still another aspect of the invention, there is provided a liquid crystal display. The LCD includes a first substrate having a pixel electrode. A switching element is formed on an inner surface of the first substrate, and turns on or off the pixel electrode. A first polarizing plate is attached on an outer surface of the first substrate. A second substrate faces the first substrate, and includes a color filter layer for displaying a color image. A common electrode is formed on the color filter layer. The second substrate converts a linear light input through one side face of the second substrate into a planar light. A liquid crystal layer is interposed between the first substrate and the second substrate. A second polarizing plate is arranged between the liquid crystal layer and the second substrate, and converts the planar light converted through the second substrate into a light vibrating in a predetermined direction. The second polarizing plate includes a polymer polarization layer arranged at a middle layer, and two supporter layers arranged at upper and lower outer faces of the polymer polarization layer, respectively.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The above and other advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0022]    [0022]FIG. 1 is a sectional view of an LCD in accordance with the conventional art;  
         [0023]    [0023]FIG. 2 is a sectional view of the polarizing plate of FIG. 1;  
         [0024]    [0024]FIG. 3 is a sectional view of an LCD in accordance with one embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a sectional view of an LCD in accordance with another embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a sectional view of a lower polarizing plate applied to various embodiments of the present invention;  
         [0027]    [0027]FIG. 6 is a sectional view of an upper polarizing plate applied to various embodiments of the present invention;  
         [0028]    [0028]FIG. 7 is a sectional view of an upper polarizing plate applied in accordance with still another embodiment of the present invention; and  
         [0029]    [0029]FIG. 8 is a schematic view for describing a driving mechanism of the upper polarizing plate shown in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Now, exemplary embodiments of the present invention will be described in detail with reference to the annexed drawings.  
         [0031]    [0031]FIG. 3 is a sectional view of an LCD in accordance with one embodiment of the present invention.  
         [0032]    Referring to FIG. 3, an upper substrate (or first substrate) having a polarizing plate (or first polarizing plate)  140  attached at an outer surface thereof is arranged at a visible side (a viewer&#39;s side) of an LCD. The first substrate includes a first transparent substrate  102 . On an inner surface of the first transparent substrate  102 , there are formed pixel electrodes  104 , and switching elements for switching the pixel electrodes  104 . An upper orientation film (or first orientation film)  106  is formed below the pixel electrode  104  and the switching element.  
         [0033]    A lower substrate (or second substrate) is arranged facing the first substrate. The lower substrate includes a second transparent substrate  112  for converting a linear light source into a planar light source. On an inner surface of the second transparent substrate  112 , there are formed a color filter layer  114 , a lower polarizing plate (or second polarizing plate)  150  for transmitting light beams vibrating only in a predetermined direction, a common electrode  118  for generating an electric field together with the pixel electrode  104  formed on the first transparent substrate  102 , and a lower orientation film (or second orientation film)  120  for determining a pretilt angle of liquid crystal molecules of a liquid crystal layer  130  together with the first orientation film  106  formed on the inner surface of the first transparent substrate  102 . A reference numeral  180  denotes a sealant.  
         [0034]    A light source  170  is arranged near one side surface and provides a linear light into the second transparent substrate  112 . There is no limitation in the type of light source emitting light beams, thus fluorescent plate, light emitting diode (LED), organic electroluminescent (EL), small sized incandescent lamp, etc may be used as the light source  170 . Among them, LED, organic EL, small sized incandescent lamp, etc., that do not need a special instrument such as an inverter circuit, etc., and are driven at a low voltage are fit for a low power consumption. Preferably, the light emitting diode (LED) capable of decreasing the size of the LCD itself is used.  
         [0035]    In order to convert the linear light that is incident from the LED  170  into a planar light, a plurality of reflective dots  160  are formed on an outer surface of the second transparent substrate  112 . Below the second transparent substrate  112 , there is disposed a reflection plate  190  for reflecting a light leaked through the second transparent substrate  112  toward the first substrate side.  
         [0036]    As the aforementioned first and second transparent substrates  102  and  112 , inorganic transparent material such as glass, polymer of sheet type or film type, or a composite thereof can be used.  
         [0037]    The reflective dots  160  that allow the second transparent substrate  112  to have a function as the light guiding plate are formed by several kinds of methods. For example, injection molding of glass material, etching of thermosetting plastic or photo-setting plastic, and attachment or formation of a dot type film or plastic layer on a transparent plastic substrate or a glass substrate, and the like can be used. In addition to a basic function for supporting the liquid crystal layer, the substrate  112  having the reflective dots  160  has the function of the light guiding plate for guiding and irradiating the light of the light source with a high efficiency.  
         [0038]    Thus, since the second transparent substrate  112  in the LCD also has the function of the light guiding plate, the overall thickness of the LCD decreases by a value corresponding to the thickness of the light guiding plate of the conventional LCD. Further, since the second polarizing plate  150  is placed between the liquid crystal layer  130  and the second transparent substrate  112 , i.e., at the inside of the LCD panel, the thickness of TAC films corresponding to TAC films  42  and  44  shown in FIG. 2 can be decreased in comparison with the conventional LCD in which the polarizing plate is attached on the outer surface of the LCD panel. Resultantly, the whole thickness of the LCD panel may be decreased.  
         [0039]    Moreover, since the second polarizing plate  150  can be formed by directly coating a polarizing material on the second transparent substrate  112 , an adhesive layer for attaching the polarizing plate on the glass substrate, as used in FIG. 2, is not necessary. Accordingly, it is possible to decrease the overall thickness of the LCD panel by the thickness of the adhesive layer.  
         [0040]    [0040]FIG. 4 is a sectional view of an LCD panel according to another embodiment.  
         [0041]    The LCD of FIG. 4 has the same structure as that of FIG. 3 except for an arrangement of the polarizing plate. Particularly, the second polarizing plate  150  is disposed between the color filter layer  114  and the transparent common electrode  118 .  
         [0042]    Upon comparing the LCD shown in FIG. 4 with the conventional LCD shown in FIG. 1, the overall thickness of the LCD of FIG. 4 decreases by a value corresponding to the thickness of the light guiding plate  60  of the conventional LCD. Further, the second polarizing plate is placed between the color filter layer  114  and the transparent common electrode layer  118 , i.e., at the inside of the LCD panel, thus the thickness of TAC films corresponding to the TAC films  42  and  44  shown in FIG. 2 can be decreased in comparison with the conventional LCD in which the polarizing plate is attached on the outer surface of the LCD panel. As a result, the overall thickness of the LCD panel may decrease.  
         [0043]    Furthermore, since it is possible to form the second polarizing plate  150  by a direct coating method of a polarization material, the adhesive layer  45  shown in FIG. 2 is excluded, so that the overall thickness of the LCD may decrease by the thickness of the adhesive layer  45 .  
         [0044]    [0044]FIG. 5 is a sectional view showing a structure of the second polarizing plate  150  shown in FIGS. 3 and 4.  
         [0045]    When comparing the second polarizing plate  150  of FIG. 5 with that of FIG. 2, it is evident that the second polarizing plate of FIG. 5 does not have an adhesive layer corresponding to the adhesive layer  45  of FIG. 2.  
         [0046]    Prior to explaining a particular structure of the second polarizing plate applied to the present invention, the structure of the conventional upper and lower polarizing plates is reviewed with reference to FIG. 2.  
         [0047]    Referring to FIG. 2, the polyvinyl alcohol layer  43  of polymer polarization medium has a thickness of approximately 30±5.0 μm, each of the supporter layers positioned at each of outer surfaces has a thickness of approximately 80±5.0 μm, the upper protection film  41  has a thickness of approximately 63±5.0 μm, the adhesive layer  45  has a thickness of approximately 25±5.0 μm, and the lower protection film  46  has a thickness of approximately 38±5.0 μm.  
         [0048]    The upper protection film  41  and the lower protection film  46  are temporarily used for protecting the polyvinyl alcohol layer  43  from foreign articles, and since they are removed during the attachment process of the polarizing plate to the substrate, the actual thickness of the polarizing plate in the LCD corresponds to a thickness of summing up the thickness of the polyvinyl alcohol layer  43 , the thickness of the two supporter layers  42  and  44 , and the thickness of the adhesive layer  45 . The total thickness approaches approximately 215 μm.  
         [0049]    Accordingly, as one example, if the second polarizing plate  150  of FIG. 5 is formed by the direct coating method, the second polarizing plate  150  may have a structure in which the thickness of the polyvinyl alcohol layer  153  of the polymer polarization medium is maintained identically to the thickness of the polyvinyl alcohol layer  43  of FIG. 2, the thickness of each of the supporter layers  152  and  154  decreases by approximately 50 μm compared with the thickness of each of the conventional supporter layers, resulting in approximately 30±5.0 μm, and the adhesive layer  45  of FIG. 2 is removed.  
         [0050]    As a result, the thickness of the second polarizing plate becomes approximately 130±5.0 μm which decreases by approximately 85 μm compared with the thickness of the conventional second polarizing plate, 215 μm.  
         [0051]    By decreasing the thickness of each of the supporter layers, a slimmed LCD in which the thickness of the second polarizing plate is no more than 130 μm can be provided.  
         [0052]    Also, as a second example, a second polarizing plate can be formed in which a lower support layer corresponding to the lower supporter layer  154  of FIG. 5 is removed. The thickness of the upper supporter layer  152  and the thickness of the polyvinyl alcohol layer  153  may be the same as those of the conventional second polarizing plate.  
         [0053]    As a result, by applying the second example to an LCD, it is possible to provide a slimmed LCD panel in which the thickness of the second polarizing plate  150  is no more than 110 μm.  
         [0054]    In a third example, a second polarizing plate in which the lower supporter layer  154  is removed from the structure of FIG. 5, the thickness of the polyvinyl alcohol layer  153  is the same as that of the conventional second polarizing plate, and the thickness of the upper supporter layer  152  is approximately 30±5.0 μm which is thinner than the thickness of the conventional upper supporter layer by 50 μm.  
         [0055]    As a result, by applying the third example to an LCD, it becomes possible to provide a slimmed LCD panel in which the thickness of the second polarizing plate  150  is no more than 60 μm.  
         [0056]    As provided in the above three examples, it is possible to make a slimmed LCD by decreasing the thickness of the second polarizing plate  150  and further to allow the second substrate  112  to have the function of the light guiding plate.  
         [0057]    Meanwhile, visible angle dependence in the LCD panel occurs because the birefringence effect of light that is incident with an oblique angle to the LCD panel differs from that of light that is incident perpendicular to the LCD panel. In this regard, an optical device in which the birefringence effect is varied with an incident angle of the light to compensate for a difference in the birefringence depending on the viewing position by overlapping the optical device on the LCD panel is necessary. For the above optical device, a phase difference film is used.  
         [0058]    The phase difference film is used in a state in which it is coupled to the polarizing plate. For an instance, between the lower supporter layer  44  of the polarizing plate and the adhesive layer  45  shown in FIG. 2, a compensation film made of discotic liquid crystal and having a thickness of approximately 2 μm is interposed. In order to support and attach the compensation film, there are sequentially arranged an adhesive layer (second adhesive layer) having a thickness of approximately 25±5.0 μm and a supporter layer (second supporter layer) having a thickness of approximately 100±10 μm.  
         [0059]    However, when a reliability test is carried out for an LCD having the polarizing plate of the above structure as the first polarizing plate, the light leakage at an edge portion, which is not generated in a general polarizing plate having no the phase difference film, is seriously generated. This is understood that when the second adhesive layer is deformed by heat concentrated on the edge portion of the LCD panel at a high temperature, the second supporter layer below the second adhesive layer and the compensation film are twisted and contracted, so that such a light leakage is generated.  
         [0060]    [0060]FIG. 6 is a sectional view showing a structure of an optical compensation polarizing plate for removal of light leakage.  
         [0061]    Referring to FIG. 6, the optical compensation polarizing plate  240  has a structure in which a protection film (hereinafter referred to as “first protection film)  247  having a thickness of approximately 38±5.0 μm, an adhesive layer  246  having a thickness of approximately 25±5.0 μm, a compensation film  245  made of discotic liquid crystal and having a thickness of approximately 2 μm, a supporter layer (hereinafter referred to as “first supporter layer”)  244  having a thickness of approximately 100±10 μm, a polarization device layer  243  having a thickness of approximately 30±5.0 μm, a supporter layer (hereinafter referred to as “second supporter layer”)  242  having a thickness of approximately 60±5.0 μm, and a protective layer (hereinafter referred to as “second protective layer”)  241  having a thickness of approximately 60±5.0 μm and having an antistatic electricity, are sequentially stacked.  
         [0062]    Compared with the conventional structure having the phase compensation film, the optical compensation polarizing plate  240  of FIG. 6 has the structure in which the lower supporter layer having a thickness of approximately 80±10 μm and the adhesive layer having a thickness of approximately 25±5 μm are removed from the conventional structure, so that the thickness of the polarizing plate decreases from 440±57 μm to 335±42 μm, producing a slimmed LCD.  
         [0063]    Also, as another embodiment, the second polarizing plate is formed at the inside of the LCD panel, and as shown in FIG. 7, the first polarizing plate  340  has a structure in which the second adhesive layer having a thickness of approximately 25±5 μm, and the second supporter layer having a thickness of approximately 100±10 μm are removed from the structure of the conventional phase difference film, and the compensation film made of discotic liquid crystal is merely interposed between the lower supporter layer  344  of the conventional polarizing plate and the adhesive layer  346 , so that it is possible to provide an LCD having a slimmed optical compensation polarizing film without a defect of a twist or a contraction. In FIG. 7, a reference numeral  341  represents the second protection layer,  342  represents the second supporting layer,  343  represents the polarization device layer, and  347  represents the first protection layer, respectively.  
         [0064]    Thus, by using the aforementioned optical compensation polarizing plate as the upper polarizing plate, the light leakage at the edge portion of the LCD panel can be prevented, a slimmed LCD is realized, and the manufacturing cost of the polarizing plate is decreased.  
         [0065]    [0065]FIG. 8 is a schematic view for describing an optical compensation principle when the optical compensation plate of the above structure is applied to an LCD. In FIG. 8, a reference numeral  530  indicates a cell region, a reference numeral  440  indicates a phase compensation region, and a reference numeral  680  indicates a boundary between the cell region  430  and the phase compensation region  440 .  
         [0066]    Liquid crystal  534  of the cell region  530  has birefringence in which the refractivity of a short axis direction differs from that of a long axis direction. Based on this birefringence, the liquid crystal is classified into a positive liquid crystal in which an extra ordinary refractivity n e  is greater than an ordinary refractivity n o , i.e., Δn=n e -n o &gt;0, and a negative liquid crystal in which the extra ordinary refractivity n e  is smaller than the ordinary refractivity n o , i.e., Δn=n e -n o &lt;0  
         [0067]    Refractivity is an important factor in optically determining the light velocity passing through a medium, and in describing a progressive mechanism of the light in the medium. Accordingly, when it is viewed at the front of the LCD panel, the light is mainly influenced by the extra ordinary refractivity while when it is viewed with an inclination about the front of the LCD panel, the light is mainly influenced by the ordinary refractivity, so that there occurs a difference in the refractivity of the light, thereby there occurs a difference in the variation rate of the polarization status while the polarization status is varied from the linearly polarized status to the planar polarized status with passing through the liquid crystal layer, and thereby the light amount and color characteristic at the front of the LCD panel become different from those at positions other than the front of the LCD panel.  
         [0068]    Thus, in order to compensate for a phase difference generated in the LC cell region  530 , it is ideal to use the phase difference film  440  of which the extra ordinary refractivity is identical to the ordinary refractivity of the LC cell region  530 , and of which the ordinary refractivity is identical to the ordinary refractivity of the LC cell region, i.e., n e =n o ′, and n o =n e ′.  
         [0069]    To compensate for the phase difference using the phase difference film  440 , retardation of the LC cell, i.e., Δn(=n e -n o )·d (thickness of the LC cell), and retardation of the phase difference film, i.e., Δn′(=n e ′-n o ′) )·d′ (thickness of the phase difference film), should be identical to each other, and thus the thickness of the LC cell, d becomes identical to the thickness of the phase difference film, d′.  
         [0070]    Thus, since a phase variation in the LC cell, Δn·d, has the same value as that in the phase difference film, Δn ‘·d’, but has an opposite direction, they compensate for phase difference each other. However, in a twisted nematic LC cell, since a degree in which liquid crystals aligned parallel with a surface of a glass substrate is vertically aligned is varied with a gray level, the phase difference film has a three dimensional refractivity, and the refractivity is varied with the wavelength of the light, the relationship of n e =n o ′, and n o =n e ′ should be modified.  
         [0071]    Accordingly, although the thickness of the phase difference film is defined within a specific range in FIGS. 6 and 7, it is not limited only to such a range. In other words, the thickness can be modified in an optimum range considering Δn, cell gap, light wavelength, etc.  
         [0072]    As described above, the LCD of the present invention has no light guiding plate. Further, the LCD has a polarizing plate at an inside of the LCD panel, thereby rendering the polarizing plate more thinner, so that a slimmed LCD can be obtained.  
         [0073]    Further, it is possible to decrease an amount of raw material used in manufacture of the light guiding plate and the polarizing plate, so that a manufacturing cost is saved.  
         [0074]    While the present invention has been described in detail, it should be understood that various changes, substitutions and alterations could be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.