Patent Publication Number: US-7710513-B2

Title: Method for illuminating liquid crystal display device, a back-light assembly for performing the same, and a liquid crystal display device using the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a Continuation of U.S. patent application Ser. No. 10/933,520, filed Sep. 3, 2004, now U.S. Pat. No. 7,397,522 which is a Continuation Application from a U.S. patent application Ser. No.: 09/874,316 filed on Jun. 6, 2001 and issued a U.S Pat. No.: 6,788,359, the contents of which are incorporated herein by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for illuminating in a liquid crystal display device, a back-light assembly for performing the same and a liquid crystal display device using the same, and more particularly, to a method for illuminating a liquid crystal display device and a liquid crystal display device using the same. The method for illuminating a liquid crystal display device enhances the front luminance of the liquid crystal display device, reduces the manufacturing process steps and renders a thin and light liquid crystal display device. 
   2. Description of the Related Art 
   Generally, a liquid crystal display device applies a voltage to a specific molecular arrangement of a liquid crystal to convert its molecular arrangement. Then, the liquid crystal display device converts the changes of the optical properties such as birefringence, optical linearity, dichroism and optical scattering characteristics of liquid crystal cells that emits light according to the molecular arrangement, and uses the modulation of the light of the liquid crystal cells. 
   The liquid crystal display device is largely sorted into a TN (Twisted Nematic) type and a STN (Super-Twisted Nematic) type. The liquid crystal display device is, according to the driving method, sorted into an active matrix display type, which uses a switching device and a TN liquid crystal, and a passive matrix type, which uses an STN liquid crystal. 
   A distinguishable difference of two types is that the active matrix display type is a TFT-LCD that drives an LCD by using a TFT and that the passive matrix display type does not have such a complicated circuit associated with a transistor because it has no transistor. 
   Since the liquid crystal display device is a passive light element incapable of emitting light itself, the images are displayed by using a back-light assembly attached to the rear plane of the liquid crystal panel. 
   Recently, there are several structures developed for contriving the slim and light products so as to have the leading edge of the competitiveness. Especially, the light-weight becomes a more important factor in view of the adoption chiefly to a portable computer, etc. 
   In such a liquid crystal display device, the role and function of the back-light assembly have been considered as increasingly important, because the dimension, light efficiency and so on differ depending on the back-light assembly structure to affect on overall mechanical/optical characteristics of the liquid crystal display device. 
   A structure of a general back-light assembly used in the liquid crystal display device is disclosed in U.S. Pat. No. 5,467,208. 
     FIG. 1  schematically shows the foregoing back-light assembly of the conventional liquid crystal display device.  FIG. 2  schematically shows an illuminating process via the back-light assembly of the conventional liquid crystal display device. 
   Referring to  FIGS. 1 and 2 , the liquid crystal display device includes a lamp  1  for generating lights, a light guide plate  4  for guiding the lights from the lamp  1 , and a lamp housing  2  installed to the side plane of the light guide plate  4  surrounding the lamp  1 . 
   The lamp  1  employs a cold cathode, and the lights generated from the lamp  1  are incident via the side plane of the light guide plate  4 . 
   The lamp housing  2  is formed with a reflection plate  6  on the inner surface thereof to reflect the lights from the lamp  1  toward the side plane of the light guide plate  4 , thereby enhancing the efficiency of the lights from the lamp  1 . 
   The light guide plate  4  is formed of a transparent material of plastics such as acrylic resin for forming a panel shape having a sloped lower plane and a horizontal upper plane (alternatively, a sloped upper plane) to allow the lights generated from the lamp  1  to advance toward an LCD panel (not shown) seated on the upper portion after passing through the upper plane of light guide plate  4 . Accordingly, the lower plane of light guide plate  4  is formed with various patterns such as a minute dotted pattern printed for changing the advancing direction of the lights from lamp  1  into toward the LCD panel side. 
   A reflection plate  6  is formed under the lower plane of light guide plate  4 , and a diffusion sheet  8 , a first prism sheet  10 , a second prism sheet  12  and a protecting film  14  are sequentially stacked on the upper plane of light guide plate  4 . 
   The reflection plate  6  again reflects the lights that are not reflected by the minute dotted pattern in the lights which have been generated from the lamp  1  to advance toward the lower plane of the light guide plate  4  toward the upper plane of the light guide plate  4 , thereby improving the consistency of the lights transmitted to the upper plane of the light guide plate  4  while decreasing the optical loss of the lights incident to the LCD panel. As described above, the light guide plate  4  and the reflection plate  6  guide the lights generated from the lamp  1  toward the upper plane of the light guide plate  4 . 
   At this time, the lights passing through the upper plane of the light guide plate  4  include the lights outgoing by being to be sloped at various angles as well as the lights outgoing perpendicularly to the upper plane thereof. 
   The diffusion plate  8  placed between the light guide plate  4  and the first prism sheet  10  scatters the incident lights from the light guide plate  4  to prevent the partial concentration of the lights. Additionally, the diffusion plate  8  decreases the sloped angle of the lights advancing toward the first prism sheet  10  with respect to the first prism sheet  10 . 
   The first prism sheet  10  and the second prism sheet  12  are formed in a manner that triangular bar-shaped prisms are formed to respective upper planes in a regular arrangement, and the prism arrangement of the first prism sheet  10  and that of the second prism sheet  12  are provided to cross each other at a predetermined angle. 
   The first and second prism sheets  10  and  12  respectively serve for condensing the lights diffused by the diffusion plate  8 . By doing so, the vertical incidence with respect to the protecting film  14  of the lights having passed through the first and second prism sheets  10  and  12  becomes most favorable. 
   For this reason, almost all lights passing through the first and second prism sheets  10  and  12  advance perpendicularly, so that the luminance distribution over the protecting film  14  is uniform. 
   The protecting film  14  formed on the second prism sheet  12  not only protects the surface of second prism sheet  12  but also diffuses the lights for uniform distribution of lights. The LCD panel  15  is installed on the protecting film  14 . 
   In other words, in the back-light assembly structure of the conventional liquid crystal display device, as shown in  FIG. 2 , the lights are generated from the lamp to be incident to the LCD panel  15  placed on the protecting film  14  of the back-light assembly via the path provided by the diffusion, condensing, condensing then to diffusion. 
   However, when applying the illuminating method of the back-light assembly, the above-mentioned conventional liquid crystal display device requires several sheets for condensing/diffusing the lights on/from the upper portion of the light guide plate to therefore complicate the assembling process. Furthermore, the plurality of sheets used for the device may render high probability of puckering to degrade reliability of products and increase the manufacturing cost. 
   In addition, in the conventional diffusion plate, a plurality of beads are distributed over the front plane and rear plane of the same density to distribute the lights incident from the light guide plate to the rear plane of the diffusion plate. 
   When constructing the diffusion plate having the aforesaid structure, the precision of condensing function conducted by the first and second prism sheets stacked on the diffusion plate is degraded to lower the front luminance when the lights incident from the lower portion of the diffusion plate are aslant. 
   Moreover, in the conventional liquid crystal display device, if the protecting film is eliminated to decrease the number of sheets stacked in the back-light assembly, the lights condensed by the first and second prism sheets having the triangularly-shaped section are concentratedly condensed only to the direction that is nearly perpendicular to the plane of the LCD panel to be incident to the LCD panel. The lights condensed as described above cause interference with the pixels of the LCD panel, which in turn incites a moiré phenomenon of producing wave pattern throughout the screen. 
   SUMMARY OF THE INVENTION 
   Therefore, one object of the present invention is to provide a method for illuminating in a liquid crystal display device capable of improving front luminance by modulating a lights processing system in a method for illuminating in a liquid crystal display device. 
   Another object of the present invention is to provide a back-light assembly of a liquid crystal display device and a liquid crystal display device using the same capable of decreasing a manufacturing process and manufacturing a slim and light product by modulating the lights processing in the method for illuminating the liquid crystal display device. 
   Still another object of the present invention is to provide a back-light assembly of a liquid crystal display device capable of reducing the number of sheets employed therein while preventing a moiré phenomenon. 
   To achieve the above objects of the present invention, there is provided a method for illuminating in a liquid crystal display device performed by allowing lights generated from a light source to be incident to a light guide plate, and then primarily diffusing the incident lights for a uniformity of the luminous flux distribution. Thereafter, the luminous flux of the primarily diffused lights is primarily condensed with respect to a first orientation component on a plane in parallel with an LCD panel, and the luminous flux of the lights condensed to have the first orientation for extending a view angle of pixels of the LCD panel is secondarily and partially diffused. The luminous flux of the secondarily diffused lights is secondly condensed with respect to a second orientation component that is perpendicular to the first orientation on the parallel plane, and the secondarily condensed lights are illuminated onto the LCD panel. 
   To achieve the above objects of the present invention, a back-light assembly of a liquid crystal display device includes a light source for generating lights and a light guide plate formed to one side of the light source for allowing the lights generated from the light source to be uniformly incident to an LCD panel side. A first diffusion unit provided to the LCD panel side of the light guide plate is formed to allow the reflectivity of the lights incident from the light guide plate to be higher than the light transmission, so that the incident lights are primarily diffused for uniformly distributing the luminous flux of the incident lights. Also included parts are a first condensing unit provided to the LCD panel side of the first diffusion unit for primarily condensing the luminous flux of the primarily diffused lights with respect to a first orientation component on a plane in parallel with the LCD panel, and a second diffusion unit for partially and secondarily diffusing the luminous flux of the lights condensed to have the first orientation to increase a view angle of pixels of the LCD panel. In addition to these, a second condensing unit secondarily condenses the luminous flux of the lights condensed to have the first orientation with respect to a second orientation perpendicular to the first orientation on the parallel plane. 
   According to the present invention, the method for illuminating in the liquid crystal display device is improved to make it possible to embody the liquid crystal display device having a high front luminance and reducing the number of sheets which form the back-light assembly of the liquid crystal display device. Therefore, overall thickness and weight of the liquid crystal display device can be decreased. 
   Further to these, the number of sheets used is reduced to decrease the manufacturing process, which in turn reduces the cost, and the overall quality of back-light assembly to contrive the higher product yield. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other objects and advantages of the present invention will become readily apparent with reference to the following detailed description, when considered in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a perspective view schematically showing a back-light assembly of a conventional liquid crystal display device; 
       FIG. 2  is a view schematically showing an illuminating path via the back-light assembly of the conventional liquid crystal display device; 
       FIG. 3  is a perspective view schematically showing a back-light assembly of a liquid crystal display device according to one embodiment of the present invention; 
       FIG. 4  is a view schematically showing an illuminating process via the back-light assembly of the liquid crystal display device according to one embodiment of the present invention; 
       FIG. 5  is a view schematically showing a diffusion sheet formation of the back-light assembly and an illuminating process thereof according to another embodiment of the present invention; 
       FIG. 6  shows graphs plotting a result of measuring the luminance according to view angles of the diffusion sheet of the conventional liquid crystal display device and of that according to the present invention; 
       FIG. 7  is a schematic view for illustrating a back-light assembly structure of the liquid crystal display device according to still another embodiment of the present invention; 
       FIG. 8  is a section view for illustrating a high condensing sheet of the liquid crystal display device according to still another embodiment of the present invention; 
       FIG. 9  is a section view for illustrating a high condensing sheet of the liquid crystal display device according to further another embodiment of the present invention; and 
       FIG. 10  is a view schematically showing an illuminating path formed by the back-light assembly of the liquid crystal display device according to further another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, with reference to the accompanying drawings, a method for illuminating a liquid crystal display device, a back-light assembly and a liquid crystal display device using the method will be described in detail. 
     FIG. 3  is a view schematically showing a structure of a back-light assembly of the liquid crystal display device according to one embodiment of the present invention.  FIG. 4  shows an illuminating path formed by the back-light assembly of the liquid crystal display device according to one embodiment of the present invention. 
   Referring to  FIGS. 3 and 4 , the back-light assembly of the liquid crystal display device according to the present invention includes a lamp unit  23  and a light guide unit  35 . 
   The lamp unit  23  includes a lamp  20  for generating lights and a lamp reflection plate  22  for surrounding the lamp  20 . 
   The lamp  20  usually employs a cold cathode, and the lights generated from the lamp  20  is incident to a light guide plate  24  of the light guide unit  35 . The lamp reflection plate  22  serves for reflecting the lights generated from the lamp  20  to the light guide plate  24 . 
   The light guide unit  35  is provided with the light guide plate  24  having a reflection plate  26  at its lower surface. A first diffusion sheet  28  is provided onto the light guide plate  24 , i.e., onto the LCD panel side of the light guide plate  24 . A first condensing sheet  30 , a second diffusion sheet  32  and a second condensing sheet  34  are sequentially stacked on the first diffusion sheet  28  in the direction toward a LCD panel  37 . 
   The LCD panel  37  is seated on the upper portion of the light guide unit  35 . 
   The light guide plate  24  is formed of a transparent material of plastic group such as an acrylic so as to form a panel shape having a slanted lower plane and a horizontal upper plane. The light guide plate  24  may be formed to have a wedge shape or a planar shape if the lamp unit  23  is formed to both sides of the light guide plate  24 . 
   The light guide plate  24  is formed with the lamp unit  23  at one side thereof, so that the lights generated from the lamp  20  are incident to the light guide plate  24  to advance to the first diffusion sheet  28  sitting on the light guide plate  24 . 
   The lower plane of the light guide plate  24  may be formed with a variety of patterns such as dotted patterns (not shown) printed for reflecting the lights generated from the lamp to make them incident toward the first diffusion sheet  28  stacked on the upper plane of the light guide plate  24 . 
   The reflection plate  26  is formed to the lower portion of the light guide plate  24 , which serves for repeatedly reflecting the lights without being reflected toward the first diffusion sheet  28  by the dotted patterns of the light guide plate  24  to make them incident toward the first diffusion sheet  28 . 
   The lights having reflected by the dotted patterns of the light guide plate  24  and the reflection plate  26  to be incident to the first diffusion sheet  28  advance in a manner to form a prescribed angle range with respect to the horizontal plane of the first diffusion sheet  28 . 
   The first diffusion sheet  28  stacked on light guide plate  24  makes the luminous flux distribution of the lights reflected from the dotted patterns of the light guide plate  24  and the reflection plate  26  consistent. The first diffusion sheet  28  is formed with beads at the plane adjacent to the light guide plate  24  and the plane adjacent to the first condensing sheet  30 . 
   Subsequently, the first condensing sheet  30  is stacked on the first diffusion sheet  28 . It condenses the luminous flux of the lights diffused by the first diffusion sheet  28  with respect to a first orientation component on the plane in parallel with the LCD panel  37 . In association with the first condensing sheet  30 , a plurality of triangular first prisms  30   a  are arranged on the upper plane of the first condensing sheet  30  in the first orientation. The first prisms  30   a  condenses the lights with respect to the first orientation component. More specifically, the first prisms  30   a  are arranged in the X-axis direction on the plane that is perpendicular to the advancing direction of the lights. 
   A second diffusion sheet  32  is stacked on the first condensing sheet  30 . It partially and secondarily diffuses the luminous flux of the condensed lights from the first condensing sheet  30  so as to have the first orientation, thereby extending the view angle of the pixels of the LCD panel  37 . 
   Also, the second diffusion sheet  32  prevents the moiré phenomenon. 
   That is, if the first condensing sheet  30  and the second condensing sheet  34  are sequentially stacked without the second diffusion sheet  32 , the lights condensed by the first and second condensing sheets  30  and  34  having the triangularly-shaped section are incident to the LCD panel  37  by being concentratedly condensed in the direction almost perpendicular to the plane of the LCD panel at the vertex portions of the first and second condensing sheets  30  and  34 . The lights condensed in this manner cause interference with the pixels of the LCD panel to produce the moiré phenomenon that involves the wave pattern throughout the screen. 
   Therefore, the second diffusion sheet  32  that diffuses the perpendicularly condensed lights is stacked on the first condensing sheet  30  to diffuse the lights concentratedly condensed at the vertex portion of the first condensing sheet  30 . After this, the lights are condensed by the second condensing sheet  34  in the direction perpendicular to the LCD panel direction to thereby prevent the foregoing moiré phenomenon. 
   The second condensing sheet  34  is stacked on the second diffusion sheet  32  for condensing the luminous flux of the lights diffused by second diffusion sheet  32  with respect to a second orientation component perpendicular to the first orientation on the horizontal plane. 
   The second condensing sheet  34  is constructed such that a plurality of triangular second prisms  34   a  are arranged in parallel with one another on the upper plane of the transparent sheet in a prescribed direction to cross the prism arrangement direction of the first condensing sheet  30  at a predetermined angle. Preferably, the second prisms  34   a  are arranged in the Y-axis direction that is perpendicular to the X-axis direction on the plane, i.e., perpendicular to the advancing direction of the lights. 
   Accordingly, the second condensing sheet  34  condenses the luminous flux of the lights secondarily condensed in the second diffusion sheet  32  to have the second orientation that is perpendicular to the first orientation on the parallel plane, and then the finally condensed lights are incident to the LCD panel  37  to improve the front luminance over the conventional one. 
   At this time, the second diffusion sheet  32  and the second condensing sheet  34  may be formed in a single sheet. 
   That is, the optical path of the conventional liquid crystal display device is formed from diffusion, condensing, condensing to diffusion. This made it impossible to manufacture a single sheet for performing the diffusion after condensing the lights. However, in the present invention, as shown in  FIG. 4 , the path of the lights incident to the LCD panel  37  is formed by the path from the diffusion, condensing, diffusion and then condensing to be capable of manufacturing a sheet consisting of a single layer that performs the condensing after the diffusion. 
   Therefore, in the back-light assembly structure of the liquid crystal display device, the number of sheets stacked on the light guide plate can be reduced from the conventional four-layered structure to the three-layered structure. Thus, the manufacturing process is shortened while minimizing the quality problem of respective sheets, thereby enhancing the manufacturing yield of the liquid crystal display device. 
     FIG. 5  is a view schematically showing the diffusion sheet formation of the back-light assembly and illuminating process according to another embodiment of the present invention.  FIG. 6  is graphs plotting the result of measuring the luminance according to the view angles of the diffusion sheets of the conventional liquid crystal display device and those according to the present invention. 
   Referring to  FIG. 5 , the back-light assembly of the liquid crystal display device according to the present invention includes a light guide plate  24  for permitting the lights generated from a lamp  20  to be uniformly incident to an LCD panel side, and a first diffusion sheet  28  stacked to the LCD panel side of the light guide plate  24 . Also included as parts are a first condensing sheet  30  provided to the LCD panel side of the light guide plate  24 , a second diffusion sheet  32  provided to the LCD panel side of the first condensing sheet  30 , and a second condensing sheet  34  provided to the LCD panel side of the second diffusion sheet  32 . 
   A plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24  and a plane  28   a  adjacent to the first condensing sheet  30  are respectively formed with beads  29   a  and  29   b . Here, the beads  29   a  and  29   b  are distributed to make a haze value of the plane  28   b  of first diffusion sheet  28  adjacent to the light guide plate  24  smaller than that of the plane  28   a  adjacent to the first condensing sheet  30 . 
   The haze value represents the degree of diffusive transmission of the incident lights, which is defined as: 
   In the above equation, the quantity of diffusive transmission denotes the quantity of the diffused lights in the lights that are incident to be transmitted, and the quantity of overall transmission denotes the quantity of the total lights that are incident to be transmitted. 
   Because the beads  29   a  and  29   b  of the first diffusion sheet  28  function for diffusing the lights, the haze value becomes high when the beads  29   a  and  29   b  are distributed in the high density, or the haze value is low when being distributed in the low density. 
   Additionally, in order to differentiate the haze values of the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24  and of the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30 , the beads respectively having different refractive indices may be used. 
   In this way, if the haze value of the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30  is higher than that of the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24 , the lights incident aslant from the light guide plate  24  to the first diffusion sheet  28  while having the incidence angle nearly perpendicular to the plane of the first diffusion sheet  28  is diffusively reflected from the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30  or the plane  28   b  thereof adjacent to the light guide plate  24 . 
   Among these, the lights reflected from the plane  28   a  adjacent to the first condensing sheet  30  are, as indicated by the optical path C, subject to a recycle process of being diffusively reflected from the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30 , and then again reflected from the plane  28   b  adjacent to the light guide plate  24  of the first diffusion sheet  28 . While being subject to the recycle process, the lights from the first diffusion sheet  28  toward the first condensing sheet  30  provided to LCD panel side, i.e., the angle changed to have a nearly right angle with respect to the plane of the LCD panel  37 , when outgoing from the first diffusion sheet  28  to the first condensing sheet  30 . 
   Furthermore, the lights reflected from the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24  are, as denoted by the optical path D, reflected by the reflection plate  26  underlying the light guide plate  24  to be again incident at the angle near to be perpendicular to the plane of the first diffusion sheet  28 , thereby outgoing to the first condensing sheet  30  while having the angle with high front luminance. 
   In other words, conventionally the lights outgoing from the light guide plate are heretofore concentrated much more onto the side rather than the front, so that the condensing rate of the lights in the vertical direction is degraded regardless of being the diffusing and condensing via the diffusion sheet and condensing sheet. For this reason, the high front luminance cannot be obtained. 
   However, according to the present invention, even if the lights outgoing from the light guide plate  24  is concentrated onto the side to be incident to have the angle almost perpendicular to the plane of the first diffusion sheet  30 , the lights are outgoing to the first condensing sheet  30  in the direction nearly perpendicular to the plane of the LCD panel  37  via the recycle process of repeated diffusion and reflection of the lights within the first diffusion sheet  30 . It improves the condensing rate at the first condensing sheet  30  and enhances the front luminance of the liquid crystal display device. 
   The graphs representing the difference of the front luminance resulting from the haze value of the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24  and that of the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30  are plotted in  FIG. 6 . 
   In  FIG. 6 , an alphabet A denotes the result obtained by measuring the front luminance with respect to the back-light assembly of the liquid crystal display device using the conventional diffusion sheet which has the haze value of the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24  similar to that of plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30 . An alphabet B denotes the result obtained by measuring the front luminance with respect to the back-light assembly of the liquid crystal display device using the diffusion sheet according to the present invention, in which the haze value of the plane  28   a  of the first diffusion sheet  28  adjacent to the first condensing sheet  30  is higher than that of the plane  28   b  of the first diffusion sheet  28  adjacent to the light guide plate  24 . 
   As shown in  FIG. 6 , it can be noted that the liquid crystal display device equipped with the diffusion sheet B using the repeated diffusion and reflection processes according to the present invention has higher front luminance over the conventional diffusion sheet A. 
   Also, the LCD panel side of the first diffusion sheet  28  is sequentially stacked with the first diffusion sheet  32  and the second condensing sheet  34 . Otherwise, the second condensing sheet  34  may be firstly stacked to LCD panel side of the first diffusion sheet  28 , and the second diffusion sheet  32  is then stacked to the LCD panel side of the second condensing sheet  34 . 
   At this time, there is a problem of degrading the front luminance due to small quantity of light transmission toward the LCD panel  37  if the haze value of the second diffusion sheet  32  is high. For this fact, the second diffusion sheet  32  has the haze value generally lower than that of the first diffusion sheet  28 . Preferably, the light transmission rate at the second diffusion sheet  32  is higher than that at the first diffusion sheet  28  by about 20˜30%. 
   The first diffusion sheet  32  and the second condensing sheet  34  may be formed of a single sheet. 
   When the second diffusion sheet  32  and the second condensing sheet  34  are constituted by the single sheet as stated above, the foregoing first diffusion sheet  28  according to the present invention having the high front luminance is employed to be able to reduce the number of the sheet used in the back-light assembly of the liquid crystal display device while maintaining the high front luminance. 
   The structure of the back-light assembly of the liquid crystal display device formed by constituting the aforementioned second diffusion sheet  32  and second condensing sheet  34  in a single sheet is illustrated in  FIG. 7 . 
   Hereinafter, the single sheet for performing the diffusion and condensing is referred to as a high luminance condensing sheet  36 . 
     FIG. 7  is a schematic view for illustrating the structure of the liquid crystal display device according to the present invention.  FIG. 8  is a sectional view for illustrating the high luminance condensing sheet of the liquid crystal display device according to the present invention. 
   Referring to  FIGS. 7 and 8 , the liquid crystal display device according to the present invention is provided with a lamp  20  and a light guide plate  24  formed to one side of lamp  20 . In addition to these, a first diffusion sheet  28 , first condensing sheet  30 , a high luminance condensing sheet  36  and a LCD panel  37  are sequentially stacked on light guide plate  24 . 
   At the other side of the lamp  20 , a lamp reflection plate  22  is formed for reflecting the lights generated from the lamp  20  to the light guide plate  24  while surrounding the lamp  20 . A reflection plate  26  is formed to the plane of the light guide plate  24  opposite to the plane formed with the first diffusion sheet  28 . 
   A high luminance condensing sheet  36  is formed by a transparent sheet  40  having a predetermined thickness, a diffusion layer  42  formed at the lower plane of transparent sheet  40 , and a condensing layer  44  formed at the upper plane of transparent sheet  40 . 
   The transparent sheet  40  is the main body of high luminance condensing sheet  36 , which is formed of polyester. The diffusion layer  42  is formed by distributing acrylic beads  42   a  over the polyester resin. The diffusion layer  42  uniformly diffuses the lights primarily condensed by the first condensing sheet  30 . 
   Moreover, the diffusion layer  42  serves for preventing the moiré phenomenon that produces the wave pattern throughout the screen as described above, which has the haze value lower than that of the first diffusion sheet  28 . 
   In the diffusion layer  42 , it is preferable that the content of acrylic beads (beads made from acrylic resin)  42   a  is approximately 10˜50 percents by weight with respect to the total amount of the overall diffusion layer. 
   When the content of the acrylic beads  42   a  is lower than 10 weight/percent, the diffusion rate is lowered to induce the moiré phenomenon. If the content of the acrylic beads  42   a  exceeds 50 percents by weight, the front luminance is degraded. 
   The condensing layer  44  is formed by the second prisms  44   a  formed of acrylic resin, which are arranged to cross perpendicularly to the arranging direction of the first prisms  30   a  of the first condensing sheet  30  underlying the high luminance condensing sheet  36 . 
   The condensing layer  44  allows the luminous flux of the secondarily diffused lights from the diffusion layer  42  to have the second orientation perpendicular to the first orientation of the plane in parallel with the LCD panel  37 . 
   Preferably, the thickness of the high luminance condensing sheet  36  is approximately 160 μm, the angle of the vertex of the prism section is approximately 90°, and the interval between respective prisms (distance between the ridgelines of one prism and adjacent prism) is roughly 50 μcm. 
   Except for the high luminance condensing sheet  36 , the lamp  20 , the lamp reflection plate  22 , the first diffusion sheet  28  and the first condensing sheet  30  are formed to have the same structures as those of the above-described embodiments. 
     FIG. 9  is a sectional view for illustrating the high luminance condensing sheet of the liquid crystal display device according to further another embodiment of the present invention 
   Referring to  FIG. 9 , the vertex portions  46   b  of the prisms  46   a  which are the condensing layer  44  of the high luminance condensing sheet  36 , i.e., the portions of being seated with the LCD panel  37  among the prisms  46   a , may be formed to be round. 
   The illuminating path via the back-light assembly of the liquid crystal display device according to above-described further another embodiment of the present invention is as shown in  FIG. 10 . 
   Referring to  FIG. 10 , it can be noted that the lights incident to the LCD panel  37  after having passed through the high luminance condensing sheet  36  according to the present invention are condensed to be more perpendicular to the plane of LCD panel  37  than the lights incident to the LCD panel  15  after being diffused from the protecting film  14  in the back-light assembly of the conventional liquid crystal display device as shown in  FIG. 2 , thereby having the higher front luminance. 
   The lights diffused and condensed by the high luminance condensing sheet  36  are incident to the LCD panel  37  seated on top of the high luminance condensing sheet  36 . 
   According to the present invention, the path of the lights that have been emitted from the back-light assembly of the liquid crystal display device to be incident to the LCD panel is formed by diffusion, condensing, diffusion then to condensing. Therefore, the condensing rate of the lights incident to the LCD panel is improved to obtain a liquid crystal display device with high front luminance. 
   At the same time, the layers for performing the diffusion and condensing of the lights are formed by the single sheet, so that the number of the sheets stacked on the light guide plate of the back-light assembly of the liquid crystal display can be decreased from the four-layered structure to the three-layered structure. 
   By decreasing the number of sheets forming the back-light assembly of the liquid crystal display device as described above, the manufacturing process is reduced and the defect of the back-light assembly caused by the quality problem of the sheet can be minimized. Consequently, the cost is economized and manufacturing yield is remarkably improved. Further to these, overall thickness and weight of the liquid crystal display device can be minimized to easily attain the light weight and slimness of the liquid crystal display device. 
   Additionally, the diffusion unit is provided between condensing sheets to prevent the generation of the moiré phenomenon due to the interference of the lights concentratedly condensed only in a certain direction with the pixels of the LCD panel. 
   Also, the diffusion sheet for regulating the incidence angle of lights of the light guide plate incident from the light guide plate at the LCD panel side to have the high front luminance with respect to the plane of the LCD panel via the repeated diffusive reflection process is provided to improve the front luminance of the liquid crystal display device. 
   As stated above, preferred embodiments of the present invention are shown and described. Although the preferred embodiments of the present invention have been described, it is understood that the present invention should not be limited to these preferred embodiments but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed.