Patent Publication Number: US-9416936-B2

Title: Prism sheet, and back light unit and liquid crystal display device therewith

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Divisional Application of U.S. patent application Ser. No. 12/477,406, filed on Jun. 3, 2009, which claims the benefit of the Patent Korean Application No. 10-2008-0052480, filed on Jun. 4, 2008, which is hereby incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present invention relates to liquid crystal display devices, and more particularly, to a prism sheet which is made to improve lamp mura, and a back light unit and a liquid crystal display device therewith. 
     2. Discussion of the Related Art 
     As an information oriented society is developed, demands for display devices have been increased in a variety of forms. To meet the demands, various flat display devices including liquid crystal display devices LCD, plasma display devices PDP, electro-luminescence display devices ELD, have been studied and some of which are used widely as display device, already. 
     Of the flat display devices, currently, the liquid crystal display device replaces the cathode ray tube CRT rapidly owing to advantages of a good picture quality, light weight, thin, low power consumption, and so on. The liquid crystal display device is developing as a monitor of a notebook computer, display panel of a TV set, and so on. 
     The liquid crystal display device has a back light unit mounted to a back side of a liquid crystal display panel, for displaying a desired image by cutting/passing a light from the back light unit by controlling a switch for each of pixels on the liquid crystal panel. 
     In the back light unit, there are an edge lighting type back light unit which has a light source at an edge of the liquid crystal display panel, and a direct lighting type back light unit which has a plurality of light sources arranged at regular intervals. 
     Presently, the direct lighting type back light unit is developing to be slim by reducing an overall thickness of the back light unit while providing a minimum gap between the lamps and sheets. 
     Since such back light unit has many advantages in view of system, producers of the liquid crystal display device put many efforts for developing a slim back light unit. 
       FIG. 1  illustrates a related art back light unit, schematically. 
     Referring to  FIG. 1 , the related art liquid crystal display device is provided with a cover bottom  10 , a reflective plate  20  formed on an inside surface of the cover bottom  10 , a plurality of lamps  30  arranged on the reflective plate  20  at regular intervals for emitting a light, a diffusion plate  40  over the lamps  30  for diffusing the light, a plurality of optical sheets  50  on the diffusion plate  40  for scattering the light diffused thus, a mold frame  60  which surrounds the cover bottom  10  and edges of the optical sheets  50 , a liquid crystal display panel  70  over the diffusion plate  40  seated on an edge of the mold frame for displaying the image, and a case top  80  for fastening the mold frame  60  and the liquid crystal display panel  70 . 
     The optical sheets  50  has two diffusion sheets, a protective sheet, and two prism sheets, and may have a polarized light separation sheet depending on products. 
     In the meantime, currently, in fabrication of the liquid crystal display device having the directing lighting type back light unit applied thereto, producers put much efforts in fabricating a larger, slimmer, and lower cost liquid crystal display device, by removing the sheets. 
     With regard to the distances between the reflective plate  20  and the lamps  30 , and between the lamps  30  and the diffusion plate  40 , lamp mura is shown if the distances are made shorter or a number of the lamps are reduced for reducing an overall thickness, and the lamp mura is shown even in a case the optical sheets are removed, selectively. 
       FIG. 2  illustrates a diagram of the prism sheet in a related art back light unit. 
     Referring to  FIG. 2 , the related art prism sheet  60  is provided with a diffusion film  62 , and a plurality of prism mountains  63  on the diffusion film  62 . 
     Each of the prism mountains  63  has a shape of a triangular column arranged such that an apex thereof faces the diffusion sheet  61 . The prism mountains  63  scatters the light from the lamps toward the diffusion sheet  61 . 
     In the meantime, the prism sheet  60  converges a light diverging in an X-axis direction better or diverging in a Y-axis direction better depending on a direction of arrangement of the prism mountains  63 . 
     In detail, the prism sheet  60  scatters the light diverging in a direction sloped surfaces of the prism mountains  63  face, i.e., in the Y-axis direction in the drawing, efficiently. Opposite to this, the prism sheet  60  scatters the light diverging in a length direction of the prism mountains  63 , i.e., in the Y-axis direction in the drawing, poorly. 
     According to this, the light passed through the prism sheet  60  spreads in the Y-axis direction less (due to good converging), and in the X-axis direction more (due to poor converging). 
     Thereafter, the light passed through the prism sheet  60  thus is incident on a back side of the liquid crystal display panel via the diffusion sheet  61 . In this instance, due to a difference of converging efficiency depending on the directions, a picture quality can be poor due to the light incident on a back side of the liquid crystal display panel. 
       FIG. 3  illustrates a diagram for explaining problems taking place in the related art back light unit. 
     Referring to  FIG. 3 , with regard to the distances between the reflective plate  20  and the lamps  30 , and between the lamps  30  and the diffusion plate  40  shown in  FIG. 1 , lamp mura is shown if the distances are made shorter or a number of the lamps are reduced for reducing an overall thickness, and the lamp mura is also shown even in a case the optical sheets are removed, selectively. 
     The lamp mura is shown in a case emission angles of the lights from the lamps overlap with each other, making an overlapped portion brighter than surroundings. That is, in order to make the emission angles not to overlap, a certain distance is required between the lamps and the diffusion plate, and if the distance is shorter, problems with respect to an outer appearance, such as the lamp mura, takes place. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, the present invention is directed to a prism sheet, and a back light unit and a liquid crystal display device therewith. 
     An object of the present invention is to provide a prism sheet, and a back light unit and a liquid crystal display device therewith, in which total thickness of a back light unit is reduced while taking place of the lamp mura is prevented, for improving a picture quality. 
     Additional advantages, objects, and features of the disclosure will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a prism sheet includes a plurality of first prism mountains formed on a supporting film at regular intervals, a resin layer filled between the first prism mountains, and a plurality of second prism mountains formed on the resin layer and the first prism mountains with at intervals greater than the intervals of the first prism mountains. 
     In another aspect of the present invention, a back light unit includes a plurality of light sources arranged at regular intervals for emission of a light, a cover bottom for supporting and housing the light sources, a diffusion plate over the light sources having opposite edges supported on the cover bottom for diffusing and forward the light from the light sources, and a prism sheet as claimed in one of claims  1  to  4  formed on the diffusion plate for scattering and spreading the light from the diffusion plate, uniformly. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the disclosure and together with the description serve to explain the principle of the disclosure. In the drawings: 
         FIG. 1  illustrates a diagram of a related art back light unit, schematically. 
         FIG. 2  illustrates a diagram of the prism sheet in a related art back light unit. 
         FIG. 3  illustrates a diagram for explaining problems taking place in the related art back light unit. 
         FIG. 4  illustrates a section of a prism sheet in accordance with a first preferred embodiment of the present invention. 
         FIG. 5  illustrates a diagram for explaining a light diffusion profile of the prism sheet of the present invention. 
         FIG. 6  illustrates a section of a prism sheet in accordance with a second preferred embodiment of the present invention. 
         FIG. 7  illustrates a section of a prism sheet in accordance with a third preferred embodiment of the present invention. 
         FIG. 8  illustrates a section of a back light unit and a liquid crystal display device having a prism sheet in accordance with one of first to third embodiments of the present invention applied thereto. 
         FIGS. 9A ˜ 9 B illustrate a section and a graph showing a pitch shift versus a diffusion ratio of a prism sheet in accordance with a first preferred embodiment of the present invention, respectively. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Reference will now be made in detail to the specific embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 4  illustrates a section of a prism sheet in accordance with a first preferred embodiment of the present invention, and  FIG. 5  illustrates a diagram for explaining a light diffusion profile of the prism sheet of the present invention. 
     Referring to  FIG. 4 , the prism sheet  100  for changing a path of a light from a lamp, and increasing brightness of the light includes first prism mountains  120  formed on a PET film  110  at regular intervals, resin layers  130  formed between the first mountains  130 , and second prism mountains  140  each formed on the resin layer  130  such that a center of a bottom side thereof is placed on one of the first prism mountains  120  and ends of the bottom side are placed on adjacent first mountains  120 . 
     The first prism mountains  120  are a plurality of triangular patterns formed at regular intervals, and the resin layer  130  is filled between adjacent first prism mountains  120  to form an inverted triangle, and the second prism mountain  140  is placed on the resin layer  130  filled thus, having, in general, a pitch of a pitch of the first prism mountain×2. 
     The pitch of the second prism mountain  140  has a size of 50 μm≦pitch≦100 μm, and the pitch of the first prism mountain  120  or the resin layer  130  has a size of ½ of the pitch of the second prism mountain  140 . That is, each of the first prism mountain  120  and the resin layer  130  has a pitch of 20˜50 μm. 
     The resin layer  130  filled between adjacent mountains  120  is filled from a bottom to an upper side by 50˜100%, and the upper side may be formed to be concave or convex. 
     Each of the first prism mountain  120  and the resin layer  130  have an apex of a regular angle, and the second prism mountain  140  has an obtuse angle greater than the regular angle. Though it is described that the second prism mountain  140  has an obtuse angle greater than the regular angle, the angle of the second prism mountain  140 , is not limited to this, but may have a regular angle or a certain curvature (3˜5 μm. 
     Since the prism sheet can separate and scatter the light, the first and second prism mountains  120  and  140  are formed for scattering a lamp image widely to attenuate the lamp mura. 
     That is, referring to  FIG. 5 , since the lamp image is sharp as shown in a profile {circle around ( 1 )}, a problem of outer appearance takes place. If the lamp image passes through the first prism mountain  120 , the lamp image is spread as shown in a profile {circle around ( 2 )} to attenuate the sharp lamp image, and if the lamp image passes through the second prism mountain  140 , the lamp image is spread widely as shown in a profile {circle around ( 3 )}, such that the lamp mura is invisible. 
       FIG. 6  illustrates a section of a prism sheet in accordance with a second preferred embodiment of the present invention. 
     Referring to  FIG. 6 , a prism sheet  100  for changing a path of a light from a lamp and increasing brightness of the light includes first prism mountains  120  formed on a PET film  110  at regular intervals, a resin layer  130  formed between the first prism mountains  120  adjacent to each other, diffusion agent  150  formed in the resin layer  130 , and a second prism mountain  140  formed on the resin layer such that a center of a bottom side thereof is placed on one of the first prism mountains  120  and ends of the bottom side respectively are placed on adjacent first prism mountains  120 . 
     The first prism mountains  120  are a plurality of triangular patterns formed at regular intervals, and the resin layer  130  is filled between adjacent first prism mountains  120  to form an inverted triangle with diffusing agent filled therein, and the second prism mountain  140  is placed on the resin layer  130  having the diffusing agent filled thus, having, in general, a pitch of a pitch of the first prism mountain×2. That is, the second prism mountain  140  has a pitch greater than the first prism mountain  120 . 
     The pitch of the second prism mountain  140  has a size of 5 μm≦pitch≦100 μm, and the pitch of each of the first prism mountain  120  and the resin layer  130  has a size of ½ of the pitch of the second prism mountain  140 . That is, each of the first prism mountain  120  and the resin layer  130  has a pitch of 20˜50 μm. 
     The resin layer  130  filled between adjacent mountains  120  is filled from a bottom to an upper side by 50˜100%, and the upper side may be formed to be concave or convex. 
     Each of the first prism mountain  120  and the resin layer  130  have an apex of a regular angle, and the second prism mountain  140  has an obtuse angle greater than the regular angle. Though it is described that the second prism mountain  140  has an obtuse angle greater than the regular angle, the angle of the second prism mountain  140 , is not limited to this, but may have a regular angle or a certain curvature (3˜5 μm. 
     Since the prism sheet  100  can separate and scatter the light, the first and second prism mountains  120  and  140  are formed, the resin layer  130  is formed between adjacent first prism mountains  120  and the diffusion agent  150  such as beads is filled in the resin layer  130  for scattering a lamp image widely to attenuate the lamp mura. 
       FIG. 7  illustrates a section of a prism sheet in accordance with a third preferred embodiment of the present invention. 
     Referring to  FIG. 7 , the prism sheet  100  for changing a path of a light from a lamp and increasing brightness of the light includes prism mountains  120  formed on a PET film  110  at regular intervals, and lenti type blocks  160  filled with resin between the prism mountains adjacent to each other projected beyond apexes of the prism mountains  120 . 
     The prism mountains  120  are a plurality of triangular patterns formed at regular intervals, and the lenti type block  160  is formed by filling the resin to form an inverted triangle. 
     Since the prism sheet  100  can separate and scatter the light, the prism mountains  120  and the blocks  160  are formed, for scattering the lamp image widely to attenuate the lamp mura. 
       FIG. 8  illustrates a section of a back light unit and a liquid crystal display device having a prism sheet in accordance with one of first to third embodiments of the present invention applied thereto. 
     Referring to  FIG. 8 , the liquid crystal display device includes a cover bottom  210 , a reflective plate  220  formed on an inside surface of the cover bottom  210 , a plurality of light sources  230  placed in and supported on the cover bottom  210  and projected over the reflective plate  220  at regular intervals for emission of lights, a diffusion plate  240  over the diffusion plate  240  for diffusing the light, a prism sheet  100  over the diffusion plate  240  having multi-layered prism mountains  120  and  140  for scattering and spreading the light diffused thus uniformly, a mold frame  260  for surrounding and holding the cover bottom  210  and edges of the prism sheet  100 , a liquid crystal display panel  270  on the prism sheet  100  seated on edges of the mold frame  260 , for displaying an image, and a case top  280  for fastening the mold frame  260  and the liquid crystal display panel  270 . 
     As described before, the prism sheet  100  includes multi-layered prism mountains  120  and  130 , and a resin layer or diffusion agent  150  filled in the resin layer formed between adjacent prism mountains at a lowest layer. The prism mountain on an upper side has a pitch greater than the prism mountain at a lower side. 
     Since the back light unit and the liquid crystal display device of the present invention can spread the light from the diffusion plate  240  uniformly owing to one sheet of prism sheet  100  having multi-layered prism mountains, the back light unit and the liquid crystal display device can prevent the lamp mura from taking place even if a distance between the diffusion plate  240  and the light source is put closer for reducing total thickness of the back light unit. 
       FIGS. 9A ˜ 9 B illustrate a section and a graph showing a pitch shift versus a diffusion ratio of a prism sheet in accordance with a first preferred embodiment of the present invention, respectively. 
     Referring to  FIG. 9 , a shift takes place at the time of formation of the first and second prisms  120  and  140  due to failure of accurate alignment coming form process conditions. 
     In this instance, pitches of the first prism mountain  120 , and the resin layer  130  set to have 25 μm respectively, the pitch of the second prism mountain  140  is set to have 50 μm, the second prism mountain  140  is shifted to a left side by 5 μm inward of 25 μm, and the prism sheet formed thus is subjected to simulation, for comparing diffusion ratios due to the left side 5 μm shift. 
     As a result of this, in can be known that the left side shift drops the diffusion ratio and attenuates the lamp mura, and a fabricating process for minimizing the shift to 5 μm is required. 
     As has been described, the prism sheet, and the back light unit and the liquid crystal display device therewith have the following advantages. 
     The multi-layered prism mountains on one prism sheet, enabling to scatter and spread the light from the diffusion plate uniformly, permits to reduce a total thickness of the back light unit while preventing the lamp mura from taking place, thereby improving a picture quality. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.