Abstract:
An optical film and a backlight unit having the same are disclosed. The optical film includes a base film and a first prism disposed on the base film, the first prism having a first peak height and a second prism disposed on the base film, the second prism having a second peak height. A difference between the first peak height and the second peak height is substantially 1 μm to 10 μm.

Description:
This application claims the benefit of Korean Patent Application No. 10-2007-0036643 filed on Apr. 13, 2007, which is hereby incorporated by reference. 
   BACKGROUND OF THE DISCLOSURE 
   1. Field of the Disclosure 
   This document relates to an optical film and a backlight unit having the same. 
   2. Description of the Background Art 
   A liquid crystal display (LCD) is an electronic device that changes various electrical information generated from various elements to visual information by using a change in a liquid crystal transmission and transfers it. 
   The general LCD comprises a liquid crystal panel that displays an image according to a drive signal and a data signal applied from an external source and a backlight unit disposed on a rear surface of the liquid crystal panel in order to illuminate the liquid crystal panel. 
   The backlight unit comprises a light source unit, a reflection sheet, and an optical film. 
   The light source generates light of a certain wavelength. 
   The reflection sheet reflects light that has not been made incident on the optical film, among light generated from the light source, to allow the light to proceed in the direction of the liquid crystal panel. 
   The optical film comprises a diffusion sheet, a prism sheet, and a protection sheet. 
   Light outputted toward the liquid crystal panel after being generated from the light source passes through the diffusion sheet. At this time, the diffusion sheet distributes the incident light, preventing the light from concentrating partially and making luminance uniform. 
   As the light passes through the diffusion sheet, its luminance is sharply degraded, so in order to prevent the degradation of the luminance, the prism sheet is used. 
     FIG. 1  is a perspective view of the prism sheet according to the related art, and  FIG. 2  is a sectional view showing a display state of the liquid crystal panel when the prism sheet in  FIG. 1  is used. 
   With reference to  FIG. 1 , the prism sheet  10  comprises a prism support unit  20  and a plurality of prism configuration parts  30  formed side by side on the entire surface of the prism support unit  20 . 
   The prism configuration parts  30  comprise side parts each with a first side  32  and a second side  34  and substantially having an equilateral triangular shape when viewed from the front side. The angle between the first and second sides  32  and  34  is generally 90°, and may vary according to a selection. 
   As the plurality of prism configuration parts  30  are continuously formed on the prism support unit  20 , there are formed valleys  38  and peaks  36  alternately. Light made incident on the prism support unit  20  of the thusly constructed prism sheet  10 , it is refracted while passing through the prism configuration parts  30 . Accordingly, the light made incident at the low angle is concentrated toward the front side, enhancing luminance within the range of an effective viewing angle. 
   However, when the prism configuration parts  30  of the related art prism sheet  10  contact with a smooth surface of a different optical film, traces remain on one surface of the optical film according to the configuration of the peaks  36  of the prism configuration parts  30 , causing a wet-out phenomenon that the optical film is damaged, which results in appearance of bright lines  40   a  that a corresponding portion is seen brighter than a peripheral portion when viewed from an outer side of the liquid crystal panel  40 . 
   In addition, when the wet-out phenomenon occurs as the prism configuration parts  30  contact with the different optical film, the configuration of the peaks  36  may be deformed because the peaks  36  come in contact with the optical film. 
   Moreover, the screen display capability deteriorates because a moiré phenomenon occurs due to interference of periodical patterns between pixels constituting the liquid crystal panel  40  and the prism configuration parts  30  of the prism sheet  10  and also because of a Newton&#39;s ring phenomenon. 
   SUMMARY OF THE DISCLOSURE 
   An aspect of this document is to provide an optical film capable of reducing or removing a wet-out phenomenon, reducing the probability that a prism configuration part is deformed, and reducing or removing a moiré phenomenon and Newton&#39;s ring phenomenon, and a backlight unit having the same. 
   In one aspect, an optical film comprises a base film and a first prism disposed on the base film, the first prism having a first peak height and a second prism disposed on the base film, the second prism having a second peak height. A difference between the first peak height and the second peak height is substantially 1 μm to 10 μm. 
   In another aspect, an optical film comprises a base film and a plurality of prisms disposed on the base film. And an average horizontal amplitude of the peak of the prism is substantially 1 μm to 10 μm. 
   In still another aspect an optical film comprises a base film and a plurality of prisms disposed on the base film. And an average horizontal wavelength of valleys of the prisms is substantially 100 μm to 500 μm. 
   In still another aspect, a backlight unit comprises a light source and an optical film on which light emitted from the light source is incident. And the optical film includes a base film and a first prism disposed on the base film, the first prism having a first height and a second prism disposed on the base film, the second prism having a second height. And a difference between the first peak height of the first prism and the peak height of the second prism is substantially 1 μm to 10 μm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated on and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a perspective view of the prism sheet according to the related art. 
       FIG. 2  is a sectional view showing a display state of a liquid crystal panel when the prism sheet in  FIG. 1  is used. 
       FIG. 3  is a perspective view showing a liquid crystal display (LCD) according to one exemplary embodiment to which this document is applied. 
       FIG. 4  is a sectional view showing a liquid crystal panel in  FIG. 3 . 
       FIG. 5  is a sectional view showing a backlight unit according another exemplary embodiment to which this document is applied. 
       FIG. 6  is a perspective view showing the prism of an optical film according to one exemplary embodiment to which this document is applied. 
       FIG. 7A  is a front view of the optical film in  FIG. 6 . 
       FIG. 7B  is a plan view of the optical film in  FIG. 6 . 
       FIG. 7C  is a side view of a portion ‘B’ in  FIG. 7B . 
       FIG. 7D  is a front view of a portion ‘A’ of the optical film in  FIG. 6 . 
       FIG. 8A  is another front view of the optical film in  FIG. 6 . 
       FIG. 8B  is a side view of a portion ‘B’ in  FIG. 7B . 
       FIG. 9  is a perspective view showing a disposition state of the optical film in use for a backlight unit according to one exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Reference will now be made in detail embodiments of the invention examples of which are illustrated in the accompanying drawings. 
   The detailed exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. 
     FIG. 3  is a perspective view showing a liquid crystal display (LCD) according to one exemplary embodiment to which this document is applied, and  FIG. 4  is a sectional view showing a liquid crystal panel in  FIG. 3 . 
   With reference to  FIGS. 3 and 4 , a liquid crystal display (LCD)  100  comprises a liquid crystal panel  110  that displays an image according to a drive signal and a data signal applied from an external source, and a backlight unit  120  disposed on a rear surface of the liquid crystal panel  110  in order to illuminate the liquid crystal panel  110 . 
   The liquid crystal panel  110  comprises an upper substrate  111   b , a lower substrate  111   a , color filters  112 , black matrixes  117 , pixel electrodes  114 , common electrodes  115 , a liquid crystal layer  116 , and TFT arrays  113 . A pair of polarizers  118   a  and  118   b  are disposed on both side of the liquid crystal panel  110 . 
   The color filters  112  comprise a plurality of pixels each comprising red, green and blue subpixels, and when light is applied thereto, the color filters generate an image corresponding to the red, green or blue. 
   In general, the pixels comprise the red, green, and blue subpixels, respectively, but without being limited thereto, red, green, blue, and white subpixels may constitute a single pixel and there may be various combinations. 
   The TFT arrays  113 , switching elements, switch the pixel electrodes  114 . 
   The common electrodes  115  and the pixel electrodes  114  change an arrangement of molecules of the liquid crystal layer  116  according to a certain voltage applied from the exterior. 
   The liquid crystal layer  116  comprises a plurality of liquid crystal molecules, and the arrangement of the liquid crystal molecules changes according to a voltage difference generated between the pixel electrodes  114  and the common electrodes  115 . Light provided from the backlight unit  120  is made incident on the color filters  112  according to the change in the arrangement of the molecules of the liquid crystal layer  116 . 
   The backlight unit  120  is positioned on a rear surface of the liquid crystal panel  110  and provides light, e.g., white light, to the liquid crystal panel  110 . 
   The backlight unit  120  may be divided into a direct type backlight unit in which a lamp is positioned below the liquid crystal panel and an edge-light type backlight unit in which a lamp is positioned at the side of a light guide plate, depending on an optical light (e.g., a CCFL (Cold Cathode Fluorescent Lamp)) installation method. 
   With reference to  FIG. 3 , the backlight unit  120  is driven according to an edge-light method, and comprises a light source unit  122 , a light guide plate  124 , a reflection sheet  126 , and an optical film  128 . 
   The light source unit  122  is positioned at the side of the backlight unit  120 , and comprises a light source  122   a  and a lamp housing  122   b.    
   As the light source  122   a , the CCFL, which provides very bright white light, may be used. 
   Besides the CCFL, a light emitting diode (LED) or an external electric fluorescent lamp (EEFL) may be used as the light source  122   a.    
   The LED may be formed with the red, green, or blue color or may be formed with a single color of white light. In case of the backlight unit  120  using the LED as the light source, the backlight unit  120  can become compact, light efficiency can be improved, and light uniformity can be maintained. 
   The EEFL has excellent luminance compared with the CCFL and is advantageous for being operated in parallel because its electrode exists outside. In particular, the EEFL can reduce the number of inverters compared with those required for the related art light source, so a unit cost based on the components and the weight of the LCD module can be reduced. 
   The lamp housing  122   b  allows the light source  122   a  to be mounted thereon and allows light emitted from the light source  122   a  to be made incident on the side of the light guide plate  124 , to thus enhance light efficiency. For this purpose, the lamp housing  122   b  is made of a material with high reflexibility and silver (Ag) may be coated on its surface. 
   The reflection sheet  126  is positioned below the light guide plate  124  and serves to reflect light emitted from the light source  122   a  toward the entire surface of the light guide plate  124 . 
   The light guide plate  124  is designed such that after light is made incident on the side, total reflection is continuously made at below a threshold angle. Because the light source  122   a  is positioned at the side of the backlight unit  120 , the light generated from the light source  122   a  is not uniformly on the entire surface of the backlight unit  120  but concentrated into the edge portions. 
   Thus, in order to uniformly transmit light to the entire surface, the light guide plate  124  is required. The light guide plate  124  is typically made of a transparent acrylic resin such as poly methyl meta acrylate (PMMA). The PMMA has high strength, which thus is not easily broken or deformed, is light, and has a high visible ray transmittance. 
   The light guide plate  124  allows light to proceed toward the liquid crystal panel  110 . 
   The optical film  128  may comprise, for example, a diffusion sheet  128   a , a prism sheet  128   b , and a protection sheet  128   c.    
   Light outputted toward the liquid crystal panel  110  from the light guide plate  124  passes through the diffusion sheet  128   a . The diffusion sheet  128   a  distributes the light made incident from the light guide plate  124 , preventing the light from being partially concentrated, making luminance uniform, and widening a viewing angle. 
   When the light passes through the diffusion sheet  128   a , its luminance sharply degrades. Thus, in order to prevent the degradation of luminance, the prism sheet  128   b  is used. The prism sheet  128   b  collects a portion of light diffused or collected by the diffusion sheet  128   a  toward the protection sheet  128   c  and reflects the remaining portion of the light toward the diffusion sheet  128   a . The detailed construction of the prism sheet  128   b  will be described later. 
   The protection sheet  128   c  is positioned on the prism sheet  128   b , prevents generation of scars on the prism sheet  128   b , and widens the viewing angle which has been reduced by the prism sheet  128   b.    
   Meanwhile, besides the above-described edge-light type backlight unit, light can be provided to the liquid crystal panel by using the direct type backlight unit. 
     FIG. 5  is a sectional view showing a backlight unit according another exemplary embodiment to which this document is applied. 
   With reference to  FIG. 5 , the backlight unit  320  is driven according to the direct light method and comprises a light source  322 , a diffusion plate  324 , a reflection sheet  326 , and an optical film  328 . 
   The light source  322  is formed as a plurality of CCFLs aggregate. The CCFLs provide very bright white light. 
   Besides the CCFLs, the LCD or the EEFL may be used as the light source  322 . 
   The reflection sheet  326  is positioned at a lower side of the diffusion plate  324  and serves to reflect light emitted from the light source  322  toward the front surface of the diffusion plate  324 . 
   Meanwhile, instead of the reflection sheet  326 , a light source reflection plate (not shown) may be positioned at a lower side of the light source  322 , the light source  322  may be mounted thereon, and light emitted from the light source  322  is made incident on the diffusion sheet  328   a , thus improving light efficiency. The light source reflection plate is made of a material with high reflexibility and silver (Ag) may be coated thereon. 
   The diffusion plate  324  allows light made incident from the light source  322  to pass therethrough. Preferably, the diffusion plate  324  is made of PMMA. 
   The optical film  328  may comprise, for example, a diffusion sheet  328   a , a prism sheet  328   b , and a protection sheet  328   c.    
   The light outputted toward the liquid crystal panel  110  from the diffusion plate  324  passes through the diffusion sheet  328   a . The diffusion sheet  328   a  distributes the light made incident from the diffusion plate  324 , preventing the light from being partially concentrated and making luminance uniform. 
   When the light passes through the diffusion sheet  328   a , its luminance sharply degrades. Thus, in order to prevent the degradation of luminance, the prism sheet  328   b  is used. The prism sheet  328   b  collects a portion of light diffused or collected by the diffusion sheet  328   a  toward the protection sheet  328   c  and reflects the remaining portion of the light toward the diffusion sheet  328   a . The detailed construction of the prism sheet  328   b  will be described later. 
   The protection sheet  328   c  is positioned on the prism sheet  328   b , prevents generation of scars on the prism sheet  328   b  and widens the viewing angle which has been reduced by the prism sheet  328   b.    
   The illumination operation of the LCD  100  will now be described. 
   With reference to  FIG. 4 , the backlight units  120  and  320  provide a plane light, white light, to the liquid crystal panel  110 . 
   Subsequently, the TFT arrays  113  switch the pixel electrodes  114 . 
   Successively, a certain voltage difference is applied between the pixel electrodes  114  and the common electrodes  115 , as accordingly, the liquid crystal layer  116  is arranged to correspond to the red subpixels, the green subpixels, and the blue subpixels. 
   In this case, when the light provided from the backlight units  120  and  320  passes through the liquid crystal layer  116 , the quantity of light is controlled, and the quantity-controlled light is provided to the color filters  112 . 
   As a result, the color filters  112  implement an image with certain gray scales. 
   In detail, the pixels comprising the red subpixels, the green subpixels, and the blue subpixels implement a certain image according to combination of light which has passed through the red subpixels, the green subpixels, and the blue subpixels. 
   The optical films  128  and  328 , in particular, the prism of the prism sheets  128   b  and  328   b , will be described as follows. 
     FIG. 6  is a perspective view showing the prism of an optical film according to one exemplary embodiment to which this document is applied. 
     FIG. 7A  and  FIG. 8A  are front view of the optical film in  FIG. 6 ,  FIG. 7B  is a plan view of the optical film in  FIG. 6 ,  FIG. 7C  and  FIG. 8B  are side view of a portion ‘B’ in  FIG. 7B , and  FIG. 7D  is a front view of a portion ‘A’ of the optical film in  FIG. 6 . 
   With reference to  FIG. 6 , the optical film  200  according to the exemplary embodiment of the present invention, for example, the prism sheets  128   b  and  328   b , comprises a base film  210  and a plurality of prisms  220  formed on the base film  210 . The plurality of prisms  220  comprises each prism  230  and each prism  230  is formed in a row to constitute the plurality of prisms  220 . 
   One side of the optical film  200  is structured as the plurality of prisms  220  while the opposite side thereof is the base film  210  which is smoothly formed. 
   The base film  210  is preferably made of a thermoplastic polymer film which is transparent and flexible and has good processibility. 
   The plurality of prisms  220  are disposed side by side on the entire surface of the base film  210  and the peaks  232  and the valleys  234  of each prism  230  form continuous curved surface. The continuous curved surface formed by the peaks  232  and the valleys  234  of each prism  230  are randomly formed. 
   The side portions  236  and  238  of each prism  230  are also bent. 
   In detail, with reference to  FIG. 7A , the height dl of the valley  234  of each prism  230  is uniform while the height h 1  of the peaks  232  may change randomly. 
   On the contrary, with reference to  FIG. 8B , the height dl of the valley  234  of each prism  230  and the height h 1  of the peaks  232  may change randomly. 
   Namely, the difference between the height h 1  of a peak  232  of one prism  230  among the plurality of prisms  220  and the height h 1  of a peak of another prism may be within the range of about 1 μm to 10 μm, and preferably, the difference may be within the range of about 1 μm to 3 μm. 
   Herein, the peak height refers to the height of the peak. 
   With reference to  FIG. 7B , the distance PI between the peaks  232  of the plurality of prisms  220  changes randomly. In addition, the left and right of the peak  232  of each prism  230  has a certain variation, and a horizontal average amplitude A 1  of the peak  232  of each prism  230  vibrates within the range of about 1 μm to 10 μm. 
   The left and right variation of the valley  234  is random with a wavelength (T) of substantially 100 μm to 500 μm. 
   With reference to  FIG. 7C , when the peak  232  of each prism  230  formed at one line of the plurality of prisms  220  is observed from the side, the height h 1  of the peak  232  from the bottom changes randomly. Namely, the height h 1  of the peak  232  of each prism  230  may change randomly. 
   On the contrary, with reference to  FIG. 8B , when the peak  232  of each prism  230  formed at one line of the plurality of prisms  220  is observed from the side, the height h 1  of the peak  232  and the height d 1  of the valley  234  from the bottom changes randomly. 
   That is, the difference between heights h 1  of the peaks  232  within each prism  230  may be within the range of about 1 μm to 10 μm, and preferably, within the range of about 1 μm to 3 μm. 
   Unlike the height of the peaks  232 , the height of the valleys  234  may be formed to be uniform. 
   With reference to  FIG. 7D , one section of the prism  230  may have a substantially triangular shape. The length (W) of the bottom side of the prism  230  is within the range of about 20 μm to 300 μm, and a vertical angle (θ) of the prism  230  may have a value within the range of about 60° to 120°. Preferably, the one section of the prism  230  has a shape of the right-angled equilateral triangle. As for the prism  230 , both sides, excluding the bottom side, may be formed as a curved surface. 
     FIG. 9  is a perspective view showing a disposition state of the optical film in use for a backlight unit according to one exemplary embodiment of the present invention. 
   In this manner, the height of the peaks  232  of the plurality of prisms  220  of the optical film  200  changes randomly and the horizontal amplitude of the peaks  232  also changes randomly. 
   Accordingly, as shown in  FIG. 9 , although the optical film  200  contacts physically with a different upper optical sheet, e.g., the protection sheets  128   c  and  328   c , the peaks  232  of the plurality of prisms  220  can be prevented from being entirely deformed, and thus, when the optical film  200  is employed for the backlight, the picture quality of the LCD is not affected. 
   In addition, because the height of the peaks  232  of the plurality of prisms  220  of the optical film  200  changes randomly, as shown by the contact states of the regions ‘C’ and ‘D’ in  FIG. 9 , the physical contact areas are reduced between the optical film  200  and the different upper optical sheet, e.g., the protection sheets  128   c  and  328   c , a wet-out phenomenon can be reduced or removed, so a defect cannot be easily detected visually. 
   Thus, because the optical film  200  according to one exemplary embodiment of the present invention has the random pattern, a moiré phenomenon or a Newton&#39;s ring phenomenon can be reduce or removed. 
   The characteristics of the optical film  200  according to the exemplary embodiment of the present invention will now be described in detail. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Horizontal average 
               Average 
               Height difference 
             
             
                 
               amplitude 
               wavelength (T) 
               of peaks 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Embodiment 1 
               2 μm 
               150 μm 
               1.5 μm   
             
             
               Embodiment 2 
               4 μm 
               200 μm 
               2 μm 
             
             
               Embodiment 3 
               1 μm 
               120 μm 
               1 μm 
             
             
               Embodiment 4 
               5 μm 
               300 μm 
               3 μm 
             
             
                 
             
           
        
       
     
   
   With reference to  FIG. 1 , in Embodiment 1, the horizontal average amplitude A 1  of the peak of the prism is 2 μm, the average wavelength (T) of the valley is 150 μm, and the height difference h 1  of the peaks is 1.5 μm. 
   In Embodiment 2, the horizontal average amplitude A 1  of the peak of the prism is 4 μm, the average wavelength (T) of the valley is 200 μm, and the height difference h 1  of the peaks is 2 μm. 
   In Embodiment 3, the horizontal average amplitude A 1  of the peak of the prism is 1 μm, the average wavelength (T) of the valley is 120 μm, and the height difference h 1  of the peaks is 1 μm. 
   In Embodiment 4, the horizontal average amplitude A 1  of the peak of the prism is 5 μm, the average wavelength (T) of the valley is 300 μm, and the height difference h 1  of the peaks is 3 μm. 
   All the prisms of the Embodiment 1 to Embodiment 4 have the bottom side with the length of 50 μm average and have the shape of the right-angled equilateral triangle with the vertical angle of 90°. 
   The backlight unit was fabricated by using the optical films according to the Embodiment 1 to Embodiment 4, and generation of the moiré phenomenon and the wet-out phenomenon in the LCD employing the backlight unit was checked. The result is as shown below. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Generation of moiré 
               Generation of Wet-out 
             
             
                 
               phenomenon 
               phenomenon 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Embodiment 1 
               No 
               Within tolerance 
             
             
                 
               Embodiment 2 
               No 
               Within tolerance 
             
             
                 
               Embodiment 3 
               No 
               Within tolerance 
             
             
                 
               Embodiment 4 
               No 
               Within tolerance 
             
             
                 
                 
             
           
        
       
     
   
   The wet-out phenomenon and the moiré phenomenon was checked such that whether a defect of the optical film was visually detected or not from the screen of the LCD and whether each phenomenon occurred or not, and whether there is a defect or not was checked based on the average eyesight of general persons. 
   According to the determination, as shown in [Table 2], the optical films according to Embodiment 1 to Embodiment 4 showed the wet-out phenomenon within the range of tolerance, and thus, any visual external defect was not detected. In addition, the moiré phenomenon did not occur. 
   Consequently, because the optical film according to the exemplary embodiment of the present invention has the random pattern, the moiré phenomenon or the Newton&#39;s ring phenomenon can be reduced or removed, and the likelihood of deformation of the plurality of prisms can be reduced. 
   In addition, because the peaks of the prisms have each different height, the contact area between the optical film and other optical sheets can be reduced, and thus, a defect can be hardly detected visually from outside and the wet-out phenomenon can be reduced or removed. 
   The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.