Abstract:
An optical sheet includes an substrate having a first surface and a second surface opposite to the first surface, a light gathering layer formed on the first surface of the substrate, and an light diffusion layer formed on the second surface of the substrate. The light diffusion layer includes a polymeric resin having a plurality of bubbles mixed therein. The optical sheet of the subject invention can be used in LCDs as a photo-diffusive brightness enhancement film.

Description:
BACKGROUND OF THE INVENTION  
       [0001]      1 . Field of the Invention  
         [0002]     This invention relates to an optical sheet, more particularly to an optical sheet including a substrate formed with a light gathering layer and a light diffusion layer on two opposite sides thereof, respectively.  
         [0003]      2 . Description of the Related Art  
         [0004]     Many approaches have been proposed to increase the range of viewing angles or brightness of a liquid crystal display (LCD). For example, it has been attempted to increase the number of lamps in the light source of the LCD so as to enhance brightness of a display panel of the LCD. However, an increase in the number of lamps will cause a waste of energy and generate a considerable amount of heat. The generated heat will accumulate inside the LCD, thereby deteriorating electronic components in the LCD and shortening the service life of the LCD.  
         [0005]     Recently, the brightness of a display panel of the LCD can be enhanced using a brightness enhancement film or a prism film.  
         [0006]     The brightness enhancement film traditionally can be manufactured by applying a layer of curable resin, such as acrylic resin, on a polyester substrate, and then patterning the layer of curable resin through imprinting or irradiating with a UV light so as to form microstructures on a surface of the layer of curable resin.  
         [0007]     WO  96 / 23649  discloses an improved method for making a brightness enhancement film. Referring to  FIG. 1 , the brightness enhancement film  1  obtained from WO  96 / 23649  includes a substrate  11  and a layer of oligomeric resin  12  formed on the substrate  11 . The substrate  11  has a smooth surface  111  opposite to the layer of oligomeric resin  12 . The layer of oligomeric resin  12  is formed with a microstructure in the form of prisms  121 . Subsequently, the microstructure is subjected to heat treatment so as to reduce deformation thereof.  
         [0008]     Although the brightness of the display panel of the LCD can be enhanced using a brightness enhancement film, uneven light beams through the display panel remains a problem. Hence, there is a need in the art to provide a dual-function optical sheet which can enhance the brightness as well as the uniformity of light through the display panel.  
       SUMMARY OF THE INVENTION  
       [0009]     Therefore, the object of the present invention is to provide an optical sheet that includes a light gathering layer and a light diffusion layer so as to enhance the brightness as well as the uniformity of light when applied to the LCDs as a photo-diffusive brightness enhancement film.  
         [0010]     According to this invention, an optical sheet includes a substrate having a first surface and a second surface opposite to the first surface, a light gathering layer formed on the first surface of the substrate, and an light diffusion layer formed on the second surface of the substrate. The light diffusion layer includes a polymeric resin having a plurality of bubbles mixed therein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment of this invention, with reference to the accompanying drawings, in which:  
         [0012]      FIG. 1  is a fragmentary side view to illustrate a conventional optical sheet;  
         [0013]      FIG. 2  is a fragmentary sectional view to illustrate the preferred embodiment of an optical sheet according to the present invention; and  
         [0014]     FIGS.  3  to  7  are fragmentary sectional views to illustrate various structural modifications of the preferred embodiment of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring to  FIG. 2 , the preferred embodiment of an optical sheet  2  according to the present invention includes a substrate  21  having a first surface  211  and a second surface  212  opposite to the first surface  211 , a light gathering layer  22  formed on the first surface  211  of the substrate  21 , and an light diffusion layer  23  formed on the second surface  212  of the substrate  21 .  
         [0016]     The thickness of the substrate  21  is determined according to the requirement for a desired optical product to be manufactured. Preferably, the substrate  21  has a thickness ranging from 50 μm to 150 μm. In addition, the substrate  21  may be made from any suitable material known in art, such as glass and a plastic material. Non-limiting examples of the plastic material suitable for making the substrate  21  include polyester resin, such as polyethylene terephthalate (PET) resin, polyacrylate resin, such as polymethylmethacrylate (PMMA) resin, polyolefinresin, such as polyethylene (PE) resin and polypropylene (PP) resin, polyimide resin, polycarbonate resin, polyurethane (PU) resin, triacetate cellulose, and mixtures thereof. Preferably, the substrate  21  is made from the plastic material selected from the group consisting of polyethylene terephthalate (PET) resin, polymethyl methacrylate (PMMA) resin, triacetate cellulose, and mixtures thereof.  
         [0017]     The light gathering layer  22  may be formed on the first surface  211  of the substrate  21  by applying a composition including a resin, a photoinitiator, and a cross-linking agent to the first surface  211  of the substrate  21 . Preferably, the light gathering layer  22  has a thickness ranging from 5 μm to 100 μm, and a refractive index ranging from 1.3 to 1.8. The resin used in the composition for forming the light gathering layer  22  can be any suitable material known in art. Non-limiting examples of the resin include polyester resin, polyacrylate resin, polycarbonate resin, and mixtures thereof. The photoinitiator used in the composition for forming the light gathering layer  22  can be any suitable material known in art, which is capable of producing free radicals when irradiated, and initiating polymerization through transfer of the free radicals. A non-limiting example of the photoinitiator is benzophenone. The cross-linking agent used in the composition for forming the light gathering layer  22  can be any suitable material known in art. A non-limiting example of the cross-linking agent includes methacylate resin having one or more functional groups. Preferably, the cross-linking agent is multi-functional methacrylate resin capable of raising the glass transition temperature of the light gathering layer  22 . In addition, the composition for forming the light gathering layer  22  may further include other additives, such as inorganic fillers, a leveling agent, an anti-foaming agent, and an anti-static agent.  
         [0018]     The light gathering layer  22  has at least one microstructure. Preferably, the microstructure of the light gathering layer  22  is selected from the group consisting of a regular prismatic pattern (see  FIG. 2 ), an irregular prismatic pattern (see  FIG. 3 ), an annular prismatic pattern (see  FIG. 4 ), a cube-corner pattern (see  FIG. 5 ), a bead-like pattern (see  FIG. 7 ) and a lens-like pattern (see  FIG. 6 ). More preferably, the microstructure of the light gathering layer  22  is a regular prismatic pattern shown in  FIG. 2 , or a beak-like pattern (not shown) The light diffusion layer  23  is formed on the second surface  212  of the substrate  21  through application of a composition including a polymeric resin to the second surface  212  of the substrate  21 . The polymeric resin suitable for use in the composition for forming the light diffusion layer  23  may include a thermosetting resin or an ultraviolet (UV) curable resin. Preferably, the polymeric resin is an acrylic UV curable resin. Non-limiting examples of the acrylic UV curable resin include methacrylate resin, urethane acrylate resin, polyester acrylate, epoxy acrylate, and mixtures thereof. Preferably, the acrylic UV curable resin is methacrylate resin. In addition, the acrylic UV curable resin may have one or more functional groups. Preferably, the acrylic UV curable resin has multiple functional groups.  
         [0019]     Preferably, the polymeric resin  231  included in the light diffusion layer  23  has a plurality of bubbles  232  mixed therein for scattering a light beam passing through the polymeric resin  231 . In this case, the composition for forming the light diffusion layer  23  further includes a blowing agent. The blowing agent used in the composition for forming the light diffusion layer  23  can produce an inert gas through heating or UV-irradiating, which results in formation of the bubbles  232  mixed in the polymeric resin  231 . The bubbles  232  may have sizes different from each other. A non-limiting example of the blowing agent is sodium carbonate (Na 2 CO 3 ).  
         [0020]     In the formation of the light diffusion layer  23  of this invention, when the initiating process (such as heating or UV-irradiating) of foaming the polymeric resin  231  with the blowing agent is the same as that of curing the resin, foaming of the polymeric resin  231  with the blowing agent can be conducted concurrently with curing of the polymeric resin  231 . On the other hand, when foaming of the polymeric resin  231  with the blowing agent and curing of the polymeric resin  231  are initiated by different processes, for example, the former being initiated by heating, the latter being initiated by UV-irradiating, foaming of the polymeric resin  231  with the blowing agent is conducted first, followed by curing of the polymeric resin  231 .  
       EXAMPLES AND COMPARATIVE EXAMPLES  
       [0000]     Preparation of a First Colloidal Solution for Forming the Light Diffusion Layer  23   
         [0021]     50% by weight of 2-phenoxyethyl acrylate (commercially available from Eternal Co., R.O.C., trademark: EM210®) was mixed with 50% by weight of aliphatic polyurethane hexapropionate (commercially available from Eternal Co., R.O.C., trademark: 6145-100®) so as to form a polymeric resin matrix. 0.5% by weight of sodium carbonate (Na 2 CO 3 , commercially available from Merck Co., U.S.A.), based on 100% by weight of the polymeric resin matrix, was added to the polymeric resin matrix with stirring so as to form the first colloidal solution for the light diffusion layer  23 .  
         [0000]     Preparation of a Second Colloidal Solution for Forming the Light Gathering Layer  22   
         [0022]     50% by weight of 2-phenoxyethyl acrylate (commercially available from Eternal Co., R.O.C., trademark: EM210®) was mixed with 49% by weight of aliphatic polyurethane hexapropionate (commercially available from Eternal Co., R.O.C., trademark: 6145-100®), and 1% by weight of benzophenone (photoinitiator, commercially available from Double bond, trademark: Chivacure®) with stirring so as to form the second colloidal solution for the light gathering layer  22 .  
       COMPARATIVE EXAMPLE 1  
       [0000]     Optical Sheet Includes the Light Diffusion Layer  23  on a Transparent Substrate  21   
         [0023]     The first colloidal solution thus obtained was applied to the transparent substrate  21  made from PET resin (commercially available from Toray company, Japan, trademark: U34®), followed by air-drying the transparent substrate  21  in an oven at a temperature of 100° C. for 20 minutes so as to permit foaming to take place in the applied first colloidal solution. Next, the transparent substrate  21  was moved out of the oven, followed by irradiating with energy rays so as to form the light diffusion layer  23  on the transparent substrate  21 . The energy ray refers to a light source with a wavelength ranging from 200 to 600 nm. Preferably, the energy ray is an ultraviolet ray. The light diffusion layer  23  thus formed includes a plurality of bubbles  232  mixed in the polymeric resin matrix of the light diffusion layer  23  and that have an irregular distribution of volumes.  
       COMPARATIVE EXAMPLE 2  
       [0000]     Optical Sheet Includes the Light Gathering Layer  22  on a Transparent Substrate  21   
         [0024]     The second colloidal solution thus formed was applied to the transparent substrate  21  made from PET resin (commercially available from Toray company, Japan, trademark: U34®) . A mold with a pattern was pressed to the applied second colloidal solution so as to transfer-print the pattern from the mold to the applied second colloidal solution, followed by curing the applied second colloidal solution on the transparent substrate  21  using UV-irradiation. The mold was then removed so as to obtain the optical sheet.  
       EXAMPLE 1  
       [0025]     The first colloidal solution thus formed was applied to a bottom surface  212  of a transparent substrate  21  made from PET resin (commercially available from Toray company, Japan, trademark: U34®), followed by air-drying the transparent substrate  21  in an oven at a temperature of 100° C. for 20 minutes so as to permit foaming to take place in the applied first colloidal solution. Next, the transparent substrate  21  was moved out of the oven, followed by irradiating with UV rays so as to form the light diffusion layer  23  on the bottom surface  212  of the transparent substrate  21 . The light diffusion layer  23  thus formed includes a plurality of bubbles  232  that are mixed in the polymeric resin matrix of the light diffusion layer  23  and that have irregular sizes.  
         [0026]     The second colloidal solution thus formed was subsequently applied to a top surface  211  of the substrate  21  opposite to the light diffusion layer  23 . A mold with a pattern was pressed to the applied second colloidal solution so as to transfer-print the pattern from the mold to the applied second colloidal solution, followed by curing the applied second colloidal solution using UV-irradiation. The mold was then removed so as to obtain the optical sheet  2 .  
         [0027]     Haze and diffusivity of the transparent substrate, the optical sheet of comparative example 1, the optical sheet of comparative example 2, and the optical sheet of example 1 were tested using NDH 2000 instrument (commercially available from NIPPON DENSHOKU Co., Japan). The test results are shown in Table 1.  
                                                 TABLE 1                                   Specimen   Haze   Diffusivity (Dfs)                                        The transparent   0.5   0.5           substrate           The optical sheet of   25   20           comparative example 1           The optical sheet of   94   5           comparative example 2           The optical sheet of   97   25           example 1                      
 
         [0028]     In view of the foregoing, the optical sheet  2  of this invention has a distinctive structure over the conventional optical sheet. In addition, when the optical sheet  2  of this invention is applied to the LCD, the light beams from a light source and passing through the optical sheet  2  are scattered in the light diffusion layer  23  first and then collimated in the light gathering layer  22 , thereby improving light-collimating and light-scattering effects of the optical sheet  2 .  
         [0029]     While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.