Abstract:
An exemplary prism sheet includes a transparent main body. The main body includes a first surface, a second surface opposite to the first surface, a plurality of elongated, curved micro-depressions formed in the first surface, and a plurality of elongated, curved micro-protrusions protruding out from the second surface. The micro-depressions extend along first imaginary circular arcs having a same curvature. The micro-protrusions extend along second arcs having a same curvature. A backlight module using the present prism sheet is also provided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to prisms, and particularly, to a prism sheet used in a backlight module. 
         [0003]    2. Discussion of the Related Art 
         [0004]    In a liquid crystal display device (LCD device), liquid crystal is a substance that does not itself illuminate light. Instead, the liquid crystal relies on light received from a light source to display information. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light. 
         [0005]      FIG. 5  depicts a typical direct type backlight module  100 . The backlight module  100  includes a housing  11 , a plurality of lamps  12  disposed above a base of the housing  11 , and a light diffusion plate  13  and a prism sheet  10  stacked on top of the housing  11  in that order. Inner walls of the housing  11  are configured for reflecting light upwards. The light diffusion plate  13  includes a plurality of dispersion particles (not shown) therein. The dispersion particles are configured for scattering light, thus enhancing the uniformity of light exiting the light diffusion plate  13 . 
         [0006]    Referring to  FIG. 6  together with  FIG. 5 , the prism sheet  10  includes a base layer  101  and a prism layer  102  formed on the base layer  101 . The prism layer  102  contains a plurality of parallel prism lenses  103  having a triangular cross section. The prism lenses  103  are configured for collimating light to a certain extent. Typically, a method of manufacturing the prism sheet  10  includes the following steps: first, a melted ultraviolet (UV)-cured transparent resin is coated on the base layer  101  to form V-shaped lenses, then the melted UV-cured transparent resin is solidified to form the prism lenses  103 . 
         [0007]    In use, unscattered light from the lamps  12  enters the light diffusion plate  13  and becomes scattered. The scattered light leaves the light diffusion plate  13  and enters the prism sheet  10 . The scattered light then travels through the prism sheet  10  before being refracted out at the prism lenses  103  of the prism layer  102 . Thus, the refracted light leaving the prism sheet  10  is concentrated at the prism layer  102  and increases the brightness (illumination) of the prism sheet  10 . The refracted light then propagates into an LCD panel (not shown) disposed above the prism sheet  10 . 
         [0008]    When the light is scattered in the light diffusion plate  13 , scattered light enters the prism sheet at different angles of incidence. Referring to  FIG. 7 , when scattered light enters the prism sheet  10  at different angles of incidence, the scattered light generally travels through the prism sheet  10  along three light paths. In the first light path (such as a 1 , a 2 ) the light enters the prism sheet at small angles of incidence and refracts out of the prism lenses with the refracted path closer to the normal to the surface of the base layer. In the second light path (such as a 3 , a 4 ) the light enters the prism sheet  10  at angles of incidence larger than the first light path and refracts out of the prism lenses  103  with the refracted path being closer to the normal to the surface of the prism lenses  103 . Both the first light path and the second light path contribute to the brightness of the LED and the light utilization efficiency of the backlight module  100 . However, in the case of the third light path (such as a 5 , a 6 ), the light enters the prism sheets at angles greater than the second light path, such that when the refracted light in the third light path leaves the prism sheet  10  at the prism lenses  103  the refracted light impinges on the surface of adjacent prism lens  103  and reenters the prism sheet  10 . Thus, light traveling along the third light path will eventually reenter the prism sheet  10  and may exit the prism sheet  10  on the same side the light entered. This third light path does not contribute to the light utilization efficiency of the backlight module  100 . Further, the third light path may interfere with or inhibit other incident light resulting in decreasing brightness of the backlight module  100 . 
         [0009]    What is needed, therefore, is a new prism sheet and a backlight module using the prism sheet that can overcome the above-mentioned shortcomings. 
       SUMMARY 
       [0010]    In one aspect, a prism sheet according to a preferred embodiment includes a transparent main body. The main body includes a first surface, a second surface opposite to the first surface, a plurality of elongated, curved micro-depressions formed in the first surface, and a plurality of elongated, curved micro-protrusions protruding out from the second surface. The micro-depressions extend along first imaginary circular arcs having a same curvature. The micro-protrusions extend along second arcs having a same curvature. 
         [0011]    In another aspect, a backlight module according to a preferred embodiment includes a plurality of lamps, a light diffusion plate and a prism sheet. The light diffusion plate is disposed above the lamps and the prism sheet is stacked on the light diffusion plate. The prism sheet is same as described in a previous paragraph. 
         [0012]    Other advantages and novel features will become more apparent from the following detailed description of various embodiments, when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present prism sheet and backlight module. Moreover, in the drawings, like reference numerals designate corresponding parts throughout several views, and all the views are schematic. 
           [0014]      FIG. 1  is an isometric view of a prism sheet according to a first preferred embodiment of the present invention. 
           [0015]      FIG. 2  is a side, cross-sectional view of the prism sheet of  FIG. 1 , taken along line II-II thereof. 
           [0016]      FIG. 3  is similar to  FIG. 2 , but taken along line III-III of the prism sheet of  FIG. 1 . 
           [0017]      FIG. 4  is a side, cross-sectional view of a backlight module using the prism sheet of  FIG. 1  according to a second preferred embodiment of the present invention. 
           [0018]      FIG. 5  is a side cross-sectional view of a conventional backlight module employing a typical prism sheet. 
           [0019]      FIG. 6  is an isometric view of the prism sheet shown in  FIG. 5 . 
           [0020]      FIG. 7  is side, cross-sectional view of the prism sheet of  FIG. 6 , taken along line VII-VII, showing light transmission paths. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Reference will now be made to the drawings to describe preferred embodiments of the present prism sheet and backlight module, in detail. 
         [0022]    Referring to  FIG. 1 , a prism sheet  20  in accordance with a first preferred embodiment of the present invention is shown. The prism sheet  20  includes a transparent main body. The main body includes a first surface  201 , a second surface  203 . The first surface  201  and the second surface  203  are on opposite sides of the main body. The first surface  201  defines a plurality of elongated, curved micro-depressions  202 . The micro-depressions  202  extend along first arcs. The micro-depressions  202  have a same curvature. A plurality of elongated, curved micro-protrusions  204  protrude out from the second surface  203 . The micro-protrusions  204  extend along second arcs. The micro-protrusions have a same curvature. 
         [0023]    In this embodiment, centers of the first arcs are aligned parallely across the line II-II (parallel to a X-axis), and centers of the second arcs are aligned parallely across the line III-III (parallel to a Y-axis). Each of the micro-depressions  202  has a semicircle cross-section taken along the line II-II. Each of the micro-protrusions  204  has a semicircle cross-section taken along line III-III. In other words, a line connecting centers of the first arcs perpendicular to a line connecting centers of the second arcs. In alternative embodiments, the line connecting centers of the first arcs may be oblique with the line connecting centers of the second arcs. 
         [0024]    Referring to  FIG. 2 , a pitch P 1  between adjacent micro-depressions  202  along the X-axis is configured to be in the range from about 0.025 millimeters to about 1.5 millimeters. A radius R 1  of the hemispherical cross-section defined by each of the micro-depression  202  is configured to be in the range satisfying the following expression: P 1 /4≦R 1 ≦2P 1 . A depth H 1  of each micro-depression  202  is configured to be in the range satisfying the following expression: 0.01 millimeters≦H 1 ≦R 1 . In this embodiment, the depth H 1  of each micro-depression  202  equals to the radius R 1 . The pitch P 1  of adjacent micro-depressions  202  equals to 2R 1 . 
         [0025]    Referring to  FIG. 3 , a pitch P 2  between adjacent micro-protrusions  204  along the Y-axis is configured to be in the range from about 0.025 millimeters to about 1.5 millimeters. A radius R 2  of the hemispherical cross-section defined by each of the micro-protrusion  204  is configured to be in the range satisfying the following expression: P 2 /4≦R 2 ≦2P 2 . A depth H 2  of each micro-protrusion  204  is configured to be in the range satisfying the following expression: 0.01 millimeters≦H 2 ≦R 2 . In this embodiment, the depth H 2  of each micro-protrusion  204  equals to the radius R 2 . The pitch P 2  of adjacent micro-protrusions  204  equals to 2R 1 . 
         [0026]    A thickness of the prism sheet  20  is preferably in the range from about 0.5 millimeters to about 3 millimeters. The prism sheet  20  can be made of transparent material selected from the group consisting of polycarbonate (PC), polymethyl methacrylate (PMMA), polystyrene (PS), copolymer of methylmethacrylate and styrene (MS), and any suitable combination thereof. 
         [0027]    Compared with the conventional prism sheet, the prism sheet  20  can be easily mass-produced via the injection molding method. Also, because the prism lenses of the conventional prism sheet is formed by solidifying the melted ultraviolet-cured transparent resin, in use, the prism lenses are easily damaged or scratched due to their poor rigidity and mechanical strength. Compared with the conventional prism sheet, the prism sheet  20  of the present invention has a better rigidity and mechanical strength. Therefore, the present prism sheet is not easily to be damaged or scratched when in use. 
         [0028]    Referring to  FIG. 4 , a backlight module  200  in accordance with a second preferred embodiment of the present invention is shown. The backlight module  200  includes the prism sheet  20 , a housing  21 , a plurality of lamps  22 , and a light diffusion plate  23 . The lamps  22  are regularly aligned above a base of the housing  21 . The light diffusion plate  23  and the prism sheet  20  are stacked on the top of the housing  21  in that order. The prism sheet  20  is stacked on the light diffusion plate  21  in a way such that the first surface  201  is adjacent to the light diffusion plate  21 , and the second surface  203  faces away from the light diffusion plate  21 . 
         [0029]    The lamps  22  can be point light sources such as light emitting diodes, or linear light sources such as cold cathode fluorescent lamps. The housing  23  is made of metal or plastic materials with a high reflectivity rate. Alternatively, an interior of the housing  23  is preferably deposited with a high reflectivity coating for improving the light reflectivity rate of the housing  23 . In this embodiment, the lamps  22  are cold cathode fluorescent lamps. The housing  23  is made of high reflective metal. 
         [0030]    In the prism sheet  20 , the micro-depressions  202  are configured for enabling the first surface  201  to converge incident light from the lamps  22  to a certain extent (hereafter first light convergence). The micro-protrusions  204  are configured for enabling the second surface  203  to converge light emitting the second surface  203  (hereafter second light convergence). In the backlight module  200 , when light enters the prism sheet  20  via the first surface  201 , the light undergoes the first light convergence at the first surface  201 . Then the light further undergoes a second light convergence at the second  202  before exiting the prism sheet  20 . Thus, a brightness of the backlight module  200  is increased. In addition, because the arrangement of the curved, elongated micro-depressions  202  and micro-protrusions  204  are not aligned with the LCD pixels, light or dark bands produced by diffraction between the prism sheet  20  with the pixel pitch of LCD panel can be decreased or even eliminated. 
         [0031]    Finally, while various embodiments have been described and illustrated, the invention is not to be construed as being limited thereto. Various modifications can be made to the embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.