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 micro-depressions formed in the first surface, a plurality of spherical micro-protrusions formed in the second surface. Each micro-depression is defined by four connecting inner sidewalls. A transverse width of each inner sidewall of each micro-depression progressively decreases with increasing distance from its bottom surface that is coplanar with the first surface of the transparent main body. A backlight module using the present prism sheet is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to nine co-pending U.S. patent applications, which are: application Ser. No. 11/938,307 and Ser. No. 11/938,308, filed on Nov. 12, 2007, and both entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”; application Ser. No. 11/940,328, filed on Nov. 15, 2007, and entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”, application Ser. No. 11/946,866 and Ser. No. 11/946,867, filed on Nov. 12, 2007, and both entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”, application Ser. No. 11/949,058, Ser. No. 11/949,059 and Ser. No. 11/949,060, filed on Dec. 3, 2007, and both entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”, and application serial no. [to be determined], with Attorney Docket No. US16427, entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”. In all the co-pending applications, the inventors are Tung-Ming Hsu and Shao-Han Chang. The co-pending applications have the same assignee as the present application. The disclosure of the above identified application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to prism sheets, and particularly, to a prism sheet used in a backlight module. 
         [0004]    2. Discussion of the Related Art 
         [0005]    In a liquid crystal display device (LCD device), liquid crystal is a substance that does not illuminate light by itself. Instead, the liquid crystal propagates 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. 
         [0006]      FIG. 8  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 , 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 certain light upwards. The light diffusion plate  13  includes a plurality of dispersion particles (not shown). The dispersion particles are configured for scattering light, thus enhancing the uniformity of light output from the light diffusion plate  13 . 
         [0007]    Referring to  FIG. 9 , the prism sheet  10  includes a base layer  101  and a prism layer  103  formed on the base layer  101 . The prism layer  103  contains a plurality of parallel prism lenses  105  having a triangular cross section. The prism lenses  105  are configured for collimating received 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 , then the melted UV-cured transparent resin is solidified into the prism lenses  105 . 
         [0008]    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 refracting out at the prism lenses  105  of the prism layer  103 . Thus, refracted light that leaves the prism sheet  10  is concentrated at the prism layer  103  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 . 
         [0009]    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. 10 , 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  105  with the refracted path being closer to normal to the surface of the prism lenses  105 . Both the first light path and the second light path contribute to the light utilization efficiency of the backlight module  100 . However, in a 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 traveling in the third light path leaves the prism sheet  10  at the prism lenses  105  the refracted light impinges on the surface of adjacent prism lens  105  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. The 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 . 
         [0010]    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 
       [0011]    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 micro-depressions formed in the first surface, a plurality of spherical micro-protrusions formed in the second surface. Each micro-depression is defined by four connecting inner sidewalls. A transverse width of each inner sidewall of each micro-depression progressively decreases with increasing distance from its bottom surface that is coplanar with the first surface of the transparent main body. 
         [0012]    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. 
         [0013]    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 
         [0014]    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. 
           [0015]      FIG. 1  is a side, cross-sectional view of a backlight module using a prism sheet according to a first preferred embodiment of the present invention. 
           [0016]      FIG. 2  is a bottom plan view of the prism sheet of  FIG. 1 . 
           [0017]      FIG. 3  is an isometric view of the prism sheet of  FIG. 1 . 
           [0018]      FIG. 4  is a bottom plan view of a prism sheet according to a second preferred embodiment of the present invention. 
           [0019]      FIG. 5  is a bottom plan view of a prism sheet according to a third preferred embodiment of the present invention. 
           [0020]      FIG. 6  is a bottom plan view of a prism sheet according to a fourth preferred embodiment of the present invention. 
           [0021]      FIG. 7  is a top plan view of a prism sheet according to a fifth preferred embodiment of the present invention. 
           [0022]      FIG. 8  is a side cross-sectional view of a conventional backlight module employing a typical prism sheet. 
           [0023]      FIG. 9  is an isometric view of the prism sheet shown in  FIG. 8 . 
           [0024]      FIG. 10  is side, cross-sectional view of the prism sheet of  FIG. 9 , taken along line X-X, showing light transmission paths. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Reference will now be made to the drawings to describe preferred embodiments of the present prism sheet and backlight module, in detail. 
         [0026]    Referring to  FIG. 1 , a backlight module  200  in accordance with a first preferred embodiment of the present invention is shown. The backlight module  200  includes a prism sheet  20 , a light diffusion plate  21 , a plurality of lamps  22 , and a housing  23 . The lamps  22  are regularly aligned above a base of the housing  23 . The light diffusion plate  21  and the prism sheet  20  are stacked on the top of the housing  23  in that order. 
         [0027]    Referring to  FIGS. 2 and 3 , the prism sheet  20  includes a transparent main body. The main body includes a first surface  201  and a second surface  202 . The first surface  201  and the second surface  202  are on opposite sides of the main body. Furthermore, the first surface  201  and the second surface  202  defines a plurality of micro-depressions  203  and a plurality of spherical micro-protrusions  204  respectively. 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  202  is away from the light diffusion plate  21 . Each micro-depression  203  has a shape like an inverted prism and is enclosed by four triangular inner sidewalls connected with each other. In the first preferred embodiment, each first micro-depression  203  is a square pyramidal void formed by four triangular sidewalls. The triangular inner sidewalls are isosceles triangles. A transverse width of each of the triangular inner sidewalls progressively decreases with increasing distance from the first surface  201 . 
         [0028]    In the first embodiment, the micro-depressions  203  are formed side by side on the first surface  201  according to a first matrix manner. The micro-depressions  203  are configured for enabling the first surface  201  to converge incident light from the lamps  22  to a certain extent (hereafter first light convergence). Rows and columns of the micro-depressions  203  in the matrix are parallel to the edges of the prism sheet  20  (along an X-axis and a Y-axis direction) correspondingly. A pitch between adjacent micro-depressions  203  along either the X-axis direction or the Y-axis direction is configured to be in the range from about 0.025 millimeters to about 1 millimeter. Again referring to  FIG. 1 , a dihedral angle θ 1 , defined between the sidewalls on opposite sides of each micro-depression  203  is configured to be in the range from about 40 degrees to about 120 degrees. In the alternative embodiment, rows or columns, of the first micro-depressions  203 , may be obliquely aligned to the sides of the prism sheet, thus having other alignments or orientations. 
         [0029]    In a first preferred embodiment, the spherical micro-protrusions  204  are arranged regularly on the second surface  202  in a matrix. Each spherical micro-protrusion is substantially a hemisphere. The spherical micro-protrusions  204  are configured for enabling the second surface  202  to converge light emitting the second surface  202  (hereafter second light convergence). 
         [0030]    In order to obtain a better optical effect, A distance P between centers of adjacent spherical micro-protrusions  204  is in the range from about 0.025 millimeters to 1.5 millimeters. A radius R of each spherical micro-protrusion  204  is in the range from about a quarter of the distance P to about double the distance P. A maximum height H of the spherical micro-protrusions  204  relative to the second surface  202  is in the range from about 0.01 millimeters to the radius R. In the first embodiment, the height H is equal to the radius R, and the distance P is equal to double the radius R. It can be understood that each spherical micro-protrusion  204  can be replaced by a similar micro-protrusion that is smaller than a hemisphere. That is, each spherical protrusion  204  can instead of a sub-hemispherical protrusion. 
         [0031]    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. 
         [0032]    Again referring to  FIG. 1 , the lamps  22  can be point light sources such as light emitting diodes or linear light sources such as cold cathode fluorescent lamps. Even though the housing  23  is made of high reflectivity material, additionally, an extra coating can be further applied on the interior. In this embodiment, the lamps  22  are cold cathode fluorescent lamps. The housing  23  is made of high reflective metal. 
         [0033]    In the backlight module  200 , when the light enter 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 surface  202  before exiting the prism sheet  20 . Thus, a brightness of the backlight module  200  is increased. In addition, due to the micro-depressions  203 , the light exiting the prism sheet  20  would mostly propagate along directions substantially parallel to the Z-direction. At the same time, less light would travel along directions parallel to the X-direction, minimizing light energy loss. Thus, the light energy utilization rate of the backlight module  200  is high. 
         [0034]    When compared with the conventional prism sheet, the prism sheet  20  is easier to mass produce because the prism lenses of the conventional prism sheet is manufactured by solidifying melted ultraviolet-cured transparent resin whereas the prism sheet  20  is manufactured by injection molding. The prism lenses made by ultraviolet-cured transparent resin are usually damaged or scratched due to poor rigidity, mechanical strength, and the abrasive properties of the transparent resin. However, the prism sheet  20  of the present invention has better rigidity, mechanical strength, and abrasive properties. Therefore, the present prism sheet is not easily damaged or scratched. 
         [0035]    Referring to  FIG. 4 , a prism sheet  30  in accordance with a second preferred embodiment of the present invention is shown. The prism sheet  30  is similar in principle to the prism sheet  20 . However, micro-depressions  303  are aligned apart on first surface  301  of the prism sheet  30  in a matrix arrangement. 
         [0036]    Referring to  FIG. 5 , a prism sheet  40  in accordance with a third preferred embodiment of the present invention is shown. The prism sheet  40  is similar in principle to the prism sheet  30 , except that each of first micro-depressions  403  of first surface  401  is a frusto-pyramidal depression, and includes four inner sidewalls  407 . Each of the inner sidewalls  407  of the first micro-depressions  403  is an isosceles trapezium. 
         [0037]    Referring to  FIG. 6 , a prism sheet  50  according to a fourth embodiment is shown. The prism sheet  50  is similar in principle to the prism sheet  30 , except that each of micro-depressions  503  of a first surface  501  is a polyhedron depression that includes four inner sidewalls. A first pair of opposite inner sidewalls of the four inner sidewalls is isosceles triangles with planar surfaces parallel to an X-axis. A second pair of opposite inner sidewalls of the four inner sidewalls is isosceles trapeziums with planar surfaces parallel to a Y-axis. 
         [0038]    Referring to  FIG. 7 , a prism sheet  60  according to a fifth embodiment is shown. The prism sheet  60  is similar in principle to the prism sheet  20 , except that spherical micro-protrusions  604  of a second surface  602  are aligned side by side in a matrix manner. 
         [0039]    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.