Patent Publication Number: US-7715132-B2

Title: Prism sheet

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a prism sheet, particularly to a prism sheet employed in a backlight module. 
   2. Description of the Related Art 
   Referring to  FIGS. 10 and 11 , a typical direct type backlight module  100  includes a frame  11 , a plurality of light sources  12 , a light diffusion plate  13 , and a prism sheet  10 . The light sources  12  are disposed in an inner side of the frame  11 . The light diffusion plate  13  and the prism sheet  10  are disposed on the light sources  12  above a top of the frame  11  in that order. The light diffusion plate  13  includes a plurality of diffusing particles (not shown) configured for diffusing light. The prism sheet  10  includes a transparent substrate  101  and a prism layer  103  formed on a surface of the transparent substrate  101 . The prism layer  103  forms a plurality of elongated V-shaped ridges  105 . 
   In use, light from the light sources  12  enters the diffusion plate  13  and becomes scattered. The scattered light leaves the diffusion plate  13  to the prism sheet  10 . The scattered light then travels through the prism sheet  10  and is refracted out at the elongated V-shaped ridges  105  of the prism sheet  10 . The refracted light leaving 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 a liquid crystal display panel (not shown) disposed above the prism sheet  10 . 
   However, although light from the light sources  12  enters the diffusion plate  13  and becomes scattered, after the light leaves the prism sheet  10 , strong light spot of the light sources  12  often occurs. 
   In order to reduce or eliminate the strong light spot of the light sources  12 , the backlight module  100  should further include an upper light diffusion film  14  disposed on the prism sheet  10 . However, although the upper light diffusion film  14  and the prism sheet  10  are in contact with each other, a plurality of air pockets may still exist around the boundaries of the light diffusion film  14  and the prism sheet  10 . When the backlight module  100  is in use, light passes through the air pockets, and some of the light undergoes total reflection by the air pockets along one or another of the corresponding boundaries. In addition, the upper light diffusion film  14  may absorb a certain amount of the light from the prism sheet  10 . As a result, a brightness of light illumination of the backlight module  100  is reduced. 
   Therefore, a new prism sheet is desired in order to overcome the above-described shortcomings. 
   SUMMARY 
   A prism sheet includes a transparent main body. The transparent main body includes a first surface and a second surface. The first surface and the second surface are on opposite sides of the main body. The first surface of transparent main body defines a plurality of first elongated arc-shaped grooves and a plurality of second elongated arc-shaped grooves in the first surface. The first elongated arc-shaped grooves intersect with the second elongated arc-shaped grooves. The second surface defining a plurality of rectangular structures. Each rectangular structure defining four adjacent triangular pyramid depressions. 
   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 
     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. Moreover, in the drawings, like reference numerals designate corresponding parts throughout several views, and all the views are schematic. 
       FIG. 1  is an isometric view of a prism sheet in accordance with a first embodiment of the present invention. 
       FIG. 2  is a side cross-sectional view taken along the line II-II of  FIG. 1 . 
       FIG. 3  is a side cross-sectional view taken along the line II-II of  FIG. 1 . 
       FIG. 4  is similar to  FIG. 1 , but showing a view of the prism sheet from another aspect. 
       FIG. 5  is a photo showing an illumination distribution test result of the prism sheet of  FIG. 11 . 
       FIG. 6  is a photo showing an illumination distribution test result of the prism sheet of  FIG. 1 . 
       FIG. 7  is a top plan view of a prism sheet in accordance with a second embodiment of the present invention. 
       FIG. 8  is a side cross-sectional view of a prism sheet in accordance with a third embodiment of the present invention. 
       FIG. 9  is a side cross-sectional view of a prism sheet in accordance with a fourth embodiment of the present invention. 
       FIG. 10  is a side cross-sectional view of a conventional backlight module. 
       FIG. 11  is an isometric view of the prism sheet in  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made to the drawings to describe embodiments of the present prism sheet in detail. 
   Referring to  FIGS. 1 through 4 , in a first embodiment, a prism sheet  20  includes a main body  21 . The main body  21  includes a first surface  201  and a second surface  203 . The first surface  201  and the second surface  203  are on opposite sides of the main body  21 . The first surface  201  defines a plurality of first parallel elongated arc-shaped grooves  204  and a plurality of second parallel elongated arc-shaped grooves  205 . The first elongated arc-shaped grooves  204  intersect with the second elongated arc-shaped grooves  205 , thereby forming an angle larger than 0 degrees. In the illustrated embodiment, the angle defined by the first elongated arc-shaped grooves  204  and the second elongated arc-shaped grooves  205  is 90 degrees. The intersecting of the first grooves  204  and the second grooves  205  forms a plurality of intersection spots  2010 . Each two adjacent spots  2010  are connected by a wall  2012  extending from the first surface  203  to the bottom of the first grooves. The second surface  203  defines a plurality of rectangular structures  207  and each rectangular structure  207  defines four adjacent triangular pyramid depressions  206 . Each side of the rectangular structure  207  is bordered with a side of another adjacent rectangular structure  207 . 
   In the illustrated embodiment, a cross-section taken along a plane perpendicular to an extending direction of the first elongated arc-shaped grooves  204  is semicircular. The first elongated arc-shaped grooves  204  are distributed side by side in the first surface  201 . A radius R defined by each first elongated arc-shaped groove  204  is preferably in the range from about 0.01 millimeters to about 2 millimeters. A pitch P of adjacent first elongated arc-shaped grooves  204  is in the range from about 0.025 millimeters to about 1 millimeter. A depth H of each first elongated arc-shaped groove  204  is in the range from about 0.01 millimeters to about 2 millimeters. Preferably, the parameters of the second elongated arc-shaped grooves  205  are the same to those of the first elongated arc-shaped grooves  204 . 
   The triangular pyramid depressions  206  are closely connected. In each rectangular structure  207 , the four triangular pyramid depressions  206  together with the sidewalls thereof cooperatively form a four-pointed star. The four-pointed stars are distributed in a matrix manner in the second surface  203 . 
   In the illustrated embodiment, corresponding sidewalls on each side of adjacent depressions  206  sharing a same edge collectively form V-shaped ridges, namely, a plurality of first V-shaped ridges, a plurality of second V-shaped ridge, a plurality of third V-shaped ridges, and a plurality of fourth V-shaped ridge. The first V-shaped ridges extend along a first direction X 1 . The second V-shaped ridges extend along a second direction X 2 . The third V-shaped ridges extend along a third direction X 3 . The fourth V-shaped ridges extend along a fourth direction X 4 . 
   The first, second, third, and fourth V-shaped ridges intersect with each other and cooperatively define the triangular pyramid depressions  206 . In addition, the first V-shaped ridges and the third V-shaped ridges intersect with each other and form a plurality of intersections. Each of the plurality of intersections also intersects the second V-shaped ridges and the fourth V-shaped ridges correspondingly. 
   The first direction X 1  and the second direction X 2  cooperatively define an angle θ 1 , the second direction X 2  and the third direction X 3  define an angle θ 2 , the third direction X 3  and the fourth direction X 4  define an angle θ 3 , and the first direction X 1  and the fourth direction X 4  define an angle θ 4 . The θ 1 , θ 2 , θ 3 , θ 4 , are 45 degrees. 
   A vertex angle β of the V-shaped ridge is in the range from about 80 degrees to about 100 degrees. A pitch of the first V-shaped ridges is in the range from about 0.025 millimeters to about 1 millimeter. In the illustrated embodiment, the relations of the first, the second, the third, and the fourth V-shaped ridges are determined by the formula: D 1 =D 3 =√{square root over (2)}D 2 =√{square root over (2)}D 4 , wherein D 1  represents a pitch of the first V-shaped ridges, D 2  represents a pitch of the second V-shaped ridges, D 3  represents a pitch of the third V-shaped ridges, and D 4  represents a pitch of the fourth V-shaped ridges. 
   A thickness T of the sheet  20  is in the range from about 0.4 millimeters to about 4 millimeters. The prism sheet  20  can be made of a material selected from the group consisting of polycarbonate, polymethyl methacrylate, polystyrene, copolymer of methyl methacrylate and styrene, and any suitable combination of those. A mold used to manufacture the prism sheet  20  includes a plurality of first elongated arc-shaped protrusions and a plurality of second elongated arc-shaped protrusions formed on a first side surface, and a plurality of triangular pyramid protrusions formed on a second side surface opposite to the first side surface. The first elongated arc-shaped protrusions and the second elongated arc-shaped protrusions are configured to form the first elongated arc-shaped grooves  204  and the second elongated arc-shaped grooves  205  on the first surface  201  of the prism sheet  20  and the triangular pyramid protrusions are configured to form the triangular pyramid depressions  206  on the second surface  203  of the prism sheet  20 , such that both sides of the prism sheet can be produced at a same time per injection molding. 
   The prism sheet  20  can be employed in a backlight module (not shown) in such manner that the first surface  201  is facing light sources of the backlight module and the second surface  203  is farther away from the light sources. Light enters the prism sheet  20  via the first surface  201 . Since the inner surfaces of the first elongated arc-shaped grooves  204  and the second elongated arc-shaped grooves  205  are curved and the inner surfaces of the triangular pyramid depressions  206  are slanted, incident light that may have been internally reflected on a flat surface, are refracted, reflected, and diffracted. As a result, light outputted from the second surface  203  is more uniform than light outputted from a light output surface of a conventional prism sheet. Strong light spots of the light sources seldom occurs. There is no need to add an extra upper light diffusion film between the prism sheet  20  and the liquid crystal display panel. Thus, the efficiency of light utilization is enhanced. 
   Referring to the table 1 below, test samples are provided. 
   
     
       
         
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Test samples 
               Condition 
             
             
                 
             
           
          
             
               1 
               LED + prism sheet 10 
             
             
               2 
               LED + prism sheet 20 
             
             
                 
             
          
         
       
     
   
   Referring to the  FIGS. 5 and 6 , which reflect the test results from the test conditions in Sheet  1 , light emitting from the prism sheet  10  will form a relatively strong light spot as shown in  FIG. 5 ; in contrast, light emitting from the prism sheet  20  will form a uniform surface light source shown in  FIG. 6 . The test result shows that light emitting from the prism sheet  20  is more uniform. Therefore, when the prism sheet  20  is employed in a backlight module, strong light spot of the light sources seldom occurs, more uniform light is achieved, and there is no need to add an upper light diffusion film between the prism sheet  20  and the liquid crystal display panel. Thus, the efficiency of light utilization is enhanced. 
   In addition, because the triangular pyramid depressions  206  form the first, the second, the third, and the fourth V-shaped ridges, light emitting from the second surface  203  would be concentrated in planes perpendicular to the first direction X 1 , the second direction X 2 , the third direction X 3 , and the fourth direction X 4  respectively, thereby increasing the brightness (illumination) of the prism sheet  10  along a direction perpendicular to the second surface  203 . 
   In addition, in contrast to the conventional prism sheet  10 , the prism sheet  20  is integrally formed by injection molding technology. The prism sheet  20  has a better rigidity and mechanical strength than the conventional prism sheet because the prism sheet is formed as a whole unit integrally. Thus the prism sheet  20  has a relatively high reliability. 
   Referring to  FIG. 7 , a prism sheet  30  in accordance with a second preferred embodiment is shown. The prism sheet  30  is similar in principle to the prism sheet  20  of the first embodiment, the second surface  303  defines a plurality of rectangular structures  307  and each rectangular structure  307  defines four adjacent triangular depressions  306 . The prism sheet  30  also defines a plurality of quadrilateral pyramids  308 . Each side of the rectangular structures  307  is bordered with one side of one quadrilateral pyramid  308 . 
   Referring to  FIG. 8 , a prism sheet  40  in accordance with a third preferred embodiment is shown. The prism sheet  40  is similar in principle to the prism sheet  20  of the first embodiment, a plurality of the triangular pyramid depressions  406  are defined in the second surface  403 . However a top end of the V-shaped ridge is flat. Since the top end of the V-shaped ridges is flat, strength of the V-shaped ridges is enhanced. As a result, the prism sheet  40  has a higher reliability than a prism sheet with the V-shaped ridges, the top of which would be easily damaged in use. 
   Referring to  FIG. 9 , a prism sheet  60  having a first surface  601  and in accordance with a fourth preferred embodiment is shown. The prism sheet  60  is similar in principle to the prism sheet  20  of the first embodiment, however a cross-section taken along a plane perpendicular to an extending direction of the first elongated arc-shaped grooves is elliptical. It can be understood that, a cross-section taken along a plane perpendicular to an extending direction of the second elongated arc-shaped grooves (not labeled) also can be elliptical. 
   Finally, while the preferred embodiment has 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.