Abstract:
A backlight includes a case, pluralities of light sources, pluralities of needles, pluralities of supporting portions, a first optical sheet, a second optical sheet, and pluralities of first microstructures. The light sources are disposed inside the case, the needles are disposed on the bottom surface of the case, and the supporting portions are disposed on the needles. The first optical sheet is supported on the top of the needles. The second optical sheet placed under the first optical sheet has pluralities of holes, and the needles penetrate the holes. The second optical sheet is supported on the supporting portions. The first microstructures are disposed on the first optical sheet or on the second optical sheet to provide a light condensing function.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a backlight module and particularly to a backlight module which has two optical sheets. 
   2. Description of the Prior Art 
   In recent years, the traditional cathode ray tube display (commonly called CRT display) has been gradually replaced by a liquid crystal display (LCD). The main reason is that the LCD releases far less radiation than the CRT display. Moreover, the production cost of the LCD has dropped significantly in recent years. In general, the LCD includes a backlight module and a liquid crystal panel. The backlight module mainly aims to provide a light source for the LCD. 
   Refer to  FIG. 1  for a conventional backlight module  100  which includes a case  110 , a cold cathode fluorescent lamp (CCFL)  120 , a diffusion plate  130 , an optical film  140  and a plurality of needles  150 . The CCFL  120  is located in the case  110  which has a reflective sheet (not shown in the drawing) on the inside surface to use light generated by the CCFL  120  more effectively. The diffusion plate  130  mainly diffuses the light generated by the CCFL  120  so that the light can be projected more evenly to the liquid crystal panel (not shown in the drawing) without creating uneven luminosity on the display of the LCD. The needles  150  aim to support the diffusion plate  130  to prevent it from warping due to its own weight. 
   In addition, the optical film  140  further includes a brightness enhancement film (BEF)  144 . As the diffusion plate  130  cannot fully overcome the uneven luminosity, a diffusion film  142  is added to evenly diffuse the light. Moreover, light exited from the diffusion film  142  has a greater light exit angle. Hence, the BEF  144  is disposed on the diffusion film  142 . The BEF  144  has a thickness about 0.062 mm to 0.375 mm and a plurality of prism structures  144   a  formed at an upper side thereof to converge light. Thus, the exit angle of the light emitting from the BEF  144  becomes smaller. Therefore, the luminosity within the visual angle of the backlight module  100  is enhanced. 
   Due to manufacturing process and material, the BEF  144  often is the most expensive item in the backlight module  100 . In order to reduce the total cost of the backlight module  100 , some producers use a less expensive prism plate to replace the BEF  144 . However, the prism plate is thicker and, thus, the total thickness of the backlight module  100  also becomes thicker. Hence, how to use the prism plate without increasing the thickness of the backlight module  100  is an issue remained to be resolved in the industry. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a backlight module that has two optical sheets without increasing the total thickness of the backlight module. 
   The backlight module according to the invention includes a case, a plurality of light sources, a plurality of needles, a plurality of supporting portions, a first optical sheet, a second optical sheet and a plurality of first microstructures. The light sources are located in the case. The needles are mounted onto the bottom surface of the case. The supporting portions are located on the needles. The first optical sheet is held at the top of the needles. The second optical sheet is located beneath the first optical sheet and has a plurality of holes run through by the needles. The second optical sheet is also held by the supporting portions. The first microstructures are located on the first optical sheet or the second optical sheet and provide a light converging function. 
   In one aspect of the backlight module, the needles and the supporting portions have a reflective material coated on the surfaces thereof. 
   In another aspect, the needles and the supporting portions are made from transparent material. 
   In yet another aspect, the backlight module further includes a plurality of latch elements to run through side walls of the case with one end pressing an upper surface of the second optical sheet. 
   In yet another aspect, each needle has a stem and a needle head. The supporting portions are located on the stem. The needle head is flexible and has a bottom spaced from the supporting portions at a distance which is the same as the thickness of the second optical sheet. When the needle does not receive an external force, the needle head has an outside diameter greater than the inner diameter of the holes of the second optical sheet. Moreover, the needle head further includes a holding plate and a plurality of warping blades. The warping blades bridge the holding plate and the stem. 
   In yet another aspect, the first microstructures are located on the first optical sheet. The first optical sheet is formed at a thickness greater than 0.5 mm with a penetrative rate at 90% or more. The second optical sheet is formed at a thickness greater than 0.5 mm with a penetrative rate between 50%-70%. The first microstructures are prisms. 
   In yet another aspect, the backlight module according to the invention includes a case, a plurality of light sources, a plurality of needles, a first optical sheet, a second optical sheet and a plurality of first microstructures. The light sources are located in the case. The needles are mounted on the bottom surface of the case and have respectively a conical surface. The first optical sheet is held at the top of the needles. The second optical sheet is located beneath the first optical sheet and has a plurality of holes run through by the needles. The second optical sheet is held by the conical surface of the needles. The first microstructures are located on the first optical sheet or the second optical sheet and provide a light converging function. The needles and the holes are coupled in a compact manner. 
   Due to the backlight module of the invention having two optical sheets, due to the second optical sheet having a plurality of holes run through by the needles, and since the second optical sheet is located in a lower chamber of the backlight module, the total thickness of the backlight module does not increase even containing the two optical sheets. 
   The foregoing, as well as additional objects, features and advantages of the invention, will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a conventional backlight module. 
       FIG. 2  is a schematic view of a first embodiment of the backlight module of the invention. 
       FIG. 3  is a schematic view of a second optical sheet of the backlight module. 
       FIG. 4  is a schematic view of a second embodiment of the backlight module of the invention. 
       FIG. 5  is a perspective view of a needle according to  FIG. 4 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Please refer to  FIGS. 2 and 3  for a first embodiment of the backlight module and the second optical sheet of the invention. The backlight module  200  includes a case  210 , a plurality of light sources  220 , a first optical sheet  270 , a second optical sheet  230  and a plurality of needles  250 . The light sources  220  are CCFLs and located in the case  210 . Of course to those skilled in the art, the CCFLs may also be substituted by light emitting diodes. In this embodiment, the needles  250  are mounted onto the bottom surface of the case  210 . Each needle  250  has a stem  254  and a base  255 . The stem  254  is conical. The first optical sheet  270  is held at the top of the needles  250  and has a plurality of first microstructures  270   a  formed thereon that are prisms to provide a light converging function. Of course to those skilled in the art, the first microstructures  270   a  may also adopt other designs to provide the light converging function, such as a conical shape. 
   Referring to  FIGS. 2 and 3 , the second optical sheet  230  has a plurality of holes  232 . Each hole  232  has a conical or cylindrical surface to mate with the stem  254  of the needles  250  to form a compact coupling. As shown in  FIG. 2 , the second optical sheet  230  is rested on the conical surface of the stem  254  of the needles  250  and held in a lower chamber  202  (not shown in  FIG. 2 ) of the backlight module  200 . Hence, compared with the backlight module  100  shown in  FIG. 1 , even if the backlight module  200  contains the second optical sheet  230 , the total thickness thereof does not increase. Moreover, as the hole  232  and the stem  254  form a compact coupling, the second optical sheet  230  does not loosen off even if the backlight module  200  is moved at different positions such as in an upright or upside down position. In addition, a plurality of latch elements  280  may be provided at lateral sides of the backlight module  200 . The latch elements  280  run through side walls of the case  210  to press an upper surface of the second optical sheet  230  to hold the second optical sheet  230  more firmly. Of course to those skilled in the art, the latch elements  280  may also be dispensed with, according to actual conditions. 
   The first optical sheet  270  and the second optical sheet  230  are made from transparent material and are blended with multiple light diffusion particles. The refractive index of the light diffusion particles is different from the transparent material. Hence, when the light passes through the first optical sheet  270  and the second optical sheet  230 , it passes through the two media of different refractive indices to generate refraction, reflection and scattering to diffuse the light. The number of the light diffusion particles in the second optical sheet  230  is greater than the first optical sheet  270  and, thus, the second optical sheet  230  has a lower penetration rate. The transparent material may be selected from Polymethyl Methacrylate (PMMA), Polycarbonate (PC), Polystyrene (PS), Methyl Methacrylate Styrene (MS), Polypropene (PP), Polythylene (PE), or Polythylene terephthalate (PET). Moreover, the first optical sheet  270  may include no light diffusion particles. In this embodiment, an optical measurement instrument (model No. NDH2000) produced by Japan KEISOKU GIKEN is used to measure the penetration rate of the first optical sheet  270  and the second optical sheet  230 . The first optical sheet  270  and the second optical sheet  230  may also be formed by extruding. 
   Referring to  FIG. 2 , as the first optical sheet  270  and the second optical sheet  230  are spaced at a distance, light can be fully mixed in the space. In the event that the first optical sheet  270  contains a smaller amount of the light diffusion particles, the light can be further diffused. Hence, the light can be evenly diffused even without including a diffusion film  142  in the backlight module  200 . As a result, the total cost of the backlight module  200  can be reduced. 
   In the embodiment set forth above, the first microstructures  270   a  are formed on the first optical sheet  270 . To those skilled in the art, the first microstructures  270   a  may also be formed on the second optical sheet  230 . Moreover, besides the light exit surface of the first microstructures  270   a , the microstructures may also be formed at other locations, such as a light incident surface of the first optical sheet  270 , a light exit surface of the second optical sheet  230  or a light incident surface of the second optical sheet  230 . In addition, in the embodiment set forth above, the first optical sheet  270  has a greater penetration rate, while the second optical sheet  230  has a lower penetration rate. To those skilled in the art, the aforesaid condition may be altered by having a higher penetration rate for the second optical sheet  230 . 
   In the first embodiment, the holes  232  and the stem  254  are coupled in a compact manner to anchor the second optical sheet  230 . Of course to those skilled in the art, other means may also be used to anchor the second optical sheet  230  as shown in a second embodiment discussed below. 
   Refer to  FIGS. 4 and 5  for the second embodiment of the invention that show the backlight module and the needle. The needle  250 ′ has a stem  254 ′, a needle head  258 ′ and a supporting portion  256 ′. The supporting portion  256 ′ is located on the stem  254 ′. The needle head  258 ′ is flexible. In the second embodiment, the needle head  258 ′ has a holding plate  258   a ′ and a plurality of warping blades  258   b ′. The warping blades  258   b ′ bridge the holding plate  258   a ′ and the stem  254 ′. The needle head  258 ′ has a bottom spaced from the supporting portion  256 ′ at a distance which is the same as the thickness of the second optical sheet  230 . When the needle head  258 ′ does not receive an external force, the needle head  258 ′ has an outer diameter greater than the inner diameter of the hole  232  of the second optical sheet  230 . As the needle head  258 ′ is flexible and during assembly of the second optical sheet  230  by pressing the needle head  258 ′, the second optical sheet  230  can be moved and held between the bottom of the needle head  258 ′ and the supporting portion  256 ′. Moreover, when the external force is absent from the needle head  258 ′, the outer diameter thereof is greater than the inner diameter of the hole  232  of the second optical sheet  230 . Hence, an anchoring effect for the second optical sheet  230  is accomplished. Therefore, even if the position of the backlight module  200 ′ is changed, the second optical sheet  230  does not loosen off. 
   Referring to  FIGS. 4 and 5 , the needle  250 ′ may further include a base  255 ′ and a tail end  252 ′. The tail end  252 ′ is constructed the same as the needle head  258 ′ and also is flexible. Moreover, the tail end  252 ′ is spaced from the base  255 ′ at a distance approximate to the thickness of the case  210 . Hence, by applying a selected external force on the tail end  252 ′, the needle  250 ′ can be anchored on the bottom surface of the case  210 . 
   In the first and second embodiments set forth above, the needles  250  and  250 ′ are made from a transparent material, or the needles  250  and  250 ′ and the supporting portions  256 ′ may be coated with a reflective material on the surfaces thereof to increase utilization of the light sources  220 . 
   While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.