Abstract:
An optical sheet with better brightness-enhancing effectiveness includes a substrate and pluralities of microstructures disposed on the substrate. The microstructures are spaced from one another at a distance d. The cross-section of the microstructure is formed in a triangle which has a base length D. Distance d and base length D satisfy the following equation: 0&lt;d/(d+D)≦0.61.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical sheet and, particularly, to a brightness enhancing optical sheet. 
   2. Description of the Prior Art 
   In recent years, the traditional cathode ray tube display (commonly called CRT display) is being gradually replaced by a liquid crystal display (LCD). This is mainly because the LCD releases far less radiation than the CRT display, and the production cost of the LCD also dropped significantly in recent years. In general, the LCD consists of two main elements, namely a backlight module and a liquid crystal panel. The backlight module mainly aims to provide light to the LCD. 
   Refer to  FIG. 1  for a conventional backlight module. The backlight module  100  mainly includes a cold cathode fluorescent lamp (CCFL)  110 , a reflection box  120 , a diffusion plate  130  and a plurality of optical films  140 . The CCFL  110  aims to generate light. The reflection box  120  aims to direct the light generated by the CCFL  110  towards the diffusion plate  130 . The optical films  140  includes a diffusion film  142  and a brightness-enhancement film (BEF)  144 . The diffusion plate  130  aims to diffuse the light generated by the CCFL  110  and project the light to a liquid crystal panel (not shown in the drawings) to make the light more uniform to prevent uneven brightness on the LCD. The diffusion plate  130  contains a plurality of light diffusion particles which lower the transmittance of the diffusion plate  130 . In general, the transmittance of the diffusion plate  130  is between 50%-70%. 
   However, the diffusion plate  130  often cannot fully overcome the problem of uneven brightness. Hence, a diffusion film  142  has to be added to diffuse the light more evenly. Moreover, as the light emission angle of the light emitted from the diffusion film  142  is larger, the BEF  144  has to be added on an upper side of the diffusion film  142 . The BEF  144  has a thickness about 0.062 mm to 0.375 mm. The BEF  144  mainly includes a substrate  144   a  and a plurality of microstructures  144   b  disposed on the substrate  144   a . The microstructures  144   b  are prism structures in a triangular shape, and each has a cross section in the form of an isosceles right triangle in the vertical direction. The BEF  144  provides a light converging effect and, thus, can enhance the brightness within the visual angle range of the backlight module  100 . 
   Because of manufacturing process and material, the BEF  144  is the most expensive in the cost of the backlight module  100 . Referring to  FIG. 2 , in order to reduce cost, a technique has been developed that forms a plurality of microstructures  130   b ′ on the diffusion plate  130 ′. The microstructures  130   b ′ are triangular struts to provide a light converging effect. Hence the diffusion plate  130 ′ can replace the BEF  144  and reduce the production cost of the backlight module  100 ′. However, the light converging effect provided by the diffusion plate  130 ′ still is not as desirable as the BEF  144 . Hence, how to improve the light converging effect of the diffusion plate is an issue remaining to be resolved in the industry. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an optical sheet with an improved light converging effect. 
   The optical sheet according to the invention includes a substrate and a plurality of microstructures disposed on the substrate. The microstructures are spaced from one another at a distance d, are formed in triangular struts, and have respectively a vertex angle of 90°. Namely the cross section of each of the microstructures is an isosceles right triangle with a base length D. Distance d and D are formed to satisfy the following equation:
 
0&lt; d /( d+D )&lt;0.61
 
   In another aspect, the optical sheet is formed with the distance d and base length D to satisfy the following equation:
 
0.03≦ d /( d+D )&lt;0.52
 
   In yet another aspect, the optical sheet is formed with the distance d and D to satisfy the following equation:
 
0.06≦ d /( d+D )&lt;0.38
 
   In yet another aspect, the optical sheet is formed with the distance d and D to satisfy the following equation:
 
0.08≦ d /( d+D )&lt;0.27
 
   In yet another aspect, the optical sheet is formed with the distance d and base length D to satisfy the following equation:
 
 d /( d+D )=0.13
 
   In yet another aspect, the optical sheet is formed with the distance d and base length D to satisfy the following equation:
 
 d /( d+D )=0.1
 
   In yet another aspect, the optical sheet is formed at a thickness between 0.5 mm and 2 mm or between 0.062 mm and 0.375 mm. 
   In yet another aspect, the optical sheet is made from material selected from the group consisting of polymethylmethacrylate, polycarbonate, polystyrene, methyl methacrylate-styrene monomers copolymer, polyvinyl, polypropylene and polyethylene terephthalate. 
   It is found that the optical sheet of the invention has a more desirable light converging effect in a condition in which the microstructures are spaced from one another at a selected distance and 0 21  d/(d+D)&lt;0.61. The light converging effect is optimum in the condition of d/(d+D)=0.13. 
   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 fragmentary enlarged view of a brightness-enhancing film. 
       FIG. 3  is a schematic view of an embodiment of the optical sheet of the invention. 
       FIGS. 4A ,  4 B and  4 C are charts showing the relationship of luminosity distribution and visual angle on the light emission surface of the optical sheet in the conditions of d/(D+d) being 0.0.1, 0.3 and 0.5. 
       FIG. 5  is a chart showing the relationship of the ratio of d/(D+d) and the luminosity on the light emission surface of the optical sheet with the visual angle being 0°. 
       FIG. 6  is a chart showing luminosity distribution on the light emission surface of the optical sheet which has a thickness of 1.376 mm with d/(D+d) being 0.1. 
       FIG. 7  is a fragmentary perspective view of another embodiment of the optical sheet of the invention. 
       FIG. 8  is a fragmentary perspective view of yet another embodiment of the optical sheet of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Refer to  FIG. 3  for an embodiment of the optical sheet of the invention. The optical sheet  230  includes a substrate  230   a  and a plurality of microstructures  230   b  disposed on the substrate  230   a . The microstructures  230   b  are spaced from one another at a selected distance d. Each microstructure  230   b  is formed in a triangular strut with a cross section of an isosceles right triangle having a vertex angle θ at 90°. The triangle has two equal lateral sides and a base length D. 
   Simulations for light converging effect are performed on the optical sheet  230 . In the simulations, incident light  10  is a uniform parallel light with an intensity of 1000 lumen. The substrate  230   a  has a length and width of 4.8 mm and a thickness of 0.6 mm. The optical sheet  230  is made from a transparent material selected from the group consisting of polymethylmethacrylate, polycarbonate, polystyrene, methyl methacrylate-styrene monomers copolymer, polyvinyl, polypropylene and polyethylene terephthalate. In this embodiment, the transparent material selected is polymethylmethacrylate. During the simulations, the sum of the base length D and the distance d is maintained a constant, namely 0.3 mm. The distance d is changed relative to the ratio of (D+d) to perform the simulations. 
   Refer to  FIGS. 4A ,  4 B and  4 C for charts which show the relationship of luminosity distribution and visual angle on the light emission surface of the optical sheet  230  in the conditions of d/(D+d) being 0, 0.1, 0.3 and 0.5. The horizontal coordinate represents the range of visual angle (−90°-90°), and the vertical coordinate represents the ratio of luminosity, namely the ratio against the brightness of the optical sheet  230  at a visual angle of 0° with no interval among the microstructures (such as the conventional diffusion plate  130 ′ with d/(D+d) being 0).  FIGS. 4A ,  4 B and C show that when the ratio of the distance d vs. (D+d) is 0.1, 0.3 and 0.5 for the microstructures  230   b , the brightness of the optical sheet  230  increases by 38%, 27% and 12% compared with the conventional diffusion plate  130 ′ on the light emission surface at the visual angle of 0°. 
   Refer to  FIG. 5  for the relationship of the ratio of d/(D+d) and the luminosity on the light emission surface of the optical sheet when the visual angle is 0°. The horizontal coordinate represents the ratio of d/(D+d), and the vertical coordinate represents the ratio of luminosity on the light emission surface of the optical sheet against the conventional diffusion plate  130 ′ (namely with d/(D+d) being 0). As shown in  FIG. 5 , when d/(D+d) is about 0.13, the optical sheet  230  has an optimum light converging effect. After the ratio of d/(D+d) is over 0.13, the light converging effect of the optical sheet  230  starts diminishing. When the ratio of d/(D+d) is over 0.61, the light converging effect of the optical sheet  230  is smaller than the conventional diffusion plate  130 ′. 
     FIG. 5  also indicates that when d/(D+d) is between 0.03 and 0.52, compared with the conventional diffusion plate  130 ′, the luminosity (with the visual angle of 0° on the light emission surface of the optical sheet  230  is higher by at least 10%. When d/(D+d) is between 0.06 and 0.38, compared with the conventional diffusion plate  130 ′, the luminosity (with the visual angle 0°) on the light emission surface of the optical sheet  230  is higher by at least 20%. When d/(D+d) is between 0.08 and 0.27, compared with the conventional diffusion plate  130 ′, the luminosity (with the visual angle of 0°) on the light emission surface of the optical sheet  230  is higher by at least 30%. When d/(D+d) is 0.13, compared with the conventional diffusion plate  130 ′, the luminosity (with the visual angle of 0°) on the light emission surface of the optical sheet  230  is higher by at least 40%. 
   In the simulations previously discussed, the thickness of the substrate  230   a  is 0.6 mm. Even if the thickness of the substrate is 1.376 mm, when the ratio of d/(D+d) is 0.1, and compared with the conventional technique (namely d/(D+d) being 0), the luminosity on the light emission surface of the optical sheet  230  is still higher by 30% (referring to  FIG. 6 ). It indicates that the light converging effect of the optical sheet  230  is mainly determined by the ratio of d/(D+d). 
   As a conclusion, when 0&lt;d/(d+D)&lt;0.61, compared with the conventional diffusion plate  130 ′, the optical sheet  230  has a more desirable light converging effect. When d/(D+d) is  0 . 13 , the optical sheet  230  has an optimum light converging effect. Moreover, the simulation outcomes indicate that even if the thickness of the substrate  230   a  decreases to 0.06 mm and when the ratio of d/(D+d) is 0.1, the luminosity on the light emission surface of the optical sheet  230  is still higher by about 40% than the conventional technique. Using the optical sheet  230  at the thickness of 0.06 mm to replace the BEF  144  in  FIG. 1  can achieve more a desirable light converging effect. 
   In short, the invention not only improves the light converging effect of the conventional diffusion plate  130  (with the thickness ranged from 0.5 mm to 2 mm), but also, can improve the light converging effect of the BEF  144  (with the thickness ranged from 0.062 mm to 0.375 mm). 
   In the embodiments set forth above, the microstructures are formed in triangular struts and have a cross section of an isosceles right triangle. To those skilled in the art, the shape of the microstructures may be changed, such as making the vertex angle other than 90°. Also, the microstructures may also be formed in a shape other than the triangular struts.  FIG. 7  illustrates another embodiment of the optical sheet  230 ′ on which the microstructures  230   b ′ are formed in a tortuous fashion on the top surface of the substrate  230   a ′.  FIG. 8  depicts yet another embodiment of the optical sheet  230 ″ on which the microstructures  230   b ″ are disposed in a tortuous fashion on a curved top surface of the substrate  230   a″.    
   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.