Patent Publication Number: US-2011058389-A1

Title: Brightness enhancement film and backlight module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98130611, filed on Sep. 10, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to an optical film and a light source module using the optical film, and more particularly, to a brightness enhancement film (BEF) and a backlight module using the BEF. 
     2. Description of Related Art 
     Along with the development of display technology, flat panel display has replaced the conventional bulky cathode ray tube (CRT) display as the mainstream of display devices. Liquid crystal display (LCD) is one of the most commonly used display among all flat panel displays. An LCD includes a liquid crystal panel and a backlight module. The liquid crystal panel may not emit light but determine the light transmittance. Thus, a backlight module may be disposed behind the liquid crystal panel as a surface light source of the liquid crystal panel. The optical quality of a surface light source is critical to the display quality of the LCD. For example, a uniform surface light source may be disposed in order to display images correctly and reduce distortion. In addition, the range of the light emitting angle of a surface light source may be restricted to reduce light loss and increase the brightness of displayed images. 
     In a conventional side-type backlight module, a lower diffuser, two prism sheets with orthogonal prisms, and an upper diffuser are sequentially disposed from bottom to top on a light guide plate. The prism sheets are used for reducing the range of the light emitting angle, and the upper diffuser and the lower diffuser are used for uniforming the light and preventing moiré produced by the contour of the prisms and the liquid crystal panel. However, because four optical films are disposed on the light guide plate, the fabricating cost of the backlight module is increased, the assembly of the backlight module is complicated, and the thickness of the backlight module may not be reduced. In addition, the adoption of four optical films may cause light loss and accordingly have difficult to improve the forward luminance of the backlight module. 
     In addition, the Taiwan patent publication No. 200911513 discloses an optical film structure disposed on a light guide plate, wherein the optical film structure has a light transmissive body and a reflective layer disposed on an incident surface of the light transmissive body, and a lens array is disposed on a light emitting surface of the light transmissive body. Besides, the reflective layer has openings corresponding to the lenses. Moreover, the U.S. patent publication No. 20070002452 also discloses such an optical film structure. However, because the LCDs on different electronic devices (for example, a cell phone, a notebook computer, a monitor, or a TV) have different requirements to brightness distribution in different directions, and the light emitting angle range of a backlight module adopting one of foregoing two optical film structures may not change with the change of directions, such a design concept is not adaptable to different types of electronic devices. Furthermore, the U.S. Pat. No. 7,374,328 and the Taiwan patent publication No. 200846774 disclose a diffusion layer disposed at the bottom of a reflective layer, so that the light is diffused. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention is directed to a brightness enhancement film (BEF), and the BEF may effectively increase the forward luminance of emitted light. 
     The invention is further directed to a backlight module, and the backlight module provides a surface light source with increased forward luminance 
     Additional aspects and advantages of the invention may be set forth in part in following descriptions. 
     In order to achieve at least one of the objectives, an embodiment of the invention provides a BEF including a light transmissive substrate, a plurality of optical structures, a reflective layer, and a prism layer. The light transmissive substrate has a first surface and a second surface opposite to the first surface. The optical structures are disposed on the first surface. The reflective layer is disposed on the second surface and has a plurality of light transmissive openings. The prism layer covers the reflective layer and the second surface and includes a plurality of prism structures protruded away from the second surface. 
     According to another embodiment of the invention, a backlight module including at least one light emitting device, the BEF described above, and an optical unit is provided. The light emitting device is capable of emitting a light beam. The BEF is disposed in the transmission path of the light beam. The optical unit is disposed in the transmission path of the light beam between the light emitting device and the BEF. 
     As described above, the embodiment or the embodiments of the invention may have at least one of the following advantages, in a BEF according to the embodiments of the invention, the prism structures of a prism layer refract an incident light so that the incident light may travel in a direction close to the normal direction of a first surface after the incident light passes through the prism structures, and when the incident light is reflected by the reflective layer and accordingly leaves the prism structures, the incident light may be refracted again by the surfaces of the prism structures and accordingly travels in a direction close to the normal direction of the first surface. Thereby, the forward luminance of the light emitted by the optical structures is increased, and a surface light source with higher brightness is provided by the backlight module according to the embodiments of the invention. 
     Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1A  and  FIG. 1B  are cross-sectional views of a backlight module in two orthogonal directions according to an embodiment of the invention. 
         FIG. 2A  is a three dimensional view of a brightness enhancement film (BEF) in  FIG. 1A . 
         FIG. 2B  is a top view of the BEF in  FIG. 2A . 
         FIG. 3  is a cross-sectional view of a backlight module according to another embodiment of the invention. 
         FIG. 4  is a cross-sectional view of a backlight module according to another embodiment of the invention. 
         FIG. 5  is a cross-sectional view of a backlight module according to another embodiment of the invention. 
         FIG. 6A  is a top view of a BEF according to another embodiment of the invention. 
         FIG. 6B  is a top view of a BEF according to another embodiment of the invention. 
         FIG. 7  is a cross-sectional view of a backlight module according to another embodiment of the invention. 
         FIG. 8  is a three dimensional view of a BEF according to another embodiment of the invention. 
         FIG. 9  is a three dimensional view of a BEF according to another embodiment of the invention. 
         FIG. 10  is a three dimensional view of a BEF according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” etc., is used with reference to the orientation of the Figure(s) being described. The components of the present invention can be positioned in a number of different orientations. As such, the directional terminology is used for purposes of illustration and is in no way limiting. On the other hand, the drawings are only schematic and the sizes of components may be exaggerated for clarity. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. Similarly, the terms “facing,” “faces” and variations thereof herein are used broadly and encompass direct and indirect facing, and “adjacent to” and variations thereof herein are used broadly and encompass directly and indirectly “adjacent to”. Therefore, the description of “A” component facing “B” component herein may contain the situations that “A” component directly faces “B” component or one or more additional components are between “A” component and “B” component. Also, the description of “A” component “adjacent to” “B” component herein may contain the situations that “A” component is directly “adjacent to” “B” component or one or more additional components are between “A” component and “B” component. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
     Referring to  FIG. 1A ,  FIG. 1B ,  FIG. 2A , and  FIG. 2B , in the embodiment, the backlight module  100  includes a light emitting device  110 , a brightness enhancement film (BEF)  200 , and an optical unit  300 . The light emitting device  110  is capable of emitting a light beam  112 . In the embodiment, the light emitting device  110  may be a cold cathode fluorescent lamp (CCFL). However, in another embodiment, the backlight module may have a plurality of light emitting devices, such as light emitting diodes (LEDs) arranged in a straight line. 
     The BEF  200  is disposed in the transmission path of the light beam  112 . The optical unit  300  is disposed in the transmission path of the light beam  112  between the light emitting device  110  and the BEF  200 . In the embodiment, the optical unit  300  includes a light guide plate  310  having a surface  312 , a surface  314  opposite to the surface  312 , and an incident surface  316  connecting the surface  312  and the surface  314 . The light emitting device  110  is disposed beside the incident surface  316 . To be specific, the light beam  112  emitted by the light emitting device  110  enters the light guide plate  310  through the incident surface  316 , and the light beam  112  is totally internally reflected by the surface  312  and the surface  314  and therefore is restricted within the light guide plate  310 . However, the microstructures  315  on the surface  314  of the light guide plate  310  destroy the total internal reflection. For example, a portion of the light beam  112  is reflected by the microstructures  315  to the surface  312  and passes through the surface  312 . Another portion of the light beam  112  passes through the microstructures  315  and reaches a reflector  320  disposed at one side of the surface  314 . The reflector  320  reflects the light beam  112  so that the light beam  112  sequentially passes through the surface  314  and the surface  312 . 
     The BEF  200  includes a light transmissive substrate  210 , a plurality of optical structures  220 , a reflective layer  230 , and a prism layer  240 . The light transmissive substrate  210  has a first surface  212  and a second surface  214  opposite to the first surface  212 . The optical structures  220  are disposed on the first surface  212 . In the embodiment, each of the optical structures  220  is a lens and has a convex surface  222  facing away from the light transmissive substrate  210 . The curvature radius of the convex surface  222  in a first direction D 1  parallel to the first surface  212  is R 1 , and the curvature radius of the convex surface  222  in a second direction D 2  parallel to the first surface  212  is R 2 , wherein R 1 ≠R 2 . However, in another embodiment, there may be R 1 =R 2 . In the embodiment, the convex surface  222  may be a smooth curved surface or may be composed of a plurality of micro straight or curved line segments. Besides, in the embodiment, the first direction D 1  is substantially perpendicular to the second direction D 2 . The reflective layer  230  is disposed on the second surface  214  and has a plurality of light transmissive openings  232 , wherein the light transmissive openings  232  are respectively located on optical axes X of the optical structures  220 . In the embodiment, the reflective layer  230  is located between the light transmissive substrate  210  and the surface  312 . The distance between a vertex T of the convex surface  222  of the optical structure  220  and the corresponding light transmissive opening  232  is L, and the refractive index of the optical structures  220  is n. In the embodiment, the BEF  200  satisfies L&lt;nR 1 /(n−1) and L&lt;nR 2 /(n−1). 
     If the light beam  112  leaves the surface  312  at a large angle, a great part of the light beam  112  is reflected by the reflective layer  230  back into the light guide plate  310  to be used again. If the light beam  112  leaves the surface  312  at a small angle, a great part of the light beam  112  passes through the light transmissive openings  232 . The optical power distribution of the light beam  112  passing through the light transmissive openings  232  may be close to Gaussian distribution, and the light beam  112  is focused by the optical structures  220  and therefore is emitted from the optical structures  220  in a direction approximately perpendicular to the first surface  212 . Thereby, unlike the conventional technique with four optical films, the backlight module  100  provided by the embodiment offers a reduced light emitting angle range, and accordingly increased brightness to a liquid crystal display (LCD), with a single optical film (i.e., the BEF  200 ). 
     Moreover, the prism layer  240  covers the reflective layer  230  and the second surface  214 , and the prism layer  240  includes a plurality of prism structures  242  protruded away from the second surface  214 . In the embodiment, each of prism structures  242  is a prism rod, such as a triangular prism. The prism structures  242  are arranged along the first direction D 1 , and each of the prism structures  242  is extended along the second direction D 2 . In the embodiment, each of the prism structures  242  has a first prism face  244  and a second prism face  246 , wherein the first prism face  244  and the second prism face  246  are extended along the second direction D 2 . In the embodiment, each of the prism structures  242  is non-mirror-symmetrical in the first direction D 1 . In other words, the normal vector N 1  of the first prism face  244  forms an angle θ 1  with the normal vector N 4  of the first surface  212 , and the normal vector N 2  of the second prism face  246  forms an angle θ 2  with the normal vector N 4  of the first surface  212 , wherein θ 1 ≠θ 2 . In the embodiment, the angle θ 1  falls within a range of 130 ˜170 degrees, and the angle θ 2  falls within a range of 90˜110 degrees. However, the invention is not limited thereto. Additionally, in the embodiment, the first surface  212  is substantially parallel to the second surface  214 , and the normal vector N 3  of the incident surface  316 , the normal vector N 1 , the normal vector N 2 , and the normal vector N 4  are coplanar. However, the invention is not limited thereto. 
     The first prism face  244  refracts the light beam  112  so that the light beam  112  is transmitted in a direction close to the normal direction of the first surface  212 . To be specific, after a portion of the light beam  112  (a partial light beam  112   a ) leaves the surface  312 , the partial light beam  112   a  is refracted by the first prism face  244  so that the partial beam  112   a  is transmitted in a direction close to the normal direction of the first surface  212 . Accordingly, the partial light beam  112   a  may be emitted toward the corresponding optical structure  220  right above a light transmissive opening  232  instead of another optical structure  220  beside the corresponding optical structure  220  after the partial light beam  112   a  passes through the light transmissive opening  232 . After the partial light beam  112   a  reaches another optical structure  220  beside the corresponding optical structure  220 , the travelling direction of the partial light beam  112   a  still greatly deviates from the normal direction of the first surface  212  and accordingly invalid light is produced. The first prism face  244  in the embodiment may effectively reduce such a problem. In the embodiment, because the light beam  112  passing through the light transmissive openings  232  is ensured to pass through the corresponding optical structures  220  and the optical structures  220  allow the light beam  112  to be emitted straightly, the BEF  200  in the embodiment may effectively increase the forward luminance and accordingly increase the brightness of the surface light source provided by the backlight module  100 . 
     On the other hand, after a partial light beam  112   b  in the light beam  112  leaves the surface  312 , the partial light beam  112   b  is refracted by the first prism faces  244  so that the partial light beam  112   b  is transmitted in a direction close to the normal direction of the first surface  212 . After that, the partial light beam  112   b  is reflected by the reflective layer  230  back to the first prism faces  244 . Then, the first prism faces  244  refract the partial light beam  112   b  again so that the partial light beam  112   b  is transmitted in a direction close to the normal direction of the first surface  212  again. After that, the partial light beam  112   b  returns to the light guide plate  310  to be used again. Accordingly, every time when the partial light beam  112   b  is reflected by the reflective layer  230  and accordingly returns to the light guide plate  310 , the transmission direction of the partial light beam  112  gets closer to the normal direction of the first surface  212 , so that the partial light beam  112   b  may quickly pass through the light transmissive openings  232 . Thus, in the backlight module  100  provided by the embodiment, the number of times that the light beam  112  is reflected between the reflective layer  230  and the reflector  320  before passing through the light transmissive openings  232  may be effectively reduced, so that the loss of optical power is reduced and the forward luminance of the backlight module  100  is increased. 
     Additionally, in the embodiment, because R 1 ≠R 2 , the BEF  200  may be applied to backlight modules having different requirements to the ranges of the light emitting angle in different directions. By appropriately setting the values of the R 1  and R 2 , the backlight module  100  adopting the BEF  200  may be applied to the displays of different electronic devices, such as the LCD of a cell phone, a notebook computer, a monitor, or a TV. 
     In the embodiment, the width of the light transmissive openings  232  in the first direction D 1  is not equal to the width of the light transmissive openings  232  in the second direction D 2 . However, in another embodiment, the width of the light transmissive openings  232  in the first direction D 1  may also be equal to the width of the light transmissive openings  232  in the second direction D 2 . In the embodiment, the width of the light transmissive openings  232  in the first direction D 1  is A 1 , the width of the light transmissive openings  232  in the second direction D 2  is A 2 , the width of the convex surfaces  222  corresponding to the light transmissive openings  232  in the first direction D 1  is P 1 , the width of the convex surfaces  222  corresponding to the light transmissive openings  232  in the second direction D 2  is P 2 , and BEF  200  satisfies 0.1&lt;A 1 /P 1 &lt;0.9 and 0.1&lt;A 2 /P 2 &lt;0.9. Thereby, the range of the light emitting angle in the first direction D 1  and the range of the light emitting angle range in the second direction D 2  may have increased variations, and accordingly the BEF  200  and the backlight module  100  may be applied more broadly. 
     In the embodiment, the light transmissive openings  232  of the reflective layer  230  may be formed through a laser drilling technique. To be specific, the reflective layer  230  entirely covers the second surface  214  before a laser drilling process is performed. Then, parallel laser beams are irradiated onto the optical structures  220  from right above the BEF  200  illustrated in  FIG. 1A  (i.e., along a direction perpendicular to the first direction D 1  and the second direction D 2 ). Through the focusing effect of the optical structures  220 , the light spots produced by the laser beams on the reflective layer  230  are the positions of the light transmissive openings  232 . Because the light spots have uniform luminance distribution, the light transmissive openings  232  having similar sizes as the light spots may be drilled on the reflective layer  230  as long as the laser beams have sufficient power, and such luminance distribution of the light spots is achieved when the BEF  200  satisfies L&lt;nR 1 /(n−1) and L&lt;nR 2 /(n−1). Thus, the light transmissive openings  232  with expected sizes and positions may be formed through a single drilling process with the parallel laser light beams. Thereby, the design of the BEF  200  in the embodiment simplifies the fabricating process and reduces the cost of the backlight module  100 . Contrarily, if the BEF  200  satisfies L&gt;nR 1 /(n−1) and L&gt;nR 2 /(n−1), the central luminance of the light spots is greater than the peripheral luminance of the light spots, and the power distribution of the light spots has no obvious boundary, so that controlling the sizes of the light transmissive openings  232  is difficult. As a result, the sizes of the obtained light transmissive openings  232  may be smaller than the sizes of the light spots and not up to expectation. Accordingly, the incident angle of the laser beams may be changed and multiple drilling processes may be performed by using the laser beams in order to obtain the light transmissive openings  232  having expected sizes and positions, so that the fabricating process may be complicated and the fabrication cost and fabrication time may be increased. 
     Besides, the light beam  112  passing through the BEF  200 , and accordingly the surface light source provided by the backlight module  100  in the embodiment, may be uniformed if the BEF  200  is made to satisfy L&lt;nR 1 /(n−1) and L&lt;nR 2 /(n−1). In order to further improve the uniformity of the light beam  112  passing through the BEF  200 , in the embodiment, the BEF  200  is further made to satisfy L&lt;0.95 nR 1 /(n−1) and L&lt;0.95 nR 2 /(n−1). 
     Referring to  FIG. 3 , the backlight module  100   a  in the embodiment is similar to the backlight module  100  illustrated in  FIG. 1A , and the main difference between the backlight module  100  and the backlight module  100   a  may be described herein. In the backlight module  100  illustrated in  FIG. 1A , each of the prism structures  242  has substantially the same size. However, in the BEF  200   a  provided by the embodiment, at least parts of the prism structures  242 ′ have different widths in the direction parallel to the second surface  214  (for example, the first direction D 1 ), and at least parts of the prism structures  242 ′ have different heights in the direction perpendicular to the second surface  214 , so that the regularity of the prism structures  242 ′ is broken, and moiré produced by the prism structures  242 ′ and the pixel array on the display panel (not shown) disposed above the backlight module  100   a  is reduced. In the embodiment, the positions of the prism structures  242 ′ may not be corresponding to the positions of the light transmissive openings  232 . However, in another embodiment, the positions of the prism structures  242 ′ may also be corresponding to the positions of the light transmissive openings  232  appropriately. 
     Referring to  FIG. 4 , the backlight module  100   b  in the embodiment is similar to the backlight module  100  illustrated in  FIG. 1A , and the difference between the backlight module  100  and the backlight module  100   b  may be described herein. In the backlight module  100   b  provided by the embodiment, two light sources  110  are respectively disposed at two opposite sides of the light guide plate  310 . Besides, in the embodiment, the prism structures  242 ″ of the BEF  200   b  are in mirror symmetry in the first direction D 1 . To be specific, the normal vector N 1 ′ of the first prism faces  244 ″ of the prism structures  242 ″ forms an angle θ 1 ′ with the normal vector N 4  of the first surface  212 , and the normal vector N 2 ′ of the second prism faces  246 ″ of the prism structures  242 ″ forms an angle θ 2  with the normal vector N 4  of the first surface  212 ′, wherein θ 1 ′=θ 2 ′. Such a design is suitable for the backlight module  100   b  with two light incident directions. In the embodiment, both the angles θ 1 ′ and θ 2 ′ fall within a range of 130˜170 degrees. However, the invention is not limited thereto. Besides, the first prism faces  244 ″ are capable of refracting the light emitted by the light emitting device  110  at the left side in  FIG. 4  so that the light is transmitted in a direction close to the normal direction of the first surface  212 , and the second prism faces  246 ″ are capable of refracting the light emitted by the light emitting device  110  at the right side in  FIG. 4  so that the light is transmitted in a direction close to the normal direction of the first surface  212 . 
     Referring to  FIG. 5 , the backlight module  100   c  in the embodiment is similar to the backlight module  100  illustrated in  FIG. 1A  and  FIG. 1B , and the main difference between the backlight module  100  and the backlight module  100   c  may be described herein. In the embodiment, the prism structures  242 ″ of the BEF  200   c  in the embodiment are polygonal pyramids, such as tetragonal pyramids. The cross section of each tetragonal pyramid in another direction is the same as the cross section illustrated in  FIG. 1A . In other words, each tetragonal pyramid includes cross sections connected to each other, such as the first prism face  244  and the second prism face  246  illustrated in  FIG. 1A  and the third prism face  248  and the fourth prism face  249  illustrated in  FIG. 5 . The prism structures  242 ″ may refract light in both the first direction D 1  and the second direction D 2  so that the light may be transmitted in a direction close to the normal direction of the first surface  212 . 
     Referring to  FIG. 6A , the BEF  200 ′ in the embodiment is similar to the BEF  200  in  FIG. 2B , and the difference between the backlight module  200  and the backlight module  200 ′ may be described herein. In the BEF  200 ′ provided by the embodiment, at least parts of the optical structures  220  have different widths P 1  in the first direction D 1 . The ratio of a maximum value among the widths P 1  of the lenses in the first direction D 1  to a minimum value among the widths P 1  of the lenses in the first direction D 1  is between 1 and 4. In addition, in the embodiment, at least parts of the optical structures  220  have different widths P 2  in the second direction D 2 . The ratio of a maximum value among the widths P 2  of the lenses in the second direction D 2  to a minimum value among the widths P 2  of the lenses in the second direction D 2  is between 1 and 4. Thereby, moiré produced by the BEF  200 ′ and a LCD panel (not shown) disposed on the BEF  200 ′ may be reduced through the irregular design of the sizes and positions of the optical structures  220 . 
     Referring to  FIG. 6A  and  FIG. 6B , the difference between the BEF  200 ″ (as shown in  FIG. 6B ) and the BEF  200 ′ (as shown in  FIG. 6A ) described above may be described herein. In the BEF  200 ′, the optical structures  220  of a same column in a direction (for example, the first direction D 1 ) have substantially the same width P 2 , and at least parts of the optical structures  220  of a same column in another direction (for example, the second direction D 2 ) have different widths P 1 . However, in the BEF  200 ″, at least parts of the optical structures  220  of a same column have different widths P 1  or P 2  in both the first direction D 1  and the second direction D 2 . The BEF  200 ″ has higher irregularity, while the BEF  200 ′ is easier to design and fabricate. 
     Referring to  FIG. 7 , the backlight module  100   d  in the embodiment is partially similar to the backlight module  100  illustrated in  FIG. 1A , and the difference between the backlight module  100  and the backlight module  100   d  may be described herein. The backlight module  100  in  FIG. 1A  is a side-type backlight module, while the backlight module  100   d  in the embodiment is a direct-type backlight module. To be specific, the optical unit  300   a  includes a diffusion plate  330  disposed between the BEF  200   b  and a plurality of light emitting devices  110 , and this is an characteristic of direct-type backlight modules. The light beams  112  emitted by the light emitting devices  110  pass through the diffusion plate  330  to reach the BEF  200  and are diffused by the diffusion plate  330 . In the embodiment, the backlight module  100   d  further includes a light box  340 , and the light emitting devices  110  are disposed in the light box  340 . The internal wall of the light box  340  has reflection function and may reflect the light beams  112  emitted by the light emitting devices  110  to the diffusion plate  330 . 
     Referring to  FIG. 8 , the BEF  200   d  in the embodiment is similar to the BEF  200  illustrated in  FIG. 2A , and the main difference between the backlight module  200  and the backlight module  200   d  is that the optical structures  220 ′ in the embodiment are lenticulars with rod-shaped convex surfaces  222 ′. 
     Referring to  FIG. 9 , the BEF  200   e  in the embodiment is similar to the BEF  200  illustrated in  FIG. 2A , and the main difference between the backlight module  200  and the backlight module  200   e  is that the optical structures  220 ″ in the embodiment are polygonal-pyramid-shaped prisms, such as tetragonal pyramid prisms. 
     Referring to  FIG. 10 , the BEF  200   f  in the embodiment is similar to the BEF  200  illustrated in  FIG. 2A , and the main difference between the backlight module  200  and the backlight module  200   f  is that the optical structures  220 ′″ in the embodiment are rod-shaped prisms, such as triangular rod prisms. 
     The type of the optical structures in the BEF is not limited in the invention, and in other embodiments, the optical structures may be any combination of lenses, lenticulars, polygonal-pyramid-shaped prisms, rod-shaped prisms, and other types of optical structures. 
     As described above, the embodiment or the embodiments of the invention may have at least one of the following advantages, in the BEF according to the embodiments of the invention, the prism structures of a prism layer may refract incident light and allow the incident light to be transmitted in a direction close to the normal direction of a first surface after the incident light passes through the prism structures, and when the incident light is reflected by a reflective layer and accordingly leaves the prism structures, the incident light is refracted by the surface of the prism structures again so that the incident light is transmitted in a direction closer to the normal direction of the first surface. Thereby, the forward luminance of the light emitted by the optical structures, and accordingly the brightness of the surface light source provided by the backlight module in the invention is increased. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like does not necessarily limit the claim scope to a specific embodiment, and the reference to particularly preferred exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.