Patent Publication Number: US-2023141295-A1

Title: Backlight module and display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of China application serial no. 202111324271.9 filed on Nov. 10, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a light source module and a display apparatus, and more particularly to a backlight module and a display apparatus. 
     Description of Related Art 
     With the increasing application of non-self-luminous displays such as liquid-crystal displays, the design of the backlight module also needs to be adjusted for different applications. In order to meet the display requirements of large size, high dynamic range (HDR), and high contrast, the backlight module needs to have the function of local dimming. Therefore, direct-lit backlight modules using light-emitting diodes as the main light source have gradually become the mainstream in the market. Since the overall thickness of this type of backlight module is thinner (for example, the optical distance is less than 10 mm), and the light-emitting diode is a point light source, the backlight module readily forms a brighter light output above the light-emitting diode, i.e., a hot spot phenomenon, causing the overall brightness uniformity of the light-emitting surface to be decreased. In order to solve this issue, a technical solution of covering the light-emitting diode with a light-shielding film having light-shielding dots is provided. However, this technique requires very high assembly accuracy of the backlight module. 
     The information disclosed in this Background section is only for enhancement of understanding of the background of the described technology and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Further, the information disclosed in the Background section does not mean that one or more problems to be resolved by one or more embodiments of the invention was acknowledged by a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The invention provides a backlight module having better light output uniformity but lower production cost. 
     The invention provides a display apparatus having better display quality. 
     Other objects and advantages of the invention may be further understood from the technical features disclosed in the invention. 
     In order to achieve one or a portion of or all of the above objectives or other objectives, an embodiment of the invention provides a backlight module. The backlight module includes a circuit substrate, a plurality of light-emitting devices, a first cholesteric liquid-crystal layer, and a second cholesteric liquid-crystal layer. The light-emitting devices are disposed on the circuit substrate and electrically connected to the circuit substrate. The first cholesteric liquid-crystal layer is disposed on the light-emitting devices and overlapped with a light-emitting surface of each of the light-emitting devices. The second cholesteric liquid-crystal layer is disposed on the first cholesteric liquid-crystal layer and overlapped with the first cholesteric liquid-crystal layer. The first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer respectively have a first chiral direction and a second chiral direction. 
     In order to achieve one or a portion of or all of the above objectives or other objectives, an embodiment of the invention provides a display apparatus. The display apparatus includes a backlight module, a display panel, and a quarter-wave plate. The backlight module includes a circuit substrate, a plurality of light-emitting devices, a first cholesteric liquid-crystal layer, and a second cholesteric liquid-crystal layer. The light-emitting devices are disposed on the circuit substrate and electrically connected to the circuit substrate. The first cholesteric liquid-crystal layer is disposed on the light-emitting devices and overlapped with a light-emitting surface of each of the light-emitting devices. The second cholesteric liquid-crystal layer is disposed on the first cholesteric liquid-crystal layer and overlapped with the first cholesteric liquid-crystal layer. The first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer respectively have a first chiral direction and a second chiral direction. The display panel is overlapped at the backlight module and has a liquid-crystal cell and a polarizer located between the liquid-crystal cell and the backlight module. The quarter-wave plate is disposed between the backlight module and the display panel. An included angle between a slow axis of the quarter-wave plate and an absorption axis of the polarizer is 45 degrees. 
     Based on the above, in the backlight module and the display apparatus of an embodiment of the invention, by arranging two cholesteric liquid-crystal layers having the same or opposite chiral directions on the light-emitting side of the light-emitting devices, a portion of the forward light emitted by the light-emitting devices may be directed to a region not overlapped with the light-emitting devices, so as to effectively improve the overall light uniformity of the backlight module. Moreover, compared with the general use of a light-shielding film technique having light-shielding dots, the cholesteric liquid-crystal layers and the light-emitting devices of the invention do not have issues in assembly accuracy. Therefore, the manufacturing process flexibility of the backlight module is better and the production cost is also lower. 
     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.  1    is a schematic cross-sectional view of a backlight module according to the first embodiment of the invention. 
         FIG.  2    is a schematic cross-sectional view of a backlight module according to the second embodiment of the invention. 
         FIG.  3    is a schematic cross-sectional view of a backlight module according to the third embodiment of the invention. 
         FIG.  4    is a schematic cross-sectional view of a display apparatus according to an embodiment of the invention. 
         FIG.  5    is a schematic top view of the polarizer and the quarter-wave plate of  FIG.  4   . 
         FIG.  6    is a schematic cross-sectional view of a backlight module according to the fourth embodiment of the invention. 
         FIG.  7    is a schematic cross-sectional view of a backlight module according to the fifth embodiment of the invention. 
         FIG.  8    is a schematic cross-sectional view of a backlight module according to the sixth embodiment of the invention. 
         FIG.  9 A  is a graph of reflectivity versus incident angle of the first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer of  FIG.  8   . 
         FIG.  9 B  and  FIG.  9 C  are graphs of reflectivity versus incident angle of the first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer according to some other modified embodiments of the backlight module of  FIG.  8   . 
     
    
    
     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. 
       FIG.  1    is a schematic cross-sectional view of a backlight module according to the first embodiment of the invention. Referring to  FIG.  1   , a backlight module  10  includes a circuit substrate  110 , a plurality of light-emitting devices  120 , a first cholesteric liquid-crystal layer  131 , and a second cholesteric liquid-crystal layer  132 . The light-emitting devices  120  are disposed on the circuit substrate  110  and electrically connected to the circuit substrate  110 . For example, the light-emitting devices  120  may be arranged in an array on the circuit substrate  110 , and the circuit substrate  110  is suitable for individually controlling the light-emitting devices  120  to emit light. In other words, the backlight module  10  of the present embodiment may have a local dimming function, but the invention is not limited thereto. In the present embodiment, the light-emitting devices  120  are, for example, mini-light-emitting diodes (mini-LEDs), but the invention is not limited thereto. In other embodiments, the light-emitting devices  120  may also be micro-LEDs. 
     The light-emitting devices  120  have a light-emitting surface  120   es  facing away from the circuit substrate  110 . At a side of the light-emitting surface  120   es  of the light-emitting devices  120 , a first cholesteric liquid-crystal layer  131  and a second cholesteric liquid-crystal layer  132  are provided. More specifically, the first cholesteric liquid-crystal layer  131  is disposed on the light-emitting devices  120  and overlapped with the light-emitting surface  120   es  of the light-emitting devices  120 . The second cholesteric liquid-crystal layer  132  is disposed on the first cholesteric liquid-crystal layer  131  and overlapped with the first cholesteric liquid-crystal layer  131 . In the present embodiment, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  each have substantially a same film thickness T and substantially a same pitch P, and the film thickness T of each of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may be greater than 10 times the pitch P, wherein the pitch P refers to the pitch of a cholesteric helix of each of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132 , so that the cholesteric liquid-crystal layer has a reflectivity close to 50% for light of a specific polarization state, but the invention is not limited thereto. In other embodiments, the film thickness T of each of the two cholesteric liquid-crystal layers may satisfy the following relationship: 3×P≤T≤20×P. In other words, the film thickness of the cholesteric liquid-crystal layers may be adjusted according to actual reflectivity or transmittance requirements. 
     Moreover, in the present embodiment, the materials of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may be optionally the same and have the same average refractive index. Accordingly, each of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may reflect light having substantially the same wavelength. However, the invention is not limited thereto. In other embodiments, the choice of materials for the two cholesteric liquid-crystal layers may also be different, for example: two cholesteric liquid-crystal materials having different average refractive indices and pitches of the cholesteric helix, as long as the product value of the average refractive index and the pitch of the cholesteric helix of each of the two cholesteric liquid-crystal layers is substantially the same. 
     It should be mentioned that, in the present embodiment, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  have a first chiral direction CD1 and a second chiral direction CD2, respectively, and the first chiral direction CD1 and the second chiral direction CD2 may be optionally different, but the invention is not limited thereto. Since the cholesteric liquid-crystal layers may reflect light having a polarization rotation state the same as its own chiral direction, a light LB1 emitted from the light-emitting devices  120  in a more forward direction (a smaller angle with the normal direction of the light-emitting surface  120   es ) respectively forms two sub-lights having different polarization rotation states after being incident on the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132 . 
     For example, after the light LB1 is incident on the first cholesteric liquid-crystal layer  131 , the light component (i.e., a sub-light LB1a) having a first circular polarization state CP1 is reflected by the first cholesteric liquid-crystal layer  131 , and the light component (i.e., a sub-light LB1b) having a second circular polarization state CP2 passes through the first cholesteric liquid-crystal layer  131  and is incident on the second cholesteric liquid-crystal layer  132 . Since the second chiral direction CD2 of the second cholesteric liquid-crystal layer  132  is the same as the polarization rotation state of the sub-light LB1b, the sub-light LB1b passing through the first cholesteric liquid-crystal layer  131  is reflected by the second cholesteric liquid-crystal layer  132 . 
     In other words, the light LB1 emitted from the light-emitting devices  120  in a more forward direction is directed by the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  to a region not overlapped with the light-emitting devices  120  (for example, the region between two adjacent light-emitting devices  120 ). Moreover, a light LB2 from the light-emitting devices  120  exiting at a greater angle with the normal direction of the light-emitting surface  120   es  may pass through two cholesteric liquid-crystal layers. In other words, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  do not substantially reflect light incident at a large angle. 
     As mentioned above, the reflectivity of the cholesteric liquid-crystal layers to light depends on their own film thickness. Therefore, the light output intensity of the backlight module  10  in the region overlapped with the light-emitting devices  120  (that is, the region directly above the light-emitting devices  120 ) may be controlled by adjusting the film thickness T of the two cholesteric liquid-crystal layers, so that the difference thereof from the light output intensity of the non-overlapped region is reduced, thereby improving the overall light output uniformity of the backlight module  10 . 
     From another perspective, compared with generally adopting a light-shielding film technique having light-shielding dots, there is no issue of assembly accuracy between the cholesteric liquid-crystal layers and the light-emitting devices  120  of the present embodiment. Moreover, the manufacturing process of the cholesteric liquid-crystal layers is simple, and the control of the reflection wavelength range is also easy. Therefore, the backlight module  10  of the present embodiment may have better process flexibility and cost advantages. 
     In the present embodiment, the backlight module  10  may also optionally include a light-transmitting substrate  140  disposed between the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132 . More specifically, the two cholesteric liquid-crystal layers may be directly disposed on two opposite surfaces of the light-transmitting substrate  140 , but the invention is not limited thereto. The material of the light-transmitting substrate  140  includes glass, polymer (for example: polyimide, polycarbonate, TAC, COP, PMMA, etc.), or other plates having high light transmittance. 
     Furthermore, the light-emitting devices  120  of the present embodiment may adopt blue light-emitting diodes to achieve white light illumination together with the wavelength conversion layer  180 . Specifically, the wavelength conversion layer  180  is disposed at a side of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  away from the light-emitting devices  120 , and overlapped with the respective light-emitting surfaces  120   es  of the light-emitting devices  120 . For example, the light LB2 passing through the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  obliquely and having a blue wavelength is absorbed after entering the wavelength conversion layer  180 , and is converted into a converted light LB2c having red light (or green light) wavelength. In other words, the wavelength conversion layer  180  of the present embodiment may include at least two wavelength conversion materials, so that the wavelength conversion layer  180  emits converted light of at least two wavelengths after absorbing blue light. 
     In the present embodiment, the backlight module  10  may further include a reflective layer  160  disposed on the circuit substrate  110  and not overlapped with the light-emitting devices  120 . More specifically, the reflective layer  160  is located between the plurality of light-emitting devices  120 . Through the arrangement of the reflective layer  160 , in addition to increasing the light-emitting efficiency of the light-emitting devices  120 , the opportunity for light to be transmitted laterally between the cholesteric liquid-crystal layers and the reflective layer  160  may also be increased to further improve the overall light output uniformity of the backlight module  10 . 
     In the following, other embodiments are provided to explain the disclosure in detail. The same members are labeled with the same reference numerals, and description of the same technical content is omitted. For the omitted parts, please refer to the above embodiments, which are not repeated herein. 
       FIG.  2    is a schematic cross-sectional view of a backlight module according to the second embodiment of the invention. Referring to  FIG.  2   , the difference between a backlight module  11  of the present embodiment and the backlight module  10  of  FIG.  1    is that in the backlight module  11 , the light-transmitting substrate  140  of the backlight module  10  is replaced by a half-wave plate  145 . Therefore, in the present embodiment, the chiral directions of the first cholesteric liquid-crystal layer  131 A and the second cholesteric liquid-crystal layer  132  may be optionally the same. For example: the two cholesteric liquid-crystal layers both have the second chiral direction CD2. However, the invention is not limited thereto. In other embodiments, the two cholesteric liquid-crystal layers may both have the first chiral direction CD1. 
     Specifically, in the present embodiment, after the light LB1 emitted from the light-emitting devices  120  in a more forward direction is incident on the first cholesteric liquid-crystal layer  131 A, the light component having the second circular polarization state CP2 (i.e., the sub-light LB1b) is reflected by the first cholesteric liquid-crystal layer  131 A, and the light component having the first circular polarization state CP1 (i.e., the sub-light LB1a) passes through the first cholesteric liquid-crystal layer  131 A. It is particularly noted that the sub-light LB1a passes through the half-wave plate  145  before being incident on the second cholesteric liquid-crystal layer  132 , so that the first circular polarization state CP1 is converted to the second circular polarization state CP2 after passing through the half-wave plate  145 . Since the second chiral direction CD2 of the second cholesteric liquid-crystal layer  132  is the same as the polarization rotation state of the sub-light LB1a, the sub-light LB1a passing through the first cholesteric liquid-crystal layer  131 A and the half-wave plate  145  is reflected by the second cholesteric liquid-crystal layer  132 . 
     Since the effect of the combination of the first cholesteric liquid-crystal layer  131 A, the half-wave plate  145 , and the second cholesteric liquid-crystal layer  132  of the present embodiment on the backlight module  11  is similar to the effect of the combination of the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  of  FIG.  1    on the backlight module  10 , for detailed description, please refer to the relevant paragraphs of the above embodiments, which is not repeated herein. In particular, the arrangement of the half-wave plate  145  may increase the material selection flexibility of the two cholesteric liquid-crystal layers. 
       FIG.  3    is a schematic cross-sectional view of a backlight module according to the third embodiment of the invention. Please refer to  FIG.  3   , the difference between a backlight module  12  of the present embodiment and the backlight module  10  of  FIG.  1    is: the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  of the backlight module  12  of the present embodiment are respectively disposed on different light-transmitting substrates. For example: the first cholesteric liquid-crystal layer  131  is disposed on the first light-transmitting substrate  141 , and the second cholesteric liquid-crystal layer  132  is disposed on the second light-transmitting substrate  142 , and an optical adhesive layer  170  may also be provided between the two cholesteric liquid-crystal layers. More specifically, the first light-transmitting substrate  141  provided with the first cholesteric liquid-crystal layer  131  and the second light-transmitting substrate  142  provided with the second cholesteric liquid-crystal layer  132  may be attached to each other via the optical adhesive layer  170 . The optical adhesive layer  170  here is, for example, an optical clear adhesive (OCA), a hardened optical clear resin (OCR), or an optical pressure sensitive adhesive (PSA). 
       FIG.  4    is a schematic cross-sectional view of a display apparatus according to an embodiment of the invention.  FIG.  5    is a schematic top view of the polarizer and the quarter-wave plate of  FIG.  4   . Please refer to  FIG.  4   , the difference between a backlight module  10 A of the present embodiment and the backlight module  10  of  FIG.  1    is that the film thickness configuration of the two cholesteric liquid-crystal layers is different. 
     In the present embodiment, a first cholesteric liquid-crystal layer  131 B and the second cholesteric liquid-crystal layer  132  of the backlight module  10 A may have different film thicknesses. For example, a film thickness T″ of the first cholesteric liquid-crystal layer  131 B may be optionally less than the film thickness T of the second cholesteric liquid-crystal layer  132 . Therefore, a portion of the light energy of the light LB1 emitted from the light-emitting devices  120  in a more forward direction may pass through the first cholesteric liquid-crystal layer  131 B and the second cholesteric liquid-crystal layer  132  in a specific polarization state. 
     Specifically, since the film thickness of the first cholesteric liquid-crystal layer  131 B is thinner than that of the second cholesteric liquid-crystal layer  132 , the reflectivity of the first cholesteric liquid-crystal layer  131 B to the light component having the first circular polarization state CP1 in the light LB1 is less than the reflectivity of the second cholesteric liquid-crystal layer  132  to the light component having the second circular polarization state CP2 in the light LB1. When the film thickness of the second cholesteric liquid-crystal layer  132  is thick enough (for example, more than ten times the pitch), the reflectivity of the second cholesteric liquid-crystal layer  132  to the light LB1 may be substantially close to 50% (that is, the reflectivity of the light component having the second circular polarization state CP2 is close to 100%). Conversely, when the first cholesteric liquid-crystal layer  131 B is thinner (for example, less than ten times the pitch), the reflectivity of the first cholesteric liquid-crystal layer  131 B to the light LB1 cannot reach 50% (that is, the reflectivity of the light component having the first circular polarization state CP1 may not be close to 100%). Therefore, a portion of the light component of the light LB1 having the first circular polarization state CP1 (i.e., a sub-light LB1c) passes through two cholesteric liquid-crystal layers. 
     In other words, the light emitted by the backlight module  10 A of the present embodiment may have the first circular polarization state CP1, but the invention is not limited thereto. In other embodiments, the relative film thickness relationship between the first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer may also be adjusted so that the light emitted by the backlight module has the second circular polarization state CP2. For example, the film thickness of the first cholesteric liquid-crystal layer is made to be greater than ten times the pitch, and the film thickness of the second cholesteric liquid-crystal layer is made to be less than ten times the pitch. 
     In the present embodiment, the backlight module  10 A may be used as the backlight source of a display panel  200 , and is overlapped with the display panel  200  to form a display apparatus  1 . The display panel  200  has a liquid-crystal cell  210  and two polarizers  221  and  222  disposed at two opposite sides of the liquid-crystal cell  210 , wherein the polarizer  221  is located between the liquid-crystal cell  210  and the backlight module  10 A, and an absorption axis AX1 of the polarizer  221  may be optionally perpendicular to an absorption axis AX2 of the polarizer  222 , but the invention is not limited thereto. In other embodiments, the axial directions of the absorption axes AX1 and AX2 of the two polarizers  221  and  222  may also be set to be parallel to each other according to the operation mode of the liquid-crystal cell  210  (for example: twisted-nematic mode, vertical alignment mode, horizontal electric field mode, optical compensation birefringence mode, etc.) 
     It should be mentioned that, in order to increase the light energy utilization rate of the display apparatus  1 , a quarter-wave plate  250  may be further provided between the polarizer  221  of the display panel  200  and the backlight module  10 A, wherein an included angle α between a slow axis SA of the quarter-wave plate  250  and the absorption axis AX1 of the polarizer  221  is 45 degrees (as shown in  FIG.  5   ). 
       FIG.  6    is a schematic cross-sectional view of a backlight module according to the fourth embodiment of the invention. Please refer to  FIG.  6   . Compared with the backlight module  10  of  FIG.  1   , a backlight module  20  of the present embodiment may further include a first optical film  151  and a second optical film  152 . The first optical film  151  is disposed between the light-emitting devices  120  and the first cholesteric liquid-crystal layer  131 . The second optical film  152  is disposed between the second cholesteric liquid-crystal layer  132  and the wavelength conversion layer  180 . In detail, the first optical film  151  has a first substrate SB1 and a plurality of first optical microstructures MS1, and the first optical microstructures MS1 are disposed on a surface SB1s of the first substrate SB1 facing the plurality of light-emitting devices  120 . The second optical film  152  has a second substrate SB2 and a plurality of second optical microstructures MS2, and these second optical microstructures MS2 are disposed on a surface SB2s of the second substrate SB2 facing the wavelength conversion layer  180 . 
     Since the configuration and functions of other members of the present embodiment are all similar to those of the backlight module  10  in  FIG.  1   , for detailed description, please refer to the relevant paragraphs of the above embodiments, which is not repeated herein. 
     In the present embodiment, the first optical microstructures MS1 and the second optical microstructures MS2 are, for example, prism structures, and respectively have a vertex angle β1 and a vertex angle β2 away from the first substrate SB1 and the second substrate SB2. Preferably, the vertex angle β1 of the first optical microstructures MS1 and the vertex angle β2 of the second optical microstructures MS2 may be less than 120 degrees. For example, in the present embodiment, the two vertex angles of the first optical microstructures MS1 and the second optical microstructures MS2 may be optionally the same, but the invention is not limited thereto. In other embodiments, the two vertex angles of the first optical microstructures MS1 and the second optical microstructures MS2 may also be designed to be different according to the requirements of the light output type or the light output uniformity of the backlight module. 
     It should be mentioned that, in the present embodiment, after passing through the first optical microstructures MS1 of the first optical film  151 , a light LB emitted in a more forward direction in the light emitted by the light-emitting devices  120  is deflected and incident on the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  at a larger angle. After the light LB passes through the second cholesteric liquid-crystal layer  132  and is incident on the second optical microstructures MS2 of the second optical film  152 , the light LB is deflected to a more forward direction and exit the second optical film  152 . The forward direction here is, for example, the normal direction of the light-emitting surface  120   es  (or the surface SB2s). 
     Via the arrangement of these two optical films, the light LB emitted by the light-emitting devices  120  in a more forward direction may be directed away from the region directly above the light-emitting devices  120  (for example: located between two adjacent light-emitting devices  120  and not overlapped with the region of the light-emitting devices  120 ). In other words, the configuration of the two optical films above may further improve the overall light output uniformity of the backlight module. 
       FIG.  7    is a schematic cross-sectional view of a backlight module according to the fifth embodiment of the invention. Please refer to  FIG.  7   , the difference between a backlight module  20 A of the present embodiment and the backlight module  20  of  FIG.  6    is that the configuration of the optical microstructures is different. In the present embodiment, in the backlight module  20 A, a surface  131   s  of the first cholesteric liquid-crystal layer  131  facing the plurality of light-emitting devices  120  may be directly provided with the plurality of first optical microstructures MS1, and a surface  132   s  of the second cholesteric liquid-crystal layer  132  facing the wavelength conversion layer  180  may be directly provided with the plurality of second optical microstructures MS2. 
     Since the effect of the optical microstructures MS1 and the optical microstructures MS2 of the present embodiment in the backlight module  20 A is similar to the effect of the optical microstructures of  FIG.  6    on the backlight module  20 , for detailed description, please refer to the relevant paragraphs of the above embodiments, which is not repeated herein. 
       FIG.  8    is a schematic cross-sectional view of a backlight module according to the sixth embodiment of the invention.  FIG.  9 A  is a graph of reflectivity versus incident angle of the first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer of  FIG.  8   .  FIG.  9 B  and  FIG.  9 C  are graphs of reflectivity versus incident angle of the first cholesteric liquid-crystal layer and the second cholesteric liquid-crystal layer according to some other modified embodiments of the backlight module of  FIG.  8   . Referring to  FIG.  8   , the difference between a backlight module  30  of the present embodiment and the backlight module  10  of  FIG.  1    is that the light-emitting surface  120   es  of the light-emitting devices  120  of the backlight module  30  may also be provided with an encapsulation pattern  190 , and the encapsulation pattern  190  may have different degrees of reflectivity. In the present embodiment, the encapsulation pattern  190  may be formed by a thinned metal layer or an encapsulation adhesive doped with a reflective particle. 
     When the encapsulation pattern  190  has a reflectivity greater than 0%, a portion of the light (for example, a light LB3) emitted by the light-emitting devices  120  is reflected by the encapsulation pattern  190  to the side of the light-emitting devices  120 . Therefore, in order to match the encapsulation pattern  190  having different degrees of reflectivity, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may have different light reflectivity distributions at different incident angles θ. For example, when the reflectivity of the encapsulation pattern  190  is greater, more light is reflected to the side, and the incident angle θ at which the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  have the maximum reflectivity is also greater. Accordingly, the chance of light transmission in the lateral direction may be increased, and the overall light output uniformity of the backlight module  30  may be improved. The incident angle θ here is, for example, the included angle between the optical path of the incident light and the normal direction of the surface  131   s  of the first cholesteric liquid-crystal layer  131  facing the light-emitting devices  120 . 
     For example, when the reflectivity of the encapsulation pattern  190  is between 0% and 20%, since most of the light (for example, the light LB1) is still emitted through the light-emitting surface  120   es  and penetrates the encapsulation pattern  190 , the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may have the maximum reflectivity for light having the incident angle θ between 0 degrees and 30 degrees, and the maximum reflectivity is between 80% and 100% (as shown in  FIG.  9 A ). Accordingly, the formation of brighter light directly above the light-emitting devices  120  causing a hot spot phenomenon may be avoided, thus helping to improve the light output uniformity of the backlight module  30 . 
     When the reflectivity of the encapsulation pattern  190  is between 20% and 50%, since the proportion of light reflected to the side of the light-emitting devices  120  is increased, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may have maximum reflectivity for light having the incident angle θ between 20 degrees and 60 degrees, and the maximum reflectivity is between 60% and 100% (as shown in  FIG.  9 B ). Therefore, the light having the incident angle θ between 20 degrees and 60 degrees may be reflected by the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  to be transmitted laterally, thereby improving the light output uniformity of the backlight module  30 . 
     When the reflectivity of the encapsulation pattern  190  is between 50% and 100%, since the proportion of light reflected to the side of the light-emitting devices  120  is increased, the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  may have maximum reflectivity for light having the incident angle θ between 40 degrees and 80 degrees, and the maximum reflectivity is between 60% and 100% (as shown in  FIG.  9 C ). Therefore, the light having the incident angle θ between 40 degrees and 80 degrees may be reflected by the first cholesteric liquid-crystal layer  131  and the second cholesteric liquid-crystal layer  132  to be transmitted laterally, thereby improving the light output uniformity of the backlight module  30 . 
     Based on the above, in the backlight module and the display apparatus of an embodiment of the invention, by arranging two cholesteric liquid-crystal layers having the same or opposite chiral directions on the light-emitting side of the light-emitting devices, a portion of the forward light emitted by the light-emitting devices may be directed to a region not overlapped with the light-emitting devices, so as to effectively improve the overall light uniformity of the backlight module. Moreover, compared with the general use of a light-shielding film technique having light-shielding dots, the cholesteric liquid-crystal layers and the light-emitting devices of the invention do not have issues in assembly accuracy. Therefore, the manufacturing process flexibility of the backlight module is better and the production cost is also lower. 
     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. Moreover, these claims may refer to use “first”, “second”, etc. following with noun or element. Such terms should be understood as a nomenclature and should not be construed as giving the limitation on the number of the elements modified by such nomenclature unless specific number has been given. 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.