Patent Publication Number: US-2013242607-A1

Title: Backlight module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 101109342, filed on Mar. 19, 2012. 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 
     1. Field of the Invention 
     The present invention relates to a backlight module, and more particularly to a backlight module in which a white light is formed by mixing polychromatic light. 
     2. Description of Related Art 
     Since light emitting diodes (LED) have advantages of low pollution, low power consumption, short response time, long service life, etc., they have been widely used in light sources of the backlight module of the display. Currently, due to the high fabricating cost of LED in color mixing with red, green and blue light, using the blue light emitting diode with the yellow fluorescent powder to form a white light has been the mainstream of the white light emitting diodes in the market. 
       FIG. 1  shows a backlight spectrum distribution diagram of a conventional light source, wherein a yellow fluorescent powder is excited by a blue light emitting diode and then a white light is formed with color mixing thereof. Referring to  FIG. 1 , the red light wavelength of 600 nm to 700 nm and the green light wavelength of 500 nm to 600 nm of the backlight spectrum is formed when the yellow fluorescent powder is excited by the blue light. Therefore, compared to the blue light wavelength of 400 nm to 500 nm, the conventional backlight spectrum has a lower energy in the red light wavelength and the green light wavelength, and the conventional backlight spectrum has no separate peak for each of the red light wavelength and the green light wavelength. 
       FIG. 2  shows a color gamut of a panel with a conventional light source. Referring to  FIG. 2 , the area of the triangle surrounded by the dashed-line  202  is 100% NTSC which is defined by CIE 1931. The area of the triangle surrounded by the solid line  204  is the color gamut of a panel with the conventional light source. Since the spectrum of the conventional light source has no separate peak corresponding to the red color filter and the green color filter, the area of the pure color is reduced and the color saturation is further affected. 
     In actual operation, backlight modules with different specifications need corresponding mixing ratios of fluorescent powder so as to achieve the desired color saturation. However, such method may increase the development duration for the product, and the resulting backlight may also become a customized product. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention provide a backlight module having a high color saturation. 
     One embodiment of the present invention provides a backlight module including a light source and a group of replaceable optical elements. The light source emits a first light. The group of replaceable optical elements receives the first light and excites a backlight light, wherein the group of replaceable optical elements includes a first replaceable optical element and a second replaceable optical element. The first replaceable optical element has a first phosphor, and the first phosphor is excited by a light and emits a second light. The second replaceable optical element has a second phosphor, and the second phosphor is excited by a light and emits a third light. 
     In an exemplary embodiment of the present invention, the first replaceable optical element is a light guide plate, a diffusion film or a prism film, and the second replaceable optical element is a light guide plate, a diffusion film or a prism film. 
     In an exemplary embodiment of the present invention, the first phosphor is disposed in the first replaceable optical element by doping, and the doping concentration of the first phosphor is related to the luminous intensity (brightness) of the second light. 
     In an exemplary embodiment of the present invention, the first phosphor is disposed on the first replaceable optical element by coating, and the thickness of the first phosphor is related to the luminous intensity of the second light. 
     In an exemplary embodiment of the present invention, the second phosphor is disposed in the second replaceable optical element by doping, and the doping concentration of the second phosphor is related to the luminous intensity of the third light. 
     In an exemplary embodiment of the present invention, the second phosphor is disposed on the second replaceable optical element by coating, and the thickness of the second phosphor is related to the luminous intensity of the third light. 
     In an exemplary embodiment of the present invention, the light source is at least a blue light emitting diode, and the first light is a blue light. 
     In an exemplary embodiment of the present invention, the second light is a red light, and the third light is a green light. 
     In an exemplary embodiment of the present invention, the backlight module further includes a third replaceable optical element, wherein the third replaceable optical element has a third phosphor, and the third phosphor is excited by a light and emits a fourth light. 
     In an exemplary embodiment of the present invention, the third phosphor is disposed in the third replaceable optical element by doping, and the doping concentration of the third phosphor is related to the luminous intensity of the fourth light. 
     In an exemplary embodiment of the present invention, the third phosphor is disposed on the third replaceable optical element by coating, and the thickness of the third phosphor is related to the luminous intensity of the fourth light. 
     In an exemplary embodiment of the present invention, the light source is at least an invisible light emitting diode, and the first light is an invisible light. 
     In an exemplary embodiment of the present invention, the second light is a red light, the third light is a green light, and the fourth light is a blue light. 
     In an exemplary embodiment of the present invention, the backlight module further includes at least a replaceable optical element with no phosphor, wherein the replaceable optical element with no phosphor is disposed between the first replaceable optical element and the second replaceable optical element. 
     In light of the above, in the backlight modules of the exemplary embodiments of the present invention, the phosphors of the replaceable optical elements can be stacked to each other, and the phosphors of different layers are excited by the first light so as to achieve a backlight spectrum with three separate color peaks, such as a backlight spectrum with a red color peak, a green color peak and a blue color peak. Therefore, the backlight spectrum of such backlight module has separate color peaks corresponding to the red color filter, green color filter and blue color filter of panel, and the color saturation of a panel with the backlight module is higher than conventional produces. In addition, since each of the phosphor of the replaceable optical elements is a phosphor with a single color, i.e., the phosphor is not obtained from mixing of multi-color, it is no need to consider the non-uniformity of the color distribution. In addition, simply and rapidly adjusting the existing replaceable optical elements according to the desired color saturation may greatly reduce the development duration and fabrication cost. 
     Other features and advantages of the invention will be further understood from the further technological features disclosed by the following embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments and, together with the description, serve to explain the principles of the disclosure. 
         FIG. 1  shows a backlight spectrum distribution diagram of a conventional light source. 
         FIG. 2  shows color gamuts of a panel with a conventional light source and the panel with a backlight module of an embodiment of the present invention. 
         FIG. 3  is a schematic cross-sectional view illustrating a backlight module according to an exemplary embodiment of the present invention. 
         FIG. 4  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
         FIG. 5  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
         FIG. 6  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
         FIG. 7  shows backlight spectrum distribution diagrams of a backlight module of exemplary embodiments of the present invention and a conventional light source. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 3  is a schematic cross-sectional view illustrating a backlight module according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the backlight module  300  of the embodiment includes a light source  310  and a group of replaceable optical elements  320 , wherein the light source  310  emits a first light L 1 , and the group of replaceable optical elements  320  receive the first light L 1  and excite a backlight light L B . More specifically, the group of replaceable optical elements  320  includes a first replaceable optical element  322   a  and a second replaceable optical element  324   a . Herein the first replaceable optical element  324   a  has a first phosphor  330  and the first phosphor  330  can be excited by a light to emit a second light L 2 . The second replaceable optical element  324   a  has a second phosphor  340 , and the second phosphor  340  can be excited by a light to emit a third light L 3 . In addition, the backlight module  300  of the embodiment can have a plurality of optical elements selectively disposed therein according to various requirements. In the embodiment, an optical element  326   a  (e.g., a prism film without a phosphor) can be selectively disposed on the second replaceable optical element  324   a.    
     The first phosphor  330  and the second phosphor  340  may be fluorescent powder, optical adhesive or any other material which can be excited by a light to emit a light with desired wavelength range. In the embodiment, the first phosphor  330  is a red fluorescent powder or any other chemical material which can be excited for emitting a red light, for example. And the second phosphor  340  is a green fluorescent powder or any other chemical material which can be excited for emitting a green light, but the present invention is not limited thereto. In other embodiments, the first phosphor can also be a green fluorescent powder or any other chemical material which can be excited to emit a green light, and the second phosphor is a red fluorescent powder or any other chemical material which can be excited for emitting a red light. 
     It has to be noted that different phosphors can be exited to emit different ranges of wavelength. In other words, the phosphors of the replaceable optical elements of the backlight module  300  of the embodiment can be adjusted according to color filters with different specifications or different light sources, so as to obtain the desired range of wavelength to achieve the required color saturation. 
     In the present embodiment, the light source  310  is a blue light emitting diode, for example, and the first light L 1  is a blue light, but not limited thereto. In addition, the first replaceable optical element  322   a  and the light source  310  are disposed in parallel to form an edge-type light source, but the present invention is not limited thereto. In other embodiments, the light source and the first replaceable optical element can be perpendicularly disposed to form a direct-type light source. 
     The first replaceable optical element  322   a  and the second replaceable optical element  324   a  can be a light guide plate, a diffusion film, a prism film or an other optical film, respectively. In the embodiment, the first replaceable optical element  322   a  is a light guide plate, for example, so as to guide the first light L 1  generated by the light source  310 , and the first phosphor  330  of the first replaceable optical element  322   a  is excited and generates a second light L 2 . 
     In addition, in order to increase the ratio of light which is reflected from the bottom of the first replaceable optical element  322   a , a reflecting sheet  350  is disposed beneath the light guide plate (the light guide plate is the first replaceable optical element  322   a  in this embodiment) of the backlight module  300  of the embodiment, so that more of the light can be reflected from the first replaceable optical element  322   a . Herein the light L 322a  from the first replaceable optical element  322   a  is a mixed color light including the blue color of the first light L 1  emitted by the light source  310  and the red color of the second light L 2  generated by the excited first phosphor  330 . Therefore, the color of the light L 322a  is close to the violet. 
     In the embodiment, the second replaceable optical element  324   a  is a diffusion film disposed on the first replaceable optical element  322   a  for example, so as to diffuse the light L 322a  from an the first replaceable optical element  322   a  and to diffuse the light L 322a  from a point light source (in which the light is concentrated as a point) into a surface light source (in which the light is uniformly distributed), and the second phosphor  340  of the second replaceable optical element  324   a  is excited and generates a third light L 3 , wherein the third light L 3  is a green light. Therefore, the light L 324a  from the second replaceable optical element  324   a  becomes a white color light because of the mixture of red, blue and green light. Specifically, the color of the light L 324a  includes the blue color of the first light L 1  emitted from the light source  310 , the red color of the second light L 2  generated by the first phosphor  330  and the green color of the third light L 3  generated by the second phosphor  340 . 
     It should be noted that, the present invention is not limited to the types of the plurality of the replaceable optical elements  320  in the embodiment. The replaceable optical elements  320  are used to illustrate that in the backlight module  300  of the embodiment, the first replaceable optical element  322   a  having the first phosphor  330  and the second replaceable optical element  324   a  having the second phosphor  340  are stacked together, and the first phosphor  330  and the second phosphor  340  which are located in different layers are excited to achieve a backlight light having separate peaks in spectrum distribution for red, green and blue color. Therefore, the backlight spectrum of the backlight module  300  has separate color spectrum peaks corresponding to a red color filter, a green color filter and a blue color filter of a panel, and the color saturation of the panel can even surpass the area of 100% NTSC (shown as the triangular area surrounded by the dash-line  206  in  FIG. 2 ) defined by CIE 1931. 
     In other embodiments, the first replaceable optical element and the second replaceable optical element are not limited to be the light guide plate or the diffusion film, and they can be any other optical element (optical film) usually disposed in the backlight module which is known by people having ordinary skill in the art of the invention field. 
     Additionally, the optical element  326   a  of the embodiment can be a prism film without phosphor for example, so as to refract the light L 324a  from from the second replaceable optical element  324   a  to the front view angle of the display device, so that the backlight light L B  of the optical element  326   a  can be concentrated to enhance the luminance. In the embodiment, since the optical element  326   a  has no phosphor, the color of the backlight light L B  and the color of the light L 324a  are similar. 
     In addition, in the embodiment, the first phosphor  330  and the second phosphor  340  are disposed on the surfaces of the first replaceable optical element  322   a  and the second replaceable optical element  324   a  respectively by coating process. Moreover, the thickness D 330  of the first phosphor  330  is related to the luminous intensity of the second light L 2 , i.e., changes the luminous intensity of L 322a . The thickness D 340  of the second phosphor  340  is related to the luminous intensity of the third light L 3 , i.e., changes the luminous intensity of L 324a , and further changes the luminous intensity of the backlight light L B  from the optical element  326   a . In other words, the luminous intensity of the peaks in the backlight spectrum of the embodiment can be changed by adjusting the thickness D 330  of the first phosphor  330  and the thickness D 340  of the second phosphor  340 . 
     In actual operating process, since the first phosphor  330  and the second phosphor  340  are located in different replaceable optical elements, it is no need to consider the non-uniformity of the phosphor distribution of the backlight module  300 . In addition, simply and rapidly adjusting the existing replaceable optical elements according to the desired color saturation may greatly reduce the development duration and fabrication cost. For instance, excited lights with various spectrum peaks or luminous intensity can be achieved by establishing a source material data base of the replaceable optical elements having different phosphors with various coating materials or doping concentration, so as to meet the demands of different light sources or color filters with different specifications. 
     In addition, the phosphors can be either disposed on the replaceable optical elements by coating, or disposed in the replaceable optical elements by doping.  FIG. 4  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , the backlight module  400  and the backlight module  300  of  FIG. 3  are similar in structure, and similar components with similar functions are represented by the same reference numbers and are not repeated therein. One difference between the backlight module  400  and the backlight module  300  is that the first phosphor  330  and the second phosphor  340  are disposed in the first replaceable optical element  322   b  and the second replaceable optical element  324   b  respectively by doping process, and the present invention is not limited thereto. In the embodiment, the doping concentration of the first phosphor  330  is related to the luminous intensity of the second light L 2 , and the doping concentration of the second phosphor  340  is related to the third light L 3 . 
     It has to be noted that, the locations of the first replaceable optical element and the second replaceable optical element are not limited in the present invention. In more detail, the first replaceable optical element and the second replaceable optical element can be adjacently stacked up and down to each other, or stacked up and down with an optical element without phosphor sandwiched therebetween.  FIG. 5  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the backlight module  500  and the backlight module  400  of  FIG. 4  are similar in structure, and similar components with similar functions are represented by the same reference numbers and are not repeated therein. One difference between the backlight module  500  and the backlight module  400  is that the optical element  326   a  without phosphor disposed therein is disposed between the first replaceable optical element  322   b  and the second optical element  324   b . Herein since the optical element  326   a  has no phosphor, the colors of the light L 326a  from the optical element  326   a  and the light L 322b  from the first replaceable optical element  322   b  are similar. 
     Naturally, the light source of the backlight module can be the above-mentioned blue light emitting diode or any other type of light source.  FIG. 6  is a schematic cross-sectional view illustrating a backlight module according to another exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the backlight module  600  and the backlight module  400  of  FIG. 4  are similar in structure, and similar components with similar functions are represented by the same reference numbers and are not repeated therein. One difference is that the light source  610  of the backlight module  600  of the embodiment is at least an invisible light emitting diode. Herein the first light L 1 ′ is an invisible light. 
     In addition, the backlight module  600  of the embodiment further includes a third replaceable optical element  326   b , wherein the third replaceable optical element  326   b  has a third phosphor  650 , and the third phosphor  650  can be excited by a light and emits a fourth light L 4 . In the embodiment, the third phosphor  650  may be a blue fluorescent powder, an optical adhesive or any other chemical material which can be excited to emit a blue light, and the fourth light L 4  is a blue light. Furthermore, the third phosphor  650  can be disposed in the third replaceable optical element  326   b  by doping, wherein the doping concentration of the third phosphor  650  is related to the luminous intensity of the fourth light L 4 . However, the present invention is not limited thereto. In other embodiments, the third phosphor can be disposed on the third replaceable optical element by coating, and the thickness of the third phosphor is related to the luminous intensity of the fourth light. 
     In the embodiment, the light source  610  emits a first light L 1 ′. The first light L 1 ′ excites the first phosphor  330  of the first replaceable optical element  322   b , and the light L 322c  from the first replaceable optical element  322   b  includes an invisible light and a red light. And then, the light L 322c  excites the second phosphor  340  of the second replaceable optical element  324   b , and the light L 324c  from the second replaceable optical element  324   b  includes an invisible light, a red light and a green light. After that, the light L 324c  excites the third phosphor  650  of the third replaceable optical element  326   b , and the light L B  from the third replaceable optical element  326   b  includes an invisible light, a red light, a green light and a blue light, and then they are mixed to form a white light. 
     It has to be illustrated that in the backlight module  600  of the embodiment, the first replaceable optical element  322   b  having the first phosphor  330 , the second replaceable optical element  324   b  having the second phosphor  340  and the third replaceable optical element  326   a  having the third phosphor  650  are stacked together, and the first phosphor  330 , the second phosphor  340  and the third phosphor  650  which are located in different layers are excited to achieve a backlight light having separate peaks in spectrum distribution for red, green and blue colors. Therefore, the backlight spectrum of the backlight module  600  has separate color peaks corresponding to a red color filter, a green color filter and a blue color filter of a panel, and the color saturation of the panel with backlight module  600  can be further improved. 
     In order to illustrate the embodiment of the present invention more clearly, a backlight module in which separate peaks of spectrum and superior color saturation can be achieved by stacking the replaceable optical elements having single color phosphors is further described with the following  FIG. 7  and Table 1. The specification of the color saturation for a panel with a backlight module is assumed to be W(x,y)=(0.313,0.329). 
       FIG. 7  shows backlight spectrum distribution diagrams of backlight modules of exemplary embodiments of the present invention and a conventional light source. In  FIG. 7 , the three curves respectively represent the backlight spectrum curve of a conventional backlight module and the backlight spectrum curves of the two embodiments (Embodiment 1 and Embodiment 2) in which the doping concentrations (quantity of particles/μm 2 ) are different. Herein the ratio of the doping concentration of phosphor of first replaceable optical element to the doping concentration of phosphor of second replaceable optical element in Embodiment 1 is 1.0X:1.0Y (wherein X, Y are positive real numbers). And in Embodiment 2 in which the doping concentrations of phosphors of first and second replaceable optical elements have been adjusted according to the specification of the color saturation, the ratio of the doping concentration of phosphor of first replaceable optical element to the doping concentration of phosphor of second replaceable optical element is 1.7X:1.5Y. Herein the doping concentrations of the phosphors can be adjusted by means of preparing the first and second replaceable optical elements with different doping concentrations (e.g., the first replaceable optical elements can be prepared to have the doping concentrations of 1.0X, 1.1X, . . . , 2.0X, and the second replaceable optical elements can be prepared to have the doping concentrations of 1.0Y, 1.1Y, . . . , 2.0Y), and then the doping concentrations of the phosphors can be adjusted by means of replacing or changing the first and second replaceable optical elements having different doping concentrations. 
     Referring to  FIG. 7 , since the backlight spectrum of the conventional backlight module is obtained by means of the yellow fluorescent powder being excited by the blue light emitting diode to form a mixed white light, there is no separate peak for each of red waveband and green waveband. On the other hand, since the blue light emitting diodes are used in Embodiment 1 and Embodiment 2, in which the two replaceable optical elements each having a single color phosphor and stacked together are excited to generate a mixed white light, each of the light spectrums has separate peaks in blue light, red light and/or green light waveband. Furthermore, in the embodiments, the luminous intensity of each light spectrum peak can be adjusted by changing the doping concentration of the phosphors. As shown in  FIG. 7 , the doping concentrations of the first phosphor and the second phosphor of Embodiment 2 are greater than the doping concentrations of the first phosphor and the second phosphor of Embodiment 1, thus the light spectrum peaks of the red light and green light wavebands of Embodiment 2 are higher than those of Embodiment 1, respectively. 
     By applying a panel to the three backlight modules, chromaticity coordinates of red, green and blue vertices of the panel with each of the three backlight modules can be calculated respectively. The red, green and blue vertices of the panel with the corresponding backlight module form a triangle and its area can be used to calculate color saturation. 
     As shown in Table 1, since the backlight modules of the embodiments uses the blue light emitting diodes to excite the red phosphor and the green phosphor to generate a mixed white light, the color saturations of the embodiments are higher. More specifically, the color saturation of the panel with the conventional backlight module in which a blue light emitting diode is used to excite the yellow fluorescent powder to generate a mixed white light is 49.04%, and the chromaticity coordinates of the white light is W(x,y)=(0.2896,0.2924). Comparatively, the panel with the backlight module of Embodiment 1 and the panel with the backlight module of Embodiment 2 have a higher color saturation. Herein, when the first replaceable optical element and the second replaceable optical element having the doping concentration ratio of 1.0X:1.0Y are used (i.e., Embodiment 1), the chromaticity coordinates of the white light is W(x,y)=(0.2674,0.2923), and the color saturation is 69.36%. In addition, when the first replaceable optical element and the second replaceable optical element having the doping concentration ratio of 1.7X:1.5Y are used (i.e., Embodiment 2), the chromaticity coordinates of the white light is W(x,y)=(0.313,0.3299), and the color saturation is 73.65%. Accordingly, the luminous intensity of the three-color backlight spectrum can be more evenly distributed by adjusting the doping concentration of the phosphor of each of the replaceable optical elements, and the color saturation can be greatly improved, for example, the color saturation is enhanced from 69.36% in Embodiment 1 to the color saturation of 73.65% in Embodiment 2. 
     On the other hand, the backlight module of the embodiment can obtain the color saturation which is close to the desired color saturation, i.e., the chromaticity coordinates of white light W(x,y)=(0.313,0.3299) of the color gamut ( FIG. 2 ). As shown in Table 1, the white light of the conventional backlight is a cool color slightly close to blue color light. Comparatively, since the backlight module of the embodiment has separate spectrum peak of each of red, green and blue color corresponding to the red color filter, green color filter and blue color filter of the panel, specifications of the desired color saturation can be rapidly achieved by adjusting the doping concentration of the phosphor of each of the replaceable optical elements. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 W(x, y) 
                 NTSC (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Conventional 
                 (0.2896, 0.2924) 
                 49.04 
               
               
                   
                 Embodiment 1 
                 (0.2674, 0.2923) 
                 69.36 
               
               
                   
                 Embodiment 2 
                 (0.3139, 0.3299) 
                 73.65 
               
               
                   
                   
               
            
           
         
       
     
     In light of the foregoing, the backlight modules of the embodiments of the present invention have replaceable optical elements stacked together and the phosphor of each of the replaceable optical elements can be excited and mixed to form a white light, thus separate spectrum peak of each blue light, red light and/or green light respectively corresponding to the color filters can be obtained, and a broader color gamut or a better color saturation can be further achieved. In addition, rapidly adjusting the existing replaceable optical elements according to the desired color saturation may greatly reduce the development duration and fabrication cost. Furthermore, according to the backlight module of the embodiment of the present invention, the material and the doping concentration of the phosphor of each of the replaceable optical elements can be changed and adjusted, so that a source material data base can be established according to the resulting luminous intensity and the wavelength range of the excited light. Accordingly, under the condition of different light sources and different color filters, the desired color saturation can be simply and conveniently achieved by changing or replacing the replaceable optical elements according to the source material data base. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.