Patent Publication Number: US-2022221761-A1

Title: Backlight device

Description:
FIELD 
     The present invention relates to a backlight device. 
     BACKGROUND 
     Various techniques are known in a direct type backlight device using light emitting elements such as a light emitting diode (LED) elements in order to increase the uniformity of brightness by reducing the unevenness of luminance. 
     For example, a technique for reducing the unevenness of luminance without reducing the luminance of the outer peripheral portion is described in JP 2016-66598 to reduce the amount of light emitted to the reflective sheet disposed around LEDs, by disposing light block members that do not transmit light emitted to the outer edge side of the LEDs. Further, a technique for reducing the unevenness of luminance of a backlight device is described in JP 2008-282744 in which dot patterns dotted with dots made of ink containing white pigment are disposed on a plate surface of a diffuser plate facing to LEDs in order to uniform the brightness distribution of a backlight device. A direct type backlight device may increase the uniformity of brightness by reducing the unevenness of luminance by the techniques described in JP 2016-66598 and JP 2008-282744. 
     SUMMARY 
     However, when a backlight device becomes thinner and the number of LEDs mounted thereon for reducing the cost for a backlight device, the unevenness of luminance in a backlight device may be increased. 
     The object of the present invention is to provide techniques that may reduce the unevenness of luminance in a backlight device. 
     The backlight device according to the embodiment has a substrate, a plurality of light emitting elements disposed on a surface of the substrate, a diffuser plate having an incident surface disposed facing to the surface of the substrate, and an emitting surface disposed on the opposite side of the incident surface, diffuses light incident on the incident surface from the plurality of light emitting elements, and emits the diffused light from the emitting surface, and a frame defining a light emitting region on which the plurality of light emitting elements are disposed, wherein a half value region is defined in which light that is equal to or more than half of a peak value of a brightness of light emitted from one of the plurality of light emitting elements is incident on the incident surface, all of the plurality of light emitting elements disposed on the light emitting region are disposed so that an overlapping ratio is 20% or less, the overlapping ratio is a ratio of an area of an overlapping region to the area of the half value region defined around the one of the light emitting elements, and the overlapping region is a region overlapping the half value region defined around the one of the light emitting elements and a half value region defined around another one of the light emitting elements adjacent to the one of the light emitting elements. 
     It is preferable that the backlight device according to the embodiment further has a plurality of reflection members disposed on portions of the incident surface respectively facing the plurality of light emitting elements so as to reflect light emitted from the light emitting element. 
     It is preferable in the backlight device according to the embodiment that all of the light emitting elements disposed along the frame are disposed so that the overlapping regions are formed among the adjacent light emitting elements disposed along the frame. 
     It is preferable in the backlight device according to the embodiment that the frame is disposed so as not to form a region less than the half value outside the light emitting elements disposed along the frame, and all of the plurality of light emitting elements are disposed so that a ratio less than the half value is 20% or less, the ratio less than the half value is a ratio of an area of the region less than the half value to an area of a surrounding region formed by connecting at least three light emitting elements disposed around the region less than the half value, and the region less than the half value is a region on which no half value regions of the plurality of light emitting elements are disposed. 
     It is preferable in the backlight device according to the embodiment that each of the plurality of light emitting elements are disposed so that three light emitting elements adjacent to each other are not aligned in a straight line. 
     The backlight device according to the embodiment has a substrate, a plurality of light emitting elements disposed on a surface of the substrate, a diffuser plate having an incident surface disposed facing to the surface of the substrate, and an emitting surface disposed on the opposite side of the incident surface, diffuses light incident on the incident surface from the plurality of light emitting elements, and emits the diffused light from the emitting surface, and a frame defining a light emitting region on which the plurality of light emitting elements are disposed, wherein the frame is disposed so as not to form a region less than the half value outside the light emitting elements disposed along the frame, and all of the plurality of light emitting elements are disposed so that a ratio less than the half value is 20% or less, the ratio less than the half value is a ratio of an area of the region less than the half value to an area of a surrounding region formed by connecting at least three light emitting elements disposed around the region less than the half value, and the region less than the half value is a region on which no half value regions of the plurality of light emitting elements are disposed. 
     A backlight device according to the present invention may reduce the unevenness of luminance therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a backlight device according to the first embodiment; 
         FIG. 2  is an exploded perspective view of the backlight device shown in  FIG. 1 ; 
         FIG. 3  is a line A-A′ cross-sectional view of  FIG. 1 ; 
         FIG. 4  is a portion enlarged view of a position of the incident surface of the first diffuser plate facing to each of a plurality of LEDs shown in  FIG. 2 ; 
         FIG. 5  is a plan view showing the arrangement of the LEDs and the pair of struts  12 ; 
         FIG. 6  is a figure for explaining the half value region; 
         FIG. 7A  is a figure showing an example in which three LEDs adjacent are aligned in a straight line, and  FIG. 7B  is a figure showing an example in which three LEDs adjacent are not aligned in a straight line; 
         FIG. 8A  is a figure showing an example of an arrangement in which an overlapping ratio between adjacent LEDs is 20% or less,  FIG. 8B  is a cross-sectional view corresponding to the arrangement shown in  FIG. 8A ,  FIG. 8C  is a figure showing an example of arrangement in which a ratio less than the half value is 20% or less, and  FIG. 8D  is a cross-sectional view corresponding to the arrangement shown in  FIG. 8C ; 
         FIG. 9A  is a figure showing a relationship between the distance from the LEDs and the brightness ratio in the arrangement shown in  FIG. 8A , and  FIG. 9B  is a figure showing a relationship between the distance from the LEDs and the brightness ratio in the arrangement shown in  FIG. 8C ; 
         FIG. 10  is a figure showing a luminance distribution in the arrangement shown in  FIG. 8C ; 
         FIG. 11A  is a perspective view of a backlight device according to the second embodiment, and  FIG. 11B  is a D-D′ line sectional view of  FIG. 11A ; 
         FIG. 12A  is a figure showing an example of the arrangement of LEDs in a backlight device according to the embodiment, and  FIG. 12B  is a figure showing the other example of the arrangement of LEDs in a backlight device according to the embodiment; 
         FIG. 13  is a figure showing a relationship between the LEDs and the half value regions according to the exemplary embodiment; 
         FIG. 14A  is a figure showing a relationship between the LEDs and the first requirement according to the embodiment, and  FIG. 14B  is a partially enlarged view of a portion corresponding to the first requirement shown in  FIG. 14A ; and 
         FIG. 15A  is a figure showing a relationship between the LEDs and the second requirement according to the embodiment,  FIG. 15B  is a partially enlarged view of a portion corresponding to the first example of the second requirement shown in  FIG. 15A , and  FIG. 15C  is a partially enlarged view of a portion corresponding to the second example of the second requirement shown in  FIG. 15A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, referring to the accompanying drawings, backlight devices according to the present invention will be described in detail. It should be noted, however, that the technical scope of the present invention is not limited to those embodiments, but extends to the inventions and their equivalents described in the claims. 
     (Outline of Backlight Device According to the Embodiment) 
     In a backlight device according to the embodiment, an overlapping ratio is 20% or less, wherein a half value region is defined in which light that is equal to or more than half of a peak value of a brightness of light emitted from one of the plurality of light emitting elements is incident on the incident surface, the overlapping ratio is an ratio of an overlapping region to a half value region defined around the one of the light emitting elements, and the overlapping region is a region overlapping a half value region defined around the one of the light emitting elements and a half value region of another one of light emitting elements adjacent to the one of the light emitting elements, the half value region is a region to be equal to or more than half the peak value of the brightness of a light emitted from a light emitting element. Further, in a backlight device according to the embodiment, a ratio less than the half value is 20% or less, wherein the ratio less than the half value is a ratio of an area of a region less than the half value to an area of a surrounding region formed by connecting at least three light emitting elements disposed around the region less than the half value, and the region less than the half value is a region on which no half value regions of the plurality of light emitting elements are disposed. The unevenness of luminance in a backlight device according to the embodiment may be reduced, by disposing the light emitting elements in accordance with the above conditions. 
     (A Backlight Device According to a First Embodiment) 
       FIG. 1  is a perspective view of a backlight device according to the first embodiment,  FIG. 2  is an exploded perspective view of the backlight device shown in  FIG. 1 , and  FIG. 3  is a line A-A′ cross-sectional view of  FIG. 1 . 
     A backlight device  1  has a mounting board  10 , a plurality of LEDs  11 , a pair of struts  12 , a reflective sheet  13 , a first diffuser plate  14 , a second diffuser plate  15 , a prism sheet  16 , a polarizing sheet  17  and a directional control sheet  18 . Further, the backlight device  1  has a back case  20 , the front case  21  and a resin frame  22 . The backlight device  1  has a circular outer shape in a planar view, a backlight device according to the embodiment may have a square outer shape in a planar view. 
     The mounting substrate  10  is, for example, a glass epoxy substrate made of glass fibers and an epoxy resin, a substrate formed of paper phenol, paper epoxy and a resin substrate based on a polyimide resin substrate. Wiring patterns (not shown) are formed on front and rear surfaces of the mounting substrate  10 . The wiring patterns on the front surface supply electric signals supplied through vias (not shown) from the wiring patterns on the rear surface to each of the plurality of LEDs  11 , and therefore each of the plurality of LEDs  11  emits light. 
     The LED  11  is an example of a light emitting element used as a light source for the backlight device  1 , and is, for example, a packaged surface mount type LED element. For example, the LED  11  has a package made of a white resin with recesses, an LED chip referred to as a die primarily mounted on a bottom surface of the recesses of the package, and a sealing member enclosed within the recesses of the package. The sealing member is made of a translucent resin material such as a silicone resin. For example, the LED  11  has a blue LED die emitting blue light and yellow phosphors such as yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG) contained in the sealing member, and emits white light. In this specification, the brightness of the light emitted from the LED  11  means the brightness of the light emitted from the LED  11  when the rated current is applied to the LED  11 . 
     Each of the pair of struts  12  is made of a white resin, is a member standing from the back case  20 , and supports each of the first diffuser plate  14  to the directional control sheet  18  from below. Although the backlight device  1  has a pair of struts  12 , a backlight device according to the embodiment may have a plurality of struts. 
     The reflective sheet  13  is a sheeted reflecting member having a light reflective surface, and is mounted or attached to a surface of the mounting substrate  10 . A plurality of openings having a shape corresponding to each of the plurality of LEDs  10  and the pair of struts  12  are formed on the reflective sheet  13 . 
     The outer edge of the first diffuser plate  14  is supported by the resin frame  22 , the vicinity of the center of the first diffuser plate  14  is supported by the pair of struts  12 , and therefore the first diffuser plate  14  is disposed above the mounting substrate  10  so that the first diffuser plate  14  covers the LEDs  11  disposed on the surface of the mounting substrate  10 . The first diffuser plate  14  has an incident surface disposed facing to the surface of the mounting substrate  10 , and an emitting surface disposed on the opposite side of the incident surface, and diffuses light incident on the incident surface from a plurality of LEDs  11  and emits the diffused light from the emitting surface. A reflection material  140  referred to as a lighting curtain layer is disposed on the incident surface of the first diffuser plate  14 . The reflection material  140  is a white light shielding member such as titanium oxide, barium sulfate that shield and reflects a part of light emitted from the LEDs  11 , and although the reflection material  140  is formed by screen printing, the reflection material  140  may be integrated with the diffuser plate by molding methods such as ejection. The reflection material  140  may be a diffusion member such as glass or silicon oxide, or ink members having a black color. The reflection material  140  is disposed directly above each of the plurality of LEDs  11  having the highest brightness of light emitted from each of the plurality of LEDs  11 , and reflects the incident light toward the reflective sheet  13 . The imbalance of the brightness of the light incident on the first diffuser plate  14  is reduced between the directly above portion and another portion of each of the plurality of LEDs  11 , since the reflection material  140  reflects the incident light to the direction of the reflecting sheet  13 . 
       FIG. 4  is a portion enlarged view of a position of the incident surface of the first diffuser plate  14  facing to each of a plurality of LEDs  11 . 
     The reflection material  140  has a hexagonal first reflector  141  and a plurality of circular second reflectors  142 . The first reflector  141  is disposed directly above the LED  11  so as to cover the entire LED  11 , and the second reflectors  142  are disposed around the first reflector  141  so as to maintain the hexagonal shape as a whole. The second reflectors  142  are formed so as to be smaller as the distances from the first reflector  141  are longer. 
     The second diffuser plate  15  is a flat member disposed so as to cover the emitting surface of the first diffuser plate  14 . The second diffuser plate  15  has an incident surface disposed opposite to the surface of the mounting substrate  10  through the first diffuser plate  14 , and an emitting surface disposed on the opposite side of the incident surface, and further diffuses the light incident to the incident surface from the first diffuser plate  14  and emits the diffused light from the emitting surface. The diffusivity of the second diffuser plate  15  is higher than that of the first diffuser plate  14 , the second diffuser plate  15  diffuses the light emitted from the LED  11  so that the diffused light has a desired spread, by further diffusing the light diffused by the first diffuser plate  14 . The diffusivity of each of the first diffuser plate  14  and the second diffuser plate  15  is adjusted by adjusting the Haze value, diffusivity and light transmittance thereof, and defining the thickness and emboss shape thereof. 
     The prism sheet  16  is a flat optical member made of a synthetic resin such as acrylic resin, and is disposed so as to cover the emitting surface of the second diffuser plate  15 . The prism sheet  16  has an incident surface facing to the emitting surface of the second diffuser plate  15 , and an emitting surface disposed on the opposite side of the incident surface, and emits light emitted from the emitting surface of the second diffuser plate  15  in the vertical direction. 
     The polarizing sheet  17  is a flat plated optical member having a synthetic resin such as polyvinyl alcohol in which Iodine and stain is dyed and adsorbed, and is disposed so as to cover an emitting surface of the prism sheet  16 . The polarizing sheet  17  has an incident surface facing to the emitting surface of the prism sheet  16 , and an emitting surface disposed on the opposite side of the incident surface thereof. The polarizing sheet  17  emits polarized light emitted from the emitting surface of the prism sheet  16  so as to align in the direction with the polarization axis of the liquid crystals of an LCD device (not shown) disposed on the backlight device  1 . 
     The directional control sheet  18  is a flat optical member made of a synthetic resin such as acrylic resin, and is disposed so as to cover the emitting surface of the polarizing sheet  17 . The directional control sheet  18  has an incident surface facing to the emitting surface of the polarizing sheet  17 , and an emitting surface disposed on the opposite side of the incident surface thereof. The directional control sheet  18  is formed so as to emit light emitted from the emitting surface of the polarizing sheet  17  in a desired direction. For example, the directional control sheet  18  controls a light direction so that the light may be easily viewed by an occupant of the vehicle such as a driver, when the backlight device  1  is used as a backlight of an on-board display device. Further, the directional control sheet  18  controls a light direction so that an image displayed on a display device is not reflected on windshields. 
     The back case  20  is made of a metal material such as aluminum and stainless steel, and is a concave member that houses the mounting substrate  10  to the directional control sheet  18  and the resin frame  22 . The front case  21  is made of a metallic material such as aluminum and stainless steel or a synthetic resin such as polycarbonate, and functions as a housing of backlight device  1  along with the back case  20 . A circular opening is formed on the front case  21 , and light is emitted from the opening formed on the front case  21 . 
     The resin frame  22  is made of a synthetic resin such as polycarbonate and a recess formed for housing the mounting substrate  10  to the reflective sheet  13 . Further, a step for supporting the first diffuser plate  14  to directional control sheet  18  is formed on the inner edge of the resin frame  22 . The resin frame  22  is a frame defining a light emitting region and a region inside the inner edge of the front case  21  and the resin frame  22  shown by a broken line arrow B in  FIG. 3  is defined as the light emitting region on which the plurality of LEDs  11  are disposed. Wiring patterns for supplying power to the LEDs  11  may be formed on a surface of the resin frame  22 . 
       FIG. 5  is a plan view showing the arrangement of the LEDs  11  and the pair of struts  12 , and  FIG. 6  is a figure for explaining the half value region. 
     Each of the plurality of LEDs  11  is disposed such that the half value regions satisfy a predetermined requirement. The half value region is a region defined by a half value angle θ which is an angle to be half the peak value of the brightness of the light, and a distance L between a surface of an LED  11  and the incident surface of the first diffuser plate  14 . Specifically, the half value region is a substantially circular region included in the distance of R=L tan θ from the center of an LED  11  in a planar view of the LED  11 . The half value region is defined with respect to a surface facing to the substrate, i.e. the incident surface of the diffuser plate. Each of the plurality of the LEDs  11  is disposed so that the half value region which is a substantially circular region included in the distance of R=L tan θ from the center of an LED  11  in a planar view of the LED  11  satisfy the following four requirements. 
     (First Requirement) 
     A plurality of LEDs  11  is disposed so that an overlapping ratio is 20% or less, wherein the overlapping ratio is a ratio of an area of an overlapping region  111  to an area of the half value region  110  defined around the one of the plurality of LEDs  11 , and the overlapping region  111  is a region overlapping a half value region  110  defined around one of the LEDs  11  and a half value region defined around another one of LEDs  11  adjacent to the one of the LEDs  11 . 
     (Second Requirement) 
     The plurality of LEDs  11  are disposed so that a ratio less than the half value is 20% or less, wherein the ratio less than the half value is a ratio of a an area of the region less than the half value  112  to an area of a surrounding region  113  formed by connecting among the plurality of LEDs  11  disposed around the region less than the half value  112 , and the region less than the half value  112  is a region on which no half value regions of the plurality of LEDs  11  are disposed. 
     (Third Requirement) 
     All of the LEDs  11  disposed along the resin frame  22  are disposed so that the overlapping regions are formed among the adjacent LEDs  11  disposed along the resin frame  22 . 
     (Fourth Requirement) 
     Each of the plurality of LEDs  11  are disposed so that three LEDs  11  adjacent to each other are not aligned in a straight line. 
     The LEDs  11  disposed along the resin frame  22  are disposed so that no regions less than the half value are formed on the outside of the LEDs. The LED  11  disposed along the resin frame  22  is disposed so that no regions less than the half value are formed between the LED and the resin frame  22 . It is preferable that the LEDs  11  disposed along the resin frame  22  are disposed so that the outer ends of the overlapping regions between the LEDs  11  disposed adjacent along the resin frame  22  are contacted with the inner edge of the resin frame  22 . 
     Although a part of the half value region defined around an LED  11  disposed along the resin frame  22  overlaps the resin frame  22 , the half value region defined around an LED  11  disposed along the resin frame  22  is defined as substantially circular in first requirement, similar to other LEDs  11 . Further, the outer side of LEDs  11  disposed along the resin frame  22  are not objects of the second requirement, since no regions less than the half value are formed between the LEDs  11  disposed along the resin frame  22  and the resin frame  22 . 
     The first requirement indicates that an overlapping ratio (Alap/Ahalf) is 20% or less, wherein the overlapping ratio (Alap/Ahalf) is a ratio of an area Alap of an overlapping region to the area Ahalf of the half value region defined around the one of the plurality of LEDs  11 , and the overlapping region is a region overlapping a half value region defined around one of the LEDs  11  and half value region defined around another one of LEDs  11  adjacent to the one of the LEDs  11 . The LEDs  11  are disposed so as to satisfy the first requirement, and therefore the area of the overlapping regions formed between the adjacent pair of the LEDs  11  is increased, and the overlapping region having a brightness higher than a desired brightness between the adjacent pair of the LEDs  11  is prevented from being formed. 
     The second requirement indicates that a ratio (Abelow/Aaround) of a less than half value is 20% or less, wherein the ratio (Abelow/Aaround) is a ratio of an area Abelow of a region less than the half value to an area Aaround of a surrounding region formed by connecting among the plurality of LEDs  11  disposed around the region less than the half value, and the region less than the half value is a region on which no half value region  110  of the plurality of LEDs  11  are disposed. The LEDs  11  are disposed so as to satisfy the second requirement, and therefore the areas of the regions less than the half value formed among the at least three LED  11  are decreased, and the regions less than the half value having a luminance lower than a desired luminance is prevented from being formed among the LEDs  11 . 
     The third requirement indicates that each of LEDs  11  disposed along the resin frame  22  is disposed so that the overlapping regions are formed between the LEDs adjacent to each other along the resin frame  22 . Thus, the third requirement indicates that each of the LEDs  11  disposed along the outer edge of the plurality of LEDs  11  are disposed so that the adjacent half value regions along the resin frame  22  are overlapped in order to form overlapping regions, as indicated by reference numeral  11   a  and  11   b  in  FIG. 5 . The LEDs  11  are disposed so as to satisfy the third requirement, and therefore overlapping regions are formed among all of the LEDs  11  disposed along the resin frame  22 , and the brightness of the outer edge of the light emitting region is prevented from being low. 
     The fourth requirement indicates that each of the plurality of LEDs  11  are disposed so that three LEDs  11  adjacent to each other are not aligned in a straight line, as indicated by reference numeral  11   c  in  FIG. 5 . Thus, the fourth requirement indicates that three LEDs  11  adjacent are not aligned in a straight line either in a first direction indicated by arrow A in  FIG. 5  and a second direction orthogonal to the first direction and indicated by arrow B in  FIG. 5 . In the present specification, an arrangement where the LEDs  11  are aligned in a straight line indicates that facing sides of the LEDs  11  disposed adjacent to the first and second directions are overlapped more than a half of a short side length of the LED  11 . On the other hand, an arrangement in which the LEDs  11  are not aligned in a straight line indicates that facing sides of the LEDs  11  disposed adjacent to the first and second directions are not overlapped more than a half of a short side length of the LED  11 . The LEDs  11  are disposed so as to satisfy the fourth requirement, and therefore linear regions having high brightness prevented from being generated in a light emitting region. 
       FIG. 7A  is a figure showing an example in which three LED  11  adjacent are aligned in a straight line, and  FIG. 7B  is a figure showing an example in which three LED  11  adjacent are not aligned in a straight line. In  FIGS. 7A and 7B , the long side length of the LED  11  in a plain view is indicated by LL, the short side length of the LED  11  in a plain view is indicated by LS. 
     When the overlapped length LF 1  of facing sides of the LEDs  11  disposed adjacent to the first and second directions is equal to or greater than the half of the short side length LS/2 of LED  11 , the adjacent LEDs  11  are aligned in a straight line. On the other hand, when the overlapped length LF 2  of the facing sides of the LEDs  11  disposed adjacent to the first direction and the second direction is less than the half of the short side LS/2 of the LED  11 , the adjacent LEDs  11  are aligned in a straight line. 
     (Effect of Operation of Backlight Device According to the First Embodiment) 
     Since a plurality of LEDs in a backlight device according to the first embodiment are disposed so that the first and second requirements are satisfied, the unevenness of luminance among adjacent LEDs may be within ±2% in which light and dark are not visually recognized by a person. 
       FIG. 8A  is a figure showing an example of an arrangement in which an overlapping ratio between adjacent LEDs is 20% or less, and  FIG. 8B  is a cross-sectional view corresponding to the arrangement shown in  FIG. 8A .  FIG. 8C  is a figure showing an example of arrangement in which a ratio less than the half value is 20% or less,  FIG. 8D  is a cross-sectional view corresponding to the arrangement shown in  FIG. 8C . 
     In the arrangement shown in  FIG. 8A , the overlapping ratio is 20% or less, and a ratio less than the half value is 0%. On the other hand, in the arrangement shown in  FIG. 8C , the ratio less than the half value is 20% or less, and the overlapping ratio is 0%. 
     In the arrangement shown in  FIG. 8A , the LEDs  11  are disposed in first arrangement interval P 1  (=18 mm), and the distance between the surface of LEDs  11  and the incident surface of the first diffuser plate  14  is L (=6.188 mm). In the arrangement shown in  FIG. 8C , the LEDs  11  are disposed in a second arrangement interval P 2  (=13 mm), and the distance between the surface of the LEDs  11  and the incident surface of the first diffuser plate  14  is L. 
       FIG. 9A  is a figure showing a relationship between the distance from the LEDs  11  and the brightness ratio in the arrangement shown in  FIG. 8A , and  FIG. 9B  is a figure showing a relationship between the distance from the LEDs  11  and the brightness ratio in the arrangement shown in  FIG. 8C . 
     In the arrangement shown in  FIG. 8A  and the arrangement shown in  FIG. 8C , since the difference in the luminance ratio corresponding to the unevenness of luminance between the adjacent LEDs  11  is both 2% or less, the unevenness of luminance between the adjacent LEDs  11  is not visible by a person. 
     Further, in a backlight device according to the first embodiment, since each of the LED  11  disposed along the outer edge is disposed so that overlapping regions are formed between the LEDs adjacent to each other, no bright linear regions in the radiant light referred to as hot spots may be formed. 
     Further, in a backlight device according to the first embodiment, since an arrangement of the LEDs  11  are determined based on the half value region in which the light having more than half the peak value of the brightness of the light emitted from LED  11  is incident on the incident surface, it is easy to uniform the brightness of the light emitted from the backlight device. 
       FIG. 10  is a figure showing a luminance distribution in the arrangement shown in  FIG. 8C . In  FIG. 10 , a broken line  101  shows a luminance distribution of light emitted from one of the LEDs  11 , a broken line  102  shows a luminance distribution of light emitted from another of the LEDs  11 , and a solid line  103  shows a luminance distribution of light combined light emitted from both LEDs  11 . 
     In the arrangement shown in  FIG. 10 , the brightness of the combined lights at the midpoint of the two LED  11  is substantially twice the half value of the peak value, and therefore the brightness thereof is substantially similar to the peak value. Further, as the brightness between the two LEDs  11  of the light emitted from one of the LEDs  11  is decreased, the brightness between the two LEDs  11  of the light emitted from another one of the LEDs  11  is increased, and therefore the brightness of the combined light is substantially similar to the peak value. In the backlight device according to the first embodiment, the arrangement of the LEDs  11  is determined based on the half value region, and therefore an arrangement of the LEDs  11  may be easily determined in order to uniform the brightness of light emitted from the backlight device. 
     (A Backlight Device According to the Second Embodiment) 
       FIG. 11A  is a perspective view of a backlight device according to the second embodiment, and  FIG. 11B  is a D-D′ line sectional view of  FIG. 11A . 
     A backlight device  2  is different from the backlight device  1  in that the backlight device  2  has a first diffuser plate  24  instead of the first diffuser plate  14 . Since components in the backlight device  2  other than the first diffuser plate  24  have the same structures and functions as those of components in the backlight device  1  having the same reference numerals, a detailed description thereof will be omitted. 
     The first diffuser plate  24  is different from the first diffuser plate  14  in that the reflection material  140  is not disposed on the incident surface. Further, the first diffuser plate  24  is different from the first diffuser plate  14  in that the first diffuser plate  24  contains reflection materials  240 . The reflection material  240  is a member for shielding and reflecting a part of the light emitted from the LED  11  similarly to the reflection material  140 . The reflection materials  240  are disposed directly above each of the plurality of LEDs  11  having the highest brightness of the light emitted from each of the plurality of LEDs  11 , and reflects the incident light toward the reflective sheet  13 . 
     The reflection material  240  is a white light shielding member such as titanium oxide, barium sulfate and etc. that shields and reflects a portion of the light emitted from LED  11 , and is formed by integrally molded with the diffuser plate by molding methods such as ejection. The reflection material  240  may be a diffusion member such as glass, silicon oxide or an ink member having a black color. 
     In a backlight device according to the embodiment, each of the plurality of LEDs  11  may be disposed so as to satisfy the first to fourth requirements, and the LEDs may be disposed such that three LEDs form a triangular shape. 
       FIG. 12A  is a figure showing an example of the arrangement of LEDs in a backlight device according to the embodiment, and  FIG. 12B  is a figure showing the other example of the arrangement of LEDs in a backlight device according to the embodiment. 
     In the arrangement shown in  FIG. 12A , the three LEDs  11  are disposed in an equilateral triangular shape. The arrangement interval among the three LEDs  11  is the third arrangement interval P 3 . The three LEDs  11  are disposed in an equilateral triangular shape at the third arrangement interval P 3 , and therefore the overlapping ratio is 0%, the ratio less than the half value is 9.3%. 
     In the arrangement shown in  FIG. 12B , the two LEDs  11  and one LED  11 ′ are disposed in an isosceles triangular shape. The rated value of the brightness of the LED  11 ′ is lower than that of the brightness of the LEDs  11 , and the radius of a half value region defined around the LED l′ is shorter than that of a half value region defined around the LEDs  11 . An arrangement interval between the two LEDs is a third arrangement spacing P 3  similarly to the arrangement example shown in  FIG. 12A . An arrangement interval between each of the two LED  11  and LED  11 ′ is shorter than the third arrangement interval P 3 . The two LEDs  11  and the LED  11 ′ are disposed in an isosceles triangular shape, and therefore the overlapping ratio is 0%, the ratio less than the half value is lower than 9.3%. 
     Further, although the reflection materials  140  and  240  are respectively disposed in backlight device  1  and  2  in addition to the first diffuser plate  14  and  24 , in a backlight device according to the embodiment, only the diffuser is disposed, and no reflection materials may be disposed therein. 
     Example 
       FIG. 13  is a figure showing a relationship between the LEDs  11  and the half value regions according to the exemplary embodiment. 
     The LEDs  11  disposed along the resin frame  22  are disposed so that no regions less than the half value are formed on the outside, when the brightness of LED  11  is the rated value. Thus, the LEDs  11  disposed along the resin frame  22  is disposed so that no regions less than the half value are formed between the LEDs and the resin frame  22 . In the exemplary embodiment shown in  FIG. 13 , the brightness of LED  11  is 20000 [cd/m2]. It is preferable that the LEDs  11  disposed along the resin frame  22  are disposed so that the outer ends of the overlapping regions among the LEDs  11  disposed adjacent along the resin frame  22  are disposed so as to contact with the inner edge of the resin frame  22 , as indicated by arrow C in  FIG. 13 . 
     Although a part of the half value regions defined around LEDs  11  disposed along the resin frame  22  overlaps the resin frame  22 , the half value regions defined around LED  11  disposed along the resin frame  22  are defined as substantially circular in the first requirement similar to other LEDs  11 . Further, since the LEDs  11  disposed along the resin frame  22  are disposed so that no regions less than the half value are formed between the LEDs and the resin frame  22 , the outer side of LED  11  disposed along the resin frame  22  is not object of the second requirement. 
       FIG. 14A  is a figure showing a relationship between the LEDs  11  and the first requirement according to the embodiment,  FIG. 14B  is a partially enlarged view of a portion corresponding to the first requirement shown in  FIG. 14A . 
     The first requirement indicates that an overlapping ratio (Alap/Ahalf) is 20% or less, wherein the overlapping ratio (Alap/Ahalf) is a ratio of an area Alap of an overlapping region  111  of LED  11  to an area Ahalf of the half value region the one of the plurality, and the overlapping region  111  is a region where a half value region  110 ′ defined around one of the LEDs  11  overlaps that of an adjacent LED  11 . In the LEDs  11  shown in  FIG. 14B , the overlapping ratio (Alap/Ahalf) is 6.0%, and the mean of the overlapping ratio (Alap/Ahalf) of all of the LEDs  11  shown in  FIG. 14A  is 7.7%. 
       FIG. 15A  is a figure showing a relationship between the LEDs  11  and the second requirement according to the embodiment.  FIG. 15B  is a partially enlarged view of a portion corresponding to the first example of the second requirement shown in  FIG. 15A , and  FIG. 15C  is a partially enlarged view of a portion corresponding to the second example of the second requirement shown in  FIG. 15A . 
     The second requirement indicates that a ratio less than the half value (Abelow/Aaround) is 20% or less, wherein the ratio less than the half value (Abelow/Aaround) is a ratio of an area Abelow of a region less than the half value to an area Aaround of a peripheral region formed by connecting among the plurality of LEDs disposed around the region less than the half value, and the region less than the half value is a region on which no half value regions  110 ′ of the plurality of LEDs  11  are disposed. In the LEDs  11  of the first example shown in  FIG. 15B , a region less than the half value  112 ′ is a region where no half value region defined around the four LED  11  are disposed, and the surrounding region  113 ′ is a region formed by connecting among the four LEDs  11  disposed around the region less than the half value  112 ′. In the LEDs  11  of the second example shown in  FIG. 15C , a region less than the half value  112 ″ is a region where no half value regions of the seven LEDs  11  are disposed, and a surrounding region  113 ″ is a region formed by connecting among the seven LEDs  11  disposed around the regions less than the half value  112 ″. In the LEDs  11  of the first example shown in  FIG. 15B , the ratio less than the half value (Abelow/Aaround) is 12.6%, in the LEDs  11  of the second example shown in  FIG. 15C , the ratio less than the half value (Abelow/Aaround) is 11.3%. Further, the mean value of the ratio less than the half value (Abelow/Aaround) of all of the LEDs  11  shown in  FIG. 15A  is 11.7%.