Patent Publication Number: US-11650455-B2

Title: Planar light source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 17/331,479, filed on May 26, 2021. This application claims priority to Japanese Patent Application No. 2020-95002 filed on May 29, 2020, and Japanese Patent Application No. 2020-207548 filed on Dec. 15, 2020. The entire disclosures of U.S. patent application Ser. No. 17/331,479 and Japanese Patent Application No. 2020-95002 and Japanese Patent Application No. 2020-207548 are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a planar light source and a liquid crystal display device. 
     Background Art 
     Planar light sources using light emitting elements such as light emitting diodes are widely used for various light sources such as a backlight for a liquid crystal display device and a display device. 
     An example of such a planar light source has a configuration including light emitting elements arranged two-dimensionally and a light diffusion plate disposed above the light emitting elements. For example, see JP 2012-221779 A. In a planar light source described in JP 2012-221779 A, the light diffusion plate contains diffusion particles for diffusing light emitted from light emitting portions of the light emitting elements. In this light diffusion plate, protrusions with a gentle curved surface shape protruding toward the light emitting elements are integrally formed at least in areas corresponding to the light emitting portions of the light emitting elements in a surface of the light diffusion plate proximate to the light emitting elements. 
     SUMMARY 
     An object of the present disclosure is to reduce unevenness in brightness generated at a peripheral region of a planar light source having an irregular planar shape. 
     A planar light source according to an embodiment of the disclosure includes a mounting substrate, a plurality of light sources, a partition member, a frame body, a wavelength conversion member, and a light diffusion plate. The light sources are arranged two-dimensionally on the mounting substrate in a plan view. The partition member includes a wall portion surrounding each of the light sources except for outermost ones of the light sources in the plan view. The wall portions are located inward of the outermost ones of the light sources. The frame body has a bottom portion and a lateral wall surrounding the mounting substrate. The wavelength conversion member is disposed on the lateral wall of the frame body. The light diffusion plate is disposed above the plurality of light sources and the wavelength conversion member. 
     A planar light source according to an embodiment of the disclosure includes a mounting substrate, a plurality of light sources, a frame body, a wavelength conversion member, and a light diffusion plate. The light sources are arranged two-dimensionally on the mounting substrate in a plan view. The frame body has a bottom portion and a lateral wall surrounding the mounting substrate. The wavelength conversion member is disposed on the lateral wall of the frame body. A distance between the wavelength conversion member and an outermost light source among the plurality of light sources is greater than a distance between two light sources among the plurality of light sources. The light diffusion plate is disposed above the plurality of light sources and the wavelength conversion member. 
     According to one embodiment of the disclosure, unevenness in brightness generated at a peripheral region can be reduced in a planar light source having an irregular planar shape. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic plan view illustrating a planar light source according to a first embodiment. 
         FIG.  2    is a schematic partially enlarged plan view of part B of  FIG.  1   . 
         FIG.  3    is a cross-sectional view taken along line A-A of  FIG.  2   . 
         FIG.  4    is a schematic plan view illustrating the arrangement of the light sources in the planar light source according to the first embodiment. 
         FIG.  5    is a schematic plan view illustrating a light diffusion plate in the planar light source according to the first embodiment. 
         FIG.  6    is a partially enlarged cross-sectional view of the vicinity of the light source of  FIG.  3   . 
         FIG.  7    is a schematic plan view for describing a width of a thin plate portion (Part 1). 
         FIG.  8    is a schematic plan view for describing the width of the thin plate portion (Part 2). 
         FIG.  9    is a diagram showing a simulation result for a light diffusion plate (Part 1). 
         FIG.  10    is a diagram showing a simulation result for the light diffusion plate (Part 2). 
         FIG.  11    is a schematic cross-sectional view illustrating an arrangement of optical members. 
         FIG.  12    is a schematic partially enlarged cross-sectional view illustrating a light diffusion plate of the planar light source according to a first modified example of the first embodiment (Part 1). 
         FIG.  13    is a schematic partially enlarged cross-sectional view illustrating a light diffusion plate of the planar light source according to the first modified example of the first embodiment (Part 2). 
         FIG.  14    is a schematic partially enlarged cross-sectional view illustrating a light diffusion plate of the planar light source according to the first modified example of the first embodiment (Part 3). 
         FIG.  15    is a schematic plan view illustrating the light diffusion plate of  FIG.  14   . 
         FIG.  16    is a schematic partially enlarged plan view of a partition member according to a second modified example of the first embodiment. 
         FIG.  17    is a cross-sectional view taken along line B-B of  FIG.  16   . 
         FIG.  18 A  is a schematic partially enlarged cross-sectional view in the vicinity of an outer edge of the planar light source (Part 1). 
         FIG.  18 B  is a schematic partially enlarged cross-sectional view in the vicinity of the outer edge of the planar light source (Part 2). 
         FIG.  19    is a schematic plan view explaining an outer shape of a substrate in a planar light source according to a third modified example of the first embodiment. 
         FIG.  20    is a configuration diagram illustrating a liquid crystal display device according to a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, certain embodiments of the present invention will be described with reference to the drawings. In the description below, terms indicating a specific direction or position (e.g., “upper”, “lower”, and other terms including those terms) are used when necessary. The use of those terms is to facilitate understanding of the invention with reference to the drawings, and the technical scope of the invention is not limited by the meaning of those terms. In addition, parts designated with the same reference numerals appearing in a plurality of drawings indicate identical or equivalent parts or members. 
     Further, the embodiments to be illustrated below exemplify planar light sources for embodying the technical concept of the present invention, and do not limit the present invention to the description below. In addition, unless otherwise specified, the dimensions, materials, shapes, relative arrangements, and the like of constituent elements described below are not intended to limit the scope of the present invention to those, but are intended to be exemplified. Also, the contents described in one embodiment can be applied to another embodiment and modified examples. Further, the size, positional relationship, and the like of the members illustrated in the drawings can be exaggerated in order to clarify the explanation. 
     First Embodiment 
     Planar Light Source  10   
       FIG.  1    is a schematic plan view illustrating a planar light source according to a first embodiment.  FIG.  2    is a schematic partially enlarged plan view of part E of  FIG.  1   .  FIG.  3    is a cross-sectional view taken along line A-A of  FIG.  2   .  FIG.  4    is a schematic plan view illustrating an arrangement of light sources in the planar light source according to the first embodiment, and is a view in which a partition member and a light diffusion plate in  FIG.  1    are not illustrated.  FIG.  5    is a schematic plan view illustrating the light diffusion plate in the planar light source according to the first embodiment.  FIG.  6    is a partially enlarged cross-sectional view of the vicinity of the light source of  FIG.  3   . 
     As illustrated in  FIGS.  1  to  6   , a planar light source  10  is a surface-emitting light emitting device including a substrate  11 , light sources  12 , a partition member  13 , and a light diffusion plate  14 . In  FIGS.  1  and  2   , the structures under the light diffusion plate  14  (i.e., the substrate  11 , the light sources  12 , and the partition member  13 ) are illustrated with solid lines in order to better show these structures. The partition member  13  is not an essential component of the planar light source  10 , but is provided when necessary. When the partition member  13  is not provided, for example, a supporting body that supports the light diffusion plate  14  can be provided. 
     In the description below, the term “in a plan view” refers to viewing a target object in the normal direction to an upper surface  11   m  of the substrate  11 , and the term “planar shape” refers to the shape of the target object viewed in the normal direction to the upper surface  11   m  of the substrate  11 . 
     A plurality of light sources  12  each including a light emitting diode are arranged on the substrate  11 , which is a mounting substrate. Any appropriate number of light sources  12  can be disposed on the substrate  11 . 
     The partition member  13  is disposed on the same side as the light sources  12  of the substrate  11 . The partition member  13  has a top portion  13   a  disposed in a grid pattern in a plan view, and a wall portion  13   b  surrounding each of the light sources  12  in a plan view, and includes a plurality of regions each surrounding a corresponding one of the light sources  12 . The wall portion  13   b  of the partition member  13 , for example, extends from the top portion  13   a  toward the substrate  11 . In a cross-sectional view, each space surrounded by corresponding portions of the wall portion  13   b  that are opposite to each other has a width reduced toward the substrate  11 . 
     A region surrounded by corresponding portions of the wall portion  13   b  (that is, the area and the space) is defined as a single section C, so that the partition member  13  includes a plurality of sections C. In the present embodiment, a single light source  12  is disposed in a single section C. However, two or more light sources  12  can be disposed in a single section C. In this case, for example, three light sources  12  of red, green, and blue can be disposed in a single section C. Alternatively, two light sources  12  of a daylight color and a bulb color can be disposed in a single section C. 
     The light diffusion plate  14  is an optical member disposed on the top portion  13   a  of the partition member  13  and disposed above the light source  12 . With the light diffusion plate  14 , the planar light source  10  can have improved the uniformity of light. The light diffusion plate  14  according to the present embodiment has a portion having a thin thickness on a peripheral region in order to reduce unevenness in brightness on the peripheral region of the planar light source  10 . 
     The outermost contour portion of each member in a plan view is referred to as an “outer edge”, and an area having a width and including the outer edge in a plan view is referred to as a peripheral region. In particular, the term “peripheral region of the light diffusion plate  14 ” refers to an area of the light diffusion plate  14  located outward of peripheral light sources  12  (outermost light sources) of the plurality of light sources  12  in a plan view. The peripheral region does not necessarily represent an annular region. 
     Hereinafter, components constituting the planar light source  10  will be described in detail. 
     Substrate  11   
     The substrate  11  is a member on which the plurality alight sources  12  are arranged, and has an irregular shape. The term “irregular shape” as used herein refers to a shape other than a rectangular shape, such as a non-rectangular shape partially or entirely modified from a perfect rectangular shape in order to correspond to a predetermined product shape. 
     As illustrated in  FIG.  6   , conductor wirings  18 A and  18 B for supplying electric power to the light sources  12  such as the light emitting elements  12   a  are disposed on the upper surface  11   m  of the substrate  11 . Portions of the conductor wirings  18 A and  18 B that are not to be electrically connected to the light emitting elements  12   a  are preferably covered with a covering member  15 . 
     The material of the substrate  11  can be any appropriate material that can insulate at least between a pair of conductor wirings  18 A and  18 B, and examples thereof include ceramics, resins, and composite materials. Examples of the ceramics include alumina, mullite, forsterite, glass ceramics, nitride-based ceramics (e.g., AlN), carbide-based ceramics (e.g, SiC), and LTCC. Examples of the resin include phenol resin, epoxy resin, polyimide resin, BT resin, polyphthalamide (PPA), and polyethylene terephthalate (PET), Examples of the composite material include the resins described. above with an inorganic filler such as glass fiber, SiO 2 , TiO 2 , or Al 2 O 3  mixed in the resins, a glass fiber reinforced resin (glass epoxy resin), and a metal substrate having an insulating layer formed on the metal member. 
     The thickness of the substrate  11  can be appropriately selected. The substrate  11  can be either a flexible substrate that can be manufactured in roll-to-roll processing or a rigid substrate. The rigid substrate can be a bendable thin rigid substrate. For the conductor wirings  18 A and  18 B, any appropriate material can be used as long as it is a conductive member, and a material commonly used for a wiring layer of a circuit board or the like can be used. A plating film, a light reflecting film, or the like can be formed on surfaces of the conductor wirings  18 A and  18 B. 
     The covering member  15  is preferably formed of an insulating material. Examples of the material of the covering member  15  include the same materials as those described above as examples of the material of the substrate  11 . Using the resin described above containing white filler or the like for the covering member  15  allows the light emitted from the light source  12  to be reflected at the covering member  15 , so that the light extraction efficiency of the planar light source  10  can be improved. 
     Light Source  12   
     The plurality of light sources  12  are disposed two-dimensionally in a first direction and in a second direction perpendicular to the first direction, on the substrate  11  in a plan view. More specifically, as shown in, for example,  FIG.  1   , the light sources  12  are arranged in a plurality of first arrays of the light sources each extending along the first direction and a plurality of second arrays of the light sources each extending along the second direction perpendicular to the first direction in the plan view. The first direction and the second direction are, for example, the X direction and the Y direction of  FIG.  1   , etc. 
     The light source  12  is a member configured to emit light, and examples of the light source  12  include a light emitting element configured to emit light, a light emitting element sealed with a light-transmissive resin or the like, and a surface-mounted light emitting device (which may also be referred to as an LED) in which the light emitting element is packaged. Examples of the light source  12  include, as illustrated in  FIG.  6   , the light emitting element  12   a  covered by a sealing member  12   b.  A single light emitting element  12   a  can be used for the light source  12 , but a plurality of light emitting elements can be used for the single light source  12 . Further, the light source  12  can have a configuration including a resin containing a light-reflective material and surrounding lateral surfaces of the light emitting element, and a light-transmissive member that covers an upper surface of the light emitting clement and an upper surface of the resin containing the light-reflective material. The light source  12  can have a configuration including a light-transmissive member covering the upper surface of the light emitting element and a resin containing a light-reflective material and surrounding the lateral surfaces of the light emitting element and lateral surfaces of the light-transmissive member. The light-transmissive member in an example herein can contain a phosphor. A light-transmissive bonding member that adheres the light emitting element and the light-transmissive member can be disposed between the light emitting element and the light-transmissive member. 
     The light source  12  can have any appropriate light distribution characteristics, but preferably has a wide light distribution in order to illuminate in each section C surrounded by corresponding portions of the wall portion  13   b  of the partition member  13  with less unevenness in brightness. In particular, each of the light sources  12  preferably has batwing light distribution characteristics. With such a light distribution, the amount of light to be emitted directly above the light source  12  can be reduced, and the light distribution of each of the light sources  12  can be spread, and the wall portion  13   b  and bottom portions  13   c  can be irradiated with the spread light, so that unevenness in brightness in each of the sections C surrounded by corresponding portions of the wall portion  13   b  can be reduced. 
     As used herein, the batwing light distribution characteristics are defined as characteristics having a light emission intensity distribution in which the light emission intensity at light distribution angles with an absolute value greater than 0° is greater than that at 0°, where an angle of an optical axis OA is 0°. The optical axis OA is defined by a line passing through the center of the light source  12  and perpendicularly intersecting the upper surface  11   m  of the substrate  11 , as illustrated in  FIG.  6   . 
     In particular, as illustrated in  FIG.  6   , an example of the light source  12  having the batwing light distribution characteristics includes the light emitting element  12   a  including a light reflecting film  12   c  on an upper surface thereof. With the light reflecting film  12   c  disposed on the upper surface of the light emitting element  12   a,  most of the upward light from the light emitting element  12   a  is reflected at the light reflecting film  12   c  to reduce the amount alight directly above the light emitting element  12   a,  so that the batwing light distribution characteristics can be obtained. The light reflecting film  12   c  can be disposed directly on the light emitting element  12   a,  so that it is not necessary to separately combine a special lens for the batwing light distribution, which allows for reducing a thickness of the light source  12 . 
     The light reflecting film  12   c  can be a metal film made of silver or copper, a resin containing white filler or the like, a combination thereof, or the like. Alternatively, the light reflecting film  12   c  can be an organic multilayer film (DBR film) and can have an incident angle dependence of reflectance for the emission wavelength of the light emitting element  12   a.  More specifically, the reflectance of the light reflecting film  12   c  is preferably set to be lower in the oblique incidence than in the vertical incidence. With such a reflectance, variation in the brightness immediately above the light emitting element  12   a  can be gradual, and it is possible to prevent the area immediately above the light emitting element  12   a  from being excessively dark, e.g., becoming a dark spot. 
     For the light source  12 , for example, the light emitting element  12   a  mounted directly on the substrate  11  has a height in a range of 100 μm to 500 μm. The light reflecting film  12   c  can have a thickness in a range of 0.1 μm to 3.0 μm. Even when the sealing member  12   b  is included, the light source  12  can have a thickness approximately in a range of 0.5 mm to 2.0 mm. 
     The plurality of light sources  12  are preferably wired on the substrate  11  such that the plurality of light sources  12  can be driven separately from each other and dimming control (e.g., local dimming or high dynamic range) can be performed for each light source  12 . 
     Light Emitting Element  12   a    
     A known light emitting element can be used for the light emitting element  12   a.  For example, a light emitting diode is preferably used for the light emitting element  12   a.  A light emitting element having any appropriate wavelength can be selected for the light emitting element  12   a.  For example, for a blue or green light emitting element, a light emitting element using a nitride-based semiconductor such as GaN, InGaN, AlGaN, and AlInGaN can be used. For a red light emitting element, GaAlAs, AlInGaP, and the like can be used. A semiconductor light emitting element made of a material other than these can also be used. The composition, light emission color, size, number, and the like of the light emitting elements to be used can be appropriately selected according to the purpose. 
     As illustrated in  FIG.  6   , the light emitting element  12   a  can be flip-chip mounted straddling a pair of positive and negative conductor wirings  18 A and  18 B on the upper surface  11   m  of the substrate  11  using bonding members  19  disposed between the light emitting element  12   a  and the substrate  11 . Alternatively, the light emitting element  12   a  can be face-up mounted instead of being flip-chip mounted. 
     The bonding member  19  is a member for securing the light emitting element  12   a  to the substrate or the conductor wirings, and examples thereof include an insulating resin and a conductive member. In a case of flip-chip mounting as illustrated in  FIG.  6   , a conductive member is used. Specific examples thereof include Au-containing alloys, Ag-containing Pd-containing alloys, In-containing alloys, Pb-Pd-containing alloy, Au—Ga-containing alloys, Au—Sn-containing alloys, Sn-containing alloys, Sn—Cu-containing alloys, Sn-Cu-Ag-containing alloys, Au—Ge-containing alloys, Au—Si-containing alloys, Al-containing alloys, Cu—In containing alloys, and mixtures of metals and fluxes. 
     Sealing Member  12   b    
     The sealing member  12   b  covers the light emitting element  12   a  for the purpose of protecting the light emitting element  12   a  from the external environment, optically controlling the light to be emitted from the light emitting element  12   a  (for example, obtaining the batwing light distribution characteristics), and the like. For the sealing member  12   b,  a light-transmissive material is used. Examples of a material of the sealing member  12   b  include a light-transmissive resin such as an epoxy resin, a silicone resin, or a resin obtained by mixing them, glass, and the like, Of these, the silicone resin is preferably used in consideration of light resistance and ease of molding. The sealing member  12   b  can contain a diffusing agent for diffusing light from the light emitting element  12   a,  a coloring agent corresponding to the light emitting color of the light emitting element  12   a,  and the like. For the diffusing agent, the coloring agent, and the like, those known in the art can be employed. 
     The sealing member  12   b  can be in direct contact with the substrate  11 . The sealing member  12   b  is adjusted to have a viscosity that allows printing, coating with a dispenser, and the like, and can be cured by heat treatment or light irradiation. Examples of the shape of the sealing member  12   b  include a substantially hemispherical shape, a vertically elongated protruding shape in a cross-sectional view, a flat protruding shape in a cross-sectional view, and a circular shape or an elliptical shape in a plan view. As used herein, the term “vertically long protruding shape” refers to a shape, in a cross-sectional view, in which the maximum length in the direction perpendicular to the upper surface  11   m  of the substrate  11  is greater than the maximum length in the direction parallel to the upper surface urn of the substrate  11 . Further, the term “flat protruding shape” as used herein refers to a shape, in a cross-sectional view, in which the maximum length in the direction parallel to the upper surface  11   m  of the substrate  11  is greater than the maximum length in the direction perpendicular to the upper surface  11   m  of the substrate  11 . The sealing member  12   b  can be disposed to serve as an underfill  12   d  between the lower surface of the light emitting element  12   a  and the upper surface  11   m  of the substrate  11 . 
     Partition Member  13   
     The wall portion  13   b  of the partition member  13  can be in a grid pattern in a plan view. In a plan view, the boundary between adjacent sections C can be regarded as the top portion  13   a.  The partition member  13  preferably includes the bottom portions  13   c  connected to the lower end of the wall portion  13   b  in respective sections C. In other words, in the partition member  13 , it is preferable that each section C is constituted by a corresponding one of the bottom portions  13   c  and corresponding portions of the wall portion  13   b.  Peripheral ones of the bottom portions  13   c  can extend to the peripheral region of the substrate  11  in a plan view. In this case, the peripheral bottom portions  13   c  can be located closer to the outer edge of the substrate  11  than peripheral portions of the wall portion  13   b.  The peripheral region of the partition member  13  can have a portion that overlaps with the peripheral region of the substrate  11 . The partition member  13  is preferably a reflective member. 
     The partition member  13  includes, for example, a through hole  13   d  in which the light source  12  is disposed at substantially the center of the bottom portion  13   c  in the section C. As illustrated in  FIG.  6   , the light source  12  is preferably disposed in the through hole  13   d.  The through hole  13   d  can have any appropriate shape and size that allows the entire light source  12  to be exposed, and the outer edge of the through hole  13   d  is preferably set to be located only in the vicinity of the light source  12 . With this structure, when the partition member  13  has light-reflectivity, the light from the light source  12  can be reflected also at the bottom portion  13   c,  so that the light extraction efficiency can be improved. 
     The top portion  13   a  is a portion of the wall portion  13   b  located at the greatest height. The top portion  13   a  can be a flat surface, but the vicinity of the top portion  13   a  preferably has a ridge shape. That is, the vertical cross section of a portion of the wall portion  13   b  constituting the top portion  13   a  preferably forms an acute-angled triangle, and more preferably an acute-angled isosceles triangle. 
     An acute angle of the acute-angled triangle or the acute-angled isosceles triangle that is an angle of the wall portion  13   b  at the top portion  13   a  (α in  FIG.  6   ) is preferably in a range of 60° to 90°, for example. With such a range, the space and area occupied by the partition member  13  can be reduced, and the height of the partition member  13  can be reduced, so that size and thickness of the planar light source  10  can be reduced. 
     A pitch P, which is an interval between the opposite portions of top portion  13   a  of the partition member  13 , can be appropriately adjusted according to the size of the light source to be used, the intended size and performance of the planar light source, and the like. The pitch P can be, for example, in a range of 1 mm to 50 mm, preferably in a range of 5 mm to 20 mm, and more preferably in a range of 6 mm to 15 mm. In each section C, surfaces of the wall portion  13   b  on the section C side inclined spreading upwardly from the bottom portion  13   c  and the vicinity of the upper surface  11   m  of the substrate  11  can constitute corresponding portions of the wall portion  13   b  that surround a corresponding light source  12 . 
     In addition, the partition member  13  preferably has a height, that is, a length between the lower surface of the bottom portion  13   c  and the top portion  13   a  of 8 mm or less, and when a thinner planar light source is configured, preferably in a range of approximately 1 mm to 4 mm. Further, the distance between the lower surface of the bottom portion  13   c  of the partition member  13  and the light diffusion plate  14  is preferably approximately 8 mm or less, and when a thinner planar light source is configured, the distance is preferably in a range of approximately 2 mm to 4 mm. Accordingly, a thickness of the backlight unit including the optical member such as the light diffusion plate  14  can be greatly reduced. The partition member  13  can have a thickness, for example, in a range of 100 μm to 300 μm. 
     The shape of each section C formed by corresponding portions of the partition member  13  surrounding the light source  12 , that is, the shape of each of regions demarcated by the wall portion  13   b  can be a quadrangular shape in a plan view, or any other appropriate shapes. For example, the shape of the section C can be circular, elliptical, or the like. In order to efficiently dispose the plurality of light sources  12 , a polygonal shape such as a triangular shape, a quadrangular shape, or a hexagonal shape is preferable. This allows for facilitating demarcating the light emitting area into any number of regions by the wall portion  13   b  according to the area dimension of the light emitting surface of the planar light source  10 , and the light emitting area can be disposed at a high density. 
     The number of sections C demarcated by the wall portion  13   b  can be set to any appropriate number, and the shape and arrangement of the wall portion  13   b,  the number of sections C, and the like can be changed according to the desired size of the planar light source. According to the number and positions of the light sources  12  disposed on the substrate  11  in a plan view, the partition member  13  can have various shapes, for example, the shapes in which three sections C are adjacent to each other and three end portions of the top portion meet at one point, four sections C are adjacent to each other and four end portions of the top portion meet as illustrated in  FIG.  2   , and six sections C are adjacent to each other and six end portions of the top portion meat at one point. When the four sections C are adjacent to each other and the four end portions of the top portion meet, the section C has a quadrangular shape in a plan view is. 
     The partition member  13  is preferably disposed on the substrate  11 , and the lower surface of the bottom portion  13   c  of the partition member  13  and the upper surface  11   m  of the substrate  11  are preferably secured to each other. In particular, these are preferably secured at the periphery of the through hole  13   d  using a light-reflective adhesive member so that the light emitted from the light source  12  does not enter a space between the substrate  11  and the partition member  13 . For example, the light-reflective adhesive member is more preferably disposed in a ring shape along the outer periphery of the through hole  13   d.  The adhesive member can be a double-sided tape, a hot melt adhesive sheet, or a resin-based adhesive such as a thermosetting resin, a thermoplastic resin, or the like. These adhesive members preferably have high flame resistance. However, the partition member  13  can be secured on the substrate  11  by screwing or the like. 
     As described above, the partition member  13  preferably has the light reflectivity. This allows light emitted from the light source  12  to be efficiently reflected at the wall portion  13   b  and the bottom portion  13   c.  In particular, when the wall portion  13   b  has the inclination as described above and is irradiated with the light emitted from the light source  12 , the light can be reflected upward. Thus, when the light is not emitted in an adjacent section C, the contrast ratio can be further improved. Further, the light can be reflected upward more efficiently. 
     The partition member  13  can be obtained by molding a resin or the like containing a reflective material composed of metal oxide particles made of titanium oxide, aluminum oxide, silicon oxide or the like, or by molding a resin containing no reflective material and then disposing a reflective member on a surface of the molded resin. Alternatively, a resin containing a plurality of fine bubbles can be used for partition member  13 . In this case, light is reflected at the interface of the bubbles. Further, examples of the resin to be used for the partition member  13  include a thermoplastic resin such as an acrylic resin, a polycarbonate resin, a cyclic polyolefin resin, polyethylene terephthalate (PET), or polyester, and a thermosetting resin such as epoxy or silicone. The partition member  13  is preferably set to have a reflectance of 70% or greater to the light emitted from the light source  12 . 
     The partition member  13  can be formed by using a molding technique using a mold, a molding technique using stereolithography, or the like. For the molding technique using the mold, molding techniques such as injection molding, extrusion molding, compression molding, vacuum forming, pressure forming, and press forming can be employed. For example, the partition member  13  in which the bottom portions  13   c  and the wall portion  13   b  are integrally formed can be formed by vacuum forming using a reflective sheet formed of PET or the like. 
     Light Diffusion Plate  14   
     The light diffusion plate  14  is a member having an irregular shape that diffuses and transmits incident light, and one light diffusion plate  14  can be disposed above the plurality of light sources  12 . The light diffusion plate  14  is preferably a flat plate-shaped member, but may alternatively have protrusions and/or recesses in a surface thereof. The light diffusion plate  14  is preferably disposed substantially parallel to the substrate  11 . 
     When the pitch between the portions of the top portion  13   a  of the partition member  13  is P [mm], the light diffusion plate  14  is preferably disposed so that a distance OD between the light diffusion plate  14  and the light source  12  is, for example, 0.3P [mm] or less, and more preferably 0.25P [mm] or less. As used herein, as illustrated in  FIG.  6   , the distance OD refers to a distance between the outermost surface of the substrate  11  (that is, when the substrate  11  has a coating layer, wiring layer, or the like at a surface thereof, the outermost surface of the coating layer, wiring layer, or the like) and the lower surface of the light diffusion plate  14 . From another perspective, the light diffusion plate  14  is preferably disposed such that a distance H between the light diffusion plate  14  and the upper surface of the bottom portion  13   c  of the partition member  13 , illustrated in  FIG.  6   , is in a range of 1.5 mm to 5 mm, more preferably in a range of 2 mm to 3 mm. 
     The light diffusion plate  14  can be made of a material having low light absorption for visible light, such as a polycarbonate resin, a polystyrene resin, an acrylic resin, or a polyethylene resin. To diffuse the incident light, a surface of the light diffusion plate  14  can have protrusions and recesses, or a material having a refractive index different from that of a base material of the light diffusion plate  14  can be dispersed in the light diffusion plate  14 . The protrusions and recesses can be made, for example, in a size of 0.01 mm to 0.1 mm. For the material having the different refractive index, for example, a polycarbonate resin, an acrylic resin, or the like can be selected to be used. 
     A thickness of the light diffusion plate  14  and the degree of light diffusion of the light diffusion plate  14  can be appropriately set, and commercially available members such as a light diffusion sheet or a diffuser film can be used for the light diffusion plate  14 . For example, the thickest portion of the light diffusion plate  14  can have a thickness in a range of 1 mm to 2 mm. 
     In the planar light source  10 , with the substrate  11  having an irregular shape, when as many light sources  12  as possible are disposed on the substrate  11  while maintaining the matrix arrangement in the first direction and the second direction, a region on which no light source  12  is disposed may be generated on the substrate  11  near the outer periphery of the substrate  11 . Then, if no measures are taken, the peripheral region of the planar light source  10  (for example, the area between the outermost periphery of the partition member  13  and the outer edge of the light diffusion plate  14 ) would become a dark portion, and the peripheral region of the planar light source may have unevenness in brightness. In  FIG.  1   , for example, the dark portions would be noticeable in regions surrounded by the long dashed double-short dashed lines F. That is, if no measures are taken, unevenness in brightness will occur in a part of the peripheral region of the planar light source  10 . In view of this, in the planar light source  10 , a part of the peripheral region of the light diffusion plate  14  is thinner than the central side of the light diffusion plate  14  to reduce the occurrence of unevenness in brightness. When necessary, the entire peripheral region of the light diffusion plate  14  can be thinner than the central side of the light diffusion plate  14 . Hereinafter, the shape of the light diffusion plate  14  will be described in detail. 
     The light diffusion plate  14  includes a thick plate portion  14   a  and a thin plate portion  14   b  having a thinner thickness than the thick plate portion  14   a,  and the thick plate portion  14   a  and the thin plate portion  14   b  are disposed adjacent to each other and integrally formed as a single body. In  FIG.  5   , for convenience, the thick plate portion  14   a  is illustrated in white and the thin plate portion  14   b  is illustrated in a dot pattern. The light diffusion plate  14  can be constituted by a single piece, or can be constituted by two or more layers. When the light diffusion plate  14  is constituted by two layers, for example, a second layer having a width narrower than that of the first layer can be provided on the first layer that is located closer to the substrate. In this structure, a region where the second layer is disposed on the first layer can serve as the thick plate portion  14   a,  and a region where the second layer is not disposed on the first layer can serve as the thin plate portion  14   b.    
     A boundary  14   c  between the thick plate portion  14   a  and the thin plate portion  14   b  is located, for example, at a position facing peripheral portions of the wall portion  13   b  of the partition member  13 . With this structure, on an outer peripheral side with respect to the peripheral portions of the wall portion  13   b  of the partition member  13 , the frequency of light diffusion can be reduced and the amount of light transmitted through the thin plate portion  14   b  can be increased. This can result in a reduction in the occurrence of unevenness in brightness at the peripheral region of the planar light source  10 . The boundary  14   c  can be located at a position facing the peripheral portions of the top portion  13   a  of the partition member  13 . 
     When the wail portion  13   b  is located at a position directly below the boundary between the thick plate portion  14   a  and the thin plate portion  14   b  of the light diffusion plate  14  ( FIG.  3   ) or its vicinity, a light emitted from the light source  12  toward the thin plate portion  14   b  of the light diffusion plate  14  is diffused in the thick plate portion  14   a  of the light diffusion plate  14 , and then a portion of the diffused light is incident on the thin plate portion  14   b.  With a smaller thickness of the thin plate portion  14   b,  the light incident on the thin plate portion  14   b  is diffused at less frequency. Accordingly, the light extraction in the thin plate portion  14   b  is improved. 
     The thin plate portion  14   b  preferably has a thickness 0.5 times or less the thickness of the thick plate portion  14   a.  With such a thickness, the amount of light extracted from the area of the thin plate portion  14   b  increases, so that the occurrence of unevenness in brightness at the peripheral region of the planar light source  10  can be reduced. 
     A lower surface  14   n  of the thick plate portion  14   a  (a surface on the light source  12  side) and a lower surface  14   t  of the thin plate portion  14   b  (a surface on the light source  12  side) are in the same plane, and an upper surface  14   m  of the thick plate portion  14   a  (a surface opposite to the surface on the light source  12  side) and an upper surface  14   s  of the thin plate portion  14   b  (a surface opposite to the surface on the light source  12  side) are located at positions different in the height direction. 
     That is, the height from the upper surface  11   m  of the substrate  11  to the lower surface  14   t  of the thin plate portion  14   b  is the same as the height from the upper surface  11   m  of the substrate  11  to the lower surface  14   n  of the thick plate portion  14   a.  On the other hand, the height from the upper surface  11   m  of the substrate  11  to the upper surface  14   s  of the thin plate portion  14   b  is smaller than the height from the upper surface  11   m  of the substrate  11  to the upper surface  14   m  of the thick plate portion  14   a.    
     The thin plate portion  14   b  is located in at least a part of the peripheral region of the light diffusion plate  14  in a plan view. As illustrated in  FIG.  5   , in the present embodiment, as an example, the thin plate portion  14   b  is provided over the entire peripheral region of the light diffusion plate  14  (the portion indicated by the dot pattern). With the thin plate portion  14   b  is provided over the entire peripheral region of the light diffusion plate  14 , the brightness of the entire peripheral region of the light diffusion plate  14  can be increased, so that the unevenness in brightness can be reduced. Alternatively, a structure in which the thin plate portion  14   b  is located only in a part of the peripheral region of the light diffusion plate  14  may be employed. 
     In the planar light source  10 , in a plan view, the greater the distance between the optical axis of a peripheral one of the light sources  12  in the X direction and a corresponding portion on the outer edge of the light diffusion plate  14  in the X direction, the greater the width of a corresponding portion of the thin plate portion  14   b  in the X direction. In  FIG.  7   , the structures under the light diffusion plate  14  (i.e., the substrate  11 , the light sources  12 , and the partition member  13 ) are illustrated with solid lines in order to better show these structures. For example, as illustrated in  FIG.  7   , the plurality of light sources  12  include one first distal-end light source  12   1 , which is an outermost one of the light sources in one of the first arrays of light sources  12  extending along the X direction, and another first distal-end light source  12   2 , which is an outermost one of the light sources in another of the first arrays of light sources  12  extending along the X direction, and a distance L 1  between an optical axis OA 1  of the one first distal-end light source  12   1  and the outer edge of the light diffusion plate  14  in the X direction is greater than a distance L 2  between an optical axis OA 2  of the another first distal-end light source  12   2  and the outer edge of the light diffusion plate  14  in the X direction. In this case, as illustrated in  FIG.  8   , in a plan view, a width W 1  of the thin plate portion  14   b  as measured along a straight line extending along the X direction from the optical axis OA 1  of the one first distal-end light source  12   1  toward the outer edge of the light diffusion plate  14  is greater than a width W 2  of the thin plate portion  14   b  as measured along a straight line extending along the X direction from the optical axis OA 2  of the another first distal-end light source  12   2  toward the outer edge of the light diffusion plate  14 . 
     Further, in a plan view, the greater the distance between the optical axis of a distal-end light source  12  in the Y direction and a corresponding portion on the outer edge of the light diffusion plate in the Y direction, the greater the width of a corresponding portion of the thin plate portion  14   b  in the Y direction. For example, as illustrated in  FIG.  7   , the plurality of light sources  12  include one second distal-end light source  12   3 , which is an outermost one of the light sources in one of the second arrays of light sources  12  extending along the Y direction, and another second distal-end light source  12   4 , which is an outermost one of the light sources in another of the second arrays of light sources  12  extending along the Y direction, and a distance L 3  between an optical axis OA 3  of the one second distal-end light source  12   3  and the outer edge of the light diffusion plate  14  in the Y direction is greater than a distance L 4  between an optical axis OA 4  of the another second distal-end light source  12   4  and the outer edge of the light diffusion plate  14  in the Y direction. In this case, as illustrated in  FIG.  8   , in a plan view, a width W 3  of the thin plate portion  14   b  as measured along a straight line extending along the Y direction from the optical axis OA 3  of the one second distal-end light source  12   3  toward the outer edge of the light diffusion plate  14  is greater than a width W 4  of the thin plate portion  14   b  as measured along a straight line extending along the Y direction from the optical axis OA 4  of the another second distal-end light source  12   4  toward the outer edge of the light diffusion plate  14 . 
       FIGS.  9  and  10    are diagrams showing the results of simulations for the light diffusion plates.  FIG.  9    shows the simulation result of the planar light source  10 , and  FIG.  10    shows the simulation result of a planar light source  10 X provided with a light diffusion plate  14 X having a constant thickness instead of the light diffusion plate  14  of the planar light source  10  (comparative example). In  FIGS.  9  and  10   , a large number of thin lines indicate light rays. 
     Comparing the portions surrounded by the broken lines in  FIGS.  9  and  10   , it can be confirmed that the planar light source  10  provided with the thin plate portion  14   b  has greater density of light rays, and allows better light extraction at the peripheral region of the light diffusion plate  14  than the planar light source  10 X not provided with the thin plate portion. That is, it can be confirmed that the unevenness in the brightness of the light emitting surface at the peripheral region of the light diffusion plate  14  can be reduced. This is because providing the thin plate portion  14   b  to the light diffusion plate  14  allows for reducing the frequency of light diffusion at a location of the thin plate portion  14   b,  which allows for increasing the amount of light transmitted through the location of the thin plate portion  14   b.  This is also because light emitted from a lateral surface of the thick plate portion  14   a  exposed toward the thin plate portion  14   b  and light passed through the thin plate portion  14   b  and reflected on a lateral surface of the thick plate portion  14   a  propagate toward the upper surface side of the light diffusion plate, which allows for increasing the density of the light on the upper surface side of the light diffusion plate. 
     For the light diffusion plate  14 , a simulation was performed with a thickness of the thick plate portion  14   a  set to 1.2 mm and a thickness of the thin plate portion  14   b  varied to 0.4 mm, 0.2 mm, and 0.1 mm, and then the brightness of the light transmitted through the thin plate portion  14   b  in a region of the thin plate portion  14   b  was calculated. As shown in Table 1, in a case in which the thin plate portion  14   b  was 0.4 mm, the brightness was increased by 1.1 times as compared with a case in which the thin plate portion  14   b  was 1.2 mm (that is, when the thin plate portion  14   b  had the same thickness as the thick plate portion  14   a ). Similarly, in a case in which the thin plate portion  14   b  was 0.2 mm, the brightness was increased by 1.12 times. Further, in a case in which the thin plate portion  14   b  was 0.1 mm, the brightness was increased by 1.17 times. 
                         TABLE 1                   BRIGHTNESS RATIO OF LIGHT           TRANSMITTED THROUGH THIN       THICKNESS OF THIN PLATE   PLATE PORTION 14b IN AREA OF       PORTION 14b (mm)   THIN PLATE PORTION 14b                                        1.2   1.00       0.4   1.10       0.2   1.12       0.1   1.17                    
Furthermore, when comparing the light leaking to the outside through a lateral surface of the light diffusion plate, it can be confirmed that the planar light source  10  provided with the thin plate portion  14   b  is less likely to leak the light compared to a case in which the planar light source  10 X not provided with the thin plate portion is used. That is, in the planar light source  10 , the light that would laterally leak in a conventional technique can be propagated to the upper surface side of the light diffusion plate  14 . Accordingly, along with a reduction in the frequency of light diffusion in the thin plate portion  14   b,  the density of the light rays on the upper surface side of the thin plate portion  14   b  can be increased.
 
     Further, in the planar light source  10 , with the thin plate portion  14   b  disposed on the light diffusion plate  14 , the amount of light leaking from the lateral surface of the light diffusion plate  14  can be reduced. 
     In the planar light source  10 , a wavelength conversion sheet adapted to convert light from the light source  12  into light having a different wavelength can be disposed above the light diffusion plate  14 . In the case in which the wavelength conversion sheet is disposed above the light diffusion plate  14 , when light leaks from the lateral surface of the light diffusion plate, an end portion of the planar light source may be seen in the emission color (for example, blue) of the light emitting element  12   a.  However, in the planar light source  10 , the light diffusion plate  14  includes the thin plate portion  14   b,  which allows for reducing the amount of light leaking from the lateral surface of the light diffusion plate  14 , so that the phenomenon that the end portion of the planar light source  10  is seen in the emission color of the light emitting element  12   a  can be reduced. That is, when disposing the wavelength conversion sheet above the light diffusion plate  14 , the phenomenon that light having a wavelength different from a wavelength resulting from conversion by the wavelength conversion sheet leaks to the outside through the lateral surface of the light diffusion plate  14  can be reduced. 
     As described above, in the planar light source  10 , with the substrate  11  having the irregular shape, when as many light sources  12  as possible are placed on the substrate  11  while maintaining the matrix arrangement in the first direction and the second direction, there is a region near the outer edge of the substrate  11  where the light source  12  is not disposed. Then, when no measures are taken, for example, the region between the outermost periphery of the partition member  13  and the outer edge of the light diffusion plate  14  becomes a dark portion. That is, when no measures are taken, unevenness in brightness will occur in a part of the peripheral region of the planar light source  10 . However, with the thin plate portion  14   b  located in the peripheral region of the light diffusion plate  14 , light extraction is prioritized over light diffusion in the thin plate portion  14   b.  Accordingly, the frequency of light diffusion can be reduced, and the amount of light transmitted through the thin plate portion  14   b  can be increased. As a result, the occurrence of unevenness in brightness at the peripheral region of the planar light source  10  can be reduced. 
     The thin plate portion  14   b  can be provided in a portion where unevenness in brightness may occur, and thus the thin plate portion  14   b  need not be provided over the entire peripheral region of the light diffusion plate  14 , and the thin plate portion  14   b  can be provided only in a part of the peripheral region of the light diffusion plate  14 . 
     In addition, for the similar reason, in both the X direction and the Y direction, it is not necessary that the greater a distance between an optical axis of a distal-end light source  12  and a corresponding portion on the outer edge of the light diffusion plate  14 , the greater the width of the thin plate portion  14   b.  That is, in at least one of the X direction and the Y direction, the greater the distance between the optical axis of a distal-end light source  12  and the outer edge of the light diffusion plate  14 , the greater a width of the thin plate portion  14   b    
     The planar light source  10  can include, above the light diffusion plate  14 , at least one type of sheet selected from the group consisting of a wavelength conversion sheet adapted to convert light from the light source  12  into light having a different wavelength, a prism sheet, and a polarizing sheet. More specifically, as illustrated in  FIG.  11   , optical members such as a wavelength conversion sheet  72 , prism sheets (a first prism sheet  73  and a second prism sheet  74 ), a polarizing sheet  75 , and the like can be disposed above the light diffusion plate  14  at a predetermined distance or directly or indirectly on the upper surface of the light diffusion plate  14 , and a liquid crystal panel can be further disposed on the optical members, so that a surface emitting light emitting device to be used as a light source for a direct backlight can be obtained. The order of layering these optical members can be set as desired. 
     Wavelength Conversion Sheet  72   
     While the wavelength conversion sheet  72  can be disposed on either the upper surface or the lower surface of the light diffusion plate  14 , as illustrated in  FIG.  11   , the wavelength conversion sheet  72  is preferably disposed on the upper surface of the light diffusion plate  14 . The wavelength conversion sheet  72  is adapted to absorb a portion of light emitted from the light source  12  and emit light having a wavelength different from the wavelength of the light emitted from the light source  12 . For example, the wavelength conversion sheet  72  is adapted to absorb a portion of blue light emitted from the light source  12  and emit yellow light, green light, and/or red light, so that the planar light source  10  adapted to emit white light can be obtained. The wavelength conversion sheet  72  is disposed spaced apart from the light emitting element  12   a  of the light source  12 , so that a phosphor or the like having poor resistance to heat or light intensity, which is difficult to use in the vicinity of the light emitting element  12   a,  can be used for the wavelength conversion sheet  72 . Consequently, the performance of the planar light source  10  as a backlight can be improved. The wavelength conversion sheet  72  has a sheet shape or a layered shape, and includes the phosphor or the like described above. The wavelength conversion sheet may be referred to as a wavelength conversion layer. 
     First Prism Sheet  73  and Second Prism Sheet  74   
     The first prism sheet  73  and the second prism sheet  74  have a shape in which a plurality of prisms extending in a predetermined direction are disposed in surfaces thereof. For example, when each of the first and second prism sheets  73  and  74  is viewed in two dimensions consisting of the X direction and the Y direction perpendicular to the X direction, the first prism sheet  73  can have a plurality of prisms extending along the Y direction, and the second prism sheet  74  can have a plurality of prisms extending along the X direction. The first prism sheet  73  and the second prism sheet  74  are configured to refract incident light, propagated in various directions, toward the display panel facing the planar light source  10 . Accordingly, light emitted from the light emitting surface of the planar light source  10  can be emitted mainly in a direction perpendicular to the upper surface of the light emitting surface, and the brightness when the planar light source  10  is viewed in a front view can be increased. 
     Polarizing Sheet  75   
     For example, the polarizing sheet  75  is configured to selectively transmit light in a polarization direction that matches the polarization direction of a polarizing plate disposed proximate to the backlight of a display panel such as a liquid crystal display panel, and to reflect polarized light in a direction perpendicular to the polarization direction toward the first prism sheet  73  and the second prism sheet  74 . A portion of the polarized light returned from the polarizing sheet  75  is reflected at the first prism sheet  73 , the second prism sheet  74 , the wavelength conversion sheet  72 , and the light diffusion plate  14 . At this time, the reflected light changes in the polarization direction, for example, is converted into polarized light having the polarization direction of the polarizing plate of the liquid crystal display panel, is again incident on the polarizing sheet  75 , and is emitted to the display panel. Accordingly, the polarization direction of the light emitted from the planar light source  10  can be aligned, and light in the polarization direction effective for enhancing the brightness of the display panel can be emitted with high efficiency. For the polarizing sheet  75 , the first prism sheet  73 , the second prism sheet  74 , and the like, commercially available optical members for backlights can be used. 
     In the planar light source  10 , instead of providing the wavelength conversion sheet  72 , the sealing member  12   b  can contain a wavelength conversion material such as phosphor adapted to absorb light emitted from the light emitting element  12   a  and to emit light having a wavelength different from that of the outputted light from the light emitting element  12   a.  With this structure, the sealing member  12   b  absorbs a portion of blue light emitted from the light source  12  and emits yellow light, green light, and/or red light, so that the planar light source  10  configured to emit white light can be obtained. 
     In addition to the wavelength conversion material, the sealing member  12   b  can contain a diffusing agent for diffusing light emitted from the light emitting element  12   a,  a coloring agent corresponding to the emission color of the light emitting element  12   a,  and the like. For the diffusing agent, the coloring agent, and the like, those known in the art can be employed. Further, instead of a structure in which the sealing member  12   b  contains the wavelength conversion material such as a phosphor, for example, a light emitting element  12   a  in which a nitride-based semiconductor is covered by a wavelength conversion material such as a phosphor, that is, the light emitting element  12   a  configured to emit white light can be used. 
     First Modified Example of First Embodiment 
     In the first modified example of the first embodiment, an example in which the cross-sectional shape of the thin plate portion of the light diffusion plate is different from that of the first embodiment will be illustrated. In description of the first modified example of the first embodiment, repetitive description of the same components as those of the embodiment described above may be omitted. 
       FIGS.  12  to  14    are schematic partially enlarged cross-sectional views illustrating different examples of light diffusion plates of the planar light source according to the first modified example of the first embodiment.  FIG.  15    is a schematic plan view illustrating the light diffusion plate of  FIG.  14   . 
     In a light diffusion plate  24  illustrated in  FIG.  12   , the height from the upper surface  11   m  of the substrate  11  to a lower surface  24   t  of a thin plate portion  24   b  is the same as the height from the upper surface  11   m  of the substrate  11  to a lower surface  24   n  of a thick plate portion  24   a.  In addition, the thickness of the thin plate portion  24   b  gradually decreases from a boundary  24   c  with the thick plate portion  24   a  toward the outer edge of the light diffusion plate  24 . 
     As described above, the boundary between the thick plate portion and the thin plate portion need not be stepped as illustrated in  FIG.  3   , and can have a shape in which the thickness varies smoothly as illustrated in  FIG.  12   . Also with this structure, in the thin plate portion  24   b,  light extraction can be prioritized over light diffusion. This allows for reducing the frequency of light diffusion, the amount of light transmitted through the thin plate portion  24   b  can be increased. Accordingly, the occurrence of unevenness in brightness at the peripheral region of the planar light source can be reduced. 
     Further, instead of the shape of the light diffusion plate  24  illustrated in  FIG.  12   , in which a thickness of the entire thin plate portion  24   b  is gradually decreased, the thin plate portion can have a shape having a portion in which the thickness gradually decreases from the boundary with the thick plate portion toward the outer edge of the light diffusion plate. For example, as in a light diffusion plate  34  illustrated in  FIG.  13   , a thin plate portion  34   b  can have a thickness gradually decreasing portion  34   b   1  in which the thickness gradually decreases from a boundary  34   c  with a thick plate portion  34   a  toward the outer edge of the light diffusion plate  34 , and can further have a thickness constant portion  34   b   2  located closer to the outer edge of the light diffusion plate  34  than the thickness gradually decreasing portion  34   b   1 . The thickness constant portion  34   b   2  has, for example, the same thickness as the thinnest portion of the thickness gradually decreasing portion  34   b   1 . 
     Also, as in a light diffusion plate  44  illustrated in  FIGS.  14  and  15   , a thin plate portion  44   b  can have a first thin plate portion  44   b   1  on a thick plate portion  44   a  side, and a second thin plate portion  44   b   2 , which is located closer to the outer edge of the light diffusion plate  44  than the first thin plate portion  44   b   1  and is thinner than the first thin plate portion  44   b   1 . With such a two-step variation in thickness of the thin plate portion  44   b,  the brightness at the end portion of the planar light source can be increased. That is, because the greater the distance from the light source  12 , the darker the portion would become, a structure in which the thickness of the light diffusion plate  14  is reduced in a direction in which the distance from the light source  12  increases allows for reduce the occurrence of unevenness in brightness at the peripheral region of the planar light source. The variation in the thickness of the thin plate portion  44   b  can be in more than two steps. Further, the thin plate portion  44   b  can have a cross-sectional shape in which the shapes illustrated in  FIGS.  12  to  14    are appropriately combined. 
     The second thin plate portion  44   b   2  is disposed, for example, closer to the outer edge of the light diffusion plate  44  than the area where the through hole  13   d  of the partition member  13  is formed in a plan view. The second thin plate portion  44   b   2  can be disposed in the section C where the light source  12  is not disposed. The boundary  44   c  between the first thin plate portion  44   b   1  and the second thin plate portion  44   b   2  can be located at a position overlapping the top portion  13   a  of the partition member  13  in a plan view. 
     It is not always necessary to dispose both the first thin plate portion  44   b   1  and the second thin plate portion  44   b   2  over the entire peripheral region of the light diffusion plate  44 , and a structure may be employed in which only the first thin plate portion  44   b   1  is disposed in a region where unevenness in brightness is not likely to occur. For example, in an area R in which the light sources  12  are linearly arranged in  FIG.  15   , unevenness in brightness is unlikely to occur, so that it is possible to dispose only the first thin plate portion  44   b   1  in the region R. Alternatively, when the unevenness in brightness hardly occurs in the region R in which the light sources  12  are linearly arranged in  FIG.  15   , the thin plate portion  44   b  need not be provided in the region R. 
     Second Modified Example of First Embodiment 
     In the second modified example of the first embodiment, an example is illustrated in which the partition member has sections having different sizes on its peripheral region. In description of the second modified example of the first embodiment, repetitive description of the same components as those of the above-described embodiment may be omitted. 
       FIG.  16    is a schematic partially enlarged plan view of a partition member according to the second modified example of the first embodiment,  FIG.  17    is a cross-sectional view taken along line B-B of  FIG.  16   . In a partition member  23  illustrated in  FIGS.  16  and  17   , portions of a top portion  23   a  and portions of a wall portion  23   b,  which constitute peripheral sections C of the plurality of sections C, are disposed in the vicinity of the outer edge of the substrate  11 . That is, in at least one of the peripheral sections C, in a plan view, the area of the area surrounded by the wall portion  23   b  is larger than that of a section C inward of the peripheral sections C. While a region in a peripheral section C surrounded by a corresponding portion of the wall portion  23   b  the example of  FIGS.  16  and  17    has an area dimension expanded in the X direction, such a region can have an area dimension expanded in the Y direction. Alternatively, the peripheral sections C may include both a peripheral section C in which a region surrounded by a corresponding portion of the wall portion  23   b  has an area dimension expanded in the X direction and a peripheral section C in which the area dimension of the region surrounded by a corresponding portion of the wall portion  23   b  of the peripheral section C is expanded in the Y direction. 
     As described above, while the area dimension of the bottom portions  23   c  each exposed in a respective section C are equal among all the sections C in a plan view in the example of  FIGS.  2  and  3   , other arrangement may alternatively be employed. As in the example of  FIGS.  16  and  17   , in a plan view, at least one of the regions surrounded by peripheral portions of the wall portion  23   b  located at the periphery of the partition member  23  can have an area dimension larger than that of the area dimension surrounded by a portion of the wall portion  23   b  located inward of the periphery of the partition member  23 . 
     In the structure of the partition member  23  illustrated in  FIGS.  16  and  17   , portions of the wall portion  23   b  constituting a peripheral section C is located in the vicinity of the outer edge of the substrate  11 , which allows for improving light extraction toward a light emitting surface. That is, in the structure of the partition member  23  illustrated in  FIGS.  16  and  17   , the wall portion  23   b  is also located on the peripheral region of the substrate  11 , so that a greater amount of light can be transmitted to the thin plate portion  14   b  of the light diffusion plate  14 . This can result in a reduction in the occurrence of unevenness in brightness at the peripheral region of the planar light source  10 . However, the wall portion  23   b  needs not be provided on at least a part of the periphery of the partition member  23 . While the wall portion  23   b  of the partition member is not disposed at the boundary between the thick plate portion  14   a  and the thin plate portion  14   b  of the light diffusion plate  14  in the example in  FIGS.  16    and  17 , the wall portion  23   b  can be provided at this position. 
     Third Modified Example of First Embodiment 
     In the third modified example of the first embodiment, modified examples of the members other than the light diffusion plate and the partition member are illustrated. In the description of the third modified example of the first embodiment, repetitive description of the same components as those of the above-described embodiment may be omitted. 
       FIG.  18 A  is a schematic partially enlarged cross-sectional view in the vicinity of the outer edge of the planar light source. As illustrated in  FIG.  18 A , the planar light source can have a frame body  26  surrounding the substrate  11  and the light diffusion plate  14 . The frame body  26  has an irregular shape such as a shape similar to that of the substrate  11 , and includes a bottom portion  26   a  that is a little larger than the substrate  11  in a plan view and a lateral wall  26   b.    
     A peripheral region of the bottom portion  26   a  is annularly exposed to the outside of the substrate  11 , and the lateral wall  26   b  is located in the exposed portion of the bottom portion  26   a  to surround the substrate  11 . A cover  27  surrounding the outer edges of the substrate  11  and the light diffusion plate  14  can be disposed on the side opposite to the bottom portion  26   a  with respect to the lateral wall  26   b.  The cover  27  is disposed at a position that does not block the light emitted from each of the light sources  12 . The frame body  26  and the cover  27  can be formed of various materials such as resins containing a reflective material, metals, and ceramics. 
     As illustrated in  FIG.  18 B , a wavelength conversion member  28  containing a phosphor can be disposed in a region between the substrate  11  and the light diffusion plate  14  on inner lateral surfaces of the lateral wall  26   b.  With this structure, a wavelength of a portion of light emitted from the light source  12  is converted by the wavelength conversion member  28  disposed on the inner lateral surfaces of the lateral wall  26   b,  and the wavelength converted light is extracted. Therefore, the phenomenon that the end portion of the planar light source  10  is seen in an emission color of the light emitting element can be reduced. When the wavelength conversion member  28  is disposed on the inner lateral surfaces of the lateral wall  26   b,  in the cross-sectional view of the planar light source, the wall portion  13   b  of the partition member  13  may or may not be disposed between the light source  12  and the lateral wall  26   b.  The wavelength conversion member  28  can be disposed on the entire inner lateral surfaces of the lateral wall  26   b  or can be disposed on the inner lateral surfaces of the lateral wall  26   b  in a region below the lower surface of the light diffusion plate  14 . When the wavelength conversion member  28  is disposed in the region below the lower surface of the light diffusion plate  14 , the thin plate portion  14   b  of the light diffusion plate  14  may or may not cover the upper part of the wavelength conversion member  28 . For the wavelength conversion member  28 , a material that emits yellow light (for example, YAG) can be used. One wavelength conversion member  28  or a plurality of wavelength conversion members  28  can be disposed on the inner lateral surfaces of the lateral wall  26   b.    
     With the frame body  26  and the cover  27  are provided in the planar light source in this arrangement, the substrate  11  and the light diffusion plate  14  can be protected from external impact or the like. The light diffusion plate  14 , the substrate  11 , and the frame body  26  can have similar irregular shapes. 
       FIG.  19    is a schematic plan view explaining an outer shape of a substrate of the planar light source according to the third modified example of the first embodiment, and illustrates only the substrate, the light sources, and the partition member. The shape of the light diffusion plate can be, for example, the same shape as that of  FIG.  5   . A substrate  21  of a planar light source  20  illustrated in  FIG.  19    has an irregular shape without a region where the light source  12  is not disposed as compared with the substrate  11  illustrated in  FIG.  1   . That is, the substrate  21  has an outermost shape corresponding to the outermost shape of the partition member  13 . In the example of  FIG.  19   , the substrate  21  is located at a position overlapping with the partition member  13  (the lower side of the partition member  13 ) in a plan view. 
     As described above, the substrate  21  to be used for the planar light source can have a shape without a region where the light source  12  is not disposed. In this case as well, using the light diffusion plate  14  having the same shape as that of  FIG.  5    allows for decreasing the frequency of light diffusion at the peripheral region of the light diffusion plate  14 , so that light to be transmitted through the light diffusion plate  14  can be increased. This can result in reduction in occurrence of unevenness in brightness at the peripheral region of the planar light source. 
     The cross-sectional shape of the planar light source  20  can be a linear shape parallel to the XY plane as illustrated in  FIG.  11    and the like, or can be a curved shape with respect to the XY plane. For example, the cross-sectional shape of the planar light source  20  can be a curved shape in which the light emitting surface is recessed in the X direction. 
     Second Embodiment 
     For a second embodiment, an example of a liquid crystal display device using the planar light source according to the first embodiment as a backlight source will be illustrated. In description of the second embodiment, repetitive description of the same components as those of the embodiment described above may be omitted. 
       FIG.  20    is a configuration diagram illustrating a liquid crystal display device according to the second embodiment. As illustrated in  FIG.  20   , a liquid crystal display device  1000  includes a liquid crystal panel  120 , an optical sheet  110 , and the planar light source  10  according to the first embodiment in this order from the upper side. In the planar light source  10 , a reference sign  70  denotes the optical members such as the light diffusion plate and the wavelength conversion sheet. In the example herein, the optical sheet  110  can be provided with a reflective polarizer film (DBEF), a brightness enhancement film (BEF), a color filter, or the like, in addition to or instead of some of the optical members. 
     The liquid crystal display device  1000  is a so-called direct-lit liquid crystal display device in which the planar light source  10  is layered below the liquid crystal panel  120 . In the liquid crystal display device  1000 , the liquid crystal panel  120  is irradiated with light emitted from the planar light source  10 . In addition to the above-described constituent members, a member such as a color filter can be further provided. 
     In general, in a direct liquid crystal display device, since the distance between the liquid crystal panel and the planar light source is short, the uneven color and the unevenness in the brightness of the planar light source can affect the uneven color and unevenness in the brightness of the liquid crystal display device. Therefore, as a planar light source for a direct liquid crystal display device, a planar light source having less uneven color and unevenness in brightness is desired. Using the planar light source  10  for the liquid crystal display device  1000  allows for reducing the unevenness in brightness to be generated on the peripheral region and reducing the overall unevenness in brightness and emission color while reducing the thickness of the planar light source  10  to 5 mm or less, 3 mm or less, 1 mm or less, or the like. 
     The number of the planar light source  10  used as the backlight for a single liquid crystal display device  1000  may be other than one, and a plurality of arranged planar light sources  10  can be used for the backlight for one liquid crystal display device  1000 . For example, producing a plurality of small planar light sources  10  and performing inspection or the like on each of them allows for improving the yield compared with a case of producing a single large planar light source  10  a large number of light sources  12  are mounted. 
     As described above, the planar light source  10  is configured to emit uniform light from the optical member  70 , and thus is preferable for use as a backlight for the liquid crystal display device  1000 . 
     The planar light source  10  can also be preferably used as a backlight devices for televisions, tablets, smartphones, smart watches, head-up displays, digital signage, bulletin boards, and the like. In addition, the planar light source  10  can also be used as a light source for lighting, and can also be used for emergency lights, linear lighting, various illuminations, vehicle instrument panels, and the like. One or more modifications illustrated in the first to third modified examples of the first embodiment can be applied to the planar light source  10 . 
     While certain embodiments and the like have been described in detail above, the present invention is not limited to the embodiments and the like described above, various modifications and substitutions can be made to the embodiments and the like described above without departing from the scope described in the claims. 
     For example, in the embodiment described above, the example in which the lower surface of the thick plate portion of the light diffusion plate (the surface proximate to the light source) and the lower surface of the thin plate portion of the light diffusion plate (surface proximate to the light source) are on the same plane. However, the upper surface of the thick plate portion of the light diffusion plate (a surface on a side opposite to a side where the light source is disposed) and the upper surface of the thin plate portion (a surface on the side opposite to the side where the light source is disposed) can be in the same plane. That is, the light diffusion plate can be thinned on its lower surface side to obtain the thin plate portion. For example, the light diffusion plate can have an inverted shape of the light diffusion plate  14  illustrated in  FIG.  6   , the light diffusion plate  24  illustrated in  FIG.  12   , the light diffusion plate  34  illustrated in  FIG.  13   , and the light diffusion plate  44  illustrated in  FIG.  14   . In these cases, while it cannot be expected that the light from a lateral surface of the thick plate portion toward the upper surface side of the light diffusion plate will increase, the frequency alight diffusion in the thin plate portion can be decreased and the amount of light transmitted through the thin plate portion can be increased, so that a certain effect of reducing unevenness in brightness of the light emitting surface at the peripheral region of the light diffusion plate can be obtained. 
     Also, the light diffusion plate can contain scattered particles such as titanium oxide particles or phosphor particles, and the concentration of scattered particles contained in the thin plate portion can be less than the concentration of scattered particles contained in the thick plate portion. Consequently, the frequency of light diffusion in the thin plate portion can be further reduced and light transmitted through the thin plate portion can be further increased, so that the effect of reducing unevenness in the brightness of the light emitting surface at the peripheral region of the light diffusion plate can be improved. Alternatively, in a case in which a scattered particle layer containing scattered particles is disposed on an upper surface and/or a lower surface of the light diffusion plate, and the concentration of scattered particles in the scattered particle layer disposed on the thin plate portion is less than the concentration of scattered particles in the scattered particle layer disposed on the thick plate portion, a similar effect can be obtained.