Patent Publication Number: US-2023151945-A1

Title: Light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/400,001, filed on Aug. 11, 2021, which is a continuation of U.S. patent application Ser. No. 16/696,494, filed on Nov. 26, 2019 (now U.S. Pat. No. 11,118,757), which is a continuation of U.S. patent application Ser. No. 15/705,057, filed Sep. 14, 2017 (now U.S. Pat. No. 10,527,257), which claims priority to Japanese Patent Application No. 2016-182079, filed on Sep. 16, 2016. The disclosures of these applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     The present disclosure relates to a light emitting device. 
     Direct-backlit light emitting devices using semiconductor light emitting elements have been proposed as backlights for use in display devices, such as liquid crystal display devices. For example, Japanese Unexamined Patent Application Publication Nos. 2012-174371 and 2012-212509 disclose light emitting devices including a plurality of light emitting diodes (LEDs) arranged in an array and a reflector combined with a half mirror whose reflectance is controlled in part. 
     SUMMARY 
     For direct-backlit type light emitting devices, a reduction of the non-uniform luminance of the light sources is desirable. Certain exemplary embodiments of the present disclosure can provide a light emitting device with reduced non-uniform luminance. 
     A light emitting device according to one embodiment includes: a mounting board; a plurality of light sources positioned on the mounting board; a light diffusion plate; a half mirror positioned between the light diffusion plate and the plurality of light sources of the mounting board; and a plurality of diffuse reflectors positioned between the mounting board and the light diffusion plate, and above at least part of each emission face of the plurality of light sources. 
     According certain embodiments of the present disclosure, a light emitting device with reduced non-uniform luminance can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of an example of the light emitting device according to an embodiment of the present disclosure. 
         FIG.  2    is a top view of a light source unit. 
         FIG.  3    is a chart showing an example of the luminous intensity distribution characteristics of the light emitted from the light source. 
         FIG.  4    is a chart showing the dependency of the reflectance and transmittance of the half mirror in the embodiment on the light distribution angle by way of example. 
         FIG.  5    is a chart showing the relationship between the reflectance spectrum of the half mirror and the emission wavelengths of the light emitting elements in the embodiment. 
         FIG.  6    is a top view showing the layout of the diffuse reflectors in the embodiment. 
         FIG.  7 A  is a top view showing another layout of the diffuse reflectors in the embodiment. 
         FIG.  7 B  is a top view showing a structure of the diffuse reflectors in the embodiment. 
         FIG.  7 C  is a cross-sectional view showing another structure of the diffuse reflectors in the embodiment. 
         FIG.  8    is a cross-sectional view of another example of the light emitting device according to an embodiment. 
         FIG.  9    is a cross-sectional view of another example of the light emitting device according to an embodiment. 
         FIG.  10    is a cross-sectional view of another example of the light emitting device according to an embodiment. 
         FIG.  11    is a cross-sectional view of another example of the light emitting device according to an embodiment. 
         FIG.  12    is a chart comparing the luminance distribution of an example and a reference sample. 
         FIG.  13    is a cross-sectional view of still another example of the light emitting device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to studies conducted by the present inventors, the reflectance of the half mirrors used in the light emitting devices disclosed in Japanese Unexamined Patent Application Publication Nos. 2012-174371 and 2012-212509 needs to be controlled at different locations depending on the display panel size and specifications. For this reason, such a half mirror must be prepared as an exclusive-use specialty member. This makes the half mirror very expensive, in particular one used in the light emitting device for a large screen LCD TV. 
     In the light emitting device disclosed in Japanese Unexamined Patent Application Publication No. 2012-212509, the half mirror is constructed by using aluminum for the reflective material. Because aluminum absorbs part of visible light, the light emitted from the light sources can be absorbed by the half mirror while repeatedly being reflected between the half mirror and the reflective plate. This can reduce the light extraction efficiency of the light emitting device. Because the reflectance of the half mirror differs depending on the location, a non-uniform luminance can be occurred along the boundaries of areas in which reflectances are different. In view of these problems, the present inventors arrived at the light emitting device having a novel structure. 
     Certain embodiments of the present disclosure will be explained in detail below with reference to the accompanying drawings. The embodiments described below are illustrations of the present disclosure, and the light emitting device according to the present disclosure is not limited to the embodiments discussed below. In the explanations below, terms indicating certain directions and positions will be used as needed (for example, “upper,” “lower,” “right,” “left,” and other terms including these). These terms are merely used for the sake of clarity of the relative directions and positions of the components in the drawings being referred to. As long as the relative directions and positions are the same as those indicated by the terms such as “upper” and “lower” in the referenced drawings, the components shown in the drawings other than the present disclosure or actual products do not need to have the same layouts as those shown in the drawings. The sizes of the constituent elements and their positional relationships shown in the drawings might be exaggerated for the sake of clarity, and might not reflect the sizes, or the magnitude relationship among the constituent elements in an actual surface emitting device. The terms “parallel” and “perpendicular”/“orthogonal” herein includes cases where the angle formed by two lines, sides, planes, or the like, respectively within the ranges of 0°±5° and 90°±5°, approximately. 
     Structure of Light Emitting Device  101   
       FIG.  1    is a cross-sectional view of the light emitting device  101  according to an embodiment. The light emitting device  101  is provided with a light source unit  10 , which includes a mounting board  11  and a plurality of light sources  20  disposed on the mounting board  11 , and a light transmissive multilayer stack  30 , which includes a half mirror  31 , a light diffusion plate  33 , and diffuse reflectors  32 . The light transmissive multilayer transmits the light from the light sources  20  of the light source unit  10 . Each constituent element will be explained in detail below. 
     Mounting Board  11   
     The mounting board  11  includes an upper face  11   a  and a lower face  11   b , and a plurality of light sources  20  are disposed on and supported by the upper face  11   a.  On the upper face  11   a  and the lower face  11   b  of the mounting board  11 , the conductor wiring layer  13  and the metal layer  12  described in detail below are disposed. On the upper face  11   a,  moreover, dividing members  15  are disposed, each surrounding individual light sources  20 . 
     For the material for the mounting board  11 , a ceramic or resin can be used, for example. A resin may be selected as the material for the mounting board  11  from the perspective of low cost and ease of molding. Examples of resins include phenol resins, epoxy resins, polyimide resins, BT resins, polyphthalamide (PPA), and polyethylene terephthalate (PET). The thickness of the mounting board can be suitably selected, and the mounting board  11  may be either a flexible mounting board, which can be manufactured by roll-to-roll processing, or a rigid mounting board. The rigid mounting board may be of a thin type which is bendable. 
     A ceramic may be selected as the material for the mounting board  11  from the perspective of good resistance to heat and light. Examples of ceramics include alumina, mullite, forsterite, glass-ceramics, nitrides (e.g., AIN), carbides (e.g., SiC), and LTCCs. 
     The mounting board  11  may alternatively be formed with a composite material. Specifically, inorganic fillers, such as glass fibers, SiO 2 , TiO 2 , Al 2 O 3 , or the like can be mixed into the resins described above. Examples include glass fiber reinforced resins (e.g., glass epoxy resin). This can increase the mechanical strength, reduce the thermal expansion coefficient, and increase the reflectance of the mounting board  11 . 
     It is sufficient to provide electrical insulation at least on the upper face  11   a  of the mounting board  11 . The mounting board  11  may have a stack structure. For example, a metal plate with an insulating layer disposed on the surface thereof may be used as the mounting board  11 . 
     Conductor Wiring Layer  13   
     The conductor wiring layer  13  is disposed on the upper face  11   a  of the mounting board  11 . The conductor wiring layer  13  includes a wiring pattern for supplying power to the plurality of light sources  20  from the outside. The material for the conductor wiring layer  13  can be suitably selected in accordance with the material used for the mounting board  11 , the manufacturing method, and the like. For example, if a ceramic is used for the mounting board  11 , the conductor wiring layer  13  is formed with, for example, a high melting point metal that can be sintered simultaneously with the ceramic. The conductor wiring layer  13  is formed with a high melting point metal, for example, tungsten, molybdenum, or the like. The conductor wiring layer  13  may have a multilayer structure. For example, the conductor wiring layer  13  may be equipped with a high melting point metal pattern formed by the method described above, and a metal layer which contains another metal such as nickel, gold, or silver, and is disposed on the pattern by plating, sputtering, or vapor deposition. 
     If a glass epoxy resin is used as the mounting board  11 , it is preferable to use a material for the conductor wiring layer  13  that can be readily processed. For example, a metal layer containing copper, nickel, or the like, formed by plating, sputtering, vapor deposition, or pressing can be used. The metal layer can be processed into a predetermined wiring pattern by printing or photolithography through masking and etching. 
     Metal Layer  12   
     The light source unit  10  may further include a metal layer  12  on the lower face  11   b  of the mounting board  11 . The metal layer  12  may be disposed across the entire lower face  11   b  for heat dissipation. The lower face of  11   b  of the mounting board  11  may alternatively have a wiring pattern. For example, the metal layer  12  may have a drive circuit pattern for driving the light sources  20 . The metal layer  12  may further include drive circuit components on the circuit pattern. 
     Light Source  20   
     The plurality of light sources  20  are arranged on the upper face  11   a  of the mounting board  11 .  FIG.  2    is a top view of the light source unit  10 . The plurality of light sources  20  are arranged one-dimensionally or two-dimensionally on the upper face  11   a  of the mounting board  11 . In the present embodiment, the plurality of light sources  20  are two-dimensionally arranged along two orthogonal directions, in other words, the x direction and the y direction where the pitch Px in the x direction and the pitch Py in the y direction, are the same. The arrangement directions, however, are not limited to these. The pitches in the x and y directions may be different, and the directions of arrangement do not have to be orthogonal to one another. The pitch is not limited to that of equal intervals, and may be of irregular intervals. For example, the light sources  20  may be arranged such that the intervals increase from the center to its periphery of the mounting board  11 . 
     Each light source  20  includes at least a light emitting element  21  including an emission face  21   a.  The light source  20  may include a cover member  22  that covers the emission face  21   a.  If the light source  20  includes a cover member  22 , the surface  22   a  of the cover member  22  is the emission face of the light source  20 . If the light source has no cover member  22 , the emission face  21   a  of the light emitting element  21  also serves as the emission face of the light source  20 . Each light source  20  may include one or one type of light emitting element  21 . In this case, it may be adapted to have the light emitting element  21  emitting white light, or have the light source  20  emitting white light as a whole by allowing the light emitted from the light emitting element  21  to transmit through the cover member  22 . The light source  20 , for example, may be a light emitting element which includes three light emitting components that individually emit red, blue, and green light, or may include three light emitting elements that individually emit red, blue, and green light, to emit white light as a result of having the red, blue, and green light be mixed. Alternatively, the light source  20  may include a light emitting element that emits white light and a light emitting element that emits another color to enhance the color rendering properties of the light emitted from the light source  20 . 
     The light emitting element  21  is a semiconductor light emitting element, and a known light emitting element such as a semiconductor laser, light emitting diode, or the like can be utilized. In the present embodiment, a light emitting diode is used for each light emitting element  21  by way of example. For the light emitting element  21 , one that emits light of any given wavelength can be selected. For example, for a blue or green light emitting element, one that employs ZnSe, nitride based semiconductor (In x Al y Ga 1-x-y N, 0≤X, 0≤Y, X+Y≤1)), or GaP can be used. For a red light emitting element, one that employs GaAlAs, AlInGaP, or the like can be used. Moreover, a semiconductor light emitting element composed of other materials than the above can alternatively be used. The compositions, emission colors, sizes, and the number of the light emitting elements used can be suitably selected in accordance with the purpose. In the case where the cover member  22  is provided with a wavelength conversion member, it is preferable for the light emitting element  21  to include a nitride based semiconductor (In x Al y Ga 1-x-y N, 0≤X, 0≤Y, X+Y≤1) capable of emitting light having a short wavelength to efficiently excite the wavelength conversion member. 
     Various emission wavelengths can be selected by adjusting the materials or mixed crystal compositions employed for the semiconductor layers. The light emitting element may have both positive and negative electrodes on the same face, or have one electrode on one face and the other electrode on the other face. 
     The light emitting element  21 , for example, has a light transmittive mounting board, and a semiconductor stack structure formed on the mounting board. The semiconductor stack structure includes an n-side semiconductor layer and a p-side semiconductor layer which interpose an active layer, and an n-side electrode and a p-side electrode are respectively electrically connected to the n-side semiconductor layer and the p-side semiconductor layer. In the present embodiment, the n-side electrode and the p-side electrode are both located on the face opposite the emission face. 
     The n-side electrode and the p-side electrode of a light emitting element  21  are electrically connected and secured to the conductor wiring layer  13  disposed on the upper face  11   a  of the mounting board  11  using the bonding members  23  described later. In other words, the light emitting elements  21  are mounted on the mounting board  11  by flip chip bonding. 
     Each light emitting element  21  may be a bare chip, or equipped with a package which includes a reflector on the lateral face side. It may further include a lens or the like to broaden the emission angle of the light emitted from the emission face  21   a.    
     The cover member  22  covers at least the emission face  21   a  of the light emitting element  21 , and is supported by the upper face  11   a  of the mounting board  11 . The cover member  22  reduces the instances of the emission surface  21   a  being exposed and damaged by external factors. For the cover member  22 , a light transmissive material such as an epoxy resin, silicone resin, a mixture of these, or glass, can be used. For the light resistance and the ease of molding of the cover member  22 , selecting a silicone resin for the cover member  22  is preferred. 
     The cover member  22  may contain a diffuser, wavelength conversion material, coloring agent, or the like. For example, a light source  20  may include a light emitting element  21  which emits blue light and a wavelength conversion material that converts blue light into yellow light, in such a manner as to emit white light by way of combining blue light and yellow light. Alternatively, the light source  20  can include a light emitting element  21  which emits blue light, a wavelength conversion material which converts blue light into green light, and a wavelength conversion material which converts blue light into red light, in such a manner as to emit white light by way of combining blue light, green light, and red light. Examples of wavelength conversion materials which convert blue light into green light include β-SiAlON phosphors, and those converting blue light into red light include fluoride-based phosphors, such as KSF-based phosphors. Containing a β-SiAlON phosphor and a fluoride-based phosphor such as KSF phosphor as wavelength conversion materials can increase color reproduction range of the light emitting device. Alternatively, a light source including a semiconductor light emitting element which emits blue light, a semiconductor light emitting element which emits green light, and a wavelength conversion material which converts blue or green light into red light may be used. 
     The cover member  22  can be formed by compression molding or injection molding in such a manner as to cover the emission face  21   a  of each light emitting element  21 . The cover member  22  can alternatively be formed by appropriately adjusting the viscosity of the material which is then dripped or drawn on the light emitting element  21  to allow the surface tension of the material itself to control its shape. In the case of employing the latter method, the cover member can be formed in a more simplified manner without requiring dies. The viscosity of the material for the cover member  22  can be adjusted by utilizing the aforementioned diffuser, wavelength conversion material, or coloring agent, besides appropriately adjusting the viscosity of the material itself. 
       FIG.  3    is a chart showing an example of the luminous intensity distribution characteristics of the light emitted from a light source  20 . It is preferable for a light source  20  to have batwing type luminous intensity distribution characteristics. This can control the amount of light emitted from the light sources  20  in the direction directly above them, and broaden the light distribution of individual light sources  20 , thereby improving on the non-uniform luminance. Broadly defined, a batwing type luminous intensity distribution characteristic means a luminous intensity distribution in which the emission intensity increases as the absolute values of distribution angles become greater than 0° when assuming the optical axis L of a light source  20  is 0°. Narrowly defined, a batwing type luminous intensity distribution characteristic means a luminous intensity distribution in which the emission intensity is highest near absolute values of distribution angles of from 45° to 90°. In other words, in the case of batwing type luminous intensity distribution characteristics, the central portion is less bright than the peripheral portion. 
     In order to achieve batwing type luminous intensity distribution characteristics, the light source  20  may include a reflective layer  24  disposed on the emission face  21   a  of the light emitting element  21 . The reflective layer  24  may be a metal film or a dielectric multilayer film. This allows the light emitted upward from the light emitting element  21  to be reflected by the light reflective layer, which reduces the quantity of light directly above the light emitting element  21 , thereby achieving batwing type luminous intensity distribution characteristics. Alternatively, the light source having batwing type luminous intensity distribution characteristics can be achieved by adjusting the outer shape of the cover member  22 . 
     Bonding Member  23   
     Bonding members  23  electrically connect and secure the light emitting elements  21  to the conductor wiring layer  13 . Examples of the bonding members  23  include an Au-containing alloy, Ag-containing alloy, Pd-containing alloy, In-containing alloy, Pb-Pd-containing alloy, Au-Ga-containing alloy, Au-Sn-containing alloy, Sn-containing alloy, Sn-Cu-containing alloy, Sn-Cu-Ag-containing alloy, Au-Ge-containing alloy, Au-Si-containing alloy, Al-containing alloy, Cu-In-containing alloy, a metal and flux mixture, and the like. 
     The bonding members  23  may be in a liquid form, paste form, or solid form (sheet, block, powder, or wire), which can be suitably selected depending on the composition, mounting board shape, or the like. These bonding members  23  may be formed using a single material, or a combination of several types. 
     The bonding members  23  do not have to electrically connect the light emitting elements  21  and the conductor wiring layer  13 . In this case, the bonding members  23  connect the areas of the light emitting element  21  other than the p-side electrode and n-side electrode to the upper face  11   a  of the mounting board  11 , while the p-side electrode and the n-side electrode are electrically connected to the conductor wiring layer  13  using wires. 
     Insulating Member  14   
     The light source unit  10  may further include an insulating member  14  that covers the areas of the conductor wiring layer  13  other than those that are electrically connected to the light emitting elements  21  and other elements. As shown in  FIG.  1   , the insulating member  14  is disposed on parts of the conductor wiring layer  13  on the upper face  11   a  side of the mounting board  11 . The insulating member  14  functions as a resist that provides insulation to the areas of the conductor wiring layer  13  other than those areas electrically connected to the light emitting elements  21  and other elements. Examples of the insulating member  14  can be a resin or resin containing a reflecting substance composed of oxide particles such as titanium oxide, aluminum oxide, silicon oxide, or the like, dispersed in the resin to reflect the light from the light emitting elements  21 . A reflective insulating member  14  reflects the light emitted from the light emitting elements  21  on the upper face  11   a  side of the mounting board  11 , and prevents or discourages the light from leaking through or being absorbed at the mounting board  11  side, thereby improving the light extraction efficiency of the light emitting device. 
     Dividing Member  15   
     The dividing members  15  include wall portions  15   ax  and  15   ay , and bottom portions  15   b . As shown in  FIG.  2   , the wall portions  15   ay  are disposed to extend in the y direction between two light sources  20  that are adjacent in the x direction, and the wall portions  15   ax  are disposed to extend in the x direction between two light sources  20  that are adjacent in the y direction. Thus, each light source  20  is surrounded by two wall portions  15   ax  extending in the x direction and two wall portions extending in the y direction. A bottom portion  15   b  is located in the region  15   r  which is surrounded by two wall portions  15   ax  and two wall portions  15   ay . In the present embodiment, the outer shape of the bottom portion  15   b  is a square because the light sources  20  are arranged at the same pitch in x and y directions. 
     A through hole  15   e  is provided in the center of each bottom portion  15   b , and the bottom portions  15   b  are disposed on the insulating member  14  so that the light sources  20  are positioned in the through holes  15   e . There are no restrictions for the shape and size of the through holes  15   e  so long as they are shaped and sized to position the light sources  20  therein. It is preferable for the outer perimeter of each through hole  15   e  to be positioned in the vicinity of a light source  20 , i.e., the gap between a through hole  15   e  and a light source  20  in a top view is small, such that the light from the light source  20  can also be reflected by the bottom portion  15   b.    
     As shown in  FIG.  1   , in a y-z cross section, each wall portion  15   ax  includes a pair of oblique faces  15   s  extending in the x direction. The pair of oblique faces  15   s  are in contact with one another at one of the two sides that extend in the x direction to form a ridge  15   c.  The other side of each oblique face is respectively connected to the bottom portions  15   b  located in two adjacent regions  15   r . Likewise, each wall portion  15   ay  includes a pair of oblique faces  15   t  extending in the y direction. The pair of oblique faces  15   t  are contacting one another at one of the two sides that extend in the y direction to form a ridge  15   c . The other side of each oblique face is respectively connected to the bottom portions  15   b  located in two adjacent regions  15   r.    
     An emission space  17  include an opening  17   a  is formed by a bottom portion  15   b , two wall portions  15   ax,  and two wall portions  15   ay .  FIG.  2    shows emission spaces  17  arranged in three rows and three columns. A pair of oblique faces  15   s  and a pairs of oblique faces  15   t  face the opening  17   a  of an emission space  17 . 
     The dividing members  15  have reflectivity, and the light emitted from the light sources  20  is reflected by the oblique faces  15   s  and  15   t  of the wall portions  15   ax  and  15   ay  towards the openings  17   a  of the emission spaces  17 . Furthermore, the light incident on the bottoms  15   b  is also reflected towards the openings  17   a  of the emission spaces  17 . This allows the light emitted from the light sources  20  to be efficiently incident on the light transmissive multilayer stack  30 . 
     An emission space  17  defined by the dividing member  15  represents the minimum unit of the emission space when the plurality of light sources  20  are independently driven. This also represents the minimum unit area of local dimming when viewing the light emitting device  101  from the upper face  30   a  of the light transmissive multilayer stack  30  as a surface emission source. In the case of independently driving the light sources  20 , it can be realized that a light emitting device can be driven by local dimming in the smallest emission space units. By simultaneously driving a plurality of adjacent light sources  20  while synchronizing the ON/OFF timing, it can be possible to drive the light emitting device by way of local dimming in larger units. 
     The dividing members  15  can be formed by using, for example, a resin containing a reflecting substance composed of particles of a metal oxide, such as titanium oxide, aluminum oxide, silicon oxide, or the like. If formed with a resin containing no reflecting substances, a reflector may be disposed on the surfaces thereof. The reflectance of the dividing members  15  for the light emitted from the light sources  20  is preferably, for example, at least 70%. 
     The dividing members  15  can be formed by molding using dies, or optical shaping. Examples of molding methods using dies include injection molding, extrusion molding, compression molding, vacuum forming, and pressure forming. For example, vacuum forming the dividing members using a reflective sheet made of PET or the like can form the dividing members  15  where the bottom potion  15   b , wall portions  15   ax , and  15   ay  are integrally formed. The thickness of the reflecting sheet can be, for example, 100 μm to 500 μm. 
     The lower faces of the bottoms  15   b  of the dividing members  15  and the upper face of the insulating member  14  are secured by an adhesive or the like. The insulating member  14  exposed at the through holes  15   e  preferably has reflectivity. It is preferable to dispose a bonding material to surround the through holes  15   e  such that the light emitted from the light sources  20  does not penetrate between the insulating member  14  and the dividing members  15 . For example, a bonding material is preferably disposed in a ring shape along the circumference of a through hole  15   e . The adhesive member may be a double-sided tape, hot melt adhesive sheet, or adhesive liquid of a thermosetting resin or thermoplastic resin. These bonding materials are preferably highly flame retardant. Alternatively, they may be secured by using screws, pins, or the like. 
     Half Mirror  31   
     The half mirror  31  of the light transmissive multilayer stack  30  is disposed above the light sources  20  in such a manner as to cover the emission spaces  17  of the light source unit  10 . 
     The half mirror  31  has the reflection and transmission characteristics where it reflects some portion of the incident light while transmitting the remaining portion of the light. The half mirror  31  has a reflectance of in a range of about 30% to about 75% for the emission spectrum of the light source. The reflectance of the half mirror  31  is substantially equal across the entire area of the half mirror  31 . Here, “substantially equal” refers to, for example, that the reflectance of light perpendicularly incident measured at any given location of the principal plane (i.e., upper face  31   a  or lower face  31   b ) is within ±5% of the average value. 
     The half mirror  31  preferably has a dielectric multilayer structure where two or more dielectric films of varying refractive indices are stacked on a light transmissive base. Specific examples of the materials for the dielectric films include metal oxide film, metal nitride film, metal fluoride film, and resins such as polyethylene terephthalate (PET). It is preferable to employ a material that does not readily absorb the light from the light sources  20  or the later described wavelength conversion layer  34 . The dielectric multilayer film reflects portion of the incident light at the boundaries of the stacked dielectric films due to a refractive index difference. The reflectance can be adjusted by the thickness of a dielectric film which changes the phase shift between the incident light and the reflected light to adjust the interference between the two. Phase adjustments based on the thickness of a dielectric film depends on the wavelength of the light passing therethrough. By stacking multiple layers of dielectric films such that each dielectric film reflects light of a different wavelength, the wavelength dependency of the reflectance can also be adjusted. Accordingly, a dielectric multilayer film can produce a half mirror  31  that does not readily absorb light while being capable of suitably adjusting reflectance characteristics. 
     When using a dielectric multilayer structure, even if a dielectric film has a uniform thickness, the optical path length differs between perpendicularly-incident light and obliquely-incident light. For this reason, it is also possible to control the reflectance of the half mirror  31  based on the angle of incidence of the incident light. By setting the reflectance to be lower for obliquely incident light than perpendicularly-incident light, the reflectance in the direction of the optical axis L of a light source  20 , i.e., in the direction perpendicular to the principal plane of the half mirror  31 , can be increased while reducing the reflectance for the light being incident at a large angle φ relative to the optical axis L. In other words, increasing the transmittance at larger angles of incidence can further reduce the non-uniform luminance across the surface of the light emitting device when observed from the outside. 
       FIG.  4    shows the dependency of the reflectance and transmittance of the half mirror  31  on the light distribution angle by way of example. Assuming that the optical axis L is 0°, where the absolute value of the light distribution angle φ in  FIG.  1   ) is in the range of up to about 40°, the reflectance is 60%. The reflectance declines while the transmittance increases in the range where the absolute value is over 40°. Providing the half mirror with such reflectance characteristics can more effectively reduce the non-uniform luminance discussed earlier. 
       FIG.  5    is a chart showing the emission spectrum of the light emitted from the light source  20  and the reflectance characteristics of the half mirror  31  by way of example. The horizontal axis represents wavelengths, and the vertical axis represents reflectance or relative emission intensity. The reflectance values are those measured in the direction perpendicular to the principal plane of the half mirror  31 . With respect to the reflectance characteristics of the half mirror  31  in the perpendicular direction, it is preferable for the reflectance spectrum on the longer side of the peak emission wavelength of the light source  20  to be broader than that on the shorter side. In the example shown in  FIG.  5   , the peak emission wavelength of the light source  20  is about 450 nm. The reflectance bandwidth where the half mirror  31  has a reflectance of at least 40%, for example, is 50 nm (i.e., 400 nm to 450 nm) on the shorter wavelength side from 450 nm, whereas is 120 nm (i.e., 450 nm to 570 nm) on the longer wavelength side from 450 nm the bandwidth. 
     In general, the reflectance spectrum of a half mirror shifts to the shorter wavelength side because the optical path length increases when light is obliquely incident as compared to perpendicularly incident. For example, even if a half mirror has a certain reflectance for light having a wavelength λ when the light is perpendicularly incident on the half mirror, the reflectance spectrum shifts to the shorter wavelength side by δ when the light is obliquely incident on the half mirror. For this reason, even though the half mirror has the same reflectance for light having wavelengths shorter than λ by the amount corresponding to the reflectance spectrum shift δ, its reflectance declines for the light having wavelength λ. 
     By designing the reflectance characteristics of the half mirror  31  in the perpendicular direction such that the reflectance bandwidth on the longer wavelength side of the peak emission wavelength of a light source  20  is broader than that on the shorter wavelength side as described above, the half mirror provided with a broad reflectance bandwidth on the longer wavelength side can maintain the same reflectance even if obliquely incident light shifts the reflectance spectrum to the shorter wavelength side by δ. Even if light obliquely enters the half mirror  31  in the aforementioned range of absolute values of light distribution angles of up to about 40°, this can more effectively prevent or discourage a decline of the reflectance and an increase of the transmittance of the light entering slightly obliquely relative to the optical axis L of the light source  20  which could otherwise emphasize the non-uniform luminance. 
     Light Diffusion Plate  33   
     The light diffusion plate  33  is positioned on the upper face  31   a  side of the half mirror  31 . The light diffusion plate  33  diffuses and transmits incident light. A material of the light diffusion plate  33  can include that does not readily absorb visible light, such as a polycarbonate resin, polystyrene resin, acrylic resin, polyethylene resin, or the like. The light diffusion plate  33  is provided with a light diffusing structure by way of projections and depressions provided on the surface, or a material having a different refractive index dispersed in the light diffusion plate  33 . For the light diffusion sheet, those commercially available generally known as a diffusion sheet, diffuser film, or the like may be used. 
     Diffuse Reflector  32   
     The diffuse reflectors  32  are positioned between the half mirror  31  and the light diffusion plate  33 , or on the lower face  31   b  of the half mirror  31 . In the present embodiment, the diffuse reflectors  32  are provided on the lower face  33   b  of the light diffusion plate  33 . If the light diffusion plate  33  is constructed with a polystyrene resin, the diffuse reflectors  32  are preferably disposed on the half mirror  31 . This can prevent or discourage misalignment between the light sources  20  and the diffuse reflectors  32  attributable to thermal expansion because the material used for constructing the half mirror  31  has a smaller linear expansion coefficient than that of polystyrene resin. 
       FIG.  6    shows the positional relationship between the diffuse reflectors  32  and the emission faces (the surfaces  22   a  of the cover members  22  or the emission faces  21   a ) of the light sources  20  when viewing the light emitting device  101  from the top. As shown in  FIG.  1    and  FIG.  6   , the diffuse reflectors  32  are positioned at least above the emission face of each light source  20 , i.e., above the optical axis of each light source  20 . The diffuse reflectors  32  scatter and reflect incident light. Since the light emitted from the light sources  20  is intense above the optical axis L, providing the diffuse reflectors  32  can attenuate the non-uniform luminance in the light emitted from the light sources  20 . Assuming that the optical axis of a light source  20  is 0°, the emission intensity is relatively lower at an absolute value of light distribution angle larger than 0°. Since there is no need to scatter light at an area above the ridges  15   c  of the dividing members  15  which are the boundaries of the emission spaces  17 , diffuse reflectors  32  may not be provided above the ridges. 
     In  FIG.  6   , the diffuse reflectors  32  are circular in shape centering around the optical axis of each light source  20 , but the shape of the diffuse reflectors is not required to a circle. The shape of the diffuse reflectors  32  can be determined in accordance with the light distribution characteristics of the light sources  20  such that light can be uniformly scattered, such as an ellipse, quadrangle, or the like. In the case where the emission intensity of a light source  20  is lower above the optical axis than the periphery of the optical axis because the light source  20  has batwing type luminous intensity distribution characteristics, or such other reasons, the diffuse reflectors  32  may have, for example, a ring shape in a top view. In other words, the diffuse reflector  32  only need to be positioned above part of the emission face of each light emitting source  20 . 
     In the case where the ridges  15   c  are in contact with the light diffusion plate  33  or the half mirror  31 , the light reflected by the light diffusion plate  33  or the half mirror  31  increases the amount of light that irradiates wall portions  15   ax  and  15   ay,  thereby making the areas near the ridges  15   c  brighter. It is thus preferable to dispose the diffuse reflectors  32  directly above the wall portions  15   ax  and  15   ay . This can attenuate the non-uniform luminance that would otherwise result from the areas near the ridges  15   c  becoming brighter. 
     The diffuse reflectors  32  include a resin, and reflecting oxide substance particles, such as titanium oxide, aluminum oxide, silicon oxide, or the like dispersed in the resin. The average particle size of the oxide particles, for example, is about 0.05 μm to about 30 μm. The diffuse reflectors  32  may further contain a pigment, light absorbing material, phosphor, or the like. In the case of the resin employs a light curable material that contains acrylate, epoxy, or the like as a major component, the diffuse reflectors  32  can be formed by irradiating, for example, with UV rays after applying a curable resin containing the reflecting substance on the lower face  33   b  of the light diffusion plate  33 . The curable resin may alternatively be cured by the light emitted from the light sources  20 . The uncured resin with a reflecting substance dispersed therein can be applied, for example, by a printing technique using a printing plate or an inkjet technique. 
     The reflecting particles for scattering light can be uniformly distributed in the diffuse reflectors  32 , or arranged in higher density in the areas of smaller absolute values of light distribution angles of the light sources  20  than the areas of larger absolute values of light distribution angles. Each diffuse reflector  32 ′ shown in  FIG.  7 A  includes a first portion  32   a  and a second portion  32   b.  The first portion  32   a  is positioned directly above an emission face  21   a,  and the second portion  32   b  is positioned in the periphery of the first portion  32   a.    
     The reflecting particle density in the second portion  32   b  is lower than the reflecting particle density in the first portion  32   a.  Particle density here, for example, can be a number density expressed by the number of particles per unit area of a plane viewed from the top view, i.e., an x-y plane. 
     For example, the diffuse reflectors  32 ′ can be formed by arranging micro areas  32   c  composed of an uncured resin in which reflecting particles are dispersed more densely in the first portion  32   a  and less densely in the second portion  32   b  by printing or inkjet as shown, in  FIG.  7 B . As shown in  FIG.  7 C , a first layer  32   d  of an uncured resin in which reflecting particles are dispersed may be formed on both the first portion  32   a  and the second portion  32   b , followed by forming a second layer  32   e  only on the second portion  32   b . The diffuse reflectors  32 ′ shown in  FIG.  7 B and  7 C  both satisfy the relationship of the reflecting particle density in the diffuse reflectors  32 ′ in an x-y plane described above. 
     The diffuse reflectors  32  may be disposed on the upper face  31   a  of the half mirror  31  as shown in  FIG.  8    or on the lower face  31   b  of the half mirror  31  as shown in  FIG.  9   . Alternatively, the diffuse reflectors  32  may be disposed on the upper face  33   a  of the light diffusion plate  33  as shown in  FIG.  10   . 
     Wavelength Conversion Layer  34   
     The light emitting device  101  may further include a wavelength conversion layer  34  in the light transmissive multilayer stack  30 . The wavelength conversion layer  34  is positioned on the light diffusion plate  33  side opposite the half mirror  31  side, i.e., on the upper face  33   a . The wavelength conversion layer  34  absorbs portion of the light emitted from the light sources  20 , and emits light having different wavelengths from those of the light emitted from the light sources  20 . 
     Since the wavelength conversion layer  34  is distant from the light emitting elements  21  of the light sources  20 , a wavelength conversion material that is less resistant to heat or light and not suitable for use near the light emitting elements  21  can also be used. This can improve the performance of the light emitting device  101  as a backlight. The wavelength conversion layer  34  is in a sheet or layer form, and contains the wavelength conversion substance described above. 
     In the case where a wavelength conversion layer  34  is employed, a dichroic layer  38  having a higher reflectance for the wavelengths of the light emitted from the wavelength conversion layer  34  than that for the emission wavelengths of the light sources  20  may be provided between the wavelength conversion layer  34  and the half mirror  31  as shown in  FIG.  11   . 
     Prism Array Layers  35  and  36 , Reflective Polarizing Layer  37   
     The light emitting device  101  may further include prism array layers  35  and  36 , and a reflective polarizing layer  37  in the light transmissive multilayer stack  30 . The prism array layers  35  and  36  have the format of arranging a plurality of prisms in a predetermined direction. For example, the prism array layer  35 , in  FIG.  1   , includes a plurality of prisms extending in the y direction and the prism array layer  36  includes a plurality of prisms extending in the x direction. The prism array layers  35  and  36  refract the light entering from various directions towards the display panel that opposes the light emitting device (i.e., the z direction). This can increase the component of the light output by the upper face  30   a  of the light transmissive multilayer stack  30  serving as the emission face of the light emitting device  101  in perpendicular direction to the upper face  30   a  (i.e., parallel to the z-axis), thereby increasing the luminance of the light emitting device  101  when viewed from the front (i.e., in the z-axis direction). 
     The reflective polarizing layer  37  selectively passes the light having the polarization direction consistent with the polarization direction of the polarizer disposed on the backlight side of a display panel, for example, an LCD panel, while reflecting the polarized light perpendicular to the polarization direction back to the prism array layers  35  and  36 . Portion of the polarized light returning from the reflective polarizing layer  37  is converted into the polarized light having the polarization direction of the polarizer of the LCD panel as its polarization direction is changed when re-reflected by the prism array layers  35  and  36 , the wavelength conversion layer  34 , and the light diffusion plate  33 , and is incident on the reflective polarizing layer  37  again to be released toward the display panel. This aligns the polarization directions of the light emitted from the light emitting device  101 , and enables a highly efficient emission of the light having the polarization direction that effectively increase the luminance of the display panel. 
     For the prism array layers  35  and  36 , and the reflective polarizing layer  37 , those commercially available optical members for backlight applications can be employed. 
     Light Transmissive Multilayer Stack  30   
     The light transmissive multilayer stack  30  is structured by stacking the half mirror  31 , the diffuse reflectors  32 , the light diffusion plate  33 , the wavelength conversion layer  34 , the prism array layers  35  and  36 , and the reflective polarizing layer  37  described above on top of one another. At least one interface of these layers may have a space where the layers are not in contact with one another. However, in order to minimize the thickness of the light emitting device  101 , it is preferable to stack the layers so that two adjacent layers are in contact with one another. 
     The light transmissive multilayer stack  30  is supported by a support at a predetermined distance from the light source unit  10 . The lower face  30   b  of the light transmissive multilayer stack  30  is preferably in contact with the ridges  15   c  of the dividing members  15 . For example, the ridges  15   c  and the lower face  31   b  of the half mirror  31  may be bonded using bonding members, or the ridges  15   c  may be connected to the half mirror  31  or other member using pins, screws, or the like. With the structure that the ridges  15   c  are in contact with the lower face  30   b  of the light transmissive multilayer stack  30 , the light emitted from the light source  20  in one emission space  17  can be less likely to enter an adjacent emission space  17 . 
     The distance OD between the half mirror  31  and the mounting board  11  is preferably set to 0.2 times the arrangement pitch P for the light sources  20  at most (OD/P≤0.2). More preferably, the distance OD is set to in a range of 0.05 times to 0.2 times the arrangement pitch P for the light sources  20  (0.5≤OD/P≤0.2). With a conventional construction, setting the distance between the light transmissive multilayer stack  30  and the mounting board on which light sources  20  are mounted to be small caused considerable non-uniform in the luminance of the light emitting device. In contrast, in the light emitting device  101  according to this disclosure, the employment of the half mirror  31  and the diffuse reflectors  32  can achieve uniform luminance distribution. 
     The light emitting device  101  can be manufactured by individually providing a light source unit  10  and a light transmissive multilayer stack  30 , and assembling the two by supporting the light transmissive multilayer stack  30  on the light source unit  10  using the aforementioned support. 
     Operation and Effect of the Light Emitting Device  101   
     The operation of the light emitting device  101 , particularly the reason why the non-uniform luminance of the light emitted from the light sources  20  can be attenuated, will be explained. In the case where the light emitting device  101  is used as a surface emitting device such as a backlight, it is preferable to minimize the non-uniform luminance at the upper face  30   a  of the light transmissive multilayer stack  30 , which is the emission face of the light emitting device  101 . 
     However, because the light source  20  is a point light source, the illuminance of the surface illuminated by the light emitted from the light source  20  is inversely proportional to the square of the distance. For this reason, the illuminance of the light incident on the lower face  30   b  of the light transmissive multilayer stack  30  is higher in areas R 1  that is immediately above the light source  20  in a top view than in areas R 2  that are positioned in the periphery of R 1 . This is because the distance between the light source  20  and the upper face  30   a  in areas R 1  is smaller than the distance between the light source  20  and the upper face  30   a  in areas R 2 . 
     In using the light emitting device  101  as a backlight, a reduction in the thickness or height is desirable because a thinner display device is in demand from the perspective of the design, beauty, and functionality of the display device. For this reason, the smaller the distance OD between the light source unit  10  and the light transmissive multilayer stack  30 , the more preferable it is. A smaller OD would increase the light that directly enters the light transmissive multilayer stack  30 . Thus, unless the intervals between the light sources  20  are reduced to the extent possible, the non-uniform luminance at the upper face  30   a  described above worsens. 
     The light emitting device  101  according to the present embodiment includes a half mirror  31  and diffuse reflectors  32 . The half mirror  31  is positioned closer to the light sources  20  than the diffuse reflectors  32  are, and reflects portion of the light emitted from the light sources  20 . The light reflected by the half mirror  31  enters the mounting board  11  that supports the light sources  20 , and is reflected at the mounting board  11  side to enter the half mirror  31  again. The reentering light which has been reflected by the half mirror  31  and at the mounting board  11  side is more diffused than the light that directly enters from the light sources  20 . Thus, by allowing portion of the light emitted from the light sources  20  to be reflected between the half mirror  31  and the mounting board  11  once or multiple times, the light from the light sources  20  can be released from a larger area than the emission faces of the light sources  20 , in other words, the light from the light sources  20  can be output planarly from the half mirror  31 . 
     The light released from the half mirror  31  is incident on and scattered by the diffuse reflectors  32  positioned at least above the emission faces  21   a  of the light sources  20 . This can selectively diffuse the high luminous flux density light near the optical axes L of the light sources  20  thereby attenuating the non-uniform luminance. 
     In the case of constructing the half mirror  31  with a dielectric multilayer film, in particular, the absorption of light by the half mirror  31  can be reduced, and thus the use efficiency of light can be increased. The reflectance of the half mirror  31  in the perpendicular direction is substantially uniform. This property can be achieved by stacking dielectric films, and a large area half mirror  31  can be produced relatively easily and inexpensively by using, for example, display panel production technology. Accordingly, a half mirror  31  having good properties can be produced inexpensively, and the production costs of the light emitting device can be reduced. 
     Setting the reflectance of the half mirror  31  to be lower for oblique incidence than perpendicular incidence allows the half mirror to reflect more light that is directly incident on the half mirror in the optical axis L direction of the light source  20  and reflect less light that is directly incident on the half mirror  31  at a larger angle relative to the optical axis L of the light source  20 . This is particularly effective in reducing the non-uniform luminance attributable to the direct light from the light sources  20 . 
     Each diffuse reflector  32 , which includes a resin and particles dispersed in the resin, may be arranged such that the density of the particles in the first portion  32   a  directly above the emission face of each light source when viewed from the top is higher than the density of the second portion  32   b  positioned in the periphery of the first portion  32   a . This can provide different degrees of scattering within each diffuse reflector  32 , in other words, increasing scattering in the higher luminous flux density area, thereby further attenuating the non-uniform luminance. 
     The diffuse reflectors  32  may be disposed on the upper  31   a  of the half mirror  31  or the lower face  33   b  of the light diffusion plate  33 . In the case where the linear expansion coefficient of the half mirror  31  is smaller than the linear expansion coefficient of the light diffusion plate  33 , and the linear expansion coefficient difference between the mounting board  11  and the light diffusion plate  33  is smaller than the linear expansion coefficient difference between the mounting board  11  and the half mirror  31 , the diffuse reflectors  32  may be disposed on the half mirror  31 . In this case, misalignment between the light sources  20  and the diffuse reflectors  32  attributable to thermal expansion/contraction can be reduced. This thus can produce a light emitting device  101  with small optical characteristic changes attributable to the heat generated during operation. 
     In general, the reflectance bandwidth of a half mirror shifts to the shorter wavelength side for obliquely incident light as compared to light perpendicularly incident on the half mirror. For this reason, by designing the reflectance characteristics of the half mirror  31  in the perpendicular direction such that the reflectance bandwidth on the longer wavelength side from the peak emission wavelength of the light sources  20  is broader than that on the shorter wavelength side, a reduction of the reflectance which can emphasize the luminance non-uniformity can be attenuated even if the reflectance spectrum of the half mirror shifted to the shorter wavelength side for the light that is incident slightly obliquely relative to the optical axis. 
     Furthermore, by providing the light sources  20  with batwing type light distribution characteristics, the illuminance at areas R 1  in  FIG.  1    can be reduced. This can further attenuate the non-uniform luminance at the upper face  30   a  of the light transmissive multilayer stack  30  that is the emission face of the light emitting device  101 . When the light sources  20  have the light distribution characteristics where the quantity of light less than the elevation angle of 20° relative to the lateral direction is at least 30% of the total quantity of light, in particular, the non-uniform luminance can be further attenuated. As described above, the light emitting device  101  according to the present disclosure can effectively attenuate the non-uniform luminance at the upper face  30   a  of the light transmissive multilayer stack  30  which is the emission face of the light emitting device  101 . 
     Furthermore, the light transmissive multilayer stack  30  may include one or more light absorbing layers  40  located at least above the light emission surface of the plurality of light sources. The light absorbing layers  40  absorbs at least a part of the light emitted from the light sources. In the case where white light is obtained by a combination of the semiconductor light emitting element and the cover member including the wavelength conversion member, the luminous intensity of may be high in a region where the absolute value of the light distribution angle is close to 0 compared a region where to the absolute value of the light distribution angle is greater. Consequently, wavelength conversion by the wavelength conversion member is not sufficiently performed in some cases. For example, when the light sources including the semiconductor light emitting element that emits blue light and a cover member that includes the wavelength conversion member that converts blue light into yellow light is used, bluish color may appear on the display surface directly above each light source in some cases. That is, color non-uniformity sometimes occurs. Also in the configuration of such the light sources, by disposing the light absorption layer  40  above the light emission surface of the light source, the blue light emitted at the light distribution angle close to 0° can be selectively absorbed by the light absorption layer  40 . Therefore, color non-uniformity can be less likely to occur. In the  FIG.  13   , the light absorption layer  40  is formed on the upper surface of the light diffusion plate  33 . 
     EXAMPLE 
     A light emitting device  101  was produced and its luminance distribution was observed as explained below. For the light sources  20 , those including nitride-based blue light emitting elements  21  and cover members  22  and light source having batwing type luminous intensity distribution characteristics were used. 
     For the half mirror  31 , PICASUS 100GH10 having a 50% transmittance manufactured by Toray Industries, Inc. was used. For the light diffusion plate  33 , a light diffusion sheet was used. For the wavelength conversion layer  34 , a phosphor sheet containing a green phosphor and a red phosphor was used. For prism array layers  35  and  36 , prism sheets were used, which were arranged so that the prism extending directions were orthogonal to one another. For the reflective polarizing layer  37 , a reflective polarizing film was used. For the diffuse reflectors  32 , a white ink composed of a resin in which titanium oxide particles were dispersed was printed on the lower face  33   b  of the light diffusion plate  33  using an inkjet printer. The light sources  20  were arranged in five rows and five columns at a pitch P of 18.8 mm. The distance OD between the mounting board  11  and the half mirror  31  was set to 1.8 mm. OD/P was 0.096. 
     For comparison purposes, a light emitting device was produced without a half mirror  31 , and by setting the distance OD between the mounting board  11  and the diffusion plate  33  to 3.8 mm (hereinafter referred to as the Reference Sample). OD/P was 0.20. 
     The light emitting devices of the Example and the Reference Sample were turned on, and the emission faces were photographed as shown in  FIG.  12   . As is understood from  FIG.  12   , by employing the half mirror  31  and the diffuse reflectors  32 , it was possible to achieve luminance distribution characteristics having equivalent or better luminance non-uniform attenuation as compared to the Reference Sample that had no half mirror  31  and OD set to 3.8 mm. In other words, even when the distance OD between the mounting board  11  and the half mirror  31  is reduced to one half of that of the Reference Sample or smaller, a light emitting device having equivalently or more uniform luminance distribution can be realized. 
     When the relative luminance was measured, the luminance of the light emitting device of the Example was about 85% of that of the Reference Sample. This is believed to be because the insertion of the half mirror  31  slightly reduced the light extraction efficiency. 
     It is understood from these results that the light emitting device according to the present disclosure can achieve uniform luminance distribution with reduced luminance non-uniformity even when the distance between the half mirror and the mounting board is 0.2 times the distance between two adjacent light sources or smaller. 
     The light emitting device according to the present disclosure can be utilized as a backlight light source for a liquid crystal display, various types of lighting fixtures, or the like.