Patent Publication Number: US-2021175394-A1

Title: Wavelength converter

Description:
TECHNICAL FIELD 
     The present disclosure relates to a wavelength converter using photoluminescence. 
     BACKGROUND ART 
     Heretofore, as an optical conversion layer using photoluminescence, there has been known an optical conversion layer composed of: a plurality of optical conversion inorganic particles which emit light by being irradiated with excitation light; and a binder layer that holds the plurality of optical conversion inorganic particles. When the optical conversion layer is formed on a surface of a substrate portion, a wavelength converter composed of the substrate portion and the optical conversion layer is obtained. It is preferable that the substrate portion and the optical conversion layer have high adhesion strength. 
     For example, in PTL 1, there is disclosed a wavelength converter including: a quartz glass substrate; and a wavelength conversion quartz glass layer that is formed on a surface of the quartz glass substrate and contains phosphor particles. Moreover, in the wavelength converter of PTL 1, a concentration of phosphor in the wavelength conversion quartz glass layer is distributed from a high concentration to a low concentration from the glass substrate toward a surface of the wavelength conversion quartz glass layer. 
       FIG. 4  is a schematic cross-sectional view of the wavelength converter according to PTL 1. As illustrated in  FIG. 4 , a wavelength converter  100 C according to PTL 1 includes: a substrate portion  10  made of glass; and a wavelength conversion quartz glass layer  130 C as an optical conversion layer. In the wavelength conversion quartz glass layer  130 C, optical conversion inorganic particles  40  are held by an inorganic binder portion  150 C. The inorganic binder portion  150 C is formed into a quartz glass layer  52  composed of an amorphous binder. In the wavelength converter  100 C, a concentration of the optical conversion inorganic particles  40  in the wavelength conversion quartz glass layer  130 C is distributed from a high concentration to a low concentration from the substrate portion  10  toward a surface of the wavelength conversion quartz glass layer  130 C. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2016-34891 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the wavelength converter disclosed in PTL 1 is required to be heated at approximately 550° C. at the time of being manufactured. Therefore, in the case where the substrate portion is made of metal such as aluminum in order to reflect light on the substrate portion, it is apprehended that the metal of the substrate portion and the optical conversion inorganic particles may be degraded by being heated as described above. 
     The present disclosure has been made in consideration of the above-described problem. It is an object of the present disclosure to provide a wavelength converter that prevents the substrate portion and the optical conversion inorganic particles from being degraded by being heated and has high adhesion between the substrate portion and the optical conversion layer. 
     Solution to Problem 
     In order to solve the above-described problem, a wavelength converter according to an aspect of the present disclosure includes: a substrate portion; and an optical conversion layer including optical conversion inorganic particles and an inorganic binder portion that mutually holds the optical conversion inorganic particles, and being formed on the substrate portion, wherein the inorganic binder portion includes: an amorphous binder, and granular binder particulates with an average particle size smaller than an average particle size of the optical conversion inorganic particles, and wherein, in a case where, in the optical conversion layer, a portion present close to the substrate portion with respect to an intermediate plane in a thickness direction of the optical conversion layer is defined as a substrate-side portion, and a portion present remote from the substrate portion with respect to the intermediate plane is defined as a non-substrate-side portion, when a ratio of an average volume concentration of the binder particulates in the substrate-side portion with respect to an average volume concentration of the optical conversion inorganic particles in the substrate-side portion is defined as a substrate-side binder particulate concentration ratio RF S . and, when a ratio of an average volume concentration of the binder particulates in the non-substrate-side portion with respect to an average volume concentration of the optical conversion inorganic particles in the non-substrate-side portion is defined as a non-substrate-side binder particulate concentration ratio RF O , the substrate-side binder particulate concentration ratio RF S  is larger than the non-substrate-side binder particulate concentration ratio RF O . 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a wavelength converter according to an embodiment and Example 1. 
         FIG. 2  is a schematic cross-sectional view of a wavelength converter according to Comparative example 1. 
         FIG. 3  is a schematic cross-sectional view of a wavelength converter according to Comparative example 2. 
         FIG. 4  is a schematic cross-sectional view of a wavelength converter according to PTL 1. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a wavelength converter according to an embodiment will be described with reference to the drawings. 
     [Wavelength Converter] 
       FIG. 1  is a schematic cross-sectional view of a wavelength converter according to the embodiment. As illustrated in  FIG. 1 , a wavelength converter  1 A ( 1 ) includes a substrate portion  10 , and an optical conversion layer  30 A ( 30 ) formed on the substrate portion  10 . When optical conversion inorganic particles  40  in the optical conversion layer  30 A receive excitation light emitted from an excitation light source (not shown), the wavelength converter  1 A radiates, to an outside thereof, light subjected to optical conversion by the optical conversion inorganic particles  40 . 
     (Substrate Portion) 
     The substrate portion  10  is a member that has functions to reinforce the optical conversion layer  30 A formed on a surface of the substrate portion  10 , and to impart good optical and thermal characteristics to the optical conversion layer  30 A by selection of a material and thickness thereof. 
     As a material of the substrate portion  10 , for example, there are used metal that is not translucent and translucent ceramics such as glass and sapphire. As the metal, for example, aluminum, copper or the like is used. The substrate portion  10  made of metal is preferable since light reflectivity thereof is excellent. Moreover, the substrate portion  10  made of ceramics is preferable since translucency thereof is excellent. As ceramics, silicon nitride, alumina or the like is used. A thermal expansion coefficient of aluminum is 23×10 −6  to 24×10−6/K, and a thermal expansion coefficient of copper is 16×10 −6  to 17×10 −6 /K. A thermal expansion coefficient of silicon nitride is 2×10 −6  to 4×10 −6 /K, and a thermal expansion coefficient of alumina is 5×10−6 to 8×10−6/K. 
     It is preferable that the material of the substrate portion  10  be metal since it is easy to improve heat dissipation. That is, if the substrate portion  10  is made of metal, then thermal conductivity of the substrate portion  10  increases. Accordingly, in the optical conversion layer  30 A, it becomes possible to efficiently remove heat generated in a process where excitation light is converted to fluorescent light  70 , and the like. Therefore, it is preferable that the substrate portion  10  be made of metal since it is easy to suppress temperature quenching of the optical conversion inorganic particles  40  and degradation and burning of an inorganic binder portion  50 A. 
     Meanwhile, in the case where the material of the substrate portion  10  is a translucent material such as translucent ceramics, it becomes possible to apply light to the optical conversion inorganic particles  40  in the optical conversion layer  30 A via the substrate portion  10 . Herein, the fact that a material has translucency means that the material is transparent with respect to the visible light (with a wavelength of 380 nm to 800 nm). Moreover, in this embodiment, being transparent means that light transmittance in a material is preferably 80% or more, more preferably 90% or more. Moreover, it is preferable that an extinction coefficient for the visible light by the material for use in the substrate portion  10  be as low as possible since it is possible to sufficiently apply light via the substrate portion  10  to the optical conversion inorganic particles  40  in the optical conversion layer  30 A. As described above, it is preferable that the substrate portion  10  be made of a translucent material since it becomes easy to construct a compact system. As described above, it is preferable that the substrate portion  10  be translucent since it becomes easy to construct a compact system. 
     Incidentally, the above-described metal usually has lower heat resistance than the above-described ceramics. Therefore, in the case where the substrate portion  10  is made of metal, it is preferable that the substrate portion  10  not be heated at a high temperature when the optical conversion layer  30 A is provided on the surface thereof. As will be described later, it is possible to form the optical conversion layer  30 A of the wavelength converter  1 A at a relatively low temperature. Therefore, the wavelength converter  1 A is preferable in the case where the substrate portion  10  is made of metal. 
     It is preferable that reflectance of a surface of the substrate portion  10 , which is close to the optical conversion layer  30 A, be 90% or more since extraction efficiency of light from the surface of the optical conversion layer  30 A increases. As a method of setting the reflectance of the surface of the substrate portion  10 , which is close to the optical conversion layer  30 A, to 90% or more, for example, a method using the substrate portion  10  made of metal in which reflectance of a surface is 90% or more is mentioned. 
     Moreover, as another method of setting the reflectance of the surface of the substrate portion  10 , which is close to the optical conversion layer  30 A, to 90% or more, there is mentioned a method of forming the substrate portion  10  into a two-layer structure including a substrate body and a reflective film formed on a surface of the substrate body, the surface having reflectance of 90% or more. In the case of this two-layer structure, a material of the substrate body is not particularly limited, and for example, metal such as aluminum and translucent ceramics can be used. 
     Moreover, when the substrate portion  10  is formed into the above-described two-layer structure, the substrate portion  10  can be formed into a three-layer structure by further forming a protective film for protecting the reflective film on the surface of the reflective film. For example, when the reflective film is an aluminum thin film, the substrate portion  10  can be formed into the three-layer structure by forming a protective film for suppressing oxidation of aluminum on the surface of the reflective film. 
     (Optical Conversion Layer) 
     The optical conversion layer  30 A ( 30 ) includes the optical conversion inorganic particles  40  and the inorganic binder portion  50 A ( 50 ) that mutually holds the optical conversion inorganic particles  40 , and is formed on the substrate portion  10 . A film thickness of the optical conversion layer  30 A is, for example, 10 μm to 1000 μm. It is preferable that the film thickness of the optical conversion layer  30 A stay within the above-described range since the obtained wavelength converter becomes one that has high thermal conductivity and light extraction efficiency and scatters light largely. 
     &lt;Optical Conversion Inorganic Particle&gt; 
     The optical conversion inorganic particles  40  are particles made of an optical conversion material that is an inorganic compound capable of photoluminescence. A type of the optical conversion inorganic particles  40  is not particularly limited as long as the optical conversion inorganic particles  40  are capable of photoluminescence. As the optical conversion inorganic particles, for example, used are particles containing a nitride-based optical conversion material activated by Eu 2+ , and crystalline particles with a garnet structure made of YAG, that is, Y 3 Al 5 O 12 . Among optical conversion inorganic particles, the particles containing the nitride-based optical conversion material activated by Eu 2+  are preferable since the excitation light is converted to light with a longer wavelength. Moreover, as the particles containing the nitride-based optical conversion material activated by Eu 2+ , for example, used are optical conversion inorganic particles containing (Sr,Ca)AlSiN 3 :Eu, silicon nitride Si 3 N 4 :Eu, and SiAlON:Eu. 
     An average particle size of optical conversion inorganic particles  40  having a large particle size is usually 100 μm or less, preferably 30 μm or less. It is preferable that the average particle size of the optical conversion inorganic particles  40  stay within the above-described range since it is possible to reduce a spot diameter of output light output from the wavelength converter  1 A because guidance of light trapped in the optical conversion inorganic particles  40  due to total reflection is limited to a range of the particle size. Moreover, it is preferable that the average particle size of the optical conversion inorganic particles  40  stay within the above-described range since it is possible to produce the optical conversion inorganic particles  40  in an inexpensive production process such as a coating method while reducing color variation of the output light of the wavelength converter  1 A. 
     The average particle size of the optical conversion inorganic particles  40  having a large particle size is obtained by observing the arbitrarily preprocessed optical conversion layer  30 A by a scanning electron microscope (SEM) or the like and obtaining an average value of diameters of particles of which number is sufficiently significant from a statistical viewpoint, for example, 100. 
     Moreover, it is possible to determine a composition of the optical conversion inorganic particles  40  by a well-known analysis method such as energy dispersive X-ray spectrometry (EDX) and X-ray diffraction analysis (XRD). 
     The optical conversion inorganic particles  40  may be made of phosphors having the same composition, or may be a mixture of phosphor particles having two or more types of compositions. 
     It is preferable that a refractive index of the optical conversion inorganic particles  40  be larger than a refractive index of the inorganic binder portion  50 A. When the refractive index of the optical conversion inorganic particles  40  is larger than the refractive index of the inorganic binder portion  50 A, light is trapped in the inside of the phosphor due to total reflection. Therefore, in in-plane guided light in the inorganic binder portion  50 A, components limited to the range of the particle size of the optical conversion inorganic particles  40  are increased. Hence, it is preferable that the refractive index of the optical conversion inorganic particles  40  be larger than the refractive index of the inorganic binder portion  50 A since it is easy to reduce the spot diameter of the output light output from the wavelength converter  1 A. 
     In the case where the optical conversion inorganic particles  40  are YAG particles, the refractive index of the optical conversion inorganic particles  40  is 1.8 to 1.9. 
     &lt;Inorganic Binder Portion&gt; 
     The inorganic binder portion  50 A ( 50 ) is a member that constitutes the optical conversion layer  30 A and mutually holds the optical conversion inorganic particles  40 . As illustrated in  FIG. 1 , the inorganic binder portion  50 A includes: an amorphous binder  52 ; and granular binder particulates  51  with an average particle size smaller than that of the optical conversion inorganic particles  40 . The substrate portion  10  and the inorganic binder portion  50 A in the optical conversion layer  30 A are bonded to each other on an interface therebetween. 
     [Binder Particulate] 
     The binder particulates  51 A are one component that constitutes the inorganic binder portion  50 A, and is composed of a granular inorganic compound with an average particle size smaller than that of the optical conversion inorganic particles  40 . The binder particulates  51  usually form fixed bodies  55  in which a large number of binder particulates  51  are fixed, thereby mutually holding the optical conversion inorganic particles  40 . Herein, the fixing means that solids such as the binder particulates  51  are fixed to one another by intermolecular force. In the case where the binder particulates  51  form the fixed bodies  55  of binder particulates, the optical conversion inorganic particles  40  are held by only the fixed bodies  55  of the binder particulates or by cooperation between the fixed bodies  55  of the binder particulates and the amorphous binder  52 . 
     Note that, in the optical conversion layer  30 A, since the amorphous binder  52  is included therein, it is not always necessary for the binder particulates  51  to form the fixed bodies  55  of the binder particulates. In this case, the binder particulates  51  mutually hold the optical conversion inorganic particles  40  by cooperation between the binder particulates  51  and the amorphous binder  52 . 
     In the case where the fixed bodies  55  of the binder particulates are formed, a shape of each of the binder particulates  51  which constitute the fixed bodies  55  of the binder particulates is maintained. Therefore, air gaps are formed between the adjacent binder particulates  51  in the fixed bodies  55  of the binder particulates. In the case where the fixed bodies  55  of the binder particulates and the amorphous binder  52  cooperate with each other to mutually hold the optical conversion inorganic particles  40 , the amorphous binder  52  adheres to peripheries of the fixed bodies  55  of the binder particulates, or is impregnated into the air gaps formed between the binder particulates  51 . That is, the amorphous binder  52  adheres to the peripheries of the fixed bodies  55  of the binder particulates, or is impregnated into the air gaps formed between the binder particulates  51 , thereby mutually holding the optical conversion inorganic particles  40  in cooperation with the fixed bodies  55  of the binder particulates. 
     Meanwhile, in the case where the fixed bodies  55  of the binder particulates are not formed, the optical conversion inorganic particles  40  are mutually held mainly by the amorphous binder  52 , and the binder particulates  51  auxiliarly performs the mutual holding of the optical conversion inorganic particles  40 , which is mainly performed by the amorphous binder  52 . 
     As will be described later, an average volume concentration of the binder particulates  51  in the optical conversion layer  30 A is not uniform, and differs depending on places in the optical conversion layer  30 A. Usually, in a non-substrate-side portion  32  in the optical conversion layer  30 A, the average volume concentration of the binder particulates decreases more than in a substrate-side portion  31 . Therefore, in such a portion as the non-substrate-side portion  32 , where the average volume concentration of the binder particulates  51  is relatively low, the fixed bodies  55  of the binder particulates are not formed in some cases. For example, in the optical conversion layer  30 A, in the non-substrate-side portion  32  to be described later, which is located remote from the substrate portion  10 , the average volume concentration of the binder particulates  51  is relatively low, and accordingly, the fixed bodies  55  of the binder particulates are not usually formed. Moreover, the fixed bodies  55  of the binder particulates are usually generated by drying a mixture of the optical conversion inorganic particles  40  and the binder particulates  51  at the time of manufacturing the optical conversion layer  30 A. Therefore, the fixed bodies  55  of the binder particulates are not usually formed in the optical conversion layer  30 A manufactured without being subjected to such a step. 
     A material of the binder particulates  51  is, for example, a metal oxide or a fluorine compound. As the metal oxide, for example, used are aluminum oxide, magnesium oxide, zinc oxide, zirconium oxide, and the like. Among them, aluminum oxide, magnesium oxide and zinc oxide are preferable since thermal conductivity of each thereof is high. The zirconium oxide is preferable since a thermal expansion coefficient thereof is relatively large to suppress peeling, which is caused by a difference in thermal expansion coefficient, in the case where a metal substrate made of aluminum or the like is used. As the fluoride, for example, used are magnesium fluoride (refractive index: 1.38); calcium fluoride (refractive index: 1.399), lithium fluoride (refractive index: 1.392) and the like. Among such fluorides, magnesium fluoride is preferable since it is a stable substance, in which a reliability is high, and a refractive index is low. It is preferable that a refractive index of the binder particulates  51  be low since a ratio at which the in-plane guided light is generated in the inorganic binder portion  50 A is reduced, and the light extraction efficiency of the wavelength converter  1 A is increased, whereby it is possible to reduce the output light spot diameter. 
     The refractive index of the binder particulates  51  is preferably 1.43 or less, more preferably less than 1.40. It is preferable that the refractive index of the binder particulates  51  stay within the above-described range since the ratio at which the in-plane guided light is generated in the inorganic binder portion  50 A is reduced, and the light extraction efficiency of the wavelength converter  1 A is increased, whereby it is possible to reduce the output light spot diameter. Usually, the refractive index of the binder particulates  51  is smaller than the refractive index of the optical conversion inorganic particles  40 . When the refractive index of the binder particulates  51  is 1.43 or less, a difference thereof from the refractive index of the optical conversion inorganic particles  40  is increased more, and the ratio at which the in-plane guided light is generated in the inorganic binder portion  50 A is reduced. Therefore, it is preferable that the refractive index of the binder particulates  51  be 1.43 or less since the light extraction efficiency of the wavelength converter  1 A is increased to make it easy to reduce the output light spot diameter. 
     An average particle size of the binder particulates  51  is usually 10 to 100 nm, preferably 10 to 50 nm, more preferably 15 to 25 nm. It is preferable that the average particle size of the binder particulates  51  stay within the above-described range since fixing strength of the fixed bodies  55  of the binder particulates is high. The average particle size of the binder particulates  51  can be obtained similarly to the average particle size of the optical conversion inorganic particles  40 . 
     The binder particulates  51  can be formed into hollow particles or mesoporous particles. Herein, the mesoporous particles mean porous particles, each of which has a large number of pores with a size from 1 nm to several ten nanometers. When the binder particulates  51  are hollow particles or mesoporous particles, the effective refractive index of the binder particulates  51  becomes a value between the refractive index of the material of the binder particulates  51  and the refractive index (1.0) of the air, and decreases more than the refractive index of the material of the binder particulates  51 . Therefore, when the binder particulates  51  are hollow particles or mesoporous particles, such a wavelength converter  1 A is obtained, in which the light extraction efficiency from the binder particulates  51  and the inorganic binder portion  50 A including the same is high and a power density of the output light is high. 
     A thermal expansion coefficient of the binder particulates  51  is usually 1×10 −6 /K or more, preferably 1×10−6 to 50×10 −6 /K. It is preferable that the thermal expansion coefficient of the binder particulates  51  stay within the above-described range since it is easy to suppress peeling between the substrate portion  10  and the binder particulates  51  in the inorganic binder portion  50 . 
     [Amorphous Binder] 
     The amorphous binder  52  is one component that constitutes the inorganic binder portion  50 A, and is composed of an amorphous inorganic compound. The amorphous binder  52  mutually holds the optical conversion inorganic particles  40  by only the amorphous binder  52  or by cooperation between the amorphous binder  52  and the fixed bodies  55  of the binder particulates. 
     As will be described later, an average volume concentration of the amorphous binder  52  in the optical conversion layer  30 A is not uniform, and differs depending on places in the optical conversion layer  30 A. Usually, in the substrate-side portion  31  in the optical conversion layer  30 A, the average volume concentration of the amorphous binder  52  decreases more than in the non-substrate-side portion  32 . However, a precursor as a raw material of the amorphous binder  52  that is still ungenerated as a product has high fluidity, and accordingly, the amorphous binder  52  is impregnated into the air gaps between the binder particulates  51 . 
     The material of the amorphous binder  52  is, for example, silica glass (refractive index: 1.44 to 1.50) using, as a precursor, at least either one of polysilazane and a polysilazane derivative. It is preferable that the material of the amorphous binder  52  be silica glass using at least either one of polysilazane and a polysilazane derivative as a precursor since the fluidity of the precursor is excellent. That is, it is preferable that the material of the amorphous binder  52  be the above-described silica glass since the inorganic binder portion  50 A that is dense is obtained because the precursor excellent in fluidity is impregnated into the air gaps between the fixed bodies  55  of the binder particulates  51 . 
     Herein, the polysilazane is a polymer having a cyclic or linear Si—N skeleton structure in which one or more Si—N bonds are continuous, where all of side chains of Si and N are H. Moreover, the polysilazane derivative is a polymer having a structure in which groups other than H, for example, hydrocarbon groups are substituted for one or more side chains or terminal groups which constitute the polysilazane. Herein, as the hydrocarbon groups, for example, alkyl groups, phenyl groups and the like are mentioned. As the polysilazane, for example, perhydropolysilazane is used. Note that the precursor means a fluidic substance before the silica glass as a product is cured. 
     By a silica conversion reaction of the following Formula (1), at least either one of the polysilazane and the polysilazane derivative, which are described above, forms silica glass by silica conversion in which at least a part of the Si—N skeleton structure changes to a three-dimensional mesh structure of SiO 4  tetrahydrons. 
       [Chem. 1] 
       (−SiH 2 NH—)+2H 2 O→(—SiO 2 —)+NH 3 +2H 2   (1)
 
     Note that, in the above-described Formula (1), (—SiH 2 NH—) represents a part present in the structures of the polysilazane and the polysilazane derivative. Each of the polysilazane and the polysilazane derivative includes one or more (—SiH 2 NH—). Moreover, in the above-described Formula (1), (—SiO 2 —) represents a part present in the structure of the silica glass after the silica conversion reaction. The silica glass includes one or more (—SiO 2 −). 
     Herein, the silica glass means a substance having a three-dimensional mesh structure in which SiO 4  tetrahydrons share oxygen on vertices, in which N is substituted for a part of O of the SiO 4  tetrahydrons according to needs, and a content of N is reduced more than in the polysilazane or the polysilazane derivative, which is a precursor. Herein, tetrahydrons obtained by substitution of N for a part of O of the SiO 4  tetrahydrons are referred to as substituted tetrahydrons. The silica glass has a three-dimensional mesh structure composed of only the SiO 4  tetrahydrons, a three-dimensional mesh structure composed of the SiO 4  tetrahydrons and the substituted tetrahydrons, or a three-dimensional mesh structure composed of only the substituted tetrahydrons. 
     As described above, the optical conversion layer  30 A includes the optical conversion inorganic particles  40  and the inorganic binder portion  50 A, and the inorganic binder portion  50 A includes the binder particulates  51  and the amorphous binder  52 . That is, the optical conversion layer  30 A includes the optical conversion inorganic particles  40 , the binder particulates  51 , and the amorphous binder  52 . However, in the optical conversion layer  30 A, the average volume concentrations of the above-described substances are not uniform, and the average volume concentrations of the above-described substances differ between a substrate portion  10 -side portion of the optical conversion layer  30 A, which is close to the substrate portion  10  in a thickness direction of the optical conversion layer  30 A, and a portion of the optical conversion layer  30 A, which is remote from the substrate portion  10  in the thickness direction. A description will be given below of the average volume concentrations of the substances in the optical conversion layer  30 A and ratios of the average volume concentrations of the substances. 
     &lt;Substrate-Side Portion and Non-Substrate-Side Portion&gt; 
     First, for the sake of convenience, the optical conversion layer  30 A is classified into the portion of the optical conversion layer  30 A, which is close to the substrate portion  10 , in the thickness direction, and the portion of the optical conversion layer  30 A, which is remote from the substrate portion  10 . Specifically, in the optical conversion layer  30 A, a portion present close to the substrate portion  10  with respect to an intermediate plane  35  in the thickness direction of the optical conversion layer is defined as the substrate-side portion  31 , and a portion present remote from the substrate portion  10  with respect to the intermediate plane  35  therein is defined as the non-substrate-side portion  32 . Herein, the intermediate plane  35  is a plane defined by assuming to be composed of an aggregate of points at which such a thickness of the optical conversion layer  30 A is halved. In  FIG. 1 , the intermediate plane  35  is illustrated. Since the optical conversion layer  30 A of  FIG. 1  has a uniform thickness, the intermediate plane  35  is a plane parallel to the surface of the substrate portion  10 . Therefore, the intermediate plane  35  is illustrated as a line in  FIG. 1 . The optical conversion layer  30 A becomes equal to the sum of the substrate-side portion  31  and the non-substrate-side portion  32 . 
     &lt;Binder Particulate Concentration Ratio RF&gt; 
     In the optical conversion layer  30 A, a ratio of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40 , that is, a substrate-side binder particulate concentration ratio RF is different between the substrate-side portion  31  and the non-substrate-side portion  32 . A description will be given below of a substrate-side binder particulate concentration ratio RF S  of the substrate-side portion  31  and a non-substrate-side binder particulate concentration ratio RF O  of the non-substrate-side portion  32 . 
     First, the ratio of the average volume concentration CF S  (vol %) of the binder particulates  51  in the substrate-side portion  31  with respect to the average volume concentration CO S  (vol %) of the optical conversion inorganic particles  40  in the substrate-side portion  31  is defined as a substrate-side binder particulate concentration ratio RF S . Herein, the binder particulates  51  also include such binder particulates  51  which constitute the fixed bodies  55  of the binder particulates. Note that the average volume concentration CO S  of the above-described optical conversion inorganic particles  40  and the average volume concentration CF S  of the binder particulates  51  are average values in the substrate-side portion  31 . Therefore, it is not necessary that the substrate-side portion  31  be uniform in a volume concentration of the optical conversion inorganic particles  40  and a volume concentration of the binder particulates  51 . Specifically, the substrate-side portion  31  may include a portion in which the volume concentration of the optical conversion inorganic particles  40  is different from the average volume concentration CO S  of the optical conversion inorganic particles  40 , and a portion in which the volume concentration of the binder particulates  51  is different from the average volume concentration CF S  of the binder particulates  51 . 
     Meanwhile, the ratio of the average volume concentration CF O  (vol %) of the binder particulates  51  in the non-substrate-side portion  32  with respect to the average volume concentration CO O  (vol %) of the optical conversion inorganic particles  40  in the non-substrate-side portion  32  is defined as a non-substrate-side binder particulate concentration ratio RF O . Herein, the binder particulates  51  also include such binder particulates  51  which constitute the fixed bodies  55  of the binder particulates. Note that the average volume concentration CO O  of the above-described optical conversion inorganic particles  40  and the average volume concentration CF O  of the binder particulates  51  are average values in the non-substrate-side portion  32 . Therefore, it is not necessary that the non-substrate-side portion  32  be uniform in the volume concentration of the optical conversion inorganic particles  40  and the volume concentration of the binder particulates  51 . Specifically, the non-substrate-side portion  32  may include a portion in which the volume concentration of the optical conversion inorganic particles  40  is different from the average volume concentration CO O  of the optical conversion inorganic particles  40 , and a portion in which the volume concentration of the binder particulates  51  is different from the average volume concentration CF O  of the binder particulates  51 . 
     In the optical conversion layer  30 A, the substrate-side binder particulate concentration ratio RF S  is larger than the non-substrate-side binder particulate concentration ratio RF O . That is, in the optical conversion layer  30 A, the ratio of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40  is larger in the substrate-side portion  31  than in the non-substrate-side portion  32 . 
     Therefore, in the optical conversion layer  30 A, since a ratio of the binder particulates  51  with respect to the optical conversion inorganic particles  40  is larger in the substrate-side portion  31  than in the non-substrate-side portion  32 , the optical conversion inorganic particles  40  are firmly held by the binder particulates  51  in the substrate-side portion  31 . Moreover, since the refractive index of the binder particulates  51  is smaller than the refractive indices of the optical conversion inorganic particles  40  and the amorphous binder  52 , light smoothly travels from the binder particulates  51  toward the optical conversion inorganic particles  40 . In the optical conversion layer  30 A, since the concentration of the binder particulates  51  is higher in the substrate-side portion  31  than in the non-substrate-side portion  32 , light smoothly travels from the binder particulates  51  toward the optical conversion inorganic particles  40  in the substrate-side portion  31 , and high light extraction efficiency is achieved. 
     &lt;Amorphous Binder Concentration Ratio RA&gt; 
     Moreover, in the optical conversion layer  30 A, it is preferable that a ratio of an average volume concentration of the amorphous binder  52  with respect to the average volume concentration of the optical conversion inorganic particles  40 , that is, an amorphous binder concentration ratio RA be different between the substrate-side portion  31  and the non-substrate-side portion  32 . A description will be given below of a substrate-side amorphous binder concentration ratio RA S  of the substrate-side portion  31  and a non-substrate-side amorphous binder concentration ratio RA O  of the non-substrate-side portion  32 . 
     First, a ratio of the average volume concentration CA S  (vol %) of the amorphous binder  52  in the substrate-side portion  31  with respect to the average volume concentration CO S  (vol %) of the optical conversion inorganic particles  40  in the substrate-side portion  31  is defined as a substrate-side amorphous binder concentration ratio RA S . Note that the average volume concentration CO S  of the above-described optical conversion inorganic particles  40  and the average volume concentration CA S  of the amorphous binder  52  are average values in the substrate-side portion  31 . Therefore, it is not necessary that the substrate-side portion  31  be uniform in a volume concentration of the optical conversion inorganic particles  40  and a volume concentration of the amorphous binder  52 . Specifically, the substrate-side portion  31  may include a portion in which the volume concentration of the optical conversion inorganic particles  40  is different from the average volume concentration CO S  of the optical conversion inorganic particles  40 , and a portion in which the volume concentration of the amorphous binder  52  is different from the average volume concentration CA S  of the amorphous binder  52 . 
     Meanwhile, the ratio of the average volume concentration CA O  (vol %) of the amorphous binder  52  in the non-substrate-side portion  32  with respect to the average volume concentration CO O  (vol %) of the optical conversion inorganic particles  40  in the non-substrate-side portion  32  is defined as a non-substrate-side amorphous binder concentration ratio RA O . Note that the average volume concentration CO O  of the above-described optical conversion inorganic particles  40  and the average volume concentration CA O  of the amorphous binder  52  are average values in the non-substrate-side portion  32 . Therefore, it is not necessary that the non-substrate-side portion  32  be uniform in the volume concentration of the optical conversion inorganic particles  40  and the volume concentration of the amorphous binder  52 . Specifically, the non-substrate-side portion  32  may include a portion in which the volume concentration of the optical conversion inorganic particles  40  is different from the average volume concentration CO O  of the optical conversion inorganic particles  40 , and a portion in which the volume concentration of the amorphous binder  52  is different from the average volume concentration CA O  of the amorphous binder  52 . 
     In the optical conversion layer  30 A, it is preferable that the non-substrate-side amorphous binder concentration ratio RA O  be larger than the substrate-side amorphous binder concentration ratio RA S . That is, in the optical conversion layer  30 A, it is preferable that the ratio of the average volume concentration of the amorphous binder  52  with respect to the average volume concentration of the optical conversion inorganic particles  40  be larger in the non-substrate-side portion  32  than in the substrate-side portion  31 . 
     Therefore, in the optical conversion layer  30 A, since the ratio of the amorphous binder  52  with respect to the optical conversion inorganic particles  40  is larger in the non-substrate-side portion  32  than in the substrate-side portion  31 , excitation light is efficiently supplied to the optical conversion inorganic particles  40  via the amorphous binder  52 . That is, in the optical conversion layer  30 A, excitation light made incident from a surface of the non-substrate-side portion  32  is sufficiently supplied to the non-substrate-side portion  32  and the substrate-side portion  31 , and accordingly, light emission efficiency of the whole of the optical conversion layer  30 A increases. 
     &lt;Manufacturing Method of Wavelength Converter&gt; 
     The wavelength converter  1 A is manufactured, for example, by the following manufacturing method. 
     First, prepared are a phosphor dispersion liquid A for fabricating the substrate-side portion  31  and a phosphor dispersion liquid B for fabricating the non-substrate-side portion  32 . For example, as the phosphor dispersion liquid A, prepared is a dispersion liquid in which the optical conversion inorganic particles  40  and the binder particulates  51  are dispersed in water. Meanwhile, as the phosphor dispersion liquid B, a dispersion liquid is prepared by mixing the optical conversion inorganic particles  40 , the binder particulates  51  and a fluidic raw material of the amorphous binder  52  with one another. 
     Next, the phosphor dispersion liquid A is applied on the surface of the substrate portion  10 , and is dried at room temperature, and the fixed bodies  55  of the binder particulates  51  are formed on the surface of the substrate portion  10 . Moreover, the phosphor dispersion liquid B is applied on the surfaces of the fixed bodies  55  of the binder particulates  51 , and a part of the phosphor dispersion liquid B is impregnated into the fixed bodies  55  of the binder particulates  51 , and is thereafter dried at room temperature. When a part of the phosphor dispersion liquid B is impregnated into the fixed bodies  55  of the binder particulates  51  and is dried, the substrate-side portion  31  in which the amorphous binder  52  is impregnated into the fixed bodies  55  of the binder particulates  51  is formed. Moreover, when the phosphor dispersion liquid B applied on the surface of the substrate-side portion  31  is dried at room temperature, the non-substrate-side portion  32  is formed on the surface of the substrate-side portion  31 . 
     In accordance with the above-described manufacturing method of the wavelength converter, the wavelength converter  1 A can be manufactured at room temperature. Therefore, in accordance with the above-described manufacturing method of the wavelength converter, the wavelength converter  1  can be provided, which prevents the substrate portion  10  and the optical conversion inorganic particles  40  from being degraded by being heated and has high adhesion between the substrate portion  10  and the optical conversion layer  30 . 
     (Function of Wavelength Converter) 
     First, for the wavelength converter  1 A illustrated in  FIG. 1 , excitation light is applied to the surface of the optical conversion layer  30 A from an excitation light source (not shown). The excitation light travels in the optical conversion layer  30 A in an order of the non-substrate-side portion  32  and the substrate-side portion  31 . Herein, in the case where the non-substrate-side amorphous binder concentration ratio RA O  is larger than the substrate-side amorphous binder concentration ratio RA S , the excitation light easily travels through the inside of the non-substrate-side portion  32 . This is because the excitation light easily travels through the inside of the amorphous binder  52  when the ratio of the concentration of the amorphous binder  52  with respect to the optical conversion inorganic particles  40  is large in the non-substrate-side portion  32  and small in the substrate-side portion  31 . 
     The excitation light that travels through the inside of the optical conversion layer  30 A in an order of the non-substrate-side portion  32  and the substrate-side portion  31  travels in the binder particulates  51  with an increased concentration in the substrate-side portion  31 . Generally, the refractive indices of the amorphous binder  52 , the binder particulates  51  and the optical conversion inorganic particles  40  increase in an ascending order. Therefore, the excitation light smoothly travels in an order of the amorphous binder  52 , the binder particulates  51  and the optical conversion inorganic particles  40  or in an order of the amorphous binder  52  and the optical conversion inorganic particles  40 . 
     When the excitation light is applied to the optical conversion inorganic particles  40 , the optical conversion inorganic particles  40  radiate fluorescence. The radiated fluorescence travels as it is through the inside of the optical conversion layer  30 A, or travels through the inside of the optical conversion layer  30 A after being reflected by the surface of the substrate portion  10 , and is radiated as emitted light from the surface of the optical conversion layer  30 A. 
     (Effect of Wavelength Converter) 
     In accordance with the wavelength converter  1 A, the wavelength converter is obtained, which prevents the substrate portion  10  and the optical conversion inorganic particles  40  from being degraded by being heated and has high adhesion between the substrate portion  10  and the optical conversion layer  30 A. Moreover, the substrate-side binder particulate concentration ratio RF S  is larger than the non-substrate-side binder particulate concentration ratio RF O , and accordingly, has high adhesion to the substrate. 
     Further, the wavelength converter  1 A can be manufactured at room temperature, and accordingly, the wavelength converter  1 A can be provided, which prevents the substrate portion  10  and the optical conversion inorganic particles  40  from being degraded by being heated and has high adhesion between the substrate portion  10  and the optical conversion layer  30 . 
     EXAMPLES 
     Hereinafter, this embodiment will be described more in detail by examples; However, this embodiment is not limited to these examples. 
     Example 1 
     (Preparation of Phosphor Dispersion Liquid P1) 
     Powder of magnesium fluoride nanoparticles (refractive index: 1.38) with an average particle size of 40 nm, which was produced by a gas phase method, was mixed with ion exchange water, and was dispersed by an ultrasonic wave, whereby 15 mass % of a magnesium fluoride dispersion liquid D1 was obtained. Moreover, YAG particles with an average particle size of 20 μm were prepared as phosphor (optical conversion inorganic particles). The magnesium fluoride dispersion liquid D1 and the YAG particles were mixed with each other in a weight ratio of 1:2, and a phosphor dispersion liquid P1 was prepared. 
     (Preparation of Phosphor Dispersion Liquid P2) 
     As the polysilazane that is the raw material of the amorphous binder, Perhydropolysilazane Aquamica (registered trademark) NL120A made by Merck Performance Materials Ltd. was used. The polysilazane, the above-described YAG particles and the magnesium fluoride dispersion liquid D1 were mixed with one another in a weight ratio of 1:2:0.01, and a phosphor dispersion liquid P2 was prepared. 
     (Fabrication of Wavelength Converter) 
     The phosphor dispersion liquid P1 was applied on a surface of a substrate portion composed of an aluminum plate by using an applicator equipped with a bar coater, and was dried by being left standing at 25° C. After the drying, a dry coating film M1 with a thickness of 50 μm was formed on the surface of the substrate portion. It was found that, as illustrated in  FIG. 1 , the dry coating film M1 has a structure in which the fixed bodies  55  of the magnesium fluoride nanoparticles which are the binder particulates  51  mutually hold the YAG particles  40  which are the optical conversion inorganic particles. 
     Next, the phosphor dispersion liquid P2 was applied on the surface of the dry coating film M1 on the substrate portion by using the above-described applicator, and was dried by being left standing at 25° C. After the drying, a dry coating film M2 with a thickness of 50 μm was formed on the surface of the dry coating film M1. 
     Thus, a wavelength converter including the substrate portion composed of an aluminum plate and an optical conversion layer was obtained. In the optical conversion layer, a portion with a thickness of 50 μm, which is derived from the dry coating film M1, and a portion with a thickness of 50 μm, which is formed by the application of the phosphor dispersion liquid P2, was laminated on the substrate portion in this order. 
     When a structure of the wavelength converter according to Example 1 was investigated, it was found that this wavelength converter has a similar structure to that of the wavelength converter  1 A illustrated in  FIG. 1 . Therefore, in  FIG. 1 , the wavelength converter according to Example 1 is illustrated as the wavelength converter  1 A, and the optical conversion layer is illustrated as the optical conversion layer  30 A. 
     Note that the optical conversion layer  30 A of the wavelength converter  1 A according to Example 1 includes the portion derived from the dry coating film M1 and the portion formed by the application of the phosphor dispersion liquid P2, both of which have the same thickness. Therefore, since the intermediate plane  35  is located on the boundary between these portions, the portion derived from the dry coating film M1 corresponds to the substrate-side portion  31 A, and the portion formed by the application of the phosphor dispersion liquid P2 corresponds to the non-substrate-side portion  32 . 
     It was found that, in the substrate-side portion  31 A in the optical conversion layer  30 A of Example 1, the polysilazane impregnated into the air gaps of the fixed bodies  55  of the magnesium fluoride nanoparticles  51 , which mutually hold the YAG particles  40 , is cured and becomes the silica glass  52  as the amorphous binder. Moreover, it was found that, in the substrate-side portion  31 A, the YAG particles  40  are mutually held by the fixed bodies  55  of the magnesium fluoride nanoparticles  51  and the silica glass  52 . 
     Meanwhile, it was found that, in the non-substrate-side portion  32 A in the optical conversion layer  30 A of Example 1, the YAG particles  40  and the magnesium fluoride nanoparticles  51  are dispersed in the silica glass  52  as the amorphous binder composed by the curing of the polysilazane. Moreover, it was found that, in the non-substrate-side portion  32 A, the YAG particles  40  are mutually held by the silica glass  52  mainly. 
     (Comparison Between Substrate-Side Binder Particulate Concentration Ratio RF S  and Non-Substrate-Side Binder Particulate Concentration Ratio RF O ) 
     Examined are the substrate-side binder particulate concentration ratio RF S  and the non-substrate-side binder particulate concentration ratio RF O  in the optical conversion layer  30 A of Example 1. 
     The binder particulate concentration ratio RF is a ratio of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40 . 
     Moreover, the substrate-side binder particulate concentration ratio RF S  is a ratio of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40  in the substrate-side portion  31 A. 
     Furthermore, the non-substrate-side binder particulate concentration ratio RF O  is a ratio of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40  in the non-substrate-side portion  32 A. 
     As seen from  FIG. 1  and the above-described test conditions, with regard to the ratio (binder particulate concentration ratio RF) of the average volume concentration of the binder particulates  51  with respect to the average volume concentration of the optical conversion inorganic particles  40 , the value thereof in the substrate-side portion  31 A is larger than the value thereof in the non-substrate-side portion  32 A. 
     As described above, in Example 1, the substrate-side binder particulate concentration ratio RF S  that is the binder particulate concentration ratio RF of the substrate-side portion  31 A became larger than the non-substrate-side binder particulate concentration ratio RF O  that is the binder particulate concentration ratio RF of the non-substrate-side portion  32 A. 
     Comparative Example 1 
     (Preparation of Phosphor Dispersion Liquid P1) 
     A phosphor dispersion liquid P1 was prepared in a similar way to Example 1. 
     (Fabrication of Wavelength Converter) 
     The phosphor dispersion liquid P1 was applied on a surface of a substrate portion composed of an aluminum plate by using an applicator equipped with a bar coater, and was dried by being left standing at 25° C. After the drying, a dry coating film M2 with a thickness of 100 μm was formed on the surface of the substrate portion. 
     Thus, a wavelength converter  100 A including the substrate portion composed of an aluminum plate and an optical conversion layer composed of the dry coating film M2 with a thickness of 100 μm was obtained. 
     When a structure of the wavelength converter  100 A according to Comparative example 1 was investigated, it was found that the optical conversion layer composed of the dry coating film M2 has a structure in which the fixed bodies  55  of the magnesium fluoride nanoparticles  51  mutually hold the YAG particles  40 . 
     Results are illustrated in  FIG. 2 .  FIG. 2  is a schematic cross-sectional view of the wavelength converter  100 A according to Comparative example 1. 
     As illustrated in  FIG. 2 , the wavelength converter  100 A ( 100 ) according to Comparative example 1 included the substrate portion  10  composed of an aluminum plate, and an optical conversion layer  130 A ( 130 ) formed on the substrate portion  10 . The optical conversion layer  130 A included the YAG particles  40  as the optical conversion inorganic particles, and an inorganic binder portion  150 A that mutually held the YAG particles  40 . It was found that the inorganic binder portion  150 A forms the fixed bodies  55  of a large number of the magnesium fluoride nanoparticles  51 , and that the optical conversion inorganic particles  40  are mutually held by the fixed bodies  55  of the magnesium fluoride nanoparticles  51 . 
     Note that, in  FIG. 2 , for the sake of convenience, in the optical conversion layer  130 A, a portion present close to the substrate portion  10  with respect to an intermediate plane  35  in a thickness direction of the optical conversion layer  130 A is displayed as a substrate-side portion  131 A, and a portion present remote from the substrate portion  10  with respect to the intermediate plane  35  therein is displayed as a non-substrate-side portion  132 A. 
     However, in the optical conversion layer  130 A of the wavelength converter  100 A according to Comparative example 1, in the thickness direction thereof, concentration gradients of the optical conversion inorganic particles  40  and the magnesium fluoride nanoparticles  51  are not substantially present. Therefore, in the optical conversion layer  130 A illustrated in  FIG. 2 , the substrate-side portion  131 A and the non-substrate-side portion  132 A have substantially the same structure. Hence, in the wavelength converter  100 A according to Comparative example 1, the substrate-side binder particulate concentration ratio RF S  and the non-substrate-side binder particulate concentration ratio RF O  are the same. 
     Comparative Example 2 
     (Preparation of Phosphor Dispersion Liquid P2) 
     A phosphor dispersion liquid P2 was prepared in a similar way to Example 1. 
     (Fabrication of Wavelength Converter) 
     The phosphor dispersion liquid P2 was applied on a surface of a substrate portion composed of an aluminum plate by using an applicator equipped with a bar coater, and was dried by being left standing at 25° C. After the drying, a dry coating film M3 with a thickness of 100 μm was formed on the surface of the substrate portion. 
     Thus, a wavelength converter  100 A including the substrate portion composed of an aluminum plate and an optical conversion layer composed of the dry coating film M3 with a thickness of 100 μm was obtained. 
     When a structure of the wavelength converter  100 B according to Comparative example 2 was investigated, it was found that, in the optical conversion layer composed of the dry coating film M3, the YAG particles  40  and the magnesium fluoride nanoparticles  51  are dispersed in the silica glass  52  as the amorphous binder composed by the curing of the polysilazane. 
     Results are illustrated in  FIG. 3 .  FIG. 3  is a schematic cross-sectional view of the wavelength converter  100 B according to Comparative example 2. 
     As illustrated in  FIG. 3 , the wavelength converter  100 B ( 100 ) according to Comparative example 2 included a substrate portion  10  composed of an aluminum plate, and an optical conversion layer  130 B ( 130 ) formed on the substrate portion  10 . The optical conversion layer  130 B included the YAG particles  40  as the optical conversion inorganic particles, and an inorganic binder portion  150 B that mutually held the YAG particles  40 . It was found that the inorganic binder portion  150 B is one in which the magnesium fluoride nanoparticles  51  are dispersed in the silica glass  52  as the amorphous binder composed by the curing of the polysilazane. 
     Note that, in  FIG. 3 , for the sake of convenience, in the optical conversion layer  130 B, a portion present close to the substrate portion  10  with respect to an intermediate plane  35  in a thickness direction of the optical conversion layer  130 B is displayed as a substrate-side portion  131 B, and a portion present remote from the substrate portion  10  with respect to the intermediate plane  35  therein is displayed as a non-substrate-side portion  132 B. 
     However, in the optical conversion layer  130 B of the wavelength converter  100 B according to Comparative example 2, in the thickness direction thereof, concentration gradients of the optical conversion inorganic particles  40 , the magnesium fluoride nanoparticles  51  and the silica glass  52  are not substantially present. Therefore, in the optical conversion layer  130 B illustrated in  FIG. 3 , the substrate-side portion  131 B and the non-substrate-side portion  32 B have substantially the same structure. Hence, in the wavelength converter  100 A according to Comparative example 2, the substrate-side binder particulate concentration ratio RF S  and the non-substrate-side binder particulate concentration ratio RF O  are the same. 
     (Evaluation of Film Adhesion Strength of Wavelength Converter) 
     For each of the wavelength converters according to Example 1 and Comparative examples 1 and 2, film adhesion strength between the substrate portion  10  and the optical conversion layer was measured. 
     Specifically, Cellotape (registered trademark) made by Nichiban Co., Ltd. was pasted onto the surface of the optical conversion layer of the wavelength converter, and thereafter, was peeled off with a fixed force, whereby a degree of collapse of the optical conversion layer in a portion from which the Cellotape was peeled was visually confirmed. 
     As a result, in Comparative example 1, observed was a state in which a surface layer was peeled in a part of the surface of the optical conversion layer  130 A, specifically, a 30% or more portion in an area of the surface of the optical conversion layer  130 A. This is presumed to be because, at the time of peeling the Cellotape, there collapsed such a surface layer portion of the fixed bodies  55  of the magnesium fluoride nanoparticles  51  in the optical conversion layer  130 A, and the magnesium fluoride nanoparticles  51  were peeled. 
     Specifically, the above-described peeling is presumed to have been caused because the fixing strength of the fixed bodies  55  of the magnesium fluoride nanoparticles  51  in the inside of the optical conversion layer  130 A is lower than the adhesin strength between the substrate portion  10  and the optical conversion layer  130 A. 
     Moreover, in Comparative example 2, observed was a state in which an entirety (800 or more of an area of the surface of the optical conversion layer  130 B) of the optical conversion layer  130 B was peeled from the substrate portion  10 . This is presumed to be because, since a bonding force between the substrate portion  10  and the optical conversion layer  130 B was low, the entirety of the optical conversion layer  130 B was peeled at the time of peeling the Cellotape. 
     Specifically, the above-described peeling is presumed to have been caused because the bonding strength between the substrate portion  10  and the optical conversion layer  130 B was lower than a bonding strength by the silica glass  52  and the like in the inside of the optical conversion layer  130 B. The reason why the bonding strength by the silica glass  52  and the like in the inside of the optical conversion layer  130 B is high is presumed to be that a composition of the optical conversion layer  130 B is substantially uniform in the optical conversion layer  130 B. 
     In contrast, in Example 1, observed was a state in which, peeled from the substrate portion  10  was a part of the optical conversion layer  30 A, specifically, 5% or less portion in the area of the surface of the optical conversion layer  130 A. However, such a peeled area was significantly small in comparison with Comparative examples 1 and 2. 
     The entire contents of Japanese Patent Application No. 2017-221095 (filed on: Nov. 16, 2017) are incorporated herein by reference. 
     Although the contents of this embodiment have been described above in accordance with the examples, it is obvious to those skilled in the art that this embodiment is not limited to the description of these and that various modifications and improvements are possible. 
     INDUSTRIAL APPLICABILITY 
     In accordance with the present disclosure, the wavelength converter can be provided, which prevents the substrate portion and the optical conversion inorganic particles from being degraded by being heated and has high adhesion between the substrate portion and the optical conversion layer. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1 A,  100 ,  100 A,  100 B,  100 C WAVELENGTH CONVERTER 
           10  SUBSTRATE PORTION 
           30 ,  30 A,  130 ,  130 A,  130 B OPTICAL CONVERSION LAYER 
           130 C WAVELENGTH CONVERSION QUARTZ GLASS LAYER 
           31 ,  131 A,  131 B,  131 C SUBSTRATE-SIDE PORTION 
           32 ,  132 A,  132 B,  132 C NON-SUBSTRATE-SIDE PORTION 
           35  INTERMEDIATE PLANE 
           40  OPTICAL CONVERSION INORGANIC PARTICLE (PHOSPHOR PARTICLE, YAG PARTICLE) 
           50 ,  50 A,  150 ,  150 A,  150 B,  150 C INORGANIC BINDER PORTION 
           51  BINDER PARTICULATE (MAGNESIUM FLUORIDE NANOPARTICLE) 
           52  AMORPHOUS BINDER (QUARTZ GLASS LAYER, SILICA GLASS) 
           55  FIXED BODY OF BINDER PARTICULATES (FIXED BODY OF MAGNESIUM FLUORIDE NANOPARTICLES)