Wavelength conversion member including phosphor that converts light from semiconductor light-emitting element into longer-wavelength light

A wavelength conversion member, comprises: a substrate; a first wavelength conversion layer on the substrate, the first wavelength conversion layer containing a first phosphor and a first matrix; and a second wavelength conversion layer containing a second phosphor, first inorganic particles, and a second matrix. The first phosphor and the second phosphor convert at least part of the excitation light incident on the second main surface into first light having longer wavelengths than the excitation light. The first light is emitted from the second main surface of the second wavelength conversion layer. A volume Vp1 of the first phosphor, a volume Vw1 of the first wavelength conversion layer, a volume Vp2 of the second phosphor, and a volume Vw2 of the second wavelength conversion layer satisfy Vp1/Vw1>Vp2/Vw2.

BACKGROUND

1. Technical Field

The present disclosure relates to a wavelength conversion member including a phosphor that converts light from a semiconductor light-emitting element into longer-wavelength light, a light source including the wavelength conversion member, and a vehicle headlamp including the light source.

2. Description of the Related Art

Hitherto, a vehicle lighting device that generates light has been reported, the vehicle lighting device including a semiconductor light-emitting element that generates light, a phosphor spaced apart from the semiconductor light-emitting element, a first optical member that collects light generated by the semiconductor light-emitting element into a phosphor, and a second optical member that has an optical center at a position where the phosphor is provided and that sends light generated by the phosphor in response to light collected by the first optical member to the outside of the lighting device (see Japanese Unexamined Patent Application Publication No. 2012-99280).

Japanese Unexamined Patent Application Publication No. 2012-99280 discloses that the luminous efficiency is improved by allowing a spot area of excitation light on a surface of the phosphor, the excitation light being emitted from the semiconductor light-emitting element, to be smaller than the area of the surface of the phosphor.

SUMMARY

One non-limiting and exemplary embodiment provides a wavelength conversion member that inhibits a reduction in luminous efficiency.

In one general aspect, the techniques disclosed here feature a wavelength conversion member including a substrate, a first wavelength conversion layer, and a second wavelength conversion layer. The first wavelength conversion layer is located on the substrate and contains a first phosphor and a first matrix. The second wavelength conversion layer has a first main surface facing the first wavelength conversion layer and a second main surface on which excitation light from a semiconductor light-emitting element is incident, and contains a second phosphor, first inorganic particles, and a second matrix. The first phosphor and the second phosphor convert the excitation light incident on the second main surface into first light having longer wavelengths than the excitation light. The first light emits from the second main surface of the second wavelength conversion layer. A volume Vp1 of the first phosphor, a volume Vw1 of the first wavelength conversion layer, a volume Vp2 of the second phosphor, and a volume Vw2 of the second wavelength conversion layer satisfy Vp1/Vw1>Vp2/Vw2.

The use of the wavelength conversion member disclosed here inhibits the reduction in luminous efficiency.

DETAILED DESCRIPTION

In the related art, there has been a demand for a method for improving luminous efficiency, improving contrast between a light-emitting portion and a non-light-emitting portion, and easily adjusting an emission color at the time of manufacture in order to meet laws and regulations governing lighting devices for vehicles. The inventors have conducted intensive studies in order to provide a method for improving contrast between a light-emitting portion and a non-light-emitting portion and easily achieving a stable emission color without reducing luminous efficiency. According to the present disclosure, main wavelength conversion from excitation light to fluorescence or phosphorescence occurs in the first wavelength conversion layer. Thus, heat generated during the wavelength conversion can be efficiently dissipated toward a substrate to inhibit a reduction in luminous efficiency. Furthermore, it is possible to improve the contrast between the light-emitting portion and the non-light-emitting portion and easily adjust the emission color by the arrangement of a reflective layer around a second wavelength conversion layer and by the control of the thickness of the second wavelength conversion layer.

A wavelength conversion member according to an embodiment of the present disclosure includes a substrate, a first wavelength conversion layer, and a second wavelength conversion layer. The first wavelength conversion layer is located on the substrate and contains a first phosphor and a first matrix. The second wavelength conversion layer has a first main surface facing the first wavelength conversion layer and a second main surface on which excitation light from a semiconductor light-emitting element is incident. The second wavelength conversion layer contains a second phosphor, first inorganic particles, and a second matrix. The first phosphor and the second phosphor convert the excitation light incident from the second main surface into first light having longer wavelengths than the excitation light. The first light is emitted from the second main surface of the second wavelength conversion layer. A volume Vp1 of the first phosphor, a volume Vw1 of the first wavelength conversion layer, a volume Vp2 of the second phosphor, and a volume Vw2 of the second wavelength conversion layer satisfy Vp1/Vw1>Vp2/Vw2. The first light may be fluorescence or phosphorescence. The first phosphor may include phosphor particles. The second phosphor may also include phosphor particles. In the case that the first phosphor consists of first phosphor particles and the second phosphors consists of second phosphor particles, Vp1 and Vp2 are calculated as follows: Vp1 is the sum of the volumes of all first phosphor particles contained in the first wavelength conversion layer; and Vp2 is the sum of the volumes of all second phosphor particles contained in the second wavelength conversion layer.

The wavelength conversion member may further include a reflective layer that is located on at least one side face of the second wavelength conversion layer and that contains second inorganic particles and a third matrix. The volume Vp1 of the first phosphor may be equal to or more than 30% and less than or equal to 80% of the volume Vw1of the first wavelength conversion layer. The volume Vp2 of the second phosphor may be equal to or more than 0.1% and less than or equal to 20% of the volume Vw2 of the second wavelength conversion layer. The first matrix may have a higher thermal conductivity than the second matrix. The first matrix may be zinc oxide. The second matrix may be a material having a siloxane bond.

A light source according to an embodiment of the present disclosure includes the semiconductor light-emitting element that emits the excitation light having a peak wavelength of 420 nm or more and 470 nm or less and the wavelength conversion member. A vehicle headlamp according to an embodiment of the present disclosure includes the light source and an exit optical system that guides light from the light source to the outside.

First Embodiment

FIG. 1schematically illustrates a structure of a light source10according to a first embodiment of the present disclosure. The light source10at least includes a semiconductor light-emitting element11and a wavelength conversion member20. The semiconductor light-emitting element11emits excitation light31. The wavelength conversion member20receives the excitation light31and emits first light32having longer wavelengths than the excitation light31and diffusively reflected light33of the excitation light31. The first light32is fluorescence or phosphorescence. The semiconductor light-emitting element11may be formed of a light-emitting diode (LED), a superluminescent diode (SLD), a laser diode (LD), or the like. For example, the semiconductor light-emitting element11in this embodiment is an LD. The semiconductor light-emitting element11may be formed of a single LD or a plurality of LD's optically coupled. The semiconductor light-emitting element11emits, for example, blue light. The blue light used in the present disclosure indicates light having a peak wavelength of 420 nm or more and 470 nm or less. The light source10further includes an entrance optical system12that may be located in an optical path of the excitation light31emitted from the semiconductor light-emitting element11. The entrance optical system12includes, for example, a lens, a mirror, an optical fiber, or any combination thereof.

As described above, the wavelength conversion member20includes a surface on which excitation light from the semiconductor light-emitting element11is incident, and emits the first light32having longer wavelengths than the excitation light31from the surface and the diffusively reflected light33having the same wavelength as the excitation light31that has not been subjected to wavelength conversion. The wavelength conversion member20includes a first wavelength conversion layer201located on at least a substrate21and a second wavelength conversion layer202on the first wavelength conversion layer201. The wavelength conversion member20further includes a reflective layer203located on the first wavelength conversion layer201and around the second wavelength conversion layer202.

The first wavelength conversion layer201includes a first main surface201aand a second main surface201b.The first main surface201afaces the substrate21. The second main surface201bis located opposite the first main surface201a.The first wavelength conversion layer201at least contains a first phosphor25and a first matrix22. The first phosphor25has a main function of partially converting the excitation light31into the first light32. The material composition of the first phosphor25is not particularly limited. For example, the first phosphor25in this embodiment is an (Y1-yGdy)3(Al1-zGaz)5O12:Ce3+[0≤y<1, z<1] phosphor. The first matrix22may be a transparent inorganic material, for example, glass, a silicone resin, an organic-inorganic hybrid material, Al2O3, or ZnO. For example, the first matrix22in this embodiment is ZnO.

In the case where ZnO is used as the first matrix22, the first phosphor25may be sealed by, for example, the following method: (1) ZnO seed crystals are formed on the substrate21by vacuum deposition, and a layer consisting of only the first phosphor25is formed by, for example, a casting method on the ZnO seed crystals; (2) the substrate21with the seed crystals and the layer consisting of the first phosphor25is immersed in, for example, an aqueous zinc nitrate (Zn(NO3)2solution containing hexamethylenetetramine (C6H12N4); and (3) the temperature of the aqueous solution is set to 80° C. Thereby, ZnO is formed by liquid-phase deposition so as to surround the first phosphor25. The ZnO serving as the first matrix22formed by this method has a thermal conductivity of 10 W/m·K.

The volume ratio of the first phosphor25in the first wavelength conversion layer201is preferably 30% or more and 80% or less. Note that, the “volume ratio of X in Y” in the present disclosure means the percentage of the volume of X, as compared to the volume of Y, which is taken as 100%. For example, the volume ratio of the first phosphor25in the first wavelength conversion layer201is 60% in this embodiment. When the volume ratio of the first phosphor25in the first wavelength conversion layer201is 30% or more, the contact area among particles of the first phosphor25is large, so that thermal conduction between the particles occurs easily. Thus, heat generated during the wavelength conversion is sufficiently dissipated to the substrate21to inhibit an increase in the temperature of the wavelength conversion member20, thereby inhibiting a reduction in luminous efficiency. When the volume ratio of the first phosphor25in the first wavelength conversion layer201is 80% or less, the breaking or the like of the particles of the first phosphor25is inhibited. Thus, a reduction in luminous efficiency due to defects caused by the break is inhibited.

The phosphor is uniformly present in the wavelength conversion layer without regularity. Thus, the volume ratio of the phosphor in the wavelength conversion layer is substantially the same as the proportion of the cross-sectional area of the phosphor with respect to the cross-sectional area of the entire wavelength conversion layer in a cross section of the wavelength conversion layer. Thus, the proportion of the cross-sectional area is used for a method of measuring the volume ratio of a phosphor in a wavelength conversion layer. For example, a cross section of the wavelength conversion layer obtained by cross-section polisher processing (hereinafter, referred to as “CP processing”) is observed with a scanning electron microscope (hereinafter, referred to as a “SEM”) to calculate the proportion of the cross-sectional area of the phosphor with respect to the cross-sectional area of the entire wavelength conversion layer. The resulting proportion of the cross-sectional area is defined as the volume ratio of the phosphor in the wavelength conversion layer.

The first wavelength conversion layer201may contain inorganic particles, for example, SiO2, Al2O3, or MgO, in a proportion lower than the volume ratio of the first phosphor25, in addition to the first phosphor25and the first matrix22. In this case, the scattering of the excitation light31and the first light32can be controlled.

The second wavelength conversion layer202includes a first main surface202aand a second main surface202b.The first main surface202afaces the second main surface201bof the first wavelength conversion layer201. The second main surface202bis located opposite the first main surface202a.The second main surface202bserves as a surface of the wavelength conversion member20on which the excitation light31is incident and also serves as a surface that emits the first light32and the diffusively reflected light33. The first main surface202aof the second wavelength conversion layer202lies on the side of the substrate21. The second main surface202bof the second wavelength conversion layer202lies on the opposite side of the substrate21. The second wavelength conversion layer202at least includes a second phosphor26, first inorganic particles27, and a second matrix23. The second phosphor26partially converts the excitation light31into the32. The first inorganic particles27have the function of generating the diffusively reflected light33having the same wavelength as the excitation light31.

The material composition of the second phosphor26is not particularly limited and may be the same as that of the first phosphor25. For example, the second phosphor26in this embodiment is an (Y1-yGdy)3(Al1-zGaz)5O12:Ce3+[0≤y<1, 0≤z<1] phosphor. The material of the first inorganic particles27is preferably a transparent material that exhibits lower absorption of light in the visible region. Examples of the material of the first inorganic particles27may include SiO2, Al2O3, MgO, ZnO, TiO2, ZrO, Ta2O5, Nb2O5, BN, AlN, and BaSO4. For example, the material of the first inorganic particles27in this embodiment is SiO2. Examples of the second matrix23may include transparent inorganic materials, such as glass, silicone resins, organic-inorganic hybrid materials, Al2O3, and ZnO. For example, the second matrix23in this embodiment is a polysilsesquioxane, which is an organic-inorganic hybrid material having a siloxane bond.

When the polysilsesquioxane is used as the second matrix23, the second phosphor26and the first inorganic particles27may be sealed by, for example, the following method: (1) the polysilsesquioxane that has been dissolved in a benzyl alcohol solvent, the second phosphor26, and the first inorganic particles27are mixed together into a paste-like mixture; and (2) the paste-like mixture is applied by, for example, a screen printing method to the first wavelength conversion layer201and thermally cured. Thereby, the second matrix23including the second phosphor26and the first inorganic particles27sealed therein is formed. The polysilsesquioxane serving as the second matrix23formed by this method has a thermal conductivity of 1 W/m·K.

The volume ratio of the second phosphor26in the second wavelength conversion layer202is preferably lower than the volume ratio of the first phosphor25in the first wavelength conversion layer201. More preferably, the volume ratio of the second phosphor26in the second wavelength conversion layer202is 0.1% or more and 20% or less. The volume ratio of the second phosphor26in the second wavelength conversion layer202may be, for example, 1%.

The first wavelength conversion layer201converts the excitation light31into fluorescence or phosphorescence having longer wavelengths than the excitation light31and emits the fluorescence or phosphorescence. The second wavelength conversion layer202converts the excitation light31into fluorescence or phosphorescence having longer wavelengths than the excitation light31and emits the fluorescence or phosphorescence. The first light32emitted from the wavelength conversion member20includes the fluorescence or phosphorescence from the first wavelength conversion layer201and the fluorescence or phosphorescence from the second wavelength conversion layer202. The second wavelength conversion layer202also emits the diffusively reflected light33having the same wavelength as the excitation light31. It is thus possible to adjust the color of light emitted from the wavelength conversion member20by changing the thickness of the second wavelength conversion layer202to change the mixing ratio of the first light32to the diffusively reflected light33.

In the case where the volume ratio of the second phosphor26in the second wavelength conversion layer202is higher than the volume ratio of the first phosphor25in the first wavelength conversion layer201, the amount of fluorescence or phosphorescence emitted from the second phosphor26varies significantly depending on the thickness of the second wavelength conversion layer202because the excitation light31is first incident on the second wavelength conversion layer202. Thus, the color of light emitted from the wavelength conversion member20varies easily depending on a slight change in the thickness of the second wavelength conversion layer202. The volume ratio of the first inorganic particles27in the second wavelength conversion layer202is not particularly limited and may be adjusted in response to the color of light obtained by combining the first light32with the diffusively reflected light33originating from the excitation light31.

The reflective layer203at least contains second inorganic particles28and a third matrix24and is located at least one side face202clocated between the first main surface202aand the second main surface202bof the second wavelength conversion layer202. The reflective layer203in this embodiment is in contact with the side face202cof the second wavelength conversion layer202and is located on the second main surface201bof the first wavelength conversion layer201. The second wavelength conversion layer202may have a circular shape, an elliptical shape, a triangular shape, a rectangular shape, or a polygonal shape in plan view (that is, when viewed from a direction perpendicular to the second main surface202b). When the second wavelength conversion layer202has a circular shape or an elliptic shape in plan view, the second wavelength conversion layer202has a single side face. When the second wavelength conversion layer202has a triangular shape, a rectangular shape, or a polygonal shape, the second wavelength conversion layer202has a plurality of side faces.

The reflective layer203has the function of suppressing the emission of the first light32from a surface other than the surface of the second wavelength conversion layer202on which the excitation light31is incident to improve the contrast between the light-emitting portion and the non-light-emitting portion. The term “contrast” used here indicates the ratio of the luminance of the second wavelength conversion layer202serving as a light-emitting portion to the luminance of the reflective layer203serving as a non-light-emitting portion (the luminance of the light-emitting portion/the luminance of the non-light-emitting portion).

The material of the second inorganic particles28is preferably a transparent material that exhibits lower absorption of light in the visible region. Examples of the material of the second inorganic particles28may include SiO2, Al2O3, MgO, ZnO, TiO2, ZrO, Ta2O5, Nb2O5, BN, AlN, and BaSO4. For example, the material of the second inorganic particles28in this embodiment is TiO2. Examples of the third matrix24may include transparent inorganic materials, such as glass, silicone resins, organic-inorganic hybrid materials, Al2O3, and ZnO. For example, the third matrix24in this embodiment is a silicone resin having a siloxane bond.

When polysilsesquioxane is used as the third matrix24, the second inorganic particles28may be sealed by, for example, the following method: (1) the polysilsesquioxane that has been dissolved in a benzyl alcohol solvent and the second inorganic particles28are mixed together into a paste-like mixture; and (2) the paste-like mixture is applied by, for example, a screen printing method to the first wavelength conversion layer201and thermally cured. Thereby, the third matrix24including the second inorganic particles28sealed therein is formed.

The volume ratio of the first phosphor25in the first wavelength conversion layer201is higher than the volume ratio of the second phosphor26in the second wavelength conversion layer202in the first embodiment of the present disclosure as described above. Thus, a main source of heat generated in the wavelength conversion member20during wavelength conversion is the first wavelength conversion layer201. The contact area among particles of the first phosphor25in the first wavelength conversion layer201is larger than the contact area among particles of the second phosphor26in the second wavelength conversion layer202. In addition, the first wavelength conversion layer201is in contact with the substrate21. This enables efficient dissipation of heat generated during wavelength conversion, thereby suppressing a reduction in luminous efficiency. The volume ratio of the second phosphor26in the second wavelength conversion layer202is lower than the volume ratio of the first phosphor25in the first wavelength conversion layer201. In addition, the second wavelength conversion layer202contains the first inorganic particles27. The ratio of the first light32to the diffusively reflected light33originating from the excitation light31varies depending on the thickness of the second wavelength conversion layer202. This enables control of the color of light emitted. The reflective layer203suppresses the emission of the first light32from a surface other than the surface of the second wavelength conversion layer202on which the excitation light31is incident, thereby improving the contrast between the light-emitting portion and the non-light-emitting portion.

Second Embodiment

FIG. 2schematically illustrates a structure of a light source40according to a second embodiment. The light source40at least includes a semiconductor light-emitting element41and a wavelength conversion member50. The semiconductor light-emitting element41emits excitation light61. The wavelength conversion member50receives the excitation light61and emits first light62having longer wavelengths than the excitation light61. The first light62is fluorescence or phosphorescence. The semiconductor light-emitting element41may be formed of an LED, a superluminescent diode (SLD), a laser diode (LD), or the like. For example, the semiconductor light-emitting element41in this embodiment is an LD. The semiconductor light-emitting element41may be formed of a single LD or a plurality of LD's optically coupled. The semiconductor light-emitting element41emits, for example, blue light. As with the first embodiment, the light source40further includes an entrance optical system42that may be located in an optical path of the excitation light61emitted from the semiconductor light-emitting element41. The entrance optical system42includes, for example, a lens, a mirror, an optical fiber, or any combination thereof.

The wavelength conversion member50includes a surface on which excitation light from the semiconductor light-emitting element41is incident, and emits the first light62having longer wavelengths than the excitation light61from the surface and diffusively reflected light63having the same wavelength as the excitation light61that has not been subjected to wavelength conversion. The wavelength conversion member50includes a first wavelength conversion layer501located on at least a substrate51and a second wavelength conversion layer502on the first wavelength conversion layer501. The wavelength conversion member50further includes a reflective layer503located on a side face of the first wavelength conversion layer501and a side face of the second wavelength conversion layer502.

The first wavelength conversion layer501includes a first main surface501a,a second main surface501b,and at least one side face501clocated between the first main surface501aand the second main surface501b.The first main surface501afaces the substrate51. The second main surface501bis located opposite the first main surface501a.The first wavelength conversion layer501at least contains a first phosphor55and a first matrix52. The first wavelength conversion layer501has a main function of converting the excitation light61into the first light62. The material composition of the first phosphor55is not particularly limited. For example, the first phosphor55in this embodiment is an (Y1-yGdy)3(Al1-zGaz)5O12:Ce3+[0≤y<1, 0≤z<1] phosphor. The first matrix52may be a transparent inorganic material, for example, glass, a silicone resin, an organic-inorganic hybrid material, Al2O3, or ZnO. For example, the first matrix52in this embodiment is ZnO.

In the case where ZnO is used as the first matrix52, the first phosphor55may be sealed by, for example, the following method: (1) ZnO seed crystals are formed by vacuum deposition on a portion of the substrate51where the first wavelength conversion layer501is to be formed, and then a layer consisting of the first phosphor55is formed by a casting method on the ZnO seed crystals; (2) the substrate51with the seed crystals and the layer consisting of the first phosphor55is immersed in an aqueous zinc nitrate (Zn(NO3)2solution containing hexamethylenetetramine (C6H12N4); and (3) the temperature of the aqueous solution is set to 80° C. Thereby, ZnO is formed by liquid-phase deposition on a specific portion of the substrate51so as to surround the first phosphor55to form the first wavelength conversion layer501. The ZnO serving as the first matrix52formed by this method has a thermal conductivity of 10 W/m·K.

The volume ratio of the first phosphor55in the first wavelength conversion layer501is 30% or more and 80% or less. For example, the volume ratio of the first phosphor55in the first wavelength conversion layer501is 50% in this embodiment. When the volume ratio of the first phosphor55in the first wavelength conversion layer501is 30% or more, the contact area among particles of the first phosphor55is large, so that thermal conduction between the particles occurs easily. Thus, heat generated during the wavelength conversion is sufficiently dissipated to the substrate51to inhibit an increase in the temperature of the wavelength conversion member50, thereby inhibiting a reduction in luminous efficiency. When the volume ratio of the first phosphor55in the first wavelength conversion layer501is 80% or less, the breaking or the like of the particles of the first phosphor55is inhibited. Thus, a reduction in luminous efficiency due to defects caused by the break is inhibited.

The first wavelength conversion layer501may contain inorganic particles, for example, SiO2, Al2O3, or MgO, in a proportion lower than the volume ratio of the first phosphor55, in addition to the first phosphor55and the first matrix52. In this case, the scattering of the excitation light61and the first light62can be controlled.

The second wavelength conversion layer502includes a first main surface502a,a second main surface502b,and at least one side face502clocated between the first main surface502aand the second main surface502b. The first main surface502afaces the second main surface501bof the first wavelength conversion layer501. The second main surface502bis located opposite the first main surface502a.The second wavelength conversion layer502at least contains a second phosphor56, first inorganic particles57, and a second matrix53. The second phosphor56partially converts the excitation light61into the first light62. The first inorganic particles57have a function of generating the diffusively reflected light63having the same wavelength as the excitation light61.

The material composition of the second phosphor56is not particularly limited and may be a different composition from that of the first phosphor55. For example, the second phosphor56in this embodiment is a (Srs,Cat)AlSiN:Eu2+[0≤s<1, 0≤t<1] phosphor. The material of the first inorganic particles57is preferably a transparent material that exhibits lower absorption of light in the visible region. Examples of the material of the first inorganic particles57may include SiO2, Al2O3, MgO, ZnO, TiO2, ZrO, Ta2O5, Nb2O5, BN, AlN, and BaSO4. For example, the material of the first inorganic particles57in this embodiment is SiO2. Examples of the second matrix53may include transparent inorganic materials, such as glass, silicone resins, organic-inorganic hybrid materials, Al2O3, and ZnO. For example, the second matrix53in this embodiment is a polysilsesquioxane, which is an organic-inorganic hybrid material having a siloxane bond.

When the polysilsesquioxane is used as the second matrix53, the second phosphor56and the first inorganic particles57may be sealed by, for example, the following method: (1) the polysilsesquioxane that has been dissolved in a benzyl alcohol solvent, the second phosphor56, and the first inorganic particles57are mixed together into a paste-like mixture; and (2) the paste-like mixture is applied by, for example, a screen printing method to the first wavelength conversion layer501and thermally cured. Thereby, the second matrix53including the second phosphor56and the first inorganic particles57sealed therein is formed. The polysilsesquioxane serving as the second matrix53formed by this method has a thermal conductivity of 1 W/m·K.

The volume ratio of the second phosphor56in the second wavelength conversion layer502is preferably lower than that of the first phosphor55in the first wavelength conversion layer501. More preferably, the volume ratio of the second phosphor56in the second wavelength conversion layer502is 0.1% or more and 20% or less. The volume ratio of the second phosphor56in the second wavelength conversion layer502may be, for example, 10%.

As with the first embodiment, the first wavelength conversion layer501converts the excitation light61into fluorescence or phosphorescence having longer wavelengths than the excitation light61and emits the fluorescence or phosphorescence. The second wavelength conversion layer502converts the excitation light61into fluorescence or phosphorescence having longer wavelengths than the excitation light61and emits the fluorescence or phosphorescence. The first light62emitted from the wavelength conversion member50includes the fluorescence or phosphorescence from the first wavelength conversion layer501and the fluorescence or phosphorescence from the second wavelength conversion layer502. The second wavelength conversion layer502also emits the diffusively reflected light63having the same wavelength as the excitation light61. It is thus possible to adjust the color of light emitted from the wavelength conversion member50by changing the thickness of the second wavelength conversion layer502to change the mixing ratio of the first light62to the diffusively reflected light63.

In the case where the volume ratio of the second phosphor56in the second wavelength conversion layer502is higher than volume ratio of the first phosphor55in the first wavelength conversion layer501, the amount of the first light62emitted from the second phosphor56varies significantly depending on the thickness of the second wavelength conversion layer502because the excitation light61is first incident on the second wavelength conversion layer502. Thus, the color of light emitted from the wavelength conversion member50varies easily depending on a slight change in the thickness of the second wavelength conversion layer502. The volume ratio of the first inorganic particles57in the second wavelength conversion layer502is not particularly limited and may be adjusted in response to the color of light obtained by combining the first light62with the diffusively reflected light63originating from the excitation light61.

The reflective layer503at least contains second inorganic particles58and a third matrix54and is located on the side face501cof the first wavelength conversion layer501and the side face502cof the second wavelength conversion layer502. The reflective layer503in this embodiment is in contact with the side face501cof the first wavelength conversion layer501and the side face502cof the second wavelength conversion layer502and is located on the substrate51. Each of the first wavelength conversion layer501and the second wavelength conversion layer502may have a circular shape, an elliptical shape, a triangular shape, a rectangular shape, or a polygonal shape in plan view (that is, when viewed from a direction perpendicular to the second main surface502b). When each of the first wavelength conversion layer501and the second wavelength conversion layer502has a circular shape or an elliptic shape in plan view, each has a single side face. When each of the first wavelength conversion layer501and the second wavelength conversion layer502has a triangular shape, a rectangular shape, or a polygonal shape, each has a plurality of side faces. The shape of the first wavelength conversion layer501and the shape of the second wavelength conversion layer502in plan view may be the same or different.

The reflective layer503has a function of suppressing the emission of the first light62from a surface other than the surface of the second wavelength conversion layer502on which the excitation light61is incident to improve the contrast between the light-emitting portion and the non-light-emitting portion. The material of the second inorganic particles58is preferably a transparent material that exhibits lower absorption of light in the visible region. Examples of the material of the second inorganic particles58may include SiO2, AL2O3, MgO, ZnO, TiO2, ZrO, Ta2O5, Nb2O5, BN, AlN, and BaSO4. For example, the material of the second inorganic particles58in this embodiment is TiO2. Examples of the third matrix54may include transparent inorganic materials, such as glass, silicone resins, organic-inorganic hybrid materials, Al2O3, and ZnO. For example, the third matrix54in this embodiment is a silicone resin having a siloxane bond.

When the silicone resin is used as the third matrix54, the second inorganic particles58may be sealed by, for example, the following method: (1) the uncured silicone resin and the second inorganic particles58are mixed together into a paste-like mixture; and (2) the paste-like mixture is applied by, for example, a screen printing method only to the substrate51and thermally cured. Thereby, the third matrix54including the second inorganic particles58sealed therein is formed.

The volume ratio of the first phosphor55in the first wavelength conversion layer501is higher than the volume ratio of the second phosphor56in the second wavelength conversion layer502in the second embodiment of the present disclosure as described above. Thus, a main source of heat generated in the wavelength conversion member50during wavelength conversion is the first wavelength conversion layer501. The contact area among particles of the first phosphor55in the first wavelength conversion layer501is larger than the contact area among particles of the second phosphor56in the second wavelength conversion layer502. In addition, the first wavelength conversion layer501is in contact with the substrate51. This enables efficient dissipation of heat generated during wavelength conversion, thereby suppressing a reduction in luminous efficiency. The volume ratio of the second phosphor56in the second wavelength conversion layer502is lower than the volume ratio of the first phosphor55in the first wavelength conversion layer501. In addition, the second wavelength conversion layer502contains the first inorganic particles57. The ratio of the first light62to the diffusively reflected light63originating from the excitation light61varies depending on the thickness of the second wavelength conversion layer502. This enables easy control of the color of light emitted. In particular, the first phosphor55and the second phosphor56have different material compositions in the second embodiment of the present disclosure. This results in different light colors and enables appropriate control of a mixed color obtained by the combination of at least three colors: the color of the diffusively reflected light63originating from the excitation light61, the color of light emitted from the first phosphor, and the color of light emitted from the second phosphor. The reflective layer503suppresses the emission of the first light62from a surface other than the surface of the second wavelength conversion layer502on which the excitation light61is incident to improve the contrast between the light-emitting portion and the non-light-emitting portion.

Third Embodiment

FIG. 3schematically illustrates a structure of a vehicle headlamp120according to a third embodiment of the present disclosure. The vehicle headlamp120in this embodiment includes the light source10in the first embodiment or the light source40in the second embodiment; and an exit optical system122that guides light from the light source toward the front. To adjust the color of light emitted, a wavelength cutoff filter121that partially absorbs or reflects light from the light source may be provided. The exit optical system122guides light from the light source10or the light source40to the outside. The exit optical system122is, for example, a projection lens. The vehicle headlamp120may be what is called a projector-type vehicle headlamp or a reflector-type vehicle headlamp. The term “vehicle” used in the present disclosure includes automobiles, two-wheeled vehicles, and special-purpose vehicles.

The vehicle headlamp according to the third embodiment provides the same advantageous effects as those obtained in the first or second embodiment. In particular, the vehicle headlamp120emits a high-contrast light beam even when the state of collection of light emitted from the light source10or40is adjusted with the projection lens because of the improved contrast between the light-emitting portion an the non-light-emitting portion of the light source10or40.

Fourth Embodiment

FIG. 4schematically illustrates a structure of a vehicle130according to a fourth embodiment of the present disclosure. The vehicle130includes the vehicle headlamp120according to the third embodiment and a power supply source131. The vehicle130may include a generator132that is rotationally driven by a driving source such as an engine to generate electric power. The electric power generated by the generator132is stored in the power supply source131. The power supply source131is, for example, a secondary battery that can be charged and discharged. The vehicle headlamp120is operated by electric power from the power supply source131. Examples of the vehicle130include automobiles, two-wheeled vehicles, and special-purpose vehicles. The vehicle130may be an engined vehicle, an electric vehicle, or a hybrid electric vehicle.

The vehicle according to the fourth embodiment also provides the same advantageous effects as those obtained in the first or second embodiment.

The wavelength conversion member according to an embodiment of the present disclosure is usable for light sources, such as special lighting devices, head-up displays, projectors, and vehicle headlamps.