Patent Publication Number: US-2019187545-A1

Title: Wavelength conversion element, light source apparatus, and projector

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a wavelength conversion element, a light source apparatus, and a projector. 
     2. Related Art 
     JP-A-2013-30380 discloses an illuminator that irradiates a phosphor with excitation light to produce fluorescence and uses the produced fluorescence as illumination light. In the illuminator, the phosphor contains scatterer particles that cause Rayleigh scattering or cause only the excitation light to be scattered. Fluorescence conversion efficiency of conversion of the excitation light into the fluorescence is thus improved. 
     JP-A-2014-186882 discloses a technology of a reflective wavelength conversion element in which the density of scatterers is higher on the side facing a base that supports a phosphor than on the side close to the light incident and emitting surface of the phosphor for prevention of occurrence of temperature quenching of the phosphor and improvement in fluorescence extraction efficiency. 
     In the invention disclosed in JP-A-2013-30380 described above, however, since the fluorescence is unlikely to be scattered, some of the fluorescence does not exit out of the phosphor. As a result, the fluorescence extraction efficiency undesirably decreases. 
     In the invention disclosed in JP-A-2014-186882 described above, the phosphor absorbs a large amount of excitation light on the side facing the base where the excitation light is strongly scattered to produce a large amount of fluorescence. On the side facing the base where the excitation light is strongly scattered, however, the amount of scattered fluorescence also increases, and the optical path length of the fluorescence therefore increases. The fluorescence therefore enters a highly excited state portion of the phosphor (base-side portion). 
     When a phosphor in the excited state absorbs fluorescence, the following phenomenon occurs: Electrons are re-excited to the conduction band, where no light emission occurs, so that the fluorescence emission efficiency lowers. This phenomenon is called optical quenching. As described above, in the invention disclosed in JP-A-2014-186882 described above, since the fluorescence enters the highly excited state portion of the phosphor, the effect of the optical quenching undesirably increases. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a wavelength conversion element capable of suppressing the effect of optical quenching with a decrease in light extraction efficiency suppressed. Another advantage of some aspects of the invention is to provide a light source apparatus including the wavelength conversion element. Another advantage of some aspects of the invention is to provide a projector including the light source apparatus. 
     According to a first aspect of the invention, there is provided a wavelength conversion element including a wavelength conversion layer having a first surface on which excitation light is incident and a second surface provided on a side opposite the first surface, the wavelength conversion layer wavelength-converting the excitation light into light having a wavelength longer than a wavelength of the excitation light, a scattering element contained in the wavelength conversion layer and having a size smaller than or equal to the wavelength of the excitation light, and an angle changing section that changes an angle of incidence of the converted light from the wavelength conversion layer with respect to the first surface or the second surface. 
     In the wavelength conversion element according to the first aspect, since the excitation light is dominantly scattered in the form of Rayleigh scattering, the converted light is scattered by the scattering element by a small degree. Therefore, since the converted light passes through the wavelength conversion layer in the excited state along a short optical path, the converted light is unlikely to be absorbed, whereby the effect of the optical quenching can be suppressed. Further, since the angle changing section changes the angle of incidence of the converted light with respect to the first or second surface, the converted light is not totally reflected off the first or second surface but can be extracted out of the wavelength conversion layer. A wavelength conversion element that suppresses the effect of the optical quenching with a decrease in the converted light extraction efficiency suppressed can therefore be provided. 
     In the first aspect described above, it is preferable that in the wavelength conversion layer, a density of the scattering element in a region facing the second surface is higher than the density of the scattering element in a region facing the first surface. 
     According to the configuration described above, the excitation light is relatively strongly diffused on the side facing the second surface of the wavelength conversion layer, whereby the converted light produced in the region facing the second surface and directed toward the first surface passes through a low excited state density region. The amount of absorbed converted light in the region facing the first surface is therefore reduced, whereby the effect of the optical quenching can be further suppressed. 
     In the first aspect described above, it is preferable that the wavelength conversion layer contains an activator, and that a concentration of the activator in a region facing the second surface is higher than the concentration of the activator in a region facing the first surface. 
     According to the configuration described above, in the wavelength conversion layer, a high excited state density region can be formed in a narrow region facing the second surface. As a result, since a large amount of converted light directed toward the first surface exits via the first surface without passing through the high excited state density region, the effect of the optical quenching can be further suppressed. 
     In the first aspect described above, it is preferable that the wavelength conversion element further includes a first reflection layer that is provided on a side facing the second surface and reflects the converted light, and that the converted light reflected off the first reflection surface exits via the first surface. 
     According to the configuration described above, a reflective wavelength conversion element that suppresses the effect of the optical quenching with a decrease in the converted light extraction efficiency suppressed can be provided. 
     In the first aspect described above, it is preferable that the wavelength conversion element further includes a second reflection layer that is provided on a side facing the first surface, transmits the excitation light, and reflects the converted light, and that the converted light reflected off the second reflection layer exits via the second surface. 
     According to the configuration described above, a transmissive wavelength conversion element that suppresses the effect of the optical quenching with a decrease in the converted light extraction efficiency suppressed can be provided. 
     According to a second aspect of the invention, there is provided a light source apparatus including the wavelength conversion element according to the first aspect described above and a light source that outputs light toward the wavelength conversion element. 
     The light source apparatus according to the second aspect, which includes the wavelength conversion element that prevents a decrease in the converted light extraction efficiency and suppresses the effect of the optical quenching, can produce bright illumination light. 
     According to a third aspect of the invention, there is provided a projector including the light source apparatus according to the second aspect, a light modulator that modulates light from the light source apparatus in accordance with image information to form image light, and a projection system that projects the image light. 
     The projector according to the third aspect, which includes the light source apparatus according to the second aspect described above, can form a high-luminance image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  shows the configuration of a projector according to a first embodiment. 
         FIG. 2  shows a schemtic configuration of an illuminator. 
         FIG. 3  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element. 
         FIG. 4  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element according to a first variation. 
         FIG. 5  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element according to a second variation. 
         FIG. 6  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element according to a third variation. 
         FIG. 7  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element according to a fourth variation. 
         FIG. 8  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element according to a second embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the invention will be described below in detail with reference to the drawings. 
     In the drawings used in the following description, a characteristic portion is enlarged for convenience in some cases for clarity of the characteristic thereof, and the dimension ratio and other factors of each component are therefore not always equal to actual values. 
     First Embodiment 
     An example of a projector according to a first embodiment of the invention will first be descried. 
       FIG. 1  shows a schematic configuration of the projector according to the present embodiment. 
     A projector  1  according to the present embodiment is a projection-type image display apparatus that displays color video images on a screen SCR, as shown in  FIG. 1 . The projector  1  includes an illuminator  2 , a color separation system  3 , a light modulator  4 R, a light modulator  4 G, a light modulator  4 B, a light combining system  5 , and a projection system  6 . 
     The color separation system  3  separates illumination light WL into red light LR, green light LG, and blue light LB. The color separation system  3  generally includes a first dichroic mirror  7   a  and a second dichroic mirror  7   b , a first total reflection mirror  8   a , a second total reflection mirror  8   b , and a third total reflection mirror  8   c , and a first relay lens  9   a  and a second relay lens  9   b.    
     The first dichroic mirror  7   a  separates the illumination light WL from the illuminator  2  into the red light LR and the other light (green light LG and blue light LB). The first dichroic mirror  7   a  transmits the separated red light LR and reflects the other light (green light LG and blue light LB). On the other hand, the second dichroic mirror  7   b  reflects the green light LG and transmits the blue light LB to separate the other light into the green light LG and the blue light LB. 
     The first total reflection mirror  8   a  is disposed in the optical path of the red light LR and reflects the red light LR having passed through the first dichroic mirror  7   a  toward the light modulator  4 R. On the other hand, the second total reflection mirror  8   b  and the third total reflection mirror  8   c  are disposed in the optical path of the blue light LB and guide the blue light LB having passed through the second dichroic mirror  7   b  to the light modulator  4 B. The green light LG is reflected off the second dichroic mirror  7   b  toward the light modulator  4 G. 
     The first relay lens  9   a  and the second relay lens  9   b  are disposed in the optical path of the blue light LB and on the downstream side of the second dichroic mirror  7   b.    
     The light modulator  4 R modulates the red light LR in accordance with image information to form image light corresponding to the red light LR. The light modulator  4 G modulates the green light LG in accordance with image information to form image light corresponding to the green light LG. The light modulator  4 B modulates the blue light LB in accordance with image information to form image light corresponding to the blue light LB. 
     The light modulators  4 R,  4 G, and  4 B are each formed, for example, of a transmissive liquid crystal panel. Polarizers (not shown) are disposed on the light incident side and the light exiting side of each of the liquid crystal panels. 
     Field lenses  10 R,  10 G, and  10 B are disposed on the light incident side of the light modulators  4 R,  4 G, and  4 B, respectively. The field lenses  10 R,  10 G, and  10 B parallelize the red light LR, the green light LG, and the blue light LB incident on the light modulators  4 R,  4 G, and  4 B, respectively. 
     The image light fluxes from the light modulators  4 R,  4 G, and  4 B enter the light combining system  5 . The light combining system  5  combines the image light fluxes corresponding to the red light LR, the green light LG, and the blue light LB with one another and causes the combined image light to exit toward the projection system  6 . The light combining system  5  is formed, for example, of a cross dichroic prism. 
     The projection system  6  is formed of a projection lens group, enlarges the combined image light from the light combining system  5 , and projects the enlarged image light toward the screen SCR. Enlarged color video images are thus displayed on the screen SCR. 
     Illuminator 
     The illuminator  2  will subsequently be described.  FIG. 2  shows a schematic configuration of the illuminator  2 . The illuminator  2  includes a light source apparatus  2 A according to an embodiment of the invention, an optical integration system  31 , a polarization conversion element  32 , and a superimposing lens  33   a , as shown in  FIG. 2 . In the present embodiment, the optical integration system  31  and the superimposing lens  33   a  form a superimposing system  33 . 
     The light source apparatus  2 A includes an array light source  21 , a collimator system  22 , an afocal system  23 , a first retardation film  28   a , a polarization separation element  25 , a first light collection system  26 , a wavelength conversion element  40 , a second retardation film  28   b , a second light collection system  29 , and a diffusive reflection element  30 . 
     The array light source  21 , the collimator system  22 , the afocal system  23 , the first retardation film  28   a , the polarization separation element  25 , the second retardation film  28   b , the second light collection system  29 , and the diffusive reflection element  30  are sequentially arranged along an optical axis ax 1 . On the other hand, the wavelength conversion element  40 , the first light collection system  26 , the polarization separation element  25 , the optical integration system  31 , the polarization conversion element  32 , and the superimposing lens  33   a  are sequentially arranged along an illumination optical axis ax 2 . The optical axis ax 1  and the illumination optical axis ax 2  are present in the same plane and perpendicular to each other. 
     The array light source  21  includes a plurality of semiconductor lasers  21   a , which each serve as a solid-state light source. The plurality of semiconductor lasers  21   a  are arranged in an array in a plane perpendicular to the optical axis ax 1 . 
     The semiconductor lasers  21   a  each output, for example, a blue light beam BL (laser light having intensity that peaks at wavelength of 460 nm, for example). The array light source  21  outputs a light beam flux formed of a plurality of light beams BL. In the present embodiment, the array light source  21  corresponds to the “light source” in the appended claims. 
     The light beams BL outputted from the array light source  21  enter the collimator system  22 . The collimator system  22  converts the light beams BL outputted from the array light source  21  into parallelized light. The collimator system  22  is formed, for example, of a plurality of collimator lenses  22   a  arranged in an array. The plurality of collimator lenses  22   a  are disposed in correspondence with the plurality of semiconductor lasers  21   a.    
     The light beams BL having passed through the collimator system  22  enter the afocal system  23 . The afocal system  23  adjusts the light flux diameter of each of the light beams BL. The afocal system  23  is formed, for example, of a convex lens  23   a  and a concave lens  23   b.    
     The light beams BL having passed through the afocal system  23  are incident on the first retardation film  28   a . The first retardation film  28   a  is, for example, a half wave plate configured to be rotatable. The light beams BL outputted from the semiconductor lasers  21   a  are each linearly polarized light. Appropriately setting the angle of rotation of the first retardation film  28   a  allows each of the light beams BL having passed through the first retardation film  28   a  to be a light beam containing an S-polarized component and a P-polarized component with respect to the polarization separation element  25  mixed with each other at a predetermined ratio. The ratio between the S-polarized component and the P-polarized component can be changed by rotating the first retardation film  28   a.    
     The light beams BL each containing the S-polarized component and the P-polarized component produced when the light beam BL passes through the first retardation film  28   a  are incident on the polarization separation element  25 . The polarization separation element  25  is formed, for example, of a polarizing beam splitter having wavelength selectivity. The polarization separation element  25  inclines by 45° with respect to the optical axis ax 1  and the illumination optical axis ax 2 . 
     The polarization separation element  25  has a polarization separation function of separating each of the light beams BL into a light beam BL S , which is formed of the S-polarized light component with respect to the polarization separation element  25 , and a light beam BL P , which is formed of the P-polarized light component with respect to the polarization separation element  25 . Specifically, the polarization separation element  25  reflects the light beams BL S , which are each formed of the S-polarized light component, and transmits the light beams BL P , which are each formed of the P-polarized light component. 
     The polarization separation element  25  further has a color separation function of transmitting fluorescence YL, which belongs to a wavelength band different from the wavelength band to which the light beams BL belong, irrespective of the polarization state of the fluorescence YL. 
     The S-polarized light beams BL S  exited from the polarization separation element  25  enter the first light collection system  26 . The first light collection system  26  collects the light beams BL S  and directs the collected light beams BL S  toward the wavelength conversion element  40 . 
     In the present embodiment, the first light collection system  26  is formed, for example, of a first lens  26   a  and a second lens  26   b . The light beams BL S  having exited out of the first light collection system  26  are incident in the form of a collected light flux on the wavelength conversion element  40 . 
     The fluorescence YL produced by the wavelength conversion element  40  is parallelized by the first light collection system  26  and then incident on the polarization separation element  25 . The fluorescence YL passes through the polarization separation element  25 . 
     On the other hand, the P-polarized light beams BL P  having exited out of the polarization separation element  25  are incident on the second retardation film  28   b . The second retardation film  28   b  is formed of a quarter wave plate disposed in the optical path between the polarization separation element  25  and the diffusive reflection element  30 . The P-polarized light beams BL P  having exited out of the polarization separation element  25  are converted by the second retardation film  28   b , for example, into right-handed circularly polarized blue light BL C   1 , which then enters the second light collection system  29 . 
     The second light collection system  29  is formed, for example, of a convex lens  29   a  and a concave lens  29   b  and causes the collected blue light BL C   1  to be incident on the diffusive reflection element  30 . 
     The diffusive reflection element  30  is disposed on the side opposite a phosphor layer  42  with respect to the polarization separation element  25  and diffusively reflects the blue light BL C   1  having exited out of the second light collection system  29  toward the polarization separation element  25 . The diffusive reflection element  30  preferably not only reflects the blue light BL C   1  in a Lambertian reflection scheme but does not disturb the polarization state of the blue light BL C   1 . 
     The light diffusively reflected off the diffusive reflection element  30  is hereinafter referred to as blue light BL C   2 . According to the present embodiment, diffusively reflecting the blue light BL C   1  results in blue light BL C   2  having a roughly uniform illuminance distribution. For example, the right-handed circularly polarized blue light BL C   1  is reflected in the form of left-handed circularly polarized blue light BL C   2 . 
     The blue light BL C   2  is converted by the second light collection system  29  into parallelized light and then incident on the second retardation film  28   b  again. 
     The left-handed circularly polarized blue light BL C   2  is converted by the second retardation film  28   b  into S-polarized blue light BL S   1 . The S-polarized blue light BL S   1  is reflected off the polarization separation element  25  toward the optical integration system  31 . 
     The blue light BL S   1  is thus used as the illumination light WL along with the fluorescence YL having passed through the polarization separation element  25 . That is, the blue light BL S   1  and the fluorescence YL exit out of the polarization separation element  25  in the same direction to form the white illumination light WL, which is the mixture of the blue light BL S   1  and the fluorescence (yellow light) YL. 
     The illumination light WL exits toward the optical integration system  31 . The optical integration system  31  is formed, for example, of a lens array  31   a  and a lens array  31   b . The lens arrays  31   a  and  31   b  are each formed of a plurality of lenslets arranged in an array. 
     The illumination light WL having passed through the optical integration system  31  is incident on the polarization conversion element  32 . The polarization conversion element  32  is formed of polarization separation films and retardation films. The polarization conversion element  32  converts the illumination light WL containing the non-polarized fluorescence YL into linearly polarized light. 
     The illumination light WL having passed through the polarization conversion element  32  enters the superimposing lens  33   a . The superimposing lens  33   a  cooperates with the optical integration system  31  to homogenize the illuminance distribution of the illumination light WL in an area illuminated therewith. The illuminator  2  thus produces the illumination light WL. 
     Wavelength Conversion Element 
       FIG. 3  is a cross-sectional view showing the configuration of key parts of the wavelength conversion element  40 . 
     The wavelength conversion element  40  includes a base  41 , a reflection layer  43 , a phosphor layer  42 , a bonding layer  44 , and a heat dissipation member  45 , as shown in  FIG. 3 . 
     The base  41  has an upper surface  41   a , which faces the first light collection system  26 , and a lower surface  41   b , which is opposite the upper surface  41   a . The heat dissipation member  45  is provided on the lower surface  41   b  of the base  41 . 
     The base  41  is preferably made of a material that has high thermal conductivity and excels in heat dissipation performance. Examples of the material of the base  41  may include aluminum, copper, silver, or any other metal or an aluminum nitride, alumina, sapphire, diamond, or any other ceramic material. In the present embodiment, the base  41  is made of copper. 
     In the present embodiment, the phosphor layer  42  is held on the upper surface  41   a  of the base  41  via the bonding layer  44 . The phosphor layer  42  converts part of the light incident thereon into the fluorescence YL, which then exits out of the phosphor layer  42 . The light beams BL S , which exit out of the first light collection system  26  and enter the phosphor layer  42 , are hereinafter referred to as excitation light BL S . 
     The phosphor layer  42  has an upper surface  42 A, on which the excitation light BL S  is incident and via which the fluorescence YL exits, and a lower surface  42 B, which is provided on the side opposite the upper surface  42 A (side facing bonding layer  44 ). In the present embodiment, the phosphor layer  42  corresponds to the “wavelength conversion layer” set forth in the appended claims, the fluorescence YL corresponds to the “converted light” set forth in the appended claims, the upper surface  42 A corresponds to the “first surface” set forth in the appended claims, and the lower surface  42 B corresponds to the “second surface” set forth in the appended claims. 
     In the present embodiment, the reflection layer  43  is formed on the upper surface  41   a  of the base  41 . The reflection layer  43  is formed on the upper surface  41   a , for example, in an evaporation process. The reflection layer  43  reflects part of the fluorescence YL traveling toward the upper surface  41   a  of the base  41  toward the upper surface  42 A of the phosphor layer  42 . The reflection layer  43  further reflects the excitation light BL S  that has not been converted into the fluorescence YL but has exited out of the phosphor layer  42  back into the phosphor layer  42  again. The reflection layer  43  corresponds to the “first reflection layer” set forth in the appended claims. 
     The bonding layer  44  bonds the base  41  and the phosphor layer  42  to each other. In the present embodiment, the bonding layer  44  is made of a light transmissive material. 
     The heat dissipation member  44  is formed, for example, of a heat sink and has a structure having a plurality of fins. The heat dissipation member  44  is provided on the lower surface  41   b  of the base  41 , which is the surface opposite the phosphor layer  42 . The heat dissipation member  44  is fixed to the base  41 , for example, by brazing-metal bonding (metal bonding). Since the wavelength conversion element  40  can dissipate heat via the heat dissipation member  44 , thermal degradation of the phosphor layer  42  can be avoided, and a decrease in the conversion efficiency of the phosphor layer  42  due to an increase in the temperature thereof can be suppressed. 
     In the present embodiment, the phosphor layer  42  is made of a ceramic phosphor formed of fired phosphor particles. A YAG (yttrium aluminum garnet) phosphor containing a Ce ion as an activator (YAG:Ce) is used as the phosphor particles that form the phosphor layer  42 . In the present embodiment, the concentration distribution of the activator (Ce ion) is uniform in the phosphor. 
     The phosphor particles may be made of one material or may be a mixture of particles made of two or more materials. The phosphor layer  42  is preferably, for example, a phosphor layer in which phosphor particles are dispersed in an inorganic binder, such as alumina, or a phosphor layer formed by firing a glass binder, which is an inorganic material, and phosphor particles. 
     In the present embodiment, the phosphor layer  42  contains a plurality of scattering particles  46 . The scattering particles  46  is uniformly distributed in the phosphor layer  42 . In the present embodiment, the scattering particles  46  correspond to the “scatter element” set forth in the appended claims. 
     The scattering particles  46  each have a size smaller than or equal to the wavelength of the excitation light BL S . For example, the scattering particles  46  are formed of pores having an average particle diameter smaller than or equal to 400 nm. The lower limit of the average particle diameter of the scattering particles  46  is desirably, for example, about one-tenth of the wavelength of the blue light beams BL. For example, in the case where the intensity of the blue light beams BL peaks at 460 nm, the average particle diameter of the scattering particles  46  preferably ranges from about 40 to 50 nm. 
     The scattering particles  46  may be made of a material having a refractive index different from that of YAG, which is the base material of the phosphor layer  42 , (for example, alumina, Y 3 Al 5 O 12 , YAlO 3 , Zirconium dioxide, Lu 3 Al 5 O 12 , or glass). 
     In the present embodiment, since the scattering particles  46  contained in the phosphor layer  42  each have a size smaller than or equal to the excitation light BL S , the excitation light BL S  is dominantly scattered in the form of Rayleigh scattering. That is, the excitation light BL S  is strongly scattered in the form of Rayleigh scattering. The scattered excitation light BL S  is likely to be absorbed because the optical path length thereof lengthens in the phosphor layer  42 . That is, since the amount of absorbed excitation light BL S  increases, an adequate amount of produced fluorescence YL can be ensured even by a thinner phosphor layer  42 . Therefore, since the thickness of the phosphor layer  42  can be reduced, the heat dissipation performance of the phosphor layer  42  is improved, whereby a decrease in the fluorescence conversion efficiency due to an increase in the temperature of the phosphor layer  42  can be suppressed. 
     When a phosphor in the excited state absorbs fluorescence, the following phenomenon occurs (optical quenching): Electrons are re-excited to the conduction band, where no light emission occurs, so that the fluorescence emission efficiency lowers. 
     In the present embodiment, the fluorescence YL, which has a wavelength longer than that of the excitation light BL S , is unlikely to be scattered by the scattering particles  46 . The fluorescence YL therefore travels roughly straight in the phosphor layer  42  and reaches the upper surface  41 A along a short optical path. Therefore, since the fluorescence YL passes through the phosphor layer  42  in the excited state along the short optical path, the fluorescence YL is unlikely to be absorbed, whereby the effect of the optical quenching described above can be suppressed. 
     Further, in the present embodiment, an irregular section  47  having minute irregularities is formed across the upper surface  42 A of the phosphor layer  42 . The irregular section  47  is formed by roughening the upper surface  42 A. The irregular section  47  is intended to extract the fluorescence YL (cause fluorescence YL to exit) out of the upper surface  42 A of the phosphor layer  42 . The irregular section  47  corresponds to the “incident angle changing section” set forth in the appended claims. 
     The effect of the irregular section  47  will now be described. If no irregular section  47  is provided, the fluorescence YL incident on the upper surface  42 A at angles greater than the critical angle is totally reflected off the upper surface  42 A and directed back into the phosphor layer  42  so that the fluorescence YL cannot exit out of the phosphor layer  42 , resulting in a decrease in the efficiency of extraction of the fluorescence YL. 
     In contrast, the wavelength conversion element  40  according to the present embodiment, which includes the irregular section  47 , can change the angle of incidence of the fluorescence YL with respect to the upper surface  42 A. That is, even the fluorescence YL incident on the upper surface  42 A at an angle greater than the critical angle in the case where no irregular section  47  is formed is incident on the surface of the irregular section  47  at an angle smaller than the critical angle. The fluorescence YL totally reflected off the upper surface with no irregular section  47  formed thereacross passes through the upper surface  42 A with the irregular section  47  formed thereacross and exits out of the phosphor layer  42 . 
     Providing the irregular section  47  therefore causes the fluorescence YL to be unlikely to be trapped in the phosphor layer  42 , whereby a decrease in the efficiency of extraction of the fluorescence YL can be suppressed. 
     As described above, the wavelength conversion element  40  according to the present embodiment can suppress the effect of the optical quenching of the fluorescence YL without a decrease in the efficiency of extraction of the fluorescence YL. Further, since the amount of absorbed excitation light BL S  increases and the thickness of the phosphor layer  42  can be reduced accordingly, the size of the wavelength conversion element  40  itself can be reduced. 
     The light source apparatus  2 A including the wavelength conversion element  40  therefore prevents a decrease in the efficiency of extraction of the fluorescence YL and suppresses the effect of the optical quenching, whereby the light source apparatus  2 A can produce bright illumination light WL. Further, the projector  1  according to the present embodiment, which includes the illuminator  2  using the light source apparatus  2 A described above, can form a high-luminance image. 
     First Variation 
     A first variation of the first embodiment will subsequently be described. Components and members common to those in the first embodiment have the same reference characters and will not be described in detail. 
       FIG. 4  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element  140  according to the present variation. 
     In the phosphor layer  42  of the wavelength conversion element  140 , the density of the scattering particles  46  in a region facing the lower surface  42 B is higher than the density of the scattering particles  46  in a region facing the upper surface  42 A, as shown in  FIG. 4 . That is, for example, provided that the scattering particles  46  all have the same size, the above configuration means that the number of scattering particles  46  contained in the region facing the lower surface  42 B of the phosphor layer  42  is greater than the number of scattering particles  46  contained in the region facing the upper surface  42 A of the phosphor layer  42 . 
     Therefore, in the wavelength conversion element  140  according to the present variation, the excitation light BL S  is more strongly scattered in a region closer to the lower surface  42 B of the phosphor layer  42 . The excitation light BL S  that enters the phosphor layer  42  via the upper surface  42 A is therefore so scattered that the amount of scattering in the region facing the upper surface  42 A is suppressed, and the excitation light BL S  hence reaches the region facing the lower surface  42 B but a large amount of excitation light BL S  is not absorbed in the region facing the upper surface  42 A. That is, the fluorescence YL can be efficiently produced because the optical path length of the excitation light BL S  in the phosphor layer  42  lengthens. 
     Also in the present variation, the fluorescence YL is unlikely to be scattered in the phosphor layer  42  and therefore reaches the upper surface  42 A along a short optical path. Therefore, since the fluorescence YL passes through the phosphor layer  42  in the excited state along the short optical path, the fluorescence YL is unlikely to be absorbed. 
     In the present variation, the excitation light BL S  is strongly scattered in the region facing the lower surface  42 B, as described above. Therefore, in the phosphor layer  42  in the present variation, an excited state density in the region facing the lower surface  42 B is higher than the excited state density in the region facing the upper surface  42 A, as shown in  FIG. 4 . The excited state density corresponds to the ratio of the volume of the phosphor particles having absorbed the excitation light BL S  to the volume of the phosphor layer  42  (volume density). 
     Therefore, the fluorescence YL produced in the region facing the lower surface  42 B and directed toward the upper surface  42 A passes through a low excited state density region, that is, a region having a relatively small number of phosphor particles having absorbed the excitation light BL S . Therefore, the wavelength conversion element  140  according to the present variation, in which the amount of absorbed fluorescence YL is further reduced in the region facing the upper surface  42 A, can further suppress the effect of the optical quenching. 
     Second Variation 
     A second variation of the first embodiment will subsequently be described. Components and members common to those in the first embodiment have the same reference characters and will not be described in detail. 
       FIG. 5  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element  240  according to the present variation. In  FIG. 5 , the number of hatched dots represents the magnitude of an activator concentration in the phosphor layer  42 . The activator concentration corresponds to the distribution of the concentration of the activator (Ce ion) that occupies the phosphor layer  42 . 
     In the phosphor layer  42  of the wavelength conversion element  240 , the concentration of the activator (activator concentration) in the area facing the lower surface  42 B is higher than the concentration of the activator in the area facing the upper surface  42 A, as shown in  FIG. 5 . 
     Further, in the present variation, the density of the scattering particles  46  in the region facing the lower surface  42 B is higher than the density of the scattering particles  46  in the region facing the upper surface  42 A, as in the first variation. 
     The wavelength conversion element  240  according to the present variation therefore allows a selective increase in the excited state density in a narrow region facing the lower surface  42 B. As a result, since a high excited state density region can be formed on the side facing the lower surface  42 B in the phosphor layer  42 , a large amount of fluorescence YL directed toward the upper surface  42 A exits out of the phosphor layer  42  without passing through the high excited state density region. 
     As described above, the wavelength conversion element  240  according to the present variation allows reduction in the amount of absorption of the fluorescence YL as compared with the configuration in the first variation, whereby the effect of the optical quenching can be further suppressed. 
     Third Variation 
     A third variation of the first embodiment will subsequently be described. Components and members common to those in the first embodiment have the same reference characters and will not be described in detail. 
       FIG. 6  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element  340  according to the present variation. 
     In the present variation, as shown in  FIG. 6 , an irregular section  147  is formed across the lower surface  42 B of the phosphor layer  42 . The irregular section  147  is formed by roughening the lower surface  42 B. 
     In the present variation, the irregular section  147  is intended to extract the fluorescence YL (cause fluorescence YL to exit) out of the upper surface  42 A of the phosphor layer  42 . The irregular section  147  corresponds to the “incident angle changing section” set forth in the appended claims. 
     The effect of the irregular section  147  will subsequently be described. If no irregular section  147  is provided, part of the fluorescence YL is reflected off the reflection layer  43  and incident on the upper surface  42 A at angles greater than the critical angle so that the fluorescence YL is totally reflected into the phosphor layer  42  and does not therefore exit out of the phosphor layer  42 . 
     In contrast, the wavelength conversion element  340  according to the present variation, which includes the irregular section  147  and in which the fluorescence YL is reflected off the surface of the irregular section  147 , can change the angle of incidence of the fluorescence YL with respect to the upper surface  42 A. Part of the fluorescence YL passes through the lower surface  42 B and is reflected off the reflection layer  43  toward the upper surface  42 A. The part of the fluorescence YL, when passing through the lower surface  42 B, is refracted and the traveling direction of the fluorescence YL therefore changes. Therefore, the fluorescence YL incident on the upper surface  42 A at an angle greater than the critical angle in the case where no irregular section  147  is formed is incident on the upper surface  42 A at an angle smaller than the critical angle after the fluorescence YL is reflected off the irregular surface of the irregular section  147  or passes through the irregular surface and is then reflected off the reflection layer  43 . The fluorescence YL totally reflected off the upper surface  42 A in the case where no irregular section  147  is formed passes through the upper surface  42 A and exits out of the phosphor layer  42 . 
     Providing the irregular section  147  therefore causes the fluorescence YL to be unlikely to be trapped in the phosphor layer  42 , whereby a decrease in the efficiency of extraction of the fluorescence YL can be suppressed. 
     Fourth Embodiment 
     A fourth variation of the first embodiment will subsequently be described. Components and members common to those in the first embodiment have the same reference characters and will not be described in detail. 
       FIG. 7  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element  440  according to the present variation. 
     The wavelength conversion element  440  according to the present variation includes the base  41 , the reflection layer  43 , a scattering layer  48 , the phosphor layer  42 , the bonding layer  44 , and the heat dissipation member  45 , as shown in  FIG. 7 . 
     In the present variation, the scattering layer  48  is formed on the lower surface  42 B of the phosphor layer  42 . The scattering layer  48  is formed of a porous layer having pores or a layer containing particles having a refractive index different from that of the base material (YAG) of the phosphor layer  42 . 
     In the present variation, the scattering layer  48  is intended to extract the fluorescence YL (cause fluorescence YL to exit) out of the upper surface  42 A of the phosphor layer  42 . The scattering layer  48  corresponds to the “incident angle changing section” set forth in the appended claims. 
     The effect of the scattering layer  48  will subsequently be described. If no scattering layer  48  is provided, part of the fluorescence YL is reflected off the reflection layer  43  and incident on the upper surface  42 A at angles greater than the critical angle so that the fluorescence YL is totally reflected into the phosphor layer  42  and does not therefore exit out of the phosphor layer  42 . 
     In contrast, the wavelength conversion element  440  according to the present variation, which includes the scattering layer  48  and in which the fluorescence YL is reflected off the scattering layer  48 , can change the angle of incidence of the fluorescence YL with respect to the upper surface  42 A. Part of the fluorescence YL passes through the scattering layer  48  and is reflected off the reflection layer  43  toward the upper surface  42 A. The part of the fluorescence YL, when passing through the scattering layer  48 , is refracted and the traveling direction of the fluorescence YL therefore changes. Therefore, the fluorescence YL incident on the upper surface  42 A at an angle greater than the critical angle in the case where no scattering layer  48  is formed is incident on the upper surface  42 A at an angle smaller than the critical angle after the fluorescence YL is reflected off the scattering layer  48  or passes through the scattering layer  48  and is then reflected off the reflection layer  43 . The fluorescence YL totally reflected off the upper surface  42 A in the case where no scattering layer  48  is formed passes through the upper surface  42 A and exits out of the phosphor layer  42 . 
     Providing the scattering layer  48  therefore causes the fluorescence YL to be unlikely to be trapped in the phosphor layer  42 , whereby a decrease in the efficiency of extraction of the fluorescence YL can be suppressed. 
     Second Embodiment 
     A wavelength conversion element according to a second embodiment of the invention will subsequently be described. The wavelength conversion element according to the present embodiment differs from the wavelength conversion element  40  according to the first embodiment in that a light transmissive wavelength conversion element is employed. 
       FIG. 8  is a cross-sectional view showing the configuration of key parts of a wavelength conversion element  540  according to the present embodiment. 
     The wavelength conversion element  540  includes a base  50 , a dichroic film  51 , a phosphor layer  52 , and an irregular section  57 , as shown in  FIG. 8 . 
     In the present embodiment, the phosphor layer  52  is disposed in a through hole  50   a  provided in the base  50 . A reflection film (not shown) is provided on the surface of the through hole  50   a.    
     The phosphor layer  52  has the same configuration as that of the phosphor layer  42  in the first embodiment. Specifically, the phosphor layer  52  has a lower surface  52 B, on which the excitation light BL S  is incident, and an upper surface  52 A, via which the fluorescence YL exits. The phosphor layer  52  contains a plurality of scattering particles  46 . 
     In the present embodiment, the phosphor layer  52  corresponds to the “wavelength conversion layer” set forth in the appended claims, the lower surface  52 B corresponds to the “first surface” set forth in the appended claims, and the upper surface  52 A corresponds to the “second surface” set forth in the appended claims. 
     The dichroic film  51  is provided on the lower surface  52 B of the phosphor layer  52 . The dichroic film  51  transmits the excitation light BL S  and reflects the fluorescence YL produced in the phosphor layer  52 . The dichroic film  51  corresponds to the “second reflection layer” set forth in the appended claims. 
     In the wavelength conversion element  540  according to the present embodiment, the excitation light BL S  is strongly scattered in the form of Rayleigh scattering. The scattered excitation light BL S  is likely to be absorbed because the optical path length thereof lengthens in the phosphor layer  52 . 
     The fluorescence YL, which is unlikely to be scattered by the scattering particles  46 , travels roughly straight in the phosphor layer  53  and therefore reaches the upper surface  52 A along a short optical path. Therefore, since the fluorescence YL passes through the phosphor layer  52  in the excited state along the short optical path, the fluorescence YL is unlikely to be absorbed, whereby the effect of the optical quenching can be suppressed. 
     In the present embodiment, the irregular section  57  is formed across the upper surface  52 A of the phosphor layer  52 . The irregular section  57  is formed by roughening the upper surface  52 A. The irregular section  57  is intended to extract the fluorescence YL (cause fluorescence YL to exit) out of the upper surface  52 A of the phosphor layer  52 . The irregular section  57  corresponds to the “incident angle changing section” set forth in the appended claims. 
     The wavelength conversion element  540  according to the present embodiment, which includes the irregular section  57 , can change the angle of incidence of the fluorescence YL with respect to the upper surface  52 A. That is, even the fluorescence YL incident on the upper surface with no irregular section  57  formed thereacross at an angle greater than the critical angle is incident on the surface of the irregular section  57  at an angle smaller than the critical angle. The fluorescence YL totally reflected off the upper surface with no irregular section  57  formed thereacross passes through the upper surface  52 A with the irregular section  57  formed thereacross and exits out of the phosphor layer  52 . 
     Providing the irregular section  57  therefore causes the fluorescence YL to be unlikely to be trapped in the phosphor layer  52 , whereby a decrease in the efficiency of extraction of the fluorescence YL can be suppressed. 
     As described above, the wavelength conversion element  540  according to the present embodiment can be a wavelength conversion element capable of suppressing the effect of the optical quenching of the fluorescence YL without a decrease in the efficiency of extraction of the fluorescence YL. 
     The invention is not limited to the contents of the embodiments described above but can be changed as appropriate to the extent that the change does not depart from the substance of the invention. 
     For example, the above-mentioned first embodiment has been described with reference to the case where the reflection layer  43  is formed on the upper surface  41   a  of the base  41 , but not necessarily in the invention. That is, the reflection layer  43  may instead be formed on the lower surface  42 B of the phosphor layer  42 . In this case, the wavelength conversion element  40  has a configuration in which the phosphor layer  42  with the reflection layer  43  formed on the lower surface  42 B is bonded to the upper surface  41   a  of the base  41  via the bonding layer  44 . 
     For example, in the first embodiment described above, in the phosphor layer  42  of the wavelength conversion element  40 , the activator concentration in the region facing the lower surface  42 B may instead be higher than the activator concentration in the region facing the upper surface  42 A. 
     For example, in the third variation of the first embodiment described above, in the phosphor layer  42  of the wavelength conversion element  340 , the activator concentration in the region facing the lower surface  42 B may be higher than the activator concentration in the region facing the upper surface  42 A, as in the second variation. Further, the density of the scattering particles  46  in the region facing the lower surface  42 B may be higher than the density of the scattering particles  46  in the region facing the upper surface  42 A. 
     For example, in the fourth variation of the first embodiment described above, in the phosphor layer  42  of the wavelength conversion element  440 , the activator concentration in the region facing the lower surface  42 B may be higher than the activator concentration in the region facing the upper surface  42 A, as in the second variation. Further, the density of the scattering particles  46  in the region facing the lower surface  42 B may be higher than the density of the scattering particles  46  in the region facing the upper surface  42 A. 
     Further, the above-mentioned first embodiment has been described with reference to the case where the light source apparatus according to the embodiment of the invention is incorporated in a projector, but not necessarily. The light source apparatus according to the embodiment of the invention may be used as a lighting apparatus, a headlight of an automobile, and other components. 
     The entire disclosure of Japanese Patent Application No. 2017-241908, filed on Dec. 18, 2017 is expressly incorporated by reference herein.