Patent Publication Number: US-2017353701-A1

Title: Light source apparatus and projector

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a light source apparatus and a projector. 
     2. Related Art 
     There is a known projector of related art including a light source apparatus, a light modulator that modulates light outputted from the light source apparatus to form an image according to image information, and a projection optical apparatus that enlarges and projects the formed image on a projection surface, such as a screen. As a projector of this type, there is a known projector including a light source apparatus including semiconductor lasers and a reflective color wheel (see Japanese Patent No. 5,429,079, for example). 
     In the projector described in Japanese Patent No. 5,429,079, the reflective color wheel has a base rotated with a motor as a rotating mechanism, and the base has one surface that is so processed as to have a mirror surface and divided into a plurality of segments at 2-degrees rotational angular intervals. Phosphor layers that emit red light, green light, and blue light when excited with excitation light fluxes incident from the semiconductor lasers are sequentially formed on the segments along the rotational direction of the base. When the thus configured base is rotated, and the phosphor layers, on which the excitation light fluxes are incident, are sequentially switched from one to another, the color light fluxes are sequentially outputted. 
     The phosphor layers on the reflective color wheel generate heat when the excitation light fluxes are incident thereon, and when the phosphor layers are heated to too high a temperature, the efficiency of conversion of the wavelength of the excitation light decreases. To avoid the situation, the reflective color wheel has a plurality of fins, which function as a heat dissipater, formed on and integrated with the rear surface of the base. Examples of the fins may include a plurality of fins concentrically formed around the center of rotation of the base and a plurality of fins radially formed from the side facing the center of rotation. Still another example of the fins is a plurality of fins helically formed around the center of rotation. 
     In general, a plurality of fins located on a rotating optical element, such as the reflective color wheel, are radially or helically formed so that a cooling gas having cooled the fins is readily discharged toward the outer circumference of the base. 
     However, when the plurality of fins are, for example, densely arranged so that the inter-fin dimension is inappropriate, the cooling air having cooled the fins are unlikely to be discharged from the side facing the center of rotation of the base toward the outer circumference thereof, undesirably resulting in a decrease in cooling efficiency. 
     Further, the life of an optical element having a plurality of fins, such as the reflective color wheel described in Japanese Patent No. 5,429,079, depends on how well the generated heat is dissipated, in other words, how high the efficiency of cooling of the optical element. 
     A structure for efficiently cooling an optical element has therefore been desired. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a light source apparatus that allows improvement in efficiency of cooling of an optical element, and another advantage of some aspects of the invention is to provide a projector. 
     A light source apparatus according to a first aspect of the invention includes a light source, an optical element on which light emitted from the light source is incident, and a rotating device that rotates the optical element. The optical element includes a substrate rotated by the rotating device, an optical element layer located on a first surface of the substrate and disposed inside an outer edge of the substrate and along a rotational direction of the substrate, with the light emitted from the light source being incident on the first surface, and a heat dissipater located on at least one of the first surface and a second surface opposite the first surface. The heat dissipater has a plurality of fins extending from a side facing a center of rotation of the optical element toward an outer circumference of the optical element and arranged along the rotational direction. Among the plurality of fins, a dimension along the rotational direction between two fins adjacent to each other in the rotational direction is so set as to fall within a predetermined dimension range. 
     A wavelength conversion element having, as the optical element layer, a wavelength conversion layer (phosphor layer, for example) that converts the wavelength of light incident on the layer and a diffusion element having, as the optical element layer, a diffusion layer that diffuses light incident on the layer can be exemplified as the optical element. 
     According to the first aspect, the dimension along the rotational direction of the optical element between two fins adjacent to each other in the rotational direction (that is, inter-fin channel width along the direction perpendicular, in a plan view, to the extension direction of a channel of a cooling gas flowing through the space between the fins) is so set as to fall within a predetermined dimension range. Therefore, in a portion where the channel width is so set as to fall within the dimension range, the fins can readily produce vortices of the cooling gas (vortices having an axis of rotation along the extension direction of the fins) when the optical element rotates, and the produced vortices are allowed to be likely to collide with the facing end surfaces of the two adjacent fins. As a result, the cooling gas is allowed to effectively collide with the fins, whereby heat in the fins can be readily transferred to the cooling gas. Therefore, heat generated in the optical element layer can be efficiently cooled, whereby the efficiency of cooling of the optical element can be improved. In addition, since the optical element is stabilized, a light source apparatus capable of stably outputting light can be configured. 
     In the first aspect, it is preferable that the dimension along the rotational direction between the adjacent two fins is so set as to fall within the dimension range at least in a portion on a side facing the outer circumference of the optical element layer. 
     In a case where heat is generated in the optical element layer, the heat is transferred from the optical element layer to the substrate and then transferred to an inner circumferential area (area facing center of rotation) of the substrate, which is the area inside the inner circumference of the optical element layer, and an outer circumferential area of the substrate, which is the area outside the outer circumference of the optical element layer. When the optical element is rotated, since the flow speed of the cooling gas flowing through the outer circumferential area is greater than the flow speed of the cooling gas flowing through the inner circumferential area, the outer circumferential area of the substrate is more likely to be cooled than the inner circumferential area of the substrate, and the heat described above is also more likely to be transferred to the outer circumferential area than to the inner circumferential area. 
     In view of the fact described above, according to the first aspect, at least in the portion outside the outer circumference of the optical element layer (outer circumferential area), the channel width is so set as to fall within the dimension range, whereby the efficiency of cooling of the portion outside the outer circumference can be reliably improved. The efficiency of cooling of the optical element can therefore be reliably improved as compared with a case where the area where the channel width is so set as to fall within the dimension range is only the inner circumferential area, which is the area inside the inner circumference of the optical element layer. 
     In the first aspect, it is preferable that the dimension along the rotational direction between the adjacent two fins is so set as to fall within the dimension range over a range from the center of rotation to an outer circumference of the substrate. 
     According to the configuration described above, the channel width is so set as to fall within the dimension range in the inner circumferential area and the outer circumferential area. The heat can therefore be further readily transferred from the fins to the cooling gas as compared with a case where the channel width is so set as to fall within the dimension range only in the outer circumferential area. The efficiency of cooling of the optical element can therefore be more reliably improved. 
     In the first aspect, it is preferable that the dimension range is set in accordance with a size of vortices of a cooling gas that are formed by the plurality of fins when the optical element rotates. 
     In a case where the channel width is relatively narrow, for example, in a case where the channel width is smaller than the size of the vortices, the vortices are unlikely to be produced even when the optical element is rotated. On the other hand, in a case where the channel width is significantly greater than the size of the vortices, the vortices, even when they are produced in the spaces between the fins, are unlikely to collide with the fins located on the side facing opposite the rotational direction, and the cooling efficiency is therefore not very high. 
     In contrast, according to the configuration described above, in which the dimension range is set in accordance with the size of the vortices, the dimension range can be so set that each of the produced vortices collides with the two fins that sandwich the vortex. Further, when the channel width is so set as to fall within the thus set dimension range, the efficiency of cooling of the optical element can be reliably improved. 
     In the first aspect, it is preferable that the dimension range is greater than or equal to 3 mm and smaller than or equal to 6 mm. 
     According to the configuration described above, not only can the vortices be reliably produced in the spaces between the plurality of fins when the optical element rotates, but also each of the vortices is allowed to reliably collide with the two fins that sandwich the vortex. The efficiency of cooling of the optical element can therefore be reliably improved. 
     In the first aspect, it is preferable that a dimension of the plurality of fins in a direction along an axis of rotation of the optical element is at least 3 mm. 
     In a case where the optical element is rotated at a rotational speed, for example, greater than or equal to 3000 rpm and smaller than or equal to 9000 rpm, and when the dimension of the fins in the direction along the axis of rotation (standing dimension of fins measured from substrate) is smaller than 3 mm, the vortices are likely to collide with the bottom surface of the substrate (surface from fins stand), so that the vortices are unlikely to be continuously produced. 
     In contrast, when the dimension of the fins is at least 3 mm, the vortices are unlikely to collide with the bottom surface of the substrate, so that the vortices are likely to be continuously produced. The efficiency of cooling of the optical element can therefore be more reliably improved. 
     In the first aspect, it is preferable that the plurality of fins include a plurality of first fins arranged along the rotational direction and a plurality of second fins that are each disposed between two first fins adjacent to each other among the plurality of first fins and are arranged along the rotational direction, that an end of each of the plurality of first fins that faces the center of rotation is located on a first virtual circle a center of which coincides with the center of rotation and which has a predetermined diameter, that an end of each of the plurality of first fins that faces the outer circumference is located on a second virtual circle a center of which coincides with the center of rotation and which has a diameter greater than the diameter of the first virtual circle, that an end of each of the plurality of second fins that faces the center of rotation is located on a third virtual circle a center of which coincides with the center of rotation and which has a diameter greater than the diameter of the first virtual circle and smaller than the diameter of the second virtual circle, that an end of each of the plurality of second fins that faces the outer circumference is located on the second virtual circle, and that among the plurality of first fins and the plurality of second fins, a dimension along the rotational direction between a first fin and a second fins adjacent to each other in the rotational direction is so set as to fall within the dimension range. 
     In a case where the fins so configured that the dimension (channel width) is so set as to fall within the dimension range are densely formed on the substrate in a cutting process, the dimension between the fins on the side facing the center of rotation could undesirably exceed the dimension range depending on the size of a cutting tool. 
     In contrast, the second fins, which are smaller than the first fins, are so provided as to be located in the area outside the outer circumference of the first virtual circle and between the first fins, whereby the above-mentioned dimension between the first fins is readily so set as to fall within the dimension range over the area from the first virtual circle to the third virtual circle, and the dimension between the first fins and the second fins is readily so set as to fall within the dimension range over the area from the third virtual circle to the second virtual circle. Therefore, not only can the fins so configured that the channel width is set as described above be reliably formed in a cutting process, but also an optical element having improved cooling efficiency can be reliably configured. 
     In the first aspect, it is preferable that an intersection angle between a line tangent to an edge of each of the fins that faces in the rotational direction and a radial direction originating from the center of rotation is so set as to fall within an angular range greater than or equal to −45° and smaller than or equal to +60°. 
     A case where the intersection angle is 0° represents that each of the fins is perpendicular, in a position where the tangent is specified, to the rotational direction of the substrate. In a case where the intersection angle is negative (has negative value), each of the fins has a shape that, for example, warps toward the side facing in the rotational direction with distance from the side facing the center of rotation toward the outer circumference, whereas in a case where the intersection angle is positive (has positive value), each of the fins has a shape that, for example, warps toward the side facing opposite the rotational direction with distance from the side facing the center of rotation toward the outer circumference. 
     The heat transfer coefficient representing heat transfer from the fins to the cooling gas flowing toward the outer circumference changes in accordance with the intersection angle. For example, when the intersection angle is a value outside the angular range, the fins are likely to follow the rotational direction, and vortices are unlikely to be produced, resulting in a decrease in the effect. 
     In contrast, according to the configuration described above, in which the intersection angle is so set as to fall within the angular range, vortices are likely to be produced, whereby the efficiency of cooling of the optical element can be reliably improved. 
     The angular range is preferably greater than 0° and smaller than or equal to +60°. 
     In the case where the intersection angle is negative (has negative value), so that the fin has a shape that warps toward the side facing in the rotational direction with distance from the side facing the center of rotation toward the outer circumference, and when the optical element is rotated, pressure that causes the cooling gas to flow from the side facing the outer circumference toward the center of rotation is produced. In this case, the cooling gas is likely to stay between the fins, and the efficiency of cooling of the optical element therefore decreases. 
     On the other hand, in the case where the intersection angle is 0°, so that the fin is perpendicular to the rotational direction of the substrate, the rotational resistance (air resistance) of the optical element increases, and the load acting on the rotating device therefore increases. 
     In contrast, when the angular range is greater than 0° and smaller than or equal to +60°, the problems described above are solved, whereby an optical element having further improved cooling efficiency can be configured. 
     A light source apparatus according a second aspect of the invention includes a light source, an optical element on which light emitted from the light source is incident, and a rotating device that rotates the optical element. The optical element includes a substrate rotated by the rotating device, an optical element layer located on a first surface of the substrate and disposed inside an outer edge of the substrate and along a rotational direction of the substrate, with the light emitted from the light source being incident on the first surface, and a heat dissipater located on at least one of the first surface and a second surface opposite the first surface. The heat dissipater has a plurality of fins extending from a side facing a center of rotation of the optical element toward an outer circumference of the optical element and arranged along the rotational direction. An intersection angle between a line tangent to an edge of each of the plurality fins that faces opposite the rotational direction and a radial direction originating from the center of rotation is so set as to fall within a predetermined angular range. 
     A wavelength conversion element having, as the optical element layer, a wavelength conversion layer that converts the wavelength of light incident on the layer and a diffusion element having, as the optical element layer, a diffusion layer that diffuses light incident on the layer can be exemplified as the optical element. 
     According to the second aspect, in which the intersection angle of each of the fins is so set as to fall within the predetermined angular range, when the optical element is rotated, a vortex of the cooling gas is likely to be produced in the space between two fins adjacent to each other in the rotational direction of the optical element. The vortex collides with the facing end surfaces of the two fins (end surface of the rotational-direction-side fin that faces opposite the rotational direction and rotational-direction-side end surface of the fin facing opposite the rotational direction), whereby heat transfer from the fins to the cooling gas can be facilitated. The efficiency of cooling of the fins to which heat generated in the optical element layer is transferred via the substrate and hence the efficiency of cooling of the optical element can therefore be improved. As a result, the life of the optical element can be prolonged. 
     In the second aspect, it is preferable that each of the plurality of fins has a radius of curvature that changes with a position thereon. 
     According to the configuration described above, the above-mentioned intersection angle of part or entirety of the fins can be readily so set as to fall within the angular range. Therefore, in a portion where the intersection angle is so set as to fall within the angular range, the vortices are likely to be produced between the fins, whereby improvement in the efficiency of cooling of the optical element and extension of the life of the optical element can be reliably achieved. 
     In the second aspect, it is preferable that each of the plurality of fins is formed in an arcuate shape having the radius of curvature that increases with distance from the side facing the center of rotation toward the outer circumference. 
     According to the configuration described above, the intersection angle can be readily so set as to fall within the angular range across the entire fins. Therefore, since the vortices can be readily produced over the entire spaces between the fins, the improvement in the efficiency of cooling of the optical element and the extension of the life of the optical element can be more reliably achieved than in a case where the vortices are produced in part of the spaces between the fins. 
     In the second aspect, it is preferable that the angular range is greater than or equal to −45° and smaller than or equal to +60°. 
     According to the configuration described above, since the fins can face in the rotational direction of the optical element, the vortices can be more readily formed on the side of each of the fins that faces opposite the rotational direction when the optical element rotates. The improvement in the efficiency of cooling of the optical element and the extension of the life of the optical element can therefore be more reliably achieved. 
     In the second aspect, it is preferable that the angular range is greater than 0° and smaller than or equal to +60°. 
     In the case where the above-mentioned intersection angle of each of the fins is 0°, that is, in the case of the fins that radially extend from the side facing the center of rotation, the load acting on the rotating device, which rotates the optical element, increases because the fins are perpendicular to the rotational direction and the air resistance (rotational resistance) therefore increases. 
     On the other hand, the fins having the intersection angle greater than or equal to −45° and smaller than 0° each have a shape that warps toward the side facing in the rotational direction with distance from the side facing the center of rotation toward the outer circumference of the substrate. When the fins have the shape, and the optical element is rotated, pressure that causes the cooling gas to flow from the side facing the outer circumference toward the center of rotation is produced. In this case, the cooling gas flowing from the side facing the center of rotation toward the outer circumference is likely to stay between the fins, and the efficiency of cooling of the optical element therefore decreases. 
     In contrast, according to the configuration described above, the cooling gas flowing through the spaces between the fins is likely to flow from the side facing the center of rotation toward the outer circumference, whereby the flow speed and flow rate of the cooling gas can be increased. Therefore, since the situation in which the cooling gas to which the heat is transferred from the fins stays in the spaces between the fins can be avoided, the efficiency of cooling of the optical element can be further improved. 
     In the second aspect, it is preferable that, among the plurality of fins, a dimension along the rotational direction between two fins adjacent to each other in the rotational direction of the optical element is so set as to fall within a predetermined dimension range. 
     In the case where the dimension, that is, the width of the channel of the cooling gas flowing through the spaces between the fins is relatively narrow, for example, in a case where the channel width is smaller than the size of the vortices, the vortices are unlikely to be produced in the spaces between the fins even when the optical element is rotated. On the other hand, in the case where the channel width is significantly greater than the size of the vortices, the vortices, even when they are produced in the spaces between the fins, are unlikely to collide with the fins located on the side facing opposite the rotational direction (in detail, rotational-direction-side end surfaces of fins), and the cooling efficiency is therefore not very high. 
     In contrast, according to the configuration described above, in which the dimension along the rotational direction between the two fins is so set as to fall within the dimension range set in accordance, for example, with the size of the vortices, each of the produced vortices is allowed to collide with a fin on the rotational direction side and a fin adjacent to the rotational-direction-side fin and facing opposite the rotational direction. The efficiency of cooling of the optical element can therefore be more reliably improved. 
     In the second aspect, it is preferable that the dimension range is greater than or equal to 3 mm and smaller than or equal to 6 mm. 
     According to the configuration described above, not only can the vortices be reliably produced in the spaces between the plurality of fins when the optical element rotates, but also each of the vortices is allowed to reliably collide with a fin on the rotational direction side and a fin adjacent to the rotational-direction-side fin and facing opposite the rotational direction. The efficiency of cooling of the optical element can therefore be reliably improved. 
     A projector according to a third aspect of the invention includes any of the light source apparatus described above, a light modulator that modulates light outputted from the light source apparatus, and a projection optical apparatus that projects the light modulated by the light modulator. 
     According to the third aspect, the same effects provided by the light source apparatus according to the first and second aspects can be provided. In addition, since the light source apparatus can stably output light, a reliable projector can be configured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view showing an exterior appearance of a projector according to a first embodiment of the invention. 
         FIG. 2  is a diagrammatic view showing the configuration of an apparatus body in the first embodiment. 
         FIG. 3  is a diagrammatic view showing the configuration of an illuminator in the first embodiment. 
         FIG. 4  is a perspective view of a wavelength conversion element viewed from the side opposite the light incident side in the first embodiment. 
         FIG. 5  is a diagrammatic view showing vortices produced by fins in the first embodiment. 
         FIG. 6  shows graphs illustrating the relationship between a channel width and a heat transfer coefficient for each rotational speed in the first embodiment. 
         FIG. 7  is a perspective view of the wavelength conversion element viewed from the light incident side in the first embodiment. 
         FIG. 8  is a perspective view of a wavelength conversion element provided in a light source apparatus of a projector according to a second embodiment of the invention and viewed from the side opposite the light incident side. 
         FIG. 9  is a plan view of the wavelength conversion element in the second embodiment viewed from the side opposite the light incident side. 
         FIG. 10  shows a graph illustrating the relationship between a channel width and a heat transfer coefficient ratio in the second embodiment. 
         FIG. 11  is a perspective view of a wavelength conversion element provided in a light source apparatus of a projector according to a third embodiment of the invention and viewed from the side opposite the light incident side. 
         FIG. 12  is a plan view of the wavelength conversion element in the third embodiment viewed from the side opposite the light incident side. 
         FIG. 13  is a perspective view of a wavelength conversion element provided in a light source apparatus of a projector according to a fourth embodiment of the invention and viewed from the side opposite the light incident side. 
         FIG. 14  is a plan view of the wavelength conversion element in the fourth embodiment viewed from the side opposite the light incident side. 
         FIG. 15  describes the intersection angle between a tangent corresponding to the position on a fin&#39;s edge facing opposite the rotational direction of a substrate and the radial direction of the substrate in the fourth embodiment. 
         FIG. 16  is a plan view of a wavelength conversion element presented as a comparative example in the fourth embodiment and viewed from the side opposite the light incident side. 
         FIG. 17  shows graphs illustrating the relationship between the angle of the fins with respect to the radial direction (tangent intersection angle) and the heat transfer coefficient for each channel width in the fourth embodiment. 
         FIG. 18  shows graphs illustrating the relationship between the angle of the fins with respect to the radial direction (tangent intersection angle) and the heat transfer coefficient ratio for each channel width in the fourth embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
     A first embodiment of the invention will be described below with reference to the drawings. 
     Schematic Configuration of Projector 
       FIG. 1  is a perspective view showing an exterior appearance of a projector  1  according to the present embodiment. 
     The projector  1  according to the present embodiment is a projection-type image display apparatus that modulates light outputted from a light source apparatus  5 , which will be described later, to form an image according to image information and enlarges and projects the formed image on a projection surface PS, such as a screen. The projector  1  includes an exterior enclosure  2 , which forms the exterior appearance of the projector  1 , and an apparatus body  3  (see  FIG. 2 ), which is accommodated and disposed in the exterior enclosure  2 , as shown in  FIG. 1 . 
     The thus configured projector  1 , which will be described later in detail, is partly characterized in that a heat dissipater  65 , which is part of a wavelength conversion element  61 , which forms the light source apparatus  5  (see  FIG. 3 ), has a plurality of fins  66 , and that the width of channels of a cooling gas flowing through the spaces between the plurality of fins  66  is adequately set. 
     The configuration of the projector  1  will be described below. 
     Configuration of Exterior Enclosure 
     The exterior enclosure  2  is formed of an upper case  2 A, a lower case  2 B, a front case  2 C, and a rear case  2 D, each of which is made of a synthetic resin and which are combined with one another into a roughly box-shaped shape, as shown in  FIG. 1 . The thus configured exterior enclosure  2  has a top surface section  21 , a bottom surface section  22 , a front surface section  23 , a rear surface section  24 , a left side surface section  25 , and a right side surface section  26 . 
     Legs  221  ( FIG. 1  shows only two legs  221 ), which come into contact with an installation surface, in a case where the projector  1  is placed on the installation surface, are provided at a plurality of locations on the bottom surface section  22 . 
     To expose an end portion  461  of a projection optical apparatus  46 , which will be described later, an opening  231 , through which an image projected by the projection optical apparatus  46  passes, is formed in a central portion of the front surface section  23 . 
     Discharge ports  232 , through which a heated cooling gas in the exterior enclosure  2  is discharged, are formed in the front surface section  23  and in positions shifted toward the left side surface section  25 , and each of the discharge ports  232  is provided with a plurality of louvers  233 . 
     On the other hand, a plurality of indicators  234 , which indicate the action state of the projector  1 , are provided on the front surface section  23  and in positions shifted toward the right side surface section  26 . 
     An introduction port  261 , through which outside air is introduced as the cooling gas into the exterior enclosure  2 , is provided in the right side surface section  26 , and a cover member  262 , which is provided with a filter (not shown), is attached to the introduction port  261 . 
     Configuration of Apparatus Body 
       FIG. 2  is a diagrammatic view showing the configuration of the apparatus body  3 . 
     The apparatus body  3  includes an image projection apparatus  4 , as shown in  FIG. 2 . The apparatus body  3  further includes, although not shown, a controller that controls the action of the projector  1 , a power supply that supplies electric parts that form the projector  1  with electric power, and a cooler that cools objects to be cooled. 
     Configuration of Image Projection Apparatus 
     The image projection apparatus  4  forms an image according to an image signal inputted from the controller described above and projects the image on the projection surface PS. The image projection apparatus  4  includes an illuminator  41 , a color separation apparatus  42 , parallelizing lenses  43 , light modulators  44 , a color combiner  45 , and the projection optical apparatus  46 . 
     Among the components described above, the illuminator  41  outputs illumination light WL, which uniformly illuminates the light modulators  44 . The configuration of the illuminator  41  will be described later in detail. 
     The color separation apparatus  42  separates the illumination light WL incident from the illuminator  41  into blue light LB, green light LG, and red light LR. The color separation apparatus  42  includes dichroic mirrors  421  and  422 , reflection mirrors  423 ,  424 , and  425 , relay lenses  426  and  427 , and an optical part enclosure  482 , which accommodates the components described above. 
     The dichroic mirror  421  transmits the blue light LB contained in the illumination light WL described above and reflects the green light LG and the red light LR contained therein. The blue light LB having passed through the dichroic mirror  421  is reflected off the reflection mirror  423  and guided to the corresponding parallelizing lens  43  ( 43 B). 
     The dichroic mirror  422  receives the green light LG and the red light LR reflected off the dichroic mirror  421  described above, reflects and guides the green light LG to the corresponding parallelizing lens  43  ( 43 G), and transmits the red light LR. The red light LR is guided to the corresponding parallelizing lens  43  ( 43 R) via the relay lens  426 , the reflection mirror  424 , the relay lens  427 , and the reflection mirror  425 . 
     Each of the parallelizing lenses  43  (reference characters  43 R,  43 G, and  43 B denote parallelizing lenses for red, green, and blue color light fluxes, respectively) parallelizes the light incident thereon. 
     The light modulators  44  (reference characters  44 R,  44 G, and  44 B denote light modulators for red, green, and blue color light fluxes, respectively) modulate the color light fluxes LR, LG, and LB described above having been parallelized and incident thereon to form images based on the color light fluxes LR, LG, and LB according to image signals inputted from the controller. Each of the light modulators  44  is formed, for example, of a liquid crystal panel that modulates a color light flux incident thereon and polarizers disposed on the light incident side and the light exiting side of the liquid crystal panel. 
     The color combiner  45  combines the images based on the color light fluxes LR, LG, and LB incident from the light modulators  44 R,  44 G, and  44 B with one another. The color combiner  45  is formed of a cross dichroic prism in the present embodiment and can instead be formed of a plurality of dichroic mirrors. 
     The projection optical apparatus  46  enlarges and projects the combined image from the color combiner  45  on the projection surface PS. As the thus configured projection optical apparatus  46 , for example, a lens unit formed of a lens barrel and a plurality of lenses disposed in the lens barrel can be employed. 
     Configuration of Illuminator 
       FIG. 3  is a diagrammatic view showing the configuration of the illuminator  41 . 
     The illuminator  41  outputs the illumination light WL toward the color separation apparatus  42 , as described above. The illuminator  41  includes the light source apparatus  5  and a homogenizing apparatus  7 , as shown in  FIG. 3 . 
     Configuration of Light Source Apparatus 
     The light source apparatus  5  outputs a light flux to the homogenizing apparatus  7 . The light source apparatus  5  includes a light source section  51 , an afocal optical element  52 , a first retardation element  53 , a homogenizer optical apparatus  54 , a light combiner  55 , a second retardation element  56 , a first light collecting element  57 , a diffuser  58 , a second light collecting element  59 , and a wavelength converter  6 . 
     Among the components described above, the light source section  51 , the afocal optical element  52 , the first retardation element  53 , the homogenizer optical apparatus  54 , the second retardation element  56 , the first light collecting element  57 , and the diffuser  58  are arranged along a first illumination optical axis Ax 1 . On the other hand, the second light collecting element  59  and the wavelength converter  6  are arranged along a second illumination optical axis Ax 2  perpendicular to the first illumination optical axis Ax 1 . The light combiner  55  is disposed in the portion where the first illumination optical axis Ax 1  and the second illumination optical axis Ax 2  intersect each other. 
     Configuration of Light Source Section 
     The light source section  51  is a light source that emits excitation light that is blue light toward the afocal optical element  52 . The light source section  51  includes a first light source section  511 , a second light source section  512 , and a light combining member  513 . 
     The first light source section  511  includes a solid-state light source array  5111 , in which solid-state light sources SS, each of which is an LD (laser diode), are arranged in a matrix, and a plurality of parallelizing lenses (not shown) corresponding to the solid-state light sources SS. The second light source section  512  similarly includes a solid-state light source array  5121 , in which solid-state light sources SS are arranged in a matrix, and a plurality of parallelizing lenses (not shown) corresponding to the solid-state light sources SS. The solid-state light sources SS emit excitation light fluxes, for example, having a peak wavelength of 440 nm but may instead emit excitation light fluxes having a peak wavelength of 446 nm. Still instead, each of the light source sections  511  and  512  may include both the solid-state light sources that emit excitation light fluxes having the peak wavelength of 440 nm and those that emit excitation light fluxes having the peak wavelength of 446 nm. The excitation light fluxes emitted from the solid-state light sources SS are parallelized by the parallelizing lenses and incident on the light combining member  513 . In the present embodiment, the excitation light emitted from each of the solid-state light sources SS is S polarized light. 
     The light combining member  513  combines the excitation light fluxes with one another by transmitting the excitation light fluxes outputted from the first light source section  511  along the first illumination optical axis Ax 1  and reflecting the excitation light fluxes outputted from the second light source section  512  along a direction that intersects the first illumination optical axis Ax 1 . In the present embodiment, the light combining member  513  is configured as a plate-shaped member in which a plurality of transmitters that transmit the excitation light fluxes from the first light source section  511  and a plurality of reflectors that reflect the excitation light fluxes from the second light source section  512  are alternately arranged. The excitation light fluxes having traveled via the thus configured light combining member  513  are incident on the afocal optical element  52 . 
     Configuration of Afocal Optical Element 
     The afocal optical element  52  adjusts the diameter of the excitation light incident from the light source section  51 . Specifically, the afocal optical element  52  includes a lens  521 , which causes the excitation light incident as parallelized light from the light source section  51  to converge so that the diameter of the excitation light decreases, and a lens  522 , which parallelizes the excitation light incident through the lens  521  and outputs the parallelized excitation light. 
     Configuration of First Retardation Element 
     The first retardation element  53  is a half-wave plate. That is, the S-polarized excitation light incident through the afocal optical element  52 , when it passes through the first retardation element  53 , is partially converted into P-polarized excitation light to form excitation light formed of S-polarized and P-polarized light and then incident on the homogenizer optical apparatus  54 . 
     Configuration of Homogenizer Optical Apparatus 
     The homogenizer optical apparatus  54  homogenizes, along with the first light collecting element  57  and the second light collecting element  59 , the illuminance distribution of the excitation light to be incident on illumination areas of the diffuser  58  and the wavelength converter  6 . The excitation light having passed through the homogenizer optical apparatus  54  is incident on the light combiner  55 . The thus configured homogenizer optical apparatus  54  includes a first multi-lens  541  and a second multi-lens  542 . 
     The first multi-lens  541  has a configuration in which a plurality of first lenses  5411  are arranged in a matrix in a plane perpendicular to the first illumination optical axis Ax 1  and divides the excitation light incident on the first multi-lens  541  into a plurality of sub-light fluxes. 
     The second multi-lens  542  has a configuration in which a plurality of second lenses  5421 , which correspond to the plurality of first lenses  5411  described above, are arranged in a matrix in a plane perpendicular to the first illumination optical axis Ax 1 . The second multi-lens  542 , in cooperation with the first light collecting element  57  and the second light collecting element  59 , superimposes the plurality of divided sub-light fluxes on one another in the illumination areas described above. The illuminance of the excitation light incident on the illumination areas is homogenized in a plane perpendicular to the center axis of the excitation light. 
     Configuration of Light Combiner 
     The light combiner  55  is a PBS (polarizing beam splitter) formed of a prism  551 , which is formed in the shape of a roughly right-angled isosceles triangular column, with a surface  552 , which corresponds to the oblique side of the triangular shape, inclining by about 45° with respect to the first illumination optical axis Ax 1  and the second illumination optical axis Ax 2 , a surface  553 , which is one of the adjacent sides  553  and  554  of the triangular shape, being roughly perpendicular to the second illumination optical axis Ax 2 , and the surface  554 , which is the other one of the adjacent sides, being roughly perpendicular to the first illumination optical axis Ax 1 . A polarization separation layer  555 , which has wavelength selectivity, is formed on the surface  552 . 
     The polarization separation layer  555  is characterized not only in that it separates the S polarized light and the P polarized light contained in the excitation light from each other but also in that it transmits fluorescence produced by the wavelength converter  6  irrespective of the polarization state of the fluorescence. That is, the polarization separation layer  555  has a wavelength-selective polarization separation characteristic as follows: The polarization separation layer  555  separates light having wavelengths within the blue light region into S polarized light and P polarized light and transmits both S polarized light and P polarized light contained in light having wavelengths within the green light region and the red light region. 
     The thus configured light combiner  55 , which receives the excitation light incident through the homogenizer optical apparatus  54 , transmits P polarized light out of the incident excitation light along the first illumination optical axis Ax 1  toward the second retardation element  56  and reflects S polarized light out of the incident excitation light along the second illumination optical axis Ax 2  toward the second light collecting element  59 . 
     Although will be described later in detail, the light combiner  55  combines the excitation light incident via the second retardation element  56  (blue light) and the fluorescence incident via the second light collecting element  59  with each other. 
     Configuration of Second Retardation Element 
     The second retardation element  56  is a quarter-wave plate, converts the P-polarized excitation light incident from the light combiner  55  into circularly polarized light, and converts the excitation light incident through the first light collecting element  57  (circularly polarized light having a polarization direction opposite the polarization direction of the initial circularly polarized light) into S polarized light. 
     Configuration of First Light Collecting Element 
     The first light collecting element  57  is an optical element that collects the excitation light having passed through the second retardation element  56  (cause the excitation light to converge) on the diffuser  58  and is formed of three lenses  571  to  573  in the present embodiment. The number of lenses that form the first light collecting element  57  is, however, not limited to three. 
     Diffuser 
     The diffuser  58  diffusively reflects the excitation light incident thereon in such a way that the reflected excitation light has the same diffusion angle as that of the fluorescence produced by and outputted from the wavelength converter  6 . The diffuser  58  includes a disc-shaped diffusive reflection element  581 , on which an annular reflection layer is formed around the center of rotation of the diffuser  58 , and a rotating device  582 , which rotates the diffusive reflection element  581 . The reflection layer reflects light incident thereon in the Lambertian reflection scheme. 
     The excitation light diffusively reflected off the thus configured diffuser  58  is incident on the second retardation element  56  again via the first light collecting element  57 . When reflected off the diffuser  58 , the circularly polarized light incident on the diffuser  58  becomes circularly polarized light having a polarization direction opposite the polarization direction of the incident circularly polarized light and passes through the second retardation element  56 , where the circularly polarized light is converted into S-polarized excitation light having a polarization direction rotated by 90° with respect to the polarization direction of the P-polarized excitation light incident from the light combiner  55 . The resultant S-polarized excitation light is reflected off the polarization separation layer  555  described above and incident as the blue light along the second illumination optical axis Ax 2  on the homogenizing apparatus  7 . 
     Configuration of Second Light Collecting Element 
     On the second light collecting element  59  is incident the S-polarized excitation light having passed through the homogenizer optical apparatus  54  and having been reflected off the polarization separation layer  555  described above. The second light collecting element  59  not only collects the excitation light incident thereon (causes excitation light to converge) on the illumination area of the wavelength converter  6  (phosphor layer  63  of wavelength conversion element  61 ) as described above but also parallelizes the fluorescence emitted from the wavelength converter  6  and outputs the fluorescence to the polarization separation layer  555  described above. The second light collecting element  59  is formed of three pickup lenses  591  to  593  in the present embodiment, but the number of lenses that form the second light collecting element  59  is not limited to three, as in the case of the first light collecting element  57  described above. 
     Configuration of Wavelength Converter 
     The wavelength converter  6  converts, in terms of wavelength, the blue excitation light incident thereon into fluorescence containing green light and red light. The wavelength converter  6  includes a rotating device  60  and the wavelength conversion element  61 . 
     Out of the two components described above, the rotating device  60  is formed, for example, of a motor that rotates the wavelength conversion element  61  formed in a flat plate shape. 
     The wavelength conversion element  61  corresponds to an optical element in an aspect of the invention. The wavelength conversion element  61  includes a substrate  62 , a phosphor layer  63  and a reflection layer  64 , which are located on a light incident surface  62 A (see  FIG. 7 ), which is an excitation light incident surface of the substrate  62 , and the heat dissipater  65 , which is located on a surface  62 B (see  FIG. 4 ), which is the surface of the substrate  62  opposite the light incident surface  62 A. Among the components described above, the configuration of the heat dissipater  65  will be described later in detail. 
     The substrate  62  is a flat-plate-shaped member formed in a roughly circular shape when viewed from the excitation light incident side. The substrate  62  can be made, for example, of a metal or ceramic material. 
     The phosphor layer  63  corresponds to an optical element layer in an aspect of the invention. The phosphor layer  63  is a layer containing a phosphor that emits, when excited with the excitation light incident thereon, fluorescence (fluorescence having a peak wavelength within a wavelength range, for example, from 500 to 700 nm), which is non-polarized light, and the phosphor layer  63  is the illumination area illuminated with the light from the homogenizer optical apparatus  54  and the second light collecting element  59  described above. Part of the fluorescence produced in the phosphor layer  63  is directed toward the second light collecting element  59 , and the other part is directed toward the reflection layer  64 . 
     The reflection layer  64  is disposed between the phosphor layer  63  and the substrate  62  and reflects the fluorescence incident from the phosphor layer  63  toward the second light collecting element  59 . 
     When the thus configured wavelength conversion element  61  is irradiated with the excitation light, the phosphor layer  63  and the reflection layer  64  diffusively output the fluorescence described above toward the second light collecting element  59 . The fluorescence is then incident via the second light collecting element  59  on the polarization separation layer  555  of the light combiner  55 , passes through the polarization separation layer  555  along the second illumination optical axis Ax 2 , and is incident on the homogenizing apparatus  7 . That is, the fluorescence, which passes through the light combiner  55 , is incident, along with the excitation light, which is the blue light reflected off the light combiner  55 , as the illumination light WL on the homogenizing apparatus  7 . 
     Configuration of Homogenizing Apparatus 
     The homogenizing apparatus  7  homogenizes the illuminance in a plane perpendicular to the center axis of the illumination light (plane perpendicular to optical axis) incident from the light source apparatus  5  and hence homogenizes the illuminance distribution in an image formation area (modulation area) or an illumination area of each of the light modulators  44  ( 44 R,  44 G, and  44 B). The homogenizing apparatus  7  includes a first lens array  71 , a second lens array  72 , a polarization conversion element  73 , and a superimposing lens  74 . These components (reference characters  71  to  74 ) are so disposed that the optical axes thereof coincide with the second illumination optical axis Ax 2 . 
     The first lens array  71  has a configuration in which a plurality of lenslets  711  are arranged in a matrix in a plane perpendicular to the second illumination optical axis Ax 2 , and the plurality of lenslets  711  divide the illumination light WL incident thereon into a plurality of sub-light fluxes. 
     The second lens array  72  has a configuration in which a plurality of lenslets  721  are arranged in a matrix in a plane perpendicular to the second illumination optical axis Ax 2 , as in the case of the first lens array  71 , and the lenslets  721  and the corresponding lenslets  711  are in a one-to-one relationship. The lenslets  721 , along with the superimposing lens  74 , superimpose the plurality of divided sub-light fluxes from the lenslets  711  on one another in the above-mentioned image formation area of each of the light modulators  44 . 
     The polarization conversion element  73  is disposed between the second lens array  72  and the superimposing lens  74  and has the function of aligning the polarization directions of the plurality of sub-light fluxes incident on the polarization conversion element  73  with one another. 
     Configuration of Heat Dissipater of Wavelength Conversion Element 
       FIG. 4  is a perspective view of the wavelength conversion element  61  viewed from the side facing the surface  62 B. 
     The wavelength conversion element  61  includes the heat dissipater  65 , which is located on the surface  62 B of the substrate  62 , as described above. The heat dissipater  65  has the plurality of fins  66 , and the fins  66  each extend from the side facing the center of rotation C of the substrate  62  along the direction toward the outer circumference of the substrate  62 , as shown in  FIG. 4 . 
     Specifically, the fins  66  radially extend from the side facing the center of rotation C toward the outer circumference of the substrate  62  and are arranged at equal intervals in the circumferential direction of the circular substrate  62 , that is, in a +D direction, which is the rotational direction of the substrate  62 . 
     The dimension of each of the fins  66  in the direction perpendicular to the extension direction of the fin  66  (thickness dimension in D direction) is roughly fixed across the entire fins  66 , but the thickness dimension may instead increase with distance from the side facing the center of rotation C toward the outer circumference. 
     Further, the fins  66  are so formed that the dimension between the fins  66  in the +D direction, that is, a channel width S between the fins  66  in the direction perpendicular, in a plan view, to the extension direction of a channel through which the cooling gas passes through the spaces between the fins  66  from the side facing the center of rotation C toward the outer circumference falls within a predetermined range. 
       FIG. 5  is a diagrammatic view showing vortices VT produced by the fins  66  when the wavelength conversion element  61  is rotated. 
     In a case where the wavelength conversion element  61  is rotated around the center of rotation C in the +D direction, negative pressure is produced on the side facing opposite the +D direction with respect to the fins  66  (−D-direction side), and the vortices VT of the cooling gas are produced, as shown in  FIG. 5 , when the channel width S described above is greater than or equal to a predetermined value. In this case, the cooling gas flows through the spaces between the fins  66  from the side facing the center of rotation C toward the outer circumference with the vortices VT produced. Each of the thus produced vortices VT swirls around an axis extending along the extension direction of the fins  66  in such a way that the vortex VT collides with the facing end surfaces of two fins  66  adjacent to each other in the +D direction (two fins  66  that sandwich vortex VT) (facing end surface are −D-direction-side end surface  66 T of +D-direction-side fin  66  and +D-direction-side end surface  66 S of −D-direction-side fin  66 ), so that the heat transferred to the fins  66  is likely to be transferred to the cooling gas, and the fins  66  are likely to be cooled. That is, the fins  66  and hence the wavelength conversion element  61  are cooled with improved efficiency. 
     Setting of Channel Width 
       FIG. 6  shows graphs illustrating the relationship between the channel width S described above and a heat transfer coefficient ratio for each rotational speed (number of revolutions per unit time) of the wavelength conversion element  61 . The heat transfer coefficient ratio is a value representing the ratio of the heat transfer coefficient for each channel width S to the largest heat transfer coefficient, provided that the rotational speed is fixed. 
     In a case where the wavelength conversion element  61  is rotated at 3000, 6000, and 9000 rpm, which are practical rotational speeds, the heat transfer coefficient ratio described above, which represents heat transfer from the fins  66  to the cooling gas, changes with the channel width S between the fins  66  described above, as shown in  FIG. 6 . The heat transfer coefficient changes in the same manner irrespective of the rotational speed of the wavelength conversion element  61 , 3000 rpm (solid line in  FIG. 6 ), 6000 rpm (broken line in  FIG. 6 ), and 9000 rpm (dotted line in  FIG. 6 ). 
     Specifically, over the range of the channel width S greater than or equal to 1 mm and smaller than or equal to 10 mm, the heat transfer coefficient ratio described above increases as the channel width S increases from 1 mm in all cases where the wavelength conversion element is rotated at the three rotational speeds described above. The heat transfer coefficient ratio is maximized when the channel width S is 3 mm and decreases as the channel width S increases from 4 mm. When the channel width S is greater than 6 mm, the heat transfer coefficient is roughly fixed. 
     That is, the heat transfer coefficient ratio and hence the heat transfer coefficient, which represents heat transfer from the fins  66  to the cooling gas, is large when the channel width S is greater than or equal to 3 mm and smaller than or equal to 6 mm, in more detail, the heat transfer coefficient is maximized when the channel width S is greater than or equal to 3 mm and smaller than or equal to 5 mm. 
     In the simulation for generating the graphs shown in  FIG. 6 , a standing dimension of the fins  66  measured from the substrate  62  (dimension in the direction along the axis of rotation of the substrate  62 ) is so set at 3 mm or greater, in more detail, 10 mm that the vortices VT described above are produced. Further, in the simulation, the diameter of the substrate  62  is set at 100 mm, and the fins  66  are formed within an area defined around the center of rotation C and having a diameter of 90 mm. 
     A high value of the heat transfer coefficient, which represents heat transfer from the fins  66  to the cooling gas, indicates that heat generated in the phosphor layer  63  and transferred to the fins  66  via the substrate  62  is transferred to the cooling gas with high efficiency, that is, the wavelength conversion element  61  is cooled with high efficiency. In other words, the above description indicates that in the case where the channel width S described above is greater than or equal to 3 mm and smaller than or equal to 6 mm, the wavelength conversion element  61  is cooled with high efficiency, and in the case where the channel width S is greater than or equal to 3 mm and smaller than or equal to 5 mm, the wavelength conversion element  61  is cooled with higher efficiency. The range of the channel width S that allows the above-mentioned improvement in cooling efficiency (greater than or equal to 3 mm and smaller than or equal to 6 mm) is hereinafter referred to as an adequate channel width range. The adequate channel width range corresponds to the predetermined dimension range in an aspect of the invention. 
     The high cooling efficiency described above is believed to be achieved because a vortex VT is produced in the spaces between two fins  66  when the wavelength conversion element  61  rotates, and the vortex VT effectively collides with the above-mentioned end surface  66 T of the +D-direction-side fin  66  and the above-mentioned end surface  66 S of the −D-direction-side fin  66 , so that the heat is likely to be transferred from the fins  66  to the cooling gas, as described above. 
     On the other hand, it is believed that when the channel width S is 1 mm, the vortex VT described above is unlikely to be produced because the channel width S is too narrow, so that the heat transfer coefficient is relatively small. 
     Further, it is believed that when the channel width S is greater than 6 mm, the vortex VT produced in the space between the fins  66  is unlikely to collide with the −D-direction-side fin  66  because the channel width S is too large, so that the heat transfer coefficient is fixed. 
     The relationship between the channel width S and the heat transfer coefficient does not depend on the diameter of the substrate  62 . 
     Area where Channel Width within Range Described Above is Set 
     As described above, the vortices VT produced when the wavelength conversion element  61  is rotated improve the efficiency of cooling of the wavelength conversion element  61 . The channel width S between the fins  66  that allows the vortices VT to be produced does not necessarily fall within the adequate channel width range described above over the entire length of the fins  66  from the ends on the side facing the center of rotation C to the ends on the side facing the outer circumference. 
       FIG. 7  is a perspective view of the wavelength conversion element  61  viewed from the light incident side. 
     The phosphor layer  63  and the reflection layer  64  described above are located on the light incident surface  62 A of the substrate  62  and in an area inside the outer edge thereof and having an annular shape around the center of rotation C, as shown in  FIG. 7 . The heat generated in the thus located phosphor layer  63  is transferred not only to an inner circumferential area  62 A 1  of the substrate  62 , which is the area inside the inner circumference of the phosphor layer  63 , but also to an outer circumferential area  62 A 2  of the substrate  62 , which is the area outside the outer circumference of the phosphor layer  63 . Since the flow speed of the cooling gas flowing through the spaces between the fins  66  when the substrate  62  rotates increases with distance from the center of rotation C toward the outer circumference, the outer circumferential area  62 A 2  is more likely to be cooled than the inner circumferential area  62 A 1 , and the heat generated in the phosphor layer  63  is more likely to be transferred to the outer circumferential area  62 A 2  than to the inner circumferential area  62 A 1 . 
     Therefore, although not shown, across the plurality of fins  66  located on the substrate  62 , when the channel width S at least in the portion corresponding to the outer circumferential area  62 A 2  is so set as to fall within the adequate channel width range described above, vortices VT produced in the portion are allowed to adequately collide with the fins  66  that sandwich the vortices VT. The fins  66  and hence the wavelength conversion element  61  can therefore be cooled with improved efficiency. 
     Further, when the channel width S in the portion corresponding to the inner circumferential area  62 A 1 , as well as the outer circumferential area  62 A 2 , is also so set as to fall within the adequate channel width range described above, the wavelength conversion element  61  can be cooled with further improved efficiency. On the other hand, it goes without saying that setting the channel width S only in the portion corresponding to the inner circumferential area  62 A 1  or the channel width S only between part of the fins  66  to fall within the adequate channel width range described above allows improvement in the cooling efficiency as compared with a wavelength conversion element in which the channel width S across the entire fins is set to a value outside the adequate channel width range. 
     Effects of First Embodiment 
     The projector  1  according to the present embodiment described above provides the following effects. 
     The dimension in the +D direction between two fins  66  adjacent to each other in the +D direction (that is, channel width S described above) is so set as to fall within the adequate channel width range described above. The setting described above allows a vortex VT of the cooling gas to be readily produced between two fins  66  when the wavelength conversion element  61  rotates and further allows the vortex VT to readily collide with the facing end surfaces  66 S and  66 T of the two fins  66  that sandwich the vortex VT. As a result, the cooling air is allowed to effectively collide with the fins  66 , whereby the heat in the fins  66  can be readily transferred to the cooling gas. Therefore, the heat generated in the phosphor layer  63  can be efficiently cooled, and the wavelength conversion element  61  can be cooled with improved efficiency. Further, since the wavelength conversion element  61  is thus stabilized, the light source apparatus  5  can stably output light, whereby the reliability of the projector  1  can be improved. 
     The heat generated in the phosphor layer  63  is likely to be transferred from the phosphor layer  63  to the outer circumferential area  62 A 2 , as described above. In correspondence with this, the fact that the channel width S between fins  66  corresponding at least to the outer circumferential area  62 A 2  is so set as to fall within the adequate channel width range described above reliably allows improvement in the efficiency of cooling of the outer circumferential area  62 A 2 . The efficiency of cooling of the wavelength conversion element  61  can therefore be reliably improved. 
     The channel width S between the fins  66  is so set as to fall within the adequate channel width range described above across the entire fins  66 . The heat transfer from the fins  66  to the cooling gas can therefore be more efficient than in a case where the channel width S only in the portion corresponding to the outer circumferential area  62 A 2  or only in the portion corresponding to the inner circumferential area  62 A 1  is so set as to fall within the adequate channel width range. The efficiency of cooling of the wavelength conversion element  61  can therefore be more reliably improved. 
     The adequate channel width range described above is set in accordance with the size of the vortices VT described above. The adequate channel width range can therefore be so set that each of the produced vortices VT collides with two fins  66  that sandwich the vortex VT. When the fins  66  are so configured that the channel width S falls within the adequate channel width range, the efficiency of cooling of the wavelength conversion element  61  can be reliably improved. 
     The adequate channel width range described above is greater than or equal to 3 mm and smaller than or equal to 6 mm. In this case, not only can the vortices VT described above be produced reliably in the space between the two fins  66  when the wavelength conversion element  61  rotates, but also each of the vortices VT is allowed to reliably collide with the two fins that sandwich the vortex VT. The efficiency of cooling of the wavelength conversion element  61  can therefore be reliably improved. 
     The dimension of the fins  66  along the axis of rotation of the wavelength conversion element  61  (standing dimension of the fins  66  measured from the substrate  62 ) is at least 3 mm. In this case, the vortices VT are allowed to be unlikely to collide with the surface  62 B, which is the bottom surface of the substrate  62 , whereby the vortices VT can be readily continuously produced. The efficiency of cooling of the wavelength conversion element  61  can therefore be more reliably improved. 
     Second Embodiment 
     A second embodiment of the invention will next be described. 
     A projector according to the present embodiment has the same configuration as that of the projector  1  described above but differs therefrom in terms of the shape of the fins with which the wavelength conversion element is provided. In the following description, the same or roughly the same portions as those having been already described have the same reference characters and will not be described. 
       FIGS. 8 and 9  are a perspective view and a plan view of a wavelength conversion element  61 A provided in the light source apparatus  5  of the projector according to the present embodiment and viewed from the side opposite the light incident side. In  FIGS. 8 and 9 , only part of fins  67  is labeled with the reference character for clarity. 
     The projector according to the present embodiment has the same configuration and function as those of the projector  1  described above except that the wavelength conversion element  61  is replaced with the wavelength conversion element  61 A. The wavelength conversion element  61 A has the same configuration and function as those of the wavelength conversion element  61  described above except that the plurality of fins  66  described above are replaced with a plurality of fins  67 . 
     The plurality of fins  67  form the heat dissipater  65  in the present embodiment. The fins  67  extend along the direction from the center of rotation C of the substrate  62  toward the outer circumference thereof and are arranged at equal intervals along the outer circumference of the substrate  62 , as in the case of the fins  66 . 
     On the other hand, the fins  67  each have a curved shape (arcuate shape) that warps toward the −D-direction side (side facing opposite +D direction, which is rotational direction of wavelength conversion element  61 A), with distance from the side facing the center of rotation C toward the outer circumference. The dimension of each of the fins  67  in the direction perpendicular to the extension direction of the fins  67  (thickness direction in D direction) also increases with distance from the end facing the center of rotation C toward the end facing the outer circumference. The shape described above is intended to make the channel width S between the fins  67  constant. 
       FIG. 10  shows a graph illustrating the relationship described above between the channel width S and the heat transfer coefficient ratio in a case where the wavelength conversion element  61 A is rotated at a speed of 6000 rpm. 
     Also in the case of the wavelength conversion element  61 A described above, the heat transfer coefficient ratio, which represents heat transfer from the fins  67  to the cooling gas, increases over the range of the channel width S described above greater than or equal to 3 mm and smaller than or equal to 6 mm (adequate channel width range) and is maximized over the range of the channel width S greater than or equal to 4 mm and smaller than or equal to 5 mm, as shown in  FIG. 11 . Although not shown, the same tendency is shown in cases where the wavelength conversion element  61 A is rotated at speeds of 3000 rpm and 9000 rpm. 
     Therefore, even in the case of the fins  67  having the shape described above, when the channel width S between the fins  67  is so set as to fall within the adequate channel width range described above, the heat can be efficiently transferred from the fins  67  to the cooling gas, whereby the wavelength conversion element  61 A can be cooled with improved efficiency. 
     In the present embodiment, in consideration of the tendency described above, the channel width S between the fins  67  is set at a fixed value of 4 mm from the end on the side facing the center of rotation C to the end on the side facing the outer circumference, but not necessarily. The channel width S between the fins  67  may be set at another fixed value or may change within a predetermined range of the channel width S (within the adequate channel width range described above, for example). In this case, the above-mentioned thickness dimension of the fins  67  may not increase with distance from the end on the side facing the center of rotation C toward the end on the side facing the outer circumference. 
     Effects of Second Embodiment 
     The projector according to the present embodiment described above can provide the following effect as well as the same effects as those provided by the projector  1  described above. 
     The fins  67  each have a curved shape (arcuate shape) that warps toward the −D-direction side with distance from the side facing the center of rotation C toward the outer circumference. Since the fins  67  are therefore not perpendicular to the +D direction, the rotational resistance (air resistance) of the wavelength conversion element  61 A can be reduced as compared with the wavelength conversion element  61 , in which the fins  66  radially extend. The load acting on the rotating device  60  can therefore be reduced. 
     Third Embodiment 
     A third embodiment of the invention will next be described. 
     A projector according to the present embodiment has the same configuration as that of the projector  1  described above but differs therefrom in that the fins with which the wavelength conversion element is provided are formed of two types of differently dimensioned fins. In the following description, the same or roughly the same portions as those having been already described have the same reference characters and will not be described. 
       FIGS. 11 and 12  are a perspective view and a plan view of a wavelength conversion element  61 B provided in the light source apparatus  5  of the projector according to the present embodiment and viewed from the side opposite the light incident side. In  FIGS. 11 and 12 , only part of fins  68  is labeled with the reference character for clarity. 
     The projector according to the present embodiment has the same configuration and function as those of the projector  1  described above except that the wavelength conversion element  61  is replaced with the wavelength conversion element  61 B. The wavelength conversion element  61 B has the same configuration as that of the wavelength conversion element  61  described above except that the plurality of fins  66  are replaced with a plurality of fins  68 . 
     The plurality of fins  68  form the heat dissipater  65  in the present embodiment. The fins  68  include a plurality of first fins  681  and a plurality of second fins  682 . 
     The first fins  681  extend along the direction from the center of rotation C of the substrate  62  toward the outer circumference thereof, are arranged at equal intervals along the outer circumference of the substrate  62  (in other words, in +D direction), and each have a curved shape (arcuate shape) that warps toward the −D-direction side with distance from the side facing the center of rotation C toward the outer circumference, as in the case of the fins  67 . The dimension of the first fins  681  in the direction perpendicular to the extension direction thereof (thickness direction) is roughly fixed. 
     The ends of the first fins  681  on the side facing the center of rotation C are located on a first virtual circle VC 1  around the center of rotation C, as shown in  FIG. 12 . On the other hand, the ends of the first fins  681  on the outer circumference side are located on a second virtual circle VC 2 , the center of which coincides with the center of rotation C and the diameter of which is greater than the diameter of the first virtual circle VC 1  but smaller than the diameter of the substrate  62 . For example, in the case where the diameter of the substrate  62  is 100 mm as described above, the diameter of the second virtual circle VC 2  is 90 mm. 
     The second fins  682  are located between the plurality of first fins  681 . The second fins  682  extend along the direction from the center of rotation C toward the outer circumference, are arranged at equal intervals along the outer circumference of the substrate  62  (in other words, in +D direction), and each have a curved shape (arcuate shape) that warps toward the −D-direction side with distance from the side facing the center of rotation C toward the outer circumference, as in the case of the first fins  681  described above. 
     The ends of the second fins  682  on the side facing the center of rotation C are located on a third virtual circle VC 3 , the center of which coincides with the center of rotation C and the diameter of which is greater than the diameter of the first virtual circle VC 1  but smaller than the diameter of the second virtual circle VC 2 . On the other hand, the ends of the second fins  682  on the side facing the outer circumference are located on the second virtual circle VC 2 . 
     The dimension of the second fins  682  in the direction perpendicular to the extension direction of the second fins  682  (thickness direction) is so set as to increase with distance toward the outer circumference so that the dimension in the +D direction between each pair of two first fins  681  that sandwich the corresponding second fin  682  in the +D direction (channel width S) is so set as to fall within the adequate channel width range described above. 
     In the thus configured wavelength conversion element  61 B, in the area from the first virtual circle VC 1  to the third virtual circle VC 3 , the channel width S between two first fins  681  adjacent to each other is so set as to fall within the adequate channel width range described above. That is, out of two first fins  681  adjacent to each other in the +D direction, the dimension in the +D direction between the −D-direction-side end surface of the +D-direction-side first fin  681  and the +D-direction-side end surface of the −D-direction-side first fin  681  is so set as to fall within the adequate channel width range described above. 
     On the other hand, in the area from the third virtual circle VC 3  to the second virtual circle VC 2 , the channel width S between each first fin  681  and a second fin  682  adjacent thereto in the +D direction is so set as to fall within the adequate channel width range described above. That is, the dimension in the +D direction between the −D-direction-side end surface of the +D-direction-side first fin  681  and the +D-direction-side end surface of the −D-direction-side second fin  682  is so set as to fall within the adequate channel width range described above. Similarly, the dimension in the +D direction between the −D-direction-side end surface of the +D-direction-side second fin  682  and the +D-direction-side end surface of the −D-direction-side first fin  681  is so set as to fall within the adequate channel width range described above. 
     As described above, in the wavelength conversion element  61 B, the channel widths S between the fins  68  are so set as to fall within the adequate channel width range described above. 
     It is noted that as long as the channel width S between each first fin  681  and a second fin  682  adjacent thereto that correspond to the outer circumferential area  62 A 2  described above is so set as to fall within the adequate channel width range described above, the channel width S between the first fins  681  that corresponds to the inner circumferential area  62 A 1  described above and the channel width S between each first fin  681  and a second fin  682  adjacent thereto that correspond to the inner circumferential area  62 A 1  may be values outside the adequate channel width range described above. On the other hand, regarding the above-mentioned effect provided by the produced vortices VT, the effect can be provided when there is a portion where the channel width S between the first fins  681  or the channel width S between each first fin  681  and a second fin  682  adjacent thereto is so set as to fall within the adequate channel width range described above. 
     Effects of Third Embodiment 
     The projector according to the present embodiment described above can provide the following effect as well as the same effects as those provided by the projectors shown in the first and second embodiments described above. 
     For example, to form fins on the substrate in a cutting process, it is difficult in some cases to set the channel width S between the fins to fall within the adequate channel width range described above depending on the size of a cutting tool. In other words, the channel width S could undesirably increase to a value beyond the adequate channel width range described above. 
     In contrast, in the case where the fins  68  including the first fins  681  and the second fins  682  are formed on the substrate  62  in a cutting process, since the dimension between the ends of the first fins  681  on the side facing the center of rotation C can be a large value to the extent the value falls within the adequate channel width range described above, the cutting tool is allowed to readily pass through the spaces between the fins. Therefore, since the fins  68  can be readily formed with the cutting tool, the substrate  62  can be readily processed, whereby an increase in manufacturing cost of the wavelength conversion element  61 B can be suppressed. 
     Fourth Embodiment 
     A fourth embodiment of the invention will next be described. 
     A projector according to the present embodiment has the same configuration as that of the projector  1  described above but differs therefrom in that the angle of the fins with respect to the radial direction of the wavelength conversion element (substrate) is adequately set. In the following description, the same or roughly the same portions as those having been already described have the same reference characters and will not be described. 
       FIGS. 13 and 14  are a perspective view and a plan view of a wavelength conversion element  61 C provided in the light source apparatus  5  of the projector according to the present embodiment and viewed from the side opposite the light incident side. In  FIGS. 13 and 14 , only part of fins  69  is labeled with the reference character for clarity. 
     The projector according to the present embodiment has the same configuration and function as those of the projector  1  described above except that the wavelength conversion element  61  is replaced with the wavelength conversion element  61 C. The wavelength conversion element  61 C has the same configuration as that of the wavelength conversion element  61  described above except that the plurality of fins  66  are replaced with a plurality of fins  69 . 
     The plurality of fins  69  form the heat dissipater  65  in the present embodiment. The fins  69  extend along the direction from the center of rotation C of the substrate  62  toward the outer circumference thereof, are arranged at equal intervals along the outer circumference of the substrate  62  (in other words, in +D direction), and each have a curved shape (arcuate shape) that warps toward the −D-direction side with distance from the side facing the center of rotation C toward the outer circumference, as in the case of the fins  67 . The dimension in the direction perpendicular to the extension direction of the fins  69  (thickness direction) is roughly fixed. 
       FIG. 15  describes the intersection angle between a tangent corresponding to a position on a −D-direction-side edge of the fin  69  and the radial direction originating from the center of rotation C. 
     The fins  69  each have the curved shape as described above, but each of the fins  69  has a radius of curvature that changes with the position on the fin in such a way that in all positions on the fin  69 , the intersection angle between the direction of the line tangent to the fin  69  and the radial direction originating from the center of rotation C is fixed. In detail, the fins  69  are each so formed that the radius of curvature increases with distance from the side facing the center of rotation C toward the outer circumference. 
     Specifically, along the −D-direction-side edge of one fin  69 , let T 1 , T 2 , and T 3  be lines tangent to the fin  69  at a point P 1  on the side facing the center of rotation C, a point P 2  roughly at the center, and a point P 3  on the side facing the outer circumference, and let L 1 , L 2 , and L 3  be straight lines that pass through the points P 1  to P 3  and extend in the radial direction originating from the center of rotation C, as shown in  FIG. 15 . Under the definitions described above, an intersection angle α 1  between the tangent T 1  and the straight line L 1 , an intersection angle α 2  between the tangent T 2  and the straight line L 2 , and an intersection angle α 3  between the tangent T 3  and the straight line L 3  are roughly equal to one another. That is, each of the fins  69  is so formed that the intersection angle described above is fixed in any position on the −D-direction-side edge of the fin  69 . 
     The intersection angle described above is hereinafter referred to as a tangent intersection angle of the fins  69 . 
       FIG. 16  is a plan view of a wavelength conversion element  61 X presented as a comparative example of the wavelength conversion element  61 C according to the present embodiment and viewed from the side opposite the light incident side. In  FIG. 16 , only part of fins  69 X is labeled with the reference character for clarity. 
     A description will now be made of the wavelength conversion element  61 X including the fins  69 X so formed as to have a fixed radius of curvature. 
     The wavelength conversion element  61 X has the same configuration as that of the wavelength conversion element  61 C except that the plurality of fins  69  are replaced with a plurality of fins  69 X, as shown in  FIG. 16 . 
     The fins  69 X extend along the direction from the center of rotation C of the substrate  62  toward the outer circumference thereof, are arranged at equal intervals along the outer circumference of the substrate  62  (in other words, in +D direction), each have a curved shape (arcuate shape) that warps toward the −D-direction side with distance from the side facing the center of rotation C toward the outer circumference, and the dimension of the fins  69 X in the direction perpendicular to the extension direction of the fins  69 X (thickness direction) is roughly fixed, as in the case of the fins  69  described above. 
     The fins  69 X, however, differ from the fins  69  in that the fins  69  are so formed that the tangent intersection angle of each of the fins is fixed, whereas the fins  69 X are formed in an arcuate shape having fixed curvature (fixed radius of curvature) so that the tangent intersection angle is not fixed. 
     When the thus configured wavelength conversion element  61 X is rotated, the intersection angle between the tangent to the fin  69 X and the radial direction originating from the center of rotation C increases in an outer-circumference-side portion of the substrate  62 . In other words, the intersection angle between the tangent to the fin  69 X and the flowing direction of the cooling gas flowing through the space between the fin  69 X and a fin  69 X adjacent thereto toward the outer circumference increases. Therefore, in the outer-circumference-side portion, the fins  69 X serve as walls against the cooling gas flowing through the space between the fins  69 X toward the outer circumference, and the cooling gas undesirably stays between the fins  69 X. 
     In this case, the cooling gas flowing through the space between the fins  69 X toward the outer circumference primarily flows through a −D-direction-side area in the channel between the fins  69 X, and the flow speed and flow rate of the cooling gas to be discharged out of the substrate  62  undesirably decrease. The efficiency of cooling of the wavelength conversion element  61 X is therefore not very high. 
     In contrast, in the wavelength conversion element  61 C described above, in which the radius of curvature changes with the position on each of the fins  69 , the fins  69  can be so located as not to serve as walls against the cooling gas flowing through the spaces between the fins  69  toward the outer circumference. Specifically, since the fins  69  are so formed that the radius of curvature thereof increases with distance from the side facing the center of rotation C toward the outer circumference, the fins  69  are so located as not to serve as walls against the cooling gas. Therefore, the situation in which the cooling gas stays between the fins can be avoided, whereby decreases in the flow speed and flow rate of the cooling gas can be suppressed, and the efficiency of cooling of the wavelength conversion element  61 C can therefore be improved. 
     Intersection Angle Between Fins and Radial Direction 
       FIG. 17  shows graphs illustrating the relationship between the tangent intersection angle of the fins  69  and the heat transfer coefficient for each channel width S described above.  FIG. 18  shows graphs illustrating the relationship between the tangent intersection angle and the heat transfer coefficient ratio described above for each channel width S described above. 
     The heat transfer coefficient and the heat transfer coefficient ratio, which represent heat transfer from a fin located on a wavelength conversion element rotated around the center of rotation C to the cooling gas, change with the tangent intersection angle of the fin, as shown in  FIGS. 17 and 18 . 
     Specifically, a plurality of wavelength conversion elements in each of which a plurality of fins having the tangent intersection angle greater than or equal to −80° and smaller than or equal to +80° are so formed that the channel width S described above between the fins is 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, and 7 mm have high values of the heat transfer coefficient and the heat transfer coefficient ratio over the range of the tangent intersection angle greater than or equal to −45° and smaller than or equal to +60°. The regions where the tangent intersection angle is negative represent that the direction in which the fins are curved faces the side opposite the side described above, that is, the fins each have a shape that warps toward the +D-direction side with distance from the side facing the center of rotation C toward the outer circumference. 
     As described above, it is believed that when a plurality of fins having the tangent intersection angle set to be greater than or equal to −45° and smaller than or equal to +60° (hereinafter referred to as adequate angle range), the vortices VT described above are likely to be produced, whereby a wavelength conversion element having a high coefficient of heat transfer to the cooling gas can be formed. 
     On the other hand, in the regions where the tangent intersection angle has values outside the adequate angle range described above, in which the fins each have a shape that follows the +D direction, it is believed that the vortices VT described above are unlikely to be produced, and the coefficient of heat transfer to the cooling gas decreases. 
     An outer-circumferential-side portion where the tangent intersection angle is greater than or equal to −45° and smaller than 0° drags the cooling gas toward the center of rotation C when the substrate  62  is rotated. As a result, pressure that causes the cooling gas to flow from the side facing the outer circumference toward the center of rotation C is produced in the portion. In the portion, the direction of the pressure that causes the cooling gas to flow toward the outer circumference and the direction of the pressure that causes the cooling gas to flow toward the center of rotation C faces each other, undesirably resulting in decreases in the flow speed and flow rate of the cooling gas that flows toward the outer circumference and exits. That is, the efficiency of cooling of the wavelength conversion element  61 C could undesirably decrease. 
     In contrast, a fin that allows the intersection angle with respect to the radial direction to be greater than 0° and smaller than or equal to +60°, that is, a fin that warps toward the −D-direction side with the distance from the side facing the center of rotation C toward the outer circumference does not drag the cooling gas toward the center of rotation C, whereby decreases in the flow speed and flow rate of the cooling gas that flows toward the outer circumference and exits can be suppressed. 
     On the other hand, when the tangent intersection angle of a fin is 0°, that is, when a fin radially extends from the side facing the center of rotation C, the fin is perpendicular to the +D direction. Therefore, the rotational resistance of the wavelength conversion element increases, and the load acting on the rotating device  60  increases, as described above. 
     In consideration of these factors, the tangent intersection angle of each fin is preferably greater than 0° and smaller than or equal to +60° (hereinafter referred to as optimum angle range). Therefore, in the wavelength conversion element  61 C in the present embodiment, the radius of curvature of each of the fins  69  changes with the position thereon, in detail, the radius of curvature increases with distance from the side facing the center of rotation C toward the outer circumference, so that the tangent intersection angle is set at +30° over the entire length of the fin  69 . 
     In the wavelength conversion element  61 C, the fins  69  are so formed that the channel width S between the fins  69  falls within the adequate channel width range described above. The wavelength conversion element  61 C can therefore be configured to provide the effects provided by the first and second embodiments described above. The wavelength conversion element  61 C is, however, not necessarily configured as described above and may be so configured that the channel width S between the fins  69  does not fall within the adequate channel width range described above. 
     Effects of Fourth Embodiment 
     The projector according to the present embodiment described above can provide the following effects as well as the same effects as those provided by the projectors shown in the first and second embodiments described above. 
     Since the tangent intersection angle of each of the fins  69  is so set as to fall within the adequate angle range described above, the vortex VT described above is likely to be produced between two fins  69  adjacent to each other in the +D direction in the wavelength conversion element  61 C when the wavelength conversion element  61 C is rotated. The vortex VT collides with the facing end surfaces of the two fins  69  that sandwich the vortex VT, whereby the heat transfer from the fins  69  to the cooling gas can be facilitated. The efficiency of cooling of the fins  69 , to which the heat generated in the phosphor layer  63  is transferred via the substrate  62 , and hence the efficiency of cooling of the wavelength conversion element  61 C can therefore be improved. The life of the wavelength conversion element  61 C can therefore be prolonged. 
     Each of the fins  69  has a radius of curvature that changes with the position thereon. As a result, the above-mentioned tangent intersection angle of part or entirety of each of the fins  69  is readily so set as to fall within the adequate angle range described above (optimum angle range, in particular). Therefore, in a portion where the tangent intersection angle falls within the adequate angle range described above, the vortices VT described above can be readily produced between the fins  69 , whereby improvement in the efficiency of cooling of the wavelength conversion element  61 C and extension of the life of the wavelength conversion element  61 C can be reliably achieved. 
     Each of the fins  69  is formed in an arcuate shape having a radius of curvature that increases with distance from the side facing the center of rotation C toward the outer circumference. The above-mentioned tangent intersection angle over the entire length of each of the fins  69  can therefore be readily so set as to fall within the adequate angle range described above (optimum angle range, in particular). Therefore, since the vortices VT described above can be readily produced over the entire spaces between the fins  69 , improvement in the efficiency of cooling of the wavelength conversion element  61 C and extension of the life of the wavelength conversion element  61 C can be more reliably achieved than in a case where the vortices VT described above are produced in part of the spaces between the fins  69 . 
     The adequate angle range is a range greater than or equal to −45° and smaller than or equal to +60°. Therefore, since the fins  69  can face in the +D direction, the vortices VT described above can be more readily produced on the −D-direction side of the fins  69  when the wavelength conversion element  61 C rotates. The improvement in the efficiency of cooling of the wavelength conversion element  61 C and the extension of the life of the wavelength conversion element  61 C can therefore be further reliably achieved. 
     In the case where the above-mentioned tangent intersection angle of each of the fins  69  is 0°, that is, in the case where the fins radially extend from the side facing the center of rotation C, the load acting on the rotating device  60  increases because the fins are perpendicular to the +D direction and the rotational resistance therefore increases, as described above. 
     On the other hand, a fin having the tangent intersection angle described above greater than or equal to −45° and smaller than 0° has a shape that warps toward the +D-direction side with distance from the side facing the center of rotation C of the substrate  62  toward the outer circumference thereof. When the fins each have the shape described above, and the wavelength conversion element described above is rotated, pressure that causes the cooling gas to flow from the side facing the outer circumference toward the center of rotation C is produced. In this case, the cooling gas flowing from the side facing the center of rotation C toward the outer circumference is likely to stay between the fins, and the efficiency of cooling the wavelength conversion element therefore decreases. 
     In contrast, when the fins  69  are so formed that the tangent intersection angle described above falls within the optimum angle range greater than 0° and smaller than or equal to +60°, the cooling gas flowing through the spaces between the fins  69  is likely to flow from the side facing the center of rotation C toward the outer circumference, whereby the flow speed and flow rate of the cooling gas can be increased. Therefore, since the situation in which the cooling gas to which the heat is transferred from the fins  69  stays in the spaces between the fins  69  can be avoided, the efficiency of cooling of the wavelength conversion element  61 C can be further improved. 
     Variations of Embodiments 
     The invention is not limited to the embodiments described above, and changes, improvements, and other modifications to the extent that the advantages described above can be achieved fall within the scope of the invention. 
     In the embodiments described above, the dimension between the fins  66  to  69  along the +D direction, that is, the width of the channel (channel width S), along which the cooling gas flowing through the spaces between the fins  66  to  69  flows, in the direction perpendicular, in a plan view, to the extension direction of the channel is so set as to fall within the adequate channel width range described above across the fins  66  to  69 , but not necessarily. The channel width of only part of the channel formed between the fins may be so set as to fall within the adequate channel width range described above. For example, the channel width S may be so set as to fall within the adequate channel width range only in the portion corresponding to the outer circumferential area  62 A 2 , or the channel width S may be so set as to fall within the adequate channel width range only in the portion corresponding to the inner circumferential area  62 A 1 , as described above. 
     In the embodiments described above, the adequate channel width range, which is an index used to set the inter-fin channel width S, is set in accordance with the size of vortices produced in the spaces between the fins when the wavelength conversion element as the optical element is rotated, but not necessarily. The range of the channel width S may instead be set on the basis of any other factor. 
     In the embodiments described above, the adequate channel width range described above is greater than or equal to 3 mm and smaller than or equal to 6 mm. In the invention, however, the adequate channel width range is not necessarily set as described above. For example, a range different from the range described above may be used as the adequate channel width range depending on the rotational speed of the wavelength conversion element, and the inter-fin channel width S may be set in accordance with the adequate channel width range having the different value. 
     Further, the standing dimension of the fins  66  to  69  measured from the substrate  62  is set at 3 mm or greater, but not necessarily, and the standing dimension may be smaller than 3 mm. 
     In the third embodiment described above, the fins  68  includes the plurality of first fins  681 , which are arranged along the rotational direction of the substrate  62 , and the plurality of second fins  682 , which are located between the plurality of first fins  681  and arranged along the rotational direction, and the second fins  682  are smaller than the first fins  681 , but not necessarily. Third fins that are sized differently from the first and second fins may further be provided. 
     Further, the above-mentioned thickness dimension of the second fins  682  increases with distance from the side facing the center of rotation C toward the outer circumference, but not necessarily. The thickness dimension of the second fins  682  may be fixed, but the thickness dimension of the first fins  681  may instead increase with distance from the side facing the center of rotation C toward the outer circumference. That is, the thickness dimensions of the fins are adjustable in accordance with the inter-fin channel width S. 
     In the fourth embodiment described above, the tangent intersection angle described above is so set as to fall within the adequate angle range described above (optimum angle range, in particular) across the fins  69 , but not necessarily. The fins  69  may be so formed that only the tangent intersection angle of part of the fins  69  is so set as to fall within the adequate angle range. For example, only the tangent intersection angle in the portion of the fins corresponding to the outer circumferential area  62 A 2  described above may be so set as to fall within the adequate angle range, and the tangent intersection angle in the other portion may be so set as not to fall within the adequate angle range. Conversely, only the tangent intersection angle in the portion of the fins corresponding to the inner circumferential area  62 A 1  described above may be so set as to fall within the adequate angle range, and the tangent intersection angle in the other portion may be so set not as to fall within the adequate angle range. 
     In the fourth embodiment described above, the radius of curvature of each of the fins  69  increases with distance from the side facing the center of rotation C toward the outer circumference, but not necessarily. The radius of curvature only needs to be set in accordance with the position of a portion where the tangent intersection angle is so set as to fall within the adequate angle range, as described above. Further, in correspondence with the above, each of the fins  69  may not have an arcuate shape and may include a straight portion. 
     In the fourth embodiment described above, the adequate angle range described above is greater than or equal to −45° and smaller than or equal to +60°, but not necessarily. Another angular range may be set on the basis of a factor other than the heat transfer coefficient. The same holds true for the optimum angle range described above. 
     In the embodiments described above, the fins  66  to  69 , which form the heat dissipater  65 , are located on the surface  62 B of the substrate  62 , which serves as a second surface and which is opposite the light incident surface  62 A of the substrate  62 , which serves as a first surface, but not necessarily. The heat dissipater having a plurality of fins may be located on the light incident surface  62 A or may be located on both the light incident surface  62 A and the surface  62 B. 
     In the embodiments described above, the wavelength conversion elements  61  and  61 A to  61 C as the optical element are each configured as a reflective wavelength conversion element that emits fluorescence produced by incidence of excitation light toward the side on which the excitation light is incident, but not necessarily. These wavelength conversion elements may each be configured as a transmissive wavelength conversion element that emits the fluorescence from the surface  62 B. In this case, the transmissive wavelength conversion element can be formed by forming the substrate  62  as a light transmissive member and disposing, in place of the reflection layer  64 , a wavelength selective reflection layer that transmits the excitation light but reflects the fluorescence on the side opposite the phosphor layer  63  with respect to the substrate  62 . 
     Further, the phosphor layer  63  and the reflection layer  64  are annually disposed around the center of rotation C, but not necessarily. At least the phosphor layer  63  may be formed in a circular shape around the center of rotation C. 
     In the embodiments described above, the image projection apparatus  4  has the configuration shown in  FIG. 2  described above, and the illuminator  41  and the light source apparatus  5  have the configurations and arrangements shown in  FIG. 3  described above, but not necessarily. The configurations and arrangements of the image projection apparatus, the illuminator, and the light source apparatus may be changed as appropriate. For example, the light source apparatus  5  may not be so configured that part of the excitation light outputted from the light source section  51  is diffusively reflected off the diffuser  58  and the other part of the excitation light is incident on the wavelength converter  6  for generation of fluorescence, followed by combination of the excitation light and the fluorescence with each other and output of the combined light. Specifically, the light source apparatus may include a wavelength converter  6  that outputs light containing blue light and the fluorescence. Still instead, the light source apparatus may have a configuration in which a light source section that outputs blue light to be combined with the fluorescence produced in the wavelength converter is provided separately from the light source section described above. Still further instead, the light outputted from the light source apparatus is not necessarily white light. 
     In the embodiments described above, the projector includes the three light modulators  44  ( 44 R,  44 G, and  44 B) each including a liquid crystal panel, but not necessarily. The invention may be applied to a projector including two or fewer light modulators or four or greater light modulators. 
     In the embodiments described above, the projector includes the light modulators  44  each including a transmissive liquid crystal panel having a light incident surface and a light exiting surface separately from each other, but not necessarily. A light modulator including a reflective liquid crystal panel having a single surface that serves as both the light incident surface and the light exiting surface may be employed. Further, a light modulator using any component other than a liquid-crystal-based component and capable of modulating an incident light flux to form an image according to image information, such as a device using micromirrors, for example, a DMD (digital micromirror device), may be employed. 
     In the embodiments described above, the light source apparatus  5  is used in a projector by way of example, but not necessarily. The light source apparatus  5  may be used in an electronic apparatus, such as a lighting apparatus. 
     Further, the wavelength conversion elements  61 ,  61 A,  61 B, and  61 C are presented as the optical element, but not necessarily. The configuration according to any of the embodiments of the invention may be applied to the diffusive reflection element  581 . 
     The present application claim priority from Japanese Patent Application No. 2016-110427 filed on Jun. 1, 2016, and No. 2016-110428 filed on Jun. 1, 2016, which is hereby incorporated by reference in its entirety.