Abstract:
A projection display device includes an optical system which modulates light based on an image signal to generate and output image light; an illumination device which has a plurality of light sources, and emits illumination light in a predetermined axis direction in parallel to an installation plane of the optical system to supply the illumination light to the optical system; a heat transfer system which transfers a heat generated in the light sources in a direction generally perpendicular to the installation plane; and a cooling device which is disposed in a direction generally perpendicular to the installation plane, and removes the heat transferred by the heat transfer system.

Description:
[0001]    This application is a continuation of International App. No. PCT/JP2009/53488, filed Feb. 26, 2009, and designating the U.S., which International Application claims priority to Japanese Pat. App. No. 2009-024213, filed Feb. 4, 2009, and Japanese Pat. App. No. 2008-058384, filed Mar. 7, 2008. The disclosures of the above applications are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a projection display device and an illumination device, and more particularly to an arrangement for use in generating illumination light by using a laser light source. 
         [0004]    2. Disclosure of Related Art 
         [0005]    Conventionally, a projection display device (hereinafter, called as a “projector”) for enlarging and projecting light modulated by an image signal onto a screen has been commercialized and widely used. The projector of this kind is loaded with an illumination device for supplying illumination light to an imager such as a liquid crystal panel. Heretofore, there has been used a lamp light source such as an ultra high pressure mercury lamp, a metal halide lamp, or a xenon lamp. 
         [0006]    On the other hand, in recent years, there has been developed a projector incorporated with a solid-state light source such as a semiconductor laser, in place of a lamp light source. A laser light source is advantageous in expressing a wide color space with high luminance and high precision, and is expected as a light source for a next-generation projector. In the case where an image is projected onto a large screen by using the projector of this kind, it is necessary to further increase the luminance of illumination light. 
         [0007]    As a method for increasing the luminance of illumination light, there is proposed an arrangement, wherein plural laser light sources are arranged in a two-dimensional array, or an arrangement, wherein laser light emitted from plural laser light sources is combined by using a prism mirror. Further, in the arrangement incorporated with the prism mirror, it is possible to reduce the cross-sectional area (light beam area) of illumination light by properly adjusting the dispositions of the laser light sources and the prism mirror, and enhance the light use efficiency based on Etendue theory. 
         [0008]    A laser light source has a characteristic that the emission intensity thereof is varied depending on a temperature change. In view of the above, in an illumination device incorporated with a laser light source as described above, it is necessary to provide a system of properly controlling an emission intensity of the laser light source by removing a heat generated in the laser light source. 
         [0009]    In the above arrangement, it is advantageous to use a method, in which a heat is transferred from a laser light source by a cooling element such as a Peltier element or a heat pipe, and the transferred heat is removed by a radiator or a heat sink, rather than using a method, in which cooling air is blown onto a laser light source, in order to smoothly adjust the temperature of the laser light source. In the above method, however, a large-scaled cooling system is required, which may resultantly increase the size of a projector main body. Further, in the above cooling system, there is used a pipe for circulating a coolant, or a heat pipe for directly transferring a heat, as a heat transfer system for transferring the heat generated in the laser light source to the radiator or the heat sink, in view of the above, in the cooling system, it is necessary to properly dispose the laser light source and the heat transfer system in order to further enhance the light use efficiency based on Etendue theory, while preventing blocking of laser light by the heat transfer system. 
       SUMMARY OF THE INVENTION 
       [0010]    A projection display device according to a first aspect of the invention includes an optical system which modulates light based on an image signal to generate and output image light; an illumination device which has a plurality of light sources, and emits illumination light in a predetermined axis direction in parallel to an installation plane of the optical system to supply the illumination light to the optical system; a heat transfer system which transfers a heat generated in the light sources in a direction generally perpendicular to the installation plane; and a cooling device which is disposed in a direction generally perpendicular to the installation plane, and removes the heat transferred by the heat transfer system. 
         [0011]    In the projection display device according to the first aspect of the invention, since the cooling device is disposed in an upper position or a lower position with respect to the optical system, it is possible to reduce the outer size of the projection display device, as compared with an arrangement, wherein a cooling device is disposed in parallel to an installation plane of an optical system. Further, it is possible to suppress elongation of the heat transfer system by disposing the cooling device at a position immediately above or immediately below the illumination device. 
         [0012]    A second aspect of the invention is directed to an illumination device provided with a plurality of light sources, and adapted to emit light from the plurality of the light sources in a first axis direction. The illumination device according to the second aspect includes a heat transfer system which transfers a heat generated in the light sources in a second axis direction perpendicular to the first axis direction, and a cooling device which is provided in a direction perpendicular to the first axis direction, and removes the heat transferred by the heat transfer system. 
         [0013]    A third aspect of the invention is directed to an illumination device provided with a plurality of light sources, and adapted to emit light from the plurality of the light sources in a first axis direction. The illumination device according to the third aspect includes a first light source which emits light in a second axis direction perpendicular to the first axis direction; a first heat transfer system which transfers a heat generated in the first light source in a third axis direction perpendicular to the first axis direction and the second axis direction; a second light source which emits light in the second axis direction, and is disposed at a forward position or a rearward position in a light emission direction of the first light source; a second heat transfer system which transfers a heat generated in the second light source in the third axis direction; a cooling device which is disposed in the third axis direction, and removes the heats transferred by the first heat transfer system and the second heat transfer system; and reflection means which guides the light emitted from the first light source and the light emitted from the second light source in the first axis direction. In this arrangement, the first light source and the second light source are disposed at such positions that the rearward light source is displaced with respect to the forward light source in a direction opposite to the heat transfer direction. 
         [0014]    In the illumination devices according to the second aspect and the third aspect of the invention, since the cooling device is disposed in an upper position or a lower position with respect to the light source group, it is possible to reduce the overall outer size of the illumination device including the cooling device, as compared with an arrangement, wherein a cooling device is disposed transversely with respect to a light source group. Further, it is possible to suppress elongation of the heat transfer system by disposing the cooling device at a position immediately above or immediately below the light source group. 
         [0015]    Further, in the illumination device according to the third aspect, since the rearward light source out of the first and the second light sources is disposed with a displacement with respect to the forward light source in the direction opposite to the heat transfer direction by a predetermined distance, there is no likelihood that the heat transfer system for the forward light source may be positioned on an optical path of light emitted from the rearward light source. Accordingly, there is no likelihood that the light emitted from the rearward light source may be blocked by the heat transfer system for the forward light source. 
         [0016]    A projection display device according to a fourth aspect of the invention includes an optical system which modulates light based on an image signal to generate and output image light; a light source which supplies the light to the optical system; a heat transfer system which transfers a heat generated in the light source; and a cooling device which removes the heat transferred by the heat transfer system. In this arrangement, the heat transfer system includes a cooling portion which is mounted with the light source, and which is internally formed with a flow channel through which a refrigerant from the cooling device is circulated. Further, the cooling portion is disposed, with a surface thereof where the light source is mounted being aligned with a gravitational force direction. 
         [0017]    In the projection display device according to the fourth aspect of the invention, since the air (air bubbles) in the flow channel is less likely to stagnate near the light source mounting surface, it is possible to suppress lowering of heat transfer (increase of thermal resistance) resulting from stagnation of the air (air bubbles). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and other objects, and novel features of the present invention will become more apparent upon reading the following detailed description of the embodiment along with the accompanying drawings. 
           [0019]      FIGS. 1A and 1B  are diagrams showing an arrangement of a projector in a first embodiment of the invention. 
           [0020]      FIGS. 2A ,  2 B,  2 C, and  2 D are diagrams showing an arrangement of the light source unit in the first embodiment. 
           [0021]      FIGS. 3A ,  3 B,  3 C, and  3 D are diagrams showing an arrangement of the light source unit in the first embodiment. 
           [0022]      FIGS. 4A ,  4 B,  4 C,  4 D,  4 E, and  4 F are diagrams explaining disposition methods of light source units in the first embodiment. 
           [0023]      FIG. 5  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0024]      FIGS. 6A and 6B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0025]      FIG. 7  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0026]      FIGS. 8A and 8B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0027]      FIG. 9  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0028]      FIGS. 10A and 10B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0029]      FIG. 11  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0030]      FIGS. 12A and 12B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0031]      FIG. 13  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0032]      FIGS. 14A and 14B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0033]      FIG. 15  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0034]      FIGS. 16A and 16B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0035]      FIGS. 17A and 17B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0036]      FIG. 18  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0037]      FIGS. 19A and 19B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0038]      FIG. 20  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0039]      FIGS. 21A and 21B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0040]      FIG. 22  is a diagram (perspective view) showing how laser light is combined in the first embodiment. 
           [0041]      FIGS. 23A and 23B  are diagrams (top plan view/front view) showing how laser light is combined in the first embodiment. 
           [0042]      FIGS. 24A and 24B  are diagrams showing an arrangement of a projector in a second embodiment. 
           [0043]      FIGS. 25A and 25B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0044]      FIGS. 26A and 26B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0045]      FIGS. 27A and 27B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0046]      FIGS. 28A and 28B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0047]      FIGS. 29A and 29B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0048]      FIGS. 30A and 30B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0049]      FIGS. 31A and 31B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0050]      FIGS. 32A and 32B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0051]      FIGS. 33A and 33B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0052]      FIGS. 34A and 34B  are diagrams (top plan view/front view) showing how laser light is combined in the second embodiment. 
           [0053]      FIGS. 35A and 35B  are diagrams showing another arrangement example of a light source unit. 
           [0054]      FIGS. 36A ,  36 B,  36 C, and  36 D are diagrams showing an arrangement of a liquid cooling jacket as another arrangement example. 
           [0055]      FIGS. 37A and 37B  are diagrams for describing a cooling operation of a laser light source to be performed by a cooling portion as another arrangement example. 
           [0056]      FIGS. 38A  an  38 B are diagrams showing modification examples of the liquid cooling jacket as another arrangement example. 
       
    
    
       [0057]    The drawings are provided mainly for describing the present invention, and do not limit the scope of the present invention. 
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0058]    In the following, embodiments of the invention are described referring to the drawings. 
       A. First Embodiment 
       [0059]      FIGS. 1A and 1B  show an arrangement of a projector embodying the invention.  FIG. 1A  is a perspective side view of the projector, and  FIG. 1B  is a perspective top plan view of the projector. 
         [0060]    Referring to  FIGS. 1A and 1B , the interior of a projector  1  is divided into a space R 1  in the upper position and a space R 2  in the lower position by a partition plate  2 . An optical system  20  for modulating light in accordance with an image signal, and an illumination device  10  for supplying illumination light to the optical system  20  are disposed in the space R 1 . The optical system  20  may be constituted of a well-known optical system such as an LCOS optical system or a DLP optical system, in place of an optical system incorporated with a liquid crystal panel as an imager. Light (image light) modulated by the optical system  20  is projected onto a projection plane (screen) through a projection lens  21 . The members constituting the optical system  20  are disposed on an installation plane in parallel to X-Z plane shown in  FIGS. 1A and 1B . 
         [0061]    A cooling device  30  is disposed immediately below the illumination device  10  in the space R 2 . The cooling device  30  is provided with a radiator  31 , a pump  32 , a fan  33 , and a plumbing pipe  34 . The plumbing pipe  34  is adapted to connect the radiator  31  and the pump  32 , and extends from an opening formed in the partition plate  2  into the space R 1  to be connected to plumbing pipes  12   d  (see  FIGS. 2A ,  2 B,  2 C, and  2 D) of a cooling portion  12  mounted on a laser light source  11  in the illumination device  10 . The radiator  31 , the pump  32 , and the cooling portion of the laser light source are connected to each other in the form of a closed loop by the plumbing pipes  34  and  12   d , whereby a flow channel of a refrigerant is formed. 
         [0062]    When the pump  32  is driven, a refrigerant is circulated through the plumbing pipes  12   d , and a heat generated in the laser light source is transferred to the radiator  31 . The heat transferred to the radiator  31  is removed by the air supplied to the radiator  31  by the fan  33 . In this way, the heat generated in the laser light source is released to the exterior, and the temperature of the laser light source is adjusted to a predetermined temperature. 
         [0063]      FIGS. 2A and 2B  are diagrams showing an arrangement example of the light source unit.  FIGS. 2C and 2D  are diagrams showing another arrangement example of the light source unit.  FIGS. 2A and 2C  are side view of the laser light source, and  FIGS. 2B and 2D  are front view of the laser light source. 
         [0064]    Referring to  FIGS. 2A and 2B , a light source unit is constituted of the laser light source  11  and the cooling portion  12 . The laser light source  11  is constituted of a reflection element  11   a  having a wavelength selectivity, a wavelength conversion element  11   b , a laser diode  11   c , and a housing  11   d  for housing the reflection element  11   a , the wavelength conversion element  11   b , and the laser diode  11   c . The laser diode  11   c  emits laser light of wavelength λ1. The wavelength conversion element  11   b  generates laser light of wavelengthλ2 (λ2&lt;λ1) from the laser light of wavelength λ1. The reflection element  11   a  transmits the laser light of wavelength λ 2 , and reflects the laser light of wavelength λ1. The laser light of wavelength λ1 repeats reflection between the reflection element  11   a  and the laser diode  11   c , and generates the laser light of wavelength λ2 by the wavelength conversion element  11   b  during the repetitive reflection. The generated laser light of wavelength λ2 is successively transmitted through the reflection element  11   a , and emitted to the exterior through an opening formed in a front surface of the housing  11   d.    
         [0065]    The cooling portion  12  is constituted of a copper plate  12   a , a Peltier element  12   b , and a liquid cooling jacket  12   c . The copper plate  12   a  is mounted on a back surface of the laser diode  11   c  to diffuse the heat generated in the laser diode  11   c . The Peltier element  12   b  is mounted on the copper plate  12   a  to transfer the heat diffused by the copperplate  12   a  to the liquid cooling jacket  12   c . The liquid cooling jacket  12   c  is internally formed with a flow channel, and the plumbing pipes  12   d  are connected to an entrance and an exit of the flow channel. A refrigerant flows in the liquid cooling jacket  12   c  from one of the two plumbing pipes  12   d , and flows out from the other of the two plumbing pipes  12   d . In this way, the refrigerant is circulated through the flow channel within the liquid cooling jacket  12   c , and the heat transferred from the Peltier element  12   b  to the liquid cooling jacket  12   c  is transferred to the refrigerant circulating in the liquid cooling jacket  12   c . As described above, the heat is transferred to the radiator  31  by the refrigerant, and removed by the air passing through the radiator  31 . 
         [0066]    In the arrangement example shown in  FIGS. 2A and 2B , the plumbing pipes  12   d  are arranged to extend downward from a lower surface of the liquid cooling jacket  12   c . Alternatively, as shown in the arrangement example in  FIGS. 2C and 2D , plumbing pipes  12   d  may be projected from a lower portion on a back surface of a liquid cooling jacket  12   c  by a predetermined length, and bent downward so that the plumbing pipes  12   d  are directed downward. The light source units shown in  FIGS. 2A ,  2 B,  2 C, and  2 D are adapted to emit laser light of a green wavelength band, and laser light of a blue wavelength band. 
         [0067]      FIGS. 3A and 3B  are diagrams showing another arrangement example of the light source unit.  FIGS. 3C and 3D  are diagrams showing modification example of the arrangement example.  FIGS. 3A and 3C  are side view of the laser light source, and  FIGS. 3B and 3D  are front view of the laser light source. The light source unit shown in  FIGS. 3A ,  3 B,  3 C and  3 D is adapted to emit laser light of a red wavelength band. 
         [0068]    In the arrangement example shown in  FIGS. 3A and 3B , a laser light source  11  is constituted of a semiconductor laser array. The semiconductor laser array is constructed in such a manner that plural laser emitting portions are arranged in left and right directions in  FIG. 3B . A copper plate  12   a  is mounted on a lower surface of the laser light source  11 , and a Peltier element  12   b  and a liquid cooling jacket  12   c  are mounted in this order. The arrangements and the functions of the Peltier element  12   b  and the liquid cooling jacket  12   c  are the same as those in the arrangement examples shown in  FIGS. 2A ,  2 B,  2 C, and  2 D. 
         [0069]    In the arrangement example shown in  FIGS. 3A and 3B , the plumbing pipes  12   d  are arranged to extend downward from a lower surface of the liquid cooling jacket  12   c . Alternatively, as shown in the arrangement example in  FIGS. 3C and 3D , plumbing pipes  12   d  may be projected from a lower portion on a back surface of a liquid cooling jacket  12   c  by a predetermined length, and bent downward so that the plumbing pipes  12   d  are directed downward. 
         [0070]    In the arrangement examples shown in  FIGS. 2A through 3D , the copper plate  12   a  is used for heat diffusion. Alternatively, a heat conductive sheet (graphite sheet), a heat diffusion sheet, a thermal grease, or a like member may be used. Further, there is a case that non-use of the copper plate  12   a  is advantageous in enhancing the cooling efficiency, depending on a heat generation area of the laser light source  11  or an area of the liquid cooling jacket  12   c . In such a case, the copper plate  12   a  may be omitted. Further alternatively, other heat transfer element may be used, in place of the Peltier element  12   b.    
         [0071]      FIGS. 4A ,  4 B,  4 C,  4 D,  4 E, and  4 F are diagrams showing disposition methods of light source units. To simplify the description,  FIGS. 4A ,  4 B,  4 C,  4 D,  4 E, and  4 F illustrate disposition methods, in the case where the light source units shown in  FIGS. 2A ,  2 B,  2 C, and  2 D are used. However, the same disposition methods may be applied to a case where the light source units shown in  FIGS. 3A ,  3 B,  3 C, and  3 D are used. 
         [0072]      FIG. 4A  shows a disposition method, wherein two light source units are arranged side by side in left and right directions.  FIG. 4B  shows a disposition method, wherein two light source units are disposed at forward and rearward positions in the light emission direction, while being partially overlapped with each other in left and right directions. In the disposition method shown in  FIG. 4B , since the light source units are partially overlapped with each other in left and right directions, the distance L 1  between the laser light sources in left and right directions is reduced, as compared with the disposition method shown in  FIG. 4A . Accordingly, as compared with the disposition method shown in  FIG. 4A , the disposition method shown in  FIG. 4B  is advantageous in reducing the overall size of a light flux obtained by combining laser light from the two light source units, and enhancing the light use efficiency based on Etendue theory. 
         [0073]      FIG. 4C  shows an arrangement example, wherein two laser light sources  11  are disposed side by side in left and right directions, and a cooling portion  12  is mounted in common between the two laser light sources  11 . In this arrangement example, a copper plate  12   a  and a Peltier element  12   b  (not shown in  FIG. 4C ) are mounted in common between the two laser light sources  11  on the back surfaces of the two laser light sources  11 , and a liquid cooling jacket  12   c  is also mounted in common between the two laser light sources  11 . In this arrangement example, since the two laser light sources  11  can be disposed closer to each other, as compared with the arrangement example shown in  FIG. 4B , the distance L 1  between the laser light sources  11  in left and right directions can be further reduced, as compared with the disposition method shown in  FIG. 4B . Accordingly, as compared with the disposition method shown in  FIG. 4B , the disposition method shown in  FIG. 4C  is more advantageous in reducing the overall size of a light flux obtained by combining laser light from the two light source units, and further enhancing the light use efficiency based on Etendue theory. 
         [0074]      FIG. 4D  shows a disposition method, wherein two light source units are arranged in upward and downward directions.  FIG. 4E  shows a disposition method, wherein two light source units are disposed at forward and rearward positions in the light emission direction, while being partially overlapped with each other in upward and downward directions. In the disposition method shown in  FIG. 4E , since the light source units are partially overlapped with each other in upward and downward directions, the distance L 2  between the laser light sources in upward and downward directions is reduced, as compared with the disposition method shown in  FIG. 4D . Accordingly, as compared with the disposition method shown in  FIG. 4D , the disposition method shown in  FIG. 4E  is advantageous in reducing the overall size of a light flux obtained by combining laser light from the two light source units, and enhancing the light use efficiency based on Etendue theory. 
         [0075]      FIG. 4F  shows an arrangement example, wherein two laser light sources  11  are disposed side by side in upward and downward directions, and a cooling portion  12  is mounted in common between the two laser light sources  11 . In this arrangement example, a copper plate  12   a  and a Peltier element  12   b  (not shown in  FIG. 4F ) are mounted in common between the two laser light sources  11  on the back surfaces of the two laser light sources  11 , and a liquid cooling jacket  12   c  is also mounted in common between the two laser light sources  11 . In this arrangement example, since the two laser light sources  11  can be disposed closer to each other, as compared with the arrangement example shown in  FIG. 4E , the distance L 2  between the laser light sources  11  in upward and downward directions can be further reduced, as compared with the disposition method shown in  FIG. 4E . Accordingly, as compared with the disposition method shown in  FIG. 4E , the disposition method shown in  FIG. 4F  is more advantageous in reducing the overall size of a light flux obtained by combining laser light from the two light source units, and further enhancing the light use efficiency based on Etendue theory. 
         [0076]    In the following, combination examples of laser light in the illumination device  10  are described. In the diagrams of  FIG. 5  and thereafter, to simplify the description, the light source units shown in  FIGS. 2A ,  2 B,  2 C, and  2 D are schematically illustrated. Each of the light source units may be replaced by the light source units shown in  FIGS. 3A ,  3 B,  3 C, and  3 D. The illumination device  10  is required to emit laser light of at least a red wavelength band, a green wavelength band, and a blue wavelength band. Accordingly, in the following combination examples, any one of the light source units serves as a light source unit for emitting laser light of a red wavelength band, a green wavelength band, or a blue wavelength band, as necessary, and the laser light of each wavelength band emitted from the respective light source units is combined by a prism mirror. In the following combination examples, a light source unit for emitting laser light of a yellow wavelength band may be added. 
         [0077]    In the diagrams of  FIG. 5  and thereafter, light source units attached with the symbols “B”, “M”, “U” respectively show light source units to be disposed in the bottom row, the middle row, and the upper row. Similarly, prism mirrors attached with the symbols “B”, “M”, and “U” respectively show prism mirrors to be disposed in the bottom row, the middle row, and the upper row. 
         [0078]    In the following, combination examples of combining light by a prism mirror are described. In any one of the following combination examples, the propagating directions of light emitted from light source units can be aligned with one direction, and high luminance of illumination light in one direction can be realized. 
       Combination Example 1-1 
       [0079]      FIG. 5 ,  FIG. 6A  and  FIG. 6B  are diagrams showing a combination example, wherein four light source units  101  through  104  are opposed to each other in X-axis direction, and laser light is reflected in Z-axis direction by two prism mirrors  151  and  152 .  FIG. 6A  is a top plan view of  FIG. 5 , and  FIG. 6B  is a front view of  FIG. 5 . 
         [0080]    In this combination example, the light source units  101  and  102  are disposed at forward and rearward positions in the light emission direction, and the rearward light source unit  101  is displaced in upward direction with respect to the forward light source unit  102  by a predetermined distance. Further, the light source units  103  and  104  are disposed at forward and rearward positions in the light emission direction, and the rearward light source unit  103  is displaced in downward direction with respect to the forward light source unit  104  by a predetermined distance. The polarization directions of laser light to be emitted from the light source units  101  through  104  are aligned with one direction. Accordingly, the polarization directions of laser light after reflection on the prism mirrors  151  and  152  are also aligned with one direction. In all the following combination examples, the polarization directions of laser light are aligned with one direction, as well as this combination example. 
         [0081]    In this combination example, the optical path lengths from the light source units  101  through  104  to mirror surfaces of the corresponding prism mirrors  151  and  152  can be made equal to each other. Accordingly, it is possible to align the beam shapes of two laser light after reflection on the prism mirror  151 , and also possible to align the beam shapes of two laser light after reflection on the prism mirror  152 . However, in this combination example, since laser light from the light source unit  103  interferes with the plumbing pipes  12   d  for the light source unit  104 , laser light from the light source unit  103  may be deteriorated. It is desirable to dispose light source units at such positions as to avoid interference between laser light and the plumbing pipes  12   d  in order to stabilize illumination light. 
       Combination Example 1-2 
       [0082]      FIG. 7 ,  FIG. 8A  and  FIG. 8B  are diagrams showing a combination example, wherein the dispositions of the light source units shown in  FIG. 5 ,  FIG. 6A , and  FIG. 6B  are adjusted to avoid interference between laser light and the plumbing pipes  12   d .  FIG. 8A  is a top plan view of  FIG. 7 , and  FIG. 8B  is a front view of  FIG. 7 . 
         [0083]    In this combination example, the light source units  103  and  104  are disposed at forward and rearward positions in the light emission direction, and the rearward light source unit  103  is displaced in upward direction with respect to the forward light source unit  104  by a predetermined distance. In the above arrangement, there is no likelihood that laser light from the light source unit  103  may be blocked by the plumbing pipes  12   d  for the light source unit  104 , and it is possible to smoothly allow incidence of laser light from all the light source units  101  through  104  into the corresponding prism mirrors  151  and  152 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the plumbing pipes  12   d.    
         [0084]    In this combination example, by disposing the light source units  101  and  102 , and disposing the light source units  103  and  104  as shown in  FIG. 4E , as described referring to  FIG. 4E , it is possible to reduce the overall size of a light flux obtained by combining laser light from the two laser light sources, and enhance the light use efficiency based on Etendue theory. 
       Combination Example 1-3 
       [0085]      FIG. 9 ,  FIG. 10A  and  FIG. 10B  are diagrams showing a combination example, wherein six light source units  101  through  106  are opposed to each other in X-axis direction, and laser light is reflected in Z-axis direction by three prism mirrors  151 ,  152  and  153 .  FIG. 10A  is a top plan view of  FIG. 9 , and  FIG. 10B  is a front view of  FIG. 9 . 
         [0086]    In this combination example, the light source units  101 ,  102  and  105  are disposed at forward and rearward positions in the light emission direction, and the rearward light source units  101  and  102  are displaced gradually in upward direction with respect to the forward light source unit  105  by a predetermined distance. Further, the light source units  103 ,  104  and  106  are disposed at forward and rearward positions in the light emission direction, and the rearward light source units  103  and  104  are displaced gradually in upward direction with respect to the forward light source unit  106  by a predetermined distance. The polarization directions of laser light to be emitted from the light source units  101  through  106  are aligned with one direction. Accordingly, the polarization directions of laser light after reflection on the prism mirrors  151 ,  152  and  153  are also aligned with one direction. 
         [0087]    In the above arrangement, there is no likelihood that laser light from the light source units  101  and  103  may be blocked by the plumbing pipes  12   d  for the light source units  102  and  104  disposed in front of the light source units  101  and  103 . Furthermore, there is no likelihood that laser light from the light source units  102  and  104  may be blocked by the plumbing pipes  12   d  for the light source units  105  and  106  disposed in front of the light source units  102  and  104 . Therefore it is possible to smoothly allow incidence of laser light from all the light source units  101  through  104  into the corresponding prism mirrors  151 ,  152  and  152 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the plumbing pipes  12   d.    
         [0088]    In this combination example, by disposing the light source units  101  and  102 , disposing the light source units  102  and  105 , disposing the light source units  103  and  104 , and disposing the light source units  104  and  106 , as shown in  FIG. 4E , as described referring to  FIG. 4E , it is possible to reduce the overall size of a light flux obtained by combining laser light from the two laser light sources, and enhance the light use efficiency based on Etendue theory. 
       Combination Example 1-4 
       [0089]      FIG. 11 ,  FIG. 12A  and  FIG. 12B  are diagrams showing a combination example, wherein light source units  111  and  112  shown in  FIG. 4F  are opposed to each other in X-axis direction, and laser light is reflected in Z-axis direction by prism mirror  161 .  FIG. 12A  is a top plan view of  FIG. 11 , and  FIG. 12B  is a front view of  FIG. 11 . 
         [0090]    In this combination example, it is possible to reduce the distance between the laser light sources  111   a  and  111   b  and the distance between the laser light sources  112   a  and  112   b , as compared with the combination example in  FIGS. 7 ,  8 A and  8 B. Therefore, as described referring to  FIG. 4F , it is possible to reduce the overall size of a light flux obtained by combining laser light from the two laser light sources, and enhance the light use efficiency based on Etendue theory. In this combination example, since one cooling portion is mounted with respect to two laser light sources, it is possible to simplify the arrangement. However, since a cooling operation is performed by two laser light sources as a pair, it is impossible to individually control the temperatures of the light sources. Accordingly, the combination example shown in  FIG. 7 ,  FIG. 8A , and  FIG. 8B  is superior in the aspect of temperature control. 
       Combination Example 1-5 
       [0091]      FIG. 13 ,  FIG. 14A , and  FIG. 14B  are diagrams showing a combination example, wherein the light source units  101  and  104 , and the prism mirror  151  in the combination example shown in  FIG. 5 ,  FIG. 6A , and  FIG. 6B  are displaced in Z-axis direction by a predetermined distance.  FIG. 14A  is a top plan view of  FIG. 13 , and  FIG. 14B  is a front view of  FIG. 13 . 
         [0092]    In this combination example, since the light source units  101  and  104 , and the prism mirror  151  are displaced in Z-axis direction by a predetermined distance, it is possible to avoid the problem in the combination example shown in  FIG. 5 ,  FIG. 6A , and  FIG. 6B , in other words, interference between laser light from the light source unit  103 , and the plumbing pipes  12   d  for the light source unit  104 . Thus, it is possible to suppress deterioration of illumination light. 
         [0093]    In addition, in this combination example, the optical path lengths from the light source units  101  through  104  to mirror surfaces of the corresponding prism mirrors  151  and  152  can be made equal to each other. Accordingly, it is possible to align the beam shapes of two laser light after reflection on the prism mirror  151 , and also possible to align the beam shapes of two laser light after reflection on the prism mirror  152 . 
         [0094]    In this combination example, the light source units  101  and  102  are partially overlapped with each other in Z-axis direction, and the light source units  103  and  104  are also partially overlapped with each other in Z-axis direction. This arrangement is advantageous in reducing the optical path difference between laser light from the light source units  101  and  103 , and laser light from the light source units  102  and  104 , as compared with an arrangement, wherein light source units are disposed without being overlapped with each other. Accordingly, it is possible to reduce the size difference between the beam shape of laser light from the light source units  101  and  103  after reflection on the prism mirror  151 , and the beam shape of laser light from the light source units  102  and  104  after reflection on the prism mirror  152 , and enhance uniformity of illumination light. 
         [0095]    In this combination example, furthermore, by partially overlapping the light source units  101  and  102  with each other in Y-axis direction, and partially overlapping the light source units  103  and  104  with each other in Y-axis direction, it is possible to reduce the overall size of a light flux obtained by combining laser light from two light source units, and enhance the light use efficiency of illumination light based on Etendue theory. 
       Combination Example 1-6 
       [0096]      FIG. 15 ,  FIG. 16A  and  FIG. 16B  are diagrams showing a combination example, wherein eight light source units  121  through  128  are opposed to each other in X-axis direction, and laser light is reflected in Z-axis direction by two prism mirrors  171  and  172 .  FIG. 16A  is a top plan view of  FIG. 15 , and  FIG. 16B  is a front view of  FIG. 15 . 
         [0097]    In this combination example, the light source units  121  and  122 , the light source units  123  and  124 , the light source units  125  and  126 , and the light source units  127  and  128  are respectively disposed at forward and rearward positions in X-axis direction. The rearward light source units  121 ,  123 ,  125  and  127  are displaced in upward direction with respect to the forward light source units  122 ,  124 ,  126  and  128  by a predetermined distance. Further, the light source units  121  and  123 , the light source units  122  and  124 , the light source units  125  and  127 , and the light source units  126  and  128  are disposed side by side in Z-axis direction. 
         [0098]    In the above arrangement, there is no likelihood that laser light from the light source units  121 ,  123 ,  125  and  127  may be blocked by the plumbing pipes  12   d  for the light source units  122 ,  124 ,  126  and  128  disposed in front of the light source units  121 ,  123 ,  125  and  127 . Therefore it is possible to smoothly allow incidence of laser light from all the light source units  121  through  128  into the corresponding prism mirrors  171  and  172 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the plumbing pipes  12   d.    
         [0099]    Further, in this combination example, the light source units  121  and  122 , the light source units  123  and  124 , the light source units  125  and  126 , and the light source units  127  and  128  are disposed in a partially overlapped state in Y-axis direction, as shown in  FIG. 4E . Accordingly, as described referring to  FIG. 4E , it is possible to reduce the overall size of a light flux obtained by combining laser light from two light source units, and enhance the light use efficiency of illumination light based on Etendue theory. Further, by partially overlapping the forward and rearward light source units with each other in Y-axis direction as described above, it is possible to reduce the sizes of the prism mirrors  171  and  172  in Y-axis direction. 
         [0100]    In this combination example, by replacing light source units disposed side by side in Z-axis direction, specifically, the light source units  121  and  123 , the light source units  122  and  124 , the light source units  125  and  127 , and the light source units  126  and  128  with the arrangement example shown in  FIG. 4C , as described referring to  FIG. 4C , it is possible to further reduce the overall size of a light flux obtained by combining laser light from two light source units, and further enhance the light use efficiency of illumination light based on Etendue theory. 
         [0101]    Further, in this combination example, the forward and rearward light source units are disposed in a partially overlapped state in Y-axis direction. Alternatively, as shown in  FIGS. 17A and 17B , it is possible to dispose the forward and rearward light source units in a partially overlapped state in X-axis direction. The modification enables to reduce the optical path difference between laser light from two light source units at forward and rearward positions in X-axis direction, and reduce the size difference between the beam shapes of laser light after reflection on the prism mirrors  171  and  172 . As a result, it is possible to enhance uniformity of illumination light. 
       Combination Example 1-7 
       [0102]      FIG. 18 ,  FIG. 19A , and  FIG. 19B  are diagrams showing a combination example, wherein eight light source units  121  through  128  are opposed to each other in X-axis direction, laser light from the light source units  121  through  128  is reflected in Z-axis direction by four prism mirrors  181  through  184 , and two light source  129  and  130  are disposed on the back surface side of the prism mirrors  181  through  184  to emit two laser light from the light source units  129  and  130  respectively through a clearance between the prism mirrors  181  and  182 , and through a clearance between the prism mirrors  183  and  184  in Z-axis direction.  FIG. 19A  is a top plan view of  FIG. 18 , and  FIG. 19B  is a front view of  FIG. 18 . 
         [0103]    In this combination example, the light source units  121  and  122 , the light source units  123  and  124 , the light source units  125  and  126 , and the light source units  127  and  128  are respectively disposed at forward and rearward positions in X-axis direction. The rearward light source units  121 ,  123 ,  125  and  127  are displaced in upward direction with respect to the forward light source units  122 ,  124 ,  126  and  128  by a predetermined distance. Further, the light source units  121  and  123 , the light source units  122  and  124 , the light source units  125  and  127 , and the light source units  126  and  128  are disposed side by side in Z-axis direction. 
         [0104]    In the above arrangement, there is no likelihood that laser light from the light source units  121 ,  123 ,  125  and  127  may be blocked by the plumbing pipes  12   d  for the light source units  122 ,  124 ,  126  and  128  disposed in front of the light source units  121 ,  123 ,  125  and  127 . Therefore it is possible to smoothly allow incidence of laser light from all the light source units  121  through  128  into the corresponding prism mirrors  181  through  184 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the plumbing pipes  12   d . Further, in this combination example, since the two light source units  129  and  130  are additionally provided, the luminance of illumination light can be further increased, as compared with the combination example shown in  FIG. 15 ,  FIG. 16A , and  FIG. 16B . 
         [0105]    Similarly to the combination example in  FIGS. 15 ,  16 A and  16 B, in this combination example, by replacing light source units disposed side by side in Z-axis direction, specifically, the light source units  121  and  123 , the light source units  122  and  124 , the light source units  125  and  127 , and the light source units  126  and  128  with the arrangement example shown in  FIG. 4C , as described referring to  FIG. 4C , it is possible to further reduce the overall size of a light flux obtained by combining laser light from two light source units, and further enhance the light use efficiency of illumination light based on Etendue theory. Further, similarly to the combination example in  FIGS. 17A and 17B , by disposing two light source units at forward and rearward positions in X-axis direction in a partially overlapped state in X-axis direction, it is possible to enhance the light use efficiency of illumination light in the optical system  20 . 
       Combination Example 1-8 
       [0106]      FIG. 20 ,  FIG. 21A , and  FIG. 21B  are diagrams showing a combination example, wherein the dispositions of the prism mirrors  181  through  184  in the combination example shown in  FIG. 18 ,  FIG. 19A , and  FIG. 19B  are modified.  FIG. 21A  is a top plan view of  FIG. 20 , and  FIG. 21B  is a front view of  FIG. 20 . 
         [0107]    This combination example is different from the combination example shown in  FIG. 18 ,  FIGS. 19A , and  19 B in the positions of the prism mirrors  182  and  184 . Specifically, laser light from the light source units  122  and  126  is reflected on the prism mirror  184 , and laser light from the light source units  124  and  128  is reflected on the prism mirror  182 . 
         [0108]    In this combination example, substantially the same advantage as in the combination example shown in  FIG. 18 ,  FIGS. 19A , and  19 B is obtained. Further, in this combination example, as well as the combination example shown in  FIG. 18 ,  FIG. 19A , and  FIG. 19B , by replacing the light source units disposed side by side in Z-axis direction with the arrangement example shown in  FIG. 4C , it is possible to enhance the light use efficiency of illumination light; and by disposing two light source units at forward and rearward positions in X-axis direction in a partially overlapped state in X-axis direction, it is possible to enhance the light use efficiency of illumination light in the optical system  20 . 
       Combination Example 1-9 
       [0109]      FIG. 22 ,  FIG. 23A  and  FIG. 23B  are diagrams showing a combination example, wherein four light source units  101  through  104  are opposed to each other in X-axis direction, and laser light is reflected in Z-axis direction by two prism mirrors  151  and  152 .  FIG. 23A  is a top plan view of  FIG. 22 , and  FIG. 23B  is a front view of  FIG. 22 . 
         [0110]    In this combination example, the light source units  101  through  104 , and the prism mirrors  151  and  152  are disposed at such positions that the optical path lengths from the light source units  101  through  104  to a plane S perpendicular to the optical axes of laser light after reflection on the prism mirror  151  and  152  are made equal to each other. Specifically, referring to  FIG. 23A , the dispositions of the light source units  101  through  104 , and the prism mirrors  151  and  152  are adjusted to satisfy a relation: P1+D=P2, where P1 is a distance from the light source units  101  and  103  to a reflection surface of the prism mirror  151 , P2 is a distance from the light source units  102  and  104  to a reflection surface of the prism mirror  152 , and D is a distance in Z-axis direction between the light source units  101  and  102 , and a distance in Z-axis direction between the light source units  103  and  104 . 
         [0111]    As described above, in this combination example, since the optical path lengths from the light source units  101  through  104  to the plane S perpendicular to the optical axes of laser light after reflection on the prism mirrors  151  and  152  are made equal to each other, it is possible to align the beam shapes of all the laser light after reflection on the prism mirrors  151  and  152 . As a result, it is possible to enhance uniformity of illumination light. 
       B. Second Embodiment 
       [0112]    This embodiment is directed to an arrangement, wherein a cooling device  30  is disposed in an upper position with respect to an optical system  20 . In this embodiment, since the cooling device  30  is disposed in an upper position with respect to the optical system  20 , a cooling device of air-cooling type is used as the cooling device  30 , and a heat pipe is used as a heat transfer system. Thus, by using a cooling device and a heat transfer system of a type other than liquid-cooling type, it is possible to avoid a drawback resulting from liquid leakage. 
         [0113]      FIGS. 24A and 24B  show an arrangement of a projector as the second embodiment.  FIG. 24A  is a perspective side view of the projector, and  FIG. 24B  is a perspective view of the projector, when viewed from a bottom side thereof. 
         [0114]    Referring to  FIGS. 24A and 24B , similarly to the first embodiment, the interior of a projector  1  is divided into a space R 1  in the upper position and a space R 2  in the lower position by a partition plate  2 . The optical system  20 , and an illumination device  10  for supplying illumination light to the optical system  20  are disposed in the space R 2 . 
         [0115]    The cooling device  30  is disposed at a position immediately above the illumination device  10  in the space R 1  . The cooling device  30  is provided with a heat pipe  35 , a heat sink  36 , and a fan  37 . The heat pipe  35  is connected to a Peltier element  12   b  (see  FIGS. 2A ,  2 B,  2 C,  2 D, and  FIGS. 3A ,  3 B,  3 C) on the side of a light source unit. Specifically, in this embodiment, the elements in the arrangements shown in  FIGS. 2A ,  2 B,  2 C,  2 D, and  FIGS. 3A ,  3 B,  3 C,  3 D except for the liquid cooling jacket  12   c  and the plumbing pipes  12   d  are provided, and the heat pipe  35  is mounted on the Peltier element  12   b . The heat pipe  35  is mounted on the Peltier element  12   b  in such a manner that the heat pipe  35  extends upward from the Peltier element  12   b.    
         [0116]    A heat generated in a laser light source is transferred to the heat sink  36  by the heat pipe  35 . The heat transferred to the heat sink  36  is removed by the air supplied to the heat sink  36  by the fan  37 . Thus, the heat generated in the laser light source is released to the exterior, and the temperature of the laser light source is adjusted to a predetermined temperature. 
         [0117]    In this embodiment, since the heat transfer direction is made upside down with respect to the arrangement example (first embodiment) shown in  FIGS. 1A and 1B , it is necessary to make the positional relation of the light source units in the combination examples shown in  FIGS. 5 through 23B  upside down, and resultantly make the positional relation of the prism mirrors upside down, in order to avoid interference between laser light and the heat pipe  35 . 
         [0118]    In the following, combination examples of this embodiment, wherein the combination examples shown in  FIGS. 5 through 23B  in the first embodiment are applied to this embodiment by making the dispositions of the light source units, and the dispositions of the prism mirrors upside down (inverted in Y-axis direction), are described one by one referring to the drawings. In the following, to simplify the description, only a top plan view and a front view of each of the combination examples are shown, and a perspective view thereof is omitted. 
       Combination Example 2-1 
       [0119]      FIGS. 25A and 25B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 5 ,  FIGS. 6A , and  6 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 5 ,  FIG. 6A , and  FIG. 6B , since laser light from the light source unit  103  interferes with the heat pipe  35  mounted on the light source unit  104 , laser light from the light source unit  103  may be deteriorated. 
       Combination Example 2-2 
       [0120]      FIGS. 26A and 26B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 7 ,  FIGS. 8A , and  8 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 7 ,  FIGS. 8A , and  8 B, there is no likelihood that laser light from the light source unit  103  may be blocked by the heat pipe  35  mounted on the light source unit  104 , and it is possible to smoothly allow incidence of laser light from all the light source units  101  through  104  into the corresponding prism mirrors  151  and  152 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the heat pipe  35 . 
         [0121]    As well as the combination example shown in  FIG. 7 ,  FIGS. 8A , and  8 B, in this combination example, it is also possible to enhance the light use efficiency of illumination light by adjusting the dispositions of the light source units at forward and rearward positions in X-axis direction, as shown in  FIG. 4E . 
       Combination Example 2-3 
       [0122]      FIGS. 27A and 27B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 9 ,  FIGS. 10A , and  10 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 9 ,  FIGS. 10A , and  10 B, there is no likelihood that laser light from the light source units  101  and  103  may be blocked by the heat pipe  35  mounted on the light source units  102  and  104  that are disposed in front of the light source units  101  and  103 . In addition, there is no likelihood that laser light from the light source units  102  and  104  may be blocked by the heat pipe  35  mounted on the light source units  105  and  106  that are disposed in front of the light source units  102  and  104 . Therefore, it is possible to smoothly allow incidence of laser light from all the light source units  101  through  106  into the corresponding prism mirrors  151 ,  152  and  153 . Thus, it is possible to supply stable illumination light to the optical system  20 . 
         [0123]    As well as the combination example shown in  FIG. 9 ,  FIGS. 10A , and  10 B, in this combination example, it is also possible to enhance the light use efficiency of illumination light by adjusting the dispositions of the light source units at forward and rearward positions in X-axis direction, as shown in  FIG. 4E . 
       Combination Example 2-4 
       [0124]      FIGS. 28A and 28B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 11 ,  FIG. 12A , and  12 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 11 ,  FIG. 12A , and  12 B, it is possible to reduce the distance between the laser light sources  111   a  and  111   b  and the distance between the laser light sources  112   a  and  112   b . Therefore, it is possible to reduce the overall size of a light flux obtained by combining laser light from the two laser light sources, and enhance the light use efficiency. In this combination example, since one cooling portion is mounted with respect to two laser light sources, it is possible to simplify the arrangement. However, since a cooling operation is performed by two laser light sources as a pair, it is impossible to individually control the temperatures of the light sources. 
       Combination Example 2-5 
       [0125]      FIGS. 29A and 29B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 13 ,  FIG. 14A , and  14 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 13 ,  FIG. 14A , and  14 B, since the light source units  101  and  104 , and the prism mirror  151  are disposed with a displacement in Z-axis direction by a predetermined distance, it is possible to avoid interference between laser light from the light source unit  104 , and the heat pipe  35  of the light source unit  103 . Accordingly, it is possible to suppress deterioration of illumination light. Further, in this combination example, substantially the same advantage as in the combination example (first embodiment) shown in  FIG. 13 ,  FIGS. 14A and 14B  is obtained. Further, this combination example may be modified in the similar manner as the combination example (first embodiment) shown in  FIG. 13 ,  FIGS. 14A and 14B . 
       Combination Example 2-6 
       [0126]      FIGS. 30A and 3013  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 15 ,  FIG. 16A , and  16 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 15 ,  FIG. 16A , and  1613 , there is no likelihood that laser light from the light source units  121 ,  123 ,  125  and  127  may be blocked by the heat pipe  35  mounted on the light source units  122 ,  124 ,  126  and  128  that are disposed in front of the light source units  121 ,  123 ,  125  and  127 . Therefore, it is possible to smoothly allow incidence of laser light from all the light source units  121  through  128  into the corresponding prism mirrors  171  and  172 . Thus, it is possible to supply stable illumination light to the optical system  20 . Further, in this combination example, substantially the same advantage as in the combination example (first embodiment) shown in  FIG. 15 , FIG.  16 A and  16 B is obtained. Further, this combination example may be modified in the similar manner as the combination example (first embodiment) shown in  FIG. 15 ,  FIGS. 16A and 16B . 
       Combination Example 2-7 
       [0127]      FIGS. 31A and 32B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIGS. 17A and 17B  is applied to this embodiment. In this combination example, as well as the combination example shown in  FIGS. 17A and 17B , it is possible to reduce the optical path difference between laser light from the two light source units, wherein light source units are disposed at forward and rearward positions in X-axis direction, and reduce the size difference between the beam shapes of the laser light after reflection on the prism mirror  171  and  172 . Thus, it is possible to enhance the light use efficiency of illumination light in the optical system  20 . 
       Combination Example 2-8 
       [0128]      FIGS. 32A and 32B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIG. 18 ,  FIG. 19A , and  19 B is applied to this embodiment. In this combination example, as well as the combination example shown in  FIG. 18 ,  FIG. 19A , and  19 B, there is no likelihood that laser light from the light source units  121 ,  123 ,  125  and  127  may be blocked by the heat pipe  35  mounted on the light source units  122 ,  124 ,  126  and  128  that are disposed in front of the light source units  121 ,  123 ,  125  and  127 . Therefore, it is possible to smoothly allow incidence of laser light from all the light source units  121  through  128  into the corresponding prism mirrors  181  through  184 . Thus, it is possible to supply stable illumination light to the optical system  20 , without deterioration of laser light resulting from interference with the heat pipe  35 . Further, in this combination example, substantially the same advantage as in the combination example (first embodiment) shown in  FIG. 18 ,  FIG. 19A and 19B  is obtained. Further, this combination example may be modified in the similar manner as the combination example (first embodiment) shown in  FIG. 18 ,  FIGS. 19A and 19B . 
       Combination Example 2-9 
       [0129]      FIGS. 33A and 33B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIGS. 20 ,  21 A, and  21 B is applied to this embodiment. As well as the combination example shown in  FIGS. 20 ,  21 A, and  21 B, this combination example is different from the combination example shown in  FIGS. 32A and 32B  in the positions of the prism mirrors  181  and  183 . In this combination example, substantially the same advantage as in the combination example shown in  FIGS. 32A and 32B  is obtained. Further, this combination example may be modified in the similar manner as the combination example (first embodiment) shown in  FIGS. 20 ,  21 A and  21 B. 
       Combination Example 2-10 
       [0130]      FIGS. 34A and 34B  are diagrams showing a combination example, wherein the combination example (first embodiment) shown in  FIGS. 22 ,  23 A and  23 B is applied to this embodiment. In this combination example, since the optical path lengths from the light source units  101  through  104  to the plane S perpendicular to the optical axes of laser light after reflection on the prism mirrors  151  and  152  are made equal to each other, it is possible to align the beam shapes of all the laser light after reflection on the prism mirrors  151  and  152 . As a result, it is possible to enhance uniformity of illumination light. 
         [0131]    As described above, in the first embodiment and the second embodiment, since the cooling device  30  is disposed in a lower position or an upper position with respect to the optical system  20 , it is possible to reduce the outer size of the projector  1 , as compared with an arrangement, wherein a cooling device  30  is disposed in parallel to an installation plane of an optical system  20 . Further, since the cooling device  30  is disposed at a position immediately below or immediately above the illumination device  10 , it is possible to suppress elongation of the plumbing pipes  12   d ,  34 , and the heat pipe  35 , thereby simplifying the arrangement of the heat transfer system and reducing the cost. 
         [0132]    Further, it is possible to avoid interference between the plumbing pipe  12   d  or the heat pipe  35 , and laser light, and supply stable illumination light to the optical system  20  by combining laser light in the illumination device  10  in accordance with the combination examples shown in  FIGS. 7 through 23B , and the combination examples shown in  FIGS. 26A through 34B . Further, as described individually in each of the combination examples, using the combination examples shown in  FIGS. 7 through 23B , and the combination examples shown in  FIGS. 26A through 34B  enables to enhance the light use efficiency of illumination light in the optical system  20 , and increase the luminance of a projection image. 
       C. Another arrangement example of Light source unit 
       [0133]      FIGS. 35A and 35B  are diagrams showing another arrangement example of a light source unit.  FIG. 35A  is a side view of the light source unit, and  FIG. 35B  is a front view of the light source unit. 
         [0134]    Referring to  FIGS. 35A and 35B , a light source unit is constituted of the laser light source  50  and the cooling portion  60 . As well as the laser light source in the first embodiment, the laser light source  50  is constituted of a reflection element having a first wavelength selectivity, a wavelength conversion element  52 , a laser diode  53 , and a housing  54  for housing the reflection element  51 , the wavelength conversion element  52 , and the laser diode  53 . 
         [0135]    The cooling portion  60  is constituted of a copper plate  61 , a Peltier element  62 , and a liquid cooling jacket  63 . The copper plate  61  is mounted on a back surface of the laser diode  53  to diffuse the heat generated in the laser diode  53 . The Peltier element  62  is mounted on the copperplate  61  to transfer the heat diffused by the copper plate  61  to the liquid cooling jacket  63 . The copper plate  61  and the Peltier element  62  are mounted on a front surface (attachment surface) of the liquid cooling jacket  63  by four screws  64 . In this arrangement, a graphite sheet or an indium sheet having a high thermal conductivity is disposed in a boundary surface between the laser diode  53  and the copper plate  61 , a boundary surface between the copper plate  61  and the Peltier element  62 , and a boundary surface between the Peltier element  62  and the liquid cooling jacket  63 . Alternatively, a thermal grease may be coated on each of the boundary surfaces, in place of using these sheets. 
         [0136]    The Peltier element  62  in the cooling portion  60  may be omitted. In the modification, the copper plate  61  is directly attached to the liquid cooling jacket  63 . 
         [0137]      FIGS. 36A ,  36 B,  36 C, and  36 D are diagrams showing an arrangement of the liquid cooling jacket  63 .  FIGS. 36A and 36B  are respectively a front view and a top plan view of the liquid cooling jacket  63 .  FIG. 36C  is a cross-sectional view taken along the line A-A′ in  FIG. 36A , and  FIG. 36D  is an inner perspective view of the liquid cooling jacket  63 , when viewed from a front side of the liquid cooling jacket  63 . 
         [0138]    The liquid cooling jacket  63  is constituted of a jacket portion  631 , an inlet portion  632  projecting from a lower surface of the jacket portion  631 , and an outlet portion  633  projecting from an upper surface of the jacket portion  631 . 
         [0139]    The liquid cooling jacket  63  is made of a material having a high thermal conductivity such as aluminum or copper. As shown in  FIG. 36C , the liquid cooling jacket  63  is formed by joining a front jacket portion F and a back jacket portion B at a central part by welding or a like process. 
         [0140]    Four screw holes  631   a  for fixing the copper plate  61  and the Peltier element  62  with respect to the jacket portion  631  by the screws  64  are formed in a front surface of the jacket portion  631 . Further, a flow channel  634  is formed in the interior of the jacket portion  631 . An entrance  634   a  is formed in a lower surface of the flow channel  634 , and an exit  634   b  is formed in an upper surface of the flow channel  634 . The entrance  634   a  is communicated with an inlet path  635  formed in the inlet portion  632 , and the exit  634   b  is communicated with an outlet path  636  formed in the outlet portion  633 . 
         [0141]    As shown in  FIG. 36D , plural straight fins  637  are disposed in the flow channel  634  with a predetermined interval (e.g. 1 mm) in left and right directions. Each of the straight fins  637  projects from a front surface of the flow channel  634  in rearward direction, and extends in up and down directions along a flow of a refrigerant in the flow channel  634 . The straight fins  637  are formed in such a manner that the laser light source  50  is disposed in an area where the straight fins  637  are disposed, when viewed from the front side of the liquid cooling jacket  63 . 
         [0142]    A slope  634   c  is formed on a lower portion of the flow channel  634  in such a manner that the flow channel  634  is gradually expanded from the entrance  634   a . A slope  634   d  is formed on an upper portion of the flow channel  634  in such a manner that the flow channel  634  is gradually narrowed toward the exit  634   b.    
         [0143]    Further, an area S 2  having the same transverse width as a disposition area S 1  where the straight fins are disposed is formed between lower ends of the straight fins  637  and the slope  634   c ; and an area S 3  having the same transverse width as the disposition area S 1  is formed between upper ends of the straight fins  637  and the slope  634   d.    
         [0144]      FIGS. 37A and 37B  are diagrams for describing a cooling operation of the laser light source  50  to be performed by the cooling portion  60 .  FIG. 37A  is a side view, with a portion corresponding to the liquid cooling jacket  63  being illustrated as a cross-sectional view.  FIG. 37B  is an inner perspective view, when viewed from the front side of the liquid cooling jacket  63 . 
         [0145]    Referring to  FIGS. 37A and 37B , the cooling portion  60  is disposed in a state that a surface (front surface of the liquid cooling jacket  63 ) where the laser light source  50  is mounted is aligned with up and down directions of the projector, in other words, a gravitational force direction. In this arrangement, the entrance  634   a  of the flow channel  634  is positioned on the lower side in the gravitational force direction, and the exit  634   b  of the flow channel  634  is positioned on the upper side in the gravitational force direction. 
         [0146]    The plumbing pipes (not shown) from the radiator  31  of the cooling device  30  shown in  FIGS. 1A and 1B  are connected to the inlet portion  632  and the outlet portion  633  of the liquid cooling jacket  63 . In this arrangement, a refrigerant flows in through the inlet portion  632 , and flows out through the outlet portion  633  via the flow channel  634 . In this way, the refrigerant is circulated through the flow channel  634  in the liquid cooling jacket  63  and the radiator  31 . Water or an ethylene-glycol-based liquid medium may be used as the refrigerant. 
         [0147]    A heat generated in the laser light source  50  is transferred to the liquid cooling jacket  63  through the copper plate  61  and the Peltier element  62 . Then, the heat transferred to the liquid cooling jacket  63  is heat-exchanged with the refrigerant flowing through the flow channel  634  at the front surface of the flow channel  634  and the straight fins  637  to be transferred to the refrigerant. The heat is then transferred to the radiator  31  by the refrigerant, and removed by the air passing the radiator  31 . 
         [0148]    There is a case that air bubbles are generated in the flow channel  634  of the liquid cooling jacket  63 , resulting from e.g. intrusion of air into the refrigerant, or evaporation of air dissolved in the refrigerant from the refrigerant. In this case, if the air bubbles stagnate in the liquid cooling jacket  63 , the heat transferred from the laser light source  50  may not be sufficiently transferred to the refrigerant by the air bubbles (due to an increase of thermal resistance), with the result that a cooling effect of the laser light source  50  may not be sufficiently obtained. In such a case, the laser light source  50  may be deteriorated (lifetime of the laser light source  50  may be reduced). 
         [0149]    In contrast, in this arrangement example, the front surface of the liquid cooling jacket  63  where the laser light source  50  is mounted is aligned with the gravitational force direction, and the exit  634   b  is formed in the upper portion of the flow channel  634 . Accordingly, as shown in  FIG. 37B , the air bubbles generated in the flow channel  634  are moved to the upper portion of the flow channel  634 , and discharged through the exit  634   b  and the outlet path  636  along with the refrigerant. 
         [0150]    Accordingly, in this arrangement example, air bubbles are less likely to stagnate on the front surface of the flow channel  634 , or the portion corresponding to the straight fins  637 , where a heat exchange between the heat from the laser light source  50 , and the refrigerant is mainly performed. As a result, since an increase of thermal resistance due to air bubbles is suppressed, a cooling effect of the laser light source  50  can be maintained. 
         [0151]    Since the width of the lower portion of the flow channel  634  is gradually increased by the slope  634   c , and the width of the upper portion of the flow channel  634  is gradually decreased by the slope  634   d , a resistance in the flow channel is reduced, and the refrigerant is allowed to flow smoothly in the flow channel  634 . Further, the air bubbles are smoothly guided and discharged to the exit  634   b  in the upper portion of the flow channel  634  by the slope  634   d.    
         [0152]    Further, the areas S 2  and S 3  are formed at positions anterior and posterior to the straight fins  637  to prevent the width of the flow channel  634  from reducing immediately from an end portion of the straight fins  637 . This arrangement further reduces a resistance in the flow channel, thereby smoothly flowing the refrigerant. Furthermore, a sufficient clearance (area S 3 ) is secured between the upper ends of the straight fins  637 , and the upper surface of the flow channel  634  at both of left and right corner ends on the upper portion of the flow channel. Accordingly, as compared with an arrangement, in which the clearance (area S 3 ) is not formed, air bubbles passing the left and right corner ends can be easily released from the straight fins  637 . Thus, discharge of air bubbles is smoothly performed. 
         [0153]    As described above, forming the entrance  634   a  and the exit  634   b  at upper and lower positions (in the gravitational force direction) of the flow channel  634 , and forming the upper surface and the lower surface of the flow channel  634  into the slopes  634   c  and  634   d  not only enables to secure a smooth flow of a refrigerant, but also enables to realize smooth discharge of air bubbles generated in the flow channel  634 . In the case where an ethylene-glycol-based liquid medium is used as a refrigerant, the viscosity of the liquid medium is increased, as compared with water. In view of the above, the above arrangement is more desirable to secure a smooth flow. 
         [0154]      FIGS. 38A and 38B  are diagrams showing modification examples of the liquid cooling jacket, specifically, inner perspective views, when viewed from a front side of the liquid cooling jacket. In the modification examples, needle fins  737  are used, in place of the straight fins  637  shown in  FIGS. 36A and 36B . The modification example shown in  FIG. 38A  is different from the modification example shown in  FIG. 38B  in the arrangement of the needle fins  737 . 
         [0155]    Referring to  FIG. 38A , The liquid cooling jacket  73  is constituted of a jacket portion  731 , an inlet portion  732  and an outlet portion  733  projecting from an upper surface of the jacket portion  731 . 
         [0156]    As well as the liquid cooling jacket  63  described above, the liquid cooling jacket  73  is made of a material having a high thermal conductivity such as aluminum or copper. The liquid cooling jacket  73  is formed by joining a front jacket portion and a back jacket portion at a central part by welding or a like process. 
         [0157]    A flow channel  734  is formed in the interior of the jacket portion  731 . A lower portion of the flow channel  734  is branched out into two sub-channels. One of the two sub-channels is communicated with an entrance  734   a , and the other thereof is communicated with an exit  734   b . An inlet path  735  formed in an inlet portion  732  is communicated with the entrance  734   a , and an outlet path  736  formed in an outlet portion  733  is communicated with the exit  734   b.    
         [0158]    The plural needle fins  737  are disposed in a matrix in the flow channel  734  with a predetermined interval (e.g. 1 mm) in up and down directions and left and right directions. The needle fins  737  project from a front surface of the flow channel  734  in rearward direction. The needle fins  737  are formed in such a manner that the laser light source  50  is disposed in an area where the needle fins  737  are disposed, when viewed from the front side of the liquid cooling jacket  73 . 
         [0159]    A space of a predetermined size devoid of the needle fins  737  is formed between the uppermost array of the needle fins  737  and an upper surface of the flow channel  734 . The space serves as an air bubble stagnating portion  734   c  for stagnating air bubbles generated in the flow channel  734 . Inner surfaces of corner portions of the flow channel  734  are formed into curved surfaces to easily flow the refrigerant, as shown in  FIGS. 38A and 38B . 
         [0160]    The liquid cooling jacket  73  is disposed in a state that a front surface where the laser light source  50  is mounted is aligned with up and down directions of the projector, in other words, a gravitational force direction. The plumbing pipes (not shown) from the radiator  31  of the cooling device  30  shown in  FIGS. 1A and 1B  are connected to the inlet portion  732  and the outlet portion  733  of the liquid cooling jacket  73 . In this arrangement, a refrigerant flows in through the inlet portion  732 , and flows out through the outlet portion  733  via the flow channel  734 . As shown by the blank arrows in  FIGS. 38A and 38B , the flow of the refrigerant in the flow channel  734  is changed from upward direction to downward direction so that the refrigerant substantially passes through a clearance between the respective two needle fins  737  arranged side by side in up and down directions. In this way, the refrigerant is circulated through the flow channel  734  in the liquid cooling jacket  73  and the radiator  31 . As well as the cooling jacket  63 , water or an ethylene-glycol-based liquid medium may be used as the refrigerant. 
         [0161]    The heat transferred to the liquid cooling jacket  73  from the laser light source  50  is heat-exchanged with the refrigerant flowing through the flow channel  734  at the front surface of the flow channel  734  and the needle fins  737  to be transferred to the refrigerant. The heat is then transferred to the radiator  31  by the refrigerant, and removed by the air passing the radiator  31 . 
         [0162]    In the above arrangement, the liquid cooling jacket  73  is disposed in a state that the front surface thereof where the laser light source  50  is mounted is aligned with the gravitational force direction, and the air bubble stagnating portion  734   c  is formed in the upper portion of the flow channel  734 . Accordingly, air bubbles generated in a flow channel  734  are moved to the air bubble stagnating portion  734   c  formed in the upper portion of the flow channel  734 , and stagnate in the air bubble stagnating portion  734   c.    
         [0163]    Accordingly, in this arrangement example, air bubbles are less likely to stagnate on the front surface of the flow channel  734 , or the portion corresponding to the needle fins  737 , where a heat exchange between the heat from the laser light source  50 , and the refrigerant is mainly performed. As a result, since an increase of thermal resistance due to air bubbles is suppressed, a cooling effect of the laser light source  50  can be maintained. 
         [0164]    The arrangement of the needle fins  737  may be modified as shown in  FIG. 38B . In the arrangement example shown in  FIG. 38B , the needle fins  737  are formed in such a manner that arrays of the needle fins  737  adjacent to each other in left and right directions are displaced from each other in up and down directions by one-half pitch. 
         [0165]    Further, in the arrangements shown in  FIGS. 38A and 38B , the inlet portion  732  and the outlet portion  733  of a refrigerant are formed in the lower portion of the liquid cooling jacket  73 . Alternatively, as shown in the arrangement in  FIGS. 36A and 36B , the inlet portion and the outlet portion of a refrigerant may be respectively formed in the lower portion and the upper portion of the liquid cooling jacket. Further alternatively, the straight fins  637  may be replaced by needle fins in the liquid cooling jacket having the arrangement shown in  FIGS. 36A and 36B . 
         [0166]      FIGS. 35A through 38B  show examples, wherein one laser light source  50  is mounted on one liquid cooling jacket. Alternatively, plural laser light sources  50  may be mounted on one liquid cooling jacket. In the modification, a fin structure may be formed individually on a surface in contact with a corresponding one of the laser light sources, or may be formed in such a manner that all the laser light sources are uniformly covered. 
         [0167]    Further, in the arrangements shown in  FIGS. 35A through 38B , the cooling portion  60  is disposed in a state that the surface (front surfaces of the liquid cooling jackets  63  and  73 ) where the laser light source  50  is mounted is aligned with up and down directions of the projector i.e. the gravitational force direction. Alternatively, the laser light source mounting surface may not be strictly aligned in parallel to the gravitational force direction, and may be slightly tilted with respect to the gravitational force direction. Even in a state that the mounting surface is slightly tilted with respect to the gravitational force direction, the air (air bubbles) in the flow channel is retracted in the upper portion of the flow channel by a buoyant force, and is less likely to stagnate near the mounting surface of the laser light source  50 . Accordingly, substantially the same effect as described above can be obtained. The expression “the cooling portion is disposed, with a surface thereof where the light source is mounted being aligned with a gravitational force direction” in the claims embraces the above case, wherein the mounting surface of the laser light source  50  is slightly tilted with respect to the gravitational force direction. 
         [0168]    The embodiments of the invention have been described as above, but the invention is not limited to the foregoing embodiments. Further, the embodiments of the invention may be changed or modified in various ways. 
         [0169]    For instance, in  FIGS. 1A and 1B , and  FIGS. 24A and 24B , illumination light is entered into the optical system  20  in one direction, and in the above combination examples, light of a red wavelength band, a green wavelength band, and a blue wavelength band is combined by a prism mirror for incidence into the optical system  20 . Alternatively, it is possible to apply the invention to an optical system, wherein light of the respective colors is individually entered in three directions into the optical system  20 . As described above, in the case where illumination light is entered into the optical system  20  in one direction, the illumination light is temporarily separated into light of a red wavelength band, a green wavelength band, and a blue wavelength band in the optical system  20 , followed by modulation of the light of the respective colors by imagers, and then, the separated light is combined by a dichroic cube for incidence into the projection lens  21 . Further, in the case where light of the respective colors is entered in three directions, the light of the respective colors is guided to imagers (liquid crystal panels) by corresponding light guiding optical systems for modulation, and then, the modulated light is combined by a dichroic cube for incidence into the projection lens  21 . In the case where light of the respective colors is entered in three directions, the illumination device in each of the above combination examples is individually disposed with respect to the light guiding optical systems of the respective colors. In this case, all the light source units in each of the above combination examples are modified to emit laser light of a same wavelength band. For instance, in the illumination device of the combination example, wherein illumination light is supplied to a light guiding optical system for green light, all the light source units emit laser light of a green wavelength band, and the emitted laser light is combined into illumination light by a prism mirror. 
         [0170]    In the forgoing embodiments, laser light is combined by using a prism mirror. Alternatively, it is possible to use two mirrors or an edge mirror, in place of the prism mirror. The embodiments of the invention may be changed or modified in various ways as necessary, as far as such changes and modifications do not depart from the scope of the claims of the invention hereinafter defined.