Patent Publication Number: US-7717568-B2

Title: Manufacturing method for cooling unit, cooling unit, optical device, and projector

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to a manufacturing method for a cooling unit, a cooling unit, an optical device, and a projector. 
   2. Related Art 
   As a cooling unit using a cooling fluid, there is one including a cooling plate in which a metal pipe serving as a cooling fluid channel is arranged between inner surfaces of a pair of metal plates combined to be opposed to each other. This cooling plate is manufactured by forming a pipe housing groove larger than the metal pipe at least in one of the pair of metal plates and integrally combining the metal pipe and the pair of metal plates. In a manufacturing process of the cooling plate, a pressurized fluid is supplied into the metal pipe after the combination and the metal pipe is expanded in diameter to cause the metal pipe to come into close contact with the pipe housing groove (see, for example, JP-A-2002-156195). 
   In the manufacturing method for a cooling unit, the pipe housing groove is formed in a reverse taper shape with respect to a mating surface of the metal plate and the metal plate and the metal pipe are combined by causing an edge portion (an undercut portion) of the groove to cut into the metal pipe at the time of the expansion of the diameter of the metal pipe. 
   However, in the manufacturing method, cutting needs to be performed using a special cutting tool for formation of the undercut portion. Thus, it is difficult to realize a reduction in cost. 
   In order to satisfactorily bring the metal pipe into close contact with the pipe housing groove, it is necessary to repeat the processing for expanding the diameter of the metal pipe plural times. This requires a great deal of time. 
   Moreover, when the metal pipe has a small diameter, it is difficult to expand the diameter of the metal pipe and an amount of deformation of the metal pipe tends to fluctuate. Thus, a gap is formed between the metal pipe and the pipe housing groove. As a result, deterioration in cooling performance of the cooling plate tends to be causes. 
   SUMMARY 
   An advantage of some aspects of the invention is to provide a manufacturing method for a cooling unit, a cooling unit, an optical device, and a projector that are suitable for a reduction in cost and a reduction in size. 
   A manufacturing method according to a first aspect of the invention is a method of manufacturing a cooling unit that includes a cooling plate in which a cooling fluid flows. The cooling plate has a cooling pipe through which the cooling fluid flows, and a pair of tabular members arranged to be opposed to each other across the cooling pipe. The manufacturing method includes: forming a groove in which the cooling pipe is housed at least in one opposed surface of the pair of tabular members; combining the pair of tabular members while housing the cooling pipe in the groove; and filling a heat conduction material in a gap between the groove and the cooling pipe. 
   In a cooling unit manufactured by the manufacturing method according to the first aspect of the invention, the tabular members and the cooling pipe are thermally connected directly in a portion where the groove of the tabular members and the cooling pipe are in contact with each other. The tabular members and the cooling pipe are thermally connected indirectly via the heat conduction material in a portion where the gap is formed. 
   In other words, in the manufacturing method according to the first aspect of the invention, it is possible to thermally connect the tabular members and the cooling pipe without expanding a diameter of the cooling pipe. Since a process for expanding the diameter of the cooling pipe is made unnecessary, it is possible to significantly reduce a manufacturing time. The manufacturing method according to the first aspect of the invention is preferably applied to a small-diameter cooling pipe as well. Therefore, the manufacturing method according to the first aspect of the invention is preferably applied to a reduction in cost and a reduction in size. 
   In the cooling unit manufactured by the manufacturing method according to the first aspect of the invention, since the groove of the tabular members and the cooling pipe are thermally connected, heat of an object to be cooled, which comes into contact with the tabular members, is removed by the cooling fluid flowing through the cooling pipe. In the structure in which the cooling pipe is disposed in the cooling plate, a risk of fluid leakage is low because only a relatively small joining portion is required for forming a channel for the cooling fluid. Further, a piping resistance is low because the channel, which is uniform and smooth in a flowing direction of the fluid, is formed. 
   A thermal conductivity of the heat conduction material is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W/(m·K). The thermal conductivity of the heat conduction material lower than 3 W/(m·K) is not preferable because heat of the tabular members less easily moves to the cooling pipe. When the thermal conductivity of the heat conduction material is equal to or higher than 5 W/(m·K), heat of the tabular members satisfactorily moves to the cooling pipe. 
   It is possible that, for example, in the manufacturing method according to the first aspect of the invention the heat conduction material includes at least one of a resin material mixed with a metal material, a resin material mixed with a carbon material, and hot-melt adhesive. 
   In this case, it is preferable that the heat conduction material has elasticity in an operating temperature range of the cooling plate. 
   Since the heat conduction material has elasticity, the heat conduction material expands and contracts according to a change of the gap between the tabular members and the cooling pipe involved in thermal deformation or the like. Thus, thermal connection between the tabular members and the cooling pipe are stably maintained. 
   It is possible that, in forming the groove, the groove is formed using a casting method or a forging method. In the casting method or the forging method, a reduction in cost is easily realized through mass production compared with the formation of the groove using the cutting. 
   It is possible that, in forming the groove, a supplementary groove, in which the heat conduction material is at least temporarily stored, is further formed in the inner surface of the groove and/or in at least the one opposed surface of the pair of tabular members. 
   With the supplementary groove, an amount of arrangement of the heat conduction material is appropriately adjusted according to a capacity of the gap between the tabular members and the cooling pipe and the thermal connection between the tabular members and the cooling pipe is stably maintained. 
   It is possible that, in filling the heat conduction material, the heat conduction material is softened and fluidized to be filled. 
   In this case, for example, the heat conduction material is softened by the heating by an object that holds the pair of tabular members and/or the flow of a high-temperature fluid in the cooling pipe. 
   Since the heat conduction material is softened and fluidized, the heat conduction material is filled in the entire area of the gap. 
   It is possible that, in combining the pair of tabular members, at least one of fastening by screws or the like, bonding, welding, and mechanical combination such as fitting is used. 
   It is possible to combine the pair of tabular members with each other by using such methods. 
   It is possible that at least a part of a combining force of the pair of tabular members is obtained by an adhesive force of the heat conduction material. 
   A manufacturing method according to a second aspect of the invention is a method of manufacturing a cooling unit including a cooling plate in which a cooling fluid flows. The cooling plate has a cooling pipe through which the cooling fluid flows, and a pair of tabular members arranged to be opposed to each other across the cooling pipe. The manufacturing method according to the second aspect of the invention includes forming a second tabular member around the cooling pipe according to molding using a material having a low melting point compared with that of the cooling pipe in a state in which the cooling pipe is arranged on a first tabular member of the pair of tabular members. 
   In the manufacturing method according to the second aspect of the invention, the second tabular member is formed around the cooling pipe according to molding. Consequently, the second tabular member and the cooling pipe are brought into close contact with each other and thermally connected to each other. Since the second tabular member is formed according to an external shape of the cooling pipe, the tabular members and the cooling pipe satisfactorily come into contact with each other and a heat transfer property between the second tabular member and the cooling pipe is improved. Thus, the manufacturing method according to the second aspect of the invention is preferably applied to a small-diameter cooling pipe as well. 
   Therefore, the manufacturing method according to the second aspect of the invention is preferably applied to a reduction in cost and a reduction in size. 
   In this case, for example, it is possible to thermally connect the respective tabular members and the cooling pipe by combining the first tabular member and the second tabular member following the molding of the second tabular member. 
   In the cooling unit manufactured by the manufacturing method according to the second aspect of the invention, as in the manufacturing method according to the first aspect of the invention, the tabular members and the cooling pipe are thermally connected and heat of an object to be cooled that comes into contact with the tabular members is removed by the cooling fluid flowing through the cooling pipe. In the structure in which the cooling pipe is disposed in the cooling plate, a risk of fluid leakage is low because only a relatively small joining portion is required for forming a channel for the cooling fluid. Further, a piping resistance is low because the channel, which is uniform and smooth in a flowing direction of the fluid, is formed. 
   It is preferable that, for example, in the manufacturing method according to the second aspect of the invention the first tabular member is formed of a metal material or a resin material and the second tabular member is formed of a resin material. 
   It is possible that, for example, the resin material includes at least one of a resin material mixed with a metal material and a resin material mixed with a carbon material. 
   In this case, it is preferable that a coefficient of thermal expansion of the cooling pipe and a coefficient of thermal expansion of each of the pair of the tabular members are substantially the same. 
   Consequently, since at least one of the tabular members is formed of a resin material having a high thermal conductivity, a reduction in weight of the cooling unit is realized. Since a coefficient of thermal expansion of the cooling pipe and a coefficient of thermal expansion of each of the tabular members are substantially the same, at the time of hardening and contraction or after molding, a gap due to a difference of an amount of thermal deformation is prevented from being formed between the respective tabular members and the cooling pipe. Thermal connection between the respective tabular members and the cooling pipe is stably maintained. 
   It is possible that the manufacturing method according to the second aspect of the invention further includes filling a heat conduction material in a gap between the cooling pipe and at least one of the pair of tabular members. 
   Consequently, a heat transfer property between the tabular members and the cooling pipe is improved by filling the heat conduction material. 
   Thermal conductivity of the heat conduction material is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W/(m·K). The thermal conductivity of the heat conduction material lower than 3 W/(m·K) is not preferable because heat of the tabular members less easily moves to the cooling pipe. When the thermal conductivity of the heat conduction material is equal to or higher than 5 W/(m·K), heat of the tabular members satisfactorily moves to the cooling pipe. 
   In this case, it is preferable that, for example, the heat conduction material includes at least one of a resin material mixed with a metal material, a resin material mixed with a carbon material, and hot-melt adhesive. 
   It is preferable that the heat conduction material has elasticity in an operating temperature range of the cooling plate. 
   Since the heat conduction material has elasticity, the heat conduction material expands and contracts according to a change of the gap between the tabular members and the cooling pipe involved in thermal deformation or the like. Thus, thermal connection between the tabular members and the cooling pipe are stably maintained. 
   It is preferable that a supplementary groove, which communicates with the gap and in which the heat conduction material is at least temporarily housed, is formed in the first tabular member. 
   With the supplementary groove, an amount of arrangement of the heat conduction material is appropriately adjusted according to a capacity of the gap between the first tabular member and the cooling pipe and the thermal connection between the first tabular member and the cooling pipe is stably maintained. 
   It is possible that the heat conduction material is softened and fluidized to be filled. 
   In this case, for example, the heat conduction material is softened by the heat at the time of molding of the second tabular member and/or the flow of a high-temperature fluid in the cooling pipe. 
   Since the heat conduction material is softened and fluidized, the heat conduction material is filled in the entire area of the gap. 
   A manufacturing method according to a third aspect of the invention is a method of manufacturing a cooling unit including a cooling plate in which a cooling fluid flows. The cooling plate has a cooling pipe through which the cooling fluid flows, and a tabular member inside which the cooling pipe is arranged. The manufacturing method according to the third aspect of the invention includes forming the tabular member around the cooling pipe according to molding using a material having a low melting point compared with that of the cooling pipe. 
   In the manufacturing method according to the third aspect of the invention, the tabular member is formed around the cooling pipe according to molding. Consequently, the tabular member and the cooling pipe are brought into close contact with each other and thermally connected to each other. Since the tabular member is formed according to an external shape of the cooling pipe, the tabular member and the cooling pipe satisfactorily come into contact with each other and a heat transfer property between the tabular member and the cooling pipe is improved. Thus, the manufacturing method according to the third aspect of the invention is preferably applied to a small-diameter cooling pipe as well. 
   Therefore, the manufacturing method according to the third aspect of the invention is preferably applied to a reduction in cost and a reduction in size. 
   In the cooling unit manufactured by the manufacturing method according to the third aspect of the invention, as in the manufacturing method according to the first aspect of the invention, the tabular member and the cooling pipe are thermally connected and heat of an object to be cooled that comes into contact with the tabular member is removed by the cooling fluid flowing through the cooling pipe. In the structure in which the cooling pipe is disposed in the cooling plate, a risk of fluid leakage is low because only a relatively small joining portion is required for forming a channel for the cooling fluid. Further, a piping resistance is low because the channel, which is uniform and smooth in a flowing direction of the fluid, is formed. 
   It is preferable that, for example, both the cooling pipe and the tabular member are formed of a metal material. 
   In this case, it is preferable that a coefficient of thermal expansion of the tabular member is high compared with that of the cooling pipe. 
   For example, it is possible that the cooling pipe is formed of a copper alloy and the tabular member is formed of an aluminum alloy or a magnesium alloy. 
   Since a coefficient of thermal expansion of the tabular member is large compared with that of the cooling pipe, an amount of contraction of the tabular member is large compared with that of the cooling pipe at the time of hardening and contraction of the tabular member. Thus, a gap is prevented from being formed between the tabular member and the cooling pipe and thermal connection between the tabular member and the cooling pipe is stably maintained. 
   It is preferable that, for example, in the manufacturing method according to the third aspect of the invention the cooling pipe is formed of a metal material and the tabular member is formed of a resin material having a high thermal conductivity. 
   In this case, it is preferable that a coefficient of thermal expansion of the cooling pipe and a coefficient of thermal expansion of the tabular member are substantially the same. 
   It is possible that, for example, the resin material includes at least one of a resin material mixed with a metal material and a resin material mixed with a carbon material. 
   Since the tabular member is formed of a resin material having a high thermal conductivity, a reduction in weight of the cooling unit is realized. Further, since a coefficient of thermal expansion of the cooling pipe and a coefficient of thermal expansion of the tabular member are substantially the same, a gap is prevented from being formed between the tabular member and the cooling pipe after molding. Thermal connection between the tabular member and the cooling pipe is stably maintained. 
   A cooling unit according to a fourth aspect of the invention is manufactured by the manufacturing method for a cooling unit according to any one of the first to the third aspects of the invention. 
   According to the cooling unit, a reduction in cost and a reduction in weight are realized. 
   An optical device according to a fifth aspect of the invention is an optical device including optical modulators that modulate light beams emitted from a light source according to image information to form an optical image. At least the optical modulators are mounted on a cooling unit that is manufactured by the manufacturing method according to any one of the first to the third aspects of the invention. 
   According to the optical device, a reduction in cost, a reduction in size, and efficiency of cooling are realized. 
   A projector according to a sixth aspect of the invention includes: a light source device; an optical device in which at least optical modulators that modulate light beams emitted from the light source device according to image information to form an optical image are mounted on a cooling unit manufactured by the manufacturing method according to any one of the first to the third aspect of the invention; and a projection optical device that magnifies and projects the optical image formed by the optical device. 
   According to the projector, a reduction in cost, a reduction in size, and efficiency of cooling are realized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
       FIG. 1A  is a plan view showing a constitution of a cooling unit. 
       FIG. 1B  is a sectional view along line A-A in  FIG. 1A . 
       FIG. 2  is a partial sectional view showing grooves of tabular members in an enlarged state. 
       FIGS. 3   a  and  3 B are diagrams for explaining an example of a manufacturing method for the cooling unit. 
       FIG. 4  is a diagram showing an example of a state at the time when the tabular members are combined. 
       FIG. 5  is a diagram showing a state of combination of the tabular members using screws. 
       FIGS. 6A and 6B  are diagrams for explaining a modification of the manufacturing method for the cooling unit. 
       FIG. 7  is a diagram showing an example of another form of supplementary grooves. 
       FIG. 8  is a diagram showing an example of still another form of the supplementary grooves. 
       FIG. 9  is a diagram showing an example in which the supplementary grooves are formed in a cooling pipe. 
       FIG. 10  is a diagram showing an example in which the supplementary grooves are formed in the cooling pipe. 
       FIG. 11  is a diagram showing an example in which the supplementary grooves are formed in the cooling pipe. 
       FIG. 12  is a sectional view showing a second cooling unit. 
       FIGS. 13A and 13B  are diagrams for explaining a manufacturing method for the second cooling unit. 
       FIG. 14  is a sectional view showing a modification of the second cooling unit. 
       FIG. 15  is a sectional view showing a modification of the second cooling unit. 
       FIG. 16  is a sectional view showing a third cooling unit. 
       FIG. 17  is a diagram for explaining a manufacturing method for the third cooling unit. 
       FIG. 18  is a diagram schematically showing a constitution of a projector. 
       FIG. 19  is a perspective view of a part inside the projector view from an upper side thereof. 
       FIG. 20  is a perspective view of an optical device and a liquid cooling unit inside the projector viewed from a lower side thereof. 
       FIG. 21  is a perspective view showing an overall constitution of the optical device. 
       FIG. 22  is a perspective view showing an entire constitution of a branching tank. 
       FIG. 23  is a perspective view showing an overall constitution of a merging tank. 
       FIG. 24  is a partial perspective view showing a panel constitution for red light in the optical device. 
       FIG. 25  is a disassembled perspective view of a liquid crystal panel holding frame. 
       FIG. 26A  is an assembled front view of the liquid crystal panel holding frame. 
       FIG. 26B  is a sectional view along line A-A in  FIG. 26A . 
       FIG. 27A  is an assembled front view of an incidence side sheet polarizer holding frame. 
       FIG. 27B  is a sectional view along line B-B in  FIG. 27A . 
       FIG. 28A  is an assembled front view of an emission side sheet polarizer holding frame. 
       FIG. 28B  is a sectional view along line C-C in  FIG. 28A . 
       FIG. 29  is a piping system diagram showing a flow of a cooling fluid in the optical device. 
       FIG. 30  is a diagram showing a modification of the piping system. 
       FIG. 31  is a diagram showing another modification of the piping system. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   First embodiment 
   A first embodiment of the invention will be hereinafter explained with reference to the accompanying drawings. In the respective figures, dimensions of components are made different from actual dimensions as required in order to set sizes of the components to be recognizable on the drawings. 
   First Cooling Unit 
     FIG. 1A  is a plan view showing a constitution of a cooling unit  10 .  FIG. 1B  is a sectional view along line A-A shown in  FIG. 1A . 
   As shown in  FIGS. 1A and 1B , the cooling unit  10  is a unit that holds a peripheral edge of a transmissive optical element  11  and cools the optical element  11 . The cooling unit  10  includes a pair of tabular members  12  and  13  that holds the optical element  11  and a cooling pipe  14  that is sandwiched by the pair of tabular members  12  and  13 . 
   As the optical element  11 , other than a liquid crystal panel and a sheet polarizer, various optical elements such as a phase plate and a viewing angle compensating plate are adoptable. The invention is applicable not only to the transmissive optical element but also to a reflective optical element. Moreover, the invention is applicable to cooling of not only the optical element but also of other objects. An example in which a cooling plate of the invention is applied to a cooling structure for a liquid crystal panel and a sheet polarizer will be explained in detail later. 
   The tabular members  12  and  13  are frames of a rectangular shape in a plan view. The tabular members  12  and  13  have rectangular openings  121  and  131  corresponding to transmission areas for light beams in the optical element  11  and grooves  122  and  132  for housing the cooling pipe  14 , respectively. The tabular members  12  and  13  are arranged to be opposed to each other across the cooling pipe  14 . A thermal good conductor made of a material having a high thermal conductivity is used as the tabular members  12  and  13 . For example, various kinds of metals are adopted other than aluminum, (234 W/(m·K)), magnesium (156 W/(m·K)), and alloys of aluminum and magnesium (an aluminum alloy (about 100 W/(m·K)), a low specific gravity magnesium alloy (about 50 W/(m·K)), etc.). The tabular members  12  and  13  are not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   The cooling pipe  14  is made of, for example, a pipe or a tube that has an annular section and extends along a center axis of the section. The cooling pipe  14  is bent according to a geometry of the grooves  122  and  132  of the tabular members  12  and  13 . As the cooling pipe  14 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum (234 W/(m·K)), copper (398 W/(m·K)), stainless steel (16 W/(m·K) (austenitic)), and alloys of aluminum, copper, and stainless steel. The cooling pipe  14  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   Specifically, as shown in  FIGS. 1A and 1B , the cooling pipe  14  is disposed on an outer side of the peripheral edge of the optical element  11  and along substantially the entire peripheral edge of the optical element  11 . In other words, on respective opposed surfaces  123  and  133  (inner surfaces or mating surfaces) of the tabular members  12  and  13 , the grooves  122  and  132  of a substantially semicircular shape in section are formed along the entire edges of the openings  121  and  131 . The groove  122  and the groove  132  are in a substantially mirror symmetrical shape relation with each other. The tabular members  12  and  13  are joined with each other in a state in which the cooling pipe  14  is housed in the grooves  122  and  132 . In this example, the cooling pipe  14  is a circular pipe and an outer diameter thereof is substantially the same as thickness of the optical element  11 . 
     FIG. 2  is a partial sectional view showing the grooves  122  and  132  of the tabular members  12  and  13  in an enlarged state. As shown in  FIG. 2 , the grooves  122  and  132  in the respective tabular members  12  and  13  and the cooling pipe  14  have external shape portions (semicircular sectional shapes) of substantially the same shapes such that the grooves  122  and  132  and the cooling pipe  14  are combined with each other. Diameters of the grooves  122  and  132  are formed to be substantially the same as or slightly larger than the external shapes of the cooling pipe  14 . For example, an inner diameter dimension of the grooves  122  and  132  are formed with a positive tolerance with respect to an outer diameter dimension of the cooling pipe  14 . A heat conduction material  140  is filled in a gap between the grooves  122  and  132  and the cooling pipe  14  formed at the time of combination or the like. 
   As the heat conduction material  140 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. Specifically, for example, a resin material mixed with a metal material, a resin material mixed with a carbon material, hot-melt adhesive, or the like is used. A thermal conductivity of the heat conduction material  140  is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W/(m·K). A thermal conductivity of hot-melt adhesive is usually equal to or higher than 5 W/(m·K)). As the resin material mixed with a metal material or a carbon material, there is a resin material having a thermal conductivity equal to or higher than 3 W/(m·K) and a resin material having a thermal conductivity equal to or higher than 10 W/(m·K). As an example, there are D2 (registered trademark) (an LCP resin mixed with a material for heat transfer), 15 W/(m·K), coefficient of thermal expansion: 10×10^−6/k) and RS007 (registered trademark) (a PPS resin mixed with a material for heat transfer, 3.5 W/(m·K), coefficient of thermal expansion: 20×10^−6/K) manufactured by Cool Polymers, Inc. 
   The tabular member  12  and the tabular member  13  are combined using at least one of fastening by screws or the like, bonding, welding, and mechanical combination such as fitting. A simple combining method is used for a reduction in cost and a reduction in size. At least a part of a combining force of the tabular member  12  and the tabular member  13  may be obtained by an adhesive force of the heat conduction material  140 . 
   Referring back to  FIGS. 1A and 1B , an inflow section (IN) for a cooling fluid is disposed at one end of the cooling pipe  14  and an outflow section (OUT) is disposed at the other end thereof. The inflow section and the outflow section of the cooling pipe  14  are connected to piping for circulation of the cooling fluid, respectively. On a path of the cooling fluid, devices for fluid circulation such as a fluid pumping unit, various tanks, and a radiator, which are not shown in the figure, are arranged. 
   The cooling fluid flowing into the cooling pipe  14  from the inflow section (IN) flows along substantially the entire peripheral edge of the optical element  11  and flows out from the outflow section (OUT). The cooling fluid deprives the optical element  11  of heat while flowing through the cooling pipe  14 . In other words, the heat of the optical element  11  is transmitted to the cooling fluid in the cooling pipe  14  via the tabular members  12  and  13  and carried to the outside. 
   In this example, the respective tabular members  12  and  13  and the cooling pipe  14  are thermally connected directly in a part where the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14  are in direct contact with each other. The tabular members  12  and  13  and the cooling pipe  14  are thermally connected indirectly via the heat conduction material  140  in a part where a gap is formed. In other words, heat transfer between the tabular members  12  and  13  and the cooling pipe  14  is supplemented by the heat conduction material  140  to realize improvement of a heat transfer property between the tabular members  12  and  13  and the cooling pipe  14 . Since the cooling pipe  14  is disposed along substantially the entire peripheral edge of the optical element  11 , enlargement of a heat transfer area is realized. Therefore, the optical element  11  is effectively cooled by the cooling fluid flowing through the cooling pipe  14 . 
   In the structure in which the cooling pipe  14  is disposed inside the frame members (the tabular members  12  and  13 ) holding the optical element  11 , a risk of fluid leakage is low because only a relatively small joining portion is required for forming a channel for the cooling fluid. Further, a piping resistance is low because the channel, which is uniform and smooth in a flowing direction of the fluid, is formed. In particular, in this example, turbulence of a flow is small because the sectional shape of the cooling pipe  14  is kept to be a substantially circular shape. Moreover, in this structure, the frame members function as both holding means and cooling means for the optical element  11 . As a result, there is an advantage that a reduction in size of an apparatus including the optical device  11  is easily realized. 
   Manufacturing method for a first cooling unit 
   A manufacturing method for the cooling unit  10  will be explained.  FIGS. 3A and 3B  are diagrams for explaining an example of the manufacturing method for the cooling unit  10 . The manufacturing method includes a groove forming step, a combining step, and a filling step. In this example, the filling step is included in the combining step. 
   First, in the groove forming step, as shown in  FIG. 3A , the grooves  122  and  132  of a substantially semicircular shape or a substantially U shape in section for housing a cooling pipe are formed in the respective opposed surfaces  123  and  133  of the pair of tabular members  12  and  13 . In this step, the tabular member  12  ( 13 ) including the groove  122  ( 132 ) is integrally formed using a casting method (a die cast method, etc.) or a forging method (cold/hot forging, etc.). In the casting method, for example, a melted material is poured into a die of a predetermined shape and coagulated to obtain a tabular member of a desired shape. In the forging method, for example, a material member is sandwiched between a pair of dies and compressed to obtain a tabular member of a desired shape. It is possible to form the tabular members  12  and  13  of such a shape easily and at low cost by using the casting method (the die cast method, etc.) or the forging method (the cold/hot forming, etc.). The groove forming step is preferable applied to a small object as well. Since a shape of the tabular members  12  and  13  are simple, it is possible to form the tabular members  12  and  13  easily and at low cost even if cutting is used. 
   Subsequently, in the combining step (the filling step), as shown in  FIG. 3B , the tabular member  12  and the tabular member  13  are arranged to be opposed to each other and the cooling pipe  14  is housed in the respective grooves  122  and  132 . In this case, as shown in  FIG. 4 , a recess  157  and a projection  158  for positioning may be provided in the tabular members  12  and  13  and combined to decide two-dimensional relative positions of the tabular member  12  and the tabular member  13 . Prior to the housing, the heat conduction material  140  is applied to inner surfaces of the grooves  122  and  132  and/or an outer surface of the cooling pipe  14 . It is possible to use various methods such as a spin coat method, a spray coat method, a roll coat method, a die coat method, a dip coat method, and a droplet jetting method for the application of the heat conduction material  140 . 
   After the application of the heat conduction material  140 , as shown in  FIG. 3B , an external force is applied to bring the opposed surface  123  of the tabular member  12  and the opposed surface  133  of the tabular member  13  into close contact with each other in a state in which the cooling pipe  14  is housed in the respective grooves  122  and  132 . Consequently, the heat conduction material  140  is filled in the gap between the grooves  122  and  132  of the respective tabular members  12  and  13  and the cooling pipe  14 . Thereafter, the tabular member  12  and the tabular member  13  are combined. It is possible to perform the combination using at least one of fastening by screws  159  shown in  FIG. 5 , bonding, welding, and mechanical combination such as fitting. When an adhesive force of the heat conduction material  140  is sufficiently large, it is also possible to omit the combination by the method other than bonding. 
   At the time of the combination, the heat conduction material  140  is softened and fluidized as required. For example, when the heat conduction material  140  is thermoplastic, the heat conduction material  140  is heated at the time of the combination. In this case, for example, the tabular members  12  and  13  are heated via an object (a jig) holding the tabular members  12  and  13  or a high-temperature fluid is caused to flow through the cooling pipe  14 . Since the heat conduction material  140  is softened and fluidized at the time of combination of the tabular members  12  and  13 , the heat conduction material  140  is filled in the entire area of the gap between the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14 . 
   Through the steps described above, a cooling structure (a cooling plate) having a structure in which the pair of tabular members  12  and  13  are arranged to be opposed to each other across the cooling pipe  14  is manufactured. 
   Thereafter, as shown in  FIGS. 1A and 1B , the cooling unit  10  is completed by fixing the optical element  11  to the tabular members  12  and  13  and connecting the cooling pipe  14  to the supply system of the cooling fluid. 
   As explained above, in the manufacturing method for the cooling unit  10  in this example, since the heat conduction material  140  is used, it is possible to thermally connect the respective tabular members  12  and  13  and the cooling pipe  14  without expanding the diameter of the cooling pipe  14 . Since the step of expanding the diameter of the cooling pipe  14  is made unnecessary, it is possible to significantly reduce the manufacturing time and preferably apply the manufacturing method to the small-diameter cooling pipe  14  as well. Therefor, according to this manufacturing method, it is possible to realize a reduction in cost and a reduction in size of the cooling unit  10  to be manufactured. 
   The heat conduction material  140  may be filled (injected) in the gap between the respective grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14  after combining the pair of tabular members  12  and  13  with each other. 
   It is preferable that the heat conduction material  140  has elasticity in an operating temperature range of the cooling plate (the tabular members  12  and  13 ). Since the heat conduction material  140  has elasticity, the heat conduction material  140  expands and contracts according to a change of the gap between the tabular members  12  and  13  and the cooling pipe  14  involved in thermal deformation or the like. Thus, thermal connection between the tabular members  12  and  13  and the cooling pipe  14  are stably maintained. 
     FIGS. 6A and 6B  are diagrams for explaining a modification of the manufacturing method shown in  FIGS. 3A and 3B . Components having functions identical with those already explained are denoted by the identical reference numerals. Explanations of the components are omitted or simplified. 
   In an example in  FIGS. 6A and 6B , plural supplementary grooves  160  in which the heat conduction material  140  is at least temporarily housed are formed in the opposed surface  133  of the tabular member  13 . 
   In the groove forming step, the groove  122  for housing the cooling pipe  14  is formed in the opposed surface  123  of one tabular member  12  and the groove  132  for housing the cooling pipe  14  and the supplementary grooves  160  provided adjacently to the groove  132  are formed in the opposed surface  133  of the other tabular member  13  ( FIG. 6A ). The supplementary grooves  160  are formed substantially parallel to the groove  132  on both the outer sides of the groove  132  in the opposed surface  133  of the tabular member  13 . The plural supplementary grooves  160  are disposed to be apart from one another. A shape and the number of the supplementary grooves  160  are appropriately set according to a material characteristic and the like of the heat conduction material  140 . It is possible to form even the tabular member  13  of such a shape easily and at low cost by using the casting method (the die cast method, etc.) or the forging method (the cold/hot forging, etc.). The same supplementary grooves may be provided in the opposed surface  123  of the tabular member  12 . 
   In the combining step (the filling step), prior to the housing of the cooling pipe  14  in the grooves  122  and  132 , the heat conduction material  140  is applied on the inner surfaces of the grooves  122  and  132  and/or the outer surfaces of the cooling pipe  14 . After the application of the heat conduction material  140 , an external force is applied to bring the opposed surface  123  of the tabular member  12  and the opposed surface  133  of the tabular member  13  into close contact with each other in a state in which the cooling pipe  14  is housed in the respective grooves  122  and  132 . Consequently, the heat conduction material  140  is filled in the gap between the grooves  122  and  132  of the respective tabular members  12  and  13  and the cooling pipe  14  ( FIG. 6B ). In this case, the heat conduction material  140  is softened and fluidized by heating or the like as required. An excess of the heat conduction material  140  flows to the supplementary grooves  160  and stored therein. Thereafter, the tabular member  12  and the tabular member  13  are combined. 
   In this example, since the supplementary grooves  160  are formed in the opposed surface  133  of the tabular member  13 , the excess of the heat conduction material  140  is stored in the supplementary grooves  160 . Since places to which the heat conduction material  140  escapes are provided, the heat conduction material  140  tends to uniformly spread. Thus, the heat conduction material  140  is more surely arranged in the entire area of the gap between the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14 . The heat conduction material  140  arranged in the supplementary grooves  160  (or the gap between the opposed surfaces  123  and  133 ) has a function of improving thermal connectivity between the tabular member  12  and the tabular member  13 . 
   When the heat conduction material  140  has an adhesive force, since an arrangement area of the heat conduction material  140  is expanded, a bonding area between the tabular member  12  and the tabular member  13  is expanded to improve the combining force between the tabular member  12  and the tabular member  13  by the heat conduction material  140 . As a result, it is possible to omit the combination by the other methods such as fastening by screws or the like. 
   The heat conduction material  140  may have fluidity in the operating temperature range of the cooling plate (tabular members  12  and  13 ). In this case, when a capacity of the gap between the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14  changes following thermal deformation or the like, the heat conduction material  140  appropriately moves between the gap and the supplementary grooves  160 . Thus, a filling state of the heat conduction material  140  in the gap is kept and the thermal connection between the tabular members  12  and  13  and the cooling pipe  14  is stably maintained. In this case, it is preferable that measures for preventing the heat conduction material  140  from leaking out to the outside are taken. For example, it is also possible that a heat conduction material other than an anaerobic type material is used and the heat conduction material is caused to harden in a part in contact with the external air and hold fluidity in the inside thereof. Alternatively, it is also possible that a heat conduction agent having fluidity in the operating temperature range is arranged on the inner side and another heat conduction material to be hardened is arranged on the outer side. 
     FIGS. 7 and 8  show examples of other forms of the supplementary grooves  160 . 
   In an example in  FIG. 7 , the supplementary grooves  160  are formed in the inner surfaces of the respective grooves  122  and  132  of the tabular members  12  and  13  to extend in an axial direction of the grooves  122  and  132 . The plural supplementary grooves  160  are disposed to be apart from one another in the circumferential direction of the grooves  122  and  132 . 
   In an example in  FIG. 8 , the supplementary grooves  160  are formed in the inner surfaces of the respective grooves  122  and  132  of the tabular members  12  and  13  to extend in the circumferential direction of the grooves  122  and  123 . The plural supplementary grooves  160  are disposed to be apart from one another in the axial direction of the grooves  122  and  132 . In  FIG. 8 , the supplementary grooves  160  may be formed such that depth thereof gradually decreases from the bottom of the groove  122  ( 132 ) toward the top thereof. 
   It is possible to form even the tabular members  12  and  13  of such a shape easily and at low cost by using the casting method (the die cast method, etc.) or the forging method (the cold/hot forging, etc.). 
   In the examples in  FIGS. 7 and 8 , the supplementary grooves  160  are formed in the inner surfaces of the respective grooves  122  and  132  of the tabular members  12  and  13 . Thus, the excess of the heat conduction material  140  easily moves to the supplementary grooves  160  when the heat conduction material  140  is filled. As a result, the heat conduction material  140  tends to uniformly spread and the heat conduction material  140  is more surely arranged in the entire area of the gap between the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14 . 
   The supplementary grooves  160  may be provided in both the grooves  122  and  132  of the tabular members  12  and  13  and the opposed surfaces  123  and  133 . 
     FIGS. 9 ,  10 , and  11  show examples in which the supplementary grooves  160  are formed in the outer surface of the cooling pipe  14 . 
   In the example in  FIG. 9 , the supplementary grooves  160  are formed in the outer surface of the cooling pipe  14  to extend in the axial direction of the cooling pipe  14 . The plural grooves  160  are disposed to be apart from one another in the circumferential direction of the cooling pipe  14 . 
   In the example in  FIG. 10 , the supplementary grooves  160  are formed in the outer surface of the cooling pipe  14  to extend in the circumferential direction of the cooling pipe  14 . The plural supplementary grooves  160  are disposed to be apart from one another in the axial direction of the cooling pipe  14 . 
   In the example in  FIG. 11 , the supplementary grooves  160  are formed in a spiral shape in the outer surface of the cooling pipe  14 . 
   In the examples in  FIGS. 9 ,  10 , and  11 , since the supplementary grooves  160  are formed in the outer surface of the cooling pipe  14 , the excess of the heat conduction material  140  easily moves to the supplementary grooves  160  when the heat conduction material  140  is filled. As a result, the heat conduction material  140  tends to uniformly spread and the heat conduction material  140  is more surely arranged in the entire area of the gap between the grooves  122  and  132  of the tabular members  12  and  13  and the cooling pipe  14 . 
   Second Embodiment 
   A second embodiment of the invention will be explained with reference to the drawings. In the respective figures, dimensions of components are made different from actual dimensions as required in order to set sizes of the components to be recognizable on the drawings. Components having functions identical with those already explained are denoted by the identical reference numerals. Explanations of the components are omitted or simplified. 
   Second Cooling Unit 
     FIG. 12  is a sectional view showing a cooling unit  105  in this embodiment. The cooling unit  105  is a unit that holds the peripheral edge of the optical element  11  and cools the optical elements  11  in the same manner as the cooling unit  10  in  FIGS. 1A and 1B . The cooling unit  105  includes the pair of tabular members  12  and  13  that hold the optical element  11  and the cooling pipe  14  that is sandwiched by the pair of tabular members  12  and  13 . 
   Unlike the cooling unit  10  in  FIGS. 1A and 1B , in the cooling unit  105  in this embodiment, one tabular member  12  is formed by insert molding. 
   A thermal good conductor made of a material having a high thermal conductivity is used as the tabular member  13  (the first tabular member). For example, various kinds of metals are adopted other than aluminum, (234 W/(m·K)), magnesium (156 W/(m·K)), and alloys of aluminum and magnesium (an aluminum alloy (about 100 W/(m·K)), a low specific gravity magnesium alloy (about 50 W/(m·K)), etc.). The tabular member  13  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   On the other hand, as the tabular member  12  (the second tabular member), a resin material having a low melting point compared with those of the tabular member  13  and the cooling pipe  14  is used. For example, a resin material mixed with a metal material, a resin material mixed with a carbon material, or the like is used. A thermal conductivity of the resin material is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W(m·K). As the resin material mixed with a metal material or a carbon material, there is a resin material having a thermal conductivity equal to or higher than 3 W/(m·K) and a resin material having a thermal conductivity equal to or higher than 10 W/(m·K). As an example, there are D2 (registered trademark) (an LCP resin mixed with a material for heat transfer), 15 W/(m·K), coefficient of thermal expansion: 10×10^−6/k) and RS007 (registered trademark) (a PPS resin mixed with a material for heat transfer, 3.5 W/(m·K), coefficient of thermal expansion: 20×10^−6/K) manufactured by Cool Polymers, Inc. 
   The cooling pipe  14  is made of, for example, a pipe or a tube that has an annular section and extends along a center axis of the section. The cooling pipe  14  is bent according to a geometry of the grooves  122  and  132  of the tabular members  12  and  13 . As the cooling pipe  14 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum (234 W/(m·K)), copper (398 W/(m·K)), stainless steel (16 W/(m·K) (austenitic)), and alloys of aluminum, copper, and stainless steel. 
   As a combination of materials of the tabular member  13  (the first tabular member), the tabular member  12  (the second tabular member), and the cooling pipe  14 , it is preferable that coefficients of thermal expansion of the materials are substantially the same. 
   As an example, there is a combination of the tabular member  13  and the cooling pipe  14  made of copper (coefficient of thermal expansion: 16.6×10^−6/K) or stainless steel (austenitic, coefficient of thermal expansion: 13.6×10^−6/K) and the tabular member  12  made of the resin material having a high thermal conductivity (coefficient of thermal expansion: 10 to 20×10^−6/K). 
   The groove  132  in which the cooling pipe  14  is housed and a through-hole  165  serving as an engaging section are provided in the opposed surface  133  of the tabular member  13 . The through-hole  165  is formed to have, near an opening on an opposite side of the opposed surface  133 , a slope  165   a  of a taper shape, an area of which increases toward the opening. An opening having a step may be provided instead of the opening of the taper shape. It is possible to arbitrarily set a shape and the number of through-holes  165 . At the time of insert molding of the tabular member  12 , a material forming the tabular member  12  is filled in the inside of the through-hole  165  of the tabular member  13 , whereby the tabular member  12  and the tabular member  13  are combined. Consequently, the tabular members  12  and  13  and the cooling pipe  14  are thermally connected to one another. 
   Manufacturing Method for the Second Cooling Unit 
   A manufacturing method for the cooling unit  105  will be explained. 
     FIGS. 13A and 13B  are diagrams for explaining an example of the manufacturing method for the cooling unit  105 . This manufacturing method includes a groove forming step and a combining step. 
   First, in the groove forming step, as shown in  FIG. 13A , the groove  132  of a substantially semicircular shape or a substantially U shape in section for housing the cooling pipe  14  and the through-hole  165  for combination are formed in the opposed surface  133  of the tabular member  13  (the first tabular member). As described above, the through-hole  165  has, near the opening on the opposite side of the opposed surface  133 , the slope  165   a  of a taper shape, an area of which increases toward the opening. In this step, the tabular member  13  including the groove  132  and the through-hole  165  is integrally formed using the casting method (the die cast method, etc.) or the forging method (cold/hot forging, etc.). In the casting method, for example, a melted material is poured into a die of a predetermined shape and coagulated to obtain a tabular member of a desired shape. In the forging method, for example, a material member is sandwiched between a pair of dies and compressed to obtain a tabular member of a desired shape. It is possible to form the tabular member  13  of such a shape easily and at low cost by using the casting method (the die cast method, etc.) or the forging method (the cold/hot forming, etc.). The groove forming step is preferable applied to a small object as well. 
   Subsequently, in the combining step, as shown in  FIG. 13B , the tabular member  12  is formed by insert molding in a state in which the cooling pipe  14  is housed in the groove  132  of the tabular member  13 . The tabular member  13  is fixed to a die  166  in a state in which the cooling pipe  14  is housed in the groove  132  of the tabular member  13 . A melted material is supplied to the inside of the die  166  (e.g., poured to be supplied or injected to be supplied) and coagulated to obtain the tabular member  12  of a desired shape. 
   In this molding step, the tabular member  12  is formed to follow external shapes of the tabular member  13  and the cooling pipe  14 . Consequently, the groove  122  having an external shape portion (a semicircular sectional shape) substantially the same as the shape of the cooling pipe  14  is formed in the opposed surface  123  of the tabular member  12 . Since the material forming the tabular member  12  is filled in the through-hole  165  of the tabular member  13 , the portion comes into an engaged state. As a result, the tabular member  12  is held in a state in which the tabular member  12  is in close contact with the tabular member  13  and the cooling pipe  14 . The tabular members  12  and  13  and the cooling pipe  14  are thermally connected to each other. 
   As a combination of materials of the tabular member  13  (the first tabular member), the tabular member  12  (the second tabular member), and the cooling pipe  14 , materials having substantially the same coefficients of thermal expansion of are used. Consequently, when the tabular member  12  is hardened and contracted or after the tabular member  12  is molded, a gap due to a difference of an amount of thermal deformation is prevented from being formed between the respective tabular members  12  and  13  and the cooling pipe  14 . Thermal connection between the tabular members  12  and  13  and the cooling pipe  14  is stably maintained. 
   As explained above, in this example, the tabular member  12  is formed around the cooling pipe  14  by insert molding. Thus, the tabular member  12  is formed to follow the external shapes of the cooling pipe  14  and the tabular member  13 . The tabular members  12  and  13  and the cooling pipe  14  satisfactorily come into contact with each other. Therefore, improvement of the heat transfer property between the respective tabular members  12  and  13  and the cooling pipe  14  is realized even in the small cooling pipe  14 . Further, since the diameter expanding step is made unnecessary, complicated processing such as cutting using a special cutting tool is unnecessary. In other words, according to this manufacturing method, it is possible to realize a reduction in cost and a reduction in size of the cooling unit  105  to be manufactured. 
   In the cooling unit, since the heat conduction material is filled in the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 , it is possible to realize improvement of a heat transfer property between the tabular member  13  and the cooling pipe  14 . 
   As the heat conduction material, a thermal good conductor made of a material having a high thermal conductivity is preferably used. Specifically, for example, a resin material mixed with a metal material, a resin material mixed with a carbon material, hot-melt adhesive, or the like is used. A thermal conductivity of the heat conduction material is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W/(m·K). A thermal conductivity of hot-melt adhesive is usually equal to or higher than 5 W/(m·K)). As the resin material mixed with a metal material or a carbon material, there is a resin material having a thermal conductivity equal to or higher than 3 W/(m·K) and a resin material having a thermal conductivity equal to or higher than 10 W/(m·K). As an example, there are D2 (registered trademark) (an LCP resin mixed with a material for heat transfer), 15 W/(m·K), coefficient of thermal expansion: 10×10^−6/k), RS007 (registered trademark) (a PPS resin mixed with a material for heat transfer, 3.5 W/(m·K), coefficient of thermal expansion: 20×10^−6/K) manufactured by Cool Polymers, Inc. 
   It is possible to fill the heat conduction material by, for example, applying the heat conduction material on the inner surface of the groove  132  of the tabular member  13  and/or the outer surface of the cooling pipe  14  prior to housing the cooling pipe  14  in the groove  132  of the tabular member  13 . It is possible to use various methods such as a spin coat method, a spray coat method, a roll coat method, a die coat method, a dip coat method, and a droplet jetting method for the application of the heat conduction material. 
   When the cooling pipe  14  is housed in the groove  132  of the tabular member  13  after the application of the heat conduction material, the tabular member  13  and the cooling pipe  14  are thermally connected directly in a part where the groove  132  of the tabular member  13  and the cooling pipe  14  are in contact with each other. The tabular member  13  and the cooling pipe  14  are thermally connected indirectly via the heat conduction material in a part where the gap is formed. In other words, heat transfer between the tabular member  13  and the cooling pipe  14  is supplemented by the heat conduction material to realize improvement of the heat transfer property between the tabular member  13  and the cooling pipe  14 . When the heat conduction material has an adhesive force, it is also possible to use the an adhesive force for a combining force or the like between the tabular member  13  and the cooling pipe  14 . 
   At the time of the combination, it is advisable to soften and fluidize the heat conduction material as required. For example, when the heat conduction material is thermoplastic, the heat conduction material is heated at the time of the combination. In this case, for example, heat at the time of molding of the tabular member  12  is used or a high-temperature fluid is caused to flow through the cooling pipe  14 . Since the heat conduction material is softened and fluidized, the heat conduction material is filled in the entire area of the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . 
   In this case, it is preferable that the heat conduction material has elasticity in an operating temperature range of the cooling plate (the tabular members  12  and  13 ). Since the heat conduction material has elasticity, the heat conduction material expands and contracts according to a change of the gap between the tabular members  12  and  13  and the cooling pipe  14  involved in thermal deformation or the like. Thus, thermal connection between the tabular members  12  and  13  and the cooling pipe  14  are stably maintained. 
     FIGS. 14 and 15  are diagrams for explaining modifications of the cooling unit  105  in  FIG. 12 . Components having functions identical with those already explained are denoted by the identical reference numerals. Explanations of the components are omitted or simplified. 
   As shown in  FIGS. 14 and 15 , in the examples, the heat conduction material  140  is filled in the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . Improvement of the heat transfer property between the tabular member  13  and the cooling pipe  14  is realized by filling the heat conduction material  140 . The supplementary grooves  160  in which the heat conduction material  140  is at least temporarily stored are formed in the inner surface of the groove  132  of the tabular member  13 . 
   In the example in  FIG. 14 , as in the example shown in  FIG. 7 , the plural supplementary grooves  160 , which extend in the axial direction of the groove  132  and are arranged to be apart from one another in the circumferential direction, are formed in the inner surface of the groove  132  of the tabular member  13 . 
   In the example in  FIG. 15 , as in the example shown in  FIG. 8 , the plural supplementary grooves  160 , which extend in the circumferential direction of the groove  132  and are arranged to be apart from one another in the axial direction, are formed in the inner surface of the groove  132  of the tabular member  13 . In the example in  FIG. 15 , the supplementary grooves  160  may have a shape in which depth thereof gradually decreases from the bottom of the groove  132  toward the top thereof. 
   In a manufacturing process for the cooling unit  105  in  FIG. 14  or  15 , in a groove forming step, the groove  132  for housing the cooling pipe  14  in the opposed surface  133  of the tabular member  13  is formed and the supplementary grooves  160  are formed in the inner surface of the groove  132 . A shape and the number of the supplementary grooves  160  are appropriately set according to a material characteristic and the like of the heat conduction material  140 . The tabular member  13  having such a shape is formed easily and at low cost by using the casting method (the die cast method, etc.) or the forging method (the cold/hot forging, etc.). 
   In a combining step, prior to housing the cooling pipe  14  in the groove  132 , the heat conduction material  140  is applied on the inner surface of the groove  132  and/or the outer surface of the cooling pipe  14 . After the application of the heat conduction material  140 , as in the example shown in  FIG. 13B , the tabular member  12  is formed by insert molding in a state in which the cooling pipe  14  is housed in the groove  132 . Consequently, the tabular member  12  and the tabular member  13  are combined and the heat conduction material  140  is filled in the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . In this case, the heat conduction material  140  is softened and fluidized by heating or the like as required. An excess of the heat conduction material  140  flows to the supplementary grooves  160  and are stored therein. When the heat conduction material  140  is thermoplastic, it is advisable to heat the heat conduction material  140  at the time of the combination. For example, heat at the time of molding of the tabular member  12  is used or a high-temperature fluid is caused to flow into the cooling pipe  14 . Since the heat conduction material is softened and fluidized, the heat conduction material is filled in the entire area of the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . 
   In this example, since the supplementary grooves  160  are formed in the inner surface of the groove  132  of the tabular member  13 , the excess of the heat conduction material  140  is stored in the supplementary grooves  160 . Since places to which the heat conduction material  140  escapes are provided, the heat conduction material  140  tends to uniformly spread. Thus, the heat conduction material  140  is more surely arranged in the entire area of the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . The heat conduction material  140  arranged in the supplementary grooves  160  improves thermal connectivity between the cooling pipe  14  and the tabular member  13 . 
   When the heat conduction material  140  has an adhesive force, according to expansion of an arrangement area of the heat conduction material  140 , a bonding area between the cooling pipe  14  and the tabular member  13  increases and the combining force between the cooling pipe  14  and the tabular member  13  by the heat conduction material  140  is improved. 
   The heat conduction material  140  may have fluidity in the operating temperature range of the cooling plate (tabular member  13 ). In this case, when a capacity of the gap between the groove  132  of the tabular member  13  and the cooling pipe  14  changes following thermal deformation or the like of the tabular member  13  and/or the cooling pipe  14 , the heat conduction material  140  appropriately moves between the gap and the supplementary grooves  160 . As a result, a filling state of the heat conduction material  140  in the gap is kept and the thermal connection between the tabular member  13  and the cooling pipe  14  is stably maintained. In this case, it is preferable that measures for preventing the heat conduction material  140  from leaking out to the outside are taken. For example, it is also possible that a heat conduction material other than an anaerobic type material is used and the heat conduction material is caused to harden in a part in contact with the external air and hold fluidity in the inside thereof. Alternatively, it is also possible that a heat conduction agent having fluidity in the operating temperature range is arranged on the inner side and another heat conduction material to be hardened is arranged on the outer side. 
   As another modification of the cooling unit  105  in  FIG. 12 , the supplementary grooves  160  may be provided in the outer surface of the cooling unit  14  as shown in  FIG. 9 ,  10 , or  11 . 
   As shown in  FIG. 9 , the plural supplementary grooves  160 , which are arranged to be apart from one another in the circumferential direction of the cooling pipe  14  and have a shape extending in the axial direction, may be formed in the outer surface of the cooling pipe  14 . 
   Alternatively, as shown in  FIG. 10 , the plural supplementary grooves  160 , which are arranged to be apart from one another in the axial direction of the cooling pipe  14  and have a shape extending in the circumferential direction, may be formed in the outer surface of the cooling pipe  14 . 
   Alternatively, as shown in  FIG. 11 , the supplementary grooves  160  having a spiral shape may be formed in the outer surface of the cooling pipe  14 . 
   Since the supplementary grooves  160  are formed in the outer surface of the cooling pipe  14 , the excess of the heat conduction material  140  easily moves to the supplementary grooves  160  when the heat conduction material  140  is filled. As a result, the heat conduction material  140  tends to uniformly spread and the heat conduction material  140  is more surely arranged in the entire area of the gap between the groove  132  of the tabular member  13  and the cooling pipe  14 . 
   Third Embodiment 
   A third embodiment of the invention will be explained. In the respective figures, dimensions of components are made different from actual dimensions as required in order to set sizes of the components to be recognizable on the drawings. Components having functions identical with those already explained are denoted by the identical reference numerals. Explanations of the components are omitted or simplified. 
   Third Cooling Unit 
     FIG. 16  is a sectional view showing the cooling unit  106  in this embodiment. The cooling unit  106  is a unit that holds the peripheral edge of the optical element  11  and cools the optical element  11  in the same manner as the cooling unit  10  in  FIGS. 1A and 1B . The cooling unit  106  includes the tabular member  12  that holds the optical element  11  and the cooling pipe  14  that is arranged inside the tabular member  12 . 
   Unlike the cooling unit  10  in  FIGS. 1A and 1B , in the cooling unit  106  in this embodiment, one tabular member  12  is formed around the cooling pipe  14  by insert molding. 
   As the tabular member  12 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum (234 W/(m·K)), magnesium (156 W/(m·K)), and alloys of aluminum and magnesium (an aluminum alloy (about 100 W/(m·K)), a low specific gravity magnesium alloy (about 50 W/(m·K)), etc.). The tabular member  12  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   The cooling pipe  14  is made of, for example, a pipe or a tube that has an annular section and extends along a center axis of the section. The cooling pipe  14  is bent according to a geometry of the grooves  122  and  132  of the tabular members  12  and  13 . As the cooling pipe  14 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum (234 W/(m·K)), copper (398 W/(m·K)), stainless steel (16 W/(m·K) (austenitic)), and alloys of aluminum, copper, and stainless steel. 
   As a combination of materials of the tabular member  12  and the cooling pipe  14 , it is preferable that a material of the tabular member  12  has a low melting point and a high coefficient of thermal expansion compared with a material of the cooling pipe  14 . 
   As an example, there is a combination of the tabular member  12  made of an aluminum alloy (melting point: 580° C., coefficient of thermal expansion: 22×10^−6/K) and the cooling pipe  14  made of copper (melting point: 1083° C., coefficient of thermal expansion: 16.6×10^−6/K) or a combination of the tabular member  12  made of a low specific gravity magnesium alloy (melting point: 650° C., coefficient of thermal expansion: 27×10^−6/K) and the cooling pipe  14  made of copper (melting point: 1083° C., coefficient of thermal expansion: 16.6×10^−6/K). 
   Since the tabular member  12  is formed around the cooling pipe  14  by molding, the tabular member  12  and the cooling pipe  14  are thermally connected to each other. 
   Manufacturing Method for the Third Cooling Unit 
   A manufacturing method for the cooling unit  106  will be explained.  FIG. 17  is a diagram for explaining an example of the manufacturing method for the cooling unit  106 . This manufacturing method includes a molding step. 
   As shown in  FIG. 17 , the tabular member  12  is formed around the cooling pipe  14  by insert molding. Specifically, the cooling pipe  14  is fixed to the mold  167 , a melted material is supplied to the inside of the mold  167  (e.g., poured to be supplied or injected to be supplied), and the material is coagulated to obtain the tabular member  12  of a desired shape. 
   In this molding step, the tabular member  12  is formed to follow an external shape of the cooling pipe  14  and a hole  168  having an external shape portion (a circular shape in section) substantially the same as the shape of the cooling pipe  14  is formed inside the tabular member  12 . As a result, the tabular member  12  and the cooling pipe  14  are held in a close-contact state and the tabular member  12  and the cooling pipe  14  are thermally connected to each other. 
   As the combination of the materials of the tabular member  12  and the cooling pipe  14 , the material of the tabular member  12  has a high coefficient of thermal expansion and a large amount of contraction compared with that of the cooling pipe  14 . Thus, a gap is prevented from being formed between the tabular member  12  and the cooling pipe  14 , which are surely brought into close contact with each other. In other words, in a process of hardening and contraction of the cooling pipe  14  and the tabular member  12 , the cooling pipe  14  is fit in the hole  168  of the tabular member  12  on the basis of a difference of an amount of thermal deformation between the cooling pipe  14  and the tabular member  12 . As a result, thermal connection between the cooling pipe  14  and the tabular member  12  is stably maintained. 
   As explained above, in this example, since the tabular member  12  is formed around the cooling pipe  14  by insert molding, the tabular member  12  is formed to follow the external shape of the cooling pipe  14 . The tabular member  12  and the cooling pipe  14  satisfactorily come into contact with each other. Therefore, improvement of the heat transfer property between the tabular member  12  and the cooling pipe  14  is realized even in the small cooling pipe  14 . Further, since the diameter expanding step is made unnecessary, complicated processing such as cutting using a special cutting tool is unnecessary. In other words, according to this manufacturing method, it is possible to realize a reduction in cost and a reduction in size of the cooling unit  106  to be manufactured. 
   As the tabular member  12 , it is possible to use a resin material having a low melting point and a high thermal conductivity compared with those of the cooling pipe  14 . For example, it is possible to use a resin material mixed with a metal material, a resin material mixed with a carbon material, or the like. A thermal conductivity of the resin material is preferably equal to or higher than 3 W/(m·K) and more preferably equal to or higher than 5 W/(m·K). As the resin material mixed with a metal material or a carbon material, there are a resin material having a thermal conductivity equal to or higher than 3 W/(m·K) and a resin material having a thermal conductivity equal to or higher than 10 W/(m·K). As an example, there are D2 (registered trademark) (an LCP resin mixed with a material for heat transfer), 15 W/(m·K), coefficient of thermal expansion: 10×10^−6/k) and RS007 (registered trademark) (a PPS resin mixed with a material for heat transfer, 3.5 W/(m·K), coefficient of thermal expansion: 20×10^−6/K) manufactured by Cool Polymers, Inc. 
   In this case, as a combination of materials of the tabular member  12  and the cooling pipe  14 , it is preferable that coefficients of thermal expansion of the materials are substantially the same. 
   As an example, there is a combination of the cooling pipe  14  made of copper (coefficient of thermal expansion: 16.6×10^−6/K) or stainless steel (austenitic, coefficient of thermal expansion: 13.6×10^−6/K) and the tabular member  12  made of the resin material having a high thermal conductivity (coefficient of thermal expansion: 10 to 20×10^−6/K). 
   As the combination of materials of the tabular member  12  and the cooling pipe  14 , materials having substantially the same coefficients of thermal expansion are used. Thus, a gap due to a difference of an amount of thermal deformation is prevented from being formed between the tabular member  12  and the cooling pipe  14  at the time of hardening and contraction or after molding of the tabular member  12 . Thermal connection between the tabular member  12  and the cooling pipe  14  is stably maintained. 
   The cooling units and the manufacturing methods for the cooling units according to the aspects of the invention are preferably applied to various optical devices that require cooling for optical elements. It is possible to realize a reduction in cost and a reduction in size of the optical devices. 
   Constitution of a Projector 
   As an example of application of the cooling units, an embodiment of a projector will be hereinafter explained with reference to the drawings. In the embodiment described below, it is possible to apply the cooling units  10 ,  105 , and  106  and the manufacturing methods for the cooling units to a liquid cooling unit  46  described later (see  FIG. 18 ). 
   In this case, the optical element  11  (see  FIGS. 1 ,  12 , and  16 ) is applied to at least one of liquid crystal panels  441 R,  441 G, and  441 B, incidence side sheet polarizers  442 , and emission side sheet polarizers  443  described later (see  FIG. 21 ). 
   Similarly, the tabular members  12  and  13  are applied to at least one of a liquid crystal panel holding frame  445  (a frame-like member  4451  and a frame-like member  4452 ), an incidence side sheet polarizer holding frame  446  (a frame-like member  4461  and a frame-like member  4462 ), and an emission side sheet polarizer holding frame  447  (a frame-like member  4471  and a frame-like member  4472 ) described later. 
   Similarly, the cooling pipe  14  is applied to element cooling pipes  463  (a liquid crystal panel cooling pipe  4631 R, an incidence side sheet polarizer cooling pipe  4632 R, and an emission side sheet polarizer cooling pipe  4633 R). 
   It is possible to realize a reduction in cost and a reduction in size of the projector by applying the cooling units and the manufacturing methods for the cooling units to the liquid crystal unit  46  described later. Moreover, it is possible to extend a durable life according to improvement of cooling performance. 
     FIG. 18  is a diagram schematically showing a constitution of a projector  1 . 
   The projector  1  modulates light beams emitted from a light source according to image information to form an optical image and magnifies and projects the optical image on a screen. The projector  1  includes an armor case  2 , an air cooling device  3 , an optical unit  4 , and a projection lens  5  serving as a projection optical device. 
   In  FIG. 18 , although not shown in the figure, it is assumed that a power supply block, a lamp driving circuit, and the like are arranged in spaces other than the air cooling device  3 , the optical unit  4 , and the projection lens  5  in the armor case  2 . 
   The armor case  2  is formed of synthetic resin or the like and, as a whole, formed in a substantially rectangular parallelepiped shape in which the air cooling device  3 , the optical unit  4 , and the projection lens  5  are housed and arranged. Although not shown in the figure, the armor case  2  includes an upper case constituting a top surface, a front surface, a rear surfaces, and sides of the projector  1  and a lower case constituting a bottom surface, the front surface, the sides, and the rear surface of the projector  1 . The upper case and the lower case are fixed to each other by screws or the like. 
   A material of the armor case  2  is not limited to synthetic resin. The armor case  2  may be formed of other materials such as metal. 
   Further, although not shown in the figure, an intake port (e.g., an intake port  22  shown in  FIG. 19 ) for leading the air into the inside of the projector  1  from the outside and an exhaust port for discharging the air warmed in the projector  1  are formed in the armor case  2 . 
   Moreover, as shown in  FIG. 18 , a partition wall  21  is also formed in the armor case  2 . The partition wall  21  is located in a side direction of the projection lens  5  and a corner part of the armor case  2  and separates a radiator  466 , an axial flow fan  467 , and the like described later of the optical unit  4  from other members. 
   The air cooling device  3  is a device that feeds a cooling air into a cooling channel formed in the projector  1  and cools heat generated in the projector  1 . The air cooling device  3  includes a sirocco fan  31  that is located in the side direction of the projection lens  5  and leads the cooling air on the outside of the projector  1  into the inside of the projector  1  from the not-shown intake port formed in the armor case  2 , a cooling fan for cooling the power supply block, the lamp driving circuit, and the like not shown in the figure, and the like. 
   The optical unit  4  is a unit that optically processes light beams emitted from the light source to form an optical image (a color image) according to image information. As shown in  FIG. 18 , as an overall shape, the optical unit  4  has a substantially L shape that extends generally along the rear surface of the armor case  2  and extends along the sides of the armor case  2 . A detailed constitution of the optical unit  4  is described later. 
   The projection lens  5  is constituted as a group lens in which plural lenses are combined. The projection lens  5  magnifies and projects the optical image (the color image) formed by the optical unit  4  on a not-shown screen. 
   Detailed Constitution of the Optical Unit 
   As shown in  FIG. 18 , the optical unit  4  includes an optical component housing  45  in which an integrator lighting optical system  41 , a color separating optical system  42 , a relay optical system  43 , an optical device  44  are housed and arranged and the liquid cooling unit  46 . 
   The integrator lighting optical system  41  is an optical system for substantially uniformly lighting an image forming area of a liquid crystal panel described later that constitutes the optical device  44 . As shown in  FIG. 18 , the integrator lighting optical system  41  includes a light source unit  411 , a first lens array  412 , a second lens array  413 , a polarization converting element  414 , and a superimposing lens  415 . 
   The light source unit  411  includes a light source lamp  416  that emits rays of a radial shape and a reflector  417  that reflects radiated light emitted from the light source lamp  416 . As the light source lamp  416 , a halogen lamp, a metal halide lamp, and a high-pressure mercury lamp are often used. In  FIG. 18 , a radiating surface mirror is adopted as the reflector  417 . However, the reflector  417  is not limited to this. The reflector  417  may be constituted by an ellipsoidal mirror and adopt, on a light beam emitting side, a paralleling concave lens that changes light beams reflected by the ellipsoidal mirror to parallel beams. 
   The first lens array  412  has a constitution in which small lenses having a substantially rectangular outline viewed from an optical axis direction are arranged in a matrix shape. The respective small lenses divide a light beam emitted from the light source unit  411  into plural partial light beams. 
   The second lens array  413  has substantially the same constitution as the first lens array  412  in which small lenses are arranged in a matrix shape. The second lens array  413  has a function of focusing images of the respective small lenses of the first lens array  412  on a liquid crystal panel described later of the optical device  44  in conjunction with the superimposing lens  415 . 
   The polarization converting element  414  is arranged between the second lens array  413  and the superimposing lens  415  and converts light from the second lens array  413  into substantially one kind of polarized light. 
   Specifically, respective partial lights converted into substantially one kind of polarized light by the polarization converting element  414  are generally superimposed on the liquid crystal panel described later of the optical device  44  finally by the superimposing lens  415 . In a projector using a liquid crystal panel of a type for modulating polarized light, since only one kind of polarized light can be used, substantially a half of light from the light source unit  411 , which emits random polarized light, cannot be used. Therefore, emitted light from the light source unit  411  is converted into substantially one kind of polarized light by using the polarization converting element  414 . Consequently, efficiency of use of light in the optical device  44  is improved. 
   As shown in  FIG. 18 , the color separating optical system  42  includes two dichroic mirrors  421  and  422  and a reflection mirror  423 . The color separating optical system  42  has a function of separating plural partial light beams emitted from the integrator lighting optical system  41  into color lights of three colors, red (R), green (G), and blue (B), using the dichroic mirrors  421  and  422 . 
   As shown in  FIG. 18 , the relay optical system  43  includes an incidence side lens  431 , a relay lens  433 , and reflection mirrors  432  and  434 . The relay optical system  43  has a function of leading blue light separated by the color separating optical system  42  to a liquid crystal panel for blue light described later of the optical device  44 . 
   In this case, the dichroic mirror  421  of the color separating optical system  42  reflects a red light component of a light beam emitted from the integrator lighting optical system  41  and transmits a green light component and a blue light component. Red light reflected by the dichroic mirror  421  is reflected by the reflection mirror  423  and passes through a field lens  418  to reach a liquid crystal panel for red light described later of the optical device  44 . The field lens  418  converts respective partial light beams emitted from the second lens array  413  into light beams parallel to a center axis of the light beams (a main ray). The field lenses  418  provided on light incidence sides of the liquid crystal panels for green light and blue light function in the same manner. 
   In green light and blue light transmitted through the dichroic mirror  421 , the green light is reflected by the dichroic mirror  422  and passes through the field lens  418  to reach the liquid crystal panel for green light described later of the optical device  44 . On the other hand, the blue light is transmitted through the dichroic mirror  422  and passes through the relay optical system  43  and the field lens  418  to reach the liquid crystal panel for blue light described later of the optical device  44 . The relay optical system  43  is used for the blue light in order to prevent deterioration in efficiency of use of light due to divergence or the like of light because length of an optical path of the blue light is longer than lengths of optical paths of other color lights. In other words, the relay optical system  43  is used for the blue light because an optical path length of partial color light made incident on the incidence side lens  431  is long. However, it is also conceivable to set an optical path length of the red light long. 
   As shown in  FIG. 18 , the optical device  44  is obtained by integrally constituting three liquid crystal panels  441  (a liquid crystal panel for red light is denoted by  441 R, a liquid crystal panel for green light is denoted by  441 G, and a liquid crystal panel for blue light is denoted by  441 B) serving as optical modulation elements, three incidence side sheet polarizers  442  and three emission side sheet polarizers  443  serving as optical conversion elements arranged on light beam incidence sides and light beam emission sides of the liquid crystal panels  441 , and a cross dichroic prism  444  serving as a color combining optical device. 
   Although not specifically shown in the figure, the liquid crystal panels  441  have a structure in which liquid crystal serving as an electro-optic material is sealed and encapsulated between a pair of transparent glass substrates. An orientation state of the liquid crystal is controlled according to a driving signal outputted from a not-shown control device. Consequently, the liquid crystal panels  441  modulate a polarization direction of polarized light beams emitted from the incidence side sheet polarizers  442 . 
   Respective color lights, polarizing directions of which are arranged in a substantially one direction by the polarization converting element  414 , are made incident on the incidence side sheet polarizers  442 . The incidence side sheet polarizers  442  transmit only polarized lights in substantially the same direction as a polarization axis of the light beams arranged by the polarization converting element  414  in the light beams made incident on the incidence side sheet polarizers  442  and absorb the other light beams (a light absorption type). 
   Although not specifically shown in the figure, the incidence side sheet polarizers  442  have a structure in which a polarizing film is stuck on a translucent substrate of sapphire glass, liquid crystal, or the like. The polarizing film of the light absorption type is formed by, for example, uniaxially stretching a film containing iodine molecules or dye molecules. The polarizing film has an advantage that an extinction ratio is relatively high and incidence angle dependency is relatively small. 
   The emission side sheet polarizers  443  have substantially the same constitution as the incidence side sheet polarizers  442 . The emission side sheet polarizers  443  transmit only light beams having a polarization axis orthogonal to a transmission axis of light beams in the incidence side sheet polarizers  442  in the light beams emitted from the liquid crystal panel  441  and absorb the other light beams (a light absorption type). 
   The cross dichroic prism  444  is an optical element that composes optical images modulated for each of color lights emitted from the emission side sheet polarizers  443  to form a color image. The cross dichroic prism  444  assumes a substantially square shape in a plan view obtained by sticking four rectangular prisms. Two dielectric multilayer films are formed on interfaces where the rectangular prisms are stuck to one another. The dielectric multilayer films reflect color lights emitted from the liquid crystal panels  441 R and  441 B and passing through the emission side sheet polarizers  443  and transmit color light emitted from the liquid crystal panel  441 G and passing through the emission side sheet polarizer  443 . In this way, the respective color lights modulated by the respective liquid crystal panels  441 R,  441 G, and  441 B are combined to form a color image. 
   The optical component housing  45  is formed of, for example, a metal member. A predetermined lighting optical axis A is set inside the optical component housing  45 . The optical components  41  to  44  are housed and arranged in predetermined positions relative to the lighting optical axis A. A material of the optical component housing  45  is not limited to the metal member. The optical component housing  45  may be formed of other materials. In particular, the optical component housing  45  is preferably formed of a heat conductive material. 
   The liquid cooling unit  46  circulates a cooling fluid to cool mainly the optical device  44 . The liquid cooling unit  46  includes a fluid pumping unit, an element cooling pipe, a branching tank, a merging tank, a pipe unit, and the like described later other than a main tank  461  that temporarily stores the cooling fluid, the radiator  466  serving as a heat radiating unit for radiating heat of the cooling fluid, and the axial flow fan  467  that blows a cooling air on the radiator  466 . 
     FIG. 19  is a perspective view of a part in the projector  1  viewed from an upper side thereof.  FIG. 18  is a perspective view of mainly the optical device  44  and the liquid cooling unit  46  in the projector  1  viewed from below. 
   In  FIG. 19 , for simplification of explanation, only the optical device  44  is shown and the other optical components  41  to  43  in the optical component housing  45  are not shown. In  FIGS. 19 and 20 , for simplification of explanation, a part of members in the liquid cooling unit  46  are not shown. 
   As shown in  FIG. 19 , the optical component housing  45  includes a component housing member  451  and a not-shown cover member that closes an opening of the component housing member  451 . 
   The component housing member  451  constitutes a bottom surface, a front surface, and sides of the optical component housing  45 . 
   In the component housing member  451 , as shown in  FIG. 19 , grooves  451 A for fitting in the optical components  41  to  44  from above in a sliding manner are formed in inner side surfaces of the sides. 
   As shown in  FIG. 19 , a projection lens setting unit  451 B for setting the projection lens  5  in a predetermined position relatively to the optical unit  4  is formed in a front surface portion of the sides. The projection lens setting unit  451 B is formed in a substantially rectangular shape in a plan view. A not-shown circular hole is formed in a substantially center portion in a plan view in association with a light beam emitting position of the optical device  44 . A color image formed by the optical unit  4  is magnified and projected by the projection lens  5  through the hole. 
   Liquid Cooling Unit 
   The liquid cooling unit  46  will be hereinafter explained in detail. 
   In  FIGS. 19 and 20 , the liquid cooling unit  46  includes the main tank  461 , a fluid pumping unit  462  ( FIG. 20 ), the element cooling pipes  463 , a branching tank  464  ( FIG. 20 ), a merging tank  465 , a radiator  466 , the axial flow fan  467 , and a pipe unit  469 . 
   As shown in  FIGS. 19 and 20 , the main tank  461  has a substantially cylindrical shape as a whole. The main tank  461  includes two container-like members made of metal such as aluminum. Openings of the two container-like members are connected to each other to temporarily store a cooling fluid inside the main tank  461 . These container-like members are connected by, for example, seal welding or interposing an elastic member such as rubber. 
   As shown in  FIG. 20 , an inflow section  461 A and an outflow section  461 B for the cooling fluid are formed in a peripheral surface of the main tank  461 . 
   The inflow section  461 A and the outflow section  461 B are formed of a tubular member and arranged to project to the inside and the outside of the main tank  461 . One end of the pipe section  469  is connected to one end of the inflow section  461 A projecting to the outer side. A cooling fluid from the outside flows into the main tank  461  via the pipe section  469 . One end of the pipe section  469  is also connected to one end of the outflow section  461 B projecting to the outer side. The cooling fluid in the main tank  461  flows out to the outside via the pipe section  469 . 
   In the main tank  461 , respective center axes of the inflow section  461 A and the outflow section  461 B are in a positional relation in which the center axes are substantially orthogonal to each other. Consequently, the cooling fluid flowing into the main tank  461  via the inflow section  461 A is prevented from immediately flowing out to the outside via the outflow section  461 B. Uniformalization of the cooling fluid and homogenization of temperature are realized by a mixing action inside the main tank  461 . The cooling fluid flowing out from the main tank  461  is sent to the fluid pumping unit  462  via the pipe section  469 . 
   As shown in  FIG. 20 , the fluid pumping unit  462  sucks the cooling fluid from the main tank  461  to the inside thereof and forcibly discharges the cooling fluid to the outside toward the branching tank  464 . The outflow section  461 B of the main tank  461  and the inflow section  462 A of the fluid pumping unit  462  are connected via the pipe section  469 . The outflow section  462 B of the fluid pumping unit  462  and the inflow section  464 A of the branching tank  464  are connected via the pipe section  469 . 
   Specifically, the fluid pumping unit  462  has, for example, a constitution in which impellers are arranged in a hollow member made of metal such as aluminum of a substantially rectangular parallelepiped shape. Under the control of the not-shown control device, when the impellers rotate, the fluid pumping unit  462  forcibly sucks the cooling fluid stored in the main tank  461  via the pipe section  469  and forcibly discharges the fooling fluid to the outside via the pipe section  469 . With such a constitution, it is possible to reduce a thickness dimension in a rotation axis direction of the impellers. A reduction in size and saving of space are realized. In this embodiment, as shown in  FIGS. 19 and 20 , the fluid pumping unit  462  is arranged below the projection lens  5 . 
   The element cooling pipes  463  are disposed to be adjacent to the liquid crystal panels  441 , the incidence side sheet polarizers  442 , and the emission side sheet polarizers  443  in the optical device  44 . Heat exchange is performed between the cooling fluid flowing through the element cooling pipes  463  and the respective elements  441 ,  442 , and  443 . 
     FIG. 21  is a perspective view showing an overall constitution of the optical device  44 . 
   In  FIG. 21 , as described above, the optical device  44  is obtained by integrally constituting the three liquid crystal panels  441  (the liquid crystal panel for red light  441 R, the liquid crystal panel for green light  441 G, and the liquid crystal panel for blue light  441 B), the sheet polarizers (the incidence side sheet polarizers  442  and the emission side sheet polarizers  443 ) arranged on the incidence sides or the emission sides of the respective liquid crystal panels  441 , and the cross dichroic prism  444 . 
   For each of the colors, red (R), green (G), and blue (B), the emission side sheet polarizers  443 , the liquid crystal panels  441 , and the incidence side sheet polarizers  442  are arranged to be superimposed on the cross dichroic prism  444  in this order. 
   The element cooling pipes  463  are disposed individually for the liquid crystal panels  441 , the incidence side sheet polarizers  442 , and the emission side sheet polarizers  443 , respectively. 
   Specifically, for red light, the element cooling pipe  463  includes a liquid crystal panel cooling pipe  4631 R disposed at the peripheral edge of the liquid crystal panel  441 R, an incidence side sheet polarizer cooling pipe  4632 R disposed at the peripheral edge of the incidence side sheet polarizer  442 , and an emission side sheet polarizer cooling pipe  4633 R disposed at the peripheral edge of the emission side sheet polarizer  443 . The cooling fluid flows into the respective pipes from inflow sections (IN) of the respective element cooling pipes  4631 R,  4632 R, and  4633 R, flows along the peripheral edges of the respective elements  441 R,  442 , and  443 , and flows out to the outside from outflow sections OUT) of the respective pipes. 
   Similarly, for green light, the element cooling pipe  463  includes a liquid crystal panel cooling pipe  4631 G disposed at the peripheral edge of the liquid crystal panel  441 G, an incidence side sheet polarizer cooling pipe  4632 G disposed at the peripheral edge of the incidence side sheet polarizer  442 , and an emission side sheet polarizer cooling pipe  4633 G disposed at the peripheral edge of the emission side sheet polarizer  443 . Further, for blue light, the element cooling pipe  463  includes a liquid crystal panel cooling pipe  4631 B disposed at the peripheral edge of the liquid crystal panel  441 B, an incidence side sheet polarizer cooling pipe  4632 B disposed at the peripheral edge of the incidence side sheet polarizer  442 , and an emission side sheet polarizer cooling pipe  4633 B disposed at the peripheral edge of the emission side sheet polarizer  443 . 
   In this embodiment, peripheral edges of the respective elements, that is, the liquid crystal panels  441 , the incidence side sheet polarizers  442 , and the emission side sheet polarizers  443 , are held by holding frames. The respective element cooling pipes  463  are disposed inside the holding frames along the peripheral edges of the respective elements. The inflow sections (IN) and the outflow sections (OUT) of the respective element cooling pipes  463  are disposed on the identical sides of the respective elements  441 ,  442 , and  443 . 
   Detailed structures of the element holding frames and the element cooling pipes  463  will be described later. 
   Referring back to  FIGS. 19 and 20 , the branching tank  464  branches a cooling fluid sent from the fluid pumping unit  462  to the respective element cooling pipes  463  as shown in  FIG. 20 . 
   As shown in  FIG. 19 , the merging tank  465  merges cooling fluids sent from the respective element cooling pipes  463  and temporarily stores the merged cooling fluids. 
   In this embodiment, the branching tank  464  is arranged on one surface of the cross dichroic prism  444  in the optical device  44  and the merging tank  465  is arranged on one surface on the opposite side of the cross dichroic prism  444 . Arrangement positions of the branching tank  464  and the merging tank  465  are not limited to thee positions and may be other positions. 
     FIG. 22  is a perspective view showing an overall constitution of the branching tank  464 .  FIG. 23  is a perspective view showing an overall constitution of the merging tank  465 . 
   As shown in  FIG. 22 , the branching tank  464  has a substantially cylindrical shape as a whole. The branching tank  464  is formed of a sealed container-like member made of metal such as aluminum to temporarily store a cooling fluid in the inside thereof. 
   An inflow section  464 A and outflow sections  464 B 1 ,  464 B 2 , . . . , and  464 B 9  for a cooling fluid are formed on a peripheral surface of the branching tank  464 . 
   The inflow sections  464 A and the outflow sections  464 B 1  to  464 B 9  are formed of a tubular member and arranged to project to the inside and the outside of the branching tank  464 . One end of the pipe section  469  is connected to one end of the inflow section  464 A projecting to the outer side. A cooling fluid from the fluid pumping unit  462  (see  FIG. 20 ) flows into the branching tank  464  via the pipe section  469 . One ends of the pipe sections  469  are also individually connected respective one ends the outflow sections  464 B 1  to  464 B 9  projecting to the outer side. The cooling fluid in the branching tank  464  flows out to the respective element cooling pipes  463  (see  FIG. 21 ) via the pipe sections  469 . 
   As shown in  FIG. 23 , the merging tank  465  has a substantially cylindrical shape as a whole and is formed of a sealed container-like member made of metal such as aluminum to temporarily store a cooling fluid in the inside thereof in the same manner as the branching tank  464 . 
   Inflow sections  465 A 1 ,  465 A 2 , . . . , and  465 A 9  and an outflow section  465 B for a cooling fluid are formed on a peripheral surface of the merging tank  465 . 
   The inflow sections  465 A 1  to  465 A 9  and the outflow section  465 B are formed of a tubular member and arranged to project to the inside and the outside of the merging tank  465 . One ends of the pipe sections  469  are individually connected to respective one ends of the inflow sections  465 A 1  to  465 A 9  projecting to the outer side. Cooling fluids from the respective element cooling pipes  463  (see  FIG. 21 ) flow into the merging tank  465  via the pipe sections  469 . One end of the pipe section  469  is also connected to one end of the outflow section  465 B projecting to the outside. The cooling fluid in the merging tank  465  flows out to the radiator  466  via the pipe section  469 . 
   Referring back to  FIGS. 19 and 20 , the radiator  466  includes a tubular member  4661  through which a cooling fluid flows and plural radiation fins  4662  connected to the tubular member as shown in  FIG. 20 . 
   The tubular member  4661  is formed of a member having a high thermal conductivity such as aluminum. The cooling fluid flowing in from the inflow section  4661 A flows inside the tubular member  4661  to the outflow section  4661 B. The inflow section  4661 A of the tubular member  4661  and the outflow section  465 B of the merging tank  465  are connected via the pipe section  469 . The outflow section  4661 B of the tubular member  4661  and the main tank  461  are connected via the pipe  469 . 
   The plural radiation fins  4662  are formed of a tabular member having a high thermal conductivity such as aluminum and arranged in parallel to one another. The axial flow fan  467  is constituted to blow a cooling air on the radiator  466  from one surface side of the radiator  466 . 
   In the radiator  466 , heat of the cooling fluid flowing in the tubular member  4661  is radiated via the radiation fins  4662 . The heat radiation is facilitated by the supply of the cooling air by the axial flow fan  467 . 
   As a material forming the pipe section  469 , for example, metal such as aluminum is used. Other materials such as resin may be used. 
   As the cooling fluid, for example, ethylene glycol, which is transparent nonvolatile liquid, is used. Other fluids may be used. The cooling fluid in some aspects of the invention is not limited to liquid and may be gas. A mixture of liquid and solid and the like may be used. 
   As explained above, in the liquid cooling unit  46 , the cooling fluid flows through the main tank  461 , the fluid pumping unit  462 , the branching tank  464 , the element cooling pipes  463 , the merging tank  465 , and the radiator  466  in this order via the pipe section  469 . The cooling fluid returns to the main tank  461  from the radiator  466  and flows through the path repeatedly to circulate. 
   In the liquid cooling unit  46 , since the cooling fluid flows through the respective element cooling pipes  463 , heat of the respective elements  441 ,  442 , and  443  in the optical device  44  generated by irradiation or the like of light beams is appropriately removed. Consequently, temperature rise in the respective element  441 ,  442 , and  443  is controlled. The heat of the respective elements  441 ,  442 , and  443  is transmitted to the cooling fluid in the respective element cooling pipes  463  via the holding frames of the respective elements. 
   Element Holding Frames and Element Cooling Pipes 
   The element holding frames and the element cooling pipes will be explained. The element holding frame and the element cooling pipe for red light will be explained as representative ones. However, those for green light and blue light are the same. 
     FIG. 24  is a partial perspective view showing a panel constitution for red light in the optical device  44 . 
   As shown in  FIG. 24 , for red light, the peripheral edge of the liquid crystal panel  441 R is held by the liquid crystal panel holding frame  445 , the peripheral edge of the incidence side sheet polarizer  442  is held by the incidence side sheet polarizer holding frame  446 , and the peripheral edge of the emission side sheet polarizer  443  is held by the emission side sheet polarizer holding frame  447 . The respective holding frames  445 ,  446 , and  447  have rectangular openings described later corresponding to an image forming area of the liquid crystal panel  441 R. Light beams pass through these openings. 
   The liquid crystal panel cooling pipe  4631 R is disposed inside the liquid crystal panel holding frame  445  along the peripheral edge of the liquid crystal panel  441 R. The incidence side sheet polarizer cooling pipe  4632 R is disposed inside the incidence side sheet polarizer holding frame  446  along the peripheral edge of the incidence side sheet polarizer  442 . The emission side sheet polarizer cooling pipe  4633 R is disposed inside the emission side sheet polarizer holding frame  447  along the peripheral edge of the emission side sheet polarizer  443 . 
     FIG. 25  is a disassembled perspective view of the liquid crystal panel holding frame  445 .  FIG. 26A  is an assembled front view of the liquid crystal panel holding frame  445  and  FIG. 26B  is a sectional view along line A-A in  FIG. 26A . 
   As shown in  FIG. 25 , the liquid crystal panel holding frame  445  includes a pair of frame-like members  4451  and  4452  and a liquid crystal panel fixing plate  4453 . 
   The liquid crystal panel  441 R is a transmission type and has a constitution in which a liquid crystal layer is sealed and encapsulated between a pair of transparent substrates. The pair of substrates include a data line for applying a driving voltage to liquid crystal, a scanning line, a switching element, a driving substrate on which a pixel electrode and the like are formed, and an opposed substrate on which a common electrode, a black matrix, and the like are formed. 
   The frame-like members  4451  and  4452  are frames of a substantially rectangular shape in a plan view. The frame-like members  4451  and  4452  include rectangular openings  4451 A and  4452 A corresponding to the image forming area of the liquid crystal panel  441 R and grooves  4451 B and  4452 B for housing the liquid crystal panel cooling pipe  4631 R. The frame-like member  4451  and the frame-like member  4452  are arranged to be opposed to each other across the liquid crystal panel cooling pipe  4631 R. As the frame-like members  4451  and  4452 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum (234 W/(m·K)), magnesium (156 W/(m·K)), and alloys of aluminum and magnesium (an aluminum die cast alloy (about 100 W/(m·K)), an Mg—Al—Zn alloy (about 50 W/(m·K)), etc.) A material of the frame-like members  4451  ad  4452  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   As shown in  FIG. 25 , the liquid crystal panel fixing plate  4453  is formed of a tabular member having a rectangular opening  4453 A corresponding to the image forming area of the liquid crystal panel  441 R. The liquid crystal panel fixing plate  4453  is fixed to the frame-like member  4452  with the liquid crystal panel  441 R sandwiched between the liquid crystal panel fixing plate  4453  and the frame-like member  4452 . As shown in  FIG. 26B , the liquid crystal panel fixing plate  4453  is arranged to be in contact with the liquid crystal panel  441 R. The liquid crystal panel fixing plate  4453  has a function of bringing the frame-like members  4451  and  4452  and the liquid crystal panel  441 R into close contact with each other and thermally connecting the same and a function of radiating heat of the liquid crystal panel  441 R. A part of heat of the liquid crystal panel  441 R is transmitted to the frame-like members  4451  and  4452  via the liquid crystal panel fixing plate  4453 . 
   The liquid crystal panel cooling pipe  4631 R is made of, for example, a pipe or a tube that has an annular section and extends along a center axis of the section. As shown in  FIG. 25 , the liquid crystal panel cooling pipe  4631 R is bent according to a shape of the grooves  4451 B and  4452 B of the frame-like members  4451  and  4452 . As the liquid crystal panel cooling pipe  4631 R, a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum, copper, stainless steel, and alloys of aluminum, copper, or stainless steel. A material of the liquid crystal panel cooling pipe  4631 R is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   Specifically, as shown in  FIGS. 26A and 26B , the liquid crystal panel cooling pipe  4631 R is disposed on the outer side of the peripheral edge of the liquid crystal panel  441 R along substantially the entire peripheral edge of the liquid crystal panel  441 R. In the respective inner surfaces (mating surfaces, opposed surfaces) of the frame-like members  4451  and  4452 , the grooves  4451 B and  4452 B having a substantially semicircular shape in section are formed along substantially the entire edges of the openings  4451 A and  4452 A. The groove  4451 B and the groove  4452 B are in a substantially mirror symmetrical shape relation with each other. The frame-like members  4451  and  4452  are joined with each other in a state in which the liquid crystal panel cooling pipe  4631 R is housed in the grooves  4451 B and  4452 B. In this embodiment, the liquid crystal panel cooling pipe  4631 R is a circular pipe and an outer diameter thereof is substantially the same as thickness of the liquid crystal panel  441 R. 
   As the joining of the frame-like member  4451  and the frame-like member  4452 , various methods such as fastening by screws or the like, bonding, welding, and mechanical joining such as fitting are adoptable. As a joining method, a method with a high heat transfer property between the liquid crystal panel cooling pipe  4631 R and the frame-like members  4451  and  4452  (or the liquid crystal panel  441 R) is preferably used. 
   An inflow section (IN) for a cooling fluid is disposed at one end of the liquid crystal panel cooling pipe  4631 R and an outflow section (OUT) is disposed at the other end thereof. The inflow section and the outflow section of the liquid crystal panel cooling pipe  4631 R are connected to the piping for cooling fluid circulation (the pipe section  469 ). 
   The cooling fluid flowing into the liquid crystal panel cooling pipe  4631 R from the inflow section (IN) flows along substantially the entire peripheral edge of the liquid crystal panel  441 R and flows out from the outflow section (OUT). The cooling fluid deprives the liquid crystal panel  441 R of heat while flowing through the liquid crystal panel cooling pipe  4631 R. In other words, the heat of the liquid crystal panel  441 R is transmitted to the cooling fluid in the liquid crystal panel cooling pipe  4631 R via the frame-like members  4451  and  4452  and carried to the outside. 
   In the liquid crystal panel holding frame  445 , as shown in  FIG. 26B , the liquid crystal panel cooling pipe  4631 R is disposed to be close to a light beam incidence surface side of the liquid crystal panel  441 R in a thickness direction of the liquid crystal panel  441 R. In the liquid crystal panel  441 R, in general, heat absorption is large on an incidence surface side where black matrixes are arranged compared with an emission surfaced side. Therefore, the liquid crystal panel cooling pipe  4631 R is disposed to be close to the incidence surface side where temperature tends to rise. Consequently, heat of the liquid crystal panel  441 R is effectively removed. 
   Moreover, since a step is provided on the side of the liquid crystal panel  441 R, an area of the emission surface is large compared with that of the incidence surface. Therefore, the liquid crystal panel cooling pipe  4631 R is disposed to be close to the incidence surface side having a small area. Consequently, efficiency of arrangement of the components and a reduction in size of the apparatus are realized. 
     FIG. 27A  is an assembled front view of the incidence side sheet polarizer holding frame  446  and  FIG. 27B  is a sectional view along B-B in  FIG. 27A . 
   The incidence side sheet polarizer holding frame  446  has generally the same constitution as the liquid crystal panel holding frame  445  (see  FIG. 25 ). As shown in  FIGS. 27A and 27B , the incidence side sheet polarizer holding frame  446  includes a pair of frame-like members  4461  and  4462  and a sheet polarizer fixing plate  4463 . 
   The incidence side sheet polarizer  442  has a structure in which a polarizing film is stuck on a translucent substrate. 
   The frame-like members  4461  and  4462  are frames of a substantially rectangular shape in a plan view. The frame-like members  4461  and  4462  include rectangular openings  4461 A and  4462 A corresponding to a light transmitting area of the incidence side sheet polarizer  442  and grooves  4461 B and  4462 B for housing the incidence side sheet polarizer cooling pipe  4632 R. The frame-like member  4461  and the frame-like member  4462  are arranged to be opposed to each other across the incidence side sheet polarizer cooling pipe  4632 R. As the frame-like members  4461  and  4462 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum, magnesium, and alloys of aluminum and magnesium. A material of the frame-like members  4461  and  4462  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K) ). 
   As shown in  FIGS. 27A and 27B , the sheet polarizer fixing plate  4463  is made of a tabular member having a rectangular opening  4463 A corresponding to the light transmitting area of the incidence side sheet polarizer  442 . The sheet polarizer fixing plate  4463  is fixed to the frame-like member  4461  with the incidence side sheet polarizer  442  sandwiched between the sheet polarizer fixing plate  4463  and the frame-like member  4461 . As shown in  FIG. 27B , the sheet polarizer fixing plate  4463  is arranged to be in contact with the incidence side sheet polarizer  442 . The sheet polarizer fixing plate  4463  has a function of bringing the frame-like members  4461  and  4462  and the incidence side sheet polarizer  442  into close contact with each other and thermally connecting the same and a function of radiating heat of the incidence side sheet polarizer  442 . A part of the heat of the incidence side sheet polarizer  442  is transmitted to the frame-like members  4461  and  4462  via the sheet polarizer fixing plate  4463 . 
   The incidence side sheet polarizer cooling pipe  4632 R is made of a seamless pipe formed by drawing or the like. The incidence side sheet polarizer cooling pipe  4632 R is bent according to a shape of the grooves  4461 B and  4462 B of the frame-like members  4461  and  4462 . As the incidence side sheet polarizer cooling pipe  4632 R, a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal is adopted other than aluminum, copper, stainless steel, and alloys of aluminum, copper, and stainless steel. A material of the incidence side sheet polarizer cooling pipe  4632 R is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   Specifically, as shown in  FIGS. 27A and 27B , the incidence side sheet polarizer cooling pipe  4632 R is disposed on the outer side of the peripheral edge of the incidence side sheet polarizer  442  and along substantially the entire peripheral edge of the incidence side sheet polarizer  442 . In the respective inner surfaces (mating surfaces, opposed surfaces) of the frame-like members  4461  and  4462 , the grooves  4461 B and  4462 B having a substantially semicircular shape in section are formed along substantially the entire edges of the openings  4461 A and  4462 A. The groove  4461 B and the groove  4462 B are in a substantially mirror symmetrical shape relation with each other. The frame-like members  4461  and  4462  are joined with each other in a state in which the incidence side sheet polarizer cooling pipe  4632 R is housed in the grooves  4461 B and  4462 B. In this embodiment, the incidence side sheet polarizer cooling pipe  4632 R is a circular pipe and an outer diameter thereof is substantially the same as thickness of the incidence side sheet polarizer  442 . 
   As the joining of the frame-like member  4461  and the frame-like member  4462 , various methods such as fastening by screws or the like, bonding, welding, and mechanical joining such as fitting are adoptable. As a joining method, a method with a high heat transfer property between the incidence side sheet polarizer cooling pipe  4632 R and the frame-like members  4461  and  4462  (or the incidence side sheet polarizer  442 ) is preferably used. 
   An inflow section (IN) for a cooling fluid is disposed at one end of the incidence side sheet polarizer cooling pipe  4632 R and an outflow section (OUT) is disposed at the other end thereof. The inflow section and the outflow section of the incidence side sheet polarizer cooling pipe  4632 R are connected to the piping for cooling fluid circulation (the pipe section  469 ). 
   The cooling fluid flowing into the incidence side sheet polarizer cooling pipe  4632 R from the inflow section (IN) flows along substantially the entire peripheral edge of the incidence side sheet polarizer  442  and flows out from the outflow section (OUT). The cooling fluid deprives the incidence side sheet polarizer  442  of heat while flowing through the incidence side sheet polarizer cooling pipe  4632 R. In other words, the heat of the incidence side sheet polarizer  442  is transmitted to the cooling fluid in the incidence side sheet polarizer cooling pipe  4632 R via the frame-like members  4461  and  4462  and carried to the outside. 
     FIG. 28A  is an assembled front view of the emission side sheet polarizer holding frame  447  and  FIG. 28B  is a sectional view along C-C in  FIG. 28A . 
   The emission side sheet polarizer holding frame  447  has the same constitution as the incidence side sheet polarizer holding frame  446  (see  FIG. 10 ). As shown in  FIGS. 28A and 28B , the emission side sheet polarizer holding frame  447  includes a pair of frame-like members  4471  and  4472  and a sheet polarizer fixing plate  4473 . 
   Like the incidence side sheet polarizer  442 , the emission side sheet polarizer  443  has a structure in which a polarizing film is stuck on a translucent substrate. 
   The frame-like members  4471  and  4472  are frames of a substantially rectangular shape in a plan view. The frame-like members  4471  and  4472  include rectangular openings  4471 A and  4472 A corresponding to a light transmitting area of the emission side sheet polarizer  443  and grooves  4471 B and  4472 B for housing the emission side sheet polarizer cooling pipe  4633 R. The frame-like member  4471  and the frame-like member  4472  are arranged to be opposed to each other across the emission side sheet polarizer cooling pipe  4633 R. As the frame-like members  4471  and  4472 , a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal are adopted other than aluminum, magnesium, and alloys of aluminum and magnesium. A material of the frame-like members  4471  and  4472  is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   As shown in  FIGS. 28A and 28B , the sheet polarizer fixing plate  4473  is made of a tabular member having a rectangular opening  4473 A corresponding to the light transmitting area of the emission side sheet polarizer  443 . The sheet polarizer fixing plate  4473  is fixed to the frame-like member  4471  with the emission side sheet polarizer  443  sandwiched between the sheet polarizer fixing plate  4473  and the frame-like member  4471 . As shown in  FIG. 28B , the sheet polarizer fixing plate  4473  is arranged to be in contact with the emission side sheet polarizer  443 . The sheet polarizer fixing plate  4473  has a function of bringing the frame-like members  4471  and  4472  and the emission side sheet polarizer  443  into close contact with each other and thermally connecting the same and a function of radiating heat of the emission side sheet polarizer  443 . A part of the heat of the emission side sheet polarizer  443  is transmitted to the frame-like members  4471  and  4472  via the sheet polarizer fixing plate  4473 . 
   The emission side sheet polarizer cooling pipe  4633 R is made of a seamless pipe formed by drawing or the like. The emission side sheet polarizer cooling pipe  4633 R is bent according to a shape of the grooves  4471 B and  4472 B of the frame-like members  4471  and  4472 . As the emission side sheet polarizer cooling pipe  4633 R, a thermal good conductor made of a material having a high thermal conductivity is preferably used. For example, various kinds of metal is adopted other than aluminum, copper, stainless steel, and alloys of aluminum, copper, and stainless steel. A material of the emission side sheet polarizer cooling pipe  4633 R is not limited to a metal material and may be other materials (a resin material, etc.) having a high thermal conductivity (e.g., equal to or higher than 5 W/(m·K)). 
   Specifically, as shown in  FIGS. 28A and 28B , the emission side sheet polarizer cooling pipe  4633 R is disposed on the outer side of the peripheral edge of the emission side sheet polarizer  443  and along substantially the entire peripheral edge of the emission side sheet polarizer  443 . In the respective inner surfaces (mating surfaces, opposed surfaces) of the frame-like members  4471  and  4472 , the grooves  4471 B and  4472 B having a substantially semicircular shape in section are formed along substantially the entire edges of the openings  4471 A and  4472 A. The groove  4471 B and the groove  4472 B are in a substantially mirror symmetrical shape relation with each other. The frame-like members  4471  and  4472  are joined with each other in a state in which the emission side sheet polarizer cooling pipe  4633 R is housed in the grooves  4471 B and  4472 B. In this embodiment, the emission side sheet polarizer cooling pipe  4633 R is a circular pipe and an outer diameter thereof is substantially the same as thickness of the emission side sheet polarizer  443 . 
   As the joining of the frame-like member  4471  and the frame-like member  4472 , various methods such as fastening by screws or the like, bonding, welding, and mechanical joining such as fitting are adoptable. As a joining method, a method with a high heat transfer property between the emission side sheet polarizer cooling pipe  4633 R and the frame-like members  4471  and  4472  (or the emission side sheet polarizer  443 ) is preferably used. 
   An inflow section (IN) for a cooling fluid is disposed at one end of the emission side sheet polarizer cooling pipe  4633 R and an outflow section (OUT) is disposed at the other end thereof. The inflow section and the outflow section of the emission side sheet polarizer cooling pipe  4633 R are connected to the piping for cooling fluid circulation (the pipe section  469 ). 
   The cooling fluid flowing into the emission side sheet polarizer cooling pipe  4633 R from the inflow section (IN) flows along substantially the entire peripheral edge of the emission side sheet polarizer  443  and flows out from the outflow section (OUT). The cooling fluid deprives the emission side sheet polarizer  443  of heat while flowing through the emission side sheet polarizer cooling pipe  4633 R. In other words, the heat of the emission side sheet polarizer  443  is transmitted to the cooling fluid in the emission side sheet polarizer cooling pipe  4633 R via the frame-like members  4471  and  4472  and carried to the outside. 
   As described above, in this embodiment, for red light, the element cooling pipes  4631 R,  4632 R, and  4633 R are disposed inside the holding frames  445 ,  446 , and  447  of the respective elements, namely, the liquid crystal panel  441 R, the incidence side sheet polarizer  442 , and the emission side sheet polarizer  443 . Heat of the respective elements  441 R,  442 R, and  443 R is appropriately removed by a cooling fluid flowing through the element cooling pipes  4631 R,  4632 R, and  4633 R. The respective elements  441 R,  442 , and  443  and the element cooling pipes  4631 R,  4632 R, and  4633 R are thermally connected via the respective holding frames  445 ,  446 , and  447 . Heat exchange is performed between the respective elements  441 R,  442 , and  443  and the cooling fluid in the element cooling pipes  4631 R,  4632 R, and  4633 R. Consequently, heat of the respective elements  441 R,  442 , and  443  is transmitted to the cooling fluid in the element cooling pipes  4631 R,  4632 R, and  4633 R via the holding frames  445 ,  446 , and  447 . Since the heat of the respective elements  441 R,  442 , and  443  moves to the cooling fluid, the respective elements  441 R,  442 , and  443  are cooled. 
   In this embodiment, the respective element cooling pipes  4631 R,  4632 R, and  4633 R are disposed along substantially the entire peripheral edges of the respective elements  441 R,  442 , and  443 . Thus, expansion of a heat transfer area is realized to efficiently cool the respective elements. 
   Moreover, since the channels (the element cooling pipes  4631 R,  4632 R, and  4633 R) for a cooling fluid are disposed along the peripheral edges of the respective elements  441 R,  442 , and  443 , light beams for image formation do not pass through the cooling fluid. Therefore, images of bubbles, dust, and the like in the cooling fluid are prevented from being included in an optical image formed on the liquid crystal panel  441 R. Fluctuation in the optical image due to a temperature distribution of the cooling fluid is prevented from occurring. 
   In this embodiment, the paths for a cooling fluid at the peripheral edges of the respective elements  441 R,  442 , and  443  are formed by the channels (the element cooling pipes  4631 R,  4632 R, and  4633 R). Thus, only a relatively small joining portion is required for formation of the channels. Since the number or an area of the joining portion is small, simplification of a constitution is realized and leakage of the cooling fluid is prevented. 
   As described above, according to this embodiment, it is possible to effectively control a temperature rise of the respective elements  441 R,  442 , and  443  while controlling occurrence of deficiencies due to the use of a cooling fluid. 
   In the structure in which the element cooling pipes  4631 R,  4632 R, and  4633 R are disposed inside the element holding frames  445 ,  446 , and  447 , the holding frames  445 ,  446 , and  447  function as both holding means and cooling means for the respective elements  441 R,  442 , and  443 . As a result, a reduction in size of the structure is easily realized. The structure is preferably applicable to a small optical element. 
   For example, in this embodiment, the element cooling pipes  4631 R,  4632 R, and  4633 R having an outer diameter substantially the same as thickness of the respective elements  441 R,  442 , and  443  are disposed on the outer side of the peripheral edges of the respective elements. Thus, expansion in a thickness direction due to inclusion of the cooling fluid channels is controlled. 
   The panel constitution for red light and the cooling structure therefor in the optical device  44  (see  FIG. 21 ) have been explained as the representative panel constitution and cooling structure. However, the panel constitution and the cooling structure are the same for green light and blue light. For green light and blue light, respective elements (a liquid crystal panel, an incidence side sheet polarizer, and an emission side sheet polarizer) are individually held in holding frames and element cooling pipes are disposed inside the holding frames. 
   In this embodiment, nine optical elements in total including the three liquid crystal panels  441 R,  441 G, and  441 B, the three incidence side sheet polarizers  442 , and the three emission side sheet polarizers  443  are individually cooled using a cooling fluid. Since the respective elements are individually cooled, occurrence of deficiencies due to a temperature rise in the respective elements is surely prevented. 
   Piping System 
     FIG. 29  is a piping system diagram showing a flow of a cooling fluid in the optical device  44 . 
   As shown in  FIG. 29 , in this embodiment, channels for a cooling fluid are provided parallel to the nine optical elements in total including the three liquid crystal panels  441 R,  441 G, and  441 B, the three incidence side sheet polarizers  442 , and the three emission side sheet polarizers  443  in the optical device  44 . 
   Specifically, the three element cooling pipes including the liquid crystal panel cooling pipe  4631 R, the incidence side sheet polarizer cooling pipe  4632 R, and the emission side sheet polarizer cooling pipe  4633 R for red light are connected to the branching tank  464  at one ends and connected to the merging tank  465  at the other ends, respectively. Similarly, the three element cooling pipes  4631 G,  4632 G, and  4633 G for green light and the three element cooling pipes  4631 B,  4632 B, and  4633 B for blue light are also connected to the branching tank  464  at one ends and connected to the merging tank  465  at the other ends, respectively. As a result, the nine element cooling pipes are arranged in parallel to one another on the channels for the cooling fluid between the branching tank  464  and the merging tank  465 . 
   The cooling fluid is branched to the nine channels in total by the branching tank  464 , three for each of the colors, and the branched cooling fluids flow through the nine element cooling pipes ( 4631 R,  4632 R,  4633 R,  4631 G,  4632 G,  4633 G,  4631 B,  4632 B, and  4633 B) in parallel to one another. Since the nine element cooling pipes are arranged in parallel to one another on the channels for the cooling fluid, cooling fluids having substantially the same temperatures flow into the respective element cooling pipes. Since the cooling fluids flow through the respective element cooling pipes along the peripheral edges of the respective elements, the respective elements are cooled and temperature of the cooling fluids flowing through the respective element cooling pipes rises. After the heat exchange, the cooling fluids merge in the merging tank  465  and cooled according to heat radiation in the radiator  466  (see  FIG. 20 ) explained above. The cooling fluid having lowered temperature is supplied to the branching tank  464  again. 
   In this embodiment, the nine element cooling pipes corresponding to the nine optical elements are arranged in parallel to one another on the channels for a cooling fluid. Thus, the channels for the cooling fluid from the branching tank  464  to the merging tank  465  are relatively short and a channel resistance due to a pressure loss on the channels is small. Therefore, even if the respective element cooling pipes have small diameters, it is easy to secure a flow rate of the cooling fluid. Further, since a cooling fluid with a relatively low temperature is supplied to each of the elements, the respective elements are effectively cooled. 
   The element cooling pipe does not have to be disposed for an element having less heat generation among the nine optical elements. For example, when the incidence side sheet polarizer  442  or the emission side sheet polarizer  443  are in a form with less absorption of light beams like an inorganic sheet polarizer, a cooling pipe does not have to be provided for the sheet polarizer. 
   All the plural element cooling pipes do not have to be arranged in parallel to one another on channels for a cooling fluid. At least apart of the element cooling pipes may be arranged in series. In this case, it is advisable to set the channels according to amounts of heat generation of the respective elements. 
     FIG. 30  shows a modification of the piping system. Components same as those in  FIG. 29  are denoted by the identical reference numerals and signs. 
   In an example in  FIG. 29 , the element cooling pipes ( 4631 R,  4632 R,  4633 R,  4631 G,  4632 G,  4633 G,  4631 B,  4632 B, and  4633 B) are disposed for the nine optical elements in total including the three liquid crystal panels  441 R,  441 G, and  441 B, the three incidence side sheet polarizers  442 , and the three emission side sheet polarizers  443  in the optical device  44 . Channels for a cooling fluid are provided in series for each of the colors. 
   Specifically, for red light, the outflow section of the branching tank  464  and the inflow section of the emission side sheet polarizer cooling pipe  4633 R are connected, the outflow section of the emission side sheet polarizer cooling pipe  4633 R and the inflow section of the liquid crystal panel cooling pipe  4631 R are connected, the outflow section of the liquid crystal panel cooling pipe  4631 R and the inflow section of the incidence side sheet polarizer cooling pipe  4632 R are connected, and the outflow section of the incidence side sheet polarizer cooling pipe  4632 R and the inflow section of the merging tank  465  are connected. In other words, from the branching tank  464  to the merging tank  465 , the emission side sheet polarizer cooling pipe  4633 R, the liquid crystal panel cooling pipe  4631 R, and the incidence side sheet polarizer cooling pipe  4632 R are arranged in series in this order. Similarly, for green light, from the branching tank  464  to the merging tank  465 , the emission side sheet polarizer cooling pipe  4633 G, the liquid crystal panel cooling pipe  4631 G, the incidence side sheet polarizer cooling pipe  4632 G are arranged in series in this order. Similarly, for blue light, from the branching tank  464  to the merging tank  465 , the emission side sheet polarizer cooling pipe  4633 B, the liquid crystal panel cooling pipe  4631 B, and the incidence side sheet polarizer cooling pipe  4632 B are arranged in series in this order. 
   A cooling fluid is branched to three channels in the branching tank  464 . For each of the colors, first, the cooling fluids flow through the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B. Subsequently, the cooling fluids flow through the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B. Finally, the cooling fluids flow through the incidence side sheet polarizer cooling pipes  4632 R,  4632 G, and  4632 B. Since the cooling fluids flow through the respective element cooling pipes along the peripheral edges of the respective elements, the respective elements are cooled and temperature of the cooling fluids flowing through the respective element cooling pipes rises. In this example, since the three element cooling pipes are arranged in series for each of the colors, temperature at the inflow of the cooling fluids (entrance temperature) is the lowest in the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B on an upstream side, second lowest in the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B, and relatively high in the incidence side sheet polarizer cooling pipes  4632 R,  4632 G, and  4632 B on a downstream side. Thereafter, the cooling fluids merge in the merging tank  465  and cooled by heat radiation in the radiator  466  (see  FIG. 20 ) explained earlier. The cooling fluid having lowered temperature is supplied to the branching tank  464  again. 
   In the liquid crystal panels  441 R,  441 G, and  441 B, simultaneously with the light absorption by the liquid crystal layers, a part of light beams are absorbed by the data line and the scanning line formed on the driving substrate and the black matrixes and the like formed on the opposed substrate. In the incidence side sheet polarizer  442 , light beams to be made incident are converted into substantially one kind of polarized light by the polarization converting element  414  (see  FIG. 18 ) on the upstream side. Most of the light beams are absorbed and absorption of the light beams is relatively small. In the emission side sheet polarizer  443 , a polarizing direction of light beams to be made incident are modulated on the basis of image information. Usually, an amount of absorption of the light beams is larger than that of the incidence side sheet polarizer  442 . 
   An amount of heat generation in the optical device  44  tends to be larger in an order of the incidence side sheet polarizer, the liquid crystal panel, and the emission side sheet polarizer (the incidence side sheet polarizer&lt;the liquid crystal panel&lt;the emission side sheet polarizer). 
   In the example in  FIG. 30 , the three element cooling pipes are arranged in series on the channel for a cooling fluid for each of the colors. Thus, a reduction in a piping space is realized compared with the constitution in which all the nine element cooling pipes are arranged in parallel to one another. 
   Since the cooling fluid is first supplied to the emission side sheet polarizer  443  having a relatively large amount of heat generation, the emission side sheet polarizer  443  is surely cooled. 
   In the example described above, the element cooling pipes are arranged in series from the upstream side in order from one having a largest amount of heat generation. However, an order of arrangement of the element cooling pipes is not limited to this. The element cooling pipes may be arranged in series from the upstream side in order from one having a smallest amount of heat generation or may be arranged in other orders. An order of arrangement is decided according to a difference of amounts of heat generation among the plural elements, cooling abilities of the element cooling pipes, and the like. 
   Moreover, all the plural element cooling pipes do not have to be arranged in series for each of the colors. Only a part of the element cooling pipes may be arranged in series as explained below. 
     FIG. 31  shows another modification of the piping system. Components same as those in  FIG. 29  are denoted by the identical reference numerals and signs. 
   In an example in  FIG. 31 , the element cooling pipes ( 4631 R,  4632 R,  4633 R,  4631 G,  4632 G,  4633 G,  4631 B,  4632 B, and  4633 B) are disposed for the nine optical elements in total including the three liquid crystal panels  441 R,  441 G, and  441 B, the three incidence side sheet polarizers  442 , and the three emission side sheet polarizers  443  in the optical device  44 . A part of channels for a cooling fluid are provided in series for each of the colors. 
   Specifically, for red light, from the branching tank  464  to the merging tank  465 , the liquid crystal panel cooling pipe  4631 R and the incidence side sheet polarizer cooling pipe  4632 R are arranged in series in this order. The emission side sheet polarizer cooling pipe  4633 R is arranged in parallel to the liquid crystal panel cooling pipe  4631 R and the incidence side sheet polarizer cooling pipe  4632 R. In other words, the outflow section of the branching tank  464  and the inflow section of the liquid crystal panel cooling pipe  4631 R are connected, the outflow section of the liquid crystal panel cooling pipe  4631 R and the inflow section of the incidence side sheet polarizer cooling pipe  4632 R are connected, and the outflow section of the incidence side sheet polarizer cooling pipe  4632 R and the inflow section of the merging tank  465  are connected. The outflow section of the branching tank  464  and the inflow section of the emission side sheet polarizer cooling pipe  4633 R are connected and the outflow section of the emission side sheet polarizer cooling pipe  4633 R and the inflow section of the merging tank  465  are connected. Similarly, for green light, from the branching tank  464  to the merging tank  465 , the liquid crystal panel cooling pipe  4631 G and the incidence side sheet polarizer cooling pipe  4632 G are arranged in series in this order. The emission side sheet polarizer cooling pipe  4633 G is arranged in parallel to the liquid crystal panel cooling pipe  4631 G and the incidence side sheet polarizer cooling pipe  4632 G. Similarly, for blue light, the liquid crystal panel cooling pipe  4631 B and the incidence side sheet polarizer cooling pipe  4632 B are arranged in series in this order. The emission side sheet polarizer cooling pipe  4633 B are arranged in parallel to the liquid crystal panel cooling pipe  4631 B and the incidence side sheet polarizer cooling pipe  4632 B. 
   A cooling fluid is branched to six channels in total, two for each of the colors, in the branching tank  464 . First, the cooling fluids flow into the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B and the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B for each of the colors. The cooling fluids flowing through the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B flow through the incidence side sheet polarizer cooling pipes  4632 R,  4632 G, and  4632 B and, then, flow to the merging tank  465 . On the other hand, the cooling fluids flowing through the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B directly flow to the merging tank  465  from the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B for each of the colors. Since the cooling fluids flow through the respective element cooling pipes along the peripheral edges of the respective elements, the respective elements are cooled and temperature of the cooling fluids flowing through the respective element cooling pipes rises. In this example, temperature at the inflow of the cooling fluids (entrance temperature) is relatively low in the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B and the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B on the upstream side and relatively high in the incidence side sheet polarizer cooling pipes  4632 R,  4632 G, and  4632 B on the downstream side. Since an amount of heat generation of the emission side sheet polarizer  443  is the highest compared with the other elements as described above, temperature at the outflow of the cooling fluids (exit temperature) in the emission side sheet polarizer cooling pipes  4633 R,  4633 G, and  4633 B is relatively high. Compared with this exit temperature, exit temperature of the liquid crystal panel cooling pipes  4631 R,  4631 G, and  4631 B is relatively low. Therefore, in the example in  FIG. 31 , entrance temperature of the incidence side sheet polarizer cooling pipes  4632 R,  4632 G, and  4632 B is low compared with that in the example in  FIG. 30 . Thereafter, the cooling fluids flowing at the peripheral edges of the respective elements merge in the merging tank  465  and cooled by heat radiation in the radiator  466  (see  FIG. 20 ) explained earlier. The cooling fluid having lowered temperature is supplied to the branching tank  464  again. 
   In the example in  FIG. 31 , the two element cooling pipes are arranged in series for each of the colors and the other one element cooling pipe is arranged in parallel to the two element cooling pipes. Thus, compared with the constitution in which all the nine element cooling pipes are arranged in parallel to one another, a reduction in a piping space is realized. 
   The cooling channels are provided for the liquid crystal panels  441 R,  441 G, and  441 B and the incidence side sheet polarizer  442  in parallel to the cooling channel for the emission side sheet polarizer  443  having a large amount of heat generation. Thus, a thermal influence of the emission side sheet polarizer  443  is prevented from being exerted on the other elements. The liquid crystal panels  441 R,  441 G, and  441 B and the incidence side sheet polarizer  442  are effectively cooled. 
   In the examples in  FIGS. 29 ,  30 , and  31 , the cooling structures for three colors, red (R), green (G), and blue (B), are the same. However, the cooling structures may be different for each of the colors. For example, it is possible that the constitution in  FIG. 30  or  31  is adopted for red light and blue light and the constitution in  FIG. 29  or  31  is adopted for green light. Further, other combinations of the constitutions may be adopted. 
   In general, since green light has a relatively high light intensity, temperature of the optical element for green light tends to rise. Therefore, a cooling structure having a high cooling effect is adopted for green light and a cooling structure with a simple constitution is adopted for red light and blue light. Consequently, a reduction in a piping space and efficiency of element cooling are realized. 
   In the examples in  FIGS. 29 ,  30 , and  31 , the branching tank  464  branches a channel for a cooling fluid to at least three channels in association with the three colors, red, green, and blue. However, branching of a channel is not limited to this. For example, the branching tank  464  may branch a channel for a cooling fluid to a system for red light and blue light and a system for green light. In this case, for example, cooling structures for red light and blue light are arranged in series and a cooling structure for green light is arranged in parallel to the cooling structures for red light and blue light. Consequently, as described above, it is possible to realize a reduction in a piping space and efficiency of element cooling. 
   In the embodiment described above, the example of the projector using three liquid crystal panels is explained. However, the invention is also applicable to a projector using only one liquid crystal panel, a projector using only two liquid crystal panels, or a projector using four or more liquid crystal panels. 
   The liquid crystal panel is not limited to the transmission liquid crystal panel and a reflection liquid crystal panel may be used. 
   The optical modulator is not limited to the liquid crystal panel. An optical modulator other than liquid crystal such as a device using a micro-mirror may be used. In this case, the sheet polarizers on the light beam incidence side and the light beam emission side do not have to be provided. 
   The invention is also applicable to a front-type projector that projects an image from a direction for observing a screen and a rear-type projector that projects an image from a side opposite to the direction for observing the screen. 
   The exemplary embodiments of the invention have been explained with reference to the accompanying drawings. However, it goes without saying that the invention is not limited to such embodiments. It is evident that those skilled in the art can arrive at various modifications and alterations within a range of the technical ideal described in the claims. It is understood that the modifications and the alterations naturally belong to the technical scope of the invention. 
   The entire disclosure of Japanese Patent Application Nos: 2005-055631, filed Mar. 01, 2005 and 2005-350449, filed Dec. 05, 2005 are expressly incorporated by reference herein.