Abstract:
An optical device includes: a light modulation element that modulates a luminous flux emitted from a light source; a casing that houses the light modulation element; a holding member that holds the light modulation element and exposes a part thereof to outside; a refrigerant housed in a housing space surrounded by the casing, the holding member and the light modulation element; and a convection start unit that is provided in the casing and causes convection of refrigerant.

Description:
BACKGROUND 
   1. Technical Field 
   This invention relates to an optical device and a projector. 
   2. Related Art 
   Recently, there has been an image display device using a light modulation element in which light emitted from a light source is cast to a light modulation element such as a liquid crystal panel, the incident light is modulated by the light modulation element, and then the modulated light is projected onto a screen. In such an image display device, the light except for the light projected onto the screen is absorbed by the light modulation element and its peripheral optical elements, generating heat. Thus, a device having a cooler using a gas for radiating the generated heat has been proposed. However, as the density of semiconductors becomes higher, the calorific value per unit volume increases and the cooling with a gas cannot realize sufficient radiation. Therefore, cooling devices using a liquid have been proposed, as disclosed in JP-A-2003-195253, JP-A-2005-227353 and JP-A-2002-131737. 
   In the projector disclosed in JP-A-2003-195253, a polarizer is cooled by using ethyleneglycol as a refrigerant. Specifically, the heat generated from the polarizer is transmitted to a refrigerant filling a refrigerant-filled part and the refrigerant heated by the heat rises because of the lowered density, causing free convection within the refrigerant. The heat transferred by this convection performs heat exchange with external air, thereby cooling the polarizer. 
   The projector disclosed in JP-A-2005-227353 has a fluid circulation member through which a refrigerant flows. A cooled fluid is sent to the fluid circulation member from a main tank and this cooling fluid is branched and thus sent into three light modulation element holding bodies by a fluid branching unit. This causes the heat generated from a liquid crystal panel and a polarizer to be transmitted to the cooled fluid in the fluid circulation member. Then heated cooling fluid is sent to a radiator, and when passing through a tubular member of the radiator, the cooling fluid is transmitted to a radiation fin and gets cooled. 
   In the projector disclosed in JP-A-2002-131737, a liquid crystal panel for red light, a liquid crystal panel for green light, a liquid crystal panel for blue light, and their respective light incident-side polarizers and light exiting-side polarizers are arranged within a rectangular box-shaped cooling container. A refrigerant is sealed in this cooling container, and as this refrigerant is forcedly circulated by a stirring unit the refrigerant absorbs the heat generated from a polarization element and a liquid crystal panel. The heat absorbed by the refrigerant moves to the inner wall surface of the cooling container and performs heat exchanges with external air by using a cooling fin provided on the outer surface of the cooling container. 
   The above techniques have the following problems. That is, in the projector disclosed in JP-A-2003-195253, since the polarizer is cooled by using free convection, it is difficult to sufficiently cool the polarizer. 
   In the projector disclosed in JP-A-2005-227353, since plural fluid circulation members are used for cooling the light modulation element, the configuration is complicated by the connection of these members. Also, the refrigerant may leak from the connecting parts of the fluid circulation n embers and dust may enter the fluid circulation members because of the connection of the fluid circulation members. 
   In the liquid display device disclosed in JP-A-2002-131737, since an entire liquid crystal display unit is immersed in a refrigerant, the refrigerant may enter the liquid crystal panel. Also, since three liquid crystal panels are arranged within one cooling container, assembly of the liquid crystal display device is difficult. Maintenance is difficult when one of the liquid crystal panels has trouble, and the other liquid crystal panels may also have trouble. 
   SUMMARY 
   An advantage of some aspects of the invention is to provide an optical device and a projector in which entry of dust or the like into the refrigerant that cools the light modulation element is prevented and in which the entire device is miniaturized. 
   According to a first aspect of the invention, an optical device includes: a light modulation element that modulates a luminous flux emitted from a light source; a casing that houses the light modulation element; a holding member that holds the light modulation element and exposes a part thereof to outside; a refrigerant housed in a housing space surrounded by the casing, the holding member and the night modulation element; and a convection start unit that is provided in the casing and causes convection of refrigerant. 
   In the optical device according to the first aspect of the invention, first, the convection start unit caused forced convection of the refrigerant in the housing space. This causes the heat generated in the light modulation element to be transmitted to the refrigerant, and the heated refrigerant flows toward the holding member. Since a part of the holding member is exposed to outside, the heat of the refrigerant is radiated outward via the holding member. After that, the refrigerant, having its heat radiated outward and thus cooled, absorbs the heat of the light modulation element again via the convection start unit, and then the heat is radiated by the holding member. As this is repeated, the light modulation element is efficiently cooled. Moreover, since the light modulation element is held by the holding member, the heat of the light modulation element can be radiated directly outward by the holding member. 
   Thus, according to this aspect of the invention, since the convection start unit and the radiator are entirely housed in the casing, the optical device with a cooling function can be easily assembled, compared with a traditional case where a convection start unit that sends out a refrigerant and a radiator that radiates heat are provided outside of a casing. Also, since heat receiving and radiation are carried out within the casing, the device has no connecting part of circulation members that circulate a refrigerant to a light modulation element, which would be necessary in the case where the convection start unit and the radiator are provided outside. Therefore, the refrigerant will not leak on its way toward the convection start unit and the radiator, and dust, bubbles and the like will not enter the refrigerant. Moreover, since no circulation members are necessary, the device is not bulky and can be miniaturized even when a liquid refrigerant is used. 
   By assembling the optical device including the refrigerant in the process of assembling a traditional light modulation element in a clean room, it is possible to cool the light modulation element with the refrigerant in which no dust, bubbles or the like are present. When this optical device is used for a display device of a projector or the like, the image quality will not be deteriorated. Therefore, the reliability of the entire device can be improved. 
   It is preferable that, in the optical device according to this aspect of the invention, the convection start unit is provided on the holding member. 
   In the optical device according to this aspect of the invention, since the convection start unit is provided on the holding member that carries out radiation and heat receiving, the refrigerant in the housing space can be efficiently circulated and the heat can be radiated by the holding member. Also, since the holding member that holds the convection start unit need not be additionally provided, the number of components can be restrained. 
   It is also preferable that, in the optical device according to this aspect of the invention, the light modulation element is a transmissive liquid crystal element, and that a refrigerant reservoir which stores a part of the refrigerant is provided around an image forming area of the light modulation element in the housing space, and the convection start unit is provided in the refrigerant reservoir. 
   In the optical device according to this aspect of the invention, in the refrigerant reservoir provided around the image forming area of the light modulation element, the heat generated from the light modulation element is transmitted to the refrigerant. In this manner, the refrigerant that is caused to flow by the convection start unit receives heat in the area around the image forming area of the light modulation element. Therefore, as the refrigerant reservoir is provided around the image forming area and the convection start unit is provided in this refrigerant reservoir, unevenness in temperature of the refrigerant in the image forming area can be restrained. That is, since the light modulation element is a transmissive liquid crystal element, an optical image having no unevenness in luminance can be formed when a luminous flux passes through the light modulation element. 
   It is also preferable that, in the optical device according to this aspect of the invention, a polarization member is provided on an outer side of a cover glass that forms the housing space. 
   In the optical device according to this aspect of the invention, since the polarization member provided on the outer side of the cover glass that forms the housing space can be cooled by the refrigerant in the housing space at the same time when the light modulation element is cooled, the polarization member can be cooled without providing any particular unit for cooling the polarization member. That is, the number of components can be reduced and the cost of the entire device can be reduced. Also, since the polarization member is provided on the casing that is outside of the housing space, the polarization member can be cooled without causing the polarization member to directly contact the refrigerant. Therefore, degradation of the polarization member due to contact with the refrigerant can be restrained. 
   It is also preferable that, in the optical device according to this aspect of the invention, a rectifying member that rectifies the flow of the refrigerant is provided in the housing space. 
   In the optical device according to this aspect of the invention, since the rectifying member is formed in the housing space, the refrigerant in the housing space can be spread therein, and the heated refrigerant can be prevented from locally staying in the housing space or from forming a deflected flow. Therefore, unevenness in temperature in the image forming area of the light modulation element can be restrained and the light modulation element can be cooled more efficiently. Thus, an optical device capable of improving the display property can be provided. 
   It is also preferable that, in the optical device according to this aspect of the invention, a driving unit that drives the convection start unit is provided outside of the casing. 
   In the optical device according to this aspect of the invention, as the driving unit is provided outside of the casing, entry of dust into the refrigerant in the housing space can be prevented even when dust is generated by the driving unit. 
   It is also preferable that, in the optical device according to this aspect of the invention, the driving unit is an electromagnetic motor or piezoelectric ultrasonic motor. 
   In the optical device according to this aspect of the invention; as an electromagnetic motor or piezoelectric ultrasonic motor is used as the driving unit, the refrigerant in the housing space can be circulated without increasing the size of the device. 
   It is also preferable that, in the optical device according to this aspect of the invention, a radiation fin is provided at the part of the holding member that is exposed outward from the casing. 
   In the optical device according to this aspect of the invention, the heat generated from the light modulation element can be efficiently radiated outward by the radiation fin provided on the holding member. 
   According to a second aspect of the invention, a projector includes: a light source that emits light; plural optical devices that include the above-described optical device; a light combining unit that has lateral side which is arranged along the plural optical devices, and combines color lights emitted from the plural optical devices; and a projection unit that projects an optical image combined by the light combining unit. 
   In the projector according to this aspect of the invention, light emitted from the light source becomes incident on each of the optical devices. The light incident on the optical devices is modulated in accordance with image information, and the modulated optical image is projected by the projection unit. Therefore, as the optical devices which suppress entry of dust into the refrigerant are provided, a projector that can project a sharper image and that has high heat resistance and reliability can be provided. 
   It is preferable that, in the projector according to this aspect of the invention, a radiation unit in contact with the holding members of the plural optical devices is provided. 
   In the projector according to this aspect of the invention, for example, if the light modulation element has a large calorific value, the radiation unit in contact with the holding members can efficiently radiate this neat outward. 
   It is also preferable that, in the projector according to this aspect of the invention, the light combining unit is a dichroic prism and the radiation unit is provided on a surface of the dichroic prism where the plural optical devices are not provided. 
   In the projector according to this aspect of the invention, as the radiation unit is provided on the surface of the dichroic prism where the plural optical devices are not provided, the radiation by the dichroic prism and the radiation by the holding members can be made common and a projector that is further miniaturized can be provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
       FIG. 1  is a plan view showing a schematic configuration of an optical device according to a first embodiment of the invention. 
       FIG. 2  is an exploded perspective view showing essential parts of the optical device according to the first embodiment of the invention. 
       FIG. 3  is a sectional view taken along line A-A of the optical device of  FIG. 1 . 
       FIG. 4  is a perspective view showing a schematic configuration of the optical device of  FIG. 1 . 
       FIG. 5  is a view taken along line B-B of the optical device  1 . 
       FIG. 6  is a plan view showing a driving unit of an optical device according to a second embodiment of the invention. 
       FIG. 7  is a side view showing the driving unit of the optical device of  FIG. 6 . 
       FIGS. 8A and 8B  show movement of the driving unit of the optical device of  FIG. 6 . 
       FIG. 9  is a sectional view of essential parts showing fixation of the optical device of  FIG. 6 . 
       FIG. 10  is a plan view showing a schematic configuration of a projector according to a third embodiment of the invention. 
       FIG. 11  is a perspective view showing an optical device used for the projector of  FIG. 10 . 
       FIG. 12  is a perspective view showing a modification of the projector according to the third embodiment of the invention. 
       FIG. 13  is a plan view showing a modification of the optical device according to each embodiment of the invention. 
       FIG. 14  is a plan view showing a modification of the optical device according to each embodiment of the invention. 
       FIG. 15  is a plan view showing a modification of the optical device according to each embodiment of the invention. 
       FIG. 16  is a plan view showing a modification of the optical device according to each embodiment of the invention. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Hereinafter, optical devices and projectors according to embodiments of the invention will be described with reference to the drawings. In the following drawings, the scale of each member is appropriately changed in order to show each member in a recognizable size. 
   First Embodiment 
   An optical device according to a first embodiment of the invention will be described with reference to  FIGS. 1 to 5 . 
     FIG. 1  is a plan view showing a schematic configuration of the optical device according to this embodiment.  FIG. 2  is an exploded perspective view showing essential parts of the optical device of  FIG. 1 .  FIG. 3  is a sectional view taken along line A-A of the optical device of  FIG. 1 .  FIG. 4  is a perspective view of the optical device of  FIG. 1 .  FIG. 5  is a view taken along line B-B of the optical device of  FIG. 1 . 
   A liquid crystal light valve unit (optical device)  1  has a liquid crystal light valve (light modulation element)  2  that modulates a luminous flux emitted from a light source, a holding unit (holding member)  20  that holds the liquid crystal light valve  2 , and a convection driving unit  30  that circulates a refrigerant (cooling medium)  31 , with the liquid crystal light valve  2 , the holding unit  20  and the convection driving unit  30  being provided within a casing  10 , as shown in  FIG. 1 and 5 . 
   The liquid crystal light valve  2  has a structure in which an area provided between a TFT board  3  and a counter-board  4  is filled with liquid crystal (not shown), as shown in  FIG. 2 . The liquid crystal light valve  2  is also provided with a light valve FPC (flexible printed circuit)  5  as a flexible board connected to a projector body (not shown). 
   The casing  10  is provided to hold the holding unit  20  from both sides, as shown in  FIG. 2 . 
   First, the holding unit will be described. 
   The holding unit  20  has a supporting frame  21  that supports the liquid crystal light valve  2 , a pair of liquid crystal panel packings  92   a ,  22   b  made of an elastic material and arranged to hold the supporting frame  21  between them, and a pair of holding frames  23   a ,  23   b  arranged to hold the liquid crystal panel packings  22   a ,  22   b  between them, as shown in  FIG. 2 . 
   The supporting frame  21  is formed by a plate unit that is rectangular in a plan view, and a rectangular aperture  21   a  that can fit with the liquid crystal light valve  2  is formed therein, as shown in  FIGS. 2 and 3 . This aperture  21   a  has a step part  21   b  formed therein. The liquid crystal light valve  2  is fixed by filling this step part  21   b  with an adhesive. The supporting frame  21  is constructed in contact with the holding frames  23   a ,  23   b , as shown in  FIG. 3 . 
   The liquid crystal panel packings  22   a ,  22   b  have apertures  22   c ,  22   d  of a size corresponding to an image forming area A, as shown in  FIG. 2 . An elastic silicon rubber can be employed for the liquid crystal panel packings  22   a ,  22   b . For the liquid crystal panel packings  22   a ,  22   b , not only a silicon rubber but also a butyl rubber or fluorine rubber having less moisture transmission may be used. 
   On the surfaces of the first and second holding frames  33   a ,  23   b  on the side where the liquid crystal light valve  2  is arranged, recessed parts  23   c ,  23   d  are formed, as shown in  FIG. 3 . The first and second liquid crystal panel packings  22   a ,  22   b  and the supporting frame  21  are arranged in these recessed parts  23   c ,  23   d , thus holding the liquid crystal light valve  2  between them. 
   On the surfaces of the first and second holding frames  23   a ,  23   b  opposite to the side where the liquid crystal light valve  2  is arranged, rectangular groove parts  23   e ,  23   f  are provided. First and second glass packings  24   a ,  24   b  are provided in these groove parts. In this manner, the first and second holding frames  23   a ,  23   b  are provided substantially symmetrically on both sides of the liquid crystal light valve  2 . 
   Thus, this holding unit  20  is constructed to prevent the refrigerant  31  from entering the liquid crystal of the liquid crystal light valve  2 . 
   The supporting frame  21  and the holding frames  23   a ,  23   b  are made of aluminum. However, they may be made of a metal material having high thermal conductivity, for example, a metal material such as Cu, Al, Fe or Mg, or an alloy containing these metals. 
   Next, the casing will be described. 
   The casing  10  has a first cover glass  11   a  provided toward the TFT board  3  of the liquid crystal light valve  2 , a second cover glass  11   b  provided toward the counter-board  4  of the liquid crystal light valve a first fixing frame  13   a  that fixes the first cover glass  11   a  and the first holding frame  23   a  via the first glass packing  24   a , and a second fixing frame  13   b  that fixes the second cover glass  11   b  and the second holding frame  23   b  via the second glass packing  24   b , as shown in  FIG. 3 . 
   In the first and second fixing frames  13   a ,  13   b , apertures  13   c ,  13   d  larger than the image forming area A of the liquid crystal light valve  2  are formed, and a luminous flux can be incident through them. 
   The liquid crystal light valve  2  is provided with a space between the first cover glass  11   a  and the second cover glass  11   b , and the lateral sides (a part) of the first and second holding frames  23   a ,  23   b  are exposed from the casing  10 , as shown in  FIG. 3 . Radiation fins  27   a ,  27   b  are formed on these outwardly exposed parts of the first and second holding frames  23   a ,  23   b , that is, the outer circumferences of the first and second holding frames  23   a ,  23   b , as shown in  FIG. 4 . That is, the first and second holding frames  23   a ,  23   b  are structured to serve also as a radiation part. The supporting frame  21  is constructed in contact with the holding frames  23   a ,  23   b . Therefore, the heat of the liquid crystal light valve  2  is transmitted to the holding frames  23   a ,  23   b  via the supporting frame  21  and then radiated outward. 
   The first and second fixing frames  13   a ,  13   b  have their fixing holes  15   a ,  15   b  fixed in a snap-fitting manner to fixing hook parts  25   a ,  25   b  formed on the first and second holding frames  23   a ,  23   b , as shown in  FIG. 4 . There is no problem in using screws to fix the first and second fixing frames  13   a ,  13   b  with the first and second holding frames  23   a ,  23   b , but snap-fitting makes it easy to assemble, disassemble and reassemble these parts. 
   The radiation fins  27   a ,  27   b  of the first and second holding frames  23   a ,  23   b  are so shaped that the parts other than the fixing hook parts  25   a ,  25   b  of the first and second holding frames  23   a ,  23   b  and the fixing holes  15   a ,  15   b  of the fixing frames  13   a ,  13   b  can contact external air as much as possible. 
   A first cooling chamber (housing space)  17   a  in which the refrigerant  31  is sealed in such a manner that it can freely flow, is provided between the first cover glass  11   a , and the first holding frame  23   a  and the TFT board  3 , as shown in  FIG. 5 . Also, a second cooling chamber (housing space)  17   b  in which the refrigerant  31  is sealed in such a manner that it can freely flow, is provided between the second cover glass  11   b , and the second holding frame  23   b  and the counter-board  4 . That is, the TFT board  3 , the counter-board  4  and the first and second holding frames  23   a ,  23   b  of the liquid crystal light valve  2  directly contact the refrigerant  31 . 
   The refrigerant  31  sealed in the first and second cooling chambers  17   a ,  17   b  is water-based, in consideration of thermal conductivity and cost. Usually, an ethylene glycol or propylene glycol solution is used as the refrigerant  31  in consideration of freezing at low temperatures. Moreover, organic compounds such as defoamer and anticorrosive are added. 
   A light incident-side polarizer (polarization member)  18  and a light exiting-side polarizer (polarization member)  19  are provided on outer surfaces (outer sides)  11   e ,  11   f  that are opposite to inner surfaces  11   c ,  11   d  contacting the refrigerant  31 , of the first cover glass  11   a  and the second cover glass  11   b.    
   In the first and second cooling chambers  17   a ,  17   b , the space formed between the first holding frame  23   a  and the first cover glass  11   a  and the space formed between the second holding frame  23   b  and the second cover glass  11   b  are refrigerant reservoirs  33   a ,  33   b . As these refrigerant reservoirs are provided, the first and second cooling chambers  17   a ,  17   b  have large volumes and a required amount of the refrigerant  31  can be stored. 
   Next, convection driving unit will be described. 
   The convection driving units  30  are provided toward the first cooling chamber  17   a  and the second cooling chamber  17   b , as shown in  FIG. 5 . As they have the same configuration, the convection driving unit  30  toward the first cooling chamber  17   a  will be described. 
   The convection driving unit  30  has a convection start plate (convection start unit)  34  provided in the refrigerant reservoir  33   a , a rotation start plate  35  provided on the outer surface  11   e  of the first cover glass  11   a  and connected to the convection start plate  34  via the first cover glass  11   a , and a driver (driving unit)  36  that is provided on an outer surface  13   e  of the first fixing frame  13   a  and drives the rotation start plate  35 . That is, the convection start plate  34  is provided within the first cooling chamber  17   a  (within the casing  10 ), and the rotation start plate  35  and the driver  36  are provided outside of the first cooling chamber  17   a  and drive the convection start plate  34  from outside. 
   Specifically, the rotation start plate  35  is a flat electromagnetic motor and is rotated by the driving force of the driver  36 . Convection start plate-side core members  37  and rotation start plate-side core member  38  are formed in the convection start plate  34  and the rotation start plate  35 . The core members  37 ,  38  on both sides are magnets. The convection start plate  34  receives the rotation force of the rotation start plate  35 , and as the convection start plate  34  follows the rotation force, it rotates about a rotation axis B formed in the first holding frame  23   a . This causes forced convection in the first cooling chamber  17   a  and circulates the refrigerant  31 . 
   The driver  36  has a driver control board  39  that controls the number of rotations of the rotation start plate  35  and the like, and the driver control board  39  is provided on the outer surface  13   e  of the first fixing frame  13   a . The driver control board  39  is provided with a connection line (driving unit FPC)  40  as a flexible board connected to a projector body (not shown). The driver control board  39  may be situated toward the projector body. 
   Next, a method of cooling the liquid crystal light valve  2  by using the liquid crystal light valve unit  1  of this embodiment having the above configuration will be described. 
   First, the driver  36  is started to drive the rotation start plate  35 . As the rotation start plate  35  rotates, the convection start plate  34  rotates. This causes the refrigerant  31  in the refrigerant reservoirs  33   a ,  33   b  to flow toward the image forming area A, and the heat of the liquid crystal light valve  2  is transmitted to the fluid. The refrigerant  31  is caused to flow toward the refrigerant reservoirs  33  by the convection generated by the convection start plate  34 . The heat of the refrigerant  31  is radiated outward by the first and second holding frames  23   a ,  23   b  and the radiation fins  27   a ,  27   b . That is, since the first and second holding frames  23   a ,  23   b  directly contact the refrigerant  31 , the heat of the refrigerant  31  is quickly transmitted to the first and second holding frames  23   a ,  23   b  that are arranged nearby, and then is radiated broadly by the radiation fins  27   a ,  27   b  of the first and second holding frames  23   a ,  23   b.    
   The refrigerant  31 , cooled by having the heat radiated outward, is caused to flow again toward the image forming area A by the convection start plate  34 , and the heat of the liquid crystal light valve  2  is transmitted to the refrigerant  31 . The heat of the liquid crystal light valve  2  is also radiated by the first and second holding frames  23   a ,  23   b  via the supporting frame  21 . At the same time when the liquid crystal light valve  2  is cooled, the light incident-side polarizer  18  and the light exiting-side polarizer  19  are cooled by the flow of the refrigerant  31  in the first and second cooling chamber  17   a ,  17   b.    
   In the optical device  1  according to this embodiment, as forced convection is generated by the convection driving Unit  30  in the first and second cooling chambers  17   a ,  17   b  in which the refrigerant  31  is sealed, for example, thermal energy due to beams from the light source can be efficiently lowered in the liquid crystal light valve  2 . That is, though the cooling efficiency is overwhelmingly low in a free convection system, the cooling efficiency leaps as the refrigerant  31  in the first and second cooling chambers  17   a ,  17   b  is caused to flow even slightly by the convection driving unit  30 . 
   Moreover, the refrigerant reservoirs  33   a ,  33   b , the convection driving unit  30 , and the first and second holding frames  23   a ,  23   b  radiating heat, are provided within the casing  10 , instead of providing a refrigerant storing unit, a convection start unit and a radiation unit separately from a liquid crystal light valve as is often the case with traditional forced convection-type liquid cooling. Therefore, the liquid crystal light valve unit  1  that is small-sized and that can avoid weight loss due to leakage of the refrigerant  31  and avoid entry of dust, bubbles and the like, can be provided. 
   As the radiation fins  27   a ,  27   b  are provided in the parts of the first and second holding frames  23   a ,  23   b  that are exposed outside of the casing  10 , the liquid crystal light valve  2  can be cooled more efficiently. That is, the liquid crystal light valve  2  can be cooled sufficiently without arranging a large external radiator. 
   The liquid crystal light valve unit  1  according to this embodiment of the invention does not need any pipe for transporting the refrigerant or connection member, and for example, the assembly of a dichroic prism used in a projector is similar to the assembly of a traditional light valve. Therefore, the assembly is easy. 
   Second Embodiment 
   Next, a second embodiment of the invention will be described with reference to  FIGS. 6 to 9 . In each of the embodiments described below, the same parts as in the configuration of the liquid crystal light valve unit  1  according to the first embodiment are denoted by the same numerals and will not be described further in detail. 
   A liquid crystal light valve unit according to this embodiment differs from the first embodiment in that a piezoelectric ultrasonic motor is used as a driver (driving unit)  50 . 
   The piezoelectric ultrasonic motor (PZT motor)  50  has a rotatable rotor (rotation start plate of the first embodiment)  51  and a stator (vibrator)  52  having a protrusion  56  contacting the lateral side of the rotor  51 , as shown in  FIG. 6 . 
   The rotor  51  is made of stainless steel and its lateral side is a driving surface. 
   The stator  52  has a configuration in which thin plate-like piezoelectric elements (piezoelectric ceramics)  53  are provided on both sides of a stainless steel shim (thin plate)  54 , as shown in  FIG. 7 , and its total thickness is 0.4 mm. These two piezoelectric elements  53  are so arranged that their directions of polarization are as shown  FIG. 7 . The planar dimension of the piezoelectric element  53  is approximately 7 mm by 2 mm. The electrode on the outer surface of the piezoelectric element  53  is divided into three in the y-direction as shown in  FIG. 6 , and of these three divided electrodes, the electrode near end parts  52   a ,  52   b  is divided into two in the x-direction. Thus, the electrode is divided into five in total. That is, the electrodes include an electrode A near the end part  52   a  and a lateral side  53   a , an electrode B near the end part  52   a  and a lateral side  53   b , a central electrode C, an electrode D near the end part  52   b  and the lateral side  53   a , and an electrode E near the end part  52   b  and the lateral side  53   b.    
   Supporting units  55  that serve for holding and continuity are provided on the lateral sides in the direction of width of the stator  52 . As a pressure toward the rotor  51  is applied via these supporting units  55 , vibrations of the stator  52  are transmitted to the rotor  51  by friction. 
   Next, the operation of the piezoelectric ultrasonic motor  50  of this embodiment having the above configuration will be described. 
   First, when an AC signal of approximately 300 kHz is applied between the electrodes A, C, E and the shim  54 , vertical linear standing waves shown in  FIG. 8A  and curved quadratic standing waves shown in  FIG. 8B  are excited. In the piezoelectric ultrasonic motor  50  with such a shape, since the difference between the vertical linear and curved quadratic resonance frequencies is as small as approximately 3 to 6 kHz, vibrations in a mixed mode occur. As a result of such vibrations, the distal end of the protrusion  56  forms an elliptic locus and realizing highly efficient driving. 
   As the voltage applying position is switched to the electrodes B, C and D, the direction of the elliptic vibrations is reversed and the rotor  51  rotates in the reverse direction. This rotation of the rotor  51  causes the convection start plate  34  to rotate. 
   In the liquid crystal light valve unit according to this embodiment, a large torque can be generated with a small size by using a piezoelectric ultrasonic motor  50 . That is, when the torque is not enough to rotate the convection start plate  34 , the piezoelectric ultrasonic motor  50  is fixed to the first fixing frame  13   a  by a fixing screw  57 , and the rotor  51  is connected directly with the convection start plate  34 , as shower in  FIG. 9 , thereby enabling provision of a liquid crystal light valve unit having a small size and a large torque. In this case, in order to prevent leakage of the refrigerant  31  from a rotary shaft C that connects the rotor  51  with the convection start plate  34 , packings (O rings)  59  are provided between a bearing  58  and the rotary shaft C and between the bearing  58  and the first cover glass  11   a . If the liquid crystal light valve unit has an enough space to form a driver therein, a thin brushless motor having a large torque or a small motor with a decelerator may be used instead of using the piezoelectric ultrasonic motor  50 . 
   Third Embodiment 
   Next, as a third embodiment of the invention, a projector having the liquid crystal light valve unit  1  of the first embodiment will be described. 
     FIG. 10  is an explanatory view of a projector  500  having the liquid crystal light valve unit  1  of the embodiment. 
   The projector  500  has light sources  512 ,  513 ,  514 , a liquid crystal light valve unit for red light  522 , a liquid crystal light valve unit for green light  523 , a liquid crystal light valve unit for blue light  524 , a dichroic prism (light combining unit)  525  that combines the color lights emitted from the respective liquid crystal light valve units  522 ,  523 ,  524 , and a projection lens  526  that projects an optical image combined by the dichroic prism  525 . The liquid crystal light valve units  522 ,  523 ,  524  are fixed to a dichroic prism fixing member  545  by positioning holes  540  along the lateral sides of the dichroic prism  525 , as shown in  FIG. 11 . In this embodiment, since it is assumed that a refrigerant of high temperatures is moved upward by convection, the refrigerant reservoirs  33   a ,  33   b  of the liquid crystal light valve units  522 ,  523 ,  524  are arranged in their upper parts. In the case where It is suspended from the ceiling, the refrigerant reservoirs  33   a ,  33   b  may be arranged In the lateral parts in the drawing. 
   The light sources  512 ,  513 ,  514  employ LED chips that emit red (R), green (G) and blue (B), respectively. As an even illumination system to provide even illuminance distribution of the light from the light sources, a rod lens or a fly-eye lens may be arranged on the rear side from each light source. 
   A luminous flux from the red light source  512  is transmitted through a superimposing lens  535 R, then reflected by a reflection mirror  517 , and becomes incident on a liquid crystal light valve for red light  522 R (not shown) of the liquid crystal light valve unit for red light  522 . A luminous flux from the green light source  513  is transmitted through a superimposing lens  535 G and becomes incident on a liquid crystal light valve for green light  523 G (not shown) of the liquid crystal light valve unit for green light  523   
   A luminous flux from the blue light source  514  is transmitted through a superimposing lens  535 B, then reflected by a reflection mirror  516 , and becomes incident on a liquid crystal light valve for blue light  524 B of the liquid crustal light valve unit for blue light  524 . As the luminous fluxes from the respective light sources pass through the superimposing lenses, the luminous fluxes are superimposed with each other in display areas of the liquid crystal light valves, and the liquid crystal light valves are illuminated evenly. Of the luminous fluxes from the light sources  512 ,  513 ,  514 , only linearly polarized light in a predetermined direction is transmitted through the light incident-side polarizer and becomes incident on each liquid crystal light valve unit  522 ,  523 ,  524 . 
   The three color lights modulated by the respective liquid crystal light valve units  522 ,  523 ,  524  become incident on the dichroic prism  525 . This prism is formed by bonding four right-angled prisms, and on its inner surfaces, a dielectric multilayer film to reflect red light and a dielectric multilayer film to reflect blue light are arranged in a cross-shape. The three color lights are combined by these dielectric multilayer films, thus forming light representing a color image. The combined light is projected onto a projection screen  527  by a projection lens  526 , which is a projection system, thus displaying an enlarged image. 
   Since the projector  500  of this embodiment has the liquid crystal light valve unit for red light  522 , the liquid crystal light valve unit for green light  523  and the liquid crystal light valve unit for blue light  524  in which entry of dust to the refrigerant is restrained, a sharper image can be projected onto the projection screen  527 . 
   As the liquid crystal light valve units  522 ,  523 ,  524  are used in the projector  500 , only the light valve FPC  5  and the connection line  40  are the connecting parts between the liquid crystal light valve unit  1  and the projector body, except for a mechanical fixing part  60  that also serves for positioning. Therefore, miniaturization of the projector  500  as a whole can be realized. 
   If the radiation fins  27   a ,  27   b  cannot be sufficiently provided on the holding frames of the liquid crystal light valve units  522 ,  523 ,  524 , or if the device is of a high-luminance type and its quantity of radiation is insufficient, a radiation unit  550  contacting the radiation fins  27   b  of the liquid crystal light valve units  522 ,  523 ,  524  and having plural radiation fins  550   a  may be provided on an upper surface (where the liquid crystal light valve units  522 ,  523 ,  524  are not provided)  525   a  of the dichroic prism  525 , as shown in  FIG. 12 . In this configuration, since contact pieces  550   b  contacting the radiation fins  27   b  are provided on the radiation unit  550  and the radiation unit  550  can thus radiate the heat of the radiation fins  27   b , the electrothermal efficiency can be improved. In this case, the radiation unit  550  is provided on the upper surface  525   a  of the dichroic prism  525 , but it is not limited to the upper surface  525   a  of the dichroic prism  525  if there is an enough space within the projector body. 
   The technical field of the invention is not limited to the embodiments. Various changes can be made without departing from the scope of the invention. 
   For example, rectifying protrusions (rectifying members)  71  that diffuses the refrigerant  31  may be provided in the first and second cooling chambers  17   a ,  17   b , as shown in  FIG. 13 . This configuration enables smoothing of the flow of the fluid around the convection start plate  34 . Thus, since the influence of a turbulent flow on the image forming area A can be restrained, the turbulence of the luminous flux transmitted through the liquid crystal light valve  2  can be reduced. It is preferable that the rectifying protrusions  71  are formed in the refrigerant reservoirs  33   a ,  33   b , which are outside of the image forming area A. However, if they are made of transparent members, they may be formed in the image forming area A. 
   The convection start plate  34  is formed in the refrigerant reservoirs  33   a ,  33   b  of the first and second cooling chambers  17   a ,  17   b . However, the convection star plate  34  may be formed in the image forming area A as long as it is formed in the first and second cooling chambers  17   a ,  17   b . In this case, it is preferable that the convection start unit  34  is a transparent member in order to restrain the influence on an image. 
   The first and second cooling chambers  17   a ,  17   b  are provided on both sides of the liquid crystal light valve  2 . However, depending on the calorific value and application, a cooling chamber may be provided only on one side. 
   The driver  36  that drives the convection start plate is provided outside, but it may be provided within the casing  10 . In this configuration, it is preferable that the driver is provided, for example, at a part of the first and second holding frames  23   a ,  23   b  in order to prevent entry of dust and the like. 
   The one convection start plate  34  is provided, but plural convection start plates may be provided. In this configuration, the convection of the refrigerant  31  can be increased. However, in consideration of the turbulence of the flow and the cost of the driving source, a smaller number of convection start plates are preferred. Magnets are used as the convection start plate-side core member  37  and the rotation start plate-side core member  38 , but one of these core members  37 ,  38  may be a magnet. Alternatively, an electrostatic force may be used instead of magnets. 
   The first cooling chamber  17   a  and the second cooling chamber  17   b  are separately provided near the TFT board  3  and near the counter-board  4 , respectively. However, a continuing path may be formed at a part of the first and second holding frames  23   a ,  23   b . Since this configuration enables movement of the refrigerant  31  between the first cooling chamber  17   a  and the second cooling chamber  17   b , it suffices to provide the convection driving unit  30  in one of the first cooling chamber  17   a  and the second cooling chamber  17   b.    
   In each of the embodiments a transmissive liquid crystal element is used as the light modulation element. However, the light modulation element is not limited to this and, for example, a liquid crystal light valve unit  600  using a DMD (digital micromirror device) element  602 , which is a reflective light modulation element, may be employed. This configuration includes the DMD element  602  held between boards  601 , a convection start plate  604  provided in a refrigerant  603 , a driver  606  provided outside of a cooling chamber  605  and connected with a driving circuit  606   a  that drives the convection start plate  604 , and a radiation member  607  in contact with the cooling chamber  605 , as shown in  FIG. 14 . The radiation member  607  may be integrated with the cooling chamber  605 . The connection of the driving circuit  606   a  of the driver  606  and the board  601  can be made common. This liquid crystal light valve unit  600  is to be fixed at a predetermined position with fixing screws  610 . 
   The light incident-side polarizer  18  and the light exiting-side polarizer  19  are cooled by the refrigerant  31  in the first cooling chamber  17   a  and the second cooling chamber  17   b . However, a transparent pipe  701  that are circulating in the refrigerant reservoirs  33   a ,  33   b  and in which convection of the refrigerant  31  is possible, and a gear pump  702  that sends the refrigerant  31  into the transparent pipe  701  may be provided, and this transparent pipe  701  may be brought in contact with the first and second cover glasses  11   a ,  11   b , thus cooling the light incident-side polarizer  18  and the light exiting-side polarizer  19 , as shown in  FIG. 15 . This configuration enables better cooling of the light incident-side and light exiting-side polarizers  18 ,  19 . 
   Moreover, a fixing frame  802  having a follow part  801  circulating to the refrigerant reservoir  33   a , and a gear pump  803  that sends the refrigerant  31  into the hollow part  801  may be provided around the image forming area A, as shown in  FIG. 16 . The liquid crystal light valve  2  may be fixed by the fixing frame  802  and the refrigerant  31  may be circulated in the hollow part  801 , thus causing forced convection. In this configuration, since the refrigerant  31  does not flow in the image forming area A, the risk of degradation in image quality can be restrained. 
   The entire disclosure of Japanese Patent Application No. 2006-059965, filed Mar. 6, 2006 is expressly incorporated by reference herein.