Abstract:
A holding mechanism for holding an image display element to a prism includes: a first fixing member fixed to the prism by an adhesive; and a second fixing member, which is fixed to the first fixing member using a screw, and which has the image display element directly or indirectly fixed to the second fixing member. A protruding section is provided at the periphery of a hole to be used for the purpose of fixing using the screw, and at the time of fixing the second fixing member to the first fixing member using the screw, the first fixing member and the second fixing member are directly or indirectly brought into contact with each other at the protruding section such that deformation due to screwing of the screw does not generate within a bonding region of the first fixing member.

Description:
[0001]    The present U.S. patent application is a U.S. National phase Application under 35 U.S.C. 371 of International Application PCT/JP2014/064075 filed on May 28, 2014. This application claims a priority under the Paris Convention of Japanese Patent Applications No. 2013-114125 filed on May 30, 2013, and No. 2013-119628 filed on Jun. 6, 2013, the entirety of each of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to an image display element holding mechanism, a prism unit, and a projector. The present invention more specifically relates to, for example, a holding mechanism for holding an image display element such as a liquid crystal panel, a digital micromirror device, or the like on a prism, a prism unit formed by integrating an image display element and a prism with each other with the holding mechanism, and a projector including the prism unit. 
       BACKGROUND ART 
       [0003]    Recent years have been observing an increasingly more widespread use of projectors configured to project image light formed by an image display element such as a liquid crystal panel, a digital micromirror device, or the like onto a screen to thereby project an image onto the screen in an enlarged manner. In such projectors, an image display element and a prism are integrated with each other with an adhesive agent, but with the use of an adhesive agent, internal stress occurs mainly at an adhesion portion due to contraction force of the adhesive agent as it hardens, and the internal stress sometimes causes distortion of a member fixing the image display element, which may give rise to displacement of pixels. As a measure to prevent this inconvenience, Patent Literature 1 listed below proposes providing a cut in a fixing frame, and Patent Literature 2 listed below proposes providing a hole in a fixing frame to reduce thermal distortion of the fixing frame caused by, for example, variation of environmental temperature. 
         [0004]    Patent Literature 3 listed below proposes integrating an image display element and a prism with each other with a member to which the prism is fixed with an adhesive agent and a member screwed on the member to which the prism is fixed. Such use of two kinds of members for holding an optical element facilitates fine adjustment of positions of a plurality of image display elements, and thus facilitates removal of the image display elements for replacement, too. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1 Japanese Patent Application Publication No. 2011-150109 
         [0006]    Patent Literature 2 Japanese Patent Application Publication No. 2006-3826 
         [0007]    Patent Literature 3 U.S. Pat. No. 7,152,979 B2 
         [0008]    As described above, in a case where two different kinds of members are used as fixing members for integrating an image display element and a prism with each other, with the configurations proposed in Patent Literatures 1 to 3, there is a risk that the screwing between the fixing members may have adverse effects on an adhesion surface. Specifically, assuming that one of the two members to which the prism adheres is a first fixing member and the other to which the image display element is fixed is a second fixing member, when the second fixing member is fixed to the first fixing member by tightening a screw into them, tightening force (axial force) is applied between the first and second fixing members, and if there is a warp or poor flatness in contact surfaces of the first and second fixing members, bending stress will be generated. There is a risk that the bending stress may cause the prism on the adhesion surface to chip, or cause the adhesive agent to come off, to reduce adhesive strength between the prism and the first fixing member. Moreover, even a product produced as a non-defective product may suffer from gradual coming-off or warping of the adhesive agent over time. Even though the measures described in Patent Literatures 1 and 2 are helpful to make improvements related to heat, it has been recognized, on replacing or adjusting a plurality of image display elements, that warping of the prism, coming-off of the adhesive agent, and the like are caused. 
       SUMMARY OF THE INVENTION 
       [0009]    One or more embodiments of the present invention provide an image display element holding mechanism configured such that bending stress generated by the tightening of a screw between fixing members is prevented from being applied to an adhesion surface of a prism with respect to a fixing member, a prism unit including the holding mechanism, and a projector. 
         [0010]    According to a first aspect of the present invention, an image display element holding mechanism for holding an image display element on a prism includes a first fixing member that is fixed to a prism with an adhesive agent, and a second fixing member that is fixed to the first fixing member with a screw, and to which an image display element is directly or indirectly fixed. Here, a protrusion is formed around a hole used for fixing with the screw, and the first fixing member and the second fixing member contact each other directly or indirectly at the protrusion so as to prevent distortion from being caused in an adhesion area of the first fixing member by tightening of the screw when fixing the first fixing member to the second fixing member with the screw. 
         [0011]    According to a second aspect of the present invention, in the image display element holding mechanism according to the first aspect of the present invention described above, the protrusion is integrally formed with at least one of the first fixing member and the second fixing member. 
         [0012]    According to a third aspect of the present invention, in the image display element holding mechanism according to the first aspect of the present invention described above, the protrusion is constituted by a spacer detachable from the first fixing member and the second fixing member. 
         [0013]    According to a fourth aspect of the present invention, in the image display element holding mechanism according to any one of the first to third aspects of the present invention described above, the protrusion has a height that is equal to or larger than a sum of a flatness of a surface of the first fixing member and a flatness of a surface of the second fixing member, the surface of the first fixing member and the surface of the second fixing member facing each other, such that the surface of the first fixing member and the surface of the second fixing member facing each other are prevented from contacting each other at positions other than a position at which the first fixing member and the second fixing member are fixed to each other with the screw. 
         [0014]    According to a fifth aspect of the present invention, in the image display element holding mechanism according to any one of the first to fourth aspects of the present invention described above, the protrusion is disposed so as to circularly surround the hole used for the fixing with the screw. 
         [0015]    According to a sixth aspect of the present invention, an image display element holding member for holding an image display element on a prism includes a first fixing member that is fixed to the prism with an adhesive agent, and a second fixing member that is fixed to the first fixing member with a screw, and to which the image display element is directly or indirectly fixed. Here, the first fixing member includes a buffer portion that absorbs bending stress generated by tightening of the screw when fixing the second fixing member to the first fixing member with the screw. 
         [0016]    According to a seventh aspect of the present invention, in the image display element holding mechanism according to the sixth aspect of the present invention described above, the first fixing member includes a gap or a cut disposed between an adhesion surface of the first fixing member with respect to the prism and a position at which the tightening of the screw is received, and the buffer portion is formed by providing the gap or the cut. 
         [0017]    According to an eighth aspect of the present invention, in the image display element holding mechanism according to the sixth aspect of the present invention described above, the first fixing member includes a thin-walled portion formed by providing a recess between an adhesion surface of the first fixing member with respect to the prism and a position at which the tightening of the screw is received, and the buffer portion is constituted by the thin-walled portion. 
         [0018]    According to a ninth aspect of the present invention, in the image display element holding mechanism according to any one of the sixth to eighth aspects of the present invention described above, a relative position of the image display element with respect to the prism is adjustable by performing the tightening of the screw against biasing force between the first fixing member and the second fixing member. 
         [0019]    According to a tenth aspect of the present invention, in a prism unit, the image display element and the prism are integrated with each other with the image display element holding mechanism according to any one of the first to ninth aspects of the present invention described above. 
         [0020]    According to an eleventh aspect of the present invention, a projector includes the prism unit according to the tenth aspect of the present invention described above. 
       ADVANTAGEOUS EFFECTS OF ONE OR EMBODIMENTS OF THE INVENTION 
       [0021]    According to an image display element holding mechanism of one or more embodiments of the present invention, it is possible to prevent bending stress generated by tightening of a screw between fixing members from being applied to an adhesion surface of a prism with respect to a fixing member. As a result, it is possible to prevent generation of a crack in the prism, degradation of adhesion strength of the prism, etc. from being caused by the bending stress generated by the tightening of the screw. Furthermore, by providing a prism unit with the image display element holding mechanism, it is possible to keep the positional relationship between the prism and the image display element in an accurate and stable manner, and by providing a projector with the prism unit, it is possible to achieve a high-quality image projection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic configuration diagram showing an embodiment of a projector in which an image display element holding mechanism is mounted on a prism unit; 
           [0023]      FIG. 2  is a perspective view showing first and fourth embodiments of a prism unit having an image display element holding mechanism; 
           [0024]      FIG. 3  is an exploded perspective view showing the first embodiment of the prism unit having the image display element holding mechanism; 
           [0025]      FIGS. 4A and 4B  are sectional views schematically showing an image display element holding mechanism for comparison; 
           [0026]      FIGS. 5A and 5B  are sectional views schematically showing a first embodiment of the image display element holding mechanism; 
           [0027]      FIGS. 6A and 6B  are sectional views schematically showing a second embodiment of the image display element holding mechanism; 
           [0028]      FIGS. 7A and 7B  are sectional views schematically showing a third embodiment of the image display element holding mechanism; 
           [0029]      FIG. 8  is an exploded perspective view showing the fourth embodiment of the prism unit having the image display element holding mechanism; 
           [0030]      FIGS. 9A, 9B, and 9C  are schematic diagrams showing a principal part of a fourth embodiment of the image display element holding mechanism; 
           [0031]      FIGS. 10A, 10B, and 10C  are schematic diagrams showing a principal part of a fifth embodiment of the image display element holding mechanism; and 
           [0032]      FIGS. 11A, 11B, and 11C  are schematic diagrams showing a principal part of a sixth embodiment of the image display element holding mechanism. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    Hereinafter, with reference to the accompanying drawings, descriptions will be given of an image display element holding mechanism, a prism unit, a projector, etc. that embody the present invention. Here, common or corresponding components and portions in embodiments, examples for comparison, and the like are identified with the same reference numerals, and overlapping descriptions thereof will be omitted where deemed appropriate. 
         [0034]      FIG. 1  shows an embodiment of a projector PJ in which an image display element holding mechanism for holding an image display element  4  is mounted on a prism unit PU. The projector PJ shown in  FIG. 1  includes a light source  1 , an illumination optical system  2 , a prism  3 , the image display element  4 , a projection lens system  5 , etc. That is, the projector PJ includes the image display element  4  that displays an image, the light source  1 , the illumination optical system  2  that directs light from the light source  1  to the image display element  4 , and the projection lens system  5  that projects the image displayed on the image display element  4  onto a screen in an enlarged manner. 
         [0035]    Light emitted from the light source  1  (which is, for example, a white light source such as a xenon lamp or the like, or a laser light source) is directed by the illumination optical system  2  and the prism  3  to the image display element  4 , where image light is formed. The illumination optical system  2  is constituted by a rod integrator, a relay optical system, a folding mirror, etc., and the prism  3  is constituted by a TIR prism that separates illumination light from projection light and a color prism (color separating/combining prism) that separates colors of RGB (red, green, and blue) from each other and combines the colors of RGB with each other. The image display element  4  is an image modulation element (for example, a digital micromirror device) which modulates light and thereby generates an image, and the projection lens system  5  is configured to project the image displayed on the image display element  4  onto a screen surface in an enlarged manner. That is, the image light formed at the image display element  4  is projected by the projection lens system  5  toward the screen surface. 
         [0036]    An embodiment of the prism unit PU is shown in  FIGS. 2 and 3 . The prism unit PU is formed by integrating three image display elements  4  respectively corresponding to the colors of RGB with the prism  3 , and the integration is achieved by using three sets of image display element holding mechanisms HK for holding the image display elements  4  on the prism  3 . Each set of the image display element holding mechanism HK includes two pairs of a first fixing member A 1  ( FIG. 3 ) that is fixed to the prism  3  with an adhesive agent and a second fixing member B 1  ( FIG. 2 ,  FIG. 3 ) that is fixed to the first fixing member A 1  with screws  9 , and to which one of the image display elements  4  is indirectly fixed. One of the two pairs of the first fixing member A 1  and the second fixing member B 1  is disposed on an upper part of the prism  3 , and the other pair is disposed on a lower part of the prism  3 . Here, the fixing with the screws  9  is achieved, as shown in  FIG. 3 , by using threaded holes Ah provided in the first fixing member A 1 , and through holes Bh provided in the second fixing member B 1 . 
         [0037]    The prism  3  is a component formed of a plurality of glass prisms joined with each other, and the first fixing member A 1  as a mechanical component is fixed with an adhesive agent to a glass surface of a color prism portion of the prism  3  ( FIG. 3 ). Each of the image display elements  4  is fixed to a third fixing member C 1  with an adhesive agent ( FIG. 2 ), and the third fixing member C 1  is fixed with respect to the second fixing member B 1  with the fastening members  11 , which are each disposed with one of pins  10  of the second fixing member B 1  penetrating therethrough ( FIG. 2 ). Specifically, the fastening members  11  fix the pins  10  of the second fixing member B 1  and the third fixing member C 1  to each other via a UV adhesive agent. Here, the fixing of the image display element  4  with respect to the second fixing member B 1  is achieved indirectly via the third fixing member C 1 , but the fixing may be achieved in a simpler manner, by fixing the image display element  4  directly with respect to the second fixing member B 1  with an adhesive agent. 
         [0038]    As has been described above, each of the image display element holding mechanisms HK for holding the image display elements  4  is composed of the first fixing members A 1  ( FIG. 3 ) to which the prism  3  is bonded with an adhesive agent, the third fixing member C 1  ( FIG. 3 ) to which the image display element  4  is bonded with an adhesive agent, and the second fixing members B 1  ( FIG. 2 ,  FIG. 3 ) that are fixed with respect to the first and third fixing members A 1  and C 1  with the screws  9  and the fastening members  11 , respectively. The fixing with the screws achieved via the second fixing members B 1  in this manner makes it possible to perform, for example, relative position adjustment between the prism  3  and the image display element  4  with ease as will be described below. 
         [0039]    In the prism unit PU, when forming one image by superimposing images each corresponding to one of the colors of RGB that have been separated from each other, it is important to align the images of the colors of RGB with each other. Even if the alignment is performed accurately, t an adhesive agent often shrinks as it hardens during the fixing, and the shrinking of the adhesive agent invites displaced adhesive fixing. If the members joining the prism  3  and the image display element  4  to each other are all fixed with an adhesive agent, it is impossible to finely adjust the positional relationship between the prism  3  and the image display element  4  afterwards. 
         [0040]    With this in mind, to make it possible to finely adjust the positional relationship between the prism  3  and the image display element  4  even after the adhesive fixing of the prism  3  to the first fixing member A 1  and the adhesive fixing of the image display element  4  to the third fixing member C 1 , the second fixing member B 1  is used as a component for joining the prism  3  and the image display element  4  to each other with the screws  9  and the fastening members  11 . Moreover, the image display element  4  is detachable together with the third fixing member C 1  to facilitate replacement of the image display element  4  if it malfunctions and needs to be replaced. 
         [0041]    However, when fixing the second fixing member B 1  to the first fixing member A 1  with the screws  9 , it is necessary to prevent axial force, which is generated when the screws  9  are tightened, from adversely affecting an adhesion surface of the prism  3  with respect to the first fixing member A 1 . If there is a warp or poor flatness in contact surfaces of the first fixing member A 1  and the second fixing member B 1 , the tightening of the screws causes one of the contact surfaces to move toward the other for full contact with each other, and this generates bending stress. The bending stress invites a crack or a chip in the prism  3  at the adhesion surface, or peels off the adhesive agent to cause degradation in strength of adhesion between the prism  3  and the first fixing member A 1 . 
         [0042]    For comparison,  FIGS. 4A and 4B  show an embodiment of the image display element holding mechanism HK of a case where the first fixing member A 1  has a warp due to part accuracy or the like.  FIGS. 4A and 4B  are sectional views each showing the prism  3  and first and second fixing members A 0  and B 0  as viewed from above,  FIG. 4A  showing a state before the first fixing member A 0  and the second fixing member B 0  are fixed to each other with screws  9 ,  FIG. 4B  showing a state after the first fixing member A 0  and the second fixing member B 0  are fixed to each other with the screws  9 . When the screws  9  are tightened, the first fixing member A 0  moves toward the second fixing member B 0  for full contact with each other, and this generates bending stress. In other words, the warp of the first fixing member A 0  is straightened, and thereby bending stress is generated which works to peel off an adhesive agent  3   a.  The bending stress is applied in a concentrated manner to an adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 0 , which may invite a crack  3   c  in the prism  3  at a boundary between the prism  3  and the adhesive agent  3   a.    
         [0043]    To prevent this, according to a first embodiment of the image display element holding mechanism HK, the first fixing member A 1  is provided with protrusions  6   a  as shown in  FIGS. 5A and 5B .  FIGS. 5A and 5B  are sectional views each showing the prism  3 , the first fixing member A 1 , and the second fixing member B 1  as viewed from above,  FIG. 5A  showing a state before the first fixing member A 1  and the second fixing member B 1  are fixed to each other with screws  9 ,  FIG. 5B  showing a state after the first fixing member A 1  and the second fixing member B 1  are fixed to each other with the screws  9 . 
         [0044]    The protrusions  6   a  are formed around the threaded holes Ah used for the fixing with the screws  9 , and the protrusions  6   a  are integrally formed with the first fixing member A 1 . The image display element holding mechanism HK of the present embodiment is configured such that the first fixing member A 1  and the second fixing member B 1  come into direct contact with each other at the protrusions  6   a  so that distortion will not be caused in an adhesion area of the first fixing member A 1  by tightening the screws  9  when fixing the second fixing member B 1  to the first fixing member A 1  with the screws  9  (for example, there are at least two fixing positions). Moreover, the protrusions  6   a  are each formed to circularly surround one of the threaded holes Ah used for the fixing with the screws  9 , and this allows the fixing with the screws to be performed in a stable manner. 
         [0045]    With this image display element holding mechanism HK, since the first fixing member A 1  and the second fixing member B 1  do not need to contact each other at portions other than the protrusions  6   a,  distortion of the first fixing member A 1  is reduced, and thus no stress is generated to peel off the adhesive agent  3   a.  That is, when the screws  9  are tightened, surfaces of the first fixing member A 1  and the second fixing member B 1  are not in contact with each other between the two protrusions  6   a,  and thus no stress is applied to the adhesion surface  3   s  from the warped surface of the first fixing member A 1 . 
         [0046]    This makes distortion less likely to occur in the first fixing member A 1  in the adhesion area located between the fixing positions, and thus, it is possible to prevent bending stress generated by the tightening of the screws  9  between the first and second fixing members A 1  and B 1  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 1 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Moreover, the adhesive agent  3   a  is allowed to exert its original characteristics to hold the prism  3 , and this helps achieve a range of usable temperature wider than has conventionally been achieved. Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0047]    In order to prevent a surface of the first fixing member A 1  and a surface of the second fixing member B 1  that face each other from contacting each other at positions other than the positions at which the first and second fixing members A 1  and B 1  are fixed to each other with the screws  9 , it may be preferable in some embodiments that the protrusions  6   a  have a height equal to or larger than a sum of flatnesses of the surface of the first fixing member A 1  and the surface of the second fixing member B 1  that face each other (the flatness of a surface is measured by the difference between the highest and lowest points over the length of the surface, and corresponds to part accuracy). For example, when the flatnesses of the facing surfaces of the first and second fixing members A 1  and B 1  are  0 .1 mm and 0.05 mm, respectively, it may be preferable in some embodiments that the height of the protrusions  6   a  be 0.15 mm or larger. 
         [0048]    Axial force Ff generated when a screw is tightened is expressed by the following formula: Ff=T/(K×d) (where T represents tightening torque (N·m); K represents a torque coefficient; and d represents a nominal diameter of the screw (mm)) For example, when an M3 screw (d=3) with a torque coefficient K of 0.2 is tightened with force having a tightening torque T of 0.63(N·m), the axial force Ff generated in the vicinity of a threaded hole Ah is 1050N. Note that, since distortion of the first fixing member A 1  in the adhesion area is reduced, no undesired stress is applied to the adhesion surface  3   s  even when the axial force Ff is applied around the threaded holes Ah (this applies also to other embodiments). 
         [0049]    By tightening the screws  9  against biasing force between the first and second fixing members A 1  and B 1 , it is possible to adjust the relative position of the image display element  4  with respect to the prism  3 . The screws  9  fix the first fixing member A 1  and the second fixing member B 1  to each other by being inserted through the through holes Bh ( FIG. 3 , etc.) formed in the second fixing member B 1  into screw-engagement with the threaded holes Ah formed in the first fixing member A 1 . In the above fixing achieved with the screws, for example, by arranging an elastic member (not shown) such as a spring or the like between the first and second fixing members A 1  and B 1  such that the screws  9  are tightened against biasing force between the first and second fixing members A 1  and B 1 , pushing and pulling between the screws  9  and the elastic member helps finely adjust the relative position of the image display element  4  with respect to the prism  3  in an optical-axis direction. This adjustment mechanism is applicable also to other embodiments. 
         [0050]    An example of a material of the first fixing member A 1  and the second fixing member B 1  (and of first fixing members A 2  and A 3 , second fixing members B 2  and B 3 , a spacer  7 , etc. which will be described later) is SUS430. In some embodiments, SUS430 may be preferable as the material in view of relationship with prism materials in terms of elasticity, linear expansion coefficient, etc. The adhesive agent  3   a,  which is used to fix the prism  3  and the first fixing member A 1  to each other, may preferably be an epoxy resin adhesive agent in some embodiments. 
         [0051]      FIGS. 6A and 6B  show a second embodiment of the image display element holding mechanism HK.  FIGS. 6A and 6B  are sectional views each showing the prism  3 , a first fixing member A 2 , and a second fixing member B 2  as viewed from above,  FIG. 6A  showing a state before the first fixing member A 2  and the second fixing member B 2  are fixed to each other with the screws  9 ,  FIG. 6B  showing a state after the first fixing member A 2  and the second fixing member B 2  are fixed to each other with the screws  9 . 
         [0052]    Protrusions  6   b  are formed around the through holes Bh used for the fixing with the screws  9 , and the protrusions  6   b  are integrally formed with the second fixing member B 2 . The image display element holding mechanism HK of the present embodiment is configured such that the first fixing member A 2  and the second fixing member B 2  come into direct contact with each other at the protrusions  6   b  so that distortion will not be caused in an adhesion area of the first fixing member A 2  by tightening the screws  9  when fixing the second fixing member B 2  to the first fixing member A 2  with the screws  9  (for example, there are at least two fixing positions). Moreover, the protrusions  6   b  are each formed to circularly surround one of the through holes Bh used for the fixing with the screws  9 , and this allows the fixing with the screws to be performed in a stable manner. The present embodiment is different from the first embodiment ( FIGS. 5A, 5B ) in that, as described above, the protrusions  6   b  are integrally formed with the second fixing member B 2 . 
         [0053]    With this image display element holding mechanism HK, since the first fixing member A 2  and the second fixing member B 2  do not have to contact each other at portions other than the protrusions  6   b,  distortion of the first fixing member A 2  is reduced, and thus no stress is generated to peel off the adhesive agent  3   a.  That is, when the screws  9  are tightened, surfaces of the first fixing member A 2  and the second fixing member B 2  are not in contact with each other between the two protrusions  6   b,  and thus no stress is applied to the adhesion surface  3   s  from the warped surface of the first fixing member A 2 . 
         [0054]    This makes distortion less likely to occur in the first fixing member A 2  in the adhesion area located between the fixing positions, and as a result, it is possible to prevent bending stress generated by the tightening of the screws  9  between the first and second fixing members A 2  and B 2  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 2 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Moreover, the adhesive agent  3   a  is allowed to exert its original characteristics to hold the prism  3 , and this helps achieve a range of usable temperature wider than has conventionally been achieved. Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0055]    As in the first embodiment described above, in order to prevent a surface of the first fixing member A 2  and a surface of the second fixing member B 2  that face each other from contacting each other at positions other than the positions at which the first and second fixing members A 2  and B 2  are fixed to each other with the screws  9 , it may be preferable in some embodiments that the protrusions  6   b  have a height equal to or larger than a sum of flatnesses of the surface of the first fixing member A 2  and the surface of the second fixing members B 2  that face each other. For example, when the flatnesses of the facing surfaces of the first and second fixing members A 2  and B 2  are 0.1 mm and 0.05 mm, respectively, it may be preferable in some embodiments that the height of the protrusions  6   a  be 0.15 mm or larger. 
         [0056]      FIGS. 7A and 7B  show a third embodiment of the image display element holding mechanism HK.  FIGS. 7A and 7B  are sectional views each showing the prism  3 , a first fixing member A 3 , and a second fixing member B 3  as viewed from above,  FIG. 7A  showing a state before the first fixing member A 3  and the second fixing member B 3  are fixed to each other with the screws  9 ,  FIG. 7B  showing a state after the first fixing member A 3  and the second fixing member B 3  are fixed to each other with the screws  9 . 
         [0057]    Around the threaded holes Ah and the through holes Bh used for the fixing with the screws  9 , there are provided spacers  7  having holes  7   h  coaxial with the threaded holes Ah and the through holes Bh. The spacers  7  are configured to be detachable from the first fixing member A 3  and the second fixing member B 3 . The image display element holding mechanism HK of the present embodiment is configured such that the first fixing member A 3  and the second fixing member B 3  come into indirect contact with each other via the spacers  7  so that distortion will not be caused in an adhesion area of the first fixing member A 3  by tightening the screws  9  when fixing the second fixing member B 3  to the first fixing member A 3  with the screws  9  (for example, there are at least two fixing positions). The present embodiment is different from the first and second embodiments ( FIG. 5A and 5B ,  FIGS. 6A and 6B ) in that, as described above, the spacers  7  which correspond to the protrusions  6   a  and protrusions  6   b  are provided between the first fixing member A 3  and the second fixing member B 3 . Here, washers are used as the spacers  7 , for example, and the holes  7   h  formed in centers of the spacers  7  may be as large as or larger than the through holes Bh. In cases including the case where washers are used as the spacers  7 , too, it may be preferable in some embodiments for stable fixing with the screws that the washers occupy 90% or more (in angle) of areas around the threaded holes Ah used for the fixing with the screws  9 . 
         [0058]    With this image display element holding mechanism HK, since the first fixing member A 3  and the second fixing member B 3  do not need to contact each other at portions other than the positions at which the first fixing member A 3  and the second fixing member B 3  contact each other via the spacers  7 , distortion of the first fixing member A 3  is reduced, and thus no stress is generated to peel off the adhesive agent  3   a.  That is, when the screws  9  are tightened, surfaces of the first fixing member A 3  and the second fixing member B 3  are not in contact with each other between the two spacers  7 , and thus no stress is applied to the adhesion surface  3   s  from the warped surface of the first fixing member A 3 . 
         [0059]    This makes distortion less likely to occur in the first fixing member A 3  in the adhesion area between the fixing positions, and as a result, it is possible to prevent bending stress generated by the tightening of the screws  9  between the first and second fixing members A 3  and B 3  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 3 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Moreover, the adhesive agent  3   a  is allowed to exert its original characteristics to hold the prism  3 , and this helps achieve a range of usable temperature wider than has conventionally been achieved. Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0060]    As in the first and second embodiments described above, in order to prevent a surface of the first fixing member A 3  and a surface of the second fixing member B 3  that face each other from contacting each other at positions other than the positions at which the first and second fixing members A 3  and B 3  are fixed to each other with the screws  9 , it may be preferable that the spacers  7  have a thickness equal to or larger than a sum of flatnesses of the surface of the first fixing member A 3  and the surface of the second fixing member B 3  that face each other. For example, when the flatnesses of facing surfaces of the first and second fixing members A 3  and B 3  are 0.1 mm and 0.05 mm, respectively, it may be preferable that the thickness of the spacers  7  be 0.15 mm or larger. 
         [0061]    Still another embodiment of the prism unit PU is shown in  FIGS. 2 and 8 . The prism unit PU is formed by integrating three image display elements  4  respectively corresponding to the colors of RGB and the prism  3  together with three sets of image display element holding mechanisms HK for holding the image display elements  4  on the prism  3 . Each of the image display element holding mechanisms HK has two pairs of a first fixing member A 4  ( FIG. 8 ) that is fixed to the prism  3  with an adhesive agent, and the second fixing member B 1  ( FIG. 2 ,  FIG. 8 ) that is fixed to the first fixing member A 4  with the screws  9  and to which one of the image display elements  4  is indirectly fixed. One of the two pairs of the first fixing member A 4  and the second fixing member B 1  is disposed on an upper part of the prism  3 , and the other pair is disposed on a lower part of the prism  3 . Here, the fixing with the screws  9  is achieved, as shown in  FIG. 8 , by using the threaded holes Ah formed in the first fixing member A 4 , and the through holes Bh formed in the second fixing member B  1 . 
         [0062]    The prism  3  is a component formed of a plurality of glass prisms joined with each other, and the first fixing member A 4  as a mechanical component is fixed with an adhesive agent to a glass surface of a color prism portion of the prism  3  ( FIG. 8 ). The image display element  4  is fixed to the third fixing member C 1  with an adhesive agent ( FIG. 2 ), and the third fixing member C 1  is fixed with respect to the second fixing member B 1  with the fastening members  11 , which are each disposed with one of pins  10  of the second fixing member B 1  penetrating therethrough ( FIG. 2 ). Specifically, the fastening members  11  fasten the pins  10  of the second fixing member B 1  and the third fixing member C 1  to each other via a UV adhesive agent. Here, the fixing of the image display element  4  with respect to the second fixing member B 1  is achieved indirectly via the third fixing member C 1 , but the fixing may be achieved in a simpler manner by fixing the image display element  4  directly with respect to the second fixing member B 1  with an adhesive agent. 
         [0063]    As has been described above, each of the image display element holding mechanisms HK for holding the image display elements  4  is composed of the first fixing members A 4  ( FIG. 8 ) to which the prism  3  is bonded with an adhesive agent, the third fixing member C 1  ( FIG. 8 ) to which the image display element  4  is bonded with an adhesive agent, and the second fixing members B 1  ( FIG. 2 ,  FIG. 8 ) that are fixed with respect to the first and third fixing members A 4  and C 1  with the screws  9  and the fastening members  11 , respectively. The fixing with screws achieved via the second fixing members B 1  in this manner makes it possible to perform, for example, relative position adjustment between the prism  3  and the image display element  4  with ease as will be described below. 
         [0064]    In the prism unit PU, in forming one image by superimposing images each corresponding to one of the colors of RGB that have been separated from each other, it is important to align the images of the colors of RGB with each other. Even if the alignment is performed accurately, an adhesive agent often shrinks as it hardens during the fixing, and the shrinking of the adhesive agent invites displaced adhesive fixing. If the members joining the prism  3  and the image display element  4  to each other are all fixed with an adhesive agent, it is impossible to finely adjust the positional relationship between the prism  3  and the image display element  4  afterwards. 
         [0065]    With this in mind, to make it possible to finely adjust the positional relationship between the prism  3  and the image display element  4  even after the adhesive fixing of the prism  3  to the first fixing member A 4  and the adhesive fixing of the image display element  4  to the third fixing member C 1 , the second fixing member B 1  is used as a component for joining the prism  3  and the image display element  4  to each other with the screws  9  and the fastening members  11 . Moreover, the image display element  4  is detachable together with the third fixing member C 1  to facilitate replacement of the image display element  4  if it malfunctions and needs to be replaced. 
         [0066]    However, when fixing the second fixing member B 1  to the first fixing member A 4  with the screws  9 , it is necessary to prevent axial force, which is generated when the screws  9  are fastened, from adversely affecting the adhesion surface of the prism  3  with respect to the first fixing member A 4 . If there is a warp or poor flatness of the contact surfaces of the first fixing member A 4  and the second fixing member B 1 , the fastening of the screws  9  causes one of the adhesion surfaces to move toward the other for full contact with the other, and this generates bending stress. The bending stress invites a crack or a chip in the prism  3  at the adhesion surface, or peels off the adhesive agent to cause degradation in strength of adhesion between the prism  3  and the first fixing member A 4 . 
         [0067]    To prevent this, according to a fourth embodiment of the image display element holding mechanism HK, the first fixing member A 4  is provided with gaps (through holes)  16  as shown in  FIGS. 9A, 9B, and 9C .  FIG. 9A  shows the prism  3  and the first fixing member A 4  in section as viewed from above, and  FIG. 9C  shows the prism  3  and the first fixing member A 4  in section as viewed from the front side.  FIG. 9B  shows a principal part P in  FIG. 9C  in an enlarged manner. Buffer portions  16   a  ( FIG. 9B ) are provided in the first fixing member A 4  for absorbing bending stress generated by tightening of the screws  9  when fixing the second fixing member B 1  to the first fixing member A 4  with the screws  9  ( FIG. 8 ). The first fixing member A 4  has the gaps  16  each disposed between an adhesion surface  3   s  with respect to the prism  3  (that is, an area where the adhesive agent  3   a  is applied) and one of positions at which the tightening of the screws  9  is received (around the threaded holes Ah), and the buffer portions  16   a  are formed by providing the gaps  16 . 
         [0068]    Without the gaps  16  in the first fixing member A 4 , the first fixing member A 4  would have a uniform shape, and thus the bending stress would be applied in a concentrated manner to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 4 . In contrast, with the image display element holding mechanism HK of the present embodiment, the buffer portions  16   a  located one at a root of the U-shaped gaps  16  are liable to be distorted when the screws  9  are tightened, and thus the bending stress is applied to the buffer portions  16   a  in a concentrated manner, and the buffer portions  16   a  bend and thereby absorb the bending stress. This happens because the buffer portions  16   a  each having a small width are the lowest in strength in areas around the threaded holes Ah. 
         [0069]    This makes it possible to prevent the bending stress generated by the tightening of the screws  9  between the first and second fixing members A 4  and B 1  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 4 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0070]    It may be preferable in some embodiments that the width of the buffer portions  16   a  at the roots of the U-shaped gap  16  be equal to or smaller than the thickness of the first fixing member A 4 . For example, when the thickness of the first fixing member A 4  is 3 mm it may be preferable in some embodiments that the width of the buffer portions  16   a  be 3 mm or smaller. If the width of the buffer portions  16   a  is larger than the thickness of the first fixing member A 4 , the bending stress is less likely to be absorbed by the buffer portions  16   a.  It may also be preferable in some embodiments for the buffer portions  16   a  to bend within a range of elastic deformation to absorb the bending stress. 
         [0071]    Axial force Ff generated when a screw is tightened is expressed by the following formula: Ff=T/(K×d) (where T represents tightening torque (N·m); K represents a torque coefficient; and d represents a nominal diameter of the screw (mm)) For example, when an M3 screw (d=3) with a torque coefficient K of 0.2 is tightened with force with a tightening torque T of 0.63(N·m), the axial force Ff generated in the vicinity of a screw hole Ah is 1050N. Here, even when the axial force Ff is applied to areas around the threaded holes Ah, the absorption of the stress by the buffer portions  16   a  helps reduce transfer of the stress to the adhesion surface  3   s.    
         [0072]    By tightening the screws  9  against biasing force between the first and second fixing members A 4  and B  1 , it is possible to adjust the relative position of the image display element  4  with respect to the prism  3 . The screws  9  fix the first fixing member A 4  and the second fixing member B 1  to each other by being inserted through the through holes Bh (FIG.  8 ) formed in the second fixing member B 1  into screw-engagement with the threaded holes Ah formed in the first fixing member A 4 . In the above fixing achieved with the screws, for example, by arranging an elastic member (not shown) such as a spring or the like between the first and second fixing members A 4  and B 1  such that the screws  9  are tightened against biasing force between the first and second fixing members A 4  and B 1 , pushing and pulling between the screws  9  and the elastic member helps finely adjust the relative position of the image display element  4  with respect to the prism  3  in an optical-axis direction. 
         [0073]    An example of a material of the first fixing member A 4  and the second fixing member B 1  is SUS430. In some embodiments, SUS430 may be preferable as the material in view of relationship with prism materials in terms of elasticity, linear expansion coefficient, etc. The adhesive agent  3   a,  which is used to fix the prism  3  and the first fixing member A 4  to each other, may preferably be an epoxy resin adhesive agent in some embodiments. 
         [0074]      FIGS. 10A, 10B, and 10C  show a fifth embodiment of the image display element holding mechanism HK.  FIG. 10A  shows the prism  3  and a first fixing member A 5  in section as viewed from above, and  FIG. 10C  shows the prism  3  and the first fixing member A 5  in section as viewed from the front side.  FIG. 10B  shows a principal part Q in  FIG. 10C  in an enlarged manner. The present embodiment is different from the fourth embodiment ( FIG. 9 ) in that the first fixing member A 5  has cuts  17  each disposed between an adhesion surface  3   s  with respect to the prism  3  and one of positions at which the tightening of the screws  9  is received, and buffer portions  17   a  are formed by providing the cuts  17 . 
         [0075]    Buffer portions  17   a  are provided in the first fixing member A 5  for absorbing bending stress generated by tightening of the screws  9  when fixing the second fixing member B 1  (the same as the one in the forth embodiment shown in  FIG. 8 ) to the first fixing member A 5  with the screws  9 . The cuts  17  formed in the first fixing member  5  are each disposed between the adhesion surface  3   s  with respect to the prism  3  (that is, an area where the adhesive agent  3   a  is applied) and one of positions at which the screws  9  are tightened (around the threaded holes Ah), and the buffer portions  17   a  are formed by providing the cuts  17 . 
         [0076]    Without the cuts  17  in the first fixing member A 5 , the first fixing member A 5  would have a uniform shape, and thus the bending stress would be applied in a concentrated manner to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 5 . In contrast, according to the image display element holding mechanism HK of the present embodiment, the buffer portions  17   a  located one between roots of two adjacent ones of the cuts  17  are liable to be distorted when the screws  9  are tightened, and thus the bending stress is applied to the buffer portions  17   a  in a concentrated manner, and the buffer portions  17   a  bend and thereby absorb the bending stress. This happens because the buffer portions  17   a  forming boundaries with respect to the adhesion surface  3   s  are the lowest in strength in areas around the threaded holes Ah. 
         [0077]    Consequently, it is possible to prevent the bending stress generated by the tightening of the screws  9  between the first and second fixing members AS and B 1  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 4 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0078]      FIGS. 11A, 11B, and 11C  show a sixth embodiment of the image display element holding mechanism HK.  FIG. 11A  shows the prism  3  and a first fixing member A 6  in section as viewed from above, and  FIG. 11C  shows the prism  3  and the first fixing member A 6  in section as viewed from the front side.  FIG. 11B  shows a principal part R in  FIG. 11A  in an enlarged manner. The present embodiment is different from the fourth embodiment ( FIGS. 9A, 9B, 9C ) in that the first fixing member A 6  has recesses  18  each disposed between an adhesion surface  3   s  with respect to the prism  3  and one of positions at which the tightening of the screws  9  is received, and buffer portions  18   a  are formed as thin-walled portions by providing the recesses  18 . 
         [0079]    The buffer portions (thin-walled portions)  18   a  are provided in the first fixing member A 6  for absorbing bending stress generated by tightening of the screws  9  when fixing the second fixing member B 1  (the same as the one in the fourth embodiment shown in  FIG. 8 ) to the first fixing member A 6  with the screws  9 . The first fixing member A 6  has the thin-walled portions each disposed between the adhesion surface  3   s  with respect to the prism  3  (that is, an area where the adhesive agent  3   a  is applied) and one of positions at which the tightening of the screws  9  is received (around the threaded holes Ah), and the thin-walled portions are formed by providing the recesses  18 , and the buffer portions  18   a  are constituted by the thin-walled portions. 
         [0080]    Without the recesses  18  in the first fixing member A 6 , the first fixing member A 6  would have a uniform shape, and thus the bending stress would be applied in a concentrated manner to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 6 . In contrast, according to the image display element holding mechanism HK of the present embodiment, the buffer portions  18   a  located one at a root of each of the recesses  18  are liable to be distorted when the screws  9  are tightened, and thus the bending stress is applied to the buffer portions  18   a  in a concentrated manner, and the buffer portions  18   a  bend and thereby absorb the bending stress. This happens because the thin-walled buffer portions  18   a  are the lowest in strength in areas around the threaded holes Ah. 
         [0081]    This makes it possible to prevent the bending stress generated by the tightening of the screws  9  between the first and second fixing members A 6  and B 1  from being applied to the adhesion surface  3   s  of the prism  3  with respect to the first fixing member A 6 . As a result, it is possible to prevent generation of a crack in the prism  3 , degradation of adhesion strength of the prism  3 , etc. from being caused by the bending stress generated by the tightening of the screws  9 . Furthermore, by providing the prism unit PU with the image display element holding mechanism HK, it is possible to keep the positional relationship between the prism  3  and the image display element  4  in an accurate and stable manner, and by providing a projector PJ with the prism unit PU, it is possible to achieve a high-quality image projection. 
         [0082]    It may be preferable in some embodiments that the thickness of the buffer portions  18   a  that absorb the bending stress by being formed as thin-walled portions be equal to or smaller than one-third of a thickness of the first fixing member A 6 . For example, when the thickness of the first fixing member A 6  is 3 mm, it may be preferable in some embodiments that the thickness of the buffer portions  18   a  be 1 mm or smaller. If the thickness of the buffer portions  18   a  is larger than the thickness of the first fixing member A 6 , the bending stress is less likely to be absorbed by the buffer portions  18   a.  It may also be preferable in some embodiments for the buffer portions  18   a  to bend within a range of elastic deformation to absorb the bending stress. 
         [0083]    Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
         PJ projector 
         PU prism unit 
         HK holding mechanism 
           1  light source 
           2  illumination optical system 
           3  prism 
           3   a  adhesive agent 
           3   s  adhesion surface 
           4  image display element 
           5  projection lens system 
           6   a,    6   b  protrusion 
           7  spacer (protrusion) 
           9  screw 
           10  pin 
           11  fastening member 
           16  gap (through hole) 
           16   a  buffer portion 
           17  cut 
           17   a  buffer portion 
           18  recess 
           18   a  buffer portion (thin-walled portion) 
         Ah threaded hole 
         Bh through hole 
         A 1 ˜A 6  first fixing member 
         B 1 ˜B 3  second fixing member 
         C 1  third fixing member