Abstract:
A projection display device includes a light source; an optical system which modulates light from the light source, and emits the modulated light; a projecting portion which enlarges and projects the light emitted from the optical system; a cabinet which houses the light source, the optical system, and the projecting portion; an opening which is formed in one surface of the cabinet to allow mounting or dismounting of an optical component constituting the optical system, or the light source; a cover which covers the opening; and a holding portion which holds the cover in such a manner that the cover is allowed to slide in a direction along the one surface.

Description:
This application claims priority under 35 U.S.C. Section 119 of a Japanese Patent Application No. 2009-225593 filed Sep. 29, 2009, entitled “PROJECTION DISPLAY DEVICE”. The disclosures of the above application are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a projection display device for modulating light from a light source, and enlarging and projecting the modulated light. 
     2. Disclosure of Related Art 
     Generally, a lamp such as a metal halide lamp or a high pressure mercury lamp is used as a light source in a projection display device such as a liquid crystal projector. These lamps are deteriorated due to a long-time operation. In the case where it is impossible to obtain a proper luminance due to deterioration of the lamp, it is necessary to replace the lamp with a new one. 
     In the projection display device, there is proposed an arrangement, wherein a cabinet is formed with an opening through which a light source is mounted or dismounted so that the light source can be replaced without disassembling the cabinet. The opening is opened and closed by a cover. 
     In the projection display device, there is proposed an arrangement, wherein a cover is completely detached from a cabinet in opening the cover. In this arrangement, it is necessary to place the detached cover in a position where replacement of alight source is not obstructed, which requires a cumbersome operation. In view of the above, there is proposed e.g. an arrangement, wherein a cover is interconnected to a cabinet by a hinge so that the cover can be opened in a state that the cover is mounted on the cabinet. 
     In the above arrangement, however, it is impossible to sufficiently open the cover, if there is not a sufficient space on the outside of a surface of the cabinet where the cover is mounted, and the cover may obstruct mounting or dismounting of the light source. 
     In the projection display device, an optical component other than the light source may be replaced. Accordingly, it is desirable to configure an arrangement, wherein an optical component other than a light source can be easily replaced, as well as the light source. 
     SUMMARY OF THE INVENTION 
     A projection display device according to a main aspect of the invention includes a light source; an optical system which modulates light from the light source, and emits the modulated light; a projecting portion which enlarges and projects the light emitted from the optical system; a cabinet which houses the light source, the optical system, and the projecting portion; an opening which is formed in one surface of the cabinet to allow mounting or dismounting of an optical component constituting the optical system, or the light source; a cover which covers the opening; and a holding portion which holds the cover in such a manner that the cover is allowed to slide in a direction along the one surface. 
     In the projection display device according to the main aspect of the invention, the opening can be opened by sliding the cover with respect to the holding portion. Accordingly, the opening can be sufficiently opened, even if the space defined on the outside of the cover is small in the state that the projection display device is installed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, and novel features of the present invention will become more apparent upon reading the following detailed description of the embodiment along with the accompanying drawings. 
         FIGS. 1A and 1B  are diagrams showing an arrangement of a projector embodying the invention. 
         FIGS. 2A and 2B  are diagrams showing an inner structure of the projector embodying the invention. 
         FIG. 3  is a diagram showing an arrangement of an optical system in the embodiment. 
         FIGS. 4A and 4B  are diagrams showing an arrangement of a prism unit in the embodiment. 
         FIG. 5  is a diagram showing an arrangement of an attachment frame on which the prism unit in the embodiment is mounted. 
         FIGS. 6A and 6B  are diagrams showing a state that the prism unit in the embodiment is fixedly mounted on the attachment frame. 
         FIGS. 7A and 7B  are diagrams showing an arrangement of a cooling device of the optical system in the embodiment. 
         FIGS. 8A and 8B  are diagrams showing an arrangement of an upper casing and first through third ducts in the embodiment. 
         FIGS. 9A and 9B  are diagrams showing an arrangement of a lower casing and fourth through sixth ducts in the embodiment. 
         FIG. 10A  is a perspective and elevational sectional view of a lamp unit in the embodiment, when viewed from a rear of the lamp unit. 
         FIG. 10B  is a perspective and transverse sectional view of the lamp unit in the embodiment, when viewed from a front of the lamp unit. 
         FIG. 11  is a perspective view of the lamp unit in the embodiment, when viewed from the front of the lamp unit. 
         FIG. 12A  is a left side view of the lamp unit in the embodiment. 
         FIG. 12B  is a rear view of the lamp unit in the embodiment. 
         FIG. 13A  is a top plan view of the lamp unit in the embodiment. 
         FIG. 13B  is a bottom plan view of the lamp unit in the embodiment. 
         FIG. 14  is a front view showing a state that the lamp unit in the embodiment is connected to a fan unit. 
         FIG. 15  is a perspective view of an upper cabinet in a state that a prism cover and a lamp cover in the embodiment are detached. 
         FIGS. 16A and 16B  are diagrams showing an arrangement of the prism cover in the embodiment. 
         FIGS. 17A and 17B  are diagrams showing an arrangement of the lamp cover in the embodiment. 
         FIGS. 18A and 18B  are cross-sectional views showing essential parts of the upper cabinet in a state that the prism cover and the lamp cover are mounted on the upper cabinet in the embodiment. 
         FIGS. 19A and 19B  are diagrams showing a state that the lamp cover in the embodiment is opened. 
         FIG. 20  is a cross-sectional view showing essential parts of the upper cabinet for describing an operation to be performed for the lamp cover, in the case where the lamp cover in the embodiment is opened. 
         FIGS. 21A and 21B  are diagrams showing a state that the prism cover in the embodiment is opened. 
     
    
    
     The drawings are provided mainly for describing the present invention, and do not limit the scope of the present invention. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following, an embodiment of the invention is described referring to the drawings. 
     In this embodiment, a projection lens  5  corresponds to a “projecting portion” in the claims. A prism cover  10  corresponds to a “cover” and a “second cover” in the claims. A lamp cover  11  corresponds to a “cover” and a “first cover” in the claims. A lamp unit  13  corresponds to a “light source” in the claims. A prism unit  16  corresponds to an “optical component” in the claims. A prism opening  602  corresponds to an “opening” and a “second opening” in the claims. A lamp opening  606  corresponds to an “opening” and a “first opening” in the claims. Guide ribs  603  correspond to a “holding portion” and a “second holding portion” in the claims. Guide grooves  609  and guide holes  609   a  correspond to a “holding portion” and a “first holding portion” in the claims. The description regarding the correspondence between the claims and the embodiment is merely an example, and the claims are not limited by the description of the embodiment. 
     Entire Arrangement of Projector 
       FIGS. 1A and 1B  are diagrams showing an arrangement of a projector.  FIG. 1A  is a perspective view of the projector when viewed from a front of the projector.  FIG. 1B  is a perspective view of the projector when viewed from a rear of the projector. 
     Referring to  FIGS. 1A and 1B , the projector includes a cabinet  1  having a substantially rectangular parallelepiped shape with a longer size in left and right directions. The cabinet  1  is constituted of a lower cabinet  2  with an upper surface thereof being opened, and an upper cabinet  3  for covering the upper surface of the lower cabinet  2 . 
     A projection opening  4  is formed in a central part on a front surface of the lower cabinet  2 . A front portion of a projection lens  5  is exposed through the projection opening  4 . 
     A left side surface of the lower cabinet  2  is constituted of an air inlet cover  6 , except for a front end and a rear end of the left side surface. The air inlet cover  6  has a hinge structure (not shown) at a lower end thereof, and is pivotally opened in the left direction about the lower end (see  FIGS. 2A and 2B ). An air inlet  7  is formed in the air inlet cover  6 . The air inlet  7  is constituted of a large number of slit holes. 
     An exhaust port  8  is formed in a right rear corner of the lower cabinet  2 . The exhaust port  8  is constituted of a large number of slit holes. An AV terminal portion  9  is formed on a rear surface of the lower cabinet  2 , and an AV (Audio Visual) signal is inputted from the AV terminal portion  9 . 
     The upper cabinet  3  has a prism cover  10  and a lamp cover  11 . The prism cover  10  is a cover for covering a prism opening formed in the upper cabinet  3 . The prism opening is used for e.g. replacement of a prism unit or adjustment of a polarizer. The lamp cover  11  is a cover for covering a lamp opening formed in the upper cabinet  3 . The lamp opening is used for replacement of a lamp unit. Upper surfaces of the prism cover  10  and the lamp cover  11  are formed flush with an upper surface of the upper cabinet  3 . An attachment structure as to how the prism cover  10  and the lamp cover  11  are attached to the upper cabinet  3  will be described later. 
     An indicator portion  12  is formed on a right-side front end of the upper cabinet  3 . The indicator portion  12  has plural LEDs. A user is notified of whether the projector is in an operation state or in a standby state, or notified of various error statuses by on/off states of the respective LEDs. For instance, the indicator portion  12  may notify the user of a timing when the lamp unit is to be replaced. 
       FIGS. 2A and 2B  are diagrams showing an inner structure of the projector.  FIG. 2A  is a perspective view of the projector in a state that the upper cabinet  3  is detached.  FIG. 2B  is a perspective view of the projector in a state that a control circuit board  26 , the AV terminal portion  9 , and an air inlet member  22  are detached from the state shown in  FIG. 2A . 
     Referring to  FIG. 2B , the lower cabinet  2  is internally provided with a lamp unit  13 , and an optical system  14  for modulating light from the lamp unit  13  to generate image light. 
     The lamp unit  13  is disposed at a central part on a right side surface of the lower cabinet  2  in such a manner that the lamp unit  13  is detachably attached from above. The lamp unit  13  is constituted of a light source lamp  300 , and a lamp holder  400  for holding the light source lamp  300  (see  FIGS. 10A and 10B ). A fan unit  15  is disposed in front of the lamp unit  13 . The fan unit  15  supplies an air to cool the light source lamp  300 . The lamp holder  400  is formed with an air duct through which the cooling air from the fan unit  15  is guided to the light source lamp  300 . The detailed arrangement of the lamp unit  13  will be described later. 
     The optical system  14  is disposed on the left of the lamp unit  13  and in a central part of the lower cabinet  2 . The optical system  14  includes a prism unit  16 . The prism unit  16  is disposed inside the lower cabinet  2  in such a manner that the prism unit  16  is detachable from above. The detailed arrangement of the optical system  14  will be described later. 
     A lens shift unit  17  is disposed in front of the optical system  14 . The projection lens  5  is mounted on the lens shift unit  17 . The projection lens  5  enlarges image light generated by the optical system  14 , and projects the enlarged image light onto a projection plane such as a screen. The lens shift unit  17  shifts the projection lens  5  in up and down directions and left and right directions by using a driving force of a motor. By performing the above operation, the position of a projected image can be adjusted. 
     A power source unit  18  is disposed behind the optical system  14 . The power source unit  18  has a power source circuit, and supplies a power source to each of the electrical components of the projector. A lamp ballast  19  is disposed at an upper portion of the power source unit  18 . The lamp ballast  19  converts a power source supplied from the power source unit  18  into a power source suitable for the light source lamp  300 , and supplies the converted power source to the light source lamp  300 . 
     The lower cabinet  2  is further internally provided with a cooling device  20 . The cooling device  20  has six cooling fans, and supplies the external air drawn in through the air inlet  7  to the exothermic components of the optical system  14  such as the prism unit  16  to cool the exothermic components. The detailed arrangement of the cooling device  20  will be described later. 
     A power source cooling fan  21  is disposed on the left of the power source unit  18  and the lamp ballast  19 . The power source cooling fan  21  supplies an air to the power source unit  18  and the lamp ballast  19  to cool the power source unit  18  and the lamp ballast  19 . An axial fan is used as the power source cooling fan  21 , for example. 
     Next, referring to  FIG. 2A , the air inlet member  22  is mounted on a left side portion of the lower cabinet  2 . The air inlet member  22  is constituted of a frame member  23 , and a filter member  24  mounted on the frame member  23 . An air inlet (not shown) is formed in a surface of the frame member  23  opposing to the filter member  24 . The filter member  24  is covered by the air inlet cover  6 . In replacing the filter member  24 , the air inlet cover  6  is opened, and the filter member  24  is detached from the frame member  23 . 
     An air flow velocity sensor (not shown) is disposed in the air inlet member  22  at a position downstream of the filter member  24 . A determination is made as to whether the filter member  24  is clogged, based on an air flow velocity to be detected by the air flow velocity sensor, and the user is notified of whether the filter member  24  is clogged by e.g. the indicator portion  12 . 
     In response to activation of e.g. the lamp unit  13 , the cooling device  20 , and the power source cooling fan  21 , the external air is drawn in through the air inlet  7  of the air inlet cover  6 , the filter member  24 , and the air inlet of the frame member  23 . 
     An exhaust fan  25  is disposed at the right rear corner of the lower cabinet  2 . The exhaust fan  25  is disposed in oblique direction with respect to the right side surface and the rear surface of the lower cabinet  2 , and an intake surface of the exhaust fan  25  is directed obliquely leftward in the front direction. An axial fan is used as the exhaust fan  25 , for example. 
     In response to activation of the exhaust fan  25 , as shown in  FIG. 2B , the cooling air which has cooled the power source unit  18  and the lamp ballast  19  is drawn in toward the exhaust fan  25  in the direction from the left side, and the cooling air which has cooled the light source lamp  300  and exited the lamp unit  13  is drawn in toward the exhaust fan  25  in the direction from the front side. Further, the cooling air which has cooled the optical system  14  is drawn in toward the exhaust fan  25  obliquely leftward from the front direction. In performing the above operation, since the intake surface of the exhaust fan  25  is directed obliquely leftward in the front direction, the cooling air to be supplied in the three directions i.e. from the side of the lamp unit  13 , the side of the power source unit  18 , and the side of the optical system  14  is easily drawn in toward the exhaust fan  25 . Thus, the above arrangement enables to smoothly discharge the cooling air which has cooled the exothermic components to the exterior of the projector, thereby advantageously cooling the exothermic components. 
     Further, since the air inlet  7  is formed in aside surface (the left side surface) opposite to the position where the exhaust fan  25  is disposed, the external air drawn in through the air inlet  7  is drawn in toward the exhaust fan  25 , after having been sufficiently used for cooling the lamp unit  13 , the power source unit  18 , and the optical system  14 . Thus, the arrangement is further advantageous in cooling the exothermic components. 
     Furthermore, the cooling air to be supplied in the three directions is not discharged immediately after exiting the exhaust fan  25 , but is discharged after having been sufficiently mixed in a space between an exhaust surface of the exhaust fan  25  and a corner of the lower cabinet  2 . The light source lamp  300  is heated to an exceedingly high temperature, as compared with the power source unit  18  or a like member. Accordingly, the cooling air from the side of the lamp unit  13  is heated to an exceedingly high temperature, as compared with the cooling air in the other directions. However, as described above, since the cooling air which has been drawn in toward the exhaust fan  25  from the side of the lamp unit  13  is discharged after having been sufficiently mixed with the cooling air in the other directions, the temperature of the discharged air can be lowered. 
     Furthermore, since the exhaust fan  25  is disposed in oblique direction, it is possible to dispose a largest possible exhaust fan in a limited space enclosed by the corner of the lower cabinet  2 , the lamp unit  13 , and the power source unit  18 . 
     Furthermore, since the exhaust port  8  is formed in the corner of the lower cabinet  2 , it is possible to increase the opening area of the exhaust port  8 . Thus, a more smooth air discharge operation can be performed. 
     The exhaust fan  25  may be disposed at a corner other than the right rear corner, depending on the dispositions of the respective constituent components in the lower cabinet  2 . 
     As shown in  FIG. 2A , the control circuit board  26  is disposed above the optical system  14  and the power source unit  18 . The control circuit board  26  is provided with a control circuit for controlling driving components such as liquid crystal panels and the light source lamp  300 . The control circuit board  26  is cut away at a position above the prism unit  16 . In this arrangement, the prism unit  16  is detachably attached from above in a state that the control circuit board  26  is mounted. 
     Arrangement of Optical System 
       FIG. 3  is a diagram showing an arrangement of the optical system  14 . 
     White light emitted from the light source lamp  300  is transmitted through a condenser lens  101 , a fly-eye integrator  102 , and a PBS array  103 . The fly-eye integrator  102  makes a light amount distribution of light of each of the colors to be irradiated to liquid crystal panels (which will be described later) uniform. The PBS array  103  aligns polarization directions of light of the respective colors toward a dichroic mirror  105  in one direction. 
     Light transmitted through the PBS array  103  is transmitted through a condenser lens  104 , and entered into the dichroic mirror  105 . 
     The dichroic mirror  105  reflects only light (hereinafter, called as “B light”) in a blue wavelength band, and transmits light (hereinafter, called as “G light”) in a green wavelength band and light (hereinafter, called as “R light”) in a red wavelength band, out of the light entered into the dichroic mirror  105 . 
     B light reflected on the dichroic mirror  105  is irradiated onto a liquid crystal panel  108  for B light in a proper irradiation state by a lens function by the condenser lens  104  and a condenser lens  106 , and reflection on a reflection mirror  107 . The liquid crystal panel  108  is driven in accordance with an image signal for B light to modulate the B light depending on a driven state of the liquid crystal panel  108 . One incident-side polarizer  109  is disposed on the incident side of the liquid crystal panel  108 . B light is irradiated onto the liquid crystal panel  108  through the incident-side polarizer  109 . Further, two output-side polarizers  110  are disposed on the output side of the liquid crystal panel  108 , and B light emitted from the liquid crystal panel  108  is entered into the output-side polarizers  110 . 
     G light and R light transmitted through the dichroic mirror  105  are entered into a dichroic mirror  111 . The dichroic mirror  111  reflects the G light and transmits the R light. 
     G light reflected on the dichroic mirror  111  is irradiated onto a liquid crystal panel  113  for G light in a proper irradiation state by a lens function by the condenser lens  104  and a condenser lens  112 . The liquid crystal panel  113  is driven in accordance with an image signal for G light to modulate the G light depending on a driven state of the liquid crystal panel  113 . One incident-side polarizer  114  is disposed on the incident side of the liquid crystal panel  113 , and G light is irradiated onto the liquid crystal panel  113  through the incident-side polarizer  114 . Further, two output-side polarizers  115  are disposed on the output side of the liquid crystal panel  113 , and G light emitted from the liquid crystal panel  113  is entered into the output-side polarizers  115 . 
     R light transmitted through the dichroic mirror  111  is irradiated onto a liquid crystal panel  121  for R light in a proper irradiation state by a lens function by the condenser lens  104 , a condenser lens  116 , and relay lenses  117  and  118 , and reflection on reflection mirrors  119  and  120 . The liquid crystal panel  121  is driven in accordance with an image signal for R light to modulate the R light depending on a driven state of the liquid crystal panel  121 . One incident-side polarizer  122  is disposed on the incident side of the liquid crystal panel  121 , and R light is irradiated onto the liquid crystal panel  121  through the incident-side polarizer  122 . Further, two output-side polarizers  123  are disposed on the output side of the liquid crystal panel  121 , and R light emitted from the liquid crystal panel  121  is entered into the output-side polarizers  123 . 
     B light, G light, and R light modulated by the liquid crystal panels  108 ,  113 , and  121  are transmitted through the output-side polarizers  110 ,  115 , and  123 , and entered into a dichroic prism  124 . The dichroic prism  124  reflects B light and R light, and transmits G light, out of the B light, the G light, and the R light, to thereby combine the B light, the G light, and the R light. Thus, image light after the color combination is projected toward the projection lens  5  from the dichroic prism  124 . 
     An imager constituting the optical system  14  may be a reflective liquid crystal panel or an MEMS device, in place of the transmissive liquid crystal panels  108 ,  113 , and  121 . Further, the optical system  14  may be constituted of e.g. a single-panel optical system incorporated with an imager and a color wheel, in place of the three-panel optical system incorporated with three imagers as described above. 
     Attachment Structure of Prism Unit 
       FIGS. 4A and 4B  are diagrams showing an arrangement of the prism unit  16 .  FIG. 4A  is a perspective view of the prism unit  16 , and  FIG. 4B  is a bottom plan view of the prism unit  16 . 
     The prism unit  16  is assembled into one unit by assembling the liquid crystal panels  108 ,  113 , and  121 , the output-side polarizers  110 ,  115 , and  123 , and the dichroic prism  124  on a prism holder  125 . The liquid crystal panels  108 ,  113 , and  121  are fixedly attached to the prism holder  125  via brackets  126 . 
     An attachment leg  127  is provided at three positions on a bottom portion of the prism holder  125 . Each of the attachment legs  127  is formed with an attachment hole  128  and a positioning hole  129 . Further, an insertion hole  130  is formed in a central part on a bottom surface of the prism holder  125 . An inwardly protruding annular flange portion  131  is formed at an entrance of the insertion hole  130 . 
       FIG. 5  is a perspective view showing an arrangement of an attachment frame  132  on which the prism unit  16  is mounted. 
     The attachment frame  132  on which the prism unit  16  is mounted is provided in the lower cabinet  2 . The attachment frame  132  is provided with three bosses  133  corresponding to the three attachment holes  128  of the prism holder  125 . The attachment frame  132  is further provided with positioning projections  134  corresponding to the three positioning holes  129  of the prism holder  125 . The attachment frame  132  is furthermore provided with a stopper pin  135  corresponding to the insertion hole  130 . 
       FIG. 6A  is a perspective view showing a state that the prism unit  16  is fixedly mounted on the attachment frame  132 .  FIG. 6B  is a cross-sectional view showing essential parts of the prism unit  16  in a state that the stopper pin  135  is received in the insertion hole  130 . 
     The prism unit  16  is placed on the attachment frame  132  in such a manner that the positioning projections  134  are received in the corresponding positioning holes  129 . Thereby, the attachment holes  128  of the prism holder  125  are aligned with the corresponding bosses  133 . In the alignment operation, the stopper pin  135  is fitted into the insertion hole  130  of the prism unit  16 . Then, by fastening the attachment legs  127  of the prism unit  125  to the bosses  133  by screws, the prism unit  16  is fixedly mounted on the attachment frame  132 . 
     As shown in  FIG. 6B , an engaging portion  136  which is flexed in the circumferential direction is formed at a lead end of the stopper pin  135 . By inserting the stopper pin  135  in the insertion hole  130 , the engaging portion  136  is engaged with the flange portion  131 . In this state, there is no likelihood that the stopper pin  135  may come out of the insertion hole  130 , even if a force substantially equal to the weight of the prism unit  16  is exerted in a direction of disengaging the stopper pin  135  from the insertion hole  130 . 
     The installation manner of a projector includes a ceiling mount, wherein a projector is suspended from a ceiling, in addition to a fixed mount, wherein a projector is mounted on a floor surface or a desk surface. In the case of the ceiling mount, a projector is mounted upside down. 
     In this embodiment, in the case where a projector is suspended from a ceiling, there is no likelihood that the prism unit  16  may come out of the attachment frame  132  by the weight thereof, even if screws are unfastened from the bosses  133 . Accordingly, mounting/dismounting operations of the prism unit  16  can be easily performed in replacing the prism unit  16 . 
     In this embodiment, the stopper pin  135  as an independent member is fixedly attached to the attachment frame  132 . Alternatively, a stopper portion formed with the engaging portion  136  may protrude from the attachment frame  132 . In the modification, the stopper portion may be integrally formed with the attachment frame  132 . 
     Arrangement of Cooling Device of Optical System 
       FIGS. 7A and 7B ,  FIGS. 8A and 8B , and  FIGS. 9A and 9B  are diagrams showing an arrangement of the cooling device  20  of the optical system  14 .  FIGS. 7A and 7B  are perspective views of the cooling device  20 . In  FIG. 7A , only the prism unit  16  and the PBS array  103  in the arrangement of the optical system  14  are shown together with the cooling device  20 .  FIGS. 8A and 8B  are respectively a top plan view and a bottom plan view of an upper casing  202 , a first duct  210 , a second duct  211 , and a third duct  212 .  FIGS. 9A and 9B  are respectively a top plan view and a bottom plan view of a lower casing  203 , a fourth duct  217 , a fifth duct  218 , and a sixth duct  219 . 
     The cooling device  20  has a fan casing  201 . The fan casing  201  is constituted of the upper casing  202  and the lower casing  203 . A rear surface and a bottom surface of each of the upper casing  202  and the lower casing  203  are opened. The lower casing  203  is mounted on the bottom surface of the lower cabinet  2 , and the upper casing  202  is mounted on the lower casing  203 . 
     The interior of the upper casing  202  is divided into three housing portions (a first housing portion  204 , a second housing portion  205 , and a third housing portion  206 ) by two partition walls  202   a  and  202   b . Likewise, the interior of the lower casing  203  is divided into three housing portions (a fourth housing portion  207 , a fifth housing portion  208 , and a sixth housing portion  209 ) by two partition walls  203   a  and  203   b.    
     The first housing portion  204 , the second housing portion  205 , and the third housing portion  206  of the upper casing  202  are respectively connected to the first duct  210 , the second duct  211 , and the third duct  212 . Each of the first duct  210 , the second duct  211 , and the third duct  212  has a bottom surface thereof opened, and extends to a position below the prism unit  16 . The first duct  210 , the second duct  211 , and the third duct  212  are integrally formed with the upper casing  202  by a resin material. 
     An air outlet  213  is formed in a lead end of the first duct  210 . The air outlet  213  is directed toward the incident-side polarizer  114  and the liquid crystal panel  113  for G light. A partition member  213   a  is formed in the middle of an exit of the air outlet  213  to guide the cooling air to each of the incident-side polarizer  114  and the liquid crystal panel  113 . An air outlet  214  is formed in a lead end of the second duct  211 . The air outlet  214  is directed toward the output-side polarizers  115  for G light. Two air outlets  215  and  216  are formed in a lead end of the third duct  212 . The air outlet  215  is directed to the incident-side polarizer  122  and the liquid crystal panel  121  for R light, and the air outlet  216  is directed to the output-side polarizers  123  for R light. 
     The fourth housing portion  207 , the fifth housing portion  208 , and the sixth housing portion  209  of the lower casing  203  are respectively connected to the fourth duct  217 , the fifth duct  218 , and the sixth duct  219 . Each of the fourth duct  217 , the fifth duct  218 , and the sixth duct  219  has a bottom surface thereof opened. The fourth duct  217  extends to a position below the PBS array  103 , and the fifth duct  218  and the sixth duct  219  each extends to a position below the prism unit  16 . The fourth duct  217 , the fifth duct  218 , and the sixth duct  219  are integrally formed with the lower casing  203  by a resin material. 
     An air outlet  220  is formed in a lead end of the fourth duct  217 . The air outlet  220  is directed toward the PBS array  103 . An air outlet  221  is formed in a lead end of the fifth duct  218 . The air outlet  221  is directed toward the incident-side polarizer  109  and the liquid crystal panel  108  for B light. An air outlet  222  is formed in a lead end of the sixth duct  219 . The air outlet  222  is directed to the output-side polarizers  110  for B light. Another air outlet  223  is formed adjacent to the air outlet  220  for the PBS array  103 . The air outlet  223  is communicated with a cooling fan (not shown) different from the cooling device  20 , and the cooling air from the cooling fan is drawn through the air outlet  223 . An air deflector  220   a  is provided at exits of the air outlets  220  and  223  to guide the air toward the PBS array  103 . 
     As shown in  FIG. 9A , bottom surface members  210   b ,  211   b , and  212   b  corresponding to the first duct  210 , the second duct  211 , and the third duct  212  are integrally formed on upper surfaces of the fourth duct  217 , the fifth duct  218 , and the sixth duct  219 . When the upper casing  202  is mounted on the lower casing  203 , the bottom surfaces of the first duct  210 , the second duct  211 , and the third duct  212  are closed by the corresponding bottom surface members  210   b ,  211   b , and  212   b , whereby an air duct having an airtight structure is formed. On the other hand, when the lower casing  203  is mounted on the lower cabinet  2 , the bottom surfaces of the fourth duct  217 , the fifth duct  218 , and the sixth duct  219  are closed by a bottom member (not shown) which is integrally formed with the lower cabinet  2 , whereby an air duct having an airtight structure is formed. 
     Cooling fans (first through sixth fans  224  through  229 ) are respectively disposed in the first through the sixth housing portions  204  through  209  of the fan casing  201 . As shown in  FIGS. 8B and 9B , air outlets  224   a  through  229   a  of the respective cooling fans  224  through  229  are connected to entrances  210   a  through  212   a  of the corresponding first through the third ducts  210  through  212 , and entrances  217   a  through  219   a  of the corresponding fourth through the sixth ducts  217  through  219 . Each of the cooling fans  224  through  229  is a centrifugal fan having the same performance, with both surfaces thereof being formed into intake surfaces. Air inlets  224   b  through  229   b  are formed on both ends of each of the cooling fans  224  through  229 . 
     Each of the first fan  224 , the second fan  225 , and the third fan  226  is fixedly fastened to an attachment boss  230  provided on the upper surface of the lower casing  203  by a screw. Likewise, each of the fourth fan  227 , the fifth fan  228 , and the sixth fan  229  is fixedly fastened to an attachment boss  231  provided on the bottom surface of the lower cabinet  2  by a screw. In a state that the first through the sixth fans  224  through  229  are fixedly attached to the attachment bosses  230  and  231 , a clearance for drawing in the external air is formed between upper end surface of each of the cooling fans  224  through  229  and the upper surface of each of the first through the sixth housing portions  204  through  209 , and a clearance for drawing in the external air is formed between the bottom surface of each of the cooling fans  224  through  229  and the bottom surface of each of the first through the sixth housing portions  204  through  209 . 
     As shown in  FIG. 2A , the rear surface of the fan casing  201  is covered by the air inlet member  22 . 
     In the above arrangement, in response to activation of the cooling fans  224  through  229 , the external air is drawn into the cabinet  1  through the air inlet member  22 . The drawn external air passes through the upper and lower clearances of the first through the sixth housing portions  204  through  209  from the rear of the fan casing  201 , and is drawn to the cooling fans  224  through  229  through the air inlets  224   b  through  229   b  formed in both end surfaces of each of the cooling fans  224  through  229 . 
     The cooling air from the first fan  224  passes through the first duct  210 , and is drawn in toward the incident-side polarizer  114  and the liquid crystal panel  113  for G light through the air outlet  213 . As a result of the above operation, the incident-side polarizer  114  and the liquid crystal panel  113  are cooled. The cooling air from the second fan  225  passes through the second duct  211 , and is drawn in toward the output-side polarizers  115  for G light through the air outlet  214 . As a result of the above operation, the output-side polarizers  115  are cooled. The cooling air from the third fan  226  passes through the third duct  212 , and is drawn in toward the incident-side polarizer  122  and the liquid crystal panel  121  for R light through the air outlet  215 , and is also drawn in toward the output-side polarizers  123  for R light through the air outlet  216 . As a result of the above operation, the incident-side polarizer  122 , the liquid crystal panel  121 , and the output-side polarizers  123  are cooled. 
     The cooling air from the fourth fan  227  passes through the fourth duct  217 , and is drawn in toward the PBS array  103  through the air outlet  220 . As a result of the above operation, the PBS array  103  is cooled. The cooling air from the fifth fan  228  passes through the fifth duct  218 , and is drawn in toward the incident-side polarizer  109  and the liquid crystal panel  108  for B light through the air outlet  221 . As a result of the above operation, the incident-side polarizer  109  and the liquid crystal panel  108  are cooled. The cooling air from the sixth fan  229  passes through the sixth duct  219 , and is drawn in toward the output-side polarizers  110  for B light through the air outlet  222 . As a result of the above operation, the output-side polarizers  110  are cooled. 
     In the prism unit  16 , concerning the light amounts to be absorbed at the time of modulation, the calorific value generated by a green imager (constituted of the liquid crystal panel  113 , the incident-side polarizer  114 , and the output-side polarizers  115 ) becomes largest, and the calorific value generated by a blue imager (constituted of the liquid crystal panel  108 , the incident-side polarizer  109 , and the output-side polarizers  110 ) becomes second largest. As compared with the calorific values generated by these imagers, the calorific value generated by a red imager (constituted of the liquid crystal panel  121 , the incident-side polarizer  122 , and the output-side polarizers  123 ) is small. As compared with the calorific values generated by the liquid crystal panels  108 ,  113 , and  121 , and the incident-side polarizers  109 ,  114 , and  122 , the calorific values generated by the output-side polarizers  110 ,  115 , and  123  are large. Thus, the calorific values are different from each other depending on the imagers. 
     In this embodiment, applied voltages to the second fan  225  and the third fan  226  are set equal to each other. This is because there is no significant difference in the required air volume between the second fan  225  and the third fan  226 . Further, applied voltages to the first fan  224 , the fifth fan  228 , and the sixth fan  229  are set equal to each other. This is because there is no significant difference in the required air volume between the first fan  224 , the fifth fan  228 , and the sixth fan  229 . 
     Further, applied voltages to the second fan  225 , the third fan  226 , and the fourth fan  227  are set higher than the applied voltages to the first fan  224 , the fifth fan  228  and the sixth fan  229 . The applied voltage to the second fan  225  is set high, because the calorific value generated by the output-side polarizers  115  for G light is largest, and it is necessary to increase the air volume for the output-side polarizers  115  for G light. The air volume for the third fan  226  is set high, because the red imager constituted of the incident-side polarizer  122 , the liquid crystal panel  121 , and the output-side polarizers  123  is cooled only by the third fan  226 , and it is necessary to increase the air volume for the red imager. 
     As described above, by setting the air volumes of the second fan  225  and the third fan  226  larger than the air volumes of the first fan  224 , the fifth fan  228  and the sixth fan  229 , it is possible to efficiently cool the output-side polarizers  115  for G light, and the red imager. 
     The applied voltage to the fourth fan  227  is set equal to the applied voltages to the second fan  225  and the third fan  226 . This is because the length of the fourth duct  217  extending to the PBS array  103  is longer than the lengths of the other air ducts, and a pressure loss of the fourth duct  217  is large. 
     As described above, in this embodiment, the cooling air from the third fan  226  is supplied to the red imager, the cooling air from the third fan  224  and the second fan  225  is supplied to the green imager, and the cooling air from the fifth fan  228  and the sixth fan  229  is supplied to the blue imager. Thus, the embodiment is configured to supply the cooling air from the individual cooling fans to each of the imagers. Accordingly, it is possible to set the air volumes of the cooling fans  224 ,  225 ,  226 ,  228 , and  229  depending on the calorific values generated by the respective imagers. This is advantageous in efficiently cooling the prism unit  16 , while reducing noises and electric power consumption. 
     Further, with respect to the green imager, the cooling air from the first fan  224  is supplied to the air outlet  213  directed toward the incident-side polarizer  114  and the liquid crystal panel  113 , and the cooling air from the second fan  225  is supplied to the air outlet  214  directed toward the output-side polarizers  115 . Thus, since the cooling air is supplied from the plural cooling fans to the green imager whose calorific value becomes largest, it is possible to secure a sufficient air volume to thereby sufficiently cool the target imager. Further, by setting the air volume of the first fan  224  depending on the calorific value generated by the incident-side polarizer  114  and the liquid crystal panel  113 , and setting the air volume of the second fan  225  depending on the calorific value generated by the output-side polarizers  115 , it is possible to efficiently cool these optical elements. 
     Similarly, with respect to the blue imager, the cooling air from the fifth fan  228  is supplied to the air outlet  221  directed to the incident-side polarizer  109  and the liquid crystal panel  108 , and the cooling air from the sixth fan  229  is supplied to the air outlet  222  directed to the output-side polarizers  110 . Thus, since the cooling air is supplied from the plural cooling fans to the blue imager whose calorific value is second largest to the green imager, it is possible to secure a sufficient air volume to thereby sufficiently cool the target imager. Further, by setting the air volume of the fifth fan  228  depending on the calorific value generated by the incident-side polarizer  109  and the liquid crystal panel  110 , and setting the air volume of the sixth fan  229  depending on the calorific value generated by the output-side polarizers  110 , it is possible to efficiently cool these optical elements. 
     In the case where the projector is configured to supply the cooling air from a single cooling fan to the two air outlets  213  and  214  ( 221  and  222 ) for the green (blue) imager through individual ducts, a large-sized cooling fan is necessary. An increase in the size of a cooling fan results in an increase in the size of an air outlet. As a result, a difference in opening area between the air outlet of the cooling fan, and the air outlets  213  and  214  ( 221  and  222 ) is increased. Then, the air flow rates of the respective air passages from the cooling fan to the air outlets  213  and  214  ( 221  and  222 ) are increased, which resultantly increases the pressure loss, and lowers the air supply rate of the cooling fan. 
     In this embodiment, since each of the cooling fans  224  and  225  ( 228  and  229 ) individually supplies the cooling air to the air outlets  213  and  214  ( 221  and  222 ) for the green (blue) imager, it is possible to reduce the size of the individual cooling fans  224  and  225  ( 228  and  229 ). Thus, since it is possible to reduce a difference in opening area between the air outlets  224   a  and  225   a  ( 228   a  and  229   a ) of the cooling fans  224  and  225  ( 228  and  229 ), and the air outlets  213  and  214  ( 221  and  222 ), it is possible to enhance the air supply rate of the cooling fans  224  and  225  ( 228  and  229 ). 
     In this embodiment, the six cooling fans  224  through  229  are divided into two groups, in each of which a required air volume is approximate to each other, and an applied voltage is set for each of the groups. Alternatively, for instance, the six cooling fans  224  through  229  may be divided into three or more groups depending on a required air volume, and an applied voltage may be set with respect to each of the groups. Further alternatively, applied voltages may be set individually with respect to all the six cooling fans  224  through  229 . 
     Further alternatively, temperatures of the optical elements of the prism unit  16 , and the PBS array  103  may be detected, and applied voltages to the cooling fans  224  through  229  may be changed, based on the detected temperatures. The modification is further advantageous in reducing noises and reducing electric power consumption. 
     Further, in this embodiment, one cooling fan (the third fan) is provided for both of the air outlets  215  and  216 . Alternatively, a cooling fan may be provided for each of the air outlets  215  and  216 . Further alternatively, three or more cooling fans may be provided for at least one of the imagers, as necessary. 
     Cooling Structure of Lamp Unit 
       FIGS. 10A and 10B ,  FIG. 11 ,  FIGS. 12A and 12B ,  FIGS. 13A and 13B , and  FIG. 14  are diagrams for describing a cooling structure of the lamp unit  13 .  FIG. 10A  is a perspective and elevational sectional view of the lamp unit  13 , when viewed from a rear of the lamp unit  13 .  FIG. 10B  is a perspective and transverse sectional view of the lamp unit  13 , when viewed from a front of the lamp unit  13 .  FIG. 11  is a perspective view of the lamp unit  13 , when viewed from the front of the lamp unit  13 .  FIGS. 12A and 12B  are respectively a left side view and a rear view of the lamp unit  13 .  FIGS. 13A and 13B  are respectively a top plan view and a bottom view of the lamp unit  13 .  FIG. 14  is a front view showing a state that the lamp unit  13  is connected to the fan unit  15 . In  FIG. 14 , the general contour of a housing  503  is shown by the dotted line so that cooling fans  501  and  502  disposed in the fan unit  15  can be seen. 
     Referring to  FIGS. 10A through 14 , the lamp unit  13  is constituted of the light source lamp  300 , and the lamp holder  400  for holding the light source lamp  300 . 
     The light source lamp  300  is provided with an arc tube  301  and a reflector  302 . In this embodiment, a metal halide lamp is used as the arc tube  301 . Alternatively, other lamp such as an ultra high-pressure mercury lamp or a xenon lamp may be used. An inner surface of the reflector  302  is formed into a paraboric shape to reflect white light emitted from the arc tube  301  on the inner surface of the reflector  302 , and guide the reflected light in the forward direction. 
     A reflector base  303  made of e.g. plaster is formed on a rear end of the reflector  302  to fixedly mount the arc tube  301  on the reflector  302 . The arc tube  301  has a seal portion  304  at an inner position of the reflector base  303 . 
     The lamp holder  400  is provided with a holder main body  401 , an upper plate  402  mounted on a rear end of an upper surface of the holder main body  401 , and a bottom plate  403  mounted on a rear end of a bottom surface of the holder main body  401 . 
     An emission window  404  through which light from the light source lamp  300  is emitted is formed in a front surface of the holder main body  401 . A heat resistant concave lens  405  is fitted in the emission window  404 . A rear surface of the holder main body  401  is opened, and the light source lamp  300  is mounted in the opening from a rear side. 
     A guide piece  406  is formed on both ends of a front portion of the holder main body  401 . A guide member (not shown) having vertically extending guide grooves is formed in the lower cabinet  2  at a housing position of the lamp unit  13 . The guide pieces  406  are fitted in the guide grooves from above in housing the lamp unit  13  in the lower cabinet  2 . 
     A first air outlet  407  is formed in the upper surface of the holder main body  401 . A first air deflector  408  extending obliquely downward in rearward direction is provided in the first air outlet  407 . Further, a second air outlet  409  is formed in the bottom surface of the holder main body  401 . A second air deflector  410  extending obliquely upward in rearward direction is provided in the second air outlet  409 . Exhaust ports  411  and  412  are formed in a right side surface and a left side surface of the holder main body  401 , respectively. Filters  411   a  and  412   a  in the form of a mesh are provided in the exhaust ports  411  and  412 , respectively, to prevent pieces of the arc tube  301  from coming out of the projector, in case that the arc tube  301  be damaged or broken. 
     A third air outlet  413  is formed in the upper plate  402  at a position substantially right above the reflector base  303 . Further, a fourth air outlet  414  is formed in the bottom plate  403  at a position substantially right below the reflector base  303 . 
     An upper duct portion  415  is mounted on an upper surface of the lamp holder  400 . As shown in  FIG. 13A , the upper duct portion  415  has a substantially T-shape in plan view to guide the cooling air that has been drawn in through an entrance  415   a  formed in a right side surface of the upper duct portion  415  to the first air outlet  407  and the third air outlet  413 . On the other hand, a lower duct portion  416  is mounted on a bottom surface of the lamp holder  400 . As shown in  FIG. 13B , the lower duct portion  416  has a substantially T-shape in plan view to guide the cooling air that has been drawn in through an entrance  416   a  formed in a right side surface of the lower duct portion  416  to the second air outlet  409  and the fourth air outlet  414 . 
     Similarly to the filters  411   a  and  412   a , filters  415   b  and  416   b  in the form of a mesh are provided in the upper duct portion  415  at a position near the first air outlet  407  and in the lower duct portion  416  at a position near the second air outlet  409 , respectively, to prevent pieces of the arc tube  301  from coming out of the projector, in case that the arc tube  301  be damaged or broken. 
     As shown in  FIG. 14 , the fan unit  15  is disposed in the housing  503  in a state that two cooling fans  501  and  502  are vertically stacked one over the other. When the lamp unit  13  is mounted in the lower cabinet  2 , the entrance  415   a  of the upper duct portion  415  is connected to an upper exit  504  of the housing  503 , and the entrance  416   a  of the lower duct portion  416  is connected to a lower exit  505  of the housing  503 . 
     In the above arrangement, in response to activation of the cooling fans  501  and  502 , cooling airs generated by the cooling fans  501  and  502  are respectively allowed to flow through the upper duct portion  415  and the lower duct portion  416 . 
     In  FIGS. 10A and 10B , flows of the cooling air are shown by the arrows. The cooling air through the upper duct portion  415  is branched out in the duct portion into a flow in the forward direction and a flow in the rearward direction. The flow of the cooling air in the forward direction is passed through the filter  415   b , drawn into the holder main body  401  through the first air outlet  407 , has its direction changed by the first air deflector  408 , and flows into the reflector  302 . Further, the cooling air through the lower duct portion  416  is branched out in the duct portion into a flow in the forward direction and a flow in the rearward direction. The flow of the cooling air in the forward direction is passed through the filter  416   b , drawn into the holder main body  401  through the second air outlet  409 , has its direction changed by the second air deflector  410 , and flows into the reflector  302 . The interior of the reflector  302  is cooled by the flows of the cooling air which have flown into the reflector  302  from both sides i.e. from the upper and lower duct portions. Thereafter, the cooling air in the reflector  302  is passed through the filters  411   a  and  412   a , and discharged to the exterior of the lamp unit  13  through the exhaust ports  411  and  412 . 
     On the other hand, the flow of the cooling air in the rearward direction in the upper duct portion  415  is drawn in through the third air outlet  413 , and impinges on the reflector base  303  of the light source lamp  300  from above. Further, the flow of the cooling air in the rearward direction in the lower duct portion  416  is drawn in through the fourth air outlet  414 , and impinges on the reflector base  303  of the light source lamp  300  from below. As a result of the above operation, the reflector base  303  is cooled from both sides i.e. from the upper and lower duct portions, and the seal portion  304  is cooled via the reflector base  303 . 
     As described above, the cooling air which has exited the lamp unit  13  is discharged to the exterior of the cabinet  1  by the exhaust fan  25 . 
     The upper portion of the light source lamp  300  is heated to a high temperature, as compared with the lower portion of the light source lamp  300  at the time of light emission, due to an influence of a gravitational force. In the case where the projector is mounted in a fixed position, the lamp unit  13  is brought to a state as shown in  FIG. 10A , and a portion of the light source lamp  300  corresponding to the upper duct portion  415  is heated to a high temperature, as compared with a portion of the light source lamp  300  corresponding to the lower duct portion  416 . On the other hand, in the case where the projector is suspended from a ceiling, the lamp unit  13  is brought to a state opposite to the state shown in  FIG. 10A , and the portion of the light source lamp  300  corresponding to the lower duct portion  416  is heated to a high temperature, as compared with the portion of the light source lamp  300  corresponding to the upper duct portion  415 . 
     In this embodiment, since the flows of the cooling air branched out by the upper duct portion  415  and the lower duct portion  416  are guided into the reflector  302  from both sides i.e. from the upper and lower duct portions, it is possible to efficiently cool a high-temperature portion of the light source lamp  300 , without depending on whether the projector is mounted in a fixed position or mounted from a ceiling. 
     In the case where the projector is mounted in a fixed position, it is desirable to set the air volume of the cooling fan  501  for supplying the air to the upper duct portion  415  higher than the air volume of the cooling fan  502  for supplying the air to the lower duct portion  416  to efficiently cool the portion of the light source lamp  300  corresponding to the upper duct portion  415 . On the other hand, in the case where the projector is mounted from a ceiling, it is desirable to set the air volume of the cooling fan  502  higher than the air volume of the cooling fan  501  to efficiently cool the portion of the light source lamp  300  corresponding to the lower duct portion  416 . 
     Further, in the light source lamp  300 , the seal portion  304  is heated to a high temperature by heat generation in the arc tube  301  resulting from light emission of the arc tube  301 . If the seal portion  304  is exceedingly heated, the seal portion  304  may be deteriorated, with the result that the performance of the light source lamp  300  may be deteriorated. Since the seal portion  304  is disposed at a position relatively away from the inner surface of the reflector  302 , the seal portion  304  may not be sufficiently cooled by the cooling air that has been draw into the interior of the reflector  302 . It may be possible to enhance the cooling performance by increasing the air volume of the cooling air. However, an enhanced cooling performance may excessively cool the arc tube  301 , which may obstruct a normal light emission. 
     In this embodiment, since the flows of the cooling air which have branched out by the upper duct portion  415  and the lower duct portion  416  are directly supplied to the reflector base  303  from both sides i.e. from the upper and lower duct portions, the entirety of the reflector base  303  is efficiently cooled, and the seal portion  304  is efficiently cooled via the reflector base  303 . Thus, it is possible to prevent lowering of the performance of the light source lamp  300  due to deterioration of the seal portion  304 . 
     Attachment Structure of Prism Cover and Lamp Cover 
       FIG. 15  is a perspective view of the upper cabinet  3  in a state that the prism cover  10  and the lamp cover  11  are detached. 
     A recess  601  in which the prism cover  10  and the lamp cover  11  are mounted is formed in an area of the upper cabinet  3  from a central part to a right side surface of the upper cabinet  3 . The recess  601  has a first area  601   a  where the prism cover  10  is mounted, and a second area  601   b  where the lamp cover  11  is mounted. 
     A prism opening  602  is formed in the first area  601   a . The prism opening  602  is formed at a position substantially right above the prism unit  16  disposed in the lower cabinet  2 , and has a size capable of mounting and dismounting the prism unit  16 . 
     Guide ribs  603  are formed at two positions on each of a front wall surface and a rear wall surface of the first area  601   a . Predetermined clearances are formed between the guide ribs  603  and a bottom surface of the recess  601 . Further, an insertion hole  604  is formed at two positions on a left wall surface of the first area  601   a . Furthermore, a nut  605  is embedded in upward direction in a central part on a right end of the first area  601   a , and a screw hole of the nut  605  faces upward. 
     A lamp opening  606  is formed in the second area  601   b . The lamp opening  606  is formed at a position substantially right above the lamp unit  13  disposed in the lower cabinet  2 , and has a size capable of mounting and dismounting the lamp unit  13 . A pair of guide portions  607  is formed on a front edge and a rear edge of the lamp opening  606 . Each of the paired guide portions  607  is constituted of two ribs arranged side by side in transverse direction with a predetermined clearance. When the lamp unit  13  is housed in the lower cabinet  2 , the guide pieces  406  of the lamp holder  400  are guided between the respective rib pairs. 
     In the second area  601   b , the lamp opening  606 , and left and right portions of the lamp opening  606  are recessed from the first area  601   a . An insertion hole  608  is formed at two positions in a wall surface corresponding to the step difference between the first area  601   a  and the second area  601   b.    
     A transversely extending guide groove  609  is formed in each of the front edge and the rear edge of the second area  601   b . A transversely extending guide hole  609   a  is formed in a side surface of each of the guide grooves  609 . Further, an opening  609   b  for passing a stem portion  808  of the lamp cover  11  is formed at an outer position substantially in the middle of each of the guide grooves  609 . 
     A nut  610  is embedded in transverse direction in a right end of the second area  601   b , and a screw hole of the nut  610  faces transversely through an attachment hole  611  formed in a side surface of the recess  601 . Further, an attachment hole  612  for screw fastening is formed in a right end of the second area  601   b  in fixedly mounting the upper cabinet  3  on the lower cabinet  2 . Furthermore, a transversely extending groove portion  613  is formed in the right end of the second area  601   b . An opening  613   a  is formed in a left end of the groove portion  613 , and a micro switch (not shown) for detecting whether the lamp cover  11  is completely closed faces the groove portion  613  through the opening  613   a.    
       FIGS. 16A and 16B  are diagrams showing an arrangement of the prism cover  10 .  FIG. 16A  is a perspective view of the prism cover  10 , when viewed from a front side of the prism cover  10 , and  FIG. 16B  is a perspective view of the prism cover  10 , when viewed from a back side of the prism cover  10 . 
     The prism cover  10  is formed into a rectangular shape, and has a thickness substantially equal to the depth of the recess  601 . A projection  701  is formed at two positions on a left end of the prism cover  10 . Further, an attachment piece  702  having an attachment hole  702   a  is provided substantially in the middle on a right end of the prism cover  10 . 
     A metal shield plate  703  is mounted on a back surface of the prism cover  10  to suppress unwanted radiation from e.g. the prism opening  602 . Further, a transversely extending guided rib  704  is formed on each of a front end and a rear end of the prism cover  10 . 
       FIGS. 17A and 17B  are diagrams showing an arrangement of the lamp cover  11 .  FIG. 17A  is a perspective view of the lamp cover  11 , when viewed from a front side of the lamp cover  11 , and  FIG. 17B  is a perspective view of the lamp cover  11 , when viewed from a back side of the lamp cover  11 . 
     The lamp cover  11  is constituted of an upper plate  801  and a side plate  802 . As shown in  FIGS. 1A and 1B , the upper surface of the upper cabinet  3  has a moderately curved shape such that the upper surface is lowered from a central part thereof in leftward and rightward directions. The upper plate  801  is moderately inclined toward the side plate  802  in conformity with the upper surface shape of the upper cabinet  3 . 
     A metal shield plate  803  is provided on a back surface of the upper plate  801 . The shield plate  803  is mounted on a holding portion  804  which is slightly bulged from the back surface of the upper plate  801 . The shield plate  803  shields the lamp opening  606  in mounting the lamp cover  11  on the upper cabinet  3 . The shield plate  803  suppresses unwanted radiation from the lamp opening  606 , and protects the lamp cover  11  from a heat generated in the lamp unit  13  (light source lamp  300 ). A projection  805  is formed at two positions on a left end of the holding portion  804 . 
     Support ribs  806  are respectively formed on a front end and a rear end on the back surface of the upper plate  801 . Because of the arrangement that each of the support ribs  806  has such a shape that a certain part thereof is cut away between a left end and a right end thereof, and the upper plate  801  is inclined, the height of each of the support ribs  806  is reduced toward the side plate  802 . The support ribs  806  support the upper plate  801  with respect to the bottom surface of the recess  601  in mounting the lamp cover  11  on the upper cabinet  3  (see  FIG. 18B ). 
     Further, an arm portion  807  is formed on each of the front end and the rear end of the back surface of the upper plate  801 . Each of the arm portions  807  has a lead end thereof bent toward the side plate  802 , and the outwardly extending stem portion  808  is formed at the lead end of each of the arm portions  807 . Further, a stopper portion  809  extending in parallel to the lead end is formed on each of the arm portions  807  (see  FIG. 20 ). 
     Further, a rib  810  to be housed in the groove portion  613  of the upper cabinet  3  is formed on the back surface of the upper plate  801 . When the lamp cover  11  is completely closed, the micro switch is pressed by the rib  810 . Then, the micro switch is turned on, and it is detected that the lamp cover  11  is completely closed. 
     An attachment hole  811  is formed in the side plate  802 . 
     Thus, when the prism cover  10  is mounted on the upper cabinet  3 , the prism cover  10  is housed in the recess  601  from the right end of the first area  601   a , and slidingly moved in leftward direction. Then, as shown in  FIG. 18A , the guided ribs  704  are housed in the clearances between the guide ribs  603  and the bottom surface of the recess  601 . This suppresses an upward movement of the prism cover  10 . 
     When the prism cover  10  is completely closed, the projections  701  are received in the insertion holes  604  of the recess  601 . This suppresses an upward movement of the left end of the prism cover  10 . Further, the attachment hole  702   a  of the prism cover  10  is aligned with the screw hole of the nut  605 . Then, by screw-fastening the nut  605 , the prism cover  10  is fixedly mounted on the upper cabinet  3 . 
     Next, as shown in  FIG. 18B , when the lamp cover  11  is mounted on the upper cabinet  2 , the arm portions  807  are housed in the guide grooves  609  from above, and the stem portions  808  are received in the guide holes  609   a . In the insertion operation, the stem portions  808  are received in the guide holes  609   a  through the openings  609   b . Thereafter, the lamp cover  11  is slidingly moved in leftward direction. Then, the stem portions  808  are moved in leftward direction along the guide holes  609   a . As a result of the above operation, an upward movement of the lamp cover  11  is suppressed by the stem portions  808  received in the guide holes  609   a  and the support ribs  806 . 
     When the lamp cover  11  is completely closed, the left end of the lamp cover  11  is placed over the right end of the prism cover  10 . As a result of the above operation, the screws of the prism cover  10  are covered by the lamp cover  11 . Further, the projections  805  of the lamp cover  11  are received in the insertion holes  608  of the recess  601 . As a result of the above operation, an upward movement of the left end of the lamp cover  11  is suppressed. Further, the attachment hole  811  of the lamp cover  11  is aligned with the screw hole of the nut  610 . Then, by screw-fastening the nut  610 , the lamp cover  11  is fixedly mounted on the upper cabinet  3 . 
     As described above, as shown in  FIGS. 1A and 1B , by performing the above operations, both of the prism cover  10  and the lamp cover  11  are mounted on the upper cabinet  3 . 
     The lamp unit  13  (light source lamp  300 ) and the prism unit  16  are deteriorated by a long-time operation. In such a case, it is necessary to replace the lamp unit  13  and the prism unit  16  with new ones. The lamp unit  13  is easily deteriorated, as compared with the prism unit  16 , and the replacement frequency of the lamp unit  13  is larger than the replacement frequency of the prism unit  16 . 
     In the case where the lamp unit  13  is replaced, the lamp cover  11  is opened to mount or dismount the lamp unit  13  through the lamp opening  606 . Replacement of the lamp unit  13  may be performed by the user. 
       FIGS. 19A and 19B  are diagrams showing a state that the lamp cover  11  is opened.  FIG. 19A  shows a state that the lamp cover  11  is halfway opened, and  FIG. 19B  shows a state that the lamp cover  11  is completely opened.  FIG. 20  is a cross-sectional view showing essential parts of the upper cabinet  3  for describing an operation to be performed for the lamp cover  11  in the case where the lamp cover  11  is opened. 
     In the case where the lamp unit  13  is replaced, the user unfastens the screw, and slidingly moves the lamp cover  11  in rightward direction. By performing the above operation, as shown in  FIG. 19A , the lamp opening  606  is gradually opened. Then, as shown by the broken line in  FIG. 20 , the user is allowed to move the stem portions  808  in rightward direction within the guide holes  609   a.    
     When the stem portions  808  reach the right end of the guide holes  609   a , the lamp cover  11  is not slidingly moved any more. In this state, a right end portion of the lamp opening  606  is still covered by the lamp cover  11 . 
     Next, the user pushes a portion of the lamp cover  11  projecting from the right end of the upper cabinet  3  in downward direction. Then, as shown by the solid line in  FIG. 20 , the lamp cover  11  is pivotally moved about the stem portions  808 . In this state, as shown in  FIG. 17B , since the support ribs  806  are cut away at a position around the arm portions  807 , there is no likelihood that the support ribs  806  may be abutted against a corner of the upper cabinet  3  in pivotally moving the lamp cover  11 . 
     As described above, as shown in  FIG. 19B , the lamp cover  11  stands upright along the right side surface of the upper cabinet  3 , and the lamp opening  606  is completely opened. In this state, as shown in  FIG. 20 , the stopper portions  809  are abutted against the right side surface of the upper cabinet  3 . 
     As shown in  FIG. 20 , a bulging projection  609   c  is formed below and at a right end of each of the guide holes  609   a . The height of the projection  609   c  is very small. Accordingly, by applying a small external force in slidingly moving the lamp cover  11 , the stem portions  808  are moved over the projections  609   c , and reach the right ends of the guide holes  609   a , respectively. In this state, left portions of the stem portions  808  are supported by the projections  609   a . Since the stem portions  808  are easily rotatable, the lamp cover  11  is smoothly and pivotally moved. 
     When the lamp opening  606  is completely opened, the user is allowed to dismount the deteriorated lamp unit  13  through the lamp opening  606 . Then, the user is allowed to house a new lamp unit  13  in the lower cabinet  2  through the lamp opening  606 . Then, the user is allowed to close the lamp cover  11 , and fasten the screw to fixedly mount the lamp cover  11  on the upper cabinet  3  by a sequence opposite to the sequence to be performed in opening the lamp opening  606 . 
     In the case where the prism unit  16  is replaced, the prism opening  10  is opened, and the prism unit  16  is mounted or dismounted through the prism opening  602 . Replacement of the prism unit  16  is performed by a serviceperson. 
       FIGS. 21A and 21B  are diagrams showing a state that the prism cover  10  is opened.  FIG. 21A  shows a state that the prism cover  10  is halfway opened, and  FIG. 21B  shows a state that the prism cover  10  is completely opened. 
     In the case where the prism unit  16  is replaced, a serviceperson opens the lamp cover  11  by the above sequence. Then, the serviceperson unfastens the screw, and slidingly moves the prism cover  10  in rightward direction. Then, as shown in  FIG. 21A , the prism cover  10  is slidingly retracted in the space of the recess  601  which is defined by opening the lamp cover  11 . Then, as shown in  FIG. 21B , by slidingly moving the prism cover  10  to the right end of the recess  601 , the prism opening  602  is completely opened. 
     When the prism opening  602  is completely opened, the serviceperson is allowed to dismount the deteriorated prism unit  16  through the prism opening  602 . Then, the serviceperson is allowed to house a new prism unit  16  in the lower cabinet  2  through the prism opening  602 . Then, the serviceperson is allowed to close the prism cover  10 , and fasten the screw to fixedly mount the prism cover  10  on the upper cabinet  3  by the sequence opposite to the sequence to be performed in opening the prism opening  602 . Lastly, the lamp cover  11  is closed. 
     As described above, in this embodiment, by slidingly moving the prism cover  10 , the prism opening  602  is opened. Further, by slidingly moving the lamp cover  11 , the lamp opening  606  is opened. In this way, even if the space defined above the cabinet  1  is small in installing the projector, the prism opening  602  and the lamp opening  606  can be sufficiently opened. 
     Further, in this embodiment, since both ends of each of the prism cover  10  and the lamp cover  11  are securely fixed so that the both ends are not moved in upward direction in slidingly moving the prism cover  10  and the lamp cover  11 , there is no or less step difference between the prism cover  10  and the lamp cover  11 , and the upper cabinet  3 . Thus, a sophisticated appearance of the projector is secured. 
     Further, the embodiment is configured to slidingly move the prism cover  10  with respect to the second area  601   b  in opening the prism cover  10 . Accordingly, there is no need of additionally forming a recess in the upper cabinet  3  to house the slidable prism cover  10 . This is advantageous in simplifying the arrangement of the upper cabinet  3 . 
     In the above case, it is necessary to open the lamp cover  11  to open the prism cover  10 . However, since the replacement frequency of the prism unit  16  is smaller than the replacement frequency of the lamp unit  13 , a burden of operation is reduced. 
     Further, in this embodiment, when the lamp cover  11  is slidingly moved to some extent, the lamp cover  11  is bent downward, thereby completely opening the lamp opening  606 . This enables to reduce the sliding amount of the lamp cover  11 , and suppress a projecting amount of the lamp cover  11  from the cabinet  1  in slidingly moving the lamp cover  11 . Thus, it is possible to reduce the space for sliding movement, which is necessary in opening or closing the lamp cover  11 . Further, even if an external force is applied from above by e.g. hitting of a user&#39;s/serviceperson&#39;s hand in a state that the lamp cover  11  is opened to the right end, the force is absorbed by pivotal movement of the lamp cover  11 . Thus, it is possible to prevent damage or breakage of the lamp cover  11 . 
     In this embodiment, the prism unit  16  is disposed in the central part of the cabinet  1 , and the lamp unit  13  is disposed near the right side surface of the cabinet  1 . Alternatively, the prism unit  16  may be disposed at a position near the side surface of the cabinet  1 , depending on the structure of the projector. In the modification, the arrangements of the prism cover  10  and the lamp cover  11  are opposite to those in the embodiment. In the modification, since it is necessary to open the prism cover in order to open the lamp cover, a burden of operation may be slightly increased. 
     Further, it is not necessary to dispose the prism opening  602  and the lamp opening  606  independently of each other. Alternatively, the prism opening  602  and the lamp opening  606  may be communicated with each other. Specifically, a single opening for covering the disposition areas of the lamp unit  13  and the prism unit  16  may be formed in the cabinet  1  so that the lamp unit  13  and the prism unit  16  can be dismounted through the single opening. In the modification, the prism cover  10  and the lamp cover  11  may be formed into one cover. 
     The embodiment of the invention has been described as above, but the invention is not limited to the foregoing embodiment. Further, the embodiment of the invention may be changed or modified in various ways as necessary, as far as such changes and modifications do not depart from the scope of the claims of the invention hereinafter defined.