Patent Publication Number: US-2006017362-A1

Title: Method of reducing the temperature difference between upper and lower portions of a lamp bulb, optical unit, projection type display apparatus and projection television apparatus

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a method of reducing the temperature difference between upper and lower portions of a lamp bulb for elongating the life of a discharge lamp used in an optical apparatus such as a projector, an optical unit, a projection type display apparatus and a projection television apparatus.  
      Such a liquid crystal projector as shown in  FIG. 11  is known and disclosed in Japanese Patent Laid-Open No. 2001-183746. Referring to  FIG. 11 , the liquid crystal projector  1001  includes an optical unit  103  installed in the inside of an outer housing  102 . The optical unit  103  includes a discharge lamp  104  serving as a light source, a UV-IR cut filter  105  disposed on an optical axis of the discharge lamp  104 , a PS conversion apparatus  106  and a mirror  107 , and dichroic mirrors  108 R and  108 G and a mirror  109  provided on an optical axis of light reflected by the mirror  107 . The optical unit  103  further includes mirrors  1111  and  112  provided on optical axes of light reflected by the dichroic mirror  108 R and the mirror  109 , respectively, and condenser lenses  113 R,  113 G and  113 B provided on the optical axes of the light reflected by the mirrors  1111 ,  108 G and  112 , respectively. The optical unit  103  further includes three spatial optical modulation elements  114 R,  114 G and  114 B for the three primary colors of R, G and B each in the form of a transmission type liquid crystal panel or the like serving as a light modulation section, a cross prism  115  for light synthesis disposed on paths of light beams emitted from the spatial optical modulation elements  114 R,  114 G and  114 B, and a projection lens  116  for projecting a full color image optically synthesized by the cross prism  115 . An ultra-high pressure mercury lamp is used for the discharge lamp  104 , and an air blower fan is used to cool around the lamp bulb of the discharge lamp  104  and the inside of a lamp reflector of the discharge lamp  104 .  
     SUMMARY OF THE INVENTION  
      In the liquid crystal projector  1001  described above, air is fed through a gap called air gap provided in the lamp reflector for a discharge lamp used in a projector and so forth, and the introduction angle of a fan duct and the voltage to the fan motor are adjusted to reduce the temperature difference between upper and lower portions of the lamp bulb. According to the mechanism, however, since air is fed through the air gap to cool the lamp bulb at a distance, it is difficult to perform precise cooling control of the temperatures at the upper and lower portions of the lamp bulb. As a result, the mechanism fails to sufficiently reduce the temperature difference between the upper and lower portions of the lamp bulb.  
      This gives rise to such a problem that the bulb of the discharge lamp is whitened or blackened, which results in reduction of the life of the bulb.  
      Such a problem as just described not only occurs where a UV-IR cut filter but also occurs similarly where an IR cut filter is used.  
      It is an object of the present invention to provide a method of reducing the temperature difference between upper and lower portions of a lamp bulb and an optical unit, a projection type display apparatus and a projection television apparatus to which the method is applied.  
      In order to attain the object described above, according to the present invention, that location of a reflecting face of a lamp reflector for a discharge lamp upon which reflected light from an IR cut filter or a UV-IR cut filter is illuminated is set specifically.  
      In particular, according to an embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in a horizontal direction in an apparatus which includes, in addition to the discharge lamp an IR cut filter disposed in front of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face in a rather downward direction toward the reflecting face by means of the IR cut filter.  
      According to another embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in a downward direction in an apparatus which includes, in addition to the discharge lamp, an IR cut filter disposed in lower side of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the downward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the downward direction by and from the reflecting face in a direction toward an outer circumference of the reflecting face by means of the IR cut filter.  
      According to a further embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in an upward direction in an apparatus which includes, in addition to the discharge lamp, an IR cut filter disposed in upper side of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the upward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the upward direction by and from the reflecting face in a direction toward an outer circumference of the reflecting face by means of the IR cut filter.  
      According to a still further embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in a horizontal direction in an apparatus which includes, in addition to the discharge lamp, a UV-IR cut filter disposed in front of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face in a rather downward direction toward the reflecting face by means of the UV-IR cut filter.  
      According to a yet further embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in a downward direction in an apparatus which includes, in addition to the discharge lamp, a UV-IR cut filter disposed in lower side of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the downward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the downward direction by and from the reflecting face in a direction toward an outer circumference of the reflecting face by means of the UV-IR cut filter.  
      According to a yet further embodiment of the present invention, there is provided a method of reducing the temperature difference between upper and lower portions of a lamp bulb of a discharge lamp disposed in an upward direction in an apparatus which includes, in addition to the discharge lamp, a UV-IR cut filter disposed in upper side of the discharge lamp. The discharge lamp includes, in addition to the lump bulb, a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the upward direction. The method includes a step of reflecting infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the upward direction by and from the reflecting face in a direction toward an outer circumference of the reflecting face by means of the UV-IR cut filter.  
      According to an additional embodiment of the present invention, there is provided an optical unit, including a discharge lamp disposed in a horizontal direction for projecting an image, and an IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the IR cut filter.  
      According to another additional embodiment of the present invention, there is provided an optical unit, including a discharge lamp disposed in a horizontal direction for projecting an image, and a UV-IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Ultraviolet rays and infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the UV-IR cut filter.  
      According to a further additional embodiment of the present invention, there is provided a projection type display apparatus including an optical unit for projecting an image on a screen. The optical unit includes a discharge lamp disposed in a horizontal direction and an IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the IR cut filter.  
      According to a still further additional embodiment of the present invention, there is provided a projection type display apparatus including an optical unit for projecting an image on a screen. The optical unit includes a discharge lamp disposed in a horizontal direction and a UV-IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Ultraviolet rays and infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the UV-IR cut filter.  
      According to a yet further additional embodiment of the present invention, there is provided a projection television apparatus, including a frame, a rear projection type screen attached to a front face of the frame, an optical unit provided on the frame for projecting a television image, and a reflecting mirror attached to the frame for reflecting a television image projection light flux emitted from the optical unit on a rear face of the rear projection type screen. The optical unit includes a discharge lamp disposed in a horizontal direction and an IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the IR cut filter.  
      According to a yet further additional embodiment of the present invention, there is provided a projection television apparatus, including a frame, a rear projection type screen attached to a front face of the frame, an optical unit provided on the frame for projecting a television image, and a reflecting mirror attached to the frame for reflecting a television image projection light flux emitted from the optical unit on a rear face of the rear projection type screen. The optical unit includes a discharge lamp disposed in a horizontal direction and a UV-IR cut filter disposed in front of the discharge lamp. The discharge lamp includes a lamp bulb, and a lamp reflector having a reflecting face disposed around the lamp bulb for reflecting light emitted from the lamp bulb and emitting the reflected light in the forward direction. Ultraviolet rays and infrared rays included in the light emitted from the lamp bulb and reflected and emitted in the forward direction by and from the reflecting face are reflected in a rather downward direction toward the reflecting face by means of the UV-IR cut filter.  
      In summary, light emitted from the lamp bulb and reflected at a comparatively high temperature portion of the reflecting face of the lamp reflector is reflected by an IR cut filter or a UV-IR cut filter so that it is illuminated toward a comparatively low temperature portion of the reflecting face or it escapes to the outer side of the reflecting face. Consequently, the light reflected by the IR cut filter or UV-IR cut filter is not returned to the comparatively high temperature portion of the reflecting face, and the temperature difference between upper and lower portions of the lamp bulb is reduced thereby.  
      The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view of a projection type display apparatus to which the present invention is applied;  
       FIG. 2  is a perspective view of a discharge lamp used in the projection type display apparatus of  FIG. 1 ;  
       FIG. 3A  is a sectional view of an attaching structure for a UV-IR cut filter and the discharge lamp of the projection type display apparatus of  FIG. 1  and  
       FIG. 3B  is a perspective view of the attaching structure for the UV-IR cut filter shown in  FIG. 3A ;  
       FIG. 4A  is a schematic view of the discharge lamp and the UV-IR cut filter relates to the embodiment of the present invention, and  FIG. 4B  is sectional view of the UV-IR cut filter;  
       FIG. 5  is a schematic view showing loci of upper and lower side reflected light fluxes where the UV-IR cut filter  21  is inclined by −4°;  
       FIG. 6  is a similar view but showing loci of upper and lower side reflected light fluxes where the UV-IR cut filter  21  is inclined by 4°;  
       FIGS. 7A, 7B  and  7 C are schematic views illustrating different methods of reversing the UV-IR cut filter;  
       FIG. 8A  is a schematic view showing another projection type display apparatus to which the present invention is applied, and  FIG. 8B  is a similar view but showing a modification to the projection type display apparatus of  FIG. 8A ;  
       FIG. 9A  is a schematic view showing a further projection type display apparatus to which the present invention is applied, and  FIG. 9B  is a similar view but showing a modification to the projection type display apparatus of  FIG. 9A ;  
       FIGS. 10A and 10B  are a front elevational view and a side elevational view, respectively, of a projection television apparatus; and  
       FIG. 11  is a schematic view showing a configuration of a conventional projector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
      Referring first to  FIG. 1 , there is shown a projection type display apparatus to which the present invention is applied. The projection type display apparatus  2  shown includes an outer housing  201 , a projection lens  90 , and an optical unit  20  accommodated in the outer housing  201  for projecting an image.  
      The optical unit  20  includes a discharge lamp  10  which forms a light source for projecting an image, an illuminating optical system  26 , a color separating optical system  30 , three liquid crystal panels  60 A,  60 B and  60 C each in the form of a transmission type liquid crystal apparatus serving as an optical modulation section (spatial optical modulation element), a cross dichroic prism  70  and a relay optical system  80 . The components mentioned of the optical unit  20  are accommodated in a housing  210  of the optical unit  20 .  
      A flux of light emitted from the discharge lamp  10  is introduced by the illuminating optical system  26  to the color separating optical system  30  and the relay optical system  80 , by which it is split into three light fluxes of red (R), green (G) and blue (B).  
      The three light fluxes of the colors are passed through the three liquid crystal panels  60 A,  60 B and  60 C (spatial optical modulation elements) which individually display image information of the three corresponding colors of red (R), green (G) and blue (B), whereupon they are modulated based on the image information. Thereafter, the three light fluxes are synthesized into a single image projection light flux by the cross dichroic prism  70 , and the image projection light flux is projected on a screen through the projection lens  90 . It is to be noted that the image information is supplied to the liquid crystal panels  60 A,  60 B and  60 C from an external apparatus such as a personal computer not shown through an electric circuit incorporated in the projection type display apparatus  2 .  
      More particularly, the discharge lamp  10  is disposed such that the optical axis thereof is directed horizontally, and the illuminating optical system  26  is disposed in front of the discharge lamp  10 .  
      The illuminating optical system  26  includes a UV-IR cut filter  21  and a PS conversion element  24  disposed on a first light path  10 A. After the flux of light from the discharge lamp  10  passes through the UV-IR cut filter  21  and the PS conversion element  24 , it is reflected by a reflecting mirror  341  and comes to a second light path  10 B of the color separating optical system  30 . The first light path  10 A and the second light path  10 B orthogonally cross with each other.  
      The color separating optical system  30  includes a dichroic mirror  301  disposed on the second light path  10 B and a dichroic mirror  302  disposed on an extension line of the second light path  10 B.  
      The two dichroic mirrors  301  and  302  are disposed such that they make an angle of 45 degrees with respect to the second light path  10 B and the extension line of the second light path  10 B.  
      The light flux emitted from the illuminating optical system  26  is reflected by the dichroic mirror  301  and passes through the liquid crystal panel  60 A past a reflecting mirror  344  and a condenser lens  351 , and then comes to the cross dichroic prism  70 . The light path from the dichroic mirror  301  to the reflecting mirror  344  is hereinafter referred to as third light path  10 C, and the light path from the reflecting mirror  344  to the cross dichroic prism  70  is hereinafter referred to as fourth light path  10 D. In the present embodiment, the second light path  10 B and the third light path  10 C orthogonally cross with each other, and also the third light path  10 C and the fourth light path  10 D orthogonally cross with each other.  
      That portion of the light flax passing through the dichroic mirror  301  which is reflected by the dichroic mirror  302  passes through the liquid crystal panel  60 B past a condenser lens  352  and comes to the cross dichroic prism  70 . The light path from the dichroic mirror  301  to the dichroic mirror  302  is hereinafter referred to as fifth light path  10 E, and the light path from the dichroic mirror  302  to the cross dichroic prism  70  is hereinafter referred to as sixth light path  10 F. In the present embodiment, the second light path  10 B and the fifth light path  10 E are disposed coaxially, and the fifth light path  10 E and the sixth light path  10 F orthogonally cross with each other.  
      The light flux passing through the dichroic mirror  301  and then passing through the dichroic mirror  302  successively passes a relay lens  381 , a reflecting mirror  342 , another relay lens  382  and a reflecting mirror  343  which form the relay optical system  80  and further passes through the liquid crystal panel  60 C past a condenser lens  353 , and then comes to the cross dichroic prism  70 . The light path from the dichroic mirror  302  to the reflecting mirror  342  is hereinafter referred to as seventh light path  10 G, and the light path from the reflecting mirror  342  to the reflecting mirror  343  is hereinafter referred to as eighth light path  10 H. Further, the light path from the reflecting mirror  343  to the cross dichroic prism  70  is hereinafter referred to as ninth light path  10 I. In the present embodiment, the seventh light path  10 G and the eighth light path  10 H orthogonally cross with each other, and also the eighth light path  10 H and the ninth light path  10 I orthogonally cross with each other.  
      A light flux emitted from an outgoing face  70 A of the cross dichroic prism  70  comes into the projection lens  90 . Then, a color image is formed on a screen through the projection lens  90 .  
      The UV-IR cut filter  21  is disposed in an inclined relationship at an end portion  210 A of the housing  210  in front of the discharge lamp  10  as seen in  FIGS. 3A and 3B .  
      The discharge lamp  10  includes a lamp housing  101 , a lamp reflector  14 , a lamp bulb  11  and a protective glass plate  15  as seen in  FIG. 2 .  
      The lamp housing  101  has a rectangular opening at a front end thereof, and the lamp reflector  14  is provided in the inside of the lamp housing  101 . A pair of cutaway portions  16 A for allowing cooling air to pass therethrough are provided at upper and lower portions of a front portion of the lamp housing  101  and the lamp reflector  14 , and a protective net  16 B is provided in each of the cutaway portions  16 A.  
      The lamp reflector  14  has a reflecting face  14 A provided on an inner face thereof and forming a rotational symmetrical paraboloid.  
      The lamp bulb  11  has a pair of discharge electrodes  12   a  and  12   b  located in an opposing relationship to each other at places corresponding to the focus of the rotational symmetrical paraboloid on a rotational symmetrical axis of the reflecting face  14 A.  
      The lamp bulb  11  is disposed such that a rear portion thereof extends through a rear end of the lamp housing  101 .  
      The protective glass plate  15  is provided so as to close up the opening at the front end of the lamp housing  101 .  
      Accordingly, light emitted by discharge between the discharge electrodes  12   a  and  12   b  is converted into parallel light by the reflecting face  14 A and is reflected forwardly through the protective glass plate  15 .  
      Now, attachment of the discharge lamp is described.  
      Referring to  FIGS. 3A and 3B , an end portion  210 A of the housing  210  in which the UV-IR cut filter  21  is accommodated has a cross section having a shape of a rectangular framework, and a housing side framework member  110  having a cross section having a shape of rectangular framework is attached to the base end of the end portion  210 A.  
      Meanwhile, a lamp side framework member  111  having a cross section having a shape of a rectangular framework is attached to a front portion of the lamp housing  101 . The lamp side framework member  111  is inserted in the inside of the housing side framework member  110 , and the framework members  110  and  111  are secured to each other by screws.  
      Accordingly, the discharge lamp  10  is supported by the housing  210  through the lamp side framework member  111  and the housing side framework member  110 .  
      Openings  18  and  19  are provided at a location of the lamp side framework member  111  and a location of the housing side framework member  110  corresponding to the upper and lower cutaway portions  16 A, and an air blowing fan  17  is provided in an opposing relationship to the lower side opening  18 . Thus, external air is fed into the discharge lamp  10  through the opening  18  on the lower side and the cutaway portion  16 A on the lower side by the air blowing fan  17  to cool the lamp bulb  11  and the lamp reflector  14 . Then, the air is discharged to the outside of the discharge lamp  10  through the cutaway portion  16 A on the upper side and the opening  19  on the upper side. Consequently, the lamp bulb  11  is cooled, and the difference in temperature between the upper and lower portions of the lamp bulb  11  is reduced.  
      It is to be noted that, since the average temperature between the upper and lower portions with respect to the lamp bulb varies depending upon the blown amount of air, voltage setting to the air blowing fan  17  is controlled so that the temperature difference between the upper and lower portions with respect to the lamp bulb  11  may be minimized.  
      Now, attachment of the UV-IR cut filter  21  is described.  
      A groove  212  is formed at a lower portion of an inner face of the end portion  210 A of the housing  210  such that the lower end of the UV-IR cut filter  21  is accommodated therein. Meanwhile, an inclined face  213  is formed at an upper portion of the inner face of the end portion  210 A such that it is engageable with the upper end of the UV-IR cut filter  21 .  
      The UV-IR cut filter  21  is disposed such that it is accommodated at the lower end thereof in the groove  212  and held down at the upper end thereof to the inclined face  213  by leaf springs  211 . The leaf springs  211  are assembled to an upper wall of the end portion  210 A.  
      The UV-IR cut filter  21  is disposed in a downwardly inclined relationship such that an upper portion of the UV-IR cut filter  21  is nearer to the discharge lamp  10  than a lower portion of the UV-IR cut filter  21  so that infrared rays and ultraviolet rays included in reflected light of light emitted from the lamp bulb  11  and reflected forwardly by the reflecting face  14 A are reflected rather downwardly toward the reflecting face  14 A.  
      Since the disposition of the UV-IR cut filter  21  in a rather downwardly inclined relationship presents significant advantages of the present embodiment, details of the same are described below.  
      Referring to  FIGS. 4A and 4B , the UV-IR cut filter  21  disposed in front of the discharge lamp  10  has a UV-IR cut film  22  provided on a light incoming face thereof for cutting ultraviolet rays and infrared rays by reflection. Further, the UV-IR cut filter  21  has an AR film  23  applied to a light outgoing face thereof for suppressing reflection of stray light from the optical unit  20 .  
      Emitted light from the lamp bulb  11  is converted into a parallel flux of light by the reflecting face  14 A and comes to the UV-IR cut filter  21 . UV-IR light (ultraviolet rays and infrared rays) included in the incoming light to the UV-IR cut filter  21  is reflected by the UV-IR cut film  22  toward the reflecting face  14 A, by which it is reflected again. Here, the UV-IR cut filter  21  is disposed in an inclined relationship by a small amount in a downward direction such that an upper portion of the UV-IR cut filter  21  is nearer to an upper portion of the reflecting face  14 A (the downward direction is hereinafter referred to as −direction) or in an upward direction such that an upper portion of the UV-IR cut filter  21  is spaced away from an upper portion of the reflecting face  14 A (the direction is hereinafter referred to as +direction) so that the UV-IR light reflected by the UV-IR cut filter  21  comes in a rather downward direction or in a rather upward direction to the reflecting face  14 A. Consequently, the UV-IR light reflected by the lamp reflector  14  comes to upper and lower portions of the lamp bulb  11  with different incoming light amounts from each other, and consequently, the temperature of the upper portion or the lower portion changes.  
      The inclination angle of the UV-IR cut filter  21  is most preferable when the temperature difference between the upper and lower portions of the lamp bulb  11  is minimum, and in the present embodiment, the inclination angle is set fixedly to −4°.  
      The following experiments were conducted in order to confirm an effect where the inclination angle of the UV-IR cut filter  21  is varied.  
      Experiment 1  
     
         
         
           
              Experiment conditions:  
              Discharge lamp: 190 W ultra-high pressure mercury lamp  
              Air gap of lamp reflector: 10 mm×10 mm  
              Voltage to fan for blowing in air through air gap: 5.5 V  
           
         
       
    
      Experiments wherein, with the angle perpendicular to the optical axis of the UV-IR cut filter  21  set as a reference angle (0°), the inclination angle of the UV-IR cut filter  21  was set to 1) 4°, 2) 0° and 3) −4° and 4) no filter was used were conducted successively, and the temperatures of an upper portion and a lower portion of the lamp bulb  11  were measured to determine the temperature difference between the upper and lower portion of the lamp bulb  11 . Results of the experiments are listed in Table 1 below:  
                                   TABLE 1                                   Inclination   Temp. of   Temp. of               angle of   bulb upper   bulb lower   Temp.           UV-IR cut filter   portion   portion   difference                                                        1   4°   860° C.   800° C.   60° C.       2   0°   850° C.   820° C.   30° C.       3   −4°     840° C.   840° C.    0° C.       4   No UV-IR cut filter   820° C.   760° C.   60° C.                  
 
      According to the experiments described above, by setting the UV-IR cut filter  21  so as to have an inclination angle of −4° from the reference inclination angle 0°, the temperature difference between the upper and lower portions of the lamp bulb  11  reduced from 30° C. to 0° C.  
      Experiment 2  
     
         
         
           
              Experiment conditions:  
              Discharge lamp: 150 W ultra-high pressure mercury lamp  
              Other conditions: same as those in the Experiment 1  
           
         
       
    
      Experiments wherein the inclination angle of the UV-IR cut filter  21  was set to 1) 4°, 2) 0° and 3) −4° and 4) no filter was used were conducted successively similarly as in the Experiment 1 above to determine the temperature difference between the upper and lower portion of the lamp bulb  11 . As a result, by setting the UV-IR cut filter  21  so as to be inclined to −4° from the reference inclination angle 0°, the temperature difference between the upper and lower portions of the lamp bulb  11  reduced from 95° C. to 35° C.  
      From the experiments described above, it has been confirmed successfully that the temperature difference of the lamp bulb  11  can be reduced by the inclination angle of the UV-IR cut filter  21  utilizing reflected light of the UV-IR cut filter  21 .  
      Now, the reason why the temperature difference between upper and lower portions of the lamp bulb  11  varies depending upon the inclination angle of the UV-IR cut filter  21  is described.  
      Light emitted from the lamp bulb  11  and coming to and reflected forwardly by an upper portion of the reflecting face  14 A is then reflected by the UV-IR cut filter  21  and comes to a lower portion of the reflecting face  14 A or is illuminated to the outer side below the reflecting face  14 A without returning to the upper portion of the reflecting face  14 A. Accordingly, the temperature of the upper portion of the reflecting face  14 A becomes lower when compared with that in the conventional arrangement and simultaneously the temperature of the lower portion of the reflecting face  14 A becomes higher than that in the conventional arrangement.  
      More detailed description is given below.  
      Illumination optical system designing and evaluation software Odis Ver. 70 was used to perform light flux tracing of returning light from the UV-IR cut filter  21  under the conditions that the UV-IR cut filter  21  was 1) inclined by −4° and 2) inclined by 4°. Light loci obtained are illustrated in  FIGS. 5 and 6 .  
      As seen in  FIG. 5 , where the UV-IR cut filter  21  is inclined by −4°, upper and lower side outgoing light fluxes a 1  and b 1  emitted from the lamp bulb  11  through the reflecting face  14 A are reflected by the UV-IR cut filter  21  inclined by −4° and make rather downwardly inclined filter reflected light beams a 2  and b 2 , respectively. The filter reflected light beams a 2  and b 2  are further reflected by the reflecting face  14 A again to make reflector reflected light fluxes a 3  and b 3 , respectively. The reflector reflected light fluxes a 3  and b 3  are further reflected by the reflecting face  14 A to make reflector reflected light fluxes a 4  and b 4  and individually escape downwardly as seen in  FIG. 5 . Consequently, it is considered that a lower portion of the lamp bulb  11  is warmed while the temperature does not stay at the upper portion of the lamp bulb  11 , and as a result, the temperature difference between the upper and lower portions of the lamp bulb  11  decreases.  
      In contrast, where the UV-IR cut filter  21  is inclined by 4°, upper and lower side outgoing light fluxes a 1  and b 1  emitted from the lamp bulb  11  through the reflecting face  14 A are reflected by the UV-IR cut filter  21  inclined by 4° and make rather upwardly inclined filter reflected light beams a 2  and b 2 , respectively, as seen in  FIG. 6 . The filter reflected light beams a 2  and b 2  are further reflected by the reflecting face  14 A again to make reflector reflected light fluxes a 3  and b 3 , respectively. The reflector reflected light fluxes a 3  and b 3  are further reflected by the reflecting face  14 A to make reflector reflected light fluxes a 4  and b 4  and individually escape upwardly as seen in  FIG. 6 . Consequently, it is considered that an upper portion of the lamp bulb  11  is warmed thereby to increase the temperature difference between the upper and lower portions of the lamp bulb  11 .  
      Therefore, according to the present embodiment, the temperature difference between upper and lower portions of the lamp bulb can be reduced only by disposing the UV-IR cut fitter in an inclined relationship without the necessity for any special apparatus, and consequently, the life of the discharge lamp can be elongated.  
      The projection type display apparatus may be used in both of two manners of use including use as a stationarily placed apparatus and use as a suspended apparatus. When compared with a case wherein the projection type display apparatus is used as a stationarily placed apparatus as seen in  FIG. 7A , where the projection type display apparatus is used as a suspended apparatus as seen in  FIG. 7B  or  7 C, an upper portion A and a lower portion B of the lamp bulb are reversed vertically, and therefore, also it is necessary to reverse the inclination angle of the UV-IR cut filter  21 . As a mechanism for reversing the inclination angle of the UV-IR cut filter  21 , a pivoting mechanism may be provided which pivots around pivots  21 P provided at left and right end faces of the UV-IR cut filter  21  as seen in  FIG. 7B . Alternatively, a displacement mechanism for displacing one of the upper and lower ends of the UV-IR cut filter  21  around the other one of the upper and lower ends as seen in  FIG. 7C  may be provided.  
     Second Embodiment  
      Now, a second embodiment of the present invention is described.  
       FIG. 8A  shows part of a projection type display apparatus according to the second embodiment.  
      The second embodiment is similar to the first embodiment except that the discharge lamp  10  is directed downwardly and that a reflecting mirror  401 A is disposed between the discharge lamp  10  and the PS conversion element  24 .  
      The discharge lamp  10  is disposed in a downward direction, and a UV-IR cut filter  21 A is disposed below the discharge lamp  10 .  
      The discharge lamp  10  has a configuration similar to that in the first embodiment. Consequently, light emitted from the lamp bulb  11  is converted into and reflected downwardly as parallel light by the reflecting face  14 A and introduced to the UV-IR cut filter  21 A. The light passing through the UV-IR cut filter  21 A is reflected by the reflecting mirror  401 A and introduced to the PS conversion element  24 .  
      In the present embodiment, the UV-IR cut filter  21 A, or more particularly the UV-IR cut film  22 , is formed so as to have a curved face convex in the upward direction as seen in  FIG. 8A .  
      In the second embodiment, light emitted from the discharge electrodes  12   a  and  12   b  and introduced to and reflected downwardly by the reflecting face  14 A is then reflected toward an outer circumference of the reflecting face  14 A by the convex curved face of the UV-IR cut filter  21 A.  
      Accordingly, reflected light reflected by the UV-IR cut filter  21 A does not return to the upper portion of the reflecting face  14 A but comes to a lower portion of the reflecting face  14 A. Therefore, the temperature of an upper portion of the reflecting face  14 A drops when compared with that of the conventional arrangement, and the temperature difference between an upper portion and a lower portion of the lamp bulb can be reduced and consequently the life of the discharge lamp can be elongated.  
      A modification to the second embodiment is shown in  FIG. 8B .  
      Referring to  FIG. 8B , in the modification, a UV-IR cut filter  21 B, more particularly the UV-IR cut film  22 , is formed from, for example, a curved face convex downwardly.  
      In the present modification, light emitted from the discharge electrodes  12   a  and  12   b  and coming to and reflected by the reflecting face  14 A is reflected toward an outer periphery of the reflecting face  14 A by the convex curved face of the UV-IR cut filter  21 B.  
      Accordingly, reflected light reflected by the UV-IR cut filter  21 B comes to a lower portion of the reflecting face  14 A without returning to the upper portion of the reflecting face  14 A. Therefore, the temperature of an upper portion of the reflecting face  14 A drops when compared with that of the conventional arrangement, and the temperature difference between an upper portion and a lower portion of the lamp bulb can be reduced and consequently the life of the discharge lamp can be elongated.  
      It is to be noted that a similar effect is exhibited also where the second embodiment is modified such that the UV-IR cut filter  21 A is formed from a conical face convex upwardly in place of the upwardly convex curved face shown in  FIG. 8A  or the UV-IR cut filter  21 B is formed from a conical face convex downwardly in place of the downwardly convex curved face shown in  FIG. 8B .  
     Third Embodiment  
       FIG. 9A  shows part of a projection type display apparatus according to a third embodiment of the present invention.  
      The third embodiment is similar to the first embodiment except that the discharge lamp  10  is disposed in an upward direction and that the a reflecting mirror  401 B is disposed between the discharge lamp  10  and the PS conversion element  24 .  
      The discharge lamp  10  is disposed in an upward direction, and a UV-IR cut filter  21 D is disposed above the discharge lamp  10 .  
      The discharge lamp  10  has a configuration similar to that in the first embodiment. Consequently, light emitted from the lamp bulb  11  is converted into and reflected upwardly as parallel light by the reflecting face  14 A and introduced to the UV-IR cut filter  21 D. The light passing through the UV-IR cut filter  21 D is reflected by the reflecting mirror  401 B and introduced to the PS conversion element  24 .  
      In the present embodiment, the UV-IR cut filter  21 D, or more particularly the UV-IR cut film  22 , is formed so as to have a curved face convex in the downward direction as seen in  FIG. 9A . In this instance, the inclined face of the curved face of the UV-IR cut filter  21 D preferably has a slope steeper than that of the UV-IR cut filter  21 A in the second embodiment.  
      In the third embodiment, light emitted from the discharge electrodes  12   a  and  12   b  and introduced to and reflected upwardly by the reflecting face  14 A is reflected toward an outer circumference of the reflecting face  14 A by the UV-IR cut filter  21 D. Meanwhile, light reflected from an upper portion of the reflecting face  14 A is reflected by the convex curved face of the UV-IR cut filter  21 D so as to escape to the outside of the lamp reflector  14  without returning to the upper portion of the reflecting face  14 A.  
      Therefore, the temperature of an upper portion of the reflecting face  14 A drops when compared with that of the conventional arrangement, and the temperature difference between an upper portion and a lower portion of the lamp bulb can be reduced and besides the life of the discharge lamp can be elongated similarly as in the first embodiment.  
      A modification to the third embodiment is shown in  FIG. 9B .  
      Referring to  FIG. 9B , in the modification, a UV-IR cut filter  21 E, more particularly the UV-IR cut film  22 , is formed from, for example, a curved face convex upwardly. In this instance, the inclined face of the curved face of the UV-IR cut filter  21 E preferably has a slope steeper than that of the UV-IR cut filter  21 B in the second embodiment.  
      In the present modification, light emitted from the discharge electrodes  12   a  and  12   b  and coming to and reflected by the reflecting face  14 A is reflected toward an outer periphery of the reflecting face  14 A by the convex curved face of the UV-IR cut filter  21 E. Accordingly, the light reflected from an upper portion of the reflecting face  14 A is reflected so as to escape to the outside of the lamp reflector  14 .  
      Therefore, the temperature of an upper portion of the reflecting face  14 A drops when compared with that of the conventional arrangement, and the temperature difference between an upper portion and a lower portion of the lamp bulb can be reduced and consequently the life of the discharge lamp can be elongated.  
      It is to be noted that a similar effect is exhibited also where the third embodiment is modified such that the UV-IR cut filter  21 D is formed from a conical face convex downwardly in place of the downwardly convex curved face shown in  FIG. 9A  or the UV-IR cut filter  21 E is formed from a conical face convex upwardly in place of the upwardly convex curved face shown in  FIG. 9B .  
      The optical units of the first to third embodiments described hereinabove can be applied, for example, also to a projection television apparatus  500  shown in  FIGS. 10A and 10B .  
      In the following, the projection television apparatus  500  is described.  
      Referring to  FIGS. 10A and 10B , the projection television apparatus  500  includes a frame in the inside thereof, and an optical unit  510  for projection of a television image, a reflecting mirror (not shown), and a rear projection type screen  550 .  
      The projection television apparatus  500  further includes an image information production circuit which decodes a received television signal into an image signal and a sound signal, performs a necessary signal process for the image signal to produce image information of the three primary colors of red (R), green (G) and blue (B) and supplies the image signals to the optical unit  510 .  
      The optical unit  510  is configured similarly to the projection type display apparatus  2  in the first embodiment and emits, when the image information is supplied to liquid crystal panels corresponding to red (R), green (G) and blue (B), a television image projection light flux.  
      The reflecting mirror is disposed above the optical unit  510  in the rear of the rear projection type screen  550  of the rear projection type and reflects the television image projection light flux emitted from the optical unit  510  toward the rear face of the rear projection type screen  550 . The rear projection type screen  550  displays a television image in the front face by projecting the television image projection light flux reflected by the reflecting mirror on the rear face.  
      The rear projection type screen  550  is formed from, for example, a Fresnel lens disposed on the image source side, and a lenticular screen disposed at the next stage to the Fresnel lens. Another screen may be provided additionally in order to reduce degradation of the contrast by external light and protect the lenticular screen.  
      The present invention can be applied also to an IR filter or a UV-IR filter disposed in an opposing relationship to the discharge lamp of the optical unit of the projection television apparatus  500  having such a configuration as described above.  
      It is to be noted that, while, in the embodiments described above, the UV-IR cut filter  21  is disposed in an opposing relationship to the lamp bulb  11 , also where an IR cut filter is used in place of the UV-IR cut filter  21 , the temperature difference between upper and lower portions of the lamp bulb can naturally be reduced by reflecting light emitted from a lamp bulb and reflected at a comparatively high temperature portion of a reflecting face by means of the IR cut filter so as to illuminate IR light (infrared rays) toward a comparatively low temperature portion of the reflecting face or to direct the IR light to escape to the outside of the reflecting face.  
      Further, while, in the embodiments described hereinabove, a liquid crystal panel formed from a transmission type liquid crystal apparatus is used as the light modulation section (spatial optical modulation element) which modulates a flux of light based on image information in the optical unit, such an optical spatial modulation element may be any element which modulates a flux of light based on image information. Thus, for example, a liquid crystal panel formed from a reflection type liquid crystal apparatus may be used, or an element called DLM (Digital Micromirror Device) which includes very small mirrors corresponding to individual pixels to reflect a flux of light may be used. It is to be noted that the DLM is used widely in projection type display apparatus and rear projection television apparatus of the DLP (Digital Light Processing) system.  
      Further, in the present invention, the discharge lamp disposed in a horizontal direction is not limited to that which is directed only in the strictly horizontal direction, but may naturally include a discharge lamp which is directed in a rather inclined relationship to an upper or lower direction from the horizontal direction.  
      While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.