Abstract:
A projection apparatus having a light source, a color separating system for separating the light from the light source into a plurality of color beams, a plurality of light modulating elements for modulating the separated color beams, based on an image signal, a color combining system for combining the modulated color beams, and a projection optical system for projecting composite light of the combined color beams, onto a screen. The color combining system incorporates a cross dichroic prism with dichroic films on joint surfaces between four prisms, each of the color beams incident to the cross dichroic prism is converted into linearly polarized light, and the following relation is met: 
     
       
         0°&lt;θ&lt;90° 
       
     
     where θ is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for projecting an image and an apparatus for observing an image and, more particularly, to those suitably applicable to liquid crystal projectors using a liquid crystal display element (liquid crystal panel) as an image display element and constructed to project an image obtained thereby through a projection lens, for example, onto a polarizing screen and to image observation systems constructed to permit observation of an enlarged and projected image from a screen of a computer, a picture of a video camera, or the like. 
     2. Related Background Art 
     A variety of proposals have been made heretofore as to the image projection devices (liquid crystal projectors) constructed to illuminate the liquid crystal panel with light from a light source, display an image on the liquid crystal panel, and enlarge and project an image based on transmitted or reflected light from the liquid crystal panel, through the projection lens onto the screen. 
     FIG. 17 is a schematic diagram of major part of a conventional image projection apparatus. In FIG. 17 reference numeral  101  designates a white light source. Numeral  102  designates a reflector. Numeral  103  represents a visible-light-transmitting filter for removing the components of light except for the visible light. 
     Numeral  104  indicates an integrator for yielding a uniform illumination area, which is comprised of fly&#39;s eye lenses  104   a ,  104   b  each consisting of an array of lenses. Numeral  105  denotes an array of polarization converting elements for converting non-polarized light into linearly polarized light polarized in a predetermined direction of polarization, each element consisting of a polarization separating surface  105   a , a reflecting surface  105   b , and a half-wave plate  105   c . 
     Numeral  106  represents a condenser lens. Numeral  107  designates a first dichroic mirror,  108  a second dichroic mirror, and  109   a  and  109   b  reflecting mirrors. Numeral  110  stands for a relay system for relaying the illumination light, which is comprised of relay lenses  110   a ,  110   b  and relay mirrors  110   c ,  110   d.    
     Symbols  111   r ,  111   g , and  111   b  are condenser lenses for images (light beams) of the colors of R (Red), G (Green), and B (Blue), respectively. Symbols  112   r ,  112   g , and  112   b  are image display elements for R, G, and B, respectively. Numeral  113  represents a cross dichroic prism DP for color composition. Numeral  114  stands for a projection lens. 
     The white light emitted from the white light source  101  is collected by the reflector  102  and then travels through the integrator  104 , the polarization converting element array  105 , and the condenser lens  106 . After that, the light is separated into the color beams of R, G, and B light by the dichroic mirrors  107 ,  108 . The first color light (B in the figure) is guided via the reflecting mirror  109   b  and condenser lens  111   b  to the image display element  112   b , the second color light (G in the figure) is guided via the condenser lens  111   g  to the image display element  112   g , and the third color light (R in the figure) is guided via the relay system  110  and condenser lens  111   r  to the image display element  112   r.    
     The color beams of R, G, and B, traveling through the image display elements  112   b ,  112   g , and  112   r  and modulated according to image signals, are then combined into one by the cross dichroic prism DP  113 , whereby the images displayed on the respective image display elements are enlarged and projected in a superimposed manner onto the screen (not illustrated) through the projection lens  114 . A discharge lamp such as a metal halide lamp, a mercury lamp, or the like is used as the white light source. 
     FIG. 18 shows an example of spectral distribution of the white light source  101 . From the white light having the continuous spectral distribution as illustrated, the dichroic mirrors DM 1 , DM 2  create the three color beams of R, G, and B, for example, having respective spectral distributions as illustrated in FIG.  19 . 
     In the conventional apparatus, these light beams are modulated by the respective image display elements  112   r ,  112   g ,  112   b  and thereafter combined by the cross dichroic prism DP. In order to avoid loss in light amount in the cross dichroic prism DP, dichroic films of the cross dichroic prism are designed so that light reflected thereby is s-polarized light components of red (R) and blue (B) while the light of green (G) transmitted by the dichroic films of the cross dichroic prism DP is a p-polarized light component. 
     The reason is that, from the characteristics of the dichroic films as illustrated in FIG. 20, a broader reflection band can be set in the case of the s-polarized light components being reflected by the dichroic films (BRs, RRs) and a broader transmission band can be set in the case of the p-polarized light component being transmitted by the dichroic films (GTp). This suppresses the loss of light amount in the dichroic prism due to the so-called incident angle characteristics of the dichroic films, which are variations in cut wavelengths of the dichroic films due to variations in angles of incidence of light to the dichroic films. 
     In order to realize this structure, where the polarization directions of the image beams emerging from the image display elements were as illustrated in FIG. 21, the apparatus was so constructed that a half-wave plate was placed in each of the three paths of the emergent beams and that the slow phase axis directions of the phase plates were set so as to make the polarization direction of G light perpendicular to the polarization direction of R and B light and so as to make the polarization direction of G light coincident with that of the p-polarized light with respect to the dichroic films of the dichroic prism DP. 
     In systems necessitating alignment of the polarization directions of projected light on the occasion of projection of image (for example, such as polarized image projection systems using the polarizing screen or stereoscopic image projection systems for projecting images for the right eye and for the left eye with beams having respective polarization directions different from each other), however, the polarization direction of G light has to be aligned with the polarization direction of R and B light by providing a polarizing means at an arbitrary position in the optical path from the dichroic prism to the polarizing screen or to the observer. 
     The reason is as follows. When the polarization direction of light reflected by the polarizing screen is set in parallel to the s-polarized light component of the dichroic prism, the color beam of green is absorbed. When the polarization direction of light reflected by the polarizing screen is set in parallel to the p-polarized light component of the dichroic prism, the color beams of red and blue are absorbed. This will result in failing to reproduce a correct color image. 
     It is then conceivable, for example, to convert the beams into the polarization directions inclined at 45° relative to the polarization direction SC of the screen by the half-wave plates as illustrated in FIGS. 22A and 22B, or to convert the polarized beams into circularly polarized light beams by quarter-wave plates as illustrated in FIGS. 23A and 23B. FIG. 22A shows the relationship between the polarization directions of the beams (R, B, and G beams) emerging from the dichroic prism and the slow phase axis direction of the phase plates (indicated by the dashed line), and FIG. 22B shows the relationship between the polarization directions of the projected beams and the transmission-axis direction of the polarizing screen. FIG. 23A shows the relationship between the polarization directions of the beams (R, B, and G beams) emerging from the dichroic prism and the slow phase axis direction of the phase plates (indicated by the dashed line), and FIG. 23B shows the relationship between the polarization directions of the projected beams and the transmission-axis direction of the polarizing screen. 
     In such use ways, however, the intensity of the projected light decreases as follows because of absorption of light by a polarizing plate on the polarizing screen. 
     
       
         cos 2 (45)=0.5 
       
     
     Therefore, this poses another problem that brightness of the projected image becomes half, and the structure is not suitable for the image projection systems requiring the alignment of polarization directions. 
     If the polarization directions of the respective color beams incident to the dichroic prism are preliminarily aligned with each other there can be little loss of brightness at the polarizing screen. However, this will narrow the widths of the reflection and transmission bands of the dichroic films, as illustrated in FIG. 24, thus decrease margins for the wavelength components of the respective color beams transmitted or reflected by the dichroic prism, and increase the loss of light amount due to the incident angle characteristics of the dichroic films. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a projection apparatus and an observation apparatus that can achieve higher utilization efficiency of light than the conventional apparatus. 
     A projection apparatus according to one aspect of the present invention is a projection apparatus comprising means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system onto a plane, wherein the color combining system comprises a plurality of dichroic films, each of the color beams incident to the cross dichroic film is a linearly polarized light, and the following relation is met: 
     
       
         0°&lt;θ&lt;90° 
       
     
     where θ is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film. 
     Another projection apparatus according to a further aspect of the present invention is a projection apparatus comprising means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system, onto a polarizing screen, wherein the color combining system comprises a plurality of dichroic films, each of the color beams incident to the dichroic film is a linearly polarized light, and the following relation is met: 
     
       
         0°&lt;θ&lt;90° 
       
     
     where θ is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film, and wherein a half-wave plate is placed in an optical path from the color combining system to the polarizing screen, and an angle between the polarization direction of the color beam component transmitted by all the dichroic films and a transmission polarization direction of the polarizing screen is substantially equal to an angle between the polarization direction of the color beam component reflected by the dichroic film and the transmission polarization direction of the polarizing screen. 
     In a preferred form of the above projection apparatus, the half-wave plate is provided at an exit side of a projection lens of the projection optical system. 
     In a preferred form of the above projection apparatus, the half-wave plate is provided between the color combining system and a projection lens of the projection optical system. 
     In a preferred form of the above projection apparatus, a slow phase axis of the half-wave plate rotates about the optical axis of the projection optical system. 
     Another projection apparatus according to a further aspect of the present invention is a projection apparatus comprising means for supplying a plurality of color light beams, a plurality of light modulating elements for modulating the respective color beams, based on an image signal, a color combining system which has a plurality of dichroic films for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system onto a plane, wherein each of the color beams incident to the dichroic film of the color combining system is converted into linearly polarized light, and the following relation is met: 
     
       
         0°&lt;θ&lt;90° 
       
     
     where θ is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film. 
     In a preferred form of the above projection apparatus, a half-wave plate is provided at an exit side of a projection lens of the projection optical system. 
     In a preferred form of the above projection apparatus, a half-wave plate is provided between the color combining system and a projection lens of the projection optical system. 
     In a preferred form of the above projection apparatus, a slow phase axis of the half-wave plate rotates about the optical axis of the projection optical system. 
     In a preferred form of the above projection apparatus, the polarization direction of the color beam component reflected by the dichroic film is s-polarized light to the dichroic films. 
     In a preferred form of the above projection apparatus, the angle e satisfies the following relation: 
     
       
         0°&lt;θ&lt;80°. 
       
     
     In a preferred form of the above projection apparatus, the angle θ satisfies the following relation: 
      0°&lt;θ&lt;60°. 
     In a preferred form of the above projection apparatus, the angle θ satisfies the following relation: 
     
       
         0°&lt;θ&lt;45°. 
       
     
     In a preferred form of the above projection apparatus, the angle θ satisfies the following relation: 
     
       
         θ=45°. 
       
     
     An observation apparatus according to a further aspect of the present invention is an observation apparatus with which an observer, wearing polarizing glasses to which light beams of polarization states different from each other are incident selectively to the left eye and to the right eye, observes a stereoscopic image from parallax images projected onto a polarizing screen, which preserves polarization directions, by first and second projection devices, wherein each of the first and second projection devices comprises means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system comprising a cross dichroic prism with dichroic films on joint surfaces between four prisms, for combining the color beams emerging from the respective light modulating elements, a projection optical system for projecting composite light of the color beams combined by the color combining system, onto the polarizing screen, and a polarizer placed in an optical path from the cross dichroic prism to the polarizing screen, the polarizer having a polarization axis directed along a direction which bisects an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film, wherein each of the color beams incident to the dichroic film is a linearly polarized light, and the following relation is satisfied: 
     
       
         0°&lt;θ&lt;90° 
       
     
     where θ is the angle between the polarization direction of the color beam component transmitted by all the dichroic films and the polarization direction of the color beam component reflected by the dichroic film, and wherein a phase plate capable of altering a polarization state of light is set at an exit position of an image projection optical system of at least one of the first and second image projection devices, whereby polarization states of light beams projected from the two projection devices are made different from each other. 
     In a preferred form of the above observation apparatus, the angle θ satisfies the following relation: 
     
       
         0°&lt;θ&lt;80°. 
       
     
     In a preferred form of the above observation apparatus, the angle θ satisfies the following relation: 
     
       
         0°&lt;θ&lt;60°. 
       
     
     In a preferred form of the above observation apparatus, the angle θ°satisfies the following relation: 
     
       
         0°&lt;θ&lt;45°. 
       
     
     In a preferred form of the above observation apparatus, the angle θ satisfies the following relation: 
     
       
         74 =45°. 
       
     
     A system according to a further aspect of the present invention is a system for projecting a video picture by either of the projection apparatus described above. 
     A system according to a further aspect of the present invention is a system for projecting an image produced by a computer, by either of the projection apparatus described above. 
     In a preferred form of the above apparatus, 
     
       
         θ=80°. 
       
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram to show the major part of Embodiment 1 of the present invention; 
     FIG. 2 is an explanatory diagram to show an enlarged view of a portion of FIG. 1; 
     FIG. 3 is an explanatory diagram to show an enlarged view of a portion of FIG. 1; 
     FIG. 4 is a diagram to explain the polarization directions of projected light in Embodiment 1 of the present invention; 
     FIG. 5 is an explanatory diagram to explain a portion in Embodiment 1 of the present invention; 
     FIG.  6 A and FIG. 6B are diagrams to explain the polarization directions of projected light in Embodiment 1 of the present invention; 
     FIG. 7 is a modification of a portion in Embodiment 1 of the present invention; 
     FIG. 8 is a schematic diagram to show the major part of a portion of Embodiment 2 of the present invention; 
     FIG. 9 is a diagram to explain the polarization directions of projected light in Embodiment 2 of the present invention; 
     FIG. 10 is a schematic diagram to show the major part of a portion of Embodiment 3 of the present invention; 
     FIG. 11 is a diagram to explain the polarization directions of projected light in Embodiment 3 of the present invention; 
     FIG. 12 is a schematic diagram to show the major part of a portion of Embodiment 4 of the present invention; 
     FIG.  13 A and FIG. 13B are diagrams to explain the polarization directions of projected light in Embodiment 4 the present invention; 
     FIG. 14 is a schematic diagram to show the major part of a portion of Embodiment 5 of the present invention; 
     FIG. 15A, FIG. 15B, and FIG. 15C are diagrams to explain the polarization directions of projected light in Embodiment 5 of the present invention; 
     FIG. 16A, FIG. 16B, and FIG. 16C are diagrams to explain the polarization directions of projected light in Embodiment 5 of the present invention; 
     FIG. 17 is a diagram to explain the structure of the conventional projection apparatus; 
     FIG. 18 is a diagram for explaining the characteristics of the color separating system in the conventional example; 
     FIG. 19 is a diagram for explaining the characteristics of the color separating system in the conventional example; 
     FIG. 20 is a diagram for explaining the characteristics of the color separating system in the conventional example; 
     FIG. 21 is a diagram to explain the polarization directions of projected light in the conventional example; 
     FIG.  22 A and FIG. 22B are drawings to explain the polarization directions of projected light to the polarizing screen in the conventional example; 
     FIG.  23 A and FIG. 23B are drawings to explain the polarization directions of projected light to the polarizing screen in the conventional example; and 
     FIG. 24 is a diagram for explaining the characteristics of the color separating system in another conventional example. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram to show the major part of Embodiment 1 of the present invention. In the figure reference numeral  101  designates a light source (lamp) such as a metal halide lamp, a mercury lamp, or the like. Numeral  102  denotes a reflector comprised of a parabolic surface or an ellipsoidal surface. 
     Numeral  103  indicates an integrator consisting of a first lens array  103   a  and a second lens array  103   b . Numeral  4  represents a polarization converting element array consisting of a plurality of polarization separating surfaces  4   a , a plurality of reflecting surfaces  4   b  corresponding to the polarization separating surfaces  4   a , and a plurality of phase plates  4   c . Numeral  5  stands for a color separating system consisting of dichroic mirrors  51 ,  52 . 
     Numerals  71  and  72  denote mirrors. Numeral  8  is a relay system having condenser lenses  81 ,  82 ,  83  and mirrors  84 ,  85 , and  1   r ,  1   g , and  1   b  are image display elements for red, for green, and for blue, using the liquid crystal. Symbols  2   r ,  2   g , and  2   b  represent sheet polarizers as analyzers of light from the image display elements  1   r ,  1   g ,  1   b , and symbols  3   r  and  3   b  half-wave plates for converting the polarization direction of light in the R light path and in the B light path, respectively. DP represents a cross dichroic prism as a color combining system. 
     Numeral  12  indicates a projection lens having a positive refracting power, for enlarging and projecting the images displayed on the respective image display elements. Numeral  6  stands for a condenser lens for condensing diffused light from the polarization converting element array  4  onto the image display elements. 
     Symbols  9 G and  9 B denote condenser lenses for condensing the illumination light onto the projection lens  12 . 
     The optical paths of FIG. 1 will be described. Beams of part of the light from the light source  101  are incident directly to the first lens array  103   a  and the other beams are reflected by the reflector  102  and then enter the first lens array  103   a . The first lens array  103   a  focuses these beams to form a plurality of secondary light source images near the second lens array  103   b.    
     Beams from the plurality of secondary light source images near the second lens array  103   b  are incident to the corresponding polarization converting elements. The polarization converting element array  4  converts the beams into beams aligned in a certain polarization direction (s-polarized light) and the beams from the array  4  are incident to the condenser lens  6 . 
     The beams from the plurality of secondary light source images formed near the second lens array  103   b  travel via the condenser lens  6  and the condenser lens  9 B (or  9 G, or the relay system  8 ) to illuminate the image display element  1   b  ( 1   g , or  1   r ) as a surface to be illuminated, in a superimposed manner thereon. 
     Here the white light from the condenser lens  6  is reflected by the mirror  71  to be made incident to the dichroic mirror  51 . The blue light is transmitted by the dichroic mirror  51 , then is reflected by the mirror  72 , and is condensed by the condenser lens  9 B, thereby illuminating the image display element  1   b  for blue. 
     Among the green light and red light reflected by the dichroic mirror  51  the dichroic mirror  52  reflects the green light but transmits the red light. 
     The green light reflected by the dichroic mirror  52  is condensed by the condenser lens  9 G to illuminate the image display element  1   g  for green. 
     The red light transmitted by the dichroic mirror  52  is condensed by the relay system  8  to illuminate the image display element  1   r  for red. The images of the respective color beams from the image display elements  1   b ,  1   g ,  1   r  are guided through each element (a polarizing plate  2 , a half-wave plate  3 ) illustrated in the enlarged view of FIG.  3  and thereafter are combined by the cross dichroic prism DP (hereinafter referred to as a dichroic prism DP). Then they are guided through a half-wave plate  34  to be enlarged and projected through the projection lens  12  onto the polarizing screen  13 . 
     The polarization converting element array  4  transmits the p-polarized light but reflects the s-polarized light out of the incident light LI, at the polarization separating surfaces  4   a  each provided with a polarization separating film, as illustrated in the enlarged view of FIG.  2 . The p-polarized light transmitted by the polarization separating surfaces  4   a  out of the incident light travels through the half-wave plates  4   c  with the polarization direction thereof being turned 90°, so that it is converted into the s-polarized light. Thus the s-polarized light emerges from the array  4 . 
     On the other hand, the s-polarized light reflected by the polarization separating surfaces  4   a  is reflected by the reflecting surfaces  4   b  to emerge from exit surfaces  4   d . According to this action, the element array  4  functions to output beams of linearly polarized light of the s-polarized light from the incident light. 
     FIG. 3 is an enlarged view to show the major part near the cross dichroic prism DP in Embodiment 1 of the present invention. 
     FIG. 3 shows the structure of each optical system from the image display element  1   r ,  1   g , or  1   b  to the dichroic prism DP. In FIG. 3 symbols  1   r ,  1   g , and  1   b  designate the image display elements for red (R), for green (G), and for blue (B), and  2   r ,  2   g , and  2   b  the polarizers as analyzers for the light from the image display elements. 
     Symbols  3   r  and  3   b  denote the half-wave plates for converting the polarization direction of the beams in the R light path and in the B light path. 
     FIG. 4 shows the directions of polarization in each of the optical elements used in the present embodiment. At the image display elements  1   r ,  1   g ,  1   b , the polarization directions of the image beams indicated by the arrows in the figure make 45° relative to the direction of the s-polarized light component (polarized light in the vertical direction on the plane of the drawing) of the dichroic films of the dichroic prism. At the polarizers  2   r ,  2   g ,  2   b , the polarization direction of transmitted light (referred to as a transmission polarization direction) indicated by the arrows in the figure is set in parallel (0°) to the polarization direction of the image beams from the image display elements. At the phase plates  3   r ,  3   b , the direction indicated by the dotted line in the figure indicates the slow phase axis direction, this direction being set at 22.5° relative to the direction of the s-polarized light component of the dichroic films of the dichroic prism. 
     Based on this arrangement, the polarization direction of the red light and blue light is converted into that of the s-polarized light component of the dichroic films of the dichroic prism DP and thereafter the s-polarized red or blue light is incident to the dichroic prism DP. Since the green light is incident to the dichroic prism without changing the polarization direction at the exit of the image display element  1   g , the polarization direction of the green light is inclined at 45° relative to the s-polarized light component of the dichroic films, so that the angle between the polarization direction of the light passing through all the dichroic films and the polarization direction of the light once reflected by the dichroic film is 45°. 
     FIG. 5 shows the optical system from the cross dichroic prism DP to the polarizing screen  13  in Embodiment 1 of the present invention. In FIG. 5 symbol DP represents the cross dichroic prism (dichroic prism),  34  the half-wave plate for converting the polarization direction of the light combined by the prism DP,  12  the projection lens, and  13  the polarizing screen. 
     In the present embodiment, the polarization direction of the red and blue light emerging from the dichroic prism DP is coincident with that of the s-polarized light with respect to the dichroic films of the dichroic prism DP, while the polarization direction of the green light emerging from the prism DP is inclined at 45° relative to the polarization direction of the red and blue light. 
     Relations of the slow phase axis direction of the half-wave plate  34  and the transmission polarization direction of the polarizing screen  13  against the polarization directions of this projected light are presented in FIGS. 6A and 6B. FIG. 6A shows the relationship between the polarization directions of the beams (R, B, and G light) emerging from the dichroic prism and the slow axis direction (dotted line) of the half-wave plate  34 , and FIG. 6B shows the relationship between the polarization directions of the beams (R, B, and G light) projected onto the polarizing screen  13  and the transmission axis direction of the polarizing screen. 
     In FIG. 6A the slow axis direction (dotted line) of the half-wave plate  34  is set at the angle of 11.25° relative to the direction of the s-polarized light in the dichroic films of the dichroic prism DP. Passing through the half-wave plate  34 , the beams of the three colors of R, G, and B from the prism DP are converted each into light of the polarization direction inclined at 22.5° relative to the direction of the s-polarized light in the dichroic films of the dichroic prism DP. Since the transmission polarization direction of the polarizing screen  13  is set in parallel to the direction of the s-polarized light in the dichroic films of the dichroic prism, the rate of the light that can be utilized for observation without being absorbed by the screen  13 , is computed as follows. 
     
       
         cos 2 (22.5)=0.853 
       
     
     This means that 85.3% of the projected light is allowed to pass through the polarizing screen  13  and to be utilized. 
     In the present embodiment the utilization efficiency of light is improved greatly as compared with 50% in the conventional apparatus. The half-wave plate  34  can be positioned anywhere between the dichroic prism DP and the polarizing screen  13 , and it may also be constructed so as to be detachably mounted on the exit side of the projection lens  12  as illustrated in FIG.  7 . 
     As described above, the present embodiment suppresses the loss of light amount occurring in the use of the beams with their polarization directions aligned, by setting the angle to 45°, smaller than 90°, between the polarization direction of the color beam component transmitted by all the dichroic films of the cross dichroic prism and the polarization direction of the color beam components once reflected by the dichroic film of the cross dichroic prism, in the color beams incident to the cross dichroic prism for color composition. 
     At this time it is preferable that the polarization direction of the color beam components once reflected by the dichroic film be coincident with that of the s-polarized light to the dichroic films and that the color beam component transmitted by all the dichroic films be inclined at the angle larger than 0° but smaller than 90° relative to the s-polarized light, because the loss of light amount due to the incident angle characteristics of the dichroic films can be suppressed more than in the case of the polarization directions of the respective color beams being aligned and because in the systems without using the polarizing screen the loss of light amount can be decreased in the dichroic films. 
     It is also preferable that the color light transmitted by all the dichroic films be the green light and the color light reflected by the dichroic film be the red and blue light, because the loss of light amount is little in the dichroic films. The angle between the two polarization components is desirably 80° or less, because the light amount is increased 17% or more. More desirably, the angle is not more than 60°, because the light amount is increased 50% or more. Still more desirably, the angle is not more than 45°, because the light amount is increased 70% or more. 
     FIG. 8 is a schematic diagram to show the major part of a portion of Embodiment 2 of the present invention. FIG. 8 shows the structure of the portion from the image display elements  11   r ,  11   b ,  11   g  to the dichroic prism DP. 
     In FIG. 8 symbols  11   r ,  11   g , and  11   b  designate the image display elements for red (R), for green (G), and for blue (B). Symbols  12   r ,  12   g , and  12   b  denote the polarizers as analyzers for the light from the image display elements. 
     Symbols  13   r ,  13   g , and  13   b  represent the half-wave plates for converting the polarization direction of the associated color light, which are placed in the optical paths of the respective colors of R, G, and B. 
     The directions of polarization in the respective optical elements used in the present embodiment are presented in FIG.  9 . At the image display elements  11   r ,  11   g ,  11   b , the polarization direction of the image light indicated by the arrows in the figure is inclined at 45° relative to the direction of the s-polarized light component in the dichroic films of the dichroic prism DP. At the polarizers  12   r ,  12   g ,  12   b , the transmission polarization direction indicated by the arrows in the figure is parallel (0°) to the polarization direction of the image light from the image display elements. At the phase plates  13   r ,  13   g ,  13   b , the directions indicated by the dotted lines in the figure are the slow axis directions, the phase plates  13   r ,  13   b  in the optical paths of the respective colors of R and B are set at 22.5° relative to the direction of the s-polarized light component in the dichroic films of the dichroic prism DP, and the phase plate  13   g  in the optical path G is set at 50°. 
     Based on this arrangement, the polarization direction of the red and blue light is converted into that of the s-polarized component in the dichroic films of the dichroic prism DP and thereafter the red and blue light is incident to the dichroic prism DP. 
     In contrast with it, the system is so set that the polarization direction of the green light is converted into the polarization direction inclined at the angle of 55° relative to the s-polarized light component in the dichroic films of the dichroic prism DP and thereafter the green light is incident to the dichroic prism. This achieves the effect similar to that in Embodiment 1. 
     FIG. 10 is a schematic diagram to show the major part of a portion of Embodiment 3 of the present invention. FIG. 10 shows the structure of the portion from the image display elements  21   r ,  21   g ,  21   b  to the dichroic prism DP. 
     In FIG. 10 symbols  21   r ,  21   g , and  21   b  denote the image display elements for red (R), for green (G), and for blue (B). Symbols  22   r ,  22   g , and  22   b  represent the polarizers as analyzers for the light from the image display elements. 
     Symbol  23   g  denotes the half-wave plate for converting the polarization direction of the green light, which is placed in the optical path of the green light. The directions of polarization in the respective optical elements used in the present embodiment are presented in FIG.  11 . At each image display element  21 , the direction indicated by the arrows in the figure is the polarization direction of the image light, this direction being parallel to the direction of the s-polarized light component in the dichroic prism. At the polarizers  22   r ,  22   b ,  22   g , the direction indicated by the arrows in the drawing is the transmission polarization direction, this direction being set in parallel (0°) to the polarization direction of the image light from the image display elements. At the half-wave plate  23   g , the direction indicated by the dotted line in the figure is the slow axis direction, this direction being set at 30° relative to the direction of the s-polarized light component in the dichroic prism. 
     Based on this arrangement, the red and blue light enters the dichroic prism while the polarization direction of the red and blue light is kept parallel to the s-polarized light component in the dichroic films of the dichroic prism DP. The polarization direction of the green light is converted by the phase plate  23   g , so that the angle is set to 60° between the polarization direction of the green light transmitted by all the dichroic films and the polarization direction of the red and blue light once reflected by the dichroic film. This accomplishes the effect similar to that in Embodiment 1. 
     FIG. 12 is a schematic diagram to show the major part of a portion of Embodiment 4 of the present invention. In FIG. 12 symbol DP represents the dichroic prism, and  44  the half-wave plate for converting the polarization direction of the composite light. Numeral  45  denotes the projection lens, and  46  the polarizing screen. 
     The present embodiment also has the structure as described in Embodiments 1 to 3; that is, the polarization direction of the red and blue light once reflected by the dichroic film and emerging from the dichroic prism DP is converted into that of the s-polarized light to the dichroic films of the dichroic prism DP, and the polarization direction of the green light is inclined at 45°, which is larger than 0° but smaller than 90°, relative to the polarization direction of the red and blue light. Relations of the slow axis direction of the half-wave plate  44  and the transmission polarization direction of the polarizing screen  46  against the polarization directions of this projected light in Embodiment 4 are presented in FIGS. 13A and 13B. FIG. 13A shows the relationship between the polarization directions of the beams (R, B, and G light) emerging from the dichroic prism and the slow axis direction (dotted line) of the half-wave plate  44 , and FIG. 13B shows the relationship between the polarization directions of the beams (R, B, and G light) projected onto the polarizing screen  46  and the transmission axis direction of the polarizing screen. 
     In FIG. 13A, the slow axis direction of the half-wave plate  44  is indicated by the dotted line, which is set at the angle of 56.25° relative to the s-polarized light direction in the dichroic films of the dichroic prism DP. Therefore, the light having passed through the half-wave plate  44  is converted into light of the polarization direction each inclined at 22.5° relative to the p-polarized light direction in the dichroic films of the dichroic prism DP. Since the transmission polarization direction of the polarizing screen  46  is set perpendicular to the s-polarized light direction in the dichroic films of the dichroic prism DP, the rate of the light that can be utilized for observation without being absorbed by the screen  46 , is computed as follows. 
     
       
         cos 2 (22.5)=0.853 
       
     
     This means that 85.3% of the projected light is allowed to pass through the polarizing screen and to be utilized. 
     In Embodiments 1 and 4 the phase plate may also be arranged to be rotatable about the axis of rotation along the direction parallel to the optical axis of the projection lens, without being fixed. This permits the polarization direction of the projected light to be converted into an optimal state no matter how the transmission polarization direction of the polarizing screen is oriented. 
     Next described is Embodiment 5 as an image observation apparatus of the present invention. The systems commonly used for observing a stereoscopic picture by use of the image projection apparatus are stereoscopic image projection systems using two image projection devices PJ 1 , PJ 2  in such structure that the projectors PJ 1 , PJ 2  project enlarged images of right eye image and left eye image (or left eye image and right eye image) onto the screen Sc having the property of preserving the polarization states thereof and that the images are observed through polarizing glasses provided with polarizing plates of polarization components perpendicular to each other for the left and right eyes. The present embodiment concerns such a system. 
     FIG. 14 is a schematic diagram to show the major part of the stereoscopic image projection system according to Embodiment 5 of the present invention. In FIG. 14 each of the image projection devices PJ 1 , PJ 2  has a color combining system of the structure as stated in Embodiments 1 to 4, in which the polarization direction of the red and blue light emerging from the dichroic prism is set to that of the s-polarized light to the dichroic films of the dichroic prism and in which the polarization direction of the green light is inclined at 45°, which is larger than 0° but smaller than 90°, relative to the polarization direction of the red and blue light. 
     A filter PF 1  or PF 2  comprised of a half-wave plate and a polarizer is located at the exit part of the projection lens of each image projector PJ 1 , PJ 2 . Relations of the slow axis direction of the half-wave plate and the transmission polarization direction of the polarizer of the filter PF 1  against the polarization directions of the projected light emerging from the dichroic prism DP in the image projector PJ 1  are presented in FIGS. 15A to  15 C. FIG. 15A shows the relationship between the polarization directions of the beams (R, B, and G light) emerging from the dichroic prism DP and the slow axis (indicated by the dashed line) of the half-wave plate of the filter PF 1 , FIG. 15B the relationship between the polarization directions of the beams (R, B, and G light) transmitted by the half-wave plate of the filter PF 1  and the transmission direction A (indicated by the dashed line) of the polarizer of the filter PF 1 , and FIG. 15C the polarization direction of the light projected to the screen Sc. 
     In FIGS. 15A to  15 C, the slow axis direction of the half-wave plate indicated by the dotted line is set at the angle of 11.25° relative to the direction of the s-polarized light in the dichroic films of the dichroic prism DP, the light transmitted by the half-wave plate is converted each into light of the polarization direction inclined at 22.5° relative to the direction of the s-polarized light in the dichroic films of the dichroic prism DP, and the transmission polarization direction A of the polarizer of the filter PF 1  is set in parallel to the direction of the s-polarized light in the dichroic films of the dichroic prism DP. Relations of the slow axis direction of the half-wave plate and the transmission polarization direction of the polarizer of the filter PF 2  against the polarization directions of the projected light emerging from the dichroic prism DP in the image projector PJ 2  are presented in FIGS. 16A to  16 C. FIG. 16A shows the relationship between the polarization directions of the beams (R, B, and G light) emerging from the dichroic prism DP and the slow axis (indicated by the dashed line) of the half-wave plate of the filter PF 2 , FIG. 16B the relationship between the polarization directions of the beams (R, B, and G light) transmitted by the half-wave plate of the filter PF 2  and the transmission direction A (indicated by the dashed line) of the polarizer of the filter PF 2 , and FIG. 16C the polarization direction of the light projected to the screen Sc. 
     In FIGS. 16A to  16 C, the slow axis direction (dotted line) of the half-wave plate is set at the angle of 56.25° relative to the s-polarized light direction in the dichroic films of the dichroic prism DP, the light transmitted by the half-wave plate is converted each into light of the polarization direction inclined at 22.5° relative to the p-polarized light direction in the dichroic films of the dichroic prism, and the transmission polarization direction A of the polarizer of the filter PF 2  is set perpendicular to the s-polarized light direction in the dichroic films of the dichroic prism DP. 
     This permits such setting that the polarization direction of the light projected from the image projector PJ 1  is perpendicular to that of the light projected from the image projector PJ 2 . The images are projected onto the screen having the property of being capable of reflecting incident light while maintaining polarization states of the incident light. When the images are observed through the polarizing glasses provided with the polarizers having the respective transmission polarization axes perpendicular to each other, for the right eye and for the left eye, the parallax image for the right eye is guided to the right eye and the parallax image for the left eye is guided to the left eye, thus permitting the observer to observe a stereoscopic image.