Abstract:
An image display device for displaying a high quality and high resolution image without causing color irregularities, adapted to be assembled simply and at low cost. The device includes a plurality of spatial light modulation elements ( 6 R,  6 G;  6 B) illuminated by lights of different colors for performing an image modulation; a color synthesizer ( 7 ) for synthesizing image lights modulated by each of the spatial light modulation elements; an incident polarized light controller ( 10 ) for converting at least one of the plurality of image lights of different colors projected to the color synthesizer, into a different polarization than those of the other image lights; and a pixel shift device ( 15 ) that includes at least one set of a polarization conversion element ( 21 ) and a double refraction plate ( 22 ). The pixel shift device ( 15 ) selectively shifts the optical paths of the image lights synthesized by the color synthesizer. A color-selective polarization converter ( 25 ) is arranged between the polarization conversion element ( 21 ) and the double refraction plate ( 22 ) forming a first set in the pixel shift device ( 15 ). The color-selective polarization converter ( 25 ) forms an integral a unit with the pixel shift device and aligning polarizations of the plurality of image lights from the polarization conversion element ( 21 ) to each other before the image lights are projected to the double refraction plate ( 22 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an image display device and, more particularly, to an image display device wherein image lights are subjected to an image modulation of different colors by a plurality of spatial light modulation elements, the modulated image lights are synthesized by a color synthesizer, and the synthesized image light is displayed while selectively shifting an optical path thereof by a pixel shift device. 
         [0003]    2. Description of Related Art 
         [0004]    As a conventional image display device, there is known a 3LCD-type image display device wherein a light from a white light source is separated into a red (R) light, a green (G) light and a blue (B) light, and the separated lights are projected to respective liquid crystal display elements as spatial light modulation elements and thereby subjected to image modulation. The modulated R, G and B lights are synthesized by a color synthesizer in the form of a dichroic prism, for example, and then displayed on a screen through a projection lens. 
         [0005]    In such a 3LCD-type image display device, the synthesized image light may be subjected to a wavelength shift due to incidence angle characteristics of a dichroic film of the dichroic prism, thereby causing color irregularities. This sort of color irregularities may occur particularly when an integrator optical system is arranged in an illumination optical system, in order to equalize the illumination distribution of an illumination light emitted from the white light source. This is because the image lights modulated by the respective liquid crystal display elements are projected to the dichroic film at different incidence angles. 
         [0006]    For eliminating such a problem, there is known a method that takes into account the polarization characteristics of the dichroic film, such that the G image light is projected to, and transmitted through the dichroic film in P-polarization, whereas the R and B image lights are projected to, and reflected on the dichroic film in S-polarization, before the R, G and B image lights are synthesized. 
         [0007]    Also, various image display devices have been proposed, wherein the optical paths of the image lights subjected to image modulation by spatial light modulation elements are selectively shifted by pixel shift device, so as to achieve a high resolution. Reference may be had, for example, to Patent Document 1 identified below. 
         [0008]    The pixel shift device includes, as shown in  FIG. 9  by way of example, a polarization conversion element  110  and a double refraction plate  111 , such as a liquid crystal panel and a quartz plate, for example. The polarization conversion element  110  selectively rotates the direction of the linear polarization of the modulated image light by 90 degrees while being synchronized with the image modulation by the spatial light modulation elements (not shown), so that the double refraction plate  111  shifts the optical path of the image light according to the direction of the linear polarization, for example, by half a pixel pitch with respect to a horizontal pixel arrangement of the spatial light modulation elements. There are also known a pixel shift device in which a plurality of sets of a polarization conversion element  110  and a double refraction plate  111  are provided so that the position of a displaying pixel can be selectively shifted in different directions, for example, in the horizontal direction, in the vertical direction, or in the oblique direction. 
         [0009]    There is also known an image display device in the form of combination of the above mentioned 3LCD image display device and the pixel shift device shown in  FIG. 9 , in which the optical path of the image light synthesized by the dichroic prism is selectively shifted by the pixel shift device so as to achieve a high resolution. 
         [0010]      FIG. 10  shows the configuration of the relevant parts of such an image display device, including a liquid crystal display element  120 R for performing an image modulation for the R light, a liquid crystal display element  120 G for performing an image modulation for the G light, a liquid crystal display element  120 B for performing an image modulation for the B light, a dichroic prism  121  for synthesizing the image lights modulated by the liquid crystal display elements  120 R,  120 G and  120 B, a pixel shift device  122  for selectively shifting the optical paths of the synthesized image lights, and a projection lens  123  for projecting the image lights from the pixel shift device  122  onto a screen (not shown).  FIG. 10  further shows an example of the pixel shift device  122 , which includes a set of a polarization conversion element  110  and a double refraction plate  111 , for performing a double-point pixel shift. 
         [0011]    In the image display device shown in  FIG. 10 , however, in an attempt to prevent color irregularities caused by incidence angle characteristics of the dichroic film of the dichroic prism  121 , if the G image light from the liquid crystal display element  120 G is projected to, and transmitted through the dichroic film in P-polarization, while the R image light and the B image light from the liquid crystal display element  120 R and the liquid crystal display element  120 B, respectively, are projected to, and reflected by the dichroic film in S-polarization, and those R, G and B image lights are synthesized so that the optical path of the synthesized image lights is selectively shifted by the pixel shift device, the display position of the G image light will be misaligned from the display positions of the R and B lights, because the directions of the linear polarizations of the R, G and B image lights projected to the pixel shift device  122  are not aligned with each other. 
         [0012]    More specifically, in the image display device shown in  FIG. 10 , the G image light is projected to the polarization conversion element  110  of the pixel shift device  122  in P-polarization, whereas the R and B image lights are projected to the polarization conversion element  110  in S-polarization. Accordingly, if the R, G and B image lights are projected to the double refraction plate  111  without changing the directions of the linear polarizations of the incident lights by the polarization conversion element  110 , the optical paths of the R and B image lights are not subjected to pixel shift, and only the optical path of the G image light is subjected to a pixel shift, as shown in  FIG. 11(   a ). Conversely, if the directions of the linear polarizations of the incident lights are rotated by 90 degrees by the polarization conversion element  110 , the G image light is projected to the double refraction plate  111  in S-polarization, whereas the R and B image lights are projected to the polarization conversion element  110  in P-polarization. Accordingly, the optical paths of the R and B image lights are subjected to a pixel shift, and only the optical path of the G image light is not subjected to pixel shift, as shown in  FIG. 11(   b ). Therefore, the display position of the G image is misaligned from the display positions of the R and B images, which is an obstacle to achievement of high resolution and which may degrade the image quality. 
         [0013]    In order to solve these problems, there has been proposed an image display device wherein a color-selective polarization plane rotation means including a laminated phase plate is arranged between the dichroic prism and the pixel shift device. With such an image display device, the polarization plane of the G light in P-polarization, for example, is rotated and converted into S-polarization, while the polarization planes of the R and B lights are not rotated so as to remain in S-polarization, so that the polarization directions of the image lights of different colors projected to the pixel shift device are aligned with S-polarization. Reference may be had, for example, to Patent Document 2 identified below. 
         [0014]    Patent Document 1: JP 2813041B2 
         [0015]    Patent Document 2: JP 2003-207747A 
         [0016]    In the case of pixel shift device such as that disclosed in JP 2813041B2 mentioned above, it is common to make the pixel shift device into a unit in order to assemble it while managing the specific characteristics inherent to the pixel shift device. 
         [0017]    Therefore, in the case of an arrangement wherein a color-selective polarization plane rotation means in the form of a laminated phase plate is simply arranged between the dichroic prism and the pixel shift device, as in the image display device disclosed in JP 2003-207747A, the pixel shift device and the laminated phase plate would be installed separately for assembling the image display device. 
         [0018]    Furthermore, since the laminated phase plate is exposed to outside, it is necessary to carry out an advance step of applying an antireflection coating to a purchased laminated phase plate or cleaning the surface of the laminated phase plate for removing dust thereon, resulting in burdensome assembling operation and increased cost. 
         [0019]    In addition, the pixel shift may cause a color mixture if, in terms of the polarization conversion characteristics of the laminated phase plate, there are overlaps of a P-polarization component and an S-polarization component in the transitional regions of the polarization conversion at upper or lower edge of the P-S conversion or, in other words, in the transitional region from the longer wavelength band of the B light to the shorter wavelength band of the G light, and the transitional region from the longer wavelength band of the G light to the shorter wavelength band of the R light, as shown in  FIG. 12 . 
         [0020]    More specifically, if there are overlaps of the P-polarization component and the S-polarization component at the boundary regions between the R and G lights and between the G and B lights, a colored light containing a P-polarization component is mixed with the R light that is supposed to be S-polarized in the shorter wavelength band of the R light (RP 1 ) in a transitional region between the P-polarization and the S-polarization, and a colored light containing a P-polarization component is mixed with the G light that is supposed to be S-polarized in the longer wavelength band of the G light (GS 1 ). Similarly, a colored light containing the P-polarization component is mixed with the G light that is supposed to be S-polarized in the shorter wavelength band of the G light (GP 1 ), and a colored light containing the P-polarization component is mixed with the B light that is supposed to be S-polarized in the longer wavelength band of the B light (BS 2 ). Thus, the pixel shift may cause color mixtures if there are mixtures of the P-polarization and S-polarization, to degrade the image quality. 
         [0021]    Such a problem may also occur also when the color synthesizer is comprised of a polarizing beam splitter. 
       DISCLOSURE OF THE INVENTION 
       [0022]    It is a therefore primary object of the present invention to provide an image display device capable of displaying a high quality and high resolution image without causing color irregularities, and adapted to be assembled easily and at low cost. 
         [0023]    To this end, a first aspect of the present invention resides in an image display device comprising: a plurality of spatial light modulation elements illuminated by lights of different colors for performing an image modulation; a color synthesizer for synthesizing image lights modulated by said spatial light modulation elements, respectively; an incident polarized light controller for converting at least one of said plurality of image lights of different colors projected to said color synthesizer, into a different polarization than those of the other image lights; a pixel shift device comprised of at least one set of a polarization conversion element and a double refraction plate, said pixel shift device being for selectively shifting the optical paths of the image lights synthesized by said color synthesizer; wherein a color-selective polarization converter is arranged between said polarization conversion element and said double refraction plate forming a first set in said pixel shift device, said color-selective polarization converter forming an integral unit together with said pixel shift device and aligning polarizations of said plurality of image lights from said polarization conversion element with each other, before the image lights are projected to said double refraction plate. 
         [0024]    A second aspect of the present invention resides in the image display device according to the first aspect, wherein the color synthesizer comprises a dichroic prism. 
         [0025]    A third aspect of the present invention relates to the image display device according to the first aspect, wherein said color synthesizer comprises a polarizing beam splitter. 
         [0026]    A fourth aspect of the present invention resides in the image display device according to any one of the first to third aspects, wherein said plurality of spatial light modulation elements comprise three spatial light modulation elements for performing an image modulation for a red light, a green light and a blue light, and said green light is projected to said color synthesizer in P-polarization, while said red light and said blue light are projected to said color synthesizer in S-polarization. 
         [0027]    A fifth aspect of the present invention resides in the image display device according to any one of the first to third aspects, wherein said plurality of spatial light modulation elements comprise a spatial light modulation element for performing an image modulation for a green light and a spatial light modulation element for selectively performing an image modulation for a red light and a blue light, and said green light is projected to said color synthesizer in P-polarization, while said red light and said blue light are projected to said color synthesizer in S-polarization. 
         [0028]    A sixth aspect of the present invention resides in the image display device according any one of the first to fifth aspects, wherein each of said plurality of spatial light modulation elements is a transmissive spatial light modulation element. 
         [0029]    A seventh aspect of the present invention resides in the image display device according to any one of the first to fifth aspects, wherein each of said plurality of spatial light modulation elements is a reflective spatial light modulation element. 
         [0030]    An eighth aspect of the present invention resides in the image display device according to any one of the first to seventh aspects, wherein said color-selective polarization converter has such polarization conversion characteristics that it performs a polarization conversion for a light in a wavelength band not being overlapped with those of said lights of different colors which are not subjected to a polarization conversion by said color-selective polarization converter. 
         [0031]    According to the present invention, since the color-selective polarization converter is arranged between the first set of the polarization conversion element and the double refraction plate of the pixel shift device so as to form an integral unit with the pixel shift device, and the color-selective polarization converter aligns the polarizations of the plurality of image lights from the polarization conversion element projected to the double refraction plate, a high quality and high resolution image can be displayed without causing color irregularities, besides that the color-selective polarization converter can be assembled in a simple manner and at low cost. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic diagram of an image display device according to a first embodiment of the present invention; 
           [0033]      FIGS. 2(   a ) and  2 ( b ) are diagrams explaining the pixel shift operation in the first embodiment; 
           [0034]      FIG. 3  is a schematic diagram of an image display device according to a second embodiment of the present invention; 
           [0035]      FIGS. 4(   a ) and  4 ( b ) are diagrams explaining the pixel shift operation in the second embodiment; 
           [0036]      FIG. 5  is a schematic diagram of an image display device according to a third embodiment of the present invention; 
           [0037]      FIGS. 6(   a ) and  6 ( b ) are diagrams explaining an image display device according to a fourth embodiment of the present invention; 
           [0038]      FIG. 7  is a diagram showing a modification of the pixel shift device; 
           [0039]      FIG. 8  is a diagram showing an example of the polarization conversion characteristics of the color-selective polarization converter; 
           [0040]      FIG. 9  is a diagram showing a basic configuration of a pixel shift device; 
           [0041]      FIG. 10  is a diagram showing a configuration of the relevant parts in a conventional image display device; 
           [0042]      FIGS. 11(   a ) and  11 ( b ) are diagrams explaining the pixel shift operation in the image display device of  FIG. 10 ; and 
           [0043]      FIG. 12  is a diagram showing an example of the polarization conversion characteristics of a laminated phase plate. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    The present invention will be explained below with reference to some preferred embodiments shown in the accompanying drawings. 
       First Embodiment 
       [0045]      FIG. 1  is a schematic diagram of an image display device according to a first embodiment of the present invention. In this embodiment, three transmissive spatial light modulation elements are used, which are in the form of transmissive liquid crystal display elements, for example, and a dichroic prism is used as a color synthesizer. With reference to  FIG. 1 , an illumination light from a white light source  1 , such as a mercury discharge lamp, is transmitted through an integrator optical system  2  and projected to a dichroic mirror  3  such that the R light is transmitted whereas the lights of other wavelengths are reflected to separate the R light. 
         [0046]    The R light separated by the dichroic mirror  3  is projected, via reflecting mirrors  4  and  5 , to a spatial light modulation element  6 R for the R light as an illumination light, so that the R light is subjected to an image modulation and projected to a dichroic prism  7  as a color synthesizer. 
         [0047]    The lights reflected on the dichroic mirror  3 , on the other hand, are projected to a dichroic mirror  8  so that the B light is transmitted whereas the G light is reflected, thereby separating the B and G lights from each other. The B light from the dichroic mirror  8  is projected, via a reflecting mirror  9 , to a spatial light modulation element  6 B for the B light as an illumination light, so that the B light is subjected to an image modulation and then projected to the dichroic prism  7 . 
         [0048]    The G light separated by the dichroic mirror  8  is projected, via a reflecting mirror  11 , to a spatial light modulation element  6 G for the G light as an illumination light after the polarization plane thereof has been rotated by 90 degrees by means of a half-wavelength plate  10  as an incident polarized light controller, so that the G light is subjected to an image modulation and projected to the dichroic prism  7 . 
         [0049]    The integrator optical system  2  may be of a type known, per se, comprised of an optical system including an integrator rod or a fly-eye lens for realizing a substantially uniform illumination intensity distribution of the illumination light for the spatial light modulation elements  6 R,  6 G and  6 B, and a P-S conversion element for converting the emitted light into a predetermined kind of linear polarization. 
         [0050]    The dichroic prism  7  reflects the R light modulated by the spatial light modulation element  6 R, and the B light modulated by the spatial light modulation element  6 B, while transmitting the G light modulated by the spatial light modulation element  6 C; in order that the images of the R, G and B lights are synthesized and then emitted. 
         [0051]    According to the present embodiment, the light emitted from the integrator optical system  2  is made into S-polarization. The spatial light modulation elements  6 R and  6 B are respectively illuminated with the R and B lights in S-polarization, and each modulated image light is then projected to the dichroic prism  7  in S-polarization. The spatial light modulation element  6 G, on the other hand, is illuminated with the G light in P-polarization since the polarization plane of the G light has been rotated by 90 degrees by means of a half-wavelength plate  10 . The modulated G light is then projected to the dichroic prism  7  in P-polarization. Accordingly, a wavelength shift may be made of the synthesized image lights due to incidence angle characteristics of a dichroic film  7   a  of the dichroic prism  7 , so that color irregularities in a displaying image is prevented 
         [0052]    The optical paths of the synthesized image lights from the dichroic prism  7  is shifted by a pixel shift device  15 , being synchronous with image modulation performed by the spatial light modulation elements  6 R,  6 G and  6 B. The image lights from the pixel shift device  15  is projected and displayed by means of a projector lens  16  onto a screen which is not illustrated. 
         [0053]    In the present embodiment, the pixel shift device  15  is of a double-point pixel shift configuration comprising a set of a polarization conversion element  21  and a double refraction plate  22 . A laminated phase plate  25  as a color-selective polarization converter is provided between the polarization conversion element  21  and the double refraction plate  22 . The polarization plane of the G light is rotated by 90 degrees by means of this laminated phase plate  25 , and the polarizations of the R, G and B image lights from the polarization conversion element  21  are aligned with each other and then projected to the double refraction plate  22 . 
         [0054]    The pixel shift device  15  is made into a unit by holding the polarization conversion element  21  and the double refraction plate  22  integrally with a holder member  31 . The laminated phase plate  25  is held with the holder member  31  together with the polarization conversion element  21  and the double refraction plate  22  to be made into a part of the unitized pixel shift device  15 . As the laminated phase plate, there may be used a “Color Select” (trade name; manufactured by Color Link Inc., USA). 
         [0055]    When the polarization conversion element  21  included in the pixel shift device  15  comprises, e.g., a liquid crystal panel as in the present embodiment, the polarization conversion element  21  transmits the incident light without any conversion of polarization in an ON state where a required voltage is applied to the polarization conversion element  21 . On the other hand, in an OFF state where the voltage to the polarization conversion element  21  is shut off, the polarization conversion element  21  transmits the incident light while rotating the polarization plane of the incident light by 90 degrees. 
         [0056]    Accordingly, in the ON state of the polarization conversion element  21 , the image lights synthesized by the dichroic prism  7  is projected to the laminated phase plate  25  without being subjected to rotation of the polarization plane at the polarization conversion element  21 , as shown in a schematic diagram in  FIG. 2(   a ). More specifically, since the G light is projected to the laminated phase plate  25  in P-polarization while the R and B lights are projected to the laminated phase plate  25  in S-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  and converted from P-polarization into S-polarization. Accordingly, the R, G and B image lights synthesized by the dichroic prism  7  is projected to the double refraction plate  22  in uniform S-polarization. Thus, each image light is transmitted through the double refraction plate  22  without being subjected, e.g., to an optical path shift. 
         [0057]    On the other hand, in the OFF state of the polarization conversion element  21 , the image lights synthesized by the dichroic prism  46  are projected to the laminated phase plate  25  while being subjected to rotation of the polarization plane by means of the polarization conversion element  21 , as schematically shown in the diagram of  FIG. 2(   b ). More specifically, since the G light is projected to the laminated phase plate  25  in S-polarization while the R and B lights are projected to the laminated phase plate  25  in P-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  and thereby converted from S-polarization into P-polarization. Accordingly, the R, G and B image lights synthesized by the dichroic prism  7  are projected to the double refraction plate  22  in uniform P-polarization. Thus, each image light is transmitted through the double refraction plate  22  while being subjected to an optical path shift. 
         [0058]    As described above, according to the present embodiment, the modulated images of the R and B lights are projected to the dichroic prism  7  in S-polarization while the modulated image of the G light is projected to the dichroic prism  7  in P-polarization, before the image lights are synthesized. The polarizations of the synthesized image lights are aligned with each other by means of the laminated phase film  25  and subjected to pixel shift. Therefore, it is possible to prevent color irregularities that may be otherwise caused by the incidence angle characteristics of the dichroic prism  7 , and to display an image of high quality and high resolution. Further, the pixel shift device  15  is made into a unit by integrally holding the polarization conversion element  21  and the double refraction plate with the holder member  31 , and the laminated phase plate  25  for aligning the polarizations of the synthesized image lights is provided between the polarization conversion element  21  and the double refraction plate  22  and held by the holder member  31  to be made into a part of the unitized pixel shift device  15 . Therefore, the polarization conversion element  21 , the laminated phase plate  25  and the double refraction plate  22  can be joined with an adhesive having a similar refractive index, which will make it possible to assemble it in a simple manner and at low cost without any need of applying an antireflective coating to the laminated phase plate  25  or cleaning the surface thereof. 
       Second Embodiment 
       [0059]      FIG. 3  is a schematic diagram of an image display device according to a second embodiment of the present invention. In the present embodiment, three reflective spatial light modulation elements are used, which are in the form of reflective liquid crystal display elements or DMDs (digital micro mirror devices), for example, and a dichroic prism is used as a color synthesizer. 
         [0060]    With reference to  FIG. 3 , an illumination light emitted from a white light source  41  is transmitted through an integrator optical system  42  and is projected to a dichroic mirror  43  in P-polarization, so that the R light is reflected while the lights of other wavelengths are transmitted, thereby separating the R light. 
         [0061]    The R light separated by the dichroic mirror  43  is projected to a polarizing beam splitter  44  and transmitted through a multiple layer  44   a  thereof. The R light from this polarizing beam splitter  44  is projected to a spatial light modulation element  45 R for the R light as an illumination light to be subjected to an image modulation by the spatial light modulation element  45 R. The R light modulated by the spatial light modulation element  45 R is converted into S-polarization since the spatial light modulation element  45 R is a reflective one, and thus the R light is reflected on the multiple layer  44   a  of the polarizing beam splitter  45  and is projected to the dichroic prism  46  as a color synthesizer. 
         [0062]    The lights transmitted through the dichroic mirror  43 , on the other hand, are projected to a dichroic mirror  48  through a reflecting mirror  47 , so that the B light is transmitted while the G light is reflected, thereby separating the B and G lights from each other. The B light separated by the dichroic mirror  48  is projected to a polarizing beam splitter  49  and transmitted through a multiple layer  49   a  thereof. The B light from the polarizing beam splitter  49  is projected to a spatial light modulation element  45 B for the B light as an illumination light, and subjected to an image modulation by the spatial light modulation element  45 B to be converted into S-polarization. The S-polarized B light modulated by the spatial light modulation element  45 B is reflected on the multiple layer  49   a  of the polarizing beam splitter  49  and projected to the dichroic prisms  46 . 
         [0063]    The polarization plane of the G light separated by the dichroic mirror  48  is rotated by 90 degrees by means of a half-wavelength plate  50  to be converted into S-polarization. The G light is then projected to a polarizing beam splitter  51  and reflected on a multiple layer  51   a  thereof. The G light from the polarizing beam splitter  51  is projected to a spatial light modulation element  45 G for the G light as an illumination light, and subjected to an image modulation by the spatial light modulation element  45 G to be converted into P-polarization. The P-polarized G light modulated by the spatial light modulation element  45 G is transmitted through the multiple layer  51   a  of the polarizing beam splitter  51  and projected to the dichroic prism  46 . 
         [0064]    At the dichroic prism  46 , the S-polarized R light modulated by the spatial light modulation element  45 R and the S-polarized B light modulated by the spatial light modulation element  45 B are reflected while the P-polarized G light modulated by the spatial light modulation element  45 G is transmitted, and the images of the R, G and B lights are synthesized and emitted. 
         [0065]    The polarizations of the image lights modulated by the spatial light modulation elements  45 R,  45 G and  45 B are aligned with each other with the use of a pixel shift device  15  and a laminated phase plate  25 . The pixel shift device  15  is of double-point pixel shift configuration and made into a unit comprising a set of a polarization conversion element  21  and a double refraction plate  22  which are integrally held by a holder member  31 . The laminated phase plate  25  is provided between the polarization conversion element  21  and the double refraction plate  22  and made into a part of the unitized pixel shift device  15 . The optical paths of the image lights are shifted thereby being synchronous with an image modulation, and the image lights are then projected by means of a projection lens  16  onto a screen which is not illustrated. 
         [0066]    In the present embodiment, in the same way as in Embodiment 1, in the ON state of the polarization conversion element  21 , the image lights synthesized by the dichroic prism  46  are projected to the laminated phase plate  25  without being subjected to rotation of the polarization plane by the polarization conversion element  21 , as shown in a schematic diagram in  FIG. 4(   a ). More specifically, since the G light is projected to the laminated phase plate  25  in P-polarization while the R and B lights are projected to the laminated phase plate  25  in S-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  to be converted from P-polarization into S-polarization. With this, the R, G and B image lights synthesized by the dichroic prism  46  are projected to the double refraction plate  22  in uniform S-polarization. Thus, each image light is transmitted through the double refraction plate  22  without being subjected, e.g., to an optical path shift. 
         [0067]    In the OFF state of the polarization conversion element  21 , on the other hand, the image lights synthesized by the dichroic prism  46  are projected to the laminated phase plate  25  while being subjected to rotation of the polarization plane at the polarization conversion element  21 , as shown in a schematic diagram in  FIG. 4(   b ). More specifically, since the G light is projected to the laminated phase plate  25  in S-polarization while the R and B lights are projected to the laminated phase plate  25  in P-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  to be converted from S-polarization into P-polarization. Accordingly, the R, G and B image lights synthesized by the dichroic prism  46  are projected to the double refraction plate  22  in uniform P-polarization. Thus, each image light is transmitted through the double refraction plate  22  while being subjected to an optical path shift. 
         [0068]    With the present embodiment, therefore, it is possible to prevent color irregularities caused by incidence angle characteristics of the dichroic prism  46  and to display an image of high quality and high resolution. Further, because the laminated phase plate  25  is made into a part of the unitized pixel shift device  15 , it can be assembled in a simple manner and at low cost without any need of applying an antireflective coating to the laminated phase plate  25  or cleaning the surface thereof. 
       Third Embodiment 
       [0069]      FIG. 5  is a schematic diagram of an image display device according to a third embodiment of the present invention. In the present embodiment, a transmissive spatial light modulation element for the G light and a transmissive spatial light modulation element shared for the R and B lights are used as spatial light modulation elements, and a polarizing beam splitter is used as a color synthesizer. 
         [0070]    With reference to  FIG. 5 , an illumination light emitted from a white light source  61  is transmitted through an integrator optical system  62  and emitted in S-polarization to be projected to a dichroic mirror  63 , whereby the G light is reflected while the lights of other wavelengths are transmitted, and the G light is thus separated. 
         [0071]    The polarization plane of the G light separated by the dichroic mirror  63  is rotated by 90 degrees by means of a half-wavelength plate  64  as an incident polarized light controller to be converted into P-polarization. The G light is then projected to a spatial light modulation means  65 G for the G light and subjected to an image modulation. The modulated G light is projected to a polarizing beam splitter  66  as a color synthesizer and is emitted through a multiple layer  66   a  thereof. 
         [0072]    The lights transmitted through the dichroic mirror  63 , on the other hand, are projected to a dichroic mirror  67  so that the R light is reflected while the B light is transmitted, to thereby separate the R and B lights from each other. The R light separated by the dichroic mirror  67  is reflected on a dichroic mirror  69  via a shutter  68 , and projected to a spatial light modulation element  65 RB that is shared for both the R and B lights. The B light separated by the dichroic mirror  67  is passed through a reflecting mirror  70 , a shutter  71  and a reflecting mirror  72 , transmitted through the dichroic mirror  69 , and projected to the spatial light modulation element  65 RB. 
         [0073]    The shutters  68  and  71  are controlled so as to be alternately opened and closed, so that the R and B lights are subjected to a time-shared image modulation and projected to the polarizing beam splitter in S-polarization. 
         [0074]    Since a P-polarized light is transmitted through the polarizing beam splitter  66  while an S-polarized light is reflected thereon, the P-polarized G light modulated by the spatial light modulation element  65 G is transmitted through the multiple layer  66   a  of the polarizing beam splitter  66  while the S-polarized R or B light modulated by the spatial light modulation element  65 RB is reflected on the multiple layer  66   a , so that the G light and the R light, or the G light and the B light, are synthesized and then emitted. 
         [0075]    The polarizations of the synthesized R and G lights or the synthesized B and G lights emitted from the polarizing beam splitter  66  are aligned with each other with the use of the pixel shift device  15  and the laminated phase plate  25 , in the same way as in the above embodiment. The pixel shift device  15  is of double-point pixel shift configuration and made into a unit comprising a set of a polarization conversion element  21  and a double refraction plate  22  which are integrally held by a holder member  31 . The laminated phase plate  25  is provided between the polarization conversion element  21  and the double refraction plate  22  and made into a part of the unitized pixel shift device  15 . The optical paths of the image lights are shifted being synchronous with an image modulation, and the image lights are then projected by means of a projection lens  16  onto a screen which is not illustrated. 
         [0076]    According to the present embodiment, in the ON state of the polarization conversion element  21 , the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  66  are projected to the laminated phase plate  25  without being subjected to rotation of the polarization plane at the polarization conversion element  21 . More specifically, since the G light is projected to the laminated phase plate  25  in P-polarization while the R or B light is projected to the laminated phase plate  25  in S-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  and thereby converted from P-polarization into S-polarization. With this, the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  64  are projected to the double refraction plate  22  in uniform S-polarization. Thus, each image light is transmitted through the double refraction plate  22  without being subjected to e.g. an optical path shift. 
         [0077]    In the OFF state of the polarization conversion element  21 , on the other hand, the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  66  are projected to the laminated phase plate  25  while being subjected to rotation of the polarization plane at the polarization conversion element  21 . More specifically, since the G light is projected to the laminated phase plate  25  in S-polarization while the R or B light is projected to the laminated phase plate  25  in P-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  and thereby converted from S-polarization into P-polarization. Accordingly, the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  66  are projected to the double refraction plate  22  in uniform P-polarization. Thus, each image light is transmitted through the double refraction plate  22  while being subjected to an optical path shift. In this way, by modulating the G light and the time-shared R and B lights synchronously with the ON/OFF states of the polarization conversion element  21  of the pixel shift device  15 , an image of higher resolution can be displayed. 
         [0078]    Accordingly, with the present embodiment, it is possible to prevent color irregularities caused by incidence angle characteristics of the polarizing beam splitter  66  and to display an image of high quality and high resolution. Further, since the laminated phase plate  25  is made into a part of the unitized pixel shift device  15 , the polarization conversion element  21 , it can be assembled in a simple manner and at low cost without any need of applying an antireflective coating to the laminated phase plate  25  or cleaning the surface thereof. 
       Fourth Embodiment 
       [0079]      FIG. 6  is a diagram for explaining an image display device according to a fourth embodiment of the present invention.  FIG. 6(   a ) shows an overall schematic configuration, while  FIG. 6(   b ) shows a configuration of an example of a rotating color filter shown in  FIG. 6(   a ). In the present embodiment, a reflective spatial light modulation element for the G light and a reflective spatial light modulation element shared for the R and B lights are used as spatial light modulation elements, and a polarizing beam splitter is used as a color synthesizer. 
         [0080]    With reference to  FIG. 6(   a ), an illumination light from a white light source  81  is transmitted through an integrator optical system  82  and emitted in S-polarization, and this S-polarized illumination light is transmitted through the rotating color filter  86  and a color-selective polarization conversion element  83  as an incident polarized light controller, and then projected to a polarizing beam splitter  84  as a color synthesizer. 
         [0081]    The rotating color filter  86  comprises, for example as shown in a plan view of  FIG. 6(   b ), equally divided six regions of a circle, which are alternately provided therein with a color filter GB for transmitting the G and B lights therethrough, or a color filter GR for transmitting the G and R lights therethrough. With rotation of the rotating color filter  86 , the GB light and the GR light are switched in a time-shared manner. Here, the color-selective polarization conversion element  83  converts the G light into P-polarization, and it may be the laminated phase plate explained in the above embodiment. 
         [0082]    P-polarized light is transmitted through the polarizing beam splitter  84  while S-polarized light is reflected thereon. G light included in GB illumination light or GR illumination light projected to the polarizing beam splitter  84  in a time-shared manner, which has been converted into P-polarization by means of the color-selective polarization conversion element  83 , is transmitted through a multiple layer  84   a  and projected to a reflective spatial light modulation element  85 G for the G light, whereby the G light is subjected to an image modulation and converted into S-polarization. The S-polarized G light modulated by the spatial light modulation element  85 G is reflected on the multiple layer  84   a  of the polarizing beam splitter  84  and emitted. 
         [0083]    The S-polarized R or B light incident on the polarizing beam splitter  84  in a time-shared manner is reflected on the multiple layer  84   a  and subjected to an image modulation by a reflective spatial light modulation element  85 RB shared for the R and B lights while being converted into P-polarization. The P-polarized R or B light modulated by the spatial light modulation element  85 RB is transmitted through the multiple layer  84   a  of the polarizing beam splitter  84  and synthesized with the G light modulated by the spatial light modulation element  85 G to be emitted. 
         [0084]    The polarizations of the synthesized R and G lights or the synthesized B and G lights emitted from the polarizing beam splitter  84  are aligned with each other with a pixel shift device  15  and a laminated phase plate  25 , in the same way as in the above embodiment. The pixel shift device  15  is of double-point pixel shift configuration and made into a unit comprising a set of a polarization conversion element  21  and a double refraction plate  22  which are integrally held by a holder member  31 . The laminated phase plate  25  is provided between the polarization conversion element  21  and the double refraction plate  22  and made into a part of the unitized pixel shift device  15 . The optical paths of the image lights are shifted thereby being synchronous with an image modulation, and the image lights are then projected by means of a projection lens  16  onto a screen which is not illustrated. 
         [0085]    According to the present embodiment, in the ON state of the polarization conversion element  21 , the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  84  are projected to the laminated phase plate  25  without being subjected to rotation of the polarization plane by the polarization conversion element  21 . More specifically, since the G light is projected to the laminated phase plate  25  in the S-polarization while the R or B light is projected to the laminated phase plate  25  in the P-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  and converted from the S-polarization into the P-polarization. In this instance, the R and G image lights or the B and G image lights synthesized at the polarizing beam splitter  84  are projected to the double refraction plate  22  in alignment with the P-polarization. Thus, each image light is transmitted through the double refraction plate  22  while being subjected, for example, to an optical path shift. 
         [0086]    In the OFF state of the polarization conversion element  21 , on the other hand, the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  84  is projected to the laminated phase plate  25  while being subjected to rotation of the polarization plane at the polarization conversion element  21 . More specifically, since the G light is projected to the laminated phase plate  25  in G-polarization while the R or B light is projected to the laminated phase plate  25  in S-polarization, only the polarization plane of the G light is rotated by 90 degrees by means of the laminated phase plate  25  to be converted from P-polarization into S-polarization. Accordingly, the R and G image lights or the B and G image lights synthesized by the polarizing beam splitter  84  are projected to the double refraction plate  22  in uniform S-polarization. Thus, each image light is transmitted through the double refraction plate  22  without being subjected to an optical path shift. In this way, by modulating the G light and the time-shared R and B lights synchronously with the ON/OFF states of the polarization conversion element  21  of the pixel shift device  15 , it is possible to display a high resolution image. 
         [0087]    With the present embodiment, as well as with the third embodiment, it is possible to prevent color irregularities that may be otherwise caused by incidence angle characteristics of the polarizing beam splitter  84 , and to display an image of high quality and high resolution. Further, since the laminated phase plate  25  is made into a part of the unitized pixel shift device  15 , the polarization conversion element  21 , it can be assembled in a simple manner and at low cost without any need of applying an antireflective coating to the laminated phase plate  25  or cleaning the surface thereof. 
         [0088]    The present invention is not limited to the embodiments described above, and various modifications or changes may be made without departing from the scope of the invention. For example, the pixel shift device  15  is not limited to the double-point pixel shift configuration having a set of a polarization conversion element  21  and a double refraction plate  22 , and there may be applied a four-point pixel shift configuration shown in  FIG. 7 , in which two sets of polarization conversion elements  21   a ,  21   b  and double refraction plates  22   a ,  22   b  are held integrally with a holder member  31  as a part of the unitized pixel shift device. Further, the pixel shift device  15  may be of a six-point pixel shift configuration in which not less than three sets of polarization conversion elements and double refraction plates are held integrally with a holder member as a part of the unitized pixel shift device, so as to allow pixel shift configurations of not less than six points. In case where the pixel shift device  15  is made into an integral unit comprising a plurality of sets of polarization conversion elements and double refraction plates, a color-selective polarization converter  25  is preferably arranged between the polarization conversion element  21   a  and the double refraction plate  22   a  of the first set of the pixel shift device  15  as a part of the unitized pixel shift device  15 , as illustrated in  FIG. 7 . 
         [0089]    Moreover, the light source of the illumination light is not limited to a white light source, and colored light sources such as LEDs for emitting R, G and B lights may also be used. The color-selective polarization converter provided integrally with the pixel shift device may convert a polarization of the R or B light instead of that of the G light. Further, the color-selective polarization converter may also be configured to have such polarization conversion characteristics that a light in a wavelength band subjected to polarization conversion is not overlapped with a light in a wavelength band not subjected to polarization conversion, as shown in  FIG. 8 . In this instance, since the P-polarized component is not mixed with the S-polarized component in the wavelength regions ranging from the longer wavelength band of the B light to the shorter wavelength band of the G light, and from the longer wavelength band of the G light to the shorter wavelength band of the R light, a pixel shift can be made without color mixtures, thereby allowing an image of higher quality to be displayed. In addition, instead of controlling polarization conversion characteristics of the color-selective polarization converter, similar effects may also be achieved by controlling the reflection characteristics or transmission characteristics of a coating applied on the optical elements such as a dichroic prism, a polarizing beam splitter or a dichroic mirror so that the wavelength bands of R, G and B are not overlapped. Further, when a colored light source such as an LED is used, similar effects may be achieved by using narrow-banded colored light sources so that the illumination wavelength bands of R, G and B are not overlapped. 
       LISTING OF REFERENCE NUMERALS 
       [0000]    
       
           1  white light source 
           2  integrator optical system 
           3 ,  8  dichroic mirror 
           4 ,  5 ,  9 ,  11  reflecting mirror 
           6 R,  6 G,  6 B spatial light modulation element 
           7  dichroic prism 
           10  half-wavelength plate 
           15  pixel shift device 
           16  projection lens 
           21 ,  21   a ,  21   b  polarization conversion element 
           22 ,  22   a ,  22   b  double refraction plate 
           25  laminated phase plate 
           31  holder member 
           41  white light source 
           42  integrator optical system 
           43 ,  48  dichroic mirror 
           44 ,  49 ,  51  polarizing beam splitter 
           44   a ,  49   a ,  51   a  multiple layer 
           45 R,  45 G,  45 B spatial light modulation element 
           66  polarizing beam splitter 
           66   a  multiple layer 
           68 ,  71  shutter 
           70 ,  72  reflecting mirror 
           81  white light source 
           82  integrator optical system 
           83  color-selective polarization conversion element 
           84  polarizing beam splitter 
           85 G,  85 R,  85 B spatial light modulation element 
           86  rotating color filter 
           91  color-selective polarization converter