Patent Publication Number: US-2006007399-A1

Title: Projector

Description:
This is a Continuation of application Ser. No. 10/805,241 filed Mar. 22, 2004. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a projector projecting an image by using spatial light modulation devices, such as liquid crystal panels.  
      Among related art rear projection projectors, a shallow rear projector is prevailing, in which image light emitted from a projection optical system disposed behind and at the lower part of a screen is eventually reflected toward the forward direction, while being reflected at at least one plane mirror to secure a light path, so as to be projected onto the screen See Japanese Unexamined Patent Application Publication No. 5-66482.  
      A projection display apparatus in which a diagonal projection having a large magnification is possible by forming its projection optical system with a plurality of concave and convex mirrors having different optical axes from each other has been proposed. See Japanese Unexamined Patent Application Publication No. 2001-255462. The projection display apparatus has a thin structure by achieving a high magnification and a diagonal projection.  
     SUMMARY OF THE INVENTION  
      According to the former method using the plane mirror, although the rear projector has a relatively thin structure, distortion occurs and the projector becomes taller. Also, not only the relatively large plane mirror makes the projector heavier but also the plane mirror incorporated in an actual projector, requires a special mechanism to adjust displacements of the projection optical system and the plane mirror, thereby increasing an adjusting step and resultantly a cost of the projector.  
      Also, according to the latter method, using the projection optical system formed by a plurality of concave and convex mirrors, although the projection display apparatus can project a relatively highly accurate image while maintaining a thin structure, and adjusting work in the case of incorporating a plane mirror can be eliminated, the projection optical system itself is hard to assemble and adjust, thereby resulting in an increased cost of the projection display apparatus.  
      In view of the above problems, the present invention provides a thin projector (projection apparatus), such as a rear projector, formed by a less expensive projection optical system which is easy to assemble and install, and performs highly accurate projection.  
      In order to address the above problems, a projector according to an aspect of the present invention includes an illumination device to emit illumination light; spatial light modulation devices illuminated with illumination light emitted from the illumination device; a projection optical system to project image light emitted from the spatial light modulation devices; a flat and rectangular screen onto which the image light passing through the projection optical system is projected; and a polarizing device to make the image light emitted from the spatial light modulation devices incident on the screen so as to serve as linearly polarized light having a polarization azimuth along a predetermined direction except for the lateral direction of the screen. Here, the term “spatial light modulation device” refers to an optical device represented by, for example, a liquid-crystal light valve, and has an embodiment including a digital mirror device.  
      In this projector, since the polarizing device makes the image light, emitted from the spatial light modulation devices, incident on the screen, so as to serve as linearly polarized light having a polarization azimuth along a predetermined direction, except for the lateral direction of the screen, the longitudinal both ends of the screen can also maintain a low reflectance, thereby reducing a loss in quantity of image light when passing through the screen. Accordingly, even when this structure is applied to, for example, a thin rear projection type projector having a large projection magnification, high luminance of an image can be achieved while maintaining the uniformity of brightness across the entire screen.  
      In a specific modification of the projector, the polarizing device makes the image light emitted from the spatial light modulation devices incident on the screen so as to serve as linearly polarized light having a polarization azimuth along the longitudinal direction of the screen. In this case, a loss in quantity of illumination light incident on the longitudinal both ends of the screen can be minimized, thereby providing a projector exhibiting relatively less unevenness of brightness as a whole.  
      Also, in a specific variation of the projector, the screen is a rear projection screen including a Fresnel lens portion disposed at the incident side thereof and a diffusing screen portion, such as a lenticular lens, disposed at the exit side thereof. Meanwhile, The Fresnel lens portion has a flat incident surface. In this case, a reflection loss of both ends of the flat incident surface having a large incident angle from the projection optical system to the flat incident surface of the Fresnel lens can be reduced.  
      In addition, in another specific variation of the projector, the polarizing device includes polarization filters disposed at the exit sides of the corresponding spatial light modulation devices. In this case, the projector has a simple structure having only the polarization filters disposed at the exit sides of the spatial light modulation devices. When the spatial light modulation devices are liquid-crystal light valves, although the polarization filters are disposed at the incident and exit surfaces of each liquid-crystal light valve, the polarization filters at the corresponding exit sides are disposed in corresponding predetermined azimuths so as to serve as the polarizing means. Also each polarization filter at the incident side is disposed such that its azimuth is turned by, for example, 90 degrees with respect to that of the corresponding polarization filter at the exit side.  
      Furthermore, in another specific variation of the projector, the projection optical system includes an L-shaped optical unit to bend a light path, having a pair of lens groups and reflecting device interposed therebetween. In this case, the projection optical system has a reduced length extending in the optical axis direction on the exit side of the optical unit, and also, optical components, such as the illumination device can be disposed at lateral sides of the optical unit. Thus, a projector or the like having, for example, a rear projection screen housed therein does not require a relatively large reflecting mirror or the like, substantially opposed to the screen to be incorporated in its housing. As a result, a shallow and thin projector can be achieved in spite of the fact that it is light and is easy to assemble and manufacture. Meanwhile, the L-shaped optical unit has a simple structure in which a reflecting device, such as a mirror, is merely incorporated therein while known lens systems being basically used, thereby making an optical design and manufacturing of the projector easy.  
      Also, another projector according to an aspect of the present invention includes an illumination device to emit illumination light; spatial light modulation devices illuminated with illumination light emitted from the illumination device; a projection optical system which includes an L-shaped optical unit to bend a light path, having a pair of lens groups and a reflecting device interposed therebetween and which projects image light emitted from the spatial light modulation devices via the optical unit; and a screen onto which the image light passing through the projection optical system is projected.  
      In this projector, since the projection optical system projects image light emitted from the spatial light modulation devices via the L-shaped optical unit to bend a light path, having the pair of lens groups and the reflecting device interposed therebetween, the projection optical system has a reduced length extending in the optical axis direction on the exit side of the optical unit. Also optical components, such as the illumination device, can be disposed at lateral sides of the optical unit. Thus, a shallow and thin projector can be achieved. Meanwhile, an optical design of the L-shaped optical unit is simple.  
      Also, in a specific variation of the projector, the screen is a rear projection screen, and the optical unit directly focuses the image light emitted from the spatial light modulation devices onto the screen. In this case, since the projector has a structure in which a relatively large reflecting mirror substantially opposed to the screen is not incorporated in the housing thereof, a thin projector can be achieved in spite of the fact that it is light and is easy to assemble and manufacture.  
      In addition, in another specific variation of the projector, the optical unit has an optical axis bent on a vertically extending plane orthogonal to the screen. In this case, optical components, such as the illumination device can be reliably disposed around an upper part and/or a lower part of a plane orthogonal to the optical axis direction, and also the structure of the optical unit can be simple.  
      Furthermore, in another specific variation of the projector, the illumination device is disposed such that the optical axis of a lamp serving as a light source to generate illumination light lies horizontally. In this case, an operation of the lamp can be stabilized.  
      Still furthermore, in another specific variation of the projector, the exit-side optical axis of the projection optical system is perpendicular to a surface of the screen extending along the central part of the screen. In this case, an image projected by the projection optical system onto the screen has less aberration, such as distortion.  
      Moreover, in another specific variation of the projector, there are provided a color modulation device including the spatial light modulation devices for a plurality of colors, for corresponding colors, each device being illuminated with corresponding illumination light emitted from the illumination device, and a light-separation modulation device which includes a light-synthesizing member to synthesize corresponding kinds of color image light emitted from the color modulation device and which emits the synthesized image light. The projection optical system projects the image light synthesized with the light-synthesizing member onto the screen. In this case, a color image having a highly uniform luminance can be projected by a shallow and thin projector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic of a projector according to an exemplary embodiment of the present invention;  
      FIGS.  2 ( a ) and  2 ( b ) are schematics of the projector;  
       FIG. 3  is a sectional schematic of a screen;  
       FIG. 4  is a schematic illustrating the structure of an optical system portion;  
       FIG. 5  is a schematic illustrating the structure of a projection optical system;  
       FIG. 6  is a graph illustrating light extinction due to the screen;  
       FIG. 7  is a graph illustrating light extinction due to another screen; and  
       FIG. 8  is an illustration of areas on the screen. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The structure of a projector according to an exemplary embodiment of the present invention will be described with reference to the accompanying drawings.  
       FIG. 1  illustrates the projector according to an exemplary embodiment, that is, an elevation view of the projector, and FIGS.  2 ( a ) and  2 ( b ) are respectively a perspective plan view and a perspective side view of the projector.  
      A projector  10  has a structure in which a main body formed by an optical system portion, an electrical circuit, and so forth is accommodated and held in a casing  12  serving as a housing.  
      The casing  12  has a screen  14  fixed across the entire front surface thereof in an embedded state. The screen  14  is a rear projection screen illuminated with projection light emitted from the inside of the casing  12  and has a rectangular shape which has a long width relative to the length. That is, which extends longer in the horizontal direction than in the vertical direction.  
       FIG. 3  is an illustration of the sectional structure of the screen  14 . The screen  14  has a triple layer structure formed by a transparent substrate  14   a , a screen film  14   b , and a Fresnel lens  14   c  laminated in that order. The transparent substrate  14   a  is made from a transparent parallel plate and has the screen film  14   b  bonded and closely contacted to the rear surface thereof. The screen film  14   b  is also referred to as a lenticular screen and has a large number of microlenses formed on the surface thereof opposed to the Fresnel lens  14   c . The shape, the size, the arrangement, and the like of the microlenses are appropriately decided taking the use of the projector, the compatibility with other optical systems, and so forth into account.  
      The Fresnel lens  14   c  is fixed to the transparent substrate  14   a  with a fixing member (not shown) in a state in which it is opposed to the screen film  14   b , having a gap GA interposed therebetween. The Fresnel lens  14   c  has a flat surface  14   f  formed on the incident side thereof and lens protrusions  14   g , each having a ring belt shape, formed on the exit side thereof. Projection light PL incident on the rear surface of the Fresnel lens  14   c  is converted into a flux substantially perpendicular to the screen  14  with the Fresnel lens  14   c  and is incident on the screen film  14   b . The projection light PL incident on the screen film  14   b , serving as image light, is scattered at appropriately distributed angles with the screen film  14   b  and is transmitted through the transparent substrate  14   a.    
      Referring back to  FIGS. 1 and 2 , behind the screen  14  and in the casing  12 , the projector  10  includes an illumination device  21  including a light source to generate illumination light, a color-separation modulation optical-system  23  to form a transmittance distribution corresponding to an image by applying spatial light modulation on illumination light emitted from the illumination device  21  and a projection optical system  25  to project the transmittance distribution formed with the color-separation modulation optical-system  23  onto the screen  14  with an appropriate magnification. An optical system portion including the illumination device  21 , the color-separation modulation optical-system  23  and the projection optical system  25  is constructed such that these components are reliably fixed in the casing  12  with respective retaining members (not shown) and that the positional relationship and the like among them can be finely adjusted if needed.  
      The projection light PL projected with the projection optical system  25  onto the screen  14  is linearly polarized light having a polarization azimuth along the longitudinal direction of the screen  14 . With this arrangement, as will be described later, the right and left ends of the rear surface of the screen  14  also maintain a low reflectance, thereby reducing a loss in quantity of projection light when passing through the screen, specifically, achieving high luminance of an image projected onto the screen  14  while maintaining its uniformity of brightness.  
      Also, an exit-side optical axis OA 1  of the projection optical system  25  perpendicularly intersects with the plane of the screen  14  extending along the central part of, that is, the center of the screen  14 . With this arrangement, an image projected with the projection optical system  25  onto the screen  14  becomes highly accurate and sharp with less aberration, such as distortion.  
      Also, the projection optical system  25  is an L-shaped optical unit, and an incident-side optical axis OA 2  thereof is perpendicular to the exit-side optical axis OA 1  and extends downwards in the vertical direction. Image light emitted from the projection optical system  25  is directly incident on the screen  14  without passing through an optical member, such as a mirror. With this arrangement, the optical system portion, including the illumination device  21  and the color-separation modulation optical-system  23 , can be arranged in directions perpendicular to the optical axis OA 1  with respect to the projection optical system  25 , that is, in a surrounding space extending. For example, downwards and sidewards from the projection optical system  25  but not rearwards from the projection optical system  25 . The size of the optical system portion including the illumination device  21 , the color-separation modulation optical-system  23  and the projection optical system  25  can be made shorter in the direction along the optical axis OA 1 , thereby achieving a relatively thin projector without using a plane mirror to secure a light path. Also, since no mirror is needed to be incorporated in the housing, distortion or displacement of an image caused by the mirror inserted in the light path can be reduced or prevented. As a result, a correction mechanism and a correction step against these problems can be eliminated. In addition, since the mirror and a mechanism accompanying the mirror can be eliminated, a shallow and thin projector can be achieved in spite of the fact that it is light and is easy to assemble and manufacture.  
       FIG. 4  is an elevation view illustrating the structure of the optical system portion including the illumination device  21 , the color-separation modulation optical-system  23  and the projection optical system  25 .  FIG. 5  is a side view illustrating the structure of the projection optical system  25 .  
      The illumination device  21  includes a light source lamp  41 , a first fly-eye lens  43 , a second fly-eye lens  45 , a polarization-conversing member  47 , and a superimposing lens  49 . Here, an example of the light source lamp  41  is a high-pressure mercury-vapor lamp including a concave mirror for collimating source light. Also, the first fly-eye lens  43 , having a plurality of element lenses arranged in a matrix pattern, divides the source light emitted from the light source lamp  41  and separately collects it with these element lenses. The second fly-eye lens  45 , also having a plurality of element lenses arranged in a matrix pattern, forms uniform divergent light from secondary light sources formed with the first fly-eye lens  43  and emits uniform illumination light to be superimposed on light valves (spatial light modulation devices), which will be described later, with these element lenses. The polarization-conversing member  47  converts the illumination light emitted from the second fly-eye lens  45  into only a polarized component orthogonal to the plane of  FIG. 4  and supplies it to the following optical system. The superimposing lens  49  converges the illumination light passing through the polarization-conversing member  47  if needed so that the light valves in the color-separation modulation optical-system  23  can perform superimposing illumination. An optical axis OA 3  of the illumination device  21  extends horizontally, and resultantly, the light axis of the light source lamp  41  also extends horizontally. As a result, the light source lamp  41  lies horizontally, whereby stable light emission can be achieved by maintaining an operating temperature and the like of the light source lamp  41  in a stable state and also the life span of the light source lamp  41  can be extended.  
      The color-separation modulation optical-system  23 , serving as a light-separation modulation device, includes first and second dichroic mirrors  51  and  52 , three field lenses  53   a  to  53   c , three light valves  54   a  to  54   c , a cross dichroic prism  55 , three pairs of polarization filters  56   a  to  56   c  arranged so as to sandwich the corresponding light valves  54   a  to  54   c . Among them, the light valves  54   a  to  54   c  and the polarization filters  56   a  to  56   c  form a color modulation device. Blue light (B light) reflected at the first dichroic mirror  51  is reflected at a reflecting mirror M 1 , passes through the field lens  53   a , and is incident on the light valve  54   a  sandwiched between the pair of polarization filters  56   a . Green light (G light), transmitted through the first dichroic mirror  51  and reflected at the second dichroic mirror  52 , passes through the field lens  53   b  and is incident on the light valve  54   b  sandwiched between the pair of polarization filters  56   b . Red light (R light), transmitted through the first and second dichroic mirrors  51  and  52 , passes through a relay lens R 1 , is reflected at a reflecting mirror M 2 , passes through a relay lens R 2 , is reflected at a reflecting mirror M 3 , passes through the field lens  53   c  and is then incident on the light valve  54   c  sandwiched between the pair of polarization filters  56   c . Each of the light valves  54   a  to  54   c  serve as a spatial light modulation device to modulate the corresponding spatial intensity distribution of illumination light incident thereon. The three kinds of color light, incident on the corresponding light valves  54   a  to  54   c , are respectively modulated thereby, are synthesized with the cross dichroic prism  55  serving as a light-synthesizing member and are then emitted from a side surface thereof. The synthesized light, emitted from the cross dichroic prism  55 , is incident on the projection optical system  25 .  
      The projection optical system  25  is formed by a front first lens group  25   a , a prism mirror  25   b  to bend a light path and a rear second lens group  25   c . Here, the first lens group  25   a  is composed of six concave and convex lens elements. Also, the second lens group  25   c  is composed of three meniscus lens elements. The prism mirror  25   b , sandwiched between the two lens groups  25   a  and  25   c , serves as a reflecting device so as to bend a vertical light path by 90 degrees into a horizontal light path. The exit-side optical axis OA 1  and the incident-side optical axis OA 2  of the projection optical system  25  are perpendicular to each other on the reflective surface of the prism mirror  25   b.    
      An operation of the projector according to the exemplary embodiment shown in FIGS.  1  to  5  will be described. The illumination device  21  serves as a white light source generating the three kinds of R, G, and B light. Illumination light, emitted from the illumination device  21 , undergoes color separation with the dichroic mirrors  51  and  52  disposed in the color-separation modulation optical-system  23  and the three kinds of separated color light are incident on the corresponding light valves  54   a  to  54   c . Each of the light valves  54   a  to  54   c , modulated in accordance with an external image signal, has a two-dimensional refractive-index distribution and modulates the illumination light incident thereon. The three kinds of illumination light, that is, image light modulated with the light valves  54   a  to  54   c , as mentioned above, are synthesized with the cross dichroic prism  55  and the synthesized image light is incident on the projection optical system  25 . The image light, incident on the projection optical system  25 , is incident on the screen  14  so as to serve as linearly polarized light having a polarization direction along the longitudinal direction of the screen  14 . In this case, the polarization direction of projection light is set in the longitudinal direction of the screen  14 , thereby achieving high luminance of an image projected onto the screen  14  while maintaining its uniformity of brightness.  
      Each of  FIGS. 6 and 7  is a graph illustrating, by simulation, the relationship between polarization direction of image light. Specifically, the projection light PL incident on the screen  14  disposed in the projector  10  shown in  FIG. 1  and the other related figures and relative light intensity, that is, light extinction rate of an image light when passing through the screen  14 . In these graphs, the horizontal and vertical axes represent a projection magnification and a relative light intensity, respectively.  
      Here, the term “projection magnification” refers to the ratio H/WD, where H represents the diagonal length of the screen  14  and WD represents the projection distance from the projection optical system  25  to the screen  14 , and the greater the projection magnification, the greater the divergent angle of image light becomes. Also, the term “relative light intensity” refers to the ratio of the quantity of light transmitted through the screen  14  to that of light incident on the same. As the relative light intensity, a transmittance is computed in accordance with the Fresnel reflection formula, taking an incident angle and a perpendicularly polarized component of the image light incident on the screen  14  as parameters. The graphs shown in  FIGS. 6 and 7  respectively correspond to the cases of using the screen  14  having an aspect ratio of 9 to 16, that is, having long width relative to the length, and the screen  14  having an aspect ratio of 3 to 4.  
       FIG. 8  illustrates areas on the screen  14 , the relative light intensities of which are compared to each other between  FIGS. 6 and 7 . The center, each of the right and left ends, each of the upper and lower ends, and each of the corners of the screen  14  are respectively defined as a central area CA, a horizontal end area HEA, a vertical end area VEA, and a diagonal end area DEA. With reference to these areas, marks shown in  FIGS. 6 and 7  will be described. Each of the solid and open square marks represents a light extinction rate of linearly polarized light incident on the horizontal end areas HEA, each of the solid and open round marks represents a light extinction rate of linearly polarized light incident on the diagonal end areas DEA, and each of the solid and open diamond marks represents a light extinction rate of linearly polarized light incident on the vertical end areas HEA. Each of the straight lines in  FIGS. 6 and 7 , which exhibits a relative light intensity of about 0.87 and does not vary, represents a light extinction rate of linearly polarized light incident on the central area CA.  
      Also, each of the solid square, round, and diamond marks represents a light extinction rate in the case where image light incident on the screen  14  is linearly polarized light polarized in the longitudinal direction, that is, in the horizontal direction, corresponding to the polarization direction of the image light in the projector according to the present exemplary embodiment. Each of the open square, round, and diamond marks represents a light extinction rate in the case where image light incident on the screen  14  is linearly polarized light polarized in the lateral direction, that is, in the vertical direction, corresponding to the polarization direction of the image light in a comparative example projector.  
      As shown in both graphs, in the horizontal end areas HEA and the diagonal end areas DEA, a reflection loss of polarized light in the horizontal direction due to the screen  14  is less than that in the vertical directionThe reflection loss of polarized light in the vertical direction due to the screen  14  is very large. Also, in the vertical end areas VEA, a reflection loss of polarized light in the vertical direction due to the screen  14  is less than that in the horizontal direction. Even in the case of polarized light in the horizontal direction, a loss due to the screen  14  does not increase so much. In summary, when the entire screen  14  is intended to be uniformly illuminated, by setting the polarization direction of image light in the longitudinal direction of the screen  14 , the image light is unlikely to suffer from bias in accordance with the Fresnel reflection formula. Although a transmittance in the vertical end areas VEA is somewhat sacrificed, transmittances in the horizontal end areas HEA and the diagonal end areas DEA become relatively higher, whereby projection light having a uniform distribution as a whole is incident on the screen.  
      In addition, this phenomenon becomes significant as the projection magnification becomes greater. Accordingly, in the projector  10  offering a greater projection magnification that is, in the projector  10  of a direct projection type in which no mirror to bend a light path is disposed between the projection optical system  25  and the screen  14 , it is very important to convert image light into polarized light in the horizontal direction as in the present exemplary embodiment from the viewpoint of achieving uniform illumination of the screen  14 . Also, the above arrangement is very important to achieve a magnified size of the screen and a thin structure of the projector at the same time.  
      Although the present invention has been described according to an exemplary embodiment, it is not limited to the foregoing exemplary embodiment. For example, when the screen  14  has long length relative to the width, image light to be incident on the screen  14  is converted into linearly polarized light in the vertical direction, that is, as in its longitudinal direction.  
      Also, image light to be incident on the screen  14  can be converted into linearly polarized light not only in its longitudinal direction but also in its diagonal direction. In other words, linearly polarized light in the lateral direction of the screen  14  may be contained as long as it is not dominant.  
      In addition, a polarizing device to make linearly polarized light in a desired direction incident on the screen  14  is not limited to the three pairs of the polarization filters  56   a  to  56   c , each pair sandwiching the corresponding light valves  54   a  to  54   c . But it may be specially formed irrelevantly to the light valves  54   a  to  54   c . In addition, the polarization filters  56   a  to  56   c  disposed on the exit sides of the corresponding light valves  54   a  to  54   c  may be single one commonly formed on the exit surface of the cross dichroic prism  55 .  
      Furthermore, although the exit-side optical axis OA 1  and the incident-side optical axis OA 2  of the projection optical system  25  are perpendicular to each other in the foregoing exemplary embodiment, the degree of bend between the optical axes of the first lens group  25   a  and the second lens group  25   c  may be changed in accordance with the application and/or the purpose of the projector if needed.  
      Still furthermore, although the exit-side optical axis OA 1  of the projection optical system  25  is orthogonal to the central part of the screen  14  in the foregoing exemplary embodiment, the optical axis OA 1  may be slightly slanted with respect to a line perpendicular to the screen  14 .  
      Moreover, although the screen  14  is formed by the transparent substrate  14   a , the screen film  14   b , and the Fresnel lens  14   c  in the foregoing exemplary embodiment, the screen is not limited to this structure and may have another optical element incorporated therein. In this case, by making divergent image light incident on a surface so as cause the polarization direction of the image light to agree with the longitudinal direction of the screen  14 , the phenomenon of a reduced transmittance at longitudinal both ends of the screen can be reduced or prevented.