Patent Publication Number: US-2005134807-A1

Title: Projection lens unit and projection system employing the same

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2003-95408, filed on Dec. 23, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to a projection lens unit that can be used in a projection system with a small thickness with respect to a screen size and a projection system employing the same.  
      2. Description of the Related Art  
      As the demand for projection systems with large screens, high resolution, and slim designs has increased, research has been actively conducted to satisfy these requirements. To achieve a slim projection system, a projection lens unit must generate a thin frustum of a bundle of light rays. However, when the projection lens unit is located at the center of a screen, the frustum of a bundle of light rays can only be reduced to a limited degree. Thus, to achieve a thin frustum, the projection lens unit should be disposed obliquely on the lower portion of the screen.  
      Other various techniques are being developed for projection systems with large screens and slim designs. However, it is difficult to realize both a large screen and a slim design. That is, since a projection lens unit that projects an image onto the screen tends to become bulkier as the size of the screen increases, it is technically difficult to reduce the thickness of a projection display while increasing the size of a screen.  
      Referring to  FIG. 1 , a conventional projection system disclosed in U.S. Patent Publication No. 2002/0071186 A1 includes a light source (not shown), a refracting lens group GL, a first mirror M 1  that reflects a beam exiting the refracting lens group GL, a second mirror M 2  that reflects the beam reflected from the first mirror M 1  upward, and a third mirror M 3  that reflects the beam reflected from the second mirror M 2  toward a screen  12 . The projection lens system also includes a first image plane  11 , a prism PR, and an aperture ST.  
      The refracting lens group GL extends parallel to the screen  12 , and the first mirror M 1  is at a 45 degree angle to the screen  12  such that a beam emerging from the refracting lens group GL is reflected 90 degrees toward the screen  12 . Typically, a screen has an aspect ratio of 4:3 or 16:9 where the longer direction and the shorter direction are a major axis direction L and a minor axis direction S, respectively.  FIG. 2  is a schematic drawing of a projection system in which the distance between the screen  12  and the third mirror M 3  is less than that of the projection system shown in  FIG. 1 .  
      Referring to  FIG. 2 , since the first mirror M 1  is inclined at 45 degrees to the major axis direction L, light reflected from the second mirror M 2  may be intercepted by the first mirror M 1 , as indicated by a portion A. This is because the first mirror M 1  has a major axis (longer axis) disposed in a path of light reflected from the second mirror M 2  toward the direction in which the first mirror M 1  is slanted, that is, the major axis direction L of the first mirror M 1 .  
      That is, since the first mirror M 1  is disposed such that its major axis is disposed in a light propagation path, the first mirror M 1  intercepts most of the light reflected from the second mirror M 2  than when the minor axis of the first mirror M 1  disposed in the light propagation path. Thus, reducing the thickness of the projection system configured by moving the screen  12  closer to the third mirror M 3  makes it more likely that the beam reflected from the second mirror M 2  is intercepted by the first mirror M 1 . A projection system having an aspect ratio of 16:9 has a higher possibility that the beam reflected from the second mirror M 2  is intercepted by the first mirror M 1  than a projection system having an aspect ratio of 4:3 because the first mirror M 1  also has an aspect ratio of 16:9 and the major axis of the first mirror M 1  is disposed in a path of the light reflected from the second mirror M 2 .  
      As described above, in the conventional projection system shown in  FIG. 2 , the beam reflected from the second mirror M 2  is obstructed by the first mirror M 1 . This problem becomes worse as the thickness of the projection system decreases. Thus, the conventional projection system has problems when its thickness is reduced.  
     SUMMARY OF THE INVENTION  
      Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and may overcome other disadvantages not listed above. The present invention provides a projection lens unit that can be used in a projection system with a slim design by reducing the thickness of the projection system with respect to a screen size as much as possible and a projection system employing the same.  
      According to an aspect of the present invention, there is provided a projection lens unit for enlarging and projecting a beam onto a screen having a major axis and a minor axis including: a refractive optical unit including a plurality of lenses that enlarge and project an image produced by a display; and a reflective optical unit including at least one reflecting mirror slanted toward the minor axis of the screen, the reflective optical unit reflecting the image passing through the refractive optical unit toward the screen.  
      Optical axes of the refractive optical unit and the reflective optical unit are coplanar. The optical axes of the refractive optical unit and the reflective optical unit are in a plane perpendicular to the major axis of the screen. Each of the at least one reflecting mirror has a short side slanted toward the minor axis of the screen.  
      The refractive optical unit includes first and second lens groups, and the reflective optical unit includes: a first reflecting mirror that is disposed in an optical path between the first and second lens groups and changes the path of a beam passing through the first lens group; and a second reflecting mirror that is slanted toward the minor axis of the screen and reflects a beam passing through the second lens group.  
      According to another aspect of the present invention, there is provided a projection system in which an image produced by a display is enlarged and projected by a projection lens unit and focused onto a screen having a major axis and a minor axis. The projection lens unit includes a refractive optical unit including a plurality of lenses that enlarge and project the image and a reflective optical unit including at least one reflecting mirror slanted toward the minor axis of the screen, the reflective optical unit reflecting the image passing through the refractive optical unit toward the screen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a schematic diagram of a projection system disclosed in U.S. Patent Publication No. 2002/0071186 A1;  
       FIG. 2  illustrates a beam being obstructed by a reflective mirror when the thickness of the projection lens unit of  FIG. 1  is reduced;  
       FIG. 3  is a side view of a projection lens unit according to an embodiment of the present invention;  
       FIG. 4  is a plan view of the projection lens unit illustrated in  FIG. 3 ;  
       FIG. 5  is a schematic diagram of a projection lens unit including the refractive and reflective optical systems arranged in a different manner than those in  FIG. 3 ; and  
       FIG. 6  is a schematic diagram of a projection system including the projection lens unit of  FIG. 3  according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 3 , a projection lens unit according to an illustrative, non-limiting embodiment of the present invention includes a refractive optical unit  20  including a plurality of lenses that enlarge an image produced by a display  15  for projection and a reflective optical unit  30  including one or more reflecting mirrors that reflect the image at an appropriate angle toward a screen SR.  
      The display  15  modulates a beam emitted from a light source  10  according to image information and produces an image. The refractive optical unit  20  may include a plurality of lenses or lens groups arranged depending on available installation space. For example, the refractive optical unit  20  may be comprised of a first lens group  20  and a second lens group  20   b . In this case, the first lens group  20   a  is arranged in a thickness direction D of a projection system including the projection lens unit and the second lens group  20   b  is arranged in a minor axis direction S of the screen SR.  
      The reflective optical unit  30  may include one or more reflecting mirrors, and  FIG. 3  shows an example in which the reflective optical unit  30  includes first through third reflecting mirrors  31  through  33 . The first reflecting mirror  31  is disposed in an optical path between the first lens group  20   a  and the second lens group  20   b  and changes the path of light emitted from the first lens group  20   a  toward the second lens group  20   b . If the first reflecting mirror  31  is properly installed according to the entire lengths of the first and second lens groups  20   a  and  20   b , the length of the refractive optical unit  20  in the thickness direction D of the projection system can be adjusted.  
      The light reflected from the first reflecting mirror  31  passes through the second lens group  20   b  and is reflected by a second reflecting mirror  32  toward a third reflecting mirror  33 . The second reflecting mirror  32  can have a shorter side tilted in the minor axis direction S. The third reflecting mirror  33  is an aspheric mirror that corrects image distortion. The light reflected from the third reflecting mirror  33  is reflected by a fourth reflecting mirror  34  and is focused onto the screen SR.  
      In the present embodiment, the refractive optical unit  20  includes the first and second lens groups  20   a  and  20   b  and the first reflecting mirror  31  is disposed in the optical path between the first and second lens groups  20   a  and  20   b . However, the projection lens unit may not include the first reflecting mirror  31  when the refractive optical unit  20  is arranged obliquely to the thickness direction D such that the light passing through the refractive optical unit  20  is directly incident onto the second reflecting mirror  32 .  
      That is, the first reflecting mirror  31  is required only when the propagation path of light must be changed according to the arrangement of the refractive optical unit  20 . When the first and second lens groups  20   a  and  20   b  are disposed as in  FIG. 3 , the first and second reflecting mirrors  31  and  32  form identical angles with the fourth reflecting mirror  34 , symmetrically about the second lens group  20   b.    
      The optical axes of the refractive optical unit  20  and the reflective optical unit  30  are coplanar. Referring to  FIG. 4 , which is a plan view of the projection lens unit of  FIG. 3 , the optical axes of the refractive optical unit  20  and the reflective optical unit  30  are in the same plane H, parallel to the minor axis of the screen SR. Since the projection lens unit is symmetric with respect to the plane H,  FIG. 4  illustrates only half of the screen SR.  
      Further, optical axes of the light source  10  and the display  15  may be disposed in the plane H, thereby making optical alignment of the projection lens unit easier.  
      While the refractive optical unit  20  is circularly symmetric, the reflective optical unit  30  is planar symmetric. The projection lens unit is symmetric about a plane perpendicular to the minor axis direction S.  
      The first and second reflecting mirrors  31  and  32  can be slanted at an angle to the minor axis directions of the screen SR, thereby preventing the reflective optical unit  30  from intercepting or obstructing the propagation of light when the thickness projection system including the projection lens unit is decreased.  
      Meanwhile, referring to  FIG. 5 , the thickness of the projection lens unit can be adjusted by adjusting the positions and angles of the first and second reflecting mirrors  31  and  32 . In this case, the first and second reflecting mirrors  31  and  32  are slanted opposite each other at the same angle. Since adjusting the positions and angles of the first and second reflecting mirrors  31  and  32  can hardly obstruct the propagation of a beam, there is no restrictions on reducing the thickness of the projection lens unit.  
      Referring to  FIG. 3 , the effective area of the second reflecting mirror  32  has the same aspect ratio as that of the display  15  and the screen SR, and a short side of the effective area of the second reflecting mirror  32  is slanted with respect to the minor axis direction S of the screen SR. The effective area refers to an area on which an effective beam is incident. The second reflecting mirror  32  has an effective area with the same aspect ratio as that of the display  15  and the screen SR.  
      Since the short side of the second reflecting mirror  32  extends in a path of a beam propagating toward the direction in which the second reflecting mirror  32  is slanted among beams reflected from the third reflecting mirror  33 , there is little possibility that the second reflecting mirror  32  obstructs the propagation of light reflected by the second reflecting mirror  32  and then reflected by the third reflecting mirror  33  into the fourth reflecting mirror  34 . Thus, the projection lens unit can be used in a projection system with a slim design without suffering from any restrictions to its construction due to the second reflecting mirror  32  even if a distance between the fourth reflecting mirror  34  and the screen SR is decreased.  
      A projection system according to an embodiment of the present invention includes the projection lens unit shown in  FIG. 3  and narrow, and will now be described in detail with reference to  FIG. 6 .  
      Referring to  FIG. 6 , the projection system includes a light source  10  located at a lower portion of a cabinet  50 , a display  15  that processes light emitted by the light source  10  according to image information and produces an image, a projection lens unit  40  that enlarges the image produced by the display  15  for projection, and a screen SR onto which the image projected through the projection lens unit  40  is focused.  
      The projection lens unit  40  includes a refractive optical unit  20  including a plurality of lenses that enlarge the image produced by the display  15  for projection and a reflective optical unit  30  including one or more mirrors that reflect the image at an appropriate angle toward a screen SR. The screen SR has a minor axis S and a major axis L, and the refractive optical unit  30  has at least one reflecting mirror  32  tilted toward the minor axis S.  
      Optical axes of the refractive optical unit  20  and the reflective optical unit  30  can be coplanar. The projection lens unit  40  has the same structure as described with references to  FIGS. 3 and 4 , so a detailed description thereof will not be given.  
      The projection system is constructed with a thickness that is related to a screen size. As described above, the projection lens unit has an improved arrangement of optical components that can allow the thickness of the projection system to be minimized. The projection lens unit is constructed such that a short side of a reflecting mirror in a reflective optical unit that reflects an image produced by a display toward a screen lies in a plane perpendicular to the major axis of the screen, thereby preventing interference between light and the reflective optical unit.  
      Thus, the projection system including the projection lens unit can have a large screen and a slim design, and is advantageous in a projection system having an aspect ratio of 16:9. Furthermore, optical axes of the refractive optical unit and the reflective optical unit in the projection lens unit are in the same plane, thereby making optical alignment and setup easier.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.