Abstract:
A rear projection optical system to enlarge and project images of an image-forming panel onto a rear side of a screen includes a front lens group disposed in a vicinity of the screen, and having first, second, and third lenses having a negative power respectively, and fourth and fifth lenses having a positive power respectively, a rear lens group disposed in a vicinity of the panel, and having a first bonding lens including a sixth lens with a positive power and a seventh lens with a negative power, a second bonding lens including an eighth lens with a negative power and a ninth lens with a positive power, and a tenth lens with a negative power, and an iris disposed between the front lens group and the rear lens group to control an amount of light. The rear projection optical system enables a wide angle-of-view (91 degrees), a high resolution, and a reduced depth of a display device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119 of Korean Application No. 2004-12407, filed Feb. 24, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety and by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present general inventive concept relates to a projection optical system useable with a projection-type image display apparatus, and more particularly, to a rear projection lens system configured to form a projection distance short enough to implement a wide angle-of-view and a high resolution.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, due to a demand for a wide angle-of-view and a high resolution of an image display apparatus, image projection devices which project and enlarge small-sized images onto a large screen by use of a projection optical system are rapidly spreading. In particular, rear projection devices, which project an image signal from behind the screen, are widely used as projection TVs.  
         [0006]     Conventional rear projection devices comprise a light source, a Liquid Crystal Display (LCD) or a Digital Micromirror Device (DMD) panel as a display device forming images by use of light emitted from the light source, a screen, and a projection optical system enlarging by a certain magnification factor and projecting the formed images of the panel onto the screen.  
         [0007]     However, as the screen onto which the projection optical system focuses images becomes gradually larger in size, a higher resolution is required, and a wider angle-of-view is also required for the thinner devices. A description of a conventional projection optical system is provided below.  
         [0008]      FIG. 1  is a view illustrating a structure of a conventional projection optical system. Referring to  FIG. 1 , the conventional projection optical system has a front lens group comprising first to fifth lenses  1 ,  2 ,  3 ,  4 , and  5 , with the first lens disposed nearest to a screen (not shown), a rear lens group comprising sixth to tenth lenses  6 ,  7 ,  8 ,  9 , and  10 , and an iris  20  disposed between the front lens group and the rear lens group. The rear lens group has a negative power and primarily reduces various kinds of aberrations of light that are incident through a prism  30 , reflected from a digital micromirror display (DMD) panel  40  and passing therethrough. Further, the aberrations are re-compensated due to a strong positive power of the fourth and fifth lenses  4  and  5 . The first lens  1  mostly compensates distorted aberrations, and the second and third lenses  2  and  3  compensate for coma-aberration, astigmatism, and spherical aberration. The front lens group is constructed to have a negative power in order to widen an angle-of-view.  
         [0009]     However, since unwanted images are displayed on the screen due to ghost images occurring by the rear lens group, the conventional projection optical system has a problem that the rear lens group must have a different curvature depending on its position. Also, since the iris  20  is disposed close to the sixth lens  6 , it is difficult to insert a new iris to control an amount of light. Further, the conventional projection optical system does not provide a sufficient angle-of-view. Accordingly, an improved projection optical system is needed.  
       SUMMARY OF THE INVENTION  
       [0010]     The present general inventive concept provides a projection optical system having a wide angle-of-view, a high resolution, and less distortion.  
         [0011]     Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
         [0012]     The foregoing and/or other aspects and advantages of the present general inventive concept are achieved by providing a rear projection optical system to enlarge and project images of an image-forming panel onto a rear side of a screen, the rear projection optical system comprising a front lens group disposed in a vicinity of the screen, and having first, second, and third lenses that have a negative power, respectively, and fourth and fifth lenses that have a positive power, respectively, a rear lens group disposed in a vicinity of the image-forming panel, and having a first bonding lens comprising a sixth lens with a positive power and a seventh lens with a negative power, a second bonding lens comprising an eighth lens with a negative power and a ninth lens with a positive power, and a tenth lens with a negative power, and an iris disposed between the front lens group and the rear lens group to control an amount of light.  
         [0013]     The tenth lens has a first surface with a positive curvature and a second surface with a negative curvature, satisfying an equation as follows:  
                     0.046   &lt;                  ∑     i   =   6     7     ⁢     p   i       +       ∑     i   =   8     9     ⁢     p   i                    Q     i   =   6       9     ⁢     n   i     ⁢     v   i         ×   1000     &lt;   0.05                     OBJ   BFL     ×   m     =   K     ,     K   ≤   0.35             ,                         
        where, p i , n i , and v i  denote a refractive power, refraction index, and dispersion of an i-th lens, respectively, OBJ (Object Distance) denotes a distance from the screen to the first lens, BFL (Back Focal Length) denotes a distance from the tenth lens to the image-forming panel, and m denotes a magnifying power of all of the lenses.        
 
         [0015]     The OBJ, BFL, and m may be set to 593, 26.5, and 0.0154, respectively.  
         [0016]     Further, the iris may be disposed spaced a predetermined distance from a center of a first surface of the sixth lens, satisfying an equation as follows:  
       1.55   &lt;           ∑     i   =   6     7     ⁢     n   i         d   11       +         ∑     i   =   8     9     ⁢     n   i         d   11         &lt;   1.58       
        where, d 11  denotes a center distance between the iris and the sixth lens, and n i  denotes a refractive power of the i-th lens.        
 
         [0018]     The d 11  and the corresponding n i  may be set as follows: 
        d 11 =4.19, n 6 =1.48749, n 7 =1.75520, n 8 =1.84666, and n 9 =1.48749.        
 
         [0020]     The fourth lens has a first and a second surface that respectively have a negative curvature, the fifth lens has a first surface with a positive curvature and a second surface with a negative curvature, the first bonding lens has a first surface, a bonding surface, and a second surface that have a positive curvature, a positive curvature, and a negative curvature, respectively, and an equation as below is satisfied:  
         7.30   &lt;         ∑     i   =   4     5     ⁢     p   i           ∑     i   =   1     2     ⁢     p     b   ⁢           ⁢   i           &lt;   7.60     ,       
        where, p i  denotes a refractive power of the i-th lens, and p bi  denotes a refractive power of the i-th bonding lens.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0023]      FIG. 1  is a view illustrating a structure of a conventional rear projection optical system;  
         [0024]      FIG. 2  is a view illustrating a structure of a rear projection optical system according to an embodiment of the present general inventive concept;  
         [0025]      FIG. 3  is a view illustrating aberration characteristics of the rear projection optical system of  FIG. 2 ; and  
         [0026]      FIG. 4  is a view illustrating a practical application of the rear projection optical system of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures.  
         [0028]      FIG. 2  is a view illustrating a structure of a rear projection optical system according to an embodiment of the present general inventive concept. The rear projection optical system of  FIG. 2  can be applied to an image projection device employing a DMD panel, such as for example, a projection TV.  FIG. 2  illustrates main components of the image projection device for the sake of explanation. Referring to  FIG. 2 , the image projection device has a screen  100 , the rear projection optical system  50 , a lamp  75 , a prism  80 , and a DMD panel  90  to form images. The rear projection optical system  50  according to the embodiment of  FIG. 2  is structured to project the images of the DMD panel  90  having an effective area of 0.79″ (20.1 mm) onto a 46″ screen and a 56″ screen when a 50″ screen is considered as a standard screen.  
         [0029]     The rear projection optical system  50  is structured with a front lens group  50   a  having five lenses  51 ,  52 ,  53 ,  54  and  55  disposed near the screen  100 , a rear lens group  50   b  having five lenses  56 ,  57 ,  58 ,  59  and  60  disposed near the DMD panel  90 , and an iris  70  disposed between the front lens group  50   a  and the rear lens group  50   b  to control an amount of light. Thus, the rear projection optical system  50  includes ten lenses  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59  and  60  in total. As shown in  FIG. 2 , reference numerals are assigned to lenses L i  so that L i  refers to an i-th lens from the screen  100 . Similarly, reference numerals are assigned to lens surfaces S i , a center thickness of a lens, and a center distance d i  between lenses in order, with reference to the screen  100 . For example, the first lens  51  is denoted as L 1 , the second lens  52  as L 2 , a first surface of the first lens  51  as S 1 , a second surface of the first lens  51  as S 2 , the center thickness of the first lens  51  as d 1 , and the center distance between the first lens  51  and the second lens  52  as d 2 .  
         [0030]     The front lens group  50   a  has five lenses including the lenses L 1 , L 2 , L 3 , L 4  and L 5  ( 51 ,  52 ,  53 ,  54  and  55 ), and characteristics of the individual lenses are as follows. The lens L 1  ( 51 ) and the lens L 2  ( 52 ) have first surfaces S 1 and S   3  and second surfaces S 2  and S 4 , respectively, formed in a positive (+) curvature, so that the lenses L 1  and L 2  ( 51  and  52 ) have a negative (−) power, and the lens L 3  ( 53 ) has a first surface S 5  formed in a negative curvature and a second surface S 6  formed in a positive curvature, so that the lens L 3  ( 53 ) has a negative power. Further, the lens L 4  ( 54 ) has first and second surfaces S 7  and S 8  that are formed in a negative curvature, so that the lens L 4  ( 54 ) has a positive power, and the lens L 5  ( 55 ) has a first surface S 9  formed in a positive curvature and a second surface S 10  formed in a negative curvature, so that the lens L 5  ( 55 ) has a positive power.  
         [0031]     The rear lens group  50   b  has five lenses including the lenses L 6 , L 7 , L 8 , L 9  and L 10  ( 56 ,  57 ,  58 ,  59 , and  60 ). The lens L 6  ( 56 ) and the lens L 7  ( 57 ) are bonded by an ultraviolet (UV) bonding, or the like, to form a first bonding lens, and the lens L 8  ( 58 ) and the lens L 9  ( 59 ) are bonded to form a second bonding lens.  
         [0032]     The lens L 6  ( 56 ) of the first bonding lens has a first surface S 12  formed in a positive curvature and a second surface S 13  formed in a negative curvature, so that the lens L 6  ( 56 ) has a positive power, and the lens L 7  ( 57 ) has a first surface S 13  formed in a negative curvature and a second surface S 14  formed in a negative curvature, so that the lens L 7  ( 57 ) has a negative power. The lens L 8  ( 58 ) of the second bonding lens has a first surface S 15  formed in a positive curvature and a second surface S 16  formed in a positive curvature, so that the lens L 8  ( 58 ) has a negative power, and the lens L 9  ( 59 ) has a first surface S 16  formed in a positive curvature and a second surface S 17  formed in a negative curvature, so that the lens L 9  ( 59 ) has a positive power. Further, the lens L 10  ( 60 ) has a first surface S 18  formed in a positive curvature and a second surface S 19  formed in a negative curvature, so that the lens L 10  ( 60 ) has a negative power.  
         [0033]     In order that the images of the DMD panel  90  pass through the rear lens group  50   b  and the front lens group  50   a , and are projected and clearly focused on the screen  100 , distortion reduction is performed. The lens L 1  ( 51 ) can be an aspheric lens composed of a plastic material to reduce distortion. Further, in order to obtain a wide angle-of-view, the lenses L 1 to L   3  ( 51 ,  52 , and  53 ), which are disposed near the screen  40 , are structured to have a negative power, and, in order to reduce an angle of light, the lens L 4  ( 54 ) and the lens L 5  ( 55 ) are structured to have a positive power. Further, since the lens L 1  to the lens L 3  ( 51 ,  52 , and  53 ) have a negative power, aberration is reduced due to a partial offset resulting from interaction among the lenses L 1 , L 2  and L 3  ( 51 ,  52  and  53 ).  
         [0034]     The first and second bonding lenses reduce aberrations not eliminated by the front lens group  50   a . In particular, the first and second bonding lenses can compensate for chromatic aberration. The first and second bonding lenses can be formed of FD-series and low-dispersion materials that are low-priced. Further, the lens L 10  ( 60 ) has a refractive power set to control a final performance correction and an angle of light incident on the DMD panel  90 . The lenses ( 51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59  and  60 ) are structured as described above so that clear images can be displayed in precise focus on the screen  100  with less distortion.  
         [0035]     The rear projection optical system  50  according to the embodiment of  FIG. 2  satisfies Equations 1-3 as follows.  
                     0.046   &lt;                  ∑     i   =   6     7     ⁢     p   i       +       ∑     i   =   8     9     ⁢     p   i                    Q     i   =   6       9     ⁢     n   i     ⁢     v   i         ×   1000     &lt;   0.05                     OBJ   BFL     ×   m     =   K     ,     K   ≤   0.35             ,           [     Equation   ⁢           ⁢   1     ]             
 
 where, p i , n i , and v i  denote a refractive power, refraction index, and dispersion of the lens L 1 , respectively, OBJ (Object Distance) denotes a distance from the screen  100  to the lens L 1  ( 51 ), BFL (Back Focal Length) denotes a distance from the lens L 10  ( 60 ) to the DMD panel  90 , and m denotes a magnifying power of all of the lenses ( 51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59  and  60 ). Here, the refractive power p i  is defined as the reciprocal of a focal length f i .  
               1.55   &lt;           ∑     i   =   6     7     ⁢     n   i         d   11       +         ∑     i   =   8     9     ⁢     n   i         d   11         &lt;   1.58     ,           [     Equation   ⁢           ⁢   2     ]             
 
 where, d 11  denotes a center distance between the iris  70  and the lens L 6  ( 56 ), and n i  denotes a refractive power of the lens L i .  
               7.30   &lt;         ∑     i   =   4     5     ⁢     p   i           ∑     i   =   1     2     ⁢     p     b   ⁢           ⁢   i           &lt;   7.60     ,           [     Equation   ⁢           ⁢   3     ]             
        where, p i  denotes a refractive power of the lens L i , and p bi  denotes a refractive power of the i-th bonding lens.        
 
         [0037]     Table 1 illustrates exemplary values of the lenses ( 51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59  and  60 ) of the rear projection optical system  50  according to an embodiment of the present general inventive concept.  
                                                     TABLE 1                       Surface   Curvature   Thickness,   Refraction           number (S i )   Radius   distance (d i )   index (n d )   Dispersion (V d )                                *1   110.638   5.21   1.49200   57.1       *2   39.691   10.99        3   103.871   2.50   1.65844   50.9        4   29.588   18.76        5   −45.121   3.97   1.65844   50.9        6   45.121   30.36        7   −822.751   10.00   1.60342   38.0        8   −63.890   1.46        9   84.950   11.00   1.62290   58.1       10   −221.455   75.00       Stop   Infinity   4.19       12   75.739   5.00   1.48749   70.4       13   −22.786   4.50   1.75520   27.5       14   −68.618   8.29       15   191.645   1.20   1.84666   23.8       16   29.267   6.99   1.48749   70.4       17   −60.727   5.11       18   54.057   5.46   1.78472   25.7       19   −74.049   2.90       Prism   Infinity   25.0   1.51680   64.2       Imager   Infinity   0.00                  
 
         [0038]     The Equation of two aspheric surfaces S 1  and S 2  of the lens L 1  ( 51 ) can be expressed below.  
                     x   =         C   ⁢           ⁢     Y   2         1   +       1   -       (     K   +   1     )     ⁢     C   2     ⁢     Y   2               +     a   ⁢           ⁢     Y   4       +     b   ⁢           ⁢     Y   6       +     c   ⁢           ⁢     Y   8       +     d   ⁢           ⁢     Y   10                     C   =     1   /   R             ,           [     Equation   ⁢           ⁢   4     ]             
 
         [0039]     where, Y denotes a distance from an optical axis, C denotes a curvature, and R denotes a radius. Table 2 illustrates a Conic constant and aspheric surface coefficients (a, b, c, and d) of the surfaces S 1  and S 2  of the lens L 1  (51).  
                                         TABLE 2                                   S 1     S 2                                      K   −0.848776   −0.407388       A     0.432770E−05     0.120734E−05       B   −0.170360E−08     0.218927E−09       C     0.655455E−12   −0.435046E−11       D   −0.710030E−17     0.148104E−14                  
 
         [0040]     Table 3 illustrates a magnifying factor, an effective focal length (EFL), an F number the distance (object distance (OBJ)) from the screen  100  to the lens L 1  (51), an angle-of-view (FOV), and wavelengths of RGB light sources.  
                                           TABLE 3                       m   EFL   F#   OBJ   FOV   R   G   B                   0.0154   9.6864   2.5   593 mm   91   640 mm   550 mm   440 mm                       degrees                  
 
         [0041]      FIG. 3  illustrates characteristics of spherical aberration (a), astigmatism (b), and distortion (c) of the rear projection optical system  50  having the exemplary values as described above. Further,  FIG. 4  illustrates that a depth of a display device can be reduced by insertion of a reflection mirror  400  between the front lens group  50   a  and the rear lens group  50   b , according to an embodiment of the present general inventive concept.  
         [0042]     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.