Abstract:
A projection lens system, in the order from a magnified side to a reduced side, includes a first lens group with negative refraction power and a second lens group with positive refraction power. The projection lens system satisfies the formulas: −1.55&lt;F1/F2&lt;−1.12; wherein F1 is the effective focal length of the first lens group, and F2 is the effective focal length of the second lens group.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to projection lens systems and, particularly, to a projection lens systems with a long back focal length. 
     2. Description of Related Art 
     Projection lens systems generally include at least two lens groups for focusing light rays reflected from a digital mirror device (DMD) on a screen. One of the at least two lens groups moves in relative to the other lens group to adjust a focal length of the projection lens system. However, when the projection lens system is designed with a lower aberration, a back focal length of the projection lens system is very short. Therefore, there is not enough room for positioning optical elements, such as DMD, in the range of the back focal length. 
     Therefore, it is desirable to provide a projection lens system which can overcome the limitations described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a projection lens system in accordance with a first exemplary embodiment. 
         FIGS. 2-4  are graphs respectively showing spherical aberration, field curvature, and distortion of the projection lens system of  FIG. 1 , when the projection lens system is in a wide-angle state. 
         FIGS. 5-7  are graphs respectively showing spherical aberration, field curvature, and distortion of the projection lens system of  FIG. 1 , when the projection lens system is in a telephoto state. 
         FIG. 8  is a schematic view of a projection lens system in accordance with a second exemplary embodiment. 
         FIGS. 9-11  are graphs respectively showing spherical aberration, field curvature, and distortion of the projection lens system of  FIG. 8 , when the projection lens system is in a wide-angle state. 
         FIGS. 12-14  are graphs respectively showing spherical aberration, field curvature, and distortion of the projection lens system of  FIG. 8 , when the projection lens system is in a telephoto state. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the disclosure will now be described in detail with reference to the accompanying drawings. 
     Referring to  FIG. 1 , a projection lens system  100 , according to a first exemplary embodiment, projects an image onto a screen (not shown). The projection lens system  100  includes, in the order from a magnified side to a reduced side, a first lens group  10  with negative refraction power, a second lens group  20  with positive refraction power, and a filter glass  30 . During focusing, the first lens group  10  and the second lens group  20  move along an optical axis of the projection lens system  100 . The projection lens system  100  can be set among a wide-angle state and a telephoto state. 
     In this embodiment, the projection lens system  100  is utilized in a digital light processing (DLP) projector. A spatial light modulator (SLM)  40 , for example, a digital micro-mirror device (DMD), modulates light rays for projecting through the projection lens system  100 . The light rays transmit through the filter glass  30 , the second lens group  20 , and the first lens group  10  in order, and then project onto the screen. 
     The first lens group  10  includes, in the order from a magnified side to a reduced side of the projection lens system  100 , a first lens  11  with positive refraction power, a second lens  12  with negative refraction power, a third lens  13  with negative refraction power, and a fourth lens  14  with positive refraction power. The first lens  11 , the second lens  12 , the third lens  13 , and the fourth lens  14  are spherical lenses. The first lens  11  is meniscus shaped and includes a convex first surface S 1  facing the magnified side, and a concave second surface S 2  facing the reduced side. The second lens  12  is meniscus shaped and includes a convex third surface S 3  facing the magnified side, and a concave fourth surface S 4  facing the reduced side. The third lens  13  is meniscus shaped and includes a convex fifth surface S 5  facing the magnified side, and a concave sixth surface S 6  facing the reduced side. The fourth lens  14  is meniscus shaped and includes a convex seventh surface S 7  facing the magnified side, and a concave eighth surface S 8  facing the reduced side. 
     The second lens group  20  includes, in the order from a magnified side to a reduced side of the projection lens system  100 , a fifth lens  21  with positive refraction power, a sixth lens  22  with positive refraction power, a seventh lens  23  with negative refraction power, an eighth lens  24  with negative refraction power, and a ninth lens  25  with positive refraction power. The fifth lens  21 , the sixth lens  22 , the seventh lens  23 , and the eighth lens  24  are spherical lenses. The ninth lens  25  is aspherical lens. The fifth lens  21  is biconvex shaped and includes a convex ninth surface S 9  facing the magnified side, and a convex tenth surface S 10  facing the reduced side. The sixth lens  22  is biconvex shaped and includes a convex eleventh surface S 11  facing the magnified side, and a convex twelfth surface S 12  facing the reduced side. The seventh lens  23  is meniscus shaped and includes a convex thirteenth surface S 13  facing the magnified side, and a concave fourteenth surface S 14  facing the reduced side. The eighth lens  24  is biconcave shaped and includes a concave fifteenth surface S 15  facing the magnified side, and a concave sixteenth surface S 16  facing the reduced side. The ninth lens  25  is biconvex shaped and includes a convex seventeenth surface S 17  facing the magnified side, and a convex eighteenth surface S 18  facing the reduced side. 
     The filter glass  30  is flat plate shaped, and includes a nineteenth surface facing the magnified side and a twentieth surface facing the reduced side. 
     The aperture stop  50  is positioned between the seventh lens  23  and the eighth lens  24 . The aperture stop  50  adjusts the light flux from the seventh lens  23  to the eighth lens  24 . In addition, the aperture stop  50  facilitates a uniform light transmission when light passes through the seventh lens  23 , to correct chromatic aberrations of the lens system  100 . 
     In this embodiment, in order to reduce the number of the lenses of the projection lens system  100 , the ninth lens  25  is a moulded lens and made of glass. In order to decrease the size of the projection lens system  100 , the projection lens system  100  consists of at most ten lenses. 
     The projection lens system  100  satisfies the formula:
 
−1.55&lt; F 1/ F 2&lt;−1.12;  (1)
 
wherein F1 is the effective focal length of the first lens group  10 , and F2 is the effective focal length of the second lens group  20 . Formula (1) is for lengthening a back focal length BFL of the projection lens system  100  in a condition of the projection lens system  100  obtains a high image quality.
 
     The projection lens system  100  further satisfies the formula:
 
0.2&lt;BFL/L&lt;0.28;  (2)
 
wherein BFL is the distance from the second lens group  20  to the SLM  40 , and L is a total length of the projection lens system  100 . Formula (2) is for decreasing the size and correcting the aberration of the projection lens system  100  in a condition of the projection lens system  100  obtains a longer back focal length BFL.
 
     The above aspherical surfaces are shaped according to the formula: 
             x   =         ch   2       1   +       1   -       (     k   +   1     )     ⁢     c   2     ⁢     h   2               +       ∑             ⁢     Aih   i               
wherein h is a height from the optical axis of the sixth lens  31  to the aspherical surface, c is a vertex curvature, k is a conic constant, and Ai are i-th order correction coefficients of the aspherical surfaces.
 
     The following symbols are used:
     R: radius of curvature,   D: distance between surfaces on the optical axis,   Nd: refractive index for the Helium d-line (587.56 nm) of lens,   Vd: Abbe number,   f: effective focal length of the projection lens system,   D3: distance between the fourth surface and the fifth surface on the optical axis,   D9: distance between the tenth surface and the eleventh surface on the optical axis,   D17: distance between the eighteenth surface and the nineteenth surface on the optical axis, and   D19: distance between the twentieth surface and the twenty-first surface on the optical axis.   

     Example 1 
     The projection lens system  100  satisfies the parameters of Tables 1-3. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 sur- 
                   
                   
                   
                   
                   
                   
               
               
                 face 
                 type 
                 R(mm) 
                 D(mm) 
                 Nd 
                 Vd 
                 Conic 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S1  
                 spherical 
                 63.28 
                 5.13 
                 1.487 
                 70.44 
                 — 
               
               
                 S2  
                 spherical 
                 496.35 
                 0.1 
                   
                   
                 — 
               
               
                 S3  
                 spherical 
                 47.84 
                 3.4 
                 1.729 
                 54.66 
                 — 
               
               
                 S4  
                 spherical 
                 18.22 
                 6.26 
                 — 
                 — 
                 — 
               
               
                 S5  
                 spherical 
                 201.36 
                 1.4 
                 1.74 
                 52.676 
                 — 
               
               
                 S6  
                 spherical 
                 21.14 
                 10.62 
                 — 
                 — 
                 — 
               
               
                 S7  
                 spherical 
                 28.59 
                 3.09 
                 1.85 
                 23.785 
                 — 
               
               
                 S8  
                 spherical 
                 40.98 
                 19.86(2.79) 
                 — 
                 — 
                 — 
               
               
                 S9  
                 spherical 
                 35.99 
                 3.67 
                 1.496 
                 81.57 
                 — 
               
               
                 S10 
                 spherical 
                 −259.59 
                 0.1 
                 — 
                 — 
                 — 
               
               
                 S11 
                 spherical 
                 18.56 
                 5.5 
                 1.67 
                 51.7585 
                 — 
               
               
                 S12 
                 spherical 
                 −536.96 
                 0.3 
                 — 
                 — 
                 — 
               
               
                 S13 
                 spherical 
                 365.99 
                 4.8 
                 1.728 
                 28.32 
                 — 
               
               
                 S14 
                 spherical 
                 28.1 
                 1.39 
                 — 
                 — 
                 — 
               
               
                 50 
                 flat 
                 infinity 
                 2.57 
                 — 
                 — 
                 — 
               
               
                 S15 
                 spherical 
                 −21.93 
                 1.4 
                 1.728 
                 28.32 
                 — 
               
               
                 S16 
                 spherical 
                 41.8 
                 0.26 
                 — 
                 — 
                 — 
               
               
                 S17 
                 aspherical 
                 24.140 
                 4.06 
                 1.68996 
                 52.81 
                 1.874858 
               
               
                 S18 
                 aspherical 
                 −20.313 
                 22.74 
                 — 
                 — 
                 −0.69899 
               
               
                 S19 
                 flat 
                 infinity 
                 1.05 
                 1.511 
                 60.40 
                 — 
               
               
                 S20 
                 flat 
                 infinity 
                 1.11 
                 — 
                 — 
                 — 
               
               
                 40 
                 flat 
                 infinity 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 aspherical coefficient 
                 S17 
                 S18 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A4 
                 −5.87979e−5 
                 3.56641e−5 
               
               
                 A6 
                 9.991033e−7 
                 1.114679e−6 
               
               
                 A8 
                 −7.624939-9 
                 −1.627e−8 
               
               
                  A10 
                 3.0030120e−10 
                 4.699e−10 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 state 
                 F 
                 F No   
                 F1 
                 F2 
                 F1/F2 
                 L 
                 BFL 
                 BFL/L 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 wide- 
                 16 
                 2.56 
                 −28.98 
                 25.37 
                 −1.142 
                 98.81 
                 24.9 
                 0.252 
               
               
                 angle 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 state 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 tele- 
                 25.6 
                 3.24 
                 −28.98 
                 25.37 
                 −1.142 
                 90.38 
                 24.9 
                 0.276 
               
               
                 photo 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 state 
               
               
                   
               
             
          
         
       
     
     The spherical aberration graph, the field curvature graph, and the distortion graph of the projection lens system  100  in the wide angle state are respectively shown in  FIGS. 2-4 . The curves presented in the  FIGS. 2-4  are F light rays (wavelength: 486 nm), d light rays (wavelength: 588 nm), c light rays (wavelength: 655 nm). The spherical aberration of visible light in  FIG. 2  is within a range of −0.10 mm to 0.10 mm. The sagittal field curvature and tangential field curvature shown in  FIG. 3  are kept within a range of −0.20 mm to 0.20 mm. The distortion in  FIG. 4  falls within a range of −3% to 3%. Obviously, the spherical aberration, field curvature, and distortion are well controlled in the projection lens system  100 . 
     The spherical aberration graph, the field curvature graph, and the distortion graph of the projection lens system  110  in the telephoto state are respectively shown in  FIGS. 5-7 . The curves presented in the  FIGS. 5-7  are F light rays (wavelength: 486 nm), d light rays (wavelength: 588 nm), c light rays (wavelength: 655 nm). Spherical aberration of visible light in  FIG. 5  is within a range of −0.20 mm to 0.20 mm. The sagittal field curvature and tangential field curvature shown in  FIG. 6  are kept within a range of −0.30 mm to 0.30 mm. The distortion in  FIG. 7  falls within a range of −3% to 3%. Obviously, the spherical aberration, field curvature, and distortion are well controlled in the projection lens system  100 . 
     Example 2 
     Tables 4-6 show a specification of a second exemplary embodiment of the projection lens system  100   a . 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 sur- 
                   
                   
                   
                   
                   
                   
               
               
                 face 
                 type 
                 R(mm) 
                 D(mm) 
                 Nd 
                 Vd 
                 Conic 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S1  
                 spherical 
                 63.28 
                 5.13 
                 1.487 
                 70.44 
                 — 
               
               
                 S2  
                 spherical 
                 496.35 
                 0.1 
                   
                   
                 — 
               
               
                 S3  
                 spherical 
                 47.84 
                 3.4 
                 1.729 
                 54.66 
                 — 
               
               
                 S4  
                 spherical 
                 18.22 
                 6.26 
                 — 
                 — 
                 — 
               
               
                 S5  
                 spherical 
                 201.36 
                 1.4 
                 1.74 
                 52.676 
                 — 
               
               
                 S6  
                 spherical 
                 21.14 
                 10.62 
                 — 
                 — 
                 — 
               
               
                 S7  
                 spherical 
                 28.59 
                 3.09 
                 1.85 
                 23.785 
                 — 
               
               
                 S8  
                 spherical 
                 40.98 
                 19.86(2.80) 
                 — 
                 — 
                 — 
               
               
                 S9  
                 spherical 
                 35.99 
                 3.67 
                 1.496 
                 81.57 
                 — 
               
               
                 S10 
                 spherical 
                 −259.59 
                 0.1 
                 — 
                 — 
                 — 
               
               
                 S11 
                 spherical 
                 18.56 
                 5.5 
                 1.67 
                 51.7585 
                 — 
               
               
                 S12 
                 spherical 
                 −536.96 
                 0.3 
                 — 
                 — 
                 — 
               
               
                 S13 
                 spherical 
                 365.99 
                 4.8 
                 1.728 
                 28.32 
                 — 
               
               
                 S14 
                 spherical 
                 28.1 
                 1.39 
                 — 
                 — 
                 — 
               
               
                 50 
                 flat 
                 infinity 
                 2.57 
                 — 
                 — 
                 — 
               
               
                 S15 
                 spherical 
                 −21.93 
                 1.4 
                 1.728 
                 28.32 
                 — 
               
               
                 S16 
                 spherical 
                 41.8 
                 0.26 
                 — 
                 — 
                 — 
               
               
                 S17 
                 aspherical 
                 24.140 
                 4.06 
                 1.69036 
                 52.86 
                 1.874858 
               
               
                 S18 
                 aspherical 
                 −20.313 
                 22.74 
                   
                   
                 −0.69899 
               
               
                 S19 
                 flat 
                 infinity 
                 1.05 
                 1.511 
                 60.40 
                 — 
               
               
                 S20 
                 flat 
                 infinity 
                 1.11 
                 — 
                 — 
                 — 
               
               
                 40 
                 flat 
                 infinity 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 aspherical coefficient 
                 S17 
                 S18 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A4 
                 −5.87979e−5 
                 3.56641e−5 
               
               
                 A6 
                 9.991033e−7 
                 1.114679e−6 
               
               
                 A8 
                 −7.624939-9 
                 −1.627e−8 
               
               
                  A10 
                 3.0030120e−10 
                 4.699e−10 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 State 
                 F 
                 F No   
                 F1 
                 F2 
                 F1/F2 
                 L 
                 BFL 
                 BFL/L 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 wide- 
                 16 
                 2.55 
                 −28.98 
                 25.36 
                 −1.143 
                 98.73 
                 24.9 
                 0.252 
               
               
                 angle 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 state 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 tele- 
                 25.62 
                 3.24 
                 −28.98 
                 25.36 
                 −1.143 
                 90.32 
                 24.9 
                 0.276 
               
               
                 photo 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 state 
               
               
                   
               
             
          
         
       
     
     The spherical aberration graph, the field curvature graph, and the distortion graph of the projection lens system  100   a  in the wide angle state are respectively shown in  FIGS. 9-11 . The curves presented in the  FIGS. 9-11  are F light rays (wavelength: 486 nm), d light rays (wavelength: 588 nm), c light rays (wavelength: 655 nm). Spherical aberration of visible light in  FIG. 9  is within a range of −0.20 mm to 0.20 mm. The sagittal field curvature and tangential field curvature shown in  FIG. 10  are kept within a range of −0.20 mm to 0.20 mm. The distortion in  FIG. 11  falls within a range of −3% to 3%. Obviously, the spherical aberration, field curvature, and distortion are well controlled in the projection lens system  110   a.    
     The spherical aberration graph, the field curvature graph, and the distortion graph of the projection lens system  110   a  in the telephoto state are respectively shown in  FIGS. 12-14 . The curves presented in the  FIGS. 5-7  are F light rays (wavelength: 486 nm), d light rays (wavelength: 588 nm), c light rays (wavelength: 655 nm). Spherical aberration of visible light in  FIG. 12  is within a range of −0.20 mm to 0.20 mm. The sagittal field curvature and tangential field curvature shown in  FIG. 13  are kept within a range of −0.20 mm to 0.20 mm. The distortion in  FIG. 14  falls within a range of −3% to 3%. Obviously, the spherical aberration, field curvature, and distortion are well controlled in the projection lens system  110   a.    
     Particular embodiments are shown and described by way of illustration only. The principles and the features of the present disclosure may be employed in various and numerous embodiments thereof without departing from the scope of the disclosure as claimed. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.