Abstract:
Disclosed is a projection optical system for projecting an original image onto a screen, including a projection lens comprising, in succession from the screen side, a first lens unit having positive refractive power, and a second lens unit having positive refractive power, and a reflecting member disposed between the first lens unit and the second lens unit of the projection lens for directing light from an illuminating light source to the original image through the second lens unit, one of a plurality of sub-lens units constituting the first lens unit being moved to thereby effect focusing.

Description:
This application is a continuation of application Ser. No. 07/873,236, filed Apr. 24, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a projection optical system for enlarging and projecting the original image of a liquid crystal panel or the like. 
     2. Related Background Art 
     As a projection optical system for enlarging and projecting an original image formed by a liquid crystal panel or the like, there is one disclosed, for example, in Japanese Laid-Open Patent Application No. 61-13885. The projection optical system of this publication, as shown in FIG. 23 of the accompanying drawings, is provided with a light source, a wide band polarizing beam splitter, reflection type liquid crystal devices and a projection lens, and is designed such that reflected light from the light source is projected onto a screen through the polarizing beam splitter and the projection lens and an enlarged image is displayed on the screen. 
     In the above-described example of the prior art, however, as shown in FIG. 23, it has been necessary to dispose a polarizing beam splitter 12 and two color resolving dichroic prisms 13 and 14 between a projection lens 22 and reflection type liquid crystal devices 16, 17, 18 and therefore, a back focal length greater than three times the width of the reflection type liquid crystal devices 16, 17, 18 has been required of the projection lens 22. Therefore, this example of the prior art is of a construction which generally cannot be made greatly compact, and it has been necessary to use a lens of the so-called retrofocus type as the projection lens, and this has caused the lens to be bulky and increased the number of lenses used in the projection lens. 
     SUMMARY OF THE INVENTION 
     In view of such a problem peculiar to the prior art, the present invention intends to make a projection optical system compact, and further intends to suppress any fluctuation of the focus of the projection optical system when the optimal system is made compact. 
     A feature of the present invention resides in a projection optical system for projecting an original image onto a screen, the optical system including a projection lens comprising, in succession from the screen side, a first lens unit having positive refractive power, and a second lens unit having positive refractive power, and a reflecting-member disposed between said first lens unit and said second lens unit of said projection lens for directing light from an illuminating light source to said original image through said second lens unit, one of a plurality of lenses constituting said first lens unit being moved to thereby effect focusing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an optical cross-sectional view of a first embodiment of the present invention. 
     FIG. 2 is an optical cross-sectional view of a second embodiment of the present invention. 
     FIG. 3 is an optical cross-sectional view of a third embodiment of the present invention. 
     FIG. 4 is an optical cross-sectional view of a fourth embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of the lens of a first numerical value embodiment of the present invention. 
     FIG. 6 shows aberrations in the first numerical value embodiment (magnification 40×). 
     FIG. 7 shows aberrations in the first numerical value embodiment (magnification 2033). 
     FIG. 8 is a cross-sectional view of the lens of a second numerical value embodiment of the present invention. 
     FIG. 9 shows aberrations in the second numerical value embodiment (magnification 40×). 
     FIG. 10 shows aberrations in the second numerical value embodiment (magnification 20×). 
     FIG. 11 is a cross-sectional view of the lens of a third numerical value embodiment of the present invention. 
     FIG. 12 shows aberrations in the third numerical value embodiment (magnification 40×). 
     FIG. 13 shows aberrations in the third numerical value embodiment (magnification 20×). 
     FIG. 14 is a cross-sectional view of the lens of a fourth numerical value embodiment of the present invention. 
     FIG. 15 shows aberrations in the fourth numerical value embodiment (magnification 40×). 
     FIG. 16 shows aberrations in the fourth numerical value embodiment (magnification 20×). 
     FIG. 17 is a cross-sectional view of the lens of a fifth numerical value embodiment of the present invention. 
     FIG. 18 shows aberrations in the fifth numerical value embodiment (magnification 40×). 
     FIG. 19 shows aberrations in the fifth numerical value embodiment (magnification 20×). 
     FIG. 20 is a cross-sectional view of the lens of a sixth numerical value embodiment of the present invention, 
     FIG. 21 shows aberrations in the sixth numerical value embodiment (magnification 40×). 
     FIG. 22 shows aberrations in the sixth numerical value embodiment (magnification 20×). 
     FIG. 23 is a cross-sectional view showing the essential portions of a projection optical system according to the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 which shows a cross-sectional view of the essential portions of a first embodiment of the present invention, the reference numeral 1 designates a negative lens constituting the front sub-lens unit of a first lens unit, the reference numeral 2 denotes a positive lens constituting the rear sub-lens unit of the first lens unit, and the reference numeral 3 designates a second lens unit having positive refractive power. The reference numeral 4 denotes a prism block having a reflecting surface 4a for reflecting, for example, S wave, the reference numeral 5 designates a conventional cross dichroic prism for effecting color resolution, the reference numeral 9 denotes a light source, and the reference numerals 6, 7 and 8 designate reflection type liquid crystal display devices which form original images. 
     Under such a construction, white illuminating light emitted from the light source 9 has its S wave alone reflected by the reflecting surface 4a and condensed by the second lens unit 3, and then is resolved into three colors by the cross dichroic prism 5, and these color lights arrive at the reflection type liquid crystal display devices 6, 7 and 8. These color lights are reflected by the original images on the reflection type liquid crystal display devices and are endowed with the gradations of the respective colors in accordance with display information formed, and are reflected again by the cross dichroic prism 5. The reflected lights are polarized into P wave by the liquid crystal display. The combined light is condensed by the second lens unit 3 and passes through the reflecting surface 4a, and is further condensed by the first lens unit 1, 2 and projected onto a distant screen S. 
     In FIG. 1, the reflecting surface 4a is constructed as a polarizing beam splitter, but alternatively, this reflecting surface may be replaced by a half mirror 10 as shown in FIG. 3, or by a dot mirror 11 having reflecting portions and transmitting portions disposed alternately as shown in FIG. 4. 
     Now, the projection lens according to the present invention is designed such that the light of the illuminating light source 9 illuminates each liquid crystal display device by way of the reflecting surface 4a and through the second lens unit, and the first lens unit is divided into a front sub-lens unit having negative refractive power and a rear sub-lens unit having positive refractive power, and the spacing between these sub-lens units is changed so as to effect focusing. 
     In the present invention, it is desirable that the following conditional expressions be satisfied to maintain the optical performance of the entire system well and to suppress aberration fluctuation when focusing is effected by the first lens unit. The arrow in FIG. 1 indicates the direction of movement of the focusing lens. 
     
         0.4&lt;|φ.sub.1f /φ|&lt;1.5            (1) 
    
     
         0.8&lt;ν.sub.1f /ν.sub.1s &lt;1.2                          (2) 
    
     where φ is the refractive power of the entire projection lens system, φ 1f  and ν 1f  are the mean values of the refractive power and Abbe number, respectively, of the sub-lens unit moved during the focusing of the first lens unit, and ν 1s  is the mean value of the Abbe number of the fixed sub-lens unit of the first lens unit. 
     Conditional expression (1) shows the ratio of the power of the focusing lens of the first lens unit to the power of the entire system, and if the lower limit value of this conditional expression is exceeded, the power of the focusing lens of the first lens unit will become too weak and the amount of movement for focusing will increase to thereby increase the diameter of the front lens, and this is not preferable. 
     If the upper limit value of conditional expression (1) is exceeded, the power of the focusing lens of the first lens unit will strengthen and therefore, the fluctuations of aberrations, particularly spherical aberration and curvature of image field, for a variation in the distance of the screen will become great, and this is not preferable. 
     Conditional expression (2) limits the ratio of the mean value of the Abbe number of the focusing lens of the first lens unit to the mean value of the Abbe number of the fixed lens of the first lens unit, and outside the range of this conditional expression, the occurrence of the chromatic aberration of magnification is remarkable, and this is not preferable. 
     Now, in the projection optical system according to the present invention, the position of the pupil of the projection lens is brought close to the position of the reflecting surface 4a so that the eclipse or darkening of the illuminating light by the reflecting surface 4a does not occur on the screen S. To enable the illuminating light to illuminate the screen well at this time, it is desirable that the following conditional expression be satisfied: 
     
         0.15&lt;d·φ.sub.2 &lt;0.30                          (3) 
    
     where d is the length of the optical path as converted into air spacing from the point of intersection between the reflecting surface or the extension plane of the reflecting surface and the optical axis of the projection lens to the lens surface of the second lens unit which is adjacent to the screen side, and φ 2  is the refractive power of the second lens unit. 
     If the lower limit value of this conditional expression (3) is exceeded, the illuminating light will be eclipsed, and this is not preferable. On the other hand, if the upper limit value of this conditional expression is exceeded, the lens diameter of the second lens unit will be increased, and this is not preferable. 
     Some numerical value embodiments of the optical system according to the present invention will be shown below. In the following numerical value embodiments, R i  represents the radius of curvature of the ith lens surface from the object side, D i  represents the thickness and air gap of the ith lens from the object side, and N i  and ν i  represent the refractive index and Abbe number, respectively, of the glass of the ith lens from the object side. 
     
         ______________________________________(Numerical Value Embodiment 1)F = 71.6  FNO = 1:2.0  2W = 39°______________________________________R1 =  128.592       D1 =  3.00 N1 =  1.60311                              ν1 =  60.7R2 =  45.749       D2 =  7.00R3 =  71.250       D3 =  10.00                  N2 =  1.60311                              ν2 =  64.1R4 =  6463.902       D4 =  10.00R5 =  ∞       D5 =  18.00                  N3 =  1.51633                              ν3 =  64.1R6 =  (stop)       D6 =  18.00                  N4 =  1.51633                              ν4 =  64.1R7 =  ∞       D7 =  5.00R8 =  73.630       D8 =  8.25 N5 =  1.77250                              ν5 =  49.6R9 =  -163.325       D9 =  0.20R10 = 81.054       D10 = 7.63 N6 =  1.77250                              ν6 =  49.6R11 = -634.459       D11 = 4.31R12 = -169.412       D12 = 2.00 N7 =  1.80518                              ν7 =  25.4R13 = 44.566       D13 = 12.42R14 = -32.515       D14 = 2.00 N8 =  1.64769                              ν8 =  33.8R15 = 311.588       D15 = 6.28R16 = -69.886       D16 = 8.31 N9 =  1.77250                              ν9 =  49.6R17 = -36.806       D17 = 0.20R18 = -3524.918       D18 = 4.21 N10 = 1.77250                              ν10 = 49.6R19 = -189.899       D19 = 0.20R20 = 83.234       D20 = 10.11                  N11 = 1.77250                              ν11 = 49.6R21 = -306.672       D21 = 4.99R22 = ∞       D22 = 56.00                  N12 = 1.51633                              ν12 = 64.1R23 = ∞|φ.sub.1f /φ| = 0.60ν.sub.1f /ν.sub.1s = 1d · φ.sub.2 = 0.226______________________________________ 
    
     
         ______________________________________(Numerical Value Embodiment 2)F = 66.2  FNO = 1:2.0  2W = 42°______________________________________R1 =  -3916.141       D1 =  3.00 N1 =  1.60311                              ν1 =  60.7R2 =  45.381       D2 =  7.00R3 =  69.227       D3 =  10.00                  N2 =  1.60311                              ν2 =  60.7R4 =  -156.151       D4 =  10.00R5 =  ∞       D5 =  18.00                  N3 =  1.51633                              ν3 =  64.1R6 =  (stop)       D6 =  18.00                  N4 =  1.51633                              ν4 =  64.1R7 =  ∞       D7 =  5.00R8 =  70.983       D8 =  8.25 N5 =  1.77250                              ν5 =  49.6R9 =  -157.324       D9 =  0.20R10 = 87.114       D10 = 7.63 N6 =  1.77250                              ν6 =  49.6R11 = -584.667       D11 = 4.02R12 = -156.887       D12 = 2.00 N7 =  1.80518                              ν7 =  25.4R13 = 44.403       D13 = 13.14R14 = -32.439       D14 = 2.00 N8 =  1.64769                              ν8 =  33.8R15 = 292.889       D15 = 5.98R16 = -76.981       D16 = 7.60 N9 =  1.77250                              ν9 =  49.6R17 = -36.391       D17 = 0.20R18 = -2928.012       D18 = 4.66 N10 = 1.77250                              ν10 = 49.6R19 = -192.366       D19 = 0.20R20 = 83.260       D20 = 10.08                  N11 = 1.77250                              ν11 = 49.6R21 = -299.570       D21 = 4.97R22 = ∞       D22 = 56.00                  N12 = 1.51633                              ν12 = 64.1R23 = ∞|φ.sub.1f /φ| = 0.82ν.sub.1f /ν.sub.1s = 1d · φ.sub.2 = 0.229______________________________________ 
    
     
         ______________________________________(Numerical Value Embodiment 3)F = 62.7  FNO = 1:2.0  2W = 44°______________________________________R1 =  -101.066       D1 =  3.00 N1 =  1.60311                              ν1 =  60.7R2 =  63.974       D2 =  6.00R3 =  85.747       D3 =  11.25                  N2 =  1.60311                              ν2 =  60.7R4 =  -85.748       D4 =  35.00R5 =  (stop)       D5 =  15.00R6 =  59.414       D6 =  8.25 N3 =  1.78590                              ν3 =  44.2R7 =  1409.977       D7 =  0.20R8 =  85.758       D8 =  6.65 N4 =  1.71299                              ν4 =  53.8R9 =  2093.315       D9 =  8.22R10 = -229.344       D10 = 2.00 N5 =  1.80518                              ν5 =  25.4R11 = 42.141       D11 = 12.31R12 = -27.829       D12 = 2.00 N6 =  1.69895                              ν6 =  30.1R13 = -514.065       D13 = 1.19R14 = -167.773       D14 = 11.40                  N7 =  1.77250                              ν7 =  49.6R15 = -38.480       D15 = 0.20R16 = 5258.723       D16 = 7.55 N8 =  1.77250                              ν8 =  49.6R17 = -89.605       D17 = 0.20R18 = 92.583       D18 = 7.90 N9 =  1.77250                              ν9 =  49.6R19 = ∞       D19 = 5.10R20 = ∞       D20 = 59.20                  N10 = 1.51633                              ν10 = 64.1R21 = ∞|φ.sub.1f /φ| = 0.98ν.sub.1f /ν.sub.1s = 1d · φ.sub.2 = 0.209______________________________________ 
    
     
         ______________________________________(Numerical Value Embodiment 4)F = 62.4  FNO = 1:2.0  2W = 44.2°______________________________________R1 =  -100.316       D1 =  3.00 N1 =  1.60311                              ν1 =  60.7R2 =  64.196       D2 =  6.00R3 =  85.747       D3 =  11.51                  N2 =  1.60311                              ν2 =  60.7R4 =  -85.748       D4 =  35.00R5 =  (stop)       D5 =  15.00R6 =  56.769       D6 =  6.40 N3 =  1.77250                              ν3 =  49.6R7 =  -1514.170       D7 =  0.20R8 =  84.532       D8 =  3.52 N4 =  1.78590                              ν4 =  44.2R9 =  237.710       D9 =  9.72R10 = -425.900       D10 = 2.00 N5 =  1.80518                              ν5 =  25.4R11 = 42.141       D11 = 12.69R12 = -27.635       D12 = 2.00 N6 =  1.64769                              ν6 =  33.8R13 = -6885.777       D13 = 1.75R14 = -171.971       D14 = 11.85                  N7 =  1.69680                              ν7 =  55.5R15 = -37.828       D15 = 0.20R16 = 1389.573       D16 = 8.38 N8 =  1.69680                              ν8 =  55.5R17 = -84.844       D17 = 0.20R18 = 83.352       D18 = 8.91 N9 =  1.69680                              ν9 =  55.5R19 = ∞       D19 = 5.00R20 = ∞       D20 = 59.20                  N10 = 1.51633                              ν10 = 64.1R21 = ∞|φ.sub.1f /φ| = 0.97ν.sub.1f /ν.sub.1s = 1d · φ.sub.2 = 0.210______________________________________ 
    
     
         ______________________________________Numerical Value Embodiment 5)______________________________________F = 76.01955  FNO = 1;  2W =______________________________________R1 =  103.747       D1 =  3.00 N1 =  1.60621                              ν1 =  60.7R2 =  47.374       D2 =  7.00R3 =  64.533       D3 =  10.00                  N2 =  1.51884                              ν2 =  64.1R4 =  773.355       D4 =  10.00R5 =  ∞       D5 =  18.00                  N3 =  1.51884                              ν3 =  64.1R6 =  (stop)       D6 =  18.00                  N4 =  1.51884                              ν4 =  64.1R7 =  ∞       D7 =  5.00R8 =  68.275       D8 =  8.25 N5 =  1.77735                              ν6 =  49.6R9 =  -241.619       D9 =  0.20R10 = 84.802       D10 = 7.63 N6 =  1.77735                              ν6 =  49.6R11 = -776.696       D11 = 4.98R12 = -147.871       D12 = 2.00 N7 =  1.81499                              ν7 =  25.4R13 = 44.373       D13 = 13.92R14 = -32.869       D14 = 2.00 N8 =  1.65364                              ν8 =  33.8R15 = 629.200       D15 = 4.98R16 = -70.218       D16 = 8.63 N9 =  1.77735                              ν9 =  49.6R17 = -37.923       D17 = 0.20R18 = -2273.108       D18 = 4.73 N10 = 1.77735                              ν10 = 49.6R19 = -154.565       D19 = 0.20R20 = 83.886       D20 = 10.22                  N11 = 1.77735                              ν11 = 49.6R21 = -304.247       D21 = 4.98R22 = ∞       D22 = 56.00                  N12 = 1.51884                              ν12 = 64.1R23 = ∞|φ.sub.1f /φ| = 0.52ν.sub.1f /ν.sub.1s = 0.95d · φ.sub.2 = 0.219______________________________________ 
    
     
         ______________________________________(Numerical Value Embodiment 6)F = 75.10962  FNO = 1:  2W =______________________________________R1 =  233.984       D1 =  3.00 N1 =  1.51884                              ν1 =  64.1R2 =  48.513       D2 =  7.00R3 =  66.115       D3 =  10.00                  N2 =  1.60621                              ν2 =  60.7R4 =  -1485.500       D4 =  10.00R5 =  ∞       D5 =  18.00                  N3 =  1.51884                              ν3 =  64.1R6 =  (stop)       D6 =  18.00                  N4 =  1.51884                              ν4 =  64.1R7 =  ∞       D7 =  5.00R8 =  72.835       D8 =  8.25 N5 =  1.77735                              ν5 =  49.6R9 =  -363.450       D9 =  0.20R10 = 82.338       D10 = 7.63 N6 =  1.77735                              ν6 =  49.6R11 = -790.300       D11 = 4.86R12 = -202.748       D12 = 2.00 N7 =  1.81499                              ν7 =  25.4R13 = 43.292       D13 = 13.61R14 = -32.656       D14 = 2.00 N8 =  1.65364                              ν8 =  33.8R15 = 1613.337       D15 = 4.30R16 = -70.069       D16 = 9.67 N9 =  1.77735                              ν9 =  49.6R17 = -38.001       D17 = 0.20R18 = -694.527       D18 = 4.65 N10 = 1.77735                              ν10 = 49.6R19 = -138.281       D19 = 0.20R20 = 83.705       D20 = 10.30                  N11 = 1.77735                              ν11 = 49.6R21 = -307.197       D21 = 5.00R22 = ∞       D22 = 56.00                  N12 = 1.51884                              ν12 = 64.1R23 = ∞|φ.sub.1f /φ| = 0.63ν.sub.1f /ν.sub.1s = 1.056d · φ.sub.2 = 0.221______________________________________ 
    
     As described above, the projection optical system according to the present invention is an optical system in which a reflecting member is provided between the positive first lens unit and the positive second lens unit to thereby cause illuminating light to enter, and that part of the first lens unit which is adjacent to the screen side is moved to thereby effect focusing, whereby there can be realized a very compact projection optical system. Also, the focusing lens unit is light in weight and this is advantageous for focusing. There is also an advantage in that the aberration fluctuation, by a variation in distance, is small.