Abstract:
A medium magnification objective for video disks comprising three or four single lenses for which the number of lenses constituting the objective is small, the working distance is large and flatness of image is high.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a medium magnification objective for video disks and, more particularly, to a medium magnification objective for reading out the signals recorded on high-density information-recording disks (video disks). 
     (B) Description of the Prior Art 
     For objectives to be used in reproducing systems for video disks, it is required to warrant resolving power of 1μ due to the fact that the objective has to read out very small signals recorded with high density. Moreover, the information read out from the disk, which rotates at high speed, contains signals for making the objective follow up the recorded track and signals for automatic focusing in addition to image information. To make the objective read out those information and signals correctly, the flatness of image focused by the objective should be high. To prevent breakage of the video disk and objective which will be caused when the objective contacts the video disk, the working distance of the objective should be long. Besides, to perform automatic focusing, the objective should be compact and light in weight. Moreover, the price of the objective should be low. 
     As the light used for the objective for video disks is generally a monochromatic light (λ = 632.8mm), it is effective for eliminating the noise at the time of amplifying the signals from a detector when transparency for the light of this wavelength is as high as possible. Therefore, to make transparency high, it is necessary to provide multi-layer anti-reflection coating on the lens surface or to make the number of lenses constituting the objective as small as possible. When this problem is considered in connection with the above-mentioned other requirements such as low price and light weight, it is more advantageous when the number of lenses constituting the objective is made as small as possible. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide a medium magnification objective for video disks for which the working distance is large, flatness of image is high and resolving power is high. 
     The objective for video disks according to the present invention comprises a first, second and third lenses as shown in FIG. 1. The first lens is a positive meniscus lens having comparatively large thickness with its planar or concave surface positioned toward the object, the second lens is a negative meniscus lens with its convex surface positioned toward the object, and the third lens is a biconvex lens. Besides, the objective for video disks according to the present invention satisfies the following conditions when reference symbol r 2  represents the radius of curvature of the surface on the image side of the first lens, reference symbol r 3  represents the radius of curvature of the surface on the object side of the second lens, reference symbol d 1  represents the thickness of the first lens, reference symbol d 2  represents the airspace between the first and second lenses, reference symbol d 4  represents the airspace between the second and third lenses, reference symbol f 1  represents the focal length of the first lens, reference symbol f 23  represents the total focal length of the second and third lenses, and reference symbol f represents the focal length of the lens system as a whole. 
     (1) 2.5 ≧ |r 2  |/d 1  ≧ 1.3 
     (2) 2 ≧ r 3  /f ≧ 1.1 
     (3) 0.3 ≧ d4/d2 ≧ 0.13 
     (4) 0.8 ≧ d2/f ≧ 0.1 
     (5) 2.3 ≧ f23/f1 ≧ 1.5 
     In the objective for video disks according to the present invention having the above-mentioned configuration, the first and third lenses serve to converge the rays from the object and the second lens serves to make the flatness of image high and to make the working distance large by shifting the front focal point toward the object. 
     As for the condition (1) out of the above-mentioned respective conditions, spherical aberration will be somewhat undercorrected if |r 2  |/d 1  bocomes |r 2  |/d 1  &gt;2.5. If it becomes |r 2  |/d 1  &lt;1.3, spherical aberration will be somewhat overcorrected in the marginal portion, coma will be aggravated and, at the same time, the working distance will become short. 
     If it becomes r 3  /f &gt;2 in the condition (2), astigmatism will be aggravated. If, on the contrary, it becomes r 3  /f &lt;1.1, the astigmatic difference will tend to become large. 
     If it becomes d 4  /d 2  &gt;0.3 in the condition (3), the astigmatic difference will become large and coma will be somewhat overcorrected. If it becomes d 4  /d 2  &lt;0.13, the astigmatic difference becomes small. However, coma of offaxial rays becomes asymmetrical in the marginal portion. 
     If it becomes d 2  /f &gt;0.8 in the condition (4), the astigmatic difference will become small. However, coma of offaxial rays will be somewhat overcorrected in the marginal portion and will become assymmetrical. If it becomes d 2  /f &lt;0.1, the astigmatic difference will become large. 
     If it becomes f 23  /f 1  &lt;1.5 in the condition (5), it becomes difficult to satisfactorily correct the coma, which is caused by the first lens, by the second and third lenses. Moreover, spherical aberration in the zonal portion will be considerably undercorrected and pin-cushion type distortion will be caused and will tend to become large. If it becomes f 23  /f 1  &gt;2.3, spherical aberration will be somewhat overcorrected in the marginal portion. 
     When refractive indices n 1 , n 2  and n 3  of respective lenses are made n 1  ≧ 1.6, n 2  ≧ 1.6 and n 3  ≧ 1.6 in addition to the above mentioned respective conditions, it is not necessary to make the radii of curvature of respective lens surfaces very small in the course of correction of aberrations and, therefore, it becomes easier to manufacture the lenses. 
     Besides, when the ratio d 2  /f is large, the back focal point, i.e., the exit pupil will be positioned within the lens system. When, on the other hand, the ratio d 2  /f is small, the exit pupil will be positioned outside the lens system. When, on the other hand, the ratio d 2  /f is small, the exit pupil will be positioned outside the lens system. When, therefore, the user requires that the exit pupil should be positioned near the final lens, the ratio d 2  /f becomes an important factor. 
     Now, FIG. 6 shows an objective for video disks arranged by improving the objective shown in FIG. 1 so that the numerical aperture becomes larger than that of the objective shown in FIG. 1, the flatness of image becomes still higher and the working distance becomes still larger. To make the numerical aperture larger, in the objective shown in FIG. 6, a negative meniscus lens having a large thickness is added on the image side of the lens system as the fourth lens by positioning its convex surface toward the object. The objective shown in FIG. 6 satisfies the following conditions when reference symbol f represents the focal length of the lens system as a whole, reference symbol f 2  represents the focal length of the second lens, reference symbols d 3  and d 7  respectively represent thicknesses of the second and fourth lenses, and reference symbol r 8  represents the radius of curvature of the surface on the image side of the fourth lens. 
     (6) 4.2 ≧ |f 2  |/f ≧ 2.2 
     (7) 0.31 ≧ d 3  /f ≧ 0.1 
     (8) 6 ≧ d 7  /d 3  ≧ 1 
     (9) 0.8 ≧ r 8  /f ≧ 0.5 
     The objective for video disks according to the present invention shown in FIG. 6 is arranged to make the numerical aperture large by adding the negative meniscus lens as the fourth lens as described in the above. Besides, the fourth lens serves in combination with the second lens to shift the front focal point toward the object so that the working distance becomes still larger and to make the flatness of image still higher. 
     Out of the above conditions (6) through (9), the condition (6) relates to the focal length f 2  of the second lens. If it becomes 4.2 &lt;|f 2  |/f in this condition, the flatness of image will be aggravated. Moreover, as the front focal point will be shifted toward the lens, the working distance tends to become small. Therefore, it becomes necessary to solve the above problems by the fourth lens. If, however, it is attempted to solve the above problems by the fourthe lens, the radius of curvature r 8  necessarily becomes small and it becomes difficult to manufacture the fourth lens. If, on the contrary, it becomes 2.2 &lt;|f 2  |/f, the flatness of image may be improved. However, divergence of rays passed through the second lens becomes large. As a result, spherical aberration will be undercorrected in the marginal portion and it becomes impossible to correct it by the third and fourth lenses. 
     If it becomes 0.31&lt;d 3  /f in the condition (7), spherical aberration and sine condition will be considerably undercorrected and it becomes very difficult to correct them favourably. Moreover, the front focal point will be shifted toward the lens and the working distance will become small. When, on the contrary, it becomes 0.1 &gt;d 3  /f, spherical aberration and sine condition may be only slightly overcorrected and this is not unfavourable. However, the thickness of the second lens will become too small. As a result, it becomes difficult to manufacture the second lens and the cost of production becomes high. 
     If it becomes 6 &lt;d 7  /d 3  in the condition (8), spherical aberration will be undercorrected and the astigmatic difference will become large. If it becomes 1 &gt;d 7  /d 3 , it becomes difficult to correct spherical aberration and coma favourably. 
     If it becomes 0.8&lt;r 8  /f in the condition (9), the flatness of image will be aggravated and it becomes very difficult to correct it by the second lens. Moreover, spherical aberration will be undercorrected. If it becomes 0.5&gt;r 8  /f, the flatness of image will become favourable. However, spherical aberration will be overcorrected. Moreover, r 8  becomes small and consequently it becomes difficult to manufacture the fourth lens. 
     In the objective shown in FIG. 6, the airspace d 4  between the second and third lenses contributes to correction of astigmatism. To correct astigmatism favourably, it is more preferable to select the airspace d 4  within the range of 0.4&lt;d 4  &lt;0.64. Besides, when refractive indices of respective lenses are higher, aberrations to be caused by respective lens surfaces become smaller and it is easier to correct them. Therefore, it is preferable to make refractive indices of respecitive lenses as nλ&gt;1.6. Especially, the fourth lens has the surface for correcting curvature of field and, therefore, the radius of curvature of that surface tends to become small. As a result, there is such tendency that coma is caused by that surface and, moreover, it will become somewhat difficult to manufacture the fourth lens. To solve these problems, it is also advantageous when the refractive index of the fourth lens is made high. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view of Embodiments 1 through 4 of the objective according to the present invention; 
     FIGS. 2A, 2B 2C and 2D respectively show graphs illustrating aberration curves of Embodiment 1; 
     FIGS. 3A, 3B, 3C and 3D respectively show graphs illustrating aberration curves of Embodiment 2; 
     FIGS. 4A, 4B, 4C and 4D respectively show graphs illustrating aberration curves of Embodiment 3; 
     FIGS. 5A, 5B, 5C and 5D respectively show graphs illustrating aberration curves of Embodiment 4; 
     FIG. 6 shows a sectional view of Embodiments 5 through 7 of the objective according to the present invention; 
     FIGS. 7A, 7B, 7C and 7D respectively show graphs illustrating aberration curves of Embodiment 5; 
     FIGS. 8A, 8B, 8C and 8D respectively show graphs illustrating aberration curves of Embodiment 6; and 
     FIGS. 9A, 9B, 9C and 9D respectively show graphs illustrating aberration curves of Embodiment 7. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the medium magnification objective for video disks according to the present invention are as shown below. 
     
         ______________________________________Embodiment 1r1 = ∞     d1 = 0.415  n1 = 1.77859 ν1 = 25.7r2 = 0.915     d2 = 0.521r3 = 1.185     d3 = 0.307  n2 = 1.77859 ν2 = 25.7r4 = 0.961     d4 = 0.143r5 = 3.330     d5 = 0.314  n3 = 1.77859 ν3 = 25.7r6 = -2.041     f = 1.0,    f1 = 1.18,   f23 = 2.09     S = 0.307,  N.A. = 0.4,  β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 2r1 = -12.859     d1 = 0.349  n1 = 1.77859 ν1 = 25.7r2 = -0.813     d2 = 0.575r3 = 1.320     d3 = 0.350  n2 = 1.77859 ν2 = 25.7r4 = 1.045     d4 = 0.129r5 = 3.452     d5 = 0.352  n3 = 1.77859 ν3 = 25.7r6 = 1.806     f = 1.0,    f1 = 1.10,   f23 = 1.98     S = 0.14,   N.A = 0.4,   β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 3r1 = -2.847     d1 = 0.25   n1 = 1.77859 ν1 = 25.7r2 = 0.591     d2 = 0.759r3 = 1.391     d3 = 3.03   n2 = 1.77859 ν2 = 25.7r4 = 1.140     d4 = 0.126r5 = 5.253     d5 = 0.385  n3 = 1.77859 ν3 = 25.7r6 = 1.695     f = 1.0,    f1 = 0.91,   f23 = 2.05     S = 0.235,  N.A = 0.4,   β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 4r1 = ∞     dl = 0.448  nl = 1.72309 νl = 28.5r2 = -0.652     d2 = 0.693r3 = 1.883     d3 = 0.356  n2 = 1.77861 ν2 = 25.7r4 = 1.289     d4 = 0.102r5 = 2.824     d5 = 0.397  n3 = 1.77861 ν3 = 25.7r6 = -1.762     f = 1.0,    fl = 0.90,   f23 = 1.94     S = 0.144,  N.A = 0.4,   β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 5r1 = -7.6008     d1 = 0.9048 n1 = 1.77861 ν1 = 25.7r2 = -1.2050     d2 = 0.0354r3 = 2.3387     d3 = 0.2257 n2 = 1.77861 ν2 = 25.7r4 = 1.2799     d4 = 0.5856r5 = 2.6429     d5 = 0.6135 n3 = 1.61655 ν3 = 36.3r6 = -2.4851     d6 = 0.0189r7 = 1.3231     d7 = 1.0313 n4 = 1.77861 ν4 = 25.7r8 = 0.7271     f = 1,      f2 = -4.002, S = 0.528     N.A. = 0.45,                 β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 6r1 = ∞     d1 = 0.4154 n1 = 1.77861 ν1 = 25.7r2 = -0.9866     d2 = 0.1901r3 = 2.0044     d3 = 0.2740 n2 = 1.77861 ν2 = 25.7r4 = 0.9722     d4 = 0.4522r5 = 3.0901     d5 = 0.2776 n3 = 1.77861 ν3 = 25.7r6 = -1.4689     d6 = 0.5103r7 = 0.9637     d7 = 0.3789 n4 = 1.77861 ν4 = 25.7r8 = 0.7378     f = 1,      f2 = -2.743, S = 0.344     N.A = 0.45, β = -20 x______________________________________ 
    
     
         ______________________________________Embodiment 7r1 = -9.2994     dl = 0.7012 n1 = 1.77861 ν1 = 25.7r2 = -1.2407     d2 = 0.0501r3 = 2.1560     d3 = 0.1140 n2 = 1.77861 ν2 = 25.7r4 = 1.0029     d4 = 0.4989r5 = 4.2726     d5 = 0.4967 n3 = 1.77861 ν3 = 25.7r6 = -1.4708     d6 = 0.0873r7 = 0.9676     d7 = 0.6206 n4 = 1.77861 ν4 = 25.7r8 = 0.6090     f = 1,      f2 = -2.517, S = 0.313     N.A = 0.45, β = -20 x______________________________________ 
    
     In the above-mentioned respective embodiments, reference symbols r1, r2, . . . respectively represent radii of curvature of respective lens surfaces, reference symbols d1, d2, . . . respectively represent thicknesses of respective lenses and airspaces between respective lenses, reference symbols n1, n2, . . . respectively represent refractive indices of respective lenses at λ= 632.8nm, reference symbols ν1, ν2, . . . respectively represent Abbe&#39;s numbers of respective lenses for d-line and reference symbol S represents the distance from the object to the first lens surface of the lens system (working distance).