JP3389266B2 - Objective optical system for endoscope - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact objective optical system for an endoscope in which chromatic aberration is well corrected. 2. Description of the Related Art Conventionally, an endoscope used for a bronchus or a biliary tract or a small-diameter endoscope for industrial use has a relatively small number of pixels, is compact, has a good curvature of field, and has a good number of constituent lenses. There is an invention of Japanese Patent Application No. 4-156218 developed by the present applicant as an objective optical system for an endoscope having a small number of problems. This optical system includes two lenses, a concave lens and a convex lens, as shown in FIG. Since the objective optical system having such a configuration has no factor for correcting axial chromatic aberration and chromatic aberration of magnification, by using a glass material having a large Abbe number to minimize chromatic aberration generated in each lens, The chromatic aberration is corrected within a few pixels of the image guide and the solid-state imaging device to ensure the performance that can be practically used. In general, in an objective optical system having large chromatic aberration of magnification, color blur is conspicuous particularly in the peripheral portion of an image. Also, an objective optical system with large on-axis chromatic aberration causes color bleeding over the entire image, which leads to deterioration of image quality and can hinder diagnosis when used as an objective optical system for medical endoscopes. Many. In recent years, with the improvement of image guide manufacturing technology and solid-state imaging device manufacturing technology, products with higher resolution have been manufactured, and accordingly, the objective optical system for an endoscope also has chromatic aberration of magnification and axial chromatic aberration. It is necessary to correct chromatic aberration sufficiently well to have a high resolution. However, in the above-mentioned objective lens, chromatic aberration of magnification and axial chromatic aberration are not sufficiently corrected, and in this configuration, since there is no factor for correcting these aberrations, it is necessary to correct these chromatic aberrations sufficiently to correct the resolving power. Have difficulty. An object of the present invention is to provide a wide-angle objective optical system for an endoscope in which the number of constituent lenses is small, and chromatic aberration of magnification and axial chromatic aberration are sufficiently sufficiently corrected. [0006] The objective optical system of the present invention comprises:
As shown in FIG. 1, in order from the object side, a front group of a diverging lens having a concave surface facing the image side, a brightness stop, and a diverging lens having a concave surface facing the image side and a biconvex converging lens were joined. It consists of a rear group which is a converging lens group of a cemented lens, and the Abbe number ν _2d of the diverging lens of the rear group and the Abbe number ν _{3d of the} converging lens have the following relationship. (1) ν _2d <ν _{3d In} general, when a surface having a large refraction of a principal ray is provided with a strong power, various aberrations occur. In order to avoid this, it is preferable to make the surface close to concentric to the aperture stop. In the present invention, the above-described lens arrangement is used. However, it is desirable to increase the positive power of the image-side convex surface of the converging lens in the rear group. When this surface has a curvature smaller than that of the object-side surface of the diverging lens and | r ₄ | ≦ | r ₆ |, the off-axis light flux passing through the aperture stop, especially the upper marginal ray, is reflected by the diverging lens of the rear group. Large coma and astigmatism occur on the object side surface, resulting in insufficient correction. Note that r ₄ and r ₆ are the radii of curvature of the object-side surface of the diverging lens in the rear group and the image-side surface of the convergent lens, respectively. In the objective optical system according to the present invention, the divergent lens unit of the front unit and the convergent lens unit of the rear unit are arranged close to each other with a brightness stop interposed therebetween in order to realize compactness. For this reason, a light beam that forms an image around the visual field enters the object side surface of the rear convergent lens unit at a large angle, and chromatic aberration of large magnification occurs on this surface. Also, since a strong positive power is given to the image side surface of the converging lens unit in the rear unit, large axial chromatic aberration occurs on this surface. In order to satisfactorily correct these chromatic aberrations of magnification and axial chromatic aberration, the center of curvature of the cemented surface of the rear group should be on the opposite side of the aperture stop, and should be converged with the Abbe number ν _2d of the diverging lens of the rear group. The Abbe number ν _{3d of the} lens has the above-described relationship. On the contrary, the relationship between the Abbe numbers is ν _2d > ν
_{In the case of 3d} , if the center of curvature of the cemented lens is arranged on the brightness stop side, axial chromatic aberration can be corrected,
Since the angle of incidence of the off-axis light beam on the joint surface cannot be made large, chromatic aberration of magnification cannot be sufficiently corrected. If the curvature of the cemented surface is increased to enhance the effect of correcting chromatic aberration, the edge thickness of the converging lens, which is the second optical element in the rear group, becomes small, causing a processing problem. If the center of curvature of the cemented surface of the rear cemented lens is positioned on the opposite side of the aperture stop as in the lens configuration of the present invention, the higher the ray height of the off-axis light beam is, the more the beam enters the cemented surface. Since the angle is increased, it is possible to favorably correct not only longitudinal chromatic aberration but also chromatic aberration of magnification, which is advantageous in widening the angle. In the diverging lens of the cemented converging lens, the effect of correcting the chromatic aberration increases as the dispersion increases, and therefore it is desirable to satisfy the following condition (2). (2) In order to better correct the chromatic aberration at ν _2d <35 and the chromatic aberration on the axis, it is desirable to reduce the dispersion of the front diverging lens, and it is desirable to satisfy the following condition (3). (3) ν _1d > 40 If the front group diverging lens satisfies the above condition, the chromatic aberration can be corrected well, so that the curvature of the cemented surface of the rear group converging lens can be reduced by that amount, which is the second optical element. Workability of the convergent lens (biconvex lens) is improved. Next, if an aspherical surface is provided on at least one surface of the objective optical system according to the present invention and the aspherical surface satisfies the following condition (4), spherical aberration, coma and the like can be corrected more favorably. Preferred above. (4) E _i ′ (n _i−1 −n _i )> 0 where E _i ′ is the fourth-order aspheric coefficient of the above aspheric surface, n
_i-1, n _i is the refractive index of the object side and the image side of the medium of each aspherical. Here, the expression of the aspherical surface in the present invention will be described. In the embodiment described later, the shape of the aspherical surface is expressed by using the following equation. Here, x and y are those where the optical axis is the x axis, the direction of the image is positive, and the direction perpendicular to the optical axis is the y axis.
The point of intersection between the plane and the optical axis is the origin. R _i is the radius of curvature in the quadric surface term, p is the conic constant, B _i , E _i , F
_i, G _i, ··· are respectively secondary, fourth, sixth, eighth, ...
The aspherical coefficient of The above equation has a high degree of freedom and is suitable for expressing an axially symmetric surface, but is unsuitable for an explanation on aberration. Therefore, to explain the function of the aspherical surface in the present invention, the following equation is preferable, and the explanation will be made based on this equation. In equation (b), r _i is the radius of curvature of the reference aspherical sphere (the sphere in contact with the aspherical surface at the top).
E _i ′, F _i ′, G _i ′,...
Eighth,... Aspherical coefficients. The conversion from the expression (a) to the expression (b) can be performed using Taylor expansion, and the conversion expression of r _i ′ and low-order coefficients up to the 12th order is as follows. . Note that the following conversion equations of r _i ′, E _i ′,..., I _i ′ are collectively referred to as equation (c). _{r i '= r i / (} 1 + 2B i r i) E i' = 0.125 {P i - (1 + 2B i r i) 3} / r i 3 +
_{E i F i '= 0.0625 {} P i 2 - (1 + 2B i r i) 5} / r i 5 +
F _i G _i ′ = 0.0390625 {P _i ³ − (1 + 2B _i r _i ) ⁷ } / r
^{_{i 7 + G i H i '}} = 0.02734375 {P i 4 - (1 + 2B i r i) 9} / r
_i ⁹ + H _i I _i ′ = 0.02050782 {P _i ⁵ − (1 + 2B _i r _i ) ¹¹ } / r
_i ¹¹ + I _{i In the} above equation (c), the first term on the right side of the equation for each aspheric coefficient is obtained by subjecting a quadratic surface term to Taylor expansion.
Since the expression obtained by expansion is an infinite series, it is approximate in the expression of finite order. However, a very good approximation can be obtained by including up to the twelfth order coefficient. In equation (a), P _i =
1, if B _i = 0, there is no need for conversion, and r _i ′ = r _i ,
_{E i '= E i, F} i' = F i, the G _i '= G _i ···. The condition (4) is a condition for favorably correcting spherical aberration, coma and the like by using an aspheric surface. The aspherical surface is effective for correcting various aberrations other than chromatic aberration and field curvature. In the present invention, the residual aberration due to the small number of components of the lens system is canceled by using the function of the aspherical surface. In order to use an aspheric surface in this way, it is necessary to know the residual aberration in the lens system. In the lens system of the present invention, negative spherical aberration and negative coma (inner coma) generally remain. Therefore, it is necessary to correct these aberrations by using an aspherical surface. Therefore, it is only necessary to generate a positive aberration for each aberration on the aspherical surface. The relationship between the fourth-order aspherical coefficient E _i ′ and the third-order aberration coefficients of spherical aberration and coma caused by asphericization is shown by the following equations (d) and (e). is there. ^{_{ΔSP i = 8h i 4 · E}} i '(n i-1 -n i) (d) ΔCM i = 8h i 3 · h ci · E i' (n i-1 -n i) (e) provided that whose ASP _i , ΔCM _i are the fourth-order coefficients E of the aspheric surface, respectively.
spherical aberration caused by the _i ', 3-order aberration coefficient of coma, h _i is the paraxial marginal ray height in the aspheric, h _ci is the paraxial chief ray height at the aspherical. [0021] In formula (d), (e), different orders of h _i and h _ci the type of aberration, difference occurs in the effect of the aberrations by the placement of the order aspheric. Paraxial marginal ray is located numerals always positive for h _i because always on the same side with respect to the optical system the optical axis, whereas the paraxial chief ray, in order to cross the optical axis at the center of the diaphragm, h _ci Has a different sign before and after the stop, and is negative before the stop and positive after the stop.
ΔSP _i , ΔC calculated using the signs of h _i and h _ci
The sign of M _i is intact aberration generated in aspheric code. To positively the whose ASP _i arranged an aspherical surface in front of the group before the throttle should be _{E i '(n i-1} -n i)> 0. At that time, ΔCM _i <0. Therefore, when trying to correct spherical aberration, the residual coma aberration is worsened by the action of the aspherical surface, and the effect of using the aspherical surface is not very favorable. However, coma generated by an aspheric surface
If the correction is performed by a spherical system and the large spherical aberration generated at that time is corrected sufficiently well by an aspherical surface, the aberration of the lens system can be improved. If E _i ′ (n _i−1 −n _i )> 0 using an aspherical surface in the rear group behind the stop, Δ
SP _i > 0 and ΔCM _i > 0, and it is possible to cancel out any residual aberration when the aspherical surface is not used. Therefore, it is desirable to provide an aspheric surface in the rear group so as to satisfy the condition (4). The position of the aspherical surface in the rear group is set at the relatively high surface of the marginal ray height and the amount of aberration generation in order to efficiently correct spherical aberration and coma which have a large effect when the NA is increased. A strong surface with a large positive power is suitable. Therefore, the image-side surface of the convergent lens, which is the second lens element in the rear group convergent lens group behind the stop, is most desirable. The condition (4) defines the sign of the aspherical coefficient, but is substituted by using the displacement of the aspherical surface from the reference spherical surface whose radius is the paraxial radius of curvature r 'of the aspherical surface. Is also good. Since the value obtained by removing the first term on the right side of Expression (b) is the aspherical displacement amount Δx (y), this Δx (y) is defined by the following Expression (f). Δx (y) = E _i 'y ⁴ + F _i ' y ⁶ + G _i 'y ⁸ +... (F) In the above equation (f), since all the orders of y are even numbers, the sign of the aspheric coefficient and its sign The sign of the displacement of Δx (y) due to the influence is the same. Therefore, the following condition (5) can be defined as an expression instead of the condition (4). (5) Δx (y) {n i- 1 -n i}> 0 wherein [Delta] x (y) is a function of y is the distance from the optical axis, correction of spherical aberration is the main object of the present invention , The height of the marginal ray (the ray from the on-axis object point passing through the periphery of the aperture stop) on the aspheric surface is hm, and y =
It is preferable that the following condition (6) is satisfied at hm. (6) Δx (hm) { n i-1 -n i}> 0 Therefore, the above condition (6) may be used in place of the condition (4). Next, embodiments of the objective optical system for an endoscope according to the present invention will be described. Example 1 f = 1.000, F-number = 3.927, Image height = 1.0139, 2ω
= 140 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.2151 n ₁ = 1.51633
ν ₁ = 64.15 r ₂ = 2.0588 d ₂ = 0.1499 r ₃ = ∞ (aperture) d ₃ = 0.0396 r ₄ = -3.4926 d ₄ = 0.0978 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.7762 d ₅ = 0.4941 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = -0.6650 PS = 0.388 Example 2 f = 1.000, F number = 3.978, Image height = 0.9862, 2ω
= 130 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.2093 n ₁ = 1.7725
0 ν ₁ = 49.66 r ₂ = 2.0904 d ₂ = 0.0613 r ₃ = ∞ (aperture) d ₃ = 0.0386 r ₄ = -4.9556 d ₄ = 0.0952 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.8480 d ₅ = 0.4808 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = −0.6447 PS = 0.403 Example 3 f = 1.000, F-number = 4.456, Image height = 0.7422, 2ω
= 110 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.1856 n ₁ = 1.7291
6 ν ₁ = 54.68 r ₂ = 1.4756 d ₂ = 0.0341 r ₃ = ∞ (aperture) d ₃ = 0.0878 r ₄ = -5.6354 d ₄ = 0.0845 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.8362 d ₅ = 0.3983 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = -0.6145 PS = 0.37 Example 4 f = 1.000, F number = 4.437, Image height = 0.8778, 2ω
= 110 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.1863 n ₁ = 1.7291
6 ν ₁ = 54.68 r ₂ = 1.5402 d ₂ = 0.0343 r ₃ = ∞ (aperture) d ₃ = 0.0993 r ₄ = 9.7981 d ₄ = 0.0848 n ₂ = 1.84666
ν ₂ = 23.78 r ₅ = 0.8996 d ₅ = 0.4000 n ₃ = 1.7291
6 ν ₃ = 54.68 r ₆ = −0.5754 PS = 0.465 Example 5 f = 0.999, F-number = 4.025, image height = 0.9926, 2ω
= 140 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.2106 n ₁ = 1.5163
3 ν ₁ = 64.15 r ₂ = 2.3331 (aspherical surface) d ₂ = 0.1489 r ₃ = ∞ (aperture) d ₃ = 0.0333 r ₄ = ∞ d ₄ = 0.0782 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.7599 d ₅ = 0.4468 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = −0.7442 Aspherical surface coefficient P = 1.0000, B = 0, E = 0.21919 × 10, F = −0.15432
× 10 ² G = 0.10528 × 10 ³ E _i '= 2.1919, F _i ' = -15.432, G _i '= 105.28, H _i ' =
0, I _i ′ = 0 E _i ′ (n _i−1 −n _i ) = 1.1317 Δx (h _m ) {n _i−1 −n _i } = 2.416 × 10 ⁻⁴ , PS
Embodiment 6 f = 1.000, F-number = 3.878, Image height = 1.0188, 2ω
= 140 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.2162 n ₁ = 1.5163
3 ν ₁ = 64.15 r ₂ = 2.4934 (aspherical surface) d ₂ = 0.1299 r ₃ = ∞ (aperture) d ₃ = 0.0342 r ₄ = -3.0474 d ₄ = 0.1241 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.7800 d ₅ = 0.4969 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = -0.6650 Aspherical surface coefficient P = 1.0000, B = 0, E = 0.116161, F = -0.11324 × 10
² G = 0.46435 × 10 ² E _i '= 0.81161, F _i ' = -11.324, G _i '= 46.435, H _i '
_{= 0, I i '= 0} E i' (n i-1 -n i) = 0.4191 Δx (h m) {n i-1 -n i} = 9.094 × 10 -5, PS
Embodiment 7 f = 1.000, F-number = 4.680, Image height = 0.8476, 2ω
= 110 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.1799 n ₁ = 1.7291
6 ν ₁ = 54.68 r ₂ = 1.2535 d ₂ = 0.0608 r ₃ = ∞ (aperture) d ₃ = 0.1109 r ₄ = ∞ (aspherical surface) d ₄ = 0.1520 n ₂ = 1.8466
6 ν ₂ = 23.88 r ₅ = 0.8776 d ₅ = 0.4203 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = −0.6499 Aspherical surface coefficient P = 1.0000, B = 0, E = −0.79869, F = 0.84299 × 10 G = −0.43452 × 10 ² E _i ′ = −0.79869, F _i ′ = 8.4299 , G _i '= -43.452, H _i '
_{= 0, I i '= 0} E i' (n i-1 -n i) = 0.6762 Δx (h m) {n i-1 -n i} = 1.097 × 10 -4, PS
Embodiment 8 f = 1.000, F-number = 4.653, Image height = 0.8507, 2ω
= 110 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.1806 n ₁ = 1.7291
6 ν ₁ = 54.68 r ₂ = 1.1780 d ₂ = 0.0541 r ₃ = ∞ (aperture) d ₃ = 0.1132 r ₄ = -10.6961 (aspherical surface) d ₄ = 0.1143 n ₂ = 1.84
666 ν ₂ = 23.88 r ₅ = 0.8869 d ₅ = 0.4080 n ₃ = 1.8160
0 ν ₃ = 46.62 r ₆ = −0.6145 Aspherical surface coefficient P = 1.0000, B = 0, E = −0.53141, F = 0.69161 × 10 G = −0.45286 × 10 ² E _i ′ = −0.53141, F _i ′ = 6.9161 , G _i '= -45.286, H _i '
_{= 0, I i '= 0} E i' (n i-1 -n i) = 0.4499 Δx (h m) {n i-1 -n i} = 7.286 × 10 -5, PS
Embodiment 9 f = 1.000, F-number = 4.724, Image height = 0.8787, 2ω
= 110 ° Object distance = ∞ r ₁ = ∞ d ₁ = 0.1865 n ₁ = 1.7291
6 ν ₁ = 54.68 r ₂ = 0.8865 d ₂ = 0.1040 r ₃ = ∞ (aperture) d ₃ = 0.0331 r ₄ = 3.5253 d ₄ = 0.2660 n ₂ = 1.8051
8 ν ₂ = 25.43 r ₅ = 0.8933 d ₅ = 0.4237 n ₃ = 1.6652
4 ν ₃ = 55.10 r ₆ = -0.5260 (aspherical surface) Aspherical surface coefficient P = 1.0000, B = 0, E = 0.32363, F = -0.14738 × 10 G = 0.10795 × 10 ² E _i ′ = 0.32363, F _i ′ _{= -1.4738, G i '= 10.795} , H i'
= 0, I _i ′ = 0 E _i ′ (n _i−1 −n _i ) = 0.2153 Δx (h _m ) ｛n _i−1 −n _i ｝ = 8.981 × 10 ^-5 , PS
= 0.358 However _{r 1, r 2, ···,} r 6 the radius of curvature of each lens _{surface, d 1, d 2, ···} , d 5 is the thickness and lens distance of each lens, n _1, n ₂ , N ₃ is the refractive index of each lens, ν
₁ , ν ₂ and ν ₃ are Abbe numbers of each lens. PS is Petzval sum. In all of the first to third embodiments, the large astigmatism generated on the image side surface of the front group diverging lens is reduced while the chromatic aberration of magnification and the axial chromatic aberration are kept good. In this case, a correction is made on the object-side surface of the optical element (1), and an ultra-wide-angle objective optical system is obtained. Example 1 has an angle of view of 140 °,
Example 2 has an angle of view of 130 °, and Example 3 has an angle of view of 110 °. The fourth embodiment is an example in which the power of the cemented surface of the rear group converging lens is increased to suppress chromatic aberration of magnification and axial chromatic aberration to a sufficiently small value, thereby realizing a wide angle. In this embodiment, since the power of the cemented surface is increased to enhance the ability to correct chromatic aberration, large astigmatism also occurs at the cemented surface. Therefore, in order to further enhance the effect of correcting astigmatism, the object-side surface of the first optical element of the rear group converging lens is made convex. Example 4
Is 110 °. Embodiments 5 and 6 are examples in which the image-side surface of the front group diverging lens is made aspherical, thereby achieving a super wide angle of view of 140 °. By making this surface an aspherical surface, the spherical aberration is further satisfactorily corrected while maintaining the chromatic aberration of magnification and the axial chromatic aberration well. In the seventh and eighth embodiments, the object side surface of the first optical element of the rear group converging lens is made aspherical to provide a wide-angle objective optical system having an angle of view of 110 °. By making this surface an aspherical surface, spherical aberration and coma are corrected more favorably while chromatic aberration of magnification and axial chromatic aberration are kept good. In the ninth embodiment, a wide-angle objective optical system having an angle of view of 110 ° is realized by making the image-side surface of the second optical element of the rear group converging lens aspheric. By making this surface aspherical, spherical aberration and coma are also corrected well while maintaining good chromatic aberration of magnification and axial chromatic aberration. According to the present invention, it is possible to realize a wide-angle endoscope objective optical system in which chromatic aberration of magnification and axial chromatic aberration are favorably corrected with a small number of constituent lenses.

[Brief description of the drawings] FIG. 1 is a sectional view of a first embodiment of the present invention. FIG. 2 is a sectional view of a second embodiment of the present invention. FIG. 3 is a sectional view of a third embodiment of the present invention. FIG. 4 is a sectional view of a fourth embodiment of the present invention. FIG. 5 is a sectional view of a fifth embodiment of the present invention. FIG. 6 is a sectional view of a sixth embodiment of the present invention. FIG. 7 is a sectional view of a seventh embodiment of the present invention. FIG. 8 is a sectional view of Embodiment 8 of the present invention. FIG. 9 is a sectional view of Embodiment 9 of the present invention. FIG. 10 is an aberration curve diagram according to the first embodiment of the present invention. FIG. 11 is an aberration curve diagram according to the second embodiment of the present invention. FIG. 12 is an aberration curve diagram according to the third embodiment of the present invention. FIG. 13 is an aberration curve diagram according to the fourth embodiment of the present invention. FIG. 14 is an aberration curve diagram according to the fifth embodiment of the present invention. FIG. 15 is an aberration curve diagram according to the sixth embodiment of the present invention. FIG. 16 is an aberration curve diagram according to the seventh embodiment of the present invention. FIG. 17 is an aberration curve diagram of the eighth embodiment of the present invention. FIG. 18 is an aberration curve diagram of the ninth embodiment of the present invention. FIG. 19 is a cross-sectional view of a conventional endoscope objective optical system.

Claims (1)

(57) [Claim 1] In order from the object side, an image-side surface comprises a negative lens concave to the image side, a brightness stop, and a positive cemented lens, and the positive cemented lens is provided. The lens has a negative lens on the object side and a positive lens on the image side, the center of curvature of the cemented surface is located on the opposite side to the aperture stop, and the object side surface and image of the positive cemented lens are Side surface is concave on the object side and
The power of the object-side surface of the negative lens of the positive cemented lens
Than the image side surface of the positive cemented lens of the positive lens
An endoscope objective which has a strong power and in which the Abbe number ν _{2d of the} negative lens of the positive cemented lens and the Abbe number ν _{3d of the} positive lens of the positive cemented lens satisfy the following relationship (1): Optical system. (1) ν _2d <ν _3d