JPH10197806A - Objective lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-resolution and low-intrusive objective lens for an endoscope which is short in lens-length and has its chromatic aberration of magnification and image plane curvature satisfactorily corrected by constituting the objective lens of a 1st group having negative power, a diaphragm and a 2nd group having positive power and including a specified diffraction optical element in the 2nd group. SOLUTION: In order from an object side, the objective lens is constituted of three pieces; the 1st group of the negative refractive lens, the brightness diaphragm and the 2nd group constituted of the 1st lens such as a positive refractive lens and a 2nd lens of a diffraction optical element (diffraction lens). Besides, the objective lens is provided with a scratch preventing plate P and a CCD cover glass. The diffraction face is arranged near the diaphragm so that light beams may be nearly horizontally made incident on the face and also almost the luminous flux may be obtained. The diffraction lens satisfies 3<=fd /f<=100, where (f) denotes the focal distance of the whole objective lens system and (fd ) denotes the focal distance of the diffraction lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an endoscope which is inserted into a human body for diagnosis and treatment.

[0002]

2. Description of the Related Art In general, an objective lens of an endoscope is a retrofocus type lens system which can provide a wide angle of view and a long back focus. In addition, it is common to arrange an end face of a CCD or an image fiber on an image plane,
In response to the demand for higher resolution, their sampling is moving toward finer resolution. Therefore, a high-performance lens system is also required for the objective lens of the endoscope.

In a conventional refraction system lens, a high refractive index glass material is used for a positive lens and a low refractive index glass material is used for a negative lens in order to reduce the Petzval sum and correct the field curvature. Since a glass material having a low ratio has a relatively small Abbe number and a large chromatic dispersion, lateral chromatic aberration remains in the rear group. Further, with respect to the chromatic aberration of magnification, it is difficult to correct the chromatic aberration of magnification because the chromatic aberration of magnification occurs on the negative side in both the front group and the rear group.

To correct this chromatic aberration of magnification, a cemented lens may be used in the rear group, but there is a disadvantage that the number of lenses increases and the cost increases.

Further, from the viewpoint of low invasiveness, an endoscope having a short hard portion and good operability is desired, and an objective lens having a small number of lenses and a short overall length is desired.

[0006] In order to solve the above problem, it is conceivable to use a diffractive lens.

In the case of a normal refractive lens, when the radii of curvature of both surfaces of the thin lens are R ₁ and R ₂ , the refractive index of the glass material is n, and the focal length is f, the following equation (a) is established. 1 / f = (n−1) (1 / R ₁ + 1 / R ₂ ) (a)

When both sides of the above equation (a) are differentiated by the set wavelength λ, the following equation (b) is obtained, and further the equation (c) is derived. df / dλ = −f (dn / dλ) / (n−1) (b) Δf = fΔn / (n−1) (c)

On the other hand, with respect to a diffractive optical element (diffractive lens), the following equation (d) holds. λf = C (constant) (d)

The following equation is derived by differentiating both sides of the equation (d) with λ. df / dλ = −C / λ ² = −f / λ

From this, the following equation (e) is derived. Δf = −f (Δλ / λ) (e)

From the equations (c) and (e), the Abbe number ν _d of the diffractive optical element is expressed by the following equation (f). ν _d = λ _d / (λ _F −λ _C ) = − 3.45 (f)

As shown in the above formula (f), the Abbe number ν _d of the diffractive optical element has an inverse dispersion property and an extraordinary dispersion property of −3.45, and a positive diffractive optical element having a moderate power to a normal refractive lens. , It is possible to obtain a large chromatic aberration correction action. Therefore, for example, by arranging a diffractive optical element in the rear group in a lens system composed of a negative front group and a positive rear group, in the case of a lens system consisting only of a refraction lens, The chromatic aberration of magnification of the group can be generated in the opposite direction, and the chromatic aberration of magnification can be favorably corrected in the front group and the rear group by canceling each other.

The diffractive optical element can have an aspherical efficiency by continuously changing the pitch of the groove, and it is effective to use this to reduce the field curvature. Also, since the function of the diffractive optical element does not depend on the glass material, the degree of freedom of the glass material to be used can be treated as a plate lens having no thickness, for example, forming the diffractive optical element on the surface of an infrared cut filter or the like. It is possible.

A plate lens having such a negative dispersion and having no thickness (a lens having a diffractive element provided on a parallel plane plate)
There is an ultra-high index method as a method of designing by replacing with an equivalent refractive lens. That is, by making the refractive index infinite, light incident on the equivalent lens is treated as if it were bent in a plane. Assuming that the refractive index is infinite, the ray tracing of the diffractive lens can be performed by ordinary refraction ray tracing. In practice, it is impossible to make the refractive index infinite, so that 1
It is desirable to set it to 000 or more.

Next, the pitch of the aspherical diffractive optical element represented by the following equation is calculated. ^{Z = ch 2 / [1+ {} 1-c 2 (K + 1) h 2} + Ah
⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ ] Here, Z is the sag value, h is the ray height, c is the curvature, K is the conic coefficient, and A, B, C, and D are the fourth, sixth, eighth, and tenth non-symbols. This is the spherical coefficient.

The above-described equation representing the aspherical surface may use a twelfth or higher order aspherical surface coefficient.

In a diffractive optical element (diffractive lens),
If the incident angle on the diffraction surface (lens) is θ and the exit angle is θ ′, the following relationship is established. sin θ−sin θ ′ = mλ / P where m is the order and P is the pitch of the diffraction surface.

On the other hand, in the Ultra High Index, the diffraction conditions are expressed as effective properties as follows. (N−1) dz / dh = (sin θ−sin θ ′)

From the above, the pitch P of the diffractive optical element (diffractive lens) can be expressed by the following equation (g). P = mλ / (n-1 ) (dz / dh) = {mλ / (n-1)} · [ch / {1-c 2 (K + 1) h 2} + 4Ah 3 + 6Bh 5 + 8Ch 7 + 10Dh 9] -1 (G)

In particular, when K = -1, the equation (g) becomes the following equation (h), and the pitch P can be easily obtained. P = {mλ / (n−1)} [ch + 4Ah ³ + 6Bh ⁵
+ 8Ch ⁷ + 10Dh ⁹ ] ^{−1 In} practice, the refractive index of the diffractive lens is 1001, 10001
It is desirable to take such a value. The ultra high index lens does not contribute to Petzval sum and does not affect the field curvature.

[0022]

SUMMARY OF THE INVENTION An endoscope objective lens is
From the viewpoints of operability, low invasiveness, cost reduction, and the like, it is required to reduce the number of lenses. As a means for reducing the number of lenses, it is conceivable to use a diffractive lens,
Since the diffractive lens has an extremely large negative dispersion, when the number of lenses is reduced by using the diffractive lens, the chromatic aberration of magnification becomes overcorrected. Therefore, it is desirable that the power of the diffractive lens used is about 1/10 that of a normal lens. However, if the power of the diffractive lens is reduced, it is difficult for a lens system having a small diameter such as an endoscope objective lens to secure the number of orbits in the on-axis and off-axis light fluxes. In particular, since the size of the central orbicular zone occupies a considerable part of the diameter of the light flux, most of the axial flux will pass through the central orbicular zone, and the effect of the blaze error due to the lens shape will be large. More easily.

On the other hand, when a certain amount of power is given to the diffractive lens in consideration of securing the number of ring zones and shortening the lens length,
Processing of the annular shape at the periphery of the diffractive lens becomes difficult, and the processing error of the annular shape leads to deterioration of diffraction efficiency, which causes deterioration of image contrast.

As described above, when a diffractive lens is used for a small-diameter lens having a small light beam diameter such as an endoscope objective lens, it is necessary to secure the number of orbicular zones in the light beam while maintaining the orbicular pitch of the diffractive lens. This is important, and it is necessary to arrange a diffraction lens having appropriate power and annular shape.

The present invention uses a diffractive lens having a power capable of effectively exhibiting a diffraction effect and a realistic shape in a retrofocus type optical system, so that the lens length is short and the chromatic aberration of magnification and the field curvature are sufficient. It is an object of the present invention to provide a high-resolution, minimally invasive objective lens for an endoscope that is well corrected.

[0026]

According to the present invention, there is provided an objective lens comprising:
The second group includes a first group having a negative power, a stop, and a second group having a positive power. The second group includes at least one diffractive optical element, and the diffractive optical element satisfies the following condition (1). It is characterized by satisfaction. (1) 3 ≦ f _d / f ≦ 100 where f _d is the focal length of the diffractive optical element, and f is the focal length of the entire objective lens system.

The objective lens according to the present invention employs a retrofocus type, which is the most common endoscope objective lens, and includes at least one lens satisfying the above condition (1) in the second lens group behind the stop. This uses two diffractive optical elements.

In the above condition (1), if the lower limit of 3 is exceeded, an excessively corrected state is obtained, which is not preferable. Also the upper limit of 100
When the value exceeds, the reverse correction capability of the diffractive optical element cannot be sufficiently exhibited. In the lens system of the present invention, it is necessary to give the diffractive optical element a certain level of power in consideration of securing the number of annular zones in the light beam of the diffractive optical element.
Is preferably set to 10. That is, it is more preferable that the following condition (1-1) is satisfied instead of the condition (1). (1-1) 3 ≦ f _d / f ≦ 10

In the objective lens of the present invention, in order to further reduce the number of lenses by using a diffractive optical element, the diffractive optical element has a certain amount of power in order to disperse power and suppress occurrence of aberration. It is desirable that the following condition (2) be satisfied. (2) f _d / f ₂ ≦ 15 where f ₂ is the focal length of the second lens unit.

If the upper limit of 15 to condition (2) is exceeded, the power of the diffractive optical element becomes too large, making it difficult to correct aberrations other than chromatic aberration of magnification.

Further, in the diffractive optical element, the pitch angle becomes smaller because the exit angle becomes larger toward the periphery thereof, but a certain pitch in the peripheral portion is required in view of the manufacturing limit. As a result, the number of zones in the on-axis light beam is limited, but a certain number of zones is required to secure the diffraction effect. Considering these points, the optimum minimum pitch P _min of the diffractive optical element used in the lens system of the present invention preferably satisfies the following condition (3). (3) 0.005 mm <P _min <0.015 mm If the lower limit of 0.005 mm of the condition (3) is exceeded, the diffraction efficiency is lowered due to processing errors, which is not preferable. When the value exceeds the upper limit of 0.015 mm of the condition (3), it is difficult to secure the number of annular zones for the light beam.

In the lens system of the present invention, in order for the diffractive optical element to satisfy the condition (2) and the condition (3), the following condition (4) must be satisfied. (4) 0.1 ≦ P _min / P _max ≦ 0.4 where P _max is the maximum pitch of the diffractive optical element. If the lower limit of 0.1 of the condition (4) is exceeded, it is difficult to secure the number of orbicular zones required for diffraction while suppressing the occurrence of aberration. 005m
It is difficult to secure Pmin of m or more.

In order to satisfy the above condition (4), it is necessary to use the aspherical effect of the diffractive lens.

In practice, the central annular zone P of the endoscope objective lens
It is desirable that _max and the aperture stop diameter d satisfy the following condition (5). (5) P _max /d≦0.5

In general, it is desirable that _{Pmax of the} endoscope objective lens satisfies the following relationship. 0.05 mm ≦ P _max ≦ 0.15 mm Here, the central orbicular zone pitch is the radius of the central orbicular zone, and the orbicular zone pitch is the distance from the front and rear orbicular zones. If the lower limit of 0.05 mm in the above equation is exceeded, the orbicular zone pitch at the periphery of the diffractive lens becomes fine, and the diffraction efficiency decreases due to processing errors. If the upper limit of 0.15 mm is exceeded, the number of annular zones at the center of the diffractive lens becomes insufficient.

[0036] Next described optimal value of the focal length f _d optimal diffractive optical element obtained from the P _max and annular number.

In order to reproduce the wavefront, the required number of ring zones is required to be at least about 5 in the effective light flux, and preferably at least 15 or more.

Here, when the axial light flux of the diffractive optical system using the m-th order diffracted light is considered, the following relationship is established. f _d ² + Y _M ² = {f _d + (N + 1) / 2λ × m} ² (a) where Y _M is the ray height of the axial ray on the diffraction surface, N is the number of orbicular zones in the light flux, and λ is The set wavelength, m, is the order.

Since f _d ≫λ, the above equation (a)
Can be approximated by the following equation. N = Y _M ² / mλf _d -1

Therefore, if N is at least 5 or more, f _d needs to satisfy the following condition (6). (6) f _d ≦ Y _M ² / 4mλ

Next, the magnitude of the off-axis pitch will be considered.

The zone pitch at the off-axis principal ray height Y _C of the diffractive optical element having the focal length f _d is represented by the following equation. P = ｛Y _C + 2mλ (f _d ² + Y _C ² ) ^1/2 ｝ ^1/2 -Y _C

[0043] When consideration also P _min a lowering of the diffraction efficiency due to the processing error of the diffractive optical element is given that 0.005mm is required, the focal length f _d of the diffractive optical element satisfies the following condition (7) It is desirable to do. (7) f _d ≧ 0.005Y _C / mλ

From the conditions (6) and (7), the focal length f _d of the diffractive optical element preferably satisfies the following condition (8). (8) 0.005Y _C / mλ ≦ f _d ≦ Y _M ² / 4mλ

Further, considering the optimization at a wavelength of 500 to 600 nm using the first-order diffracted light to secure the number of ring zones on the axis, it is desirable that f _d satisfies Expression (9). (9) 8Y _C ≦ f _d ≦ 500Y _M ²

The diffractive optical element is desirably arranged near the aperture stop in consideration of securing the incident angle and the light flux of the light beam on the diffractive surface. Here, the distance from the aperture stop to the diffraction plane is D ₁ , and the distance from the aperture stop to the image plane is D ₂
Then, Y _C can be easily expressed by the following equation. Y _C = D ₁ × IH / D ₂

From the above equation, the condition (9) is obtained by the following condition (10).
Can be expressed as (10) 8D ₁ × IH / D ₂ ≦ f _d ≦ 150d ²

In the case of an endoscope, a solid-state image sensor or an image fiber is arranged on the image plane. When obliquely incident light is incident on them, color shading or transmission loss occurs. Therefore, the endoscope objective lens needs to maintain telecentricity to some extent. However, it is not actually a complete telecentric optical system, and C has a tilt of 0 to 20 °.
Light is incident on the CD surface. In this case, Y _C is equal to or greater than IH / 2, and the condition (11) described below may be satisfied. (11) 4IH ≦ f _d ≦ 150d ²

In the condition (8), in the case of a retrofocus type lens system having a refractive lens in the front group (first group) and at least one diffractive optical element in the rear group (second group) before the stop, Rear group (2nd
The front focus position of the group is located near the stop. When a diffractive optical element is used for the first lens of the rear group (second group), the following relationship is satisfied. h _C ≧ 0.7 × IH × D ₁ / f ₂ where h _C is an axial marginal ray and f ₂ is a rear group (second
Group).

When a diffractive optical element is used for the second lens of the rear group (second group), the following relationship is satisfied. H _C ≧ 0.5IH

Here, when a diffractive optical element is arranged on the first lens of the rear group (second group), the axial marginal ray height h _M
Is expressed as follows when the object distance is set to infinity for simplicity. h _M = d (−f ₁ + D ₃ + D ₁ ) / (− f ₁ + D ₃ ) where D ₃ is the distance from the front group (first group) to the aperture stop.

When a diffractive optical element is used for the second lens of the rear group (second group), the ray height H _M is as follows. H _M = D ₄ / 2F _{NO where} F _NO is the F number of the objective lens, and D ₄ is the distance from the image plane to the diffractive optical element.

Therefore, from the condition (11), the rear group (second
When a diffractive optical element is used for the first lens of the group, it is desirable that the focal length f _d of the diffractive optical element satisfies the following condition (12). (12) 0.003 IH × D ₁ / (f ₂ · m · λ) ≦ f _{d
^{≦ d 2 (-f 1 + D}} 3 + D 1) 2 / 4mλ (-f 1 +
D ₃₎ ²

Similarly, when a diffractive optical element is used for the second lens of the rear group (second group), the focal length f _d of the diffractive optical element is used.
Is as follows. (13) 0.002 × IH / mλ ≦ f d ≦ D 4 2 / 16m
λF _NO ²

In the optical system of the present invention, considering the reduction of flare due to unnecessary orders, the set wavelength is 500 to 600.
Near nm is desirable. It is desirable to use the primary light as the order used for diffraction in order to secure an annular zone in the axial light beam.

Therefore, when a diffractive optical element is used for the first lens of the rear group (second group), it is desirable that the focal length f _d of the diffractive optical element satisfies the following condition (14). (14) 6 × IH × D ₁ / f ₂ ≦ f _d ≦ 500d ²
(−f ₁ + D ₃ + D ₁ ) ² / (− f ₁ + D ₃ ) ^{2 When a} diffractive optical element is used for the second lens of the rear group (second group), the focal length f _d of the diffractive optical element is The following condition (15)
It is desirable to satisfy (15) 4 × IH ≦ f d ≦ 125D 4 2 / F NO 2

Actually, when the above condition is satisfied, the axial chromatic aberration near the optical axis is reduced in the -z direction and the chromatic aberration of magnification by the strong reverse color correction capability of the diffractive optical element when the optical axis direction is the z direction. Occurs in the -y direction. In particular, when trying to shorten the lens length of the objective lens, the angle of incidence on the diffractive optical element becomes large, and the diffractive optical element is over-corrected.

For the above reasons, it is effective to use the aspherical effect of the diffractive optical element to suppress the incident angle of the incident light beam of the lens system off-axis, thereby suppressing excessive correction by the diffractive optical element. That is, with respect to off-axis rays, the off-axis and on-axis aberrations are corrected well by using the aspherical effect of the diffractive optical element to bring both the chromatic aberration of magnification and the on-axis chromatic aberration in the plus direction. Can be. For this purpose, particularly when the distance from the center of the diffractive optical element to the minimum pitch end is h, the following condition (16) can be satisfied. (16) 0.5 ≦ h / f ≦ 0.85 If the above condition (16) is satisfied, chromatic aberration can be favorably corrected both on-axis and off-axis. Exceeding the lower limit of 0.5 to condition (16) will result in an overcorrected state, which is not preferable. On the other hand, when the value exceeds the upper limit of 0.85, the reverse correction capability of the diffraction lens cannot be sufficiently exhibited.

In order to further reduce the number of lenses in the lens system, it is necessary to generate chromatic aberration of magnification with a refractive lens other than the diffractive optical element. Therefore, by reducing the chromatic dispersion of the concave lens located on the object side of the aperture stop and using a lens with large chromatic dispersion for the convex lens on the image side of the aperture stop, chromatic aberration can be corrected well. That is, the Abbe number of the concave lens on the object side of the aperture stop is ν _d1 , and the Abbe number of the convex lens on the image side of the aperture stop is ν _d2.
It is desirable that the following condition be satisfied. ν _d1 > ν _d2

In particular, the Abbe number ν _d of the concave lens of the first lens of the lens system (the lens closest to the object in the lens system) is 40
It is desirable that this is the case. That is, it is desirable to satisfy the following conditions. ν _d > 40

[0061]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the objective lens for an endoscope according to the present invention will be described based on respective examples. Example 1 r ₁ = ∞ d ₁ = 0.3100 n ₁ = 1.51633 ν ₁ = 64.15 r ₂ = 0.9130 d ₂ = 0.5100 r ₃ = ∞ (aperture) d ₃ = 0.2000 r ₄ = ∞ d ₄ = 0.7100 n ₂ = 1.84666 _{ν 2 = 23.78 r 5 = -1.0180} d 5 = 0.0300 r 6 = 5300.0000 d 6 = 0.0010 n 3 = 1.0 × 10 3 ν 3 = -3.45 (DOE) r 7 = ∞ d 7 = 0.5000 n 4 = 1.45846 ν 4 = 67.70 r ₈ = ∞ d ₈ = 0.4100 r ₉ = ∞ d ₉ = 0.5000 n ₅ = 1.51633 ν ₅ = 64.15 r ₁₀ = ∞ d ₁₀ = 0.7500 n ₆ = 1.53172 ν ₆ = 48.91 r ₁₁ = ∞ aspheric coefficient K = -1.0000, A = -6.9550 × 10 ^-5 , B = 6.5146 × 10 ^-6

Example 2 r ₁ = ∞ d ₁ = 0.3100 n ₁ = 1.51633 ν ₁ = 64.15 r ₂ = 1.3279 d ₂ = 0.2726 r ₃ = ∞ (aperture) d ₃ = 0.0060 r ₄ = ∞ d ₄ = 0.5863 n ₂ = 1.45846 ν ₂ = 67.70 r ₅ = ∞ d ₅ = 0.0010 n ₃ = 1.0 × 10 ³ ν ₃ = -3.45 (DOE) r ₆ = 3805.8900 d ₆ = 0.0300 r ₇ = ∞ d ₇ = 0.6000 n ₄ = 1.88300 ν ₄ = 40.78 r ₈ = -1.0072 d ₈ = 0.4921 r ₉ = ∞ d ₉ = 0.5000 n ₅ = 1.51633 ν ₅ = 64.15 r ₁₀ = ∞ d ₁₀ = 0.7500 n ₆ = 1.53172 ν ₆ = 48.91 r ₁₁ = ∞ non Spherical coefficient K = 0, A = 3.2406 × 10 ^-4 , B = -2.1849 × 10 ^-4

As described above, the objective lens for an endoscope of the type according to the present invention uses a diffractive optical element in order to shorten the overall length of the lens system from the viewpoint of operability.

In the first embodiment, in order to shorten the total length of the lens system, in order from the object side, a first group of negative refractive lenses, a stop, a first positive refractive lens, and a diffractive optical element ( (Diffraction lens) and a second lens unit including a second lens. P and C are a flat plate for scratch prevention and a CCD cover glass, respectively. The diffractive surface is arranged near a stop where light rays are incident substantially perpendicular to the surface and a large light flux can be obtained. Further, by disposing a negative lens and a positive lens before and after the stop, aberrations other than chromatic aberration of magnification are corrected.

The positive spherical aberration generated on the image-side surface of the negative refractive lens of the front group (first group) is changed to the negative spherical aberration generated by the positive refractive lens which is the first lens of the rear group (second group). It cancels out by spherical aberration.

The Petzval sum of the diffractive lens is almost 0, and the field curvature is corrected by the refracting surface of the front group (first group) and the refracting surface of the rear group. Off-axis aberrations are corrected by a diffraction lens.

The longitudinal chromatic aberration is canceled by the front group (first group) and the rear group (second group), but the power of the diffractive lens is increased to some extent in order to secure the number of ring zones. Therefore, on paraxial, the axial chromatic aberration occurs in the -z direction, and the chromatic aberration of magnification occurs in the -y direction, that is, in the direction opposite to the chromatic aberration of the ordinary refractive lens. In this embodiment, the angle of incidence on the diffractive lens becomes tight in order to shorten the overall length of the lens system, and the off-axis aberration is overcorrected by the diffractive lens. In this embodiment, the aberration in the overcorrected state is made to have an aspherical action on the diffractive lens, and the power of the diffractive lens in the periphery is weakened by using the action of the aspherical face to reduce the incident angle of the light beam. The chromatic aberration in the z-direction off the axis is relaxed in the positive direction. By the action of the aspherical surface, the chromatic aberration at the overcorrected magnification is also corrected in a favorable manner by taking it in the positive direction. By providing the diffractive lens with an aspherical surface function, good off-axis performance is maintained.

Further, a glass material having a small dispersion is used for the glass material of the refractive lens of the front group (first group), and a material having a large dispersion is used for the glass material of the refractive lens which is the first lens of the rear group (second group). As a result, chromatic aberration of magnification occurring in portions other than the diffractive lens is generated in the positive direction, and the excessive correction capability of the diffractive lens is canceled. For this purpose, it is particularly desirable that the Abbe number of the refracting lens (first lens) in the rear group be 40 or less. That it is desirable to satisfy the following condition when the Abbe number of the first lens and [nu _d. ν _d > 40

Embodiment 2 is also an embodiment of an endoscope objective lens, and includes, in order from the object side, a front group (first group) of a negative refractive lens.
, An aperture stop, and a rear group (second group) including a first lens of a positive diffractive lens and a second lens of a positive refraction lens. Flat plate P, CC
D cover glass C is arranged. The diffractive surface is arranged near a stop where light rays are incident substantially perpendicular to the surface and a large light flux can be obtained. This embodiment is an objective lens which emphasizes shortening the overall length of the lens system, and the power of the diffractive lens is made stronger with respect to the focal length of the entire system.

The objective lens according to the present invention achieves the object of the present invention in addition to the lens systems having the structures described in the claims and the lens systems described in the following claims.

(1) Claims 1, 2, and 3 of the claims
Alternatively, in the lens system described in 4, the diffractive optical element satisfies the following condition (4). (4) 0.1 ≦ Pmin / Pmax ≦ 0.4

(2) Claims 1, 2, and 3 of the claims
Or 4 or the lens system described in the above item (1), wherein the objective lens satisfies the following condition (5). (5) Pmax / d ≦ 0.5

(3) Claims 1, 2, and 3 of the claims
Or 4 or the lens system described in (1) or (2) above, wherein the diffractive optical system satisfies the following condition. 0.05mm ≦ Pmax ≦ 0.15mm

(4) Claims 1, 2, and 3 of the claims
Or 4 or the lens system described in (1), (2) or (3) above, wherein the diffractive optical system satisfies the following condition (9). (9) 8Y _c ≦ f _d ≦ 500Y _M ²

(5) Claims 1, 2, and 3 of the claims
Or 4 or the lens system described in (1), (2) or (3) above, wherein the diffractive optical system satisfies the following condition (10):
An objective lens characterized by satisfying the following. (10) 8D ₁ × IH / D ₂ ≦ f _d ≦ 150d ²

(6) Claims 1, 2, and 3 of the claims
Or 4 or the lens system described in (1), (2) or (3) above, wherein the diffractive optical system satisfies the following condition (11):
An objective lens characterized by satisfying the following. (11) 4IH ≦ f _d ≦ 150d ²

(7) Claims 1, 2, and 3 of the claims
Or 4 or the above (1), (2), (3),
In the lens system described in (4), (5) or (6), the first group is composed of one refracting lens and the second group is composed of one lens.
An objective lens comprising: one diffraction lens and one refraction lens.

(8) In the lens system described in the above (7), the second group includes one diffractive lens and one refractive lens in order from the object side, and the diffractive lens satisfies the following condition (14). An objective lens characterized by satisfying (1). (14) 6IH × D ₁ / f ₂ ≦ f _d ≦ 500d ² (−f
_{1 + D 3 + D 1)} 2 / (- f 1 + D 3) 2

(9) In the lens system described in the above (7), the first group includes one refractive lens, and the second group includes one refractive lens and one lens in order from the object side. An objective lens comprising a number of diffraction lenses and satisfying the following condition (15). _{(15) 4IH ≦ f d ≦} 125D 4 2 / F NO 2

(10) The above (7), (8) or (9)
Wherein the first group comprises one lens, the second group comprises one refractive lens and one diffractive optical element, and the Abbe of the second group of refractive lenses. An objective lens, wherein the number satisfies the following condition. ν _d > 40

(11) In the lens system described in the above item (7), (8), (9) or (10), the Abbe number of the refracting lens of the first group and the refraction of the second group. An objective lens, wherein the Abbe number of the lens satisfies the following condition. ν _d1 > ν _d2

(12) Claims 1 and 2 of the claims
3 or 4 or the above (1), (2), (3),
An objective lens according to (4), (5), (6), (7), (8) or (9), which satisfies the following expression. 0.5 ≦ h / f ≦ 0.85

(13) An objective lens according to the item (12), wherein the diffraction lens comprises an infrared cut filter.

[0084]

According to the present invention, it is possible to obtain a low-invasive, high-performance endoscope objective lens with reduced chromatic aberration, short overall length, and excellent operability.

[Brief description of the drawings]

FIG. 1 is a sectional view of Embodiment 1 of an objective lens according to the present invention.

FIG. 2 is a cross-sectional view of Embodiment 2 of the objective lens of the present invention.

Claims (4)

[Claims] A first power source having a negative power in order from an object side;
A group, a diaphragm, and a second group having a positive power,
An objective lens, wherein the second group includes at least one diffractive optical element, and the diffractive optical element satisfies the following condition (1). (1) 3 ≦ f _d / f ≦ 100 f is the focal length of the entire objective lens system, and f _d is the focal length of the diffractive optical element. 2. The diffractive optical element satisfies the following condition (1-1) instead of the condition (1).
Objective lens. (1-1) 3 ≦ f _d / f ≦ 10 3. The objective lens according to claim 1, wherein said diffractive optical element satisfies the following condition (2). (2) f _d / f ₂ ≦ 15 where f ₂ is the focal length of the second lens unit. 4. The objective lens according to claim 1, wherein said diffractive optical element satisfies the following condition (3). (3) 0.005 mm ≦ Pmin ≦ 0.015 mm where Pmin is the minimum annular zone pitch of the diffractive optical element.