阅读说明：本技术 内窥镜物镜光学系统 (Endoscope objective optical system ) 是由 小山雄高 森田灯 于 2018-08-31 设计创作，主要内容包括：提供一种对于支气管、鼻腔、咽喉、泌尿器官、子宫等脏器的观察而言较佳的内窥镜物镜光学系统,该内窥镜物镜光学系统为细径,光学系统全长短,视角广,并且良好地校正色像差、像面弯曲等各像差。内窥镜物镜光学系统的特征在于,包括从物体侧起依次配置的具有负折射力的前(GF)组、亮度光圈(S)以及具有正折射力的后组(GR),前组(GF)具有负透镜(L1),负透镜(L1)为负的单透镜,后组(GR)具有从物体侧起配置的正透镜(L2)以及一个以上的接合透镜(CL1),正透镜(L2)为正的单透镜,该内窥镜物镜光学系统满足下面的条件式(1)、(2)、(3)。1.95<ndCLn (1) 35<ΔνdCL (2) 1.1<fL2/ft<1.6 (3)。(Provided is an endoscope objective optical system which is small in diameter, short and long in total, has a wide angle of view, and can satisfactorily correct various aberrations such as chromatic aberration and field curvature, and which is suitable for observation of organs such as bronchi, nasal cavities, throats, urinary organs, and uterus. An endoscope objective optical system is characterized by comprising a front Group (GF) having negative refractive power, a bright diaphragm (S) and a rear Group (GR) having positive refractive power, which are arranged in order from the object side, wherein the front Group (GF) has a negative lens (L1), the negative lens (L1) is a negative single lens, the rear Group (GR) has a positive lens (L2) and one or more cemented lenses (CL1) arranged from the object side, and the positive lens (L2) is a positive single lens, and the endoscope objective optical system satisfies the following conditional expressions (1), (2) and (3). 1.95< ndCLn (1) 35< Δ ν dCL (2) 1.1< fL2/ft <1.6 (3).)

1. An endoscope objective optical system characterized in that,

consists of a front group with negative refractive power, a diaphragm and a rear group with positive refractive power which are arranged in sequence from the object side,

the front group has a negative lens and the front group has a negative lens,

the negative lens is a negative single lens,

the rear group has a positive lens and one or more cemented lenses arranged from the object side,

the positive lens is a positive single lens,

the endoscope objective optical system satisfies the following conditional expressions (1), (2) and (3),

1.95<ndCLn (1)

35<ΔνdCL (2)

1.1<fL2/ft<1.6 (3)

in this case, the amount of the solvent to be used,

ndCLn is the refractive index for the d-line of the negative lens in the cemented lens of the rear group,

Δ ν dCL is the difference between the abbe number of the positive lens and the abbe number of the negative lens in the cemented lens of the rear group,

ft is the focal length of the entire system of the endoscope objective optical system,

fL2 is the focal length of the positive singlet lens in the back group.

2. The endoscope objective optical system according to claim 1,

the positive single lens of the rear group satisfies the following conditional expression (4),

1.75＜ndL2 (4)

in this case, the amount of the solvent to be used,

ndL2 is the refractive index for the d-line of the positive singlet lens of the rear group.

3. The endoscope objective optical system according to claim 1,

the positive single lens in the rear group satisfies the following conditional expressions (5), (6),

-7<R1/ft<-3.6 (5)

0.8<|R2/Ls2|<1.2 (6)

in this case, the amount of the solvent to be used,

r1 is the radius of curvature of the object side of the positive singlet lens in the rear group,

ft is the focal length of the entire system of the endoscope objective optical system,

r2 is the radius of curvature of the image side surface of the positive singlet lens in the rear group,

ls2 is the distance along the optical axis from the aperture stop to the image side surface of the positive single lens.

4. The endoscope objective optical system according to claim 1,

the front group has parallel flat plates,

the endoscope objective optical system satisfies the following conditional expression (7),

1.4＜Lgr/Lgf＜2 (7)

in this case, the amount of the solvent to be used,

lgf is a distance along an optical axis from an object side surface of the negative single lens of the front group to the brightness aperture,

lgr is a distance along the optical axis from the aperture stop to a surface closest to the image side of the rear group lens.

5. The endoscope objective optical system according to claim 1,

the front group comprises only the negative singlet lens,

the endoscope objective optical system satisfies the following conditional expression (8),

1.g＜Lgr/ft＜2.6 (8)

in this case, the amount of the solvent to be used,

lgr is a distance along the optical axis from the aperture stop to a surface closest to the image side of the rear group of lenses,

ft is the focal length of the whole system of the endoscope objective optical system.

6. The endoscope objective optical system according to claim 4 or 5,

the negative einzel lens of the front group is a plano-concave lens with a plane facing the object side,

the endoscope objective optical system satisfies the following conditional expression (9),

0.5＜D1/R1L1＜0.9 (9)

in this case, the amount of the solvent to be used,

R1L1 is the radius of curvature of the image-side surface of the negative singlet lens of the front group,

d1 is the radius of a concave portion, which is a spherical cutout portion of the image-side surface of the negative single lens of the front group, in the direction perpendicular to the optical axis.

7. The endoscope objective optical system according to claim 4 or 5,

the cemented lens is composed of a positive lens and a negative lens arranged in this order from the object side,

the endoscope objective optical system satisfies the following conditional expression (10),

-2＜R1CL/ft＜-1.1 (10)

in this case, the amount of the solvent to be used,

r1CL is the radius of curvature of the cemented surface of the cemented lens of the rear group,

ft is the focal length of the whole system of the endoscope objective optical system.

Technical Field

The present invention relates to an objective optical system of an endoscope, and mainly relates to an objective optical system of a medical endoscope.

Background

In recent years, with the increase in the number of pixels of an image pickup device, the size of one pixel has been reduced. Accordingly, reduction of various aberrations occurring in the objective optical system of the endoscope is desired. In particular, an objective optical system in which magnification chromatic aberration, axial chromatic aberration, and field curvature are corrected well is required. On the other hand, endoscopes are used for observation in narrow body cavities. Thus, the optical system needs to be small in size. The size of the optical system is determined by the diameter of the lens and the total length of the lens. Further, there is a demand for further miniaturization of an objective optical system of an endoscope for observing organs such as digestive organs, bronchi, nasal cavities, throats, urinary organs, and uteruses.

For example, patent documents 1 to 5 below disclose small-sized endoscope objective optical systems with reduced aberrations.

Disclosure of Invention

Problems to be solved by the invention

In all the embodiments of the endoscope objective optical system disclosed in patent document 1, the chromatic aberration of magnification is large. Therefore, it cannot be said that the magnification chromatic aberration is sufficiently suppressed for the current image pickup element of high pixel.

In examples 5 and 6 of the endoscope objective optical system disclosed in patent document 2, the field curvature of the endoscope objective optical system is large. Therefore, the focal position of the central portion of the image is deviated from the focal position of the peripheral portion. As a result, there is a possibility that an obstacle may occur in observation with an endoscope. In examples other than examples 5 and 6, the chromatic aberration of magnification was large. Therefore, the magnification chromatic aberration is not sufficiently suppressed for the current image pickup element of high pixel.

In all the embodiments of the endoscope objective optical system disclosed in patent document 3, the diameter of the rear group lens is larger than that of the front group lens, and it is difficult to reduce the diameter. In addition, the chromatic aberration of magnification is large. The magnification chromatic aberration is not sufficiently suppressed for the current image pickup element of high pixel.

In all the embodiments of the endoscope objective optical system disclosed in patent document 4, the distance from the object-side first surface of the endoscope objective optical system to the image plane exceeds 8 times the focal length. Therefore, it is difficult to miniaturize the endoscope.

In all the embodiments of the endoscope objective optical system disclosed in patent document 5, the distance from the object-side first surface of the endoscope objective optical system to the image plane exceeds 10 times the focal length. Therefore, it is difficult to miniaturize the endoscope.

An endoscope objective optical system for observing bronchi, nasal cavities, throats, urinary organs, uterus, and the like is required to satisfy the following requirements (1), (2), (3), and (4) at the same time.

(1) The optical system is of a small diameter;

(2) the optical system is full-length;

(3) the visual angle is wide;

(4) various aberrations such as chromatic aberration and field curvature are corrected well.

The endoscope objective optical systems disclosed in patent documents 1 to 5 described above cannot satisfy the above requirements (1) to (5) at the same time.

The present invention has been made in view of the above problems, and an object thereof is to provide an endoscope objective optical system having a small diameter, a short optical system length, and a wide angle of view, and in which various aberrations such as chromatic aberration and field curvature are corrected well.

Means for solving the problems

In order to solve the above-mentioned problems and achieve the object, an endoscope objective optical system of the present invention includes a front group having negative refractive power, a diaphragm, and a rear group having positive refractive power, which are arranged in order from an object side,

the front group has a negative lens and the rear group has a negative lens,

the negative lens is a negative single lens,

the rear group has a positive lens and one or more cemented lenses arranged from the object side,

the positive lens is a positive single lens,

the endoscope objective optical system satisfies the following conditional expressions (1), (2) and (3),

1.95＜ndCLn (1)

35＜ΔvdCL (2)

1.1＜fL2/ft＜1.6 (3)

in this case, the amount of the solvent to be used,

ndCLn is the refractive index for the d-line (wavelength 587.6nm) of the negative lens in the rear set of cemented lenses,

Δ ν dCL is the difference between the abbe number of the positive lens and the abbe number of the negative lens in the cemented lens of the rear group,

ft is the focal length of the entire system of the endoscope objective optical system,

fL2 is the focal length of the positive singlet lens in the back group.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide an endoscope objective optical system which has a small diameter, a short optical system, a wide angle of view, and which can correct chromatic aberration, field curvature, and the like well.

Drawings

Fig. 1 (a) is a lens cross-sectional view of the endoscope objective optical system according to the first embodiment. (b) Is a lens cross-sectional view of the endoscope objective optical system of the second embodiment.

Fig. 2 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 1. (b) The terms (c), (d) and (e) are aberration diagrams showing the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT) and magnification chromatic aberration (CC) in the present example, respectively.

Fig. 3 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 2. (b) The terms (c), (d) and (e) are aberration diagrams showing the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT) and magnification chromatic aberration (CC) in the present example, respectively.

Fig. 4 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 3. (b) The terms (c), (d) and (e) are aberration diagrams showing the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT) and magnification chromatic aberration (CC) in the present example, respectively.

Fig. 5 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 4. (b) The terms (c), (d) and (e) are aberration diagrams showing the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT) and magnification chromatic aberration (CC) in the present example, respectively.

Fig. 6 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 5. (b) The aberration diagrams of the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT), and magnification chromatic aberration (CC) in the present example are shown in (c), (d), and (e), respectively.

Fig. 7 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 6. (b) The terms (c), (d) and (e) are aberration diagrams showing the Spherical Aberration (SA), Astigmatism (AS), distortion aberration (DT) and magnification chromatic aberration (CC) in the present example, respectively.

Detailed Description

Next, an endoscope objective optical system according to an embodiment will be described in detail with reference to the drawings.

The reason and operation of the endoscope objective optical system according to the first embodiment will be described below with reference to the drawings. The present invention is not limited to the endoscope objective optical system according to the following embodiments.

A basic configuration of the endoscope objective optical system of the first embodiment will be described. An endoscope objective optical system of a basic configuration includes a front group having negative refractive power, a diaphragm, and a rear group having positive refractive power, which are arranged in order from an object side. The front group has a negative singlet lens of the singlet lens. The rear group includes a positive single lens of a single lens arranged from the object side and one or more cemented lenses.

The endoscope objective optical system of the present embodiment requires a back focus having a length to obtain a wide angle of view and to be easily adjusted through an assembly process. Therefore, a so-called reverse-distance type structure is adopted for the basic structure.

Fig. 1 (a) is a lens cross-sectional view showing an example of the basic configuration of the endoscope objective optical system according to the present embodiment. In the basic configuration, an optical system is configured by a front group GF having negative refractive power, a bright diaphragm S, and a rear group GR having positive refractive power, which are arranged in this order from the object side.

In the basic configuration, the rear group GR has a cemented lens CL1 including a positive lens L3 and a negative lens L4. Thereby correcting chromatic aberration.

The rear group GR has a positive single lens L2 (first single positive lens L2). In order to secure a wide viewing angle, it is necessary to make the front group GF have a large negative refractive power. In order to form positive refractive power by the entire system of the optical system, it is necessary to arrange large positive refractive power in the rear group GR. A large positive refractive power is ensured by the positive single lens L2 of the rear group GR. When the positive single lens L2 is not disposed immediately after the stop S but the positive single lens L2 is disposed on the rear side (image plane side) so as to sandwich 1 or more lenses, the position through which the off-axis light beam passes is shifted from the optical axis AX. Therefore, it is undesirable to cause the effective diameter of the lens to become large. Therefore, as in the present embodiment, the positive single lens L2 after the bright stop S is a single lens and has a positive refractive power.

As described above, the basic configuration of the endoscope objective optical system according to the present embodiment is preferable for the reduction in diameter, that is, the reduction in the outer diameter of the lens, the shortening of the entire optical system length, the correction of chromatic aberration, and the like.

The front group GF is composed of a negative single lens L1 (first single negative lens L1) and an optical filter F. The rear group GR is composed of a positive single lens L2 and a cemented lens CL 1. The cemented lens CL1 is composed of a positive lens L3 and a negative lens L4.

The brightness aperture S is disposed between the front group GF and the rear group GR. The bright stop S may be provided on the lens surface. Examples of a method for providing the brightness stop S on the lens surface include a method in which a metal light-shielding film is applied to the lens surface and an opening is formed by etching, and a method in which an annular thin metal sheet is sandwiched between the lens and the frame.

In fig. 1 (a), the optical filter F is disposed between the negative single lens L1 and the aperture stop S.

The optical filter F is, for example, an infrared cut filter or a color temperature conversion filter. These optical filters F are used for sensitivity correction of an image pickup device such as a CCD.

In addition, a laser cut filter or a special function filter may be disposed in the optical system. The laser cut filter is a filter for cutting off laser light such as YAG laser light or semiconductor laser light, for example. The special function filter is, for example, a notch filter for cutting off light of a specific wavelength range.

The optical filter F may be an absorption filter, a reflection filter, or a composite filter obtained by combining these filters. In addition, a filter to which an antireflection film is applied may be used.

The rear group GR is composed of a positive single lens L2 and a cemented lens CL 1. The cemented lens CL1 is composed of a positive lens L3 and a negative lens L4.

The object-side lens surface of the positive single lens L2 in the rear group GR is concave. The image-side lens surface of the positive single lens L2 is convex. Thus, the positive single lens L2 is a meniscus lens.

The glass block CG is disposed on the image side of the rear group GR. The glass block CG is assumed to be cover glass of the solid-state imaging element. On the image side surface of the glass block CG, an image of an object having an image height IH is formed on the image pickup plane I. The image side surface of the glass block CG coincides with the image pickup plane I of the image pickup element.

Next, the endoscope objective optical system according to the first embodiment will be described.

The endoscope objective optical system according to the first embodiment is characterized by having the basic configuration described above and satisfying the following conditional expressions (1), (2), and (3).

1.95＜ndCLn (1)

35＜ΔvdCL (2)

1.1＜fL2/ft＜1.6 (3)

In this case, the amount of the solvent to be used,

ndCLn is the refractive index for the d-line (wavelength 587.6nm) of the negative lens L4 in cemented lens CL1 of the rear group GR,

Δ ν dCL is the difference between the abbe number of the positive lens L3 and the abbe number of the negative lens L4 in the cemented lens CL1 of the rear group GR,

ft is the focal length of the entire system of the endoscope objective optical system,

fL2 is the focal length of the positive single lens L2 in the rear group GR.

The conditional expression (1) specifies conditions for correcting the field curvature satisfactorily. A high refractive index glass material was used in the negative lens L4 of the cemented lens CL 1. Thereby reducing the absolute value of the total value (petzval sum) of the values obtained by dividing the refractive power of each lens by the refractive index of the glass material.

If the value of conditional expression (1) is less than the lower limit value, the absolute value of the petzval sum becomes large, that is, the field curvature becomes large, and a favorable observed image cannot be obtained over the entire field of view.

In an endoscope objective optical system having a wide angle of view, the petzval sum is mostly negative. Therefore, in order to reduce the petzval sum, it is necessary to use a high refractive index glass material in the negative lens or a low refractive index glass material in the positive lens. However, when a low refractive index glass material is used in the positive lens, spherical aberration, coma aberration, and the like are caused to occur, so that image quality deteriorates. Therefore, it is desirable to use a high refractive index glass material in the negative lens.

In the present embodiment, it is also considered to use a high refractive index glass material for the negative single lens L1 of the front group GF. However, the negative einzel 1 exposed at the distal end of the endoscope is limited in options of glass materials that can be used from the viewpoint of biocompatibility. Therefore, a high refractive index glass material is used in the negative lens L4 of the cemented lens CL1 of the rear group GR.

The conditional expression (2) is a conditional expression for correcting the chromatic aberration of magnification satisfactorily. In the present embodiment, chromatic aberration is corrected by the cemented lens CL1 of the rear group GF. When the difference in abbe numbers between the positive lens L3 and the negative lens L4, that is, the value of conditional expression (2) is lower than the lower limit value, the correction of the chromatic aberration of magnification is insufficient, and the image quality is deteriorated in the periphery of the field of view.

The conditional expression (3) is a conditional expression relating to the positive single lens L2 of the rear group GR for shortening the total lens length of the endoscope objective optical system. When the value exceeds the upper limit value of the conditional expression (3), the refractive power of the positive single lens L2 decreases. Therefore, the total length of the lens becomes long, and it is difficult to miniaturize the lens.

When the value of conditional expression (3) is lower than the lower limit value, the positive refractive power becomes excessively large, and spherical aberration, off-axis coma aberration become large, which is undesirable.

In the endoscope objective optical system according to the first embodiment, the positive single lens L2 included in the rear group GR preferably satisfies the following conditional expression (4).

1.75＜ndL2 (4)

In this case, the amount of the solvent to be used,

ndL2 is the refractive index for the d-line of the positive single lens L2 of the rear group GR.

When the value of conditional expression (4) is lower than the lower limit value, the curvature radius of the image-side surface of the positive single lens L2 of the rear group GR becomes too small, and the off-axis principal ray is bent largely, so that off-axis coma aberration becomes large.

In the endoscope objective optical system according to the first embodiment, the positive single lens L2 in the rear group GR preferably satisfies the following conditional expressions (5) and (6).

-7＜R1/ft＜-3.6 (5)

0.8＜|R2/Ls2|＜1.2 (6)

In this case, the amount of the solvent to be used,

r1 is the radius of curvature of the object-side surface of the positive single lens L2 in the rear group GR,

ft is the focal length of the entire system of the endoscope objective optical system,

r2 is the radius of curvature of the image-side surface of the positive single lens L2 in the rear group GR,

ls2 is the distance along the optical axis AX from the stop S to the image side surface of the positive single lens L2.

When the value of conditional expression (5) exceeds the upper limit value, the refractive power of the positive single lens L2 of the rear group GR decreases. By increasing the positive refractive power of the cemented lens CL1, the focal length and the angle of view of the entire objective optical system can be made equal, but in this case, the total lens length is also increased, which is undesirable.

If the lower limit of conditional expression (5) is exceeded, the total lens length will still be increased. This is because the position of the intersection of the image side surface and the optical axis (top surface) is closer to the image than the position of the principal point of the positive single lens L2 in the rear group GR, and the restriction arises that the next lens is disposed closer to the image side.

The following are known: when the principal point is positioned uniformly, the top of the image-side surface of the plano-convex lens is closer to the object side than the top of the image-side surface of the biconvex lens, and the top of the image-side surface of the meniscus lens having a concave surface on the object side is closer to the object side. Therefore, using a meniscus lens satisfying such a condition can further increase the degree of freedom of lens arrangement on the image side. Specifically, another optical element can be disposed close to the positive single lens L2.

Conditional expression (6) is a conditional expression for suppressing occurrence of off-axis coma aberration on the object side surface of the positive single lens L2 in the rear group GR. When the value of conditional expression (6) exceeds the upper limit value, large coma aberration is generated, and the image quality in the periphery of the field of view deteriorates. On the other hand, if the value of conditional expression (6) is lower than the lower limit value, coma aberration with opposite signs is also generated, which is undesirable.

In the endoscope objective optical system according to the first embodiment, the front group GF preferably has the parallel flat plate F and satisfies the following conditional expression (7).

1.4＜Lgr/Lgf＜2 (7)

In this case, the amount of the solvent to be used,

lgf is the distance along the optical axis AX from the object side surface of the negative single lens L1 of the front group GF to the diaphragm S,

lgr is a distance along the optical axis AX from the aperture stop S to the most image-side surface of the lenses of the rear group GR.

If the value of conditional expression (7) exceeds the upper limit, the height of the light beam increases at the cemented lens CL1 of the rear group GR, and therefore the lens diameter increases, making it difficult to miniaturize the objective optical system.

When the value is lower than the lower limit value of conditional expression (7), conversely, the light beam height becomes high at the object side surface of the negative single lens L1 of the front group GF. The negative single lens L1 is undesirable because it has a large lens diameter and makes it difficult to miniaturize the endoscope objective optical system.

In the endoscope objective optical system according to the present embodiment, the negative single lens L1 of the front group GF is preferably a plano-concave lens having a flat object side surface and a concave surface facing the image side.

In the endoscope objective optical system according to the present embodiment, the negative single lens L1 of the front group GF is preferably a plano-concave lens having a plane directed to the object side surface, and satisfies the following conditional expression (9).

0.5＜D1/R1L1＜0.9 (9)

In this case, the amount of the solvent to be used,

R1L1 is the radius of curvature of the image-side surface of the negative single lens L1 of the front group GF,

d1 is the radius of the spherical notch (concave) portion on the image-side surface of the negative single lens L1 of the front group GF in the direction perpendicular to the optical axis AX.

When the value of conditional expression (9) exceeds the upper limit value, it is difficult to miniaturize the endoscope objective optical system. When the value of D1 is large, the lens diameter of the negative single lens L1 becomes large. On the other hand, when the value of R1L1 becomes small, the spherical cutout portion becomes deep with an effective diameter through which light passes being ensured. This increases the total thickness from the object side surface to the image side surface of the negative single lens L1, making it difficult to shorten the total lens length.

When the value of conditional expression (9) is lower than the lower limit value, the lens diameter still becomes large. This is because the negative refractive power becomes smaller due to an increase in the value of R1L1, and the light height passing through the periphery of either the object side surface or the image side surface increases, and the lens diameter becomes larger. When it is desired to achieve the good image quality required for endoscope objective optics, it is desirable to avoid diffraction blur. For this reason, the beam diameter through the image side cannot be reduced significantly (which determines D1).

In the endoscope objective optical system according to the present embodiment, the cemented lens CL1 is preferably composed of a positive lens L3 and a negative lens L4 arranged in this order from the object side, and satisfies the following conditional expression (10).

-2＜R1CL/ft＜-1.1 (10)

In this case, the amount of the solvent to be used,

r1CL is the radius of curvature of the cemented surface of the rear group GR cemented lens CL1,

ft is the focal length of the whole system of the endoscope objective optical system.

When the value of conditional expression (10) exceeds the upper limit value, the refractive power acting at the joint surface becomes excessively strong, and therefore off-axis coma aberration increases, which is undesirable. In addition, the positive lens L3 and the negative lens L4 are both difficult to process and increase in cost, which is undesirable.

If the value of conditional expression (10) is less than the lower limit value, the refractive power of the light beam acting off-axis at the joint surface is insufficient, and the chromatic aberration of magnification cannot be sufficiently corrected, which is undesirable.

Next, the reason and operation of the endoscope objective optical system according to the second embodiment will be described with reference to the drawings.

Fig. 1 (b) is a lens cross-sectional view showing an example of the basic configuration of the endoscope objective optical system according to the present embodiment.

The endoscope objective optical system of the second embodiment includes a front group GF having negative refractive power, a bright diaphragm S, and a rear group GR having positive refractive power, which are arranged in this order from the object side, and has a cemented lens CL1 including a positive lens L3 and a negative lens L4.

In the endoscope objective optical system according to the second embodiment, the front group GF preferably includes only the negative single lens L1, and satisfies the following conditional expression (8).

1.9＜Lgr/ft＜2.6 (8)

In this case, the amount of the solvent to be used,

lgr is a distance along the optical axis AX from the aperture stop S to the most image-side surface of the lenses of the rear group GR,

ft is the focal length of the whole system of the endoscope objective optical system.

If the value of conditional expression (8) exceeds the upper limit, the light beam height at the junction lens CL1 of the rear group GR increases, and therefore the lens diameter increases, making it difficult to miniaturize the objective optical system.

When the value of conditional expression (8) is lower than the lower limit value, each aberration such as coma aberration and chromatic aberration cannot be sufficiently corrected by the rear group GR, and the image quality deteriorates, which is undesirable.

The structures and conditional expressions of the negative single lens L1 of the front group GF and the rear group GR in the endoscope objective optical system according to the second embodiment are the same as those in the endoscope objective optical system according to the first embodiment. Therefore, redundant description is omitted.

In the endoscope objective optical system according to the first embodiment and the endoscope objective optical system according to the second embodiment, the negative single lens L1 of the front group GF is preferably a plano-concave lens whose object side surface is a flat surface.

By making the object side surface flat, damage to the lens surface can be reduced. In addition, since water droplets are less likely to accumulate in the peripheral portion of the lens surface, drainage is improved, and the range in which the lens surface can be observed is not narrowed.

In the endoscope objective optical systems according to the first and second embodiments, it is preferable that nd of the positive lens L3 of the cemented lens CL1 is 1.65 or less.

In the endoscope objective optical systems according to the first and second embodiments, the abbe number of the negative lens L4 of the cemented lens CL1 is preferably 20 or less.

Further, the above-described endoscope objective optical system may satisfy a plurality of configurations at the same time. This is preferable in obtaining a good endoscope objective optical system. In addition, the combination of preferable structures is arbitrary. In addition, each conditional expression may be further limited only by an upper limit value or a lower limit value of a numerical range of the limited conditional expression.

Next, examples will be explained. In the lens sectional views of the respective embodiments, light rays incident from the object side and passing through the center of the aperture stop S and both ends of the inner section of the aperture stop S are shown. In each aberration diagram, the abscissa axis represents the aberration amount. Regarding spherical aberration and astigmatism, the unit of the aberration amount is mm. In addition, regarding distortion aberration, the unit of the aberration amount is%. In addition, IH is the maximum image height in mm, and FNO is the aperture value. The unit of the wavelength of the aberration curve is nm.

(example 1)

An endoscope objective optical system according to example 1 will be described. Fig. 2 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 1. Fig. 2 (b) shows Spherical Aberration (SA), fig. 2 (c) shows Astigmatism (AS), fig. 2 (d) shows distortion aberration (DT), and fig. 2 (e) shows magnification chromatic aberration (CC).

As shown in fig. 2 (a), the endoscope objective optical system of example 1 includes a negative refractive power front group GF, a bright diaphragm S, and a positive refractive power rear group GR, which are arranged in this order from the object side.

The front group GF includes a plano-concave negative first lens L1 whose object side is a plane, and an optical filter F.

The rear group GR includes a positive second meniscus lens L2 whose object side is concave, a biconvex positive third lens L3, and a negative fourth meniscus lens L4 whose convex surface faces the image side. Here, a positive refractive power cemented lens CL1 is formed by the biconvex positive third lens L3 and the negative fourth meniscus lens L4.

The bright aperture S is disposed on the image side of the optical filter F. The front group GF is provided with an optical filter F. The optical filter F is disposed between the plano-concave negative first lens L1 and the aperture stop S. On the image side of the rear group GR, a glass block CG is disposed to provide a cover glass of the solid-state imaging element.

The optical design is as follows: the joint lens CL1 and the glass block CG are spaced apart from each other by a focus adjustment interval, and a sufficient adjustment width can be secured.

Next, the characteristics of the negative first lens L1 will be described. With respect to the negative first lens L1, the object side is made planar. This structure is generally used as an endoscope front end structure. In an endoscope, when the side surface of an object is convex, illumination light is directly incident, and therefore, it is necessary to adopt a light shielding structure at the distal end portion of the endoscope. Therefore, in embodiment 1, there are the following advantages: it is not necessary to perform light shielding at the negative first lens L1 or the frame structure for directly incident stray light from an illumination system not shown.

In addition, since the object side of the negative first lens L1 is a plane, there is no protrusion (convex shape). Therefore, even if something hits the lens surface from the object side, the probability of the negative first lens L1 being damaged is lower than that in the case of the convex surface.

Preferably, the glass material of the negative first lens L1 is sapphire having excellent mechanical durability. By using sapphire, the damage is not easily captured in the image, and stray light due to the damage is not easily generated. However, the glass material of the negative first lens L1 is not limited to sapphire.

The optical filter F is, for example, a color correction filter. The color correction filter is formed of an absorbing material that attenuates the wavelength range from the long wavelength side of the visible light region to the near infrared wavelength region. However, in the applications of digestive organs, respiratory organs, urinary organs, and otorhinolaryngological organs, Nd: YAG laser is sometimes used for the treatment of tumors and the like. Therefore, it is desirable to apply a multilayer optical interference film having a reflectance of approximately 100% with respect to the wavelength of the Nd: YAG laser light in advance to one surface or both surfaces of the color correction filter.

The cemented lens CL1 is composed of a positive third lens L3 of a low refractive index glass material and a negative fourth meniscus lens L4 of a high refractive index glass material. Further, by providing the negative refractive power to the joint surface, astigmatism and coma aberration are corrected. In addition, it is considered that the difference between the object-side refractive index and the image-side refractive index of the joint surface is increased so that the radius of curvature of the joint surface is not excessively small. This suppresses aberration variation caused by decentering. A lens capable of correcting chromatic aberration of magnification is not present on the object side of the cemented lens CL 1. Therefore, using ultra-high dispersion glass in the negative fourth meniscus lens L4 of the cemented lens CL1, chromatic aberration of magnification is corrected collectively by the cemented lens CL 1.

The chromatic aberration of magnification of the endoscope objective optical system of example 1 will be described. The maximum image height IH is 0.652 mm. As shown in fig. 2 (e), the magnification chromatic aberration is 1.5 μm or less, which corresponds to 0.23% of one side of the diagonal. In general, if the magnification chromatic aberration is 3 pixels or less, the image quality is not affected. When an image pickup element of full high definition (FULLHD) (1920 pixels in the horizontal direction × 1080 pixels in the vertical direction) is used, 1101 pixels are half of the diagonal line, 2.5 pixels are 0.23%, and 3 pixels or less are assumed.

The entire length of the endoscope objective optical system of example 1 will be described. The distance from the object side first surface of the lens to the image plane is 5.38 times the focal length, which is shorter than that of the prior art document. Therefore, the objective optical system is suitable for an endoscope whose distal end portion is small and has high image quality.

(example 2)

Next, an endoscope objective optical system according to example 2 will be described. Fig. 3 (a) is a lens cross-sectional view of the endoscope objective optical system according to example 2. Fig. 3 (b) shows Spherical Aberration (SA), fig. 3 (c) shows Astigmatism (AS), fig. 3 (d) shows distortion aberration (DT), and fig. 3 (e) shows magnification chromatic aberration (CC).

As shown in fig. 3 (a), the endoscope objective optical system of example 2 includes a negative refractive power front group GF, a bright diaphragm S, and a positive refractive power rear group GR, which are arranged in this order from the object side.

In the endoscope objective optical system according to example 2, a glass material having nd of 1.816 is used for the positive second meniscus lens L2, and a glass material having nd of 1.569 is used for the negative fourth meniscus lens L4 of the cemented lens CL 1.