Endoscope and variable power optical system for the same

The present invention relates a variable power optical system for an endoscope and an endoscope provided with such a variable power optical system.

In a medical field, an endoscope (a fiber scope or an electronic scope) is generally known as a device for observing a body cavity of a patient and has been practically used. Among endoscopes of this type, there is an endoscope provided with a variable power optical system having the variable power function for conducting fine observation for lesions.

For example, Japanese Patent Publication No. JP4819969B (hereafter, referred to as a “patent document 1”) describes a concrete configuration of a variable power optical system for an endoscope. The variable power optical system for an endoscope described in the patent document 1 includes a first lens group having a negative power, a second lens group having a positive power and a third lens group having a positive power which are arranged in this order from the object side, and is configured to perform focusing by moving the second lens group in accordance with change of the object distance without changing the entire length defined as a distance from the first lens group to the image plane.

However, a lens configuration of a negative-leading type (a type in which a most object side lens is a negative lens) exemplified in the patent document 1 has a drawback that the effective diameter of the most object side negative lens needs to be increased to suppress occurrence of aberrations while securing an angle of view at the wide angle end. A variable power optical system of such a negative-leading type decreases a degree of freedom regarding disposing of components (e.g., a light guide, a forceps, an air supply or water supply channel) to be installed in a tip portion of an endoscope, and is hard to be installed in a fine tip portion of an endoscope. Furthermore, in the lens configuration exemplified in the patent document 1, a degree of change of power is small although focusing can be achieved in response to change of the object distance. Furthermore, since fluctuation of aberration becomes larger when the observing magnification is changed, it is difficult to keep the adequate optical performance over the entire region from the wide angle end to the telephoto end.

The present invention is made in view of the above described circumstances. That is, the object of the present invention is to provided an endoscope and a variable power optical system for an endoscope capable of suppressing the size in a radial direction even if a lens configuration is the negative-leading type, capable of keeping the adequate optical performance over the entire region from the wide angle end to the telephoto end, and capable of securing an adequate observing magnification.

A variable power optical system for an endoscope according to an embodiment of the invention comprises a first lens group having a negative power, a second lens group having a positive power and a third lens group arranged in this order from an object side. The variable power optical system for an endoscope is configured to change magnification for an optical image by moving the second lens group in a direction of an optical axis with respect to the first lens group and the third lens group which are fixed lens groups, while keeping a distance from a most object side lens surface of the first lens group to an image plane constant. The first lens group includes at least a negative lens having a concave surface pointing to an image side and a positive meniscus lens having a concave surface pointing to the object side, arranged in this order from the object side. The second lens group includes at least a meniscus lens having a convex surface pointing to the object side and a cemented lens formed by cementing together a negative lens and a positive lens, arranged in this order from the object side. The third lens group includes at least a positive lens having a convex surface pointing to the object side.

When a focal length of the meniscus lens which the first lens group includes is defined as f s1 (unit: mm), an overall focal length of the first to third lens groups at a wide angle end is defined as f w (unit: mm), and the overall focal length of the first to third lens groups at a telephoto end is defined as f t (unit: mm), the variable power optical system for an endoscope according to an embodiment of the invention satisfies following two conditions:

20< f s1 /f w <50

1.2< f t /f w <1.5.

When a focal length of the first lens group is defined as f 1 (unit: mm), the above described condition (20<f s1 /f w <50) may be replaced with the following condition:

5<| f s1 /f 1 |<20.

When a focal length of the second lens group is defined as f 2 (unit: mm), the variable power optical system for an endoscope according to an embodiment of the invention may be configured to satisfy the following condition:

2< f 2 /f w <5.

When a focal length of the third lens group is defined as f 3 (unit: mm), the variable power optical system for an endoscope according to an embodiment of the invention may be configured to satisfy the following condition:

5< f 3 /f w <20.

When a focal length of the first lens group is defined as f 1 (unit: mm), the variable power optical system for an endoscope according to an embodiment of the invention may be configured to satisfy the following condition:

−4< f 1 /f w <−2.

The variable power optical system for an endoscope according to an embodiment of the invention may further comprise an aperture stop disposed between the first lens group and the second lens group to move along the optical axis integrally with the second lens group.

In the variable power optical system for an endoscope according to an embodiment of the invention, an angle of view of the variable power optical system for an endoscope is, for example, 120 degrees or more.

An endoscope according to an embodiment of the invention comprises the above described variable power optical system for an endoscope installed in a tip portion of the endoscope.

According to the embodiments of the invention, an endoscope and a variable power optical system for an endoscope capable of suppressing the size in a radial direction even if a lens configuration is the negative-leading type, capable of keeping the adequate optical performance over the entire region from the wide angle end to the telephoto end, and capable of securing an adequate observing magnification are provided.

FIG. 1 is an external view illustrating an outer appearance of an electronic scope according to an embodiment of the invention.

FIG. 2 is a lens arrangement diagram illustrating a configuration of a variable power optical system for an endoscope according to an example 1 of the invention.

FIG. 3 illustrates various aberration diagrams of the variable power optical system for an endoscope according to the example 1 of the invention.

FIG. 4 is a lens arrangement diagram illustrating a configuration of a variable power optical system for an endoscope according to an example 2 of the invention.

FIG. 5 illustrates various aberration diagrams of the variable power optical system for an endoscope according to the example 2 of the invention.

FIG. 6 is a lens arrangement diagram illustrating a configuration of a variable power optical system for an endoscope according to an example 3 of the invention.

FIG. 7 illustrates various aberration diagrams of the variable power optical system for an endoscope according to the example 3 of the invention.

FIG. 8 is a lens arrangement diagram illustrating a configuration of a variable power optical system for an endoscope according to an example 4 of the invention.

FIG. 9 illustrates various aberration diagrams of the variable power optical system for an endoscope according to the example 4 of the invention.

FIG. 10 is a lens arrangement diagram illustrating a configuration of a variable power optical system for an endoscope according to an example 5 of the invention.

FIG. 11 illustrates various aberration diagrams of the variable power optical system for an endoscope according to the example 5 of the invention.

In the following, explanation is given for a variable power optical system for an endoscope according to an embodiment of the invention and an electronic scope in which such a variable power optical system for an endoscope is installed, with reference to the accompanying drawings.

FIG. 1 is an external view illustrating an outer appearance of an electronic scope 1 according to the embodiment of the invention. As shown in FIG. 1 , the electronic scope 1 includes an elastic insertion tube 11 covered with a sheath 11 a having elasticity. A tip portion (a bending part 14 ) of the elastic insertion tube 11 is configured to be bent in response to remote control (specifically, a rotating operation for a bending operation knob 13 a ) from a hand operating unit 13 coupled to the proximal end of the elastic insertion tube 11 . A mechanism for such bending is a known mechanism installed in a general endoscope, and is configured to bend the bending part 14 through drawing of operation wires in conjunction with the rotating operation for the bending operation knob 13 a . To a tip of the bending part 14 , the proximal end of a tip portion 12 covered with a resin housing having rigidity is connected. In response to change of the direction of the tip portion 12 due to the bending motion caused by the rotating operation for the bending operation knob 13 a , an imaging area of the electronic scope 1 moves.

In the inside of the resin housing of the tip portion 12 , a variable power optical system 100 for an endoscope (a block indicated by a dashed line in FIG. 1 ) is installed. The variable power optical system 100 for an endoscope converges light from a subject onto a light-receiving surface of a solid-state image pickup device (not shown) so as to pick up image data of the subject in the imaging area. As the solid-state image pickup device, a CCD (Changed Coupled Device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor may be used.

FIG. 2 is a cross sectional view illustrating disposition of the variable power optical system 100 for an endoscope and optical components disposed on the rear side of the variable power optical system 100 for an endoscope, according to an example 1 of the invention (which is described in detail later). FIGS. 2( a ) and 2( b ) illustrate lens arrangements when the position of the variable power is at the wide angle end and the telephoto end, respectively. In the following, the variable power optical system 100 for an endoscope according to the embodiment of the invention is explained in detail with reference to FIG. 2 .

As shown in FIG. 2 , the variable power optical system 100 for an endoscope includes a first lens group G 1 having a negative power, an aperture stop S, a second lens group G 2 having a positive power and a third lens group G 3 having a positive power or a negative power. The variable power optical system 100 for an endoscope is configured to change the overall focus length (i.e., a combined focus length defined from the first lens group G 1 to the third lens group) while keeping the focused state and thereby to change magnification for an optical image, by letting the second lens group G 2 move in an optical axis AX direction with respect to the first lens group G 1 and the third lens group G 3 which are fixed lens groups, while keeping the distance (i.e., the entire length of the variable power optical system 100 for an endoscope) from the most object side lens surface of the first lens group G 1 to the image plane constant. The variable power optical system 100 for an endoscope has the angle of view of 120 degrees or more (the half angle of view of 60 degrees or more) at the wide angle end. Each optical lens constituting each of the first to third lens groups G 1 to G 3 has a rotationally-symmetrical shape about the optical axis AX. On the rear side of the third lens group G 3 , a color correction filter F for the solid-state image pickup device is disposed. The color correction filter F is adhered to a cover glass CG for protecting the solid-state image pickup device.

The first lens group G 1 includes at least a negative lens having a concave surface pointing to the image side (a lens L 1 in the example shown in FIG. 2 ), a positive meniscus lens having a concave surface pointing to the object side (a lens L 3 in the example shown in FIG. 2 ) which are arranged in this order from the object side. The first lens group G 1 is a lens group having a negative power disposed on the object side with respect to the aperture stop S. The expression “includes at least” used above means that a lens group may be additionally provided with another optical component, such as a parallel plate, within the scope of the present invention. For the same reasons, the expression “includes at least” is also used in explanations about the second lens group G 2 and the third lens group G 3 .

By thus disposing, in the first lens group G 1 , a meniscus lens which has a concave surface pointing to the object side (in other words, which has a convex surface pointing to the image side) and has a positive power, aberrations caused by the negative lens having the concave surface pointing to the image side can be canceled out. As a result, fluctuation of the aberrations in the entire optical system can be suppressed, and thereby the adequate optical performance can be maintained over the entire region from the wide angle end to the telephoto end.

The second lens group G 2 is a lens group which is disposed on the rear side of the aperture stop S and has a positive power. In order to suppress occurrence of chromatic aberration, the second lens group G 2 is configured to include, in the order from the object side, at least a meniscus lens (a lens L 4 in the example shown in FIG. 2 ) having a convex surface pointing to the object side, and a cemented lens (a cemented lens CL 1 formed by cementing together a lens L 5 and a lens L 6 ) formed by cementing together positive and negative two lenses. In the example shown in FIG. 2 , the cemented lens CL 1 is formed by disposing the negative lens (the lens L 5 ) on the object side and disposing the positive lens (the lens L 6 ) on the image side; however, in another embodiment the cemented lens may be configured by disposing a positive lens on the object side and disposing a negative lens on the image side.

The second lens group G 2 moves integrally with the aperture stop S in the optical axis AX direction in order to change magnification for the optical image formed on the light-receiving surface of the solid-state image pickup device. By moving the second lens group G 2 together with the aperture stop S, astigmatism caused at the telephoto end can be effectively suppressed.

The aperture stop S may be a plate-like member having a circular hole centered at the position of the optical axis AX or may be a light shielding film which is formed on a lens surface (an object side surface r 8 in the example shown in FIG. 2 ) nearest to the aperture stop S in the second lens group G 2 to coating a region other than a circular region centering at the position of the optical axis AX. The thickness of the aperture stop S is extremely small relative to the thickness of each lens constituting the variable power optical system 100 for an endoscope, and may be neglected when the optical performance of the variable power optical system 100 for an endoscope is calculated. Therefore, in the following, explanation is given assuming that the thickness of the aperture stop S is zero.

The third lens group G 3 is a lens group having a positive or negative power, and includes at least a positive lens (a lens L 8 in the example shown in FIG. 2 ) having a convex surface pointing to the object side, in order to suppress the exit angle of light proceeding from the variable power optical system 100 for an endoscope to the solid-state image pickup device.

When the focal length of the meniscus lens of the first lens group G 1 is defined as f s1 (unit: mm), the overall focal length at the wide angle end is defined as f w (unit: mm), and the overall focal length at the telephoto end is defined as f t (unit: mm), the variable power optical system 100 for an endoscope is configured to satisfy the following conditions (1) and (2):

20< f s1 /f w <50  (1)

1.2< f t /f w <1.5  (2)

By satisfying the condition (1), it becomes possible to suppress the effective diameter of the negative lens in the first lens group G 1 which has the greatest tendency to become large in the variable power optical system 100 for an endoscope, and thereby it becomes possible to suppress the size in the radial direction of the entire variable power optical system 100 for an endoscope. Therefore, the variable power optical system 100 for an endoscope is suitable for installing into the inside of the tip portion 12 of the electronic scope 1 designed to have a small diameter. Furthermore, since the size in the radial direction of the entire variable power optical system 100 for an endoscope is suppressed, a degree of freedom regarding disposing of other components (e.g., a light guide, a forceps, an air supply or water supply channel) to be installed into the tip portion 12 is enhanced.

Furthermore, by satisfying the condition (1), aberrations caused in the first lens group G 1 can be suppressed, and thereby it becomes possible to more suitably maintain the suitable optical performance over the region from the wide angle end to the telephoto end.

When, regarding the condition (1), f s1 /f w gets larger than or equal to 50 (the value of the right term), the positive power in the first lens group G 1 becomes too small and thereby it becomes difficult to cancel out the aberration caused by the negative lens in the first lens group G 1 . Furthermore, when, regarding the condition (1), f s1 /f w gets larger than or equal to 50 (the value of the right term), the angle of view needs to be narrowed in order to suppress the aberration caused in the first lens group G 1 to an appropriate amount.

When, regarding the condition (1), f s1 /f w gets smaller than or equal to 20 (the value of the left term), the positive power becomes too strong in the first lens group G 1 and therefore the effective diameter of the negative lens in the first lens group G 1 needs to be increased in order to suppress the aberration caused in the first lens group G 1 to an appropriate amount.

When considering usability of the electronic scope 1 for observing a body cavity, it is preferable that the best object distance becomes shorter as the focal length gets closer from the wide angle end to the telephoto end, and becomes shortest when the focal length reaches the telephoto end. By satisfying the condition (2), change of the object distance with respect to the observation magnification becomes appropriate. Furthermore, the observation magnification can be adequately secured.

When, regarding the condition (2), f t /f w gets larger than equal to 1.5, the value of the right term, change of F number according to change of the magnification becomes too large and thereby resolution at the telephoto end decreases.

When, regarding the condition (2), f t /f w gets smaller than equal to 1.2, the value of the left term, the magnification at the telephoto end becomes too small and thereby the adequate observation by an operator becomes difficult.

It should be noted that when the focal length of the first lens group G 1 is defined as f 1 (unit: mm), the condition (1) may be replaced with the following condition (3):

5<| f s1 /f 1 |<20  (3)

When the conditions (2) and (3) are satisfied, the same advantageous effects as those achieved by satisfying the conditions (1) and (2) can be achieved.

When the focal length of the second lens group G 2 is defined as f 2 (unit: mm), the variable power optical system 100 for an endoscope is configured to satisfy the following condition (4):

2< f 2 /f w <5  (4)

By satisfying the condition (4), it becomes possible to secure the moving amount of the second lens group G 2 required for the variable power while suppressing the entire length of the variable power optical system 100 for an endoscope.

When, regarding the condition (4), f 2 /f w gets larger than equal to 5, the value of the right term, the power of the second lens group G 2 becomes too small, and thereby the moving amount of the second lens group G 2 required for the variable power becomes large and the entire length of the variable power optical system 100 for an endoscope becomes large. As a result, it becomes necessary to lengthen the entire length of the tip portion 12 of the electronic scope 1 being the rigid part in order to accommodate the variable power optical system 100 for an endoscope having the long entire length. Furthermore, when, regarding the condition (4), f 2 /f w gets larger than equal to 5, the value of the right term, the observation magnification becomes too high, and thereby the operability of the electronic scope 1 when the observation is conducted at the telephoto end decreases. For example, in the observation, the tip portion 12 of the electronic scope 1 moves largely in response to a slight operation, and therefore it becomes difficult for the operator to bring an area which the operator wants to observe within the angle of view.

When, regarding the condition (4), f 2 /f w gets smaller than equal to 2, the value of the left term, the power of the second lens group G 2 becomes too strong, and therefore Petzval's sum becomes large in the negative direction and the curvature of filed occurs largely although the moving amount of the second lens group G 2 required for the variable power can be suppressed.

When the focal length of the third lens group G 3 is defined as f 3 (unit: mm), the variable power optical system 100 for an endoscope is configured to satisfy the following condition (5):

5< f 3 /f w <20  (5)

By satisfying the condition (5), the exit angle of light proceeding from the variable power optical system 100 for an endoscope to the solid-state image pickup device can be suppressed over the region from the wide angle end to the telephoto end.

When, regarding the condition (5), f 3 /f w gets larger than equal to 20, the value of the right term, the power of the third lens group G 3 becomes weak and thereby change of the exit angle of light according to the variable power becomes small whereas coma and chromatic aberration occur largely.

When, regarding the condition (5), f 3 /f w gets smaller than equal to 5, the value of the left term, the power of the third lens group G 3 becomes too large and thereby change of the exit angle of light according to the variable power becomes large.

The variable power optical system 100 for an endoscope is configured to satisfy the following condition (6):

−4< f 1 /f w <−2  (6)

By satisfying the condition (6), the effective diameter of the first lens group G 1 can be suppressed.

When, regarding the condition (6), f 1 /f w gets larger than equal to −2, the value of the right term, power of the negative lens disposed on the most object side in the first lens group G 1 becomes too strong, and therefore coma is caused largely.

When, regarding the condition (6), f 1 /f w gets smaller than equal to −4, the value of the left term, it becomes necessary to increase the effective diameter of the negative lens disposed on the most object side in the first lens group G 1 to secure a negative power of the first lens group G 1 .

Hereafter, five concrete numerical examples of the variable power optical system 100 for an endoscope described above will be explained. The variable power optical system 100 for an endoscope according each of the numerical examples 1 to 5 is disposed in the tip portion 12 of the electronic scope 1 shown in FIG. 1 .