JP2009297401A - Image pickup lens and capsule type endoscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image pickup optical system for a capsule type endoscope, wherein the image pickup optical system has well corrected magnification ratio and chromatic aberration while it is a compact and wide-angled system. <P>SOLUTION: The image pickup optical system 31 for a capsule type endoscope is used by being swallowed by a subject, and produces on its plane surface an image of the subject's opposing insides of their curved surfaces. The system comprises a dome 32, an image pickup lens 33 and a cover glass 34. The dome 32 forms the outer shape of a capsule type endoscope and defines the capsule's transparent dome end. The cover glass 34 is a flat glass plate located in parallel with the imaging surface of the image pickup device and covering the imaging surface. The surface S8 of the image pickup lens 33 is a diffraction surface that is provided with a concentric relief for diffracting incident light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an imaging optical system used for an endoscope that images a living body, and more particularly to an imaging optical system used for a capsule endoscope that is swallowed and used.

  Conventionally, in the medical field, an insertion type endoscope has been widely used. The insertion type endoscope inserts a flexible insertion portion into the body of the subject, and images the inside of the subject with an imaging portion provided at the distal end of the insertion portion. The insertion portion can be freely bent according to the shape of the body cavity of the subject, and the inorganic content of the tip portion provided with the imaging portion can be freely changed. For this reason, the insertion-type endoscope can accurately capture an image of an arbitrary observation site, which helps accurate diagnosis.

  Diagnosis using an insertion-type endoscope requires a long load to be inserted into the body for a long period of time, so the load on the subject is very large. For this reason, in recent years, a capsule endoscope in which an imaging device is housed in a capsule that can be easily swallowed is known in order to image the inside of a body cavity with as little load on the subject as possible. The capsule endoscope is used by being swallowed by a subject, and images the body cavity while moving naturally within the subject's body. While this capsule endoscope can greatly reduce the load on the subject, it is difficult to control the position and posture within the subject.

  For this reason, the capsule endoscope is equipped with a wide-angle imaging lens so that an image can be captured in as wide a range as possible without controlling the posture in the body cavity. Furthermore, since the capsule endoscope is required to be configured as compactly as possible to facilitate swallowing, the imaging lens mounted on the capsule endoscope is not only wide-angle but also configured with two elements in two groups For example, a small number of lenses can be used to fit in a small capsule.

  As a capsule endoscope as described above, there is known a capsule with a reduced capsule size while ensuring a wide imaging range (Patent Document 1). In addition to a compact and wide-angle, a capsule endoscope that can obtain an optimum depth of field (Patent Document 2), and a capsule-type endoscope that can illuminate a cylindrical subject satisfactorily. An endoscope (Patent Document 3) is known.

  By the way, a lens that diffracts light by providing concentric reliefs (so-called annular zones) on the surface at intervals of about the wavelength of light used for imaging (hereinafter referred to as a diffractive lens). It has been known.

A diffractive lens is an optical pickup that handles optical disks with different wavelengths, such as CDs and DVDs, with one optical system, and is used as an objective lens that limits the aperture according to the wavelength used. In addition, it is known that a diffractive lens is used as a lens for correcting chromatic aberration among imaging lenses by utilizing the fact that the Abbe number becomes negative (Patent Documents 4 and 5).
Japanese Patent Laid-Open No. 2005-80789 JP 2005-80790 A JP 2006-61438 A Japanese Patent Laid-Open No. 10-186223 JP 2007-86485 A

  The larger the angle of view, the more easily the effects of aberrations such as distortion and chromatic aberration of magnification occur in the periphery of the acquired image, so the wide-angle imaging lens corrects these aberrations by combining multiple lenses. Like to do. However, since the capsule endoscope is required to have a compact capsule size, an imaging lens mounted on the capsule endoscope is inevitably configured with a small number of lenses even though it is a wide-angle lens. For this reason, it is difficult for an imaging lens for a capsule endoscope to sufficiently correct various aberrations such as lateral chromatic aberration.

  When an imaging lens that does not sufficiently correct various aberrations that become noticeable at wide angles is installed in a capsule endoscope, the peripheral part of the acquired image is distorted to a smaller extent than the actual area to be imaged, or color blurring occurs. This may cause oversight or misunderstanding of the lesion, and may prevent an appropriate diagnosis because the imaging lens has a wide angle.

  Further, in diagnosis performed based on an image obtained from a capsule endoscope, not only the shape and size of a region to be imaged but also its color is an important determination factor. For this reason, the imaging lens mounted on the capsule endoscope is required not only to have a wide angle but also to have various aberrations (particularly chromatic aberration) corrected well.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide an imaging optical system for a capsule endoscope in which chromatic aberration is well corrected while being compact and wide-angle. . It is another object of the present invention to provide a capsule endoscope that can easily obtain an image suitable for diagnosis by mounting this imaging optical system.

  The imaging optical system of the present invention is an imaging optical system for a capsule endoscope that is used by being swallowed by a subject and images the inside of the body cavity of the subject, and has an outer shape of the capsule endoscope. None, a transparent dome-shaped dome cover, an imaging lens for imaging light incident from the body of the subject through the end of the capsule on the imaging device, and an imaging surface of the imaging device A diffractive surface on any one of the dome cover, the imaging lens, and the cover glass.

  The diffractive surface is provided on the imaging lens.

  The imaging lens includes, in order from the object side, a first group having a negative refractive power, a diaphragm, and a second group having a positive refractive power.

The diffractive surface is provided in the lens of the first group, and satisfies B ₂ > 0 when a second-order coefficient of a phase difference function representing the shape of the diffractive surface is B _2. To do.

The diffractive surface is provided in the lens of the second group, and satisfies B ₂ <0 when a second-order coefficient of a phase difference function representing the shape of the diffractive surface is B _2. To do.

  In addition, the imaging lens includes two lenses.

  In addition, the imaging lens includes three lenses.

  Further, all the lenses constituting the imaging lens are made of a resin material.

  The field angle of the imaging lens is 100 degrees or more.

  A capsule endoscope according to the present invention includes the above-described imaging optical system.

  According to the present invention, it is possible to provide an imaging optical system for a capsule endoscope that is compact and has a wide angle and has a well corrected chromatic aberration. In addition, by mounting this imaging lens, it is possible to provide a capsule endoscope that can easily obtain an image of a wide range in which an appropriate diagnosis can be performed while being compact.

  As shown in FIG. 1, a capsule endoscope 11 is an endoscope in which a small imaging device is housed in a capsule 12, and is used by being swallowed by a subject. While moving to (2), the inside of the subject (hereinafter referred to as the subject 13) is imaged.

  The capsule 12 is a hollow container in which both ends of a cylindrical shape are formed in a dome shape, and is provided in a size that can be easily swallowed by a subject. In the capsule 12, an imaging device including an imaging optical system 17 including an imaging lens 16 and an illumination element (not shown), an imaging element 18, a battery for driving the imaging device, and a captured image are externally provided. The transmitting antenna and the like are stored in a watertight manner.

  The front end portion of the capsule 12 is not only formed in a dome shape, but also has a substantially uniform thickness, and a sufficiently wide range is made of a transparent material in accordance with the angle of view of the imaging lens 16. Illumination light from the illumination element is irradiated to the subject 13 through the transparent portion of the capsule 12. In addition, light from the subject 13 enters the imaging lens 16 through this transparent portion. For this reason, the front end portion of the capsule 12 not only forms the outer shape of the capsule endoscope 11 but also greatly affects optical performance. Therefore, the transparent front end portion of the capsule 12 is hereinafter referred to as a dome (dome cover) 21. Will be described as a part of the imaging optical system 17 of the capsule endoscope 11.

  The imaging optical system 17 of the capsule endoscope 11 includes a dome 21, an imaging lens 16 that forms an image of light from a subject on the imaging surface of the imaging device 18 that is a plane, and the imaging device 18 so as to cover the imaging surface. The cover glass 22 is provided. As described above, the dome 21 is the front end portion of the capsule 12, and the inner surface and the outer surface thereof are formed in a spherical shape. The cover glass 22 is a parallel plate glass and is provided integrally with the image sensor 18.

  The imaging lens 16 is provided so as to be a wide-angle lens having an angle of view 2ω of 100 degrees or more, and includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group having a positive refractive power. Consists of G2. Further, the lenses and the aperture stop constituting the imaging lens 16 are arranged in order of the first group G1, the aperture stop S6, and the second group G2 from the subject 13 side (object side). Further, all the lenses constituting the imaging lens 16 are made of a resin material.

  Further, the surfaces of the lenses constituting the imaging lens 16 are all curved surfaces, but the rearmost (most image sensor 18 side) surface of the lenses of the second group G2 has a wavelength of visible light. Concentric concavities and convexities (hereinafter referred to as relief) are provided around the optical axis at intervals of. For this reason, when the light from the subject 13 passes through a surface provided with a relief (hereinafter referred to as a diffraction surface), it is not only refracted but also diffracted according to the curved surface shape.

  When light is bent on a surface having a positive refractive power where no relief is provided, the Abbe number becomes a positive value because light with a longer wavelength is usually collected at a position farther from the lens. However, a lens provided with a relief (hereinafter referred to as a diffractive lens) collects light at a position closer to the lens as light having a longer wavelength has a negative value. For this reason, the diffractive lens plays a role of correcting chromatic aberration due to other lenses of the imaging lens 16, the dome 21, the cover glass 22, and the like.

The shape of each surface of the lens constituting the imaging lens 16 is expressed as follows using a curvature c, a conic constant K, a distance ρ (ρ ² = x ² + y ² ) from the optical axis, and an i-order aspheric coefficient A _i. It is represented by the formula 1. Further, the shape of the relief is expressed by a phase difference function φ using a distance ρ (ρ ² = x ² + y ² ) from the optical axis and a phase difference coefficient B _2j as shown in _Equation ² below. Accordingly, the detailed shape of the entire diffractive surface is represented by the sum of z in Equation 1 and φ in Equation 2.

In order to better correct the chromatic aberration of the imaging optical system 17 by providing the diffractive surface, it is necessary to appropriately determine the coefficient of the phase difference function φ according to the position of the diffractive surface with respect to the aperture stop S6. In the case where the second-order coefficient B ₂ of the phase difference function φ is positive (B ₂ > 0), it is preferable that the second-order coefficient B ₂ is positive (B ₂ > 0). On the other hand, in the case where than S6 aperture stop diffractive surface provided on the rear (the image pickup element 18 side) is preferably determined as quadratic coefficient B ₂ of the phase difference function φ is negative (B 2 _<0) .

In the imaging lens 16, since the diffractive surface is provided behind the aperture stop S6 (on the imaging element 18 side) as described above, the value of the second-order coefficient B ₂ (j = 1) of the phase difference function φ is It is determined to be negative (B ₂ <0).

  Hereinafter, as examples of the imaging optical system 17 described above, examples in which the imaging lens 16 is configured by three lenses are examples 1 to 3, and examples in which the imaging lens 16 is configured by two lenses are examples 4 to 4. This will be described in FIG.

<Example 1>
As shown in FIG. 2, the imaging optical system 31 includes a dome 32, an imaging lens 33, and a cover glass 34, and is configured to form an image of a spherical subject 13 on a flat imaging surface. The dome 32 is made of resin, and is formed into a spherical surface on both the subject 13 side and the image side (image pickup element 18 side). The cover glass 34 is a parallel flat glass plate and is provided in close contact with the imaging surface of the imaging element 18. For this reason, the image side surface of the cover glass 34 coincides with the imaging surface.

  The imaging lens 33 includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group G2 having a positive refractive power. The first group G1 of the imaging lens 33 includes a first lens L1 having a negative refractive power. The first lens L1 is made of resin and has a meniscus shape convex toward the subject 13 side. The second group G2 includes a second lens L2 having a positive refractive power and a third lens L3 having a positive refractive power. The second lens L2 and the third lens L3 are arranged in this order from the subject 13 side. ing. The second lens L2 is made of resin and has a biconvex shape, and the third lens L3 is made of resin and has a meniscus shape that is convex toward the subject 13 side.

In the following, the surface of the object 13 is S1, the object 13 side (object side) surface of the dome 32 is S2, the image side surface of the dome 32 is S3, the object 13 side surface of the first lens L1 is S4, and the first lens. The image side surface of L1 is S5, the aperture stop is S6, the surface of the second lens L2 on the subject 13 side is S7, the image side surface of the second lens L2 is S8, and the surface of the third lens L3 on the subject 13 side. In S9, the image side surface of the third lens L3 is S10, the surface of the cover glass 34 on the subject 13 side is S11, the image side surface of the cover glass 34 is S12, and the surface Si (surface number i = 1 to 12). To express. Further, the distance on the optical axis between the surface Si and the image-side surface Si + 1 adjacent thereto is represented by a surface distance D _i (i = 1 to 11). It should be noted that the manner in which the surfaces of the subject 13 and the imaging optical system 31 and the distance between the surfaces are expressed is the same in the second and third embodiments described later.

As data of the imaging optical system 31 configured as described above, the curvature radius R _i (mm) of each surface Si, the distance between the surfaces D _i (mm), the refractive index Ne for the e-line (wavelength 546.1 nm), and The Abbe number νe is shown in Table 1. Abbe number [nu _e shown here, which is calculated using the refractive index _N e of the e-line, the refractive index _N F of the F-line, the refractive index _{N C} of C line, [nu _e = _(N e -1 ) / (N _F −N _C ). The lower part of Table 1 shows the angle of view 2ω of the imaging lens 33. The lens data of the imaging lens 33 shown in Table 1 is lens data when standardized so that the focal length f of the entire system is 1.0. The method of representing data of the imaging optical system is the same for Examples 2 to 5 described later.

  As indicated by * in the surface number column of Table 1, in the imaging lens 33, the surfaces S4 and S5 of the first lens L1, the surface S8 of the second lens L2, and the surfaces S9 and S10 of the third lens L3 are aspheric. The surface S7 of the second lens L2 is a spherical surface. Further, in the imaging lens 33, the third lens L3 is a diffractive lens, and as shown by # in the surface number column of Table 1, a diffractive surface is provided on the image side surface S10 of the third lens L3. Therefore, the image-side surface S10 of the third lens L3 is an aspheric surface and also a diffractive surface.

Since the shape of each surface of the imaging optical system 31 is expressed by the above-described equation 1 and equation 2, the surface of the imaging optical system 31 that is an aspherical surface or a diffractive surface. The aspheric coefficient A _i is shown in Table 2, and the phase difference coefficient B _2j is shown in Table 3.

  Furthermore, the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging optical system 31 are shown in FIG. As for spherical aberration, the e-line (wavelength 546.1 nm) is indicated by a solid line, the F-line (wavelength 486.13 nm) is indicated by a broken line, and the C-line (wavelength 656.27 nm) is indicated by a two-dot chain line. Astigmatism, the sagittal direction is indicated by a solid line, and the tangential direction is indicated by a broken line. The lateral chromatic aberration is indicated by a broken line for the F line and by a two-dot chain line for the C line. It should be noted that these aberration diagrams are represented in the same manner in Examples 2 to 5 described later.

<Example 2>
As shown in FIG. 4, the imaging optical system 41 includes a dome 42, an imaging lens 43, and a cover glass 44, and is configured to form an image of the spherical subject 13 on a flat imaging surface. The dome 42 is made of resin, and is formed into a spherical surface on both the subject 13 side and the image side. The cover glass 34 is a parallel flat glass plate, and is provided in close contact with the imaging surface of the imaging element 18 of the imaging element 18. For this reason, the image side surface of the cover glass 44 coincides with the imaging surface.

  The imaging lens 43 includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group G2 having a positive refractive power. The first group G1 of the imaging lens 43 includes a first lens L1 having a negative refractive power. The first lens L1 is made of resin and has a meniscus shape convex toward the subject 13 side. The second group G2 includes a second lens L2 having a positive refractive power and a third lens L3 having a positive refractive power. The second lens L2 and the third lens L3 are arranged in this order from the subject 13 side. Yes. The second lens L2 and the third lens L3 are both made of resin and have a biconvex shape.

  Table 4 shows data of the imaging optical system 41 configured as described above. As indicated by * in the surface number column of Table 4, in the imaging lens 43, the surfaces S4 and S5 of the first lens L1, the surface S8 of the second lens L2, and the surfaces S9 and S10 of the third lens L3 are aspheric. The surface S7 of the second lens L2 is a spherical surface. Further, in the imaging lens 43, the second lens L2 is a diffractive lens, and the surface S8 on the image side of the second lens L2 is a diffractive surface, as indicated by # in the surface number column of Table 4. Therefore, the image-side surface S8 of the second lens L2 is an aspheric surface and also a diffractive surface.

Since the shape of each surface of the imaging optical system 41 is expressed by the above-described formulas 1 and 2, among the surfaces of the imaging optical system 41, those that are aspherical surfaces or diffractive surfaces are The spherical coefficient A _i is shown in Table 5, and the phase difference coefficient B _2j is shown in Table 6. Furthermore, the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging optical system 41 are shown in FIG.

<Example 3>
As shown in FIG. 6, the imaging optical system 51 includes a dome 52, an imaging lens 53, and a cover glass 54, and is configured to form an image of a spherical subject 13 on a flat imaging surface. The dome 52 is made of resin, and is formed into a spherical surface on both the subject 13 side and the image side. The cover glass 54 is a parallel flat glass plate and is provided in close contact with the imaging surface of the imaging element 18. For this reason, the image side surface of the cover glass 54 coincides with the imaging surface.

  The imaging lens 53 includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group G2 having a positive refractive power. The first group G1 of the imaging lens 53 includes a first lens L1 having a negative refractive power. The first lens L1 is made of resin and has a meniscus shape convex toward the subject 13 side. The second group G2 includes a second lens L2 having a positive refractive power and a third lens L3 having a positive refractive power. The second lens L2 and the third lens L3 are arranged in this order from the subject 13 side. Yes. The second lens L2 and the third lens L3 are both made of resin and have a biconvex shape.

  Table 7 shows data of the imaging optical system 51 configured as described above. As indicated by * in the surface number column of Table 7, in the imaging lens 53, the surfaces S4 and S5 of the first lens L1, the surface S8 of the second lens L2, and the surfaces S9 and S10 of the third lens L3 are aspherical. The surface S7 of the second lens L2 is a spherical surface. Further, in the imaging lens 53, the first lens L1 is a diffractive lens, and the surface S4 on the subject 13 side of the first lens L1 is a diffractive surface, as indicated by # in the surface number column of Table 7. . Therefore, the surface S4 on the subject 13 side of the first lens L1 is an aspheric surface and also a diffractive surface.

Since the shape of each surface of the imaging optical system 51 is expressed by the above-described formulas 1 and 2, among the surfaces of the imaging optical system 51, those that are aspherical surfaces or diffractive surfaces are The spherical coefficient A _i is shown in Table 8, and the phase difference coefficient B _2j is shown in Table 9. Furthermore, FIG. 7 shows spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging optical system 41.

<Example 4>
As shown in FIG. 8, the imaging optical system 61 includes a dome 62, an imaging lens 63, and a cover glass 64, and is configured to form an image of a spherical subject 13 on a flat imaging surface. The dome 62 is made of resin and has a spherical surface on both the subject 13 side and the image side. The cover glass 64 is a parallel flat glass plate, and is provided in close contact with the imaging surface of the imaging element 18. For this reason, the image side surface of the cover glass 64 coincides with the imaging surface.

  The imaging lens 63 includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group G2 having a positive refractive power. The first group G1 of the imaging lens 63 includes a first lens L1 having a negative refractive power. The first lens L1 is made of resin and has a meniscus shape convex toward the subject 13 side. The second group G2 includes a second lens L2 having a positive refractive power. The second lens L2 is made of resin and has a biconvex shape.

In the following, the surface of the subject 13 is S1, the surface of the dome 62 on the subject 13 side (object side) is S2, the image side surface of the dome 62 is S3, the surface of the first lens L1 on the subject 13 side is S4, the first. The image side surface of the lens L1 is S5, the aperture stop is S6, the surface of the second lens L2 on the subject side S7, the image side surface of the second lens L2 is S8, and the surface of the cover glass 64 on the subject 13 side. S9, the image side surface of the cover glass 64 is denoted by S10, and is represented by a surface Si (surface number i = 1 to 10). Further, representing the distance on the optical axis between the surface Si + 1 on the image side adjacent to the surface _{S i} in surface spacing _D i (i = _1~9). It should be noted that the surface of the subject 13 and the imaging optical system 61 and how to represent the surface intervals are the same as in Example 5 described later.

  Table 10 shows data of the imaging optical system 61 configured as described above. As indicated by * in the surface number column of Table 10, in the imaging lens 63, both surfaces S4, S5, S7, and S8 of the first lens L1 and the second lens L2 are all aspherical surfaces. Further, in the imaging lens 63, the first lens L1 is a diffractive lens, and the surface S4 on the subject 13 side of the first lens L1 is a diffractive surface, as indicated by # in the surface number column of Table 10. . Therefore, the surface S4 on the subject 13 side of the first lens L1 is an aspheric surface and also a diffractive surface.

Since the shape of each surface of the imaging optical system 61 is expressed by the formulas 1 and 2, the surface of the imaging optical system 61 that is an aspherical surface or a diffractive surface is not The spherical coefficient A _i is shown in Table 11, and the phase difference coefficient B _2j is shown in Table 12. Further, FIG. 9 shows spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging optical system 61.

<Example 5>
As shown in FIG. 10, the imaging optical system 71 includes a dome 72, an imaging lens 73, and a cover glass 74, and is configured to form an image of a spherical subject 13 on a flat imaging surface. The dome 72 is made of resin, and is formed into a spherical surface on both the subject 13 side and the image side. The cover glass 74 is a parallel flat glass plate and is provided in close contact with the imaging surface of the imaging element 18. For this reason, the image side surface of the cover glass 74 coincides with the imaging surface.

  The imaging lens 73 includes a first group G1 having a negative refractive power, an aperture stop S6, and a second group G2 having a positive refractive power. The first group G1 of the imaging lens 73 includes a first lens L1 having a negative refractive power. The first lens L1 is made of resin and has a meniscus shape convex toward the subject 13 side. The second group G2 includes a second lens L2 having a positive refractive power. The second lens L2 is made of resin and has a biconvex shape.

  Table 13 shows data of the imaging optical system 71 configured as described above. As indicated by * in the surface number column of Table 13, in the imaging lens 73, all of both surfaces S4, S5, S7, and S8 of the first lens L1 and the second lens L2 are aspherical surfaces. Further, in the imaging lens 73, the second lens L2 is a diffractive lens, and the surface S8 on the image side of the second lens L2 is a diffractive surface as indicated by # in the surface number column of Table 13. Therefore, the image-side surface S8 of the second lens L2 is an aspheric surface and also a diffractive surface.

Since the shape of each surface of the imaging optical system 71 is expressed by the above-described equations (1) and (2), the aspheric coefficient A of the imaging optical system 71 that is an aspherical surface or a diffractive surface is used. _i is shown in Table 14, and the phase difference coefficient B _2j is shown in Table 15. Furthermore, the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging optical system 71 are shown in FIG.

  As can be seen from the above-described embodiments and examples, by providing a diffractive surface in the imaging optical system 17 of the capsule endoscope 11, a compact and wide-angle imaging optical system can be formed with a small number of two or three lenses. In addition, it is possible to easily correct chromatic aberration and distortion, particularly lateral chromatic aberration that becomes remarkable at a wide angle.

  The position where the diffractive surface is provided is not limited to the imaging lens, but may be provided at an arbitrary position of the imaging optical system including the dome and the cover glass. Since it is not necessary to interfere with the manufacture of other parts such as the element 18, it is most preferable that the diffractive surface is provided on the imaging lens as in the imaging optical systems of the above-described embodiments and examples.

  Further, as can be seen from the above-described embodiments and examples, the imaging lens not only corrects chromatic aberration satisfactorily by the diffractive surface, but also in order from the subject 13 side, a first group having a negative refractive power, an aperture stop, By being configured by the second group having positive refractive power, spherical aberration, astigmatism, and distortion can be easily and satisfactorily corrected.

Further, as shown in Examples 1, 2, 5 described above, the imaging optical system, when providing a diffraction surface to the rear of the aperture stop S6, the second-order coefficient B ₂ of the phase difference function φ is negative Since the value is determined to be a value, correction of chromatic aberration by the diffractive surface can be further improved. Similarly, as shown in Examples 3 and 4, in the case of providing the diffractive surface in front of the aperture stop S6, by determining the second-order coefficient B ₂ phase difference function φ to a positive value, the diffraction surface It is possible to further improve the correction of chromatic aberration due to the above.

  Resin materials can be easily molded into complex shapes such as aspherical shapes, but the range of selectable refractive indexes is narrow, and some resin materials exhibit birefringence, and are used as lenses. The resin material that can be produced is less than that of glass. For this reason, if it is going to comprise all the imaging lenses from a resin material, selection of a material is difficult, and it is difficult to correct various aberrations with sufficient balance rather than the case where a glass lens is used, and chromatic aberration tends to remain. However, as in the above-described embodiments and examples, in an imaging optical system provided with a diffractive surface, it is possible to easily and satisfactorily correct even chromatic aberration even if each lens of the imaging lens is made of a resin material. In addition, the imaging lens can be configured to be inexpensive, lightweight, and compact. At the same time, by using a lens made of a resin material for the imaging lens, when providing a diffractive surface on the imaging lens, it is possible to easily form a diffractive surface that has to have a complicated shape.

  Further, as shown in the above-described embodiments and examples, since the lens is mounted on the capsule endoscope 11, it is preferable that the angle of view of the imaging lens is as wide as possible while being compact. More preferably, the angle of view 2ω is 100 degrees or more as in the imaging lens described in the above-described embodiment and examples. Furthermore, it is particularly preferable that the angle of view 2ω is 130 degrees or more.

  Further, as described in the first to third embodiments, since the imaging lens is a lens mounted on the capsule endoscope 11, it is desired to be configured to be compact while having a wide angle. It is preferable to configure with three lenses as in Examples 1 to 3, and it is particularly preferable to configure with two lenses as in Examples 4 to 5 described above.

  In the above-described embodiment, an example of an imaging optical system that is optimal when the subject 13 is a spherical surface has been described. However, the present invention is not limited to this, and the actual imaging system in the body cavity of the subject viewed from the capsule endoscope 11 is more practical. An imaging optical system suitable for the case where the subject 13 has another curved surface shape that matches a specific shape may be used.

  In the above-described embodiments and examples, an example in which one diffractive surface is provided in the imaging optical system has been described. However, the present invention is not limited thereto, and a plurality of diffractive surfaces may be provided. However, as the number of diffractive surfaces increases, the imaging optical system becomes more expensive and flare is more likely to occur. Therefore, it is preferable to provide one diffractive surface as in the above-described embodiments and examples.

  In the above-described embodiment and examples, an example in which the cover glass 22 is a glass plate has been described. However, the present invention is not limited thereto, and the cover glass may be made of resin.

  In the above-described embodiments and examples, a description has been given of an example in which a relief is simply provided as a diffractive surface. However, the present invention is not limited to this, and in order to suppress generation of unnecessary light, irregularities are fitted to each other. Alternatively, a pair of lenses provided with a relief may be bonded to a diffractive lens, and the bonded surface may be a diffractive surface.

  In the above-described embodiments and examples, an example in which diffractive surfaces are provided at various positions of the imaging lens has been described. However, in order to better correct lateral chromatic aberration, the diffractive surface is located as far as possible from the aperture stop. Is preferably provided. Therefore, in the case where a diffractive surface is provided on the imaging lens and the surface on the object side of the aperture stop S6 is a diffractive surface, it is preferable to provide the diffractive surface on the most object side surface of the first group. Similarly, in the case where a diffractive surface is provided on the imaging lens and the image side surface of the aperture stop S6 is a diffractive surface, it is preferable to provide the diffractive surface on the most image side surface of the second group. . The same applies to the case where a diffractive surface is provided on the dome or cover glass.

  In the above-described embodiments and examples, the example in which the diffractive surface is formed by the relief of the concavo-convex shape has been described, but not limited to this, instead of the concavo-convex shape, a concentrically roughened slit is provided, The diffractive surface may be formed by forming a refractive index distribution concentrically. For this reason, “relief” does not only mean a physical uneven structure, but also means relief and distribution of optical properties that cause diffraction. Further, the diffractive surface is not limited to be provided on the surface of a lens or the like, and may be one that is processed by a laser inside the lens or the like.

  In the above-described embodiments and examples, an example in which a subject is a spherical surface and forms an image on a plane will be described, but the present invention is not limited to this. The imaging optical system and the imaging lens may be suitable for other shapes (cylindrical shape or the like) according to the shape in the body cavity of the subject. Further, the surface to be imaged may be an arbitrary shape instead of a flat surface.

  In the above-described embodiment, an example has been described in which the imaging lens is a lens having a two-group configuration with a diaphragm interposed therebetween. However, the present invention is not limited to this, and other group configurations such as another three-group configuration are used. It may be.

It is explanatory drawing which shows the structure of a capsule type endoscope roughly. 1 is a cross-sectional view illustrating a configuration of an imaging optical system according to Example 1. FIG. FIG. 6 is an aberration diagram of the image pickup optical system according to the first example. 6 is a cross-sectional view illustrating a configuration of an imaging optical system according to Example 2. FIG. FIG. 6 is an aberration diagram of the image pickup optical system according to the second embodiment. 6 is a cross-sectional view illustrating a configuration of an imaging optical system according to Example 3. FIG. 10 is an aberration diagram of the image pickup optical system according to Example 3. FIG. 6 is a cross-sectional view illustrating a configuration of an imaging optical system according to Example 4. FIG. FIG. 10 is an aberration diagram of the image pickup optical system according to the fourth embodiment. 10 is a cross-sectional view illustrating a configuration of an imaging optical system according to Example 5. FIG. FIG. 10 is an aberration diagram of the image pickup optical system according to the fifth embodiment.

Explanation of symbols

11 Capsule Endoscope 12 Capsule 13 Subject 16, 33, 43, 53, 63, 73 Imaging Lens 17, 31, 41, 51, 61, 71 Imaging Optical System 18 Imaging Device 21, 32, 42, 52, 62, 72 Dome (dome cover)
22, 34, 44, 54, 64, 74 Cover glass G1 1st group G2 2nd group L1 1st lens L2 2nd lens L3 3rd lens S6 Aperture stop

Claims (10)

In an imaging optical system for a capsule endoscope that is swallowed by a subject and used to image the inside of the body cavity of the subject,
The outer shape of the capsule endoscope, and a dome cover that is transparent and formed in a dome shape;
An imaging lens that forms an image on the imaging element of light incident from the body of the subject through the dome cover;
A parallel flat cover glass covering the imaging surface of the imaging element;
An imaging optical system comprising a diffractive surface on one of the surfaces of the dome cover, the imaging lens, and the cover glass.   The imaging optical system according to claim 1, wherein the diffractive surface is provided in the imaging lens.   The imaging optical system according to claim 1, wherein the imaging lens includes, in order from the object side, a first group having a negative refractive power, a diaphragm, and a second group having a positive refractive power. The diffractive surface is provided on the lens of the first group,
The imaging optical system according to claim 3, wherein B ₂ > 0 is satisfied when a second-order coefficient of a phase difference function representing the shape of the diffractive surface is B ₂ . The diffractive surface is provided on the lens of the second group,
The imaging optical system according to claim 3, wherein B ₂ <0 is satisfied when a second-order coefficient of a phase difference function representing the shape of the diffractive surface is B ₂ .   The imaging optical system according to claim 1, wherein the imaging lens includes two lenses.   The imaging optical system according to claim 1, wherein the imaging lens includes three lenses.   The imaging optical system according to claim 1, wherein all of the lenses constituting the imaging lens are made of a resin material.   9. The imaging optical system according to claim 1, wherein an angle of view of the imaging lens is 100 degrees or more.   A capsule endoscope comprising the imaging optical system according to claim 1.

Images