JP2009300796A - Imaging lens and capsule type endoscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens for capsule type endoscope, the imaging lens being sufficiently corrected in magnification color aberration and distortion and compact despite a wide-angle lens. <P>SOLUTION: The imaging lens 21 is a lens that images light from a spherical subject 13 onto the imaging surface of a flat imaging element. The imaging lens includes: a first lens L1 of negative refractive power; an aperture stop S6; a second lens L2 of positive refractive power; a third lens L3 of negative refractive power; a fourth lens L4 of positive refractive power; and a fifth lens L5 of positive refractive power. The lenses L1 to L5 and aperture stop S6 composing the imaging lens 21 are shaped and disposed taken into account of the shape of the front end of a capsule 12 and a cover glass 18 of the imaging element 17. They are disposed in the following order from the subject 13 side: the first lens L1, aperture stop S6, second lens L2, third lens L3, fourth lens L4, and fifth lens L5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an imaging lens used for an endoscope for imaging the inside of a living body, and more particularly to an imaging lens used for a capsule endoscope used by swallowing.

    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 a subject, and images the inside of the subject with an imaging portion provided at the distal end of the insertion portion. The insertion portion is freely curved according to the shape of the body cavity of the subject, and the direction of the distal end 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.

  By the way, in the diagnosis using the insertion type endoscope, since a long insertion portion must be continuously inserted into the body for a long time, the load on the subject is very large. Therefore, in recent years, in order to reduce the load on the subject, the imaging device is housed in a capsule that can be easily swallowed, and the capsule swallowed by the subject naturally 2. Description of the Related Art A capsule endoscope that captures an image of the inside of a subject while moving is known.

  When a capsule endoscope is swallowed, it moves naturally within the subject's body, so the burden on the subject is greatly reduced compared to the insertion endoscope, but the position within the subject's body It is difficult to control the posture. For this reason, the imaging device mounted on the capsule endoscope is mounted with a wide-angle imaging lens so that it can capture the widest possible range without changing the posture.

  It is also desirable to make the capsule size as small as possible so that the capsule endoscope can be swallowed easily. For this reason, the imaging lens mounted on many capsule endoscopes is not only wide-angled, but also has a two-group, two-lens configuration, so that the imaging lens is made compact with a small number of lenses. To fit in a small capsule.

As such a capsule endoscope, one having a smaller capsule size while ensuring a wide imaging range is known (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.
Japanese Patent Laid-Open No. 2005-80789 JP 2005-80790 A JP 2006-61438 A

  The larger the angle of view, the more noticeable the effects of aberrations such as distortion and lateral chromatic aberration occur in the periphery of the acquired image, but with a wide-angle imaging lens that consists of only a few lenses to make the capsule size compact. It is difficult to sufficiently correct these various aberrations.

  In addition, when a medical examination is performed based on an image taken with an imaging lens that is wide-angle but distortion and magnification chromatic aberration are not sufficiently corrected, the portion displayed in the periphery of the acquired image is slightly distorted or color blurring occurs. Therefore, it is easy to cause oversight or misidentification of the lesion, and the imaging lens may have a wide angle, which may hinder an appropriate diagnosis.

  The present invention has been made in view of the above-mentioned problems, and provides a compact imaging lens for a capsule endoscope that has a wide angle, sufficiently corrects aberrations in the peripheral portion of an acquired image, and is compact. For the purpose.

  The imaging lens of the present invention is an imaging lens for a capsule endoscope disposed in a dome-shaped capsule, and in order from the subject side, a first lens having a negative refractive power, a diaphragm, and a positive refractive power. A second lens having a negative refracting power, a fourth lens having a positive refracting power, and a fifth lens having a positive refracting power, and imaging a curved object on a plane. And

  Further, when the half angle of view is ω (degrees), 2ω> 135 is satisfied.

  Further, when the focal length of the entire system is f and the focal length of the first lens is f1, | f1 / f | <5 is satisfied.

  Further, the first lens is characterized in that at least one surface is aspherical.

  The fifth lens is characterized in that at least one surface is aspherical.

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

  An object of the present invention is to provide a compact imaging lens for a capsule endoscope that has a wide angle, sufficiently corrects aberrations even in the peripheral portion of an acquired image. In addition, by mounting this imaging lens, it is possible to easily perform an appropriate diagnosis without overlooking or misidentifying a lesion, and to provide a compact capsule endoscope that is easy to swallow.

  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. The subject 13) is imaged.

  The capsule 12 is a hollow container in which both ends of a cylinder are formed in a dome shape, and is provided in a size that can be swallowed easily. In the capsule 12, an imaging lens 16, an imaging element 17, an illumination element (not shown) constituting the imaging apparatus, a battery for driving the imaging apparatus, an antenna for transmitting the captured image to the outside, and the like are stored in a watertight manner. It has been. Further, the front end of the capsule 12 is not only formed in a dome shape, but is made of a transparent material in a sufficiently wide range in accordance with the angle of view of the imaging lens 16. Illumination light from the illumination element irradiates the subject 13 through the transparent portion of the capsule 12. Further, light from the subject 13 enters the imaging lens 16 through this transparent portion.

  The imaging lens 16 focuses light from the spherical subject 13 on the imaging surface of the imaging element 17 that is a plane. The imaging lens 16 includes a first lens L1 having a negative refractive power, a stop S6, a second lens L2 having a positive refractive power, a third lens L3 having a negative refractive power, and a first lens having a positive refractive power. The first lens L1, the diaphragm S6, the second lens L2, the third lens L3, the fourth lens L4, and the fifth lens L5 are configured from the fourth lens L4 and the fifth lens L5 having positive refractive power. Arranged in this order. By adopting such a lens configuration for the imaging lens 16, the chromatic aberration of magnification and distortion are sufficient even in the peripheral portion of the acquired image even though the lens is compact and is a wide-angle lens suitable for the capsule endoscope 11. Is corrected well. In particular, since the stop S6 is disposed between the first lens L1 and the second lens L2, it is easy to construct a compact while maintaining good optical performance.

  The imaging lens 16 is a wide-angle lens that satisfies 2ω> 135 when the half angle of view is ω (degrees). For this reason, even in the capsule endoscope 11 whose posture is difficult to control, the body of the subject can be imaged without omission.

  Further, the imaging lens 16 is configured to satisfy | f1 / f | <5, where f is the focal length of the entire system and f1 is the focal length of the first lens L1. This conditional expression reduces the focal length of the first lens L1 in order to sufficiently and easily correct various aberrations such as lateral chromatic aberration and distortion in the peripheral portion of the acquired image, while making the entire imaging lens 16 compact. It is a limitation. Therefore, if the value of | f1 / f | exceeds the upper limit, it is difficult to satisfactorily correct various aberrations in the peripheral portion of the imaging range (acquired image) even if the imaging lens 16 can be widened. become.

  In addition, the first lens L1 is formed such that at least one of the object side (subject 13 side) surface or the image side (imaging device 17 side) surface is an aspherical surface. Similarly, in the fifth lens L5, at least one of the object side surface and the image side surface is an aspherical surface. This makes it easy to sufficiently correct various aberrations even in the peripheral portion of the acquired image.

  Examples 1 to 3 will be described below as an example of the imaging lens 16 having specific numerical values.

<Example 1>
As shown in FIG. 2, the imaging lens 21 includes a first lens L1 having a negative refractive power, an aperture stop S6, a second lens L2 having a positive refractive power, a third lens L3 having a negative refractive power, a positive A fourth lens L4 having a refractive power of 5 and a fifth lens L5 having a positive refractive power. The lenses L1 to L5 and the aperture stop S6 are arranged in order of the first lens L1, the aperture stop S6, the second lens L2, the third lens L3, the fourth lens L4, and the fifth lens L5 from the subject 13 side. The third lens L3 and the fourth lens L4 are bonded lenses. In addition, the surface shapes and arrangement of the lenses L1 to L5 take into account the shape of the front end of the capsule 12 and the cover glass 18 of the image sensor 17.

  Therefore, the spherical subject surface is S1, the surface of the capsule 12 on the subject 13 side is S2, the surface of the capsule 12 on the image side is S3, the surface of the first lens L1 on the subject 13 side is S4, and the surface of the first lens L1. The surface on the image side is S5, the aperture stop is S6, the surface on the subject 13 side of the second lens L2 is S7, the surface on the image side of the second lens L2 is S8, the surface on the subject 13 side of the third lens L3 is S9, The bonding surface of the third lens L3 and the fourth lens L4 is S10, the image side surface of the fourth lens L4 is S11, the surface of the fifth lens L5 on the subject side S12, and the image side surface of the fifth lens L5. Is S13, the surface of the cover glass 18 on the subject 13 side is S14, the image side surface of the cover glass 18 is S15, and is represented by a surface Si (surface number i = 1 to 15). Also, Di (i = 1 to 14) represents an interval (surface interval) on the optical axis between the surface Si and the image-side surface Si + 1 adjacent thereto. Note that the image pickup surface of the image pickup device 17 coincides with S15 which is the image side surface of the cover glass 18. In addition, the manner of expressing each surface and the surface interval of the imaging lens is the same in the second and third embodiments described later.

  As lens data of the imaging lens 21 configured as described above, the curvature radius Ri (mm) of each surface Si, the distance between surfaces Di (mm), the refractive index Nd and the Abbe number νd with respect to the d-line (wavelength 587.6 nm). Table 1 shows. At the same time, the lower part of Table 1 also shows the F value of the imaging lens 21, the focal length f of the entire system, the focal length f1 of the first lens L1, and the angle of view 2ω (degrees). The lens data of the imaging lens 21 shown in Table 1 is lens data when standardized so that the focal length f of the entire system is 1.0.

Further, as indicated by * in the surface number column of Table 1, both surfaces S4 and S5 of the first lens L1, both surfaces S7 and S8 of the second lens L2, and both surfaces S12 and S13 of the fifth lens L5 are aspheric. ing. These aspherical shapes are expressed by using the curvature (reciprocal of the paraxial radius of curvature R) c, the conic constant K, the distance ρ (ρ ² = x ² + y ² ) from the optical axis, and the i-th order aspheric coefficient Ai. It is represented by the following formula 1. Table 2 shows the conic constant K and the aspheric coefficient Ai of the surfaces S4, S5, S7, S8, S12, and S13. Note that the notation of lens data and the expression of Formula 1 that determines the shape of the aspherical surface are the same in Example 2 and Example 3 described later.

  Furthermore, the spherical aberration, astigmatism, lateral chromatic aberration, and distortion of the imaging lens 21 are shown in FIG. In each of these aberration diagrams, the capsule 12 and the cover glass 18 are arranged. The spherical aberration is indicated by a solid line for the d-line (wavelength 587.6 nm), a broken line for the F-line (wavelength 486.13 nm), and a two-dot chain line for the C-line (wavelength 656.27 nm). Astigmatism is indicated by a solid line in the sagittal direction and by a broken line in the tangential direction. 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. The notation of these aberration diagrams is the same for Example 2 and Example 3 described later.

  For distortion, a lens that forms an image of a planar object on a plane usually has a difference between the actual image height H and the difference ΔH between the actual image height H and the ideal image height without distortion, ΔH / H × It is expressed in 100 (%). However, since the imaging lens 21 forms an image of the spherical subject 13 on a plane and originally forms an image by changing the shape of the subject 13, the distortion of the imaging lens 21 is evaluated in the above-described evaluation. The normal way of expressing distortion is not suitable. For this reason, in FIG. 3, (Y (ω + Δω) −Y (ω)) / (Y (0 + Δω) −Y (0)) = (Y (ω + Δω) −Y (ω)) / Y (Δω) is shown as distortion of the imaging lens 21. Here, Y (ω) represents an image height at a certain imaging angle ω from the optical axis, and Δω represents a minute change amount of the imaging angle ω. This distortion is expressed in the same way in Example 2 and Example 3 described later.

  As described above, the imaging lens 21 has a wide angle, aberrations are sufficiently corrected even in the peripheral portion of the acquired image, and is configured compactly, and is an imaging lens suitable for the capsule endoscope 11. ing.

  As shown in Table 1, the imaging lens 12 is a wide-angle lens having an angle of view 2ω of as much as 170 degrees, and satisfies the condition of 2ω> 135 (degrees). Therefore, it is suitable for a capsule endoscope whose posture is difficult to control in the body of the subject.

  Furthermore, as can be seen from Table 1, the imaging lens 21 satisfies | f1 / f | = 1.6, which satisfies the condition of | f1 / f | <5. For this reason, the imaging lens 21 can correct the lateral chromatic aberration and distortion particularly well.

  In addition, the imaging lens 21 facilitates correction of lateral chromatic aberration and distortion because the first lens L1 has a surface configuration including an aspherical surface. Similarly, the imaging lens 21 makes it easy to correct lateral chromatic aberration and distortion because the fifth lens L5 has a surface configuration including an aspherical surface. Further, since the third lens L3 and the fourth lens L4 are a cemented lens, the lateral chromatic aberration is corrected more favorably.

<Example 2>
As shown in FIG. 4, the imaging lens 31 includes a first lens L1 having a negative refractive power, an aperture stop S6, a second lens L2 having a positive refractive power, a third lens L3 having a negative refractive power, a positive A fourth lens L4 having a refractive power of 5 and a fifth lens L5 having a positive refractive power. The lenses L1 to L5 and the aperture stop S6 are arranged in order of the first lens L1, the aperture stop S6, the second lens L2, the third lens L3, the fourth lens L4, and the fifth lens L5 from the subject 13 side. The third lens L3 and the fourth lens L4 are bonded lenses. In addition, the surface shapes and arrangement of the lenses L1 to L5 take into account the shape of the front end of the capsule 12 and the cover glass 18 of the image sensor 17.

  Table 3 shows lens data of the imaging lens 31. As indicated by * in the surface number column in Table 3, both surfaces S4 and S5 of the first lens L1 and both surfaces S12 and S13 of the fifth lens L5 are aspheric. Table 4 shows the conic constant K and the aspheric coefficient Ai of each aspherical surface Si. Further, FIG. 5 shows aberration diagrams of the imaging lens 31. The notation of each surface Si and the surface interval Di and the aberration diagram of the imaging lens 31 are the same as those in the first embodiment.

  As can be seen from Tables 3 and 4 and FIG. 5, the imaging lens 31 has a wide angle, aberration is sufficiently corrected even in the peripheral portion of the acquired image, and is configured compactly. It is an imaging lens suitable for.

  As shown in Table 3, the imaging lens 31 is a wide-angle lens having an angle of view 2ω of 170 degrees and satisfies the condition of 2ω> 135 (degrees). Therefore, it is suitable for a capsule endoscope whose posture is difficult to control in the body of the subject.

  Further, as can be seen from Table 3, the imaging lens 31 satisfies | f1 / f | = 1.6, which satisfies the condition of | f1 / f | <5. For this reason, the imaging lens 31 can correct the lateral chromatic aberration and distortion particularly well.

  In addition, the imaging lens 31 facilitates correction of lateral chromatic aberration and distortion because the first lens L1 has a surface configuration including an aspherical surface. Similarly, the imaging lens 31 facilitates correction of lateral chromatic aberration and distortion by the fifth lens L5 having a surface configuration including an aspherical surface. Further, since the third lens L3 and the fourth lens L4 are a cemented lens, the lateral chromatic aberration is corrected more favorably.

<Example 3>
As shown in FIG. 6, the imaging lens 41 includes a first lens L1 having negative refractive power, an aperture stop S6, a second lens L2 having positive refractive power, a third lens L3 having negative refractive power, a positive A fourth lens L4 having a refractive power of 5 and a fifth lens L5 having a positive refractive power. The lenses L1 to L5 and the aperture stop S6 are arranged in order of the first lens L1, the aperture stop S6, the second lens L2, the third lens L3, the fourth lens L4, and the fifth lens L5 from the subject 13 side. ing. The surface shapes and arrangement of these lenses L1 to L5 are in consideration of the shape of the front end of the capsule 12 and the cover glass 18 of the image sensor 17.

  Unlike the first and second embodiments described above, in the imaging lens 41, the third lens L3 and the fourth lens L4 are not bonded lenses. S2 is the side surface, S3 is the image side surface of the capsule 12, S4 is the surface on the subject 13 side of the first lens L1, S5 is the image side surface of the first lens L1, S6 is the aperture stop, and the second lens L2 The surface on the subject 13 side is S7, the image side surface of the second lens L2 is S8, the surface on the subject 13 side of the third lens L3 is S9, the image side surface of the third lens L3 is S10, and the fourth lens L4. The surface of the subject 13 side is S11, the image side surface of the fourth lens L4 is S12, the surface of the fifth lens L5 on the subject 13 side is S13, the image side surface of the fifth lens L5 is S14, and the cover glass 18 The surface on the subject 13 side is S15 and the cover glass 18 is And S16 the surface on the side, expressed in terms Si (surface number i = 1 to 16). Also, Di (i = 1 to 15) represents an interval (surface interval) on the optical axis between the surface Si and the image-side surface Si + 1 adjacent thereto. The image pickup surface of the image pickup device 17 coincides with S <b> 16 that is the image side surface of the cover glass 18.

  Table 5 shows lens data of the imaging lens 41. Further, as indicated by * in the surface number column of Table 5, both surfaces S4 and S5 of the first lens L1 and both surfaces S13 and 14 of the fifth lens L5 are aspherical shapes represented by the above-described equation (1). ing. Table 6 shows the conic constant K and the aspheric coefficient Ai of the imaging lens 41. Further, FIG. 7 shows aberration diagrams of the imaging lens 41. The notation of the aberration diagram is the same as in Example 1.

  As can be seen from Tables 5 and 6 and FIG. 7, the imaging lens 41 has a wide angle, aberrations are sufficiently corrected even in the peripheral portion of the acquired image, and is configured to be compact. The capsule endoscope 11 It is an imaging lens suitable for.

  As shown in Table 5, the imaging lens 12 is a wide-angle lens having an angle of view 2ω of 170 degrees and satisfies the condition of 2ω> 135 (degrees). Therefore, it is suitable for a capsule endoscope whose posture is difficult to control in the body of the subject.

  Furthermore, as can be seen from Table 5, the imaging lens 41 satisfies | f1 / f | = 1.6, which satisfies the condition of | f1 / f | <5. For this reason, the imaging lens 41 can correct the lateral chromatic aberration and distortion particularly well.

  In addition, the imaging lens 41 facilitates correction of lateral chromatic aberration and distortion because the first lens L1 has a surface configuration including an aspherical surface. Similarly, in the imaging lens 41, the fifth lens L5 has a surface configuration including an aspheric surface, thereby facilitating correction of lateral chromatic aberration and distortion.

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

It is explanatory drawing which shows the structure of a capsule type endoscope roughly. 2 is a cross-sectional view of an imaging lens of Example 1. FIG. FIG. 3 is an aberration diagram of the imaging lens of Example 1. 6 is a cross-sectional view of an imaging lens of Example 2. FIG. 6 is an aberration diagram of the imaging lens of Example 2. FIG. 6 is a cross-sectional view of an imaging lens of Example 3. FIG. 6 is an aberration diagram of the imaging lens of Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Capsule type endoscope 12 Capsule 13 Subject 16, 21, 31, 41 Imaging lens 17 Imaging element L1 1st lens S6 Aperture stop L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens 18 Cover glass

Claims (6)

In an imaging lens for a capsule endoscope placed in a dome-shaped capsule,
In order from the subject side, a first lens having negative refractive power, a diaphragm, a second lens having positive refractive power, a third lens having negative refractive power, a fourth lens having positive refractive power, and positive refraction An imaging lens comprising a fifth lens having power and imaging a curved object on a plane. When the half angle of view is ω (degrees),
2ω> 135
The imaging lens according to claim 1, wherein: When the focal length of the entire system is f and the focal length of the first lens is f1,
| F1 / f | <5
The imaging lens according to claim 1 or 2, wherein:   The imaging lens according to claim 1, wherein at least one surface of the first lens is an aspherical surface.   The imaging lens according to claim 1, wherein at least one surface of the fifth lens is an aspheric surface.   A capsule endoscope comprising the imaging lens according to claim 1.

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