JP2009297307A - Imaging lens and capsule type endoscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens for a capsule type endoscope which achieves sufficient corrections in magnification chromatic aberrations and distortion while being a compact and wide-angle lens. <P>SOLUTION: The imaging lens 21 forms an image of light from a spherical subject 13 on an imaging surface of a planar image pickup device and comprises a first lens L1 having negative refractive power, a second lens L2 having positive refractive power, an aperture diaphragm S8, a third lens L3 having positive refractive power, and a fourth lens L4 having positive refractive power. A shape of the distal end of an capsule 12 and a cover glass 18 for the image pickup device 17 are taken into consideration in the shapes and arrangements of the lenses L1-L4 and the aperture diaphragm S8 constituting the imaging lens 21, and the first lens L1, wherein the second lens L2, the aperture diaphragm S8, the third lens L3, and the fourth lens L4 are disposed in this order from the side of the subject 13. <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-described problems, and provides a compact and wide-angle imaging lens for a capsule endoscope in which distortion and lateral chromatic aberration are sufficiently corrected even in a peripheral portion of an acquired image. 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 and a first lens having a positive refractive power. It comprises two lenses, a diaphragm, a third lens having a positive refractive power, and a fourth lens having a positive refractive power, and forms an object on a curved surface on a plane.

  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 fourth 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.

  According to the present invention, it is possible to provide an imaging lens for a capsule endoscope that is compact and has a wide angle and in which distortion and lateral chromatic aberration are sufficiently corrected even in a peripheral portion of an acquired image. In addition, by mounting this imaging lens, it is possible to provide a capsule endoscope that can obtain an image that can easily perform an appropriate diagnosis without overlooking or misidentifying a lesion.

  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. In addition, 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 has a first lens L1 having a negative refractive power, a second lens L2 having a positive refractive power, an aperture stop S8, a third lens L3 having a positive refractive power, and a positive refractive power. The fourth lens L4 includes a first lens L1, a second lens L2, an aperture stop S8, a third lens L3, and a fourth lens L4 in this order from the subject 13 side. When the imaging lens 16 has such a lens configuration, the chromatic aberration of magnification and the distortion can be corrected sufficiently satisfactorily and easily even though the imaging lens 16 is a wide-angle lens suitable for the capsule endoscope 11.

  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 limits the focal length of the first lens L1 in order to sufficiently correct the chromatic aberration of magnification and distortion. If the value of | f1 / f | exceeds the upper limit value, these should be corrected sufficiently. Becomes difficult.

  In addition, the first lens L1 is formed such that at least one of the object side (subject 13 side) surface and the image side (imaging element 17 side) surface is an aspherical surface. Similarly, in the fourth lens L4, at least one of the object side surface and the image side surface is formed as an aspherical surface. This makes it easy to sufficiently correct lateral chromatic aberration and distortion 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, a second lens L2 having a positive refractive power, an aperture stop S8, a third lens L3 having a positive refractive power, a positive lens The first lens L1, the second lens L2, the aperture stop S8, the third lens L3, and the fourth lens L4 are arranged in this order from the subject 13 side. Further, the surface shapes and arrangements of these lenses L1 to L4 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 subject 13 side (object side) surface of the capsule 12 is S2, the image side surface of the capsule 12 is S3, the subject 13 side surface of the first lens L1 is S4, The image side surface of one lens L1 is S5, the surface of the second lens L2 on the subject side S6, the image side surface of the second lens L2 is S7, the aperture stop is S8, and the third lens L3 is on the subject 13 side. The surface of the third lens L3 on the subject 13 side is S10, the surface of the fourth lens L4 on the subject 13 side is S11, the image side surface of the fourth lens L4 is S12, and the cover glass 18 is on the subject 13 side. The surface is S13, the image side surface of the cover glass 18 is S14, and is represented by a surface Si (surface number i = 1 to 14). Also, the distance (surface distance) on the optical axis between the surface Si and the image-side surface Si + 1 adjacent thereto is represented by Di (i = 1 to 13). Note that the image pickup surface of the image pickup element 17 coincides with a surface S14 that is the image side surface of the cover glass 18. In addition, the notation of each surface and surface interval of the imaging lens is the same for Example 2 and Example 3 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, the lenses L1 to L4 constituting the imaging lens 21 are both aspheric on both sides. 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 each surface Si. It should be noted that the notation of lens data and the equation of Formula 1 that defines the shape of the aspheric surface are the same for 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, although the imaging lens 21 is a compact and wide-angle lens, the chromatic aberration of magnification and distortion are sufficiently corrected, and the imaging lens 21 is an imaging lens suitable for the capsule endoscope 11.

  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 | = 1.2 and 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 fourth lens L4 has a surface configuration including an aspherical surface.

<Example 2>
As shown in FIG. 4, the imaging lens 31 includes a first lens L1 having a negative refractive power, a second lens L2 having a positive refractive power, an aperture stop S8, a third lens L3 having a positive refractive power, a positive lens The first lens L1, the second lens L2, the aperture stop S8, the third lens L3, and the fourth lens L4 are arranged in this order from the subject 13 side. The surface shape and arrangement of these lenses L1 to L4 are in consideration of 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, all of the lenses L1 to L4 constituting the imaging lens 31 have an aspherical shape represented by the formula 1 described above. Table 4 shows the conic constant K and the aspheric coefficient Ai of each surface Si of the imaging lens 31. 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 is a compact and wide-angle lens and has a sufficiently corrected lateral chromatic aberration and distortion, and is suitable for the capsule endoscope 11. It has become.

  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 | = 3.0 and satisfies the condition of | f1 / f | <5. For this reason, the imaging lens 31 can correct the lateral chromatic aberration and distortion particularly well.

  The imaging lens 31 has all aspheric surfaces on both surfaces of the lenses L1 to L4. However, since the first lens L1 has a surface configuration including an aspheric surface, lateral chromatic aberration and distortion are corrected. Making it easy. Similarly, in the imaging lens 31, the fourth lens L4 has a surface configuration including an aspheric surface, thereby facilitating correction of lateral chromatic aberration and distortion.

<Example 3>
As shown in FIG. 6, the imaging lens 41 includes a first lens L1 having a negative refractive power, a second lens L2 having a positive refractive power, an aperture stop S8, a third lens L3 having a positive refractive power, a positive lens The first lens L1, the second lens L2, the aperture stop S8, the third lens L3, and the fourth lens L4 are arranged in this order from the subject side. Further, the surface shapes and arrangements of these lenses L1 to L4 take into account the shape of the front end of the capsule 12 and the cover glass 18 of the image sensor 17.

  Table 5 shows lens data of the imaging lens 41. As indicated by * in the surface number column of Table 5, all of the lenses L1 to L4 constituting the imaging lens 41 have an aspherical shape represented by the formula 1 described above. Table 6 shows the conic constant K and the aspherical coefficient Ai of each surface Si of the imaging lens 41. Further, FIG. 7 shows aberration diagrams of the imaging lens 41. The notation of each surface Si and the surface interval Di and the aberration diagram of the imaging lens 41 are the same as those in the first embodiment.

  As can be seen from Tables 5 and 6 and FIG. 7, the imaging lens 41 is a compact and wide-angle lens and has a sufficiently corrected lateral chromatic aberration and distortion, and is suitable for the capsule endoscope 11. It has become.

  As shown in Table 5, the imaging lens 12 is a wide-angle lens having an angle of view 2ω of 150 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.

  As can be seen from Table 5, the imaging lens 41 satisfies | f1 / f | = 2.7, 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.

  The imaging lens 41 has both aspheric surfaces on all the lenses L1 to L4. In particular, the first lens L1 has a surface configuration including an aspheric surface, thereby correcting the lateral chromatic aberration and distortion. Making it easy. Similarly, in the imaging lens 41, the fourth lens L4 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 Image pick-up lens 17 Image pick-up element L1 1st lens L2 2nd lens S8 Aperture stop L3 3rd lens L4 4th 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 a negative refractive power, a second lens having a positive refractive power, a diaphragm, a third lens having a positive refractive power, and a fourth lens having a positive refractive power are provided. An imaging lens characterized in that an image of a subject is formed 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 fourth lens is an aspherical surface.   A capsule endoscope comprising the imaging lens according to claim 1.

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