JP2009015253A - Optical device, optical system with the same and endoscope using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive optical device capable of imaging an omnidirectional image in the direction substantially orthogonally crossing with a central axis on an imaging device in a simple constitution, and being a small type having less strain in image and favorable F-θ characteristics; to provide an optical system with the same; and to provide an endoscope using the same. <P>SOLUTION: The optical device consists of a transparent medium L1 having a field angle of one side approximately 90° about the direction orthogonal to the central axis 2 in the cross section including the central axis 2, being rotation symmetry about the central axis 2 and having a refraction index larger than 1, wherein the transparent medium L1 has a first transmission face 11 disposed in the outermost circumference from the central axis, a first reflecting surface 12 disposed closer to the central axis 2 than the first transmission face 11, a second reflecting surface 13 disposed in the opposite side of an image plane 5 relative to the first reflecting surface 12, and a second transmission face 14 disposed closer to the image plane 5 than the second reflecting surface 13. The optical device is characterized in that an optical flux entering the transparent medium L1 passes the first transmission face 1, the first reflecting face 12, the second reflecting face 13 and the second transmission face 14 in the order of tracing a normal light and outgoes to the side of image plane 5, wherein the optical path A consists of one side alone relative to the central axis 2. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical element, an optical system including the optical element, and an endoscope using the optical element, and in particular, an imaging optical system having a function of forming an image around a rotationally symmetric axis as an annular image on an image sensor. The present invention relates to a projection optical system.

Patent Document 1 discloses an imaging optical system in which a refractive optical system, a reflection optical system, and an imaging optical system are arranged and has two optical paths and can capture panoramic images and axial images. Similarly, there is Patent Document 2 as an endoscope having two optical paths. Further, there is Patent Document 3 as an endoscope that can observe all surrounding directions, and Patent Document 4 as a capsule endoscope that can observe all surrounding directions. Further, Patent Document 5 is an imaging apparatus capable of simultaneously imaging all surrounding directions and the front. Further, there are Patent Documents 6 to 13 as omnidirectional optical systems.
Special Table 2003-042743 US Patent Publication No. 2004-0254424 JP 60-42728 A JP 2001-174713 A JP 2002-341409 A U.S. Pat. No. 3,505,465 US Pat. No. 5,854,713 US Pat. No. 6,115,193 US Pat. No. 6,175,454 US Pat. No. 6,356,296 US Pat. No. 6,392,687 US Pat. No. 6,449,103 US Pat. No. 6,597,520

  However, the optical systems described in any of the patent documents cannot obtain an image with a small size and good resolution.

  The present invention has been made in view of such a situation in the prior art, and an object of the present invention is to be able to take an image of an omnidirectional image in a direction substantially orthogonal to the central axis on an image sensor with a simple configuration. It is an object of the present invention to provide a compact optical element with low image distortion and good F-θ characteristics, high resolving power and low cost, an optical system including the optical element, and an endoscope using the optical element.

  The optical element of the present invention that achieves the above object has an angle of view of about 90 ° on one side about a direction orthogonal to the central axis in a cross section including the central axis, and a refractive index that is rotationally symmetric around the central axis. Is made of a transparent medium greater than 1, the transparent medium is a first transmission surface disposed on the outermost periphery with respect to the central axis, a first reflection surface disposed on the central axis side from the first transmission surface, A second reflecting surface disposed on the opposite side of the image surface from the first reflecting surface; and a second transmitting surface disposed on the image surface side of the second reflecting surface, and is incident on the transparent medium. The luminous flux enters the transparent medium through the first transmission surface in the order of forward ray tracing, is reflected on the opposite side to the image surface by the first reflection surface, and is reflected on the image surface side by the second reflection surface. A substantially Z-shaped optical path exiting from the transparent medium to the image plane side through the second transmission surface, and the optical path is the central axis Characterized in that it is composed of only one side for.

When the angular magnification of the meridional section of the optical element is βω,
0.01 <βω <0.5 (1)
It satisfies the following condition.

Further, when the power of the meridional section of the second transmission surface is negative, the power of the second transmission surface is P2, and the power of the entire meridional section of the central principal ray is Pm,
−10 <P2 / Pm <−1 (2)
It satisfies the following condition.

  The optical element is formed of a transparent medium having a refractive index of 1.5 or more.

  Further, at least one of the first reflection surface and the second reflection surface has a total reflection function.

  Further, at least one of the first reflecting surface and the second reflecting surface is composed of an extended rotation free-form surface formed by rotating an arbitrary-shaped line segment having no symmetry plane around the central axis. It is characterized by being.

  In addition, at least one of the surfaces of the transparent medium is formed of an extended rotation free-form surface that is formed by rotating an arbitrary-shaped line segment including an odd-numbered term around the central axis. .

  Furthermore, an optical system including the optical element of the present invention that achieves the above object is disposed between the front group, the rear group disposed on the image plane side from the front group, and the front group and the rear group. The optical element is disposed in the front group and forms an image of an object disposed so as to surround the central axis, or projects an image of the object in a direction radiating from the central axis It is characterized by that.

  The optical system is characterized in that an image of an annular object around the central axis is formed in a plane orthogonal to the central axis.

  The first reflecting surface and the second reflecting surface may be disposed with a concave surface facing the opening.

  Further, the second transmission surface is arranged with a concave surface facing the opening side.

  Further, the optical system does not form an intermediate image in the optical path.

  The optical element may be disposed on the opposite side of the image plane with respect to the aperture.

  The optical element has a direct-view optical path for observing immediately before the optical element and a side-view optical path for observing a direction perpendicular to the central axis, and connects the images of the direct-view optical path and the side-view optical path in the same plane. It is characterized by imaging.

When the outer diameter of the optical element is D and the outer diameter of the image is Dr,
D / Dr <10 (3)
It satisfies the following condition.

The outer diameter D of the optical element is
D <20mm (4)
It satisfies the following condition.

  Furthermore, the present invention for achieving the above object is an endoscope using the optical system.

  In the above optical system of the present invention, it is possible to obtain a compact optical system with good resolving power that can be observed in a different direction or projected an image in a different direction with a simple configuration and in which aberrations are well corrected.

  Hereinafter, an optical element of the present invention and an optical system including the optical element will be described based on examples.

  FIG. 3 is a cross-sectional view taken along the central axis (rotation symmetry axis) 2 of the optical system 1 of Example 1 described later. In the following description, the imaging optical system will be described. However, it can be used as a projection optical system with the optical path reversed.

    The optical system 1 according to the first exemplary embodiment includes a front group Gf having a negative power and rotational symmetry with respect to a central axis 2, an aperture S, and a rear group Gb having a positive power. This is an optical system 1 that forms or projects an image without forming the image. The parallel flat plate near the image plane 5 is a cover glass C of the image sensor or the like.

  Also, by setting the front group Gf to be negative and the rear group Gb to be positive, a so-called retrofocus type is obtained, which is particularly effective when a wide observation angle of view is desired.

  The optical element of the present invention has an angle of view of about 180 ° on one side about a direction orthogonal to the central axis 2 within a cross section including the central axis 2, and a rotationally symmetric refractive index around the central axis 2. The transparent medium L1 is composed of a larger transparent medium L1. The transparent medium L1 includes a first transmission surface 11 disposed on the outermost periphery with respect to the central axis 2, and a first reflection surface 12 disposed on the central axis side from the first transmission surface. The second reflecting surface 13 disposed on the opposite side of the image surface 5 with respect to the first reflecting surface 12, and the second transmitting surface 14 disposed on the image surface 5 side with respect to the second reflecting surface 13. The light beam incident on the transparent medium L1 enters the transparent medium L1 via the first transmission surface 11 in the order of forward ray tracing, and is reflected by the first reflection surface 12 on the side opposite to the image surface 5 to be second reflection surface. 13 constitutes a substantially Z-shaped optical path A which is reflected to the image plane 5 side and exits from the transparent medium L1 to the image plane 5 side through the second transmission surface 14. Optical path A is configured on only one side with respect to the central axis 2.

  With this configuration, it is possible to make the incident angle of the optical path A to the first reflecting surface 12 and the second reflecting surface 13 relatively small while having a large angle of view of about 90 ° on one side, and eccentricity generated on the reflecting surface. Occurrence of aberration can be reduced. Further, by configuring the optical path A only on one side of the central axis, the optical path in the optical element does not straddle the central axis 2, and the optical element can be made thin.

Furthermore, when the angular magnification of the meridional section of the optical element of the present invention is βω,
0.01 <βω <0.5 (1)
It is preferable that this condition is satisfied. This condition represents the reduction angle magnification in the meridional section, and if the lower limit of the conditional expression (1) is exceeded, the burden on the optical element of the present invention increases, and the optical element Becomes abnormally large, or requires expensive glass having an abnormally high refractive index. If the upper limit of conditional expression (1) is exceeded, the effect of reducing the angle of view of the optical element is lost, and the angle of view incident on the rear group becomes wider, increasing the burden on the rear group.

More preferably, 0.02 <βω <0.2 (1-1)
It is better to satisfy the following conditions.

When the paraxial power of the second transmission surface 14 is P, and the power of the meridional section of the entire optical system of the central principal ray is Pm,
−10 <P2 / Pm <−1 (2)
It is preferable to satisfy the following conditions. Conditional expression (2) represents the ratio of the second transmission surface 14 to the total power. When the lower limit of conditional expression (2) is exceeded, the negative power of the second transmission surface 14 becomes stronger. However, it is possible to reduce the angular magnification, but if it is too small, coma and astigmatism occurring on this surface will become too large, and correction on other surfaces becomes impossible. . If the upper limit of conditional expression (2) is exceeded, the negative power of the second transmission surface 14 becomes too small, and the angular magnification of the meridional section cannot be increased (the reduction ratio is increased) similarly to conditional expression (1). The burden on the group increases and the optical system becomes large.

  Further, when the optical element is a medium having a refractive index of 1.5 or more and the reflecting surface is configured by an internal reflecting surface, the occurrence of aberration is reduced as compared with the reflecting surface. With this configuration, it is possible to configure a small and high-resolution optical system, and at the same time, it is possible to configure the two reflecting surfaces integrally, which is preferable in terms of assembly and adjustment.

  More preferably, it is preferably composed of a medium having a refractive index of 1.7 or more. Increasing the refractive index can decrease the curvature of the internal reflecting surface (increase the radius of curvature). Especially when it is configured as a decentration optical system as in the present invention, the occurrence of decentration aberrations can be reduced and the resolution is improved. To do.

  More preferably, by setting the refractive index to 1.8 or more, the critical angle becomes 33 degrees. In particular, the first reflection surface can be configured as a total reflection surface, and it is not necessary to perform a reflection coating. It is preferable in terms of processing and loss of light amount.

  In addition, since at least one of the first reflecting surface 12 and the second reflecting surface 13 has a total reflection function, it is not necessary to attach a reflecting film, and the manufacturing becomes easy and the reflectance becomes 100%. Bright images can be taken.

  In addition, at least one of the first reflecting surface 12 and the second reflecting surface 13 is composed of an extended rotation free-form surface formed by rotating an arbitrary-shaped line segment having no symmetry plane around the central axis 2. By doing so, it becomes possible to correct the distortion around the angle of view.

  In addition, at least one of the surfaces of the transparent medium L1 is formed of an extended rotation free-form surface that is formed by rotating an arbitrary-shaped line segment including an odd-order term around the central axis 2. It is possible to give a vertically asymmetric shape with respect to the corner center, which is preferable in terms of aberration correction.

  In this embodiment, the two reflecting surfaces having a reflecting action both have a concave surface facing the opening S. As a result, the power arrangement of the reflecting surface becomes a negative-positive arrangement, and the power arrangement of the reflecting surface of the optical element itself becomes a so-called retrofocus type, which makes it easy to obtain a wide angle of view and reduce the occurrence of coma aberration. It becomes.

  The second transmission surface is preferably a surface having a strong negative power with the concave surface facing the opening side. By reducing the angle of view emitted from the optical element through this surface, the angle of view incident on the rear group is reduced, reducing the burden of aberration correction on the rear group, and the overall optical system is small and has a small number of components. Can be achieved.

  In addition, by arranging the optical element on the object side from the opening, it becomes easy to balance with the rear group, and good results can be obtained for miniaturization and simplification of the optical system. In the vicinity of the aperture, the central ray of the meridional section and the ray having a large angle of view are close to each other. Therefore, if an optical element is disposed in the vicinity of the aperture, a high angular magnification cannot be obtained, and the burden on other lenses increases. If an optical element is disposed on the image side of the aperture, it becomes impossible to obtain a wider angle of view in the front group on the object side than the aperture.

  FIG. 6 is a cross-sectional view taken along the central axis (rotation symmetry axis) 2 of the optical system 1 of Example 2 described later. In this example, in addition to the side view optical path for obtaining a side image viewed from the central axis, as described in the first example, the front along the central axis of the optical system passes through the center of the optical element. A direct-view optical path for obtaining the image of As described above, when the optical element is configured to take the two optical paths of the side-viewing optical path A and the direct-viewing optical path B, it is possible to reduce the number of optical elements by partially sharing other optical elements. The system can be made small. In addition, by projecting images of both optical paths on the same plane, it becomes possible to focus on both images simultaneously with a single image sensor and to capture a clear image.

More preferably, when the outer diameter of the optical element is D and the outer diameter of the image is Dr,
D / Dr <10 (3)
It is preferable to satisfy the following conditions.

  If the upper limit of conditional expression (3) is exceeded, the outer diameter of the optical system becomes larger with respect to the image sensor. The preferred size of the image sensor is determined to some extent by the price and noise of the image sensor. For example, when a high-resolution image sensor that is relatively inexpensive and easily available, such as 1/3 inch or 1/4 inch, is used, if the upper limit of conditional expression (3) is exceeded, the entire optical system may be made compact. It becomes impossible.

More preferably, D / Dr <5 (3-1)
It is preferable to satisfy the following conditions.

More preferably, the outer diameter D of the optical element is
D <20mm (4)
It is preferable to satisfy the following conditions.

  In particular, when used as an imaging system for an endoscope, satisfying conditional expression (4) is preferable in terms of reducing the burden on the subject.

More preferably, D <10 mm (4-1)
It is preferable to satisfy the following conditions.

  Examples 1 to 4 of the optical system of the present invention will be described below. The configuration parameters of these optical systems will be described later.

  In the forward ray tracing, for example, as shown in FIG. 1, the coordinate system uses, as the origin O of the decentered optical surface, the point where the extension of the central principal ray from the object surface 3 toward the first surface intersects the central axis 2. 3, the direction orthogonal to the central axis 2 opposite to the central axis 2 is defined as the Y-axis positive direction, and the plane of FIG. 1 is defined as the YZ plane. The direction on the image plane 5 side in FIG. 1 is the Z axis positive direction, and the Y axis, the Z axis, and the axis constituting the right-handed orthogonal coordinate system are the X axis positive direction.

  For the decentered surface, the decentering amount from the origin O of the optical system 1 of the coordinate system in which the surface is defined (X-axis direction, Y-axis direction, and Z-axis direction are X, Y, and Z, respectively), and the optical system 1 The inclination angles (α, β, γ (°), respectively) of the coordinate system defining the respective planes centered on the X axis, the Y axis, and the Z axis of the coordinate system defined by the origin O are given. . In this case, positive α and β mean counterclockwise rotation with respect to the positive direction of each axis, and positive γ means clockwise rotation with respect to the positive direction of the Z axis. Note that the α, β, and γ rotations of the central axis of the surface are performed by rotating the coordinate system defining each surface counterclockwise around the X axis of the coordinate system defined at the origin of the optical system. Then rotate it around the Y axis of the new rotated coordinate system by β and then rotate it around the Z axis of another rotated new coordinate system by γ. It is.

  Further, among the optical action surfaces constituting the optical system of each embodiment, when a specific surface and a subsequent surface constitute a coaxial optical system, a surface interval is given, in addition, the curvature radius of the surface, The refractive index and Abbe number of the medium are given according to conventional methods.

  In addition, a term relating to an aspheric surface for which no data is described in the constituent parameters described later is zero. The refractive index and the Abbe number are shown for the d-line (wavelength 587.56 nm). The unit of length is mm. As described above, the eccentricity of each surface is expressed by the amount of eccentricity from the reference surface.

  The aspheric surface is a rotationally symmetric aspheric surface given by the following definition.

^{Z = (Y 2 / R)} / [1+ {1- (1 + k) Y 2 / R 2} 1/2]
+ AY ⁴ + bY ⁶ + cY ⁸ + dY ¹⁰ +...
... (a)
However, Z is taken as an axis, and Y is taken in a direction perpendicular to the axis. Here, R is a paraxial radius of curvature, k is a conic constant, a, b, c, d,... Are fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, respectively. The Z axis of this defining formula is the axis of a rotationally symmetric aspherical surface.

  The extended rotation free-form surface is a rotationally symmetric surface given by the following definition.

  First, as shown in FIG. 2, the following curve (b) passing through the origin on the YZ coordinate plane is determined.

^{Z = (Y 2 / RY)} / [1+ {1- (C 1 +1) Y 2 / RY 2} 1/2]
+ C ₂ Y + C ₃ Y ² + C ₄ Y ³ + C ₅ Y ⁴ + C ₆ Y ⁵ + C ₇ Y ⁶
+ ··· + C ₂₁ Y ²⁰ + ··· + C _{n + 1} Y ⁿ + ····
... (b)
Next, a curve F (Y) obtained by rotating the curve (b) in the positive direction of the X-axis and turning it counterclockwise to be positive is determined. This curve F (Y) also passes through the origin on the YZ coordinate plane.

  The curve F (Y) is translated in the Y positive direction by a distance R (Y negative direction if negative), and then the rotationally symmetric surface is rotated by rotating the translated curve around the Z axis. Let it be a free-form surface.

  As a result, the extended rotation free-form surface becomes a free-form surface (free-form curve) in the YZ plane and a circle with a radius | R | in the XY plane.

  From this definition, the Z-axis becomes the axis of the extended rotation free-form surface (rotation symmetry axis).

Where RY is the radius of curvature of the spherical term in the YZ section, C ₁ is the conic constant, C ₂ , C ₃ , C ₄ , C ₅ . Aspheric coefficient.

A conical surface having the Z axis as the central axis is given as one of the extended rotation free-form surfaces, and RY = ∞, C ₁ , C ₂ , C ₃ , C ₄ , C ₅ ,... = 0, and θ = ( The inclination angle of the conical surface), R = (the radius of the bottom surface in the XZ plane).

  In addition, a term relating to an aspheric surface for which no data is described in the constituent parameters described later is zero. The refractive index and the Abbe number are shown for the d-line (wavelength 587.56 nm). The unit of length is mm. As described above, the eccentricity of each surface is expressed by the amount of eccentricity from the reference surface.

  A cross-sectional view taken along the central axis 2 of the optical system 1 of Example 1 is shown in FIG. Further, FIG. 4 shows a lateral aberration diagram of the entire optical system of this example, and FIG. 5 shows an image height with respect to an angle of view. In this lateral aberration diagram, the angle shown at the center indicates (horizontal field angle, vertical field angle), and the lateral aberrations in the Y direction (meridional direction) and X direction (sagittal direction) at that field angle. Show. Note that a negative field angle means a clockwise angle in the Y-axis positive direction for the horizontal field angle, and a clockwise angle in the X-axis positive direction for the vertical field angle. same as below.

  In this embodiment, the transparent surface and the reflective surface of the transparent medium having a refractive index larger than 1 and concentric with the central axis 2 of the optical system 1 are all configured as different surfaces without being commonly used in the optical path. It is.

  The optical system 1 is arranged coaxially on the central axis 2 between the front group Gf and the rear group Gb, and the front group Gf rotationally symmetric about the central axis 2, the rear group Gb rotationally symmetric about the central axis 2. The rear group Gb includes the first group G1 and the second group G2.

  The front group is made of a transparent medium L1 having a refractive index that is rotationally symmetric about the central axis 2 and greater than 1. The transparent medium L1 is opposed to the object surface and is disposed outside and includes a first transmission surface 11 including a cylindrical surface formed parallel to the central axis 2 and the first transmission surface 11 formed inside the transparent medium L1. Is disposed on the central axis 2 side, is formed of a toric surface and has a negative power, and is formed inside the transparent medium L1, and is opposite to the image surface 5 with respect to the first reflective surface 12. A second reflecting surface 13 having a toric surface and having a positive power, and a second transmitting surface having an aspherical surface and having a negative power, which is disposed closer to the image surface 5 than the second reflecting surface 13. 14

  The first group includes a cemented lens of a negative meniscus lens L2 and a biconvex positive lens L3 having a concave surface facing the image surface 5 side, and is disposed on the image surface 5 side from the third transmission surface 21 and the third transmission surface 21. And a fourth transmission surface 31 disposed on the image plane 5 side with respect to the bonding surface 23.

  The second group includes a cemented lens of a biconvex positive lens L4 and a biconcave negative lens L5, a fifth transmission surface 41, a cemented surface 45 disposed on the image plane 5 side with respect to the fifth transmission surface 41, and a cemented lens. A sixth transmission surface 51 is disposed on the image surface 5 side with respect to the surface 45.

  The optical system 1 forms an optical path A. In the optical path A, a light beam incident from the object plane 3 of the optical system 1 passes through the front group Gf and the rear group Gb in order and forms an image in an annular shape outside the center axis 2 of the image plane 5 perpendicular to the center axis 2. Form.

  The light beam incident on the optical system 1 as the optical path A enters the transparent medium L1 of the front group Gf via the first transmission surface 11, is reflected by the first reflection surface 12 on the side opposite to the image surface 5, and is reflected by the second reflection surface. 13 has a substantially Z-shaped optical path that is reflected to the image plane 5 side and exits from the transparent medium L1 through the second transmission surface 14.

  After that, a cemented lens of the negative meniscus lens L2 of the first group of the rear group Gb and the biconvex positive lens L3 is disposed between the front group Gf and the rear group Gb through an aperture S that is coaxially arranged on the central axis 2 and forms a stop. It enters through the third transmission surface 21 on the opposite side across the central axis 2 and exits from the fourth transmission surface 31 through the joint surface 23 and the biconvex positive lens L4 of the second group and the biconcave negative The lens L5 enters the cemented lens through the fifth transmitting surface 41, passes through the cemented surface 45, exits from the sixth transmitting surface 51, and is connected to a predetermined radial position away from the central axis 2 of the image surface 5. Image.

The specification of this Example 1 is
Angle of view -89 ° -89 °
Image size φ0.48 to φ2.37
F number 3.97
It is.

  A sectional view taken along the central axis 2 of the optical system 1 of Example 2 is shown in FIG. Further, FIG. 7 shows a lateral aberration diagram of the side viewing optical path of the entire optical system of this embodiment, FIG. 8 shows a lateral aberration diagram of the direct viewing optical path, and FIG. 9 shows a diagram representing the image height with respect to the angle of view.

  In this embodiment, the transmission surface and the reflection surface of a transparent medium having a refractive index larger than 1 that is concentric with the central axis 2 of the optical system 1 are all different surfaces without being commonly used in the side-view optical path. This is an example.

  The optical system 1 is arranged coaxially on the central axis 2 between the front group Gf and the rear group Gb, and the front group Gf rotationally symmetric about the central axis 2, the rear group Gb rotationally symmetric about the central axis 2. The rear group Gb includes a first group G1 and a second group G2.

  The front group Gf is composed of a transparent medium L1 having a rotational symmetry around the central axis 2 and a refractive index larger than 1, and is an optical path combining optical system that combines the side viewing optical path A and the direct viewing optical path B.

  The transparent medium L1 is opposed to the side object surface 3 and is disposed outside, and is formed inside the cylindrical side view first transmission surface 11 parallel to the central axis 2 and the transparent medium L1. Formed on the side of the central axis 2 from the transmission surface 11, is formed of a toric surface, has a negative side power first reflection surface 12, and is formed inside the transparent medium L 1, with respect to the side reflection first reflection surface 12. Arranged on the opposite side of the image plane 5, consisting of a toric surface, having a positive power side-viewing second reflecting surface 13, arranged on the image plane 5 side from the side-viewing second reflecting surface 13, consisting of an aspherical surface, It has the 2nd permeation | transmission surface 14 with a negative power. Further, a direct-view third transmitting surface 15 having a spherical power and having a positive power, and a direct-view fourth transmitting surface 16 having an aspherical surface and having a negative power disposed on the image plane 5 side of the direct-viewing third transmitting surface 15. It has. The side view second transmission surface 14 and the direct view fourth transmission surface 16 are the same surface.

  The first group includes a cemented lens of a negative meniscus lens L2 and a biconvex positive lens L3 having a concave surface facing the image surface 5 side, and is disposed on the image surface 5 side from the third transmission surface 21 and the third transmission surface 21. And a fourth transmission surface 31 disposed on the image plane 5 side with respect to the bonding surface 23.

  The second group includes a cemented lens of a biconvex positive lens L4 and a biconcave negative lens L5, a fifth transmission surface 41, a cemented surface 45 disposed on the image plane 5 side with respect to the fifth transmission surface 41, and a cemented lens. A sixth transmission surface 51 is disposed on the image surface 5 side with respect to the surface 45.

  The optical system 1 forms a side viewing optical path A and a direct viewing optical path B. In the side viewing optical path A, the light beam incident from the side viewing object plane 3 on the side of the optical system 1 deviates from the central axis 2 of the image plane 5 perpendicular to the central axis 2 through the front group Gf and the rear group Gb in this order. An image is formed in an annular shape on the outside. In the direct-view optical path B, the light beam incident from the direct-view object surface 4 in the vicinity of the central axis 2 of the optical system 1 passes through the front group Gf and the rear group Gb in this order, and the central axis 2 of the image plane 5 perpendicular to the central axis 2. An image is formed in a circle in the vicinity.

  A light beam incident from the side of the optical system 1 as the side viewing optical path A enters the transparent medium L1 of the front group Gf through the first transmission surface 11 as viewed from the side, and is opposite to the image surface 5 at the first reflection surface 12 as viewed from the side. A substantially Z-shaped optical path that is reflected to the side, reflected to the image plane 5 side by the side-view second reflection surface 13, and exits from the transparent medium L <b> 1 through the side-view second transmission surface 14.

  After that, a cemented lens of the negative meniscus lens L2 of the first group of the rear group Gb and the biconvex positive lens L3 is disposed between the front group Gf and the rear group Gb through an aperture S that is coaxially arranged on the central axis 2 and forms a stop. It enters through the common third transmission surface 21 on the opposite side of the central axis 2 and exits from the common fourth transmission surface 31 through the joint surface 23 to both the biconvex positive lens L4 of the second group. The concave negative lens L5 enters the cemented lens through the common fifth transmitting surface 41, passes through the cemented surface 45, exits from the common sixth transmitting surface 51, and deviates from the central axis 2 of the image plane 5. An image is formed at a predetermined position.

  Further, the light beam incident on the optical system 1 as the direct-view optical path B enters the transparent medium L1 of the front group Gf through the direct-view first transmission surface 15, and is directly viewed from the first-view first transmission surface 15 on the image plane 5 side. It goes out of the transparent medium L1 through the second transmission surface 16.

  After that, a cemented lens of the negative meniscus lens L2 of the first group of the rear group Gb and the biconvex positive lens L3 is disposed between the front group Gf and the rear group Gb through an aperture S that is coaxially arranged on the central axis 2 and forms a stop. It enters through the common third transmission surface 21 on the opposite side of the central axis 2 and exits from the common fourth transmission surface 31 through the joint surface 23 to both the biconvex positive lens L4 of the second group. The light enters the cemented lens of the concave negative lens L5 through the common fifth transmission surface 41, exits from the common sixth transmission surface 51 through the cemented surface 45, and forms an image on the central axis 2 of the image plane 5.

The specification of Example 2 is
Angle of view (side view) -89 ° to 89 °
(Direct view) 0 ° -60 °
Image size (side view) φ0.87 to φ2.76
(Direct view) φ0.66
F number (side view) 4.26
(Direct view) 4.24
A sectional view taken along the central axis 2 of the optical system 1 of Example 3 is shown in FIG. Further, FIG. 11 shows a lateral aberration diagram of the entire optical system of this example, and FIG. 12 shows a diagram showing the image height with respect to the angle of view. In this lateral aberration diagram, the angle shown at the center indicates (horizontal field angle, vertical field angle), and the lateral aberrations in the Y direction (meridional direction) and X direction (sagittal direction) at that field angle. Show. Note that a negative field angle means a clockwise angle in the Y-axis positive direction for the horizontal field angle, and a clockwise angle in the X-axis positive direction for the vertical field angle. same as below.

  In this embodiment, the transparent surface and the reflective surface of the transparent medium having a refractive index larger than 1 and concentric with the central axis 2 of the optical system 1 are all configured as different surfaces without being commonly used in the optical path. It is.

  The optical system 1 is arranged coaxially on the central axis 2 between the front group Gf and the rear group Gb, and the front group Gf rotationally symmetric about the central axis 2, the rear group Gb rotationally symmetric about the central axis 2. The front group Gf consists of the first group G1 and the second group G2, and the rear group Gb consists of the third group G3 and the fourth group G4.

  The first group is made of a transparent medium L1 having a refractive index that is rotationally symmetric about the central axis 2 and greater than 1. The transparent medium L1 is opposed to the object surface and is disposed outside and includes a first transmission surface 11 including a cylindrical surface formed parallel to the central axis 2 and the first transmission surface 11 formed inside the transparent medium L1. The first reflecting surface 12 is arranged on the central axis 2 side, is composed of an extended rotation free-form surface, has negative power, and is formed inside the transparent medium L1, and has an image plane 5 with respect to the first reflecting surface 12. Is disposed on the opposite side of the second reflecting surface 13 having a positive rotation power and having a positive power. The second reflecting surface 13 is disposed on the image surface 5 side of the second reflecting surface 13 and has an aspherical surface and has a negative power. A second transmission surface 14 is provided.

  The second group includes a positive meniscus lens L2 having a convex surface facing the image surface 5 side, and includes a third transmission surface 21 and a fourth transmission surface 31 disposed on the image surface 5 side with respect to the third transmission surface 21.

  The third group includes a cemented lens of a negative meniscus lens L3 and a biconvex positive lens L4 having a concave surface facing the image surface 5 side, and is disposed on the image surface 5 side from the fifth transmission surface 31 and the fifth transmission surface 31. And a sixth transmission surface 41 disposed closer to the image plane 5 than the bonding surface 34.

  The fourth group includes a cemented lens of a biconvex positive lens L5 and a biconcave negative lens L6, a seventh transmission surface 51, a cemented surface 56 disposed closer to the image plane 5 than the seventh transmission surface 51, and a cemented lens. An eighth transmission surface 61 is provided on the image plane 5 side with respect to the surface 56.

  The optical system 1 forms an optical path A. In the optical path A, a light beam incident from the object plane 3 of the optical system 1 passes through the front group Gf and the rear group Gb in order and forms an image in an annular shape outside the center axis 2 of the image plane 5 perpendicular to the center axis 2. Form.

  The light beam incident on the optical system 1 as the optical path A enters the first group of transparent media L1 of the front group Gf through the first transmission surface 11, is reflected by the first reflection surface 12 on the opposite side to the image surface 5, It has a substantially Z-shaped optical path that is reflected by the second reflecting surface 13 toward the image surface 5 and goes out of the transparent medium L1 through the second transmitting surface 14. Subsequently, the light enters the positive meniscus lens L <b> 2 of the second group through the third transmission surface 21 and exits from the fourth transmission surface 31.

  After that, a cemented lens of the negative meniscus lens L3 of the third group of the rear group Gb and the biconvex positive lens L4 passes through an aperture S that is disposed coaxially with the central axis 2 between the front group Gf and the rear group Gb and forms a stop. It enters through the fifth transmission surface 31 on the opposite side across the central axis 2 and exits from the sixth transmission surface 41 through the cementing surface 34 and the biconvex positive lens L5 of the fourth group and the biconcave negative The lens L6 enters the cemented lens through the seventh transmission surface 51, passes through the cemented surface 56, exits from the eighth transmission surface 61, and is connected to a predetermined radial position away from the central axis 2 of the image plane 5. Image.

The specification of this Example 3 is
Angle of view -89 ° -89 °
Image size φ0.44 to φ2.38
F number 5.9
It is.

Further, the angle magnification of the meridional section of the optical element is βω, the power of the meridional section of the surface having the second transmission action is P2, the power of the entire optical system of the meridional section of the central principal ray is Pm, and the outer diameter of the optical element Is D and the outer diameter of the image is Dr,
Example 1 Example 2 Example 3
βω 0.099 0.090 0.129
P2 / Pm -3.022 -2.427 -3.932
D 7.000 7.000 8.000
Dr 2.366 2.756 2.381
D / Dr 2.959 2.528 3.360
It is.

  The configuration parameters of Examples 1 to 4 are shown below. In the table below, “ASS” indicates an aspherical surface, “ERFS” indicates an extended rotation free-form surface, and “RE” indicates a reflecting surface.

Example 1
Surface number Curvature radius Surface spacing Eccentricity Refractive index Abbe number Object surface ∞ ∞ Eccentricity (1)
1 ERFS [1] Eccentricity (2) 1.8348 42.7
2 ERFS [2] (RE) Eccentricity (3) 1.8348 42.7
3 ERFS [3] (RE) Eccentricity (4) 1.8348 42.7
4 ASS [1] Eccentricity (5)
5 ∞ 0.50 Eccentricity (6) 1.5163 64.1
6 ∞ (Aperture) 0.35
7 2.82 0.30 1.8467 23.8
8 1.28 1.20 1.7440 44.8
9 -1.85 0.10
10 1.84 1.40 1.4971 69.3
11 -1.14 0.30 1.8467 23.8
12 20.09 0.50
13 ∞ 0.40 1.5163 64.1
14 ∞ 0.10
Image plane ∞
ERFS [1]
RY ∞
θ 90.00
R -3.50
ERFS [2]
RY 4.52
θ 39.68
R -2.18
ERFS [3]
RY 5.72
θ 10.80
R -1.62
ASS [1]
R 0.70
k -1.9423E-01
Eccentric [1]
X 0.00 Y 0.00 Z 0.00
α 90.00 β 0.00 γ 0.00
Eccentric [2]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [3]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [4]
X 0.00 Y 0.00 Z -2.95
α 0.00 β 0.00 γ 0.00
Eccentric [5]
X 0.00 Y 0.00 Z -1.46
α 0.00 β 0.00 γ 0.00
Eccentric [6]
X 0.00 Y 0.00 Z -0.10
α 0.00 β 0.00 γ 0.00

Example 2
Side-viewing optical path number of curvature radius Surface spacing Eccentricity Refractive index Abbe number Object surface ∞ ∞ Eccentricity (1)
1 ERFS [1] Eccentricity (2) 1.8348 42.7
2 ERFS [2] (RE) Eccentricity (3) 1.8348 42.7
3 ERFS [3] (RE) Eccentricity (4) 1.8348 42.7
4 ASS [1] Eccentricity (5)
5 ∞ 0.50 Eccentricity (6) 1.5163 64.1
6 ∞ (Aperture) 0.35
7 2.77 0.30 1.8467 23.8
8 1.39 1.20 1.7012 44.8
9 -1.79 0.10
10 2.01 1.40 1.4875 70.4
11 -1.25 0.30 1.8467 23.8
12 10.29 0.50
13 ∞ 0.40 1.5163 64.1
14 ∞ 0.10
Image plane ∞
ERFS [1]
RY ∞
θ 90.00
R -3.50
ERFS [2]
RY 3.88
θ 39.86
R -2.34
ERFS [3]
RY 5.61
θ 13.60
R -1.86
ASS [1]
R 0.87
k -1.8327E-01
Eccentric [1]
X 0.00 Y 0.00 Z 0.00
α 90.00 β 0.00 γ 0.00
Eccentric [2]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [3]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [4]
X 0.00 Y 0.00 Z -2.64
α 0.00 β 0.00 γ 0.00
Eccentric [5]
X 0.00 Y 0.00 Z -1.55
α 0.00 β 0.00 γ 0.00
Eccentric [6]
X 0.00 Y 0.00 Z 0.13
α 0.00 β 0.00 γ 0.00
Direct-view optical path number Curvature radius Surface spacing Eccentricity Refractive index Abbe number Object surface ∞ ∞ Eccentricity (1)
1 5.00 Eccentricity (7) 1.8348 42.7
2 ASS [1] Eccentricity (5)
3 ∞ 0.50 Eccentricity (6) 1.5163 64.1
4 ∞ (Aperture) 0.35
5 2.82 0.30 1.8467 23.8
6 1.28 1.20 1.7440 44.8
7 -1.85 0.10
8 1.84 1.40 1.4971 69.3
9 -1.14 0.30 1.8467 23.8
10 20.09 0.50
11 ∞ 0.40 1.5163 64.1
12 ∞ 0.10
Image plane ∞
ASS [1]
R 0.70
k -1.9423E-01
Eccentric [1]
X 0.00 Y 0.00 Z 0.00
α 90.00 β 0.00 γ 0.00
Eccentric [7]
X 0.00 Y 0.00 Z -2.85
α 0.00 β 0.00 γ 0.00
Eccentric [5]
X 0.00 Y 0.00 Z -1.55
α 0.00 β 0.00 γ 0.00
Eccentric [6]
X 0.00 Y 0.00 Z 0.13
α 0.00 β 0.00 γ 0.00

Example 3
Surface number Curvature radius Surface spacing Eccentricity Refractive index Abbe number Object surface ∞ ∞ Eccentricity (1)
1 ERFS [1] Eccentricity (2) 1.8348 42.7
2 ERFS [2] (RE) Eccentricity (3) 1.8348 42.7
3 ERFS [3] (RE) Eccentricity (4) 1.8348 42.7
4 ASS [1] Eccentricity (5)
5 -1.45 0.50 Eccentricity (6) 1.5163 64.1
6 -1.21 0.10
7 ∞ (Aperture) 0.70
8 2.51 0.30 1.8467 23.8
9 1.50 0.80 1.5831 62.4
10 -2.32 1.47
11 2.02 1.60 1.6204 60.3
12 -1.38 0.30 1.8467 23.8
13 ∞ 0.31
14 ∞ 0.40 1.5163 64.1
15 ∞ 0.10
Image plane ∞
ERFS [1]
RY ∞
θ 90.00
R -4.00
ERFS [2]
RY 4.80
θ 40.86
R -2.38
C4 -1.2352E-02
ERFS [3]
RY 6.57
θ 12.25
R -1.84
C4 -3.4641E-03
ASS [1]
R 0.58
k -1.3453E-01
Eccentric [1]
X 0.00 Y 0.00 Z 0.00
α 90.00 β 0.00 γ 0.00
Eccentric [2]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [3]
X 0.00 Y 0.00 Z 0.00
α 0.00 β 0.00 γ 0.00
Eccentric [4]
X 0.00 Y 0.00 Z -3.73
α 0.00 β 0.00 γ 0.00
Eccentric [5]
X 0.00 Y 0.00 Z -1.77
α 0.00 β 0.00 γ 0.00
Eccentric [6]
X 0.00 Y 0.00 Z -0.71
α 0.00 β 0.00 γ 0.00
In the above embodiment, the transmission surface and the reflection surface of a transparent medium having a refractive index larger than 1 concentric with the central axis 2 of the optical system 1 are designed as extended rotation free-form surfaces. When the rotational free-form surface is orthogonal to the rotationally symmetric surface and does not use a higher-order term, the configuration is equivalent to a spherical surface.

  In addition, the reflecting surface and the refracting surface of the front group Gf are designed with an extended rotation free-form surface that is formed by rotating a line segment of an arbitrary shape around the central axis 2 and does not have a top on the central axis 2. However, each may be replaced with an arbitrary curved surface.

  For example, the first transmission surface 11 needs to be formed on the outer peripheral portion of the transparent medium L1, but the shape is not limited to the cylindrical surface, and may be a toric surface as shown in FIG. . Further, it may be a conical surface as shown in FIG.

  In addition, the optical system of the present invention uses an equation that includes an odd-order term in an expression that defines a line segment of an arbitrary shape that forms a rotationally symmetric surface, so that the inclination of the image plane 5 caused by decentering or back projection of the stop The pupil aberration is corrected.

  Also, by using the transparent medium that is rotationally symmetric around the central axis 2 constituting the front group Gf of the present invention as it is, it is possible to shoot or project an image having an angle of view of 360 ° in all directions. By cutting the cross section including the central axis 2 to make it half, one third, two thirds, etc., the angle of view around the central axis 2 is 180 °, 120 °, 240 °, etc. May be taken or projected.

  Further, the optical system of the present invention may be an imaging or observation optical system that obtains an image with an angle of view of 360 ° in all directions (all circumferences) including the zenith with the central axis (rotation symmetry axis) 2 in the vertical direction. Further, the present invention is not limited to the photographing optical system and the observation optical system, but can be used as a projection optical system that projects an image on 360 ° omnidirectional (all circumference) angles of view including the zenith with the optical path reversed. The endoscope can also be used as an all-round observation optical system of an in-tube observation apparatus.

  FIG. 15 shows an arrangement example of an image and an image sensor in the present embodiment. FIG. 15A shows an example in which an image sensor having a screen ratio of 16: 9 is used. When an image in the vertical direction is not used, it is preferable to match the size of the image sensor 50 with the left and right positions of the image A1 in the side viewing optical path A. FIG. 15B is an example in which the image sensor 50 having a screen ratio of 4: 3 is used, and the size of the image sensor 50 is matched with the image B1 in the direct-view optical path B, as in FIG. Shows the case where the image in the vertical direction is not used. FIG. 15C illustrates an example in which the image sensor 50 having a screen ratio of 4: 3 is used, and the size of the image sensor 50 is matched with the image A1 in the side viewing optical path A. Thus, when arranged, both the image A1 of the side viewing optical path A and the image B1 of the direct viewing optical path B can be captured.

  Hereinafter, as an application example of the optical system 1 of the present invention, a usage example of the photographing optical system 101 or the projection optical system 102 will be described. FIG. 16 is a diagram for illustrating an example in which the photographing optical system 101 according to the present invention is used as the photographing optical system at the distal end of the endoscope. FIG. 16A illustrates the present invention at the distal end 101 of the rigid endoscope 110. This is an example of imaging and observing 360 ° omnidirectional images by attaching the photographing optical system. FIG. 16B shows a schematic configuration of the tip. Around the entrance surface 11 of the front group Gf of the panoramic imaging optical system 101 according to the present invention, a flare stop 107 made of a casing or the like having an opening 106 extending in a slit shape in the circumferential direction is arranged so that flare light enters. It is preventing. FIG. 16C shows a panoramic imaging optical system 101 according to the present invention attached to the tip of the flexible electronic endoscope 113 in the same manner, and the image captured on the display device 114 is subjected to image processing to correct distortion. This is an example of displaying.

  FIG. 17 shows an example in which the photographing optical system 101 according to the present invention is attached to the capsule endoscope 120 and images of 360 ° omnidirectional images are taken and observed. The aperture 106 extending in a slit shape in the circumferential direction around the side-view first transmission surface 11 of the front group Gf in the side-view optical path A of the photographing optical system 101 according to the present invention, and the direct view of the front group Gf in the direct-view optical path B A flare stop 107 is formed in a casing or the like having a circular opening 106 in front of the first transmission surface 15 to prevent flare light from entering.

  As shown in FIGS. 16 and 17, by using the imaging optical system 101 for the endoscope, it is possible to image and observe images behind the imaging optical system 101, and to image and observe various parts from angles different from the conventional ones. can do.

  FIG. 18 (a) shows an image obtained by attaching a photographic optical system 101 according to the present invention as a photographic optical system in front of an automobile 130, and performing image processing on an image photographed through each photographic optical system 101 on a display device in a vehicle. FIG. 18B is a diagram showing an example in which distortion is corrected and simultaneously displayed, and FIG. 18B shows the photographing optical system 101 according to the present invention as a photographing optical system at each corner of the automobile 130 and the top of the pole of the head portion. It is a figure which shows the example which attached the plurality and displayed the image image | photographed through each imaging | photography optical system 101 on the display apparatus in a vehicle, performing image processing and correct | amending distortion simultaneously. In this case, as shown in FIG. 18A, it is preferable to match the size of the image sensor 50 to the left and right positions of the image A1 in the side viewing optical path A, because the left and right images can be captured widely.

  Further, FIG. 19 uses the projection optical system 102 according to the present invention as the projection optical system of the projection apparatus 140, displays a panoramic image on a display element arranged on the image plane 5, and 360 ° in all directions through the projection optical system 102. This is an example in which a 360 ° omnidirectional image is projected and displayed on the arranged screen 141.

  Further, FIG. 20 shows that the photographing apparatus 151 using the photographing optical system 101 according to the present invention is attached to the outside of the building 150, and the projection apparatus 151 using the photographing optical system 101 according to the present invention is disposed indoors. It connects so that the imaged image may be sent to the projection device 140 via the electric wire 152. In such an arrangement, an outdoor 360 ° omnidirectional subject P is photographed by the photographing device 151 via the photographing optical system 101, and the video signal is sent to the projection device 140 via the electric wire 152 and disposed on the image plane. In this example, the image is displayed on the display element, and the image P ′ of the subject P is projected and displayed on an indoor wall surface or the like through the projection optical system 102.

It is a figure for demonstrating the coordinate system of the optical system of this invention. It is a figure which shows the principle of an extended rotation free-form surface. It is sectional drawing taken along the central axis of the optical system of Example 1 of this invention. 2 is a transverse aberration diagram for the whole optical system of Example 1. FIG. 2 is a diagram illustrating an F-θ diagram of the entire optical system according to Example 1. FIG. It is sectional drawing taken along the central axis of the optical system of Example 2 of this invention. 6 is a lateral aberration diagram of a side-view optical path of the entire optical system according to Example 2. FIG. FIG. 6 is a diagram illustrating a lateral aberration diagram of a direct-view optical path of the entire optical system according to Example 2. FIG. 6 is an F-θ diagram of the entire optical system according to Example 2. It is sectional drawing taken along the central axis of the optical system of Example 3 of this invention. FIG. 6 is a transverse aberration diagram for the whole optical system of Example 3. FIG. 6 is an F-θ diagram of the entire optical system of Example 3. It is a figure which shows another Example. It is a figure which shows another Example. It is a figure which shows the example of arrangement | positioning of the image of the optical system of this invention, and an image pick-up element. It is a figure which shows the example which used the optical system of this invention as an imaging | photography optical system of the endoscope front-end | tip. It is a figure which shows the example which used the optical system of this invention as the imaging | photography optical system of a capsule endoscope. It is a figure which shows the example which used the optical system of this invention as the imaging | photography optical system of a motor vehicle. It is a figure which shows the example which used the optical system of this invention as a projection optical system of a projection apparatus. It is a figure which shows the example which used the optical system of this invention as an imaging | photography optical system which image | photographs the outdoor to-be-photographed object.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical system 2 ... Center axis 3 ... Object surface 5 ... Image surface

Claims (17)

  A transparent medium having an angle of view of about 90 ° on one side and a rotational index around the central axis and having a refractive index greater than 1 in a cross section including the central axis, the direction being perpendicular to the central axis; The medium includes a first transmission surface disposed on the outermost periphery with respect to the central axis, a first reflection surface disposed on the central axis side with respect to the first transmission surface, and a side opposite to the image plane from the first reflection surface. A second reflection surface disposed on the image plane side of the second reflection surface, and a light beam incident on the transparent medium is arranged in the order of tracking the forward ray. The transparent medium enters the transparent medium through one transmission surface, is reflected on the opposite side to the image surface by the first reflection surface, is reflected on the image surface side by the second reflection surface, and passes through the second transmission surface. A substantially Z-shaped optical path going out to the image plane side from the image plane, and the optical path is configured only on one side with respect to the central axis. Optical element to be. When the angular magnification of the meridional cross section of the optical element is βω,
0.01 <βω <0.5 (1)
The optical element according to claim 1, wherein the following condition is satisfied. When the power of the meridional section of the second transmission surface is negative, the power of the second transmission surface is P2, and the power of the entire optical system of the meridional section of the central principal ray is Pm,
−10 <P2 / Pm <−1 (2)
The optical element according to claim 1, wherein the following condition is satisfied.   The optical element according to claim 1, wherein the optical element is made of a transparent medium having a refractive index of 1.5 or more.   5. The optical element according to claim 1, wherein at least one of the first reflecting surface and the second reflecting surface has a total reflection function.   At least one of the first reflecting surface and the second reflecting surface is formed of an extended rotation free-form surface formed by rotating an arbitrary-shaped line segment having no symmetry plane around the central axis. The optical element according to any one of claims 1 to 5, wherein:   The at least one surface among the surfaces of the transparent medium is configured by an extended rotation free-form surface formed by rotating an arbitrary-shaped line segment including an odd-numbered term around a central axis. The optical element according to claim 1.   A front group, a rear group disposed closer to the image plane than the front group, and an aperture disposed between the front group and the rear group, and the optical element is disposed in the front group, 8. The optical system according to claim 1, wherein an image of an object arranged so as to surround a central axis is formed, or an image of the object is projected in a direction radiating from the central axis. An optical system equipped with an element.   The optical system according to claim 8, wherein the optical system forms an image of an annular object around the central axis in a plane orthogonal to the central axis.   10. The optical system according to claim 8, wherein the first reflecting surface and the second reflecting surface are disposed with a concave surface facing the opening.   The optical system according to claim 8, wherein the second transmission surface is disposed with a concave surface facing the opening.   The optical system according to claim 8, wherein the optical system does not form an intermediate image in the optical path.   The optical system according to claim 8, wherein the optical element is disposed on the opposite side of the image plane with respect to the opening.   The optical element has a direct-view optical path for observing immediately before the optical element and a side-view optical path for observing a direction orthogonal to the central axis, and forms images of the direct-view optical path and the side-view optical path in the same plane. The optical system according to claim 8, wherein the optical system is an optical system. When the outer diameter of the optical element is D and the outer diameter of the image is Dr,
D / Dr <10 (3)
The optical system according to claim 8, wherein the following condition is satisfied. The outer diameter D of the optical element is
D <20mm (4)
The optical system according to claim 15, wherein the following condition is satisfied.   An endoscope using the optical system according to any one of claims 8 to 16.

Images