JP2008298866A - Imaging optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high speed and wide angle imaging optical system by reducing astigmatism and improving imaging performance. <P>SOLUTION: The imaging optical system has three reflection mirrors of a first reflection mirror 1, a second reflection mirror 2 and a third reflection mirror 3, which are arranged in the described order on the optical path of incident light, and the light beams reflected on the three reflection mirrors form an imaging plane 4, wherein a plane including the incident center main light beam to the first reflection mirror and the reflection center main light beam to the first reflection mirror is defined as a plane A, a plane including the incident center main light beam to the second reflection mirror and the reflection center main light beam to the second reflection mirror is defined as a plane B, a plane including the incident center main light beam to the third reflection mirror and the reflection center main light beam to the third reflection mirror is defined as a plane C, respectively, in terms of the central main light beams defined as the main light beams which are focused on the center of the imaging plane, the plane A and the plane C do not coincide, and the angle difference Δν= ¾ν1-ν2¾ is 45° or smaller, where ν1 stands for the angle between the plane A and the plane B, and ν2 stands for the angle between the plane C and the plane B. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical system that can be used over a wide wavelength band using a reflecting mirror, and more particularly to an optical system that is optimal for an imaging apparatus that captures an image of a subject such as a camera.

  A function required for an optical system used in an imaging apparatus is to form an image of a subject on an image plane by bending and collecting light incident from the subject. Examples of such an optical element having a function of bending light include a lens that bends light using a difference in refractive index and a reflector that bends light using reflection.

  Regarding the lens, since light is transmitted through the inside, it is necessary to use a material having a sufficiently large transmittance with respect to a desired wavelength band. In addition, depending on special wavelength bands such as ultraviolet rays and infrared rays, the material is limited to an expensive material, which causes a problem in terms of cost.

  Furthermore, since the refractive index of the lens material generally has chromatic aberration due to the difference in size depending on the wavelength of light, in order to obtain a constant imaging performance over a wide wavelength band, lenses having different refractive index changes with respect to the wavelength are used. Complicated correction such as combining more than one sheet, so-called achromatic, must be performed.

  With respect to the reflecting mirror, any material can be used as long as the reflecting surface can be coated with a reflecting material having sufficient performance. Therefore, an inexpensive optical system can be obtained for any wavelength band. Further, since the reflection action does not depend on the wavelength of light, an optical system free from chromatic aberration can be easily obtained over a wide wavelength band.

  However, in the reflection type optical system, the incident light on the reflecting surface and the reflected light appear on the same side with respect to the reflecting surface, so that the position of the reflecting mirror on the next surface is also on the same side as the incident light. For this reason, a phenomenon in which the reflecting mirror on the next surface blocks incident light, that is, so-called vignetting is likely to occur. When vignetting occurs, the amount of incident light is reduced, and thus a bright optical system or a wide-angle optical system cannot be obtained.

  In order to avoid vignetting, for example, a method of setting the reflected light beam so as not to overlap the incident light beam by causing the light beam to enter the reflecting mirror obliquely is used.

  In the conventional reflection-type imaging optical system, the reflecting mirrors are arranged so as to be decentered so that the incident light rays are obliquely incident so that no vignetting occurs. In addition, the three reflecting mirrors have a plane-symmetric structure with a common symmetry plane, and the central principal ray intersects more than once in the optical system and has an intermediate imaging point to achieve miniaturization. Yes. The fact that the optical system is plane symmetric has the effect of suppressing aberrations because aberrations having non-plane symmetric characteristics do not occur. Each reflecting mirror is decentered on the symmetry plane, and the incident angle of the central chief ray on each mirror is set to be 8.5 degrees or more. The angle of view of the optical system is approximately 12 degrees in the direction on the symmetry plane (see, for example, Patent Document 1).

JP 2003-5074 A

  However, the conventional reflective imaging optical system as described above has a plane-symmetric structure in which the direction of decentering is limited to one surface, and thus there is a restriction on the degree of freedom in arranging the reflecting mirrors. As a result, it has been a limit in designing bright optical systems and wide-angle optical systems. For example, in the reflective optical system according to Patent Document 1, incident light is sandwiched between the second and third reflecting mirrors, or the second reflecting mirror and the image plane, and a wider angle can be obtained without causing vignetting. Larger diameters cannot be made.

  On the other hand, when the structure of the optical system is non-symmetrical, the degree of freedom in arranging the reflecting mirrors increases, and it is possible to design bright and wide-angle optical systems by moving the reflecting mirrors in a direction where vignetting does not occur. Become. For example, in the reflection optical system according to Patent Document 1, it is possible to widen the angle and increase the diameter without using vignetting by using a light beam inclined from a symmetry plane. However, since aberrations with non-plane symmetry characteristics are newly generated by making the optical system non-plane symmetrical, this aberration must be suppressed to obtain an optical system with practical imaging performance. Is required.

  The present invention has been made to solve the above-described problems. It is an object of the present invention to obtain a bright and wide-angle imaging optical system by reducing astigmatism and improving imaging performance.

  The imaging optical system according to the present invention includes three reflecting mirrors arranged in the order of the first reflecting mirror, the second reflecting mirror, and the third reflecting mirror in the order of the optical path of the incident light, and the three reflecting mirrors are arranged. In an imaging optical system in which a light beam reflected by a mirror forms an imaging surface, a central principal ray defined as a principal ray of a light beam that forms an image at the center of the imaging surface is applied to the first reflecting mirror. A plane including the incident center chief ray and the reflection center chief ray at the first reflecting mirror is defined as plane A, and the incident center chief ray on the second reflecting surface and the reflection center chief ray at the second reflecting surface are When the plane including the plane C is a plane including the plane principal ray incident on the third reflecting surface and the center principal ray reflected on the third reflecting surface, the plane A and the plane C are The angle between the plane A and the plane B is ν1, and the plane C and the plane B When the angle formed and .nu.2, the angular difference Δν = | ν1-ν2 | is equal to or is less than 45 °.

  According to the present invention, it is possible to improve imaging performance by reducing astigmatism and to obtain a bright and wide-angle imaging optical system.

Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing an imaging optical system according to Embodiment 1 of the present invention. As shown in FIG. 1, the light beam propagated from the subject first enters the first reflecting mirror 1 by the optical system, and the light beam reflected by the first reflecting mirror 1 then enters the second reflecting mirror 2. The light beam reflected by the second reflecting mirror 2 then enters the third reflecting mirror 3, and the light beam reflected by the third reflecting mirror 3 forms an image of the subject on the image plane 4. Reference numeral 5 denotes a central principal ray that indicates the optical path of the principal ray of the light beam that forms an image at the center of the imaging plane 4.

  Unlike the reflective optical system such as the Cassegrain type optical system, the optical system according to the first embodiment does not have a central shielding inside the optical system with respect to the incident light, and therefore forms an image on the image plane without losing the incident light. can do.

  In the optical system of the first embodiment, the central principal ray 5 is not on a single plane. FIG. 2 shows the relationship between each plane formed by the central principal ray incident on each reflecting mirror and the central principal ray reflected by each reflecting mirror. In FIG. 2, a plane formed by the central principal ray 5 incident on the optical system and the central principal ray 5 reflected by the first reflecting mirror 1 is defined as plane A, and the central principal ray 5 incident on the second reflecting mirror 2 and reflected. The plane formed by the plane principal plane 5 is defined as plane B, and the plane formed by the central principal ray 5 incident on and reflected by the third reflecting mirror 3 is defined as plane C. Further, the central principal ray 5 is incident on the first reflecting mirror 1 by 5a, the second reflecting mirror 2 by 5b, the third reflecting mirror 3 by 5c, and the image plane 4 Let 5d be the portion of the light beam incident on.

  The three reflecting mirrors are on the plane B, but the central principal ray 5a incident on the optical system is not on the plane A. That is, since the incident light beam is not positioned between the reflecting mirrors as in the optical system having a plane-symmetric structure, vignetting hardly occurs even if the incident light beam spreads. For this reason, according to the first embodiment, it is possible to increase the diameter and the angle of the optical system.

  According to the first embodiment, the central principal ray 5d incident on the imaging plane 4 from the optical system is not on the plane B. Therefore, vignetting is unlikely to occur even when the light rays incident on the imaging surface 4 spread, so that the optical system can have a large aperture and a wide angle.

  An optical system having a structure that does not have a symmetric surface generates non-plane symmetric aberration. Among the non-plane-symmetrical aberrations, the dominant aberration is an aberration whose focal position changes depending on the direction on the pupil plane of the optical system. This is similar to the effect of astigmatism in the coaxial optical system. Light rays obliquely incident on the reflecting mirror are collected near the corresponding focal plane, but considering aberrations, it can be approximated by focusing on different focal planes in the orthogonal direction on the pupil plane, so-called radial direction and tangential direction. In order to obtain an optical system in which aberration is suppressed, the difference in focal plane between the reflecting mirrors in the radial direction and the tangential direction cancels each other, and finally the focal planes in the two directions almost coincide with each other. Become.

  In order for the difference in focal plane between the two directions to cancel each other, the radial direction and the tangential direction of the focal plane in each reflector need to be substantially coincident. This can be satisfied by suppressing the difference between the inclination of the plane A with respect to the plane B and the inclination of the plane C with respect to the plane B.

  An angle formed by the plane A and the plane B is ν1, an angle formed by the plane C and the plane B is ν2, and a difference between the angles is Δν = | ν1−ν2 |. FIG. 3 shows the result (point spot diameter / (optical system focal length / viewing angle)) of the imaging performance with respect to the tilt angle difference Δν of the design sample of the reflection optical system having the non-plane symmetry structure. The imaging performance was expressed as an amount obtained by normalizing the average value of the rms spot diameter of the point image on the image plane by the focal length and the viewing angle of the optical system. The smaller the evaluation value, the smaller the image blur and the higher the imaging performance. As can be seen from FIG. 3, for example, if the imaging performance is defined to be 0.0001 or less in this evaluation value, the angle difference Δν may be set to 45 degrees or less.

  In the first embodiment, the optical system can be made compact by providing the second reflecting mirror 2 with an aperture stop that limits the diameter of the incident light beam. The spread of light within the optical system expands around the stop. Therefore, the third reflecting mirror 3 becomes large when the stop is arranged in the vicinity of the first reflecting mirror 1, and the first reflecting mirror 1 becomes large when the stop is arranged in the vicinity of the third reflecting mirror 3, and the optical system becomes large.

  Therefore, by disposing the aperture stop on the second reflecting mirror 2 that is substantially in the middle of the optical system, it is possible to suppress an increase in size of the other reflecting mirror and obtain a compact optical system. As a result, since the size of the reflecting mirror can be suppressed, the size of the entire optical system can be made compact. At this time, the aperture stop may be provided separately from the second reflecting mirror 2, and the same effect can be obtained as an aperture stop using the light reflection area of the second reflecting mirror 2.

  In the first embodiment, some or all of the three reflecting mirrors may have a free-form surface having no rotational symmetry axis. The optical system of the first embodiment has a non-rotationally symmetric structure. Although aberrations similar to astigmatism due to non-rotational symmetry can be suppressed by the conditions for the planes A, B, and C, higher-order aberrations may need to be further reduced depending on use conditions. . At this time, by using a free-form surface as the shape of the reflecting mirror, it is possible to easily reduce non-rotationally symmetric aberrations that appear as higher-order aberrations.

  Further, in the first embodiment, a high-precision reflector can be produced with high productivity by molding the reflector by using a mold to transfer the shape thereof. Die creation requires high-accuracy 3-axis control such as cutting and grinding. However, if one die is created, many reflectors can be created, resulting in low cost. Can be mass-produced at Examples of the creation method for transferring the mold shape include press molding, injection molding, and mold molding. By using a polymer material as the material of the reflecting mirror, the moldability is high and the cost of the material can be kept low. A sufficient reflectance can be obtained by coating or plating the reflective surface with a metal having a high reflectance in the wavelength band targeted by the optical system.

  Further, the imaging optical system of the first embodiment may be used as an optical system in the infrared region. In the infrared region, since a special and expensive material such as germanium or silicon is generally used as a lens material, the cost can be reduced by using an optical system composed only of the reflecting mirror of the first embodiment. An infrared reflecting mirror can be produced by coating or plating a metal having high reflectivity such as aluminum. Since such a metal is generally available and processing is not special, the cost can be reduced.

Embodiment 2. FIG.
FIG. 4 is a schematic diagram showing an imaging optical system according to Embodiment 2 of the present invention. 4, the same or corresponding parts as those in the imaging optical system according to Embodiment 1 shown in FIGS. 1 and 2 are denoted by the same reference numerals, and the description thereof is omitted.

  In FIG. 4, a central principal ray 5 b incident on the optical system and a central principal ray 5 d incident on the imaging plane 4 are formed by central principal rays 5 b and 5 c propagating from the first reflecting mirror 1 to the third reflecting mirror 3. Offset in the same direction relative to the plane. As a result, since the incident light beam is not positioned between the reflecting mirrors as in the optical system having a plane-symmetric structure, vignetting hardly occurs even if the incident light beam spreads. In addition, since the light beam incident on the imaging surface 4 is not positioned between the reflecting mirrors, vignetting is not easily generated even if the incident light beam spreads. Therefore, according to the second embodiment, as in the first embodiment, the optical system can have a large aperture and a wide angle.

  An optical system having a structure that does not have a symmetric surface generates non-plane symmetric aberration. The dominant aberration among the non-plane-symmetrical aberrations is an aberration whose focal position changes depending on the direction on the pupil plane of the optical system, as in the first embodiment. Therefore, in order to obtain an optical system in which aberration is suppressed, the difference in focal plane between the radial and tangential reflecting mirrors cancels each other, and finally the focal planes in the two directions almost coincide with each other. .

  In order for the difference in focal plane between the two directions to cancel each other, the radial direction and the tangential direction of the focal plane in each reflector need to be substantially coincident. FIG. 5 shows a plane formed by the central principal ray 5 as in FIG. In the second embodiment, in order to suppress non-rotationally symmetric aberration similar to astigmatism, it is necessary to reduce the difference between the inclination of the plane A with respect to the plane B and the inclination of the plane C with respect to the plane B. The relationship between the imaging performance and the angle difference Δν shown in FIG. 3 can also be applied to the angle difference Δν in the second embodiment.

  Therefore, for example, in order to make the evaluation value obtained by normalizing the average value of the rms spot diameter of the point image on the imaging plane 4 by the focal length and the viewing angle of the optical system to be 0.0001 or less, the angle formed by the plane A and the plane B Is ν1, the angle between plane C and plane B is ν2, and the difference in angle is Δν = | ν1−ν2 |, the angle difference Δν may be set to 45 degrees or less.

1 is a schematic diagram showing an imaging optical system according to Embodiment 1 of the present invention. It is a figure which shows the relationship between each plane which the center chief ray which injects into each reflector shown in FIG. 1, and the center chief ray reflected by each reflector make. It is a figure which shows the result (point image spot diameter / (optical system focal length * viewing angle)) which evaluated the imaging performance with respect to the inclination-angle difference (DELTA) v about the design sample of the reflection optical system of a non-plane symmetry structure. It is the schematic which shows the imaging optical system which concerns on Embodiment 2 of this invention. It is a figure which shows the relationship between each plane which the center chief ray which injects into each reflector shown in FIG. 4 and the center chief ray reflected by each reflector make.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 1st reflective mirror, 2nd reflective mirror, 3rd reflective mirror, 4 imaging surface, 5 center principal ray, A center incident principal ray to 1st reflector, and reflection center principal ray in 1st reflector B, a plane including the incident central chief ray on the second reflecting surface and the reflecting central chief ray on the second reflecting surface, and C, the incident central chief ray on the third reflecting surface and the third reflecting surface. A plane including the reflection center principal ray, an angle formed by ν1 plane A and plane B, and an angle formed by ν2 plane C and plane B.

Claims (5)

Having three reflecting mirrors arranged in the order of the first reflecting mirror, the second reflecting mirror, and the third reflecting mirror in the order of the optical path of the incident light beam;
In the imaging optical system in which the light beam reflected by the three reflecting mirrors forms an imaging surface,
With respect to the central principal ray defined as the principal ray of the light beam imaged at the center of the imaging plane,
A plane including the incident central chief ray on the first reflecting mirror and the reflecting central chief ray on the first reflecting mirror is defined as plane A,
The plane including the incident central chief ray on the second reflecting surface and the reflecting central chief ray on the second reflecting surface is defined as plane B,
When a plane including the central light ray incident on the third reflecting surface and the central light ray reflected on the third reflecting surface is a plane C,
The plane A and the plane C do not coincide with each other,
When the angle between the plane A and the plane B is ν1, and the angle between the plane C and the plane B is ν2, the angle difference Δν = | ν1−ν2 | is 45 ° or less. A characteristic imaging optical system. The imaging optical system according to claim 1,
The imaging optical system, wherein the second reflecting mirror has an aperture stop that limits a beam diameter of incident light. The imaging optical system according to claim 1 or 2,
Any one or all of said 1st reflective mirror, said 2nd reflective mirror, and said 3rd reflective mirror are the free-form surface shape which does not have a rotational symmetry axis. The imaging optical system characterized by the above-mentioned. The imaging optical system according to any one of claims 1 to 3,
An imaging optical system, wherein the three reflecting mirrors are manufactured by molding. The imaging optical system according to claim 4,
An imaging optical system, wherein the material of the three reflecting mirrors is a polymer material.

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