JP2007199360A - Imaging optical system and imaging optical unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging optical system or the like used in a high-specification (mega-pixel) imaging device. <P>SOLUTION: The imaging optical system OS has at least three or more optical prisms PR making light from an object side pass, and at least one of the plurality of prism optical elements PR has positive power. At least one of optical acting surfaces included in the plurality of optical prisms PR is arranged to be eccentric. When light advances consecutively to the different optical prisms PR in the plurality of optical prisms PR, a light emitting surface in one optical prism PR and a light incident surface in the other optical prism PR are opposed to each other. At least one of combinations comprising two optical prisms PR included in the plurality of optical prisms PR satisfies a predetermined conditional expression, and also at least two transmission surfaces of the optical acting surfaces are made non-rotational symmetric surfaces. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an imaging optical system and an imaging optical unit that are mounted on an imaging apparatus that captures a subject image with an imaging element or the like.

  In recent years, digital devices such as a mobile phone and a personal digital assistant (PDA) have incorporated a digital camera or the like (imaging device) for capturing an image. In such a digital device, downsizing (thinning) is desired from the viewpoint of portability, while increasing the number of pixels (high specification) of the image sensor is also demanded from the viewpoint of improving image quality.

  Usually, when the imaging device becomes high-spec, it is necessary to improve the resolving power of an optical system (imaging optical system) for forming a subject image on the imaging device. Therefore, for example, an optical system (a coaxial optical system) called a straight type increases the size of the optical system itself or increases the number of lenses constituting the optical system in order to increase the resolution and the like. For this reason, it can be said that a digital device employing such a coaxial optical system is difficult to satisfy both demands for downsizing and high performance (high resolution, high aberration correction capability, etc.) at the same time. Therefore, unlike a coaxial optical system, there is an increasing demand for a compact and high-performance optical system.

In order to meet this demand, in recent years, various eccentric optical systems different from coaxial optical systems (for example, optical systems using optical prisms having a reflecting surface inclined with respect to the incident optical axis; see Patent Documents 1 to 8). Proposed.
Japanese Patent Laid-Open No. 10-11525 (see FIG. 1) Japanese Patent Laid-Open No. 9-329747 (see FIG. 1) Japanese Patent Laid-Open No. 11-271618 (refer to configuration parameters) JP-A-8-292368 (see FIG. 41) JP 2001-174705 A (see FIG. 11) Japanese Patent Laid-Open No. 10-39209 (see FIG. 1) JP 2002-90692 A (see FIG. 41) JP 2003-84200 A (refer to numerical data)

  Since the optical systems of Patent Documents 1 and 2 include only one optical prism, the size of the optical system itself is small. However, such an optical prism has a sufficient number of other optical working surfaces required for correcting chromatic aberration (axial chromatic aberration and lateral chromatic aberration) caused by two transmission surfaces (incident surface / exit surface). It will not be prepared. In other words, the optical systems of Patent Documents 1 and 2 can be said to be optical systems in which chromatic aberration is likely to occur. For this reason, it is difficult for this optical system to achieve high performance due to the high specification of the image sensor.

  On the other hand, the optical system of Patent Document 3 includes two optical prisms, and the number of optical action surfaces that can be used for chromatic aberration correction is relatively large. However, in this optical system, since two optical prisms having the same Abbe number are used, chromatic aberration correction using the difference in Abbe number cannot be performed. For this reason, it is difficult to say that such an optical system has been subjected to sufficient chromatic aberration correction, and it is difficult to achieve high performance due to high specifications.

  The optical systems of Patent Documents 4 and 5 include three optical prisms, and the optical systems of Patent Documents 6 and 7 include four optical prisms. In these optical systems, the number of optical working surfaces increases as the number of optical prisms is relatively large. Therefore, such an optical system is effective for correcting monochromatic aberration. However, in these optical systems, chromatic aberration tends to occur with an increase in the optical action surface (transmission surface) having a transmission function.

  However, similar to the optical system of Patent Document 3, these optical systems use a plurality of optical prisms having the same Abbe number and power, and therefore chromatic aberration correction using the difference in Abbe number cannot be performed. Therefore, it is difficult to say that these optical systems are sufficiently corrected for chromatic aberration, and it is difficult to realize high performance due to the high specification of the image sensor.

  On the other hand, the optical system of Patent Document 8 includes two optical prisms having different Abbe numbers, thereby performing chromatic aberration correction using the difference in Abbe numbers. In addition, looking at the power distribution in this optical system, the reflecting surface bears a relatively large power, and the transmitting surface bears a relatively small power. Therefore, it can be said that the occurrence of chromatic aberration is suppressed.

  However, when the specification of the image sensor is increased, the size of the image sensor may increase, and in this case, the image height also increases. As a result, the focal length of the optical system becomes longer due to the higher image height. Such a long focal length leads to an increase in the size of the optical system, so it is preferable to place power on the transmission surface as well as the reflection surface. However, it can be said that the optical system of Patent Document 8 with a relatively small power load on the transmission surface is not suitable for downsizing.

  In such an optical system, the reflecting surface in an eccentric arrangement bears a relatively large power, so that there are also specific aberrations (such as eccentric coma aberration and eccentric astigmatism) caused by the eccentric arrangement. Likely to happen. However, since the transmission surface does not bear a sufficient power burden, it is difficult to correct eccentric coma using the power of the transmission surface. Conversely, if power is applied to the transmission surface in order to correct decentration coma and the like, chromatic aberration may occur remarkably. That is, the imaging optical system of Patent Document 8 cannot be a small and high-performance optical system because the balance of power burden between the transmission surface and the reflection surface is lost.

  The present invention has been made in view of the above situation, and an object thereof is to provide an optical system and the like that can be used for a high-spec image sensor. More specifically, the object of the present invention is to provide a balanced balance of power on the transmission surface and the reflection surface of a plurality of optical prisms, so that a small size and high performance {high correction capability for various aberrations etc. It is an object of the present invention to provide an imaging optical system and the like that can exhibit high chromatic aberration correction capability).

  In the optical system (imaging optical system) of the present invention, the prism optical element that allows light from the object side to pass can suppress an increase in the size of the imaging optical system while securing an optical action surface required for correcting various aberrations. The number is at least 3 or more. In addition, in this imaging optical system, at least one of the plurality of prism optical elements exhibits positive power (composite positive power) as a whole of the prism optical elements in order to suppress various aberrations. Yes.

  Furthermore, in the imaging optical system of the present invention, in order to reduce the size of the imaging optical system by, for example, bending the optical path, at least one of the optical action surfaces (for example, reflecting surfaces) included in the plurality of prism optical elements is decentered. It is arranged. In addition, in order to further reduce the size of the imaging optical system, the imaging optical system is configured such that light continuously travels to different prism optical elements in the plurality of prism optical elements, and the light in one prism optical element. And the light incident surface of the other prism optical element face each other to narrow the space between the prism optical elements.

  In particular, in the imaging optical system of the present invention, at least one combination of two prism optical elements included in the plurality of prism optical elements satisfies the following conditional expression (1), and the imaging optical system: At least two transmission surfaces of the inner optical action surface are non-rotation symmetric surfaces.

5.0 <| νd (α) −νd (β) | <80.0 Conditional expression (1)
However,
νd (α): One prism optical element in a set of two prism optical elements is
Abbe number with respect to d-line νd (β): the other prism light different from one prism optical element constituting the above set
This is the Abbe number for the d-line of the scientific element.

  Conditional expression (1) defines a preferable range of Abbe number differences when a combination of prism optical elements having different Abbe numbers is included in the imaging optical system in order to correct chromatic aberration. Therefore, if it is within the range of conditional expression (1), chromatic aberration correction utilizing the difference in Abbe number can be achieved. Therefore, even if the transmission surface bears a certain amount of power, chromatic aberration is unlikely to occur, and an imaging optical system that can bear power suitable for both the transmission surface and the reflection surface is realized.

  In addition, since at least two transmission surfaces of the optical action surface are non-rotationally symmetric surfaces, various effects that cannot be removed only by correction using the Abbe number difference due to the influence of the optical action surface of the eccentric arrangement. Aberrations (eccentric chromatic coma, eccentric chromatic astigmatism, etc.) can also be corrected. Therefore, the present invention is an imaging optical system that includes a plurality of prism optical elements and is small in size and can exhibit high performance. As a result, the present invention can be said to be an imaging optical system that can be used for a high-spec imaging device.

  By the way, various power arrangements in the imaging optical system are assumed. For example, in the case of a telephoto type power arrangement (in the case of a power arrangement of positive power first and then negative power), at least one set in the imaging optical system is configured. It is desirable that the two prism optical elements satisfy the following conditional expression (2).

νd (F) −νd (L)> 5 Conditional expression (2)
However,
νd (F): prism light received first in a set of two prism optical elements
Abbe number with respect to d-line possessed by scientific element νd (L): prism light received later in a set of two prism optical elements
This is the Abbe number for the d-line of the scientific element.

  When the conditional expression (2) is satisfied, the Abbe number of the prism optical element that receives light first is larger than the Abbe number of the prism optical element that receives light later. Therefore, an imaging optical system desirable for chromatic aberration correction is realized.

  In the imaging optical system of the present invention, it is desirable that at least one prism optical element in the plurality of prism optical elements satisfies the following conditional expression (3).

0.2 <Σ | φREFR | / Σ | φREFL | <100.0 Conditional expression (3)
However,
.SIGMA..vertline..phi.REFR.vertline .... absolute value of power of each transmission surface included in one prism optical element.
Sum Σ | φREFL |: Absolute value of the power of each reflecting surface included in one prism optical element
Is the sum of

  Conditional expression (3) defines power distribution for the transmission surface and the reflection surface in the prism optical element. Therefore, within the range of conditional expression (3), the occurrence of various aberrations due to the excessive power load on the transmission surface is suppressed, while the processing error due to the excessive power load on the reflecting surface is suppressed. Variation in performance (for example, generation of various aberrations caused by processing errors on the reflecting surface) is suppressed. Therefore, a high-performance imaging optical system capable of suppressing various aberrations is realized within the range of conditional expression (3).

  In the imaging optical system of the present invention, it is desirable that each prism optical element having a positive power satisfies the following conditional expression (4).

0.01 <φp / φALL <10.0… Conditional expression (4)
However,
φp: Horizontal power and vertical power in prism optical element with positive power
Average power φALL: Horizontal power and vertical power in the imaging optical system (all systems)
It is an average power.

  Conditional expression (4) defines a power range in which downsizing of the imaging optical system using the positive power can be realized while suppressing various aberrations caused by the positive power. Therefore, as long as it is within the range of conditional expression (4), the present invention is a high-performance imaging optical system that is compact but suppresses various aberrations.

  In the imaging optical system of the present invention, it is desirable that at least one of the optical action surfaces included in the plurality of prism optical elements is a free-form surface from the viewpoint of correcting chromatic aberration.

  Further, from the viewpoint of miniaturization, in the imaging optical system of the present invention, at least one prism optical element in the plurality of prism optical elements receives light and reflects light traveling from the incident surface. It is desirable to have a simple structure having one surface and one emission surface for emitting light reflected from the reflection surface.

  By the way, the material of the prism optical element in the imaging optical system of the present invention is not particularly limited. For example, at least one of the plurality of prism optical elements may be formed of resin. However, the resin has the property of changing the refractive index depending on the temperature change. Therefore, it is desirable that the resin of the optical element in the imaging optical system of the present invention has a characteristic that suppresses optical transition depending on temperature (for example, refractive index change of resin depending on temperature).

  This is because if the prism optical element includes such a resin (referred to as an athermal resin), the imaging optical system of the present invention can exhibit functions such as high aberration correction even under various temperature changes. In addition, because of the resin, the imaging optical system is reduced in weight.

  In particular, the athermal resin includes a base material and a base material, and the first property of the base material is altered by the second property of the base material, so that optical transition is suppressed. May be.

  Needless to say, an imaging optical unit including the imaging optical system described above and an imaging element that receives light from the imaging optical system also exhibits the above-described effects.

  According to the present invention, chromatic aberration correction using the difference in Abbe number among a plurality of prism optical elements can be performed, so that a certain amount of power can be applied to the transmission surface that causes chromatic aberration. Therefore, power suitable for the transmission surface and the reflection surface in the imaging optical system can be borne. As a result, it is possible to realize a high-performance and compact imaging optical system that corrects chromatic aberration caused by the power load on the transmission surface and suppresses various aberrations caused by the excessive power load on the reflection surface.

  First, various optical systems are assumed as the optical system (imaging optical system) of the present invention. For example, there are an imaging optical system in which only optical prisms (prism optical elements) are combined, and an imaging optical system in which a reflecting mirror and a lens are added in addition to the optical prism. Therefore, in the following, an example of various assumed imaging optical systems will be described.

  The optical prism used in the imaging optical system is an optical element having at least one incident surface on which light (light rays) is incident, a reflecting surface that reflects light, and an emission surface that emits light. A unit including an imaging optical system and an imaging element that receives light from the imaging optical system is expressed as an imaging optical unit. In addition, the imaging optical system may be expressed as an imaging optical system from the point of forming an optical image on the imaging element, or has a decentered optical working surface, so that it is a non-axis optical system (eccentric optical system). System).

[Embodiment 1]
[1. Configuration of Imaging Optical Unit (Example 1) (see FIGS. 1 to 5)]
FIG. 1 shows an optical cross-sectional view of the imaging optical unit OSU of Embodiment 1 of the present invention. As shown in FIG. 1, the imaging optical unit OSU includes an imaging optical system OS (first optical prism PR1 to third optical prism PR3) and an imaging element SR. In addition, in order to express each surface (si) in the first optical prism PR1 to the third optical prism PR3 and the image surface (si) of the imaging element SR, light from the object side to the image side (image surface side) The numbers are assigned according to the incident order (i-th; i = 1, 2, 3,...). An asterisk (*) is attached to a surface that is a free-form surface.

<1-1. Imaging optical system and image sensor>
As shown in FIG. 1, the imaging optical system OS includes a first optical prism PR1, a second optical prism PR2, and a third optical prism PR3. The first optical prism PR1 is an optical prism through which light from the object side first passes, and the second optical prism PR2 is an optical prism through which light emitted from the first optical prism PR1 continues to enter. The third optical prism PR3 is an optical prism that passes (transmits) the light emitted from the second optical prism and guides it to the image plane (imaging element SR).

<< 1-1-1. About the first optical prism >>
The first optical prism PR1 has three optical action surfaces (s2 to s5). Note that the first surface s1 is a dummy surface (reference surface; a surface for representing each surface vertex position) in the design using global coordinates. Therefore, in FIG. 1, even if the light (light flux) from the object side is the first incident surface, it is not described as the first surface s1 (displayed in parentheses).

  Therefore, the surface that first receives and transmits light from the object side, that is, the incident surface is denoted as the second surface s2 in FIG. The third surface s3 is a reflective surface that reflects the light transmitted (passed) through the second surface s2 toward the fourth surface s4.

  The fourth surface s4 is a reflecting surface that reflects the reflected light from the third surface s3 toward the fifth surface s5. As shown in FIG. 1, the second surface s2 and the fourth surface s4 are TIR (Total Internal Reflection) surfaces having both transmission and reflection functions. Further, the position of the light beam incident on the second surface s2 is different from the position of the light beam incident on the fourth surface s4.

  The fifth surface s5 is an emission surface (transmission surface) that emits (transmits) the reflected light from the fourth surface s4 toward the second optical prism PR2. In addition, this 5th surface s5 and the below-mentioned 6th surface s6 are the arrangement | positioning relationship (facing arrangement | positioning) which faced each other.

<< 1-1-2. About the second optical prism >>
On the other hand, the second optical prism PR2 guides light that has passed through the first optical prism PR1 to the third optical prism PR3. The second optical prism PR2 has three optical action surfaces (s6 to s8).

  The sixth surface s6 is an incident surface that first receives and transmits light from the first optical prism PR1. The seventh surface s7 is a reflecting surface that reflects light (transmitted light) that has passed through the sixth surface s6 toward the eighth surface s8. Further, the eighth surface s8 is an exit surface that emits light (reflected light) from the seventh surface s7 toward the third optical prism PR3.

  The sixth surface s6 is provided with an optical aperture ST (for example, an optical aperture having a circular aperture shape). Further, the eighth surface s8 and a ninth surface s9, which will be described later, are opposed to each other.

  The second optical prism PR2 has a positive power [converging force (+); the power is defined by the reciprocal of the focal length]. The power of the optical prism PR includes power for light in the horizontal direction (referred to as X direction) in the light beam and power for light in the vertical direction (referred to as Y direction) in the light beam. Therefore, the optical prism PR having a positive power means that the average of the power in the horizontal direction and the vertical direction is positive.

<< 1-1-3. About the third optical prism >>
The third optical prism PR3 guides light that has passed through the second optical prism PR2 to the imaging element SR (image surface s12). The third optical prism PR3 has three optical action surfaces (s9 to s11).

  The ninth surface s9 is an incident surface that first receives and transmits light from the second optical prism PR2. The tenth surface s10 is a reflecting surface that reflects the light that has passed through the ninth surface s9 toward the eleventh surface s11. Furthermore, the eleventh surface s11 is an emission surface that emits light from the tenth surface s10 toward the imaging element SR (image surface s12).

<< 1-1-4. About image sensor >>
The imaging element SR receives light (light image) that has passed through the optical prisms PR1 to PR3 on the imaging surface s12 and converts it into an electrical signal (electronic data). For example, a CCD (Charge Coupled Device) area sensor, a CMOS (Complementary Metal Oxide Semiconductor) sensor, and the like can be cited.

<1-2. About construction data>
Here, construction data in the imaging optical unit OSU of the first embodiment will be described with reference to Tables 1 to 4.

  “Si” in Table 1 indicates the i-th surface according to the incident order of light counted from the object side, as described above. “Ri” indicates a radius of curvature [unit: mm] on each surface (si). “Ni” and “υi” are refractive indices (Nd) with respect to the d-line (587.56 nm) of the medium located at the axial upper surface distance between the i-th surface (si) and the i + 1-th surface (si + 1). ) · Abbe number (νd). The Abbe number (νd) is obtained from the following equation.

νd = (nd−1) / (nF−nc)
However,
nd: refractive index with respect to d-line (wavelength 587.57 nm) nF: refractive index with respect to F-line (wavelength 486.13 nm) nc: refractive index with respect to c-line (wavelength 656.28 nm)

  In Table 2, “surface vertex coordinates” and “rotation angle” in each surface (si) are shown. The surface vertex coordinates (surface data: [unit; mm]) are expressed based on the right-handed orthogonal coordinates (X coordinate, Y coordinate, Z coordinate) shown in FIG. (X axis); thumb, Y coordinate (Y axis); index finger, Z coordinate (Z axis); middle finger}.

  Specifically, a light beam that passes from the center of the object plane to the center of the aperture stop (center of the optical aperture) and travels toward the center of the image plane is defined as a base beam, and the intersection of the base beam and the first surface s1 is defined as the origin (0, 0, 0). The Z-axis direction is a direction in which the base ray passes through the origin from the center of the object surface toward the first surface s1, and the direction is <positive (positive direction)>. Then, the X-axis direction is a direction perpendicular to the paper surface in FIG. 1, and the direction toward the back surface side of the paper surface is <positive (positive direction)>. On the other hand, the Y-axis direction is a direction parallel to the paper surface, and the direction toward the upper side of the paper surface is <positive (positive direction)>.

  Further, the rotation angle (rotation angle data: [unit; °]) is expressed by an inclination centered on the coordinate position (surface vertex position) of the surface vertex defined by the right-handed XYZ orthogonal coordinates. Yes.

  Specifically, the rotation angle (X rotation, Y rotation, Z rotation) around each axis (X coordinate, Y coordinate, Z coordinate) around the surface vertex of each surface (si) is expressed. ing. It should be noted that the counterclockwise direction with respect to the positive direction on the X and Y axes is positive X rotation and positive Y rotation. That is, the rotation angle is defined as the positive direction (positive). On the other hand, the clockwise direction with respect to the positive direction on the Z axis is defined as the positive Z rotation.

Table 3 shows the free-form surface coefficient of each surface. Specifically, the free-form surface is defined by the following definition formula (I) using local orthogonal coordinates (x, y, z) whose origin is the surface vertex.
Therefore, Table 3 shows the free-form surface coefficients used in the following defining formula (I).

It should be noted that the coefficient of the term not described is 0 (k = 0 for all free-form surfaces), and E−n = × 10 ⁻ⁿ for all data.

…定義式（I）

... Definition formula (I)

However, in the definition formula (I),
z: Displacement in the z-axis direction at the position of height h (based on the surface vertex)
h: height in a direction perpendicular to the z-axis (h ² = x ² + y ² )
c: Paraxial curvature (= 1 / radius of curvature)
k: conic coefficient C _j : free-form surface coefficient, and the free-form surface term is represented by the following defining formula (II).

…定義式（II）

... Definition formula (II)

  Table 4 shows the focal length [unit; mm], the F number [Fno], and the radius [unit; mm] of the optical aperture stop ST (in the case of a circle) in the entire imaging optical system OS. Table 4 shows the horizontal angle (unit: °) in the horizontal direction (X direction) and vertical direction (Y direction), and the horizontal direction (long side; long side) and vertical direction (short side) of the image plane size. The length [unit: mm] is also shown.

<1-3. About aberration diagrams>
2 (FIGS. 2A to 2F), FIG. 3 (FIGS. 3A to 3F), FIG. 4 (FIGS. 4A to 4F), and FIG. 5 (FIGS. 5A to 5F) are the imaging optical unit in the first embodiment. It is a lateral aberration diagram of OSU. Specifically, FIGS. 2 and 3 show lateral aberrations (ΔX · ΔY) in the X direction (horizontal direction) of the light beam, and FIGS. 4 and 5 show lateral aberrations (ΔX · ΔY) in the Y direction (vertical direction) of the light beam. Is shown. These lateral aberration diagrams show the lateral aberration [unit: mm] with respect to the d-line at the image height [unit: mm] represented by local orthogonal coordinates (x, y) on the image plane IS in FIG. Yes.

  That is, (A) to (C) in FIGS. 2 to 5 are three positions on the positive side in the x direction in the local orthogonal coordinate system (x, y) with the center of the image plane IS as the origin o {image plane It corresponds to three locations (positions of circle A to circle C)} on one short side in IS. 2 (D) to 5 (F) show three locations on both positive and negative sides in the y direction including the origin o {three locations including the center in the image plane IS and along the y direction (circles D to F). )}. The scales of the lateral aberration diagrams in FIGS. 2 to 5 are the vertical axis [−0.050 to 0.050] representing the aberration amount and the horizontal axis [−1.0 to 1.0] representing the image height. ing.

[2. Examples of various features of the present invention]
As described above, the imaging optical system OS according to the first exemplary embodiment includes three optical prisms that allow light from the object side to pass therethrough (that is, imaging optical in which the total number of optical prisms PR is at least three or more). It can also be said to be a system OS). Such a number of about three optical prisms is preferable as the number of optical prisms included in the imaging optical system OS.

  This is because when the number of optical prisms PR is one or two, the number of optical action surfaces is insufficient, and the performance of the imaging optical system OS cannot be improved (for example, high aberration correction capability cannot be exhibited). If a small number of optical prisms PR include a large number of optical action surfaces, the optical prism PR itself (and consequently the imaging optical system OS) becomes large.

  On the other hand, when the number of the optical prisms PR is excessively increased, the number of optical action surfaces is increased, but the imaging optical system OS is increased in size. Therefore, the imaging optical system OS including at least three optical prisms PR is relatively small (thin) while exhibiting high aberration correction capability and the like by including an appropriate number of optical working surfaces.

  In addition, it is desirable that the imaging optical system OS has a positive power in order to form an image of light from an object on the imaging element SR. Therefore, the imaging optical system OS of the present invention includes the second optical prism PR2 having a positive power (that is, at least one of the optical prisms PR included in the imaging optical system OS has a positive power). Can also be said to have).

  In particular, the second optical prism PR2 in Embodiment 1 does not exhibit positive power only with one optical action surface in the optical prism PR2, but passes through a plurality of optical action surfaces in one optical prism PR2. , Positive power (composite positive power) is to be demonstrated.

  For example, when the optical prism is designed so as to exhibit positive power with only one optical action surface in the optical prism, the optical action surface passes through a plurality of optical action surfaces and is combined with the optical prism PR that exhibits combined positive power. Strong positive power is required compared to one surface of the optical working surface. Therefore, relatively various aberrations are likely to occur due to such an optical working surface that exhibits such a strong positive power. In particular, spherical aberration increases and image surface tilting or the like significantly occurs.

  However, in the imaging optical system OS of the present invention, light converges through a plurality of optical action surfaces in one optical prism PR. For this reason, in the present invention, the power on each optical action surface is weakened by distributing the positive power to a plurality of surfaces to be borne. As a result, the occurrence of aberrations in the optical prism PR, and hence the imaging optical system OS (entire system), is reduced. Further, when the combined power of the entire optical prism PR is positive, it becomes possible to cancel out aberrations even when a negative power (divergence) optical working surface is provided on the optical prism PR. The occurrence of aberrations can be suppressed.

  In addition, at least one of the optical action surfaces included in the plurality of optical prisms (PR1 to PR3) included in the imaging optical system OS is eccentrically arranged. Here, “eccentricity” means not only a 45 ° reflection surface such as a right-angle prism but also an optical action surface having various angles.

  In the case of such an imaging optical system (decentered optical system) OS, light from the object side reaches the image side while being refracted and reflected by the decentered surface. Therefore, the imaging optical system OS of the present invention cannot be configured to extend in one direction unlike a straight type optical system (coaxial optical system). That is, by bending the optical path, the imaging optical system OS of the present invention can be made smaller and thinner than a straight type optical system.

  Further, since the light reaches the image plane while being refracted and reflected, the optical path in the imaging optical system OS is relatively long. When the optical path length is thus increased, the imaging optical system OS can effectively correct and suppress various aberrations by using the long optical path. Moreover, even if a manufacturing error occurs in the optical prism PR or the like, such an imaging optical system OS can suppress a change in performance due to the manufacturing error to be small because of a relatively long optical path length.

  Furthermore, in the imaging optical system OS of the present invention, when light continuously travels to different optical prisms PR in the plurality of optical prisms PR, the light exit surface of one optical prism PR and the other optical The light incident surface of the prism PR faces each other.

  For example, in the case of Example 1, the fifth surface s5 and the sixth surface s6, and the eighth surface s8 and the ninth surface s9 are opposed to each other. With such an opposed arrangement, adjacent optical prisms PR (and thus the entire imaging optical system OS) can be accommodated in a small size. Further, if the opposing faces are substantially parallel, the arrangement is such that both faces are very close to each other. The imaging optical system OS is realized. As a result, the size of the imaging optical system OS itself of the present invention tends to be small.

  Incidentally, the optical prism PR includes a transmission surface (incident surface / exit surface) and a reflective surface. When the transmission surface has power, chromatic aberration that inevitably deteriorates the imaging performance of the imaging optical system OS is inevitably generated due to the power. For example, a phenomenon of color blur on the imaging surface of the imaging element SR. Appears. In order to avoid such a phenomenon, there is a measure for adjusting the power sharing between the transmission surface and the reflection surface in the optical prism PR. For example, there is a measure for burdening the reflecting surface with a larger power than the transmitting surface.

  In the case of such a measure, although chromatic aberration can be suppressed due to a reduction in the power burden on the transmission surface, it also has a negative effect. For example, when the reflecting surface is excessively burdened with power and is eccentrically arranged, the excessive power burden and eccentricity are combined, and various aberrations peculiar to eccentricity (eccentric coma, eccentric astigmatism, etc.) Is likely to occur. Such various aberrations peculiar to eccentricity significantly deteriorate the imaging performance of the imaging optical system OS. For this reason, such measures cannot achieve high performance (high resolution, high aberration correction capability, etc.) due to the high specification of the image sensor SR.

  Then, applying a certain amount of power to the transmission surface leads to suppression of various aberrations due to the eccentric reflection surface. However, when the transmission surface has power, chromatic aberration occurs as described above. That is, some aberrations occur even when the reflection surface bears power or when the transmission surface bears power.

  Therefore, the imaging optical system OS of the present invention suppresses various aberrations peculiar to eccentricity so as not to impose excessive power on the reflecting surface, while it imposes a certain amount of power on the transmitting surface, but does not compensate for the generated chromatic aberration. It is corrected by the difference in Abbe number of different optical prisms PR. Specifically, it is desirable that at least one combination of two optical prisms PR included in the plurality of optical prisms PR satisfies the following conditional expression (1).

5.0 <| νd (α) −νd (β) | <80.0 Conditional expression (1)
However,
νd (α): One prism optical element in the set of two optical prisms PR
Abbe number with respect to d-line νd (β): the other optical prism different from the one optical prism PR constituting the set
Is the Abbe number for the d line of the PR.

  Conditional expression (1) defines an appropriate range when chromatic aberration correction is performed by including a combination (set) of optical prisms PR having different Abbe numbers in the imaging optical system OS.

  For example, when the value of the conditional expression (1) is less than or equal to the lower limit value, the difference in Abbe number within the combination of the optical prisms PR included in the imaging optical system OS is too small to sufficiently correct chromatic aberration. On the other hand, when the value of the conditional expression (1) is equal to or higher than the upper limit value, for example, the Abbe number of the optical action surface that exhibits a relatively positive high power and the Abbe number of the optical action surface that exhibits a relatively negative low power. When the difference is too large, other aberrations such as image plane correction and astigmatism cannot be corrected due to the difference.

  However, within the range of the conditional expression (1), the imaging optical system OS can bear a certain amount of power on the transmission surface while correcting the chromatic aberration, and also correct other aberrations. I can plan. In addition, since the transmission surface bears power, for example, an excessive power load does not occur on the eccentric reflection surface. Therefore, the occurrence of aberrations peculiar to eccentricity due to the reflecting surface such as the eccentric arrangement is relatively suppressed. Therefore, the present invention can be said to be an imaging optical system OS that realizes higher performance (higher aberration correction capability can be achieved) due to higher specifications of the imaging element SR.

  The values of conditional expression (1) in Example 1 are as shown in Table 5 below.

Further, it is desirable that the range of the following conditional expression (1a) is satisfied among the range defined by the conditional expression (1).
15.0 <| νd (α) −νd (β) | <60.0 Conditional expression (1a)

  By the way, when the reflecting surface etc. which became eccentric arrangement | positioning are contained in imaging optical system OS, an advantageous effect also arises from a viewpoint of the difference of Abbe number. Therefore, this effect will be described in detail.

  First, the relationship between the Abbe number and chromatic aberration in a coaxial optical system (an optical system that does not use a decentered reflecting surface) will be described. For example, in the case of a coaxial optical system, when the entire system has a positive power, the following relational expression (A) is derived. This relational expression (a) is derived from the fact that the power of the entire system is defined as “positive” when the sum of the absolute values of the powers of the transmission surfaces is “positive”.

| Φ (+) |> | φ (−) |
However,
| Φ (+) |: absolute value of transmission surface having positive power | φ (−) |: absolute value of transmission surface having negative power.

  Further, from the power and Abbe number of each transmission surface, the following relational expression (b) serving as a measure of chromatic aberration is derived.

| Φ (+) / ν (+) + φ (−) / ν (−) |
However,
ν (+): Abbe number of the material constituting the transmission surface having a positive power ν (−): Abbe number of the material constituting the transmission surface having a negative power.

In order to suppress chromatic aberration from the relational expressions (a) and (b) {that is, to reduce the absolute value of the relational expression (b) (preferably to “0” (zero)). )}, The following relational expression (c) is derived.
ν (+)> ν (−) ... Relational expression (c)

  Then, it can be seen from the relational expression (c) that it is desirable that the Abbe number of the constituent material on the positive power transmission surface is low dispersion and the Abbe number of the constituent material on the negative power transmission surface is high dispersion.

  Here is an example. For example, if ν (−) = 25, | φ (+) | = 2, | φ (−) | = 1, if the value of the relational expression (b) is to be close to “0”, It is desirable that ν (+) is 50 {ν (+) = 50} (example a). If ν (−) = 25, | φ (+) | = 3, and | φ (−) | = 1, if the value of the relational expression (b) is to be close to “0”, It is desirable that ν (+) is 75 {ν (+) = 75}.

  Then, in the case of chromatic aberration correction in a coaxial optical system, the ratio similar to the ratio relationship (power ratio relationship) between the absolute value of the power of the transmission surface having positive power and the absolute value of the power of the transmission surface having negative power The relationship is also necessary in the ratio relationship (abbe number ratio relationship) between the Abbe number of the constituent material on the positive power transmitting surface and the Abbe number of the constituent material on the negative power transmitting surface.

  However, the present invention is an imaging optical system OS including an optically acting surface (reflecting surface) arranged eccentrically. For this reason, the Abbe number ratio relationship required for chromatic aberration in the coaxial optical system is not necessarily required (that is, the degree of freedom in setting the Abbe number increases). For example, as in example a, even in the case of ν (−) = 25, | φ (+) | = 2, | φ (−) | = 1, in the case of the imaging optical system OS of the present invention, Ν (+) is not necessarily 50.

  This is because the power of the reflecting surface arranged eccentrically also affects the chromatic aberration. More specifically, when the reflecting surface exhibits appropriate positive power or negative power, it can contribute to correction of chromatic aberration. Therefore, the imaging optical system OS of the present invention is effective even if the difference between the Abbe number of the constituent material on the positive power transmitting surface and the Abbe number of the constituent material on the negative power transmitting surface is small regardless of the power ratio relationship. Chromatic aberration correction.

  Conversely, it can be said that the Abbe number ratio relationship does not depend on the power ratio relationship. Therefore, the power ratio relationship may change regardless of the Abbe number ratio relationship (abbe number difference). For example, as the difference in Abbe number is large, the power of the positive power transmission surface may increase, or the power of the positive power transmission surface may increase despite the small difference in Abbe number. .

  As described above, the imaging optical system OS of the present invention includes the reflecting surface and the like arranged in an eccentric arrangement, so that the Abbe number ratio relationship necessary for chromatic aberration in the coaxial optical system is not necessarily required. It can be said that the degree of freedom of setting is increased.

  However, when the imaging optical system OS has an optical surface with an eccentric arrangement, eccentric coma and eccentric astigmatism may occur even at the center of the image plane (on the axis). In addition, the chromatic aberration may not be completely removed only by correction using the difference in Abbe number due to the influence of the optically acting surface of the eccentric arrangement. Therefore, eccentric chromatic coma aberration, eccentric chromatic astigmatism, etc. will arise. In order to correct such eccentric chromatic coma and eccentric chromatic astigmatism, which are a combination of decentration-specific aberrations and chromatic aberration, in addition to the appropriate setting of the Abbe number of the optical prism PR in the imaging optical system OS. It is desirable that at least two transmission surfaces of the optical action surface in the imaging optical system OS are non-rotation symmetric surfaces.

  If this is the case, for example, even if an eccentric color coma aberration occurs due to a certain non-rotationally symmetric transmission surface, another transmission surface is made a non-rotationally symmetric surface, and the generated eccentric color coma is generated. An eccentric chromatic coma in the opposite direction to the aberration can be generated. Then, the eccentric chromatic coma aberrations cancel each other. Therefore, in such an imaging optical system OS, it can be said that another non-rotationally symmetric transmission surface can correct an eccentric chromatic coma aberration or the like caused by one non-rotationally symmetric transmission surface.

  By the way, various types of power arrangement in the imaging optical system OS (entire system) are assumed. For example, there are a retrofocus type that can increase the back focus when the focal length of the entire system is short, a telephoto type that can shorten the total length of the imaging optical system OS when the focal length of the entire system is long, and the like.

  And, as in the present invention, a telephoto type is desirable in an imaging optical system OS that can cope with high specification (high pixel count) of the imaging element SR. This is because when the image pickup element SR becomes high-spec, the size of the image pickup element SR may increase accordingly, and in this case, the image height (Y ′) also increases. More specifically, when the image height is increased in this way, the imaging optical system OS does not increase the focal length (f) as compared with the case of a low image height even when acquiring the light rays having the same angle of view (θ). Don't be. Therefore, a telephoto type imaging optical system OS having a long focal length of the entire system is desirable.

  In the case of the telephoto type imaging optical system OS, positive power is arranged at the front (front group) and negative power is arranged at the rear (rear group). Then, from the viewpoint of chromatic aberration correction, if the optical prism PR that receives light first has low dispersion and the optical prism PR that receives light later has high dispersion, an imaging optical system OS that can efficiently correct chromatic aberration is realized. Will do.

  Therefore, in the imaging optical system OS of the present invention, it is desirable that at least two optical prisms PR constituting one set satisfy the following conditional expression (2).

νd (F) −νd (L)> 5 Conditional expression (2)
However,
νd (F): an optical prism that receives light first in a set of two optical prisms PR
Abbe number νd (L) for the d-line of the optical system PR: an optical prism that receives light later in a set of two optical prisms PR
Is the Abbe number for the d line of the PR.

  When this conditional expression (2) is satisfied, the Abbe number of the optical prism PR that receives light first increases in the set, while the Abbe number of the optical prism PR that receives light later decreases, so that an imaging optical system desirable for chromatic aberration correction is desired. The OS will be realized.

  Further, as in the present invention, in the case of the imaging optical system OS that bends the optical path in the vertical direction, for example, with respect to the incident optical axis, the relationship between the beam width and the thickness of the imaging optical system OS is high. Then, as in the conditional expression (2), when the Abbe number of the optical prism PR that receives light first is increased, the light beam width is further reduced due to the reduction of the light beam width due to low dispersion and the convergence of the light beam due to the positive power. Miniaturize. Therefore, the imaging optical system OS is effectively downsized (thinned). Accordingly, the eccentric imaging optical system OS as in the present invention is suitable for the high-spec imaging element SR because it performs sufficient color correction and is downsized when the conditional expression (2) is satisfied. It can be said.

  The values of conditional expression (2) in Example 1 are as shown in Table 6 below.

Further, it is desirable that the range of the following conditional expression (2a) is satisfied among the range defined by the conditional expression (2).
νd (F) −νd (L)> 14 Conditional expression (2a)

More specifically, it is more desirable to satisfy the range of the following conditional expression (2b) among the range defined by the conditional expression (2a).
νd (F) −νd (L)> 20 Conditional expression (2b)

Moreover, it is even more desirable that the range of the following conditional expression (2c) is satisfied among the range defined by the conditional expression (2b).
νd (F) −νd (L)> 25 Conditional expression (2c)

  By the way, the power of the optical prism PR is the sum of the powers of the transmission surface and the reflection surface. However, if this power distribution is not set appropriately, a sufficiently high-performance image sensor SR cannot be realized. For example, the present invention is an imaging optical system OS that corrects chromatic aberration by using the difference in the Abbe number in different optical prisms PR, although it imposes a certain amount of power on the transmission surface. However, since the Abbe number of known materials is limited, the range of Abbe number differences is limited to some extent. Therefore, there is a limit to chromatic aberration correction using the difference in Abbe number. For example, if the power of the transmission surface is excessive, there may be a case where chromatic aberration correction cannot be performed.

  Further, when the power of the transmission surface (bending surface) is excessive, another adverse effect may occur. The adverse effect will be described using a relational expression indicating a general power in the coaxial optical system. In the coaxial optical system, the power of the transmission surface is represented by the following relational expression (H), while the power of the reflection surface is represented by the following relational expression (I).

(Nn ') / r (REFR) ... Relational expression (H)
2n '/ r (REFL) ... Relational expression (ri)
However,
n: Refractive index of medium before light is bent (or reflected) n ′: Refractive index of medium after light is bent (or reflected) r (REFR): Radius of curvature of transmission surface r (REFL): The radius of curvature of the reflecting surface.

When the power of the transmission surface and the power of the reflection surface are the same {when the values of the relational expression (h) and the relational expression (i) are the same}, the curvature radius of the transmission surface and the curvature radius of the reflection surface The ratio relationship is determined as in the following relational expression (nu).
r (REFR) / r (REFL) = (1/2) * (1-n / n ') ... Relational expression (nu)

  Then, for example, when n = 1 and n ′ = 2, the value of the relational expression (nu) is ¼. When n = 1 and n ′ = 1.5, the value of the relational expression (nu) is 1/6. In view of these results, when the power of the transmission surface and the power of the reflection surface are the same, the curvature radius {r (REFL)} of the transmission surface is smaller than the curvature radius {r (REFL)} of the reflection surface. . This means that the curvature of the transmissive surface is tighter than the curvature of the reflective surface when the reflective surface tries to exhibit the same power as the transmissive surface. Therefore, it can be said that various aberrations that occur when the power of the transmission surface is excessive are more likely to occur than the various aberrations due to the power of the reflection surface (the various aberrations are likely to increase).

  Such a phenomenon also applies to the case of the decentered imaging optical system OS as in the present invention. Therefore, the imaging optical system OS of the present invention prevents the power burden on the transmission surface from becoming excessive. Specifically, in the imaging optical system OS of the present invention, it is desirable that at least one optical prism PR in the plurality of optical prisms PR satisfies the following conditional expression (3).

0.2 <Σ | φREFR | / Σ | φREFL | <100.0 Conditional expression (3)
However,
Σ | φREFR |: Absolute value of power of each transmission surface included in one optical prism PR
Σ | φREFL |: Absolute value of power of each reflecting surface included in one optical prism PR
Is the sum of

  Conditional expression (3) defines power distribution for the transmission surface and the reflection surface in the optical prism PR. For example, when the value of conditional expression (3) is equal to or greater than the upper limit value, the power burden on the transmission surface is excessive, and chromatic aberration correction using the Abbe number difference cannot be sufficiently performed. In addition, various aberrations due to the power of the transmission surface are likely to occur, and a high-performance imaging optical system OS cannot be realized.

  On the other hand, when the value of conditional expression (3) is less than or equal to the lower limit value, the power burden on the reflecting surface is excessive. In such a case, adverse effects due to processing errors occur. This is because the error in the radius of curvature of the reflecting surface due to the processing error has an effect of about four times compared to the performance fluctuation due to the error in the radius of curvature of the transmissive surface. That is, when the power burden on the reflecting surface becomes excessive, performance fluctuations due to processing errors tend to appear remarkably, and for example, assembly (manufacture) of the imaging optical system OS with various aberrations suppressed cannot be realized.

  Then, if it is within the range of conditional expression (3), the transmission surface and the reflection surface in the imaging optical system OS can be harmonized (balanced), and the imaging optical system OS in which chromatic aberration correction and other various aberrations are suppressed. Is realized. Therefore, the present invention can be said to be an imaging optical system OS that realizes higher performance (higher aberration correction capability can be achieved) due to higher specifications of the imaging element SR.

  Conditional expression (3) will be established independently in both the horizontal direction (X) and the vertical direction (Y). Therefore, the conditional expression (3) can also be expressed as the following conditional expression (3sub1) or conditional expression (3sub2).

0.2 <Σ | φREFR (X) | / Σ | φREFL (X) | <100.0 Conditional expression (3sub1)
0.2 <Σ | φREFR (Y) | / Σ | φREFL (Y) | <100.0 Conditional expression (3sub2)
However,
When the light that passes through the center of the object from the center of the object and goes to the center of the image plane is the base ray,
Σ | φREFR (X) |… The sum of the absolute values of the horizontal power that each transmission surface has at the intersection with the base ray Σ | φREFL (X) |… The horizontal direction that each reflection surface has at the intersection with the base ray Sum of absolute values of power Σ | φREFR (Y) |… Sum of absolute values of vertical power that each transmission surface has at the intersection with the base ray Σ | φREFL (Y) | It is the sum of the absolute values of the power in the vertical direction at the intersection.

  The values of conditional expression (3) {conditional expression (3sub1) or conditional expression (3sub2)} in Example 1 are as shown in Table 7 below.

Of the range defined by conditional expression (3), it is desirable to satisfy the range of conditional expression (3a) below.
0.5 <Σ | φREFR | / Σ | φREFL | <50.0 Conditional expression (3a)
However, this conditional expression (3a) can also be the following conditional expression (3sub1a) or conditional expression (3sub2a).
0.5 <Σ | φREFR (X) | / Σ | φREFL (X) | <50.0 ... Conditional expression (3sub1a)
0.5 <Σ | φREFR (Y) | / Σ | φREFL (Y) | <50.0 ... Conditional expression (3sub2a)

More specifically, it is more desirable to satisfy the range of the following conditional expression (3b) among the range defined by the conditional expression (3a).
1.0 <Σ | φREFR | / Σ | φREFL | <30.0 Conditional expression (3b)
However, this conditional expression (3b) can also be the following conditional expression (3sub1b) or conditional expression (3sub2b), as described above.
1.0 <Σ | φREFR (X) | / Σ | φREFL (X) | <30.0 ... Conditional expression (3sub1b)
1.0 <Σ | φREFR (Y) | / Σ | φREFL (Y) | <30.0 ... Conditional expression (3sub2b)

  By the way, in the imaging optical system OS of the present invention, at least one optical prism PR (the optical prism PR2 in the first embodiment) in the plurality of optical prisms (PR1 to PR3) has positive power. Thus, when a positive power optical prism is included, it is necessary to appropriately distribute the positive power in the imaging optical system OS.

  For example, when an optical prism having a positive power is present in the imaging optical system OS, an under field curvature due to the positive power occurs. In order to correct such curvature of field, negative power corresponding to positive power (optical working surface having negative power) is required in the imaging optical system OS.

  However, if the positive power is not appropriate, for example, if it is too strong, the negative power must be increased corresponding to the intensity. However, since the power of the entire system needs to be positive, the negative optical working surface is arranged near the stop having a low light beam height. In such a case, a large coma aberration occurs due to a relatively strong negative optical working surface, and the imaging performance of the imaging optical system is significantly deteriorated. In addition, since the decentered astigmatism generated due to the decentered imaging optical system is also remarkably generated, the imaging performance of the imaging optical system is further deteriorated. Under such circumstances, setting the power of the positive optical prism PR appropriately can be said to be a requirement for a high-performance imaging optical system OS. Therefore, it is desirable that each optical prism PR having a positive power satisfies the following conditional expression (4).

0.01 <φp / φALL <10.0… Conditional expression (4)
However,
φp: horizontal power and vertical power in the optical prism PR having positive power
Power that is averaged with power φALL: Horizontal power and vertical power in the imaging optical system OS (entire system)
Is the average power.

  Conditional expression (4) defines the power ratio of the positive power optical prism PR with respect to the power of the entire system. Conditional expression (4) defines a range for realizing downsizing and high performance of the imaging optical system OS based on the positive power (composite positive power) of the optical prism PR. .

  Specifically, when the value is equal to or greater than the upper limit value of conditional expression (4), the power of the positive optical prism PR becomes too strong, and coma and astigmatism occur. As a result, the performance of the imaging optical system OS deteriorates. On the other hand, when the value is less than or equal to the lower limit value of the conditional expression (4), the power of the positive optical prism PR becomes too weak, and the power contribution of the positive optical prism PR to the entire system becomes small. This makes it difficult to reduce the size of the imaging optical system OS. Therefore, within the range of the conditional expression (4), the present invention provides an imaging optical system that is small in size and suppresses the occurrence of aberrations (high performance).

  Note that the value of the conditional expression (4) in the first embodiment, that is, the value of the conditional expression (4) corresponding to the optical prism PR2 is “1.01”.

Further, it is desirable that the range of the following conditional expression (4a) is satisfied among the range defined by the conditional expression (4).
0.05 <φp / φALL <5.0 Conditional expression (4a)

More specifically, it is more desirable to satisfy the range of the following conditional expression (4b) among the range defined by the conditional expression (4a).
0.2 <φp / φALL <3.0 Conditional expression (4b)

Further, it is even more desirable that the range of the following conditional expression (4c) is satisfied among the range defined by the conditional expression (4b).
0.5 <φp / φALL <2.0 Conditional expression (4c)

  In order to perform more effective aberration correction, at least one of the optical action surfaces of the plurality of optical prisms PR may be a free-form surface. For example, if there is a free-form surface in which the shape in the horizontal direction and the shape in the vertical direction on the optical action surface are different, it is possible to effectively correct on-axis astigmatism generated due to the decentered optical system. .

  Further, from the viewpoint of miniaturization, at least one optical prism PR (second optical prism PR2 in the first embodiment) in the plurality of optical prisms PR of the imaging optical system OS proceeds from an incident surface and an incident surface that receive light. It is desirable to have one reflecting surface for reflecting the incoming light and one exit surface for emitting the light reflected from the reflecting surface.

  Such an optical prism PR can contribute to miniaturization of the imaging optical system OS because of its simple structure. Further, because of its simple structure, it is also an optical prism PR that can simplify manufacturing and reduce costs. Therefore, the imaging optical system OS using such an optical prism PR can also be simplified in production and cost reduced.

  By the way, the material of the optical prism PR included in the imaging optical system OS of the present invention is not particularly limited. That is, the material of the optical prism PR may be glass or resin (plastic material or the like) as long as it is a material used as an optical material. However, a material with less dependence on temperature (heat) or the like is preferable. Therefore, when a resin material is used for the optical prism PR, the imaging optical system OS of the present invention uses a resin (athermal resin) having low temperature dependency.

  More specifically, the optical prism PR includes an athermal resin that has a relatively small refractive index change (optical transition; including changes in the Abbe number and the like) due to temperature. May be included partially or entirely). Moreover, you may mix athermal resin from which a characteristic differs. This is because an effect of canceling changes due to temperature can be obtained.

  When such a resin material with little change in refractive index is included in the optical prism PR, in the imaging optical system OS, a change in image point position due to a change in refractive index based on a temperature change is suppressed. Further, the imaging optical system OS of the present invention has an eccentric optical working surface. For this reason, astigmatism or the like is likely to occur on the axis. However, an optical prism made of a resin (athermal resin) in which a change in refractive index is suppressed can effectively suppress astigmatic difference and the like.

An example of such an athermal resin is a resin (base material) in which particles {child material; for example, niobium oxide (Nb ₂ O ₅ )} having a maximum length of 30 nm or less are dispersed (Japanese Patent Application Laid-Open (JP-A)). 2005-55852 gazette). In such a resin (mixed resin), a decrease in the refractive index due to the resin accompanying an increase in temperature and an increase in the refractive index of the particles accompanying an increase in temperature occur simultaneously. Therefore, both temperature dependencies (refractive index decrease / refractive index increase) cancel each other, and the refractive index change is less likely to occur.

  Here, an example will be described in detail regarding the offset due to the temperature-dependent refractive index decrease and the temperature-dependent refractive index increase. The refractive index change depending on temperature is expressed by the following refractive index temperature change equation by differentiating the refractive index nd with the temperature t based on the Lorentz-Lorentz equation.

… 屈折率温度変化式
ただし、
α ：線膨張係数
[Ｒ]：分子屈折
である。

… Refractive index temperature change formula
α: Linear expansion coefficient
[R]: Molecular refraction.

  The values of the refractive index temperature change equation (temperature change A = dnd / dt) in several resins (base materials) and inorganic fine particles (child materials) are obtained as shown in Tables 8 and 9 below ( The unit is [/ ° C.].

  The optical prism of the imaging optical system OS of the present invention is not limited to the mixed material in which niobium oxide is dispersed, but may be composed of the mixed material in which the inorganic fine particles in Table 9 are dispersed in the resin in Table 8 above. (For example, it may be composed of a mixed material in which aluminum oxide is dispersed in a polyolefin-based resin).

  Then, in the mixed material (mixed resin), the resin having the symbol A (−) and the inorganic fine particles having the symbol A (+) are mixed. That is, the resin / inorganic fine particles having opposite signs are mixed. Therefore, in the optical prism, it can be seen that the refractive index decrease (first property) due to the resin with the temperature increase and the refractive index increase (second property) of the inorganic fine particles with the temperature increase are effectively offset. . In particular, since such cancellation occurs, the ratio of the inorganic fine particles to the resin is at least sufficiently suppressed from changing the refractive index of the optical prism.

  Further, in the mixed material, the mixing ratio of the resin having the symbol A (−) and the inorganic fine particle having the symbol A (+) is variously adjusted, so that the mixed resin becomes the resin A (−). Or, although it is a mixed resin, it may have a sign A of (+), unlike a resin having a sign (-) of A. Further, by using such a resin material as a part of the optical system, it is possible to cancel the influence of the temperature change in each optical element in the entire system. In such a case, it is possible to reduce the image point movement and the increase in the astigmatic difference due to the temperature change in the entire optical system.

  In addition, a new property change may occur in the athermal resin by appropriately adjusting the amount of inorganic fine particles dispersed in the resin. For example, a property change in which the linear expansion coefficient of the resin, and thus the athermal resin, becomes relatively small by mixing inorganic fine particles is an example.

  Note that the method for producing such a property change and the property of reducing the refractive index change due to the temperature dependency described above is not limited to the adjustment of the dispersion amount. For example, inorganic particles having a relatively large absolute value {A sign (+)} of “A” may be dispersed in the resin material. Moreover, you may disperse | distribute other materials (organic fine particles etc.) provided with such a property of "A".

  By the way, even if the sign A of the resin and the inorganic fine particles is the same, the change in the refractive index of the optical prism accompanying the temperature change can be reduced. For example, in the case of inorganic fine particles having the same sign but the absolute value of A being smaller than that of the resin, the refractive index change of the confused resin containing the inorganic fine particles is smaller than the refractive index change of the resin alone. That is, by including the inorganic fine particles, the confusion resin can reduce the refractive index change depending on the temperature change smaller than the resin alone. However, the amount of dispersion can be reduced by dispersing the inorganic fine particles having the sign A different from that of the resin as compared with the case of dispersing the inorganic fine particles having the sign A same as the resin.

[Embodiment 2]
A second embodiment of the present invention will be described. In addition, about the member which has the same function as the member used in Embodiment 1, the same code | symbol is attached and the description is abbreviate | omitted.

  In the imaging optical unit OSU of Example 1, the optical aperture stop ST is provided on the optical action surface in the imaging optical system OS. However, the present invention is not limited to this. For example, an imaging optical unit OSU (imaging optical system OS) in which an independent optical aperture ST is interposed between the optical prisms PR without providing the optical aperture ST on the optical working surface may be used.

[1. Other Examples (Examples 2 to 4)]
<1-1. Imaging Optical Units of Examples 2 to 4 (see FIGS. 8 to 22)>
Therefore, such an imaging optical unit OSU, for example, an imaging optical unit OSU including the first optical prism PR1, the optical aperture stop ST, the second optical prism PR2, the third optical prism PR3, and the imaging element SR (Examples 2 to 4). ) Will be described with reference to FIGS. Specifically, Example 2 will be described with reference to FIGS. 8 to 12, Example 3 with reference to FIGS. 13 to 17, and Example 4 with reference to FIGS. 18 to 22. 8, 13, and 18 are optical cross-sectional views of Example 2, Example 3, and Example 4, and FIGS. 9 to 12, 14 to 17, and 19 to 22 are implemented. The lateral aberration diagram of Example 2, Example 3, and Example 4 is shown.

<About the first optical prism>
The first optical prism PR1 has four optical action surfaces (s2 to s5). The first surface s1 is a dummy surface (reference surface) as in the first embodiment. Therefore, a surface (incident surface) that first receives and transmits light from the object side is denoted as a second surface s2 in FIGS. The third surface s3 is a reflecting surface that reflects the light transmitted (passed) through the second surface s2 toward the fourth surface s4.

  The fourth surface s4 is a reflecting surface that reflects the light (reflected light) reflected by the third surface s3 toward the fifth surface s5. As shown in FIGS. 8, 13, and 18, the second surface s2 and the fourth surface s4 are TIR surfaces having both functions of transmission and reflection.

  The fifth surface s5 is an emission surface (transmission surface) that emits (transmits) the reflected light from the fourth surface s4 toward the second optical prism PR2. In addition, this 5th surface s5 and the 7th surface s7 mentioned later are opposing arrangement | positioning.

<About optical aperture>
The optical diaphragm ST has a diaphragm shape such as a circle, and is provided between the fifth surface s5 of the first optical prism PR1 and the seventh surface s7 of the second optical prism PR2. The optical aperture stop ST is also referred to as a sixth surface s6 through which light passes.

<About the second optical prism>
The second optical prism PR2 guides light that has passed through the first optical prism PR1 and partially blocked by the optical aperture stop ST to the third optical prism PR3. The second optical prism PR2 has three optical action surfaces (s7 to s9).

  The seventh surface s7 is an incident surface that first receives and transmits light from the first optical prism PR1. The eighth surface s8 is a reflecting surface that reflects light (transmitted light) that has passed through the seventh surface s7 toward the ninth surface s9. Furthermore, the ninth surface s9 is an exit surface that emits light (reflected light) from the eighth surface s8 toward the third optical prism PR3. Note that the ninth surface s9 and a later-described tenth surface s10 are opposed to each other.

<About the third optical prism>
The third optical prism PR3 guides light that has passed through the second optical prism PR2 to the imaging element SR (image surface s13). The third optical prism PR3 has three optical action surfaces (s10 to s12).

  The tenth surface s10 is an incident surface that first receives and transmits light from the second optical prism PR2. The eleventh surface s11 is a reflecting surface that reflects the light that has passed through the tenth surface s10 toward the twelfth surface s12. Furthermore, the twelfth surface s12 is an exit surface that emits light from the eleventh surface s11 toward the image sensor SR (image surface s13).

<1-2. About Construction Data of Examples 2 to 4>
Here, construction data in the imaging optical units OSU of Examples 2 to 4 are shown in Tables 10 to 21. Table 10 to Table 13 show Example 2, Table 14 to Table 17 show Example 3, and Table 18 to Table 21 show Example 4. Table 10, Table 14, Table 18 are in Table 1, Table 11, Table 15, Table 19 are in Table 2, Table 12, Table 16, Table 20 are in Table 3, Table 13, Table 17, Table 21 are in Table 3. Table 4 shows a corresponding similar expression.

  << Construction Data of Example 2 >>

  << Construction Data of Example 3 >>

  << Construction Data of Example 4 >>

<1-3. About aberration diagrams of Examples 2 to 4>
9 to 12 showing the lateral aberration of Example 2, FIGS. 14 to 17 showing the lateral aberration of Example 3, and FIGS. 19 to 22 showing the lateral aberration of Example 4 are the lateral ones of Example 1. This is the same expression as that shown in FIGS.

[2. Examples of various features in the present invention]
The imaging optical unit OSU (imaging optical system OS) of Examples 2 to 4 as described above has all the various features described in the first embodiment. Therefore, the effects corresponding to these features are also exhibited in the imaging optical units OSU of the second to fourth embodiments.

  Therefore, Tables 22 to 25 show the results of Examples 2 to 4 corresponding to the conditional expressions (1) to (4). In these tables, the results of Example 1 are also shown for convenience.

<< Result of Conditional Expression (1) >>

<< Result of Conditional Expression (2) >>

<< Result of Conditional Expression (3) >>

<< Result of Conditional Expression (4) >>

  As shown in Table 25, the imaging optical system OS of Example 2 is an optical prism PR having all positive powers. On the other hand, in the imaging optical systems OS of Examples 3 and 4, the optical prisms PR1 and PR2 have positive power.

[Other embodiments]
The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. However, the imaging optical system OS of the present invention performs chromatic aberration correction based on the difference in Abbe number. Therefore, even if the Abbe number of an optical element (such as a plane parallel plate) having an optical working surface having no power such as a plane is different from the Abbe number of another optical element (such as an optical prism), the Abbe number It can be said that the effect of correcting chromatic aberration due to the difference is low.

[1. (Reflectivity of reflective surface)
As an example of various changes, for example, it is preferable that all of the reflection surfaces included in the imaging optical system OS have a reflectance of 80% or more. This is because the reflectance of the imaging optical system (entire system) OS is obtained by multiplying each reflecting surface. However, in other words, in order to improve the reflectance of the imaging optical system OS, it can be said that at least one of the plurality of reflecting surfaces only needs to have a reflectance of 80% or more. This is because if at least one surface has a reflectance of 80% or more, it greatly contributes to an improvement in the reflectance of the imaging optical system OS.

[2. (Regarding the reflective surface area)
Further, the reflection surface of the optical prism PR may include a reflection region and an absorption region, or may include a reflection region and a light shielding region. Alternatively, the reflection surface of the optical prism PR may include a reflection area and a transmission area. That is, the reflection surface of the optical prism PR may include a reflection region and a non-reflection region (absorption region, light shielding region, transmission region, etc.).

  With such a configuration, for example, the position of the non-reflective area of the reflecting surface and the end (edge) of the reflecting surface can be made to correspond to each other. Then, stray light caused by reflection at the edge cannot occur. In addition, the non-reflective area of the reflecting surface can be made to function as a holding portion for attaching the optical prism PR to an imaging device or the like.

<2-1. About the characteristics of the reflection area>
Note that the reflection region on the reflection surface may have various characteristics. For example, the reflection area may be in a mirror state. With such a configuration, there are no irregularities or wavy shapes (ripples) on the reflective region. For this reason, stray light due to ripples or the like is not generated on the reflection region, and the reflection efficiency is further improved.

  Further, the reflective region may be formed by applying a reflective coat or the like. With such a configuration, only a desired position can function as a reflection region. For example, only a portion corresponding to the effective diameter (within the effective range) of the optical aperture stop ST or the like can be set as the reflection region.

In addition, various coatings are mentioned as a reflective coat given to a reflective area | region. Therefore, some coatings and their characteristics are listed below.
・ Aluminum-deposited coating This coating exhibits relatively high reflectivity. In addition, it is a relatively inexpensive coating.
• Aluminum-enhanced reflective coatings These coatings are relatively expensive but exhibit higher reflectivity than aluminum-deposited coatings.
• Dielectric coating and silver deposition coating Both coatings are relatively expensive but exhibit very high reflectivity. For this reason, even when the light is repeatedly reflected a plurality of times, the light amount loss is suppressed.

  Then, when all the reflecting surfaces in the optical prism PR are formed of an aluminum vapor deposition coating surface, the cost of forming the reflecting surface is suppressed, and the cost of the optical prism PR itself is also suppressed. However, the reflectance of the optical prism PR as a whole is lower than that of the optical prism PR composed of other coating surfaces (dielectric coating surface or the like). On the contrary, when all the reflecting surfaces in the optical prism PR are formed of a dielectric coating surface, the reflectance of the entire optical prism PR becomes high due to the extremely high reflectance. However, because of the expensive coating, the cost of the optical prism PR itself increases.

  Therefore, in the imaging optical system OS of the present invention, the reflective surface of the above-mentioned coating (aluminum deposition coating surface, aluminum enhanced reflection coating surface, dielectric coating surface, or silver deposition coating surface) is mixed as the reflection surface of the optical prism PR. You may make it do. With such a configuration (for example, a configuration in which a plurality of types are selected from four types of surfaces), an optical prism PR with improved reflectivity can be realized while reducing costs. .

<2-2. Characteristics of non-reflective areas>
In addition, the non-reflective region (particularly the absorption region and the light-shielding region) on the reflective surface may have various characteristics. For example, the non-reflective region may be formed by rough grinding. For rough grinding, for example, a curve generator is used. Therefore, a desired area of the reflective surface can be finished into a non-reflective area relatively easily. In addition, there is an advantage that the cost of rough grinding itself is low.

  Further, the non-reflective region may be formed by roughening. The rough surface processing is performed by, for example, a die press. Specifically, it is performed by press working in which a part of the mold is roughened. Therefore, a desired area of the reflecting surface is finished into a non-reflecting area relatively easily and inexpensively. The rough surface processing may be performed by rough polishing (for example, polishing without an abrasive; polishing without finishing).

  For example, in the above methods (rough grinding, rough surface processing), the non-reflective region is formed by making the surface of the surface uneven. Therefore, when such a method is used, a non-reflective region in which small pieces (raised pieces; for example, quadrangular pyramids such as pyramids) raised from the reflecting surface are scattered may be formed. That is, a non-reflective region including a plurality of raised pieces that scatter light may be formed.

  In such a non-reflective region, light is attenuated in the vicinity of the raised piece, so that there is an advantage that stray light can be suppressed. However, the method of forming the non-reflective region of the type including the raised pieces is not limited to the above method (rough grinding, rough surface processing). Note that if a non-reflective region including a raised piece is formed by an inexpensive die press or the like, the optical prism PR can be incorporated into the imaging optical system OS without providing a separate member for preventing stray light. There is also.

  By the way, the above non-reflective area | region is comprised by providing a protrusion, such as an unevenness | corrugation, in a reflective surface. However, the non-reflective region is not limited to such a type. For example, the non-reflective region may be formed by blackening a part of the reflective surface. In such a case, the surface of the non-reflective area itself is not deformed. Therefore, the surface of the non-reflective region can function as an attachment position reference for the optical prism PR.

  Further, the non-reflective region may be formed by a chemical reaction with an organic solvent. In the case of a chemical reaction, a plurality of reflecting surfaces can be immersed together in an organic solvent, or an organic solvent can be applied to a plurality of reflecting surfaces at once. Therefore, there is an advantage that a large amount of processing (production) can be performed at one time. In addition, if the non-reflective region is formed by changing the properties of the optical prism material, the surface of the non-reflective region itself is not deformed as described above. Therefore, in such a case, the same effect as the non-reflective region due to black dyeing is produced.

[3. Preferred coating)
Incidentally, in the imaging optical system OS (imaging optical unit OSU) as in the present invention, light in various wavelength ranges is incident. And in these lights, unnecessary light (for example, infrared light) is also included by the point of image-forming light. However, the imaging element SR such as a CCD has sensitivity to such a wavelength range (long wavelength range) of infrared light. For this reason, the infrared light may adversely affect the light receiving surface (imaging surface) of the image sensor SR.

  Therefore, in the present invention, a coating that absorbs light in a long wavelength region may be applied to any of the surfaces (transmission surface or reflection surface) of the optical prism PR. With such a configuration, for example, it is not necessary to arrange a plane parallel plate or the like that functions as an IR cut filter in front of the image sensor SR. As a result, it is possible to realize a high-performance (for example, high resolution) imaging optical system OS in addition to cost reduction.

[4. (About optical aperture)
The arrangement of the optical aperture stop ST can be arranged on the working surface of the optical prism PR and between the optical prisms PR. That is, the present invention is not limited wherever the optical aperture stop ST is disposed. Further, the shape of the optical aperture stop ST is not particularly limited. For example, it may be circular or elliptical. Further, the optical aperture stop ST may have a polygonal shape or an asymmetric shape.

1 is an optical cross-sectional view of an imaging optical unit (Example 1) including an imaging optical system of the present invention. FIG. 4 is a lateral aberration diagram in the X direction (however, ΔX) in the imaging optical unit of Example 1. FIG. 6 is a lateral aberration diagram in the X direction (however, ΔY) in the imaging optical unit of Example 1. FIG. 6 is a lateral aberration diagram in the Y direction (however, ΔX) in the imaging optical unit of Example 1. FIG. 6 is a lateral aberration diagram in the Y direction (however, ΔY) in the imaging optical unit of Example 1. It is explanatory drawing of a right-handed system XYZ coordinate. It is explanatory drawing of the local orthogonal coordinate in the image surface IS. It is an optical sectional view of an image pick-up optical unit (Example 2) containing the image pick-up optical system of the present invention. FIG. 12 is a lateral aberration diagram in the X direction (however, ΔX) in the imaging optical unit of Example 2. FIG. 6 is a lateral aberration diagram in the X direction (however, ΔY) in the imaging optical unit of Example 2. FIG. 10 is a transverse aberration diagram in the Y direction (however, ΔX) in the image pickup optical unit of Example 2. FIG. 6 is a lateral aberration diagram in the Y direction (however, ΔY) in the imaging optical unit of Example 2. It is an optical sectional view of an image pick-up optical unit (Example 3) containing the image pick-up optical system of the present invention. FIG. 12 is a lateral aberration diagram in the X direction (however, ΔX) in the imaging optical unit of Example 3. FIG. 12 is a lateral aberration diagram in the X direction (however, ΔY) in the imaging optical unit of Example 3. FIG. 12 is a lateral aberration diagram (provided by ΔX) in the Y direction in the imaging optical unit according to Example 3. FIG. 12 is a lateral aberration diagram in the Y direction (however, ΔY) in the imaging optical unit of Example 3. It is an optical sectional view of an image pick-up optical unit (Example 4) containing the image pick-up optical system of the present invention. FIG. 10 is a lateral aberration diagram in the X direction (however, ΔX) in the image pickup optical unit of Example 4. FIG. 12 is a lateral aberration diagram in the X direction (however, ΔY) in the imaging optical unit of Example 4. FIG. 12 is a lateral aberration diagram (provided by ΔX) in the Y direction in the imaging optical unit according to Example 4. FIG. 12 is a lateral aberration diagram in the Y direction (however, ΔY) in the imaging optical unit according to Example 4;

Explanation of symbols

OS Imaging optical system OSU Imaging optical unit PR Optical prism (prism optical element)
PR1 First optical prism (prism optical element)
PR2 Second optical prism (prism optical element)
PR3 Third optical prism (prism optical element)
ST Optical aperture SR Image sensor IS Image surface si Optical action surface * Free-form surface

Claims (11)

There are at least three or more prism optical elements that allow light from the object side to pass through,
At least one of the plurality of prism optical elements has a positive power;
further,
At least one of the optical working surfaces included in the plurality of prism optical elements is in an eccentric arrangement,
In the case where light continuously travels to different prism optical elements in the plurality of prism optical elements, the light exit surface of one prism optical element and the light incident surface of the other prism optical element are: Opposite,
in addition,
At least one combination of two prism optical elements included in the plurality of prism optical elements satisfies the following conditional expression (1), and
An imaging optical system, wherein at least two transmission surfaces of the optical action surface are non-rotationally symmetric surfaces;
5.0 <| νd (α) −νd (β) | <80.0 Conditional expression (1)
However,
νd (α): One prism optical element in a set of two prism optical elements is
Abbe number with respect to d-line νd (β): the other prism light different from one prism optical element constituting the above set
This is the Abbe number for the d-line of the scientific element. The imaging optical system according to claim 1, wherein the two prism optical elements constituting at least one set satisfy the following conditional expression (2):
νd (F) −νd (L)> 5 Conditional expression (2)
However,
νd (F): prism light received first in a set of two prism optical elements
Abbe number with respect to d-line possessed by scientific element νd (L): prism light received later in a set of two prism optical elements
This is the Abbe number for the d-line of the scientific element. The imaging optical system according to claim 1 or 2, wherein at least one prism optical element in the plurality of prism optical elements satisfies the following conditional expression (3):
0.2 <Σ | φREFR | / Σ | φREFL | <100.0 Conditional expression (3)
However,
.SIGMA..vertline..phi.REFR.vertline .... absolute value of power of each transmission surface included in one prism optical element.
Sum Σ | φREFL |: Absolute value of the power of each reflecting surface included in one prism optical element
Is the sum of The imaging optical system according to any one of claims 1 to 3, wherein each of the prism optical elements having positive power satisfies the following conditional expression (4):
0.01 <φp / φALL <10.0… Conditional expression (4)
However,
φp: Horizontal power and vertical power in prism optical element with positive power
The average power of the power φALL: The average power of the horizontal and vertical power in the imaging optical system
It is a word.   The imaging optical system according to any one of claims 1 to 4, wherein at least one of the optical action surfaces included in the plurality of prism optical elements is a free-form surface. At least one prism optical element in the plurality of prism optical elements described above,
An incident surface for receiving light,
A reflecting surface that reflects light traveling from the incident surface; and
An exit surface for emitting light reflected from the reflective surface;
The imaging optical system according to any one of claims 1 to 5, wherein each of the imaging optical systems has one surface.   The imaging optical system according to claim 1, wherein at least one of the plurality of prism optical elements is made of resin.   The imaging optical system according to claim 7, wherein the resin has a characteristic of suppressing optical transition depending on temperature. The resin includes a base material and a child material,
9. The imaging optical system according to claim 8, wherein the first optical property of the base material is altered by the second property of the child material, whereby the optical transition is suppressed.   10. The imaging optical system according to claim 8, wherein the optical shift is a change in refractive index of the resin depending on temperature.   An imaging optical unit comprising: the imaging optical system according to claim 1; and an imaging element that receives light from the imaging optical system.

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