JP2008164887A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens which has wide viewing angle, in which aberration is excellently corrected in the wide wavelength range from a visible range to a near-infrared range and which is suitable for a camera for observation which is mounted on a satellite for instance and performs multi-band observation in the wide wavelength range. <P>SOLUTION: The imaging lens comprises a first group G1, an aperture diaphragm St, a second group G2 and a third group G3 in order from an object side. The second group G2 includes a cemented lens comprising a positive meniscus lens L21 having a concave surface on the object side and a negative meniscus lens L22 having a concave surface on the object side. The third group G3 includes positive lenses L31 and L32 having an Abbe number of 80 or more and satisfies a conditional expression (1):¾f/f1¾<0.5, a conditional expression (2):¾f/f2¾<0.8, a conditional expression (3):0.8<f/f3<2.0, and a conditional expression (4):0.15<DA/f, where f is a focal length of an entire system, fi is the focal length of an i-th group Gi and DA is the air spacing between the second group G2 and the third group G3. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is mounted on a refraction type imaging lens used in a wide wavelength range from a visible range to a near infrared range (for example, a wavelength of 350 nm to 900 nm), in particular, an artificial satellite, and observes the earth in a plurality of observation wavelength ranges. It is related with the imaging lens suitable for.

Conventionally, a reflection optical system is often employed for an earth observation camera mounted on an artificial satellite because of the advantage of chromatic aberration and the like. As the reflective satellite optical system, those described in the following patent documents are conventionally known.
JP 2002-214530 A US Pat. No. 4,598,981

  However, the optical system described in Patent Document 1 has a narrow viewing angle of 0.1 ° × 1.2 °. Further, the optical system described in Patent Document 2 has a wide viewing angle but a large distortion. If the distortion is large, the image area that can be acquired differs greatly between the center of the field of view and the edge of the field of view, which is inconvenient for observation purposes. In order to widen the viewing angle, it is necessary to use an off-axis reflecting optical system such as the optical system described in Patent Document 2, but the overall size of the optical system is significantly smaller than that of a refractive optical system. growing.

  On the other hand, when a refractive optical system is used, distortion is easy to correct, but chromatic aberration is a problem. Here, as a method for observing the earth, for example, a one-dimensional line sensor is arranged on the imaging plane of the optical system, and the camera is moved in a direction orthogonal to the longitudinal direction of the line sensor to thereby two-dimensional the ground surface. There is a so-called push bloom method for obtaining an image. In this case, a two-dimensional image can be obtained for each of a plurality of observation wavelength regions by arranging a plurality of line sensors having different spectral sensitivities in parallel. In such a system, when chromatic aberration of magnification occurs, the photographing position of the ground surface differs for each observation wavelength region. For this reason, the chromatic aberration of magnification needs to be a sufficiently small amount for one pixel. In order to ensure good performance in all observation wavelength ranges, it is necessary to design the aberration shape in each wavelength range as much as possible. For this reason, conventionally, a refracting optical system for multiband observation used in a wide wavelength range has been used by dividing the optical system for each range in which chromatic aberration can be corrected. That is, it is necessary to use a plurality of imaging lenses as a refractive optical system cannot be handled by a single imaging lens.

  The present invention has been made in view of such problems, and its purpose is a wide viewing angle (for example, 20 degrees or more), and aberrations in a wide wavelength range from the visible range to the near infrared range (for example, a wavelength of 350 nm to 900 nm). An object of the present invention is to provide an imaging lens that is well corrected and suitable for an observation camera that is mounted on an artificial satellite and performs multiband observation in a wide wavelength range.

The imaging lens according to the present invention includes, in order from the object side, a first group, an aperture stop, a second group, and a third group. The first group is in order from the object side, the positive lens, and the object side. A positive meniscus lens having a convex surface facing the object side and a negative meniscus lens having a convex surface facing the object side. The second group includes at least a positive meniscus lens having a concave surface facing the object side and a negative meniscus lens having a concave surface facing the object side. The third lens group includes a cemented lens including a meniscus lens, and includes a positive lens having an overall positive refractive power and an Abbe number of 80 or more. Moreover, the following conditional expression is satisfied.
│f / f1│ <0.5 (1)
│f / f2│ <0.8 (2)
0.8 <f / f3 <2.0 (3)
0.15 <DA / f (4)
Where f is the focal length of the entire system, f1 is the focal length of the first group, f2 is the focal length of the second group, f3 is the focal length of the third group, and DA is between the second group and the third group. Air spacing.

Since the imaging lens according to the present invention is a refractive optical system, it is advantageous in correcting a distortion aberration while ensuring a wide viewing angle while suppressing the overall size as compared with a reflective optical system. In addition, by making the shape, refractive power, and the like of each lens appropriate, aberrations are favorably corrected in a wide wavelength range from the visible range to the near infrared range. In particular, satisfying each conditional expression optimizes the power distribution and lens arrangement of each group, and corrects various aberrations satisfactorily.
In the imaging lens according to the present invention, the optical performance is further improved by satisfying the following preferable conditions as appropriate.

In the imaging lens according to the present invention, the angle θ _A at which the principal ray at the maximum angle of view enters each surface is:
It is preferable that | θ _A | ≦ 30 °. However, θ _A is an angle with respect to the normal of each surface at the light incident position.
As a result, the refraction of light rays on each surface is suppressed to be relatively small, and the aberration shape (aberration tendency) for each wavelength region can be prevented from changing greatly over a wide wavelength range.

Moreover, it is preferable that the transmittance | permeability in wavelength lambda = 350nm -900nm and thickness 10mm is 70% or more about the material of each lens which comprises each group.
This is advantageous for use in a wide wavelength range from the visible range to the near infrared range.

Moreover, it is preferable that the lens closest to the object is made of quartz glass.
The lens on the most object side is made of quartz glass to prevent cosmic rays from entering the optical system when mounted on an artificial satellite, and a quartz plate filter for cutting cosmic rays is separately provided in front of the optical system. There is no need to place them. This prevents the lens from being colored by cosmic rays without separately providing a quartz plate filter or the like.

  According to the imaging lens of the present invention, a refracting optical system having a three-group configuration as a whole is satisfied, and the shape and refractive power of each lens are optimized to satisfy an appropriate condition. Since it has a configuration that is advantageous for correcting aberrations in the wavelength range, it has a wide viewing angle while suppressing the overall size compared to the reflective optical system, and corrects aberrations in a wide wavelength range from the visible range to the near infrared range. For example, an optical system suitable for an observation camera mounted on an artificial satellite and performing multiband observation in a wide wavelength range can be obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a first configuration example of an imaging lens according to an embodiment of the present invention. This configuration example corresponds to the lens configuration of a first numerical example (FIG. 8) described later. FIG. 2 shows a second configuration example, which corresponds to a lens configuration of a second numerical example (FIG. 9) described later. FIG. 3 shows a third configuration example, which corresponds to a lens configuration of a third numerical example (FIG. 10) described later. FIG. 4 shows a fourth configuration example, which corresponds to a lens configuration of a fourth numerical example (FIG. 11) described later. FIG. 5 shows a fifth configuration example, which corresponds to a lens configuration of a fifth numerical example (FIG. 12) described later. FIG. 6 shows a sixth configuration example, which corresponds to a lens configuration of a sixth numerical example (FIG. 13) described later. In FIG. 1 to FIG. 6, the symbol Ri is the curvature of the i-th surface that is numbered sequentially so as to increase toward the image side (imaging side), with the surface of the component closest to the object side first. Indicates the radius. A symbol Di indicates a surface interval on the optical axis Z1 between the i-th surface and the i + 1-th surface. 1 to 6 show a light beam at a field angle of zero degrees and a principal light beam C1 at a maximum field angle.

  This imaging lens can be applied to various imaging devices using an imaging element such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). In particular, it is most suitable as an imaging lens used for imaging from the visible range to the near-infrared range (for example, a wavelength of 350 nm to 900 nm), such as an artificial satellite and mounted on an observation device that performs earth observation in a plurality of observation wavelength ranges. It is. The imaging lens includes, in order from the object side along the optical axis Z1, a first group G1, an aperture stop St, a second group G2, and a third group G3. Note that, on the rear side of the aperture stop St, the second group G2 and the third group G3 are separated by a portion having the longest air gap DA between the lenses. That is, on the rear side of the aperture stop St, the object side is the second group G2 and the image side is the third group G3 across the longest air gap DA.

  An imaging element such as a CCD (not shown) is disposed on the image plane Simg of the imaging lens. Various optical members LF are arranged between the third group G3 and the image sensor in accordance with the configuration of the apparatus on which the lens is mounted. For example, an optical filter, a cover glass, or the like is disposed as the optical member LF.

  The first group G1 includes, in order from the object side, a positive lens L11, a positive meniscus lens L12 having a convex surface facing the object side, and a negative meniscus lens L13 having a convex surface facing the object side. This first group G1 has a negative refractive power as a whole in the configuration examples of FIGS. 1 to 3 and FIGS. 5 and 6, and has a positive refractive power as a whole in the fourth configuration example of FIG. ing. The positive lens L11 is a biconvex lens in the first and second configuration examples of FIGS. 1 and 2, and in the third to sixth configuration examples of FIGS. 3 to 6, a positive meniscus lens having a convex surface facing the object side. It is said that. In addition, it is preferable that the positive lens L11 which is a lens arrange | positioned at the most object side of this imaging lens is quartz glass.

  The second group G2 includes at least a cemented lens including a positive meniscus lens L21 having a concave surface facing the object side and a negative meniscus lens L22 having a concave surface facing the object side. In the first to fourth configuration examples of FIGS. 1 to 4, the second group G2 includes only a cemented lens including a positive meniscus lens L21 and a negative meniscus lens L22. In the fifth and sixth configuration examples of FIGS. 5 and 6, in order from the object side, a total of 3 cemented lenses including a positive meniscus lens L21 and a negative meniscus lens L22, and a meniscus lens L23 having a concave surface facing the object side. It consists of a single lens. The second group G2 has a positive refracting power as a whole in the configuration examples of FIGS. 1 to 3 and FIGS. 5 and 6, and has a negative refracting power as a whole in the fourth configuration example of FIG. ing.

  The third group G3 has a positive refractive power as a whole. The third group G3 includes at least a positive lens having an Abbe number of 80 or more. Preferably, a positive lens having an Abbe number of 90 or more is included. In the first and second configuration examples of FIGS. 1 and 2, the third group G3 is configured by two single lenses L31 and L32. In the third to fifth configuration examples of FIGS. 3 to 5, the third group G3 includes a total of three lenses including one single lens L31 and a cemented lens including a positive lens L32A and a negative lens L32B. Has been. In the sixth configuration example of FIG. 6, the third group G3 includes three single lenses L31, L32, and L33.

This imaging lens satisfies the following conditions. In the formula, f is the overall focal length, f1 is the focal length of the first group G1, f2 is the focal length of the second group G2, and f3 is the focal length of the third group G3.
│f / f1│ <0.5 (1)
│f / f2│ <0.8 (2)
0.8 <f / f3 <2.0 (3)

This imaging lens also satisfies the following conditional expression (4). Moreover, it is preferable that the following conditional expression (5) is satisfied. However, DA shows the air space (the axial distance from the rear vertex of the second group G2 to the front vertex of the third group G3) between the second group G2 and the third group G3. DB is the sum of the air gap DA between the second group G2 and the third group G3 and the total thickness of the third group G3 (on the axis from the rear vertex of the second group G2 to the rear vertex of the third group G3) Distance) (see FIG. 1).
0.15 <DA / f (4)
0.35 <DB / f <0.85 (5)
Conditional expression (4) is preferably in the range of the following expression (4A).
0.2 ≦ DA / f <0.5 (4A)

In this imaging lens, the angle θ _A at which the principal ray C1 at the maximum angle of view enters each surface is
It is preferable that | θ _A | ≦ 30 °.
Here, θ _A is an angle with respect to the normal 10 of each surface at the incident position of the principal ray C1, as shown in FIG.

  Moreover, in this imaging lens, it is preferable that the material of each lens constituting each group has a transmittance of 70% or more at a wavelength λ = 350 nm to 900 nm at a thickness of 10 mm.

  Next, operations and effects of the imaging lens configured as described above will be described.

  Since this imaging lens is a refractive optical system, it is advantageous in correcting a distortion aberration while ensuring a wide viewing angle while suppressing the overall size as compared with a reflective optical system. In addition, by making the shape, refractive power, and the like of each lens appropriate, aberrations are favorably corrected in a wide wavelength range from the visible range to the near infrared range. In particular, satisfying each conditional expression optimizes the power distribution and lens arrangement of each group, and corrects various aberrations satisfactorily.

  Conditional expression (1) defines an appropriate refractive power of the first group G1. If the refractive power of the first lens group G1 increases beyond the upper limit of conditional expression (1), coma aberration occurs and it becomes difficult to correct the aberration. Conditional expression (2) defines an appropriate refractive power of the second group G2. When the refractive power of the second group G2 increases beyond the upper limit of conditional expression (2), it becomes difficult to correct aberrations. In particular, spherical aberration and lateral chromatic aberration increase. Conditional expression (3) defines an appropriate refractive power of the third lens group G3. When the refractive power of the third group G3 increases beyond the upper limit, coma aberration on the short wavelength side and the long wavelength side increases. When the refractive power of the third lens unit G3 becomes weaker beyond the lower limit, the power of the second lens unit G2 increases accordingly, and aberration correction becomes difficult.

  Conditional expressions (4) and (4A) define an appropriate value of the air gap DA between the second group G2 and the third group G3. Conditional expression (5) defines a value obtained by adding the total thickness of the third group G3 to the air gap DA. If the air gap DA between the second group G2 and the third group G3 exceeds the lower limit of the conditional expression (4), it is difficult to correct the image surfaces on the short wavelength side and the long wavelength side, which is not preferable. The lower limit of the air interval DA is preferably the lower limit indicated by the conditional expression (4A). A longer air gap DA is advantageous for aberration correction, but exceeding the upper limit of conditional expression (4A) is not preferable because the lens system becomes larger. Similarly, if the lower limit of conditional expression (5) is exceeded, correction of the image surfaces on the short wavelength side and the long wavelength side becomes difficult, which is not preferable. On the other hand, if the upper limit is exceeded, the lens system becomes large, which is not preferable.

  In this imaging lens, it is preferable that the lens closest to the object side (the positive lens L11 of the first group G1) is made of quartz glass. When mounted on an artificial satellite, coloring due to aging, that is, attenuation of transmittance is observed in ordinary optical glass due to the penetration of cosmic rays into the optical system. By making the lens on the most object side thick quartz glass that does not pass cosmic rays, cosmic rays are prevented from entering the optical system. Thereby, coloring of the lens by cosmic rays can be prevented without separately arranging a quartz plate filter for cutting cosmic rays in front of the optical system.

  In this imaging lens, with respect to the material of each lens constituting each group, the transmittance at a wavelength λ = 350 nm to 900 nm is 70% or more (thickness 10 mm), so that a wide wavelength range from the visible range to the near infrared range. It is advantageous for use in.

  In this imaging lens, a positive lens having an Abbe number of 80 or more in the third group G3, preferably a low refractive index / low dispersion positive lens made of a material having a refractive index of 1.5 or less and an Abbe number of 80 or more. By using this, it is advantageous for correcting lateral chromatic aberration. When the Abbe number is preferably 90 or more, it is advantageous to correct lateral chromatic aberration.

Further, in this imaging lens, when the absolute value of the angle θ _A at which the chief ray C1 at the maximum angle of view is incident on each surface is 30 ° or less, the refraction of the light beam on each surface can be suppressed to be relatively small. It is possible to prevent the aberration shape (aberration tendency) for each wavelength region from changing greatly over a wide wavelength range. Thereby, aberration correction suitable for multiband observation in a wide wavelength range can be performed.

  As described above, the imaging lens according to the present embodiment has a three-group refractive optical system as a whole, and optimizes the shape and refractive power of each lens while satisfying appropriate conditions. Since it has a configuration that is advantageous for aberration correction over a wide wavelength range from the visible range to the near infrared range, it has a wide viewing angle while suppressing the overall size compared to the reflective optical system, and from the visible range to the near infrared range. It is possible to obtain an optical system suitable for an observation camera in which aberration is favorably corrected in a wide wavelength range, for example, mounted on an artificial satellite and performing multiband observation in a wide wavelength range.

  Next, specific numerical examples of the imaging lens according to the present embodiment will be described. Hereinafter, the first to sixth numerical examples will be described together.

Specific lens data corresponding to the configuration of the imaging lens shown in FIG. 1 is shown as Example 1 in FIG. In the field of the surface number Si in the lens data shown in FIG. 8, the surface of the component on the most object side is the first, and the number of the i-th surface that is sequentially increased toward the image side. Is shown. In the column of the curvature radius Ri, the value (mm) of the curvature radius of the i-th surface from the object side is shown in correspondence with the reference symbol Ri in FIG. Similarly, the column of the surface interval Di indicates the interval (mm) on the optical axis between the i-th surface Si and the i + 1-th surface Si + 1 from the object side. In the column Ndj, the value of the refractive index with respect to the d-line (wavelength 587.6 nm) of the j-th optical element from the object side is shown. The column of νdj shows the Abbe number value for the d-line of the j-th optical element from the object side. The θ _A i column shows the incident angle of the principal ray C1 at the maximum field angle on the i-th surface from the object side. The value of the incident angle θ _A is an air-side converted value. In the case of the joint surface, the angle is on the light incident side.

  FIG. 9 shows specific lens data corresponding to the configuration of the imaging lens shown in FIG. 2 as Example 2, in the same manner as the imaging lens according to Example 1 described above. Similarly, specific lens data corresponding to the configuration of the imaging lens shown in FIG. 3 is shown as Example 3 in FIG. Similarly, specific lens data corresponding to the configuration of the imaging lens shown in FIG. 4 is shown as Example 4 in FIG. Similarly, specific lens data corresponding to the configuration of the imaging lens shown in FIG. 5 is shown in FIG. Similarly, specific lens data corresponding to the configuration of the imaging lens shown in FIG. 6 is shown as Example 6 in FIG.

As can be seen from the lens data, for each example, the incident angle θ _A of the principal ray C1 is
| Θ _A | ≦ 30 °.
In each example, each lens material constituting each group has a transmittance of 70% or more at a wavelength λ = 350 nm to 900 nm and a thickness of 10 mm. The most object side lens is made of quartz glass.

  In FIG. 14, the values regarding the above-described conditional expressions (1) to (5) are collectively shown for the respective examples. As various data, FNo. The values of (F value), f (focal length of the entire system), and 2ω (field angle) are collectively shown for each example. As can be seen from FIG. 14, the value of each example is within the numerical range of each conditional expression.

  FIGS. 15A to 15D show spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration in the imaging lens according to Example 1, respectively. Each aberration diagram shows aberrations with a wavelength of 530 nm as a reference wavelength. The spherical aberration diagram and the lateral chromatic aberration diagram also show aberrations for the wavelength of 380 nm and the wavelength of 865 nm. In the astigmatism diagram, the solid line indicates the sagittal direction and the broken line indicates the tangential direction. FNo. Indicates an F value, and ω indicates a half angle of view.

  Similarly, various aberrations in the imaging lens according to Example 2 are shown in FIGS. 16 (A) to 16 (D). Similarly, various aberrations in the imaging lens according to Example 3 are shown in FIGS. 17A to 17D, and various aberrations in the imaging lens according to Example 4 are shown in FIGS. 18A to 18D. 19A to 19D show aberrations in the imaging lens according to Example 5, and FIGS. 20A to 20D show aberrations in the imaging lens according to Example 6. Show.

  As can be seen from the above numerical data and aberration diagrams, in each example, a refractive type imaging lens having a viewing angle of 20 degrees or more and aberrations corrected satisfactorily from the visible region to the near infrared region, for example, multiband observation An imaging lens suitable for satellite use can be realized.

  In addition, this invention is not limited to the said embodiment and each Example, A various deformation | transformation implementation is possible. For example, the radius of curvature, the surface interval, and the refractive index of each lens component are not limited to the values shown in the above numerical examples, and may take other values.

It is a lens sectional view corresponding to the imaging lens concerning Example 1 of the present invention. It is lens sectional drawing corresponding to the imaging lens which concerns on Example 2 of this invention. It is lens sectional drawing corresponding to the imaging lens which concerns on Example 3 of this invention. It is lens sectional drawing corresponding to the imaging lens which concerns on Example 4 of this invention. It is a lens sectional view corresponding to the image pick-up lens concerning Example 5 of the present invention. It is lens sectional drawing corresponding to the imaging lens which concerns on Example 6 of this invention. It is explanatory drawing about the incident angle of a light ray. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 1 of this invention. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 2 of this invention. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 3 of this invention. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 4 of this invention. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 5 of this invention. It is a figure which shows the basic lens data of the imaging lens which concerns on Example 6 of this invention. It is the figure which showed collectively the value regarding conditional expressions etc. about each Example. FIG. 4 is an aberration diagram showing various aberrations of the imaging lens according to Example 1 of the present invention, in which (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, and (D) shows lateral chromatic aberration. It is an aberration diagram which shows the various aberrations of the imaging lens which concerns on Example 2 of this invention, (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, (D) shows lateral chromatic aberration. It is an aberration diagram which shows the various aberrations of the imaging lens which concerns on Example 3 of this invention, (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, (D) shows lateral chromatic aberration. It is an aberration diagram which shows the various aberrations of the imaging lens which concerns on Example 4 of this invention, (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, (D) shows lateral chromatic aberration. It is an aberration diagram which shows the various aberrations of the imaging lens which concerns on Example 5 of this invention, (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, (D) shows lateral chromatic aberration. It is an aberration diagram which shows the various aberrations of the imaging lens which concerns on Example 6 of this invention, (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, (D) shows lateral chromatic aberration.

Explanation of symbols

  C1 ... principal ray, G1 ... first group, G2 ... second group, G3 ... third group, LF ... optical member, St ... aperture stop, Ri ... radius of curvature of the i-th lens surface from the object side, Di ... The distance between the i-th lens surface and the (i + 1) -th lens surface from the object side, Z1... The optical axis.

Claims (4)

In order from the object side, the first group, the aperture stop, the second group, and the third group,
The first group includes, in order from the object side, a positive lens, a positive meniscus lens having a convex surface facing the object side, and a negative meniscus lens having a convex surface facing the object side.
The second group includes at least a cemented lens including a positive meniscus lens having a concave surface facing the object side and a negative meniscus lens having a concave surface facing the object side,
The third group includes a positive lens having positive refractive power as a whole and at least an Abbe number of 80 or more,
An imaging lens characterized by satisfying the following conditional expression.
│f / f1│ <0.5 (1)
│f / f2│ <0.8 (2)
0.8 <f / f3 <2.0 (3)
0.15 <DA / f (4)
However,
f: focal length of the entire system f1: focal length of the first group f2: focal length of the second group f3: focal length of the third group DA: an air interval between the second group and the third group. The angle θ _A at which the chief ray at the maximum angle of view enters each surface is
The imaging lens according to claim 1, wherein | θ _A | ≦ 30 °.
However,
θ _A : Angle relative to the normal of each surface at the light incident position. About the material of each lens that constitutes each group,
The imaging lens according to claim 1, wherein the transmittance at a wavelength λ = 350 nm to 900 nm and a thickness of 10 mm is 70% or more. The imaging lens according to any one of claims 1 to 3, wherein the lens closest to the object is made of quartz glass.

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