JP2008197541A - Optical image forming system, optical device and imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical image forming system capable of changing a projection system while maintaining a viewing angle, an optical device, and to provide an imaging apparatus that uses the same. <P>SOLUTION: The optical image forming system is equipped with a basic optical system, having an image-forming function as a single body, and a dome-type lens arranged on the object side of the basic optical system, turning its convex surface to the object side, and made variable in terms of its relative position in an optical axis direction to the basic optical system. Then, the thickness of the dome-type lens may be set substantially uniform, or at least one surface of the dome-type lens may be set to be aspherical. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging optical system, an optical apparatus, and an imaging apparatus that are applied to an imaging apparatus such as a surveillance camera.

In order to improve straight line reproducibility, a projection method represented by the following equation (1) is adopted as a normal imaging lens projection method.
y = ftanθ (1)
In Expression (1), f is the focal length of the imaging lens, θ is the angle (object angle) formed by the light from the object (object light) and the optical axis, and y is the image height of the image of the object. The condition represented by the equation (1) is called “normal image condition” and is known as a condition for acquiring a natural image.

However, since it is difficult for an actual imaging lens to strictly satisfy the normal image condition, some distortion is generated. The distortion amount D is expressed by the following equation (2), where y _P is the actual image height of an image and y ₀ is the ideal image height of the image.
D (%) = 100 × (y _P −y ₀ ) / y ₀ (2)
On the other hand, the fish-eye lens has an angle of view close to 180 ° unlike a normal imaging lens. However, in this case, if the normal image condition (formula (1)) is to be satisfied, the image is increased as the object angle θ approaches 90 °. The high y increases remarkably, and y = ∞ when θ = 90 °, which is unrealistic. For this reason, another projection method that allows the generation of distortion is adopted for the fisheye lens. Of these, the two representative projection methods are as follows.

The first projection method is an equidistant projection method in which the object angle θ and the image height y are in a proportional relationship, and is represented by the following equation (3).
y = fθ (3)
The second projection method is an orthogonal projection method in which the image height y is proportional to sin of the object angle θ, and is expressed by the following equation (4).

y = fsinθ (4)
Of these, the equidistant projection method (Equation (3)) is employed for a circular fisheye lens that produces a circular image with a 180 ° angle of view, for example, and the orthographic projection method (Equation (4)) is for example a diagonal direction of a rectangular screen. It is used for a diagonal fisheye lens with an angle of view of 180 °.
Based on the above, an imaging lens suitable for surveillance cameras will be studied.

If an imaging lens that satisfies the normal image condition (formula (1)) is used for the surveillance camera, it may be an ultra-wide angle lens, but the image size is reduced although the distortion of the image to be monitored is reduced. The visibility is poor because of the small size.
On the other hand, when a fisheye lens of the equidistant projection method (formula (3)) or the orthographic projection method (formula (4)) is used for the surveillance camera, the size of the image to be monitored increases, but the image distortion increases. It is hard to say that the visibility is good.

Therefore, instead of modifying the equidistant projection method (Equation (3)) or the orthographic projection method (Equation (4)) and allowing the visibility of the periphery of the monitoring range to deteriorate, A projection method with better visibility was proposed. A lens employing this projection method is called a “foveal lens” or the like (see Patent Document 1). Incidentally, a portion having good visibility in the center of the image plane of the foveal lens is called a “fovea”. This foveal lens is considered effective in enhancing the performance of the surveillance camera.
JP 2004-354572 A

However, since the monitoring target of the monitoring camera may move, it is desirable that the projection method of the imaging lens changes dynamically according to the situation. If the projection method is variable, for example, the entire monitoring range can be monitored uniformly during normal times, and when the monitoring target enters the center of the monitoring range, the monitoring target can be monitored in detail. This is because it becomes possible.
Incidentally, there is a zoom lens as a lens having a variable function of the projection method. However, since the angle of view of the zoom lens is narrowed by zooming up, there is a problem that the monitoring range cannot be maintained.

Although the variable function of the projection method can be realized by image processing, pixel interpolation is indispensable in image processing, so that there is a problem that image quality deteriorates and the authenticity of the image cannot be guaranteed.
The present invention has been made in view of the above problems, and provides an imaging optical system, an optical apparatus, and an imaging apparatus using the same that can change the projection method while maintaining the angle of view. With the goal.

The imaging optical system of the present invention is a basic optical system having an imaging function as a single unit, and is disposed on the object side of the basic optical system, with a convex surface facing the object side, and relative to the basic optical system in the optical axis direction. And a dome-shaped lens whose position is variable.
It is desirable that the thickness of the dome lens is substantially uniform.
Moreover, it is desirable that at least one surface of the dome lens is an aspherical surface.

Further, it is desirable that the dome-shaped lens has a sufficiently large aperture so as not to narrow the angle of view of the basic optical system.
In addition, the angle of view of the imaging optical system of the present invention is desirably 90 ° or more.
The optical device of the present invention includes the imaging optical system of the present invention, a moving mechanism that moves the dome-shaped lens in the optical axis direction, and a focusing mechanism that changes the focal position of the basic optical system. It is characterized by that.

An imaging apparatus according to the present invention includes the optical apparatus according to the present invention and an imaging element that detects an image formed by the imaging optical system.
The imaging apparatus of the present invention preferably further includes control means for controlling the optical device in accordance with a change in an image acquired by the imaging element.

  According to the present invention, it is possible to realize an imaging optical system, an optical apparatus, and an imaging apparatus using the same that can change the projection method while maintaining the angle of view.

[Embodiment of Imaging Optical System]
The imaging optical system of this embodiment includes a basic optical system having an imaging function as a single unit (see FIG. 1). The specifications (field angle, focal length, F value, chromatic aberration, total length, etc.) of the basic optical system satisfy the specifications required by the surveillance camera. For example, the angle of view of the basic optical system alone is wide, 90 ° or more, preferably 140 ° or more.

At the head of the imaging optical system, a dome-shaped lens having a convex surface facing the object side and movable in the optical axis direction is disposed (see FIG. 4).
The dome lens is a meniscus lens having a substantially uniform thickness. Therefore, when attention is paid to each part of the dome-shaped lens, it can be regarded as a substantially parallel plate. Therefore, the insertion of the dome-shaped lens hardly changes the overall aberration (aberration other than distortion) of the imaging optical system. That is, the entire aberration (aberration other than distortion) of the imaging optical system is substantially the same as the single aberration (aberration other than distortion) of the basic optical system.

  However, each object light incident on each position of the dome lens causes a lateral shift when passing through the dome lens. Therefore, when an object is viewed from the basic optical system through the dome lens, the position of each object is laterally shifted according to the object angle. In addition, the relationship between the object angle and the lateral shift amount changes depending on the position of the dome-shaped lens in the optical axis direction. Hereinafter, the position of the dome lens in the optical axis direction is simply referred to as “position”.

  Therefore, when the position of the dome-shaped lens is made to coincide with a reference position where the lateral shift amount is substantially zero, the overall projection method of the imaging optical system is substantially the same as the single projection method of the basic optical system. However, when the position of the dome lens is gradually changed from the reference position, the overall projection method of the imaging optical system can be gradually changed from the same one as the single projection method of the basic optical system.

  For example, if the projection system of the basic optical system is the same projection system as the fovea lens or a projection system similar to the fovea lens, the position of the dome lens is gradually changed from the reference position, so that It is also possible to gradually change the balance between the amount of distortion of the image of the object located and the amount of distortion of the image of the object located around the angle of view. Such an imaging optical system is suitable for a surveillance camera.

In the imaging optical system of the present embodiment, the aperture size of the dome lens needs to be secured sufficiently large in advance (see FIG. 4). The aperture size is such that the angle of view of the imaging optical system is maintained equal to the angle of view of the basic optical system regardless of the position of the dome lens.
Further, in the imaging optical system of the present embodiment, a slight distribution may be provided instead of making the thickness of the dome-shaped lens completely uniform. If the thickness distribution of the dome lens is adjusted, the relationship between the position change amount of the dome lens and the change amount of the projection method (the change degree of the projection method) can be adjusted.

In the imaging optical system of this embodiment, the entire aberration (aberration other than distortion aberration) of the imaging optical system may be corrected by making at least one surface of the dome-shaped lens an aspherical surface.
[Embodiment of Optical Device]
The optical apparatus according to the present embodiment includes the above-described imaging optical system, a moving mechanism (such as a helicoid type lens barrel) that moves the dome-shaped lens in the optical axis direction, and a focusing mechanism. Of these, when the moving mechanism is driven, only the projection method of the imaging optical system can be changed while maintaining the angle of view of the imaging optical system.

  In the optical device according to the present embodiment, the moving mechanism and the focusing mechanism may be electrically driven by a motor. Further, when the focusing mechanism is motorized, known autofocus control can be applied. Further, it is desirable that the focusing mechanism is driven every time the moving mechanism is driven. This is because when the position of the dome lens changes, the image plane of the imaging optical system also shifts.

[Embodiment of surveillance camera]
The surveillance camera according to the present embodiment includes the above-described optical device, an image sensor disposed near the image plane of the imaging optical system, and a control unit that controls the optical device and the image sensor. This control unit monitors whether or not the monitoring target is within the monitoring range of the monitoring camera based on the video acquired by the image sensor, and drives the moving mechanism of the optical device as necessary.

For example, the projection method of the basic optical system is the same projection method as the foveal lens, or a projection method similar to the foveal lens, and the amount of distortion of the image of the object located at the center of the angle of view by driving the moving mechanism And the balance between the distortion amount of the image of the object located at the periphery of the angle of view, the control unit can perform the following flexible monitoring operation.
In other words, the control unit equalizes the balance between the distortion amount of the image of the object located in the central part of the monitoring range and the distortion amount of the image of the object located in the peripheral part of the monitoring range in normal times. At this time, the monitoring camera can uniformly monitor the entire monitoring range. On the other hand, when the monitoring target approaches the central part of the monitoring range, the control unit determines the distortion amount of the image of the object located in the central part of the monitoring range and the distortion amount of the image of the object located in the peripheral part of the monitoring range. Smaller than. At this time, the monitoring camera can monitor the monitoring target in more detail. Even if the control unit operates in this way, the angle of view of the imaging optical system is maintained, so that the monitoring range of the monitoring camera is not narrowed.

If the monitoring camera of this embodiment is applied, for example, when a person is monitored, it is possible to capture in detail the face and clothes of the person who has moved within the monitoring range. Alternatively, when a vehicle is monitored, it is possible to photograph a driver of the vehicle passing through the monitoring range in detail or to read the license plate of the vehicle.
[Others]
Further, the above-described surveillance camera function may be installed in another type of digital camera, and the possibility of video expression may be expanded by implementing an unprecedented deformation effect or forming an image different from human vision. Further, the imaging optical system and the optical device described above can be applied to special effect lenses, image sensors for automobiles and robots, and the like.

An embodiment of the above-described imaging optical system will be described.
The specifications of the basic optical system are as follows. This basic optical system is designed to have a wide angle of view of 140 ° and a relatively large negative distortion (barrel-shaped distortion) assuming use with a surveillance camera.
・ Focal distance f = 5.6 mm,
-F value = 2.8,
-Angle of view = 140 °,
Detailed data of the basic optical system is shown in Table 1. In Table 1, nC is the refractive index of the C line (656.27 nm), nd is the refractive index of the d line (587.56 nm), and nF is the refractive index of the F line (486.13 nm).
[Table 1]
Surface number Curvature radius (mm) Surface spacing (mm) nC nd nF
0 (object surface) ∞ ∞
1 137.373 12.000 1.876625 1.883000 1.898172
2 42.875 29.819
3 -229.892 75.883 1.876625 1.883000 1.898172
4 30.751 58.135
5 -1423.298 13.105 1.836523 1.846660 1.871977
6 -79.860 11.743
7 184.885 72.419 1.767827 1.772500 1.783411
8 -200.589 5.701
9 (Aperture) ∞ 3.451
10 -226.467 12.000 1.836523 1.846660 1.871977
11 21.020 12.992 1.767827 1.772500 1.783411
12 -164.470 0.100
13 34.011 12.725 1.615008 1.618000 1.624759
14 -218.680 0.100
15 54.841 12.077 1.876625 1.883000 1.898172
16 54.664 0.252
17 230.816 25.853 1.495520 1.497820 1.502969
18 ∞ 0
19 (image plane) ∞ 0
FIG. 1 shows an optical path diagram of the basic optical system, FIG. 2 shows a distortion diagram of the basic optical system, and FIG. 3 shows a spot diagram of the basic optical system.

In FIG. 1, some lenses close to the image plane are omitted for the convenience of the paper size (for reference, all the lenses are shown without being omitted in FIGS. 4, 7, and 10 described later). .)
Also, the horizontal axis (distortion amount) in FIG. 2 is a value related to the d-line (587.56 nm), and each scale on the horizontal axis is −80 °, −40 °, 0 °, + 40 °, + 80 ° in order from the left. It is.
Also, the vertical axis in FIG. 2 is the normalized image height normalized by the maximum image height, and each scale is 100%, 75%, 50%, and 25% in order from the top. However, the numerical values in the figure are obtained by converting the standardized image height y _s into the object angle θ, and are 70 °, 64.11 °, 53.95 °, and 34.48 ° in order from the top. Incidentally, the conversion formula is expressed by the following formula (5) when the maximum object angle is θ _max . Note that the notation method of FIG. 2 is common to the following similar drawings.

θ = arctan (y _s tan θ _max ) (5)
In the spot diagram of FIG. 3, the results of C line (656.27 nm), d line (587.56 nm), and F line (486.13 nm) are superimposed. The notation method of FIG. 3 is common to the following similar drawings.
In the imaging optical system of the present embodiment, a dome-shaped lens having the specifications shown in Table 2 is installed on the object side of the above basic optical system. In Table 2, ν is the Abbe number.
[Table 2]
Surface number Curvature radius (mm) Surface spacing (mm) nd ν
1 130.000 10.000 2.5000 90
2 (Aspherical) 100.000 Variable The aspherical coefficient of the second surface is as follows. This aspherical coefficient is determined by trial and error or numerical optimization for the purpose of correcting the spread of the spot diagram accompanying the change in the position of the dome-shaped lens.

Fourth-order coefficient a: 6.19619 × 10 ⁻⁸
6th order coefficient b: -1.20776 × 10 ^-11
8th order coefficient c: -1.44282 × 10 ^-17
By these aspheric coefficients a, b, and c, the aspheric amount y (= displacement from the spherical surface) at the radial position h of the second surface is expressed as follows.

y = ah ⁴ + bh ⁶ + ch ⁸ (6)
In this embodiment, the position of the dome lens (the distance from the object-side surface of the basic optical system to the image-side surface of the dome lens) is changed in four steps of 40 mm, 100 mm, 120 mm, and 150 mm. The optical path, distortion, and spot diagram of the image optical system were calculated.

An optical path diagram, a distortion diagram, and a spot diagram when the position of the dome-shaped lens is 40 mm are shown in FIGS.
FIGS. 7, 8, and 9 show optical path diagrams, distortion diagrams, and spot diagrams when the position of the dome-shaped lens is 100 mm.
An optical path diagram, a distortion diagram, and a spot diagram when the position of the dome lens is 120 mm are shown in FIGS.

An optical path diagram, a distortion diagram, and a spot diagram when the position of the dome lens is 150 mm are shown in FIGS.
First, when the dome-shaped lens is not inserted (see FIG. 1), the distortion aberration of the imaging optical system is as shown in FIG. 2, and it can be seen that it is a barrel-shaped distortion aberration.
From this state, when the dome-shaped lens is inserted and the position is set to 40 mm (see FIG. 4), the distortion aberration of the imaging optical system is as shown in FIG. 5, and the distortion when the dome-shaped lens is not inserted. It can be seen that there is almost no change with the aberration (see FIG. 2). Therefore, the 40 mm position can be regarded as the reference position of the dome-shaped lens.

When the position of the dome-shaped lens is changed to 100 mm (see FIG. 7), the distortion aberration of the imaging optical system is as shown in FIG. 8, and it can be seen that the amount of distortion at the object angle θ ≦ 50 ° is reduced.
Further, when the position of the dome-shaped lens is changed to 120 mm (see FIG. 10), the distortion aberration of the imaging optical system is as shown in FIG. 11, and it can be seen that the distortion at the object angle θ ≦ 40 ° is further reduced.

Further, when the position of the dome-shaped lens is changed to 150 mm (see FIG. 13), the distortion aberration of the imaging optical system is as shown in FIG. 14, and the distortion at the object angle θ ≦ 40 ° is further reduced, being substantially zero. I understand that That is, it can be seen that the normal image condition is substantially satisfied within a limited range of the object angle θ ≦ 40 °, and the straight line reproducibility is good.
Further, in this embodiment, when the position of the dome-shaped lens is 40 mm (see FIG. 4), since negative distortion has occurred (see FIG. 5), the image of the imaging optical system is distorted into a barrel shape. The image magnification is smaller than the image magnification of the imaging optical system that satisfies the normal image condition. However, if the position of the dome lens is changed from 40 mm to 150 mm, the distortion in the range of the object angle θ ≦ 40 ° is obtained. Since the amount is small (see FIGS. 8, 11, and 14), the image magnification of the imaging optical system is equivalent to the image magnification of the imaging optical system that satisfies the normal image condition.

Therefore, in the imaging optical system of the present embodiment, if the position of the dome-shaped lens is changed in the direction of 40 mm → 150 mm, the distortion amount of the image at the center of the angle of view can be reduced and the image magnification can be increased.
In addition, the field angle of the imaging optical system of the present embodiment is 140 ° and does not change regardless of the position of the dome-shaped lens. That is, the compression ratio of the image around the angle of view changes depending on the position of the dome-shaped lens, but the image around the angle of view is not lost.

  Moreover, as can be confirmed from the spot diagrams (FIGS. 6, 9, 12, and 15) when the dome lens is in each position, the spot size is kept small regardless of the position of the dome lens. . Therefore, the change in the position of the dome lens does not affect the imaging performance of the imaging optical system.

It is an optical path figure of a basic optical system. This is the distortion of the basic optical system. It is a spot diagram of a basic optical system. It is an optical path figure of an image formation optical system when the position of a dome shape lens is 40 mm. This is a distortion aberration of the imaging optical system when the position of the dome-shaped lens is 40 mm. It is a spot diagram of an imaging optical system when the position of a dome shape lens is 40 mm. It is an optical path diagram of an imaging optical system when the position of a dome-shaped lens is 100 mm. This is a distortion aberration of the imaging optical system when the position of the dome lens is 100 mm. It is a spot diagram of the imaging optical system when the position of the dome-shaped lens is 100 mm. It is an optical path diagram of the imaging optical system when the position of the dome-shaped lens is 120 mm. This is a distortion aberration of the imaging optical system when the position of the dome-shaped lens is 120 mm. It is a spot diagram of an imaging optical system when the position of a dome shape lens is 120 mm. It is an optical path diagram of the imaging optical system when the position of the dome lens is 150 mm. This is a distortion aberration of the imaging optical system when the position of the dome-shaped lens is 150 mm. It is a spot diagram of an imaging optical system when the position of a dome shape lens is 150 mm.

Claims (8)

A basic optical system with a single imaging function;
A dome-shaped lens that is disposed on the object side of the basic optical system, has a convex surface facing the object side, and the relative position in the optical axis direction with respect to the basic optical system is variable;
An imaging optical system comprising: The imaging optical system according to claim 1,
An imaging optical system characterized in that the thickness of the dome-shaped lens is substantially uniform. The imaging optical system according to claim 1 or 2,
An imaging optical system, wherein at least one surface of the dome-shaped lens is an aspherical surface. In the imaging optical system according to any one of claims 1 to 3,
The aperture diameter of the dome-shaped lens is secured sufficiently large so as not to reduce the angle of view of the basic optical system. In the imaging optical system according to any one of claims 1 to 4,
An imaging optical system having an angle of view of 90 ° or more. The imaging optical system according to any one of claims 1 to 5,
A moving mechanism for moving the dome-shaped lens in the optical axis direction;
A focusing mechanism for changing the focal position of the basic optical system;
An optical apparatus comprising: An optical device according to claim 6;
An imaging device comprising: an imaging device that detects an image formed by the imaging optical system. The imaging apparatus according to claim 7,
An imaging apparatus, further comprising control means for controlling the optical device in accordance with a change in an image acquired by the imaging element.

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