JP2020030436A - Optical system and imaging system - Google Patents

Abstract

To provide a new optical system having a plurality of wide-angle lenses.SOLUTION: The optical system includes a plurality of wide-angle lenses 10b, 10c, in which each wide-angle lens has a lens peripheral region III and a lens inner region ranging from the optical axis to the lens peripheral region, and subject image portions imaged by the lens peripheral regions of the respective wide-angle lenses overlap each other, while subject image portions imaged by the lens inner regions do not overlap. The lens inner region has a first region I from the optical axis toward the lens peripheral region, and a second region II from the first region to the lens peripheral region. In the first region, an image magnification of a subject image per unit angle of view is constant; in the lens peripheral region, an image magnification of a subject image per unit angle of view decreases; and in the second region, an image magnification of a subject image per angle of view continuously varies from the first region to the lens peripheral region.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical system and an imaging system.

As an imaging system that uses a plurality of wide-angle lenses, overlaps the peripheral portions of the subject images formed by the wide-angle lenses with each other, and joins each subject at the overlapping portion to obtain a wide-field image, for example, a so-called “ For use in an “omnidirectional imaging system”.
As the wide-angle lens, which is also called a “fish-eye lens”, four types of projection methods known as an orthographic projection method, an equal solid angle projection method, an equidistant projection method, and a stereoscopic projection method are known.
Conventionally, the orthographic projection method and the equal solid angle projection method have been widely used for fisheye lenses for silver halide cameras.
In recent digital cameras and the like, with the improvement of image processing technology, the stereoscopic projection method is suitable for emphasizing the resolution of the peripheral portion, and to improve the balance between on-axis and off-axis resolution, Patent Document 1 discloses that equidistant projection is suitable.
However, the fisheye lens described in Patent Literature 1 is used alone, and is not intended to be used in combination with a plurality of fisheye lenses, such as the above-described spherical imaging system. Absent.

  An object of the present invention is to realize a novel optical system having a plurality of wide-angle lenses.

  The optical system according to the present invention is an optical system having a plurality of wide-angle lenses, and each wide-angle lens has a lens peripheral region and a lens inner region from the optical axis to the lens peripheral region. The subject image portions in which the lens peripheral region forms an image overlap each other, the subject image portions in which the lens inner region forms an image do not overlap with each other, and the lens inner region extends from the optical axis toward the lens peripheral region. A first region and a second region from the first region to the lens peripheral region. In the first region, the imaging magnification of a subject image per unit angle of view is constant. The imaging magnification of the subject image per unit angle of view decreases, and in the second area, the imaging magnification of the subject image per unit angle of view continuously changes from the first area to the lens peripheral area. .

  According to the present invention, a novel optical system having a plurality of wide-angle lenses can be realized.

FIG. 1 is a diagram illustrating an embodiment of an imaging system. FIG. 4 is a spherical aberration diagram of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 3 is an astigmatism diagram of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 4 is a coma aberration diagram of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 3 is an MTF diagram of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 3 is a diagram illustrating a projection method of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 3 is a diagram of an imaging magnification per unit angle of view of a specific example of a wide-angle lens used in an optical system of an imaging system. FIG. 9 is a diagram of an imaging magnification per unit angle of view of a comparative example wide-angle lens. It is a figure which shows the value of the "spatial frequency: MTF value with respect to 200 LP / mm" with respect to a half angle of view in a specific example wide-angle lens and a comparative example wide-angle lens. It is a figure which shows "the resolution per unit pixel on the light-receiving surface of an image sensor" of a specific example wide-angle lens and a comparative example optical lens.

Hereinafter, embodiments will be described.
FIG. 1 shows an embodiment of an imaging system.
In FIG. 1, reference numeral 1b indicates an “imaging system”.
The imaging system 1b is an “omnidirectional imaging system” having two wide-angle lenses 10b and 10c.
As shown in FIG. 1, the imaging system 1b includes a wide-angle lens 10b, a cover glass CGb, an image sensor ISb, a wide-angle lens 10c, a cover glass CGc, and an image sensor ISc.
The wide-angle lenses 10b and 10c are of the "same specifications", and the wide-angle lens 10b will be described below. In the description, the reference numerals in parentheses relate to the wide-angle lens 10c.

  The wide-angle lens 10b (10c) includes a “front group, a rear group, a stop, and a prism”.

The “front group” includes two negative meniscus lenses L1b (L1c) and L2b (L2c). The group has negative refractive power and captures light rays with a wide angle of view exceeding 180 degrees.
The “rear group” includes five lenses, that is, a lens L3b (L3c), a lens L4b (L4c), a lens L5b (L5c), a lens L6b (L6c), and a lens L7b toward the image side (imaging element side). (L7c).
A prism PSb (PSc) is arranged between the front group and the rear group, and bends the optical axis OA2b (OA2c) of the front group by 90 degrees. Accordingly, the optical axis of the rear group is perpendicular to the optical axis OA2b (OA2c) of the front group, and is orthogonal to the light receiving surface of the image sensor ISb (ISc).
The prism PSb (PSc) is a right-angle prism and has a reflection surface MSb (MSc) formed on an inclined surface, and the prism PSb and the prism PSc are integrated with each other by bringing the reflection surfaces MSb and MSc into close contact.
A stop 4b (4c) is provided in close contact with the lens surface on the prism PSb (PSc) side of the lens L3b (L3c) closest to the prism PSb (PSc) among the lenses constituting the rear group.

Of the lenses L3b (L3c) to L7b (L7c) constituting the rear group, the lens L4b (L4c) is a “biconvex lens”.
The lens L5b (L5c) is a "biconvex lens" and the lens L6b (L6c) is a "biconcave lens", and these biconvex and biconcave lenses are joined.
The lens L7b (L7c) closest to the image is a “biconvex lens”.
Table 1 shows an example of lens data of the wide-angle lens 10b (10c) shown in FIG.

“Surface number” in Table 1 indicates the number of the surface counted from the object side, and surface number 7 is “aperture surface”. The “type” is the type of the lens surface, which is “spherical” or “aspherical”. The term "thickness" refers to the spacing between surfaces, and the "refractive index" and "Abbe number" are the refractive index and Abbe number of the lens material with respect to the sodium D line. The “radius of curvature” of a type-aspheric surface is a “paraxial radius of curvature”. The unit of the quantity having the dimension of length is “mm” unless otherwise specified.
In the wide-angle lens whose data is shown in Table 1, the lens surfaces of surface numbers 3, 4, 10, 11, 15, and 16 are aspherical. The data of these aspheric surfaces are shown below.
The aspheric surface is represented by the following equation.

^{X = (H 2 / R)} / [1+ {1-K (H / r) 2} 1/2]
+ C4 · H ⁴ + C6 · H ⁶ + C8 · H ⁸ + C10 · H ¹⁰ + ...
In this equation, X is “the height from the optical axis with reference to the surface vertex: displacement in the optical axis direction at the position of H”, K is “cone coefficient”, and C4, C6, C8, C10 Is the “aspheric coefficient”. R is “paraxial radius of curvature”.
Each of the six aspherical surfaces employed in the wide-angle lens whose data is shown in Table 1 has a conic constant: K = 0.
X = (H ² / 2R) + C4 · H ⁴ + C6 · H ⁶ + C8 · H ⁸ + C10 · H ¹⁰ +...
And the shape of the aspheric surface: X is determined by the “paraxial radius of curvature” given in Table 1 and each order aspheric surface coefficient. The data of each aspheric surface is shown below.

"Aspheric data"
Surface number: 3 C4 = 0.002491, C8 = 4.63 × 10 ⁻⁷
Surface number: 4 C4 = 0.003451 and C6 = 0.000504
C10 = 1.45 × 10 ⁻⁵
Surface number: 10 C4 = -0.00235, C6 = -0.00025
Surface number: 11 C4 = -0.00217, C6 = -0.00028
Surface number: 15 C4 = -0.00167, C6 = -0.00031
C8 = 3.07 × 10 ⁻⁵ , C10 = 3.12 × 10 ⁻⁵
Surface number: 16 C4 = 0.004209, C6 = -0.00141
C8 = 0.000474, C12 = 2.02 × 10 ⁻⁶
In each of the surface numbers, the “aspherical coefficient of an order not described” is all “0”.

The wide-angle lens having the data shown in Table 1 and the above-mentioned aspherical data will be referred to as a “specific example wide-angle lens”.
Specific Example The wide-angle lens 10b (10c) has a full angle of view: 200 degrees (half angle of view: 100 degrees), and a half angle of view area of a half angle of view: 90 to 100 degrees is a "lens peripheral area". In the “half field angle region: 90 ° to 100 °” portion of the wide-angle lenses 10b and 10c, the subject image portions formed on the light receiving surfaces of the image sensors ISb and ISc overlap each other.
Specific examples of various aberrations of the wide-angle lens are shown in FIGS. 2 is a spherical aberration diagram, FIG. 3 is an astigmatism diagram, and FIG. 4 is a coma aberration diagram. As is clear from these figures, the wide-angle lens of the specific example has good performance.
FIG. 5 shows the MTF characteristics of the specific example wide-angle lens with respect to the half angle of view in the lens inner region: 30, 60, and 90 degrees. The horizontal axis indicates the spatial frequency (0 to 200 in LP / mm (line pair / mm)).
The MTF characteristics are shown in the tangential direction (indicated by “T” in the figure) and the sagittal direction (indicated by “S” in the figure).
Specific Example Although the MTF characteristics of the wide-angle lens are good, as the spatial frequency increases, the MTF decreases as the half angle of view increases. That is, the resolution tends to decrease.

Now, in the plurality of wide-angle lenses constituting the optical system of the present invention, the lens inner area includes a first area from the optical axis toward the lens peripheral area, and a second area from the first area to the lens peripheral area. Having.
In the first area, “the imaging magnification of the subject image per unit angle of view is constant”, in the peripheral area of the lens, “the imaging magnification of the subject image per unit angle of view decreases”, and in the second area, “the imaging magnification of the subject image per unit angle of view” decreases. The imaging magnification of the subject image per corner continuously changes from the first area to the lens peripheral area. "

This will be described below.
The optical system being described has two wide-angle lenses 10b and 10c having a full angle of view exceeding 180 degrees, and these two wide-angle lenses 10b and 10c have the same specifications.

FIG. 6 shows the “relationship between the half angle of view and the image height” of the specific example wide-angle lens.
The figure plots the relationship between the half angle of view and the image height for the “equidistant projection method” and the “optical system in the form of the real image”.
The “optical system in the form of the real image” is a specific example of a wide-angle lens.
In the equidistant projection method, the image height increases linearly as the “half angle of view” on the horizontal axis increases.

The "relationship between half field angle and image height" of the specific example wide-angle lens is almost the same as that of the equidistant projection method, but there is a slight shift. Specific Example The wide-angle lens is designed to be “equidistant projection method in the lens inner area”, but the actually manufactured lens is “slightly deviated from the equidistant projection method”.
Assuming that the image height on the vertical axis in FIG. 6 is “Y” and the half angle of view on the horizontal axis is “θ”, “imaging magnification per unit angle of view (change in image height per unit angle of view)” is “ dY / dθ ”.
FIG. 7 shows “imaging magnification per unit angle of view: dY / dθ” in the specific example wide-angle lens.

This curve is obtained by differentiating the curve representing the “optical system in the form of the real image” in FIG. 6 with the angle of view: θ.
In FIG. 7, the region: I is the first region, the region: II is the second region, and the region: III (half angle of view: the region of 90 to 100 degrees) is the peripheral lens region. Inner region ".

As shown in this figure, the lens inner region includes a first region I extending from the optical axis (horizontal axis: 0) to the lens peripheral region III, and a second region II from the first region I to the lens peripheral region III. Have. In the first area I, “the imaging magnification of the subject image per unit angle of view is constant”, and in the lens peripheral area III, “the imaging magnification of the subject image per unit angle of view decreases”. In the second region II, the imaging magnification of the subject image per unit angle of view continuously changes from the first region I to the lens peripheral region III.
Strictly speaking, dY / dθ in the first region I gradually decreases toward the second region II, and is not exactly constant. However, such a “case that can be regarded as substantially constant” is also referred to as “the imaging magnification of the subject image per unit angle of view in the first region I is constant”.

As a comparative example with respect to this specific example wide-angle lens, a wide-angle lens (hereinafter, referred to as “comparative example wide-angle lens”) in which dY / dθ changes from the optical axis to the entire area including the lens peripheral area as shown in FIG. Specific Example Following the case of a wide-angle lens, a first area I, a second area II, and a lens peripheral area III are used as shown in the figure.
A wide-angle lens, such as a comparative example wide-angle lens, is known from US Pat.
The dY / dθ of the comparative example wide-angle lens increases in a first area I by drawing a “convex downward curve”, decreases monotonously in the lens peripheral area III, and decreases in the second area II from the first area I to the lens. It changes continuously toward the peripheral region III.
That is, in the comparative example wide-angle lens, the first area I of the lens inner area is of the “stereoscopic projection method”.

  FIG. 9 shows the values of the “MTF value (vertical axis) for a spatial frequency: 200 LP / mm (vertical axis)” with respect to the half angle of view (horizontal axis) in the specific example wide-angle lens and the comparative example wide-angle lens. The solid line is for the comparative example wide-angle lens, and the broken line is for the specific example wide-angle lens.

Hereinafter, the MTF value of the spatial frequency: 200 LP / mm is abbreviated as “MTF200”.
Then, FIG. 9 shows a change accompanying a change in the half angle of view of the MTF 200 in the case of the comparative example wide-angle lens (solid line) and the case of the specific example wide-angle lens (dashed line). This diagram can be roughly described as “a state in which the resolution changes with the half angle of view”.
Referring to FIG. 9, the MTF 200 at a high spatial frequency of 200 LP / mm is “decreased with an increase in the half angle of view” in the wide-angle lens of the comparative example, and “in a favorable state up to the lens maximum angle of view” in the wide-angle lens of the specific example. Is maintained.

Here, the “imaging magnification per unit angle of view in the specific example wide-angle lens: dY / dθ” shown in FIG. 7 is multiplied by the change due to the half angle of view of “MTF200” shown by the broken line in FIG. It becomes something like the broken line of.
On the other hand, when the “imaging magnification per unit angle of view in the comparative example wide-angle lens: dY / dθ” shown in FIG. 8 is multiplied by the change due to the half angle of view of “MTF200” shown by the solid line in FIG. It becomes something like the solid line of 10.
“Product of dY / dθ and MTF 200” corresponds to “resolution per unit pixel” on the light receiving surface of the image sensor.

In the comparative example wide-angle lens, the “resolution per unit pixel” is substantially constant and stable from the optical axis to the outermost part of the lens peripheral region, and is slightly larger where the half angle of view is large. That is, the comparative example wide-angle lens has a performance in which “resolution of the peripheral portion” is emphasized, and it can be said that the performance is good.
On the other hand, the “resolution per unit pixel” in the specific example wide-angle lens monotonically decreases from the optical axis to the outermost part of the lens peripheral area. However, the magnitude of “resolution per unit pixel” is larger than that of the comparative example wide-angle lens over substantially the entire area inside the lens.
That is, the specific example wide-angle lens has a large resolution in the entire region of the actually connected image. Considering the resolution and projection system of an optical system using a wide-angle lens in total, it is preferable that "the resolution is higher over the entire light receiving surface of the image sensor".
From this, it can be said that the optical system of the present invention is superior in basic performance to the optical system using the comparative example wide-angle lens.

As shown in FIG. 10, the “resolution per unit pixel” is lower in the specific example wide-angle lens than in the comparative example wide-angle lens in the lens peripheral area.
However, since the images in the lens peripheral area overlap with each other and "are not used for the actual spherical image", this point is not a problem.
Since the images formed in the lens peripheral area overlap each other, "double information amount" is obtained in this part with respect to the same image part. That is, for an image formed by the lens peripheral region, the same result as an increase in “light receiving area” of the light receiving element is obtained, which is the same as an increase in magnification per unit angle of view. Accordingly, even if the imaging magnification per unit angle of view monotonically decreases as shown by the broken line in FIG. 10, sufficient information amount compensation is possible, and "the whole sky with uniform image quality over the whole sphere" Spherical image ".

Further, when an image formed by two wide-angle lenses is connected to form an omnidirectional image, an image formed in a lens peripheral region is used for image connection (stitching) as “reference data representing the same image”. Can be
As described above, according to the present invention, the following optical system and imaging system can be realized.

  [1] An optical system having a plurality of wide-angle lenses 10b and 10c, wherein each wide-angle lens has a lens peripheral region III and a lens inner region from the optical axis to the lens peripheral region. The subject image portions formed by the lens peripheral region III overlap each other, the subject image portions formed by the lens inner region do not overlap each other, and the lens inner region goes from the optical axis to the lens peripheral region. It has a first region I and a second region II from the first region I to the lens peripheral region III. In the first region I, the imaging magnification of a subject image per unit angle of view is constant. In the lens peripheral region III, the imaging magnification of the subject image per unit angle of view decreases, and in the second region II, the imaging magnification of the subject image per unit angle of view decreases from the first region to the lens. Perimeter area Continuously changes (FIG. 7) optical system.

[2]
[1] The optical system according to [1], wherein a lens inner region of each wide-angle lens is an equidistant projection system.
[3]
The optical system according to [1] or [2], comprising two wide-angle lenses 10b and 10c having a total angle of view of 180 degrees or more.
[4]
[3] The optical system according to [3], wherein the two wide-angle lenses 10b and 10c have the same specifications, and the lens peripheral region III is a half angle of view: an angle of view larger than 90 degrees.

[5]
[4] The optical system according to [4], wherein the two wide-angle lenses 10b and 10c having the same specifications have a front group having a negative refractive power and a rear group having a positive refractive power.
[6]
[5] The optical system according to [5], wherein the two wide-angle lenses 10b and 10c having the same specifications respectively reflect between the front group and the rear group to bend the light flux from the front group toward the rear group. An optical system having transparent bodies PSb and PSc having surfaces MSb and MSc, wherein the reflecting surfaces of the transparent bodies are arranged close to or in close contact with each other.
[7]
[6] The optical system according to [6], wherein the transparent bodies having the reflecting surfaces MSb and MSc are prisms PSb and PSc.

[8]
An imaging system comprising: the optical system according to any one of [1] to [7]; and imaging elements ISb and ISc that convert light that has passed through the optical system into an electric signal to generate a captured image. (Fig. 1).
[9]
[8] The imaging system according to [8], wherein the images captured by the wide-angle lenses are connected to each other and displayed using overlapping portions of the subject images formed by the wide-angle lenses.

As described above, the preferred embodiments of the present invention have been described. However, the present invention is not limited to the specific embodiments described above, and unless otherwise specified in the above description, the invention described in the claims. Various modifications and changes are possible within the scope of the spirit of the present invention.
The effects described in the embodiments of the present invention merely enumerate preferred effects resulting from the invention, and the effects of the invention are not limited to those described in the embodiments.

  The form described above as “optical system using two wide-angle lenses” can be extended to a general case. That is, it has a total angle of view: A (degree) greater than 360 / n, where n is a natural number of 2 or more, and “the first area where the magnification per unit angle of view is substantially constant, the lens peripheral area that decreases, An optical system including a combination of n or more wide-angle lenses having a second region in which the magnification changes continuously between them can be provided.

  For example, using an optical system in which three wide-angle lenses having an angle of view larger than 360 degrees / 3 = 120 degrees: A (for example, 140 degrees) and arranged radially in the same plane, an image sensor is provided for each of the wide-angle lenses. When configured in combination, an imaging system can capture a 360-degree horizontal panoramic image.

  The image obtained in this case is not a “spherical image”, but an imaging system capable of capturing such a horizontal panoramic image can be satisfactorily implemented as a vehicle-mounted camera or a security camera.

  If a wide-angle imaging system using four wide-angle lenses having an angle of view of A = 140 degrees, spatially radially, and a tetrahedron-type optical system is used, a solid angle of 4π radians is obtained. A celestial sphere image can be captured.

10b, 10c Wide-angle lens
I First area
II Second Area
III Lens peripheral area
PSb, PSc transparent body (prism)
ISb, ISc Image sensor

JP 2006-17837 A JP 2013-45089 A

Claims (9)

An optical system having a plurality of wide-angle lenses,
Each wide-angle lens has a lens peripheral region and a lens inner region extending from the optical axis to the lens peripheral region,
The subject image portions where the lens peripheral regions of each wide-angle lens form an image overlap each other, and the subject image portions where the lens inner regions form an image do not overlap each other,
The lens inner area has a first area from an optical axis toward the lens peripheral area, and a second area from the first area to the lens peripheral area,
In the first area, the imaging magnification of the subject image per unit angle of view is constant,
In the lens peripheral area, the imaging magnification of the subject image per unit angle of view decreases,
An optical system in which an imaging magnification of a subject image per unit angle of view continuously changes from the first area to the lens peripheral area in the second area. The optical system according to claim 1, wherein
The inner area of each wide-angle lens is an optical system that uses the equidistant projection method. The optical system according to claim 1 or 2,
An optical system having two wide-angle lenses having a total angle of view of 180 degrees or more. The optical system according to claim 3, wherein
An optical system in which the two wide-angle lenses have the same specifications, and the lens peripheral area is a half angle of view: an angle of view area larger than 90 degrees. The optical system according to claim 4, wherein
Two wide-angle lenses of the same specification are an optical system having a front group having a negative refractive power and a rear group having a positive refractive power. The optical system according to claim 5, wherein
Each of the two wide-angle lenses having the same specifications has a transparent body between the front group and the rear group, which has a reflecting surface that bends a light beam from the front group toward the rear group. An optical system that is placed close to or close to a surface. The optical system according to claim 6, wherein
An optical system in which a transparent body having a reflecting surface is a prism. An optical system according to any one of claims 1 to 7,
An image sensor that converts light that has passed through the optical system into an electric signal to generate a captured image. The imaging system according to claim 8,
An imaging system for connecting and displaying images captured by respective wide-angle lenses by using overlapping portions of subject images formed by the respective wide-angle lenses.

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