WO2017002335A1 - Imaging apparatus - Google Patents

Abstract

An imaging apparatus includes an imaging optical system, an imaging device, and a light-guiding member configured to guide light from the imaging optical system to the imaging device. The light-guiding member includes first and second light-guiding portions each including a plurality of optical waveguides. An inclination angle, with respect to an optical axis of the imaging optical system, of an optical waveguide farthest from the optical axis in the first light-guiding portion is different from an inclination angle, with respect to the optical axis, of an optical waveguide farthest from the optical axis in the second light-guiding portion. Each of the optical waveguides of the first light-guiding portion has a smaller diameter at an incident surface than at an emitting surface. In the first light-guiding portion, an optical waveguide nearest to the optical axis is shorter than the optical waveguide farthest from the optical axis.

Description

IMAGING APPARATUS

The present invention relates to an imaging apparatus.

There is a known imaging apparatus that includes an imaging optical system configured to form an image of an object, and an optical-fiber bundle (a light-guiding member) including a plurality of optical fibers (optical waveguides) that guide light from the imaging optical system.

An imaging apparatus disclosed by PTL 1 includes a light-guiding member including a plurality of optical waveguides. The optical waveguides each have an incident surface and an emitting surface that are of different sizes.

The imaging apparatus according to PTL 1 is designed with no consideration of distortion that occurs in the imaging optical system. The optical waveguides of this imaging apparatus all have the same size ratio between the incident surface and the emitting surface. Hence, the light-guiding member is incapable of correcting the distortion.

Japanese Patent Laid-Open No. 7-087371

In view of the above, the present invention provides an imaging apparatus that includes a light-guiding member configured to compensate for distortion occurring in an imaging optical system.

An imaging apparatus includes an imaging optical system configured to form an image of an object; an imaging device configured to take the image of the object; and a light-guiding member configured to guide light from the imaging optical system to the imaging device. The light-guiding member includes a first light-guiding portion and a second light-guiding portion, the first light-guiding portion including a plurality of optical waveguides that transmit the light from the imaging optical system, the second light-guiding portion including a plurality of optical waveguides that transmit the light from the first light-guiding portion. An inclination angle, with respect to an optical axis of the imaging optical system, of a center axis of an optical waveguide that is farthest from the optical axis in the first light-guiding portion is different from an inclination angle, with respect to the optical axis, of a center axis of an optical waveguide that is farthest from the optical axis in the second light-guiding portion. Each of the optical waveguides of the first light-guiding portion has a smaller diameter at an incident surface than at an emitting surface. In the first light-guiding portion, an optical waveguide that is nearest to the optical axis is shorter than the optical waveguide that is farthest from the optical axis.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1A is a schematic diagram of an imaging apparatus according to a general embodiment. Fig. 1B is an enlarged sectional view of an optical-fiber bundle included in the imaging apparatus. Fig. 2 is a schematic diagram illustrating the distance between centers of adjacent optical fibers. Fig. 3A is a schematic diagram of a light-guiding member included in an imaging apparatus according to a first embodiment. Fig. 3B is an enlargement of the light-guiding member included in the imaging apparatus according to the first embodiment. Fig. 4A is a graph of data on distortion occurring in the imaging apparatus according to the first embodiment. Fig. 4B is another graph of data on distortion occurring in the imaging apparatus according to the first embodiment. Fig. 4C is yet another graph of data on distortion occurring in the imaging apparatus according to the first embodiment. Fig. 4D is yet another graph of data on distortion occurring in the imaging apparatus according to the first embodiment. Fig. 5 is a schematic diagram of an imaging apparatus according to a fourth embodiment. Fig. 6A is a graph of data on distortion occurring in the imaging apparatus according to the fourth embodiment. Fig. 6B is another graph of data on distortion occurring in the imaging apparatus according to the fourth embodiment. Fig. 7A is a schematic diagram of an imaging apparatus according to a fifth embodiment. Fig. 7B illustrates a part of a light-receiving surface of an imaging device included in the imaging apparatus according to the fifth embodiment. Fig. 8A is a graph of data on distortion occurring in the imaging apparatus according to the fifth embodiment. Fig. 8B is another graph of data on distortion occurring in the imaging apparatus according to the fifth embodiment. Fig. 8C is yet another graph of data on distortion occurring in the imaging apparatus according to the fifth embodiment. Fig. 8D is yet another graph of data on distortion occurring in the imaging apparatus according to the fifth embodiment. Fig. 9 is a schematic diagram of a light-guiding portion included in an imaging apparatus according to a sixth embodiment. Fig. 10A is a graph of data on distortion occurring in the imaging apparatus according to the sixth embodiment. Fig. 10B is another graph of data on distortion occurring in the imaging apparatus according to the sixth embodiment. Fig. 10C is yet another graph of data on distortion occurring in the imaging apparatus according to the sixth embodiment. Fig. 10D is yet another graph of data on distortion occurring in the imaging apparatus according to the sixth embodiment. Fig. 11 is a schematic diagram of a light-guiding portion included in an imaging apparatus according to a seventh embodiment. Fig. 12 is a schematic diagram of an imaging apparatus according to an eighth embodiment. Fig. 13 is a schematic diagram of a light-guiding portion included in the imaging apparatus according to the eighth embodiment. Fig. 14 is a schematic diagram of a light-guiding portion included in an imaging apparatus according to a ninth embodiment. Fig. 15 is a schematic diagram of a light-guiding portion included in an imaging apparatus according to a tenth embodiment. Fig. 16 is a schematic diagram of a light-guiding portion included in an imaging apparatus according to an eleventh embodiment.

General Embodiment

An imaging apparatus according to a general embodiment will now be described with reference to Figs. 1A and 1B. According to the general embodiment, distortion occurring in an imaging optical system of the imaging apparatus is reduced. Furthermore, the level of distortion reduction realized by each image-transmitting member is freely settable. Furthermore, the distortion is reduced while the shape of the incident surface and/or the emitting surface of the image-transmitting member is determined flexibly.

Fig. 1A is a schematic diagram of an imaging apparatus 1. The imaging apparatus 1 includes an imaging optical system (focusing optical system) 2, a light-guiding member 30, and a complementary-metal-oxide-semiconductor (CMOS) sensor 4 (hereinafter referred to as "sensor 4"). The sensor 4 serves as an imaging device. The light-guiding member 30 includes a first optical-fiber bundle 31 serving as a first light-guiding portion (a first optical-waveguide unit), and a second optical-fiber bundle 32 serving as a second light-guiding portion (a second optical-waveguide unit). The imaging optical system 2, the first optical-fiber bundle 31, the second optical-fiber bundle 32, and the sensor 4 are arranged such that light from an object (an object image) that is focused by the imaging optical system 2 is transmitted to the sensor 4 through the optical- fiber bundles 31 and 32.

The object image received by the sensor 4 of the imaging apparatus 1 is magnified with an imaging magnification (photographing magnification) obtained by multiplying the focusing magnification of the imaging optical system 2 by the image-transmitting magnification of the light-guiding member 30. The photographing magnification of the imaging apparatus 1 varies with the height in the image (hereinafter referred to as "image height"). Here, the ratio of the photographing magnification at each of different heights in the image with respect to the photographing magnification at the center of the image is denoted as local magnification Mb. As given in Expression (1) below, the local magnification Mb is the result of differentiating distortion Dist by image height Y. The distortion is compensated for by setting the local magnification Mb to a desired value.

The first optical-fiber bundle 31 includes a plurality of optical fibers 31c (optical waveguides, see Fig. 2) that guide light from the imaging optical system 2 to the second optical-fiber bundle 32. Specifically, the optical fibers 31c receive image light BM through the imaging optical system 2 and guide the image light BM to the second optical-fiber bundle 32 while propagating the image light BM in the optical fibers 31c. The first optical-fiber bundle 31 is a bundle of tapered optical fibers 31c whose core diameters change from one end thereof to the other end thereof.

A tapered optical fiber refers to an optical fiber having a peripheral surface inclined with respect to a center axis VF thereof within an angle that is greater than 0.0 degrees and smaller than 90.0 degrees. Herein, the angle of inclination of the peripheral surface of the optical fiber with respect to the center axis VF of the optical fiber is referred to as taper angle. Accordingly, a straight optical fiber refers to an optical fiber having a taper angle of 0 degrees. The definition of the center axis VF will be given later.

Letting the core diameter of an optical fiber at the incident surface be Da, and the core diameter of the optical fiber at the emitting surface be Db, the ratio of the core diameter Db to the core diameter Da (i.e., Db/Da) is denoted as taper ratio Rt. The plurality of optical fibers 31c included in the first optical-fiber bundle 31 each desirably have a taper ratio Rt that is greater than 1.

The second optical-fiber bundle 32 includes a plurality of straight optical fibers 32c (optical waveguides) that guide light from the first optical-fiber bundle 31 to the sensor 4. Specifically, the optical fibers 32c receive the image light BM emitted from the first optical-fiber bundle 31 and guide the image light BM to pixels of the sensor 4 while propagating the image light BM in the optical fibers 32c. The image light BM is expected to be focused on a specific point by the imaging optical system 2, to pass through an opening of an aperture stop 2c, and to be incident on the optical fibers 31c.

An incident surface 31a of the first optical-fiber bundle 31 is flat. More specifically, the incident surface 31a has a flat shape that substantially conforms to the image surface of the imaging optical system 2. An emitting surface 31b of the first optical-fiber bundle 31 is curved and is convex when seen from the imaging optical system 2. The emitting surface 31b of the first optical-fiber bundle 31 forms a smooth optical surface by having undergone spherical grinding, as with the surface of a glass lens. By giving such a grinding process, scattering at the emitting surface 31b is suppressed.

An incident surface 32a of the second optical-fiber bundle 32 is convex when seen from the imaging optical system 2. That is, the incident surface 32a has substantially the same shape as the emitting surface 31b of the first optical-fiber bundle 31. The second optical-fiber bundle 32 is positioned such that the incident surface 32a thereof is closely in contact with the emitting surface 31b of the first optical-fiber bundle 31. Hence, the image is transmitted without blur from the emitting surface 31b of the first optical-fiber bundle 31 to the incident surface 32a of the second optical-fiber bundle 32.

An emitting surface 32b of the second optical-fiber bundle 32 is flat. The second optical-fiber bundle 32 is positioned such that the emitting surface 32b thereof is closely in contact with the incident surface of the sensor 4. The incident surface 32a of the second optical-fiber bundle 32 forms a smooth optical surface, as with the emitting surface 31b, by having undergone flat grinding. Thus, the closeness between the two surfaces 31b and 32a is improved.

Each of the optical fibers 31c of the first optical-fiber bundle 31 is connected to one or more optical fibers 32c of the second optical-fiber bundle 32. Specifically, an inclination angle, with respect to an optical axis AX of the imaging optical system 2, or the taper angle of each optical fiber 31c provided far from the optical axis AX is different from the inclination angle or the taper angle of each of any optical fibers 32c that are connected to that optical fiber 31c. In the general embodiment, optical fibers 31c provided on the periphery of the first optical-fiber bundle 31 are inclined with respect to the optical axis AX, whereas all the optical fibers 32c of the second optical-fiber bundle 32 are parallel to the optical axis AX. Note that the first optical-fiber bundle 31 of the light-guiding member 30 may include optical fibers 31c that are connected to optical fibers 32c inclined at the same angle as those optical fibers 31c.

The inclination angle of each of the optical fibers 31c is set such that the image light BM incident on that optical fiber 31c undergoes total internal reflection in that optical fiber 31c. Hence, the reduction in the transmittance of the optical fibers 31c on the periphery of the first optical-fiber bundle 31 is suppressed.

The optical axis AX of the imaging optical system 2 is a straight line that passes through the center of the exit pupil of the imaging optical system (lens) 2 and is perpendicular to a light-receiving surface of the sensor 4. The optical axis AX also passes through the center of the incident surface 31a of the first optical-fiber bundle 31. That is, a straight line that connects the center of the exit pupil of the imaging optical system 2 and the center of the incident surface 31a of the first optical-fiber bundle 31 to each other coincides with the optical axis AX.

Fig. 1B is an enlarged sectional view of the second optical-fiber bundle 32 in a plane parallel to the light-receiving surface of the sensor 4. In the sectional view, core portions 32co are arranged in a triangular-lattice pattern, with cladding 32cl filling the space among the core portions 32co. That is, each of the optical fibers 32c includes the core portion 32co and the cladding 32cl provided around the core portion 32co. While Fig. 1B illustrates the core portions 32co arranged in a triangular-lattice pattern, the pattern of the core portions 32co is not limited to the triangular-lattice pattern.

For example, the core portions 32co may be arranged in any lattice pattern such as a tetragonal-lattice pattern or an orthorhombic-lattice pattern. Moreover, as long as the cladding 32cl is present among the core portions 32co, the core portions 32co may be arranged in a random pattern. Alternatively, the pattern of the core portions 32co may be a combination of a lattice pattern and a random pattern. Likewise, each of the optical fibers 31c of the first optical-fiber bundle 31 includes a core portion 31co and cladding 31cl provided around the core portion 31co. The cladding 31cl may be provided with a light-absorbing layer. Alternatively, the optical fiber 31c may be an optical waveguide that does not include cladding and may guide light while confining light therein with the refractive index thereof varying in a plane perpendicular to the direction of light guiding by the optical fiber 31c.

The optical fibers 32c of the second optical-fiber bundle 32 may be provided for the respective pixels of the sensor 4 or otherwise. For example, a portion of the image light BM that is propagated through the optical fibers 32c may be received by a certain pixel of the sensor 4, while another portion of the image light BM may be received by other pixels. Alternatively, a certain pixel of the sensor 4 may receive a portion of the image light BM that is propagated through a plurality of optical fibers 32c.

According to the general embodiment, the emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 are closely in contact with each other, and the joint surface (hereinafter also denoted by 31b) between the optical- fiber bundles 31 and 32 is convex when seen from the imaging optical system 2. That is, the optical fibers 31c and 32c have different lengths in accordance with the distance from the optical axis AX. Specifically, the length of the optical fibers 31c of the first optical-fiber bundle 31 becomes longer as the distance from the optical axis AX increases, whereas the length of the optical fibers 32c of the second optical-fiber bundle 32 becomes shorter as the distance from the optical axis AX increases. That is, in the first optical-fiber bundle 31, an optical fiber 31c that is nearest to the optical axis AX is shorter than an optical fiber 31c that is farthest from the optical axis AX. In contrast, in the second optical-fiber bundle 32, an optical fiber 32c that is nearest to the optical axis AX is longer than an optical fiber 32c that is farthest from the optical axis AX.

Now, terms used in this specification will be defined with reference to Fig. 2. Fig. 2 is a schematic diagram of three optical fibers 31c that are arranged side by side in a meridional direction of the first optical-fiber bundle 31. The following definitions are not limited to factors regarding the first optical-fiber bundle 31 and are also applicable to factors regarding the second optical-fiber bundle 32.

The term "meridional direction" refers to a radial direction around the optical axis AX and is indicated by an arrow Y in Fig. 2. An arrow Z illustrated in Fig. 2 indicates a direction parallel to the optical axis AX. As illustrated in Fig. 2, the distance between the centers of respective incident surfaces 31ca of a pair of optical fibers 31c that are adjacent to each other in the Y direction is denoted by Pa, and the meridional component of the distance Pa is denoted by Pa'. Furthermore, the distance between the centers of respective emitting surfaces 31cb of the pair of optical fibers 31c that are adjacent to each other in the Y direction is denoted by Pb, and the meridional component of the distance Pb is denoted by Pb'. In the general embodiment, since the incident surface 31a is a flat surface that is perpendicular to the optical axis AX, the distance Pa and the meridional component Pa' are equal to each other.

Furthermore, the ratio of the distance Pb between the centers of the emitting surfaces 31cb of the pair of optical fibers 31c that are adjacent to each other in the Y direction with respect to the distance Pa between the centers of the incident surfaces 31ca of the pair of optical fibers 31c (i.e., the ratio Pb/Pa) is denoted by Rp. Likewise, the value represented by a ratio Pb'/Pa' is denoted by Rp'. Furthermore, in the following description, the Y-direction distance between the centers of each pair of optical fibers 31c that are adjacent to each other in the Y direction in a section defined by points at the same position in the direction of the optical axis AX is referred to as fiber pitch (pitch of arrangement). Furthermore, the meridional component (Pa' or Pb') of the distance between the centers of the pair of adjacent optical fibers 31c of the first optical-fiber bundle 31 is hereinafter referred to as the first value, and the value (Rp') representing the ratio of the first value at the emitting surface 31b with respect to the first value at the incident surface 31a is hereinafter referred to as the second value.

Referring to Figs. 3A and 3B, an inclination angle α of each of the optical fibers 31c and 32c with respect to the optical axis AX of the imaging optical system 2 will now be described. Note that the following description, which concerns the optical fibers 31c, also applies to the optical fibers 32c. Referring to Fig. 3A, the inclination angle α is an angle formed between the center axis VF of each optical fiber 31c and the optical axis AX and is greater than 0.0 degrees and smaller than 90.0 degrees. The inclination angle α formed between the center axis VF of the optical fiber 31c that is farthest from the optical axis AX and the optical axis AX is defined as maximum inclination angle αmx.

According to the general embodiment, the size of the core portion 31co of each optical fiber 31c varies with the position. The optical fiber 31c (or 32c) may have a linear shape or a curved shape. The optical fibers 31c according to the general embodiment include those that are parallel to the optical axis AX and those that are not parallel to the optical axis AX. Therefore, referring to Fig. 3B, the center axis VF of each optical fiber 31c is defined as a straight line connecting a sectional center (centroid) A of the core portion 31co of the optical fiber 31c at the incident surface 31ca and a sectional center (centroid) B of the core portion 31co of the optical fiber 31c at a section SB parallel to the incident surface 31ca. The section SB is defined at a distance, corresponding to the core diameter Da and in the direction of the optical axis AX, from the sectional center A.

The center axes VF of the respective optical fibers 31c of the first optical-fiber bundle 31 all meet the optical axis AX at a point Pt. The fiber pitch of the first optical-fiber bundle 31 is constant in a section defined by points that are at the same position in the direction of the optical axis AX, and becomes smaller as the distance from the point Pt becomes shorter. The joint surface 31b is designed to be convex when seen from the imaging optical system 2, whereby optical fibers 31c that are provided farther from the optical axis AX are longer. Hence, the longer the distance from the optical axis AX, the greater the second value Rp'. Accordingly, at the emitting surface 31b of the first optical-fiber bundle 31, as the distance from the optical axis AX increases, the effect of magnifying the image becomes stronger. Such an increase in the effect of magnifying the image reduces negative distortion occurring in the imaging optical system 2 and negative distortion occurring when the image is transmitted to the flat light-receiving surface of the sensor 4 through a curved image surface.

The optical fibers 32c, which are of a straight type, become shorter as the distance from the optical axis AX increases. Regarding each of the straight-type optical fibers 32c, since the core diameter Da at an incident surface 32ca and the core diameter Db at an emitting surface 32cb are equal, the taper ratio Rt is 1. That is, the second value Rp' is 1. Accordingly, the second optical-fiber bundle 32 does not affect the magnification/reduction of the image. The image magnified by each of the optical fibers 31c is emitted from the emitting surfaces 32cb of the optical fibers 32c without being magnified/reduced. Consequently, an image with less negative distortion is emitted from the second optical-fiber bundle 32.

According to the general embodiment, the joint surface 31b between the first optical-fiber bundle 31 and the second optical-fiber bundle 32, i.e., the emitting surface 31b of the first optical-fiber bundle 31, contains a curved part. In such a configuration, the optical fibers included in at least one of the first optical-fiber bundle 31 and the second optical-fiber bundle 32 that is responsible for the magnification/reduction of the image have different lengths at different distances from the optical axis AX. That is, the second value Rp' of the first optical-fiber bundle 31 varies with the distance from the optical axis AX. Hence, the local magnification with which the image is magnified/reduced varies in accordance with the distance from the optical axis AX. Thus, an imaging apparatus that causes less distortion is provided. Note that an area of the emitting surface 31b of the first optical-fiber bundle 31 where distortion does not need to be reduced may be flat or may be curved in another way, not in the way described above.

According to such a principle of the general embodiment, the distortion occurring in the imaging optical system 2 is compensated for. Furthermore, distortion that occurs with a light-guiding portion including optical waveguides whose inclination angles are smaller than the angle of view is also compensated for in good manner.

Distortion can be compensated for, i.e., reduced, by means of image processing. To reduce distortion by image processing, however, it takes time for the calculation of distortion reduction. Accordingly, it may take time for displaying an image even if the user wants to check the image immediately after the shooting. Particularly, in a case of a moving image, which is typically taken at a frame rate of 30 fps or 60 fps, the time period taken for image processing intended for distortion reduction affects the performance of the apparatus to a higher extent. Moreover, in recent years, the number of pixels of an image has been increasing. The larger the number of pixels, the longer the time period taken for the calculation. In such a situation, it takes more time for image processing.

In contrast, according to the general embodiment, the sensor 4 receives an image having less distortion with the aid of the optical- fiber bundles 31 and 32 serving as light-guiding portions. Hence, the time period taken for distortion reduction is shorter than in a case where distortion is reduced by image processing. Accordingly, the image thus taken can be displayed in real time even if the image is a moving image or an image composed of many pixels. Thus, according to the general embodiment, resolution deterioration is less than in the case of distortion reduction by image processing, and a high-quality image with a high resolution is provided.

First Embodiment

An imaging apparatus according to a first embodiment is the same as the imaging apparatus 1 illustrated in Fig. 1. The imaging optical system 2 has a maximum angle of view of ±60 degrees and focuses light from an object (not illustrated) on the image surface thereof. Table 1 below summarizes specifications of the optical- fiber bundles 31 and 32 and the sensor (imaging device) 4 according to the first embodiment.

Fig. 3A is a schematic diagram of the first optical-fiber bundle 31 and the second optical-fiber bundle 32 according to the first embodiment. The first optical-fiber bundle 31 includes a plurality of tapered optical fibers 31c and has a spherical emitting surface 31b that is convex when seen from the object side.

Referring to Figs. 4A to 4D, a method of compensating for distortion occurring in the imaging optical system 2 by using the first optical-fiber bundle 31 will now be described. Fig. 4A is a graph illustrating the fiber pitch of the optical fibers 31c at the emitting surface 31b of the first optical-fiber bundle 31 according to the first embodiment. The fiber pitch of the optical fibers 32c of the second optical-fiber bundle 32 is 6.0 μm both at the incident surface 32a and at the emitting surface 32b. The joint surface between the optical- fiber bundles 31 and 32 has a curvature radius of +36.6 mm.

In the graph illustrated in Fig. 4A, the horizontal axis represents the image-height ratio. The image-height ratio is defined as follows. When the image height on the center axis of the first optical-fiber bundle 31 at the emitting surface 31b (coinciding with the optical axis AX of the imaging optical system 2) is referred to as "center image height" and the image height at a position farthest from the optical axis AX is referred to as "marginal image height," the center image height is defined as "0" and the marginal image height is defined as "1." Note that the center image height and the marginal image height each originally refer to the image height at the sensor 4 but herein each refer to the position at the incident surface 31a or the emitting surface 31b of the first optical-fiber bundle 31 where the principal ray of the light reaching the position of the sensor 4 at the center or marginal image height passes.

The vertical axis of the graph illustrated in Fig. 4A represents the fiber pitch. The fiber pitch of the optical fibers 31c of the first optical-fiber bundle 31 at the emitting surface 31b increases from the position at the center image height (the center of the image surface) toward the position at the marginal image height (the periphery of the image surface). Specifically, the fiber pitch is 3.9 μm at the center image height and 8.2 μm at the marginal image height. The fiber pitch of the optical fibers 31c of the first optical-fiber bundle 31 at the incident surface 31a is constant at 3.0 μm. Therefore, the second value Rp' is greater than 1 for all of the optical fibers 31c. Hence, an image focused by the imaging optical system 2 is magnified when being transmitted through the first optical-fiber bundle 31.

Fig. 4B is a graph illustrating the effect of local-magnification reduction produced by the first optical-fiber bundle 31 according to the first embodiment. In the graph illustrated in Fig. 4B, the horizontal axis represents the image-height ratio, and the vertical axis represents the effect of local-magnification reduction (MHi) produced by the first optical-fiber bundle 31. The effect of local-magnification reduction is defined as the ratio of a magnification Mi at each of different image heights with respect to a magnification Mo at the center image height (MHi = Mi/Mo) that is established when the first optical-fiber bundle 31 transmits light from the incident surface 31a to the emitting surface 31b. In the first embodiment, the magnification Mo is ×1.3, and the magnification Mi at the marginal image height is ×2.7.

As graphed in Fig. 4B, the effect of local-magnification reduction is greater for optical fibers 31c that are farther from the optical axis AX. Specifically, the effect of local-magnification reduction is ×1.0 at the center image height and ×2.1 at the marginal image height. That is, an image transmitted through an optical fiber 31c that is farther from the optical axis AX is magnified at a higher rate. Therefore, negative distortion that lowers the rate of magnification of the image in a direction from the position at the center image height toward the position at the marginal image height is reduced.

Fig. 4C is a graph illustrating the effect of distortion reduction (distortion compensation). The effect of distortion reduction is calculated from the above effect of local-magnification reduction MHi. Letting the number of pieces of data obtained within a range from the position at the center image height to a position at an arbitrary image height be denoted by i, an actual image height Yi at the arbitrary image height is calculated as follows:

Furthermore, letting the ideal image height be denoted by Yo and the actual image height be denoted by Yi, distortion Dist is defined as follows, in general:

Then, the ideal image height Yo is expressed as follows:
Yo = i ... (4)

According to Expressions (2) to (4), an effect of distortion reduction DH at each of different image heights is calculated as follows:

As graphed in Fig. 4C, the effect of distortion reduction produced by the first optical-fiber bundle 31 according to the first embodiment gradually increases from the position at the center image height to the position at the marginal image height. Specifically, the effect of distortion reduction is +3.8% at a 50% image height (an image-height ratio of 0.5), +11.8% at an 80% image height (an image-height ratio of 0.8), and +23.7% at a 100% image height (an image-height ratio of 1.0). Thus, a satisfactory effect of distortion reduction is produced.

Fig. 4D is a graph illustrating the distortion, represented by a dotted line, occurring in the imaging optical system 2. The graph shows that negative distortion increases from the position at the center image height to the position at the marginal image height. Specifically, the distortion increases significantly as follows: -3.4% at a 50% image height, -12.4% at an 80% image height, and -21.4% at the marginal image height.

The line with circles illustrated in the graph in Fig. 4D represents the distortion of an image that has been emitted from the first optical-fiber bundle 31. The above values representing the effect of distortion reduction at different image heights of the first optical-fiber bundle 31 are almost the same as the values representing the distortion occurring in the imaging optical system 2 but are of the opposite sign. With such an effect of distortion reduction, the first optical-fiber bundle 31 compensates for the distortion occurring in the imaging optical system 2. Consequently, as graphed in Fig. 4D, the distortion occurring at the incident surface 31a of the first optical-fiber bundle 31 is reduced when the image is emitted from the first optical-fiber bundle 31.

To compensate for the distortion occurring in the imaging optical system 2, the distortion occurring at the image surface of the imaging optical system 2 and the effect of distortion reduction produced by the first optical-fiber bundle 31 need to offset each other. To establish such a situation, it is important that a local magnification ML of distortion occurring at an arbitrary position on the image surface of the imaging optical system 2 and an effect of reduction in local magnification MHi produced by the first optical-fiber bundle 31 offset each other. In such an "offset" relationship, one of the two values is the reciprocal of the other; that is, one of the two values multiplied by the other comes to 1, as in the following expression:

Herein, a state where the distortion has been reduced to 20% or less is regarded as good distortion reduction. To reduce the distortion to 20% or less, a local magnification of about ×2 at maximum is allowable. Hence, the distortion is substantially compensated for in good manner as long as the local magnification, denoted by MLi, satisfies the following conditional expression:

The distortion is also compensated for in good manner when the following conditional expression is satisfied:

where P1i denotes the first value of a pair of optical fibers 31c at the incident surface 31a, and P2i denotes the first value of the pair of optical fibers 31c at the emitting surface 32b.

The local magnification MLi of the distortion occurring at the image surface of the imaging optical system 2 is determined by a vertical component YV at the image height in the image surface and a section pitch ΔYVo representing an ideal f×tanω characteristic. Specifically, the local magnification ML is calculated as follows:

The section pitch ΔYVo representing the ideal f×tanω characteristic is expressed as follows:

where f denotes the focal length of the imaging optical system 2, ωmax denotes the maximum angle of view of the imaging optical system 2, and n denotes the number of sections included in a range from a position on the optical axis AX to a position at the maximum image height.

Thus, the shape of the curved joint surface 31b between the first optical-fiber bundle 31 and the second optical-fiber bundle 32 can be determined from the local magnification ML of the distortion occurring at the image surface. By joining the straight-type second optical-fiber bundle 32 to the first optical-fiber bundle 31, the shape of the emitting surface 31b of the first optical-fiber bundle 31 can be set as desired. That is, the emitting surface (joint surface) 31b of the first optical-fiber bundle 31 can be shaped suitably for distortion reduction. The shapes of the incident surface 31a of the first optical-fiber bundle 31 and the emitting surface 32b of the second optical-fiber bundle 32 can be determined in accordance with the shape of the image surface of the imaging optical system 2 or the shape of the image-sensing surface of the sensor 4 such that the image does not blur at the incident surface 31a and the emitting surface 32b. Since the shape of the joint surface 31b between the first optical-fiber bundle 31 and the second optical-fiber bundle 32 can be set to a shape suitable for distortion reduction as described above, the effect of distortion reduction is enhanced.

Second Embodiment

An imaging apparatus according to a second embodiment includes a second optical-fiber bundle 32 including straight optical fibers, as in the first embodiment. However, the fiber pitch of the second optical-fiber bundle 32 is different from that of the first embodiment. Specifically, the fiber pitch of the second optical-fiber bundle 32 according to the second embodiment is smaller than the fiber pitch of the second optical-fiber bundle 32 according to the first embodiment.

According to the second embodiment, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is about 3.9 to 8.2 μm, and the fiber pitch of the second optical-fiber bundle 32 is about 3.0 μm. That is, the fiber pitch of the second optical-fiber bundle 32 is smaller than the fiber pitch of any pair of optical fibers 31c of the first optical-fiber bundle 31 at the emitting surface 31b. With such a difference in the fiber pitch between the first optical-fiber bundle 31 and the second optical-fiber bundle 32, moire becomes less visible, and a high-quality image is provided.

Furthermore, the centers of the emitting surfaces 31cb of the optical fibers 31c included in the first optical-fiber bundle 31 do not necessarily coincide with the centers of the incident surfaces 32ca of the optical fibers 32c included in the second optical-fiber bundle 32. Therefore, when the image is transmitted through the joint surface 31b, the resolution (lateral resolution) or the modulation transfer function (MTF, i.e., contrast) of the image may be deteriorated. However, since the second optical-fiber bundle 32 according to the second embodiment has a small fiber pitch at the incident surface 32a, such deterioration in the resolution or MTF of the image is suppressed. Therefore, an image with higher quality and higher resolution than in the first embodiment is provided.

The sensor (imaging device) 4 according to the second embodiment is a CMOS sensor that is capable of sensing a color image. The imaging device 4 includes a pixel structure in which each of pixels is provided with a green, red, or blue color filter. Since each of the optical fibers 31c and 32c of the light-guiding member 30 propagates all of the light beams of red, green, and blue wavelength bands, a single optical fiber 31c or 32c can transmit an image to a plurality of pixels that receive light beams of different colors.

In the case of a color image, the length of a straight line connecting the centers of pixels for green is the shortest and is two times the pixel pitch. In the second embodiment, the pixel pitch is 7.5 μm, and the pixels for green are arranged at a pitch of 15 μm.

According to the second embodiment, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is 3.9 to 8.2 μm, whereas the fiber pitch of the second optical-fiber bundle 32 is 3.0 μm. The fiber pitches of the first and second optical- fiber bundles 31 and 32 are both shorter than the pitch of the pixels for green. Therefore, the light-guiding member 30 has a satisfactory number of samples to be sensed by the imaging device 4, and the distortion is reduced with no deterioration in the resolution (lateral resolution).

Third Embodiment

An imaging apparatus according to a third embodiment differs from the imaging apparatus 1 according to the first embodiment in that a single optical fiber serves as the optical fiber 31c of the first optical-fiber bundle 31 and as the optical fiber 32c of the second optical-fiber bundle 32. Specifically, the light-guiding member 30 according to the third embodiment is a bundle of optical fibers each extending continuously from the incident surface 31a to the emitting surface 32b.

Each of the optical fibers has a tapered shape that broadens from the side of the incident surface 31a, but the shape of the optical fiber changes into a straight shape at a halfway point between the incident surface 31a and the emitting surface 32b. With such a change in the shape, the inclination angle α changes. In the third embodiment, the tapered portions of the optical fibers are collectively denoted as a first light-guiding portion 31, the straight portions of the optical fibers are collectively denoted as a second light-guiding portion 32, and a surface obtained by connecting points of change in the inclination angles α of the respective optical fibers is denoted as an emitting surface (joint surface) 31b of the first light-guiding portion 31. That is, the portion on the object side with respect to and inclusive of the surface obtained by connecting the points of change in the inclination angles α of the optical fibers is denoted as the first light-guiding portion 31, and the portion on the sensor side (imaging-device side) with respect to the same surface is denoted as the second light-guiding portion 32. The joint surface 31b contains a curved part, as in the first embodiment. Therefore, in the imaging apparatus according to the third embodiment, negative distortion occurring in the imaging optical system 2 is reduced.

If such a light-guiding member 30 as a bundle of optical fibers each having an inclination angle and a taper angle that change at a halfway point is employed as in the third embodiment, the effect of distortion reduction can be calculated from the value Rp, which is the ratio of the fiber pitch at the emitting surface 32b with respect to the fiber pitch at the incident surface 31a. Furthermore, in the light-guiding member 30 according to the third embodiment, since light is propagated through optical fibers each extending continuously from the incident surface 31a to the emitting surface 32b, the loss of light is reduced.

Fourth Embodiment

An imaging apparatus 51 according to a fourth embodiment will now be described with reference to Fig. 5. The imaging apparatus 51 differs from the imaging apparatus 1 according to the first embodiment in the shape of the emitting surface 32b of the second optical-fiber bundle 32, in including a relay optical system 6 that focuses the light emitted from the emitting surface 32b of the second optical-fiber bundle 32 on the sensor 4, and in the effect of distortion reduction produced by the first optical-fiber bundle 31. Table 2 summarizes specifications of the optical- fiber bundles 31 and 32 and the sensor (imaging device) 4 according to the fourth embodiment.

The emitting surface 32b of the second optical-fiber bundle 32 has a spherical shape that is convex with a curvature radius of 56.6 mm when seen from the imaging optical system 2. The spherical shape is determined in conformity with the field curvature of the relay optical system 6. Therefore, the relay optical system 6 focuses the image transmitted from the second optical-fiber bundle 32 on the sensor 4.

Fig. 6A is a graph illustrating different kinds of distortion. Distortion occurring in the imaging optical system 2 is represented by a solid line. Distortion occurring in the relay optical system 6 is represented by a line with crosses. The distortion occurring in the imaging optical system 2 is the same as that observed in the first embodiment. The distortion occurring in the relay optical system 6 increases from the position at the center image height to the position at the marginal image height: specifically, -0.5% at a 50% image height, -4.1% at an 80% image height, and -11.9% at a 100% image height.

In the graph illustrated in Fig. 6A, the effect of distortion reduction produced by the first optical-fiber bundle 31 is represented by a dotted line. The first optical-fiber bundle 31 according to the fourth embodiment produces an effect of compensating the distortion toward the positive side, i.e., an effect of increasing positive distortion. That is, the amount of distortion reduction by the first optical-fiber bundle 31 increases from the position at the center image height toward the position at the marginal image height. Specifically, the amount of distortion reduction is +4.6% at a 50% image height, +15.3% at an 80% image height, and +39.6% at a 100% image height. According to the fourth embodiment, the effect of distortion reduction produced by the first optical-fiber bundle 31 offsets the sum of the distortion occurring in the imaging optical system 2 and the distortion occurring in the relay optical system 6. Thus, the distortion at the image-sensing surface of the sensor 4 is reduced as graphed by a line with circles in Fig. 6A.

In the graph illustrated in Fig. 6A, the distortion remaining in the image formed by the relay optical system 6 is +0.7% at a 50% image height, -1.3% at an 80% image height, and +6.3% at a 100% image height. That is, the distortion is reduced in good manner to a low level of 10% or below at all image heights.

Fig. 6B is a graph illustrating the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b according to the fourth embodiment. The fiber pitch of the optical fibers 31c of the first optical-fiber bundle 31 varies from 3.9 μm to 15.0 μm. Hence, the joint surface 31b between the first optical-fiber bundle 31, which is inclined with respect to the optical axis AX, and the second optical-fiber bundle 32, which has an inclination angle different from that of the first optical-fiber bundle 31, contains a curved part. Such a configuration reduces the distortion occurring in the imaging optical system 2 and the distortion occurring in the relay optical system 6. Employing the optical- fiber bundles 31 and 32 according to the fourth embodiment realizes a good reduction in the distortion occurring in a plurality of focusing optical systems.

The shape of the joint surface 31b is settable regardless of the shape of the incident surface 31a of the first optical-fiber bundle 31 that is determined by the shape of the image surface of the imaging optical system 2 and regardless of the shape of the emitting surface 32b of the second optical-fiber bundle 32 that is determined by the shape of the image-sensing surface of the sensor 4. Therefore, the effect of distortion reduction and the effect of field-curvature reduction are produced simultaneously. That is, even if distortion and field curvature occur simultaneously, such distortion and field curvature can be both reduced, providing an in-focus high-quality image.

In the related art, to simultaneously correct different kinds of aberration, such as distortion and field curvature, by adjusting the configuration of the focusing optical system, many lenses need to be used in combinations. In contrast, in the fourth embodiment employing the optical- fiber bundles 31 and 32, the focusing optical system can have a simple configuration, providing some other benefits such as a reduction in the number of lenses, a reduction in the diameter of each of the lenses, a reduction in the total length of the apparatus, and an increase in the resolution.

Fifth Embodiment

Referring to Fig. 7A, an imaging apparatus 71 according to a fifth embodiment will now be described. Fig. 7A is a schematic diagram of the imaging apparatus 71. The fifth embodiment differs from the first embodiment in employing a ball lens as the imaging optical system 2 and in that the incident surface 31a of the first optical-fiber bundle 31 has a spherical shape that is concave when seen from the ball lens 2.

The incident surface 31a of the first optical-fiber bundle 31 has a concave shape that substantially conforms to the image surface of the ball lens 2. The incident surface 31a of the first optical-fiber bundle 31 forms a smooth optical surface by being subjected to spherical grinding that is in general performed on glass lenses. In the fifth embodiment, the image light BM is expected to pass through an opening of an aperture stop 2c and to be incident on the optical fibers 31c. The image light BM contains a principal ray PR that passes through the center of the opening of the aperture stop 2c, and an upper marginal ray NR and a lower marginal ray MR that are defined by the upper and lower edges, respectively, of the opening of the aperture stop 2c.

The first optical-fiber bundle 31 is provided such that the center axes VF of the respective optical fibers 31c all meet the optical axis AX at a point Pt. The optical axis AX and the center axes VF of the optical fibers 31c that are at different distances from the optical axis AX form different inclination angles α.

In the fifth embodiment also, the joint surface 31b between the first optical-fiber bundle 31 and the second optical-fiber bundle 32 forms a curved surface. Regarding the first optical-fiber bundle 31, which is of a tapered type, the core diameters of the optical fibers 31c are all the same at a specific point in the direction of the optical axis AX, as described above. Therefore, distortion is reduced by setting the emitting surfaces 31cb of the respective optical fibers 31c at different points in the direction of the optical axis AX so that the joint surface 31b forms a curved surface.

According to the fifth embodiment, since the incident surface 31a of the first optical-fiber bundle 31 is concave when seen from the ball lens 2, the positions of the incident surfaces 31ca of the respective optical fibers 31c in the direction of the optical axis AX vary with the distance from the optical axis AX. More specifically, the distance from the point Pt to the incident surfaces 31ca is the longest for the optical fiber 31c that is on the optical axis AX and becomes shorter as the distance from the optical axis AX to the optical fibers 31c increases.

Furthermore, the length of the core portion 31co of each of the optical fibers 31c in the vertical direction (Y direction) that is perpendicular to the optical axis AX (i.e., the fiber pitch) affects distortion reduction. If the incident surface 31a is set to a spherical shape that is concave when seen from the ball lens 2, the line normal to each incident surface 31ca is inclined with respect to the optical axis AX. Under such a situation, a fiber pitch Pa' of the optical fibers 31c is expressed as follows:
Pa' = Pa×cosβ ... (11)
where β denotes the angle formed between the line normal to the incident surface 31ca and the optical axis AX, and Pa denotes the distance between the centers of adjacent ones of the optical fibers 31c.

As described above, the fiber pitch Pa' of the optical fibers 31c varies with the inclinations of the lines normal to the incident surfaces 31ca. Specifically, the fiber pitch Pa' becomes smaller from the point on the optical axis AX toward the periphery of the first optical-fiber bundle 31.

Since the incident surface 31a has a spherical shape that is concave when seen from the ball lens 2, the fiber pitch of the first optical-fiber bundle 31 at the incident surface 31a becomes smaller from the point on the optical axis AX toward the periphery of the first optical-fiber bundle 31, as graphed by a dotted line in Fig. 8A. Therefore, the second value Rp' of each of the optical fibers 31c becomes much larger, contributing to further distortion reduction.

Specifications of the imaging apparatus 71 will now be described. The ball lens 2 has a maximum angle of view of ±60 degrees and focuses light from an object (not illustrated) on the image surface thereof. Table 3 summarizes the specifications of the optical- fiber bundles 31 and 32 and the sensor (imaging device) 4 according to the fifth embodiment.

The inclination angle α of one of the optical fibers 31c of the first optical-fiber bundle 31 that is farthest from the optical axis AX is 34.7 degrees. The incident surface 31a of the first optical-fiber bundle 31 has a spherical shape that conforms to the image surface of the ball lens 2, with a curvature radius of -10.7 mm and being concave when seen from the object side. The emitting surface (joint surface) 31b of the first optical-fiber bundle 31 has a spherical shape with a curvature radius of +30.0 mm and being convex when seen from the object side.

Referring to Figs. 8A to 8D, a method of compensating for the distortion occurring in the ball lens (imaging optical system) 2 by using the first optical-fiber bundle 31 will now be described. Fig. 8A is a graph illustrating the fiber pitch of the first optical-fiber bundle 31 according to the fifth embodiment at the incident surface 31a, represented by a dotted line, and at the emitting surface 31b, represented by a solid line, with the horizontal axis representing the image-height ratio and the vertical axis representing the fiber pitch. The fiber pitches of the second optical-fiber bundle 32 at the incident surface 32a and at the emitting surface 32b are both 6.0 μm. The joint surface between the optical- fiber bundles 31 and 32 has a curvature radius of +36.6 mm.

As graphed by the dotted line in Fig. 8A, the fiber pitch of the first optical-fiber bundle 31 at the incident surface 31a becomes smaller as the distance from the optical axis AX increases: specifically, 4.22 μm at the point on the optical axis AX, and 2.47 μm at the point farthest from the optical axis AX.

On the other hand, as graphed by the solid line in Fig. 8A, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b becomes larger as the distance from the optical axis AX increases: specifically, 4.89 μm at the point on the optical axis AX, and 6.94 μm at the point farthest from the optical axis AX.

Fig. 8B is a graph illustrating the effect of local-magnification reduction produced by the first optical-fiber bundle 31 according to the fifth embodiment, with the horizontal axis representing the image-height ratio and the vertical axis representing the effect of local-magnification reduction produced by the first optical-fiber bundle 31.

At the center image height, the fiber pitch at the incident surface 31a is 4.22 μm, the fiber pitch at the emitting surface 31b is 4.89 μm, and the magnification Mo is ×1.16. On the other hand, at the marginal image height, the fiber pitch at the incident surface 31a is 2.47 μm, the fiber pitch at the emitting surface 31b is 6.94 μm, and the magnification Mo is ×2.43.

As graphed in Fig. 8B, the effect of local-magnification reduction is higher for optical fibers 31c that are farther from the optical axis AX. Therefore, the rate of magnification of the image transmitted through those optical fibers 31c that are farther from the optical axis AX is higher. Consequently, negative distortion that reduces a part of the image that is farther from the optical axis AX to higher extent is reduced.

Fig. 8C is a graph illustrating the effect of distortion reduction, with the horizontal axis representing the image-height ratio and the vertical axis representing the effect of distortion reduction. The effect of distortion reduction is calculated from the effect of local-magnification reduction graphed in Fig. 8B and on the basis of Expression (5) given above. The amount of distortion reduction increases from the position at the center image height to the position at the marginal image height, reaching +48.3%.

Fig. 8D is a graph illustrating different kinds of distortion, with the horizontal axis representing the image-height ratio and the vertical axis representing the distortion. In Fig. 8D, distortion occurring at the image surface of the imaging optical system 2 is represented by a solid line, and distortion occurring at the sensor (imaging device) 4 is represented by a line with circles. The distortion at the position farthest from the optical axis AX is -51.4% at the image surface of the imaging optical system 2 but is reduced to -3.0% at the sensor 4. That is, the distortion is reduced by the first optical-fiber bundle 31 and the second optical-fiber bundle 32. The maximum distortion at the sensor 4 is -8.1%, which shows that the distortion is reduced over the entire range of the image height.

As described above, in the imaging apparatus 71, the incident surface 31a of the first optical-fiber bundle 31 is set to a curved surface that is concave when seen from the imaging optical system 2 while the emitting surface 31b of the first optical-fiber bundle 31 is set to a curved surface that is convex when seen from the imaging optical system 2, whereby the effect of distortion reduction is enhanced. Thus, even if an imaging optical system causing a significant negative distortion (barrel-shaped distortion) of over -50% is used, such a distortion is reduced to an allowable level. Consequently, an image in which the distortion is compensated for in good manner is obtained.

In general, depending on the level of distortion occurring in the imaging optical system 2, the photographing magnification may become smaller as the distance from the optical axis AX increases. Consequently, some parts of the image may be jammed. In the fifth embodiment, however, since the fiber pitch of the first optical-fiber bundle 31 at the incident surface 31a becomes smaller as the distance from the optical axis AX increases as graphed in Fig. 8A, the sampling pitch of the sensor 4 becomes smaller as the distance from the optical axis AX increases. The imaging optical system 2 is capable of focusing an image while realizing a satisfactory level of contrast (MTF) even at a frequency corresponding to the smallest sampling pitch. Therefore, if the sampling pitch is made smaller, a higher resolution (lateral resolution) is realized, which suppresses the reduction in the number of samples to be sensed by the sensor 4 occurring in distortion reduction.

In the fifth embodiment, the pixel pitch of the sensor 4 is set to 6.0 μm. The imaging apparatus 71 according to the fifth embodiment acquires a color image, and the pixels of the sensor 4 of the imaging apparatus 71 are provided with color filters. Fig. 7B illustrates a part of the light-receiving surface of the sensor 4. The sensor 4 includes a plurality of pixels 72, which are each provided with any of those color filters. Reference numerals 1 to 3 given to the respective pixels 72 illustrated in Fig. 7B denote the kinds of the color filters. The color filters are arranged in a Bayer pattern. Reference numeral 1 is given to green filters, reference numeral 2 is given to red filters, and reference numeral 3 is given to blue filters. The green filters 1 are arranged in a staggered pattern so as not to be positioned side by side. The shortest distance between the centers of any two of the pixels 72 provided with the green filters 1 is about 8.5 μm (corresponding to a length of 1.4 pixels). The red filters 2 and the blue filters 3 are each provided to every two pixels 72. The shortest distance between the centers of any two of the pixels 72 provided with the red filters 2 and the shortest distance between the centers of any two of the pixels 72 provided with the blue filters 3 are both about 12.0 μm (corresponding to a length of 2.0 pixels).

The optical fibers 31c and 32c each transmit all of light beams of red, green, and blue wavelength bands. Hence, as long as at least one optical fiber is allocated to one pixel 72 for each of the three colors, there is no chance of resolution deterioration. As graphed in Fig. 8A, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b ranges from 4.9 to 6.9 μm. That is, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is smaller than the pixel pitch for each of the three colors. Therefore, distortion can be reduced with no resolution deterioration. Such a configuration is also applicable to any of the other embodiments described above and embodiments to be described below.

The second optical-fiber bundle 32 is of a straight type. The maximum inclination angle αmx of the second optical-fiber bundle 32 is 0.0 degrees. The incident surface 32a of the second optical-fiber bundle 32 has a spherical shape that substantially conforms to the emitting surface 31b of the first optical-fiber bundle 31. The incident surface 32a and the emitting surface 31b are joined together, forming a joint surface. On the other hand, the emitting surface 32b of the second optical-fiber bundle 32 has a flat shape and is closely in contact with the sensor 4. Since the second optical-fiber bundle 32 having such a shape is joined to the first optical-fiber bundle 31, the shapes of the emitting surfaces 31b and 32b of the respective optical- fiber bundles 31 and 32 are optimized so that the blur occurring when an image is transmitted to a device provided immediately after the optical- fiber bundles 31 and 32 is reduced as much as possible. Thus, distortion and image blur are reduced simultaneously.

If the device provided immediately after the optical- fiber bundles 31 and 32 is the sensor 4 as in the fifth embodiment, the emitting surface 32b of the second optical-fiber bundle 32 is set to a flat surface. If an optical-fiber bundle including optical fibers that are not inclined (with a maximum inclination angle αmx of 0.0 degrees) is employed as the second optical-fiber bundle 32 as in the fifth embodiment, the effect of distortion reduction produced by the second optical-fiber bundle 32 can be set to almost zero. Hence, the influence of the shape of the joint surface between the first optical-fiber bundle 31 and the second optical-fiber bundle 32 is substantially eliminated. Moreover, such a straight optical-fiber bundle is manufacturable at a lower cost than a tapered optical-fiber bundle.

In the fifth embodiment, the fiber pitch of the second optical-fiber bundle 32 at the incident surface 32a is 3.0 μm, which is smaller than the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b. Thus, the resolution deterioration occurring when the image is transmitted through the joint surface 31b is suppressed to a low level.

Furthermore, the fiber pitch of the second optical-fiber bundle 32 at the incident surface 32a is smaller than 3/4 of the fiber pitch of the second optical-fiber bundle 32 at the emitting surface 32b. With such a large difference in the fiber pitch, moire becomes less visible.

The emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 may be fixedly bonded to each other so that the optical- fiber bundles 31 and 32 are integrated with each other. Alternatively, the optical- fiber bundles 31 and 32 may be movable relative to each other, with the emitting surface 31b and the incident surface 32a being only in contact with each other. In either case, the advantageous effects described in the fifth embodiment are fully exerted.

To summarize, according to the fifth embodiment, distortion can be reduced even if the image surface of the imaging optical system 2 has a curved shape and the image formed by such an imaging optical system 2 is transmitted to the sensor 4 having a flat surface. Moreover, distortion occurring when the image is converted from that formed on the curved surface to that formed on the flat surface can be reduced.

Sixth Embodiment

Fig. 9 illustrates first and second optical- fiber bundles 31 and 32 included in an imaging apparatus according to a sixth embodiment. The imaging apparatus according to the sixth embodiment differs from the imaging apparatus 71 according to the fifth embodiment in that the emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 each have an aspherical shape defined by the following expression:

where c denotes the curvature (1/curvature radius), h denotes the distance from the optical axis AX, and A to E denote the aspherical coefficients for respective terms that are raised to the second to tenth power. The other details are the same as in the first embodiment.

Table 4 summarizes the specifications of the optical- fiber bundles 31 and 32 and the sensor (imaging device) 4 according to the sixth embodiment. Table 5 summarizes the aspherical coefficients representing the shape of the emitting surface (joint surface) 31b of the first optical-fiber bundle 31.

Fig. 10A is a graph illustrating the fiber pitch of the first optical-fiber bundle 31 according to the sixth embodiment. In Fig. 10A, the fiber pitch of the first optical-fiber bundle 31 at the incident surface 31a is represented by a dotted line, and the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is represented by a solid line. The fiber pitch at the incident surface 31a is the same as that of the fifth embodiment and becomes larger as the distance from the optical axis AX increases as in the fifth embodiment, except the following. The fiber pitch in a range from a 20% image-height ratio to an 80% image-height ratio is larger than that of the fifth embodiment. The gradient of the fiber pitch from the position corresponding to the 20% image-height ratio to the position corresponding to the 100% image-height ratio is linear; that is, the increment of the fiber pitch with respect to the image-height ratio is constant.

Fig. 10B is a graph illustrating the effect of local-magnification reduction produced by the first optical-fiber bundle 31. The effect of local-magnification reduction is larger than that of the fifth embodiment in the range from the position corresponding to the 20% image-height ratio to the position corresponding to the 80% image-height ratio. Furthermore, the effect of local-magnification reduction increases linearly from the position corresponding to the 20% image-height ratio to the position corresponding to the 100% image-height ratio.

Fig. 10C is a graph illustrating the effect of distortion reduction produced by the first optical-fiber bundle 31. The effect of distortion reduction is the accumulation of the effects of local-magnification reduction. Generally, a higher effect of distortion reduction is produced in the sixth embodiment than in the fifth embodiment.

Fig. 10D is a graph illustrating different kinds of distortion. Distortion occurring at the image surface of the imaging optical system 2 is represented by a solid line, and distortion occurring at the sensor (imaging device) 4 is represented by a line with circles. The distortion at the image surface is -51.4% at maximum but is reduced to 1.0% or below before the image is transmitted to the sensor 4.

According to the sixth embodiment, the distortion is reduced as described above. Furthermore, since the emitting surface 31b of the first optical-fiber bundle 31 has an aspherical surface, the characteristic of the effect of distortion reduction can be set freely in accordance with the distortion occurring in the imaging optical system 2. Thus, the distortion is reduced effectively.

In the sixth embodiment, the maximum inclination angle αmx of the first optical-fiber bundle 31 is 34.7 degrees, which is smaller than the angle of view of the imaging optical system 2 of 60.0 degrees. Even in such a configuration, the distortion occurring at the image surface of the imaging optical system 2 is reduced, as graphed in Fig. 10D. Hence, even in such a case where the light-guiding portion includes optical waveguides (optical fibers 31c) whose inclination angles are smaller than the angle of view of the imaging optical system 2, the distortion can be reduced. In the related art, it is impossible to employ a light-guiding portion including optical waveguides whose inclination angles are the same as the angle of view of the imaging optical system. However, such a light-guiding portion is manufacturable if optical waveguides each having a small inclination angle but being configured to reduce distortion are employed.

While the sixth embodiment concerns a case where the emitting surface 31b of the first optical-fiber bundle 31 has an aspherical shape, the present invention is not limited to such a case. For example, the first optical-fiber bundle 31 may have a hyperboloidal shape expressed as follows:

where h denotes the distance from the optical axis AX, and a and b denote coefficients. Table 6 summarizes the coefficients representing the hyperboloidal shape.

The above hyperboloidal shape is substantially the same as the aspherical shape summarized in Table 5 and therefore provides substantially the same characteristic as that produced in the sixth embodiment.

Seventh Embodiment

An imaging apparatus according to a seventh embodiment will now be described with reference to Fig. 11. Fig. 11 is a schematic diagram of first and second optical- fiber bundles 31 and 32 included in the imaging apparatus according to the seventh embodiment.

The imaging apparatus according to the seventh embodiment differs from the imaging apparatus 1 according to the first embodiment in the configuration of the first and second optical- fiber bundles 31 and 32. Specifically, in the seventh embodiment, the first optical-fiber bundle 31 is of a straight type, whereas the second optical-fiber bundle 32 is of a tapered type. The other details are the same as those described in the first embodiment, and detailed description thereof is omitted. Instead, details of the first and second optical- fiber bundles 31 and 32 will be described.

The first optical-fiber bundle 31 is of a straight type and is incapable of magnifying/reducing an image. The second optical-fiber bundle 32 includes a plurality of optical fibers 32c each having a taper ratio Rt smaller than 1 and a value Rd' smaller than 1. That is, the second optical-fiber bundle 32 reduces the image. The point Pt where the center axes VF of the respective optical fibers 32c of the second optical-fiber bundle 32 meet is at a position nearer to the sensor 4 than the emitting surface (joint surface) 31b of the first optical-fiber bundle 31, with a maximum inclination angle αmx of the optical fibers 32c of 31.0 degrees.

The incident surface 31a of the first optical-fiber bundle 31 is flat. The emitting surface 31b of the first optical-fiber bundle 31 is spherical and is convex when seen from the imaging optical system 2. The emitting surface 32b of the second optical-fiber bundle 32 is flat.

The joint surface 31b is spherical and is convex when seen from the imaging optical system 2. Therefore, regarding the distance in the direction of the optical axis AX from the point Pt to an arbitrary point on the joint surface 31b, a distance Lo from the point Pt to the center of the joint surface 31b is the longest, and a distance Li from the point Pt to the peripheral edge of the joint surface 31b is the shortest. Hence, the fiber pitch of the second optical-fiber bundle 32 at the incident surface 32a is largest on the optical axis AX and decreases as the distance from the optical axis AX increases.

The emitting surface 32b of the second optical-fiber bundle 32 is flat, and the fiber pitch of the second optical-fiber bundle 32 is constant over the entirety of the emitting surface 32b. Hence, the image transmitted through the second optical-fiber bundle 32 from the incident surface 32a to the emitting surface 32b is reduced generally, with the reduction rate being highest at the point on the optical axis AX and becoming lower as the distance from the optical axis AX increases. To describe such a situation on the basis of the ratio of the reduction rate at each image height with respect to the reduction rate at the image height on the optical axis AX (i.e., at the center image height), a part of the image nearer to the position at the marginal image height is magnified to a higher extent than a part of the image at the center image height. Therefore, even if a reduction-type optical-fiber bundle is employed as in the seventh embodiment, the negative distortion occurring in the imaging optical system 2 can be reduced.

While the seventh embodiment concerns a case where the first optical-fiber bundle 31 is of a straight type and the second optical-fiber bundle 32 is of a reduction type, the present invention is not limited to such a case. The first optical-fiber bundle 31 may be of a reduction type and the second optical-fiber bundle 32 may be of a straight type. In that case, the point Pt where the center axes VF of the respective optical fibers 31c of the first optical-fiber bundle 31 meet is at a position nearer to the sensor 4 than the joint surface 31b, with a maximum inclination angle αmx of the optical fibers 31c being 31.0 degrees. In such a configuration, to make the reduction rate smaller with the increase in the distance from the optical axis AX, the length of the optical fibers 31c of the first optical-fiber bundle 31 is made larger as the distance from the optical axis AX becomes shorter. That is, the joint surface 31b has a spherical shape that is concave when seen from the imaging optical system 2. In such a configuration also, negative distortion can be reduced.

Alternatively, the first optical-fiber bundle 31 may be of a straight type and the second optical-fiber bundle 32 may be of a magnification type, as in the first embodiment. In such a configuration, the magnification rate is desired to be increased with the increase in the distance from the optical axis AX. Hence, the length of the optical fibers 32c is set to increase as the distance from the optical axis AX increases. Therefore, the joint surface 31b has a spherical shape that is concave when seen from the imaging optical system 2. In such a configuration also, negative distortion can be reduced.

Eighth Embodiment

Referring to Fig. 12, an imaging apparatus according to an eighth embodiment will now be described. Fig. 12 is a schematic diagram of the imaging apparatus according to the eighth embodiment. The imaging apparatus according to the eighth embodiment differs from the imaging apparatus 1 according to the first embodiment in the characteristic of distortion occurring in the imaging optical system 2 and in the configuration of the optical- fiber bundles 31 and 32. Specifically, the imaging optical system 2 according to the eighth embodiment has a characteristic of causing positive distortion, which makes the image formed on the image surface of the imaging optical system 2 distort into a pincushion-like shape.

Fig. 13 is a schematic diagram of the first and second optical- fiber bundles 31 and 32 according to the eighth embodiment. The first optical-fiber bundle 31 is of a magnification type and includes a plurality of tapered optical fibers 31c. The second optical-fiber bundle 32 is of a straight type. Since the first optical-fiber bundle 31 is of a magnification type, the point Pt where the center axes VF of the respective optical fibers 31c meet is at a position nearer to the imaging optical system 2 than the joint surface 31b between the first optical-fiber bundle 31 and the second optical-fiber bundle 32, with a maximum inclination angle αmx of 31.0 degrees.

The incident surface 31a of the first optical-fiber bundle 31 is flat. The emitting surface (joint surface) 31b of the first optical-fiber bundle 31 is spherical and is concave when seen from the imaging optical system 2. The emitting surface 32b of the second optical-fiber bundle 32 is flat.

The joint surface 31b is spherical and is concave when seen from the imaging optical system 2. Therefore, regarding the distance in the direction of the optical axis AX from the point Pt to an arbitrary point on the joint surface 31b, a distance Lo from the point Pt to the center of the joint surface 31b is the longest, and a distance Li from the point Pt to a point of the joint surface 31b that is farthest from the center of the joint surface 31b is the shortest, as illustrated in Fig. 13. Hence, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is largest at the center image height and decreases toward the position at the marginal image height.

The incident surface 31a of the first optical-fiber bundle 31 is flat, and the fiber pitch of the first optical-fiber bundle 31 is constant over the entirety of the incident surface 31a.

Hence, the image transmitted through the first optical-fiber bundle 31 from the incident surface 31a to the emitting surface 31b is magnified generally, with the magnification rate being highest at the point on the optical axis AX (i.e., at the center image height) and becoming lower as the distance from the optical axis AX increases. Accordingly, the image is magnified to the highest extent at the position corresponding to the center image height. To describe such a situation on the basis of the ratio of the reduction rate with respect to that at the center image height, a part of the image nearer to the position corresponding to the marginal image height is reduced to a higher extent than a part of the image at the position corresponding to the center image height. Such a phenomenon compensates for the positive distortion occurring in the imaging optical system 2.

As described above, the joint surface 31b is set to a shape containing a curved part, and the orientation of the concave/convex surface and the curvature of that surface are set in accordance with the characteristic of the distortion occurring in the imaging optical system 2. Thus, the distortion is reduced.

Ninth Embodiment

Fig. 14 is a schematic diagram of first and second optical- fiber bundles 31 and 32 included in an imaging apparatus according to a ninth embodiment. The imaging apparatus according to the ninth embodiment differs from the imaging apparatus according to the eighth embodiment in the configuration of the first and second optical- fiber bundles 31 and 32. Specifically, while the eighth embodiment employs a magnification-type optical-fiber bundle as the first optical-fiber bundle 31 and a straight-type optical-fiber bundle as the second optical-fiber bundle 32, the ninth embodiment employs a straight-type optical-fiber bundle as the first optical-fiber bundle 31 and a reduction-type optical-fiber bundle as the second optical-fiber bundle 32. In the ninth embodiment also, the first optical-fiber bundle 31 and the second optical-fiber bundle 32 are joined together.

Since the second optical-fiber bundle 32 according to the ninth embodiment is of a reduction type, the point Pt where the center axes VF of the respective optical fibers 32c meet is at a position nearer to the sensor 4 than the emitting surface (joint surface) 31b of the first optical-fiber bundle 31, with a maximum inclination angle αmx of the optical fibers 32c being 31.0 degrees.

The incident surface 31a of the first optical-fiber bundle 31 is flat. The joint surface 31b is spherical and is concave when seen from the imaging optical system 2. The emitting surface 32b of the second optical-fiber bundle 32 is flat.

The joint surface 31b has a spherical shape that is concave when seen from the imaging optical system 2. Therefore, regarding the distance in the direction of the optical axis AX from the point Pt to an arbitrary point on the joint surface 31b, a distance Lo from the point Pt to the center of the joint surface 31b is the shortest, and a distance Li from the point Pt to the point of the joint surface 31b that is farthest from the optical axis AX is the longest. Hence, the fiber pitch of the second optical-fiber bundle 32 at the incident surface (joint surface) 32a is smallest at the point on the optical axis AX and increases as the distance from the optical axis AX increases.

The emitting surface 32b of the second optical-fiber bundle 32 is flat, and the fiber pitch of the second optical-fiber bundle 32 is constant over the entirety of the emitting surface 32b. Hence, the image transmitted from the incident surface 31a of the first optical-fiber bundle 31 to the emitting surface 32b of the second optical-fiber bundle 32 is reduced generally, with the reduction rate being lowest at the point on the optical axis AX (i.e., at the center image height) and becoming higher from the position corresponding to the center image height to the position corresponding to the marginal image height. To describe such a situation on the basis of the ratio of the reduction rate with respect to that at the center image height, a part of the image nearer to the position at the marginal image height is reduced to a higher extent than a part of the image at the center image height. Therefore, according to the ninth embodiment, the positive distortion occurring in the imaging optical system 2 can be reduced.

While the ninth embodiment concerns a case where the first optical-fiber bundle 31 is of a straight type and the second optical-fiber bundle 32 is of a reduction type, the present invention is not limited to such a case. The first optical-fiber bundle 31 may be of a reduction type and the second optical-fiber bundle 32 may be of a straight type. In that case, the point Pt where the center axes VF of the respective optical fibers 31c of the first optical-fiber bundle 31 meet is at a position nearer to the sensor 4 than the joint surface 31b, with a maximum inclination angle αmx of 31.0 degrees. To make the reduction rate smaller with the increase in the distance from the optical axis AX in such a case, the length of the optical fibers 31c of the first optical-fiber bundle 31 is made shorter as the distance from the optical axis AX becomes shorter. That is, the joint surface 31b has a spherical shape that is convex when seen from the imaging optical system 2. In such a configuration also, positive distortion can be reduced.

Alternatively, the first optical-fiber bundle 31 may be of a straight type and the second optical-fiber bundle 32 may be of a magnification type. In such a case, the magnification rate is desired to be reduced with the increase in the distance from the optical axis AX. Hence, the length of the optical fibers 32c of the second optical-fiber bundle 32 is set to decrease as the distance from the optical axis AX increases. That is, the joint surface 31b has a spherical shape that is convex when seen from the imaging optical system 2. In such a configuration also, positive distortion can be reduced.

Tenth Embodiment

Fig. 15 is a schematic diagram of first and second optical- fiber bundles 31 and 32 included in an imaging apparatus according to a tenth embodiment.

The tenth embodiment differs from the first embodiment in the configuration of the first and second optical- fiber bundles 31 and 32. In each of the first to ninth embodiments, one of the first and second optical- fiber bundles 31 and 32 is of a straight type while the other is of a tapered type. In the tenth embodiment, both of the first and second optical- fiber bundles 31 and 32 are of a tapered type. Specifically, according to the tenth embodiment, the first optical-fiber bundle 31 is of a magnification-tapered type, and the second optical-fiber bundle 32 is of a reduction-tapered type. In the tenth embodiment also, the emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 are joined together.

A point Pt1 where the center axes VF1 of the respective optical fibers 31c of the first optical-fiber bundle 31 meet is at a position nearer to the imaging optical system 2 than the emitting surface (joint surface) 31b of the first optical-fiber bundle 31, with a maximum inclination angle αmx1 being 31.0 degrees.

A point Pt2 where the center axes VF2 of the respective optical fibers 32c of the second optical-fiber bundle 32 meet is at a position nearer to the sensor 4 than the joint surface 31b, with a maximum inclination angle αmx2 being 16.7 degrees.

In the tenth embodiment, the optical fibers 31c of the first optical-fiber bundle 31 have larger inclination angles than the optical fibers 32c of the second optical-fiber bundle 32. Therefore, the first optical-fiber bundle 31 is basically responsible for distortion reduction. The incident surface 31a of the first optical-fiber bundle 31 is flat. The joint surface 31b is spherical and is convex when seen from the imaging optical system 2. The emitting surface 32b of the second optical-fiber bundle 32 is flat.

The joint surface 31b is spherical and is convex when seen from the imaging optical system 2. Therefore, as illustrated in Fig. 15, the distance in the direction of the optical axis AX from the point Pt1 of the first optical-fiber bundle 31 to an arbitrary point on the joint surface 31b is the shortest at the point on the optical axis AX and increases as the distance from the optical axis AX increases. Hence, the fiber pitch of the first optical-fiber bundle 31 at the emitting surface 31b is smallest at the point on the optical axis AX and increases as the distance from the optical axis AX increases. The incident surface 31a of the first optical-fiber bundle 31 is flat, and the fiber pitch of the first optical-fiber bundle 31 is constant over the entirety of the incident surface 31a.

Hence, the image transmitted through the first optical-fiber bundle 31 from the incident surface 31a to the emitting surface 31b is magnified generally, with the magnification rate being lowest at the point on the optical axis AX and becoming higher as the distance from the optical axis AX increases. To describe such a situation on the basis of the ratio of the magnification rate with respect to that at the center image height, a part of the image nearer to the position corresponding to the marginal image height is magnified to a higher extent than a part of the image at the position corresponding to the center image height. Thus, the first optical-fiber bundle 31 produces an effect of compensating for the negative distortion occurring in the imaging optical system 2.

Furthermore, since the joint surface 31b is spherical and is convex when seen from the imaging optical system 2, the distance in the direction of the optical axis AX from the point Pt2 of the second optical-fiber bundle 32 to an arbitrary point on the joint surface 31b is the longest at the point on the optical axis AX and decreases as the distance from the optical axis AX increases. Hence, the fiber pitch of the second optical-fiber bundle 32 at the incident surface (joint surface) 32a is largest at the point on the optical axis AX and decreases as the distance from the optical axis AX increases. The emitting surface 32b of the second optical-fiber bundle 32 is flat, and the fiber pitch of the second optical-fiber bundle 32 is constant over the entirety of the emitting surface 32b.

Hence, the image transmitted through the second optical-fiber bundle 32 from the incident surface 32a to the emitting surface 32b is reduced generally, with the reduction rate being highest at the point on the optical axis AX and becoming lower as the distance from the optical axis AX increases. To describe such a situation on the basis of the ratio of the magnification rate with respect to that at the center image height, a part of the image nearer to the position corresponding to the marginal image height is magnified to a higher extent than a part of the image at the position corresponding to the center image height. Thus, the second optical-fiber bundle 32 produces an effect of compensating for the negative distortion occurring in the imaging optical system 2.

To summarize, according to the tenth embodiment, negative distortion can be reduced. Furthermore, if a magnification-type optical-fiber bundle and a reduction-type optical-fiber bundle are employed and the joint surface between the two is set to a curved surface that is convex when seen from the imaging optical system 2, the effect of reducing the negative distortion is enhanced.

Using the theory employed by the imaging apparatus according to the tenth embodiment, an imaging apparatus that causes less positive distortion can also be provided. To do so, a magnification-type optical-fiber bundle is employed as the first optical-fiber bundle 31, and a reduction-type optical-fiber bundle is employed as the second optical-fiber bundle 32, as in the tenth embodiment. However, to reduce positive distortion, the joint surface forming the emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 is set to a spherical shape that is concave when seen from the imaging optical system 2. In such a configuration, the magnification rate of the first optical-fiber bundle 31 that is of a magnification type becomes lower as the distance from the optical axis AX increases, whereas the reduction rate of the second optical-fiber bundle 32 that is of a reduction type becomes higher as the distance from the optical axis AX increases. Thus, both the first optical-fiber bundle 31 and the second optical-fiber bundle 32 can produce an effect of reducing positive distortion.

Eleventh Embodiment

Fig. 16 is a schematic diagram of first and second optical- fiber bundles 31 and 32 included in an imaging apparatus according to an eleventh embodiment. The imaging apparatus according to the eleventh embodiment differs from the imaging apparatus according to the tenth embodiment in the configuration of the first and second optical- fiber bundles 31 and 32. While the tenth embodiment concerns a case where the first optical-fiber bundle 31 is of a magnification type and the second optical-fiber bundle 32 is of a reduction type, the eleventh embodiment concerns a case where the first optical-fiber bundle 31 is of a reduction type and the second optical-fiber bundle 32 is of a magnification type. In the eleventh embodiment also, the emitting surface 31b of the first optical-fiber bundle 31 and the incident surface 32a of the second optical-fiber bundle 32 are joined together.

The incident surface 31a of the first optical-fiber bundle 31 is flat. The emitting surface (joint surface) 31b of the first optical-fiber bundle 31 is spherical and is concave when seen from the imaging optical system 2. The emitting surface 32b of the second optical-fiber bundle 32 is flat.

The reduction rate of the first optical-fiber bundle 31 is highest at the point on the optical axis AX and becomes lower as the distance from the optical axis AX increases. To describe such a situation on the basis of the ratio of the magnification rate with respect to that at the center image height, a part of the image nearer to the position corresponding to the marginal image height is magnified to a higher extent than a part of the image at the position corresponding to the center image height. In such a configuration, the first optical-fiber bundle 31 produces an effect of compensating for the negative distortion occurring in the imaging optical system 2.

The magnification rate of the second optical-fiber bundle 32 is lowest at the point on the optical axis AX and becomes higher as the distance from the optical axis AX increases. To describe such a situation on the basis of the ratio of the magnification rate with respect to that at the center image height, a part of the image nearer to the position corresponding to the marginal image height is magnified to a higher extent than a part of the image at the position corresponding to the center image height. In such a configuration, the second optical-fiber bundle 32 produces an effect of compensating for the negative distortion occurring in the imaging optical system 2.

Thus, negative distortion can be reduced by employing a reduction-tapered optical-fiber bundle and a magnification-tapered optical-fiber bundle and setting the joint surface 31b between the two to form a curved surface that is concave when seen from the imaging optical system 2.

Furthermore, with a combination of optical-fiber bundles that are each capable of reducing negative distortion, the effect of distortion reduction is enhanced. If the joint surface 31b is set to a curved surface that is convex when seen from the imaging optical system 2, positive distortion can also be reduced.

While exemplary embodiments of the present invention have been described above, the present invention is not limited to those embodiments. Various changes and modifications can be made to the above embodiments within the scope of the present invention. For example, the shape of the emitting surface (joint surface) 31b of the first optical-fiber bundle 31 is not limited to any of those described in the above embodiments, and may be a shape containing a curved surface such as an elliptical surface, a conical surface, a hyperboloidal surface, an aspherical surface, or the like. The shape of the curved surface can be determined in accordance with the characteristic of distortion that is desired to reduce. Furthermore, to reduce distortion, the emitting surface 31b of the first optical-fiber bundle 31 only needs to contain a curved part and may additionally contain a flat part or another type of curved part, as described above.

While the above embodiments each concern a case where the light-guiding member 30 includes the first optical-fiber bundle 31 and the second optical-fiber bundle 32, the present invention is not limited to such a case. A combination of a plurality of light-guiding portions may be employed, with a plurality of joint surfaces.

The imaging apparatus according to any of the above embodiments is also applicable to an imaging apparatus including an imaging unit for infrared rays (at a wavelength of 0.7 μm to 15 μm). In that case, the imaging optical system, the light-guiding portion, and the imaging device are to be operable with infrared rays.

The present invention is applicable to a product including an imaging apparatus such as a digital camera, a digital video camera, a mobile-phone camera, a surveillance camera, and a fiber scope.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2015-132143, filed June 30, 2015, which is hereby incorporated by reference herein in its entirety.

Claims (20)

1. An imaging apparatus comprising:
   an imaging optical system configured to form an image of an object;
   an imaging device configured to take the image of the object; and
   a light-guiding member configured to guide light from the imaging optical system to the imaging device,
   wherein the light-guiding member includes a first light-guiding portion and a second light-guiding portion, the first light-guiding portion including a plurality of optical waveguides that transmit the light from the imaging optical system, the second light-guiding portion including a plurality of optical waveguides that transmit the light from the first light-guiding portion,
   wherein an inclination angle, with respect to an optical axis of the imaging optical system, of a center axis of an optical waveguide that is farthest from the optical axis in the first light-guiding portion is different from an inclination angle, with respect to the optical axis, of a center axis of an optical waveguide that is farthest from the optical axis in the second light-guiding portion,
   wherein each of the optical waveguides of the first light-guiding portion has a smaller diameter at an incident surface than at an emitting surface, and
   wherein, in the first light-guiding portion, an optical waveguide that is nearest to the optical axis is shorter than the optical waveguide that is farthest from the optical axis.
2. The imaging apparatus according to Claim 1, wherein, in the first light-guiding portion, the diameter of the emitting surface of the optical waveguide that is nearest to the optical axis is smaller than the diameter of the emitting surface of the optical waveguide that is farthest from the optical axis.
3. The imaging apparatus according to Claim 1, wherein a pitch of arrangement of the optical waveguides at an incident surface of the first light-guiding portion is smaller than a pitch of arrangement of the optical waveguides at an emitting surface of the first light-guiding portion.
4. The imaging apparatus according to Claim 1, wherein an emitting surface of the first light-guiding portion is convex when seen from the object.
5. The imaging apparatus according to Claim 1, wherein an incident surface of the first light-guiding portion is concave when seen from the object.
6. The imaging apparatus according to Claim 1, wherein an image surface of the imaging optical system is concave when seen from the object.
7. The imaging apparatus according to Claim 1, wherein the light-guiding member has a lower local magnification at a position corresponding to a center image height than at a position corresponding to a marginal image height.
8. The imaging apparatus according to Claim 1, wherein an emitting surface of the second light-guiding portion is flat.
9. The imaging apparatus according to Claim 1, wherein, in the second light-guiding portion, a diameter of each of the optical waveguides at an incident surface is equal to a diameter of each of the optical waveguides at an emitting surface.
10. The imaging apparatus according to Claim 1, wherein, in the first light-guiding portion, letting a meridional component of a distance between centers of a pair of optical waveguides that are adjacent to each other in a meridional direction be denoted as a first value and a ratio of the first value at an emitting surface with respect to the first value at an incident surface be denoted as a second value, the second values at first and second positions that are at different distances from the optical axis are different.
11. The imaging apparatus according to Claim 10, wherein the following expression is satisfied at each of different image heights:
    

    

    where MHi denotes a local magnification of the light-guiding member, MLi denotes a local magnification of distortion occurring at an image surface of the imaging optical system, P1i denotes the first value of the pair of optical waveguides at the incident surface, and P2i denotes the first value of the pair of optical waveguides at the emitting surface.
12. The imaging apparatus according to Claim 10, wherein the second value of the first light-guiding portion increases as the distance from the optical axis increases.
13. The imaging apparatus according to Claim 10, wherein the first value at the emitting surface of one of the first and second light-guiding portions that includes an optical waveguide having a largest inclination angle with respect to the optical axis in the first and second light-guiding portions is larger than the first value at the emitting surface of an other light-guiding portion.
14. The imaging apparatus according to Claim 1, wherein a pitch of arrangement of the optical waveguides at an emitting surface of the first light-guiding portion is different from a pitch of arrangement of the optical waveguides at an incident surface of the second light-guiding portion.
15. The imaging apparatus according to Claim 1, wherein the following expression is satisfied at each of different image heights:
    

    

    where MHi denotes a local magnification of the light-guiding member, and MLi denotes a local magnification of distortion occurring at an image surface of the imaging optical system.
16. An imaging apparatus comprising:
    an imaging optical system configured to form an image of an object;
    an imaging device configured to take the image of the object; and
    a light-guiding member configured to guide light from the imaging optical system to the imaging device,
    wherein the light-guiding member includes a first light-guiding portion and a second light-guiding portion, the first light-guiding portion including a plurality of optical waveguides that transmit the light from the imaging optical system, the second light-guiding portion including a plurality of optical waveguides that transmit the light from the first light-guiding portion,
    wherein an inclination angle, with respect to an optical axis of the imaging optical system, of a center axis of an optical waveguide that is farthest from the optical axis in the first light-guiding portion is different from an inclination angle, with respect to the optical axis, of a center axis of an optical waveguide that is farthest from the optical axis in the second light-guiding portion,
    wherein each of the optical waveguides of the second light-guiding portion has a larger diameter at an incident surface than at an emitting surface, and
    wherein, in the second light-guiding portion, an optical waveguide that is nearest to the optical axis is longer than the optical waveguide that is farthest from the optical axis.
17. The imaging apparatus according to Claim 16, wherein, in the second light-guiding portion, the diameter of the emitting surface of the optical waveguide that is nearest to the optical axis is larger than the diameter of the emitting surface of the optical waveguide that is farthest from the optical axis.
18. The imaging apparatus according to Claim 16 or 17, wherein a pitch of arrangement of the optical waveguides at an incident surface of the second light-guiding portion is larger than a pitch of arrangement of the optical waveguides at an emitting surface of the second light-guiding portion.
19. The imaging apparatus according to any of Claims 16 to 18, wherein the incident surface of the second light-guiding portion is convex when seen from the object.
20. The imaging apparatus according to any of Claims 16 to 19, wherein a pitch of arrangement of the optical waveguides at the emitting surface of the first light-guiding portion is different from the pitch of arrangement of the optical waveguides at the incident surface of the second light-guiding portion.

Images