WO2017010086A1

WO2017010086A1 - Imaging apparatus

Info

Publication number: WO2017010086A1
Application number: PCT/JP2016/003293
Authority: WO
Inventors: Keiichiro Ishihara
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-07-16
Filing date: 2016-07-12
Publication date: 2017-01-19
Also published as: JP2017028378A

Abstract

An imaging apparatus includes an imaging optical system configured to form an image of an object, an imaging element configured to pick up the image of the object, and optical waveguides configured to guide light from the imaging optical system to the imaging element. A refractive index of a medium adjacent to an incident surface of each of the optical waveguides is higher than that of a center part of the optical waveguide. The following condition is satisfied: 0 < α < θ_p < β where α represents an angle formed by an axis of the optical waveguide and an optical axis of the imaging optical system, θ_p represents an angle formed by a principal ray of a light beam incident on the optical waveguide and the optical axis, and β represents an angle formed by a surface normal to a center of the incident surface and the optical axis.

Description

IMAGING APPARATUS

The present invention relates to an imaging apparatus.

There is known an imaging apparatus that includes an optical fiber bundle (light guide) composed of a plurality of optical fibers (optical waveguides) and that guides light from an imaging optical system to an imaging element by the light guide.

PTL 1 discloses an imaging apparatus in which a plurality of tapered optical fibers having open ends at an image surface of a ball lens are bundled to form an optical fiber bundle and in which longitudinal directions of all of the optical fibers coincide with the directions toward a ball center of the ball lens. In this imaging apparatus, field curvature of the imaging optical system is reduced by shaping an incident surface of the optical fiber bundle to match the image surface of the imaging optical system.

The shape of an image surface in a wide-angle imaging optical system tends to be greatly curved. For this reason, the imaging apparatus described in PTL 1 needs optical fibers greatly inclined with respect to the optical axis of the imaging optical system to achieve a wide angle of view.

However, it is not easy to obtain optical fibers greatly inclined with respect to the optical axis. For this reason, the inclination of the principal ray of a light beam incident on the optical fibers with respect to the optical axis is sometimes larger than the inclination with respect to the axes of the optical fibers. In this case, the emission angle of the light beam emitted from emission surfaces of the optical fibers becomes large, and this may reduce light receiving sensitivity of the imaging element.

Japanese Patent Laid-Open No. 2005-338341

The present invention suppresses deterioration of light receiving sensitivity of an imaging element in an imaging apparatus that guides light from an imaging optical system to the imaging element by a light guide.

An imaging apparatus includes an imaging optical system configured to form an image of an object, an imaging element configured to pick up the image of the object, and a plurality of optical waveguides configured to guide light from the imaging optical system to the imaging element. A refractive index of a medium adjacent to an incident surface of each of the optical waveguides is higher than a refractive index of a center part of the optical waveguide. The following condition is satisfied:
0 < α < θ_p < β
where α represents an angle formed by an axis of the optical waveguide and an optical axis of the imaging optical system, θ_p represents an angle formed by a principal ray of a light beam incident on the optical waveguide and the optical axis, and β represents an angle formed by the optical axis and a surface normal to a center of the incident surface of the optical waveguide.

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 view illustrating a structure of an imaging apparatus according to an embodiment. Fig. 1B is a schematic view illustrating the structure of the imaging apparatus according to the embodiment. Fig. 2 is a schematic view illustrating a structure of an imaging optical system in the embodiment. Fig. 3A is a schematic view illustrating the structures of the imaging optical system and a light guide in the embodiment. Fig. 3B is a schematic view illustrating the structures of the imaging optical system and the light guide in the embodiment. Fig. 4 is a schematic view illustrating refraction of a light beam at an image surface in the embodiment. Fig. 5 is a schematic view illustrating a structure of an imaging apparatus according to a second example. Fig. 6 is a schematic view illustrating a structure of an imaging optical system in the second example. Fig. 7 is a schematic view illustrating the structures of the imaging optical system and a light guide in the second example. Fig. 8 is a schematic view illustrating a structure of a fifth lens in the second example. Fig. 9 is a schematic view illustrating a structure of an imaging apparatus according to a third example. Fig. 10 is a schematic view illustrating a structure of an imaging optical system in the third example. Fig. 11A is a schematic view illustrating a structure of an optical waveguide. Fig. 11B is a schematic view illustrating another structure of the optical waveguide.

Description of Embodiment

Embodiment

An imaging apparatus (image device) 1 according to an embodiment will be described with reference to Fig. 1A and 1B. Figs. 1A and 1B are schematic views illustrating the structure of the imaging apparatus 1.

The imaging apparatus 1 includes an imaging optical system 2, a light guide (image transfer member) 3, and an imaging element (image sensor) 4 (hereinafter referred to as a "sensor 4"). The imaging optical system 2, the light guide 3, and the sensor 4 are arranged to transfer an image formed by the imaging optical system 2 to the sensor 4 by the light guide 3.

The imaging optical system 2 is an image forming optical system that forms an image of an unillustrated subject on an image surface (imaging surface) 20. The image of the subject formed by the imaging optical system 2 is shaped like a curved surface that is convex to the sensor 4. That is, the image surface 20 of the imaging optical system 2 is a curved surface. Fig. 2 is a schematic view illustrating the structure of the imaging optical system 2.

The imaging optical system 2 has a substantially point-symmetrical structure, and includes five spherical lenses, that is, a first lens 21, a second lens 22, a third lens 23, a fourth lens 24, and a fifth lens 25, and an aperture stop 29. An exemplary structure of the imaging optical system 2 will be described with reference to Fig. 2. Fig. 2 is a schematic view illustrating the exemplary structure of the imaging optical system 2.

The first lens 21 is a meniscus lens that is convex to the object side, and has an incident surface 21a serving as an R1 surface and an emission surface 21b serving as an R2 surface with a radius of curvature shorter than that of the R1 surface. The second lens 22 is a plano-convex lens that is convex to the object side, and has an incident surface 22a serving as the R2 surface and an emission surface 22b serving as a planar STO surface. The third lens 23 is a meniscus lens that is convex to the image side (a side of the image surface 20), and has an incident surface 23a serving as the planar STO surface and an emission surface 23b serving as an R3 surface that is convex to the image side. The fourth lens 24 is a meniscus lens that is convex to the image side, and has an incident surface 24a serving as the R3 surface and an emission surface 24b serving as an R4 surface that has a radius of curvature longer than that of the R3 surface and is convex to the image side. The fifth lens 25 is a meniscus lens that is convex to the image side, and has an incident surface 25a serving as an R5 surface that is convex to the image side and an emission surface 25b serving as an IMG surface that has a radius of curvature longer than that of the R5 surface and is convex to the image side. The power of the fifth lens 25 is positive.

The aperture stop 29 is disposed at contact surfaces (22b and 23a) of the second lens 22 and the third lens 23. A light shielding member is disposed outside an effective area of the aperture stop 29 so that a light beam can pass only through the effective area.

The imaging optical system 2 is also structured so that the emission surface 25b of the fifth lens 25 nearly matches the image surface 20. The shape of the emission surface 25b of the fifth lens 25 is set in accordance with the shape of the image surface 20. The imaging optical system 2 is an image forming optical system including the image surface 20 that is formed by a curved surface and has a wide angle of view. However, the structure of the imaging optical system 2 is not limited thereto.

The light guide 3 is composed of a plurality of optical waveguides (light waveguide members) 3c that guide light from the imaging optical system 2 to the sensor 4. Specifically, each of the optical waveguides 3c receives imaging light (light beam) BM through the imaging optical system 2, propagates the imaging light BM therein, and guides the imaging light BM to pixels in the sensor 4. The imaging light BM includes a principal ray PR passing through the center of an exit pupil of the imaging optical system 2, an upper marginal ray NR, and a lower marginal ray MR.

Fig. 1B illustrates a part of a cross section of the light guide 3 taken parallel to a light receiving surface of the sensor 4. In this cross section, core parts 3co are arranged in a triangular lattice form, and cladding parts 3cl are arranged between the core parts 3co. In this way, each optical waveguide 3c is composed of the core parts (center parts) 3co and the cladding parts (peripheral parts) 3cl disposed around the core parts 3co. While the core parts 3co are arranged in a triangular lattice form in Fig. 1B, the structure is not limited thereto. For example, the core parts 3co may be arranged in an arbitrary lattice form such as a square lattice form or a diagonal lattice form. Alternatively, the core parts 3co may be arranged at random as long as the cladding parts 3cl are arranged between the core parts 3co. Further alternatively, it is possible to use an optical waveguide in which regions including the core parts 3co arranged in a lattice form and regions including the core parts 3co arranged at random are mixed.

Figs. 11A and 11B are schematic views illustrating the structures of each optical waveguide 3c. As illustrated in Fig. 11A, a part (center part) having a refractive index N higher than that of a peripheral part in the optical waveguide 3c is referred to as a core part 3co, and a part that covers the core part 3co and has a refractive index N lower than that of the core part 3co is referred to as a cladding part 3cl.

Further, for example, as illustrated in Fig. 11B, the optical waveguide 3c may confine and guide light by providing a refractive index distribution in a plane perpendicular to the light guide direction thereof. In this case, the core part 3co also refers to a part that exists in a center part of the optical waveguide 3c and has a refractive index N higher than that of the peripheral part. While the refractive index of the core part 3co changes from the center toward the periphery thereof in the optical waveguide having the refractive index distribution, a refractive index N_co of the core part 3co refers to a refractive index of the center (center of gravity) of an incident surface 3ca of the optical waveguide 3c. In general, the refractive index N_co of the core part 3co is the highest in the optical waveguide 3c. Further, a refractive index N_cl of the cladding part 3cl to be described later refers to a refractive index of a part on the outermost side (outermost part) of the optical waveguide 3c. When the optical waveguide 3c does not include the cladding part 3cl, the lowest refractive index in the optical waveguide 3c is taken as the refractive index N_cl of the cladding part 3cl.

The optical waveguides 3c may or may not be in a one-to-one correspondence with the pixels in the sensor 4. For example, a part of the imaging light BM propagating through the optical waveguides 3c may be received by a certain pixel in the sensor 4, and the other part of the light may be received by the other pixels. Alternatively, a certain pixel in the sensor 4 may receive the imaging light BM propagating through a plurality of optical waveguides 3c.

An incident surface 3a of the light guide 3 is shaped like a curved surface equal to the image surface 20 of the imaging optical system 2, and is disposed to coincide with the position of the image surface 20. An emission surface 3b of the light guide 3 is flat, and is in close contact with the sensor 4. As the optical waveguides 3c, for example, optical fibers can be used.

Here, the definition of an inclination angle α of the optical waveguides 3c will be described with reference to Figs. 3A and 3B. Figs. 3A and 3B illustrate the structures of the light guide (optical waveguide group, optical fiber bundle) 3 and the fifth lens 25 disposed closest to the light guide 3 among the plurality of lenses included in the imaging optical system 2.

The inclination angle α of each optical waveguide 3c is an angle formed by an optical axis AX of the imaging optical system 2 and the axis of the optical waveguide 3c. In other words, the inclination angle α of the optical waveguide 3c is an inclination of the longitudinal direction of the optical waveguide 3c with respect to the optical axis AX of the imaging optical system 2. The inclination angle α of each optical waveguide 3c is more than or equal to 0.0 degrees and less than 90.0 degrees. The axis of the optical waveguide 3c is defined as follows. The axis of the optical waveguide 3c is a straight line that connects a center point A of a cross section of the core part 3co on the incident surface 3ca of the optical waveguide 3c and a center point B of a cross section of the core part 3co shifted inward from the center point A by an amount L corresponding to the radius of the core part 3co on the incident surface 3ca.

The light guide 3 of the embodiment includes tapered sections in which the thickness of the core part 3co differs according to the position and straight sections which are connected in series to the tapered sections and in which the thickness of the core part 3co is constant. However, alternatively, only straight optical waveguides 3c in which the thickness of the core part 3co is constant may be used, or only tapered optical waveguide 3c may be used. The shape of the optical waveguides 3c may be straight or curved.

In Fig. 3A, an angle that is formed by the optical axis AX of the imaging optical system 2 and the principal ray PR of the light beam BM incident on each optical waveguide 3c and is more than or equal to 0.0 degrees and less than 90.0 degrees is designated as an inclination angle θ_p of the light beam BM. Further, an angle that is formed by the optical axis AX and a surface normal to the incident surface 3ca of the optical waveguide 3c at the center point A of the cross section of the core part 3co and is more than or equal to 0.0 degrees and less than 90.0 degrees is designated as an inclination angle β of the surface normal.

In the optical waveguide 3c of the light guide 3 far from the optical axis AX, the inclination angle θ_p of the light beam BM is larger than the inclination angle α of the optical waveguide 3c. That is, the light guide 3 includes the optical waveguide 3c that is far from the optical axis AX and satisfies Expression 1. As the distance of the optical waveguide 3c from the optical axis AX increases, the difference between the inclination angle α of the optical waveguide 3c and the inclination angle θ_p of the light beam BM increases.
0 < α < θ_p (1)

In this imaging apparatus 1, the emission angle of the light beam BM emitted from the emission surface 3cb of the optical waveguide 3c is larger than when the inclination angle θ_p of the light beam BM coincides with the inclination angle α of the optical waveguide 3c. Each pixel in the sensor 4 has a sensitivity characteristic with respect to the incident angle θ_s of light incident on the pixel. The sensitivity characteristic with respect to the incident angle θ_s is the highest at vertical incidence (incident angle θ_s on the sensor 4 = 0 degrees), and decreases as the incident angle θ_s increases. When the incident angle θ_s becomes ±15 degrees, the sensitivity decreases to about half the sensitivity at vertical incidence. When the incident angle θ_s exceeds ±25 degrees, the sensitivity falls to be lower than or equal to 10% of the sensitivity at vertical incidence.

To decreases the the incident angle θ_s of the light beam BM on the sensor 4, the imaging apparatus 1 has the following structure. As the structure, the refractive index N_co of the core part 3co of the optical waveguide 3c that satisfies Expression 1 is lower than a refractive index N_img of a medium in which light propagates just before incidence on the light guide 3 (a medium adjacent to the incident surface of the optical waveguide 3c). In the embodiment, the medium in which light propagates just before incidence on the light guide member 3 is the fifth lens 25 disposed closest to the incident surface 3a of the light guide 3 among the plurality of lenses included in the imaging optical system 2. For this reason, a refractive index N₅ of the fifth lens 25 is set to be higher than the refractive index N_co of the core part 3co in the optical waveguide 3c.

In the optical waveguide 3c that satisfies Expression 1, the inclination angle θ_p of the light beam BM incident on the incident surface 3ca of the optical waveguide 3c and the inclination angle β of the surface normal to the incident surface 3ca of the optical waveguide 3c satisfy Expression 2. The inclination angle θ_p of the light beam BM, the inclination angle β of the surface normal, and the inclination angle α of the optical waveguide 3c can each differ among the optical waveguides 3c.
0 < θ_p < β (2)

According to this structure, deterioration of the light receiving sensitivity of the sensor 4 can be suppressed in the optical waveguide 3c whose inclination angle α is smaller than the inclination angle θ_p of the light beam BM. The optical waveguide 3c disposed on the optical axis AX does not satisfy Expressions 1 and 2. In the embodiment, the optical waveguides 3c disposed at the positions far from the optical axis AX, that is, all of the optical waveguides 3c that do not exist on the optical axis AX satisfy Expressions 1 and 2. By combining Expression 1 and Expression 2, it is possible to express that 0 < α < θ_p <β.

At this time, the refractive index N_img of the medium can be determined so that the relationship among the inclination angle θ_p of the light beam BM, the inclination angle α of the optical waveguide 3c, and the inclination angle β of the surface normal satisfies Expression 3 and Expression 4. Expression 3 shows a condition that the marginal ray MR of the light beam BM on the side of the optical axis AX can be totally reflected and can propagate without any loss in the optical waveguide 3c and that the decrease in light amount of the light beam BM due to propagation in the optical waveguide 3c can be limited to about 50%. Expression 4 shows a condition that the principal ray PR of the light beam BM can be totally reflected and can propagate without any loss in the optical waveguide 3c. That is, Expression 4 shows a condition that all light components of the light beam BM are totally reflected and the decrease in light amount can be minimized. An angle formed by the principal ray PR of the light beam BM and the marginal ray MR on the side of the optical axis AX is designated as ρ, and a refractive index of the cladding part 3cl is designated as N_cl. The marginal ray MR on the side of the optical axis AX refers to a marginal ray closer to the optical axis AX in the cross section including the optical axis AX (cross section illustrated in Fig. 3A), in other words, to a marginal ray closest to the optical axis AX in the light beam BM.

The structures of the fifth lens 25 and the light guide 3 will be described in more detail with reference to Fig. 3A. The emission surface 25b of the fifth lens 25 and the incident surface 3a of the light guide 3 are structured to have almost the same shape. The emission surface 25b of the fifth lens 25 is bonded in contact with the incident surface 3a of the light guide 3. This bonded surface coincides with the image surface 20 of the imaging optical system 2. According to this structure, an image formed on the image surface 20 of the imaging optical system 2 can be obtained at the incident surface 3a of the light guide 3 without being blurred. The image incident from the incident surface 3a of the light guide 3 is transferred to the emission surface 3b of the light guide 3 by the optical waveguides 3c, and is picked up by the sensor 4.

As illustrated in Fig. 3A, an exit pupil position Pe of the imaging optical system 2 is located on the object side. Here, the exit pupil position Pe refers to an intersection of the principal ray PR of the light beam BM incident on the image surface 20 (emission surface 252 of the fifth lens 25) and the optical axis AX of the imaging optical system 2. The image surface 20 is spherical, and a curvature center Cimg of the image surface 20 is located on the optical axis AX.

As described above, the light guide 3 includes the optical waveguides 3c that are far from the optical axis AX and satisfy Expression 1. An intersection Pwg of an axis 311 of each optical waveguide 3c from the image surface 20 and the optical axis AX of the imaging optical system 2 is located closer to the object side than the exit pupil position Pe of the imaging optical system 2.

The curvature center Cimg of the image surface 20 is located closer to the image side than the exit pupil position Pe of the imaging optical system 2. That is, a distance L_ci from the intersection Cimg of a normal 2521 to the image surface 20 and the optical axis AX of the imaging optical system 2 to the image surface 20 is set to be shorter than a distance L_pe from the exit pupil position Pe of the imaging optical system 2 to the image surface 20. This relationship is expressed by Expression 5:
L_ci < L_pe (5)

When the image surface 20 is spherical as in the embodiment, the distance L_ci can be replaced with the curvature radius of the image surface 20 or the distance from the image surface 20 to the curvature center Cimg.

The structure of each optical waveguide 3c will be described with reference to Fig. 4. Fig. 4 is a schematic view illustrating refraction of the light beam BM at the image surface 20.

A section closer to the object side than the incident surface 3a is filled with a glass material having the refractive index N₅, a section closer to the image side than the incident surface 3a is filled with a glass material having the refractive index N_co, and the refractive index N₅ of the fifth lens 25 is different from the refractive index N_co of the core part 3co in the optical waveguide 3c. The emission surface 25b of the fifth lens 25 is bonded in contact with the incident surface 3a of the core part 3co of the optical fiber 3c, and this bonded surface 25b coincides with the image surface 20 of the imaging optical system 2. Hence, the refractive index N_img of the medium is the refractive index N₅ of the fifth lens 25.

The principal ray PR of the light beam BM from the imaging optical system 2 enters the bonded surface 25b at an incident angle θ_i to the normal 2521 to the bonded surface 25b, and is emitted at an emission angle θ_o. The incident angle θ_i and the emission angle θ_o satisfy the Snell's law expressed by Expression 6. The incident angle θ_i and the emission angle θ_o are angles formed by the surface normal to the incident surface 3ca of the optical waveguide 3c and the principal ray PR.
N_img × sinθ_i = N_co × sinθ_o (6)

When the refractive index N_img of the medium in which light to be incident on the incident surface 3a of the light guide 3 propagates is higher than the refractive index N_co of the core part 3co in the optical waveguide 3c, that is, when the condition N_img > N_co is satisfied, the light incident on the optical waveguide 3c is refracted. At this time, when the optical waveguide 3c disposed at the position far from the optical axis AX satisfies Expression 1 and Expression 2, the emission angle θ_o and the incident angle θ_i have a relationship expressed by Expression 7. That is, the emission angle θ_o is larger than the incident angle θ_i. In other words, an angle Δθ_o formed by a straight line 2522 extended from the principal ray PR incident on the bonded surface 25b and the principal ray PR just emitted from the bonded surface 25b (an angle difference between the emission angle θ_o and the incident angle θ_i) is larger than zero.

When Expression 7 is satisfied, the inclination of the propagating direction of the principal ray PR just after incidence on the optical waveguide 3c with respect to the axis of the optical waveguide 3c is smaller than the inclination of the propagating direction of the principal ray PR just before incidence on the optical waveguide 3c. That is, the inclination of the principal ray PR with respect to the optical axis AX can be made close to the inclination angle α of the optical waveguide 3c. This structure decreases the emission angle of light emitted from the emission surface 3cb of the optical waveguide 3c.

In the light guide 3 formed by bundling the optical waveguides 3c together, when the light is not refracted by the incident surfaces 3ca of the optical waveguides 3c, the emission angle is determined according to the incident angle of the light beam BM incident on each optical waveguide 3c. For example, in the case of a straight optical waveguide in which the diameter of the optical waveguide is constant, the emission angle is equal to the incident angle. In the case of a tapered optical waveguide that can enlarge an image, in which the diameter of the optical waveguide gradually increases from the incident surface to the emission surface, the emission angle is smaller than the incident angle according to the taper ratio.

For example, when the incident surface 3ca and the emission surface 3cb of the optical waveguide 3c are perpendicular to the axis, a ratio (taper ratio) Tf of the diameter of the incident surface of the tapered optical waveguide to the diameter of the emission surface is expressed by Expression 8. At this time, the incident angle on the optical waveguide 3c is designated as θ_fi, and the emission angle from the optical waveguide 3c is designated as θ_fo. The relationship such that the emission angle decreases as the incident angle decreases is similar to the straight optical waveguide.

In the embodiment, a refractive index difference is formed between the core part 3co of the optical waveguide 3c and the image side of the incident surface 3ca of the optical waveguide 3c. According to this structure, the propagating direction in which the light incident on the incident surface 3ca is refracted and propagates in the optical waveguide 3c can be made close to the axial direction of the optical waveguide 3c without changing the incident angle of the light on the incident surface 3ca. For this reason, the incident surface 3a of the light guide 3 and the image surface 20 can have almost the same shape. This can transfer the light beam BM to the sensor 4 while reducing the field curvature of the imaging optical system 2.

According to the imaging apparatus 1, the inclination angle θ_po of the principal ray PR of the light beam BM propagating in the core part 3ca with respect to the optical axis AX can be made closer to the inclination angle of the optical waveguide 3c than when there is no refractive index difference between the object side and the image side of the incident surface 3a. That is, the propagating direction of the principal ray PR of the light beam BM propagating in the core part 3ca can be made close to the axial direction of the optical waveguide 3c. As a result, the emission angle of the light beam BM from the optical waveguide 3c decreases, and this can suppress deterioration of the light receiving sensitivity of the sensor 4. In particular, loss of the incident light beam BM in the optical waveguide 3c can be reduced by satisfying Expression 3 and Expression 4.

As the angle of view of the imaging optical system 2 increases and the distance of the optical waveguide 3c from the optical axis AX increases, the difference between the inclination angle α of the optical waveguide 3c and the inclination angle θ_p of the light beam BM increases. However, according to the structure of the imaging apparatus 1, even when the imaging apparatus 1 has a superwide angle of view larger than ±45 degrees, deterioration of the light receiving sensitivity can be suppressed.

As illustrated in Fig. 3A, in the imaging apparatus 1, the position of a curvature center Cr5 of the incident surface 25a of the fifth lens 25 is located closer to the object side than the position of the curvature center Cimg of the emission surface 252 of the fifth lens 25. Thus, the light beam propagating inside the optical waveguide 3c can be made closer to the longitudinal direction of the optical waveguide 3c than when the fifth lens 25 is not disposed.

Further, the position of the curvature center Cr5 of the incident surface 25a of the fifth lens 25 is located closer to the object side than the exit pupil position Pe of the imaging optical system 2. Thus, the incident surface 25a of the fifth lens 25 has the function of making the propagating direction of the principal ray PR propagating inside the optical waveguide 3c close to the axial direction of the optical waveguide 3c. This can increase the effect of suppressing deterioration of the light receiving sensitivity.

By increasing the refractive index N_img of the medium just in front of the incident surface 3a, the spot diameter of the light beam BM on the incident surface 3a can be reduced. For this reason, high resolution can be achieved.

First Example

In a first example, the structure of the imaging apparatus 1 according to the above-described embodiment will be described. As described above, the imaging apparatus 1 includes the imaging optical system 2, the light guide 3, and the sensor 4. The imaging apparatus 1 is configured so that an image formed on the curved image surface 20 by the imaging optical system 2 is obtained at the incident surface 3a of the light guide 3, is transferred to the emission surface 3b, and is obtained by the sensor 4 to pick up an image of a subject. As the light guide 3, an optical fiber bundle having a plurality of optical fibers serving as optical waveguides 3c is used.

The imaging optical system 2 includes the first lens 21, the second lens 22, the third lens 23, the fourth lens 24, the fifth lens 25, and the aperture stop 29, and has a substantially point-symmetrical structure. Exemplary structures of the first to fifth lenses 21 to 25 are shown in Table 1. While the imaging optical system 2 of the first example is a superwide-angle optical system such that the focal length f is 10.0 mm, the F-number is F/1.4, and the maximum angle ω of view is ±60 degrees, it is not limited thereto.

The first lens 21 is a meniscus lens that is convex to the object side, and has the incident surface 21a serving as the R1 surface and the emission surface 21b serving as the R2 surface. The second lens 22 is a plano-convex lens that is convex to the object side, and has the incident surface 22a serving as the R2 surface and the emission surface 22b serving as the STO surface. The third lens 23 is a meniscus lens that is convex to the image side, and has the incident surface 23a serving as the STO surface and the emission surface 23b serving as the R3 surface. The fourth lens 24 is a meniscus lens that is convex to the image side, and has the incident surface 24a serving as the R3 surface and the emission surface 24b serving as the R4 surface. The fifth lens 25 is a meniscus lens that is convex to the image side, and has the incident surface 25a serving as the R5 surface and the emission surface 25b serving as the IMG surface.

Table 2 shows parameters relating to the optical waveguide 3c located at the farthest position from the optical axis AX of the imaging apparatus 1 of the first example. Here, "beam angle" is defined as the angle formed by the optical axis AX of the imaging optical system 2 and the principal ray PR of the light beam BM propagating in a certain medium, that is, the inclination of the principal ray PR with respect to the optical axis AX. Further, the distance along the optical axis AX from the intersection Pwg of the axis of the optical waveguide 3c and the optical axis AX to the image surface 20 is designated as L_wg.

As shown in Table 2, the inclination angle α of the optical waveguide 3c with respect to the optical axis AX of the imaging optical system 2 is 35.00 degrees. In contrast, the inclination angle θ_p of the principal ray PR of the light beam BM incident on the optical waveguide 3c is 59.92 degrees. That is, the inclination angle α of the optical waveguide 3c is smaller than the inclination angle θ_p of the light beam BM. Further, the inclination angle β of the surface normal to the incident surface 3ca of the optical waveguide 3c is 64.91 degrees. That is, the light guide 3 includes the optical waveguide 3c that is far from the optical axis AX and satisfies Expression 1 and Expression 2. Hence, the intersection Pwg of the axis 311 of the optical waveguide 3c from the image surface 20 and the optical axis AX of the imaging optical system 2 is located closer to the object side than the exit pupil position Pe of the imaging optical system 2. Specifically, the distance L_wg from the image surface 20 to the intersection Pwg of the longitudinal axis 311 of the optical waveguide 3c and the optical axis AX of the imaging optical system 2 is 8.88 mm, and the distance L_pe from the image surface 20 to the exit pupil position Pe of the imaging optical system 2 is 5.23 mm.

As shown in Table 2, the distance L_pe from the exit pupil position Pe of the imaging optical system 2 to the image surface 20 is 5.23 mm, and the distance L_ci from the intersection Cimg of the normal 2521 to the image surface 20 and the optical axis AX of the imaging optical system 2 to the image surface 20 is 4.75 mm. Hence, Expression 5 is satisfied.

According to Table 2, in the imaging apparatus 1 of the first example, the refractive index N₅ of the fifth lens 25 (= N_img) is 1.9108, and the refractive index N_co of the core part 3co of the optical waveguide 3c is 1.8200. Hence, Expression 6 is satisfied. The emission angle θ_o (=5.24 degrees) from the image surface 20 is larger than the incident angle θ_i (=4.99 degrees) on the image surface 20, and the angle difference Δθ can be a positive value of +0.25 degrees.

Thus, the inclination angle θ_po of the principal ray PR just after incidence on the optical waveguide 3c is 59.67 degrees. The inclination angle θ_p of the principal ray PR at incidence on the incident surface 3ca of the optical waveguide 3c is 59.92 degrees. That is, the inclination angle of the principal ray PR can be made closer to the direction α of the axis 311 of the optical waveguide 3c (inclination angle of optical waveguide 3c) than before incidence on the optical waveguide 3c. For this reason, the angle of the principal ray PR of the light beam BM emitted from the emission surface 3cb of the optical waveguide 3c can also be kept down.

Hence, according to the imaging apparatus 1 of the first example, deterioration of the light receiving sensitivity of the imaging element can be suppressed even when the angle of the longitudinal direction of the optical waveguide in the light guide is smaller than the angle of the light beam incident from the imaging optical system on the optical waveguide.

The difference between the inclination angle of the optical waveguide 3c and the inclination angle of the principal ray PR increases as the angle of view increases. Hence, the effect of the structure of the first example is exerted particularly in a superwide-angle imaging apparatus whose angle of view exceeds ±45 degrees.

As illustrated in Fig. 3A, in the imaging apparatus 1 of the first example, the position of the curvature center Cr5 of the incident surface 251 of the fifth lens 25 is located closer to the object side than the position of the curvature center Cimg of the emission surface 252 of the fifth lens 25. For this reason, the light beam propagating inside the optical waveguide can be made closer to the longitudinal direction of the optical waveguide than when the fifth lens 25 is not disposed.

Further, in the imaging apparatus 1 of the first example, the position of the curvature center Cr5 of the incident surface 251 of the fifth lens 25 is located closer to the object side than the exit pupil position Pe of the imaging optical system 2. For this reason, the incident surface 251 itself of the fifth lens 25 can also make the light beam propagating inside the optical waveguide close to the longitudinal direction of the optical waveguide.

In the imaging apparatus 1 of the first example, the refractive index N_img of the medium just in front of the image surface 20, that is, of the fifth lens 25 is set at a very high refractive index of 1.9108. Hence, the spot diameter of the light beam BM on the image surface 20 can be reduced. For this reason, high resolution can be achieved.

Second Example

The structure of an imaging apparatus 51 according to a second example will be described with reference to Fig. 5. Fig. 5 is a schematic view illustrating the structure of the imaging apparatus 51. The second example is different from the first example in the structure of an imaging optical system and the shape of an incident surface of a light guide.

Fig. 6 is a structural view of an imaging optical system 52 of the second example. The imaging optical system 52 includes a first lens 521, a second lens 522, a third lens 523, a fourth lens 524, a fifth lens 525, and an aperture stop 529. As the specifications of the imaging optical system 52, the focal length f is 10.0 mm, the F-number is F/1.4, and the maximum angle ω of view is ±60 degrees. Thus, the imaging optical system 52 has a superwide angle of view.

The first lens 521 is a meniscus lens that is convex to the object side, and has an incident surface 521a serving as an R1 surface and an emission surface 521b serving as an R2 surface. The second lens 522 is a plano-convex lens that is convex to the object side, and has an incident surface 522a serving as the R2 surface and an emission surface 522b serving as an STO surface. The third lens 523 is a meniscus lens that is convex to the image side, and has an incident surface 523a serving as the STO surface and an emission surface 523b serving as an R3 surface. The fourth lens 524 is a meniscus lens that is convex to the image side, and has an incident surface 524a serving as the R3 surface and an emission surface 524b serving as an R4 surface. The fifth lens 525 is an aspherical lens having an incident surface 525a serving as an R5 surface and an emission surface 525b serving as an IMG surface. Table 3 shows exemplary structures of the lenses 521 to 525 in the imaging optical system 52. Table 4 shows examples of aspheric coefficients of the fifth lens 525.

The incident surface 525a and the emission surface 525b of the fifth lens 525 are rotation-symmetric aspherical surfaces centered on the optical axis AX. A sag amount z (mm) of the aspherical shape of the fifth lens 525 along the optial axis AX is expressed by a polynomial expression of Expression 9. Here, the curvature on the optical axis AX is designated as c (1/mm), the radial distance of the aspherical shape from the optical axis AX is designated as r (mm), and four-, six-, eight-, and ten-order coefficients are designated as A, B, C, and D, respectively.

Fig. 7 is a schematic view illustrating the structure of the fifth lens 525. The imaging apparatus 51 is also configured so that the emission surface 525b of the fifth lens 525 coincides with an image surface 20. The image surface 20 has an aspherical shape. In the second example, the aspherical shape of the fifth lens 525 in the second example is such that the curvature decreases from the optical axis AX toward the off-axis side with respect to the base spherical surface.

This structure can reduce the radius of curvature of the image surface 20, and a normal 5251 to an incident surface 3ca of an optical waveguide 3c outside the axis is greatly inclined with respect to the optical axis AX of the imaging optical system 2 so that the incident angle of an off-axis ray (a light beam BM far from the optical axis AX) on the optical waveguide 3c becomes large. Thus, the angle of the light beam propagating inside the optical waveguide 3c after incidence on the optical waveguide 3c can be made closer to a longitudinal direction 311 of the optical waveguide 3c.

Table 5 shows example of parameters relating to the optical waveguide 3c disposed at the farthest position from the optical axis AX of the imaging apparatus 51.

An inclination angle α of the axis 311 of the optical waveguide 3c with respect to the optical axis AX of the imaging optical system 52 is 35.00 degrees. An inclination angle θ_p of the principal ray PR of the light beam BM incident on the optical waveguide 3c is 47.80 degrees. The inclination angle α of the optical waveguide 3c in the light guide 3 is smaller than the angle of the incident light beam. That is, Expression 1 is satisfied. Further, an inclination angle β of the surface normal to the incident surface 3ca of the optical waveguide 3c is 73.05 degrees. That is, the optical waveguide 3c satisfies Expression 2. From the above, the light guide 3 includes the optical waveguide 3c that is far from the optical axis AX and satisfies Expression 1 and Expression 2.

An intersection Pwg of the axis 311 of the optical waveguide 3c from the image surface 20 and the optical axis AX of the imaging optical system 52 is located closer to the object side than an exit pupil position Pe of the imaging optical system 52. Specifically, a distance L_wg along the optical axis AX from the intersection of the image surface 20 and the optical axis AX to the intersection Pwg is 4.28 mm, and a distance L_pe along the optical axis AX from the image surface 20 to the exit pupil position Pe of the imaging optical system 52 is 3.28 mm.

In the imaging apparatus 51, the distance L_pe along the optical axis AX from the image surface 20 to the exit pupil position Pe of the imaging optical system 52 is 3.28 mm, and a distance L_ci from the image surface 20 to the intersection of the normal to the incident surface 3ca of the optical waveguide 3c and the optical axis AX is 2.124 mm. This satisfies Expression 5. Further, a ratio Ri (=L_ci/L_pe) of the distance L_ci to the distance L_pe is 64.80%.

When the ratio Ri is lower than or equal to 95%, the emission angle of light emitted from the emission surface 3cb of the optical waveguide 3c can be decreased. As the value of the ratio Ri decreases, the effect of decreasing the emission angle of light emitted from the emission surface 3cb of the optical waveguide 3c increases. In the second example, the value of the ratio Ri is decreased and the light emission angle is further decreased by greatly shortening the distance L_ci.

In the second example, the refractive index N_img of a medium just in front of the image surface 20 is N₅ = 1.9108, and the refractive index N_co of the core part 3co of the optical waveguide 3c is 1.8200. Thus, the condition N_img > N_co is satisfied.

According to this structure, the incident angle θ_i on the image surface 20 is 25.25 degrees, and the emission angle θ_o is 27.97 degrees. An angle difference Δθ_o therebetween is +2.72 degrees, which satisfies Expression 7. Thus, the inclination angle θ_po of the principal ray PR inside the optical waveguide 3c is 45.08 degrees. Since the inclination angle θ_p of the principal ray PR at incidence on the incident surface 3a is 47.78 degrees, the angle θ_po (beam angle) formed by the optical axis AX and the principal ray PR after incidence on the optical waveguide 3c approaches the inclination angle α of the optical waveguide 3c. Hence, the emission angle of the light beam emitted from the emission surface 3cb of the optical waveguide 3c can be decreased.

Table 6 shows parameters relating to the light beam BM before and after passing through the fifth lens 525 in the second example. Fig. 8 is a schematic view illustrating the structure of the fifth lens 525.

The principal ray PR of the light beam BM is incident on an arbitrary point Pir5 on the incident surface 525a of the fifth lens 525, and an intersection of the principal ray PR of the light beam BM and the optical axis AX is designated as Pr5. At this time, an angle formed by the optical axis AX and the principal ray PR of the light beam BM is designated as θ_pr5. An intersection of a surface normal 5251 to the arbitrary point Pir5 on the incident surface 525a of the fifth lens 525 and the optical axis AX is designated as Cr5. At this time, an angle (surface normal angle) formed by the optical axis AX and the surface normal 5251 is designated as β_r5.

The incident surface 525a (R5 surface) of the fifth lens 525 is configured so that the normal angle β_r5 is smaller than the beam angle θ_pr5 of the principal ray PR of the incident light beam BM. According to Table 6, the beam angle θ_pr5 is 59.43 degrees, whereas the normal angle β_r5 is 36.68 degrees. A distance L_cr5 along the optical axis AX between the intersection Cr5 and the incident surface 525a is longer than a distance L_pr5 along the optical axis AX between the intersection Pr5 and the incident surface 525a.

The length L_cr5 (= 3.06 mm) along the optical axis AX from the incident surface 525a of the fifth lens 525 to the intersection Cr5 is longer than the length L_pe (= 1.71 mm) from the incident surface 525a of the fifth lens 525 to the exit pupil position Pe. That is, the intersection Cr5 is located closer to the object side than the exit pupil position Pe.

According to this structure, the beam angle θ_p of the principal ray PR that is incident from the incident surface 525a of the fifth lens 525 and passes inside the fifth lens 525 can be made smaller than the beam angle of the principal ray PR before incidence on the fifth lens 525. Specifically, as shown in Table 6, the beam angle θ_pr5 at incidence on the incident surface 525a of the fifth lens 525 is 59.43 degrees, whereas the beam angle θ_p of the principal ray PR propagating inside the fifth lens 525 is 47.80 degrees. The beam angle θ_p is smaller than the beam angle θ_pr5.

In this way, the incidence angle on the image surface 20 can be set small by setting the surface normal angle to the incident surface of the lens just in front of the image surface 20 to be smaller than the angle of the principal ray of the light beam incident on the incident surface. Further, the light receiving sensitivity of the imaging element can be increased by decreasing the emission angle from the light guide.

Third Example

An imaging apparatus 91 according to a third example will be described with reference to Fig. 9. Fig. 9 is a schematic view illustrating the structure of the imaging apparatus 91. The imaging apparatus 91 includes an imaging optical system 92, a light guide 3, and a sensor 4. The imaging apparatus 91 is different from the first example in the structure of the imaging optical system 92 and the inclination angle of optical waveguides 3c in the light guide 3. Detailed descriptions of structures similar to those adopted in the first example are skipped.

Fig. 10 is a structural view of the imaging optical system 92 in the third example. The imaging optical system 92 includes a first lens 921, a second lens 922, a third lens 923, a fourth lens 924, a fifth lens 925, a sixth lens 926, and an aperture stop 929. As the specifications of the imaging optical system 92, the focal length f is 7.0 mm, the F-number is F/2.0, and the maximum angle ω of view is ±80 degrees. That is, the angle of view is superwide.

The first lens 921 is a spherical lens having an incident surface 921a serving as an R1 surface and an emission surface 921b serving as an R2 surface. The second lens 922 is a spherical lens having an incident surface 922a serving as the R2 surface and an emission surface 922b serving as an R3 surface. The third lens 923 is a plano-convex lens having an incident surface 923a serving as the R3 surface and an emission surface 923b serving as an STO surface. The fourth lens 924 is a plano-convex lens having an incident surface 924a serving as the STO surface and an emission surface 924b serving as an R4 surface. The fifth lens 925 is a spherical lens having an incident surface 925a serving as the R4 surface and an emission surface 925b serving as an R5 surface. The sixth lens 926 is a spherical lens having an incident surface 926a serving as an R6 surface and an emission surface 926b serving as an IMG surface. All of the first to sixth lenses 921 to 926 are spherical lenses. Table 7 shows an exemplary structure of the imaging optical system 92.

In the imaging apparatus 91, the emission surface 926b of the sixth lens 926 disposed closest to the image side among a plurality of lenses also serves as an image surface 20. Further, the emission surface 926b of the sixth lens 926 nearly coincides with an incident surface 3a of the light guide 3. While the imaging optical system 92 has an angle of view wider than that of the first example, it is characterized in that the angle of the principal ray of the light beam incident on the image surface 20 is much smaller than the angle of view.

Table 8 shows parameters of the imaging apparatus 91.

In the third example, an inclination α of an axis 311 of an optical waveguide 3c located at the farthest position from the optical axis AX of the imaging optical system 92 (inclination angle of the optical waveguide 3c) with respect to the optical axis AX is 30.00 degrees. An inclination angle θ_p of the principal ray PR of the light beam BM incident on the optical waveguide 3c is 36.16 degrees. An inclination angle β of a surface normal to an incident surface 3ca of the optical waveguide 3c is 56.39 degrees. That is, the light guide 3 includes the optical waveguide 3c that is far from the optical axis AX and satisfies Expression 1 and Expression 2.

An intersection Pwg of the longitudinal axis 311 of the optical waveguide 3c from the image surface 20 and the optical axis AX of the imaging optical system 92 is located closer to the object side than an exit pupil position Pe of the imaging optical system 92. Specifically, a distance L_wg along the optical axis AX from the image surface 20 to the intersection Pwg of the axis of the optical waveguide 3c and the optical axis AX of the imaging optical system 92 is 7.06 mm, and a distance L_pe along the optical axis AX from the image surface 20 to the exit pupil position Pe of the imaging optical system 92 is 5.93 mm. Hence, Expression 5 is satisfied. Further, a ratio Ri is 63.05%. Here, the emission angle of light emitted from each optical waveguide 3c in the light guide 3 is also decreased by shortening the distance L_ci to decrease the ratio Ri, similarly to the second example.

According to Table 8, a refractive index N_img (= N₆) of the sixth lens 926 serving as a medium just in front of the image surface 20 is 2.0027, and a refractive index N_co of a core part 3co of the optical waveguide 3c is 1.8200. Thus, Expression 6 is satisfied.

An incident angle θ_i on the incident surface 3a of the light guide 3 is 20.23 degrees, whereas an emission angle θ_o from the incident surface 3a is 22.37 degrees, which is larger than the incident angle θ_i. An angle difference Δθ_o is +2.13 degrees. That is, Expression 7 is satisfied.

Thus, a beam angle θ_p at incidence of the light beam BM on the incident surface 3a is 36.16 degrees, whereas a beam angle θ_po of the light beam BM propagating inside the optical waveguide 3c is 34.02 degrees. In this way, the propagating direction of the principal ray of the light beam propagating inside the optical waveguide 3c can be made close to the axial direction α of the optical waveguide 3c. For this reason, the angle of the light beam emitted from the emission surface of the optical waveguide 3c can be kept down.

Table 9 shows parameters relating to the light beam BM before and after passing through the sixth lens 926 of the third example.

A surface normal angle β_r6 of the incident surface 926a (R6 surface) of the sixth lens 926 in the third example is set to be smaller than a beam angle θ_pr6 of the principal ray PR of the incident light beam BM. Specifically, according to Table 9, the beam angle θ_pr6 is 42.0 degrees, whereas the normal angle β_r6 is 30.4 degrees.

A length L_r6 from the incident surface 926a of the sixth lens 926 to an intersection Cr6 of the normal to the incident surface 926a of the sixth lens 926 and the optical axis AX is 5.43 mm, and is longer than a distance L_pe of 3.80 mm from the incident surface 926a of the sixth lens 926 to the exit pupil position Pe. That is, the intersection Cr6 of the surface normal to the incident surface 926a of the sixth lens 926 and the optical axis AX is located closer to the object side than the exit pupil position Pe.

This arrangement allows the beam angle θ_p of the light beam emitted from the incident surface 926a of the sixth lens 926 and passing through the sixth lens 926 to be set small.

As shown in Table 9, the beam angle θ_p inside the sixth lens 926 is 36.2 degrees, and is smaller than the beam angle θ_pr6 of 42.0 degrees at the time of incidence on the incident surface 926a of the sixth lens 926.

By thus setting the surface normal angle of the incident surface of the lens disposed just in front of the image surface 20 to be smaller than the angle of the principal ray of the light beam incident on the incident surface, the incident angle on the image surface 20 can be set small. Further, the light receiving sensitivity of the sensor can be increased by decreasing the emission angle from the light guide.

While the embodiment and examples of the present invention have been described above, the present invention is not limited to these embodiment and examples, and various modifications and changes can be made within the scope of the invention. For example, the shape of the emission surface (connecting surface) 3b of the light guide 3 is not limited to the shape adopted in the above-described embodiment and examples, and it is only necessary that the shape of the emission surface 3b should be a curved surface such as an elliptic surface, a conical surface, a hyperboloidal surface, or an aspherical surface. The shape of the curved surface can be changed according to distortion to be reduced.

While the light guide includes the tapered section and the straight section in the embodiment and examples described above, a single light guide or a combination of a plurality of light guides may be used.

The imaging apparatuses of the embodiment and examples are also applicable to an imaging apparatus using an imaging unit for infrared light (wavelength of 0.7 to 15 μm). At this time, an imaging optical system, a light guide, and an imaging element that are adaptable to infrared light can be used.

The imaging apparatus according to the present invention can be applied to products using an imaging apparatus, for example, a digital camera, a digital video camera, a camera for a mobile phone, a monitoring camera, and a fiberscope.

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.2015-142509, filed July 16, 2015, which is hereby incorporated by reference herein in its entirety.

Claims

An imaging apparatus comprising:
an imaging optical system configured to form an image of an object;
an imaging element configured to pick up the image of the object; and
a plurality of optical waveguides configured to guide light from the imaging optical system to the imaging element,
wherein a refractive index of a medium adjacent to an incident surface of each of the optical waveguides is higher than a refractive index of a center part of the optical waveguide, and
wherein the following condition is satisfied:
0 < α < θ_p < β
where α represents an angle formed by an axis of the optical waveguide and an optical axis of the imaging optical system, θ_p represents an angle formed by a principal ray of a light beam incident on the optical waveguide and the optical axis, and β represents an angle formed by a surface normal to a center of the incident surface of the optical waveguide and the optical axis.
The imaging apparatus according to Claim 1, wherein an intersection of the surface normal to the center of the incident surface of the optical waveguide and the optical axis is located closer to an image side than an exit pupil of the imaging optical system.
The imaging apparatus according to Claim 1, wherein the imaging optical system forms the image of the object on the incident surface of each of the plurality of optical waveguides.
The imaging apparatus according to Claim 1, wherein the following condition is satisfied:

where N_img represents the refractive index of the medium, N_co represents the refractive index of the center part, N_cl represents a refractive index of a peripheral part of the optical waveguide, and ρ represents an angle formed by the principal ray of the light beam incident on the optical waveguide and a marginal ray of the light beam on a side of the optical axis.
The imaging apparatus according to Claim 4, wherein the following condition is satisfied:
The imaging apparatus according to Claim 1, wherein an exit pupil of the imaging optical system is located closer to an image side than an intersection of the axis of the optical waveguide and the optical axis.
The imaging apparatus according to Claim 1, wherein the medium is a lens disposed closest to an image side in the imaging optical system.
The imaging apparatus according to Claim 7, wherein the imaging optical system forms the image of the object on an emission surface of the lens disposed closest to the image side.
The imaging apparatus according to Claim 7, wherein the incident surface of each of the plurality of optical waveguides adjoins an emission surface of the lens disposed closest to the image side.
The imaging apparatus according to Claim 7, wherein the lens disposed closest to the image side has a positive power.
The imaging apparatus according to Claim 7, wherein a curvature center of an incident surface of the lens disposed closest to the image side is located closer to an object side than a curvature center of an emission surface of the lens disposed closest to the image side.
The imaging apparatus according to Claim 7, wherein the lens disposed closest to the image side is a meniscus lens with a convex surface facing toward the image side.
The imaging apparatus according to Claim 12, wherein a radius of curvature of an emission surface of the meniscus lens is larger than a radius of curvature of an incident surface of the meniscus lens.
The imaging apparatus according to Claim 7, wherein the lens disposed closest to the image side includes an aspherical surface.
The imaging apparatus according to Claim 14, wherein a curvature center of an incident surface of the lens disposed closest to the image side is located closer to an object side than an intersection of the principal ray of the light beam incident on the optical waveguide and the optical axis.
The imaging apparatus according to Claim 7, wherein an incident surface and an emission surface of the lens disposed closest to the image side are spherical.
The imaging apparatus according to Claim 16, wherein an intersection of a surface normal to the incident surface of the lens disposed closest to the image side and the optical axis is located closer to an object side than an exit pupil of the imaging optical system.
The imaging apparatus according to Claim 16, wherein the angle formed by the principal ray of the light beam incident on the optical waveguide and the optical axis is larger than an angle formed by the optical axis and a surface normal to a position on the incident surface of the lens disposed closest to the image side where the principal ray is incident.