JP2001330799A - Optical low-pass filter and optical apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical low-pass filter which is small in thickness and with which a good low-pass effect may be effectively obtained and good optical performance may be easily obtained and an optical apparatus using the same. SOLUTION: This low-pass filter is formed by combining and joining plural double refractive plates of lithium niobate single crystals in such a manner that the optical axes of the plural double refractive plates face the directions varying from each other and constituting these plates to a parallel flat plate form.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical low-pass filter and an optical apparatus (photographing apparatus) using the same, and is suitable for an electrophotographic camera, a video camera, a digital camera and the like using a solid-state photographing element such as a CCD. Things.

[0002]

2. Description of the Related Art In an imaging apparatus such as a digital still camera or a video camera using a two-dimensional solid-state imaging device such as a CCD or a MOS, a component having a high spatial frequency is sampled in order to sample a subject image at each pixel pitch. When photographing a held object, a false resolution signal in which a folded image of a high-frequency component is output as a low-frequency component is generated, which causes a reduction in the resolution of the object image. Similarly, in an image pickup apparatus using a single-plate color solid-state image pickup device, when an object having a high spatial frequency component is photographed, a false color signal determined by an arrangement of a color filter arranged in front of each pixel is generated. This is a factor that reduces the color reproducibility of the subject image.

Conventionally, there is an optical low-pass filter as a means provided in an optical path of a photographing system to reduce a false resolution signal or a false color signal due to such a high frequency component of a subject image.
Various proposals have been made. As the most typical optical low-pass filter, there is an optical low-pass filter composed of a single crystal parallel plate of quartz. Generally, when the normal to the incident surface and the optical axis (Z-axis) of the crystal are arranged at a predetermined angle,
The light beam incident on the parallel plate of quartz, which is a uniaxial crystal, is Z
It shows anisotropy in the direction of the axis and separates it into ordinary rays and extraordinary rays.
The separated light beams exit from the parallel plate in parallel. At this time, the separation width between the ordinary ray and the extraordinary ray is determined by the angle between the normal to the plane of incidence of the parallel plate and the Z axis of the crystal and the thickness of the parallel plate.

Optical low-pass filters utilizing the optical action of quartz are disclosed in, for example, Japanese Utility Model Publication No. 47-18688, Japanese Utility Model Publication No. 47-18689, and Japanese Patent Application Laid-Open No. Sho 59-7.
No. 5222 and Japanese Patent Application Laid-Open No. 60-164719.

Japanese Utility Model Publication No. 47-18688, Japanese Utility Model Publication No. 4
In Japanese Patent Application Laid-Open No. 7-18689, a stripe-shaped filter is assumed as a color filter. In order to reduce a false color signal generated when the spatial frequency of a subject is synchronized with the color filter, a birefringent material such as quartz is used. There is disclosed a configuration in which a light beam is separated into an ordinary light beam and an extraordinary light beam by a parallel plate having an image and is formed on an imaging surface. In particular,
Japanese Patent Publication No. 18689 discloses a configuration in which a single crystal of quartz is cut out and used so that its optical axis (Z-axis) forms an angle of approximately 45 ° with respect to an input / output surface of a parallel flat plate.

In Japanese Patent Application Laid-Open Nos. 59-75222 and 60-164719, for example, a Bayer-arrayed filter as shown in FIG. 4 is assumed as a color filter, and a plurality of birefringent plates are used. By combining them, the light beam is separated into an ordinary light beam and an extraordinary light beam, and the subject image is separated into a plurality of images shifted by a predetermined pitch to form an image, and a false resolution signal or a false color signal generated by a high frequency component of the subject is generated. A configuration for effectively reducing the occurrence is disclosed.

Further, examples of using a single crystal plate other than quartz as a birefringent plate are disclosed in JP-A-9-212222 and JP-A-11-218612. JP-A-9-212222, JP-A-11-218612
Japanese Patent Application Laid-Open Publication No. H11-157210 proposes an optical low-pass filter using a plurality of birefringent plates and using lithium niobate for at least one of the birefringent plates. A single crystal of lithium niobate is a uniaxial crystal like a single crystal of quartz, but since the difference between the refractive index of the ordinary ray and that of the extraordinary ray is larger than that of quartz,
There is a feature that the thickness of the birefringent plate required to obtain a predetermined beam separation width can be reduced.

[0008]

In the prior art in which one or more quartz birefringent plates are combined and used as an optical low-pass filter, the difference in the refractive index between the ordinary ray and the extraordinary ray is small. In order to secure a predetermined amount, it was necessary to increase the thickness of the birefringent plate to a certain degree or more.

In general, when a parallel flat plate of a uniaxial crystal is formed so that the angle between the normal to the plane of incidence and the optical axis (Z-axis) is θ, when circularly polarized light is incident perpendicularly, The angle φ between the traveling direction of the ordinary ray and the traveling direction of the extraordinary ray in the flat plate is expressed by the following equation.

[0010] ^{_{tanφ = (n o 2 -n e}} 2) sinθcosθ / (n e
² cos ² θ + n. ² sin ² θ)) ·· (a) The expression a represents the separation width between the ordinary ray and the extraordinary ray per unit thickness of the birefringent plate. When θ = 45 °, this value becomes the maximum.

[0011] In the crystal of the single crystal, with respect to D lines, the ordinary ray refractive index n _o = 1.544, the refractive index n _e = 1 for the extraordinary ray.
533, if θ = 45 °, tanφ ≒ −0.
0058. Therefore, for example, assuming that the pixel pitch Ph in the long side direction of the solid-state imaging device is 10 μm, an extraordinary ray is shifted by 10 μm in this direction by a quartz crystal birefringent plate in order to remove a false resolution signal generated in this direction. , It is necessary that the thickness d of the birefringent plate of quartz be at least about 1.7 mm. As described above, when a crystal having a small difference in the refractive index between the ordinary ray and the extraordinary ray is used as the birefringent plate, the optical low-pass filter becomes thicker,
Space issues occur.

Further, in the case where a parallel flat plate exists in the optical path of the photographing optical system, there is a problem as described below in addition to a space problem.

The light beam imaged by the ideal lens and passing through the parallel plate is refracted at a refraction angle in accordance with the well-known Snell's law of refraction according to the incident angle α to the parallel plate and exits in parallel with the incident light beam. The expected focal plane of the ideal lens is determined by paraxial theory using the approximation of sinα ≒ α to determine the optical path length in the parallel plate, and the air-equivalent optical path length d expressed using the plate thickness d and the refractive index N of the parallel plate. / N is set at a position shifted in the optical axis direction by the amount replaced by / N. However, the angle of incidence of the light beam incident on the parallel plate becomes large,
If the approximation of α ≒ α does not match the actual state, the light beam emitted from the ideal lens will not form an image on the predetermined image formation plane.
Specifically, astigmatism occurs in which the spherical aberration is over the optical axis and the meridional image plane is more over the sagittal image plane outside the optical axis.

FIG. 9 is an explanatory diagram for explaining such a phenomenon. F2.
Birefringence in the case where an ideal lens having an exit pupil of 0 is arranged, and a crystal birefringent plate made of a parallel flat plate having a thickness of 5 mm and having an entrance / exit surface perpendicular to the optical axis is arranged between the ideal lens and the ideal image plane. FIG. 4 is a diagram illustrating ray aberration generated by a plate. In FIG. 9A, reference numeral 41 denotes an ideal lens having a focal length of 50 mm, reference numeral 42 denotes a birefringent plate of quartz having a thickness of 5 mm, and reference numeral 43 denotes a planned focal plane on paraxial calculation. Reference numeral 44 denotes the vicinity of the image plane at the image height of 0 mm, and reference numeral 45 denotes the vicinity of the image plane at the image height of 20 mm.

The light beam formed by the ideal lens 41 and passing through the parallel plate 42 travels to form an image near the predetermined focal plane 43 as shown in FIG. However, when the image plane vicinity 44 and 45 are enlarged, they become as shown in FIG. 9B and FIG. 9C, respectively. That is, the best image plane position at an image height of 0 mm is 46, and the image height is 20 m.
The best image plane position at m is the position indicated by 47, and spherical aberration is over at the center of the screen, and astigmatism is over at the periphery of the screen, especially on the meridional image plane.

As described above, when the parallel flat plate is inserted between the photographing optical system and its expected focal plane, there is a property that the above-described ray aberration occurs. Therefore, in a photographing optical system on the premise that a parallel flat plate such as a crystal birefringent plate is arranged, it is general practice to design a photographing optical system in consideration of aberrations generated in the parallel flat plate. However, an image pickup apparatus (for example, a single-lens reflex type) having a replaceable photographing optical system so that various interchangeable lenses as a photographing optical system, particularly an interchangeable lens system completed for a silver halide camera can be used as the photographing optical system. Of course, such a countermeasure is not possible in digital still cameras).

Incidentally, in a single crystal of lithium niobate, which is a uniaxial crystal similar to quartz, the refractive index n of ordinary light with respect to d-line. = 2.300, the refractive index of extraordinary ray n _e =
Assuming 2.215, when θ = 45 °, tan φ ≒ 0.
0376.

Here, as in the case of quartz, the pixel pitch Ph in the long side direction of the solid-state imaging device is assumed to be 10 μm.
To remove spurious resolution signals generated in this direction, an extraordinary ray is shifted in this direction by a birefringent plate of lithium niobate.
Assuming that the structure is shifted by 0 μm, since the lithium niobate single crystal has tan φ ≒ 0.0376, it can be understood that the thickness d of the birefringent plate may be about 0.27 mm. Compared to the case of using the above-mentioned single crystal of quartz,
The thickness of the birefringent plate can be reduced to about 0.16 times,
By applying a lithium niobate birefringent plate to the imaging system as an optical low-pass filter, it is possible to solve the space problem and realize a smaller imaging system and to reduce the problem of ray aberration caused by the parallel plate. Is also possible.

The above-mentioned JP-A-9-212222 and JP-A-11-218612 focus on such properties of a lithium niobate single crystal.

Here, a pixel pitch of 10 μm, an aspect ratio of 2: 3, and an effective pixel number of about 2.5 million pixels
Assuming an imaging device using a three-dimensional solid-state imaging device, the number of pixels in the horizontal direction is about 1950 pixels, the number of pixels in the vertical direction is about 1300 pixels, and the size of the effective pixels of the solid-state imaging device is 19. It is about 5 mm, and about 13.0 mm in the vertical direction. Considering that an optical low-pass filter composed of a parallel plate is disposed relatively close to the front of the solid-state imaging device (subject side), the size of the parallel plate is determined by the solid angle of a light beam incident on the solid-state imaging device. Is added to the area for holding the parallel flat plate itself in consideration of the effective light area, and at least in the horizontal direction.
It needs to be about 0 mm and about 14.5 mm in the vertical direction.

On the other hand, when it is desired to separate extraordinary rays by a pixel pitch of 10 μm in the horizontal direction, the angle θ between the optical axis (Z axis) of the single crystal of lithium niobate and the normal to the plane of incidence of the parallel plate is 45 °. As described above, the thickness of the parallel flat plate is about 0.27 mm as described above. In addition, real Kimiaki 4
As disclosed in Japanese Patent Application Laid-Open No. 7-18688, in consideration of removing the influence of a false signal due to a half frequency component of a pixel pitch, an abnormality is made in a direction forming an angle of about 45 ° with a horizontal line of a screen. If a parallel plate for separating light beams is used, the thickness of the parallel plate is about 0.19 mm.
The thickness of such a parallel flat plate is about 1% or less as compared with the diagonal length determined by the above-mentioned external dimensions, and becomes extremely fragile due to insufficient mechanical strength.

The above-mentioned JP-A-9-212222 and JP-A-11-218612 propose an optical low-pass filter in which lithium niobate and quartz are joined. These conventional examples can be made thinner than an optical low-pass filter made entirely of quartz, but are still not enough to solve the above-mentioned space problem and the problem of the optical performance of the photographing optical system. Was enough.

An object of the present invention is to propose an optical low-pass filter suitable for reducing generation of a false signal based on periodic sampling of a solid-state imaging device and an optical apparatus using the same. In particular, the present invention is suitable for configuring a single-lens reflex type digital camera that can effectively utilize various interchangeable lenses prepared as being mounted on a single-lens reflex camera using a silver halide film.

One of the objects of the present invention is to make the structure sufficiently thin so that it can be provided in the body of a single-lens reflex type digital camera, and not to reduce the resolving power of the interchangeable lens more than necessary. Another object of the present invention is to provide an optical low-pass filter which does not cause ghost or flare when used in a photographing system.

The interchangeable lens of a single-lens reflex camera using a silver halide film maintains a sufficiently long distance (back focus) from the lens surface closest to the image side to the focal plane, and
It is designed to secure space for a rotating mirror and space for a focal plane shutter to realize a TTL finder without parallax.

In order to realize a single-lens reflex digital camera using a solid-state imaging device instead of a silver halide film, it is sufficient to arrange the imaging surface of the solid-state imaging device at the position of the silver halide film and at a transparent position. However, when an optical low-pass filter using a plurality of crystal birefringent plates is arranged in a camera as disclosed in the first conventional example, the optical low-pass filter interferes with a rotating mirror or a focal plane shutter. The problem occurs.

On the other hand, in an interchangeable lens of a single-lens reflex camera using a silver halide film, it is generally not assumed that there is a reflecting member between the lens surface closest to the image side and the focal plane. It is a target. That is, if an optical member having a reflectance of a predetermined value or more in the visible wavelength region is disposed between the imaging optical system and its focal plane, harmful light, such as surface reflection ghosts and flare spots, which lowers the contrast of the subject image. There is a problem that it is likely to be a cause of occurrence.
Therefore, it is necessary to make the reflectivity of the optical low-pass filter in the visible wavelength range sufficiently low.

Another object of the present invention is to overcome the above problems and to provide an optical low-pass filter suitable for being provided in a single-lens reflex digital camera using an interchangeable lens prepared mainly for a silver halide camera. Is to make it happen.

[0029]

According to a first aspect of the present invention, there is provided an optical low-pass filter comprising a plurality of lithium niobate single crystal birefringent plates such that the optical axes of the plurality of birefringent plates are oriented in different directions. It is characterized by being combined and joined to form a parallel plate.

According to a second aspect of the present invention, in the first aspect of the present invention, at least two of the plurality of lithium niobate single crystal birefringent plates have their optic axes parallel to the plane of incidence or the plane of incidence of the parallel plate. It is characterized in that the projections are set at an angle of 45 degrees with each other.

According to a third aspect of the present invention, when the plurality of lithium niobate single crystal birefringent plates are joined to each other, the surface of the birefringent plate is provided with an adhesive and a birefringent. The present invention is characterized in that a dielectric thin film having an anti-reflection effect for light in the visible wavelength region at the interface of the plate is added.

According to a fourth aspect of the present invention, in the first, second or third aspect of the present invention, the optical axis of the plurality of lithium niobate single crystal birefringent plates and the normal to the entrance plane or exit plane of the parallel flat plate. Is defined as θo, at least one birefringent plate satisfies one of the following conditional expressions: 10 ° <θo <27 ° 61 ° <θo <80 °.

An optical low-pass filter according to a fifth aspect of the present invention is an optical low-pass filter in which three sheets of lithium niobate single crystal birefringent plates are joined to form a parallel flat plate. The birefringent plate of the crystal is characterized in that the orthogonal projection of its optical axis onto the plane of incidence or the plane of emergence of the parallel plate makes an angle of 45 degrees with each other.

According to a sixth aspect of the present invention, in the invention of the fifth aspect, the three lithium niobate single crystal birefringent plates are first, second, and third birefringent plates in order from the incident and incident sides. The separation widths of the ordinary ray and the extraordinary ray at the exit surface of each of the first birefringent plate, the second birefringent plate, and the third birefringent plate are Dl and D, respectively.
When D2 and D3 are satisfied, a conditional expression of D1 ≒ D3> D2 is satisfied.

According to a seventh aspect of the present invention, when the angle between the optical axis of the second birefringent plate and the normal of the plane of incidence or exit of the parallel plate is θ2 in the fifth or sixth aspect of the invention. , 10 ° <θ2 <27 °, 61 ° <θ2 <80 °.

According to an eighth aspect of the present invention, there is provided an optical apparatus using the optical low-pass filter according to any one of the first to seventh aspects.

According to a ninth aspect of the present invention, there is provided an optical apparatus for forming an image on a photographing device through an optical low-pass filter, wherein the optical low-pass filter includes a plurality of lithium niobate single crystal birefringent plates. The orthogonal projection of the optical axes of the birefringent plates of the plurality of lithium niobate single crystals onto the entrance surface or the exit surface of the parallel plate forms an angle of approximately 45 ° with each other. At least one of the orthogonal projections is configured to form an angle of approximately 45 ° with the long side direction of the imaging device.

An optical apparatus according to a tenth aspect of the present invention is an optical apparatus for forming an image on a photographing element via an optical low-pass filter, wherein the optical low-pass filter comprises three lithium bicarbonate single crystal birefringent plates. The birefringent plate of lithium niobate single crystal is formed by joining the birefringent plate of lithium niobate, and the orthographic projection of the optical axis of the birefringent plate on the incident surface or the exit surface of the parallel plate is substantially the same as the long side direction of the image sensor. 45 ° first angle
A birefringent plate, a second birefringent plate forming an angle of approximately 45 ° with the orthographic projection of the first birefringent plate, and a birefringent plate forming an angle of approximately 90 ° with the orthographic projection of the first birefringent plate. A third birefringent plate and a third birefringent plate are combined in this order.

According to an eleventh aspect of the present invention, in the invention of the tenth aspect, the separation of the ordinary ray and the extraordinary ray at the exit surface of each of the first birefringent plate, the second birefringent plate, and the third birefringent plate. When the widths are D1, D2, and D3, respectively, the configuration is such that the conditional expression of D1 ≒ D3> D2 is satisfied.

According to a twelfth aspect of the present invention, when the angle between the optical axis of the second birefringent plate and the normal to the entrance surface or the exit surface of the parallel plate is θ2, , 10 ° <θ2 <27 °, 61 ° <θ2 <80 °.

According to a thirteenth aspect of the present invention, in the first aspect of the present invention, the plurality of lithium niobate single crystal birefringent plates have an optical axis and an incident surface of the parallel plate,
Alternatively, when at least one birefringent plate has an angle of θo with respect to the normal line of the exit surface, 10 ° <θo <27 ° 61 ° <θo <80 ° is characterized.

[0042]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical low-pass filter according to the present invention is arranged in an optical path between a photographing optical system 1 and an image pickup device 3, and comprises a plurality of lithium niobate single crystal birefringent plates. Are joined together so that their optical axes face different directions to form a parallel plate.

As described above, in the present invention, each of the birefringent plates of the optical low-pass filter having a function of separating a subject image in a predetermined direction is made of a single crystal of lithium niobate, so that the processing can be performed within a processable range. A sufficiently thin parallel plate can be provided in the main body of a single-lens reflex digital camera, and can be used without deteriorating the optical performance of an interchangeable lens prepared for a silver halide camera.

Further, when the optical low-pass filter of the present invention is applied to an optical apparatus having a photographing element, the incident surface of the parallel plate of the optical axis of the plurality of birefringent plates of lithium niobate single crystal; Alternatively, the orthogonal projection on the exit surface is configured to form an angle of approximately 45 ° with each other, and at least one of the orthogonal projections is approximately 45 ° with the long side direction of the image sensor.
It is configured to form an angle of °.

In a solid-state image pickup device such as a CCD, it is general that individual pixels are regularly arranged two-dimensionally in a horizontal direction and a vertical direction. False signals are prominent in this arrangement direction. At the same time, when the horizontal pitch and the vertical pitch of the individual pixels are substantially equal, a clear pixel periodicity is also seen in a direction that forms an angle of approximately 45 ° with respect to these two directions. On the other hand, it is necessary to lower the resolution at a predetermined frequency.

Of course, in a special case where the horizontal and vertical pitches of individual pixels are greatly different, the angle at which the periodicity in the oblique direction occurs will be different. Therefore, in order to efficiently reduce spurious signals generated when there is a subject having periodicity in each direction in the shooting screen when using a general solid-state imaging device, an optical low-pass filter configuration is used. As described above.

With such a configuration, the intensity ratio of the ordinary ray to the extraordinary ray separated by one birefringent plate becomes 1: 1. The effect as a low-pass filter becomes uniform over the entire screen, and it becomes easy to realize a desirable one.

Assuming that such a general solid-state image pickup device is used, the optical low-pass filter of the present invention is formed in a parallel plate shape by joining three lithium niobate single crystal birefringent plates. are doing. When applied to an optical apparatus, the birefringent plate of the lithium niobate single crystal is disposed in an optical path between a photographing optical system and an image pickup device, and an entrance surface or an exit surface of the parallel plate having an optical axis thereof. A first birefringent plate whose orthogonal projection forms an angle of approximately 45 ° with the long side direction of the image sensor, and an orthographic projection of the first birefringent plate which is approximately 4 °.
A second birefringent plate forming an angle of 5 ° and a third birefringent plate forming an angle of approximately 90 ° with the orthogonal projection of the first birefringent plate are combined in this order.

Further, in the present invention, the first birefringent plate,
Assuming that the separation width of the ordinary ray and the extraordinary ray on the exit surface of each of the second birefringent plate and the third birefringent plate is D1, D2, and D3, the following conditional expression (1) is satisfied. The imaging device is configured to realize a thin optical low-pass filter and reduce a false signal in combination with the low-pass filter in signal processing.

D1 ≒ D3> D2 (1) Conditional expression (1) makes the separation widths of the subject images in two directions at 45 ° to the long side direction of the image sensor substantially equal. While
9 is a conditional expression that discloses a configuration in which the separation width in the long side direction is set to be large (within a range of ± 20%). As described above, in a single-panel solid-state imaging device having a general color filter array as shown in FIG. 4, the pixel pitch in the long side direction and the short side direction of the imaging device corresponds to the pixel pitch. At the spatial frequency, a remarkable false luminance signal and color signal are generated, and in the oblique direction, the pixel pitch

[0051]

(Equation 1)

At the spatial frequency corresponding to the above, a remarkable false color signal is generated.

An optical low-pass filter having a configuration satisfying the conditional expression (1) is configured to reduce a false color signal generated in an oblique direction among these false signals. It is assumed that false color signals generated at specific spatial frequencies in the direction and the vertical direction are reduced by a low-pass filter in signal processing. At this time, although it is somewhat difficult to reduce a false luminance signal generated at a specific spatial frequency in the long side direction and the short side direction of the image sensor, it is difficult to reduce the false luminance signal. An image can be taken. At this time, the second birefringent plate that separates the ordinary ray and the extraordinary ray in the long side direction of the image sensor substantially functions as a phase plate that rotates the phase of the ray emitted from the birefringent plate. Thus, the third birefringent plate acts to realize separation of the ordinary ray and the extraordinary ray.

At this time, when the pixel pitch is P, the separation widths D1 and D3 of the ordinary ray and the extraordinary ray by the first birefringent plate and the third birefringent plate are as follows.

[0055]

(Equation 2)

It is desirable that the separation width D2 between the ordinary ray and the extraordinary ray by the second birefringent plate is D2 ≦ P / 2. Note that approximately equal here is ± 20%
Means within.

However, in practice, the separation widths D1 and D3 are determined from the above comparison from the comparison between the reduction of the resolution of the captured image and the effect of reducing the false signal.

[0058]

(Equation 3)

In general, it is considered to be slightly smaller, and it is considered that D2 should be set in consideration of the difficulty of machining.

When the plurality of lithium niobate single crystal birefringent plates are joined to each other, the surface of the birefringent plate has an effect of preventing reflection of light in the visible wavelength region at the interface between the adhesive and the birefringent plate. When a dielectric thin film having the following is added, a favorable embodiment can be realized in that generation of ghost and flare is reduced.

The refractive index of the lithium niobate single crystal with respect to the d-line of ordinary light is n. = 2.300, which is higher than that of quartz or optical glass, and therefore, the reflectance at the interface tends to be high. For example, in an optical glass having a refractive index of about 1.6, the reflectance at the interface with air is about 5.3%, and the reflectance at the interface with an adhesive having a refractive index of about 1.5 is about 1.0. %. On the other hand, in a lithium niobate single crystal, the reflectance at the interface with air is about 15.5%, and the reflectance at the interface with an adhesive having a refractive index of about 1.5 is about 4.4%.
Becomes

As described above, the single crystal of lithium niobate has a high reflectivity for light rays in the visible wavelength range. Therefore, if it is disposed in the optical path without adding an antireflection film such as a dielectric thin film, ghosts and flares are generated. Since the contrast of the photographed image is reduced, it is preferable to perform a process for preventing reflection at least on the interface with the air.

Further, also in the case where lithium niobate single crystals are joined to each other, the reflectance at the interface with the adhesive can be reduced by setting the refractive index of the adhesive to be substantially the same as that of the lithium niobate single crystal. However, in reality, there is no good transparent adhesive having such a high refractive index.

Therefore, in a preferred embodiment of an optical low-pass filter using a single crystal of lithium niobate, an anti-reflection film is added to an interface with an adhesive when a plurality of birefringent plates are joined. good. In this case, the antireflection film is made of a dielectric having an intermediate refractive index between the refractive index of lithium niobate single crystal and that of the adhesive, whose optical thickness is reduced to 1/4 of the visible center wavelength. If there is no dielectric thin film material having an appropriate refractive index, it is preferable to add a multilayer antireflection film having an appropriate optical film thickness.

In the optical low-pass filter of the present invention, the angle between the optical axis of the plurality of birefringent plates of lithium niobate single crystal and the normal to the plane of incidence or the plane of emission of the parallel plate is θo. In at least one birefringent plate, any one of the following conditional expressions: 10 ° <θo <27 ° (2) 61 ° <θo <80 ° (3) It is good to satisfy.

According to this, in the optical low-pass filter of the present invention, the angle between the optical axis of the second birefringent plate and the normal of the plane of incidence or the plane of emission of the parallel plate is θ2. At this time, it is preferable that the zoom lens be configured so as to satisfy one of the following conditional expressions (45).

10 ° <θ2 <27 ° (4) 61 ° <θ2 <80 ° (5) Conditional expressions (2), (3) and (4) , (5) are numerical values of the features of the configuration for preventing the lithium niobate birefringent plate from being thin and difficult to manufacture, and include the Z axis of uniaxial single crystal and the incidence of the birefringent plate. This is an expression representing an appropriate setting range of the angle θ0 or θ2 between the surface and the normal to the exit surface. The range specified by the conditional expressions (2), (4), (3) and (5) is the above-mentioned expression (6) representing the separation width of the ordinary ray and the extraordinary ray per unit thickness of the birefringent plate. In FIG. 8, the angle is set to satisfy the conditional expression (2), (4), (3), or (5), and θ0 or θ2 = 45 ° The thickness of the birefringent plate can be increased by 1.2 to 3 times as compared with the case of setting.

When θ exceeds the upper limit of conditional expressions (2) and (4) or exceeds the lower limit of conditional expressions (3) and (5), θ
When the value of 2 approaches 45 °, the substantial difference from that set at θ0 or θ2 = 45 ° becomes small, and the birefringent plate becomes thin, resulting in insufficient mechanical strength. Conversely, when the values of θ0 and θ2 exceed the lower limits of conditional expressions (2) and (4), the values of 0 °
Or the value of θ0 or θ2 approaches 180 ° beyond the upper limit of conditional expressions (3) and (5), the Z-axis of the single crystal and the normal of the plane of incidence of the parallel plate are formed. The change in the separation width of the light beam with respect to the change in the angle becomes large, and it becomes difficult to stably process a parallel plate as a birefringent plate capable of obtaining a desired separation width. FIG. 8 shows the relationship between the thickness of the birefringent plate of lithium niobate single crystal and the separation width between the ordinary ray and the extraordinary ray.

The range defined by the conditional expressions (2) and (4) or (3) and (5) of the present invention is the range of the hatched portion in FIG. When configured, the thickness of the birefringent plate can be set sufficiently thicker than when the angle between the normal line of the input / output surface of the birefringent plate and the optical axis of the single crystal is 45 °.

When the value of θ0 or θ2 is conditional expression (2),
When a parallel flat plate is formed from lithium niobate so that the angle satisfies the range defined by (4) or (3) or (5), for example, when it is set so that θ2 = 70 °, this parallel The separation width of the ordinary ray and the extraordinary ray by the flat plate is 0.0235 mm per 1 mm of the parallel flat plate. Therefore, the thickness of the parallel plate required to obtain a separation width of 10 μm is about 0.43 mm. This is θ2 =
As compared with the case of 45 °, the thickness becomes about 1.6 times thicker, and it is possible to solve problems in mechanical strength and machining. Also, the thickness of the parallel plate is about 0.25 times sufficiently thin compared to the case where a crystal of θ2 = 45 ° is used, so that the optical performance due to the insertion of the optical low-pass filter into the optical path is improved. Adverse effects can be minimized.

As described above, the second birefringent plate has a smaller separation width than the first birefringent plate and the third birefringent plate so as to satisfy the conditional expression (1). Since it acts as a phase plate, it only needs to be of a thickness that can be manufactured and that has the function of converting the polarization direction.

The optical low-pass filter of the present invention is formed by joining a plurality of birefringent plates of lithium niobate single crystal, and the birefringent plates have different orthogonal projections of the Z-axis onto the incidence plane. Joining in the same direction also has the following strength effect.

A parallel flat plate made of a single crystal such as lithium niobate has a drawback that cleavage tends to occur in a predetermined direction due to the properties of the single crystal. Therefore, if a plurality of birefringent plates are joined such that the orthogonal projection of the single crystal on the incident surface of the Z axis is directed in different directions as in the present invention, it becomes difficult to break and the handling becomes easy.

As described above, in an image pickup apparatus using a two-dimensional solid-state image pickup device such as a CCM or a MOS, a solid-state image pickup optical system and a solid-state image pickup device for the purpose of reducing a false resolution signal or a false color signal due to a high frequency component of a subject image. An optical low-pass filter to be disposed between the imaging elements is realized by joining a birefringent plate of lithium niobate single crystal having an appropriate thickness to a suitable optical low-pass filter. It has realized a single-lens reflex digital still camera that can use the interchangeable lens system for salt cameras as it is.

Next, a specific embodiment of a photographing apparatus to which the optical low-pass filter of the present invention is applied will be described. FIG. 1 is a schematic diagram of a first main part of an optical apparatus having an optical low-pass filter according to an embodiment of the present invention.

This embodiment shows an example in which an optical low-pass filter is mounted on a single-lens reflex digital still camera. In FIG. 1, reference numeral 1 denotes an interchangeable photographing lens (photographing optical system), which has a common mount so that an interchangeable lens for a silver halide camera can be used. 2 is a plurality of single crystal birefringent plates of lithium niobate,
In the present embodiment, an optical low-pass filter composed of three parallel plates (shown in FIGS. 21, 22, and 23) is connected, and 3 is a solid-state imaging device. In the camera according to the present embodiment, the optical low-pass filter 2 is disposed immediately before the light incident side of the solid-state imaging device 3, and the subject image is separated into eight images by the action of the three birefringent plates 21, 22, and 23. I am imaging.

FIG. 2 is an explanatory diagram of three birefringent plates 21, 22, and 23 of lithium niobate single crystal constituting the optical low-pass filter 2 shown in FIG. The optical low-pass filter 2 of the present embodiment is configured by bonding three single-crystal birefringent plates 21, 22, and 23 of lithium niobate. FIG. 2A is a view of the optical low-pass filter 2 as viewed from the optical axis direction of the photographing lens 1, and includes Z1a, Z2a,
Z3a represents the orthogonal projection of the optical axis (Z axis) of the birefringent plates 21, 22, 23 onto the plane of incidence or the plane of emission of the parallel plate.

As shown in FIG. 2A, in the present embodiment, the orthogonal projection z1a of the Z axis of the birefringent plate 21 is oriented in a direction forming an angle φ1 with the long side of the optical low-pass filter 2.
The orthogonal projection z2a of the Z axis of the birefringent plate 22 is directed to the long side of the optical low-pass filter 2 (parallel to the long side), and the orthogonal projection Z3a of the Z axis of the birefringent plate 23 is
In the direction that forms an angle φ3 with the long side of. In this embodiment, φ1 = + 45 ° and φ3 = −45 °.

The size of the optical low-pass filter 2 is determined not only by the size of the effective pixel area of the
Although it is determined in consideration of the solid angle of the light beam incident from the lens and the retention of the optical low-pass filter 2 itself, in the present embodiment, it is slightly larger than the effective pixel area of the image sensor 3. As a matter of course, the long side direction of the optical low-pass filter 2 is arranged so as to substantially coincide with the long side direction of the image sensor.

The optical low-pass filter 2 of this embodiment is
FIG. 2B is a diagram viewed from the direction A in FIG. 2A, and FIG. 2C is a partially enlarged view thereof. FIG. 2 (b) and FIG. 2 (c)
In FIG. 2, z1 represents the Z axis of the birefringent plate 21, and as shown in FIG. 2C, the Z axis z1 of the birefringent plate 21 and the normal PL of the entrance / exit surface of the optical low-pass filter 2 are at an angle. θ1 is formed.

In this embodiment, θ1 = 45 °.
FIG. 2A shows the optical low-pass filter 2 of the present embodiment.
2 (d) shows a diagram viewed from the B direction, and FIG. 2 (e) shows a partially enlarged view thereof. In FIGS. 2D and 2E, z2 represents the Z axis of the birefringent plate 22, and FIG.
As shown in (2), the Z-axis z2 of the birefringent plate 22 and the normal PL of the entrance / exit surface of the optical low-pass filter 2 are configured to form an angle θ2. In the present embodiment, θ2 = 65 °.

The optical low-pass filter 2 of the present embodiment
2 (f) is a view of FIG. 2 (a) as viewed from the direction C, and FIG. 2 (g) is a partially enlarged view thereof. FIG. 2 (f) and FIG.
2 (g), z3 represents the Z axis of the birefringent plate 23, and as shown in FIG. 2 (g), the Z axis z3 of the birefringent plate 23 and the normal PL of the input / output surface of the optical low-pass filter 2. Are configured to form an angle θ3. In the present embodiment, θ3 = 45 °. Furthermore, birefringent plates 21, 22, 2
3, the birefringent plate 21 has d1 = 0.19 and the birefringent plate 22 has d2 = 0.multidot.
19. The birefringent plate 23 has d3 = 0.19, and is sufficiently thin as far as it can be processed.

In the present embodiment, the birefringent plates 21, 22, and 23 are configured as described above, so that the extraordinary ray is emitted at an angle of about 7.2 μm in the direction obliquely upward at an angle of 45 ° with respect to the ordinary ray. 5.4 [mu] m in the horizontal direction (long side direction) and approximately 7.2 [mu] m in a direction 45 [deg.] Downward with respect to the horizontal to form a total of eight subject images.
FIG. 3 shows the state of separation of the subject image.

In FIG. 3, the incident light beam L is separated by the first birefringent plate 21 into a light beam L1 and a light beam L11. Second birefringent plate 2
2, the light beam L1 is separated into the light beam L1 and the light beam L12, and the light beam L11 is separated into the light beam L11 and the light beam L112.

The third birefringent plate 23 moves the light beam L1 to the light beams L1 and L13, moves the light beam L2 to the light beams L12 and L123, moves the light beam L11 to the light beams L11 and L113, and moves the light beam L112 to the light beams L112 and L1123. .

As a result, one light flux L as a whole is
Are separated into two luminous fluxes.

By the way, the image pickup device 3 used in this embodiment
Is a solid-state imaging device having a two-dimensional array of a plurality of pixels each having a square of 10 μm square as one pixel, and having a color filter array having a repeating pattern as shown in FIG. 1950 pixels in the direction and 1300 pixels in the vertical direction. As a result, a two-dimensional color solid-state imaging device having more than 2.5 million effective pixels is configured. In FIG. 4, G is green light,
B is a color filter that transmits blue light, and R is a color filter that transmits red light.

In a solid-state image pickup device in which square pixels having color filters as shown in FIG. 4 are provided two-dimensionally, when P is a pixel sampling pitch n, the pixel arrangement direction, that is, the horizontal direction , And in the vertical direction, it is known that moire occurs in a subject image having a spatial frequency of n / p, and a false color signal occurs in a subject image having a spatial frequency of (2n-1) / 2p. Also, since there is a periodicity of pixels in the 45 ° oblique direction, the spatial frequency

[0089]

(Equation 4)

It is well known that false color signals occur in the subject image. Therefore, some kind of low-pass filter that suppresses the MTF value at these spatial frequencies to a low level is required.

The optical low-pass filter of the present embodiment is used for efficiently reducing the false resolution signal and the false color signal generated based on the arrangement of the color filters as shown in FIG. 4 and the sampling pitch p of the pixels. As shown in FIG. 3, the first birefringent plate 21 allows
Abnormal rays in 5 ° direction

[0092]

(Equation 5)

The extraordinary ray is separated by about p / 2 in the horizontal direction by the second birefringent plate 22, and the third birefringent plate 23 further lowers the extraordinary ray by 45 ° with respect to the horizontal direction.
Abnormal rays are omitted in the ° direction.

[0094]

(Equation 6)

[0095] The object image is divided into eight parts as a whole. In the present embodiment, since an image sensor having a pixel pitch of 10 μm is assumed, the first birefringent plate 21 is inclined obliquely upward 45 degrees with respect to the horizontal direction.
About 7.2 μm in the ° direction, ie, the pixel pitch in this direction

[0096]

(Equation 7)

The above-described false color signal is mainly reduced by separating the image at a distance substantially half the distance of the second birefringent plate 22, and the second birefringent plate 22 is about 5.2 μm in the horizontal direction, that is, the pixel pitch is 10.
The third birefringent plate 2 acts to rotate the phase of each point image while separating the image at a distance of about half the μm.
3 is about 7.2μ in a direction 45 ° below the horizontal direction.
m, the pixel pitch in this direction

[0098]

(Equation 8)

The above false color signal is mainly reduced by separating the image at a distance which is approximately half the distance.

In the optical low-pass filter of this embodiment,
By using the three birefringent plates 21, 22, and 23 in this manner, the height determined by the pixel pitch p and arranged at an angle of approximately 45 ° with respect to the horizontal direction of the photographing screen. To reduce the false color signal generated when shooting a subject with frequency components,

[0101]

(Equation 9)

The MTF value of the above high spatial frequency is configured to be suppressed low. The false color signal generates a magenta false signal at the spatial frequency 1 / p determined by the pixel pitch p in the pixel arrangement direction, that is, the horizontal direction and the vertical direction. To be removed by an optical low-pass filter,
Since it is inevitable to lower the contrast of the high-frequency component and the adverse effects such as insufficient resolution of the photographed image are revealed, in the present embodiment, the false color signal generated in these directions is used as the color of the photographed image. Using the information and the spatial frequency information, a low-pass filter for signal processing is added so that only the MTF value of the false signal is reduced.

FIG. 5 shows the spatial frequency characteristics of the optical low-pass filter of this embodiment. 5A shows a horizontal direction (long side direction of the image sensor), FIG. 5B shows a vertical direction (short side direction of the image sensor), and FIG. 5C shows a 45 ° oblique direction.

Further, the optical low-pass filter of the present embodiment has not only a function as a low-pass filter but also a function as an infrared cut filter by adding a dielectric thin film so that the spectral sensitivity of the solid-state image sensor substantially matches the visibility. Used together.

Further, as described above, since lithium niobate has a high refractive index with respect to ordinary light, the reflectance of the surface of lithium niobate is not only at the interface with air but also with the adhesive generally used. It is also quite high at the interface. Therefore, in this embodiment, when the birefringent plates 21, 22, and 23 are joined to each other,
The structure is such that a dielectric thin film is added to the bonding surface side of each birefringent plate to reduce the reflectance in the visible wavelength region at the interface with the adhesive.

FIG. 6A shows the spectral transmittance characteristics of the entire optical low-pass filter of this embodiment. In order to obtain such spectral transmittance characteristics, in the optical low-pass filter of the present embodiment, the spectral reflectance characteristics after bonding at the interface between each birefringent plate and the adhesive are shown in FIG. 6B. An anti-reflection coating made of a thin dielectric thin film is added.

The antireflection coating of the present embodiment realizes the characteristics shown in FIG. 6 (b) by using two dielectric layers having a small optical film thickness. If the refractive index of the body thin film material is appropriately selected, a normal single-layer film having an optical thickness of 1/4 of the central wavelength of antireflection can be obtained.

In this case, the adhesive has a refractive index of 1.5.
It is assumed that about 6 UV-curable adhesives are used. FIG.
(C) shows the spectral reflectance characteristics after bonding at the interface when the antireflection coating is not added to the interface with the adhesive.

As shown in FIG. 6C, in a configuration in which about 4% of light is reflected per one bonding surface of the lithium biobate birefringent plate in the visible wavelength region, the bonding surface has four surfaces. In the optical low-pass filter of this embodiment,
As shown in FIG. 6A, the transmittance in the visible wavelength range is 90%.
% Or more cannot be obtained.

As described above, according to the present embodiment,
Three birefringent plates that are sufficiently thin within a processable range of a thickness of 0.19 mm are joined so that their optic axes face different directions, and a dielectric thin film is added to each interface including a joining surface. As a result, while effectively reducing spurious signals that adversely affect the captured image, it also has a visibility correction function, and can reduce the generation of harmful light rays that cause ghosts and flares. An optical low-pass filter suitable for being provided in a single-lens reflex digital camera using an interchangeable lens prepared for a camera is realized.

FIG. 7 is a schematic view of a main part of an embodiment of the optical low-pass filter 4 of the present invention.

As in the first embodiment, the optical low-pass filter 2 of the present invention comprises three single crystal birefringent plates 121, 122 and 123 of lithium niobate. The optical low-pass filter 2 of the present embodiment is used by being arranged in the optical path between the taking lens 1 and the solid-state imaging device 3 as in the image pickup system shown in FIG.
Is functionally equivalent to The following description focuses on differences from the optical low-pass filter 2 of the first embodiment.

FIG. 7A is a view of the optical low-pass filter 2 viewed from the direction of the optical axis of the photographing lens 1, where z1a,
z2a and z3a represent the orthogonal projection of the optical axis (Z axis) of the birefringent plates 21, 22, and 23, respectively. As shown in FIG. 7A, in the present embodiment, the orthogonal projection z1a of the birefringent plate 21 onto the entrance plane or exit plane of the Z-axis parallel flat plate forms a direction that forms an angle φ1 with the long side of the optical low-pass filter 2. The orthogonal projection z2a of the Z axis of the birefringent plate 22 is oriented in the direction of the long side of the optical low-pass filter 2 (parallel to the long side), and the orthogonal projection z3a of the Z axis of the birefringent plate 23 is optical. It is directed in a direction forming an angle φ3 with the long side of the low-pass filter 2. In this embodiment, φ1 = + 45 °, φ3 = −4
5 °.

The size of the optical low-pass filter 2 of the present embodiment is one size larger than the effective pixel area of the image sensor 3, similarly to the optical low-pass filter 2 of the first embodiment. In addition, the direction of the long side of the optical low-pass filter 2 is also configured to substantially coincide with the long side direction of the image sensor 3.

FIG. 7B shows the optical low-pass filter 2 of the present embodiment as viewed from the direction A in FIG. 7A, and FIG. 7C shows a partially enlarged view thereof. 7 (b) and 7 (c), z1 represents the Z axis of the birefringent plate 21, and FIG.
As shown in (c), the Z-axis zl of the birefringent plate 21 and the normal to the input / output surface of the optical low-pass filter 2 are configured to form an angle θ1. FIG. 7A is a view of the optical low-pass filter 2 of the present embodiment viewed from the direction B in FIG.
FIG. 7D is a partially enlarged view of FIG. FIG.
7 (d) and FIG. 7 (e), z2 represents the Z-axis of the birefringent plate 22, and as shown in FIG. 7 (e), the Z-axis z2 of the birefringent plate 22 and the input of the optical low-pass filter 2. The normal to the exit surface is configured to form an angle θ2. FIG. 7F shows the optical low-pass filter 2 of the present embodiment viewed from the direction C in FIG. 7A, and FIG.
(G). 7F and 7G, z3 represents the Z-axis of the birefringent plate 23. As shown in FIG. 7G, the Z-axis z3 of the birefringent plate 23 and the optical low-pass filter 2 Are formed so that the normal to the entrance / exit surface forms an angle θ3. In the present embodiment, the thickness of each birefringent plate is minimized by setting θ1 = θ2 = θ3 = 45 °. In the present embodiment, the thickness d1 of the birefringent plate 21 = 0.1
9, thickness d2 of birefringent plate 22 = 0.14, birefringent plate 23
Has a thickness d3 = 0.19.

The optical low-pass filter 2 of the present embodiment is
By configuring the birefringent plates 21, 22, and 23 in this way, similarly to the first embodiment, extraordinary rays are respectively emitted to the normal ray by about 7.2 μm in an obliquely upward 45 ° direction with respect to the horizontal.
An image is formed by shifting about 5.3 μm in a horizontal direction (long side direction) and about 7.2 μm in a 45 ° diagonally downward direction with respect to the horizontal,
The subject image is divided into eight in total.

The present embodiment is an embodiment on the assumption that the lithium niobate constituting the birefringent plate can be processed to a sufficiently small thickness. However, it is needless to say that the difficulty in processing the thin plate is taken into consideration. Naturally, it is conceivable to set the thickness of each birefringent plate to be larger. For example, if the thickness of each birefringent plate is required to be 0.25 mm in order to process each birefringent plate into a high-precision thin plate at a low cost, the Z-axis of each birefringent plate and the insertion of an optical low-pass filter are required. The angles θ1, θ2, and θ3 formed by the normal to the exit surface are respectively approximately θ1 = θ3 = 64 °, θ2 = 72 °, or approximately θ.
By setting 1 = θ3 = 24 ° and θ2 = 17 °, a similar optical low-pass filter can be realized.

As described above, by appropriately setting the angle of the optical axis of each birefringent plate, it is possible to control the thickness of the birefringent plate according to the difficulty of processing. Can be made sufficiently thin as compared with a conventional optical low-pass filter using quartz or the like.
In particular, when an image sensor having a small pixel pitch is used and processing of the birefringent plate becomes difficult, the birefringent plate may be adjusted so as to satisfy the conditional expression (4) or (5) of the present invention. It is effective to make the thickness of the birefringent plate slightly larger by appropriately setting the angle of the optical axis. In any case, as described in the first embodiment of the present invention, the addition of a dielectric thin film to the surface of the birefringent plate of lithium niobate to prevent reflection in the visible wavelength range is not possible with ghost or flare. This is effective in reducing the amount of light.

In each of the embodiments of the present invention described above, the separation direction of the ordinary ray and the extraordinary ray by each birefringent plate does not necessarily have to be as described above. For the purpose of reducing a false color signal generated in an oblique 45 ° direction as an optical low-pass filter as in each embodiment of the present invention, for example, the orthogonal projection of the Z-axis of the first birefringent plate is performed by the optical low-pass filter. The second birefringent plate is oriented so as to be oriented at an angle of + 45 ° with the long side, and the orthogonal projection of the Z-axis of the second birefringent plate is oriented at an angle of + 90 ° with the long side of the optical low-pass filter. It is also possible to set the orthogonal projection of the Z-axis of the plate so as to face a direction forming an angle of + 135 ° with the long side of the optical low-pass filter.

In the first and second embodiments, the plurality of birefringent plates are bonded with the respective Z-axes oriented in different directions, because the MTF value of the high frequency component is reduced as described above. In addition to the action, the purpose is to obtain the effect of overcoming the disadvantage of the lithium niobate single crystal, which is easily cleaved in a predetermined direction.

As described above, according to the present embodiment,
2. Description of the Related Art In an imaging apparatus using a two-dimensional solid-state imaging device such as a CCD or a MOS, the imaging device is disposed between an imaging optical system and an individual imaging device for the purpose of reducing false resolution signals and false color signals due to high frequency components of a subject image. A suitable optical low-pass filter can be realized by forming an optical low-pass filter by bonding lithium niobate single crystal birefringent plates having an appropriate thickness to each other, and particularly, an interchangeable lens system for a silver halide camera. , A single-lens reflex digital still camera that can be used satisfactorily as it is can be realized.

[0122]

According to the present invention, the thickness is small, a good low-pass effect is effectively obtained, and high optical performance is easily obtained.

An optical low-pass filter and an optical device using the same can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of Embodiment 1 of an imaging system when an optical low-pass filter of the present invention is mounted on a single-lens reflex camera.

FIG. 2 is an explanatory diagram of Embodiment 1 of the optical low-pass filter of the present invention.

FIG. 3 is an explanatory diagram of a state of a subject image separated by the optical low-pass filter according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram of an arrangement of color filters configured on an image sensor used in the present invention.

FIG. 5 is an explanatory diagram of a spatial frequency characteristic of the optical low-pass filter of the present invention.

FIG. 6 is an explanatory diagram of the spectral characteristics of the optical low-pass filter of the present invention.

FIG. 7 is an explanatory diagram of Embodiment 2 of the optical low-pass filter of the present invention.

FIG. 8 is an explanatory diagram showing a relationship between an angle formed by an optical axis of a uniaxial single crystal and a normal line of an entrance / exit surface of a parallel plate and a separation width between an ordinary ray and an extraordinary ray.

FIG. 9 is an explanatory diagram of ray aberration generated in a parallel plate.

[Explanation of symbols]

 1, photographing lens 2, optical low-pass filter 3, image sensor 21, 22, 23, birefringent plate of lithium niobate single crystal

Claims (13)

[Claims] 1. A plurality of birefringent plates of lithium niobate single crystal are combined in such a manner that the optical axes of the plurality of birefringent plates are directed in different directions, and are joined to form a parallel plate. An optical low-pass filter. 2. At least two of said plurality of birefringent plates of lithium niobate single crystal are such that their orthogonal projections of their optical axes onto a plane of incidence or a plane of incidence of a parallel plate make an angle of 45 ° with each other. 2. The optical low-pass filter according to claim 1, wherein: 3. When joining the plurality of birefringent plates of lithium niobate single crystal to each other, the surface of the birefringent plate includes
3. The optical low-pass filter according to claim 1, further comprising a dielectric thin film having an action of preventing reflection of light in a visible wavelength region at an interface between the adhesive and the birefringent plate. 4. When the angle between the optical axis of the plurality of birefringent plates of lithium niobate single crystal and the normal to the plane of incidence or the plane of emission of the parallel plate is θo, at least 1
4. The optical low-pass filter according to claim 1, wherein two birefringent plates satisfy one of the following conditional expressions: 10 ° <θo <27 ° 61 ° <θo <80 °. 5. 5. An optical low-pass filter formed by joining three birefringent plates of lithium niobate single crystal to form a parallel plate, wherein the birefringent plate of three lithium niobate single crystals is An optical low-pass filter, wherein an orthogonal projection of an optical axis onto an entrance surface or an exit surface of the parallel plate is set to form an angle of 45 degrees with each other. 6. The three birefringent plates of lithium niobate single crystal are referred to as first, second, and third birefringent plates in this order from the incident and incident sides, and the first birefringent plate and the second birefringent plate are arranged. When the separation width of the ordinary ray and the extraordinary ray on the exit surface of each of the refraction plate and the third birefringent plate is D1, D2, and D3, respectively, the conditional expression of D1 ≒ D3> D2 is satisfied. An optical low-pass filter according to claim 5. 7. An angle θ2 between an optical axis of the second birefringent plate and a normal to an entrance surface or an exit surface of the parallel plate.
7. The optical low-pass filter according to claim 5, wherein any one of the following conditional expressions is satisfied: 10 ° <θ2 <27 ° 61 ° <θ2 <80 °. 8. An optical apparatus which forms an image on a photographic element via the optical low-pass filter according to claim 1. Description: 9. An optical device for forming an image on an imaging element via an optical low-pass filter, wherein the optical low-pass filter is formed of a parallel plate in which a plurality of lithium niobate single crystal birefringent plates are joined. The plane of incidence of the parallel plate of the plurality of lithium niobate single crystal birefringent plate optical axis,
Alternatively, the orthogonal projection to the exit surface is configured to form an angle of approximately 45 ° with each other, and at least one of the orthogonal projections is configured to form an angle of approximately 45 ° with the long side direction of the imaging element. Characteristic optical equipment. 10. An optical device for forming an image on a photographic element via an optical low-pass filter, wherein the optical low-pass filter is formed by joining three lithium biobate single crystals of lithium niobate to form a parallel plate. The birefringent plate of lithium niobate single crystal is an entrance surface of the parallel plate of its optical axis,
Alternatively, an orthographic projection on the exit surface forms a first birefringent plate at an angle of approximately 45 ° with the long side direction of the image sensor, and an orthographic projection of the first birefringent plate at an angle of approximately 45 °. An optical apparatus comprising: a second birefringent plate; and a third birefringent plate forming an angle of about 90 ° with the orthogonal projection of the first birefringent plate in this order. 11. When the separation width of an ordinary ray and an extraordinary ray at the exit surface of each of the first birefringent plate, the second birefringent plate, and the third birefringent plate is D1, D2, and D3, respectively. The optical apparatus according to claim 10, wherein the optical system is configured to satisfy the following conditional expression: D1 ≒ D3> D2. 12. An angle between an optical axis of the second birefringent plate and a normal to an entrance surface or an exit surface of the parallel plate is θ.
The optical device according to claim 10, wherein when 2 is satisfied, any one of the following conditional expressions is satisfied: 10 ° <θ2 <27 ° 61 ° <θ2 <80 °. 13. When at least one of the plurality of lithium niobate single crystal birefringent plates has an angle θo between an optical axis of the birefringent plate and a normal of an incident surface or an exit surface of the parallel plate, at least one birefringent plate is used. The optical apparatus according to claim 9, wherein the refractive plate satisfies a conditional expression of 10 ° <θo <27 ° 61 ° <θo <80 °.