JP4545859B2 - Imaging device - Google Patents

Description

だけ分離させるように構成し、図４に示されるように被写体像を４つに分離させている。本実施例においては、画素ピッチ１０μｍの撮像素子を想定しているため、複屈折板２１は水平方向に画素ピッチ１０μｍと略等距離の像の分離を行なうことによって主として前述のモアレを低減させるように作用させ、複屈折板２２は斜め４５°方向に約７μｍ、即ち、水平方向及び垂直方向に画素ピッチ１０μｍの約１／２に相当する距離５μｍの像の分離を行なうことによって主として前述の偽色信号を低減させるように作用させている。
【００５６】
本実施例では、このように複屈折板２１，２２の２枚の複屈折板を用いることによって，画素ピッチｐによって決定される遮断周波数１／２ｐ以上の高い空間周波数のＭＴＦ値を低く抑制し、偽解像信号や偽色信号の発生を低減させる光学ローパスフィルタを効率的に構成している。
【００５７】
本実施例の光学ローパスフィルタの空間周波数特性を図６に示す。図６（ａ）は水平方向（撮像素子の長辺方向）、図６（ｂ）は垂直方向（撮像素子の短辺方向）を表わしている。
【００５８】
（第２の実施例）
図７は、第２の実施例の光学ローパスフィルタ１０２を構成するニオブ酸リチウムの単結晶の複屈折板１２１，１２２の説明図である。本実施例の光学ローパスフィルタ１０２は、図１に示した撮像系と同様に撮影レンズ１と固体撮像素子３の間の光路中に配置して使用され、第１の実施例の光学ローパスフィルタ２と機能的には等価なものである。第１の実施例の光学ローパスフィルタ２と異なる点について以下に説明する。
【００５９】
図７（ａ）は、光学ローパスフィルタ１０２を撮影レンズ１の光軸方向から見た図であって、ｚ１ａ，ｚ２ａは、それぞれ複屈折板１２１，１２２の光学軸（Ｚ軸）の正射影を表わしている。図７（ａ）に示されるように、本実施例では複屈折板１２１のＺ軸の正射影ｚ１ａは光学ローパスフィルタ１０２の長辺方向を向いており（長辺に平行）、複屈折板１２２のＺ軸の正射影ｚ２ａは光学ローパスフィルタ１０２の長辺と角度φ２をなす方向を向いている。なお本実施例でも、φ２＝４５°としている。
【００６０】
本実施例の光学ローパスフィルタ１０２の大きさは、第１の実施例の光学ローパスフィルタ２と同様に、撮像素子３の有効画素領域より一回り大きく構成されている。なお、やはり光学ローパスフィルタ１０２の長辺の方向は、撮像素子３の長辺方向とほぼ一致するように構成されている。
【００６１】
本実施例の光学ローパスフィルタ１０２を、図７（ａ）のＡ視方向から見た図を図７（ｂ）に、その一部拡大図を図７（ｃ）に示す。図７（ｂ）及び図７（ｃ）においてｚ１は複屈折板１２１のＺ軸を表わしており、図７（ｃ）に示されるように、複屈折板１２１のＺ軸ｚ１と光学ローパスフィルタ１０２の入出射面の法線は角度θ１をなすように構成されている。本実施例ではθ１＝１５°としている。また本実施例の光学ローパスフィルタ１０２を図７（ａ）のＢ視方向から見た図を図７（ｄ）に、その一部拡大図を図７（ｅ）に示す。図７（ｄ）及び図７（ｅ）においてｚ２は複屈折板１２２のＺ軸を表わしており、図７（ｅ）に示されるように、複屈折板１２２のＺ軸ｚ２と光学ローパスフィルタ１０２の入出射面の法線は角度θ２をなすように構成されている。本実施例ではθ２＝３５°としている。本実施例では、複屈折板１２１の厚さｄ１＝０．３０、複屈折板１２２の厚さｄ２＝０．３０となっている。
【００６２】
本実施例の光学ローパスフィルタ１０２は、複屈折板１２１，１２２をこのように構成することによって、第１の実施例と同様に、常光線に対して異常光線を各々水平方向に約１０μｍ、斜め４５°方向に約７μｍずらして結像させ、被写体像を４つに分離させている。
【００６３】
本実施例では、複屈折板を構成するニオブ酸リチウムのＺ軸と光学ローパスフィルタの入出射面の法線とのなす角θが第１の実施例と異なる例を示したが、条件式（１）又は（２）を満足する範囲内で角θを適切に設定することによって、第１の実施例と同様に機械加工上の問題点や取り扱い上の問題点を克服した十分に薄型の光学ローパスフィルタを実現することができる。
【００６４】
なお、第１及び第２実施例において、複数枚の複屈折板の各Ｚ軸を異なる方向に向けた上で貼り合わせているのは、前述のように高い周波数成分のＭＴＦ値を低減させる作用の他に、所定方向にへき開しやすいというニオブ酸リチウム単結晶の欠点を克服するという効果を得るためのものでもある。
【００６５】
また、光学ローパスフィルタにニオブ酸リチウム単結晶の複屈折板を用いた上で、赤外カットフィルタとして赤外波長域の光を吸収する色ガラスフィルタではなく、誘電体多層膜からなる赤外域の光を反射させるダイクロイックフィルタを併用すれば、赤外カットフィルタとしての平行平板も不必要になり、小型化及び光学性能の一層の向上が図れる。このときダイクロイックフィルタはニオブ酸リチウム単結晶の複屈折板の少なくとも１面に蒸着すればより効果的である。
【００６６】
この他、ニオブ酸リチウム単結晶の特徴として、水晶や撮影レンズに用いられる光学ガラス等に対して屈折率が高いという点がある。このためニオブ酸リチウム単結晶の平行平板の空気との界面は反射率が高く、表面を露出したままでは反射光がフレアーとなって画質を著しく低下させる要因となり得た。また、複屈折板同士を接合する際の接合面との界面に関しても、接着剤としてこれらの複屈折板と同程度の屈折率を持った接着剤を使用して接合すれば界面反射の問題は少ないのだが、そのような接着剤で透明なものはあまり一般的となっていない。そこで、少なくともこれらの複屈折板の空気との界面に反射防止のための膜を蒸着することが好ましく、更に複屈折板同士の接合面側にも接着剤との界面における反射を低減させる反射防止のための膜を蒸着することがより好ましい。このようにニオブ酸リチウムの単結晶の複数枚の複屈折板による光学ローパスフィルタを構成する際には、複屈折板の各面に所定の蒸着膜を付加し、接着するように構成することが望ましい。
【００６７】
また、第１、第２の実施例の説明においては、光学ローパスフィルタを２枚の複屈折板によって構成した例を示したが、３枚の複屈折板を貼り合わせ被写体像の高周波数成分による偽解像信号や偽色信号をより良好に補正することも当然のごとく考えられる。
【００６８】
（その他の実施形態）
次に第１の実施例や第２の実施例の光学ローパスフィルタを固体撮像素子の保護板とした実施形態について図８を用いて説明する。図８において、９２は第１又は第２の実施例で説明した光学ローパスフィルタ、９４はパッケージ、９５は撮像部を表わしている。図８の撮像素子は、ＣＣＤやＭＯＳ等の２次元の固体撮像素子から成る撮像部９５をパッケージ９４内に配設し、パッケージ９４を透明な平行平板で覆うタイプの撮像素子であって、図８の実施形態では、この透明な平行平板に光学ローパスフィルタを適用し、保護板として機能を持たせている。
【００６９】
通常、保護板としての平行平板には低価格の板ガラスを用いることが多いが、光路中に平行平板が存在することは、前述した理由によってスペース上及び撮影光学系の光学性能上の両面から望ましくない。そこで、図８に示した実施形態では、撮像素子を用いた光学系において必須に近い光学ローパスフィルタを撮像部の保護板として利用し、光路中に存在する平行平板をなるべく少なくしている。
【００７０】
図９は、第１の実施例や第２の実施例の光学ローパスフィルタを固体撮像素子の保護板とした利用した図８とは異なる実施形態であり、撮像部を基板上に配設することによって、撮像ユニットそのものの薄型化を図ったものである。図９の撮像素子は、基板９６の両面に撮像部９７と保護板としての光学ローパスフィルタ９８を接着した構成となっており、これにより、撮像ユニットそのものの厚さを薄くするというこの撮像ユニットの目的をより良好に達成するものである。
【００７１】
なお、図８や図９に示した固体撮像素子の実施形態において、保護板としての光学ローパスフィルタと撮像部は直接接触していないが、より小型化を図るために撮像部に光学ローパスフィルタを直接貼り合わせてもよい。
【００７３】
【発明の効果】
以上説明したように、本発明によれば、複屈折板の厚さが適切になるよう結晶の光学軸と入射面の法線の関係を適切に設定することで、光学性能上、また機械加工上における不具合を除去した光学ローパスフィルタを有する撮像装置を実現できる。
【図面の簡単な説明】
【図１】ニオブ酸リチウム単結晶の光学軸と平行平板の入出射面の法線のなす角と、常光線と異常光線の分離巾の関係を説明するための図である。
【図２】第１の実施例の光学ローパスフィルタを一眼レフレックスカメラに搭載した際の撮像系の概略断面図である。
【図３】第１の実施例の光学ローパスフィルタの説明図である。
【図４】常光線と異常光線による被写体像の分離の様子の説明図である。
【図５】撮像素子上に用いるカラーフィルタ配列の説明図である。
【図６】第１の実施例の光学ローパスフィルタの空間周波数特性を示す説明図である。
【図７】第２の実施例の光学ローパスフィルタの説明図である。
【図８】光学ローパスフィルタを保護板として配設した撮像ユニットの説明図である。
【図９】光学ローパスフィルタを保護板として配設したその他の撮像ユニットの説明図である。
【図１０】平行平板で発生する光線収差の説明図である。
【符号の説明】
１ 撮影レンズ
２，１０２ 光学ローパスフィルタ
３ 撮像素子
２１，２２，１２１，１２２ ニオブ酸リチウム単結晶の複屈折板[0001]
BACKGROUND OF THE INVENTION
  The present inventionImaging deviceIn particular, digital still cameras and video cameras that use two-dimensional solid-state image sensors such as CCD and MOSConcerningIs.
[0002]
[Prior art]
In an imaging device such as a digital still camera or a video camera that uses a two-dimensional solid-state imaging device such as a CCD or MOS, in order to sample the subject image at every pixel pitch, the subject is photographed with a high spatial frequency component. In some cases, a false resolution signal is generated in which a high-frequency component aliasing image is output as a low-frequency component, which is a factor that lowers the resolution of the subject image. Similarly, in an image pickup apparatus using a single-plate color solid-state image pickup device, a false color signal determined by the arrangement of color filters arranged in front of each pixel is generated when shooting a subject having a component having a high spatial frequency. This is a factor that reduces the color reproducibility of the subject image.
[0003]
Conventionally, various optical low-pass filters have been proposed for the purpose of reducing false resolution signals and false color signals due to such high-frequency components of the subject image. Among them, the most typical method is a method using a single crystal parallel plate of quartz. When the normal of the incident surface and the optical axis (Z axis) of the crystal are tilted by a predetermined angle, the light incident on the parallel plate of quartz that is a uniaxial crystal exhibits anisotropy in the direction of the Z axis and is an ordinary ray. Are separated into extraordinary rays, and the separated rays are emitted in parallel from a parallel plate. At this time, the separation width of the ordinary ray and the extraordinary ray is determined by the angle formed by the normal line of the plane of incidence of the parallel plate and the Z axis of the crystal and the thickness of the parallel plate.
[0004]
As a conventional example of such an optical low-pass filter using the action of quartz, for example, Japanese Utility Model Publication No. 47-18688, Japanese Utility Model Publication No. 47-1889, Japanese Patent Application Laid-Open No. 59-75222, Japanese Patent Application Laid-Open No. 60-164719. Publications and the like are known.
[0005]
In Japanese Utility Model Publication No. 47-18688 and Japanese Utility Model Publication No. 47-1889, a stripe-shaped filter is assumed as a color filter, and a false color signal generated when the spatial frequency of a subject is synchronized with this color filter. In order to reduce this, a configuration is disclosed in which a light beam is separated into an ordinary ray and an extraordinary ray by a parallel plate having birefringence, such as quartz, and an image is formed on the imaging surface. In particular, Japanese Utility Model Publication No. 47-18689 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 incident / exit plane of a parallel plate. ing.
[0006]
In JP-A-59-75222 and JP-A-60-164719, for example, a filter having a Bayer arrangement as shown in FIG. 5 is assumed as a color filter, and a plurality of birefringent plates are combined. Separating rays into ordinary rays and extraordinary rays, separating the subject image into multiple images shifted by a predetermined pitch, and forming false images and false color signals generated by high-frequency components of the subject. The structure which reduces automatically is disclosed.
[0007]
Examples of using a single crystal plate other than quartz as the birefringent plate are disclosed in Japanese Patent Application Laid-Open Nos. 9-212222 and 11-218612. Japanese Patent Application Laid-Open Nos. 9-212122 and 11-218612 propose an optical low-pass filter using a plurality of birefringent plates and using at least one of them as lithium niobate. A single crystal of lithium niobate is a uniaxial crystal similar to a single crystal of crystal, but the difference between the refractive index of ordinary light and the refractive index of extraordinary light is larger than that of quartz, so that a predetermined beam separation width is obtained. Therefore, the thickness of the birefringent plate necessary for the purpose can be reduced.
[0008]
[Problems to be solved by the invention]
In the conventional example in which one or a plurality of quartz birefringent plates are combined and used as an optical low-pass filter, the refractive index difference between ordinary rays and extraordinary rays is small. It was necessary to increase the thickness of the refracting plate to some extent.
[0009]
In general, when a parallel plate of uniaxial crystal is made so that the angle between the normal of the incident surface and the optical axis (Z-axis) is θ, The angle φ formed by the traveling direction of the ordinary ray and the traveling direction of the extraordinary ray is expressed by the following equation.
[0010]
tanφ = (no²−ne²) Sinθcosθ / (ne²cos²θ + no²sin²θ) (4)
[0011]
Equation (4) represents the separation width of the ordinary ray and the extraordinary ray per unit thickness of the birefringent plate, and this value is maximum when θ = 45 °. Assuming that the ordinary light refractive index no = 1.544 and the extraordinary light refractive index ne = 1.553 with respect to the d-line of the crystal single crystal, tan φ≈−0.0058 when θ = 45 °.
[0012]
Here, assuming that the pixel pitch Ph in the long side direction of the solid-state imaging device is 10 μm, the extraordinary ray is shifted by 10 μm in this direction by a crystal birefringent plate in order to remove the false resolution signal generated in this direction. Assuming that it is configured, as described above, tan φ≈−0.0058 in a crystal single crystal, the thickness d of the birefringent plate needs to be at least about 1.7 mm. As described above, when quartz having a small refractive index difference between ordinary rays and extraordinary rays is used as a birefringent plate, there is a problem that the optical low-pass filter becomes thick and the optical system becomes large.
[0013]
Further, when a parallel plate is present in the optical path of the photographing optical system, the following problems are included in addition to the space problem.
[0014]
The light beam formed by the ideal lens and passing through the parallel plate is refracted at a refraction angle according to the well-known Snell's refraction law according to the incident angle α to the parallel plate and is emitted in parallel with the incident light beam. The planned focal plane of the ideal lens is determined by paraxial theory using the approximation of sin α≈α, the optical path length in the parallel plate, and is expressed in terms of air thickness d and refractive index N of the parallel plate. The position is shifted in the optical axis direction by the amount replaced by / N. However, if the incident angle of light incident on the parallel plate increases and the approximation of sin α≈α does not match the actual condition, the light emitted from the ideal lens will not form an image on the planned image plane. Specifically, astigmatism occurs in which the spherical aberration is over on the optical axis and the meridional image surface is more over the sagittal image surface outside the axis.
[0015]
FIG. 10 is an explanatory diagram for explaining such a phenomenon. An ideal lens having an exit pupil of F2.0 is arranged at a position 50 mm away from the ideal image plane, and the optical axis and the ideal image plane are arranged. It is the figure which showed the light ray aberration which generate | occur | produces in a birefringent plate at the time of arrange | positioning the quartz birefringent plate which consists of a parallel plate of thickness 5mm with a vertical entrance / exit surface. In FIG. 10A, 41 is an ideal lens having a focal length of 50 mm, 42 is a quartz birefringent plate having a thickness of 5 mm, and 43 is a predetermined focal plane in paraxial calculation. 44 represents the vicinity of the image plane having an image height of 0 mm, and 45 represents the vicinity of the image plane having an image height of 20 mm.
[0016]
The light beam imaged by the ideal lens 41 and passed through the parallel plate 42 proceeds to form an image in the vicinity of the planned focal plane 43 as shown in FIG. However, when the image plane neighborhoods 44 and 45 are enlarged, they are as shown in FIGS. 10 (b) and 10 (c), respectively. That is, the best image plane position at an image height of 0 mm is 46, the best image plane position at an image height of 20 mm is a position shown by 47, and the spherical aberration is over at the center of the screen, and over the meridional image plane in the periphery of the screen. Astigmatism occurs.
[0017]
As described above, when a parallel plate is inserted between the photographing optical system and its planned focal plane, the above-described ray aberration is generated. Therefore, in a photographic optical system based on the premise that a parallel plate such as a quartz birefringent plate is arranged, it is a common practice to design the photographic optical system in consideration of aberrations generated on the parallel plate. However, an imaging device (for example, a single-lens reflex type) configured so that the photographic optical system can be exchanged so that various photographic optical systems can be used as the photographic optical system. Of course, this is not possible with digital still cameras.
[0018]
By the way, in a single crystal of lithium niobate, which is a uniaxial crystal similar to quartz, assuming that the ordinary light refractive index no = 2.300 and the extraordinary light refractive index ne = 2.215 with respect to the d-line, θ = 45. When the angle is tan φ≈0.0376.
[0019]
Here, as in the case of quartz, assuming that the pixel pitch Ph in the long side direction of the solid-state imaging device is 10 μm, an extraordinary ray is emitted by a lithium niobate birefringent plate in order to remove the false resolution signal generated in this direction. It is understood that the thickness d of the birefringent plate may be about 0.27 mm because the lithium niobate single crystal is tan φ≈0.0376. Compared with the case of using the above-mentioned quartz single crystal, the thickness of the birefringent plate can be reduced to about 0.16 times, and the birefringent plate of lithium niobate can be applied to the imaging system as an optical low-pass filter. For example, it is possible to solve the space problem and realize a smaller imaging system, and to reduce the problem of light aberration caused by the parallel plate.
[0020]
The above-mentioned Japanese Patent Application Laid-Open Nos. 9-211222 and 11-218612 focus on such properties of single crystals of lithium niobate.
[0021]
Here, assuming an image pickup apparatus using a two-dimensional solid-state image pickup device having a pixel pitch of 10 μ, an aspect ratio of 2: 3, and an effective pixel number of about 2.5 million pixels, the number of pixels in the horizontal direction is about 1950 pixels and vertical. The number of pixels in the direction is about 1300 pixels, and the size occupied by the effective pixels of the solid-state imaging device is about 19.5 mm in the horizontal direction and about 13.0 mm in the vertical direction. Considering that an optical low-pass filter made of a parallel plate is placed relatively close to the front (subject side) of such a solid-state image sensor, the size of the parallel plate is the solid angle of light incident on the solid-state image sensor. It is necessary to add the effective ray area considering the above and the area for holding the parallel plate itself to at least about 21.0 mm in the horizontal direction and about 14.5 mm in the vertical direction.
[0022]
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 formed by the optical axis (Z axis) of the lithium niobate single crystal and the normal of the plane of incidence of the parallel plate is 45 °. In this case, as described above, the thickness of the parallel flat plate is about 0.27 mm. Further, as disclosed in Japanese Utility Model Publication No. 47-18688, an angle of about 45 ° with respect to the horizontal line of the screen is set in consideration of removing the influence of the false signal due to the frequency component of ½ of the pixel pitch. If a parallel plate that separates extraordinary rays in the direction formed is used together, the thickness of the parallel plate is about 0.19 mm. The thickness of such a parallel plate is about 1% or less as compared with the diagonal length determined by the above-mentioned outer dimensions, and the mechanical strength is insufficient, and it becomes very easy to crack. That is, even if lithium niobate having a large refractive index difference between ordinary rays and extraordinary rays is simply used as a birefringent plate, it is difficult to produce a parallel plate itself because it is necessary to polish it very thinly. There was a problem that it was easy to break carelessly at the time and after handling.
[0023]
Japanese Patent Application Laid-Open Nos. 9-211222 and 11-218612 propose a configuration of an optical low-pass filter in which lithium niobate and crystal are bonded. These conventional examples can be made thinner than optical low-pass filters that are all made of quartz, but are still unsatisfactory in solving the above-mentioned space problems and the optical performance problems of photographing optical systems. It was enough.
[0024]
  The present invention is an optical low-pass that eliminates defects in optical performance and machining by appropriately setting the relationship between the optical axis of the crystal and the normal of the incident surface so that the thickness of the birefringent plate is appropriate. filterAn imaging device comprisingIt is intended to provide.
[0025]
[Means for Solving the Problems]
  To achieve the above objective,An imaging device according to the present invention includes an optical low-pass filter having a biaxial plate of a uniaxial single crystal of lithium niobate having a refractive index difference of 0.02 or more between ordinary rays and extraordinary rays, and an imaging device having a rectangular imaging surface The birefringent plate separates the subject image in the long side direction of the imaging surface, and the uniaxial single crystal optical axis of the birefringent plate and the surface normal of the birefringent plate Is θ, the pixel pitch in the long side direction of the imaging surface is p, and the thickness of the birefringent plate is d,
10 ° <  θ  <30 °              ... (1)
60 ° <  θ  <80 °              ... (2)
One of the conditional expressions,
0.012 <  p / d  <0.033  ... (3)
It satisfies the following conditional expression.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
First, an outline of an embodiment of the present invention will be described.
[0030]
The optical low-pass filter of the present embodiment is an optical low-pass filter including a birefringent plate disposed in an optical path between a solid-state imaging device such as a CCD or MOS and a photographing optical system, and the birefringent plate is made of lithium niobate or the like. It is composed of a uniaxial single crystal that has a large refractive index difference between ordinary and extraordinary rays compared to the quartz crystal, and the angle between the crystal's optical axis (Z-axis) and the normal of the incident surface of the birefringent plate is θ When
10 ° <θ <30 ° (1)
60 ° <θ <80 ° (2)
Any one of the following conditional expressions is satisfied.
[0031]
In this embodiment, the thickness of the parallel plate as the birefringent plate is set by appropriately setting the angle between the Z-axis of the uniaxial single crystal constituting the birefringent plate and the normal line of the incident surface. It has an appropriate thickness that can be processed without any problems in processing, and secures mechanical strength. Furthermore, since the birefringent plate can be made sufficiently thin compared to the case of using quartz, space efficiency is good, and the light aberration generated by inserting a parallel plate can be sufficiently reduced. An optical low-pass filter that is particularly suitable for an imaging apparatus that uses the prepared interchangeable lens as a photographing optical system can be realized.
[0032]
Conditional expressions (1) and (2) are numerical values of the characteristics of the configuration of the optical low-pass filter of the present invention, and the refractive index difference between ordinary rays and extraordinary rays is larger than that of quartz such as lithium niobate. 6 is an expression representing an optimum setting range of an angle θ formed by a Z axis of a uniaxial single crystal and a normal line of an incident surface of a birefringent plate. The range specified by conditional expressions (1) and (2) is a hatched line in FIG. 1 showing the relationship of the above-described expression (4) representing the separation width of the ordinary ray and extraordinary ray per unit thickness of the birefringent plate. When the angle is set to satisfy the conditional expression (1) or (2), the birefringent plate can be made thicker than when θ = 45 °.
[0033]
When the upper limit value of conditional expression (1) is exceeded or the lower limit value of conditional expression (2) is exceeded and the value of θ approaches 45 °, the substantial difference from that set to θ = 45 ° is small. Thus, the birefringent plate becomes thin and the mechanical strength is insufficient. Conversely, if the value of θ approaches 0 ° beyond the lower limit of conditional expression (1), or the value of θ approaches 180 ° beyond the upper limit of conditional expression (2), the Z axis of the single crystal The change in the beam separation width with respect to the change in the angle between the parallel plate and the normal of the plane of incidence of the parallel plate increases, making it difficult to stably process the parallel plate as a birefringent plate that can obtain the desired separation width. It becomes.
[0034]
When parallel plates are formed with lithium niobate so that the value of θ satisfies the range defined by conditional expressions (1) and (2), for example, when θ is set to 70 °, The separation width of the ordinary ray and the extraordinary ray by the parallel plate is 0.0235 mm per 1 mm of the parallel plate. Therefore, the thickness of the parallel plate necessary for obtaining a separation width of 10 μm is about 0.43 mm. This is about 1.6 times thicker than the case of θ = 45 °, and it is possible to solve the problems in mechanical strength and machining. Compared to the case of using a crystal with θ = 45 °, the thickness of the parallel plate is about 0.25 times and is sufficiently thin. Therefore, the optical performance by inserting an optical low-pass filter in the optical path. The adverse effects on can be minimized.
[0035]
In addition, it is more practical if a plurality of birefringent plates are joined to form an optical low-pass filter so that the orthogonal projections of the plurality of birefringent plates on the Z-axis incident surface face in different directions. This is desirable.
[0036]
This is because a parallel plate made of a single crystal such as lithium niobate is difficult to handle because it tends to cleave in a predetermined direction due to the properties of the single crystal. Therefore, as described above, if a plurality of birefringent plates are joined so that the orthogonal projections of the single crystal onto the Z-axis incident surface are directed in different directions, it is difficult to break and the handling becomes easy. Of course, the orthogonal projection of the single crystal onto the Z-axis entrance surface coincides with the extraordinary ray separation direction, and false resolution generated by sampling at each pixel pitch of the solid-state imaging device using this ray separation. It is desirable to configure the signal and the false color signal to be appropriately reduced in both the horizontal and vertical directions.
[0037]
Further, when the optical low-pass filter is composed of a plurality of birefringent plates, a first birefringent plate in which the orthogonal projection of the single crystal on the Z-axis incident surface faces the substantially long side direction of the solid-state imaging device; It is desirable to include two birefringent plates, that is, a second birefringent plate that faces a direction that forms an angle of approximately 45 ° with the long side direction of the solid-state imaging device.
[0038]
Although the method of generating a false resolution signal and a false color signal due to sampling at each pixel pitch of the solid-state image sensor varies depending on the arrangement of the pixels, the most general pixel with a substantially square opening is in the horizontal and vertical directions. In the arranged two-dimensional solid-state imaging device, those generated by high frequency components in the horizontal direction, the vertical direction, and the oblique 45 ° direction are easily noticeable. Therefore, in order to efficiently reduce these false resolution signals and false color signals, the optical low-pass filter may be configured to generate image separation in at least a substantially horizontal direction and a substantially oblique 45 ° direction. desirable.
[0039]
In addition, the optical low-pass filter of the present embodiment can also be used as a protective plate for the image sensor. For example, in a solid-state imaging device of a type that forms an imaging unit by enclosing the imaging unit in a package, the imaging unit may be protected by bonding the optical low-pass filter of the present embodiment to the front surface of the package. it can. In a solid-state image sensor of the type that forms an imaging unit by bonding an imaging unit on a substrate, the optical low-pass filter of this embodiment is bonded to the front surface of the substrate on which the imaging unit is disposed using the outer peripheral portion. The imaging unit can be enclosed and protected.
[0040]
By the way, in an imaging apparatus using a solid-state imaging device, an infrared cut filter is generally arranged in the optical path for the purpose of correcting the difference between the spectral sensitivity and the visibility of the solid-state imaging device. Therefore, if a multilayered film of a dielectric material having an infrared cut function is deposited on at least one surface of the optical low-pass filter of the present embodiment so as to function as an infrared cut filter, the system can be further downsized. desirable.
[0041]
  Further, in the imaging apparatus of the present embodiment, the optical low-pass filter includes a birefringent plate having an action of separating a subject image in the long side direction of the two-dimensional solid-state imaging element, and the thickness of the birefringent plate is d, solid When the pixel pitch in the long side direction of the image sensor is p,
0.012<P / d <0.033  ... (3)
To satisfy the following conditional expression.
[0042]
Conditional expression (3) is the relationship between the horizontal pixel pitch that causes the false resolution signal and the false color signal, and the thickness of the birefringent plate of the uniaxial single crystal having the effect of separating the light rays in the horizontal direction. It is a formula that defines the relationship. If the configuration satisfying conditional expression (3) is assumed, when a solid-state imaging device having a predetermined number of pixels is assumed, the ratio between the external dimension and thickness of the birefringent plate is substantially set appropriately. It is possible to easily solve the processing problems and reduce the handling problems.
[0043]
At this time, the angle formed by the optical axis (Z-axis) of the uniaxial single crystal constituting the optical low-pass filter and the normal line of the incident surface of the birefringent plate is θ, whichever of the conditional expressions (1) or (2) It is desirable to configure to satisfy either one. The meanings of conditional expression (1) and conditional expression (2) are as described above.
[0044]
As described above, in an imaging apparatus using a two-dimensional solid-state imaging device such as a CCD or MOS, the imaging optical system and the solid-state imaging device are used for the purpose of reducing false resolution signals and false color signals due to high-frequency components of the subject image. By appropriately setting the thickness of the optical low-pass filter disposed between them, a suitable optical low-pass filter that eliminates functional and machining problems can be realized, and such an optical low-pass filter can be effectively used. It is possible to realize the provided imaging device. Furthermore, it is possible to realize a single-lens reflex digital still camera in which an interchangeable lens system for a silver salt camera can be used satisfactorily.
[0045]
Next, specific examples of the photographing apparatus to which the optical low-pass filter of the present invention is applied will be described.
[0046]
(First embodiment)
FIG. 2 is a schematic cross-sectional view of an imaging system when the optical low-pass filter of this embodiment is mounted on a single-lens reflex digital still camera. In FIG. 2, reference numeral 1 denotes an imaging lens (imaging optical system) configured to be interchangeable, and a mount is shared so that an interchangeable lens for a silver salt camera can also be used. Reference numeral 2 denotes an optical low-pass filter composed of parallel plates obtained by joining two single crystal birefringent plates of lithium niobate (21 and 22 in the figure), and 3 denotes a solid-state imaging device. In the camera of the present embodiment, the optical low-pass filter 2 is disposed immediately in front of the solid-state image sensor 3, and the subject image is separated into four images by the action of the two birefringent plates 21 and 22.
[0047]
FIG. 3 is an explanatory view of single crystal birefringence plates 21 and 22 of lithium niobate constituting the optical low-pass filter 2 of the present embodiment shown in FIG. The optical low-pass filter 2 of this embodiment is configured by bonding two single-crystal birefringent plates 21 and 22 of lithium niobate. 3A is a diagram of the optical low-pass filter 2 viewed from the optical axis direction of the photographing lens 1, and z1a and z2a are orthogonal projections of the optical axes (Z-axis) of the birefringent plates 21 and 22, respectively. It represents. As shown in FIG. 3A, in this embodiment, the orthogonal projection z1a of the Z-axis of the birefringent plate 21 faces the long side direction of the optical low-pass filter 2 (parallel to the long side), and the birefringent plate 22 The Z-axis orthographic projection z2a of the optical axis of the optical low-pass filter 2 is oriented in a direction that forms an angle φ2. In this embodiment, φ2 = 45 °.
[0048]
The size of the optical low-pass filter 2 is determined in consideration of the solid angle of light incident from the photographing lens 1 and the holding of the optical low-pass filter 2 itself, in addition to the size of the effective pixel region of the image sensor 3. In this embodiment, it is slightly larger than the effective pixel area of the image sensor 3. Of course, the direction of the long side of the optical low-pass filter 2 is arranged so as to substantially coincide with the direction of the long side of the image sensor.
[0049]
FIG. 3B shows a view of the optical low-pass filter 2 of this embodiment as viewed from the direction A in FIG. 3A, and FIG. 3C shows a partially enlarged view thereof. 3 (b) and 3 (c), z1 represents the Z-axis of the birefringent plate 21, and as shown in FIG. 3 (c), the Z-axis z1 of the birefringent plate 21 and the optical low-pass filter 2 The normal of the incident / exit surface is configured to form an angle θ1. In this embodiment, θ1 = 75 °. FIG. 3 (d) shows a view of the optical low-pass filter 2 of this embodiment viewed from the direction B in FIG. 3 (a), and FIG. 3 (e) shows a partially enlarged view thereof. 3 (d) and 3 (e), z2 represents the Z axis of the birefringent plate 22, and as shown in FIG. 3 (e), the Z axis z2 of the birefringent plate 22 and the optical low-pass filter 2 The normal of the incident / exit surface is configured to make an angle θ2. In this embodiment, θ2 = 75 °. Further, regarding the respective thicknesses d1 and d2 of the birefringent plates 21 and 22, the birefringent plate 21 has d1 = 0.45, and the birefringent plate 22 has d2 = 0.30.
[0050]
In this embodiment, by configuring the birefringent plates 21 and 22 in this way, extraordinary rays with respect to ordinary rays are each about 10 μm in the horizontal direction (long side direction), and about 7 μm in a 45 ° oblique direction with respect to the horizontal. The images are shifted and the subject image is separated into four.
FIG. 4 shows the subject image separation state. In FIG. 4, p is a pixel pitch in the long side direction (horizontal direction) of the image sensor 3.
[0051]
Incidentally, the image pickup device 3 of the present embodiment is a solid-state image pickup device having a 10 μm square as one pixel shape, and an array of color filters having a repeating pattern as shown in FIG. 5 is provided in front of each pixel. The two-dimensional color solid-state imaging device having about 2.5 million effective pixels is configured by using 1950 pixels in the horizontal direction and 1300 pixels in the vertical direction. In FIG. 5, G is a color filter that transmits green light, B is blue light, and R is red light.
[0052]
In the present embodiment, the relationship between the thickness d of the birefringent plate 21 having the function of separating the subject image in the long side direction of the solid-state image sensor 3 and the pixel pitch p in the long side direction of the solid-state image sensor 3 is
p / d = 0.222
Thus, the above-described conditional expression (3) is satisfied.
[0053]
In a square pixel two-dimensional solid-state imaging device having a color filter as shown in FIG. 5, when n is an integer, moire occurs in a subject image having a spatial frequency n / p, and the spatial frequency (2n−1) ) / 2p is known to generate a false color signal in a subject image, and an optical low-pass filter that suppresses at least the MTF value at these spatial frequencies is required.
[0054]
The optical low-pass filter of the present embodiment is birefringent in order to efficiently reduce false resolution signals and false color signals generated based on the arrangement of color filters as shown in FIG. 5 and the pixel sampling pitch p. The plate 21 separates the extraordinary ray by p in the horizontal direction, and the birefringent plate 22 emits the extraordinary ray in the oblique 45 ° direction.
[0055]
[Outside 1]

The subject images are separated into four as shown in FIG. In this embodiment, since an imaging device with a pixel pitch of 10 μm is assumed, the birefringent plate 21 mainly reduces the above-described moire by separating images at an equidistance from the pixel pitch of 10 μm in the horizontal direction. The birefringent plate 22 separates an image of about 7 μm obliquely in a 45 ° direction, that is, a distance of 5 μm corresponding to about ½ of a pixel pitch of 10 μm in the horizontal and vertical directions. It acts to reduce the color signal.
[0056]
In this embodiment, by using the two birefringent plates 21 and 22 as described above, the MTF value of the high spatial frequency of the cutoff frequency 1 / 2p or more determined by the pixel pitch p is suppressed to be low. An optical low-pass filter that reduces the generation of false resolution signals and false color signals is efficiently constructed.
[0057]
FIG. 6 shows the spatial frequency characteristics of the optical low-pass filter of this example. 6A shows the horizontal direction (the long side direction of the image sensor), and FIG. 6B shows the vertical direction (the short side direction of the image sensor).
[0058]
(Second embodiment)
FIG. 7 is an explanatory diagram of single crystal birefringent plates 121 and 122 of lithium niobate constituting the optical low-pass filter 102 of the second embodiment. The optical low-pass filter 102 of the present embodiment is used by being disposed in the optical path between the photographing lens 1 and the solid-state imaging device 3 in the same manner as the imaging system shown in FIG. 1, and the optical low-pass filter 2 of the first embodiment. Is functionally equivalent. Differences from the optical low-pass filter 2 of the first embodiment will be described below.
[0059]
FIG. 7A is a diagram of the optical low-pass filter 102 viewed from the optical axis direction of the photographic lens 1, and z1a and z2a are orthogonal projections of the optical axes (Z-axis) of the birefringent plates 121 and 122, respectively. It represents. As shown in FIG. 7A, in this embodiment, the orthogonal projection z1a of the Z-axis of the birefringent plate 121 faces the long side direction of the optical low-pass filter 102 (parallel to the long side), and the birefringent plate 122 is shown. The Z-axis orthogonal projection z2a of the optical axis of the optical low-pass filter 102 is directed in a direction that forms an angle φ2. In this embodiment, φ2 = 45 °.
[0060]
The size of the optical low-pass filter 102 of the present embodiment is configured to be slightly larger than the effective pixel area of the image sensor 3, similarly to the optical low-pass filter 2 of the first embodiment. Note that the direction of the long side of the optical low-pass filter 102 is also configured to substantially coincide with the direction of the long side of the image sensor 3.
[0061]
FIG. 7B shows a view of the optical low-pass filter 102 of this embodiment as viewed from the direction A in FIG. 7A, and FIG. 7C shows a partially enlarged view thereof. 7B and 7C, z1 represents the Z-axis of the birefringent plate 121. As shown in FIG. 7C, the Z-axis z1 of the birefringent plate 121 and the optical low-pass filter 102. The normal of the incident / exit surface is configured to make an angle θ1. In this embodiment, θ1 = 15 °. FIG. 7 (d) shows a view of the optical low-pass filter 102 of this embodiment viewed from the direction B in FIG. 7 (a), and FIG. 7 (e) shows a partially enlarged view thereof. 7D and 7E, z2 represents the Z-axis of the birefringent plate 122, and as shown in FIG. 7E, the Z-axis z2 of the birefringent plate 122 and the optical low-pass filter 102. The normal line of the incident / exit surface is configured to form an angle θ2. In this embodiment, θ2 = 35 °. In the present embodiment, the thickness d1 of the birefringent plate 121 is 0.30, and the thickness d2 of the birefringent plate 122 is 0.30.
[0062]
In the optical low-pass filter 102 of this embodiment, the birefringent plates 121 and 122 are configured in this manner, so that the extraordinary rays are each approximately 10 μm in the horizontal direction with respect to the ordinary rays as in the first embodiment. The image is shifted by about 7 μm in the 45 ° direction, and the subject image is separated into four.
[0063]
In the present embodiment, an example in which the angle θ formed by the Z axis of lithium niobate constituting the birefringent plate and the normal line of the light incident / exit surface of the optical low-pass filter is different from that of the first embodiment is shown. By sufficiently setting the angle θ within a range satisfying 1) or (2), a sufficiently thin optical system that overcomes problems in machining and handling as in the first embodiment. A low-pass filter can be realized.
[0064]
In the first and second embodiments, the bonding is performed with the Z axes of the plurality of birefringent plates oriented in different directions, as described above, to reduce the MTF value of the high frequency component. In addition to this, it is also for obtaining the effect of overcoming the drawback of the lithium niobate single crystal that is easily cleaved in a predetermined direction.
[0065]
In addition, using a birefringent plate of lithium niobate single crystal as an optical low-pass filter, it is not a colored glass filter that absorbs light in the infrared wavelength region as an infrared cut filter, but an infrared region composed of a dielectric multilayer film. If a dichroic filter that reflects light is used in combination, a parallel plate as an infrared cut filter becomes unnecessary, and the size and optical performance can be further improved. At this time, the dichroic filter is more effective if it is deposited on at least one surface of the birefringent plate of lithium niobate single crystal.
[0066]
In addition, as a feature of the lithium niobate single crystal, there is a point that the refractive index is high with respect to optical glass or the like used for a crystal or a photographing lens. For this reason, the interface between the parallel plates of the lithium niobate single crystal and the air has high reflectivity, and if the surface is exposed, the reflected light becomes flare and may be a factor of remarkably degrading the image quality. Also, regarding the interface with the joint surface when joining birefringent plates, if the adhesive having the same refractive index as these birefringent plates is used as the adhesive, the problem of interface reflection will be Although few, such adhesives that are transparent are not very common. Therefore, it is preferable to deposit an antireflection film at least on the interface of these birefringent plates with the air, and also to reduce the reflection at the interface with the adhesive on the bonding surface side of the birefringent plates. More preferably, a film for is deposited. Thus, when configuring an optical low-pass filter using a plurality of birefringent plates of a single crystal of lithium niobate, a predetermined vapor deposition film may be added to each surface of the birefringent plate and bonded. desirable.
[0067]
In the description of the first and second embodiments, an example in which the optical low-pass filter is configured by two birefringent plates has been described. However, the three birefringent plates are bonded to each other depending on the high frequency component of the subject image. Naturally, it is conceivable to correct the false resolution signal and the false color signal better.
[0068]
  (Other embodiments)
  Next, an embodiment in which the optical low-pass filter of the first example or the second example is used as a protection plate of a solid-state imaging device will be described with reference to FIG. In FIG. 8, 92 represents the optical low-pass filter described in the first or second embodiment, 94 represents a package, and 95 represents an imaging unit. The image pickup device in FIG. 8 is an image pickup unit composed of a two-dimensional solid-state image pickup device such as a CCD or MOS.95The package94In the embodiment shown in FIG. 8, an optical low-pass filter is applied to the transparent parallel plate so as to function as a protective plate. ing.
[0069]
Usually, a low-priced glass plate is often used for the parallel plate as the protective plate, but the presence of the parallel plate in the optical path is desirable from the viewpoints of space and the optical performance of the photographing optical system for the reasons described above. Absent. Therefore, in the embodiment shown in FIG. 8, an optical low-pass filter that is almost essential in an optical system using an image sensor is used as a protective plate of the image pickup unit, and the number of parallel plates existing in the optical path is reduced as much as possible.
[0070]
FIG. 9 is an embodiment different from FIG. 8 in which the optical low-pass filter of the first embodiment or the second embodiment is used as a protective plate of a solid-state image sensor, and the image pickup unit is disposed on the substrate. Thus, the imaging unit itself is made thinner. 9 has a configuration in which an image pickup unit 97 and an optical low-pass filter 98 as a protection plate are bonded to both surfaces of a substrate 96, whereby the thickness of the image pickup unit itself is reduced. The objective is better achieved.
[0071]
In the embodiment of the solid-state imaging device shown in FIG. 8 or FIG. 9, the optical low-pass filter as the protective plate and the imaging unit are not in direct contact, but an optical low-pass filter is provided in the imaging unit for further miniaturization. You may stick together.
[0073]
【The invention's effect】
  As described above, according to the present invention, by appropriately setting the relationship between the optical axis of the crystal and the normal line of the incident surface so that the thickness of the birefringent plate is appropriate, the optical performance and the machining are improved. Optical low-pass filter that eliminates the above problemsImaging device havingCan be realized.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a relationship between an angle formed by an optical axis of a lithium niobate single crystal and a normal line of an incident / exit surface of a parallel plate and a separation width of an ordinary ray and an extraordinary ray.
FIG. 2 is a schematic sectional view of an imaging system when the optical low-pass filter of the first embodiment is mounted on a single-lens reflex camera.
FIG. 3 is an explanatory diagram of an optical low-pass filter according to the first embodiment.
FIG. 4 is an explanatory diagram showing how a subject image is separated by ordinary rays and extraordinary rays.
FIG. 5 is an explanatory diagram of a color filter array used on the image sensor.
FIG. 6 is an explanatory diagram showing spatial frequency characteristics of the optical low-pass filter of the first embodiment.
FIG. 7 is an explanatory diagram of an optical low-pass filter according to a second embodiment.
FIG. 8 is an explanatory diagram of an imaging unit in which an optical low-pass filter is provided as a protection plate.
FIG. 9 is an explanatory diagram of another imaging unit in which an optical low-pass filter is disposed as a protection plate.
FIG. 10 is an explanatory diagram of light aberration occurring on a parallel plate.
[Explanation of symbols]
1 Photo lens
2,102 Optical low-pass filter
3 Image sensor
21, 22, 121, 122 Birefringent plate of lithium niobate single crystal

Claims (2)

An imaging apparatus comprising an optical low-pass filter having a biaxial plate of a uniaxial single crystal of lithium niobate having a refractive index difference of 0.02 or more between ordinary rays and extraordinary rays, and an imaging element having a rectangular imaging surface. The birefringent plate separates the subject image in the long side direction of the imaging surface, and the angle formed by the optical axis of the uniaxial single crystal of the birefringent plate and the surface normal of the birefringent plate is θ, When the pixel pitch in the long side direction of the imaging surface is p and the thickness of the birefringent plate is d,
10 ° < θ <30 ° ... (1)
60 ° < θ <80 ° ... (2)
One of the conditional expressions,
0.012 < p / d <0.033 ... (3)
An image pickup apparatus satisfying the following conditional expression: The optical low-pass filter joins a plurality of uniaxial single crystal birefringent plates, and the orthogonal projections of the uniaxial single crystal optical axes of the birefringent plates on the surfaces of the birefringent plates are in different directions. The imaging apparatus according to claim 1, configured as described above.

Images