JP3792991B2 - Optical low-pass filter and optical apparatus using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical low-pass filter and an optical device using the same, and is suitable for an electrophotographic camera, a digital camera, a video camera, or the like that uses a two-dimensional solid-state image sensor such as CCD or MOS.
[0002]
[Prior art]
In an optical apparatus using a solid-state imaging device in which pixels are arranged two-dimensionally at an equal pitch, such as CCD and MOS, the subject image is sampled at each pixel pitch. If there are two or more high-frequency components, moire (moire fringes) or false colors occur, and the image is significantly deteriorated.
[0003]
2. Description of the Related Art Conventionally, an optical low-pass filter using a plurality of birefringent plates such as quartz is known as a means for reducing moire and false colors provided in an imaging system. An optical low-pass filter is used to shift the abnormal light of the optical image to form an image, thereby blurring the optical image and cutting off high-frequency components that cause generation of moire and false colors, thereby obtaining a good image.
[0004]
The present applicant has proposed an imaging apparatus that can obtain a good image using an optical low-pass filter, for example, in Japanese Patent Laid-Open Nos. 2-19821 and 3-31813.
[0005]
In recent years, an optical low-pass filter using lithium niobate having high birefringence has been proposed. For example, Japanese Patent Laid-Open No. 11-218612 proposes an optical low-pass filter having a structure in which a quarter λ plate of quartz is provided between two lithium niobates and bonded. When lithium niobate with high birefringence is used in an optical low-pass filter, the same image separation can be achieved with a thin optical low-pass filter, so that space can be used effectively and when an interchangeable lens for a silver salt camera is attached. There is an advantage that the generation of aberration due to the parallel plate can be reduced.
[0006]
[Problems to be solved by the invention]
The conventional optical low-pass filter using lithium niobate has the above-mentioned advantages, but on the other hand, lithium niobate is a thin member, and it is configured to be bonded to both sides (light incident side) of quartz. ,
-When bonding thin and warped surfaces to both sides of quartz, it is difficult to bond and bond while maintaining high surface accuracy of lithium niobate.
・ Since the stress remains even after bonding to the crystal after correcting the warpage of lithium niobate, it is unstable and easy to cause peeling due to stress in the environment such as high temperature and high humidity, thermal shock, etc.・ Thin lithium niobate, which is thin and difficult to process, is prone to scratches during processing.
There was a problem such as.
[0007]
The present invention provides an optical low-pass filter that has a small thickness (optical path length) for performing predetermined image separation, generates little aberration even when provided in an optical system, and can easily obtain a predetermined low-pass effect. The purpose is to provide optical equipment.
[0008]
[Means for Solving the Problems]
The optical low-pass filter according to the first aspect of the invention includes, in order from the light incident side, a first birefringent plate A that separates light in a first direction, a second birefringent plate B that separates light in a second direction, In an optical low-pass filter formed by joining a third birefringent plate C that is separated in the direction of 3,
NoA, NoB, and NoC are the first, second, and third birefringent plates A, B, and C, respectively, and the refractive index with respect to the d-line normal light, and NeA, NeB, and NeC are the first, second, and third, respectively. When the refractive index of the birefringent plates A, B, and C with respect to the extraordinary light of the d-line is | (NeA-NoA) / NoA | <| (NeB-NoB) / NoB |
| (NeC-NoC) / NoC | <| (NeB-NoB) / NoB |
It is characterized by satisfying the following conditions.
[0009]
According to a second aspect of the present invention, when the thicknesses (unit: mm) of the first, second, and third birefringent plates A, B, and C are TA, TB, and TC, respectively, in the first aspect of the present invention, the. 7 <TA <2
0.004 <| (NeA-NoA) / NoA | <0.01
0.1 <TB <0.4
0.025 <| (NeB-NoB) / NoB | <0.12
0.7 <TC <2
0.004 <| (NeC-NoC) / NoC | <0.01
It is characterized by satisfying the following conditions.
[0010]
An optical low-pass filter according to a third aspect of the invention includes, in order from the light incident side, a first birefringent plate A that separates light in a first direction, a second birefringent plate B that separates light in a second direction, In an optical low-pass filter formed by joining a third birefringent plate C that is separated in the direction of 3,
NoA, NoB, and NoC are the first, second, and third birefringent plates A, B, and C, respectively, and the refractive index with respect to the d-line normal light, and NeA, NeB, and NeC are the first, second, and third, respectively. The refractive indexes of the birefringent plates A, B, and C with respect to the extraordinary light of the d line , TA, TB, and TC are the thicknesses of the first, second, and third birefringent plates A, B, and C (unit: mm), respectively. ) 0.7 <TA <2
0.004 <| (NeA-NoA) / NoA | <0.01
0.1 <TB <0.4
0.025 <| (NeB-NoB) / NoB | <0.12
0.7 <TC <2
0.004 <| (NeC-NoC) / NoC | <0.01
It is characterized by satisfying the following conditions.
[0011]
According to a fourth aspect of the present invention, in the first, second or third aspect of the invention, the material of the first and third birefringent plates A and C is quartz, and the material of the second birefringent plate B is lithium niobate. It is characterized by being.
[0012]
According to a fifth aspect of the invention, in the invention according to any one of the first to fourth aspects, an infrared cut coating is applied to a light emitting surface of the first birefringent plate or the third birefringent plate. It is a feature.
[0013]
The invention of claim 6 is characterized by the fact that in the invention of any one of claims 1 to 4, an infrared cut coating is provided on the light incident side surface of the first birefringent plate.
[0014]
An optical apparatus according to a seventh aspect of the invention is characterized by including the optical low-pass filter according to any one of the first to sixth aspects.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
FIG. 1A is a cross-sectional view of a main part of an optical low-pass filter according to Embodiment 1 of the present invention, and FIG. 1B is an explanatory view of an optical axis of a birefringent plate in FIG. FIG. 7 is a schematic diagram of a main part of an optical apparatus such as a video camera or a digital camera having the optical low-pass filter of the present invention. FIG. 7 shows a state in which an image of the object OB is formed on the imaging means IP via the optical low-pass filter 1 by the imaging system (imaging lens) LA.
[0016]
FIG. 2 is an explanatory view showing a state of separation of incident light by the optical low-pass filter of the present invention, and FIG. 3 is an explanatory view showing a frequency trap on the image pickup means surface in an optical apparatus using the optical low-pass filter of the present invention.
[0017]
As shown in FIG. 1A, the optical low-pass filter 1 according to the present embodiment includes a first crystal plate 2 as a first birefringent plate in order from the imaging lens LA toward the imaging plate IP, that is, from the light incident side. Lithium niobate plate 3 as the second birefringent plate, which is thinner than the quartz plate 1, and the second quartz plates 4 and 5 as the third birefringent plate are infrared cut coatings and the first quartz It is provided on the light incident surface side of the plate 2 . An antireflection coating 6 is provided on the light emitting surface side of the second quartz plate 4.
[0018]
As shown in FIG. 1B, the projection in the planar direction of the optical axis 2a of the first crystal plate 2 is oriented upward 45 degrees with respect to the long side of the imaging screen IP, and is a lithium niobate plate material. The projection in the plane direction of the optical axis 3a of 3 is directed to the right horizontal direction with respect to the long side of the imaging screen IP, and the projection in the plane direction of the optical axis 4a of the second crystal plate 4 is the long side of the imaging screen IP. With respect to the lower 45 degrees.
[0019]
The light transmitted through the first crystal plate 2 is separated in the direction of 45 degrees upward with respect to the long side of the imaging screen IP (light L1 and light L11). Separated in the right horizontal direction with respect to the long side (light L11 and light L112, light L1 and light L12), and each of which is transmitted through the second crystal plate 4 and then in the direction of 45 degrees below the long side of the imaging screen (Light L1 and light L13, light L12 and light L123, light L11 and light L113, light L112 and light L1123), and separated into 8 points as a whole.
[0020]
The direction of the optical axis may be directed to any one as long as it is separated into 8 points as a whole.
[0021]
In the optical low-pass filter of the present invention, NoA, NoB, and NoC are the first, second, and third birefringent plates 2, 3, and 4, respectively. The first birefringent plate 2, the second birefringent plate 3, and the third birefringent when the refractive indexes of the second and third birefringent plates 2, 3, 4 to the d-line extraordinary light are used. Plate 4 is
| (NeA-NoA) / NoA | <| (NeB-NoB) / NoB | (1)
| (NeC-NoC) / NoC | <| (NeB-NoB) / NoB | (2)
The two conditions are satisfied.
[0022]
As a result, a good low-pass effect is obtained.
[0023]
Conditional expressions (1) and (2) mean that the birefringence of the second birefringent plate is larger than the birefringence of the first and third birefringent plates, and such birefringence By using a combination of plates, the thickness of the birefringent plates 2 and 4 on the outer side (light incident / exit surface side) is increased, and the thickness of the middle birefringent plate 3 is decreased. Has achieved a good optical low-pass filter.
[0024]
Note that the optical low-pass filter of the present invention alone, or under the conditions (1) and (2) described above, TA, TB, and TC are changed to the first, second, and third birefringent plates A, B, and C, respectively. Of the first birefringent plate 2, the second birefringent plate 3, and the third birefringent plate 4,
0.7 <TA <2 (3)
0.004 <| (NeA-NoA) / NoA | <0.01 (4)
0.1 <TB <0.4 (5)
0.025 <| (NeB-NoB) / NoB | <0.12 (6)
0.7 <TC <2 (7)
0.004 <| (NeC-NoC) / NoC | <0.01 (8)
It is preferable to configure so as to satisfy the following conditions.
[0025]
The thicknesses of the first, second, and third birefringent plates are determined so as to satisfy the conditional expressions (3), (5), and (7), and the conditional expressions (4), (6), and (8) are satisfied. Thus, if the magnitude of the birefringence of the first, second and third birefringent plates is determined, the separation of the optical image, the generation of aberration, the processing of each birefringent plate, and the workability of joining are improved. be able to. That is, if the thicknesses of the first and third birefringent plates are reduced beyond the lower limit values of the conditional expressions (3) and (7), it is difficult to obtain a sufficient optical image separation, and the false color is easily noticeable. Further, if the thickness of the second birefringent plate is reduced beyond the lower limit value of the conditional expression (5), the polishing process of the second birefringent plate tends to be difficult. On the other hand, if the thicknesses of the first, second and third birefringent plates increase beyond the upper limit values of the conditional expressions (3), (5) and (7), they are generated in the optical low-pass filter when used in the photographing system. Astigmatism and spherical aberration are increased. Conditional expressions (4), (6), and (8) If the birefringence of the first, second, and third birefringent plates is small beyond the lower limit value, it is difficult to obtain a sufficient separation of optical images. When the birefringence of the first, second, and third birefringent plates increases beyond the upper limit of (4), (6), and (8), the plates of the first, second, and third birefringent plates It means that the thickness is reduced, and it becomes difficult to obtain good workability.
[0026]
It should be noted that the first, second, and third thicknesses when the total thickness of the first, second, and third birefringent plates is T instead of the conditional expressions (3), (5), and (7). Thickness TA, TB, TC of birefringent plate is 0.38 <TA / T <0.49
0.024 <TB / T <0.2
0.38 <TC / T <0.49
If set as described above, the above-described optical low-pass filter of the present invention can be obtained.
[0027]
In this embodiment, the plate thickness of the first crystal plate 2 is 1.1 mm, the plate thickness of the lithium niobate plate 3 is 0.22 mm, and the plate thickness of the second crystal plate 4 is 1.1 mm. It is set.
[0028]
The first quartz plate 2 constituting the optical low-pass filter of the present invention is provided with an infrared cut coating on the most object side surface (light incident surface). The lithium niobate plate 3 is bonded to the quartz plate on both sides, but lithium niobate has a very high refractive index of 2.3002, and the refractive index of the adhesive is about 1.5. As compared with the above, since the difference in refractive index is large and reflection occurs at the interface with the adhesive, antireflection films are provided on both sides. An antireflection film is provided on the outer surface (light emitting surface) of the second crystal plate 4. The interface between the first crystal plate 2 and the adhesive of the second crystal plate 4 has a small difference in refractive index and a small reflection at the interface, so an antireflection film may be attached, but there is no significant effect even if it is not attached. In this embodiment, a structure in which a birefringent plate having a large birefringence is sandwiched between two birefringent plates having a birefringence similar to that of quartz is warped with lithium niobate alone. Even if it remains, it is possible to stably manufacture an optical low-pass filter with good surface accuracy by adhering from both sides with a thick quartz plate and restraining it without difficulty to suppress warping well. In addition, an optical low-pass filter having a stable form is realized with less resistance to peeling due to stress in the environment such as high temperature and high humidity and thermal shock. Further, lithium niobate is a material that is difficult to process and easily causes scratches, but the lithium niobate is buried in an adhesive so that the scratches are not noticeable.
[0029]
In the present embodiment, since the number of layers of the film is large, an infrared cut coat that is likely to be warped or the like is provided on the thick plate 2 so that the occurrence of warpage in the coating process is reduced and the surface accuracy is kept good. .
[Embodiment 2]
FIG. 4A is a cross-sectional view of an essential part of Embodiment 2 of the optical low-pass filter of the present invention,
FIG. 4B is an explanatory diagram of the optical axis of the birefringent plate of FIG.
[0030]
FIG. 5 is an explanatory diagram showing a state of separation of incident light by the optical low-pass filter of the present invention, and FIG. 6 is an explanatory diagram showing frequency traps on the imaging means surface in an optical apparatus using the optical low-pass filter of the present invention.
[0031]
As shown in FIG. 4A, the optical low-pass filter 1 of the present embodiment includes a first crystal plate 2 as a first birefringent plate, in order from the imaging lens LA toward the imaging plate IP, that is, in order from the light incident side. The lithium niobate plate 3 as the second birefringent plate thinner than the first quartz plate 2 and the second quartz plate 4 as the third birefringent plate are joined together with an adhesive in this order. .
[0032]
An infrared cut coating 5 is provided on the light incident surface side of the first quartz plate 2.
[0033]
An antireflection coating 6 is provided on the light emitting surface side of the second quartz plate 4.
[0034]
As shown in FIG. 4B, the projection in the planar direction of the optical axis 2a of the first quartz plate 2 is directed horizontally with respect to the long side of the imaging screen IP, and the optical material of the lithium niobate plate 3 is optical. The projection in the plane direction of the axis 3a is directed upward 45 degrees with respect to the long side of the imaging screen IP, and the projection in the plane direction of the optical axis 4a of the second crystal plate 4 is relative to the long side of the imaging screen IP. It is arranged facing upwards.
[0035]
Therefore, as shown in FIG. 5, the light transmitted through the first crystal plate 2 is separated in the horizontal direction with respect to the long side of the imaging screen (light L1 and light L11), and each of them is transmitted through the lithium niobate plate 3 Later, it is separated in the direction of 45 degrees upward with respect to the long side of the imaging screen (light L11 and light L112, light L1 and light L12), and each of them is transmitted through the second quartz plate 4 with respect to the long side of the imaging screen. The light L1 and the light L13, the light L12 and the light L123, the light L11 and the light L113, and the light L112 and the light L1123 are separated into eight points as a whole.
[0036]
The direction of the optical axis may be any direction as long as it is separated into 8 points as a whole.
[0037]
In this embodiment, lithium niobate has a structure in which a birefringent plate having a large birefringence is sandwiched between two birefringent plates having a birefringence of the order of quartz as in the case of lithium niobate. Even if the warp remains alone, it is possible to stably manufacture an optical low-pass filter with good surface accuracy by adhering from both sides with a thick quartz plate and restraining it without difficulty. In addition, an optical low-pass filter having a stable form is realized with less resistance to peeling due to stress in the environment such as high temperature and high humidity and thermal shock. Further, lithium niobate is a material that is difficult to process and easily causes scratches, but the lithium niobate is buried in an adhesive so that the scratches are not noticeable.
[0038]
The optical low-pass filter of the present invention can also serve as a cover glass for the image sensor package. In this case, it is preferable to provide an IR cut coat on the side far from the imaging surface, because the influence of ghost and flare due to reflection on the imaging surface can be reduced, and high image quality can be obtained.
[0039]
As described above, in the optical low-pass filter of the present embodiment, a birefringent plate having a large birefringence like lithium niobate is sandwiched between two birefringent plates having a birefringence of the order of quartz. By bonding and providing an infrared cut coat on the crystal surface,
A three-element optical low-pass filter capable of sufficiently removing moire and false colors can be achieved with a thin shape with little occurrence of spherical aberration and astigmatism.
[0040]
・ Even if warpage remains with lithium niobate alone, thick quartz plates are adhered and regulated from both sides, so it is possible to stably produce an optical low-pass filter with good pressing surface accuracy. . In addition, there is little occurrence of peeling due to stress in the environment such as high temperature and high humidity and thermal siege, and it is possible to realize a component having a stable form.
[0041]
-Since the surface is protected because the birefringent plate having a high birefringence index and a weak environment resistance property is sandwiched, the environment resistance property such as high temperature and high humidity is improved.
[0042]
-Lithium niobate is a material that is difficult to process and easily causes scratches, but it is buried in an adhesive so that the scratches are not noticeable.
[0043]
-Since the number of layers of the film is such that the lR cut coat, which is likely to be warped, is provided on the thick plate material, the surface accuracy can be kept good.
[0044]
Etc. are obtained.
[0045]
【The invention's effect】
According to the present invention, an optical low-pass filter that has a small thickness (optical path length) for performing predetermined image separation, generates little aberration even when provided in an optical system, and can easily obtain a predetermined low-pass effect, and the optical low-pass filter An optical instrument using can be achieved.
[Brief description of the drawings]
FIG. 1 is a diagram showing the configuration of an optical low-pass filter according to Embodiment 1 of the present invention. FIG. 2 is an explanatory diagram of point image separation of the optical low-pass filter according to Embodiment 1 of the present invention. FIG. 4 is a diagram showing a frequency trap according to the first embodiment. FIG. 4 is a diagram showing a configuration of an optical low-pass filter according to a second embodiment of the present invention. 6 is a diagram showing a frequency trap of Embodiment 2 of the optical low-pass filter of the present invention. FIG. 7 is a schematic diagram of an optical apparatus having the optical low-pass filter of the present invention.
DESCRIPTION OF SYMBOLS 1 ... Optical low pass filter 2 ... Crystal plate 3 ... Lithium niobate 4 ... Crystal plate 5 ... IR cut coat 6 ... Antireflection coat LA ... Imaging system IP- ..Imaging means OB ... object

Claims (7)

In order from the light incident side, a first birefringent plate A that separates light in a first direction, a second birefringent plate B that separates light in a second direction, and a third birefringent plate B that separates light in a third direction. In an optical low-pass filter configured by joining a refractive plate C,
NoA, NoB, and NoC are the first, second, and third birefringent plates A, B, and C, respectively, and the refractive index with respect to the d-line ordinary light is NeA, NeB, and NeC are the first, second, and third, respectively. When the refractive index of the birefringent plates A, B, and C with respect to the extraordinary light of the d-line is | (NeA-NoA) / NoA | <| (NeB-NoB) / NoB |
| (NeC-NoC) / NoC | <| (NeB-NoB) / NoB |
An optical low-pass filter that satisfies the following condition. When the plate thickness (unit: mm) of the first, second, and third birefringent plates A, B, and C is TA, TB, and TC, respectively, 0.7 <TA <2
0.004 <| (NeA-NoA) / NoA | <0.01
0.1 <TB <0.4
0.025 <| (NeB-NoB) / NoB | <0.12
0.7 <TC <2
0.004 <| (NeC-NoC) / NoC | <0.01
The optical low-pass filter according to claim 1, wherein the following condition is satisfied. In order from the light incident side, a first birefringent plate A that separates light in a first direction, a second birefringent plate B that separates light in a second direction, and a third birefringent plate B that separates light in a third direction. In an optical low-pass filter configured by joining a refractive plate C,
NoA, NoB, and NoC are the first, second, and third birefringent plates A, B, and C, respectively, and the refractive index with respect to the d-line normal light, and NeA, NeB, and NeC are the first, second, and third, respectively. The refractive indexes of the birefringent plates A, B, and C with respect to the extraordinary light of the d line , TA, TB, and TC are the thicknesses of the first, second, and third birefringent plates A, B, and C (unit: mm), respectively. ) 0.7 <TA <2
0.004 <| (NeA-NoA) / NoA | <0.01
0.1 <TB <0.4
0.025 <| (NeB-NoB) / NoB | <0.12
0.7 <TC <2
0.004 <| (NeC-NoC) / NoC | <0.01
An optical low-pass filter that satisfies the following condition.   4. The material of the first and third birefringent plates A and C is quartz, and the material of the second birefringent plate B is lithium niobate. Optical low-pass filter. The infrared cut coating is provided on the light exit surface of the first birefringent plate or the third birefringent plate, according to any one of claims 1 to 4. Optical low-pass filter.   The optical low-pass filter according to any one of claims 1 to 4, wherein an infrared cut coating is provided on a light incident side surface of the first birefringent plate.   An optical apparatus comprising the optical low-pass filter according to claim 1.

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