JP2005062524A - Optical filter and optical instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an inexpensive and thin optical filter using optical anisotropy due to a fine structure.
SOLUTION: Two optical elements 1 and 2 having a light beam separating action and an optical element 3 disposed between these two optical elements and having a depolarizing action have at least one of the three optical elements. An optical filter is configured as a fine structure element having a light beam separation action or a depolarization action by optical anisotropy having a structure finer than the wavelength of incident light.
[Selection] Figure 1

Description

  The present invention relates to an optical filter, and more particularly to an optical filter suitable as a low-pass filter used in an optical apparatus such as a digital still camera or a video camera.

  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 CMOS sensor, a subject image is sampled for each pixel pitch. For this reason, when shooting an object with a component with a high spatial frequency, an image caused by aliasing distortion of the high frequency component (if a frequency higher than the sampling pitch is input, a false answer that occurs when the cutoff frequency is used as the boundary) Image) is generated as an unnecessary resolution signal having a low frequency component, which is a factor for reducing the resolution of the subject image.

  Similarly, in an image pickup apparatus using a single-plate color solid-state image pickup device, unnecessary color signals determined by the arrangement of color filters arranged in front of each pixel when shooting a subject having a component having a high spatial frequency. Occurs, which is a factor of reducing the color reproducibility of the subject image.

  Conventionally, various optical low-pass filters have been proposed for the purpose of reducing unnecessary signals due to high frequency components of the subject image.

  The most typical of them uses a single crystal parallel plate such as quartz. When the normal of the incident surface and the optical axis (Z axis) of the crystal are tilted by a predetermined angle, a light beam incident on a parallel plate of quartz that is a uniaxial crystal exhibits anisotropy in the direction of the Z axis, Separate into ordinary and extraordinary rays. The separated light beams are emitted in parallel from the 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 of the incident surface of the parallel plate and the Z axis of the crystal and the thickness of the parallel plate. This phenomenon is called the birefringence action of a uniaxial crystal material like quartz, and it affects the light rays of the crystal when polarized light with a vibration component along the crystal axis and polarized light with a vibration component perpendicular to the crystal axis. Caused by different refractive indices.

  Polarized light having a vibration direction along the crystal axis is called an ordinary ray, and polarized light having a vibration direction perpendicular to the crystal axis is called an extraordinary ray. Moreover, the refractive index with respect to each light ray differs in a uniaxial crystal.

  As conventional examples of the optical low-pass filter using the action of such a crystal, there are those disclosed or proposed in Patent Documents 1 to 4.

  The optical low-pass filter disclosed in Patent Document 1 and Patent Document 2 assumes a striped filter as a color filter, and generates an unnecessary color signal generated when the subject's spatial frequency is synchronized with the color filter. In order to reduce this, light rays are separated into ordinary rays and extraordinary rays by a parallel plate having birefringence, such as quartz crystal, and formed on the imaging surface.

  In particular, in the optical low-pass filter disclosed in Patent Document 1, a single crystal of crystal is cut out and used so that its optical axis (Z-axis) forms an angle of approximately 45 ° with respect to an incident / exit surface of a parallel plate.

  The optical low-pass filters proposed in Patent Document 3 and Patent Document 4 assume a Bayer array filter 100 as shown in FIG. 10 as a color filter, for example, and combine light beams by combining a plurality of birefringent plates. Is separated into ordinary rays and extraordinary rays, and the subject image is separated into a plurality of images shifted by a predetermined pitch, and the generation of unnecessary resolution signals and unnecessary color signals due to the high frequency components of the subject is effectively reduced. I am letting.

  Also, Patent Documents 5 and 6 propose examples in which a single crystal plate other than quartz is used as the birefringent plate.

The optical low-pass filter proposed in Patent Documents 5 and 6 uses a plurality of birefringent plates, and at least one of them uses lithium niobate. A single crystal of lithium niobate is a uniaxial crystal, similar to a crystal single crystal, but the difference between the refractive index of ordinary rays and the refractive index of extraordinary rays is larger than that of quartz, so the separation width of a given ray The thickness of the birefringent plate necessary for obtaining the thickness can be reduced.
Japanese Utility Model Publication No. 47-18688 (claims, FIG. 3 etc.) Japanese Utility Model Publication No. 47-18689 (claims, FIG. 3 etc.) JP-A-59-75222 (claims, page 2, lower left column, line 5 to lower right column, line 7, FIG. 4, 5 etc.) JP-A-60-164719 (claims, page 2, lower right column, lines 8 to 20, line 4 etc.) JP-A-9-212222 (Claims, paragraphs 0011 to 0012, FIG. 2, etc.) JP-A-11-218612 (paragraphs 0011 to 0012, FIG. 3B, etc.)

  In a conventional example in which one or a plurality of quartz birefringent plates are combined and used as an optical low-pass filter, the difference in refractive index between ordinary rays and extraordinary rays is small. It is necessary to increase the thickness of the birefringent plate to a certain extent.

  In general, when a parallel flat 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.

tanφ = (no ² −ne ² ) sin θ cos θ / (ne ² cos ² θ + no ² sin ² θ) (1)
Expression (1) 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 °.

  If the refractive index of ordinary light with respect to the d-line of a crystal single crystal is no = 1.544 and the refractive index of extraordinary light is ne = 1.553, then tan φ≈−0.0058 when θ = 45 °.

  Here, 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 birefringent plate in order to remove unnecessary resolution signals generated in this direction. It is assumed that In this case, as described above, since tanφ≈−0.0058 in the single crystal crystal, the thickness d of the birefringent plate needs to be at least about 1.7 mm.

  As described above, when a crystal having a small refractive index difference between ordinary light and extraordinary light is used as a birefringent plate, the optical low-pass filter becomes thick and the optical system becomes large. At the same time, crystals such as crystals are expensive, and the filters become more expensive due to the need for expensive and thick crystals.

  Further, if a crystal having a large difference in refractive index between ordinary light and extraordinary light is used, such as lithium niobate, the thickness can be reduced. However, the crystal itself is expensive and difficult to process.

  An object of the present invention is to provide an inexpensive and thin optical filter that suppresses the use of a crystal plate as much as possible.

  In order to achieve the above object, the present invention realizes an inexpensive and thin optical filter by using a phenomenon called optical anisotropy of a fine structure, that is, structural birefringence or form anisotropy.

  In other words, the optical low-pass filter of the present invention has two optical elements having a light beam separating action and an optical element that is disposed between the two optical elements and has a depolarizing action. Here, at least one of the three optical elements is a microstructure element having a light beam separation action or a depolarization action by optical anisotropy having a structure finer than the wavelength of incident light.

  As described above, according to the present invention, an optical filter that is thinner than a conventional optical filter can be realized by using an optical element having a fine structure. Further, since the number of expensive optical crystal plates can be reduced, the optical filter can be configured at low cost.

  Before describing the embodiments of the present invention, structural birefringence will be described.

  By artificially controlling and producing a fine structure finer than the wavelength of incident light, it becomes possible to provide optical anisotropy, that is, birefringence.

  As shown in FIG. 9, it is known that the effective refractive index varies depending on the polarization direction of incident light in a structure in which layers having different refractive indexes n1 and n2 are alternately stacked. Since the refractive index varies depending on the polarization direction, optical anisotropy occurs. The refractive index is called an effective refractive index, and is generated by the directionality of the structure, and is sometimes called structural birefringence.

  When the layer pitches (thicknesses) a and b in FIG. 9 are sufficiently smaller than the wavelength of the incident light, the effective refractive index for the light is given by the following equation depending on the angle of polarization with respect to the optical axis of the incident light. It is done.

For polarized light with an angle of 0 degrees parallel to the layer,

... (2)
For polarized light at an angle of 90 degrees perpendicular to the layer,

... (3)
It becomes.

  From these equations, the effective refractive index varies between n1 and n2 depending on the ratio of the thicknesses a and b of each layer, and even if each layer is made of an isotropic material, the optical anisotropy depends on the polarization direction. It turns out that produces.

  Here, like the crystal having a uniaxial optical axis, the optical axis can be defined in the stacking direction. For example, consider the case where an isotropic material having a refractive index of 1.500 and air are laminated. Assuming that the pitch is an equal interval such that a = b, n0 = 1.275 (ordinary ray) and n90 = 1.177 (abnormal ray) from the equations (2) and (3). This means that the refractive index difference between ordinary and extraordinary rays reaches 0.1. This is about 17 times that of quartz, indicating that even a thin layer can sufficiently separate light.

  If calculated by applying the equation (1), tanφ = 0.0798 is obtained when the angle of the optical axis is 45 °. For example, when an attempt is made to obtain a separation width of 10 μm (0.01 mm), the layer thickness can be as thin as 0.125.

  In the present embodiment, a structure suitable as an optical low-pass filter is realized by using such optical anisotropy to cause structural birefringence.

  FIG. 1 shows the configuration of an optical low-pass filter that is Embodiment 1 of the present invention.

  In this embodiment, the light beam (light beam) is separated by the light beam separating action of two uniaxial crystal plates 1 and 2 such as crystal whose optical axis is inclined with respect to the incident light beam. An optical element having optical anisotropy by a periodic structure (fine periodic structure) finer than the wavelength of incident light is disposed as a phase plate (λ / 4 plate) 3 that is a depolarization layer between 1 and 2. .

  The fine structure of the phase plate 3 is formed on the back surface of the crystal plate 1 (the inner surface facing the crystal plate 2). It is assumed that the line segments obtained by projecting the optical axes of the crystal plates 1 and 2 into the crystal plate plane are orthogonal.

  Here, the incident light beam L1 is separated by the crystal plate 1 into two polarized light beams L2 and L3 whose polarization directions are orthogonal. These two polarized light beams L2 and L3 are converted into circularly polarized light by the depolarizing action of the phase plate 3, and then enter the crystal plate 2 and polarized light beams L21, L22 and L31 having polarization directions orthogonal to each other. , L32. This achieves four-point separation of the incident light beam.

  This will be described with reference to FIG. FIG. 2A is a cross-sectional view of the light beam L1 before entering the low-pass filter of the present embodiment. A circle indicates a cross section of the light beam L1, and an arrow direction in the circle indicates a polarization direction. In the stage before incidence on the low-pass filter shown in FIG. 2A, two polarization components exist orthogonally to the light beam L1.

  When the light beam L1 in such a state is incident on the crystal plate 1 of FIG. 1 as the first light beam separation layer, the light beam L1 has two light beams L2 having different polarization directions as shown in FIG. , L3.

  The two circles shown in FIG. 2B indicate two separated light beams L2 and L3, and each of the light beams L2 and L3 has one polarization component (the polarization components of the light beams L2 and L3 are orthogonal to each other). Existing. A long arrow 1 a shown in the center of FIG. 2B indicates a direction in which the crystal optical axis of the crystal plate 1 is projected onto the plane of the crystal plate 1.

  The two light beams L2 and L3 emitted from the crystal plate 1 are incident on the phase plate 3 which is a depolarization layer, and are depolarized as shown in FIG. 2C to be converted into circularly polarized light beams L2 ′ and L3 ′. Converted.

  A long arrow 3 a in FIG. 2C is an optical axis showing optical anisotropy caused by the fine periodic structure of the phase plate 3, and exists in the plane of the phase plate 3. This fine periodic structure will be described in detail later.

  The light beams L2 ′ and L3 ′ converted into circularly polarized light by the action of the phase plate 3 are each composed of polarization components orthogonal to each other by the crystal plate 2 as the second light beam separation layer, as shown in FIG. It is separated into two (total four) polarized light beams L21, L22 and L31, L32. Thereby, the four-point separation of the light beam L1 incident on the low-pass filter is achieved. A long arrow 2 a in FIG. 2D indicates the direction in which the crystal optical axis of the crystal plate 2 is projected onto the plane of the crystal plate 2.

  Next, the fine periodic structure of the phase plate 3 as the depolarizing layer shown in FIG. 1 will be described with reference to FIG.

  In order to convert linearly polarized light into circularly polarized light, a phase plate generally called a λ / 4 plate is used. This is because linearly polarized light is incident at 45 ° between the directions of the orthogonal fast axis and the slow axis caused by the anisotropy of the uniaxial optical crystal with respect to the polarization direction, and λ / 4 is applied to the polarization components in the two axial directions. This is realized by causing a phase shift.

  FIG. 3 shows the fine periodic structure of the phase plate 3 in an enlarged manner. The pitches a and b of the periodic structure in FIG. 3 are sufficiently smaller than the wavelength of incident light. The portion having the pitch (width) a is made of a medium having optical isotropy, and the portion having the pitch (air interval) b is an air layer.

  The effective refractive index in the direction of the fast axis and the slow axis shown in the figure can be easily obtained by the above formulas (2) and (3), and from the refractive index difference α generated there, λ / 4 The thickness L in the transmission direction of a light beam having a fine structure that causes a phase difference can be calculated by the following equation.

L = (λ / 4) / α (4)
When the light source is natural light and the coherency is weak, by making L sufficiently large, the same treatment as circularly polarized light can be performed without strictly satisfying the expression (4).

  According to the present embodiment, by using the phase plate 3 having a very thin structure compared to the crystal plate, the thickness is increased by stacking three crystal plates, compared with the conventional optical low-pass filter. The whole can be reduced in thickness. In addition, an inexpensive optical low-pass filter can be realized as compared with the case where three expensive crystal plates are used.

  The fine periodic structure may be obtained by patterning a glass substrate or the like by a photolithography technique and then forming a fine structure by etching or by injection molding a transparent resin material that can be precisely molded. May be.

  Further, in this embodiment, as shown in FIG. 1, the phase plate 3 having a fine structure is provided on the inner surface of the crystal plate 1, and the crystal plate 2 is formed of a sealing material 4 that also functions as a spacer. Sealed with an air gap between them. This makes it easy to protect the fine structure from disturbance. Further, since the fine phase plate 3 and the crystal plate 2 are not in contact with each other, the fine structure can be protected.

  Note that it is possible to add a function as a color filter to the isotropic medium constituting the fine structure by using a wavelength selective resin or the like.

  FIG. 4 shows a camera system having a camera (digital still camera or video camera) 11 provided with the optical low-pass filter 13 of this embodiment and an interchangeable lens 16.

  In the camera 11, reference numeral 12 denotes an image sensor (light receiving element) made up of a CCD, a CMOS sensor, or the like, and an optical low-pass filter 13 is disposed in front of the image sensor 12.

  In the interchangeable lens 16, reference numeral 15 denotes a photographing optical system. The light beam from the subject incident from the photographing optical system 15 is separated at four points by the optical low-pass filter 13 as described above, and is incident on the image sensor 12 having an RGB color filter as shown in FIG. By guiding the light beam separated by four points to the R part, two G parts, and the B part of the color filter, it is possible to suppress generation of unnecessary resolution signals and color signals.

  Note that the optical low-pass filter of this embodiment can also be applied to a photographic lens integrated camera (optical apparatus).

  FIG. 5 shows the configuration of an optical low-pass filter that is Embodiment 2 of the present invention. This optical low-pass filter is also mounted on the camera 11 shown in FIG.

  In FIG. 5, reference numerals 5 and 6 denote parallel flat plates formed of glass which is an optically isotropic medium. Here, a glass plate is used.

  Reference numeral 51 denotes a first fine plate formed on the surface (outer surface) of the first glass plate 5 to have a fine structure having an optical axis inclined obliquely to the incident light beam and having a structural birefringence action. It is a structural element.

  Reference numeral 52 denotes a second microstructure element as a depolarization layer having the same microstructure as that of the phase plate 3 shown in FIG. 1 and formed on the back surface (inner surface) of the first glass plate 5. Has been.

  Reference numeral 61 denotes a third microscopic structure formed on the back surface (inner surface) of the second glass plate 6 and having an optical axis inclined obliquely with respect to the incident light beam and having a structural birefringence action. It is a structural element.

  A sectional shape of the first microstructure element 51 is shown in FIG. The first microstructure element 51 is formed by laminating fine layers having different refractive indexes on the surface of the first glass plate 5 that is an isotropic medium. In the figure, a black slanted strip-shaped portion 51a indicates a layer having a first refractive index (high refractive index), and a white slanted strip-shaped portion 51b has a second refractive index (low refractive index) lower than the first refractive index. Rate) layer. Here, the low refractive index layer may be an air layer or the like.

  Thereby, the first microstructure element 51 as the light beam separation layer having optical anisotropy having the optical axis inclined in the direction indicated by the arrow 53 in FIG. 6 can be formed. The characteristics can be easily calculated from the refractive index of the laminated material and the laminated pitch using the equations (1), (2), and (3). The optical axis is defined in the stacking direction.

  The second fine structure element 61 as the light beam separation layer is also formed on the back surface of the second glass plate 6 in the same manner as the first fine structure element 51.

  The optical axes of the first and third microstructure elements 51 and 61 in FIG. 5 are in the same direction as the crystal plates 1 and 2 described in FIG. 1, and the optical action is also the same. Therefore, the first and third fine structure elements 51 and 61 of the present embodiment have the same light beam separating action as the crystal plates 1 and 2 of the first embodiment.

  In FIG. 5, the incident light beam L1 is separated by the first fine structure element 51 into two polarized light beams L2 and L3 whose polarization directions are orthogonal. Then, the polarized light beams L2 and L3 are converted into circularly polarized light by the second fine structure element 52, and then enter the third fine structure element 61.

  The two circularly polarized light beams incident on the third fine structure element 61 are separated into two (total four) polarized light beams L21, L22 and L31, L32 whose polarization directions are orthogonal to each other. Point separation is achieved.

  Further, in this embodiment, the second and third fine structure elements 52 and 61 are provided on the inner surfaces of the first and second glass plates 5 and 6, respectively, and the second and third fine structure elements 52 are provided. 61 are sealed using a sealing material 4 that also functions as a spacer, with a space between each other. This makes it easy to protect the second and third microstructure elements 52 and 61 from disturbance. Further, since the second and third microstructure elements 52 and 61 do not contact each other, they can be protected.

  Further, according to the present embodiment, an optical low-pass filter can be configured without using an expensive crystal plate, and an inexpensive optical low-pass filter can be realized. Further, when the large image pickup device 12 is used for the camera 11 shown in FIG. 4, it is necessary to increase the size of the optical low-pass filter correspondingly, but it is difficult to increase the size of the crystal plate. On the other hand, the glass plate used in the present embodiment can be easily enlarged. Therefore, the optical low-pass filter of the present embodiment is particularly suitable for a camera using a large image sensor.

  Furthermore, this embodiment also uses a microstructure element that is extremely thin compared to the crystal plate, and uses only the minimum number (two) of glass plates as the substrate for forming three microstructure elements. As a result, the entire optical low-pass filter can be reduced in thickness as compared with a conventional stack of three crystal plates.

  Here, although the case where light is incident from the first fine structure element 51 side has been described with reference to FIG. 5, light is incident from the second glass plate 6 side (third fine structure element 61 side). May be. In this case, the present optical low-pass filter may be arranged on the camera 11 shown in FIG. 4 so that the second glass plate 6 faces the subject and the first fine structure element 51 faces the image sensor 12. . Thereby, it is possible to prevent the user from touching the first fine structure element 51 with the hand while the interchangeable lens 15 is detached from the camera 11, and to protect the first fine structure element 51.

  If a resin having a wavelength selective absorption effect is used for the isotropic medium constituting the first and third microstructure elements 51 and 61, a function as a color filter can be added.

  FIG. 7 shows the configuration of an optical low-pass filter that is Embodiment 3 of the present invention. This optical low-pass filter is also mounted on the camera 11 shown in FIG.

  7 and 8 in FIG. 7 are first and second parallel flat plates made of a medium such as glass having optical isotropy. Here, a glass plate is used.

  Reference numeral 71 denotes a first fine plate formed on the surface (outer surface) of the first glass plate 7 in a fine structure having an optical axis inclined obliquely with respect to the incident light beam and having a structural birefringence action. It is a structural element.

  Reference numeral 81 denotes a third fine plate formed on the surface (outer surface) of the second glass plate 8 in a fine structure having an optical axis inclined obliquely with respect to the incident light beam and having a structural birefringence action. It is a structural element. The first and third fine structure elements 71 and 81 are formed in the same manner as the fine structure element 51 described with reference to FIG. 6, and both have a light beam separation function.

  Reference numeral 72 denotes a second microstructure element formed in a microstructure having a λ / 4 plate action (polarization cancellation action) as described with reference to FIG. The second microstructure element 72 is formed on the back surface (inner surface) of the first glass plate 7 in the present embodiment, but on the back surface (inner surface) of the second glass plate 8. It may be formed.

  In this embodiment, the inner surface of the second glass plate 8 is in close contact with the second microstructure element 72.

  The optical low-pass filter according to the present embodiment also has a four-point separation function for incident light flux as in the second embodiment.

  That is, the incident light beam L1 is separated by the first fine structure element 71 into two polarized light beams L2 and L3 whose polarization directions are orthogonal. Next, the polarized light beams L2 and L3 are respectively converted into circularly polarized light by the second fine structure element 72, and then enter the third fine structure element 81, and the two polarization directions are orthogonal to each other (four in total). The polarized light beams L21, L22 and L31, L32 are separated, and four-point separation of the incident light beam L1 is achieved.

  According to this embodiment, an optical low-pass filter can be configured without using an expensive crystal plate, and an inexpensive optical low-pass filter can be realized. In addition, since the glass plate used in this embodiment can be easily increased in size, the optical low-pass filter can be adapted to a camera using a large image sensor.

  Furthermore, this embodiment also uses a microstructure element that is extremely thin compared to the crystal plate, and uses only the minimum number (two) of glass plates as the substrate for forming three microstructure elements. As a result, the entire optical low-pass filter can be reduced in thickness as compared with a conventional stack of three crystal plates.

  Also in the present embodiment, it is possible to add a function as a color filter by using a resin having a wavelength-selective absorption effect for the glass plates 7 and 8 which are isotropic media.

  FIG. 8 shows the configuration of an optical low-pass filter that is Embodiment 4 of the present invention. This optical low-pass filter is also mounted on the camera 11 shown in FIG.

  Reference numeral 9 in FIG. 8 denotes a phase plate formed of a uniaxial optical crystal such as quartz and has a depolarizing action.

  91 and 92 are formed on both surfaces of the phase plate 9, and as shown in FIG. 6, the first and second microstructures having a structural birefringence function having an optical axis inclined with respect to the incident light. It is an element and has a light beam separation effect.

  With such a configuration, the optical low-pass filter of the present embodiment has a four-point separation function for incident light flux, as in the other embodiments.

  The incident light beam L1 is separated by the first fine structure element 91 into two light beams L2 and L3 whose polarization directions are orthogonal. Next, the light beams L2 and L3 are respectively converted into circularly polarized light by the phase plate 9, and then enter the second microstructure element 92, and two (total four) light beams L21 and L22 whose polarization directions are orthogonal to each other. And L31 and L32 are separated, and four-point separation of the incident light beam L1 is achieved.

  According to the present embodiment, by using the optical elements 91 and 92 having a very thin structure compared to the crystal plate, the conventional optical low-pass filter whose thickness is increased by stacking three crystal plates is used. In comparison, the whole can be reduced in thickness. In addition, an inexpensive optical low-pass filter can be realized as compared with the case where three expensive crystal plates are used.

  The configuration of each of the embodiments described above is merely an example, and two optical elements having a light beam separating action are combined with an optical element having a depolarizing action arranged between these two optical elements. In the optical low-pass filter, as long as at least one optical element is constituted by a fine structure element, all are examples of the present invention.

  In each of the above embodiments, the optical low-pass filter has been described. However, the present invention can also be applied to an optical filter having other uses.

  As described above, the optical filter is disposed between two optical crystal plates having a light beam separation function and these two optical crystal plates, and has a depolarization function due to optical anisotropy having a structure smaller than the wavelength of incident light. By having a structure having a fine structure element, an optical element having a very thin structure compared to the optical crystal plate can be used, and the thickness is increased by stacking three optical crystal plates. Compared with the conventional optical filter, the whole can be reduced in thickness. Moreover, it can be made cheaper than a conventional optical filter in which three optical crystal plates are stacked.

  Further, since the fine structure element is not exposed on the filter surface, the fine structure element can be protected.

  Furthermore, by providing a spacer between the fine structure element and the other optical crystal plate, it is possible to protect the fine structure element by avoiding these direct contacts. Further, the fine structure element can be protected from disturbance by sealing the fine structure element with the spacer.

  Further, the optical filter is formed on two flat plates having optical isotropy and an outer surface of one of the two flat plates, and has an optical anisotropy having a structure smaller than the wavelength of incident light. A first fine structure element having a light beam separation action and a second fine structure formed on the inner surface of the one flat plate and having a depolarization action due to optical anisotropy of a structure finer than the wavelength of incident light An element and a third microstructure element formed on one surface of the other of the two flat plates and having a light beam separation action by optical anisotropy having a structure finer than the wavelength of incident light. Thus, an optical filter can be configured without using an expensive optical crystal plate, and an inexpensive optical filter can be realized. Further, since a material that is difficult to increase in size, such as an optical crystal plate, is not used, the optical filter can be easily increased in size.

  Further, since the microstructure element is extremely thin as compared with the quartz plate and only the minimum number (two) of flat plates is used as a substrate for forming the three microstructure elements, the entire optical filter is conventionally used. It can be made thinner than

  Further, since the second microstructure element is not exposed on the filter surface by forming the third microstructure element on the inner surface of the other flat plate, the second and third microstructures are not exposed. The element can be protected.

  Further, by providing a spacer between the second and third microstructure elements, direct contact between the second and third microstructure elements can be prevented, and these microstructure elements can be protected. it can. By sealing the fine structure element with the spacer, the fine structure element can be protected from disturbance.

  Further, in an optical apparatus including the optical filter and a light receiving element that receives light that has passed through the optical filter, the optical filter is disposed so that the first fine structure element faces the light receiving element. Contact to the first microstructure element facing the light receiving element can be prevented, and the first microstructure element can also be protected.

It is sectional drawing of the optical low-pass filter which is Example 1 of this invention. It is a figure explaining the light beam separation effect | action and depolarization effect | action in the optical low-pass filter of the said Example 1. FIG. It is a schematic diagram which shows the structure of the phase plate by the fine structure in the optical low-pass filter of the said Example 1. FIG. It is the schematic of the camera system provided with the optical low-pass filter of the said Example 1. FIG. It is sectional drawing of the optical low-pass filter which is Example 2 of this invention. It is sectional drawing which shows the structure of the fine structure element in the optical low-pass filter of the said Example 2. FIG. It is sectional drawing of the optical low-pass filter which is Example 3 of this invention. It is sectional drawing of the optical low-pass filter which is Example 4 of this invention. It is explanatory drawing of the anisotropy by a fine structure. It is a figure which shows the structural example of a color filter.

Explanation of symbols

1,2,9 Crystal plate 3,52,72 Fine structure element 51, 61, 71, 81 Fine structure element having depolarization action 4 Sealing material 11 Camera 12 Imaging element 13 Optical low pass filter

Claims (17)

Two optical elements having a beam separation action;
An optical element disposed between these two optical elements and having a depolarizing action,
An optical filter characterized in that at least one of the three optical elements is a fine structure element having a light beam separation action or a depolarization action by optical anisotropy having a structure finer than the wavelength of incident light. Two optical crystal plates with a beam separation action;
An optical filter comprising: a microstructure element disposed between these two optical crystal plates and having a depolarizing effect due to optical anisotropy having a structure finer than the wavelength of incident light. The optical filter according to claim 2, wherein the microstructure element is formed on one of the two optical crystal plates. The optical filter according to claim 2, further comprising a spacer for forming an air gap between the microstructure element and the other optical crystal plate of the two optical crystal plates. The optical filter according to claim 4, wherein the spacer is a sealing member that seals the microstructure element between the two optical crystal plates. Two fine structure elements having a light beam separation action by optical anisotropy of a structure finer than the wavelength of incident light;
An optical filter comprising an optical crystal plate disposed between these two optical elements and having a depolarizing action. The optical filter according to claim 6, wherein the two microstructure elements are formed on the optical crystal plate. The microstructure element is composed of a medium having optical isotropy,
The optical filter according to claim 1, wherein the medium has a wavelength selection filter function. An optical apparatus comprising the optical filter according to claim 1. Two flat plates having optical isotropy;
A first microstructure element formed on the outer surface of one of the two flat plates and having a light beam separation action by optical anisotropy of a structure finer than the wavelength of incident light;
A second microstructure element formed on the inner surface of the one flat plate and having a depolarization effect due to optical anisotropy of a structure finer than the wavelength of incident light;
An optical system comprising: a third microstructure element formed on one surface of the other of the two flat plates and having a light beam separation action by optical anisotropy of a structure finer than a wavelength of incident light. filter. The optical filter according to claim 10, wherein the third microstructure element is formed on an inner surface of the other flat plate. The optical filter according to claim 11, further comprising a spacer for forming an air gap between the second and third microstructure elements. The optical filter according to claim 12, wherein the spacer is a sealing member that seals the microstructure element between the two flat plates. The optical filter according to claim 10, wherein the optically isotropic medium forming the flat plate has a wavelength selection filter function. The microstructure element is composed of a medium having optical isotropy,
14. The optical filter according to any one of inventions 10 to 13, wherein the medium has a wavelength selective filter action. An optical apparatus comprising the optical filter according to claim 10. The optical filter according to claim 11, and a light receiving element that receives light that has passed through the optical filter,
The optical apparatus, wherein the optical filter is disposed so that the first fine structure element faces the light receiving element.