Optical filter and infrared sensor and optical filter manufacturing method

Abstract

This optical filter (1) comprises: a base material (10) including a fluoride polycrystal continuous phase (11); and an absorbent (20) dispersed in the base material (10). The optical filter (1) contains 0.1-10 mass% of the absorbent (20) which absorbs light in the infrared region from more than 7.5 μm to at most 9 μm. In a target wavelength band from at least 3 μm to at most 7 μm, the wavelength bandwidth for which the linear transmittance per 1 mm thickness is at least 30% is at least 50 nm. In a target wavelength band from more than 7.5 μm to at most 25 μm, the maximum linear transmittance per 1 mm thickness is at most 10%. At least 80 mass% of an inorganic substance is included, and at least 50 mass% of the fluoride polycrystal is included. The porosity is at most 30% and the pore diameter median value is at most 500 nm.

Description

Optical filter, infrared sensor, and method of manufacturing optical filter

[0001] The present invention relates to an optical filter, an infrared sensor, and a method for manufacturing an optical filter.

[0002] Conventionally, infrared-transmitting windows based on inorganic fluorides have been used for special applications in the fields of academic research and industry, such as window materials for scientific and chemical instruments. ２ , BaF ２ , MgF ２ The document discloses a Fourier transform infrared spectrophotometer that includes a measurement cell made of one material selected from the group consisting of LiF, LiF, and ZnSe, and that measures fluorine-based gases in a sample that contains corrosive gases.

[0003] International Publication No. 2019 / 176624

[0004] On the other hand, there is known a technique for sensing various gases and flames using an infrared detection element and an infrared-transmitting optical filter. However, the above-mentioned materials may transmit, for example, unintended wavelengths, and may not be suitable as an optical filter for an infrared sensor, for example.

[0005] The present invention has been made in view of the problems associated with the prior art, and an object of the present invention is to provide an optical filter suitable for an infrared sensor for detecting, for example, a flame, and an infrared sensor and optical filter using the same.

[0006] In order to solve the above problems, an optical filter according to a first aspect of the present invention comprises a base material including a continuous phase of fluoride polycrystals and an absorber dispersed within the base material. The optical filter contains 0.1 to 10 mass % of an absorber that absorbs light in the infrared region greater than 7.5 μm and equal to or less than 9 μm. In a target wavelength band of 3 μm or greater and equal to or less than 7 μm, the optical filter has a wavelength bandwidth of 50 nm or greater, in which the linear transmittance per mm of thickness is 30% or greater. In a target wavelength band of greater than 7.5 μm and equal to or less than 25 μm, the optical filter has a maximum linear transmittance per mm of thickness of 10% or less. The optical filter contains 80 mass % or greater of an inorganic substance. The optical filter contains 50 mass % or greater of fluoride polycrystals. The optical filter has a porosity of 30% or less. The median pore diameter of the optical filter is 500 nm or less.

[0007] An infrared sensor according to a second aspect of the present invention includes an optical filter.

[0008] A method for producing an optical filter according to a third aspect of the present invention includes a step of pressurizing a mixture containing a fluoride polycrystalline material and an absorbent at a temperature of 250°C or less. The optical filter contains 0.1 to 10 mass% of an absorbent that absorbs light in the infrared region greater than 7.5 μm and less than 9 μm. In a target wavelength band of 3 μm or more and 7 μm or less, the optical filter has a wavelength bandwidth of 50 nm or more, in which the linear transmittance per mm of thickness is 30% or more. In a target wavelength band of greater than 7.5 μm and less than 25 μm, the optical filter has a maximum linear transmittance per mm of thickness of 10% or less. The optical filter contains 80 mass% or more of an inorganic substance. The optical filter contains 50 mass% or more of a fluoride polycrystalline material. The optical filter has a porosity of 30% or less. The median pore diameter of the optical filter is 500 nm or less.

[0009] FIG. 1 is a cross-sectional view schematically showing an example of an optical filter according to the present embodiment. FIG. 2 is a cross-sectional view schematically showing an enlarged portion of the optical filter shown in FIG. 1. FIG. 3 is a cross-sectional view schematically showing another example of an optical filter according to the present embodiment. FIG. 4 is a cross-sectional view schematically showing an enlarged portion of the optical filter shown in FIG. 3. FIG. 5 is a cross-sectional view schematically showing an example of an infrared sensor according to the present embodiment. FIG. 6 is an infrared absorption spectrum of a fluororesin used in the examples. FIG. 7 is an infrared absorption spectrum of PVDF used in the examples. FIG. 8 is the linear transmittance of a test sample according to Example 1. FIG. 9 is the linear transmittance of a test sample according to Example 2. FIG. 10 is the linear transmittance of a test sample according to Example 3. FIG. 11 is the linear transmittance of a test sample according to Example 4. FIG. 12 is a secondary electron image of a cross section of a test sample according to Example 1. FIG. 13 is a backscattered electron image of a cross section of a test sample according to Example 1. FIG. 14 is a binarized image of the backscattered electron image of FIG. 13. FIG. 15 shows XRD patterns of powders obtained by pulverizing the test samples according to the respective examples.

[0010] The optical filter, the infrared sensor, and the method for manufacturing the optical filter according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions.

[0011] [Optical Filter] Fig. 1 is a cross-sectional view schematically showing an example of an optical filter 1 according to this embodiment. Fig. 2 is a cross-sectional view schematically showing an enlarged portion of the optical filter 1 shown in Fig. 1. As shown in Figs. 1 and 2, the optical filter 1 of this embodiment includes a base material 10 and an absorbent 20. The base material 10 contains a continuous phase 11 of a fluoride polycrystal. The optical filter 1 of this embodiment is a ceramic structure.

[0012] The fluoride may contain at least one of an alkali metal and an alkaline earth metal. The alkali metal may include at least one metal selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium. The alkaline earth metal may include at least one metal selected from the group consisting of magnesium, calcium, strontium, and barium.

[0013] The fluoride may include a complex fluoride containing an alkali metal. The complex fluoride may include at least one alkali metal selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium, for example. Alkali metal fluorides have high solubility in water, but complex fluorides containing alkali metals have low solubility in water. The solubility of alkali metal fluorides in 100 g of water at 25°C is, for example, 0.134 g for LiF, 4.13 g for NaF, 102 g for KF, 300 g for RbF, and 573 g for CsF. On the other hand, the solubility of complex fluorides containing alkali metals in 100 g of water at 25°C is, for example, 0.134 g for NaF, 4.13 g for NaF, 102 g for KF, 300 g for RbF, and 573 g for CsF. ３ AlF ６ 0.042 g, NaMgF ３ Therefore, the optical filter 1 using a complex fluoride containing an alkali metal has higher water resistance than that using an alkali metal fluoride.

[0014] The complex fluoride may contain fluorine, an alkali metal, and an additional metal as main components. The additional metal may include at least one metal selected from the group consisting of alkaline earth metals, aluminum, gallium, indium, zinc, and yttrium. The alkaline earth metal may include at least one metal selected from the group consisting of magnesium, calcium, strontium, and barium. Such complex fluorides have low refractive index anisotropy and can therefore improve infrared light transmittance. Furthermore, inorganic substances containing these metal elements can easily be formed into the optical filter 1 by a pressurized and heated method, as described below. Note that the term "main component" as used herein means that the total content of fluorine, alkali metal, and additional metal contained in the complex fluoride is 80% or more in molar ratio. The total content may be 85% or more, 90% or more, 95% or more, or even 100%.

[0015] The complex fluoride is specifically A ３ AlF ６ and ABF ３ (wherein, in the above composition formula, A represents one or more of the above-mentioned alkali metals, and B represents one or more of the above-mentioned alkaline earth metals). ３ AlF ６ For example, Li ３ AlF ６ , Li ２ NaAlF ６ , Li ２ KAlF ６ , Na ３ AlF ６ , Na ２ LiAlF ６ , Na ２ KAlF ６ , K. ３ AlF ６ , K. ２ LiAlF ６ and K. ２ NaAlF ６ ABF ３ For example, LiMgF ３ , NaMgF ３, KMgF ３ , LiCaF ３ , NaCaF ３ and KCaF ３ The complex fluoride may contain at least one selected from the group consisting of: ３ AlF ６ , K. ２ NaAlF ６ and NaMgF ３ It may contain at least one selected from the group consisting of:

[0016] The complex fluoride may contain a compound having a cryolite crystal structure. Examples of the complex fluoride having a cryolite crystal structure include Na ３ AlF ６ and K. ２ NaAlF ６ The complex fluoride may have a perovskite crystal structure. Examples of complex fluorides having a perovskite crystal structure include NaMgF ３ The complex fluorides having these crystal structures are preferable from the viewpoint of infrared transmission because they are materials with small refractive index anisotropy due to crystal orientation.

[0017] The complex fluoride may contain fluorine, an alkali metal, and aluminum as main components. The term "main component" as used herein means that the total content of fluorine, alkali metal, and aluminum contained in the complex fluoride is 80% or more in molar ratio. The total content may be 85% or more, 90% or more, 95% or more, or even 100%. The complex fluoride may be, for example, sodium hexafluoroaluminate (Na ３ AlF ６ ) may be included. ３ AlF ６ is also known as a component of cryolite. ３ AlF ６ The anisotropy of the refractive index of is small, so it has excellent infrared light transmittance.

[0018] A part of the constituent anions of the complex fluoride may be substituted with hydroxide ions or oxide ions. For example, in the case of liquid-phase synthesized complex fluoride particles, a part of the complex fluoride ions may be substituted with at least one of hydroxide ions and oxide ions. When the complex fluoride has hydroxyl groups, the hydroxyl groups absorb infrared light of a specific wavelength. Therefore, by cutting unnecessary wavelengths, the sensor sensitivity can be improved. Furthermore, when the optical filter 1 contains hydroxyl groups, the transmittance fluctuation due to humidity is reduced, and the sensor becomes less susceptible to the influence of humidity.

[0019] The optical filter 1 contains 80% by mass or more of an inorganic substance. Therefore, compared to a filter using a resin, the optical filter 1 is less likely to deteriorate over time and has high infrared transmittance. The inorganic substance contains the above-mentioned fluoride. The optical filter 1 may contain 85% by mass or more, 90% by mass or more, or 95% by mass or more of the inorganic substance. The optical filter 1 may contain 99.9% by mass or less of the inorganic substance.

[0020] The optical filter 1 contains 50% by mass or more of a polycrystalline fluoride. The polycrystalline fluoride provides excellent gas barrier properties or durability. Furthermore, by containing 50% by mass or more of a polycrystalline fluoride, the resulting optical filter 1 is an optical filter that can easily utilize the properties of fluoride. Furthermore, a high fluoride content reduces the proportion of areas with different refractive indices within the optical filter 1, thereby suppressing light scattering and increasing the infrared transmittance of the optical filter 1. The fluoride content in the optical filter 1 may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or even 95% by mass or more.

[0021] As shown in FIG. 2 , the base material 10 may include a plurality of fluoride particles 12. The plurality of fluoride particles 12 may be dispersed within the base material 10. A continuous phase 11 may be present between adjacent fluoride particles 12. The continuous phase 11 may bond each of the plurality of fluoride particles 12. The continuous phase 11 may be in direct contact with the fluoride particles 12. Furthermore, the continuous phase 11 may cover at least a portion of the surface of each of the plurality of fluoride particles 12, or may cover the entire surface of each of the plurality of fluoride particles 12. The plurality of fluoride particles 12 may be bonded to each other. The fluoride particles 12 may be in point contact with each other, or may be in surface contact in which the surfaces of the fluoride particles 12 are in contact with each other.

[0022] The fluoride particles 12 may be composed of the same material as the fluoride constituting the continuous phase 11 as described above. The inorganic material constituting the fluoride particles 12 may be crystalline or amorphous. From the viewpoint of gas barrier properties or durability, the inorganic material constituting the fluoride particles 12 is preferably crystalline. From the viewpoint of light transmittance, the inorganic material constituting the fluoride particles 12 is preferably amorphous. When the inorganic material is crystalline, the fluoride particles 12 may be single crystal or polycrystalline.

[0023] The average particle diameter of the plurality of fluoride particles 12 may be 10 nm or more and 50 μm or less. When the average particle diameter of the fluoride particles 12 is 50 μm or less, the light transmittance of the optical filter 1 is increased. Furthermore, when the average particle diameter of the fluoride particles 12 is within this range, the fluoride particles 12 are firmly bonded to each other, thereby increasing the strength of the optical filter 1. Furthermore, when the average particle diameter of the fluoride particles 12 is within this range, the proportion of pores present inside the optical filter 1 is 30% or less, as described below. Furthermore, the size of pores generated between the fluoride particles 12 is reduced. Therefore, the strength of the optical filter 1 can be increased. From the viewpoint of improving the light transmittance of the optical filter 1, the average particle diameter of the plurality of fluoride particles 12 is more preferably 10 μm or less, even more preferably 2 μm or less, particularly preferably 1 μm or less, and most preferably 500 nm or less. The average particle diameter of the plurality of fluoride particles 12 may be, for example, 1 nm or more, or 10 nm or more. In this specification, the value of "average particle diameter" is, unless otherwise specified, a value calculated as the average value of particle diameters of particles observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0024] The optical filter 1 may contain impurities generated during the manufacturing process. The impurities may be, for example, raw materials for generating fluoride and may contain elements that constitute fluoride. The elements contained in the impurities may include, for example, at least one element selected from the group consisting of fluorine, alkali metals, and the additional metals described above. The impurities may include a chalcogen compound including an oxide composed of one type of metal element, a halide, a hydroxide, a nitride, or a carbide. The impurities may be crystalline or amorphous.

[0025] The refractive index difference between the fluoride and the impurity may be 0.1 or less. When the refractive index difference is 0.1 or less, light scattering can be suppressed to provide an optical filter with high transmittance. The lower limit of the refractive index difference is not particularly limited. The refractive index difference may be 0.08 or less, or may be 0.04 or less.

[0026] The absorbent 20 is dispersed within the base material 10. As shown in Fig. 2, the absorbent 20 may be a coating covering the surface of the fluoride particles 12. Alternatively, as shown in Fig. 3, the absorbent 20 is a particle, and does not have to be a coating covering the surface of the fluoride particles 12 as shown in Fig. 4. Alternatively, the absorbent 20 may include a coating covering the surface of the fluoride particles 12 as shown in Fig. 2 and particles as shown in Fig. 3. Alternatively, the absorbent 20 may be uniformly dispersed within the continuous phase 11.

[0027] The absorbent 20 absorbs light in the infrared region of more than 7.5 μm and not more than 9 μm. By absorbing light in this infrared region, it is possible to cut light of wavelengths that cannot be blocked by fluoride. Therefore, it is possible to provide an optical filter 1 that can block light of a specific wavelength region that causes noise, for example.

[0028] The absorbent 20 may contain a fluororesin. Fluororesin is also known as a fluorine-based resin. Fluororesin has a C—F bond and absorbs light in the 8 μm to 9 μm wavelength band, so it can cut light in the specific wavelength range described above. The fluororesin may contain at least one selected from the group consisting of, for example, a resin having a perfluoroalkyl group, polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), perfluoroalkoxy fluororesin (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE). In particular, resins having a perfluoroalkyl group and polyvinylidene fluoride (PVDF) have many C—F bonds, so they can effectively cut light in the specific wavelength range.

[0029] When the absorbent 20 is in the form of particles, the average particle diameter of the absorbent 20 may be 4 μm or less. When the average particle diameter of the absorbent 20 is 4 μm or less, the mechanical strength of the optical filter 1 can be maintained high. Furthermore, the average particle diameter of the absorbent 20 may be 1 μm or less, or 0.1 μm or less. There is no particular restriction on the lower limit of the average particle diameter of the absorbent 20, but the average particle diameter of the absorbent 20 may be 1 nm or more. The average particle diameter of the absorbent 20 may be 1 μm or more, or 10 μm or more.

[0030] The content of the absorbent 20 in the optical filter 1 is 0.1% by mass or more and 10% by mass or less. When the content of the absorbent 20 is 0.1% by mass or more, the amount of light absorbed by the optical filter 1 can be increased. Furthermore, when the content of the absorbent 20 is 10% by mass or less, the mechanical properties of the optical filter 1 can be improved. The content of the absorbent 20 may be 0.5% by mass or more, 1% by mass or more, 2 parts by mass or more, or 3 parts by mass or more. Furthermore, the content of the absorbent 20 may be 6% by mass or less, 4% by mass or less, 3 parts by mass or less, or 2 parts by mass or less.

[0031] The optical filter 1 may contain an organic material such as resin particles in addition to the absorbent 20. As will be described later, the optical filter 1 can be obtained by applying pressure while heating to 100 to 300°C, so a material with low heat resistance can be added to the optical filter 1. Furthermore, the optical filter 1 is not limited to a material with low heat resistance such as an organic material, and may contain an inorganic compound other than fluoride and impurities to impart functionality.

[0032] The porosity of the cross section of the optical filter 1 is 30% or less. That is, the porosity of pores exceeding 0 nm in the cross section of the optical filter 1 is 30% or less. Specifically, when the cross section of the optical filter 1 is observed, the average ratio of pores per unit area is 30% or less. When the porosity is 30% or less, the number of pores inside the optical filter 1 is reduced, resulting in a solidified body with high strength. Note that the porosity of the cross section of the optical filter 1 is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. The smaller the porosity of the cross section of the optical filter 1, the more cracks originating from the pores are suppressed, thereby enabling the strength of the optical filter 1 to be increased. Furthermore, the smaller the porosity of the cross section of the optical filter 1, the more light scattering due to the pores is suppressed, thereby enabling the transmittance of the optical filter 1 to be increased.

[0033] In this specification, the porosity can be determined as follows. First, the cross section of the optical filter 1 is observed to distinguish between pores and non-pore areas. Then, the area of ​​the pores in a unit area is measured to determine the proportion of pores per unit area. The porosity is defined as the average of the proportions of pores per unit area determined at multiple locations. When observing the cross section of the optical filter 1, an optical microscope, a scanning electron microscope (SEM), or a transmission electron microscope (TEM) can be used. The unit area and the area of ​​pores in that unit area may also be measured by binarizing an image observed with a microscope.

[0034] The median pore diameter of the optical filter 1 is 500 nm or less. When the median pore diameter is 500 nm or less, light scattering by the pores is suppressed, resulting in an optical filter 1 with high transmittance. The median pore diameter is preferably 300 nm or less, more preferably 200 nm or less, and even more preferably 100 nm or less. In this specification, the median pore diameter refers to the pore diameter when the cumulative value of the pore diameter distribution on an area basis is 50%. The pore diameter refers to the diameter of a perfect circle when the shape of the pores in the cross section of the optical filter 1 is assumed to be a perfect circle.

[0035] The optical filter 1 is infrared transmissive. In a target wavelength band, the optical filter 1 may have a wavelength bandwidth in which the linear transmittance is equal to or greater than a predetermined value. The linear transmittance of the optical filter 1 can be obtained by measuring by transmission using an FT-IR (Fourier transform infrared spectroscopy) device. The linear transmittance of an infrared transmission spectrum is converted based on the Beer-Lambert law so as to be the linear transmittance at a thickness of 1 mm. The wavelength bandwidth can be obtained by measuring a wavelength bandwidth in which the linear transmittance is equal to or greater than a predetermined value in the target wavelength band. The wavelength bandwidth may be a wavelength bandwidth in which the linear transmittance is equal to or greater than a predetermined value continuously, or may be the total wavelength bandwidth of wavelength bandwidths in which the linear transmittance is equal to or greater than a predetermined value continuously and intermittently. In other words, the wavelength bandwidth is the total wavelength bandwidth in the target wavelength band in which the linear transmittance is equal to or greater than a predetermined value. Therefore, when the linear transmittance is equal to or greater than a predetermined value intermittently, the wavelength bands in which the linear transmittance is equal to or greater than a predetermined value may be spaced apart from one another in the target wavelength band. Preferably, the wavelength bandwidth is a wavelength bandwidth in which the linear transmittance is equal to or greater than a predetermined value continuously.

[0036] In a target wavelength band of 3 μm or more and 7 μm or less, the optical filter 1 has a linear transmittance per mm of thickness of 30% or more, and the wavelength bandwidth is 50 nm or more. Such an optical filter 1 is suitable for an infrared sensor for detecting, for example, a flame. The target wavelength band may be 3.4 μm or more, or 3.8 μm or more. The target wavelength band may be 6 μm or less, 5 μm or less, or 4.4 μm or less. The linear transmittance may be 40% or more, or 50% or more. The linear transmittance may be 100% or less. The wavelength bandwidth may be 0.5 μm or more, 1 μm or more, 1.5 μm or more, or 2 μm or more. The wavelength bandwidth may be 4 μm or less, or 3 μm or less.

[0037] In the target wavelength band of more than 7.5 μm and not more than 25 μm, the maximum linear transmittance per mm of thickness of the optical filter 1 is 10% or less. Such an optical filter 1 has excellent shielding properties in the target wavelength band and is therefore suitable for, for example, an infrared sensor for detecting flames, etc. The maximum linear transmittance means the highest linear transmittance in the target wavelength band.

[0038] The optical filter 1 may be an infrared transmission filter. By using the optical filter 1 as an infrared transmission filter, it is possible to provide an infrared sensor, a light emitting device, and the like that can suppress diffuse reflection of infrared light by pores. If the optical filter 1 is capable of transmitting light in a specific infrared region as a single layer, it is possible to transmit the specific infrared light without stacking multiple dielectrics as in, for example, an interference filter, or providing a separate light-shielding filter to improve sensor sensitivity.

[0039] The thickness t of the optical filter 1 is 10 μm or more. The optical filter 1 of this embodiment is formed by a pressurized and heated method, as described below. Therefore, a bulk body having a thickness of 10 μm or more can be easily formed without forming the optical filter 1 by laminating thin films. The thickness of the optical filter 1 may be 100 μm or more, 500 μm or more, 1 mm or more, or 1 cm or more. The upper limit of the thickness of the optical filter 1 is not particularly limited, but may be, for example, 50 cm.

[0040] When the optical filter 1 is used as an infrared transmission filter, its shape is not particularly limited. It may be flat, concave, or convex. When the optical filter 1 has a lens shape, it becomes possible to allow light rays to be incident perpendicularly on the dielectric laminated bandpass filter, thereby canceling the dependence of the dielectric laminated bandpass filter on the angle of incidence of light rays. In addition, a microstructure having an anti-reflection function may be formed on the surface of the optical filter 1.

[0041] [Infrared Sensor] Next, the infrared sensor 100 according to this embodiment will be described with reference to Fig. 5. As shown in Fig. 5, the infrared sensor 100 includes the optical filter 1 described above. The infrared sensor 100 also includes an infrared detection element 110, an IC element 120, a substrate 130, and a metal case 150.

[0042] The infrared detection element 110 and the IC element 120 are mounted on a substrate 130 by a die bond material 131. The infrared detection element 110 and the IC element 120 are electrically connected to each other by wires 140. The infrared detection element 110 is also connected to electrical circuit wiring (not shown) of the substrate 130 by the wires 140.

[0043] The infrared detection element 110 receives infrared rays, converts the thermal energy of the received infrared rays into electrical energy, and outputs an electrical signal corresponding to the amount of the received infrared rays to the IC element 120. The infrared detection element 110 may be a thermal infrared detection element such as a pyroelectric element, a thermopile infrared detection element, or a bolometer infrared detection element, or a quantum infrared detection element.

[0044] The IC element 120 includes an amplifier circuit that amplifies the electrical signal output from the infrared detection element 110, and a determination circuit that determines that a flame is present when the electrical signal amplified by the amplifier circuit exceeds a threshold value.

[0045] The metal case 150 is attached to the substrate 130. The metal case 150 surrounds the infrared detection element 110 and the IC element 120, and the infrared detection element 110 and the IC element 120 are sealed by the substrate 130 and the metal case 150. The metal case 150 includes a top wall 151 and a side wall 152. An opening is provided in the top wall 151, and the opening is covered by an optical filter 1. The optical filter 1 is disposed so as to face the infrared detection element 110, and the infrared detection element 110 is provided on the substrate 130 so as to receive infrared light that has passed through the optical filter 1. The side wall 152 connects the edge of the top wall 151 to the edge of the substrate 130.

[0046] Infrared rays emitted from a flame or the like pass through the optical filter 1 and are received by the infrared detection element 110. The infrared detection element 110 outputs an electrical signal corresponding to the amount of infrared rays received to the IC element 120. The IC element 120 determines whether a flame is present based on the electrical signal. In this way, the infrared sensor 100 can detect a flame by utilizing the infrared rays emitted from the flame.

[0047] In addition, SO ２ , CO ２ , P.H. ３ , N ２ Gas molecules such as inorganic gases like O and NO have characteristic absorption properties in the mid-infrared region. Furthermore, these gas molecules emit corresponding infrared rays at high temperatures. Therefore, the optical filter 1 according to this embodiment can absorb CO ２ The optical filter 1 according to this embodiment is useful as an infrared transmission window, because it has a high transmittance of infrared light.

[0048] In the present embodiment, an example has been described in which the infrared sensor 100 is used as a flame sensor for detecting flames, but the use of the infrared sensor 100 is not limited to this form. The infrared sensor 100 can also be used as a human presence sensor, a biological sensor, a security sensor, a gas sensor, a non-contact thermometer, a solid-state imaging device, a camera module, or the like.

[0049] [Manufacturing Method of Optical Filter] Next, a manufacturing method of the optical filter 1 according to this embodiment will be described. The optical filter 1 can be manufactured by pressurizing and heating a raw material containing fluoride particles. By using such a pressurizing and heating method, the fluoride particles are deformed and bonded to each other, making it possible to form an optical filter 1 with a small porosity.

[0050] The fluoride may be one of those described above. The fluoride may be a crystal, and the optical filter 1 may be manufactured by pressurizing a mixture containing a polycrystalline fluoride and an absorbent. In this embodiment, an example in which complex fluoride particles are used as the fluoride particles will be described. The raw material containing complex fluoride particles may include a step of generating a plurality of complex fluoride particles, a step of separating them, and a step of heating them.

[0051] Specifically, in the step of producing a plurality of complex fluoride particles, complex fluoride particles are prepared by first mixing and reacting a first metal compound, which is an alkali metal fluoride, with a second metal compound other than fluoride. The method for mixing the alkali metal fluoride and the second metal compound is not particularly limited, and either dry mixing or wet mixing may be used. The solutions may be mixed by adding a second metal compound solution to an alkali metal fluoride solution, or by adding an alkali metal fluoride solution to a second metal compound solution. Furthermore, equal amounts of the two liquids may be mixed simultaneously using a microchannel or the like. For example, water may be used as the solvent.

[0052] The concentration of the alkali metal fluoride and the second metal compound when mixed can be adjusted appropriately. The concentration of the alkali metal fluoride may be 30 mM or more, or 50 mM or more. The concentration of the alkali metal fluoride may be 1000 mM or less. The concentration of the second metal compound may be 20 mM or more, or 40 mM or more. The concentration of the second metal compound may be 200 mM or less. The higher the concentration of these raw materials, the smaller the particle size tends to be, and therefore the transmittance of the optical filter 1 can be improved.

[0053] Examples of alkali metals contained in the alkali metal fluoride include lithium, sodium, potassium, rubidium, and cesium. Specifically, the alkali metal fluoride may contain at least one alkali metal selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.

[0054] The second metal compound may contain at least one metal salt selected from the group consisting of metal chlorides, metal nitrates, metal sulfates, and metal organic acid salts, excluding metal fluorides. The metal contained in the second metal compound may be at least one metal selected from the group consisting of alkaline earth metals, aluminum, gallium, indium, zinc, and yttrium. The second metal compound may contain, for example, aluminum chloride.

[0055] In the separation step, the generated multiple complex fluoride particles are separated. When the second metal compound is a metal chloride, metal nitrate, metal sulfate, or metal organic acid salt, when the alkali metal fluoride and the second metal compound are mixed, in addition to the complex fluoride, alkali metal chloride, nitrate, sulfate, or metal organic acid salt is generated as a by-product. Therefore, a washing operation to remove the by-product may be performed. The alkali metal compound may be removed by washing with a solvent such as water during filtration or centrifugation, for example.

[0056] In the heating step, the separated multiple complex fluoride particles are heated at 200°C or higher. By setting the heating temperature to 200°C or higher, the water content inside the complex fluoride particles is reduced. As a result, the absorption of infrared light by hydroxyl groups and water inside the optical filter 1 is reduced, and the transmittance of the optical filter 1 can be increased. The heating temperature of the complex fluoride particles may be 300°C or higher, 350°C or higher, or 400°C or higher. The heating temperature of the complex fluoride particles may be 900°C or lower, 800°C or lower, 700°C or lower, 600°C or lower, or 550°C or lower. The heating time of the complex fluoride particles may be 0.5 hours or longer, 1 hour or longer, or 1.5 hours or longer. The heating time of the complex fluoride particles may be 24 hours or shorter, 12 hours or shorter, 6 hours or shorter, or 3 hours or shorter.

[0057] In the method for producing a complex fluoride powder, by-products may be removed as necessary to separate the complex fluoride particles, followed by drying. The drying temperature is not particularly limited, but can be a temperature below the melting point of the complex fluoride. The drying temperature may be, for example, 40°C or higher, 80°C or higher, 100°C or higher, or 200°C or higher. The drying temperature may be, for example, 600°C or lower, 400°C or lower, 300°C or lower, or 200°C or lower. The drying time for the complex fluoride particles may be 0.5 hours or more, 1 hour or more, or 1.5 hours or more. The drying time for the complex fluoride particles may be 24 hours or less, 12 hours or less, 6 hours or less, or 3 hours or less.

[0058] The raw material may contain water of hydration in addition to the complex fluoride. The raw material may also contain by-products such as simple fluorides, oxides, and hydroxides containing a single metal element as unavoidable impurities.

[0059] Next, the raw material containing the complex fluoride particles and the absorbent is filled into the interior of the mold. If necessary, the raw material may contain a solvent such as water. The water content in the raw material may be 1% by mass or more, 5% by mass or more, 10% by mass or more, or 15% by mass or more. The water content in the raw material may be 80% by mass or less, 70% by mass or less, or 60% by mass or less. After the raw material is filled into the mold, the mold may be heated as needed. Then, by applying pressure to the raw material inside the mold, the interior of the mold becomes a high-pressure state. At this time, the complex fluoride particles are densified and simultaneously bonded to each other.

[0060] The heating and pressurizing conditions are not particularly limited as long as they are conditions that promote densification of the complex fluoride particles. For example, it is preferable to heat the raw material to 50 to 300°C and then pressurize it at a pressure of 10 to 600 MPa. The temperature at which the raw material is heated is more preferably 80 to 250°C, and even more preferably 100 to 200°C. The pressure at which the raw material is pressed is more preferably 50 to 600 MPa.

[0061] Then, the molded body is removed from the mold to obtain the optical filter 1. It is believed that by applying pressure to a raw material containing complex fluoride particles containing at least one of hydroxyl groups and water molecules while heating, the complex fluoride particles react with each other via the hydroxyl groups or water molecules to form a continuous phase 11 of a polycrystalline body of a complex fluoride containing an alkali metal. This allows the above-mentioned optical filter 1 to be formed.

[0062] The above-described reaction by pressure and heat can produce an optical filter 1 having a dense structure. Another possible method for forming an aggregate of inorganic particles is to press only inorganic particle powder to form a compact, which is then sintered at a high temperature (e.g., 1700°C or higher). However, even if the compact of inorganic particles is sintered at a high temperature, the resulting optical filter 1 contains many pores, resulting in insufficient mechanical strength and insufficient transmittance due to light scattering. Furthermore, sintering inorganic particles at a high temperature requires precise temperature control, which increases manufacturing costs.

[0063] In contrast, in the manufacturing method of this embodiment, a raw material containing complex fluoride particles containing at least one of hydroxyl groups and water molecules is pressurized while being heated, thereby making it possible to obtain a dense optical filter 1 with excellent transmittance. Furthermore, since the manufacturing method of this embodiment can be achieved by applying pressure while being heated at 50°C to 300°C, precise temperature control is not required, and it is possible to reduce manufacturing costs.

[0064] As described above, the method for manufacturing the optical filter 1 of this embodiment includes a step of pressurizing and heating the raw material containing the complex fluoride and the absorbent. According to the method for manufacturing the optical filter 1 of this embodiment, the above-mentioned optical filter 1 can be manufactured by a simple low-temperature process.

[0065] The optical filter 1 is not limited to the above method, and can also be manufactured, for example, by pressurizing and heating a raw material containing first fluoride particles containing an alkali metal, second fluoride particles containing a metal different from the alkali metal, an absorbent, and a solvent such as water. By using such a pressurizing and heating method, the fluoride particles bond to each other while forming a complex fluoride, thereby making it possible to form an optical filter 1 with low porosity. The optical filter 1 can also be manufactured, for example, by pressurizing and heating a raw material containing sodium fluoride (NaF), aluminum fluoride, an absorbent, and water.

[0066] The optical filter 1 can also be manufactured by pressurizing and heating a raw material containing complex fluoride particles, first fluoride particles containing an alkali metal, second fluoride particles containing a metal different from the alkali metal, an absorbent, and a solvent such as water.

[0067] Hereinafter, the present embodiment will be described in more detail with reference to examples and comparative examples, but the present embodiment is not limited to these examples.

[0068] (Example 1) First, sodium hexafluoroaluminate (Na ３ AlF ６ Specifically, 6.047 g of sodium fluoride (NaF) powder (Fujifilm Wako Pure Chemical Industries, Ltd., special grade reagent) was dissolved in 500 mL of ion-exchanged water to prepare a sodium fluoride aqueous solution. In addition, 4.3457 g of aluminum chloride (AlCl ３ ・6H ２ An aluminum chloride aqueous solution was prepared by dissolving powder of NaCl (Fujifilm Wako Pure Chemical Industries, Ltd., special grade reagent) in 100 mL of ion-exchanged water. The sodium fluoride aqueous solution and the aluminum chloride aqueous solution were mixed and stirred at 25°C for 20 hours. The stirred solution was suction filtered using a membrane filter with a pore size of 0.1 μm, and the residue was dried and crushed in an agate mortar to obtain a dry powder. The dry powder was fired at 400°C for 2 hours to obtain polycrystalline Na ３ AlF ６ Na containing as the main phase ３ AlF ６A powder was obtained.

[0069] Synthetic Na ３ AlF ６ 1 g of the powder was immersed in 5 mL of a fluorine-based resin-containing liquid (KTS-3000HL manufactured by AGC Seimi Chemical Co., Ltd.), and the resulting mixture was suction filtered and dried to obtain Na ３ AlF ６ The fluororesin contained in the fluororesin-containing liquid was a resin having a perfluoroalkyl group.

[0070] Next, 0.12 g of the mixed powder and 12 μL of ion-exchanged water were mixed and poured into a cylindrical molding die (Φ8) having an internal space. The mixture was then heated and pressurized at 150°C, 400 MPa, and 10 minutes to obtain a cylindrical sample. The cylindrical sample was then dried at 150°C for 2 hours to obtain the test sample of this example.

[0071] The obtained test sample was crushed in an agate mortar and subjected to TG (thermogravimetry) of the crushed product. The weight loss rate between 200°C and 400°C was approximately 3.9%. Therefore, the fluororesin content of the test sample was approximately 3.9% by mass. TG was measured by placing 10 mg of the test sample in an aluminum container, introducing air at a rate of 50 mL / min, and heating from 100°C to 600°C at a rate of 10°C / min.

[0072] The fluororesin obtained by drying the fluororesin-containing liquid was measured by a transmission method using an FT-IR device, and as shown in Figure 6, the fluororesin absorbed light in the infrared region of more than 7.5 μm and not more than 9 μm.

[0073] Example 2 A test sample was obtained in the same manner as in Example 1, except that KTS-3000HL was diluted twice with a solvent (SFcoat SCV Solvent) to form the fluororesin-containing liquid of Example 1. When the weight loss rate between 200°C and 400°C was measured in the same manner as in Example 1, the weight loss rate was 1.7%, and therefore the fluororesin content of the test sample of Example 2 was 1.7% by mass.

[0074] (Example 3) Sodium hexafluoroaluminate (Na ３ AlF ６ A mixture of 0.12 g of PVDF powder (Sigma-Aldrich), 0.003 g of PVDF powder, and 0.12 g of ion-exchanged water was placed in a cylindrical molding die (Φ8) with an internal space. The average particle diameter of the PVDF particles contained in the PVDF powder was approximately 200 nm. The mixture was then heated and pressurized at 150°C, 400 MPa, and 10 minutes to obtain a cylindrical sample. The cylindrical sample was then dried at 150°C for 2 hours to obtain the test sample. The PVDF content calculated from the mass of the test sample and the PVDF mixing ratio was approximately 2.4% by mass.

[0075] When the PVDF powder was measured by a transmission method using an FT-IR device, as shown in FIG. 7, the PVDF powder absorbed light in the infrared region of more than 7.5 μm and not more than 9 μm.

[0076] Example 4 First, sodium hexafluoroaluminate (Na ３ AlF ６ Next, the synthesized sodium hexafluoroaluminate (Na ３ AlF ６ A mixture of 0.12 g of PVDF powder (Sigma-Aldrich) and 0.12 g of ion-exchanged water was placed in a cylindrical molding die (Φ8) having an internal space. A cylindrical sample was then fabricated under the same conditions as in Example 3. The PVDF content calculated from the mass of the test sample and the mixing ratio of PVDF was approximately 1.2% by mass.

[0077] (Linear transmittance) Each test sample was measured by a transmission method using an FT-IR device to obtain an infrared transmission spectrum. The results are shown in Figures 8 to 11. The linear transmittance of the infrared transmission spectrum was converted based on the Beer-Lambert law so as to be the linear transmittance at a thickness of 1 mm.

[0078] 8, the linear transmittance of the test sample of Example 1 was 30% or more at wavelengths of 3.54 μm to 5.64 μm, 40% or more at wavelengths of 3.62 μm to 5.62 μm, and 50% or more at wavelengths of 4.38 μm to 5.46 μm. Furthermore, in the target wavelength band of more than 7 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 1 was 14.2%. Furthermore, in the target wavelength band of more than 7.5 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 1 was 1.0%.

[0079] 9, the linear transmittance of Example 2 was 30% or more at wavelengths of 3.52 μm to 5.66 μm, 40% or more at wavelengths of 3.62 μm to 5.64 μm, and 50% or more at wavelengths of 4.30 μm to 5.60 μm. Furthermore, in the target wavelength band of more than 7 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 2 was 24.8%. Furthermore, in the target wavelength band of more than 7.5 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 2 was 7.2%.

[0080] 10, the linear transmittance of Example 3 was 30% or more in the wavelength range of 3.58 μm to 5.82 μm and 40% or more in the wavelength range of 4.06 μm to 5.76 μm. Furthermore, in the target wavelength band of more than 7 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 3 was 7.1%. Furthermore, in the target wavelength band of more than 7.5 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 3 was 3.7%.

[0081] 11, the linear transmittance of Example 4 was 30% or more in the wavelength range of 3.76 μm to 5.76 μm and 40% or more in the wavelength range of 5.14 μm to 5.56 μm. Furthermore, in the target wavelength band of more than 7 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 4 was 8.2% μm. Furthermore, in the target wavelength band of more than 7.5 μm and up to 25 μm, the maximum linear transmittance of the test sample of Example 4 was 5.1%.

[0082] (Porosity) First, a cross section of the test sample of Example 1 was subjected to cross-section polisher processing (CP processing). Next, a scanning electron microscope (SEM) was used to observe a secondary electron image and a backscattered electron image of the cross section of the test sample at a magnification of 10,000 times. Fig. 12 is a secondary electron image of Example 1. Fig. 13 is a backscattered electron image of Example 1.

[0083] Next, the backscattered electron image obtained as described above was binarized to clarify the pores. Fig. 14 shows a binarized image of the backscattered electron image of Example 1. The porosity was then calculated from the binarized image by calculating the area ratio of pores with a pore diameter of 50 nm or more, and the porosity was found to be 15.12%.

[0084] The bulk density was calculated from the mass and volume of the test sample of Example 1, and the bulk density was 2.66 g / cm ３ As described above, the porosity calculated from the binarized image of the cross-sectional SEM image was about 15%, and the relative density was about 85%. From this result, the true density of the material constituting the test sample was about 3.13 g / cm ３ It was estimated that:

[0085] The bulk density calculated from the mass and volume of the test sample of Example 2 was 2.35 g / cm ３ Since the true density of the test sample was close to that of Example 1, the relative density was calculated from the true density estimated in Example 1, and the relative density was found to be approximately 75%. Furthermore, the porosity was calculated from the relative density value, and the porosity was found to be approximately 25%.

[0086] The bulk density calculated from the mass and volume of the test sample of Example 3 was 2.54 g / cm ３ The theoretical true density was 2.92 g / cm ３ Therefore, the relative density was about 87%. Furthermore, when the porosity was calculated from the relative density value, the porosity was about 13%.

[0087] The bulk density calculated from the mass and volume of the test sample of Example 4 was 2.58 g / cm ３ The theoretical true density was 2.95 g / cm３ Therefore, the relative density was about 87%. Furthermore, when the porosity was calculated from the relative density value, the porosity was about 13%.

[0088] (Pore diameter) From the binarized image of FIG. 14, the cumulative value of the pore diameter distribution on an area basis was 50% (d ５０ In the case of sodium hexafluoroaluminate (Na ), the pore diameter of Example 1 was determined, and the pore diameter d50 was found to be 175 nm. ３ AlF ６ ) and prepared under similar conditions, it is assumed that the pore diameters are similar.

[0089] Next, the powder obtained by pulverizing the test sample of each example was measured using a powder X-ray diffraction (XRD) device to obtain an XRD pattern. The results are shown in Figure 15. Furthermore, when the obtained XRD pattern was subjected to Rietveld analysis, it was found that Na ３ AlF ６ The mass ratio of was 90.5% in Example 1, 90.6% in Example 2, 95.3% in Example 3, and 93.6% in Example 4.

[0090] As described above, it is clear that the test sample according to this example can be used as an optical filter suitable for an infrared sensor for detecting, for example, a flame.

[0091] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0092] (Technology 1) An optical filter comprising a base material including a continuous phase of fluoride polycrystals and an absorbent dispersed within the base material, wherein the optical filter contains 0.1 to 10 mass % of the absorbent that absorbs light in the infrared region of more than 7.5 μm and not more than 9 μm, the optical filter has a wavelength bandwidth of 50 nm or more and a linear transmittance per mm of thickness of 30% or more in a target wavelength band of 3 μm or more and not more than 7 μm, the optical filter has a maximum linear transmittance per mm of thickness of 10% or less in a target wavelength band of more than 7.5 μm and not more than 25 μm, the optical filter contains 80 mass % or more of an inorganic substance, and the optical filter contains 50 mass % or more of fluoride polycrystals, the optical filter has a porosity of 30% or less, and the median pore diameter of the optical filter is 500 nm or less.

[0093] With this configuration, the optical filter 1 transmits infrared light in a specific region and blocks infrared light in another specific region, making the optical filter 1 suitable for an infrared sensor for detecting, for example, a flame.

[0094] (Technology 2) The optical filter according to Technology 1, wherein the fluoride includes a complex fluoride containing an alkali metal. While alkali metal fluorides have high solubility in water, complex fluorides containing alkali metals have low solubility in water. Therefore, such an optical filter is less soluble in water and has high water resistance.

[0095] (Technology 3) The optical filter according to Technology 2, wherein the complex fluoride includes a compound having a cryolite crystal structure. A complex fluoride having such a crystal structure is a material with small refractive index anisotropy due to crystal orientation. Therefore, the infrared transmittance of the optical filter 1 can be improved.

[0096] (Technology 4) The optical filter according to any one of Technologies 1 to 3, wherein the absorbent contains a fluororesin. Fluororesin has a C—F bond and absorbs light in the wavelength range of 8 μm to 9 μm. Therefore, it is possible to easily cut off light in the specific wavelength range described above.

[0097] (Technology 5) The optical filter according to Technology 4, wherein the fluororesin includes a resin having a perfluoroalkyl group. Such a fluororesin has many C—F bonds. Therefore, it can easily and effectively cut off light in a specific wavelength range.

[0098] (Technology 6) An infrared sensor comprising the optical filter according to any one of Technologies 1 to 5. With this configuration, the infrared sensor can easily detect, for example, flames.

[0099] (Technology 7) A method for manufacturing an optical filter, comprising the step of pressurizing a mixture containing a fluoride polycrystalline material and an absorbent at a temperature of 250°C or less, wherein the optical filter contains 0.1 to 10 mass% of the absorbent that absorbs light in the infrared region greater than 7.5 μm and less than 9 μm, the optical filter has a wavelength bandwidth of 50 nm or more in which the linear transmittance per mm of thickness is 30% or more in a target wavelength band of 3 μm to 7 μm, the optical filter has a maximum linear transmittance per mm of thickness of 10% or less in a target wavelength band of greater than 7.5 μm and less than 25 μm, the optical filter contains 80 mass% or more of an inorganic material, the optical filter contains 50 mass% or more of a fluoride polycrystalline material, the optical filter has a porosity of 30% or less, and the median pore size of the optical filter is 500 nm or less. This configuration allows the optical filter 1 to be manufactured at a low temperature, thereby suppressing thermal deterioration of the absorbent. Therefore, the above-described optical filter 1 can be easily manufactured.

[0100] The entire contents of Japanese Patent Application No. 2023-112348 (filing date: July 7, 2023) are incorporated herein by reference.

[0101] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment.

[0102] According to the present disclosure, it is possible to provide an optical filter suitable for an infrared sensor for detecting, for example, a flame, an infrared sensor using the same, and a method for manufacturing the optical filter.

[0103] 1 Optical filter 10 Base material 11 Continuous phase 20 Absorbent

Claims

1. A base material containing a continuous phase of polycrystalline fluoride, The absorbent dispersed within the base material, An optical filter comprising, The optical filter contains 0.1 to 10% by mass of the absorbent that absorbs light in the infrared region greater than 7.5 μm and less than or equal to 9 μm. In the target wavelength band of 3 μm to 7 μm, the wavelength bandwidth of the optical filter having a linear transmittance of 30% or more per 1 mm thickness is 50 nm or more. In the target wavelength band greater than 7.5 μm and less than or equal to 25 μm, the maximum linear transmittance per 1 mm thickness of the optical filter is 10% or less. The optical filter contains 80% by mass or more of an inorganic substance. The optical filter contains 50% by mass or more of a polycrystalline fluoride, The porosity of the optical filter is 30% or less. An optical filter having a median pore size of 500 nm or less.

2. The optical filter according to claim 1, wherein the fluoride includes a complex fluoride containing an alkali metal.

3. The optical filter according to claim 2, wherein the complex fluoride comprises a compound having a cryolite-type crystal structure.

4. The optical filter according to any one of claims 1 to 3, wherein the absorbent comprises a fluororesin.

5. The optical filter according to claim 4, wherein the fluororesin comprises a resin having a perfluoroalkyl group.

6. An infrared sensor comprising an optical filter according to any one of claims 1 to 3.

7. A method for manufacturing an optical filter, comprising the step of pressurizing a mixture containing a polycrystalline fluoride and an absorbent at a temperature of 250°C or lower, The optical filter contains 0.1 to 10% by mass of the absorbent that absorbs light in the infrared region greater than 7.5 μm and less than or equal to 9 μm. In the target wavelength band of 3 μm to 7 μm, the wavelength bandwidth of the optical filter having a linear transmittance of 30% or more per 1 mm thickness is 50 nm or more. In the target wavelength band greater than 7.5 μm and less than or equal to 25 μm, the maximum linear transmittance per 1 mm thickness of the optical filter is 10% or less. The optical filter contains 80% by mass or more of an inorganic substance. The optical filter contains 50% by mass or more of a polycrystalline fluoride, The porosity of the optical filter is 30% or less. A method for manufacturing an optical filter, wherein the median pore size of the optical filter is 500 nm or less.