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

Abstract

An optical filter (1) is provided with: a base material (10) containing a continuous phase (11) of a fluoride polycrystal; and an absorbent (20) dispersed in the base material (10), the absorbent (20) containing 0.1-10 mass% of the absorbent (20) that absorbs light in the infrared region exceeding 7.5 [mu] m and 9 [mu] m or less, the wavelength bandwidth at which the linear transmittance per 1 mm thickness is 30% or more in the target wavelength band of 3 [mu] m or more and 7 [mu] m or less is 50 nm or more, and the linear transmittance per 1 mm thickness is 30% or more in the target wavelength band exceeding 7.5 [mu] m and 25 [mu] m or less in the target wavelength band of 7.5 [mu] m and 25 [mu] m or less. The present invention relates to a polycrystalline glass which has a maximum linear transmittance of 10% or less per 1 mm thickness, contains 80% by mass or more of an inorganic substance and 50% by mass or more of a fluoride, has a porosity of 30% or less, and has a median value of a pore diameter of 500 nm or less.

Description

Technical Field

[0001] This invention relates to optical filters, infrared sensors, and methods for manufacturing optical filters. Background Technology

[0002] Previously, infrared transmission windows based on inorganic fluorides were used in special applications such as window materials for physical and chemical equipment in academic research and industry. Patent Document 1 discloses a Fourier transform infrared spectrophotometer that has a measuring chamber with a window composed of one of the following: CaF2, BaF2, MgF2, LiF, and ZnSe, for measuring fluorine-based gases in a sample containing corrosive gases.

[0003] Existing technical documents Patent documents Patent Document 1: International Publication No. 2019 / 176624 Summary of the Invention

[0004] On the other hand, techniques for sensing various gases and flames using infrared detection elements and infrared transmission optical filters are known. However, the aforementioned materials may transmit wavelengths that are not intended for use, and may be unsuitable as optical filters for infrared sensors.

[0005] This invention was made in view of the problems inherent in the prior art. Furthermore, the object of this invention is to provide an optical filter suitable for, for example, an infrared sensor used for detecting flames, an infrared sensor using the optical filter, and a method for manufacturing the optical filter.

[0006] To address the aforementioned issues, the optical filter according to the first aspect of the present invention comprises: a substrate containing a continuous polycrystalline phase of fluoride; and an absorber dispersed within the substrate. The optical filter contains 0.1 to 10% by mass of an absorber that absorbs light in the infrared region exceeding 7.5 μm and below 9 μm. In the target wavelength range of 3 μm to 7 μm, the optical filter has a linear transmittance of 30% or more per 1 mm thickness and a wavelength bandwidth of 50 nm or more. In the target wavelength range of exceeding 7.5 μm and below 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 inorganic material. The optical filter contains 50% by mass or more of a polycrystalline fluoride. The porosity of the optical filter is 30% or less. The central value of the pore size of the optical filter is 500 nm or less.

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

[0008] The third aspect of the present invention relates to a method for manufacturing an optical filter, comprising a step of pressurizing a mixture of a polycrystalline material containing a fluoride and an absorbent at a temperature below 250°C. The optical filter contains 0.1 to 10% by mass of an absorbent that absorbs light in the infrared region exceeding 7.5 μm and below 9 μm. In a target wavelength range of 3 μm to 7 μm, the optical filter has a linear transmittance of 30% or more per 1 mm thickness and a wavelength bandwidth of 50 nm or more. In a target wavelength range exceeding 7.5 μm and below 25 μm, the maximum linear transmittance of the optical filter per 1 mm thickness is 10% or less. The optical filter contains 80% by mass or more of inorganic material. The optical filter contains 50% by mass or more of a polycrystalline material containing a fluoride. The porosity of the optical filter is 30% or less. The central value of the pore size of the optical filter is 500 nm or less. Attached Figure Description

[0009] Figure 1 This is a cross-sectional view that schematically illustrates an example of the optical filter involved in this embodiment.

[0010] Figure 2 It is Figure 1 A magnified schematic cross-sectional view of a portion of the optical filter shown.

[0011] Figure 3 This is a cross-sectional view that schematically illustrates another example of the optical filter involved in this embodiment.

[0012] Figure 4 It is Figure 3 A magnified schematic cross-sectional view of a portion of the optical filter shown.

[0013] Figure 5 This is a cross-sectional view that schematically illustrates an example of the infrared sensor involved in this embodiment.

[0014] Figure 6 The infrared absorption spectrum is shown in the examples of the fluorinated resin used.

[0015] Figure 7 This is the infrared absorption spectrum of the PVDF used in the examples.

[0016] Figure 8 It is the linear transmittance of the test sample involved in Example 1.

[0017] Figure 9 It is the linear transmittance of the test sample involved in Example 2.

[0018] Figure 10 It is the linear transmittance of the test sample involved in Example 3.

[0019] Figure 11It is the linear transmittance of the test sample involved in Example 4.

[0020] Figure 12 It is a secondary electron image of the cross-section of the test sample involved in Example 1.

[0021] Figure 13 It is a reflected electron image of the cross section of the test sample involved in Example 1.

[0022] Figure 14 Yes Figure 13 The image after binarizing the reflected electron image.

[0023] Figure 15 It is the XRD pattern of the powder after the test samples involved in each example are crushed. Detailed Implementation

[0024] The optical filter, infrared sensor, and manufacturing method of the optical filter according to this embodiment will be described in detail below with the help of the accompanying drawings. It should be noted that, for ease of explanation, the scale of the drawings is exaggerated and may sometimes differ from the actual scale.

[0025] [Optical Filter] Figure 1 This is a cross-sectional view that schematically shows an example of the optical filter 1 according to this embodiment. Figure 2 It is Figure 1 A partial magnified schematic cross-sectional view of the optical filter 1 shown. Figure 1 and Figure 2 As shown, the optical filter 1 of this embodiment includes a substrate 10 and an absorbent 20. The substrate 10 contains a continuous polycrystalline phase 11 of fluoride. The optical filter 1 of this embodiment is a ceramic structure.

[0026] Fluorides may contain at least one of alkali metals and alkaline earth metals. Alkali metals may contain at least one selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium. Alkaline earth metals may contain at least one metal selected from the group consisting of magnesium, calcium, strontium, and barium.

[0027] Fluorides can include complex fluorides containing alkali metals. Complex fluorides can contain at least one alkali metal, for example, selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium. Alkali metal fluorides have high solubility in water, but complex fluorides containing alkali metals have low solubility in water. For example, the solubility of alkali metal fluorides in 100g of water at 25°C is 0.134g for LiF, 4.13g for NaF, 102g for KF, 300g for RbF, and 573g for CsF. On the other hand, the solubility of complex fluorides containing alkali metals in 100g of water at 25°C is, for example, 0.042g for Na3AlF6 and 0.025g for NaMgF3. Therefore, the optical filter 1 using a complex fluoride containing an alkali metal exhibits higher water resistance compared to alkali metal fluorides.

[0028] The composite fluoride may contain fluorine, an alkali metal, and an additional metal as main components. The additional metal may contain at least one metal selected from the group consisting of alkaline earth metals, aluminum, gallium, indium, zinc, and yttrium. The alkaline earth metal may contain at least one metal selected from the group consisting of magnesium, calcium, strontium, and barium. Such a composite fluoride exhibits low anisotropy of refractive index, thus improving infrared light transmittance. Furthermore, as described later, inorganic substances containing these metal elements can be readily formed into an optical filter 1 by a pressure heating method. It should be noted that the term "main component" here refers to the total content of fluorine, alkali metal, and additional metal in the composite fluoride, expressed as a molar ratio of 80% or more. This total content may be 85% or more, 90% or more, 95% or more, or 100%.

[0029] Specifically, the complex fluoride may contain at least one of A3AlF6 and ABF3 (here, in the above composition, A represents one or more alkali metals, and B represents one or more alkaline earth metals). A3AlF6 may, for example, contain at least one selected from the group consisting of Li3AlF6, Li2NaAlF6, Li2KAlF6, Na3AlF6, Na2LiAlF6, Na2KAlF6, K3AlF6, K2LiAlF6, and K2NaAlF6. ABF3 may, for example, contain at least one selected from the group consisting of LiMgF3, NaMgF3, KMgF3, LiCaF3, NaCaF3, and KCaF3. More specifically, the complex fluoride may contain at least one selected from the group consisting of Na3AlF6, K2NaAlF6, and NaMgF3.

[0030] Complex fluorides can include compounds with cryolite-type crystal structures. Examples of complex fluorides with cryolite-type crystal structures include Na3AlF6 and K2NaAlF6. Complex fluorides can also have perovskite-type crystal structures. Examples of complex fluorides with perovskite-type crystal structures include NaMgF3. Complex fluorides with these crystal structures are preferred from the viewpoint of infrared transmission because they are materials with low refractive index anisotropy based on crystal orientation.

[0031] Complex fluorides can contain fluorine, alkali metals, and aluminum as main components. It should be noted that "main components" here refers to the total content of fluorine, alkali metals, and aluminum in the complex fluoride, expressed as a molar ratio of 80% or more. This total content can be 85% or more, 90% or more, 95% or more, or even 100%. Complex fluorides can contain, for example, sodium hexafluoroaluminate (Na3AlF6). Na3AlF6 is also known to be a component of cryolite. Na3AlF6 has low anisotropy in its refractive index, therefore exhibiting excellent infrared light transmittance.

[0032] A portion of the constituent anions of the complex fluoride can be replaced by hydroxide ions or oxide ions. For example, in the case of complex fluoride particles synthesized in the liquid phase, sometimes a portion of the complex fluoride ions is replaced by at least one of hydroxide ions and oxide ions. When the complex fluoride has a hydroxyl group, the hydroxyl group absorbs infrared light of a specific wavelength. Therefore, by blocking unwanted wavelengths, the sensor sensitivity can be improved. Furthermore, when the optical filter 1 contains a hydroxyl group, the transmittance variation caused by humidity is reduced, and the sensor is less susceptible to humidity effects.

[0033] The optical filter 1 contains 80% or more of inorganic material. Therefore, compared to using resin, an optical filter 1 that is less prone to degradation over time and has high infrared transmittance can be obtained. It should be noted that the inorganic material includes the aforementioned fluoride. The optical filter 1 may contain 85% or more, 90% or more, or 95% or more of inorganic material. The optical filter 1 may contain 99.9% or less of inorganic material.

[0034] The optical filter 1 is a polycrystalline material containing 50% by mass or more of fluoride. From the viewpoint of gas barrier properties and durability, polycrystalline materials containing fluoride are superior. Furthermore, by using a polycrystalline material containing 50% by mass or more of fluoride, the resulting optical filter 1 becomes an optical filter that readily utilizes the properties of fluoride. Additionally, the high proportion of fluoride reduces the ratio of different refractive indexes within the optical filter 1, thereby suppressing light scattering and improving the infrared transmittance of the optical filter 1. In the optical filter 1, the proportion of fluoride can be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more.

[0035] like Figure 2 As shown, the substrate 10 may contain a plurality of fluoride particles 12. The plurality of fluoride particles 12 may also be dispersed within the substrate 10. A continuous phase 11 may exist between adjacent fluoride particles 12. The continuous phase 11 may also bond the plurality of fluoride particles 12 individually. 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 it 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 they may be in surface contact with each other.

[0036] The fluoride particles 12 can be composed of the same substance as the fluoride constituting the continuous phase 11 described above. The inorganic substance constituting the fluoride particles 12 can be crystalline or amorphous. From the viewpoint of gas barrier properties or durability, the inorganic substance constituting the fluoride particles 12 is preferably crystalline. Furthermore, from the viewpoint of light transmittance, the inorganic substance constituting the fluoride particles 12 is preferably amorphous. It should be noted that, in the case of crystalline material, the fluoride particles 12 can be monocrystalline or polycrystalline.

[0037] The average particle size of the plurality of fluoride particles 12 can be 10 nm or more and 50 μm or less. By making the average particle size of the fluoride particles 12 50 μm or less, the light transmittance of the optical filter 1 increases. Furthermore, by keeping the average particle size of the fluoride particles 12 within this range, the fluoride particles 12 are firmly bonded together, which improves the strength of the optical filter 1. Additionally, by keeping the average particle size of the fluoride particles 12 within this range, as described later, the proportion of pores present inside the optical filter 1 is 30% or less. Furthermore, the size of the pores generated between the fluoride particles 12 is reduced. Therefore, the strength of the optical filter 1 can be improved. From the viewpoint of improving the light transmittance of the optical filter 1, the average particle size of the plurality of fluoride particles 12 is more preferably 10 μm or less, further preferably 2 μm or less, particularly preferably 1 μm or less, and most preferably 500 nm or less. The average particle size of the plurality of fluoride particles 12 can, for example, be 1 nm or more, or 10 nm or more. It should be noted that, in this specification, the value of "average particle size" is, unless otherwise specified, calculated using observation methods such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM), and is the average value of the particle size observed in several to tens of fields of view.

[0038] The optical filter 1 may also contain impurities generated during manufacturing. These impurities may be, for example, raw materials used to generate fluorides, or elements that constitute fluorides. 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 aforementioned additional metals. The impurities may include chalcogenides, halides, hydroxides, nitrides, or carbides comprising an oxide of a single metallic element. The impurities may be crystalline or amorphous.

[0039] The difference in refractive index between the fluoride and the impurities can be 0.1 or less. When the difference in refractive index is 0.1 or less, light scattering can be suppressed, and an optical filter with high transmittance can be manufactured. There is no particular limitation on the lower limit of the difference in refractive index. The difference in refractive index can be 0.08 or less, or it can be 0.04 or less.

[0040] The absorbent 20 is dispersed within the substrate 10. For example... Figure 2 As shown, the absorbent 20 can be a film covering the surface of the fluoride particles 12. Additionally, as... Figure 3 As shown, absorbent 20 is in the form of particles, such as... Figure 4 As shown, the film may not be covering the surface of the fluoride particles 12. Additionally, the absorbent 20 may also contain, for example, a film covering the surface of the fluoride particles 12. Figure 2 The film covering the surface of the fluoride particles 12 shown and such Figure 3 The particles shown. Additionally, the absorbent 20 can also be uniformly dispersed within the continuous phase 11.

[0041] The absorber 20 absorbs light in the infrared region, exceeding 7.5 μm but less than 9 μm. By absorbing light in such an infrared region, wavelengths of light not completely blocked by the fluoride can be blocked. Therefore, an optical filter 1 can be provided, for example, to block wavelengths of light in specific regions that are a cause of noise.

[0042] The absorber 20 may comprise a fluoropolymer. Fluoropolymers are also known as fluorinated resins. Fluoropolymers have CF bonds and absorb light in the 8μm to 9μm wavelength range, thus blocking light in the specific wavelength range described above. Fluoropolymers may, for example, comprise at least one selected from the group consisting of resins having perfluoroalkyl groups, polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), perfluoroalkoxy fluoropolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE). In particular, resins having perfluoroalkyl groups and polyvinylidene fluoride (PVDF) have more CF bonds, thus effectively blocking light in the specific wavelength range.

[0043] When the absorber 20 is in the form of particles, the average particle size of the absorber 20 can be 4 μm or less. When the average particle size of the absorber 20 is 4 μm or less, the mechanical strength of the optical filter 1 can be maintained relatively well. Alternatively, the average particle size of the absorber 20 can be 1 μm or less, or even 0.1 μm or less. There is no particular limitation on the lower limit of the average particle size of the absorber 20; the average particle size of the absorber 20 can be 1 nm or more. The average particle size of the absorber 20 can be 1 μm or more, or even 10 μm or more.

[0044] The absorber 20 in the optical filter 1 has a content of 0.1% by mass or more and 10% by mass or less. By making the content of absorber 20 0.1% by mass or more, the amount of light absorbed by the optical filter 1 can be increased. Furthermore, by making the content of absorber 20 10% by mass or less, the mechanical properties of the optical filter 1 can be improved. The content of absorber 20 can be 0.5% by mass or more, 1% by mass or more, 2% by mass or more, or 3% by mass or more. Alternatively, the content of absorber 20 can be 6% by mass or less, 4% by mass or less, 3% by mass or less, or 2% by mass or less.

[0045] In addition to the absorbent 20, the optical filter 1 may also contain organic materials such as resin particles. As described later, the optical filter 1 can be obtained by heating to 100~300°C while applying pressure, thus allowing the addition of components with low heat resistance to the optical filter 1. Furthermore, it is not limited to components with low heat resistance such as organic materials; to impart functionality, the optical filter 1 may also contain inorganic compounds other than fluorides and impurities.

[0046] The porosity of the cross-section of the optical filter 1 is 30% or less. That is, the porosity of pores larger than 0 nm in the cross-section of the optical filter 1 is 30% or less. Specifically, when observing the cross-section of the optical filter 1, the average proportion of pores per unit area is 30% or less. With a porosity of 30% or less, the number of pores inside the optical filter 1 is reduced, thus resulting in a solidified body with high strength. It should be noted 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 better it can suppress cracks originating from pores, thus improving the strength of the optical filter 1. Furthermore, the smaller the porosity of the cross-section of the optical filter 1, the more light scattering caused by pores is suppressed, thus improving the transmittance of the optical filter 1.

[0047] In this specification, porosity can be calculated as follows. First, observe the cross-section of the optical filter 1 to distinguish between pores and other areas. Then, measure the area of ​​pores per unit area and calculate the proportion of pores per unit area. The porosity is set as the average of the proportions of pores per unit area calculated 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. Alternatively, the unit area and the area of ​​pores within that unit area can also be determined by binarizing the image observed under a microscope.

[0048] The central value of the pore diameter of the optical filter 1 is 500 nm or less. When the central value of the pore diameter is 500 nm or less, light scattering caused by the pores is suppressed, thus resulting in an optical filter 1 with high transmittance. The central value of the pore diameter is preferably 300 nm or less, more preferably 200 nm or less, and even more preferably 100 nm or less. It should be noted that, in this specification, the central value of the pore diameter refers to the pore diameter when the cumulative value of the pore diameter distribution under an area reference 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.

[0049] Optical filter 1 has infrared light transmittance. In the target wavelength band, the linear transmittance of optical filter 1 can also have a wavelength bandwidth exceeding a specified value. The linear transmittance of optical filter 1 can be obtained by measuring the transmittance using an FT-IR (Fourier Transform Infrared Spectroscopy) device. The linear transmittance of the infrared transmittance spectrum is converted to linear transmittance with a thickness of 1 mm based on the Lambert-Beer law. Furthermore, the wavelength bandwidth can be obtained by measuring the wavelength bandwidth in the target wavelength band where the linear transmittance is above a specified value. The wavelength bandwidth can be the wavelength bandwidth where the linear transmittance continuously exceeds a specified value, or it can be the combined wavelength bandwidth where the linear transmittance continuously and intermittently exceeds a specified value. That is, the wavelength bandwidth is the combined wavelength bandwidth where the linear transmittance exceeds a specified value in the target wavelength band. Therefore, even when the linear transmittance intermittently exceeds a specified value, the wavelength bands where the linear transmittance exceeds a specified value can be separated separately in the target wavelength band. It should be noted that the wavelength bandwidth is preferably the wavelength bandwidth where the linear transmittance continuously exceeds a specified value.

[0050] In the target wavelength range of 3 μm to 7 μm, the optical filter 1 with a linear transmittance of 30% or more per 1 mm thickness has a wavelength bandwidth of 50 nm or more. Such an optical filter 1 is suitable for, for example, infrared sensors used for detecting flames. It should be noted that the target wavelength range can be 3.4 μm or more, or 3.8 μm or more. Furthermore, the target wavelength range can be 6 μm or less, 5 μm or less, or 4.4 μm or less. The linear transmittance can be 40% or more, or 50% or more. Furthermore, the linear transmittance can be 100% or less. The wavelength bandwidth can be 0.5 μm or more, 1 μm or more, 1.5 μm or more, or 2 μm or more. Furthermore, the wavelength bandwidth can be 4 μm or less, or 3 μm or less.

[0051] In the target wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance per 1 mm thickness of the optical filter 1 is less than 10%. Such an optical filter 1 exhibits excellent shielding characteristics in the aforementioned target wavelength range, and is therefore suitable for use in infrared sensors, for example, for detecting flames. It should be noted that the maximum linear transmittance refers to the highest linear transmittance in the aforementioned target wavelength range.

[0052] Optical filter 1 can also be an infrared transmission filter. By using optical filter 1 as an infrared transmission filter, it is possible to provide infrared sensors and light-emitting devices that can suppress diffuse reflection of infrared light caused by pores. When optical filter 1 can transmit light in a specific infrared region in a single layer, specific infrared light can be transmitted even without stacking multiple dielectrics as in, for example, an interference filter, or by setting up a separate light-shielding filter to improve sensor sensitivity.

[0053] The thickness t of the optical filter 1 is 10 μm or more. As described later, the optical filter 1 of this embodiment is formed by a pressure heating method. Therefore, even without laminating thin films, a bulk material with a thickness of 10 μm or more can be easily formed. It should be noted that the thickness of the optical filter 1 can be 100 μm or more, 500 μm or more, 1 mm or more, or 1 cm or more. There is no particular upper limit to the thickness of the optical filter 1; for example, it can be 50 cm.

[0054] When the optical filter 1 is used as an infrared transmission filter, its shape is not particularly limited. It can be flat, concave, or convex. By giving it a lens shape, light can be incident perpendicularly onto the dielectric-layered bandpass filter, thus eliminating the light incident angle dependence of the dielectric-layered bandpass filter. Additionally, a microstructure with anti-reflective properties can be formed on the surface of the optical filter 1.

[0055] Infrared sensor Next, use Figure 5 The infrared sensor 100 involved in this embodiment will be described. For example... Figure 5 As shown, the infrared sensor 100 includes the aforementioned optical filter 1. Additionally, the infrared sensor 100 includes an infrared detection element 110, an IC element 120, a substrate 130, and a metal housing 150.

[0056] Infrared detection element 110 and IC element 120 are mounted on substrate 130 via chip bonding material 131. Infrared detection element 110 and IC element 120 are electrically connected to each other via wires 140. In addition, infrared detection element 110 is connected to circuit wiring (not shown) on substrate 130 via wires 140.

[0057] Infrared detection element 110 receives infrared radiation, converts the thermal energy of the received infrared radiation into electrical energy, and outputs an electrical signal corresponding to the amount of infrared radiation received to IC element 120. Infrared detection element 110 can be a thermoelectric element, a thermopile type infrared detection element, a calorimeter type infrared detection element, or a quantum type infrared detection element.

[0058] IC element 120 includes: an amplifier circuit that amplifies the electrical signal output from infrared detection element 110; and a determination circuit that determines the presence of a flame when the electrical signal amplified by the amplifier circuit exceeds a threshold.

[0059] A metal housing 150 is mounted on a substrate 130. The metal housing 150 surrounds the infrared detection element 110 and the IC element 120, which are sealed by the substrate 130 and the metal housing 150. The metal housing 150 includes an upper wall 151 and a side wall 152. An opening is provided in the upper wall 151, which is covered by an optical filter 1. The optical filter 1 is configured to face the infrared detection element 110, which is disposed on the substrate 130 to receive infrared light that has passed through the optical filter 1. The side wall 152 connects the edge of the upper wall 151 to the edge of the substrate 130.

[0060] Infrared radiation emitted from flames or the like passes through optical filter 1 and is received by infrared detection element 110. Infrared detection element 110 outputs an electrical signal corresponding to the amount of infrared radiation received to IC element 120. IC element 120 determines the presence of a flame based on the electrical signal. In this way, infrared sensor 100 can detect flames using infrared radiation emitted from flames.

[0061] It should be noted that inorganic gases such as SO2, CO2, PH3, N2O, and NO have characteristic absorption properties in the mid-infrared region. Furthermore, these gas molecules emit corresponding infrared radiation at high temperatures. Therefore, the optical filter 1 according to this embodiment can also be used as a sensor for gases, not limited to CO2, that have characteristic absorption properties in the infrared region. Specifically, the optical filter 1 according to this embodiment has high infrared transmittance, and is therefore useful as an infrared transmission window.

[0062] It should be noted that in this embodiment, the infrared sensor 100 is described as an example of a flame sensor used to detect flames, but the application of the infrared sensor 100 is not limited to this method. The infrared sensor 100 can also be used in human sensors, biosensors, anti-theft sensors, gas sensors, non-contact thermometers, solid-state imaging devices, or camera modules, etc.

[0063] [Manufacturing methods for optical filters] Next, the 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 pressurization and heating method, the fluoride particles deform and bond together, thus forming an optical filter 1 with low porosity.

[0064] The fluoride can be the aforementioned fluoride. The fluoride can be crystalline, and the optical filter 1 can be manufactured by pressurizing a mixture containing a polycrystalline fluoride and an absorbent. It should be noted that, in this embodiment, an example using composite fluoride particles as fluoride particles will be described. The raw materials containing composite fluoride particles may include a process for generating multiple composite fluoride particles, a separation process, and a heating process.

[0065] In the process of generating multiple complex fluoride particles, specifically, firstly, a first metal compound, which is an alkali metal fluoride, and a second metal compound different from the fluoride are mixed and reacted to prepare complex fluoride particles. The mixing method of the alkali metal fluoride and the second metal compound is not particularly limited; it can be dry mixing or wet mixing. These solutions can be mixed by adding a solution of the second metal compound to an alkali metal fluoride solution, or vice versa. Alternatively, the two liquids can be mixed simultaneously in equal amounts using a microfluidic system. Water can be used as a solvent, for example.

[0066] The concentrations of the alkali metal fluoride and the second metal compound can be adjusted appropriately. The concentration of the alkali metal fluoride can be 30 mM or higher, or 50 mM or higher. The concentration of the alkali metal fluoride can be below 1000 mM. The concentration of the second metal compound can be 20 mM or higher, or 40 mM or higher. The concentration of the second metal compound can be below 200 mM. Since there is a tendency for smaller particle sizes with higher concentrations of these raw materials, the transmittance of the optical filter 1 can be improved.

[0067] Examples of alkali metals contained in alkali metal fluorides include lithium, sodium, potassium, rubidium, and cesium. Specifically, alkali metal fluorides may contain at least one alkali metal selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.

[0068] The second metal compound is at least one selected from the group consisting of metal chlorides, metal nitrates, metal sulfates, and organometallic acid salts, and may also contain metal salts other than metal fluorides. The metal contained in the second metal compound may be at least one selected from the group consisting of alkaline earth metals, aluminum, gallium, indium, zinc, and yttrium. The second metal compound may, for example, contain aluminum chloride.

[0069] In the separation process, multiple complex fluoride particles are separated. When the second metal compound is a metal chloride, metal nitrate, metal sulfate, or organometallic acid salt, alkali metal fluorides are mixed with the second metal compound, and in addition to the complex fluorides, alkali metal chlorides, nitrates, sulfates, and organometallic acid salts are also generated as byproducts. Therefore, a washing operation to remove these byproducts can also be performed. Alkali metal compounds can be removed, for example, by washing with solvents such as water during filtration or centrifugation.

[0070] In the heating process, the separated composite fluoride particles are heated to above 200°C. By setting the heating temperature to above 200°C, the water content inside the composite fluoride particles is reduced. As a result, the absorption of infrared light caused by the hydroxyl groups and water inside the optical filter 1 is reduced, thereby improving the transmittance of the optical filter 1. The heating temperature of the composite fluoride particles can be above 300°C, above 350°C, or above 400°C. The heating temperature of the composite fluoride particles can be below 900°C, below 800°C, below 700°C, below 600°C, or below 550°C. The heating time of the composite fluoride particles can be above 0.5 hours, above 1 hour, or above 1.5 hours. Alternatively, the heating time of the composite fluoride particles can be below 24 hours, below 12 hours, below 6 hours, or below 3 hours.

[0071] In the method for manufacturing composite fluoride powder, the composite fluoride particles can be separated by removing byproducts as needed, followed by drying. The drying temperature is not particularly limited and can be carried out at a temperature below the melting point of the composite fluoride. For example, the drying temperature can be 40°C or higher, 80°C or higher, 100°C or higher, or 200°C or higher. Alternatively, the drying temperature can be 600°C or lower, 400°C or lower, 300°C or lower, or 200°C or lower. The drying time for the composite fluoride particles can be 0.5 hours or more, 1 hour or more, or 1.5 hours or more. Alternatively, the drying time for the composite fluoride particles can be 24 hours or less, 12 hours or less, 6 hours or less, or 3 hours or less.

[0072] In addition to complex fluorides, the raw materials may also contain hydrated water. Furthermore, the raw materials may also contain byproducts such as elemental fluorides, oxides, and hydroxides, which contain a single metal element as an unavoidable impurity.

[0073] Next, the raw material containing composite fluoride particles and an absorbent is filled into the mold. Depending on the requirements, the raw material may also contain a solvent such as water. The water content in the raw material can be 1% or more by mass, 5% or more by mass, 10% or more by mass, or 15% or more by mass. Alternatively, the water content in the raw material can be less than 80% by mass, less than 70% by mass, or less than 60% by mass. After filling the mold with the raw material, the mold can be heated as needed. Furthermore, by applying pressure to the raw material inside the mold, the interior of the mold becomes a high-pressure state. At this time, the composite fluoride particles become dense, and the composite fluoride particles bond together with each other.

[0074] The heating and pressurizing conditions are not particularly limited as long as they are for densifying the composite fluoride particles. For example, it is preferable to heat the raw material to 50-300°C and then pressurize it at 10-600 MPa. It should be noted that the temperature when heating the raw material is more preferably 80-250°C, and even more preferably 100-200°C. In addition, the pressure when pressurizing the raw material is more preferably 50-600 MPa.

[0075] Then, by removing the molded body from inside the mold, the optical filter 1 can be obtained. It is believed that by heating and pressurizing a raw material containing composite fluoride particles containing at least one of hydroxyl and water molecules, the composite fluoride particles react with each other via hydroxyl or water molecules to form a polycrystalline continuous phase 11 of a composite fluoride containing an alkali metal. Thus, the aforementioned optical filter 1 can be formed.

[0076] By performing a pressurization and heating reaction as described above, an optical filter 1 with a dense structure can be obtained. Alternatively, as a method for forming aggregates of inorganic particles, it is also possible to press inorganic particle powder to form a pressed powder body, and then sinter it at a high temperature (e.g., above 1700°C). However, even when sintering the pressed powder body of inorganic particles at high temperatures, the resulting optical filter 1 contains a relatively large number of pores, resulting in insufficient mechanical strength and insufficient transmittance due to light scattering. Furthermore, sintering inorganic particles at high temperatures requires precise temperature control, thus increasing manufacturing costs.

[0077] In contrast, in the manufacturing method of this embodiment, a raw material containing composite fluoride particles comprising at least one of hydroxyl and water molecules is heated and pressurized simultaneously, thus obtaining a dense optical filter 1 with excellent transmittance. Furthermore, the manufacturing method of this embodiment can be achieved by heating and pressurizing at 50 to 300°C, thus eliminating the need for precise temperature control and reducing manufacturing costs.

[0078] Thus, the manufacturing method of the optical filter 1 in this embodiment includes steps of pressurizing and heating a raw material containing a composite fluoride and an absorbent. According to the manufacturing method of the optical filter 1 in this embodiment, the optical filter 1 can be manufactured using a simple low-temperature process.

[0079] It should be noted that the optical filter 1 is not limited to the method described above. For example, it can also be manufactured 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 pressurized heating method, the fluoride particles combine with each other while forming a composite fluoride, thus forming 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.

[0080] Alternatively, the optical filter 1 can also be manufactured by pressurizing and heating a raw material containing composite fluoride particles, a first fluoride particle containing an alkali metal, a second fluoride particle containing a metal different from the alkali metal, an absorbent, and a solvent such as water.

[0081] Example

[0082] The present embodiment will be described in more detail below through examples and comparative examples, but the present embodiment is not limited to these examples.

[0083] (Example 1) First, sodium hexafluoroaluminate (Na3AlF6) was synthesized. Specifically, an aqueous solution of sodium fluoride (NaF) powder (from Fujifilm and Koko Pure Chemical Industries, Ltd., reagent grade) was prepared by dissolving 6.047 g of sodium fluoride (NaF) powder in 500 mL of deionized water. An aqueous solution of aluminum chloride (AlCl3·6H2O) powder (from Fujifilm and Koko Pure Chemical Industries, Ltd., reagent grade) was prepared by dissolving 4.3457 g of aluminum chloride (AlCl3·6H2O) powder in 100 mL of deionized water. The sodium fluoride and aluminum chloride solutions were mixed and stirred at 25°C for 20 hours. The stirred solution was filtered using a 0.1 μm membrane filter, and the residue was dried and pulverized using an agate mortar to obtain a dry powder. This dry powder was then calcined at 400°C for 2 hours to obtain Na3AlF6 powder containing polycrystalline Na3AlF6 as the main phase.

[0084] One g of synthesized Na3AlF6 powder was impregnated in 5 mL of a fluorinated resin containing liquid (manufactured by AGC Seimi Chemical Co., Ltd., KTS-3000HL). The resulting mixture was filtered and dried to obtain a mixed powder of Na3AlF6 and fluorinated resin. The fluorinated resin contained in the fluorinated resin containing liquid is a resin with perfluoroalkyl groups.

[0085] Next, 0.12 g of the mixed powder and 12 μL of deionized water were added inside a cylindrical molding die (Φ8) with an internal space. The mixture was then heated and pressurized at 150 °C and 400 MPa for 10 minutes to obtain a cylindrical sample. This cylindrical sample was then dried at 150 °C for 2 hours, thus obtaining the test sample of this example.

[0086] The obtained test sample was subjected to TG (thermogravimetric analysis) to obtain a powder pulverized from an agate mortar. The results showed a weight reduction rate of approximately 3.9% between 200°C and 400°C. Therefore, the fluorinated resin content of the test sample was approximately 3.9% by mass. It should be noted that TG was determined by placing 10 mg of the test sample in an aluminum container, allowing air to flow in at a rate of 50 mL / min, and heating from 100°C to 600°C at a rate of 10°C / min.

[0087] Fluorine resins obtained by drying a fluorine-containing liquid using an FT-IR apparatus were analyzed by transmission method. The results are as follows: Figure 6 As shown, fluorinated resins absorb light in the infrared region exceeding 7.5 μm and below 9 μm.

[0088] (Example 2) As the fluorinated resin containing liquid of Example 1, KTS-3000HL was diluted 2 times with solvent (Efcoat SCV Solvent), and otherwise, the test sample was obtained in the same manner as in Example 1. It should be noted that the weight loss rate between 200°C and 400°C was measured in the same manner as in Example 1, and the result was a weight loss rate of 1.7%. Therefore, the fluorinated resin content of the test sample of Example 2 was 1.7% by mass.

[0089] (Example 3) A mixture of 0.12 g of sodium hexafluoroaluminate (Na3AlF6), synthesized in the same manner as in Example 1, 0.003 g of PVDF powder (manufactured by Sigma-Aldrich), and 0.12 g of deionized water was placed into a cylindrical molding die (Φ8) with an internal space. The average particle size of the PVDF particles contained in the PVDF powder was approximately 200 nm. The mixture was then heated and pressurized at 150 °C and 400 MPa for 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. The PVDF content, calculated from the mass of the test sample and the mixing ratio of PVDF, was approximately 2.4% by mass.

[0090] The PVDF powder was measured using a transmission method with an FT-IR apparatus, and the results are as follows: Figure 7 As shown, PVDF powder absorbs light in the infrared region exceeding 7.5 μm and below 9 μm.

[0091] (Example 4) First, sodium hexafluoroaluminate (Na3AlF6) was synthesized in the same manner as in Example 1, except that the firing temperature was set to 300°C. Next, a mixture of 0.12 g of the synthesized sodium hexafluoroaluminate (Na3AlF6), 0.0015 g of PVDF powder (Sigma-Aldrich), and 0.12 g of deionized water was placed into a cylindrical molding die (Φ8) with an internal space. Then, cylindrical samples were prepared 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.

[0092] (Linear transmittance) Infrared transmission spectra of each test sample were obtained using a Fourier Transmission Infrared (FT-IR) instrument. The results are presented below. Figures 8-11 The linear transmittance of the infrared transmission spectrum is converted to linear transmittance with a thickness of 1 mm based on the Lambert-Beer law.

[0093] like Figure 8 As shown, the linear transmittance of the test sample in Example 1 is ≥30% in the wavelength range of 3.54 μm to 5.64 μm, ≥40% in the wavelength range of 3.62 μm to 5.62 μm, and ≥50% in the wavelength range of 4.38 μm to 5.46 μm. Furthermore, in the target wavelength range exceeding 7 μm but below 25 μm, the maximum linear transmittance of the test sample in Example 1 is 14.2%. Additionally, in the target wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance of the test sample in Example 1 is 1.0%.

[0094] like Figure 9As shown, the linear transmittance of Example 2 is ≥30% in the wavelength range of 3.52μm to 5.66μm, ≥40% in the wavelength range of 3.62μm to 5.64μm, and ≥50% in the wavelength range of 4.30μm to 5.60μm. Furthermore, in the target wavelength range exceeding 7μm but below 25μm, the maximum linear transmittance of the test sample of Example 2 is 24.8%. Additionally, in the target wavelength range exceeding 7.5μm but below 25μm, the maximum linear transmittance of the test sample of Example 2 is 7.2%.

[0095] like Figure 10 As shown, the linear transmittance of Example 3 is ≥30% in the wavelength range of 3.58 μm to 5.82 μm and ≥40% in the wavelength range of 4.06 μm to 5.76 μm. Furthermore, in the target wavelength range exceeding 7 μm but below 25 μm, the maximum linear transmittance of the test sample of Example 3 is 7.1%. Additionally, in the target wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance of the test sample of Example 3 is 3.7%.

[0096] like Figure 11 As shown, the linear transmittance of Example 4 is 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 range exceeding 7 μm but below 25 μm, the maximum linear transmittance of the test sample of Example 4 is 8.2%. Additionally, in the target wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance of the test sample of Example 4 is 5.1%.

[0097] (porosity) First, the cross-section of the test sample from Example 1 was subjected to cross-sectional polishing (CP processing). Then, using a scanning electron microscope (SEM), the secondary electron image and reflected electron image of the cross-section of the test sample were observed at 10,000x magnification. Figure 12 This is a secondary electron image of Example 1. Figure 13 This is the reflected electron image of Example 1.

[0098] Next, the pore portion is clarified by binarizing the reflected electron image obtained as described above. Figure 14 This is a binarized image of the reflected electron image from Example 1. Then, based on the binarized image, the area ratio of pores with a diameter of 50 nm or more is calculated, and the porosity is determined to be 15.12%.

[0099] The bulk density was calculated from the mass and volume of the test sample in Example 1, and the result was 2.66 g / cm³. 3Furthermore, as mentioned above, the porosity calculated from the binarized image of the cross-sectional SEM image is approximately 15%, therefore the relative density is approximately 85%. Based on this result, the true density of the material constituting the test sample is estimated to be approximately 3.13 g / cm³. 3 .

[0100] Furthermore, the bulk density calculated from the mass and volume of the test sample in Example 2 was 2.35 g / cm³. 3 The true density of the test sample was close to that of Example 1. Therefore, the relative density was calculated based on the true density estimated in Example 1, and the result was that the relative density was approximately 75%. In addition, the porosity was calculated from the value of the relative density, and the result was that the porosity was approximately 25%.

[0101] Furthermore, the bulk density calculated based on the mass and volume of the test sample in Example 3 was 2.54 g / cm³. 3 The theoretical value of true density is 2.92 g / cm³. 3 Therefore, the relative density is approximately 87%. Furthermore, the porosity is calculated from the relative density value, and the result is approximately 13%.

[0102] Furthermore, the bulk density calculated based on the mass and volume of the test sample in Example 4 was 2.58 g / cm³. 3 The theoretical value of true density is 2.95 g / cm³. 3 Therefore, the relative density is approximately 87%. Furthermore, the porosity is calculated from the relative density value, and the result is approximately 13%.

[0103] (pore diameter) according to Figure 14 The binarized image has a cumulative value of 50% (d) for the pore size distribution on an area basis. 50 In the case of [example 1], the pore size was determined, and the result was that the pore size d50 was 175 nm. For Examples 2 to 4, sodium hexafluoroaluminate (Na3AlF6) was also used to prepare test samples under the same conditions, so it is presumed that the same pore size was used.

[0104] Next, using a powder X-ray diffraction (XRD) apparatus, the powders after pulverizing the test samples for each example were analyzed to obtain XRD patterns. These results are presented below. Figure 15 In addition, Rietveld analysis of the obtained XRD patterns showed that the mass ratio of Na3AlF6 was 90.5% in Example 1, 90.6% in Example 2, 95.3% in Example 3, and 93.6% in Example 4.

[0105] As described above, the test sample involved in this embodiment can be used as an optical filter suitable for, for example, infrared sensors used to detect flames.

[0106] (appendix)

[0107] The following technology has been disclosed through the above description of the embodiments.

[0108] (Technology 1) An optical filter comprising: a substrate comprising a continuous phase of a polycrystalline fluoride; and an absorber dispersed within the substrate, wherein the optical filter contains 0.1 to 10% by mass of the absorber which absorbs light in the infrared region exceeding 7.5 μm and below 9 μm, and the optical filter has a wavelength bandwidth of 50 nm or more, having a linear transmittance of 30% or more per 1 mm thickness in a target wavelength range of 3 μm or more and 7 μm or less, and a maximum linear transmittance of 10% or less per 1 mm thickness in the optical filter in a target wavelength range exceeding 7.5 μm and below 25 μm, the optical filter comprising 80% or more of an inorganic material, the optical filter comprising 50% or more of a polycrystalline fluoride, the optical filter having a porosity of 30% or less, and the optical filter having a central value of pore diameter of 500 nm or less.

[0109] According to this structure, the optical filter 1 allows infrared light to pass through a specific area while blocking infrared light from other specific areas. Therefore, the optical filter 1 is suitable for, for example, infrared sensors used to detect flames.

[0110] (Technology 2) The optical filter according to Technology 1, wherein the fluoride comprises a complex fluoride containing an alkali metal. Alkali metal fluorides have high solubility in water, but complex fluorides containing alkali metals have low solubility in water. Therefore, such an optical filter is difficult to dissolve in water and has high water resistance.

[0111] (Technology 3) The optical filter according to Technology 2, wherein the composite fluoride comprises a compound having a cryolite-type crystal structure. The composite fluoride having such a crystal structure is a material with low refractive index anisotropy based on crystal orientation. Therefore, the infrared transmission of the optical filter 1 can be improved.

[0112] (Technology 4) An optical filter according to any one of Technologies 1 to 3, wherein the absorber comprises a fluoropolymer. The fluoropolymer has CF bonds and absorbs light in the 8 μm to 9 μm wavelength range. Therefore, it is possible to easily block light in the specific wavelength region described above.

[0113] (Technology 5) The optical filter according to Technology 4, wherein the fluororesin comprises a resin having a perfluoroalkyl group. Such a fluororesin has many CF bonds. Therefore, it is possible to effectively and easily block light in a specific wavelength range.

[0114] (Technology 6) An infrared sensor comprising the optical filter described in any one of Technologies 1 to 5. With such a structure, the infrared sensor can easily detect, for example, flames.

[0115] (Technology 7) A method for manufacturing an optical filter, comprising a step of pressurizing a mixture of a polycrystalline material containing a fluoride and an absorbent at a temperature below 250°C, wherein the optical filter contains 0.1 to 10% by mass of the absorbent that absorbs light in the infrared region exceeding 7.5 μm and below 9 μm; the optical filter has a wavelength bandwidth of 50 nm or more, with a linear transmittance of 30% or more per 1 mm thickness in a target wavelength range of 3 μm or more and 7 μm or less; the optical filter has a maximum linear transmittance of 10% or less per 1 mm thickness in a target wavelength range exceeding 7.5 μm and below 25 μm; the optical filter contains 80% or more of inorganic material; the optical filter contains 50% or more of a polycrystalline material containing a fluoride; the optical filter has a porosity of 30% or less; and the central value of the pore diameter of the optical filter is 500 nm or less. With this structure, the optical filter 1 can be manufactured at low temperatures, thus suppressing the deterioration of the absorbent due to heat. Therefore, the above-described optical filter 1 can be easily manufactured.

[0116] This document quotes the full contents of Japanese Special Appeal No. 2023-112348 (application date: July 7, 2023).

[0117] The above describes this embodiment, but this embodiment is not limited thereto, and changes can be made without departing from the spirit of this embodiment.

[0118] Industrial availability According to this disclosure, it is possible to provide an optical filter suitable for, for example, an infrared sensor used to detect flames, an infrared sensor using the optical filter, and a method for manufacturing the optical filter.

[0119] Explanation of reference numerals in the attached figures 1: Optical filter; 10: Substrate; 11: Continuous phase; 20: Absorbent.

Claims

1. An optical filter comprising: Substrate, comprising a continuous polycrystalline phase of fluoride; and The absorbent is dispersed within the substrate. in, The optical filter contains 0.1 to 10% by mass of an absorber that absorbs light in the infrared region exceeding 7.5 μm and below 9 μm. For objects with a thickness of 3μm or more but less than 7μm, the optical filter with a linear transmittance of 30% or more per 1mm thickness has a wavelength bandwidth of 50nm or more. In the object wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance per 1 mm thickness in the optical filter is less than 10%. The optical filter contains more than 80% by mass of inorganic material. The optical filter contains polycrystalline material comprising more than 50% by mass of fluoride. The porosity of the optical filter is less than 30%. The central value of the aperture of the optical filter is below 500 nm.

2. The optical filter according to claim 1, wherein, The fluorides include complex fluorides containing alkali metals.

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

4. The optical filter according to any one of claims 1 to 3, wherein, The absorbent contains a fluoropolymer.

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

6. An infrared sensor comprising the optical filter according to any one of claims 1 to 5.

7. A method for manufacturing an optical filter, comprising a step of pressurizing a mixture of a polycrystalline material containing fluoride and an absorbent at a temperature below 250°C, wherein, The optical filter contains 0.1 to 10% by mass of an absorber that absorbs light in the infrared region exceeding 7.5 μm and below 9 μm. For objects with a thickness of 3μm or more but less than 7μm, the optical filter with a linear transmittance of 30% or more per 1mm thickness has a wavelength bandwidth of 50nm or more. In the object wavelength range exceeding 7.5 μm but below 25 μm, the maximum linear transmittance per 1 mm thickness in the optical filter is less than 10%. The optical filter contains more than 80% by mass of inorganic material. The optical filter contains polycrystalline material comprising more than 50% by mass of fluoride. The porosity of the optical filter is less than 30%. The central value of the aperture of the optical filter is below 500 nm.

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