Barium fluoride sintered compact, lanthanum fluoride-doped barium fluoride sintered compact, manufacturing method of barium fluoride particle, manufacturing method of barium fluoride sintered compact, manufacturing method of lanthanum fluoride-doped barium fluoride particle, manufacturing method of lanthanum fluoride-doped barium fluoride sintered compact, optical element, optical system, interchangeable lens, and optical apparatus

Abstract

To provide a transparent material having similar optical transmissivity to that of monocrystalline of barium fluoride.SOLUTION: Barium fluoride sintered compact has transmissivity of 85% or more to light of 550 nm wavelength and transmissivity of 80% or more to light of a range of 340 nm or longer and 2000 nm or shorter and to light of a range of 4.5 μm or longer and 10.5 μm or shorter.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a barium fluoride sintered body, a lanthanum fluoride-doped barium fluoride sintered body, a method for producing barium fluoride particles, a method for producing a barium fluoride sintered body, a method for producing lanthanum fluoride-doped barium fluoride particles, a method for producing a lanthanum fluoride-doped barium fluoride sintered body, an optical element, an optical system, an interchangeable lens, and an optical device. [Background technology]

[0002] Conventionally, single crystals of barium fluoride have been used in optical systems that utilize visible to mid-infrared and far-infrared rays (for example, Patent Document 1). However, the production of barium fluoride single crystals has problems in that it takes a long time to produce and is expensive to produce. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-183096 Summary of the Invention

[0004] According to the first aspect, the barium fluoride sintered body has a transmittance of 85% or more for light with a wavelength of 550 nm, and a transmittance of 80% or more for light with a wavelength in the range of 340 nm to 2000 nm and light with a wavelength in the range of 4.5 μm to 10.5 μm. According to a second aspect, there is provided a lanthanum fluoride-doped barium fluoride sintered body having a transmittance of 85% or more for light with a wavelength of 550 nm, a transmittance of 80% or more for light with a wavelength in the range of 350 nm to 700 nm, and a transmittance of 70% or more for light with a wavelength in the range of 3.5 μm to 7.5 μm. According to a third aspect, a method for producing barium fluoride particles includes a production step of reacting a barium compound and a fluorine compound in an aqueous solution to produce a barium fluoride particle dispersion, and a separation step of separating barium fluoride particles from the barium fluoride particle dispersion, wherein in the production step, the aqueous solution contains nitric acid, and the molar volume concentration obtained by dividing the molar amount of the nitric acid by the volume of water is 0.06 mol / L or more and 0.6 mol / L or less, and the aqueous solution contains 2 to 2.5 fluorine ions per 1 barium ion. [Brief explanation of the drawings]

[0005] [Figure 1] 1 is a flowchart illustrating a method for producing a lanthanum fluoride-doped barium fluoride sintered body according to the present embodiment. [Figure 2] 1 is a perspective view showing an example of an imaging device according to an embodiment. [Figure 3] FIG. 10 is a front view showing another example of the imaging device according to the embodiment. [Figure 4] FIG. 10 is a rear view showing another example of the imaging device according to the embodiment. [Figure 5] FIG. 1 is a block diagram showing an example of a multiphoton microscope according to an embodiment. [Figure 6] FIG. 2 is a diagram showing the transmittance of a barium fluoride sintered body in Example 1. [Figure 7] FIG. 10 is a diagram showing the evaluation results of barium fluoride particles and a barium fluoride sintered body in Example 2. [Figure 8] FIG. 10 is a diagram showing the evaluation results of the barium fluoride sintered body in Example 3. [Figure 9] FIG. 10 is a diagram showing the evaluation results of a barium fluoride sintered body and a lanthanum fluoride-doped barium fluoride sintered body in Example 4. [Figure 10] FIG. 10 is a diagram showing the transmittance of a barium fluoride sintered body and a lanthanum fluoride-doped barium fluoride sintered body in Example 4. [Figure 11] FIG. 10 is a diagram showing the refractive indexes of a barium fluoride sintered body and a lanthanum fluoride-doped barium fluoride sintered body in Example 4. [Figure 12] FIG. 10 is a diagram showing the measurement results of XRD (X-ray diffraction) patterns of a barium fluoride sintered body and a lanthanum fluoride-doped barium fluoride sintered body in Example 4. [Figure 13] FIG. 10 is a diagram showing the measurement results of XRD patterns of barium fluoride particles and lanthanum fluoride-doped barium fluoride particles before sintering in Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0006] Hereinafter, the embodiments will be described in detail with reference to the drawings. The barium fluoride (BaF2) sintered body of this embodiment has a transmittance of 85% or more for light with a wavelength of 550 nm, and a transmittance of 80% or more for light with wavelengths ranging from 340 nm to 2000 nm and from 4.5 μm to 10.5 μm. A lanthanum fluoride-doped barium fluoride (BLF) sintered body obtained by doping this barium fluoride sintered body with lanthanum fluoride (LaF3) has a transmittance of 85% or more for light with a wavelength of 550 nm, a transmittance of 80% or more for light with wavelengths ranging from 350 nm to 700 nm, and a transmittance of 70% or more for light with wavelengths ranging from 3.5 μm to 7.5 μm.

[0007] With reference to FIG. 1, a method for producing the above-mentioned sintered body of lanthanum fluoride-doped barium fluoride (hereinafter referred to as BLF) will be described. First, in step S1 of Fig. 1, a barium compound and a fluorine compound are reacted in an aqueous solution to produce a barium fluoride particle dispersion in which barium fluoride (BaF2) particles are dispersed (barium fluoride particle dispersion production step (hereinafter referred to as the first production step)). In step S2, the barium fluoride particles are separated from the barium fluoride particle dispersion (barium fluoride particle separation step (hereinafter referred to as the first separation step)). In step S3, the barium fluoride particles, a lanthanum compound, and a fluorine compound are reacted in water to produce a BLF particle dispersion in which lanthanum fluoride-doped barium fluoride (BLF) particles are dispersed (BLF particle dispersion production step (hereinafter referred to as the second production step)). In step S4, the BLF particles are separated from the BLF particle dispersion (BLF particle separation step (hereinafter referred to as the second separation step)). In step S5, the separated BLF particles are washed and dried (washing and drying step). In step S6, the BLF particles are molded to obtain a molded body (molding step). In step S7, the BLF particle molded body is sintered to produce a BLF sintered body (sintering step). 1, steps S3 and S4 are omitted, and step S5 is performed after step S2. In this case, in step S5 (cleaning and drying step), barium fluoride particles are cleaned and dried to obtain barium fluoride particles, in step S6 (molding step), a compact of the barium fluoride particles is molded, and in step S7 (sintering step), the compact of the barium fluoride particles is sintered to produce a barium fluoride sintered body.

[0008] Each of the above processes will now be described in detail. (1) Barium fluoride particle dispersion liquid production process (first production process) Barium fluoride particles are prepared by reacting a barium compound with a fluorine compound in an aqueous solution. The reaction between the barium compound and the fluorine compound is carried out by dissolving each compound in an aqueous solution and gradually injecting the aqueous solution of the fluorine compound into the aqueous solution of the barium compound at room temperature and atmospheric pressure.

[0009] Suitable barium compounds are salts that dissolve well in water. Examples of barium compound salts include organic salts of barium, such as acetate, lactate, oxalate, ascorbate, alginate, benzoate, carbonate, citrate, gluconate, pantothenate, salicylate, stearate, tartrate, glycerate, and trifluoroacetate, as well as inorganic salts such as barium chloride and hydroxide. However, barium sulfate is unsuitable because it is almost insoluble in water. Barium nitrate is also unsuitable because it is poorly soluble in water. The fluorine compound may be, for example, hydrofluoric acid. By using hydrofluoric acid as the fluorine compound, impurity ions are less likely to remain in the barium fluoride particles.

[0010] The reaction between a barium compound and a fluorine compound is carried out under conditions where the amount of impurity ions, such as rare earth ions and transition metal ions, present in the mixture of the aqueous barium compound solution and the aqueous fluorine compound solution (hereinafter referred to as the reaction mixture) is minimized. When these impurity ions coexist with barium ions and fluorine ions in the reaction mixture, other ions are incorporated into the formed barium fluoride particles, making it difficult for barium fluoride crystals to grow, and the fine particles tend to aggregate. As a result, during the sintering process described below, voids form between the aggregated particles, resulting in problems such as low transmittance and the inability to obtain a sintered body with high density and high transmittance even with slight changes in sintering conditions. Thus, the presence of impurity ions other than barium ions and fluorine ions in the reaction mixture sensitively affects the properties of the resulting sintered body, so it is necessary to minimize the content of impurity ions as much as possible.

[0011] When the barium compound and the fluorine compound are reacted, the amount of the fluorine compound added is in excess of the chemical equivalent when producing the barium compound (for example, the chemical equivalent when producing barium fluoride). This allows the formation of barium fluoride particles with little fluorine deficiency and high crystallinity, and suppresses the aggregation of fine particles.

[0012] The ratio of barium ions to fluorine ions in the reaction mixture is preferably 1:1 barium ion to 2:2.5 fluorine ions. Adding hydrofluoric acid so that the fluorine ions exceed this ratio produces barium hydrogen fluoride (BaHF3). Barium hydrogen fluoride consists of coarse, polygonal columnar particles with diameters exceeding 10 μm and lengths exceeding 100 μm. Furthermore, when heated to 200°C or higher, it decomposes and turns into coarse barium fluoride particles. The presence of such coarse particles has adverse effects, such as a low density of the resulting sintered compact. Furthermore, large voids remain around the coarse particles even after the sintering process (HIP treatment) described below, preventing the sintered compact from becoming transparent.

[0013] In this embodiment, the reaction between the barium compound and the fluorine compound is carried out in the presence of nitric acid. Nitric acid remains in or on the surface of the generated barium fluoride particles. Trace amounts of nitric acid remain even after the subsequent barium fluoride particle separation process. This nitric acid is removed by degreasing through heating in the atmosphere, but any nitric acid remaining after this process is oxidized and removed along with trace amounts of other organic impurities and carbon impurities in the high-temperature environment during vacuum sintering. As a result, light absorption due to organic impurities is reduced and sinterability is improved.

[0014] Nitric acid also promotes the growth of barium fluoride crystals during the hydrothermal reaction in the barium fluoride particle separation process (first separation process) described below. Without nitric acid, barium fluoride particles grown by the hydrothermal reaction process become aggregates of microcrystals of 100 nm or less. These microcrystals tend to strongly agglomerate, and once agglomerated, they do not dissociate or deform even under the load applied during molding, easily creating voids in the molded sintered body. Furthermore, such agglomerated particles make it difficult to sufficiently remove organic matter and moisture through degreasing, which can lead to a decrease in the transmittance of the molded sintered body. Therefore, the reaction between the barium compound and the fluorine compound must be carried out in the presence of an appropriate amount of nitric acid. In this embodiment, the nitric acid present during the reaction between the barium compound and the fluorine compound is adjusted to a molar volume concentration of 0.06 mol / L or more and 0.6 mol / L or less, calculated by dividing the molar amount of nitric acid by the volume of pure water used to prepare the barium acetate compound aqueous solution.

[0015] Nitric acid may be added to a barium compound aqueous solution prepared by dissolving a barium compound in pure water, or to a reaction mixture prepared by mixing a barium compound aqueous solution with a fluorine compound aqueous solution. However, if an excessive amount of nitric acid is added, barium nitrate will be produced. That is, if the barium compound aqueous solution reacts with a fluorine compound to produce barium fluoride, barium nitrate crystals will be produced in addition to barium fluoride particles. As a result, not only will the yield of the obtained barium fluoride particles decrease, but barium nitrate crystals will be mixed with the barium fluoride particles. When such particles are sintered, the barium nitrate will decompose to produce barium oxide. That is, barium oxide will remain as a foreign substance in the sintered body, preventing it from becoming a homogeneous, transparent body.

[0016] When the fluorine compound aqueous solution is injected into the barium compound aqueous solution, the barium compound aqueous solution and the fluorine compound aqueous solution are mixed and stirred, and the mixed solution is continued to be stirred even after the injection is completed, thereby preventing the aggregation of the barium fluoride crystal particles that are produced. If the barium fluoride particles in the mixed solution are strongly agglomerated, they cannot be dissociated even by heating and pressurizing in the sintering and clarification processes, and as a result, the molded sintered body does not have high transmittance. To avoid this situation, it is necessary to thoroughly stir the mixed solution during barium fluoride crystallization.

[0017] (2) Barium fluoride particle separation process (first separation process) As described above, after reacting the barium compound with the fluorine compound in an aqueous solution at room temperature and normal pressure, the reaction mixture is placed in a sealed container and subjected to a hydrothermal reaction treatment in which the reaction mixture is heated and pressurized at a temperature of 100°C or higher and 200°C or lower in a sealed state. The reason for the hydrothermal reaction treatment is as follows: In an aqueous solution at room temperature and pressure, the reaction between the barium compound and the fluorine compound does not go to completion, and the barium fluoride crystals that are produced have many fluorine deficiencies. In other words, the chemical equivalent of the barium fluoride particles obtained under the above conditions is less than two fluorine ions per one barium ion. Such barium fluoride crystal particles are difficult to grow and are prone to aggregation.

[0018] Therefore, after reacting the barium compound with the fluorine compound at room temperature and atmospheric pressure, a hydrothermal reaction treatment is further carried out in which heating and pressurization are carried out at 100°C to 200°C to complete the reaction between the barium compound and the fluorine compound. The container used for the hydrothermal reaction treatment is not particularly limited, but for example, a sealed container such as a polytetrafluoroethylene autoclave is used. The temperature in the hydrothermal reaction treatment is 100°C or higher and 200°C or lower, and the pressure is 1.0 MPa or higher and 1.65 MPa or lower, which is the saturated vapor pressure of water in the temperature range of 100°C or higher and 200°C or lower.

[0019] The above-mentioned hydrothermal reaction treatment allows the chemical equivalence of barium fluoride particles to be substantially 2 fluorine ions per 1 barium ion, thereby forming highly crystalline barium fluoride particles. As a result, the surface energy of the barium fluoride particles can be reduced, and the cohesive force between the particles can be reduced. As a result, a high-density barium fluoride sintered body can be obtained even at a relatively low temperature. According to the above-mentioned production method, for example, barium fluoride particles having an average particle size of 100 nm or more and 3 μm or less can be obtained.

[0020] (3) BLF particle dispersion generation process (second generation process) Next, we will explain the steps required to produce BLF particles, which are barium fluoride particles doped with lanthanum fluoride (LaF). Note that this step can be omitted when producing barium fluoride particles that are not doped with lanthanum fluoride.

[0021] After the first separation step, the barium fluoride particles are precipitated in the hydrothermal vessel and separated from the supernatant liquid. The supernatant liquid is then removed using a pump or the like, and the barium fluoride particle precipitate is separated. An appropriate amount of pure water is added to the separated barium fluoride particle precipitate to obtain a barium fluoride suspension (barium fluoride slurry).

[0022] Next, the preparation of an aqueous solution of a lanthanum compound to be added to the above-mentioned barium fluoride suspension will be described. The aqueous solution of a lanthanum compound is prepared by dissolving an organic salt of a lanthanum compound, such as lanthanum acetate, or an inorganic salt of a lanthanum compound, such as lanthanum chloride, in pure water. In this case, it is not preferable to use lanthanum nitrate, a nitrate, as the lanthanum compound. If lanthanum nitrate is used as the lanthanum compound, a large amount of nitric acid is generated during the production of BLF particles, which results in the formation of barium nitrate crystals that become mixed into the BLF particles. When such BLF particles are sintered, as mentioned above, the barium nitrate decomposes to produce barium oxide, which remains as a foreign substance in the sintered body, preventing the sintered body from becoming homogeneously transparent. When dissolving lanthanum acetate in pure water to produce an aqueous solution, it is preferable to add an appropriate amount of nitric acid. However, if too much nitric acid is added, as mentioned above, barium nitrate is produced and incorporated into the BLF particles, making it impossible to obtain a homogeneous, transparent sintered body. Therefore, the amount of nitric acid added must be kept to the minimum required to dissolve lanthanum acetate.

[0023] The prepared lanthanum acetate aqueous solution is added to the aforementioned barium fluoride suspension (barium fluoride slurry) while stirring. Furthermore, hydrofluoric acid is injected as a fluorine compound, as in the case of producing barium fluoride particles. This allows the fluorine in the injected hydrofluoric acid to react with the lanthanum compound in excess. The amount of hydrofluoric acid to be injected is preferably in the range of 3 to 3.15 fluorine ions per barium ion in the barium fluoride slurry. If more fluorine is present than this range, the fluorine will react with the barium fluoride in the slurry to produce barium hydrogen fluoride (BaHF3). Therefore, the amount of hydrofluoric acid to be injected must be set so that the ratio of barium ions to fluorine ions is within the above range.

[0024] The doping concentration of lanthanum can be adjusted by the amount of the lanthanum compound solution, and it is preferable to dope the lanthanum in an amount of 0.01 to 0.4 in terms of the chemical equivalent ratio to the sum of the chemical equivalents of barium and lanthanum.

[0025] A specific lanthanum compound aqueous solution is added to the barium fluoride slurry, and after hydrofluoric acid is added, the slurry is thoroughly stirred. Stirring homogenizes the lanthanum and hydrogen fluoride concentrations in the slurry, which in turn homogenizes the reaction between barium fluoride, lanthanum ions, and hydrogen fluoride. As a result, a dispersion of barium fluoride particles doped with lanthanum fluoride (BLF particle dispersion) with a uniform composition is obtained.

[0026] (4) BLF particle separation process (second separation process) The BLF particles are separated from the BLF particle dispersion by a hydrothermal reaction treatment (second separation step). Note that this step can be omitted when producing barium fluoride particles that are not doped with lanthanum fluoride. The BLF particle dispersion is placed in a sealed container and heated and pressurized at a temperature of 100°C to 200°C in a sealed state. The pressure of the hydrothermal reaction treatment is 1.0 MPa to 1.65 MPa, which is the saturated vapor pressure of water in the temperature range of 100°C to 200°C.

[0027] (5) Washing and drying process The barium fluoride particle dispersion or BLF particle dispersion subjected to hydrothermal treatment in the first or second separation step is in the form of a slurry in which crystals of these particles are dispersed in a strongly acidic aqueous solution. This slurry is subjected to solid-liquid separation using a centrifuge or the like. That is, it is separated into a precipitate and a dispersion medium. Next, the dispersion medium is replaced with pure water (pure water replacement). By appropriately performing this pure water replacement, residual hydrofluoric acid, nitric acid, organic acids such as acetic acid generated when organic acid salts are used as raw materials contained in the dispersion medium, and other impurities can be removed, resulting in a highly pure slurry.

[0028] The pure water-substituted slurry is again subjected to solid-liquid separation by centrifugation or other methods to obtain a precipitate. This precipitate is then dried at a temperature above room temperature and below 200°C to obtain dried particles. In this way, the strongly acidic aqueous solution is substituted with pure water through solid-liquid separation and drying, making it easier to handle during subsequent processing and storage. Furthermore, the incorporation of impurities into the slurry can be suppressed.

[0029] (6) Molding process The molding process in this embodiment includes a pressure molding process and a degreasing process, each of which will be described below. (6-1) Pressure molding process The dried particles obtained in the washing and drying process are weighed and filled into a mold of the desired shape. The particles filled into the mold are compressed by applying pressure using, for example, a uniaxial press, and processed into a green body. The forming method is not limited to the above-mentioned forming method, and any commonly used forming method can be used. For example, a slip casting method, in which particles are dispersed in an appropriate medium to prepare a slurry, which is then poured into a mold and dehydrated to obtain a green body, or cold isostatic pressing (CIP) can be used.

[0030] (6-2) Degreasing When organic acid salts are used as raw materials, the resulting compact contains a large amount of organic matter as impurities. Hydrated water and hydroxyl groups also exist on the particle surfaces. Sintering such compacts as they are makes it difficult to obtain a high-density sintered compact due to the poor sinterability of the particles. Furthermore, when compacts containing such impurities are sintered to a transparent state in a subsequent sintering process, these impurities absorb light, causing coloration and reducing transmittance. Therefore, compacts are fired at high temperatures in the atmosphere to oxidize and decompose the organic matter and remove water through a dehydration reaction. This process is called degreasing. The firing temperature during this process must be high enough to adequately remove impurities such as organic matter and water. However, if the temperature is too high, fluorides may react with oxygen in the atmosphere and oxidize. Furthermore, particles in the compact may begin to sinter before the impurities are fully removed, potentially leaving impurities behind depending on the compact's shape. Therefore, the degreasing process requires appropriate settings for the treatment temperature and the treatment time (the time for maintaining the temperature). The treatment temperature can be 350°C or higher and 650°C or lower, and for example, when producing a barium fluoride sintered body, it is preferably 400°C or higher and 650°C or lower, and when producing a lanthanum fluoride-doped barium fluoride sintered body, it is preferably 350°C or higher and 450°C or lower. The treatment time can be 2 hours or higher and 6 hours or lower.

[0031] (7) Sintering process The compact, which has been degreased during the degreasing process during the molding process, is sintered in a vacuum sintering furnace. By sintering the compact through vacuum sintering, the pores in the compact are closed, resulting in a high density. The density of the sintered compact is approximately 95% or more but less than 100% of the theoretical density, resulting in a white, opaque sintered compact. The sintering temperature can be, for example, 900°C or more and 1100°C or less.

[0032] The resulting white, opaque sintered body is maintained at high temperature and pressure by hot isostatic pressing (HIP). Closed pores within the sintered body are removed to the outside as sintering progresses in a high-temperature, high-pressure atmosphere. As a result, the density of the sintered body can be increased to a level that nearly matches the density of a barium fluoride single crystal, or the theoretical density estimated assuming the additivity of barium fluoride and lanthanum fluoride. This results in a colorless, transparent barium fluoride sintered body or BLF sintered body.

[0033] An embodiment of an imaging device including an optical element made of the barium fluoride sintered body or BLF sintered body manufactured as described above will be described. FIG. 2 is a perspective view of an imaging device according to this embodiment. The imaging device 1 is a so-called digital single-lens reflex camera (interchangeable lens camera), and its photographing lens 103 (optical system) includes an optical element whose base material is the barium fluoride sintered body or BLF sintered body according to this embodiment. A lens barrel 102 is detachably attached to a lens mount (not shown) of a camera body 101. Light passing through the photographing lens 103 of the lens barrel 102 forms an image on a sensor chip (solid-state imaging element) 104 of a multi-chip module 106 disposed on the rear side of the camera body 101. This sensor chip 104 is a bare chip such as a so-called CMOS image sensor, and the multi-chip module 106 is, for example, a COG (chip-on-glass) type module in which the sensor chip 104 is bare-chip mounted on a glass substrate 105.

[0034] FIG. 3 is a front view of another example of an imaging device including an optical element made of a barium fluoride sintered body or a BLF sintered body according to this embodiment, and FIG. 4 is a rear view of the imaging device of FIG. This imaging device CAM is a so-called digital still camera (non-interchangeable lens camera), and its taking lens WL (optical system) includes an optical element whose base material is the barium fluoride sintered body or BLF sintered body according to this embodiment. When the power button (not shown) of the imaging device CAM is pressed, the shutter (not shown) of the taking lens WL is opened, and light from the subject (object) is collected by the taking lens WL and focused on an image sensor located at the image plane. The subject image focused on the image sensor is displayed on an LCD monitor LM located behind the imaging device CAM. After determining the subject image composition while looking at the LCD monitor LM, the photographer presses the release button B1 to capture the subject image with the image sensor, which is then recorded and saved in memory (not shown). The imaging device CAM is equipped with an auxiliary light emitter EF that emits auxiliary light when the subject is dark, a function button B2 used to set various parameters for the imaging device CAM, and other devices.

[0035] Note that optical devices applicable to the present embodiment are not limited to the imaging device described above, but may also include, for example, a projector, etc. The optical element is also not limited to a lens, but may also include, for example, a prism, etc.

[0036] Next, a multiphoton microscope equipped with an optical element using the barium fluoride sintered body or BLF sintered body of this embodiment will be described. 5 is a block diagram showing an example of the configuration of a multiphoton microscope 2 according to this embodiment. The multiphoton microscope 2 includes an objective lens 206, a condenser lens 208, and an imaging lens 210. At least one of the objective lens 206, the condenser lens 208, and the imaging lens 210 includes an optical element having a base material of the barium fluoride sintered compact or the BLF sintered compact according to this embodiment. The following description will focus on the optical system of the multiphoton microscope 2.

[0037] The pulsed laser device 201 emits ultrashort pulsed light, for example, near-infrared (approximately 1000 nm) with a pulse width in femtosecond units (e.g., 100 femtoseconds). The ultrashort pulsed light immediately after being emitted from the pulsed laser device 201 is generally linearly polarized light with the vibration direction of the electric field in a predetermined direction. The pulse splitter 202 splits the ultrashort pulsed light and emits the ultrashort pulsed light with a higher repetition frequency.

[0038] The beam adjusting unit 203 has a function of adjusting the beam diameter of the ultrashort pulsed light incident from the pulse splitter 202 to match the pupil diameter of the objective lens 206, a function of adjusting the focusing and divergence angles of the ultrashort pulsed light to correct the on-axis chromatic aberration (focus difference) between the wavelength of the multiphoton excitation light emitted from the sample S and the wavelength of the ultrashort pulsed light, and a pre-chirp function (group velocity dispersion compensation function) of imparting inverse group velocity dispersion to the ultrashort pulsed light to correct the pulse width of the ultrashort pulsed light being widened by group velocity dispersion while passing through the optical system.

[0039] The repetition frequency of the ultrashort pulsed light emitted from the pulsed laser device 201 is increased by the pulse dividing device 202, and the above-mentioned adjustment is performed by the beam adjusting unit 203. The ultrashort pulsed light emitted from the beam adjusting unit 203 is reflected by the dichroic mirror 204 toward the dichroic mirror 205, passes through the dichroic mirror 205, and is collected by the objective lens 206 to be irradiated onto the sample S. At this time, the ultrashort pulsed light may be scanned over the observation surface of the sample S by using a scanning means (not shown).

[0040] For example, when observing the fluorescence of the sample S, the fluorescent dye with which the sample S is stained is excited by multiphotons in the region of the sample S irradiated with the ultrashort pulsed light and in the vicinity thereof, and fluorescence (hereinafter referred to as observation light) having a wavelength shorter than that of the infrared wavelength of the ultrashort pulsed light is emitted. The observation light emitted from the sample S in the direction of the objective lens 206 is collimated by the objective lens 206, and is either reflected by or passes through the dichroic mirror 205 depending on its wavelength.

[0041] The observation light reflected by the dichroic mirror 205 enters the fluorescence detection unit 207. The fluorescence detection unit 207 is configured with, for example, a barrier filter, a PMT (Photo Multiplier Tube), etc., and receives the observation light reflected by the dichroic mirror 205 and outputs an electrical signal according to the amount of light. Furthermore, the fluorescence detection unit 207 detects the observation light across the observation surface of the sample S as the ultrashort pulsed light scans the observation surface of the sample S.

[0042] Meanwhile, the observation light transmitted through the dichroic mirror 205 is descanned by a scanning means (not shown), transmitted through the dichroic mirror 204, collected by a condenser lens 208, passes through a pinhole 209 provided at a position approximately conjugate with the focal position of the objective lens 206, and then passes through an imaging lens 210 to enter a fluorescence detection unit 211. The fluorescence detection unit 211 is composed of, for example, a barrier filter, a PMT, etc., and receives the observation light imaged on the light receiving surface of the fluorescence detection unit 211 by the imaging lens 210, and outputs an electrical signal according to the amount of light. Furthermore, the fluorescence detection unit 211 detects the observation light across the observation surface S of the sample S in synchronization with the scanning of the observation surface S of the sample S with the ultrashort pulsed light. It is also possible to remove the dichroic mirror 205 from the optical path so that all of the observation light emitted from the sample S in the direction of the objective lens 206 is detected by the fluorescence detection unit 211 .

[0043] Furthermore, observation light emitted from the sample S in the direction opposite to the objective lens 206 is reflected by the dichroic mirror 212 and enters the fluorescence detection unit 213. The fluorescence detection unit 213 is configured with, for example, a barrier filter, a PMT, etc., and receives the observation light reflected by the dichroic mirror 212 and outputs an electrical signal according to the amount of light. Furthermore, the fluorescence detection unit 213 detects the observation light across the observation surface of the sample S in accordance with the scanning of the observation surface of the sample S with the ultrashort pulsed light.

[0044] The electrical signals output from the fluorescence detection units 207, 211, and 213 are input to, for example, a computer (not shown). The computer generates an observation image based on the input electrical signals, and can display the generated observation image and store data of the observation image.

[0045] According to the above-described embodiment, the following effects can be obtained. (1) A barium fluoride sintered body made by sintering barium fluoride particles has a transmittance of 85% or more for light with a wavelength of 550 nm, and a transmittance of 80% or more for light with wavelengths in the range of 340 nm to 2000 nm and light with wavelengths in the range of 4.5 μm to 10.5 μm. A BLF sintered body made by sintering barium fluoride particles doped with lanthanum fluoride has a transmittance of 85% or more for light with a wavelength of 550 nm, a transmittance of 80% or more for light with a wavelength in the range of 350 nm to 700 nm, and a transmittance of 70% or more for light with a wavelength in the range of 3.5 μm to 7.5 μm. In polycrystalline barium fluoride and BLF, it is expected that the optical anisotropy and anisotropy of the photoelastic effect seen in single crystals will be averaged out. Furthermore, by polycrystallizing, the cleavage caused by the slippage of the crystal plane seen in single crystals will be suppressed by the grain boundaries, and it is expected that the mechanical strength will be isotropic and improved. In addition, there will be no processing marks that are characteristic of the plane orientation during polishing or film formation.

[0046] (2) In a first production step, a barium compound and a fluorine compound are reacted in an aqueous solution to produce a barium fluoride particle dispersion, and in a first separation step, the barium fluoride particles are separated from the barium fluoride particle dispersion to produce barium fluoride particles.The barium fluoride particles are placed in a mold and then sintered by hot isostatic pressing to produce a barium fluoride sintered body. In the second production step, the barium fluoride particles produced as described above are reacted with a lanthanum compound and a fluorine compound in water to produce a BLF particle dispersion doped with lanthanum fluoride, and in the second separation step, the lanthanum fluoride-doped BLF particles are separated from the BLF particle dispersion.The BLF particles are placed in a mold and sintered by hot isostatic pressing to produce a BLF sintered body. The above method makes it possible to obtain a transparent material that maintains optical transmittance in the ultraviolet to far-infrared region without growing a single crystal of barium fluoride. The polycrystalline body is produced by synthesizing barium fluoride or BLF crystals, molding them, and then heat-treating them at a low temperature below their melting point. Therefore, compared to growing a single crystal, which takes about a month, polycrystalline bodies can be produced in about a week, which reduces the amount of electricity and labor required for production, enabling significant cost savings.

[0047] (3) During the production of barium fluoride particles, a barium fluoride particle dispersion is produced in the presence of nitric acid. This promotes the growth of barium fluoride crystals during the hydrothermal reaction process. Furthermore, nitric acid has the effect of oxidizing and removing trace amounts of organic and carbon impurities remaining in the barium fluoride particles even after the degreasing process at high temperatures during sintering, thereby reducing light absorption due to organic impurities and improving sinterability.

[0048] (4) The concentration of nitric acid used in the first production step is 0.06 mol / L or more and 0.6 mol / L or less. This prevents the barium and nitric acid from reacting to produce barium nitrate, which reduces the weight of the barium fluoride particles and reduces the yield. Furthermore, this prevents the barium nitrate crystals from being mixed in with the barium fluoride particles, and the barium nitrate decomposes during sintering to barium oxide, leaving the barium oxide as a foreign substance in the sintered body, preventing the production of a uniformly transparent body.

[0049] (5) When producing a barium fluoride particle dispersion, the amount of fluorine compound added is set to 2 to 2.5 parts fluorine per 1 part barium. This prevents the production of barium hydrogen fluoride, which can reduce the density of the compact and inhibit the densification of the sintered body.

[0050] Below, examples of barium fluoride and examples of BLF according to the embodiment will be described. [Example 1] The barium fluoride sintered body of Example 1 was produced as follows. 130 g of barium acetate (special reagent grade) was weighed out and placed in a beaker. 175 g of ion-exchanged water was added to this beaker, and the barium acetate was dissolved by stirring with a stirrer to prepare a barium acetate aqueous solution. While stirring, 2.2 g of nitric acid (69 wt%, special reagent grade) was weighed out and added to the beaker. A colorless, transparent aqueous solution (barium compound aqueous solution) was obtained in about 30 minutes. The molar amount of nitric acid added was calculated based on the volume of 175 cm of ion-exchanged water used to prepare the barium acetate aqueous solution. 3 When expressed as a molar volume concentration divided by , this corresponds to 0.14 mol / l.

[0051] The above aqueous solution was transferred to a plastic beaker. 172 g of high-purity hydrofluoric acid (12.5 wt%) was weighed and injected into the plastic beaker using a tube pump, and reacted with barium acetate. After the hydrofluoric acid was injected, stirring was continued for 1 hour, and a cloudy barium fluoride slurry was obtained.

[0052] The slurry was transferred to a stainless steel autoclave vessel equipped with a polytetrafluoroethylene inner cylinder, which was then sealed. The autoclave vessel was then placed in an electric furnace and heated at 100°C for 24 hours. The cooled autoclave container was removed from the electric furnace, and a precipitate of barium fluoride particles and a supernatant liquid were obtained in the autoclave container. This was stirred to form a homogeneous slurry, which was then transferred to a centrifuge. After centrifugation, the supernatant liquid was discarded, and a precipitate was obtained. 200 g of ion-exchanged water was added to the precipitate and stirred to obtain a homogeneous barium fluoride slurry. This slurry was centrifuged for solid-liquid separation, and the supernatant liquid was discarded. This process was repeated three times for washing, and refined barium fluoride precipitate particles were obtained. The chemical equivalent ratio of fluorine ions to barium ions (F / Ba) in this barium fluoride was 2.1.

[0053] The barium fluoride precipitate particles were transferred to an electric furnace and heated at 160° C. for 18 hours to obtain a dry cake, which was then taken out and pulverized in an agate mortar to obtain approximately 84 g of white dry particles. Approximately 5 g of these white dry particles were filled into a mold with a 20 mm diameter cavity of uniform depth and press-molded using a uniaxial press. The pressing force was 2 kN, equivalent to a pressure of 6.4 MPa. The resulting cylindrical molded body had sufficient shape retention to withstand manual breaking.

[0054] This compact was degreased in an electric furnace at a temperature of 400° C. to 650° C. for 2 hours to 6 hours. The degreasing did not significantly deform the compact, but it did harden it slightly. The degreased green body was placed in a vacuum sintering furnace, held at 1000°C for 1 hour, and then allowed to cool to obtain a sintered body. The sintered body was white and cylindrical, with a diameter and height that had shrunk by approximately 20% compared to the green body, indicating that sintering had progressed. The bulk density of the sintered body was 4.6 g / cm. 3 This was approximately 93% of the density of a barium fluoride single crystal.

[0055] Next, the sintered body was subjected to hot isostatic pressing (HIP). The HIP treatment conditions were a pressure of 98 MPa and a temperature of 1060°C, and the body was maintained in this state for 2 hours. The sintered body after HIP treatment (HIP sintered body) was placed in a carbon crucible, heated using a molybdenum heater (Mo heater), and then allowed to cool. After cooling, the HIP sintered body had become transparent. The density of this HIP sintered body was almost the same as that of barium fluoride single crystal, indicating that a high-density sintered body had been obtained.

[0056] FIG. 6 shows the results of measuring the transmittance of the barium fluoride sintered body in Example 1. FIG. 6(a) shows the transmittance in the wavelength range of 200 nm to 2000 nm, and FIG. 6(b) shows the transmittance in the wavelength range of 2.5 μm to 15.5 μm. The barium fluoride sintered body used for the transmittance measurement had a parallel plate shape with a thickness of 4.6 mm and was polished on both sides. The transmittance in the wavelength range of 200 nm to 2000 nm (ultraviolet to near-infrared region) was measured using a Hitachi High-Tech UH4150 spectrophotometer, and the transmittance in the wavelength range of 2.5 μm to 15.5 μm (infrared region) was measured using an FT-IR spectrophotometer manufactured by ThermoFisher. In FIG. 6, the transmittance of the barium fluoride sintered body in Example 1 is shown by a solid line (symbol L1), and the transmittance of a 6 mm-thick single-crystal barium fluoride as a comparative example is shown by a solid line (symbol L2).

[0057] As shown in Figure 6, the barium fluoride sintered body of Example 1 exhibited transmittance in the infrared region equivalent to that of single-crystal barium fluoride, and also exhibited high transmittance in the ultraviolet to near-infrared region. That is, the transmittance for light in the wavelength range of 340 nm to 2000 nm and for light in the wavelength range of 4.5 μm to 10.5 μm was 80% or more. 80 The transmittance at a wavelength of 550 nm was 87%.

[0058] [Example 2] The barium fluoride sintered body in Example 2 was produced in the same manner as in Example 1, except for the following changes to Example 1. That is, in the barium fluoride particle dispersion liquid production step (first production step), the amount of hydrofluoric acid was set to five different values, including the conditions in Example 1, so that five types of barium fluoride particles were produced with chemical equivalent ratios of fluorine ions to barium ions (F / Ba) of 1.66 (Sample 1-1), 2.1 (Sample 1-2), 2.5 (Sample 1-3), 3 (Sample 1-4), and 4 (Sample 1-5). FIG. 7 shows the evaluation results of the barium fluoride particles of each sample and the sintered bodies produced using them.

[0059] The crystalline phase of the resulting barium fluoride particles was identified by X-ray diffraction (XRD). Particles with F / Ba ratios of 2.1 and 2.5 yielded nearly spherical particles with a size of several micrometers, composed solely of single-phase barium fluoride particles, suggesting their suitability for sintering. Particles with F / Ba ratios of 1.66 also contained single-phase barium fluoride particles, but the particles consisted of ultrafine particles several hundred nanometers in size and particles several micrometers in size, suggesting their unsuitability for sintering. Particles with F / Ba ratios of 3 and 4 did not have a single-phase crystalline layer, but consisted solely of barium hydrogen fluoride (BaHF3) or a mixture of barium fluoride and barium hydrogen fluoride. The barium hydrogen fluoride particles were coarse, polygonal pillar-shaped particles several micrometers in length and width, suggesting their unsuitability for sintering.

[0060] When the particles of each sample were pressed using a uniaxial press, the compacts containing barium hydrogen fluoride were too brittle to proceed to the sintering process. Sintered bodies were produced using the particles of each sample except for samples 1-4 and 1-5, and then clarified. As a result, sintered bodies produced using the particles of samples 1-2 and 1-3, which had F / Ba ratios of 2.1 and 2.5, were transparent, but the sintered body produced using the particles of sample 1-1 was not transparent. Furthermore, sintered bodies could not be obtained from the particles of samples 1-4 and 1-5.

[0061] [Example 3] The barium fluoride sintered body in Example 3 was produced in the same manner as in Example 1, except that the following changes were made to the barium fluoride particle dispersion liquid production step (first production step). That is, Samples 2-1, 2-2, 2-3, 2-4, 2-5, 2-6, 2-7, 2-8, and 2-9 were produced by adding different concentrations of nitric acid in the barium fluoride particle dispersion liquid production step (first production step).

[0062] Figure 8 shows the conditions in the first production step applied to each of Samples 2-1 to 2-9, and the evaluation results of the barium fluoride sintered body obtained using the particles of each sample. The nitric acid concentration applied to the production of each sample is shown as a molar volume concentration obtained by dividing the molar amount of nitric acid added in the first production step by the sum of the volume of the ion-exchanged water used to prepare the barium acetate aqueous solution and the volume of water in the added nitric acid. Figure 8 shows the following: The sintered compacts sintered using particles from Samples 2-4, 2-5, 2-6, and 2-7, which had nitric acid concentrations greater than 0.062, 0.14, 0.28, and 0.54 but less than 0.75 mol / L, were colorless and transparent. The sintered compact sintered using particles from Sample 2-1, which contained no nitric acid, was black and opaque. The sintered compacts sintered using particles from Samples 2-2 and 2-3, which had nitric acid concentrations of 0.016 mol / L and 0.031 mol / L, were white and opaque. The sintered compacts sintered using particles from Samples 2-8 and 2-9, which had nitric acid concentrations of 0.75 and 1.0 mol / L, were white and opaque. Although all of these sintered compacts were transparent after the clarification treatment, only the sintered compacts sintered using particles from Samples 2-4, 2-5, 2-6, and 2-7 achieved high transparency and a homogeneous state.

[0063] When the particles in Samples 2-1 to 2-9 were observed under an electron microscope, the particle size of the barium fluoride particles increased as the nitric acid concentration increased. When the nitric acid concentration was 0.031 mol / L or less (Samples 2-1 to 2-3), the particles generated were fine particles with a size of 1 μm or less. When the nitric acid concentration was 0.75 mol / L or more (Samples 2-8 and 2-9), the particles generated were aggregates of fine particles with a size of approximately 2 μm or more.

[0064] [Example 4] The BLF sintered body of Example 4 was produced as follows. In Example 4, lanthanum fluoride-doped barium fluoride (BLF) particles were produced, and the particles were sintered to produce a sintered body. To this end, barium acetate, hydrofluoric acid, and lanthanum acetate were used as raw materials to produce the BLF particles.

[0065] 100 g of barium acetate (special reagent grade) was weighed out and placed in a beaker. 135 g of ion-exchanged water was added to the beaker and stirred with a stirrer to dissolve the barium acetate solution. While stirring, 1.7 g of nitric acid (69 wt%, special reagent grade) was weighed out and added to the beaker. A colorless, transparent solution was obtained in about 30 minutes.

[0066] The above aqueous solution was transferred to a plastic beaker. 66.1 g of high-purity hydrofluoric acid (25 wt%) (corresponding to a chemical equivalence ratio of F / Ba of 2.1) was weighed and poured into the plastic beaker using a tube pump while stirring, and reacted with the barium acetate aqueous solution. After pouring the hydrofluoric acid, stirring was continued for 1 hour, and a cloudy barium fluoride slurry was obtained.

[0067] The barium fluoride slurry was transferred to a stainless steel autoclave vessel equipped with a polytetrafluoroethylene inner cylinder, which was then sealed. The autoclave vessel was then placed in an electric furnace and heated at 100°C for 24 hours. The cooled autoclave container was removed from the electric furnace, and a precipitate of barium fluoride particles and a supernatant liquid were obtained in the autoclave container. After the supernatant liquid was sucked out using a pump, 100 g of pure water was added to the precipitate to obtain a thick barium fluoride slurry. This slurry was transferred to a plastic beaker and stirred.

[0068] Next, we will explain the production of lanthanum fluoride for doping barium fluoride particles. 57.6 g of lanthanum acetate (special grade) was weighed and placed in a beaker. 290 g of ion-exchanged water was added to the beaker and stirred with a stirrer to dissolve the lanthanum acetate. While stirring, 10.4 g of nitric acid (69 wt%, special grade) was weighed and added to the beaker. A colorless, transparent lanthanum acetate aqueous solution was obtained in approximately 30 minutes. This lanthanum acetate aqueous solution was added to the plastic beaker in which the barium fluoride slurry was being stirred. 42.5 g of high-purity hydrofluoric acid (25 wt%) (corresponding to an F / La ratio of 3.15) was then injected into the plastic beaker with a tube pump while stirring, and reacted with the lanthanum acetate to produce BLF. After the hydrofluoric acid was added, stirring was continued for approximately 30 minutes, resulting in a cloudy BLF slurry.

[0069] The BLF slurry was transferred to a stainless steel autoclave vessel equipped with a polytetrafluoroethylene inner cylinder, which was then sealed. The autoclave vessel was then placed in an electric furnace and heated at 180°C for 24 hours. The autoclave vessel was allowed to cool and removed from the electric furnace, leaving a BLF particle precipitate and a supernatant liquid in the autoclave vessel. This was stirred to form a uniform slurry and transferred to a centrifuge. The supernatant liquid produced by centrifugation was discarded, leaving a BLF particle precipitate. 200 g of ion-exchanged water was added to this BLF particle precipitate and stirred to obtain a homogeneous BLF slurry. This BLF slurry was centrifuged for solid-liquid separation, and the supernatant liquid was discarded. This process was repeated three times for washing, yielding purified BLF precipitate particles.

[0070] The BLF precipitate particles were heated in an electric furnace at 160°C for 18 hours to obtain a dry cake, which was then taken out and pulverized in an agate mortar to obtain approximately 88 g of white dry particles. Approximately 8 g of these white dried particles were packed into a mold with a uniform depression of 25 mm in diameter and press-molded using a uniaxial press. The pressing force was 2.6 kN, equivalent to a pressure of 5.3 MPa. The resulting cylindrical molded body had sufficient shape retention to withstand manual breaking.

[0071] The compact was vacuum sealed in a thin resin bag and subjected to cold isostatic pressing (CIP). The CIP conditions were a pressure of 50 MPa and a holding time of 1 minute. It was confirmed that the compact shrunk and became denser after the CIP treatment compared to before the treatment.

[0072] This compact was degreased by holding it in an electric furnace for 6 hours at a temperature of 350° C. to 450° C. The compact was not significantly deformed by degreasing, but it became slightly hard. The degreased green body was placed in a vacuum sintering furnace, held at 1000°C for 1 hour, and then allowed to cool to obtain a sintered body. The sintered body was white and cylindrical, with a diameter and height that had shrunk by approximately 20% compared to the green body, indicating that sintering had progressed. The bulk density of the sintered body was approximately 5.4 g / cm. 3 It was.

[0073] Next, the sintered body was subjected to hot isostatic pressing (HIP). The HIP treatment conditions were a pressure of 98 MPa and a temperature of 1060°C, and the body was maintained in this state for 2 hours. After HIP treatment, the sintered body was placed in a carbon crucible, heated using a Mo heater, and then allowed to cool. The HIP treatment made the sintered body transparent.

[0074] In the above-mentioned BLF slurry production process, the doping concentration of lanthanum can be adjusted by varying the blending ratio of barium acetate and lanthanum acetate as raw materials. Herein, the doping concentration x of lanthanum is defined as x = (chemical equivalent of lanthanum) / (total chemical equivalent of barium and chemical equivalent of lanthanum). Samples 3-2 (x = 0.01), 3-3 (x = 0.50), 3-4 (x = 0.1), 3-5 (x = 0.2), 3-6 (x = 0.3), 3-7 (x = 0.4), 3-8 (x = 0.45), and 3-9 (x = 0.5) were prepared as BLF particles with different concentrations x.

[0075] Sintered bodies were produced using the BLF particles of Samples 3-2 to 3-9. The evaluation results of each of the produced sintered bodies are shown in Fig. 9. In Fig. 9, the same sample as in Example 1 above is shown as Sample 3-1. In other words, Sample 3-1 is a sample that is not doped with lanthanum (x = 0). As shown in Figure 9, Sample 3-8, with a density x of 0.45, and Sample 3-9, with a density x of 0.5, were opaque, while the other Samples 3-1 to 3-7 were colorless and transparent. Both sides of Samples 3-1 to 3-7 were polished to form parallel plates, and the transmittance of each was measured. As a result, the sintered bodies of the particles of each sample exhibited good properties in the ultraviolet to infrared range.

[0076] FIG. 10 shows the transmittance of the sintered bodies corresponding to the particles of Samples 3-1, 3-4 to 3-7, where FIG. 10(a) shows the transmittance in the wavelength range of 250 nm to 700 nm, and FIG. 10(b) shows the transmittance in the wavelength range of 2.5 μm to 14.5 μm. The thicknesses of the sintered bodies corresponding to the particles of Samples 3-1, 3-4 to 3-7 are as shown in FIG. 9. Although not shown in FIG. 10, the sintered bodies corresponding to the particles of Samples 3-2 and 3-3 with concentrations x of 0.01 and 0.05 exhibit transmittance in the infrared region that is almost equivalent to the transmittance of barium fluoride single crystal, and λ 80 showed almost the same values.

[0077] As can be seen from Figure 10, in the ultraviolet region, the transmittance characteristics fluctuated significantly with changes in concentration x. It was found that the sintered body corresponding to sample 3-4, with a concentration x of 0.1, exhibited high transmittance in the short wavelength region, such as wavelengths below 270 nm. In addition, the transmittance in the visible region was approximately 87 to 90% for the sintered bodies corresponding to all samples.

[0078] The refractive indexes of the sintered bodies corresponding to the particles of Samples 3-1, 3-2, 3-4 to 3-7 were measured using the prism coupler method and the V-block method. The Metricon Model 2010 / M was used for the prism coupler method, and the refractive indexes were measured at wavelengths of 473.594 and 654 nm. This refractive index was approximated using Cauchy's dispersion formula, and the refractive index nd at the d-line (587.6 nm) was calculated. The Shimadzu KPR-2000 was used for the V-block method, and the refractive indexes at the g-line (436 nm), F-line (486.1 nm), d-line, and C-line (656.3 nm) were measured. The refractive index at the d-line is shown in Figure 9.

[0079] Figure 11 is a graph showing the refractive index for the d-line. The refractive index increases almost linearly with increasing concentration x, suggesting that the entire amount of lanthanum fluoride is dissolved in barium fluoride, resulting in compositional additivity of the refractive index.

[0080] FIG. 12 shows the measurement results of the XRD patterns of the sintered bodies corresponding to the particles of Samples 3-1, 3-2, 3-4, 3-5 and 3-6. The BLF sintered bodies of Samples 3-2 and 3-4 to 3-6 exhibited the same fluorite structure pattern as barium fluoride, and no peaks of the added lanthanum fluoride were detected. In other words, it was confirmed that a solid solution with a fluorite structure was formed when the concentration x was 40 or less. Furthermore, for Samples 3-2 and 3-4 to 3-6, the pattern peaks shifted continuously and linearly to the higher wavenumber side, indicating that the size of the crystal lattice contracted linearly as the solid solution progressed.

[0081] FIG. 13 shows the results of measuring the XRD patterns of the particles of each sample before sintering. For particles with a concentration x of less than 0.3 (Samples 3-1 to 3-5), a barium fluoride peak and a subpeak at a higher wavenumber were detected. Based on the relationship between the solid solution concentration and the peak position, this subpeak was determined to be a solid solution particle with a concentration x of approximately 0.3 (Sample 3-6). Therefore, the particles are considered to be a mixture of barium fluoride and BLF30. For particles with a concentration x exceeding 0.3 (Samples 3-7 and 3-8), no barium fluoride pattern was detected, indicating that the BLF solid solution was the main component of the particles. In other words, it was found that the BLF solid solution was precipitated and synthesized as particles from the time of particle synthesis. This is characteristic of the fabrication method described in the above-mentioned embodiment. Therefore, compared to a reactive sintering method in which barium fluoride and lanthanum fluoride are mixed in a fixed composition ratio and then sintered, the use of solid solution particles from the beginning suppressed the inhomogeneity of the composition that occurs during sintering, resulting in a uniform sintered body. This is presumably why the high transmittance was observed over a wide wavelength range.

[0082] The present invention is not limited to the above-described embodiments, and other forms that can be considered within the scope of the technical idea of ​​the present invention are also included within the scope of the present invention, as long as the features of the present invention are not impaired. [Explanation of symbols]

[0083] 1...imaging device, 2...multiphoton microscope, 103...photographic lens, 206...objective lens, 208... condensing lens, 210... imaging lens, CAM: Imaging device, WL: Camera lens

Claims

1. A barium fluoride polycrystalline body having a transmittance of 85% or more but less than 94% for light with a wavelength of 550 nm per 4.6 mm thickness.

2. The barium fluoride polycrystalline body according to claim 1, A barium fluoride polycrystalline body containing lanthanum in a chemical equivalent ratio of lanthanum to the sum of the chemical equivalents of barium and lanthanum of 0.4 or less.

3. A barium fluoride polycrystalline body having a transmittance of 80% or more but less than 94% for light having a wavelength in the range of 340 nm or more and 2000 nm or less per 4.6 mm thickness.

4. A barium fluoride polycrystalline body having a transmittance of 80% or more but less than 94% for light having a wavelength in the range of 4.5 μm or more and 10.5 μm or less per 4.6 mm thickness.

5. A barium fluoride polycrystalline body containing lanthanum, The lanthanum is contained in a ratio of a chemical equivalent of lanthanum to the sum of the chemical equivalents of barium and lanthanum of 0.4 or less; A barium fluoride polycrystalline body having a transmittance of 80% or more but less than 94% for light having a wavelength in the range of 350 nm or more and 700 nm or less per 4.6 mm thickness.

6. A barium fluoride polycrystalline body containing lanthanum, The lanthanum is contained in a ratio of a chemical equivalent of lanthanum to the sum of the chemical equivalents of barium and lanthanum of 0.4 or less; A barium fluoride polycrystalline body having a transmittance of 70% or more but less than 94% for light having a wavelength in the range of 3.5 μm or more and 7.5 μm or less per 4.6 mm thickness.

7. 7. The barium fluoride polycrystalline body according to claim 2, 5 or 6, containing lanthanum in a chemical equivalent ratio of lanthanum to the sum of the chemical equivalents of barium and lanthanum of 0.01 or more.

8. The barium fluoride polycrystalline body according to any one of claims 1 to 7, wherein the refractive index for d line is greater than 1.475 and less than 1.

550.

9. Including lanterns, A barium fluoride polycrystalline body having a refractive index for the d-line of greater than 1.475 and not greater than 1.

550.

10. An optical element using the barium fluoride polycrystalline material according to any one of claims 1 to 9.

11. An optical system comprising the optical element according to claim 10.

12. An interchangeable lens comprising the optical system according to claim 11.

13. An optical device comprising the optical system according to claim 11.

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