Calcium fluoride sintered body, method for producing calcium fluoride particles, method for producing calcium fluoride sintered body, optical element, optical system, interchangeable lens, and optical device

Abstract

To provide a calcium fluoride sintered compact, a method for manufacturing calcium fluoride particles, a method for a manufacturing a calcium fluoride sintered compact, an optical element, an optical system, an interchangeable lens, and an optical device.SOLUTION: A calcium fluoride sintered compact has an internal transmittance of 98% or more for light at a wavelength of 550 nm per 10 mm of thickness.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] A manufacturing method for obtaining a calcium fluoride sintered body by hot pressing has been proposed (for example, Patent Document 1). However, in order to use it as an optical component, high transmittance is required. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2003-300777 Summary of the Invention

[0004] According to the first aspect, the calcium fluoride sintered body has an internal transmittance of 98% or more per 10 mm thickness of light with a wavelength of 550 nm. According to a second aspect, a method for producing calcium fluoride particles includes a producing step of producing a dispersion liquid containing calcium fluoride particles by reacting a calcium compound with a fluorine compound in a solution, a mixing step of mixing the calcium fluoride particles contained in the dispersion liquid with hydrofluoric acid, and a separation step of separating the calcium fluoride particles from the hydrofluoric acid after the mixing step. According to a third aspect, a method for producing a calcium fluoride sintered body includes a molding step of forming a molded body by molding calcium fluoride particles produced by the method for producing calcium fluoride particles of the second aspect, and a sintering step of sintering the molded body in an inert atmosphere to produce a sintered body. According to a fourth aspect, an optical element uses the calcium fluoride sintered body of the first aspect. According to a fifth aspect of the present invention, an optical system comprises the optical element of the fourth aspect. According to a sixth aspect, an interchangeable lens comprises the optical system of the fifth aspect. According to a seventh aspect, an optical device comprises the optical system of the fifth aspect. [Brief explanation of the drawings]

[0005] [Figure 1] 1 is a flowchart illustrating a method for producing a calcium fluoride sintered body according to an embodiment. [Figure 2] FIG. 1 is a diagram schematically illustrating an example of a stirring device used when reacting an aqueous calcium compound solution with an aqueous fluorine compound solution. [Figure 3] 1 is a perspective view showing an example of an imaging device according to an embodiment. [Figure 4] FIG. 10 is a front view showing another example of the imaging device according to the embodiment. [Figure 5] FIG. 10 is a rear view showing another example of the imaging device according to the embodiment. [Figure 6] FIG. 1 is a block diagram showing an example of a multiphoton microscope according to an embodiment. [Figure 7] 1 shows the conditions for producing calcium fluoride particles of an example, and the measurement results of the internal transmittance and optical distortion of a calcium fluoride sintered body for light with a wavelength of 550 nm. [Figure 8] 1 shows the conditions for producing calcium fluoride particles of an example, and the measurement results of the internal transmittance and optical distortion of a calcium fluoride sintered body for light with a wavelength of 550 nm. [Figure 9] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in an example. [Figure 10] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in an example. [Figure 11] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in an example. [Figure 12] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in an example. [Figure 13] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in an example. [Figure 14] FIG. 1 is a diagram showing the results of measuring the spectral transmittance of a calcium fluoride sintered body in Comparative Example 1. [Figure 15] FIG. 10 is a diagram showing the results of measuring the spectral transmittance of the calcium fluoride sintered body in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0006] A calcium fluoride (CaF2) sintered body, a method for producing calcium fluoride particles, and a method for producing calcium fluoride sintered body according to embodiments will be described with reference to the drawings. The calcium fluoride sintered body of this embodiment has an internal transmittance of 98% or more for light with a wavelength of 550 nm per 10 mm of thickness. The calcium fluoride sintered body of this embodiment may have an internal transmittance of 90% or more for light with a wavelength of 380 nm to 780 nm per 10 mm of thickness. The calcium fluoride sintered body of this embodiment may have an internal transmittance of 90% or more for light with a wavelength of 3 μm to 7 μm per 10 mm of thickness. The calcium fluoride sintered body of this embodiment has an internal transmittance of 80% or more for light with a wavelength IRλ in the infrared region per 10 mm of thickness. 80 The calcium fluoride sintered body of the present embodiment may have an optical distortion of 10 nm / cm or less. The calcium fluoride sintered body of the present embodiment may have a relative density of 98% or more with respect to the density of calcium fluoride single crystal.

[0007] With reference to FIG. 1, a method for producing the above-mentioned calcium fluoride sintered body and a method for producing calcium fluoride particles that are the material for the calcium fluoride sintered body will be described. In step S1, a calcium compound (e.g., high-purity calcium acetate hydrate, high-purity calcium carbonate, high-purity calcium nitrate, etc.) is dissolved in distilled water to prepare an aqueous calcium compound solution. When using an organic salt such as calcium acetate, it is preferable to add nitric acid as an oxidizing agent. In step S2, a fluorine compound aqueous solution is prepared by adding distilled water to high-purity hydrofluoric acid (hydrofluoric acid) to dilute it to an appropriate concentration, or by dissolving ammonium fluoride or the like in distilled water.

[0008] In step S3, the calcium compound aqueous solution and the fluorine compound aqueous solution are reacted (i.e., the calcium compound and the fluorine compound are reacted in the aqueous solution) to produce a dispersion containing calcium fluoride particles (production step). Specifically, while stirring the calcium compound aqueous solution, a fluorine compound aqueous solution is poured into the calcium compound aqueous solution at a molar ratio of 2.4 to 5.0. In this case, a stirring rod 31 (with a blade diameter of 10 cm) of the stirring device 3 shown in FIG. 2 is rotated at 300 rpm, and the fluorine compound aqueous solution is slowly poured into the calcium compound aqueous solution while stirring. Inlets 33 and 34 for the fluorine compound aqueous solution are attached to the side of a plastic beaker 32 of the stirring device 3, and the fluorine compound aqueous solution is poured into the calcium compound aqueous solution from a container (not shown) containing the fluorine compound aqueous solution using a roller tube pump (not shown) over a period of, for example, about 1 hour. After the pouring of the fluorine compound aqueous solution into the calcium compound aqueous solution is completed, stirring is continued for, for example, 2 to 6 hours. This suppresses aggregation of calcium fluoride particles, thereby producing calcium fluoride particles with small particle sizes. After the fluorine compound aqueous solution is poured, stirring is carried out, for example, using a water bath while maintaining the temperature at 5 to 10°C. Stirring at a low temperature can increase the transmittance of the calcium fluoride sintered body produced using the generated calcium fluoride particles.

[0009] In step S4, the dispersion containing calcium fluoride particles is simultaneously heated and pressurized to promote the reaction between the calcium compound and the fluorine compound, thereby growing the calcium fluoride particles and increasing their crystallinity (heating and pressurizing step). Specifically, the dispersion containing calcium fluoride particles (a dispersion (slurry) in which calcium fluoride microparticles are suspended) is heated and pressurized in a sealed container (e.g., an autoclave equipped with a Teflon (registered trademark) container) for 10 to 24 hours, for example, while maintaining a heating temperature of 100°C to 180°C. After the heating and pressurizing step, when the temperature of the sealed container drops to room temperature, the supernatant liquid is removed by suction, leaving the calcium fluoride particles to be separated. In step S5, the separated calcium fluoride particles are mixed with, for example, 0.1 to 20% hydrofluoric acid and stirred (mixing step).

[0010] In step S6, the calcium fluoride particles and hydrofluoric acid mixture produced in the mixing step is transferred to a centrifuge tube, and the centrifuge tube is centrifuged to separate the mixture into a solid (calcium fluoride particles) and a liquid (hydrofluoric acid) (separation step). In this case, for example, the centrifuge is rotated at 1,000 rpm for 10 minutes. After the solid and liquid are separated, the supernatant is removed, and distilled water is then poured into the centrifuge tube containing the calcium fluoride particles to thoroughly disperse the calcium fluoride particles. Using a shaker for this dispersion can prevent the inflow of foreign matter from the outside. After shaking for approximately 30 minutes with the shaker until the calcium fluoride particles no longer precipitate, the calcium fluoride particles are again separated into a solid and a liquid with the centrifuge, the supernatant is removed, and distilled water is poured in again to thoroughly disperse the calcium fluoride particles. The process of adding distilled water to disperse the calcium fluoride particles and the process of separating the solid and liquid with the centrifuge are repeated until the concentration of hydrofluoric acid in the supernatant is 200 ppm or less. If the number of distilled water injections is small, the calcium fluoride sintered body produced using the resulting calcium fluoride particles will have low permeability, and numerous white spots of about 0.1 mm, which are aggregates of tiny bubbles, will be observed inside. Furthermore, as the number of distilled water injections increases, the calcium fluoride particles will break down and their particle size will become smaller, making them less likely to settle. Therefore, the rotation speed of the centrifuge should be gradually increased, for example, from 1000 rpm to 1200 rpm, 1400 rpm, 1600 rpm, 1800 rpm, and 2000 rpm. The processes in steps S1 to S6 described above are the processes in the method for producing calcium fluoride particles in this embodiment.

[0011] In step S7, the calcium fluoride granules classified into those with a predetermined particle size or smaller are molded to form a molded body (molding step) from among the granules obtained by pulverizing the dried body (cake) made of calcium fluoride particles manufactured by the above-mentioned manufacturing method of calcium fluoride particles. The classification is performed by placing the calcium fluoride particles manufactured by the above-mentioned manufacturing method of calcium fluoride particles in a Teflon container, drying them at 160°C for about 10 hours, and then removing larger granules using a 1 mm sieve, for example. There are two molding methods, for example: In the first molding method, classified calcium fluoride particles are press-molded using a mold having a predetermined shape to form a molded body. In the second molding method, a slurry containing calcium fluoride particles produced by the above-mentioned method for producing calcium fluoride particles is placed in, for example, a dish-shaped container and dried at 70 to 300°C for about 10 hours to form a molded body.

[0012] In step S8, the green compact (with a relative density of 35-50%) formed by the first or second forming method is sintered to produce a sintered body (white sintered body) (sintering process). In the sintering process, the white green compact is sintered (initial sintering) for 2-6 hours at, for example, 400-700°C to produce a white sintered body with a relative density of approximately 40-70%. If the relative density of the green compact before sintering is too high, the white sintered body will not be transparent in the subsequent process. If the sintering temperature is too high, the initial sintering will proceed, reducing the driving force for sintering in the subsequent process and causing increased optical distortion (i.e., deterioration of optical properties). If the sintering temperature is too low, organic matter will remain in the sintered body, preventing high transmittance. Next, the mixture is held in an inert atmosphere (for example, a vacuum, argon, or nitrogen atmosphere) at, for example, 900 to 1000°C for 1 to 2 hours to obtain a white sintered body with a relative density of approximately 98%.

[0013] In step S9, the white sintered body is subjected to a heating and pressurizing treatment using, for example, a hot isostatic pressing (HIP) apparatus to make it transparent and produce a transparent sintered body (transparentization step). Specifically, the white sintered body is heated to, for example, 1000 to 1100°C in an inert atmosphere (for example, an argon atmosphere) while maintaining a pressure of 100 MPa, thereby pushing out pores remaining inside the white sintered body and producing a transparent sintered body (i.e., the calcium fluoride sintered body of this embodiment). That is, the processes of steps S7 to S9 described above are the processes in the method for producing a calcium fluoride sintered body of this embodiment.

[0014] After the transparentization step, an annealing step may be performed to anneal the transparent sintered body, if necessary. The annealing step is performed, for example, in an inert atmosphere for a heating time of 40 hours or more at a temperature range of 600°C to 800°C. This allows the optical distortion of the transparent sintered body to be further reduced, for example, to 2 nm / cm or less.

[0015] An embodiment of an imaging device including an optical element made of the calcium fluoride sintered body manufactured as described above will be described. 3 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 taking lens 103 (optical system) includes an optical element whose base material is the calcium fluoride 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 taking 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.

[0016] FIG. 4 is a front view of another example of an imaging device including an optical element made of calcium fluoride sintered body according to this embodiment, and FIG. 5 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 calcium fluoride 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 a subject (object) is collected by the taking lens WL and focused on an imaging element arranged on the image plane. The subject image formed on the imaging element is displayed on an LCD monitor LM arranged behind the imaging device CAM. After the photographer decides on the composition of the subject image while looking at the LCD monitor LM, he or she presses the release button B1 to capture the subject image with the imaging element, which is then recorded and saved in memory (not shown).

[0017] The imaging device CAM is provided with an auxiliary light emitting section EF that emits auxiliary light when the subject is dark, and a function button B2 that is used to set various conditions for the imaging device CAM. Optical systems used in such digital cameras are required to have higher resolution, lighter weight, and smaller size. To achieve these, it is effective to use glass with a high refractive index for the optical system. In particular, glass with a high refractive index and a low specific gravity (S g ) and has high press moldability, there is a high demand. From this viewpoint, the calcium fluoride sintered body of the present embodiment is suitable as a member of an optical device. 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.

[0018] Next, a multiphoton microscope equipped with an optical element using the calcium fluoride sintered body of this embodiment will be described. 6 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 made of calcium fluoride sintered compact according to this embodiment. The following description will focus on the optical system of the multiphoton microscope 2.

[0019] 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.

[0020] 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.

[0021] 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).

[0022] 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.

[0023] 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.

[0024] 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 . In this case, the observation light is descanned by a scanning means (not shown), passes through a dichroic mirror 204, is focused 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, passes through an imaging lens 210, and enters a fluorescence detection unit 211. The fluorescence detection unit 211 is composed of, for example, a barrier filter, a PMT, etc., receives the observation light that has been 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.

[0025] 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.

[0026] 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.

[0027] According to the above-described embodiment, the following effects can be obtained. (1) A method for producing calcium fluoride particles includes a producing step of producing a dispersion liquid containing calcium fluoride particles by reacting a calcium compound with a fluorine compound in a solution, a mixing step of mixing the calcium fluoride particles contained in the dispersion liquid with hydrofluoric acid, and a separation step of separating the calcium fluoride particles from the liquid component after the mixing step. This makes it possible to produce calcium fluoride particles that can be used to produce calcium fluoride sintered bodies with high transmittance.

[0028] (2) The concentration of hydrogen fluoride in the hydrofluoric acid solution used in the mixing step is between 0.1% and 20%, which prevents the formation of white spots of about 0.1 mm, which are aggregates of fine bubbles, inside the sintered body obtained by sintering the calcium fluoride particles.

[0029] (3) A method for producing a calcium fluoride sintered body includes a molding step of forming a molded body by molding the calcium fluoride particles produced by the above-mentioned method for producing calcium fluoride particles, and a sintering step of sintering the molded body in an inert atmosphere to produce a sintered body, thereby making it possible to produce a calcium fluoride sintered body with high transmittance.

[0030] (4) In the molding step, calcium fluoride particles having a predetermined particle size or less are molded to form a molded body, thereby obtaining a molded body of calcium fluoride particles to be used for producing a calcium fluoride sintered body having high transmittance.

[0031] (5) In the manufacturing method of calcium fluoride sintered bodies, a molded body with a relative density of 35% to 50% is sintered (initial sintering) at 400°C to 700°C for 2 to 6 hours, and then sintered in an inert atmosphere at 900°C to 1000°C for 1 to 2 hours. Initial sintering at 400°C to 700°C reduces the driving force for sintering in subsequent processes, preventing an increase (worsening) in optical distortion during grain growth, as occurs when sintering at too high a temperature. It also prevents organic components from remaining in the raw materials and reducing transmittance, which occurs when the temperature is too low.

[0032] (6) The method for producing calcium fluoride sintered bodies includes a clarification step in which the sintered body is heated to 1000°C or higher and 1100°C or lower under a pressure of 100 MPa in an inert atmosphere after the sintering step to make the sintered body transparent. This allows the production of transparent calcium fluoride sintered bodies.

[0033] Examples of the calcium fluoride sintered body according to the above-described embodiment will now be described. [Example] The calcium fluoride sintered body in the examples was produced according to the process shown in the flowchart of Figure 1. In the examples, calcium acetate hydrate was used as the calcium compound, and hydrofluoric acid was used as the fluorine compound. In this example, 20 samples of calcium fluoride sintered bodies (transparent sintered bodies) were prepared under different conditions for producing and sintering calcium fluoride particles, and the internal transmittance of light with a wavelength of 550 nm and the optical distortion of each sample were measured after polishing both sides. The optical distortion was measured using a fully automatic strain eye LSM-9000s (manufactured by Luceo Co., Ltd.).

[0034] Figure 7 shows the manufacturing and sintering conditions of calcium fluoride particles used for calcium fluoride sintered body samples 1 to 12 of the example, as well as the measurement results of the internal transmittance and optical distortion of the calcium fluoride sintered body at a wavelength of 550 nm. Figure 8 shows the manufacturing and sintering conditions of calcium fluoride particles used for calcium fluoride sintered body samples 13 to 24 of the example, as well as the measurement results of the internal transmittance and optical distortion of the calcium fluoride sintered body at a wavelength of 550 nm. Note that the "transmittance" in Figures 7 and 8 refers to the internal transmittance of light at a wavelength of 550 nm per 10 mm of sample thickness, in other words, the internal transmittance per 10 mm of the distance traveled by the light in the sample. The "F / Ca ratio" refers to the molar ratio of the fluorine compound aqueous solution (hydrofluoric acid) injected into the calcium compound aqueous solution (calcium acetate aqueous solution) to the calcium compound aqueous solution in the production process of step S3 in Figure 1. The "hydrofluoric acid concentration" refers to the concentration of hydrofluoric acid mixed with calcium fluoride particles in the mixing process of step S5 in Figure 1. The "hydrothermal temperature" refers to the temperature at which calcium fluoride particles undergo grain growth and crystallization in the heating and pressurizing process of step S4 in Figure 1. The "number of water washes" refers to the number of times the process of mixing and stirring calcium fluoride particles with distilled water, separating the solid from the liquid, and removing the supernatant is repeated in step S6 in Figure 1. The "drying temperature" refers to the sintering temperature at which the green body is sintered to produce a white sintered body with a relative density of approximately 40 to 70% in the sintering process of step S8 in Figure 1. The "inert atmosphere sintering temperature" refers to the sintering temperature at which a white sintered body with a relative density of 98% is produced in the sintering process of step S8 in Figure 1. The "HIP temperature" refers to the heating temperature used in the HIP process in the "clarification process" of step S9 in Figure 1.

[0035] 7 and 8, the calcium fluoride sintered bodies of Samples 1 to 24 in the examples had an internal transmittance of 98% or more for light with a wavelength of 550 nm. Furthermore, the calcium fluoride sintered bodies of Samples 1 to 22 had an optical distortion of 10 nm / cm or less.

[0036] Figures 9 to 13 show the results of measuring the spectral transmittance of calcium fluoride sintered compacts of Samples 1 to 24. The transmittances shown in Figures 9 to 13 are all internal transmittances per 10 mm of sample thickness. In other words, they are the internal transmittances per 10 mm of the distance that light actually travels through the sample. Figure 9(a) shows the measurement results of the spectral transmittance of Samples 1 to 6 for light with wavelengths from 200 nm to 800 nm, indicated by L1 to L6. Figure 9(b) shows the measurement results of the spectral transmittance of Samples 7 to 12 for light with wavelengths from 200 nm to 800 nm, indicated by L7 to L12. Figure 10(a) shows the measurement results of the spectral transmittance of Samples 13 to 17 for light with wavelengths from 200 nm to 800 nm, indicated by L13 to L17. Figure 10(b) shows the measurement results of the spectral transmittance of Samples 18 to 22 for light with wavelengths from 200 nm to 800 nm, indicated by L18 to L22. Figure 11(a) shows the measurement results of the spectral transmittance of samples 1 to 6 for light with wavelengths from 3000 nm (3 μm) to 13,000 nm (13 μm), labeled M1 to M6. Figure 11(b) shows the measurement results of the spectral transmittance of samples 7 to 12 for light with wavelengths from 3000 nm (3 μm) to 13,000 nm (13 μm), labeled M7 to M12. Figure 12(a) shows the measurement results of the spectral transmittance of samples 13 to 17 for light with wavelengths from 3000 nm (3 μm) to 13,000 nm (13 μm), labeled M13 to M17. Figure 12(b) shows the measurement results of the spectral transmittance of samples 18 to 22 for light with wavelengths from 3000 nm (3 μm) to 13,000 nm (13 μm), labeled M18 to M22. Figure 13 shows the measurement results of the spectral transmittance of samples 23 and 24 for light with wavelengths from 3000 nm (3 μm) to 13000 nm (13 μm), indicated by M23 and M24. As shown in Figures 9 and 10, the calcium fluoride sintered body of the example has an internal transmittance of 90% or more for light with wavelengths from 380 nm to 780 nm. As shown in Figures 11, 12, and 13, the internal transmittance of the calcium fluoride sintered body of the example is 90% or more for light with wavelengths from 3000 nm (3 μm) to 7000 nm (7 μm). Furthermore, as shown in Figures 11, 12, and 13, in the infrared region, there is a wavelength IRλ at which the internal transmittance of the calcium fluoride sintered body is 80% or more. 80 is 8000 nm (8 μm) or more.

[0037] The calcium fluoride sintered bodies of Samples 23 and 24 were subjected to an annealing process under the conditions of a starting temperature of 800°C, a temperature decrease rate of 5°C / h, and a finishing temperature of 600°C. As a result, the optical distortion of the calcium fluoride sintered bodies of Samples 23 and 24 was 1.5 nm / cm, respectively.

[0038] [Comparative Example 1] The calcium fluoride sintered body in Comparative Example 1 uses calcium acetate hydrate as the calcium compound and hydrofluoric acid as the fluorine compound, as in the Examples. The calcium fluoride sintered body in Comparative Example 1 was produced without the step described as step S6 above. That is, the calcium fluoride sintered body was produced using calcium fluoride particles produced without the step of stirring calcium fluoride particles and distilled water to separate the solid from the liquid, removing the supernatant, and then adding distilled water and stirring. In Comparative Example 1, a transparent calcium fluoride sintered body was obtained by HIP treatment, but many white spots about 0.1 mm in size, which were aggregates of fine bubbles, were observed in the sintered body.

[0039] FIG. 14 shows the results of measuring the spectral transmittance of the calcium fluoride sintered compact of Comparative Example 1. The transmittance shown in FIG. 14 is the internal transmittance per 10 mm of sample thickness. In other words, it is the internal transmittance per 10 mm of the distance that light actually travels through the sample. FIG. 14(a) shows the results of measuring the spectral transmittance of light with wavelengths from 200 nm to 800 nm, and FIG. 14(b) shows the results of measuring the spectral transmittance of light with wavelengths from 3000 nm (3 μm) to 13000 nm (13 μm). The calcium fluoride sintered compact of Comparative Example 1 had an internal transmittance of 97.7% for light with a wavelength of 550 nm per 10 mm of thickness, which was less than 98%.

[0040] Comparative Example 2 The calcium fluoride sintered body of Comparative Example 2 uses calcium acetate hydrate as the calcium compound and hydrofluoric acid as the fluorine compound, as in the examples. The calcium fluoride sintered body of Comparative Example 2 was produced using calcium fluoride particles produced without performing the mixing step described as step S5 above. Foreign matter was generated in the obtained calcium fluoride sintered body of Modification Example 2.

[0041] FIG. 15 shows the results of measuring the spectral transmittance of the double-side-polished calcium fluoride sintered compact of Comparative Example 2. The transmittance shown in FIG. 15 is the internal transmittance per 10 mm of sample thickness. In other words, it is the internal transmittance per 10 mm of the distance that light actually travels through the sample. FIG. 15(a) shows the results of measuring the spectral transmittance for light with wavelengths from 200 nm to 800 nm, and FIG. 15(b) shows the results of measuring the spectral transmittance for light with wavelengths from 3000 nm (3 μm) to 13000 nm (13 μm). As shown in FIG. 15, the range of light wavelengths over which the internal transmittance is 90% or higher is narrower than in the Examples. Furthermore, the calcium fluoride sintered compact of Comparative Example 2 had an internal transmittance of 87.7% for light with a wavelength of 550 nm per 10 mm of thickness, which was less than 98%.

[0042] The present invention is not limited to the above-described embodiments, and other forms that are conceivable 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]

[0043] 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. The internal transmittance of light with a wavelength of 550 nm per 10 mm of thickness is 98% or more, A calcium fluoride sintered body having an optical distortion of 10 nm / cm or less.

2. A calcium fluoride sintered body as described in claim 1, having an internal transmittance of 90% or more for light with wavelengths from 380 nm to 780 nm per 10 mm thickness.

3. A calcium fluoride sintered body as described in claim 1 or 2, having an internal transmittance of 90% or more for light with wavelengths from 3 μm to 7 μm per 10 mm thickness.

4. An optical element comprising the calcium fluoride sintered body described in any one of claims 1 to 3.

5. An optical system including the optical element described in claim 4.

6. An interchangeable lens including the optical system described in claim 5.

7. An optical device including the optical system described in claim 5.

8. A production step of reacting a calcium compound with a fluorine compound in a solution to obtain a dispersion liquid containing calcium fluoride particles; a mixing step of mixing the calcium fluoride particles with hydrofluoric acid to obtain a mixed solution; a separation step of separating the calcium fluoride particles and the hydrofluoric acid after the mixing step; a molding step of molding the calcium fluoride particles after the separation step to obtain a molded body; an initial sintering step of heating the compact at 400°C or higher and 700°C or lower for 2 hours or higher and 6 hours or lower; a sintering step of sintering the compact after the initial sintering step in an inert atmosphere at a temperature of 900°C or higher and 1000°C or lower to obtain a sintered body; a transparentizing step of heating the sintered body at 1000°C or higher and 1100°C or lower while applying pressure in an inert atmosphere after the sintering step to obtain a transparent sintered body.

9. A method for producing calcium fluoride sintered body as described in Claim 8, wherein the temperature of the solution in the production process is not less than 5°C and not more than 10°C.

10. A method for producing a calcium fluoride sintered body as described in claim 8 or 9, which comprises heating and pressurizing the dispersion liquid obtained in the production process at a temperature of 100°C or higher and 180°C or lower for 10 hours or longer and 24 hours or shorter.

11. A method for producing a calcium fluoride sintered body described in any one of claims 8 to 10, wherein in the molding step, the molded body is obtained by press-molding the calcium fluoride particles.

12. A method for producing a calcium fluoride sintered body described in any one of claims 8 to 10, wherein in the molding process, the calcium fluoride particles are placed in a container and dried at a temperature of 70°C or higher and 300°C or lower to obtain the molded body.

13. A method for producing a calcium fluoride sintered body described in any one of claims 8 to 12, which includes heating the sintered body in an inert atmosphere at 600°C or higher and 800°C or lower for 40 hours or more after the transparency process.

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