Optical System

Abstract

To provide a lens unit for an infrared region that can easily generate a tilt error.SOLUTION: A lens unit (4) is a lens unit used for an infrared region including a wavelength in the 7-14 μm range, and includes: at least one lens (1, 2, 3) with a coating film formed thereon; a lens-barrel (6) with holes (61, 62, 63) in which the lens is to be fit; and ring parts (65, 66, and 67) in contact with the circumferential parts of the lens (1c, 2c, 3c) for locating the lens fit in the holes to a predetermined position on the optical axis of the lens-barrel (6) or fixing the lens to a predetermined position of the optical axis of the lens-barrel. The lens has a region to which the coating film is not attached, in a part of the circumferential parts which is in contact with at least one of the lens-barrel and the ring parts.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a lens unit used in a spectroscopic characteristic measuring device or the like. [Background technology]

[0002] Patent Document 1 discloses a near-infrared spectroscopic characteristic measuring device that includes an objective lens that converts signal light from a specimen into parallel light, a phase shifter provided in the parallel light, and an imaging lens that forms an image of the specimen on a detection unit. Patent Document 2 discloses an imaging system that includes an optical system (objective lens) that converts light from an object into parallel light, multiple imaging lenses that form an image of part of the parallel light on a detection unit, and filters provided on each imaging lens, and describes the design of an objective lens in the visible region. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5637488 [Patent Document 2] Japanese Patent Publication No. 2020-064165 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, it is known that absorption due to molecular vibrations specific to substances occurs in the infrared region (e.g., wavelengths of 7 to 14 μm), which is longer than the near-infrared region. Therefore, for further development of spectroscopic measurement technology, it is expected that spectroscopic characteristic measurement devices such as hyperspectral cameras will also be deployed in this infrared region. However, a problem with lens units in finite-system spectroscopic characteristic measurement devices, where the measurement object is at a close distance, is that a deviation in the lens mounting angle relative to the lens barrel member (tilt error) is more likely to have a significant effect on the optical characteristics than a lens unit in infinite-system spectroscopic characteristic measurement devices, where the object is at a distant location.

[0005] In view of the above-mentioned problems, an object of the present invention is to provide a lens unit in the infrared region that can easily suppress the occurrence of tilt errors. [Means for solving the problem]

[0006] In order to solve the above problem, a lens unit according to one embodiment of the present invention is a lens unit used in the infrared region including at least one wavelength in the range of 7 to 14 μm, and comprises one or more lenses having a coating film formed on their surfaces, a lens barrel having holes into which the lenses are fitted, and a ring portion that is a member that contacts the peripheral portion of the lenses and is used to position the lenses fitted in the holes at a required position on the optical axis of the lens barrel or to fix them at a required position on the optical axis of the lens barrel, and the lenses have an area on the peripheral portion that contacts at least either the lens barrel or the ring portion where the coating film is not attached. [Effects of the Invention]

[0007] According to a lens unit according to one aspect of the present invention, the occurrence of tilt errors can be easily suppressed. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view taken along an optical axis, showing the configuration of a main part of an optical system to which a lens unit according to an embodiment is applied. [Figure 2] 1 is a cross-sectional view taken along an optical axis, showing the configuration of a main part of a lens unit according to an embodiment. [Figure 3] FIG. 2 is a cross-sectional view showing a state in which the lens unit according to the embodiment is housed in and fixed to a lens barrel. [Figure 4] FIG. 1 is a front view showing a lens unit according to an embodiment. [Figure 5] FIG. 10 is a cross-sectional view showing a state in which a lens unit according to a modified example is housed and fixed in a lens barrel. [Figure 6] 1 is a cross-sectional view showing a spectroscopic characteristic measuring device to which a lens unit according to an embodiment is applied. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Embodiment] <Optical system> 1 is a cross-sectional view taken along an optical axis, showing the configuration of the main parts of an optical system 100 to which a lens unit according to an embodiment is applied. The optical system 100 forms an image of an object surface T on an image plane S on which an image sensor (detector) or the like corresponding to a wavelength region of the infrared region (7 μm to 14 μm) can be disposed. In the following, unless otherwise specified, the infrared region refers to the region of wavelengths of 7 μm to 14 μm.

[0010] The optical system 100 includes a first lens unit 4 and a second lens unit 5 according to an embodiment of the present invention, and an aperture 8. The first lens unit 4 is an objective lens that converts light incident from the object plane T side into parallel light. The first lens unit 4 is configured by arranging a first lens 1, a second lens 2, and a third lens 3 in this order from the aperture 8 side toward the object plane T side.

[0011] The second lens unit 5 is an imaging lens that converges parallel light onto the image plane S, and has a configuration similar to that of the first lens unit 4. Specifically, the second lens unit 5 is configured by arranging, in order from the diaphragm 8 side toward the image plane S side, a first lens 1, a second lens 2, and a third lens 3.

[0012] The second lens unit 5 has a similar configuration to the first lens unit 4, and is arranged in the optical system 100 so as to be symmetrical to the first lens unit 4 with respect to the aperture 8. The apertures of each lens unit are located on the parallel light side of the first lens 1. In the optical system 100, the apertures 8 of each unit are configured to be common to both units.

[0013] The optical system 100 can be applied to a multispectral camera or a hyperspectral camera by, for example, placing a filter or a phase shifter near the position of the aperture 8. Note that the optical system 100 does not necessarily have to include the aperture 8.

[0014] The optical system 100 includes the lens unit according to the embodiment, which makes it easy to suppress the occurrence of tilt errors. This suppresses the decrease in MTF (Modulation Transfer Function) that accompanies the occurrence of tilt errors, reduces the RMS radius of light collection on the image plane, and suppresses aberrations. In addition, the optical system 100 has good resolution.

[0015] [MTF of optical system] At a spatial frequency of 41.7 cycles / mm, the MTF in the wavelength range of 7 to 14 μm is preferably 0.35 or more within the image circle, and more preferably 0.40 or more. This configuration improves the resolution at the image plane S. The spatial frequency of 41.7 cycles / mm is the Nyquist frequency f of the image sensor with a pixel pitch of 12 μm. N is equivalent to

[0016] <Lens unit> The following describes a lens unit according to one embodiment of the present invention. The second lens unit 5 has the same configuration as the first lens unit 4, and therefore, unless otherwise specified, the first lens unit 4 will be described. Furthermore, the first lens unit 4 may be referred to as lens unit 4, and the second lens unit 5 may be referred to as lens unit 5.

[0017] Fig. 2 is a cross-sectional view taken along the optical axis, showing the configuration of the main parts of a lens unit according to an embodiment. Fig. 3 is a cross-sectional view showing a state in which a lens unit 4 according to an embodiment is housed in and fixed to a lens barrel 6. Fig. 4 is a front view showing a lens unit 4 according to an embodiment. The lens unit 4 includes a first lens 1, a second lens 2, a third lens 3, a lens barrel 6, a first ring portion 65, a second ring portion 66, and a third ring portion 67.

[0018] [Coating film] A coating film (not shown) is formed on the surface of each lens. The coating film is formed for the purposes of reducing reflection on the surface to improve transmittance, protecting the surface, etc. The coating film is preferably made of at least one material selected from germanium (Ge), silicon (Si), fluoride, zinc selenide (ZnSe), zinc sulfide (ZnS), and diamond-like carbon.

[0019] The lens barrel 6 has a first hole 61, a second hole 62, and a third hole 63. The first hole 61 is connected to the second hole 62 and has a larger diameter than the second hole 62. The second hole 62 is connected to the third hole 63 and has a larger diameter than the third hole 63. The third hole 63 has an edge 63a on the opening side that protrudes relative to the axis.

[0020] The third lens 3 is fitted into the third hole 63. In the second hole 62, in order from the third lens 3 side, a third ring portion 67, a second lens 2, a second ring portion 66, and a first lens 1 are fitted. Specifically, the second lens 2 is fitted with the third ring portion 67 interposed between it and the third lens 3, and the first lens 1 is fitted with the second ring portion 66 interposed between it and the second lens 2. A first ring portion 65 that presses the first lens 1 is fitted into the first hole 61. Each of the ring portions 65, 66, and 67 positions each lens at a required position in the lens barrel 6 or fixes it at a required position on the optical axis of the lens barrel 6. Specifically, the first ring portion 65 presses the peripheral portion (first surface 1a) of the first lens 1 that is not in contact with the second ring portion 66.

[0021] Each lens is fixed with its peripheral edge 1c, 2c, or 3c in contact with the lens barrel 6 and / or at least one of the ring portions 65, 66, or 67. The peripheral edge is a region including the flange, and includes the first surfaces 1a, 2a, and 3a, the second surfaces 1b, 2b, and 3b, and the lens end faces. The peripheral edge may also include a portion of the optical surface. In this embodiment, the third lens 3 is fixed with its second surface 3b engaged with the edge 63a of the lens barrel 6 and its first surface 3a in contact with the third ring portion 67. The second lens 2 is fixed with its second surface 2b in contact with the third ring portion 67 and its first surface 2a in contact with the second ring portion 66. The first lens 1 is fixed with its second surface 1b in contact with the second ring portion 66 and its first surface 1a pressed by the first ring portion 65. That is, each lens of the lens unit 4 is housed and fixed in the lens barrel 6 by being pressed by the first ring portion 65 from the first lens 1 side.

[0022] The first and second surfaces are, for example, within 10 mm, 8 mm, 5 mm, 3 mm, or 2 mm from the end face (side face) of the lens. The lower limit is, for example, 0.5 mm or more, or 1 mm or more.

[0023] The peripheral portions 1c, 2c, and 3c have regions (non-adhesion regions) where no coating film is attached in the portions that contact at least one of the lens barrel 6 and / or each ring portion. For example, the non-adhesion region is provided on at least one of the first surfaces 1a, 2a, and 3a and the second surfaces 1b, 2b, and 3b.

[0024] However, if the lens has coating film misalignment, uneven thickness, or smearing, the lens may tilt relative to the lens barrel 6 when it is housed and secured in the lens barrel 6. Such tilt can result in tilt errors due to the coating film. In particular, lens units used in finite-field spectral characteristic measurement devices, where the measurement target is located at a close distance, are more susceptible to tilt errors affecting their optical characteristics than lens units used in infinite-field spectral characteristic measurement devices, where the measurement target is located at a distant distance. Meanwhile, the lens unit 4 according to one embodiment of the present invention has a non-adhesive area on its periphery where no coating film is attached. Direct contact between this non-adhesive area and the lens barrel 6 and / or ring portion can suppress lens tilt due to the coating film. In other words, one embodiment of the present invention makes it easier to suppress tilt errors due to the coating film.

[0025] It is preferable to have a non-adhesive area on the periphery of the lens surface that comes into contact with (is locked to) the lens barrel 6. In this embodiment, the periphery of the lens surface that is locked to the lens barrel 6 is the second surface 3b. This makes it easier to suppress the occurrence of tilt errors.

[0026] It is particularly preferable that each of the first surfaces 1a, 2a, 3a and the second surfaces 1b, 2b, 3b has a non-adhesion area, which makes it easier to effectively suppress the occurrence of tilt errors.

[0027] The thickness of the coating film is preferably 1 μm or more. The thicker the coating film, the more likely tilt errors occur, so the above configuration can effectively suppress the occurrence of tilt errors. The thickness of the coating film is preferably 5 μm or less.

[0028] The outer diameter of the lens is preferably 10 mm or more and 100 mm or less. Lenses with such outer diameters are more susceptible to tilt errors affecting their optical characteristics, making them more likely to benefit from the effects of the present invention. With lenses with an outer diameter of less than 10 mm, it is difficult to ensure a non-adhesive area while maintaining the effective optical diameter. With lenses with an outer diameter of more than 100 mm, the non-adhesive area reduces the tilt error suppression effect.

[0029] It is preferable that a non-adhesive region is formed around the entire periphery of the lens. Here, "a non-adhesive region is formed around the entire periphery" means that the non-adhesive region is continuous around the periphery. This configuration effectively suppresses tilt errors when attaching the lens.

[0030] The non-adhesion region is preferably 1 mm or more, more preferably 1.5 mm or more in the radial direction from the end face (side face) of the lens. By having the non-adhesion region in this region, the occurrence of tilt errors can be effectively suppressed.

[0031] The flange of the lens preferably has a radial width of 2 mm or more, more preferably 5 mm or more. This configuration makes it easier to fix the lens to the lens barrel 6 and / or ring, and helps to prevent tilt errors during installation. The upper limit can be, for example, 10 mm or less, or 9 mm or less.

[0032] [Telescope barrel material] The material of the lens barrel 6 is preferably an aluminum alloy, such as A5052 or A5056. Such aluminum alloys may also be subjected to a satin finish (roughening treatment). Furthermore, they may be used in a black anodized state. The material of the lens barrel 6 is not limited to an aluminum alloy, and may be, for example, SUS304 (austenitic stainless steel), with a surface layer of black trivalent chromium plating.

[0033] [Lens optical axis thickness and spacing] In this embodiment, the effective diameter of the first lens 1 is larger than the effective diameter of the third lens 3. In addition, the optical axial thickness t3 of the third lens 3 is larger than the optical axial thickness of both the first lens 1 and the second lens 2. With these configurations, when the lens unit 4 is used as an imaging lens, good resolution is achieved.

[0034] The optical axis thickness t3 of the third lens 3 is preferably 0.5 to 2 times the second distance d2, which is the distance on the optical axis (the distance between the opposing surfaces) between the second lens 2 and the third lens 3. In this case, the MTF in the tangential and sagittal directions at a spatial frequency of 41.7 cycles / mm of the optical system 100 is high.

[0035] That is, the lens unit 4 has a good resolution that can be used with an image sensor in the infrared region that has a pixel pitch on the order of the wavelength. A high MTF at the above-mentioned spatial frequency means that the lens unit 4 has a good resolution that can be used with an image sensor with a narrow pitch on the order of the wavelength.

[0036] It is preferable that the first distance d1, which is the distance on the optical axis between the first lens 1 and the second lens 2, is smaller than the second distance d2. In this case, the NA (numerical aperture) on the image side becomes larger, and the RMS radius of the collected light on the image plane can be reduced. It is also preferable that the thickness of the third ring portion 67 is larger than the thickness of the second ring portion 66. In this case, tilt errors are less likely to occur when assembling the lenses into the lens barrel 6.

[0037] The ratio of the second distance d2 to the first distance d1 is preferably 9 or less.

[0038] [Lens material] Examples of lens materials include germanium (Ge), silicon (Si), chalcogenide glass, zinc selenide (ZnSe), and zinc sulfide (ZnS). The chalcogenide glass preferably contains, in mole percent, 20 to 90% tellurium (Te) and at least one of 0 to 50% germanium (Ge) and 0 to 50% gallium (Ga). The Te content is preferably 30 to 88%, 40 to 84%, 50 to 82%, or particularly 60 to 80%. The chalcogenide glass exhibits extremely low optical absorption over a wide wavelength range in the infrared region, such as wavelengths from 7 to 14 μm, and tends to have good internal transmittance at least in this wavelength range. Internal transmittance refers to the transmittance within the material and does not include reflection losses at the material surface. Specifically, the internal transmittance at a thickness of 2 mm can be 90% or more, particularly 95% or more, at a wavelength of 10 μm. Such chalcogenide glass was developed by the present applicant (see International Publication WO2020 / 105719A1).

[0039] The chalcogenide glass preferably has an Abbe number at a wavelength of 10 μm of 100 or more, 150 or more, and particularly 200 or more. The Abbe number (ν10) is calculated using the following formula: This makes it possible to suppress chromatic aberration. ν10 = (Refractive index at wavelength 10 μm - 1) / (Refractive index at wavelength 8 μm - Refractive index at wavelength 12 μm)

[0040] The refractive index of the chalcogenide glass at a wavelength of 10 μm is preferably 2.5 to 4.0, 2.74 to 3.92, 2.8 to 3.8, and particularly preferably 2.9 to 3.7. A low refractive index requires a smaller radius of curvature for the lens than a material with a high refractive index, which increases the difficulty of lens processing. In addition, the thickness in the optical axis direction may increase, which may impair optical freedom.

[0041] It is preferable that the chalcogenide glass does not contain toxic substances such as As, Se, Tl, etc. This reduces the environmental impact.

[0042] The third lens 3 is preferably made of a material whose internal transmittance at a wavelength of 10 μm is equal to or higher than that of the material making up the second lens 2 and equal to or higher than that of the material making up the first lens 1. This configuration improves the resolution of the image formed by the optical system 100. For example, the internal transmittance of the material making up the third lens 3 at a thickness of 2 mm at a wavelength of 10 μm is preferably 90% or higher, and particularly 95% or higher. For example, the third lens 3 is preferably made of the chalcogenide glass described above.

[0043] The third lens 3 is preferably made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. With the above configuration, the resolution of the image formed by the optical system 100 is improved.

[0044] The second lens 2 is preferably made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. With the above configuration, the resolution of the image formed by the optical system 100 becomes better.

[0045] The first lens 1 is preferably made of germanium, which improves the durability and hardness of the first lens 1.

[0046] [Lens shape] It is preferable that each lens in the lens unit 4 has the following configuration. It is preferable that the first lens 1 has positive power and has a meniscus shape with a concave surface facing the second lens 2. It is preferable that the second lens 2 has negative power. It is preferable that the third lens 3 has positive power and has a meniscus shape with a convex surface facing the second lens 2. These configurations make it possible to make the lens unit compact.

[0047] Aperture The first lens 1 may have an aperture 8 (see FIG. 1) on the opposite side of the second lens 2 in the optical axis direction. The ratio of the diameter of the aperture 8 to the effective diameter of the image is preferably 3 to 4.5. With this configuration, when the lens unit 4 is an imaging lens (first lens unit 4), the NA on the image side is large. When the lens unit 4 is an objective lens (second lens unit 5), the NA on the object side is large.

[0048] The first lens 1 has at least one spherical surface. On the surface Since the first lens 1 is likely to be disposed in a position where positional deviation is likely to occur, the above configuration makes it easier to suppress the occurrence of tilt errors.

[0049] It is preferable that at least one optical surface of the second lens 2 and / or the third lens 3 is aspherical. Specifically, it is preferable that at least one of the optical surfaces on the second surface 2b, 3b side is aspherical. The optical surfaces on the second surface 2b, 3b side are in a position where misalignment is least likely to occur, making tilt errors less likely to occur. Furthermore, the non-adhesive areas on the second surfaces 2b, 3b make tilt errors even less likely to occur. Therefore, even if these optical surfaces are aspherical, degradation of optical characteristics due to tilt errors is less likely to occur. Note that the optical surfaces on the first surface 2a, 3a side may also be aspherical.

[0050] If the second lens 2 has an aspherical surface, the aspherical surface may have a diffractive surface. In this case, chromatic aberration can be easily reduced. The unevenness of the diffractive surface preferably has a step height of 1 μm to 10 μm. In this case, chromatic aberration can be more easily suppressed. Note that if an optical surface other than the second lens 2 is aspherical, the aspherical surface may include a diffractive surface.

[0051] [NA] When the lens unit 4 is an imaging lens, it is preferable that the NA on the image side is equal to or greater than 0.4. Since the NA on the image side is directly linked to the resolution, the resolution of the optical system 100 is improved.

[0052] <Modification of lens unit> The following describes a modified lens unit 12. Figure 5 is a cross-sectional view showing a modified lens unit 12 housed in and fixed to the lens barrel 7. Lens unit 12 includes a first lens 1, a second lens 2, a third lens 3, the lens barrel 7, a first ring portion 75, and a second ring portion 76.

[0053] The lens barrel 7 has a first hole 71, a second hole 72, a third hole 73, and a fourth hole 74. The first hole 71 is connected to the second hole 72 and has a larger diameter than the second hole 72. The first hole 71 has a locking portion 71a. The second hole 72 is a tapered hole that connects the first hole 71 and the third hole 73. The third hole 73 is connected to the fourth hole 74 and has a smaller diameter than the fourth hole 74. The third hole 73 has a locking portion 73a.

[0054] A second ring portion 76 is fitted into the fourth hole 74. A third lens 3 is fitted into the third hole 73. The second lens 2, the first lens 1, and the first ring portion 75 are fitted into the first hole 71, in that order from the third lens 3 side. Each ring portion 75, 76 positions each lens at a required position in the lens barrel 7 or fixes it at a required position on the optical axis of the lens barrel 7. Specifically, the first ring portion 75 presses against the peripheral portion (first surface 1a) of the first lens 1 that is not in contact with the second lens 2. The second ring portion 76 presses against the peripheral portion (second surface 3b) of the third lens 3 that is not in contact with the lens barrel 7.

[0055] Each lens is fixed with its peripheral edge 1c, 2c, 3c in contact with at least one of the lens barrel 7 and / or each ring portion 75, 76. In this modified example, the third lens 3 is fixed with its second surface 3b pressed by the second ring portion 76 and its first surface 3a locked by the locking portion 73a. The second lens 2 is fixed with its second surface 2b locked by the locking portion 71a and its first surface 2a in contact with the first lens 1. The first lens 1 is fixed with its second surface 1b in contact with the second lens 2 and its first surface 1a pressed by the first ring portion 75. In other words, each lens of the lens unit 12 is housed in and fixed to the lens barrel 7 by being pressed by the first ring portion 75 from the first lens 1 side and by being pressed by the second ring portion 76 from the third lens 3 side.

[0056] The peripheral edge of each lens has a non-adhesive area in a portion that contacts at least one of the lens barrel 7 and each ring portion. Specifically, the non-adhesive area is provided on at least one of the first surfaces 1a, 2a, 3a and the second surfaces 1b, 2b, 3b of each lens.

[0057] This modification also has a non-adhesive area on the periphery where no coating film is attached. Direct contact between this non-adhesive area and the lens barrel 7 and / or ring portion can suppress tilt of the lens caused by the coating film. In other words, this modification makes it easier to suppress tilt errors caused by the coating film.

[0058] In this modification, it is also preferable to have a non-adhesive area at least on the periphery of the lens surface (pressed surface) that is locked to the lens barrel 7, and it is more preferable to have a non-sealed area on the periphery of all pressed surfaces. In this modification, the periphery of the lens surface that is locked to the lens barrel 7 is the first surface 3a and the second surface 2b. This makes it easier to suppress the occurrence of tilt errors.

[0059] It is particularly preferable that each of the first surfaces 1a, 2a, 3a and the second surfaces 1b, 2b, 3b has a non-adhesion area, which makes it easier to effectively suppress the occurrence of tilt errors.

[0060] The optical surface on the side of the surface having the non-adhesive region may be aspherical. As described above, the occurrence of tilt errors is suppressed by having a non-adhesive region, so even if the optical surface is aspherical, the decrease in MTF due to the occurrence of tilt errors can be suppressed and the RMS radius of light collection on the image plane can be reduced.

[0061] <Spectral characteristics device> 6 is a cross-sectional view showing a spectroscopic characteristic measurement device 200 to which a lens unit according to an embodiment is applied. The spectroscopic characteristic measurement device 200 includes a lens unit 4, a lens unit 5, a sample support plate 10, a detection unit 11, and a phase shifter 9. The configurations of the lens unit 4 and the lens unit 5 of the spectroscopic characteristic measurement device 200 are similar to the configurations of the lens unit 4 and the lens unit 5 of the optical system 100.

[0062] In the spectroscopic characteristic measuring apparatus 200, the third lens 3 of the lens unit 4 is arranged to face the sample support plate 10, and the third lens 3 of the lens unit 5 is arranged to face the detection unit 11. A phase shifter 9 is arranged between the lens unit 4 and the lens unit 5. The optical axis of the lens unit 4 and the optical axis of the lens unit 5 are perpendicular to each other at the phase shifter 9. In the spectroscopic characteristic measuring apparatus 200, the optical axis is bent vertically by the reflective phase shifter 9, but the basic optical configuration is the same as that of the above-mentioned transmissive optical system 100. The phase shifter 9 is arranged near the position of the aperture 8 of the optical system 100. In other words, the lens unit 4 and the lens unit 5 are arranged symmetrically with respect to the phase shifter 9.

[0063] In this embodiment, the spectroscopic characteristic measurement apparatus 200 uses a reflective phase shifter 9. The phase shifter 9 includes a fixed mirror portion 91, a movable mirror portion 92, and a driver 93. The fixed mirror portion 91 and the movable mirror portion 92 are arranged side by side in a direction perpendicular to the plane of FIG. 6 (the x-axis direction), with the movable mirror portion 92 positioned further back in the x-axis direction than the fixed mirror portion 91. The fixed mirror portion 91 and the movable mirror portion 92 are arranged so as to be tilted at an angle of α degrees (approximately 45 degrees) with respect to the optical axis of the lens unit 4. The fixed mirror portion 91 and the movable mirror portion 92 are arranged so as to be tilted at an angle of β degrees (approximately 45 degrees) with respect to the optical axis of the lens unit 5. The movable mirror portion 92 is configured to be movable in a direction perpendicular to the surface of the movable mirror portion 92. This generates a phase difference between the first light beam reflected by the fixed mirror portion 91 and the second light beam reflected by the movable mirror portion 92. The phase shifter 9 is not limited to a reflective type, and a transmissive phase shifter may also be used.

[0064] With a sample (not shown) supported on a sample support plate 10, infrared light is irradiated onto the sample from a light source (not shown). The infrared light is scattered by various components of the sample, and the scattered light is incident on the third lens 3 of the lens unit 4. The scattered light is collimated by the lens unit 4 and reaches the fixed mirror portion 91 and the movable mirror portion 92 of the phase shifter 9. A portion of the light is reflected by the fixed mirror portion 91 as a first light beam, and the remaining light is reflected by the movable mirror portion 92 as a second light beam, both of which are incident on the first lens 1 of the lens unit 5. The first and second light beams incident on the lens unit 5 form an image on the light-receiving surface of the detection unit 11, and an interferogram (a composite waveform spectrum of changes in image intensity (changes in interference light intensity)) is formed.

[0065] By moving the movable mirror unit 92 and imparting a phase difference between the first light beam and the second light beam, an interferogram waveform is obtained. By performing a Fourier transform on the interferogram, the spectral characteristics of the sample are obtained. Since the spectroscopic characteristic measuring device 200 includes a lens unit according to one embodiment of the present invention, tilt errors that occur when attaching the lens can be suppressed, the resolution of the image formed on the detection unit 11 is improved, and the spectral characteristics of the sample can be obtained.

[0066] <Summary> A first aspect of the present invention provides a lens unit for use in an infrared region that includes at least one wavelength in the 7 to 14 μm range, comprising one or more lenses having a coating film formed on their surfaces, a lens barrel having holes into which the lenses are fitted, and a ring portion that contacts the periphery of the lenses and positions the lenses fitted in the holes at a desired position on the optical axis of the lens barrel or fixes them at a desired position on the optical axis of the lens barrel, and the lenses have an area on the periphery that contacts at least one of the lens barrel and the ring portion where the coating film is not attached. This configuration reduces tilt errors caused by the coating film.

[0067] In a second aspect of the present invention, the coating film has a thickness of 1 μm or more. When the coating film has a thickness of 1 μm or more, tilt errors are likely to occur due to the coating film, and therefore, by providing an area on the lens where no coating film is attached, the occurrence of tilt errors can be effectively suppressed.

[0068] In a third aspect of the present invention, the coating film is made of at least one material selected from the group consisting of Ge, Si, fluoride, ZnSe, ZnS, and diamond-like carbon.

[0069] In a fourth aspect of the present invention, the outer diameter of the lens is 10 mm or more and 100 mm or less. Lenses with this outer diameter are more likely to be affected by tilt errors caused by coatings and therefore more likely to enjoy the effects of the present invention with the above configuration.

[0070] According to a fifth aspect of the present invention, the peripheral portion of the lens surface that contacts the lens barrel has an area where the coating film is not attached. This configuration makes it easier to suppress the occurrence of tilt errors.

[0071] According to a sixth aspect of the present invention, the lens has an area where the coating film is not attached over the entire periphery of the lens. According to the above configuration, tilt errors occurring when the lens is attached can be effectively suppressed.

[0072] In a seventh aspect of the present invention, the coating film is not attached to an area of 1 mm or more in the radial direction from the end face of the lens. Since the end face side of the peripheral edge is prone to movement during installation, this configuration effectively suppresses the occurrence of tilt errors.

[0073] In an eighth aspect of the present invention, the lens comprises a first lens, a second lens, and a third lens, the first lens, the second lens, and the third lens are housed and fixed in the lens barrel in that order, the effective diameter of the first lens is larger than the effective diameter of the third lens, and the optical axial thickness of the third lens is larger than the optical axial thickness of either the first lens or the second lens. With this configuration, tilt error during lens installation can be suppressed, the resolution of the image formed on the detection unit is improved, and the spectral characteristics of the sample can be obtained.

[0074] According to a ninth aspect of the present invention, the optical axial thickness of the third lens is 0.5 to 2 times the second distance, which is the distance on the optical axis between the second lens and the third lens. With this configuration, the NA on the image side is increased, and the RMS radius of the collected light can be reduced. Also, chromatic aberration can be reduced.

[0075] In a tenth aspect of the present invention, a first distance, which is the distance on the optical axis between the first lens and the second lens, is shorter than a second distance, which is the distance on the optical axis between the second lens and the third lens. With this configuration, the NA on the image side is increased, and the RMS radius of the collected light can be reduced.

[0076] In aspect 11 of the present invention, the lens barrel has a first hole, a second hole, and a third hole as the holes, and a first ring portion, a second ring portion, and a third ring portion as the ring portions, the third lens is fitted into the third hole, the second hole is connected to the third hole and has a larger diameter than the third hole, the second lens is fitted with the third ring portion interposed between it and the third lens, and the first lens is fitted with the second ring portion interposed between it and the second lens, the first hole is connected to the second hole and has a larger diameter than the second hole, and the first ring portion is fitted to press against the peripheral portion of the surface of the first lens that is not in contact with the second ring portion, and at least the peripheral portion of the surface of the third lens that is not in contact with the third ring portion, the peripheral portion of the surface of the second lens that is in contact with the third ring portion, and the peripheral portion of the surface of the first lens that is in contact with the second ring portion have areas where the coating film is not attached. The above configuration can reduce the occurrence of tilt errors during installation.

[0077] In a twelfth aspect of the present invention, the lens barrel has a first hole, a second hole, a third hole, and a fourth hole as the holes, and a first ring portion and a second ring portion as the ring portion, the first hole and the third hole have locking portions that lock the peripheral portions of the lenses, the fourth hole is connected to the third hole and has a larger diameter than the third hole, the second ring portion that presses the third lens is fitted in a state where the peripheral portion of the third lens is locked in the locking portions, the second hole connects the first hole and the third hole, the first hole is connected to the second hole and has a larger diameter than the second hole, the second lens, the first lens, and the first ring portion are fitted in this order from the second hole side, and at least the peripheral portion of the third lens that contacts the locking portions and the peripheral portion of the second lens that is not in contact with the first lens have areas where the coating film is not attached. With the above configuration, it is possible to reduce the occurrence of tilt errors during installation.

[0078] In a thirteenth aspect of the present invention, the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. According to the above configuration, the third lens has high transmittance.

[0079] In a fourteenth aspect of the present invention, the chalcogenide glass contains, in mol %, 20% to 80% of Te.

[0080] [Additional notes] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0081] 1 First lens 2 Second lens 3 Third lens 1a, 2a, 3a 1st page 1b, 2b, 3b 2nd side 1c, 2c, 3c Periphery 4 First lens unit 5 Second lens unit 6, 7 Telescope tube 61, 71 Hole 1 62, 72 2nd hole 63, 73 3rd hole 63a Edge 74 Hole 4 65, 75 1st Ring 66, 76 Second Ring Section 67 Third Ring 71a, 73a Locking part 8 aperture 9 Phase Shifter 91 Fixed mirror part 92 Movable mirror part 93 Drive unit 10 Sample support plate 11 Detection unit 12 Lens unit 100 Optical System 200 Spectral property measuring device S image plane T object surface d1 First distance d2 2nd distance t3 Optical axis thickness of the third lens

Claims

1. An optical system for use in an infrared region including at least any wavelength in the range of 7 to 14 μm, It has two lens units arranged symmetrically around the aperture, Each of the lens units comprises: one or more lenses having a coating film formed on a surface thereof; a lens barrel having a hole into which the lens is fitted; a ring portion that is a member that contacts the peripheral edge of the lens and that positions the lens that is fitted into the hole at a required position on the optical axis of the lens barrel, or fixes the lens at a required position on the optical axis of the lens barrel; the lens comprises a first lens, a second lens, and a third lens; the first lens, the second lens, and the third lens are accommodated and fixed in the lens barrel in this order from the aperture stop side, the effective diameter of the first lens is larger than the effective diameter of the third lens; an optical axis thickness of the third lens is larger than an optical axis thickness of either the first lens or the second lens; An optical system, wherein the lens has an area where the coating film is not attached at a portion of the peripheral portion that contacts at least one of the lens barrel and the ring portion.

2. The optical system according to claim 1 , wherein the coating film has a thickness of 1 μm or more.

3. 3. The optical system according to claim 1, wherein the coating film is made of at least one material selected from the group consisting of Ge, Si, fluoride, ZnSe, ZnS, and diamond-like carbon.

4. 4. The optical system according to claim 1, wherein the outer diameter of the lens is between 10 mm and 100 mm.

5. 5. The optical system according to claim 1, wherein the peripheral portion of the lens surface in contact with the lens barrel has an area where the coating film is not attached.

6. 6. The optical system according to claim 1, wherein the lens has an area where the coating film is not attached over the entire periphery of the lens.

7. 7. The optical system according to claim 1, wherein the coating film is not attached to an area of 1 mm or more in a radial direction from the end face of the lens.

8. The optical system according to any one of claims 1 to 7, wherein the optical axis thickness of the third lens is 0.5 to 2 times the second distance, which is the distance on the optical axis between the second lens and the third lens.

9. 9. The optical system of claim 1, wherein a first distance, which is a distance on an optical axis between the first lens and the second lens, is smaller than a second distance, which is a distance on an optical axis between the second lens and the third lens.

10. the lens barrel has a first hole, a second hole, and a third hole as the holes, The ring portion includes a first ring portion, a second ring portion, and a third ring portion, the third lens is fitted into the third hole; the second hole is connected to the third hole and has a larger diameter than the third hole, the second lens is fitted between the second hole and the third lens with the third ring portion interposed therebetween, and the first lens is fitted between the second hole and the third lens with the second ring portion interposed therebetween, the first hole is connected to the second hole, has a diameter larger than that of the second hole, and has the first ring portion fitted therein to press against a peripheral edge portion of a surface of the first lens that is not in contact with the second ring portion; 10. The optical system of claim 1, wherein the coating film has an area not adhered to at least the peripheral portion of the surface of the third lens that does not contact the third ring portion, the peripheral portion of the surface of the second lens that contacts the third ring portion, and the peripheral portion of the surface of the first lens that contacts the second ring portion.

11. the lens barrel has a first hole, a second hole, a third hole, and a fourth hole as the holes, The ring portion includes a first ring portion and a second ring portion, the first hole and the third hole have a locking portion that locks a peripheral edge of a lens; the fourth hole is connected to the third hole and has a diameter larger than that of the third hole, and the second ring portion that presses the third lens is fitted into the fourth hole with the peripheral edge of the third lens locked in the locking portion; the second hole connects the first hole and the third hole; the first hole is connected to the second hole and has a larger diameter than the second hole, and the second lens, the first lens, and a first ring portion are fitted into the first hole in this order from the second hole side; 10. The optical system according to claim 1, wherein at least the peripheral portion of the third lens that contacts the engaging portion and the peripheral portion of the surface of the second lens that does not contact the first lens have an area where the coating film is not attached.

12. 12. The optical system according to claim 1, wherein the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm.

13. 13. The optical system according to claim 12, wherein the chalcogenide glass contains, in mole percent, 20% to 80% Te.

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