Infrared imaging lens

Abstract

To provide a telephoto lens with superior resolution.SOLUTION: An infrared imaging lens (1) comprises a first group (G1) consisting of multiple lenses, and a second group (G2) consisting of multiple lenses, each lens being made of chalcogenide glass having a refractive index of 2.5-4.0 for a wavelength of 10 μm. Each of the first group and the second group includes lenses made of the chalcogenide glass having different Abbe numbers. A focal length of the entire system is five times an image circle diameter or greater.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an infrared imaging lens.

Background Art

[0002] Infrared cameras that photograph subjects with infrared rays in the far-infrared region, particularly in the wavelength range of 10 μm suitable for biological detection, are applied to surveillance cameras, security cameras, in-vehicle night vision, etc. Specifically, these infrared cameras can be applied to various fields such as intrusion monitoring of facilities, poacher monitoring, traffic network monitoring, obstacle monitoring on the road, forest fire source detection, and maritime monitoring, and an expansion in demand is expected. Infrared imaging lenses applied to such infrared cameras are known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For applications typified by long-distance monitoring at night, so-called telephoto lenses with a relatively long focal length are required. For example, imaging lenses with a very long focal length such that the half angle of view is about 5 degrees or less are also required depending on the installation location. There is a demand for realizing an infrared imaging lens that has excellent resolution capable of corresponding to a small image sensor having a pixel pitch on the order of the wavelength, has a wide wavelength band, and is low-cost and usable as a consumer product.

[0005] One aspect of the present invention focuses on the above problems, and an object thereof is to realize an infrared imaging lens that is a telephoto lens having excellent resolution capable of corresponding to an image sensor having a pixel pitch on the order of the wavelength and is usable for consumer applications.

Means for Solving the Problems

[0006] To solve the above problems, one aspect of the present disclosure is an infrared imaging lens used in the infrared region including at least one wavelength in the range of 8 to 14 μm, wherein a first group consisting of a plurality of lenses and a second group consisting of a plurality of lenses are arranged sequentially from the object side toward the image plane side, each of the lenses constituting the first group and the second group is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and each of the first group and the second group includes lenses made of the chalcogenide glass having different Abbe numbers defined in a predetermined wavelength range in the range of 8 to 14 μm, and the total system focal length is 5 times or more the diameter of the image circle.

[0007] Furthermore, in order to solve the above problems, another aspect of the present disclosure is an infrared imaging lens used in the infrared region including at least one wavelength in the range of 8 to 14 μm, wherein a first group consisting of a plurality of lenses and a second group consisting of a plurality of lenses are arranged in order from the object side toward the image plane side, each of the lenses constituting the first group and the second group is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and each of the first group and the second group includes lenses made of the chalcogenide glass having different Abbe numbers defined in a predetermined wavelength range in the range of 8 to 14 μm, and the half-angle of view is 5° or less. [Effects of the Invention]

[0008] According to the above embodiment of the present invention, an infrared imaging lens can be realized that is a telephoto lens usable for consumer applications, which can be used with an image sensor having a pixel pitch on the order of wavelength, and has excellent resolution. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view showing the configuration of the main part of an infrared imaging lens according to an embodiment of the present invention. [Figure 2]This is an aberration diagram showing spherical aberration, astigmatism, and distortion of an infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 3] This is an aberration diagram showing the coma aberration of an infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 4] This graph shows the image height dependence of the relative intensity of the infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 5] This graph shows the spatial frequency dependence of the MTF of the infrared imaging lens according to Numerical Example 1 of the present invention in the wavelength range of 8 to 14 μm. [Figure 6] This graph shows the spatial frequency dependence of the MTF of the infrared imaging lens according to Numerical Example 1 of the present invention in the wavelength range of 8 to 14 μm. [Figure 7] This graph shows the focus shift dependence of the MTF of an infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 8] This graph shows the effect of temperature on the focus shift dependence of the MTF of an infrared imaging lens according to numerical example 1 of the present invention. [Modes for carrying out the invention]

[0010] [Embodiment] <Overview of infrared imaging lenses> The infrared imaging lens 1 according to this embodiment is a lens system that forms an image of a subject on the image plane S of an image sensor or the like, which corresponds to the infrared region that includes at least one wavelength within the range of 8 to 14 μm. This wavelength region is also referred to as the far-infrared wavelength region.

[0011] The infrared imaging lens 1 according to the present embodiment targets a telephoto lens such that the overall focal length fL is 5 times or more the diameter φs of the image circle. That is, the infrared imaging lens 1 according to the present embodiment targets an infrared imaging lens that enables an imaging device capable of magnifying and observing a distant object by including the infrared imaging lens 1. Note that the diameter of the image circle is designed to be approximately equal to or slightly larger than the diagonal length of the image sensor to be used. Specifically, the diameter of the image circle is designed to be 1 to 1.1 times the diagonal length of the image sensor to be used.

[0012] FIG. 1 is a cross-sectional view along the optical axis showing the configuration of the main part of the infrared imaging lens 1. FIG. 1 shows the configuration and light beam of the infrared imaging lens 1 in a state focused on an object at an infinite distance. The infrared imaging lens 1 is configured by arranging a first lens group G1, which is a lens group composed of a plurality of lenses, and a second lens group G2, which is a lens group composed of a plurality of lenses, in order from the object side toward the image plane S side.

[0013] Each of the lenses constituting the first lens group G1 and the second lens group G2 is made of chalcogenide glass having a refractive index N10 at a wavelength of 10 μm of 2.5 to 4.0. Here, the symbol N10 represents that it is specifically the refractive index at a wavelength of 10 μm. Also, each of the first lens group G1 and the second lens group G2 includes a lens made of the above chalcogenide glass having different Abbe numbers defined in a predetermined wavelength range within 8 to 14 μm. During focusing, the lenses constituting the first lens group G1 and the second lens group G2 move uniformly in the optical axis direction.

[0014] Each of the first lens group G1 and the second lens group G2 includes a lens having a positive power (refractive power) (convex lens) and a lens having a negative power (refractive power) (concave lens), and is configured to have a positive power. More specifically, the first lens group G1 is composed of a first lens L1 having a positive power and a second lens L2 having a negative power in order from the object side toward the image plane S side. The second lens group G2 is composed of a third lens L3 having a positive power and a fourth lens L4 having a negative power in order from the object side toward the image plane S side.

[0015] As shown in FIG. 1, a parallel plate P is disposed between the fourth lens L4 and the image plane S. The parallel plate P is an optical window hermetically sealed to an image sensor, and silicon, low-oxygen silicon, or germanium is used.

[0016] As indicated by symbol AP in FIG. 1, the effective diameter of the object-side surface (the first surface) of the first lens L1 corresponds to the aperture stop of the infrared imaging lens 1. Anti-reflection (AR) coatings are applied to the surfaces of the first lens L1, the second lens L2, the third lens L3, the fourth lens L4, and the parallel plate P. Appropriate known techniques can be applied to such anti-reflection coatings in the far-infrared region.

[0017] <Details of the configuration of each lens> The infrared imaging lens 1 of the present embodiment is an infrared imaging lens that can be used in an infrared region including at least any wavelength within the range of 8 to 14 μm. By applying chalcogenide glass having a refractive index N10 of 2.5 to 4.0 at a wavelength of 10 μm as the lens material for each lens, excellent characteristics can be obtained over a wide wavelength range of 8 to 14 μm. Details of the chalcogenide glass applied to the infrared imaging lens 1 will be described later.

[0018] In order to realize an imaging device capable of magnifying and observing a distant object, the infrared imaging lens 1 of the present embodiment is configured such that the overall focal length fL is 5 times or more the diameter φs of the image circle. This may be defined as the semi-field angle being 5° or less. In particular, in the infrared imaging lens 1 of the present embodiment, it is preferable that the overall focal length fL is configured within the range of 5 to 10 times the diameter φs of the image circle. The value of the overall focal length fL is preferably 20 to 100 mm.

[0019] According to this embodiment, a bright imaging lens can be realized in such a telephoto lens, with an F-number in the range of 0.9 to 1.1. Furthermore, it is possible to realize an imaging lens in which light absorption by the lens glass material is small over a wide wavelength range of at least 8 to 14 μm. Therefore, combined with a small F-number of about 1, a bright telephoto imaging lens can be realized.

[0020] Furthermore, the infrared imaging lens 1 of this embodiment can be configured in detail as follows. In the infrared imaging lens 1, the focal length of the first group fG1 The focal length of the second group is fG2 It is desirable to reduce this. By configuring the infrared imaging lens 1 in this way, a high resolution can be obtained that is compatible with image sensors having a pixel pitch on the order of wavelength, while maintaining good aberration characteristics.

[0021] In particular, the focal length fG1 of the first group and the total focal length fL are, 2.0 ≤ fG1 / fL ≤ 3.0 The relationship satisfies the following conditions: the focal length fG2 of the second group and the total focal length fL are equal to, 0 <fG2 / fL≦1.0 It is preferable that the configuration satisfies the following relationship:

[0022] Furthermore, it is desirable that each lens has the following configuration: The first lens L1 has positive power and a meniscus shape with its convex surface facing the object. The second lens L2 has negative power and a meniscus shape with its convex surface facing the object. The third lens L3 has positive power and a meniscus shape with its convex surface facing the object. The fourth lens L4 has negative power and a meniscus shape with its convex surface facing the object. By configuring and arranging each lens in this way, the increase in the Petzval sum can be suppressed, field curvature can be suppressed, and the flatness of the image plane can be maintained.

[0023] The focal length f1 of the first lens, the focal length f2 of the second lens, the focal length f3 of the third lens, and the focal length f4 of the fourth lens are 1.1 ≤ |f² / f⁻¹| ≤ 2.0 1.1 ≤ |f4 / f3| ≤ 2.0 It is preferable to configure the lens to satisfy the given relationship. By configuring it in this way, good achromatic characteristics can be obtained, and a telephoto lens with excellent resolution can be realized.

[0024] In particular, with respect to the Abbe number defined in a predetermined wavelength range within the range of 8 to 14 μm, it is preferable that the second lens L2 having negative power is smaller than the first lens L1 having positive power in the first group G1. More specifically, it is desirable that the second lens L2 having negative power has an Abbe number that is 5% or more smaller than that of the first lens L1 having positive power. Furthermore, with respect to the refractive index N10 at a wavelength of 10 μm, it is preferable that the second lens L2 having negative power is smaller than the first lens L1 having positive power.

[0025] Similarly, with respect to the Abbe number defined in a predetermined wavelength range within the range of 8 to 14 μm, it is preferable that the fourth lens L4, which has negative power, is smaller than the third lens L3, which has positive power, in the second group G2. More specifically, it is desirable that the fourth lens L4, which has negative power, has an Abbe number that is 5% or more smaller than that of the third lens L3, which has positive power. Furthermore, with respect to the refractive index N10 at a wavelength of 10 μm, it is preferable that the fourth lens L4, which has negative power, is smaller than the third lens L3, which has positive power. By configuring it in this way, particularly good achromatic characteristics can be obtained.

[0026] Here, the Abbe number defined in a predetermined wavelength range within the range of 8 to 14 μm is appropriately defined around 10 μm in the case of an infrared imaging lens 1 with a primary design wavelength of 10 μm. Thus, the predetermined wavelength range for defining the Abbe number should be selected according to the primary design wavelength to which the infrared imaging lens 1 is applied. The Abbe number ν10 defined with a predetermined wavelength range of 8 to 12 μm is one example. Details of the definition of the Abbe number ν10 are shown in Numerical Example 1 described later.

[0027] In the infrared imaging lens 1, it is preferable to use an aperture diaphragm as the effective diameter of the object-side surface (first surface) of the first lens L1. This configuration makes it possible to ensure a relative light intensity of nearly 100% throughout the entire image circle. Furthermore, it is possible to reduce the outer diameter and volume of the infrared imaging lens 1 compared to inserting an aperture diaphragm between the lenses.

[0028] The first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 may each be spherical lenses. This results in small variations in spherical aberration and astigmatism of the infrared imaging lens 1 with respect to image height at each wavelength. Therefore, an infrared imaging lens 1 with practical resolution characteristics and low image height dependence of the MTF (Modulation Transfer Function) can be realized. However, any of the lenses may be aspherical lenses, or aspherical lenses with diffractive surfaces may also be used.

[0029] The miniaturization of image sensors in the far-infrared region has progressed, and the pixel pitch has reached a narrow-pitch limit comparable to the wavelength. Such miniaturized image sensors can be produced at a lower cost compared to large-area sensors. Furthermore, the imaging lenses used in these sensors can also be made smaller in diameter to match the sensor's area, further reducing costs.

[0030] Therefore, by applying such image sensors and imaging lenses to infrared cameras, it is possible to achieve low costs suitable for consumer use, and infrared cameras can be deployed in various fields. As for narrow-pitch image sensors in the 10 μm wavelength range, those with a pixel pitch of 12 to 17 μm are commercially available. The infrared imaging lens 1 of this embodiment is a telephoto lens with a resolution that is sufficient to handle infrared cameras using image sensors with a pixel pitch of about 12 to 17 μm in the wavelength range of about 8 to 14 μm.

[0031] <Glass material of each lens> The chalcogenide glass components of each lens in the infrared imaging lens 1 are described below. The chalcogenide glass used in the infrared imaging lens 1 is chalcogenide glass with a refractive index of 2.5 to 4.0 at a wavelength of 10 μm.

[0032] Such chalcogenide glass with a high refractive index in the far-infrared region was developed by the present applicant (see International Publication WO2020 / 105719A1). More specifically, the refractive index N10 of this glass material at a wavelength of 10 μm is in the range of 2.74 to 3.92. For example, a refractive index N10 at a wavelength of 10 μm is preferably 2.74 to 3.92, 2.8 to 3.8, and particularly 2.9 to 3.7. If the refractive index N10 is too low, the focal length of the infrared imaging lens tends to become too long.

[0033] This glass material exhibits extremely low light absorption over a wide wavelength range in the far-infrared region, at least from 7 to 14 μm. In particular, this glass material is characterized by low light absorption even in the far-infrared region of 10 to 26 μm. In chalcogenide glass, "infrared absorption edge wavelength" and "internal transmittance" can be used as indicators to show excellent light transmittance in the far-infrared region.

[0034] Here, the infrared absorption edge wavelength refers to the absorption edge wavelength in the region of wavelengths 8 μm or greater, and is defined as the wavelength at which the light transmittance is 20% at a material thickness of 2 mm. Note that internal transmittance refers to the transmittance within the material and does not include reflection loss at the material surface. The chalcogenide glass used as the glass material for each lens has an infrared absorption edge wavelength of 18 μm or greater.

[0035] Therefore, the chalcogenide glass transmits infrared light with wavelengths exceeding 10 μm and exhibits good transmittance over a wavelength range of at least 7 to 14 μm. Furthermore, the internal transmittance of the chalcogenide glass with a thickness of 2 mm is 90% or more at a wavelength of 10 μm. Thus, the infrared imaging lens 1 of this embodiment makes it possible to realize an imaging lens with low light absorption by the lens glass material over a wide wavelength range of at least 7 to 14 μm.

[0036] Furthermore, using the above-mentioned chalcogenide glass facilitates mass production of lenses by press molding. Mass production of aspherical lenses is also possible through press molding. In this specification, aspherical includes diffracted surfaces. Preferably, the glass material has a low glass transition temperature of 200°C or less, making press molding easier. Crystalline materials such as silicon (Si), germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe), which are used as materials that transmit far-infrared regions, cannot be press molded. Therefore, there are limitations to the production efficiency of lenses.

[0037] In each group of infrared imaging lenses 1 of this embodiment, the chalcogenide glass suitable for constituting the lens with a larger refractive index N10 and Abbe number (hereinafter referred to as lens A) is preferably, for example, a chalcogenide glass with a refractive index N10 of 3.45 or higher and an Abbe number ν10 of 250 or higher. Specifically, it is preferably a chalcogenide glass containing 20-90% tellurium (Te) by mol%. More specifically, it is preferably a chalcogenide glass containing 20-90% Te and 0-50% Ge+Ga by mol%.

[0038] In this specification, unless otherwise specified, "%" means "mol%". In this specification, "A1+A2+..." means the total amount of each applicable component. However, this description means the content including at least one component selected from the group consisting of each applicable component, and the composition may not include a specific component from the above group. For example, if the composition is "A1+A2+A3+A4+A5 p~q% is preferred", the composition may be "A1+A2+A3+A4 p~q% (but A5 is not included)".

[0039] The following describes preferred compositions of chalcogenide glass suitable for lens A.

[0040] Te forms a glass skeleton and is a component that easily increases internal transmittance in the wavelength range of 10 μm or more. Te is also a component that easily increases the refractive index. The Te content is preferably 20-90%, 30-88%, 40-84%, 50-82%, and especially 60-80%. If the Te content is too low, vitrification becomes difficult. If the Te content is too high, Te-based crystals tend to precipitate. In addition, the other chalcogen elements Se and S tend to decrease internal transmittance in the wavelength range of 10 μm or more than Te. Therefore, the Se and S content is preferably 0-10%, 0-5%, 0-3%, and especially 0-1%, respectively.

[0041] A chalcogenide glass suitable for lens A preferably contains at least one of Ge and Ga in addition to Te. Specifically, the combined amount of Ge + Ga (Ge and Ga) is preferably 0-50%, 1-40%, 3-35%, 5-30%, and particularly 10-30%. Including these components broadens the vitrification range and enhances the thermal stability (vitrification stability) of the glass. The preferred ranges for each component of Ge and Ga are as follows.

[0042] Ge is a component that broadens the vitrification range and enhances the thermal stability of the glass. The preferred Ge content is 0-50%, 1-40%, 3-35%, 5-30%, 8-25%, and especially 10-20%. If the Ge content is too high, Ge-based crystals tend to precipitate easily, and raw material costs tend to increase.

[0043] Ga (Ga) is a component that broadens the vitrification range and enhances the thermal stability of the glass. The Ga content is preferably 0-50%, 1-30%, 2-20%, 3-15%, and especially 4-10%. If the Ga content is too high, Ga-based crystals tend to precipitate easily, and raw material costs tend to increase.

[0044] Furthermore, from the viewpoint of improving the stability of vitrification, it is preferable to have a high total content of Ge, Ga, and Te. Specifically, it is preferable that the Ge+Ga+Te content be 50% or more, 60% or more, 70% or more, and especially 80% or more. However, in order to introduce other components, the upper limit of Ge+Ga+Te may be 98% or less, 96% or less, 95% or less, and especially 90% or less.

[0045] In addition to the components listed above, chalcogenide glass suitable for lens A may also contain various other components as shown below.

[0046] Ag is a component that enhances the thermal stability and refractive index of glass. The preferred Ag content is 0-50%, over 0-50%, 1-45%, 2-40%, 3-35%, 4-30%, 5-25%, and particularly 5-20%. Too much Ag content makes vitrification difficult.

[0047] Si is a component that enhances the thermal stability of glass. The Si content is preferably 0-50%, greater than 0-50%, 1-45%, 2-40%, 3-35%, 4-30%, 5-25%, and particularly preferably 5-20%. If the Si content is too high, infrared absorption due to Si is more likely to occur, making it difficult for infrared rays to pass through. However, since Si is a component that tends to lower the Abbe number, from the viewpoint of increasing the Abbe number, the Si content is preferably 5% or less, 1% or less, 0.5% or less, and particularly preferably less than 0.1%.

[0048] Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn are components that enhance the thermal stability of glass without reducing its infrared transmission properties. The content of Al+Ti+Cu+In+Sn+Bi+Cr+Sb+Zn+Mn (the total amount of Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn) is preferably 0-40%, 2-35%, 4-30%, and especially 5-25%. If the content of Al+Ti+Cu+In+Sn+Bi+Cr+Sb+Zn+Mn is too high, vitrification becomes difficult.

[0049] Furthermore, the content of each component, Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn, is preferably 0-40%, 1-40%, 1-30%, 1-25%, and particularly 1-20%, respectively. Among these, Al, Cu, and / or Sn are preferred because they have a particularly large effect in improving the thermal stability of the glass. However, since Al and Sn are components that tend to lower the Abbe number, from the viewpoint of increasing the Abbe number, the content of Al and Sn is preferably 5% or less, 1% or less, 0.5%, and particularly less than 0.1%, respectively.

[0050] F, Cl, Br, and I are also components that enhance the thermal stability of glass. The content of F+Cl+Br+I (total amount of F, Cl, Br, and I) is preferably 0-40%, 2-35%, 4-30%, and particularly 5-25%. If the content of F+Cl+Br+I is too high, vitrification becomes difficult and weather resistance tends to decrease. The content of each component of F, Cl, Br, and I is preferably 0-40%, 1-40%, 1-30%, 1-25%, and particularly 1-20%, respectively. Among these, I is preferred because it can be used as an elemental raw material and has a particularly large effect in enhancing the thermal stability of glass.

[0051] Furthermore, from the viewpoint of particularly reducing the burden on the environment, it is especially preferable that the product is substantially free of Se and As. In this invention, "substantially free" means that the content is less than 0.1 mol%. It is also preferable that the product is substantially free of Cd, Tl, and Pb. In this way, the impact on the environment can be minimized.

[0052] Thus, the chalcogenide glass suitable for lens A has a larger refractive index N10 and a larger Abbe number compared to the chalcogenide glass suitable for lens B, which will be described later. In this invention, as a specific example of the above chalcogenide glass, chalcogenide glass CG1 is used, which has a refractive index N10 of 3.465 and an Abbe number ν10 of 253.

[0053] Furthermore, in each group of infrared imaging lenses 1 of this embodiment, the chalcogenide glass suitable for constituting the lens with the smaller refractive index N10 and Abbe number (hereinafter referred to as lens B) is preferably, for example, a chalcogenide glass with a refractive index N10 of less than 3.45 and an Abbe number ν10 of less than 250. Specifically, it is preferable that it contains, in mol%, S+Se+Te 25-90%, Sn 0.1-30%, Ag 0.1-15%, and Ge+Sn 1-30%, and that (Ge+Sn) / (S+Se+Te) is 0.3 or less. In this invention, "x / y" refers to the value obtained by dividing the content of component x by the content of component y.

[0054] The following describes preferred compositions of chalcogenide glass suitable for lens B.

[0055] S, Se, and Te are components that form the glass skeleton. The content of S+Se+Te (total amount of S, Se, and Te) is preferably 25-90%, 30-89%, 40-89%, 50-85%, 50-82%, and especially preferably 50-80%. If the content of S+Se+Te is too low, vitrification becomes difficult. If the content of S+Se+Te is too high, S-based, Se-based, or Te-based crystals will precipitate, and the internal transmittance will tend to decrease. The preferred ranges for the content of each component are as follows.

[0056] The sulfur content is preferably 0-90%, 10-90%, 20-89%, 30-89%, 40-88%, 50-88%, 50-80%, and particularly 50-75%. However, sulfur is a component that tends to reduce internal transmittance at wavelengths of 10 μm or more. Therefore, from the viewpoint of improving internal transmittance in the infrared region, the sulfur content is preferably 30% or less, 20% or less, 10% or less, 5% or less, 3% or less, and particularly 1% or less.

[0057] The Se content is preferably 0-90%, 10-90%, 20-89%, 30-89%, 40-88%, 50-88%, 50-80%, and particularly preferably 50-75%. However, Se is a toxic component. Therefore, from the viewpoint of reducing the burden on the environment, it is preferable that the Se content be 40% or less, 30% or less, 20% or less, 10% or less, 5% or less, 3% or less, 1% or less, and particularly preferably substantially absent.

[0058] The Te content is preferably 0-90%, 1-90%, 10-90%, 20-89%, 30-89%, 40-88%, 50-88%, 50-80%, and particularly preferably 50-75%. Te is also a component that easily increases the refractive index. If the Te content is too high, vitrification becomes difficult. In addition, Te-based crystals tend to precipitate, reducing the internal transmittance.

[0059] While it is sufficient to contain at least one component from S, Se, and Te, it is particularly preferable to contain at least Te in order to increase the internal transmittance in the wavelength range of 10 μm or more. Including Te makes it easier to set the infrared absorption edge wavelength to 20 μm or more.

[0060] Sn is a component that easily reduces the Abbe number of glass materials. It also has the effect of easily increasing the refractive index. The Sn content is preferably 0.1-30%, 0.1-28%, 0.1-26%, 0.3-26%, 0.3-24%, and especially 0.5-22%. If the Sn content is too low, the Abbe number tends to be high. If the Sn content is too high, vitrification becomes difficult. Furthermore, in terms of particularly increasing the internal transmittance, the Sn content is preferably 20% or less, 15% or less, 12% or less, 10% or less, 8% or less, and especially 5% or less.

[0061] The Te+Sn content (total amount of Te and Sn) is preferably 0.1-99%, 1-99%, 10-99%, 20-99%, 30-99%, 30-95%, and particularly preferably 30-90%. Too little Te+Sn makes vitrification difficult. Too much Te+Sn tends to reduce internal transmittance.

[0062] Ag (silver) is a component that broadens the vitrification range and easily improves the thermal stability of glass. It is also a component that easily increases the internal transmittance and refractive index of glass. The preferred Ag content is 0.1-15%, 0.1-14%, 0.3-14%, and especially 0.5-14%. If the Ag content is too low, the internal transmittance of the glass tends to decrease. If the Ag content is too high, vitrification becomes difficult.

[0063] The Sn+Ag content (total amount of Sn and Ag) is preferably 1-30%, 1-28%, 1-25%, 3-25%, and particularly 3-23%. When the Sn+Ag content satisfies the above values, vitrification becomes easier. If the Sn+Ag content is too low, the vitrification range tends to be narrower and the Abbe number tends to be higher. If the Sn+Ag content is too high, vitrification becomes difficult. In terms of particularly increasing the internal transmittance, the upper limit of the Sn+Ag content is preferably 20% or less, 15% or less, 12% or less, 10% or less, 8% or less, and particularly 5% or less.

[0064] From the viewpoint of reducing the Abbe number in particular, it is preferable that Sn / (Sn+Ag) be 0.01 or higher, 0.1 or higher, 0.2 or higher, and especially 0.3 or higher. On the other hand, if Sn / (Sn+Ag) is too high, vitrification becomes difficult. Also, the internal transmittance tends to decrease. For this reason, it is preferable that its upper limit be 0.99 or lower, and especially 0.98 or lower.

[0065] From the viewpoint of increasing the internal transmittance of the glass material, it is preferable that Ag / (Sn+Ag) be 0.01 or higher, 0.1 or higher, 0.2 or higher, and especially 0.25 or higher. On the other hand, if Ag / (Sn+Ag) is too large, the Abbe number tends to increase. Therefore, it is preferable that its upper limit be 0.99 or lower, 0.9 or lower, 0.8 or lower, and especially 0.75 or lower.

[0066] Ge is a component that forms the glass skeleton. It is also a component that easily enhances the thermal stability of glass. The preferred Ge content is 0-30%, 0.1-30%, 0.1-25%, 0.3-25%, 0.3-24%, and especially 0.5-22%. If the Ge content is too high, the internal transmittance tends to decrease, and the raw material cost tends to increase.

[0067] The Ge+Sn content (total amount of Ge and Sn) is preferably 1-30%, 1-28%, 1-26%, 3-25%, 3-24%, 5-24%, and particularly 8-23%. When Ge+Sn satisfies the above values, vitrification becomes easier. Furthermore, from the viewpoint of stabilizing vitrification, it is preferable to contain both Ge and Sn components at 0.1% or more, 0.3% or more, and particularly 0.5% or more.

[0068] (Ge+Sn) / (S+Se+Te) is preferably 0.3 or less, 0.29 or less, 0.28 or less, and especially 0.25 or less. When (Ge+Sn) / (S+Se+Te) satisfies the above values, vitrification becomes easier. The lower limit of (Ge+Sn) / (S+Se+Te) is, for example, 0.04 or more.

[0069] Furthermore, when Te is included as an essential component, it is preferable that the (Ge+Sn) / Te ratio is 0.3 or less, 0.29 or less, 0.28 or less, and particularly 0.25 or less. When (Ge+Sn) / Te satisfies the above values, vitrification becomes easier. The lower limit of (Ge+Sn) / Te is, for example, 0.04 or more.

[0070] In addition to the components listed above, chalcogenide glass suitable for lens B may also contain various other components as shown below.

[0071] Ga, Sb, and Bi are components that broaden the vitrification range and easily improve the thermal stability of the glass. The content of Ga+Sb+Bi (total amounts of Ga, Sb, and Bi) is preferably 0-50%, 0.1-50%, 0.3-40%, 0.5-30%, and particularly preferably 1-30%. If the content of Ga+Sb+Bi is too high, vitrification becomes difficult. The content of each component, Ga, Sb, and Bi, is preferably 0-50%, 0.1-50%, 0.1-40%, 0.1-30%, 0.1-25%, 0.3-25%, 0.5-25%, and particularly preferably 1-25%. Furthermore, the presence of Ga is particularly preferable in terms of facilitating vitrification.

[0072] The Ga / (S+Se+Te) ratio is preferably 2 or less, 1.9 or less, 1.8 or less, 1.5 or less, 1 or less, 0.7 or less, and especially preferably 0.5 or less. It is also preferably 0 or more, 0.04 or more, 0.05 or more, and especially preferably 0.1 or more. When the Ga / (S+Se+Te) ratio satisfies the above values, vitrification becomes easier.

[0073] Furthermore, when Te is included as an essential component, the Ga / Te ratio is preferably 2 or less, 1.9 or less, 1.8 or less, 1.5 or less, 1 or less, 0.7 or less, and particularly 0.5 or less. Also, it is preferably 0 or more, 0.04 or more, 0.05 or more, and particularly 0.1 or more. When the Ga / Te ratio satisfies the above values, vitrification becomes easier.

[0074] The Ge+Ga content (total amount of Ge and Ga) is preferably 0-60%, 0.1-60%, 0.1-55%, and particularly preferably 0.5-55%. When the Ge+Ga content satisfies the above values, vitrification becomes easier, and the thermal stability of the glass is enhanced. If the Ge+Ga content is too high, Ge-based or Ga-based crystals will precipitate, which tends to reduce the internal transmittance. Furthermore, from the viewpoint of stabilizing vitrification, it is preferable to contain 0.1% or more of both Ge and Ga components.

[0075] The ratio (Ge+Ga) / (S+Se+Te) is preferably 2 or less, 1.9 or less, 1.8 or less, 1.5 or less, 1.2 or less, and especially 1 or less. Furthermore, the ratio (Ge+Ga) / (S+Se+Te) is preferably 0.04 or more, 0.05 or more, and especially 0.1 or more. When (Ge+Ga) / (S+Se+Te) satisfies the above values, vitrification becomes easier.

[0076] Furthermore, when Te is included as an essential component, the (Ge+Ga) / Te ratio is preferably 2 or less, 1.9 or less, 1.8 or less, 1.5 or less, 1.2 or less, and particularly 1 or less. It is also preferably 0.04 or more, 0.05 or more, and particularly 0.1 or more. When the (Ge+Ga) / Te ratio satisfies the above values, vitrification becomes easier.

[0077] From the perspective of particularly increasing internal transmittance, it is preferable that Bi / (Ga+Sb+Bi) is less than 1, 0.5 or less, 0.3 or less, and especially 0.1 or less.

[0078] F, Cl, Br, and I are components that broaden the vitrification range and easily improve the thermal stability of glass. They are also components that easily increase internal transmittance. The content of F+Cl+Br+I (total amount of F, Cl, Br, and I) is preferably 0-20%, 0-15%, 0-10%, 0-5%, 0-4%, 0-3%, 0-2%, 0-1%, and especially preferably 0.1-1%. If the content of F+Cl+Br+I is too high, vitrification becomes difficult, and weather resistance tends to decrease.

[0079] Furthermore, the Cl+I content (total amount of Cl and I) is preferably 0-20%, 0-15%, 0-10%, 0-5%, 0-4%, 0-3%, 0-2%, 0-1%, and particularly preferably 0.1-1%. The content of each component F, Cl, Br, and I is preferably 0-20%, 0-15%, 0-10%, 0-5%, 0-4%, 0-3%, 0-2%, 0-1%, and particularly preferably 0.1-1%.

[0080] Al and Si form a glass skeleton and are components that easily reduce the Abbe number and improve glass dispersion. The Al+Si content (total amount of Al and Si) is preferably 0-50%, 0-40%, 0-30%, 0-20%, 0-15%, and particularly preferably 0-10%. If the Al+Si content is too high, the internal transmittance tends to decrease. The content of each component, Al and Si, is preferably 0-50%, 0-40%, 0-30%, 0-20%, 0-15%, and particularly preferably 0-10%.

[0081] Zn, In, and Cu are components that broaden the vitrification range and easily improve the thermal stability of the glass. The content of Zn+In+Cu (total amount of Zn, In, and Cu) is preferably 0-50%, 0-40%, 0-30%, 0-25%, 0-20%, 0-15%, 0-12%, 0-10%, and particularly preferably 0-5%. If the content of Zn+In+Cu is too high, vitrification becomes difficult. The content of each component, Zn, In, and Cu, is preferably 0-50%, 0-40%, 0-30%, 0-25%, 0-20%, 0-15%, 0-12%, 0-10%, and particularly preferably 0-5%.

[0082] It may contain B, C, Cr, Mn, Ti, Fe, etc. The content of B+C+Cr+Mn+Ti+Fe (total amount of B, C, Cr, Mn, Ti, and Fe) is preferably 0-40%, 0-30%, 0-20%, 0-10%, 0-5%, 0-1%, and especially less than 0-1%. If the content of these components is too high, it may be difficult to obtain the desired optical properties. The content of each component B, C, Cr, Mn, Ti, and Fe is preferably 0-10%, 0-5%, 0-1%, and especially less than 0-1%.

[0083] As is a component that enhances the thermal stability of glass. However, since As is a toxic component, from the viewpoint of reducing the burden on the environment, it is preferable that the As content be 30% or less, 25% or less, 20% or less, 10% or less, 5% or less, 1% or less, or especially substantially absent. Furthermore, from the viewpoint of particularly reducing the burden on the environment, it is especially preferable that Se be substantially absent. It is also preferable that Cd, Tl, and Pb be substantially absent. In this way, the impact on the environment can be minimized.

[0084] Thus, the chalcogenide glass suitable for lens B has a smaller refractive index N10 and a smaller Abbe number compared to the chalcogenide glass suitable for lens A. In this invention, as a specific example of the above chalcogenide glass, chalcogenide glass CG2 is used, which has a refractive index N10 of 3.419 and an Abbe number ν10 of 226. Because chalcogenide glass CG2 has the above characteristics, it is possible to obtain sufficient achromatic properties by applying it to the first group G1 and the second group G2 of the infrared imaging lens 1.

[0085] Next, we investigated the temperature dependence of the refractive index N10. The temperature dependence of the refractive index N10 of chalcogenide glass CG1, dN10 / dT, is 294 × 10⁻⁶. -6 The temperature dependence of the refractive index N10 of chalcogenide glass CG2, dN10 / dT, is 244 × 10⁻¹⁴. -6 The values ​​were / K. These values ​​are similar. Therefore, even when an infrared imaging lens 1 is constructed by combining a lens made of chalcogenide glass CG1 and a lens made of chalcogenide glass CG2, the variation in the image height of the focal position does not increase with temperature, and the temperature dependence of the focal characteristics is good.

[0086] <Numerical Example 1> Numerical examples of infrared imaging lens 1 are shown below. A cross-sectional view of the infrared imaging lens according to Numerical Example 1 is shown in Figure 1. In Numerical Example 1, r represents the radius of curvature, d represents the lens thickness on the optical axis or the distance between surfaces, and ED represents the effective diameter. The unit of length is (mm). Basic lens data and various other data are shown below.

[0087] [Table 1]

[0088] The definitions of refractive index and Abbe number ν¹⁰ are as follows: N8: Refractive index at a wavelength of 8 μm N10: Refractive index at a wavelength of 10 μm N12: Refractive index at a wavelength of 12 μm ν10 = (N10-1) / (N8-N12)

[0089] [Table 2]

[0090] The first lens L1 and the third lens L3 use the aforementioned chalcogenide glass CG1, which has a refractive index N10 of 3.465 and an Abbe number ν10 of 253. The second lens L2 and the fourth lens L4 use the aforementioned chalcogenide glass CG2, which has a refractive index N10 of 3.419 and an Abbe number ν10 of 226. The infrared imaging lens 1 has a four-lens configuration and can be made lightweight. Each lens is a simple spherical lens, and combined with the fact that it can be manufactured by press molding, the infrared imaging lens 1 can be manufactured at a low cost that makes it suitable for consumer applications.

[0091] The parallel plate P is made of silicon (Si). Optical properties were simulated using known literature values ​​for the refractive index dispersion of silicon. The parallel plate P is provided to protect the image sensor. The back focus BFL = 12.5 mm is the actual distance, ensuring sufficient distance.

[0092] The maximum image height at the image plane S is 4.1 mm, and therefore the diameter of the image circle φs is 8.2 mm. Consequently, the infrared imaging lens 1 is applicable to QVGA class image sensors such as 384 × 288 pixels with a pixel pitch of 17 μm and a diagonal length of 8.16 mm. Furthermore, the infrared imaging lens 1 can cover the pixel area of ​​QVGA class image sensors, including QVGA (320 × 240 pixels) and QVGA+ (345 × 240 pixels) with a pixel pitch of 17 μm.

[0093] It goes without saying that the infrared imaging lens 1 can cover the pixel area of ​​a QVGA class image sensor with a pixel pitch of 12 μm, including 320 x 256 pixels. Furthermore, for configurations such as 384 x 288 pixels and 320 x 256 pixels with a pixel pitch of 12 μm, an effective pixel count of QVGA (320 x 240 pixels) can be ensured even if the optical axis center of the lens does not perfectly coincide with the center of the image sensor.

[0094] The ratio of the total focal length fL of the infrared imaging lens 1 to the diameter φs of the image circle is: fL / φs = 6.1 Therefore, the infrared imaging lens 1 is a telephoto lens. Furthermore, its maximum half-angle of view is 4.7°, which is within the range of 5° or less, making it a long-focus telephoto lens. Despite being such a narrow-angle telephoto lens, the infrared imaging lens 1 is an extremely bright imaging lens with an F-number of 1.0.

[0095] The focal lengths are as follows: the first lens has a focal length of f1 = 42.44 mm, the second lens has a focal length of f2 = -57.03 mm, the third lens has a focal length of f3 = 22.73 mm, and the fourth lens has a focal length of f4 = -44.47 mm. Therefore, the infrared imaging lens 1 is configured such that the power of the third lens L3 is stronger than the power of the first lens L1. In addition, the infrared imaging lens 1 is configured such that the power of the fourth lens L4 is stronger than the power of the second lens L2.

[0096] In the first group, the ratio of the focal length of the second lens L2 to the first lens L1 is: |f2 / f1|=1.34 It is within the range of 1.1 to 2.0. In the second group, the ratio of the focal length of the fourth lens L4 to the third lens L3 is, |f4 / f3|=1.96 The ratio is within the range of 1.1 to 2.0. These ratios are configured to ensure good achromaticity of the infrared imaging lens 1.

[0097] The focal lengths of each group are fG1 = 126.84 (mm) for group 1 and fG2 = 34.07 (mm) for group 2. The ratio of the focal length of each group to the total focal length fL is: fG1 / fL = 2.54 fG2 / fL=0.68 By assigning the focal lengths of each group to the overall focal length fL in this manner, the infrared imaging lens 1 achieves well-balanced and excellent optical characteristics overall. This is reflected in the MTF characteristics described later (Figures 5 and 6).

[0098] Figures 2 to 7 show the various performance characteristics of Numerical Example 1 of the infrared imaging lens 1. Figures 2 and 3 are aberration diagrams of the infrared imaging lens 1. Figure 2 shows spherical aberration, astigmatism, and distortion. Graphs for each wavelength in the range of 8 to 14 μm are shown for each. Figure 3 is an aberration diagram showing coma aberration at each image height Y from image height 0 mm to maximum image height, separated into tangential (meridional) and sagittal (radical) directions. As shown in Figures 2 and 3, the infrared imaging lens 1 according to Numerical Example 1 shows good correction of various aberrations over a wide wavelength range of 8 to 14 μm.

[0099] In long-focus telephoto lenses where the total focal length fL is 5 times or more the diameter of the image circle φs, or where the half-angle of view is 5 degrees or less, chromatic aberration becomes a major problem in improving resolution. If all lenses constituting the imaging lens are made of the same material, the distance obtained by multiplying the total focal length fL by the reciprocal of the Abbe number of that material corresponds to the limit value Δf of residual paraxial chromatic aberration. For example, if all lenses constituting infrared imaging lens 1 are made of a single material with an Abbe number ν10 of 253, defined using the refractive index difference at wavelengths of 8 to 12 μm, then the limit value Δf of residual paraxial chromatic aberration is estimated to be approximately 0.2 mm.

[0100] According to the spherical aberration diagram in Figure 2, the difference in paraxial chromatic aberration between wavelengths of 8 μm and 12 μm is 0.1 mm near the optical axis, which corresponds to the amount of paraxial chromatic aberration. Thus, in the infrared imaging lens 1, which is constructed by combining chalcogenide glasses with different Abbe numbers ν10 in each group based on appropriate design, the amount of residual paraxial chromatic aberration is significantly improved compared to the value expected in an infrared imaging lens using a single chalcogenide glass, and the overall chromatic aberration is improved.

[0101] Figure 4 is a graph showing the relative intensity of the infrared imaging lens 1 in numerical example 1 with respect to image height Y. Here, relative intensity refers to the ratio of illuminance to the maximum illuminance at the image plane S. As shown in Figure 4, in the range up to the maximum image height, the relative intensity at a wavelength of 10 μm is 0.95 or higher, which is almost 1, and an extremely uniform light intensity distribution is obtained within the image circle.

[0102] Figure 5 is a graph showing the spatial frequency dependence of the MTF in the wavelength range of 8-14 μm. The Nyquist frequency f of the image sensor with a pixel pitch of 17 μm and 384 × 288 pixels is shown. N The frequency is 29.4 cycles / mm, and the maximum image height is 4.08 mm. Figure 5 shows the results for the spatial frequency range from 0 to 30 cycles / mm and the image height range from 0 to 4.08 mm.

[0103] In the MTF characteristics over a wide wavelength range of 8 to 14 μm, the Nyquist frequency f N At 29.4 cycles / mm, the MTF at the center of the image is 0.28, and an MTF > 0.25 (simple average in the tangential and sagittal directions) is ensured within the region of the image sensor. Furthermore, when applied to such an image sensor, the infrared imaging lens 1 exhibits extremely small image height dependence of its MTF characteristics, providing practical properties.

[0104] Figure 6 is also a graph showing the spatial frequency dependence of the MTF in the wavelength range of 8-14 μm. The Nyquist frequency f of the image sensor with a pixel pitch of 12 μm and 320 × 256 pixels. N The frequency is 41.7 cycles / mm, and the maximum image height is 2.46 mm. Figure 6 shows the results for the spatial frequency range from 0 to 50 cycles / mm and the image height range from 0 to 2.46 mm.

[0105] In the MTF characteristics over a wide wavelength range of 8 to 14 μm, the Nyquist frequency f N At 41.7 cycles / mm, the MTF at the center of the image is 0.19, and an MTF > 0.177 (simple average in the tangential and sagittal directions) is ensured within the region of the image sensor. Furthermore, the infrared imaging lens 1 exhibits extremely low image height dependence of its MTF characteristics when applied to such an image sensor, providing practical properties.

[0106] Thus, the infrared imaging lens 1 can achieve good resolution within the range of a QVGA class image sensor with a pixel pitch of approximately 12 to 17 μm. In this way, the infrared imaging lens 1 can achieve good resolution within the wavelength range of 8 to 14 μm.

[0107] In the infrared imaging lens 1, both the first and second lens groups consist of a combination of a positive-power lens (convex lens) and a negative-power lens (concave lens), and the ratios fG1 / fL and fG2 / fL are designed to be approximately as described above. As a result, the increase in the Petzval sum is suppressed, and well-balanced MTF characteristics are achieved at each image height.

[0108] Figure 7 is a graph showing the change in MTF in the wavelength range of 8-14 μm at a spatial frequency of 15 cycles / mm with respect to focus shift. Figure 8 is a graph showing the temperature dependence of the change in MTF at a wavelength of 10 μm at a spatial frequency of 15 cycles / mm with respect to focus shift. As shown in Figure 8, the focal position for each image height does not vary over a wide temperature range from -40°C to 80°C, showing extremely good temperature dependence of the focal characteristics. Specifically, the temperature dependence dp' / dT of the best focus position was -3.95 μm / °C. In addition, since the best focus position maintains very good linearity with respect to temperature changes, it becomes possible to implement a mechanism that prevents the focus position from shifting mechanically by utilizing the thermal expansion of metals or resins.

[0109] As described above, the infrared imaging lens 1 of the numerical embodiment 1 has good resolution in the wavelength range of 8 to 14 μm, which is sufficient to support QVGA class image sensors with a pixel pitch of about 12 to 17 μm. Furthermore, despite being a telephoto lens with a long focal length (fL / φs: 5 to 10), the infrared imaging lens 1 has an extremely bright F-number of 1.0, and therefore possesses performance that is sufficient to support image sensors with such small pixel pitches. Thus, according to this embodiment, an infrared imaging lens with unprecedented and superior characteristics as a telephoto lens can be realized.

[0110] 〔summary〕 One aspect of the present disclosure is an infrared imaging lens used in an infrared region including at least one wavelength in the range of 8 to 14 μm, wherein a first group consisting of a plurality of lenses and a second group consisting of a plurality of lenses are arranged sequentially from the object side toward the image plane side, each of the lenses constituting the first group and the second group is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and each of the first group and the second group includes lenses made of the chalcogenide glass with different Abbe numbers defined in a predetermined wavelength range in the range of 8 to 14 μm, and the total system focal length is 5 times or more the diameter of the image circle.

[0111] According to the above configuration, it is possible to realize an infrared imaging lens, which is a telephoto lens with excellent resolution that can be used with an image sensor having a pixel pitch on the order of wavelengths.

[0112] The infrared imaging lens according to Embodiment 2 of this disclosure is characterized in that, in Embodiment 1, each of the first group and the second group includes a lens with positive power and a lens with negative power, and is configured to have positive power. With the above configuration, it is possible to realize an infrared imaging lens that is a telephoto lens with particularly excellent resolution, which can be used with an image sensor having a pixel pitch on the order of wavelength.

[0113] The infrared imaging lens according to Embodiment 3 of this disclosure is characterized in that, in Embodiment 1, the first group has positive power and is composed of a first lens having positive power and a second lens having negative power, arranged in order from the object side to the image plane side, and the second group has positive power and is composed of a third lens having positive power and a fourth lens having negative power, arranged in order from the object side to the image plane side. With the above configuration, an infrared imaging lens that is a telephoto lens can be realized that can maintain excellent resolution even with respect to temperature changes.

[0114] The infrared imaging lens according to aspect 4 of the present disclosure is characterized in that, in aspect 3, all of the first lens, second lens, third lens, and fourth lens are meniscus lenses that are convex toward the object. With the above configuration, the increase in the Petzval sum can be suppressed, field curvature can be suppressed, and the flatness of the image plane can be maintained.

[0115] The infrared imaging lens according to aspect 5 of this disclosure is characterized in that, in aspect 3 or 4, all of the first lens, second lens, third lens, and fourth lens are spherical lenses. With this configuration, an infrared imaging lens 1 with practical resolution characteristics and low image height dependence of MTF can be realized.

[0116] The infrared imaging lens according to aspect 6 of the present disclosure is characterized in that, in aspects 3 to 5 above, the chalcogenide glass constituting the second lens has a smaller refractive index and Abbe number than the chalcogenide glass constituting the first lens, and the chalcogenide glass constituting the fourth lens has a smaller refractive index and Abbe number than the chalcogenide glass constituting the third lens. With the above configuration, good achromatic characteristics can be obtained, and a telephoto lens with excellent resolution can be realized.

[0117] The infrared imaging lens according to aspect 7 of this disclosure is characterized in that, in aspects 3 to 6 above, the chalcogenide glass constituting the second lens has an Abbe number defined in the wavelength range of 8 to 12 μm that is 5% or more smaller than that of the chalcogenide glass constituting the first lens, and the chalcogenide glass constituting the fourth lens has an Abbe number defined in the wavelength range of 8 to 12 μm that is 5% or more smaller than that of the chalcogenide glass constituting the third lens. With the above configuration, particularly good achromatic characteristics can be obtained, and a telephoto lens with excellent resolution can be realized.

[0118] The infrared imaging lens according to aspect 8 of this disclosure, in aspects 3 to 7 above, has the focal length f1 of the first lens, the focal length f2 of the second lens, the focal length f3 of the third lens, and the focal length f4 of the fourth lens, 1.1 ≤ |f² / f⁻¹| ≤ 2.0 1.1 ≤ |f4 / f3| ≤ 2.0 The configuration is characterized by satisfying the given relation. With the above configuration, particularly good achromatic characteristics can be obtained, and a telephoto lens with excellent resolution can be realized.

[0119] The infrared imaging lens according to aspect 9 of this disclosure is characterized in that, in aspects 3 to 8 above, the effective diameter of the image-plane side surface of the first lens is an aperture diaphragm. With the above configuration, it is possible to reduce the outer diameter and volume of the infrared imaging lens compared to inserting an aperture diaphragm between the lenses.

[0120] Aspect 10 of the present disclosure is an infrared imaging lens used in an infrared region including at least one wavelength in the range of 8 to 14 μm, wherein a first group consisting of a plurality of lenses and a second group consisting of a plurality of lenses are arranged sequentially from the object side toward the image plane side, each of the lenses constituting the first group and the second group is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and each of the first group and the second group includes lenses made of the chalcogenide glass having different Abbe numbers defined in a predetermined wavelength range in the range of 8 to 14 μm, and the half-angle of view is 5° or less.

[0121] According to the above configuration, it is possible to realize an infrared imaging lens, which is a telephoto lens with excellent resolution that can be used with an image sensor having a pixel pitch on the order of wavelengths.

[0122] The infrared imaging lens according to aspect 11 of this disclosure, in aspects 1 to 10 above, has the total system focal length fL and the focal length fG1 of the first group, 2.0 ≤ fG1 / fL ≤ 3.0 The configuration is characterized by satisfying the given relation. According to the above configuration, an infrared imaging lens 1 with practically excellent resolution characteristics and low image height dependence of MTF can be realized.

[0123] The infrared imaging lens according to aspect 12 of this disclosure, in aspects 1 to 11 above, has the total system focal length fL and the focal length fG2 of the second group, 0 <fG2 / fL≦1.0 The configuration is characterized by satisfying the given relation. According to the above configuration, an infrared imaging lens 1 with practically excellent resolution characteristics and low image height dependence of MTF can be realized.

[0124] The infrared imaging lens according to aspect 13 of this disclosure is characterized in that, in aspects 1 to 12 above, the total system focal length is within the range of 5 to 10 times the diameter of the image circle. With the above configuration, an infrared imaging lens that is a telephoto lens with excellent aberration characteristics and resolution can be realized.

[0125] The infrared imaging lens according to aspect 14 of this disclosure is characterized in that, in aspects 1 to 13 above, the F-number is in the range of 0.9 to 1.1. According to the above configuration, it is possible to realize a bright infrared imaging lens with a small F-number, which is a telephoto lens with excellent aberration characteristics and resolution.

[0126] The infrared imaging lens according to aspect 15 of this disclosure is characterized in that, in aspects 1 to 14 above, the chalcogenide glass has an infrared absorption edge wavelength of 18 μm or more at which the light transmittance at a thickness of 2 mm is 20%. With the above configuration, it is possible to construct an infrared imaging lens with very low light absorption in the wavelength range of at least 8 to 14 μm.

[0127] In the infrared imaging lens according to embodiment 16 of the present invention, in embodiments 1 to 15 above, the chalcogenide glass is refracted at a wavelength of 10 μm. Occasionally The temperature dependence of the rate is 200 × 10 -6 ~350×10 -6It is characterized by being / K. With the above configuration, it is possible to realize an infrared imaging lens, which is a telephoto lens, that can maintain particularly excellent resolution even with temperature changes.

[0128] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the specification are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in the specification. [Explanation of Symbols]

[0129] 1. Infrared imaging lens G1 Group 1 L1 First Lens L2 Second Lens G2 2nd group L3 3rd lens L4 4th lens P parallel plate S image plane AP aperture diaphragm

Claims

1. An infrared imaging lens used in the infrared region, which includes at least one wavelength within the range of 8 to 14 μm, From the object side toward the image plane side, a first group consisting of multiple lenses and a second group consisting of multiple lenses are arranged in order. Each of the lenses constituting the first and second groups is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. Each of the first and second groups includes lenses made of chalcogenide glass with different Abbe numbers defined in a predetermined wavelength range of 8 to 14 μm. An infrared imaging lens in which the total focal length is five times or more the diameter of the image circle.

2. The infrared imaging lens according to claim 1, wherein each of the first group and the second group includes a lens having positive power and a lens having negative power, and is configured to have positive power.

3. The first group has positive power, It consists of a first lens with positive power and a second lens with negative power, arranged in order from the object side towards the image plane side. The aforementioned second group has positive power, The infrared imaging lens according to claim 1, comprising, in order from the object side toward the image plane side, a third lens having positive power and a fourth lens having negative power.

4. The infrared imaging lens according to claim 3, wherein all of the first lens, the second lens, the third lens, and the fourth lens are meniscus lenses that are convex toward the object.

5. The infrared imaging lens according to claim 3, wherein all of the first lens, the second lens, the third lens, and the fourth lens are spherical lenses.

6. The chalcogenide glass constituting the second lens has a lower refractive index and Abbe number than the chalcogenide glass constituting the first lens. The infrared imaging lens according to claim 3, wherein the chalcogenide glass constituting the fourth lens has a smaller refractive index and Abbe number than the chalcogenide glass constituting the third lens.

7. The chalcogenide glass constituting the second lens has an Abbe number that is 5% or more smaller than that of the chalcogenide glass constituting the first lens, defined in the wavelength range of 8 to 12 μm. The infrared imaging lens according to claim 6, wherein the chalcogenide glass constituting the fourth lens has an Abbe number defined in the wavelength range of 8 to 12 μm that is 5% or less smaller than that of the chalcogenide glass constituting the third lens.

8. The focal length f1 of the first lens, the focal length f2 of the second lens, the focal length f3 of the third lens, and the focal length f4 of the fourth lens are 1.1≦|f2 / f1|≦2.0 1.1≦|f4 / f3|≦2.0 An infrared imaging lens according to claim 3, satisfying the relationship.

9. The infrared imaging lens according to claim 3, wherein the effective diameter of the image plane side surface of the first lens is set as an aperture diaphragm.

10. An infrared imaging lens used in the infrared region, which includes at least one wavelength within the range of 8 to 14 μm, From the object side toward the image plane side, a first group consisting of multiple lenses and a second group consisting of multiple lenses are arranged in order. Each of the lenses constituting the first and second groups is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. Each of the first and second groups includes lenses made of chalcogenide glass with different Abbe numbers defined in a predetermined wavelength range of 8 to 14 μm. An infrared imaging lens with a half-angle of view of 5° or less.

11. The total focal length fL and the focal length fG1 of the first group are 2.0 ≤ fG1 / fL ≤ 3.0 An infrared imaging lens according to any one of claims 1 to 10, satisfying the relationship.

12. The overall system focal length fL and the focal length fG2 of the second group are: 0<fG2 / fL≦1.0 An infrared imaging lens according to claim 11 that satisfies the relationship.

13. An infrared imaging lens according to any one of claims 1 to 10, wherein the total system focal length is in the range of 5 to 10 times the diameter of the image circle.

14. An infrared imaging lens according to any one of claims 1 to 10, wherein the F-number is in the range of 0.9 to 1.

1.

15. The infrared imaging lens according to any one of claims 1 to 10, wherein the chalcogenide glass has an infrared absorption edge wavelength of 18 μm or more at which the light transmittance at a thickness of 2 mm is 20%.

16. The chalcogenide glass has a temperature dependence of 200 × 10⁻¹⁰ －６ ~350 x 10 －６ An infrared imaging lens according to any one of claims 1 to 10, wherein the value is / K.

Images