Infrared imaging lens

Abstract

To achieve a telephoto lens having excellent resolution.SOLUTION: An infrared imaging lens (1) includes a first lens (L1), a second lens (L2), and a third lens (L3), each made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. The total system focal length is at least twice the diameter of the image circle. The first lens and the third lens are each a meniscus lens having positive power, and the second lens is a meniscus lens having negative power.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Infrared cameras that capture images of subjects using infrared rays in the mid-far infrared region, particularly in the 10 μm wavelength range that is suitable for detecting living organisms, are used in surveillance cameras, security cameras, in-vehicle night vision systems, etc. These infrared cameras can be used in a variety of fields, such as monitoring intruders, monitoring illegal activities such as poaching, detecting the source of forest fires, traffic monitoring, and obstacle detection, and demand is expected to expand.

[0003] For applications such as nighttime long-distance surveillance, infrared imaging lenses applied to infrared cameras require so-called telephoto lenses with a relatively long focal length relative to the image circle diameter of the image plane. The applicant has proposed a telephoto lens with a small F-number, high brightness, and excellent resolution that can be used with a small image sensor with a pixel pitch of about the wavelength as an infrared imaging lens. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. WO2023 / 008148 Summary of the Invention [Problem to be solved by the invention]

[0005] With the expansion of applications of infrared cameras, there is a demand for infrared imaging lenses with various characteristics. For example, it is also desirable to realize an infrared imaging lens that maintains brightness and resolution while further reducing distortion (distortion aberration) compared to the conventional technology of Patent Document 1. One aspect of the present invention focuses on the above problem and aims to realize an infrared imaging lens that is a telephoto lens that can be used for consumer applications, has excellent resolution, further reduces distortion, and is compatible with an image sensor with a pixel pitch on the order of the wavelength. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, one aspect of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in this order from an object side to an image surface side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, a total system focal length fL being equal to or greater than twice the diameter of an image circle, the first lens and the third lens being meniscus lenses having positive power, and the second lens being a meniscus lens having negative power.

[0007] In order to solve the above-mentioned problems, another aspect of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in this order from the object side to the image surface side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, the first lens and the third lens being meniscus lenses having positive power, the second lens being a meniscus lens having negative power, and having a half angle of view of 14° or less. Effect of the Invention

[0008] According to the above aspects of the present invention, it is possible to realize an infrared imaging lens that is a telephoto lens usable for consumer applications, which has excellent resolution and reduced distortion and can be used with an image sensor having a pixel pitch on the order of a wavelength. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a configuration of a main part of an infrared imaging lens according to an embodiment of the present invention. [Diagram 2] 4A to 4C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the infrared imaging lens according to Numerical Example 1 of the present invention. [Diagram 3] FIG. 2 is an aberration diagram showing coma aberration of the infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 4] 1 is a graph showing the image height dependency of relative illuminance of the infrared imaging lens according to Numerical Example 1 of the present invention. [Diagram 5] 1 is a graph showing the spatial frequency dependence of MTF in the wavelength range of 7 to 14 μm for an infrared imaging lens according to Numerical Example 1 of the present invention. [Figure 6] 1 is a graph showing the focus shift dependency of the MTF of the infrared imaging lens according to Numerical Example 1 of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Embodiment] <Outline of infrared imaging lens> The infrared imaging lens 1 according to the embodiment is a lens system that forms an image of a subject on an image plane S on which an image sensor or the like is arranged, corresponding to a mid-far infrared wavelength region. The infrared imaging lens 1 according to the embodiment is intended for a telephoto lens whose total system focal length fL is twice or more the diameter φs of the image circle. Alternatively, the infrared imaging lens 1 according to the embodiment is intended for a telephoto lens whose half angle of view ω (maximum half angle of view) is 14° or less.

[0011] In this application, the image circle refers to the range of the effective diameter of the optical system on the image plane S of the imaging lens. In other words, the image circle is a circular range on the image plane S in which the image height Y is up to the maximum image height. Here, the image circle diameter φs refers to the diameter of such a circular range, as in the case of expressions such as "lens diameter". In other words, the image circle diameter φs is equal to twice the maximum image height.

[0012] As is known in the field of imaging lenses, the half angle of view ω of an imaging lens is determined by the total focal length fL of the imaging lens, the diameter φs of the image circle, and ω=arctan[φs / (2fL)] Here, arctan is the arctangent function. The total system focal length fL being twice or more the diameter φs of the image circle (fL / φs≧2) corresponds to the half angle of view ω being 14° or less.

[0013] An imaging lens whose total focal length fL is twice or more the diameter φs of the image circle, or whose half angle of view ω is 14° or less, is an imaging lens whose focal length in 35 mm conversion is 85 mm or more and is generally called a telephoto lens. Note that when the focal length is converted to 35 mm conversion, it is assumed that the diagonal length of the image sensor is approximately equal to the diameter φs of the image circle and that the image sensor effectively utilizes the image circle. In this way, this embodiment is directed to an infrared imaging lens that enables the realization of an imaging device capable of magnifying and observing a distant object.

[0014] 1 is a cross-sectional view taken along the optical axis, showing the configuration of the main parts of the infrared imaging lens 1. The infrared imaging lens 1 is configured by arranging a first lens L1, a second lens L2, and a third lens L3 in this order from the object side toward the image surface S side. During focusing, the first lens L1 to the third lens L3 all move uniformly in the optical axis direction.

[0015] The first lens L1, the second lens L2, and the third lens L3 are each made of chalcogenide glass having a refractive index N10 of 2.5 to 4.0 at a wavelength of 10 μm. Here, the symbol N10 particularly indicates the refractive index at a wavelength of 10 μm. The first lens L1, the second lens L2, and the third lens L3 may all be made of the same glass material, that is, chalcogenide glass. In particular, the first lens L1, the second lens L2, and the third lens L3 are each preferably made of chalcogenide glass having a refractive index at a wavelength of 10 μm in the range of 3.0 to 4.0, and more preferably 3.0 to 3.7.

[0016] The first lens L1 has positive refractive power and is in a meniscus shape. The second lens L2 has negative refractive power and is in a meniscus shape. The third lens L3 has positive refractive power and is in a meniscus shape. By configuring and arranging each lens in this way, it is possible to reduce the Petzval sum, and to realize a telephoto lens having excellent resolution and further reduced distortion. In particular, it is preferable that the first lens L1, the second lens L2, and the third lens L3 all have a meniscus shape convex toward the object side.

[0017] As shown in Fig. 1, a parallel plate P is disposed between the third lens L3 and the image surface S. The parallel plate P is an optical window that is hermetically sealed on the image surface S side, and is made of silicon, low-oxygen silicon, or germanium. The material and thickness can be determined depending on the type of image sensor to be adopted.

[0018] 1, the effective diameter of the object-side surface (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, and the parallel plate P. Appropriate known techniques can be applied to such anti-reflection coatings in the mid-far infrared region.

[0019] <Glass material for each lens> The following describes the chalcogenide glass that constitutes each lens of the infrared imaging lens 1. The chalcogenide glass applied to the infrared imaging lens 1 is chalcogenide glass with a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. From the viewpoint of realizing an infrared imaging lens 1 with a bright F-number (F value) of about 1.0 and excellent resolution compatible with an image sensor with a pixel pitch equivalent to wavelengths in the mid-far infrared region of 7 to 14 μm, it is preferable that the refractive index of the chalcogenide glass be 2.5 to 4.0.

[0020] Such chalcogenide glass with a high refractive index in the far-infrared region has been 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. In addition, if the refractive index of the chalcogenide glass constituting the first lens L1, the second lens L2, and the third lens L3 is low, it is difficult to configure a compact telephoto lens. Therefore, it is particularly preferable that the refractive index of the chalcogenide glass at a wavelength of 10 μm is in the range of 3.0 to 4.0.

[0021] The Abbe number (ν10) of the chalcogenide glass is preferably 100 or more, 120 or more, 150 or more, 180 or more, and particularly preferably 220 or more. The definition of the Abbe number (ν10) will be described later. If the Abbe number is too low, chromatic aberration tends to become large. There is no particular upper limit to the Abbe number, but in the above chalcogenide glass, it is practically 350 or less.

[0022] This glass material has 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 has the characteristic of having low light absorption even in the far-infrared region of 10 to 26 μm. In chalcogenide glass, the "infrared absorption edge wavelength" and "internal transmittance" can be used as indicators of excellent light transmittance in the far-infrared region.

[0023] Here, the infrared absorption edge wavelength refers to the absorption edge wavelength in the wavelength range of 8 μm or more, 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 inside the material and does not include reflection losses 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 more.

[0024] Therefore, the chalcogenide glass transmits infrared rays with wavelengths exceeding 10 μm, and has good transmittance at least over the wavelength range of 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, with the infrared imaging lens 1 of this embodiment, an imaging lens with low light absorption by the lens glass material over a wide wavelength range of at least 7 to 14 μm can be realized.

[0025] Furthermore, by using the chalcogenide glass, it is easy to mass-produce lenses by press molding. In addition, it is also possible to mass-produce aspherical lenses by press molding. In this specification, aspherical surfaces include diffractive surfaces. It is preferable that the glass transition temperature of the glass material is low, at 200° C. or less, so that press molding is easier.

[0026] In the infrared imaging lens 1 of this embodiment, at least one of the lenses is an aspherical lens, so that aberration is suppressed. If an aspherical lens cannot be applied, the configuration of the infrared imaging lens 1 for suppressing aberration would require an increased number of lenses, which would increase the weight and size. This would also result in a high cost and an imaging lens that is not suitable for consumer use. In this specification, aspherical surfaces include diffractive surfaces.

[0027] The chalcogenide glass also makes it possible to press-mold lenses having surfaces with particularly complicated shapes, such as diffractive surfaces. Therefore, in the infrared imaging lens 1, by using the chalcogenide glass to make the surface (second surface) of the first lens L1 on the image side a diffractive surface, it becomes possible to satisfactorily suppress aberrations over a wide wavelength range of 7 to 14 μm.

[0028] In particular, chalcogenide glass having a refractive index of 3.0 to 3.7 at a wavelength of 10 μm is suitable for mass production, and the present applicant is currently working on industrial production of the same. Furthermore, chalcogenide glass having a refractive index of 3.0 to 3.7 is also excellent in processability when forming lenses. Therefore, it is particularly preferable that the chalcogenide glass constituting the first lens L1, second lens L2, and third lens L3 be composed of chalcogenide glass having a refractive index of 3.0 to 3.7 at a wavelength of 10 μm.

[0029] Crystalline materials such as silicon (Si), germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe), which are used as materials capable of transmitting the mid-far infrared region, are not glass. Unlike glass, such crystalline materials cannot be press-molded by utilizing softening caused by heating. Therefore, it is difficult to mass-produce aspherical lenses with complex shapes using such crystalline materials. Therefore, it is difficult to realize low-cost aspherical lenses for consumer use using these crystalline materials.

[0030] Specifically, the chalcogenide glass preferably contains, in mole percent, 20 to 90% tellurium (Te).More specifically, the chalcogenide glass preferably contains, in mole percent, 20 to 90% Te and 0 to 50% Ge+Ga.

[0031] In this specification, "%" means "mol %" unless otherwise specified. In this specification, "A1+A2+···" means the total amount of each corresponding component. However, this description means the content including at least one component selected from the group consisting of each corresponding component, and may be a composition that does not include a specific component from the above group. For example, a composition "A1+A2+A3+A4+A5: p~q% is preferred" includes a composition "A1+A2+A3+A4: p~q% (excluding A5)".

[0032] A preferred composition of the chalcogenide glass will be described below. Te forms a glass skeleton and is a component that easily increases the internal transmittance in the wavelength range of 10 μm or more. Te is also a component that easily increases the refractive index. The content of Te is preferably 20 to 90%, 30 to 88%, 40 to 84%, 50 to 82%, and particularly preferably 60 to 80%.

[0033] If the content of Te is too low, vitrification becomes difficult. If the content of Te is too high, Te-based crystals are likely to precipitate. In addition, other chalcogen elements Se and S tend to have lower internal transmittance in the wavelength range of 10 μm or more than Te. Therefore, the contents of Se and S are preferably 0 to 10%, 0 to 5%, 0 to 3%, and particularly 0 to 1%, respectively.

[0034] The chalcogenide glass preferably contains at least one of Ge and Ga in addition to Te. That is, Ge+Ga (the total amount of Ge and Ga) is preferably 0-50%, 1-40%, 3-35%, 5-30%, and particularly preferably 10-30%. By containing these components, the vitrification range can be expanded and the thermal stability (vitrification stability) of the glass can be improved. The preferred ranges of each of the Ge and Ga components are as follows:

[0035] Ge is a component that expands the vitrification range and enhances the thermal stability of glass. The Ge content is preferably 0 to 50%, 1 to 40%, 3 to 35%, 5 to 30%, 8 to 25%, and particularly preferably 10 to 20%. If the Ge content is too high, Ge-based crystals tend to precipitate and the raw material cost tends to increase.

[0036] Ga is a component that expands the vitrification range and enhances the thermal stability of glass. The Ga content is preferably 0 to 50%, 1 to 30%, 2 to 20%, 3 to 15%, and particularly preferably 4 to 10%. If the Ga content is too high, Ga-based crystals tend to precipitate and the raw material cost tends to increase.

[0037] From the viewpoint of increasing the stability of vitrification, it is preferable that the total content of Ge, Ga, and Te is large. Specifically, it is preferable that Ge+Ga+Te is 50% or more, 60% or more, 70% or more, and particularly 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 particularly 90% or less.

[0038] The chalcogenide glass may contain various components shown below in addition to the components described above.

[0039] Ag is a component that increases the thermal stability and refractive index of glass. The Ag content is preferably 0 to 50%, more than 0 to 50%, 1 to 45%, 2 to 40%, 3 to 35%, 4 to 30%, 5 to 25%, and particularly preferably 5 to 20%. If the Ag content is too high, vitrification becomes difficult.

[0040] Si is a component that enhances the thermal stability of glass. The Si content is preferably 0 to 50%, more than 0 to 50%, 1 to 45%, 2 to 40%, 3 to 35%, 4 to 30%, 5 to 25%, and particularly preferably 5 to 20%. If the Si content is too high, infrared absorption due to Si is likely to occur, making it difficult for infrared rays to transmit. However, since Si is a component that tends to reduce 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%.

[0041] Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn are components that enhance the thermal stability of glass without reducing infrared transmission properties. The content of Al+Ti+Cu+In+Sn+Bi+Cr+Sb+Zn+Mn (the total content of Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn) is preferably 0 to 40%, 2 to 35%, 4 to 30%, and particularly preferably 5 to 25%. If the content of Al+Ti+Cu+In+Sn+Bi+Cr+Sb+Zn+Mn is too high, vitrification becomes difficult.

[0042] The content of each of Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn is preferably 0 to 40%, 1 to 40%, 1 to 30%, 1 to 25%, and particularly preferably 1 to 20%. Among them, it is preferable to use Al, Cu, and / or Sn because they have a particularly large effect of increasing the thermal stability of the glass. However, since Al and Sn are components that tend to reduce 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 preferably less than 0.1%, respectively.

[0043] 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 to 40%, 2 to 35%, 4 to 30%, and particularly preferably 5 to 25%. If the content of F+Cl+Br+I is too high, vitrification becomes difficult and weather resistance is likely to decrease. The contents of each of the components F, Cl, Br, and I are preferably 0 to 40%, 1 to 40%, 1 to 30%, 1 to 25%, and particularly preferably 1 to 20%, respectively. Among them, I is preferable in that an elemental raw material can be used and it has a particularly large effect of enhancing the thermal stability of glass.

[0044] From the viewpoint of reducing the burden on the environment, it is particularly preferable that Se and As are not substantially contained. In the present invention, "substantially not contained" means that the content is less than 0.1 mol%. It is preferable that Cd, Tl and Pb are not substantially contained. In this way, the impact on the environment can be minimized.

[0045] <Details of each lens configuration> Furthermore, the infrared imaging lens 1 of this embodiment can have the following detailed configuration of each part.

[0046] In order to realize an imaging device capable of magnifying and observing a distant object, the infrared imaging lens 1 of this embodiment is configured so that the total system focal length fL is at least twice the diameter φs of the image circle, which may be defined as a half angle of view ω of 14° or less.

[0047] In particular, in the infrared imaging lens 1 of this embodiment, it is preferable that the total system focal length fL is within a range of 3 to 6 times the diameter φs of the image circle. This may be defined as a half angle of view ω being within a range of 5 to 10°. Such an infrared imaging lens 1 may be called a telephoto lens with a focal length in a range of 130 to 260 mm in 35 mm format. The absolute value of the total system focal length fL is preferably 20 to 50 mm.

[0048] The infrared imaging lens 1 of this embodiment is an infrared imaging lens that can be used in the infrared region including wavelengths in the range of 7 to 14 μm. By using the above-mentioned chalcogenide glass as the glass material of each lens, excellent characteristics can be obtained over the wide wavelength range of 7 to 14 μm.

[0049] According to this embodiment, it is possible to realize a bright imaging lens with an F-number in the range of 0.9 to 1.1 in such a telephoto lens. Also, it is possible to realize a telephoto lens in which light absorption by the lens glass material is small over a wide wavelength range of at least 7 to 14 μm. Therefore, combined with a small F-number of about 1, a bright imaging lens can be realized.

[0050] It is particularly preferable that the absolute values ​​of the powers of the lenses are arranged in descending order from largest to smallest as the third lens L3, the first lens L1, and the second lens L2. The third lens L3, which is closest to the image surface, has the largest power, and the infrared imaging lens 1 is configured so that all three lenses with such power settings have a meniscus shape with a convex surface facing the image surface S, thereby reducing astigmatism.

[0051] In the infrared imaging lens 1, the focal length f1 of the first lens and the focal length fL of the entire system are expressed as follows: 1.5≦f1 / fL≦2.5 It is preferable that the infrared imaging lens 1 is configured so as to satisfy the relational expression: In other words, it is preferable that the contribution of the first lens L1 to the total system focal length fL is not so large, and that the third lens L3 also contributes to the total system focal length fL. By configuring the infrared imaging lens 1 in this way, it is possible to obtain a high resolution compatible with an image sensor having a pixel pitch of about the wavelength while maintaining good aberration characteristics.

[0052] In the infrared imaging lens 1, the total optical length TTL, which is the distance on the optical axis from the object side surface (first surface) of the first lens L1 to the image surface S, and the total system focal length fL are 1.2≦TTL / fL≦2.0 It is preferable that the total optical length TTL is configured to satisfy the relational expression below. In other words, it is preferable that the total optical length TTL is configured to be slightly larger than the total system focal length fL. By configuring the infrared imaging lens 1 in this way, it is possible to obtain a high resolution compatible with an image sensor having a pixel pitch of about the wavelength while maintaining good aberration characteristics.

[0053] In the infrared imaging lens 1, it is preferable that the effective diameter of the object-side surface (first surface) of the first lens L1 is the aperture stop. This configuration reduces vignetting of peripheral light flux and improves the peripheral light amount. In particular, it is possible to ensure a relative illuminance of 94% or more even at the maximum image height, and coupled with small distortion, it is possible to configure an excellent imaging lens that can obtain natural images. Also, it is possible to reduce the outer diameter and volume of the infrared imaging lens 1 compared to inserting an aperture stop between the lenses.

[0054] In the infrared imaging lens 1, the back focus BFL and the total system focal length fL are 0.2≦BFL / fL It is desirable to configure the lens so as to satisfy the following relational expression. With this configuration, it is possible to realize a telephoto lens that is excellent in aberration characteristics and resolution while ensuring the back focus BFL.

[0055] At least one of the first lens L1 and the second lens L2 is preferably an aspherical lens. This makes it possible to reduce spherical aberration and astigmatism of the infrared imaging lens 1. In particular, it is preferable that both the first lens L1 and the second lens L2 are aspherical lenses. The object side surface (third surface) of the second lens L2 can be aspherical, and the image side surface (fourth surface) can be spherical. The third lens L3 may be a spherical lens.

[0056] Furthermore, it is preferable that any surface of the first lens L1 is a diffractive surface. This makes it possible to generate negative dispersion and reduce axial chromatic aberration and lateral chromatic aberration. In particular, it is preferable that the surface (second surface) on the image side of the first lens L1 is a diffractive surface. By making the concave surface a diffractive surface in this way, it becomes easier to form the diffractive surface by pressing.

[0057] The infrared imaging lens 1 is configured so that the modulation transfer function (MTF) in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm is 0.3 (30%) or more at an image height of 2.5 mm on the image surface S. Here, the MTF value is a simple average value in the tangential direction and the sagittal direction. The reason for focusing on the spatial frequency of 41.7 cycles / mm and the image height of 2.5 mm will be explained below.

[0058] As image sensors in the mid-to-far infrared region become smaller, the pixel pitch has reached the narrowest possible pitch limit of about the wavelength. Such miniaturized image sensors can be produced at lower cost than larger area sensors. Moreover, the imaging lens applied to the image sensor can also be made smaller in diameter to match the area of ​​the image sensor, reducing costs.

[0059] Therefore, by applying such an image sensor and imaging lens to an infrared camera, it is possible to realize low cost suitable for consumer use, and infrared cameras can be deployed in various fields. Image sensors with a pixel pitch of 12 μm are now commercially available for the 10 μm wavelength band. The spatial frequency of 41.7 cycles / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 12 μm.

[0060] An MTF of 0.3 or more at an image height of 2.5 mm indicates that a sufficient resolution of MTF 0.3 or more can be obtained over the entire area of ​​an image sensor with a pixel pitch of 12 μm and 320 × 256 pixels arranged on the image surface S. In other words, the infrared imaging lens 1 is a telephoto lens that is fully compatible with infrared cameras that use a miniaturized QVGA (320 × 240 pixels) class image sensor that corresponds to a wavelength range of about 7 to 14 μm.

[0061] Furthermore, the infrared imaging lens 1 is configured so that the MTF in the wavelength range of 7 to 14 μm at a spatial frequency of 29.4 cycles / mm is 0.45 (45%) or more at an image height of 4.1 mm on the image surface S. The spatial frequency of 29.4 cycles / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 17 μm.

[0062] Furthermore, the MTF of 0.45 or more at an image height of 4.1 mm and a spatial frequency of 29.4 cycles / mm indicates that a good resolution of MTF of 0.45 or more can be obtained over the entire area of ​​the image sensor with a pixel pitch of 17 μm and 384 × 288 pixels arranged on the image surface S. In other words, the infrared imaging lens 1 is a telephoto lens with a resolution sufficient for infrared cameras using image sensors with a pixel pitch of about 12 to 17 μm in a wavelength range of about 7 to 14 μm.

[0063] <Numerical Example 1> An example of an infrared imaging lens 1 is shown below. A cross-sectional view of the infrared imaging lens according to Numerical Example 1 is as shown in FIG. 1. In Numerical Example 1, r represents the radius of curvature, d represents the lens thickness on the optical axis or the distance between the surfaces, and ED represents the effective diameter (diameter). The unit of length is (mm). An asterisk (*) after the surface number indicates that the surface is aspheric, and DOE indicates that the surface is diffractive. Basic lens data, aspheric surface data, diffractive surface data, and various data are shown below. [Table 1] The refractive index and the Abbe number ν are defined 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) [Table 2] The definition of an aspheric shape is as follows:

number

number

number

number

[0064] Silicon (Si) is used for the parallel plate P. The parallel plate P can be an optical window associated with an image sensor arranged on the image surface S. The position of the parallel plate P can be shifted from the value in Table 1 without affecting the optical characteristics of the infrared imaging lens 1. In other words, the distance between the sixth and seventh surfaces (d6=4.00 mm in Table 1) and the distance between the eighth surface and the image surface S (d8=2.93 mm in Table 1) can be changed while keeping their total values ​​the same. The back focus BFL=7.93 mm is the actual distance.

[0065] The maximum image height on the image plane S is 4.1 mm, and therefore the diameter φs of the image circle is 8.2 mm. Therefore, the infrared imaging lens 1 is applicable to a QVGA class image sensor with a diagonal length of 8.16 mm and a pixel pitch of 17 μm and 384×288 pixels. The infrared imaging lens 1 can also cover the pixel area of ​​a QVGA class image sensor, including QVGA (320×240 pixels) and QVGA+ (345×240 pixels) with a pixel pitch of 17 μm.

[0066] 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 × 256 pixels. Note that with a configuration of 384 × 288 pixels, 320 × 256 pixels, etc., the effective pixel count can be secured as QVGA (320 × 240 pixels) even if the optical axis center of the lens does not completely coincide with the center of the image sensor.

[0067] The ratio of the focal length fL of the infrared imaging lens 1 to the diameter φs of the image circle is fL / φs=3.41 That is, the infrared imaging lens 1 is a telephoto lens. The half angle of view ω is 8.4°, which is within the range of 14° or less that can be considered a telephoto lens. Although the infrared imaging lens 1 is a telephoto lens with such a narrow angle of view, it is an extremely bright imaging lens with an F-number of 1.0.

[0068] The infrared imaging lens 1 is compact, with a total optical length TTL (total lens length) of 38.5 mm from the object side surface (first surface) of the first lens L1 to the image surface S, and a maximum effective diameter on the optical path of 28.0 mm. Such a compact configuration is achieved by the first lens L1 having positive power, the second lens L2 having negative power, and the third lens L3 having positive power each being a meniscus lens having a meniscus shape convex toward the image surface side.

[0069] In addition, the infrared imaging lens 1 is lightweight because it is made up of three lenses. Each lens can be manufactured by press molding, and the infrared imaging lens 1 can be manufactured at a low cost suitable for consumer use. Furthermore, an infrared camera using the infrared imaging lens 1 can be made compact, making it possible to realize an infrared camera that can be easily installed in a variety of locations.

[0070] The focal length f1 of the first lens L1 is 45.42 mm. Therefore, the ratio of the focal length f1 of the first lens L1 to the focal length fL of the entire system of the infrared imaging lens 1 is f1 / fL=1.62 The focal length f2 of the second lens L2 is -369.0 mm. The focal length f3 of the third lens L3 is 27.99 mm. Therefore, the infrared imaging lens 1 is configured so that the absolute value of the power of the third lens L3 is the largest, and the absolute value of the power of the first lens L1 is next largest.

[0071] The ratio of the total optical length TTL (lens length) of the infrared imaging lens 1 to the total system focal length fL is TTL / fL=1.38 The back focus BFL is 7.93 mm (actual distance), which is sufficient distance. The ratio of the total optical length TTL (total lens length) to the back focus BFL is: TTL / BFL=4.85 The ratio of the back focus BFL to the focal length fL of the entire system is BFL / fL=0.28 and 0.2≦BFL / fL is satisfied.

[0072] Various performances of Numerical Example 1 of the infrared imaging lens 1 are shown in Figs. 2 to 6. Figs. 2 and 3 are aberration diagrams of the infrared imaging lens 1. Fig. 2 shows spherical aberration, astigmatism, and distortion. In each of these diagrams, graphs are shown for each wavelength in the range of 7 to 14 µm. Fig. 3 is an aberration diagram showing coma aberration at each image height Y from 0 mm to the maximum image height, divided into the tangential (meridional) direction and the sagittal (radial) direction.

[0073] 2 and 3, in the infrared imaging lens 1 according to Numerical Example 1, various aberrations are well corrected over a wide wavelength range of 7 to 14 μm. In particular, in the infrared imaging lens 1, the distortion is extremely small at 0.37% or less over the entire image circle at each wavelength in the range of 7 to 14 μm, and excellent characteristics are obtained.

[0074] Fig. 4 is a graph showing the relative illuminance versus image height Y in Numerical Example 1 of the infrared imaging lens 1. Here, the relative illuminance refers to the ratio of illuminance to the area on the optical axis (the central area of ​​the image surface) on the image surface S. As shown in Fig. 4, even at the maximum image height of 4.1 mm, the relative illuminance is 0.94, and a nearly uniform light amount distribution is obtained within the image circle.

[0075] Figure 5 is a graph showing the spatial frequency dependence of MTF in the wavelength range of 7 to 14 μm. The Nyquist frequency f N is 41.7 cycles / mm, and the maximum image height is 2.46 mm. N At 41.7 cycles / mm, the MTF at the center of the image is 0.46, and an MTF of >0.37 (simple average of the tangential and sagittal directions) is ensured within the area of ​​the image sensor.

[0076] In addition, the Nyquist frequency f N is 29.4 cycles / mm, and the maximum image height is 4.08 mm. N At 0.45, the MTF at the center of the image is 0.58, and within the area of ​​the image sensor, an MTF of >0.49 (simple average of the tangential and sagittal directions) is ensured.

[0077] Thus, the infrared imaging lens 1 ensures good resolution even when evaluated by MTF over a wide wavelength range of 7 to 14 μm within the region of a QVGA class image sensor with a pixel pitch of about 12 to 17 μm.

[0078] It goes without saying that the infrared imaging lens 1 can provide good characteristics in any wavelength range, such as 7 to 12 μm, 8 to 12 μm, 8 to 10 μm, etc., as long as the wavelength range is within 7 to 14 μm. Figure 6 is a graph showing the change in MTF in the wavelength range of 7 to 14 μm with respect to focal point movement.

[0079] As described above, the infrared imaging lens 1 of Numerical Example 1 can cover a wavelength range of 7 to 14 μm and has a satisfactory resolution that is fully compatible with a QVGA class image sensor having a pixel pitch of about 12 to 17 μm. Moreover, the distortion is extremely small, at 0.37% or less, over the entire image circle.

[0080] The infrared imaging lens 1 of Numerical Example 1 has a bright F-number of 1.0, a lens total length (total optical length TTL) of 38.5 mm, and a compact maximum effective lens diameter of 28.0 mm. Thus, according to this embodiment, an infrared imaging lens that is a telephoto lens that is compact and has excellent characteristics, which has not been seen in the past, can be realized.

[0081] 〔summary〕 A first aspect of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in this order from the object side to the image surface side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, a focal length fL of the entire system being equal to or greater than twice the diameter of an image circle, the first lens and the third lens being meniscus lenses having positive power, and the second lens being a meniscus lens having negative power.

[0082] According to the above configuration, it is possible to realize an infrared imaging lens that is a telephoto lens that has excellent resolution and small distortion and is compatible with an image sensor having a pixel pitch on the order of a wavelength.

[0083] A second aspect of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in this order from the object side to the image side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, the first lens and the third lens being meniscus lenses having positive power, the second lens being a meniscus lens having negative power, and a half angle of view being 14° or less.

[0084] According to the above configuration, it is possible to realize an infrared imaging lens that is a telephoto lens that has excellent resolution and small distortion and is compatible with an image sensor having a pixel pitch on the order of a wavelength.

[0085] In the infrared imaging lens according to aspect 3 of the present invention, in the above aspect 1 or 2, the first lens, the second lens, and the third lens each have a meniscus shape convex toward the object side. With the above configuration, it is possible to realize an infrared imaging lens that is a compact telephoto lens with excellent aberration characteristics.

[0086] According to a fourth aspect of the present invention, in the infrared imaging lens of any one of the first to third aspects, the effective diameter of the object-side surface of the first lens is an aperture stop. With the above configuration, vignetting of peripheral light flux is reduced, and the amount of peripheral light can be improved.

[0087] An infrared imaging lens according to a fifth aspect of the present invention is the infrared imaging lens according to any one of the first to fourth aspects, wherein at least one of the first lens and the second lens is an aspheric lens. With the above configuration, it is possible to realize an infrared imaging lens with particularly excellent aberration characteristics.

[0088] The infrared imaging lens according to aspect 6 of the present invention is the infrared imaging lens according to any one of aspects 1 to 5, wherein any surface of the first lens is a diffractive surface. With the above configuration, it is possible to reduce axial chromatic aberration and lateral chromatic aberration.

[0089] An infrared imaging lens according to a seventh aspect of the present invention is an infrared imaging lens in which the image-side surface of the first lens is a diffractive surface in any one of the above-mentioned aspects 1 to 6. With the above configuration, it is easy to form a diffractive surface, and it is therefore possible to realize an infrared imaging lens that can reduce axial chromatic aberration and chromatic aberration of magnification.

[0090] An infrared imaging lens according to aspect 8 of the present invention is the infrared imaging lens of any one of aspects 1 to 7, in which the focal length f1 of the first lens and the focal length fL of the entire system satisfy the relational expression 1.5≦f1 / fL≦2.5. With the above configuration, it is possible to obtain high resolution compatible with an image sensor having a pixel pitch on the order of the wavelength while maintaining good aberration characteristics.

[0091] An infrared imaging lens according to a ninth aspect of the present invention is an infrared imaging lens in any one of the first to eighth aspects, in which a total optical length TTL, which is the distance on the optical axis from the object-side surface of the first lens to the image plane, and a total system focal length fL satisfy the relational expression of 1.2≦TTL / fL≦2.0. With the above configuration, it is possible to obtain a high resolution compatible with an image sensor having a pixel pitch on the order of the wavelength while maintaining good aberration characteristics.

[0092] An infrared imaging lens according to Aspect 10 of the present invention is the same as any one of Aspects 1 to 9, in which the back focus BFL and the total system focal length fL satisfy the relational expression of 0.2≦BFL / fL. With the above configuration, it is possible to realize a telephoto lens that is excellent in aberration characteristics and resolution while ensuring the back focus BFL.

[0093] An infrared imaging lens according to an eleventh aspect of the present invention is an infrared imaging lens in which the relative illuminance on the image plane is 94% or more within the image circle in any one of the above aspects 1 to 10. With the above configuration, it is possible to realize an infrared imaging lens in which a sufficient amount of peripheral light is ensured.

[0094] An infrared imaging lens according to a twelfth aspect of the present invention, in any one of the above aspects 1 to 11, has a modulation transfer function of 0.3 or more in a wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm at an image height of 2.5 mm. With the above configuration, it is possible to realize an infrared imaging lens that can provide good resolution over the entire region of an image sensor having a pixel pitch of approximately the wavelength.

[0095] An infrared imaging lens according to Aspect 13 of the present invention is an infrared imaging lens having a total system focal length fL within a range of 3 to 6 times the diameter of the image circle in any one of Aspects 1 to 12. With the above configuration, it is possible to realize an infrared imaging lens that is a telephoto lens excellent in aberration characteristics and resolution.

[0096] An infrared imaging lens according to aspect 14 of the present invention has an F-number in the range of 0.9 to 1.1 in any one of aspects 1 to 13. With 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.

[0097] An infrared imaging lens according to Aspect 15 of the present invention is the infrared imaging lens of any one of Aspects 1 to 13, wherein the first lens, the second lens, and the third lens are each made of chalcogenide glass having a refractive index of 3.0 to 4.0 at a wavelength of 10 μm. According to the above configuration, it is possible to realize an infrared imaging lens that is a compact telephoto lens with excellent resolution and small distortion that can be used with an image sensor having a pixel pitch on the order of the wavelength.

[0098] An infrared imaging lens according to Aspect 16 of the present invention is any one of Aspects 1 to 14, 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%. With the above configuration, it is possible to configure an infrared imaging lens with very little light absorption at least in the wavelength range of 7 to 14 μm.

[0099] The present invention is not limited to the above-mentioned embodiment, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in the specification. Furthermore, new technical features can be formed by combining the technical means disclosed in the specification. [Explanation of symbols]

[0100] 1. Infrared imaging lens L1 First lens L2 Second lens L3 Third lens P parallel plate S image plane AP Aperture

Claims

1. An infrared imaging lens for use in an infrared region including at least any wavelength in the range of 7 to 14 μm, A first lens, a second lens, and a third lens are arranged in this order from the object side to the image plane side, each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm; The focal length fL of the entire system is equal to or greater than twice the diameter of the image circle, The infrared imaging lens, wherein the first lens and the third lens are meniscus lenses having positive power, and the second lens is a meniscus lens having negative power.

2. An infrared imaging lens for use in an infrared region including at least any wavelength in the range of 7 to 14 μm, A first lens, a second lens, and a third lens are arranged in this order from the object side to the image plane side, each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm; the first lens and the third lens are meniscus lenses having positive power, and the second lens is a meniscus lens having negative power; An infrared imaging lens having a half angle of view of 14° or less.

3. The infrared imaging lens according to claim 1 , wherein each of the first lens, the second lens, and the third lens has a meniscus shape convex toward an object side.

4. 3. The infrared imaging lens according to claim 1, wherein an effective diameter of the object-side surface of said first lens is an aperture stop.

5. The infrared imaging lens according to claim 1 or 2, wherein at least one of the first lens and the second lens is an aspheric lens.

6. The infrared imaging lens according to claim 5 , wherein any surface of the first lens is a diffractive surface.

7. The infrared imaging lens according to claim 6 , wherein the surface of the first lens on the image side is a diffractive surface.

8. The focal length f1 of the first lens and the focal length fL of the entire system are 1.5≦f1 / fL≦2.5 The infrared imaging lens according to claim 1 or 2, which satisfies the following relational expression:

9. a total optical length TTL, which is the distance on the optical axis from the object side surface of the first lens to the image plane, and a total system focal length fL, 1.2≦TTL / fL≦2.0 The infrared imaging lens according to claim 1 or 2, which satisfies the following relational expression:

10. The back focus BFL and the total system focal length fL are 0.2≦BFL / fL The infrared imaging lens according to claim 1 or 2, which satisfies the following relational expression:

11. 3. The infrared imaging lens according to claim 1, wherein the relative illuminance on the image plane is 94% or more within an image circle.

12. 3. The infrared imaging lens according to claim 1, wherein a modulation transfer function in a wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm is 0.3 or more at an image height of 2.5 mm.

13. 3. The infrared imaging lens according to claim 1, wherein a total system focal length fL is within a range of 3 to 6 times the diameter of the image circle.

14. The infrared imaging lens according to claim 1 or 2, wherein the F-number is in the range of 0.9 to 1.

1.

15. 3. The infrared imaging lens according to claim 1, wherein each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 3.0 to 4.0 at a wavelength of 10 μm.

16. 3. The infrared imaging lens according to claim 1, wherein the chalcogenide glass has an infrared absorption edge wavelength of 18 [mu]m or more at which the light transmittance at a thickness of 2 mm is 20%.

Images