Infrared imaging lens

Abstract

A telephoto lens with excellent resolution is achieved. In the infrared imaging lens (1), each of a first lens (L1), a second lens (L2), and a third lens (L3) is composed of chalcogenide glass having a refractive index of 2.5-4.0 at a wavelength of 10 [mu] m, the total system focal length is at least two times the diameter of an imaging circle, the first lens and the third lens are meniscus lenses having positive refractive power, and the refractive index of the first lens (L1), the second lens (L2), and the third lens (L3) is at least two times the diameter of the imaging circle. The second lens is a meniscus lens with negative focal power.

Description

Technical Field

[0001] This invention relates to an infrared imaging lens. Background Technology

[0002] Infrared cameras, which use infrared light in the mid-to-far infrared region, especially the 10μm wavelength range suitable for biological detection, to image subjects, are used in surveillance cameras, anti-theft cameras, and vehicle-mounted night vision systems. These infrared cameras can be applied in various fields such as intruder surveillance, monitoring of illegal activities like illegal fishing, forest fire source detection, traffic monitoring, and obstacle detection, and the demand is expected to expand further.

[0003] As infrared imaging lenses used in infrared cameras, telephoto lenses, which have a relatively long focal length relative to the diameter of the image circle of the image plane, are required for applications such as long-distance nighttime surveillance. As such an infrared imaging lens, the applicant proposes a telephoto lens with excellent resolution, a small and bright F-number, and compatibility with small image sensors with pixel pitch at the wavelength level.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: International Publication No. WO2023 / 008148 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] With the expanding applications of infrared cameras, there is a demand for infrared imaging lenses with various characteristics. For example, compared to the prior art in Patent Document 1, there is a desire to achieve infrared imaging lenses that further reduce distortion (aberration) while maintaining brightness and resolution. One aspect of the present invention addresses the above-mentioned issues, with the aim of realizing an infrared imaging lens that can be used as a telephoto lens, compatible with image sensors with wavelength-level pixel pitch, possessing excellent resolution, further reducing distortion, and suitable for civilian applications.

[0009] means for solving problems

[0010] To address the aforementioned issues, one aspect of the present invention is an infrared imaging lens used in an infrared region encompassing at least any wavelength within the range of 7–14 μm, wherein a first lens, a second lens, and a third lens are sequentially arranged from the object side toward the image plane side. Each of the first lens, the second lens, and the third lens is made of chalcogenide glass with a refractive index of 2.5–4.0 at a wavelength of 10 μm, and the total system focal length fL is more than twice the diameter of the imaging ring. The first lens and the third lens are meniscus lenses with positive optical power, and the second lens is a meniscus lens with negative optical power.

[0011] In addition, to solve the aforementioned problem, another aspect of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength in the range of 7 to 14 μm, wherein a first lens, a second lens, and a third lens are arranged sequentially 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 with 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 with positive optical power, the second lens is a meniscus lens with negative optical power, and the half field of view is less than 14°.

[0012] The effects of the invention

[0013] According to the above-described technical solution of the present invention, an infrared imaging lens that can be used as a telephoto lens, which is compatible with image sensors with wavelength-level pixel pitch, has excellent resolution, further reduces distortion, and can be used for civilian purposes, can be realized. Attached Figure Description

[0014] Figure 1 This is a cross-sectional view showing the structure of the main part of the infrared imaging lens according to an embodiment of the present invention.

[0015] Figure 2 This is an aberration diagram showing the spherical aberration, astigmatism, and distortion of the infrared imaging lens involved in the numerical embodiment 1 of the present invention.

[0016] Figure 3 This is an aberration diagram representing the coma of the infrared imaging lens involved in numerical embodiment 1 of the present invention.

[0017] Figure 4 This is a graph showing the image height dependence of the relative illumination of the infrared imaging lens involved in the numerical embodiment 1 of the present invention.

[0018] Figure 5 This is a graph showing the spatial frequency dependence of the MTF of the infrared imaging lens in the wavelength range of 7 to 14 μm according to the numerical embodiment 1 of the present invention.

[0019] Figure 6 This is a graph showing the focus shift dependence of the MTF of the infrared imaging lens according to numerical embodiment 1 of the present invention. Detailed Implementation

[0020] [Implementation Method]

[0021] <Overview of Infrared Imaging Lenses>

[0022] The infrared imaging lens 1 described in this embodiment is a lens system that images an image of a subject on an image plane S equipped with an image sensor or the like, corresponding to the mid- and far-infrared wavelength region. The infrared imaging lens 1 described in this embodiment uses the entire system focal length fL as the diameter of the imaging circle. The target is a telephoto lens that is twice or more the length of s. Alternatively, the infrared imaging lens 1 according to this embodiment is a telephoto lens with a half-field angle ω (maximum half-field angle) of 14° or less.

[0023] In this application, the imaging circle refers to the effective diameter range of the optical system on the image plane S of the imaging lens. That is, the imaging circle refers to the circular range from the image height Y to the maximum image height on the image plane S. Here, the diameter of the imaging circle... Similar to expressions like "lens diameter," 's' represents the diameter of such a circular range. That is, the diameter of the imaging circle. s is equal to twice the maximum image height.

[0024] As is known in the field of imaging lenses, the half field of view ω of an imaging lens is related to the total focal length fL of the imaging lens system and the diameter of the image circle. s has: ω=arctan[ s / (2fL)] The relationship is as follows: where arctan is the arctangent function. The focal length of the entire system, fL, is the diameter of the imaging circle. More than twice that of s (fL / (s≥2), which is equivalent to a half-angle ω of less than 14°.

[0025] The total system focal length fL is the diameter of the imaging circle. An imaging lens with a focal length greater than twice that of the image sensor (s) or less, or a half-field-of-view (ω) of less than 14°, is generally referred to as a telephoto lens, having an equivalent focal length of 85mm or more in a 35mm format. Furthermore, in this 35mm format equivalent conversion, it is assumed that the diagonal length of the image sensor is approximately the same as the diameter of the imaging circle, and that the image sensor effectively utilizes the imaging circle. Thus, this embodiment focuses on an infrared imaging lens for a camera device capable of magnifying and observing distant objects.

[0026] Figure 1 This is a cross-sectional view along the optical axis showing the structure of the main parts of the infrared imaging lens 1. The infrared imaging lens 1 is composed of a first lens L1, a second lens L2, and a third lens L3 arranged sequentially from the object side to the image plane S side. During focusing, the first lens L1 to the third lens L3 move along the optical axis.

[0027] Each of the first lens L1, the second lens L2, and the third lens L3 is made of chalcogenide glass with a refractive index N10 of 2.5 to 4.0 at a wavelength of 10 μm. Here, the symbol N10 specifically represents the refractive index at a wavelength of 10 μm. The first lens L1, the second lens L2, and the third lens L3 can all be made of chalcogenide glass as the same glass material. In particular, it is preferable that each of the first lens L1, the second lens L2, and the third lens L3 is made of chalcogenide glass with a refractive index in the range of 3.0 to 4.0, and more preferably 3.0 to 3.7, at a wavelength of 10 μm.

[0028] The first lens L1 has positive optical power and a meniscus shape. The second lens L2 has negative optical power and a meniscus shape. The third lens L3 has positive optical power and a meniscus shape. By configuring and arranging the lenses in this way, the Petzval sum can be reduced, resulting in excellent resolution, and a telephoto lens with further reduced distortion can be achieved. In particular, it is preferable that the first lens L1, the second lens L2, and the third lens L3 all have a meniscus shape that bulges towards the object side.

[0029] like Figure 1 As shown, a parallel plate P is disposed between the third lens L3 and the image plane S. The parallel plate P is an optical window mounted on the image plane S side in a hermetically sealed manner, and is made of silicon, low-oxygen silicon, or germanium. The material and thickness can be determined according to the type of image sensor used.

[0030] like Figure 1As indicated by the symbol AP, 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. An anti-reflection coating is applied to the surfaces of the first lens L1, the second lens L2, the third lens L3, and the parallel plate P. Such an anti-reflection coating in the mid-to-far infrared region can be achieved using appropriate known techniques.

[0031] <Glass material of each lens>

[0032] The following describes the chalcogenide glass used in each lens constituting the infrared imaging lens 1. The chalcogenide glass used in the infrared imaging lens 1 is a 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 refractive index of 2.5 to 4.0, the infrared imaging lens 1 has an F-number of about 1.0, is bright, and has excellent resolution suitable for image sensors with pixel pitches equivalent to 7 to 14 μm in the mid-to-far infrared region.

[0033] Such chalcogenide glasses with high refractive index in the far-infrared region have been developed by the applicant (see International Publication No. WO2020 / 105719A1). More specifically, the refractive index N10 of this glass material at a wavelength of 10 μm ranges from 2.74 to 3.92. Furthermore, 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 compactly construct a telephoto lens. Therefore, the refractive index of the aforementioned chalcogenide glass at a wavelength of 10 μm is particularly preferably in the range of 3.0 to 4.0.

[0034] Furthermore, the Abbe number (ν10) of chalcogenide glasses is preferably 100 or higher, 120 or higher, 150 or higher, or 180 or higher, and particularly preferably 220 or higher. The definition of Abbe number (ν10) will be described later. When the Abbe number is too low, chromatic aberration tends to increase. Moreover, there is no particular upper limit to the Abbe number; in the aforementioned chalcogenide glasses, it is practically 350 or lower.

[0035] This glass material exhibits minimal light absorption over a wide wavelength range in the far-infrared region, at least from 7 to 14 μm. In particular, it possesses the characteristic of low light absorption even in the far-infrared region from 10 to 26 μm. In chalcogenide glasses, the "infrared absorption wavelength" and "internal transmittance" can be used as indicators of excellent light transmittance in the far-infrared region.

[0036] The infrared absorption wavelength refers to the absorption wavelength in the region with a wavelength of 8 μm or more, and is defined as the wavelength at which the light transmittance is 20% when the material thickness is 2 mm. Furthermore, internal transmittance refers to the transmittance within the material, excluding reflection losses at the material surface. The infrared absorption wavelength of the chalcogenide glass, which constitutes each lens, is 18 μm or more.

[0037] Therefore, the aforementioned chalcogenide glass also transmits infrared light with wavelengths exceeding 10 μm, exhibiting good transmittance at least within a wavelength range of 7 to 14 μm. Furthermore, the internal transmittance of the aforementioned chalcogenide glass with a thickness of 2 mm is over 90% at a wavelength of 10 μm. Thus, in the infrared imaging lens 1 of this embodiment, an imaging lens with low light absorption of the lens glass material can be achieved, at least within a wide wavelength range of 7 to 14 μm.

[0038] Furthermore, by using the aforementioned chalcogenide glass, mass production of lenses based on molding is readily achievable. Additionally, mass production of aspherical lenses is also possible through molding. In this specification, aspherical lenses include diffractive surfaces. Moreover, the glass transition temperature of the preferred glass material is preferably below 200°C, making molding even easier.

[0039] In the infrared imaging lens 1 of this embodiment, aberrations are suppressed by making at least one of the lenses as an aspherical lens. When aspherical lenses cannot be used, the structure of the infrared imaging lens 1 used to suppress aberrations becomes a structure with an increased number of lens elements, resulting in increased weight and larger size. Furthermore, this increases cost, making it unsuitable for civilian use. In this specification, aspherical includes diffractive surfaces.

[0040] Furthermore, the aforementioned chalcogenide glass can also be used to mold lenses with particularly complex shapes, such as diffraction surfaces. Therefore, in the infrared imaging lens 1, by using the aforementioned chalcogenide glass to make the image-side surface (second surface) of the first lens L1 a diffraction surface, aberrations can be well suppressed over a wide wavelength range of 7 to 14 μm.

[0041] In particular, chalcogenide glasses with a refractive index of 3.0 to 3.7 at a wavelength of 10 μm exhibit excellent mass production properties, and the applicant is currently advancing their industrial production. Furthermore, such chalcogenide glasses with a refractive index of 3.0 to 3.7 also exhibit excellent processability when forming lenses. Therefore, the chalcogenide glasses constituting the first lens L1, the second lens L2, and the third lens L3 are particularly preferably composed of chalcogenide glasses with a refractive index of 3.0 to 3.7 at a wavelength of 10 μm.

[0042] Materials with crystalline systems such as silicon (Si), germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe), which are capable of transmitting light in the mid-to-far infrared region, are not glass. Unlike glass, these crystalline materials cannot be molded using methods that soften them through heating. Therefore, it is difficult to mass-produce aspherical lenses with complex shapes using these crystalline materials. Consequently, it is difficult to achieve low-cost aspherical lenses suitable for civilian use using these crystalline materials.

[0043] Specifically, the chalcogenide glass mentioned above is preferably a chalcogenide glass containing 20 to 90% tellurium (Te) in mol%. More specifically, it is preferably a chalcogenide glass containing 20 to 90% Te and 0 to 50% Ge + Ga in mol%.

[0044] Furthermore, unless otherwise specified, "%" in this specification refers to "molar percentage". Additionally, in this specification, "A1+A2+..." refers to the total amount of each corresponding component. This description indicates that the composition includes at least one component selected from the group consisting of the corresponding components, or it may include a composition that does not include a specific component from the aforementioned group. For example, a composition of "preferred A1+A2+A3+A4+A5: p~q%" includes a composition such as "A1+A2+A3+A4: p~q% (excluding A5)".

[0045] The preferred composition of the chalcogenide glass described above will be explained below. Te is a component that forms the glass framework and easily improves the internal transmittance in the wavelength region of 10 μm and above. In addition, Te is also a component that easily improves the refractive index. The Te content is preferably 20-90%, 30-88%, 40-84%, 50-82%, and particularly preferably 60-80%.

[0046] If the Te content is too low, vitrification is difficult. If the Te content is too high, Te-based crystals tend to precipitate easily. Furthermore, compared to Te, other chalcogenides, Se and S, tend to have lower internal transmittance in the wavelength region above 10 μm. Therefore, the contents of Se and S are preferably 0–10%, 0–5%, and 0–3%, respectively, and particularly preferably 0–1%.

[0047] The aforementioned chalcogenide glasses preferably contain at least one of Ge and Ga in addition to Te. Specifically, the Ge+Ga (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 glass transition range can be broadened, and the thermal stability (glass transition stability) of the glass can be improved. Furthermore, the preferred ranges for each of the Ge and Ga components are described below.

[0048] Ge is a component that expands the glass transition range and improves the thermal stability of glass. The preferred Ge content is 0-50%, 1-40%, 3-35%, 5-30%, 8-25%, and particularly preferred is 10-20%. If the Ge content is too high, Ge-based crystals are more likely to precipitate, and there is a tendency for raw material costs to increase.

[0049] Ga is a component that expands the glass transition range and improves the thermal stability of glass. The preferred Ga content is 0-50%, 1-30%, 2-20%, 3-15%, and particularly preferred is 4-10%. If the Ga content is too high, Ga-based crystals are more likely to precipitate, and there is a tendency for raw material costs to increase.

[0050] Furthermore, from the viewpoint of improving the stability of vitrification, a higher total content of Ge, Ga, and Te is preferred. Specifically, the content of Ge + Ga + Te is preferably 50% or more, 60% or more, 70% or more, and particularly preferably 80% or more. However, in order to introduce other components, the upper limit of Ge + Ga + Te can be set to 98% or less, 96% or less, 95% or less, and especially 90% or less.

[0051] In addition to the components mentioned above, the chalcogenide glasses may also contain various components as shown below.

[0052] Ag is a component that improves the thermal stability and refractive index of glass. The preferred Ag content is 0-50%, more than 0 but less than 50%, 1-45%, 2-40%, 3-35%, 4-30%, 5-25%, and particularly preferred to be 5-20%. When the Ag content is too high, it becomes difficult to vitrify.

[0053] Si is a component that improves the thermal stability of glass. The preferred Si content is 0–50%, more than 0 but less than 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 caused by Si easily occurs, making it difficult for infrared rays to pass through. However, Si tends to reduce the Abbe number; therefore, from the viewpoint of increasing the Abbe number, the Si content is preferably less than 5%, less than 1%, less than 0.5%, and particularly preferably less than 0.1%.

[0054] Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn are components that improve the thermal stability of glass without reducing its infrared transmission characteristics. 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%, or 4–30%, and particularly preferably 5–25%. If the content of Al+Ti+Cu+In+Sn+Bi+Cr+Sb+Zn+Mn is too high, vitrification will be difficult.

[0055] Furthermore, the contents of each component Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn are preferably 0–40%, 1–40%, 1–30%, 1–25%, and particularly preferably 1–20%, respectively. Among these, Al, Cu, and / or Sn are preferred for their particularly significant effect on improving the thermal stability of the glass. However, Al and Sn are components that tend to decrease the Abbe number; therefore, from the viewpoint of increasing the Abbe number, the contents of Al and Sn are preferably 5% or less, 1% or less, 0.5%, and particularly preferably less than 0.1%, respectively.

[0056] F, Cl, Br, and I are also components that improve the thermal stability of glass. The content of F+Cl+Br+I (the total amount of F, Cl, Br, and I) is preferably 0-40%, 2-35%, and 4-30%, particularly preferably 5-25%. If the content of F+Cl+Br+I is too high, vitrification will be difficult, and weather resistance will easily decrease. Furthermore, the content of each component of F, Cl, Br, and I is preferably 0-40%, 1-40%, 1-30%, 1-25%, and particularly preferably 1-20%. Among them, I can be used as a raw material in its elemental form, and it is preferred in terms of its particularly significant effect on improving the thermal stability of glass.

[0057] Furthermore, from the viewpoint of particularly reducing environmental impact, it is especially preferable to have substances substantially free of Se and As. In this invention, "substantially free of" means that their content is less than 0.1 mol%. Preferably, it is substantially free of Cd, Tl, and Pb. In this way, the environmental impact can be minimized.

[0058] <Detailed information about the structure of each lens>

[0059] Furthermore, the infrared imaging lens 1 of this embodiment can be configured with the following detailed information.

[0060] In order to realize a camera device capable of magnifying and observing objects at a distance, the infrared imaging lens 1 in this embodiment is configured such that the focal length fL of the entire system is the diameter of the imaging circle. More than twice that of s. This can be defined as a half-field angle ω of less than 14°.

[0061] In particular, in the infrared imaging lens 1 of this embodiment, it is preferably configured such that the focal length fL of the entire system is within the diameter of the imaging circle. The range is 3 to 6 times that of s. This can be defined as a half-field angle ω in the range of 5-10°. Such an infrared imaging lens 1 can also be referred to as a telephoto lens with a 35mm equivalent focal length in the range of 130-260mm. As the absolute value of the total system focal length fL, it is preferably 20-50mm.

[0062] The infrared imaging lens 1 of this embodiment is an infrared imaging lens that can be used in the infrared region with wavelengths ranging from 7 to 14 μm. By applying the aforementioned chalcogenide glass as the glass material for each lens, excellent properties can be obtained in a wide wavelength range of 7 to 14 μm.

[0063] According to this embodiment, such a telephoto lens can achieve a bright imaging lens with an F-number in the range of 0.9 to 1.1. Furthermore, it is possible to achieve a telephoto lens with low light absorption due to the lens glass material in a wavelength range of at least 7 to 14 μm. Therefore, combined with a small F-number of approximately 1, a bright imaging lens can be achieved.

[0064] Furthermore, it is particularly preferred that the absolute values ​​of the optical power of each lens, in descending order, are the third lens L3, the first lens L1, and the second lens L2. The third lens L3, located closest to the image plane, has the largest optical power. The three lenses with such optical power are configured to form the infrared imaging lens 1 in such a manner that all have a meniscus shape with the convex surface facing the image plane S side, thereby reducing astigmatism.

[0065] In the infrared imaging lens 1, the focal length f1 of the first lens and the focal length fL of the entire system preferably satisfy the following: 1.5 ≤ f1 / fL ≤ 2.5 The relationship is as follows. That is, the first lens L1 does not contribute significantly to the overall system focal length fL; therefore, it is preferable that the third lens L3 contributes to the overall system focal length fL. By configuring the infrared imaging lens 1 in this way, aberration characteristics can be well maintained, and a high resolution comparable to that of an image sensor with wavelength-level pixel spacing can be obtained.

[0066] Furthermore, in the infrared imaging lens 1, it is preferable that the distance along the optical axis from the object-side surface (first surface) of the first lens L1 to the image plane S, i.e., the total optical length TTL, and the total system focal length fL, satisfy the following: 1.2≤TTL / fL≤2.0 The relationship is as follows. In other words, the optical total length TTL is preferably configured to be slightly larger than the total system focal length fL. By configuring the infrared imaging lens 1 in this way, aberration characteristics can be well maintained, and high resolution corresponding to image sensors with wavelength-level pixel spacing can be obtained.

[0067] In the infrared imaging lens 1, it is preferable to use the effective diameter of the object-side surface (first surface) of the first lens L1 as the aperture stop. This configuration reduces vignetting of the peripheral beam and increases the amount of peripheral light. In particular, it ensures relative illumination of over 94% even at the maximum image height, and combined with low distortion, it creates an excellent imaging lens that produces natural images. Furthermore, compared to inserting an aperture stop between lenses, the outer diameter and volume of the infrared imaging lens 1 can be reduced.

[0068] In the infrared imaging lens 1, the preferred configuration is such that the back focal length BFL and the total system focal length fL satisfy the following: 0.2≤BFL / fL The relationship is as follows. This configuration ensures a back focal length (BFL) and achieves a telephoto lens with excellent aberration characteristics and resolution.

[0069] At least one of the first lens L1 and the second lens L2 can be an aspherical lens. This reduces spherical aberration and astigmatism in the infrared imaging lens 1. It is particularly preferred 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 can be a spherical lens.

[0070] Furthermore, it is preferable that any surface of the first lens L1 is a diffraction surface. This allows for negative dispersion, thereby reducing on-axis chromatic aberration and magnification chromatic aberration. In particular, it is preferable that the image-plane side (second surface) of the first lens L1 is a diffraction surface. By using the concave side as the diffraction surface in this way, it becomes easier to form the diffraction surface using molding.

[0071] In the infrared imaging lens 1, the modulation transfer function (MTF) for the wavelength range of 7–14 μm at an image height of 2.5 mm on the image plane S and a spatial frequency of 41.7 cycles / mm is configured to be 0.3 (30%) or higher. The MTF value is the arithmetic mean of the values ​​in the tangential and sagittal directions. The reason for focusing on a spatial frequency of 41.7 cycles / mm and an image height of 2.5 mm is explained below.

[0072] Miniaturization of image sensors in the mid-to-far infrared region continues to advance, with pixel pitch reaching the wavelength-level narrow pitch limit. This miniaturization allows for lower-cost production compared to large-area image sensors. Furthermore, the imaging lenses used in these sensors can also be made smaller in diameter to match the sensor's size, further reducing costs.

[0073] Therefore, by applying such image sensors and imaging lenses to infrared cameras, it is possible to achieve low-cost operation suitable for civilian applications, expanding infrared cameras to various fields. Image sensors with a pixel pitch of 12μm, operating in the 10μm wavelength band, are already commercially available. The spatial frequency of 41.7 periods / mm corresponds to the Nyquist frequency of the 12μm pixel pitch image sensor.

[0074] With an MTF of 0.3 or higher at an image height of 2.5 mm, this indicates that a sufficient resolution of 0.3 or higher can be obtained across the entire area of ​​an image sensor with a pixel pitch of 12 μm and a resolution of 320 × 256 pixels, located on the image plane S. In other words, the infrared imaging lens 1 is a telephoto lens that is fully compatible with infrared cameras employing miniaturized QVGA (320 × 240 pixels) level image sensors that correspond to a wavelength range of approximately 7–14 μm.

[0075] Furthermore, in the infrared imaging lens 1, at an image height of 4.1 mm on the image plane S, the MTF (Mean Transmission Frequency) with a wavelength range of 7–14 μm at a spatial frequency of 29.4 periods / mm is configured to be 0.45 (45%) or higher. The spatial frequency of 29.4 periods / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 17 μm.

[0076] Furthermore, at an image height of 4.1 mm, the MTF (Mean Transmission Count) at a spatial frequency of 29.4 periods / mm is greater than 0.45. This indicates that a good resolution of MTF greater than 0.45 can be obtained across the entire area of ​​an image sensor with a pixel pitch of 17 μm and 384 × 288 pixels disposed on the image plane S. In other words, the infrared imaging lens 1 is a telephoto lens with a resolution that fully corresponds to that of an infrared camera using an image sensor with a wavelength range of approximately 7–14 μm and a pixel pitch of approximately 12–17 μm.

[0077] <Numerical Example 1>

[0078] This illustrates an embodiment of the infrared imaging lens 1. A cross-sectional view of the infrared imaging lens involved in Numerical Embodiment 1 is shown below. Figure 1As shown. In numerical embodiment 1, r represents the radius of curvature, d represents the lens thickness on the optical axis or the distance between two surfaces, and ED represents the effective diameter. The unit of length is (mm). The numbers following the surface number... An asterisk (*) indicates an aspherical surface, and DOE indicates a diffractive surface. Below are the basic lens data, aspherical surface data, diffractive surface data, and various other data.

[0079] [Table 1]

[0080] The definitions of refractive index and Abbe number ν10 are as follows: N8: Refractive index at a wavelength of 8μm 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]

[0081] The definition of an aspherical shape is as follows: [Mathematical Expression 1]

[0082] h: Height from the optical axis

[0083] r: Radius of curvature at the vertex

[0084] κ: Conic constant

[0085] An: Aspheric coefficients of degree n (n: even number)

[0086] Z: Distance from a point on the aspherical surface at position h to the tangent plane at the vertex of the aspherical surface.

[0087] [Table 3]

[0088] The diffraction plane is defined as follows: [Mathematical Expression 2]

[0089] [Mathematical Expression 3]

[0090] [Mathematical Expression 4]

[0091] Φ: Phase difference function

[0092] P1, P2: Phase coefficients

[0093] Z dif Optical path function

[0094] Z DOE : Arc vector of the diffraction surface

[0095] λ: Design center wavelength (set to 10μm)

[0096] [Table 4]

[0097] The first lens L1, the second lens L2, and the third lens L3 are made of chalcogenide glass with a refractive index N10 of 3.465 and an Abbe number of 253 at a wavelength of 10 μm. The image-side surface (second surface) of the first lens L1 is a diffraction surface with step-shaped kinoform sagittas on a sphere. The depth of each sagitta ranges from 0 to the equivalent of the design center wavelength λ.

[0098] The parallel plate P is made of silicon (Si). The parallel plate P can be an optical window attached to the image sensor positioned on the image plane S. Even if the position of the parallel plate P shifts before or after the values ​​in Table 1, it will not affect the optical characteristics of the infrared imaging lens 1. That is, 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 plane S (d8 = 2.93 mm in Table 1) can be changed while keeping their total value the same. The back focal length BFL = 7.93 mm is the actual distance.

[0099] The maximum image height on image plane S is 4.1 mm; therefore, the diameter of the imaging ring is... The diameter (s) is 8.2 mm. Therefore, the infrared imaging lens 1 can be applied to a 384×288 pixel QVGA-level image sensor with a diagonal length of 8.16 mm and a pixel pitch of 17 μm. Furthermore, the infrared imaging lens 1 can cover the pixel area of ​​QVGA-level image sensors, including QVGA (320×240 pixels) and QVGA+ (345×240 pixels) sensors with a pixel pitch of 17 μm.

[0100] The infrared imaging lens 1 can also cover the pixel area of ​​image sensors of QVGA level, including those with 320×256 pixels and a pixel pitch of 12μm. Furthermore, in structures such as 384×288 pixels and 320×256 pixels, even assuming that the optical axis center of the lens is not perfectly aligned with the center of the image sensor, the effective pixel count can still be ensured to be QVGA (320×240 pixels).

[0101] The total system focal length fL and the diameter of the imaging circle of infrared imaging lens 1 The ratio of s is: fL / s=3.41.

[0102] That is, infrared imaging lens 1 is a telephoto lens. Furthermore, its half-field of view ω is 8.4°, falling within the range of 14° or less, which is acceptable for a telephoto lens. Infrared imaging lens 1 is both a telephoto lens with such a narrow field of view and a very bright imaging lens at an F-number of 1.0.

[0103] The infrared imaging lens 1 has an optical total length (TTL) of 38.5 mm from the object-side surface (first surface) of the first lens L1 to the image plane S, and a maximum effective diameter of 28.0 mm in the optical path, exhibiting compactness. This compact structure is achieved by each of the first lens L1 with positive optical power, the second lens L2 with negative optical power, and the third lens L3 with positive optical power being a meniscus lens with a crescent shape convex toward the image plane.

[0104] Furthermore, the infrared imaging lens 1 has a three-lens structure, making it very lightweight. Each lens can be manufactured by molding, thus enabling the infrared imaging lens 1 to be manufactured at a low cost suitable for civilian use. Moreover, since an infrared camera using the infrared imaging lens 1 can be compactly constructed, an infrared camera that can be easily installed in various locations can be realized.

[0105] 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 total focal length fL of the infrared imaging lens 1 is: f1 / fL=1.62.

[0106] 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 such that the absolute value of the optical power of the third lens L3 is the largest, and the absolute value of the optical power of the first lens L1 is the second largest.

[0107] The ratio of the total optical length TTL (lens total length) of infrared imaging lens 1 to the total system focal length fL is: TTL / fL=1.38.

[0108] The back focal length (BFL) is 7.93mm (actual distance), ensuring sufficient distance. The ratio of optical total length (TTL, lens total length) to back focal length (BFL) is: TTL / BFL = 4.85.

[0109] The ratio of the back focal length (BFL) to the total system focal length (fL) is: BFL / fL=0.28, It satisfies 0.2≤BFL / fL.

[0110] Figures 2 to 6 The numerical values ​​represent the various performance parameters of the infrared imaging lens 1 in Example 1. Figure 2 and Figure 3 This is an aberration diagram of infrared imaging lens 1. Figure 2 This represents spherical aberration, astigmatism, and distortion. For each case, graphs are shown for wavelengths in the range of 7–14 μm. Figure 3 It is an aberration diagram that represents the coma in each image height Y from 0 mm to the maximum image height, divided into tangential (meridian) and sagittal (radial) directions.

[0111] like Figure 2 and Figure 3 As shown, in the infrared imaging lens 1 of numerical embodiment 1, aberrations are well corrected in a wide wavelength range of 7–14 μm. In particular, in this infrared imaging lens 1, distortion is less than 0.37% across the entire imaging circle at each wavelength in the range of 7–14 μm, resulting in extremely small and excellent characteristics.

[0112] Figure 4 This is a graph showing the relative illumination of the infrared imaging lens 1 as a function of image height Y, according to numerical embodiment 1. Relative illumination refers to the ratio of illumination on the image plane S to that on the optical axis (the central region of the image plane). Figure 4 As shown, even at the maximum image height of 4.1 mm, the relative illumination is 0.94, and a roughly uniform light distribution is obtained within the imaging circle.

[0113] Figure 5 This is a graph showing the spatial frequency dependence of MTF (Mean Transmission Frequency) in the wavelength range of 7–14 μm. The Nyquist frequency f0 of a 320×256 pixel image sensor with a pixel pitch of 12 μm is also shown. N The period is 41.7 cycles / mm, and the maximum image height is 2.46mm. At the Nyquist frequency f... N =41.7 cycles / mm, the MTF in the center of the image is 0.46, and within the region of this image sensor, the MTF is ensured to be >0.37 (arithmetic mean in the tangential and sagittal directions).

[0114] Additionally, the Nyquist frequency f of an image sensor with a pixel pitch of 17μm and a resolution of 384×288 pixels is... N The period is 29.4 cycles / mm, and the maximum image height is 4.08mm. At the Nyquist frequency f... N =29.4 cycles / mm, the MTF in the center of the image is 0.58, and within the region of this image sensor, the MTF is ensured to be >0.49 (arithmetic mean in the tangential and sagittal directions).

[0115] Thus, the infrared imaging lens 1 ensures good resolution even when evaluated with MTF over a wide wavelength range of 7 to 14 μm within the area of ​​a QVGA-level image sensor with a pixel pitch of approximately 12 to 17 μm.

[0116] As long as the infrared imaging lens 1 is within the wavelength range of 7 to 14 μm, such as any wavelength range like 7 to 12 μm, 8 to 12 μm, or 8 to 10 μm, it can certainly obtain good characteristics. Figure 6 It is a graph showing the change in MTF over a wavelength range of 7–14 μm relative to the focus shift.

[0117] As described above, the infrared imaging lens 1 of numerical embodiment 1 can cover a wavelength range of 7–14 μm and has a good resolution that fully corresponds to a QVGA-level image sensor with a pixel pitch of about 12–17 μm. Moreover, the distortion is extremely small throughout the entire imaging circle, below 0.37%.

[0118] The infrared imaging lens 1 of numerical embodiment 1 has an F-number of 1.0 and is bright, with a total lens length (optical total length TTL) of 38.5 mm and a maximum effective diameter of 28.0 mm, making it very compact. Thus, according to this embodiment, a compact and high-performance telephoto lens, i.e., an infrared imaging lens, which was not available before, can be realized.

[0119] [Summarize]

[0120] Embodiment 1 of the present invention is an infrared imaging lens used in an infrared region of at least any wavelength in the range of 7 to 14 μm, wherein a first lens, a second lens, and a third lens are arranged sequentially 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 with a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. The focal length fL of the entire system is more than twice the diameter of the imaging circle. The first lens and the third lens are meniscus lenses with positive optical power, and the second lens is a meniscus lens with negative optical power.

[0121] Based on the above structure, an infrared imaging lens that can be used as a telephoto lens can be realized, which is compatible with image sensors with wavelength-level pixel pitch, has excellent resolution and low distortion.

[0122] Embodiment 2 of the present invention is an infrared imaging lens used in an infrared region of at least any wavelength in the range of 7 to 14 μm, wherein a first lens, a second lens, and a third lens are arranged sequentially 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 with 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 with positive optical power, the second lens is a meniscus lens with negative optical power, and the half field of view is less than 14°.

[0123] Based on the above structure, an infrared imaging lens that can be used as a telephoto lens can be realized, which is compatible with image sensors with wavelength-level pixel pitch, has excellent resolution and low distortion.

[0124] In the infrared imaging lens according to Embodiment 3 of the present invention, in Embodiments 1 or 2 above, each of the first lens, the second lens, and the third lens is a meniscus shape convex toward the object side. Based on the above structure, an infrared imaging lens that is a compact telephoto lens with excellent aberration characteristics can be realized.

[0125] In the infrared imaging lens according to Embodiment 4 of the present invention, in any of Embodiments 1 to 3 described above, the effective diameter of the object-side surface of the first lens is set as the aperture stop. According to the above structure, vignetting of the peripheral beam can be reduced, and the amount of peripheral light can be increased.

[0126] In the infrared imaging lens according to Embodiment 5 of the present invention, in any of Embodiments 1 to 4 described above, at least one of the first lens and the second lens is an aspherical lens. Based on the above structure, an infrared imaging lens with particularly excellent aberration characteristics can be realized.

[0127] In the infrared imaging lens according to Embodiment 6 of the present invention, in any of Embodiments 1 to 5 described above, any side of the first lens is a diffraction surface. According to the above structure, on-axis chromatic aberration and magnification chromatic aberration can be reduced.

[0128] In the infrared imaging lens according to Embodiment 7 of the present invention, in any of Embodiments 1 to 6 described above, the image plane side of the first lens is a diffraction surface. Based on the above structure, an infrared imaging lens that facilitates the formation of the diffraction surface and thereby reduces on-axis chromatic aberration and magnification chromatic aberration can be achieved.

[0129] In the infrared imaging lens according to Embodiment 8 of the present invention, in any of Embodiments 1 to 7, the focal length f1 of the first lens and the focal length fL of the entire system satisfy the relationship 1.5 ≤ f1 / fL ≤ 2.5. Based on the above structure, aberration characteristics can be well maintained, and a high resolution corresponding to image sensors with wavelength-level pixel pitch can be obtained.

[0130] In the infrared imaging lens according to Embodiment 9 of the present invention, in any of Embodiments 1 to 8 described above, the optical total length (TTL) and the total system focal length (fL) along the optical axis from the object-side surface of the first lens to the image plane satisfy the relationship 1.2 ≤ TTL / fL ≤ 2.0. Based on the above structure, aberration characteristics can be well maintained, and a high resolution corresponding to image sensors with wavelength-level pixel pitch can be obtained.

[0131] In the infrared imaging lens according to embodiment 10 of the present invention, in any of embodiments 1 to 9 described above, the back focal length BFL and the total system focal length fL satisfy the relationship 0.2 ≤ BFL / fL. Based on the above structure, a telephoto lens that ensures a back focal length BFL and exhibits excellent aberration characteristics and resolution can be realized.

[0132] The infrared imaging lens according to embodiment 11 of the present invention, in any of embodiments 1 to 10 described above, has a relative illumination of 94% or more within the imaging circle on the image plane. Based on the above structure, an infrared imaging lens that sufficiently ensures peripheral light intensity can be realized.

[0133] The infrared imaging lens according to embodiment 12 of the present invention, in any of embodiments 1 to 11 described above, satisfies a modulation transfer function of 0.3 or higher at a wavelength range of 7 to 14 μm at a spatial frequency of 41.7 periods / mm and an image height of 2.5 mm. Based on the above structure, an infrared imaging lens capable of providing good resolution throughout the entire area of ​​an image sensor with wavelength-level pixel pitch can be realized.

[0134] In the infrared imaging lens according to embodiment 13 of the present invention, in any of embodiments 1 to 12 described above, the total system focal length fL is in the range of 3 to 6 times the diameter of the imaging circle. Based on the above structure, an infrared imaging lens that is a telephoto lens with excellent aberration characteristics and resolution can be realized.

[0135] The infrared imaging lens according to embodiment 14 of the present invention has an F-number in the range of 0.9 to 1.1 in any of embodiments 1 to 13 described above. Based on the above structure, an infrared imaging lens with a small F-number and excellent brightness, serving as a telephoto lens with excellent aberration characteristics and resolution, can be realized.

[0136] In the infrared imaging lens according to Embodiment 15 of the present invention, in any of Embodiments 1 to 13, each of the first lens, the second lens, and the third lens is made of chalcogenide glass with a refractive index of 3.0 to 4.0 at a wavelength of 10 μm. Based on the above structure, an infrared imaging lens that is compatible with image sensors having wavelength-level pixel pitch, has excellent resolution, low distortion, and is compact as a telephoto lens can be realized.

[0137] In the infrared imaging lens according to Embodiment 16 of the present invention, in any of Embodiments 1 to 14 described above, the infrared absorption wavelength of the chalcogenide glass with a transmittance of 20% when the thickness is 2 mm is 18 μm or more. Based on the above structure, an infrared imaging lens with very low light absorption in a wavelength range of at least 7 to 14 μm can be constructed.

[0138] This invention is not limited to the embodiments described above, and various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the specification are also included within the technical scope of this invention. Furthermore, new technical features can be formed by combining the technical means disclosed in the specification.

[0139] Explanation of reference numerals in the attached figures

[0140] 1. Infrared Imaging Lens

[0141] L1 First Lens

[0142] L2 Second Lens

[0143] L3 Third Lens

[0144] P parallel plate

[0145] S image plane

[0146] AP Aperture Stop

Claims

1. An infrared imaging lens, used in an infrared region comprising at least any wavelength in the range of 7–14 μm, wherein, The first lens, the second lens, and the third lens are arranged sequentially from the object side toward the image plane side. Each of the first lens, the second lens, and the third lens is made of chalcogenide glass with a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. The total system focal length fL is more than twice the diameter of the imaging circle. The first lens and the third lens are meniscus lenses with positive optical power, and the second lens is a meniscus lens with negative optical power.

2. An infrared imaging lens, used in an infrared region comprising at least any wavelength in the range of 7–14 μm, wherein, The first lens, the second lens, and the third lens are arranged sequentially from the object side toward the image plane side. Each of the first lens, the second lens, and the third lens is made of chalcogenide glass with 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 with positive optical power, and the second lens is a meniscus lens with negative optical power. The half field of view is less than 14°.

3. The infrared imaging lens according to claim 1 or 2, wherein, Each of the first lens, the second lens, and the third lens is a crescent shape that bulges toward the object side.

4. The infrared imaging lens according to claim 1 or 2, wherein, The effective diameter of the object-side surface of the first lens is set as the 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 aspherical lens.

6. The infrared imaging lens according to claim 5, wherein, Any one side of the first lens is a diffraction surface.

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

8. The infrared imaging lens according to claim 1 or 2, wherein, The focal length f1 of the first lens and the focal length fL of the entire system satisfy: The relationship is 1.5≤f1 / fL≤2.

5.

9. The infrared imaging lens according to claim 1 or 2, wherein, The distance along the optical axis from the object-side surface of the first lens to the image plane, i.e., the total optical length TTL, and the total system focal length fL satisfy the following: The relationship is 1.2≤TTL / fL≤2.

0.

10. The infrared imaging lens according to claim 1 or 2, wherein, The back focal length (BFL) and the total system focal length (fL) satisfy the following: The relationship 0.2≤BFL / fL.

11. The infrared imaging lens according to claim 1 or 2, wherein, The relative illumination in the image plane meets or exceeds 94% within the imaging circle.

12. The infrared imaging lens according to claim 1 or 2, wherein, At a spatial frequency of 41.7 periods / mm, the modulation transfer function in the wavelength range of 7–14 μm satisfies a value greater than 0.3 at an image height of 2.5 mm.

13. The infrared imaging lens according to claim 1 or 2, wherein, The focal length fL of the entire system is in the range of 3 to 6 times the diameter of the imaging 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. The infrared imaging lens according to claim 1 or 2, wherein, Each of the first lens, the second lens, and the third lens is made of chalcogenide glass with a refractive index of 3.0 to 4.0 at a wavelength of 10 μm.

16. The infrared imaging lens according to claim 1 or 2, wherein, The chalcogenide glass has a transmittance of 20% and an infrared absorption wavelength of 18μm or higher when the thickness is 2mm.

Images