Infrared imaging lens and infrared camera

Abstract

The present invention provides a normal lens which exhibits excellent resolution.　This infrared imaging lens (1) is configured by arranging a plurality of lenses (L1-L3) which each comprise glass having a refractive index of 2.8-4.0 at a wavelength of 10μm, and exhibits an image circle diameter which is 0.7-1.3 times the focal length.

Description

[Technical Field]

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

[0002] Infrared cameras that capture images of subjects using infrared light in the 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. Infrared imaging lenses that are used in these infrared cameras are known. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication WO2016 / 027786A1 [Patent Document 2] Japanese Patent Application Publication No. 2011-128538 [Patent Document 3] Japanese Patent Publication No. 62-109014 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for an infrared imaging lens that can be used in such a far-infrared region, has excellent resolution, and can be used as a standard lens. In particular, there is a demand for an infrared imaging lens that has excellent resolution compatible with image sensors with pixel pitches on the order of wavelengths.

[0005] One aspect of the present invention addresses the above-described problem and aims to provide an infrared imaging lens that can be used as a standard lens, has excellent resolution, and is compatible with image sensors having pixel pitches on the order of wavelengths. [Means for solving the problem]

[0006] In order to solve the above problems, one aspect of the present invention is an infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, and the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.17 or more within the image circle.

[0007] In order to solve the above problems, another aspect of the present invention is an infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, and the diameter of the image circle is 0.7 to 1.3 times the focal length.

Advantages of the Invention

[0008] According to the above aspect of the present invention, it is possible to realize an infrared imaging lens that can be used as a standard lens, which can be compatible with an image sensor having a pixel pitch on the order of wavelength and has excellent resolution.

Brief Description of the Drawings

[0009]

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Mode for Carrying Out the Invention

[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 such as an image sensor, corresponding to the wavelength range of far infrared rays. FIG. 1 is a cross-sectional view along the optical axis showing the configuration of the main part of the infrared imaging lens 1. FIG. 2 is an optical path diagram showing the optical path together with the cross-sectional view of the main part of the infrared imaging lens 1.

[0011] The infrared imaging lens 1 is configured by arranging a first lens L1, a second lens L2, and a third lens L3 in order from the object side toward the image plane S side. During focusing, the first lens L1 to the third lens L3 all move uniformly in the optical axis direction.

[0012] The first lens L1, the second lens L2, and the third lens L3 are each made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm. More specifically, the first lens L1, the second lens L2, and the third lens L3 are made of chalcogenide glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm. Any of the first lens L1, the second lens L2, and the third lens L3 may be made of the same glass material.

[0013] As shown in FIGS. 1 and 2, a parallel plate P is disposed between the third lens L3 and the image plane S. The parallel plate P is an optical window hermetically sealed on the image plane S side, and silicon, low-oxygen silicon, or germanium is used. The material and thickness can be determined according to the type of image sensor employed.

[0014] As indicated by the symbol AP in FIG. 1, the effective diameter of the object-side surface (the first surface) of the first lens L1 corresponds to the aperture stop of the infrared imaging lens 1. Anti-Reflection (AR) coatings are applied to the surfaces of the first lens L1, the second lens L2, the third lens L3, and the parallel plate P. Appropriate known techniques can be applied to such anti-reflection coatings in the far-infrared region.

[0015] <Glass material of each lens> In particular, the above chalcogenide glass preferably contains 20 to 90% of tellurium (Te) in mol% and has an Abbe number of 100 or more at a wavelength of 10 μm. The definition of the Abbe number in this specification is described in the numerical examples below. Further, the above chalcogenide glass preferably contains at least one of 0 to 50% of germanium (Ge) or 0 to 50% of gallium (Ga) in mol%.

[0016] Such a chalcogenide glass having a high refractive index in the far-infrared region, which is a glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, was developed by the applicant (see International Publication WO2020 / 105719A1). More specifically, the refractive index of this optical material at a wavelength of 10 μm is in the range of 2.74 to 3.92. For example, the refractive index at a wavelength of 10 μm is preferably 2.74 to 3.92, 2.8 to 3.8, particularly 2.9 to 3.7. If the refractive index is too low, the focal length tends to become too long.

[0017] Also, the Abbe number (V10) of the chalcogenide glass is preferably 100 or more, 120 or more, 150 or more, 180 or more, particularly 220 or more. The definition of the Abbe number (V10) will be described later. If the Abbe number is too low, chromatic aberration tends to increase. The upper limit of the Abbe number is not particularly limited, but is realistically 350 or less.

[0018] This optical material has extremely small light absorption over a wide wavelength range in the far-infrared region, such as at least wavelengths of 7 to 14 μm. In particular, this optical material has the characteristic of having small light absorption even in the region exceeding a wavelength of 10 μm despite being a chalcogenide. In chalcogenide glass, the "infrared absorption edge wavelength" and the "internal transmittance" can be used as indicators showing excellent light transmittance in the far-infrared region.

[0019] Here, the infrared absorption edge wavelength refers to the absorption edge wavelength in the far-infrared region of wavelengths 8 μm or more, and is defined as the wavelength at which the light transmittance at a material thickness of 2 mm is 20%. Note that internal transmittance refers to the transmittance inside the material and does not include reflection loss at the material surface. The chalcogenide glass used as the glass material for the first lens L1, second lens L2, and third lens has an infrared absorption edge wavelength of 18 μm or more.

[0020] Therefore, this 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 this chalcogenide glass at a thickness of 2 mm is 90% or more at a wavelength of 10 μm.

[0021] Furthermore, this glass material is glass, and can be press-molded to form lenses with aspherical surfaces. Therefore, lenses using this glass material can be easily mass-produced. Preferably, the glass material has a low glass transition temperature of 200°C or less, making press molding easier. In the infrared imaging lens 1, aberrations are suppressed by making at least one of the lenses an aspherical lens.

[0022] If an aspherical lens cannot be applied, the configuration of an infrared imaging lens to suppress aberrations would require an increased number of lenses, which would increase the weight and size of the lens. This would also result in a high cost and an imaging lens that is not suitable for consumer use. In this specification, the term "aspherical surface" includes a diffractive surface.

[0023] Furthermore, the chalcogenide glass can also be used to form lenses with particularly complex surfaces, such as diffractive surfaces. Therefore, by using the chalcogenide glass to form at least one lens surface into a diffractive surface in the infrared imaging lens 1, it becomes possible to effectively suppress aberrations over a wide wavelength range from 7 to 14 μm.

[0024] Crystalline materials such as silicon (Si), germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe), which are used as materials that transmit light in the far-infrared region, cannot be press-molded. This makes it difficult to mass-produce aspherical lenses with complex shapes. This means that it is difficult to create low-cost aspherical lenses for consumer use using these crystalline materials.

[0025] <Imaging surface> On the image plane S of the infrared imaging lens 1, the diameter φs of the image circle is equal to the focal length f of the infrared imaging lens 1. In other words, the infrared imaging lens 1 is a standard lens. More specifically, the diameter φs of the image circle is in the range of 0.7 to 1.3 times the focal length f. In other words, 0.7≦φs / f≦1.3 Alternatively, the infrared imaging lens 1 may be defined as a standard lens when the half angle of view of the infrared imaging lens 1 is 21 to 36 degrees.

[0026] In the infrared imaging lens 1, within such an image circle, the modulation transfer function (MTF) at a spatial frequency of 41.7 cycles / mm is 0.17 (17%) or more. Here, the reason for focusing on the spatial frequency of 41.7 cycles / mm will be explained below.

[0027] As image sensors in the far-infrared region become more compact, the pixel pitch has reached the limit of narrow pitch, which is about the wavelength. Image sensors in the far-infrared region, which are 7 to 14 μm wavelength, with a pixel pitch of 12 μm are now commercially available. A spatial frequency of 41.7 cycles / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 12 μm.

[0028] Furthermore, an MTF of 0.17 or higher within the image circle indicates that sufficient resolution is obtained throughout the entire image circle. In other words, the infrared imaging lens 1 is a standard lens compatible with far-infrared cameras employing miniaturized image sensors in the wavelength range of approximately 7 to 14 μm.

[0029] As the image sensor, image sensors with 640 x 480 pixels (VGA: Video Graphics Array) or 640 x 512 pixels (VGA+) have been developed. The effective diagonal length of these image sensors is approximately 9.8 mm. Therefore, the diameter φs of the image circle of the infrared imaging lens 1 is approximately 9.8 mm or larger. Therefore, based on the relationship between the image circle diameter φs and the focal length f described above, the focal length f of the infrared imaging lens 1 is in the range of 7.8 to 11.8 mm.

[0030] The infrared imaging lens 1 of this embodiment is an infrared imaging lens configured by arranging a plurality of lenses, each of which is made of the above-mentioned glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm. Therefore, in the infrared imaging lens 1, at least one of the plurality of lenses can be an aspherical lens.

[0031] This allows for the realization of a standard lens that is compatible with a wide wavelength range of 7 to 14 μm, has an F-number as small as about 1, and has excellent resolution, something that was previously impossible to achieve. With regard to resolution in particular, a standard lens with exceptionally excellent characteristics is realized, with an MTF of 0.17 or greater at a spatial frequency of 41.7 cycles / mm in the wavelength range of 7 to 14 μm within the image circle. Furthermore, a standard lens with low light absorption by the lens glass material can be realized over at least the wide wavelength range of 7 to 14 μm. This, combined with a small F-number of about 1, allows for the realization of a bright imaging lens.

[0032] Furthermore, in the infrared imaging lens 1 of the present embodiment, the MTF can be 0.50 or more at the spatial frequency corresponding to 1 / 2 of the above Nyquist frequency of 41.7 cycles / mm. This indicates that the MTF is good not only at the spatial frequency corresponding to the above Nyquist frequency, but also over the entire range from a spatial frequency of 0 cycles / mm to the above Nyquist frequency.

[0033] Furthermore, in the infrared imaging lens 1 of the present embodiment, the relative illuminance on the image plane can be 40% or more within the image circle. This indicates that the peripheral light quantity is well ensured.

[0034] <Details of the configuration of each lens> Furthermore, the infrared imaging lens 1 of the present embodiment can be configured in detail as follows.

[0035] The first lens L1 has a positive refractive power and has a meniscus shape with a convex surface facing the object side. The first lens L1 can be an aspherical lens. The surface on the object side (the first surface) can be spherical, and the surface on the image plane S side (the second surface) can be aspherical.

[0036] As described above, the effective diameter of the surface on the object side (the first surface) of the first lens L1 is used as the aperture stop of the infrared imaging lens 1. In this case, the subsequent lenses will become larger. Then, the outer diameter and volume of the infrared imaging lens 1 will become large. Therefore, it is preferable to make the first lens L1 have a meniscus shape with a convex surface facing the object side and to impose the following limitation on the power of the first lens L1.

[0037] The focal length f1 of the first lens L1 is in the range of 1.0 to 2.9 times the focal length f of the infrared imaging lens 1. That is, 1.0 ≦ f1 / f ≦ 2.9 It is preferable to satisfy the relational expression.

[0038] By configuring in this way, it becomes possible to reduce the outer diameter and volume of the infrared imaging lens 1 compared to inserting an aperture stop between the lenses. Further, by setting the effective diameter of the object-side surface (the first surface) of the first lens L1 as the aperture stop of the infrared imaging lens 1 and satisfying this relational expression, the vignetting of the peripheral light beam is reduced and the peripheral light quantity is improved.

[0039] The second lens L2 has a positive refractive power and has a meniscus shape with a convex surface facing the image plane S side. The second lens L2 is an aspherical lens, the object-side surface (the third surface) thereof is aspherical, and the image-plane S-side surface (the fourth surface) can be a diffractive surface.

[0040] The third lens L3 has a positive refractive power and has a meniscus shape with a convex surface facing the image plane S side. The third lens L3 is an aspherical lens, the object-side surface (the fifth surface) thereof is spherical, and the image-plane S-side surface (the sixth surface) can be aspherical.

[0041] As described above, the infrared imaging lens 1 is configured by arranging the first lens L1, the second lens L2, and the third lens, and it is preferable that all of the first lens L1, the second lens L2, and the third lens have a positive refractive index. Among them, it is preferable to configure the infrared imaging lens 1 such that the power of the third lens L3, which is the closest to the image plane side, is the largest.

[0042] Thus, since both the second lens L2 and the third lens L3 have a meniscus shape with a convex surface facing the image plane S side, an increase in the Petzval sum can be suppressed, field curvature can be suppressed, and planarity can be maintained. Also, it is particularly preferable that the order of the power strength is the third lens L3, the first lens L1, and the second lens L2 from the largest.

[0043] By configuring the infrared imaging lens 1 such that the power of the third lens L3, which is the closest to the image plane side, is the largest and the third lens L3 has a meniscus shape with a convex surface facing the image plane S side, spherical aberration is reduced.

[0044] Further, as described above, at least one of the first lens L1, the second lens L2, and the third lens may be an aspherical lens. This makes it possible to reduce the spherical aberration and astigmatism of the infrared imaging lens 1. It is particularly preferable that all of the first lens L1, the second lens L2, and the third lens are aspherical lenses.

[0045] Furthermore, it is preferable that the surface on the image plane S side (the fourth surface) of the second lens L2 or the surface on the object side (the fifth surface) of the third lens L3 is a diffractive surface. This makes it possible to generate negative dispersion and reduce the chromatic aberration of magnification.

[0046] <Other preferred embodiments> In the infrared imaging lens 1 of the present embodiment, in order to achieve excellent resolution capable of corresponding to a pixel pitch of about the same level as the wavelength of about 7 to 14 μm targeted by the infrared imaging lens 1, the F-number is preferably in the range of 1.0 to 1.2. Also, by setting the F-number within such a range, a bright standard lens used in the infrared region with a wavelength of about 7 to 14 μm is realized.

[0047] In the infrared imaging lens 1 of the present embodiment, it is desirable to configure the focal length f3 of the third lens L3 to be about the same as the focal length f of the infrared imaging lens 1. Specifically, the focal length f3 is preferably in the range of 0.8 to 1.2 times the focal length f. That is, 0.8 ≦ f3 / f ≦ 1.2 It is preferable to satisfy the relational expression. By configuring to satisfy this relational expression, good resolution can be obtained over a wide image circle range. That is, as an image sensor used in the infrared region with a wavelength of about 7 to 14 μm, good resolution can be obtained over the entire area of an image sensor with a wide detection surface, a pixel pitch of about 12 μm, and in the classes of 640×480 pixels (VGA) or 640×512 pixels (VGA+).

[0048] The fact that the focal length f3 of the third lens L3 is approximately the same as the focal length f of the infrared imaging lens 1 indicates that the contributions of the first lens L1 and the second lens L2 to the overall focal length f of the entire system are very small. By configuring only the third lens L3 to contribute significantly to the focal length f and the first lens L1 and the second lens L2 to mainly contribute to aberration correction, an infrared imaging lens can be realized that provides good resolution across the entire surface of such a wide detection surface.

[0049] More preferably, the focal length f3 of the third lens L3 is preferably in the range of 0.9 to 1.2 times the focal length f of the infrared imaging lens, that is, 0.9 ≤ f3 / f ≤ 1.2 It is more preferable that the first lens L1, the second lens L2, and the third lens L3 are configured to satisfy the relational expression.

[0050] Furthermore, in the infrared imaging lens 1 of the present embodiment, it is preferable that the overall lens length L of the infrared imaging lens is within the range of 1.0 to 2.5 times the focal length f of the infrared imaging lens. That is, 1.0 ≤ L / f ≤ 2.5 It is advisable to satisfy the relational expression. Here, the overall lens length is the actual distance along the optical axis from the object-side end of the lens closest to the object side within the effective diameter to the image plane S.

[0051] More preferably, the overall lens length L of the infrared imaging lens is preferably in the range of 1.1 to 2.3 times the focal length f of the infrared imaging lens, that is, 1.1 ≤ L / f ≤ 2.3 It is more preferable that the first lens L1, the second lens L2, and the third lens L3 are configured to satisfy the relational expression. By configuring in this way, a compact standard lens can be realized.

[0052] In this way, while making the focal length f3 of the third lens L3 approximately the same as the focal length f of the infrared imaging lens 1, the overall lens length L is configured not to be too much larger than the focal length f. By configuring in this way, an infrared imaging lens that is a standard lens and is excellent in various characteristics such as aberration characteristics, resolution, and peripheral light quantity, and is compact can be realized.

[0053] In the infrared imaging lens 1 of this embodiment, it is preferable that the back focus BF is set to be 0.5 times or more of the focal length f of the infrared imaging lens 1. That is, 0.5 ≦ BF / f It is better to satisfy the relational expression. More preferably, the back focus BF is set to be 0.65 times or more of the focal length f of the infrared imaging lens 1. That is, 0.65 ≦ BF / f It is more preferable that the first lens L1, the second lens L2, and the third lens L3 are configured to satisfy the relational expression. By configuring in this way, a sufficient back focus can be ensured, and a compact standard lens can be realized.

[0054] It is desirable that the infrared imaging lens 1 of this embodiment is configured such that the focal length of the infrared imaging lens is specifically within the range of 7 to 12 mm. By configuring in this way, while making various characteristics such as aberration characteristics, resolution, and peripheral light quantity excellent, an infrared imaging lens that is a compact standard lens and can be widely used for civilian purposes can be realized.

[0055] Also, it is desirable that the infrared imaging lens 1 of this embodiment is configured such that the overall lens length of the infrared imaging lens is specifically 30 mm or less. By configuring in this way, while making various characteristics such as aberration characteristics, resolution, and peripheral light quantity excellent, an infrared imaging lens that is a compact standard lens and can be widely used for civilian purposes can be realized.

[0056] <Configuration of Infrared Camera> The infrared camera of the present disclosure includes the above-mentioned infrared imaging lens 1 and an image sensor that is compatible with an infrared region that includes at least any wavelength in the range of 7 to 14 μm. The image sensor is positioned so that its imaging surface is at the position of the image plane S of the infrared imaging lens 1.

[0057] The pixel pitch of the image sensor is preferably 7 to 14 μm, which is approximately the wavelength of infrared light, in order to achieve performance that matches the resolution of the infrared imaging lens 1. In particular, the pixel pitch is preferably 9 to 12 μm. The diagonal length of the image sensor is preferably 7 to 11 mm in order to efficiently utilize the image circle diameter φs of the infrared imaging lens 1. However, it goes without saying that the infrared camera of the present disclosure can also be configured using an image sensor with a diagonal length smaller than this.

[0058] <Numerical Example 1> Numerical examples of an infrared imaging lens 1 are 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 distance between surfaces on the optical axis, and ED represents the effective diameter (diameter). The unit of length is (mm). An * (asterisk) after the surface number indicates that the surface is aspherical. Basic lens data, aspherical surface data, diffractive surface data, and various other data are shown below. [Table 1] The refractive index and Abbe number V10 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 V10 = (N10 - 1) / (N8 - N12) [Table 2] The definition of the aspheric shape is as follows:

number

Table 3

Number

Number

Number

Table 4

[0059] The focal length f1 of the first lens L1 is 24.35 mm. Therefore, the ratio to the focal length f of the infrared imaging lens 1 is f1 / f = 2.7 ​​It is as follows. The focal length f2 of the second lens L2 is 37.87 mm. The focal length f3 of the third lens L3 is 9.96 mm. Therefore, the power of the third lens L3 is the strongest, and then the power of the first lens L1 is strong.

[0060] The maximum image height at the image plane S is 4.92 mm. Therefore, the diameter φs of the image circle is 9.84 mm. The ratio of the diameter φs of the image circle of the infrared imaging lens 1 to the focal length f is φs / f = 1.09 That is, the infrared imaging lens 1 is a standard lens. Also, the semi-field angle is 29.8°, which is within the range of 21 to 36° that can be called a standard lens.

[0061] The infrared imaging lens 1 is compact with a total length from the first surface to the image plane of 20.9 mm and a maximum effective diameter on the optical path of 14.8 mm. Also, the infrared imaging lens 1 has a three-lens configuration and can be made lightweight. Combined with the fact that each lens can be manufactured by press molding, the infrared imaging lens 1 can be manufactured at low cost and applied to consumer applications.

[0062] The performances of the infrared imaging lens 1 of Numerical Example 1 are shown in FIGS. 3 to 7. FIG. 3 is an aberration diagram of the infrared imaging lens 1. FIG. 3 shows spherical aberration, astigmatism, and distortion. In each case, graphs for each wavelength in the range of 7 to 14 μm are shown. FIG. 4 is an aberration diagram showing the coma aberration at each image height Y from 0 mm to the maximum image height of 4.92 mm, separated into the tangential (meridional) direction and the sagittal (radial) direction. As shown in FIGS. 3 and 4, 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.

[0063] FIG. 5 is a graph showing the relative illuminance of the infrared imaging lens 1 of Numerical Example 1 with respect to the image height Y. Here, the relative illuminance refers to the ratio of the illuminance at the image plane S with respect to the region on the optical axis (the central region of the image plane). As shown in FIG. 5, even at the maximum image height of 4.92 mm, a relative illuminance of 0.68 and sufficient peripheral light quantity are obtained.

[0064] When using an image sensor with 640 x 512 pixels (VGA+) and an effective diagonal length of 9.84 mm, the image height Y corresponding to the center of the left and right edges of the image pickup surface is 3.84 mm, and the relative illuminance there is 0.80.In addition, the image height Y corresponding to the center of the top and bottom edges of the image pickup surface in this case is 3.07 mm, and the relative illuminance there is 0.87.

[0065] Figure 6 is a graph showing the spatial frequency dependence of MTF in the wavelength range of 7 to 14 μm. As mentioned above, the Nyquist frequency f N is 41.7 cycles / mm, as shown in Figure 6. Its half value f N / 2 (20.85 cycles / mm) is also shown in the figure.

[0066] At a spatial frequency of 41.7 cycles / mm, the MTF at each image height Y is 0.24 or greater, well above 0.17. At an image height Y of 0 mm, i.e., at the center of the image, the MTF is 0.43, providing good resolution. At a spatial frequency of 20.85 cycles / mm, the MTF at each image height Y is 0.56 or greater, well above 0.50. The infrared imaging lens 1 according to Numerical Example 1 achieves good resolution across the entire image circle, compatible with image sensors with a narrow pitch on the order of the wavelength.

[0067] 7 is a graph showing the change in MTF in the wavelength range of 7 to 14 μm with respect to focal point movement. As described above, the infrared imaging lens 1 of Numerical Example 1 can cover the wavelength range of 7 to 14 μm and has a satisfactory resolution that is fully compatible with image sensors with a pixel pitch of about 12 μm. Furthermore, the infrared imaging lens 1 of Numerical Example 1 is bright and compact, with an F-number of 1.0. As described above, according to this embodiment, an infrared imaging lens that is compact and has excellent characteristics, which has not been seen in the past, can be realized.

[0068] In Numerical Example 1, the ratio of the focal length f3 of the third lens L3 to the focal length f of the infrared imaging lens is f3 / f = 1.1 That is, the focal length f3 of the third lens L3 is set to be approximately the same as the focal length f of the infrared imaging lens.

[0069] In Numerical Example 1, the ratio of the overall length L of the infrared imaging lens to the focal length f of the infrared imaging lens is L / f = 2.3 so that the overall length L does not become excessively large with respect to the focal length f.

[0070] In Numerical Example 1, the ratio of the back focus BF to the focal length f of the infrared imaging lens is BF / f = 0.77 so that sufficient back focus is ensured.

[0071] <Numerical Example 2> Another numerical example of the infrared imaging lens is shown. For the sake of convenience in explanation, members having the same functions as those described in the above example are denoted by the same reference numerals, and their explanations are not repeated. The cross-section of the infrared imaging lens 2 according to Numerical Example 2 is shown in the optical path diagram of FIG. 8.

[0072] The infrared imaging lens 2 according to Numerical Example 2 is an imaging lens with an F-number of 1.0, which is optimized to form good images of infrared rays in the wavelength range of 7.5 to 13.5 μm. The design center wavelength is 10 μm. The basic lens data, aspherical data, diffractive surface data, and various other data are shown below.

Table 5

Table 6

Table 7

Table 8

[0073] The focal length f1 of the first lens L1 is 24.60 mm. Therefore, the ratio of this to the focal length f of the infrared imaging lens 2 is f1 / f=2.7 The focal length f2 of the second lens L2 is 39.15 mm. The focal length f3 of the third lens L3 is 9.88 mm. Therefore, the third lens L3 is configured to have the strongest power, followed by the first lens L1. The ratio of the focal length f3 of the third lens L3 to the focal length f of the infrared imaging lens is f3 / f=1.1 The focal length f3 of the third lens L3 is approximately the same as the focal length f of the infrared imaging lens.

[0074] The maximum image height on the image plane S is 4.92 mm, and therefore the diameter φs of the image circle is 9.84 mm. The ratio of the diameter φs of the image circle of the infrared imaging lens 2 to the focal length f is φs / f=1.09 In other words, the infrared imaging lens 2 is a standard lens. The half angle of view is 29.9°, which is within the range of 21 to 36° that can be considered a standard lens.

[0075] The infrared imaging lens 2 is compact, with a total lens length L from the first surface to the image plane of 20.6 mm and a maximum effective diameter on the optical path of 14.5 mm. The ratio of the total lens length L of the infrared imaging lens to the focal length f of the infrared imaging lens is L / f=2.3 and the total lens length L is designed not to be excessively large relative to the focal length f.

[0076] In addition, the infrared imaging lens 2 has a three-lens configuration and can be made lightweight. Coupled with the fact that each lens can be manufactured by press molding, the infrared imaging lens 2 can be manufactured at low cost and applied to consumer applications.

[0077] In Numerical Example 2, the ratio of the back focus BF to the focal length f of the infrared imaging lens is BF / f = 0.77 and sufficient back focus is ensured.

[0078] The performances of the infrared imaging lens 2 in Numerical Example 2 are shown in FIGS. 9 to 13. FIG. 9 is an aberration diagram of the infrared imaging lens 2. FIG. 9 shows spherical aberration, astigmatism, and distortion. In each case, graphs for each wavelength in the range of 7.5 to 13.5 μm are shown. FIG. 10 is an aberration diagram showing the coma aberration at each image height Y from 0 mm to the maximum image height of 4.92 mm, separated into the tangential (meridional) direction and the sagittal (radial) direction. As shown in FIGS. 9 and 10, in the infrared imaging lens 2 according to Numerical Example 2, various aberrations are well corrected over a wide wavelength range of 7.5 to 13.5 μm.

[0079] FIG. 11 is a graph showing the relative illuminance of the infrared imaging lens 2 in Numerical Example 2 with respect to the image height Y. As shown in FIG. 11, even at the maximum image height of 4.92 mm, a relative illuminance of 0.69 and sufficient peripheral light quantity are obtained.

[0080] When an image sensor with 640 × 512 pixels (VGA+) and an effective diagonal length of 9.84 mm is used, the image height Y corresponding to the center of the left and right ends of the imaging surface is 3.84 mm, and the relative illuminance there is 0.81. Also, in that case, the image height Y corresponding to the center of the upper and lower ends of the imaging surface is 3.07 mm, and the relative illuminance there is 0.87.

[0081] FIG. 12 is a graph showing the spatial frequency dependence of MTF in the wavelength range of 7.5 to 13.5 μm. At the spatial frequency of 41.7 cycles / mm corresponding to the pixel pitch of 12 μm, the MTF at each image height Y is ensured to be 0.30 or more, which sufficiently exceeds 0.17, as the simple average in the tangential direction and the sagittal direction. At this time, when the image height Y is 0 mm, that is, at the image center, the MTF is 0.45 and the resolution is good.

[0082] Also, at the spatial frequency of 20.85 cycles / mm, the MTF at each image height Y is ensured to be 0.60 or more, which sufficiently exceeds 0.50, as the simple average in the tangential direction and the sagittal direction. In the infrared imaging lens 2 according to Numerical Example 2, good resolution that can correspond to an image sensor with a narrow pitch of about the wavelength is obtained over the entire surface within the image circle.

[0083] FIG. 13 is a graph showing the change in MTF in the wavelength range of 7.5 to 13.5 μm with respect to focus shift. As described above, the infrared imaging lens 2 of Numerical Example 2 can cover the wavelength range of 7.5 to 13.5 μm and has good resolution sufficient to correspond to an image sensor with a pixel pitch of about 12 μm. Furthermore, the infrared imaging lens 2 of Numerical Example 2 has an F number of 1.0, is bright, and is compact. Thus, according to Numerical Example 2, an infrared imaging lens having unprecedented compact and excellent characteristics can be realized.

[0084] <Numerical Example 3> The cross section of the infrared imaging lens 3 according to Numerical Example 3 is shown in the optical path diagram of FIG. 14. The infrared imaging lens 3 according to Numerical Example 3 is an imaging lens with an F number of 1.2 optimized to favorably image infrared rays in the wavelength range of 7 to 14 μm. The design center wavelength is 10 μm. The basic lens data, aspherical data, diffractive surface data, and various data are shown below.

Table 9

Table 10

[0085] The focal length f1 of the first lens L1 is 24.16 mm. Therefore, the ratio of this to the focal length f of the infrared imaging lens 3 is f1 / f=2.7 The focal length f2 of the second lens L2 is 44.44 mm. The focal length f3 of the third lens L3 is 9.77 mm. Therefore, the third lens L3 is configured to have the strongest power, followed by the first lens L1. The ratio of the focal length f3 of the third lens L3 to the focal length f of the infrared imaging lens 3 is f3 / f=1.1 is.

[0086] The maximum image height on the image plane S is 4.92 mm, and therefore the diameter φs of the image circle is 9.84 mm. The ratio of the diameter φs of the image circle of the infrared imaging lens 3 to the focal length f is φs / f=1.09 In other words, the infrared imaging lens 3 is a standard lens. The half angle of view is 29.9°, which is within the range of 21 to 36° that can be considered a standard lens.

[0087] The infrared imaging lens 3 is compact, with a total lens length L from the first surface to the image plane S of 20.6 mm and a maximum effective diameter on the optical path of 14.5 mm. The ratio of the total lens length L of the infrared imaging lens to the focal length f of the infrared imaging lens is L / f=2.3 and the overall length L of the lens is made not to be excessively large with respect to the focal length f.

[0088] Further, the infrared imaging lens 3 has a three-lens configuration and can be made lightweight. Combined with the fact that each lens can be manufactured by press molding, the infrared imaging lens 3 can be manufactured at low cost applicable to consumer applications.

[0089] In Numerical Example 3, the ratio of the back focus BF to the focal length f of the infrared imaging lens is BF / f = 0.79 and sufficient back focus is ensured. The performances of the infrared imaging lens 3 of Numerical Example 3 are shown in FIGS. 15 to 19. FIG. 15 is an aberration diagram of the infrared imaging lens 3. FIG. 15 shows spherical aberration, astigmatism, and distortion. In each case, graphs for each wavelength in the range of 7 to 14 μm are shown. FIG. 16 is an aberration diagram showing the coma aberration at each image height Y from 0 mm to the maximum image height of 4.92 mm, separated into the tangential (meridional) direction and the sagittal (radial) direction. As shown in FIGS. 15 and 16, in the infrared imaging lens 3 according to Numerical Example 3, various aberrations are well corrected over a wide wavelength range of 7 to 14 μm.

[0090] FIG. 17 is a graph showing the relative illuminance of the infrared imaging lens 3 of Numerical Example 3 with respect to the image height Y. As shown in FIG. 17, even at the maximum image height of 4.92 mm, a relative illuminance of 0.68 and sufficient peripheral light quantity are obtained.

[0091] When an image sensor of 640 × 512 pixels (VGA+) and an effective diagonal length of 9.84 mm is used, the image height Y corresponding to the center of the left and right ends of the imaging surface is 3.84 mm, and the relative illuminance there is 0.80. Also in that case, the image height Y corresponding to the center of the upper and lower ends of the imaging surface is 3.07 mm, and the relative illuminance there is 0.87.

[0092] FIG. 18 is a graph showing the spatial frequency dependence of the MTF in the wavelength range of 8 to 14 μm. At a spatial frequency of 41.7 cycles / mm corresponding to a pixel pitch of 12 μm, the MTF at each image height Y ensures a value of 0.21 or more, which sufficiently exceeds 0.17, as the simple average of the tangential direction and the sagittal direction. At this time, when the image height Y is 0 mm, that is, at the image center, the MTF is 0.36 and the resolution is good.

[0093] Also, at a spatial frequency of 20.85 cycles / mm, the MTF at each image height Y ensures a value of 0.55 or more as the simple average of the tangential direction and the sagittal direction, which sufficiently exceeds 0.50. In the infrared imaging lens 3 according to Numerical Example 3, a good resolution is obtained as an infrared imaging lens with an F-number of 1.2 that can correspond to an image sensor with a narrow pitch on the order of the wavelength over the entire area within the image circle.

[0094] FIG. 19 is a graph showing the change in the MTF in the wavelength range of 7 to 14 μm with respect to focus shift. The infrared imaging lens 3 of Numerical Example 3 has a design with an F-number of 1.2, which is larger than that of the infrared imaging lens of Numerical Example 1, and a deeper depth of focus can be obtained.

[0095] As described above, the infrared imaging lens 3 of Numerical Example 3 can cover the wavelength range of 7 to 14 μm and has a good resolution that sufficiently corresponds to an image sensor with a pixel pitch of about 12 μm. Furthermore, the infrared imaging lens 3 of Numerical Example 3 is bright and compact with an F-number of 1.2. Thus, according to Numerical Example 3, an infrared imaging lens with unprecedentedly compact and excellent characteristics can be realized.

[0096] 〔Summary〕 Aspect 1 of the present invention is an infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, and the diameter of the image circle is 0.7 to 1.3 times the focal length. According to the above configuration, it is possible to realize an infrared imaging lens, which is a standard lens having excellent resolution and can be compatible with an image sensor having a pixel pitch on the order of the wavelength.

[0097] The infrared imaging lens according to Aspect 2 of the present invention may have a configuration in which, in the above Aspect 1, the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.17 or more within the image circle. According to the above configuration, the relationship between the pixel pitch on the order of the wavelength and the resolution is more specifically limited.

[0098] Aspect 3 of the present invention is an infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, and the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.17 or more within the image circle. According to the above configuration, it is possible to realize an infrared imaging lens, which is a standard lens having excellent resolution and can be compatible with an image sensor having a pixel pitch on the order of the wavelength.

[0099] The infrared imaging lens according to Aspect 4 of the present invention may have a configuration in which, in the above Aspects 1 to 3, the first lens arranged closest to the object side has a positive power and is in a meniscus shape convex on the object side. According to the above configuration, it is possible to realize an infrared imaging lens having excellent aberration characteristics.

[0100] The infrared imaging lens according to Aspect 5 of the present invention is such that, in the above Aspect 4, the focal length f1 of the first lens and the focal length f of the infrared imaging lens are 1.0 ≦ f1 / f ≦ 2.9 It may have a configuration that satisfies the relational expression. According to the above configuration, the outer diameter and volume of the infrared imaging lens can be made compact.

[0101] The infrared imaging lens according to Aspect 6 of the present invention may have a configuration in which, in the above Aspect 4 or 5, the effective diameter of the object-side surface of the first lens is an aperture stop. According to the above configuration, the divergence of the peripheral light beam is reduced, and the peripheral light quantity can be improved.

[0102] The infrared imaging lens according to Aspect 7 of the present invention may have a configuration in which, in the above Aspects 4 to 6, the first lens, the second lens, and the third lens are arranged in order from the object side toward the image plane side. According to the above configuration, while maintaining high resolution, the configuration of the infrared imaging lens can be made compact and lightweight.

[0103] The infrared imaging lens according to Aspect 8 of the present invention may have a configuration in which, in the above Aspect 7, both the second lens and the third lens have positive power. According to the above configuration, an infrared imaging lens excellent in aberration characteristics can be realized.

[0104] The infrared imaging lens according to Aspect 9 of the present invention may have a configuration in which, in the above Aspect 8, among the first lens, the second lens, and the third lens, the power of the third lens is the strongest, and the third lens has a convex meniscus shape on the image plane side. According to the above configuration, spherical aberration is reduced.

[0105] The infrared imaging lens according to Aspect 10 of the present invention may have a configuration in which, in the above Aspects 7 to 9, the second lens has a convex meniscus shape on the image plane side. According to the above configuration, an increase in Petzval sum can be suppressed, and field curvature can be suppressed.

[0106] An infrared imaging lens according to Aspect 11 of the present invention is the infrared imaging lens of Aspects 7 to 10, and may be configured such that at least one of the image-side surface of the second lens and the object-side surface of the third lens is a diffractive surface. This configuration makes it possible to reduce chromatic aberration of magnification.

[0107] An infrared imaging lens according to Aspect 12 of the present invention is, in any of Aspects 7 to 10, characterized in that the focal length f3 of the third lens and the focal length f of the infrared imaging lens are: 0.8≦f3 / f≦1.2 According to the above-described configuration, good resolution can be obtained over a wide image circle area.

[0108] The infrared imaging lens according to Aspect 13 of the present invention may have a configuration in which the half angle of view is 21 to 36° in accordance with Aspects 1 to 12. This configuration allows the infrared imaging lens to have a preferred angle of view as a standard lens.

[0109] An infrared imaging lens according to Aspect 14 of the present invention may be configured in any of Aspects 1 to 13 above, such that the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 20.85 cycles / mm is 0.50 or greater within the image circle. This configuration makes it possible to realize an infrared imaging lens that exhibits good MTF over the entire spatial frequency range from 0 cycles / mm to 41.7 cycles / mm, which corresponds to the target Nyquist frequency.

[0110] An infrared imaging lens according to a fifteenth aspect of the present invention may be configured such that the relative illuminance on the image plane is 40% or more within the image circle in any of the above-mentioned aspects 1 to 14. This configuration makes it possible to realize an infrared imaging lens that ensures a sufficient amount of peripheral light.

[0111] An infrared imaging lens according to Aspect 16 of the present invention may have a configuration in which the glass is chalcogenide glass in any of Aspects 1 to 15. This configuration makes it possible to configure an infrared imaging lens using a lens having an aspherical surface including a diffractive surface, and to manufacture an imaging lens with particularly excellent aberration characteristics and resolution.

[0112] An infrared imaging lens according to Aspect 17 of the present invention may be configured in accordance with Aspect 16, wherein the chalcogenide glass has an infrared absorption edge wavelength of 18 μm or more at which the optical transmittance at a thickness of 2 mm is 20%. This configuration makes it possible to configure an infrared imaging lens with very little optical absorption in the target wavelength range.

[0113] An infrared imaging lens according to an eighteenth aspect of the present invention may have an F-number of 1.0 to 1.2 in the above-mentioned aspects 1 to 17. With this configuration, a bright standard lens can be configured. Alternatively, with this configuration, a standard lens with excellent resolution can be realized.

[0114] An infrared imaging lens according to Aspect 19 of the present invention may be configured as in Aspects 1 to 18 above, wherein the focal length of the infrared imaging lens is 7 to 12 mm. This configuration makes it possible to configure an infrared imaging lens that is compact and has excellent properties such as resolution and brightness. Alternatively, this configuration makes it possible to configure an infrared imaging lens that has excellent properties and is low-cost and suitable for consumer use.

[0115] An infrared imaging lens according to Aspect 20 of the present invention may have a configuration in which the total lens length is 30 mm or less in Aspects 1 to 19 above. With this configuration, it is possible to configure an infrared imaging lens that is compact and has excellent properties such as resolution and brightness. Alternatively, with this configuration, it is possible to configure an infrared imaging lens that has excellent properties and is low-cost and suitable for consumer use.

[0116] The infrared camera according to Embodiment 21 of the present invention includes any one of the infrared imaging lenses of Embodiments 1 to 20 and an infrared image sensor. According to the above configuration, an infrared camera that is compact and excellent in various characteristics such as resolution and brightness can be realized. Alternatively, according to the above configuration, a low-cost infrared camera that is excellent in various characteristics and can be used for various applications in the consumer market can be realized.

[0117] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Embodiments obtained by appropriately combining the technical means disclosed in the specification are also included in the technical scope of the present invention. Furthermore, by combining the technical means disclosed in the specification, new technical features can be formed.

[0118] In the embodiment, an example in which the infrared imaging lens is configured by arranging three lenses is shown, but the application of the present invention is not limited to this, and it may be configured by arranging a plurality of lenses other than three.

Explanation of Reference Numerals

[0119] 1, 2, 3 Infrared imaging lens L1 First lens L2 Second lens L3 Third lens P Parallel plate S Image plane

Claims

1. An infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, the diameter of the image circle is 0.7 to 1.3 times the focal length, in order from the object side toward the image plane side, a first lens, a second lens, and a third lens are arranged, the first lens arranged closest to the object side has positive power and is a meniscus shape convex on the object side, both the second lens and the third lens have positive power, among the first lens, the second lens, and the third lens, the third lens has the strongest power, and the third lens is a meniscus shape convex on the image plane side, the infrared imaging lens.

2. The infrared imaging lens according to claim 1, wherein the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.17 or more within the image circle.

3. An infrared imaging lens configured by arranging a plurality of lenses, wherein each of the plurality of lenses is made of glass having a refractive index of 2.8 to 4.0 at a wavelength of 10 μm, the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.17 or more within the image circle, in order from the object side toward the image plane side, a first lens, a second lens, and a third lens are arranged, the first lens arranged closest to the object side has positive power and is a meniscus shape convex on the object side, both the second lens and the third lens have positive power, among the first lens, the second lens, and the third lens, the third lens has the strongest power, and the third lens is a meniscus shape convex on the image plane side, the infrared imaging lens.

4. The focal length f1 of the first lens and the focal length f of the infrared imaging lens satisfy 1.0 ≦ f1 / f ≦ 2.9 the relational expression, the infrared imaging lens according to any one of claims 1 to 3.

5. The infrared imaging lens according to any one of claims 1 to 4, wherein the effective diameter of the object-side surface of the first lens is used as the aperture stop.

6. The infrared imaging lens according to any one of claims 1 to 5, wherein the second lens is a meniscus shape convex on the image plane side.

7. The infrared imaging lens according to any one of claims 1 to 6, wherein at least one of the image-side surface of the second lens and the object-side surface of the third lens is a diffractive surface.

8. The focal length f3 of the third lens and the focal length f of the infrared imaging lens satisfy 0.8 ≤ f3 / f ≤ 1.2 The infrared imaging lens according to any one of claims 1 to 7, which satisfies the relational expression.

9. The infrared imaging lens according to any one of claims 1 to 8, having a half field angle of 21 to 36°.

10. The infrared imaging lens according to any one of claims 1 to 9, wherein the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 20.85 cycles / mm satisfies 0.50 or more within the image circle.

11. The infrared imaging lens according to any one of claims 1 to 10, wherein the relative illuminance on the image plane satisfies 40% or more within the image circle.

12. The infrared imaging lens according to any one of claims 1 to 11, wherein the glass is chalcogenide glass.

13. The infrared imaging lens according to claim 12, 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%.

14. The infrared imaging lens according to any one of claims 1 to 13, having an F number of 1.0 to 1.

2.

15. The infrared imaging lens according to any one of claims 1 to 14, having a focal length of the infrared imaging lens of 7 to 12 mm.

16. The infrared imaging lens according to any one of claims 1 to 15, having an overall lens length of 30 mm or less.

17. An infrared camera comprising the infrared imaging lens according to any one of claims 1 to 16 and an infrared image sensor.

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