Infrared imaging lens

Abstract

To realize a telephoto lens with an excellent resolution.SOLUTION: In an infrared imaging lens (1), a first lens (L1), a second lens (L2), and a third lens (L3) are each made of chalcogenide glass which has an index of refraction of 2.5 to 4.0 when the wavelength is 10 μm and the total system focal point distance is at least two times the diameter of an image circle.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Infrared cameras that photograph subjects with infrared rays in the mid- to far-infrared region, particularly in the wavelength range of the 10-μm band suitable for biological detection, are applied to surveillance cameras, security cameras, in-vehicle night vision, etc. These infrared cameras can be applied to various fields such as intrusion monitoring of important facilities, monitoring of illegal fishermen, traffic network monitoring, warehouse monitoring, obstacle monitoring on the route, detection of the origin of forest fires, in-tunnel monitoring, and maritime monitoring, and an expansion of demand is expected. Infrared imaging lenses applied to such infrared cameras are known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For applications typified by long-distance monitoring at night, so-called telephoto lenses with a relatively long focal length are required. In particular, there is a demand for realizing an infrared imaging lens that has excellent resolution capable of corresponding to a small image sensor having a pixel pitch of about the wavelength, has a wide wavelength band, and is available at low cost for consumer use.

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

Means for Solving the Problems

[0006] In order to solve the above problems, one aspect of the present invention is an infrared imaging lens used in the infrared region including at least any wavelength within the range of 7 to 14 μm. The first lens, the second lens, and the third lens are arranged in order from the object side to the image plane side. Each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and the overall focal length fL is not less than twice the diameter of the image circle.

[0007] In addition, in order to solve the above problems, another aspect of the present invention is an infrared imaging lens used in the infrared region including at least any wavelength within the range of 7 to 14 μm. The first lens, the second lens, and the third lens are arranged in order from the object side to the image plane side. Each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and the semi-field angle is 14° or less.

Advantages of the Invention

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

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

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 region of mid-far infrared. The infrared imaging lens 1 according to the present embodiment targets a telephoto lens such that the overall focal length fL is at least twice the diameter of the image circle. That is, the infrared imaging lens 1 according to the present embodiment is an infrared imaging lens that enables the realization of an imaging device capable of magnifying and observing a distant object by providing the infrared imaging lens 1.

[0011] FIG. 1 is a cross-sectional view along the optical axis showing the configuration of the main part of the infrared imaging lens 1. The infrared imaging lens 1 is configured by arranging a first lens L1, a second lens L2, and a third lens L3 in order from the object side toward the image plane S side. During focusing, the first lens L1 to the third lens L3 all move in the optical axis direction uniformly.

[0012] The first lens L1, the second lens L2, and the third lens L3 are each made of chalcogenide glass having a refractive index of 2.5 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 chalcogenide glass as the optical 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 what kind of image sensor is adopted.

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

[0015] <Refractive materials of each lens> In particular, the above-mentioned 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-mentioned 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 mid-infrared region, which is a glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, has been developed by the applicant (see International Publication WO2020 / 105719A1). More specifically, the refractive index of this refractive 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 is likely to become too long.

[0017] Also, the Abbe number (ν10) 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 (ν10) will be described later. If the Abbe number is too low, chromatic aberration tends to be large. Although the upper limit of the Abbe number is not particularly limited, it is realistically 350 or less.

[0018] This optical material has extremely low light absorption over a wide wavelength range in the mid- to far-infrared region, such as at least wavelengths of 7 to 14 μm. In particular, this optical material has the characteristic that, despite being a chalcogenide, it also has low light absorption in the region beyond a wavelength of 10 μm. 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 region with a wavelength of 8 μm or more, and is defined as the wavelength at which the light transmittance at a material thickness of 2 mm becomes 20%. The internal transmittance refers to the transmittance inside the material and does not include the reflection loss at the material surface. The chalcogenide glass as the optical material constituting the first lens L1, the second lens L2, and the third lens has an infrared absorption edge wavelength of 18 μm or more.

[0020] Therefore, the above chalcogenide glass transmits infrared rays with a wavelength exceeding 10 μm and has good transmittance over at least the range of wavelengths of 7 to 14 μm. Also, the internal transmittance of the above chalcogenide glass at a thickness of 2 mm is 90% or more at a wavelength of 10 μm. Thus, in the infrared imaging lens 1 of this embodiment, an imaging lens with low light absorption by the lens optical material can be realized over a wide wavelength range of at least 7 to 14 μm.

[0021] Furthermore, the above chalcogenide glass can be molded into a lens having an aspherical surface by press molding. Therefore, mass production of aspherical lenses using the above chalcogenide glass is easy. Preferably, the glass transition temperature of the optical material is as low as 200 °C or lower, making press molding easier. In the infrared imaging lens 1, aberration is suppressed by using at least one aspherical lens.

[0022] When an aspherical lens cannot be applied, the configuration of the infrared imaging lens for suppressing aberration will have an increased number of lenses, resulting in an increase in weight and size. Therefore, it becomes a high-cost imaging lens that is not suitable for consumer use. In this specification, an aspherical surface includes a diffractive surface.

[0023] In addition, with the above chalcogenide glass, it is also possible to press-mold a lens having a surface with a particularly complex shape such as a diffractive surface. Therefore, in the infrared imaging lens 1, by using the above chalcogenide glass to make at least one lens surface a diffractive surface, aberration can be well suppressed over a wide range of wavelengths from 7 to 14 μm.

[0024] Crystal materials such as silicon (Si), germanium (Ge), zinc sulfide (ZnS), and zinc selenide (ZnSe), which are used as materials transmitting in the far-infrared region, cannot be press-molded. Therefore, it is difficult to mass-produce aspherical lenses having complex shapes. Thus, it is difficult to realize low-cost aspherical lenses for consumer use with these crystal materials.

[0025] <Details of the configuration of each lens> Furthermore, the infrared imaging lens 1 of this embodiment can be configured with the details of each part as follows.

[0026] In order to realize an imaging device capable of magnifying and observing a distant object, the infrared imaging lens 1 of the present embodiment is configured such that the overall focal length fL is at least twice the diameter φs of the image circle. This may be defined as having a half field angle of 14° or less.

[0027] In particular, in the infrared imaging lens 1 of the present embodiment, it is preferable that the overall focal length fL is configured to be within the range of 3 to 6 times the diameter of the image circle. As the absolute value of the overall focal length fL, it is preferably 15 to 30 mm.

[0028] The infrared imaging lens 1 of the present embodiment is an infrared imaging lens that can be used in the infrared region including wavelengths in the range of 7 to 14 μm. By applying the above-mentioned chalcogenide glass as the glass material of each lens, excellent characteristics can be obtained over a wide wavelength range of 7 to 14 μm.

[0029] According to the present embodiment, in such a telephoto lens, it is possible to realize a bright imaging lens having an F-number within the range of 0.9 to 1.1. In addition, it is possible to realize a telephoto lens with low light absorption due to the lens glass material over at least a wide wavelength range of 7 to 14 μm. Therefore, combined with the small F-number of about 1, a bright imaging lens can be realized.

[0030] The first lens L1 has a positive refractive power and has a meniscus shape with a convex surface facing the object side. The second lens L2 has a positive refractive power and has a meniscus shape with a convex surface facing the image plane side. The third lens L3 has a positive refractive power and has a meniscus shape with a convex surface facing the object side. By configuring and arranging each lens in this way, an increase in the Petzval sum can be suppressed, field curvature can be suppressed, and the flatness of the imaging plane can be maintained.

[0031] Further, it is particularly preferable that the power strengths of the respective lenses, in descending order, are the third lens L3, the first lens L1, and the second lens L2. The power of the third lens L3, which is the closest to the image plane side, is the largest, and the infrared imaging lens 1 is configured such that the third lens L3 has a meniscus shape with a convex surface facing the image plane S side, thereby reducing spherical aberration.

[0032] In the infrared imaging lens 1, the focal length f1 of the first lens and the overall focal length fL of the entire system satisfy 1.6 ≦ f1 / fL ≦ 2.5 It is desirable to be configured to satisfy the relational expression. That is, the contribution degree of the first lens L1 to the overall focal length fL is not so large, and it is desirable that the second lens L2 and the third lens L3 also contribute to the overall focal length fL. By configuring the infrared imaging lens 1 in this way, while maintaining good aberration characteristics, a high resolution can be obtained that can correspond to an image sensor having a pixel pitch on the order of the wavelength.

[0033] Also, in the infrared imaging lens 1, the optical total length TTL, which is the distance on the optical axis from the object side surface (the first surface) of the first lens L1 to the image plane S, and the overall focal length fL satisfy 1.2 ≦ TTL / fL ≦ 2.0 It is desirable to be configured to satisfy the relational expression. That is, it is desirable that the optical total length TTL is configured to be slightly larger than the overall focal length fL. By configuring the infrared imaging lens 1 in this way, while maintaining good aberration characteristics, a high resolution can be obtained that can correspond to an image sensor having a pixel pitch on the order of the wavelength.

[0034] In the infrared imaging lens 1, it is preferable that the effective diameter of the object side surface (the second surface) of the first lens L1 is used as the aperture stop. By configuring it in this way, the vignetting of the peripheral light beam is reduced and the peripheral light quantity is improved. In particular, it is possible to ensure a relative illuminance close to 100% even at the maximum image height. Also, it is possible to reduce the outer diameter and volume of the infrared imaging lens 1 compared to inserting the aperture stop between the lenses.

[0035] In the infrared imaging lens 1, Overall focal length fL the back focus BFL and satisfy the relational expression of 0.2 ≦ BFL / fL It is desirable to be configured as such. By being configured in this way, while ensuring the back focus BFL, a telephoto lens excellent in aberration characteristics and resolution can be realized.

[0036] It is preferable that at least one of the first lens L1, the second lens L2, and the third lens is an aspherical lens. By this, it becomes possible to reduce the spherical aberration and astigmatism of the infrared imaging lens 1. In particular, it is preferable that the first lens L1 is an aspherical lens. The surface (the first surface) on the object side of the first lens L1 can be spherical, and the surface (the second surface) on the image plane S side can be aspherical.

[0037] Furthermore, it is preferable that one of the surfaces of the second lens L2 or the third lens L3 is a diffractive surface. By this, it becomes possible to generate negative dispersion and reduce the axial chromatic aberration and the magnification chromatic aberration. In particular, it is preferable that the surface (the third surface) on the object side of the second lens L2 is a diffractive surface. By making the concave surface a diffractive surface in this way, the formation of the diffractive surface by press working becomes easier.

[0038] In the infrared imaging lens 1, at an image height of 2.5 mm on the image plane S, the modulation transfer function (MTF: Modulation Transfer Function) in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm is configured to be 0.3 (30%) or more. Here, the value of the MTF is the simple average value in the tangential direction and the sagittal direction. The reason for paying attention to the spatial frequency of 41.7 cycles / mm and the image height of 2.5 mm will be explained below.

[0039] Miniaturization of image sensors in the mid- and far-infrared regions has advanced, and the pixel pitch has reached a narrow pitch limit of about the wavelength. Such miniaturized image sensors can be produced at a lower cost than large-area ones. Moreover, the imaging lenses applied to the image sensors can be reduced in aperture according to the area of the image sensors, enabling cost reduction.

[0040] Therefore, by applying such image sensors and imaging lenses to infrared cameras, cost reduction suitable for consumer applications can be achieved, and infrared cameras can be deployed in various fields. As an image sensor in the region of the 10-μm wavelength band, one with a pixel pitch of 12 μm is commercially available. The spatial frequency of 41.7 cycles / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 12 μm.

[0041] The fact that the MTF is 0.3 or more at an image height of 2.5 mm indicates that sufficient resolution with an MTF of 0.3 or more can be obtained over the entire area of an image sensor with a pixel pitch of 12 μm and 320×256 pixels arranged on the image plane S. That is, the infrared imaging lens 1 is a telephoto lens that can sufficiently support an infrared camera to which a miniaturized QVGA (320×240 pixels) class image sensor corresponding to a wavelength range of about 7 to 14 μm is applied.

[0042] Furthermore, the infrared imaging lens 1 is configured such that at an image height of 4.1 mm on the image plane S, the MTF in the wavelength range of 7 to 14 μm at a spatial frequency of 29.4 cycles / mm is 0.45 (45%) or more. The spatial frequency of 29.4 cycles / mm corresponds to the Nyquist frequency of an image sensor with a pixel pitch of 17 μm.

[0043] Also, at an image height of 4.1 mm, the MTF at a spatial frequency of 29.4 cycles / mm being 0.45 or more indicates that good resolution with an MTF of 0.45 or more can be obtained over the entire area of an image sensor with a pixel pitch of 17 μm and 384 × 288 pixels arranged on the image plane. That is, the infrared imaging lens 1 is a telephoto lens having a resolution sufficient to fully support an infrared camera to which an image sensor with a pixel pitch of about 12 to 17 μm in a wavelength region of about 7 to 14 μm is applied.

[0044] <Numerical Example 1> A numerical example of the infrared imaging lens 1 is shown. The cross-sectional view of the infrared imaging lens according to Numerical Example 1 is as shown in FIG. 1. In Numerical Example 1, r represents the radius of curvature, d represents the lens thickness on the optical axis, or the distance between surfaces, and ED represents the effective diameter (diameter). The unit of length is (mm). An asterisk (*) after the number of the surface number indicates an aspherical surface, and DOE indicates a diffractive surface. Basic lens data, aspherical data, diffractive surface data, and various data are shown below.

[0045]

Table 1

[0046] The definitions of the refractive index and Abbe number ν10 are as follows: N8: Refractive index at a wavelength of 8 μm N10: Refractive index at a wavelength of 10 μm N12: Refractive index at a wavelength of 12 μm ν10 = (N10 - 1) / (N8 - N12)

[0047]

Table 2

[0048] The definition of the aspherical shape is as follows:

[0049]

Number

[0050] h: Height from the optical axis r: Radius of curvature at the vertex κ: Conic constant An: n-th non- Sphere Surface coefficient (n: even) Z: Distance from a point on the aspherical surface at h to the tangent plane of the aspherical vertex

[0051]

Table 3

[0052] The definition of the diffractive surface is as follows:

[0053]

Equation

[0054]

Equation

[0055]

Equation

[0056] Φ: Phase difference function P1, P2: Phase coefficients Z dif : Optical path function Z DOE : Sag amount of the diffractive surface λ: Design center wavelength (assumed to be 10μm)

[0057]

Table 4

[0058] For the first lens L1, the second lens L2, and the third lens L3, chalcogenide glass with a refractive index N10 of 3.465 at a wavelength of 10 μm is used. For the parallel plate P, silicon (Si) is used. The object-side surface (the third surface) of the second lens L2 is a diffractive surface provided with a sag in the shape of a quinophore on the spherical surface. The depth of each sag ranges from 0 to the design center wavelength λ. The back focus BFL = 7.84 mm is the actual distance.

[0059] The maximum image height at the image plane S is 4.1 mm. Therefore, the diameter φs of the image circle is 8.2 mm. Thus, the infrared imaging lens 1 is applicable to a QVGA-class image sensor with a pixel pitch of 17 μm and a diagonal length of 8.16 mm, such as 384×288 pixels. Also, the infrared imaging lens 1 can cover the pixel area of a QVGA-class image sensor including QVGA (320×240 pixels) and QVGA+ (345×240 pixels) with a pixel pitch of 17 μm.

[0060] Needless to say, the infrared imaging lens 1 can cover the pixel area of a QVGA-class image sensor with a pixel pitch of 12 μm including 320×256 pixels. In addition, for configurations such as 384×288 pixels and 320×256 pixels, even if the optical axis center of the lens does not completely coincide with the center of the image sensor, the effective pixel number can ensure QVGA (320×240 pixels).

[0061] The ratio of the overall focal length fL of the infrared imaging lens 1 to the diameter φs of the image circle is fL / φs = 3.41 That is, the infrared imaging lens 1 is a telephoto lens. Also, the half field angle is 8.4°, which is within the range of 14° or less, making it a telephoto lens. The infrared imaging lens 1 is such a telephoto lens with a narrow field angle, and has an F-number of 1.0 and is an extremely bright imaging lens.

[0062] The infrared imaging lens 1 is compact with an overall optical length TTL (lens overall length) from the object side surface (first surface) of the first lens L1 to the image plane S of 46.8 mm and a maximum effective diameter on the optical path of 28.2 mm. Also, the infrared imaging lens 1 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 1 can be manufactured at low cost and is applicable to consumer applications.

[0063] The focal length f1 of the first lens L1 is 50.50 mm. Therefore, the ratio of the focal length f1 of the first lens L1 to the overall focal length fL of the infrared imaging lens 1 is, f1 / fL = 1.80 That is. The focal length f2 of the second lens L2 is 121.51 mm. The focal length f3 of the third lens L3 is 41.25 mm. Therefore, the infrared imaging lens 1 is configured such that the power of the third lens L3 is the strongest, and then the power of the first lens L1 is strong.

[0064] The ratio of the overall optical length TTL (lens overall length) of the infrared imaging lens 1 to the overall focal length fL is, TTL / fL = 1.67 That is. The back focus BFL is 7.84 mm (actual distance), ensuring a sufficient distance. The ratio of the overall optical length TTL (lens overall length) to the back focus BFL is, TTL / BFL = 5.97 That is.

[0065] The various performances of the numerical example 1 of the infrared imaging lens 1 are shown in FIGS. 2 to 7. FIGS. 2 and 3 are aberration diagrams of the infrared imaging lens 1. FIG. 2 shows spherical aberration, astigmatism, and distortion. In each case, graphs for each wavelength in the range of 7 to 14 μm are shown. FIG. 3 is an aberration diagram showing the coma aberration at each image height Y from an image height of 0 mm to the maximum image height, separated into the tangential (meridional) direction and the sagittal (radical) direction. As shown in FIGS. 2 and 3, in the infrared imaging lens 1 according to the numerical example 1, various aberrations are well corrected over a wide wavelength range of 7 to 14 μm.

[0066] Figure 4 is a graph showing the relative illuminance with respect to the image height Y in Numerical Example 1 of the infrared imaging lens 1. Here, the relative illuminance refers to the ratio of the illuminance with respect to the region on the optical axis (the central region of the image plane) on the image plane S. As shown in Figure 4, even at the maximum image height of 4.11 mm, the relative illuminance is almost 1, and an extremely uniform light quantity distribution is obtained within the image circle.

[0067] Figure 5 is a graph showing the spatial frequency dependence of the MTF in the wavelength range of 7 to 14 μm. The Nyquist frequency f N of an image sensor with a pixel pitch of 12 μm and 320 × 256 pixels is 41.7 cycles / mm, and the maximum image height is 2.46 mm. At the Nyquist frequency f N = 41.7 cycles / mm, the MTF at the center of the image is 0.41, and MTF > 0.36 (simple average in the tangential direction and sagittal direction) is ensured within the region of the image sensor.

[0068] Also, the Nyquist frequency f N of an image sensor with a pixel pitch of 17 μm and 384 × 288 pixels is 29.4 cycles / mm, and the maximum image height is 4.08 mm. At the Nyquist frequency f N = 29.4 cycles / mm, the MTF at the center of the image is 0.57, and within the region of the image sensor, MTF > 0.49 (simple average in the tangential direction and sagittal direction) is ensured.

[0069] Thus, even when evaluated by the MTF over a wide wavelength range of 7 to 14 μm within the region of a QVGA-class image sensor with a pixel pitch of about 12 to 17 μm, the infrared imaging lens 1 ensures good resolution.

[0070] Figure 6 is a graph showing the spatial frequency dependence of the MTF in the wavelength range of 8 to 12 μm. For an image sensor with a pixel pitch of 12 μm and 320 × 256 pixels, the Nyquist frequency f NAt a spatial frequency of 41.7 cycles / mm, the MTF at the image center is 0.47, and within the area of the image sensor, MTF > 0.43 (simple average in the tangential and sagittal directions) is ensured.

[0071] Regarding an image sensor with a pixel pitch of 17 μm and 384 × 288 pixels, the Nyquist frequency f N At a spatial frequency of 29.4 cycles / mm, the MTF at the image center is 0.62, and within the area of the image sensor, MTF > 0.55 (simple average in the tangential and sagittal directions) is ensured. Thus, the infrared imaging lens 1 ensures good resolution within the area of a QVGA-class image sensor with a pixel pitch of about 12 to 17 μm.

[0072] As described above, it goes without saying that the infrared imaging lens 1 has good characteristics within any wavelength range, such as 7 to 12 μm, 8 to 12 μm, or 8 to 10 μm, as long as it is within the wavelength range of 7 to 14 μm. FIG. 7 is a graph showing the change in MTF with respect to focus shift within the wavelength range of 7 to 14 μm.

[0073] As described above, the infrared imaging lens 1 of Numerical Example 1 can cover the wavelength range of 7 to 14 μm and has good resolution sufficient to correspond to a QVGA-class image sensor with a pixel pitch of about 12 to 17 μm. The infrared imaging lens 1 has a bright F-number of 1.0 and is compact. Thus, according to this embodiment, an infrared imaging lens, which is a telephoto lens with unprecedentedly compact and excellent characteristics, can be realized.

[0074] 〔Summary〕 Aspect 1 of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength within the range of 7 to 14 μm, in which a first lens, a second lens, and a third lens are arranged in order from the object side toward the image plane side, and each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index at a wavelength of 10 μm of 2.5 to 4.0, and the overall focal length fL is not less than twice the diameter of the image circle.

[0075] According to the above configuration, an infrared imaging lens, which is a telephoto lens having excellent resolution and can be compatible with an image sensor having a pixel pitch of about the wavelength, can be realized.

[0076] Aspect 2 of the present invention is an infrared imaging lens used in an infrared region including at least any wavelength within the range of 7 to 14 μm, in which a first lens, a second lens, and a third lens are arranged in order from the object side toward the image plane side, and each of the first lens, the second lens, and the third lens is made of a chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and the semi-field angle is 14° or less.

[0077] According to the above configuration, an infrared imaging lens, which is a telephoto lens having excellent resolution and can be compatible with an image sensor having a pixel pitch of about the wavelength, can be realized.

[0078] The infrared imaging lens according to Aspect 3 of the present invention is, in the above Aspect 1 or 2, each of the first lens, the second lens, and the third lens is a meniscus lens having a positive power. According to the above configuration, an infrared imaging lens excellent in aberration characteristics can be realized.

[0079] The infrared imaging lens according to Aspect 4 of the present invention is, in the above Aspect 3, each of the first lens and the third lens has a meniscus shape convex on the object side, and the second lens has a meniscus shape convex on the image plane side. According to the above configuration, an increase in Petzval sum can be suppressed, and field curvature can be suppressed.

[0080] The infrared imaging lens according to Aspect 5 of the present invention is, in the above Aspects 1 to 4, configured to use the effective diameter of the image plane side surface of the first lens as an aperture stop. According to the above configuration, vignetting of the peripheral light beam can be reduced, and the peripheral light quantity can be improved.

[0081] The infrared imaging lens according to Aspect 6 of the present invention is, in the above Aspects 1 to 5, wherein the first lens is an aspherical lens. According to the above configuration, an infrared imaging lens with particularly excellent aberration characteristics can be realized.

[0082] The infrared imaging lens according to Aspect 7 of the present invention is, in the above Aspects 1 to 6, wherein either the surface of the second lens or the third lens is a diffractive surface. According to the above configuration, it becomes possible to reduce axial chromatic aberration and magnification chromatic aberration.

[0083] The infrared imaging lens according to Aspect 8 of the present invention is, in the above Aspects 1 to 7, wherein the focal length f1 of the first lens and the overall focal length fL of the entire system satisfy the relational expression 1.6 ≦ f1 / fL ≦ 2.5. According to the above configuration, while maintaining good aberration characteristics, a high resolution can be obtained that can accommodate an image sensor having a pixel pitch on the order of the wavelength.

[0084] The infrared imaging lens according to Aspect 9 of the present invention is, in the above Aspects 1 to 8, wherein the optical total length TTL, which is the distance on the optical axis from the object side surface of the first lens to the image plane, and the overall focal length fL of the entire system satisfy the relational expression 1.2 ≦ TTL / fL ≦ 2.0. According to the above configuration, while maintaining good aberration characteristics, a high resolution can be obtained that can accommodate an image sensor having a pixel pitch on the order of the wavelength.

[0085] The infrared imaging lens according to Aspect 10 of the present invention is, in the above Aspects 1 to 9, Overall focal length fL and the back focus BFL satisfy the relational expression 0.2 ≦ BFL / fL. According to the above configuration, while ensuring the back focus BFL, a telephoto lens excellent in aberration characteristics and resolution can be realized.

[0086] The infrared imaging lens according to Aspect 11 of the present invention is, in the above Aspects 1 to 10, wherein the relative illuminance at the image plane satisfies 98% or more within the image circle. According to the above configuration, an infrared imaging lens with sufficient peripheral light quantity can be realized.

[0087] The infrared imaging lens according to Embodiment 12 of the present invention satisfies the condition that the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm is 0.3 or more at an image height of 2.5 mm in Embodiments 1 to 11 above. According to the above configuration, it is possible to realize an infrared imaging lens that can provide good resolution over the entire area of an image sensor having a pixel pitch on the order of the wavelength.

[0088] The infrared imaging lens according to Embodiment 13 of the present invention is the infrared imaging lens according to any one of claims 1 to 12, wherein in Embodiments 1 to 12 above, the overall focal length fL is within the range of 3 to 6 times the diameter of the image circle. According to the above configuration, it is possible to realize an infrared imaging lens that is a telephoto lens excellent in aberration characteristics and resolution.

[0089] The infrared imaging lens according to Embodiment 14 of the present invention has an F-number within the range of 0.9 to 1.1 in Embodiments 1 to 13 above. According to the above configuration, it is possible to realize a bright infrared imaging lens with a small F-number, which is a telephoto lens excellent in aberration characteristics and resolution.

[0090] The infrared imaging lens according to Embodiment 15 of the present invention has an infrared absorption edge wavelength of 18 μm or more such that the light transmittance at a thickness of 2 mm is 20% for the chalcogenide glass in Embodiments 1 to 14 above. According to the above configuration, it becomes possible to configure an infrared imaging lens with very small light absorption in at least the wavelength range of 7 to 14 μm.

[0091] 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.

Description of Reference Numerals

[0092] 1 Infrared imaging lens L1 First lens L2 Second lens L3 Third lens P Parallel plate S Image plane AP Aperture stop

Claims

1. An infrared imaging lens used in the infrared region including at least any wavelength within the range of 7 to 14 μm, wherein a first lens, a second lens, and a third lens are arranged in order from the object side toward the image plane side, each of the first lens, the second lens, and the third lens is a meniscus lens having a positive power, each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and an infrared imaging lens in which an overall focal length fL is not less than twice the diameter of the image circle.

2. An infrared imaging lens used in the infrared region including at least any wavelength within the range of 7 to 14 μm, wherein a first lens, a second lens, and a third lens are arranged in order from the object side toward the image plane side, each of the first lens, the second lens, and the third lens is a meniscus lens having a positive power, each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, and an infrared imaging lens in which a semi-field angle is 14° or less.

3. Each of the first lens and the third lens has a meniscus shape convex toward the object side, and the infrared imaging lens according to claim 1 or 2, wherein the second lens has a meniscus shape convex toward the image plane side.

4. The infrared imaging lens according to any one of claims 1 to 3, wherein an effective diameter of a surface on the image plane side of the first lens is an aperture stop.

5. The infrared imaging lens according to any one of claims 1 to 4, wherein the first lens is an aspherical lens.

6. The infrared imaging lens according to any one of claims 1 to 5, wherein any surface of the second lens or the third lens is a diffractive surface.

7. A relationship between a focal length f1 of the first lens and an overall focal length fL satisfies 1.6 ≤ f1 / fL ≤ 2.5 and the infrared imaging lens according to any one of claims 1 to 6.

8. A relationship between an overall optical length TTL, which is a distance on the optical axis from a surface on the object side of the first lens to the image plane, and an overall focal length fL satisfies 1.2 ≤ TTL / fL ≤ 2.0 and the infrared imaging lens according to any one of claims 1 to 7.

9. A relationship between an overall focal length fL and a back focus BFL satisfies 0.2 ≤ BFL / fL The infrared imaging lens according to any one of claims 1 to 8, which satisfies the relational expression.

10. The infrared imaging lens according to any one of claims 1 to 9, wherein the relative illuminance on the image plane satisfies 98% or more within the image circle.

11. The infrared imaging lens according to any one of claims 1 to 10, wherein the modulation transfer function in the wavelength range of 7 to 14 μm at a spatial frequency of 41.7 cycles / mm satisfies 0.3 or more at an image height of 2.5 mm.

12. The infrared imaging lens according to any one of claims 1 to 11, wherein the overall focal length fL is in the range of 3 to 6 times the diameter of the image circle.

13. The infrared imaging lens according to any one of claims 1 to 12, wherein the F-number is in the range of 0.9 to 1.

1.

14. The infrared imaging lens according to any one of claims 1 to 13, 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%.

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