Infrared imaging lens

Abstract

The present invention achieves a telephoto lens having excellent resolution. An infrared imaging lens (1) is configured as follows. Each of a first lens (L1), a second lens (L2), and a third lens (L3) is made of chalcogenide glass having a refractive index of 2.5-4.0 at a wavelength of 10 μm. The total system focal length is at least twice the diameter of an image circle. The first lens and the third lens are each a meniscus lens having a positive power, and the second lens is a meniscus lens having a negative power.

Description

Infrared imaging lens

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

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

[0003] For applications such as nighttime long-distance surveillance, infrared imaging lenses used in infrared cameras require so-called telephoto lenses, which have a relatively long focal length relative to the image circle diameter of the image plane. The present applicant has proposed a telephoto lens with a small F-number, high brightness, and excellent resolution that can be used with small image sensors with a pixel pitch on the order of the wavelength.

[0004] International Publication No. WO2023 / 008148

[0005] As the applications of infrared cameras expand, there is a demand for infrared imaging lenses with various features. It is also desirable to realize an infrared imaging lens that further reduces distortion (distortion aberration) while maintaining brightness and resolution, as compared to the conventional technology of Patent Document 1, for example. One aspect of the present invention addresses the above-mentioned problem and aims to realize an infrared imaging lens that is a telephoto lens usable for consumer applications, which has excellent resolution and further reduces distortion and is compatible with image sensors having a pixel pitch on the order of the wavelength.

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

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

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

[0009] FIG. 1 is a cross-sectional view showing the configuration of a main part of an infrared imaging lens according to an embodiment of the present invention. FIG. 2 is an aberration diagram showing spherical aberration, astigmatism, and distortion of an infrared imaging lens according to Numerical Example 1 of the present invention. FIG. 3 is an aberration diagram showing coma aberration of an infrared imaging lens according to Numerical Example 1 of the present invention. FIG. 4 is a graph showing the image height dependency of relative illuminance of an infrared imaging lens according to Numerical Example 1 of the present invention. FIG. 5 is a graph showing the spatial frequency dependency of MTF in the wavelength range of 7 to 14 μm of an infrared imaging lens according to Numerical Example 1 of the present invention. FIG. 6 is a graph showing the focus movement dependency of MTF of an infrared imaging lens according to Numerical Example 1 of the present invention.

[0010] [Embodiments] <Outline of Infrared Imaging Lens> The infrared imaging lens 1 according to the embodiment is a lens system that corresponds to the mid-to-far infrared wavelength region and forms an image of a subject on an image plane S on which an image sensor or the like is disposed. The infrared imaging lens 1 according to the present embodiment is intended for use as a telephoto lens in which the total system focal length fL is equal to or greater than twice the image circle diameter φs. Alternatively, the infrared imaging lens 1 according to the present embodiment is intended for use as a telephoto lens in which the half angle of view ω (maximum half angle of view) is equal to or less than 14°.

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

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

[0013] An imaging lens whose total system focal length fL is twice or more the diameter φs of the image circle, or whose half angle of view ω is 14° or less, has a focal length of 85 mm or more in 35 mm equivalent, and is generally referred to as a telephoto lens. Note that the 35 mm equivalent here assumes that the diagonal length of the image sensor is approximately equal to the diameter φs of the image circle, and that the image sensor effectively utilizes the image circle. As such, this embodiment is directed to an infrared imaging lens that enables the realization of an imaging device capable of magnifying and observing objects at long distances.

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

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

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

[0017] As shown in Figure 1, a parallel plate P is disposed between the third lens L3 and the image plane S. The parallel plate P is an optical window mounted on the image plane S side with a hermetic seal, and is made of silicon, low-oxygen silicon, or germanium. The material and thickness can be determined depending on the type of image sensor to be used.

[0018] 1, the effective diameter of the object-side surface (first surface) of the first lens L1 corresponds to the aperture stop of the infrared imaging lens 1. Anti-reflection (AR) coatings are applied to the surfaces of the first lens L1, the second lens L2, the third lens L3, and the plane-parallel plate P. Appropriate known techniques can be applied to such anti-reflection coatings in the mid-to-far infrared region.

[0019] <Glass Material of Each Lens> The following describes the chalcogenide glass that constitutes each lens of the infrared imaging lens 1. The chalcogenide glass used in the infrared imaging lens 1 is chalcogenide glass with a refractive index of 2.5 to 4.0 at a wavelength of 10 μm. From the perspective of realizing an infrared imaging lens 1 that is bright with an F-number (F-value) of about 1.0 and has excellent resolution compatible with image sensors with pixel pitches that correspond to wavelengths in the mid-to-far-infrared region, such as 7 to 14 μm, it is preferable that the refractive index of the chalcogenide glass be 2.5 to 4.0.

[0020] Chalcogenide glass with a high refractive index in the far-infrared region has been developed by the present applicant (see International Publication WO 2020 / 105719 A1). The refractive index N10 of this glass material at a wavelength of 10 μm, more specifically, is in the range of 2.74 to 3.92. Furthermore, if the refractive index of the chalcogenide glass constituting the first lens L1, second lens L2, and third lens L3 is low, it is difficult to construct a compact telephoto lens. Therefore, it is particularly preferable that the refractive index of the chalcogenide glass at a wavelength of 10 μm be in the range of 3.0 to 4.0.

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

[0022] This glass material has extremely low optical absorption over a wide wavelength range in the far-infrared region, at least from 7 to 14 μm. In particular, this glass material is characterized by low optical absorption even in the far-infrared region from 10 to 26 μm. In chalcogenide glass, the "infrared absorption edge wavelength" and "internal transmittance" can be used as indicators of excellent optical transmittance in the far-infrared region.

[0023] Here, the infrared absorption edge wavelength refers to the absorption edge wavelength in the wavelength range of 8 μm or more, and is defined as the wavelength at which the light transmittance at a material thickness of 2 mm is 20%. Note that the 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 each lens has an infrared absorption edge wavelength of 18 μm or more.

[0024] Therefore, the chalcogenide glass transmits infrared rays with wavelengths exceeding 10 μm, and has good transmittance over at least the wavelength range of 7 to 14 μm. Furthermore, the internal transmittance of the chalcogenide glass at a thickness of 2 mm is 90% or higher at a wavelength of 10 μm. Thus, the infrared imaging lens 1 of this embodiment can realize an imaging lens with low light absorption by the lens glass material over a wide wavelength range of at least 7 to 14 μm.

[0025] Furthermore, the use of the chalcogenide glass facilitates mass production of lenses by press molding. Press molding also enables mass production of aspherical lenses. In this specification, the term "aspherical surface" includes a diffractive surface. Preferably, the glass transition temperature of the glass material is low, at 200°C or less, making press molding easier.

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

[0027] Furthermore, the chalcogenide glass also makes it possible to press-mold lenses having particularly complex surfaces, such as diffractive surfaces. Therefore, in the infrared imaging lens 1, by using the chalcogenide glass to form the image-plane-side surface (surface No. 2) of the first lens L1 into a diffractive surface, it becomes possible to effectively suppress aberrations over a wide wavelength range from 7 to 14 μm.

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

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

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

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

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

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

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

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

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

[0037] From the viewpoint of enhancing the stability of vitrification, it is preferable that the total content of Ge, Ga, and Te is large. Specifically, it is preferable that Ge + Ga + Te is 50% or more, 60% or more, 70% or more, and particularly 80% or more. However, in order to introduce other components, the upper limit of Ge + Ga + Te may be 98% or less, 96% or less, 95% or less, and particularly 90% or less.

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

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

[0040] Si is a component that enhances the thermal stability of glass. The Si content is preferably 0 to 50%, more than 0 to 50%, 1 to 45%, 2 to 40%, 3 to 35%, 4 to 30%, 5 to 25%, and particularly preferably 5 to 20%. If the Si content is too high, infrared absorption due to Si is likely to occur, making it difficult for infrared light to transmit. However, since Si is a component that tends to reduce the Abbe number, from the viewpoint of increasing the Abbe number, the Si content is preferably 5% or less, 1% or less, 0.5% or less, and particularly preferably less than 0.1%.

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

[0042] The content of each of the components Al, Ti, Cu, In, Sn, Bi, Cr, Sb, Zn, and Mn is preferably 0 to 40%, 1 to 40%, 1 to 30%, or 1 to 25%, and particularly preferably 1 to 20%. Among these, Al, Cu, and / or Sn are preferably used because they are particularly effective in enhancing the thermal stability of the glass. However, since Al and Sn are components that tend to decrease the Abbe number, from the viewpoint of increasing the Abbe number, the content of Al and Sn is preferably 5% or less, 1% or less, 0.5%, and particularly preferably less than 0.1%, respectively.

[0043] F, Cl, Br, and I are also components that enhance the thermal stability of glass. The content of F + Cl + Br + I (total amount of F, Cl, Br, and I) is preferably 0 to 40%, 2 to 35%, 4 to 30%, and particularly 5 to 25%. If the content of F + Cl + Br + I is too high, vitrification becomes difficult and weather resistance tends to decrease. The contents of each of the components F, Cl, Br, and I are preferably 0 to 40%, 1 to 40%, 1 to 30%, 1 to 25%, and particularly 1 to 20%, respectively. Among these, I is preferred because elemental raw materials can be used and it has a particularly large effect of enhancing the thermal stability of glass.

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

[0045] <Details of the Configuration of Each Lens> Furthermore, the infrared imaging lens 1 of this embodiment can be configured in detail as follows.

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

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

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

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

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

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

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

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

[0054] The infrared imaging lens 1 is preferably configured so that the back focus BFL and the total system focal length fL satisfy the relational expression: 0.2≦BFL / fL. By configuring it in this way, it is possible to realize a telephoto lens that is excellent in aberration characteristics and resolution while ensuring the back focus BFL.

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

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

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

[0058] As image sensors in the mid-to-far infrared region have become smaller, the pixel pitch has reached the limit of narrow pitch, which is roughly the wavelength of light. These miniaturized image sensors can be produced at lower cost than larger-area sensors. Furthermore, the diameter of the imaging lens used in the image sensor can be reduced to match the area of ​​the image sensor, reducing costs.

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

[0060] An MTF of 0.3 or greater at an image height of 2.5 mm indicates that a sufficient resolution of 0.3 or greater 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. In other words, the infrared imaging lens 1 is a telephoto lens that is fully compatible with infrared cameras that use a miniaturized QVGA (320 × 240 pixels) class image sensor that corresponds to a wavelength range of approximately 7 to 14 μm.

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

[0062] Furthermore, the MTF being 0.45 or greater at an image height of 4.1 mm and a spatial frequency of 29.4 cycles / mm indicates that a good resolution of 0.45 or greater 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 S. In other words, the infrared imaging lens 1 is a telephoto lens with a resolution sufficient for use with infrared cameras that use image sensors with a pixel pitch of approximately 12 to 17 μm in a wavelength range of approximately 7 to 14 μm.

[0063] <Numerical Example 1> An example of an infrared imaging lens 1 is shown below. A cross-sectional view of the infrared imaging lens according to Numerical Example 1 is as shown in FIG. 1. In Numerical Example 1, r represents the radius of curvature, d represents the lens thickness on the optical axis or the distance between surfaces, and ED represents the effective diameter (diameter). The unit of length is (mm). An asterisk (*) after the surface number indicates that the surface is aspherical, and DOE indicates that the surface is diffractive. Basic lens data, aspherical surface data, diffractive surface data, and various data are shown below. The refractive index and Abbe number ν10 are defined as follows: N8: refractive index at a wavelength of 8 μm N10: refractive index at a wavelength of 10 μm N12: refractive index at a wavelength of 12 μm ν10=(N10-1) / (N8-N12) The definition of the aspheric shape is as follows: h: height from the optical axis; r: radius of curvature at the vertex; κ: conic constant; An: nth-order aspheric coefficient (n: even number); Z: distance from a point on the aspheric surface at h to the tangent plane of the aspheric surface vertex. The definition of the diffractive surface is as follows: Φ: Phase difference function P １ ,P ２ : Phase coefficient Z ｄｉｆ : Optical path function Z ＤＯＥ : Sag amount of the diffractive surface λ: Design central wavelength (assumed to be 10 μm) The first lens L1, second lens L2, and third lens L3 are made of chalcogenide glass with a refractive index N10 of 3.465 at a wavelength of 10 μm and an Abbe number of 253. The image-side surface (surface No. 2) of the first lens L1 is a diffractive surface with a kinoform-shaped sag on a spherical surface. The depth of each sag ranges from 0 to a value corresponding to the design central wavelength λ.

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

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

[0066] It goes without saying that the infrared imaging lens 1 can cover the pixel area of ​​a QVGA class image sensor with a pixel pitch of 12 μm, including 320 × 256 pixels. Note that with configurations such as 384 × 288 pixels and 320 × 256 pixels, the effective pixel count can be QVGA (320 × 240 pixels) even if the optical axis center of the lens does not perfectly coincide with the center of the image sensor.

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

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

[0069] In addition, the infrared imaging lens 1 has a three-lens configuration, making it lightweight. Coupled with the fact that each lens can be manufactured by press molding, the infrared imaging lens 1 can be manufactured at a low cost suitable for consumer use. Furthermore, an infrared camera incorporating the infrared imaging lens 1 can be configured compactly, making it easy to install in a variety of locations.

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

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

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

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

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

[0075] 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 is 41.7 cycles / mm, and the maximum image height is 2.46 mm. Ｎ At 41.7 cycles / mm, the MTF at the center of the image is 0.46, and an MTF of >0.37 (simple average of the tangential and sagittal directions) is ensured within the area of ​​the image sensor.

[0076] In addition, the Nyquist frequency f of the image sensor with a pixel pitch of 17 μm and 384 × 288 pixels is Ｎ is 29.4 cycles / mm, and the maximum image height is 4.08 mm. ＮAt 29.4 cycles / mm, the MTF at the center of the image is 0.58, and within the area of ​​the image sensor, an MTF of >0.49 (simple average of the tangential and sagittal directions) is ensured.

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

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

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

[0080] The infrared imaging lens 1 of Numerical Example 1 is bright with an F-number of 1.0, has a total lens length (total optical length TTL) of 38.5 mm, and is compact with a maximum effective lens diameter of 28.0 mm. As described above, this embodiment makes it possible to realize an infrared imaging lens that is a compact telephoto lens with excellent characteristics, which has not been seen in the past.

[0081] [Summary] Aspect 1 of the present invention is an infrared imaging lens used in an infrared region that includes at least any wavelength in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in this order from the object side to the image plane side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, a focal length fL of the entire system being equal to or greater than twice the diameter of an image circle, the first lens and the third lens being meniscus lenses with positive power, and the second lens being a meniscus lens with negative power.

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

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

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

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

[0086] According to Aspect 4 of the present invention, in any one of Aspects 1 to 3, the infrared imaging lens has an effective diameter of the object-side surface of the first lens as an aperture stop. With this configuration, vignetting of peripheral light flux is reduced, and the amount of peripheral light can be improved.

[0087] An infrared imaging lens according to Aspect 5 of the present invention is the infrared imaging lens of any one of Aspects 1 to 4, wherein at least one of the first lens and the second lens is an aspherical lens. With the above configuration, an infrared imaging lens with particularly excellent aberration characteristics can be realized.

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

[0089] An infrared imaging lens according to Aspect 7 of the present invention is the infrared imaging lens of any one of Aspects 1 to 6, in which the image-plane-side surface of the first lens is a diffractive surface. With the above configuration, it is easy to form a diffractive surface, thereby making it possible to realize an infrared imaging lens that can reduce axial chromatic aberration and chromatic aberration of magnification.

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

[0091] An infrared imaging lens according to Aspect 9 of the present invention is the infrared imaging lens of any one of Aspects 1 to 8, wherein a total optical length TTL, which is the distance on the optical axis from the object-side surface of the first lens to the image plane, and a total system focal length fL satisfy the relational expression 1.2≦TTL / fL≦2.0. With the above configuration, it is possible to obtain high resolution compatible with an image sensor having a pixel pitch on the order of the wavelength, while maintaining good aberration characteristics.

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

[0093] An infrared imaging lens according to Aspect 11 of the present invention is the same as any one of Aspects 1 to 10, in which the relative illuminance on the image plane is 94% or more within the image circle. With this configuration, an infrared imaging lens with sufficient peripheral light intensity can be realized.

[0094] An infrared imaging lens according to Aspect 12 of the present invention is the same as that of Aspects 1 to 11 above, in which 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 greater at an image height of 2.5 mm. This configuration makes it possible to realize an infrared imaging lens that can provide good resolution across the entire area of ​​an image sensor having a pixel pitch on the order of the wavelength.

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

[0096] An infrared imaging lens according to Aspect 14 of the present invention is the infrared imaging lens of any one of Aspects 1 to 13, wherein the F-number is in the range of 0.9 to 1.1. With the above configuration, it is possible to realize a bright infrared imaging lens with a small F-number, which is a telephoto lens with excellent aberration characteristics and resolution.

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

[0098] A sixteenth aspect of the present invention provides an infrared imaging lens in any one of the first to fourteenth aspects, 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 at least in the wavelength range of 7 to 14 μm.

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

[0100] 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 for use in an infrared region including at least any one of wavelengths in a range of 7 to 14 μm, comprising a first lens, a second lens, and a third lens arranged in that order from the object side to the image surface side, each of the first lens, the second lens, and the third lens being made of chalcogenide glass having a refractive index of 2.5 to 4.0 at a wavelength of 10 μm, a total system focal length fL being equal to or greater than twice the diameter of an image circle, the first lens and the third lens being meniscus lenses with positive power, and the second lens being a meniscus lens with negative power.

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

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

4. The infrared imaging lens according to claim 1 or 2, wherein the effective diameter of the object-side surface of said first lens is an aperture stop.

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

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

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

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

5.

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

0.

10. The infrared imaging lens according to claim 1 or 2, wherein a back focus BFL and a focal length fL of the entire system satisfy the following relation: 0.2≦BFL / fL.

11. The infrared imaging lens according to claim 1 or 2, wherein the relative illuminance on the image plane is 94% or more within the image circle.

12. The infrared imaging lens according to claim 1 or 2, wherein 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.

13. The infrared imaging lens according to claim 1 or 2, wherein the total system focal length fL is within a range of 3 to 6 times the diameter of the image circle.

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

1.

15. An infrared imaging lens according to claim 1 or 2, wherein each of the first lens, the second lens, and the third lens is made of chalcogenide glass having a refractive index of 3.0 to 4.0 at a wavelength of 10 μm.

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

Images