Infrared lens and thermal imager

Abstract

The invention discloses an infrared lens and a thermal imager. The infrared lens comprises a first lens, a second lens, a third lens and a fourth lens which are sequentially arranged along an optical axis A from an object side to an image side. Wherein the first lens is a positive meniscus lens, the first surface of the first lens is a convex surface facing the object side, and the second surface of the first lens is a concave surface facing the image side; the second lens is a biconvex lens and is provided with two convex surfaces facing the object side and the image side; the third lens is a negative meniscus lens and is provided with a concave surface facing the object side and a convex surface facing the image side; the fourth lens is a biconvex lens and also comprises two convex surfaces. The overall focal length of the infrared lens is f, the focal lengths of all the lenses are f1, f2, f3 and f4 respectively, and the focal lengths meet the following focal length relations:-2 < f1 / f <-1, 2 < f2 / f < 3,-11 < f3 / f <-10, and 2 < f4 / f < 3. Through the above structure and focal length matching, the infrared lens can achieve excellent imaging performance and good optical correction, and is suitable for a high-performance infrared imaging system.

Description

Technical Field

[0001] The present application relates to the field of optical devices, and in particular to infrared lenses and thermal imagers. Background Art

[0002] Infrared imaging technology is an imaging method that achieves image observation by detecting the infrared radiation emitted by the target itself. This technology features passive operation, no need for external lighting, and non-contact measurement. It has excellent environmental adaptability and can operate stably in harsh conditions such as darkness, rain, snow, and electromagnetic interference. It is widely used in nighttime surveillance, industrial inspection, and reconnaissance.

[0003] The increasing demand for infrared imaging applications places higher demands on the performance of imaging systems. This is especially true for applications such as wide-area monitoring and rapid response, where infrared lenses require a wider field of view to enable real-time observation of large-scale scenes. However, early infrared lenses generally suffered from a narrow field of view and limited coverage. For close-range observations of 1 to 5 meters, they often relied on mechanical rotating platforms or multi-lens stitching systems, resulting in complex system structures, increased costs, and reduced reliability.

[0004] Furthermore, to achieve good image quality and temperature adaptability, traditional infrared optical systems often require complex thermal compensation structures, resulting in large, heavy, and expensive lenses. Especially in wide-angle imaging applications at close ranges (e.g., 1-meter object distance), achieving the right balance between miniaturization, lightweighting, and cost-effectiveness while maintaining high imaging performance has become a key issue in infrared lens design.

[0005] Therefore, it is necessary to design an ultra-large field-of-view infrared lens optimized for close distances (such as 1-meter object distance), which should be low-cost, lightweight, and passively athermal while ensuring imaging clarity, so as to meet the needs of efficient and stable close-range wide-angle imaging in complex environments. Summary of the Invention

[0006] In order to overcome or alleviate at least one of the above-mentioned shortcomings of the prior art, one object of the present application is to provide an infrared lens to solve the problem that the infrared lens has a narrow field of view and is difficult to cover a large range of scenes. Another object of the present application is to provide a thermal imager.

[0007] In order to achieve the above-mentioned purpose of the invention, the present application may adopt the following technical solutions.

[0008] The present application provides an infrared lens, which includes:

[0009] A first lens, which is a positive meniscus lens, includes a first surface and a second surface, wherein the first surface is convex and faces the object side, and the second surface is concave and faces the image side;

[0010] a second lens, which is a biconvex lens, including a third surface and a fourth surface, wherein the third surface is convex and faces the object side, and the fourth surface is convex and faces the image side;

[0011] a third lens, which is a negative meniscus lens, including a fifth surface and a sixth surface, wherein the fifth surface is concave and faces the object side, and the sixth surface is convex and faces the image side;

[0012] a fourth lens, which is a biconvex lens, including a seventh surface and an eighth surface, wherein the seventh surface is convex and faces the object side, and the eighth surface is convex and faces the image side;

[0013] Wherein: the overall focal length of the infrared lens is f;

[0014] The focal length of the first lens is f1, which satisfies: -2<f1 / f<-1;

[0015] The focal length of the second lens is f2, which satisfies: 2<f2 / f<3;

[0016] The focal length of the third lens is f3, which satisfies: -11<f3 / f<-10;

[0017] The focal length of the fourth lens is f4, which satisfies: 2<f4 / f<3.

[0018] In at least one embodiment, the overall focal length is 1.9 mm.

[0019] In at least one embodiment, at least one of the first surface, the second surface, the third surface, the fourth surface, the fifth surface, the sixth surface, the seventh surface, and the eighth surface is an aspherical surface.

[0020] In at least one embodiment, the profile of the aspheric surface on the optical axis satisfies:

[0021]

[0022] Wherein, z is the sag of a point on the aspheric surface in the axial direction of the optical axis, r is the shortest distance between the point and the optical axis, c is the radius of curvature at the vertex of the aspheric surface, k is the conic coefficient of the aspheric surface, α i is the aspheric coefficient of the aspheric surface, i is the sequence number of the polynomial terms in the aspheric surface, and N is the total number of polynomial terms in the aspheric surface;

[0023] in:

[0024] The first surface satisfies: 185 mm ≤ c ≤ 190 mm;

[0025] The second surface satisfies: 0 mm ≤ c ≤ 5 mm, k = 0, α2 = 1.10e-3, α3 = 1.3e-4, α4 = -2.70e-5, α5 = 4.90e-6, α6 = 1.9e-9;

[0026] The third surface satisfies the following conditions: 15 mm ≤ c ≤ 20 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8;

[0027] The fourth surface satisfies: -20 mm ≤ c ≤ -15 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8;

[0028] The fifth surface satisfies: -10mm≤c≤-5mm;

[0029] The sixth surface satisfies: -10mm≤c≤-5mm;

[0030] The seventh surface satisfies the following conditions: 15 mm ≤ c ≤ 20 mm, k = -24, α2 = -7.67e-4, α3 = -5.23e-5, α4 = 8.1e-7, α5 = -1.43e-8, α6 = 2.0e-8;

[0031] The eighth surface satisfies: -10mm≤c≤-5mm, k=5.4, α2=-2.0e-4, α3=1.1e-5, α4=-1.3e-7, α5=-1.43e-8, α6=2.0e-8.

[0032] In at least one embodiment, the eighth surface is a diffraction surface.

[0033] The diffraction surface satisfies

[0034]

[0035] Wherein, Φ is the phase of the diffraction surface, M is the diffraction order of the diffraction surface, A i is the coefficient of each term in the diffraction surface, ρ is the normalized radial aperture coordinate of the diffraction surface, i is the sequence number of the polynomial terms in the diffraction surface, N is the total number of polynomial terms in the diffraction surface,

[0036] The eighth surface satisfies M=100, A1=-23128.619, A2=9733206.4, A3=-1.1787688e+10, A4=8.2302806e+12, and A5=-2.1632627e+15.

[0037] In at least one embodiment, the first lens and the third lens are made of chalcogenide glass of IRG 202 , and the second lens and / or the fourth lens are made of chalcogenide glass of IRG 206 .

[0038] In at least one embodiment, the optical parameters of the infrared lens meet the following conditions:

[0039] The overall focal length is 1.9 mm;

[0040] The aperture factor is 1.0;

[0041] Field of view greater than or equal to 172°;

[0042] The total optical length is less than or equal to 19mm;

[0043] The optical back focus is greater than or equal to 3.68mm;

[0044] The operating wavelength range is 8μm to 14μm.

[0045] In at least one embodiment, in the direction of the optical axis,

[0046] The center thickness of the first lens is 1.0 μm;

[0047] The center thickness of the second lens is 2.0 mm;

[0048] The center thickness of the third lens is 1.0 mm;

[0049] The center thickness of the fourth lens is 4.0 mm; and

[0050] The air gap between the first lens and the second lens is 4 mm;

[0051] The air gap between the second lens and the third lens is 2.1 mm;

[0052] The air gap between the third lens and the fourth lens is 0.75 mm.

[0053] In at least one embodiment, the infrared lens further includes a flat glass, which is located on the image side of the fourth lens; and on the optical axis, an air gap between the flat glass and the fourth lens is 2.6 mm.

[0054] The present application also provides a thermal imager, which includes a photosensitive element and the above-mentioned infrared lens.

[0055] By adopting the above technical solution, the present application provides an infrared lens and thermal imager. By rationally configuring the first to fourth lenses, the infrared lens can effectively achieve close-range wide-angle imaging while ensuring a large field of view. In addition, by optimizing the focal length configuration of each lens, the infrared lens exhibits excellent imaging quality, thermal stability, and image surface flatness. At the same time, the optical structure design of the lens simplifies traditional thermal compensation requirements, reduces weight and cost, improves system reliability, and is suitable for efficient and stable operation in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 1 is a schematic structural diagram of an infrared lens according to an embodiment of the present application;

[0057] Figure 2 is an optical modulation transfer function curve of an infrared lens at 20° C. according to one embodiment of the present application;

[0058] Figure 3 is a graph showing an optical modulation transfer function of an infrared lens at -40°C according to an embodiment of the present application;

[0059] Figure 4 is a graph showing an optical modulation transfer function of an infrared lens at 60° C. according to an embodiment of the present application;

[0060] Figure 5 is a field curvature diagram of an infrared lens according to an embodiment of the present application;

[0061] Figure 6 is a distortion diagram of an infrared lens according to an embodiment of the present application;

[0062] Figure 7 4 is a relative illumination diagram of an infrared lens according to an embodiment of the present application.

[0063] Description of Reference Numerals

[0064] 11 first lens; 12 second lens; 13 third lens; 14 fourth lens; 15 flat glass;

[0065] 20 photosensitive elements;

[0066] S1 first surface; S2 second surface; S3 third surface; S4 fourth surface; S5 fifth surface; S6 sixth surface; S7 seventh surface; S8 eighth surface;

[0067] A-axis DETAILED DESCRIPTION

[0068] The following describes exemplary embodiments of the present application with reference to the accompanying drawings. It should be understood that these specific descriptions are only used to teach those skilled in the art how to implement the present application, and are not intended to exhaust all possible methods of the present application, nor to limit the scope of the present application.

[0069] In this application, some numerical values ​​are expressed in scientific notation. For example, 1.40e-7 means 1.40×10 -7 Unless otherwise specified, “from one numerical value to another numerical value” should be understood to include the two endpoint values ​​themselves.

[0070] In this application, Figure 1 The dotted line in is used to represent the optical axis A of the infrared lens, wherein the left side of the optical axis A is the object side, and the right side is the image side.

[0071] In the following equations involving ratios, the units of each parameter should be consistent. For example, if the numerator is in millimeters (mm), the denominator should also be in millimeters (mm) to ensure physical consistency of the ratio.

[0072] In addition, the positive and negative directions of the radius of curvature of the optical surface are agreed upon as follows: if the optical surface (including the object-side surface or the image-side surface) is convex toward the object side, its radius of curvature is a positive value; if the optical surface is convex toward the image side, which is equivalent to being concave toward the object side, its radius of curvature is a negative value.

[0073] It should be noted that the shape of the lens and the degree of concavity and convexity of the surfaces on both sides of the object and image shown in the drawings are only schematic representations to assist understanding and do not constitute any limitation to the specific embodiments of the present application.

[0074] The present application is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0075] like Figure 1 As shown, an embodiment of the present application provides an infrared lens, which may include a plurality of optical elements arranged in sequence from the object side to the image side along the optical axis A, including: a first lens 11, a second lens 12, a third lens 13, a fourth lens 14 and a flat glass 15.

[0076] The first lens 11 may be a positive meniscus lens, which may include a first surface S1 and a second surface S2. The first surface S1 may be a convex surface facing the object side, and the second surface S2 may be a concave surface facing the image side.

[0077] The second lens 12 may be a biconvex lens, which may include a third surface S3 and a fourth surface S4. The third surface S3 may be a convex surface facing the object side, and the fourth surface S4 may be a convex surface facing the image side.

[0078] The third lens 13 may be a negative meniscus lens, which may include a fifth surface S5 and a sixth surface S6. The fifth surface S5 may be a concave surface facing the object side, and the sixth surface S6 may be a convex surface facing the image side.

[0079] The fourth lens 14 may be a biconvex lens, which may include a seventh surface S7 and an eighth surface S8. The seventh surface S7 may be convex and face the object side, and the eighth surface S8 may be convex and face the image side.

[0080] The flat glass 15 may be located on the image side of the fourth lens 14 for sealing, protection or other optical functions.

[0081] In this embodiment, the overall focal length of the infrared lens is denoted as f, the focal length of the first lens 11 is denoted as f1, the focal length of the second lens 12 is denoted as f2, the focal length of the third lens 13 is denoted as f3, and the focal length of the fourth lens 14 is denoted as f4, and the following relationship is satisfied:

[0082] The focal length of the first lens 11 is denoted as f1, which satisfies -2<f1 / f<-1;

[0083] The focal length of the second lens 12 is denoted as f2, which satisfies 2<f2 / f<3;

[0084] The focal length of the third lens 13 is denoted as f3, which satisfies -11<f3 / f<-10;

[0085] The focal length of the fourth lens 14 is denoted as f4, which satisfies 2<f4 / f<3.

[0086] In some preferred embodiments, the overall focal length f of the infrared lens may be 1.9 mm.

[0087] Furthermore, it can be understood that the relative positions of the above-mentioned optical elements are as follows: the first lens 11 is located on the object side of the second lens 12, the third lens 13 is located on the image side of the second lens 12, the fourth lens 14 is located on the image side of the third lens 13, and the flat glass 15 is located on the image side of the fourth lens 14.

[0088] Furthermore, in this embodiment, the center thickness of each lens element (i.e., the thickness in the direction of the optical axis A) is as follows:

[0089] The center thickness of the first lens 11 may be 1.0 mm;

[0090] The center thickness of the second lens 12 may be 2 mm;

[0091] The center thickness of the third lens 13 may be 1 mm;

[0092] The center thickness of the fourth lens 14 may be 4 mm.

[0093] Furthermore, the air spacing between lens elements along the optical axis A can be set as:

[0094] The air gap between the first lens 11 and the second lens 12 may be 4 mm;

[0095] The air gap between the second lens 12 and the third lens 13 may be 2.1 mm.

[0096] The air gap between the third lens 13 and the fourth lens 14 may be 0.75 mm;

[0097] The air space between the fourth lens 14 and the flat glass 14 may be 2.6 mm.

[0098] Preferably, the first lens 11, the second lens 12, the third lens 13, and the fourth lens 14 may include aspheric optical surfaces. Specifically, at least one of the optical surfaces S1 to S8 is an aspheric surface. In a preferred embodiment, all eight optical surfaces S1 to S8 are aspheric surfaces.

[0099] Aspheric morphology can satisfy the following general relationship:

[0100]

[0101] Among them, z is the sag of a point on the aspheric surface in the axial direction of the optical axis A, r is the shortest distance between the point and the optical axis A, c is the radius of curvature at the vertex of the aspheric surface, k is the conic coefficient of the aspheric surface, α i is the aspheric coefficient of the aspheric surface, i is the serial number of the polynomial terms in the aspheric surface, and N is the total number of polynomial terms in the aspheric surface.

[0102] For example, in this embodiment, the parameter range corresponding to each optical surface S1 to S8 can be set as follows:.

[0103] The first surface S1 satisfies: 185mm≤c≤190mm;

[0104] The second surface S2 satisfies the following conditions: 0 mm ≤ c ≤ 5 mm, k = 0, α2 = 1.10e-3, α3 = 1.3e-4, α4 = -2.70e-5, α5 = 4.90e-6, α6 = 1.9e-9;

[0105] The third surface S3 satisfies the following conditions: 15 mm ≤ c ≤ 20 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8;

[0106] The fourth surface S4 satisfies the following conditions: -20 mm ≤ c ≤ -15 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8;

[0107] The fifth surface S5 satisfies: -10mm≤c≤-5mm;

[0108] The sixth surface S6 satisfies: -10 mm ≤ c ≤ -5 mm;

[0109] The seventh surface S7 satisfies the following conditions: 15 mm ≤ c ≤ 20 mm, k = -24, α2 = -7.67e-4, α3 = -5.23e-5, α4 = 8.1e-7, α5 = -1.43e-8, α6 = 2.0e-8;

[0110] The eighth surface S8 satisfies the following conditions: -10 mm ≤ c ≤ -5 mm, k = 5.4, α2 = -2.0e-4, α3 = 1.1e-5, α4 = -1.3e-7, α5 = -1.43e-8, α6 = 2.0e-8;

[0111] Furthermore, the fourth lens 14 may include a diffraction surface to optimize the chromatic aberration of the lens. For example, the eighth surface S8 of the fourth lens 14 may be designed as a diffraction surface, and its phase function satisfies the following relationship:

[0112]

[0113] Among them, Φ is the phase function of the diffraction surface, M is the diffraction order of the diffraction surface, A i is the coefficient of each term in the diffraction surface, ρ is the normalized radial aperture coordinate, i is the sequence number of the polynomial terms in the diffraction surface, and N is the total number of polynomial terms in the diffraction surface.

[0114] For example, in some preferred embodiments, the parameters of the eighth surface S8 may be set to:

[0115] M=100, A1=-23128.619, A2=9733206.4, A3=-1.1787688e+10,

[0116] A4=8.2302806e+12, A5=-2.1632627e+15.

[0117] In this embodiment, the first lens 11 and the third lens 13 may be made of chalcogenide glass of the IRG 202 , and the second lens 12 and the fourth lens 14 may be made of chalcogenide glass of the IRG 206 .

[0118] Furthermore, in this embodiment, the infrared lens can have an overall optical length of less than or equal to 19mm, and an optical back focus of greater than or equal to 3.68mm, making it suitable for integrated installation in compact infrared imaging modules. The infrared lens can have an f-number (F-number) of 1.0, providing high light throughput and high-sensitivity imaging capabilities, while also offering a field of view greater than or equal to 172°, enabling near-panoramic infrared observation.

[0119] Figures 2 to 4 Figure 2 shows the modulation transfer function (MTF) curves of the infrared lens provided in this embodiment at different temperatures. The MTF curve shows how the lens's ability to reproduce detail on the image plane changes as spatial frequency changes. The horizontal axis represents spatial frequency in line pairs per millimeter (lp / mm), and the vertical axis represents the optical transfer function (OTF) coefficient.

[0120] Reference Figure 2 , which shows the MTF curves of the infrared lens in the meridian and sagittal directions at different image plane heights (0.384mm, 0.768mm, 1.152mm, 1.536mm, 1.92mm) at 20°C and at 42 line pairs of spatial frequencies at working wavelengths from 8μm to 12μm. Figure 2 As can be seen from the figure, the MTF values ​​in the meridian and sagittal directions at the center field of view (image height is 0mm) are both higher than 0.40, and the MTF values ​​in the other fields of view are also higher than 0.33.

[0121] It can be understood that the modulation transfer function is an important indicator to measure the imaging quality of the lens. The higher the value, the stronger the image contrast and detail restoration ability, which reflects that the infrared lens has good imaging performance and field uniformity.

[0122] Therefore, from Figure 2 It can be seen that the infrared lens maintains a high MTF performance at room temperature (20°C), indicating that it has good imaging quality.

[0123] See also Figure 3 , which shows the MTF curves of the infrared lens in the meridian and sagittal directions at different image plane heights (0.384mm, 0.768mm, 1.152mm, 1.536mm, 1.92mm) at a working wavelength of 8μm to 12μm and a spatial frequency of 42 line pairs at -40°C. Figure 3As can be seen, the MTF values ​​in both the meridian and sagittal directions at the center field of view (image plane height 0mm) are both greater than 0.40, and the MTF values ​​in the remaining fields of view are also higher than 0.27. Overall, this infrared lens maintains high MTF performance even in low-temperature environments (-40°C), demonstrating its excellent low-temperature imaging quality.

[0124] See also Figure 4 , which shows the MTF curves of the infrared lens in the meridian and sagittal directions at different image plane heights (0.384mm, 0.768mm, 1.152mm, 1.536mm, 1.92mm) at 8μm to 12μm working wavelengths and 42 line pairs of spatial frequencies at 60°C. Figure 4 As can be seen, the MTF values ​​in both the meridian and sagittal directions at the center field of view (image plane height 0mm) are both above 0.40, and the MTF values ​​in the remaining fields of view are also above 0.30. Overall, this shows that the lens still has good imaging performance and temperature stability in high temperature environments (60°C).

[0125] See also Figure 5 , which shows the field curvature of the infrared lens at different working wavelengths (8μm, 10μm, 12μm), evaluating the difference in imaging quality between the center and edge of the lens imaging surface. The horizontal axis represents the field curvature value of the point on the image surface, in mm, and the vertical axis is the field of view. Figure 5 It can be seen that no matter what wavelength, the field curvature values ​​in the meridian and sagittal directions are controlled within the range of -0.05mm to 0.05mm, indicating that the infrared lens has good image plane flatness and can ensure clear and consistent imaging.

[0126] See also Figure 6 The figure shows the distortion characteristics of an infrared lens. The horizontal axis represents the distortion percentage, and the vertical axis represents the field of view angle. As can be seen from the figure, the maximum distortion of this infrared lens is 93%, which can effectively expand the field of view angle and meet the requirements of large field of view applications.

[0127] See also Figure 7 The figure shows the relative illumination distribution of an infrared lens. Relative illumination is the ratio of center to edge brightness. The horizontal axis represents the image plane height (in mm), and the vertical axis represents the normalized illuminance. As can be seen, the center relative illumination of this lens is 1, indicating no energy loss in the central region of the optical system. The edge relative illumination is also greater than 0.87, indicating that the overall illumination distribution of the lens is uniform and exhibits good lighting balance.

[0128] comprehensive Figures 3 to 7 The analysis results show that the infrared lens provided in this embodiment has excellent performance in imaging quality, thermal stability, image plane flatness, field of view coverage and illumination uniformity, and can meet the application requirements of high-performance infrared imaging systems in complex environments.

[0129] The infrared lens provided in the embodiment of the present application is optimized for close distances (such as 1 meter object distance). Through the above-mentioned optical structure configuration, an ultra-large field of view is achieved while ensuring imaging clarity. The infrared lens does not need to rely on a focusing mechanism, and can effectively meet the optical power, achromatism and athermalization requirements of the optical system, ensuring efficient and stable close-range wide-angle imaging in complex environments. This design achieves optical passive athermalization, which means that the lens can maintain stable performance at different temperatures without the need for an additional focusing mechanism. This design is not only simple in structure, but also improves reliability and has good process adaptability, making it suitable for large-scale integration and industrial applications.

[0130] An embodiment of the present application further provides a thermal imager, which may include the above-mentioned infrared lens and a photosensitive element 20 arranged on the optical axis A.

[0131] Preferably, see Figure 1 The photosensitive element 20 can be an uncooled long-wave infrared detector, which is arranged on the optical axis A and located on the image side of the flat glass 15. Specifically, the resolution of the photosensitive element 20 can reach 256×192, the pixel pitch can be 12μm, and the operating wavelength range of the infrared lens can cover 8μm to 14μm.

[0132] It should be noted that the infrared lens provided in the embodiments of the present application is not only suitable for thermal imagers, but can also be widely used in electronic imaging devices such as smart phones, tablet computers, surveillance cameras, etc., which are not listed one by one here.

[0133] It should be understood that the above embodiments, examples, or examples are merely exemplary and are not intended to limit the present application. Those skilled in the art may make various modifications and changes to the above embodiments, examples, or examples based on the teachings of the present application without departing from the scope of the present application.

Claims

1. An infrared lens, characterized in that: It includes the following components arranged in sequence from the object side to the image side along the optical axis (A): a first lens (11) which is a positive meniscus lens and includes a first surface (S1) and a second surface (S2), wherein the first surface (S1) is convex and faces the object side, and the second surface (S2) is concave and faces the image side; a second lens (12) which is a biconvex lens and includes a third surface (S3) and a fourth surface (S4), wherein the third surface (S3) is a convex surface facing the object side, and the fourth surface (S4) is a convex surface facing the image side; a third lens (13) which is a negative meniscus lens and includes a fifth surface (S5) and a sixth surface (S6), wherein the fifth surface (S5) is concave and faces the object side, and the sixth surface (S6) is convex and faces the image side; a fourth lens (14) which is a biconvex lens and includes a seventh surface (S7) and an eighth surface (S8), wherein the seventh surface (S7) is a convex surface facing the object side, and the eighth surface (S8) is a convex surface facing the image side; Wherein: the overall focal length of the infrared lens is f; The focal length of the first lens (11) is f1, which satisfies: -2<f1 / f<-1; The focal length of the second lens (12) is f2, which satisfies: 2<f2 / f<3; The focal length of the third lens (13) is f3, which satisfies: -11<f3 / f<-10; The focal length of the fourth lens (14) is f4, which satisfies: 2<f4 / f<3.

2. The infrared lens according to claim 1, characterized in that: The overall focal length is 1.9 mm.

3. The infrared lens according to claim 1, wherein: At least one of the first surface (S1), the second surface (S2), the third surface (S3), the fourth surface (S4), the fifth surface (S5), the sixth surface (S6), the seventh surface (S7) and the eighth surface (S8) is an aspherical surface.

4. The lens according to claim 3, wherein: The profile of the aspheric surface on the optical axis (A) satisfies: Wherein, z is the sag of a point on the aspheric surface in the axial direction of the optical axis (A), r is the shortest distance between the point and the optical axis (A), c is the radius of curvature at the vertex of the aspheric surface, k is the conic coefficient of the aspheric surface, α i is the aspheric coefficient of the aspheric surface, i is the sequence number of the polynomial terms in the aspheric surface, and N is the total number of polynomial terms in the aspheric surface; in: The first surface (S1) satisfies: 185mm≤c≤190mm; The second surface (S2) satisfies: 0 mm ≤ c ≤ 5 mm, k = 0, α2 = 1.10e-3, α3 = 1.3e-4, α4 = -2.70e-5, α5 = 4.90e-6, α6 = 1.9e-9; The third surface (S3) satisfies: 15 mm ≤ c ≤ 20 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8; The fourth surface (S4) satisfies: -20 mm ≤ c ≤ -15 mm, k = 0, α2 = -1.0e-4, α3 = -5.3e-6, α4 = 9.0e-7, α5 = 2.0e-8, α6 = 2.0e-8; The fifth surface (S5) satisfies: -10mm≤c≤-5mm; The sixth surface (S6) satisfies: -10mm≤c≤-5mm; The seventh surface (S7) satisfies: 15 mm ≤ c ≤ 20 mm, k = -24, α2 = -7.67e-4, α3 = -5.23e-5, α4 = 8.1e-7, α5 = -1.43e-8, α6 = 2.0e-8; The eighth surface (S8) satisfies: -10mm≤c≤-5mm, k=5.4, α2=-2.0e-4, α3=1.1e-5, α4=-1.3e-7, α5=-1.43e-8, α6=2.0e-8.

5. The infrared lens according to any one of claims 1 to 4, characterized in that: The eighth surface (S8) is a diffraction surface, The diffraction surface satisfies Wherein, Φ is the phase of the diffraction surface, M is the diffraction order of the diffraction surface, A i is the coefficient of each term in the diffraction surface, ρ is the normalized radial aperture coordinate of the diffraction surface, i is the sequence number of the polynomial terms in the diffraction surface, N is the total number of polynomial terms in the diffraction surface, The eighth surface (S8) satisfies M=100, A1=-23128.619, A2=9733206.4, A3=-1.1787688e+10, A4=8.2302806e+12, and A5=-2.1632627e+15.

6. The infrared lens according to any one of claims 1 to 5, characterized in that: The first lens (11) and the third lens (13) are made of chalcogenide glass of IRG202, and the second lens (12) and / or the fourth lens (14) are made of chalcogenide glass of IRG206.

7. The infrared lens according to any one of claims 1 to 5, characterized in that: The optical parameters of the infrared lens meet the following conditions: The overall focal length is 1.9 mm; The aperture factor is 1.0; Field of view greater than or equal to 172°; The total optical length is less than or equal to 19mm; The optical back focus is greater than or equal to 3.68mm; The operating wavelength range is 8μm to 14μm.

8. The infrared lens according to claim 1, wherein: In the direction of the optical axis (A), The center thickness of the first lens (11) is 1.0 μm; The center thickness of the second lens (12) is 2.0 mm; The central thickness of the third lens (13) is 1.0 mm; The center thickness of the fourth lens (14) is 4.0 mm; and The air gap between the first lens (11) and the second lens (12) is 4 mm; The air gap between the second lens (12) and the third lens (13) is 2.1 mm; The air gap between the third lens (13) and the fourth lens (14) is 0.75 mm.

9. The infrared lens according to any one of claims 1 to 8, characterized in that: The infrared lens further comprises a flat glass (15), wherein the flat glass (15) is located on the image side of the fourth lens (14); as well as On the optical axis (A), the air gap between the flat glass (15) and the fourth lens (14) is 2.6 mm.

10. A thermal imager, characterized in that: The invention comprises a photosensitive element (20) and the infrared lens according to any one of claims 1 to 9.

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