Low-cost broadband large-relative-aperture infrared imaging optical system

Abstract

The invention relates to a low-cost broadband large-relative-aperture infrared imaging optical system which is a transmission-type optical system and adopts a coaxial optical structure. Four lenses mainly made of chalcogenide glass and zinc selenide are arranged, the optical system sequentially comprises a first positive lens, a first negative lens, a second negative lens and a second positive lens in the light incidence direction, the refractive indexes of all the lenses of the optical system sequentially correspond to n1, n2, n3 and n4 in the light incidence direction, and the refractive indexes of all the lenses of the optical system correspond to n1, n2, n3 and n4 in the light incidence direction. The corresponding value ranges are 2.4 < = n1 < = 3.2, 2.4 < = n2 < = 2.8, 2.3 < = n3 < = 2.7 and 2.4 < = n4 < = 2.8, and light rays are converged through the lenses, pass through a protection window of the detector and then enter a focal plane of the detector. The invention provides an optical system which is small in size, compact in structure and suitable for medium-long wave broadband of 3-14 microns and has medium-long wave broadband confocal plane imaging capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of infrared imaging optical systems, and in particular relates to a low-cost, wide-band, large-relative-aperture infrared imaging optical system. Background Art

[0002] Chinese patent CN102980657B discloses a 3μm-12μm spectral imaging system with a focal length of 60mm and an F-number of 1.0. This system compensates for defocus in narrow bands by replacing bandpass filters of varying thicknesses. However, the optical lens itself does not achieve a full-band confocal design for the 3μm-14μm range. Chinese patent application CN120065481A also suffers from the same issue.

[0003] The dual-band infrared system disclosed in Chinese patent CN105319669B uses six lenses and operates in the 3.7μm-4.8μm and 8μm-12μm bands, but does not cover the 3μm-14μm band. U.S. Patent US7369303B2 discloses an uncooled dual-band optical system operating in the 3μm-5μm and 8μm-12μm bands, similarly failing to cover the 3μm-14μm wide band.

[0004] Chinese patents CN 104297908B and CN202794679U, as well as U.S. Patent US6423969B1, all address cooled medium- and long-wave infrared systems with small relative apertures, making them difficult to match the large relative aperture requirements of uncooled detectors. While U.S. Patent US4871219A is designed for uncooled detectors, its large F-number makes it unsuitable for large relative aperture optical systems.

[0005] The off-axis three-mirror optical system disclosed in Optical Engineering, Vol. 56, No. 6, 2013, pp. 061308-1-11, and the off-axis four-mirror optical system disclosed in Chinese patent application CN 112213847A, both use reflective optical structures and have high requirements for the processing, assembly, and inspection of the optical systems.

[0006] Chinese patent CN112882210B discloses an uncooled medium- and long-wavelength wide-band confocal infrared optical system with an operating band of 3μm to 14μm. It has a focal length of 50mm and an F-number of 1.1, achieving a common aperture and confocal surface in a wide band. However, the optical system uses a total of five lenses, which is a large number of lenses and contains expensive germanium materials, which further increases the production cost of the entire lens.

[0007] Existing technologies for medium- to long-wave infrared detection equipment operating in the 3μm to 14μm wavelength range primarily consist of transmissive split-aperture systems and reflective common-aperture systems. The former, however, offers extremely limited optical material options within the wide 3μm to 14μm wavelength range, while traditional lens materials with high applicability, such as germanium, are prohibitively expensive, leading to complex design and high lens costs. While the latter's reflective optical structure inherently possesses achromatic and athermal properties, it is difficult to manufacture and assemble, making engineering implementation challenging. Summary of the Invention

[0008] The present invention provides a low-cost, wide-band, large relative aperture infrared imaging optical system. The technical problem to be solved is how to reduce the cost of a medium- and long-wavelength, wide-band transmission optical system with an operating band of 3μm to 14μm.

[0009] The technical solution adopted by the present invention to solve its technical problems is: to provide a low-cost, wide-band, large-relative-aperture infrared imaging optical system, the optical system is a transmissive optical system, and the optical structure is a coaxial structure. The optical system is composed of a first positive lens, a first negative lens, a second negative lens, and a second positive lens in the direction of incidence of light. Along the direction of incidence of light, the refractive index of each lens of the optical system corresponds to n1, n2, n3, and n4 in sequence, and the corresponding value ranges are 2.5≤n1≤3.2, 2.4≤n2≤2.8, and 2.3≤n3≤2.7, 2.4≤n4≤2.8, the light is converged by the lenses, passes through the detector protection window, and is incident on the focal plane of the detector; along the incident direction of the light, the focal lengths of the lenses are assuming to be f1, f2, f3, and f4, respectively, and their value ranges relative to the normalized value of the focal length f of the optical system are 0.48≤|f1 / f|≤0.85, 0.94≤|f2 / f|≤1.45, 0.45≤|f3 / f|≤0.92, and 0.36≤|f4 / f|≤0.87, respectively.

[0010] Preferably, the operating wavelength band of the optical system is 3 μm to 14 μm.

[0011] Preferably, the maximum relative aperture of the optical system is F / 1.0.

[0012] Preferably, the optical system can adapt to a focal plane diameter of 14 mm.

[0013] Preferably, the surface of the first positive lens close to the object plane is a stop of the optical system.

[0014] Preferably, the materials of the first positive lens, the first negative lens and the second positive lens are chalcogenide glass, and the material of the second negative lens is zinc selenide.

[0015] The beneficial effect is that the present invention provides an optical system with a small size, compact structure, suitable for the medium and long wavelength wide band of 3μm~14μm, and having the medium and long wavelength wide band confocal plane imaging capability.

[0016] Compared with the existing technology, the present invention does not use germanium material but adopts a combination of zinc selenide and chalcogenide glass to manufacture lenses. Both materials are common infrared optical materials, which are low-priced and easily available, ensuring the maturity and continuity of the technology. They can well correct the aberrations associated with large apertures. On the basis of limited materials, confocal imaging in a wide band range of 3μm to 14μm in the medium and long-wave infrared and a relative aperture of not less than F / 1.0 are achieved through the design of the focal length of the lens. The optical system has high imaging quality and good stability. Compared with the existing technology, one lens is reduced, and it is easy to manufacture and inexpensive.

[0017] like Figures 2 to 5 , gives the imaging quality evaluation results of the optical system provided by the present invention in terms of MTF, spot diagram, relative distortion, relative illumination, etc. It can be seen that the full-field MTF of the optical system of the embodiment is not less than 0.5@30lp / mm, the RMS diffuse spot is no more than 19μm, the relative distortion is better than 0.5%, and the relative illumination is better than 95%, indicating that the present invention has excellent imaging performance.

[0018] In addition, the present invention has a longer back working distance, and a filter switching mechanism can be placed in front of the focal plane to achieve multi-band imaging; it can be applied to various industrial detection, aerial remote sensing, astronomical observation and other purposes.

[0019] In practical applications, if this optical system is installed in a medium- and long-wave wide-band uncooled infrared imager, the working band will cover 3~14μm, and a single snapshot can capture both medium-wave (3~5μm) and long-wave (8~14μm) radiation, combining the advantages of both and making up for their shortcomings; in power inspections, the same frame of image can not only detect medium-wave hotspots at wire joints, but also locate long-wave leakage areas at insulators, reducing repeated flights; in the petroleum and petrochemical fields, the wide band can simultaneously monitor the 3.3μm absorption peak of hydrocarbon gas and the background 8~10μm radiation, and the detection capability is one order of magnitude higher than that of a single band; in medical care, the wide band takes into account both the long-wave radiation of the skin and the medium-wave characteristics of respiratory gas, realizing non-invasive and simultaneous measurement of body temperature and CO2 concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the lens structure of a low-cost, wide-band, large relative aperture infrared imaging optical system; Figure 2 Schematic diagram of MTF evaluation of a low-cost, wide-band, large relative aperture infrared imaging optical system; Figure 3This is a schematic diagram of the spot diagram evaluation of a low-cost, wide-band, large relative aperture infrared imaging optical system; Figure 4 Schematic diagram of field curvature and distortion evaluation for a low-cost, wide-band, large relative aperture infrared imaging optical system; Figure 5 Schematic diagram of relative illumination evaluation of a low-cost, wide-band, large relative aperture infrared imaging optical system; Among them, 1 is the first positive lens, 2 is the first negative lens, 3 is the second negative lens, 4 is the second positive lens, 5 is the detector protection window, and 6 is the detector focal plane. DETAILED DESCRIPTION

[0021] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0022] like Figure 1 As shown, the present invention provides a low-cost, wide-band, large-relative-aperture infrared imaging optical system. The optical system is a transmissive optical system with a coaxial optical structure. The optical system comprises a first positive lens 1, a first negative lens 2, a second negative lens 3, and a second positive lens 4, coaxially arranged in sequence along the incident direction of light. Light is converged by the lenses 1-4 and, after passing through a detector protective window 5, is incident on a detector focal plane 6. Along the incident direction of light, the focal lengths of the lenses are denoted by f1, f2, f3, and f4, respectively. The normalized ranges of these focal lengths relative to the focal length f of the optical system are 0.48 ≤ |f1 / f| ≤ 0.85, 0.94 ≤ |f2 / f| ≤ 1.45, 0.45 ≤ |f3 / f| ≤ 0.92, and 0.36 ≤ |f4 / f| ≤ 0.87, respectively.

[0023] Along the incident direction of light, the refractive index of each lens of the optical system corresponds to n1, n2, n3, and n4, and the corresponding value ranges are 2.5≤n1≤3.2, 2.4≤n2≤2.8, 2.3≤n3≤2.7, and 2.4≤n4≤2.8, respectively.

[0024] It is well known to those skilled in the art that lens materials that meet the refractive index requirements of the present invention are relatively inexpensive chalcogenide glass and zinc selenide, while lenses made of germanium typically have a refractive index of around 4.0. Specifically, in this embodiment, the first positive lens 1 utilizes chalcogenide glass IRG209, the first negative lens 2 utilizes chalcogenide glass IRG207, the second negative lens 3 utilizes zinc selenide, and the second positive lens 4 utilizes chalcogenide glass IRG207. The chalcogenide materials employed can be replaced with glass or crystal materials with similar performance. In this case, simply modifying the curvature radius, thickness, and spacing between the lenses in this optical structure can achieve optical performance similar to or superior to that of the present invention.

[0025] The aperture of the present invention is located on the surface of the first positive lens 1 close to the object side, and is imaged at a long distance on the image side through the rear objective lens group and the imaging lens group, forming a quasi-image-side telecentric optical path, which can ensure that the entire image surface has a uniform relative illumination distribution.

[0026] The optical system adopts a four-piece structure, with an operating band of 3μm~14μm, a focal length of 50mm, a relative aperture of F / 1.0, and an adapted focal plane diameter of 14mm.

[0027] By simultaneously satisfying or satisfying multiple of the above conditions, the present invention can realize an optical system with a small size, compact structure, and medium- and long-wavelength wide-band confocal imaging capability.

[0028] Tables 1 to 3 below provide various numerical data related to a specific embodiment of a low-cost, wide-band, large relative aperture infrared imaging optical system.

[0029]

[0030] Table 1 Specific parameters of each lens in the optical system of this embodiment (unit: mm)

[0031] Table 2 Aspheric parameters of optical system Among them, the expression of aspheric surface is:

[0032] Where: z is the aspheric surface sag, c is the aspheric surface curvature, k is the quadratic surface coefficient, r is the radial height of the lens, A2i is the expansion term coefficient; N is the number of terms.

[0033]

[0034] Table 3 Parameters of the optical system of this embodiment like Figures 2 to 5, gives the imaging quality evaluation results of the optical system of this embodiment from the aspects of modulation transfer function (MTF), spot diagram, relative distortion, relative illumination, etc. Figure 2 The full-field modulation transfer function (MTF) of the optical system of this embodiment is not less than 0.5@30lp / mm; see Figure 3 , the RMS diffuse spot of the optical system of this embodiment is no more than 19μm; see Figure 4 , the relative distortion of the optical system of this embodiment is better than 0.5%; see Figure 5 The relative illumination of the optical system of this embodiment is better than 95%, indicating that the optical system of the embodiment has excellent imaging performance.

[0035] In this embodiment, the total length from the surface of the first positive lens 1 closest to the object plane to the detector focal plane 6 is no greater than 75 mm. Each lens has a maximum aperture of 50 mm and a focal length of 50 mm. An aperture of F / 1.0 is employed. This results in a compact size, light weight, and a large relative aperture.

[0036] The above data demonstrates that the optical structure of the present invention, using only four lenses made of common infrared optical materials, can achieve confocal imaging over a wide wavelength range of 3μm to 14μm, with a relative aperture of at least F / 1.0. Due to the reduced number of lenses, the optical system provided by the present invention is easy to manufacture, and offers high imaging quality and excellent stability.

Claims

1. A low-cost, wide-band, large relative aperture infrared imaging optical system, characterized in that: The optical system is a transmissive optical system, and adopts a coaxial optical structure. The optical system comprises a first positive lens, a first negative lens, a second negative lens, and a second positive lens in sequence along the incident direction of light. Along the incident direction of the light, the refractive index of each lens of the optical system corresponds to n1, n2, n3, and n4, and the corresponding value ranges are 2.5≤n1≤3.2, 2.4≤n2≤2.8, 2.3≤n3≤2.7, and 2.4≤n4≤2.8, respectively. The light is converged by the lenses, passes through the detector protection window, and is incident on the focal plane of the detector; Along the incident direction of the light, the focal lengths of the lenses are denoted as f1, f2, f3, and f4, respectively, and their normalized values ​​relative to the focal length f of the optical system are in the range of 0.48≤|f1 / f|≤0.85, 0.94≤|f2 / f|≤1.45, 0.45≤|f3 / f|≤0.92, and 0.36≤|f4 / f|≤0.87, respectively.

2. The low-cost, wide-band, large relative aperture infrared imaging optical system according to claim 1, characterized in that: The operating wavelength range of the optical system is 3 μm to 14 μm.

3. The low-cost, wide-band, large relative aperture infrared imaging optical system according to claim 1, characterized in that: The maximum relative aperture of the optical system is F / 1.

0.

4. The low-cost, wide-band, large relative aperture infrared imaging optical system according to claim 1, characterized in that: The optical system can adapt to a focal plane diameter of 14 mm.

5. The low-cost, wide-band, large relative aperture infrared imaging optical system according to claim 1, characterized in that: The surface of the first positive lens close to the object plane serves as a stop of the optical system.

6. The low-cost, wide-band, large relative aperture infrared imaging optical system according to claim 1, characterized in that: The first positive lens, the first negative lens, and the second positive lens are made of chalcogenide glass, and the second negative lens is made of zinc selenide.

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