An adjustable focus super long wave infrared optical imaging system under low temperature condition

Abstract

This invention discloses a focusable ultra-long-wave infrared optical imaging system for low-temperature conditions. It comprises a coaxial catadioptric secondary imaging structure consisting of a coaxial primary imaging group (1) and a secondary imaging lens group (2). Utilizing the band applicability and chromatic aberration-free advantage of the coaxial primary imaging group (1), and correcting residual aberrations using the secondary imaging lens group (2) with high transmittance in the ultra-long-wave band, the system expands the field of view of the coaxial primary imaging group (1), thereby enabling target detection in the 8–16 μm ultra-long-wave infrared band. The secondary imaging lens group (2), including at least two non-edge imaging lenses, forms a focusing group, enabling focusing functionality and thus achieving target detection under low-temperature conditions of 50–300 K. This system is compact, rationally laid out, and lightweight, allowing for miniaturization.

Description

Technical Field

[0001] This invention belongs to the field of optical systems, and specifically relates to a focusable ultra-long-wave infrared optical imaging system for low-temperature conditions. Background Technology

[0002] In physics, electromagnetic waves with wavelengths between 0.75 and 1000 μm are defined as infrared radiation. Based on their generation mechanism, radiation applications, and transmission characteristics in the Earth's atmosphere, infrared radiation is divided into four bands: near-infrared (0.75–1 μm), short-wave infrared (1–3 μm), mid-infrared (3–5 μm), and long-wave infrared (8–12 μm).

[0003] Long-wave infrared imaging technology is widely used in security fields such as forest fire prevention, road monitoring, and airport surveillance. Long-wave cooled infrared imaging technology, due to its high sensitivity, has important applications in fields such as deep space exploration. Especially for the sky survey requirements in deep space exploration, long-wave infrared radiation imaging can break through the 8–12 μm band, extending to 8–16 μm or even longer bands, and the optical imaging system operates in a varying low-temperature environment.

[0004] Existing transmission-type long-wave infrared optical imaging systems are mostly limited to the 8-12μm band. Therefore, most sky survey telescopes adopt total internal reflection structures. However, different total internal reflection structures have certain limitations. The coaxial two-mirror structure is difficult to perfectly correct off-axis aberrations other than spherical aberration and coma, and the effective field of view is small. Patent document CN105278089A discloses a coaxial three-mirror optical system, in which the coaxial three mirrors have secondary field of view obstruction. Patent document CN110764241A discloses an off-axis three-mirror imaging optical system. This system has an asymmetrical structure, is large in size, and is not conducive to the overall lightweight design of the camera. Summary of the Invention

[0005] In view of the above, the purpose of this invention is to provide an adjustable focus ultra-long wave infrared optical imaging system for low temperature conditions, which expands the detection band range and can operate in a certain low temperature range. It has a compact structure, reasonable layout, and small size.

[0006] To achieve the above-mentioned invention objectives, the embodiment provides an adjustable focus ultra-long wave infrared optical imaging system for low temperature conditions, including a coaxial reflection primary imaging group (1), a secondary imaging lens group (2), a cooled detector window (3), a cold aperture (4), and a focal plane (5) arranged sequentially along the optical axis, wherein the coaxial reflection primary imaging group (1) and the secondary imaging lens group (2) form a coaxial catadioptric secondary imaging structure.

[0007] The secondary imaging lens group (2) includes at least four imaging lenses on the same optical axis, and at least two non-edge imaging lenses form a focusing group, which is driven by a motor to move along the optical axis to achieve focusing.

[0008] The secondary imaging lens group (2) uses a high transmittance material in the infrared ultra-long wave band, and the transmittance is not less than 97% in the wavelength range of 8 to 12 μm, not less than 70% in the wavelength range of 12 to 14 μm, and not less than 90% in the wavelength range of 14 to 16 μm.

[0009] The imaging system satisfies the following conditions:

[0010] 0.950 <f1 / f<1.120

[0011] 0.050 <f2 / f<0.175

[0012] Where f represents the focal length of the optical imaging system, f1 represents the focal length of the coaxial reflection primary imaging group, and f2 represents the focal length of the secondary imaging lens group.

[0013] In one embodiment, the coaxial reflection primary imaging group (1) includes a primary mirror (11) and a secondary mirror (12), with the primary image plane located between the primary mirror (11) and the secondary mirror (12), and satisfying the following conditions:

[0014] 0.630 <f 11 / f<0.708

[0015] 0.385 <f 12 / f<0.462

[0016] Among them, f 11 f represents the focal length of the primary mirror (11). 12 This indicates the focal length of the secondary mirror (12).

[0017] In one embodiment, the aperture D1 of the primary reflector (11) and the aperture D2 of the secondary reflector (12) satisfy the following relationship: 0.15 <D2 / D1<0.35。

[0018] In one embodiment, the primary reflector (11) and the secondary reflector (12) are both aspherical and are made of infrared crystal material, metal material, glass material, silicon carbide composite material, or carbon fiber / silicon carbide composite material.

[0019] In one embodiment, the secondary imaging lens group (2) is a four-element refractive mirror group, including a secondary imaging lens (21), a secondary imaging lens (22), a secondary imaging lens (23), and a secondary imaging lens (24) arranged along the optical axis, and satisfies the following conditions:

[0020] 0.251 <f 21 / f<0.314

[0021] 0.045 <f 22 / f<0.124

[0022] -0.096 <f 23 / f<-0.015

[0023] 0.031 <f 24 / f<0.072

[0024] Among them, f 21 f represents the focal length of the secondary imaging lens (21). 22 The focal length and f of the secondary imaging lens (22) are indicated. 23 f represents the focal length of the secondary imaging lens (23). 24 This indicates the focal length of the secondary imaging lens (24).

[0025] In one embodiment, the object-side surface of the secondary imaging lens (21), the object-side surface of the secondary imaging lens (23), the object-side surface of the secondary imaging lens (24) and the image-side surface are designed with even-order aspherical surfaces, and the secondary imaging lens (22) and the secondary imaging lens (23) are used as a focusing group and are controlled by a motor to perform focusing.

[0026] In one embodiment, the secondary imaging lens group uses HWS series materials.

[0027] In one embodiment, the F-number of the cooling detector ranges from 2 to F and from 3 to 3.

[0028] In one embodiment, the system further includes a lens barrel made of a material with a coefficient of thermal expansion of 9.41 × 10⁻⁶. -6 / ℃~10.03×10 -6 Titanium alloy at / ℃.

[0029] In one embodiment, the entrance pupil of the system is located on the primary reflector (11), and the exit pupil must coincide with the cold aperture (4).

[0030] Compared with the prior art, the beneficial effects of the present invention include at least the following:

[0031] A coaxial catadioptric secondary imaging structure is formed by a coaxial primary imaging group (1) and a secondary imaging lens group (2). The wide-band applicability and chromatic aberration-free advantage of the coaxial primary imaging group (1) are utilized, while the secondary imaging lens group (2), which has high transmittance in the 8–16 μm band, corrects the remaining off-axis aberrations, thereby expanding the field of view of the coaxial primary imaging group (1) and enabling the detection of targets in the 8–16 μm ultra-long-wave infrared band. The secondary imaging lens group (2) includes at least two non-edge imaging lenses forming a focusing group, enabling focusing. Through focusing, the imaging quality of the optical system can meet the requirements under low-temperature conditions of 50–300 K. This system has a compact structure, reasonable layout, small size and weight, and can be miniaturized. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the structure of an optical imaging system provided in an embodiment of the present invention;

[0034] Figure 2 The bounding circle energy curve of an optical imaging system provided in an embodiment of the present invention;

[0035] Figure 3 This is a schematic diagram of the optical path of the focusing group of an optical imaging system provided in an embodiment of the present invention;

[0036] Figure 4 The displacement curve of the focusing group of an optical imaging system provided in an embodiment of the present invention as a function of temperature;

[0037] Figure 5 The optical transfer function curve of an optical imaging system at 300K is provided in an embodiment of the present invention.

[0038] Figure 6 The optical transfer function curve of an optical imaging system at 175K is provided in an embodiment of the present invention;

[0039] Figure 7 The optical transfer function curve of an optical imaging system at 50K is provided in an embodiment of the present invention;

[0040] Reference numerals: 1-Coaxial reflection primary imaging group, 2-Secondary imaging lens group, 3-Cooled detector window, 4-Cold aperture and 5-Focal plane, 11-Primary reflector, 12-Secondary reflector, 21-Secondary imaging lens first lens, 22-Secondary imaging lens second lens, 23-Secondary imaging lens third lens, 24-Secondary imaging lens fourth lens. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the scope of protection of this invention.

[0042] like Figure 1 As shown in the embodiment of the present invention, the adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions includes a coaxial reflection primary imaging group 1, a secondary imaging lens group 2, a cooled detector window 3, a cold aperture 4, and a focal plane 5 arranged sequentially along the optical axis. The coaxial reflection primary imaging group 1 and the secondary imaging lens group 2 form a coaxial catadioptric secondary imaging structure. Incident light rays converge to the primary image plane after passing through the coaxial reflection primary imaging group 1. After passing through the primary image plane, the light rays are projected onto the secondary imaging lens group 2, where they are converged again, passing sequentially through the cooled detector window 3 and the cold aperture 4, and finally converging on the focal plane 5.

[0043] The focal length f1 of coaxial reflection single-image group 1 satisfies: 0.950 <f1 / f<1.120;

[0044] The focal length f2 of the secondary imaging lens group 2 satisfies: 0.050 <f2 / f<0.175;

[0045] Where f represents the focal length of the optical imaging system.

[0046] In this embodiment, the coaxial reflection primary imaging group 1 includes a primary reflector 11 and a secondary reflector 12. Both the primary reflector 11 and the secondary reflector 12 are aspherical and are made of infrared crystal materials, metal materials, glass materials, or other composite materials. Specifically, they can be made of materials such as single-crystal silicon, aluminum, microcrystalline glass, silicon carbide composite materials, or carbon fiber / silicon carbide composite materials.

[0047] The aperture D1 of the primary reflector 11 and the aperture D2 of the secondary reflector 12 satisfy the following relationship: 0.15 <D2 / D1<0.35;

[0048] The focal length f of the primary reflecting mirror 11 11 Satisfy: 0.630 <f 11 / f<0.708;

[0049] The focal length f of the secondary reflector 1212 Satisfies: 0.385 <f 12 / f<0.462;

[0050] In the optical imaging system, the primary image plane is located between the primary mirror 11 and the secondary mirror 12, which shortens the length of the optical system, making the system small and compact, and realizing the miniaturization of the lens design. At the same time, taking advantage of the mirror's chromatic aberration-free and large-aperture characteristics, the secondary imaging lens group 2 uses an ultra-long-wavelength material transmission mirror to correct the remaining aberrations, increasing the imaging field of view of the coaxial reflection primary imaging system, which is a reasonable layout.

[0051] In this embodiment, the secondary imaging lens group 2 is a four-element refractive mirror group, including secondary imaging lens 21, secondary imaging lens 22, secondary imaging lens 23, and secondary imaging lens 24. The object-side surface of secondary imaging lens 21, the object-side surface of secondary imaging lens 23, the object-side surface of secondary imaging lens 24, and the image-side surface are all made of even-order aspherical surfaces. Secondary imaging lenses 21, 22, 23, and 24 are all made of high-transmittance materials in the ultra-long infrared band, with a transmittance of not less than 97% in the 8–12 μm wavelength range, not less than 70% in the 12–14 μm wavelength range, and not less than 90% in the 14–16 μm wavelength range; specifically, they are made of HWS series materials.

[0052] The focal length f of the secondary imaging lens 21 21 Satisfies: 0.251 <f 21 / f<0.314;

[0053] The focal length f of the secondary imaging lens 22 22 Satisfies: 0.045 <f 22 / f<0.124;

[0054] The focal length f of the secondary imaging lens 23 23 Satisfies: -0.096 <f 23 / f<-0.015;

[0055] The focal length f of the secondary imaging lens 24 24 Satisfies: 0.031 <f 24 / f<0.072.

[0056] In such Figure 1 In the secondary imaging lens group 2 shown, secondary imaging lenses 22 and 23 serve as a focusing group, synchronously moving along the optical axis to the calibration position under motor control, thereby achieving infrared ultra-long wave imaging under different low-temperature conditions. Specifically, within the operating temperature range of 50–300K, such as… Figure 3 As shown, when the temperature decreases, the focusing assembly moves to the left as a whole; when the temperature increases, the focusing assembly moves to the right as a whole. The displacement curve of the focusing assembly with temperature change is shown in the figure. Figure 4 As shown, analysis Figure 4 It can be seen that the focusing assembly maintains a basically linear displacement as the temperature increases. For example... Figure 2 The diagram shows the enclosing circle energy curves of an optical imaging system. A circle is drawn with the principal ray or central ray on the image plane as the center and the distance from this point as the radius. The ratio of the energy falling into this circle to the total energy represents the energy concentration of the optical system. The enclosing circle energy curves of each field of view of this system are close to the diffraction limit, indicating that the energy concentration of each field of view is good. Within a circle with a radius of 38μm for the detector pixel, the energy concentration of each field of view is ≥0.7. Figures 5-7 The figure shows the optical transfer function curves of several key points in the optical imaging system within the temperature range of 300K to 50K. It can be seen that under different temperature conditions, the MTF of each field of view is ≥0.18, which meets the imaging requirements of the optical system under different low temperature conditions.

[0057] In this embodiment, the cooled detector window 3 receives the converged light from the secondary imaging lens group 2, and the F-number of the cooled detector ranges from 2 to 3. The optical imaging system provided in this embodiment achieves active thermal aberration reduction within an operating temperature range of 50–300K, including: controlling the focusing group to move along the optical axis to a calibrated position via a motor; furthermore, to improve the low-temperature stability of the optical imaging system, the lens barrel included in the system uses a material with a thermal expansion coefficient of 9.41 × 10⁻⁶. -6 / ℃~10.03×10 -6 A titanium alloy with a temperature of / ℃ has a small coefficient of thermal expansion, making it less affected by temperature changes. This can reduce the impact of thermal expansion and contraction of the lens barrel on the optical system, thereby improving image quality and stability.

[0058] The optical imaging system adopts a coaxial catadioptric secondary imaging structure, with the exit pupil coinciding with the cold aperture 4, achieving 100% cold aperture efficiency, thereby reducing beam energy loss and improving system sensitivity. At the same time, the system entrance pupil is located on the primary mirror 11, effectively reducing the size of the primary mirror 11 and thus reducing the overall size of the optical system. The aperture of the primary mirror in this system is less than or equal to 150 mm.

[0059] The aforementioned optical system can operate within a temperature range of 50–300K and is suitable for the 8–16μm infrared ultra-long-wave band. It employs a coaxial catadioptric secondary imaging structure, with the primary image plane located between the primary and secondary mirrors, and the entrance pupil located on the primary mirror. This system features a compact structure and strong detection capabilities. Specifically, the system utilizes the advantages of the mirrors' lack of chromatic aberration and large aperture, and uses an ultra-long-wave material transmission mirror to correct residual aberrations, achieving target detection in the infrared ultra-long-wave range. The secondary imaging lenses 22 and 23, used as focusing groups, ensure that the system imaging meets requirements under different low-temperature conditions.

[0060] The following table provides an example of the parameters of the coaxial reflection primary imaging group 1, secondary imaging lens group 2, cooled detector window 3, cold stop 4, and focal plane 5 in an optical imaging system with a focal length f of 300mm and an F-number of 2, as shown in Table 1:

[0061] Table 1

[0062]

[0063] The primary reflector 11 has an aperture D1 of 150 mm, and the secondary reflector 12 has an aperture D2 of 38 mm. Both the primary reflector 11 and the secondary reflector 12 adopt aspherical surface shapes. The higher-order aspherical coefficients of the secondary reflector 12 are a4 = 6.755E-007, a6 = -4.383E-010, and a8 = 1.136E-013.

[0064] Secondary imaging lenses 21, 23, and 24 all employ even-order aspherical surface types. Specifically, the higher-order aspherical coefficients of the object-side surface of secondary imaging lens 21 are a4 = 1.629E-006, a6 = 2.512E-009, and a8 = 2.217E-011, while the higher-order aspherical coefficients of the object-side surface of secondary imaging lens 23 are a4 = 6.757E-006 and a6 = -1.948E. -008, a8 = 1.794E-011, the higher order aspherical coefficients of the object side of the secondary imaging lens 24 are a4 = -4.121E-005, a6 = 2.884E-009, a8 = -4.033E-010, and the higher order aspherical coefficients of the image side of the secondary imaging lens 24 are a4 = -6.953E-007, a6 = -1.799E-007, a8 = 1.871E-010.

[0065] The cooled detector has an F-number of 2 and a pixel array of 320×256 on the focal plane 5. When the focal length f is 300mm, the field of view is ±1.17°. By moving the focusing group, it can detect targets in the 8-16μm infrared ultra-long wave range under temperature conditions of 50-300K.

[0066] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A focusable ultra-long-wave infrared optical imaging system for low-temperature conditions, characterized in that, It includes a coaxial reflection primary imaging group (1), a secondary imaging lens group (2), a cooled detector window (3), a cold aperture (4), and a focal plane (5) arranged sequentially along the optical axis. The coaxial reflection primary imaging group (1) and the secondary imaging lens group (2) form a coaxial catadioptric secondary imaging structure. The secondary imaging lens group (2) adopts a four-piece refractive mirror group, including secondary imaging lens 21, secondary imaging lens 22, secondary imaging lens 23 and secondary imaging lens 24 arranged along the optical axis, and satisfying the positive-positive-negative-positive combination method. Secondary imaging lens 22 and secondary imaging lens 23 serve as a focusing group and are synchronously moved along the optical axis to the calibration position under the control of the motor, thereby realizing infrared ultra-long wave imaging under different low temperature conditions. The secondary imaging lens group (2) uses a high transmittance material in the infrared ultra-long wave band, with a transmittance of not less than 97% in the 8~12μm wavelength range, not less than 70% in the 12~14μm wavelength range, and not less than 90% in the 14~16μm wavelength range; The imaging system satisfies the following conditions: ， ， in, Indicates the focal length of an optical imaging system. Indicates the focal length of the coaxial reflection primary imaging group. This indicates the focal length of the secondary imaging lens group.

2. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, The coaxial reflection primary imaging group (1) includes a primary mirror (11) and a secondary mirror (12). The primary image plane is located between the primary mirror (11) and the secondary mirror (12) and satisfies the following conditions: ， ， in, This indicates the focal length of the primary reflector (11). This indicates the focal length of the secondary mirror (12).

3. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 2, characterized in that, The aperture of the primary reflector (11) The aperture of the secondary mirror (12) The following relationship must be satisfied: .

4. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 2, characterized in that, The primary reflector (11) and secondary reflector (12) are both aspherical and are made of infrared crystal material, metal material, glass material, silicon carbide composite material, or carbon fiber / silicon carbide composite material.

5. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, The positive-positive-negative-positive combination of the four-element refractive mirror group satisfies the following conditions: ， ， ， ， in, This indicates the focal length of the secondary imaging lens 21. Indicates the focal length of the secondary imaging lens 22, This indicates the focal length of the secondary imaging lens 23. This indicates the focal length of the secondary imaging lens 24.

6. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, The object-side surface of the secondary imaging lens 21, the object-side surface of the secondary imaging lens 23, the object-side surface of the secondary imaging lens 24, and the image-side surface are designed with even-order aspherical surfaces.

7. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, The secondary imaging lens group uses HWS series materials.

8. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, The F-number of the cooling detector is in the range of 2 ≤ F ≤ 3.

9. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 1, characterized in that, Also included is a lens barrel, the lens barrel material is selected from titanium alloys with a coefficient of thermal expansion of 9.41 x 10 -6 / °C to 10.03 x 10 -6 / °C.

10. The adjustable-focus ultra-long-wave infrared optical imaging system for low-temperature conditions according to claim 2, characterized in that, The entrance pupil of the system is located on the primary reflector (11), and the exit pupil must coincide with the cold aperture (4).

Images