A common-path mid-wavelength wave dual-color athermalized refrigeration type infrared catadioptric optical system

Abstract

The application discloses a common-optical-path middle-wavelength wave dual-color athermalization refrigeration type infrared catadioptric optical system, and belongs to the technical field of infrared optics.The application solves the problem that the existing common-optical-path dual-band optical system cannot simultaneously remove thermal color and chromatic aberration.The application comprises a main reflector, a secondary reflector, a lens group, a cold light diaphragm window and an infrared dual-color detector;all the components are coaxially arranged and jointly form an imaging system;the optical system adopts a structure form with an intermediate real image, and the real image plane is located between the secondary reflector and the lens group;the application adopts the main and secondary reflectors to perform first imaging, adds the lens group to perform second imaging and correct off-axis aberration after the first image plane, and can be applied to the fields of infrared imaging equipment, night vision devices, infrared thermal imagers and space infrared target detection, and can be used for detecting and imaging thermal radiation objects in the infrared band.

Description

Technical Field

[0001] This invention patent relates to the field of infrared optics technology, specifically to a cooled infrared catadioptric optical system. This technology can be applied to infrared imaging equipment matched with dual-color cooled detectors, night vision devices, infrared thermal imagers, and space infrared target detection, etc. It can be used to detect and image thermally radiating objects in the infrared band, and can remain passively calorimetric over a wide temperature range, thus possessing broad application value. Background Technology

[0002] With the continuous development of modern technology, infrared imaging equipment is increasingly being used in military, security, and medical fields. Since the principle of infrared optical systems is to detect the temperature and emissivity differences between the target and the background, and different targets and backgrounds have different temperatures, resulting in characteristic differences manifested in different wavelength bands, dual-color infrared imaging has always been a research hotspot in various countries. In recent years, domestic and international companies have launched cooled detectors that can simultaneously respond to mid-wavelength radiation, and the conditions for developing common-path dual-color cooled infrared optical systems are now mature.

[0003] Furthermore, the thermal effects of temperature on optical systems mainly fall into two categories: first, the change in refractive index with temperature; and second, the thermal expansion caused by temperature-induced changes in the dimensions and thickness of optical elements or mechanical structures. Both changes in refractive index and dimensions / thickness lead to variations in the image quality of the optical system, primarily manifested as image plane defocusing. Correction of temperature-induced defocusing is called optical anechoication, also known as thermal aberration design. Among various anechoication techniques, passive optical anechoic compensation technology is widely used due to its simple structure, small size, and light weight.

[0004] Currently, optical systems that simultaneously possess pyrometric ablation and dual-band infrared imaging characteristics are mainly refractive, such as the pyrometric infrared lens and optical system with a large target surface and dual-band co-aperture described in CN112629669B. This system employs a special surface shape like a diffraction surface, which is difficult and costly to manufacture, and also results in a relatively long overall system length. While the infrared dual-band wide-angle pyrometric ablation confocal surface optical system described in application number CN 113866937A does not employ a diffraction surface, the system is also not compact enough and is unsuitable for lightweight and miniaturized infrared system applications.

[0005] Compared to refractive optical systems, catadioptric optical systems are smaller, with the total system length kept within 50% of the focal length. They are commonly used in airborne and aerospace payload scenarios. A typical catadioptric cooled infrared optical system uses two mirrors to form a primary image of the target, followed by a lens group to perform a secondary image formation on the primary image plane, achieving matching between the cooled aperture and the exit pupil. However, achromatic and thermal aberration reduction have always been challenging aspects of developing such optical systems. For example, the invention described in CN103207452A only achieves dual-band common-path, common-focal-plane imaging, without actually achieving thermal aberration reduction.

[0006] Another solution is to add a beam splitter between the reflective and refractive elements, separating the optical paths of the two bands to correct chromatic aberration and thermal aberration independently, as illustrated in "Design and Implementation of a Visible / Infrared Dual-Color Co-Aperture Optical System" (doi: 10.3788 / gzxb20215005.0511002). While this type of optical system can achieve good imaging results, the overall optical path is relatively complex, which is also detrimental to the miniaturization and cost control of the optical system. Summary of the Invention

[0007] To address the problem that existing common-path dual-band optical systems cannot simultaneously eliminate thermal color and chromatic aberration, this invention provides a common-path mid-wavelength dual-color chromatic aberration-eliminating cooled infrared catadioptric optical system.

[0008] The present invention discloses a mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system with a common optical path, comprising a primary mirror 101, a secondary mirror 102, a lens group 103, a cold aperture window 104, and an infrared dual-color detector 105; all components are arranged coaxially to form an imaging system; the optical system adopts a structure with an intermediate real image, the real image plane being located between the secondary mirror 102 and the lens group 103.

[0009] The main reflector 101 is provided with a central through hole, and the lens group 103 is disposed in front of the central through hole of the main reflector 101.

[0010] The aperture stop of the optical system is located at the cold stop window 104, which is disposed at the light output end of the lens group 103;

[0011] Light rays from a target at near infinity are reflected by the primary mirror 101 and the secondary mirror 102, forming an intermediate real image before reaching the lens group 103. After being refracted by the lens group 103, the image passes through the cold aperture window 104 and finally images the scene onto the infrared dual-color detector 105, achieving dual-band common-path imaging. The primary and secondary mirrors form a primary image of the object at infinity, and the lens group 103 performs a secondary imaging on the primary image plane, thereby adjusting the exit pupil position. The position of the cold aperture window 104 is the system exit pupil, and the cold aperture efficiency is 100%.

[0012] An optical system must meet the following conditions to achieve adiabatic and chromatic aberration:

[0013]

[0014] In the formula, h is the incident height of the lens group, h i Let be the fractional incident height of the i-th lens in the thin lens group, i.e., the normalized incident height; i = 1, 2, ..., k, where k is the number of lenses in the lens group;

[0015] αm —The coefficient of thermal expansion of the lens barrel material;

[0016] L—Length of the lens barrel of the lens group;

[0017] The optical power of the lens group. Let i be the optical power of the i-th lens;

[0018] C is the chromatic aberration coefficient of the lens group. i Let i be the chromatic aberration system of the i-th lens;

[0019] T is the thermal difference coefficient of the lens group, T i Let be the thermal difference system of the i-th lens.

[0020] Preferably, the lens group 103 has k=5 lenses coaxially arranged inside the lens barrel in the direction of light propagation. The first lens 111 is made of silicon and has positive optical power; the second lens 112 is made of infrared glass AMTIR1 and has positive optical power; the third lens 113 is made of germanium and has negative optical power; the fourth lens 114 is made of silicon and has positive optical power; and the fifth lens 115 is made of zinc selenide and has negative optical power. The lens barrel is made of titanium alloy.

[0021] If the optical power of the lens group is considered The optical power ranges of the five lenses are as follows:

[0022] Preferably, R1 and R2 correspond to the vertex radii of curvature of the primary reflector 101 and the secondary reflector 102, respectively, with R1 ranging from -300 to -500 mm and R2 ranging from -80 to -150 mm.

[0023] Preferably, under the condition of focal length normalization, the interval between the primary mirror 101 and the secondary mirror 102 is -0.25 to -0.4; the total length of the optical system is 0.3 to 0.5; the rear surfaces of the primary mirror 101, the secondary mirror 102 and the fourth lens 114 in the lens group are aspherical, and the surfaces of other optical elements are spherical or planar.

[0024] Preferably, the infrared dual-color detector 105 has a specification of 256×256 pixels, and the size of a single pixel is 30 micrometers.

[0025] Preferably, the distance between the cold aperture window 104 and the infrared dual-color detector 105 is 15-25 mm.

[0026] Preferably, an optical filter or beam splitter is added between the secondary reflector 102 and the lens group 103.

[0027] Preferably, light shields are provided in front of the primary reflector 101, at the position of the secondary reflector 102, at the position of the primary focal plane, and at the intervals between the front and rear of the lens group 103; and a light-blocking ring is provided on the light shield in front of the primary reflector 101.

[0028] The beneficial effects of this invention are as follows: This invention proposes a common-path dual-color thermally ablated cooled infrared catadioptric optical system. By employing primary and secondary mirrors for primary imaging, and then adding a lens group after the primary image plane for secondary imaging and correction of off-axis aberrations, this invention achieves dual-band common-path imaging for both mid-wave (3.7–4.8 μm) and long-wave (7.7–9.5 μm) wavelengths. Simultaneously, by optimizing materials, passive thermally ablation is achieved between -40° and +60°. This system simultaneously eliminates thermal and chromatic aberration.

[0029] The optical system provided by this invention has a compact and simple structure compared with other optical systems with similar application scenarios. The total length of the system can be controlled within 30% to 50% of the focal length. The processing and assembly technology is mature, which is conducive to miniaturization. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of the mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system with common optical path described in this invention;

[0031] Figure 2 The detailed structure of the lens group according to an embodiment of the present invention;

[0032] Figure 3 MTF curve of the optical system provided in the embodiment of the present invention at 4.2 micrometers and -40°C;

[0033] Figure 4 MTF curve of the optical system provided in the embodiment of the present invention at 4.2 micrometers and +60°C;

[0034] Figure 5 MTF curve of the optical system provided in the embodiment of the present invention at 7.7 micrometers and -40°C;

[0035] Figure 6 MTF curve of the optical system provided in the embodiment of the present invention at 7.7 micrometers and +60°C;

[0036] Figure 7 The color defocus curve of the optical system provided in the embodiments of the present invention;

[0037] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0038] 101. Primary reflector; 102. Secondary reflector; 103. Lens group; 104. Cold aperture window; 105. Infrared dual-color detector.

[0039] Lens group 103 specifically includes: 111, lens number one; 112, lens number two; 113, lens number three; 114, lens number four; and 115, lens number five. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0042] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0043] This invention achieves dual-band chromatic aberration and thermal aberration control using only refractive and reflective elements. The imaging bands are mid-infrared (3.7–4.8 μm) and long-infrared (7.7–9.5 μm), and light from both bands can converge onto detector 105 via the same optical path. Furthermore, this optical system maintains passive thermal aberration within a temperature range of -40°C to +60°C. The system's field of view reaches 1°–2°, with an F-number of 2–3 and a cold stop efficiency of 100%.

[0044] Specific Implementation Method 1: The following is combined with... Figures 1 to 7 This embodiment describes a common-path mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system, which includes a primary mirror 101, a secondary mirror 102, a lens group 103, a cold aperture window 104, and an infrared dual-color detector 105. All components are coaxially arranged to form an imaging system. The optical system adopts a structure with an intermediate real image, and the real image plane is located between the secondary mirror 102 and the lens group 103.

[0045] The main reflector 101 is provided with a central through hole, and the lens group 103 is disposed in front of the central through hole of the main reflector 101;

[0046] The aperture stop of the optical system is located at the cold stop window 104, which is set at the light output end of the lens group 103;

[0047] Light rays from a target at near infinity are reflected by the primary mirror 101 and the secondary mirror 102, forming an intermediate real image before reaching the lens group 103. After being refracted by the lens group 103, the image passes through the cold aperture window 104 and finally images the scene onto the infrared dual-color detector 105, achieving dual-band common-path imaging. The primary and secondary mirrors form a primary image of the object at infinity, and the lens group 103 performs a secondary imaging on the primary image plane, thereby adjusting the exit pupil position. The position of the cold aperture window 104 is the system exit pupil, and the cold aperture efficiency is 100%.

[0048] The optical power, thermal coefficient, and chromatic aberration coefficient of an optical system satisfy the following relationship:

[0049]

[0050] Chromatic aberration in an optical system is a quantity that depends only on the materials of the optical system, and its compensation does not require consideration of mechanical materials. Therefore, the condition for achieving achromatic aberration is to satisfy the chromatic aberration coefficient C = 0. However, the thermal aberration of the system is related to the mechanical materials. Thus, when the object is at infinity, the optical power, T, and C of the optical system can simultaneously achieve achromatic and thermal aberration by satisfying the following three equations:

[0051]

[0052] In the formula, h is the incident height of the lens group, h i Let be the fractional incident height of the i-th lens in the thin lens group, i.e., the normalized incident height; i = 1, 2, ..., k, where k is the number of lenses in the lens group;

[0053] α m —The coefficient of thermal expansion of the lens barrel material;

[0054] L—Length of the lens barrel of the lens group;

[0055] The optical power of the lens group. Let i be the optical power of the i-th lens;

[0056] C is the chromatic aberration coefficient of the lens group. i Let i be the chromatic aberration system of the i-th lens;

[0057] T is the thermal difference coefficient of the lens group, T i Let be the thermal difference system of the i-th lens.

[0058] Lens group 103 has k=5 lenses coaxially arranged inside the lens barrel in the direction of light propagation. The first lens 111 is made of silicon and has positive optical power; the second lens 112 is made of infrared glass AMTIR1 and has positive optical power; the third lens 113 is made of germanium and has negative optical power; the fourth lens 114 is made of silicon and has positive optical power; and the fifth lens 115 is made of zinc selenide and has negative optical power. The lens barrel is made of titanium alloy.

[0059] If the optical power of the lens group is considered The optical power ranges of the five lenses are as follows:

[0060] R1 and R2 correspond to the vertex curvature radii of the primary reflector 101 and the secondary reflector 102, respectively. The value of R1 ranges from -300 to -500 mm, and the value of R2 ranges from -80 to -150 mm.

[0061] Under the condition of focal length normalization, the interval between the primary mirror 101 and the secondary mirror 102 is -0.25 to -0.4; the total length of the optical system is 0.3 to 0.5; the rear surfaces of the primary mirror 101, the secondary mirror 102 and the fourth lens 114 in the lens group are aspherical, and the surfaces of other optical elements are spherical or planar.

[0062] The infrared dual-color detector 105 has a specification of 256×256 pixels, and the size of a single pixel is 30 micrometers.

[0063] The distance between the cold aperture window 104 and the infrared dual-color detector 105 is 15–25 mm. The system is very compact.

[0064] Example:

[0065] The external parameters of the optical system are: aperture 180mm, F number 2.5, system field of view 1.6°, operating wavelength 3.6-4.8 micrometers in mid-infrared and 7.7-9.5 micrometers in long-infrared, ambient temperature -40 to +60℃, and cold stop efficiency 100%.

[0066] The system's internal parameters are shown in the table below.

[0067] Table 1 Optical System Parameters

[0068]

[0069] In Table 1, 1-2 correspond to the front and rear surfaces of the primary and secondary mirrors, respectively; 3-12 correspond to the ten surfaces (front and rear) of the five lenses in the lens group; and 13-14 correspond to the front and rear surfaces, radius, thickness, and half-aperture of the cold aperture window, all in mm. The parameters for the even-order aspherical surface of the primary mirror are: fourth-order coefficient 1.841E-10, sixth-order coefficient -3.563E-14, eighth-order coefficient 1.053E-17, tenth-order coefficient -8.275E-22, and all other coefficients are 0. The parameters for the even-order aspherical surface of the secondary mirror are: fourth-order coefficient -6.968E-6, sixth-order coefficient 1.894E-8, eighth-order coefficient -3.437E-11, tenth-order coefficient 2.711E-14, and all other coefficients are 0.

[0070] See appendix Figure 1 This is a schematic diagram of a cooled infrared coaxial four-reflector optical system with an F-number of 1 provided in this embodiment. Following the direction of light incidence, target light rays from approximately infinity are reflected by the primary mirror 101 and secondary mirror 102, forming an intermediate real image before reaching the lens group 103. After refraction by 103, the image passes through the cold aperture window 104, ultimately imaging the scene onto the infrared dual-color detector 105. The total length of the optical system is 185 mm, approximately 0.41 times the focal length.

[0071] See appendix Figure 2 In the embodiments provided by the present invention, the lens group 103 includes 5 lenses, which, according to the direction of light propagation, are as follows: lens 111, made of silicon, with positive optical power; lens 112, made of infrared glass AMTIR1, with positive optical power; lens 113, made of germanium, with negative optical power; lens 114, made of silicon, with positive optical power; and lens 115, made of zinc selenide, with negative optical power.

[0072] See appendix Figure 3 In the embodiments provided by the present invention, the Nyquist frequency of the MTF curve is 16.7 lp / mm. At -40℃ and a wavelength of 4.2 micrometers, the center field of view of the MTF curve is greater than 0.50 and the edge field of view is greater than 0.45, indicating good imaging quality.

[0073] See appendix Figure 4 In the embodiments provided by the present invention, at +60℃ and a wavelength of 4.2 micrometers, the MTF curve full field value is greater than 0.50, indicating good imaging quality.

[0074] See appendix Figure 5 In the embodiments provided by the present invention, at -40℃ and 7.7 micrometer wavelength, the center field of view value of the MTF curve is greater than 0.40 and the edge field of view value is greater than 0.35, indicating good imaging quality.

[0075] See appendix Figure 6In the embodiments provided by this invention, at +60℃ and a wavelength of 7.7 micrometers, the MTF curve full-field value is greater than 0.40, indicating good imaging quality.

[0076] See appendix Figure 7 The color defocus curve provided in the embodiment of the present invention illustrates that the color difference between mid-wave infrared and long-wave infrared is well corrected in the embodiment provided in the present invention.

[0077] Specific Implementation Method Two: This implementation method further explains Implementation Method One by adding an optical filter or beam-splitting component between the secondary reflector 102 and the lens group 103. The application scenario can be freely changed within the specified wavelength range, including but not limited to adding beam-splitting elements or using a single-band detector.

[0078] Specific implementation method three: This implementation method further explains implementation method one. Light shields are respectively set in front of the main reflector 101, at the position of the secondary reflector 102, at the position of the primary focal plane, and at the intervals between the front and rear of the lens group 103; and a light blocking ring is set in the light shield in front of the main reflector 101.

[0079] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A common-path mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system, characterized in that, It includes a primary reflector (101), a secondary reflector (102), a lens group (103), a cold aperture window (104), and an infrared dual-color detector (105); all components are arranged coaxially to form an imaging system; the optical system adopts a structure with an intermediate real image, and the real image plane is located between the secondary reflector 102 and the lens group (103); The main reflector (101) is provided with a central through hole, and the lens group (103) is provided in front of the central through hole of the main reflector (101); The aperture stop of the optical system is located at the cold stop window (104), which is set at the light output end of the lens group (103); The target light rays from approximately infinity are reflected by the primary mirror 101 and the secondary mirror 102, forming an intermediate real image before reaching the lens group (103). After being refracted by the lens group (103), the image passes through the cold aperture window (104) and finally images the scene onto the infrared dual-color detector (105), achieving dual-band common-path imaging. The primary and secondary mirrors form a primary image of the object at infinity, and the lens group (103) performs a secondary imaging on the primary image plane, thereby adjusting the exit pupil position. The position of the cold aperture window (104) is the system exit pupil, and the cold aperture efficiency is 100%. An optical system must meet the following conditions to achieve adiabatic and chromatic aberration: In the formula The incident height of the lens group, Let be the fractional incident height of the i-th lens in the thin lens group, i.e., the normalized incident height; i = 1, 2, ..., k, where k is the number of lenses in the lens group; —The coefficient of thermal expansion of the lens barrel material; —The length of the lens barrel of the lens group; The optical power of the lens group. Let i be the optical power of the i-th lens; The chromatic aberration coefficient of the lens group. Let i be the chromatic aberration system of the i-th lens; The thermal difference coefficient of the lens group, For the thermal difference system of the i-th lens; The lens group (103) has k=5 lenses coaxially arranged in the direction of light propagation inside the lens barrel. The first lens (111) is made of silicon and has a positive optical power; the second lens (112) is made of infrared glass AMTIR1 and has a positive optical power; the third lens (113) is made of germanium and has a negative optical power; the fourth lens (114) is made of silicon and has a positive optical power; and the fifth lens (115) is made of zinc selenide and has a negative optical power. The lens barrel is made of titanium alloy. If the optical power of the lens group is considered The optical power ranges of the five lenses are as follows: , , , , ; Under the condition of focal length normalization, the interval between the primary mirror (101) and the secondary mirror (102) is -0.25 to -0.4; the total length of the optical system is 0.3 to 0.5; the rear surfaces of the primary mirror (101), the secondary mirror (102) and the fourth lens 114 in the lens group are aspherical, and the surfaces of other optical elements are spherical or planar.

2. The mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system according to claim 1, characterized in that, R1 and R2 correspond to the vertex curvature radii of the primary reflector (101) and secondary reflector (102), respectively. The value of R1 ranges from -300 to -500 mm, and the value of R2 ranges from -80 to -150 mm.

3. The mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system according to claim 1, characterized in that, The infrared dual-color detector (105) has a specification of 256×256 pixels, and the size of a single pixel is 30 micrometers.

4. The mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system according to claim 1, characterized in that, The distance between the cold aperture window (104) and the infrared dual-color detector (105) is 15~25mm.

5. The mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system according to claim 1, characterized in that, An optical filter or beam splitter is added between the secondary reflector (102) and the lens group (103).

6. The mid-wavelength dual-color thermochromic cooling infrared catadioptric optical system according to claim 1, characterized in that, A light shield is set in front of the primary reflector (101), at the position of the secondary reflector (102), at the position of the primary focal plane, and at the intervals between the front and rear of the lens group (103); and a light-blocking ring is set in front of the light shield of the primary reflector (101).

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