CN114994877A - Large-aperture small-pixel large-target-surface ultra-wide-temperature composite heat dissipation differential thermal imaging lens - Google Patents

Abstract

The invention discloses a large-aperture small-pixel large-target-surface ultra-wide-temperature composite athermal imaging lens, and an optical system uses a germanium lens which is made of an optical material and uses a binary diffraction surface. Meanwhile, the material of high molecular weight polyethylene with high linear expansion coefficient and self-lubricating is utilized, the temperature change characteristic of the material and the structural design of a multi-turn flat-end corrugated spring are utilized, the passive temperature compensation quantity is carried out on the relative displacement of the lens barrel and the focal plane of the detector during temperature change, and the infrared thermal imaging lens with large relative aperture, small pixel, large target surface and good image quality of the whole passive athermalized optical system in a wide temperature range of minus 50 ℃ to plus 80 ℃ is realized.

Description

Large aperture, small pixel, large target surface, ultra-wide temperature composite athermal imaging lens

technical field

The invention relates to an infrared thermal imaging lens athermalization design, in particular to the lens athermalization of a working scene that requires a large relative aperture, the target surface is getting larger, the pixel size is getting smaller and smaller, and the working temperature range is ultra-wide application.

Background technique

The operating environment of the infrared optical system is under normal temperature and pressure, and the influence of factors such as temperature change on the imaging quality of the optical system is not considered. However, for special-purpose infrared optical systems, the ambient temperature will vary greatly. When the temperature changes, due to the thermal instability of the optical material and the structural material, when the ambient temperature changes, the curvature, thickness and spacing of the optical element will change, and the refractive index of the element material will also change, causing the system focal length to change. , the image plane is displaced, resulting in a sharp drop in system performance and deterioration in image quality. Therefore, in the case of military infrared optics, space infrared optics, and extreme environments and industrial application scenarios, it is necessary to meet the design requirements of athermalized infrared thermal imaging lenses that meet the ambient operating temperature.

Especially with large relative aperture lenses, the depth of field is relatively small. And with the development of infrared thermal imaging detectors, the pixel size has developed from 35μm, 25μm, 17μm to the current pixel size of 10-12μm, and the imaging target surface has developed from the traditional 160×120, 320×240, 384×288 to 640×512 , 1240×768 or even 1280×1024 stage. Therefore, there is a greater test for the design requirements of infrared thermal imaging athermal lenses.

At present, most of the athermal imaging lens applications are designed with large pixel size in the early stage, which can meet the limited operating temperature range, and basically focus on satisfying the pixel larger than 17 microns, 640 × 512 target size detectors, and the system operating temperature is -20 ℃ ~﹢40℃ is more.

There are two main methods of heat dissipation difference design currently used.

Adopt opto-mechanical active:

In this method, the focus adjustment mechanism is controlled by the motor, and the focus adjustment of the lens is carried out under different temperature environments to meet the imaging quality. The main problem with this method is the large size and the need to add a complex focusing mechanism, as well as components such as motors. Increases the overall size and weight of the system. At the same time, the factor of the consistency of the optical axis during the focusing process is increased, which is likely to cause adverse effects such as the optical axis shaking during the focusing process.

With mechanical passive and optical passive:

The mechanical passive method uses a special alloy material as the frame structure of the lens barrel. In the optical design, the expansion coefficient of the lens barrel is considered for the overall design. Usually, the selected material is an abnormal material, the high temperature material shortens, and the low temperature material expands. Only in this way can the focal plane move forward at high temperature, shortening the distance with the lens; when the temperature is low, the focal plane moves backward, increasing the distance with the lens, and the cost of using this material is extremely high.

Optical passive type is usually designed with two or more optical materials such as germanium, zinc selenide, chalcogenide glass, etc., and the cost is high. And for chalcogenide glass, an oxygen-free infrared glass usually formed by three elements of S, Se, Te and other metal elements such as Ge, Ga, As, Sb, etc., is usually formed by hot pressing and other methods. It is not very large, and it is difficult to process large-diameter optical components, and usually doping elements contain highly toxic substances such as arsenic (As), which are unfriendly and environmentally friendly.

For the above two passive design methods, the lens design is applied to small relative aperture, such as F/1.2 F/1.4 and other small aperture and small focal length lenses, but the design can only meet the temperature difference range of -20℃~+40℃. And the design can basically only meet the application of infrared detectors below the target size of 640×512 and larger than the pixel size of 17μm.

SUMMARY OF THE INVENTION

In order to overcome the inability to achieve a large relative aperture in the prior art, it is difficult to meet the needs of small pixels, large target surface design, and the inability to achieve a wide temperature range of -50 ° C ~ + 80 ° C without motor, small size athermalized infrared thermal imaging To solve the technical problem, the present invention provides a way of combining special materials with multi-ring structure design and binary optical design using a single germanium material, which realizes the relative aperture F/#=1.0 and the ambient temperature -50°C. ~+80℃, for the pixel size of 10 microns, the large target infrared thermal imaging adaptive passive athermal optical lens design of 1280×1024 array makes the imaging quality in the whole temperature range close to the diffraction limit.

The technical scheme adopted by the present invention to solve its technical problems is:

The athermal design of the infrared optical system requires that the axial chromatic aberration of the optical element is zero, and the defocus generated by the optical element and the defocus generated by the mechanical structure cancel each other when the temperature changes, so that the entire system does not generate temperature defocus. The optical passive athermalization system composed of all-refractive elements requires at least three or more materials, and the system is relatively complex. However, the present invention adopts a single germanium single crystal material, and at the same time adopts the unique temperature characteristics and dispersion characteristics of diffractive optical elements, and realizes that the external optical system can realize the athermalization design within a certain temperature range with a simpler structure.

The temperature characteristic of an optical element is represented by the coefficient of photothermal expansion, defined as the normalized change in focal length due to changes in lens temperature:

For a refractive element, its coefficient of thermal expansion of light can be obtained by using the thin lens model:

In the formula, α is the linear expansion coefficient of the optical element, n is the refractive index of the optical element, "n ₀ is the refractive index of the ambient medium, and dn/dT is the temperature coefficient of refractive index of the material.

The optical thermal expansion coefficient of the diffractive element is:

The temperature characteristic of the refractive element is determined by the expansion coefficient of the material and the temperature coefficient of the refractive index of the material, while the temperature characteristic of the diffractive element is only determined by the expansion coefficient of the material and has nothing to do with the temperature coefficient of the refractive index of the material.

The optical thermal expansion coefficient of the diffractive element is relatively small compared to the refractive element, but its dispersion factor is much larger than that of the refractive element, which can undertake the achromatic function of the system. Therefore, the diffractive-diffractive hybrid infrared optical system can use the diffractive element to achromatically distribute the refractive power of the refraction element for athermal aberration, which reduces the design difficulty of the athermalized infrared optical system and simplifies the system structure. The system has great design freedom.

The single use of the diffractive binary optical design can optimize the imaging quality very well, and can meet the needs of small pixels and large target surfaces within a certain ambient temperature range. However, in the wide temperature range of -50℃~+80℃ with large relative pore size, it cannot meet the design of heat dissipation difference in the whole temperature range.

According to the optical design analysis, when the temperature rises, the focal plane moves forward relatively, and the distance between the lens and the focal plane needs to be reduced. Conversely, when the temperature decreases, the focal plane moves relatively backward, and the distance between the focal plane and the lens needs to be increased.

When the temperature changes, the relative position of the image surface and the lens changes linearly. When the temperature changes, the change length of the relative position of the image surface and the lens L=ɑT, T is the temperature difference between normal temperature and high temperature or low temperature, and ɑ is the linear expansion coefficient of the expansion material .

Therefore, it is necessary to select materials that meet the high thermal expansion coefficient, and at the same time have a certain lubricating function, and the modified materials can meet the high and low temperature extreme temperature working environment. The material is PE-UHMW ultra-high molecular weight polyethylene, which is a thermoplastic engineering plastic with a molecular weight of 2.5 million. Impact resistance, good corrosion resistance, excellent chemical resistance, lower density than other thermoplastics, low friction coefficient, moderate mechanical strength, rigid creep resistance, excellent machinability, good resistance to high energy radiation performance. At the same time, PE-UHMW can work normally under the ambient temperature of -150℃~90℃. More importantly, the disadvantage of PE-UHMW's high thermal expansion coefficient is just the huge advantage of increasing the overall movement of the lens barrel, which can achieve a larger expansion and compression distance with a smaller length. The design adopts an adapter ring connection method, so that the cumulative amount of expansion or compression changes can be realized in a short space, and the size of the lens can be minimized. The adapter ring is made of invar with low expansion coefficient, which reduces the self-expansion factor brought by the adapter link. In order to make the lens move as a whole, the inner sleeve of the movable lens barrel needs to be evenly distributed, and the spring adopts a multi-turn flat-end corrugated spring. When the temperature rises, the accumulation of the expanding material increases, and the expansion retaining ring connected with the sliding lens barrel is pressed to move backward, and the sliding lens barrel moves backward as a whole, which shortens the distance between the focal plane and the lens when the temperature rises. . When the temperature decreases, the accumulation of the expanding material shortens, and the expansion retaining ring connected with the sliding lens barrel is pressed to move forward, and the sliding lens barrel moves forward as a whole, so as to increase the distance between the focal plane and the lens when the temperature decreases. The temperature change length of the intumescent material and the defocus amount of the optical design temperature change are combined to design the overall length of the intumescent material.

The beneficial effect of the present invention is that the present invention adopts a design technique that combines the diffractive binary optical design and the self-adjusted athermalization difference of the multi-ring passive structure. It realizes a modern high-precision infrared optical system with small size, light weight, high reliability and large operating temperature range, while meeting the requirements of large relative aperture, small pixel and large target surface. It effectively solves the problem that the existing technology cannot meet the large relative aperture, small pixel, and large target surface infrared optical system through a single germanium single crystal to achieve clear imaging in a wide temperature range of -50°C to +80°C by passive means.

Description of drawings: Fig. 1 is the schematic diagram of the present invention;

In the picture: 1. germanium lens 1; 2. germanium lens 2; 3. germanium lens 3 (including binary diffractive surface); 4. high expansion ultra-high molecular weight polyethylene; 5. Invar adapter ring; 6. high expansion UHMWPE; 7. Expansion retaining ring; 8. Multi-turn flat-end corrugated spring; 9. Sliding inner tube; 10. Outer tube; 11. Detector imaging surface.

Detailed ways :

The large-aperture, small-pixel, large-target-surface, ultra-wide-temperature composite athermalization thermal imaging lens includes a germanium lens 1, a germanium lens 2, and a germanium lens 3 with a binary diffractive surface. Through the combination of binary diffractive optical design and multi-ring passive structure self-adjusting adiabatic design technology, the infrared thermal imaging optical system with large relative aperture and small pixel and large target surface is realized at 50℃~+80℃ A wide temperature range for clear imaging purposes. On the premise of using the binary diffractive surface germanium lens 3 to ensure good imaging quality in a small ambient temperature range near the normal temperature environment, the relative displacement between the entire lens and the detector imaging surface 11 under different ambient temperatures is simulated. According to the selected high-expansion ultra-high molecular weight polyethylene linear expansion coefficient, calculate the required high linear expansion coefficient and self-lubricating high molecular weight polyethylene material length, the way of using the collar is to reduce the axial space size, do for the purpose of minimizing size. When the temperature rises, the lengths of the expansion materials 4 and 6 increase cumulatively, and the expansion stop ring 7 connected with the sliding lens barrel is pressed to move backward, and the sliding lens barrel moves backward as a whole, so that the shortening of the focal plane and the distance between lenses. When the temperature decreases, the accumulation of the expansion materials 4 and 6 shortens, and the expansion stop ring 7 connected with the sliding lens barrel is forced to move forward, and the sliding lens barrel moves forward as a whole, and the distance between the focal plane and the lens is increased when the temperature decreases. distance effect. The expansion retaining ring 7 is rigidly connected with the inner lens barrel 9, and the adapter ring 5 is made of invar material with a thermal expansion coefficient that tends to zero. The axial pressure is uniform, and the inner lens barrel 9 and the outer lens barrel 10 can move smoothly. Finally, an optical system with overall passive athermalization is achieved.

Claims (3)

1. The large-aperture small-pixel large-target-surface ultra-wide-temperature composite heat dissipation differential thermal imaging lens comprises a germanium lens 1, a germanium lens 2, a germanium lens 3 containing a binary diffraction surface, high-expansion ultrahigh molecular weight polyethylene 4, an invar adapter ring 5, high-expansion ultrahigh molecular weight polyethylene 6, an expansion check ring 7, a multi-coil flat-end corrugated spring 8 and a sliding inner lens barrel 9 and an outer lens barrel 10, and is characterized in that: the germanium lens 3 with a binary diffraction surface is used, the lantern ring 5 made of invar steel is used, the compensation quantity of the materials 4 and 6 made of high molecular weight polyethylene with high linear expansion coefficient and self-lubrication to temperature change acts on the inner lens cone 9, the pressure of the multi-turn flat-end corrugated spring 8 acting on the expansion check ring 7 is used for pushing the inner lens cone to move axially, the large relative aperture is achieved, the small pixel and the large target surface are achieved, and the whole passive athermalization-free optical system continuously has good image quality in a wide temperature range of minus 50 to plus 80 ℃.

2. The large-aperture small-pixel large-target-surface ultra-wide-temperature composite athermal imaging lens as claimed in claim 1, wherein a binary diffractive surface germanium lens 3, a high-expansion self-lubricating ultrahigh molecular weight polyethylene 4, an invar adapter ring 5, a high-expansion ultrahigh molecular weight polyethylene 6, and an optical and structural composite athermal design are used to meet the environment temperature of-50 ℃ to +80 ℃.

3. The large-aperture small-pixel large-target-surface ultra-wide-temperature composite athermal differential thermal imaging lens as defined in claim 1, wherein a lantern ring 5 made of invar and a multi-turn flat-end corrugated spring 8 are used.

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