A large-aperture infrared imaging system transfer calibration method

Abstract

The present application relates to the technical field of infrared quantitative measurement, and particularly relates to a transfer calibration method for a large-aperture infrared imaging system. The method first calibrates a thermal imager by using a surface source blackbody, and then calibrates the large-aperture infrared imaging system by using the calibrated thermal imager, covers the sky deep cold background reflected by the large-aperture infrared imaging system by using different emissivity radiation panels, and then uses the thermal imager to jointly observe, controls the radiation brightness of the entrance pupil of the large-aperture infrared imaging system by changing the emissivity of the radiation panel, and simultaneously collects images by using the calibrated thermal imager, inverts the radiation brightness of the radiation panel, and realizes the transfer calibration of the large-aperture infrared imaging system. The calibration temperature range of the method covers the low temperature to normal temperature section, makes up for the problem that the existing standard source is lacking, and has the advantages of full optical path calibration, no need for refrigeration / heating source, high calibration efficiency, and low cost.

Description

Technical Field

[0001] This invention relates to the field of infrared quantitative measurement technology, and in particular to a calibration method for large-aperture infrared imaging systems. Background Technology

[0002] Radiometric calibration of infrared imaging systems is the prerequisite and foundation for achieving quantitative measurements. With the increasing demand for measuring weak targets such as those with low infrared radiation characteristics, the aperture of infrared imaging systems is gradually increasing (above 2m) to improve measurement capabilities. This increase in aperture makes it difficult to develop calibration sources that cover the entire aperture, especially for low-temperature calibration, for which no satisfactory solution has been found in existing technologies.

[0003] Currently, some proposed methods in engineering practice still have various problems and are difficult to meet the requirements. For example, using standard stars as reference sources has significant limitations. On the one hand, there are many stars in the visible to mid-infrared range, but fewer infrared stars in the long-wave infrared band. For ground-based large-aperture infrared imaging systems, there is insufficient observation quantity for calibration in the long-wave range, making calibration impossible. On the other hand, some infrared stars are variable stars, and their brightness fluctuates over time. Using them as calibration sources will lead to low calibration accuracy. In addition, some methods use cold sky backgrounds at different elevation angles for radiometric calibration. However, the deep cold sky background radiation at different elevation angles is essentially atmospheric path radiation at different thicknesses, with very low brightness in the long-wave band. For observation points at higher altitudes, the equivalent temperature of this method is around -60℃ to -40℃, resulting in a narrow dynamic range. Summary of the Invention

[0004] The purpose of this invention is to provide a calibration method for large-aperture infrared imaging systems, enabling low-temperature calibration, and simultaneously solving the problem that large-aperture infrared imaging systems cannot be calibrated or have an excessively small calibration dynamic range due to the lack of a suitable radiation source.

[0005] To achieve the above objectives, the present invention provides a transfer calibration method for a large-aperture infrared imaging system, which uses a thermal imager and a large-aperture infrared imaging system for transfer calibration, and includes the following steps:

[0006] S1. Absolute radiometric calibration of thermal imager:

[0007] Radiation calibration is performed using a surface-source blackbody and the direct surface-source calibration method.

[0008] By setting different integration times for the thermal imager, images of a blackbody at different temperatures are acquired using the thermal imager at different integration times, and the radiance L of the blackbody is established. b The linear response relationship with the thermal imager's grayscale value DN is as follows:

[0009] Lb =G0×DN+b0

[0010] Wherein, the calibration coefficients G0 and b0 are the gain and bias of the thermal imager, respectively;

[0011] S2. Calibration of large-aperture infrared imaging systems is divided into low-temperature range and low-temperature to room-temperature range:

[0012] After the absolute radiometric calibration is completed, the relative positions of the thermal imager and the large-aperture infrared imaging system are fixed so that the thermal imager and the large-aperture infrared imaging system can simultaneously observe the cloudless, deep, cold background of the sky in the same area.

[0013] During low-temperature calibration, the thermal imager and the large-aperture infrared imaging system simultaneously measure the cryogenic background of the sky at different elevation angles. A linear response relationship is established between the grayscale value DN1 of the large-aperture infrared imaging system and the sky radiance L1 obtained from the thermal imager calibration coefficients using the cryogenic background radiation of the sky.

[0014] L1 = G1 × DN1 + b1

[0015] Wherein, G1 and b1 are the system gain and bias of the large-aperture infrared imaging path, respectively;

[0016] During the transfer calibration from low temperature to room temperature, radiating plates with different emissivity are placed at an angle in front of the entrance pupil of the large-aperture infrared imaging system. The radiating plates cover the field of view of both the thermal imager and the large-aperture infrared imaging system. The reflective part of the radiating plates is entirely against the deep, cold background of the sky. By changing the radiating plates with different emissivity, the proportion of reflected energy and self-radiated energy is controlled. Then, the radiant brightness of the radiating plates is obtained by inversion using the thermal imager, thereby achieving the transfer calibration from low temperature to room temperature.

[0017] Alternatively, the steps for absolute radiometric calibration of a thermal imager are as follows:

[0018] S11. Based on the measurement requirements, set the thermal imager integration time and perform non-uniform calibration on the measurement equipment;

[0019] S12. Set the surface source blackbody temperature to -70℃ as the starting temperature. After the surface source blackbody temperature reaches the set temperature and stabilizes, start the calibration test.

[0020] S13. The thermal imager acquires multiple frames of infrared images of the surface source blackbody as calibration data.

[0021] S14. After the acquisition is completed, the blackbody temperature is increased by 10°C each time, and steps S12 and S13 are repeated to complete the acquisition of all calibration data under the integration time. If the image of the measuring device becomes saturated at a certain surface source blackbody temperature, the surface source blackbody temperature is taken as the maximum temperature and the set minimum temperature is taken as the minimum temperature. At least five temperature points are set at equal intervals within this temperature range and the data is acquired respectively.

[0022] S15. Change the integration time and repeat steps S11 to S14 to complete the calibration test for different integration times.

[0023] S16. Calculate the blackbody radiance of the surface source using Planck's formula, and establish the blackbody radiance L. b The linear response relationship between the thermal imager's grayscale value DN and the absolute radiometric calibration of the thermal imager is determined.

[0024] Optionally, if the temperature measurement points corresponding to multiple integration times overlap, the integration time can be switched at the surface source blackbody temperature to collect calibration data separately.

[0025] Optionally, in step S13, the thermal imager acquires at least twenty frames of infrared images of the surface source blackbody as calibration data.

[0026] Optionally, during the calibration of a large-aperture infrared imaging system, the calibration steps for the low-temperature range are as follows:

[0027] S21. The thermal imager and the large-aperture infrared imaging system collect the cloudless deep cold background radiation of the sky in the same area at the same azimuth, elevation angle and time.

[0028] S22. By adjusting the elevation angle of the large-aperture infrared imaging system, select three or four different elevation angles of the deep cold sky background and repeat step S21.

[0029] S23. Calculate the deep cold background radiance of the sky at different elevation angles using a calibrated thermal imager, and fit the deep cold background radiance of the sky at different elevation angles with the grayscale values ​​measured by the large-aperture infrared imaging system to complete the low-temperature radiometric calibration.

[0030] Optionally, during the calibration process of a large-aperture infrared imaging system, when calibrating in the low-temperature to room-temperature range:

[0031] S24. The thermal imager and the large-aperture infrared imaging system simultaneously acquire multiple frames of images of the radiating plate for calibration.

[0032] S25. Replace with radiation plates of different emissivity and repeat step S24;

[0033] S26. Calculate the radiance of radiating plates with different emissivity using a thermal imager, and fit the radiance of radiating plates with different emissivity to the grayscale values ​​of a large-aperture infrared imaging system to complete the radiometric calibration from low temperature to normal temperature.

[0034] Optionally, a radiating plate with an emissivity greater than 0.9 can be used to achieve calibration near room temperature;

[0035] Calibration at approximately -35℃ to 10℃ is achieved using a radiation plate with an emissivity of 0.2 to 0.5. The radiation surface coating is made of metal micro powder and is polished to achieve mirror-level flatness.

[0036] A high-reflectivity mirror panel with an emissivity of less than 0.1 was used as the radiating plate to achieve low-temperature calibration of -50℃ to -40℃.

[0037] Optionally, the radiating plate with an emissivity greater than 0.9 has a thermal insulation layer, and its radiating surface coating is made of carbon nanotubes;

[0038] The radiating plate has an emissivity of 0.2 to 0.5. Its radiating surface coating is made of metal micro powder and polished to achieve mirror-level flatness.

[0039] Radiation panels with an emissivity of less than 0.1 are made of 8K or 10K mirror-finish stainless steel.

[0040] Optionally, in step S2:

[0041] When performing low-temperature calibration, the calibration temperature range is -60℃ to -45℃;

[0042] When performing calibration from low temperature to normal temperature, the calibration temperature range is -50℃ to normal temperature.

[0043] Optionally, the calibration temperature range of the surface source blackbody is -70℃ to 50℃.

[0044] The above-mentioned technical solution of the present invention has the following advantages: The large-aperture infrared imaging system transfer calibration method provided by the present invention first uses a surface source blackbody to perform absolute radiometric calibration of the thermal imager, and then uses the calibrated thermal imager to perform transfer calibration of the large-aperture infrared imaging system. Radiation plates with different emissivity are used to cover the field of view of the large-aperture infrared imaging system, reflecting the cold background of the sky. Then, the thermal imager is used for joint observation. By changing the emissivity of the radiation plate material, the entrance pupil radiance of the large-aperture infrared imaging system is controlled. Simultaneously, the calibrated thermal imager is used to acquire images, and the radiance of the radiation plate is inverted, thus achieving transfer calibration of the large-aperture infrared imaging system. This method covers a calibration temperature range from low temperature to room temperature, making up for the problem of the lack of a standard source in existing methods. It also has the advantages of full-optical-path calibration, no need for cooling / heating sources, high calibration efficiency, and low cost. Attached Figure Description

[0045] The accompanying drawings are provided for illustrative purposes only, and the proportions and quantities of the components in the drawings may not be consistent with the actual product.

[0046] Figure 1 This is a schematic diagram illustrating the calibration implementation of a ground-based large-aperture infrared telescope in this embodiment of the invention.

[0047] Figure 2 This is a schematic block diagram of the calibration transfer method for a large-aperture infrared imaging system in an embodiment of the present invention;

[0048] Figure 3This is an optical schematic diagram of the direct surface source calibration method in an embodiment of the present invention;

[0049] Figure 4 This is a schematic diagram of radiation temperatures with different emissivity in an embodiment of the present invention;

[0050] Figure 5 This is a schematic diagram of temperature control for radiating plates with different emissivity in an embodiment of the present invention.

[0051] In the picture:

[0052] 1: Thermal imager;

[0053] 2: Ground-based large-aperture infrared telescope;

[0054] 3: Telescope stand;

[0055] 4: Radiation panel;

[0056] 5: Radiation panel support;

[0057] 6: Overhead crane;

[0058] 7: Dome. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, 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.

[0060] See Figure 1 As shown, this embodiment of the invention takes a ground-based large-aperture infrared telescope (primary mirror ≥ 2.4m) as an example to further illustrate the calibration method for large-aperture infrared imaging systems.

[0061] See Figure 2 The diagram shown illustrates a transfer calibration method for a large-aperture infrared imaging system. This transfer calibration method is achieved through a combination of absolute radiometric calibration of the thermal imager and calibration of the large-aperture infrared imaging system.

[0062] First, perform absolute radiometric calibration of the thermal imager. See [link / reference] Figure 3 As shown, A is the optical system of the thermal imager, B is the infrared focal plane detector of the thermal imager, and C is a blackbody that serves as a radiation calibration source, placed near the entrance pupil of the thermal imager's optical system. The thermal imager's optical system is focused to infinity, and the aperture of the blackbody should be slightly larger than that of the thermal imager to ensure that the infrared focal plane detector is completely covered by the uniformly irradiated portion of the image.

[0063] In a preferred embodiment, the absolute radiometric calibration of the thermal imager is carried out in the following steps:

[0064] (1) Based on the measurement requirements, set the thermal imager integration time and perform non-uniform calibration on the measurement equipment.

[0065] (2) Set the surface source blackbody temperature to -70℃ as the starting temperature, and start the calibration test after the surface source blackbody temperature reaches the set temperature and stabilizes.

[0066] (3) The thermal imager collects infrared images of the blackbody as calibration data, and a total of 20 frames of data are collected.

[0067] (4) After the acquisition is completed, increase the surface source blackbody temperature by 10°C and repeat steps (2) and (3) to complete the acquisition of all calibration data under this integration time. If the image of the measuring device becomes saturated at a certain surface source blackbody temperature, take this surface source blackbody temperature as the maximum temperature and the minimum temperature as the minimum temperature. Set at least 5 temperature points at equal intervals within this temperature range and acquire data respectively.

[0068] (5) Change the integration time and repeat steps (1) to (4) to complete the calibration test for different integration times. Preferably, if the temperature measurement points corresponding to multiple integration times are repeated, the integration time can be switched at the blackbody temperature to collect calibration data separately.

[0069] (6) Calculate the blackbody radiance of the surface source using Planck's formula, and establish the blackbody radiance L. b The linear response relationship with the grayscale value DN of the thermal imager is shown in the following specific calculation process provided in this embodiment:

[0070]

[0071] In the formula, M λ It is the blackbody spectral radiative exitance, c1 = 2πhc 2 =3.7415×10 8 (W·μm 4 / m 2 Let be the first radiation constant, h be Planck's constant, and c² = hc / k = 1.43879 × 10⁻⁶. 4 (μm·K) is the second radiation constant, k is the Boltzmann constant, T is the absolute temperature, and λ is the wavelength.

[0072] For the imaging band of a thermal imager, the radiative exitance is the integral of that band:

[0073]

[0074] Among them, M Δλ ε is the radiative exitance of the band. λε is the spectral emissivity. For a surface-source blackbody, ε λ It is a constant. Furthermore, the surface source blackbody is a standard Lambert radiator, obeying Lambert's radiation law; therefore, the relationship between radiance and radiant exitance can be expressed as:

[0075] M λ =πL λ

[0076] The radiance value of the blackbody source in the imaging band of the thermal imager is:

[0077]

[0078] By changing the surface source blackbody temperature, multiple sets of radiance values ​​L were recorded. b The correspondence between the grayscale value (DN) of the thermal imager and the actual grayscale value (DN) is obtained through linear fitting of the two:

[0079] L b =G0×DN+b0

[0080] For the transfer calibration of a ground-based large-aperture infrared telescope, the first step is to perform calibration in the low-temperature range. An optimal procedure is as follows:

[0081] (1) Fix the relative positions of the calibrated thermal imager and the infrared telescope so that they can achieve synchronous observation.

[0082] (2) The thermal imager and the infrared telescope collect the same area of ​​cloudless sky deep cold background radiation at the same azimuth, elevation angle and time. The elevation angle of the large aperture infrared imaging system is adjusted, and the deep cold background of the sky at 25°, 40°, 60° and 80° is measured respectively for calibration.

[0083] (3) The radiance of the cold background sky at different elevation angles was calculated using a calibrated thermal imager, and the radiance of the cold background sky at different elevation angles was fitted with the grayscale values ​​of the infrared telescope to complete the radiometric calibration in the low-temperature range. It should be noted that the calculation of the cold background sky at different elevation angles using a thermal imager is an existing technology.

[0084] In this embodiment, the key to transfer calibration lies in the fact that the system to be calibrated and the reference system must observe the same target, and the reference system achieves calibration of the system to be calibrated through the same entrance pupil radiance. Therefore, the following two points are particularly important to note:

[0085] 1) Ensure that the sky observed by the infrared telescope and the thermal imager is the same area, and ensure that there are no clouds in the field of view.

[0086] 2) The infrared telescope and thermal imager must start measuring at the same time to ensure consistency in the time domain.

[0087] Since the deep-space cold background radiation is essentially atmospheric path radiation under a deep-space cold background, its radiance is very low, and the corresponding calibration temperature range is very small, only about -60℃ to -45℃. Therefore, in order to improve the calibration range, calibration is performed in the low-temperature to room-temperature range. The preferred calibration procedure provided in this embodiment is as follows:

[0088] By tilting radiating plates with different emissivity in front of the entrance pupil of an infrared telescope, the radiant energy entering the telescope is divided into two parts: one part is the radiation emitted by the radiating plate itself, and the other part is the energy reflected from other radiation sources. By placing the radiating plates at an appropriate angle, ensuring that the entire reflected portion is against the deep, cool background of the sky, a uniform distribution of radiant brightness is achieved. By fixing the angle and changing the emissivity of the radiating plates, the proportion of reflected energy and radiated energy can be flexibly controlled. Figure 4 As shown, the variation of the radiation temperature of the radiating plate with emissivity in the long-wavelength band (7.7–9.5 μm) under a cryogenic background radiation temperature of -60℃ was calculated. Based on the above analysis, it can be seen that by utilizing the infrared radiation characteristics of the material and changing the emissivity of the passive radiating material, the entrance pupil radiation temperature of the system to be calibrated can be controlled.

[0089] See Figure 5 As shown, three radiating plates with different emissivity are used to control the temperature from low to normal. The high-emissivity radiating plate (>0.9) is used for calibration near ambient temperature. The surface coating of this plate uses carbon nanotubes, and a multi-layered structure (coating, insulation, and plate body) is employed to improve temperature uniformity. The medium-low emissivity radiating plate (0.2–0.5) is used for calibration from approximately -35℃ to 10℃. Due to the large temperature range, two radiating plates with emissivity around 0.2 and 0.5 are selected for calibration. These plates also employ a multi-layered structure, with a surface coating of metal micropowder, and are polished to achieve mirror-like smoothness. The low-temperature radiating plate uses a high-reflectivity mirror panel (<0.1) to achieve calibration from -50℃ to -40℃. Because its own radiation is very low, 8K or 10K mirror-finish stainless steel is sufficient to meet the requirements.

[0090] To compensate for the low dynamic range based on the deep cold background radiation of the sky, different emissivity radiation plates are used to control the entrance pupil radiation during the calibration of the low temperature to normal temperature range. The radiation plates achieve calibration of the low temperature sky to normal temperature range by reflecting the low temperature background of the sky plus their own radiation, and cover the field of view of the infrared telescope and thermal imager.

[0091] The specific implementation method is as follows: Figure 1As shown, the ground-based large-aperture infrared telescope 2 is adjustablely installed via telescope bracket 3, located inside dome 7. Thermal imager 1 is mounted on the outer wall of the telescope tube of ground-based large-aperture infrared telescope 2. To achieve optimal calibration results and reduce the length of the radiating plate 4, it can be fixed at a 45° angle. The radiating plate 4 is selected to be 3m × 4m in size. It is mounted and fixed in front of the entrance pupil of ground-based large-aperture infrared telescope 2 using either the radiating plate bracket 5 or a gantry crane 6. After the radiating plate 4 is fixed, calibration is performed. The specific process is as follows:

[0092] (1) Mount the thermal imager 1 onto the outer wall of the tube of the ground-based large-aperture infrared telescope 2 so that it can achieve synchronous observation.

[0093] (2) The thermal imager and the large-aperture infrared imaging system simultaneously acquire 20 frames of images of the radiation plate for calibration.

[0094] (3) Replace with other emissivity radiating plates and repeat step (2) to complete four types of emissivity radiating plates with emissivity of 0.05, 0.2, 0.5 and 0.95.

[0095] (4) Calculate the radiance of radiating plates with different emissivity using a thermal imager, and fit the radiance of radiating plates with different emissivity to the gray value of a large-aperture infrared imaging system to complete the radiometric calibration from low temperature to normal temperature.

[0096] In summary, this invention addresses the problem of large-aperture infrared imaging systems being unable to be calibrated or having an excessively small calibration dynamic range due to the lack of suitable radiation sources. It constructs a low-temperature calibration system for large-aperture infrared imaging systems, consisting of a radiating plate, a cryogenic sky background, and a pre-calibrated thermal imager. Radiating plates with different emissivities are used to cover the aperture of the large-aperture infrared imaging system against the cryogenic sky background, and then the thermal imager is used for joint observation. The entrance pupil radiance of the large-aperture infrared imaging system is controlled by changing the emissivity of the radiating plate material. Simultaneously, the pre-calibrated thermal imager is used for image acquisition. The radiance of the radiating plate is retrieved based on the calibration coefficients and the grayscale values ​​of the thermal imager, thus achieving transfer calibration of the large-aperture infrared imaging system. This method covers a calibration temperature range from low temperature to room temperature (-60℃ to room temperature), overcoming the problem of the lack of a standard source in existing systems. It also has advantages such as all-optical-path calibration, no need for cooling / heating sources, high calibration efficiency, and low cost.

[0097] Any aspects of this invention not described in detail are common knowledge or existing technology in the field.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that not every embodiment contains only one independent technical solution, and in the absence of conflict between solutions, the various technical features mentioned in each embodiment can be combined in any way to form other implementation methods that can be understood by those skilled in the art.

[0099] Furthermore, without departing from the scope of the present invention, modifications to the technical solutions described in the foregoing embodiments, or equivalent substitutions of some of the technical features, shall not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A calibration method for a large-aperture infrared imaging system, characterized in that, The calibration process using a thermal imager and a large-aperture infrared imaging system includes the following steps: S1. Absolute radiometric calibration of thermal imager: Radiation calibration is performed using a surface-source blackbody and the direct surface-source calibration method. By setting different integration times for the thermal imager, images of a blackbody at different temperatures are acquired using the thermal imager at different integration times, and the radiance L of the blackbody is established. b The linear response relationship with the thermal imager's grayscale value DN is as follows: L b =G0×DN+b0 Wherein, the calibration coefficients G0 and b0 are the gain and bias of the thermal imager, respectively; S2. Calibration of large-aperture infrared imaging systems is divided into low-temperature range and low-temperature to room-temperature range: After the absolute radiometric calibration is completed, the relative positions of the thermal imager and the large-aperture infrared imaging system are fixed so that the thermal imager and the large-aperture infrared imaging system can simultaneously observe the cloudless, deep, cold background of the sky in the same area. During low-temperature calibration, the thermal imager and the large-aperture infrared imaging system simultaneously measure the cryogenic background of the sky at different elevation angles. A linear response relationship is established between the grayscale value DN1 of the large-aperture infrared imaging system and the sky radiance L1 obtained from the thermal imager calibration coefficients using the cryogenic background radiation of the sky. L1 = G1 × DN1 + b1 Wherein, G1 and b1 are the system gain and bias of the large-aperture infrared imaging path, respectively; During the transfer calibration from low temperature to room temperature, radiating plates with different emissivity are placed at an angle in front of the entrance pupil of the large-aperture infrared imaging system. The radiating plates cover the field of view of the thermal imager and the large-aperture infrared imaging system. The reflective part of the radiating plates is entirely against the deep cold background of the sky. By changing the radiating plates with different emissivity, the proportion of reflected energy and self-radiated energy is controlled. Then, the radiant brightness of the radiating plates is obtained by inversion through the thermal imager, thereby realizing the transfer calibration from low temperature to room temperature.

2. The large-aperture infrared imaging system calibration method according to claim 1, characterized in that: The steps for absolute radiometric calibration of the thermal imager are as follows: S11. Based on the measurement requirements, set the thermal imager integration time and perform non-uniform calibration on the measurement equipment; S12. Set the surface source blackbody temperature to -70℃ as the starting temperature. After the surface source blackbody temperature reaches the set temperature and stabilizes, start the calibration test. S13. The thermal imager acquires multiple frames of infrared images of the surface source blackbody as calibration data; S14. After the acquisition is completed, the blackbody temperature is increased by 10°C each time, and steps S12 and S13 are repeated to complete the acquisition of all calibration data under the integration time. If the image of the measuring device becomes saturated at a certain surface source blackbody temperature, the surface source blackbody temperature is taken as the maximum temperature and the set minimum temperature is taken as the minimum temperature. At least five temperature points are set at equal intervals within this temperature range and the data is acquired respectively. S15. Change the integration time and repeat steps S11 to S14 to complete the calibration test for different integration times. S16. Calculate the blackbody radiance of the surface source using Planck's formula, and establish the blackbody radiance L. b The linear response relationship between the thermal imager's grayscale value DN and the absolute radiometric calibration of the thermal imager is determined.

3. The calibration method for a large-aperture infrared imaging system according to claim 2, characterized in that: If multiple integration times correspond to the same temperature measurement points, the integration time should be switched at the same blackbody temperature to collect calibration data separately.

4. The calibration method for a large-aperture infrared imaging system according to claim 2, characterized in that: In step S13, the thermal imager acquires at least twenty frames of infrared images of the surface source blackbody as calibration data.

5. The calibration method for a large-aperture infrared imaging system according to claim 1, characterized in that: The calibration steps for the low-temperature range during the calibration of a large-aperture infrared imaging system are as follows: S21. The thermal imager and the large-aperture infrared imaging system collect the deep cold background radiation of the sky in the same area at the same azimuth, elevation angle and time. S22. By adjusting the elevation angle of the large-aperture infrared imaging system, select three or four different elevation angles of the deep cold sky background and repeat step S21. S23. Calculate the deep cold background radiance of the sky at different elevation angles using a calibrated thermal imager, and fit the deep cold background radiance of the sky at different elevation angles with the grayscale values ​​measured by the large-aperture infrared imaging system to complete the low-temperature radiometric calibration.

6. The large-aperture infrared imaging system calibration method according to claim 5, characterized in that: During the calibration of large-aperture infrared imaging systems, the calibration process from low temperature to room temperature is as follows: S24. The thermal imager and the large-aperture infrared imaging system simultaneously acquire multiple frames of images of the radiating plate for calibration. S25. Replace with radiation plates of different emissivity and repeat step S24. S26. Calculate the radiance of radiating plates with different emissivity using a thermal imager, and fit the radiance of radiating plates with different emissivity to the grayscale values ​​of a large-aperture infrared imaging system to complete the radiometric calibration from low temperature to normal temperature.

7. The large-aperture infrared imaging system calibration transfer method according to claim 1, characterized in that: Calibration was achieved near room temperature using a radiating plate with an emissivity greater than 0.

9. Calibration at approximately -35℃ to 10℃ is achieved using a radiation plate with an emissivity of 0.2 to 0.

5. The radiation surface coating is made of metal micro powder and is polished to achieve mirror-level flatness. A high-reflectivity mirror panel with an emissivity of less than 0.1 was used as the radiating plate to achieve low-temperature calibration of -50℃ to -40℃.

8. The large-aperture infrared imaging system calibration method according to claim 7, characterized in that: Radiation panels with an emissivity greater than 0.9 have an insulation layer, and their radiation surface coating is made of carbon nanotubes. The radiating plate has an emissivity of 0.2 to 0.

5. Its radiating surface coating is made of metal micro powder and polished to achieve mirror-level flatness. Radiation panels with an emissivity of less than 0.1 are made of 8K or 10K mirror-finish stainless steel.

9. The large-aperture infrared imaging system calibration method according to claim 1, characterized in that: In step S2: When performing low-temperature calibration, the calibration temperature range is -60℃ to -45℃; When performing calibration from low temperature to normal temperature, the calibration temperature range is -50℃ to normal temperature.

10. The large-aperture infrared imaging system calibration transfer method according to claim 1, characterized in that: The calibration temperature range of the surface source blackbody is -70℃ to 50℃.

Images