Laboratory calibration method and system for thermal stability of optical axis of linear array camera

Abstract

The invention provides a line-scan digital camera optical axis thermal stability laboratory calibration method and system, and relates to the field of aerospace and aviation optical remote sensing imaging, and the method comprises the steps: carrying out the imaging of a geometric target collimated by a collimator through a to-be-measured camera, recording the image data of the geometric target and the temperature data of the camera at all moments, and obtaining the thermal stability of the optical axis of a linear array camera according to the characteristics of the geometric target; and determining the coordinates of the mass center position of the geometric target image, calculating the temperature drift amount of the line-scan digital camera optical axis in the along-rail direction and the vertical-rail direction, fitting to obtain the thermal stability coefficients in the two directions, and realizing the laboratory calibration of the thermal stability of the line-scan digital camera optical axis. The method solves the problem that the thermal stability of the optical axis of the linear array camera is difficult to calibrate at high precision in a laboratory, and the calibration precision is at a sub-pixel level. The system is simple in structure and high in adaptability, the static MTF testing function can be expanded, linear array camera optical axis thermal stability laboratory calibration and static MTF testing are carried out at the same time, and the development period is shortened.

Description

Technical Field

[0001] The present invention relates to the field of aerospace and aviation optical remote sensing imaging. Specifically, it relates to a method and system for laboratory calibration of the thermal stability of the line array camera's optical axis, which is applicable to the laboratory calibration of the thermal stability of the optical axis of aerospace cameras and aviation cameras using line array detectors. Background Art

[0002] Line array cameras are widely used in the field of aerospace and aviation optical remote sensing imaging. By using a Time Delay Integration (TDI) detector to push-scan (or cross-track scan) along the orbit for imaging, ground object remote sensing images are obtained, which have the advantages of large swath width, high signal-to-noise ratio, and clear images.

[0003] Geometric positioning accuracy is an important indicator in optical remote sensing and aerial photography. With the continuous development of commercial remote sensing, the demand for high geometric positioning accuracy is becoming more and more urgent. Due to the influence of the camera's thermal stress deformation, the pointing of the line array camera's optical axis will drift with the change of the camera's temperature, which greatly affects the positioning accuracy. That is, the thermal stability of the line array camera's optical axis is the core factor restricting the geometric positioning accuracy of satellites in orbit and aircraft in flight. Due to the particularity of line-of-sight imaging, for a long time, there has been a lack of an effective laboratory calibration method for the thermal stability of the line array camera's optical axis. Usually, only finite element and optical software are used for simulation analysis, and their accuracy and confidence are relatively low. At the same time, due to the numerous influencing factors in the full link of geometric positioning accuracy, it is difficult to completely separate the influence of other factors in orbit (or in flight), resulting in the unclear thermal stability of the line array camera's optical axis, which seriously restricts the ground prediction and in-orbit (or in-flight) realization of high geometric positioning accuracy. Therefore, it is urgent to develop a laboratory calibration method for the thermal stability of the line array camera's optical axis to accurately calibrate the change of the line array camera's optical axis pointing with factors such as the temperature of the focal plane assembly and the temperature of the optomechanical body, fit the temperature drift coefficient of the line array camera's optical axis pointing, and use it to evaluate the in-orbit (or in-flight) positioning accuracy and also for ground geometric correction.

[0004] In order to conduct laboratory tests and calibrations on the geometric performance of cameras, related technologies have been proposed:

[0005] Patent document CN104655153A (application number: 201510072488.3) discloses a method for calibrating the internal orientation elements of a mapping camera based on matrix orthogonality. This method uses the camera to image a star point board collimated by a collimator, changes the photographing angle through a turntable, obtains star point board images at multiple angles, and uses the orthogonality of the rotation matrix to solve the internal orientation elements of the camera. This method does not require recording the angle value of the turntable, eliminates the influence of the turntable accuracy on the calibration result, and reduces the calibration cost, but it is only applicable to the calibration of the internal orientation elements of area array cameras.

[0006] Patent document CN117011392A (application number: 202310775515.8) discloses a calibration method for the interior orientation elements of a planar array camera that improves the precise angle measurement method. This method arranges measurement equipment according to the precise angle measurement method, the camera images each measurement line, records the angles between each measurement line and the camera image coordinate axes, the measurement point coordinates, and the corresponding two-dimensional turntable rotation angles, constructs a distortion model, and uses a non-linear optimization algorithm to solve for the interior orientation elements. This method reduces the parallel accuracy requirements between the parallel light and the camera image coordinate axes, but is only applicable to the calibration of the interior orientation elements of a planar array camera.

[0007] Patent document CN115311367A (application number: 202210875082.9) discloses a calibration method and system for the internal parameters of an airborne hyperspectral linear array combined large field-of-view camera. This method equivalentizes three cameras into a virtual camera, comprehensively describes the influence of various geometric distortions using an empirical model, creates a digital calibration field using high-resolution airborne camera data with known parameters, and obtains the control point cloud through matching to complete the calibration of the camera interior orientation elements. This method gets rid of the dependence on a high-precision ground calibration field, effectively reduces the calibration cost, and improves the calibration accuracy and reliability. However, this method belongs to on-orbit calibration of a linear array camera and is not suitable for laboratory calibration of the interior orientation elements of a linear array camera.

[0008] Patent document CN116962665A (application number: 202310756168.4) discloses a general method for testing the interior orientation elements and geometric distortions of a camera. By building a test system, leveling the camera to be tested and the collimator, the camera takes pictures, and using the calculation data of the measurement records and centroid results, the principal point, principal distance, and distortion are calculated by the least squares method. This method only needs to change the light source of the test equipment and is a relatively general calibration method for interior orientation elements. However, this method does not consider the thermal stability of the line array camera optical axis and cannot be used for laboratory calibration of the thermal stability of the line array camera optical axis.

[0009] Patent document CN109724623B (application number: 201811597198.0) discloses a two-dimensional calibration method and device for the interior orientation elements of a mapping camera, and the literature "Two-Dimensional High-Precision Calibration Method for the Interior Orientation Elements of a Linear Array Camera" (Optics and Precision Engineering, Vol. 27, No. 8, 2019) discloses a two-dimensional calibration method and device for the interior orientation elements of a mapping camera. This calibration method and device use parallel light to irradiate and image at the geometric center position of the linear array detector in the mapping camera. By changing the azimuth angle and pitch angle of the mapping camera, recording the image point position coordinates in the imaging of the linear array detector, and calculating the interior orientation elements of the mapping camera based on multiple sets of measurement data. This method realizes the calibration of the interior orientation elements of a linear array mapping camera. However, this technology does not consider the thermal stability of the line array camera optical axis and is limited by the measurement accuracy of the camera azimuth angle and pitch angle, and cannot be used for high-precision laboratory calibration of the thermal stability of the line array camera optical axis.

[0010] None of the above-mentioned existing patents and literature can effectively solve the problem of high-precision calibration of the thermal stability of the line array camera's optical axis in the laboratory. Summary of the Invention

[0011] Aiming at the deficiencies in the prior art, the purpose of the present invention is to provide a laboratory calibration system and method for the thermal stability of the line array camera's optical axis.

[0012] A laboratory calibration system for the thermal stability of the line array camera's optical axis according to the present invention includes: a line array camera to be measured 1, a line array camera support structure 3, a collimator 4, a collimator support structure 6, a geometric target 5, and a fixed reference 7;

[0013] The line array camera to be measured 1 and the collimator 4 are respectively fixed on the fixed reference 7 through the line array camera support structure 3 and the collimator support structure 6;

[0014] The geometric target 5 is located on the focal plane of the collimator 4, and the line array camera to be measured 1 is aligned with the collimator 4 to image the geometric target 5.

[0015] Preferably, the size of the geometric target 5 matches the maximum allowable target size of the collimator 4.

[0016] Preferably, the geometric target 5 includes vertical stripes 9 and inclined stripes 8;

[0017] The absolute value of the inclination angle of the inclined stripe 8 is arbitrarily selected within the range of 5° to 85°.

[0018] Preferably, the geometric target 5 includes n groups of vertical stripes 9 and inclined stripes 8; among them, each group of vertical stripes 9 and inclined stripes 8 are arranged arbitrarily left and right; and the n groups of vertical stripes 9 and inclined stripes 8 are completely the same, completely different, or partially the same, and are arranged arbitrarily up, down, left, and right among multiple groups.

[0019] Preferably, the width D of the inclined stripe 8 matches the low integration level of the line array camera to be measured, and the width D of the inclined stripe 8 and the width d of the vertical stripe 9 on the geometric target 5 satisfy the following relationship:

[0020]

[0021] 0 < L up ≤ 8d pixel

[0022] 1d pixel < d cam ≤ 8d pixel

[0023] Among them, γ represents the absolute value of the inclination angle of the inclined stripes on the geometric target, M represents the integration series, d pixel represents the pixel size, f col represents the focal length of the collimator, f cam represents the focal length of the camera to be measured, L up represents the width of the overlapping rectangular area between the TDI detector and the image of the inclined stripes along the direction of the TDI linear array, d cam represents the width of the image of the vertical stripes.

[0024] Preferably, the vertical stripes of the geometric target 5 are multiplexed with the MTF test target.

[0025] According to a method for calibrating the thermal stability of the line array camera optical axis in the laboratory provided by the present invention, the following steps are performed using the above-mentioned system for calibrating the thermal stability of the line array camera optical axis in the laboratory:

[0026] Step S1: Adjust the direction of the geometric target 5 so that the image of the vertical stripes of the geometric target 5 is perpendicular to the linear array direction;

[0027] Step S2: Set the imaging conditions to meet the preset requirements, and use the line array camera 1 to be measured to image the geometric target 5, and continuously record the geometric target image data and camera temperature data at each moment;

[0028] Step S3: Use the geometric target image data at each moment to calculate the temperature drift amounts of the optical axis of the line array camera 1 to be measured in the along-track direction and the cross-track direction;

[0029] Step S4: Plot the change curves of the drift amounts with the camera temperature, and use the fitting method to obtain the thermal stability coefficients of the line array camera optical axis in the along-track direction and the cross-track direction respectively.

[0030] Preferably, the temperature drift amount of the optical axis of the line array camera 1 to be measured in the along-track direction in step S3 includes:

[0031] α = (A t B t -A0B0)cotγ

[0032] Among them, γ represents the absolute value of the inclination angle of the inclined stripes on the geometric target, A0 and B0 respectively represent the centroid coordinates of the vertical stripes and the inclined stripes on the image plane at the 0 moment, A t , B t respectively represent the centroid coordinates of the vertical stripes and the inclined stripes on the image plane at the t moment.

[0033] Preferably, the temperature drift amount of the optical axis of the line array camera 1 to be measured in the cross-track direction in step S3 includes:

[0034] β = OA t -OA0

[0035] Among them, O is the coordinate of the reference pixel, A0 represents the centroid coordinate of the vertical stripe on the image plane at time 0, and A t represents the centroid coordinate of the vertical stripe on the image plane at time t.

[0036] Preferably, the step S4 includes: performing fitting using a first-degree polynomial to obtain the thermal stability coefficients k α and b α in the along-track direction of the line array camera's optical axis, and the thermal stability coefficients k β and b β in the cross-track direction.

[0037]

[0038] Among them, T is the camera temperature, α(T) is the drift amount of the optical axis in the along-track direction when the camera temperature is T, and β(T) is the drift amount of the optical axis in the cross-track direction when the camera temperature is T.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. The present invention solves the problem that it is difficult to calibrate the thermal stability of the line array camera's optical axis with high precision in the laboratory. By using the present invention, high-precision calibration of the thermal stability of the line array camera's optical axis can be achieved in the laboratory, and the calibration accuracy is sub-pixel level. The calibration result can be used to evaluate the on-orbit (or in-flight) positioning accuracy of the line array camera, and release technical risks in advance; the obtained calibration coefficients of the thermal stability of the optical axis can be used for ground correction of the thermal drift of the camera's optical axis, improving the geometric positioning accuracy of the system.

[0041] 2. The structure of the laboratory calibration system for the thermal stability of the line array camera's optical axis proposed by the present invention is simple and has strong adaptability. It can expand the static MTF test function of the line array camera, realize the simultaneous laboratory calibration of the thermal stability of the line array camera's optical axis and the static MTF test, and shorten the development cycle of the line array camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] By reading the following detailed description of the non-limiting embodiments with reference to the accompanying drawings, other features, objects, and advantages of the present invention will become more apparent:

[0043] Figure 1 is the working flow chart of the method for laboratory calibration of the thermal stability of the line array camera's optical axis;

[0044] Figure 2 is the composition diagram of the laboratory calibration system for the thermal stability of the line array camera's optical axis;

[0045] Figure 3 is the basic configuration diagram of the geometric target;

[0046] Figure 4The working principle diagram for measuring the axis drift of a linear array camera using a geometric target in the basic configuration;

[0047] Figure 5 The configuration diagram for the functional expansion of the geometric target;

[0048] Figure 6 The deformed diagram for improving the accuracy of the geometric target;

[0049] Figure 7 shows various deformed diagrams of the geometric target;

[0050] Among them, 1 - the linear array camera to be measured; 2 - the focal plane of the linear array camera; 3 - the support structure of the linear array camera; 4 - the collimator; 5 - the geometric target; 6 - the support structure of the collimator; 7 - the fixed reference; 8 - the inclined stripe; 9 - the vertical stripe; 10 - the TDI detector; 11a - the panchromatic MTF test stripe target; 11b - the multispectral MTF test stripe target. Specific implementation mode

[0051] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0052] Embodiment 1

[0053] A linear array camera axis thermal stability laboratory calibration system provided according to the present invention includes:

[0054] The linear array camera 1 to be measured, the support structure 3 of the linear array camera, the collimator 4, the support structure 6 of the collimator, the geometric target 5, and the fixed reference 7;

[0055] The linear array camera 1 to be measured and the collimator 4 are respectively fixed on the fixed reference 7 through the support structure 3 of the linear array camera and the support structure 6 of the collimator;

[0056] The geometric target 5 is located on the focal plane of the collimator 4, and the linear array camera 1 to be measured is aligned with the collimator 4 to image the geometric target 5.

[0057] Specifically, the geometric target 5 simultaneously includes the vertical stripe 9 and the inclined stripe 8;

[0058] The absolute value of the inclination angle of the inclined stripe 8 on the geometric target 5 can be arbitrarily selected within the range of 5° to 85°;

[0059] The vertical stripe 9 of the geometric target 5 can be multiplexed with the MTF test target;

[0060] To improve the centroid positioning accuracy, the width of the inclined stripe 8 on the geometric target 5 needs to match the low integration level of the linear array camera, and the width of the vertical stripe 9 should not be too large.

[0061] The width D of the inclined stripe 8 and the width d of the vertical stripe 9 on the geometric target 5 satisfy the following relationship:

[0062]

[0063] 0 < L up ≤ 8d pixel

[0064] 1d pixel < d cam ≤ 8d pixel

[0065] In the formula, γ represents the absolute value of the inclination angle of the inclined stripe on the geometric target, M represents the integration level, d pixel represents the pixel size, f col represents the focal length of the collimator, f cam represents the focal length of the camera to be measured, L up represents the width of the overlapping rectangular area between the TDI detector and the image of the inclined stripe along the TDI linear array direction, d cam represents the width of the image of the vertical stripe.

[0066] Specifically, to ensure a sufficient field of view, the size of the geometric target 5 needs to match the maximum allowable target size of the collimator 4.

[0067] Specifically, on the premise of ensuring that both the inclined stripe 8 and the vertical stripe 9 are included, the geometric target 5 can have various configurations, including but not limited to:

[0068] The vertical stripe 9 and the inclined stripe 8 on the geometric target 5 can be arranged arbitrarily left and right;

[0069] The geometric target 5 can include multiple groups of vertical stripes 9 and inclined stripes 8 at the same time. The multiple groups can be exactly the same, completely different, partially the same, and the multiple groups can be arranged arbitrarily up, down, left, and right.

[0070] According to a method for laboratory calibration of the thermal stability of the optical axis of a linear array camera provided by the present invention, the following steps are performed using the laboratory calibration system for the thermal stability of the optical axis of the linear array camera, including:

[0071] Step S1: Adjust the direction of the geometric target 5 so that the image of the vertical stripe 9 of the geometric target 5 is perpendicular to the linear array direction, that is, parallel to the TDI integration direction;

[0072] Step S2: Set the imaging conditions, adjust the brightness of the geometric target 5, and the line array camera 1 to be measured images the geometric target 5, continuously record the geometric target image data and the camera temperature data at each moment;

[0073] Step S3: Use the geometric target image data at each moment to calculate the temperature drift amounts of the line array camera's optical axis in the along-track direction and the cross-track direction;

[0074] Step S4: Plot the curve of the drift amount changing with the camera temperature, and respectively obtain the thermal stability coefficients of the line array camera's optical axis in the along-track direction and the cross-track direction by using the fitting method.

[0075] Specifically, in the step S2, different imaging conditions can be set according to the test purpose.

[0076] In this embodiment, when the test purpose is to calibrate the thermal stability of the camera's optical axis with respect to the temperature of the focal plane assembly, set the focal plane assembly to be powered on for a long time, and do not change the temperature of the line array camera's opto-mechanical body and the ambient temperature;

[0077] When the test purpose is to calibrate the thermal stability of the camera's optical axis with respect to the temperature of the opto-mechanical body, set the focal plane assembly to be powered on for a short time, and actively change the temperature of the line array camera's opto-mechanical body or the ambient temperature.

[0078] Specifically, in the step S3, the calculation formula for the drift amount α (unit: pixel) of the line array camera's optical axis in the along-track direction is as follows:

[0079] α = (A t B t - A0B0)cotγ

[0080] In the formula, γ represents the absolute value of the inclination angle of the inclined stripe on the geometric target, A0 and B0 respectively represent the centroid coordinates of the vertical stripe and the inclined stripe on the image plane at the 0 moment, A t , B t respectively represent the centroid coordinates of the vertical stripe and the inclined stripe on the image plane at the t moment.

[0081] Specifically, in the step S3, the calculation formula for the drift amount β (unit: pixel) of the line array camera's optical axis in the cross-track direction is as follows:

[0082] β = OA t - OA0

[0083] In the formula, O is the reference pixel coordinate, A0 represents the centroid coordinate of the vertical stripe on the image plane at the 0 moment, A t represents the centroid coordinate of the vertical stripe on the image plane at the t moment.

[0084] Specifically, in the step S4, the data measured multiple times can be processed together to obtain the thermal stability coefficients of the line array camera's optical axis in the along-track direction and the cross-track direction by fitting.

[0085] Embodiment 2

[0086] Embodiment 2 is a preferred example of Embodiment 1

[0087] A certain line array camera adopts the TDI push-broom imaging system, with a focal length of 3.5 m. The focal plane consists of 3 pieces of 8k detectors, the pixel size is 3.5 μm, and the typical integration levels are 8, 16, 32, and 64 levels. The focal length of the collimator is 14.0 m, and the maximum allowable size of the target is 14 mm × 14 mm.

[0088] The following is to apply the present invention to determine the laboratory calibration scheme for the thermal stability of the line array camera's optical axis with respect to the temperature of the focal plane assembly. The implementation process is as Figure 1 shown:

[0089] Step S1: Build a laboratory calibration system for the thermal stability of the line array camera's optical axis, and adjust the direction of the geometric target so that the image of the vertical stripe of the geometric target is perpendicular to the line array direction, that is, parallel to the TDI integration direction;

[0090] Step S2: Set the imaging conditions, adjust the brightness of the geometric target, and the line array camera images the geometric target, continuously recording the geometric target image data and the camera temperature data at each moment;

[0091] Step S3: Use the geometric target image data at each moment to calculate the temperature drift amounts of the line array camera's optical axis in the along-track direction and the cross-track direction;

[0092] Step S4: Plot the curve of the drift amount changing with the camera temperature, and use the fitting method to obtain the thermal stability coefficients of the line array camera's optical axis in the along-track direction and the cross-track direction respectively.

[0093] The laboratory calibration system for the thermal stability of the line array camera's optical axis described in Step S1 is as Figure 2 shown, including:

[0094] The line array camera 1 to be measured and its support structure 3, the collimator 4 and its support structure 6, and the geometric target 5;

[0095] The line array camera 1 to be measured and the collimator 4 are respectively fixed on the same reference 7 through their support structures 3 and 6;

[0096] The geometric target 5 is located on the focal plane of the collimator 4. The line array camera 1 to be measured is aligned with the collimator 4 to image the geometric target 5, and the geometric target 5 is imaged on the focal plane 2 of the line array camera.

[0097] The geometric target 5 described in the laboratory calibration system for the thermal stability of the line array camera's optical axis in Step S1 is asFigure 3 As shown in the figure, the features are as follows:

[0098] The geometric target 5 is composed of inclined stripes 8 and vertical stripes 9;

[0099] The absolute value of the inclination angle of the inclined stripes 8 on the geometric target 5 can be arbitrarily selected within the range of 5° to 85°. Specifically, the inclination angle of the inclined stripes 8 is selected to be 45°;

[0100] The width D of the inclined stripes 8 on the geometric target 5 is matched with the low integration level M of the linear array camera, and the width d of the vertical stripes 9 should not be too large. Specifically, the integration level M = 8 is set, the width D of the inclined stripes 8 = 99 μm, and the width d of the vertical stripes 9 = 56 μm.

[0101] The length L of the inclined stripes 8 and the length l of the vertical stripes 9 on the geometric target 5 are matched with the maximum allowable target size of the collimator. Specifically, the length L of the inclined stripes 8 = 18.808 mm and the length l of the vertical stripes 9 = 13.300 mm are set, within the range of the maximum allowable target size of 14 mm × 14 mm.

[0102] The imaging conditions described in step S2 are set as follows: keeping the ambient temperature constant, the linear array camera is turned on for long-term imaging, the integration level M = 8, and the camera temperature data is the temperature data of the focal plane assembly.

[0103] The target brightness described in step S3 is set as follows: the DN value of the geometric target image exceeds half saturation and is not saturated.

[0104] The DN value characteristics of the image formed by the geometric target 5 described in step S3 are as Figure 4 shown. The DN value of the image formed by the inclined stripes 8 on the TDI detector 10 changes in a trapezoidal shape with the change of the pixel number, and the DN value of the image formed by the vertical stripes 9 on the TDI detector 10 changes in a rectangular shape with the change of the pixel number. Specifically, the bottom width of the trapezoid is 18 pixels, the top width is 2 pixels, and the width of the rectangle is 4 pixels.

[0105] In the said step S3, the calculation formula for the drift amount α (unit: pixel) of the line array camera optical axis in the along-track direction is as follows:

[0106] α = (A t B t -A0B0)cotγ

[0107] In the formula, γ = 45°, representing the inclination angle of the inclined stripes 9 on the geometric target 5, A0 and B0 respectively represent the centroid coordinates of the vertical stripes 9 and the inclined stripes 8 on the image plane at time 0, and A t 、B t respectively represent the centroid coordinates of the vertical stripes 9 and the inclined stripes 8 on the image plane at time t.

[0108] In the step S3, the calculation formula of the drift amount β (unit: pixel) of the line array camera's optical axis in the cross-track direction is as follows:

[0109] β = OA t - OA0

[0110] In the formula, O is the reference pixel coordinate, A0 represents the centroid coordinate of the vertical stripe 9 on the image plane at time 0, and A t represents the centroid coordinate of the vertical stripe 9 on the image plane at time t.

[0111] In the step S4, the multiple measurement data are processed together to fit and obtain the thermal stability coefficients of the line array camera's optical axis in the along-track direction and the cross-track direction. Specifically, a first-degree polynomial is used for fitting to obtain the thermal stability coefficients k α , b α of the line array camera's optical axis in the along-track direction, and the thermal stability coefficients k β , b β in the cross-track direction.

[0112]

[0113] Among them, T is the temperature of the focal plane assembly, α(T) is the drift amount of the optical axis in the along-track direction when the temperature of the focal plane assembly is T, and β(T) is the drift amount of the optical axis in the cross-track direction when the temperature of the focal plane assembly is T.

[0114] Embodiment 3

[0115] Embodiment 3 is a preferred example of Embodiment 1

[0116] A certain line array camera adopts the TDI push-broom imaging system, with a focal length of 3.5 m. The focal plane consists of 3 pieces of 8k detectors, the pixel size is 3.5 μm, and the typical integration levels are 8, 16, 32, and 64 levels. The focal length of the collimator is 14.0 m, and the maximum allowable size of the target is 14 mm × 14 mm.

[0117] To save time, it is necessary to simultaneously perform the thermal stability calibration of the camera's optical axis with respect to the temperature of the focal plane assembly and the static MTF test of the camera.

[0118] Next, the present invention is applied to determine the laboratory calibration of the thermal stability of the line array camera's optical axis with respect to the temperature of the focal plane assembly and the static MTF test scheme of the camera. The implementation process is as Figure 1 shown:

[0119] Step S1: Build a laboratory calibration system for the thermal stability of the line array camera's optical axis, and adjust the direction of the geometric target so that the image of the vertical stripe of the geometric target is perpendicular to the line array direction, that is, parallel to the TDI integration direction;

[0120] Step S2: Set the imaging conditions, adjust the brightness of the geometric target, and the linear array camera images the geometric target, continuously recording the geometric target image data and camera temperature data at each moment;

[0121] Step S3: Using the geometric target image data at each moment, calculate the temperature drift amounts of the linear array camera's optical axis in the along-track direction and the cross-track direction;

[0122] Step S4: Plot the change curves of the drift amounts versus the camera temperature, and respectively obtain the thermal stability coefficients of the linear array camera's optical axis in the along-track direction and the cross-track direction by using the fitting method.

[0123] Specifically, the geometric target 5 in the linear array camera optical axis thermal stability laboratory calibration system described in step S1 is as Figure 5 shown, and its features are as follows:

[0124] The geometric target 5 is composed of inclined stripes 8 and an MTF test stripe target 11;

[0125] The MTF test stripe target 11 may include a panchromatic MTF test target 11a and a multispectral MTF test target 11b.

[0126] The MTF test stripe target 11 is independently used for the MTF test of the linear array camera and also serves as a vertical stripe 9 to jointly complete the calculation of the temperature drift amounts of the linear array camera's optical axis in the along-track direction and the cross-track direction with the inclined stripes 8.

[0127] The remaining settings and operations are the same as those in Embodiment 2.

[0128] Embodiment 4

[0129] Embodiment 4 is a preferred example of Embodiment 1

[0130] A certain linear array camera adopts a TDI push-broom imaging system, with a focal length of 3.5 m, a focal plane composed of 3 pieces of 8k detectors, a pixel size of 3.5 μm, and typical integration levels of 8, 16, 32, and 64 levels. The focal length of the collimator is 14.0 m, allowing a maximum target size of 14 mm × 14 mm.

[0131] Next, apply the present invention to determine the laboratory calibration scheme for the thermal stability of the linear array camera's optical axis with respect to the opto-mechanical body temperature. The implementation process is as Figure 1 shown:

[0132] Step S1: Set up the linear array camera optical axis thermal stability laboratory calibration system, and adjust the direction of the geometric target so that the image of the vertical stripes of the geometric target is perpendicular to the linear array direction, that is, parallel to the TDI integration direction;

[0133] Step S2: Set the imaging conditions, adjust the brightness of the geometric target, and the linear array camera images the geometric target, continuously recording the geometric target image data and camera temperature data at each moment;

[0134] Step S3: Calculate the temperature drift of the linear array camera's optical axis in the along-track direction and cross-track direction by using the geometric target image data at each moment.

[0135] Step S4: Plot the curve of the drift against the camera temperature, and use the fitting method to obtain the thermal stability coefficients of the linear array camera's optical axis in the along-track direction and cross-track direction respectively.

[0136] To ensure that the temperature of the opto-mechanical main body is uniform and stable, in Step S1, the thermal stability laboratory calibration system of the linear array camera's optical axis is placed in a vacuum chamber; in Step S2, the imaging conditions are: set different thermal equilibrium temperature points, and after the temperature of the camera opto-mechanical main body reaches thermal equilibrium, the focal plane assembly is powered on for a short time.

[0137] In the said Step S4, a first-degree polynomial is used for fitting to obtain the thermal stability coefficient k α , b α in the along-track direction of the linear array camera's optical axis, and the thermal stability coefficient k β , b β in the cross-track direction.

[0138]

[0139] Wherein, T is the temperature of the camera main body, α(T) is the along-track direction optical axis drift when the temperature of the camera main body is T, and β(T) is the cross-track direction optical axis drift when the temperature of the camera main body is T.

[0140] The remaining settings and operations are the same as those in Embodiment 2.

[0141] Embodiment 5

[0142] Embodiment 5 is a preferred example of Embodiment 1

[0143] On the premise of ensuring that both inclined stripes and vertical stripes are included, the geometric target can have various configurations.

[0144] The deformation diagram of the geometric target accuracy improvement is as Figure 6 shown. The geometric target 5 is composed of two groups of inclined stripes 8 with opposite inclination angles and two groups of vertical stripes 9. The advantage of this geometric target is that it provides two sets of measurement values, and the measurement accuracy of the linear array camera's optical axis drift can be improved by taking the average. At the same time, by using this geometric target, the drift α (unit: pixel) of the linear array camera's optical axis in the along-track direction can be determined by measuring the change in the distance between points B and D, and the calculation formula is as follows:

[0145]

[0146] As Figure 7aAs shown, the geometric target 5 is composed of a set of inclined stripes and a set of vertical stripes, and the two form a "V" shape. This geometric target can be used for the thermal stability calibration of the line array camera's optical axis;

[0147] As Figure 7b shown, the geometric target 5 is composed of a "cross" shaped target. The upper and lower widths of the vertical stripes are different, which is used to distinguish whether the line array camera's optical axis is located in the upper half area or the lower half area. This geometric target can be used for the thermal stability calibration of the line array camera's optical axis;

[0148] As Figure 7c shown, the geometric target 5 is composed of inclined stripes and upper and lower groups of MTF test stripes. The left and right positions of the two groups of MTF test stripes are different, which is used to distinguish whether the line array camera's optical axis is located in the upper half area or the lower half area. This geometric target can be used for both the thermal stability calibration of the line array camera's optical axis and static MTF testing.

[0149] As Figure 7d shown, the geometric target is composed of a "K" shaped stripe and a group of MTF test stripes. The lower part of the vertical line of the "K" shaped stripe is missing, which is used to distinguish whether the line array camera's optical axis is located in the upper half area or the lower half area. This geometric target can be used for both the thermal stability calibration of the line array camera's optical axis and static MTF testing.

[0150] As Figure 7e shown, the geometric target is composed of a "V" shaped stripe, inclined stripes and a group of MTF test stripes. The upper and lower parts of the geometric target are different, which is used to distinguish whether the line array camera's optical axis is located in the upper half area or the lower half area. This geometric target can be used for both the thermal stability calibration of the line array camera's optical axis and static MTF testing.

[0151] As Figure 7f shown, the geometric target is composed of upper and lower groups of inclined stripes and a group of MTF test stripes. Through continuous observation, it can be distinguished whether the line array camera's optical axis crosses regions (from the upper half area to the lower half area, or from the lower half area to the upper half area) during the calibration process. This geometric target can be used for both the thermal stability calibration of the line array camera's optical axis and static MTF testing.

[0152] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments. Those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of this application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A laboratory calibration system for the thermal stability of the optical axis of a linear array camera, characterized in that, Including: A linear array camera to be measured (1), a linear array camera support structure (3), a collimator (4), a collimator support structure (6), a geometric target (5), and a fixed reference (7); The linear array camera to be measured (1) and the collimator (4) are respectively fixed on the fixed reference (7) through the linear array camera support structure (3) and the collimator support structure (6); The geometric target (5) is located on the focal plane of the collimator (4), and the linear array camera to be measured (1) is aligned with the collimator (4) to image the geometric target (5).

2. The line array camera optical axis thermal stability laboratory calibration system according to claim 1, characterized in that, The size of the geometric target (5) matches the maximum allowable target size of the collimator (4).

3. The line array camera optical axis thermal stability laboratory calibration system according to claim 1, wherein, The geometric target (5) includes vertical stripes (9) and inclined stripes (8); The absolute value of the inclination angle of the inclined stripe (8) is arbitrarily selected within the range of 5° to 85°.

4. The line array camera optical axis thermal stability laboratory calibration system according to claim 3, wherein The geometric target (5) includes n groups of vertical stripes (9) and inclined stripes (8); among them, each group of vertical stripes (9) and inclined stripes (8) are arranged arbitrarily left and right; and the n groups of vertical stripes (9) and inclined stripes (8) are completely the same, completely different, or partially the same, and the multiple groups are arranged arbitrarily up, down, left, and right.

5. The line array camera optical axis thermal stability laboratory calibration system according to claim 3, characterized in that, The width D of the inclined stripe (8) matches the low integration level of the linear array camera to be measured, and the width D of the inclined stripe (8) and the width d of the vertical stripe (9) on the geometric target (5) satisfy the following relationship: Among them, γ represents the absolute value of the inclination angle of the inclined stripes on the geometric target, M represents the integration series, d pixel represents the pixel size, f col represents the focal length of the collimator, f cam represents the focal length of the camera to be measured, L up represents the width of the overlapping rectangular area between the TDI detector and the image of the inclined stripes along the TDI linear array direction, d cam represents the width of the image of the vertical stripes.

6. The line array camera optical axis thermal stability laboratory calibration system according to claim 3, characterized in that, The vertical stripes of the geometric target (5) are multiplexed with the MTF test target.

7. A laboratory calibration method for the thermal stability of the optical axis of a linear array camera, characterized in that, Using the linear array camera optical axis thermal stability laboratory calibration system according to any one of claims 1 to 6, perform the following steps: Step S1: Adjust the direction of the geometric target (5) so that the image of the vertical stripes of the geometric target (5) is perpendicular to the linear array direction; Step S2: Set the imaging conditions to meet the preset requirements, use the linear array camera to be measured (1) to image the geometric target (5), and continuously record the geometric target image data and camera temperature data at each moment; Step S3: Use the geometric target image data at each moment to calculate the temperature drift amounts of the optical axis of the linear array camera to be measured (1) in the along-track direction and the cross-track direction; Step S4: Plot the change curve of the drift amount with the camera temperature, and respectively obtain the thermal stability coefficients of the linear array camera optical axis in the along-track direction and the cross-track direction by using the fitting method.

8. The method for calibrating the thermal stability of the line array camera's optical axis in a laboratory according to claim 7, characterized in that, The temperature drift amount of the optical axis of the linear array camera to be measured (1) in the along-track direction in step S3 includes: α=(A t B t -A0B0)cotγ Among them, γ represents the absolute value of the inclination angle of the inclined stripes on the geometric target, A0 and B0 respectively represent the centroid coordinates of the vertical stripes and the inclined stripes on the image plane at time 0, A t , B t respectively represent the centroid coordinates of the vertical stripes and the inclined stripes on the image plane at time t.

9. The on-axis thermal stability laboratory calibration method for a linear array camera according to claim 7, wherein The temperature drift amount of the optical axis of the linear array camera to be measured (1) in the cross-track direction in step S3 includes: β = OA t -OA0 Among them, O is the coordinate of the reference pixel, and A0 represents the centroid coordinate of the vertical stripe on the image plane at time 0, and A t represents the centroid coordinate of the vertical stripe on the image plane at time t.

10. The method for laboratory calibration of the thermal stability of the line array camera optical axis according to claim 7, characterized in that The step S4 includes: performing fitting using a first-degree polynomial to obtain the thermal stability coefficients k α , b α in the along-track direction of the line array camera's optical axis, and the thermal stability coefficients k β , b β . Where, T is the camera temperature, α(T) is the along-track direction optical axis drift amount when the camera temperature is T, and β(T) is the cross-track direction optical axis drift amount when the camera temperature is T.

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