A method and system for measuring sky polarization angle based on vortex wave plate

Abstract

The present invention discloses a method and system for measuring the sky polarization angle using a vortex wave plate in the field of sky polarization navigation technology. The method for measuring the sky polarization angle using a vortex wave plate comprises the following steps: building a sky polarization angle measurement system based on a vortex wave plate; calibrating the color error of sky wide spectrum light incident on the vortex wave plate; measuring and calibrating the sky polarization angle; and outputting the heading angle of the sky polarization angle measurement system. The method and system for measuring the sky polarization angle using a vortex wave plate have a simple optical path structure and strong compatibility, high measurement accuracy, simple operation process, and low cost, and realize fast and high-precision measurement of the sky polarization angle.

Description

Technical Field

[0001] The present invention relates to the technical field of sky polarized light navigation, and in particular to a method and system for measuring sky polarization angles by using a vortex wave plate. Background Art

[0002] Navigation using the polarization distribution of scattered light from the sky originates from the polarization navigation mechanism of organisms such as sand ants. It is a navigation technology with no cumulative error, strong anti-interference and low cost, and can be used as an important auxiliary autonomous navigation method. The detection technology of sky polarization pattern is the core of polarized light navigation. According to whether the polarization state of the sky in multiple directions can be obtained during measurement, the current sky polarization pattern detection technology can be divided into single-point and multi-point types: the single-point type can only be used to measure the polarization state of a single point in a certain direction in the sky, while the multi-point type can measure the polarization state from multiple directions in the sky. According to the different structures of the polarization sensors used during detection, the existing sky polarization pattern detection devices can be divided into two types: unit detectors (such as photodiodes) and array detectors (such as cameras): the measurement method based on unit detectors is mostly used in conjunction with linear polarizers for polarization detection, which is generally single-point; the measurement method based on array detectors can be used in conjunction with linear polarizers, processed polarization masks, or directly using polarization cameras for sky polarization pattern detection, which is generally multi-point.

[0003] Sky polarization pattern detection devices based on planar array detectors are generally more robust and adaptable than polarization detection devices based on unit detectors because they can simultaneously perform polarization measurements in multiple directions of the sky, have a large field of view, and provide more measurement information. For detection devices used with linear polarizers, polarization detection at three or more angles is required, and generally a time-sequentially rotating polarizer or multiple cameras are used, which results in long measurement times or difficulty in coaxial calibration. For detection devices used with polarization masks or polarization cameras, the sensors need to solve the problem of precise alignment between camera pixels and masks in the early processing stage, which is difficult to process, costly, and has low polarization extinction. When the above-mentioned polarization detection devices use linear polarizers to detect polarization states, most of them need to use "time division" or "space division" methods to perform polarization measurements at multiple angles, which inevitably complicates the structure or operation process.

[0004] It was reported in the literature and papers with publication numbers CN 105203102 A and CN 109059898 A (Fan Y, Zhang R, Liu Z, et al. A Skylight Orientation Sensor Based on S-Waveplate and LinearPolarizer for Autonomous Navigation [J]. IEEE Sensors Journal, 2021, 21 (20): 23551-23557) that a polarization detection method based on vector light field spatial modulation was introduced into sky polarization navigation, and radial polarizers (composed of an S-waveplate and a linear polarizer) were used to replace traditional linear polarizers. There is no need to use "time division" or "space division" methods, which improves the stability of the structure and the real-time performance of the measurement. However, due to the need to image and obtain spatial polarization modulation images, the optical imaging structures of the existing methods are mostly complex: Patent No. CN 105203102 A reports that the diffuse light spot formed by the S wave plate is imaged by the microlens array of the light field camera, and the polarization state of the sky in multiple directions can be imaged, but the spatial resolution of the image is sacrificed, so the polarization angle measurement accuracy is not high; Fan et al. reported that a fisheye lens and a relay lens are used to image the spatial modulation image formed by the S wave plate onto the camera photosensitive surface, but the introduction of the relay lens will lead to a complex optical path structure and significantly increase the volume of the device, and the introduction of polarization conversion devices such as the S wave plate into the imaging optical path of the lens will change the original flange distance of the lens, making the imaging effect worse. In addition, the manufacturing process of the S wave plate is complex, the cost is high, and the difficulty of mass production is relatively large. The vortex wave plate, as a spatial modulation polarization device with similar functions to the S wave plate, is made of liquid crystal materials, the process is relatively mature, the cost is low, and the spatial modulation effect formed by the combination of the linear polarizer is excellent.

[0005] Based on the above analysis, it is necessary to study a method and system for measuring the sky polarization angle using vortex wave plates, which has a simple structure, low cost, easy use, high measurement accuracy and fast speed, so as to promote the popularization and application of sky polarization light navigation technology. Summary of the invention

[0006] In order to solve the above problems, the purpose of the present invention is to provide a method and system for measuring the sky polarization angle using a vortex wave plate, so as to solve the technical problems of the existing sky polarization mode measurement method in the prior art, such as complex operation process, poor mechanical structure stability, poor compatibility of optical path structure, high cost, low precision, etc., without changing the optical path structure of the traditional imaging system, only need to install a polarization conversion device in front of the conventional imaging system, the optical path structure is simple and compatible, the measurement accuracy is high, the operation process is simple, and the cost is low.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] As one aspect of the present invention, a method for measuring sky polarization angle using a vortex wave plate is provided, comprising the following steps:

[0009] S1. Build a sky polarization angle measurement system based on vortex wave plates;

[0010] S2. Calibrate the complex color error of the vortex wave plate with wide spectrum sky light incident on it;

[0011] S3. Measure and calibrate the sky polarization angle;

[0012] S4. Output the heading angle of the sky polarization angle measurement system.

[0013] As a method for measuring sky polarization angle using a vortex wave plate according to the above aspect of the present invention, S1 includes the following steps:

[0014] S11. Installing a polarization conversion device: Installing a set of polarization conversion devices in front of a conventional imaging device, and installing the polarization conversion device in front of a lens without disassembling and changing the original optical path structure;

[0015] S12. Install a high-precision turntable: Install the sky polarization angle measurement system based on the vortex wave plate on a high-precision turntable, and use the angle of the turntable as the accuracy reference for the angle measurement for subsequent error calibration.

[0016] As a method for measuring sky polarization angle using a vortex wave plate in the above aspect of the present invention, S2 includes the following steps:

[0017] S21. Collect multiple sky polarization images at different polarization angles;

[0018] S22. Calculate the polarization angle to obtain the complex color error;

[0019] S23. Calibrate the complex color error of incident sky wide spectrum light.

[0020] As a method for measuring sky polarization angle using a vortex wave plate in the above aspect of the present invention, S21 includes the following steps:

[0021] S211. When a wide-spectrum partially linearly polarized light such as sky polarized light passes through a vortex wave plate and a linear polarizer, a modulation pattern with a wedge-shaped light-dark distribution is formed, whose light intensity changes with the polarization angle of the incident light. The light intensity modulation function is:

[0022]

[0023] Where D represents the polarization degree of the incident light, α represents the angle between the polarization direction of the incident light and the horizontal direction, represents the azimuth angle on the vortex wave plate; K is a constant related to the spectral distribution of the incident light. It is a fixed value when the incident light is under the same light source condition and always satisfies -1≤K≤1;

[0024] S212. The light intensity modulation function is a function that changes with the polarization direction α with a period of 180°. During calibration, it is only necessary to quickly rotate the turntable 180°, and the camera cooperates to collect images at different polarization angles.

[0025] As a method for measuring sky polarization angle using a vortex wave plate according to the above aspect of the present invention, S22 includes the following steps:

[0026] S221. Process and solve the collected multi-angle polarization images to obtain the calculated polarization angle α Cal ;

[0027] S222. Due to the actual polarization angle α Real and calculate the polarization angle α Cal There is a certain deviation, which is expressed by the color error δα, that is, δα=α Cal -α Real .

[0028] As a method for measuring sky polarization angle using a vortex wave plate according to the above aspect of the present invention, S23 includes the following steps:

[0029] S231. During the calibration process, the polarization angle α 45° , α 135° As the center, the rotation angle θ of the turntable is taken as the reference, and the color error δα=α Cal -α 45° / 135° -θ;

[0030] S232. Calibrate the polychromatic errors of the 180° polarization angle collected in step S221, and fit the polychromatic error calibration function Δ(α Cal ).

[0031] As a method for measuring sky polarization angle using a vortex wave plate according to the above aspect of the present invention, S3 includes the following steps:

[0032] S31. Take a polarized image of the sky;

[0033] S32. Based on the polarization angle calculation algorithm, the captured sky polarization image is preliminarily processed and calculated to obtain α Cal ;

[0034] S33. Using the complex color error calibration function Δ(αCal ), calibrate and obtain the final measured polarization angle α=α Cal -Δ(α Cal ).

[0035] As a method for measuring sky polarization angle using a vortex wave plate according to the above aspect of the present invention, S4 includes the following steps:

[0036] S41. Calculate the angle between the sky polarization direction and the north direction according to the distribution law of the sky polarization pattern;

[0037] S42. In combination with the sky polarization angle α measured in step S33, output the heading angle of the sky polarization angle measurement system.

[0038] As another aspect of the present invention, a system for measuring the sky polarization angle using a vortex wave plate is provided, comprising a conventional imaging device and a polarization conversion device installed in front of the conventional imaging device; the conventional imaging device comprises a lens and an image sensor; the polarization conversion device comprises a vortex wave plate and a polarizer.

[0039] By adopting the above technical solution, the present invention has the following advantages:

[0040] The present invention provides a method and system for measuring the sky polarization angle using a vortex wave plate. The optical path structure is simple and has strong compatibility. It only needs to add a polarization conversion device to a conventional optical imaging system without deconstructing and reconstructing the original optical path, which simplifies the operation of polarization measurement and reduces the system cost. The sky polarization angle measurement system using a vortex wave plate only includes a polarizer, a vortex wave plate, a lens and a camera, and the distance between each device is compact. The overall weight and volume of the system are small, the structure is simple, and it is easy to integrate. The sky polarization angle measurement method proposed by the present invention utilizes the polarization state change of the incident vortex wave plate of wide-spectrum light and the law of the spatial distribution of light intensity, and realizes the rapid and high-precision measurement of the sky polarization angle through the method combining polarization angle solution with complex color error calibration, which has high application value in the navigation field of miniaturized unmanned systems (such as drones, unmanned vehicles, etc.). BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a schematic diagram of a sky polarization angle measurement system using a vortex wave plate according to the present invention;

[0042] Figure 2 It is an operation flow chart of the method for measuring the sky polarization angle using a vortex wave plate of the present invention;

[0043] Figure 3 This is a schematic diagram of the distribution of the first-order vortex wave plate in the fast axis direction;

[0044] Figure 4Schematic diagram of the effect of incident light with different polarization directions passing through the first-order vortex half-wave plate and polarizer;

[0045] Figure 5 This is a sky polarization image actually taken by the sky angle measurement system based on vortex wave plates;

[0046] Figure 6 This is the curve of the orientation error changing with the turntable angle before and after the complex color error calibration. DETAILED DESCRIPTION

[0047] The technical solution of the present invention is described in detail below in conjunction with the drawings in the specification. It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0048] A schematic diagram of a system for measuring sky polarization angle using a vortex wave plate is shown in Figure 1. Figure 1 As shown, it includes a conventional imaging device and a polarization conversion device installed in front of the conventional imaging device; the polarization conversion device is installed on the conventional imaging device (lens + image sensor), and the conventional imaging device records the light intensity modulated image formed on the polarization conversion device in front, and further analyzes and processes it to obtain the sky polarization angle. The polarization conversion device is generally composed of a vortex wave plate and a polarizer. The imaging distance (object distance) of the imaging system should generally be as small as possible to ensure that the polarization conversion device can also be installed close to the imaging lens. Clear imaging can be achieved. According to the incident sequence of sky polarized light, the various parts of the sky polarization angle measurement system using a vortex wave plate are a vortex wave plate, a polarizer, a lens, and an image sensor. After passing through the vortex wave plate and the polarizer as an analyzer, part of the polarized light from the sky works together to form a polarization modulated image with a wedge-shaped light and dark distribution of light intensity; the polarization modulated image is imaged onto the photosensitive surface of the image sensor through the lens.

[0049] In some embodiments, the vortex wave plate can be adjusted according to the measurement band. For example, in clear weather conditions, since the scattering of sunlight by the atmosphere is mainly Rayleigh scattering, a vortex wave plate in the blue-violet band can be considered. In some embodiments, if the distance between the polarization conversion device and the lens is difficult to adjust or the lens focus is difficult to meet the requirements of clear imaging, a gasket of appropriate thickness can be added between the lens and the image sensor (i.e., the image distance is increased), and the flange distance between the two is slightly changed to meet the requirements of clear imaging, and there is no effect on the system volume and structural stability.

[0050] An embodiment of the present invention provides a schematic diagram of a sky polarization angle measurement system based on a vortex wave plate. Figure 1 As shown, the sky polarization angle measurement system includes: a vortex wave plate 1, a polarizer 2, a lens 3, a gasket 4, a camera 5, and a computer 6.

[0051] A method for measuring sky polarization angle using a vortex wave plate is as follows: Figure 2 As shown, the following steps are included:

[0052] S1. Build a sky polarization angle measurement system based on vortex wave plates;

[0053] Wherein, S1 comprises the following steps:

[0054] S11. Install the polarization conversion device: Install a set of polarization conversion device in front of the conventional imaging device. Use the threaded interface of the lens and the processed matching connector to install the polarization conversion device in front of the lens. There is no need to disassemble and change the original optical path structure. The operation is simple and the structure is stable.

[0055] S12. Install a high-precision turntable: Install the sky polarization angle measurement system based on the vortex wave plate on a high-precision turntable, and use the angle of the turntable as the accuracy reference for the angle measurement for subsequent error calibration.

[0056] S2. Calibrate the complex color error of the vortex wave plate with wide spectrum sky light incident on it;

[0057] Wherein, S2 includes the following steps:

[0058] S21. Collect multiple sky polarization images at different polarization angles;

[0059] Wherein, S21 comprises the following steps:

[0060] S211. When a wide-spectrum partially linearly polarized light such as sky polarized light passes through a vortex wave plate and a linear polarizer, a modulation pattern with a wedge-shaped light-dark distribution is formed, whose light intensity changes with the polarization angle of the incident light. The light intensity modulation function is:

[0061]

[0062] Where D represents the polarization degree of the incident light, α represents the angle between the polarization direction of the incident light and the horizontal direction, represents the azimuth angle on the vortex wave plate; K is a constant related to the spectral distribution of the incident light. It is a fixed value when the incident light is under the same light source condition and always satisfies -1≤K≤1;

[0063] In formula (1), if the incident light is broad-spectrum light, K is generally not equal to -1. Only when α = 45° / 135°, the third term in formula (1) is 0, and the light intensity modulation function can satisfy the law of cosine variation. However, the existing related polarization angle calculation algorithms are basically based on the cosine law variation, and the calculated polarization angle will have a certain deviation from the actual angle, which is defined as the polychromatic error. Before formally measuring the sky polarization angle, it is necessary to calibrate the polychromatic error through the angle reference of the high-precision turntable.

[0064] S212. Because the sky polarization angle changes slowly and can be considered unchanged in a short period of time, the same effect can be achieved by using a turntable to quickly drive the sky polarization angle measurement system to rotate. The light intensity modulation function in formula (1) is a function that changes with the polarization direction α with a period of 180°, so during calibration, it is only necessary to quickly rotate the turntable 180°, and at the same time, the camera cooperates to collect images at different polarization angles.

[0065] S22. Calculate the polarization angle and obtain the complex color error;

[0066] Wherein, S22 comprises the following steps:

[0067] S221. Processing and calculating the collected multi-angle polarization images to obtain the polarization angle α Cal On the one hand, it is difficult to directly calculate the specific value of K in formula (1) due to the complexity of the spectrum and polarization distribution of sky scattered light; on the other hand, the algorithm for calculating the polarization angle based on the cosine law is simple to implement and fast, and the accuracy of the calculated polarization angle can reach within ±0.1°, but it is still different from the actual polarization angle α. Real There is a certain deviation.

[0068] S222. Due to the actual polarization angle α Real and calculate the polarization angle α Cal There is a certain deviation, which is expressed by the color error δα, that is, δα = α Cal -α Real .

[0069] S23. Calibrate the complex color error of incident sky wide spectrum light.

[0070] Wherein, S23 comprises the following steps:

[0071] S231. When α = 45° / 135°, the light intensity modulation function completely conforms to the law of cosine variation, so the polarization angle calculated at this time does not have a complex color error (δα = 0), that is, α Cal =α Real Therefore, during the calibration process, the polarization angles closest to 45° and 135° (α 45 °、α 135° ) as the center and the rotation angle θ of the turntable as the reference, the color error δα=α Cal -α 45° / 135° -θ.

[0072] S232. Calibrate the polychromatic errors of the 180° polarization angle collected in step S221, and fit the polychromatic error calibration function Δ(α Cal ), which is used for calibration in subsequent actual measurements of sky polarization angles.

[0073] S3. Measure and calibrate the sky polarization angle;

[0074] Among them, S3 includes the following steps:

[0075] S31. Take a polarized image of the sky;

[0076] S32. Based on the polarization angle calculation algorithm, the captured sky polarization image is preliminarily processed and calculated to obtain α Cal ;

[0077] S33. Using the complex color error calibration function Δ(α Cal ), calibrate and obtain the final measured polarization angle α=α Cal -Δ(α Cal ).

[0078] S4. Calculate the angle between the sky polarization direction and the true north direction according to the distribution law of the sky polarization pattern, and then combine it with the sky polarization angle α measured in step S33 to output the system heading angle.

[0079] Formula (1) has theoretically analyzed the polarization state and distribution law of modulated light intensity after the wide-spectrum linear polarized light passes through the vortex wave plate. Figure 3 and Figure 4 The principle of polarization angle detection using vortex wave plates is explained more intuitively and briefly. The vortex wave plate has a constant delay over the entire clear aperture, but its fast axis rotates continuously over the optical region, and the direction of the fast axis can be expressed as:

[0080]

[0081] Where ψ is the fast axis direction of the vortex wave plate, is the azimuth angle of the vortex wave plate, It is the initial In the fast axis direction, m is the order of the vortex wave plate. Since the linear polarization component accounts for the majority of the sky polarized light, its polarization state can be detected using a first-order vortex half-wave plate.

[0082] Figure 3 It shows the distribution of the fast axis direction of the first-order vortex wave plate; Figure 4 The image shows the difference between the polarization state and modulated light intensity of incident linear polarized light with different polarization directions after passing through the vortex half-wave plate. Linear polarized light with three polarization directions of α = 0° (horizontal direction), α = 90° (vertical direction), and α = 45° will become three different polarization vector light fields after passing through the vortex half-wave plate; after passing through the polarizer for polarization analysis, a spatial modulation image with light intensity distribution characteristics related to the polarization direction will be formed, and the polarization angle of the incident light wave can be further calculated through image processing.

[0083] In order to further illustrate the advantages of the present invention, the sky polarization angle measurement experiment is combined to illustrate the feasibility and high precision of the sky polarization angle measurement system and method proposed in the present invention.

[0084] S1: Build a sky polarization angle measurement system based on a vortex wave plate. On the basis of a conventional imaging system, install a polarization measurement device consisting of a vortex wave plate, a linear polarizer, and a high-precision electric turntable.

[0085] In this embodiment, the vortex wave plate used is a vortex half-wave plate of order 1 produced by Jingcui Optics with a working wavelength of 532nm, model LFV1-532; the polarizer used is a thin film linear polarizer with a working wavelength of 400nm-700nm produced by Lubang Technology, model FLP20-VIS, and its extinction ratio can reach 10000:1 at a wavelength of 532nm; the wide-angle lens model is HC0516-5MP, and the effective focal length is 5mm; the camera is a Grasshopper3 series model GS3-U3-41C6M-C monochrome camera produced by FILR, with a resolution of 2048*2048, an imaging target surface size of 1 inch, and a frame rate of up to 90fps; the turntable is a high-precision model X-RST120AK-DE50 from ZABER, which can rotate 360° and the angle positioning accuracy can reach 0.01°.

[0086] S2: Calibrate the complex color error of the sky polarization angle measurement system based on vortex wave plate when incident with wide spectrum sky light.

[0087] First, the electric turntable is controlled to rotate 180°. At the same time, the camera collects polarization images of the sky every 5°. A total of 37 polarization images are collected, and 37 polarization direction angles can be obtained by calculation. The polarization angle closest to 45° and 135° is α 45° =45.5651°、α 135° =135.4634°. Calibration is performed based on the above two angles, and according to the method in S23, the calibration function Δ(α Cal ), the expression is shown in formula (3).

[0088]

[0089] S3: Measure and calibrate the sky polarization angle.

[0090] Using the sky polarization angle measurement system, a sky polarization image is taken. Figure 5 As shown; the image processing method is used to calculate the preliminary polarization direction angle α Cal =117.0823°; further calculate the α Cal Substituting into formula (3) we can obtain δα=Δ(117.0823°)=-0.1125°. After calibrating the complex color error δα, we can obtain the final measured sky polarization angle α=117.1948°.

[0091] S4: Based on the characteristic that the sky polarization angle will not change substantially in a short period of time, the rotation of the turntable is used in the experiment of this embodiment to simulate the change of the heading angle of the carrier, so as to evaluate the orientation accuracy of the sky polarization angle measurement system proposed in the present invention.

[0092] The turntable is controlled to rotate 360° in a short time, and the camera takes a polarization image of the sky every 5°. In this process, 73 polarization images of the sky can be taken, and 73 sky polarization angles can be calculated by the same method as in S3. The difference in polarization angles is the change in the measured heading angle.

[0093] In order to quantitatively describe the orientation accuracy of the sky polarization angle measurement system, the orientation error is defined by the difference in polarization angle between the two images and the difference in the rotation angle of the turntable, represented by ε. For N measurement results, the mean μ and standard deviation σ of the orientation error are defined as:

[0094] In the experiment of this embodiment, the curve of the orientation error before and after the complex color error calibration and the change of the turntable angle is as follows: Figure 6 As shown, from Figure 6It can be seen that when the polychromatic error exists, the orientation error fluctuates with a period of 90°, and the orientation error is within the range of ±0.07°; after the polychromatic error is calibrated, the orientation error of the system can be controlled within ±0.03°. The statistical values ​​of the orientation error before and after the polychromatic error calibration in the experiment of this embodiment are shown in Table 1 below:

[0095] Table 1 Statistical values ​​of orientation error before and after color correction

[0096]

[0097]

[0098] It can be seen from Table 1 that after the complex color error calibration, the statistical quantities such as the mean, standard deviation and maximum error are significantly improved. The mean is close to 0° and the standard deviation is 0.0113°, indicating that the orientation stability and repeatability of the sky polarization angle measurement system are high.

[0099] Finally, it should be pointed out that although the present invention has been described with reference to the current specific embodiments, ordinary technicians in this technical field should realize that the above embodiments are only used to illustrate the present invention and are not used as a limitation of the present invention. Various equivalent changes or substitutions can be made without departing from the concept of the present invention. Therefore, as long as the changes and modifications to the above embodiments are within the essential spirit of the present invention, they will fall within the scope of the claims of the present invention.

Claims

1. A method for measuring sky polarization angle using a vortex wave plate, characterized in that: The following steps are involved: S1. Build a sky polarization angle measurement system based on vortex wave plates; S2. Calibrate the complex color error of the vortex wave plate with wide spectrum sky light incident on it; The S2 comprises the following steps: S21. Collect multiple sky polarization images at different polarization angles; The S21 comprises the following steps: S211. When a wide-spectrum partially linearly polarized light such as sky polarized light passes through a vortex wave plate and a linear polarizer, a modulation pattern with a wedge-shaped light-dark distribution is formed, whose light intensity changes with the polarization angle of the incident light. The light intensity modulation function is: Where D represents the polarization degree of the incident light, α represents the angle between the polarization direction of the incident light and the horizontal direction, represents the azimuth angle on the vortex wave plate; K is a constant related to the spectral distribution of the incident light. It is a fixed value when the incident light is under the same light source condition and always satisfies -1≤K≤1; S212. The light intensity modulation function is a function that changes with the polarization direction α with a period of 180°. During calibration, it is only necessary to quickly rotate the turntable 180°, and the camera cooperates to collect images at different polarization angles. S22. Calculate the polarization angle to obtain the complex color error; The S22 comprises the following steps: S221. Process and solve the collected multi-angle polarization images to obtain the calculated polarization angle α Cal ; S222. Due to the actual polarization angle α Real and calculate the polarization angle α Cal There is a certain deviation, which is expressed by the color error δα, that is, δα = α Cal -α Real ; S23. Calibrate the color error of incident sky wide spectrum light; The S23 comprises the following steps: S231. During the calibration process, the polarization angle α 45° , α 135° As the center, the rotation angle θ of the turntable is taken as the reference, and the color error δα=α Cal -α 45° / 135° -θ; S232. Calibrate the polychromatic errors of the 180° polarization angle collected in step S221, and fit the polychromatic error calibration function Δ(α Cal ); S3. Measure and calibrate the sky polarization angle; S4. Output the heading angle of the sky polarization angle measurement system.

2. The method for measuring sky polarization angle using a vortex wave plate according to claim 1, characterized in that: The S1 comprises the following steps: S11. Installing a polarization conversion device: Installing a set of polarization conversion devices in front of a conventional imaging device, and installing the polarization conversion device in front of a lens without disassembling and changing the original optical path structure; S12. Install a high-precision turntable: Install the sky polarization angle measurement system based on the vortex wave plate on a high-precision turntable, and use the angle of the turntable as the accuracy reference for the angle measurement for subsequent error calibration.

3. The method for measuring sky polarization angle using a vortex wave plate according to claim 2, characterized in that: The S3 comprises the following steps: S31. Take a polarized image of the sky; S32. Based on the polarization angle calculation algorithm, the captured sky polarization image is preliminarily processed and calculated to obtain α Cal ; S33. Using the complex color error calibration function Δ(α Cal ), calibrate and obtain the final measured polarization angle α=α Cal -Δ(α Cal ).

4. The method for measuring sky polarization angle using a vortex wave plate according to claim 3, characterized in that: The S4 comprises the following steps: S41. Calculate the angle between the sky polarization direction and the north direction according to the distribution law of the sky polarization pattern; S42. In combination with the sky polarization angle α measured in step S33, output the heading angle of the sky polarization angle measurement system.

5. A system for measuring sky polarization angle using a vortex wave plate, which is used to implement a method for measuring sky polarization angle using a vortex wave plate as claimed in claim 1, characterized in that: The invention comprises a conventional imaging device and a polarization conversion device installed in front of the conventional imaging device; the conventional imaging device comprises a lens and an image sensor; the polarization conversion device comprises a vortex wave plate and a polarizer.

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