A method for 3D imaging reconstruction of polarization-modulated lidar based on phase calibration

Abstract

The present invention relates to the field of photoelectric detection and imaging technology, and specifically discloses a polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration. A polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration includes: step S01: obtaining a calibration polarization image, performing phase calculation on the pixel points on the calibration polarization image, and obtaining a phase value; step S02: drawing a phase-distance curve according to the phase value, and selecting an area showing a linear change according to the phase-distance curve; step S03: fitting the phase values ​​corresponding to all pixel points in the area showing a linear change, calculating the polynomial coefficients corresponding to each pixel point, and obtaining a fitting phase-distance curve; step S04: solving the target image to be measured according to the fitting phase-distance curve, obtaining a target distance image, and completing three-dimensional imaging reconstruction.

Description

Technical Field

[0001] The present invention relates to the field of photoelectric detection and imaging technology, and in particular to a polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration. Background Art

[0002] Traditional 3D reconstruction methods for polarization-modulated LiDAR generally use a four-step phase shift method to perform phase calculation using phase images of different polarization directions captured by a polarization camera. After obtaining the phase, the laser flight time can be obtained based on the relationship between the modulation phase, voltage, and time in electro-optical modulation, and then the distance to the target can be obtained to achieve 3D reconstruction.

[0003] At present, there is a distortion phenomenon when using this classic method for three-dimensional reconstruction. The reason for this phenomenon is that during the modulation process of the crystal, due to the conoscopic interference characteristics of the crystal, the modulated phase is different for incident light at different angles. Its phase change is nonlinear within a certain field of view, that is, the phase change curve of each point on the detector is different. The traditional method does not take into account the difference in the phase change trend of each point. In the process of modulating the crystal, if the incident light in all directions is calculated according to the same phase, it will cause a large ranging error. When performing three-dimensional reconstruction under a larger field of view, a large distance error will occur. Therefore, it is necessary to propose a ranging method that can correct the ranging error caused by the interference characteristics of the crystal. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a three-dimensional imaging reconstruction method of polarization modulation laser radar based on phase calibration. The different change trends of the phase curve of each pixel point on the detector of the laser radar modulated by electro-optical crystal are taken into account. Before three-dimensional reconstruction, the phase of each pixel point is calibrated, and the phase curve is fitted using relevant mathematical methods to obtain a phase-distance curve. By directly bringing the phase obtained using the traditional four-step phase shift method into the phase-distance curve, the distance can be solved and three-dimensional reconstruction can be achieved. Reconstruction using this method can improve the distance accuracy of three-dimensional reconstruction under a larger field of view, and effectively suppress the distortion caused by traditional methods. Compared with traditional reconstruction methods, it can have a larger effective field of view and higher distance accuracy.

[0005] The first technical solution adopted by the present invention is: a polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration, comprising:

[0006] Step S01: obtaining a calibration polarization image, performing phase calculation on pixel points on the calibration polarization image, and obtaining a phase value;

[0007] Step S02: drawing a phase-distance curve according to the phase value, and selecting a region showing a linear change according to the phase-distance curve;

[0008] Step S03: fitting the phase values ​​corresponding to all the pixels in the area showing linear changes, calculating the polynomial coefficients corresponding to each pixel, and obtaining a fitted phase-distance curve;

[0009] Step S04: solving the target image to be measured according to the fitted phase-distance curve to obtain the target range image and complete the three-dimensional imaging reconstruction.

[0010] Preferably, a four-step phase shift algorithm is used to calculate the phase value of every four adjacent pixels on the calibration polarization image, and when the phase values ​​of all the pixels on the calibration polarization image are calculated, the phase value acquisition is completed.

[0011] Preferably, the calculation formula of the phase value is:

[0012] ;

[0013] in, , , , These are the light intensities received by the CCD pixels behind the micro-polarizers with polarization directions of 0°, 45°, 90°, and 135° on the micro-polarizer array.

[0014] Furthermore, the fitting in step S03 is a 3rd order polynomial fitting or an Nth order polynomial fitting.

[0015] Preferably, the polynomial coefficients are:

[0016] ;

[0017] in, is the phase value of the pixel, is the fitting coefficient, Measure the distance to the target.

[0018] Preferably, the method for acquiring the image of the target to be measured is to perform gated imaging on the target to be measured, with the gated distance being the target distance, to obtain the image of the target to be measured.

[0019] Furthermore, it also includes performing phase calculation on the target image to be measured to obtain a phase image.

[0020] Preferably, the method for obtaining the calibration polarization image includes using a laser to emit laser light to irradiate the calibration plate, which is received by a receiving system, and after modulation and calculation, the calibration polarization image is obtained.

[0021] Preferably, after each calibration polarization image is obtained, the voltage applied to the crystal is delayed by 0.75 ns, and the image is taken again to obtain another calibration polarization image.

[0022] Beneficial effects of the above technical solution:

[0023] The method provided by the present invention obtains the phase-distance curve of the laser radar system by calibrating the flat target. After the phase diagram is calculated using the captured polarization image, it is brought into the phase-distance curve to obtain a three-dimensional image of the target. Compared with the traditional method, the advantage of the present invention is that it can eliminate the distortion phenomenon caused by the crystal interference characteristics in the traditional method, can achieve very significant correction of the distance error, and significantly improve the distance accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A flow chart of the polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration provided by the present invention;

[0025] Figure 2 A schematic diagram of using adjacent pixels to calculate phase according to an embodiment of the present invention;

[0026] Figure 3 A phase-distance curve provided for one embodiment of the present invention;

[0027] Figure 4 A three-dimensional distance map calculated using the method provided by the present invention;

[0028] Figure 5 It is the three-dimensional distance map directly calculated using the phase formula;

[0029] Figure 6 An original image in mono format taken by a polarization camera provided in one embodiment of the present invention;

[0030] Figure 7 A phase map calculated from a 1024×768 screenshot of a central area provided by an embodiment of the present invention;

[0031] Figure 8 A three-dimensional distance top view of an electric fan provided by an embodiment of the present invention;

[0032] Fig. 9 A three-dimensional distance side view of an electric fan provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The following is a further detailed description of the implementation methods of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than an exhaustive list of all the embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0034] The terms "first", "second", etc. (if any) in the specification and claims are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way are interchangeable where appropriate so that the embodiments described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products, or devices.

[0035] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0036] Embodiment 1

[0037] Figure 1 The process of the polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration provided by the present invention is shown. It includes:

[0038] 1. Acquisition of calibration polarization images.

[0039] When performing phase calibration, the laser first emits a laser beam to the calibration plate. After being received by the receiving system, it is modulated and calculated to obtain the polarization image of the calibration plate. After each shot, the crystal voltage delay is increased by 0.75ns (equivalent to moving the calibration plate backward 11.25cm), and the shot is taken again (15cm corresponds to a laser flight time of 1ns) until the cumulative moving distance reaches 6m. A total of 41 calibration images are obtained.

[0040] 2. Calculation of phase image.

[0041] Schematic diagram of using adjacent pixels to calculate phase Figure 2 As shown, for every four adjacent pixels of the polarization image, the phases of these four points can be calculated through a four-step phase shift algorithm.

[0042] ;

[0043] in, , , , They represent the light intensity received by the CCD pixel behind the micro-polarizer with polarization directions of 0°, 45°, 90°, and 135° on the micro-polarizer array. When calculating the phase of scattered light, it is necessary to use four adjacent pixel points and substitute them into formula (1.1) to calculate the phase values ​​corresponding to the four pixel center points. Similarly, the phase values ​​corresponding to the center points of four different adjacent pixels are calculated. The phases of all pixels in the entire image are calculated according to the above method, and the overall phase can be calculated in the end.

[0044] 3. Calculate the linear region of the phase diagram.

[0045] For each pixel point in the phase image, a phase-distance curve is drawn with its phase value as the ordinate and the distance (gating time) as the abscissa.

[0046] Figure 3 In the figure, the horizontal axis represents the distance between the calibration plate and the system; the vertical axis represents the phase of a certain pixel in the phase image; the calculation result of the phase value corresponding to a certain pixel when the calibration plate is placed at a specific distance is obtained; the corresponding curve represents the area where the phase-distance curve changes monotonically as a whole; the corresponding curve represents the area where the phase-distance curve changes linearly as a whole; the third-order polynomial fitting of the curve corresponding to the yellow area.

[0047] 4. Perform third-order or higher polynomial fitting on the phase-distance curve with good linearity.

[0048] After selecting the linear region range of all pixels, for each pixel position, these phase values ​​are brought into the third-order polynomial or N-order polynomial fitting to obtain the polynomial coefficients corresponding to each pixel.

[0049] ;

[0050] in, is the phase value of the pixel, is the fitting coefficient, Measure the distance to the target. Finally, bring in the target image to be solved, and bring the phase of each point into the polynomial corresponding to the pixel point for solution. Because the linear monotonic region is selected, there is a unique solution, which is the distance of this point. The distance range corresponding to this area with good linearity is the system's three-dimensional imaging depth of field.

[0051] 5. Input polarization image.

[0052] For any target, the gated distance should be roughly the target distance. After obtaining the polarization image of the target, calculate its phase image according to the method in step 2 for subsequent calculations.

[0053] 6. Calculate the distance based on the calibration results.

[0054] Substitute the phase value of the pixel point in the phase image of the target into the high-order polynomial obtained in step 4 for solution. Since the phase-distance curve obtained by fitting is monotonic, a unique solution for the distance can be obtained, which is the distance value corresponding to this pixel point. Substitute all pixel points into their corresponding polynomials for solution to obtain the target range image.

[0055] Figure 4 and Figure 5 is the three-dimensional imaging result of the flat target, where Figure 4 The three-dimensional distance map calculated by the method of the present invention has an overall distance measurement mean square error (MSE) of 0.004 meters. Figure 5 The three-dimensional distance map obtained by direct calculation has been severely distorted, and the plane has been twisted into a curved surface, making it impossible to achieve high-precision three-dimensional imaging of the target under a specific field of view angle.

[0056] Simulation results of the method of the present invention

[0057] In order to verify whether the method of the present invention can image a high-speed moving target, a three-dimensional imaging experiment was conducted on a high-speed rotating fan. The imaging results are shown in Figure 2. Figures 6 to 9 shown. Figure 6 The original mono format image taken by the polarization camera, Figure 7 The phase map calculated from the screenshot of the 1024×768 range in the center area, Figure 8 3D distance diagram of electric fan (top view), Fig. 9 3D distance map of electric fan (side view). It can be seen that the application of this method can achieve good 3D imaging of high-speed moving fans without blur, indicating that 3D imaging of moving targets at a certain speed can be achieved.

[0058] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made on the basis of the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A polarization modulation laser radar three-dimensional imaging reconstruction method based on phase calibration, characterized in that: include: Step S01: obtaining a calibration polarization image, performing phase calculation on pixel points on the calibration polarization image, and obtaining a phase value; Step S02: drawing a phase-distance curve according to the phase value, and selecting a region showing a linear change according to the phase-distance curve; Step S03: fitting the phase values ​​corresponding to all the pixels in the area showing linear changes, calculating the polynomial coefficients corresponding to each pixel, and obtaining a fitted phase-distance curve; Step S04: solving the target image to be measured according to the fitted phase-distance curve to obtain the target range image and complete the three-dimensional imaging reconstruction; Performing a four-step phase shift algorithm on every four adjacent pixels on the calibration polarization image to calculate the phase value, and completing the phase value acquisition after the phase value calculation is performed on all the pixels on the calibration polarization image; The calculation formula of the phase value is: Among them, I0, I 45 ,I 90 ,I 135 These are the light intensities received by the CCD pixels behind the micro-polarizers with polarization directions of 0°, 45°, 90°, and 135° on the micro-polarizer array.

2. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 1 is characterized in that: The fitting in step S03 is a 3rd order polynomial fitting or an Nth order polynomial fitting.

3. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 1, characterized in that: The polynomial coefficients are: in, is the phase value of the pixel, B i is the fitting coefficient, and D is the target measurement distance.

4. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 1, characterized in that: The method for acquiring the image of the target to be measured is to perform gated imaging on the target to be measured, with the gated distance being the target distance, to obtain the image of the target to be measured.

5. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 1, characterized in that: The method also includes performing phase calculation on the target image to be measured to obtain a phase image.

6. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 1, characterized in that: The method for obtaining the calibration polarization image includes: using a laser to emit laser light to irradiate the calibration plate, which is received by a receiving system, and after modulation and calculation, the calibration polarization image is obtained.

7. The method for 3D imaging reconstruction of polarization modulation laser radar based on phase calibration according to claim 6, characterized in that: After each calibration polarization image is obtained, the voltage applied to the crystal is delayed by 0.75 ns, and the image is taken again to obtain another calibration polarization image.

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