A method for calibrating the field of view of a multi-field lidar

Abstract

This invention discloses a method for calibrating the field of view of a multi-field lidar system. Based on the fact that the overlap factor varies with the angle between the transmitting and receiving optical axes, the field of view of the receiving system is calculated, thereby achieving accurate calculation of the field of view of a multi-field marine lidar system. This facilitates the inversion of optical and biological parameters of water bodies using multi-field lidar. The method of this invention can achieve field of view calibration measurements in a laboratory environment, cleverly overcoming the difficulty in field of view calculation caused by the difficulty in measuring the field stop and focal length. It is simple to operate, has high measurement accuracy, and obtaining high-precision multi-field angle information further improves the inversion accuracy of multi-field marine lidar.

Description

Technical Field

[0001] This invention belongs to the field of marine lidar technology, and in particular relates to a method for calibrating the field of view of a multi-field lidar. Background Technology

[0002] The ocean covers 71% of the Earth's surface and contains abundant and diverse resources and energy. To expand our living and development space and solve the problem of resource shortage, we must explore the ocean, learn about marine science, and fully develop and utilize marine resources.

[0003] Current ocean exploration methods include in-situ instruments, passive remote sensing, and active remote sensing. Among these, lidar in the field of active remote sensing has gained widespread attention and become an important ocean exploration tool due to its advantages such as day and night detection capabilities, latitude-independent operation, and the ability to acquire vertical ocean profile information. Ocean lidar utilizes the flight time and energy intensity of laser pulses to obtain water parameters. The flight time of the pulse reflects the seawater depth, and the energy intensity reflects the inherent optical properties of the water body. However, due to the complexity and density of seawater, laser light undergoes strong multiple scattering, resulting in temporal and spatial pulse broadening. This causes the lidar attenuation to deviate from ideal conditions, posing problems for the inversion of the inherent optical properties of the water body. In future work, by setting up multiple field-of-view receiving channels and extracting signals from these channels, multiple scattering processes can be analyzed. This will not only obtain higher-precision water optical parameters but also enable the detection of characteristics such as particle size distribution and chlorophyll content, making it one of the key areas for future development.

[0004] In the use and inversion process of multi-field-of-view (MFVR) lidar, the field of view of the system needs to be calibrated. In existing research, Chinese patent document CN107976686A discusses the selection of the field of view for marine lidar operating in the blue-green laser band based on a parameter sensitivity evaluation method. However, this method only optimizes the setup of multi-field-of-view marine lidar and does not mention how to calculate the field of view. Chinese patent CN107831485A proposes a data inversion method for shipborne multi-field-of-view lidar, but it assumes that the field of view is known and does not consider the difficulty of measuring the field of view.

[0005] Therefore, there is an urgent need to propose a field-view calibration method that can be applied to multi-field-of-view lidar, so as to provide accurate system field-view parameters for the inversion of multi-field-of-view lidar. Summary of the Invention

[0006] To address the issue that existing multi-field lidar inversion methods for probing inherent optical properties of ocean water require prior acquisition of the field-view angles of different fields of view, this invention provides a multi-field lidar field-view angle calibration method that enables high-precision measurement of the field-view angle of the system's receiving channel in a laboratory environment.

[0007] A method for calibrating the field of view of a multi-field lidar includes the following steps:

[0008] (1) Prepare a calibration surface for the receiving field of view of the multi-field lidar and place it in front of the lidar system;

[0009] (2) Adjust the laser emission direction of the laser by using an adjustable reflector, calculate the vertical distance between the laser emission position and the calibration surface, and denot it as d;

[0010] (3) Open the receiving channel that needs to be calibrated, and move the position of the laser spot using the adjustable reflector. If the signal is within the field of view, the amplitude of the laser waveform signal will not change. Record the amplitude of the signal at this time as V. m ;

[0011] (4) Move the laser spot from within the field of view along a certain direction to the edge of the field of view. When the signal amplitude begins to decrease, slow down the movement of the laser spot. When the signal amplitude drops to 0.5V... m At that moment, record the position of the laser spot and mark it as point A1;

[0012] (5) Move the light spot back into the field of view and continue moving the light spot in a different direction; find two new edge points in the same way as in step (4) and mark them as points B1 and C1;

[0013] (6) Measure the distances between points A1, B1, and C1, and denot them as a1, b1, and c1 respectively. Then, the field of view (FOV1) of the receiving field of view to be calibrated is expressed as:

[0014]

[0015] Where r is the projection radius of the receiving field of view onto the calibration plane, calculated from a1, b1, and c1:

[0016]

[0017]

[0018] (7) Repeat steps (4)-(6) to measure three sets of data in total. Take the average value of the measured field of view to obtain the final field of view calibration result.

[0019] (8) Let the size of the other receiving field of view of the lidar be FOV. i, i = 2, 3...; Change the field of view and repeat the above steps (3)-(7) to achieve field angle calibration for multiple fields of view.

[0020] This invention calculates the field of view of the receiving system based on the change of the overlap factor as the angle between the transmitting and receiving optical axes changes, thereby realizing the accurate calculation of the field of view of the multi-field marine lidar system, which helps to realize the inversion of water optical characteristic parameters and biological parameters of multi-field lidar.

[0021] Optionally, in step (1), a large flat plate or a flat wall is used as the calibration surface. The calibration surface should be as flat as possible and as perpendicular to the laser emission direction as possible to improve the accuracy of the field of view calibration.

[0022] In step (2), the reflector is adjusted using Thorlabs' right-angle optical adjustment frame KCB1E / M to achieve fine adjustment of the laser beam up, down, left, and right.

[0023] The specific process for calculating the vertical distance between the laser emission position and the calibration surface is as follows:

[0024] When the peak value of the echo signal received by the detector is closest to the zero mark, record the corresponding coordinate axis position n2; use the zero-meter calibration plate to calibrate the laser emission position n1, and the vertical distance d between the laser emission position and the calibration surface is expressed as:

[0025]

[0026] Where c represents the speed of light.

[0027] In step (4), the change in signal amplitude is based on the overlap factor principle of the lidar system. When the laser spot is within the lidar's field of view, the detector receives the laser signal and has a response waveform. At this time, the overlap factor is 1. At the edge of the field of view, as the spot gradually moves out of the edge, the overlap factor will change, and the signal amplitude will rapidly decay from the peak value. The position of the spot corresponding to the decrease of half the signal amplitude is used as the marked position of the edge of the field of view.

[0028] The overlap factor is a parameter describing the coupling efficiency between the echo signal and the receiver, and it is affected by the angle between the laser optical axis and the detector optical axis. Moving the laser spot changes the angle between the laser and detector optical axes. At the edge of the field of view, as the laser spot moves, the overlap factor changes, and the signal also changes accordingly. This invention uses an overlap factor model to describe the process of signal change.

[0029] During the calibration process in steps (4) and (5), the calibration surface needs to be kept as far away from the device as possible, so that the radius r of the field of view projected onto the calibration surface is more than 100 times the diameter of the laser spot. At the same time, appropriately increasing the vertical distance d between the laser emission position and the calibration surface and using a flatter calibration surface can effectively increase the calibration accuracy of the field of view.

[0030] In step (8), multiple field angles can be calibrated through multiple experiments, or multiple channel field angles can be calibrated simultaneously.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. This invention enables field angle calibration and measurement of multiple fields of view in a laboratory environment.

[0033] 2. The field of view measurement method of the present invention cleverly overcomes the problem of difficulty in field of view calculation caused by the difficulty in measuring the field of view aperture and focal length. It is easy to operate and has high measurement accuracy.

[0034] 3. This invention obtains high-precision multi-field-of-view information through high-precision field-of-view calibration, which helps to improve the inversion accuracy of multi-field-of-view marine lidar. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of a multi-field-of-view lidar field-of-view calibration method according to the present invention;

[0036] Figure 2 This is a flowchart of a multi-field-of-view lidar field-of-view calibration method according to the present invention;

[0037] Figure 3 This is a schematic diagram illustrating the vertical distance between the laser emission position and the calibration surface based on the laser ranging principle.

[0038] Figure 4 This is a schematic diagram illustrating how the signal amplitude changes as the light spot moves during the calibration process.

[0039] Figure 5 The result diagram shows the marking of the edge position of the calibration surface field of view in the calibration experiment. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0041] like Figure 1The diagram illustrates the marine lidar calibrated by this invention. The receiving system has three fields of view designed to be 40 mrad, 80 mrad, and 200 mrad. The laser and telescope are fixed in the same plane. The lidar's orientation is adjusted to keep the telescope's optical axis horizontal, i.e., perpendicular to the calibration plane. After receiving the echo signal, the telescope detects the signal strength using a PMT (Precision Measurement Machine). Connecting a PC to the data processing system allows for a visual observation of the signal's changing trend on the computer screen, enabling adjustments to the laser spot's movement accordingly.

[0042] The central idea of ​​this invention is that when the laser spot moves on the calibration surface of the receiving field of view, the overlap factor between the lidar echo signal and the receiving channel will change. The receiving channels of different fields of view will receive lidar echo signals in different ranges. This can be used to calculate the field of view range of the receiving system, and further realize high-precision calibration of the field of view angle of multi-field lidar under laboratory conditions.

[0043] The specific operation process of this invention is as follows: Figure 2 As shown, the multi-field-of-view lidar field-of-view calibration method is implemented through the following steps:

[0044] Step S1: Prepare a large flat panel or a flat wall as the calibration surface for the multi-field lidar receiving field of view, and place it in front of the lidar system.

[0045] Step S2: Adjust the device angle so that the laser is emitted horizontally. Use a lidar light source to measure the distance and obtain the following results: Figure 3 The signal amplitude changes over time. When the peak value of the received echo signal waveform is closest to the zero mark, record the horizontal coordinate position n2 at this moment; using a zero-meter calibration plate, calibrate the laser emission position n1. The vertical distance d between the laser emission position and the calibration surface can be expressed as:

[0046]

[0047] Where c represents the speed of light.

[0048] Step S3: Open the receiving channel that needs to be calibrated. Use Thorlabs' right-angle optical adjustment frame KCB1E / M to move the spot position. If the signal is within the field of view, the amplitude of the laser waveform signal will not change. Record the amplitude of the signal at this time as V. m .

[0049] Step S4: Move the laser spot from within the field of view along a certain direction to the edge of the field of view. When the signal amplitude begins to decrease, slow down the movement of the laser spot. When the signal amplitude drops to 0.5V... m At that moment, record the position of the laser spot and mark it as point A1. The position of the spot at the edge and the corresponding signal amplitude change are as follows: Figure 4 As shown.

[0050] The change in signal amplitude is based on the overlap factor principle of the lidar system. The overlap factor is a parameter describing the coupling efficiency between the echo signal and the receiver. When the laser spot is within the lidar's field of view, the detection system can receive the laser signal and has a response waveform. At this time, the overlap factor is 1, and the signal amplitude remains at its maximum value. As the spot gradually moves to the edge, the signal amplitude rapidly attenuates from its peak. This invention utilizes the overlap factor principle to describe the signal amplitude attenuation process. Theoretically, signal attenuation is a very fast process, with the corresponding spot movement distance on the order of millimeters (related to the laser spot diameter). However, in actual calibration, due to the influence of diffuse reflection from the calibration surface, the actual spot movement distance is approximately 1-2 cm. The specific value is determined by the distance d between the laser emission position and the calibration surface, as well as the flatness of the calibration surface. The theoretical relationship between spot movement and signal amplitude change in actual experiments is as follows: Figure 4 As shown. According to the overlap factor theory model, the position where the signal amplitude drops by half will not change due to diffuse reflection. Therefore, this invention uses the spot position corresponding to the signal amplitude dropping by half as the marker position at the edge of the field of view. When moving at the edge, the speed of the spot movement needs to be slowed down in order to accurately find the spot position corresponding to the signal amplitude dropping by half.

[0051] Step S5: Move the light spot back into the field of view, and continue moving the light spot in a different direction; find two new edge points in the same way and mark them as points B1 and C1.

[0052] Step S6: Measure the distances between points A1, B1, and C1, denoted as a1, b1, and c1. Then, the field of view (FOV1) of the receiving field of view to be calibrated can be expressed as:

[0053]

[0054] Where r is the projection radius of the receiving field of view on the calibration plane, which can be calculated from a1, b1, and c1:

[0055]

[0056]

[0057] Step S7: Repeat steps S4-S6 to measure a total of three sets of data. Take the average value of the measured field of view to obtain the final field of view calibration result.

[0058] Step S8: Record the other receiving field of view sizes of the lidar as FOV2 and FOV3. Change the field of view and repeat the above steps S3-S7 to achieve field of view angle calibration for multiple fields of view.

[0059] The final field-of-view edge marking results are as follows Figure 5 As shown, three sets of data were measured for each field of view, resulting in nine marker points at the edge of each field of view. The distances between each set of marker points are shown in Table 1 below.

[0060] Table 1

[0061]

[0062] The relative errors between the calibrated field of view and the theoretically designed field of view in this invention are 1.6%, 4.0%, and 8.5%, respectively. These relative errors mainly originate from measurement errors, including deviations in the position of the marked edge points and errors in the spacing between them. Increasing the spacing between the selected mark points and using a more precise measurement method to measure the length between them can effectively reduce these measurement errors. During laser scanning, the laser spot diameter must be much smaller than the radius r of the field of view projected onto the calibration surface. Simultaneously, appropriately increasing the vertical distance d between the laser emission position and the calibration surface, and using a flatter calibration surface, can effectively increase the calibration accuracy of the field of view.

[0063] In this embodiment, the multi-field lidar field angle calibration method of the present invention enables the calibration and measurement of multiple fields of view in a laboratory environment. It cleverly overcomes the difficulty in calculating the field of view caused by the difficulty in measuring the field stop and focal length. The method is simple to operate and has high measurement accuracy, proving the effectiveness and convenience of the present invention.

[0064] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calibrating the field of view of a multi-field lidar, characterized in that, Includes the following steps: (1) Prepare a calibration surface for the receiving field of view of the multi-field lidar and place it in front of the lidar system; (2) Adjust the laser emission direction of the laser by using an adjustable reflector, calculate the vertical distance between the laser emission position and the calibration surface, and denot it as d; (3) Open the receiving channel that needs to be calibrated, and move the position of the laser spot using the adjustable reflector. If the signal is within the field of view, the amplitude of the laser waveform signal will not change. Record the amplitude of the signal at this time as V. m ; (4) Move the laser spot from within the field of view along a certain direction to the edge of the field of view. When the signal amplitude begins to decrease, slow down the movement of the laser spot. When the signal amplitude drops to 0.5V... m At that moment, record the position of the laser spot and mark it as point A1; (5) Move the light spot back into the field of view and continue moving the light spot in a different direction; Find two new edge points using the same method as step (4), and mark them as points B1 and C1; (6) Measure the distances between points A1, B1, and C1, and denot them as a1, b1, and c1 respectively. Then, the field of view (FOV1) of the receiving field of view to be calibrated is expressed as: Where r is the projection radius of the receiving field of view onto the calibration plane, calculated from a1, b1, and c1: (7) Repeat steps (4)-(6) to measure three sets of data in total. Take the average value of the measured field of view to obtain the final field of view calibration result. (8) Let the size of the other receiving field of view of the lidar be FOV. i , i = 2, 3, ...; Change the field of view and repeat the above steps (3)-(7) to achieve field angle calibration for multiple fields of view.

2. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, In step (1), a large flat plate or a flat wall is used as the calibration surface, and the calibration surface is marked with scale.

3. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, In step (2), the reflector is adjusted using Thorlabs' right-angle optical adjustment frame KCB1E / M to achieve fine adjustment of the laser beam up, down, left, and right.

4. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, In step (2), the specific process of calculating the vertical distance between the laser emission position and the calibration surface is as follows: When the peak value of the echo signal received by the detector is closest to the zero mark, record the corresponding coordinate axis position n2; use the zero-meter calibration plate to calibrate the laser emission position n1, and the vertical distance d between the laser emission position and the calibration surface is expressed as: Where c represents the speed of light.

5. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, In step (4), when the laser spot is within the field of view of the lidar, the detector receives the laser signal and has a response waveform. At this time, the overlap factor is 1, and the signal amplitude remains at its maximum value. At the edge of the field of view, as the spot gradually moves out of the edge, the overlap factor will change, and the signal amplitude will rapidly decrease from the peak value. The position of the spot corresponding to the decrease of half the signal amplitude is used as the mark position of the edge of the field of view.

6. The multi-field-of-view lidar field-of-view calibration method according to claim 5, characterized in that, The overlap factor is a parameter describing the coupling efficiency between the echo signal and the receiver, and it is affected by the angle between the laser optical axis and the detector optical axis; moving the spot means changing the angle between the laser optical axis and the detector optical axis.

7. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, During the calibration process in steps (4) and (5), the calibration surface needs to be kept as far away from the device as possible so that the radius r of the field of view projected onto the calibration surface is more than 100 times the diameter of the laser spot.

8. The multi-field-of-view lidar field-of-view calibration method according to claim 1, characterized in that, In step (8), multiple field angles are calibrated through multiple experiments, or multiple channel field angles are calibrated simultaneously.

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