Multispectral laser radar system and NDVI calculation method based on multispectral laser radar system

Abstract

The invention provides a multispectral laser radar system and an NDVI calculation method based on the multispectral laser radar system. The multispectral laser radar system comprises a laser, a transmitting light path, a receiving light path and a single APD. The emitting light path comprises a light splitting light path, a time delay light path and a beam combining light path, the laser generates white light, the white light is divided into two paths through the light splitting light path, one path is emitted into the beam combining light path through time delay of the time delay light path, the other path is directly emitted into the beam combining light path, and the two paths of laser are emitted to the surface of a target after being combined and reflected by the target. And the received light path returns to the single APD for photoelectric detection. The system provided by the invention is low in optical loss, low in complexity and low in cost, the vegetation index can be efficiently obtained, and the provided method can play an important role in agricultural and forestry detection and precise management.

Description

Technical Field

[0001] The present invention relates to the technical field of multispectral laser radar, and in particular to a multispectral laser radar system and an NDVI calculation method based thereon. Background Art

[0002] Multi-wavelength echoes can obtain more intensity information and spectral information, which can provide a more detailed understanding of the characteristics of the target. However, as the number of wavelengths increases, not only will the optical loss increase, but the demand for detectors will also increase, making the system complex and expensive. In order to meet the needs of multiple wavelengths, existing multi-spectral lidars use multiple laser light sources and photodetectors at the same time. There are problems such as large optical loss, high complexity and high cost.

[0003] The Normalized Difference Vegetation Index (NDVI) is one of the most widely used vegetation indices. It is considered one of the best indicators of vegetation and background changes, especially in areas with dense vegetation. Since the spectral vegetation index can be linked to plant pigment concentration and leaf water content, by generating vertical profiles of the spectral index, it is possible to measure plant physiological characteristics and leaf water content, as well as to separate leaves from trees. The efficient acquisition of vegetation NDVI using multispectral lidar is of great significance for agricultural and forestry applications such as precise monitoring and high-precision classification.

[0004] Therefore, it is very meaningful to design a multispectral lidar system and an NDVI calculation method based on it. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, an object of the present invention is to provide a multispectral lidar system and an NDVI calculation method based thereon.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A first aspect of the present invention provides a multi-spectral laser radar system, comprising: a laser, a transmitting optical path, a receiving optical path and a single APD;

[0008] The emission optical path includes a splitting optical path, a delay optical path and a beam combining optical path. The laser generates white light, which is divided into two paths through the splitting circuit. One path is delayed by the delay optical path and is emitted into the beam combining optical path, and the other path is directly emitted into the beam combining optical path. After the two laser beams are combined, they hit the target surface, and after being reflected by the target, they return to the single APD through the receiving optical path for photoelectric detection.

[0009] Preferably, the splitting optical path is a dichroic mirror, and the white light generated by the laser is divided into two paths by the dichroic mirror. One path is a laser with a wavelength of 705-1080nm, which is delayed by the delayed optical path and injected into the combined optical path, and the other path is a laser with a wavelength of 400-660nm, which is injected into the combined optical path.

[0010] Preferably, the delay light path is composed of multiple mirrors, which are used to perform multiple spatial reflections on the laser with a wavelength of 705-1080nm to obtain a time delay. After the laser with a wavelength of 705-1080nm passes through the delay light path, it is injected into an 800nm ​​narrow-band filter to obtain a laser with a wavelength of 800nm, and then injected into the beam combining light path for beam combining.

[0011] Preferably, the laser light with a wavelength of 400-660 nm passes through a 600 nm narrow-band filter before entering the beam combining optical path to obtain the laser light with a wavelength of 600 nm, and then enters the beam combining optical path for beam combining.

[0012] Another aspect of the present invention provides a method for calculating NDVI based on a multispectral laser radar system, comprising:

[0013] Build a multispectral lidar system;

[0014] Photoelectric detection is performed based on a multispectral lidar system, and echo information is collected through an oscilloscope;

[0015] Calibrate the collected echo information;

[0016] Test the ranging performance of the multispectral lidar system;

[0017] Based on the multispectral radar system, the leaves, branches and soil in different health states are detected to obtain their respective NDVI;

[0018] Based on the multispectral radar system, the green plants are scanned as a whole to obtain their 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map.

[0019] Preferably, the collected echo information is calibrated, including:

[0020] Select a reference target with Lanbertian scatterer characteristics and known reflectivity, perform detection, and obtain the echo intensity of the reference target;

[0021] Based on the collected echo intensity and reflectivity of the reference target, the collected echo information is exchanged to obtain the reflectivity that can characterize the physical characteristics of the collected echo information, which is:

[0022]

[0023] Where α is the incident angle, is the target echo signal power, is the reference target echo signal power, ρ ref is the reflectivity of the reference target;

[0024] Calibration is performed based on the collected echo information and reflectivity.

[0025] Preferably, the ranging performance of the multispectral laser radar system is tested, including:

[0026] Place the foam board with the blade fixed on it at a distance of 10m from the laser to collect the echo information of the blade;

[0027] The distance between the blades is calculated using TOF, with the trigger time as the start time and the time corresponding to the echo peak as the end time. The distance is calculated in combination with the speed of light, which is:

[0028]

[0029] Where c is the speed of light, t stop is the time corresponding to the echo peak, t start for the triggering moment;

[0030] Move the blades backward, collect echoes every 15 cm, and obtain echo information at 5 positions;

[0031] Use a handheld rangefinder to obtain a reference distance, compare the system distance measurement result with the distance measurement result of the rangefinder, obtain the relative error of the system, and make adjustments based on the relative error.

[0032] Preferably, the leaves, branches and soil in different health states are detected based on the multispectral radar system to obtain their respective NDVI, specifically:

[0033] Prepare several healthy green leaves, dry leaves, diseased leaves, green branches, yellow branches and soil, and detect the above targets respectively based on the multi-spectral radar system, collecting ten sets of echo information for each target;

[0034] The collected echo information is calibrated, and the respective NDVI is calculated based on the reflectivity for comparison. The calculation method of NDVI is:

[0035]

[0036] In the formula, ρ 800 is the target reflectivity under 800nm ​​echo, ρ 600 is the target reflectivity under 600nm echo;

[0037] Preferably, the green plants are scanned as a whole based on the multispectral radar system to obtain their 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map, specifically:

[0038] The multispectral radar system is set on the motor, and the motor is controlled to rotate based on the control system to drive the multispectral radar system to rotate, scan the green plants as a whole, collect echo information, and record the corresponding motor angle information;

[0039] The collected echo information is preprocessed, and the three-dimensional coordinates of each scanned point and the echo intensity values ​​corresponding to the wavelengths of 600nm and 800nm ​​are obtained in combination with the motor angle information;

[0040] Separate the information of the green plant reflected echo based on the preset threshold;

[0041] The points that generate the secondary echo are processed to obtain the first echo corresponding to each wavelength, and the NDVI is obtained. The 600nm intensity point cloud map and the 800nm ​​intensity point cloud map are drawn respectively;

[0042] According to the 600nm intensity point cloud map and the 800nm ​​intensity point cloud map, the DVI of all valid points is calculated to generate the NDVI point cloud map.

[0043] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0044] The invention provides a multi-spectral laser radar system and an NDVI calculation method based thereon. The system comprises a laser, a transmitting optical path, a receiving optical path and a single APD, and has the advantages of low optical loss, low complexity and low cost. The multi-spectral laser radar system can efficiently obtain NDVI and is easy to use. The system uses a supercontinuum laser and a single pixel detector to realize the detection of dual-wavelength 800nm ​​and 600nm echoes, effectively saving system cost and reducing complexity. The actual scene is analyzed in combination with the radar equation, and the influence of the incident angle on the target reflectivity is studied. Since the constructed system can collect two wavelength echoes in sequence under a single pulse, the incident angle has no effect on its NDVI value. Therefore, the index can be used to effectively characterize the physical characteristics of the target. On this theoretical basis, the ranging accuracy of the system is tested, and NDVI is obtained for different targets to realize the distinction between leaves, branches and soil. In combination with a motor and a collection device, plants are scanned to obtain their NDVI point cloud map, and different parts of the plants are visualized. The effectiveness and efficiency of the system are verified, indicating that it will play an important role in agricultural and forestry detection and precise management. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0046] Figure 1 A schematic diagram of the system structure provided by an embodiment of the present invention;

[0047] Figure 2 It is a flowchart of the NDVI calculation method based on the multispectral lidar system;

[0048] Figure 3 A schematic diagram of the process of generating the overall NDVI point cloud of green plants based on the multispectral radar system;

[0049] Figure 4 This is the NDVI point cloud map. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] The purpose of the present invention is to provide a multispectral lidar system and an NDVI calculation method based thereon. The system has low optical loss, low complexity, and low cost, and can efficiently obtain vegetation index. The method can play an important role in agricultural and forestry detection and precise management.

[0052] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] Figure 1 A schematic diagram of the system structure provided by an embodiment of the present invention, such as Figure 1 As shown, the present invention provides a multi-spectral laser radar system, including: a laser, a transmitting optical path, a receiving optical path and a single APD;

[0054] The emission optical path includes a splitting optical path, a delay optical path and a beam combining optical path. The laser generates white light, which is divided into two paths through the splitting circuit. One path is delayed by the delay optical path and is emitted into the beam combining optical path, and the other path is directly emitted into the beam combining optical path. After the two laser beams are combined, they hit the target surface, and after being reflected by the target, they return to the single APD through the receiving optical path for photoelectric detection.

[0055] The splitting optical path is a dichroic mirror D1. The white light generated by the laser is divided into two paths by the dichroic mirror. One path is a laser with a wavelength of 705-1080nm, which is delayed by the delayed optical path and then injected into the beam combining optical path. The other path is a laser with a wavelength of 400-660nm, which is injected into the beam combining optical path.

[0056] The delay light path is composed of a plurality of reflectors (M1, M2, M3, M4), as shown in FIG. Figure 1 As shown, multiple mirrors are used to perform multiple spatial reflections on the laser with a wavelength of 705-1080nm to obtain a time delay. After passing through the delay optical path, the laser with a wavelength of 705-1080nm is injected into the 800nm ​​narrow-band filter F2 to obtain a laser with a wavelength of 800nm, and then injected into the beam combining optical path for beam combining.

[0057] The laser with a wavelength of 400-660nm passes through a 600nm narrowband filter F1 before entering the beam combining optical path to obtain a laser with a wavelength of 600nm, which is then injected into the beam combining optical path for beam combining.

[0058] like Figure 2 As shown, the present invention also provides a NDVI calculation method based on a multispectral laser radar system, comprising:

[0059] Step 100: Build a multispectral lidar system;

[0060] Step 200: performing photoelectric detection based on a multispectral laser radar system and collecting echo information through an oscilloscope;

[0061] Step 300: calibrating the collected echo information;

[0062] Step 400: testing the ranging performance of the multispectral laser radar system;

[0063] Step 500: Detect leaves, branches and soil in different health states based on a multispectral radar system to obtain their respective NDVIs;

[0064] Step 600: Scan the entire green plant based on the multispectral radar system to obtain its 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map.

[0065] In step 300, the collected echo information is calibrated, including:

[0066] Select a reference target with Lanbertian scatterer characteristics and known reflectivity, perform detection, and obtain the echo intensity of the reference target;

[0067] Based on the collected echo intensity and reflectivity of the reference target, the collected echo information is exchanged to obtain the reflectivity that can characterize the physical characteristics of the collected echo information, which is:

[0068]

[0069] Where α is the incident angle, is the target echo signal power, is the reference target echo signal power, ρ ref is the reflectivity of the reference target;

[0070] Calibration is performed based on the collected echo information and reflectivity.

[0071] In step 400, the ranging performance of the multispectral laser radar system is tested, including:

[0072] Place the foam board with the blade fixed on it at a distance of 10m from the laser to collect the echo information of the blade;

[0073] The distance between the blades is calculated using TOF, with the trigger time as the start time and the time corresponding to the echo peak as the end time. The distance is calculated in combination with the speed of light, which is:

[0074]

[0075] Where c is the speed of light, t stop is the time corresponding to the echo peak, t start for the triggering moment;

[0076] Move the blades backward, collect echoes every 15 cm, and obtain echo information at 5 positions;

[0077] Use a handheld rangefinder to obtain a reference distance, compare the system distance measurement result with the distance measurement result of the rangefinder, obtain the relative error of the system, and make adjustments based on the relative error.

[0078] In step 500, the leaves, branches and soil in different health states are detected based on the multispectral radar system to obtain their respective NDVIs, specifically:

[0079] Prepare several healthy green leaves, dry leaves, diseased leaves, green branches, yellow branches and soil, and detect the above targets respectively based on the multi-spectral radar system, collecting ten sets of echo information for each target;

[0080] The collected echo information is calibrated, and the respective NDVI is calculated based on the reflectivity for comparison. The calculation method of NDVI is:

[0081]

[0082] In the formula, ρ 800 is the target reflectivity under 800nm ​​echo, ρ 600 is the target reflectivity under 600nm echo;

[0083] like Figure 3As shown, in step 600, the green plant is scanned as a whole based on the multispectral radar system to obtain its 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map, specifically:

[0084] The multispectral radar system is set on the motor, and the motor is controlled to rotate based on the control system to drive the multispectral radar system to rotate, scan the green plants as a whole, collect echo information, and record the corresponding motor angle information;

[0085] The collected echo information is preprocessed, and the three-dimensional coordinates of each scanned point and the echo intensity values ​​corresponding to the wavelengths of 600nm and 800nm ​​are obtained in combination with the motor angle information;

[0086] Separate the information of the green plant reflected echo based on the preset threshold;

[0087] The points that generate the secondary echo are processed to obtain the first echo corresponding to each wavelength, and the NDVI is obtained. The 600nm intensity point cloud map and the 800nm ​​intensity point cloud map are drawn respectively;

[0088] According to the 600nm intensity point cloud map and the 800nm ​​intensity point cloud map, the DVI of all valid points is calculated to generate the NDVI point cloud map, where the point cloud map is as follows: Figure 4 shown.

[0089] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0090] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A multispectral laser radar system, characterized in that: include: Laser, transmitting optical path, receiving optical path and single APD; The emission optical path includes a splitting optical path, a delay optical path and a beam combining optical path. The laser generates white light, which is divided into two paths through the splitting circuit. One path is delayed by the delay optical path and is emitted into the beam combining optical path, and the other path is directly emitted into the beam combining optical path. After the two laser beams are combined, they hit the target surface, and after being reflected by the target, they return to the single APD through the receiving optical path for photoelectric detection.

2. The system according to claim 1, characterized in that The splitting optical path is a dichroic mirror. The white light generated by the laser is divided into two paths by the dichroic mirror. One path is a laser with a wavelength of 705-1080nm, which is delayed by the delayed optical path and injected into the beam combining optical path. The other path is a laser with a wavelength of 400-660nm, which is injected into the beam combining optical path.

3. The system according to claim 2, characterized in that The delay light path is composed of a plurality of reflectors, and is used to perform multiple spatial reflections on the laser with a wavelength of 705-1080nm to obtain a time delay. After passing through the delay light path, the laser with a wavelength of 705-1080nm is injected into an 800nm ​​narrow-band filter to obtain a laser with a wavelength of 800nm, and is injected into the beam combining light path for beam combining.

4. The system according to claim 3, characterized in that The laser with a wavelength of 400-660nm passes through a 600nm narrowband filter before entering the beam combining optical path to obtain the laser with a wavelength of 600nm, which is then injected into the beam combining optical path for beam combining.

5. A NDVI calculation method based on a multispectral lidar system, characterized in that: include: Build a multispectral lidar system; Photoelectric detection is performed based on a multispectral lidar system, and echo information is collected through an oscilloscope; Calibrate the collected echo information; Test the ranging performance of the multispectral lidar system; Based on the multispectral radar system, the leaves, branches and soil in different health states are detected to obtain their respective NDVI; Based on the multispectral radar system, the green plants are scanned as a whole to obtain their 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map.

6. The method according to claim 5, characterized in that Calibrate the collected echo information, including: Select a reference target with Lanbertian scatterer characteristics and known reflectivity, perform detection, and obtain the echo intensity of the reference target; Based on the collected echo intensity and reflectivity of the reference target, the collected echo information is exchanged to obtain the reflectivity that can characterize the physical characteristics of the collected echo information, which is: Where α is the incident angle, is the target echo signal power, is the reference target echo signal power, ρ ref is the reflectivity of the reference target; Calibration is performed based on the collected echo information and reflectivity.

7. The method according to claim 5, characterized in that Test the ranging performance of the multispectral lidar system, including: Place the foam board with the blade fixed on it at a distance of 10m from the laser to collect the echo information of the blade; The distance between the blades is calculated using TOF, with the trigger time as the start time and the time corresponding to the echo peak as the end time. The distance is calculated in combination with the speed of light, which is: Where c is the speed of light, t stop is the time corresponding to the echo peak, t start for the triggering moment; Move the blades backward, collect echoes every 15 cm, and obtain echo information at 5 positions; Use a handheld rangefinder to obtain a reference distance, compare the system distance measurement result with the distance measurement result of the rangefinder, obtain the relative error of the system, and make adjustments based on the relative error.

8. The method according to claim 5, characterized in that Based on the multispectral radar system, the leaves, branches and soil in different health states are detected to obtain their respective NDVI, specifically: Prepare several healthy green leaves, dry leaves, diseased leaves, green branches, yellow branches and soil, and detect the above targets respectively based on the multi-spectral radar system, collecting ten sets of echo information for each target; The collected echo information is calibrated, and the respective NDVI is calculated based on the reflectivity for comparison. The calculation method of NDVI is: In the formula, ρ 800 is the target reflectivity under 800nm ​​echo, ρ 600 is the target reflectivity under 600nm echo.

9. The method according to claim 5, characterized in that Based on the multispectral radar system, the green plants are scanned as a whole to obtain their 600nm intensity point cloud map, 800nm ​​intensity point cloud map and NDVI point cloud map, specifically: The multispectral radar system is set on the motor, and the motor is controlled to rotate based on the control system to drive the multispectral radar system to rotate, scan the green plants as a whole, collect echo information, and record the corresponding motor angle information; The collected echo information is preprocessed, and the three-dimensional coordinates of each scanned point and the echo intensity values ​​corresponding to the wavelengths of 600nm and 800nm ​​are obtained in combination with the motor angle information; Separate the information of the green plant reflected echo based on the preset threshold; The points that generate the secondary echo are processed to obtain the first echo corresponding to each wavelength, and the NDVI is obtained. The 600nm intensity point cloud map and the 800nm ​​intensity point cloud map are drawn respectively; According to the 600nm intensity point cloud map and the 800nm ​​intensity point cloud map, the DVI of all valid points is calculated to generate the NDVI point cloud map.

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