An underwater particulate matter detection lidar detection system and detection method based on Shack-Hartmann imaging technology

Abstract

The present invention discloses an underwater particulate matter detection lidar detection system and a detection method based on Shack imaging technology, including a transmitting system, a receiving system, and a detection system. The transmitting system includes a continuous-wave 532nm laser, a laser collimator, and a half-wave plate. The receiving system includes a receiving mirror, a focusing lens, and a band-pass color filter. The detection system includes a CMOS black-and-white camera. The underwater small-angle backscattering lidar detection system based on Shack imaging technology of the present invention adopts a multi-polarization information acquisition scheme to simultaneously measure multiple parameters in the water body components. Compared with traditional water body detection, it has a simple and compact structure and can be used for portable on-water observation; combined with a non-dispersive optical structure, multi-polarization information detection is completed through a special CMOS detector.

Description

Technical Field

[0001] The present invention relates to the technical field of marine exploration, and particularly to an underwater particulate matter detection lidar detection system and detection method based on Schlieren imaging technology. Background Art

[0002] For water body detection, the detection of water quality and suspended particulate matter is particularly important. There is a complex coupling relationship between water quality and suspended particulate matter. On the one hand, the characteristic changes of various suspended particulate matters in the water body will cause changes in water quality, and on the other hand, the changes in water quality will trigger complex evolutions of the components of water body suspended particulate matter. However, rivers, river mouths and the ocean provide necessary production and domestic water for humans. Therefore, the real-time detection of water body quality and suspended particulate matter is very important. Since lidar belongs to an active detection technology, its vertical profile measurement characteristics can provide effective data support for all-weather high spatio-temporal resolution detection of water bodies.

[0003] Generally speaking, to achieve large-scale and rapid measurement of water bodies, remote sensing measurement technology needs to be used. The profiling method is only suitable for water bodies with a water depth greater than 10 meters, and the instrument is expensive and the operation is complex; the method above the water surface is greatly affected by sunlight and cannot obtain the characteristic information of underwater particulate matter well.

[0004] In view of the above problems, the present invention provides an underwater particulate matter detection lidar detection system and detection method based on Schlieren imaging technology, which has a simple and compact structure, small volume, and is portable for observation. Summary of the Invention

[0005] The present invention provides an underwater particulate matter detection lidar detection system and detection method based on Schlieren imaging technology, which has a simple and compact structure, small volume, and is portable for observation.

[0006] The object of the present invention is to provide an underwater particulate matter detection lidar detection system based on Schlieren imaging technology, including a transmitting system, a receiving system and a detection system. The transmitting system includes a continuous-wave 532nm laser, a laser collimator and a half-wave plate. The receiving system includes a receiving mirror, a focusing lens and a band-pass color filter. The detection system includes a CMOS black-and-white camera.

[0007] Further, a regularly combined four-direction linear polarizer is covered in front of the photosensitive element of the CMOS black-and-white camera, and the four-direction polarizer is in the directions of 0°, 45°, 90° and 135°.

[0008] An underwater particulate matter detection lidar detection method based on Schlieren imaging technology, the method includes the following steps:

[0009] Step 1: Use a continuous-wave 532nm laser to generate a 532nm wavelength linearly polarized laser light source with a high degree of linear polarization;

[0010] Step 2: Use a laser collimator to shape the continuous linearly polarized light source with a wavelength of 532 nm into collimated light, reduce the laser divergence angle, and then incident it into the water body;

[0011] Step 3: Use a half-wave plate to rotate the polarization direction of the 532-nm linearly polarized laser to ensure that the laser polarization direction is aligned with the 0° polarization direction of the detection system;

[0012] Step 4: Use a focusing lens to receive the backscattered light signals from the water body and suspended particles and project them onto the detection system;

[0013] Step 5: Use a color filter to block background interference light such as sunlight, receive the backscattered signals within a fixed wavelength range in the detection range, and improve the signal-to-noise ratio;

[0014] Step 6: Use a CMOS camera to separate the polarization information of the received backscattered light, and separately obtain the linearly polarized laser scattering echo signal images in the directions of (0°, 45°, 90°, 135°), separate the polarization characteristics of the backscattered light signals of the water body and suspended particles, and convert the light intensity into a digital matrix signal.

[0015] The present invention has the following advantages:

[0016] The underwater small-angle backscattering lidar detection system of the present invention adopts a multi-polarization information acquisition scheme through Shack imaging technology, which can simultaneously measure multiple parameters in the water body components. Compared with traditional water body detection, it has a simple and compact structure and can be used for portable on-water observation; combined with a non-dispersive optical structure, a special CMOS detector is used to complete the detection of multi-polarization information.

[0017] The present invention can effectively monitor the pollutant concentration, distribution, and change trend of the water body through a multi-polarization information measurement scheme, which helps to timely discover and handle water body pollution problems, protect the health and stability of the water environment, can realize all-weather, high spatio-temporal resolution monitoring of the water body, helps to timely discover emergencies such as water pollution, algal blooms, and abnormal suspended sediment, and gives early warnings and responses in advance to reduce disaster losses. The system provides new technical means and methods for water body detection, which helps to promote scientific research and technological innovation in related fields; the invention is based on the Shack imaging principle, has the advantages of simple and compact structure, small volume, and portable observation, can reduce equipment costs and usage thresholds, and improve the popularity and efficiency of water body monitoring. By accurately monitoring the suspended particles and other pollutants in the water body, it can help formulate more effective environmental protection policies and measures and promote the restoration and maintenance of ecological balance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a flowchart for processing beam width data of the present invention. Detailed implementation mode

[0019] The present invention provides an underwater particulate matter detection lidar detection system based on Schlieren imaging technology, including a transmitting system, a receiving system and a detection system. The transmitting system includes a continuous-wave 532nm laser, a laser collimator and a half-wave plate. The receiving system includes a receiving mirror, a focusing lens and a band-pass color filter. The detection system includes a CMOS black-and-white camera.

[0020] In this embodiment, a regularly combined four-direction linear polarizer is covered in front of the photosensitive element of the CMOS black-and-white camera, and the four-direction polarizer is in the directions of 0°, 45°, 90° and 135°.

[0021] An underwater particulate matter detection lidar detection method based on Schlieren imaging technology, the method includes the following steps:

[0022] Step 1: Use a continuous-wave 532nm laser to generate a 532nm wavelength linearly polarized laser light source with a high degree of linear polarization;

[0023] Step 2: Use a laser collimator to shape the continuous 532nm wavelength linearly polarized light source into collimated light, reduce the laser divergence angle, and then incident it into the water body;

[0024] Step 3: Use a half-wave plate to rotate the polarization direction of the 532nm wavelength linearly polarized laser to ensure that the laser polarization direction is aligned with the 0° polarization direction of the detection system;

[0025] Step 4: Use a focusing lens to receive the backscattered light signal of the water body and suspended particulate matter and project it onto the detection system;

[0026] Step 5: Use a color filter to block background interference light such as sunlight, receive the backscattered signal in a fixed wavelength range within the detection range, and improve the signal-to-noise ratio;

[0027] Step 6: Use a CMOS camera to separate the polarization information of the received backscattered light, respectively obtain the linearly polarized laser scattered echo signal images in the directions of (0°, 45°, 90°, 135°), separate the polarization characteristics of the backscattered light signal of the water body and suspended particulate matter, and convert the light intensity into a digital matrix signal.

[0028] In this embodiment, the traditional marine lidar transmits a signal through the transmitting system, interacts with the detection object to generate an echo signal, and the echo signal is collected and processed by the receiving system to obtain data information. The basic working principle of the marine Schlieren lidar is roughly the same as that of the traditional marine lidar. A laser beam with a wavelength of λ0 is emitted by the laser, collimated and expanded, and then vertically incident into the detection object. When the laser passes through the water body, it is scattered and attenuated, and its backscattered light is received by the receiving system. The backscattered signal at a distance z can be expressed by the radar equation as follows:

[0029] where P s (λ,z) is the energy of the received scattered signal, P0(λ) is the energy of the initial laser beam, O(z) is the geometric overlap factor, C is a constant, β(λ,z) is the backscattering coefficient, is the attenuation coefficient, and is the distance from the lidar system.

[0030] In this embodiment, the relationship between the pixel sequence of the image sensor and the detection distance is obtained by differentiating the range resolution. The relationship between the range resolution and the detection distance is:

[0031]

[0032] The Schlieren radar technology also has unique advantages in signal processing. Substituting it into the lidar equation, it cancels out the 1 / z 2 term in the lidar equation.

[0033] In this embodiment, when the CMOS black-and-white camera receives an image, the entire laser beam can be imaged in the receiving field of view without being truncated, and the measurement range overlap factor O(z) is 1. The Schlieren lidar equation can be simplified as: where K is the system constant. As the square term of the distance in the lidar equation is canceled out, the lidar echo signal does not attenuate with the square of the distance. Compared with the traditional lidar, its dynamic range is greatly reduced, making the system structure more compact and simple. By comparing with the traditional lidar equation, we can obtain: where K is obtained from known constants, P0 is the initial laser energy, A0 is the size of the receiving field of view, R is the detection distance, d is the effective backscattering length, and F is the correction coefficient.

[0034] In this embodiment, since the Schlieren lidar measurement is out of the water body, the refraction effect of the water-air interface needs to be considered. As Figure 1 shown, using similar triangles, the solution of the receiving field of view can be calculated as: where R w is the distance from the detection point to the water-air interface, R A is the distance from the system receiver to the water-air interface, As $A$ is the receiving area at the water-gas interface, $A_0$ is the receiving area at the detector, and $n$ is the refractive index of water, which is 1.33.

[0035] In this embodiment, for the Schlidt lidar equation, two methods, namely the slope method and the Klett method, are used for solving. When using the slope method to solve $\alpha(z)$, it is considered that the attenuation coefficient of the water body in the water tank is a constant. The least squares method is used to fit the signal curve, and the slope of the curve is the attenuation coefficient of the homogeneous water body. Then, $\alpha(z)$ is substituted into it, and the backscattering coefficient $\beta(z)$ is further solved. The Klett method is generally used for the case of non-uniform water bodies. A certain boundary position $z$ in the water body is selected. m , the attenuation coefficient at the corresponding position is obtained by using the slope method, and then the attenuation coefficients at different detection distances are obtained by substituting into the formula. The attenuation coefficient $\alpha(z)$ is substituted into the formula to obtain $\beta(\lambda,z)$ at different detection distances $z$. The specific calculation formula is: where $p(z$ m ) is the backscattering signal energy at position $z$ m , and $p(z)$ corresponds to the backscattering signal energy at different detection distances $z$.

[0036] Although the specific implementation manners of the present invention have been described in detail with reference to the accompanying drawings, it should not be construed as a limitation on the protection scope of this patent. Within the scope described in the claims, various modifications and deformations that can be made by those skilled in the art without creative efforts still fall within the protection scope of this patent.

Claims

1. An underwater particulate matter detection lidar detection system based on Shack imaging technology, comprising a transmitting system, a receiving system and a detection system, characterized in that: The emission system includes a continuous-wave 532 nm laser, a laser collimator, and a half-wave plate. The receiving system includes a receiving mirror, a focusing lens, and a band-pass color filter. The detection system includes a CMOS black-and-white camera; The continuous-wave 532 nm laser emits linearly polarized light. In front of the photosensitive element of the CMOS black-and-white camera, a regularly combined four-direction polarizer is covered. The four-direction polarizer is in the directions of 0°, 45°, 90°, and 135°.

2. A detection method for underwater particulate matter detection lidar based on Schlieren imaging technology, characterized in that: The method includes the following steps: Step 1: Generate a linearly polarized laser light source with a wavelength of 532 nm through a continuous-wave 532 nm laser; Step 2: Use a laser collimator to shape the continuous linearly polarized light source with a wavelength of 532 nm into collimated light and reduce the laser divergence angle; Step 3: Use a half-wave plate to rotate the polarization direction of the linearly polarized laser with a wavelength of 532 nm to ensure that the laser polarization direction is aligned with the 0° polarization direction of the detection system; Step 4: Receive the backscattered light signal of the water body and suspended particulate matter through a focusing lens; Step 5: Use a color filter to block background interference light and receive the backscattered signal within a fixed wavelength range in the detection range to improve the signal-to-noise ratio; Step 6: Separate the polarization information of the received backscattered light through a CMOS camera: Obtain the backscattered light signal of the water body and suspended particulate matter, and then respectively obtain the linearly polarized laser reflection echo signals in the directions of 0°, 45°, 90°, and 135° through the four-direction polarizer and perform polarization characteristic separation, and convert the light intensity into a digital matrix signal.

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