An underwater high-repetition-rate pulsed laser line scanning imaging system and method

Abstract

This invention discloses an underwater high-repetition-rate pulsed laser line scanning imaging system and method, relating to the field of underwater laser imaging technology. The system includes a laser emission module for emitting high-repetition-rate pulsed lasers, comprising a 532nm high-repetition-rate pulsed laser and a beam shaping module; the beam shaping module consists of a biaxial conical mirror and a cylindrical mirror. A laser imaging module is used for large-depth-of-field imaging of target reflected echoes within a set time window, including a high-speed gated camera, an imaging lens, and a narrowband filter, constructed based on the Sham imaging principle. A synchronization control module is used for power supply, communication, and gating timing control to achieve timing synchronization between the laser and the camera in the system. This invention combines laser line scanning with range-gated imaging technology, making it suitable for long-range, large-area environmental detection as well as high-precision 3D reconstruction of close-range targets, effectively improving the clarity, signal-to-noise ratio, and applicability of underwater imaging.

Description

Technical Field

[0001] This invention relates to the field of underwater laser imaging technology, specifically to an underwater high-repetition-rate pulsed laser line scanning imaging system and method. Background Technology

[0002] Oceans cover approximately 71% of the Earth's surface and are a vital foundation for sustaining life on Earth. However, to date, about 95% of the ocean remains unexplored and underdeveloped. The rapid development of underwater vehicle technologies such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) has enabled humans to perceive the deep-sea environment with high precision. In various underwater exploration missions, high-resolution imaging is a crucial means of achieving environmental perception, target identification, and mission execution. Currently, underwater imaging mainly includes two categories: acoustic imaging and optical imaging. While acoustic imaging has strong penetrating power, its resolution is limited, and it is susceptible to multi-echo interference near the seabed or target area, leading to image blurring and loss of detail. Optical imaging offers even higher resolution, but due to the strong absorption and scattering of visible light by seawater, the imaging distance is typically only a few meters, making it difficult to meet the needs of long-distance exploration.

[0003] To overcome the limitations of traditional optical imaging, two active laser imaging schemes have been developed in recent years: laser range-gated imaging and laser line scan (LLS). The former uses a high-speed gated detector to receive only the target's reflected signal within a set time window, effectively suppressing scattered background light from non-target areas and significantly improving the image signal-to-noise ratio. The latter leverages the physical characteristic that backscattered light intensity in water decays rapidly along the direction away from the optical axis, achieving spatial separation between the target and scattered light and enhancing image contrast. LLS systems often use continuous lasers as the light source, which, while creating more concentrated linear illumination, is inevitably affected by noise from overlapping fields of view and ambient scattered light. Therefore, the laser and detector are often arranged separately to reduce the overlapping field of view and decrease backscattering interference. Furthermore, by replacing the continuous laser source with a pulsed laser and combining it with a high-speed gated detector with time-gated capabilities, a pulsed laser line scan (PLLS) imaging system can be constructed, achieving stronger scattering suppression capabilities in both spatial and temporal dimensions.

[0004] Despite the progress made, current technology still faces several limitations in terms of system architecture and imaging capabilities:

[0005] (1) Existing systems mostly use low repetition rate, high energy pulsed lasers, resulting in large system size, high power consumption and low frame rate, which makes it difficult to meet the fast real-time imaging requirements in dynamic scenes;

[0006] (2) Traditional line beam shaping methods result in uneven light intensity distribution, especially at long distances, where energy is concentrated in the center of the beam, and the brightness of the target area is inconsistent, affecting the uniformity of imaging;

[0007] (3) Due to the limitation of depth of field, it is difficult to form a clear image over a large range. Although reducing the lens aperture can increase the depth of field to a certain extent, it will lead to increased image noise and reduced brightness.

[0008] (4) Most systems only support two-dimensional imaging and lack the ability to reconstruct three-dimensional shapes, making it difficult to meet the comprehensive application needs of underwater mapping and target recognition. Summary of the Invention

[0009] The purpose of this invention is to provide an underwater high-repetition-rate pulsed laser line scanning imaging system and method to solve the problems mentioned in the background art.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0011] In a first aspect, an underwater high-repetition-rate pulsed laser line scanning imaging system includes:

[0012] The laser emitting module, used to emit high-repetition-rate pulsed lasers, includes a high-repetition-rate pulsed laser with a working wavelength of 532nm and a beam shaping module. The beam shaping module consists of a biaxial conical mirror and a cylindrical mirror. The biaxial conical mirror adjusts the spatial energy distribution of the original laser beam, radially expanding the Gaussian-distributed laser beam to transition the high-energy central region to the edge region, thus improving the energy distribution. The cylindrical mirror further expands the shaped laser beam into a narrow linear laser beam for illuminating the target area.

[0013] The laser imaging module, used for large depth-of-field imaging of target reflections within a set time window, includes a high-speed gated camera, an imaging lens, and a narrowband filter, and is built based on the Sham imaging principle. The high-speed gated camera has nanosecond-level gating capability, and through synchronous control with the laser, it only receives the reflection signal from a target at a specific distance within a selected time window, effectively suppressing background scattering interference. The imaging lens and camera are arranged at an angle to form a Sham imaging system, where the subject's focal plane, image plane, and lens focal plane intersect on a straight line. By expanding the focal plane range, clear target imaging under large depth-of-field conditions is achieved. The center wavelength of the narrowband filter matches the laser wavelength to suppress stray light and further improve the system's signal-to-noise ratio.

[0014] The synchronization control module is used for power supply communication and gating timing control to achieve timing synchronization between the laser and the camera in the system. It has a built-in high-precision clock signal source that can output precise synchronization trigger pulses to control the laser to emit laser pulses and the gating camera to open and expose. Its parameters can be programmed and modified, including pulse trigger frequency, gating delay and gate width settings, to adapt to the imaging requirements of targets within different distance ranges.

[0015] The data processing module is used to post-process the data collected by the system, and has two functions: two-dimensional image stitching and three-dimensional point cloud reconstruction. Two-dimensional image stitching can perform temporal alignment and spatial registration on continuously acquired laser line scan images to realize large-scale underwater scene detection and target recognition. Three-dimensional point cloud reconstruction, combined with system calibration parameters, extracts the three-dimensional coordinate points corresponding to each laser line to realize three-dimensional reconstruction of underwater targets.

[0016] The variable baseline module is used to adjust the baseline length and installation angle of the laser transmitter and receiver according to the imaging mode. Shorter baselines are more suitable for long-distance 2D imaging, while longer baselines result in higher 3D ranging resolution. The adjustable connecting bracket enables flexible configuration of the baseline and angle, providing optimal geometric conditions for 2D image stitching and 3D point cloud reconstruction.

[0017] Secondly, an underwater high-repetition-rate pulsed laser line scanning imaging method, based on the aforementioned underwater high-repetition-rate pulsed laser line scanning imaging system, includes the following steps:

[0018] S1: A 532nm high-repetition-rate pulsed laser is used to emit high-repetition-rate pulsed laser, which is then shaped by a biaxial conical mirror and a cylindrical mirror to form a narrow line laser beam with uniform energy distribution, which is used for line scanning illumination of underwater target areas.

[0019] S2: The imaging lens and the high-speed gated camera are tilted so that the subject's focusing plane, the image plane, and the lens's focal plane intersect in a straight line, forming a SAM imaging system, which achieves clear imaging of the target under large depth of field conditions.

[0020] S3: It has both 2D and 3D imaging modes. The 2D imaging mode prioritizes the overall coverage of the target scene and is suitable for long-distance, large-area imaging. The 3D imaging mode aims to obtain spatial depth information and is suitable for close-range, high-precision reconstruction.

[0021] S4: In two-dimensional imaging mode, the baseline of the laser transmitter and receiver is adjusted to a shorter configuration to adapt to long-distance detection; high-repetition-rate laser line scanning is used to continuously illuminate the target area, and a high-speed gated camera synchronously collects laser reflection signals to obtain sequential images. The images are then stitched together to construct a large-scale underwater two-dimensional image, enabling overall environmental detection and rapid target identification.

[0022] S5: In 3D imaging mode, the baseline of the laser transmitter and receiver is set to a longer configuration to improve ranging resolution; acquire laser line scan image sequences, extract the center position of the laser stripes, and combine the system calibration parameters to convert the center position corresponding to each laser line into 3D coordinates, thereby constructing a high-precision point cloud and realizing 3D reconstruction of underwater targets.

[0023] A further improvement to the technical solution of the present invention is that: S1 specifically includes:

[0024] The laser is a 532nm pulsed laser with high repetition rate and low single pulse energy. Its repetition rate is preferably greater than 1kHz and its pulse width is less than 10ns, which is suitable for rapid real-time imaging in underwater dynamic environments. The single pulse energy is controlled within 1mJ to effectively reduce the overall size and power consumption of the system, improve the system's lightweight and integration, and facilitate its installation on various underwater vehicle platforms.

[0025] After being emitted, the laser beam is shaped sequentially by a beam shaping module consisting of a biaxial conical mirror and a cylindrical mirror. First, the laser beam passes through the biaxial conical mirror and, with the help of its transmission characteristics, optimizes the energy distribution by appropriately offsetting the center of the laser beam from the optical axis center of the biaxial conical mirror. This allows the high-energy region at the center to smoothly extend to the edge, resulting in a more uniform light intensity distribution.

[0026] After being optimized by the biaxial conical mirror, the laser beam is shaped into a narrow line beam by the cylindrical mirror. The line beam has good directionality in the underwater environment, which can effectively suppress angular diffusion and energy dispersion, improve the effective illumination intensity of the target area, and facilitate stable propagation over longer distances in water, thereby improving the system's long-distance imaging capability and image quality.

[0027] A further improvement to the technical solution of the present invention is that: S2 specifically includes:

[0028] The high-speed gated camera adopts a miniaturized split design, which is convenient for underwater integration; the exposure mode is global exposure, which is beneficial for dynamic target imaging; the acquisition frame rate is greater than 100FPS, which can be efficiently synchronized with a high repetition rate laser to meet the requirements of fast real-time scanning and high-sensitivity imaging.

[0029] Furthermore, in order to achieve clear imaging under large depth of field conditions, the tilted arrangement of the imaging lens and the camera not only satisfies Scherm's law, that is, the subject's focusing plane, the image plane, and the lens's focal plane intersect on a straight line in space; it also incorporates the hinge law, by adjusting the angle between the lens and the image plane, so that the focal plane and the object plane remain consistent over a larger range, thereby making the underwater target as clear as possible.

[0030] Furthermore, the system employs a line scan illumination method, where the laser beam forms an optical scan line with the target surface. This optical line coincides precisely with the sharp imaging line determined by the hinge law, ensuring that the line scan imaging process naturally satisfies the combined conditions of Schahm's law and the hinge law, thus maintaining high imaging sharpness and a large depth of field throughout the entire scanning process.

[0031] A further improvement to the technical solution of the present invention is that: S3 specifically includes:

[0032] Select the appropriate imaging mode according to the imaging requirements, configure the baseline of the laser transmitter and receiver, use a 532nm high repetition rate pulse laser to emit laser, and shape it into a uniform energy line beam through a biaxial conical mirror and cylindrical mirror to scan and illuminate the underwater target area. The high-speed gated camera and the laser achieve strict time synchronization through a synchronization control module to ensure that the target reflection signal is received within the set time window.

[0033] A receiving system based on the Sham imaging principle is used to acquire laser stripe images under large depth of field conditions. In two-dimensional imaging mode, the information of the center of the stripe and its neighborhood of multiple frames is stitched together in time sequence, and after weighted fusion and contrast enhancement, a large-scale two-dimensional image is output. In three-dimensional imaging mode, the pixel coordinates of the stripe center are converted into a three-dimensional spatial point cloud by combining the system calibration parameters.

[0034] A further improvement of the technical solution of the present invention is that the two-dimensional imaging mode and the three-dimensional imaging mode can be configured and adjusted according to the task requirements and external environmental conditions, thereby improving the applicability and flexibility of the system in different underwater scenarios; in complex scenarios, it can achieve partitioned imaging: for distant areas, the two-dimensional imaging mode can be used to obtain the overall layout, and for key targets, the local area can be switched to the three-dimensional imaging mode for fine reconstruction, taking into account both long-distance large-scale detection and local high-precision measurement.

[0035] A further improvement to the technical solution of the present invention is that: S4 specifically includes:

[0036] In two-dimensional imaging mode, the laser transmitter and receiver adopt a short baseline configuration to reduce parallax and improve the coverage of distant target signals. According to the timing control of the synchronization control module, the laser signal reflected by the target is collected and imaged frame by frame to ensure strict synchronization between laser transmission and reception.

[0037] The acquired laser stripe images are preprocessed by using Gaussian filtering to suppress noise and using a threshold method to filter out stray light interference from the water body. This improves the overall signal-to-noise ratio while preserving weak target signals, resulting in preprocessed images that are more suitable for subsequent feature extraction and cross-frame accumulation.

[0038] The gray-scale centroid method is used to extract the features of laser stripes from the preprocessed image, and the center line information of the laser stripes in each frame image is obtained, that is, the center point of the line laser on each line is determined to ensure the stability and accuracy of stripe extraction.

[0039] Under short baseline conditions, laser stripes hardly undergo significant geometric deformation on the imaging plane, making it difficult to support high-precision three-dimensional reconstruction. Therefore, in the two-dimensional imaging mode, three-dimensional coordinate calculation is not performed, but the centerline information is directly used to construct the two-dimensional mosaic.

[0040] To avoid missing target information caused by extracting only the center line, the illumination area and its neighborhood are selected as the effective area, so that the weak scattering signals of the central stripe and the neighboring area can participate in the image restoration together, thereby enhancing the fidelity of stripe details and preserving the complete outline and texture features of the target object as much as possible.

[0041] The center of each frame stripe and its neighborhood are arranged and stitched according to the time sequence of laser scanning, and the overlapping areas are weighted and averaged to eliminate the brightness difference and seam effect between adjacent frames, so as to ensure the continuity and uniformity of the stitched image.

[0042] A contrast-limited adaptive histogram equalization method is adopted to perform brightness equalization and contrast enhancement processing on the stitching results. While improving the dynamic range of brightness globally, it reduces local brightness unevenness and improves the overall contrast, resulting in continuous and clear large-scale underwater two-dimensional stitched images.

[0043] A further improvement to the technical solution of the present invention is that: S5 specifically includes:

[0044] In 3D imaging mode, the laser transmitter and receiver adopt a long baseline configuration to increase parallax and improve 3D ranging resolution; according to the timing control of the synchronization control module, the laser signal reflected by the target is collected and imaged frame by frame to ensure strict synchronization between laser transmission and reception.

[0045] Before three-dimensional imaging, the underwater high repetition rate pulsed laser line scanning imaging system is calibrated. First, the camera is calibrated using the Zhang Zhengyou calibration method and a lens tilt model based on the Sham imaging principle. In the system calibration, the laser plane equation is determined based on the homography matrix and cross ratio invariance.

[0046] The acquired laser stripe images are preprocessed by using Gaussian filtering to suppress noise and threshold segmentation to coarsely locate the laser stripe regions. The segmented binary images are then subjected to morphological processing to remove interference from small scattering particles while maintaining the stripe shape and continuity.

[0047] The gray-scale centroid method was used to extract the features of laser stripes from the preprocessed image, and the center line coordinates of the laser stripes in each frame were obtained.

[0048] By combining the system calibration parameters, the pixel coordinates of the stripe center are converted into three-dimensional spatial coordinates of the target surface to obtain the original point cloud;

[0049] Radius filtering is used to remove noise points and outliers from the original point cloud, and moving least squares interpolation is used to densify the point cloud to improve its density and continuity.

[0050] Due to the adoption of the above technical solution, the technical progress achieved by this invention compared to the prior art is as follows:

[0051] 1. This invention provides an underwater high repetition rate pulsed laser line scanning imaging system and method, which combines laser line scanning with range gating technology, and uses a high repetition rate pulsed laser and a high-speed gated camera to work together to achieve miniaturization, lightweighting and rapid real-time imaging in dynamic scenes; at the same time, it achieves uniform laser illumination and large depth-of-field imaging through biaxial conical and cylindrical mirror beam shaping and Sham imaging, effectively improving the clarity and signal-to-noise ratio of underwater imaging.

[0052] 2. This invention provides an underwater high-repetition-rate pulsed laser line scanning imaging system and method, which has both two-dimensional and three-dimensional dual imaging modes and combines a variable baseline design to achieve flexible switching; by constructing an adapted two-dimensional image stitching algorithm and a three-dimensional point cloud reconstruction algorithm, it can achieve large-scale environmental detection and target recognition at a long distance and achieve high-precision three-dimensional measurement at a short distance, effectively improving the applicability and flexibility of the system in different underwater mission scenarios. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0054] Figure 1 This is a schematic diagram illustrating the workflow of an underwater high-repetition-rate pulsed laser line scanning imaging system and method according to the present invention;

[0055] Figure 2 This is a schematic diagram of the underwater high repetition rate pulsed laser line scanning imaging system of the present invention;

[0056] Figure 3 A schematic diagram of the shaping optical path using a combination of a biaxial conical mirror and a cylindrical mirror;

[0057] Figure 4 This is a schematic diagram of the optical path for Sham imaging;

[0058] Figure 5 This is a diagram showing the image stitching effect in two-dimensional imaging mode;

[0059] Figure 6 This is a diagram showing the point cloud reconstruction effect in 3D imaging mode.

[0060] Figure 7 This is a schematic diagram of a variable baseline structure;

[0061] In the diagram: 1-Imaging chamber clamp, 2-Laser chamber clamp, 3-Fixed bracket, 4-Rotating bracket, 5-Connecting plate, 6-Clamp connector. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] Example 1, such as Figures 1 to 7 As shown, the present invention provides an underwater high-repetition-rate pulsed laser line scanning imaging system, comprising:

[0064] The laser emitting module, used to emit high-repetition-rate pulsed lasers, includes a high-repetition-rate pulsed laser with a working wavelength of 532nm and a beam shaping module. The beam shaping module consists of a biaxial conical mirror and a cylindrical mirror. The biaxial conical mirror adjusts the spatial energy distribution of the original laser beam, radially expanding the Gaussian-distributed laser beam to transition the high-energy central region to the edge region, thus improving the energy distribution. The cylindrical mirror further expands the shaped laser beam into a narrow linear laser beam for illuminating the target area.

[0065] The laser imaging module, used for large depth-of-field imaging of target reflections within a set time window, includes a high-speed gated camera, an imaging lens, and a narrowband filter, and is built based on the Sham imaging principle. The high-speed gated camera has nanosecond-level gating capability, and through synchronous control with the laser, it only receives the reflection signal from a target at a specific distance within a selected time window, effectively suppressing background scattering interference. The imaging lens and camera are arranged at an angle to form a Sham imaging system, where the subject's focal plane, image plane, and lens focal plane intersect on a straight line. By expanding the focal plane range, clear target imaging under large depth-of-field conditions is achieved. The center wavelength of the narrowband filter matches the laser wavelength to suppress stray light and further improve the system's signal-to-noise ratio.

[0066] The synchronization control module is used for gating timing control to achieve timing synchronization between the laser and the camera in the system; it has a built-in high-precision clock signal source that can output precise synchronization trigger pulses to control the laser to emit laser pulses and the gating camera to open and expose; it has programmable parameters, including pulse trigger frequency, gating delay and gate width settings, to adapt to the imaging needs of targets within different distance ranges.

[0067] The data processing module is used to post-process the data collected by the system, and has two functions: two-dimensional image stitching and three-dimensional point cloud reconstruction. Two-dimensional image stitching can perform temporal alignment and spatial registration on continuously acquired laser line scan images to realize large-scale underwater scene detection and target recognition. Three-dimensional point cloud reconstruction, combined with system calibration parameters, extracts the three-dimensional coordinate points corresponding to each laser line to realize three-dimensional reconstruction of underwater targets.

[0068] The variable baseline module is used to adjust the baseline length and installation angle of the laser transmitter and receiver according to the imaging distance and resolution requirements. Among them, a shorter baseline is more suitable for long-distance two-dimensional imaging, while the longer the baseline, the higher the resolution of three-dimensional ranging. The adjustable connecting bracket enables flexible configuration of the baseline and installation angle, providing optimal geometric conditions for two-dimensional image stitching and three-dimensional point cloud reconstruction.

[0069] Among them, the 532nm high repetition rate pulsed laser is used to emit pulsed lasers with a pulse width of less than 10ns, a pulse frequency of more than 1000Hz, and a single pulse energy of no more than 1mJ, so as to achieve the light source characteristics of high repetition rate and low single pulse energy, and meet the requirements of underwater rapid real-time imaging and miniaturization.

[0070] The beam shaping module consists of a biaxial conical mirror and a cylindrical mirror. The biaxial conical mirror optimizes the spatial energy distribution of the laser beam, while the cylindrical mirror further shapes it into a linear laser beam. Figure 3 As shown, the laser beam is transformed into a ring beam after passing through the conical mirror group. The energy originally concentrated at the center of the beam is evenly distributed to the outer edge of the ring, while the energy at the outer edge converges to the inner side of the ring. Subsequently, the laser beam is focused by a cylindrical mirror to form a narrow strip beam, the energy at any point of which is the result of the integration of the ring beam in the longitudinal direction.

[0071] The high-speed gated camera receives and images the laser signal reflected from the target within a set time window. The exposure mode is global exposure, and the acquisition frame rate is greater than 100 FPS, ensuring efficient synchronization with high repetition rate laser pulses and supporting high-sensitivity, fast real-time underwater imaging.

[0072] The imaging lens collects the laser signal reflected from the target and achieves sharp imaging under large depth-of-field conditions based on Scherm's law and the hinge law. Scherm's law states that a tilted subject plane can be imaged clearly when the subject's focal plane, image plane, and lens focal plane intersect on the same straight line in space. The hinge law states that the imaginary plane in front of the lens node (PTF plane) and the front focal plane intersect on a straight line (the hinge line). Combining these two laws uniquely determines the sharp focal plane, such as... Figure 4 As shown, the image plane and the focal plane form a certain angle, and the optical axis is no longer perpendicular to the image plane; the tilt angle of the lens relative to the image plane is expressed by the formula... Calculate, where, Focal length The physical distance between the Hinge line and the front node of the lens;

[0073] Narrowband filters allow only selected laser wavelengths to pass through, while blocking other wavelengths. They have a center wavelength of 532nm, a half-width at half-maximum of 10nm, and a transmittance of over 80%. They are used to suppress stray light and improve the signal-to-noise ratio of imaging.

[0074] The synchronization control module is used for gating timing control. It uses the laser trigger signal as the time reference and sends a trigger signal to the high-speed gating camera according to the underwater transmission time of the laser to precisely control the exposure timing and ensure that the camera only turns on when the target echo arrives.

[0075] The variable baseline module is used to adjust the baseline length and installation angle of the laser transmitter and receiver according to the imaging mode to adapt to different imaging distance and resolution requirements; specifically, such as... Figure 7 As shown, the imaging cabin clamp 1 and the laser cabin clamp 2 are used to fix the laser imaging module and the laser emission module, respectively; the fixed bracket 3 is located in the middle of the system, providing load-bearing capacity and baseline reference for the overall structure, and is also used to install other modules; the rotating bracket 4 is set on both sides of the fixed bracket 3, and the connecting plate 5 is used to strengthen the connection between the fixed bracket 3 and the rotating bracket 4; the clamp connector 6 is used to reliably fix the rotating bracket 4 to the clamp.

[0076] The baseline length can be adjusted by installing brackets of different lengths between the fixed bracket 3 and the rotating bracket 4. The bolt holes can be fixed with screws and nuts, so that the baseline length can be continuously adjusted within a certain range. The installation angle can be adjusted by the slotted holes on the rotating bracket 4. The clamp connector 6 can be adjusted along the slotted holes, thereby changing the installation angle of the imaging cabin clamp 1 and the laser cabin clamp 2.

[0077] Example 2, as Figures 1 to 7 As shown, based on Example 1, the present invention also provides an underwater high-repetition-rate pulsed laser line scanning imaging method, specifically including the following two imaging modes:

[0078] Two-dimensional imaging mode is suitable for large-scale environmental detection and target identification at long distances. The specific implementation method is as follows:

[0079] Step 1.1: The laser transmitter and receiver are configured with a short baseline;

[0080] Step 1.2: The 532nm high-repetition-rate pulsed laser emits pulsed laser light;

[0081] Step 1.3: The laser beam is shaped by passing through a biaxial conical mirror and a cylindrical mirror in sequence to obtain a narrow line laser beam with uniform energy distribution, which then illuminates the underwater target area.

[0082] Step 1.4: Using a SAM imaging system consisting of a high-speed gated camera, an imaging lens, and a narrowband filter, perform large depth-of-field imaging of the target's reflected echo within a set time window;

[0083] Step 1.5: Preprocess the acquired laser stripe image by using Gaussian filtering to suppress noise. Gaussian filtering involves calculating a weighted average of the gray-level distribution within the Gaussian filtering window. The gray-level value of each pixel within the window is obtained by weighted averaging of its own gray-level value and the gray-level values ​​of its neighboring pixels. The convolution kernel for the Gaussian convolution operation is defined by a Gaussian function, and its one-dimensional form is... ,in, It is the mean. It is the variance; after Gaussian filtering, the gray values ​​of the laser stripes are more consistent with the Gaussian distribution.

[0084] Step 1.6: Remove stray light interference from the water body using a threshold method, i.e., set an intensity threshold. For each pixel grayscale The system determines whether a signal's grayscale value is below a threshold and whether it is directly suppressed as background noise; if the grayscale value is above a threshold, it is retained as a valid signal.

[0085] Step 1.7: Use the grayscale centroid method to extract laser stripe features from the preprocessed image, obtaining the centerline information of the laser stripes in each frame; specifically, for the first frame of the image... The grayscale distribution of the row, the center position of its stripes It can be derived from the formula Calculated;

[0086] Step 1.8: Expand the neighborhood range to both sides based on the center line, and select the central stripe and its neighboring pixels as the effective area; The center coordinates of the stripes are The effective area is defined as follows: ,in The neighborhood radius;

[0087] Step 1.9: Arrange and stitch the stripe centers and their neighborhoods of each frame according to the temporal sequence of the laser scanning, and use a weighted average fusion strategy for the overlapping area; the pixel gray values ​​of the two frames in the overlapping area are respectively and The fusion result is ,in, These are weighting coefficients, and are adaptively adjusted using linear interpolation based on the pixel's position in the overlapping region. ,in, and These represent the distances of the pixel to the non-overlapping boundaries of the first and second frames, respectively. Pixels closer to the first frame are given greater weight, and pixels closer to the second frame are given greater weight, thus achieving a smooth transition.

[0088] Step 1.10: Employ a contrast-limited adaptive histogram equalization method to perform brightness equalization and contrast enhancement on the stitched image. Specifically, the stitched image is divided into several local windows, and a histogram is calculated and equalized within each local window. The final two-dimensional image stitching result is shown below. Figure 5 As shown.

[0089] The three-dimensional imaging mode is suitable for achieving high-precision three-dimensional measurement at relatively close distances. The specific implementation method is as follows:

[0090] Step 2.1: The laser transmitter and receiver are configured with a long baseline;

[0091] Step 2.2: There is a certain baseline and angle between the laser and the camera. When the baseline and angle are adjusted, there are different correspondences between the projection position of the laser line in the image plane and the actual spatial depth of the target. In order to accurately establish the conversion relationship between pixel coordinates and actual three-dimensional depth, calibration needs to be completed before the system runs.

[0092] Among them, the Zhang Zhengyou calibration method is used for camera calibration. This invention adopts the Sham imaging design, and there is a certain angle between the image plane and the focal plane. Based on the traditional camera calibration model, tilt correction is introduced. During calibration, the following should be noted: the plane calibration board used is a circle center calibration, the number of images acquired is not less than 18, and the calibration board should appear in different positions and have different tilts. During the calibration, all camera-related hardware should not be changed.

[0093] During the system calibration process, a laser plane calibration method based on the homography matrix and cross-ratio invariance is adopted. Feature points on the left side of the calibration plate are used to calculate the homography matrix, and four straight lines with known spacing are arranged on the right side as references. After the laser line is projected into this area, it intersects with the straight lines. The cross-ratio invariance theorem can be used to determine the spatial coordinates of the laser line intersection point and deduce the spatial equation of the laser line. Multiple sets of data are obtained by changing the position and angle of the calibration plate, and the least squares method is used to fit and obtain the accurate equation of the laser plane.

[0094] Step 2.3: The 532nm high-repetition-rate pulsed laser emits pulsed laser light;

[0095] Step 2.4: The laser beam is shaped by passing through a biaxial conical mirror and a cylindrical mirror in sequence to obtain a narrow line laser beam with uniform energy distribution, which then illuminates the underwater target area.

[0096] Step 2.5: Using a SAM imaging system consisting of a high-speed gated camera, an imaging lens, and a narrowband filter, perform large depth-of-field imaging of the target's reflected echo within a set time window;

[0097] Step 2.6: Preprocess the acquired laser stripe image by using Gaussian filtering to suppress noise; coarsely locate the laser stripe region by threshold segmentation, i.e., setting an intensity threshold T and segmenting the grayscale of each pixel. The result of the segmentation is determined by the judgment. The pixel values ​​are either 0 or 1; morphological processing is performed on the segmented binary image, specifically by first eroding the image and then dilating it.

[0098] Step 2.7: Extract laser stripe features from the preprocessed image using the gray-scale centroid method to obtain the coordinates of the stripe center line in each frame; combine the system calibration parameters to convert the pixel coordinates of the stripe center into three-dimensional spatial coordinates of the target surface to obtain the original point cloud;

[0099] Step 2.8: Remove noise points and outliers using the radius filtering method. Select any point in the point cloud as the center and set a threshold radius. Count the number of neighboring points within this radius. If satisfied ,in, If the minimum number of neighborhood points is set as a threshold, then the point is determined to be a discrete point and is removed.

[0100] Step 2.9: The point cloud is densified using moving least squares interpolation. Any point and its neighborhood are selected in the point cloud. A local surface is fitted using weighted least squares, and additional sampling is performed on this surface to generate new points, thus increasing the point density on the original point cloud. The final 3D reconstructed point cloud result is shown below. Figure 6 As shown.

[0101] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An underwater high-repetition rate pulsed laser line scanning imaging system, characterized in that, The application relates to a high-repetition-frequency laser imaging system. The application comprises the following parts: A laser emission module for emitting high-repetition-frequency pulsed laser, which comprises a high-repetition-frequency pulsed laser with a working wavelength of 532 nm and a beam shaping module; wherein the beam shaping module is composed of a biaxial conical mirror and a cylindrical mirror, and the laser beam passes through the biaxial conical mirror to obtain uniform light intensity distribution and then enters the cylindrical mirror to be shaped into a narrow linear laser beam; A laser imaging module for large-landscape-depth imaging of a target reflection echo within a set time window, which comprises a high-speed gated camera, an imaging lens and a narrow-band filter, wherein the imaging lens and the camera are arranged in an inclined manner to form a Scheimpflug imaging system, that is, a main focusing plane, an image plane and a lens focal plane intersect at a straight line, the focal plane range is expanded, the central wavelength of the narrow-band filter matches the laser wavelength, stray light is suppressed, a receiving system based on the Scheimpflug imaging principle is adopted, laser fringe images are collected under the condition of large-landscape-depth, a large-range two-dimensional image is output after a plurality of frames of fringe center and neighborhood information are spliced in time sequence, weighted fusion and contrast enhancement; in the three-dimensional imaging mode, the system calibration parameters are combined to convert the fringe center pixel coordinates into three-dimensional space point clouds; A synchronous control module for power supply communication and gating time sequence control, realizing time sequence synchronization of the laser and the camera in the system, and having a built-in high-precision clock signal source; A data processing module for post-processing of the data collected by the system, having two-dimensional image splicing and three-dimensional point cloud reconstruction functions; 2. An underwater high-repetition-rate pulsed laser line scanning imaging method, based on the underwater high-repetition-rate pulsed laser line scanning imaging system of claim 1, characterized in that, A variable baseline module for adjusting the baseline length and installation angle of the laser emission end and the receiving end according to the imaging mode. The application comprises the following steps: S1: a 532 nm high-repetition-frequency pulsed laser is used to emit high-repetition-frequency pulsed laser, which is sequentially shaped by a biaxial conical mirror and a cylindrical mirror to form a narrow linear laser beam with uniform energy distribution; S2: the imaging lens and the high-speed gated camera are arranged in an inclined manner, so that the main focusing plane, the image plane and the lens focal plane intersect at a straight line to form a Scheimpflug imaging system; S3: the two-dimensional imaging mode and the three-dimensional imaging mode are combined, the two-dimensional imaging mode aims at overall coverage of a target scene, and the three-dimensional imaging mode aims at obtaining spatial depth information; S4: in the two-dimensional imaging mode, the baseline between the laser emission end and the receiving end is adjusted to a short configuration; high-repetition-frequency laser line scanning is used to continuously irradiate a target region, a high-speed gated camera synchronously collects laser reflection signals, sequence images are obtained, and a large-range two-dimensional image under water is constructed through image splicing, so that the environment is overall detected and the target is identified; 3. The method according to claim 2, wherein: S5: in the three-dimensional imaging mode, the baseline between the laser emission end and the receiving end is set to a long configuration; a laser line scanning image sequence is collected, laser fringe center positions are extracted, three-dimensional coordinates corresponding to the center positions of each laser line are converted in combination with system calibration parameters, high-precision point clouds are constructed, and three-dimensional reconstruction of underwater targets is completed. The S1 specifically comprises the following steps: Pulsed laser is emitted by a 532 nm high-repetition-frequency pulsed laser, wherein the repetition frequency is greater than 1 kHz, the pulse width is less than 10 ns, and the single-pulse energy is controlled within 1 mJ. The laser beam is shaped by a beam shaping module composed of a biaxial conical mirror and a cylindrical lens after being emitted.

4. The method according to claim 2, wherein: The S2 specifically comprises: The imaging lens and the high-speed gated camera are arranged in a tilt manner, so that a main body focusing plane, an image plane and a lens focal plane intersect at a straight line, forming a Sharm imaging system; Further combining the hinge law, the angle between the lens and the image plane is adjusted, so that the focal plane and the target plane remain consistent in a wide range, and the arrangement is naturally matched with the line scanning illumination; The timing of the laser illumination and the camera exposure is consistent through a synchronous control module, and stray light is suppressed by a narrowband filter.

5. The method according to claim 4, wherein: The high-speed gated camera adopts a miniaturized split design, the exposure mode is global exposure, and the acquisition frame rate is greater than 100 FPS.

6. The method according to claim 2, wherein: The S3 specifically comprises: According to the imaging requirements, a corresponding imaging mode is selected, a baseline of the laser emission end and the receiving end is configured, the emitted laser is shaped into a linear light beam with uniform energy by a biaxial conical mirror and a cylindrical lens, and the underwater target area is scanned and illuminated.

7. The method according to claim 6, wherein: The two-dimensional imaging mode and the three-dimensional imaging mode are configured and adjusted according to task requirements and external environmental conditions; in a complex scene, zoned imaging is realized: the two-dimensional imaging mode is used to obtain the overall layout of the long-distance area, and the three-dimensional imaging mode is switched to for fine reconstruction of the local key target, taking into account long-distance wide-range detection and local high-precision measurement.

8. The method according to claim 2, wherein: The S4 specifically comprises: In the two-dimensional imaging mode, the laser emission end and the receiving end are configured with a short baseline, and the laser emission and reception are synchronized by a synchronous control module; The collected laser stripe images are preprocessed, Gaussian filtering is used to suppress noise, and threshold method is used to screen out water body stray light interference; The gray gravity center method is used to extract the features of the laser stripes of the preprocessed images, and the center line information of the laser stripes in each frame image is obtained; The illumination area and its neighborhood are selected as the effective area, so that the center stripe and the weak scattering signal of the adjacent area participate in image restoration together; The center of each frame stripe and its neighborhood are arranged and spliced in the time sequence of laser scanning, and the overlapping areas are weighted and averaged and fused; The limited contrast adaptive histogram equalization method is used to process the splicing result for brightness equalization and contrast enhancement.

9. The method according to claim 2, wherein: The S5 specifically comprises: In the three-dimensional imaging mode, the laser emission end and the receiving end are configured with a long baseline, and the laser emission and reception are synchronized by a synchronous control module; Before three-dimensional imaging, the underwater high-repetition pulse laser line scanning imaging system is calibrated; The collected laser stripe images are preprocessed, Gaussian filtering is used to suppress noise, and threshold segmentation is used to coarsely position the laser stripe area, and the segmented binary image is subjected to morphological processing; The gray gravity center method is used to extract the features of the laser stripes of the preprocessed images, and the center line coordinates of the laser stripes in each frame image are obtained; combined with the system calibration parameters, the pixel coordinates of the stripe center are converted into three-dimensional space coordinates of the target surface, and the original point cloud is obtained; The original point cloud is filtered by radius filtering to remove noise points and outliers, and the point cloud is encrypted by a moving least squares interpolation method, and a continuous and high-precision underwater target three-dimensional point cloud is obtained.

10. The method according to claim 9, wherein: The underwater high-repetition pulse laser line scanning imaging system is calibrated, and specifically includes the following steps: A camera calibration is performed by using Zhang Zhengyou calibration method and a lens tilt model based on Sharm imaging principle; In the system calibration, a laser plane equation is determined based on homography matrix and cross ratio invariance; An underwater calibration is completed by combining an underwater camera internal participation light plane refraction compensation method.

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