An underwater dual-mode laser range-gated imaging system and method

Abstract

The present invention discloses an underwater dual-mode laser range-gated imaging system and method, relating to the field of underwater laser imaging technology. The system includes a laser illumination unit, a laser imaging unit, and a synchronization control unit, and supports full-field illumination and scanning illumination operating modes. The full-field illumination operating mode achieves uniform illumination through a circular beam expansion optical path, with a relatively fast imaging speed. Combined with image noise reduction and enhancement, it is suitable for high signal-to-noise ratio detection at long distances and small fields of view underwater. The scanning illumination operating mode improves laser energy utilization through a linear beam scanning optical path. Combined with image fusion and downsampling strategies, it can improve the imaging speed and resolution of long-range and large-scale underwater imaging. The present invention combines dual-mode illumination with range-gated imaging technology, which can meet diverse needs from underwater long-range target recognition to large-scale scene detection, and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater laser imaging, and in particular to an underwater dual-mode laser range-gated imaging system and method. Background Art

[0002] Imaging technology plays a vital role in the exploration of marine resources. Clearly observing and identifying underwater targets can maximize the development of my country's abundant marine resources, but this also places high demands on detection range and resolution. Unlike optical imaging in atmospheric environments, traditional underwater optical imaging is severely affected by water absorption and scattering, resulting in a short range. With the development of laser and photoelectric sensor technologies, underwater laser imaging technology has rapidly developed. By utilizing the unique spectral, temporal, and spatial characteristics of blue-green lasers in seawater to detect underwater targets, it can effectively reduce the effects of seawater absorption and scattering, enabling long-range underwater imaging.

[0003] Among them, range-gated imaging is the most representative of underwater laser imaging. This technology uses a controllable pulsed laser and a gating camera that can be opened and closed at high speed to separate the reflected light and backscattered light of the target at different distances from a temporal perspective, so that the laser pulse signal reflected by the target reaches the camera and forms an image within the gating time, thereby eliminating most of the influence of backscattered light and background noise, and improving the imaging distance and signal-to-noise ratio.

[0004] However, traditional underwater laser range gating technology usually uses a beam-expanded pulsed laser to perform single-shot illumination imaging of the target. Due to the limitations of laser power and water attenuation, it can only perform small-field-of-view imaging during underwater long-distance detection, resulting in a limited detection target range. It cannot meet the needs of large-scale detection applications such as submarine pipeline inspection, seabed topography mapping, underwater archaeology, and searches for wrecked ships and aircraft. Summary of the Invention

[0005] The present invention aims to provide an underwater dual-mode laser range-gated imaging system and method to solve the problems raised in the above-mentioned background technology.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] In a first aspect, an underwater dual-mode laser range-gated imaging system comprises:

[0008] The laser illumination unit, including a 532nm pulsed laser and an emission optical unit, is used to generate pulsed lasers and control the optical path, performing time-sharing multiplexing of full-field-of-view illumination and scanning illumination. The emission optical unit includes a switching mirror and a galvanometer. Based on different imaging requirements, the switching mirror can quickly switch between full-field-of-view illumination mode and scanning illumination mode, providing uniform or scanning illumination for underwater targets and improving laser energy utilization to meet a variety of complex application requirements, such as underwater long-distance, small-field-of-view, high-signal-to-noise ratio detection, and large-field-of-view scene detection.

[0009] The laser imaging unit, including a gated camera, an imaging lens, and a narrowband filter, is used to receive pulsed laser signals reflected by the target and form an image. The gated camera can be turned on at a specific time to ensure that only the laser pulse signal reflected by the target is received, effectively reducing interference from water scattered light and background noise. The imaging lens can collect reflected light and focus it onto the camera's photosensitive element to achieve clear imaging of the target. The narrowband filter can select light of a specific wavelength band to pass, further improving imaging quality, enhancing the system's anti-interference ability, and making imaging clearer and more accurate;

[0010] The synchronous control unit is electrically connected to the 532nm pulse laser, the gated camera, the switching mirror, and the galvanometer, respectively, and is used to transmit control signals, perform gated imaging timing control, galvanometer control, and lighting switching. It can accurately control the triggering timing of the gated camera to ensure that it is turned on when the laser pulse reflected by the target arrives, thereby realizing distance-gated imaging and improving the imaging distance and signal-to-noise ratio. At the same time, the scanning angle and rotation of the galvanometer are precisely controlled to ensure orderly scanning of the light beam in the scanning illumination mode and seamless connection of the light beams of two adjacent scans, thereby achieving complete scanning imaging of the entire imaging field of view, improving imaging speed and resolution, and being able to quickly switch between the two working modes, so that the system can flexibly adapt to different underwater detection tasks;

[0011] The data processing unit, including image noise reduction, enhancement and fusion algorithms, is used to pre-process the obtained images, provide high-quality target detection image sources for subsequent computer terminals, effectively reduce the impact of background noise such as water scattering on imaging, remove noise components in the image through differential noise reduction, median filtering and other methods, improve image clarity and contrast, and enhance the details and brightness of the image through enhancement algorithms such as dual-platform histogram equalization, making the target more prominent and easier to identify. For multi-frame images in scanning illumination mode, threshold average fusion and other methods are used for splicing and fusion to eliminate splicing traces and obtain complete and high-quality underwater target images, providing a reliable data basis for underwater target identification and analysis, and significantly improving the continuity, clarity and measurement accuracy of underwater target images.

[0012] In a second aspect, an underwater dual-mode laser range-gated imaging method is implemented based on the above-mentioned underwater dual-mode laser range-gated imaging system, comprising the following steps:

[0013] S1: The pulsed laser is emitted by a 532nm pulsed laser, which is expanded and collimated by a collimating lens group to form a collimated parallel beam, thereby improving the uniformity and directionality of the beam.

[0014] S2: By switching the rotating mirror to the circular beam expansion optical path or the linear beam shaping optical path, the full field illumination or scanning illumination working mode can be switched to meet different detection requirements;

[0015] S3: Under the full-field illumination working mode, collect the scattered background noise image of the water body at the non-target position, perform differential processing on the collected target image and the background noise image, and perform dual-platform histogram equalization enhancement processing on the differential target image to obtain an enhanced image;

[0016] S4: In scanning illumination mode, noisy image signals are collected and all scanned images are stitched together to form a complete field of view image. Stitching artifacts are eliminated through threshold averaging and fusion. Furthermore, a downsampling scanning strategy is used to increase the galvanometer angle, further improving the imaging speed and recovering detailed information from the downsampling scanned image set.

[0017] S5: Integrate images in full-field illumination mode or scanning illumination mode to output high-quality target detection images.

[0018] A further improvement of the technical solution of the present invention is that the full-field illumination working mode and the scanning illumination working mode are specifically: the full-field illumination working mode uses a circular beam expansion optical path, and the scanning illumination working mode uses a linear beam shaping optical path. The switching of the two working modes is controlled by a synchronous control unit to switch the rotating mirror to control the optical path switching.

[0019] A further improvement of the technical solution of the present invention is that: S1 specifically includes:

[0020] The 532nm pulsed laser emits pulsed laser light with a pulse width of less than 20ns and a pulse frequency of greater than 20Hz. This high-frequency, short-pulse laser can effectively reduce the absorption and scattering of laser light by water, improving the signal-to-noise ratio of imaging. The emitted pulsed laser light has high energy density and short pulse duration, making it suitable for underwater long-distance imaging.

[0021] The emitted pulsed laser enters a collimating lens group consisting of two plano-convex lenses. The first plano-convex lens focuses the laser, and the second plano-convex lens adjusts the focused beam to become a parallel beam. The function of the collimating lens group is to adjust the divergent laser beam into a parallel beam, improving the uniformity and directionality of the beam, ensuring that the laser can propagate a longer distance underwater and the energy distribution is uniform.

[0022] The light beam adjusted by the collimating lens group becomes a collimated parallel beam, and its spot size and divergence angle are controlled to meet subsequent lighting needs. The collimated parallel beam has high directionality and low divergence angle, and can maintain a longer propagation distance underwater, while reducing the diffusion of the light beam in the water, improving lighting efficiency and imaging quality.

[0023] A further improvement of the technical solution of the present invention is that: S2 specifically includes:

[0024] The switching mirror is in the initial position. The pulsed laser forms a collimated parallel beam after passing through the collimating lens group. The collimated parallel beam is ready to enter the subsequent optical path switching link. According to the detection requirements, the switching mirror is driven by a stepper motor to move. The switching mirror consists of a movable and a fixed reflector.

[0025] If the optical path is switched to the circular beam expansion path, the movable reflector is removed, and the collimated parallel light beam directly enters the circular beam expansion path. If the optical path is switched to the linear beam shaping path, the movable reflector is moved in, and the collimated parallel light beam is reflected by the movable reflector to the fixed reflector and then enters the linear beam shaping path. The movement of the switching mirror realizes the rapid switching of the optical path, and the switching time is less than 1 second, ensuring that the system can flexibly switch between the full field of view illumination mode and the scanning illumination mode.

[0026] For the circular beam expansion optical path, the collimated parallel light beam passes through a concave lens and expands into a circular beam with a divergence angle, illuminating the entire imaging field of view and providing uniform full-field illumination. It is suitable for underwater long-distance, small-field-of-view high signal-to-noise ratio detection. For the line beam shaping optical path, the collimated parallel light beam passes through a cylindrical mirror and a galvanometer to form a line beam with a divergence angle, illuminating a local strip imaging field of view. The direction of the line beam is quickly adjusted by the galvanometer to achieve scanning illumination of the entire field of view. It is suitable for large-field-of-view scene detection and improves imaging speed and resolution.

[0027] A further improvement of the technical solution of the present invention is that: S3 specifically includes:

[0028] Under the full-field illumination working mode, the water scattered background noise image at the non-target position is collected, and the stability of the background noise image is improved by multi-frame averaging for subsequent differential processing;

[0029] According to the timing control of the synchronous control unit, the noisy image signal reflected by the target is collected and imaged, and the target image at a distance of 30m underwater is acquired. The image contains both the target reflection signal and the background noise scattered by the water body;

[0030] The collected target image is differentially processed with the water scattered background noise image to correct the background noise and obtain a corrected image. The corrected target image intensity is calculated by combining the collected target image intensity, the collected water scattered background noise image intensity and the transmittance of the water body to reduce the interference of water scattering and improve the contrast of the target image.

[0031] The target image after difference is enhanced by dual-platform histogram equalization to enhance the image details and clarity, and improve the signal-to-noise ratio of underwater long-distance imaging;

[0032] Based on dual-platform histogram equalization, the statistical histogram of the original image is obtained, the statistical histogram is corrected, and the corrected statistical histogram is accumulated to obtain a cumulative histogram. The cumulative histogram is then used to calculate the image grayscale conversion equation, and the image grayscale conversion equation is used to convert the original image to obtain an enhanced image.

[0033] A further improvement of the technical solution of the present invention is that the calculation expression of the corrected image is:

[0034] ;

[0035] Where I is the intensity of the acquired target image, is the intensity of the collected water body scattered background noise image, is the transmittance of water, is the target image intensity after correction;

[0036] The expression for correcting the statistical histogram is:

[0037] ;

[0038] Where, is the corrected statistical histogram, is the statistical histogram of the original image, k is the grayscale value, is the lower threshold, is the upper threshold, when Between 0 and Between Adjust to ,when Between and between, Remain unchanged when Greater than ,Will Adjust to ;

[0039] The calculation expression of the cumulative histogram is:

[0040] ;

[0041] Where, is the cumulative histogram;

[0042] The cumulative histogram is given by the formula Calculate the image grayscale conversion equation.

[0043] A further improvement of the technical solution of the present invention is that: S4 specifically includes:

[0044] When scanning the entire scene, set the galvanometer step angle to twice the laser divergence angle, so that the beams of two adjacent scans are seamlessly connected;

[0045] According to the timing control of the synchronization control unit, the galvanometer gradually scans the entire field of view, collecting and imaging the noisy image signals reflected by the target at different positions;

[0046] Each frame of the collected image is subjected to noise reduction and dual-platform histogram equalization enhancement processing through median filtering;

[0047] All scanned images are stitched together to form a complete field of view image. The threshold averaging fusion method is used to eliminate stitching traces and improve the signal-to-noise ratio and brightness uniformity of the image.

[0048] Through the downsampling scanning strategy, the galvanometer angle is increased to further improve the imaging speed. After image noise reduction, enhancement and fusion processing, the image detail information is restored from the downsampling scanning image set, the large field of view imaging resolution is improved, and high-quality scanned images are obtained.

[0049] A further improvement of the technical solution of the present invention is that: the median filtering specifically includes: determining a filtering window and a position, and sorting the pixel values ​​in the window according to the grayscale, and then taking the median to replace the pixel value in the center of the original window;

[0050] The threshold averaging fusion specifically includes:

[0051] Calculate the column average grayscale curve and set a variable threshold to extract the illuminated area and its neighborhood to avoid interference from the black background;

[0052] The maximum position is determined by smoothing the curve with mean filtering, and 10% of the pixels on both sides are taken, and the minimum column mean is used as the threshold for adaptive adjustment;

[0053] The pixel columns above the threshold in all images are accumulated and the number of accumulations is recorded. The accumulated result is divided by the number of accumulations for normalization and restored to the original grayscale range. Finally, the target image after scanning fusion is obtained. Compared with the single-frame image, the fused image achieves brightness balance and detail enhancement, making the texture details in the illuminated area clearer, and obtaining a complete and high-quality underwater target image.

[0054] A further improvement of the technical solution of the present invention is that the specific steps of the downsampling scan are as follows:

[0055] Determine the step angle of the galvanometer when scanning the entire scene , set the new step angle according to the downsampling factor n Initialize the galvanometer to ensure that the starting position is fixed and the step angle is set. Rotate the reflector gradually so that the line beam scans the target area in sequence;

[0056] The image after each illumination is recorded by a strobe camera to obtain a downsampled scan image set, and then the downsampling coefficient is determined to complete the image restoration. Downsampling can effectively increase the imaging speed while maintaining the main structure and visual information of the image.

[0057] The downsampling coefficient uses structural similarity to evaluate the similarity between the downsampled scan image x and the full scene scan image y, as given by the formula calculate;

[0058] in, and are the average values ​​of image x and y respectively, and are the variances of images x and y, respectively, is the covariance of images x and y, and It is a constant used to prevent the denominator from being zero. The range of structural similarity is from -1 to 1. The closer the value is to 1, the more similar the structures of the two images are. It is preferred to select the maximum downsampling coefficient within the range of SSIM>0.8.

[0059] Due to the adoption of the above technical solution, the present invention has the following technical advancements compared to the prior art:

[0060] The present invention provides an underwater dual-mode laser range-gated imaging system and method, which combines dual-mode illumination with range-gated imaging technology to overcome the problem that range-gated imaging is difficult to balance imaging distance and field of view. The full-field-of-view illumination mode is suitable for high signal-to-noise ratio detection in underwater long-distance and small fields of view, while the scanning illumination mode significantly improves the imaging field of view and resolution, expands the system's applicable scenarios, and meets diverse needs from underwater long-distance target recognition to large-scale scene detection.

[0061] The present invention provides an underwater dual-mode laser range-gated imaging system and method, constructing a set of noise reduction, enhancement and fusion algorithms suitable for dual-mode range-gated imaging, including differential noise reduction, dual-platform histogram equalization and threshold average fusion, which significantly improves the continuity, clarity and measurement accuracy of underwater target images, and provides high-quality imaging guarantee for underwater long-distance and large-field-of-view detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments described in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0063] Figure 1 This is a structural schematic diagram of an underwater dual-mode laser range-gated imaging system according to the present invention;

[0064] Figure 2 This is the imaging effect diagram under the full field of view illumination working mode;

[0065] Among them, (a) water scattering background noise image; (b) target image at a distance of 30m underwater; (c) target image after background noise correction; (d) target image after dual-platform histogram equalization enhancement;

[0066] Figure 3 Enhance histograms at each stage of the dual-platform histogram equalization process;

[0067] Among them, (a) original histogram; (b) modified histogram; (c) cumulative histogram; (d) equalized histogram;

[0068] Figure 4 This is the imaging effect diagram under the scanning lighting working mode;

[0069] Among them, (a) target image at a distance of 30m underwater; (b) target image after median filtering; (c) target image after dual-platform histogram equalization enhancement; (d) target image after scanning fusion;

[0070] Figure 5 Schematic diagram of median filtering principle;

[0071] Figure 6 The calculation result of the column average grayscale;

[0072] Among them, (a) original column average grayscale curve; (b) smoothed column average grayscale curve;

[0073] Figure 7 This is the downsampling scanning imaging effect diagram;

[0074] Among them, (a) the target image after scanning and fusion when the downsampling coefficient is 2; (b) the target image after scanning and fusion when the downsampling coefficient is 3; (c) the target image after scanning and fusion when the downsampling coefficient is 4; (d) the target image after scanning and fusion when the downsampling coefficient is 5;

[0075] Figure 8 The results of structural similarity calculation for downsampled scanned images. DETAILED DESCRIPTION

[0076] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of 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. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0077] Example 1, as Figures 1 to 8 As shown, the present invention provides an underwater dual-mode laser range-gated imaging system, comprising:

[0078] The laser illumination unit, including a 532nm pulsed laser and an emission optical unit, is used to generate pulsed lasers and control the optical path, performing time-sharing multiplexing of full-field-of-view illumination and scanning illumination. The emission optical unit includes a switching mirror and a galvanometer. Based on different imaging requirements, the switching mirror can quickly switch between full-field-of-view illumination mode and scanning illumination mode, providing uniform or scanning illumination for underwater targets and improving laser energy utilization to meet a variety of complex application requirements, such as underwater long-distance, small-field-of-view, high-signal-to-noise ratio detection, and large-field-of-view scene detection.

[0079] The laser imaging unit, including a gated camera, an imaging lens, and a narrowband filter, is used to receive pulsed laser signals reflected by the target and form an image. The gated camera can be turned on at a specific time to ensure that only the laser pulse signal reflected by the target is received, effectively reducing interference from water scattered light and background noise. The imaging lens can collect reflected light and focus it onto the camera's photosensitive element to achieve clear imaging of the target. The narrowband filter can select light of a specific wavelength band to pass, further improving imaging quality, enhancing the system's anti-interference ability, and making imaging clearer and more accurate;

[0080] The synchronous control unit is electrically connected to the 532nm pulse laser, the gated camera, the switching mirror, and the galvanometer, respectively, and is used to transmit control signals, perform gated imaging timing control, galvanometer control, and lighting switching. It can accurately control the triggering timing of the gated camera to ensure that it is turned on when the laser pulse reflected by the target arrives, thereby realizing distance-gated imaging and improving the imaging distance and signal-to-noise ratio. At the same time, the scanning angle and rotation of the galvanometer are precisely controlled to ensure orderly scanning of the light beam in the scanning illumination mode and seamless connection of the light beams of two adjacent scans, thereby achieving complete scanning imaging of the entire imaging field of view, improving imaging speed and resolution, and being able to quickly switch between the two working modes, so that the system can flexibly adapt to different underwater detection tasks;

[0081] The data processing unit includes image noise reduction, enhancement, and fusion algorithms, which are used to pre-process the obtained images and provide a high-quality target detection image source for subsequent computer terminals. It effectively reduces the impact of background noise such as water scattering on imaging. It removes noise components in the image through methods such as differential noise reduction and median filtering to improve image clarity and contrast. It enhances image details and brightness through enhancement algorithms such as dual-platform histogram equalization, making the target more prominent and easier to identify. For multi-frame images in scanning illumination mode, threshold average fusion and other methods are used for splicing and fusion to eliminate splicing marks and obtain complete and high-quality underwater target images. This provides a reliable data basis for underwater target identification and analysis, and significantly improves the continuity, clarity, and measurement accuracy of underwater target images.

[0082] Among them, the 532nm pulse laser is used to emit pulsed laser with a pulse width of less than 20ns and a pulse frequency greater than 20Hz;

[0083] The transmitting optical unit includes a collimating lens group, a switching mirror, a circular beam expansion optical path and a linear beam shaping optical path;

[0084] The collimating lens group consists of two plano-convex lenses, which are used to expand and collimate the pulsed laser;

[0085] The beam splitter has a splitting ratio of 10:90 (R:T) and an operating wavelength of 400-700 nm. 10% of the pulsed laser is allocated to the high-speed photodetector, and the remaining 90% of the pulsed laser is used to illuminate the target.

[0086] High-speed photodetector, which receives a small portion of the pulsed laser and generates a pulsed voltage signal, which is used to trigger the strobe camera and galvanometer after a delay;

[0087] The switching mirror adopts a dual-mirror structure, with mirror 1 being movable and mirror 2 being fixed. A stepper motor and linear slide rail are used to achieve optical path switching through small linear movements, with a switching time of less than 1s. The stepper motor drives mirror 1 along a linear track to achieve optical path switching. When mirror 1 is moved away, the pulsed laser directly enters the circular beam expansion optical path; when mirror 1 is moved in, the pulsed laser is reflected by it to mirror 2 and then enters the linear beam shaping optical path. The mirror uses a high-reflectivity dielectric film with a reflectivity greater than 99%, reducing laser energy loss.

[0088] The circular beam expansion optical path can emit a circular beam through a concave lens for full field of view illumination. The field of view angle is ;

[0089] The line beam shaping optical path includes a cylindrical mirror and a galvanometer. The line beam can be emitted through a concave lens. The direction of the line beam can be quickly adjusted by controlling the galvanometer to achieve scanning illumination of the beam in one-dimensional direction. The field of view angle is ;

[0090] A gated camera is used to image the pulsed laser signal reflected from the target.

[0091] The imaging lens is used to collect the pulsed laser signal reflected by the target;

[0092] Narrowband filters are used to pass light in the selected laser band and cut off light outside the passband. The center wavelength is 532nm, the half-maximum width is 10nm, and the transmittance is greater than 80%;

[0093] The strobe imaging timing control in the synchronous control unit is used to send a trigger signal to the strobe camera based on the underwater transmission time of the laser pulse, using the laser trigger signal as the time reference, to control the camera's exposure time and ensure that the camera is turned on only when the laser pulse reflected by the target arrives;

[0094] The galvanometer control in the synchronous control unit is used to adjust the galvanometer scanning angle, and uses the laser trigger signal as the time reference to send a trigger signal to the galvanometer to complete the rotation before the next trigger signal.

[0095] Example 2, as Figures 1 to 8 As shown, based on Example 1, the present invention further provides an underwater dual-mode laser range-gated imaging method. The underwater dual-mode laser range-gated imaging system of the present invention has two working modes:

[0096] In the full-field illumination mode, the laser illumination field of view is the same as the laser imaging field of view. Uniform illumination is achieved through the circular beam expansion optical path, and the imaging speed is fast. Combined with image noise reduction and enhancement methods, the signal-to-noise ratio of underwater long-distance and small-field imaging can be improved. The specific implementation methods are as follows:

[0097] Step 1.1: 532 nm pulse laser emits pulse laser;

[0098] Step 1.2: The pulsed laser is expanded and collimated by the collimating lens group to form a collimated parallel beam, which improves the uniformity and directionality of the beam. The rotating mirror is switched to adjust the circular beam expansion optical path. The collimated beam is expanded into a circular beam by the concave lens, illuminating the entire imaging field of view.

[0099] Step 1.3: Collect the water scattered background noise image at the target-free position and improve the stability of the background noise image by multi-frame averaging for subsequent differential processing;

[0100] The collected water body scattered background noise image is as follows Figure 2 As shown in (a), the noise is mainly water scattering. Scattering refers to the phenomenon that the propagation direction of light changes when passing through water. Backscattering will form background noise, interfering with the detection of effective signals. Forward scattering will reduce the imaging resolution and make the image blurred. Traditional filtering algorithms have difficulty in effectively separating the target signal from the background noise.

[0101] Step 1.4: Collect and image the noisy image signal reflected by the target object according to the timing control of the synchronization control unit;

[0102] The target image collected at a distance of 30m underwater is as follows Figure 2 As shown in (b), the image contains both the target reflection signal and the water scattered background noise;

[0103] Step 1.5: Perform differential processing on the acquired target image and the water scattering background noise image. By correcting the background noise, the interference of water scattering is reduced and the contrast of the target image is improved.

[0104] The rectified image is given by the formula Calculate, where I is the acquired target image intensity, is the intensity of the collected water body scattered background noise image, is the transmittance of water, is the intensity of the corrected target image. The corrected target image is as follows Figure 2 As shown in (c);

[0105] Step 1.6: Perform dual-platform histogram equalization on the target image after differentiation to enhance image detail and clarity, and improve the signal-to-noise ratio of underwater long-distance imaging.

[0106] The basic steps of dual-platform histogram equalization are as follows: First, calculate the statistical histogram of the original image ,like Figure 3 As shown in (a); by formula Correct the statistical histogram, the result is as follows Figure 3 As shown in (b), where k is the grayscale value, is the lower threshold, is the upper threshold, when Between 0 and Between Adjust to ,when Between and between, Remain unchanged when Greater than ,Will Adjust to ;

[0107] Then the corrected statistical histogram is accumulated, and the formula Calculate the cumulative histogram ,like Figure 3 As shown in (c) in the figure; using the cumulative histogram, the formula Calculate the image grayscale conversion equation, and finally use the conversion equation to convert the original image to obtain the equalized histogram as follows Figure 3 As shown in (d), the corresponding enhanced image is Figure 2 As shown in (d), the image contrast and brightness are enhanced, and the effective imaging area and target details are significantly improved;

[0108] In the scanning illumination working mode, the galvanometer quickly and continuously changes the emission angle of the line beam to complete the scanning imaging of the entire imaging field of view. The laser energy utilization rate is high. Combined with the image fusion method and downsampling scanning strategy, it can improve the imaging speed and resolution of underwater long-distance and large imaging. The specific implementation method is as follows:

[0109] Step 2.1: 532 nm pulse laser emits pulse laser;

[0110] Step 2.2: The pulsed laser is expanded and collimated by the collimating lens group to form a collimated parallel beam. The rotating mirror is switched to adjust the line beam shaping optical path. The collimated beam passes through the cylindrical mirror and the galvanometer mirror to form a line beam, illuminating the local strip imaging field of view.

[0111] The target image collected at a distance of 30m underwater is as follows Figure 4 As shown in (a), the image contains both the target reflection signal and the water scattered background noise;

[0112] Step 2.3: Set the galvanometer step angle to twice the laser divergence angle during full scene scanning, so that the beams of two adjacent scans are seamlessly connected;

[0113] Step 2.4: Based on the timing control of the synchronization control unit, the galvanometer gradually scans the entire field of view, collecting and imaging the noisy image signals reflected by the target at different positions;

[0114] Step 2.5: Perform noise reduction and dual-platform histogram equalization enhancement processing on each frame of the acquired image through median filtering;

[0115] The median filter principle is as follows Figure 5 As shown in , first determine the filter window and position, then sort the pixel values ​​in the window by grayscale, and finally take the median to replace the pixel value in the center of the original window. The target image after median filtering is as follows Figure 4 As shown in (b);

[0116] The dual-platform histogram equalization enhancement process is the same as in step 1.6 above, so I will not go into details here. The enhanced target image is as follows Figure 4 As shown in (c), the target details in the dark area of ​​the image are more obvious;

[0117] Step 2.6: All scanned images are stitched together to form a complete field of view image. Thresholding averaging fusion method is used to eliminate stitching marks and improve the signal-to-noise ratio and brightness uniformity of the image.

[0118] The steps of thresholding average fusion are as follows: First, calculate the column average grayscale curve, such as Figure 6 As shown in (a), a variable threshold is set to extract the illuminated area and its neighborhood to avoid interference from the black background. The maximum position is determined by smoothing the curve with a mean filter, and 10% of the pixel range is taken on both sides. The minimum column mean is used as the threshold to achieve adaptive adjustment. The smoothed column average grayscale curve is shown in Figure 6 As shown in (b); then accumulate the pixel columns above the threshold in all images, record the number of accumulations, divide the accumulation result by the number of accumulations for normalization, and restore to the original grayscale range, and finally obtain the target image after scanning fusion as shown in Figure 4 As shown in (d), compared with single-frame images, the fused image achieves brightness balance and detail enhancement, making the texture details in the illuminated area clearer and obtaining a complete and high-quality underwater target image;

[0119] Step 2.7: Using a downsampling scanning strategy, the galvanometer angle is increased to further improve imaging speed. After image noise reduction, enhancement, and fusion processing, more detailed information is recovered from the downsampling scan image set, thereby improving the large field of view imaging resolution.

[0120] The specific steps of downsampling scanning are as follows: First, determine the step angle of the galvanometer when scanning the full scene , set the new step angle according to the downsampling factor n , then initialize the galvanometer to ensure that the starting position is fixed and the step angle is set The reflector is rotated step by step to make the line beam scan the target area in sequence. At the same time, the gated camera records the image after each illumination, thereby obtaining a downsampled scan image set. Finally, the image restoration is completed through steps 2.5 and 2.6. When the downsampling coefficient n is 2, 3, 4, and 5, the image results after scanning fusion are as follows: Figure 7 As shown in Figure 2, downsampling can effectively improve the imaging speed while maintaining the main structure and visual information of the image;

[0121] In order to determine the appropriate downsampling coefficient, structural similarity is used to evaluate the similarity between the downsampled scan image x and the full scene scan image y, as shown in the formula Calculate, where and are the average values ​​of image x and y respectively, and are the variances of images x and y, respectively, is the covariance of images x and y, and is a constant used to prevent the denominator from being zero. The range of structural similarity is from -1 to 1. The closer the value is to 1, the more similar the structures of the two images are. It is preferred to select the maximum downsampling coefficient within the range of SSIM>0.8. The structural similarity results calculated when the downsampling coefficient n is 2, 3, 4, and 5 are as follows Figure 8 shown.

[0122] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. An underwater dual-mode laser range-gated imaging method, characterized in that: The following steps are involved: S1: The pulsed laser is emitted by a 532nm pulsed laser, which is expanded and collimated by a collimating lens group to form a collimated parallel beam; S2: Switching between the circular beam expansion optical path or the linear beam shaping optical path by switching the rotating mirror to switch between the full field illumination and scanning illumination working modes. S2 specifically includes: The switching mirror is in the initial position. The pulsed laser forms a collimated parallel beam after passing through the collimating lens group. The collimated parallel beam is ready to enter the subsequent optical path switching link. According to the detection requirements, the switching mirror is driven by a stepper motor to move. The switching mirror consists of a movable and a fixed reflector. If the optical path is switched to the circular beam expansion path, the movable reflector is removed, and the collimated parallel beam directly enters the circular beam expansion path. If the optical path is switched to the linear beam shaping path, the movable reflector is moved in, and the collimated parallel beam is reflected by the movable reflector to the fixed reflector, and then enters the linear beam shaping path. For the circular beam expansion optical path, the collimated parallel light beam passes through a concave lens and expands into a circular beam with a divergence angle, illuminating the entire imaging field of view. For the line beam shaping optical path, the collimated parallel light beam passes through a cylindrical mirror and a galvanometer to form a line beam with a divergence angle, illuminating a local strip imaging field of view. S3: Under the full-field illumination working mode, collect the scattered background noise image of the water body at the non-target position, perform differential processing on the collected target image and the background noise image, and perform dual-platform histogram equalization enhancement processing on the differential target image to obtain an enhanced image; S4: In scanning illumination mode, noisy image signals are collected and all scanned images are stitched together to form a complete field of view image. Stitching artifacts are eliminated through threshold averaging fusion. Furthermore, a downsampling scanning strategy is used to further improve imaging speed and recover detailed information from the downsampling scanned image set. S5: Integrate images in full-field illumination mode or scanning illumination mode to output high-quality target detection images.

2. The underwater dual-mode laser range-gated imaging method according to claim 1, characterized in that: The full-field illumination working mode and the scanning illumination working mode are specifically as follows: the full-field illumination working mode uses a circular beam expansion optical path, and the scanning illumination working mode uses a linear beam shaping optical path. The switching between the two working modes is controlled by a synchronous control unit to switch the rotating mirror to control the optical path switching.

3. The underwater dual-mode laser range-gated imaging method according to claim 2, characterized in that: Said S1 specifically includes: Emits pulsed laser light through a 532nm pulsed laser, wherein the pulse width is less than 20ns and the pulse frequency is greater than 20Hz; The emitted pulsed laser enters a collimating lens group consisting of two plano-convex lenses. The first plano-convex lens focuses the laser, and the second plano-convex lens adjusts the focused beam to become a parallel beam. The light beam adjusted by the collimating lens group becomes a collimated parallel light beam, and its spot size and divergence angle are controlled.

4. The underwater dual-mode laser range-gated imaging method according to claim 3, characterized in that: The S3 specifically includes: Under the full-field illumination working mode, the image of water scattered background noise at the non-target position is collected; According to the timing control of the synchronous control unit, the noisy image signal reflected by the target is collected and imaged, and the target image at a distance of 30m underwater is acquired. The image contains both the target reflection signal and the background noise scattered by the water body; Perform differential processing on the collected target image and the water scattered background noise image to correct the background noise and obtain the corrected image. Combine the collected target image intensity, the collected water scattered background noise image intensity and the transmittance of the water body to calculate the corrected target image intensity. Perform dual-platform histogram equalization enhancement processing on the target image after difference to enhance the details and clarity of the image; Based on dual-platform histogram equalization, the statistical histogram of the original image is obtained, the statistical histogram is corrected, and the corrected statistical histogram is accumulated to obtain a cumulative histogram. The cumulative histogram is then used to calculate the image grayscale conversion equation, and the image grayscale conversion equation is used to convert the original image to obtain an enhanced image.

5. The underwater dual-mode laser range-gated imaging method according to claim 4, characterized in that: The calculation expression of the corrected image is: ； Where I is the intensity of the acquired target image, is the intensity of the collected water body scattered background noise image, is the transmittance of water, is the target image intensity after correction; The expression for correcting the statistical histogram is: ； Where, is the corrected statistical histogram, is the statistical histogram of the original image, k is the grayscale value, is the lower threshold, is the upper threshold, when Between 0 and Between Adjust to ,when Between and between, Remain unchanged when Greater than ,Will Adjust to ; The calculation expression of the cumulative histogram is: ； Where, is the cumulative histogram; The cumulative histogram is given by the formula Calculate the image grayscale conversion equation.

6. The underwater dual-mode laser range-gated imaging method according to claim 1, characterized in that: The S4 specifically includes: When scanning the entire scene, set the galvanometer step angle to twice the laser divergence angle, so that the beams of two adjacent scans are seamlessly connected; According to the timing control of the synchronization control unit, the galvanometer gradually scans the entire field of view, collecting and imaging the noisy image signals reflected by the target at different positions; Each frame of the collected image is subjected to noise reduction and dual-platform histogram equalization enhancement processing through median filtering; All scanned images are stitched together to form a complete field of view image, and the stitching traces are eliminated by threshold averaging fusion method; Through the downsampling scanning strategy, the galvanometer angle is increased to further improve the imaging speed. After image denoising, enhancement and fusion processing, the image detail information is restored from the downsampling scan image set to obtain a high-quality scanned image.

7. The underwater dual-mode laser range-gated imaging method according to claim 6, characterized in that: The threshold averaging fusion specifically includes: Calculate the column average grayscale curve and set a variable threshold to extract the illuminated area and its neighborhood to avoid interference from the black background; The maximum position is determined by smoothing the curve with mean filtering, and 10% of the pixels on both sides are taken, and the minimum column mean is used as the threshold for adaptive adjustment; Accumulate the pixel columns above the threshold in all images and record the number of accumulations. Divide the accumulated result by the number of accumulations for normalization and restore it to the original grayscale range to finally obtain the target image after scanning fusion.

8. The underwater dual-mode laser range-gated imaging method according to claim 6, characterized in that: The specific steps of downsampling scanning are as follows: Determine the step angle of the galvanometer when scanning the entire scene , set the new step angle according to the downsampling factor n , , initialize the galvanometer and follow the set step angle Rotate the reflector gradually so that the line beam scans the target area in sequence; The image after each illumination is recorded by the strobe camera to obtain a downsampled scan image set, and then the downsampling coefficient is determined to complete the image restoration; The downsampling coefficient uses structural similarity to evaluate the similarity between the downsampled scan image x and the full scene scan image y, as given by the formula calculate; in, and are the average values ​​of image x and y respectively, and are the variances of images x and y, respectively, is the covariance of images x and y, and It is a constant used to prevent the denominator from being zero. The range of structural similarity is from -1 to 1. The closer the value is to 1, the more similar the structures of the two images are. The maximum downsampling coefficient is selected in the range of SSIM>0.

8.

9. An underwater dual-mode laser range-gated imaging system, used to implement the underwater dual-mode laser range-gated imaging method according to any one of claims 1 to 8, characterized in that: include: The laser illumination unit includes a 532nm pulsed laser and an emission optical unit, which is used to generate pulsed lasers and control the optical path, and perform time-sharing multiplexing of full-field illumination and scanning illumination. The emission optical unit includes a switching mirror and a galvanometer. A laser imaging unit, comprising a gated camera, an imaging lens, and a narrowband filter, is used to receive the pulsed laser signal reflected by the target and form an image; A synchronization control unit is electrically connected to the 532nm pulse laser, the strobe camera, the switching mirror, and the galvanometer, and is used to transmit control signals, perform strobe imaging timing control, galvanometer control, and illumination switching; The data processing unit, including image noise reduction, enhancement and fusion algorithms, is used to pre-process the obtained images and provide high-quality target detection image sources for the computer terminal.

Images