Underwater dual-mode laser range gating imaging system and method

Abstract

The invention discloses an underwater dual-mode laser range gating imaging system and method, and relates to the technical field of underwater laser imaging, the system comprises a laser illumination unit, a laser imaging unit and a synchronous control unit, and supports a full-field illumination working mode and a scanning illumination working mode; according to the full-view-field illumination working mode, uniform illumination is achieved through a circular light beam expanding light path, the imaging speed is high, image noise reduction and enhancement are combined, and the method is suitable for underwater long-distance and small-view-field high-signal-to-noise-ratio detection. According to the scanning illumination working mode, the laser energy utilization rate is improved through a linear light beam scanning light path, and the imaging speed and resolution of underwater long-distance and large-scale imaging can be improved by combining image fusion and a down-sampling strategy. According to the invention, the dual-mode illumination and the range gating imaging technology are combined, the diversified requirements from underwater long-distance target identification to large-range scene detection can be met, and the method has a wide application prospect.

Description

Technical Field

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

[0002] In the process of exploring marine resources, imaging technology plays an important role. Clearly observing and identifying underwater targets can maximize the development of China's rich marine resources, but at the same time, there are high requirements for detection range and resolution. Different from optical imaging in the atmospheric environment, affected severely by water absorption and scattering, the working distance of traditional underwater optical imaging is relatively short. With the development of laser technology and optoelectronic sensor technology, underwater laser imaging technology has developed rapidly. By using the unique spectral characteristics, time characteristics, and spatial characteristics of blue-green lasers in seawater to detect underwater targets, the influence of seawater absorption and scattering can be effectively reduced, and long-distance underwater imaging can be achieved.

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

[0004] However, traditional underwater laser range-gated technology usually uses an expanded pulsed laser to illuminate the target for single-shot imaging. Limited by laser power and water attenuation, only small-field imaging can be performed during underwater long-distance detection, resulting in a limited detection target range and unable to meet the application requirements of large-range detection such as submarine pipeline detection, seabed topography mapping, underwater archaeology, and searching for sunken ships and aircraft. Summary of the Invention

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

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows: In a first aspect, an underwater dual-mode laser range-gated imaging system includes: A laser illumination unit, comprising a 532nm pulsed laser and an emission optical unit, is used to generate pulsed laser and control the optical path, enabling time-division multiplexing of full-field illumination and scanning illumination. Among them, the emission optical unit includes a switching mirror and a galvanometer mirror, which can quickly switch between the full-field illumination mode and the scanning illumination mode according to different imaging requirements through the switching mirror, providing uniform or scanned illumination for underwater targets, improving the utilization rate of laser energy, and meeting various complex application requirements such as long-distance, small-field-of-view high signal-to-noise ratio detection and large-field-of-view scene detection underwater; 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 image it. The gated camera can be turned on within a specific time to ensure that only the laser pulse signal reflected by the target is received, effectively reducing the interference of water body scattered light and background noise. The imaging lens can collect the reflected light and focus it on the photosensitive element of the camera to achieve clear imaging of the target. The narrowband filter can select light of a specific wavelength band to pass through, further improving the imaging quality, enhancing the anti-interference ability of the system, and making the imaging clearer and more accurate; A synchronization control unit, electrically connected to the 532nm pulsed laser, the gated camera, the switching mirror, and the galvanometer mirror respectively, is used to transmit control signals, perform gated imaging timing control, galvanometer mirror control, and illumination switching. It can precisely 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 achieving range-gated imaging, improving the imaging distance and signal-to-noise ratio. At the same time, it precisely controls the scanning angle and rotation of the galvanometer mirror to ensure the orderly scanning of the beam in the scanning illumination mode and the seamless connection of the beams in two adjacent scans, realizing the complete scanning imaging of the entire imaging field of view, improving the imaging speed and resolution, and can quickly switch between the two working modes, enabling the system to flexibly adapt to different underwater detection tasks; A data processing unit, including image denoising, enhancement, and fusion algorithms, is used to preprocess the obtained images, providing a high-quality target detection image source for the subsequent computer terminal, effectively reducing the impact of background noise such as water body scattering on imaging. It removes the noise components in the image through methods such as differential denoising and median filtering, improving the clarity and contrast of the image. Through enhancement algorithms such as dual-platform histogram equalization, it enhances the details and brightness of the image, making the target more prominent and distinguishable. For multiple frames of images in the scanning illumination mode, methods such as threshold averaging fusion are used for stitching and fusion to eliminate stitching traces, obtaining a complete and high-quality underwater target image, providing a reliable data basis for the recognition and analysis of underwater targets, and significantly improving the continuity, clarity, and measurement accuracy of underwater target images.

[0007] In a second aspect, an underwater dual-mode laser range-gated imaging method is realized based on the above-mentioned underwater dual-mode laser range-gated imaging system, including the following steps: S1: Emitting pulsed laser through a 532nm pulsed laser, expanding and collimating it through a collimating lens group to form a collimated parallel light beam, improving the uniformity and directivity of the light beam; S2: Switching to the circular beam expansion optical path or the linear beam shaping optical path through a switching rotating mirror to switch between the two working modes of full-field illumination or scanning illumination, meeting different detection requirements; S3: In the full-field illumination working mode, collecting the water body scattering background noise image at the target-free position, performing differential processing on the collected target image and the background noise image, and performing dual-platform histogram equalization enhancement processing on the differential target image to obtain an enhanced image; S4: In the scanning illumination working mode, collecting the noisy image signal, stitching all the scanned images to form a complete field of view image, eliminating the stitching traces through the threshold averaging fusion method, and then increasing the galvanometer angle through the decimation scanning strategy to further improve the imaging speed and recover the detailed information from the decimation scanned image set; S5: Integrating the images in the full-field illumination working mode or the scanning illumination working mode and outputting a high-quality target detection image.

[0008] 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 the circular beam expansion optical path, and the scanning illumination working mode uses the linear beam shaping optical path. The switching between the two working modes is controlled by a synchronous control unit to control the optical path switching of the switching rotating mirror.

[0009] A further improvement of the technical solution of the present invention is that: the S1 specifically includes: Emitting pulsed laser through a 532nm pulsed laser, wherein the pulse width is less than 20ns and the pulse frequency is greater than 20Hz. This kind of high-frequency and short-pulse laser can effectively reduce the absorption and scattering of the laser by the water body, improve the signal-to-noise ratio of imaging. The emitted pulsed laser has a high energy density and a short pulse duration, which is suitable for long-distance underwater imaging; The emitted pulsed laser enters a collimating lens group composed of two plano-convex lenses. Among them, the first plano-convex lens focuses the laser, and the second plano-convex lens adjusts the focused light beam again to make it a parallel light beam. The function of the collimating lens group is to adjust the divergent laser beam into a parallel light beam, improving the uniformity and directivity of the light beam, ensuring that the laser can propagate farther underwater and the energy distribution is uniform; The light beam adjusted by the collimating lens group becomes a collimated parallel light beam, and its spot size and divergence angle are controlled to meet the subsequent illumination requirements. The collimated parallel light beam has high directivity and low divergence angle, can maintain a long propagation distance underwater, reduce the diffusion of the light beam in water at the same time, and improve the illumination efficiency and imaging quality.

[0010] A further improvement of the technical solution of the present invention lies in that: The S2 specifically includes: Switch the rotating mirror to 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 rotating mirror is driven to move by a stepper motor. The switching rotating mirror consists of a movable and a fixed mirror; If switching to the circular beam expansion optical path, remove the movable mirror among them, and the collimated parallel beam directly enters the circular beam expansion optical path. If switching to the linear beam shaping optical path, move in the movable mirror. The collimated parallel beam is reflected by the movable mirror to the fixed mirror and then enters the linear beam shaping optical path. The movement of the switching rotating 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 illumination mode and the scanning illumination mode; For the circular beam expansion optical path, the collimated parallel beam passes through a concave lens and expands into a circular beam with a divergence angle to illuminate the entire imaging field of view, providing uniform full-field illumination, which is suitable for high signal-to-noise ratio detection at long distances and small fields of view underwater. For the linear beam shaping optical path, the collimated parallel beam passes through a cylindrical lens and a galvanometer mirror to form a linear beam with a divergence angle to illuminate the local strip imaging field of view. The direction of the linear beam is quickly adjusted by the galvanometer mirror to realize the scanning illumination of the entire field of view, which is suitable for large-field-of-view scene detection and improves the imaging speed and resolution.

[0011] A further improvement of the technical solution of the present invention lies in that: The S3 specifically includes: Based on the full-field illumination working mode, collect the water body scattering background noise image at the target-free position, and improve the stability of the background noise image through multi-frame averaging for subsequent differential processing; According to the timing control of the synchronization control unit, collect and image the noisy image signal reflected by the target object, and collect the target image at a distance of 30m underwater. This image contains both the target reflection signal and the water body scattering background noise; Perform differential processing on the collected target image and the water body scattering background noise image to correct the background noise, obtain the corrected image, and calculate the corrected target image intensity in combination with the intensity of the collected target image, the intensity of the collected water body scattering background noise image, and the transmittance of the water body, reducing the interference of water body scattering and improving the contrast of the target image; Perform dual-platform histogram equalization enhancement processing on the differential target image to enhance the details and clarity of the image and improve the signal-to-noise ratio of long-distance underwater imaging; 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 the cumulative histogram. Then, the image gray conversion equation is calculated using the cumulative histogram, and the original image is converted using the image gray conversion equation to obtain the enhanced image.

[0012] A further improvement of the technical solution of the present invention is that: the calculation expression of the corrected image is: ; In the formula, I is the intensity of the target image collected, is the intensity of the water body scattering background noise image collected, is the transmittance of the water body, is the intensity of the corrected target image; The expression for correcting the statistical histogram is: ; In the formula, is the corrected statistical histogram, is the statistical histogram of the original image, k is the gray value, is the lower threshold, is the upper threshold. When is between 0 and , is adjusted to . When is between and , remains unchanged. When is greater than , is adjusted to ; The calculation expression of the cumulative histogram is: ; In the formula, is the cumulative histogram; The cumulative histogram is used to calculate the image gray conversion equation by the formula .

[0013] A further improvement of the technical solution of the present invention is that: the specific steps of S4 include: Set the step angle of the galvanometer during full-scene scanning to 2 times the laser divergence angle, so that the light beams of adjacent scans are seamlessly connected; According to the timing control of the synchronization control unit, the galvanometer scans the entire field of view step by step, and collects and images the noisy image signals reflected by the target objects at different positions; For each frame of the acquired image, noise reduction is performed through median filtering and enhancement processing is carried out through dual-platform histogram equalization; All scanned images are stitched to form a complete field-of-view image. The stitching traces are eliminated through the thresholding average fusion method to improve the signal-to-noise ratio and brightness uniformity of the image; 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 detailed information of the image is restored from the downsampled scanning image set, the large-field imaging resolution is improved, and high-quality scanning images are obtained.

[0014] A further improvement in the technical solution of the present invention lies in that: the median filtering specifically includes: determining the filtering window and position, sorting the pixel values within the window according to the gray level, and then taking the median value to replace the pixel value at the center of the original window; The thresholding average fusion specifically includes: Calculating the column average gray curve, setting a variable threshold to extract the illumination area and its neighborhood to avoid interference from the black background; After smoothing the curve through mean filtering, determining the maximum value position, taking 10% pixel ranges on both sides, using the minimum column mean as the threshold, and performing adaptive adjustment; Accumulating the pixel columns higher than the threshold in all images, recording the accumulation times, normalizing the accumulation result by dividing it by the accumulation times, restoring it to the original gray level range, and finally obtaining the target image after scanning and fusion. Compared with a single-frame image, the fused image realizes brightness balance and detail enhancement, making the texture details in the illumination area clearer and obtaining a complete and high-quality underwater target image.

[0015] A further improvement in the technical solution of the present invention lies in that: the specific steps of the downsampling scanning are as follows: Determine the stepping angle of the galvanometer during full-scene scanning , set a new stepping angle according to the downsampling coefficient n , initialize the galvanometer to ensure that the starting position is fixed, and rotate the reflecting mirror step by step according to the set stepping angle to sequentially scan the target area with the line beam; The gated camera records the image after each illumination, thereby obtaining a downsampled scanning image set, and then determining the downsampling coefficient to complete image restoration. Downsampling can effectively improve the imaging speed while maintaining the main structure and visual information of the image; The downsampling coefficient uses the structural similarity to evaluate the similarity between the downsampled scanning image x and the full-scene scanning image y, and is calculated by the formula ; where, and are the average values of images 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.

[0016] Due to the adoption of the above technical solution, the technical progress achieved by the present invention compared with the prior art is as follows: 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 make up for the problem that it is difficult for range-gated imaging to balance imaging distance and field of view. The full-field illumination mode is suitable for high signal-to-noise ratio detection at long underwater distances and small fields of view, while the scanning illumination mode significantly improves the imaging field of view and resolution, expands the applicable scenarios of the system, and meets the diverse needs from underwater long-distance target recognition to large-scale scene detection.

[0017] The present invention provides an underwater dual-mode laser range-gated imaging system and method, and constructs 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 thresholded 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 detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0019] Figure 1 is a schematic structural diagram of an underwater dual-mode laser range-gated imaging system of the present invention; Figure 2 is an imaging effect diagram in the full-field illumination working mode; Among them, (a) is the water body scattering background noise map; (b) is the target map at a distance of 30 m underwater; (c) is the target map after background noise correction; (d) is the target map enhanced by dual-platform histogram equalization; Figure 3 are the histograms at each stage in the dual-platform histogram equalization enhancement process; Among them, (a) is the original histogram; (b) is the corrected histogram; (c) is the cumulative histogram; (d) is the equalized histogram; Figure 4It is the imaging effect diagram in the scanning illumination working mode; Among them, (a) is the target diagram at a distance of 30m underwater; (b) is the target diagram after median filtering; (c) is the target diagram after dual-platform histogram equalization enhancement; (d) is the target diagram after scanning fusion; Figure 5 It is the schematic diagram of the median filtering principle; Figure 6 It is the calculation result of the column average gray value; Among them, (a) is the original column average gray value curve; (b) is the smoothed column average gray value curve; Figure 7 It is the imaging effect diagram of downsampling scanning; Among them, (a) is the target diagram after scanning fusion when the downsampling factor is 2; (b) is the target diagram after scanning fusion when the downsampling factor is 3; (c) is the target diagram after scanning fusion when the downsampling factor is 4; (d) is the target diagram after scanning fusion when the downsampling factor is 5; Figure 8 It is the calculation result of the structural similarity of the downsampling scanning imaging image. Specific implementation mode

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] Embodiment 1, as Figures 1 to 8 shown, the present invention provides an underwater dual-mode laser range-gated imaging system, including: A laser illumination unit, including a 532nm pulsed laser and a transmitting optical unit, is used to generate pulsed laser and control the optical path, and perform time-division multiplexing of full-field illumination and scanning illumination. Among them, the transmitting optical unit includes a switching rotating mirror and a galvanometer mirror, and can quickly switch between the full-field illumination mode and the scanning illumination mode according to different imaging requirements through the switching rotating mirror, providing uniform or scanning illumination for underwater targets, improving the utilization rate of laser energy, and meeting various complex application requirements such as long-distance underwater, small-field high-signal-to-noise ratio detection, and large-field scene detection; The laser imaging unit, including a gated camera, an imaging lens, and a narrowband filter, is used to receive the pulsed laser signal reflected by the target and image it. The gated camera can be turned on within a specific time to ensure that only the laser pulse signal reflected by the target is received, effectively reducing the interference of water body scattered light and background noise. The imaging lens can collect the reflected light and focus it on the photosensitive element of the camera to achieve clear imaging of the target. The narrowband filter can select light of a specific band to pass through, further improving the imaging quality, enhancing the anti-interference ability of the system, and making the imaging clearer and more accurate; The synchronization control unit is electrically connected to the 532nm pulsed laser, the gated camera, the switching rotating mirror, and the galvanometer respectively, and is used to transmit control signals for gated imaging timing control, galvanometer control, and illumination 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, so as to achieve range-gated imaging, improve the imaging distance and signal-to-noise ratio. At the same time, it accurately controls the scanning angle and rotation of the galvanometer to ensure the orderly scanning of the beam in the scanning illumination mode and the seamless connection of the beams in two adjacent scans, realizing the complete scanning imaging of the entire imaging field, improving the imaging speed and resolution, and can quickly switch between two working modes, enabling the system to flexibly adapt to different underwater detection tasks; The data processing unit includes image denoising, enhancement, and fusion algorithms, and is used to preprocess the obtained images to provide a high-quality target detection image source for the subsequent computer terminal. It effectively reduces the influence of background noise such as water body scattering on imaging, removes the noise components in the image through methods such as differential denoising and median filtering, improves the clarity and contrast of the image, and enhances the details and brightness of the image through enhancement algorithms such as dual-platform histogram equalization, making the target more prominent and distinguishable. For multiple frames of images in the scanning illumination mode, methods such as threshold averaging fusion are used for stitching and fusion to eliminate stitching traces and obtain a complete and high-quality underwater target image, providing a reliable data basis for the recognition and analysis of underwater targets, and significantly improving the continuity, clarity, and measurement accuracy of underwater target images; Among them, the 532nm pulsed laser is used to emit pulsed laser with a pulse width less than 20ns and a pulse frequency greater than 20Hz; The emission optical unit includes a collimating lens group, a switching rotating mirror, a circular beam expanding optical path, and a linear beam shaping optical path; The collimating lens group consists of two plano-convex lenses and is used to expand and collimate the pulsed laser; The beam splitter has a splitting ratio of 10:90 (R:T), a working wavelength of 400 - 700nm. 10% of the pulsed laser is allocated to the high-speed photodetector, and the other 90% of the pulsed laser irradiates the target; The high-speed photodetector receives a small part of the pulsed laser, generates a pulsed voltage signal, and is used to trigger the gated camera and the galvanometer after a delay; The switching rotating mirror adopts a double - mirror structure. Mirror 1 is movable and mirror 2 is fixed. The optical path switching is realized by a small - amplitude linear movement using a stepper motor and a linear slide rail, and the switching time is less than 1 s. The stepper motor drives mirror 1 to move along a straight track to achieve optical path switching. When mirror 1 moves away, the pulsed laser directly enters the circular beam expansion optical path. When mirror 1 moves in, the pulsed laser is reflected by it to mirror 2 and then enters the linear beam shaping optical path. The mirrors adopt high - reflectivity dielectric films with a reflectivity greater than 99%, reducing the loss of laser energy. The circular beam expansion optical path can emit a circular beam through a concave lens for full - field illumination, and the field of view angle is ; The linear beam shaping optical path includes a cylindrical lens and a galvanometer. It can emit a linear beam through a concave lens. By controlling the galvanometer, the direction of the linear beam can be quickly adjusted to achieve scanning illumination of the beam in one - dimensional direction, and the field of view angle is ; The gated camera is used to image the pulsed laser signal reflected by the target. The imaging lens is used to collect the pulsed laser signal reflected by the target. The narrow - band filter is used to allow the light of the selected laser band to pass through and cut off the light outside the pass - band. The central wavelength is 532 nm, the full width at half maximum is 10 nm, and the transmittance is greater than 80%. In the synchronous control unit, the gated imaging timing control is used to take the laser trigger signal as the time reference, and according to the underwater transmission time of the laser pulse, send a trigger signal to the gated camera to control the exposure time of the camera, ensuring that the camera only turns on when the laser pulse reflected by the target arrives. In the synchronous control unit, the galvanometer control is used to adjust the scanning angle of the galvanometer and take the laser trigger signal as the time reference, and send a trigger signal to the galvanometer to complete the rotation before the next trigger signal.

[0022] Example 2, as Figures 1 to 8 shown, on the basis of Example 1, the present invention also 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: In the full - field illumination working mode, the laser illumination field of view and the laser imaging field of view are the same. Uniform illumination is achieved through the circular beam expansion optical path, and the imaging speed is relatively fast. Combining image noise reduction and enhancement methods can improve the imaging signal - to - noise ratio for long - distance and small - field - of - view underwater imaging. The specific implementation is as follows: Step 1.1: The 532 - nm pulsed laser emits pulsed laser. Step 1.2: The pulsed laser is expanded and collimated by a collimating lens group to form a collimated parallel beam, improving the uniformity and directivity of the beam. The switching rotating mirror is adjusted to the circular beam expansion optical path, and the collimated beam is expanded into a circular beam by a concave lens to illuminate the full imaging field. Step 1.3: Collect the water body scattering background noise images at the non-target positions, and improve the stability of the background noise images through multi-frame averaging for subsequent differential processing; The collected water body scattering background noise images are as shown in Figure 2 (a) in. The noise is mainly water body scattering. Scattering refers to the phenomenon that the propagation direction of light changes when passing through the water body. Among them, backscattering will form background noise, interfering with the detection of effective signals, and forward scattering will reduce the imaging resolution, making the image blurred. Traditional filtering algorithms are difficult to effectively separate the target signal from the background noise; Step 1.4: According to the timing control of the synchronization control unit, collect and image the noisy image signals reflected by the target object; The target image collected at a distance of 30 m underwater is as shown in Figure 2 (b) in. This image contains both the target reflection signal and the water body scattering background noise; Step 1.5: Perform differential processing on the collected target image and the water body scattering background noise image. By correcting the background noise, reduce the interference of water body scattering and improve the contrast of the target image; The corrected image is calculated by the formula where I is the intensity of the collected target image, is the intensity of the collected water body scattering background noise image, is the transmittance of the water body, is the intensity of the corrected target image. The corrected target image is as shown in Figure 2 (c) in; Step 1.6: Perform dual-platform histogram equalization enhancement processing on the differential target image to enhance the details and clarity of the image and improve the signal-to-noise ratio of underwater long-distance imaging; The basic steps of dual-platform histogram equalization are as follows: First, calculate the statistical histogram of the original image , as shown in Figure 3 (a) in; Correct the statistical histogram through the formula , and the result is as shown in Figure 3 (b) in, where k is the gray value, is the lower threshold, is the upper threshold. When is between 0 and , adjust to . When is between and , remains unchanged. When is greater than , adjust Adjust to ; Then, accumulate the corrected statistical histogram, and obtain the cumulative histogram by calculating with the formula as shown in (c) of ; Calculate the image grayscale conversion equation using the cumulative histogram with the formula Figure 3 as shown in (c) of ; Finally, use the conversion equation to convert the original image, and obtain the equalized histogram as shown in (d) of Figure 3 ; The corresponding enhanced image is as shown in (d) of Figure 2 . The image contrast and brightness are enhanced, and the effective imaging area and target details are significantly improved; In the scanning illumination working mode, the emission angle of the line beam is rapidly and continuously changed by the galvanometer to complete the scanning imaging of the entire imaging field of view. The laser energy utilization rate is high. Combining the image fusion method and the downsampling scanning strategy can improve the imaging speed and resolution of long-distance and large imaging underwater. The specific implementation method is as follows: Step 2.1: The 532nm pulsed laser emits pulsed laser light; Step 2.2: The pulsed laser light is expanded and collimated by the collimating lens group to form a collimated parallel beam. Switch the rotating mirror and adjust it to the line beam shaping optical path. The collimated beam forms a line beam through the cylindrical lens and the galvanometer to illuminate the local strip imaging field of view; The target image at a distance of 30m underwater is collected as shown in (a) of Figure 4 . This image contains both the target reflection signal and the water body scattering background noise; Figure 4 Step 2.3: Set the stepping angle of the galvanometer during full-scene scanning to be twice the laser divergence angle to make the beams of adjacent scans seamlessly connect; Step 2.4: According to the timing control of the synchronous control unit, the galvanometer gradually scans the entire field of view, collects and images the noisy image signals reflected by the target objects at different positions; Step 2.5: Denoise each frame of the collected image by median filtering and perform dual-platform histogram equalization enhancement processing; The principle of median filtering is as shown in Figure 5 . First, determine the filtering window and position, then sort the pixel values within the window according to the grayscale size, and finally take the median value to replace the pixel value at the center of the original window. The target image after median filtering is as shown in (b) of Figure 4 ; The dual-platform histogram equalization enhancement processing is the same as that in step 1.6 above and will not be elaborated here. The enhanced target image is as shown in (c) of Figure 4 . The target details in the dark area of the image are more obvious; Figure 4 Step 2.6: Stitch all the scanned images to form a complete field-of-view image, and eliminate the stitching traces through the threshold averaging fusion method to improve the signal-to-noise ratio and brightness uniformity of the image; The steps of threshold averaging fusion are as follows: First, calculate the column average gray curve, as shown in (a) of Figure 6 , set a variable threshold to extract the illumination area and its neighborhood to avoid interference from the black background; after smoothing the curve by mean filtering, determine the maximum value position, and take 10% pixel ranges on both sides, and use the minimum column mean as the threshold to achieve adaptive adjustment, and smooth the column average gray curve as shown in (b) of Figure 6 ; then accumulate the pixel columns higher than the threshold in all images, and record the accumulation times. Normalize the accumulation result by dividing it by the accumulation times and restore it to the original gray range. Finally, obtain the target image after scanning fusion as shown in (d) of Figure 4 . Compared with the single-frame image, the fused image realizes brightness balance and detail enhancement, making the texture details in the illumination area clearer and obtaining a complete and high-quality underwater target image; Step 2.7: By adopting a decimation scanning strategy, increase the galvanometer angle to further improve the imaging speed. After image denoising, enhancement and fusion processing, recover more detailed information from the decimation scanned image set to improve the large field-of-view imaging resolution; The specific steps of decimation scanning are as follows: First, determine the stepping angle of the galvanometer during full-scene scanning , set a new stepping angle according to the decimation factor n , then initialize the galvanometer to ensure that the starting position is fixed, and rotate the reflecting mirror step by step according to the set stepping angle , so that the line beam scans the target area in turn, and at the same time, the gated camera records the image after each illumination, so as to obtain a decimation scanned image set. Finally, complete the image restoration through Steps 2.5 and 2.6. The image results after scanning fusion when the decimation factor n is 2, 3, 4, 5 are as shown in Figure 7 . Decimation can effectively improve the imaging speed while maintaining the main structure and visual information of the image; To determine the appropriate decimation factor, structural similarity is used to evaluate the similarity between the decimation scanned image x and the full-scene scanned image y, which is calculated by the formula , where and are the average values of images 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, or 5 are as Figure 8 shown.

[0023] As described above, it is only the specific implementation manner of this application, but the protection scope of this application is not limited thereto. Any person skilled in the art within the technical scope disclosed by this application can easily think of changes or substitutions, which should all be covered within the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

Claims

1. An underwater dual-mode laser range-gated imaging system, characterized in that: include: A laser illumination unit, including a 532nm pulsed laser and an emission optical unit, is used to generate pulsed laser and control the optical path, and perform time-division multiplexing of full-field illumination and scanning illumination, wherein the emission optical unit includes a switching mirror and a galvanometer; A laser imaging unit, including a gated camera, an imaging lens and a narrow-band filter, is used to receive the pulsed laser signal reflected by the target object and image it; A synchronous control unit is electrically connected to the 532nm pulse laser, the gated camera, the switching mirror and the galvanometer, respectively, for transmitting control signals, performing gated 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 acquired images and provide a high-quality target detection image source for the computer terminal.

2. An underwater dual-mode laser range-gated imaging method, implemented based on the underwater dual-mode laser range-gated imaging system described in claim 1, characterized in that: The following steps are involved: S1: The pulsed laser is emitted through a 532nm pulsed laser, which is expanded and collimated by a collimating lens group to form a collimated parallel beam; S2: Switch to the circular beam expansion optical path or the linear beam shaping optical path by switching the rotating mirror, and switch between the two working modes of full-field illumination and scanning illumination; 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, perform dual-platform histogram equalization enhancement processing on the differential target image, and obtain an enhanced image; S4: In the scanning illumination working mode, noisy image signals are collected, all scanned images are stitched together to form a complete field of view image, and the stitching traces are eliminated by the threshold average fusion method. Then, the imaging speed is further improved by the downsampling scanning strategy, and the detail information is recovered from the downsampling scanned image set; S5: Integrate images in full-field illumination working mode or scanning illumination working mode to output high-quality target detection images.

3. The underwater dual-mode laser range gating imaging method according to claim 2, characterized in 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.

4. The underwater dual-mode laser range gating imaging method according to claim 3, characterized in that: The 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 composed of two plano-convex lenses, wherein the first plano-convex lens focuses the laser, and the second plano-convex lens adjusts the focused beam again to make it 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.

5. The underwater dual-mode laser range gating imaging method according to claim 4, characterized in that: The 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 the local strip imaging field of view.

6. The underwater dual-mode laser range gating imaging method according to claim 5, 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 scattered background noise of the water body; The collected target image and the water scattered background noise image are subjected to differential processing to correct the background noise to obtain a corrected image, and the intensity of the corrected target image is calculated by combining the intensity of the collected target image, the intensity of the collected water scattered background noise image and the transmittance of the water body; Perform dual-platform histogram equalization enhancement processing on the target image after difference to enhance the details and clarity of the image; Based on the 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 the cumulative histogram, and then the image grayscale conversion equation is calculated using the cumulative histogram, and the image grayscale conversion equation is used to convert the original image to obtain the enhanced image.

7. The underwater dual-mode laser range gating imaging method according to claim 6, 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 corrected target image intensity; The expression for correcting the statistical histogram is: ； In the formula, is the corrected statistical histogram, is the statistical histogram of the original image, k is the gray 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: ； In the formula, is the cumulative histogram; The cumulative histogram is given by the formula Calculate the image grayscale conversion equation.

8. The underwater dual-mode laser range gating imaging method according to claim 5, characterized in that: The S4 specifically includes: When scanning the entire scene, set the step angle of the galvanometer to twice the laser divergence angle, so that the beams of two adjacent scans are seamlessly connected; According to the timing control of the synchronous control unit, the galvanometer gradually scans the entire field of view, and collects and images the noisy image signals reflected by the target at different positions; Each frame of the collected image is subjected to noise reduction through median filtering and enhanced through dual-platform histogram equalization; All scanned images are stitched together to form a complete field of view image, and the stitching traces are eliminated by thresholding and 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 scanning image set to obtain a high-quality scanned image.

9. The underwater dual-mode laser range gating imaging method according to claim 8, characterized in that: The threshold average 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 pixel range is taken on both sides, and the minimum column mean is used as the threshold for adaptive adjustment; 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, restore to the original grayscale range, and finally obtain the target image after scanning and fusion.

10. The underwater dual-mode laser range-gated imaging method according to claim 8, 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 The reflector is rotated gradually so that the line beam scans the target area in sequence; The image after each illumination is recorded by a gated camera, thereby obtaining a downsampled scan image set, and then determining the downsampling coefficient to complete 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, which is 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. It is preferred to select the maximum downsampling factor within the range of SSIM>0.8.

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