Underwater imaging technology plays a crucial role in various applications such as underwater target detection, surface material analysis, and marine geological engineering. It has attracted growing attention from researchers worldwide due to its significant potential. Unlike airborne imaging, which operates in air, underwater imaging faces unique challenges caused by the optical properties of water, including strong light scattering and rapid absorption. These factors limit image clarity and range when using conventional cameras directly submerged in water.
To overcome these limitations, laser-based imaging techniques have been developed, significantly improving imaging distance and quality. Recent advancements in non-traditional imaging and laser technologies have enabled more effective underwater observation. This paper explores several key underwater imaging methods, explaining their principles and recent developments.
One of the most widely used techniques is laser scanning imaging. This method relies on a narrow laser beam and a detector with a limited field of view, minimizing the impact of backscattered light. By synchronizing scanning techniques, images are reconstructed pixel by pixel, enhancing signal-to-noise ratios and extending operational range.
Another important technique is range-gated imaging, which uses pulsed lasers and a gated camera. The camera shutter opens for a short time after the laser pulse, capturing only the first return photons that experience minimal scattering. This approach allows for better depth resolution and quasi-3D imaging. By adjusting delay times, different layers of an object can be captured, offering enhanced spatial information.
Laser 3D imaging, particularly using striped tube technology, provides even more detailed three-dimensional data. A pulsed laser illuminates the scene, and the reflected light is captured by a time-resolved stripe tube. This system enables large-area imaging with high speed and accuracy, making it suitable for wide-ranging underwater surveys.
Polarized imaging is another promising technique. Water particles scatter light in specific ways, and polarized light sources can help reduce background noise. By using polarization filters, the contrast between the object and the surrounding medium can be improved, resulting in clearer images. This method is especially effective in turbid waters where traditional imaging struggles.
In terms of hardware development, lasers play a central role. Continuous-wave argon-ion lasers are commonly used for scanning systems, while flash-pumped Nd:YAG lasers are preferred for gating and 3D imaging. These lasers offer high power and long-range capabilities, though miniaturization and cost reduction remain ongoing goals.
Receivers must also be highly sensitive and capable of handling low-light conditions. Advanced devices like ICCD (Intensified Charge-Coupled Device) and EMCCD (Electron-Multiplying CCD) are being developed to improve image quality and reduce noise. These technologies are critical for achieving clear, detailed underwater images.
Globally, countries like the United States have made significant progress in underwater imaging systems. Commercial systems are already in use for submarine surveys and remote sensing. Meanwhile, research in China is advancing, with institutions working on improving device performance and system integration.
Overall, underwater imaging remains a complex system engineering challenge, requiring both advanced hardware and intelligent software algorithms. Future developments may include the integration of distance coding, polarization filtering, and image enhancement techniques to further improve performance and expand application possibilities.
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