Technical principle and development of underwater imaging technology

The lower imaging technology has extensive and important application value in underwater target discovery, sea surface material detection and marine geography engineering, and is receiving increasing attention from researchers in various countries. Unlike the airborne imaging technology that we usually see, the characteristics of the aqueous medium are strong scattering effects and fast absorption power attenuation, so the camera is directly applied to the water. Due to the strong scattering effect, the image is very noisy and the distance is limited. The use of lasers has solved the problem of imaging distance to some extent. In the past few years, the imaging distance and image quality have been greatly improved. These advances are due to the use of non-traditional imaging technology and laser technology. This paper analyzes several main underwater imaging technologies and discusses their respective technical principles and developments.

From the above, compared with atmospheric imaging technology, the focus of underwater imaging technology is to reduce the strong scattering effect of water and the absorption power attenuation characteristics of the specific medium. Imaging technology has been applied in practice and achieved good results.

Conventional underwater imaging technology

Conventional underwater imaging techniques include laser scanning underwater imaging and range gated laser underwater imaging. Among them, laser scanning underwater imaging is the principle that the backscattered light intensity of water is rapidly reduced relative to the central axis. In this system, the detector is placed separately from the laser beam. The laser emitter uses a narrow beam of continuous lasers, while using a narrow field of view receiver, there is only a small overlap between the two fields of view. The scattered light received by the small detector. The image is reconstructed pixel by pixel using synchronous scanning techniques. Therefore, this technology mainly relies on high-sensitivity detectors to track and receive target information in a narrow field of view, thereby greatly reducing the influence of backscattered light on imaging, thereby improving the system signal-to-noise ratio and the range of action.

The range gated imaging system uses a pulsed laser, and during image-enhanced CCD imaging with gating functions, the laser backscatter from the target back to the detector is reduced by gating the receiver aperture. In this system, a very short laser pulse illuminates the object, the camera shutter is opened for a certain time relative to the laser emission time of the illuminated object, and the shutter is open for a short period of time during which the detector receives the object. The returned beam, which excludes most of the backscattered light. Since the first photon returned from the object experiences minimal scattering, the gating receives the first returned photon beam for the best imaging results. If you want to obtain three-dimensional information of an object, you can use different detectors to set different delay times to obtain information of different levels of the object, so it provides the ability to image quasi-three-dimensional information of the object.

2. Underwater laser 3D imaging technology

The above two technologies cannot provide perfect 3D information capabilities, and the striped tube underwater laser 3D imaging technology can provide good 3D information. The striped tube underwater laser three-dimensional imaging technique uses a pulsed laser, and the receiving device is a time-resolved stripe tube. The emitter emits an off-axis fan beam that is then imaged on the slit photocathode of the stripe tube. The photoelectrons escaping from the photocathode are accelerated, focused and deflected by parallel plate electrodes. At the same time, there is a scanning voltage perpendicular to the direction of the fan beam to control the beam deflection in real time, so that the distance and orientation images of each laser pulse can be obtained. These distance and azimuth images are digitally stored using conventional CCD technology, so that the pulse repetition frequency of the system is synchronized with the advance speed of the platform, and the scanning route is swept by a pressure brush. In this imaging configuration, each laser pulse produces an image throughout the fan beam that is used to provide a larger width. Therefore, using current laser and CCD techniques and relatively modest pulse repetition frequencies, higher search speeds can be achieved.

3. Polarized underwater imaging technology

Among the 97% of the marine water bodies, the quantitatively dominant scattering particles are small particles having a diameter of less than 1 Lm, and the relative refractive index is 1100 to 1115. They generally follow the Rayleigh or Mie scattering theory. If illuminated with a polarized light source underwater, most of the backscattered light will also be polarized, which allows the backscattered light to be suppressed with an appropriately oriented analyzer to enhance image contrast. Polarization imaging technology uses the difference in polarization characteristics of reflected light and backscattered light of an object to improve the resolution of imaging. According to the scattering theory, the degree of depolarization of the reflected light of the object is greater than the degree of depolarization of the scattered light of the water. If the laser emits horizontally polarized light, when the linear polarizer in front of the detector is in the horizontal polarization direction, the reflected light energy and the scattered light energy of the object are approximately equal, the contrast is minimal, and the image is blurred; when the polarization direction of the linear polarizer and the polarization direction of the light source When it is vertical, the reflected light energy of the received object is much larger than the scattered light energy of the light source, so the contrast is the largest and the image is clear.

Among the above technologies, we believe that the distance gate strobe is an important one, because in fact, the principle of distance gating is also used in the three-dimensional imaging technology, but the method is different, so their performance requirements for the imaging detector Are the same.

Laser technology

Laser scanning underwater imaging systems mostly use argon-ions continuous wave lasers with an output power of less than 5W. The argon ion continuous wave laser has the characteristics of good beam quality, high resolution and stable image. Since the larger laser power does not significantly increase the imaging distance, the argon ion continuous wave laser is a better choice for laser scanning underwater imaging systems.

Most of the lasers for gating underwater imaging and other three-dimensional underwater imaging systems use flash-pumped Nd:YAG lasers. The device is mature in technology, low in cost, and has an output wavelength of 1106Lm. After multiplying, 532nm green light can be obtained. At present, the average power can reach several tens of watts, and the working distance can reach several kilometers. In order to improve underwater imaging quality, increasing the power of the illumination laser and miniaturizing the laser are the current development directions.

For Nd:YAG lasers, as the cost decreases, the future trend is to use laser diode pumping and continuous diode laser pumping. The reason is simple. Diode lasers are small and lightweight, making them ideal for underwater operation.

2. Receiver technology

The underwater imaging environment requires the imaging system to have low-light imaging capability and gating characteristics to eliminate backscattering interference. This requires the receiver to have high-speed external triggering, high resolution, high sensitivity, low noise, and sufficient gain dynamics. range. Therefore, a low-light imaging camera is generally used. Typical low-light imaging cameras include: Silicon Enhanced Target (SIT) cameras, Enhanced Silicon Enhanced Target (ISIT) cameras, Charge Load Device (CCD) cameras, and Enhanced Charge Load Device (ICCD) cameras. Compared with these devices, ICCD is the first choice for receiving devices due to its high sensitivity.

In addition, devices under development include electron bombardment CCD (E2BCCD) imaging devices and electron multiplying CCDs (EMCCD). Due to the influence of MCP, fiber optic panel window and fluorescent screen, the attenuation of noise and MTF through multiple transmission links deteriorates the image quality. In 1996, the United States developed an electron bombardment CCD, which placed the CCD on the electron imaging surface of the vacuum tube and used the electronic gain of high-energy electrons to achieve electronic enhancement of the image. Therefore, the device is not affected by the microchannel plate and the fiber optic panel, and is highly Sensitivity and almost noise-free gain. The EM CCD is an imaging device currently manufactured using the latest CCD production process. It inherits the advantages of the CCD device and has similar sensitivity to the ICCD device.

The United States has actually started research on laser scanning underwater imaging systems more than a decade ago, and there is no official report for confidentiality reasons. At present, various types of laser scanning underwater imaging systems have been developed abroad, and some have been successfully used in submarine survey, search and imaging. Westinghouse Underwater Laser Systems and Applied Telemetry Technologies have developed their own synchronous scanning laser underwater imaging systems. The effective field of view scanning time of Westinghouse's laser underwater imaging system is 011ms. The system is laid under the submarine or towed behind the surface ship. When the ship sails forward, the system images a two-dimensional image of the seabed, and the underwater observation and imaging distance can reach four attenuation lengths (the attenuation of light in water varies with the wavelength of light and water quality), the image Both resolution and clarity are high. The underwater imaging distance of Telemetry's synchronous scanning system can reach 5 attenuation lengths. Telemetry's system uses a fast-rotating prism to control laser beam scanning, which is currently equipped with submarines.

Research on gating laser underwater imaging technology has been relatively early, and the United States began research in 1966. The laboratory simulation results reported by Sperry Marine Co., Ltd. in the United States recently show that in a water body with an attenuation coefficient of 0.1/m, the observation distance is close to the theoretical calculation, up to 160 m, and a distance of 30 m can be observed in the turbid near-shore water body. The light source used is a frequency-doubled Nd:YAG laser, and the receiver is an enhanced CCD camera manufactured by the company. Sparta's successful distance-gated laser underwater imaging system uses a flash-pumped Nd:YAG laser. The laser output is multiplied to produce 532nm green light. The repetitive operating frequency is 10Hz, and the electrical efficiency converted to green is 1 %, power consumption is 250W, and the system field of view is up to 12°. The imaging distance of this system in the port waters is five times that of the 500W bulb illumination system. In addition, the ranging system using the frequency doubled Nd:YAG laser also achieves an imaging distance of 30 to 50 m.

In the research of 3D imaging technology, the American Tucson Arete Association conducted a preliminary experiment on the high-resolution 3D imaging system based on stripe tube technology in the laboratory with the support of the Naval Research Bureau, and determined the lateral and range resolution of the stripe tube receiver. . Achieve high-resolution 3D images with the collaboration of the University of Arizona Optical Science Center. Using a frequency-doubled Q-switched mode-locked Nd:YAG laser, 35 532 nm pulses are generated per second. In general waters, the experimental results in three-dimensional directions are higher than 6 135 mm; in deep water pools, the distance resolution is experimentally obtained. It is 91144m, the lateral resolution is about 6135mm, and the range resolution is limited by the 9ns pulse width of the Q-switched laser. More accurate distance resolution can be obtained if a collision-pulsed mode-locked dye laser producing three 580 nm pulses per second is used. In turbid seawater, high-resolution pulsed lasers are needed to overcome the effects of seawater attenuation and scattering if high-resolution images are required at distances of 10 to 30 m. In order to improve the range resolution, LietCycles Co., Ltd. has developed a Raman compression laser. The use of this 2 ns laser can greatly improve the range resolution, and can clearly image the target of 432 mm underwater.

In the field of polarized underwater imaging technology, many simulation experiments have been done abroad since the 1960s. The results of the British Naval Diving Medical Research Laboratory in the swimming pool show that when the brightness of the linear polarized light source is constant, the diver's visual sharpness and observation distance are better than those without the analyzer when wearing the analyzer. Free-flowing divers often have to adjust the polarization orientation of their analyzers, which is very inconvenient. The use of circularly polarized illumination does not have the above inconvenience, and some experiments have shown that most diffuse targets tend to depolarize circularly polarized light better than depolarization of linearly polarized light. The US Naval Underwater Research and Development Center and the Naval Weapons Test Station conducted simulation experiments on the use of circularly polarized illumination to improve visibility, and obtained positive results, and measured the polarization targets of diffuse targets and water bodies.

Domestically, in recent years, Xi'an Institute of Optics and Mechanics, Changchun Institute of Optics, Shanghai Institute of Optics and Technology, Tianjin Television Technology Research Institute, Beijing Institute of Technology, Huazhong University of Science and Technology, Southeast University and other units have conducted research on underwater imaging systems. However, there is still a big gap compared with the international advanced level. Beijing Institute of Technology is developing a strobe-type ICCD device suitable for underwater imaging systems, and adopts a domestic high-performance super-second generation image intensifier, which is expected to promote the development of underwater imaging systems in China.

This paper outlines the working principle, characteristics and development of underwater laser imaging technology, and describes the related technologies for underwater image processing. From the current state of the art, underwater imaging technology is a system engineering, which depends not only on the corresponding device performance, but also on the overall system design. In addition, some advanced recognition technologies, such as distance coding, polarization filtering, image extraction, etc., are expected to be further applied in underwater imaging systems.