| In recent years, with the development of marine resource and underwater robotics, underwater laser imaging technology plays an important role in the fields of underwater physiognomy drawing, underwater archeology, sea rescue, underwater operation, exploring dam's hidden trouble, and detecting sunken ships and debris of aircrafts, etc. In addition, underwater laser imaging technology can be an important aid for sonar detection technology, to make up for low resolution of sonar imaging, is very suitable for detecting the close quarters target by dint of some small platforms like underwater robots (ROV, AUV, etc.), and has a certain flexibility.It is much more complex that transmit light in seawater than in the air, due to various factors. Effect of observation and photography in water mainly depends on the propagation characteristics of light in water. Seawater has not only a large number of inorganic substances, but also a variety of organic substances with different sizes, shapes, and performance. These components have random distributions, so light would be scattered. The attenuation of light in seawater is caused by the absorption and scattering. If a media only has absorption but no scattering, the visibility will only be a function of light source intensity and receiver sensitivity, increasing either one of these two factors can widen the visibility range. However, due to the scattering in seawater, too intense light will contrary reduce the image contrast. Therefore, this paper, which is focused on the method of "improving the contrast of underwater laser images", carries out some works in following aspects:1.The optical properties of water are experimentally studied particularly; transmission spectra of seawater and tap water are measured, respectively; refractive indexes at room temperature of pure water, tap water and seawater, and the change of the refractive indices with temperature are measured, respectively; attenuation coefficients of the seawater and the tap water are measured, respectively, at the wavelengthλ=532nm; differences of the tap water and the seawater in the aspects of the spectrum characteristics, refractive index, and attenuation characteristics, etc., are compared. All these works provide a basis to the theoretical and the experimental analysis in the subsequent chapters.2. According to the transmission characteristics of underwater light waves and basing on the optical scattering theory, scattering characteristics of the particles in seawater are systematically analyzed and calculated; the formula about the contrast is derived; factors impacting the contrast of underwater laser images are analyzed deeply.3. The method to improve the contrast of underwater laser images is proposed. Analyses of both theoretical and experimental studies have shown that, using symmetrical laser is an effective way to improve the contrast.4. According to thin-film optical theory, anti-reflective films for air - optical glass interface and seawater - optical glass interface are designed to enhance the contrast, and reflection curves and the calculation results are obtained.5. Experiments are carried out in a small water tank in darkroom, with blue-green laser ofλ=532nmas the light source, and tap water mixed with dust as the scattering particles in seawater. The experimental results are obtained and compared with the theoretical results. According to the characteristics of underwater optical images, the underwater images gotten from the experiments are processed. Approaches in processing underwater images are introduced and summarized.After summarized the research results, this paper looked forward to the next work.
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