The Design Of Portable Underwater Imaging System With Continuous Illumination And The Research On Enhancement Technique Of Imaging Quality

The technique of underwater imaging with continuous illumination is widely used in engineering and scientific research fields that are performed in the ocean, inland rivers and lakes. The imaging systems designed using this technique are shipped underwater vehicles as their machine visions. But the imaging range and quality of the system are seriously restricted by the absorption and scattering of water for the light. Though the imaging range of imaging systems reaches above 6 times of attenuation lengths using synchronous scanning technique or ranged-gated technique, the systems are too huge and expensive to be applied in flexible underwater detection. Designing an underwater imaging system with continuous illumination possessing portability and excellent imaging performance has wide application prospects. This dissertation is sponsored by the national scientific research project. Considering all of the aspects of designing imaging system works, a portable underwater imaging system with continuous illumination is designed. Then the polarization imaging technique is proved to be effective for enhancing imaging quality by experiments.In this dissertation, the imaging process is analyzed by using of classics underwater imaging model at first, and the characteristics of the receive signals are obtained by this theoretical model. According to the system structure of the above analysis, the existing Monte Carlo underwater detection model of our research group is improved with polarization property to fit this application. The improved underwater imaging model is used to simulate the underwater imaging and polarization imaging processes. Then for objectively evaluating the imaging quality of the simulation and experimental systems, contrast and modulation transfer function(MTF) are presented as the evaluation criterions for the imaging quality of the system. The modified slant-edge method is introduced to calculate MTF from the underwater images based on the active illumination structure. This process will provide accurate evaluation criterions for the following simulation and experimental results.According to the established model, a preliminary system design is presented using comprehensive noise suppression technique. Simulation research is carried out on the underwater imaging process. From the simulation results, the relationship between system configuration and imaging performance is summarized, and the optimized system configuration is defined. After that, a prototype is set up to certify the validity of the theoretical analysis. The prototype is tested in different water conditions, and the test results show that the system configuration defined by the simulation is reasonable. The prototype reached about 4 times of attenuation lengths in laboratory, and about 2 times in actual environments. Nonetheless, the definition and the prototype volume do not match the design objective.Based on the experiences of the prototype experiments, the system is improved in most parts, including light source, imaging device, system structure, water tightness and so on. After improvements, the volume is changed as 1/3 of the prototype volume, and the system configuration is more reasonable. This improved system is experimented under water in the laboratory and typical water conditions. The portability is realized from the experimental process. The imaging range reaches 4 times of attenuation lengths in laboratory, and 2.4 times in the typical water conditions. This performance is close to that of international congeneric classic products. The resolution of the imaging device is no longer the choke point at the resolution performance. However, the suppression for the scattering is limited from the optimizing on the system configuration, and the above analysis does not involve the resolution enhancement.This dissertation regards the contrast and MTF as evaluation criterions of imaging quality. For distinguishing desired signals and scattering, polarization imaging technique is introduced into underwater imaging system, and investigated whether the polarization imaging enhances both contrast and resolution. The Monte Carlo model with polarization property is used to simulate the imaging process, and the preliminary simulation results show the effectiveness of the resolution enhancement. Depending on the theoretical analysis and the structure of the prototype system, an underwater imaging experimental system with polarization illumination is set up, and two targets with different polarization properties are designed at the same time. A great number of experimental results show that the change rules of imaging quality are different using two targets under different polarization detections. The contrast and MTF results present that orthogonal polarization imaging method is more advantageous for enhancing imaging quality with the low degree of polarization target, and polarization difference method is more effective for enhancing imaging quality with the high degree of polarization target. Meanwhile, according to the results of Non-orthogonal polarization difference algorithm, it is obtained that the optimal difference methods will refer to system characteristics and actual environments. From the above conclusion, polarization imaging technique can suppress both back scattering and forward scattering to enhance the contrast and resolution in underwater imaging application. This conclusion is effective for both linear and circular polarization illumination conditions. This study provides a simple and potential technological method for improving performance of underwater imaging system.