Imaging turbulence characteristic analysis to underwater dynamic temperature field

The underwater dynamic temperature field is a common situation in the deep-sea resource exploration, underwater operation and maintenance of nuclear facilities and so on. Due to the fact that dynamic temperature field will lead to the dynamic non-uniformity of the refractive index of the imaging medium, it brings difficulties to underwater optical imaging. The most intuitive imaging effects include blurring image edge, drifting image pixel, increasing image noise and so on. These direct factors greatly limit the application of high-resolution imaging technology in the field.

To overcome the limitation of imaging resolution on underwater thermal field, a variety of methods based on digital image are applied in this case, such as the combination of bispectrum analysis technology for atmospheric turbulence and lucky block selection, which can solve the phase problem of distorted image to a certain extent; the method based on the combination of improved classical turbulence mode and polarization technology also has effect on blurring image; in addition, in order to considering the improvement of image clarity and computational efficiency, there are also a combination of sequential iterative image registration and optimal block selection methods. However, due to the lack of research on the characteristics of the underwater thermal field, image processing methods are difficult to solve all kinds of problems. Therefore, the research on the rules of thermal field and imaging influence is of great significance. And similar to the air medium, the research work based on image pixel gray level similarity, location similarity, ray tracing algorithm and so on has also been carried out by some scholars.

In recent years, Feng Chang, a researcher with Special Optoelectronic Intelligent Equipment Research Center, Institute of Optics and Electronics, Chinese Academy of Sciences, led the team to study high resolution imaging technology of underwater dynamic temperature field. On the basis of advanced technology achievements, the research on the impact rules of thermal field and underwater imaging was carried out. The experimental platform is shown in Figure 1, which can be used to analyze many factors, such as thermal power, different working object distance, large field of view and others. The experiment can provide data for further study of turbulence characteristics.

Fig. 1 underwater thermal field experimental platform

In this study, parameter calibration of the underwater camera is used to eliminate the influence of system distortion. The longitudinal light path and multiple combination of heating device are set to realize the rapid adjustment of heat dissipation rate of heat field. Finally, the gray distribution, structure similarity image measurement (SSIM) and the normalized maximum gray gradient definition evaluation function are used to analyze the radial and axial distortion of the target image and the degree of distortion in each kind of condition, and the rules of underwater thermal turbulence on optical imaging is obtained.

Based on the theory of underwater imaging degradation and the quantitative analysis of the data collected by the experimental platform, it provides the basis for further underwater digital image processing.

Fig. 2 the acquisition data of thermal field

Fig. 3 SSIM and normalized definition value with respect to different object distances

About The Group

The research team led by Feng Chang, a researcher with Special Optoelectronic Intelligent Equipment Research Center, Institute of Optics and Electronics, Chinese Academy of Sciences, has been committed to the research and development of photoelectric detection and robot technology in underwater nuclear radiation and other complex environments. Especially in the irradiation environment, the team has gradually formed their own research characteristics and technical expertise on video imaging technology, special industrial robot technology, underwater photoelectric detection technology, machine vision technology, special underwater lighting technology and other aspects. The relevant research has been supported by national, provincial and ministerial funding, and the relevant achievements have been recognized by peers and recognized by the state. The relevant indicators have reached the international equivalent or domestic leading level.

Relying on the research results of the team,  the high resolution imaging device suitable for underwater dynamic temperature field
has been tested in the underwater operation and maintenance of nuclear facilities, laying the foundation for further engineering application and field promotion.

Article

Wang Congzheng, Hu Song, Gao Chunming, et al. Study on optical imaging distortion of underwater thermal disturbance[J]. Opto-Electronic Engineering, 2019, 46(10): 180438.

DOI:10.12086/oee.2019.180438