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Overview: Underwater optical imaging is widely used in industry, agriculture, scientific research, and other fields. When there are heat sources in the imaging light path, the target itself is a heat source, or there are disturbances caused by other reasons in the water environment, due to the non-uniformity of the imaging light field, image distortion and defocusing will occur in the underwater image. Therefore, it is very necessary to study the problem of imaging distortion under the condition of underwater thermal disturbance.
In order to study the influence of underwater thermal disturbance, an experimental platform with heat sources and thermal convection is designed. The underwater imaging platform can be adjusted along the axis to change the distance between the camera and the target to be L1(500 mm), L2(1000 mm) and L3(1500 mm), respectively. The distortion level of target image is evaluated through the gray scale distribution, structural similarity image measurement (SSIM), and normalized maximum gray-scale gradient definition evaluation function of underwater images. For gray scale distribution, the trend of disturbance influence is analyzed based on statistics, which provides a basis for image sequence restoration and correction under thermal disturbance environment. For SSIM, it is an effective method for image distortion analysis, and its evaluation index includes the brightness, contrast, as well as structure of the image. For the evaluation of the fuzzy index, the normalized maximum gray gradient clarity evaluation function based on edge information is adopted, that is, the closer the gray value of the whole pixel of the image is to that of the reference image, the smaller the ambiguity degree of the image is.
Experimental results show that with the increase of the axial distance between the imaging system and the target, the level of image distortion and blurring becomes larger and larger. When the axial distance L1=500 mm, the SSIM is better than 0.7 and the normalized definition is better than 0.8. When the axial distance L3=1500 mm, the SSIM is lower than 0.2 and the normalized definition is less than 0.6. In addition, when the axial distance is L1, the drift of the edges will be greater as the imaging area comes closer the heating source in the radial direction, that is, the imaging distortion is more serious. Furthermore, under the same axial and radial conditions, the conclusion that the SSIM and normalized definition values of the target images are different at different times can provide a reference for further underwater image restoration.
Direction templates. (a) 0°; (b) 45°; (c) 90°; (d) 135°; (e) 180°; (f) 225°; (g) 270°; (h) 315°
Structure diagram of the experimental platform
Experimental site. (a) Experimental equipment; (b) Working diagram of imaging system
Collected image with no thermal disturbance
Collected image with thermal disturbance
Image gray-scale distributions of L1. (a) Column 1; (b) Column 2; (c) Column 3; (d) Column 4; (e) Column 5
Image gray-scale distributions of L2. (a) Column 1; (b) Column 2; (c) Column 3; (d) Column 4; (e) Column 5
Image gray-scale distributions of L3. (a) Column 1; (b) Column 2; (c) Column 3; (d) Column 4; (e) Column 5
Comparison of SSIM among different objective distances
Comparison of clarity among different objective distances