A 3D measurement method of bubbles based on edge gradient segmentation of light field images

Abstract

Based on the light field imaging technology, a three-dimensional(3D) measurement method of bubbles through the use of edge gradient segmentation (TMBEGS) is proposed in this paper. From the characteristic that the bubble edge image has great gray gradient, the coordinates of the bubble edge pixels are determined. The depths of the edge pixels are further derived by evaluating the sharpness of the edge pixels in the refocused light field images. On this basis, the depth coordinate of the bubble is calculated by averaging the depths of all edge pixels, and the 3D measurement of the bubble is then achieved by combining the centroid coordinate and equivalent diameter of the bubble. Finally, experiments are carried out to evaluate TMBEGS on a calibration setup and a bubbling bed experimental device. Results indicated that the relative errors of the bubble diameter and depth by TMBEGS are less than 1.18 % and 5.93 %, which are better than 2.99 % and 17.51 % by the existing method, respectively. On the bubbling bed experimental device, the equivalent diameter of bubbles is increased by 5.69 % compared with the existing method, and the standard deviation of the bubble depth is less than 1.78 mm, which is 21.3 % lower than the existing method. With the increase of the bubble equivalent diameter, the bubble surface deformation intensifies under the effects of the bubble surface tension, viscous force, and inertia force, which affects the imaging quality of the bubble, and reduces the measurement accuracy of the bubble edge and depth. These results illustrated that TMBEGS has high measurement accuracy of the bubbles and is capable of charactering the bubbles in the gas–liquid two-phase flow system.