准确地提取出掺气水流中的气泡，是利用图像测量方法进行水流特性分析的前提。由于掺气水流中的气泡是无色透明的、对比度低，而且由于实验过程的光照不均，从而给图像高精度分割带来困难。由于光照不均，致使掺气水流图像中，气泡信息强弱不均，此时又会因强信息淹没弱信息而导致气泡漏提取或提取气泡缺损，为解决此问题，提出了一种双层动态阈值的分割方法，即首先在第1层完成对气泡所在局部区域的定位，再通过贴标签处理确定出每一个气泡的局部区域；然后在第2层的精分割中，对所有局部区域进行精细分割，以实现气泡区域的精确提取。实验结果证明，该方法可以大大提高气泡提取的精度。

Bubbles extraction in the images of aerated water flows accurately is the precondition to analyze the characteristics of water flows by image detection. Because bubbles are colorless and transparent in the aerated water flows and the contrast of object and background is too low and at the same time there are un-even illuminations during experimentation. It is difficult to segment bubbles accurately. In order to solve the problems of miss-extraction or deformity of bubbles which caused by un-uniformity of bubbles resulted from the un-balance illumination of the image, a method based on two layer dynamic thresholding is proposed in this paper. At first, the local areas which containing bubbles are obtained dynamically by labeling in the first layer processing. Then, bubbles are extracted accurately in the second layer with fine segmentation based on the labeled bubbles' area in the first layer. Experimental results show that the proposed method is very efficient in solving the bad illumination and can highly improve the accuracy of bubble extraction.