  The paper presents a hybrid bubble hologram processing approach for measuring
the size and 3D distribution of bubbles over a wide range of size and shape.
The proposed method consists of five major steps, including image enhancement,
digital reconstruction, small bubble segmentation, large bubble/cluster
segmentation, and post-processing. Two different segmentation approaches are
proposed to extract the size and the location of bubbles in different size
ranges from the 3D reconstructed optical field. Specifically, a small bubble is
segmented based on the presence of the prominent intensity minimum in its
longitudinal intensity profile, and its depth is determined by the location of
the minimum. In contrast, a large bubble/cluster is segmented using a modified
watershed segmentation algorithm and its depth is measured through a
wavelet-based focus metric. Our processing approach also determines the
inclination angle of a large bubble with respect to the hologram recording
plane based on the depth variation along its edge on the plane. The accuracy of
our processing approach on the measurements of bubble size, location and
inclination is assessed using the synthetic bubble holograms and a 3D printed
physical target. The holographic measurement technique is further implemented
to capture the fluctuation of instantaneous gas leakage rate from a ventilated
supercavity generated in a water tunnel experiment. Overall, our paper
introduces a low cost, compact and high-resolution bubble measurement technique
that can be used for characterizing low void fraction bubbly flow in a broad
range of applications.
