Digital in-line holography (DIH) with a divergent beam is used to measure size and concentration of cavitation bubbles (6-100 μm) in hydrodynamic facilities. A sampling probe is directly inserted in the cavitation tunnel, and the holograms of the bubbles are recorded through a transparent test section specially designed for DIH measurements. The recording beam coming from a fiber-coupled laser diode illuminates the sample volume, and holograms are recorded by a CMOS camera. From each hologram, the sampling volume can be reconstructed slice by slice by applying a wavelet-based reconstruction method. Because of the geometry of the recording beam, a magnification ratio must be introduced for recovering the 3D location and size of each bubble. The method used for processing holograms recorded in such a configuration is presented. Then, statistical results obtained from 5000 holograms recorded under different pressures in the cavitation tunnel are compared and discussed.

The 50-year life span of Applied Optics covers also approximately the time I have been engaged in optics. I started in 1962 [1] with the Institute for Optics and Spectroscopy, which was one of several Academy Institutes (mission statement: "theoria cum praxi," G. Leibniz) located in Berlin-Adlershof on the area of the first airfield in Berlin dating back to the beginning of the 20th century.

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