A high sampling rate digital holographic imager instrument for the in situ measurements of hydrometeors
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A novel digital in-line holographic imaging instrument designed for acquiring properties of individual hydrometeors in situ is presented. The instrument has a large measurement volume of 670 cm3. This combined with fast frame rate imaging and software controlled multi-exposure capabilities results in a representative sampling of rain and snowfall events. Hydrometeors are measured and analyzed from the in-focus images with microscopic resolution, and their 3D locations inside the measurement volume are determined. The instrument is designed to operate in cold climates and to produce reliable measurements also during strong winds. The imaging rate of the instrument was designed to be adequately high to observe the dynamic nature of rain and snow falls. By recording multi-exposure holograms, the effective frame rate can be increased. This allows the measurements of the velocities of the fast-falling hydrometeors. The instrument and the hologram processing are described; as well as results from laboratory tests and the first field measurements are shown. As a result, the resolving power of the instrument was measured to vary between 11 and 18 microns inside the measurement volume near the center of the field-of-view. Velocity vectors were measured both from multi-exposure and high frame rate holograms. The measured velocities ranged from 0.1 to 4 m/s. In addition, the projections of a flat-shaped and rotating snowflake imaged at different locations inside the measurement volume demonstrated the possibility to estimate the shape of the hydrometeor from multiple viewing angles.
KeywordsLaser Imaging systems Meteorology Optical instruments Disdrometry
The authors would like to thank Mr. Timo Arstila for help in preparing the instrument and the USAF1951 holograms, Mr. Timo Kananen for the instrument and server software programming, Mr. Ilkka Leinonen, Mr. Lasse Rajakangas and Mr. Veijo Sutinen for the mechanical design and implementation of the instrument and both the University of Helsinki and the Finnish Meteorological Institute for the possibility to install our instrument in the measurement field at Hyytiälä Forestry Field Station.
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