Cloud Measurement with SmHOLIMO Holographic Imager

Researchers at ETH Zurich have unveiled SmHOLIMO (Small Holographic Imager for Microscopic Objects), a cutting-edge holographic imager designed for high-resolution in situ measurements of cloud droplets as small as 3.7 µm. The findings were published in the journal of Atmospheric Measurements Techniques.

Unlike traditional probes, SmHOLIMO operates in an open-path configuration, eliminating the need for inlets. Traditional probes with inlets can distort measurements because droplets may evaporate, collide, or change speed when passing through the inlet. This leads to inaccurate data, especially for very small droplets. However, since SmHOLIMO uses open-path design, air flows freely through the measurement area. This reduces sampling bias and preserves the natural state of droplets under variable wind conditions.

For the holographic imaging system the group utilised the Cobolt 06-MLD 405 nm laser at 150 mW as the coherent light source.

After calibrating SmHOLIMO in the laboratory, the instrument was deployed in the field, connected to balloon system, testing it on a dissipating low stratus cloud. The scientists were able to measure the cloud microphysical properties at high spatio-temporal resolution, proving that SmHOLIMO can outperform previous instruments in detecting small droplets.