Holography of fast-moving cloud droplets

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Abstract
In order to test the feasibility of measuring the size distribution of cloud particles from an aircraft with a holographic technique, small water droplets (diameters 15-100 μm) were recorded holographically in the laboratory using Gabor's original lensless `in-line' technique. Using a Q-switched ruby laser (pulse duration similar 10-20 ns) it was found that droplets could be reconstructed satisfactorily even when moving at speeds comparable to those of an aircraft in cloud (similar 100 m s−1). Various factors affecting the resolution and depth of sample volume obtainable with this technique are considered and it is shown that the spatial coherence of the pulsed laser is likely to be the most serious limitation. Nevertheless, in practice droplets originally located at distances up to about 13 cm from the recording plane were satisfactorily reconstructed, which suggests that it should be possible to achieve a sample volume in excess of 051 with each hologram.

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