Discrimination and Measurement of Droplet and Ice Crystal by Combining Digital Inline Holography With Interferometric Particle Imaging With Single Color Camera

The discrimination and measurement of complex particle field mixing spherical liquid droplets and other solid irregular particles are challenging but of significance in industry, e.g., droplets and ice crystals in icing wind tunnel or in mixing phase cloud during airplane icing flight test. An approach is proposed by combining digital inline holography (DIH) and interferometric particle imaging (IPI) techniques, and a setup is elaborately designed using a color camera to simultaneously record DIH and IPI signals of the same droplet/ice crystal in the red and green channels of an red-green-blue (RGB) image, respectively. Water droplets and ice crystals generated in a cooling chamber are experimentally tested. The results show that IPI signals tune from regular interferometric fringes to random speckles as water droplet converts into ice crystal, and it is an obvious and robust indicator to discriminate water droplet and ice crystal. Water droplet sizes in the range from 100 to
$300 ~\mu \text{m}$
measured by DIH and IPI coincide with each other, with relative deviations within ±5%. Concerning ice crystal, the 2-D Fourier transform of IPI speckle pattern agrees well in both overall shape and size with 2-D autocorrelation of DIH reconstructed in-focus image, with size discrepancy within 10%. With the capability of phase discrimination and size and shape measurement, the proposed method combining DIH and IPI has the potential to be instrumented and applied to cloud particle fields.