Simultaneous interferometric in-focus and out-of-focus imaging of ice crystals

Using a freezing column, dendrite-like ice crystals are generated and characterized simultaneously using in-focus imaging and interferometric out-of-focus imaging. This simultaneous analysis allows a validation of size measurements made from the analysis of the 2D-autocorrelation of speckle-like interferometric out-of-focus patterns of ice crystals. Measurements of the same particles by in-focus and out-of-focus techniques are in good agreement for 75% of the particles tested. Simulations of out-of-focus patterns are in very good agreement with experimental images. The analysis of the 2D-Fourier transform of the speckle-like patterns confirms that it is possible to evaluate the 2D-autocorrelation of the global shape of the particle (i.e. its 2D-projection on the plane of the CCD sensor).