Optical EngineeringNew method of obtaining particle diameter by the fast Fourier transform pattern of the in-line hologram
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A new method of determining particle diameter from a farfield in-line hologram is proposed. The method is based on the following properties of an in-line hologram of small particles. The fringe interval of the hologram decreases with radius from the symmetrical axis. Therefore, the corresponding spatial frequency continuously increases with radius. The Fourier transformed (FT) pattern of the hologram preserves the properties similar to those of the original fringe pattern. In the FT plane, the uniform background and scattering term contribute only to the very low frequency region. The interference term, therefore, can be figured out, and it is the only contribution to the higher frequency region. It is shown that the FT pattern has a white ring zone, which corresponds to the zero point of the envelope function of the original fringe pattern. Since the position of the ring contains information on the particle diameter, it is demonstrated that measurement of the diameter of this ring provides a powerful and reliable way of obtaining particle diameter.