Particle Measurement With Digital Holography: Computational Simulation And Experiment

In many fields of science and technology, it is very important to acquire the size, position and velocity of particles or droplets. There are plenty of techniques to get these parameters. Of all the techniques, optical in-line holography has some particular advantages: non-destructive, wide size-range of the measured particles, large depth of field, direct reconstruction of 3-D particle field, high resolution and simple set-up. Digital holography is a novel imaging technique which uses high-resolution CCD-cameras as hologram recording medium and reconstructs the image with numerical method. It involves optical holography, computer science, electronic technology and digital image processing. In recent years, particle analysis with digital in-line holography is an active research field.The main contents of this thesis are:(1) Two numerical reconstruction methods-Direct Fourier transform method and envelope method, are presented. These methods are deduced on the base of Fresnel diffraction integral formula, which is under the condition that particle field is illuminated by plane wave. In the case of spherical wave, the numerical reconstruction methods are similar to the previous methods. The results indicate that the envelope method is more effective in actual application. And the recording distance limitations due to the stated pixel number, pixel size and laser wavelength are discussed in detail.(2) Computational simulations on in-line holography of single particle, multi-particle in one plane and particle field(multi-layer) are performed. A technique to find the focal plane is given according to the gray-gradient curve of the particle image. An improved numerical reconstruction method is proposed, which can eliminate the border diffraction.(3) In multi-particle simulation, if the particles are not too dense, the reconstruction images of particles are clear. When the particles density reaches some degree, the definition of the reconstruction images will decrease along with the particles density growing. This phenomenon is explained in this thesis.(4) In static particle field (multi-layer) simulation, the improved numerical reconstruction method which can eliminate the border diffraction are applied and get satisfying result.(5) Digital holograms of static particles and particle field(multi-layer) are recorded in the experiment. And the reconstructed particle images are gained using the envelope method with two different reconstructed waves-plane wave and spherical wave, respectively.