Abstract: Size, position, and velocity of particles or droplets in 3D- space can be measured contactlessly by in-line holography. In digital in-line holography the holograms are recorded by a CCD-array and stored digitally, the reconstruction of the complex wavefields is performed numerically in the computer. By an arrangement allowing the laser beam to pass several times through the particle field before hitting the CCD, several angular views are recorded simultaneously. In the reconstruction stage the viewed are separated by numerical focusing to the different distances, and by a tomography- related method the 3D-positions of the particles are determined. Double pulse recording enables to determine also the velocity of the particles. The paper gives a short introduction to digital holography, explains several holographic arrangements for multipass in-line holography, presents some experimental results of position and velocity measurements, and discusses some typical errors.!11
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