Multi-dimensional particle sizing techniques

Two techniques for multi-dimensional sizing of spherical particles are discussed and compared with one another: interferometric particle imaging (IPI) and a novel technique known as global phase Doppler (GPD). Whereas the IPI technique is known from various previous studies and uses a laser light sheet illumination of the particle field, the GPD technique is a new method and employs two intersecting laser light sheets. The resulting far-field interference pattern arises from the interference of like scattering orders from the particle, similar to the phase Doppler technique. A description of this far-field interference is given for both techniques. Both multi-dimensional particle sizing techniques sample the scattered light in the far-field by means of a defocused imaging system. The diameter of each droplet illuminated by the laser light sheet(s) is determined by measuring the angular frequency of the interference fringes in the defocused images. Combined with a pulsed laser, the technique also allows the velocity of the particle to be determined, similar to particle tracking velocimetry (PTV). However, the size of the defocused image of each particle also depends on the position of the particle perpendicular to the laser sheet, hence, with appropriate calibration, the third component of velocity is also accessible. The two techniques, IPI and GPD, are compared to one another in terms of implementation and expected accuracy. Possibilities of combining the two techniques are also discussed. Some novel approaches for the signal processing have been introduced and demonstrated with simulated and real signals.