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Particle Analysis/Planar Imaging Particle Analyzer

Particle Analysis/Planar Imaging Particle Analyzer

Under an SBIR program from the Army Research Office MetroLaser developed a technique to measure the size distribution of droplets in a plane. By measuring droplets on a plane, instead of point by point, the user is able to map the distribution of droplets of the entire plane in a single measurement. Multiple measurements of the same plane, normally needed to improve on statistics, can easily be obtained with a high repetition rate pulsed laser. The technique referred to as Planar Imaging Particle Analyzer (PIPA), uses a pulsed laser to form a laser sheet to illuminate a large number of droplets that are typically moving in a flow (such as a spray). The particle images are composed of two scattering components (refracted and reflected light) and appear either as two point light sources (the two-spot mode) or as a fringe pattern (the fringe mode). The particle diameter of each droplet in the illumination plane may be obtained from either the separation between the two spots or from the number of fringes contained within each image.

Figure 1 illustrates this effect. A particle of diameter d moves from right to left and refracts and reflects light as it intercepts the laser beam.

Figure 1. Schematic of the Planar Imaging Particle Analyzer
Figure 1. Schematic of the Planar Imaging Particle Analyzer

Recording the scattered light with holography as indicated in Figure 2 offers the choice between analyzing the particle images at their focus (where they appear as two spots) or out of focus (where they appear as a set of fringes). The choice depends on the resolution of the recording medium, the particle size, and the particle concentration.