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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.

Figure 2. Holographic Configuration to record the light scattered by particles
Figure 2. Holographic Configuration to record the light scattered by particles

Figure 3 illustrates the two-spot mode showing the foci of monodisperse droplets flowing vertically out of a droplet generator. The separation between the two spots provides an accurate measure of the droplet diameter.

The fringe mode regime is illustrated in Figure 4 where droplets produced by a household humidifier are represented by a set of fringes in each image and the number of fringes correlates accurately with the size of each droplet.

Holography provides a unique opportunity to choose the best measurement strategy for each particle. That is, the large particles could be measured from the separation between the two spots while the small particles could be measured from their fringe pattern.

This method may be used for applications such as automobile fuel spray, gas turbine fuel spray, fire extinguisher spray, and agricultural sprinklers.

Figure 3. Refraction and reflection spots of monodisperse droplets of 145 Ám diameter flowing out of a droplet generator.
Figure 3. Refraction and reflection spots of monodisperse droplets of 145 μm diameter flowing out of a droplet generator.

Figure 4. Images corresponding to small drops (2 to 12 Ám) produced by ultrasonic humidifier.
Figure 4. Images corresponding to small drops (2 to 12 μm) produced by ultrasonic humidifier.

Experimental Results of Calibrated Glass Beads

Holograms of glass reference spheres (40μm – 2.8μm NIST traceable glass micro-spheres) dispersed in moving air were acquired and analyzed using the two-spot technique and the fringe method. Both of these methods are possible on the same holographic data by zooming in and out of focus. A 1951 USAF resolution target was placed in the probe volume and digitized. This provided the means to obtain accurate calibration against a known standard. The following table provides a sample of the results.

Table

Tabulated data of 40 μm glass micro-spheres.

NIST Dia. (μm) Measured Spot Mode Measured Fringe Mode
40 ± 2.8 38.9 36.9
40 ± 2.8 38.9 38.7
40 ± 2.8 40.1 42.2
40 ± 2.8 40.1 42.2
40 ± 2.8 38.9 38.7
40 ± 2.8 40.1 40.4
40 ± 2.8 42.4 45.7
40 ± 2.8 40.1 40.4
40 ± 2.8 40.1 40.4
40 ± 2.8 40.1 42.2
40 ± 2.8 41.2 43.9
40 ± 2.8 40.1 42.2
40 ± 2.8 38.9 38.7
40 ± 2.8 40.1 42.2
40 ± 2.8 38.9 38.7
40 ± 2.8 40.1 42.2