Coarse-Mode Particle Characterization With Digital Holograph

Abstract:

The recent availability of high resolution optoelectronic sensors has revived holography as a useful technique to study aerosol particles. By placing a two-dimensional detector in a collimated laser beam, the interference pattern produced by this light and that forward-scattered by a particle in the beam can be easily measured. This pattern is the particle’s in-line hologram and information can be extracted from it directly. For example, applying a Fourier-transform operation to the hologram yields a silhouette-like image of the particle, thus revealing its size and shape without a priori information. In this sense, digital holography “solves” the classic inverse problem in applied light scattering. Moreover, this measurement can be done in situ and applied to flowing aerosol particles using pulsed illumination. In recent work, we have discovered there is also an inherent link between a particle’s extinction cross section and the integral of the hologram. Using Mie theory, we have shown this relationship for a variety of spherical particles. The generality of the concept, however, suggests that it applies to nonspherical particles as well. In this presentation we will show that, indeed, the extinction cross section can be extracted from the holograms produced by such particles. Specifically, we investigate prolate and oblate spheroids and cubical particles in the wavelength-size range. Using both the discrete dipole approximation and the T-Matrix method, when applicable, we are able to simulate the holograms and then perform a simple integration to yield the cross sections. We will also present our ongoing experimental work applying this technique to coarse-mode aerosol particles.