Digital Holography of Aerosol Particles
Methods to determine the physical properties of aerosol particles is important in a vast array of scientific and applied contexts. Due in part to the difficulty of collecting such particles, a variety of contact-free techniques have been developed that infer information about the particles in an indirect manner. A popular example is elastic light-scattering where the angular pattern of light scattered from a particle is analyzed to estimate particle properties like shape and size. This approach is often called the inverse problem, as there is generally no way to know if the inferred information is correct. Fundamentally, this is due to the loss of optical phase information in such measurements. An alternative approach is to image the particles using holography. By placing a digital image-sensor in an optical beam containing a particle, the interference pattern produced by the scattered and unscattered light can be easily measured. The pattern constitutes a digital hologram of the particle and useful information can be extracted from it directly since phase information is encoded in the hologram. For example, applying a Fourier-transform operation yields a silhouette-like image of the particle, thus revealing its shape and size without any need for a priori information. The extinction cross section can also be obtained from the hologram. Thus, digital holography “solves” the classic inverse problem. This talk will present our recent work in this area and its future applications, including plans for field measurements of coarse-mode atmospheric aerosols.