Tutorial: Aerosol characterization with digital in-line holography

Abstract

The purpose of this tutorial is to teach the method of digital in-line holography (DIH) as a means to characterize aerosol particles in an in-situ manner. The scope is limited to the simplest, yet powerful, lens-less implementation, which is the most intuitive and forgiving of the varieties of digital holography. By illuminating free-flowing aerosol particles with a pulsed, expanded, collimated laser beam, one can record the interference pattern of scattered and unscattered light on an image sensor. The resulting digital hologram can then be used in a diffraction calculation to render a silhouette-like image of the particle, or particles, present in the beam. A key advantage of imaging with DIH over conventional imaging, e.g. microscopy, is that particles located at different positions can be brought into focus from a single hologram measurement. Useful information can be obtained from such images including the number of particles present, their size, shape, and orientation. Notably, this is all achieved without the need to collect or otherwise trap particles. The inherent motion of aerosol particles imposes specific constraints on the design and performance of DIH imaging, and in particular, restricts the resolution in comparison to applications involving stationary or otherwise controlled particles. As such, DIH is currently most effective for particles larger than several micrometers in size. The tutorial will explain the theoretical basis of DIH, illustrate how particle images are generated computationally from hologram measurements, provide code in the Mathematica language to do so, and discuss several limitations imposed by hardware. Also reviewed will be a method to approximate a particle’s extinction cross section and two-dimensional angular light scattering pattern.