Measuring spectral extinction with digital holography.

The optical extinction caused by a small particle, such as an aerosol particle, is an important measurable quantity. Understanding the influence of atmospheric aerosols on the climate, assessing visibility in urban environments, and remote sensing applications such as lidar all need accurate measurements of particle extinction. While multiple methods are known to measure extinction, digital in-line holography (DIH) features the unique ability to provide contact-free images of particles simultaneously with estimates for the extinction cross section. This is achieved through an integration of a measured hologram followed by an extrapolation. By means of a supercontinuum laser, we investigate the measurement of the cross section via DIH for stationary particles across a broad spectrum, from 440 nm to 1040 nm. The particles considered include a 50 µm glass microsphere, a volcanic ash particle, and an iron(III) oxide particle. The results show the ability to estimate a particle's cross section to within 10% error across portions of the spectrum and approximately 20% error otherwise. An examination of the accompanying hologram-derived particle images reveals details in the images that evolve with wavelength. The behavior suggests a basic means to resolve whether absorption or scattering dominates a particle's extinction.