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.

Measuring extinction with digital holography: noisy holograms.

Digital in-line holography is a versatile method to obtain lens-less images of small particles, such as aerosol particles, ranging from several to over one hundred microns in size. It has been shown theoretically, and verified by measurement, that a particle's extinction cross section can also be obtained from a digital hologram. The process involves a straightforward integration, but if noise is present it fails to give accurate results. Here we present a method to reduce the noise in measured holograms of single particles for the purpose of rendering the cross-section estimation more effective. The method involves masking the complex-valued particle image-amplitude obtained from a noisy hologram followed by a Fresnel transformation to generate a new noise-reduced hologram. Examples are given at two wavelengths, 440 nm and 1040 nm, where the cross section is obtained for a micro-sphere particle and several non-spherical particles approximately 50 microns in size.

Multispectral small-angle light scattering from particles.

Using a supercontinuum laser, reflective optics, and a spatial filter, we measure two-dimensional small-angle light-scattering patterns for a variety of microparticles including spheres, salt, sand, and volcanic dust. The measurements are done at 13 wavelengths from 450-850 nm, where the absence of refractive optical elements minimizes the effects of chromatic aberration. Qualitative particle-material sensitivity is demonstrated by layering differently colored patterns. Last, the multispectral capability of our device demonstrates a new possibility to probe different q-space regimes for a given particle in a single measurement.

Two-dimensional small-angle scattering from single particles in infrared with a lensless technique.

An experiment is described where two-dimensional small-angle light scattering (2D-SALS) patterns from single particles are measured in the infrared through a lens-free approach. Spatial filtering is employed to separate scattered light from unscattered light to within approximately one degree from the forward direction. Non-planar reflective elements are used in the filtering process, permitting 2D-SALS measurements to be done without chromatic aberrations over a broad spectral range and from 0.8 to 8 degrees in the polar scattering angle and zero to 360 degrees in the azimuthal angle. Patterns from spherical microparticles are presented along with nonspherical particles including volcanic ash and salt. An asymmetry analysis is applied to demonstrate an ability to differentiate spherical from nonspherical particles from the 2D-SALS patterns.