Sizing submicron suspended particles by using oblique illumination to measure optical scatter

As is widely known, microbes play a very important role in marine ecology such as in the microbial loop and the subsequent incorporation into the food chain. As such, sizing and counting microbes is a fundamental element of the study of the ocean ecology. Despite this, the detection and sizing of submicron particles has been problematic. In order to provide a technology to solve this problem we have developed a method to size submicron particles suspended in water. The method is based on an optical scattering measurement that combines an oblique incidence structure and high numerical aperture lenses. The data is then analyzed using a model that incorporates the physics of scatter from these small particles. The Oblique Illumination Scattering Meter (OLSM) measures scatter from a single particle over both the entire forward and backward directions. The scattered intensities are integrated respectively to calculate a ratio (R) of forward to backward scatter. Based on this ratio and its relationship to particle scatter, the particle diameter (D) can be estimated. Both theory and experimental results from a lab setup show that R monotonically increases with D from 0.1 to 0.8 microns. Results indicate that the size distribution of polystyrene beads down to 0.2 micro-meter diameter can be accurately retrieved from the experimental data. Currently, simulations, find that the relationship is valid for a variety of different refractive index of the particles, smaller than 1.47 and is independent of particle absorption.