Determining Bulk Aerosol Absorption from Off Axis Backscattering using Rayleigh Beacon Laser Pulses

Aerosol absorption and scattering can play a key role in degrading high energy laser performance in the form of thermal blooming and beam attenuation. Aerosol absorption properties are not completely understood, and thus affect how we are able to quantify expected high energy laser weapon performance. The Air Force Institute of Technology Center for Directed Energy (AFIT CDE) developed both Laser Environmental Effects Definition and Reference (LEEDR) and the High Energy Laser End-to-End Operational Simulation (HELEEOS) code to characterize atmospheric radiative transfer effects and evaluate expected directed energy weapon system performance. These packages enable modeling of total irradiance at given off-axis locations through the use of an off-axis scattering algorithm, which uses scattering phase functions to predict the amount of radiation scattered from the beam toward a particular observation location. Laser energy scattered from a Rayleigh beacon illuminator at 527 nm, located at the John Bryan Observatory (JBO) in Yellow Springs, Ohio, is measured using a high resolution Mini- Shortwave Infrared (SWIR) snapshot camera. Aerosol characterization information was gathered using a Condensation Particle Counter (CPC), a Scanning Mobility Particle Sizer (SMPS) spectrometer, and a Continuous Light Absorption Photometer (CLAP). The differences in the measured versus predicted phase functions can help draw conclusions relative to bulk aerosol absorption properties, and lead to better quantification of degradation of laser performance due to aerosols