Backscattering

Backscattering is the process by which a portion of incident radiation or particles is scattered in a direction opposite to the incoming beam. It occurs in many wave or particle systems, including light, sound, X-rays, electrons, and neutrons, as they encounter interfaces, particles, or inhomogeneous media. The strength and angular distribution of backscattered energy depend on wavelength, the size and composition of scatterers, and the structure of the medium.

Quantities commonly used include the differential backscattering cross section, dσ/dΩ(θ), evaluated near θ = 180°, and the backscattering coefficient or backscatter ratio, which compare backscattered intensity to the incoming flux. In optical contexts, backscattering arises from Rayleigh and Mie scattering, as well as coherent backscattering, an interference enhancement in the exact back direction for multiple scattering in disordered media.

Applications span remote sensing, imaging, and diagnostics. In atmospheric lidar, backscattered laser light reveals aerosol and cloud properties. Radar and sonar systems exploit backscatter from objects to detect and characterize targets. In biomedical optics, backscattered light from tissue provides contrast for techniques such as optical coherence tomography. Backscatter X-ray imaging uses scattered X-rays to form images and is used in security screening. In seismology, backscattered seismic waves illuminate heterogeneities within the Earth.

Challenges include separating single- from multiple-scattering contributions, interpreting backscatter in heterogeneous media, and calibrating instruments to distinguish intrinsic backscattering from instrumental effects.