Scattering of partially coherent electromagnetic beams by water droplets and ice crystals

- The Lorenz-Mie theory is generalized for a case when the light source is partially coherent.
- The generalized Mueller matrix is studied analytically using vector incident field.
- Analytical results are validated by DDA simulation.
- The formalism is applied to cases of water droplets and ice crystals in clouds.
- We found backscattering is sensitive to the coherence length of the incident light.

The conventional Lorenz-Mie theory is generalized for a case when the light source is partially spatially coherent. The influence of the degree of coherence of the incident field on the generalized Mueller matrix and the spectral degree of coherence of the scattered light is analytically studied by using the vector field instead of the scalar field to extend previous results on the angular intensity distribution. The results are compared with the Mueller matrix obtained from the Discrete Dipole Approximation (DDA) method, which is an average over an ensemble of stochastic incident beams. Special attention is paid to the Mueller matrix elements in the backward direction, and the results show some Mueller matrix elements, such as P22, depend monotonically on the coherence length of the incident beam. Therefore, detecting back scattering Mueller matrix elements may be a promising method to measure the degree of coherence. The new formalism is applied to cases of large spherical droplets in water clouds and hexagonal ice crystals in cirrus clouds. The corona and glory phenomena due to spheres and halos associated with hexagonal ice crystals are found to disappear if the incident light tends to be highly incoherent.