Near-field scattering from red pigment particles: Absorption and spectral dependence

When film coatings are made of pigment particles embedded in a transparent resin, the optical characteristics of the resulting film are determined not only by the bulk optical properties of the constituent materials, but also by the spatial distribution of the light scattered from small particles. I f the particles are separated by distances comparable to their diameter, as is the case for high particle concentrations or agglomerated systems, the near-field interactions between the radiation fields of the particles can s trongly influence the resulting far-field intensity distribution. In this w ork we have used full-field finite element solutions of Maxwell's equations to calculate the near- and far-field scattering patterns for single 500 nm quinacridone spheres and for pairs of particles. We find that the scattere d intensity forms a forward-directed plume that extends far beyond the part icle surface, especially at short wavelengths and where the absorption is l arge. This results in near-field interactions between pairs of particles th at can either increase or decrease the scattering (both the total scatterin g and the fraction of the scattered light that is directed into the backwar d hemisphere), depending on the orientation of the particle pair relative t o the direction of the incident light. In some cases, particularly if the p articles are aligned along the incident direction, the two spheres can prod uce a far-field scattering distribution that is approximately that of a sin gle, larger (sometimes much larger) sphere. If the particles are aligned pe rpendicular to the incident direction, the strength of the scattering per p article volume is roughly the same as for a single particle, but the scatte ring is more forward directed. These interaction effects occur even though the surface-to-surface separation of the particles is larger than the dista nce for which a single particle shows significant scattered intensity. Thes e near-field and far-field phenomena are beyond the limitations of single-s cattering and independent multiple-scattering approaches, and the near-fiel d interactions can have a significant effect on the scattering of light fro m films containing such particles, especially if they tend to form oriented clusters. (C) 2001 American Institute of Physics.