Backscattering of light from disklike particles: is fine-scale structure or gross morphology more important?

The backscattering of light from disklike objects possessing periodic structures (e.g., resembling a wheel with spokes, hereafter called a pinwheel) or an object with a wavelength-sized deviation from a flat disk (e.g., a spherical cap) has been computed by using the discrete dipole approximation. The disks ranged in diameter from 1.5 to 2.7 microm with thicknesses from 0.04 to 0.15 microm. The goal of the study was to obtain some understanding of the differences between the backscattering of a collection of such objects in random orientation and a collection of randomly oriented homogeneous disks of the same size, i.e., the conditions under which the gross morphology (e.g., disklikeness) of these objects determines their backscattering. The computations for pinwheels showed that their backscattering cross sections were nearly identical to those of homogeneous disks of similar size (but with reduced effective refractive indices that are easily estimated) as long as the maximum separations between the spokes was less than one quarter of the wavelength. In this regime the backscattering is totally governed by the particle's gross morphology and effective index. For larger spoke separation, departures from a homogeneous disk are observed and manifest as significant increases (many times) in backscattering. In the case of spherical caps with the same projected area as the associated disk, the computations again show a complete similarity in their backscattering, and when the disks are sufficiently thin (with thickness divided by wavelength<0.15 to 0.25) there is very little difference between the backscattering of a cap and the associated disk.