  An in-line hologram of a colloidal sphere can be analyzed with the Lorenz-Mie
theory of light scattering to measure the sphere's three-dimensional position
with nanometer-scale precision while also measuring its diameter and refractive
index with part-per-thousand precision. Applying the same technique to
aspherical or inhomogeneous particles yields the position, diameter and
refractive index of an effective sphere that represents an average over the
particle's geometry and composition. This effective-sphere interpretation has
been applied successfully to porous, dimpled and coated spheres, as well as to
fractal clusters of nanoparticles, all of whose inhomogeneities appear on
length scales smaller than the wavelength of light. Here, we combine numerical
and experimental studies to investigate effective-sphere characterization of
symmetric dimers of micrometer-scale spheres, a class of aspherical objects
that appear commonly in real-world dispersions. Our studies demonstrate that
the effective-sphere interpretation usefully identifies dimers in holographic
characterization studies of monodisperse colloidal spheres. The
effective-sphere estimate for a dimer's axial position closely follows the
ground truth for its center of mass. Trends in the effective-sphere diameter
and refractive index, furthermore, can be used to measure a dimer's
three-dimensional orientation. When applied to colloidal dimers transported in
a Poiseuille flow, the estimated orientation distribution is consistent with
expectations for Brownian particles undergoing Jeffery orbits.
