  The geometric substructure of the particle-emitting source has been
characterized via two-particle interferometry by the STAR collaboration for all
energies and colliding systems at RHIC. We present systematic studies of
charged pion interferometry. The collective nature of the source is revealed
through the $m_T$ dependence of HBT radii for all particle types. Preliminary
results suggest a scaling in the pion HBT radii with overall system size, as
central Au+Au collisions are compared to peripheral collisions as well as with
Cu+Cu and even with d+Au and p+p collisions, naively suggesting comparable flow
strength in all systems. To probe this issue in greater detail,
multidimensional correlation functions are studied using a spherical
decomposition method. This allows clear identification of source anisotropy
and, for the light systems, the presence of significant long-range
non-femtoscopic correlations.
