Production of transparent ceramics has become a topic of resurgent interest in recent years, with its promise of near-net shaping appealing to applications ranging from biomedicine to solar energy. However, the mechanisms governing ceramic transparency, translucency, and opaqueness are not entirely understood. Models of both grain boundary and pore scattering have been proposed, but too often without sufficient experimental corroboration. An extensive experimental analysis of transparent alumina samples is presented, establishing a first direct link between the observed transparency, defect size, and porosity. Given the unprecedented experimental detail from the full 3D pore reconstruction from the FIB tomography, how to correctly interpret the experimentally observed transparency is additionally shown. The unprecedented experimental and theoretical agreement for the first time identifies the relative contributions of different scattering mechanisms, thereby paving the way forward for microstructural tuning of transparent polycrystalline alumina.