  It is known that compaction of granular matter can lead to anisotropic
mechanical properties. Recent work has confirmed the link to pore space
anisotropy, but the relation between compression, mechanical properties and
material microstructure remains poorly understood and new diagnostic tools are
needed. By studying the temporal and spatial characteristics of short optical
pulses diffusively transmitted through compacted granular materials, we show
that powder compaction can also give rise to strongly anisotropic diffusion of
light. Investigating technologically important materials such as
microcrystalline cellulose, lactose and calcium phosphate, we report increasing
optical anisotropy with compaction force and radial diffusion constants being
up to 1.7 times the longitudinal. This open new and attractive routes to
material characterization and investigation of compression-induced structural
anisotropy. In addition, by revealing inadequacy of isotropic diffusion models,
our observations also have important implications for quantitative spectroscopy
of powder compacts (e.g., pharmaceutical tablets).
