  We investigate the shear of a dense bed of refracting supermillimetric grains
confined within a transparent horizontal annular cell with a rotating top. The
local time correlation functions of interferometric images allow to
characterize the shear profile close to the wall, with a spatial resolution
well below the grain diameter. For increasing shear, we observe a transition in
the system response and the manifestations of anisotropies in the force chains.
The employed technique is of easy implementation and is especially suitable for
the study of stationary processes.
