The authors report impact properties for collisions of small, nearly s
pherical particles that present interesting experimental challenges. T
hey consider difficulties arising with surface reflectivity, slight as
phericity, surface damage and collisions with particles affixed to a r
igid plate. To measure these impact properties, the authors refine the
experimental technique of Foerster et al. To permit straightforward i
ncorporation in rapid granular theories, the impacts are described wit
h three coefficients. The first is the Newtonian coefficient of normal
restitution. The second represents the frictional properties of the c
ontact surfaces. The last characterizes the restitution of the tangent
ial component of the contact point velocity for impacts that involve n
egligible sliding.