  We describe here how to apply the three dimensional imaging technique of
refrecative index matched scanning to hydrogel spheres. Hydrogels are water
based materials with a low refractive index, which allows for index matching
with water-based solvent mixtures. We discuss here various experimental
techniques required to handle specifically hydrogel spheres as opposed to other
transparent materials. The deformability of hydrogel spheres makes their
identification in three dimensional images non-trivial. We will also discuss
numerical techniques that can be used in general to detect contacting,
non-spherical particles in a three dimensional image. The experimental and
numerical techniques presented here give experimental access to the stress
tensor of a packing of deformed particles.
