If a crystal is cut along a certain plane, then the atoms near the surface may well be disturbed from their equilibrium positions in the bulk. This leads to changes in the relative positions of the surface and near surface atoms (surface reconstruction). Such changes can be explored with LEED. The surface crystallographic structure can be determined by bombarding the surface with low energy electrons (approx. 10-200 eV) and observe diffracted electrons as spots on a phosphorescent screen. A Retarding Field Analyser (RFA) is used to detect the electrons. The relative position of the spots on the screen shows the surface crystallographic structure. The diffracted spots will move as the energy of the incident electrons changes, and the intensity of the spots as a function of incident electron energy reveals much about surface reconstructions. In order to determine the true surface structure, much computer analysis needs to be done.