Electrons from an electron gun diffract off the sample and head back towards the electron gun and the grids surrounding it. The two middle grids are set at an energy slightly less than the electron beam to prevent any inelastically scattered electrons from reaching the screen. The screen is biased positive between 0 and 7 keV accelerating the electrons towards it. The screen is coated in phosphor which glows green when electrons hit it. The resulting LEED pattern is viewed from the rear end of the apparatus.
