 Once sufficient thermal vibrational energy is attained electrons may overcome this electrostatic energy barrier, allowing them to travel into vacuum and accelerate down the lens column to the gun potential (because the lenses are at ground).In field emission, rather than heating the tip to vibrationally excite electrons from the surface, the source tip (usually tungsten) is sharpened to a small point such that when large electric fields are applied, they concentrate at the tip, lowering the barrier to escape the surface as well as making tunneling of electrons from the tip to vacuum level more feasible.Condenser/illumination optics are used to focus electrons leaving the electron gun and manipulate and/or translate the illumination electron beam.