 Generally, these measurements involve fitting to Richardson's law, and so they must be carried out in a low temperature and low current regime where space charge effects are absent.In order to move from the hot emitter to the vacuum, an electron's energy must exceed the emitter Fermi level by an amount
determined simply by the thermionic work function of the emitter.If an electric field is applied towards the surface of the emitter, then all of the escaping electrons will be accelerated away from the emitter and absorbed into whichever material is applying the electric field.