Electron emission is the liberation of electrons from the surface of a substance. It can occur as a result of thermionic emission, light (photoelectric effect, high electric field (field emission), or bombardment by ions or other electrons (secondary emission). It may also occur from radioactive decay (see radioactivity). In all cases, the electrons must acquire energy from the outside source in excess of the work function of the substance. Most electron tubes and cathode-ray tubes depend on thermionic emission, whereas photoelectric cells rely on photoelectric emission. Field emission is important in the field-emission microscope and secondary emission is the principle used by electron multipliers and storage tubes.