Editing Photoelectric effect (section)

===Night vision devices===
Photons hitting a thin film of alkali metal or [[semiconductor]] material such as [[gallium arsenide]] in an [[image intensifier]] tube cause the ejection of photoelectrons due to the photoelectric effect. These are accelerated by an [[electrostatic field]] where they strike a [[phosphor]] coated screen, converting the electrons back into photons. Intensification of the signal is achieved either through acceleration of the electrons or by increasing the number of electrons through secondary emissions, such as with a [[micro-channel plate]]. Sometimes a combination of both methods is used. Additional kinetic energy is required to move an electron out of the conduction band and into the vacuum level. This is known as the [[electron affinity]] of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the [[band gap]] model. Some materials such as gallium arsenide have an effective electron affinity that is below the level of the conduction band. In these materials, electrons that move to the conduction band all have sufficient energy to be emitted from the material, so the film that absorbs photons can be quite thick. These materials are known as negative electron affinity materials.{{citation needed|date=November 2023}}