A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect.  Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams.  Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier, phototube and image intensifier.