1. 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. == Important Properties == === Quantum Efficiency (QE) === Quantum efficiency is a unitless number that measures the sensitivity of the photocathode to light. It is the ratio of the number of electrons emitted to the number of incident photons. This property depends on the wavelength of light being used to illuminate the photocathode. For many applications, QE is the most important property as the photocathodes are used solely for converting photons into... from wikipedia.org