photocathode

A photocathode is a type of instrument (or instrument part) specifically to react to incident EMR with the photoelectric effect, as a means of detecting light or to serve as a source of free electrons. Before photodiodes and CCDs were practical for many applications, photocathodes were more commonly used, e.g., in television cameras. Photocathodes have served as an element of image intensifiers, devices that make images brighter, but essentially the same technology (still termed an image intensifier) is also used for devices whose purpose is to shift the wavelengths of the image, e.g., to make an X-ray image visible, or to adapt sensors that sense one spectral band to sense another.