Photomultiplier tube is an extremely sensitive electric eye vacuum tube. Most of these tubes are more sensitive to light than is the human eye. Photomultipliers are used to measure very weak light. For example, physicists use them in scintillation counters to measure the light given off by cosmic rays, nuclear particles, gamma rays, or X rays.
A photomultiplier has a photocathode, which gives off electrons when light strikes it. These electrons then strike the first of a series of plates called dynodes. As the electrons bounce from plate to plate, they knock an ever-increasing number of electrons from each plate. Several million electrons may leave the tube for every electron given off by the photocathode. The tube thus multiplies the effect of the light that strikes it and enables the brightness of the light to be measured with extreme accuracy.
