Photomultiplier tube (PMT) is a vacuum tube that is used for extremely sensitive light detection. When light strikes the photocathode, free electrons are produced a consequence of the photoelectric effect. These electrons are accelerated in the electric field and directed to a series of dynodes, where electrons are multiplied through the effect of secondary emission. When the electrons reach the anode, a current spike is produced and detected. Shown here is a circular-cage type PMT produced by Hamamatsu.