A photomultiplier tube (abbreviated to PMT or photomultiplier) is a type of device to sense electromagnetic radiation that works well for visible light and adjoining portions of ultraviolet and infrared. They were first devised before the advent of solid state devices (e.g., transistors), and were commonly used in photometry (photoelectric photometry or PEP), now being less common with the advent and development of CCDs, but are still used in some situations.
A PMT consists of a vacuum tube including a light-sensitive surface that emits an electron when a photon strikes it (photoelectric effect), and mechanisms that emit multiple electrons for each that they absorb. A series of such mechanisms repeatedly increase the number of electrons until a detectable voltage is generated. The quantum efficiency (percent of photons thus sensed) can be 30% or more, an improvement over photographic plates which were generally 10% or less, but specialized CCDs can do much better.
An image dissector (or image dissector photomultiplier) is essentially a PMT with added features to sense images, i.e., in two dimensions rather than a single pixel. The image is focused on the light-sensitive surface, and electrons from just a portion of that surface are focused and beamed using electromagnets into the series of electron-multiplication mechanisms. Manipulating the electromagnets allows selection of which portion of the image is being sensed, and the whole image can be scanned. The technology is related to CRT-based television. These were especially useful over the decade prior CCDs becoming ubiquitous.