Photomultiplier Tube

A type of phototube, an electronic device which emits electrons when exposed to light. The phototube and the principle of photoemission are based on quantum mechanics, specifically the science of dealing with the light as discrete "quanta," or "particles" of energy (in contrast to the sciences of color and photography, which treat light in terms of waves). In a phototube, a "photon" of light strikes a metallic surface, which contains electrons. As the photon hits an electron, it disappears, imparting all of its energy to the electron. This additional energy causes the electron to flow from the metallic surface. And it is the flow of electrons that produces an electric current. The electrons are collected by an anode which then allows the electric current produced to accomplish whatever it was designed to accomplish. In a photomultiplier tube, the electrons released by a phototube hit an additional electrode called a "dynode," which generates a set of secondary electrons. This secondary set of electrons is much greater in number than the original number of electrons hitting it. These secondary electrons hit a second dynode, generating even more electrons, and so on for several stages, until finally being collected by an anode. Essentially, then, the photomultiplier tube converts a small amount of light into a strong electric current.

In digital imaging devices, in particular drum scanners, photomultiplier tubes are used to sense very low levels of light and amplify them into strong electronic signals, which can then be converted to digital information describing color values. It is the high sensitivity of the photomultiplier tube that allows drum scanners to excel at producing color separation, as even small amounts of tonal variation can be detected.

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