Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically vacuum phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 10... Found on http://en.wikipedia.org/wiki/Photomultiplier
An electronic tube which generates an electrical current roughly proportional to the light intensity impinging on the tube. Photomultiplier tubes are typically used to measure small light signals from a scintillator or Cerenkov counter through which charged particles have passed.
Found on http://www-bdnew.fnal.gov/operations/accgloss/gloss.html