photomultiplier

Instrument that detects low levels of electromagnetic radiation (usually visible light or infrared radiation) and amplifies it to produce a detectable electrical signal.

One type resembles a photocell with an additional series of coated electrodes (dynodes) between the cathode and anode. Radiation striking the cathode releases electrons (primary emission) which hit the first dynode, producing yet more electrons (secondary emission), which strike the second dynode. Eventually this produces a measurable signal up to 100 million times larger than the signal produced in the first stage by the time it leaves the anode. Similar devices, called image intensifiers, are used in television camera tubes that ‘see’ in the dark.