Photomultipliers are extremely sensitive detectors of
light in the ultraviolet, visible and near infrared.
They are a type of vacuum tube in which photons
produce electrons in a photocathode in consequence of
the photoelectric effect and these electrons are
subsequently amplified by multiplication on the surface
of dynodes. A signal is produced on the anode of the device.
Amplification can be as much as 10^8 meaning that measurable
pulses can be obtained from single photons. The combination
of high gain, low noise, high frequency response and large
area of collection have meant that these devices still find
applications in particle physics, astronomy and
medical imaging. Their replacement by semiconductor devices
has long been predicted but has never been achieved.
