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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.