Photodetection devices based on the photoelectric effect and charge multiplication by secondary emission.
Photomultipliers are vacuum tubes, where Light absorbed on a photocathode generates free electrons, which are subsequently accelerated with a high voltage, generate secondary electrons on other electrodes, and finally a usable photocurrent. Due to this avalanche process, the Photocurrent can be orders of magnitude higher than from a photodiode. Therefore, photomultipliers can be used for single photon counting. Photomultiplier tubes can be highly sensitive detectors with high linearity, high speed, and a wide dynamic range.